SUBJECT 03 – AIRCRAFT MATERIALS

1.5.3 Disadvantage of titanium when subjected to very high temperatures.The ultimate and yield strength of titanium drop fast above 800˚F. The absorption of oxygen and nitrogen from the air at temperatures above 1,000°F makes the metal so brittle on long exposure that it soon becomes worthless.

2. Testing of Materials

2.1 Testing Terminology

2.1.1 Explain tensile and compression testing of metals with particular regard to determining by calculation:

a. Ultimate tensile stress - is the maximum resistance to fracture.   It is equivalent to the maximum load that can be carried by one square inch of cross-sectional area when the load is applied as simple tension.   It is expressed in pounds per square inch: UTS = “maximum load” divided by “area of original cross section”

b. Yield point - The yield point, determined by the divider method, involves an observer with a pair of dividers watching for visible elongation between two gage marks on the specimen. When visible stretch occurs, the load at that instant is recorded, and the stress corresponding to that load is calculated.

c. Percentage elongation – is calculated as the change in length divided by the original length.

d. Young’s Modulus of elasticity - Young's  Modulus  (sometimes  referred  to as  Modulus  of  Elasticity,  meaning  "measure"  of elasticity). It is the ratio of  stress  to  strain  (the  measure  of  resistance  to  elastic deformation). To calculate Young's Modulus, stress (at any point) below the proportional limit is divided by corresponding strain.   It  can  also  be calculated  as the  slope  of  the  straight-line portion of the  stress-strain curve.

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2.1.2 Calculate the tensile strength of a material under test from given data:Ultimate Tensile Strength = Max Load divided by the area of the original cross section area.

2.2.5 Identify on the Rockwell Scale the notation for hard and soft material and how the Rockwell number is determined for a specimen under test.

For hard steels, the hardness is read on the C scale. When this reading is recorded, the letter C must precede the number indicated by the pointer. If the metal is softer than C-20, the B-scale setup is used, and the hardness is read on the B scale.

2.3 Strength versus HardnessThe relationship between tensile strength and hardness is directly related.

2.5 Testing of Aluminium

2.5.1:

3. Common Properties of Metals

3.1 Terminology

a. Hardness - the resistance of a material to deformation, particularly permanent deformation, indentation, or scratching.

b. Malleability – the ability of the material to be stretched or shaped by rolling or hammering it. Malleable materials do not crack when they are formed in this way e.g. Gold and Lead.

c. Ductility – the property of the material that allows it to be drawn into a thin section without breaking e.g Copper has great deal of ductility.

d. Elasticity - that property of materials that causes them to return to their original form or condition after the applied force is removed.

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e. Toughness- the characteristic of a material that allows it to be deformed by twisting, pulling, hammering or bending without its breaking.

f. Density – mass per unit volume.

g. Brittleness - the tendency of a material to break at a very low strain, elongation, or deflection, and to exhibit a clean fracture surface with no indications of plastic deformation. Opposite of malleability.

h. Fusibility – the ability of metal to be joined by heating and melting is defined as fusibility. Correct term is fusion joining or welding.

i. Conductivity – property which enables a metal to carry heat or electricity.

j. Contraction - linear strain associated with a decrease in length.

k. Thermal Expansion – the property of the metal to expand when heated and shrink when cooled.

l. Electrical & Magnetic properties – ability of the material to allow electron flow.

m. Metal Cladding – a method of protecting aluminum alloys from corrosion by rolling a coating of pure Al onto the surface of the alloy.

n. Typical Melting points –

3.1.2 - ************************

4. Factors affecting the selection of Aircraft Material

4.1 Properties of Materials.

a. Strength - the property of a material that resists deformation induced by external forces.

b. Strength to Weight Ratio – the relationship between the strength of a material and its weight per cubic inch, expressed as a ration. This ratio forms a basis for comparing the desirability of various materials for use in airframe construction and repair.

c. Compression Strength – ability of a material to resists the forces that try to squeeze the ends of an object together.

d. Tensile Strength – the strength of the material that opposes the stresses which try to stretch or lengthen it.

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e. Yield Strength – the ability of a material to resist deformation, e.g. when a tensile load is applied to a material, the material resists any deformation until its yield point is reached.

f. Shear Strength – describes a metal’s ability to resist opposing forces, e.g. a rivet holding two sheet-metal together.

g. Bearing Strength – is the ability of a joint to withstand any form of crushing or excessive compressive distortion.

h. Fatigue Strength - maximum stress on an externally threaded fastener which can be tolerated for a specified number of repeated cycles prior to failure.

i. Metal Stresses – 1. Tension – uniaxial force tending to cause the stretching of a material.2. Compression – a resultant of two forces, acting in the sam plane, but in the opposite directions, toward each other.3. Shear – to cut by causing the parts to slide over each other.4. Bending – natural bending.5. Torsion – twisting force.

j. Failure Modes –

k. Corrosion Resistance – ability to resist corrosion.

5. Heat-Treatment of Aircraft Steels and Light Alloys

5.1 Steel

a. Hardening -

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Oil provides the slowest quench and brine the most rapid. Hardening increases the hardness and strength of the steel but makes it less ductile.

b. Hardening Precautions –

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c. Tempering –

reduces the brittleness imparted by hardening. Tempering always follows, never precedes, the hardening operation. Tempering softens the steel. It is always conducted at temperatures below the low critical point of the

steel. Tempering differs from annealing, normalizing, or hardening, all of which require temperature above the upper critical point.

Generally, the rate of cooling from the tempering temp has no effect on the resulting structure; therefore, steel is usually cooled in still air after being removed form the furnace.

d. Annealing –

Produces a soft, ductile metal without internal stresses or strains. In the annealed state, Steel has its lowest strength. Is the opposite of Hardening.

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Is accomplished by heating the metal to just above the upper critical point, soaking at that temp and cooling very slowly in the furnace.

Soaking time is approximately 1 hour per inch of thickness of material. Slow cooling is achieved by cooling in the furnace (with the heat source

removed); cooling in still air; bury the metal in ash, sand or other substance that does not conduct heat readily.

e. Normalizing –

Removes the internal stress set up by forging, welding, casting or forming. Because of better physical properties, aircraft steel is used in the

normalized state, but seldom, if ever in the annealed state. Is accomplished by heating the steel above the upper critical point and

cooling in still air. Rapid cooling in still air results in a harder and stronger material than that obtained by annealing.

Most important uses of normalizing in aircraft work is on welded parts.

f. Casehardening by Nitriding or carburizing

Produces a hard, wear resistant surface or case over a strong, tough core. Ideal for parts which require a wear resistant surface and at the same time

must be tough enough internally to withstand the applied loads. Steels best suited to casehardening are the low carbon and low alloy

steels. If high carbon steels are casehardened, the hardness penetrates the core and causes brittleness.

In casehardening, the surface of the metal is changed chemically by introducing a high carbide or nitride content. The core is unaffected chemically. When heat treated, the surface responds to hardening while the core toughens

Common forms of casehardening are carburizing and nitriding. Carburizing – In this process, carbon is added to the surface of low carbon

steel. Thus, a carburized steel has a high carbon surface and a low-carbon interior.

Nitriding – is unlike other processes in that, before Nitriding, the part is heat treated to produce definite physical properties. Thus, parts are hardened and tempered before being nitrided. Most steels can be nitrided, but special alloys are required for best results. These special alloys contain aluminum as one of the alloying elements and are called “nitroalloys”. Nitriding can be accomplished with a minimum of distortion.

g. Parkerizing – a process in which a steel part is covered with a hard oxide film that prevents the oxygen from reaching the metal. The oxide film is formed by soaking the steel in a solution of phosphoric acid and manganese or zinc dioxide.

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h. Behaviour of Steel during heating and cooling – Changing the internal structure of a ferrous metal is accomplished by

heating to a temperature above it’s critical point, holding at that temperature for a time sufficient to permit certain internal changes to occur, and then cooling to atmospheric temperature under predetermined, controlled conditions.

i. Heating and Soaking –Heating – the objective is to transform pearlite to austenite as the steel is

heated through the critical range.Soaking – The temperature of the furnace must be held constant during

the soaking period, since it is during this period that re-arrangement of the internal structure of the steel takes place. Heavier parts require longer soaking to ensure equal heating throughout.

j. Protective Atmospheres – It is often necessary or desirable to protect steel or cast iron from surface

oxidation (scaling) and loss of carbon from the surface layers (decarburization). Many surfaces are equipped with atmosphere control.

k. Quenching and Cooling – The rate of cooling through the critical range determines the form that the

steel will retain. Still air is a slow cooling medium, but is much faster than furnace cooling. Liquids are the fastest cooling media and are therefore used in hardening steels. There are three commonly used quenching liquids – brine, water and oil. Brine is the most severe (fastest) and oil is the least severe (slowest). Generally, an oil quench is used for alloy steels, and brine or water for carbon steels. Quenching solutions act only through their ability to cool the steel and have no beneficial chemical action on the quenched steel and impart no unusual properties.L – N: Omit

5.1.2

At ordinary temperatures, the carbon in steel exists in the form of particles of iron carbide scattered throughout an iron-matrix known as “ferrite”. At elevated temperatures, the carbon is dissolved in the iron matrix in the form of a solid solution called “austenite”. The fact that the carbide particles can be dissolved in austenite is the basis of the heat treatment of steel

5.1.3

The temperatures at which the transformation takes place during heat treatment of steel are called “critical points”. The element having the greatest influence is Carbon. These critical points are the temperature at which the internal structure of metal takes on a crystalline form.

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5.1.4

Heat treatment is a series of operations involving the heating and cooling of metals in their solid state. Its purpose is to make the metal more useful, serviceable and safe for a definite purpose. By heat treating the metal can be made harder, stronger and more resistant to impact.

5.2 Non-Ferrous Metals

5.2.1 Methods used and reasons for heat-treatment of alloys, magnesium alloys and Titanium.

a. Mg Alloys – there are two types of heat-treatment: Solution Heat Treatment and Precipitation (aging) heat treatment. Mg develops a negligible change in its properties when allowed to age naturally at room temp. Mg alloys are solution heat treated to improve tensile strength, ductility, and shock resistance. After solution heat treatment, Mg alloys may be given an aging treatment to increase hardness and yield strength. Generally, the aging treatments are used to relieve stress and stabilize the alloys in order to prevent dimensional changes later.

b. Al alloys – there are two types of heat treatments applicable to Al alloys: Solution Heat Treatment and Precipitation heat treatment. The Al alloys are in a comparatively soft state immediately after quenching from a solution heat treating temperature and to obtain their maximum strengths, they must be either naturally aged or precipitation hardened. Precipitation hardening produces a great increase in strength and hardness of the material with corresponding decreases in the ductile properties. The process used to obtain the desired strength is therefore known as aging, or precipitation hardening.

c. Titanium – is heat treated for the following purposes:i. relief of stresses set up during cold forming or machining.ii. annealing after hot-working or cold workingiii. thermal hardening to improve strength.

5.2.2 Limitations on heat-treating clad Al (ALCLAD)

The number of heat treatments allowed for clad sheet is limited due to increased diffusion of core and cladding with each re-heating. Existing specifications allow one to three re-heat treatments of clad sheet depending upon cladding thickness. Clad parts should be heated as quickly and carefully as possible, since long exposure to heat tends to cause some of the constituents of the core to diffuse into the cladding and this reduces the corrosion resistance of the cladding.

5.2.3

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5.2.4

5.3 Treatment Process for Non-Ferrous Alloys.

a. Hardening – the hardening of an Al alloy by heat treatment consists of 4 distinct steps:

1. Heating to a pre-determined temperature2. Soaking at that temp for a specified length of time.3. Rapidly quenching to a relatively low temperature.4. Aging or precipitation hardening either spontaneously at room temp, or as a result of a low temp thermal treatment.

The first three steps above are known as “solution heat treatment” (shorter term: heat treatment). Room temperature hardening is known as “natural aging”, while hardening done at moderate temperatures is called artificial aging or “precipitation heat treatment”.

b. Hardening of non heat-treatable Al alloys – 1100, 3003 and 5052 are non heat-treatable Al alloys and their strength is increased by cold-working.

c. Cold-Working or work-hardening – cold working refers to mechanical deformation of a metal at temperatures below its re-crystallization temperature. Cold-Working ranges from the manual bending of sheet metal for skin repairs to drawing seamless tubing and wire. The strength and hardness, as well as the elastic limit are increased; but the ductility decreases. Since this makes the metal more brittle, it must be heated from time to time during certain operations to remove the undesirable effects of the working.

1. Cold-Rolling – refers to the working of metal at room temperature. In this operation, the materials that have been rolled to approximate sizes, after which they are passed through chilled finishing rolls. This gives a smooth surface and also brings the pieces to accurate dimensions. The principal forms of cold-rolled stocks are sheets, bars and rods.

2. Cold-Drawing – is used in making seamless tubing, wire, streamlined tie rods, and other forms of stock. Because the drawings reduces the ductility of the wire, it must be annealed from time to time before further drawings can be accomplished. Although cold working reduces the ductility, it increases the tensile strength of the wire.

d. Heat treatable alloys – solution heat treatment requirements

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The process of heating certain Al alloys to allow the alloying element to mix with the base metal is called solution heat treating. The temperatures must be controlled to within ± 10°F to obtain specified properties. If the temperature is too low, maximum strength will not be obtained, if excessive temperature is used, there is danger of melting, discolouration, increases the quenching strains.

e. Toughening -

f. Strain and Age-hardening

Precipitation hardening produces a great increase in the strength and hardness of the material with corresponding decrease in the ductile properties. The process used to obtain the desired increase in strength is therefore known as aging, or precipitation hardening.

g. Artificial Aging – is aging above the room temperature.

h. Softening – is same as annealing.

i, j, k, l, m, n, o and p. refer printed pages 213-218 from A&P Mechanics General.

5.3.2 Problems possible if incorrect heat treatment and procedures are used.

5.4 Heat-Treatment Equipment

5.4.1, 5.4.2 and 5.4.3 – Refer pages 205-206 from A&P Mechanics General Handbook.

5.4.4 Quenching of Sheet-Metal

5.4.5 Quenching of irregular shaped objects.

a. The part should never be thrown into the quenching bath.b. The part should be agitated slightly to destroy the coating of vapor which

might prevent it from cooling rapidly.c. The part should be immersed in such a way that the heavy end enters the

bath first.

5.4.6 – Refer 5.3.1 (o)

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6. Metal Working Processes

6.1.1

a. Hot-Working – is the process of forming metal at an elevated temperature when it is in its annealed or soft condition.

b. Cold-Working – is performed well below a metal’s critical temperature.c. Extruding – is the process of forcing metal through a die which imparts a

required cross-section to the metal.d. Casting – metal object, at or near dimensions shape, produced by

introducing molten metal into a mold or a die and allowing it to solidify. die casting-casting produced by introducing molten metal under substantial pressure into a metal die. permanent mould casting, n--casting produced by introducing molten metal by gravity or low pressure into a mould

e. Forging - metal part worked to a predetermined shape by one or more processes such as hammering, upsetting, pressing, rolling, and so forth.

f. Drawing –

7. Identification of Materials

7.1 Classification of Steels

7.1.1 – SAE Classification of Alloy Steels

Alloy Steels SAE Designationa Carbon 1XXXb Plain carbon 10XXc Free Cutting 11XXd Manganese 13XXe Nickel 2XXXf Nickel chromium 3XXXg Molybdenum 40XXh Chrome molybdenum 41XXi Nickel, chrome, molybdenum 43XXj Chromium 5XXXK Chrome vanadium 6XXXl Silicon manganese 9XXX

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7.1.2 – SAE code meaning:First Digit – Principal alloying elementSecond Digit – the % of this alloying elementThird/Fourth digit – percentage in 100ths of a % of carbon in the steel.

7.1.3 – SAE stands for “Society of Automotive Engineers” and the SAE code is used to identify the chemical composition of the structural steels.

7.2 Identification of Aluminium and Al alloys

7.2.1 – Wrought Aluminium High degree of resistance to corrosion. Easily formed into intricate shapes Relatively low in strength. Most widely used in A/C construction, being used for stringers,

bulkheads, skin, rivets and extruded sections.Cast Aluminium

These are suitable for casting in sand, permanent mold, or die castings.

7.2.2 – Difference between heat treatable and non-heat treatable Al alloy.

Non Heat treatable alloy – The strength of these alloys is initially produced by alloying the aluminum with additions of other elements.  These alloys consist of the pure aluminum alloys (1xxx series), manganese alloys (3xxx series), silicon alloys (4xxx series) and magnesium alloys (5xxx series).  A further increase in strength of these alloys is obtained through various degrees of cold working or strain hardening.  Cold working or strain hardening is accomplished by rolling, drawing through dies, stretching or similar operations where area reduction is obtained.  Regulating the amount of total reduction in area of the material controls its final properties.  Material which has been subjected to a strain-hardening temper, may also be given a final, elevated temperature treatment called “stabilizing”, to ensure that the final mechanical properties do not change over time. The letter “H” followed by numbers denotes the specific condition obtained from strain hardening.

Heat Treatable Alloy – The initial strength of these alloys is also produced by the addition of alloying elements to pure aluminum.  These elements include copper (2xxx series), magnesium and silicon, which is able to form the compound magnesium silicide (6xxx series), and zinc (7xxx series).  When present in a given alloy, singly or in various combinations, these elements exhibit increasing solid solubility in aluminum as the temperature increases.  Because of this reaction, it is possible to produce significant additional strengthening to the heat-treatable alloys by subjecting them to an elevated thermal treatment, quenching, and, when applicable, precipitation heat-treatment known also as artificial aging.

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Note:  Because of additions of magnesium and or copper, there are also a number of silicon (4xxx series) alloys that are heat-treatable.In solution heat-treatment, the material is typically heated and this causes the alloying elements within the material to go into solid solution.  Rapid quenching, usually in water, which freezes or traps the alloying elements in solution, follows this process. Precipitation heat-treatment or artificial aging is used after solution heat-treatment.  This involves heating the material for a controlled time at a lower temperature. This process, used after solution heat-treatment, both increases strength and stabilizes the material.

7.2.3 – Four digit code for identifying Al alloys

First Digit – identifies the major alloying element used in the formation of the alloySecond Digit – represents a specific alloy modification, e.g. if the digit is zero, it indicates that there were no special controls over individual properties. However, a digit from 1 to 9 indicates the number of controls over impurities in the metal.Last two digits – for the 1XXX series it is used to indicate the hundreths of 1 % above the original 99% pure Al, e.g if the last two digits are 75, the alloy contains 99.75%

7.2.4. Aluminum alloys commonly used in aircraft construction.

7.2.5 Four-digit code to identify pure Al and Al alloys with the following alloying agents:

a. Copper – 2XXXb. Manganese – 3XXXc. Silicon – 4XXXd. Magnesium – 5XXXe. Magnesium and Silicon – 6XXXf. Zinc – 7XXX

7.2.6 Effects of the parent metal of the above alloying agents:

a. Copper – 2XXX produces greater strengths major drawback is their susceptibility to inter-granular corrosion when

improperly heat treated. Commonly used in the construction of skins and rivets, e.g. 2017, 2024.

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b. Manganese – 3XXX Generally considered non-heat treatable Most common variation is the 3003, which offers moderate strength and

has good working characteristics.

c. Silicon – 4XXX Silicon lowers the metal’s melting temperature This results in alloys that work well for welding and brazing.

d. Magnesium – 5XXX Possess good welding and corrosion resistant characteristics. However if the metal is exposed to high temperatures or excessive cold

working, it’s susceptibility to corrosion increases.

e. Magnesium and Silicon – 6XXX The Si and Mg forms magnesium silicide, which makes the alloy heat

treatable. Has medium strength with good forming and corrosion resistance

properties.

f. Zinc – 7XXX Are more harder and stronger due to the addition of Zinc. Widely used forms are 7075 and 7178.

7.2.7 Correct means of identification marking Al alloy sheets. Al alloy sheets are marked with the specification number on approximately

every square foot of the material

7.2.8 Use of Caustic Soda to identify certain Al alloy sheets

It is possible to identify the heat treatable alloys by immersing a sample of the material in a 10% solution of caustic soda (NaOH). The heat treatable alloys will turn black due to the copper content, whereas the others will remain bright. In the case of a clad material, the surface will remain bright, but there will be dark areas in the middle when viewed form the edge.

7.2.9 Al alloys deemed to be weldable

4XXX (Silicon) and 5XXX (Magnesium)

7.3 Heat Treatment Identification

7.3.1 Hardness conditions relating to Al alloys (heat treatable alloys)

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-T = Solution heat treated.-T1 = Cooled from an elevated temperature shaping process (such as

extrusion or casting) and naturally aged to a substantially stable condition.-T2 = Annealed (castings only).-T3 = solution heat treated followed by strain hardening. Different amounts of strain hardening are indicated by the second digit, e.g. –T36 indicates that the material has been solution heat treated and has had its thickness reduced 6% by cold rolling.-T4 = solution heat treated followed by natural aging at room temperature to a stable condition.-T5 = artificially aged only (castings only).-T6 = Solution heat-treated and artificially aged.-T7 = solution heat treated and then stabilized to control its growth and distortion.-T8 = solution heat treated, strain hardened and then artificially aged.-T9 = solution heat treated, artificially aged and then strain hardened.-T10 = artificially aged and then cold worked.-W = solution heat treated, unstable temper.

7.3.2 – refer 7.3.1

7.3.3 Strain hardness or temper of Al alloys (non heat treatable alloys):

-F = as fabricated (wrought alloys); as cast (cast alloys)-O = annealed, re-crystallized (for wrought products only)-H = Strain hardened.-H1 = Strain hardened only.-H2 = Strain hardened and partially annealed.-H3 = Strain hardened and stabilized.

7.3.4 The digit following the designations H1, H2 and H3 indicate the degree of strain hardening, e.g the number 8 represents the max tensile strength while O indicates an annealed state:

-H32 = Strain-hardened and then stabilized. Final temper is one-quarter hard.-H34 = Strain-hardened and then stabilized. Final temper is one-half hard.-H36 = Strain-hardened and then stabilized. Final temper is three-quarters hard.-H38 = Strain-hardened and then stabilized. Final temper is full hard.-H39 = Extra hard.

7.3.5

7.3.6 Describe the identification of clad aluminum and state what the cladding and core material may be composed of:

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7.3.7 Identify common aircraft materials by their physical characteristics.

7.3.8 Describe how a heat treatable alloy can be identified using a physical test.Refer 7.2.8

7.3.9 Describe the meaning of the temper designation (-W) when indicating the material condition:

It indicates the material has been solution heat treated and quenched but not aged.

7.4 Heat Treatment of Magnesium Alloys

7.4.1 Describe the heat treatment processes relating to Mg and it’s alloys including: F, O, H24, T4, T5 and T6.

Is the world’s lightest structural metal. Silvery white in colour and weighing only 2/3 of Al.

In pure form, it does not possess sufficient strength for structural uses, but when alloyed with Zn, Al, and Mn, it produces an alloy having the highest strength-to-weight ratio.

Possess good casting characteristics. Mg alloys are subject to such treatments as annealing, quenching,

solution heat treatment, aging and stabilizing. The solution heat treatment is used to put as much of the alloying

ingredients as possible into solid solution, resulting in high tensile strength and maximum ductility.

Aging is applied after heat treatment where maximum hardness and yield strength are dedired.

Mg embodies fire hazards of an unpredictable nature. It will not burn until the melting point is reached. Mg fires can be extinguished by use of an extinguishing powder such as powdered soapstone or graphite powder.

Mg Temper Conditions:F – as fabricatedO – AnnealedH24 – strain hardened and partially annealed.T4 – Solution heat treated. T5 – artificially aged only T6 – solution heat treated only.

7.5 Identification of Mg alloys

7.6 Titanium

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7.6.1 Identification, composition and characteristics of titanium and titanium alloys

Are light weight metals with very high strength and is a metallic element Have excellent corrosion resistance. Since the metal is sensitive to

oxygen and nitrogen, it must be converted to titanium dioxide and chlorine gas and a reducing agent before it can be used.

Titanium is classified as alpha, beta and alpha-beta, based on specific chemical compositions.

Alpha alloys have medium strengths and good elevated temp strengths. Alpha-Beta alloys are the most versatile of the Ti alloys. They have

medium strength in the annealed condition and much higher strength when heat treated. This form of Ti is generally not weldable but it has good forming characteristics.

Beta alloys have medium strength, excellent forming characteristics, and contain large quantities of high density alloying elements. Because of this beta alloys can be heat treated to a very high strength.

7.6.2 Describe a test to identify titanium sheet1. Spark Test – titanium gives off a brilliant white trace ending in a brilliant

white burst.2. also by moistening the titanium and using it to draw a line on a piece of

glass. This will leave a dark line similar in appearance to a pencil mark.

7.6.3Titanium is mostly used in the construction of fuselage skins, engine shrouds, longerons and is also used for making compressor blades and vanes, through bolts turbine housings and liners, etc.

7.6.4Iron, Molybdenum, and chromium will improve the ductility of titanium alloys.

7.7 Substitution of Materials

In selecting substitute metals for the repair and maintenance of aircraft, it is very important to check the appropriate structural manual. Aircraft manufacturers design structural members to meet a specific load requirement for a particular aircraft. The methods of repairing these members, apparently similar in construction, will thus vary with different aircraft.Four requirements must be kept in mind when selecting substitute metals. The first and most important of these is:

1. maintaining the original strength of the structure2. maintaining the contour or aerodynamic smoothness,3. keeping weight to a minimum, if possible, or keeping added weight to a

minimum.4. maintaining the original corrosion-resistant properties of the metal.

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7.8 Selection of Materials in Aircraft construction

8. Identification of Defects in Materials

8.1 Common Defects

8.1.1

a. Tensile Failure –

b. Compression Failure –

c. Shear Failure –

d. Torsional failure and torsional/tensile failure –

e. Brinelling – occurrence of shallow, spherical depressions in a surface, usually produced by a part having a small radius in contact with surface under heavy load.

f. Galling – breakdown (or build-up) of metal surfaces due to excessive friction between two parts having relative motion. Particles of the softer metal are torn loose and “welded” to the harder.

g. Spalling - the separation of macroscopic particles from a surface in the form of flakes or chips, usually associated with rolling element bearings and gear teeth, but also resulting from impact events.

h. Fretting – is caused by one object rubbing against another (small amplitude oscillatory motion) and wearing part of it away.

i. Burnishing – polishing of one surface by sliding contact with smooth, harder surface. Usually no displacement or removal of metal.

j. Overheating – heating to an excessive high temperature such that the properties/structure undergoes modification in grain structure.

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k. Overloading –

l. Incorrect heat treatment –

m. Lack of lubrication –

n. Weld decay – Inter-granular corrosion, usually of stainless steels or certain nickel-base alloys that occurs as the result of sensitization in the heat-affected zone during the welding operation.

o. Hydrogen Embrittlement – results when a chemical reaction takes place which produces hydrogen gas that is absorbed into the metal. This reduces the metal’s ductility and allows the formation of cracks and stress corrosion.

p. Fatigue Stress and fatigue failure –

q. Erosion of rotating componentry (as in prop or turbine compressor)

r. Inclusions –

8.1.4 Describe the stresses associated with the fastening or operation of A/C hardware and components such as: bolts, nuts, studs, rivets, pins, shafts, discs, blades, connecting rods, gears and struts.

8.2 Identification of Failure Debris

8.2.1

8.2.2 Debris test/identification methods

8.2.3 Typical gear and bearing failures

8.2.4 Purpose of SOAP sampling programs interpretation of SOAP sample results.

9. Corrosion

9.1 Corrosion Chemistry

9.1.1 Describe the chemistry between the various forms of corrosion found on the A/C

Corrosion is the deterioration of the metal by chemical or electrochemical attack and can take place internally as well as on the surface. Corrosion is simply a process wherein metals return to a natural state. There are two types of

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corrosion, chemical and electro-chemical; however both types involve two simultaneous changes. The metal that is attacked or oxidized suffers an anodic change, and the corrosive agent is reduced and suffers a cathodic change.

9.1.2

9.1.3 Common a/c materials in their order of electrical potential.

Magnesium, Zinc, Clad 7075 Al Alloy, Commercially Pure Al (1100), Clad 2024 Al alloy.Corrosion is an electrochemical action in which one metal is changed into a chemical salt. When two dissimilar metals are in contact with each other in the presence of some electrolyte, such as HCl acid or plain water, the less active metal acts as the cathode and attracts electrons from the anode. As the electrodes are pulled away from the anode the metal corrodes.An anode is an electrode where oxidation is the principal reaction. It is also the electrode where corrosion usually occurs and from where metal ions enter into solution.A Cathode is a negatively charged electrode where reduction is the principal reaction.

9.1.4 Susceptibility of various a/c materials to the common types of corrosion:

9.1.5 Describe why pure Al is considered to be corrosion resistant

9.2 Classification of Corrosion

9.2.1 Describe the two general classifications of corrosion:a. Direct Chemical Attack – aka pure chemical corrosion, is an attack

resulting from a direct exposure of a bare surface to caustic liquid or gaseous agents. Unlike in electrochemical attack, the changes in direct attack are occurring simultaneously at the same point. The most common agents causing direct chemical attack on a/c are:

Spilled battery acid or fumes from batteries Residual flux deposits resulting from inadequately cleaned, welded,

brazed or soldered joints. Entrapped caustic cleaning solutions.

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b. Electrochemical attack – may be likened chemically to the electrolyte reaction which takes place in electroplating, anodizing, or in a dry cell battery. The reaction in this corrosive attack requires a medium, usually water, which is capable of conducting electricity. When a metal comes in contact with a corrosive agent and connected by a liquid or gaseous path through which electrons may flow, corrosion begins as the metal decays by oxidation. During the attack, the quantity of the corrosive agent is reduced and, if not renewed or removed, may completely react with the metal (become neutralized).Exposure of the alloy surface to a conductive, corrosive medium causes the more active metal to become cathodic, thereby establishing conditions for corrosion. These are called local cells. The greater the difference in electrical potential between the two metals, the greater will be the severity of a corrosive attack, if the proper conditions are allowed to develop. Conditions for these corrosion reactions are a conductive fluid and metals having a difference in potential. If by regular cleaning and surface refinishing, the medium is removed and the minute electrical circuit eliminated, corrosion cannot occur; this is the basis for effective corrosion control.The electrochemical attack is responsible for most forms of corrosion on a/c structure and components parts.When two dissimilar metals are placed in an electrolyte, an electrical potential exists. This potential forces electrons in the more negative material (anode) to flow to the less negative material (cathode) when a conductive path is provided.

9.3 Corrosive Agents

Are substances that are capable of causing corrosive reaction.

9.3.1 Describe the effects of the following corrosive agents on a/c structure:

a. Acids and Alkalis – Form effective electrolytes as they react with metals to form metallic salts. Ferrous metals are subject to damage from both acids & alkalis, but Al is

more vulnerable to strong alkaline solutions than it is to acids.

b. Salts – Many compounds other than NaCl fall into the category of salts. In general, salts are the result of a metallic element combining with a non-

metal and the resulting compound is almost always a good electrolyte. Mg is particularly vulnerable to corrosive attack from an electrolyte formed

by salt solutions.

d. Air (moisture, humidity, acid rain, volcanic fallout) Marine atmosphere and air above industrial areas hold large

concentrations of salts.

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Pure water reacts with metals to form corrosion or oxidation, but water holding a concentration of salts or other contaminants causes much more rapid corrosion.

e. Organic Growths – Jet a/c use a high viscosity fuel which holds more water in suspension

than other aviation fuels Water in fuel tanks, contains microscopic animal and plant life called

microbes. These microbes live in water and feed on the hydrocarbon fuel.

9.4 Types of Corrosion

9.4.1 Describe the following types of corrosion, the casual factors, how each affects the base metal, and how each type of corrosion is treated/neutralized.

a. Oxidation (ferrous oxide) – One of the simpler forms of corrosion aka “dry” corrosion, and generally

known as oxidation. When a metal such as iron is exposed to a gas containing O2, a chemical

reaction takes place on the surface between the metal and the gas. Two atoms of Fe join three atoms of O2 to form iron oxide, or rust (Fe2O3).

There is one big difference between aluminum oxide and iron oxide. The film of Al2O3 is unbroken and, therefore, once it has formed, further reaction with O2 slows dramatically. Fe2O3 on the other hand, forms a porous, interrupted film, since the film is not airtight; the metal continues to react with the O2 in the air until the metal is completely eaten away.

The best way to protect iron form oxidation is to keep oxygen from coming into contact with its surface. This is done temporarily by covering the surface with oil, grease, or permanently with a coat of paint.

Al alloy can be protected from oxidation by the formation of an oxide film on its surface. This film insulates the Al from any electrolyte, and prevents further reaction with oxygen.

b. Uniform Surface Corrosion – When an area of unprotected metal is exposed to an atmosphere

containing corrosive agents, a uniform attack over the surface occurs. The dulling of the surface is caused by microscopic amounts of the metal

being converted into corrosion salts.

c. Pitting Corrosion – Pitting is a likely result of uniform surface corrosion left untreated. Is usually detected by the appearance of clumps of white powder on the

surface.

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The pits penetrate deeply into the metal and cause damage completely out of proportion.

d. Galvanic Corrosion (dissimilar metal corrosion) Occurs any time when two dissimilar metals make electrical contact in the

presence of an electrolyte. Corrosion is much more rapid when the anodic metal is smaller than that

of the cathodic metal. The reason for this is the greater area of the cathode allows a higher rate of electron flow, accelerating the speed of the reaction.

e. Concentration Cell corrosion – Aka crevice corrosion, is corrosion of metals in a metal-to-metal joint,

corrosion at the edge of a joint even though the metals are identical, or corrosion of the spot on a metal’s surface covered by a foreign material.

Oxygen concentration cells, metal ion concentration cells and active-passive cells are the three general types of concentration cell corrosion.

f. Oxygen concentration cell corrosion – Can form when water covers the surface of an Al a/c skin and seeps into

the cracks between lap joints. The unusual characteristic of this type of corrosion is that it forms in the

area where there is a deficiency of oxygen. Can occur on Al, Mg or on ferrous metals. It forms under the marking tape of ferrules on Al tubing, beneath sealer

that has loosened, and under bolt or screw heads.

g. Metal Ion concentration cell corrosion – The electrode potential within a metal is dependant on the different metals

that make up the alloy. However, a potential difference can occur if an electrolyte having a non-

uniform concentration of metal ions covers the surface. Note the difference between the two types of concentration cell corrosion.

The metallic ion concentration cell corrosion forms on the open surface, while oxygen concentration cell corrosion forms in the closed areas between the faying surfaces.

h. Active-Passive Cells Metals which depend on a tightly adhering passive film for corrosion

protection, such as corrosion resistant steel, are prone to rapid corrosive attack by active-passive cells.

The corrosive action usually starts as oxygen concentration cell corrosion. If the passive film is broken beneath this, the active metal beneath the film will be exposed to corrosive attack. An electrical potential develops

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between the large area of the cathode (passive film) and the small area of the anode (active metal)

i. Filiform Corrosion – Is a special form of oxygen concentration cell corrosion or crevice

corrosion, which occurs on metal surfaces having an organic coating system.

It is recognized by its fine thread like lines under a polyurethane enamel finish.

It often results when the wash primer used on a metal has not been properly cured. A wash primer is two-part metal preparation material in which phosphoric acid converts the surface of the metal into a phosphate film that protects the metal from corrosion & provides good bond for paint. This conversion process relies on moisture in the air and if there is not enough moisture to convert all of the acid, some acid remains on the metal. If a dense polyurethane finish is then applied, the acid becomes trapped and reacts with the Al alloy to form corrosion.

It shows itself as a puffiness under the paint film and is first noticed around rivet heads and along the lap joints of skins.

Treatment: strip the paint; remove the corrosion; treat the metals surface; refinish the A/C.

j. Intergranular corrosion – Is an attack along the grain boundaries of a material. The grain boundary and the grain center can react with each other as

anode and cathode when in contact with an electrolyte. Spot or seam welding through localized heating, can also cause grain

enlargement that leaves the metal susceptible to Intergranular corrosion. Difficult to detect without ultrasonic or eddy current equipment. Remedy for Intergranular carrion is replacement of the part.

k. Exfoliation Corrosion – Is an extreme case of Intergranular corrosion. It occurs chiefly in extruded materials, such as channels or angles, where

the grain structure is more laminar than in rolled sheets or castings. This type of corrosion occurs along the grain boundaries and causes the

metal to delaminate. Remedy is to replace the part.

l. Stress Corrosion – Occurs when the metal is subjected to a tensile stress in the presence of a

corrosive environment. The stresses in the metal can come from improper quenching after heat

treatment, or from an interference fit of a fastener. Can be transgranular or Intergranular in nature.

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To prevent stress corrosion in some heat treated Al alloy parts is to “shot-peen” the surface to provide a uniform compressive stress on the surface.

Common locations for stress corrosion are between rivets in a stressed skin, around pressed-in bushings, and tapered pipe fittings.

Dye penetrant inspection is required to find the actual extent of the crack.

m. Corrosion Fatigue – the process in which a metal fractures prematurely under conditions of simultaneous corrosion and repeated cyclic loading at lower stress levels or fewer cycles than would be required in the absence of the corrosive environment.

n. Fretting Corrosion – When two surfaces fit tightly together but can move relative to one

another, corrosion occurs. This type of corrosion is the result of the abrasive wear caused by the two

surfaces rubbing (fretting) against each other. This rubbing prevents the formation of the protective oxide film, exposing

active metal to the atmosphere. By the time this type of corrosion makes it appearance on the surface, the

damage is usually done and the parts must be replaced. If the contact areas are small and sharp, deep grooves resembling brinell

markings or pressure indentations are can be worn in the rubbing surface. As a result, this type of corrosion is called “false brinelling”.

Fretting corrosion occurs around rivets in a skin and is known as “smoking rivets”. Rivets showing this kind of fretting must be drilled out and replaced.

9.4.2 Describe the general identifying characteristics of corrosion on the following metals.

a. Steel – reddish rust.b. Copper – greenish film.c. Aluminium – appears as surface pitting and etching, often combined with

grey or white powdery deposits.d. Brass – e. Magnesium – appears as surface pitting and etching, often combined with

grey or white powdery deposits.f. Titanium – g. Lead –

9.5 Contributory Factors

9.5.1 Describe how the following factors contribute to corrosion.

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a. Size and type of metal – thick structural sections are more susceptible to corrosive attack than thin sections.

b. Foreign Materials – soil, oil, grease, salt water, battery spills. It is important that the a/c be kept clean.

c. Fluids and abrasives – d. Electrical potential difference – e. Lack of cleanliness – refer (b) above.f. Stress –

9.6 Corrosion Detection

Describe corrosion detection methods and how results may be interpreted. Dye penetrant inspection. The main limitation is the fact that it can fail to

detect cracks that are so full of corrosion that the dye cannot penetrate. Ultrasonic equipment. There are two types of ultrasonic indications used

for corrosion detection; “pulse-echo” and “resonance”. In the pulse-echo method, a pulse of ultrasonic energy is directed into the structure and this energy travels through the material to its opposite side and then bounces back. The resonance method is based on the principle that for any given thickness of the material, there is a specific frequency of ultrasonic energy that resonates. If the metal has been eaten by away by corrosion, its resonant frequency will change.

X-ray. Requires extensive training and experience for proper interpretation of the results

9.7 Corrosion prone areas on Aircraft

9.7.1 Describe the following corrosion prone areas, the likely types of corrosion found in these areas, and the possible causes:

a. Engine exhaust tail areas – cracks and seams in the exhaust track are prime areas for corrosion. Fairings on the nacelles, hinges and inspection door fasteners contain crevices which invite the formation of corrosion.

b. Battery compartments and vents – battery fumes and electrolyte can cause corrosion.

c. Lavatories and food service areas – organic materials such as food and human waste (acidic) are highly corrosive to Al surfaces.

d. Wheel wells and Landing Gear – on T/O and landing debris from the runway is thrown into the wheel well and this can be especially troublesome in the winter when chemicals are used on the runway for ice control. Furthermore, abrasion can remove protective lubricants and

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coatings, and water and mud can freeze and cause damage. Bolt heads and nuts on the Mg wheels are susceptible to galvanic corrosion and concentration cell corrosion can form under the marking tape or ferrules on the Al tubing.

e. External skin areas – concentration cell corrosion frequently appears in the seams and lap joints

f. Engine inlet areas – abrasion by high velocity air and contaminants in the air tends to remove the protective coating. Therefore abrasion strips along the leading edge of intake ducts help protect these areas.

g. Cooling air vents –

h. Fuel tanks – are highly susceptible to corrosion formation. Organic growth is the primary cause of corrosion in fuel tanks that hold turbine fuel. If ignored this organic growth can grow into the water holding scum which attaches to the Al alloy skin. Corrosion under the sealant is extremely difficult to detect and must usually be found with x-ray or ultrasonic inspection from the outside of the wing.

i. Piano hinges – located on the control surfaces and access doors are ideal locations for dissimilar metal corrosion to develop. The reason being that the hinge body is made of Al alloy while the pin is made of hard carbon steel. To help prevent this, the hinge must be kept clean and dry and should be lubricated with a spray which displaces water.

j. Control surface recesses – some airplanes have areas in the wing or empennage where the movable surfaces recess back into the fixed structure. Hinges are buried back into the recess and difficult to lubricate.

k. Bilge areas – the bottom of the fuselage below the floor is an area where water and all forms of liquid and solid debris can accumulate and cause corrosion. Airplanes having areas prone to accumulate water are typically provided with drain holes. However, dirt and other debris also collect here, and drain holes often become clogged. Drain holes should always be clean.

l. Landing Gear boxes – although this area is well protected, water can collect if the drain holes become plugged.

m. Engine mount structure – when a piston engine is started, the heavy current from the starter must return to the battery through the engine mount. This current flows through joints in the mount and creates the potential difference required for corrosion to form in these areas. To

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protect welded steel tubes mounts from internal corrosion the tubing should be periodically filled with hot linseed oil or other type of tubing oil.

n. Control cables – if carbon steel cable is left unprotected and water is allowed to get between the cable strands, the cable will start to corrode. The corrosion that forms on the inside of the cable is difficult to detect.

o. Welded areas – Al torch welding requires the use of a flux to exclude oxygen from the weld. These flux contain chemicals that are extremely corrosive to Al and all traces of the flux must be removed after welding is completed.

p. Electronic equipment – circuit boards are typically protected by sealing the wiring and circuit boards with a transparent film which excludes oxygen and moisture from the air.

9.8 Corrosion effects on specific materials.

9.8.1 Describe specific corrosion problems relating to:

a. Ferrous metal components – atmospheric oxidation of the steel, which results in rust.

b. Al and Al alloys – three forms of attack on Al alloys are particularly serious: 1. the penetrating pit type corrosion.2. stress corrosion-cracking of materials under sustained stress.3. Intergranular corrosion of certain improperly heat treated Al alloys.

c. Magnesium – is the most chemically active of the metals used in the a/c construction and is, therefore the most difficult to protect. Mg attack is probably the easiest type of corrosion to detect in the early stages since Mg corrosion products occupy several times the volume of the original Mg metal destroyed. The beginning of the attack shows as a lifting of the paint films and white spots on the Mg surface, and these rapidly develop into snow-like moulds or even “white whiskers”.

d. Copper –

e. Titanium and Ti alloys – attack on Ti is generally difficult to detect. Ti, by nature is highly corrosion resistant, but it may show deterioration by the presence of salt deposits and metal impurities particularly at high temperatures. If Ti surfaces require cleaning, hand polishing with Al polish or a mild abrasive is permissible.

9.9 Anti Corrosion treatments

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9.9.1 Describe the following treatments relating to Al alloys

a. Mechanical corrosion removal – after the paint is removed from a corroded area, all traces of corrosion must be removed from the surface. Very mild corrosion may be removed by using “Scotchbrite” pads. More severe corrosion can be removed by brushing with Al wool or with an Al wire brush. Steel wire brush should not be used since traces of steel can become embedded in the Al and lead to severe corrosion. Blasting the surface with glass beads smaller than 500 mesh can be used to remove corrosion from pits. After using abrasives or brushing, examine the metal with a 5 to 10 power magnifying glass to ensure that all traces of the corrosion have been removed.

b. Chemical neutralization – after removing all corrosion, treat the surface with a 5% chromic acid solution to neutralize any remaining corrosion salts. After the acid has been on the surface for at least 5 minutes, it should be washed offoff with water and allowed to dry. Alodine treatment will also neutralize corrosion, as well as form a protective film on the metal’s surface.

c. Cladding – once Al is alloyed, the alloying agent creates the possibility of dissimilar metal corrosion. However, Al alloys can be protected from corrosion and at the dame time made attractive in appearance by coating them with a layer of pure Al, this is known as cladding. In manufacture of clad Al, pure Al is rolled onto the surface of an Al alloy and accounts for 5 – 10% of the total sheet thickness. The cladding material is anodic as compared to the core material, and any corrosion that takes place attacks the cladding and not the core.

d. Surface Oxide film/anodizing – in areas where cladding is not practical, metallurgists have found other ways of forming the films on metal surfaces that are hard, decorative, waterproof, and airtight. Furthermore, these films have the added benefit of acting as a base for the paint finishes to adhere to. The process of applying an oxide film is performed in the factories by an electrolytic process known as anodizing. In this process a part is bathed in a lead vat containing a solution of chromic acid and water. This process forms an oxide film on the part that protects the alloy from further corrosion. The anodic film on the Al alloy is normally a light grey colour, varying to a darker grey for some of the alloys. The anodic layer also acts as an electrical insulator. When the protective anodizing film has been damaged/removed, the part can have a protective film applied through chemical rather than an electrolytic process. This process is known as aAlodizing and uses a chemical that meets spec MIL-C-5541 and is available under names such as “Alodine 1021”. If a powder appears on the surface after the material is dried, it is an indication of poor rinsing or failure to keep the surface wet during the time the chemical was working. If the part shows up, the part must be re-treated.

f. Organic Film – Paint adherence is a not a problem on porous surface, but on smooth surfaces, such as those found on Al, the surface must be prepared in

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order for the paint to have a rough surface to which it can adhere. An Al surface is typically roughened with a mild chromic acid etch, or by the formation of an oxide film through anodizing or alodizing. The surface can be mechanically roughened by carefully sanding it with 400-grit sandpaper. When sandpaper is used, it is necessary to remove every bit of sanding dust be removed with a damp rag before the primer is applied.Zinc Chromate primer has been used for years with laquer and enamel. It is inhibiting primer, meaning that the film is slightly porous and water can enter causing chromate ions to be released and held on the surface of the metal. This ionized surface prevents electrolytic action and inhibits the formation of corrosion. It has a yellow-green or dark green colour. It is thinned with toluol or some other reducers made especially for zinc chromate. Prior to applying zinc chromate, the surface to be painted is cleaned of all fingerprints and oil. Then a thin, wet coat of zinc chromate is applied with a spray gun. (Zinc chromate si toxic).A Wash Primer is used in aircraft factories for priming new aircraft before they are painted. This 2 part primer consists of a resin and an alcohol-phosphoric acid catalyst. The material is mixed and then sprayed onto the surface with a very light tack coat, followed by a full bodied wet coat. It cures quickly enough.Epoxy primers are one of the most popular primers for use under polyurethane finishes because they provide maximum corrosion protection. A typical epoxy primer consists of 2 part materials that produce a tough coat between the finish and the surface. Epoxy primers can be used on Al, Mg or steel. For maximum corrosion protection they can be applied over a wash primer.

g. Protection of integral fuel tanks for microbiological corrosion – the most successful solution to this problem has been to use an additive in fuel which kills these organic growths and prevents the formation of corrosion forming scum

h. Shot-Peening - a process whereby hard, small spherical objects (such as metallic shot) are propelled against a metallic surface for the purpose of introducing compressive stresses into that surface, hardening it or obtaining decorative effects.

9.9.2 Describe the following treatments for ferrous metals.

a/b/c. Mechanical Corrosion removal – unlike Al, the oxide film that forms on ferrous metals is porous and attracts moisture. The most effective means of removing rust is by mechanical means. Abrasive paper and wire brushes can be used, but the most thorough means of removing all of corrosion form un-plated steel parts is by abrasive blasting. Abrasive blasting is typically done using sand, Al oxide or glass beads (exercise caution, if the part has been plated either with cadmium or chromium. Highly stressed steel parts (e.g. in landing gear) should not be cleaned with a wire brush, since this can cause minute scratches which can produce stress concentrations that can potentially weaken the part. If

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abrasive blasting is used, it must be done with caution, using a very fine grit abrasive or glass beads.After all the corrosion has been removed, any rough edges caused by pitting must be faired with a fine stone or a 400 grit abrasive paper. The surface should then be primed as soon as possible. A dry, clean surface is ideal for corrosion. Zinc chromate primer is used to protect most freshly cleaned stainless steel surfaces.

d/e. Nickel or chrome plating – one way to protect ferrous metals is through chrome plating. This plating produces an airtight coating over the surface that excludes moisture from the base metal. There are 2 types of chrome plating used on the a/c: decorative and hard chrome.Decorative chrome is used primarily for its appearance and surface protection.Hard chrome is used to form wear resistant surface on piston rods, cylinder walls, and other parts subject to abrasion.

f/g. Cadmium plating – almost all steel a/c hardware is cadmium plated. It provides an attractive finish and protects against corrosion. When the Cd plating on a part is scratched through to the steel, galvanic action takes place and Cd corrodes. The oxides that form on the surface of Cd are similar to those on the Al surface (dense, air and water tight). This means that no further corrosion takes place once the initial film has formed. This type of protection is known as sacrificial corrosion.

h. Galvanizing – steel parts e.g. firewalls are typically treated with a coating of zinc in a process called galvanizing (sacrificial corrosion protection). Zinc will corrode and form an airtight oxide film.

i. Metal Spraying – a/c engine cylinders are sometimes protected from corrosion by spraying molten Al on their surface. To accomplish this process, a steel cylinder barrel is sand-blasted absolutely clean, then Al wire is fed to into an acetylene flame where the wire is melted and blown onto the surface by high-pressure compressed air. (sacrificial corrosion protection).

j. Organic Coatings – most common organic coating used to protect ferrous metal is paint. However, like Al the surface must be properly prepared to ensure a good bond. Parts which have been cadmium plated must normally have their surface etched with a 5% solution of chromic acid before the primer adheres. After surface preparation, a thin, wet coat of zinc chromate primer is sprayed on and allowed to dry.

k. Zinc Chromate priming – refer 9.9.1 (f)

l. Sherardizing –

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m. Parkerized – chemically treated to provide iron and steel with dark corrosion-resistant protective coating by boiling in solution of phosphoric acid and manganese or zinc dioxide and subsequently applying coating of paraffin oil.Bonderized - phosphate coated (refer above)Granodizing –

n. Enamels –

o. Chlorinated rubber compounds –

9.9.3 Describe the following treatments for Mg alloys:

Mg is one of the most active metals used in a/c construction. Mg alloys do not naturally form a protective film on their surfaces the way Al does.

a. Mechanical Corrosion removal – when Mg corrodes, the corrosion products occupy more space than the metal. Therefore, Mg corrosion typically raises the paint or, if it forms between lap joints, it swells the joints.Since Mg is anodic to almost all of the commonly used a/c structural metals, corrosion should not be removed with metallic tools. Stiff non-metallic bristle brushes or nylon scrubbers are used to remove the corrosion. If corrosion exists in the form of deep pits the corrosion must be cut out with sharp carbide-tipped cutting tools or scrapers. If abrasive blasting is used to remove corrosion from Mg, use only glass beads which have been used for nothing else but Mg.

b. Decarbonization – process used to remove the carbon deposits and contaminants in engine compartments.A decarbonizing unit consists of a heated tank and a decarbonizing agent, either water soluble or hydrocarbon based. Parts are immersed in the heated liquid which loosens the accumulated carbon. Complete removal sometimes requires brushing, scraping, or grit blasting. Mg parts and steel parts must not be placed in the decarbonizing unit together.

c/d/e. Surface Treatments – after all the corrosion has been removed, a chromic acid pickling solution, which confirms to MIL-M-3171A Type 1 (Dow No. 1), is applied. A satisfactory substitute for this solution may be made by adding about 50 drops of sulfuric acid to a gallon of 10% chromic acid solution. Apply this to the surface with rags and allow it to stand for about 10 – 15 mins, then rinse the part thoroughly with hot water.A treatment which forms a more protective film is a dichromate conversion treatment such as Dow # 7. This solution is applied to the metal and allowed to stand until golden brown oxide film forms uniformly on the surface, rinse the

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surface with cold water and dry it with compressed air. The oxide film is extremely soft when wet.Anodizing Mg with Dow # 17 produces a hard, surface oxide film which serves as a good base for further protection by a coat of paint.

f. Dichromate conversion –

g.Stannate immersion –

9.9.4 Describe acceptable methods of anti-corrosion treatment applied after the in-service repair or restoration of a/c structure or components with particular regard to:

a. Steel parts – b. Painted surfaces – c. Anodized parts – d. Mg parts – e. Al parts – f. Plated parts –

9.10 Specific Corrosion Prevention methods

9.10.1 Describe corrosion prevention methods such as:

a. Dissimilar metal insulation – the areas to be joined are sprayed with two coats of zinc chromate primer, and a strip of pressure-sensitive vinyl tape is placed between the surfaces before they are assembled.

b. Powerplant external preservation – when preparing an engine for storage, the engine must be drained of oil and refilled with a suitable preservative. The propeller should be wiped with an oily cloth. Never use alkaline or acid cleaners on metal props.

c. Fasteners – when steel fasteners are used in an Al structure, the holes should be drilled and countersunk, treated with a conversion coating material such as Alodine, and then primed with zinc chromate. Fasteners should be coated with primer and installed wet.

9.10.2 Effects of improper heat treatment on the corrosion of base metal.

9.10.3 Preventative methods to avoid corrosion

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9.10.4 Corrosion prevention and treatment methods for bonded metal honeycomb structure including prevention of corrosion around rivets.

9.10.5

9.10.6 Corrosion proofing methods for seaplanes

9.10.7 Corrosion proofing methods employed during a/c construction or fabrication.By use of Wash Primers

9.10.8 Surface restoration methods for damaged protective films.

9.10.9 Procedures for handling a/c recovered from salt water.

9.10.10 Treatment of structure after electrolyte spills from the various types of a/c battery commonly found in service.

9.10.11 Protection of Titanium after light corrosion removal.

Titanium is, by nature, highly corrosion resistant, but it may show deterioration form the presence of salt deposits and metal impurities, particularly at high temperatures. Use of steel wool, iron scrapers, or steel brushes is strictly prohibited. If titanium surfaces require cleaning, hand polishing with Al polish is permissible. Wipe the treated surface with dry cloths to remove excess solution, but do not use a water rinse.

9.10.12 Describe how corrosion-resistant steel parts in exhaust system should be blast cleaned

9.10.13 Describe how the internal surfaces of steel tubing is best protected from corrosion.

9.10.14When preparing the engine for storage, the engine must be drained of oil and refilled with a suitable preservative. Spark plugs are removed and the cylinders are sprayed with preservative oil. When all cylinders are coated, desiccant plugs are installed in the spark plugs hole to absorb any moisture. It is important not to move the prop after applying the preservative oil, since the pistons will break the preservative seal and allow corrosion to form.

9.11 Corrosion Repair Techniques and Limits

9.11.1 Describe corrosion repair techniques and repair limits

9.11.2

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9.12 Mercury Contamination

9.12.1 Describe the effects of mercury contamination on a/c structure, methods used to remove the contamination and precaution to be observed

Mercury attacks Al by a chemical reaction known as amalgamation. In this process, the Mercury attacks along the grain boundaries within the alloy, and in a very short time completely destroys it. Mercury is very slippery and flows through the tiny cracks to get to the lowest part of the structure where it causes extensive damage. Mercury and its vapors are very poisonous to humans.If mercury is spilled, remove every particle with a vacuum cleaner having a mercury trap in the suction line or with a rubber suction bulb or medicine dropper. Never attempt to remove mercury by use of compressed air.10. Non-Destructive Testing

10.1 Common NDT Methods

10.1.1 to 10.1.4

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10.2 NDT Operator Qualification and CertificationDescribe NDT operator certification and qualification or re qualification requirements as per (AC 43).

Reference Air Transport Association (ATA) Specification 105 – Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods. Level 1 Special –

Initial classroom hours and on-the-job training shall be sufficient enough to qualify an individual for certification for a specific task.

Pass the following: vision and colour perception exam; general exam dealing with standards and NDT procedures, and a practical exam conducted by a Level 2 or Level 3 certified person.

Level 1/Level 2 – The individual shall have an FAA A&P Mechanic Certificate, complete the

required classroom hours and complete an exam.

Level 3 – Must have a 4 year university degree in engineering or science, plus 1

year of minimum experience in NDT, comparable to that of Level 2 OR: The individual must have 2 years of engineering or science study at a

technical school, plus 2 years of experience as a Level 2 OR: The individual must have 4 years of experience working as a Level 2 in

the applicable NDT methods and complete an examination.

10.5 Visual Inspection

10.5.1 – Visual inspection is the oldest and most common form of NDI for aircraft. Visual inspection provides a means of detecting and examining a wide

variety of component and material surface discontinuities, such as cracks, corrosion, contamination, surface finish, weld joints, solder connections, and adhesive dis-bonds.

10.6 Liquid Penetrant Testing

a. It is important to ensure that parts are thoroughly cleaned and dried before doing penetrant inspection. All surfaces to be inspected should be free of contaminants, paint, and other coatings that could prevent penetrant from entering discontinuities.

b. Penetrant inspection is used on nonporous metal and nonmetal components to find material discontinuities that are open to the surface and may not be evident to normal visual inspection. The basic purpose of penetrant inspection is to increase the visible contrast between a discontinuity and its background.

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c. Apply the penetrant by spraying, brushing, or by completely submerging the part in a container of penetrant. Wait the recommended amount of time after the penetrant has been applied to allow it to enter any discontinuities.

d. Excess penetrant must be removed from the part’s surface to prevent a loss of contrast between indications of discontinuities and the background during the inspection. Removal may require actually washing or spraying the part with a cleansing liquid, or may simply require wiping the part clean with a solvent moistened cloth. The removal method is determined by the type of penetrant used.

e. Apply developer after excess penetrant is removed and, where required, the surface is dried. Apply the developer in a thin uniform layer over the surface to be inspected. Developer acts like a blotter to assist the natural capillary action bleed-out of the penetrant from discontinuities. After the developer is applied, allow sufficient time for the penetrant to be drawn out of any discontinuities.

f. Black light (UV) operation and its advantages: UV light increases the visibility of the defect.

g. Inspection area – After the penetrant has sufficiently developed, visually inspect the surface for indications from discontinuities.

h. Indications – the developer will dry white, and if there is any dye trapped in a crack the developer will pull it out, staining the developer red. Any red areas in the white developer will indicate a crack or defect.

i. Post Inspection Cleaning - Remove inspection material residues from parts after completion of penetrant inspection. This residue could interfere with subsequent part processing, or if left on some alloys, it could increase their susceptibility to hydrogen embrittlement, Intergranular corrosion, and stress corrosion during service.

j. Factors affecting the success of Liquid Penetrant Inspection – the surfaces should be clean, and not too rough. All excessdye should be removed before the developer is applied. Use extreme care not to contaminate the developer with the dye.

k. System monitoring -

l. Health & Safety Considerations – Avoid long exposure to the fumes. Avoid excessive exposure of dye or developer to hands.

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Use fire precautions as the cleaners are volatile.

10.7 Magnetic Particle Testing

Magnetic particle inspection is a method for detecting cracks, laps and discontinuities in ferro-magnetic materials. Magnetic particle inspection can be used only on ferro- magnetic materials (iron and steel). Magnetic particles are applied over a surface either dry, as a powder, or wet, as particles in a liquid carrier such as oil or water.

a/b. Magnetism of materials/Testing theory – Magnetic particle inspection uses the tendency of magnetic lines of force, or flux, of an applied field to pass through the metal rather than through the air. A defect at or near the metal’s surface distorts the distribution of the magnetic flux and some of the flux is forced to pass out through the surface. The field strength is increased in the area of the defect and opposite magnetic poles form on either side of the defect. Fine magnetic particles applied to the part are attracted to these regions and form a pattern around the defect. The pattern of particles provides a visual indication of a defect.

Certain characteristics inherent in the magnetic particle method may introduce errors in examination results. Nonrelevant errors are caused by magnetic field distortions due to intentional design features, such as sharp radii, keyways, drilled holes.The particles used in magnetic particle inspection are finely divided ferro-magnetic materials that have been treated with colour or fluorescent dyes to improve visibility against the various surface backgrounds of the parts under inspection. Magnetic particle inspection materials for use on a specific part or component will generally be specified by the aircraft or component manufacturer or the FAA.

c. Surface Preparation – Unless otherwise specified, magnetic particle examination should not be performed with coatings in place that could prevent the detection of surface defects in the ferro-magnetic substrate. Parts should be free of grease, oil, rust, scale, or other substances which will interfere with the examination process.Magnetic particle examination generally consists of: the application of magnetic particles; magnetization; determination of field strength; special examination techniques; and demagnetization and post-examination cleaning.

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d. Magnetisation – Magnetic particles can be applied by: Wet Continuous method, Dry Continuous method and Residual Magnetisation method. Dry method is not recommended for use on aerospace components because of its lower sensitivity level. The wet method is considered most satisfactory.Circular Magnetisation - is induced in the part by the central-conductor method or the direct-contact method. (Transverse, i.e. perpendicular cracks will not show)Longitudinal magnetization – is induced in a part by placing the part in a strong magnetic field, such as the center of a coil or between the poles of an electromagnetic yoke.

Wash the parts in a clean suspension vehicle only enough to remove the weakly held particle accumulations causing the non-relevant indications. Particles at true cracks will be more strongly held and will persist if the washing is gently done.j. Post test cleaning – When oil suspensions are used, solvent clean or remove the part until all magnetic particles and traces of oil are removed.When parts or materials have been examined using water suspension methods, completely remove the water by any suitable means, such as an air blast, to ensure that the parts are dried immediately after cleaning. Thoroughly rinse the part with a detergent base cleaner until all magnetic particles are removed. Then rinse in a solution of water and rust inhibitor.

10.8 Eddy Current Testing

Eddy current inspection is a testing method that requires little or no part preparation and can detect surface and sub-surface flaws in most metals. Furthermore, it can differentiate between among metals and alloys, as well as a metal’s heat treat condition. Eddy current inspection is based on the principle of current acceptance.

a. Testing Theory - Eddy currents are induced in a test article when an alternating current is applied to a test coil (probe). The alternating current in the coil induces an alternating magnetic field in the article which causes eddy currents to flow in the article. Flaws in or thickness changes of the test-piece influence the flow of eddy currents and change the impedance of the coil accordingly. Instruments display the impedance changes either by impedance plane plots or by needle deflection.The following are typical eddy current equipment requirements for surface crack inspections: Instruments must meet the liftoff and sensitivity requirements of the applicable NDI procedures. The frequency requirement is generally 100 Hz to 200 kHz.

10.9 Ultra Sonic Testing

Ultrasonic inspection is an NDI technique that uses sound energy moving through the test specimen to detect flaws. When a body vibrates, it produces

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sound waves that are transmitted by the air surrounding the body. Under normal condition these sound waves propagate longitudinally from the source of vibration and are called Longitudinal waves. A second type of wave propagation occurs at right angles to the direction of the sound, and this type of wave propagation occurs only in material made of tightly bonded molecules, such as solids are called Transverse waves or Shear waves.Ultrasonic waves used for NDI vary in frequency from 200kHz to 25MHz, and are reflected, focused or refracted. This sound energy propagates through a solid or liquid material with little loss in wave energy.Two basic ultrasonic inspection techniques are employed: Pulse-Echo and Through-Transmission.

Pulse Echo - This process uses a transducer to both transmit and receive the ultrasonic pulse. The received ultrasonic pulses are separated by the time it takes the sound to reach the different surfaces from which it is reflected. The size (amplitude) of a reflection is related to the size of the reflecting surface. The pulse-echo ultrasonic response pattern is analyzed on the basis of signal amplitude and separation.

Through Transmission - This inspection employs two transducers, one to generate and a second to receive the ultrasound. A defect in the sound path between the two transducers will interrupt the sound transmission. The magnitude (the change in the sound pulse amplitude) of the interruption is used to evaluate test results. Through transmission inspection is less sensitive to small defects than is pulse-echo inspection.

Fabrication of NDI Reference Standards: Every ultrasonic test unit should have sample materials that contain unbonds equal to the sizes of the minimum rejectable unbonds for the test parts. Information on minimum rejectable unbond sizes for test parts should be obtained from the OEM’s manuals

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10.10 Radiographic Testing

This method allows a photographic view inside a structure. In other words, this method uses certain sections of the electromagnetic spectrum to photograph an objects interior. The amount of energy these x-rays contain is related inversely related to their wavelength – the shorter the wavelength the greater the energy. They have no electrical charge or mass and travel in straight lines at the speed of light and are able to penetrate matter. The depth of the penetration is dependant upon the rays energy.An x-ray generator consists of a tube containing a heavy insulating envelope. A coil at one end of the tube serves as a cathode that emits electrons when it is heated with electric current. At the other end of the tube is an anode on which a target is mounted.A sheet of photographic film is placed as close to the specimen as possible and is oriented so that the radiation penetrates and passes an amount of radiation proportional to the specimen’s density. The denser the specimen, the less radiation passes through, and the less the film is exposed.The best shielding against radiation is a layer of lead. Persons working around x-ray should wear radiation monitoring film badges, or dosimeters. Dark spots on x-ray film indicates defects. The denser the material, the less the film is exposed.

11. Aircraft Welding

11.1 Various Welding Process

11.1.1 Describe the following welding processes giving examples of where each would be used.

a. Gas Tungsten Arc (GTAW) or TIG – a method of electric arc welding in which the electrode in the torch is a fine non-consumable tungsten wire. The arc is developed in a flow of an inert gas such as argon or helium and this prevents the formation of oxides in the puddle. A filler rod is manually fed in the molten puddle. DC current is used to welding mild steel, stainless steel and titanium. AC current is used for welding Aluminium and magnesium.

b. Gas Metal Arc Welding (GMAW) or MIG – a method of electric arc welding in which the electrode is an expandable wire and is fed into the torch. An inert gas such as Argon, Helium or CO2 flows out around the wire to protect the puddle from oxygen. Low-voltage high-current DC is used almost exclusively with GMAW welding. GMAW is used more for large volume production work than for a/c repair.

c. Oxyacetylene Welding (conventional gas welding) – A fuel gas such as acetylene or hydrogen is mixed inside a welding torch with oxygen to produce a

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flame which is used to melt the materials to be welded. A filler rod is melted into the puddle of molten metal to reinforce the weld.

d. Electric arc welding – a group of welding processes wherein coalescence is produced by heating with an arc or arcs, with or without the application of pressure and with or without the use of filler metal (i.e. GTAW and GMAW)

e. Electrical resistance welding – Many thin sheet metal parts for aircraft, especially stainless steel parts, are joined by one of the forms of electric resistance welding, either spot welding or seam welding.

f. Electron Beam welding – Electron beam welding uses a very high-energy electron beam to produce deep, narrow penetration. The electron beam has a higher energy content than a laser beam, and is also smaller. Welding has to be performed in a vacuum, as the electron beam is absorbed by air. This complicates the process when changing the workpiece. On the other hand, the absence of air is good for the welding process, as there can be no reactions between air and the metal of the weld or workpiece.

g. Plasma Arc welding – The plasma welding method employs an inner plasma gas and outer shielding gas. The plasma gas flows around a retracted centered electrode, which is usually made of tungsten. The shielding gas flows through the outer jet, serving the same purpose as in TIG welding. Resistor R limits the current in the pilot arc which can be ignited also when the torch is apart from the workpiece.

h. Thermal Spraying – Thermal spraying is used for applying metallic or ceramic layers to metals, for such purposes as producing a corrosion-resistant or wear-resistant layer on low-alloy steel.

i. Laser Welding – Laser light possesses several unique properties, among which are the fact that it is parallel and highly concentrated. It can therefore be conducted, by mirrors or glass fibers, to a welding position that is remote from the power unit. It is also monochromatic, i.e. at a single definite wavelength, which depends on the type of laser used.

11.2 Welding SymbolsA welding symbol on a drawing consists of:

An arrow line (1) One or two reference lines (2) An elementary symbol (3) Possible supplementary symbols Dimensions of the weld

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11.3 Weld Joints

11.3.1 Describe the various types of welded joint design, the preparation & the relative advantages, disadvantages and limitations of each.

Butt Joint – is made by placing two pieces of material edge to edge, so that there is no overlapping, and then welded.

Tee Joint – is formed when the edge of end of one piece is welded to the surface of another. These joints are very common in aircraft work, particularly in tubular structures.

Edge Joint – may be used when two pieces of sheet metal must be fastened together and load stresses are not important

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Corner Joint – is made when two pieces of metal are brought together so that their edges form a corner of a box or enclosure.

Lap Joint – is seldom used in a/c structures when welding with oxyacetylene, but is commonly used when spot welding.

11.3.2 Describe the depth of weld penetration that is considered desirable for each of the welded joints.

11.3.3 Describe why the edges of metal sheets may be beveled and notched prior to butt welding:

It is necessary to bevel the edges so that the heat from the torch can penetrate completely through the metal.

11.4 Welding Positional Techniques

11.4.1 Techniques used while welding in different positions:

11.4.2 Desired electrode angle when advancing the weld during arc welding.The electrode angle consists of two positions: Work angle is the angle from the horizontal measured at right angles to the direction of welding (fig, 7-15).  Travel angle is the angle in the direction of welding.

11.5 Welder Testing and Qualification

11.6 Surface Preparation prior to welding

Describe the methods of surface preparation for each welding process relative to various metal types. Include the requirement to remove such things as:

When preparing an a/c part for welding, remove all dirt, grease or oil, and any protective coating such as cadmium plating, enamel, paint, or varnish. Such coatings not only hamper welding, but also mingle with the weld and prevent good fusion.

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11.7 Weld Defects

11.7.1 Identify weld defects relating to each of the common aeronautical welding processes and an acceptable means of inspection for each.

Include misshapen and incorrectly sized welds, variable cap width and height, weld face roughness, incomplete weld fill and asymmetry of fillet welds.

SURFACE POROSITY— Surface porosity usually results from atmospheric contamination. It can be caused by a clogged nozzle, shielding gas set too low or too high, or welding in a windy area.CRATER POROSITY— Crater porosity usually results from pulling the torch and gas shield away before the crater has solidified.COLD LAP— Cold laps often result when the arc does not melt the base metal sufficientlyLACK OF PENETRATION— Lack of penetration usually results from too little heat input in the weld zone.

11.7.2 Identify indications of where excessive heat has been used in the welding processes.

Burn-through (too much penetration) is caused by having too much heat input in the weld zone. You can correct this problem by reducing the wire-feed speed, which, in turn lowers the welding amperage.Excessive heat causes excessive penetration and results when the metal melts or falls through the weld joint. Excessive heat can eventually cause the base metal to melt through the joint.

11.7.3 Describe how overheating, burning and buckling is overcome in the various welding process:

1. Proper edge Preparation and Fit-up – edges are properly beveled and spacing is adequate.

2. Control the heat input – the faster a weld is made, the less the heat is absorbed by the base metal.

3. Pre-heat the metal.

11.7.4 Specific heat treatment processes to be applied to components after welding has taken place.

After weld is completed, part is given a final stress relief or heat treated.  For best results, stress relief  or  heat  treatment  should  be  accomplished  immediately after  welding  before  material  is  allowed  to  cool  below minimum preheat

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temperature  (for  some  alloys  this  is  a  mandatory requirement),  and  in  any case  before material is allowed to cool to room or atmosphere temperature.

11.8 Welding Equipment

11.8.1 Welding equipment for each welding process:

Gas Metal Arc Welding equipment basically consists of four units: the power supply, the wire feeding mechanism, the welding gun (also referred to as the torch), and the gas supply.

11.8.2 Peening is a procedure  that  involves  lightly  hammering a weld as it cools. This process aids in relieving built-up stresses and preventing surface cracking in the joint area; however, peening should be done with care because excess hammering can work harden and in- crease stresses in the weld. This condition leads to weld embrittlement and early failure.

11.8.5 De-slagging equipment -

11.9 Safety and Health Issues

11.9.2: Usually, a CO2 extinguisher is adequate. If the space is small or if the access is only a small opening, CO2 is not the extinguishing agent to use. If CO2 is not recommended, the use ofwater spray from a fog nozzle is preferredUse a Class D fire extinguisher or a sodium chloride base dry powder to fight magnesium fires.

11.10 Metal Cutting

Describe the gas cutting; process, techniques, equipment and precautions:

The common methods used in cutting metal are oxy-gas flame cutting. When using the oxy-gas cutting process, you heat a spot on the metal to the kindling or ignition temperature. The term for this oxy-gas flame is the PREHEATING FLAME.  Next, you direct a jet of pure oxygen at the heated metal by pressing a lever on the cutting torch. The oxygen causes a chemical reaction known as OXIDATION to take place rapidly.  When oxidation occurs rapidly, it is called COMBUSTION or BURNING.An oxy-gas cutting outfit usually consists of a cylinder of acetylene or MAPP gas, a cylinder of oxygen, two regulators, two lengths of hose with fittings, and a cutting torch with tips.

11.11 Filler Rods, Fluxes, Wires and Electrodes

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11.11.1 Use of Filler Rods and Fluxes

The use of Filler Rod not only adds reinforcement to the weld area, but also adds desired properties to the finished weld. By selecting the proper type of rod, either tensile strength or ductility can be secured in a weld. Similarly, rods can be selected that will help retain the desired amount of corrosion resistanceThe use of the proper flux in welding aluminium is extremely important. Aluminium welding flux is designed to remove the aluminium oxide by chemically combining with it. In gas welding, the oxide forms rapidly in the molten metal. It must be removed or a defective weld will result.

11.11.2 Common electrode and wire types for various welding processes.

Two types of welding rods available for gas welding aluminium alloys are the 1100 and 4043 rods. The 1100 rod is used when maximum resistance to corrosion and high ductility are of primary importance. The 1100 rod is used for welding 1100 and 3003 type aluminium alloys only. The 4043 rod is used for greater strength and minimizes the tendency for cracking. It also is used for all other wrought aluminium alloys and castings.

11.11.3 Factors to be considered when selecting a welding rod for a apecific welding operation.

The diameter of the rod used is governed by the thickness of the metals being joined. If the rod is too small, it will not conduct heat away from the puddle rapidly enough, and a burned weld will result. A rod that is to large will chill the puddle. As in selecting the proper size welding torch tip, experience will enable the welder to select the proper diameter welding rod.

11.12 Welding Terminology

Heat Affected Zone - The heat-affected zone (HAZ) is the portion of steel immediately adjacent to the weld that has been metallurgically modified by the heat of the welding. The microstructure has been changed, and the mechanical properties typically have been degraded with reduced ductility and toughness, but with increased strength.

Total Weld –

Deoxidisers - A substance that can be added to molten metal to remove either free or combined oxygen.

Arc Stream –

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11.13 Welding Gasses

Describe the nature and purpose of the various gasses used in the welding processes:

11.14 Flame types and their applications

Carburizing flame - The carburizing flame, produced by burning an excess of acetylene, may be recognized by its three distinct colours. There is a bluish-white inner core, a white intermediate cone, and a light-blue outer flame.  It may be recognized also by the feather at the tip of the inner cone. The degree of carburization can be judged by the length of the feather.

Oxidising flame – Produced by burning an excess of oxygen. It has the general appearance

of the neutral flame, but the inner cone is shorter, slightly pointed and has a purplish tinge.

Flame burns with a hissing sound. When welding ferrous metals, you can recognise an oxidising flame by the numerous sparks thrown off as the metal melts and by the foam that forms on the surface.

Neutral Flame – Does not alter the composition of the base metal to any great extent;

therefore it is the flame best suited for most applications. Is divided into distinct zones: inner zone consists of a white, round,

smooth cone. Outer zone is made up of completely burned oxygen and acetylene, is blue with purple tinge at the point and edges.

Melts metal without changing the its properties, and it leaves the metal clear and clean.

11.14.2 Identify the flame type that would be most suitable for silver soldering.Neutral flame

11.15 Welding aircraft structure and components

11.15.2 Purpose of back-up strips when carrying out overhead butt welds.A backup strip should be used to prevent weld-metal drop through.

11.15.3 Describe how distortion can be minimized while butt welding metal platesRefer 11.7.3

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11.16 Soldering and Brazing

11.16.1: The melting point of the alloy is lowered when tin is added to lead.

11.16.2:Purpose of flux: flux is a cleaning agent to remove oxidation during soldering.

There are two classes of flux: corrosive and noncorrosive. Zinc chloride, hydrochloric acid, and sal ammoniac are corrosive fluxes. Corrosive flux should NEVER be used in electrical or electronic repair work. Use only rosin fluxes. Any flux remaining in the joint corrodes the connection and creates a defective circuit. Rosin is a noncorrosive flux and is available in paste, liquid, or powder form.

11.16.3:

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12. Bonded Honeycomb Structure

12.1 Terminology

12.1.1 With regards to metal honeycomb structure, describe the following:

a. Bond Line – the layer of adhesive which attaches two adherends.

b. Core – a centrally located layer or composite component of a sandwich construction which separates and stabilizes the facings and transmits shear between them.

c. Facing sheets – the outermost layer or composite component of a sandwich construction.

d. Nodes - the bonded portion of the honeycomb flat sheet material; the honeycomb cell's double wall.

e. Cell Edge –

f. Ribbon dimension & direction –

g. Fatigue elimination -

12.2 Use of Metal Bonded Honeycomb

12.2.1 – Metal bonded honeycomb is mostly used in areas such as bulkheads, control surfaces, fuselage panels, wing panels, empennage skins and radomes.

12.2.2 – Advantages: superior strength-to-weight ratio; better able to withstand sonic vibration; relatively low cost when compared with fastener cost; greatly reduces sealing problem while increasing aerodynamic smoothness.

12.3 Non-Metal Bonded Honeycomb Structure

12.3.1 – 12.3.2 –

12.4 Radomes

12.5 Repair Processes

12.5.1 Describe the repair processes relating to metal bonded honeycomb structure with particular regard to the following.

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c. Damage evaluation and classification – External damage – punctures, delamination or impact. Delamination – can be detected by tapping of the surface. Impact damage – is generally hard to detect since the real danger is not

seen on the surface but is propagated to the other side of the laminate. Internal defects – such as voids, distorted core or crushed core can only

be detected by NDI. Bondline degradation – evaluation of adhesive degradation can be made

by NDI. Skin cracks, fuel and oil contamination; corrosion. Moisture – moisture in a structure can be determined by x-ray and by

some thermal techniques.

d. Restoration of original strength –

e. Tools & Equipment – Tools for cutting Prepeg & Fabric materials » Hand Shears; Power

Shears; Rotary cutter; knives; Die Cutters. Tools for working on Composite laminates » Drill Bit (modified twist drill);

Drill bit (single flute drill); Router Bits; Countersinking and Counterboring tool;

f. Routers and routing of damaged areas – a power tool used to cut the core from a honeycomb material. A router consists of a high speed motor spinning a tool whose cutting edges are on its side. A guide is used to control the depth to which the cutting tool can penetrate the material being routed.

12.6 Testing Methods

Visual NDI It is the observation of the material to detect gross defects.

Optical NDI Used for inspecting internal components of a composite structure.

Ultrasonic NDI uses pulsed ultrasound at 2.25 and 10MHz. is adversely effected by destructive wave interference, which is caused by

varying adherend or adherend thickness.

13. Transparent Plastics

13.1 – Types of Transparent Plastics

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13.1.1 - Plastics cover a broad field of organic synthetic resin and may be divided into two main classifications – thermoplastics and thermosetting plastics.

a. Thermoplastics – may be softened by heat and can be dissolved in various organic solvents and are commonly employed in windows, canopies, etc.

b. Thermosetting Plastics - These plastics do not soften appreciably under heat but may char and blister at temperatures of 240 to 260 °C. Once formed they retain that shape and cannot be re-molded or re-shaped.

13.1.2 Characteristics of acrylic and cellulose acetate plastics: Acrylic plastics are known by the trade names of Lucite or Plexiglas and by theBritish as Perspex. Cellulose acetate was used in the past but since it is dimensionally unstable and turns yellow after it has been installed for a time, it has just about passed from the scene and is not considered an acceptable substitute for acrylic.

13.1.3 Uses and characteristics of laminated plastics:Installed in pressurized a/c because of their superior shatter resistance and greater resistance to explosive decompression.

13.1.4 Describe optical considerations relating to plastics:Optical qualities of the transparent plastic used in the a/c must be as good as those of the best quality glass. Other advantages of plastics over glass is that plastics break in large dull-edged pieces; they have low water absorption and they do not readily fatigue-crack from vibration

13.1.5 Advantages of stretched acrylic plastic for a/c windows:

13.1.6 In-situ tests to distinguish between acrylic and acetate plastics:

Rub an area on the plastic with a solution of acetone, then blow on the area. If the plastic is acrylic, it will turn white; if it is acetate, it will soften but not change colour.A drop of zinc chloride placed on the acetate base plastic will turn the plastic milky, but will have no effect on acrylic plastic.

13.1.7 Construction and installation of windows and windshields:

13.1.8 Identify various types of plastics by way of edge colour and MIL Spec.Very few of the transparent plastics are color clear when viewed from the edge; some are practically clear. The cellulose plastics have a yellowish tint when viewed from the edge.

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13.2 Forming and Fabricating

13.2.1 Describe forming and fabricating procedures for transparent plastics including the following:

a. Cutting – scribing and edge sanding is the cutting method most generally used on flat sections or 2D curved pieces. The sheet is first cut to approximate shape on a band saw, cutting approx 1/16” oversize.

b. Drilling – use drills with slow-spiral polished flutes.

c. Cementing – it is possible to obtain a cemented joint which approximates the original plastic in strength. Cementing of transparent acrylic plastics depends on the intermingling of the two surfaces of the joint. The most common method of cementing transparent plastics is the “soak method”.

d. Heat Treatment – transparent plastics can be heated to their respective forming temperatures and be formed to almost any shape; and on cooling, the material retains the shape to which it was formed, except for a small contraction.

f. Removal of masking paper adhesive – moisten the paper with aliphatic naphtha. This will loosen the adhesive. Sheets so treated should be washed immediately with clear water.

13.2.2 Common installation procedures and precautions relating to transparent plastics:

a. Since plastics expand and contract approximately three times as much as metal, suitable allowance for dimensional changes with temperature must be made.

b. Bolt and rivet mounting – In bolt installations, spacers, collars, shoulders, or stop nuts should be used to prevent excessive tightening of the bolt. To ensure long service give special considerations to the following factors:

1. Use as many bolts or rivets practical.2. Distribute the total stresses as equally as possible.3. Make sure the holes in the plastic are sufficiently larger than the

diameter of the bolt to permit expansion and contraction of the plastic relative to the frame.

c. Synthetic fibre edge attachment – modern edge attachments to transparent plastic assemblies are made of synthetic fibres specially impregnated with plastic resins. Reinforced laminated edge attachments are the preferred type, especially when mounting by bolts or rivets. The edges have the advantage of more efficiently distributing the load and reducing failures caused by differential thermal expansion.

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13.2.4 – 13.2.7: Omit (due general stuff)

13.2.5 Methods of finishing acrylic components by sanding, buffing and polishing

Clean the plastic by washing it with plenty of water and mild soap, using a clean, soft, grit free cloth, sponge or bare hands.

Plastics should not be rubbed with a dry cloth since this is likely to cause scratches.

Do not attempt hand polishing or buffing until the surface is clean. A soft, open type cotton or flannel buffing wheel is suggested.

13.2.6 – Refer above 13.2.5

13.2.7 Properties of various types of clear plastics used for windows and windshields

13.3 Handling and Storage

13.3.1 Storage and protection procedures for transparent plastics. Store them in cool, dry place. Paper-masked transparent sheets must be kept out of the direct rays of

the sun, because sunlight will accelerate deterioration of the adhesive. Plastic sheets should be stored with the masking paper in place, in bins

that are tilted at a ten-degree angle from the vertical. This will prevent their buckling.

Formed sections should be stored with ample support so they will not lose their shape. Vertical nesting should be avoided.

13.3.2 Describe how transparent plastics are polished

13.3.3 –

14. Composite Materials

14.1 Terminology

a. A-Stage – an early stage in the preparation of certain thermosetting resins in which the material is still soluble in certain liquids, and may be liquid or capable of becoming liquid upon heatingb. B-Stage – c. C-Stage – d. Accelerator – a material that increases the rate at which chemical reactions would otherwise occur.

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e. Autoclave - a closed vessel for producing an environment of fluid pressure, with or without heat, to an enclosed object undergoing a chemical reaction or other operation.f. Balanced Laminate – a composite laminate in which all laminate occur in pairs symmetric about the mid-plane (but not necessarily adjacent to each other).g. Bi-directional fabric – h. Bleeder – a nonstructural layer of material used in the manufacture of composite assemblies to allow the escape of excess gas and resin during cure.i. Blocking – undesired cohesion or adhesion that interferes with the satisfactory and efficient use of the material.j. Bridging - vacuum bagging material spanning tool or part surfaces.k. Catalyst - a substance whose presence initiates or changes the rate of a chemical reaction, but does not itself enter into the reaction.l. Caul Plate - a flat or contoured tool used to distribute pressure and to define a surface for the top of the laminate during laminate consolidation or cure.m. Core Separation – n. Core Splicing – o. Crazing – fine hairline cracks.p. Cure – to develop the strength properties of an adhesive by chemical reaction.q. Cure Stress – r. Exothermic Reaction – a chemical reaction in which heat is evolved.s. Fabric-plain, satin and twill weave – w. Finish – the final treatment or coating of a surface.x. Gel condition – y. Glass (E & S) – z. Carbon FRP – aa. Glass FRP- bb. Inhibitor - a substance used to reduce the rate of a chemical or electrochemical reaction, commonly corrosion or pickling.cc. Kevlar (Aramid) – dd. Kevlar FRP – ee. Lamination sequence (stacking or nesting) – the arrangement of ply orientations and material components in a laminate specified with respect to some reference direction.ff. Lay-Up – a description of the component materials, geometry, and so on of a laminate.gg. Matrix – the continuous constituent of a composite material, which surrounds or engulfs embedded filler or reinforcement.hh. Micro Cracking - a crack too small to be seen with the unaided eye.ii. Mould Release agent – jj. Peel ply – a removable ply molded onto the surface of a laminate to provide a chemically clean surface for bonding or painting after removal.kk. Ply orientation – the acute angle (theta) including 90 ° between a reference direction and the ply principal axis. The ply orientation is positive if measured counterclockwise from the reference direction and negative if measured clockwise.

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mm. Post Cure – further treatment by time or temperature, or both, of an adhesive to modify specific properties.nn. Pot Life – period of time during which a multi-part adhesive can be used after mixing the components.oo. Prepreg – a type of composite material in which the reinforcing fibers are encapsulated in an uncured resin. Prepreg must be kept refrigerated to prevent the resin curing before it is cured. Prepreg = preimpregnated fabric.pp. Reinforced Plastic – a plastic with high strength fillers imbedded in the composition, resulting in some mechanical properties superior to those of the base resin.qq. Resin Content - the amount of matrix present in a composite expressed either as percent by weight or percent by volume.rr. Resin Richness – ss. Resin Starvation – tt. Tracer – uu. Unidirectional Fabric - ww. Warp - any variation from a true or plane surface. Warp includes bow, crook, cup, and twist, or any combination thereof.xx. Warp Clock -

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