welding

Slide 1

WELDING

2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Overview of Welding TechnologyThe Weld JointPhysics of WeldingFeatures of a Fusion Welded Joint2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Joining and Assembly DistinguishedJoining welding and adhesive bondingThese processes form a permanent joint between partsAssembly - mechanical methods (usually) of fastening parts togetherSome of these methods allow for easy disassembly, while others do not 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

WeldingJoining process in which two (or more) parts are coalesced at their contacting surfaces by application of heat and/or pressureMany welding processes are accomplished by heat alone, with no pressure appliedOthers by a combination of heat and pressureStill others by pressure alone with no external heatIn some welding processes a filler material is added to facilitate coalescence2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Why Welding is ImportantProvides a permanent jointWelded components become a single entity Usually the most economical way to join parts in terms of material usage and fabrication costs Mechanical fastening usually requires additional hardware (e.g., screws) and geometric alterations of the assembled parts (e.g., holes)Not restricted to a factory environmentWelding can be accomplished "in the field" 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Limitations and Drawbacks of WeldingMost welding operations are performed manually and are expensive in terms of labor cost Most welding processes utilize high energy and are inherently dangerous Welded joints do not allow for convenient disassembly Welded joints can have quality defects that are difficult to detect 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Types of Welding ProcessesSome 50 different types of welding processes have been catalogued by the American Welding Society (AWS)Welding processes can be divided into two major categories:Fusion weldingSolid state welding2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Fusion WeldingJoining processes that melt the base metalsIn many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and added strength to the welded joint A fusion welding operation in which no filler metal is added is called an autogenous weld 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Some Fusion Welding ProcessesArc welding (AW) melting of the metals is accomplished by an electric arc Resistance welding (RW) melting is accomplished by heat from resistance to an electrical current between faying surfaces held together under pressure Oxyfuel gas welding (OFW) melting is accomplished by an oxyfuel gas such as acetylene2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Arc WeldingBasics of arc welding: (1) before the weld; (2) during the weld, the base metal is melted and filler metal is added to molten pool; and (3) the completed weldment


Solid State WeldingJoining processes in which coalescence results from application of pressure alone or a combination of heat and pressureIf heat is used, temperature is below melting point of metals being welded No filler metal is added in solid state welding 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Some Solid State Welding ProcessesDiffusion welding (DFW) coalescence is by solid state fusion between two surfaces held together under pressure at elevated temperatureFriction welding (FRW) coalescence by heat of friction between two surfaces Ultrasonic welding (USW) coalescence by ultrasonic oscillating motion in a direction parallel to contacting surfaces of two parts held together under pressure2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Principal Applications of Welding Construction - buildings and bridgesPiping, pressure vessels, boilers, and storage tanksShipbuildingAircraft and aerospaceAutomotive Railroad 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Welder and FitterThe welder manually controls the path or placement of welding gun Often assisted by second worker, called a fitter, who arranges the parts prior to welding Welding fixtures and positioners are used to assist in this function 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

The Safety IssueWelding is inherently dangerous to human workers High temperatures of molten metalsIn gas welding, fuels (e.g., acetylene) are a fire hazard Many welding processes use electrical power, so electrical shock is a hazard 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Special Hazards in Arc WeldingUltraviolet radiation emitted in arc welding is injurious to human visionWelder must wear special helmet with dark viewing windowFilters out dangerous radiation but welder is blind except when arc is struck Sparks, spatters of molten metal, smoke, and fumesVentilation needed to exhaust dangerous fumes from fluxes and molten metals 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Arc WeldingA manual arc welding operationWelder must wear protective clothing and helmet


Automation in WeldingBecause of the hazards of manual welding, and to increase productivity and improve quality, various forms of mechanization and automation are used Machine welding mechanized welding under supervision and control of human operatorAutomatic welding equipment performs welding without operator control Robotic welding - automatic welding implemented by industrial robot2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

The Weld JointThe junction of the edges or surfaces of parts that have been joined by welding Two issues about weld joints: Types of jointsTypes of welds used to join the pieces that form the joints 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Five Types of Joints(a) Butt joint, (b) corner joint, (c) lap joint, (d) tee joint, and (e) edge joint


Types of WeldsEach of the preceding joints can be made by welding Other joining processes can also be used for some of the joint types There is a difference between joint type and the way it is welded the weld type 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Fillet WeldUsed to fill in the edges of plates created by corner, lap, and tee joints Filler metal used to provide cross section in approximate shape of a right triangle Most common weld type in arc and oxyfuel weldingRequires minimum edge preparation2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

(a) Inside single fillet corner joint; (b) outside single fillet corner joint; (c) double fillet lap joint; (d) double fillet tee joint (dashed lines show the original part edges)Fillet Welds


Groove WeldsUsually requires part edges to be shaped into a groove to facilitate weld penetrationEdge preparation increases cost of parts fabricationGrooved shapes include square, bevel, V, U, and J, in single or double sidesMost closely associated with butt joints2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

(a) Square groove weld, one side; (b) single bevel groove weld; (c) single Vgroove weld; (d) single Ugroove weld; (e) single Jgroove weld; (f) double Vgroove weld for thicker sections (dashed lines show original part edges)Groove Welds


Plug Weld and Slot Weld(a) Plug weld and (b) slot weld


Fused section between surfaces of two sheets or plates: (a) spot weld and (b) seam weld Used for lap joints Closely associated with resistance welding Spot Weld and Seam Weld


Flange Weld and Surfacing Weld(a) Flange weld and (b) surfacing weld used not to join parts but to deposit filler metal onto surface of a base part


Physics of WeldingFusion is most common means of achieving coalescence in weldingTo accomplish fusion, a source of high density heat energy must be supplied to the faying surfacesResulting temperatures cause localized melting of base metals (and filler metal, if used)For metallurgical reasons, it is desirable to melt the metal with minimum energy but high heat densities2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Power DensityPower transferred to work per unit surface area, W/mm2 (Btu/secin2)If power density is too low, heat is conducted into work, so melting never occurs If power density too high, localized temperatures vaporize metal in affected region There is a practical range of values for heat density within which welding can be performed2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Comparisons Among Welding ProcessesOxyfuel gas welding (OFW) develops large amounts of heat, but heat density is relatively low because heat is spread over a large areaOxyacetylene gas, the hottest OFW fuel, burns at a top temperature of around 3500C (6300F)Arc welding produces high energy over a smaller area, resulting in local temperatures of 5500 to 6600C (10,000 to 12,000F)2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Power Densities for Welding ProcessesWelding processW/mm2(Btu/sec-in2) Oxyfuel10(6) Arc50(30) Resistance1,000(600) Laser beam9,000(5,000) Electron beam10,000(6,000)2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Power Density Power entering surface divided by corresponding surface area:

where PD = power density, W/mm2 (Btu/secin2); P = power entering surface, W (Btu/sec); and A = surface area over which energy is entering, mm2 (in2)


Unit Energy for MeltingQuantity of heat required to melt a unit volume of metal Um is the sum of: Heat to raise temperature of solid metal to melting point Depends on volumetric specific heatHeat to transform metal from solid to liquid phase at melting point Depends on heat of fusion2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Heat Transfer Mechanisms in WeldingNot all of the input energy is used to melt the weld metalHeat transfer efficiency f1 - actual heat received by workpiece divided by total heat generated at source Melting efficiency f2 - proportion of heat received at work surface used for meltingThe rest is conducted into work metal2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Heat Transfer Mechanisms in Welding


Heat Available for WeldingHw = f1 f2 H

where Hw = net heat available for welding; f1 = heat transfer efficiency; f2 = melting efficiency; and H = total heat generated by welding process2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Heat Transfer Efficiency f1 Proportion of heat received at work surface relative to total heat generated at sourceDepends on welding process and capacity to convert power source (e.g., electrical energy) into usable heat at work surfaceOxyfuel gas welding processes are relatively inefficientArc welding processes are relatively efficient2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Melting Efficiency f2 Proportion of heat received at work surface used for melting; the rest is conducted into the workDepends on welding process but also thermal properties of metal, joint shape, and work thicknessMetals with high thermal conductivity, such as aluminum and copper, present a problem in welding because of the rapid dissipation of heat away from the heat contact area 2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Energy Balance EquationNet heat energy into welding operation equals heat energy required to melt the volume of metal weldedHw = Um V

where Hw = net heat energy delivered to operation, J (Btu); Um = unit energy required to melt the metal, J/mm3 (Btu/in3); and V = volume of metal melted, mm3 (in3)2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Cross section of a typical fusion welded joint: (a) principal zones in the joint, and (b) typical grain structureTypical Fusion Welded Joint


Features of Fusion Welded JointTypical fusion weld joint in which filler metal has been added consists of: Fusion zoneWeld interfaceHeat affected zone (HAZ) Unaffected base metal zone2010 John Wiley & Sons, Inc. M P Groover, Principals of Modern Manufacturing 4/e SI Version

Heat Affected ZoneMetal has experienced temperatures below melting point, but high enough to cause microstructural changes in the solid metalChemical composition same as base metal, but this region has been heat treated so that its properties and structure have been altered Effect on mechanical properties in HAZ is usually negativeIt is here that welding failures often occur

IBR-1950

High Pressure WelderTube welding ProcedureTIG WeldingDEFINITIONGas tungsten arc welding(GTAW), also known astungsten inert gas(TIG)welding, is anarc weldingprocess that uses a non-consumabletungstenelectrodeto produce theweld. The weld area is protected from atmospheric contamination by aninertshielding gas(argonorhelium), and afiller metalis normally used. Aconstant-currentwelding power supplyproduces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as aplasma.

OPERATION.

OPERATIONManual gas tungsten arc welding is often considered the most difficult of all the welding processes commonly used in industry. Because the welder must maintain a short arc length, great care and skill are required to prevent contact between the electrode and the workpiece. hand while manipulating the welding torch in the other

OPERATIONSimilar to torch welding, TIG normally requires two hands, since most applications require that the welder manually feed a filler metal into the weld area with oneUSETIG is most commonly used to weld thin sections ofstainless steeland non-ferrous metals such asaluminum,magnesium, andcopperalloys. The process grants the operator greater control over the weld than competing processes such asshielded metal arc weldingandgas metal arc welding, allowing for stronger, higher quality welds. USEHowever, TIG is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process,plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated

Applications

While the aerospace industry is one of the primary users of gas tungsten arc welding, the process is used in a number of other areas. Many industries use TIG for welding thin workpieces, especially nonferrous metals. APPLICATIONSIt is used extensively in the manufacture of space vehicles, and is also frequently employed to weld small-diameter, thin-wall tubing such as those used in the bicycle industry. In addition, TIG is often used in piping of various sizes.

QUALITY

QUALITYGas tungsten arc welding, because it affords greater control over the weld area than other welding processes, can produce high-quality welds when performed by skilled operators. Maximum weld quality is assured by maintaining cleanlinessall equipment and materials used must be free from oil, moisture, dirt and other impurities, as these cause weld porosity and consequently a decrease in weld strength and quality.

QUALITYTo remove oil and grease, alcohol or similar commercial solvents may be used, while a stainless steel wire brush or chemical process can remove oxides from the surfaces of metals like aluminum.EQUIPMENT

TIG torch with various electrodes, cups, collets and gas diffusersEQUIPMENT

TIG torch, disassembledEQUIPMENTThe equipment required for the gas tungsten arc welding operation includes a welding torch utilizing a non-consumable tungsten electrode, a constant-current welding power supply, and a shielding gas source.

Power supply

Gas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of TIG are manual or semiautomatic, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficultPower SUPPLY

POWER SUPPLYThe electrode used in TIG is made of tungsten or a tungsten alloy, because tungsten has the highest melting temperature among pure metals, at3,422 C(6,192F). As a result, the electrode is not consumed during welding, though some erosion (called burn-off) can occur. Electrodes can have either a clean finish or a ground finishclean finish electrodes have been chemically cleaned, while ground finish electrodes have been ground to a uniform size and have a polished surface, making them optimal for heat conduction. SHIELDING GASShielding gasesare necessary in TIG to protect the welding area from atmospheric gases such asnitrogenandoxygen, which can cause fusion defects, porosity, and weld metalembrittlementif they come in contact with the electrode, the arc, or the welding metal. The gas also transfers heat from the tungsten electrode to the metal, and it helps start and maintain a stable arc.

SHIELDING GAS

TIG system setupMaterials

Gas tungsten arc welding is most commonly used to weld stainless steel and nonferrous materials, such as aluminum and magnesium, but it can be applied to nearly all metals, with a notable exception beingzincand its alloys. Its applications involving carbon steels are limited not because of process restrictions, but because of the existence of more economical steel welding techniques, such as gas metal arc welding and shielded metal arc weldingMATERIALS. Furthermore, TIG can be performed in a variety of other-than-flat positions, depending on the skill of the welder and the materials being weldedMaterials

A TIG weld showing an accentuated AC etched zoneALUMINIUM AND MAGNESIUMAluminum and magnesium are most often welded using alternating current, but the use of direct current is also possible, depending on the properties desired. Before welding, the work area should be cleaned and may be preheated to 175to200C (347to 392F) for aluminum or to a maximum of150 C(302F)for thick magnesium workpieces to improve penetration and increase travel speed

STEELFor TIG ofcarbonand stainless steels, the selection of a filler material is important to prevent excessive porosity. Oxides on the filler material and workpieces must be removed before welding to prevent contamination, and immediately prior to welding, alcohol or acetone should be used to clean the surface

Dissimilar metals

Welding dissimilar metals often introduces new difficulties to TIG welding, because most materials do not easily fuse to form a strong bond. However, welds of dissimilar materials have numerous applications in manufacturing, repair work, and the prevention ofcorrosionandoxidation

Welding Parameters

Regardless of the technology, efficiency or variability, these are the list of parameters that affect the quality and outcome of the weld. When these parameters are improperly configured or out of range for the equipment or materials, this can lead to a variety of problems.Current

Too much current can lead to splatter and workpiece damage. In thin materials, it can lead to a widening of the material gap. Too little current can lead to sticking of the filler wire

Welding Voltage

This can be fixed or adjustable depending on the equipment. Some metals require a specific voltage range to be able to work

Vacuum welding

For industrial applications, superior results can be achieved by eliminating the effects of absorbed gasses at the weld. This can lead to reduced oxidation, reduced workpiece heat carried by convection and stronger materials due to dissolved or reacted gases such as oxygen, nitrogen and hydrogen.

Safety

Like other arc welding processes, TIG can be dangerous if proper precautions are not taken.Welderswearprotective clothing, including heavyleatherglovesand protective long sleeve jackets, to avoid exposure to extreme heat and flames. Due to the absence of smoke in TIG, the electric arc can seem brighter than inshielded metal arc welding, making operators especially susceptible toarc eyeand skin irritations not unlikesunburn.SAFETYHelmetswith dark face plates are worn to prevent this exposure toultraviolet light, and in recent years, new helmets often feature aliquid crystal-type face plate that self-darkens upon exposure to high amounts of UV light. Transparent welding curtains, made of apolyvinyl chlorideplastic film, are often used to shield nearby workers and bystanders from exposure to the UV light from the electric arc

SAFETY

welding

Documents