classification of welding processes
TRANSCRIPT
Minia University
Faculty of Engineering
Department of Production Engineering and Mechanical Design
Welding processes
Prepared by
Ahmed Sami Akl
Under the supervision of
Prof. Dr / Hamed Mahmoud Aboul-
Enein
Classification of welding processes
Welding is a process of joining two metal pieces as a result of significant diffusion of
the atoms of the welded pieces into the joint (weld) region. Welding is carried out by
heating the joined pieces to melting point and fusing them together (with or without
filler material) or by applying pressure to the pieces in cold or heated state.
Advantages of welding:
• Strong and tight joining;
• Cost effectiveness;
• Simplicity of welded structures design;
• Welding processes may be mechanized and automated.
Disadvantages of welding:
• Internal stresses, distortions and changes of micro-structure in the weld region;
• Harmful effects: light, ultra violate radiation, fumes, high temperature.
Applications of welding:
• Buildings and bridges structures;
• Automotive, ship and aircraft constructions;
• Pipe lines;
• Tanks and vessels;
• Railroads;
• Machinery elements.
Welding processes
• Arc welding
o Carbon Arc Welding;
o Shielded Metal Arc Welding (SMAW);
o Submerged Arc Welding (SAW);
o Metal Inert Gas Welding (MIG, GMAW);
o Tungsten Inert Gas Arc Welding (TIG, GTAW);
o Electro slag Welding (ESW);
o Plasma Arc Welding (PAW);
• Resistance Welding (RW);
o Spot Welding (RSW);
o Flash Welding (FW);
o Resistance Butt Welding (UW) ;
o Seam Welding (RSEW);
• Gas Welding (GW);
o Oxyacetylene Welding (OAW);
o Oxyhydrogen Welding (OHW);
o Pressure Gas Welding (PGW);
• Solid State Welding (SSW);
o Forge Welding (FOW);
o Cold Welding (CW);
o Friction Welding (FRW);
o Explosive Welding (EXW);
o Diffusion Welding (DFW);
o Ultrasonic Welding (USW);
• Thermit Welding (TW);
• Electron Beam Welding (EBW);
• Laser Welding (LW).
Principles of arc welding
Arc welding is a welding process, in which heat is generated by an electric arc struck
between an electrode and the work piece.
Electric arc is luminous electrical discharge between two electrodes through ionized
gas.
Any arc welding method is based on an electric circuit consisting of the following
parts:
• Power supply (AC or DC);
• Welding electrode;
• Work piece;
• Welding leads (electric cables) connecting the electrode and work piece to the
power supply.
Electric arc between the electrode and work piece closes the electric circuit. The arc
temperature may reach 10000°F (5500°C), which is sufficient for fusion the work
piece edges and joining them.
When a long join is required the arc is moved along the joint line. The front edge of
the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies
forming the joint.
Types of weld joints are shown in the figure:
When a filler metal is required for better bonding, filling rod (wire) is used either as
outside material fed to the arc region or as consumable welding electrode, which
melts and fills the weld pool. Chemical compositions of filler metal is similar to that
of work piece.
Molten metal in the weld pool is chemically active and it reacts with the surrounding
atmosphere. As a result weld may be contaminated by oxide and nitride inclusions
deteriorating its mechanical properties. Neutral shielding gases (argon, helium)
and/or shielding fluxes are used for protection of the weld pool from atmospheric
contamination. Shields are supplied to the weld zone in form of a flux coating of the
electrode or in other forms.
Carbon Arc Welding
Carbon Arc Welding (CAW) is a welding process, in which heat is generated by an
electric arc struck between an carbon electrode and the work piece. The arc heats and
melts the work pieces edges, forming a joint.
Carbon arc welding is the oldest welding process.
If required, filler rod may be used in Carbon Arc Welding. End of the rod is held in
the arc zone. The molten rod material is supplied to the weld pool.
Shields (neutral gas, flux) may be used for weld pool protection depending on type of
welded metal.
Advantages of Carbon Arc Welding:
• Low cost of equipment and welding operation;
• High level of operator skill is not required;
• The process is easily automated;
• Low distortion of work piece.
Disadvantages of Carbon Arc Welding:
• Unstable quality of the weld (porosity);
• Carbon of electrode contaminates weld material with carbides.
Carbon Arc Welding has been replaced by Tungsten Inert Gas Arc Welding (TIG,
GTAW) in many applications.
Modification of Carbon Arc Welding is Twin Carbon Electrode Arc Welding,
utilizing arc struck between two carbon electrodes.
Work piece is not a part of welding electric circuit in Twin Carbon Electrode Arc
Welding; therefore the welding torch may be moved from one work piece to other
without extinguishing the arc.
Shielded Metal Arc Welding (SMAW)
Shielded metal arc welding (Stick welding, Manual metal arc welding) uses a
metallic consumable electrode of a proper composition for generating arc between
itself and the parent work piece. The molten electrode metal fills the weld gap and
joins the work pieces.
This is the most popular welding process capable to produce a great variety of welds.
The electrodes are coated with a shielding flux of a suitable composition. The flux
melts together with the electrode metallic core, forming a gas and a slag, shielding the
arc and the weld pool. The flux cleans the metal surface, supplies some alloying
elements to the weld, protects the molten metal from oxidation and stabilizes the arc.
The slag is removed after Solidification.
Advantages of Shielded Metal Arc Welding (SMAW):
• Simple, portable and inexpensive equipment;
• Wide variety of metals, welding positions and electrodes are applicable;
• Suitable for outdoor applications.
Disadvantages of Shielded Metal Arc Welding (SMAW):
• The process is discontinuous due to limited length of the electrodes;
• Weld may contain slag inclusions;
• Fumes make difficult the process control.
Submerged Arc Welding (SAW)
Submerged Arc Welding is a welding process, which utilizes a bare consumable
metallic electrode producing an arc between itself and the work piece within a
granular shielding flux applied around the weld.
The arc heats and melts both the work pieces edges and the electrode wire. The
molten electrode material is supplied to the surfaces of the welded pieces, fills the
weld pool and joins the work pieces.
Since the electrode is submerged into the flux, the arc is invisible. The flux is partially
melts and forms a slag protecting the weld pool from oxidation and other atmospheric
contaminations.
Advantages of Submerged Arc Welding (SAW):
• Very high welding rate;
• The process is suitable for automation;
• High quality weld structure.
Disadvantages of Submerged Arc Welding (SAW):
• Weld may contain slag inclusions;
• Limited applications of the process - mostly for welding horizontally located
plates.
Metal Inert Gas Welding (MIG, GMAW)
Metal Inert Gas Welding (Gas Metal Arc Welding) is a arc welding process, in
which the weld is shielded by an external gas (Argon, helium, CO2, argon + Oxygen
or other gas mixtures).
Consumable electrode wire, having chemical composition similar to that of the parent
material, is continuously fed from a spool to the arc zone. The arc heats and melts
both the work pieces edges and the electrode wire. The fused electrode material is
supplied to the surfaces of the work pieces, fills the weld pool and forms joint.
Due to automatic feeding of the filling wire (electrode) the process is referred to as a
semi-automatic. The operator controls only the torch positioning and speed.
Advantages of Metal Inert Gas Welding (MIG, GMAW):
• Continuous weld may be produced (no interruptions);
• High level of operators skill is not required;
• Slag removal is not required (no slag);
Disadvantages of Metal Inert Gas Welding (MIG, GMAW):
• Expensive and non-portable equipment is required;
• Outdoor application are limited because of effect of wind, dispersing the
shielding gas.
Tungsten Inert Gas Arc Welding (TIG, GTAW)
Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding) is a welding
process, in which heat is generated by an electric arc struck between a tungsten non-
consumable electrode and the work piece.
The weld pool is shielded by an inert gas (Argon, helium, Nitrogen) protecting the
molten metal from atmospheric contamination.
The heat produced by the arc melts the work pieces edges and joins them. Filler rod
may be used, if required.
Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux
is not used in the process.
Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):
• Weld composition is close to that of the parent metal;
• High quality weld structure
• Slag removal is not required (no slag);
• Thermal distortions of work pieces are minimal due to concentration of heat in
small zone.
Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):
• Low welding rate;
• Relatively expensive;
• Requires high level of operators skill.
Electroslag Welding (ESW)
Electroslag Welding is a welding process, in which the heat is generated by an
electric current passing between the consumable electrode (filler metal) and the work
piece through a molten slag covering the weld surface.
Prior to welding the gap between the two work pieces is filled with a welding flux.
Electroslag Welding is initiated by an arc between the electrode and the work piece
(or starting plate). Heat, generated by the arc, melts the fluxing powder and forms
molten slag. The slag, having low electric conductivity, is maintained in liquid state
due to heat produced by the electric current.
The slag reaches a temperature of about 3500°F (1930°C). This temperature is
sufficient for melting the consumable electrode and work piece edges. Metal droplets
fall to the weld pool and join the work pieces.
Electroslag Welding is used mainly for steels.
Advantages of Electroslag Welding:
• High deposition rate - up to 45 lbs/h (20 kg/h);
• Low slag consumption (about 5% of the deposited metal weight);
• Low distortion;
• Unlimited thickness of work piece.
Disadvantages of Electroslag welding:
• Coarse grain structure of the weld;
• Low toughness of the weld;
• Only vertical position is possible.
Plasma Arc Welding (PAW)
Plasma Arc Welding is the welding process utilizing heat generated by a constricted
arc struck between a tungsten non-consumable electrode and either the work piece
(transferred arc process) or water cooled constricting nozzle (non-transferred arc
process).
Plasma is a gaseous mixture of positive ions, electrons and neutral gas molecules.
Transferred arc process produces plasma jet of high energy density and may be used
for high speed welding and cutting of Ceramics, steels, Aluminum alloys, Copper
alloys, Titanium alloys, Nickel alloys.
Non-transferred arc process produces plasma of relatively low energy density. It is
used for welding of various metals and for plasma spraying (coating). Since the work
piece in non-transferred plasma arc welding is not a part of electric circuit, the plasma
arc torch may move from one work piece to other without extinguishing the arc.
Advantages of Plasma Arc Welding (PAW):
• Requires less operator skill due to good tolerance of arc to misalignments;
• High welding rate;
• High penetrating capability (keyhole effect);
Disadvantages of Plasma Arc Welding (PAW):
• Expensive equipment;
• High distortions and wide welds as a result of high heat input.
Resistance Welding (RW)
Resistance Welding is a welding process, in which work pieces are welded due to a
combination of a pressure applied to them and a localized heat generated by a high
electric current flowing through the contact area of the weld.
Heat produced by the current is sufficient for local melting of the work piece at the
contact point and formation of small weld pool (”nugget”). The molten metal is then
solidifies under a pressure and joins the pieces. Time of the process and values of the
pressure and flowing current, required for formation of reliable joint, are determined
by dimensions of the electrodes and the work piece metal type.
AC electric current (up to 100 000 A) is supplied through copper electrodes connected
to the secondary coil of a welding transformer.
The following metals may be welded by Resistance Welding:
• Low carbon steels - the widest application of Resistance Welding
• Aluminum alloys
• Medium carbon steels, high carbon steels and Alloy steels (may be welded, but
the weld is brittle)
Advantages of Resistance Welding:
• High welding rates;
• Low fumes;
• Cost effectiveness;
• Easy automation;
• No filler materials are required;
• Low distortions.
Disadvantages of Resistance Welding:
• High equipment cost;
• Low strength of discontinuous welds;
• Thickness of welded sheets is limited - up to 1/4” (6 mm);
Resistance Welding (RW) is used for joining vehicle body parts, fuel tanks, domestic
radiators, pipes of gas oil and water pipelines, wire ends, turbine blades, railway
tracks.
The most popular methods of Resistance Welding are:
• Spot Welding (RSW)
• Flash Welding (FW)
• Resistance Butt Welding (UW)
• Seam Welding (RSEW)
Spot Welding (RSW)
Spot Welding is a Resistance Welding (RW) process, in which two or more
overlapped metal sheets are joined by spot welds.
The method uses pointed copper electrodes providing passage of electric current. The
electrodes also transmitt pressure required for formation of strong weld.
Diameter of the weld spot is in the range 1/8” - 1/2” (3 - 12 mm).
Spot welding is widely used in automotive industry for joining vehicle body parts.
Flash Welding (FW)
Flash Welding is a Resistance Welding (RW) process, in which ends of rods (tubes,
sheets) are heated and fused by an arc struck between them and then forged (brought
into a contact under a pressure) producing a weld.
The welded parts are held in electrode clamps, one of which is stationary and the
second is movable.
Flash Welding method permitts fast (about 1 min.) joining of large and complex parts.
Welded part are often annealed for improvement of Toughnesstoughness of the weld.
Steels, Aluminum alloys, Copper alloys, Magnesium alloys, Copper alloys and Nickel
alloys may be welded by Flash Welding.
Thick pipes, ends of band saws, frames, aircraft landing gears are produced by Flash
Welding.
Resistance Butt Welding (UW)
Resistance Butt Welding is a Resistance Welding (RW) process, in which ends of
wires or rods are held under a pressure and heated by an electric current passing
through the contact area and producing a weld.
The process is similar to Flash Welding, however in Butt Welding pressure and
electric current are applied simultaneously in contrast to Flash Welding where electric
current is followed by forging pressure application.
Butt welding is used for welding small parts. The process is highly productive and
clean. In contrast to Flash Welding, Butt Welding provides joining with no loss of the
welded materials.
Seam Welding (RSEW)
Seam Welding is a Resistance Welding (RW) process of continuous joining of
overlapping sheets by passing them between two rotating electrode wheels. Heat
generated by the electric current flowing through the contact area and pressure
provided by the wheels are sufficient to produce a leak-tight weld.
Seam Welding is high speed and clean process, which is used when continuous tight
weld is required (fuel tanks, drums, domestic radiators)
Gas Welding (GW)
Gas Welding is a welding process utilizing heat of the flame from a welding torch.
The torch mixes a fuel gas with Oxygen in the proper ratio and flow rate providing
combustion process at a required temperature. The hot flame fuses the edges of the
welded parts, which are joined together forming a weld after Solidification.
The flame temperature is determined by a type of the fuel gas and proportion of
oxygen in the combustion mixture: 4500°F - 6300°F (2500°C - 3500°C). Depending
on the proportion of the fuel gas and oxygen in the combustion mixture, the flame
may be chemically neutral (stoichiometric content of the gases), oxidizing (excess of
oxygen), carburizing (excess of fuel gas).
Filler rod is used when an additional supply of metal to weld is required. Shielding
flux may be used if protection of weld pool is necessary.
Most of commercial metals may be welded by Gas Welding excluding reactive metals
(titanium, zirconium) and refractory metals (tungsten, molybdenum).
Gas Welding equipment:
• Fuel gas cylinder with pressure regulator;
• Oxygen cylinder with pressure regulator;
• Welding torch;
• Blue oxygen hose;
• Red fuel gas hose;
• Trolley for transportation of the gas cylinders.
Advantages of Resistance Welding:
• Versatile process;
• Low cost, portable equipment;
• Electricity supply is not required.
Disadvantages of Resistance Welding:
• High skill operator is required;
• Flame temperature is lower, than in arc welding;
• Fumes evolved by shielding fluxes;
• Some metals cannot be welded (reactive and refractory metals).
The most popular methods of Gas Welding are:
• Oxyacetylene Welding (OAW)
• Oxyhydrogen Welding (OHW)
• Pressure Gas Welding (PGW)
Oxyacetylene Welding (OAW)
Oxyacetylene Welding is a Gas Welding process using a combustion mixture of
acetylene (C2H2) and oxygen (O2) for producing gas welding flame.
Oxyacetylene flame has a temperature of about 6000°F (3300°C). Combustion of
acetylene proceeds in two stages:
1. Inner core of the flame. C2H2 + O2 = 2CO + H2
2. Outer envelope of the flame: CO + H2 + O2 = CO2 + H2O
Acetylene is safely stored at a pressure not exceeding 300 psi (2000 kPa) in special
steel cylinders containing acetone. Outside of cylinder acetylene is used at a absolute
pressure not exceeding 30 psi (206 kPa). Higher pressure may cause explosion.
Oxyhydrogen Welding (OHW)
Oxyhydrogen Welding is a Gas Welding process using a combustion mixture of
Hydrogen (H2) and oxygen (O2) for producing gas welding flame.
Oxyacetylene flame has a temperature of about 4500°F (2500°C).
Combustion reaction is as follows:
2H2 + O2 = 2H2O
Oxyhydrogen Welding is used for joining metals with low melting points, like
aluminum, magnesium, etc.
Pressure Gas Welding (PGW)
Pressure Gas Welding is a Gas Welding, in which the welded parts are pressed to
each other when heated by a gas flame.
The process is similar to Resistance Butt Welding.
Pressure Gas Welding does not require filler material.
Pressure gas welding is used for joining pipes, rods, railroad rails.
Solid State Welding (SSW)
Solid State Welding is a welding process, in which two work pieces are joined under
a pressure providing an intimate contact between them and at a temperature
essentially below the melting point of the parent material. Bonding of the materials is
a result of diffusion of their interface atoms.
Advantages of Solid State Welding:
• Weld (bonding) is free from microstructure defects (pores, non-metallic
inclusions, segregation of alloying elements)
• Mechanical properties of the weld are similar to those of the parent metals
• No consumable materials (filler material, fluxes, shielding gases) are required
• Dissimilar metals may be joined (steel - aluminum alloy steel - copper alloy).
Disadvantages of Solid State Welding:
• Thorough surface preparation is required (degreasing, oxides removal,
brushing/sanding)
• Expensive equipment.
The following processes are related to Solid State welding:
• Forge Welding (FOW)
• Cold Welding (CW)
• Friction Welding (FRW)
• Explosive Welding (EXW)
• Diffusion Welding (DFW)
• Ultrasonic Welding (USW)
Forge Welding (FOW)
Forge Welding is a Solid State Welding process, in which low carbon steel parts are
heated to about 1800°F (1000°C) and then forged (hammered).
Prior to Forge Welding, the parts are scarfed in order to prevent entrapment of oxides
in the joint.
Forge Welding is used in general blacksmith shops and for manufacturing metal art
pieces and welded tubes.
Advantages of Forge Welding:
• Good quality weld may be obtained;
• Parts of intricate shape may be welded;
• No filler material is required.
Disadvantages of Forge Welding:
• Only low carbon steel may be welded;
• High level of the operators skill is required;
• Slow welding process;
• Weld may be contaminated by the coke used in heating furnace.
Cold Welding (CW)
Cold Welding is a Solid State Welding process, in which two work pieces are joined
together at room temperature and under a pressure, causing a substantial deformation
of the welded parts and providing an intimate contact between the welded surfaces.
As a result of the deformation, the oxide film covering the welded parts breaks up,
and clean metal surfaces reveal. Intimate contact between these pure surfaces provide
a strong and defectless bonding.
Aluminum alloys, Copper alloys, low carbon steels, Nickel alloys, and other ductile
metals may be welded by Cold Welding.
Cold Welding is widely used for manufacturing bi-metal steel - aluminum alloy strips,
for cladding of aluminum alloy strips by other aluminum alloys or pure aluminum
(Corrosion protection coatings). Bi-metal strips are produced by Rolling technology.
Presses are also used for Cold Welding.
Cold Welding may be easily automated.
Friction Welding (FRW)
Friction Welding is a Solid State Welding process, in which two cylindrical parts are
brought in contact by a friction pressure when one of them rotates. Friction between
the parts results in heating their ends. Forge pressure is then applied to the pieces
providing formation of the joint.
Carbon steels, Alloy steels, Tool and die steels, Stainless steels, Aluminum alloys,
Copper alloys, Magnesium alloys, Nickel alloys, Titanium alloys may be joined by
Friction Welding.
Explosive Welding (EXW)
Explosive Welding is a Solid State Welding process, in which welded parts (plates)
are metallurgically bonded as a result of oblique impact pressure exerted on them by a
controlled detonation of an explosive charge.
One of the welded parts (base plate) is rested on an anvil, the second part (flyer plate)
is located above the base plate with an angled or constant interface clearance.
Explosive charge is placed on the flyer plate. Detonation starts at an edge of the plate
and propagates at high velocity along the plate.
The maximum detonation velocity is about 120% of the material sonic velocity.
The slags (oxides, nitrides and other contaminants) are expelled by the jet created just
ahead of the bonding front.
Most of the commercial metals and alloys may be bonded (welded) by Explosive
Welding.
Dissimilar metals may be joined by Explosive Welding:
• Copper to steel;
• Nickel to steel;
• Aluminum to steel;
• Tungsten to steel;
• Titanium to steel;
• Copper to aluminum.
Advantages of Explosive Welding
• Large surfaces may be welded;
• High quality bonding: high strength, no distortions, no porosity, no change of
the metal microstructure;
• Low cost and simple process;
• Surface preparation is not required.
Disadvantages of Explosive Welding:
• Brittle materials (low ductility and low impact toughness) cannot be processed;
• Only simple shape parts may be bonded: plates, cylinders;
• Thickness of flyer plate is limited - less than 2.5” (63 mm);
• Safety and security aspects of storage and using explosives.
Explosive Welding is used for manufacturing clad tubes and pipes, pressure vessels,
aerospace structures, heat exchangers, bi-metal sliding bearings, ship structures, weld
transitions, corrosion resistant chemical process tanks.
Diffusion Welding (DFW)
Diffusion Welding is a Solid State Welding process, in which pressure applied to two
work pieces with carefully cleaned surfaces and at an elevated temperature below the
melting point of the metals. Bonding of the materials is a result of mutual diffusion of
their interface atoms.
In order to keep the bonded surfaces clean from oxides and other air contaminations,
the process is often conducted in vacuum.
No appreciable deformation of the work pieces occurs in Diffusion Welding.
Diffusion Welding is often referred more commonly as Solid State Welding (SSW).
Diffusion Welding is able to bond dissimilar metals, which are difficult to weld
by other welding processes:
• Steel to tungsten;
• Steel to niobium;
• Stainless steel to titanium;
• Gold to copper alloys.
Diffusion Welding is used in aerospace and rocketry industries, electronics, nuclear
applications, manufacturing composite materials.
Advantages of Diffusion Welding:
• Dissimilar materials may be welded (Metals, Ceramics, Graphite, glass);
• Welds of high quality are obtained (no pores, inclusions, chemical segregation,
distortions).
• No limitation in the work pieces thickness.
Disadvantages of Diffusion Welding:
• Time consuming process with low productivity;
• Very thorough surface preparation is required prior to welding process;
• The mating surfaces must be precisely fitted to each other;
• Relatively high initial investments in equipment.
Ultrasonic Welding (USW)
Ultrasonic Welding is a Solid State Welding process, in which two work pieces are
bonded as a result of a pressure exerted to the welded parts combined with application
of high frequency acoustic vibration (ultrasonic).
Ultrasonic vibration causes friction between the parts, which results in a closer
contact between the two surfaces with simultaneous local heating of the contact area.
Interatomic bonds, formed under these conditions, provide strong joint.
Ultrasonic cycle takes about 1 sec. The frequency of acoustic vibrations is in the
range 20 to 70 KHz.
Thickness of the welded parts is limited by the power of the ultrasonic generator.
Ultrasonic Welding is used mainly for bonding small work pieces in electronics, for
manufacturing communication devices, medical tools, watches, in automotive
industry.
Advantages of Ultrasonic Welding:
• Dissimilar metals may be joined;
• Very low deformation of the work pieces surfaces;
• High quality weld is obtained;
• The process may be integrated into automated production lines;
• Moderate operator skill level is enough.
Disadvantages of Ultrasonic Welding:
• Only small and thin parts may be welded;
• Work pieces and equipment components may fatigue at the reciprocating loads
provided by ultrasonic vibration;
• Work pieces may bond to the anvil.
Thermit Welding (TW)
Thermit Welding is a welding process utilizing heat generated by exothermic
chemical reaction between the components of the thermit (a mixture of a metal oxide
and aluminum powder). The molten metal, produced by the reaction, acts as a filler
material joining the work pieces after Solidification.
Thermit Welding is mainly used for joining steel parts, therefore common thermit is
composed from iron oxide (78%) and aluminum powder (22%).
The proportion 78-22 is determined by the chemical reaction of combustion of
aluminum:
8Al + Fe3O4 = 9Fe + 4Al2O3
The combustion reaction products (iron and aluminum oxide) heat up to 4500°F
(2500°C). Liquid iron fills the sand (or ceramic) mold built around the welded parts,
the slag (aluminum oxide), floating up , is then removed from the weld surface.
Thermit Welding is used for repair of steel casings and forgings,for joining railroad
rails, steel wires and steel pipes, for joining large cast and forged parts.
Advantages of Thermit Welding:
• No external power source is required (heat of chemical reaction is utilized);
• Very large heavy section parts may be joined.
Disadvantages of Resistance Welding:
• Only ferrous (steel, chromium, nickel) parts may be welded;
• Slow welding rate;
• High temperature process may cause distortions and changes in Grain structure
in the weld region.
• Weld may contain gas (Hydrogen) and slag contaminations.
Electron Beam Welding (EBW)
Electron Beam Welding is a welding process utilizing a heat generated by a beam of
high energy electrons. The electrons strike the work piece and their kinetic energy
converts into thermal energy heating the metal so that the edges of work piece are
fused and joined together forming a weld after Solidification.
The process is carried out in a vacuum chamber at a pressure of about 2*10-7
to 2*10-6
psi (0.00013 to 0.0013 Pa). Such high vacuum is required in order to prevent loss of
the electrons energy in collisions with air molecules.
The electrons are emitted by a cathode (electron gun). Due to a high voltage (about
150 kV) applied between the cathode and the anode the electrons are accelerated up to
30% - 60% of the speed of light. Kinetic energy of the electrons becomes sufficient
for melting the targeted weld. Some of the electrons energy transforms into X-ray
irradiation.
Electrons accelerated by electric field are then focused into a thin beam in the
focusing coil. Deflection coil moves the electron beam along the weld.
Electron Beam is capable to weld work pieces with thickness from 0.0004” (0.01 mm)
up to 6” (150 mm) of steel and up to 20” (500 mm) of aluminum. Electron Beam
Welding may be used for joining any metals including metals, which are hardly
weldable by other welding methods: refractory metals (tungsten, molybdenum,
niobium) and chemically active metals (titanium, zirconium, beryllium). Electron
Beam Welding is also able to join dissimilar metals.
Advantages of Electron Beam Welding (EBW):
• Tight continuous weld;
• Low distortion;
• Narrow weld and narrow heat affected zone;
• Filler metal is not required.
Disadvantages of Electron Beam Welding (EBW):
• Expensive equipment;
• High production expenses;
• X-ray irradiation.
Laser Welding (LW)
Laser Welding (LW) is a welding process, in which heat is generated by a high
energy laser beam targeted on the work piece. The laser beam heats and melts the
work pieces edges, forming a joint.
Energy of narrow laser beam is highly concentrated: 108-10
11 W/in
2 (10
8-10
10
W/cm2), therefore diminutive weld pool forms very fast (for about 10
-6 sec.).
Solidification of the weld pool surrounded by the cold metal is as fast as melting.
Since the time when the molten metal is in contact with the atmosphere is short, no
contamination occurs and therefore no shields (neutral gas, flux) are required.
The joint in Laser Welding (Laser Beam Welding) is formed either as a sequence of
overlapped spot welds or as a continuous weld.
Laser Welding is used in electronics, communication and aerospace industry, for
manufacture of medical and scientific instruments, for joining miniature components.
Advantages of Laser Welding:
• Easily automated process;
• Controllable process parameters;
• Very narrow weld may be obtained;
• High quality of the weld structure;
• Very small heat affected zone;
• Dissimilar materials may be welded;
• Very small delicate work pieces may be welded;
• Vacuum is not required;
• Low distortion of work piece.
Disadvantages of Carbon Arc Welding:
• Low welding speed;
• High cost equipment;
• Weld depth is limited.
Brazing
Brazing is a method of joining two metal work pieces by means of a filler material at
a temperature above its melting point but below the melting point of either of the
materials being joined.
Flow of the molten filler material into the gap between the work pieces is driven by
the capillary force. The filler material cools down and solidifies forming a strong
metallurgical joint, which is usually stronger than the parent (work piece) materials.
The parent materials are not fused in the process.
Brazing is similar to Soldering. The difference is in the melting point of the filler
alloy: brazing filler materials melt at temperatures above 840°F (450°C); soldering
filler materials (solders) melt at temperatures below this point.
The difference between brazing and welding processes is more sufficient: in the
welding processes edges of the work pieces are either fused (with or without a filler
metal) or pressed to each other without any filler material; brazing joins two parts
without melting them but through a fused filler metal.
• Surface cleaning and brazing fluxes
• Brazing filler materials
• Brazing methods
• Advantages of brazing
• Disadvantages of brazing
Surface cleaning and brazing fluxes
Capillary effect (Fundamentals of adhesive bonding&Wetting|wettability) is achieved
by both: a proper Surface preparation and use of a flux for wetting and cleaning the
surfaces to be bonded.
Contaminants to be removed from the part surface are: mineral oils, miscellaneous
organic soils, polishing and buffing compounds, miscellaneous solid particles, oxides,
scale, smut, rust.
The work pieces are cleaned by means of mechanical methods, soaking cleaning and
chemical cleaning (acid etching).
A brazing flux has a melting point below the melting point of the filler metal, it melts
during the heating stage and spreads over the joint area, wetting it and protecting the
surface from oxidation.
It also cleans the surface, dissolving the metal oxides.
It is important that the surface tension of the flux is: 1. Low enough for wetting the
work piece surface; 2. Higher than the surface tension of the molten filler metal in
order to provide displacement of the flux by the fused brazing filler. The latter
eliminates the flux entrapment in the joint.
The flux is applied onto the metal surface by brushing, dipping or spraying.
Brazing filler materials
• Copper filler alloys: BCuP-2 (Cu-7P), BCuP-4 (Cu-6Ag-7P). Used for brazing
Copper alloys, steels, Nickel alloys.
• Aluminum filler alloys: Al-4Cu-10Si, Al-12Si, Al-4Cu-10Si-10Zn, 4043 (Al-
5.2Si), 4045 (Al-10Si). Used for brazing Aluminum alloys.
• Magnesium filler alloys: BMg-1 (Mg-9Al-2Zn), BMg-2 (Mg-12Al-5Zn). Used
for brazing Magnesium alloys.
• Nickel filler alloys: BNi-1 (Ni-14Cr-4Si-3.4B-0.75C), BNi-2 (Ni-7Cr-4.5Si-
3.1B-3Fe), BNi-3 (Ni-4.5Si-3.1B). Used for brazing Nickel alloys, cobalt
alloys, Stainless steels.
• Silver filler alloys: BAg-4 (40Ag-30Cu-28Zn-2Ni), BAg-5 (45Ag-30Cu-
25Zn), BAg-6 (50Ag-34Cu-16Zn), BAg-7 (56Ag-22Cu-17Zn-5Sn). Used for
most of metals and alloys except aluminum and magnesium alloys.
Brazing methods
• Torch brazing utilizes a heat of the flame from a torch. The torch mixes a
fuel gas with Oxygen or air in the proper ratio and flow rate, providing
combustion process at a required temperature.
The torch flame is directed to the work pieces with a flux applied on their surfaces.
When the work pieces are heated to a required temperature, filler alloy is fed into the
flame. The filler material melts and flows to the gap between the joined parts.
Torch brazing is the most popular brazing method.
Torch brazing equipment:
- Fuel gas cylinder with pressure regulator;
- Oxygen cylinder with pressure regulator;
- Welding torch;
- Blue oxygen hose;
- Red fuel gas hose;
- Trolley for transportation of the gas cylinders.
• Furnace brazing uses a furnace for heating the work pieces.
• Vacuum brazing is a type of furnace brazing, in which heating is performed
in vacuum.
• Induction brazing utilizes alternating electro-magnetic field of high
frequency for heating the work pieces together with the flux and the filler
metal placed in the joint region.
• Resistance brazing uses a heat generated by an electric current flowing
through the work pieces.
• Dip brazing is a brazing method, in which the work pieces together with the
filler metal are immersed into a bath with a molten salt. The filler material
melts and flows into the joint.
• Infrared brazing utilizes a heat of a high power infrared lamp.
Advantages of brazing
• Low thermal distortions and residual stresses in the joint parts;
• Microstructure is not affected by heat;
• Easily automated process;
• Dissimilar materials may be joined;
• High variety of materials may be joined;
• Thin wall parts may be joined;
• Moderate skill of the operator is required.
Disadvantages of brazing
• Careful removal of the flux residuals is required in order to prevent corrosion;
• No gas shielding may cause porosity of the joint;
• Large sections cannot be joined;
• Fluxes and filler materials may contain toxic components;
• Relatively expensive filler materials.
Soldering
Soldering is a method of joining two metal work pieces by means of a third metal
(solder) at a relatively low temperature, which is above the melting point of the solder
but below the melting point of either of the materials being joined.
Flow of the molten solder into the gap between the work pieces is driven by the
capillary force. The solder cools down and solidifies forming a joint. The parent
materials are not fused in the process.
Soldering is similar to Brazing. The difference is in the melting point of the filler
alloy: solders melt at temperatures below 840°F (450°C); brazing filler materials melt
at temperatures above this point.
The difference between soldering and welding processes is more sufficient: in the
welding processes edges of the work pieces are either fused (with or without a filler
metal) or pressed to each other without any filler material; soldering joins two parts
without melting them but through a soft low melting point solder.
Soldering joints have relatively low tensile strength of about 10000 psi (70 MPa).
• Surface cleaning and soldering fluxes
• Tin-lead solders
• Lead-free solders
• Soldering methods
• Advantages of soldering
• Disadvantages of soldering
Surface cleaning and soldering fluxes
Capillary effect (Fundamentals of adhesive bonding#Wetting|wettability) is achieved
by both: a proper Surface preparation and use of a flux for wetting and cleaning the
surfaces to be bonded.
Contaminants to be removed from the part surface are: mineral oils, miscellaneous
organic soils, polishing and buffing compounds, miscellaneous solid particles, oxides,
scale, smut, rust.
The work pieces are cleaned by means of mechanical methods, soaking cleaning and
chemical cleaning (acid etching).
A soldering flux has a melting point below the melting point of the solder, it melts
during the preheating stage and spreads over the joint area, wetting it and protecting
the surface from oxidation. It also cleans the surface, dissolving the metal oxides.
It is important that the surface tension of the flux is: 1. Low enough for wetting the
work piece surface; 2. Higher than the surface tension of the molten solder in order to
provide displacement of the flux by the fused solder. The latter eliminates the flux
entrapment in the joint.
The flux is applied onto the metal surface by brushing, dipping, spraying, in form of a
gas-flux foam or by a flux wave (flowing flux forms a wave and the printed circuit
board moves over the apex of the wave).
Flux is acidic therefore its residuals may cause corrosion if not removed.
Tin-lead solders
Traditional lead containing solders consist of tin (Sn) and lead (Pb).
The most popular alloy in this group is eutectic composition 63Sn-37Pb (commonly
called 63/37). The melting point of this alloy is lowest of all Sn-Pb alloys: 361°F
(183°C). This solder is used for joining electronic components, to which minimum
heat may be applied (computers, telecommunication devices). The 63/37 alloy may be
modified by addition of 1.4% of silver (Ag) for improvement of the joint Creep
resistance.
Low tin solders such as 5Sn-95Pb (5/95), 10Sn-90Pb (10/90), 15Sn-85Pb (15/85) are
used mainly for sealing containers and radiators, joining and coating metal parts
working at increased temperatures (above 250°F/121°C).
The alloy 70Sn-30Pb (70/30) is used for coating parts before soldering.
The advantages of tin-lead alloys:
• Non-expensive;
• Simple equipment (soldering iron, torch);
• Low skill of operator is enough;
• Low melting point.
The main disadvantage of these alloys is toxicity of lead.
Lead-free solders
Most of lead-free solders are tin base alloys: 96.5Sn-3Ag-0.5Cu, 99.3Ag-0.7Cu,
95Sn-5Sb.
The alloy 96.5Sn-3Ag-0.5Cu has a composition very close to the eutectic. Its melting
point is 423°F (217°C). Fatigue strength of the alloy is similar to that of SnPb solders,
however its wettability is poorer. Addition of 1-3% of bismuth (Bi) to the alloy
improves its wettability and decreases the melting point but the fatigue resistance
deteriorates. The alloy is now used for wave soldering, reflow and hand soldering.
The alloy 99.3Ag-0.7Cu with the melting point 441°F (227°C) is low a cost
alternative of the silver containing alloy. It is used for wave soldering.
When a low melting point is required, the alloy 42Sn-58Bi is used. Its melting point is
280°F (138°C). Fatigue strength, tensile strength and ductility of the alloy are
relatively low but may be improved by some addition of silver (Ag).
The melting point of the alloy 95Sn-5Sb is 450°F (232°C). The solder is used in the
plumbing works.
Soldering methods
• Hand soldering
Iron soldering utilizes a heat generated by a soldering iron.
Torch soldering utilizes a heat of the flame from a torch. The torch mixes a fuel gas
with oxygen or air in the proper ratio and flow rate, providing combustion process at a
required temperature.
The torch flame is directed to the work pieces with a flux applied on their surfaces.
When the work pieces are heated to a required temperature, solder is fed into the joint
region. The solder melts and flows to the gap between the joined parts.
Hand soldering is used in repair works and for low volume production.
• Wave soldering
The method uses a tank full with a molten solder. The solder is pumped, and its flow
forms awave of a predetermined height. The printed circuit boards pass over the wave
touching it with their lower sides.
The method is used for soldering through-hole components on printed circuit boards.
• Reflow soldering
In this method a solder paste (a mix of solder and flux particles) is applied onto the
surface of the parts to be joined and then are heated to a temperature above the
melting point of the solder. The process is conducted in a continuous furnace, having
different zones: preheating, soaking, reflow and cooling. The joint forms when the
solder cools down and solidifies in the cooling zone of the furnace.
Advantages of soldering
• Low power is required;
• Low process temperature;
• No thermal distortions and residual stresses in the joint parts;
• Microstructure is not affected by heat;
• Easily automated process;
• Dissimilar materials may be joined;
• High variety of materials may be joined;
• Thin wall parts may be joined;
• Moderate skill of the operator is required.
Disadvantages of soldering
• Careful removal of the flux residuals is required in order to prevent corrosion;
• Large sections cannot be joined;
• Fluxes may contain toxic components;
• Soldering joints can not be used in high temperature applications;
• Low strength of joints.