Advanced Welding Technology - Comprehensive Notes

Welding Introduction

• Welding joins metal pieces by applying heat.
• It's the least expensive and widely used fabrication process.
• Welding joins different metals using various processes, with heat supplied electrically or by gas torch.
• Applications include automobile bodies, structural work, tanks, and machine repair.
• Industries use welding in refineries and pipeline fabrication.
• Welding can be termed a secondary manufacturing process.
• Modern welding began in the late 19th century with methods for generating high temperatures to melt materials.

Welding and Allied Processes

• Welding processes
  • Cast weld process
  • Fusion weld process
    • Thermit
    • Carbon arc
    • Shielded metal arc
    • Electroslag
    • Submerged arc
    • Gas metal arc
    • Gas tungsten arc
    • Plasma arc
    • Electrogas
    • Laser beam
    • Electron beam
    • Oxy-fuel gas
  • Resistance weld process
    • Spot
    • Projection
    • Seam
    • H. F. resistance
    • H.F. induction
    • Resistance butt
    • Flash butt
  • Solid state weld process
    • Low heat input
      • Ultrasonic
      • Friction
      • Cold pressure
      • Forge
    • High heat input
      • Diffusion
      • Explosion
  • Metal depositing process
    • Weld surfacing
    • Metal spraying
• Allied processes
  • Soldering
  • Brazing
  • Adhesive bonding

Brazing vs. Soldering

• Brazing
  • Filler metal melting point above $400^{\circ}C$.
  • More stable joints.
  • High pressure and temperature do not affect the joint.
  • Equipment cost is higher.
• Soldering
  • Filler metal melting point below $400^{\circ}C$.
  • Less stable joints.
  • Joints affected by high temperature and pressure.
  • Equipment cost is very low.

Welding Processes Overview

• Arc Welding
• Resistance Welding
• Oxyfuel Gas Welding
• Other Fusion Welding Processes
• Solid State Welding
• Weld Quality
• Weldability
• Design Considerations in Welding

Two Categories of Welding Processes

• Fusion welding
  • Coalescence achieved by melting parts to be joined and sometimes adding filler metal.
  • Examples: arc welding, resistance spot welding, oxyfuel gas welding.
• Solid state welding
  • Heat and/or pressure used for coalescence, without melting base metals or adding filler metal.
  • Examples: forge welding, diffusion welding, friction welding.

Arc Welding (AW)

• Fusion welding process where coalescence is achieved by heat from an electric arc between an electrode and the work.
• Electric energy from the arc produces temperatures around $5500^{\circ}C$, which is hot enough to melt any metal.
• Most AW processes add filler metal to increase the volume and strength of the weld joint.

Electric Arc

• An electric arc is a discharge of electric current across a gap in a circuit.
• It is sustained by an ionized column of gas (plasma) through which the current flows.
• To initiate the arc in AW, the electrode is brought into contact with the work and then quickly separated by a short distance.

Arc Welding Process

• A pool of molten metal forms near the electrode tip. As the electrode moves along the joint, the molten weld pool solidifies.

Manual Arc Welding and Arc Time

• Problems with manual welding include weld joint quality and productivity.
• Arc Time = (time arc is on) / (hours worked), also called “arc-on time.”
• Manual welding arc time is approximately $20\%$.
• Machine welding arc time is approximately $50\%$.

Types of AW Electrodes

• Consumable Electrodes:
  • Consumed during the welding process.
  • Serves as a source of filler metal in arc welding.
• Non-Consumable Electrodes:
  • Not consumed during the welding process.
  • Filler metal must be added separately.

Consumable Electrodes Details

• Forms of consumable electrodes
  • Welding rods: 9 to 18 inches long, $3/8$ inch or less in diameter, and must be changed frequently.
  • Weld wire: Continuously fed from spools with long lengths of wire, avoiding frequent interruptions
• In both rod and wire forms, the electrode is consumed by the arc and added to the weld joint as filler metal.

Nonconsumable Electrodes Details

• Made of tungsten, which resists melting.
• Gradually depleted during welding by vaporization.
• Any filler metal must be supplied by a separate wire fed into the weld pool.

Arc Shielding

• At high temperatures in AW, metals are chemically reactive to oxygen, nitrogen, and hydrogen in the air.
• Reactions can degrade the mechanical properties of the joint.
• To protect the operation, the arc must be shielded from the surrounding air.
• Arc shielding is accomplished by:
  • Shielding gases (e.g., argon, helium, $CO_2$).
  • Flux to prevent oxidation of the base and filler materials. Tin-lead solder attaches well to copper but poorly to copper oxides.

Power Source in Arc Welding

• Direct current (DC) vs. Alternating current (AC)
  • AC machines are less expensive to purchase and operate but are generally restricted to ferrous metals.
  • DC equipment can be used on all metals and is noted for better arc control.

Consumable Electrode AW Processes

• Shielded Metal Arc Welding (SMAW)
• Gas Metal Arc Welding (GMAW)
• Flux-Cored Arc Welding (FCAW)
• Electrogas Welding (EGW)
• Submerged Arc Welding (SAW)

Shielded Metal Arc Welding (SMAW)

• Consumable electrode welding process
• Electrode coating provides protective gas and forms slag.
• Molten weld metal solidifies to form the joint.

Welding Stick in SMAW

• Composition of filler metal (aluminum, tin, or lead) is usually close to the base metal.
• Coating: powdered cellulose mixed with oxides and carbonates, held together by a silicate binder.
• Welding stick is clamped in an electrode holder connected to a power source.
• Disadvantages of stick welding:
  • Sticks must be periodically changed.
  • High current levels may melt the coating prematurely.

SMAW Applications

• Used for steels, stainless steels, cast irons, and certain nonferrous alloys.
• Not used or rarely used for aluminum and its alloys, copper alloys, and titanium.

Gas Metal Arc Welding (GMAW)

• Uses a consumable bare metal wire as an electrode with shielding by flooding the arc with a gas.
• Wire is fed continuously and automatically from a spool through the welding gun.
• Shielding gases include argon and helium for aluminum welding and $CO_2$ for steel welding.
• Bare electrode wire plus shielding gases eliminate slag on the weld bead, so there is no need for manual grinding and cleaning of slag.

GMAW Advantages over SMAW

• Better arc time due to continuous wire electrode, unlike SMAW where sticks must be periodically changed.
• Better use of electrode filler metal than SMAW, where the end of the stick cannot be used.
• Higher deposition rates.
• Eliminates the problem of slag removal.
• Can be readily automated.

Flux-Cored Arc Welding (FCAW)

• Adaptation of shielded metal arc welding to overcome limitations of stick electrodes
• Two versions:
  • Self-shielded FCAW - core includes compounds that produce shielding gases
  • Gas-shielded FCAW - uses externally applied shielding gases
• Electrode is a continuous consumable tubing (in coils) containing flux and other ingredients (e.g., alloying elements) in its core

Flux-Cored Arc Welding Variation

• Presence or absence of externally supplied shielding gas distinguishes:
  • Self-shielded: The core provides ingredients for shielding.
  • Gas-shielded: Uses external shielding gases.

Electrogas Welding (EGW)

• Uses a continuous consumable electrode, flux-cored wire, or bare wire with externally supplied shielding gases and molding shoes to contain molten metal.
• When flux-cored electrode wire is used and no external gases are supplied, it's a special case of self-shielded FCAW.
• When a bare electrode wire is used with shielding gases from an external source, it's a special case of GMAW.
• Electrogas welding using flux-cored electrode wire involves a front view with the molding shoe removed for clarity and a side view showing molding shoes on both sides.

Submerged Arc Welding (SAW)

• Uses a continuous, consumable bare wire electrode with arc shielding by a cover of granular flux.
• Electrode wire is fed automatically from a coil.
• Flux is introduced into the joint slightly ahead of the arc by gravity from a hopper.
• This process completely submerges the operation, preventing sparks, spatter, and radiation.

SAW Applications and Products

• Steel fabrication of structural shapes (e.g., I-beams)
• Seams for large diameter pipes, tanks, and pressure vessels
• Welded components for heavy machinery
• Most steels (except high C steel)
• Not good for nonferrous metals

Nonconsumable Electrode Processes

• Gas Tungsten Arc Welding
• Plasma Arc Welding
• Carbon Arc Welding
• Stud Welding

Gas Tungsten Arc Welding (GTAW)

• Uses a non-consumable tungsten electrode and an inert gas for arc shielding.
• Melting point of tungsten = $3410^{\circ}C$.
• Also known as Tungsten Inert Gas (TIG) welding.
• In Europe, it's called "WIG welding."
• Used with or without a filler metal.
• When filler metal is used, it is added to the weld pool from a separate rod or wire.
• Applications: mostly aluminum and stainless steel.

Advantages and Disadvantages of GTAW

• Advantages:
  • High quality welds for suitable applications
  • No spatter because no filler metal goes through the arc
  • Little or no post-weld cleaning because no flux is used
• Disadvantages:
  • Generally slower and more costly than consumable electrode AW processes

Plasma Arc Welding (PAW)

• Special form of GTAW in which a constricted plasma arc is directed at the weld area.
• The tungsten electrode is contained in a nozzle that focuses a high-velocity stream of inert gas (argon) into the arc region to form a high-velocity, intensely hot plasma arc stream.
• Temperatures in PAW reach $28,000^{\circ}C$, due to constriction of the arc, producing a plasma jet of small diameter and very high energy density.

Advantages and Disadvantages of PAW

• Advantages:
  • Good arc stability and excellent weld quality
  • Better penetration control than other AW processes
  • High travel speeds
  • Can be used to weld almost any metals
• Disadvantages:
  • High equipment cost
  • Larger torch size than other AW processes, which tends to restrict access in some joints

Resistance Welding (RW)

• A group of fusion welding processes that use a combination of heat and pressure to accomplish coalescence.
• Heat generated by electrical resistance to current flow at the junction to be welded.
• The principal RW process is resistance spot welding (RSW).

Components in Resistance Spot Welding

• Parts to be welded (usually sheet metal)
• Two opposing electrodes
• A means of applying pressure to squeeze parts between electrodes
• A power supply from which a controlled current can be applied for a specified time duration

Advantages and Drawbacks of Resistance Welding

• Advantages:
  • No filler metal required
  • High production rates possible
  • Lends itself to mechanization and automation
  • Lower operator skill level than for arc welding
  • Good repeatability and reliability
• Disadvantages:
  • High initial equipment cost
  • Limited to lap joints for most RW processes

Resistance Spot Welding (RSW)

• Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at one location by opposing electrodes.
• Used to join sheet metal parts.
• Widely used in mass production of automobiles, metal furniture, appliances, and other sheet metal products.
• A typical car body has approximately 10,000 spot welds.
• Annual production of automobiles in the world is measured in tens of millions of units.

Resistance Seam Welding (RSEW)

• Uses rotating wheel electrodes to produce a series of overlapping spot welds along a lap joint.
• Can produce air-tight joints.
• Applications:
  • Gasoline tanks
  • Automobile mufflers
  • Various sheet metal containers

Resistance Projection Welding (RPW)

• A resistance welding process in which coalescence occurs at one or more small contact points on the parts.
• Contact points are determined by the design of the parts to be joined and may consist of projections, embossments, or localized intersections of parts.
• When current is applied, weld nuggets similar to spot welding are formed at the projections.

Other Resistance Projection Welding Operations

• Welding of a fastener on sheet metal
• Cross-wire welding

Oxy-fuel Gas Welding (OFW)

• A group of fusion welding operations that burn various fuels mixed with oxygen.
• OFW employs several types of gases, which is the primary distinction among the members of this group.
• Oxyfuel gas is also used in flame-cutting torches to cut and separate metal plates and other parts.
• The most important OFW process is oxyacetylene welding.

Oxyacetylene Welding (OAW)

• Fusion welding performed by a high-temperature flame from the combustion of acetylene and oxygen.
• The flame is directed by a welding torch.
• Filler metal is sometimes added, and its composition must be similar to the base metal.
• The filler rod is often coated with flux to clean surfaces and prevent oxidation.

Acetylene ($C<em>2H</em>2$)

• The most popular fuel among the OFW group because it is capable of higher temperatures than any other, up to $3480^{\circ}C$.
• Two-stage reaction of acetylene and oxygen:
  • First stage reaction (inner cone of flame): $C<em>2H</em>2 + O<em>2 \rightarrow 2CO + H</em>2 + \text{heat}$
  • Second stage reaction (outer envelope): $2CO + H<em>2 + 1.5O</em>2 \rightarrow 2CO<em>2 + H</em>2O + \text{heat}$
• Maximum temperature is reached at the tip of the inner cone, while the outer envelope spreads out and shields the work surface from the atmosphere.

Oxyacetylene Torch

• The neutral flame of an oxyacetylene torch indicates the temperatures achieved.

Safety Issue in OAW

• Acetylene and oxygen are highly flammable.
• $C<em>2H</em>2$ is colorless and odorless; therefore, it is processed to have a characteristic garlic odor.

OAW Safety Issue

• $C<em>2H</em>2$ is physically unstable at pressures much above $15 \frac{lb}{in^2}$ (approximately 1 atm).
• Storage cylinders are packed with porous filler material saturated with acetone ($CH<em>3COCH</em>3$).
• Acetone dissolves about 25 times its own volume of acetylene.
• Different screw threads are standard on $C<em>2H</em>2$ and $O_2$ cylinders and hoses to avoid accidental connection of wrong gases.

Alternative Gases for OFW

• Methylacetylene-Propadiene (MAPP)
• Hydrogen
• Propylene
• Propane
• Natural Gas

Other Fusion Welding Processes

• FW processes that cannot be classified as arc, resistance, or oxyfuel welding.
• Use unique technologies to develop heat for melting.
• Applications are typically unique.
• Processes include:
  • Electron beam welding
  • Laser beam welding
  • Electroslag welding
  • Thermit welding

Electron Beam Welding (EBW)

• Fusion welding process in which heat for welding is provided by a highly focused, high-intensity stream of electrons striking the work surface.
• Electron beam gun operates at:
  • High voltage (e.g., 10 to 150 kV typical) to accelerate electrons
  • Beam currents are low (measured in milliamps)
• Power in EBW is non exceptional.

EBW Vacuum Chamber

• When first developed, EBW had to be carried out in a vacuum chamber to minimize disruption of the electron beam by air molecules, which was a serious inconvenience in production.
• Pump-down time can take as long as an hour.

Three Vacuum Levels in EBW

• High-vacuum welding: Welding in the same vacuum chamber as beam generation to produce the highest quality weld.
• Medium-vacuum welding: Welding in a separate chamber, but a partial vacuum reduces pump-down time.
• Non-vacuum welding: Welding done at or near atmospheric pressure, with the work positioned close to the electron beam generator - requires a vacuum divider to separate work from the beam generator.

Advantages and Disadvantages of EBW

• Advantages:
  • High-quality welds, deep and narrow profiles
  • Limited heat-affected zone, low thermal distortion
  • No flux or shielding gases needed
• Disadvantages:
  • High equipment cost
  • Precise joint preparation and alignment are required
  • Vacuum chamber required
  • Safety concern: EBW generates x-rays

Laser Beam Welding (LBW)

• Fusion welding process in which coalescence is achieved by the energy of a highly concentrated, coherent light beam focused on a joint.
• LBW is normally performed with shielding gases to prevent oxidation.
• Filler metal is not usually added.
• High power density in a small area, so LBW is often used for small parts.

Comparison: LBW vs. EBW

• No vacuum chamber is required for LBW.
• No x-rays are emitted in LBW.
• Laser beams can be focused and directed by optical lenses and mirrors.
• LBW is not capable of the deep welds and high depth-to-width ratios of EBW.
• The maximum LBW depth is approximately $19 \text{ mm}$, whereas EBW depths are $50 \text{ mm}$.

Thermit Welding (TW)

• FW process in which heat for coalescence is produced by superheated molten metal from the chemical reaction of thermite.
• Thermite = a mixture of Al and $Fe<em>3O</em>4$ fine powders that produces an exothermic reaction when ignited.
• Also used for incendiary bombs.
• Filler metal is obtained from liquid metal.
• The process is used for joining but has more in common with casting than welding.

TW Applications

• Joining of railroad rails
• Repair of cracks in large steel castings and forgings
• The weld surface is often smooth enough that no finishing is required

Solid State Welding (SSW)

• Coalescence of part surfaces is achieved by:
  • Pressure alone, or
  • Heat and pressure
• If both heat and pressure are used, the heat is not enough to melt work surfaces.
• For some SSW processes, time is also a factor.
• No filler metal is added.
• Each SSW process has its own way of creating a bond at the faying surfaces.

Success Factors in SSW

• Essential factors for a successful solid-state weld are that the two faying surfaces must be:
  • Very clean
  • In very close physical contact with each other to permit atomic bonding

SSW Advantages over FW Processes

• If no melting occurs, then there is no heat-affected zone, so the metal around the joint retains its original properties.
• Many SSW processes produce welded joints that bond the entire contact interface between two parts rather than at distinct spots or seams.
• Some SSW processes can be used to bond dissimilar metals without concerns about relative melting points, thermal expansions, and other problems that arise in FW.

Solid State Welding Processes

• Forge welding
• Cold welding
• Roll welding
• Hot pressure welding
• Diffusion welding
• Explosion welding
• Friction welding
• Ultrasonic welding

Forge Welding

• Welding process in which components to be joined are heated to a hot working temperature range and then forged together by hammering or similar means.
• Has historic significance in the development of manufacturing technology, dating from around 1000 B.C. when blacksmiths learned to weld two pieces of metal.
• Of minor commercial importance today except for its variants.

Cold Welding (CW)

• SSW process done by applying high pressure between clean contacting surfaces at room temperature.
• Cleaning is usually done by degreasing and wire brushing immediately before joining.
• No heat is applied, but deformation raises the work temperature.
• At least one of the metals, preferably both, must be very ductile; soft aluminum and copper are suited to CW.
• Applications: making electrical connections.

Roll Welding (ROW)

• SSW process in which pressure sufficient to cause coalescence is applied by means of rolls, either with or without external heat.
• Variation of either forge welding or cold welding, depending on whether heating of workpieces is done prior to the process.
  • If no external heat, it is called cold roll welding.
  • If heat is supplied, it is hot roll welding.

Roll Welding Applications

• Cladding stainless steel to mild or low alloy steel for corrosion resistance
• Bimetallic strips for measuring temperature
• "Sandwich" coins for the U.S. Mint

Diffusion Welding (DFW)

• SSW process uses heat and pressure, usually in a controlled atmosphere, with sufficient time for diffusion and coalescence to occur.
• Temperatures are less than or equal to $0.5 \cdot T_m$.
• Plastic deformation at surfaces is minimal.
• The primary coalescence mechanism is solid-state diffusion.
• Limitation: the time required for diffusion can range from seconds to hours.

DFW Applications

• Joining of high-strength and refractory metals in the aerospace and nuclear industries
• Can be used to join either similar or dissimilar metals
• For joining dissimilar metals, a filler layer of a different metal is often sandwiched between base metals to promote diffusion

Explosion Welding (EXW)

• SSW process in which rapid coalescence of two metallic surfaces is caused by the energy of a detonated explosive.
• No filler metal is used.
• No external heat is applied.
• No diffusion occurs because the time is too short.
• Bonding is metallurgical, combined with mechanical interlocking that results from a rippled or wavy interface between the metals.

Explosive Welding Configuration

• Commonly used to bond two dissimilar metals, e.g., to clad one metal on top of a base metal over large areas

Friction Welding (FRW)

• SSW process in which coalescence is achieved by frictional heat combined with pressure.
• When properly carried out, no melting occurs at faying surfaces.
• No filler metal, flux, or shielding gases are normally used.
• The process yields a narrow HAZ (heat affected zone).
• Can be used to join dissimilar metals.
• Widely used commercial process, amenable to automation and mass production.

Applications and Limitations of Friction Welding

• Applications:
  • Shafts and tubular parts
  • Industries: automotive, aircraft, farm equipment, petroleum, and natural gas
• Limitations:
  • At least one of the parts must be rotational
  • Flash must usually be removed (extra operation)
  • Upsetting reduces the part lengths (which must be taken into consideration in product design)

Friction Stir Welding (FSW)

• SSW process in which a rotating tool is fed along a joint line between two workpieces, generating friction heat and mechanically stirring the metal to form the weld seam.
• Distinguished from FRW because heat is generated by a separate wear-resistant tool rather than the parts themselves.
• Applications: butt joints in large aluminum parts in aerospace, automotive, and shipbuilding.

Advantages and Disadvantages of Friction Stir Welding

• Advantages:
  • Good mechanical properties of the weld joint
  • Avoids toxic fumes, warping, and shielding issues
  • Little distortion or shrinkage
  • Good weld appearance
• Disadvantages:
  • An exit hole is produced when the tool is withdrawn
  • Heavy-duty clamping of parts is required

Ultrasonic Welding (USW)

• Two components are held together, and oscillatory shear stresses of ultrasonic frequency are applied to the interface to cause coalescence.
• The oscillatory motion breaks down any surface films to allow intimate contact and strong metallurgical bonding between the surfaces.
• Temperatures are well below $T_m$.
• No filler metals, fluxes, or shielding gases are used.
• Generally limited to lap joints on soft materials.

USW Applications

• Wire terminations and splicing in the electrical and electronics industry (eliminates the need for soldering).
• Assembly of aluminum sheet metal panels.
• Welding of tubes to sheets in solar panels.
• Assembly of small parts in the automotive industry.

Weld Quality

• Concerned with obtaining an acceptable weld joint that is strong and absent of defects.
• Also concerned with the methods of inspecting and testing the joint to assure its quality.
• Topics:
  • Residual stresses and distortion
  • Welding defects
  • Inspection and testing methods

Residual Stresses and Distortion

• Rapid heating and cooling in localized regions during FW result in thermal expansion and contraction that cause residual stresses.
• These stresses, in turn, cause distortion and warpage.
• The situation in welding is complicated because:
  • Heating is very localized
  • Melting of base metals occurs in these regions
  • The location of heating and melting is in motion (at least in AW)

Techniques to Minimize Warpage

• Welding fixtures to physically restrain parts
• Heat sinks to rapidly remove heat
• Tack welding at multiple points along the joint to create a rigid structure prior to seam welding
• Selection of welding conditions (speed, amount of filler metal used, etc.) to reduce warpage
• Preheating base parts
• Stress relief heat treatment of welded assembly

Welding Defects

• Cracks
• Cavities
• Solid inclusions
• Imperfect shape or unacceptable contour
• Incomplete fusion
• Miscellaneous defects

Welding Cracks

• Fracture-type interruptions either in the weld or in the base metal adjacent to the weld.
• A serious defect because it is a discontinuity in the metal that significantly reduces strength.
• Caused by embrittlement or low ductility of the weld and/or base metal combined with high restraint during contraction.
• In general, this defect must be repaired.

Cavities

• Two defect types, similar to defects found in castings:
  • Porosity: small voids in weld metal formed by gases entrapped during solidification, caused by the inclusion of atmospheric gases, sulfur in the weld metal, or surface contaminants.
  • Shrinkage voids: cavities formed by shrinkage during solidification.

Solid Inclusions

• Nonmetallic material entrapped in weld metal.
• The most common form is slag inclusions generated during AW processes that use flux. Instead of floating to the top of the weld pool, globules of slag become encased during solidification.
• Other forms: metallic oxides that form during welding of certain metals such as aluminum, which normally has a surface coating of $Al<em>2O</em>3$.

Incomplete Fusion

• A weld bead in which fusion has not occurred throughout the entire cross-section of the joint.
• Several forms of incomplete fusion.

Weld Profile in AW

• (a) Desired profile for single V-groove weld joint
• (b) Undercut: a portion of the base metal melted away
• (c) Underfill: a depression in the weld below the adjacent base metal surface
• (d) Overlap: weld metal spills beyond the joint onto the part surface, but no fusion occurs

Inspection and Testing Methods

• Visual inspection
• Nondestructive evaluation
• Destructive testing

Visual Inspection

• The most widely used welding inspection method.
• A human inspector visually examines for conformance to dimensions, warpage, cracks, cavities, incomplete fusion, and other surface defects.
• Limitations:
  • Only surface defects are detectable.
  • The welding inspector must also decide if additional tests are warranted.

Non Destructive Testing (NDT)

• Also known as Nondestructive Evaluation (NDE).

Nondestructive Evaluation (NDE) Tests

• Ultrasonic testing: high-frequency sound waves through the specimen to detect cracks and inclusions.
• Radiographic testing: x-rays or gamma radiation provide a photograph of internal flaws.
• Dye-penetrant and fluorescent-penetrant tests: to detect small cracks and cavities at the part surface.
• Magnetic particle testing: iron filings sprinkled on the surface reveal subsurface defects by distorting the magnetic field in the part.

Destructive Testing

• Tests in which the weld is destroyed either during testing or to prepare a test specimen.
• Mechanical tests: the purpose is similar to conventional testing methods, such as tensile tests, shear tests, etc.
• Metallurgical tests: preparation of metallurgical specimens (e.g., photomicrographs) of the weldment to examine metallic structure, defects, the extent and condition of the heat-affected zone, and similar phenomena.

Mechanical Tests in Welding

• Tension-shear test
• Fillet break test
• Tension-shear of spot weld
• Peel test for spot weld

Weldability

• The capacity of a metal or combination of metals to be welded into a suitable structure, and for the resulting weld joint(s) to possess the required metallurgical properties to perform satisfactorily in intended service.
• Good weldability is characterized by:
  • Ease with which welding is accomplished
  • Absence of weld defects
  • Strength, ductility, and toughness in the welded joint

Weldability Factors - Welding Process

• Some metals or metal combinations can be readily welded by one process but are difficult to weld by others.
• Example: stainless steel is readily welded by most AW and RW processes but difficult to weld by OFW.

Weldability Factors - Base Metal

• Some metals melt too easily (e.g., aluminum).
• Metals with high thermal conductivity transfer heat away from the weld, causing problems (e.g., copper).
• High thermal expansion and contraction in the metal causes distortion problems.
• Dissimilar metals pose problems in welding when their physical and/or mechanical properties are substantially different.

Other Factors Affecting Weldability

• Filler metal
  • Must be compatible with the base metal(s).
  • In general, elements mixed in a liquid state that forms a solid solution upon solidification do not cause a problem.
• Surface conditions
  • Moisture can result in porosity in the fusion zone.
  • Oxides and other films on metal surfaces can prevent adequate contact and fusion.

Design Considerations in Welding

• Design for welding: the product should be designed from the start as a welded assembly, not as a casting or forging or other formed shape.
• Minimum parts: welded assemblies should consist of the fewest number of parts possible. For example, it is usually more cost-efficient to perform simple bending operations on a part than to weld an assembly from flat plates and sheets.

Arc Welding Design Guidelines

• Good fit-up of parts to maintain dimensional control and minimize distortion; machining is sometimes required to achieve satisfactory fit-up.
• The assembly must allow access for the welding gun to reach the welding area.
• The design of the assembly should allow flat welding to be performed as much as possible, as this is the fastest and most convenient welding position.

Arc Welding Positions

• Welding positions defined here for groove welds:
  • Flat
  • Horizontal
  • Vertical
  • Overhead

Design Guidelines - RSW

• Low-carbon sheet steel up to $0.125 \text{ in } (3.2 \text{ mm})$ is the ideal metal for RSW.
• Strength and stiffness can be obtained in large flat sheet metal components by spot welding reinforcing parts into them and forming flanges and embossments.
• The spot-welded assembly must provide access for electrodes to reach the welding area.
• Sufficient overlap of sheet metal parts is required for the electrode tip to make proper contact.