Welding - Part 4: Solid-State Welding, Quality, and Weldability

Solid-State Welding

  • Definition: Coalescence achieved by:
    • Pressure alone.
    • Combination of heat and pressure.
    • Time can also be a factor.
  • Heat applied is insufficient to cause melting by itself.
  • Filler metal is not added.

General Considerations in Solid-State Welding

  • Metallurgical bond created with little or no melting of base metals.
  • Atomic Bonding:
    • Metals must be brought into intimate contact for cohesive atomic forces to attract.
    • Chemical films, gases, oils, etc., prohibit intimate contact.
    • Films and contaminants must be removed for atomic bonding to succeed.
  • Fusion Welding: Films are dissolved or burned away by high temperatures.
  • Solid-State Welding: Films and contaminants must be removed by other means.
  • Essential Ingredients:
    • Very clean surfaces.
    • Very close physical contact.

Advantages of Solid-State Welding Over Fusion Welding

  • No heat-affected zone; metal retains original properties.
  • Welded joints comprise the entire contact interface.
  • Applicable to bonding dissimilar metals without concerns about relative thermal expansions and conductivities.

Forge Welding

  • Components are heated to hot working temperatures and then forged together.
  • Of minor commercial importance today.

Cold Welding

  • Solid-state welding process applying high pressure between clean surfaces at room temperature.
  • Faying surfaces must be exceptionally clean (degreasing, wire brushing).
  • At least one metal must be very ductile and free of work hardening (e.g., soft aluminum, copper).
  • Compression forces result in cold working, reducing thickness up to 50%.
  • Localized plastic deformation at contacting surfaces results in coalescence.
  • Small parts: hand-operated tools. Heavier work: powered presses.
  • No external heat is applied, but deformation raises the temperature somewhat.
  • Applications include making electrical connections.

Roll Welding

  • Pressure applied by rolls with or without external heat.
  • No external heat: cold-roll welding.
  • Heat supplied: hot-roll welding.
  • Applications include: cladding stainless steel to mild or low alloy steel for corrosion resistance and making bimetallic strips for measuring temperature.

Hot Pressure Welding

  • Coalescence occurs from heat and pressure, causing considerable deformation.
  • Deformation disrupts surface oxide film, leaving clean metal for bonding.
  • Usually carried out in a vacuum chamber or shielding medium.
  • Time must be allowed for diffusion.
  • Principal applications in the aerospace industry.

Diffusion Welding

  • Heat and pressure applied in a controlled atmosphere with sufficient time for diffusion and coalescence.
  • Temperatures well below melting points (about 0.5Tm0.5 T_m is the maximum).
  • Plastic deformation at surfaces is minimal.
  • Primary mechanism is solid-state diffusion (migration of atoms across the interface).
  • Applications include joining high-strength and refractory metals in aerospace and nuclear industries.
  • Used to join similar and dissimilar metals; filler layer may be used to promote diffusion in dissimilar metals.
  • Diffusion time can be significant (more than an hour).

Explosion Welding

  • Rapid coalescence caused by the energy of a detonated explosive.
  • Commonly used for bonding two dissimilar metals, particularly cladding.
  • Applications include production of corrosion-resistant sheet and plate stock.
  • No filler metal or external heat is used.
  • No diffusion occurs.
  • Metallurgical bond combined with mechanical interlocking due to rippled/wavy interface, strengthening the bond by increasing contact area.

Explosion Welding Process

  • Two plates in parallel configuration, separated by a gap, with explosive charge above the flyer plate.
  • Buffer layer (e.g., rubber, plastic) often used between explosive and flyer plate.
  • Lower plate (backer metal) rests on an anvil.
  • Detonation propagates from one end of the flyer plate to the other.
  • High-pressure zone propels flyer plate to collide with backer metal at high velocity, taking on an angular shape as the explosion advances.
  • High-speed collision causes surfaces at the point of contact to become fluid, expelling surface films forward.
  • Colliding surfaces are chemically clean, and fluid behavior (some interfacial melting) provides intimate contact, leading to metallurgical bonding.

Friction Welding

  • Coalescence achieved by frictional heat and pressure.
  • Friction induced by mechanical rubbing (usually rotation).
  • Parts driven together to form a metallurgical bond.
  • Compression force upsets parts, producing a flash.
  • Surface films are expunged.
  • No melting occurs at faying surfaces when properly carried out.
  • No filler metal, flux, or shielding gases are normally used.

Friction Welding - Drive Systems

  • Continuous-drive friction welding:
    • One part driven at constant speed, forced into contact with stationary part.
    • Friction heat generated at interface.
    • Braking applied to stop rotation abruptly, and pieces are forced together at forging pressures.
  • Inertia friction welding:
    • Rotating part connected to a flywheel, brought up to speed.
    • Flywheel disengaged from motor, parts forced together.
    • Kinetic energy dissipated as friction heat.
    • Total cycle is about 20 seconds.

Friction Welding - Additional Details

  • Machines resemble engine lathes.
  • Short cycle times lend to mass production.
  • Applications in welding shafts and tubular parts (automotive, aircraft, farm equipment, etc.).
  • Yields a narrow heat-affected zone.
  • At least one part must be rotational.
  • Flash must usually be removed.
  • Upsetting reduces part lengths.
  • Can join dissimilar metals.
  • Linear friction welding: Linear reciprocating motion generates friction heat; eliminates the rotational requirement.

Friction Stir Welding

  • Rotating tool fed along joint line, generating friction heat and mechanically stirring the metal.
  • Friction heat generated by a separate wear-resistant tool, not the parts themselves.
  • Tool is stepped, with a cylindrical shoulder and smaller pin.
  • Shoulder rubs against top surfaces, generating friction heat.
  • Pin mixes metal along butt surfaces.
  • Probe geometry facilitates mixing action.
  • Heat softens metal to a highly plastic condition without melting it.
  • Leading surface of rotating probe forces metal around it and into its wake, forging the metal into a weld seam.
  • Shoulder constrains the plasticized metal.
  • Applications in aerospace, automotive, railway, and shipbuilding industries, especially butt joints on large aluminum parts. Other metals, polymers, and composites can also be joined.

Advantages of Friction Stir Welding

  1. Good mechanical properties of the weld joint.
  2. Avoidance of toxic fumes, warping, shielding issues, and other problems associated with arc welding.
  3. Little distortion or shrinkage.
  4. Good weld appearance.

Disadvantages of Friction Stir Welding

  1. An exit hole is produced when the tool is withdrawn from the work.
  2. Heavy-duty clamping of the parts is required.

Ultrasonic Welding

  • Components held together under modest clamping force.
  • Oscillatory shear stresses of ultrasonic frequency applied to the interface.
  • Motion breaks down surface films for intimate contact and metallurgical bonding.
  • Temperatures well below the melting point. No filler metals, fluxes, or shielding gases are required.
  • Oscillatory motion transmitted by a sonotrode coupled to an ultrasonic transducer.
  • Transducer converts electrical power into high-frequency vibratory motion.

Ultrasonic Welding - Parameters and Applications

  • Typical frequencies: 15 to 75 kHz, amplitudes of 0.018 to 0.13 mm.
  • Clamping pressures are well below cold welding pressures.
  • Welding times less than 1 second.
  • Limited to lap joints on soft materials (plastics, aluminum, copper).
  • Harder materials cause rapid wear of the sonotrode.
  • Work parts should be relatively small, welding thicknesses less than 3 mm.
  • Applications include wire terminations and splicing, assembly of aluminum sheet-metal panels, welding of tubes to sheets in solar panels, and small parts assembly.

Weld Quality

  • Joining two or more components into a single structure.
  • Physical integrity of the structure depends on the weld quality.
  • Discussion primarily focuses on arc welding.
  • Rapid heating and cooling in localized regions during fusion welding cause residual stresses, distortion, and warping.

Residual Stresses and Distortion - Butt Welding Example

  • Butt welding of two plates by arc welding.
  • Molten pool forms and quickly solidifies behind the moving arc.
  • Adjacent regions become hot and expand, while distant regions remain cool.
  • Shrinkage occurs across the weldment as it cools.
  • Weld seam left in residual tension, and reactionary compressive stresses set up in regions away from the weld.
  • Longitudinal tensile stresses remain in the weld bead.
  • Transverse and longitudinal stresses cause warping.

Minimizing Warping in Weldments

Following are some techniques to minimize warping in a weldment:

  • Welding fixtures can be used to physically restrain movement of the parts during welding.
  • Heat sinks can be used to rapidly remove heat from sections of the welded parts to reduce distortion.
  • Tack welding at multiple points along the joint can create a rigid structure prior to continuous seam welding.
  • Welding conditions (speed, amount of filler metal used, etc.) can be selected to reduce warping.
  • The base parts can be preheated to reduce the level of thermal stresses experienced by the parts.
  • Stress relief heat treatment can be performed on the welded assembly in a furnace for small weldments or using methods that can be used in the field for large structures.
  • Proper design of the weldment can reduce the degree of warping.

Welding Defects

  • Cracks: Fracture-type interruptions in the weld or base metal, caused by embrittlement or low ductility combined with high restraint during contraction.
  • Cavities:
    • Porosity: Small voids formed by entrapped gases during solidification (spherical/blow holes or elongated/worm holes).
    • Shrinkage voids: Cavities formed by shrinkage during solidification.
  • Solid Inclusions:
    • Slag inclusions: Nonmetallic solid materials trapped inside the weld metal; generated during arc-welding processes that use flux.
    • Metallic oxides: Form during the welding of metals such as aluminum (Al<em>2O</em>3Al<em>2O</em>3).
  • Incomplete fusion: Weld bead in which fusion has not occurred throughout the entire cross section of the joint.
  • Lack of penetration: Fusion has not penetrated deeply enough into the root of the joint.
  • Imperfect shape or unacceptable contour: The weld should have a certain desired profile for maximum strength.
  • Arc strikes: Welder accidentally allows the electrode to touch the base metal next to the joint, leaving a scar.
  • Excessive spatter: Drops of molten weld metal splash onto the surface of the base parts.

Inspection and Testing Methods

  • Visual Inspection:
    • Conformance to dimensional specifications.
    • Warping.
    • Cracks, cavities, incomplete fusion, and other visible defects.
    • Limitation: Only surface defects are detectable.
  • Nondestructive Evaluation:
    • Dye-penetrant and fluorescent-penetrant tests: Detecting small surface defects (cracks, cavities).
    • Magnetic particle testing: Limited to ferromagnetic materials; subsurface defects distort the magnetic field, concentrating particles on the surface.
    • Ultrasonic testing: High-frequency sound waves (>20 kHz) detect discontinuities by losses in sound transmission.
    • Radiographic testing: X-rays or gamma radiation detect internal flaws, providing a photographic record.
  • Destructive Testing:
    • Mechanical tests: Tensile and shear tests on weld joints.
    • Metallurgical tests: Examination of metallic structure, defects, heat-affected zone, and other elements.

Weldability

  • A material's ability to be welded, characterized by ease of welding, absence of defects, and acceptable mechanical properties.

Factors Affecting Weldability

  1. Welding process: Some metals are easier to weld with certain processes.
  2. Base metal properties: Melting point, thermal conductivity, and coefficient of thermal expansion are important.
  3. Filler metal: Must be compatible with base metal(s).
  4. Surface conditions: Moisture, oxides, and other films can prevent adequate contact and fusion.

Base Metal Properties and Weldability

  • Lower melting point might seem easier, but some metals melt too easily (e.g., aluminum).
  • High thermal conductivity can make metals hard to weld (e.g., copper).
  • High thermal expansion causes distortion problems.
  • Dissimilar metals pose problems when physical and/or mechanical properties are substantially different.
  • Differences in melting temperature, strength, or thermal expansion can lead to high residual stresses and cracking.

Filler Metal and Surface Conditions' Impact on Weldability

  • Filler metal must be compatible with the base metal(s).
  • Elements mixed in the liquid state that form a solid solution upon solidification will not cause a problem. Embrittlement in the weld joint may occur if the solubility limits are exceeded.
  • Surface conditions of the base metals can adversely affect the operation.
  • Moisture can result in porosity in the fusion zone.
  • Oxides and other solid films on the metal surfaces can prevent adequate contact and fusion from occurring.