Solid State Welding Notes

Solid State Welding

  • Solid state welding involves joining processes where coalescence results from applying pressure alone or a combination of heat and pressure.
  • If heat is used, the temperature remains below the melting point of the metals being welded.
  • No filler metal is added in solid state welding.
  • This method is suitable for joining refractory metals at temperatures that don't affect their metallurgical properties.
  • It is used in industries like aerospace, automotive, and electronics.

Objectives

  • Introduce solid state welding processes and principles.
  • Understand the materials and surface conditions involved.
  • Know the advantages and limitations of solid-state welding.

Casting Metals

  • Issues include reactivity with air and mould materials.
  • Gas solubility, such as H2H_2 gas in Aluminum, is a concern.
  • Safety considerations are important due to metal fires with materials like Magnesium, Niobium, Chromium, Tantalum, Tungsten, and Rhenium.

Principles of Solid State Welding

  • Principles involve diffusion, pressure, and relative interfacial movements.
  • Movement between surfaces aids the process; heat can be generated by friction or ultrasonic vibration.
  • Surface preparation is critical, typically requiring:
    • Chemical cleaning: degreasing, chemical etching
    • Mechanical abrasion: erosion, abrasion, lapping, polishing using sand, diamond powder, or silicon carbide

Advantages of Solid-State Welding (SSW)

  • No melting occurs, which means no heat-affected zone. Thus, the metal around the joint retains its original properties, reducing the chance of defects associated with fusion welding (FW).
  • Many SSW processes bond the entire contact interface between two parts rather than at distinct spots or seams.
  • Some SSW processes can bond dissimilar metals without concerns about relative melting points or thermal expansions.

Disadvantages of Solid-State Welding (SSW)

  • Expensive equipment is required.
  • Significant preparation of parts is needed.
  • Most SSW methods are limited to certain joint designs and thin materials.

Solid State Welding Processes

  • Roll welding
  • Friction welding
  • Diffusion welding
  • Ultrasonic welding
  • Explosion welding
Roll Welding (Cladding)
  • Pressure is applied to two or more sheets through rolls until sufficient plastic deformation occurs to produce solid-state welds.
  • Both cold (without external heat) and hot (using external heat) roll welding methods are used.
  • It's equivalent to forming processes; the material must be ductile.
  • Similar metals work best.
  • Surface preparation includes degreasing, wire-brushing, and cleaning to remove oxide smudges.
Roll Bonding
  • Roll bonding introduces nascent surfaces—new metal-to-metal surfaces formed during plastic deformation.
  • Asperities are collapsed, and oxides are broken up.
  • Increased nascent surfaces result in increased weld strength.
  • A sufficient amount of nascent surfaces is required to achieve metallic bonding or a solid-state weld.
Friction Welding (FRW)
  • SSW process where coalescence is achieved by frictional heat combined with pressure.
  • When properly carried out, no melting occurs at faying surfaces, eliminating solidification-related defects.
  • No filler metal, flux, or shielding gases are normally used.
  • Can be used to join dissimilar metals.
  • It is a widely used commercial process that can be automated.
Weld Zone in Friction Welding
  • The weld zone is confined to a narrow region.
  • High pressure and rotational symmetry of parts (at least one) are required.
  • Surfaces are extruded out, leaving clean surfaces at high temperatures which bond together.
  • Excess material (flash) is usually removed after welding.
  • Pressure force and rotational speed are critical to the shape of the final weld zone.
Friction Stir-Welding (FSW)
  • A third body, a rotating non-consumable probe (Æ5-6mm), is rubbed against the two surfaces to be joined.
  • Contact pressures cause frictional heating to reach 230-260°C.
  • Aluminum-alloy plates up to 75mm (3 in.) thick can be welded by this process.
  • Used in the automotive, aircraft, farm equipment, petroleum, and natural gas industries, as well as for shafts and tubular parts.
Advantages of Friction Welding
  • No melting means no chance for solidification-related defects.
  • No consumables (e.g., filler materials or electrodes) are needed.
  • Very few process variables result in a very repeatable process.
  • Can be used in a production environment (mainly the Continuous Drive Friction Welding process).
  • The fine grain structure of friction welds typically exhibits excellent mechanical properties relative to the base metal, especially when welding aluminum.
  • No special joint preparation or welding skill is required.
Disadvantages of Friction Welding
  • Equipment is very expensive.
  • 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).
  • Limited joint designs, and in the case of Continuous Drive and Inertia Friction Welding, parts must be symmetric.
Diffusion Welding
  • Relies on diffusion to create a weld through a combination of heat and pressure.
  • Diffusion bonding depends on:
    • Absence of contamination and adequate surface finish
    • Ability of at least one component to undergo sufficient plastic flow to contact the interface
    • Sufficient time for diffusion to occur in the interface region for microstructural stability
  • Strength depends on pressure, temperature, time of contact, and cleanliness of surfaces.
  • Atoms from each surface diffuse into each other to create the bond, flattening asperities by microplastic deformation due to high pressures.
Applications
  • Joining of high-strength and refractory metals in 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.
Advantages of Diffusion Welding
  • High strength joints.
  • Versatility - It can be used with dissimilar metals, ceramics, and composites.
  • Good for reactive metals like Ti, Mg, Be, and Zr.
  • Minimal degradation to the base metal.
  • No distortion or deformation.
Disadvantages of Diffusion Welding
  • High cost.
  • Extremely long weld times.
  • Significant surface preparation is required.
  • Does not work well with all metals (e.g., Ni).
Ultrasonic Welding (USW)
  • Produces a weld through the localized application of high‐frequency vibratory energy combined with moderate static pressure.
  • Static normal force and oscillating shearing (tangential) movement create heat at the interface between workpieces.
  • Temperature can reach 0.3-0.5 TmT_m.
Applications
  • Works for dissimilar metals (bi-metallic strips).
  • 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.
Advantages of Ultrasonic Welding
  • Fast welding speeds.
  • Very low heat input and minimal part distortion (other than surface marking).
  • No consumables.
  • Good for automation.
  • Versatile.
Disadvantages of Ultrasonic Welding
  • At least one of the parts being welded must be very thin.
  • Mostly limited to soft metals (e.g., Cu, Al).
  • Limited to lap joints.
  • Equipment cost.
Explosion Welding (EXW)
  • Welding of two metallic surfaces is accomplished by accelerating one of the components at extremely high velocity using the energy of a detonated explosive.
  • The explosive is distributed over the top surface of the prime component.
  • Upon detonation, a high-velocity collision occurs between the prime and base component.
  • Joining happens continuously by local plastic deformation.
Features
  • 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.
  • Can produce almost any dissimilar metal combination, e.g., to clad one metal on top of a base metal over large areas.
  • Clad plate is typically used in the chemical and petrochemical industries as a tube sheet for heat exchangers.
Advantages of Explosion Welding
  • Versatile - It can bond a wide range of dissimilar metals in many applications.
  • Strong Bonding.
  • Cost-Effective - It does not require expensive equipment.
  • Reduced Heat Affected Zone.
Disadvantages of Explosion Welding
  • Not conducive to a production environment.
  • Limited joint designs.
  • Surface Preparation can be time-consuming and costly to achieve.
  • Safety concerns - Extensive knowledge of explosives is required to carry out the process safely. The use of high explosives is commonly regulated, requiring special licensing.

Summary

  • Solid state welding has been widely used for joining engineering components.
  • Surface preparation and cleanliness are important in these joining processes.
  • In solid-state welding, pressure is applied, and heat may be generated externally or internally.
  • A number of solid state welding processes have been introduced.

Concept Questions

  1. Describe the advantages and limitations of explosion welding.
  2. What is the fundamental difference between fusion welding and solid-state welding?