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Manufacturing Processes > Joining processes > Solid state welding


Solid state welding


Process description

A range of methods utilizing heat, pressure and/or high energy to plastically deform the material at the joint area in order to create a solid phase mechanical bond.


  • Cold Welding (CW): Ductile metals such as carbon steels, aluminum, copper and precious metals.
  • Friction Welding (FRW): can weld many material types and dissimilar metals effectively, including aluminum to steel. Also thermoplastics and refractory metals.
  • Ultrasonic Welding (USW): can be used for most ductile metals, such as aluminum and copper alloys, carbon steels and precious metals, and some thermoplastics. Can bond dissimilar materials readily.
  • Explosive Welding (EXW): carbon steels, aluminum, copper and titanium alloys. Welds dissimilar metals effectively.
  • Diffusion bonding (DFW): stainless steel, aluminum, low alloy steels, titanium and precious metals. Occasionally copper and magnesium alloys are bonded.
Solid State Welding

Process variations

  • CW: process is performed at room temperature using high forces to create substantial deformation (up to 95 per cent) in the parts to be joined. Surfaces require degreasing and scratch-brushing for good bonding characteristics.
  • Cold pressure spot welding: for sheet-metal fabrication using suitably shaped indenting tools.
  • Forge welding: the material is heated in a forge or oxyacetylene ring burners. Hand tools and anvil used to hammer together the hot material to form a solid state weld. Commonly associated with the blacksmith’s trade and used for decorative and architectural work.
  • Thermocompression bonding: performed at low temperatures and pressures for bonding wires to electrical circuit boards.
  • USW: hardened probe introduces a small static pressure and oscillating vibrations at the joint face disrupting surface oxides and raising the temperature through friction and pressure to create a bond. Can also perform spot welding using similar equipment.
  • Ultrasonic Seam Welding (USEW): ultrasonic vibrations imparted through a roller traversing the joint line.
  • Ultrasonic soldering: uses an ultrasonic probe to provide localized heating through high frequency oscillations. Eliminates the need for a flux, but requires pre-tinning of surfaces.
  • Ultrasonic insertion: for introducing metal inserts into plastic parts for subsequent fastening operations.
  • Ultrasonic staking: for light assembly work in plastics.
  • FRW: the two parts to be welded, one stationary and one rotating at high speed (up to 3000 rpm), have their joint surfaces brought into contact. Axial pressure and frictional heat at the interface create a solid state weld on discontinuation of rotation and on cooling.
  • Friction stir welding: uses the frictional heat to soften the material at the joint area using a wear resistant rotating tool.
  • EXW: uses explosive charge to supply energy for a cladding sheet-metal to strike the base sheetmetal causing plastic flow and a solid state bond. Bond strength is obtained from the characteristic wavy interlocking at the joint face. Can also be used for tube applications.
  • DFW: The surfaces of the parts to be joined are brought together under moderate loads and temperatures in a controlled inert atmosphere or vacuum. Localized plastic deformation and atomic interdiffusion occurs at the joint interface, creating the bond after a period of time.
  • Superplastic diffusion bonding: can integrate DFW with superplastic forming to produce complex fabrications.

Economic considerations

  • Production rates varying: high for CW and FW (30 s cycle time), moderate for USW and low for EXW and DFW.
  • Lead times low typically.
  • Material utilization excellent. No scrap generated.
  • High degree of automation possible with many processes (except EXW).
  • No filler materials needed.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs low to moderate.
  • Equipment costs low (CW, EXW) to high (USW, FRW, DFW).
  • Direct labor costs low to moderate. Some skilled labor maybe required.
  • Finishing costs low. Cleaning of welds not necessary typically, except with FRW, which requires machining or grinding to remove excess material.

Typical applications

  • CW: welding caps to tubes, electrical terminations and cable joining
  • USW: for sheet-metal fabrication, joining plastics, electrical equipment and light assembly work
  • FRW: for welding hub-ends to axle casings, welding valve stems to heads and gear assemblies
  • EXW: used mainly for cladding, or bonding one plate to another, to improve corrosion resistance in the process industry, for marine parts and joining large pipes in the petrochemical industry
  • DFW: for joining high strength materials in the aerospace and nuclear industries, biomedical implants and metal laminates for electrical devices.

Design aspects

  • Typical joint designs: lap (CW, USW, USEW, EXW, DFW), edge (USEW), butt (CW, FRW, ESW), T-joint (DFW), flange (EXW).
  • Access to joint area important.
  • Unequal thicknesses possible with CW, USW, EXW, DFW.
  • CW: thicknesses ranging 5–20 mm.
  • USW: thicknesses ranging 0.1–3 mm.
  • EXW: thicknesses ranging 20–500mm and maximum surface area =20m2.
  • FRW: diameters ranging between Ø2 and Ø150mm and maximum surface area =0.02m2. Parts must have rotational symmetry.
  • DFW: thicknesses ranging 0.5–20 mm.

Quality issues

  • Little or no deformation takes place (except EXW).
  • No weld spatter and no arc flash.
  • Alignment of parts crucial for consistent weld quality.
  • Parts must be able to withstand high forces and torques to create bond over long period of times.
  • Safety concerns for EXW include explosives handling, noise and provision for controlled explosion.
  • Welds as strong as base material in many cases.
  • Surface preparation important to remove any contaminates from the weld area such as oxide layers, paint and thick films of grease and oil.
  • Possibility of galvanic corrosion when welding some material combinations.
  • Surface finish of the welds good.
  • Fabrication tolerances vary from close for DFW, moderate for FRW, CW, USW and low dimensional accuracy for EXW.