9416 Solid State Welding

Solid-state welding is a group of welding processes that produce bonding by the application of pressure at a temperature below the melting temperatures of the base metal and filler.

Explosion welding (EXW) uses a controlled detonation to force parts together at such high pressure that they coalesce. Explosion welding has two applications for aluminum: It has been used to splice natural-gas distribution piping in rural areas where welding equipment and skilled labor is hard to come by and to bond aluminum to other metals like copper, steel, and stainless steel to make bimetallic plates.

Ultrasonic welding (USW) produces coalescence by pressing overlapping parts together and applying high-frequency vibrations that disperse the oxide films at the interface. Ultrasonic welding is very well suited to aluminum: Spot welds join aluminum wires to themselves or to terminals, ring welds are used to seal containers, line and area welds are used to attach mesh, and seam welds are used to join coils for the manufacture of aluminum foil. Welds between aluminum and copper are readily made for solid-state ignition systems, automotive starters, and small electric motors. The advantages of the process are that it requires less surface preparation than other methods, is automatic, fast (usually requiring less than a second), and joint strengths approach that of parent material. Joint designs are similar to resistance spot welds, but edge distance and spot spacing requirements are much less restrictive.

Diffusion welding uses pressure, heat, and time to cause atomic diffusion across the joint and produce bonding, usually in a vacuum or inert gas environment. Pressures can reach the yield strength of the alloys and times may be in the range of a minute. Sometimes a diffusion aid such as aluminum foil is inserted in the joint. Diffusion welding has been useful to join aluminum to other metals or to join dissimilar aluminum alloys. Welds are of high quality and leak tightness.

Pressure welding uses pressure to cause localized plastic flow that disperses the oxide films at the interface and causes coalescence. When performed at room temperature, it is called cold welding (CW); when at elevated temperature, it is termed hot pressure welding (HPW). Cold welding is used for lap or butt joints. Butt welds are made in wire from 0.015 in. (0.4 mm) to | in. (10 mm) in diameter, rod, tubing, and simple extruded shapes. Lap welds can be made in thicknesses from foil to \ in. (6 mm). The 5xxx alloys with more than 3% magnesium, 2xxx and 7xxx alloys, and castings fracture before a pressure weld can be made and so are not suitable for this process. Hot pressure welding is used to make alclad sheet.

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