A solid-state welding process in which pressure is used to produce a weld at room temperature with substantial deformation at the weld. See also DIFFUSION WELDING,
FORGE WELDING, and HOT PRESSURE WELDING.
A characteristic of the cold welding process is the absence of heat, either applied externally or generated by the welding process itself. A fundamental requisite for satisfactory cold welds is that at least one of the metals to be joined is highly ductile and does not
exhibit extreme work-hardening. Both butt and lap joints can be cold welded.
Cold welding involves two concurrent steps:
(1) distorting the contact surfaces of two ductile metals to rupture their surface oxide
layers, thus exposing clean metal, and
(2) applying enough pressure across those surfaces to allow interatomic bonding. The oxides and other surface contaminants become scattered as minute particles within the joint. Although most commonly used to join sheets of nonferrous metals such as
aluminum and copper, cold welding also allows dissimilar metals and other shapes to be joined.
In all cases, however, the contacting surfaces must be clean of surface contaminants and then deformed sufficiently to force the surface oxides to rupture and intimate contact of the surfaces to be made. Since work hardening is inevitable, the joints are somewhat stronger than might be expected. When joining sheet metals these objectives can be accomplished with dies.
The ends of bars also can be joined by using strong clamping shoes, powerful hydraulic forces and containment dies to deform the ends. Even tubes can be cold welded by positioning one tube inside the other and pulling them between a drawing die and a mandrel to cause the needed surface deformation.
Materials for Cold Welding
Materials with face-centered cubic (FCC) lattice structure are best suited for cold welding, provided they do not work-harden rapidly. Soft tempers of metals such as aluminum and copper are most easily cold welded. It is more difficult to weld cold worked or heat treated alloys of these metals. Other FCC metals that may be cold welded readily are gold, silver, palladium and platinum.
Dissimilar Metal Welds
Joining copper to aluminum by cold welding is a good application of the process, especially where aluminum tubing or electrical conductor grade aluminum is joined to short sections of copper to provide transition joints between the two metals. Such cold welds are characterized by substantially greater deformation of the aluminum than the copper because of the difference in the yield strengths and work-hardening behaviors of the two metals.
Numerous dissimilar metals may be joined by cold welding, whether or not they are soluble in one another. In some cases, the two metals may combine to form intermetallic compounds. Since cold welding is carried out at room temperature, there is no significant
diffusion between dissimilar metals during welding. The alloying characteristics of the metals being joined do not affect the manner in which the cold welding operation is carried out. However, the interdiffusion at elevated temperatures can affect the choice of postweld thermal treatments and the performance of the weld in service.
Welds made between metals that are essentially insoluble in each other are usually stable. Diffusion can form an intermetallic compound at elevated service temperatures. In some cases, this intermetallic layer can be brittle and cause a marked reduction in the ductility of the weld. Such welds are particularly sensitive to bending or impact loading after an intermetallic layer has formed.
Applications
Butt Joints. Cold welding is commonly used to produce butt joints in wire, tubing, and simple extruded shapes of like and unlike metals. A major application is in the manufacture of aluminum, copper, gold, silver, and platinum wire. The most common use is to join successive reels of wire for continuous drawing to smaller diameters. Diameters ranging from 0.06 to
12.7 mm (0.0025 to 0.50 in.) have been successfully welded.
Lap Welds. Lap welds can be used for joining aluminum sheet or foil to itself and also to copper sheet or foil. Commercial uses of lap welding include packaging applications, as well as electrical applications, which is probably the major use for cold welding. It is especially useful in the fabrication of electrical devices in which a transition from aluminum windings to copper terminations is required. The range of electrical applications covers large distribution transformers to small electronic devices. A variation of cold lap welding is applied to the sealing of commercially pure aluminum, copper, or nickel tubing.
The contacting surfaces must be clean of surface contaminants. Dirt, absorbed gas, oils, (even finger- prints) or oxide films on the surfaces interfere with metal-to-metal contact and must be removed to obtain strong welds. Rotary brushes of 0.1 mm (0.004 in.)
diameter stainless steel wire, brushing at a surface speed of about 15 m/s (3000 ft/min) is recommended. Chemical and abrasive cleaning methods are not satisfactory because the chemical residue or abrasive particles in or on the surface may prevent the formation of a sound weld.
Equipment
Pressure for welding may be applied to overlapped or butted surfaces with hydraulic or mechanical presses, rolls, or special manually or pneumatically operated tools. A hand tool of the toggle cutter type is suitable for very light work; common manually operated presses can be used for medium size work. Heavy work requires power operated machines. The rate of pressure application does not usually affect the strength or quality of the weld.
Pressure required to effect a weld depends on the working area of the dies. Pressures are generally slightly above the flow point of the material, and range from 186 to 276 MPa (27 000 to 40 000 psi) for aluminum, and from two to four times as much for copper. Time during which the pressure is applied is not critical; good welds can be made with either slow squeeze or impact. Hand welding by impact on an anvil is quite feasible, provided correct penetration of the die can be achieved.
Given a suitable arrangement of workpieces and dies, the application of pressure forces the work surfaces into close contact while the flow takes place, welding them solidly together. The work hardening that necessarily takes place is an advantage, because it tends to balance the loss in strength resulting from the decrease in the cross section.
The term cold welding is also applied to the self-diffusion property of a material. For example, two sheets or strips of silver in contact with one another will adhere at temperatures ranging from 200 to 400°C (400 to 750″) at pressures up to 310 MPa (45 000 psi). Lead and other materials have this same quality.