COLD WELDING
Cold pressure welding is a form of solid state welding that is unique because it is performed at room temperature. Other forms of solid state welding are conducted at elevated temperatures.
Cold welding is a bonding process during which two solids are forced to form a single piece by applying suitable pressure, as shown in Figure 3.2. In other words, cold welding is a solid-state welding process in which the joining of metals occurs without melting/heating at the interface of two parts to be welded. Cold welding is also known as contact welding. In fusion welding processes, no liquid or molten phase is present in the joint. In cold welding, metals are joined without the use of flux.
The parts are welded using extreme high pressure or high vacuum contact without applying heat. Pressure is applied to the points to be welded at a temperature lower than the recrystallization temperature of the metals to be welded. This applied pressure causes the atoms at the interface to be welded to come into such close contact that they diffuse across the interface and a cold pressure weld is made. Metal atoms are held together by metallic bonds. The metallic bond can be described as a 'cloud' of free and negatively charged atoms in a unit as a result of attractive forces.
cold weldingAll metals are surrounded by surface layers (oxide) that must be broken if they need to be welded. Cold welding performed at room temperature relies on the use of high compressive pressure of 1400 to 2800 N/mm2 for aluminum and at least double that value for copper. It provides interfacial strains of 60% to 80% that break down the oxide layers to expose fresh, uncontaminated metal contact. In this state, interatomic forces produce the weld.
Features of cold welding
(i) A cold solder is generally stronger than the original material and has the same electrical characteristics.
(ii) At least one of the metals must be ductile without excessive hardening.
(iii) There is a total absence of applied heating.
(iv) Surface preparation is important.
(v) Both pieces can be made of similar or different metals.
(vi) Both work pieces must be cleaned.
(vii) Short sections on the workpieces should be cut.
Advantages, Limitations and Applications of Cold Welding
Advantages of cold working:
1. There are no thermal effects on the parts to be joined and the process is quick.
2. As the process is carried out at room temperature, there are no thermal effects on the parts to be joined.
3. The weld zone is not only metallurgically homogeneous, but the metal is hardened and stronger than the adjacent areas.
4. It is simple and cheap to operate after the dies have been produced.
5. The process is quick.
6. There is practically no deformation.
7. The ends of the wire or rod do not require surface preparation for welding and the alignment of the two ends is automatic as the material is placed in the die.
8. Parts are joined without contamination by sparks, dust and vapors.
Limitations of cold working:
1. Because welds are made in a solid state, they are difficult to inspect.
2. The thickness of parts is significantly reduced in welding, where the contact surfaces are sheared.
3. It is a highly specialized type of welding in relation to the joint design and the materials to be welded.
4. While speed is an advantage for assemblers, it can also be a limitation.
5. When a body moving so fast encounters another, it will try to displace it.
Cold working applications:
1. It is used for joining wires, foils and wires, bimetallic wires and fouling of heat-sensitive containers such as those containing explosives.
2. Rod coils are butt welded to allow continuity in post-weld drawing at smaller diameters.
3. It is used to join components where heating is not possible, such as magnets.
4. In the electronics industry, cold welding processes are used to seal tinned steel crystal cans and copper packaging for heat-sensitive semiconductor devices.
