Cold Welding | Diagram , Advantages and Disadvantages
Cold pressure welding is a form of solid state welding which is unique because it is carried out at ambient temperature. Other forms of solid state welding are conducted at elevated temperature.
Cold welding is a bonding process during which two solids are forced to form a single piece by applying adequate pressure as shown in Figure 3.2. In other words, cold welding is a solid-state welding process in which joining of metals takes place without fusion/heating at the interface of two parts to be welded. Cold welding is also known as contact welding. In fusion-welding precesses, no liquid or molten phase is present at the joint. In cold welding, metals are joined together without using flux.
The welding of parts is done by extreme high pressure or by contact in a high vacuum with no application of heat. Pressure is applied to points to be welded at temperature below the recrystallization temperature of metals to be welded. This applied pressure brings the atoms on the interface to be welded into such close contact that they diffuse across the interface and a cold pressure weld is made. The atoms of metals are held together by metallic bond. The metallic bond can be described as a ‘cloud’ of free and negatively charged atoms into a unit as a result of attractive forces.
All metals are surrounded by surface layers (oxide) which must be disrupted if they need to be welded. Cold pressure welding carried out at ambient temperature relies.upon the use of high compressive pressure 1400 to 2800 N/mm2 for Aluminium and at least double that value for copper. It provides interfacial deformations of 60% to 80% that break the oxide layers to expose fresh and uncontaminated metal makes contact. In this state, take over to interatomic forces produce the weld.
Characteristics of Cold welding
(i) A cold weld is generally stronger than the parent material and has the same electrical characteristics.
(ii) At least one of the metals must be ductile without excessive work-hardening.
(iii) Total absence of applied heating occurs.
(iv) Surface preparation is important.
(v) Both workpieces can be similar or dissimilar metals.
(vi) Both workpieces should be cleaned.
(vii) Short sections on the workpieces should sheared off.
Advantages, Limitations and Applications of Cold Welding
Advantages of Cold Working :
1. There are no thermal effects on the parts being joined and the process is fast.
2. As the process is performed at ambient temperature, there are no thermal effects on the parts being joined.
3. The weld zone is not only metallurgically homogeneous but the metal is work hardened and stronger than the adjacent areas.
4. It is simple and inexpensive to operate once dies have been produced.
5. The process is fast.
6. It is virtually no deformation.
7. The ends of the wire or rod need no surface preparation to weld and the alignment of the two butt ends is automatic as the material is placed on the die.
8. Parts are joined without contamination from sparks or dusts and vapours.
Limitations of Cold Working :
1. As the welds are made in the solid state, they are difficult to inspect.
2. The thickness of the parts is reduced significantly at the weld where the contact surfaces are sheared together.
3. It is highly specialized type of welding with respect to joint design and materials to be welded.
4. While the speed is an advantage to assemblers, it can also be a limitation.
5. When a body moving that fast meets another, it will try to displace it.
Applications of Cold Working :
1. It is used for joining of wire, foil to wire, wire is bi-metals and scaling of heat sensitive containers such as those containing explosives.
2. Rod coils are butt welded to permit continuity in post-weld drawing to smaller diameters.
3. It is used for joining components where heating is not possible such as magnets.
4. In the electronics industry, cold welding processes are used to seal tin plated steel crystal cans and copper packages for heat sensitive semiconductor devices.
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