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Improvement of impact strength of friction‐welded joints by weld interface shape optimisation

Copper (Cu) offering superior electric conductivity is used in a wide variety of electrical systems. Cu, however, has high specific gravity and greatly adds to the weight of these systems, making it desirable to implement weightreducing measures for large-sized systems. For this purpose, aluminium (Al) or Al alloys with low specific gravity and electric conductivity second only to that of Cu are widely used. Al, however, has a low melting point, which sets limits on its application for components heated at elevated temperature, such as contact parts, etc, and it is therefore desirable to combine the advantages of both metals. Friction welding of Cu and Al is one available technique which has already been extensively researched.' The results of friction welding experiments under a variety of conditions have shown that, when the weld interface is heated at a temperature over 821 K, a melt with the eutectic point composition is formed and that the solidification layer of this eutectic point composition and the 6 phase (CuAl2 intermetallic compound) formed adjacent to the latter are extremely brittle in such a way as to cause loss of joint strength. The r¡2 phase (CuAl) and y2 phase (Cu9Al4) are formed in addition to the 6 phase under annealing treatment at a temperature over 723 K. These phases are also reported to cause some loss of joint strength. The authors have previously shown that the thickness of these intermetallic compounds strongly affects the impact strength properties of the welded joints. The most important problem facing application of Cu/Al friction-welded joints to power systems is improvement of the impact strength properties of welded joints. In this context, consideration has also been given to the free-edge stress singularities at the weld interface edge, and it has been shown that, through the adoption of a circular arc-like interface shape causing these singularities to disappear, the impact strength properties of Cu/Al friction-welded joints can be vastly improved. From the perspective of technique simplicity, however, it is more practical to control the weld interface edge shape by simple bevelling rather than to adopt a circular arc-like interface shape.