Indian Journal of Engineering & Materials Sciences Vol. 24, June 2017, pp. 207-214 Characterisation of residual stresses in dissimilar friction stir welded aluminium and copper E T Akinlabi, I Dinaharan* & S A Akinlabi Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park Kingsway Campus, Johannesburg 2006, South Africa Received 21 August 2015; accepted 13 February 2017 In the present work, dissimilar friction stir welding of aluminium alloy 5754 and C11000 copper plates were carried out and the residual stresses were characterised. Hole drilling technique was employed to measure the residual stress at various directions. The dissimilar joints were produced by varying the major process parameters such as tool rotational speed and traverse speed. A heterogeneous microstructure consisting of intercalation of both the materials was observed in the weld zone. The measurements indicated that the distribution of the residual stress along the thickness of the welded plates was not uniform. The nature of the residual stress was compressive in most of the joints. The influence of tool rotational speed and traverse speed on the distribution of residual stress was further reported. Keywords: Friction stir welding, Copper, Aluminum alloys, Residual stress Friction stir welding (FSW) is a solid state welding process invented and patented by The Welding Institute (TWI), United Kingdom in 1991 for joining ferrous and non-ferrous materials 1 . The basic concept of FSW is remarkably simple; a non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of the sheets and plates to be joined and subsequently traversed along the joint line, thereby creating a solid phase joint. The process has been successfully applied in joining different types of materials ranging from metals to plastics. In recent years, considerable research has been conducted in joining dissimilar materials and especially, aluminium and copper 2-4 . Aluminium (Al) and copper (Cu) are particularly difficult to join by conventional methods but in practical applications, the joint of both materials exhibit excellent combinations of mechanical, electrical and thermal properties. Combining these two materials also offers various engineering benefits, particularly in the electrical and heat exchanger applications. Heat input is low in FSW compared to the conventional arc welding techniques. The localized heating of the FSW process is generated by friction and plastic deformation 5-8 . The localized heating softens the material around the pin and combined with the tool rotation and translation, leads to the movement of the material from the front to the back of the pin, thus filling the hole in the tool wake as the tool moves forward 7-9 . Because of the various geometrical features of the tool, material movement around the pin can be complex with gradients in the strain, temperature, and the strain rate 10,11 . FSW is a solid state joining technique which allows successful welding of materials with considerably different melting points. Consequently, the product of the non-homogeneous plastic deformation occurring during the mechanical, thermal and metallurgical processes in joining of dissimilar materials often result in residual stresses in the welds. Although, the residual stresses in friction stir welds can be considered low compared to other fusion welding processes but these residual stresses can be sufficiently high to prevent industrial applications in some cases 12 . However, the numerical and experimental investigation of Li et al. 13 shows that there are many opportunities for misfit that could arise in FSW of dissimilar materials. Therefore, the residual stresses in the welds (at the interface) maybe due to the differences in the elastic behaviour, plastic deformation and the coefficients of thermal expansion of the parent materials joined. Nevertheless, the heat distribution and the effect of the associated —————— *Corresponding author (email: dinaweld2009@gmail.com )