Microstructure and hardness of friction stir welded aluminium–copper matrix-based composite reinforced with 10 wt-% SiCp A. Kumar, C. Veeresh Nayak, M. A. Herbert * and S. S. Rao In the present work, an attempt has been made to join aluminium–copper matrix-based composite reinforced with 10 wt-% SiC p , by the friction stir welding technique, at different combinations of tool rotational speed (710, 1000 and 1400 rev mm -1 ) and welding speed (50, 63 and 80 mm min -1 ) using square profiled friction stir welding tool. Welding parameters play a predominant role in improving the mechanical strength by minimising the defects. A good number of defect free joints were obtained at various combinations of rotational speed and welding speed. It has been observed that, rotational speed and welding speed have strong influence on microstructure, Vickers hardness and quality of welds. Keywords: Friction stir welding, Metal matrix composite, Microstructure, Hardness, Nugget zone Introduction Metal matrix composites (MMCs) have gained promi- nence in everyday life because of their light weight, high stiffness, increased strength/weight and improved wear resistance. 1–4 It has applications in almost all the fields like aerospace, locomotive, shipbuilding, etc. 5–7 Even though there is an improvement in terms of mech- anical characteristics while performing welding using conventional fusion welding techniques, defects such as air holes, cracks have been observed, and deleterious phases are formed which in turn deteriorate mechanical properties. 8 At high temperature, in silicon carbide (SiC) reinforced composites, SiC reacts with liquid alu- minium and forms aluminium carbide (Al 4 C 3 ) which is extremely hard and brittle in nature. 9 Similarly in alumina (Al 2 O 3 ) reinforced composite, the Al 2 O 3 decom- poses into aluminium and gas, on coming into contact with molten aluminium. 8,10 Studies reveal that the for- mation of Al 4 C 3 can be minimised by certain welding conditions but it cannot be completely avoided using fusion welding process. 8 On the other hand, a novel well-proven technology known as friction stir welding (FSW) has been developed by TWI in 1991 to join alu- minium alloys. 11 In this process, the temperature of the weld zone is less than the melting point of the base metal. 12 No filler rod is used in FSW and the process does not require any controlled environment. Nowadays, this process has been successfully adapted to weld steel, magnesium, copper, MMCs and dissimilar metals. The joint strength reported in the literature is up to 90% of the base material. 12,13 Friction stir welding process has its own advantages and disadvantages. The disadvantages are the defects like flash, pin hole, worm hole, tunnel and kissing bonds. 14–16 The process parameters used to eliminate the defects are rotational speed of the tool, welding speed, thrust force, tool tilt angle and geometry of the tool. 17–19 The process parameters, namely, rotational speed of the tool, welding speed and thrust force play a predominant role in optimising the process to obtain defect free weld. Flash defect can be eliminated by apply- ing proper amount of thrust force with suitable shoulder diameter. 19 Pin hole and worm holes are produced because of inadequate heat generation during welding process. 18 Tunnel defects are produced because of higher rotational speed of the tool, which soften the material and produce turbulence during material flow. 16 During FSW, the insufficient heat input conditions result in incomplete break up of oxide layer, which is the reason for kissing bonds. 14,18 These defects can be eliminated by generating the required amount of heat through optimised values of process parameters. 17–19 The tool geometry is also a key parameter to reduce the defect, because it affects the heat generation and material flow. 13 In the present work, more attention has been given to investigate the contribution of rotational speed and welding speed on the evolution of microstructure as well as production of defect free welds. Experimental procedure AA6061–4·5 wt-% Cu–10 wt-% SiC p composite was pre- pared by stir casting method. 5 The cast components were machined to 100 × 50 × 6 mm size. The chemical compo- sition of composite is shown in Table 1. A non- Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal 575025, India *Corresponding author, email merhertoma@gmail.com © W. S. Maney & Son Ltd 2014 DOI 10.1179/1432891714Z.0000000001016 Materials Research Innovations 2014 VOL 18 SUPPL 6 S6-84