Original article Ductilizing of cast hypereutectic Al–17%Si alloy by friction stir processing Nitin Saini, Chandan Pandey and Dheerendra Kumar Dwivedi Abstract In present research work, multi-pass friction stir processing was utilized for the as-cast Al–17%Si alloy. The multi-pass friction stir processing reproduced the aluminum (Al) matrix with a uniform distribution of the ultra-fine silicon (Si) particles. The multi-pass friction stir processing also resulted in the formation of refining cast microstructure with negligible porosity. The significant reduction in silicon particle size was measured and it reduced from 204 mm to 0.86 mm after the first pass and up to 0.30 mm after two pass friction stir processing. The frequency of fine Si particles was increased after two pass friction stir processing as compared to single pass friction stir processing. The engineering stress–strain curves revealed a significant enhancement in ductility and strength after one and two passes of friction stir processing as compared to the as-cast alloy. After one and two passes of friction stir processing, the ultimate tensile strength of as-cast alloy was enhanced by 24% and 31% and ductility was increased by 300% and 500%, respectively. The secondary electron micrograph of fracture surfaces of tensile specimens was taken before and after friction stir pro- cessing. The fractographs revealed the transformation from brittle mode to ductile dimples fracture after the multi-pass friction stir processing process. Keywords Al–Si alloy, friction, processing, microstructure, fracture, casting, characterization Date received: 30 June 2017; accepted: 21 September 2017 Introduction The structural materials are generally known for their superior tensile strength, toughness, and ductility. 1 For the designing of structural material, all these properties play an important role at appropriate stages. The increase in strength and hardness at any stage of designing can lead to the poor ductility of material. 2,3 The cast Al–Si alloys have a wide poten- tial for industrial applications in the field of automo- biles, aerospace, and defense industries due to its low weight to strength ratio and low thermal expansion coefficient. 4–6 The cast Al–Si alloys are also used in AC compressors, cylinder blocks, and pistons. 7 In Al–Si alloys, the main problem is a catastrophic failure due to inherent brittleness of coarse primary Si phases or particles. 8,9 However, the microstructural refinement of cast Al–Si alloys resulted in the increase ductility and strength. Both strength and ductility were enhanced as the grain size reduces. 10–12 For the refinement of primary Si, various microstructural refinement techniques were used by the researchers in the as-cast Al–Si alloys, which includes melt modifi- cation, 13–18 spray forming, 19,20 friction stir processing (FSP), 2,21,22 and squeeze casting. 23 After examining the research papers, the effect of microstructural refinement due to a reduction in primary silicon par- ticles on tensile properties is tabulated in Table 1. It was observed that the refinement of primary silicon (Si) particles was limited to 4 mm with minimal poros- ity. The effect of grain refinement on tensile properties of as-cast alloy using multi-pass FSP was also not readily available in the literature. In contrast to this, FSP is selected to refine the microstructure and to reduce the Si size much below the above mentioned limit in the present work. The schematic of FSP is shown in Figure 1. The precursor of this technique is friction stir welding. 24,25 The mechanism of FSP is started with a non-consumable tool. A rotated tool is plunged into a workpiece and traversed opposite to the processed region. 3,26 The remarkable microstructural changes were noticed in the processed zone due to thermal exposure of mater- ial, mixing, and intense plastic deformation. 27–29 Proc IMechE Part E: J Process Mechanical Engineering 0(0) 1–6 ! IMechE 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954408917737735 journals.sagepub.com/home/jpme MIED, IIT Roorkee, Roorkee, India Corresponding author: Chandan Pandey, IIT Roorkee, S-40 Azad Bhawan, Roorkee, Haridwar, Uttaranchal 247667, India. Email: chandanpy.1989@gmail.com