Parametric investigation for single sided and double sided friction stir welding of aluminium alloy 6061-t6 and s275jr mild steel butt joint

Friction stir welding (FSW) is a solid-state joining technique capable of joining aluminum and steel together, which is beneficial to multiple industries such as the automotive, aerospace and chemical industry is attributed by the combined high strength and toughness of steel as well as the corrosion resistance and low density of aluminum alloys. However, joining these two materials together has proven to be a difficult undertaking due to a strong tendency to form large amounts of brittle and hard intermetallic compounds (IMC) at the interface as a result of mutual diffusion during welding. While the growth of IMC layer is minimized in FSW due to the low process temperature, investigation on the influence of welding parameters on IMC layer formation as well as other joint properties such as tensile strength and hardness is required. The appearance of FSW-related joint defects such as tunnel defects, insufficient welding and root defects are also of interest due to its negative effect on joint strength. The influence of double sided FSW (DS-FSW) on joint properties of aluminum-to-steel joints as well as its ability to fix root defects also require evaluation. Several welding parameters were studied and their effects on joint properties investigated: tool offset, tool rotation direction, tool plunge depth, tool travel speed and tool tilt angle. Joints were evaluated based on their microstructural characterizations such as weld zones, IMC layer thickness, grain size in the stir zone and properties such as hardness and tensile strength. Defects formed on the joint such as excessive material flash, insufficient bonding, tunnel defect and root defect were scrutinized. The thickness of IMC layer at the top and middle region of the joint were measured for all joints. Tool plunge depth was seen to substantially influence heat generation and downwards pressure, evidenced by the appearance of tunnel defects at 0.1 mm and thick IMC layers at 0.3 mm and 0.5 mm tool plunge depth. At very low (30 mm/min) and very high (110 mm/min) tool travel speed, tunnel defects were formed. At 1° low tool tilt angle, welding was successfully performed. IMC layer formed in the joint was thinner than ones in a joint produced at 3° tool tilt angle, however at the topmost region of the joint it was thicker. Root defect was observed in all the welded joints. Removing the root defect from tensile specimens of several joints by machining was found to substantially increase the joint strength as fractures no longer initiate at the root defect. However, several joints without root defect still fractured at the interface, which was determined to be caused by thick IMC layer at the crown. Double sided FSW (DS-FSW) was investigated as a possible method to eliminate root defects without material removal by machining. It was found that using DS-FSW with a 4.5 mm and 4.0 mm tool pin length was successful in removing the root defect, however joints still fractured at the interface. It was found that thick IMC layers were formed at the 1st side crown of the joint. By performing DS-FSW using shorter tool pins of 3.5 mm and 3.0 mm, root defects were successfully removed and the joint fractured away from the interface, signifying satisfactory bonding. The IMC layer thickness along the depth of joints were measured to investigate DS-FSW’s effects. It was observed that longer tool pins resulted in thicker IMC layer and vice versa.