Microstructures and mechanical properties evolution during friction stir welding of SK4 high carbon steel alloy

Stir-in-plate welds were produced on 2 mm thick plates of SK4 high carbon steel alloy (0.95% C) using a load controlled friction stir welding (FSW) machine. The welding was carried out at a constant welding speed of 100 mm/min and different rotation speeds varied between100 and 400 rpm. The microstructure and mechanical properties of the weld metals were investigated. When FSW was carried out at 100 rpm, a duplex structure of spheroidal cementite and fine ferrite was formed and homogenously distributed in the entire stir zone. On the other hand, when the FSW was carried out at a rotation speed higher than 100 rpm, very fine pearlite and martensite structures were formed in the upper part of the stir zone and increased with the increasing rotation speed. Thin film of retained austenite was found in the microstructure of the weld metal stirred at 200 rpm and its volume fraction increased with further increasing in the rotation speed. Hardness values measured in the stir zone formed at 100 rpm were slightly higher than that of the base metal and homogenously distributed throughout the entire stir zone. Increasing rotation speed more than 200 rpm led to a sharp increase in the hardness values of the stir zone and maximum values around 800-820 Hv were attained at 400 rpm below the tool shoulder. Yield and ultimate tensile strengths of the weld metals increased with the increasing rotation speed, while elongation decreased. Fracture surface of the weld metal formed at 100 was similar to the base metal (BM) and exhibited only a microvoid coalescence dimple fracture, while that formed at 200-400 rpm showed a mixed mode of quasi-cleavage transgranular fracture and ductile dimple fracture mode.