Effect of travel speeds on microstructures and mechanical properties of friction-stir welded 5754 aluminum alloy sheets

The 3.0 mm thick cold-rolled sheets of 5754 aluminum alloy were prepared by friction stir welding (FSW) at a rotation rate of 800 r/min. The microstructures and mechanical properties of FSW 5754 joints with different travel speeds (100 ~ 400 mm/min) were studied. Results showed that the cross section morphologies of FSW 5754 joints were "basin" type. With the travel speed increased, the area of both nugget zone (NZ) and stir-shoulder zone (SSZ) of FSW 5754 joints reduced. While the area of stir-pin zone (SPZ) increased initially and decreased afterwards, peaking at 6.66 mm² with the speed of 300 mm/min. At that speed, the area ratio of SSZ to SPZ zone was 0.97, and the strength coefficient of FSW 5754 aluminum alloy joint was 97.5%. These were mainly resulting from the increased interface area between NZ zone and HAZ zone, caused by the similar area between SSZ and SPZ zone. Tensile specimens were all fractured in heat affected zone (HAZ) or base material (BM) with a ductile model fracture. When the travel speed reached 400 mm/min, the strength coefficient of FSW 5754 aluminum alloy joint was dropped to 58.8%. Tensile specimens were all fractured at NZ zone with a brittle model fracture.