Influence of travel speed on the microstructural evaluation and mechanical characteristics of bobbin tool friction stir-welded thick AA5456-H112 plates

Abstract

5456-H112 aluminum alloy plates with a thickness of 20 mm were joined using floating-bobbin tool friction stir welding at different travel speeds from 28 to 160 mm/min and a fixed rotational speed of 90 rpm. The results of microstructural analysis indicated that the grain size is decreased at a constant travel speed (at each of the speeds of 28, 40, 56, 80, 112, and 160 mm/min), from the retreating side to the advancing side of the joints whereas the density and the size of the intermetallic compounds are increased. Increasing the travel speed also reduces heat input and recrystallized grain size of the stirred region leading to increased hardness of the joints. The tensile test results showed that the ultimate tensile strength, yield strength, and elongation of the welded samples initially increase by increasing the travel speed and then start to decrease due to the formation of defects in the stir zone. The fracture of all transverse tensile test specimens occurs on the advancing side of the joints. The fracture location of the best specimen is located at the adjacent of the base metal with the heat-affected zone. The maximum mechanical properties are achieved at a travel speed of 40 mm/min, and the welding efficiency in that condition is approximately 97%.