Characteristic of friction welding weld joint of AA6061 on elevated environmental temperature

Abstract

Rotary Friction Welding is a solution in addressing the poblems in welding of material difficult to weld by fusion welding, such as AA6061 aluminium alloy. Fusion welding is difficult to apply to aluminium alloy because of porosity and crack which often occur during solidification. From the research on friction welding of AA6061, it can be conclude that excessive welding temperature of friction welding may result in thermal softening in weld zone and HAZ. The excessive cooling rate after friction welding run will affect to formation of hard and brittle of weld zone. Increase of environmental temperature above room temperature will accelerate workpiece to achieve solid state condition and lowering difference between weld peak temperature and inital weld temperature (ΔT) which mean decreasing of coooling rate. In this research, environmental temperature were varied of 27 °C (room temperature), 50 °C, 75 °C, 100 °C, 125 °C dan 150 °C. Some parameters are set constant such as rotation speed of 1600 rpm, friction pressure of 65 bar or 65 kg.cm−2, friction time (tf) of 6 seconds, forging pressure of 325 bar or 325 kg.cm−2, forging time of 60 seconds, contact diameter of workpiece of 15 mm and the workpiece chamfer angle of 15°. The contact area is divided into three zones, which are the plastic deformation zone (Zpl), the partial deformation zone (Zpd) and the non-deformed zone (Zud). The higher the environmental temperature will produces the wider plastic zone and plastic deformation zone (Zpl + Zpd). The higher the environmental temperature will causes the yield strength of the AA 6061 friction weld joint to increase which modelled as y = 3E-05x2 + 0.0033x + 16.582. Likewise, the tensile strength of the AA 6061 friction weld joint also increases which modelled as y = 3E-05x2 + 0.0026x + 18.119. When comparing the hardness of rotating workpiece side (spin) with workpiece side pressing (press), the previous one is higher at all variations of the temperature. From the photograph of the microstructure of base metal of Al 6061, dark particles (Mg2Si) and gray particles (Fe3SiAl12) are present in the Al matrix. The grain structure of Zud is bigger than Zpd and Zpl. Zpd microstructures at all environmental temperatures form smaller granular structures than base metal grains. Zpl microstructure in workpieces with environmental temperature of 27 °C, 50 °C, 75 °C and 100 °C were formed in small granular structure with Mg2Si (black) and Fe3SiAl12 (gray) structure which spread evenly. However Zpl microstructure with environmental temperature of 125 °C and 150 °C seems to enlarge, especially the structure of Fe3SiAl12 (gray), due to overheat and excessive softening.