Factors affecting the properties of Friction Stir Welds between aluminum and magnesium alloys

Extruded Al–0.5Mg–0.3Si (6063) aluminum and rolled Mg–3Al–1Zn (AZ31B) magnesium alloy sheets were joined by Friction Stir Welding. The dissimilar metal weld joints exhibit tortuous interfaces. The nugget grain size on both the Al and Mg sides monotonically increase as the tool rotational speed increases. The midplane microhardness traverses show fluctuating hardness peaks due to the presence of different microstructural phases in the nugget zone. The maximum tensile strength of the dissimilar weld joint is 68% of the 6063-T5 base metal with a maximum elongation of 1%. The low ductility is attributed to the formation of brittle intermetallic phases at the Al–Mg interface in the weld joint. Transverse tensile test results are correlated with several interface features: (1) actual interface length, (2) extent of interpenetration between the aluminum and magnesium base metals, (3) maximum intermetallic layer thickness, and (4) area fraction of micro-void coalescence on the tensile fracture surfaces. Results indicate that maximizing the extent of interpenetration and hence promoting mechanical interlocking between the metallic phases, is the key to attaining reasonable transverse strength in Al–Mg dissimilar metal welds. The welding process control variables that promote higher mechanical interlocking of the weld joints are discussed. The process response variables (welding torque, power, x-axis force and the nugget grain size) are presented for a range of welding parameters.