Effect of shielding gas on microstructure and mechanical properties in AA6061-T6 alloy MIG welding

Due to its properties such as high strength/weight ratio, enhanced corrosion resistance, low density, AA6061-T6 aluminum alloy welding is widely used in structural, automotive and rail industry. In this study, AA6061-T6 alloy was welded with robotic metal inert gas technique using ER5356 filler wire. The effects of different shielding gas composition ratios (argon/helyum) on the macro / microstructre, mechanical properties (hardness, tensile strength) of the weld joint were investigated. Welding porosity decreased with the addition of helium gas to argon gas. Accordingly, the tensile strength of welded joints increased from 190 MPa to 221 MPa with the addition of helium gas. The strength of the welded joints (190-221 MPa) was obtained lower than that of the base material (290 MPa) due to the changes in the microstructure as a result of the weld thermal cycle and grain coarsening in the heat affected zone. Dentritic, columnar and coarse shaped grains were observed along the weld section, respectively in the weld bead, partially melted zone and heat affected zone. The differentiation of the shielding gas composition did not cause these regions to change, but the heat affected zone expanded with the increase in the helium content. The change in hardness of the cross section of welded joint has increased from around 60 HV to 90 HV from the welding area to the base material. 25% Argon-75% Helium gas mixtures provided optimum combination in terms of microstructure, mechanical properties and cost.