Studies on the effect of process parameters on mechanical properties of CMT welded aluminium/steel joint

Abstract

Cold Metal Transfer (CMT) welding is a modified MIG welding process based on the short-circuiting transfer process. The Cold Metal Transfer machine detects a short circuit which sends a signal that retracts the welding filler material, giving the weld time to cool before each drop is placed, resulting in a drop-by-drop deposit of weld material. However, the technique can be modified or combined with suitable addons to give better weld properties such as using supercooling with CMT welding to give better weld properties. In this study, different processing parameters were varied, such as the welding current, cooling rate and feed rate to obtain the best possible weld characteristics like weld bead appearance, microstructure features and mechanical properties named tensile strength and hardness. Aluminium alloy (Al6061) similar and Al6061-Galvanized Steel (Q235) dissimilar were joined using AlSi5 (ER4043) filler wire in CMT welding. Microstructural characterization was done using an optical microscope, and comparing the optimized weld parameters to find the best possible tensile strength and hardness was done. The use of supercooling assisted welding significantly improves the weld properties. This was related to the development of finer grains and a more homogenous distribution of the strengthening particles as supercooling was applied. It also results in the development of grain refinement. This grain refinement led to the increment of hardness and toughness. The results were discussed based on the microstructural features observed and mechanical properties measured. The optimized weld parameters were identified for the maximum improvement in the tensile strength of the joint. Maximum strength for Aluminium similar joint achieved was 154.7MPa at 120A welding current, 8-10mm arc length with welding speed 4.1mm/sec using normal cooling setup. This joint strength was improved by 164.4MPa at 130A welding current using copper cooled backplate. Maximum strength for Al-Steel dissimilar joint achieved was 158.3MPa at 130A welding current, 8-10 mm arc length, 4.1mm/sec welding speed using normal cooling setup.