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Microstructure and Mechanical Properties of Double Pass Laser Welds on 6061-T6 Aluminium Alloy
D. Narsimhachary, R.N. Bathe, J. Dutta Majumdar, G. Padmanabham and A. Basu

The present study is focused on improving metallurgical characteristics and mechanical properties of 6061-T6 aluminium alloy sheet butt welds obtained by common single pass CO2 laser by refining the microstructure with extra laser pass/remelting. Screening experiments were performed initially to enumerate the power density requirement for proper coupling of the laser with the material to develop a defect free weld zone. Recognized process window (3.0 kW power, 3 m/min welding speed for single pass, 2.5 kW power, 6 m/min welding speed for double pass and 18 l/min He flow rate for both) obtained from initial trial was used in butt welding experiment and the weld zones were characterized by macro and microscopic observations, microhardness study and tensile test. The microstructure reveals macro-porosity and crack free welds. The hardness data confirmed presence of heat affected zone (HAZ) and drop in overall hardness in the HAZ of both single and double pass welds. The tensile behaviour shows a decrease in tensile strength and formability as compared to the base material which is attributed to the presence of micro porosity in the single and double pass welds. The fatigue life of the double pass welds was marginally higher compared to the base material and single pass welds. The potentiodynamic corrosion tests showed least resistance to corrosion of single pass weld in a 3.56 wt.% NaCl solution due to the presence of defects in the microstructure and unfavourable residual stress values.

Keywords: CO2 laser, 6061-T6 aluminium alloy, laser welding, double pass, hardness, fatigue, corrosion, formability

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