COMPARATIVE STUDY OF MECHANICAL AND MICROSTRUCTURE PROPERTIES OF AA6082-T6 USING CMT AND FSP OVER CMT WELDING

Abstract

Being light weight, malleable and ductile, good corrosion resistance, good conductor of heat and electricity, aluminium alloys have wide range of application in various types of industries but they are difficult to weld. FSW and CMT are widely accepted techniques for welding of aluminium alloys due to their low heat input and different mechanism. This research focuses on comparative study of CMT welding process and a hybrid welding process which is FSP over CMT. 5 mm thick AA6082 aluminium alloy was selected as base material while looking at compatibility, ER 5356 was selected as a filler wire. Mechanical properties, hardness distribution, and microstructure were studied for both the processes. For FSP, circular profile tool was used with constant 1° tilt angle in clockwise direction. Experiments were conducted on different process parameters with the help of DoE and same were optimized with the help of Taguchi optimization technique. ANOVA was used to find out the percentage contribution of an individual parameter. From optimization, welding current has more effect (67.47% contribution) on the strength of CMT welded specimen compared to other process parameter while tool rotation speed was more effecting (50.45% contribution) on UTS in case of FSP over CMT welded samples. After the tensile test, for CMT sample, the maximum UTS was (192 MPa) 75.88% of the base metal (253 MPa) while for FSP over CMT sample, maximum UTS was (168 MPa) 66.40% of base metal. More UTS was observed (sample no. 7, 8, & 9) at higher current in CMT while UTS was in range of 155-168 MPa for FSP over CMT samples. Hardness test for CMT sample shows that it have the maximum value at HAZ v followed by base metal and weld zone forming M-pattern while for FSP over CMT sample, hardness at TMAZ was highest followed by stir zone and base metal. Moreover, FSP increased the hardness value by 2%. Microstructure analysis for FSP over CMT sample shows that at higher tool rotation speed, tunnel defect was there which was lowering the value of UTS for the same sample. Onion rings can be seen in the stir zone while very fine equiaxed grain structure was there in stir zone. Due to dynamic recrystallization, fine grains were there in the stir region and hence, FSP was successful in increasing the hardness of the CMT welded specimen. Microstructure of CMT samples shows coarser grain structure at HAZ while elongated fine grain structure at weld bead zone. For some samples, lower value of UTS was observed because of gas porosity defects and lack of fusion between the plates. Moreover, FSP successfully removes the gas porosity defects and lack of fusion from the CMT welded sample. Thus conforming the strength not to be lower than its range.