MECHANICAL AND MICROSTRUCTURAL CHARACTERIZATION OF DISSIMILAR JOINT OBTAINED BY GMAW USING COLD METAL TRANSFER (CMT)

Abstract

This research work focuses on a comparative study on weld bead geometries of three different welding techniques: Cold Metal Transfer (CMT), Metal Inert Gas Pulse Synergic (MIG P) and MIG Manual Standard (MIG M). Bead-on-plate tests were performed using ER4043 (AlSi5%) as a filler material on the 3.18 mm thick plates of AA6061-T6. Current (80 A, 100 A and 120 A) and welding speed (7.5, 10.5 and 13.5 mm/sec) were used as input process parameters while shielding gas flow rate and contact tip to workpiece distance (CTWD) were maintained constant as 15 l/min and 10mm respectively. The weld beads processed by all the three techniques are compared by analysing the weld bead geometry. Microstructural characterization is carried out using optical microscopy and Field Emission Scanning Electron Microscope (FESEM). CMT has high dilution and penetration with low heat input. Compared to MIG P and MIG M, CMT shows a drastic reduction in residual stresses. Multi-response mathematical model is established for prediction of weld bead geometry in CMT, MIG P and MIG M welding of AA6061-T6 using ER4043 (AlSi5%) as a filler material. Central composite face-centered design (CCFCD) under response surface methodology (RSM) is employed to develop the design matrix for conducting the experiments. The developed model is employed in finding the optimal process parameters for good weld bead aesthetics. Current (I) and welding speed (S) are opted as input process parameters for response output such as penetration, dilution and heat input. This model is proficient to forecast the main effects and interactive effects of two factor of the opted welding process parameters. Results show that higher current values with low welding speed results in deeper penetration, high amount of dilution with higher heat input and vice versa. With lower heat input, CMT has high dilution and penetration with respect to MIG P and MIG M welding. The optimal process parameters are 92.518A and 7.50mm/sec for CMT, 109.418A and 10.873mm/sec for MIG P, 110.847A and 11.527mm/sec for MIG M with 61.11%, 68.80% and 72.6% desirability, respectively. Predicted output values generated from regression model equation obtained from welding process parameters are very close and sometimes overlaid on actual output that obviously demonstrates the suitability of the second order regression equations. A

vi good amount of penetration and dilution with low heat input is required for better joint efficiency.

The requirements projected by many industries for stronger, lighter, more efficient and cost-effective combined alloys in the welding of two dissimilar materials or dissimilar thickness. The current industry trend is the coalescence of various aluminium alloys of varying thicknesses. CMT welding process was used for joining of AA6061-T6 and AA6082-T6 using ER4043 filler wire and inspected the effect of different process parameters on mechanical properties of welded butt joints. Current (I), welding speed or travel speed (TS) and gas flow rate (Q) are the input welding process parameters that are to be optimized. Different heat input is studied w.r.t welding speed, current and gas flow rate. Heat inputs ranging from 100+, 200+ and 300+ J/mm is achieved at constant welding speed of 9, 7 and 5 mm/sec respectively at variable currents and flow rates. Bead geometry variables such as penetration (P), reinforcement (R) and contact angle (CA) are distinguished at different heat inputs. Mechanical properties such as tensile test and microhardness for different heat input were investigated. Microstructural characterization of base metal (BM), fusion line (FL) and weld metal (WM) is carried out. High-Resolution X-ray Diffraction (HR-XRD) technique based on cosα method is used for residual stress measurements at different heat inputs. Tensile fractured surfaces were examined by FESEM and energy-dispersive X-ray spectroscopy (EDX). Butt joints of various different process parameters were fabricated with the help of full factorial CCFCD under RSM to optimize the tensile properties, microhardness and residual stresses. Grey relation analysis (GRA) with Principal component analysis (PCA) is incorporated with CCFCD for finding out the optimal process parameter by considering multi-response parameters simultaneously. ANOVA was executed to interpret the impact of process parameters on the mechanical properties of the weldments. Results showed that the most dominant process parameter was found to be the welding speed. The optimal process parameter obtained via GRA-PCA technique is I3-TS1-Q1 (I - 100 A, TS - 5 mm/sec and Q - 14 L/min having heat input 352 J/mm) which produces 226 MPa of ultimate tensile strength, 12.6 % of elongation, 68.7 HV of microhardness and -152.3 MPa of compressive residual

vii stress. Desirability of optimality level obtained through CCFCD was 65.99 % and significantly improved to 97.07 % through GRA-PCA.

Nowadays, to enhance the structural efficiency, ultrasonic vibrations are combined with other manufacturing processes such as welding. It gives considerable advantages in terms of improved mechanical properties, adequate surface strength, improved material flow and uniform grain growth etc. Ultrasonic assisted cold metal transfer (U-CMT) welding is performed to fabricate the joints and improvements in mechanical properties and microstructural modifications are studied. Non-destructive technique (NDT) such as radiography technique (RT) is used to test weld consistency. Results revealed improved weld bead geometry with the aid of ultrasonic vibrations for the same welding parameters. The tensile strength and micro-hardness are enhanced. Samples with ultrasonic vibration experiences grain refining as compared to without vibration samples. As compared with CMT, U-CMT joints are rich in Al-Si eutectic structure. Al-Si structure is in globular form with reduced porosity level.