The effect of thermal frequency and current amplitude on weldability, microstructural evolution and mechanical properties of AA7075 alloy joint in DP-GMAW process

Abstract

In this study, the effect of dual-pulsed gas metal arc welding (DP-GMAW) parameters such as thermal frequency and current amplitude on weldability, microstructural evolution, and mechanical properties of AA7075 alloy joint was investigated. ER53356 filler metal was employed to make AA7075 alloy joints. Thermal frequency in three levels of 1, 2, and 3 Hz was used during the welding process. And current amplitudes of 9, 27, and 45 A were used in three levels as well. After the DP-GMAW process, the welded samples were first checked visually, by a liquid penetrant method, to make sure of their welding soundness. No surface defect or crack was observed. An optical microscope (OM) and a scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS) were used for microstructural analyses. The hardness of the base metal (BM) and different areas of the welded parts was measured by a microhardness testing method. Results showed that no visual defects were visible on the surface of the joints made under different PD-GMAW variables. As well, microstructural analyses showed that increasing the thermal frequency and current amplitude not only reduced the grain size of the weld metal (WM) and the heat-affected zone (HAZ) but also increased the hardness of these regions. The most refined structure was obtained by employing 2 Hz as frequency and 45 A as current amplitude. The average grain size under these conditions was 71 μm and 52 μm, respectively. It was also observed that increasing the current amplitude had the highest impact in reducing the grain size of the WM. The hardness of the WM and the HAZ, under the optimum frequency, was 91 HVN and 153 HVN, respectively. These values were 85 HVN and 38 HVN under the optimum current amplitude. Moreover, hardness was increased in the HAZ compared with the base metal in all welding conditions. Increasing the thermal frequency not only decreased the distance between the ripples but also increased the number of peak waves created in the fusion line. This was particularly the case in the longitudinal section of the weld and reduced the distance between these waves. On the other hand, increasing the frequency and current amplitude has led to the reduction of porosity in the WM. Finally, increasing the frequency from 1 to 3 Hz led to a 33% reduction in the porosity of the WM. This is while 66% reduction of porosity in the WM was obtained by increasing the current amplitude from 9 to 45 A.