SOME STUDIES ON ENHANCEMENT OF PROPERTIES OF DISSIMILAR WELDMENTS OF AA7075 AND AA6061 ALLOYS

Abstract

Historically, aluminum (Al) alloys were primarily utilized in the automotive industry, but their applications have expanded into aviation and military domains due to their advantageous attributes such as lightweight nature, high strength-to-weight ratio, low density, robust corrosion resistance, and toughness. A lot of studies have been investigated on 7xxx and 6xxx series aluminium alloys for diverse applications, mainly due to their eutectic structure at elevated temperatures. The AA7075 alloy stands out for its superior mechanical performance, corrosion resistance, and low density, making it a preferred choice for manufacturing upper wing structural components, airplane tails, fins, and aviation control surfaces. Conversely, AA6061 exhibits exceptional welding capabilities and characteristics, commonly used for traditional structural parts, machinery, marine, and process equipment applications. The combination of both series of aluminium alloys holds the potential to enhance the performance of structural components in both aircraft and automotive sectors. Nevertheless, a significant challenge lies in the amalgamation of dissimilar Al alloys, as variations in chemical, mechanical, and physical properties may lead to the formation of intermetallic phases in the fusion zone. This study seeks to join dissimilar AA7075 and AA6061 aluminium alloys using various welding methods. The first phase is dedicated to investigating the effect of various process parameters on the microstructure evolution and mechanical properties of dissimilar welded joints through friction stir welding (FSW) techniques. The assessment of the welded joint revealed enhanced Vickers hardness, yield strength (YS), ultimate tensile strength (UTS), and % elongation (% El.). Microstructural characteristics of the dissimilar AA7075/AA6061 welded joint, such as grain orientation spread map, image quality with grain boundary distribution, misorientation angle distribution, kernel average misorientation, and grain size distribution structure, were examined using electron backscattered diffraction (EBSD). Additionally, the confirmation of precipitation particles and the elemental composition of aluminium alloys was achieved through field emission scanning electron microscopy (FE-SEM) with energy dispersive spectroscopy (EDS) analysis and area mapping, respectively. transmission electron microscopy (TEM) was employed to examine the distribution of precipitation particles. In this study, it was observed that as the tool traverse speed increased from 20 to 60 mm/min, there was a corresponding decrease in the average grain size of the stir zone. The dissimilar joints displayed a superior ultimate tensile strength of 251 MPa for the friction stir welded specimen generated at a traverse speed of 60 mm/min. However, the welded specimen demonstrated a certain reduction in micro-hardness in the nugget zone compared to the base metal of AA7075. Hardness measurements in the nugget zone exhibited a consistent increasing trend with an increment in traverse rate. The maximum hardness at nugget zone was 128 HV. The lowest hardness was recorded in the heat-affected zone (HAZ) of the softer side material AA6061 at the lowest speed (20 mm/min). The examined fracture surfaces of the dissimilar welded samples revealed a shear ductile fracture. The second phase of the study focused on investigating the impact of filler wire and process parameters on the mechanical properties and microstructural characterization of dissimilar tungsten inert gas (TIG) and TIG combined with friction stir processing (TIG+FSP) welded joints. TIG welding techniques are most commonly utilized in joining aluminum alloys. However, TIG welding can result in defects such as micro-cracks, coarse grain structures, and porosity. To remove these defects, the FSP method is applied to an already welded joint by TIG welding to enhance the weld quality by optimizing the processing parameters of FSP. The evaluation of the welded joint demonstrated improved Vickers hardness, yield strength (YS), and ultimate tensile strength (UTS). Microstructural features of the dissimilar AA7075/AA6061 welded joints was examined by optical microscopy (OM), FESEM and EBSD scanned images such as grain orientation spread map, image quality with grain boundary distribution, misorientation angle distribution, and kernel average misorientation. Furthermore, the confirmation of precipitation particles and the elemental composition of aluminum alloys were verified through FE-SEM with EDS analysis. transmission electron microscopy was employed to investigate the distribution of precipitation particles. To analyze the mechanical properties of TIG and TIG+FSP welded joints, the tensile strength of TIG welded joints with fillers ER4043 and ER5356 was measured at 162 MPa and 170 MPa, respectively. After friction stir processing on the TIG welded joint, the TIG+FSP welded joints exhibited maximum tensile strength (252 MPa), and microhardness (128 HV) with filler ER4043. Similarly, the TIG+FSP welded joints achieved maximum tensile strength (292 MPa), and microhardness (130 HV) with filler ER5356. The third phase is dedicated to investigating the effect of various process parameters on the microstructure evolution and mechanical properties of dissimilar welded joints through metal inert gas welding (MIG) and MIG combined with friction stir processing (MIG+FSP). To remove the defects that occurred during MIG welding like micro-crack, coarse grain structure, and porosity, the FSP method is applied to an already joined through MIG welding method The evaluation of the welded joint demonstrated improved Vickers hardness, yield strength (YS), and ultimate tensile strength (UTS). Microstructural features of the dissimilar AA7075/AA6061 welded joints was examined by optical microscopy (OM), FESEM and EBSD scanned images such as grain orientation spread map, image quality with grain boundary distribution, misorientation angle distribution, and kernel average misorientation. Furthermore, the confirmation of precipitation particles and the elemental composition of aluminum alloys were verified through FE-SEM with EDS analysis. transmission electron microscopy was employed to investigate the distribution of precipitation particles. It was observed that MIG+FSP joints exhibit fine and equiaxed grain compared to MIG welded joints. In this study, tensile strength of MIG welded joints was observed at 151 MPa. After friction stir processing on MIG welded joint, the maximum tensile strength (228 MPa), microhardness (126 HV) was observed at tool rotational speed of 930 rpm, traverse speed of 40 mm/min with tilt angle 1°.