Correlative analysis between sampling direction and the mechanical properties of the friction stir welded AA5083/AA6082 dissimilar joints

Abstract

Welding is one of the most popular and established methods of joining metals. However, it can be difficult to weld aluminium (Al) and its alloys, or to create various joints between different types of aluminium, or between aluminium and other metals, by employing conventional welding techniques. Conventional techniques cause a variety of issues, including weak joints and changes in the mechanical characteristics of the materials. This makes it difficult for a number of cutting-edge and modern ideas to develop as a consequence. Friction stir welding (FSW), a method of solid-state material joining, provides a solution to these problems of weak joints and alterations of the mechanical properties by necessitating minimum heat to fuse materials. The FSW technique, in using a non-consumable tool, does not rely on additional welding consumables such as flux, filler metals, or post-welding treatment and can be applied on heat sensitive materials such as aluminium and its alloys. FSW and its influence on the mechanical properties of various materials forming comparable and dissimilar joints has been the subject of several studies. This investigation intended to assess the relationship between the mechanical characteristics of the friction stir-welded AA5083/AA6082 dissimilar joints and the sample direction. A modified vertical milling machine was employed to weld two aluminium alloys, AA5083 and AA6082, each with a 6 mm thickness. With the aid of high-pressure water technology (waterjet), samples were collected while taking note of the start, middle and end locations in both the traversing and retreating directions. To evaluate the joint's quality and characterisation, visual tests, metallographic tests (including macro-structural and microstructural analyses and fractography), as well as mechanical tests (including tensile testing, three-point flexural tests and micro-hardness tests) were all performed. The results of the traverse, longitudinal and parent materials proved comparable despite the defects identified in the joint. The macrostructure analysis of the traverse samples revealed all four major FS welded joint zones: the parent material zone (PM), heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and stir zone (SZ). An inter-material flow pattern was observed, an evident indication that the material in the joint had plasticised, albeit not sufficiently judging by the tunnel defect present. As a result, there were no onion rings in any of the samples from any of the locations which explain the underdeveloped stir zone. Micro-voids were also discovered in the samples, predominantly on the advancing side, right under the material bands. The longitudinal examination of the samples revealed material stacking, as well as a tunnel flaw that extended across the sample.

The microstructure of the traverse samples revealed that the materials were subjected to considerable stress and high temperatures, resulting in dynamic recrystallisation of the joint materials. AA6082 possessed minute grains that resembled shards of glass, but AA5083 had no discernible grains. The mean grain size of the traverse start sample was 10.328 μm; the mean grain size of the middle sample was 11.884 μm and the mean grain size of the traverse end sample was 7.618 μm. The grain was measured in the longitudinal start samples at 15.608 μm; the middle sample measured 19.881 μm; and in the end sample at 9.187 μm. There was no obvious association between the microstructure and the location of the samples. The stir zone was identified as the point of failure in each sample after tensile test analysis of the traversing samples. All samples showed a meandering tear, a slight necking, and a cup-cone fracture mode, a unique plastic deformation mode common in ductile materials. Samples taken at the start, middle, and end of the joint were recorded maximum ultimate tensile strength of 152.722 MPa, 130.694 MPa, and 122.278 MPa, respectively, at strains of 5.35%, 9.72% and 9.80%, respectively. At distances of 5, 11, and 10 mm from the bottom edge of the gripping end, the longitudinal tensile samples failed along the gauge. The longitudinal samples obtained from the joint's start location resulted in maximum ultimate tensile strength values of 137.417 MPa, 127.833 MPa, and 109.500 MPa, respectively, all at stresses of 13.8%, 8.3%, and 12.7%. The findings of the traverse and longitudinal samples indicated a weakening of the sample. The traverse samples that underwent face testing respectively measured 6.414 MPa, 47.513 MPa, and 78.575 MPa, indicating an increase in proportion to location while the longitudinal samples measured respective strengths of 218.05 MPa, 272.125 MPa, and 176.313 MPa. The traverse samples attained angles of 40°, 46°, and 150º, whilst the longitudinal samples reached an overall bending angle of 140º. The root tested traverse samples measured 196.438 MPa, 47.075 MPa, and 239.05 MPa, respectively. The maximum deflection angles for the traversal samples were 5, 10, and 20º. The longitudinal root tested samples from the SME locations had strength readings of 108.250 MPa, 198.888 MPa, and 196.438 MPa, respectively. The start, middle, and end samples of the longitudinal samples had bend angles of 140° and 135°, respectively. The traversal start sample's mean HV0.2 value was determined by micro-hardness assessment to be 79.70. The mean values for the samples' longitudinal halves, AA5083 and AA6082, were 100.59 HV0.2 and 104.84 HV0.2, respectively. After accounting for the mean micro-hardness values derived from both the parent samples and the welded samples, the joints had a greater micro-hardness than that calculated from the parent materials. While the AA5083 and AA6082 sections both registered 73.385 HV0.2, the traverse middle sample registered 75.152 HV0.2. When compared to the PMs results, the mean micro-hardness values of the samples revealed a drop. The mean micro-hardness of the traverse end sample was determined to be 78.79 HV0.2. The mean micro-hardness values were 79.555 and 76.641 HV0.2 for the longitudinal AA5083 and AA6082 sections, respectively. The hardness and grain sizes are correlated, according to the Orowan mechanism and the Hall-Petch. The start, middle, and end traverse samples' fractographic analysis revealed micro-voids and tunnel defects. The image at high magnification shows several dimples of various sizes. The samples also had cleavage facets and voids. The material's ductility is confirmed by the emergence of large dimples, which indicates that the material has gone through a suitably malleable course. The bigger dimples at the bottom were found to have bases of shattered particles, which was determined to be the reason for the joint's fragility. Surface cleavages and micro-voids on the longitudinal sample suggested ductile and brittle fractures. The picture showed portions with smooth surfaces and others with dimples of various sizes at extreme magnification. Similar to the traverse samples, the dimples contained particles embedded at the bottom of them. No trend was observed to ascertain that longitudinal or traverse samples performed better than parent materials over a range of locations. The study will advance the knowledge of the impact of friction stir welding on aluminium and its alloys and deepen the comprehension of those effects. It will also open new avenues for the investigation of cutting-edge solutions to contemporary issues in many sectors.