Characterisation of mechanical properties of friction stir processed AA5083/AA6082 dissimilar joints reinforced with coal

Abstract

The aerospace, automotive, and transportation industries have increasingly recognised the importance of lightweight planes and automobiles. The greater the mass of vehicles, the more power they require for motion and acceleration. Reducing their mass not only lowers fuel consumption but also curtails carbon emissions. Consequently, the adoption of lightweight materials like aluminium alloys has become paramount. Yet while aluminium exhibits certain excellent qualities, it falls short in applications where lightweight materials with great strength are required. To address this limitation, modifying the mechanical properties of aluminium, particularly its surface microstructure, can enhance its performance. One effective approach is to reinforce aluminium with particles, resulting in surface composites. The properties of these composites are influenced by various factors, including the method used to alter their mechanical properties. Friction stir processing (FSP) is among the methods used to produce surface composites. FSP aims to modify the local microstructure of workpieces as opposed to welding them together. While reports on the friction stir processing of particle-reinforced composites abound in the literature, the use of coal as a reinforced composite remains unexplored. This study focused on characterising the influence of friction stir processing on dissimilar joints (AA5083/AA6082) when reinforced with coal powder. The dissimilar plates were first joined using the friction stir welding (FSW) technique. Subsequently, the friction stir welded (FSWed) joints were subjected to FSP with and without the addition of reinforcing coal. The impact of employing coal reinforcement on the dissimilar joints was assessed through various tests, including microstructural analysis, tensile tests, bending tests, micro-hardness and fractographic analysis. The following comparative analysis of the results was observed. The test results of the microstructural analysis showed that the mean grain size obtained from friction stir welding (FSW) was measured at 19.7 μm. When friction stir processing (FSP) was employed, the average grain size for joints decreased dramatically to 9.63 μm. The FSP reinforced with Coal technique (FSP+Coal), which involved partial reinforcement with coal powder, achieved a remarkable grain refinement of 8.75 μm, surpassing the conventional FSP method. The results clearly indicate that FSP+Coal outperformed both FSW and conventional FSP joints in terms of grain refinement. Additionally, the microstructural analysis revealed that an increasing number of passes led to smaller grain sizes in the processed zone, resulting in a more uniform distribution of grains. In flexural tests of face specimens, FSW face specimens failed at a maximum strain of 12.7% and a flexural stress of 535 MPa. In comparison, FSP face specimens displayed a lower maximum strain of 10.81% but higher flexural stress of 545.6 MPa. Conversely, the FSP+Coal face specimens produced a substantially lower performance, failing at a maximum strain of only 3% and a flexural stress of 222 MPa. These results indicate that the inclusion of coal in the FSP process significantly affected the mechanical properties of the specimens, leading to lower maximum strain and flexural stress values than the standard FSW and FSP specimens. Some of the processed joints in the FSP+Coal group showed deflection and cracks, particularly in the AA6082 TMAZ (thermo-mechanically affected zone) side regions, while others remained free from cracks there but exhibited cracks at the AA5083 weld end TMAZ. The tensile properties of the joints were then evaluated for FSW, FSP, and FSP+Coal. For FSWed joints, the maximum ultimate tensile strength (UTS) achieved was 145.9 MPa at a tensile strain rate of 9.43%, with the minimum UTS recorded at 93.43 MPa at a tensile strain rate of 7.02%. In the FSPed joints, the maximum UTS obtained was 170.9 MPa at a tensile strain rate of 9.13%, and the minimum UTS reached 126 MPa at a tensile strain rate of 7.38%. For the FSP+Coal joints, the maximum UTS was 142 MPa at a tensile strain rate of 9.28%, while the minimum UTS was 104.06 MPa at a tensile rate of 4.63%. It is evident that the introduction of coal particle reinforcement resulted in a reduction in UTS compared to FSWed and FSPed joints, indicating a trade-off between the presence of coal particles and tensile properties. Yet despite the reduction in UTS, the FSP+Coal method had positive effects on the properties of the AA6082-T651 material. Both FSWed and FSPed samples experienced fracture at the AA5083-H111 side, signifying different failure characteristics for the welding methods used. Regarding average hardness, the FSP+Coal joints exhibited a hardness of 70.74 HV at the nugget zone. In comparison, FSP resulted in a hardness of 67.72 HV at the nugget zone, and FSW displayed a hardness of 64.64 HV at the nugget zone. The regions near the tool pin and tool shoulder positions on the AA6082-T651 side exhibited slightly lower hardness values than other positions. However, the nugget zone demonstrated a significant increase in hardness values along the entire length of the welded joint, surpassing the hardness of the AA6082-T651 HAZ (heat -affected zone) side. It is important to consider that the AA6082 alloy is a precipitate-hardened alloy, and temperatures exceeding 200°C can significantly affect its particles, particularly on the HAZ side, compared to the AA6082-T651 base material. The data obtained from this research clearly demonstrates that reinforcement with coal powder particles in friction stir processing has a significant impact on the mechanical properties of the joints. This study has embraced the spirit of scientific exploration and innovation in advancing the understanding of dissimilar aluminium alloy joints. It paves the way for future developments in the field, including the promotion of a more sustainable and efficient approach to materials joining. Moreover, the findings have potential applications in the motor industry, including crumple zone and brake disc design.