Thermo-mechanical stability of severely plastic deformed aluminum joined by friction stir welding and graphite/Al۲O۳ hybrid powder: The effect of rotation speed to traverse speed ratio and number of welding passes

In this study, thermo-mechanical stability of two-pass constrained groove pressing (CGP) AA۱۰۵۰ sheets towards friction stir welding (FSW) employing hybrid powder (۵۰%vol. micrometric graphite powder+۵۰%vol. α-Al۲O۳ nanoparticles) was investigated by examining its microstructural evolutions and mechanical properties. FSW was carried out via different process variables in order to reach the highest ultimate mechanical properties of joints. The welding variables employed in this study were single-pass and multi-pass FSW, and different rotation speed to traverse speed ratios (ω/v) were. In order to appraise the powder effect on mechanical properties in the fabricated hybrid metal matrix composite (HMMC), some CGPed sheets were also welded with no powder.Besides optical microscopy and field emission scanning electron microscopy (FESEM) observations, Vickers microhardness and transverse tensile tests were conducted to examine mechanical properties of the weld zone. It was revealed that the effect of graphite powder as a solid lubricant was substantially influenced by the welding variables. More precisely, graphite powder contributed to the thermo-mechanical stability of CGPed aluminum and thus, enhanced mechanical properties. On the other hand, graphite powder can be responsible for mechanical properties drop due to deteriorating material flow. In addition, different strengthening mechanisms, including grain boundary Zener-pinning and particulate stimulated nucleation (PSN) mechanism, were provided and governed by both powders. However, increasing the ω/v ratio was a practical approach to obtain uniform powder distribution, and consequently, to attainultimate mechanical properties. Moreover, weld soundness was perceived to be achievable by increasing the number of FSW passes due to eliminating the cavities and improved material flow, reaching the optimum efficiency of ~ ۸۰% in three-pass FSW at ω/v=۲۵.

کلیدواژه ها:

Severe plastic deformation ، Friction stir welding ، Hybrid metal matrix composite ، Solid lubricant ، Thermo-mechanical stability ، Mechanical properties.