In this study, friction stir spot welding (FSSW) is applied to join the TiAl6V4 titanium alloy with 1.5 mm thickness and then the effect of rotational speed and tool dwell time on microstructure and mechanical properties is investigated. In this regard, the speed of the tool rotation was considered as 800, 1000, and 1200 rpm, as well as the tool dwell time was set at 7 and 12s. Microstructural evaluation was carried out using optical microscopy (OM) and scanning electron microscopy (SEM). In addition, tensile-shear and hardness studies were performed to analyze mechanical properties. The obtained results from microstructural evaluation show that the welded joints consist of two regions, namely the SZ and the HAZ-regions. Additionally, microstructure of the SZ-region was identified in the form of α/β layer within the initial β-phase. The results of tensile/shear tests and micro-hardness test indicated that the joint strength and hardness are enhanced with increasing the rotational speed and dwell time. The tensile/shear strength is increased from 2.7 to 15 KN with increasing the rotational speed at constant dwell time of 7s, and also is increased from 7.3 to 17.25 KN with increasing the rotational speed at constant dwell time of 12s. The maximum tensile/shear strength was achieved for the welded joint with the dwell time of 12s and rotational speed of 1250 rpm. The hardness of SZ, HAZ regions and base metal are measured around 380 to 420, 340 to 380, and 300 to 340, respectively. Manuscript profile

Various parameters can affect shot peening such as the number of particles and distance between particles and the surface layer. In this computational study, these parameters' effects on the creation of residual stress and mechanical behavior of Ti-6Al-4V alloy were described. For this purpose, Molecular Dynamics (MD) method is applied in two main steps. First, the simulated titanium surface was equilibrated for 1 ns. Next, the shot peening process was done on the equilibrated surface by using the various numbers of particles and distance. MD simulation results indicated, that by increasing the number of particles from 1 to 5, the mechanical behavior of the titanium surface was improved, and residual stress and hardness of the surface increased and reached 452.02 MPa and 494.46 HV in model 1 (Lj potential between particle and titanium surface), respectively. Furthermore, the results indicated that decreasing the distance from 15 Å to 5 Å led to increasing compressive residual stress and hardness of titanium surface mechanical. Numerically, by decreasing the shot peening distance from 15 Å to 5 Å, residual stress and hardness of titanium surface layer increased and reached -419.63 MPa and 510.83 HV in model 1, respectively. Manuscript profile

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