A novel FRICTION STIR WELDING method called projection FRICTION STIR SPOT WELDING (PFSSW) was introduced to produce safe and reliable welds by using a pinless tool and a specially-designed projection on the surface of a backing anvil. This projection along with the tool rotation speed plays an important role in having a reliable joint with excellent mechanical properties and good surface appearance. This WELDING technique can be widely developed in oil and gas, as well as in automotive, aerospace, and transportation, industry. The effect of tool rotation speed (1000, 1600, 2000 rpm) on the hardness, microstructure, and mechanical properties of 2024 aluminum alloy sheets was investigated. The surface appearance of the WELDING zone showed that the keyhole was not formed, and the appearance of the weld was almost smooth. Fracture surfaces of the failed specimens present the interfacial fracture at the tool rotation speed of 1000 rpm and circumferential fracture at tool rotation speed of 1600 and 2000 rpm.

Two main problems exist with FRICTION STIR SPOT welded joints; remaining of a keyhole after WELDING and low strength of joints. In this paper, a novel method is proposed to address both problems in a simple and cost-effective way. This process is named “ Reinforced FRICTION STIR SPOT WELDING” or “ RFSSW” which is based on recently introduced “ TFSSW” process. SiC powder was added to the FRICTION STIR SPOT joints of polyethylene sheets with a thickness of 3 mm. First, the sheets were welded using conventional FRICTION STIR SPOT WELDING tool with a cylindrical pin. Then, the keyhole was filled with SiC powder. In the second stage, for STIRring of SiC particles in the nugget and refilling the keyhole as well, a pinless tool was utilized. A homogenized distribution of reinforcing powder was obtained in the nugget. The effect of WELDING parameters including refilling tool shoulder diameter, refilling dwell time, and refilling tool rotational speed were evaluated in both TFSSW and RFSSW. In both processes, the refilling tool shoulder diameter was the most effective parameter. The strength was increased by 40% applying TFSSW and a further increase by 20% was obtained by reinforcing. Optimized parameter levels are refilling tool shoulder diameter of 24 mm, refilling tool rotational speed of 800 rpm, and refilling dwell time of 50s which result in shear strength of 1079 N.

FRICTION STIR SPOT WELDING of dissimilar aluminum and copper sheets was investigated in details using Taguchi approach in this study. Analysis of variance was conducted to identify the effective parameters and their contributions on the mechanical performance. The findings revealed that the outputs followed normal distribution. The results indicated that the rotational speed with the contribution of 87.7% was the most effective parameter on the maximum tensile force tolerated by the welded samples. Dwell time and penetration depth were in second and third ranks from effectiveness viewpoint with the contributions of 8.8 and 2.9%, respectively. The results revealed that the maximum tensile force was significantly improved with the rotational speed. The maximum tensile force was enhanced by almost 228% by increasing the rotational speed from 550 rpm to 1500 rpm. Increasing the dwell time from 10 to 20 s led to improve the maximum tensile force by 31%.The sequence of the parameters was as rotational speed, dwell time and penetration depth from standpoint of influence on the maximum force according to the results of signal to noise ratio analysis. The signal to noise ratio analysis showed that the rotational speed of 1500 rpm, the penetration depth of 2.85 mm, and the dwell time of 20 s were the optimum conditions.

With the successful application of the protrusion SPOT FRICTION STIR WELDING technology to 2024 aluminum alloy and low carbon steel plate with a thickness of 1 mm, this technique was expanded to the SPOT WELDING of 1 mm thick 6061 aluminum alloy in this study. The effects of the tool dwell time (6, 12, 18 s) and plunging depth (0. 1, 0. 14 and 0. 18 mm) were investigated on the microstructure and mechanical properties of samples. Surface appearance of the WELDING zone showed that the keyhole was not formed and the appearance of weld was relatively smooth. Microstructure and mechanical results indicated that the WELDING zone with uniform and refine structure due to dynamic recrystallization presents higher hardness and strength than base metal while can be affected by dwell time. Increasing the plunging depth increases the tensile strength and also decreases the thickness of the upper sheet. Fracture surfaces of the failed specimens present the shear fracture and the interfacial fracture.

The FRICTION STIR SPOT WELDING has shown great potential for joining low-ductility aluminum to high-strength steels. In the last decade, wide researches were done to achieve high strength and tool life enhancement in FRICTION STIR SPOT WELDING. The electroplastic effect, with its environmentally friendly nature and high efficiency, has resulted in a reduction of flow stress and tool wear, improvement of plasticity and material flow for various processes. On the other hand, adding nanoparticles to the FRICTION STIR SPOT welded area joint increased the tool wear despite the improved strength. In this paper, the joint strength and spindle output power are investigated in electroplastic FRICTION STIR SPOT WELDING process for joining of AA6061-T6 with 1 mm thickness to DP590 steel sheet of 1.5 mm. A 2K design was used for statistical analysis considering four parameters of rotational velocity (1000, 2000 rpm), dwell time (2, 4s), electrical current (250, 500A), and adding SiC reinforcing nanoparticles. A quantitative study of the current density was performed by the finite element code with thermal-electric coupling. Results showed that electroplastic effect had a compatible impact with nanoparticles on strength improvement by accelerating the occurrence of dynamic recovery and recrystallization, and neutralized the negative effect of nanoparticles on tool life since created joints with failure loads above 7 kN.

Cast AZ91 alloy is one of the most used magnesium alloys, which is sensitive to liquation in the fusion weld method, s and FRICTION STIR SPOT WELDING due to low eutectic temperature and the presence of the intermetallic compound in eutectic temperature. In this research, the liquation behavior of AZ91 alloy during FRICTION STIR SPOT WELDING was investigated. The process was carried out at two rotation speeds of 1000 and 2500 rpm and a dwell time of 1 second on the plate of AZ91 with a 10 mm thickness. Microstructural characterization was carried out using optical and scanning electron microscopes. The results showed that at low rotation speed (1000 rpm), mechanical grinding redistribution and dissolution of-Mg17Al12 precipitates occurred. While at high rotation speed (2500 rpm), the predominant phenomenon was liquation. In this condition, the liquation initiated around the phase, and then the liquid re-solidified, leading to the typical eutectic structure instead of initial precipitates. Moreover, the liquation intensified by approaching the STIRred zone. Also, the presence of liquid film along grain boundaries resulted in decreased grain boundary strength and liquation cracking.

In the present paper, two-stage refilled FRICTION STIR SPOT WELDING is used to join mild steel sheets with a thickness of 2 mm. Meanwhile, alumina and titanium oxide nanopowders are also introduced to the nugget to achieve superior mechanical properties. Two non-consumable tools are used with and without pins to weld and refill the keyhole, respectively. Before refilling, the keyhole is filled with nanopowders which are then distributed by the pinless refilling tool. Three parameters are investigated, the rotational speed of the WELDING tool and the rotational speed and dwell time of the refilling tool. The tensile test is used to evaluate the strength of the joints. The microhardness is measured in the WELDING zone to evaluate the powder distribution. The results suggest an increase in the joint strength by 42% and 18% with alumina and TiO2 as reinforcement, respectively. With a 31. 94% contribution, the refilling tool rotational speed is the most effective input parameter affecting the joint strength. Considering the microstructure analysis and the microhardness test, the material flow pattern is mainly downward which results in the accumulation of the reinforcing powder in the lower sheet especially when a lower refilling tool rotational speed is used.

In this study, effects of process parameters of FRICTION STIR SPOT WELDING were investigated on Al2024T3 which has poor weldability. Several SPOT welds were performed by the FSSW process on the 2mm aluminium sheets. The effects of main process parameters such as tool Rotational Speed (RS), Normal Plunge Depth (NPD), and Dwell Tim e (DT) on the joint strength were investigated. By increasing the tool rotational speed, the joint strength increased, consequently. The mean failure load improved 52% when the tool rotational speed increased from 800 rpm to 1120 rpm, whereas increasing th e rotational speed from 1120 to 1600 did not have significant effect on the failure load. The results showed, increasing the normal plunge depth from 1. 5 to 1. 75 millimetre led to an increase in the failure load of SPOT welds by about 1. 62 times. Also, the 3 seconds dwell time showed higher failure load compared to 2 and 5 seconds dwell time.

The present study investigates the effect of rotational speed and dwell time on the strength of Al-5083/St-12 FRICTION STIR SPOT joints. The welds were made using a newly developed FSSW process in which the tool pin did not penetrate into the lower steel sheet. Rotational speeds of 900 and 1100 rpm were applied in association with the dwell times in the range of 5-15 s. Results showed that the strength was first improved and then declined with increasing the dwell time. Furthermore, stronger joints were achieved at 900 rpm compared to the rotational speed of 1100 rpm.