Refill friction stir spot welding (RFSSW) is an emerging technology, capable of joining thin sheets of aluminum alloys. The present thesis comprises two studies which were conducted to address two challenges faced by RFSSW: the long cycle time traditionally associated with welding and the poor life of existing RFFSW tools. In the first study, welds were made in AA5052-H36, at various cycle times and with various process parameters. It was shown that RPM, cycle time, and material thickness, all have an effect on the machine response. Decreasing RPM or weld duration leads to increased force and torque response during welding. Welds with cycle times below one second were successfully made without severely impacting joint quality, suggesting that prior work may have been limited by machine capabilities rather than by phenomena inherent to the process. On average, the sub-one second welds caused a peak probe force of 9.81 kN, a plunge torque of 26.3 N*m, and showed average lap-shear strengths of 7.0kN; compared to a peak probe force of 5.14 kN, a plunge torque of 17.3 N*m, and lap-shear strength of 6.89kN for a more traditional four-second welding condition. In the second study, the life of a steel toolset was quantified as consecutive welds were made in AA5052-H36 until the toolset seized from material accumulation/growth. At a one-second welding condition, the toolset was only capable of producing 53 welds before seizure. At a two-second welding condition, the toolset was only capable of producing 48 welds. In subsequent temperature experiments, thermocouples were embedded into welding coupons at various locations near weld center, allowing novel temperature data to be collected for welds with varying cycle times and parameters. The collected temperature data shows that as cycle time increases, so does weld temperature. At weld center, temperatures in excess of 500°C were observed in welds with 4 second durations. At these temperatures, Fe-Al intermetallic growth is anticipated as a mechanism limiting the tool life observed. The results suggest that steel is not an appropriate choice for RFSSW tools, and future evaluation of other materials is merited.

The majority of Refill Friction Stir Spot Welding (RFSSW) is used to join two materials together oriented in a lap joint configuration. In this study, RFSSW was investigated and tested using an unconventional configuration setup, a hole/plug insertion approach. RFSSW was tested as a means of repairing a cracked rivet hole due to excessive use conditions. This was done by inserting a plug into a hole and using the RFSSW process to bond the plug to the base material. Machine and tool limits were investigated to determine if a refilled plug repair was possible and if complete mixing between plug/hole interface was attainable. Plug/hole homogenization was assessed via metallographic polishing of weld cross sections. Properties of the repaired aluminum alloy including both dynamic and and quasi-static tensile tests were also evaluated.

Historically, automotive body panels have been made of steel and joined by a process called resistance spot welding (RSW). However, in efforts to reduce vehicle weight to improve the energy efficiency of the vehicle, automotive manufactures have begun substituting aluminum in place of steel. While aluminum can be joined with RSW, a myriad of challenges arise from doing so. These challenges result in less consistent weld quality and accelerated electrode wear. Refill friction stir spot welding (RFSSW) is an emerging joining technology that could replace RSW as it is believed to be capable of creating superior joints in thin sheet aluminum. This research's goal is to compare RFSSW and RSW for joining aluminum automotive body panels. To accomplish this goal two studies were conducted and reported on in this thesis. The first focused on evaluating the manufacturing performance of RFSSW and RSW while the second focused on comparing the microstructure and mechanical performance of RFSSW and RSW joints. To improve the relevance of the study, a Toyota automated welding cell was used as a case study. The cell utilizes AA6061-T4 in 8 unique stack-ups to create door frames. This cell served as the base for the manufacturing performance comparison while also providing the three stack-ups used to compare microstructure and mechanical performance. The study compares manufacturing performance utilized a digital twin to compare how each technology would interact within the manufacturing cell. Parameters such as joining time and maintenance time were considered while overall output of the manufacturing cell was recorded. The results showed that RFSSW and RSW could produce the same number of parts in the given manufacturing cell. However, as modifications were made to the cell to increase output RFSSW proved to be capable of greater output due to its longer tool life. Concluding that RFSSW is a viable option from a manufacturing performance view. The second study conducted a comparison of the microstructure and mechanical properties of RFSSW and RSW. This study found that each technology created unique surface topographies and grain structures. Mechanical performance testing found that depending on the stack-up RFSSW joints were between 16% and 73% stronger than RSW joints in tensile loading conditions. RFSSW also showed improved fatigue life, in one test surviving 2600% more cycles. Concluding that RFSSW joints have superior mechanical performance over RSW joints.

As light-weighting becomes a priority for the automotive industry, refill friction stir spot welding emerges with enormous potential to supplement or replace conventional spot joining processes. This thesis addresses the limitations of current tooling options by examining materials beyond steel for use in RFSSW. Contained herein is an analysis of weld quality as a function of tool material, a production evaluation of RFSSW using various tool materials, and an assessment of long-term performance of a tungsten carbide tool. Over the course of this research, tungsten carbide emerged as a viable candidate for long-lasting RFSSW tooling.

Refill Friction Stir Spot Welding (RFSSW) is a new solid-state joining technology, which is suitable for joining similar and dissimilar overlap sheets connections, particularly in aluminium and magnesium alloys. This welding method is expected to have wide applications in joining of body parts in the automotive industry. In the present study, RFSSW has been used to join 1.0 mm gauge sheets of two material combinations: similar AA6111-T4 automotive aluminium alloy joints and a dissimilar aluminium AA6111-T4 to magnesium AZ31-H24 alloy combinations. The performance of the joints was investigated in terms of the effect of the welding parameters (including tool rotation rate, sleeve plunge depth, and welding time etc.) to improve current understanding and allow optimisation of the process for short welding-cycles when joining similar and dissimilar light alloys. The results of the investigations on similar AA6111 welds showed the ability to use a wide window of process parameters that resulted in joints with a successfully refilled keyhole and flat weld surface, even when using a welding time as short as 0.5 s. The joints in the as-welded condition showed strengths as high as 4.2 kN, when using welding parameters of 1500 rpm, 1.0 mm with a range of welding times from 0.55 to 2.0 s. All joints showed a nugget pull-out failure mode when using a sleeve plunge depth of 0.8 mm or more, as a result of increasing the joint area. The strength of the joints further improved and reached peak loads of 5.15 and 6.43 kN after natural and artificial ageing, respectively, for welds produced using optimised welding parameters of a 2500 rpm tool rotation rate, a 1.5 s welding time and a 1.0 mm plunge. This improvement in strength resulted from the improvement in the local mechanical properties in the HAZ and other regions, which results from a minimal HAZ due to the rapid weld cycle and the re-precipitation of GPZs and clustering on natural ageing, or β on artificial ageing. A modification to the RFSSW process was developed in this project to solve the problems faced when dissimilar welding Mg to Al. This modified process involved adding a final brief pin plunge stage to consolidate refill defects and it was successful in producing nearly defect-free joints with improved mechanical properties, using a wide range of the process parameters. The average peak load of the joints increased with increasing tool rotation rate, to reach a maximum value at 2500 rpm due to eliminating the weld defects by increasing the material plasticity. However, increasing the tool rotation rate further to 2800 rpm led to a decrease in the average peak failure load due to eutectic melting at the weld interface. The optimum welding condition was thus found to be: 2500 rpm, 1.0 s, and 1.0 mm, which gave an average peak failure load of 2.4 kN and average fracture energy of 1.3 kN.mm. These values represent an improvement of about 10 % and 27 %, respectively, compared to welds produced with the conventional RFSSW process, and about 112 % and 78 % of the Mg-Mg similar joints produced using the same welding conditions. A FE model developed in this project was successful in increasing understanding of the behaviour of the RFSSW joints when subjected to lap tensile-shear loading. The stress and strain distribution in the modelled samples showed that the highest concentration occurring in the region of the confluence of the SZ with the two sheets. With increasing extension, these regions of highest stress and strain propagated to the outer surfaces of the two sheets and then annularly around the weld nugget. This annular ring of high strain concentration agreed well with the failure path and results in the full plug pull-out fracture mode shown by the experimentally tested samples. The predicted force-extension curves showed high agreement with the experimental results, especially when including the effect of the hook defect and correction of compliance in the experimental results.