Abstract
Refill friction stir spot welding (RFSSW) is a novel solid-state joining technology being developed primary as a replacement of mass-adding mechanical fastening processes. The appropriate choice of tool material and geometry can influence the tool life-expectancy and may impact the mechanical properties of the joint. Despite the extensive research, there is a lack of publicly available experimental data regarding the effect of different tool designs and tool materials on the microstructural and mechanical properties of RFSSW lapped joints.The aim of this research is to further understand the relationship between RFSSW tool design and material with the mechanical performance and microstructural features of the joint produced. The main objectives are: 1) to quantify the influence of process parameters on the mechanical performance; 2) to select and benchmark the most promising tool material candidates for RFSSW based on industrially relevant criteria (e.g. process repeatability, tool life, joint mechanical properties); 3) to characterise the effect of different tool materials and geometries on the mechanical performance of the weld; 4) to compare the material flow and microstructural features of welds produced with tools with different geometries.
The key findings are summarised as follows. Firstly, the influence of RFSSW process parameters on the weld mechanical strength was determined for various aluminium alloys using a standard RFSSW tool. This allowed determining the most effective process parameter combination for each alloy. Plunge depth had the greatest impact on mechanical strength, while welding conditions promoting lower heat input tended to improve the shear strength. Furthermore, residual stress measurements on RFSSW specimens with multiple spot-weld were performed using the contour method. The results suggest that the increasing number of spot-welds translates into lower peak stress values. RFSSW in the presence of aerospace grade sealant was also investigated. The results showed an increase in mechanical performance, which was attributed to the adhesiveness of the sealant at preventing secondary bending of the specimen.
Secondly, the tool material benchmarking investigation suggests that surface treated specimens produced intermetallic compounds at the internal surface of the tool specimen per plunge, which lead to persistent clogging. M42 high speed steel showed to be the most suitable tool material for RFSSW with the best cost-benefit ratio.
Finally, the impact of different tool materials and geometries in the microstructure and mechanical performance of the weld was determined. The main findings from this investigation suggest that the left-handed features present on the surface of the plunging component enhanced the stirring action, accentuating downward material flow and enlarging the welded area. Welds made with a conventional design RFSSW tool and made from M42 high speed steel, exhibited decreased mechanical performance. This can be attributed to the use of a tool material with a lower thermal conductivity coefficient, producing a weld with a higher peak temperature and promoting softening of the base material.
This thesis contributed to the further development of RFSSW, establishing both a theoretical and technical basis for new researchers or industrial users searching for alternative single point joining methods. Technological strong points and limitations are discussed, aiming to identify the most promising fields of application.
| Date of Award | Mar 2021 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | National Structural Integrity Research Centre , The Welding Institute (TWI) & Coventry University |
| Supervisor | Xiang Zhang (Supervisor) & Bilal Ahmad (Supervisor) |