Probeless friction stir welding of dissimilar metal welds

In recent years, the number of lightweight parts made of high-strength aluminium alloys and advanced high strength steels (AHHS) has increased fivefold to further enhance engine efficiency and reduce CO2 emissions in automotive sector. Furthermore, lightweight assemblies increasingly utilize dissimilar structures combining both AHSS and high-strength aluminium alloys. Widespread in automotive industry fusion method of resistance spot welding (RSW) triggers notable alterations in resolidified seam microstructure from its original state. Moreover, high thermal conductivity of aluminum alloys lead to rapid electrode wear, thereby enhance the formation of pores, voids, solidification and liquation cracking. Joining of dissimilar materials suffers even more challenges due to additional disparities in their electrical and thermal properties. At last, the formation of brittle intermetallic compounds (IMCs) detrimentally effects on the final mechanical properties. Temperature and holding time are two critical parameters that influence the formation of above-mentioned defects and formation of IMCs. Thus, reducing the amount of heat input and time of the welding process become an inevitable demand for both similar and dissimilar materials. Being a solid-state welding method, Friction Stir Spot Welding (FSSW) involves joining between materials by plunging a non-consumable rotating tool involving a shoulder and a probe into the workpiece with further material heating and stirring. However, the crucial drawback of FSSW is the formation of a permanent central keyhole due to presence of probe, which significantly reduces the effective cross-sectional area. Among all variations being invented to eliminate the keyhole, Probeless Friction Stir Spot Welding (P-FSSW) gains significant attention due to its process simplicity, high efficiently, and negligible tool wear. The weld formation is primarily influenced by the combined effects of vertical compression and horizontal material, which are exclusively driven by the shoulder. Thus, the contact between shoulder and workpiece surface plays a vital role in the heating and following material plasticization. The featureless flat shoulder of P-FSSW has been primarily served as a reference point, while it demonstrated promising results in forming strong joints among different featured shoulders. Notably, it possesses minimal tool wear and contamination from plasticized material compared to other shoulder designs. Its main drawback is solely the requirement for a longer dwell time to attain the same level of strength as those achieved with featured shoulders. The objective of this study is to investigate the feasibility of creating robust spot joints between dissimilar aluminum-aluminum and aluminum-steel materials using Probeless Friction Stir Spot Welding (P-FSSW) assisted by flat featureless shoulder. The study systematically evaluates heat generation, material flow, microstructural evolution and mechanical properties, aiming to optimize parameters and expand the scope of this technology. Four sets of experiments on P-FSSW with different combination of dissimilar aluminum-aluminum and aluminum-steel material couples were performed at Hybrid Welding Laboratory at DMMM Polytechnic University of Bari. Optical microscopy, microhardness tests, tensile test, and scanning electron microscopy (SEM) equipped with the electron backscatter diffraction (EBSD) technique were employed to provide fundamental insight into the microstructural development. In regard to provide a better understanding of P-FSSW, the experimental findings were coupled with numerical results of thermal fields and material flow. Finite element model was developed in assist with Simufact Forming® 2021 software.