Diffusion Bonding of Al-Mg-Si Alloy and 301L Stainless Steel by Friction Stir Lap Welding Using a Zn Interlayer

Abstract

Friction stir lap welding (FSLW) is expected to join the hybrid structure of aluminum alloy and steel. In this study, the Al-Mg-Si aluminum alloy and 301L stainless steel were diffusion bonded by FSLDW with the addition of 0.1 mm thick pure Zn interlayer, when the tool pin did not penetrate the upper aluminum sheet. The characteristics of lap interface and mechanical properties of the joint were analyzed. Under the addition of Zn interlayer, the diffusion layer structure at lap interface changed from continuous to uneven and segmented. The components of the diffusion layer were more complex, including Fe-Al intermetallic compounds (IMCs), Fe-Zn IMCs and Al-Zn eutectic. The largely changed composition and thickness of uneven and segmented diffusion layer at the lap interface played a significant role in the joint strength. The tensile shear load of Zn-added joint was 6.26 kN, increasing by 41.3% than that of Zn-not-added joint. These two joints exhibited interfacial shear fracture, while the Zn interlayer enhanced the strength of diffusion bonding by extending the propagation path of cracks.

Improved Interface Morphology and Failure Load of Ultrasonic-Assisted Friction Stir Lap Welding Joint of 2024 Aluminum Alloy to 304 Stainless Steel

“…This phenomenon results from the welding tool not plunging into the lower steel plate and no mechanical interlocking formed at the Al/steel lap interface in Ref. [33]. Combined with the results in Refs.…”

“…The atom interdiffusion between the aluminum alloy and the steel is a prerequisite for metallurgical bonding, so the atom interdiffusion near the typical Al/steel interface regions was observed using the EDS method and the results are displayed in Figure 5. Similar to the reported literature [16,18,33], this study analyzes the atom diffusion of main elements including Al and Fe. Thereinto, the regions D1, D2, and D3 scanned by the EDS method are located outside the stir zone (SZ) and the distances between the scan line and the hook structure are nearly the same, while the regions B1, B2, and B3 scanned by the EDS method are located in the middle of concavedown lap interface in the SZ.…”

“…Geng et al [44] reported that an IMC layer thickness lower than 1.5 µm at the Al/steel interface was beneficial to heightening the strength of lap joint. According the discussions reported by Dong et al [33] and Liu et al [30], the IMC layer thickness lower than 2 µm is positive on the loading capacity of dissimilar aluminum and steel alloys FSLW joint. In this study, the IMC thickness is necessarily smaller than 1.5 µm (Figures 5-7) since the largest distance of atom interdiffusion in Figures 5 and 7 is only 1.4 µm, which is the suitable value from the viewpoint for heightening the FSLW joint strength.…”

“…As is known, for the butt joint by FSW, the steel plate is always put at the advancing side (AS) based on the difference in the thermal flow behaviors of these two materials [24][25][26]. For the lap joint, the aluminum alloy plate is often served as the upper plate in order to greatly decrease and even avoid the tool wear, and can be placed at the AS [30,33] or the retreating side (RS) [16,19]. In this study, the upper aluminum plate was placed at the AS, as displayed in Figure 1a.…”

“…In this study, the failure loads of FSLW joints under different welding conditions were tested, and the results are presented in Figure 10a. In order to better explain the advantages of ultrasound addition during the FSLW process of aluminum to steel, the failure strengths of FSLW joints in this study and the reported literature [9,16,33,37,44] are compared, as displayed in Figure 10b. Therein, the failure strength in Figure 10b means the failure strength is obtained by dividing the maximum failure load by the width of test specimen and the thickness of the upper plate.…”