UFSW tool pin profile effects on properties of aluminium-steel joint

Highlights

• Effects of dissimilar underwater FSW tool pin profiles were studied on aluminum-steel joints.

• Relation between FSW tool pin profile and mechanical properties of AA5182 alloy and St-12 steel dissimilar joint evaluated.

• Thermal history, microstructure analysis, Intermetallic compounds, and hardness of joint lines were assessed.

Abstract

Effects of underwater friction stir welding tool pin profiles were studied on mechanical properties of AA5182 alloy and St-12 steel dissimilar joint. For this purpose, frustum, triangle, cubic, and hexagonal pin profiles were selected. Thermal history, microstructure analysis, tensile strength, and hardness of joint lines were assessed. The obtained results indicated that due to the high cooling rate of water environment conditions, the presence of edges in the tool pin shape (triangle, cubic, and hexagonal) improves internal material flow. Also, edged pin profiles increase the generation of frictional heat, the thickness of Intermetallic Compounds (IMC), and mechanical interlocking at base materials' interface. The hexagonal pin profile had the most significant favourable influence on the aluminium-steel joint properties among selected tool profiles. The chemical composition of IMCs at the materials interface was FeAl₃ and Fe₂Al₅, and the thickest IMC was formed near 6.5 μm. According to the results, the joint's highest tensile strength was ~69% of aluminium alloy strength, which was obtained with a hexagonal pin profile.

Introduction

Hybrid structures are great opportunities to industries for production low weight constructions with tailored dissimilar material properties and high efficiency [[1], [2], [3], [4], [5], [6]]. Manufacturing and repair of dissimilar structures at water environments have nonetheless remained challenging, mostly with respect to welding and joining technologies [[7], [8], [9], [10]]. Herein, low-energy, mutable and simple welding methods with advanced traits applicable under different environments are required [[11], [12], [13], [14], [15]]. Friction stir welding (FSW) is relatively new process for the production of hybrid structures. Among various dissimilar joints, FSW of aluminium (Al)-steel (St) structures is very challenging owing to the sensitivity of mechanical properties affected by intermetallic compounds (IMC) at base metals interface [[16], [17], [18]]. Many researches have been performed on FSW of Al-St joints, aiming to optimize of IMC chemical composition and thickness formation through controlling the FSW parameters (at air cooling condition) [19]. Underwater (submerged) FSW is a newly developed process for the fabrication of Al/steel structures in marine environments for control of cooling rate and IMC as a pre-processing condition [20]. Also a few researches show the water environment significantly affects microstructural features and mechanical strength that presents below:

Zhang and Liu demonstrated with optimum underwater friction stir welding (UFSW) parameter, the AA2219 aluminium alloy tensile strength reached 80% of base material and the hardness of stir zone (SZ) has also improved [21,22]. Sabari et al. [23,24] indicated by optimizing of the UFSW parameters, the size of the thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ) could be minimized in AA2519-T87 aluminium joint. Heirani et al. [25] studied UFSW of Al5083 alloy joint, and showed at UFSW condition, the final joint has higher tensile strength and hardness compared with air-processed FSW joints. Liang et al. [26] proved that strength of UFSWed AA7055 aluminium affected by the change of water temperature. Using high UFSW tool rotational speeds at warm water, AA7055 aluminium alloy joint strength could be improved significantly compared with cold and room temperature water [27,28]. Papahn et al. [29] UFSWed AA7075-T6 alloy sheets and showed that the maximum heat in joint line decreased (near 40%) compare with conventional FSW. They reported UFSW joints were more brittle with higher tensile strength due to the higher cooling rates of water compare air cooling condition. Tan et al. [30] showed that the size of recrystallized grains and the amount of second-phase particles in the SZ was reduced with decreasing the temperature water during UFSW of AA3003 aluminium alloy. Due to available literature, seems UFSW of dissimilar materials are more challenging than similar joints. Zhao et al. [31,32] reported UFSWed AA6013/AZ31 joints strength (~152 MPa) improves 21 MPa compare air-processed FSW specimens with a strength of 131 MPa. They also reported that the formation of Mg₂Al₃ and Mg₁₇Al₁₂ at the interface of UFSW joint and detected on the fracture surface investigation. Zhang et al. [33] reported the Al–Cu intermetallic compounds thickness could be reduced under UFSW condition. Mahto et al. [34] shown the thickness of IMC at the interface of AA6061-T6 aluminium alloy and AISI 304 steel FSWed joint decreased from 9.5 μm to 0.5 μm in UFSWed sample. The strength of the UFSWed joint revealed improvement due to the formation of finer grains structure in the weld zone compared to FSW condition. Aghajani derazkola and khodabakhshi [18] investigated UFSW of Al–Mg aluminium alloy and A441 AISI steel under various water mediums temperatures. They reported maximum tensile strength (~310 MPa) and elongation (~13%) produced at the room temperature cooling medium.

Limited literatures have been considered effects of UFSW pin profile parameters on Al/steel joints. The aim of this article is to evaluate the mechanical properties and microstructural features of UFSW dissimilar joints between AA5182 aluminium alloy and St-12 steel. The obtained results may pave a way to implement this process for the fabrication of lightweight aluminium/steel hybrid structures.