With the worsening global climate change, environmental pollution, and energy crisis, lightweight automobiles have become essential for the automotive industry to save energy and reduce emissions [1–3]. Aluminum alloys have numerous advantages, such as low density, high specific strength, corrosion resistance, and good thermal conductivity [4–6]. Moreover, aluminum alloy and steel composite structures are increasingly used in the body [7, 8]. However, these composite structures present a huge challenge to the existing body welding process while achieving a lightweight body [9, 10].
Owing to the large physical and chemical property differences between aluminum alloy and steel [11, 12], such as thermal expansion coefficient, thermal conductivity, and specific heat capacity, the joining of aluminum and steel has consistently become a research focus in dissimilar joining techniques [13]. Although such methods as fusion welding and brazing can realize the joining of aluminum and steel [14], internal stress, cracks, and pores in joints often lead to poor joint quality and mechanical properties [15, 16].
Friction stir welding (FSW) is a promising technique for joint dissimilar materials, such as aluminum and steel, which effectively eliminate existing problems in fusion welding and brazing techniques [17]. In 1993, Mazda Corporation of Japan developed a friction stir spot welding (FSSW) technology [18], and the first dissimilar material FSSWed joint of aluminum alloy and steel was obtained successfully in 2005 [19]. To date, FSW and FSSW have gradually developed into one of the most advanced joint technologies in aerospace, shipbuilding, automobile, and other industrial fields, which will replace traditional joint technologies, such as resistance spot welding and riveting [20–22]. Given that the traditional FSSW joint has keyhole and hook defects [23–25], the GKSS Research Center of Germany developed the refill friction stir spot welding process. Although keyhole defects can be eliminated, this process is complicated and requires high equipment rigidity and control accuracy [26–28].
Bakavos et al. achieved a high lap shear strength joint without keyhole defect of a thin AA6111-T4 sheet using the pinless friction stir spot welding (P-FSSW) process. P-FSSW mainly depends on the sufficient plastic flow of the material at the welding point under the friction and extrusion action of the pinless tool shoulder [29]. Similarly, the solid-state joining of a 38.1-mm thick AA6061 to 12.7-mm thick steel plate has been achieved using a new process called friction stir dovetailing (FSD) by Md. Reza-E-Rabby et al. [30], who studied the effects of welding parameters on joint strength and microstructure.
Plasticized aluminum alloy material was extruded to the groove pre-processed on the titanium sheet by FSW. Evans et al. obtained a metallurgical bonding joint with mechanical lock of aluminum alloy and titanium sheet [31]. Jarrell et al. also obtained T-joints of low carbon steel and AA6061 sheet by extruded aluminum into a concave groove cut into the top edge of a steel sheet using friction stir extrusion (FSE) [32]. To avoid hard and brittle intermetallic compounds during FSW of aluminum alloy and steel, Wang et al. proposed a novel friction stir rivet welding (FSRW) process for spot joining AA6061 and DP600 galvanized steel [33]. This process is attributed to the fact that plasticized aluminum alloy is extruded into the pre-fabricated hole on a steel sheet, thereby forming an “aluminum rivet.” However, this process uses a low die with cavity, and the exterior protrusion on the bottom surface of the FSRWed joint has greatly limited its applications in the visible areas and functional surfaces.
The preceding studies have mainly focused on the principle of friction stir welding process and the influence of process parameters on the mechanical properties of joints. Whether the dovetail groove or hole is pre-fabricated on the steel sheet, the influence of its geometric size on the joint mechanical properties has been rarely discussed. Moreover, the FSRWed joint obtained by Wang et al. has a exterior protrusion at the bottom, thereby affecting its practicability. Therefore, the current study proposes a probeless friction stir extrusion joining process for aluminum and steel dissimilar metals, in which the bottom of the joint is flat. The microstructure distribution of the joint is analyzed using electron back scattered diffraction (EBSD). The influence of rotational speed and diameter of the pre-fabricated threaded hole on the joint mechanical properties are substantially discussed. Lastly, different joint failure modes are examined using scanning electron microscope (SEM).