**1. Introduction**

Friction stir spot welding (FSSW) is a type of solid-state welding process that combines friction stir welding (FSW) and spot welding. FSSW is widely used for the connection of similar Al alloys [1] and Mg alloys [2] and dissimilar Al/steel [3], Al/Cu [4], Al/Mg [5] and Mg/steel [6] materials in the transportation and aerospace industries. In order to realize lightweight automobiles, Al and Mg alloys are widely used as substitutes for steel, which can effectively reduce the overall weight of the automobile body, and achieve energy conservation and environmental protection. However, the melting points of Al and steel are quite different, which makes them difficult to weld.

Compared with solid state welding, it is easier to produce gas pockets and intermetallic compounds (IMCs) in dissimilar Al/steel joints during fusion welding, which results in a decline in the mechanical properties of joints. Similarly, there are some inevitable defects in FSW and FSSW, such as keyholes [7], hook defects [8–10], and IMCs [11–13]. These defects seriously affect the forming quality, tensile-shear strength, impact toughness, and fatigue life of welded joints. Therefore, a deeper understanding of dissimilar Al/steel FSSW is necessary. In order to overcome the above welding defects, keyhole-free FSSW techniques have been developed based on traditional FSSW technology. The remaining keyhole is the weakest area of the weld, and the joint first form cracks at the edge of the keyhole. Kubit et al. [7] found that keyholes and alclads are the main weld defects that worsen the joint quality in the FSSW process. Refilled friction stir spot welding (RFSSW) can fill the keyhole to avoid structural defects. In addition, FSSW joints can be divided into four zones: the weld nugget zone

(WNZ), thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ), and base metal (BM). Reimann et al. [14] found partial recrystallization in the stir zone for the first time using the stop-action technique. Chen [10] and Cao [15] et al. observed fine grains and shear texture in the stir zone and proved that the microstructure's evolution was related to recrystallization in the RFSSW of the 6061-T6 alloy.

The hook defect is formed by the plunge of the tool pin, causing a large extrusion deformation of the material during FSW. Garg and Bhattacharya [9] found that hook formation is related to the length of the tool pin. A pinless tool can obscure hook formation and improve joint strength. Chen et al. [10] investigated the microstructure and mechanical properties of the RFSSW of a 6061 Al alloy to transformation-induced plasticity (TRIP) steel. They proved that RFSSW increases the joint's strength by 56.33% compared with that of conventional FSSW joints. In contrast, they thought that the hook structure generated in the regular FSSW step is indispensable for a strong joint.

IMCs, as brittle phases, are also one of the main causes of joint cracking. Hsieh et al. [11] obtained dissimilar lap joints for a low carbon steel (SS400) sheet on a 6061-T6 Al alloy sheet, which were achieved by FSSW with a welding tool that had an independent tool pin and shoulder. They found that two IMC layers, Fe2Al5 and Fe4Al13, were formed at this surface, and the failure load was related to the thickness of the IMC layer. Bozzi et al. [12] also studied the IMCs of joints of 6016 Al alloy to IF-steel performed by FSSW and reached the same conclusion. They noted, at the same time, that the presence of IMCs depends on the welding conditions. Dong et al. [13] studied dissimilar lap joints of a Novelist AC 170 PX Al alloy and 1.2 mm thick ST06 Z galvanized steel sheets with RFSSW. They found that the IMC layer of ZnO, which was as thin as 0.68 μm in the sleeve-plunging zone, was the weakest part of the structure. Although the diameter of the welded spot was 9 mm, the maximum tensile/shear fracture load was only 4.5 kN. They also predicted the material flow during the Refilled FSSW/sleeve plunging process by the distribution of stirred zinc coating.

A large number of numerical simulation analyses of FSSW have been carried out. The temperature field, the stress and strain fields, the welding force, the residual stress, the formability, the material flow, and grain size of welding have become the focus of numerical simulations of FSSW [16–24]. At the same time, many articles have reported the shear-tensile strength and fatigue properties of FSSW joints [25–28]. However, there are few publications that study the impact properties of spot-welding structures, especially keyhole-free FSSW joints. Several studies on impact toughness mainly focused on the friction stir butt-welded joints of thick plates [29–31]. In this work, the microstructure and interface behavior of dissimilar Al/steel keyhole-free FSSW joints are studied and analyzed in combination with the keyhole-free FSSW process. In order to determine the relationship between the microstructure and the mechanical properties of the keyhole-free FSSW joints, an impact test was carried out on the dissimilar Al/steel keyhole-free FSSW joint.

## **2. Experimental Procedures**

#### *2.1. Materials and Fabrication Process*

AA6082-T4 Al alloy plates and DP600 galvanized steel plates were used in the experiment. Their dimensions were 150 mm × 50 mm × 2 mm and 150 mm × 50 mm × 1 mm, respectively. Tables 1 and 2 show the chemical composition of the 6082 Al alloy and DP600 galvanized steel.


**Table 1.** The chemical composition of the 6082 Al alloy (in wt%).

**Table 2.** The chemical composition of DP600 galvanized steel (in wt%).


The joints of the dissimilar 6082 Al alloy and the DP600 galvanized steel were welded by a retractile keyhole-free FSSW machine. The welding tool was made of a nickel-based super alloy with a tool shoulder of 20 mm in diameter and a tool pin of 5 mm in diameter and 2.1 mm in length. The pin can move up and down and rotate freely in the inner hole of the shoulder [32]. The spot-welded joints adopt the overlap form of positioning the steel plate on Al plate. This is different from another form of Al plate on the top of the joint [33], although in terms of the cost of the welding tool, it can improve the life of the welding tool. The overlapping form of steel on top can make the tool pin penetrate the steel plate and mix the steel plate and the aluminum plate thoroughly. At the same time, it increases the welding temperature and the thermoplastic deformation of the steel side, which can improve the quality of the dissimilar Al/steel spot-welding joint. The welding process of the dissimilar Al/steel retractile keyhole-free FSSW is shown in Figure 1. In the welding process, the pin and shoulder rotated simultaneously, and the rotational speeds of the welding tool ω*<sup>r</sup>* were 800 rpm, 1000 rpm, and 1200 rpm, respectively. When the rotational speed of the welding tool reaches a steady state, it begins to plunge, and the plunge speed *vp* was 5 mm/min (in Figure 1a). The shoulder plunge depth *ds* was 0.3 mm after the tool shoulder reached the surface of the workpiece (in Figure 1b) [33,34]. Then, the welding tool was moved forward at a speed *vf* of 3 mm/min, and the tool pin moved upward at a speed *vu* of 5 mm/min (in Figure 1c). When the pin was lifted by 2.1 mm relative to the shoulder, the bottom of the pin was level with the bottom of the shoulder. At that point, the pin stopped moving upward, and the shoulder stopped moving forward. This is equivalent to the tool pin moving forward approximately 1.5 mm. With the stirring, retracting, and advancing of the stirring pin, the thermoplastic metal near the stirring pin could gradually fill the rear keyhole under the action of the shoulder. Then, the welding tool moved upward (in Figure 1d). When the welding tool was moved to a safe distance, the tool shoulder and tool pin simultaneously reset and the keyhole-free FSSW joint was obtained (in Figure 1e). At the same time, the thermocouples are used to measure the temperature of dissimilar Al/steel joints during keyhole-free FSSW. Figure 2 shows the schematic diagram of thermocouple distribution around the dissimilar Al/steel joint during keyhole-free FSSW. The welded structure adopts the overlap form, and the rotating direction is shown in Figure 2. Four thermocouples are evenly placed around the spot-welded joints on the steel plate, and the welding temperature is measured in real time through the four channels.

**Figure 1.** The welding process of the dissimilar Al/steel retractile keyhole-free friction stir spot welding (FSSW), (**a**) welding initial stage I, (**b**) warming-up stage II, (**c**) welding stage III, (**d**) welding end stage IV, (**e**) cooling stage V.

**Figure 2.** Schematic diagram of thermocouple distribution around the dissimilar Al/steel keyhole-free FSSW joint.

#### *2.2. Microstructure Characterization*

The cross-sectional specimens of the keyhole-free FSSW joints, with dimensions of 40 mm × 3 mm × 15 mm, were cut off by wire cutting. They were used for microscopic metallographic and interface analyses. The cross-sectional specimens were ground with sandpaper, polished with a polishing cloth, and then cleaned with anhydrous alcohol. The Al layer of the cross-sectional specimens was etched with Keller's etchant (2.5 mL HNO3, 1.5 mL HCl, 1.0 mL HF, and 95 mL distilled water) for 20 s, and the steel layer was etched with a 4% nitric acid alcohol etchant for 4 s.

The microstructure of the keyhole-free FSSW joint was observed using an MeF3 large metallographic microscope (Leica Corporation, Wetzlar, Germany). The interfacial behavior, metal fluidity, and impact fracture of the keyhole-free FSSW joints were studied on a JSM-5600LV low-vacuum scanning electron microscope (SEM) (Japan Electron Optics Laboratory Company, Tokyo, Japan), and the interface element diffusion was measured with an X-ray energy-dispersive spectrometer (EDS) (Oxford Instruments, Oxford, UK). The IMCs of the keyhole-free FSSW joint were analyzed by X-ray diffractometry (XRD) (BRUKER-AXS Corporation, Billerica, Mass, Germany).

#### *2.3. Impact Tests*

The cantilever method was used to carry out impact experiments on the dissimilar Al/steel keyhole-free FSSW joints with different welding parameters. There were three specimens in each group of experiments. The cantilever impact test was performed on a CIEM-30D-CPC modified oscillographic impact testing machine (Tokyo Testing Machine MFG. CO. LTD, Tokyo, Japan). Figure 3 shows the cantilever impact schematic of the dissimilar Al/steel keyhole-free FSSW joints. One end of the sample was fixed on the impact fixture, and the other end was connected to an extension body with a long screw and nut, as shown in Figure 3. The extension body is separated from the impact fixture and can move freely. Aluminum and steel filler pieces of the same thickness and material are installed on both sides of the sample to keep the direction of force unchanged. The falling pendulum hammer hit the extension body horizontally, resulting in the fracture of the keyhole-free FSSW joint. The impact energy was calculated by the start and end angles before and after the pendulum impact on the dial. The oscilloscope recorded and displayed the impact load-displacement curve.

**Figure 3.** Cantilever impact schematic of the dissimilar Al/steel keyhole-free FSSW joints.

## **3. Results and Discussion**

#### *3.1. Microstructure and Interface Behavior*

#### 3.1.1. Welding Temperature Curve

Figure 4 shows the real-time temperature–time curves and histogram of the maximum welding temperature of the dissimilar Al/steel keyhole-free FSSW process. It can be seen that the temperature variation curves of the four thermocouple channels around the spot-welded joints are slightly different, but the curves are identical in shape, which shows that the temperature variation is in good agreement with the welding stages. According to the temperature curve, the welding process can be divided into five stages as follows, as shown in Figure 4a.

**Figure 4.** The real-time temperature–time curves (**a**) and histogram of maximum welding temperature (**b**) of dissimilar Al/steel keyhole-free FSSW process.

The welding initial stage (I): The point O is the starting point of welding, and the temperature of the sample is about 20 ◦C at room temperature. Starting from point O, the welding tool begins to rotate and plunge in stage Oa, but it is not in contact with the workpiece, so the sample temperature has not changed.

The warming-up stage (II): The temperature of the sample begins to rise rapidly in the ab stage. At this time, the tool pin begins to plunge into the workpiece, and the shoulder then contacts the surface of the steel plate. The temperature of the workpiece rises sharply due to friction heat generated by friction action. The higher heating rate *v*<sup>c</sup> = dT/dt indicates that the temperature rises very fast in this stage, and the maximum temperature reaches about 500 ◦C.

The welding stage (III): this stage, bc, is the stable stage of welding, and the temperature is still maintained at a maximum of about 500 ◦C and is almost constant. At this time, when the tool pin is retracted back, the contact between the shoulder and the workpiece is still stable. The friction heat is almost invariant, and the welding temperature is maintained near 500 ◦C.

The welding end stage (IV): The welding tool is lifted in stage cd, and the shoulder begins to detach from the workpiece surface. The workpiece temperature drops rapidly. The cooling rate *v*<sup>d</sup> = dT/dt is almost the same as the heating rate *v*c. This indicates that the friction heat between the shoulder and the workpiece surface is an important factor of the temperature rise in the FSSW welding process.

The cooling stage (V): This stage, de, is the cooling period of the specimen. At this stage, the FSSW welding process has ended and the workpiece temperature begins to decrease naturally. However, the cooling mode is mainly the heat conduction between the workpiece and the worktable and the heat convection and radiation between the specimen and the air. Therefore, the cooling rate is relatively slow, and the time is relatively long.

With the agitation of the mixing head and the increase of the welding temperature, the top steel plate and the bottom aluminum plate begin to deform and gradually soften under the action of the external force and friction heat. When the peak temperature of the workpiece on the outer side of the shoulder in the stable stage of welding reaches 500 ◦C, the temperature inside the joint will be higher during the dissimilar Al/steel FSSW process. At this time, the welding temperature is close to the melting point *T*<sup>m</sup> = 660 ◦C of the Al alloy, which makes the Al alloy completely reach the thermoplastic state. With the stirring of the tool pin at such a high temperature, a plastic mixing of Al and steel components can be formed, which will lead to element diffusion, transition layers, and new phases. However, this mixing is only in the stirring area, as it is not mixed sufficiently in the action area of the shoulder. Instead, the bonding between metals is formed under the action of the shoulder extrusion force, similar to the rolling or extrusion process. At the same time, as can be found in Figure 4b, the maximum welding temperature increases gradually with the increase of rotational speed. Although a high welding temperature can increase the mixing and diffusion of the dissimilar Al/steel joint, too high a temperature can also lead to defects, such as excessive transition layer thickness and brittle phase formation.
