*3.2. Impact Properties*

### 3.2.1. Impact Energy

The impact tests on the keyhole-free FSSW specimens with welding parameters of 800 rpm, 1000 rpm, and 1200 rpm were carried out to measure the impact toughness of the dissimilar Al/steel keyhole-free FSSW joint. The calculation formula for impact energy is as follows:

$$E = P \times D \times (\cos \rho - \cos \alpha) \tag{1}$$

where *E* is the impact energy, *P* is the weight of the pendulum hammer, *D* is the distance between the center of the shaft and the center of gravity of the pendulum hammer, α is the starting angle of the pendulum hammer, and ϕ is the end angle of the pendulum hammer.

Parameters *P*, *D* and α are the fixed constants given by the equipment, and their values are 26.63 kgf, 0.6340 m and 60◦, respectively. The impact test parameters and the impact energy are shown in Table 3. Table 3 shows that the impact toughness of the specimen with the welding parameter of 1000 rpm is the best, with an impact energy of approximately 42 J. However, the impact energy of the other two specimens is similar, approximately 32 J.


**Table 3.** The impact test parameters and the impact energy.

#### 3.2.2. Load-Displacement Curves of Impact

Figure 13 shows the load-displacement curves and the histogram of the impact properties of the dissimilar Al/steel keyhole-free FSSW joint. The maximum load of the curve is the maximum impact load *Fm*, as the vertex of the dotted line in Figure 13a. The area integral of the left half of the dotted line is the crack generation energy *Ei*, as shown in the yellow area in Figure 13a. The area integral of the right half of the dotted line is the crack propagation energy *Ep*, as shown in the blue area in Figure 13a. The area integral of the load-displacement curves of impact is the impact energy *E* = *Ei* + *Ep*. It can be found in Figure 13a that the maximum impact loads of the three specimens are almost the same, approximately 55 kN. The specimen with a welding parameter of 1000 rpm had the largest impact deformation and the best impact toughness, which illustrates that the factor affecting the size of the impact energy is not the maximum impact load but the maximum impact deformation. In addition, the integral impact energy of the load-displacement curve approximately agrees with the calculated impact energy. At the same time, it can be found in Figure 13b that the maximum impact deformation directly reflects the crack propagation energy. The pre-crack generation energy is almost the same and cannot affect the maximum impact energy to a great extent. However, the post-crack propagation energy substantially affects the impact toughness. There is an internal relationship between the crack propagation energy and impact deformation. The greater the crack propagation energy is, the longer the crack propagation, the greater the impact deformation, the better the impact toughness, and vice versa.

#### 3.2.3. Impact Fracture

This impact test is based on the lap-shear test, as show in Figure 3. Although the welding parameters and impact energy of each group of samples are different, the fracture morphology rules under different welding parameters are consistent. The joint fracture of the Al side, with welding parameters at 1000 rpm, was selected as the object of observation and analysis. Figure 14 shows the macroscopic morphology of impact fracture of the dissimilar Al/steel keyhole-free FSSW joint. It can be found that the impact fracture morphology was diverse and uneven [13]. The magnification figure of the white rectangle area in Figure 14 is shown at the bottom of Figure 14. According to the welding process and connection mode, the impact fracture morphology is divided into the three zones noted as the non-action zone (NAZ), shoulder action zone (SAZ), and the stir zone (TSZ). The NAZ is weakly connected, and the fracture shows a shallow dimple-like morphology [11,42–45], which is not affected by the plunge force of the shoulder. This is the edge of the keyhole-free FSSW joint shown in Figure 8b, where the IMCs were not completely filled. Therefore, the cracks develop from the gaps and propagate into the weld [37,42–45]. SAZ has a deeper dimple and stronger connection [42–45], which is affected by the dual effects of the friction heat and the plunge force of the shoulder. That is, the interface junction area of the keyhole-free FSSW joint in Figure 9b is in this zone. TSZ has the strongest connection strength and is affected by the stirring and heat effect of the pin, i.e., the cloud cluster-like microstructure of the keyhole-free FSSW joint shown in Figure 11.

**Figure 13.** Load-displacement curves (**a**) and histogram of impact properties (**b**) of the dissimilar Al/steel keyhole-free FSSW joint.

**Figure 14.** Macroscopic morphology of impact fracture of dissimilar Al/steel keyhole-free FSSW joint.

Figure 15 shows the microscopic fracture morphology for the TSZ of a dissimilar Al/steel keyhole-free FSSW joint. Figure 15a shows the whole picture of the fracture morphology of the TSZ. It can be found that the crack started at a point in the brittle phase and expanded outward along the blue line in the TSZ, as shown in Figure 15a [10]. After the crack was generated, it first extended along the brittle phase. During the large deformation during impact, the large impact energy causes the brittle phase to crack, as shown in the upper left corner of Figure 15a. When there is no brittle phase in the crack extension zone, the crack can continue to expand along the plastic phase [11]. Figure 15b shows the brittle–ductile transition zone of the impact fracture in the TSZ. This zone illustrates the transition of the fracture mode from a brittle fracture to a ductile fracture. Further magnification of the brittle and ductile areas is shown in Figure 15c,d, respectively. Figure 15c shows a brittle cleavage fracture, whereas Figure 15d shows a ductile fracture with typical shear dimples [40,42]. It can be inferred that the impact fracture mode of the keyhole-free FSSW joint is the mixed ductile and brittle fracture mode [40,42]. Most of the impact energy is absorbed by the ductile fracture.

**Figure 15.** Microscopic fracture morphology of TSZ of dissimilar Al/steel keyhole-free FSSW joint: (**a**) whole picture, (**b**) brittle–ductile transition, (**c**) brittle cleavage fracture, and (**d**) ductile fracture.

Figure 16 shows the EDS surface-scanning results of the impact fracture in the TSZ. The elements were unevenly distributed on the fracture surface, as shown in Figure 16. Therefore, many brittle phases existed at the fracture surface, as discussed above, such as AlZn*x*, FeAl3, and FeAl3Zn*<sup>x</sup>* [13,37,38]. This is another important cause of crack formation.

**Figure 16.** EDS of the surface-scanning analysis of the impact fracture in STZ.

From the above analysis, it can be seen that during the impact fracture of dissimilar Al/steel joints, the crack first starts from the weak zone of the joint, such as brittle phase and the gap. At the same time, the crack first propagates along the brittle zone, forming a dissociation fracture, and then propagates along the ductile zone, forming a ductile fracture. The intermetallic transition layer provides the environment for brittle crack growth, while the Al and steel matrix provide the buffer for ductile crack growth. Therefore, the impact fracture of the dissimilar Al/steel keyhole-free FSSW joint is the mixed ductile and brittle fracture mode, in which the plastic fracture mode improves the fracture impact energy.

### **4. Conclusions**

The keyhole-free spot-welding joints of the dissimilar 6082 Al alloy and DP600 galvanized steel were successfully fabricated by retractile keyhole-free FSSW. The keyhole was eliminated by pin retraction technology. The interface behavior and impact performance of the dissimilar Al/steel keyhole-free FSSW joints were studied and analyzed in combination with the keyhole-free FSSW process. Important conclusions are as follows:


**Author Contributions:** Z.Z., Y.Y. and X.W. designed and planned the experiment. Y.Y. made the tests and wrote the paper. Z.Z. modified the paper. H.Z. provided the funding acquisition.

**Funding:** This research was funded by the Aviation Science Fund of China (No. 201611U2001) and Major Science and Technology Project of Gansu Province (No. 18ZD2GC013).

**Acknowledgments:** This work was funded by the Aviation Science Fund of China (No. 201611U2001) and Major Science and Technology Project of Gansu Province (No. 18ZD2GC013).

**Conflicts of Interest:** The authors declare no conflict of interest.
