*3.5. Parameter Optimization for Spot Welding of Hot-Stamped Hardened Steels*

In case of spot welding of hot-stamped hardened steel, splash is an important problem demanding a solution. This is due to its ultra-high hardness, which requires a relatively large electrode force to keep the two overlapped sheets in close contact. Therefore, liquid metal tends more to be squeezed out from the molten nugget and thus splash occurs. In this case, a larger plastic metal ring around the molten nugget is beneficial for avoiding splash in spot welding. In order to optimize the technology of spot welding of hot-stamped hardened steel, three current modes were used in this investigation—direct current (DC), mid-frequency alternative current (AC), mid-frequency AC with a 6 ms cooling interval every other cycle (named AC-6)—as shown in Figure 10.

For the AC-6, the time interval was inserted between two neighboring cycles aiming to slow down the expanding rate of the molten metal nugget, and at the same time give more time for the plastic ring areas to expand outward. This is beneficial for the molten metal to be included in the plastic ring and prevent splash happening. Large enough plastic ring areas are important for avoiding splash in spot welding, which can ensure the molten metal is enclosed through close contact of the two overlapped sheets at the plastic areas under great pressure from electrode forces. The length of the time interval should be long enough to ensure the plastic ring expands faster than the molten metal nugget. If the length of the interval is too long, the nugget size would be too small and more welding time would be needed for a joint. After experimenting repeatedly, 6 ms was determined for the experimental ultra-high strength steel of 1.5 mm thickness.

**Figure 10.** Current modes applied in the present investigation: (**a**) DC: direct current, (**b**) AC: mid-frequency alternating current, (**c**) AC-6: mid-frequency alternating current with a 6 ms cooling interval every other cycle.

Under the two spot welding conditions of AC (AC and AC-6), it was found that the frequency of 110 Hz was the most suitable parameter for the present steel through a series of orthogonal tests by changing current value and frequency. Therefore, frequency of 110 Hz was used here. Under each welding condition, the most suitable current was determined and used, as shown in Table 2. Spot welding test results are summarized in Table 2, showing the diameter of the nugget, splash ratio, and maximum bearable tensile load. It was AC-6 that can solve perfectly the splash problem in spot welding, and only 5% of weld joints were splashed.


**Table 2.** Spot welding test results using three different current modes.

It is known that a plastic metal ring around the nugget is important for avoiding splash during spot welding, which can encircle the molten nugget and prevent it from splashing out. Therefore, keeping a large enough plastic ring during welding is important for avoiding splash. At the end of heating, the liquid molten nugget has the largest volume, and at the same time, the surrounding metal is in austenite condition and exhibits good ductility. Under the large electrode force, the austenite zone is extruded significantly, leading to the two overlapped sheets compressed against each other closely. Therefore, the HAZ with a large width is more favorable in order to avoid splash in spot welding. The distance from edge of the nugget to the nearest white arc line was measured for the three welding conditions using different current modes, as shown in Figure 11. The results, as shown in Figure 11d, indicate that the distance from edge of the nugget to the nearest white arc line is the largest under the AC-6 condition, which proves that the austenite zone around the nugget plays an important role in avoiding splash.

From Table 2, it is noted that the welded joints obtained under the AC-6 condition exhibit the highest mechanical property (maximum tensile load). In tensile shearing tests, the welded joints under AC-6 always fractured with a button-shaped mode, yet interfacial fractures may occur in samples obtained under AC and DC conditions. Microstructural examination of the welded joints obtained under three different current modes was carried out. Defects like shrinkage holes, micro cracks, and Al-Si inclusions were found in DC and AC specimens, as shown in Figure 12a–d. These defects probably result from the splash during the welding process. Splash leads to the reduction of liquid metal in the nugget, resulting in the solidification shrinkage holes and cracks. In addition, some molten Al-Si coating was pushed back into the liquid nugget when splash occurred and led to Al-Si inclusions in the nugget, as shown in Figure 12a,c, which significantly decreases the load-carrying capacity of the spot welding joint. Microstructural examinations on the spot welding joint of the AC-6 specimen showed that the microstructure in the nugget was in good condition, and there were no obvious weld defects.

It can be concluded from the above results that applying AC with 6 ms intervals between two neighboring cycles is a good method to address the splash problem in spot welding of hot-stamped hardened steels, and thus increase evidently the tensile shearing property.

**Figure 11.** Comparison of plastic zone size under three conditions with different current modes: (**a**) AC-6, (**b**) AC, (**c**) DC, (**d**) results summarization.

**Figure 12.** Various defects produced in spot welded specimens applying AC and DC, showing shrinkage holes, Al-Si inclusions, and cracks: (**a**) and (**b**) AC condition, (**c**) and (**d**) DC condition, (**e**) and (**f**) AC-6 condition.

#### **4. Conclusions**

The microstructure of spot welded joints of hot-stamped hardened steel was studied by establishing a CCT diagram and microstructural examination. Nugget microstructure completely consisted of martensite within columnar crystals and HAZ was composed of coarse martensite, fine martensite, acicular ferrite, granular bainite, and tempered martensite, in the order from nugget edge to outside. The nugget exhibited the highest microhardness and the acicular ferrite zone was the softest area.

The boundary between the nugget and the coarse martensite zone was the weakest area and fracture was prone to occur in the boundary when the tensile shearing load exceeded its limit. Therefore, for a qualified spot welding joint, a button-shape fracture was generally obtained. If a joint has a lot of welding defects, i.e., shrinkage holes, cracks, and Al-Si inclusions, interfacial fracture tends to occur, which is characterized by a shearing fracture.

Splash is liable to produce various defects in a spot welded nugget, resulting in interfacial fracture of the joint. The splash problem in the spot welding of hot-stamped hardened steel was solved perfectly by using a specific mid-frequency AC current input mode, in which a 6 ms cooling cycle was inserted between every two neighboring current pulses. Under traditional mid-frequency AC and DC heat input modes, splash easily occurs in spot welded joints, resulting in defects in the nugget, and significantly decreasing the load-carrying capacity of welded joints.

**Author Contributions:** Writing—original draft preparation, Z.Q. and H.L.; investigation, L.L.; formal analysis, X.R. and L.F.

**Funding:** This research was funded by the Key Technologies R & D Program of Tianjin (Grant No. 18YFZCGX00050), the National Natural Science Foundation of China (Grant No. 51204121), and the Natural Science Foundation of Tianjin City (Grant No. 16JCTPJC48900).

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