**1. Introduction**

Hot-stamped ultra-high strength steels have been widely used as automobile structural parts, due to the increasing requirements for safety and weight reduction of automobiles. The hot-stamped steel sheet is first heated to the austenite zone and then transferred quickly into deforming dies and hot pressed into structural components with required shapes, such as A-pillar, B-pillar, and bumpers, and finally is rapidly cooled to room temperature by flowing cooling water in dies. During the cooling process, essentially 100% martensite is produced, and the tensile strength can be up to 1500 MPa and the yield strength can reach 1100 MPa [1–3]. To avoid oxidation at high temperatures during hot stamping, the as-received steel sheets are usually pre-coated by Al-Si [4]. Resistance spot welding is one of the most important joining methods in automobile manufacture. However, the ultra-high strength steel exhibits a poor weld-ability during the spot welding process. Molten metal splash tends to occur in welding, which is related to the high hardness of the steel. On the one hand, the two overlapping steel sheets are hard to press together, and on the other, the contact resistance between the two ultra-high strength steel sheets is much higher than that of plain-carbon steels, which result in the faster melting of metal. Under the combined action of these two factors, splash is a common issue in the spot welding of ultra-high strength steel. Splash tends to cause many kinds of welding defects in the nugget and decreases the tensile capacity significantly. In order to optimize spot welding parameters of automobile steels, researchers have studied the influence of spot welding parameters, especially current, electrode force, and welding time, on the microstructure and mechanical properties

of spot welding joints [5–7]. In recent years, alternating current (AC) spot welding machines with variable frequency have been developed and more widely used in resistance spot welding in the automobile industry [8–10]. In this case, except for the above mentioned three traditional parameters, some other parameters like frequency and pulse duration would influence the spot welding process, and thereby the mechanical properties of the joints. However, studies have seldom considered the effect of austenite pre-strain, which results from electrode force during welding on the phase transformation of the heating affected zone (HAZ) after spot welding. In this work, the damage mechanism of spot welding joints of hot-stamped strengthened steels was studied in detail, and then spot welding experiments were performed with different current input modes in order to optimize the parameters of resistance spot welding and decrease the splashing ratio in welding.
