Abstract
Hot-stamped ultra-high strength steels have been widely used in automobile structural parts. Considering the high splash tendency in resistance spot welding due to their extremely high hardness, in this work, microstructural characteristics and mechanical performance of the resistance spot welded ultra-high strength steels are investigated. The results indicate that the interface between the nugget and heat-affected zone (HAZ) is the weakest zone where fractures initiate. In tensile shearing tests, a qualified spot welding joint failed with a button-shaped fracture. Welding defects would significantly decrease the load-carrying capacity and lead to interfacial fracture, except for a button-shaped fracture. In spot welding, it was found that a specific mid-frequency alternating current (AC) input mode, in which a 6 ms cooling cycle was inserted between every two neighboring current pulses, can avoid the splash in the spot welding of hot-stamped hardened steels.
Highlights
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
Under the slow cooling rate of 1.5 ◦ C/s, as shown in Figure 4a, the microstructure consisted of polygonal ferrite and a small amount of pearlite, which was formed in the range 603~678 ◦ C, as seen from Figure 3
The microstructure of spot welded joints of hot-stamped hardened steel was studied by establishing a continuous cooling transformation (CCT) diagram and microstructural examination
Summary
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. 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. The damage mechanism of spot welding joints of hot-stamped strengthened steels was studied in detail, and 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
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
