# Research on Microstructure and Mechanical Properties of Laser Welding of BR1500 HS Joints
## 1 Introduction  
Nowadays, as people pay more and more attention to environmental protection, how to reduce the energy consumption and exhaust emissions of vehicles has become an important trend in vehicle research and development [1; 2; 3]. Structural lightweight is an important means to achieve this goal. There are two ways to realize the lightweight of automobile structure: optimization of automobile structure design and lightweight of body material [4; 5; 6]. High-strength steel is a relatively mature, lightweight material with great application and development prospects. The use of high-strength steel can reduce the thickness of the steel plate on the basis of ensuring the strength and stiffness of the body so as to effectively reduce the weight of the body and then greatly reduce the fuel consumption, ultimately achieving energy saving and environmental protection [7; 8; 9; 10]. BR1500 HS high-strength steel is a kind of high-performance material that can realize the lightweight of an automobile body and also meet the safety performance of automobile crashes [11; 12]. This kind of steel has good formability, high-temperature plasticity, and low resilience [13]. After hot stamping and quenching, the tensile properties and hardness of the material have been greatly improved, and it is widely used in automobile reinforced structures and high-strength parts [14]. Due to the high strength of the high-strength steel, the biggest problem in the application is the connection strength of the weld. To give full play to the advantages of high-strength steel, advanced welding methods must be used to obtain welded joints with qualified mechanical properties. Laser welding technology is more and more widely used in high-strength steel and aluminum alloy materials, which can improve the strength of the weld, reduce the width of the heat-affected zone and reduce the softening of the heat-affected zone, so the weldformed by laser welding has better mechanical properties [15; 16; 17]. The good performance of welded joints depends on the appropriate welding process, so the selection of welding process parameters is very important [18]. At present, some research achievements have been made in laser welding of high-strength steel. [PERSON] et al. [19] explored the influence of different oscillation modes on the properties and microstructure of ultra-high-strength steel and found that laser welding with the circular oscillation mode has the best tensile properties and microhardness among the four explored oscillation modes. [PERSON] et al. [20] investigated the double lap joints of LS and UHS steels fabricated by laser welding at different energy densities and found that fatigue cracks nucleated at the intersection of the HAZ and the plate boundary in the welded zone. [PERSON] et al. [21] researched different joints of the third generation AHSS by characterizing the mechanical behavior and the evolution of microstructure and found that because of local tempering of initial martensite in the material structure, there is severe HAZ softening within the HAZ on the TRIP1100 and 22 MnB5 sides of the joint. [PERSON] et al. [22] investigated the influence of welding parameters on the mechanical behavior and microstructure changes of ultra-high-strength dual-phase steels. [PERSON] et al. [23] studied the mechanical behavior and microstructural evolution of dissimilar joints obtained under different heat inputs by laser welding. At present, the research on the welding of BR1500 HS high-strength steels mainly adopts resistance spot welding, while the research on laser welding is very limited and only stays on the rough exploration of process parameters. Therefore, in this paper, BR1500 HS high strength steel was laser welded under various process parameters. The welded samples were heated in a furnace for 5 min with a temperature of 900 \({}^{\circ}\)C and then water-quenched.