Objective For hot stamping high strength boron steel B1500HS and Q235 steel, dissimilar materials laser welding tailored blanks combine the excellent properties of the two materials and can meet the special performance requirements of the structure. For example, B-pillar of automobile body structure requires that the upper and lower sections of B-pillar have low strength, while the middle section has high strength. However, due to the difference in mechanical properties of the materials connected at the end of the welded joint interface, the stress singularity and other mechanical mismatching effects are caused in the interface among weld, heat-affected zone (HAZ), and base metal (BM), forming a plastic gradient. So far, there are few researches on controlling the plastic gradient in each region of the welded joint during laser welding, so it is very necessary to find a method to control the plastic gradient of the welded joint. In this paper, welding with synchronous thermal field (WSTF) method is proposed in order to regulate the cooling rate of the joint, intervene the structural transformation of each region of the welded joint, reduce the plastic gradient and coordinate the plastic of each region. A comparative investigation of 2 mm boron steel/Q235 steel laser welded joint prepared by conventional laser welding and the WSTF conditions is carried out to systematically evaluate the differences about plasticity between them, and further provide a beneficial reference for the selection of controlling the plastic gradient of dissimilar material laser welded joint in practical engineering applications.

Methods Boron steel/Q235 steel laser welding tailor blanks are performed using YAG laser device. The welding conditions are conventional laser welding with 300, 450, 600 ℃ thermal fields, respectively. After that, high temperature tensile test, fracture morphology, and microstructure are observed by using electronic high temperature tensile testing machine, scanning electron microscope, and metallographic microscope, respectively. The high temperature tensile tests are conducted under 700, 750, 800, 850, 900 ℃, respectively. By comparing the above results, the plastic gradient difference of boron steel/Q235 steel welded joint under conventional laser welding and WSTF conditions is obtained.

Results and Discussions By comparing the high temperature tensile test and microstructure of boron steel/Q235 steel laser welded joint under conventional laser welding with 300, 450, 600 ℃ synchronous thermal field, respectively, it can be found that: 1) elongation: compared with the conventional condition, the elongation of 300, 450, 600 ℃ increases by 19.70%、 20.69%、 21.21% respectively (Table 5). 2) fracture angle: under conventional laser welding conditions, the fracture Angle of the joint is only 115°. Under the synchronous thermal field conditions of 300, 450, 600 ℃, the fracture angle increases by 13.04%, 21.74% and 41.74%, respectively(Fig. 8). 3) plastic strain: when the thermal field temperature is 600 ℃, compared with the conventional conditions, the plastic strain in the weld increases by 31.47%, the HAZ of Q235 steel increases by 28.23% and the HAZ of boron steel increases by 28.61%(Fig. 10). The plasticity of each region is more gradual and harmonious. 4) microstructure: under conventional conditions, the majority of the weld and the HAZ of boron steel are martensite, while the content of martensite in the HAZ of Q235 steel is relatively small, and ferrite and pearlite account for the majority. Under the condition of 600 ℃ thermal field, the microstructure of the weld is mostly ferrite and pearlite, while the microstructure of the HAZ of boron steel is mostly ferrite and the difference with the ferrite microstructure of the weld is smaller. There is almost no martensite in the HAZ of Q235 steel, but all ferrite and pearlite, and the fusion line interface transition is more uniform (Fig. 13).

Conclusions The results show that through synchronously applying the preset thermal field during the laser welding process and laser heat source for boron steel/Q235 steel welded tailor blanks, under the synchronous effect of thermal field and laser heat source, the WSTF method improves the microstructure transformation of weld and HAZ of boron steel and Q235 steel. The method can effectively improve the plasticity of integrated boron steel/Q235 steel welded joint, significantly reduce the plastic gradient in each region of the joint and make it to be gradual and harmonious. For integrated boron steel/Q235 steel laser welded joint, the higher the temperature of the thermal field, the higher the elongation of the integrated welded joint, and the more flat the fracture surface. For each region of boron steel/Q235 steel laser welded joint, the plasticity of the weld is obviously improved, the difference of stress-strain relationship among the weld, the HAZ and the BM is reduced, and the deformation of each region of the welded joint is more coordinated. Due to the reduction of cooling rate of weld and HAZ after welding, the temperature is higher and the retention time is longer, the formation of the brittle organization is avoided. The difference between the weld and its both sides is smaller, the transition at the interface of each region is more uniform, and the plastic gradient is greatly reduced. Therefore, the plasticity of boron steel /Q235 steel laser welded joint is higher than that of conventional laser welding under WSTF, and the plasticity is consistent and coordinated.