激光拼焊异种超高强钢组织和力学性能

 汽车轻量化后对安全性和碰撞吸能性提出了更高要求,从而促进了高强、吸能材料及其拼接技术的发展。以汽车安保件之一的汽车B柱为研究对象,采用能满足要求的DP980双相钢和22MnB5热冲压成型钢异种材料进行激光拼焊,研究焊接热输入对焊接接头显微组织与力学性能的影响。通过保持激光输出功率不变(1.3 kW)改变焊接速度的方法控制焊接热输入,考察焊接热输入与拼焊接头组织和力学性能之间的关系。利用光学显微镜、扫描电子显微镜、显微硬度测试仪和拉伸试验机研究接头不同亚区的组织和性能。结果表明,当焊接速度为16~26 mm/s时,均获得了完整而无缺陷的熔化区组织;随着焊接速度的提高,不仅焊缝表面凹陷逐步改善,并且焊接热影响区宽度也随之减小。硬度测试表明,接头中存在明显的软化区域,主要分布在DP980侧热影响区的回火区和不完全相变区,而DP980侧热影响区的细晶区、粗晶区、22MnB5侧热影响区以及焊缝金属区的硬度则有所增加,形成了焊接接头的硬化区。拼焊接头在能形成完整接头的条件下抗拉强度保持为576~597 MPa,断裂均发生在22MnB5侧的母材区,断裂时有明显的颈缩现象;接头断后伸长率为11.9%~15.5%,介于DP980母材(11%)和22MnB5(22%)母材的断后伸长率之间;研究还表明,焊接热输入越大,焊接接头相同区域的组织越粗大。

Microstructures and mechanical properties of laser tailor- welded dissimilar super-high strength steel sheets

Automotive safety and crash energy absorption have increased the demand for higher strength,energy-absorbing materials and their joining technology. Laser tailor-welded of DP980 dual phase steel and 22MnB5 hot-forming steel is used to investigate the effect of welding heat input on the microstructure and mechanical properties. By keeping the laser output power constant(1.3 kW) and varying the welding speed to control the welding heat input,the relationship between the welding heat input and the microstructure and mechanical properties of the tailor-welded joints was investigated. The microstructure and properties of the different sub-regions of the joint were examined using optical microscopy(OM),scanning electron microscopy(SEM),micro-hardness tester and tensile test machine. The results show that when the welding speed varies between 16 and 26 mm/s,a complete and defect-free fusion zone is obtained; as the welding speed increases,not only does the weld surface depression gradually improve,and the width of the weld heat affected zone(HAZ) also decreases. Hardness tests indicated the presence of softened zones in the joint,mainly in the tempered zone and incomplete phase transformation region of the heat affected zone on the DP980 side,while the fine grain zone, coarse grain zone,heat affected zone of 22MnB5 and weld metal zone of DP980 together formed the hardened area of the joint. Furthermore,the tensile strength of the welded joints remained between 576 and 597 MPa,with the corresponding fractures occurring in the base metal zone of 22MnB5 and accompanied by significant necking. The post-break elongation of the joints ranged from 11.9% to 15.5% between the post-break elongation of the DP980(11%) and the 22MnB5(22%). The study also showed that the greater the heat input during welding,the coarser the microstructure of the same area of the joint.