中文摘要：

研究了应变速率对DP780钢激光焊接接头的拉伸性能及变形行为的影响规律和机制．结果表明，与母材相比，DP780钢激光焊接接头的变形行为对应变速率更敏感．随应变速率的增加，DP780钢激光焊接接头的强度提高，塑性呈现整体下降趋势．在较低应变速率（〈10^1S^-1）条件下，随应变速率增加，焊接接头的强度有所提高，但变化幅度不大，塑性降低较明显；当应变速率超过10^1S^-1后，强度的提高幅度增大，而塑性在应变速率10^1--10^1S^-1范围内有所恢复后再降低．DP780钢激光焊接接头拉伸变形过程中宏观力学行为的应变速率敏感性主要取决于DP780钢母材在不同应变速率下变形行为及机制的改变．随应变速率的增加，DP780钢激光焊接接头断裂位置距焊缝中心线的距离显著降低，断裂位置由母材区转移至热影响区的软化区．动态载荷下，DP780钢激光焊接接头不同区域组织塑性变形行为应变速率依存性存在差异，是焊接接头断裂位置表现出明显应变率效应的本质原因．

英文摘要：

Dual phase （DP） steels have good combinations of strength and ductility, and are being increasingly used in vehicle body structures to meet enhanced government regulations and safety standards. The use of DP steels in automotive industries involves laser welding, which would lead to changes in local material properties and create potential safety and reliability issues under dy- namic loads. The present work aimed to study the effects of strain rate on tensile properties and deformation behavior of laser welded DP780 steel joints. The results showed that the deformation behavior of laser welded joints was more sensitive to strain rate as compared to base metal of DP780 steel. The strength of DP780 steel joint increased with increasing strain rate, while the ductility de- creased first with increasing strain rate from 10-3 to 101 s-1, and then increased up to a strain rate of 102 s-1. The strain rate sensitivity of the deformation behavior of DP780 steel joints was mainly dependent on the change of deformation behavior and its mechanisms of base metal at various strainrates. The distance of the tensile failure location from the weld centerline decreased obviously with the increase of strain rate. And the failure location changed from the base metal to the softened heat affected zone （HAZ） as strain rate increased. The mechanism for changing failure location can be related to the strain rate dependence of the plastic deformation behaviors of microstructures in various regions across a ioint.