基于铝硅镀层原位改性的热成形钢激光焊缝马氏体化机理研究

热成形钢激光拼焊板是实现汽车轻量化的重要途径，但是铝硅镀层进入激光焊熔池导致焊缝为马氏体和第二相的混合组织，严重损害接头的力学性能。在保留镀层的前提下抑制热成形钢激光焊缝中的第二相是该领域具有挑战性的研究课题。本课题提出对待焊区域内的镀层通过激光扫描镀层原位改性的方法，制备Al-Si-Fe镀层从而克服Al元素对焊缝金属组织的影响。采用热力学理论计算结合镀层改性试验、热模拟试验、激光焊接实验、微区组织表征等研究手段，研究激光扫描镀层原位改性制备Al-Si-Fe镀层时元素的扩散行为、化合物相形成机制；阐述Al-Si-Fe镀层在热成形钢激光焊缝金属凝固过程的组织演化历程；解析焊缝中Al、Si元素的存在形式及分布规律，揭示焊缝中第二相的形成机理并发展抑制方法。本项目成果为铝硅镀层热成形钢激光焊缝组织马氏体化调控提供了科学依据和理论支持。

Automotive light weighting is the inevitable trend with energy saving and emission reduction. Laser tailor-welded blank (TWB) of hot stamping steel is one of the important ways for car lightweight. Previous research involving the laser welding of Al-Si coated hot stamping steels showed that the microstructures in the fusion zone are martensite and the second phase, which reduce hot stamping TWBs performance. The effect of the Al-Si coating on the weld seam, as well as the existing form of coating elements, has not been clarified thus far. ..In this study, the laser weld seam martensitizing mechanism based on the in-situ coating modification is investigated. Thermodynamic calculation, thermophysical simulation and laser welding experiments are developed. The metallurgical characteristics of the weld seam are analyzed by OM, SEM, TEM, XRD, EBSD and so on. The following scientific tasks are included: the coating elements diffusion behavior and the formation mechanism of new Al-Si-Fe intermetallic compounds under different laser scan pre-heating in-situ modification and the non-equilibrium solidification process of laser welding based on Al-Si-Fe intermetallic compounds coating modification. The distribution characteristics of Al and Si element in weld seam and the interactive mechanism are analyzed. Besides the fully martensite regulation strategy by laser scanning pre-heat in-situ coating modification, the reasons of second phase and the corresponding suppression method are proposed for good quality and high efficiency. ..Accordingly, the theoretic foundation and scientific evidence would be established for laser welded Al-Si coated hot stamping steel.