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

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.

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。

热成形钢具有超高强度与无回弹等优点，采用热成形钢激光拼焊板制造汽车零部件是实现汽车轻量化的重要手段之一。热成形钢表面的铝硅镀层可防止钢板在热成形过程中的氧化与脱碳。然而在激光焊接时铝硅镀层熔化进入焊缝，带来铝元素的偏析，严重影响焊缝的组织及性能。本课题针对Al-Si镀层热成形钢激光焊接接头铝元素偏析、组织不均匀问题，获得了一种利用镀层原位改性实现保留镀层直接激光焊接新方法，揭示了激光焊接过程中匙孔穿透对底部熔池流动和下表面镀层的作用机制，发现调控焊缝成形，可以改善接头的力学性能。获得了预热处理过程参数对镀层组织结构和元素分布的影响规律，发现镀层铝元素与基体的铁元素发生相互扩散及相变反应，原始的铝硅共晶组织转变为具有多层结构的金属间化合物组织。基于焊接接头显微组织形貌、物相表征与力学性能测试，建立了焊缝组织偏析与力学性能的内在联系，阐明了接头的断裂机理。研究结果表明：经过预热处理后，镀层由铝硅共晶相转变为多层结构的金属间化合物组织，主要为FeAl3，Fe3Al2Si4，Fe2Al5等，预热处理的扩散作用明显降低镀层中铝元素浓度，降低镀层与基材之间的铝含量差距，更有益于后续激光焊接焊缝中铝含量的均匀分布。偏析的铝元素对铁素体形成有促进作用，热成形状态下未经预热处理的样品中焊缝组织为马氏体和连续块状铁素体，其中铁素体的含量达到64.9%，因此造成焊缝中的力学薄弱点，焊缝的抗拉强度降低；经过预热处理后，无论是焊态还是热成形态的焊缝中铝元素分布更均匀，焊缝区域中的平均铝浓度降低，焊缝微观结构为马氏体基体上弥散分布的小尺寸铁素体，焊接接头的平均抗拉强度提升至1523MPa，与去除镀层的焊接接头强度相当。对镀层采用连续直线循环激光扫描预热处理，经激光扫描预热处理后，镀层转变为均匀的金属间化合物，镀层中铝元素浓度降低且分布均匀。扫描改性后的热成形钢激光焊接焊缝组织为马氏体基体上分布的小尺寸铁素体，接头抗拉强度提升了约20%，拉伸断口为脆性断裂。预热处理的方法能有效抑制激光焊缝中大尺寸铁素体的形成，从而提升焊接接头的抗拉强度。