Effect of laser power on microstructure and mechanical properties of laser heat conduction lap welded joint between AZ31B magnesium alloy and DP780 galvanized steel

Abstract

In this work, laser heat conduction lap welding (LHCLW) of AZ31B magnesium alloy sheet and DP780 galvanized steel sheet was carried out by the defocused laser beam. The effects of laser power on the microstructure and mechanical properties of the joint were studied. The pros and cons of the joint were identified and evaluated by measuring the tensile shear strength, microhardness and microstructure observation. The formation mechanism of various phases at the Mg/steel interface was analyzed. The results indicated that the galvanized layer could promote the metallurgical bonding between magnesium alloy and steel by improving the diffusion ability of molten magnesium alloy at the steel interface and reacting with Mg, so as to enhance the strength of the joint. A continuous dense layered eutectic structure (α-Mg+MgZn) was formed at the interface of the joint, while MgZn₂ and MgZn phase was formed at the weld edge zone and heat affective zone (HAZ), whereas no reaction layer was generated between the uncoated steel and magnesium alloy. A sound joint could be obtained at 2.5 kW, and the corresponding tensile shear strength reached the maximum value of 42.9 N/mm. The strength was slightly reduced at 2.6 kW due to the existence of microcracks in the eutectic reaction layer.

摘要

采用离焦激光束对AZ31B镁合金和DP780镀锌钢进行了激光热传导搭接焊(LHCLW)。研究了激 光功率对接头组织和力学性能的影响，通过检测接头的拉伸剪切强度和显微硬度、观察分析微观组织 等方法来评价接头的优劣，并且分析了镁/钢界面各种物相的形成机理。结果表明，镀锌层通过改善熔 融镁合金在钢界面上的扩散能力并与Mg发生反应，促进镁合金与钢的冶金结合，从而提高接头的强 度。观察发现在接头界面处形成了连续致密的层状共晶组织(α-Mg+MgZn)，焊缝边缘处和热影响区 (HAZ)形成了MgZn₂和MgZn 相，而无镀层钢与镁合金之间没有发生冶金反应。当功率为2.5 kW时， 接头的拉伸剪切强度能够达到最大值，约为42.9 N/mm。然而，在2.6 kW时由于共晶反应层中产生了 微裂纹而导致接头强度略有降低。