镁合金焊缝中碳纳米管和激光束扫描耦合组织增韧及其冷变形规律研究

The weld toughness of magnesium (Mg) alloy is poor because of coarse-grained embrittlement, strong basal texture, reticular second phase and defect of pores, which is the main technical bottleneck for continuous cold rolling of the Mg alloy. Therefore, synergistic toughening employing carbon nanotubes and laser beam oscillation is proposed basing on the solidification and crystallization laws of the laser-arc hybrid welded Mg alloy. By using laser beam with high-frequency oscillating to stir the molten pool and modulate the heat-mass transfer, the basal texture can be weakened, the distribution of the second phase can be controlled, and the pores can be finally cleared. More importantly, the stirring effect can fully develop grain refinement by suppressing aggregation of the CNTs, and the crack propagation is then slowed down because of the "reinforced concrete" structure formed by the embedded CNTs in the Mg substrate. This project aims to study the toughness improvement mechanisms induced by the coupling of the Mg substrate and the CNTs within the multiple thermal fields driven by the oscillated laser beam. On the basis of the above researches, the microstructure evolutions and cold rolling behaviors of the non-equilibrium weld with CNTs skeleton are investigated to achieve balance between stress-strain and weld toughness. Theoretical system of the rolling of the non-equilibrium microstructure is finally established to provide guidance for continuous rolling of the Mg alloy.

镁合金焊缝容易出现粗晶脆化、基面织构强、第二相网状析出和气孔等系列问题，导致焊缝韧性不足，是镁合金连续冷轧面临的主要技术瓶颈。为此，本项目基于镁合金激光-电弧复合焊缝凝固结晶规律，采用碳纳米管（CNTs）和激光束扫描协同改善镁合金焊缝韧性。高频扫描激光束可增强熔池搅拌效应，调控传热传质和凝固结晶行为，从而弱化织构、调控第二相分布和消除气孔。更为重要的是，该搅拌效应可抑制CNTs团聚，充分发挥CNTs细化晶粒效果，并在其嵌入镁基体形成“钢筋混凝土”结构后减缓裂纹扩展，实现CNTs和激光束扫描协同增韧。本项目将重点研究激光扫描驱动的CNTs在多重焊接热场中与镁基体耦合诱发的焊缝增韧规律；在此基础上揭示含有非平衡组织和CNTs骨架的镁合金焊缝在轧制过程中的组织演变和冷变形机制，解决应力应变和焊缝韧性的平衡匹配，从而建立完善的非平衡组织轧制理论体系，为镁合金连续冷轧提供理论依据。

镁合金焊缝容易出现粗晶脆化、基面织构强、第二相网状析出和气孔等系列问题，导致焊缝韧性不足，是镁合金连续冷轧面临的主要技术瓶颈。为此，本项目基于已经掌握的镁合金激光-电弧复合焊缝凝固结晶规律，提出碳纳米管（CNTs）和激光束扫描协同改善镁合金焊缝韧性的思想，重点研究激光扫描驱动的CNTs在多重焊接热场中与镁基体耦合诱发的焊缝增韧规律，在此基础上揭示含有非平衡组织和CNTs骨架的镁合金焊缝冷变形机制。研究结果表明，在所有工艺参数下，焊缝电弧区平均晶粒尺寸均较激光区更为细小，这是因为电弧作用区域受到的热量较激光区更低。在激光-电弧复合焊接中加入CNTs后，CNTs可以增加激光吸收率，增大焊缝熔深。焊缝中添加CNTs可以细化晶粒，增加大角度晶界占比，减弱{0001}面的织构强度。当涂覆的CNTs醇分散液中质量分数为3%时，焊缝抗拉强度与延伸率达到了244MPa与12.5%，相较于无CNTs焊缝分别提高7%与43.7%。整体而言，激光束扫描能够通过搅拌效应抑制CNTs在熔池中团聚，实现焊缝晶粒协同细化。研究结果对于拓宽对镁合金焊接性的认识，促进其在金属冶金、交通运输和航空航天领域的应用具有重要推动意义。