纳米强化钢激光-GMA复合焊接热影响区的相变机理及性能调控

As the research frontier of optical manufacture, the hybrid fiber laser gas metal arc (GMA) welding provides a new jointing method for the new type hot-rolled nano-scale precipitate strengthened steel (whose yield strength range is 600-700MPa), giving a solution to the problem of the plummeting of anti-fatigue and toughness of the heat affected zone for matal active gas (MAG) welding. By studying the phase transformation mechanisms of microstructure at the heat affected zone of the hybrid fiber laser-GMA welding, the regularities of re-dissolution, coarsening, and re-precipitation of nano-scale precipitate strengthened phase (Nb,Ti)C is revealed, thus identifying the respective areas of re-dissolution, coarsening, and reprecipitation at the heat affected zone of the nano-scale precipitate strengthened phase (Nb,Ti)C. By learning the micro-scale mechanical properties of the heat affected zone in the welded joints, this thesis links the variations of microstructure, especially the nano-scale precipitate strengthened phase (Nb,Ti)C to the macro-scale mechanical properties, thereby revealing the inherent regularities and correlations between the microstructure and the mechanical properties, and identifying the optimal microstructure of heat affected zone and related technological process of welding. The implementation of the project is expected to improve the welding quality of the new type hot-rolled nano-scale precipitate strengthened steel, and develop wider applied areas for the hybrid fiber laser-GMA welding, in order to improve the research levels on the frontier areas in this field for China.

作为光制造领域的研究前沿，光纤激光-GMA复合焊接技术为具有广阔应用前景的新型热轧纳米强化钢（屈服强度600-700MPa级）提供了一种新的连接方法，解决常规MAG焊接接头热影响区韧性及抗疲劳性能骤降的问题。通过光纤激光-GMA复合焊接过程中焊接接头热影响区微观组织相变机理的研究，揭示纳米强化相(Nb,Ti)C在热影响区内的回溶、粗化及再析出规律，确定纳米强化相(Nb,Ti)C在热影响区中的回溶区、粗化区及再析出区；通过焊接接头热影响区微观尺度力学性能的研究，将焊接接头热影响区微观组织尤其是纳米强化相(Nb,Ti)C与宏观尺度力学性能联系起来，揭示微观组织与力学性能的相关性和内在的规律性；最终确定热影响区最优微观组织及相应焊接工艺制度。项目的实施，有望提升新型热轧纳米强化钢焊接质量，拓展光纤激光-GMA复合焊接技术应用领域，从而有力提升我国在该前沿领域的研究水平。

新型热轧纳米强化钢(NPS钢)是一种依靠纳米析出相的析出强化和细晶强化的超高强度钢，其抗拉强度可达到700MPa以上。针对采用传统的CO2气保焊焊接方法对新型热轧纳米强化钢进行焊接时出现热影响区强韧性骤降的问题，本项目提出利用光纤激光/光纤激光-GMA复合焊接技术进行该钢的连接，通过研究NPS钢热影响区微观组织尤其是纳米强化相(Nb,Ti)C的相变机理、焊接工艺参数对熔深及热影响区微观组织的影响、焊接接头尤其是热影响区微观组织与微观/宏观尺度力学性能之间的本质关系，确定可实现热影响区韧性及疲劳性能与母材持平或微降的微观组织，寻求新型热轧纳米强化钢的最优焊接技术。. 研究发现，母材中的纳米强化相(Ti, Nb)C在焊接热循环过程中的相变机理如下：在焊缝区，由于焊接过程中经历熔化与快速凝固的过程，纳米强化相(Ti, Nb)C发生了回溶，并在随后的冷却过程中与氧结合形成少量对性能无害的氧化物，大部分的Ti和Nb元素在焊缝区被固溶；在热影响区内，由于焊接热循环的峰值温度低于纳米强化相(Ti, Nb)C的溶解温度，纳米强化相(Ti, Nb)C只发生了粗化，原母材中的3~20nm之间(Ti, Nb)C粒子尺寸增大至10~100nm之间。通过研究焊接工艺参数对组织性能的影响规律发现，该钢的激光焊接接头具有良好的力学性能，是该类型钢连接的首选焊接方法，尤其是在薄规格（<5mm）的纳米强化钢的焊接方面具有广阔的应用前景；而激光-GMA复合焊接是中等厚度（5~15mm）纳米强化钢的有效技术，焊接接头强度高于母材，焊接接头韧性略有降低，但优于传统的CO2气体保护焊接接头的综合性能。. 本研究工作的开展为解决新型热轧纳米强化钢的高效优质连接问题提供了重要的理论基础和基础数据，同时也为激光、激光-GMA复合焊接技术在高品质钢材的应用上奠定了坚实基础。