基于原位微锻造的冷喷涂增材制造超细晶镁锂合金的组织形成及控性机理研究

The high chemical activity and preferential evaporation of alloying elements (Zn, Mg and Li etc.) in molten alloy makes Magnesium Lithium alloy of high quality a big challenge to be additively manufactured by laser and electron beam based high energy techniques with high heat input. To solve this problem, in this study an in-situ micro-forging assisted cold spray is firstly developed to additively manufacture Magnesium Lithium alloys at low temperature. The in-situ micro-forging is not only capable to densify the deposit but also promoting the dynamic recrystallization during deposition. The unique deposition mechanism in micro-forging assisted cold spray makes the microstructure, strengthening and toughening mechanisms of cold spray additively manufactured Magnesium Lithium alloys totally different from those in conventional metallurgical bulk counterparts. In this study alpha and beta dual-phase LA91 MgLi alloy and beta single-phase LA141 MgLi alloy are taken as the examples. Effect of the intrinsic properties of the materials, particle velocity and temperature as well as micro-forging on deposition behavior and microstructure of the resultant deposits will be systematically investigated. Since the cold sprayed metallic deposits usually shows a metastable microstructure showing anisotropic properties, the microstructure evolution of the deposit during the heat treatment will be experimentally studied. The static and dynamic mechanical properties of the as-sprayed and heat-treated deposits will be examined in terms of uniaxial tensile test and fatigue test. Detailed microstructure of as-deposited, heat-heat treated and failed specimens will be characterized by SEM, EDS, EBSD and TEM to make clear of the strengthening and toughening mechanisms as well as failure mode of the cold spray additively manufactured ultra-fine grained MgLi alloy. It is aimed to establish the correlation among microstructure of the as-deposited MgLi-heat treatment condition-microstructure of the heat-treated MgLi-mechanical properties. This program is potential to provide a new approach for additively manufacturing high quality light metals of low density such as MgLi alloys.

镁锂合金化学活性极高、熔融状态下合金元素易蒸发且氧化物易夹杂，因此激光、电子束等高热输入增材技术在镁锂合金制造上面临困境。为了实现高强韧超轻镁锂合金的增材制造，本项目首次提出固态颗粒高速碰撞沉积冷喷涂加后热处理的组合策略，并通过引入原位微锻造新机制，来实现沉积体的高致密化和动态再结晶晶粒细化，进而实现高致密、高强韧的高性能超轻镁锂合金的增材制造。本项目以高锂含量hcp+bcc双相和bcc单相两种镁锂合金作为典型对象，首先，研究材料特性、碰撞条件及微锻造效应对沉积体微结构的影响规律，揭示镁锂合金的组织形成原理；其次，针对冷喷涂沉积形成的非平衡及各向异性组织，阐明后热处理中沉积态组织的遗传性和演变规律；进一步，结合沉积态及热处理态镁锂合金的力学行为研究，建立沉积态组织-热处理工艺-热处理组织-性能图谱并阐明强韧化机制。本项目可为高性能镁锂合金的增材制造提供理论依据并完善我国轻合金增材技术体系。

项目针对现有高能束增材技术在低熔点、高激光反射率材料增材制造方面的困境，项目提出以镁锂合金、高强铝合金等材料为研究对象，进行冷喷涂固态增材制造控性研究。研究了冷喷涂粒子碰撞过程中粉末粒子表面氧化膜的演变过程以及对粒子间结合质量的影响机制；基于数值模拟研究氧化膜在粒子碰撞变形过程中的破裂过程机制。采用有限元模拟，研究冷喷涂高速颗粒碰撞过程中初始粉末颗粒表面氧化膜和粒子界面法向应力的演变，提出了冷喷涂粒子变形预测的高精度材料模型。针对冷喷涂金属内粒子界面结合有限，粒子内部晶粒尺寸不均匀的问题，以LA91镁锂合金为原料，通过在喷涂粉末中加入硬度较高且具有磁性的大粒不锈钢喷丸颗粒的方式在沉积过程中引入原位的微锻造效应。揭示了固态颗粒沉积镁锂合金显微组织的形成原理，确立了冷喷涂制备晶粒尺寸<1μm的镁锂合金沉积体的优化方法。以MF-CS AA6061合金为对象，研究了冷喷涂沉积体内部粒子间界面、高密度位错、超细再结晶晶粒在热处理过程中的演变规律。揭示了热处理条件对颗粒间结合状态、位错密度、晶粒尺寸与取向特征及析出相尺寸及分布的影响规律。通过与沉积态组织的对比揭示后热处理中沉积体组织的遗传性及其演变规律，阐明热处理工艺对增材制造MF-CSAA6061合金性能的影响规律。细晶特征使得制备态的MF-CS制备的AA6061合金的强度高达310MPa，但延伸率不足1%，优化热处理后粒子间界面结合质量的提升、位错密度的降低以及纳米析出相的产生使得沉积体强度不发生下降的同时，延伸率提高的9%，高于激光增材制造的同成分材料。探索了原位微锻造辅助冷喷涂技术在镁合金表面制备防腐涂层的可行性。结果表明，采用MF-CS方法可在镁合金表面制备液体腐蚀介质不渗透的Al基与Ni基耐腐蚀涂层，Ni涂层的抗盐雾腐蚀寿命>3000 h，优化条件下制备的Al合金涂层中的超细晶粒与较高水平的残余压应力可在同时提高耐腐蚀性能与疲劳性能，打破了涂层防护后镁合金耐腐蚀性能与疲劳寿命此消彼长的行业瓶颈。依托本项目共发表SCI收录期刊论文13篇，会议论文5篇，申请发明专利6件，培养硕士生6名，博士生2名。