激光增材制造钛合金的动态力学行为及变形破坏机制

Synchronous powder feeding laser clad deposition is an advanced digital laser multi-layer additive manufacturing technology of the metallic parts, which can realize the direct manufacturing of the metallic components and the high-performance repair of the damaged parts. This technology has a wide development and application prospect in the field of engineering. When laser additive manufacturing components and parts service under Special working condition and extreme environments in the field of aerospace engineering, it may be subject to high speed impact by space fragments and shedding and bearing high strain rate. But now research efforts have little focused on understanding of dynamic mechanical behaviors, deformation and damage mechanism on laser additive manufacturing titanium alloys. It is urgent to carry out the research on microstructural evolution during deformation process at wide strain rate rang. In this project, by using experimental measurements and microstructural observation, a comparative study on directional solidification structure formed during laser deposition and basketweave microstructure by forging process. The mechanical response and strain rate sensitivity of different microstructure morphology will be determined by mechanical property testing under quasi-static and dynamic condition. Based on the uniaxial in-situ SEM tensile experiments and multi-stage loading by controlling deformation amount under quasi-static and dynamic condition respectively, the microstructure evolution during plastic deformation will be obtained. The Failure mode and influence rules by strain rate will be also explored. Finally, it will be proposed to reveal the deformation and failure mechanism of laser additive manufacturing titanium alloys based on experimental results and microstructure observation. The research achievements of this project will be of great scientific meaning for our full understand the mechanical behavior and deformation mechanism of laser additive manufacturing metal parts. The research results will also play a guiding role for security design and its application in engineer structure.

同步送粉激光熔覆沉积是一项先进的数字化增材制造技术，能够实现复杂结构致密金属零件的直接制造，具有广阔的发展和应用前景。在航空航天领域某些极端环境下，增材制造零部件服役过程中可能受到空间碎片的高速冲击，材料承受很高的应变率。然而目前对激光增材制造合金在动态冲击载荷下的力学行为、变形机制及破坏机理认识还很不全面，亟待开展高应变率下的力学响应和变形破坏过程中微观组织演化规律相关的研究工作。本项目以激光增材制造Ti-6Al-4V合金为研究对象，采用SEM加载系统和Hopkinson拉杆系统进行宽应变率范围内的力学性能测试，探讨合金在不同应变率下的力学响应; 获得准静态和动态条件下塑性变形过程中的微观组织演化，阐明其塑性变形行为的应变率敏感性及依赖性，揭示变形破坏机理。研究结果不仅对于全面掌握激光增材制造金属零件的力学行为和变形机理具有重要的科学意义，而且对其在工程结构应中的安全性设计具有指导作用。

项目以激光增材制造钛合金为研究对象，针对该技术制造的复杂薄壁结构件、支架、框、梁等承力构件在起飞或发射、着陆过程中可能受到离散源或脱落物的高速碰撞或打击等特殊工况和极端环境，研究了材料在高应变率下的动态力学行为和破坏机理，以期作为评价增材制造材料在航空航天领域服役性能的一个重要标准。项目根据激光增材制造Ti-6Al-4V钛合金激光熔覆沉积过程中定向凝固而形成原始β晶粒外延生长的特点，将试样分为沿着晶粒生长方向（L）和垂直于晶粒生长方向（T）两类，通过准静态（SEM原位加载系统）和动态（分离式Hopkinson杆）条件下的力学性能测试，研究了宽应变率范围内激光增材制造Ti-6Al-4V合金的力学响应对应变率的敏感性和依赖性。采用扫描电镜原位拉伸实验，通过控制加载位移实现拉伸过程不同阶段的实时SEM组织图像拍摄，分别对比了激光增材制造TC4钛合金L方向和T方向原位拉伸时试样的微观变形及断裂特征。采用光滑试样分析原位拉伸过程中滑移带的形成和演化与显微组织中α和β相形态及尺度的关系；从微观角度实时观测了激光增材制造TC4钛合金变形损伤破坏过程，获得了激光立体成形Ti-6Al-4V合金在拉伸过程中的组织形态演化。研究结果表明：1). 不同应变率下两个方向的压缩应力应变曲线结果表明，激光增材制造钛合金由于原始β晶粒呈现外延柱状生长的特点，力学性能也表现为各向异性。相同的应变率条件下，无论是沉积态还是热处理试样，L方向的流动应力均大于T方向，L方向平均压缩强度比T方向高出10%左右。2). 随着应变率的增加，材料的应变硬化行为呈现下降的趋势，这是由于高应该率下材料自身应变硬化与剪切效应引起的应变软化相互竞争的结果。相比沉积态试样，热处理后试样的应变率敏感性有所增大，即随着应变率的增加，流动应力增大，高应变率下材料的塑性也相应增大。3). 通过SEM原位拉伸观察发现，塑性变形阶段随着载荷的增加，微裂纹首先出现在与拉伸方向成45°的两相界面上，之后错动和起伏。L方向试样晶界上没有明显的变化，而T方向试样上观察到沿晶界开裂的微裂纹，性能测试结果也表明T方向塑性比L方向差。4). 激光沉积态试样抗裂纹扩展的能力较差，微裂纹萌生于α与β相界，最终沿与着拉伸方向呈45°的相界开裂然后扩展，晶界没有表现出较强抗裂纹扩展的能力，层间缺陷是裂纹产生的又一主要因素。