选择性激光熔化成形TC4钛合金开裂行为及其机理研究

裂纹是选择性激光熔化(Selective laser melting, SLM)成形Ti6-Al-4V(TC4)钛合金过程中最常见、破坏性最大的一种缺陷。采用逐行交替的扫描策略制备TC4钛合金试样，利用扫描电子显微镜及X射线能谱分析等检测方法，研究SLM成形TC4钛合金过程中裂纹的开裂行为及其形成机理。结果表明，SLM成形TC4钛合金所产生的裂纹为冷裂纹，具有典型的穿晶开裂特征。采用逐行交替扫描策略成形的TC4钛合金，其组织为网篮状马氏体组织。SLM成形过程中高温度梯度导致制件内部存在较高的残余应力，抗裂强度较差的马氏体组织在残余应力的作用下而产生裂纹，粗大的裂纹最终分解为较小的裂纹而终止扩展。进一步分析认为，通过调整成形工艺参数，可以改变制件组织，同时削弱残余应力，从而达到减弱或消除裂纹的目的。

Cracking Behavior and Formation Mechanism of TC4 Alloy Formed by Selective Laser Melting

Crack is the most common and great destructive defect in selective laser melting (SLM) Ti6Al4V (TC4) alloy process. A TC4 alloy test specimen is fabricated by progressive alternative scan strategy in SLM process. The cracking behavior and formation mechanism is discussed in TC4 alloy SLM processing using scanning electron microscopy and X-ray energy spectrum analysis. The experimental results show that the crack is considered as cold cracking in SLM TC4 alloy part, which has typical characteristic of the transgranular fracture. By using the progressive alternative scan strategy, the basket-like martensitic structure is formed in SLM process. The high residual stress is caused by high temperature in forming process of SLM part. Under the residual stress, the crackle will occur in the martensitic structure because it has poor cracking strength. Finally, the coarse crack decomposes into smaller cracks in the process of expansion and the extension is terminated. Further research shows that by improving the process parameters, the microstructure of TC4 alloy will be changed and the residual stress will be weakened. At the same time, achieving to the purpose of weakening or eliminating cracks.