激光选区熔化钛铝合金裂纹形成机理及抑制研究

Study on Cracking Mechanism and Inhibiting Process of TiAl Alloys Fabricated by Selective Laser Melting

裂纹是激光选区熔化成形TiAl合金时最常见、破坏性最大的一种缺陷。以Ti-47Al-2Cr-2Nb合金粉末为成形材料,利用X射线衍射、扫描电子显微镜、X射线能谱分析及残余应力检测等检测方法,研究SLM成形TiAl合金过程中裂纹的形成机理及抑制方法。研究结果表明:SLM成形TiAl合金的裂纹为冷裂纹,具有穿晶断裂的特征。由于SLM成形过程中的快速加热冷却,成形件内部残余应力高于材料的抗拉强度,导致了裂纹的形成。裂纹多起源于试样侧面边缘粉末黏结、缺口等缺陷存在的地方,此外,合金试样内部含有大量的α2脆性相及孔隙,也有利于裂纹产生和拓展。随着基板预热温度逐渐提升到300℃,一方面温度梯度减小使得材料热胀冷缩减弱,试样内部的残余应力由(267.2±13.4)MPa降低到(172.6±8.6)MPa;另一方面,预热导致β相稳定性提高,α2脆性相的含量减少,最终使得试样中的裂纹数量和尺寸明显得到控制,试样的相对致密度也由87.64%上升到93.84%。

Crack is the most common and destructive defect in TiAl alloys processed by selective laser melting(SLM).The Ti-47Al-2Cr-2Nb alloy specimens are fabricated by SLM,detected by X-ray diffraction(XRD),scanning electron microscope(SEM),X-ray EDS Analysis and residual stress detection,the cracking mechanism and Inhabiting is designed.The experimental results showed that:The crack is considered as cold cracks in samples,which has typical characteristics of the transgranular fracture.Due to the rapid heating and cooling during the SLM,the residual stress inside the formed part is higher than the tensile strength of the material,resulting in the formation of cracks.The cracks originate from the existence of defects such as powder sticking and notch on the side edge of the samples.In addition,the samples contain a large amount of α2 brittle phase and pores,which is also beneficial to the generation and expansion of cracks.As the temperature of substrate preheating is gradually increased to 300℃,on the one hand,the temperature gradient is reduced,the thermal expansion and contraction of the material is weakened,and the residual stress inside the sample is reduced from(267.2±13.4)MPa to(172.6±8.6)MPa.On the other hand,β phase is more stable because of preheating,and the content of α2 brittle phase is reduced.Finally,the number and size of cracks in the sample are obviously controlled,and the relative density of the sample is also increased from 87.64%to 93.84%.