应力水平对线弧增材制造TA15钛合金疲劳行为的影响

Influence of Cyclic Stress on the Fatigue Behavior of TA15 Titanium Alloy by Wire-arc Additive Manufacturing

以原位轧制线弧增材制造TA15合金为研究对象,研究了退火态TA15合金的疲劳性能与疲劳断裂失效行为。结果表明,退火态TA15合金的轧向和纵向组织均以α/β片层交叠形成的网篮组织为主,局部存在等轴晶粒分布。通过对S-N曲线进行拟合,TA15钛合金疲劳极限为311 MPa。试样断口包含疲劳裂纹源区、疲劳裂纹扩展区和瞬断区。裂纹均从表面萌生,同时裂纹扩展区存在大量的疲劳辉纹和二次裂纹,瞬断区由等轴状的韧窝构成,呈现典型的准解理断裂特征。在低循环应力下,裂纹主要穿过或沿着α/β片层组织进行扩展,裂纹扩展路径较长;在高循环应力下,裂纹开始沿着原始β晶界扩展,二次裂纹扩展路径较短但数量较多。疲劳裂纹容易沿着组织中相对薄弱的位置进行扩展,并且二次裂纹周围有裂纹分支生成。

Taking in-situ rolling wire-arc additive manufacturing TA15 titanium alloy as object,the fatigue performance and fatigue fracture failure behavior of annealed TA15 alloy were investigated.The results indicate that the rolling and longitudinal structure of the annealed TA15 alloy is dominated by the basket structure formed by overlapping ofα/β lamellas,and equiaxed grains are locally distributed.By fitting the S-N curve,the fatigue limit of TA15 titanium alloy isσ_(f)=311 MPa.The fracture is divided into fatigue crack source zone,fatigue crack propagation zone and instantaneous fracture zone,where the cracks all initiate from the surface.Meanwhile,the crack propagation zone exists a mass of fatigue striations and secondary cracks,and the instantaneous fracture zone is mainly composed of equiaxed dimples,which is characterized by typical quasi-cleavage fracture.Under low cyclic stress,cracks tend to propagate along or through the lamellar structure with long path.Under high cyclic stress,cracks propagate along the originalβgrain boundary,and secondary crack propagation path is short with large quantities.The fatigue crack is prone to propagate along the relatively weak position in the structure,and crack branches are generated around the secondary cracks.