超声深滚作用力对Ti-6Al-4V合金微观组织及表面性能的影响

Effects of Ultrasonic Deep Rolling Force on Microstructure and Surface Properties of Ti-6Al-4V

目的改善Ti-6Al-4V合金的微观组织及表面性能。方法用不同作用力(150、250、350、450 N)的超声深滚工艺对Ti-6Al-4V进行表面处理。通过金相显微镜观察、透射电镜分析、维氏硬度测试和残余应力测试,分别评价表面处理后材料的显微组织变化、显微硬度分布和残余应力分布。结果随着超声深滚作用力的增加,表面塑性变形层厚度增大,但增大趋势随作用力的增加而趋缓,450 N超声深滚作用后,塑性变形层厚度最大。超声深滚处理后,材料表面出现纳米晶,随作用力的增加,纳米晶会逐渐转为非晶态。350 N深滚后,距表面220μm处会出现少量的孪晶及大量位错堆积。随着塑性应变的加剧,孪晶逐渐消失,晶粒不断细化。材料近表面硬度提高明显,提高幅度同作用力呈正相关。材料基体的硬度为328 HV。在150 N力作用后,材料距表面50μm处的硬度能达到343 HV,而在450 N力作用后可以达到381 HV。随着超声深滚作用力的增加,材料近表面的残余压应力总体呈现增加趋势,最大残余应力可达到-875 MPa;残余应力的分布范围随力的增加,从0.40 mm增长到0.58 mm。另外,不同作用力作用后,材料最大残余压应力均出现在距表面20~40μm处。结论超声深滚作用可有效改变TC4合金的表面微观组织及性能,进而影响其结构完整性和服役寿命。

The work aims to improve microstructure and surface properties of Ti-6Al-4V.Ti-6Al-4V was treated by ultrasonic deep rolling with different rolling forces(150,250,350,450 N).Microstructure,micro-hardness and residual stress of TC4 alloy after the treatment were examined with micro-hardness tester,X-ray diffractometer,optical microscope(OM)and transmission electron microscope(TEM).With the increase of ultrasonic deep rolling force,thickness of plastic deformation layer increased.However,the increasing tendency slowed down as the force increased.The maximum thickness 338μm appeared after the 450 N ultrasonic deep rolling treatment.Nanocrystals appeared on the surface of treated samples,and gradually transformed into amorphous state as the force increased.After being treated by 350 N deep rolling,a small amount of twin crystals and massive dislocations formed at the depth of 220μm from top surface.With the intensification of plastic strain,the twin crystals disappeared,and grains were continuously refined to reach nanometer level eventually.Near-surface hardness increased obviously,and increase amplitude was positively correlated with the force.Compared with the hardness of material matrix(328HV),final micro-hardness after 150 and 450 N treatment,at a distance of 50μm from the surface,could reach 343 and 381HV,respectively.Residual compressive stress of near-surface,and the maximum residual compressive stress could reach?875 MPa;and distribution range of compressive stress also increased as the force increased,and depth of affected distance increased from 0.40 mm to 0.58 mm.In addition,the maximum residual compressive stress appeared at the distance of 20~40μm from the surface.Ultrasonic deep rolling can improve surface microstructure and properties of TC4 alloy,and further influence its structure integrity and service life.