喷丸工艺对TC4钛合金梯度纳米晶结构的作用规律

目的通过改变喷丸的压力或时间,在钛合金表面制备出剧烈塑性变形(SPD)层较厚、硬度较高的梯度纳米晶结构。方法改变喷丸压力(0.3~0.6 MPa)或喷丸时间(15~60 min),调控TC4钛合金表面梯度纳米晶结构的变形层厚度和纳米晶晶粒尺寸。利用金相显微镜观察塑性变形层截面的组织形貌,通过X射线衍射仪(XRD)和透射电子显微镜(TEM)确定喷丸表面纳米晶的晶粒尺寸,通过显微硬度计对塑性变形层的截面硬度进行研究。结果一定喷丸压力(0.6MPa)下,SPD层和总变形层厚度分别在喷丸25、30 min时达到饱和值78μm和143μm。一定喷丸时间(25 min)下,SPD层和总变形层的厚度随喷丸压力的增加而增厚,在0.4 MPa时达到饱和,分别为78μm和120μm。当SPD层厚度进入饱和阶段后,表层晶粒大小和硬度强化程度都趋于稳定;在0.6 MPa下,当表面α相细化至稳定阶段时,晶粒尺寸为30~90 nm,表面硬度提高约30%。结论喷丸SPD层及总变形层的厚度随喷丸时间的延长或喷丸压力的增大而增厚,当SPD层厚度趋于饱和后,表面晶粒尺寸和硬度强化程度都已饱和。

Effect of Shot Peening Parameters on the Gradient Nanocrystalline Structure of TC4 Titanium Alloy

A gradient nanocrystalline structure of high hardness with a thick severe plastic deformation(SPD)layer was prepared on the surface of the titanium alloy by changing the pressure or time of shot peening.The deformation layer thickness and surface grain size of the gradient nanocrystalline TC4 titanium alloy were adjusted by changing the shot peening pressure from 0.3 MPa to 0.6 MPa or the shot peening time from 15 min to 60 min.Metallographic microscopy was performed to observe the cross-sectional microstructure of plastic deformation layer.The grain size of the surface nanocrystals was determined by X-ray diffraction(XRD)and transmission electron microscopy(TEM).The hardness of plastic deformation layer was studied by microhardness tester.The experiment results show that under certain shot peening pressure(0.6 MPa),the thickness of the SPD layer and the total deformation layer reached their saturation value of 78μm and 143μm respectively after 25 min and 30 min.Comparing the results shot peened under different pressure for 25 min,the thickness of both the SPD layer and the total deformation layer increased with rising pressure and reached their maximum value of 78μm and 120μm at 0.4 MPa,respectively.When the SPD layer was saturated,the surface grain size and hardness enhancement tended to be stable.When the surfaceαphase was refined into a stable state at 0.6 MPa,the surface grain size ranged from 30 nm to 90 nm,and the surface hardness was increased by about 30%.In summary,the thickness of both the SPD layer and total deformation layer increased in a step wise manner,with the increasing shot peening time or pressure,before they reached the saturation values.It should also be noted that both the surface grain refinement and surface hardness reached a saturated state before the thickness of the SPD layer was saturated.