Characterization And Accuracy Analysis Of X-ray Diffraction Of Residual Stresses For Shot Peened Titanium Alloy

Wet shot peening is a surface treatment used to improve the fatigue performance in many applications,such as aeronautics,railway welding,bearings,gears,coatings,heat treating of the steel product etc.The compressive residual stress formed in the surface layers plays an important role in the improvement of the fatigue life.The Sin2? method is the most widely used X-ray diffraction method for the residual stress measurement.The accuracy of residual stress measured by the Sin2? method is the focus of relevant researchers in the industry,which affects formulation of the shot peening process parameters and the service reliability of reinforced parts.Accuracy of measurement consists of correctness and uncertainty.In this paper,the high-speed X-ray diffraction residual stress analyzer designed by Proto Manufacturing Ltd was used to test the residual stresses of the shot peened titanium alloy samples.A self-designed in-situ tension/compression device is adopted to evaluate the correctness of residual stress measurement in titanium alloy and a calculation method in multiple convolution is given to evaluate the uncertainty of residual stress measurement.Meantime,the surface residual stress field is characterized and its uniformity is analyzed.The calibration experiments of the X-ray elastic constant show that the X-ray elastic constant of TC4 titanium alloy with 0.18%oxygen content,1/2S2213,is higher than that offered by software system.After calibration of 1/2S2213,the correctness of residual stress measurement increases by more than 5%.In addition,1/2S2234 is not sensitive to plastic deformation and tissue changes caused by shot peening,but it is greatly affacted by oxygen content in titanium alloy.1/2S2213 is endowed to 12.7881 × 10-6MPa-1?12.4409 x 10-6MPa-1?12.1552 × 10-6MPa-1 in oxygen content levels of 0.13%?0.18%?0.23%after calibration.With oxygen content increases,1/2S2213 decreases.The multi-convolutional approach to calculate the uncertainty of residual stress measurement doesn't need to know about any part uncertainty which is complicated to calculate,but only to know the distribution range of measured results.For example,measurement uncertainty in the case of 95%confidence level is U(95%)=0.47a and the confidence interval for the mean is[m-0.47a,m + 0.47a]for 5 repeated stress tests where m is the mean of measurements and a is half width of measurement interval.Experiments show that the uncertainty of stress measurement is closely related to surface condition of samples.The higher the shot peening intensity,the greater the surface damage of samples.As roughness increases,measurement uncertainty increases.The characterization experiments of residual stress field caused by wet shot peening show that shots don't strike the surface uniformly and therefore uneven residual stress field is formed.From the center to the edge of the wet peened samples,residual stress gradually decreases.And the lower the shot peening coverage,the more uneven the stress distribution.With increase of shot peening coverage,the difference between center and edge of residual stress is reduced,and the uniformity of stress distribution is improved.With shot peening coverage increases,residual stress in the center of samples increases first,then decreases and moves in circles.This is because that as dislocations appear,increase,annihilate during shot peening,residual stress is generated,increased,released,and repeatedly cycled.