Investigation On Ultrasonic Testing Technology Of Residual Stress In SLM Metal Additive Manufacturing

Selective laser melting(SLM)metal additive manufacturing technology has the advantages of directly forming complex structural parts and forming samples with excellent performance,and is widely used in aerospace,automotive and medical fields.SLM additive samples usually have residual stresses,which can cause defects such as deformation and cracking,so it is very important to develop accurate,efficient and non-destructive testing methods.However,there are relatively few related studies,especially the use of ultrasonic surface wave to accurately detect the residual stress of SLM additive samples.This study focuses on the detection of residual stress of SLM additive samples by ultrasonic surface wave.Firstly,the anisotropic characteristics of the surface wave velocity caused by texture of the specimen and the effect of such characteristics on the detection of residual stress by ultrasonic surface wave were studied.Then the method of correcting the acoustic elastic coefficient,reducing the frequency and correcting the surface wave calculation formula were taken to eliminate the anisotropic effect.Based on the above investigations,the surface wave generated by laser ultrasound was used to measure the residual stress of an anisotropic SLM additive sample.The research results show that the surface wave has anisotropic characteristics when propagating in the 316L rolling sample with{101}texture.The wave velocity symmetrically distributes around the axis of 180°and the wave speed decreases first and then increase from0°to 180°(parallel to the rolling direction)with the smallest wave velocity at 90°(perpendicular to the rolling direction).As the frequency decreases,the anisotropy of the wave speed gradually becomes insignificant,and the fluctuation of the wave speed also gradually decreases.Due to the existence of texture,the acoustic elastic coefficients in different directions of the same plane and at different depths are different,so that the stress should be consistent with the corresponding acoustic elastic coefficient and the frequency of surface waves should be same when measuring residual stress using surface waves.In addition,the method of reducing frequency can effectively reduce the effect of anisotropy on the detection.The residual stress error obtained by using the 2.5MHz frequency probe is the smallest,and the maximum error value is 4.7MPa when the surface wave propagation direction is parallel to the loading direction and 3.6MPa when vertical.At the same frequency,the accuracy of residual stress measurement by surface wave can be effectively improved by modification formula considering the anisotropic effect.Based on the above research,the residual stress of the anisotropic SLM additive sample was measured by the correction method.The results show that the surface wave velocity in the 316L additive sample obtained by different printing methods is different.The average wave velocity is 2928.7m/s in unidirectional printing specimen but the volatility is large(-13.419%).The average wave velocity is 2906.5m/s in the strip-shaped and 45°printed sample.The average wave velocity is 2962.5m/s and 2936.4m/s respectively in orthogonal printing and 45°printing samples with the smallest volatility of 0%.In addition,the measurement depth is different if the surface wave frequency is different.The measurement depth is about 0.8mm under 5MHz surface wave frequency and about 1.6mm under 2.5MHz.The residual stress at different depth of the sample can be calculated by establishing residual stress gradient detection model.Finally,the residual stress of the 316L additive sample is measured by surface wave generated by laser ultrasonic.The main stress?₁,?₂ of the 316L additive sample is unevenly distributed.The residual stress at the edge part is tensile stress with the maximum value of 164.9MPa and 90.8MPa respectively while that the central area is compressive stress with the maximum value of -141.6MPa and -74.1MPa respectively.