Research On Evaluation Method Of Grain Size In Metal Additive Manufactured Parts Based On Surface Wave

Metal additive manufacturing technology has been widely used in aerospace,medical,automotive and other fields due to its characteristics of strong special-shaped structure forming ability and high material utilization rate.Due to the complex forming process,the grain size of the metal additive manufactured parts will be affected by the temperature gradient during the melting and solidification process and thus directly affects the performance.Generally,metallographic method is used for the detection of grain size,but this method requires multiple fields of view to measure,and the detection period is long.Therefore,there is important application value to develop an accurate,efficient,and non-destructive testing method.In the present paper,the grain size evaluation method of metal additive parts based on laser ultrasonic surface waves is studied and the influence of detection process parameters,surface roughness and specimen anisotropy on the grain size evaluation is systematically analyzed.On this basis,the relationship between the ultrasonic signal and the grain size is established,and the nondestructive evaluation of the grain size of the metal additive part based on the laser ultrasonic surface wave technology is completed.The research results show that the laser incident energy has a linear relationship with the surface wave amplitude.Under the experimental conditions of this article,the optimal laser setting value and laser energy are 55% and 728?J,respectively,and the current is 41 A.Considering the peak-to-peak value of the surface wave signal and the signal attenuation,the detection distance is selected as 4mm.Considering the influence of grain morphology and grain size,any angle can be used when detecting the scanning surface of the additive product,and when detecting the printing surface,the detection angle is selected to be along the 0° direction.As the surface roughness of the sample increases,the attenuation coefficient of the ultrasonic surface wave gradually increases.When surface wave is used to detect crystal grains,the surface roughness of the sample should be reduced and kept within 1?4?m as much as possible.The anisotropy of the material has a certain influence on the grain size detection,and the attenuation coefficient has a certain relationship with the grain growth direction.The optimal detection angles are 0° or 180°,that is,perpendicular to the deposition direction,so that the direction results are less affected by anisotropy.Based on the systematic analysis of the influence of the laser ultrasonic inspection process and material characteristics on the grain size detection,the grain size of the additively manufactured Ti-6Al-4V alloy was evaluated by laser ultrasonic surface wave.The results show that the relationship between the surface wave attenuation coefficient and the average grain size conforms to the cubic polynomial calculation model,and the maximum error of the grain size detection of the additively manufactured Ti-6Al-4V alloy is 5.7%.The relationship between surface wave frequency band energy and average grain size conforms to the linear calculation model,and the error of the detection of the grain size of the additively manufactured Ti-6Al-4V alloy is 3.7%.