Study On Laser Ultrasonic Detection Of Hole Defects In 316L Stainless Steel By Selective Laser Melting

Additive manufacturing(AM)technology,also known as 3D printing,combines computer-aided design and material forming technology to manufacture various parts by stacking and solidifying metal powder materials layer by layer based on 3D data models.Compared with traditional subtractive manufacturing methods,AM technology has many advantages such as a short processing cycle,low cost,no need for tools and fixtures,and no design restrictions,and theoretically can achieve true "free manufacturing".Therefore,it has been widely used in many fields such as biomedicine,chemical and nuclear energy,aerospace,and automotive industries.Among them,selective laser melting(SLM)is one of the most promising technologies in AM.The technology is based on the idea of discrete-stacking,using high-density energy lasers to scan the metal powder bed layer by layer,and melting and solidifying it in a very short time to achieve metallurgical bonding and manufacture complete parts.Due to the fact that the metal powder undergoes solid-liquid-solid phase changes in a short period of time,the laser energy output is unstable and the processing environment is complex,defects such as residual stress,cracks,and pores are easily produced inside the manufactured parts,which can damage their mechanical properties.Therefore,it is necessary to propose a defect detection method for the SLM process technology that is efficient,high-resolution,versatile,and low-cost to ensure its further application and development.This article focuses on the SLM technology and uses 316 L stainless steel as the research object.Artificial defects with a diameter of 0.6 mm were pre-manufactured inside the material,and different roughness surfaces were generated through mechanical polishing.A multi-physics coupling simulation model was established by combining simulation and experimental verification of the pulse laser beam and metal material thermophysical model.A laser ultrasonic testing(LUT)experimental platform was constructed,and offline laser ultrasonic defect detection was performed on 316 L stainless steel parts with different surface roughness.The study investigated the mechanism of pulse laser-induced ultrasonic waves of different shapes,analyzed the impact of surface roughness on ultrasonic wave propagation,and explored the temporal changes and correlation of the ultrasonic displacement field inside the 316 L parts with different surface roughness.The amplitude of the detected ultrasonic surface wave signals increased first and then decreased with the increase of surface roughness.The amplitude of the detected longitudinal wave and its echo gradually increased with the increase of surface roughness.The noise signal detected gradually increased with the increase of surface roughness.The experimental and simulation results were consistent.Different signal characterization methods were used to obtain defect information,and a 0.6 mm diameter hole defect was successfully detected.The results of this study provide a reference for the further development of laser ultrasonic testing technology.