Study On The Laser Ultrasonic Testing Technology Of Defects In PBF Additive Manufacturing

The powder bed fusion(PBF)method of Additive manufacturing(AM)is increasingly used in various industrial manufacturing fields due to its high formability for parts with complex structures,design freedom and material utilization.However,due to the nature characteristics of discrete-collecting principle of AM,defects such as porosity,crack and less-fusion can hardly be avoided inside the parts.These small and randomly distributed defects notably affect the quality of products.Without stable and reliable detective technology available at present for the on-line inspection of defects,the producted parts have to be tested after production.This offline inspection technology is confronting the problem of low detection rate and even if the defects inside the part can be detected,it is still very diffcult to be fixed.Therefore,if the key techonolgy for on-line detecting in AM process can be breakthrough,it will significantly promote the quality improvement,cost reduction and popularization of PBF metal additive manufacturing.Laser ultrasonic testing(LUT)can function under the complex environments of radiation,temperature,pressure and corrosion due to its non-contact characteristics.Therefore,LUT has shown great potential in the on-line inspection of AM.In this paper,the LUT technology in detecting of metal AM defects is studied carefully.Firstly,a finite element simulation model was established to explore the interaction mechanism between laser ultrasonic wave and defects.Then a system of LUT was established to evaluate the method applicability for defects detection of PBF parts.And the LUT system for AM on-line inspection and testing project wre designed.The main results are as follows:(1)The model of interaction between laser ultrasound and defects was established.The process of point-source laser generating generating ultrasonic wave on the surface of 316 L stainless steel and the effect of laser parameters on the ultrasonic field were studied.The interaction mechanism between laser ultrasonic wave and surface defects,sub-surface defects were deeply explored.And the effect of defect size changes on the ultrasonic signals were also explored.(2)A system of LUT was established to detect surface and near-surface defects of a 316 L stainless steel sample fabricated by L-PBF using laser ultrasonic wave.The influence of the surface roughness of AM sample on the LUT was analyzed.The quantitative detection theme for detecting of sub-millimeter-level crack defects and hole defects with a diameter of 50 μm was realized by processing the A-scan,B-scan,and C-scan results.(3)The LUT system for AM on-line inspection and testing project wre designed,and an on-line inspction experimental platform was established.The on-line scanning technology of quick scanning to locate defects and C-scan to quantify defects was proposed.