Measurement of residual stress in laser additive manufacturing TC4 titanium alloy with the laser ultrasonic technique

Laser additive manufacturing (LAM) has wide prospects in the titanium alloy for aerospace applications because of its low consumption, high forming efficiency and digital manufacturing process. However, the problems of deformation and cracking caused by residual stress restrict its rapid development. In this paper, laser ultrasonic technology, as an advanced nondestructive testing method, is applied to measure residual stress in TC4 titanium alloy produced by LAM for the first time. The surface wave induced by pulse laser is applied to inverse the longitudinal and transverse residual stress. The result shows that the residual stress level in the LAM specimens belongs to low residual stress domain. The residual stress parallel to the direction of laser scanning is obviously greater than it perpendicular to the direction of laser scanning. Then, the effect of process parameters on residual stress is discussed. The residual stress is positively correlated with the laser power, and it is negatively correlated with the scanning speed and powder feeding rate. Finally, orthogonal experiment design and multiple linear regression analysis are used to establish empirical formula for residual stress evaluation of LAM titanium alloy parts.