Study on the Effect of Scanning Strategy on Residual Stress in Laser Additive Manufacturing with the Laser Ultrasound Technique

BackgroundDuring the laser additive manufacturing (LAM) process large temperature gradients can form, generating a mismatch in deformation that can lead to high level of residual stress. The stress can have irreversible effects such as warping and cracking of parts during and post manufacturing.ObjectiveOne of the most important LAM parameters that should be controlled carefully in order to effectively manage residual stress is the scanning strategy. This study presents an evaluation of six different scanning strategies, namely reciprocating, 90° reciprocating, line, screwing, reciprocating overlapping and island scanning strategies with respect to their effect on residual stress.MethodsLaser ultrasound technology, as an advanced nondestructive testing method, is applied to measure the residual stress distribution under different scanning strategies for the first time. The surface wave generated by laser is used to evaluate the plane stress state within the surface layer of the specimen.ResultsThe results show that the island scanning strategy is found to contribute to the least average residual stress, and lowered residual stress by up to 45% relative to the line scanning strategy. The overall stress level is island < reciprocation overlapping < screwing < 90° reciprocating < reciprocating < line. Further to this, the effect of inspection path on the results of residual stress measurement is discussed and the concept of optimal inspection path is proposed for the first time.ConclusionsLaser ultrasound method can effectively measure the residual stress in laser additive manufacturing, and the research can serve as useful guidelines for engineers and designers to restrain and evaluate residual stress in LAM.