In-situ thermal control-assisted laser directed energy deposition of curved-surface thin-walled parts

Due to the single track-multilayered deposition structure and complex curvature, the curved-surface thin-walled components fabricated by laser directed energy deposition (LDED) would suffer from serious thermal accumulation effect problem. The slower cooling rate of melt pool leads to the collapse of components, which further affect the forming quality and overall performance. Therefore, an in-situ thermal control (ISTC) process was proposed to mitigate the thermal effect by using the melt pool width as a visualization target and then varying the laser power. An adaptive integral-separated proportional-integral-derivative (PID) controller was improved to solve the steady state error and improve the control accuracy. In this work, thirty sets of AISI 316 L SS curved-surface thin-walled specimens were prepared by ISTC-assisted LDED process using laser power and curvature as parameter variables, respectively. A mathematical model of the thermal control effect on the melt pool width was derived and the effects of in-situ thermal control on the surface roughness, microstructure and tensile properties of the specimens were systematically analysed. The results showed that the ISTC process had a significant role in solving the thermal effect problem for the largest curvature (K5) and the highest power (2400 W) components. The primary dendrite arm spacing (PDAS) of 316 L was reduced by increasing the cooling rate, and the dimensional accuracy of the components was improved by up to 60.2%. In addition, twin-induced plastic deformation (TWIP) was promoted by mitigating the thermal accumulation effect, and the elongation of the specimens successfully reached quasi-superplasticity after control.