Influence mechanism of laser defocusing amount on surface texture in direct metal deposition

Abstract

Direct metal deposition is a promising additive manufacturing, but the surface quality of as-built parts has been a prominent issue limiting its immediate application. The laser beam exerts different influence mechanisms on the surface texture of the top surface and sidewalls, which are the main contour surfaces of as-built parts during multi-axis deposition. In this paper, the deposition height deviation and the deposition height instability were proposed to characterize the surface texture on the top surface, and the surface maximum height was used for the sidewalls. The correlation between the laser defocusing amount and the surface texture parameters was quantitatively studied. Results show that the deposition height deviation under the positive laser defocusing state is smaller than that under the negative laser defocusing state, although the laser defocusing distance is the same. The deposition height presents a self-stable phenomenon under the positive laser defocusing state. The surface quality on sidewalls is better under the negative laser defocusing state than that under the positive laser defocusing state. In addition, the influence mechanism of the laser defocusing amount was analyzed on the formation of multiple overlapped tracks on the top surface. Three formation mechanisms of adhered protrusions on sidewalls were revealed through a high-speed camera, namely particle adhesion, powder adhesion, and agglomeration adhesion. The research can be utilized to guide the dynamic adjustment of the laser defocusing amount to improve the forming quality. • The deposition height presents a self-stable phenomenon under the positive laser defocusing state. • The surface maximum height is suitable to characterize the surface texture on sidewalls. • A large laser defocusing distance under the NLDS is preferred to obtain a better surface quality on sidewalls. • Adhered protrusions are categorized into particle adhesion, powder adhesion, and agglomeration adhesion.