Quality target-based control of geometrical accuracy and residual stresses in laser metal deposition

Abstract

Laser metal deposition (LMD) connected with milling processes offers the opportunity for an efficient, resource conserving manufacturing of large structural components made from Ti-6Al-4V. The hindrance for a broader use of this technology is highlighted in various published studies. These publications show anisotropic properties as well as a lack in tolerance definition of the part geometry regarding structures manufactured with LMD. In addition to the poor geometrical accuracy, the occurrence of residual stresses is still a common cause for failure of parts or even the entire build job, as their prediction and determination is complicated. In this paper, a quality aim-controlled process chain is introduced to manufacture complexly shaped three-dimensional parts in given tolerance bands. In order to control the quality aims, an in-depth analysis of the interdependencies of different exposure patterns, three-dimensional process strategies, and part shapes is conducted. A simulation-based approach is chosen to develop a strategy for further optimization of the exposure. The influence of the local geometry, e.g., in terms of heat accumulation, is taken into account for this approach. For validation, the crack compliance method has successfully been used in thorough experiments to determine the local distribution of residual stresses in multiple LMD specimens. The results show a significant variation of residual stresses, with varied process parameters, as expected by theory. It is shown that the intricate correlation between process parameters, part geometry, and resulting quality can be controlled with a developed process-relationship model in LMD manufacturing of Ti-6Al-4V parts. Finally, based on the summarized results, an outlook is given on tolerances for the two quality aims, geometrical accuracy and residual stresses. This gathered knowledge is used to demonstrate the developed process-relationship model to manufacture a 2.5D prototype application as well as a complex shaped 3D aerospace application.