Direct Laser Deposition (DLD) is a type of laser-based additive manufacturing process used to create functional metal components layer by layer using a sliced 3D CAD (computer aided drawing) file. Unlike Selective Laser Melting which utilizes a bed of powder metal that is ‘selectively’ melted via a laser, DLD is based on melting feedstock (blown powder or wire) at the focus point of a laser source. In this post, we address the residual stresses occurring during the build of metal components with DLD technology .
Thermal dynamics during DLD
Residual stress is defined as the “stress in a body which is at rest and in equilibrium and at uniform temperature in the absence of external and mass forces” .
The DLD process is based on transient melting and resolidification of metals powders or wires. As the component gets built, large heating/cooling rates along the part generates temperature build up and dynamic temperature distribution in the component. These high thermal gradients and repetitious/rapid local heat transfer rates are known to cause residual stresses in DLD parts [3,4].
In addition to residual stresses throughout the part, this thermal history results in non-uniform anisotropic microstructures and directly affect the material properties such as tensile strength and fatigue resistance. The presence of residual stresses can reduce the strength or life of mechanical parts and can also result in dimensional inaccuracies due to warping [5,6].
Factors influencing residual stress formation in DLD metal parts [3-8]
Material properties (thermal conductivity, CTE, elastic modulus, yield stress) and phase transformation
Part geometry, Process parameters and scanning pattern during fabrication
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 F. Liu, X. Lin, G. Yang, M. Song, J. Chen, W. Huang, Microstructure and residual stress of laser rapid formed Inconel 718 nickel-base superalloy, Opt. Laser Technol. 43 (2011) 208–213.
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 J. Beuth, N. Klingbeil, The role of process variables in laser-based direct metal, in: Solid Freeform Fabrication Symp., 2001.
 M.L. Griffith, M.E. Schlienger, L.D. Harwell, M.S. Oliver, M.D. Baldwin, M.T. Ensz, et al., Understanding thermal behavior in the LENS process, Mater. Des. 20 (1999) 107–113.
 Z. Shuangyin, L. Xin, C. Jing, H. Weidong, Influence of heat treatment on residual stress of Ti-6Al-4V alloy by laser solid forming, Rare Met. Mater. Eng. 38 (2009) 774–778.
 C. Selcuk, Laser metal deposition for powder metallurgy parts, Powder Metall. 54 (2011) 94–99.
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