Residual stress in metal components built using laser direct deposition

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 [1].

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” [2].

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

• The magnitude of local residual stresses can be up to 75% of nominal yield strength [3].
  
• The material stress–strain relationship and strain mismatch during the cooling phase affect the amplitude of resultant residual stresses.
  
• residual stresses with higher magnitude usually occur in materials with higher Young’s modulus (modulus of elasticity) and yield stress
  
• Residual stresses of In718 (yield strength=1100MPa) >> 316SS (yield strength=450MPa) [3].
  
• The yield stress–temperature curve of a material system is an important factor. As the yield stress decreases rapidly with increasing temperature, the strain mismatch is low at high temperatures.
  
• As these volume changes can be accommodated by plastic flow [3], materials with high yield strength at high temperatures (ex: In718) produce greater residual stresses [3].

Part geometry, Process parameters and scanning pattern during fabrication

• The highest residual stress value tend to be compressive and occur along the build direction (normal to the build plate) [3].
  
• Generally, increasing the build height increases the level of residual stresses [9].
  
• Within the building plane, residual stresses are typically aligned with the laser scanning direction. They are compressive at the center and tensile at the edges of the part [3,7,8].
  
• residual stresses are lower at the starting location of laser scanning and reach their maximum value at the end of the laser scanning path [8].
  
• Compressive residual stresses are larger near the substrate and transit to tensile stresses with lower magnitudes toward the top of parts as the number of layers increases [8].

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Reference
[1] N. Shamsaei, A. Yadollahi, L. Bian, S.M. Thompson, An overview of Direct Laser Deposition for additive manufacturing; Part II: Mechanical behavior, process parameter optimization and control, Additive Manufacturing 8 (2015) 12–35
[2] A.S.T.M. Standard, ASTM E6-09be1 Standard terminology relating to methods of mechanical testing, 2009.
[3] P. Rangaswamy, M.L. Griffith, M.B. Prime, T.M. Holden, R.B. Rogge, J.M. Edwards, et al., Residual stresses in LENS® components using neutron diffraction and contour method, Mater. Sci. Eng. A 399 (2005) 72–83.
[4] 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.
[5] K. Dai, L. Shaw, Distortion minimization of laser-processed components through control of laser scanning patterns, Rapid Prototyp. J. 8 (2002) 270–276.
[6] J. Beuth, N. Klingbeil, The role of process variables in laser-based direct metal, in: Solid Freeform Fabrication Symp., 2001.
[7] 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.
[8] 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.
[9] C. Selcuk, Laser metal deposition for powder metallurgy parts, Powder Metall. 54 (2011) 94–99.