Limiting residual stresses during fabrication of metal components by SLM

Residual stresses remain inside a material after processing, when it has reached equilibrium with its environment. Rapid cooling of high localised thermal gradients lock in stresses that arise from localised compressive and tensile stresses introduced during rapid thermal expansion of material around the melt pool.  The immediate effect of thermal strains during the build process is distortion. How can you limit the residual stresses in SLM?

SLM is known to introduce large amounts of residual stress. Two mechanisms can be distinguished which cause residual stresses. 

Localised high temperature gradient 

The first mechanism results from the large thermal gradients that occur around the laser spot. Rapid heating of the upper surface by the laser beam combined with the slow heat conduction gives rise to a steep localised temperature variation. As the temperature rises, the material strength simultaneously reduces. In SLM, the underlying resolidified layers inhibit expansion of the heated top layer and elastic compressive strains are formed. When the material’s yield strength is reached, the top layer is plastically compressed.

Thermal gradient effect ref[1].

Cooling phase and shrinking

The cool-down phase of the molten top layers also induces residual stresses. The molten areas tend to shrink due to the thermal contraction. This shrinkage is also inhibited by the underlying material and introduces tensile stress in the added top layer and compressive stress below.

Limiting residual stresses during SLM

A few factors influence the amount of residual stresses locked in the material:

- Number of layers (build height and size)

The stress reduces with decreasing height of the component. Stress profiles before removal of finished product from the base plate consist of a large zone of tensile stress at the upper zone of the part being built. The maximum stress is reached at the surface of the part (equal to the yield stress). 

Influence of number of layers on residual stress profile ref[1].

- Base plate geometry and thickness

The heat platform (ie substrate = base plate) usually exhibits different stiffness from the component. This results in deformations and stress levels revealed by a stress step profile at the junction between substrate and component. The thicker the base plate, the smaller the resulting residual stresses will be (for a fixed part thickness). A very thick base plate results in a large shrinkage in build (xy) plane, while the bending deformation becomes smaller. A thin base plate height results in high residual stresses in the part and in a high bending deformation.

influence of base plate height on residual stress profile ref[1].

- Materials properties

The higher the yield strength, the higher the stresses being developed. The stresses after part removal are also larger.

- Influence of the exposure strategy and influence of the sector scanning order

Division of the in-layer in smaller sectors (islands or chessboard scanning) generally yields lower stress level than continuous patterns. 

Key take-aways

• Parts connected to the base plate contain very high stress levels (in the range of the material’s yield strength). 
  
• Parts removed from the base plate contain much lower stress levels but suffer from deformation during part removal. 

• Residual stresses magnitude depends on the part height and the stiffness and height of the base plate. 
• The exposure strategy has a large influence on the residual stress levels being developed. Islands or chessboard patter scans result in a lower maximum stress value.
• Heating the substrate plate reduces the stress level since temperature gradients are reduced. 

• Stress-relief post heat treatment (more on that later) can be used before removing components from the build substrate. 

References
[1] Peter Mercelis and Jean-Pierre Kruth, Residual stresses in selective laser sintering and selective laser melting, Rapid Prototyping Journal, Volume 12 · Number 5 · 2006 · 254–265
[2] L. Parry, I. Ashcroft, D. Bracket, R. D. Wildman,  Investigation of Residual Stresses in Selective Laser Melting, Key Engineering Materials Vol. 627 (2015) pp 129-132