Additive’s idiosyncrasies — producing functional parts
The test was rigged, Simpson says. The part was designed with internal and external features in so many opposing directions that no single DMLS build could generate all of them perfectly. What the test shows, he says, is the kind of thinking that designers have to put into an additive-manufactured part. Even dimensional requirements are not necessarily helpful. Design engineers instead must prioritise which part features are more critical and which features can be left free to slightly depart from the model. For example, every model hole through a part is likely to be defined with a circular cross section, because designers are accustomed to holes being drilled. Simpson points to this as an example of a basic design assumption that now needs to be examined. That is: Which holes truly need to be circular?
He adds that the orientation of the part relates to more than just the direction of layers of material. On the DMLS machine, there is a recoater blade that might affect a delicate feature of the part or might leave it unharmed, depending on the orientation of this feature with respect to the blade. The figure on (1 and 2 in the picture gallery) shows this.
4. Residual Stress
Alternatively, an additive-manufactured part might depart from the CAD model not because of any effect of its orientation, but instead because of residual stresses that accumulate as the part solidifies during the build.
The metallurgical formation of an additive part is complex, Dickman says. It is not precisely true that a given layer is added to the part as a liquid and becomes a solid thereafter. Instead, a given layer is affected by the heat of the layer added directly above it, somewhat less by the heat of the layer above that one, and on and on. Any layer of material thus continues to heat and cool, settling into its final integration with the rest of the part only gradually. The heating and cooling affects the material microstructure and creates residual stresses that build up in the part. Modeling this behavior so as to predict potential distortion from these stresses is one of the more important areas of CIMP-3D’s additive manufacturing research.