Additive Manufacturing Simulation - The Inherent Strain Method Simply Explained.
Updated: Mar 13, 2019
Metal-deposition Additive Manufacturing (AM) uses a complex thermo-mechanical process to produce a wide variety of parts and applications. In a layer-by-layer fashion a moving heat source melts deposited material, which solidifies immediately as soon as the heat source moves on. Around the melt pool of the heat source, material expands locally during the heating phase and contracts afterwards in the cool down (solidification) phase.
In an ideal situation the shape changes caused by the expansions are reversed by the contractions, however, in reality the expansions irreversibly compress the material in the peripheral regions of the melt pool. These regions remain compressed even when the material contracts as the heat source moves on. (See the illustration below).
These remaining, compressed regions hinder the material locally to go back to its original shape and introduce an imbalance. As a consequence entire designs start to deform to counteract this imbalance. In technical terms the compressed regions can be expressed as regions with residual strains (= mathematical expression of the shape changes) that cause localized, and subsequently, global part deformations when accumulated.
If one takes a step back and looks at the complex, thermo-mechanical process of AM from the outside, then they can describe it as follows: If a given design is manufactured with a thermo-mechanical process, it causes thermal expansions and contractions which lead to residual strains, and at some point, to a deformed shape of the original design. One could even go a step further and look at this process via black box approach: If a given design is manufactured with a certain process (black box), it leads to residual strains that will lead to a deformed shape of the original design.
In other words, it can be of lesser importance to know in depth what is going on in this black box as long as a proper way can be found of utilizing residual strains for transforming the original shape into the deformed one.
On a high level this is exactly what the Eigen- or Inherent-Strain (IS) method does. It employs strains that are inherited from the thermo-mechanical process and allows for calculation of the deformed shape. The absolute beauty of the IS method is that it reduces the complex thermo-mechanical simulation to a sole mechanical one, which makes the simulation much faster and more convenient while having a limited trade off in accuracy. The trade off commonly affects stresses that may be predicted less accurately in some cases than they would be with an in-depth thermo-mechanical analysis.
A common question that is raised by manufacturers is: Can this simplified method consider for machine specific settings? Yes, to a certain extend. Machine specific settings can be considered best if they introduce strains that cause the same part deformations independent of the part location on the building plate. This is commonly achieved with well randomized scanning patterns which allow for a more general application of the inherent strains to simulate deformations independent a parts geometry, orientation and location on the building platform.
The remaining question is, how does one obtain the strains for machine specific settings to simulate the AM process via IS method? For this there are several options but one of the most commonly used approach is via calibration procedure. The idea is to perform an AM process to produce a defined geometry, often a cantilever shape, measure the deformations and find the inherent strains that lead to the measured deformations. This needs to be done once for given machine settings and used material.
AdditiveLab software utilizes the Inherent Strain method to allow the user to efficiently predict regions that suffer from localized and global deformations and may lead to build failure or unsuitability of designs for service. Due to the reduction of the complex thermo-mechanical problem into a simple mechanical one, this method can produce realistic predictions within short moments. Furthermore, in AdditiveLab the calibration procedure is fully automated and allows for efficient and accurate estimation of the inherent strains based on measured deformations of manufactured designs. All that the user needs to do is manufacture a test geometry, measure the deformations and provide the measured values to AdditiveLab. After the automatic determination, the user can use the determined inherent strain values to precisely predict their manufacturing outcome under consideration of their machine-specific settings.
Example of a simulation via the Inherent Strain Method in AdditiveLab:
Geometry of a seatmast topper manufactured in Ti6Al4V (courtesy S. Wright, pencerw.com)