Additive Manufacturing Simulation - Build Failure Risk Assessment.
Updated: Feb 15, 2019
The workflow for Additive Manufacturing (AM) simulations can be quite simplified. In AdditiveLab for example, AM simulations of part-support configurations can be performed with only a couple of clicks. However, having generated simulation results often is not enough to determine whether a build configuration will fail.
Traditional Engineering Assessments vs. Additive Manufacturing Process Assessments
In traditional structural engineering, simulation-based assessments of potential failures are commonly done based on defined and standardized strength assessment strategies. For example, strength evaluations guidelines like the FKM and the Eurocode have been successfully developed and applied in structural engineering for decades. To determine whether structures fail during service, simulation results are compared to a defined criteria (e.g. maximum stresses); if the simulated values are below defined criteria, the structure will be able to bare the presented loads. (Of course, the details of such assessment strategies are more complicated but to get the gist of it, one can break it down to the above).
On the contrary, assessing whether structures will fail during the AM process is a much more tedious task. Firstly, there are no standardized assessment strategies as there are for structural engineering disciplines, and secondly, additional factors such as machine-part interaction (e.g. re-coater-structure), changing material states (e.g. powder-liquid-solid) and post-processing (e.g. heat treatment) make it much harder to perform strength evaluations and subsequently develop failure assessments. Therefore, in additive manufacturing engineers have to develop their own simulation-based strategies and apply their best engineering knowledge to determine whether the build process could potentially fail.
Simulation Challenges For Additive Manufacturing Assessments
The theoretical and best utilization of simulation to aid in build risk failure assessment would be to perform micro-scale resolution computations for entire build configuration under consideration of all existing non-linear effects. A micro-scale resolution means that the model discretization (e.g. via mesh) needs to be very fine so that the melt pool and its subsequent effects during the building process can be correctly simulated everywhere for the entire build platform. Then, nonlinear effects, such as material property and state changes, geometrical non-linearities need to be considered. Such simulation approaches would allow the user for very detailed insight into the process and enable them to assess their chances of failure during the AM process in depth.
However, due to current computational limitations, it is not possible to apply such simulations, particularly not in an industrial setting. Therefore simplified simulation approaches such as the Inherent Strain Method and the Multi-Volume Method have been introduced to allow for faster simulation. These simulations can provide the user with the following results to give them more insight into the manufacturing process:
Stresses and Strains
(Depending on the type of simulation there are additional results that can be obtained, but for the sake of simplicity the above are mentioned).
AdditiveLab Assessment Strategy
These results can be utilized, and with some AM experience and engineering-judgment used to determine critical areas that may lead to build failure or to failure of parts during service. Below I have summarized strategies that we at AdditiveLab apply and recommend to our users to evaluate build configurations based on AM process simulation results.
The deformations occurring during and after the AM process are the easiest results to assess the risk of build failure and compromised part qualities. Mostly because these results are easy to relate to and engineers can see and measure deformations on the manufactured parts as well.
Simulated deformations can be used to assess the risks of the following:
Final part quality (geometrical tolerances)
Part-support and building plate-support interface rupture
The picture below shows the deformations of a build configuration of a hip-stem implant. The color plot indicates the deformations (scaled x10) that are present after the manufacturing process. The red areas indicate zones that experience large deformations and may cause issues during the printing process.
Stresses and strains
The stresses and strains occurring during and after the AM process are good indicators for strength and rupture related failures that may cause damages during the printing and during the service of a manufactured part. Stresses and strains are a little harder to understand for inexperienced engineers because efficiently measuring them is often not feasible. Furthermore, a proper stress assessment relies on a suitable simulation mesh to resolve stress gradients and allow for better assessment.
Stresses and strains help to assess the risks of the following:
Part-support and building plate-support interface rupture
Regions of elevated risk for failure during service
Guidance for strength assessment
The picture below shows the stresses of a build configuration of a seatmast-topper. The color plot indicates the stresses that are present after the manufacturing process. The red areas indicate zones that experience localized stress concentrations and may cause issues during the printing process; the high stress concentrations occur near the part-support interface increasing the risk of interface rupture:
Thermal histories can provide valuable information for the AM process on different scales. For example, simulating the laser scanning trajectories on a macro-scale model or for entire parts via layer-by-layer strategy.
Thermal histories can be utilized to assess the risks of the following:
Regions that suffer from heat removal
Lack of support structures to enable heat transfer
Occurrence of regions with high stresses
Certainties about failure assessment
One thing that needs to be addressed is certainty and the exactness of the simulation-based build failure assessment. In our experience, deformations can be very well predicted and used as valuable and reliable information to determine build failure. Nevertheless, it is hard to say exactly at what deformation a build may fail during the manufacturing process. The same applies in a similar fashion to stresses; localized stresses and strains can be predicted well and used to compare it to material strength parameters to get an idea of whether rupture or de-lamination may occur. For stresses, engineers have to keep in mind that the simulation mesh needs to be able to capture localized stress concentrations to allow for proper stress and strain predictions and subsequent assessments.
These are a few examples of how we at AdditiveLab use simulation results to assess entire build configurations. If you are interested in having your design evaluated by us, or having trainings to help you increase your knowledge in simulation-based assessment of build configurations, please get in touch with us.