Additive manufacturing (AM) is revolutionizing manufacturing, allowing complex parts to be constructed that are not readily fabricated by traditional techniques. Although there has been significant interest and investment in AM, the fraction of this investment devoted to modeling and simulation is relatively small and is not focused on developing high-fidelity predictive models but instead on reduced-order models for industry use. The Exascale Additive Manufacturing project (ExaAM) represents a unique opportunity to use exascale simulation to enable the design of AM components with location-specific properties and the acceleration of performance certification.
ExaAM aims to develop the Integrated Platform for Additive Manufacturing Simulation (IPAMS), a suite of exascale-optimized capabilities that directly incorporate microstructure evolution and the effects of microstructure within AM process simulation. In AM, a geometric description of the part is processed into 2D slices. A feedstock material is melted, and the part is built layer by layer. In metal AM, the feedstock is often in wire or powder form, and the energy source is a laser or electron beam. ExaAM focuses on powder bed processes in which each layer is approximately 50 µm. For example, a part that is 1 cm tall would require 200 layers, each requiring the spreading of new feedstock powder and one or more passes of the laser or electron beam to sinter and/or melt the powder in appropriate locations.
The physical processes involved in AM are similar to those of welding—a field with decades’ worth of experimental, modeling, simulation, and characterization research. Although calibrated and approaching predictive capability, the simulation tools developed for welding and other similar processes are unfortunately inadequate for AM processes, as demonstrated by the inability to predict the failure rate for new AM parts, which can be as high as 80%. This is believed to be largely because the process-structure-property-performance relationship is traditionally modeled in an uncoupled manner, relying on tabular databases that cannot adequately capture the implicit, dynamic, nonequilibrium nature of AM processes.
One goal of ExaAM is to remove those limitations by integrating high-fidelity mesoscale simulations within continuum process simulations to determine the microstructure and properties by using local conditions. Sub-mesoscale physics is upscaled from detailed science simulations. Typically, thermomechanical finite element models are employed at the macroscopic part scale; finite volume or finite element models are used at millimeter scales for fluid dynamics and heat transfer to capture the melt pool dynamics and solidification; mesoscale approaches (e.g., discrete elements, cellular automata, kinetic Monte Carlo, or phase-field models) are used at the micron scale to simulate melting, solidification, and microstructure formation; and polycrystal plasticity models are used to develop the microscale mechanical property relationships.
The ExaAM IPAMS suite is a collection of simulation capabilities for performing process-aware performance modeling of additively manufactured parts by using locally accurate properties predicted from microstructures that develop based on local processing conditions. ExaAM will demonstrate this capability by simulating the complex bridge structure developed for the 2018 National Institute of Standards and Technology AM-Bench Conference, known as AMB2018-01. The simulation will be performed where experimental observations were taken (e.g., “cut locations” for transverse and longitudinal scanning electron microscope [SEM] specimens for microstructure images).
This demonstration challenge problem will be instantiated on the exascale computer as a workflow involving four stages. Initially, a coarse-scale simulation of the AMB2018-01 build is performed via Diablo to determine the thermomechanical state everywhere in the build. These simulations are validated with x-ray measurement of residual stress. Next, a continuum powder layer melt-refreeze model, TruchasPBF, is used to simulate the laser scan pattern for several layers within this thermomechanical state. The resulting thermal histories are used to drive simulations of the development of microstructure via ExaCA. These microstructures are unique to the AM process and require careful validation with the SEM observations. The location-specific properties of these unique AM microstructures are calculated by using a finite element model, ExaConstit, which resolves the grain structure and uses a crystal model for the behavior of the individual grains. In this way, a process-structure-property correlation is developed between the AM processing conditions and the desired location-specific properties.