Researchers at the Fraunhofer Institute for Mechanics of Materials (IWM), Freiburg, Germany, have developed a method for simulating the Laser Beam Powder Bed Fusion (PBF-LB) Additive Manufacturing process at the microstructure level, in order to identify direct correlations between the workpiece properties and the selected process parameters. The method is the result of combining a number of different simulation methods, and will enable users to identify optimal process parameters, such as the scan speed or power of the laser.
The microstructure of metallic grains is particularly important for the components’ mechanical properties: these have certain orientations, sizes and shapes which have an impact on important mechanical properties. It can, however, be difficult to determine optimal process parameters during the PBF-LB process, especially when components are made of a mixture of materials which form different microstructures.
Using the discrete element method, researchers from Fraunhofer IWM first simulated how the individual powder particles are spread in the building chamber with the aid of a special tool, namely the doctor blade. Next, the way in which the powder particles melt was simulated using the smoothed particle hydrodynamics method – both the laser interaction and heat conduction were calculated, as well as the surface tension that causes the melt to flow (the calculation also accounted for gravity and the recoil pressure that occurs when the material vaporises).
The simulation must also describe the microstructure of the material in order to predict mechanical material properties.
“To analyse this microstructure, we have incorporated another simulation method, known as cellular automaton. This describes how the metallic grains grow as a function of the temperature gradient,” stated Dr Claas Bierwisch, team leader at Fraunhofer IWM.
This is because temperatures can reach up to 3,000ºC where the laser meets the powder, while material only a few millimetres away will be cool. Furthermore, the laser moves over the powder bed at a speed of up to several meters per second. As a result, the material heats up extremely quickly, then cools again within milliseconds. All of this has an impact on how the microstructure is formed.
The final step is the finite element simulation. The research team uses this to perform tensile tests in different directions on a representative volume element of the material in order to find out how the material reacts to these loads.
“In the experiment, we can only study the final result, whereas in the simulation, we can watch what happens in real time – In other words, we create a process-structure-property relationship,” added Dr Bierwisch. “For example, if we increase the laser power, the microstructure changes. This, in turn, significantly affects the yield stress of the material. The quality of this is completely different to what is possible in an experiment. You can detect interrelationships in an almost investigative way.”