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Aimed at minimising trial-and-error and optimising parameters for individual prints, the process was developed by researchers at the Fraunhofer Institute for Mechanics of Materials in Freiburg, Germany, and is focused on laser powder bed fusion (LPBF).
Additive manufacturing using the LPBF process “offers a great number of advantages”, the researchers said. “It is economical, precise and allows for customised solutions.”
In LPBF, a powder bed up to 50 microns thick is heated with pinpoint accuracy by a laser. The powder liquefies, the particles fuse and the melt pool solidifies as soon as the laser moves on. In areas where the laser beam does not touch the powder, no fusion occurs. This process is repeated many times, causing the component to grow layer by layer.
Finished components need 100% density with no pores, the Fraunhofer researchers said, with each layer firmly bound to the one below. This is achieved by adjusting the process parameters, such as the scan speed and power of the laser, with the resulting microstructure having a significant impact on the mechanical properties of the finished piece.
It can be difficult to determine the optimal process parameters, however, so the Fraunhofer team simulated the entire process at the microstructure level.
“Because the laser powder bed fusion process is becoming increasingly complex due to new materials and requirements, we have decided to simulate the entire process chain,” said team leader Dr Claas Bierwisch. “This enables us not only to minimise trial-and-error cycles, but also to quickly and effectively evaluate variations in the overall process and eliminate undesirable effects during manufacturing.”
First, the discrete element method simulates how the individual powder particles are spread in the building chamber. Next, the way in which the powder particles melt is simulated using the smoothed particle hydrodynamics method. Both the laser interaction and heat conduction are calculated, as well as the surface tension that causes the melt to flow. The calculation also accounts for gravity and the recoil pressure that happens when the material vaporises.
The simulation also needs to describe the microstructure of the material to predict its mechanical 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,” said Dr Bierwisch.
Temperatures can reach up to 3,000ºC where the laser meets the powder, but the material is cool only a few millimetres away. The laser can also move over the powder bed at several metres per second, so the material heats up extremely quickly before cooling down again within milliseconds, all of which affects how the microstructure is formed.
The final step is finite element simulation, which performs tensile tests in different directions to find out how the material reacts to loads.
“In the simulation, we can watch what happens in real time,” said Dr Bierwisch. “The quality of this is completely different to what is possible in an experiment… you can detect interrelationships in an almost investigative way.”
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.