By Liz Nickels

Some SLM applications require parts that are very dense, with less than 1% porosity, as the pores or voids are the weakest part of the material and most likely would result in failure. However, building functional parts and components to specific standards and performance specifications can be challenging because a large number of parameters must be set appropriately. Some of these key parameters include laser power, laser speed, distance between laser scan lines, scanning strategy and powder layer thickness. As a result, there is a need for a reliable and cost effective approach to determine the right parameters to develop parts with the desired properties.

LLNL researchers have developed an approach, based on computational simulations and experiments, to identify optimal parameters to print 3D high-density metal parts. The simulations are used to compute the dimensions of the melt pool, which is the pool of liquid formed when the laser melts the metal powder particles.

"We mine the simulation output to identify important SLM parameters and their values such that the resulting melt pools are just deep enough to melt through the powder into the substrate below," said Chandrika Kamath, an LLNL researcher who is the lead author of the article. "By using the simulations to guide a small number of single-track experiments, we can quickly arrive at parameter values that will likely result in high-density parts."

Finding the right parameters

Kamath and her colleagues use simulations at various scales to gain insight into the SLM process.

"We found that the metal density reduces if the speed is too low, due to voids created as a result of keyhole mode laser melting, where the laser drills into the material," Kamath said in a paper which covers the research. "At the same time, too high a speed results in insufficient melting. The key is to find the right parameters where the melting is just enough."

The LLNL team found that the use of different powders affected densities at lower power, but not at higher power.

"Furthermore, for 316L stainless steel, at higher powers, the density is high over a wider range of scan speeds, unlike at lower powers," the paper said. "This would indicate that higher powers could provide greater flexibility in choosing process parameters that optimize various properties of a manufactured part."

Other powders

Although 316L stainless steel was used in this experiment, Kamath said the team's approach can be applied to other metal powders as well.

The paper, entitled "Density of additively-manufactured, 316L SS parts using laser powder-bed fusion at powers up to 400W" was recently published in the International Journal of Advanced Manufacturing Technology.