Material Differences in Metal Additive Manufacturing

For an established manufacturer, adopting additive manufacturing entails something even more than a fundamental shift in thinking about how to make the part. It also entails different thinking about material.

John Hunter has a perspective on this. He is the general manager of the newly opened U.S. office of LPW Technology, a company dedicated to supplying metal powder engineered for additive manufacturing. Founded in the U.K., LPW works with additive manufacturing machine makers to tailor powders to their machines, and also supplies powder to the end users of this equipment. I spoke with Mr. Hunter at the new U.S. facility in Pittsburgh.

Starting out, he says, shops that adopt additive manufacturing typically buy powder from the machine OEM. This source can provide a particular formulation of an alloy along with proven machine parameters for that formulation. But later, the shop might become comfortable enough at fine-tuning machine parameters that it is willing to try obtaining comparable stock (or even a novel formulation) from a third party.

Experienced with all of the major metal additive machines, LPW can counsel customers on which particle size distribution (PSD), for example, might work better for a particular machine type. Some anecdotes from Mr. Hunter illustrate other material considerations that can determine additive manufacturing’s success, including:

• Purity. He says one customer came to LPW because aluminum alloy parts were cracking. Powder and parameters were supplied by the OEM—the build should have been successful. However, micrographs revealed the culprits. Just a half-dozen particles of Inconel 625 were the impurities from which the cracks propagated. The customer was not cleaning its machine thoroughly enough between jobs.
• Flow. In additive manufacturing, the powder metal has to move. LPW once tested powder from four sources that was all identical in terms of measured specs, but still not identical in performance. At a given energy, only one sample delivered a fully dense part. Thus, some different spec was needed to predict performance. The answer proved to be powder flow rate. Like pharmaceutical firms, LPW now uses rheometers to gage this parameter.
• Change over time. The metal powder changes with use, Hunter notes. Specifically, since smaller grains are consumed in the first few additive builds, the material’s PSD is prone to change.

In 2014, LPW introduced a service called PowderSolve to address the challenge of changing material properties. Users enter key measurements of their own powder samples into online software in order to track changes and predict performance. Thus, instead of guessing when a batch of material has reached its limits, the aim is for users to be able to determine this for certain. That is, the aim is to let the user perhaps employ 90 percent of a batch of powder instead of stopping at only 70 percent. In certain cases, Mr. Hunter says, powder that has gone past the acceptable limits for some applications might even still be useful for applications that have different requirements.