3 Ways Measuring a Laser Powder Bed Fusion Part Is Unlike Measuring a Part from Another Process

Additive manufacturing is the one industrial part-making process invented after modern quality systems thinking was fully embraced by industry.

This, by itself, makes the challenge of validating an additive part different from validating a part made another way. Every other part-making process had an opportunity early on to validate itself by doing. A robust measurement standard applied to castings, when this first began to happen, had to contend with the fact that cast parts were already in use and had already succeeded enough to earn trust. The measurement standard thus had to accept or concede to what had already been proven. By contrast, additive is the first such process to confront a modern measurement regime from its beginning, without a legacy of use cases behind it. In effect, this leaves additive held to a higher standard.

Ankit Saharan of EOS and I talk about this in the video below, about the challenge or realities of measurement and qualification for laser powder bed fusion (LPBF) additive manufacturing. This video is part of a series in which he and I explore myths and misconceptions around LPBF (also called direct metal laser solidification or DMLS). See other installments from this series in the form of our conversations on the repeatability and the speed of laser powder bed fusion.

From the video above, here are three ways part measurement is different if the part happens to have been made via LPBF:

1. Additive Makes Material and Geometry at the Same Time, and Both Have to Be Measured

To anyone accustomed with additive, particularly LPBF, the point above is obvious.

Still, how different this is. In machining, the metal properties are locked into the bar or billet before any cut is made. Machining at best changes surface properties, otherwise leaves properties untouched. Casting, by contrast, involves a phase change of the metal, but the geometry is defined by the mold. Nothing about the way the material solidifies will change the geometry (unless the mold is not filled).

Measurement in additive manufacturing—and by extension, process control in additive—therefore (among other things) seeks to account for the interplay between external geometry and internal properties, and how thermodynamics and other factors of the build may affect either or both. This can lead to measurement considerations for which other part-making processes do not offer precedent.

2. The Qualification Standard Derives from the Reason for Using Additive

In an intricate metal part made additively, there is potentially too much to measure. Yet not every measurement is meaningful to the application.

Laser powder bed fusion has a range of application niches that is diverse compared to other metalworking processes it competes or coexists with. LPBF might offer substitutes for parts made through other processes when this delivers the lead-time win. Or, it might fill an application by creating a part that could be made no other way.

If parts made by more established processes seem easier to qualify, this is partly because the application windows for those processes are better known and defined. Qualifying the additive part should focus on the use of that part, Saharan says, likely deriving from the reason why additive was chosen in the first place. What performance outcomes are necessary or valuable? The answer to this question should suggest the qualification requirement. There is not, and may never be, a universal way that all production LPBF parts are measured.

3. In Additive, We Are Measuring Too Much

…and we can’t help it. Since additive is the process invented after the maturation of modern industrial measurement systems, it gets subject to all the measurement we can bring to it.

And in most cases, there is not a history sufficient to pare the measurement down. Again, other manufacturing processes have the legacy of use cases before modern measurement systems began.

One specific way this plays out is in material property test coupons made to validate laser powder bed fusion builds. It is easy to plausibly say that industry is overproducing test coupons. However, by how much it is overproducing them is impossible to say. With much more confidence making LPBF production parts, it will become clearer when the test coupons are wasted metal.

All of this is ultimately to additive’s benefit. “It’s good that additive is held to such high standards because it helps ensure that we don’t leave any box unchecked,” Saharan says. The overmeasurement (if that is what it is) presents LPBF and additive in general with high bars or a multiplicity of bars to clear. This is a challenge today, but it will make additive that much stronger as it clears them.

Ankit and I discuss all of this in the video: