Successful Metal-AM Builds Are No Accident: The Value of In-Process Metrology

By: Zach Murphree

Zach Murphree is vice president of technical partnerships at Velo3D, Campbell, CA;

Wednesday, November 11, 2020

If you can’t measure it, you can’t control it—and controlling as much of the 3D-printing process as possible directly relates to end-product quality. But what exactly should manufacturers measure during a metal-AM build, and how? And in what ways can companies leverage metrology technology to ensure that they produce perfect, in-spec parts?

A series of 3D-printed mission-critical parts for industrial applications, from left to right: cross-section of a small engine, super-critical CO2 impeller, microturbine component and two types of heat exchangers.
Here we’ll discuss three key metrics to help ensure successful metal-AM production: meltpool status, atmospheric composition and powder-bed height.

Monitoring the Meltpool

As metal-AM powder transforms to solid metal in the build chamber, a lot can happen to affect the quality of the base material. Excess laser energy can cause the material to vaporize, while too little energy results in unmelted powder. Both scenarios can lead to porosity and negatively impact the mechanical integrity of the printed parts.

Meltpool monitoring of the active micro-welding area, where the laser meets the powder, can gather data related to the laser beam, the temperature of the liquid metal and even the surface tension of the meltpool. When detecting anomalies outside of control limits, the AM machine can alert the operator to interrupt a build and change settings, or even stop the build completely. Some systems rely on a photodiode for these measurements, while others incorporate mirrors and a high-resolution camera for optimal performance.

Next-generation AM systems go beyond just reporting such data by providing integrated, in-situ process metrology to detect individual meltpool instabilities. They then can enable closed-loop energy adjustments in real time during the build. Then, during post-production, correlating in-system metrology data with part outcomes can help identify probabilities of porosity for any given region in a part.

Analyzing the Atmosphere

Next-generation AM systems can detect variations in layer height and identify any signs of unwanted protrusions; crossing the warning threshold will alert the user.
When manufacturers print metal parts at very high temperatures in a less-than-perfect build-chamber environment, any unwanted atoms or molecules in the atmosphere can become incorporated into the material as it solidifies. Oxygen in the chamber can lead to oxidation, causing pores to occur in the build material. Hydrogen can react with and embrittle the metal. And, humidity of any kind must be monitored, because metal powder acts like a desiccant and will absorb water vapor. Moisture absorbed by the powder bed will be ionized when the laser hits it, leading to other complications in addition to oxidation.

Humidity in the build chamber also causes quality issues when printing at very low angles, where the process window is much smaller than at the middle of a part where printing is technically easier. In these low-angle areas, excess oxygen or humidity can cause instability and affect the mechanical properties of the part.

For these reasons, manufacturers must maintain an extremely pure and controlled atmosphere during the metal-AM process. To accomplish this, metal-AM systems typically replace the build-chamber atmosphere with argon, a nonreactive noble gas that provides an inert atmosphere for the build. These systems constantly monitor humidity and oxygen levels and compare them against control limits, so that the data become meaningful and actionable.

Proper Powder-Bed Height

Layer-height detection technology, visualized here in Velo3D’s Assure software, provides true Z-height quantitative powder-bed and part metrology. Note the sections of parts in process, with red lobes indicating metal protruding >300 m above the powder bed but still below control limits.
Throughout a metal-AM build, it is essential to maintain the proper powder-bed height, and avoid ridges, waves or other variations in the powder bed. The most frequent and challenging problem that manufacturers face: protuberances of solid metal through the powder bed, which can interfere with the recoater arms used to spread the layers of powder.

Many traditional metal-AM setups rely on photographs of the powder bed to conduct qualitative analyses of the build process; next-generation systems offer both a noncontact recoater arm and the use of integrated metrology, ensuring constant and quantitative measurement and monitoring of the topology of the powder bed. In these cases, mapping the layer height not only detects variations in the powder bed, but also can instruct the recoater to lay down the next layer of powder in a way that corrects for a problem that has been detected. The process can be repeated—in closed-loop fashion—until the system indicates that the situation has been corrected.

Layer-height data also can be used to analyze the build behavior of an individual part. Details of a build can be color-coded in the scan for each layer according to how far powder-bed height deviates from the specified nominal value. Blue images, for example, may indicate a minor deviation that may have minimal effect, whereas red images can report areas of greater concern. Manufacturers can use this visualization to track trends in the build process and identify design weaknesses that may require modification.

A Marriage of Data Science and Material Science

Even AM systems that employ relatively simple metrology suites can in some cases alert operators when things go wrong, as sensors can indicate off-nominal measurements. But with these systems, operators usually lack all of the information needed to allow them to go back and determine what caused an inconsistency, at the time it happened. In these cases, a field engineer often must download system logs so they can be sent out and analyzed—an inefficient and time-consuming process.

To ensure print accuracy, many AM systems require a custom instruction file for every machine serial number, even when printing the same part from the same material—a herculean task, particularly when it comes to part qualification. With the advanced metrology and quality-assurance software now available with next-generation metal-AM systems, every machine will produce the same results with the same material, regardless of where the machine is located. Operators gain complete visibility into real-time machine performance and, should an issue arise, they can query system logs for data that help them determine root cause.

Because these state-of-the-art machines make thousands of measurements per build layer, the resulting database consists of millions of data points. Manufacturers can collect and save this vast amount of data, analyze it, and use the confluence of material science and data science to ensure that every part meets the required specs, all the way to the end of the build. Quality control performed in this way—during the manufacturing process—ensures reproducible and repeatable outcomes. 3DMP


See also: Velo3D

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