Removing the flaws from 3D printing metal alloys

An electron micrograph of one of the nickel powder alloys used in the study. Image: Raiyan Seede/Texas A&M Engineering.
An electron micrograph of one of the nickel powder alloys used in the study. Image: Raiyan Seede/Texas A&M Engineering.

In the past few decades, metal 3D printing has spearheaded efforts to create custom parts with intricate shapes and high functionality. But as additive manufacturers have utilized more metal alloys for their 3D printing needs, so they have faced challenges in creating uniform, defect-free parts.

Now, in a new study, researchers at Texas A&M University have created superior metal parts by refining a 3D-printing method called laser powder bed fusion. Using a combination of machine learning and single-track 3D printing experiments, they were able to identify the favorable alloy chemistries and process parameters, like laser speed and power, required to print parts with uniform properties at the microscale.

“Our original challenge was making sure there are no pores in the printed parts because that's the obvious killer for creating objects with enhanced mechanical properties,” said Raiyan Seede, doctoral student in the Department of Materials Science and Engineering. “But having addressed that challenge in our previous work, in this study, we take deep dives into fine-tuning the microstructure of alloys so that there is more control over the properties of the final printed object at a much finer scale than before.” The researchers report their findings in a paper in Additive Manufacturing.

Like other 3D-printing methods, laser powder bed fusion builds 3D metal parts layer-by-layer. The process starts with rolling a thin layer of metal powder on a base plate and then melting the powder by scanning a laser beam along tracks that trace the cross-sectional design of the intended part. Then another layer of the powder is applied and the process is repeated, gradually building up the final part.

Alloy metal powders used for additive manufacturing can be quite diverse, containing a mixture of metals, such as nickel, aluminum and magnesium, at different concentrations. During printing, these powders cool rapidly after being heated by the laser beam. Since the individual metals in the alloy powder have very different cooling properties and solidify at different rates, this can create a type of microscopic flaw called microsegregation.

“When the alloy powder cools, the individual metals can precipitate out,” explained Seede. “Imagine pouring salt in water. It dissolves right away when the amount of salt is small, but as you pour more salt, the excess salt particles that do not dissolve start precipitating out as crystals. In essence, that’s what is happening in our metal alloys when they cool quickly after printing.” This can produce tiny pockets with slightly different concentrations of the metal ingredients than other regions of the printed part, compromising its mechanical properties.

To try to prevent this happening, the research team investigated the solidification of four alloys containing nickel and one other metal ingredient. In particular, for each of these alloys, they studied the physical states or phases present at different temperatures for increasing concentrations of the other metal in the nickel-based alloy. This produced detailed phase diagrams, from which the researchers could determine the chemical composition of the alloy that would lead to minimum microsegregation during additive manufacturing.

Next, they melted a single track of the alloy metal powder at different laser settings and determined the process parameters that would yield porosity-free parts. They then combined the information gathered from the phase diagrams with the results from the single-track experiments to get a consolidated view of the laser settings and nickel alloy compositions that would yield a porosity-free printed part without microsegregation.

Lastly, they went a step further and trained machine-learning models to identify patterns in their single-track experiment data and phase diagrams, in order to develop an equation for microsegregation applicable to any other alloy. According to Seede, this equation is designed to predict the extent of segregation given the solidification range, material properties, and laser power and speed.

“Our methodology eases the successful use of alloys of different compositions for additive manufacturing without the concern of introducing defects, even at the microscale,” said Ibrahim Karaman, professor and head of the Department of Materials Science and Engineering. “This work will be of great benefit to the aerospace, automotive and defense industries that are constantly looking for better ways to build custom metal parts.”

Research collaborators Raymundo Arroyavé and Alaa Elwany added that the uniqueness of their methodology is its simplicity, which means the methodology can easily be adapted by industries to build sturdy, defect-free parts with an alloy of choice. They noted that their approach contrasts with prior efforts that have primarily relied on expensive, time-consuming experiments for optimizing processing conditions.

This story is adapted from material from Texas A&M University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.