Texas A&M publishes research on cooling properties for non-porous AM parts

September 30, 2021

New method developed by Texas A&M researchers optimises alloy properties and process parameters to create superior metal AM parts. Shown here is a colorised electron micrograph of a nickel powder alloy used in the study (Courtesy of Raiyan Seede)

A new study by Texas A&M University researchers, and published in the journal Additive Manufacturing, has further refined the process of creating superior metal parts using Laser Beam Powder Bed Fusion (PBF-LB) Additive Manufacturing techniques. By using a combination of machine learning and single-track AM experiments, researchers have identified the favourable alloy chemistries and process parameters, like laser speed and power, needed to additively manufacture 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,” stated 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.”

Alloy metal powders used for AM can be quite diverse, containing a mixture of metals – nickel, aluminium, magnesium, etc. – at different concentrations. Since the individual metals in the alloy powder have very different cooling properties and consequently solidify at different rates, this mismatch in cooling times can create a type of microscopic flaw called microsegregation.

“When the alloy powder cools, the individual metals can precipitate out,” Seede added. “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.”

He said this defect appears as tiny pockets containing a slightly different concentration of the metal ingredients throughout the additively manufactured part. These inconsistencies compromise the mechanical properties of the object.

To rectify this microdefect, 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-base alloy. Hence, from detailed phase diagrams, they 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 for different laser settings and determined the process parameters that would yield porosity-free parts. Then, they combined the information gathered from the phase diagrams with that from the single-track experiments to get a consolidated view of the laser settings and nickel alloy compositions that would yield a porosity-free additively manufactured part without microsegregation.

A scanning electron microscope image of a single laser scan cross-section of a nickel and zinc alloy. Here, dark, nickel-rich phases interleave lighter phases with uniform microstructure. A pore can also be observed in the melt pool structure (Courtesy of Raiyan Seede)

Last, the researchers went a step further and trained machine-learning models to identify patterns in their single-track experiment data and phase diagrams to develop an equation for microsegregation applicable to any other alloy. Seede said the 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,” stated Dr Ibrahim Karaman, Chevron Professor I and head of the materials science and engineering department. “This work will be of great benefit to the aerospace, automotive and defence industries that are constantly looking for better ways to build custom metal parts.”

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

The research is supported by the United States Army Research Office and the National Science Foundation.

www.tamu.edu

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