Tweaking alloy microchemistry for flawless metal 3D printing

Date:
September 28, 2021
Source:
Texas A&M University
Summary:
In the last few decades, metal 3D printing has spearheaded the
efforts in creating custom parts of intricate shapes and high
functionality. But as additive manufacturers have included more
alloys for their 3D printing needs, so have the challenges in
creating uniform, defect-free parts.



FULL STORY ==========================================================================
In the last few decades, metal 3D printing has spearheaded the efforts
in creating custom parts of intricate shapes and high functionality. But
as additive manufacturers have included more alloys for their 3D printing needs, so have the challenges in creating uniform, defect-free parts.


==========================================================================
A new study by Texas A&M University researchers has further refined the
process of creating superior metal parts using laser powder bed fusion
3D printing techniques. By using a combination of machine learning and single-track 3D printing experiments, they have identified the favorable
alloy chemistries and process parameters, like laser speed and power,
needed 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 have published their
findings in the journal Additive Manufacturing.

Like other 3D-printing methods, laser powder bed fusion also 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 with 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 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 a laser beam. Since the individual metals in the
alloy powder have very different cooling properties and consequently
solidify at different rates, this mismatch can create a type of
microscopic flaw called microsegregation.



========================================================================== "When the alloy powder cools, the individual metals can precipitate out,"
said 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." He said this defect appears as tiny pockets containing a slightly different concentration of the metal ingredients
than other regions of the printed part.

These inconsistencies compromise the mechanical properties of the
printed 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-based 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 printed part without microsegregation.

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," said 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 defense industries that are constantly looking for
better ways to build custom metal parts." Research collaborators,
Dr. Raymundo Arroyave' 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 optimizing processing conditions.

Arroyave' is a professor in the materials science and engineering
departments and Elwany is an associate professor in the Wm Michael Barnes
'64 Department of Industrial and Systems Engineering. Other contributors
to this research include Austin Whitt and William Trehern from the
materials science and engineering department and Jiahui Ye from the
industrial and systems engineering department.

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

========================================================================== Story Source: Materials provided by Texas_A&M_University. Original
written by Vandana Suresh.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Raiyan Seede, Jiahui Ye, Austin Whitt, William Trehern, Alaa Elwany,
Raymundo Arroyave, Ibrahim Karaman. Effect of composition and
phase diagram features on printability and microstructure in laser
powder bed fusion: Development and comparison of processing maps
across alloy systems. Additive Manufacturing, 2021; 47: 102258 DOI:
10.1016/ j.addma.2021.102258 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210928112446.htm

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