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Manufacturing Aluminum Alloys With 3D Printing

August 10, 2020

The trend toward replacing or complementing subtractive manufacturing with additive manufacturing (AM) processes shows no signs of letting up. Example: the growing interest in using 3D printing in metalworking, where flooding the metal part with coolant leading to metal removal is a commonly used subtractive process.

Fig. 1—3D printing of Type 6000 and 7000 aluminum alloys has been achieved by the addition of hydrogen stabilized zirconium particles to the powdered aluminum alloy prior to processing. Figure courtesy of HRL Laboratories, LLC.
In contrast, 3D printing systematically builds metal components by adding metal layers. “Building a metal structure layer by layer, using an electronic beam to melt alloy powders, is analogous to welding,” says Hunter Martin, technical staff member at HRL Laboratories, LLC, in Malibu, CA.

However, that’s not to say that alloy powders, especially aluminum alloys, are necessarily amenable to welding. Finding a procedure to 3D print aluminum alloys represents a good demonstration platform for other non-weldable alloys.

“Aluminum alloys offer the worst-case scenario for 3D printing because they exhibit a large solidification range that is difficult to weld,” Martin says. “We consider Type 6061 aluminum to be the most difficult alloy to weld, with 7075 a close second.

“In attempting to weld one Type 7075 aluminum plate to another,” he continues, “large cracks form between the materials, leading to microstructures with columnar grains and cracks present in successive print layers. The resulting metal suffers from inferior physical and mechanical properties related to strength, fatigue life and fracture toughness.”

The microstructure of these materials contains dendritic structures similar to snowflakes. These structures form due to a breakdown at the solid-liquid interface.

The challenges described by Martin recently drove research and development of an approach to 3D print with 6000 and 7000-series aluminum that lead to less material shrinkage, with the expectation that this will apply to other alloys.

Nanoparticle Grain Refiners

Martin and his colleagues successfully 3D printed types 6061 and 7075 aluminum by adding nanoparticle grain refiners to the metal powders prior to processing. Fig. 1 shows an image of an aluminum alloy powder mixed with nanoparticles being readied for 3D printing.

“We found,” Martin says, “that adding the nanoparticle grain refiners facilitates the nucleation of new aluminum grains. The result: formation of a fine equiaxial grain structure that minimizes the formation of columnar grains and cracks. In effect, we used a 70-year-old nucleation theory to solve a 100-year-old problem.”

For More on This Aluminum Alloy Research…

The researchers developed a database to determine the best nanoparticle alloy to combine with the aluminum alloys. More than 4500 powder-nanoparticle combinations were analyzed before the researchers found that hydrogen-stabilized zirconium particles represented the best fit because they result in the formation of a favorable Al3Zr nucleation phase.

“We used the database to find the right type of nanoparticle grain refiner to induce epiaxial nucleation that helps all of the atoms to line up in the same space lattice with the appropriate crystal structure,” Martin says.

Aluminum alloys were prepared by adding pre-alloyed spherical aluminum powders with an average powder size of 45 microns, coated with 1 percent by volume of hydrogen-stabilized zirconium nanoparticles. AM was accomplished using selective laser melting.

Using microstructure analysis and physical properties testing, nanoparticle-functionalized aluminum alloys were evaluated. Analysis revealed crack-free microstructures with grain sizes estimated to be 5 microns. This represents a 100-times reduction in grain sizes compared to unmodified aluminum powder.

Tensile testing indicated that the nanoparticle-functionalized alloys displayed properties consistent with wrought alloys prepared by conventional techniques. For Type 7075 aluminum, the ultimate strength difference and yield strength were not within acceptable ranges.

“Compositional analysis of the nanoparticle-functionalized alloys showed that zinc and magnesium were lost in the 3D-printing process due to evaporation,” Martin says. “We will need to add these two metals back into the alloy but have not yet optimized the parameters for 3D printing.”

Future work includes optimization of this process and examination of other difficult-to-weld alloys. 3DMP

Industry-Related Terms: Alloys, Aluminum Alloy, Form, LASER, Layer, Plate, Wrought
View Glossary of Metalforming Terms


See also: Society of Tribologists and Lubrication Engineers (STLE)



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