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Selective laser melting 3D prints an Al alloy with superior performance

By Lauren Borja June 26, 2018
a-b-selective-laser-melting
Backscatter scanning electron microscope micrographs showing the Al-3.60Mg-1.18Zr wt.% alloy. (a) The cross section of as-fabricated created with the highest laser energy density of 247 J/mm3. Build direction is from bottom to top. Coarse, elongated grains are evident in the top half of the image and fine, equiaxed grains in the bottom half. Cuboidal Al3Zr precipitates (white) are located in the center of most fine grains, but none are present in the elongated coarser grains. Oxide particles (dark) are present throughout. (b) A cuboidal, sub-micrometer Al3Zr precipitate (white) in the center of a micrometer-size grain which also contains sub-200 nm oxide particles (black rounded particles, labeled “o”). Grain boundaries are darker, using Z-contrast, consistent with Mg and/or nanosize oxide enrichment. Credit: Elsevier Science and Technology Journals

A research team has demonstrated a new high-strength, heat-tolerant, ductile aluminum alloy that can be made using selective laser melting (SLM), a three-dimensional (3D) printing additive manufacturing technique. This work was a collaborative effort between NanoAl, a US company specializing in high-performance aluminum alloys; Empa, the Swiss Federal Laboratories for Materials Science and Technology; and Northwestern University. As reported in a recent issue of Acta Materialia, the 3D-printed alloy could be used to create damage- and corrosion-resistant components.

Additive manufacturing methods are lucrative to companies since they offer flexibility during the design process because different prototypes can be explored and produced in small quantities. Selective laser melting is a method to form metal alloys where a high-powered laser 3D prints a structure from a powder of constituent materials. Because the metal alloy cools rapidly once formed after interacting with the laser, an SLM-produced alloy often has a markedly different microstructure and consequently different properties than one produced using conventional processes.

“The alloy in this study was designed specifically for this type of process and takes advantage of the rapid solidification inherent to SLM,” says Joseph Croteau, a materials engineer at NanoAl and first author of the recent publication. Two bulk aluminum-magnesium alloys with varying amounts of zirconium, Al-3.60Mg-1.18Zr and Al-3.66Mg-1.5Zr (wt.%), were consolidated from powders using SLM. The zirconium bonded with the aluminum during the melting to form nano- and microsized precipitates and influenced the growth of the grains in the microstructure of the alloy. “This prevented the formation of cracks and yielded [an alloy] with high tensile strength and ductility,” says Nesma Aboulkhair, a research fellow at the Centre for Additive Manufacturing at the University of Nottingham. Aboulkhair is not affiliated with the research published in Acta Materialia.

The hardness of the alloys were further improved by aging, a simple post-processing treatment where the alloy is gently heated to mimic aging over time. Aging relieves the internal stress built up by the rapid cooling during SLM while preserving the distinct microstructure. The two alloys were heated at 400°C for as long as 144 hours; peak hardness was achieved for both alloys after about 10 hours of heating. Here again, the presence of aluminum-zirconium precipitates inside the grains of the alloy was beneficial. “The trapped zirconium forms nanoscale precipitates [during aging],” Croteau says, “which are very effective [for] strengthening the alloy.”

Aluminum alloys can be hardened by incorporating scandium, but its exorbitant cost stands in the way of its widespread use. In comparison, the price of zirconium is a small fraction of that of scandium; this is an important consideration when using an expensive additive manufacturing technique like SLM.

The performance of these aluminum-magnesium-zirconium alloys could make them beneficial for many different applications. Conventionally manufactured aluminum-magnesium alloys are already used for their strength and corrosion-resistance in high-pressure vessels and in marine applications, but they are not suitable for high temperatures. This new Al-Mg-Zr alloy could be used for applications that require these characteristics and thermal stability, such as heat exchangers. “We are continuing to investigate the properties of these alloys under extreme conditions, such as high temperatures and dynamic loads,” Croteau says.

“The approach in this study is very relevant to one of the growing aspects of SLM: developing new alloys specifically tailored for the technology,” Aboulkhair says. By understanding the unique microstructure created by the SLM process, scientists can target alloy compositions beyond those used in conventional manufacturing to create new materials with novel properties. Croteau and his colleagues hope to use electron and x-ray imaging to observe the formation of the microstructure during SLM in real time. “This will provide fundamental insights into the underlying mechanisms, and will support new and highly focused strategies of multiscale alloy design,” Croteau says.  

Read the abstract in Acta Materialia.