Researchers at Caltech have developed a novel approach to metallurgy that enables precise control over both the chemical composition and microstructure of metallic materials, significantly enhancing their mechanical resilience. Traditionally, however, metallurgy has followed a very different path. “If you look at how metallurgy has been done for centuries, in broad strokes, you nearly always start with a raw ore, which is then thermally and/or chemically treated and refined, to produce the desired metal or alloy,” materials science professor Julia R. Greer said. “And basically, the mechanical properties of the metals produced this way are limited.”
The new approach challenges previous methods by integrating advanced 3D printing techniques into the process. It builds on work Greer led in 2018, in which her team used additive manufacturing to construct intricate metal microstructures. The process, known as hydrogel-infusion additive manufacturing (HIAM), is described in their paper as “a microscale printing technique distinguished by its forming of parts throughout phase transformations and crystal growth.” Now, the researchers have taken HIAM a step further by learning to infuse more than one metal at a time. This advancement allows them to create copper–nickel alloys with custom-tailored ratios of each element—variations that lead to significant changes in the material’s mechanical properties.
The process for making copper-nickel alloys using HIAM
How Does HIAM Work?
The HIAM process stands out for its complexity and precision. It begins with 3D printing an organic hydrogel to form a soft, gel-like scaffold. To do this, the researchers used a technique called digital light processing. Once the scaffold is printed, a liquid solution of metallic salts is applied, allowing metal ions to infuse into the structure. The next step is calcination: the printed object is burned in the presence of oxygen, which burns away all organic material and leaves metal oxides behind. Following this, the structure undergoes reductive annealing—a process in which it is exposed to high temperatures in a hydrogen-rich environment. This causes most of the oxygen to diffuse out of the solid, reacting with hydrogen to form water vapor. What effect does this complex phase evolution have? These carefully controlled thermal steps result in the formation of intricate microstructures within the 3D-printed, metal-infused gels. In the end, HIAM produces a metallic structure of the intended shape—now an alloy composed of the two infused metals.
Once fabricated, the scientists can analyze the microstructure of the alloy, including the orientation of individual crystal grains and the boundaries between them. They can also mechanically test the material, yielding insights into how the HIAM process influences performance. “This lays the groundwork for thinking about 3D-printed alloy design in a unique way from other microscale additive manufacturing techniques,” one of the researchers, Rebecca Gallivan, said. “We see that the processing environment leads to very different microstructures in comparison to other methods.”
A graphic visualisation of HIAM.
Analysis with a Transmission Electron Microscope
The Caltech researchers used a transmission electron microscope (TEM) to reveal that alloys produced using the HIAM method form more homogeneously, exhibiting higher degrees of symmetry throughout their crystal structures. According to lead author Thomas T. Tran, the shape, size, and orientation of metal grains are influenced by the transition from oxide to metal during reductive annealing. As temperatures rise, water vapor escapes, creating pores that slow grain growth. The growth is also affected by the specific types of oxides present in the metals.
Another notable finding from the study challenges conventional thinking. It shows that the strength of alloys created through HIAM depends not only on the size of the grains, as previously believed, but also on their chemical composition. Furthermore, the process leaves behind tiny oxide inclusions that contribute to the alloys’ exceptional strength. “Because of the complex ways in which metal is formed during this process, we find nanoscale structures rich with metal–oxide interfaces that contribute to the hardening of our alloys by up to a factor of four,” Tran said.
The research was supported by the US Department of Energy’s Basic Energy Sciences program and by a National Science Foundation graduate fellowship. To learn more about the research, find the Caltech study, titled “Multiscale Microstructural and Mechanical Characterization of Cu–Ni Binary Alloys Reduced During Hydrogel Infusion-Based Additive Manufacturing (HIAM),” here.
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*All Photo Credits: Thomas Tran/Caltech