Tailoring Microstructure and Corrosion Performance of In-Situ Alloyed Ti-5Cu-1Si Alloy via Laser Powder Bed Fusion: Impact of Volumetric Energy Density

Titanium is a metal that engineers and doctors love for its strength and its ability to live peacefully inside the human body. It is the material of choice for hip replacements and dental implants because it resists the body's fluids and does not rust easily. However, making titanium parts, especially those with complex shapes for individual patients, is often difficult and expensive using traditional methods like casting or cutting. In recent years, a technique called laser powder bed fusion has emerged as a powerful alternative. This process works like a high-precision 3D printer, using a focused laser beam to melt tiny particles of metal powder layer by layer, building a solid object from the bottom up. While this method offers incredible design freedom, the rapid heating and cooling involved can leave the metal with a microscopic structure that is not perfectly uniform, sometimes making it weaker or more prone to corrosion than desired.

Researchers have been exploring ways to improve these printed metals by mixing different elements directly into the powder before printing. One promising combination involves adding copper and silicon to titanium. Copper is known for its ability to fight bacteria, which is a major benefit for medical implants, while silicon can help strengthen the metal. The challenge lies in getting these different elements to mix perfectly during the printing process. If they do not blend well, tiny pockets of unmixed material can form, creating weak spots where corrosion might start. The key to solving this puzzle is understanding how the energy delivered by the laser affects the final structure of the metal.

In a recent study, a team of researchers investigated how changing the amount of energy used during the printing process influences the quality of a new titanium alloy containing copper and silicon. They focused on a specific measure called volumetric energy density, which essentially describes how much laser power is applied to a given volume of material. By printing samples of this new alloy using three different energy levels, they were able to observe how the metal's internal structure changed and how those changes affected its ability to resist corrosion in a salty environment, similar to the fluids found in the human body.

The researchers began by printing small cubes of pure titanium and the new titanium-copper-silicon alloy. They used a laser to melt the powder, carefully adjusting the speed of the laser and the power it delivered to create three distinct energy conditions. The lowest energy setting was the baseline, while the other two settings provided significantly more heat. They also printed pure titanium under the lowest and highest energy settings to see how the alloying elements specifically altered the metal's behavior compared to pure titanium. After printing, they examined the samples under powerful microscopes and tested them in a saltwater solution to measure how well they resisted rusting.

What they found was that the amount of energy used made a dramatic difference in the metal's internal architecture. At the lowest energy level, the metal was not perfectly mixed. The microscope revealed that some of the original titanium powder particles had not fully melted, and the copper and silicon had not spread evenly throughout the material. Instead, these elements had clumped together in separate regions, creating a patchwork of different compositions. This unevenness is problematic because different parts of the metal can react differently to water and salt, leading to localized corrosion. However, when the researchers increased the energy density, the metal became much more uniform. The higher heat allowed the powder particles to melt completely and gave the copper and silicon atoms enough time to spread out evenly, eliminating the clumps and creating a homogeneous mixture.

This change in mixing also transformed the shape and size of the metal's grains, which are the tiny crystals that make up the solid metal. In the low-energy samples, the grains were large and stretched out in long columns. As the energy increased, these columns broke down into smaller, more rounded grains that were distributed evenly throughout the material. The average size of these grains shrank significantly, dropping from nearly five micrometers in the low-energy sample to less than two micrometers in the high-energy sample. This refinement is important because smaller, more uniform grains generally make a metal stronger and more resistant to damage.

The study also looked at the different phases, or states, of the metal that formed during cooling. Titanium can exist in different structural forms depending on how fast it cools down. The low-energy samples, which cooled very quickly, contained a high amount of a non-equilibrium phase that is rich in dissolved copper and silicon but is structurally unstable. This phase tends to be more reactive and prone to corrosion. In contrast, the high-energy samples cooled more slowly, allowing the copper and silicon to settle into a more stable arrangement. This resulted in a higher proportion of a stable phase that is naturally more resistant to corrosion. The researchers measured the internal stress within the metal's crystal lattice and found that the high-energy samples had much less internal strain, indicating a more relaxed and stable structure.

When these samples were tested in saltwater, the difference in their performance was clear. The samples printed with the higher energy density showed a much stronger ability to resist corrosion. Their corrosion potential, a measure of how likely the metal is to start rusting, shifted to a more positive value, indicating greater stability. More importantly, the rate at which the metal corroded dropped significantly. The high-energy samples lost material at a rate that was more than five times slower than the low-energy samples. This improvement was linked directly to the formation of a better protective film on the surface of the metal. The uniform structure and stable phases allowed a thin, protective oxide layer to form more effectively, acting as a shield against the salty water.

The researchers concluded that simply increasing the energy used during the printing process was enough to transform the metal's properties. By providing enough heat to ensure complete melting and mixing, and by allowing a slightly slower cooling rate, they could eliminate the weak spots caused by unmixed elements and unstable phases. This approach did not require any additional heat treatment after printing, which saves time and cost. The findings suggest that by carefully controlling the energy input, manufacturers can produce titanium implants that are not only strong and antibacterial but also highly resistant to the corrosive environment of the human body, potentially leading to longer-lasting and safer medical devices.