In science-fiction movies, advanced robots make copies of themselves by building complex objects of any shape and design from scratch. This is approaching reality with the rapid development of additive manufacturing, or 3D printing, where powders or filaments of input material are melted and converted into computer-aided designs.
Additive manufacturing allows thinner metal structures to be mass-produced with ease, inspiring manufacturers to replace solid metal components with thinner struts or lattices. Computer models can simulate the performances of lattices using standard material properties, and ensure that they can withstand the same mechanical load while using less material—or so the theory goes.
However, new research from A*STAR suggests that surface roughness can unexpectedly weaken 3D-printed metal parts, suggesting the need for caution. According to Pan Wang, a Scientist in the Metal and Ceramic Forming Group at A*STAR's Singapore Institute of Manufacturing Technology (SIMTech), few researchers have studied the mechanical properties of additively-manufactured metal products.
Wang and his team have been studying a technique called electron beam melting (EBM), where a high-power electron beam is used to build an object layer by layer, by melting selected spots on a bed of metal powder. “While the hot process produces parts with no residual stress, and the vacuum environment remains clean and highly controlled, we wanted to know if and how the process of EBM can jeopardize the strength of the final product,” Wang said.
To address these concerns, the researchers used the EBM technique to manufacture two car suspension wishbones, one using a conventional design and the other with thinner, computer-optimized walls. Mechanical testing revealed that the computer-optimized design was less stiff than the conventional design—and while the conventional design was strong enough to withstand a weight of over ten tons, the computer-optimized design was only able to bear 73 percent of the maximum load.
Further tests revealed that EBM samples of two millimeters or less in thickness had lower per-area stiffness and strength than thicker EBM samples under as-built conditions. This differs markedly from standard computer models, which assume that such material properties remain constant regardless of thickness. “Under the microscope, we found that although thinner EBM samples had finer microstructure, which implies a high strength, the rougher surfaces led to local flaws that weakened the metal overall,” Wang said.
As Wang explained, these findings indicate that any new EBM designs need careful attention and testing before mass production, especially for the design of thin walls and struts. “We are preparing new databases for thickness-dependent mechanical properties, and updating simulation software to take these limitations into account,” Wang said. Improving the surface finishing of 3D-printed metal can help to reduce local flaws and make full use of high strength microstructure of thin wall and struts, ultimately smoothing the path to end-user adoption.
The A*STAR researchers contributing to this research are from the Singapore Institute of Manufacturing Technology (SIMTech).