New surface treatment strategies for 3D printed metals

THE PROBLEM

Surface roughness is often the limiting factor in the fatigue performance of metals produced with additive manufacturing (AM). Unfortunately, the typical methods to smoothen metal surfaces do not work well for AM parts. For example, shot blasting and laser peening need line-of-sight access, which is not possible for many complex AM parts. Furthermore, liquid-based approaches, such as etching and electropolishing, remove material unevenly around the part, creating "weak links" in AM microlattices.

OUR WORK

To address the need for ways to smooth the surfaces on complex AM parts, our team is laying the basic scientific groundwork for an approach that selectively remelts the outer surface of AM parts in a highly uniform way. To do so, we apply a thin, conformal coating via atomic layer deposition (ALD). The coating is targeted to activate surface remelting via eutectic reactions that selectively lower the melting point of the surface. For example, titanium — commonly used in 3D printed metals — melts at 1668°C. With the right proportion of copper, however, the melting point of the Ti/Cu combination drops to 880°C. As Ti-6Al-4V is often heat treated at 900°C, the typical heat treatment schedule will activate surface remelting, thereby rewetting and smoothening the part surface, including within complex features that are beyond line of sight.

Zooming in on the fatigue of shape memory alloys

Shape memory alloys (SMAs) break the mold for metals — thanks to their "superelasticity," they can recover about 20 times more deformation than typical metals. Also, they can be used for solid-state actuation and multifunctional structures through their "shape memory effect." However, a number of aspects about the fatigue of SMAs is not well understood. For example, the durability of cardiovascular implants made of NiTi SMA is often limited by surface defects called inclusions. However, the process of damage initiation at these inclusions, as well as the nature of the "weakest link" of inclusions, has not been well characterized. Through advanced experimental techniques such as scanning electron microscope digital image correlation (SEM-DIC), we are zooming in on the role of inclusions in the fatigue performance of SMAs. Furthermore, with a custom capability for combined thermal-mechanical fatigue cycling with rapid thermal cycling rates, we are probing the damage mechanisms of NiTi beyond inclusions.

Advancing experimental techniques for characterizing mechanical behavior

As part of the DIC Challenge, we are working to understand and advance the measurements from scanning electron microscope digital image correlation (SEM-DIC). In these efforts, our group is leading an international round robin of scientists to better understand the source of error in SEM-DIC and further the best practices for this exciting experimental technique.

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