A new surface engineering approach for fatigue-resistant AM microlattices

Sponsor: Office of Naval Research, Aerospace Structures and Materials
PI: William LePage (Tulsa)
Co-PI: Michael Sangid (Purdue)
Award: FA9550-21-1-0212
Dates: May 2021 to May 2024

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.

BIG: Building Intelligent Gradients in additively manufactured space engine materials via a multiscale predictive framework

Sponsor: NASA Space Technology Mission Directorate
PI: Jerard Gordon (Michigan)
Co-PI: Wenda Tan (Michigan) and William LePage (Tulsa)
Award: 22-ESI-0060
Dates: Jan 2023 to Jan 2026

Scalable manufacturing of high-performance propulsion and cryogenic fluid management systems remains a major challenge for space applications. To meet this need, functionally graded materials (FGMs) made by metals additive manufacturing (AM) show great potential for transformative impact. However, very limited information exists on the influence of processing conditions on composition, microstructure, defect densities, and property gradients in AM-FGMs. Additionally, models capable of accurately predicting the fundamental process outcomes such as local microstructural evolution and defect formation in these materials are largely nonexistent. We will employ an integrated experimental-computational framework to guide the design, synthesis, and optimization of AM-FGMs using a novel approach of surrounding brittle intermetallic compounds with ductile phases in the gradient regions, thus reducing their negative impact on thermomechanical performance. The objective of this work is to connect local thermophysical behaviors at the melt-pool scale with microstructure and property gradients in AM-FGM model alloy systems relevant to NASA needs.

Zooming in on the fatigue of shape memory alloys

Sponsor: SMST Founder's Grant
PI: William LePage (Tulsa)
Dates: May 2021 through May 2023

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