Spatial Control of Functional Response in 4D-Printed Active Metallic Structures

Abstract

We demonstrate a method to achieve local control of 3-dimensional thermal history in a metallic alloy, which resulted in designed spatial variations in its functional response. A nickel-titanium shape memory alloy part was created with multiple shape-recovery stages activated at different temperatures using the selective laser melting technique. The multi-stage transformation originates from differences in thermal history, and thus the precipitate structure, at various locations created from controlled variations in the hatch distance within the same part. This is a first example of precision location-dependent control of thermal history in alloys beyond the surface, and utilizes additive manufacturing techniques as a tool to create materials with novel functional response that is difficult to achieve through conventional methods.

Figure 1

(a) Multi-stage shape recovery in a U-shaped additively manufactured NiTi build piece using Selective Laser Melting. Two “arms” of the piece activate their shape recovery at different temperatures, creating a location-dependent active response; (b) The location-dependent active response is created by changing the SLM processing parameters (shown in the DSC curves on the right) at different sections of the build, which results in differences in the transformation temperatures in corresponding sections.

Figure 2. The time-temperature plot shows the temperature history of a probe point during the initial and two adjacent laser passes as recorded by a pyrometer.

The sample built with 120 μm hatch distance experiences lower temperatures during adjacent laser passes. This is caused by the larger distance between adjacent tracks in the 120 μm hatch distance sample. The melt pool (bottom figure from a pyrometer capture, and illustrated as the circled area on the schematic on the right) from adjacent tracks in the 35 μm hatch distance sample experiences significant overlap, resulting in repeated re-melting and reheating.

Figure 3

(a) Simulated temperature history of a probe point in a build as additional layers above it are created for a 35 μm hatch distance sample. The curves show the history when the laser is directly above the point or when it is one hatch spacing (35 μm) or two hatch spacing (70 μm) away; (b) boundary of the locations, as compared to the surface, for which the temperatures are suitable for precipitation nucleation and growth for samples of hatch distances of 35 μm and 120 μm.

Figure 4

(a) Optical and TEM micrographs of the 35 μm and 120 μm hatch distance samples, illustrating differences in the grain size and structure, dislocation density, and second phase and sub-grain structures; (b) high resolution image of the 120 μm hatch distance sample showing small precipitates approximate 1–3 nm in size.