Characterization of Nitinol Produced by Laser Powder Bed Fusion for Mechanical Metamaterial Applications

Abstract

This study investigates the relationship between the process parameters of the laser powder bed fusion technology and the functional properties of nitinol metamaterial for morphing actuator applications. Using an extraordinary wide range of laser powers (40-400 W) and scanning speeds (175-3000 mm s-1) provides the most comprehensive assessment of resulting morphologies, allowing identification of defect-free configurations, especially for low energy densities. The assessment is done with respect to porosity, thin-wall dimensional accuracy, crystallography, austenite-martensite phase transformation, and recoverability under cyclic loading. The results show that low volumetric energy density of 41-55 J mm-3 can lead to an internal porosity of less than 0.1%, although brittle cracking may occur. The cyclic compression tests show a variable quasilinear pseudoelasticity with low hysteresis. The highest total strain after 50 cycles is 5.43% with an associated cumulative residual strain of 2.79%, stabilizing after approximately 30 cycles. The recoverable strain decreases with increasing load, most significantly from 53.4% at 800 MPa to 35.1% at 1200 MPa. The computational estimation of metamaterial morphing capability provides reliable results if the linear assumption after the fifth cycle is adopted by the material model and geometrical thickness deviations are reflected.