MnCu alloys, a shape memory alloy (SMA) type, exhibit good mechanical bearing performance, shape memory effect, damping performance, and low manufacturing cost. Furthermore, the alloys are easy to process. Therefore, MnCu-based SMA can potentially replace NiTi-based alloys in specific applications. Moreover, the alloy has application prospects in vibration reduction in aerospace, ships, vehicles, and machinery manufacturing. High manganese-type MnCu alloy (atomic fraction of Mn>70%) with high functional properties exhibits poor elongation. It is also readily brittle and oxidizes during high-temperature manufacturing. In the conventional manufacturing methods for MnCu alloys, complex structures such as lattice, inner channels, truss, and thin walls cannot be effectively completed. Hence, we select selective laser melting (SLM) to process MnCu alloy. SLM is a typical metal additive manufacturing technology. Different SLM manufacturing parameters and heat treatment methods significantly influence the properties of MnCu alloy samples. Therefore, in this study, we first study the influence of varying scanning speeds on the relative density of Mn-30%Cn alloy samples made from SLM to find a process window suitable for processing MnCu alloys. Second, we study the effects of SLM process parameters and cyclic heat treatment on the grain size, phase composition, phase structure, chemical composition, and microhardness of MnCu alloys. Finally, we obtain the comprehensive effects of SLM process parameters and cyclic heat treatment on the sample.

In this study, a planetary ball mill employs Al₂O₃ as a grinding medium to mix the Mn and Cu powders with 70:30 mass ratio. Then, we prepare cubic samples based on the mixed powders using the self-developed SLM instrument at different scanning speeds. Furthermore, we divide the Mn-30%Cu alloy prepared using SLM into two groups; one has cyclic heat treatment, and the other does not. The scanning tracks and grains on the surface of the sample are observed using an optical and electron microscope at different magnifications. Subsequently, the scanning track width and grain size are measured and compared. An X-ray fluorescence spectrometer is used to analyze and compare the phase composition of the samples before and after heat treatment. Moreover, the micro Vickers hardness tester is used to calculate the microhardness of the sample surface. Grain orientation difference is measured using electron backscattering diffraction.

This study innovatively investigates the effect of cyclic heat treatment on Mn-30%Cu alloy samples. Compared with conventional heat treatment, abnormal grain growth is observed in the samples with epitaxial mixed grains with two types of grain sizes (2‒5 μm and 140‒240 μm) in the samples after cyclic heat treatment . The coarse grain size is approximately two orders larger than the fine grain size. In addition, a significant gap in hardness between coarser and finer grains in Mn-30%Cu alloy is observed. Furthermore, the microhardness of the finer grains is in the range of 145‒156 HV, which is close to the hardness of the alloy without heat treatment. In contrast, the microhardness of large grains is 130‒135 HV, which is significantly lower than that of small grains. Therefore, it ultimately decreases the microhardness of the Mn-30%Cu alloy samples after heat treatment.

When the laser power is 196 W, hatching distance is 0.06 mm, layer thickness is 0.02 mm, and scanning speed is 500‒600 mm/s, the relative density of Mn-30%Cu alloy reaches its highest (>99.7%). The pores form when the scanning speed is lower than 500 mm/s; incomplete fusion forms when the scanning speed is higher than 600 mm/s. The Mn-30%Cu alloy formed by SLM mainly comprises γ-(Mn, Cu) phase grains with grain sizes of 0.5‒1.6 μm. As the scanning speed increases from 300 mm/s to 700 mm/s, the grain size decreases first and then increases. During cyclic heat treatment, the Mn-30%Cu alloy expands abnormally, resulting in equiaxed mixed grains with 2‒5 μm and 140‒240 μm sizes. In addition, a small amount of γ- (Mn, Cu) undergoes martensitic transformation to produce γ´-(Mn, Cu) or precipitate a small amount of α-Mn phase. The Mn-30%Cu alloy will exhibit Mn burning phenomenon during SLM. The burning ratio of Mn element decreases from 16.4% to 3.9% with the increase of scanning speed (i.e., the decrease of laser energy density). The microhardness of SLM-formed Mn-30%Cu alloy is mainly influenced by the relative density, chemical composition, phase composition, and grain size. After heat treatment, the microhardness of Mn-30%Cu alloy decreases from 143‒153 HV to 137‒145 HV.