Electron Microscopy Investigation of Binder Saturation and Microstructural Defects in Functional Parts Made by Additive Manufacturing

Abstract

Metallic foams of ferromagnetic Ni-Mn-Ga Heusler alloy show an 8.7% magnetic field induced strain (MFIS) [1]. The large MFIS in foams are orders of magnitude higher compared to bulk polycrystalline Ni-Mn-Ga, revealing that porosity is responsible for the increase in MFIS. Additive manufacturing (AM) techniques such as powder-bed binder-jet technique, known as 3D printing, are suitable for producing near-net shaped parts with controlled porosity. AM is a relatively new method of engineering that produces near-net shape components, one layer at a time, based on a computer aided design (CAD). The purpose of this work is to manufacture functional near-net shape parts with predetermined porosity from pre-alloyed ferromagnetic Ni-Mn-Ga powders using powder-bed binder-jet 3D printing technique. 3DP resemble ink-jet printing, but with multiple passes to build the materials upwards into a threedimensional form. This method has been proved successful in additive manufacturing near-net shape parts made from pre-alloyed Ti-Ni-Hf powders [2]. Bulk Ti-Ni-Hf alloy is a known high-temperature shape memory alloy [3].Pre-alloyed spherical Ni-Mn-Ga powders used in this experiment have been produced by spark-erosion in argon and nitrogen dielectrics. Recommended printing parameters (layer thickness, binder saturation, spread speed) for metallic powders by 3D printer manufacturer (ExOne, North Huntington, PA) did not prove successful for Ni-Mn-Ga spark-eroded powders. Binder saturation is a major parameter in the printing process. Binder saturation level affects both the breaking strength, as well as the dimensional tolerance of the green printed part. Insufficient binder reduces the mechanical strength of the green part, which can affect the integrity of the part when removed from the printer, or manipulated in between the printer and curing oven. A higher binder saturation level can produce lateral spreading, therefore affecting the dimension and surface quality of the final product. A water-based binder, provided by ExOne, was used in the experiment. In order to find the optimum binder saturation for Ni-Mn-Ga powders, a method similar to the bench top test was used [4]. Cured samples impregnated with various amounts of binder were investigated using SEM, Figure 1. The 2.0 µL of binder impregnation seemed to provide an adequate amount of binder for the printing process. Using the optimized printing parameters, green Ni-Mn-Ga parts were obtained. Figure 2(a) shows a green part after binder curing at 463K for 4 h.The green Ni-Mn-Ga parts were sintered in controlled atmosphere at various temperatures for varying periods of time. Figure 2(b) shows a low magnification SEM micrograph of the sintered part. It can be easily notice, voids are visible and the part is porous. The martensitic transformation behavior of sintered Ni-Mn-Ga parts has been analyzed by XRD, DSC, SEM, FIB, and TEM. The XRD investigation shows that the printed material displays Ni 2 MnGa Heusler phase. Based on the DSC investigation the sintered material shows revers...

Additive Manufacturing from the Point of View of Materials Research

“…Preheating was required [97] E-PBF process resulted in better tensile properties than L-DED and L-PBF [97,122] PAD Pre-alloyed powder Linear superelasticity [123] Quasi-linear superelasticity with narrow hysteresis [123,124] WAAM Wire High hardness and tensile strength [125] Cu-Al-Ni L-PBF Elemental powders High aluminum content led to dendrites and high hardness [126] Cu-Al-Ni-Mn Gas atomized powder High relative density (>92%) achieved [127][128][129] Reversible martensitic transformation with the formation of β 1 '-martensite [127][128][129] Large strain recovery after unloading (up to 18%) [127] Strong distribution of pores produced by the L-PBF sample [127,128] Additional re-melting led to smaller grain size and yielded a deformability of 14% [129] Higher strength and improved plasticity was observed for both samples (Cu-Al-Ni-Mn and Cu-Al-Ni-Mn-Zr) [128][129][130] For the Cu-Al-Ni-Mn-Zr sample, Zr-rich phase was found to precipitate at the grain boundaries during the annealing process [130] Cu-Al-Ni-Mn-Zr Cu-Al-Ni-Ti Copper alloy with Ti addition had a high hardness of about 280 HV due to the grain refinement. The relative density exceeded 99% [131] Fe-Mn-Al-Ni Reversible martensitic transformation and pseudo-elastic effect [132] MSMA Ni-Mn-Ga 3D ink printing Ink with elemental powders Reversible martensitic transformation [133] Martensitic twins [134] Binder jetting Mechanically produced powder Martensitic twins and reversible martensitic transformation after post-processing [135,136] Binder jetting produced a complex-shaped porous Ni-Mn-Ga geometry with a reversible martensitic transformation [137][138][139] Sintering of Ni-Mn-Ga powder was shown to produce net-shaped porous structures…”

“…All the scientific literature available at the time focuses on the additive manufacturing of Ni-Mn-Ga-based MSMAs. The most common approaches on additive manufacturing of Ni-Mn-Ga have concentrated on 3D ink printing [133,134] and binder jetting [135][136][137][138][139][140]. However, also a few investigations into manufacturing of polycrystalline Ni-Mn-Ga using L-DED [141] and L-PBF [142][143][144][145][146] have recently been published.…”

Modeling and characterization of binder jet 3D printed NiMnGa components using X-ray microscopy

“…Binder jetting additive manufacturing provides major benefits for obtaining NiMnGa components displaying magnetic shape memory effect (MSME) [1][2][3]. In the case of NiMnGa alloys it was proved that cast porous materials with bimodal pore-size distributions shows a magnetic field induced strain (MFIS) up to 8.7% [4].…”

Grain Growth, Porosity, and Hardness Changes in Sintered and Annealed Binder-jet 3D Printed Ni-Mn-Ga Magnetic Shape Memory Alloys

“…Lázpita et al showed that fine-grained and randomly textured Ni-Mn-Ga samples exhibit hardly any MFIS under an external magnetic field due to internal constraints on twin boundary motion [4]. Recently, there has been interest in manufacturing Ni-Mn-Ga systems containing a porous network over a completely dense polycrystalline sample in order to enhance the MFIS [5]. According to Müllner et al, the MFIS is highly diminished in solid polycrystalline Ni-Mn-Ga as the grain boundaries inhibit the movement of twin boundaries.…”