Microstructure, Mechanical Properties, and Martensitic Transformation in NiTi Shape Memory Alloy Fabricated Using Electron Beam Additive Manufacturing Technique

Abstract

The electron beam additive manufacturing (EBAM) method was applied in order to fabricate rectangular-shaped NiTi component. The process was performed using an electron beam welding system using wire feeder inside the vacuum chamber. NiTi wire containing 50.97 at.% Ni and showing martensitic transformation near room temperature was used. It allowed to obtain a good quality material consisting of columnar grains elongated into the built direction growing directly from the NiTi substrate, which is related to the epitaxial grain growth mechanism. As manufactured material showed martensitic and reverse transformations diffused over the temperature range from −10 to 44 °C, the applied aging at 500° C moved the transformation to higher temperatures and transformation peaks became sharper. The highest recoverable strain of about 3.5% was obtained in the as-deposited sample deformed along the deposition direction. In the case of deformation of the alloy aged at 500 °C for 2h, the formation of martensite occurs at significantly lower stress; however, at about 2.5% the stress begins to increase gradually and only a small shape recovery was observed due to a higher martensitic transformation temperature. In situ SEM tensile deformation in the direction perpendicular to deposition direction showed that the martensite began to appear at the surface of the sample and at the grain boundaries due to heterogeneous nucleation. In situ studies allowed to determine the following crystallographic relationships between B2 and B19’ martensite: (100)B2||(100)B19’ and (100) B2 || (011)B19’; (011)B2|| (001)B19’ and $${(011)}_{\mathrm{B}2}||{\left(11\bar{1 }\right)}_{\mathrm{B}1{9}^{\mathrm{^{\prime}}}}$$ ( 011 ) B 2 | | 11 1 ¯ B 1 9 ′ . Samples aged at 500 °C exhibited fully austenitic microstructure; however, with increasing degree of deformation, the formation of martensite was observed. The majority of needles were tilted about 45° with respect to the tensile direction, and the presence of type I (11 $$\bar{1 }$$ 1 ¯ ) invariant twin boundaries was observed at higher degrees of deformation.

Additive Manufacturing of Binary Ni–Ti Shape Memory Alloys Using Electron Beam Powder Bed Fusion: Functional Reversibility Through Minor Alloy Modification and Carbide Formation

“…A recent study reporting on PBF-EB/M manufacturing of a Ni-Ti SMA also revealed similar columnar grained microstructures [15]. However, the microstructures shown in other studies focusing on Ni-Ti even feature a higher texture component in the h001idirection [15,16]. Zhou et al [15] comprehensively discussed the microstructural evolution of PBF-EB/M processed Ni-Ti.…”

“…Furthermore, the microstructure was found to be different compared to well-known PBF-LB/M manufactured Ni-Ti SMAs [11,15]. Recent studies demonstrate that upon PBF-EB/M of Ni-Ti strongly textured microstructures appear, which can be rationalized by epitaxial solidification [15,16].…”

“…In recent years, additive manufacturing (AM) techniques, such as laser beam powder bed fusion of metals (PBF-LB/M) and electron beam powder bed fusion of metals (PBF-EB/M), came into focus of research, since solid metallic components can be directly fabricated from a computer-aided design (CAD) file by melting successive layers of a metallic powder feedstock [8][9][10][11][12][13][14][15][16][17]. AM techniques are also referred to as 3D-printing processes.…”

“…Only recently, PBF-EB/M was used in preliminary studies to process alloyed [14,15], pre-mixed Ni-Ti powders [14,15] and also pre-mixed powders [31] as well as wires [16]. Besides processing under vacuum atmosphere, one of the major advantages of PBF-EB/M (in contrast to PBF-LB/ M) is the high process temperature usually applied.…”

Heat Input Effect on Microstructure and Mechanical Properties of Electron Beam Additive Manufactured (EBAM) Cu-7.5wt.%Al Bronze

“…Varying the heat input as a function of the built-up wall height makes it possible to control cooling and therefore the solidification rate. However, no approach for forming the desired grain structures is guaranteed, especially in materials inclined to form large columnar grains such as nickel superalloys [30], titanium [31], copper [32], bronze [33], nickel titanium [34,35], etc. In connection with this, finding out the limiting maximum and minimum values of the heat input for deposition of each material is an important task.…”