Abstract
Two different methods of rapid manufacturing—electron beam additive manufacturing (EBAM) and laser-engineered net shaping (LENS)—were used in order to fabricate NiTi elements. Microstructure and martensitic transformation temperatures of initial materials in the form of wire or spherical powder were established. The samples fabricated using LENS technique showed martensitic transformation temperature (MTT) at − 26 °C (represented by maximum martensite peak maximum in DSC) which was lower in comparison with raw powder. In the case of samples fabricated using EBAM, the MMT reached − 19 °C. The peaks of martensite and reverse transformations were diffuse due to differences in grain size and composition across the sample. Aging at 500 °C for 2 h caused not only separation of R-phase during cooling of both samples, but also formation of sharper and higher transformation peaks as well as shift of MTT to higher temperatures. Microstructural investigation showed columnar grains, near the interface of deposited element and base plate, growing perpendicular to the plate surface. The grains showed axial fiber texture <001> along the growth direction. STEM micrographs revealed the presence of elongated particles enriched in Ti. Formation of Ti-rich particles during the process led to the depletion of Ti in the matrix and contributed to increase in MTT in comparison with initial NiTi powder. LENS-deposited sample additionally contained higher dislocation density in the austenite. Compression stress/strain curves of EBAM-deposited sample revealed deformation of martensite only, while the LENS-deposited one showed almost complete superelastic effect in compression mode up to 3%.
Highlights
Additive manufacturing (AM) has been gaining more attention in the recent years due to the possibility of fabricating metallic components of complex shapes, which are difficult to obtain using conventional techniques, simultaneously saving energy and materials (Ref [1, 2])
In addition two further temperatures were calculated—Mp and Ap, which represented the temperatures corresponding to the maximum rate of martensitic and reverse transformations
The curves obtained for powder in as-supplied state showed diffuse maxima of martensitic and reverse transformations with the following characteristic temperatures: martensite start (Ms)= 17.9 °C, martensite finish (Mf) = À 21.3 °C, austenite start (As)= 19.5 °C, austenite finish (Af)= 45 °C, Mp= 2.5 °C, Ap= 35.3 °C
Summary
Additive manufacturing (AM) has been gaining more attention in the recent years due to the possibility of fabricating metallic components of complex shapes, which are difficult to obtain using conventional techniques, simultaneously saving energy and materials (Ref [1, 2]). An additive material in the form of powder or wire which is melted using a highly focused laser or electron beam. Based on the type of the additive material, AM techniques can be divided into powder bed-like selective laser melting (SLM), powderfeed as laser engineered net shaping (LENS) and wire-feed, e.g., electron beam additive manufacturing (EBAM) (Ref 2). In the case of powder-based techniques, spherical, pre-alloyed powders are typically used (Ref 3), the use of a mixture of elemental powders is possible (Ref 4). In the powder-feed techniques, like LENS, the process is usually conducted in a glove box under Ar atmosphere (Ref 7), while in the methods using an electron beam, like EBAM, a high vacuum has to be applied (Ref 8). The atmosphere influences the surface quality, density and mechanical properties of the final material (Ref [1, 9])
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