Electron beam powder bed fusion of Nitinol : A development from production process window towards delicate structures
Sammanfattning: Electron beam powder bed fusion (PBF-EB) is increasingly attracting attention for manufacturing the near-net shape parts due to its incomparable merits, such as free residual stress and superior mechanical performance. Nickel Titanium (NiTi) as the most widely used functional alloy, has not been systematically explored for manufacturing using PBF-EB despite the perfect vacuum and high temperature manufacturing environment. Therefore, this research explores the various aspects of PBF-EB for enabling the manufacturing of NiTi parts.The first section, the critical role of powder pre-heating in PBF-EB and its relation to smoking and sintering issues when using highly susceptible-to-smoke NiTi powder is studied. The research includes assessments of the electron beam spot size and its impact on smoking. In addition, this study investigates the influence of defocused electron beams on smoking, with negative defocusing mitigating smoke compared to positive defocusing that may increase the smoking phenomenon. Processing windows for pre-heating NiTi powder are developed based on smoke tests and sintering levels, showing three modes: smoke-heating, melting-heating, and healthy-heating. Accordingly, the healthy-heating processing window is chosen to manufacture the dense NiTi parts.Further, to produce high density and healthy components, the research focuses on investigating the effects of different PBF-EB parameter sets when manufacturing dense NiTi parts, including beam current, scan speed, and cooling conditions. After manufacturing, densest parts with different parameter sets are divided into three groups: i) high power with high scan speed and vacuum slow cooling, ii) low power with low scan speed and vacuum slow cooling and iii) low power with low scan speed and medium cooling rate in helium gas. A combination of low power and low scan speed leads to denser parts. This is attributed to lower electrostatic repulsive forces from lower number density of the impacting electrons. Different cooling conditions are proven to significantly affect phase transformation temperatures. The slower cooling rate leads to a higher Af and Ms temperatures and a wider phase transformation window than those from the parts with the medium cooling rate due to the formation of Ni4Ti3 precipitates. Afterwards, the pseudoelasticity of all the as-built parts is evaluated and illustrated, which shows that correct control of the process can produce components with recoverable strains as high as 8%.The final part of this thesis the quality and accuracy of manufacturing delicate NiTi parts using PBF-EB is studied. Thin cylinders, thin walls, and lattice structures with various designs are manufactured using different scan strategies. The research reveals that both continuous melting and spot melting modes achieve a dense part in delicate structures. As-built lattice structures exhibit excellent spring-back, with the channel structure displaying the most deformation recoverability. The compressive strength and ultimate compressive strength increase with higher volume fractions. Spot melting is demonstrated as a valuable engineering tool for customizing delicate beam-shaped structures with superior pseudoelasticity.
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