Electron-beam additive manufacturing of high-temperature metals
- PDF / 1,190,406 Bytes
- 6 Pages / 585 x 783 pts Page_size
- 61 Downloads / 212 Views
roduction Electron-beam melting (EBM) has recently become one of the standard additive manufacturing technologies for metals. Commercial EBM systems are available to a limited extent, currently dominated by Arcam AB, Sweden. The Arcam EBM system, illustrated schematically in Figure 1, relies on gravity feed of an optimized metal (or alloy) powder (Figure 1 inset), usually prepared by rapid solidification processing. Powders in cassettes flow by gravity onto a raked bed where the electron beam is selectively scanned using computer-aided design (CAD) software to heat and melt each raked layer; these usually consist of roughly three powder particle diameters in thickness. As each raked layer is selectively melted, the build platform drops down by a layer thickness (usually 50–100 μm) and a new powder bed is formed; the process is then repeated. In the Arcam system, the build volume (in vacuum) is limited to about 0.03 m3, which allows only small product fabrication. In addition, many high-temperature metal products require pre-alloyed powders, and fabricated products requiring intricate internal structures must be designed to allow the unmelted powder to be conveniently removed. Consequently, closed cellular structures cannot be fabricated since unmelted powder remains within the closed cells. In this article, we examine the fundamental aspects of additive EBM fabrication, including process optimization
and limitations. Selective examples of EBM processing of high-temperature metals and alloys include (with their melting points in parentheses) Ti-6Al-4V (1630°C), the Ni-based superalloy René 142 (1375°C), and Fe (1537°C). These metals represent hexagonal close-packed (hcp), face-centered-cubic (fcc), and body-centered-cubic (bcc) crystal structures, respectively. These metals are used in a range of products and applications, including aerospace components (Fe, Ti-6Al-4V and Ni-based superalloys), orthopedic appliances (Ti-6Al-4V), turbine system components (René 142), and special property components such as magnetic materials (Fe).
Fundamental issues in contemporary EBM: Process control and optimization The melt (L)–solidification (S) process characteristics of powder bed-layer additive manufacturing by EBM are shown in Figure 2. The local temperature in the layer thickness (t) can be represented generally by: T = Q (1 − R) /ρ Cpt ,
(1)
where Q is the power density, R is the electron reflectivity, Cp is the heat of fusion, and ρ is the powder bed density, which increases with melting of the scanned layer. It can be envisioned that EBM process optimization will involve the beam scan
Lawrence E. Murr, Department of Metallurgical, Materials and Biomedical Engineering, The University of Texas at El Paso, USA; [email protected] Shujun Li, Institute of Metal Research, Chinese Academy of Sciences, China; [email protected] doi:10.1557/mrs.2016.210
752
• www.mrs.org/bulletin MRShttp:/www.cambridge.org/core. BULLETIN • VOLUME 41 • OCTOBER 2016Stony © 2016 Materials Research Society Downloaded from SUNY Brook, on 13 Dec 2016 at 21:
Data Loading...
