Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718
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INTRODUCTION
ELECTRON beam melting (EBM) is a powder bed fusion (PBF) process in which a high power electron beam is used to melt metal powder layer by layer. Compared to the laser-based processes, EBM has a higher productivity and a lower amount of residual stresses in the built components. It is potentially well suited for the aerospace industry, especially for production of high-end, structurally optimized components made from expensive materials where traditional manufacturing is inefficient and challenging, as for the Ni-Fe-base superalloy Alloy 718.[1–4] Currently, Alloy 718 (also known as INCONEL 718 or IN718) is the predominant superalloy in the world. It is extensively used in the hot sections in aerospace engines as well as in the automotive, oil and gas industries. Alloy 718 stands out for its high strength, corrosion resistance, a wide operating temperature spectrum (from cryogenic up to around 650 C), and
HANS GRUBER, COSMINA LUCHIAN, EDUARD HRYHA and LARS NYBORG are with the Department of Industrial and Materials Science, Chalmers University of Technology, Rnnv. 2A, 412 96 Gothenburg, Sweden. Contact e-mail: [email protected] Manuscript submitted July 6, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
notably good welding characteristics.[5,6] However, such high-performance materials often suffer from difficulties during fabrication. Castings, for example, typically exhibit coarse, segregated, and porous microstructures which require homogenization and densification to reach the requirements for critical components.[7,8] In addition, its high strength and ductility make subsequent forging and machining operations inconvenient.[9,10] Furthermore, an intrinsic problem in heavily alloyed superalloys is the tendency to form non-metallic inclusions, which may cluster into larger sized defects during liquid metal processing.[11] Such clustered non-metallic inclusions are known to have a strong effect on the fatigue life of rotating parts.[1,12,13] As very little modification of non-metallic inclusions is possible through subsequent heat treatments, the number, size, and distribution of non-metallic inclusions need to be controlled in the melting process. Additive manufacturing is characterized by a smallsized, rapidly solidified feedstock material,[13] high solidification rates (usually in the range of 104-106 K/s for laser and electron beam powder bed fusion,[14] compared to 100-102 K/s for any bulky casting process[2]), as well as a comparably small amount of material in liquid state at any one time.[2] These are all attributes that limit the size of non-metallic inclusion defects that may form during the process. It is in this
sense a potential candidate for production of high-performance components. However, it has been reported that oxide inclusions may occur in components fabricated by both laser[15–17] and electron beam[18] additive manufacturing. Their presence may inhibit proper wetting in-between successive layers and act as fatigue crack nucleation sites.[18] Furthermore, the cost-effectiv
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