Microstructure Formation and Tailoring of the Intermetallic TiAl Alloy Produced by Direct Laser Deposition
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termetallic c-TiAl alloys have low density, high specific strength, and excellent high-temperature properties, which makes them one of the most promising candidates for replacing nickel-based super alloys in low-pressure turbines of aircraft jet engines.[1–9] Ti-48Al2Cr-2Nb alloy was applied in General Electric’s GEnex engine for the Boeing 787 Dreamliner, which further inspired development of this type of alloy.[5] However, TiAl alloys lack sufficient ductility at ambient temperature, which is a major drawback that severely restricts their fabrication and also increases the costs involved
XINYU ZHANG, HAOZHANG ZHONG, and XUDONG YANG are with the Institute of Materials Modification and Modelling, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. CHUANWEI LI is with the Materials Genome Initiative Center, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] JIANFENG GU is with the Institute of Materials Modification and Modelling, School of Materials Science and Engineering, Shanghai Jiao Tong University and with the Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] Manuscript submitted 5 August, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
when traditional manufacturing technologies are employed.[2,3] Additive manufacturing (AM) is useful for fabricating near-fully dense and complex metal parts directly from computer-aided design models, and as such, it is a novel method that can be used to fabricate quality TiAl alloys with complex geometries at an acceptable cost.[10,11] In addition, its rapid solidification process is advantageous for acquiring exceedingly fine grains and is thus superior to a traditional casting process. Some preliminary attempts at producing near-fully dense TiAl alloys have been made by different AM process. Murr[12] fabricated a TiAl alloy via electron beam melting (EBM) and obtained an equiaxed small-grained structure with lamellar colonies. Li[13,14] changed several processing parameters, such as energy density input and substrate preheating, to fabricate TiAl alloy by means of selected laser melting (SLM) and explored their influence on phase transformation and crystallographic texture. In addition, Thomas et al.[15] utilized direct laser deposition (DLD) to optimize processing parameters and effectively suppressed the generation of cracks. However, it is necessary to control the microstructure when AM is employed, not only because AM uses various processing parameters (such as laser power, scanning speed, powder feed rate, defocus distance, and substrate temperature), but also the solidification conditions are complicated and multiple thermal cycles are involved. Although some studies have been conducted, the formation of the microstructure has not been adequately elucidated because of vast variations in as-deposited microstructures. For example, Srivastava[16,17] used the DLD approach to deposit
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