Abstract
• 90W-7Ni-3Fe interlayers were used for the joining of RAFM steel and W-Cr coatings by LMD. • The interlayer with high dilution rate is effective in relieving residual stress. • W-Cr coatings have reliable anti-oxidation performance compared to pure W. As promising plasma facing materials considering the loss of coolant accident (LOCA) in fusion reactors, W-Cr coatings need to be joined to the structural components manufactured with reduced activation ferritic/martensitic (RAFM) steel. Laser melting deposition (LMD) is a promising additive manufacturing technique to deposit W-Cr coatings on the RAFM steels. In order to reduce the stress concentration at the coating/substrate interface, 90W-7Ni-3Fe interlayers were deposited on the RAFM steel by LMD before the deposition of W-Cr coatings. Two interlayers with similar thickness of about 750 μm were prepared with different dilution rates from the RAFM steel substrates. Both interlayers have improved the joining of RAFM steels and W-Cr coatings by eliminations of cracks, which originate at the substrate/coating interfaces without interlayers. No pores or cracks are observed at the substrate/interlayer interfaces, or the interlayer/coating interfaces. The top and mid-top regions of coatings have unique microstructures showing W particles embedded in Cr based matrix. Compared to the high hardness at the substrate/interlayer and interlayer/coating interfaces for the high dilution rate interlayer, the interface hardness is much lower for the low dilution rate interlayer. Moreover, sharp decreases of hardness happen at the interfaces for the low dilution rate interlayer. Pores are formed at the W-Cr coatings due to the lack of fusion, residual of delivered gas, or evaporations of low melting point elements. For W-Cr coatings deposited on the low dilution rate interlayers, cracks can be observed. The thermogravimetric analysis under air atmosphere indicates reliable anti-oxidation performance of the W-Cr coatings compared to the pure W.
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