Alloy design for powder bed fusion - laser beam: Grain boundary and segregation engineering in a novel molybdenum-boron-lanthanum alloy

Abstract

Molybdenum components manufactured using Powder Bed Fusion-Laser Beam (PBF-LB) currently cannot achieve the mechanical properties and quality of powder-metallurgically produced parts. This is a result of the process characteristics of steep thermal gradients and high solidification rates in PBF-LB, which promote the development of a coarse-grained, columnar microstructure. It has been observed that segregated oxygen impurities have a negative impact grain boundary cohesion, resulting in intergranular cracking and pore formation. This study aims to investigate the effect of a grain boundary and segregation engineering alloying approach for molybdenum using boron and lanthanum additions, and simultaneous adjustment of the PBF-LB process conditions.

Mo-B-La alloy specimens were manufactured from a Mo-1.5 wt% LaB₆ powder blend by in-situ alloying in PBF-LB using varying substrate plate temperatures of 200 °C, 500 °C and 800 °C. The results show that all specimens exhibit a fine-grained microstructure. A grain size of between 15 and 21 μm was achieved owing to the solute rejection effect of B and La during solidification. The specimens exhibit a mixed cellular and columnar dendritic subgrain structure of α-Mo cells or dendrites surrounded by intercellular or interdendritic Mo₂B filaments. La is found to support grain boundary purification from oxygen impurities owing to the formation of nanometer sized oxidic particles located at the interface between α-Mo and Mo₂B. 60–80 % of the added La evaporates during the process, which leads to the formation of spherical gas pores with a size <Ø30 μm and limits the specimen's density to a maximum of 98.4 ± 0.4 %. The formation of cracks is significantly affected by the substrate plate temperature during the manufacturing process. Specimens manufactured at ≥500 °C are crack-free, while those produced at 200 °C exhibit cold cracks. Therefore, the bending strength of specimens manufactured at 200 °C is limited to 445 MPa, whereas those manufactured at 500 °C and 800 °C reach a bending strength of 1107 ± 117 MPa and 1210 ± 125 MPa, respectively, at room temperature. At elevated test temperatures, specimens demonstrate excellent properties, with bending strengths of 2151 ± 272 MPa and 1954 ± 510 MPa achieved at 400 °C and 600 °C, respectively.

The study concludes that the grain boundary and segregation engineering approach by alloying Mo with B and La, along with the simultaneous optimization of the process conditions by substrate plate heating, makes it possible to suppress the formation of the main quality inhibiting defects and to produce specimens with excellent mechanical properties in PBF-LB.