Abstract
Co based superalloy Mar M 509 having excellent high temperature oxidation and hot corrosion resistance is studied via the laser powder bed fusion (LPBF) process. The microstructure and mechanical properties of Mar M 509 in the as-printed (AsP) and heat-treated (HT) condition are compared, as a function of two build orientations (longitudinal (L) and transverse (T)), to establish a working range for application of the alloy. The AsP condition has a distinct cellular microstructure (500–600 nm) with 50–60 nm carbide particles decorating the cell boundaries. The L build orientation displays a strong <001> texture, has columnar grains with a grain size of 8–35 μm (along major axis) and a grain aspect ratio of 4, while the T orientation displays a more equiaxed, but bi-modal microstructure with a grain size of 5–28 μm. The room temperature mechanical properties show variability between L and T with T having 15% higher hardness and 34% higher 0.2% yield strength (YS), 30% lower elongation than L. After a short cycle heat treatment at 1250°C, the weld bead structure and cellular boundaries are broken down and there is substantial grain growth in both L (25–33 μm along major axis) and T orientations (5–42 μm), along with coarsening of carbides (250–350 nm). The dislocation density reduces substantially, indicating recrystallisation, and the lattice parameter of the matrix drops significantly, suggesting solute depletion that contributes to precipitate growth and enrichment of the carbides. There is a drop in the yield strength from 860 MPa to 740 MPa in L and from 1150 MPa to 840MPa in T and an increase in ductility from 14% to 23% in L.
