Abstract
Under the advanced materials and manufacturing technologies program, two Ni-based alloys fabricated by laser powder bed fusion (LPBF) have been evaluated, gamma’ strengthened Haynes 282 and solution strengthened 625. Large printing defects were observed in the LPBF 282 alloy fabricated using a Renishaw 250 machine, likely due to particle spattering during printing. Annealing at 1h 1180°C followed by 4h at 800°C in a high density of 50nm ’ strengthening precipitates, and partial recrystallization resulted in a bi-modal grain distribution. Creep testing performed at 750°C revealed lower creep life and ductility for the LPBF 282 compared with wrought 282. X-ray Computed tomography combined with optical and SEM microstructural characterization revealed cracks formation during creep testing, initiated either from printing defects or from creep cavitation at grain boundaries. Printing defects were the likely reason for the lower creep performance of LPBF 282 and could be suppressed by optimizing build configuration.
Printing of 625 on an EOS M290 machine using the recommended EOS parameters resulted in a very low defect density. Superior creep strength at 725°C was observed for the as printed LPBF 625 along the build direction when compared with wrought 625. No defect larger than ~50µm could be detected by XCT in the as printed conditions and after creep testing for 1000h at 725°C, 150MPa. Small needle like phase precipitates were observed after creep testing for 100h at 725°C, 200MPa. The rapid formation of phase is directly related to the initial Nb and Mo segregation in the LPBF 625 cell walls and its impact on the alloy high temperature performance will be evaluated. Solution annealing was carried out at 1150°C for 1h and led to the full recrystallization of the alloy. A decrease in strength and increase in ductility was observed after solution annealing and the alloy recrystallization resulted in isotropic tensile properties at room temperature, contrary to what was observed for the as printed LPBF 625.
