2011 Volume 52 Issue 6 Pages 1269-1275
Complex and thin-walled refractory metallic parts including molybdenum (Mo) rocket nozzle and crucible were fabricated using plasma spray forming (PSF) followed by hot isostatic pressing (HIPing). Optical microscope (OM), scanning electron microscope (SEM), Archimedes method, Vickers hardness and tensile tests have been employed to study microstructure, density, micro-hardness and mechanical properties of the parts. A lamellar structure consisting of vertical columnar grains, micron-sized pores, partially melted particles and rough interlamellar contacts with gaps of sub-micron sizes between lamellae were found in PSF Mo deposits. Relative density, micro-hardness and ultimate tensile strength (UTS) of the deposits were about 89%, 150 HV0.025 and 44 MPa, respectively. After low-pressure and two-step HIPing, those changed up to about 92%, 250∼400 HV0.025 (interior ∼ exterior layers), 93 MPa, and 97%, 325 HV0.025 and 170 MPa, respectively. Moreover, a four-stage mechanism of HIPing for PSF parts including heating up, recrystallization, lamellae movement, plastic yielding and creep has been proposed and discussed in detail.
