The essential Processes in liquid phase sintering of W-Ni are investigated. Powders are mixed by stirring in a slurry state and compacted into cylindrical shape under 50MPa. Compacts are heated at the rate of about 50℃/min and sintered in $H_2$ atmosphere for various time.
When mixture of 10㎛ W and 4.6㎛ Ni powders is sintered at 1550℃, Ni initially coagulates at the center of specimen forming a region of high density. This phenomenon can occur without any local variation of green density, and may be attributed to the tendency to decrease the total liquid-vapor interfacial area. In case of finer (1.23㎛) W powder and/or high Ni content over 6 wt.%, Ni coagulates even below the Ni melting point during heating-up stage.
Upon further sintering the inner dense region and Ni spread outward, leaving sometimes isolated pores. The driving force of the outward liquid flow may be the tendency to reduce the liquid-vapor interfacial area in the outer porous rim. And the kinetics of the outward liquid flow is expected to be determined by the rate of shape accomodation of solid particles in the liquid coagulate.
The isolated pore elimination process is investigated with artificially created large spherical pores. The artificial pores are created by sintering the mixtures of W powders of 1.23㎛, 5.4㎛, or 10㎛ in average size by Fisher Subsieve Sizer, and large spherical Ni particles of 30㎛, 125㎛, or 220㎛ in diameter at 1550℃. Upon melting, Ni flows into W particle matrix completely leaving spherical pores of nearly the same size surrounded by fully densified region. After prolonged sintering, these large spherical pores are filled with liquid squeezed out from the surrounding dense region, while the skeleton of W particles remains intact.
Solid grains around the pore develop negative curvature, but once the pore is filled with liquid, the grains grow into the liquid with positive curvature. When the liquid content is high, the solid grains may also move into the liquid pool. These processes are expected to facilitate liquid flow into the other pores.
The effects of pore size, liquid content, initial porosity and W particle size are also investigated in this pore filling process. The driving force of the liquid flow into the isolated pore may also be the tendency to decrease the liquid-vapor interfacial area forming a pore. The kinetics of the liquid flow is expected to be determined by the rate of shape accomodation of solid particles in the dense region around the pore.
Based on these experimental observations, it is suggested that liquid phase sintering of W-Ni occurs by liquid coagulation, outward liquid flow along with outward expansion of the dense region, and liquid flow into isolated pores.
