Effect of the difference between the atmospheric pressure and the pressure in isolated pores, $ΔP(P_{ext}-P_p)$, on the liquid-filling of pores has been investigated in 98.5W-1.05Ni-0.45Fe(Wt.%) at 1460℃. Large isolated pores were created in the specimens by using spherical Ni particles 100㎛ in diameter.
In the first series of experiments, specimens were sintered for 1h in $H_2$(latm) and resintered for various times in 1 atmospheric pressure of $1H_2-12Ar$(volume ratio). During resintering, $H_2$ gas in the isolated pores was rapidly degassed; the pressure in the pores became lower(1/13 atm) than the external one, resulting in the positive pressure difference(Δp>0). Due to the positive pressure difference, the liquid-filling of pores occured in shorter sintering time than in usual sintering without any pressure difference.
In the second series, specimens were initially sintered for 1h in various $H_2-Ar$ and resintered for various times in $H_2$. During resintering, $H_2$ gas in the sintering atmosphere diffused into the pores; the pressure difference became negative. The starting time of pore-filling was more retarded than in usual sintering. After pore-filling, small pores were still remained in the liquid pockets formed due to entrapped Ar. The size of remaining gas bubble increased with the entrapped Ar pressure in pores.
The acceleration and the retardation of pore-filling due to atmospheric change during sintering have been analized theoretically by the balance of the liquid pressure at the menisci of the specimen surface and at the menisci of the pores. The results from this analysis agreed semi-quantitatively with the experimental one.
Spherical, residual pores, frequently observed in commercial W-Ni-Fe heavy alloy products, were proposed to remain in the specimen when nearly non-diffusing gas (0.03-0.8atm) was entrapped in initial large isolated pores. Irregular pores were found to be formed by both solidification shrinkage(3-6%) and the evolution of $H_2$ during solidification of the melt.
