Characteristic Constants in Kawakita's Powder Compression Equation

Kawakita's experimental equation has been introduced from the relationship observed between pressure and volume in the case of piston compression.Although the cheracteristic constants a and b in Kawakita's eq. can be obtained by a graphical method, any clear relationship between these constants and physical properties has not been found out.The authors have carried out an experimental work on the hydrostatic compaction using several kinds of powders, i.e. electrolytic copper, electrolytic iron, reduced iron, atomized iron, molybdenum and tungsten.From the data obtained, it is found out that the constant a in Kawakita's eq. is nearly equal to the value of volume reducticn at infinitely large pressure, that is the initial porosity.On all the powders tested, two stages are recognized of the curve of Pb (V-V∞)/(Vao-V∞) versus (Vao-V)/(Vao-V∞), where P is the compacting pressure, b the constant, V-V∞ the volume of residual porosity, Vao-V∞ the volume of initial porosity and Vao-V the volume of reduced porosity.In the first stage that is the linear part of the curve, the constant b has a constant value. In this region, the constant b appears to be related with the yield strength σ0 of the powder particles and apparent initial porosity Nao.In the secondary stage that is the curved portion of the figure (Fig. 3), the constant b has not a constant value. In this region, the constant b appears to be related with σ0*, Nao and (Vao-V)/(Vao-V∞).The relationship between σt/P and 1/b has a clear linearity, and then this fact means that the constant b is related indirectly to the strength of powder particles.