Prediction of sintered density for bimodal powder mixtures
- PDF / 1,261,571 Bytes
- 11 Pages / 613 x 788.28 pts Page_size
- 66 Downloads / 345 Views
I.
INTRODUCTION
M O N O S I Z E D spherical powders will approach a fractional packing density of 0.64 in a random arrangement. Mixtures of powders with differing sizes give improved packing densities. Since the efforts of Furnas, ul various models have been advanced to predict the density v s composition for powder mixtures, assuming certain knowledge about the two constituent powders. Recent models include corrections for the particle size ratio and inhomogeneity of the mixtures, t2~ However, the sintered density for a bimodal powder mixture is less predictable and requires experimentation. This article provides a basis for predicting the sintered density for any bimodal mixture from knowledge of the sintering behavior of the constituent powders. Consider two powders of the same chemical composition but having two distinctly different sizes designated "L" and "S" for large and small. Figure 1 schematically illustrates the variation in packing density f with composition X (measured by the percentage of large particles, assuming here both powders are of the same chemistry) for a bimodal mixture. Five possible conditions are sketched: (1) 100 pct small particles, (2) majority of small particles with some added large particles, (3) maximum packing density mixture, (4) majority of large particles with some small particles, and (5) 100 pct large particles. The packing density improves in the terminal region rich in small particles, because the addition of large particles substitutes dense regions for porous clusters of small particles. Alternatively, for regions rich in large particles, the density improves because the small particles are able to fill the interstices between the large particles. The packing density peaks at X*, which corresponds to a dense packing of large particles with small particles in all of the interstices. The kinetics of sintering scale to an inverse power of the particle size; thus, small particles exhibit faster densification with higher final densities. The addition of large R.M. G E R M A N , Professor and Brush Chair in Materials, is with the Engineering Science and Mechanics Department, Pennsylvania State University, University Park, PA 16802. Manuscript submitted July 1, 1991. METALLURGICAL TRANSACTIONS A
particles to a matrix of small particles will increase the initial packing density but decrease the densification in sintering. If small particles are added to a matrix of large particles, then there will be an increase in packing density as long as the small particles fit into the interstitial spaces. However, the influence of the small particles on sintering densification is not clear. Under ideal conditions, Coble t31 suggested interstitial small particles might increase the sintered density of the large particles. However, in most situations, the small particles have a negligible effect on the sintering response of the large particle skeletal structure, because the system rigidity is far greater than the sintering stress. ~4"51Depending on the sintering conditions, either the pac
Data Loading...
