Regulation of shrinkage anisotropy during sintering of Cu-Al2O3 compacts
Aleksander Cyunczyk Politechnika Rzeszowska, Katedra Materiałoznawstwa, ul. Pola 2, 35-959 Rzeszów
Annals 3 No. 8, 2003 pages 397-401
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abstract It has often been stated that during sintering of metal powder compacts anosotropy of the linear shrinkage may occur. This anisotropy is observed in two directions perpendicular to each other in the sintered compact. One of these directions is parallel to the direction of pressing the powder prior to sintering. Usually a mass of metal powders is compacted into a cylindrical shape for sintering examination (Fig. 1), and therefore these directions commonly correspond to the radial and axial direction. The ratio radial shrinkage/axial shrinkage (R/A) may have a value of 1, if both percentage linear shrinkage are equal; it may have a value greater than 1 in some cases and in other cases a value smaller than 1. In the previous own papers an hypothesis has been put forward that the anisotropy of shrinkage is the result of the differences in the degree of mechanical deformations occuring on the surface of irregular powder particles in the direction of pressing and crosswise during the previous pressing process. The relative linear shrinkage of compact is of higher value in the direction in which smaller deformation of the surface of powder paricle takes place during pressing. In soft-metal powder compacts this direction is perpendicular to the pressing one. The reversal of the state described above may take place in compacts pressed from hard-material powder or particles with hard-deforming coating (Fig. 2). In the present study the verification of the above model of sintering has been undertaken with reference to composite powders Cu-Al2O3. Dispersion strengthened Cu-Al2O3 composite powders included 0.1; 0.18 and 0.3 vol.% alumina were prepared by co-precipitation of copper and aluminium salts from water solution. The precipitates were dried-ignited to the oxides and selective reduced in hydrogen. One third of each powder was immediately compacted, the second part was exposed to air at room temperature for a period of 720 h (30 days), and remaining part was stored under the same conditions for a period of 2880 h (120 days). The powders have been undergone surface oxidation during storage (Tab. 1). The structure of powder particles is sensitive to alumina content. Compacts sintered from Cu+0.18%Al2O3 have fine, equiaxed grains, approximately 5 microns in diameter. This structure is stable at temperature 1000○C. Sintered compacts with lower and higher alumina content have greater metal grains (Fig. 3). In sintered compacts made from powders Cu+0.1%Al2O3 and Cu+0.3%Al2O3 the value of R/A decreases with an increasing degree of surface oxidation of metal powder, and can vary from >1 to <1 (Fig. 4). The equalization of radial and axial shrinkages will take place for powder Cu+0.18%Al2O3. In this case the grain boundary sliding (analogous to superplastic flow) is principally responsible mechanism for material transport. Key words: dispersion strengthening, sintering, linear shrinkage