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10iSHS 2019
Moscow, Russia
STRUCTURE FORMATION AT CONSECUTIVE PORTION PACKING OF MONODISPERSED GRANULES IN SQUARE MATRICES WITH SMOOTH
AND PROFILED BOTTOM
M. A. Ponomarev*" and V. E. Loryan"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia
*e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10127
The spatial arrangement of the components in the initial powder reaction mixtures significantly affects the course of SHS and characteristics of the synthesis product - microstructure, density distribution, and pore morphology in the final product [1]. When solving the problem of formation of a certain macro- and microstructure in the synthesis product [1, 2], it is advisable to use powder systems with the components and corresponding reaction cells located in the mixture volume as regularly as possible [3, 4]. In connection with the earlier studies of structural ordering of the components in the model heterogeneous systems [5-7], in this paper we study specific features of structured billets formation in successive compaction of monodispersed granules depending on the nature of the initial distribution of the mixture in the matrix volume and the state of the pressing punches surface. The investigation results are relevant for preparation of composite materials by the SHS method from the mixtures with one of the components consisting of large spherical particles [5, 6].
The compaction regularities of the backfills of the model system of large steel granules in matrix molds with a square cross section (side of the square L = 14.2 mm) are investigated. A consecutive portion compaction was used. The density of the filling was taken as the ratio of the total mass of all portions of the granules, at each stage of the portion compaction, to the total volume occupied by them in the matrix between the planes of the upper and lower punch. The granules were calibrated balls of monodispersed composition (d = 1 mm), which allowed simulating the compaction of multicomponent SHS mixtures including narrow fractions of large spherical particles - metal, composite, mechanoactivated and clad ones - along with fine components [1, 5, 6].
The peculiarities of structural ordering in the model system using different ways of filling the original portions of the granules into the matrix were considered. They can be practically used for pressing long billets for the subsequent obtaining of the items by SHS. To achieve a preset uneven distribution of the granules in the cross section of the mold during the portion loading we used special inserts into the matrix - dies. Four different cases of each portion distribution by the cross-section of the matrix before the compaction procedure are considered: (1) the filling components fill the matrix corners; (2) the filling is located near the middle section of the side walls of the matrix; (3) the filling is in the central part of the matrix; (4) the filling is evenly distributed throughout the matrix. The ratio of openings areas in the dies (Si) for the granules to the total cross-section area of the matrix (S) were: S1/S = 21.5%; S2/S = 22%; S3/S = 17%; S4/S = 18.5%. During the compaction with the punch, redistribution of the granules in the mold volume occurred.
The lower fixed punches with the ends of two types were used as the base of the matrix: (1) with a smooth polished surface; (2) with a surface evenly covered with holes (196 ones) obtained by drilling with a spiral drill of 1 mm in diameter with a sharpening angle of 135° to a depth of 0.8 mm. The position of the centers of the holes corresponded to the nodes of a
regular cubic lattice of the size 14 by 14. Pre-filling of the holes with steel balls gave a profiled matrix bottom, covered with a monolayer of balls, which corresponded to one layer of cubic, octahedral and rhombohedral packing [4].
Structure formation peculiarities of multilayered compacts at consecutive portion compaction of the fillings was studied; the number of granules in the portions (SNo) could vary (for small fillings: 5No = 5 for N = 196-725, 5No = 10 when N = 725-2200; for large fillings: SNo = 196 odd monolayers and SNo = 169 for even monolayers). The number of the balls in the compacts (N) reached 2200. Low compaction pressure (P = 1 MPa) did not allow the granules to deform significantly. The filling density increased due to the movement of the granules in the volume (Vc) limited by the matrix walls and the planes of the punches. With embedding the punch we measured the height of the compacted layer of granules with a micrometer. The distance between the upper punch and the plane corresponding to the position of the lower point of the granule in the first monolayer was taken as the height (H) of the compact. The relative density of the samples (p0) was defined as p0 = p/ps (where ps is the density of the granules; p is the density of the compact - the ratio of the granules mass to the volume they occupy of the height H: p = (p*VvN)IVc, where Vb is the spherical granule volume), and calculated by the formula p0 = rcd3NI(6HL2).
Figures 1-4 demonstrate the packing density of the granules in the matrices with smooth and profiled bottom as well as their structure changes (observed upper monolayers) with an increasing height of the compacts. The plots of experimental dependences p0 = fN) have a number of density maxima - from 5 to 10 (Figs. 1, 3).
0 500 1000 1500 N 0 500 1000 1500 N 0 500 1000 1500 N
Fig. 1. Dependences of the relative density of compacts on the number of balls (the matrix with the profiled bottom, SNo = 5-10): (a) the filling is evenly distributed; (b) the filling in the center; (c) the filling is in the corners of the matrix; 1-3 the calculated density of the structures at consecutive packing of the balls in the tetrahedral (1). octahedral (2) and rhombohedral (3) packaging; 4 experimental plots.
For the compacts in the matrices with the profiled bottom when small backfills were used, SN0 = 5-10 after 6-7 density maxima, we observed smoothing of the dependencies. and the extremes became fuzzy (Figs. 1a, 1b and Fig. 3a). In the photo of the upper layer of the compacts, it is seen that with the growth of N, the number and area of the fragments with octahedral packing decreases, and with tetrahedral - increases. On the experimental curves (Figs. 1a, 1b, 3a), we observe a shift in the density extremums along the x-axis from the respective calculated values of the density of octahedral packing (curves 2) in the direction of the density extremums of tetrahedral packing (curves 1). After 6-7 density maximums, the number of ordered fragments decreases, and the structure becomes disordered. The decrease in ordering is apparently connected with the increase in the spread of granules from one monolayer in the coordinate arrangement in the direction of the matrix axis, that is, the gradual
ISHS 2019
Moscow, Russia
degeneration of the layer-by-layer structure of the compact. This phenomenon is facilitated by the formation of stable fragments of rhombohedral packing near the walls along the perimeter of the matrix. The distance between the granule centers in the rhombohedral packing is greater than in the tetrahedral and octahedral, so the punch, resting on the granules near the walls, exerts force on the granules remote from the walls, which is not enough to align their position in the transverse plane and the formation of fragments of octahedral and tetrahedral packing.
Fig. 2. The upper monolayer of granules (the matrix with the profiled bottom, SNo = 5-10): (a) the filling is evenly distributed; (b) the filling is in the center; (c) the filling is in the corners of the matrix.
Fig. 3. Dependences of the relative density of compacts on the number of balls: (a) the filling is at the walls of the matrix (SNo = 5-10), 1-3 the calculated density of the structures (in the matrix) at consecutive packing of the balls in the tetrahedron (1), octahedral (2) and rhombohedral (3) packaging, 4, 5 experimental plots (4 smooth bottom, 5 profiled); (b, c) large filling SNo = 169-196, matrix with smooth (b) and profiled (c) bottom: 1 filling at the walls of the matrix; 2 filling in the center.
In the matrices with the smooth bottom, clear density maximums were observed for all the values of N (Fig. 3), mainly, there were fragments of the tetrahedral packing (Fig. 4). The average density was higher than that in the case of granule compacting in the matrix with the profiled bottom. In the case of large backfills (Figs. 3b, 3c), the relative density of the compacts is comparable to the result at small 5No.
Fig. 4. The upper monolayer of granules (filling at the walls of the matrix SNo = 5-10): (a) matrix with smooth bottom; (b) with profiled bottom.
Thus, the consecutive portion pressing of non-deformable spherical granules into the square matrix with the bottom shaped as a monolayer of octahedral packing granules (L/d = 14) does not result in the stable formation of the structure with extensive fragments of octahedral or tetrahedral packing for more than 6-7 monolayers of the granules. In the square matrix with the smooth bottom there was a steady formation of extensive fragments of tetrahedral packing, on the boundaries of which there were elements of octahedral packing. A thin wall layer of the granules with rhombohedral packing along the perimeter of the matrix can prevent compaction and formation of fragments of denser and more stable packing - octahedral and tetrahedral. The obtained results can be useful in the formation of homogeneous compact billets from multicomponent mixtures for the production of composite materials by the SHS method.
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