Robert Mehrabian

Rheological behavior of Sn-15 pct Pb in the crystallization range

Rheological behavior of Sn-15 pct Pb alloy in the solidification range has been investigated using a Couette type viscometer. In samples partially solidified before shearing, deformation is localized and primarily intergranular. Samples containing more than about 0.15 fraction solid exhibit an “apparent yield point” which is on the order of 106 dyne per sq cm and increases with increasing fraction solid. When shearing is conducted continuously while the alloy is cooled from above the liquidus to the desired final fraction solid, shear stresses required for flow are reduced by about three orders of magnitude. The solid-liquid mixture now behaves as a fluid slurry. Structural examination shows that shear takes place throughout the cross section of the specimen and that the solid is present as a fine grained particulate suspension. Flow behavior can be described by a viscosity which depends on fraction solid, decreases with increasing shear rate and exhibits hysteresis when shear rate is changed. For shear rates of 200 sec−1, at 0.40 fraction solid, viscosity is about 5 poise which is equivalent to that of heavy machine oil at room temperature. The fact that the slurry is highly fluid at large fractions solid suggests potential applications in new and existing metal casting processes.

Preparation and casting of metal-particulate non-metal composites

A new process for the preparation and casting of metal-particulate non-metal composites is described. Particulate composites of ceramic oxides and carbides and an Al-5 pet Si-2 pct Fe matrix were successfully prepared. From 10 to 30 wt pct of A12O3, SiC, and up to 21 wt pct glass particles, ranging in size from 14 to 340 ώ were uniformly distributed in the liquid matrix of a 0.4 to 0.45 fraction solid slurry of the alloy. Initially, the non-wetted ceramic particles are mechanically entrapped, dispersed and prevented from settling, floating, or agglomerating by the fact that the alloy is already partially solid. With increasing mixing times, after addition, interaction between the ceramic particles and the liquid matrix promotes bonding. Efforts to mix the non-wetted particles into the liquid alloy above its liquidus temperature were unsuccessful. The composite can then be cast either when the metal alloy is partially solid or after reheating to above the liquidus temperature of the alloy. End-chilled plates and cylindrical slugs of the composites were sand cast from above the liquidus temperature of the alloy. The cylindrical slugs were again reheated and used as starting material for die casting. Some of the reheated composites possessed “thixotropy.” Distribution of the ceramic particles in the alloy matrix was uniform in all the castings except for some settling of the coarse, 340ώ in size, particles in the end-chilled cast plates.

Specific permeability of partially solidified dendritic networks of Al-Si alloys

Porous dendritic networks of Al-4 pet Si and Al-4 pct Si-0.25 pet Ti alloys with volume fraction solid,gsA > 0.628 were prepared by removing the segregated interdendritic liquid from partially solidified samples of the alloys. In the equiaxed samples, available channels for flow were predominately between the grains. Specific permeabilities of the porous dendritic networks were measured with a triaxial cell permeameter. Measured values of specific permeability were 1 × 10-9 to 3.5 × 10-11 cm2 in the Al-4 pet Si alloy for volume fractions solid of 0.655 to 0.94, respectively. Specific permeabilities in Al-4 pct Si-0.25 pct Ti alloy were 4.82 × 10-10 to 7.6 × 10-11 cm2 for volume fractions solid of 0.628 to 0.837, respectively. For equivalent volume fractions solid, the measured specific permeabilities were consistently lower for the grain refined samples. Flow through the porous dendritic networks obeys D’Arcy’s law and equations derived from the capillaric flow model for volume fraction liquid less than ∼0.35.

Effect of homogenization on sulfide inclusions in ferrous alloys

The effect of high temperature homogenization on sulfide inclusions was studied in directionally solidified AISI 4340 low alloy steel of various sulfur contents and in directionally solidified iron-carbon-manganese-sulfur alloys. The morphological modifications of the various types of inclusions caused by coarsening were investigated versus homogenization time. The decrease of the number of inclusions per unit volume of matrix, and that of the sulfide-matrix interface area per unit volume of sulfide, and the increase of the mean over-all inclusion size with homogenization time were quantitatively established. These variations were compared in a specific case with analytical predictions based on a coarsening model.

The effect of homogenization treatment and hot isostatic pressing on microporosity in cast steel

The effect of high temperature homogenization treatment and hot isostatic pressing on microporosity in unidirectionally solidified AISI 4330 low alloy steel ingots was investigated. The volume fraction and size distribution of micropores in the as-cast, homogenized, and hot isostatically pressed conditions were determined using X-ray microradiography and optical metallography. The vol pct of spherical, nonspherical and total microporosity increased with distance from the chill in a linear fashion in both the as-cast and the homogenized conditions. The vol pct of microporosity decreased with increasing homogenization time. Nonspherical micropores gradually spheroidized during the treatment. Elimination of microporosity was substantially accelerated by carrying out homogenization under high isostatic pressure. Measured vol pct microporosity at 3 cm from the chill before and after vacuum homogenization at 1315°C for 30 h were 2 × 10−2 and 1 × 10−2, respectively. All observable microporosity was eliminated by hot isostatic pressing for 1 h at 1260 and 1O38°C with corresponding pressures of 29,000 and 27,000 psi, respectively. It is proposed that vacancy diffusion to grain boundaries is the rate controlling mechanism.

A new process for atomization of metal alloys

A laboratory apparatus was constructed to atomize metal alloys by forcing them through sintered ceramic disc filters. The process combines filtration of oxides and/or undesirable second phase particles with atomization. Sintered Al2O3 and SiO2 disc filters with average pore sizes in the range of 36 to 150 Μm were used. The effectiveness of various filters in removing “synthetic inclusions” from Al-Ti-B melts was studied. 2 to 10 Μm size TiB2 particles were successfully filtered out. Atomization experiments were done with pure aluminium and 7075 aluminium alloy. Spherical powders, 150 to 2000 Μm in size, of 7075 aluminium alloy with secondary dendrite arm spacings between 4 to 8 Μm, were cold compacted and extruded into a billet. Room temperature longitudinal properties of the billet in T6 condition were: YS 80×103 psi, UTS 94.3×103 psi, elongation 15% and R.A. 42%.