B. K. Dhindaw

Behavior of ceramic particles at the solid- liquid metal interface in metal matrix composites

Directional solidification experiments have been conducted to document SiC particle behavior at the solid-liquid interface in Al-2 pct Mg (cellular interface) and Al-6.1 pct Ni (eutectic interface) alloys. Particle size ranged from 20 to 150 μm diameter. Although predictions based on the thermodynamic approach suggest that no engulfment is possible, it was demonstrated that particles can be entrapped in the solid if adequate solidification rates and temperature gradients are used. The main factors responsible for this behavior are considered to be the difference between the thermal conductivities of particles and metal, the buildup of volume fraction of particles at the interface, and the morphological instability of the interface induced by the particles. A model including the contribution of drag and thermal conductivity has been proposed. Calculation with this model produced numbers for the critical velocity slightly higher than those evaluated experimentally. Various factors which can account for this discrepancy are discussed.

The influence of interfacial characteristics between SiCp and Mg/Al metal matrix on wear, coefficient of friction and microhardness

Abstract The aim of the present investigation is to characterize the interface between the SiCp as the reinforcement and Al and Mg metals of the metal matrix composites (MMCs) prepared through vacuum infiltration technique. The weight loss as an index of abrasive wear using pin-on-disc apparatus, the coefficient of friction, the microhardness value and the interparticle distance were determined under dry conditions and these results were correlated to characterize the interface as a function of properties of metal and the reinforcement. The results of the investigation indicate that interparticle distance in cast infiltrated composites strongly influences the tribological properties of these composites. Microhardness on the reinforcement particles and interparticle distance are good indicators of the strength of the interface between particle and the matrix. Magnesium base composites in general show better wettability as compared to the aluminium base composites. Coating of SiCp reinforcements with Ni and Cu generally leads to good quality interface characteristics in Al matrix composite as both microhardness and wear properties are improved. Oxidized SiCp reinforcements behave in complex manner in influencing the interfacial characteristics in aluminium/magnesium matrix composites possibly due to the formation of reaction product at the interface.

The influence of buoyant forces and volume fraction of particles on the particle pushing/entrapment transition during directional solidification of Al/SiC and Al/graphite composites

Directional solidification experiments in a Bridgman-type furnace were used to study particle behavior at the liquid/solid interface in aluminum metal matrix composites. Graphite or siliconcarbide particles were first dispersed in aluminum-base alloysvia a mechanically stirred vortex. Then, 100-mm-diameter and 120-mm-long samples were cast in steel dies and used for directional solidification. The processing variables controlled were the direction and velocity of solidification and the temperature gradient at the interface. The material variables monitored were the interface energy, the liquid/particle density difference, the particle/liquid thermal conductivity ratio, and the volume fraction of particles. These properties were changed by selecting combinations of particles (graphite or silicon carbide) and alloys (Al-Cu, Al-Mg, Al-Ni). A model which considers process thermodynamics, process kinetics (including the role of buoyant forces), and thermophysical properties was developed. Based on solidification direction and velocity, and on materials properties, four types of behavior were predicted. Sessile drop experiments were also used to determine some of the interface energies required in calculation with the proposed model. Experimental results compared favorably with model predictions.

Directional solidification of Cu-Pb and Bi-Ga monotectic alloys under normal gravity and during parabolic flight

Cu-Pb and Bi-Ga monotectic alloys of nominal hypermonotectic compositions were directionally solidified under various furnace translation rates, temperature gradients, and gravity levels. Gravity was varied by solidifying the alloys under ground conditions and in the furnace aboard NASA KC-135 aircraft, flying on parabolic trajectories. High translation rates, high gradients, high gravity levels, and higher density and lower thermal conductivity of the L2 phase favored the formation of fiber composite structure, while the opposite conditions resulted in structures consisting of L2 droplets in α matrix. A modified particle engulfment theory as originally enunciated by Ulhmannet al. is proposed to explain these observations.

Melt convection effects on the critical velocity of particle engulfment

Liquid convection ahead of the solidifying interface alters particle behavior in the vicinity of the interface. This effect has not been quantified to date. Relevant directional solidification experiments were conducted using samples of varying thicknesses, as well as normal and low-gravity experiments. A mixture of transparent biphenyl matrix and spherical glass particles, as well as one of succinonitrile matrix with polystyrene particles were used. Two experimental setups were used: a horizontal gradient heating facility (HGF) for horizontal solidification, and a Bridgman-type furnace (BF) for vertical solidification. The convection level during solidification in the HGF was varied by changing the distance between the glass slides containing the composite sample. The BF was used on ground and during parabolic flights, and thus the convection level was changed by alternating low-gravity and high-gravity solidified regions. It was found that the convection level and/or particle buoyancy significantly influences the critical velocity for particle engulfment. At higher natural convection during solidification the critical velocity increases by up to 40%. At very high convection levels engulfment may become impossible because particles fail to interact with the interface. A systematic analysis of some theoretical models was performed in an attempt to evaluate the present level of theoretical understanding of the problem. Methods of evaluating the surface energies required for model validation are also presented.

Processing of Aluminum-Graphite Particulate Metal Matrix Composites by Advanced Shear Technology

To extend the possibilities of using aluminum/graphite composites as structural materials, a novel process is developed. The conventional methods often produce agglomerated structures exhibiting lower strength and ductility. To overcome the cohesive force of the agglomerates, a melt conditioned high-pressure die casting (MC-HPDC) process innovatively adapts the well-established, high-shear dispersive mixing action of a twin screw mechanism. The distribution of particles and properties of composites are quantitatively evaluated. The adopted rheo process significantly improved the distribution of the reinforcement in the matrix with a strong interfacial bond between the two. A good combination of improved ultimate tensile strength (UTS) and tensile elongation (ε) is obtained compared with composites produced by conventional processes.

Nodular graphite formation in vacuum melted high purity Fe-C-Si alloys

This paper describes a study of the cast structure of vacuum melted high purity Fe-C-Si alloys with emphasis on hypoeutectic and eutectic compositions. Nodular graphite was observed to form at high cooling rates and coral graphite at low cooling rates. This result was also confirmed by a limited study on directional solidification of alloys prepared from the same starting materials. The formation of nodular graphite at the high cooling rates was suppressed to near zero by changing the starting iron from 99.94 pct electrolytic iron to an ultra-pure zone refined iron, or by holding the melt at a low super-heat prior to cooling. Chemical analysis showed only that the impurity responsible for nodular formation was present at the low ppm level. An attempt is made to explain the appearance of the various microstructures in terms of the nucleation and growth of nodular graphite, coral graphite and the carbide structure of white iron.

Twin-Roll Casting of Aluminum Alloys – An Overview

Since its invention by Sir Henry Bessemer in 1865, twin-roll casting (TRC) has been the subject of extensive research, not only to develop the technology but also to achieve an understanding of microstructural evolution. The present review confines itself to the literature on process aspect, modeling, and quality issues. Initially, the principles of the process are outlined. Modeling of fluid flow, heat transfer, and microstructural evolution, surface and internal defects in TRC of aluminum alloys are next discussed. The role of process parameters on solidification during casting is reviewed. The controls of grain structure by melt treatment are also discussed in brief.

In Situ Observations of Interaction Between Particulate Agglomerates and an Advancing Planar Solid/Liquid Interface: Microgravity Experiments

Results are reported of directional solidification experiments on particulate agglomerate pushing and engulfment by a planar solid/liquid (s/l) interface. These experiments were conducted on the Space Shuttle Columbia during the United States Microgravity Payload 4 (USMP-4) Mission. It was found that the pushing to engulfment transition velocity, Vcr, for agglomerates depends not only on their effective size but also their orientation with respect to the s/l interface. The analytical model for predicting Vcr of a single particle was subsequently enhanced to predict Vcr, of the agglomerates by considering their shape factor and orientation.

Scanning electron microscopy studies on tensile rupture of rubber

Tensile rupture of natural rubber (NR) and styrene-butadiene rubber (SBR), vulcanized by sulphur and peroxide systems, both with and without fillers, has been studied by scanning electron microscopy (SEM). NR gum fracture surfaces show evidence of straininduced crystallization, which is absent in SBR. The fracture surfaces of filler-reinforced NR and SBR vulcanizates are characterized by their roughness and by the presence of short and curved tear lines. Increase of cross-link density changes the fracture mode. Peroxide-cured SBR undergoes brittle fracture, whereas sulphur-cured SBR shows a smooth surface with a few straight tear lines.