M. Makhlouf

Optimum conditions for pressureless infiltration of SiCp preforms by aluminum alloys

Abstract The optimum parameters for processing aluminum silicon carbide metal matrix composites via pressureless infiltration of porous SiC p preforms are described. These optimum parameters were obtained by using a carefully designed experiment in which the effect of critical processing parameters on pressureless infiltration was investigated quantitatively. The parameters examined included: SiC particle size, infiltration time, preform height, % SiC in the preform, and Si coating on the SiC particles. The contribution of each of these parameters and their interactions to variation in density, modulus of elasticity, and modulus of rupture of the composites were determined and used to project the optimum processing parameters. Al/SiC p composites were produced using the conditions that were projected to produce optimum modulus of elasticity and their modulus of elasticity was measured. The results show excellent agreement between the projected and measured values.

The role of silicon in wetting and pressureless infiltration of SiCp preforms by aluminum alloys

Silicon plays an important role in the production of Al/SiC metal matrix composites. As an alloying element in aluminum, silicon retards the kinetics of the chemical reactions that result in the formation of the unwanted intermetallics Al4C3 and Al4SiC4. As a thin coating on silicon carbide, silicon becomes an active participant in a thermally activated chemical reaction that enhances wetting of silicon carbide by aluminum alloys. Consequently, Al/SiC composites made with siliconized silicon carbide and silicon rich aluminum alloys show mechanical properties that are significantly different from those of similar composites produced with unsiliconized silicon carbide or with aluminum alloys that do not contain silicon. It is shown that a silicon coating on SiC significantly enhances wetting of SiC particles by aluminum alloys, reduces porosity, does not affect the modulus of elasticity, but decreases the modulus of rupture of Al/SiC metal matrix composites.

Low Cost and Energy Efficient Methods for the Manufacture of Semi-Solid (SSM) Feedstock

The SSM Consortium (now ACRC) at WPI has been carrying out fundamental, pre-competitive research in SSM for several years. Current and past research (at WPI) has generated many results of fundamental and applied nature, which are available to the SSM community. These include materials characterization, yield stress effects, alloy development, rheological properties, process modeling/simulation, semi-solid slurry formation, etc. Alternative method to produce SSM slurries at lower processing costs and with reduced energy consumption is a critical need. The production of low cost SSM feedstock will certainly lead to a dramatic increase in the tonnage of castings produced by SSM, and will provide end users such as the transportation industry, with lighter, cheaper and high performance materials. In this program, the research team has addressed three critical issues in semi-solid processing. They are: (1) Development of low cost, reliable slurry-on-demand approaches for semi-solid processing; (2) Application of the novel permanent grain refining technology-SiBloy for the manufacture of high-quality SSM feedstock, and (3) Development of computational and modeling tools for semi-solid processing to enhance SSM process control. Salient results from these studies are summarized and detailed in our final technical report.

Microstructures and properties of aluminum die casting alloys

This document provides descriptions of the microstructure of different aluminum die casting alloys and to relate the various microstructures to the alloy chemistry. It relates the microstructures of the alloys to their main engineering properties such as ultimate tensile strength, yield strength, elongation, fatigue life, impact resistance, wear resistance, hardness, thermal conductivity and electrical conductivity. Finally, it serves as a reference source for aluminum die casting alloys.

CDS Method for Casting Aluminium-Based Wrought Alloy Compositions: Theoretical Framework

A novel process named Controlled Diffusion Solidification (CDS) has been developed to circumvent problems that are typically associated with casting wrought aluminum alloy compositions into near net shaped components. The process involves bringing two precursor alloys of precisely controlled composition, temperature, and quantity into intimate contact, and then casting the resultant alloy using a conventional casting process to yield a component of predetermined composition with a microstructure that is similar to that of semi-solid processed alloys. Describing the many interactions that occur during solidification of aluminum alloys in a consistent manner is virtually impossible without the use of computational tools that are based on thermodynamic models. In this paper, we describe how the CALPHAD method, which allows calculating all the necessary data from thermodynamic model parameters, was used along with theoretical calculations and empirical rules to allow describing the Gibbs free energy of each phase in the alloy system and yield quantitative data that guided the development and optimization of the CDS method.

Precipitation Strengthening in Al-Ni-Mn Alloys

Precipitation hardening of eutectic and hypoeutectic Al-Ni alloys by 2 to 4 wt pct. manganese is investigated with focus on the effect of the alloys’ chemical composition and solidification cooling rate on microstructure and tensile strength. Within the context of the investigation, mathematical equations based on the Orowan Looping strengthening mechanism were used to calculate the strengthening increment contributed by each of the phases present in the aged alloy. The calculations agree well with measured values and suggest that the larger part of the alloy’s yield strength is due to the Al3Ni eutectic phase, this is closely followed by contribution from the Al6Mn particles, which precipitate predominantly at grain boundaries.

Grain growth at the free surface of WC-Co materials

The grain growth rate at the free surface of a WC-Co material was measured at different high temperatures and the microstructure and elemental composition of the material were characterized at various stages of the grain growth process. It was found that free surface grains grew at an abnormally fast rate, and this fast growth rate coincided with the vaporization of the binder phase from the free surface. It is suggested that this abnormal rate of growth is related to a change in the growth mechanism from interfacial reaction limited growth in the bulk of the material to a surface diffusion rate limited growth at the free surface. It is shown that the contact points between grains provide bridges for atomic transport from the high free-energy regions (the small grains) to the low free-energy regions (the large grains); hence, the contiguity of the material strongly influences the rate of growth. It is believed that vaporization of the binder phase allows for an increased atomic mobility at the surface, a reduction in the energy barrier to chemisorption and consequently an accelerated grain growth.

Castable Aluminium Alloys for High Temperature Applications

Most traditional aluminium casting alloys are based on the aluminium-silicon eutectic system because of its excellent casting characteristics. However, the solidus in this system does not exceed 577 °C and the major alloying elements used with silicon in these alloys have high diffusivity in aluminium. Therefore, while these elements enhance the room temperature strength of the alloy, they are not useful at elevated temperatures. Considering nickel-base superalloys, whose mechanical properties are retained up to temperatures that approach 75% of their melting point, it is conceivable that castable aluminium alloys can be developed on the same basis so that they are useful at temperatures approaching 300 °C. In this publication, we present the thought process behind developing a new castable aluminum alloy that is designed specifically for such high temperature applications and we present the alloy’s measured castability characteristics and its elevated temperature tensile properties.

Characterization of the Flow Behavior of Near Eutectic Composition Aluminum-Silicon Alloys

The cone and plate method is used to quantify the effect of shearing rate, shearing time, and molten alloy temperature on the rheological properties of strontium modified and unmodified near eutectic Al-Si alloys. It is found that in the temperature range 583°C (1,081°F) to 598°C (1,108°F) at relatively low shear rates, these alloys behave as Non-Newtonian fluids. In addition, they exhibit shear-thinning, and at all the shear rates tested, the viscosity of the Sr-modified alloys is higher than that of the unmodified alloys.

The Al-Al3Ni Eutectic Reaction: Crystallography and Mechanism of Formation

The characteristics of the Al-Al3Ni eutectic structure are examined with emphasis on its morphology and crystallography. Based on these examinations, the mechanism of formation of this technologically important eutectic is postulated. It is found that a thin shell of α-Al forms coherently around each Al3Ni fiber. The excellent thermal stability of the Al-Al3Ni eutectic may be attributed to the presence of this coherent layer.