Microstructure and Tensile Properties of Magnesium (AM60)/Aluminum Oxide Metal Matrix Composites with Varying Volume Fractions of Reinforcement

Magnesium alloy AM60 matrix-based composite reinforced with 7%, 9%, 11%, 22% and 35% of Al2O3 fibers were squeeze casted. The microstructure and mechanical properties were investigated in comparison with the matrix alloy AM60. The results of tensile testing indicated that the addition of Al2O3 fibres to magnesium alloy AM60 led to a significant improvement in mechanical properties. As the fiber volume fraction increased, the strengths and moduli of the composites were enhanced considerably. However, the notably increase in strengths was at sacrifice in elongation. Microstructural analyses via Scanning Electron Microscopy (SEM) revealed that the grain size decreases with increasing volume fraction of reinforcement. The restriction of grain growth by the limited inter-fiber spacing could be the primary mechanism for a reduction in the grain size of the matrix alloy. The corrosion test showed an increasing in corrosion rates as fibers were added to the matrix alloy AM60

HEAT TRANSFER IN SQUEEZE CASTING OF LIGHT ALLOYS

In automotive industry, the weight reduction in vehicles can be achieved by using new designed lighter engineering materials such as aluminum or magnesium alloys. To maintain the same performance as reducing the weight of the vehicles, high strength material has to be used. This study was aimed to develop a solution for casting high strength wrought aluminum alloys and magnesium alloys. Some critical process parameters need to be precisely pre-determined. The interfacial heat transfer coefficient is one of the most important factor. At beginning of this study, an experiment has been carried out to characterize the pressure distribution in the die cavity during squeeze casting of magnesium alloy AM50. This experiment aimed to reveal the changes of pressure distribution with the cavity geometry as well as the local cavity pressure at various locations during the solidification process. To understand the solidification and microstructure refining phenomena, squeeze casting of magnesium alloy AJ62 were performed under an applied pressure 60 MPa by using the simple cylindrical mold. A more complex shape casting mold with five different section thicknesses (2, 4, 8, 12 and 20 mm) was then developed. Wrought aluminum alloys 5083, 7075 and magnesium alloy AM60, AJ62 were squeeze casted under different applied pressures of 30, 60 and 90 MPa. With measured temperature, heat fluxes and interfacial heat transfer coefficients were determined using the inverse method. By observing the IHTC versus time curve profiles, the IHTC peak values of each step were found to increase accordingly as the applied pressure increased. In comparison with the thinner steps, the relatively thicker steps attained higher heat fluxes IHTCs values due to high local pressures and high melt temperature. Finally, the empirical equation relating IHTCs to the local pressures and solidification temperature at the casting surface were derived for wrought aluminum alloy 7075 and magnesium alloy AM60. For magnesium alloy AM60, the calculated IHTC values by using the inverse method were integrated into the casting simulation software (MAGMAsoft) to simulate the solidification process of the 5-step casting. The results indicated that the numerical calculated temperatures were in good agreement with the experimental measured temperatures

Dense Granular Flow as Heat Transfer Media: A New Type of High Power Target Design

High power target systems require reliable high-temperature heat transfer media. Dense granular flow materials have potential to work as heat transfer media, especially at high temperature. In this chapter, a brief review of dense granular and their heat transfer properties is introduced, including basic concepts of heat transfer, thermal behavior of these materials, and factors that affect this behavior. The implementation of these materials as targets is addressed, where two targets were designed, constructed, and tested based on the concept of dense granular flow. The results of the application of a hopper flow-type target in 2.5 MW accelerator-driven system (ADS) and a chute flow-type target in the material irradiation facility will be presented

Section thickness-dependent tensile properties of squeeze cast magnesium alloy AM60

The development of alternative casting processes is essential for the high demand of light weight magnesium components to be used in the automotive industry, which often contain different section thicknesses. Squeeze casting with its inherent advantages has been approved for the capability of minimizing the gas porosity in magnesium alloys. For advanced engineering design of light magnesium automotive applications, it is critical to understand the effect of section thickness on mechanical properties of squeeze cast magnesium alloys. In this study, magnesium alloy AM60 with different section thicknesses of 6, 10 and 20 mm squeeze cast under an applied pressure of 30 MPa was investigated. The prepared squeeze cast AM60 specimens were tensile tested at room termperature. The results indicate that the mechanical properties including yield strength (YS), ultimate tensile strength (UTS) and elongation (A) decrease with an increase in section thickness of squeeze cast AM60. The microstructure analysis shows that the improvement in the tensile behavior of squeeze cast AM60 is primarily attributed to the low-gas porosity level and fine grain strucuture which result from the variation of cooling rate of different section thickness. The numerical simulation (Magmasoft? was employed to determine the solidification rates of each step, and the simulated results show that the solidification rate of the alloy decreases with an increase in the section thickness. The computed solidification rates support the experimental observation on grain structural development

Numerical study of Louver cooling scheme on gas turbine airfoils

This work presents the performance of a louver film-cooling scheme under different operating conditions. The louver cooling scheme consists of a bend by which the coolant going through the flow passage is redirected from vertical to horizontal direction before being injected into the mainstream through an expanded exit. Not only is the momentum of the coolant converted to the mainstream direction, but it is also reduced by the expanded exit before injection. The impingement of the coolant on the blade surface inside the bend also enables further cooling on the targeted surface. The louver cooling scheme was tested under a variety of conditions, from a flat plate to airfoils, from low speed incompressible flows to transonic flows, from a stationary airfoil to a rotating airfoil, and from the leading edge to the middle of an airfoil. Unsteady analysis using a DES (Detached Eddy Simulation) model was also carried out to evaluate its ability to accurately simulate film cooling by comparing with steady state analysis. In general, the louver cooling scheme has been proved to provide enhanced cooling protection to the targeted surface in comparison with other cooling schemes in all conditions tested. At low speed incompressible flow conditions, a higher blowing ratio led to a higher cooling effectiveness. At transonic flow conditions, a moderately higher blowing ratio also proved helpful with a higher cooling effectiveness. Very high blowing ratios, however, proved to be detrimental to the cooling performance since strong detached shock wave structures due to high blowing ratios caused boundary layer separation, rendering the coolant virtually ineffective. The rotation of blade was found to have a significant impact on the level of cooling effectiveness at the leading edge of an airfoil. With regard to the cooling performance, blowing ratio was the dominant factor at low rotational speeds and the rotational speed was the dominant factor at high blowing ratios for circular holes. For the louver scheme as jet liftoff was avoided, effectiveness increased with rotating speed. Results also showed that, unsteady analysis was not significantly more accurate than steady analysis. The unsteady analysis did capture the coolant lateral spreading better, with a high cost of computing, however. Results in this work show that shock waves encountered on transonic airfoils had a significant impact on film cooling effectiveness on any shaped holes. Therefore, experimental data obtained under low speed test should be used with great caution in real design of turbine blade cooling. There are fundamental differences in film cooling between at the leading edge and elsewhere on an airfoil in that a slight incidence shifting due to turbine rotating speed may cause a sudden decrease in cooling effectiveness level at high blowing ratios for circular hole. This could lead to a catastrophic failure if the blade is already in a weak and stressed state. Using of shaped holes with expanded exits may prevent this from happening

An advanced-Louver cooling scheme for gas turbines : adiabatec effectiveness and heat transfer performance

The thermal performance of a novel film-cooling scheme for a high temperature gas turbine application is introduced. The new jet, with both forward and laterally diffused exit, enables the coolant to spread wider and more uniform over the downstream surface, when compared with the traditional circular hole. As a result, the coolant momentum is reduced in the normal direction, and thus the occurrence of jet lift-off is avoided. This novel film-cooling scheme is superior to traditional cooling scheme since less amount of coolant can provide the same protection under the same conditions, making more efficient use of the coolant air. Systematic simulations have been carried out on two benchmark cases. The performances of different turbulence models as well as different wall treatments have been isolated and evaluated. Turbulence was modeled using four classes of turbulence models, namely k-[varepsilon] (including its 3 variants), k-}, Reynolds-Stress, and Spalart-Allmaras. Three-dimensional simulations were carried out by numerically solving the Reynolds-averaged Navier-Stokes equations. For the first time, to the best of author's knowledge, the jet lift-off effect is clearly captured in the simulations at high blowing ratios, and the results are in excellent agreement with experimental data. The new methodology established in the two benchmark cases has been applied to the new scheme. (Abstract shortened by UMI.

Microstructure and tensile properties of Mg(AM60)/Al2O3 metal matrix composites with varying volume fractions of reinforcement

Magnesium alloy AM60 matrix-based composite reinforced with 7%, 9%, 11%, 22% and 35% of Al2O3 fibers were squeeze casted. The microstructure and mechanical properties were investigated in comparison with the matrix alloy AM60. The results of tensile testing indicated that the addition of Al2O3 fibres to magnesium alloy AM60 led to a significant improvement in mechanical properties. As the fiber volume fraction increased, the strengths and moduli of the composites were enhanced considerably. However, the notably increase in strengths was at sacrifice in elongation. Microstructural analyses via Scanning Electron Microscopy (SEM) revealed that the grain size decreases with increasing volume fraction of reinforcement. The restriction of grain growth by the limited inter-fiber spacing could be the primary mechanism for a reduction in the grain size of the matrix alloy. The corrosion test showed an increasing in corrosion rates as fibers were added to the matrix alloy AM60