Staff

Abstract:
Functionally graded metal matrix composites have attracted the attention of various industries as materials with tailorable properties due to spatially varying composition of constituents. This research work was inspired by an application, such as automotive brake disks, which requires advanced materials with improved wear resistance on the outer surface as combined with effective heat flux dissipation of the graded system. To this end, graded AlSi12/Al2O3 composites (FGMs) with a stepwise gradient in the volume fraction of alumina reinforcement were produced by hot pressing and spark plasma sintering techniques. The thermal conductivities of the individual composite layers and the FGMs were evaluated experimentally and simulated numerically using 3D finite element (FE) models based on micro-computed X-ray tomography (micro-XCT) images of actual AlSi12/Al2O3 microstructures. The numerical models incorporated the effects of porosity of the fabricated AlSi12/Al2O3 composites, thermal resistance, and imperfect interfaces between the AlSi12 matrix and the alumina particles. The obtained experimental data and the results of the numerical models are in good agreement, the relative error being in the range of 4 to 6 pct for different compositions and FGMstructure. The predictive capability of the proposed micro-XCT-based FE model suggests that this model can be applied to similar types of composites and different composition gradients.

Abstract:
Metal-ceramic composites by their nature have thermal residual stresses at the micro-level, which can compromise the integrity of structural elements made from these materials. The evaluation of thermal residual stresses is therefore of continuing research interest both experimentally and by modeling. In this study, two functionally graded aluminum alloy matrix composites, AlSi12/Al2O3 and AlSi12/SiC, each consisting of three composite layers with a stepwise gradient of ceramic content (10, 20, 30 vol%), were produced by powder metallurgy. Thermal residual stresses in the AlSi12 matrix and the ceramic reinforcement of the ungraded and graded composites were measured by neutron diffraction. Based on the X-ray micro-computed tomography (micro-XCT) images of the actual microstructure, a series of finite element models were developed to simulate the thermal residual stresses in the AlSi12 matrix and the reinforcing ceramics Al2O3 and SiC. The accuracy of the numerical predictions is high for all cases considered, with a difference of less than 5 % from the neutron diffraction measurements. It is shown numerically and validated by neutron diffraction data that the average residual stresses in the graded AlSi12/Al2O3 and AlSi12/SiC composites are lower than in the corresponding ungraded composites, which may be advantageous for engineering applications.

Keywords:
Finite element modeling,Micro-XCT,Thermal residual stress,Hot pressing,Aluminum matrix composites

Abstract:
Lightweight materials with high wear resistance, good thermal conductivity and enhanced mechanical properties are desired for modern brake discs in the automotive industry. One way to achieve this target is to use functionally graded metal-ceramic composite materials (FGMs). Besides improving the main properties, an FGM must ensure proper thermal conductivity of the system to release heat generated during brake operation and keep the residual stresses at acceptable levels.
Two ceramic materials, SiC and Al2O3, were used as the reinforcing phase of the AlSi12 matrix composites fabricated by powder metallurgy with a stepwise composition gradient (layered composites). The hot-pressing technique was employed to consolidate the powder mixtures with the volume fraction of the ceramics phase ranging from 10 to 30%. High relative density of the composite layers (above 99%) was obtained. Fracture toughness and flexural strength in a four-point bending ranged from 8.7 to 12.94 MPa√m and from 412 to 717 MPa, respectively. In-situ tensile tests under SEM allowed to analyze deformation and crack growth mechanisms on the microscale. Wear tests evidenced high wear resistance of the manufactured materials as compared with the reference material (grey cast iron). Results of the neutron diffraction experiments showed a desired effect of the FGM structure on decreasing the processing-induced residual stresses.
In parallel, FEM simulations based on the actual material microstructure reconstructed from micro-CT images were performed for thermal conductivity and thermal residual stresses to optimize the FGM structure and to answer the question which reinforcement (SiC or Al2O3) better serves the intended application.