Excellent thermal conductivity and lower density make Al–Si alloys a suitable alternative for cast iron in the fabrication of engine components. The increase in the maximum operation temperature and pressure of engines necessitates improving the thermomechanical fatigue performance of Al–Si alloys. This paper has two major parts focussing on the use of Al–Si based alloys in cylinder heads and engine blocks.In the first part, the structural stress–strain and material property requirements of cylinder heads are discussed. In addition, the physical and mechanical properties of different competing materials used in the manufacture of engine components are reviewed. The physical metallurgy, solidification sequence and thermal conductivity of Al–Si based alloys are reviewed. Also discussed is the effect of microstructural features on thermomechanical fatigue lifetime. This part also includes an overview of the strengthening mechanisms of cast Al–Si alloys, by dispersed phases and heat treatment.Demands ...

Application of cast Al–Si alloys in internal combustion engine components

The aluminum–silicon eutectic reaction: mechanisms and crystallography

Microcracks at the device level in bulk solar cells are the current subject of substantial research by the photovoltaic (PV) industry. This review paper addresses nondestructive testing techniques that are used to detect microfacial and subfacial cracks. In this paper, we mainly focused on mono- and polycrystalline silicon PV devices and the root causes of the cracks in solar cells are described. We have categorized these cracks based on size and location in the wafer. The impact of the microcracks on electrical and mechanical performance of silicon solar cells is reviewed. For the first time, we have used the multi-attribute decision-making method to evaluate the different inspection tools that are available on the market. The decision-making tool is based on the analytical hierarchy process and our approach enables the ranking of the inspection tools for PV production stages, which have conflicting objectives and multi-attribute constraints.

Review of Microcrack Detection Techniques for Silicon Solar Cells

Effect of high-intensity ultrasonic irradiation on the modification of solidification microstructure in a Si-rich hypoeutectic Al–Si alloy

The combined and separate effects of microstructural scale and silicon phase morphology on the mechanical properties of Al–Si eutectic alloys are investigated here. The Bridgman-type gradient-zone directional solidification method is employed to produce as-cast structures characteristic of the full range of practical (i.e. casting) growth velocities, and the corresponding mechanical properties are characterized by uniaxial tension testing. The results are analyzed in light of previously reported microstructural changes associated with the flake to fiber or “quench modification” transition [1] . Both tensile strength and elongation were found to increase with solidification rate. Application of the Nan–Clarke [2] micromechanical analysis to the Al–Si composite structure, incorporating the strengthening effects of reinforcement-induced dislocations, suggests that the decreasing microstructural scale alone is sufficient to account for the increase in tensile strength with solidification rate. However, the flake to fiber transition was found to have a particular relevance with regard to the fracture behavior of the alloy, increasing tensile elongation and decreasing the overall variability of tensile properties. A maximum in elongation was observed at approximately 600 μm/s, corresponding to the upper threshold of the flake to fiber transition associated with complete disappearance of the flake morphology and dominance of the fibrous structure. These results emphasize the importance of understanding and controlling the flake to fiber transition that occurs with increasing solidification rate in Al–Si eutectic alloys.

The effect of the flake to fiber transition in silicon morphology on the tensile properties of Al–Si eutectic alloys

The microstructure, hardness, tensile properties and fracture have been studied for the non-modified and modified aluminium (Al) silicon (Si) commercial hypoeutectic alloy. Three modifiers were used being sodium (Na), antimony (Sb) and strontium (Sr). The Sb-modified structure revealed small plate-like Si morphology. The Na and Sr-modified structures exhibited fibrous Si. A slight increase in the hardness values (HV) due to modification was observed. A general increase in the tensile properties was observed due to modification. The tensile properties of the sand mould Sr-modified alloy were significantly higher than those of the Na-modified alloy by 12.7% in proof stress, 16.3% in ductility and 33.3% in toughness. For the metal mould ingots the increase in tensile properties of Sr-modified alloy were respectively: 16.7%, 32.5% and 41.7% compared to a Na-modified alloy. Optical fractography on longitudinal sections near the fracture surfaces of the modified alloys revealed that the crack propagates in the eutectic thus, circumventing the Al-dendrites. The dimple and smooth ripple patterns observed by scanning electron microscope (SEM) on the fracture surface of the Na and Sr-modified alloys suggest a transgranular type of fracture across the grains of the eutectic matrix.

Effect of microstructure on the mechanical properties and fracture of commercial hypoeutectic Al-Si alloy modified with Na, Sb and Sr

Spheroidal graphite (SG) irons with a variety of matrix-structure have been produced. The correlation between tensile properties, impact toughness, hardness and pearlite content is investigated. The pearlite content is varied from 0 to about 95 per cent by the use of different heat treatment processes. The apparent variation in the properties with the pearlite level reveals the remarkable consistency in the relationships between mechanical properties and pearlite content. The study of the tensile properties showed that the yield and ultimate tensile strengths are increased with increasing pearlite level in the matrix structure. For matrix structure with 94.6% pearlite, the increases are about 91% and 98%, respectively, compared with those of the ferritic matrix material. The impact toughness of SG-iron is influenced significantly by matrix microstructure. Energy of about 230 × 103J/m2 is required to fracture a ferritic matrix SG-iron. On the other hand, when the matrix structure approaches a fully pearlitic matrix the fracture energy is decreased by an amount of 75.5%. The Brinell hardness value is found to increase with increasing pearlite content in the matrix structure of the present material. It increases from about 128 for a fully ferritic matrix to about 258 as the matrix structure approaches a fully pearlitic condition. This change in the hardness value reflects the change in the mechanical properties presented in this study.

Mechanical properties of SG-iron with different matrix structure

The static fracture toughness of a series of eutectic Al-Si casting alloy with different microstructural features has been evaluated. The dominant influence of eutectic silicon in controlling the fracture toughness is thus clarified. The relationship between the fracture toughness and the microstructure was established. Fracture toughness was found to be strongly associated with the size and morphology of silicon particles. The other feature which greatly influences the fracture toughness is the ratio (λ/DE)Si, i.e. the silicon particle spacing divided by the equivalent particle diameter, rather than the silicon particle spacing, λSi. Fracture toughness also correlates well with the void growth parameter, VGP (=σy (λ/DE)Si), proposed by the authors. The results of the present work can be used to develop an understanding of the variation of fracture toughness with the microstructural features of eutectic Al-Si alloys.

M. Hafiz

Papers

Effect of microstructure on the mechanical properties and fracture of commercial hypoeutectic Al-Si alloy modified with Na, Sb and Sr

Mechanical properties of SG-iron with different matrix structure

Fracture toughness of eutectic Al-Si casting alloy with different microstructural features