Al-Si alloys are applicable in a wide area in the industry because of their good mechanical properties, low mass and good castability. Their mechanical properties can be enhanced with modification which causes the eutectic silicon morphology to transform from a needle shape (acicular morphology) to small and rounded particles. One of the most used modifying agent is strontium. Other modifying agents used for eutectic modification are sodium and antimony. Authors attribute the formation of porosity to the following effects: volumetric shrinkage, increase in hydrogen content in the melt, increase in oxide content, decrease in surface tension, decrease in solubility of hydrogen in the solid phase and increase in viscosity because of a wider solidification interval. This work will focus on dimensional changes and changes in the solidification interval of the process of modification of the eutectic silicon.
Most metals and alloys contract during solidification. During solidification volumetric changes cause a drop of pressure in the rest of the melt because of the volume difference of the liquid and solid phase. Porosity will form in these areas if the feeding of interdendritic areas is poor during these changes. According to some sources contraction is greater with strontium addition but according to others smaller contraction was determined.
The addition of the modification agent causes changes in the characteristic temperatures and the solidification interval. This will cause longer diffussion times of hydrogen which causes the occurence of porosity.
This research will be conducted in two parts. The first part will consist of pouring of AlSi12 alloy, AlSi12 alloy with addition of strontium and sodium and AlSi12 alloy with addition of strontium and degassing. Measurements of volume changes with dilatometric analysis and temperature changes with simple thermal analysis will be performed during solidification. The effect of strontium and sodium on dimensional changes will be determined in conjunction with porosity formation and in correlation to the characteristic temperatures. In the second part of the research, the effect of modifying agents on the shape of the eutectic silicon will be observed with optical microscopy and image analysis.
|
