Microstructural Changes and Quality Improvement of Al7Si0.2Mg (356) Alloy by Die Vibration
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00408-3
Abstract The influence of amplitude and frequency of die vibration during solidification on microstructural evolution of Al– 7Si–0.2Mg (356, LM-25) alloy was studied. The amplitude of die vibration was varied from 0.0 to 1.05 mm at 50 Hz frequency, and the frequency was changed from 30 to 50 Hz at a amplitude of 0.75 mm. Structural examination and quality of the casting were evaluated in terms of porosity at various processing conditions. Vibration modified and refined structure during gravity die casting of the alloy. Macrostructure of casting prepared in vibrating die consisted of fine equiaxed grains. In contrast, macrostructure of casting produced in stationary die typically consisted of columnar grains at the periphery and equiaxed grains at the center. Die vibration resulted in microstructure of mixed type comprising of globular and dendritic primary a-Al with interdendritic eutectic Si particles. On the contrary, microstructure of casting produced
in stationary die consisted of dendritic a-Al structure and eutectic Si particles. In addition, die vibration reduced secondary dendritic arms spacing (SDAS) to 18.98 lm from 34.38 lm obtained without vibration. Since SDAS is a measure of cooling rate, its reduction due to die vibration implies an increase in cooling rate of casting. This is attributed to the forced convection effect generated by die vibration. Consequently, the higher cooling rate owing to the die vibration reduced microsegregation of Si and Mg in the casting. Further, structural modification and refinement due to die vibration improved the quality of casting significantly in terms of porosity.
Introduction
Quality of the casting is improved by minimizing defects in the casting such as inclusions, porosity, and hot tears. Among various defects, porosity is considered as an important quality factor responsible for high rejection rate of castings used for critical applications. There are two types of porosity present in Al casting, namely shrinkage porosity and gas porosity. Reduction in shrinkage porosity in casting is achieved by using Al alloys with high fluidity, optimizing gating and riser design, grain refinement, imposing vibration during solidification, and controlling heat transfer rates.3 The other type of porosity in Al casting, namely gas porosity, is caused by hydrogen absorption in molten Al above liquidus temperature owing to its high solubility at higher temperature and entrapment of air in the mold during casting. Gas porosity is conventionally
The Al–Si–Mg cast alloy is widely considered as the workhorse in automotive industries owing to its high specific strength, high thermal conductivity, and relatively low cost as compared to cast iron.1 Since last couple of decades, there is an increasing trend to switchover from ferrous components to cast Al alloys for improving fuel efficiency and performance of automobiles.2 Various applications of Al–Si–Mg alloy in automotive industries are engine blo
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