Effects of Amplitude of Die Vibration on Cast Structure of Al4.5Cu Alloy
The effect of mechanical vibration on structural evolution during gravity die casting of Al4.5Cu (LM11) alloy was studied. Two types of die were used to cast alloy, namely top gating die and bottom gating die. The preheated die was placed on a vibrating table prior to casting. The frequency of die vibration was 50 Hz. The amplitude of die vibration was varied to 0.5 mm, 0.75 mm, and 1.05 mm to understand the role of vibration on structural evolution. When vibration amplitude was increased to greater than 1.05 mm, it generated higher degree of turbulence which resulted in splashing of molten metal out of the die and consequently it was difficult to cast the alloy. For comparative purpose, castings were produced without vibrating the die. Microscopic examination showed progressive microstructural transformation from predominantly dendritic structure in casting without vibration to greater degree of globularized primary α-Al structure in casting produced with die vibration. In addition, casting produced in vibrating die showed microstructural refinement and reduction in microsegregation of Cu in the matrix. The average grain size of casting produced in the top gating die under vibration with 1.05 mm of amplitude at 50 Hz was 0.75 mm, whereas that of casting produced in stationary die was 2.1 mm. Further, vibration of die reduced the size of eutectic Al2Cu phase from 10.23 μm (without vibration) to 6.75 μm (with vibration at 50 Hz and amplitude of 1.05 mm). The refinement of grain and eutectic phase in castings produced under vibration is because it caused forced convection in melt that increased cooling rate during solidification. The evidence of high cooling rate in casting produced in vibrating die is noted from secondary dendritic arms spacing (SDAS) values. The SDAS value of casting produced in die vibrating at 50 Hz and 1.05 mm of amplitude was less as compared to that of casting produced in stationary die, and correspondingly the calculated cooling rate of casting produced in vibrating die was greater than that of casting produced in stationary die. Significant reduction in shrinkage porosity was observed in casting produced in vibrating die, and consequently their density was higher as compared to those produced without vibrating the die. This is attributed to the increase in melt flowability due to the fragmentation of primary α-Al dendrites to form higher amount of globularized microstructure and grain refinement during solidification of casting in vibrating die.