| In this study, the large ZL205 A alloy cylindrical castings produced by low-pressure casting were selected for investigation. In order to study the typical defects, such as macro segregation and shrinkage appeared in cylindrical castings, the effect of casting structures on solidification characteristics and defects of ZL205 A cylindrical castings were studied by production and experimental research combined with numerical simulation technology. The formation mechanism and different feeding behaviors during the formation of typical macro segregation were revealed. The prediction criterion of linear macro segregation was developed. And the control method and the prediction criterion of shrinkage porosity were also suggested.Through classifying the typical ZL205 A alloy cylindrical castings, the effect of different structures on modulus and solidification characteristics of castings was investigated. According to the experimental and the simulation data, results show that, the effect of the sizes of cylindrical castings and internal structures on modulus is small, for cylindrical castings with the same wall thickness. Thus, the solidification characteristics of cylindrical castings can be represented by its modulus. The cylindrical castings which have similar moduli will have similar solidification characteristics.The macro segregations of ZL205 A alloy cylindrical castings were always found in a fixed position. Linear macro-segregation usually appeared at the wall of casting in circumferential direction connected with slit gating system, and mainly at bottom of the casting. And nebulous macro-segregation usually appeared in the casting wall between the silt gating system at middle of the casting. The primary defect of cylindrical castings without internal structure is macro segregation, and the primary defect of cylindrical castings with internal structures is shrinkage porosity, and some of them have linear macro-segregation. The investigation of solidification characteristics at the position of defects shows that there is a relationship between the formation of nebulous macro-segregation and shrinkage porosity.At the final solidification stage of ZL205 A cylindrical castings, due to the peculiarly technological structure of casting and the hot spot formed in solidification, great stress concentration formed at the wall of casting in circumferential direction connected with slit gating system, leading to the formation of hot cracks in this position when the temperature entering into the hot-crack sensitive temperature range. However, by investigating the feeding angle, temperature distribution, temperature gradient and solid fraction distribution, the slit gating system still have good conditions for feeding, even if the solid fraction at the front of the hot spot reached 90%. Associating with the results of solute concentration gradient formed in the slit gating systems in the direction of gravity, the linear macro segregation would be formed by ―healing‖ the cracks through feeding behavior. According to the formation mechanism of linear macro segregation, the prediction criterion and the critical value of linear macro segregation(linear macro segregation sensitivity) were proposed by logistic regression method.The vibration of the designed casting was measured during solidification, and the experimental results show that the burst feeding process does exist during solidification process in the designed casting. And the burst feeding behavior is likely to occur randomly and sporadically. Through the theoretical analysis of the macro and micro burst feeding behavior, it is found that the smaller feeding angle, sudden changes of channel size, higher solid fraction are key factors acting on the macro burst feeding behavior. And the kinetic and thermodynamic analysis of micro burst feeding behavior shows that the micro burst feeding is mainly affected by pressure difference on feeding distance, temperature gradient in feeding direction and cooling rate at solidification stages of eutectic. Although the reason of channel blocking for macro / micro burst feeding is not the same, the main driving force of macro / micro burst feeding is both provided by pumping action due to negative pressure in the isolated zone of liquid.Due to the influence of burst feeding process, the remaining melt with hi gher solute concentration can feed into the isolated zone, leading to the formation of nebulous macro segregation in this region. The investigation results of t wo kinds of typical macro-segregation of ZL205 A alloy cylindrical castings show that the formation of linear macro segregation and the formation of nebulous macro segregation have close relationship with hot crack, shrinkage porosity, feeding behavior and solute distribution in the melt. The burst feeding behavior existed in both formations of linear macro segregation and nebulous macro segregation. The hot crack or shrinkage porosity defects would appear instead of macro-segregation when the macro / micro burst feeding behavior did not happen.The experimental results of pressure fluctuation show that compared with solidification under gravity and constant pressure, castings solidified under fluctuation of pressure had better internal quality. The fluctuation of pressure implemented during solidification could lead to elastic wave in the mushy zone of ZL205 A alloy, which was similar to the one caused by appling the vibration. And this enhanced and promoted the burst feeding behavior. This achieved the complete feeding of the inside part of the casting which could not be fed formerly. Finally this could reduce the tendency of shrinkage porosity defect in the castin g.Through characterized ZL205 A cylindrical castings produced by low pressure die casting by modulus, a quick prediction model of shrinkage porosity was established by numerical simulation and engineering statistical analysis method. And, it was validated by actual pouring test. |
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