Fluidity of Al-and Fe- based eutectic alloys at high velocity in thin sections

• Main Author: Habibollahzadeh, Ali
• Published: University of Birmingham 2001
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.566226

In this investigation, the fluidity test was adopted to study (a) the flow behaviour of liquid metals at high velocity in thin channels and (b) the maximum fluidity for two important casting eutectics: grey iron and Al-12% Si. The effects of section thickness, pouring temperature, mould binder (for cast iron) and the design of filtering system were also investigated. In the first step of studies, some fluidity tests were carried out with cast iron and the results and the quality of the fluidity strips were examined. The quality of the fluidity strips revealed that due to the high velocity of liquid during casting, the liquid suffered from air entrainment and the flow washed sand inclusions into the stream. SEM microscopy studies indicates the presence of a surface film on liquid cast iron. This film was a carbon film when the melt was poured in resin-bonded moulds. In the atmosphere of sodium silicate moulds a complex silicate film was detected. It seems that, in comparison with the silicate film, the carbon film was strong enough to prevent the liquid flow from disintegration at high velocities of about 2 mls. In the next step, investigations were carried out by the use of real-time x-ray radiography to design an improved filter system to incorporate in the fluidity channel. Among five different filter systems, the one that produced the best quality casting was chosen. Another series of fluidity tests incorporating this filter system was carried out. In addition, two software packages (MAGMAsoft and FLOW3D) were used to simulate the fluid flow and heat transfer conditions inside the filter system. The measurements from both the real-time x-ray radiography technique and the simulation results indicate that a mixed laminar and inertial dependant flow dominates the flow inside the filter system. In this step, the fluidity of cast iron and Al-Si alloy liquids and effects of superheat and modulus were studied and results were in fairly good agreement with theoretical predictions. The results revealed that the fluidity of metal at high velocities, similar to the fluidity at normal velocity, increases with superheat and is proportional to the square of the modulus of the fluidity channel. In the final stage of this investigation, the flow of liquid in the fluidity channel was studied to find out the mechanism of flow endurance in the channel in both alloys. The mechanism was the formation of a flow channel and the remelting of solid particles at the flow front. Also, quantitative image analysis on the shape and distribution of phases was carried out to evaluate the possible arresting mechanism of the fluidity samples. The evidence indicates that in cast iron with low cooling rates, accumulation of eutectic particles in the flow tip is the main feature that acts to stop the flow. At high cooling rates, the mechanism of flow arrest in liquid cast iron is the formation of vein at some distance behind the flow tip. When this vein is closed due to the growth of grains from walls, the flow will stop. In AI-Si alloy stream, it seems that a-AI dendrites and/or primary silicon particles are carried down stream. Their accumulation is the reason for the arrest of flow.