Research On Rheological Filling And Rheological Feeding Ability Of Squeeze Casting Aluminum Alloy

With the development of green manufacturing and low-carbon economy,casting products are transforming to high-precision,lightweight,complex and large-scale.In order to reduce the formation of shrinkage defects,pressurized rheoforming method such as squeeze casting,is particularly important.And pressurized rheoforming method is a process of filling,feeding and solidification of alloy melt containing liquid and solid-liquid mixture with rheological property under applied pressure.In this process,the coupling effect of rheology and solidification occurs in the alloy melt under applied pressure,which leads to the beneficial effects of microstructure refinement,feeding and segregation reduction.However,the influence of the coupling effect of rheology and solidification on the filling and feeding behavior has not been accurately understood at present,let alone effectively controlled.Based on the stopping rheological mechanism and condition under pressurized solidification,the rheological filling and feeding behavior of squeeze casting aluminum alloy were further studied in this paper.It can provide theoretical guidance for die design and process design of large castings produced by squeeze casting,so as to effectively reduce the formation of shrinkage defects.The main conclusions are as follows:(1)The Archimedes spiral experiment of squeeze casting was designed to study the rheological filling and feeding behavior of aluminum alloy in a round pipe.The variations of primary ?-Al phase and eutectic structure along spiral length direction were analyzed.The results showed that with the increase of spiral length,the primary?-Al phase was transformed from fine rosettelike or granular structure to coarse platelet-like structure,and its volume fraction increased from 45.57 % to 70.35 %.The solid fraction of stopping rheology reached 70.35 %,which proved that the experimental pressure promoted the rheological ability of the alloy melt containing liquid and solid-liquid mixture;The eutectic Si phase varied from a fine granular or wormlike structure to a coarse platelet-like or needle-like structure,and the dispersion of eutectic Si particles was also varied along the spiral length direction.(2)According to the rheological properties and mechanical analysis of different rheological bodies,it was revealed that the stopping rheological mechanism of squeeze casting aluminum alloy was as follows.With the progress of solidification,the temperature of alloy melt was gradually decreased,so the rheological resistance was gradually increased.While the rheological driving pressure was gradually decreased due to friction consumption.When the rheological driving pressure could not overcome the rheological resistance,the rheological velocity was rapidly decreased and the alloy melt stopped rheology.(3)By solving the one-dimensional coupled heat conduction and convection equation with harmonic method,the law of temperature attenuation along the path was acquired,then the law of rheological resistance increasing along the path was obtained;on the other hand,the law of pressure attenuation along the path was solved.Finally,the mathematical model of rheological filling distance for squeeze casting aluminum alloy was established by combining the stopping rheological mechanism and conditions of alloy melt.The verification experiment showed that the maximum relative deviation between the calculated results and the experimental results was only8.2 %.Compared with the existing filling capacity model,the rheological filling distance model in this paper has some progress in the prediction accuracy and application scope.(4)According to the solidification square root law,the variation rule of feeding channel diameter along the path was derived for the rod and frame parts with large length diameter ratio.Combined with the pressure loss in the rheological process,the rheological resistance of non-Newtonian fluid and the condition of stopping rheological feeding,the mathematical model of rheological feeding distance for squeeze casting rod and frame parts was established.The verification experiment showed that the maximum relative deviation between the theoretical calculation results and the experimental results was only 6.61 %.However,the prediction model of rheological feeding distance for squeeze casting rod frame parts has not been reported.(5)Considering the heat conduction in the height direction and the solidification square root law,when the flow driving force in the radius direction could not overcome the rheological resistance,the alloy melt stopped rheology.The mathematical model of rheological feeding range for squeeze casting disks parts was established according to the stopping rheological condition.The experimental results showed that the rheological feeding distance calculated by the model was in good agreement with the experimental results,and the maximum relative deviation was only 11.8 %.However,there is no report about the prediction model of rheological feeding range for squeeze casting disks parts.(6)According to the theoretical model of rheological filling distance and rheological feeding distance,the influences of main process parameters on filling defects and density were revealed: appropriately increasing the applied pressure and rheological velocity could improve the rheological properties of alloy melt containing liquid and solid-liquid mixture,thus increasing the rheological filling distance and rheological feeding distance of alloy melt.In addition,increasing the initial diameter of feeding channel could reduce the pressure drop and increase the rheological feeding distance.Finally,the compact castings with complete filling,smooth surface and no shrinkage defects were obtained.(7)The application of rheological feeding model for squeeze casting disc parts in actual production was further studied.The variations of density and mechanical properties along radius direction were mainly analyzed to verify the pressurized rheological feeding law.The results showed that the material density,hardness and tensile properties decreased with the increase of the rheological radius.These results fully revealed that the rheological feeding driving pressure decreased gradually along the rheological radius direction,but the material rheological resistance increased gradually.With the increase of the rheological radius,the difference between the rheological feeding driving pressure and the material rheological resistance became smaller and smaller,or even negative.Therefore,complete feeding became incomplete feeding during this process.