Propagation Of Mechanical Wave During Solidification Of ZL205A Alloy And Its Promoting Behavior On Feeding

ZL205A Alloy has been widely used in industry because of its high strength and low density.However,the shrinkage defects are easy to appear because of the paste solidification characteristics of ZL205A alloy.There are two ways to eliminate shrinkage defects:increasing pressure and unblocking feeding channel.If the feeding channel is blocked,increasing the feeding pressure has limited improvement on feeding,so the vibration is applied to improve the feeding capacity by unlocking the feeding channel.Vibration acts on the alloy melt in the form of mechanical wave,so it is very important to explore the propagation law of mechanical wave in the alloy during solidification.At present,there is little research on the problem,and the action behavior of vibration on feeding is not clear.Therefore,we established a unified rheological model in the whole temperature range and explore the propagation law of mechanical wave in complex non-uniform viscoelastic media with variable temperature and variable structure.The continuity of structure and properties of alloy in solidification can lead to the continuous constitutive relationship.However,the stress-strain relationship described by different models in different solidification stages right now.The results of rheological experiment show that the viscoelastic model of ZL205A alloy in the solid-liquid region is the H-K model（Hooke-Kelvin）.Based on the H-K model,we construct a unified constitutive model in the whole temperature range according to studying the variation law of parameters,and we obtain the value of parameters by experiment and derivation.The unified model reveals the constitutive relationship and the variation law of model parameters of alloy as a viscoelastic medium during its continuous transformation from liquid to solid.The results show that the stress-strain relationship of ZL205A alloy in the whole temperature range can be described by H-K model uniformly.The shear rheological parameters areμ₂→0 andμ₁→∞when the temperature is higher than the de ndrite coherency temperature,and the volumetric rheological parameters is K₁→∞when the temperature is higher than the liquidus.According to the unified constitutive model,the thermoviscoelastic wave equations are established in elastic,viscoelastic and viscous media.The comparison between numerical solution and experiment al results shows that the thermoviscoelastic wave equations with thermal mechanical coupling effect can describe the propagation law of mechanical wave in alloy melt better.The longitudinal wave velocity predict ed by the equation is in good agreement with the experimental values.The difference equations of the thermodynamically coupled H-K viscoelastic wave equations are established by the high-order staggered grid finite difference method.The treatment method of complex free boundary in wave propagation calculation is proposed.The numerical platform which can stably simulate the viscoelastic wave equations under any complex structure and any temperature field is found by Fortran.The numerical simulation of mechanical wave propagation in arbitrary inhomogeneous and unsteady viscoelastic media is realized.Taking account the shear wave and longitudinal wave,the propagation law of mechanical wave in uniform and non-uniform viscoelastic media is studied numerically.The propagation behavior of mechanical wave at the solid-liquid interface and mold-casting interface is discussed,and the effects of vibration source and waveguide rod on the wave field are analyzed.The resul ts show that the wave velocity and the stress amplitude decrease and the displacement amplitude increases when the mechanical wave propagates from the elastic medium to the viscous medium of alloy melt.When the mechanical wave propagates through the steel mold to th e alloy melt,the energy flow has large transmission coefficient and small reflection coefficient.The fundamental reason why the mechanical wave can hardly propagate through the sand mold to the casting is the ablation of the interface of sand mold and casting by the high temperature melt,and the air gap appears at the interface,which isolates the propagation of vibration.Propagation of mechanical vibration through metal waveguide rod into casting is an effective means to solve the vibration transmission problem in sand casting.The conical waveguide rod with large to small cross-section has the best effect of transmitting vibrati on to the casting and the minimum vibration loss.Under the complete free boundary,the displacement caused by vibration in the casting and the normal stress gradi ent along the vibration direction are the largest.The less the restriction on the mold in the actual casting,the higher th e utilization of vibration.The higher the concentration of vibration source,the higher the efficiency of vibration propagation to the casting,and the greater the fluctuation caused in the casting.Based on the study of the propagation law of mechanical wave in viscoelastic medium,the action behavior of S-wave and P-wave on burst feeding and seepage feeding is revealed.The effect of vibration on feeding is verified by low-pressure vibration casting test and seepage experiment.Finally,the effectiveness of vibration promoting feeding of vertical cylinder structure is verified by low-pressure ring-shaped casting.The results show that the S-wave propagates the shear stress,which is easy to form stress concentration at the weakest part of the channel,and destroys the blocking structure to form the burst feeding.The P-wave propagats tensile and compressive stress,which expands the feeding channel and increases the feeding pressure along the vibration direction,to promote seepage feeding.Vibration can obviously promote the feeding of vertical cylinder structure and help to eliminate the shrinkage defects on large thin-walled cylindrical castings.