Coupling Prediction Of Porosity,segregation And Hot Tearing In The Solidification Of Al Alloys

Aluminum alloys have been widely used in aerospace,train and vehicle industry due to their good castability.With the rapid development of national defense technology and manufacture industry,and the increasing demand for environmental protection and energy conservation and emission reduction,large-scale,complex,precise,thin-walled,integrated and lightweight aluminium alloy castings are greatly needed.The major problem for the casting process will be insufficient liquid feeding at the thin wall,intense liquid flow at the abrupt change of cross-section and stress concentration at the hot spot,which will lead to the formation of porosity,segregation and hot tearing in castings.Numerous efforts have been put into investigation on porosity,solute segregation and hot tearing separatly since the 50s of last century.However,few work has been done to for predicting the three defects simultaneously,whereas they often interact with each other.In this thesis,we are trying to establish a theoretical model for coupling prediction of the three kinds of defects,based on the analysis on the influence of porosity on interdendritic feeding flow,the impact of porosity on solute redistribution behavior and the role of porosity during the evolution of hot tearing.As for the prediction of porosity in the mushy zone,first the expression of hydrogen solubility in the melt was established as a function of temperature,solute concentration and the pressure evolution.Then a comprehensive porosity model was proposed by combing the effects of hydrogen segregation and the pressure drop induced by solidification shrinkage.The fraction of porosity can be obtained by analysis on the redistribution behavior of hydrogen among liquid phase,solid phase and porosity at the solid/liquid interface.Besides,the gas precipitation during the eutectic reaction has also been taken into account.The application of this novel porosity model to the unidirectional solidification of Al-4.5 wt%Cu alloy with columanr dendritic morphology predicts a relative smaller porosity fraction when compared to the model proposed by Poirier.Furthermore,the microstructure size,especially the primary dendrite arm spacing dependence of the porosity fraction shows that a coarse microstructure tends to promote the formation of porosity.As for the influence of porosity on solute segregation,the solidification shrinkage in the mushy zone during the solidification of primary phase and eutectic was first analyzed.Then the expressions of feeding flow in the mushy zone of columnar dendritic solidification structure were established with the presence of porosity.By analyzing the effect of porosity on solute redistribution behavior in the mushy zone,the well known"local solute redistribution equation"was modified to take into account both the precipitation of porosity and the variation of liquid density in the solidification process.Application of this model to the unidirectionally solidified Al-4.5 wt%Cu alloy with columnar dendritic structure.The results show that with the increase of initial hydrogen content,the volume fraction of porosity increases accordingly,while the backward feeding flow and the solid and liquid fraction decreases,resulting in a decrease of average alloy composition in the final solidification structure.The unidirectional solidification process of Al-Cu alloys was simulated by MATLAB software with implicit finite difference method.The equivalent heat capacity method is used to treat with the latent heat for solution of heat transfer with phase change,and the temperature emendation method has been utilized to treat with nodes where the temperature drop across two adjacent temperature regions in one time step.Both the numerical prediction of evolution of the mushy zone for unidirectional solidification of Al-4.5 wt%Cu alloy,and the prediction of cooling curves for unidirectional solidification of Al-6.2 wt%Cu alloy are in good agreement with the corresponding experimental measurement obtained from literature.The solidification parameters,including the moving velocity of solidus isotherm and liquidus isotherm,the temperature gradient at the dendrite tips and the local solidification time were calculated.With which the microstructure size,especially the primary dendrite arm spacing and secondary dendrite arm spacing were predicted.Besides,the feeding flow was modified to account for the three special regions of unidirectional solidification:initial transient,complete mushy zone and final transient.The porosity fraction and average alloy composition distribution along the ingot were obtained,the results show that with the increase in initial hydrogen content,the porosity fraction increases,while the average alloy composition decreases,indicating that the porosity can effectively reduce the inverse segregation appearing near the surface of castings.The classical RDG hot tearing model was modified to include the effect of porosity precipitation on the pressure drop in the mushy zone.Based on the summary of previous investigation on hot tearing and the observation of fracture surface,a novel hot tearing criterion based on the occupation of porosity in the interdendritic space was proposed.A critical hot tearing susceptibility H_cr is further proposed for columnar dendrite solidification structure.When the hot tearing susceptibility index is greater than this critical value,porosity tends to coalesce with the nearby ones to form a local hot tear.Meanwhile,the initial hydrogen content and tensile strain rate dependence of the hot tearing susceptibility of Al-Cu alloys were studied separately.It was found that both factors elevate the hot tearing susceptibility,especially for alloys with 0.5%?2%Cu.The hot tearing susceptibility of Al-x Cu alloys(x=0.5?1.3?1.5?2.0?4.0 wt%)were further calculated.The results show that Al-1.3 wt%Cu alloys is more vulnerable to hot tearing than the other alloys.Al-4 wt%Cu alloy is not likely to experience hot tearing at the current hydrogen content and strain rate because of the high permeability of its solid network,interdendritic feeding channels remains open at the end of solidification and the efficient backward liquid feeding prevents the formation of hot tearing.Al-0.5 wt%Cu alloy has a low tendency of hot tearing because the dendrite coalescence and solid bridging will transfer stress and strain to the solidified parts.Therefore,porosity increases the hot tearing tendency by weakening the solid bridging and coalescence between dendrites for dilute alloys,or by reducing the effective feeding flow for alloys with high solute concentration.