Solidification Liquid Film Characterization Of Al-Cu Alloys With Spherical Structure And Its Effect Mechanism On Formation Of Hot Cracking

Al-Cu alloys,as the potential materials replacing the iron and steel materials to promote the development of lightweight industry,has the advantage of low density,high specific strength,good plasticity,easy processing,excellent thermal conductivity and corrosion resistance.However,Al-Cu alloys has high thermal cracking tendency in the casting process due to the wide range of the solidification temperature and large solidification shrinkage,which limits its application.Although many targeted studies have been conducted at home and abroad to improve the hot cracking of Al-Cu alloys and certain progress has been achieved,and a series of hot cracking criterions have been put forward to successfully predict the hot cracking tendency of Al-Cu alloys,the effect mechanism of solidified liquid film on hot cracking under the condition of multiple complex factors has not been clearly defined.Meanwhile,the present hot crack criterions can only predict the tendency of hot cracking but are unable to quantitatively calculate whether hot cracks occur or not.Therefore,based on the prominent effect of solidified liquid film on hot cracking,the effect mechanism of liquid film on the hot cracking initiation was systematically studied in this paper to achieve the characterization of liquid film characteristics.A new criterion for hot cracking initiation was further proposed,and the relationship between alloying elements,liquid film characteristics and the hot cracking initiation was established to enrich the condition for the hot cracking formation.A system for measuring the liquid film shrinkage force at the end of solidification was designed and developed.The system can reproduce a initial solidification state of solid-liquid two-phase coexistence composed of the liquid film and spherical structure by secondary remelting method,so as to exclude the influence of dendrite growth on the hot cracking initiation,and realize the measurement of liquid film properties and the analysis of the hot cracking mechanism.The liquid film stress during the hot cracking initiation was detected using the device,and the hot cracking behavior of Al-4.0Cu alloys with spherical structure in different initial solidification states was analyzed.The relationship between initial solidification temperature,liquid film volume fraction and hot cracking behavior at the late stage of solidification was established.The results show that at the same temperature the larger liquid film volume fraction,the more beneficial it is to reduce the thermal cracking tendency at the late stage of solidification of Al-4.0Cu alloys with spherical structure.At the same temperature the increase of the liquid film volume fraction could change the characteristics of liquid film,so as to increase the filling capacity of liquid film,thus the formed hot cracking was successfully healed,which effectively prevent the initiation of hot cracks.Molecular dynamics（MD）and finite element analysis（FEA）were used to characterize the liquid film characteristics based on the liquid film flow.The influence of liquid film volume fraction and liquid film composition on the liquid film characteristics was studied and the effect mechanism of the liquid film characteristics on the hot cracking initiation was analyzed at the different temperatures.The results show that the intrinsic characteristics of the liquid film was regulated through the liquid film volume fraction.The change of the liquid film volume fraction made no change on the structural characteristics of the liquid film,while altered the dynamic characteristics of the liquid film.At the same temperature the increase of liquid film volume fraction reduced the viscosity of liquid film and enhanced the flow performance of the liquid film.Meanwhile,at the same temperature the increase in liquid film volume fraction resulted in a widening of the feeding channel,a reduction in liquid film flow resistance,and an enhancement of the feeding capacity of the liquid film flow.When the volume fraction of the liquid film was 36.68%,the viscosity（η）of the liquid film equaled to0.91m Pa·s,and the flow pressure drop(P_drop)of the liquid film was 0.0939Pa.Currently,the formation of hot cracks could be avioded due to the liquid film fluidity was optimal and the flow resistance is minimized.In addition,with the increase of the liquid film volume fraction,the diffusion coefficient of Cu elements contained in the liquid film increased at the same temperature,thus the subcooling degree（ΔT_c）decreased and the liquid film feeding zone（Δx）was prolonged,which was conducive to the formation of cell crystals after solidification of Al-4.0Cu alloys,and furthered increased the feeding time,eventually reduced the tendency of the hot cracking initiation.Under the experimental conditions of this paper,the fundamental cause of the hot cracking was the different volume shrinkage of solid-liquid two-phase according to the theory of solidification shrinkage and the feeding of the liquid film,and a hot cracking initiation criterion based on the characteristics of liquid films and microstructure was proposed in this paper.The primary advantage of this criterion lied in its ability to quantitatively determine the potential for hot cracking during solidification under specific conditions.The hot cracking initiation of Al-4.0Cu alloys under different solidification conditions was predicted by the criterion,which was basically consistent with the experimental results,further verified the accuracy of the criterion.By comparing the calculation results of RDG criterion and Kou criterion,it was demonstrated that this criterion yielded more consistent results with experimental data.This further substantiated its accuracy and underscored its advantages.The criterion offered significant guidance for the composition of alloys,microstructure development,and casting process condition control during solidification.According to the expression of the criterion,a lower liquid film viscosity（η）,a smaller intergranular flow pressure difference（ΔP^*）,a longer length（l^*）,wider width（r^*）and an increased number of the feeding channel（n）were advantageous for increasing the volume feeding of the liquid film(V_feeding^*)and thus reduced the tendency of the hot cracking.The solidification behavior of Al-4.0Cu-x Mg（x=0.5-2.5 wt.%）alloys and the hot cracking susceptibility of Al-4.0Cu-x Mg（x=0.5-2.5 wt.%）alloys with spherical structure in different initial solidification states were systematically investigated,with a particular focus on the influence of Mg contents on the liquid film characteristics and hot cracking initiation.The results indicated that the grain size initially decreased and subsequently increased with Mg contents under different initial solidification conditions,exhibiting a"V"-shaped trend.The minimum value was achieved at 1.5 wt.%Mg contents.From the perspective of shrinkage,the correlation between solid shrinkage volume（V_s^*）and Mg additions was consistent with the grain size,while the liquid film shrinkage volume（V_L^*）decreased linearly with the increase of Mg addition.In terms of the liquid film flow feeding,when the volume fraction of the initial solidified liquid film was the same,although too much Mg element would lead to the increase of liquid filmη,the change in r^*and l^*with Mg contents was consistent with the grain size,and V_feeding^* reached its maximum value at minimum grain size.Therefore,the Al-4.0Cu-1.5Mg alloy exhibited high resistance to the hot cracking.Two kinds of hot cracking morphologies were existed in Al-4.0Cu-x Mg（x=0.5-2.5 wt.%）alloys under different initial solidification conditions.When the Mg content ranged from 0.5to 1.5 wt.%,the hot cracks occurred due to the formation of a closed zone beforehand and insufficient filling capacity for the liquid film.The other one kind of the hot cracking morphology was that the hot cracks formed by intermetallic compounds with unstable surface when Mg content was greater than 1.5 wt.%.The prediction results of the criterion showed that the hot cracks formed due to the solid-liquid two-phase shrinkage and insufficient liquid film feeding,which was consistent with the experimental results,which furthered verified the accuracy of the criterion proposed in this paper.The interfacial stability of different intermetallic compounds calculated by MD method indicated that the separation work required for the interfacial separation of Al Cu Mg phase as matrix was less than that required for S-Al₂Cu Mg phase,regardless of whether S-Al₂Cu Mg and Al Cu Mg formed interfaces withα-Al orθ-Al₂Cu.When the content of Mg elements was greater than 1.5 wt.%,more unstable Al Cu Mg phases formed,resulting in the increasing of hot cracking sources in the alloy.