Relationship between amounts of low-melting-point eutectics and hot tearing susceptibility of ternary Al−Cu−Mg alloys during solidification

A systematical study on the relationship between the amounts of different eutectic phases especially the low-melting-point(LMP)eutectics and the hot tearing susceptibility of ternary Al−Cu−Mg alloys during solidification was performed.By controlling the concentrations of major alloying elements(Cu,Mg),the amounts of LMP eutectics at the final stages of solidification were varied and the corresponding hot tearing susceptibility(HTS)was determined.The results showed that the Al−4.6Cu−0.4Mg(wt.%)alloy,which contained the smallest fraction of LMP eutectics among the investigated alloys,was observed to be the most susceptible to hot tearing.With the amount of total residual liquid being approximately the same in the alloys,the hot tearing resistance is considered to be closely related to the amounts of LMP eutectics.Specifically,the higher the amount of LMP eutectics was,the lower the HTS of the alloy was.Further,the potential mechanism of low HTS for alloys with high amounts of LMP eutectics among ternary Al−Cu−Mg alloys was discussed in terms of feeding ability and permeability as well as total viscosity evolution during solidification.

Prediction of hot tearing susceptibility of direct chill casting of AA6111 alloys via finite element simulations

To predict hot tearing susceptibility(HTS)during solidification and improve the quality of Al alloy castings,constitutive equations for AA6111 alloys were developed using a direct finite element(FE)method.A hot tearing model was established for direct chill(DC)casting of industrial AA6111 alloys via coupling FE model and hot tearing criterion.By applying this model to real manufacture processes,the effects of casting speed,bottom cooling,secondary cooling,and geometric variations on the HTS were revealed.The results show that the HTS of the billet increases as the speed and billet radius increase,while it reduces as the interfacial heat transfer coefficient at the bottom or secondary water-cooling rate increases.This model shows the capabilities of incorporating maximum pore fraction in simulating hot tearing initiation,which will have a significant impact on optimizing casting conditions and chemistry for minimizing HTS and thus controlling the casting quality.

Effects of cooling rates on microporosity in DC casting Al-Li alloy

During the direct chill(DC)casting process,primary cooling from the mold and bottom block,and secondary cooling from the waterjets produce a concave solid shell.The depth of this liquid pocket and mushy zone not only depends on the solidification range of the alloy but also the boundary conditions such as cooling rates.Al-Li alloys solidify in a long solidification range increasing the susceptibility of porosity nucleation in the semi-solid region.In this study,the effects of cooling rate on the porosity formation were quantified for the large ingot casting using X-ray computed tomography(XCT).By characterizing pore size distributions at four different cooling conditions,the correlation between the mechanical properties at both room and high temperatures and the microstructure features was identified.The constitutive equations were constructed.It is found that increasing the cooling rate reduces the grain size,increases the number density of micropores,and minimizes the number of large pores,thereby improving the mechanical performance.Therefore,long mushy zones and deep liquid pockets in Al-Li alloys can be effectively controlled by controlling the boundary conditions of the DC casting solidification process,thereby obtaining castings with excellent mechanical properties.

Phase-field simulation of superconductor vortex clustering in the vicinity of ferromagnetic domain bifurcations

Superconductors and ferromagnets are highly non-compatible materials due to the natures of their respective electronic states.But when artificially brought together,they develop interesting characteristics,one of which,vortex clustering,is discussed here in this paper.Phase-field and micromagnetic simulations are performed to investigate the superconductor and ferromagnet bilayer,respectively.The ferromagnet with uniaxial anisotropy is observed to develop the maze domain,whereas the superconductor subjected to the influence of the ferromagnetic stray field displays a vortex pattern.Clustered vortices in superconductors at certain locations are observed to be precisely located over magnetic domain bifurcations.The enhanced out-of-plane stray field at bifurcations around the curved domain walls and the convergent Lorentz force due to screening currents in superconductor are attributed to the formation of clusters at bifurcation sites.Segregation of the inter-vortex spacing between straight and bifurcated domain is clearly observed.More importantly,inter-vortex spacing is predicted to serve as a precise tool to map local ferromagnet domain shapes.