Homogenization annealing of the 7050/6009 bimetal slab prepared by direct-chill casting was investigated and its effects on microstructural evolution, composition distribution and mechanical properties in the interfacial region of the bimetal were studied. The results show that the optimized homogenization annealing process was 460℃for 24 h. After homogenization annealing, the Zn-rich phases and Al15（FeMn）3Si2phases were precipitated at the interface of the bimetal. The diffusion layer thickness of homogenized bimetal increased by 30 μm from 440 to480℃for 24 h, while it increased by 280 μm from 12 to 36 h at 460℃. The Vickers hardnessesat 6009 alloy side and interface of the bimetal decreased after homogenized annealing and grain coarsening was considered asthedominating softening mechanism.The hardness variation at 7050 alloy side was complicated due to the combined action of solution strengthening, dispersion strengthening and dissolution of reinforced phases.
High entropy alloy has attracted increasing attentions.However,to enhance the alloy strength often leads to impairment of the ductility,or vice versa.Here we reported a heat treatment approach on AlCrFeNi2Ti0.5 high entropy alloy,which can elevate the strength and ductility simultaneously.An ingot of AlCrFeNi2Ti0.5 weighing 2.5 kg was firstly fabricated by medium frequency induction melting.Then samples from the same height of the bulk ingot were annealed for 6 h at 600,700,800 and 1000 ℃,respectively.After 1000 ℃ annealing,an optimal microstructure was obtained by using our approach which can make some precipitation particles distribute homogeneously in the dendrite interior while keep the interdendrite structure as a single solid solution phase.The mechanical test on this AlCrFeNi2Ti0.5 alloy sample showed that,the compressive fracture strength σbc was increased by about600 MPa and the plastic strain ep was doubled,compared with those of the as-cast sample.Our approach can be readily adapted to large-scale industrial production of high entropy alloys with high strength and ductility by proper annealing treatment.
Li JiangHui JiangYiping LuTongmin WangZhiqiang CaoTingju Li
The mold filling and solidification process of a marine propeller hub were simulated using ProCAST. Three casting processes–gravity casting, centrifugal casting and low pressure casting–were compared in order to get the best process. The heat transfer coefficient of the casting/mold interface was determined using a reverse method. The simulated results of velocity, temperature and shrinkage porosity distribution were discussed in detail for the three casting processes. A smooth filling was found in all three casting processes, especially the low pressure casting exhibiting a better filling performance than the other two, but the solidification processes were different. The casting did not experience the sequential solidification, and the feeding paths were blocked, leading to shrinkage porosity defects in the riser and the bottom of the casting in gravity casting and in the upper zone of the casting in low pressure casting. While, the sequential solidification was well controlled in the solidification process of centrifugal casting, and majority of the shrinkage porosity defects can only be observed in the riser. It could be concluded that the centrifugal casting process is the most suitable casting process for the production of propeller hub body. The casting experiments verified the simulation results, and a defect-free propeller hub was obtained by centrifugal casting with a rotational speed of 150 r·min-1.
Wang TongminLi JiaWu LiJie JinchuanCao ZhiqiangKang Huijun
A modified horizontal continuous casting process under the electromagnetic field was proposed for preparing AA3003/ AA4045 clad composite hollow billets. To investigate the effect of electromagnetic field on this process, a comprehensive three-dimensional model was developed. Two cases with and without electromagnetic field were compared using the simulations. When rotating electromagnetic stirring is applied, the flow pattern of fluid melt is greatly modified; the mushy zone becomes much wider, the temperature profile becomes more uniform, and the solid fraction decreases for both the external and internal alloy melt layers. These modifications are beneficial for the formation of a bimetal interface and fine and uniform grain structure of the clad composite hollow billet. Experiments conducted using the same electrical and casting parameters as the simulations verify that under the electromagnetic field the microstructure of the clad composite hollow billet becomes fine and the diffusion of the elements at the interface is promoted.
The three dimensional(3D) microstructures of Al-Bi alloys with different grain refiners(Al-5Ti-B, Al-3B and Al-3Ti) have been studied using synchrotron X-ray microtomography. The relationships between the microstructures and the corresponding wear behavior are verified through the friction and wear tests. The worn surfaces of the samples with grain refiners tested under 15 and 60 N loads are analyzed using Scanning Electron Microscopy(SEM). The results indicate that the addition of grain refiners is beneficial to the size and distribution of the Bi-rich particles in Al-Bi alloys. Compared with Al-5Ti-B and Al-3B grain refiners, the Bi-rich particles are more uniformly distributed and spherical with finer size with the addition of Al-3Ti grain refiner. The refinement renders the Al-20wt%Bi alloy refined by Al-3Ti the superior wear resistance with respect to those refined by Al-5Ti-B and Al-3B grain refiners, corresponding to the microstructures with fine and uniformly distributed Bi-rich particles in the Al matrix.
WANG TongMinCAO FeiCHEN ZongNingKANG HuiJunZHU JingFU YaNanXIAO TiQiaoLI TingJu
The distribution coefficient (LB) of boron between CaO?SiO2?BaO?CaF2 slag and silicon was investigated usingelectromagnetic induction melting for the purpose of improving the boron removal fraction. The dependence of the borondistribution coefficient between slag and silicon on the fundamental parameters of CaO to SiO2 mass ratio and refining time and theadditions of BaO and CaF2 to the slag was discussed. The results show that LB can be increased by adding BaO and CaF2 toCaO?SiO2 slag. The maximum value of LB (6.94) is obtained when the CaO to SiO2 mass ratio is 1.1:1 and the contents of BaO andCaF2 are fixed at 15% and 20%, respectively. Increasing the refining time increases the LB. After the slag treatment is performedtwice, the boron content of the silicon is successfully reduced from 3.5×10?5 to 3.7×10?6, and the removal fraction of boron reaches89.4%.
Jin-ling SUNJin-chuan JIEQing-chuan ZOULi-juan GUOZhi-qiang CAOTong-min WANGTing-ju LI
A nickel-based coating was deposited on the pure Al substrate by immersion plating,and the Al/Cu bimetals were prepared by diffusion bonding in the temperature range of 450-550 ℃.The interce microstructure and fracture surface of Al/Cu joints were studied by scanning electron microscopy（SEM） and X-ray diffraction（XRD）.The mechanical properties of the Al/Cu bimetals were measured by tensile shear and microhardness tests.The results show that the Ni interiayer can effectively eliminate the formation of Al-Cu intermetallic compounds.The Al/Ni interface consists of the Al3Ni and Al3Ni2 phases,while it is Ni-Cu solid solution at the Ni/Cu interce.The tensile shear strength of the joints is improved by the addition of Ni interiayer.The joint with Ni interiayer annealed at 500 ℃ exhibits a maximum value of tensile shear strength of 34.7 MPa.
The eutectic CoFeNi2V0.5Nb0.75 high entropy alloys （HEAs） were heated at 500, 600, 700, 800 and 1000 ℃, respectively for 6 h and subsequently quenched in the water to investigate their thermal stability and phase transformation at high temperature. The microstructure and mechanical properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, compressive and hardness tests. It was found that the as-cast CoFeNi2V0.5Nb0.75 HEAs showed a eutectic microstructure with alternating fcc solid solution phase and Fe2Nb-type Laves phase. The NbNi4-type intermetallic phase appeared when the heat-treated temperature was higher than 600 ℃. With increasing quenching temperature, the volume fraction of the NbNi4-type intermetallic phase increased while that of the eutectic regions decreased. The sample quenched at 800 ℃ showed the most excellent comprehensive mechanical properties; its fracture strength, yield strength and plastic strain were as high as 2586.76 MPa, 2075.18 MPa and 16.73%, respectively. Moreover, the eutectic CoFeNi2V0.5Nb0.75 HEAs exhibited apparent age hardening, especially quenched at 700 ℃, the hardness reached up to the maximum value of HV 727.52.
