Phase-field simulation of secondary dendrite growth in directional solidification of binary alloys

Phase field method was used to simulate the effect of grains orientation angle θ_(11) and azimuth θ_A of non-preferentially growing dendrites on the secondary dendrites of preferentially growing dendrites. In the simulation process, two single-factor influence experiments were designed for columnar crystal structures. The simulation results showed that, when θ_(11) < 45o and θ_A < 45o, as θ_(11) was enlarged, the growth direction of the secondary dendrites on the preferentially growing dendrites at the converging grain boundary(GB) presented an increasing inclination to that of preferentially growing dendrites; with increasing θ_A, the growth direction of the secondary dendrites on the preferentially growing dendrites at the converging GB exhibited greater deflection,and the secondary dendrites grew with branches; the secondary dendrites on the preferentially growing dendrites at diverging GBs grew along a direction vertical to the growth direction of the preferentially growing dendrites.When θ_A = 45o and θ_(11) = 45o, the secondary dendrites grew in a direction vertical to the growth direction of preferentially growing dendrites. The morphologies of the dendrites obtained through simulation can also be found in metallographs of practical solidification experiments. This implies that the effect of a grain's orientation angle and azimuth of non-preferentially growing dendrites on the secondary dendrites of preferentially growing dendrites does exist and frequently appears in the practical solidification process.

Effects of chill casting processes on secondary dendrite arm spacing and densification of Al-Si-Mg alloy

Based on the resin bonded sand casting process, the effects of chill processes on the secondary dendrite arm spacing(SDAS) and densification of Al-Si-Mg alloy were studied. The influences of the chill thickness and effective distance of chill operating on the SDAS were researched; and the effect of chill heat capacity on SDAS was investigated. The result reveals that, SDAS decreases with increasing the thickness of chill but the effect of chill is finite. The effective distance of chill operating for the chill with different thickness were obtained, and the functional relations among modulus, length of castings and thickness of chill were discussed, and the synthetical network chart of the relation among them was plotted. The relationship between local solidification rate and SDAS was defined by means of quadratic polynomial regression.

Evaluation of the microstructure, secondary dendrite arm spacing, and mechanical properties of Al–Si alloy castings made in sand and Fe–Cr slag molds

The microstructure and mechanical properties of as-cast A356（Al–Si） alloy castings were investigated. A356 alloy was cast into three different molds composed of sand, ferrochrome（Fe–Cr） slag, and a mixture of sand and Fe–Cr. A sodium silicate–CO_2 process was used to make the necessary molds. Cylindrical-shaped castings were prepared. Cast products with no porosity and a good surface finish were achieved in all of the molds. These castings were evaluated for their metallography, secondary dendrite arm spacing（SDAS）, and mechanical properties, including hardness, compression, tensile, and impact properties. Furthermore, the tensile and impact samples were analyzed by fractography. The results show that faster heat transfer in the Fe–Cr slag molds than in either the silica sand or mixed molds led to lower SDAS values with a refined microstructure in the products cast in Fe–Cr slag molds. Consistent and enhanced mechanical properties were observed in the slag mold products than in the castings obtained from either sand or mixed molds. The fracture surface of the slag mold castings shows a dimple fracture morphology with a transgranular fracture nature. However, the fracture surfaces of the sand mold castings display brittle fracture. In conclusion, products cast in Fe–Cr slag molds exhibit an improved surface finish and enhanced mechanical properties compared to those of products cast in sand and mixed molds.

Recent progresses in the suppression method based on the growth mechanism of lithium dendrite

Lithium secondary batteries（LSBs） with high energy densities need to be further developed for future applications in portable electronic devices, electric vehicles, hybrid electric vehicles and smart grids. Lithium metal is the most promising electrode for next-generation rechargeable batteries. However, the formation of lithium dendrite on the anode surface leads to serious safety concerns and low coulombic efficiency.Recently, researchers have made great efforts and significant progresses to solve these problems. Here we review the growth mechanism and suppression method of lithium dendrite for LSBs’ anode protection. We also establish the relationship between the growth mechanism and suppression method. The research direction for building better LSBs is given by comparing the advantages and disadvantages of these methods based on the growth mechanism.

Antitumor immunity by a dendritic cell vaccine encoding secondary lymphoid chemokine and tumor lysate on murine prostate cancer

Aim： To investigate the antitumor immunity by a dendritic cell （DC） vaccine encoding secondary lymphoid chemokine gene and tumor lysate on murine prostate cancer. Methods： DC from bone marrow of C57BL/6 were transfected with a plasmid vector expressing secondary lymphoid chemokine （SLC） cDNA by Lipofectamine2000 liposome and tumor lysate. Total RNA extracted from SLC＋lysate-DC was used to verify the expression of SLC by reverse transcriptase-polymerase chain reaction （RT-PCR）. The immunotherapeutic effect of DC vaccine on murine prostate cancer was assessed. Results： We found that in the prostate tumor model of C57BL/6 mice, the adminstration of SLC＋lysate-DC inhibited tumor growth most significantly when compared with SLC-DC, lysate-DC, DC or phos- phate buffer solution （PBS） counterparts （P 〈 0.01）. Immunohistochemical fluorescent staining analysis showed the infiltration of more CD4＋, CD8＋ T cell and CD11c＋ DC within established tumor treated by SLC＋lysate-DC vaccine than other DC vaccines （P 〈 0.01）. Conclusion： DC vaccine encoding secondary lymphoid chemokine and tumor lysate can elicit significant antitumor immunity by infiltration of CD4＋, CD8＋ T cell and DC, which might provide a potential immunotherapy method for prostate cancer.

Grain and dendrite refinement of A356 alloy with Al-Ti-C-RE master alloy

Commercial A356 alloy was refined with a homemade A1-5Ti-0.25C-2RE master alloy, and the microstructure and macrostructure of the refined alloy were investigated. The results show that the grain refining effect of A356 is poor by the addition level of 0.5 wt% master alloy, but when the level reaches 3.0 wt% the grain can get a satisfactory refining effect. Dendrite of A356 can be effectively refined by addition of 0.5 wt% master alloy; however, the refining effect is not significantly improved by further increasing the addition of master alloy. Grain and dendrite refining effects are compared in this article, and the results show that the grain and dendrite exhibit different refining effects with the same addition level of master alloy. Dendrite is easier to reach the optimal refining effect than grain.

锶变质近共晶Al-Si合金力学性能与枝晶α-Al特征参数的相关性

测量了用不同Sr加入量变质的近共晶Al Si合金中枝晶α Al的一次分枝间距和二次枝晶臂间距 ,考察了枝晶α Al特征参数的变化对合金力学性能的影响。研究结果表明 :Sr的加入可以细化枝晶 ,表现为一次分枝间距和二次枝晶臂间距随合金中Sr含量的增加而减小 ;枝晶细化可以提高合金的力学性能 ,合金的铸态力学性能与枝晶α

Effect of strontium on columnar growth of dendritic α phase in near-eutectic Al-11.6%Si alloys

For Al-11.6%Si alloy,the influence of the addition of Sr on the morphology of the dendrite α phase was investigated,and the characteristic parameters of the dendrite α phase,the primary dendrite spacing and the secondary dendr ite arm spacing,were also measured. The addition of strontium promotes the colu mnar dendrite growth and leads to a decrease of both the primary dendrite spacin g and secondary dendrite arm spacing with the increase of the content of stronti um in the modified near-eutectic Al-Si alloys. It is thought that the addition of Sr leads to a reduction of the solid-liquid interfacial energy of the dendr ite α phase,consequently resulting in a decrease of the growth undercoolin g of dendrite tips. And hence,the nucleation of the equiaxed grains in the liqu id in front of the columnar dendrite tips is restrained,thus the addition of st rontium in Al-Si alloys promotes the growth of the columnar dendrites. The redu ction of the solid-liquid interfacial energy also leads to the decreases in the primary dendrite spacing and the secondary dendrite arm spacing.

Composite corrosion inhibitors for secondary alkaline zinc anodes

The corrosion inhibition property of PEG600 and In（OH）3 as composite corrosion inhibitors for （second-）（ary） alkaline zinc electrodes was studied, and the inhibition efficiency was determined as 81.9%. The research focused on the mechanism by the methods of electrochemical impedance spectroscopy, polarization curves and IR spectroscopy. The results indicate that the corrosion inhibition effectiveness is attributed to the joint inhibition of anodic zinc dissolution and cathodic hydrogen evolution. And the anodic process is depressed to a greater extent than the cathodic process. The synergistic mechanism of the composite inhinbitors proves to be the enhancement of adsorption of PEG600 by In（OH）3. Potentiostatic experiment results and SEM images verify the inhibition of dendritic growth by the composite inhibitors.

镍基单晶高温合金近服役环境性能研究进展

航空发动机与燃气轮机作为国之重器,对保障国防安全和能源安全具有重要意义。随着发动机效率的不断提升,涡轮进气口温度不断提高。先进叶片往往采用高代次单晶高温合金材料制备,具有薄壁、多孔复杂冷却结构,且表面涂覆先进涂层,因而先进单晶叶片结构越来越精细复杂,服役过程中叶片不同部位温度场、应力场分布变化更大,其损伤机制无法通过传统棒状试样的变形损伤机制完全体现。基于先进单晶叶片的结构特点及服役环境特点,综述了叶片结构(薄壁、气膜孔)、二次枝晶取向(二次取向)及先进涂层等单一因素及多因素耦合作用对单晶合金拉伸、蠕变及疲劳等典型性能的影响规律及损伤机理研究,并对镍基单晶高温合金近服役环境性能研究的发展方向进行展望。

冷却速率对ZG40CrNi2Mo大型铸件凝固组织及元素偏析的影响

为了研究铸件凝固过程中不同冷却速率对其凝固组织与元素偏析的影响,利用铸件凝固过程数值模拟方法和自研的高通量凝固过程热模拟实验装置,开展了ZG40CrNi2Mo优质调质钢在不同冷却速率下的凝固模拟实验。结果表明:ZG40CrNi2Mo调质钢的二次枝晶臂间距λ_(2)与局部凝固时间t的关系符合λ_(2)=36.03 t0.26;随着冷却速率的降低,Cr元素偏析呈现上升趋势,Ni元素偏析先上升后降低;Mo元素偏析最为严重,但是对于冷却速率的变化不敏感。

BP Neural Network of Continuous Casting Technological Parameters and Secondary Dendrite Arm Spacing of Spring Steel

The continuous casting technological parameters have a great influence on the secondary dendrite arm spacing of the slab, which determines the segregation behavior of materials. Therefore, the identification of technological parameters of continuous casting process directly impacts the property of slab. The relationships between continuous casting technological parameters and cooling rate of slab for spring steel were built using BP neural network model, based on which, the relevant secondary dendrite arm spacing was calculated. The simulation calculation was also carried out using the industrial data. The simulation results show that compared with that of the traditional method, the absolute error of calculation result obtained with BP neural network model reduced from 0. 015 to 0. 0005, and the relative error reduced from 6, 76 % to 0.22 %. BP neural network model had a more precise accuracy in the optimization of continuous casting technological parameters.

Relationship between secondary dendrite arm spacing and local solidification time of 30Cr2Ni4MoV alloy at slow cooling rates

Solidification process of 231 t 30Cr2Ni4MoV ingot manufactured by slow cooling process was studied using experimental and numerical simulations, which tackled the problems of high cost and long period in large ingot studying. Based on the numerical results of large ingot, five characteristic locations under different temperature gradients and cooling rates chosen from the large ingot were simulated. The experiments were performed under the same temperature conditions as in numerical simulations with specialized instrument. The influences of temperature gradient in the solid-liquid interface and solidification rate on the size and morphology of solidification structure were analyzed at cooling rate ranging from 10-3 to 10 2℃ s-1. Solidification macrostructure and microstructure showed that no columnar dendrite was found in any specimen. The grain size and secondary dendrite arm spacing decreased at larger cooling rate, and the relationship between secondary dendrite arm spacing and local solidification time or cooling rate was determined.

Sr对近共晶Al-Si合金中α枝晶生长行为的影响

针对Al-11.6％Si和Al-11.6％Si-0.4％Mg两种近共晶成分铸造合金,考察了Sr的加入对组织中α枝晶形态的影响,测量了完全变质合金中α枝晶相数量及α枝晶的特征参数:一次分枝间距和二次枝臂间距.结果表明,Sr的加入促进了α枝晶柱状化生长并导致α枝晶相数量大幅度提高;在完全变质合金中,随着合金中Sr量增加.一次分枝间距和二次枝臂间距都降低,Sr对α枝晶起一定的细化作用.Sr加入到Al-Si合金中引起合金液的性质发生了显著变化,Sr的存在降低了α枝晶固液界面能,导致枝晶尖端的生长过冷度降低,抑制液相中等轴晶的形核,因此Sr的加入促进了柱状枝晶的形成.固液界面能的降低也使得柱状枝晶的二次枝臂间距和一次分枝间距随着Sr量增多而降低.

EFFECTS OF MISCHMETAL ADDITION ON THE CAST MICROSTRUCTURE OF 6063 ALLOYS

Effect of mischmetal addition on the cast microstructure of 6063 alloys has been investigated by means of optical microscopy, TEM and anode filming etc. The results show that there is a critical content of fining dendrite structure by adding mischmetal to 6063 alloys. This critical mischmetal content is about 0.15%. Only when the mischmetal content is above 0.15%, the secondary dendrite arm spacing decreased and eutectic structure fined. The cast grain is obviously refined when the content of mischmetal is lower. Consideration from the cast structure, the suitable mischmetal content in 6063 alloys is 0.20%.

稀土熔剂对A356合金二次枝晶臂间距的影响

将含有稀土的熔剂JDN -Ⅰ用于覆盖处理A35 6合金。实验结果表明 ,该稀土熔剂对A35 6合金的二次枝晶臂间距dDAS值有着较大的影响。无熔剂覆盖时 ,水淬获得的dDAS值比空冷下降 30 .8% ;有JDN -Ⅰ熔剂覆盖时 ,水淬获得的dDAS值比空冷下降 88.2 % ;水淬时 ,有熔剂覆盖的dDAS值比无熔剂覆盖低 77.7% ;空冷时 ,有熔剂覆盖的dDAS值比无熔剂覆盖高 2 3.3%。分析认为 ,存在 2个临界冷速v1和v2 :当冷速v <v1时 ,dDAS值由液相线斜率决定 ;当v >v2 时 ,dDAS值由JDN -Ⅰ熔剂中的稀土含量决定 ;当v1<v <v2 时 。

可充锌电极存在的问题及解决途径

综述了在碱性溶液中可充锌电极存在的形变、枝晶、腐蚀和钝化等问题的形成机制,总结和分析了解决上述问题的方法和途径。

不同冷速作用下K424镍基高温合金相变及组织形成规律

使用差示扫描量热仪,研究了不同冷却速率作用下K424镍基高温合金的相变过程。使用光学显微镜(OM)、扫描电镜(SEM)及能谱分析仪(EDS)对K424合金二次枝晶间距及成分偏析随冷速的变化规律进行了分析。结果表明,提高冷却速率使合金二次枝晶间距及碳化物的尺寸减小。枝晶组织的细化及相变速率的加快,导致空间上残余液相的分散分布及时间上初生相的不充分生长,从而减弱了枝晶间Ti,Nb等正偏析元素的富集,降低了碳化物的析出温度及转变程度,甚至可抑制其二次析出。

Zr、RE对含Ti 5182铝合金铸态组织及性能的影响

采用金相显微镜、扫描电镜、X射线衍射等分析手段,研究Zr、富铈混合稀土(RE)对Ti含量为0.03%的5182铝合金铸态组织及性能的影响。研究发现Zr细化5182铝合金晶粒能力强于RE;而RE细化5182铝合金二次枝晶的能力强于Zr。0.20%的Zr和0.20%的RE复合细化的合金具有Zr及RE细化合金的某些优点,不仅硬度最高,同时晶粒最细。Ti联合RE可使5182铝合金中析出相的形态及分布发生改变,析出相的球化程度明显增加。Zr细化的合金中,出现含Al、Ti、Zr的规则块状析出相,起到异质形核作用;添加RE的合金中,出现含Al、Ti、RE的规则块状相。

2099铝锂合金均匀化处理工艺

利用光学显微镜、扫描电镜、透射电镜、韦氏硬度计及MTS试验机研究2099铝锂合金的均匀化处理工艺。研究结果表明:2099铝锂合金适宜的均匀化处理制度为530℃/36 h。铸态合金树枝晶发达,大量的Cu元素在晶界偏析形成以Al2Cu为主的粗大骨状第二相,同时含有一定量的Zn,Mg,Fe和Mn。经530℃/36 h均匀化处理后,合金组织中的树枝晶消失,绝大部分粗大骨状第二相回溶到基体,残余第二相粒径较小(<5μm),为含有一定量的Cu,Fe和Mn的点状或条状结构。