Nucleation and growth of eutectic cell in hypoeutectic Al-Si alloy

The nucleation and growth of eutectic cell in hypoeutectic Al-Si alloy was investigated using optical microscopy and scanning electron microscopy equipped with electron backscattering diffraction（EBSD）.By revealing the eutectic cells and analyzing the crystallographic orientation,it was found that both the eutectic Si and Al phases in an eutectic cell were not single crystal,representing an eutectic cell consisting of small ＇grains＇.It is also suggested that the eutectic nucleation mode can not be determined based on the crystallographic orientation between eutectic Al phases and the neighboring primary dendrite Al phases.However,the evolution of primary dendrite Al phases affects remarkably the following nucleation and growth of eutectic cell.The coarse flake-fine fibrous transition of eutectic Si morphology involved in impurity elements modification may be independent of eutectic nucleation.

Grain refining mechanism of Al-3B master alloy on hypoeutectic Al-Si alloys

Al-3B master alloy is a kind of efficient grain refiner for hypoeutectic Al-Si alloys. Experiments were carried out to evaluate the effect of undissolved AlB2 particles in Al-3B master alloy on the grain refinement of Al-7Si. It is found that the number and the settlement of AlB2 particles in the melt all have effect on the grain refining efficiency. On the basis of experiments and theoretical analysis, a new grain refinement mechanism was proposed to explain the grain refinement action of Al-3B on hypoeutectic Al-Si alloys. The formation of ＇Al-AlB2＇ shell structure is the direct reason for grain refinement and the undissolved AlB2 particles is the indirect nucleating base for subsequent α（Al） phase.

Microstructure and room temperature mechanical properties of NiAl-Cr(Mo)-(Hf,Dy) hypoeutectic alloy prepared by injection casting

The NiA1 Cr（Mo） （Hf, Dy） hypoeutectic alloys were prepared by conventional casting and injection casting techniques respectively, and their microstructure and room temperature mechanical properties were investigated. The results reveal that with the addition of Hf and Dy, the Ni2AIHf Heusler phase and NisDy phase form along the NiAI/Cr（Mo） phase boundaries in intercellular region. By the injection casting method, some Ni2AIHf Heusler phase and NisDy phase transform into Hf and Dy solid solutions, respectively. Moreover, the microstructure of the alloy gets good optimization, which can be characterized by the fine interlamellar spacing, high proportion of eutectic cell area and homogeneously distributed fine Ni2AIHf, NisDy, Hf solid solution and Dy solid solutions. Compared with conventional-cast alloy, the room temperature mechanical properties of injection-cast alloy are improved obviously.

Research on semi-solid slurry of a hypoeutectic Al-Si alloy prepared by low superheat pouring and weak electromagnetic stirring

The semi-solid slurry of a hypoeutectic Al-Si alloy was manufactured by low superheat pouring and weak electromagnetic stirring. The effects of pouting temperature and stirring power on the semi-solid slurry were investigated. The results indicated that the semi-solid slurry to satisfy rheocasting can be manufactured by low superheat pouring and weak electromagnetic stirring. The pouring temperature （or superheat） and the stirring power remarkably affected the morphology of primary α-Al and the size of primary α-Al, and there is no obvious effect of stirring time on primary α-Al. Compared with the samples made by low superheat pouring with no stirring, the nucleation rate, particle morphology and grain size of primary α-Al in A356 were markedly improved by low superheat pouring and weak electromagnetic stirring. On the condition of weak electromagnetic stirring, the pouring temperature with low superheat can be suitably raised to reach the effectiveness obtained from the lower pouring temperature without stirring.

Manufacture technique of semi-solid slurry of hypoeutectic Al-Si alloy by low superheat pouring and weak electromagnetic stirring

The semi-solid slurry of hypoeutectic Al-Si alloy was manufactured by low superheat pouring and weak electromagnetic stirring. The effects of pouring temperature and stirring power on the semi-solid slurry making process were investigated. The results indicate that the semi-solid slurry to satisfy rheocasting requirement can be made by a combination of low superheat pouring and weak electromagnetic stirring. The pouring temperature (or superheat) and the stirring power significantly affect the morphology and the size of primary α-Al, while there is no obvious effect of the stirring time on primary α-Al. Compared with the samples made by low superheat pouring without stirring, the nucleation rate, particle morphology and grain size of primary α-Al in A356 Al alloy are markedly improved by a process of applying both low superheat pouring and weak electromagnetic stirring. Under the condition of weak electromagnetic stirring applied, the pouring temperature with low superheat can be equivalently to reach the effectiveness obtained from the even lower pouring temperature without stirring.

Effect of molybdenum on the microstructure and wear resistance of hypoeutectic Fe-Cr-B-C hardfacing alloys

Hypoeutectic Fe-Cr-B-C hardfacing alloys with different molybdenum( Mo) contents( The design content was 0,2,3. 3 and 4. 5 wt. %,respectively) were deposited using the flux-cored wire by means of metal active gas arc welding. The effects of Mo on the refinement of microstructures,eutectic microstructure changes and improvement of wear resistance were investigated. The main results were shown as follows: the added Mo could increase the volume fraction of eutectic microstructure and reduce the size of coarse primary austenite as well as the volume fraction. The carboboride of M3( B,C) could be observed in hypoeutectic Fe-Cr-B-C hardfacing alloys at the Mo design content of ≤2. 0 wt. %,while that of M23( B,C)6was formed when 3. 3 wt. % Mo was added.Additionally,the wear resistance of alloys was increased with the increase in Mo content. Specifically,the highest wear resistance of alloys was achieved at 4. 5 wt. % Mo design content,which was 113. 7% higher than that in alloys without Mo.

Effect of Sr modification on microstructure and thermal conductivity of hypoeutectic Al−Si alloys

Trace amount of Sr(0.05 wt.%)was added into the hypoeutectic Al−Si(3−12 wt.%Si)alloys to modify their microstructure and improve thermal conductivity.The results showed that the thermal conductivity of hypoeutectic Al−Si alloys was improved by Sr modification,and the increment and increasing rate of the thermal conductivity gradually increased with Si content increasing.The improvement of thermal conductivity was primarily related to the morphology variation of eutectic Si phases.In Sr-modified Al−Si alloys,the morphology of eutectic Si phases was a mixed morphology of fiber structure and fine flaky structure,and the proportion of the fine flaky eutectic Si phases gradually decreased with Si content increasing.Under the Si content reaching 9 wt.%,the proportion of fine flaky eutectic Si phases was nearly negligible in Sr-modified alloys.Correspondingly,the increment and increasing rate of thermal conductivity of Sr-modified alloys reached the maximum and tended to be stable.

Effects of vibration and grain refiner on microstructure of semisolid slurry of hypoeutectic Al-Si alloy

The effects of vibration and grain refiner on the microstructure of semisolid slurry of hypoeutectic Al-Si alloy were studied. The impact of vibration on the convection of liquid was conducted by using a system of water-particle tracer. The 356 melt at temperature of 630-660 ℃ with or without grain refiner Al-5%Ti-1%B was poured into a metal cup as the vibrating vessel, then it was cooled to 590-610 ℃ in the semisolid zone and kept for some time, subsequently vibration with different frequencies was applied. The results show that the primary α(Al) particles become finer and rounder with the increase of vibration frequency. The slurry with primary α(Al) equivalent particle diameter(EPD) of about 90 μm and average shape coefficient(ASC) of about 0.5 can be prepared under vibration of 20 Hz. With the combined action of vibration and grain refiner Al-5Ti-B, even smaller and rounder spheroids with EPD of about 85 μm and ASC of about 0.6 are obtained.

Microstructural evolution and microsegregation in directional solidification of hypoeutectic Al−Cu alloy:A comparison between experimental data and numerical results obtained via phase-field model

The objective of the work is focused on predictions of microsegregation,solidification speed,dendritic arm spacings and dendrite morphology by phase-field model.The numerical results were compared with experimental data.The experimental values for cooling rates and effective partition coefficient were adopted during calculations.The results of microsegregation through phase-field model show excellent agreement with the experimental data.Such excellent agreement is because cooling rates,effective partition coefficient and back-diffusion of solute are considered in the model.For solidification speed,the calculation results show good agreement with the experimental data.Tertiary dendritic arm spacing calculated with phase-field model is compared with experimental data.The results show good agreement between them.The dendrite arm spacing varies with position because high cooling rates are responsible for the refinement effect on microstructure.Finally,two-dimensional simulation produced a dendrite that is similar to that found in the experiment.

Refining Effect of Boron on Hypoeutectic Al-Si Alloys

Several concepts of the grain refinement mechanism of B on hypoeutectic Al-Si alloys have been adopted: the refining effect of B on the a-AI and eutectic Si with the different additions of Al-B master alloys made at 850℃ was investigated; and the Al-B master alloys formed under different temperature conditions have been studied to explore the morphologies of AIB2 particles; slowly cooled sample with addition of Al-B was made to explore the refinement mechanism. AI-B master alloy can refine not only a-AI, but eutectic Si. Theoretical analysis indicates that, although AIB2 does not take part directly in the nucleation process in pure Al in the presence of Si, it provides a substrate for precipitation of a small content of Si from which a-At will grow without any undercooling. When the temperature decreases to eutectic line, AIB2 subsequently nucleates eutectic Si; AIB2 particles appear in two different morphologies, namely, hexagonal platelet and tetradehedron morphology which depend on the processing temperature conditions.

Effect of eutectic phase on damping and mechanical properties of as-cast Mg-Ni hypoeutectic alloys

Dynamic mechanical analysis (DMA) was applied to systematically investigate the low frequency damping properties of as-cast hypoeutectic Mg-Ni alloys. The results show that the as-cast hypoeutectic Mg-Ni alloys exhibit high damping capacities. The strain amplitude dependent damping curve has its own special characteristic, in which the damping is strongly related to the strain amplitude. The effect of the eutectic phase on damping and the mechanical properties of as-cast hypoeutectic Mg-Ni alloys were also discussed in detail.

Effect of slow shot speed on externally solidified crystal,porosity and tensile property in a newly developed high-pressure die-cast Al-Si alloy

The effect of slow shot speed on externally solidified crystal(ESC),porosity and tensile property in a newly developed high-pressure die-cast Al-Si alloy was investigated by optical microscopy(OM),scanning electron microscopy(SEM)and laboratory computed tomography(CT).Results showed that the newly developed AlSi9MnMoV alloy exhibited improved mechanical properties when compared to the AlSi10MnMg alloy.The AlSi9MnMoV alloy,which was designed with trace multicomponent additions,displays a notable grain refining effect in comparison to the AlSi10MnMg alloy.Refining elements Ti,Zr,V,Nb,B promote heterogeneous nucleation and reduce the grain size of primaryα-Al.At a lower slow shot speed,the large ESCs are easier to form and gather,developing into the dendrite net and net-shrinkage.With an increase in slow shot speed,the size and number of ESCs and porosities significantly reduce.In addition,the distribution of ESCs is more dispersed and the net-shrinkage disappears.The tensile property is greatly improved by adopting a higher slow shot speed.The ultimate tensile strength is enhanced from 260.31 MPa to 290.31 MPa(increased by 11.52%),and the elongation is enhanced from 3.72%to 6.34%(increased by 70.52%).

Effects of Si Content and the Addition Amount of Al-3B Master Alloy on the Solidification Structures of Hypoeutectic Al-Si Alloys

Effects of Si content and the addition amount of Al-3B master alloy on the solidification structures of hypoeutectic Al-Si alloys were studied. The addition amounts of the master alloy were 0.2%, 0.4%, 0.7% and 1% (mass fraction, so as the follows), respectively. The Si content of Al-Si binary alloys investigated varied from 1% to 11%. The observation of macrostructures of non-refined samples showed that 3% Si constitutes a transition point at which the minimum grain size can be obtained. It was also found that Al-3B master alloy can shift the transition point towards a higher Si value when its addition amount increases, making this point appear at 4%, 5% and 6% Si as its addition amount increases up to 0.4%, 0.7% and 1%, respectively.

Grain refinement of hypoeutectic Al-Si alloy prepared with ELTA by A1-4B master alloy

Electrolytic low-titanium aluminum (ELTA) was produced by adding TiO2 powder to an industrial aluminum electrolyzer. The grain refining effect of A1-4B master alloy in the hypoeutectic Al-Si alloy prepared by using ELTA was investigated, and compared with those of Al-5Ti, A1-5Ti-1B and Al-4B master alloys in the similar alloy prepared by using pure Al. The results indicate that when Al-4B is added to the melt of the alloy prepared by using ELTA in terms of the Ti/B mass ratio of 5:1, the grain refining effect is better than those of Al-5Ti, Al-5Ti-1B and Al-4B master alloys. Thus, using Al-4B to refine the grain of Al-Si alloys prepared by using ELTA will possibly become a feasible way of obtaining Al-Si alloy with homogeneous and fine microstructure.

形变热处理对亚共晶Fe-C-B合金组织和性能的影响

采用形变热处理的方法来改善含0.36%C、1.48%B的亚共晶Fe-C-B合金的组织和性能。结果表明,高温形变后,合金中的含硼硬质相由连续网状分布转变为块状聚集态分布,α-Fe中形成了(110)晶面的择优取向。形变后的试样经1050℃奥氏体化淬火后,硬质相聚集现象消失,转变为条块状弥散分布在基体中,基体完全转变成马氏体,α-Fe中(110)晶面的择优取向消失。经过形变热处理后,合金硬度达58 HRC,冲击韧性与形变热处理前有较大提高,达到9.3 J/cm2。

Influence of chemical modification by Y2O3 on eutectic Si characteristics and tensile properties of A356 alloy

The modification of A356 aluminum-silicon alloy using yttrium oxide (Y2O3) was studied. Addition levels of up to 2.5 wt.% Y2O3 were investigated. A premixed powder of Al-30wt.%Y2O3 was added to the melt at about 750℃ using vortex method. Samples were then poured in sand mold. The results showed that evident modification was obtained using the Y2O3 addition. The optimum level was 1.5 wt.%, and was corresponding to a eutectic temperature depression from 568 to 557℃. The eutectic Si particles were refined in length from 44.8 to 8.3 μm, and modified in aspect ratio from 6.8 to 0.98. Higher additions of Y2O3 caused de-modification of the eutectic Si particles. The ductility of the modified specimens was enhanced by more than 20% compared to the unmodified ones. This was associated with a gradual transfer from cleavage to a more ductile mode of fracture.

Mg-Sn共晶合金的凝固组织演化及晶体生长机理

采用金相显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)和能谱仪(EDS),研究自由凝固下Mg-Sn共晶合金在不同凝固阶段的组织、相的生长形态及相组成等。探讨Mg-Sn共晶合金的晶体生长机制以及冷却速度对合金显微组织的影响。结果表明:Mg-Sn亚共晶合金中的凝固组织为六角蔷薇花状的初生α-Mg相和共晶Mg/Mg_(2)Sn层片组织的混合结构;Mg-Sn过共晶合金中的凝固组织为棱角分明的初生Mg_(2)Sn金属间化合物相和共晶Mg/Mg_(2)Sn层片组织的混合结构;在亚共晶成分范围内,随着Sn含量的增加,初生α-Mg相含量减少,共晶相含量增多;随着冷却速率的提高,合金组织得到了明显的细化;初生α-Mg相为非小平面相,初生Mg_(2)Sn金属间化合物相为小平面相。

母合金成分和混合温度对受控扩散凝固组织的影响

采用强制对流混合方式的受控扩散凝固技术制备了亚共晶Al-8%Si合金,研究了不同的母合金成分和混合温度对亚共晶Al-8%Si合金组织中初生α-Al相的形貌和尺寸的影响。结果表明,在纯Al与Al-12%Si合金混合后制备的Al-8%Si合金中初生α-Al相为细小圆整的球状晶,而且在纯Al的混合温度为665℃时,初生α-Al相的细化效果最好,晶粒尺寸可以达到43μm。随着母合金中Si含量的增加与纯Al混合温度的升高,初生α-Al相的细化效果变差。

铝合金复合板钎焊层的初晶硅控制

钎焊用铝合金复合板外层使用的是亚共晶铝硅合金,钎焊时粗大初晶硅熔蚀芯材,降低铝合金复合板的成形性能。分析了亚共晶铝硅合金的初晶硅产生原理,研究了细化初晶硅的方法。降低原材料磷含量和增加铸造冷却强度有助于减少初晶硅,且不影响钎焊性能。

Effect of a High Magnetic Field on the Microstructure During Directionally Solidified Sn-20wt.%Pb Hypoeutectic Alloy

The microstructures of Sn-20wt.%Pb hypoeutectic alloy directionally solidified under a longitudinal magnetic field were investigated.The results show that the application of a high magnetic field has a great influence on the morphology of primary β-Sn phase at a temperature gradient of G_L=52 K/cm.At a certain growth speed,with the increase of magnetic field intensity,the magnetic field causes the primary β-Sn phase irregular and to be deformed,further,the magnetic field promotes the columnar to equaixed transition(CET).Further,the thermoelectric magnetic force(TEMF) imposed on the dendrite under a high magnetic field has been calculated and the results show that the numerical magnitude of the TEMF during directional solidification under a 10 T high magnetic field is about 10~4N/m^3 and this force should be responsible for the occurrence of the CET in the Sn-Pb alloy.This may act as an experimental proof that the coupling of temperature gradient and high magnetic field will induce the occurrence of the CET in Sn-Pb alloy.Above phenomena may be attributed to the thermoelectric magnetic force(TEMF)in solid.