基于CAFE模型研究过冷度/形核数对H13钢微观组织的影响

基于CA-FE方法对工业H13钢进行凝固过程数值模拟,建立了宏观温度场、浓度场和微观生长过程耦合的凝固组织模拟模型。以元胞自动机模型为基础,耦合有限元模型,在晶粒尺度上模拟了其凝固过程。应用建立的微观组织模型,研究了过冷度、形核数对凝固组织的影响。结果表明,过冷度处于0.1~5℃,晶区主要是中间等轴晶区为主,柱状晶区占有比极小,且生长趋势受到等轴晶的抑制;当过冷度处于5~7℃,柱状晶生长速度增大,占有比增加,但是中间等轴晶内部组织粗化,形状因子增大,晶粒尺寸和晶粒度偏差表现为先增大,后减小;然而,过冷度达到7~15℃,柱状晶生长受到内部等轴晶的抑制,柱状晶占有优势,且随过冷度增加,晶粒尺寸得到细化;且形核数增加,柱状晶区减小,等轴晶区增大,同时晶粒先增大后减小。     

Mg含量对金属型铸造Al-Mg合金的微观组织和枝晶形貌的影响

利用OM、EM研究了Mg含量对金属型铸造Al-Mg合金微观组织和枝晶形貌的影响,并用EBSD(电子背散射衍射)研究了组织中柱状晶的生长取向.结果表明:不同成分的合金其组织主要由柱状晶区和等轴晶区组成,纯铝的柱状晶区最大,随着Mg含量的增加柱状晶区的宽度逐渐减小,当Mg含量达到15％时柱状晶区完全消失;一次枝晶间距则随Mg含量的增加而持续增加,且枝晶形貌由胞状晶转变为柱状树枝晶,最后变为等轴晶;Mg元素对Al-Mg合金中初生晶的生长取向有一定的影响,在Mg含量为2％时,枝晶生长取向以[100]晶向为主,同时还伴有[011]、[120]、[230]晶向,而Mg含量为10％时,枝晶的生长取向为[001]晶向.     

焊接熔池快速凝固过程的微观组织演化数值模拟

基于晶粒形成原理和枝晶生长动力学特点,建立了焊接熔池凝固过程中的形核、枝晶生长、溶质再分配及扩散的二维数学物理模型,对焊接熔池快速凝固过程中柱状晶向等轴晶的转变以及不同的冷却速度对这一转变过程的影响进行了模拟.结果表明,焊接熔池在快速冷却凝固过程中,溶质再分配与扩散明显;柱状晶向等轴晶转变时,熔池中心等轴晶凝固排出的溶质使柱状晶尖端浓度急剧升高,抑制了柱状晶的生长;冷却速度越大,柱状晶越容易向等轴晶转变,且转变所需时间越短.     

枝晶尖端生长速度对凝固组织数值模拟的影响研究——Ivantsov函数近似方式的确定

利用ProCAST的CAFE模块模拟铝合金的凝固组织,比较不同生长动力学系数下的模拟结果来说明枝晶生长速度对数值模拟结果的影响。对比发现,相同凝固条件下,枝晶生长速度越小,凝固组织中柱状晶区比例越小,等轴晶区比例越大。计算过程表明,采用不同的方法处理描述枝晶尖端稳态扩散的Ivantsov函数,得到的枝晶生长速度不同。通过分析Al-7%Si及Al-4.15%Mg在不同的Ivantsov函数近似下的枝晶生长动力学,本文认为,应用KGT模型时,采用Ivantsov函数的二级近似比较合适。     

钢锭中柱状晶向等轴晶区域转变的机理

<正> 本文研究开发了柱状晶区域向等轴晶区域转变的新理论。本文提出,枝晶生长的前端成长为自由晶;由于自然对流,这些结晶在熔融的液体中生长。这些自由晶迅速长大或迅速增加的话,则将会随着凝固的前沿阻止柱状晶的生产。铸型的冷却速度和凝固工艺的动力学决定了自由晶的生长速度。根据理论推定,冷却速度愈快,转变愈容易完成。本文还将上述情     

Fe-0.04%C合金焊缝熔池凝固过程中枝晶生长及溶质浓度分布模拟

基于枝晶生长的扩散界面模型,改进元胞自动机法,并结合有限差分法,综合考虑浓度场、温度场以及熔池形状,建立Fe-0.04%C(质量分数)合金枝晶生长和溶质浓度分布模型。模拟单个等轴晶的生长形貌及枝晶尖端生长速度随时间的变化关系、多个等轴晶的生长形貌和溶质浓度分布、柱状晶的生长形貌和耦合温度场后的柱状晶-等轴晶转变过程,并与实验进行对比。结果表明:取向角对枝晶形貌有一定的影响;枝晶尖端生长速度随时间的延长最后趋于稳定;熔池形状影响柱状晶生长形貌;溶质主要富集在枝晶根部及晶界处。模拟结果与实验结果吻合较好。     

基于三相模型的Al-4%Cu合金三维半连铸圆锭的宏观偏析数值模拟

本文采用基于 Eulerian-Eulerian方法的等轴晶、柱状晶以及熔体三相完全混合的凝固模型计算了三维半连铸Al-4%Cu铝合金圆锭的宏观偏析。基于热溶质对流的基础上,模型考虑了等轴晶的移动,以及柱状晶对浮游等轴晶的捕获,等轴晶和柱状晶的相互竞争生长行为。模拟结果表明铸锭底部出现了明显的锥形负偏析区(CET转变区域),铸锭中心正偏析带,毗邻中心的负偏析区,以及铸锭1/2半径处正偏析带,总的偏析形态呈现W型,与铸锭实际情况基本一致。此外,铸造速度相较于浇注温度对铸锭宏观偏析具有更大的影响。     

对流作用下纯物质枝晶生长的相场法模拟

基于Tong和Beckermann提出的耦合流场和噪声场的相场模型，采用有限差分法对控制方程进行数值模拟求解，得出结论：逆流和顺流侧枝晶将不再对称，逆流侧枝晶尖端生长速度明显比顺流侧枝晶生长速度快，垂直方向几乎不受流动的影响。分析了热噪声、过冷度、对流流速和各向异性强度对枝晶主枝和侧向分支的影响。     

激光-电弧复合焊接热源布置方式对焊缝组织和性能的影响

以低合金高强钢为试验材料,采用Nd:YAG激光-MAG电弧复合焊接的方法,研究了激光与电弧的前后布置方式对焊缝形貌、组织和性能的影响。结果表明:电弧引导时焊缝主要以柱状晶为主,焊缝中心有少量的树枝晶,电弧区的柱状晶生长出二次枝晶,二次枝晶非常细小和致密,激光区的二次晶枝较粗且稀疏。激光引导的电弧区为柱状晶和树枝晶,但柱状晶较短并具有明显的层状特征,激光区中心为等轴晶,两侧为树枝晶和柱状晶,晶枝及晶枝臂粗大,晶枝臂间的间隙较小,焊缝区冲击功平均值比电弧引导时提高了33%,冲击断口的形貌表明:电弧引导时,韧窝较浅,并且存在较多的穿晶断裂区,而激光引导时韧窝较深,韧窝边缘较锐利。电弧引导比激光引导的热影响区窄,板条马氏体的晶粒相对较小。     

离子镀铝青铜膜的XTEM研究

本文利用透射电镜对离子镀铝青铜膜进行横截面显微组织研究结果表明:整个膜层分为细等轴晶、细纤维状及柱状晶三种不同形态的生长区并且柱状晶主要以孪晶方式生长膜中存在NiAIl Cu_9Al_4合金相,与基体取向关系为:讨论了膜层成核和生长过程发现气相沉积膜层中存在生长条纹     

Effects of Post-weld Heat Treatment on Microstructure,Mechanical Properties and the Role of Weld Reinforcement in 2219 Aluminum Alloy TIG-Welded Joints

In as-welded state, each region of 2219 aluminum alloy TIG-welded joint shows diff erent microstructure and microhardness due to the diff erent welding heat cycles and the resulting evolution of second phases. After the post-weld heat treatment, both the amount and the size of the eutectic structure or θ phases decreased. Correspondingly, both the Cu content in α-Al matrix and the microhardness increased to a similar level in each region of the joint, and the tensile strength of the entire joint was greatly improved. Post-weld heat treatment played the role of solid solution strengthening and aging strengthening. After the post-weld heat treatment, the weld performance became similar to other regions, but weld reinforcements lost their reinforcing eff ect on the weld and their existence was more of an adverse eff ect. The joint without weld reinforcements after the post-weld heat treatment had the optimal tensile properties, and the specimens randomly crack in the weld zone.     

The First Phase of the SRIF Industrialized Base in ＂West Area＂ Has Been Put into Operation

After nearly two years＇ tense construction, the first phase of industrialized base of Shenyang Research Institute of Foundry （SRIF）, located at the Tiexi Casting and Forging Industrial Park in the west of Tiexi District, has now been completed and formally put into operation.     

Thermodynamics Study on the Decomposition of Chromite with KOH

Institute of Process Engineering, Chinese Academy of Sciences, China, has proposed a method for oxidative leaching of chromite with potassium hydroxide. Understanding the mechanism of chromite decomposition, especially in the potassium hydroxide fusion, is important for the optimization of the operating parameters of the oxidative leaching process. A traditional thermodynamic method is proposed and the thermal decomposition and the reaction decomposition during the oxidative leaching of chromite with KOH and oxygen is discussed, which suggests that chromite is mainly destroyed by reactions with KOH and oxygen. Meanwhile, equilibrium of the main reactions of the above process was calculated at different temperatures and oxygen partial pressures. The stable zones of productions, namely, K2CrO4 and Fe2O3, increase with the decrease of temperature, which indicates that higher temperature is not beneficial to thermodynamic reactions. In addition, a comparison of the general alkali methods is carried out, and it is concluded that the KOH leaching process is thermodynamically superior to the conventional chromate production process.     

Influence of Heat Treatment on Damping Behaviour of the Magnesium Wrought Alloy AZ61

The effect of isochronal heat treatments for 1h on variation of damping, hardness and microstructural change of the magnesium wrought alloy AZ61 was investigated. Damping and hardness behaviour could be attributed to the evolution of precipitation process. The influence of precipitation on damping behaviour was explained in the framework of the dislocation string model of Granato and Lücke.     

Effect of Rare Earth Doping on Impedance,Modulus and Conductivity Properties of Multiferroic Composites:0.5(BiLa_xFe_(1-x)O_3)–0.5(PbTiO_3)

The Lanthanum-doped bismuth ferrite–lead titanate compositions of 0.5(Bi LaxFe1-xO3)–0.5(Pb Ti O3)(x = 0.05,0.10,0.15,0.20)(BLxF1-x-PT) were prepared by mixed oxide method.Structural characterization was performed by X-ray diffraction and shows a tetragonal structure at room temperature.The lattice parameter c/a ratio decreases with increasing of La(x = 0.05–0.20) concentration of the composites.The effect of charge carrier/ion hopping mechanism,conductivity,relaxation process and impedance parameters was studied using an impedance analyzer in a wide frequency range(102–106Hz) at different temperatures.The nature of Nyquist plot confirms the presence of bulk effects only,and non-Debye type of relaxation processes occurs in the composites.The electrical modulus exhibits an important role of the hopping mechanism in the electrical transport process of the materials.The ac conductivity and dc conductivity of the materials were studied,and the activation energy found to be 0.81,0.77,0.76 and 0.74 e V for all compositions of x = 0.05–0.20 at different temperatures(200–300 °C).     

Using Finite Element and Contour Method to Evaluate Residual Stress in Thick Ti-6Al-4V Alloy Welded by Electron Beam Welding

This work was to reveal the residual stress profile in electron beam welded Ti-6Al-4V alloy plates(50 mm thick) by using finite element and contour measurement methods.A three-dimensional finite element model of 50-mmthick titanium component was proposed,in which a column–cone combined heat source model was used to simulate the temperature field and a thermo-elastic–plastic model to analyze residual stress in a weld joint based on ABAQUS software.Considering the uncertainty of welding simulation,the computation was calibrated by experimental data of contour measurement method.Both test and simulated results show that residual stresses on the surface and inside the weld zone are significantly different and present a narrow and large gradient feature in the weld joint.The peak tensile stress exceeds the yield strength of base materials inside weld,which are distinctly different from residual stress of the thin Ti-6Al-4V alloy plates presented in references before.     

Microstructures and Tensile Properties of Ultrafine-Grained Ni–(1–3.5) wt% SiC_(NP) Composites Prepared by a Powder Metallurgy Route

Silicon carbide nanoparticle-reinforced nickel-based composites(Ni–Si CNP),with a Si CNPcontent ranged from1 to 3.5 wt%,were prepared using mechanical alloying and spark plasma sintering.In addition,unreinforced pure nickel samples were also prepared for comparative purposes.To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–Si CNPcomposites transmission electron microscopy(TEM) was used,while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests.TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample,whereas,for the Ni–Si CNPcomposites,the presence of nano-dispersed Si CNPand twinning crystals was observed.These homogeneously distributed Si CNPwere found located either within the matrix,between twins or on grain boundaries.For the Ni–Si CNPcomposites,coerced coarsening of the Si CNPassembly occurred with increasing Si CNPcontent.Furthermore,the grain sizes of the Ni–Si CNPcomposites were much finer than that of the unreinforced pure nickel,which was considered to be due to the composite ball milling process.In all cases,the Ni–Si CNPcomposites showed higher strengths and hardness values than the unreinforced pure nickel,likely due to a combination of dispersion strengthening(Orowan effects) and particle strengthening(Hall–Petch effects).For the Ni–Si CNPcomposites,the strength increased initially and then decreased as a function of Si CNPcontent,whereas their elongation percentages decreased linearly.Compared to all materials tested,the Ni–Si CNPcomposite containing 1.5% Si C was found more superior considering both their strength and plastic properties.     

Directional Growth of Tin Crystals Controlled by Combined Solute Concentration Gradient Field and Static Magnetic Field

A new method was introduced to achieve directional growth of Sn crystals. Microstructures in liquid(Pb)/liquid(Sn) diffusion couples were investigated under various static magnetic fields. Results show that the β-Sn crystals mainly reveal an irregular dendritic morphology without or with a relatively low static magnetic field(B0.3 T). When the magnetic field is increased to 0.5 T, the β-Sn dendrites close to the final stage of growth begin to show some directional character. With a further increase in the magnetic field to a higher level(0.8–5 T), the β-Sn dendrites have an enhanced directional growth character, but the dendrites show a certain deflection. As the magnetic field is increased to 12 T, the directional growth of the β-Sn dendrites in the center of the couple is severely destroyed. The mechanism of the directional growth of the β-Sn crystals and the deflection of the β-Sn crystals with the application of static magnetic field was tentatively discussed.     

Quantitative Analysis of the Crystallographic Orientation Relationship Between the Martensite and Austenite in Quenching–Partitioning–Tempering Steels

The orientation relationships(ORs)between the martensite and the retained austenite in low-and medium-carbon steels after quenching–partitioning–tempering process were studied in this work.The ORs in the studied steels are identified by selected-area electron diffraction(SAED)as either K–S or N–W ORs.Meanwhile,the ORs were also studied based on numerical fitting of electron backscatter diffraction data method suggested by Miyamoto.The simulated K–S and N–W ORs in the low-index directions generally do not well coincide with the experimental pole figure,which may be attributed to both the orientation spread from the ideal variant orientations and high symmetry of the low-index directions.However,the simulated results coincide well with experimental pole figures in the high-index directions{123}_(bcc).A modified method with simplicity based on Miyamoto’s work was proposed.The results indicate that the ORs determined by modified method are similar to those determined by Miyamoto’method,that is,the OR is near K–S OR for the low-carbon Q–P–T steel,and with the increase of carbon content,the OR is closer to N–W OR in medium-carbon Q–P–T steel.     

Growth Kinetics of Single Inclusion Particle in Molten Melts

On the basis of the single-particle framework, a new theory on inclusion growth in metallurgical melts is developed to study the kinetics of inclusion growth on account of reaction and collision. The studies show that the early growth of inclusion depends on reaction growth and Brawnian motion collision, and where the former is decisive, the late growth depends on turbulence collision and Stokes＇ collision, and where the former is dominant; collision growth is very quick during the smelting process, lessened in the refining process, but nearly negligible in the continuous casting process.