半固态A356合金重熔加热时初生相的组织演变

利用低过热度浇注技术制备了半固态A356铝合金坯料,研究了半固态温度区间重熔加热时半固态A356铝合金坯料的初生相形貌的转变过程.结果表明,在半固态两相区保温,半固态A356合金的初生相逐渐团球化,该过程随保温温度的升高而加快.半固态A356铝合金晶粒的圆度与保温温度和保温时间的关系不大,但晶粒的尺寸随着保温温度和保温时间的增加而增大.半固态A356合金试样重熔加热最佳工艺制度为583℃下保温30min,其晶粒平均等积圆直径为80μm,晶粒平均圆度为0.83.     

Rheology of A356 Alloy During Solidification Under Stirring

This work presents an experimental investigation on the rheology of A356 alloy in semisolid state using a high temperature Couette type viscometer. The molten liquid, resides in the annular space between the cylinders, is stirred and cooled continuously during experiments. The stirring results in fragmentation of dendrites which are transported into bulk liquid and form a semisolid slurry. The viscosity of the slurry is distinct in nature, which depends on microstructure of the suspended dendrites after coarsening. Hence, in the work, the variation of viscosity and microstructure is captured during cooling under different process parameters such as shear rate and cooling rate. Angular velocity of the inner cylinder and torque applied to stir the slurry are recorded to determine the apparent viscosity of the slurry. Temperature of the slurry is recorded to calculate the fraction of solids present in the slurry. For micrograph analysis, a vacuum quartz tube is used to remove the slurry-samples during experiments and they are quenched them in water.     

Study on Rheological Behavior of Semisolid A356 Alloy During Solidification

In the present work, a model is developed to predict the rheological behavior of an Al-alloy (A356) in semisolid state where the alloy is sheared between two parallel plates during continuous cooling. The flow field is represented by the momentum conservation equation where the non-Newtonian behavior of the semisolid alloy is incorporated considering the Herschel?CBulkley model. In the slurry, the agglomeration and de-agglomeration phenomena of the suspended particles under shear are represented using a time dependent structural parameter. The temperature field during cooling is predicted considering the transient energy conservation equation, and hence the fraction of solid and the yield stress of the semisolid alloy are continuously updated. Considering an apparent viscosity of the semisolid alloy as a function of structural parameter, shear stress and shear rate, the governing equations are solved analytically. Finally, the work predicts the variation of the apparent viscosity of the semisolid A356 alloy with fraction of solid. At first, the present prediction is validated against an available experimental data and, thereafter, the work predicts the effect of process parameters such as shear rate and cooling rate on the apparent viscosity.     

Microstructural Evolution of A356 Al Alloy During Flow Along a Cooling Slope

Cooling slope (CS) has been used in this study to prepare semi-solid slurry of A356 Al alloy, keeping in view of slurry generation on demand for Rheo-pressure die casting process. Understanding the physics of microstructure evolution during cooling slope slurry formation is important to satisfy the need of semi-sold slurry with desired shape, size and morphology of primary Al phase. Mixture of spherical and rosette shaped primary Al phase has been observed in the samples collected during melt flow through the slope as well as in the cast (mould) samples compared to that of dendritic shape, observed in case of conventionally cast A356 alloy. The liquid melt has been poured into the slope at 650?°C temperature and during flow it falls below the liquidus temperature of the said alloy, which facilitates crystallization of ??-Al crystals on the cooling slope wall. Crystal separation due to melt flow is found responsible for nearly spherical morphology of the primary Al phase.     

Multiphase Model of Semisolid Slurry Generation and Isothermal Holding During Cooling Slope Rheoprocessing of A356 Al Alloy

Metallurgical and Materials Transactions B - In the present paper, we present an experimentally validated 3D multiphase and multiscale solidification model to understand the transport processes...     

Modeling of Microporosity Size Distribution in Aluminum Alloy A356

Porosity is one of the most common defects to degrade the mechanical properties of aluminum alloys. Prediction of pore size, therefore, is critical to optimize the quality of castings. Moreover, to the design engineer, knowledge of the inherent pore population in a casting is essential to avoid potential fatigue failure of the component. In this work, the size distribution of the porosity was modeled based on the assumptions that the hydrogen pores are nucleated heterogeneously and that the nucleation site distribution is a Gaussian function of hydrogen supersaturation in the melt. The pore growth is simulated as a hydrogen-diffusion-controlled process, which is driven by the hydrogen concentration gradient at the pore liquid interface. Directionally solidified A356 (Al-7Si-0.3Mg) alloy castings were used to evaluate the predictive capability of the proposed model. The cast pore volume fraction and size distributions were measured using X-ray microtomography (XMT). Comparison of the experimental and simulation results showed that good agreement could be obtained in terms of both porosity fraction and size distribution. The model can effectively evaluate the effect of hydrogen content, heterogeneous pore nucleation population, cooling conditions, and degassing time on microporosity formation.     

变形铝合金可挤压性的分析与评价

本文较全面系统地介绍了挤压管、棒、线型材用铝合金的化学成分、特性与典型用途，深入论述了变形铝合金的可挤压性，并对各种铝合金的可挤压性进行了评价，列出了铝合金型材挤压难易程度排序表。     

A356铝合金的铸造方法对其组织和性能的影响

通过电磁搅拌＋连续铸造、人工搅拌＋连续铸造和常规铸造3种方式制备了A356合金，重点研究了铸造方式对合金组织和性能的影响。研究结果表明，与常规铸造相比，电磁或人工搅拌＋水冷热顶铸造工艺制备的材料晶粒得到了较大的细化，针孔度和Si偏析都有显著的改善，针孔度由常规铸造的2～3级提高到1级，而Si偏析由常规铸造的0．10％～0．54％降低到0．08％～0．16％。电磁搅拌或人工搅拌＋连续铸造得到材料的硬度和冲击韧性较常规铸造也都有一定的提高，分别提高了0．2倍和3倍以上。     

铝合金车轮铸造用A356合金的熔体处理

本文详细讨论了铝合金车轮用A356合金的精炼除气、晶粒细经及变质处理的机理，并介绍了本公司所采用的熔体处理方法及实际效果。     

Synchrotron Tomographic Characterization of Damage Evolution During Aluminum Alloy Solidification

Fast synchrotron X-ray microtomography was used to directly observe damage accumulation in a semi-solid Al-15 wt pct Cu alloy with a solid fraction of ~0.75 during isothermal tensile deformation. The evolution of damage was quantified in terms of size distribution of internal and surface-connected damage, strain mapping, and volume change to provide an insight into hot tear formation. A combination of existing void growth, void nucleation, and void coalescence all contribute to the final failure, although each dominates during different stages of deformation. Specifically, internal voids are shown to grow and coalesce from the region of high triaxiality at the center of the gage length outward and prove to be the contributing factor to final failure caused by insufficient liquid feeding.     

二次加热控制高硅铝合金中初生硅形态的研究

用光学显微镜(OM)及Image Pro Plus金相分析软件对半固态二次加热处理后的过共晶铝硅合金的初生硅组织特征进行分析。结果表明,在过共晶铝硅合金中,通过磷盐变质处理可以让初生硅相细化,平均直径由117μm降至31μm,并通过半固态二次加热使初生硅的形态得到显著的改善,其形状因子为0.67。     

ZK60合金多道次ECAP晶粒组织模拟和实验分析

采用刚粘塑性有限元软件对ZK60合金四道次等通道转角挤压(ECAP)过程进行数值模拟。对一至四道次ECAP试样进行晶粒组织模拟,观察晶粒细化程度的分布和变化规律。通过多道次ECAP实验,利用金相显微观察试样头部和尾部的晶粒尺寸的变化以及动态再结晶形成机理。对比有限元数值模拟与实验组织分析结果,探索利用有限元模拟与实验分析相结合的方法,研究镁合金ECAP成形过程的晶粒组织变化规律。     

Microstructural Inclusion Influence on Fatigue of a Cast A356 Aluminum Alloy

We examine the dependence of fatigue properties on the different size scale microstructural inclusions of a cast A356 aluminum alloy in order to quantify the structure-property relations. Scanning electron microscopy (SEM) analysis was performed on fatigue specimens that included three different dendrite cell sizes (DCSs). Where past studies have focused upon DCSs or pore size effects on fatigue life, this study includes other metrics such as nearest neighbor distance (NND) of inclusions, inclusion distance to the free surface, and inclusion type (porosity or oxides). The present study is necessary to separate the effects of numerous microstructural inclusions that have a confounding effect on the fatigue life. The results clearly showed that the maximum pore size (MPS), NND of gas pores, and DCS all can influence the fatigue life. These conclusions are presumed to be typical of other cast alloys with similar second-phase constituents and inclusions. As such, the inclusion-property relations of this work were employed in a microstructure-based fatigue model operating on the crack incubation and MSC with good results.     

The Effect of Inner Corner Radius of ECAP Die on Strain Distribution and Damage Accumulation in Deformed Sample

In the present research, the effect of inner corner radius of ECAP die on the material flow characteristic and the strain distribution inside sample were analyzed using 2D plain strain finite element simulation. Results showed that increase in inner corner radius results in the formation of smaller corner gap and narrow deformation zone. Consequently, the amount of plastic strain in regions at the bottom side of sample is increased. It is also concluded that the amount of damage factor in the upper regions of sample is higher than bottom regions and therefore cracks may initiate from these regions. In addition, the pressing force was raised by increasing inner corner radius.     

热轧板坯加热温度场数值模拟及应用

 建立了三段步进梁式加热炉板坯加热过程数学模型,用全隐式有限差分法对数学模型进行离散化,开发了板坯温度场计算模型。采用该模型重点研究了板坯宽度对板坯中心温度变化过程的影响规律。研究结果表明,对于厚度为200 mm的板坯,当板坯宽度大于600 mm时,板坯中心温度变化过程与板坯宽度无关。以此为根据,优化了板坯加热工艺,达到了提高生产效率并节能减排的目的。     

Microstructure Evolution during Semi-solid Isothermal Holding of Liquidus Cast Strontium-Modified A356 Alloy

An attempt has been made to understand the microstructural evolution during semi-solid isothermal holding of liquidus cast Sr-modified A356 alloy. It was observed that the liquidus casting is effective in producing non-dendritic microstructure with average sphericity of 0.8 and 0.82 for unmodified and modified alloys, respectively. Isothermal holding of the samples at a temperature corresponding to solid fraction of 0.3 for 15 min enhanced the sphericity of unmodified and modified alloys to 0.84 and 0.85, respectively. Further increase in holding resulted grain coarsening and reduction in sphericity. The grain coarsening rate (K) of samples was analysed with Lifshitz–Slyozov–Wanger equation, and the values of K were found to be 58.3 and 98.3 µm³/s for unmodified and modified alloys, respectively. The grain coarsening in unmodified alloy was dominated by Ostwald ripening, whereas in case of modified alloy, grain coarsening was governed by grain coalescence.     

Factors Affecting the Nucleation Kinetics of Microporosity Formation in Aluminum Alloy A356

Metal cleanliness is one of the most critical parameters affecting microporosity formation in aluminum alloy castings. It is generally acknowledged that oxide inclusions in the melt promote microporosity formation by facilitating pore nucleation. In this study, microporosity formation under different casting conditions, which aimed to manipulate the tendency to form and entrain oxide films in small directionally cast A356 samples was investigated. Castings were prepared with and without the aid of argon gas shielding and with a varying pour surface area. Two alloy variants of A356 were tested in which the main difference was Sr content. Porous disc filtration analysis was used to assess the melt cleanliness and identify the inclusions in the castings. The porosity volume fraction and size distribution were measured using X-ray micro-tomography analysis. The measurements show a clear increment in the volume fraction, number density, and pore size in a manner consistent with an increasing tendency to form and entrain oxide films during casting. By fitting the experimental results with a comprehensive pore formation model, an estimate of the pore nucleation population has been made. The model predicts that increasing the tendency to form oxide films increases both the number of nucleation sites and reduces the supersaturation necessary for pore nucleation in A356 castings. Based on the model predictions, Sr modification impacts both the nucleation kinetics and the pore growth kinetics via grain structure.     

A356合金初生相在固液区间等温时形貌演变

在选择适当的浇注温度、等温温度的前提下,本文主要研究半固态等温时间对金相组织演变的影响规律。