Solidification in continuous casting of aluminum

During casting of aluminum ingot by a new process utilizing level pouring, variations in cooling rate within the surface band occur as a result of the discontinuous nature of metal flow. Associated segregation contributes to a marked cyclic pattern of the macrostructure in that region. The kinetics of metal flow into the surface zone are explained graphically using a sequence of macrographs. The proposed mechanism is also supported by results of thermal measurements and Electron Microprobe analyses.     

大直径铝锭热顶铸造中超声施振深度的细晶机制

在直径为650 mm的铝合金热顶半连续铸造过程中施加双源超声振动系统, 研究3种超声辐射杆浸入深度对铸锭宏观凝固组织的影响.基于铝合金铸锭凝固组织形貌的检测结果以及ANSYS等有限元软件对铸造过程中声场的仿真结果, 深入探讨了超声辐射杆在不同的施振深度下对铝合金铸锭凝固组织细化机制的影响.结果表明: 随着超声辐射杆施振深度的增加, 铸锭截面组织整体进一步细化, 晶粒形状由发达的枝晶变为等轴枝晶; 由于超声辐射杆端面以及柱面存在几个固定位置处振动波峰, 在铝熔体中不同的超声施振深度下存在不同的超声空化范围, 进而导致凝固组织的细化机制也不同.      

Performance analysis of the aluminum casting furnace

The casting furnace plays a central role in the production of aluminum. Its design and operation are complex and involve some 450 parameters. There is a need for a model to predict and analyze its performance. We propose a simplified model in which each main component of the furnace is treated as a 1-D heat conduction medium. Based on the equations of conservation of mass, energy, and chemical species, complemented by the equations of conduction and the Hottel’s formulation of radiative heat transfer, this dynamic model can simulate any sequence of operations such as loading, heating, stirring, skimming … that constitutes a batch, and can take into account other operational details such as the opening of doors. It is validated on a real furnace, then used to predict furnace performance in other modes of operation, and also to determine an optimal fuel flow that minimizes a chosen cost function.     

Performance analysis of the aluminum casting furnace

The casting furnace plays a central role in the production of aluminum. Its design and operation are complex and involve some 450 parameters. There is a need for a model to predict and analyze its performance. We propose a simplified model in which each main component of the furnace is treated as a 1-D heat conduction medium. Based on the equations of conservation of mass, energy, and chemical species, complemented by the equations of conduction and the Hottel’s formulation of radiative heat transfer, this dynamic model can simulate any sequence of operations such as loading, heating, stirring, skimming … that constitutes a batch, and can take into account other operational details such as the opening of doors. It is validated on a real furnace, then used to predict furnace performance in other modes of operation, and also to determine an optimal fuel flow that minimizes a chosen cost function.     

Numerical simulation of solidification in an aluminum casting

This work describes the computer simulation of the solidification pattern in an aluminum casting. The Finite Element Method (FEM), adopting nonmoving mesh, was used to calculate the transient thermal field during phase transition. Integral averaging techniques were employed to calculate both the nonlinear heat conductionK(T) and specific heatC _p (T) functions for the elements of the domain; a temperature dependent functionC _p (T) accounts for the energy of the phase change. A complete computer program, with pre- and postprocessors, for Computer Aided Design of castings is also presented. Anad hoc experiment has been performed to verify the accuracy of numerical results.     

Microstructure formation in high pressure die casting

In order to optimise the high pressure die casting (HPDC) process, more understanding of microstructure and defect formation is essential. This article gives an overview of the key microstructural features and the mechanisms of microstructure formation in this process. The incavity solidified grain size in HPDC can be as low as 10 μm, but externally solidified crystals (ESCs) as large as 200 μm are often also present. The eutectic microstructure is very fine with an interlamellar spacing around 500 nm. Bands of positive macrosegregation and sometimes with cracks/porosity, so-called defect bands, are also often observed. Intensification pressure (IP) is one of the major factors governing the porosity level in the casting. At high IP, defect bands form in the gate region and appear to be assisting the feeding during the intensification stage.     

Interface heat transfer in investment casting of aluminum alloys

Via design of experiments and using a newly developed inverse method, the heat-transfer boundary conditions in the investment casting process have been studied. It has been shown in the past that these conditions, expressed as interface heat transfer coefficients (HTCs), vary during alloy solidification and cooling. In this work, the authors have studied the additional effects of alloy solidification range, metallostatic head, investment shell thickness, preheat, and interface geometry. This provides an improved set of relationships from which to build realistic boundary conditions into computer simulations of shape casting. Using axisymmetric solidification experiments and numerical inverse analysis, it is shown that the effect of metallostatic head is only significant for long freezing-range alloys. Increasing shell mold thickness and preheat also have effects that are alloy-dependent, and significant differences in thermal behavior are reported between the alloy/mold interface and the alloy/core interface. The four alloys used in the experiments are aluminum-based and vary from short freezingrange commercially pure to an alloy with a freezing range of 120 °C.     

Modeling of microporosity formation in A356 aluminum alloy casting

A numerical model for predicting microporosity formation in aluminum castings has been developed, which describes the redistribution of hydrogen between solid and liquid phases, the transport of hydrogen in liquid by diffusion, and Darcy flow in the mushy zone. For simulating the nucleation of hydrogen pores, the initial pore radius is assumed to be a function of the secondary dendrite arm spacing, whereas pore growth is based on the assumption that hydrogen activity within the pore and the liquid are in equilibrium. One of the key features of the model is that it uses a two-stage approach for porosity prediction. In the first stage, the volume fraction of porosity is calculated based on the reduced pressure, whereas, in the second stage, at fractions solid greater than the liquid encapsulation point, the fraction porosity is calculated based on the volume of liquid trapped within the continuous solid network, which is estimated using a correlation based on the Niyama parameter. The porosity model is used in conjunction with a thermal model solved using the commercial finite-element package ABAQUS. The parameters influencing the formation of microporosity are discussed including a means to describe the supersaturation of hydrogen necessary for pore nucleation. The model has been applied to examine the evolution of porosity in a series of experimental samples cast using unmodified A356 in which the initial hydrogen content was varied from 0.048 to 0.137 (cc/100 g). A comparison between the model predictions and the experimental measurements indicates good agreement in terms of the variation in porosity with distance from the chill and the variation resulting from initial hydrogen content.     

铝硅系铸造铝合金的原位统计分布分析

The element distribution analysis for the Al-Si casting aluminum alloys has been done by metal original position analyzer(OPA). The quantitative distribution analysis results of Si, Fe, Cu, Mn and Ti have been obtained by selecting suitable spectrum reference materials and optimizing the instrumental parameters. The content results of five elements by OPA method have good coincidence with the values determined byspark source atomic emission spectrometer. The scanning analysis results show that the distribution of Si, Cu and Ti is homogeneous and some obvious segregation of Fe and Mn has been found in two real aluminum alloys and the content distributions of two elements are very similar. It can be concluded thatthere are some compound of Fe and Mn in casting aluminum alloys by the result of multi-element spectrum channel combination analysis. The conclusion also has good agreement with the results by scanning electron microscope and energy spectrum analysis.     

Modeling of microporosity formation in A356 aluminum alloy casting

A numerical model for predicting microporosity formation in aluminum castings has been developed, which describes the redistribution of hydrogen between solid and liquid phases, the transport of hydrogen in liquid by diffusion, and Darcy flow in the mushy zone. For simulating the nucleation of hydrogen pores, the initial pore radius is assumed to be a function of the secondary dendrite arm spacing, whereas pore growth is based on the assumption that hydrogen activity within the pore and the liquid are in equilibrium. One of the key features of the model is that it uses a two-stage approach for porosity prediction. In the first stage, the volume fraction of porosity is calculated based on the reduced pressure, whereas, in the second stage, at fractions solid greater than the liquid encapsulation point, the fraction porosity is calculated based on the volume of liquid trapped within the continuous solid network, which is estimated using a correlation based on the Niyama parameter. The porosity model is used in conjunction with a thermal model solved using the commercial finite-element package ABAQUS. The parameters influencing the formation of microporosity are discussed including a means to describe the supersaturation of hydrogen necessary for pore nucleation. The model has been applied to examine the evolution of porosity in a series of experimental samples cast using unmodified A356 in which the initial hydrogen content was varied from 0.048 to 0.137 (cc/100 g). A comparison between the model predictions and the experimental measurements indicates good agreement in terms of the variation in porosity with distance from the chill and the variation resulting from initial hydrogen content.     

Modeling of microporosity formation in A356 aluminum alloy casting

A numerical model for predicting microporosity formation in aluminum castings has been developed, which describes the redistribution of hydrogen between solid and liquid phases, the transport of hydrogen in liquid by diffusion, and Darcy flow in the mushy zone. For simulating the nucleation of hydrogen pores, the initial pore radius is assumed to be a function of the secondary dendrite arm spacing, whereas pore growth is based on the assumption that hydrogen activity within the pore and the liquid are in equilibrium. One of the key features of the model is that it uses a two-stage approach for porosity prediction. In the first stage, the volume fraction of porosity is calculated based on the reduced pressure, whereas, in the second stage, at fractions solid greater than the liquid encapsulation point, the fraction porosity is calculated based on the volume of liquid trapped within the continuous solid network, which is estimated using a correlation based on the Niyama parameter. The porosity model is used in conjunction with a thermal model solved using the commercial finite-element package ABAQUS. The parameters influencing the formation of microporosity are discussed including a means to describe the supersaturation of hydrogen necessary for pore nucleation. The model has been applied to examine the evolution of porosity in a series of experimental samples cast using unmodified A356 in which the initial hydrogen content was varied from 0.048 to 0.137 (cc/100 g). A comparison between the model predictions and the experimental measurements indicates good agreement in terms of the variation in porosity with distance from the chill and the variation resulting from initial hydrogen content.     

Die soldering: Mechanism of the interface reaction between molten aluminum alloy and tool steel

Die soldering is the result when molten aluminum sticks to the surface of the die material and remains there after the ejection of the part; it results in considerable economic and production losses in the casting industry, and is a major quality detractor. In order to alleviate or mitigate die soldering, one must have a thorough understanding of the mechanism by which the aluminum sticks to the die material. A key question is whether the die soldering reaction is diffusion controlled or interface controlled. A set of diffusion couple experiments between molten aluminum alloy and the ferrous die was carried out. The results of the diffusion couple experiments showed that soldering is a diffusional process. When aluminum comes in contact with the ferrous die material, the iron and the aluminum atoms diffuse into each other resulting in the formation of a series of intermetallic phases over the die material. Initially iron and aluminum react with each other to form binary iron-aluminum intermetallic phases. Subsequently, these phases react with the molten aluminum to further form ternary iron-aluminum-silicon intermetallic phases. Iron and aluminum have a great affinity for each other and the root cause of die soldering is the high reaction kinetics, which exists between iron and aluminum. Once the initial binary and ternary intermetallic phase layers are formed over the die material, the aluminum sticks to the die due to the abnormally low thermal conductivity of the intermetallic phases, and due to favorable interface energies between the intermetallic layers and aluminum. The experimental details, the results of the interface reactions, and the analysis leading to the establishment of the mechanism giving rise to die soldering are reviewed discussed.     

Experimental study of continuous electromagnetic casting of aluminum alloys

This work begins with a summary of the characteristics of electromagnetic casting and the present knowledge of the subject. Also described is the use of new local measurement techniques for velocity, magnetic field, current density, and phase difference, which allow experimental investigation of the flow of molten metal in industrial equipment (up to 700 °C) in the presence or absence of an induction magnetic field. Next, these methods are applied to the study of electromagnetic and hydrodynamic phenomena, inside the sumps of both rectangular and circular cross-section ingots of aluminum alloys cast in electromagnetic molds. The important shield effect as a function of the screen location is studied by means of a mercury pool simulating electromagnetic casting.     

反重力渗流铸造法制备开孔泡沫铝材料

采用反重力渗流铸造法成功制备了开孔泡沫铝材料,分析了材料的准静态压缩性能及主要制备工艺参数与开孔泡沫铝空隙度之间的关系。研究结果表明：采用反重力渗流铸造法所制备的泡沫铝几乎没有宏观缺陷,孔洞分布均匀、孔壁结构完整;泡沫铝的空隙度对其压缩性能的影响很大,泡沫铝的屈服强度与平台应力均随空隙度的降低而升高;提高粒子预热温度、增大保压压力或延长保压时间,均有助于降低空隙度。     

Mathematical modeling of an aluminum casting furnace combustion chamber

A mathematical model has been developed for the combustion chambers of aluminum casting furnaces by combining the fluid flow code PHOENICS with a zone model for the radiative heat transfer analysis and a simplified flame model. It offers flexibility in specifying the size and the combustion and heat transfer characteristics of the furnace. Thus, the model can be used to study a combustion chamber under different operating conditions and for different design op-tions. This paper presents the model and describes the coupling mechanism between PHOENICS and the zone method. Various case studies have been carried out for a 72-ton melter-holder. Results are presented which show the negative effect of ambient air inleakage on furnace per-formance as an application example. T. BOURGEOIS, Formerly Graduate Student.     

Characterization of the flow in the molten metal sump during direct chill aluminum casting

A recent analytical model for the liquid aluminum flow in a direct chill (DC) casting sump has been investigated and the scaling coefficients evaluated. The magnitudes of flow-field features, such as the depth of the temperature stratification in the sump and the velocity of the metal in the thermal boundary layer close to the solidification front, have been calculated. The results broadly agree with recent full numerical calculations of the flow in the sump. The variation of these essential flow features has been investigated across a range of typical ingot sizes, casting speeds, and superheats, and critical macro-casting-parameter combinations have been identified. The limitations of the model are discussed and the possible effects the identified structure has on macrosegregation are briefly explored. Finally, the influence on the flow field of the method of feeding the ingot is investigated, and it is concluded that the model and these results are not invalidated if the feeding is nonuniform over the top surface of the sump.     

连铸结晶器石英水口蚀损机理分析

在前人研究的基础上,总结了石英水口制备工艺、选用以及在钢水浇铸过程的作用。针对本厂下线石英水口的状况,通过测量下线水口断面尺寸,分析了钢水成分、保护渣和水口对中等因素对石英水口蚀损的影响,得知石英水口蚀损主要有内部扩颈和外部缩颈两种形式,其中以保护渣侵蚀造成缩颈为主,平均速率为15.86 mm/h。水口在使用过程中如果未良好对中,会造成水口薄厚不均,影响水口正常使用。针对上述问题提出了在保证水口对中的前提下,改变石英水口浸入金属液面深度来延长其使用寿命的方法。