Effect of superheat,mold, and casting materials on the metal/mold interfacial heat transfer during solidification in graphite-lined permanent molds

Heat transfer during the solidification of an Al-Cu-Si alloy (LM4) and commercial pure tin in single steel, graphite, and graphite-lined metallic (composite) molds was investigated. Experiments were carried out at three different superheats. In the case of composite molds, the effect of the thickness of the graphite lining and the outer wall on heat transfer was studied. Temperatures at known locations inside the mold and casting were used to solve the Fourier heat conduction equation inversely to yield the casting/mold interfacial heat flux transients. Increased melt superheats and higher thermal conductivity of the mold material led to an increase in the peak heat flux at the metal/mold interface. Factorial experiments indicated that the mold material had a significant effect on the peak heat flux at the 5% level of significance. The ratio of graphite lining to outer steel wall and superheat had a significant effect on the peak heat flux in significance range varying between 5 and 25%. A heat flux model was proposed to estimate the maximum heat flux transients at different superheat levels of 25 to 75 °C for any metal/mold combinations having a thermal diffusivity ratio (α _R) varying between 0.25 and 6.96. The heat flow models could be used to estimate interfacial heat flux transients from the thermophysical properties of the mold and cast materials and the melt superheat. Metallographic analysis indicated finer microstructures for castings poured at increased melt superheats and cast in high-thermal diffusivity molds.     

Regularized determination of interfacial heat transfer coefficient during ZL102 solidification process

The interfacial heat transfer coefficient（IHTC） between the casting and the mould is essential to the numerical simulation as one of boundary conditions. A new inverse method was presented according to the Tikhonov regularization theory. A regularized functional was established and the regularization parameter was deduced. The functional was solved to determine the interfacial heat transfer coefficient by using the sensitivity coefficient and Newton-Raphson iteration method. The temperature measurement experiment was done to ZL102 sand mold casting, and the appropriate mathematical model of the IHTC was established. Moreover, the regularization method was used to determinate the IHTC. The results indicate that the regularization method is very efficient in overcoming the ill-posedness of the inverse heat conduction problem（IHCP）, and ensuring the accuracy and stability of the solutions.     

Equivalent heat transfer coefficient at casting/Cu mould interface and temperature field simulation

1　INTRODUCTIONThesolidifiedmicrostructuresofalloysdependontheirsolidifyingprocesswhoseprimarycharacteristicsarethetemperaturedropofthesuperheatedmeltandthere leaseofthelatentheat.Sothestudyontheheattransferduringthesolidificationprocessistheessentialprobleminthesolidificationtheorystudy .Theresearchersworkingonthenumericalsimulationofthesolidificationprocessallknowthattheinterfacialheattransfercoefficientatthecasting/mouldisavariablechangingwithtime .Thusthedeterminationoftheinterfacialh…     

22MnB5钢表面微观形貌对界面换热系数的影响

为了探究冲压压强和样件表面粗糙度对22MnB5钢板的界面换热系数(Interfacial heat transfer coefficient,以下简称IHTC)的影响,自主设计了圆台试验模型,利用Beck非线性估算法,求解热成形工艺中高温样件与低温模具间界面换热系数。实验中通过调整冲压压强和改变样件表面粗糙度的方式,进而改变模具与样件间接触表面微观形貌,探究其对IHTC的影响。研究表明:压强与IHTC存在高度近似的正幂函数关系;粗糙度也会对IHTC产生影响,并且当粗糙度大于1μm时,IHTC值会随粗糙度的增大而明显减小,当粗糙度小于1μm时,由于表面氧化皮的影响,IHTC将随粗糙度值的减小而增大放缓,甚至趋于定值而不再增加。     

Spatial interfacial heat transfer and surface characteristics during gravity casting of A356 alloy

As one of the key boundary conditions during casting solidification process, the interfacial heat transfer coefficient (IHTC) affects the temperature variation and distribution. Based on the improved nonlinear estimation method (NEM), thermal measurements near both bottom and lateral metal-mold interfaces throughout A356 gravity casting process were carried out and applied to solving the inverse heat conduction problem (IHCP). Finite element method (FEM) is employed for modeling transient thermal fields implementing a developed NEM interface program to quantify transient IHTCs. It is found that IHTCs at the lateral interface become stable after the volumetric shrinkage of casting while those of the bottom interface reach the steady period once a surface layer has solidified. The stable value of bottom IHTCs is 750 W/(m²·°C), which is approximately 3 times that at the lateral interface. Further analysis of the interplay between spatial IHTCs and observed surface morphology reveals that spatial heat transfer across casting-mold interfaces is the direct result of different interface evolution during solidification process.     

Determination of interfacial heat transfer coefficient and its application in high pressure die casting process

In this paper,the research progress of the interfacial heat transfer in high pressure die casting（HPDC）is reviewed.Results including determination of the interfacial heat transfer coefficient（IHTC）,influence of casting thickness,process parameters and casting alloys on the IHTC are summarized and discussed.A thermal boundary condition model was developed based on the two correlations：（a）IHTC and casting solid fraction and（b）IHTC peak value and initial die surface temperature.The boundary model was then applied during the determination of the temperature field in HPDC and excellent agreement was found.     

Measurement of temperature inside die and estimation of interfacial heat transfer coefficient in squeeze casting

As an advanced near-net shape technology, squeeze casting is an excellent method for producing high integrity castings. Numerical simulation is a very effective method to optimize squeeze casting process, and the interfacial heat transfer coefficient (IHTC) is an important boundary condition in numerical simulation. Therefore, the study of the IHTC is of great significance. In the present study, experiments were conducted and a"plate shape" aluminum alloy casting was cast in H13 steel die. In order to obtain accurate temperature readings inside the die, a special temperature sensor units (TSU) was designed. Six 1 mm wide and 1 mm deep grooves were machined in the sensor unit for the placement of the thermocouples whose tips were welded to the end wall. Each groove was machined to terminate at a particular distance (1, 3, and 6 mm) from the front end of the sensor unit. Based on the temperature measurements inside the die, the interfacial heat transfer coefficient (IHTC) at the metal-die interface was determined by applying an inverse approach. The acquired data were processed by a low pass filtering method based on Fast Fourier Transform (FFT). The feature of the IHTC at the metal-die interface was discussed.     

RHCM模具的三维瞬态传热建模与分析

研究RHCM模具的三维有限元瞬态传热建模,分析进入稳态工作状态的模具和熔体之间的耦合传热过程,提出了模具和熔体接触热阻计算模型。案例对比分析发现:三维模型可克服目前简化二维模型存在的加热时间预测偏短,精密模具型腔边缘与模具中心温差误差大的问题;接触热阻导致热传导效率大大降低,冷却时间增长,可通过减小热阻来解决;减少加热时间可避免冷却阶段初期型腔壁面温度继续上升。     

Zr-4合金与模具钢的界面换热行为研究

界面换热系数是锆合金塑性成形模拟的重要边界条件之一。本文测定了界面有、无玻璃润滑剂条件下Zr-4合金和H13模具钢的界面接触温度随接触时间的变化曲线,在此基础上分析了界面换热特征,获得了界面换热系数随初始界面温度变化的函数式。结果表明,玻璃润滑剂可有效减缓Zr-4合金与H13钢的界面传热,当Zr-4合金和H13钢的初始界面温度分别为700℃和470℃时,有玻璃润滑剂时Zr合金表面温度达到稳定的时间约为16.3s,该时间段内相应的界面换热系数随实验时间的延长由226 W/( m₂?℃)增大到2166 W/( m₂?℃),无润滑剂时Zr合金表面温度达到稳定的时间约为7.7s,该时间段内界面换热系数由250 W/( m₂?℃)增大到2700 W/( m₂?℃)。采用本文确定的换热系数随温度变化的关系式进行热交换模拟可以获得较高的模拟精度,模拟与实验结果的最大误差约为4.5%。     

Study on interfacial heat transfer coefficient at metal/die interface during high pressure die casting process of AZ91D alloy

The high pressure die casting (HPDC) process is one of the fastest growing and most efficient methods for the production of complex shape castings of magnesium and aluminum alloys in today's manufacturing industry. In this study, a high pressure die casting experiment using AZ91D magnesium alloy was conducted, and the temperature profiles inside the die were measured. By using a computer program based on solving the inverse heat problem, the metal/die interfacial heat transfer coefficient (IHTC) was calculated and studied. The results show that the IHTC between the metal and die increases right after the liquid metal is brought into the cavity by the plunger, and decreases as the solidification process of the liquid metal proceeds until the liquid metal is completely solidified, when the IHTC tends to be stable. The interfacial heat transfer coefficient shows different characteristics under different casting wall thicknesses and varies with the change of solidification behavior.     

The deformation of the chill in experiments to determine the interfacial heat transfer coefficient during casting solidification

Interfacial heat transfer coefficients during casting solidification are often measured in experiments in which unidirectional heat transfer is assumed. Finite element modelling of the chill in these experiments has shown that the chill surface deforms elastically into a convex shape, the extent of which generally decreases with time. Examples from several different unidirectional solidification experiments are given. The deformation of the chill surface was also calculated with the assumption of nonuniform temperature boundary conditions. The deformation of the chill could be such that only in the central region of the casting-chill interface would the two surfaces be in contact, with a localised gap between them at their periphery. The extent of this deformation could be sufficient that heat transfer through the interface may not be unidirectional as assumed in the experiment and, depending on the location of the thermocouples placed close to the interface, the accuracy of the heat transfer coefficients calculated from the data collected at these points may therefore be affected.     

不同间隙下硼钢板-模具界面换热系数的反演

确定不同条件下板料-模具界面的换热系数对控制硼钢板在热成形中的局部冷却速率、获得组织性能呈区域性分布的构件具有重要意义,还可为热成形过程精确数值仿真提供可靠的热边界条件。设计了简单、有效的实验装置以模拟硼钢板热成形中不同单边间隙下板料与模具间的传热状态,其中测温点处的温度变化仅与板料-模具界面换热系数相关,从而为界面换热系数的准确反演提供了保障。建立了界面换热系数的有限元优化模型,反演获得了不同间隙下B1500HS板与H13模具钢之间的换热系数,发现其随间隙的增大呈指数函数式减小,并对反演获得的界面换热系数进行了实验验证。     

方圆坯连铸结晶器传热分析及温度场仿真研究

连铸结晶器的工作物理环境极其复杂,其温度场难以确定和分析。本文依据经典传热理论,探讨分析了结晶器的传热现象,建立了描述结晶器温度场的数学和物理模型,并根据实际工况确定了相关参数,对方坯结晶器和圆坯结晶器两种不同简化模型进行了温度场有限元仿真分析,其结果可为结晶器的结构设计和监控提供理论依据和参考数据。     

Effect of oxide scale on temperature-dependent interfacial heat transfer in hot stamping process

An optimization-based numerical procedure was developed to determine the temperature-dependent interfacial heat transfer coefficient (IHTC). The effects of temperature, pressure and oxide scale thickness were analyzed, for oxide thickness between 9 μm and 156 μm and pressure from 8 MPa to 42 MPa. Oxide scales and contact pressure both show distinctive effects on IHTC in the cooling process. The average IHTC decreases about 2461 W/(m² °C) with the increase of oxide scale thickness and increases 2620 W/(m² °C) with the increase of pressure. Based on the two-way ANOVA, the effect of contact pressure influences the IHTC most. Their mutual interaction is negligible. The IHTC decreases when the average temperature between the blank and die surface is above 250 °C and increases when the latent heat release.     

Heat flux distributions and convective heat transfer in deep grinding

By using experimental data including the monitored temperature and power signals, combined with detailed theoretical analysis, the relationship between the undeformed grinding chip thickness and specific grinding energy has been studied and used to derive the heat flux distribution along the wheel-work contact zone. The relationship between the grinding chip thickness and specific grinding energy (SGE) has been shown to follow an exponential trend over a wide range of material removal rates. The distribution of the total grinding heat flux, q_t, along the grinding zone does not follow a simple linear form. It increases at the trailing edge with sharp gradients and then varies nearly linearly for the remainder of the contact length. The heat flux entering into the workpiece, q_w, is estimated by matching the measured and theoretical grinding temperatures, and it has been found that the square law heat flux distribution seems to give the best match, although the triangular heat flux is good enough for most cases to generate accurate temperature predictions. With the known heat flux distributions of q_t and q_w, the heat flux to the grinding fluid can then be estimated once the heat partitioning to the grinding wheel is determined by the Hahn model for a grain sliding on a workpiece. The convective heat transfer coefficient of the grinding fluid has been shown to vary along the grinding zone. An understanding of this variation is important in order to optimise the grinding fluid supply strategy, especially under deep grinding conditions when contact lengths are large. It has been demonstrated that the down grinding mode can provide a beneficial fluid supply condition, in which the fluid enters the grinding zone at the position of highest material removal where a high convective cooling function is needed.     

铝铸锭凝固边界热交换规律及温度场模拟

研究铝铸锭凝固边界热交换的变化规律及数学模型,并对不同浇注温度下凝固过程的温度场进行模拟。利用实时数据采集系统获得凝固过程中铸锭和金属模温度变化历史数据,采用非线性反算法和一维传热差分法对试验数据处理,建立界面换热模型并将其应用于凝固温度场模拟中。结果表明：在铸锭表面凝固前后凝固界面热流密度可分段用指数函数来描述其变化规律,而所建立的热交换系数与边界温度的对应关系可更好地反映实际的传热情况。模拟结果与实验测温结果相符,验证了该铸件/铸型边界热交换规律的可靠性。     

带有玻璃润滑剂的P92耐热钢与H13模具钢间的界面传热系数

在玻璃润滑热挤压工艺的模拟中,界面传热系数是非常重要的一项参数,然而目前已有文献中所使用的数值缺乏实验依据。该文根据一维稳态传热原理,搭建了界面传热系数的测试平台,测试了P92耐热钢与H13模具钢之间的界面传热系数。P92与H13之间用玻璃润滑剂隔开,研究不同温度、不同玻璃润滑剂厚度和不同界面压力对传热系数的影响,并提出了应用于工艺数值模拟的界面传热系数的合理数值。     

不同构造窗户的传热系数和设计应用

窗是房屋建筑中的主要围护构件之一,是建筑物得失热的主要部位。在建筑节能的研究领域内,窗户的传热系数是一个极其重要的物理量。本文探讨了不同构造窗户的传热系数及其设计运用。     

一种铝合金水冷界面换热系数反求方法的研究

针对水冷金属界面换热系数影响因素多,测量与求解难的问题,以温度场数学模型为基础,以实测温度曲线为基准,通过数值模拟迭代计算和自动寻优,实现了铝合金水冷界面换热系数随温度变化定量关系的反求.反求得到的铝合金换热系数结果表明:在浸入式水冷过程中,铝合金界面换热系数随表面温度由低到高呈现出先升后降的单峰形状特征,降低冷却水的温度会使换热系数的峰值点升高,但不会改变峰值点出现的温度范围,换热系数的最大值出现在200～230℃.金属与冷却水之间热交换的强度主要取决于界面温度,将界面温度控制在200～230℃会使强化传热效果达到最佳.     

Effect of chill cooling conditions on cooling rate,microstructure and casting/chill interfacial heat transfer coefficient for sand cast A319 alloy

A combination of experiments and numerical analyses were used to examine the cooling conditions, solidification microstructure and interfacial heat transfer in A319 cast in a chilled wedge format. Both solid copper chills and water cooled chills, with and without a delay in water cooling, were examined in the study. Various chill preheats were also included. The goal of the investigation is to explore methods of limiting heat transfer during solidification directly beside the chill and increasing cooling rates during solidification away from the chill. Within the range of conditions examined in the study, chill preheat was found to have only a small effect on cooling rates between 5 and 50 mm from the chill/casting interface, pour superheat a moderate effect and water cooling a significant effect. In comparison to the results for the solid chill, the solidification time at 5 mm with water cooling applied at the beginning of mould filling is reduced from 56 to 15 s and at 50 mm from 588 to 93·5 s. Furthermore, the average cooling rate during solidification is increased from 1·9 to 7·06°C s^?1 at 5 mm and from 0·18 to 1·13°C s^?1 at 50 mm. At 50 mm, for example, the increased cooling rate achieved with water translates into a reduction in secondary dendrite arm spacing from 40 to 25 μm or ～40%. Delaying the water cooling by 10 s facilitated slow cooling rates at 5 mm (similar to those achieved with a solid chill) and high cooling rates 50 mm from the chill. A temperature based correlation was found to be suitable for characterising the behaviour of the interfacial heat transfer coefficient in the solid shill castings, whereas a time based correlation was needed for the water cooled castings.