High strength and integrity investment castings

The paper briefly outlines the way in which the ancient method of investment [“lost wax”] casting of metal components has been transformed into a near nett shape forming technique offering the designer at the forefront of metal casting economy and flexibility. Following a brief outline of the process and its benefits, recent development trends in steel, aluminium alloy and superalloy investment casting are reviewed. Data on preferred design features for investment casting are included.     

Effect of Processing on Microstructure and Physical Properties of Three Nickel‐Based Superalloys with Different Hardening Mechanisms

The nickel‐based superalloys Inconel alloy 600, Udimet alloy 720, and Inconel alloy 718 were produced by electron beam melting (EBM), casting, and directional solidification (DS). The distance between dendrites and the size of the precipitates indicated the difference in solidification rates between the three processes. In this study, the solidification rate was fastest with EBM, closely followed by casting, whereas it was much slower with DS. In the directional solidified materials the <100> direction was the fastest and thus, preferred growth direction. The EBM samples show a sharp (001)[100] texture in the building direction and in the two scanning directions of the electron beam. Macrosegregation was observed in some cast and directionally solidified samples, but not in the EBM samples. The melting temperatures are in good agreement with literature and the narrow melting interval of IN600 compare to UD720 and IN718 might reduce the risk of incipient melting during EBM processing. Porosity was observed in the EBM samples and the reasons are discussed. However, EBM seems to be a feasible process route to produce nickel‐based superalloys with well‐defined texture, no macrosegregation and a rapidly solidified microstructure.     

Continuum model for predicting macrosegregation in dendritic alloys

“Macrosegregation” represents a class of defects in cast products of serious concern to both alloy producers and users. Many types of macrosegregation result from thermosolutal convection in the solid plus liquid and all-liquid regions of a solidifying alloy, and this has spurred modeling and simulations, which treat the solid plus liquid region (i.e., the mushy zone) as a porous medium of variable porosity and permeability. Simulations include scenarios in which the convection is strong enough to make channels in the mushy zone region, and these channels lead to localized segregates known as “freckles”. Using Pb-10 wt.% Sn as a model alloy, we simulated vertical solidification with various solidification rates. By sufficiently increasing the cooling rate at the bottom surface, convection can be suppressed enough to prevent the formation of freckles. The simulation is an example of relating microstructural metrics to a macroscopic property of the porous medium used in continuum theory. In this case, the property is the permeability, which is governed by two microstructural metrics: the volume fraction of liquid and a characteristic length in the dendritic microstructure. Permeability data, relevant to columnar dendritic solidification, are reviewed, and recommendations for future work on determining the permeability in terms of microstructural metrics are given.     

SCM435钢大方坯凝固过程组织的模拟

为了模拟不同冷却状态下的连铸坯的凝固组织,利用反算确定了SCM435钢325 mm×280 mm连铸坯的换热系数,采用有限元法模拟了连铸传热过程,获得了连铸坯的温度场及冷却速率,在此基础上与元胞自动机耦合模拟了连铸坯的凝固组织.研究发现,表面细晶区很大,且在连铸结晶器中完成形核并长大形成,而柱状晶开始形核于结晶器末端.降低形核数,晶粒密度、最大晶粒面积、平均半径存在不同程度的改变,其中晶粒的最大截面积增加了2.7倍,而微调成分对晶粒密度与平均半径影响较小,但同样凝固条件下晶粒不均匀程度有所加剧.     

Effect of process parameters on structure and macrosegregation upon direct chill casting of Mg–Nd–Zn–Zr alloy

Abstract

Direct chill (DC) semicontinuous casting process has been successfully used to produce sound Mg–3·0Nd–0·4Zn–0·4Zr (NZ30K) billets. The influence of process parameters such as casting speed, casting temperature on the microstructure and macrosegregation was studied. The results show that the casting speed affects the macrosegregation greatly while it has a slight influence on the grain size of the billet; the casting temperature has a slight influence on macrosegregation of the billet while the grain size of the billet increases as the casting temperature increases. The optimal process parameters have been experimentally determined as follows: casting temperature 700°C and casting speed 90 mm min^?1. The ultimate tensile strength, yield strength and elongation of billets cast at the optimal casting parameters are 196 MPa, 125 MPa and 16·5% respectively.     

Fatigue life enhancement of aluminium joints through mechanical and thermal prestressing

This paper presents some simple and flexible methods to enhance the fatigue life of welded aluminium components. Besides enhancing the fatigue life, the proposed methods can easily be implemented into manufacturing processes. The key element of the methods is to change residual stresses from tension to compression at locations vulnerable to fatigue. This is accomplished by mechanical prestressing using elastic pre-deformation or by thermal prestressing using induction heating. The specimens tested are welded aluminium rectangular hollow section T-joints. Prior to fatigue testing, welding FE-simulations were carried out to verify the magnitude and pattern of the residual stress fields (through process modeling). Fatigue testing was later carried out on four different batches. One batch was produced using elastically pre-deformed chords, two batches were treated by means of thermal prestressing (induction heating), and one batch was “as welded” representing a “reference case”. Based on statistical evaluation of S–N data, the introduction of superimposed compressive stress fields results in a significantly improved fatigue life. Among the different batches, induction heating turned out to be the most promising method with a fatigue strength improvement factor of 1.5 on stress, compared to “as welded” components.     

Deposition and etch processes: continuum film evolution in microelectronics

Aspects of modeling and simulation of topography evolution during deposition and etch processes used in the fabrication of integrated circuits are discussed. Overall, we hope to demonstrate that combined simulation and experimental studies of film profiles and composition profiles inside features is a valuable tool in efforts to arrive at useful kinetic and transport models. In particular, conformality limitations and film composition variations inside features for films deposited at low pressures are explained using examples of studies that combine transport and reaction simulations of deposition processes and carefully designed experimental work. The technical presentation is divided into three major parts. In the first section, we demonstrate that thermal systems can be modeled without “calibrating” the transport and reaction models used, though calibration can still be useful. The process considered in this section is the thermal deposition of SiO₂ from TEOS (tetraethoxysilane). We discuss the use of film profile information to decide between two kinetic models for the deposition process, then we discuss one way to integrate reactor scale and feature scale models in order to capture “microloading” due to changes in local pattern density. The second section demonstrates the state of topography simulation for plasma processes. We demonstrate the use of physically motivated models that in general require calibration from experimental data for a given set of operating conditions. As our first plasma example, we use the sputter deposition of Ti–W to demonstrate the existence of composition profiles inside features. We then use etch simulations to show how simulations can be used to test proposed chemical and/or physical phenomena. The last major section is a case study on plasma enhanced deposition of SiO₂ from TEOS and oxygen (PETEOS) that demonstrates the roles of “3d/2d” and “3d/3d” (transport dimensionality/surface dimensionality) topography simulators in “virtual wafer fabs”. The same methodology would apply to most topography relevant processes, including thin film flow processes.     

Modelling mixed columnar-equiaxed solidification with melt convection and grain sedimentation – Part II: Illustrative modelling results and parameter studies

A volume-averaging multiphase solidification model was introduced in Part I. In Part II, illustrative simulations are made for two benchmarks, a unidirectional solidification benchmark and a cylindrical ingot casting, using a binary Al–Cu alloy. For the case of unidirectional solidification the competing growth of columnar and equiaxed structures, evolution of different phase regions, solute redistribution, and the influence of grain sedimentation and melt convection are analyzed in detail. The columnar-to-equiaxed transition (CET) is investigated, with important insights derived from the CET prediction. The new features of the model and its applicability to industrial-type castings are demonstrated with simulations of a cylindrical ingot casting. This is done in both a 2D axisymmetric and a full 3D geometric domain to demonstrate the ability of the model to produce consistent results. The main features of the model that are verified include tracking of the columnar primary dendrite tip, nucleation of equiaxed grains ahead of the columnar tip front, hydrodynamic and solutal interactions between the equiaxed and columnar structures, the columnar-to-equiaxed transition (CET), melt convection and grain sedimentation, and macrosegregation and the final macrostructure. With appropriate modelling parameters the typical columnar-equiaxed macrostructure observed in experiments can be reproduced. Uncertainties due to model parameters and assumptions are addressed and discussed.     

Measurement of plastic zones associated with small fatigue cracks by selected area channeling patterns

The selected area electron channeling pattern method has been used to measure the plasticity associated with “growing” small fatigue cracks in a 7000 series aluminium alloy, allowing study of plastic zone development as well as investigation of the mechanisms by which small fatigue cracks and their plastic zones interact with grain boundaries. It was found that both plastic zone size and shape were dependent on both crack length and growth morphology. Naturally initiated Stage I fatigue cracks were predominantly crystallographic in nature and were accompanied by a relatively long, slender plastic zone shape. However, during subsequent growth over 1–2 grain diameters, this shape evolved, first to a semicircular shape, and finally to the lobed configuration typically found associated with long cracks.     

A new approach to improving the macrosegregation in zirconium‐containing magnesium‐lithium alloy

In the preparation process of zirconium‐containing magnesium alloy, although zirconium is introduced into alloy by magnesium‐zirconium master alloy, the settling of zirconium particles has always been a key problem. In this study, the magnesium‐30wt.%zirconium master alloy was added into magnesium‐14wt.%lithium‐zinc alloy melt in the form of the block (about 20 mm) and the particles (about 20 μm), respectively, and then magnesium‐14wt.%lithium‐zinc with 0.5 wt.% zirconium alloy were prepared using stir‐casting process. Macrosegregation of major element (zirconium), microstructure and microhardness at different casting positions were examined to investigate the effect of zirconium addition methods on macrosegregation of magnesium‐14wt.%lithium‐zinc alloy. The results show that, for the block magnesium‐zirconium master alloy addition, there is obvious macrosegregation in alloy ingots, the zirconium contents at the top position of ingot are higher than that at the bottom by nearly 200 %. The method of particles master alloy addition can effectively improve macrosegregation, the difference in zirconium contents between the top and bottom is less than 16 %.     

Mechanism of grain refinement of an Al–Zn–Mg–Cu alloy prepared by low-frequency electromagnetic casting

Low-frequency electromagnetic casting (LFEC) process had been developed and is being used for the past several years with the application of an induction coil placed outside the conventional direct chill (DC) casting mould. It has been demonstrated that the LFEC process has a significant grain refining effect on aluminium alloys. In the present study, temperature measurement and direct quenching from liquid and/or semi-solid were carried out to study the temperature field during casting process and to understand the mechanism of the grain-refining effect of the LFEC process. The experimental results showed that in contrast to the conventional DC casting process, the liquid melt from the launder, during the LFEC process, is cooled with very high cooling rate directly to 3–6 °C below the liquidus, and the temperature field of the entire melt in the mould, and the hot top is quite uniform, which results in the enhanced heterogeneous nucleation and improved survival rate of the nuclei. This is believed to be the main reason why the LFEC process can significantly refine the grain size of aluminium alloys.     

Computer-aided characterization of solification processes

The directional solidification process is used to make columnar grained and single crystal turbine blades for the hot section of advanced gas turbine engines. These blades are fabricated using a family of alloys known as the superalloys. The microstructural features of these alloys are dependent upon the thermal characteristics of the solidification process. In this article it will be shown how an improved understanding of such processing has been achieved using computer simulation and advanced graphics techniques. This has led to new insight into cause and effect relative to grain quality and microstructural details concerning dendritic features. Improved processing methods have evolved based on the application of scientific methodology to what has traditionally been the art of casting.     

Segregation in cast products

Microsegregation gets eliminated significantly if subsequent hot working and/or annealing are done on cast products. Macrosegregation however persists, causing problems in quality, and hence, has to be attended to. Microsegregation is a consequence of rejection of solutes by the solid into the interdendritic liquid. Scheil’s equation is mostly employed. However, other equations have been proposed, which take into account diffusion in solid phase and/or incomplete mixing in liquid. Macrosegregation results from movements of microsegregated regions over macroscopic distances due to motion of liquid and free crystals. Motion of impure interdendritic liquid causes regions of positive macrosegregation, whereas purer solid crystals yield negative macrosegregation. Flow of interdendritic liquid is primarily natural convection due to thermal and solutal buoyancy, and partly forced convection due to suction by shrinkage cavity formation etc. The present paper briefly deals with fundamentals of the above and contains some recent studies as well. Experimental investigations in molten alloys do not allow visualization of the complex flow pattern as well as other phenomena, such as dendrite-tip detachment. Experiments with room temperature analogues, and mathematical modelling have supplemented these efforts. However, the complexity of the phenomena demands simplifying assumptions. The agreement with experimental data is mostly qualitative. The paper also briefly discusses centreline macrosegregation during continuous casting of steel, methods to avoid it, and the, importance of early columnar-to-equiaxed transition (CET) as well as the fundamentals of CET.     

Effect of Low Frequency Electromagnetic Field on Macrosegregation of Horizontal Direct Chill Casting Aluminum Alloys

The horizontal direct chill (HDC) casting process is a well-established production route for aluminum alloy ingot but the ingot may suffer from macrosegregation sometimes. In order to control the defect, a low frequency electromagnetic field has been applied in HDC casting process and the relevant influence has been studied. The results show that application of low frequency electromagnetic field can reduce macrosegregation in HDC casting process; and two main parameters of electromagnetic field density and frequency, have great influences on the solution distribution along the diameter of ingot. Moreover, the mechanisms of reduction of macrosegregation by electromagnetic field have been discussed.     

Modeling of Liquid Metal Infiltration of Porous Fiber Preform During Squeeze Casting

The present work adopts a new approach to the analytical modeling of infiltration of porous fiber preforms by liquid metal in the squeeze casting of metal matrix composites, with the assumption that the process is adiabatic and that the flow is unidirectional. Fluid dynamics is described on the basis of Darcy's law, while separate equations are derived to explain the thermal behavior of the liquid metal and the fiber, assuming that the thermal interactions between the two are interfacial. Unlike earlier models, this approach does not consider the thermal behavior of a “composite,” but instead studies the behavior of the liquid metal and the fiber preform separately. In addition to the conventional application of heat balance techniques and development of partial differential equations involving temperatures, this work introduces supplementary conditions for temperature calculations, specifically at the entry and front points during infiltration. Differential equations are solved by a method of finite differences, and the problem of additional unknowns (preform temperature) at the infiltration front position is overcome using the “virtual point” concept. Simple expressions are derived for the calculation of process parameters like total time for complete infiltration and time for solidification, on the basis of which the occurrence of complete infiltration is predicted. A novel attempt in generating the profiles of the preform and liquid temperatures at specific instants during infiltration has also been made. The relative influence of the liquid superheat temperature, the preform preheat temperature, and the squeeze pressure on the infiltration mechanism is analyzed by studying the infiltration characteristics for various squeeze conditions.     

An electron metallographic study of the commercial zinc-based pressure-die-cast alloy 3

The solidification structure of pressure-die-cast commercial alloy 3 was examined using scanning electron microscopy and thin foil transmission electron microscopy techniques. It was found that the rapidly cooled alloy commenced solidification by the formation of small rounded primary zinc (η) particles followed by pseudo-primary β particles and then a fine eutectic of β + η. The high temperature β phase subsequently decomposed into -aluminum and η, the phases stable at ambient temperature, but an intermediate transitional phase was found in alloys examined shortly after casting. Chemical analysis by energy-dispersive spectroscopy showed that this phase contained zinc with 11.8 wt.% Al, suggesting that it was a transitional phase since none of the stable phases in the Al---Zn system has that composition. This transitional phase had almost entirely disappeared after aging for a period of 5 years.

Precipitates had formed during or immediately after casting within the central regions of the zinc-rich primary dendrites. These were identified as the aluminium-rich solid solution with f.c.c. structure. The orientation relationship between the phases was determined as

[0001][111]

Each grain contained two families of precipitates with a common (0001)_η habit plane, each adopting one of the two non-equivalent variants of the orientation relationship.

Since this work had shown that aluminium precipitation from η was completed rapidly, the long-term dimensional changes found in zinc alloy castings on aging are considered to be due to the gradual replacement of the zinc-rich metastable phase by equilibrium and η.     

Microstructure characterization and tensile properties of direct squeeze cast and gravity die cast 2017A wrought Al alloy

Squeeze casting is a pressurized solidification process wherein finished components can be produced in a single process from molten metal to solid utilizing re-useable die tools. This one activates different physical processes which have metallurgical repercussions on the cast material structure. Desirable features of both casting and forging are combined in this hybrid method. 2017A aluminium alloy, conventionally used for wrought products, has been successfully cast using direct squeeze casting. Squeeze casting with an applied pressure removes the defects observed in gravity die cast samples. Tensile properties and microstructures are investigated. The results show that the finer microstructure was achieved through the squeeze casting. Furthermore, higher pressures improved the fracture properties and decreased the percentage of porosity of the cast alloy. The ultimate tensile strength, the yield strength and the elongation of the squeezed cast samples improved when the squeeze pressure increased.     

电磁搅拌作用下CoCrMo合金熔模铸件凝固细晶研究

目的 研究电磁搅拌对CoCrMo合金熔模铸件晶粒尺寸的影响,解决熔模铸造CoCrMo合金铸件晶粒粗大的问题。方法 将CoCrMo合金熔化后,在其凝固过程中分别施加不同工艺参数的电磁搅拌,并对其凝固后的组织进行表征分析。同时,采用有限元法对电磁搅拌在金属熔体中的电磁场和流场进行数值模拟。结果 在不同的电磁搅拌参数下,CoCrMo合金铸件凝固组织出现了不同程度的细晶效果,浇道处的细晶效果优于铸件试棒处的。铸件试棒处的晶粒尺寸最小能控制在1 mm以下,等轴晶率最高能提升至31%。数值模拟结果表明,在电磁搅拌过程中,铸件试棒的磁场、电流和洛伦兹力都呈周期性变化,铸件试棒内部的流速随搅拌时间的延长而增大,最后趋于稳定。结论 电磁搅拌对CoCrMo合金的凝固组织产生了明显的细化效果,促进了柱状晶向等轴晶转变。电磁搅拌的时间越长,铸件凝固组织的细化效果越好,铸件厚大部位的细晶效果越显著。结合实验结果和数值模拟结果发现,在电磁搅拌过程中,熔体流动引发枝晶断裂是晶粒细化的主要原因,而电磁场促进异质形核为次要原因。     

Effect of combined application of electromagnetic fields on horizontal direct chill casting of 7050 aluminium alloy

Abstract

Influence of combined electromagnetic field application on horizontal direct chill casting of 7050 aluminium alloy is investigated. Temperature measurement and structure observation are carried out to analyse the casting process under different electromagnetic fields. Combined electromagnetic field can reduce the harmful gravitational thermal effect in the horizontal direct chill casting process and improve the ingot properties. With application of combined electromagnetic field, temperature distribution in the melt is more uniform, sump of the ingot becomes flat and symmetric, surface quality of ingot improves markedly, grain morphology changes from feathery grains to equiaxed grains and grain size decreases. Distribution of copper and zinc in the transverse section of the ingot becomes more uniform.