Numerical Modelling of Semi‐Solid Flow under Processing Conditions

During the industrial process of semi‐solid forming (or thixoforming) of alloy slurries, typically the operation of die filling takes around 0.1s. During this time period the alloy slug is transformed from a solid‐like structure capable of maintaining its shape, into a liquid‐like slurry able to fill a complex die cavity: this involves a decrease in viscosity of some 6 orders of magnitude. Many attempts to measure thixotropic breakdown experimentally in alloy slurries have relied on the use of concentric cylindrical viscometers in which viscosity changes have been followed after shear rate changes over times above 1s to in excess of 1000 s, which have little relevance to actual processing conditions and therefore to modelling of flow in industrial practice. The present paper is an attempt to abstract thixotropic breakdown rates from rapid compression tests between parallel plates moving together at velocities of around 1m/s, similar to industrial conditions. From this analysis, a model of slurry flow has been developed in which rapid thixotropic breakdown of the slurry occurs at high shear rates.     

Effects of Pre‐Processing on Thixoformability of Steel Grade 100Cr6

Thixoforming is a new manufacturing technology which has been widely investigated for aluminium alloys. The thixoformability of steel and especially the impact of different pre‐processing is subject of this paper. For the thixoforming process it is of special interest to obtain the required fraction liquid content between 20 and 40%, a globulitic microstructure and low process temperatures. Therefore, steel primary material from three different processing routes was compared. The primary materials are rolled bars, laboratory cast billets and laboratory cast billets with liquid core reduction. The melting behaviour of the different materials was investigated by using Differential Thermal Analysis (DTA). Furthermore the materials were reheated into the solid‐liquid range and quenched to monitor the development of the microstructure. No significant differences between the three different pre‐processing routes could be determined with regard to thixoforming. Thus it seems possible to start thixoforming with as‐cast primary material.     

Thixoforging of Steel Using Ceramic Tool Materials

Today thixoforming processes are mainly established in the production of parts made of aluminium alloys. Compared to high melting alloys the process temperature is low and thus the process is easier to handle. Because of process temperatures up to 1470°C thixoforming of steels demands a sophisticated process control and adapted tool materials. In this field there is still a large need for research. This paper deals with experiences made at the IFUM concerning the thixoforging of steel. The use of ceramic tool materials for thixoforging of steel is presented. For this purpose hybrid dies where a ceramic insert is prestressed with a hot working steel shrink ring as well as different stamp geometries were developed and built up. Different ceramic materials from Si₃N₄ to ZrSiO₄ were tested and evaluated. To prevent an unwanted cooling and oxidation of the slug during its transfer, the transfer is carried out encapsulated in high temperature resistant crucibles that can provide a protective atmosphere. The suitability (e.g. thermo shock resistance) of different materials for the use as transport crucibles was also subject of this research work. The forming experiments were carried out with the institute's hydraulic press. The process logic controller of this press offers the appropriate flexibility and various control modes needed for a sophisticated and reproducible filling of the die. In this way the die filling process of different steels can be tested.     

立弯式浇道对半固态A356铝合金浆料组织的影响

采用立弯式浇道制备A356铝合金半固态浆料,研究了浇注温度对A356铝合金半固态浆料组织的影响,并考察了浆料经过均热处理后的组织。结果表明,采用立弯式浇道可以制备出半固态浆料,在本实验条件下,获得理想的半固态A356铝合金浆料的浇注温度为630℃和640℃,而且随着浇注温度的降低,初生相的形态由树枝状→蔷薇状→颗粒状转变;经过感应均热后,初生相更圆整,且尺寸、分布都很均匀。     

矿化模型技术的新进展

本文总结了矿化模型技术的发展过程,重点介绍了实体矿化模型的研究现状和实体矿化模型对采矿计算机辅助设计技术的发展所起的巨大促进作用,并对如何进行矿化模型的研究提出了一些看法.     

Experimental Investigations and Computer Simulation of the Liquid Fraction Evolution during the Solidification of Alloys Suitable for Semi‐Solid Processing

One important parameter for the processing of materials by semi‐solid forming is the actual distribution of the solid and liquid phases in the semi‐solid range. This parameter defines the process stability for the forming step. Therefore it is necessary to obtain information about the materials behaviour in the semi‐solid state for different materials grades. This kind of information can be obtained by experimental studies in the interesting temperature range or by calculations with simulation programs using thermodynamic data validated by experiments. This work shows the results of experimental studies and thermodynamic calculations of the solidification and heat treatment behaviour of the aluminium alloy A319 and the steel X210CrW12. The experimental studies of solidification and heat treatment of these alloys were carried out using a differential thermal analysis system (DTA). The theoretical fraction of liquid content was calculated from the DTA signal by using a software module called Corrdsc. The experimental data obtained were used to validate the thermodynamic simulations of the solidification of semi‐solid alloys. The simulations of the solidification process were carried out for equilibrium conditions, with the Scheil‐Gulliver model as well as with diffusion calculations. The equilibrium and Scheil‐Gulliver calculations were performed by the program Thermo‐Calc, and the diffusion by the program DICTRA. The required thermodynamic and mobility data for multicomponent systems were taken from the data bases COST 507 light alloys, TCFE2000 Steel/Alloys and MOB2 mobility and from newly added data. The comparison of calculated phase transformations and fractions of liquid content with experimental data revealed a good agreement.     

Theoretical and Experimental Damage Prediction in Cold and Semi‐Hot Bulk Forming of Ductile Steels

Bulk forging is among the most important manufacturing methods in metal forming, due to its wide applicability from some ounces to several tons of steel in a high diversity of shapes and forming conditions. Economical constraints demand for further optimisation and cost‐effective production. This requires the application of suitable finite elements simulation software, in order to support the already digitalised construction processes. Ductile damage is one of the most severe problems to arise during the production sequences, not only in cold but also in semi‐hot forging operations. Mathematical approaches exist for the modelling and simulation of ductile fracture in steel. In this paper some widespread used damage models are introduced and discussed. Their damage prediction quality has been verified by experiments, the tensile test and the collar specimen upsetting with several different steels under cold and semi‐hot forging conditions. The methods for the experimental fracture detection are introduced as well. In cold forging the passive ultrasonic testing with integrated statistical filtering algorithms is used. As this method is not applicable to semi‐hot forging experiments, optical fracture detection by means of a high‐speed camera is used instead. A very interesting material behaviour of the steels tested has been identified in the semi‐hot upsetting of collar specimen. For every steel a distinct temperature crossover interval exists, in which the forging process abruptly changes from damaged to undamaged state. This interval amounts to some degrees Celsius only for each of the seven materials investigated. Among the damage models proposed, the Model of Effective Stresses by Lemaitre is chosen for the application to a cold and a semi‐hot forging operation. These industrial processes of an axle end (cold) and a journal bearing (semi‐hot) are susceptible to damage for reasons to be discussed in this paper. It will be shown that the internal fracture of the axle end (chevrons) and the surface fissures of the journal bearing can be predicted with high accuracy. Moreover, the application of the damage model in the finite element software MSC.SuperForm 2004 offers a promising approach for process optimisation. Several possibilities could be tested for their suitability of reducing the calculated damage: geometry variation of the forming tools, process annealing, different materials. The use of damage models in finite element simulation can be regarded as a further step towards an optimal process design.     

DTA‐Measurements to Determine the Thixoformability of Steels

Semi‐solid metallurgy (SSM), also known as “thixoforming” or “thixoprocessing”, is of special interest as a new potential manufacturing technology for components in the automobile, machine and electronic industries. The aim of this technology is to produce complex shapes which cannot be produced with conventional processing methods. An important process step of semi‐solid processing (SSP) is the reheating and isothermal holding of the billet within the solid‐liquid range in order to obtain the required fraction liquid content and the desired globular microstructure. Aside from the investigation of billet heating and the development of a suitable tool design, the development and evaluation of adequate microstructures over a wide temperature area is very important. The focus of this paper is to determine the semi‐solid area of different steels through Differential Thermal Analysis (DTA) measurements. To determine a process window for handling the alloys in the semi‐solid state, the DTA‐results can be combined with microstructure parameters. Subsequent quenching experiments show the development of the microstructure parameters (e.g. grain size, phase distribution, volume fraction, shape factor, matrix character, contiguity, and particle density of the primary solid and liquid phases). A comparison of the slopes of the determined solid‐liquid areas for different steels show the width of the melting or freezing intervals to evaluate the possible process windows. DTA‐experiments performed at different heating rates show the influence of faster heating and cooling rates on the solidus‐liquidus interval. To evaluate the suitability for the thixoforming processes, this paper describes, and then compares, the semi‐solid intervals of different steel grades, which have been investigated in the Department of Ferrous Metallurgy at the RWTH Aachen University. The tool steel HS 6‐5‐3 and the cold work tool steel X210CrW12 have a wide semi‐solid area, which can be explained due to the dissolution of different carbides. In contrast to this, the steels C45, 42CrMo4, 16MnCr5, 34CrNiMo4, 100Cr6, X220CrVMo13‐4 and the Alloy 33 show a much smaller semi‐solid area.     

Two‐Phase Simulations as a Development Tool for Thixoforming Processes

Semi‐solid metal alloys are employed in Thixoforming processes. In the mushy state alloys, consisting of spherical solid particles suspended in a liquid matrix, show shear thinning thixotropic behaviour and exhibit a yield stress. Beyond this the flow behaviour is influenced by the solid fraction, which is dependent on the slurry temperature. Due to the complex flow behaviour of metallic suspensions new concepts of die and process design are required to guarantee a reproducible high quality of the produced parts. To this end a non‐isothermal two‐phase model has been developed and implemented into a proprietary developed FEM software. The model bases on a modified Herschel‐Bulkley approach and comprises the entire non‐Newtonian behaviour of semi‐solid alloys. A specially designed die filling experiment is used to validate the numerical simulation. The experimental set‐up allows to observe the moving flow front during die filling and to measure the pressure. The filling experiments are carried out with various filling velocities and die temperatures.     

Casting of High Alloy Steels in the Mushy State

In order to broaden the field of application for the innovative thixocasting process, much research is dedicated to the thixocasting of high melting point alloys. The wide property range of modern high alloy steels combined with the productive semi‐solid die casting process opens up new fields of application. The Foundry Institute of the Aachen University has therefore been concentrating on the research of the possibilities and limits of high pressure die casting of high alloy steels in the semi‐solid state. This paper gives an overview of the current work dedicated to thixocasting of steel alloys by a high pressure die casting machine at the Foundry‐Institute of the Aachen University of Technology. In order to understand and describe the material properties in the semi‐solid state, basic test specimens have been investigated. Weak points of tool preheating as well as directional solidification of the produced parts can be controlled by numerical simulation of the temperature distribution inside the dies. In consideration of the outstanding flow properties of semi‐solid steels more complex geometries with accurately defined applications are now being investigated. Extensive metallographical analyses of the pre‐material, the reheated billets and the produced parts have been done to evaluate the viability of the process. The mechanical properties of the specimens outline the outstanding potential of the thixocasting process.     

镁合金半固态材料成形与浆料制备工艺研究

由一对带有螺纹的内外桶的旋转，在其间的缝隙形成剧烈的剪切应力场的作用原理自行研制的镁合金浆料制备装置。内外桶缝隙控制在1～3mm。通过控制桶的转速可以获得0～100s^-1剪切速率，剪切时的温度在50～900℃之间任意调节。通过计算分析：AZ91合金的半固态流变成形温度区间为550～590℃，同时得到了，AZ91合金的固相体积分数fs(固相率)随温度变化的关系曲线。此外还确定了转速与剪切速率的关系。在温度为570℃，固相体积分数为0．41时，不同的剪切速率条件下镁合金浆料制备与压铸的实验结果表明：利用该装置制备出的半固态浆料晶粒圆整，组织均匀，并随着剪切速率的提高固相颗粒更加细小均匀。     

Semi‐Solid Processing of Metal Alloys

Semi‐solid metal casting is an innovative technology for the production of near‐net‐shape parts with demanding mechanical properties. The paper describes different processing routes and materials for semi‐solid‐metal casting (SSM), which have been investigated and also partially developed at the Foundry‐Institute of Aachen University. The standard thixocasting process for aluminium, highly reactive magnesium alloys and steel alloys with high melting points was investigated under variation of a wide range of process parameters. Specially adapted pre‐material production and reheating methods were developed for different materials and their application and future potential is pointed out. The thixocasting experiments were executed on a modified high pressure die‐casting machine with a specially designed “step‐die” providing wall thicknesses from 0.5 to 25 mm. The mechanical properties were tested in dependence of the wall thickness and the metal velocity. The results of these examination show high tensile strength values in combination with very good elongations. The rheocasting process is a new SSM‐forming method with liquid melt as feed‐stock and a high recycling potential. The research results of RCP‐technology (Rheo‐Container‐Process) invented at the Foundry‐Institute and of the Cooling‐Channel‐Process for aluminium and magnesium alloys are promising and are presented in this paper. Studies on semi‐solid processing of magnesium alloys and mixtures of them were conducted by Thixomolding^TM. To establish the most adequate process parameters, the temperature and the mixture relations were varied. Using a mould for tensile test specimens, the mechanical properties and the microstructure evolution could be evaluated. The chemical composition of the different phases was determined using SEM and EDX technologies. Evaluations of the flowing properties were conducted using a spiral mould with a total length of 2m and a cross section of 20mm x 1.5mm.     

Evolution of Globular Microstructure in New Rheocasting and Super Rheocasting Semi‐Solid Slurries

Based on the solidification theory for metal alloys, a simple recipe for the controlled processing of globular microstructures without external stirring is presented: Firstly, small solidification nuclei must be distributed homogeneously throughout a melt. In New Rheocasting (NRC) these nuclei are formed by forced homogeneous nucleation due to partial quenching of the melt, while in Super Rheocasting (SRC) the nuclei are “second phase particles” in specially designed alloys, which are grown in a controlled fashion in a certain temperature range. Potential alloy compositions for SRC are provided. Secondly, given these melts with small particles in them, globular growth can be assured by utilizing the Gibbs‐Thomson “self healing effect” and slow further cooling to allow diffusion in the melt and to suppress constitutional supercooling. This simple recipe is applicable to various ferrous and non‐ferrous alloys. If an SRC alloy is cooled more rapidly than necessary for globular growth of the primary phase, but is held sufficiently long in the SRC range for dispersoid formation, these dispersoids can act as potent grain refiners and possibly enhance elevated temperature properties. A combination of both processes by using SRC alloys in the NRC equipment may lead to pressure tight castings with low porosity and finer grain structure than can be achieved with NRC on its own, and consequently, better mechanical properties.     

固体废物的固化/稳定化研究现状

阐述了固体废物的定义、特点、分类,指出固体废物对环境造成的危害,总结出国内外水泥固化、化学药剂稳定化、熔融固化、塑性材料固化、石灰固化和自胶结固化处理固体废弃物的固化/稳定化技术的研究现状,对目前各种固化/稳定化技术存在的问题及今后研究工作提出建议.     

The Rheological Behaviour of Metallic Suspensions

The knowledge of the rheological behaviour of metallic suspensions is an important issue when simulating die filling in thixoforming processes. Therefore rheological experiments like step‐change in shear rate tests, shear stress ramps and oscillation experiments have been carried out to characterise the flow behaviour of semi‐solid alloys exemplarily for A356 and Sn‐15%Pb. The material exhibits a yield stress and shows shear‐thinning and thixotropic flow behaviour, which is strongly influenced by the solid fraction, the particle shape and size and the degree of particle agglomeration. The instantaneous response on a change in shear rate was observed and pretends shear‐thickening properties. All experiments were carried out in a Couette rheometer. The experimentally gained data are well fitted to a model approach, which consists of a modified Herschel‐Bulkley law and accounts for the time‐dependent effects by introducing a structural parameter κ.     

Selection of Suitable Tool Materials and Development of Tool Concepts for the Thixoforging of Steels

This paper describes the results of the European project “THIXOCOMP” within the 5th framework programme concerning the material selection and development of tools for the thixoforging of steels. Due to high process temperatures, special requirements are necessary regarding the tool material and the tool concept. Special tool coatings (High Velocity Oxi Fuel, HVOF and Plasma Spray, CAPS) with high strength, high corrosion and oxidation resistance were deposited on substrate materials 1.2367, 1.4841 and 2.4631 to improve the properties of the tool materials. Different laboratory tests were performed to investigate the suitability of the tool systems. Pull tests, micro‐hardness measurements, high temperature corrosion tests and spelling resistance tests were performed to investigate the adhesion of the coatings. The investigation of the thermal shock loading on the coated samples indicated a clear dependence on the base material. Both coatings on the base materials 1.2367 and 1.4841 were removed completely in the spelling resistance tests, so that even in the coated state, they are not suitable as tool materials for the thixoforming of steel. The combination of 2.4631+HVOF provided the best results. Afterwards, thixoforging trials were performed with the nickel‐based alloy, 2.4631. After 30 trials at 1290°C (HS6‐5‐3), the tool showed no macroscopic damages, whereas a deformation of the press channel was already visible after eight forming trials at 1430°C (100Cr6).     

半固态加工技术及其应用

半固态加工技术具有高效、节能、近净形生产以及制成品显微组织细化均匀、机械性能好等诸多优点，是一种很有发展前景的加工方法。介绍了半固态金属原料制备技术、重熔工艺和成形技术。综述了半固态加工技术的应用领域，并展望了其应用前景。