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.     

Advanced Processing and Properties of Thixotropic Formed Components Based on Partially Melted Magnesium

Contrary to the manufacture of aluminium components in the semi‐solid, thixotropic state the production of magnesium based components by semi‐solid techniques is still uncommon. For this reason, the advantage of this production method is analysed with regard to the commercial magnesium alloy AZ80. The objective of this research is semi‐solid‐casting (SSC) of AZ80 for the production of a light weight component in near‐net‐shape quality and with advanced properties. Using extruded primary feedstock material, the behaviour and the advantages are investigated. Billets with a weight of up to 2 kg are heated up into a semi‐solid state. To avoid any risk of self‐ignition of the material an automatic, temperature controlled induction heating system is used. To achieve an optimum homogeneous grain structure the induction heating power is varied making use of a process control system based on power‐time‐curves. The heated billets are transported in the soft semi‐solid condition from the induction heating system to a die casting machine to produce components with wall thickness’ between 2 and 10 mm. After forming of the components, the influence of heat treatment on the grain structure and especially on the mechanical properties is determined to provide parts with optimised characteristics. To compare the properties of the special globular grain and microstructure, the results of various static and dynamic tests are analysed. It is found that components can be manufactured with a magnesium alloy in a thixotropic state in near‐net‐shape quality, with low porosity and with excellent mechanical properties like elongation of up to 15%.     

Thermodynamic Criteria for the Selection of Alloys Suitable for Semi‐Solid Processing

In this work the suitability of alloys for semi‐solid processing was investigated using numeric simulation. The simulation was based on equilibrium calculations, Scheil‐Gulliver calculations and, when necessary, diffusion simulations. For this purpose a new parameter was introduced in addition to the commonly used selection criteria. With the new parameter, the thixo ranges ΔT^40–60 and ΔT^20–40, the specific demands of the different semi‐solid processes thixocasting and thixoforging can be considered. On the basis of thermodynamic simulation, the conventional aluminium alloys A356, AA6082 and A319, the steels 100Cr6, HS6‐5‐2 and X210CrW12 and a number of experimental aluminium‐lithium based alloys were evaluated according to the selection criteria. The thermodynamic calculations showed a large sensitivity of the course of the solidification with respect to variations in the contents of the alloying elements. This shows the necessity of keeping a tight composition control on alloys for semi‐solid processing. For aluminium alloys in particular silicon has to be monitored closely and for steels carbon and chromium.     

Semi‐Solid Rheoforging of Steel

To produce steel components with complex shapes excessive machining is necessary frequently since high pressure die casting of steel is not industrially applied. Forming steel in the semi‐solid state can in principle produce new components and geometries which cannot be realised by conventional closed die forging. Semi‐solid forging of steel combines the possibility of producing geometries not conventionally forgeable in one forming operation and of adding further functions during the same operation. In previous investigations on thixoforming of steels, the semi‐solid steel was generated by reheating precursor material billets. An alternative approach for generating semi‐solid steel from the liquid state with subsequent forging operation is presented in this paper for the first time. The steel grades X210CrW12 cold work tool steel and 100Cr6 bearing steel are molten and driven into a globular semi‐solid state using a cooling slope and a cup. By cooling the steel into the semi‐solid range instead of heating it, the required process temperatures are lower than in the process route via heating. Therefore, the load on the dies in a semi‐solid forging operation is decreased. Suggestions for the respective layout of the process are made for both steel grades. Future potentials and challenges to be solved are discussed, showing advantages especially in the field of high melting point alloys such as steels. This technique enables to produce pre‐shaped semi‐solid billets to optimise the materials flow and the homogeneity of the mechanical properties.     

Thixocasting Steel Hand Tools using Al2O3‐coated Steel and Molybdenum Dies

Forging is state‐of‐the‐art for producing hand tools on an industrial scale. Due to high demands on the stiffness and the fracture toughness, high‐strength forging steels are used to provide cavity‐free components with high mechanical load capacity. Moreover, forging is a cost‐effective mass production process but, in spite of all its advantages, it has its limitations, e.g. in the freedom of designs. However, because of the extreme thermal loading (particularly with regard to permanent moulds) and the frequently unavoidable casting defects, hand tools are not cast. By means of thixocasting steel, technical difficulties can be reduced and new options are provided which allow the manufacturing of components with much higher complexity than that using forging. Through near‐net shape production, manufacturing steps and costs can be reduced. Furthermore, steels, which are difficult to forge but nonetheless have high potential for specific applications (such as high strength or corrosion resistant steels), can also be processed. In cooperation with industrial partners, X39CrMo17 stainless steel size 17 combination spanners were thixocast. Forming dies were designed and optimized by simulation, the hot forming X38CrMoV5 tool steel as well as the molybdenum alloy TZM were selected as the tool alloys. The dies were treated by a plasma nitriding process and subsequently coated with crystalline Al₂O₃ protective coatings by plasma‐enhanced chemical vapor deposition (PECVD). During the experiments, combination spanners were successfully cast in the semi‐solid state. Cast parts were heat‐treated to enhance the components' toughness, which was subsequently measured by Charpy impact and tensile tests.     

半固态铝合金设计与试验研究

在半固态加工的基本原理基础上采用合金热力学计算方法设计了Al-Si-Mg系半固态新合金，并进行了初步的试验验证。结果表明，新合金在铸态和半固态下的组织和力学性能特点与预测结果基本一致，并且发现适当增加Mg，si含量可以改善铝合金半固态加工性能和提高合金的力学性能。这对进一步开发新型的实用半固态合金具有一定的指导作用。     

连铸保护渣与氧化铝的交互作用研究现状

保护渣中Al2O3质量分数变化是渣钢交互作用的结果,包括吸附钢液中Al2O3夹杂和渣金反应产物。氧化铝对保护渣成分与性能稳定性影响突出,进而影响铸坯质量。分析指出,影响保护渣吸附Al2O3夹杂的因素包括接触角、润湿速率和界面张力、熔渣成分和性能等。熔渣与夹杂物的交互作用行为决定了其对上浮夹杂的去除率。高铝钢连铸过程渣金反应尤为突出,保护渣的工作性能将发生明显改变。基于近年关于Al2O3夹杂的吸附溶解试验研究,以及高铝钢浇铸渣金反应及其保护渣性能变化的研究结果,探讨了通过合理设计保护渣成分或更换渣系等措施来实现该类钢种保护渣工作性能的相对稳定性。     

Strain induced melt activation — an effective route to produce high performance alloy plates and sheets

Processing of alloys in the semi solid state, within the liquidus and solidus temperature, has several potential advantages over casting and forging, such as reduction of macrosegregation, porosity and improved properties. A356 alloy produced by means of conventional gravity die casting, rheocasting and strain induced melt activation (SIMA) process has been investigated and their microstructure, mechanical and tribological properties were compared. The microstructure of conventional cast sample is fully dendritic in contrast to spheroidal morphology in rheocast and SIMA samples. The mechanical properties of the SIMA samples are considerably higher than that of conventional cast and rheocast samples. The volumetric wear loss and co-efficient of friction in SIMA samples are always less than those in conventional cast samples at all loads.     

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.     

Formation of defect bands in high pressure die cast magnesium alloys

Die cast magnesium components are being increasingly used worldwide because of the excellent castability and properties that magnesium alloys offer. High pressure die casting of thin-walled components is particularly suitable because of the excellent flow characteristics of molten magnesium alloys. Typical automotive applications for thin-walled castings include components such as instrument panels, steering wheels, door frames and seat frames. These applications require optimisation of the quality and performance of the castings. It has been found that bands of porosity or segregation which follow contours parallel to the surface of the casting are formed under certain casting conditions in thin-walled magnesium high pressure die castings. The presence of this type of defect can have a significant effect on the mechanical properties. This paper investigates the effect of varied casting conditions on casting integrity and the appearance of the bands. A rationale for understanding the origin of these defects is related to the solidification behaviour, the mushy zone rheological properties and the filling pattern of the casting with associated shearing of the mushy zone. Methods to optimise the process parameters to control the occurrence of the banded defects, and thereby optimise the quality of high pressure die cast magnesium components, are outlined.     

Pulsed‐Magnetic Hot Joining ‐ New Solutions for Modern Lightweight Structures

To further significantly reduce the car weight, new and innovative production technologies are necessary which allow to join aluminium and – in the future – magnesium profiles. Then it will be possible to connect components made of aluminium or magnesium base alloy with steel parts to one group. Such a new technology is pulsed‐magnetic joining. In order to lower the process forces necessary for pulsed‐magnetic joining of high strength aluminium alloys and in order to establish a technologically useful forming of magnesium alloys, joining at increased temperatures appears to be a viable solution. As part of the research a tool for pulsed‐magnetic hot joining has been developed that combines the processes of heating by induction and pulsed‐magnetic joining. The applicability and the potentials of the tool have been demonstrated with hot joining of aluminium and magnesium profiles. The combination of the techniques allows for the first time to join magnesium profiles with forming by using pulsed‐magnetic forming at high temperatures. Regarding aluminium the combination of the techniques allows a significant reduction of the magnetic pressure that is necessary for the forming process; it thereby increases the operating life of the tools.     

铝、镁合金流变压铸成形技术及应用

流变压铸成形技术在提高铸件力学性能、减少内部缺陷方面具有明显的优势,是一种先进的材料成形工艺．文中重点介绍了作者近年来开发的铝、镁合金半固态浆料制备装置和技术,以及TBR、FCR流变压铸成形技术在应用方面的研究进展与取得的成果．采用模拟仿真技术,对比了流变浆料和液态金属的压铸充型行为,结果表明：流变压铸充型平稳,可以有效减少铸件的缩孔疏松缺陷．     

Rupture Strength of some Mould Flux Slag Films relevant to Swedish Continuous Casting Practice

In this paper, the high temperature rupture strengths of some solid mould flux slag films in Swedish continuous casting processes are presented. The apparatus, designed and constructed in the present laboratory is based on the principle that the solidified film of the mould flux slag is subjected to elongation stress at temperatures below the solidus points of the fluxes. The rupture strength tests were conducted in air. The results indicate that the rupture strength is dependent on the extent of polymerization of the silicate network. The results of Raman spectra studies of the mould flux slags used in the present work were corroborated by estimations of the NBO/T ratios of the slags. It was found that lower NBO/T ratios in the slag led to higher rupture strengths. Earlier measurements of the viscosities, thermal diffusivities as well as stress relaxation results for the same slags followed a similar trend indicating the dependency of these properties on the slag structure and a clear link between thermophysical and mechanical properties of slags. The results are expected to provide some basic guidelines to develop the next‐generation mould flux and modelling of the continuous casting process.     

Effect of Melting Process on the Microstructure and Mechanical Properties of Fe–7 wt% Al Alloy

This article presents the effect of melting process on chemical composition, microstructure and mechanical properties of Fe–7 wt% Al alloy. The alloy ingot was prepared by air induction melting (AIM), air induction melting with flux cover (AIMFC) and vacuum induction melting (VIM) and cast into 50 mm diameter split cast iron mould. These cast ingots were hot-forged and hot-rolled at 1,373 K to 2 mm thick sheet. Hot-rolled alloys were characterized with respect to chemical composition, microstructure and mechanical properties. Ingots produced by AIM, AIMFC and VIM were free from gas porosity, however AIM ingots exhibited higher concentration of hydrogen as compared to AIMFC and VIM. The recovery of aluminium as well as reduction of oxygen during AIM is very poor as compared to AIMFC and VIM. AIMFC ingots exhibit low level of sulphur as compared to AIM and VIM ingots. The alloys produced by AIMFC and VIM exhibited superior tensile ductility compared to the alloys produced by AIM. The tensile properties of alloys produced by AIMFC are comparable to the alloys produced by VIM.     

Effect of cerium addition on microstructure and mechanical properties of Al-Zn-Mg-Cu alloy

Effect of cerium addition on the microstructure and mechanical properties of an Al-Zn-Mg-Cu alloy have been investigated. In this study, aluminum alloys with up to 0.4% cerium content have been prepared by melting, metal mould casting followed by thermo-mechanical processing. The alloys were extensively characterized by optical and transmission electron microscopy, followed by mechanical property examination by tensile tests as well as nanoindentation tests. It was observed that cerium addition results in up to 5% grain refinement of the cast dendritic structure as well as up to 38% refinement of the heat treated microstructure. Transmission electron microscopy (TEM) has revealed the uniform distribution of fine GP zones and some semi coherent β(MgZn₂) precipitates in the Al rich matrix. Further TEM results show that when the Ce content was changed from 0.1% to 0.4%, precipitate size increased from 5 to 50nm and the precipitate morphology changed from spherical to needle shape. Evaluation of mechanical properties through tensile and nano-indentation tests have exhibited that both Young’s modulus and tensile strength increases with Ce addition up to 0.3% and subsequently decrease.     

Microstructure Formation in AlSi7Mg Alloys Directionally Solidified in a Rotating Magnetic Field

To produce high stressed automotive components like engine frames and cylinder heads in foundry industry often AlSi7Mg alloys are used. During mould filling and casting melt flow affects the development of the microstructure, which defines the mechanical properties. In this paper the microstructure formation in AlSi7Mg0.3 and AlSi7Mg0.6 alloys during directional solidification is investigated. To induce a forced melt flow a rotating magnetic field is applied. For that purpose a Bridgman‐type gradient furnace is equipped with a rotary ring magnet. For detailed investigation of the shape of the solid‐liquid interface and the primary dendrite spacing a decanting device is used. As a result, the forced melt flow substantially changes the dendritic solidification microstructure. The rotating magnetic field generates a radial secondary flow in and ahead of the mushy zone, which causes an enrichment of eutectics in the centre of the samples. At lower solidification velocities this locally leads to the transition to mixed columnar‐equiaxed or even to equiaxed growth. In that case the solid‐liquid interfaces of the decanted samples show a significant depression in the centre part. In the out‐of‐centre region columnar growth still exists and the primary dendrite spacing decreases with increasing melt flow.     

Microstructural investigations in the semi‐solid state of the steel X210CrW12

Thixoforming is an emerging young technology to produce complex structural parts and near net shape components. Thixoforming stands for the forming of materials in the semi‐solid state. One precondition for the thixoformability of materials is the minimum temperature range for the solidus‐liquidus interval and the globulitic formation of the solid phase during the thixoforming process. Besides this other parameters like shape factor, contiguity, matrix character, melting interval, and phase distribution are important process parameters. Aluminium and magnesium alloys are the objectives of numerous investigations, but research activities concerning the thixoformability of steel alloys have been commenced recently. This article provides metallographic information on the relevant parameters of the steel X210CrW12, taking into account the microstructural evolution and the establishment of a parameter field for forming this material in the semi‐solid state.     

Development of the as-cast microstructure in magnesium–aluminium alloys

This paper presents an overview of several projects undertaken at CAST to increase our understanding of the solidification characteristics of Mg–Al alloys. With the increased use of magnesium alloys, and with casting dominating as a production route, there is a need for a more comprehensive understanding of the mechanisms of solidification and defect formation to allow further optimisation of alloys and casting processes. The paper starts with considering the formation of the primary magnesium dendrites and the means for grain refinement of magnesium–aluminium alloys. The Mg–Al system is then shown to display a range of eutectic morphologies for increasing aluminium content, ranging from a divorced structure, through several intermediate structures, to a fully lamellar structure at the eutectic composition. The eutectic also influences discontinuous precipitation which occurs in the aluminium-rich regions of the magnesium phase. The paper concludes with a section on porosity formation as a function of aluminium content and an outline of the mechanism responsible for the formation of banded defects in magnesium alloys, particularly in products made in pressure assisted casting processes.     

Thixo‐Forging and Thixo‐Joining of an Integrated Product

In an effort to investigate thixo‐joining of aluminium alloy AISi7Mg with bolts of different metals, a series of experiments was carried out. An aluminium master part of high geometric complexity was thixo‐forged. Then thixo‐joining experiments were conducted. While the thixo‐forging of the aluminium master part being completed, bolts of brass, copper, plain carbon steel and stainless steel were integrated into the thixo‐forged part in one step. Evaluations of the produced parts were performed with X‐ray inspection, microstructure and segregation analyses. The experimental results confirm that the aluminium alloy A356 has good formability in semi‐solid state and that the thixo‐joining of the metal bolts with the aluminium part in one step is feasible.     

Developments in Solidification Processing of Functionally Graded Aluminium Alloys and Composites by Centrifugal Casting Technique

Centrifugal casting is one of the potential solidification processing techniques used for producing near-net shaped symmetrical cast components with improved properties. The emergence of new class of functionally graded materials has propelled this technique for the fabrication of engineering components and structures with graded property. The specific properties obtained by the use of functionally graded metal matrix composites (FGMMC) are high temperature surface wear resistance, surface friction and thermal properties, adjustable thermal mismatching, reduced interfacial stresses, increased adhesion at metal?Cceramic interface, minimized thermal stresses and increased fracture toughness and crack retardation. Among various processing techniques available for the fabrication of FGMMC, centrifugal casting has emerged as the simplest and cost effective technique for producing large size engineering components of FGMMC. The present paper gives an overview on the developments in use of centrifugal processing technique for processing various functionally graded aluminium alloys and composites. The influence of various process and solidification parameters on microstructure and properties of graded alloys and composites are described.