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.     

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.     

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.     

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.     

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.     

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).     

Advances in the Optimization of Thin Strip Cast Austenitic 304 Stainless Steel

This study is about the latest advances in the optimization of the microstructure and properties of thin strip cast austenitic stainless steel (AISI 304, 1.4301). Concerning the processing steps the relevance of different thin strip casting parameters, in‐line forming operations, and heat treatments for optimizing microstructure and properties have been studied. The microstructures obtained from the different processing strategies were analysed with respect to phase and grain structures including the grain boundary character distributions via EBSD microtexture measurements, the evolution of deformation‐induced martensite, the relationship between delta ferrite and martensite formation in austenite, and the texture evolution during in‐line deformation. It is observed that different process parameters lead to markedly different microstructures and profound differences in strip homogeneity. It is demonstrated that the properties of strip cast and in‐line hot rolled austenitic stainless steels are competitive to those obtained by conventional continuous casting and hot rolling. This means that the thin strip casting technique is not only competitive to conventional routes with respect to the properties of the material but also represents the most environmentally friendly, flexible, energy‐saving, and modern industrial technique to produce stainless steel strips.     

Structural Morphologies and Deformaiton Characteristics of Semi‐solid Type 304 Stainless Steel during Solidificaiton and Remelting

Microstructural evolutions of type 304 stainless steel and the related mechanical property of flow stress in semi‐solid state are investigated. The evolutions of microstructure during solidification, partial remelting of a hot‐rolled billet and partial remelting of a cast billet are compared with respect to structural morphologies in the semi‐solid state. Various structural morphologies, such as the linear and multilayered liquid/austenite/δ‐ferrite structure, globular liquid/δ‐ferrite structure and dendrite structure, are characterized using optical micrographs and an EPMA (electron probe microanalyzer). The various structural morphologies in the semi‐solid state are influenced not only by the phase transformation but also by the previous treatment of type 304 steel, such as hot rolling and casting. Furthermore, a series of hot compression tests are conducted for various combinations of deformation rate and deformation temperature in the semi‐solid state, to measure the flow stress and the change in microstructure resulting from plastic deformation. Flow stress, phase segregation, microfracture and distortion of solid particles during and after the hot compression test are strongly affected by the structural morphology in the semi‐solid state, such as the dendrite structure, nonglobular structure and globular structure. Semi‐solid type 304 stainless steel with dendrite structure exhibits the highest flow stress, which is about three times that of steel with globular structure, although the testing temperature and deformation rate are controlled to be the same. This is a result of the higher bonding force between solid particles and lower fluidity of the liquid phase of the dendrite structure than those of the globular structure, which exhibits excellent fluidity of the liquid phase and rotation of solid particles.     

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.     

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%.     

Microstructural Development of HP9/4/30 Steel During Partial Remelting

Semi‐solid processing, also known as thixoforming, is a forming process that shapes metal components in their semi‐solid state. Prior to forming, the microstructure of the alloy consists preferably of solid metal spheroids in a liquid matrix. This paper describes the microstructural development within the semi‐solid zone of a typically banded high performance HP9/4/30 steel through a direct partial remelting process from as‐received and as‐deformed conditions. Partial remelting was carried out at temperatures between 1430 and 1470°C. Liquation occurred initially at the grain boundaries, then also along the segregation bands. With increasing time and hold temperature, these “columns” broke down into shorter, more equiaxed segments, offering a better chance of being thixoformed. The microstructures revealed distinct polygonal cells at 1430°C that changed to more rounded solid grains with diminishing sharp edges at 1450°C, followed by smaller truncated cell structures due to the liquation of the bands at 1460°C and 1470°C. The partial remelting procedures carried out in this study are from material that is in a recrystallised state. Thixoforming from this recrystallised state is shown to be successful. This indicates a widening of the range of potential routes to thixoformable microstructures.     

Experimental Testing and Computer Simulations of Ductile Fracture in Tool Steels

Ductility was determined in experimental four‐point bending tests of smooth specimens of tool steel. The tool steels had different contents of carbides and carbide sizes and with a hardness of approximately 60HRc. Two of the materials tested were produced powder metallurgically, one was spray formed and one was conventionally uphill ingot cast. Carbide size distribution analysis was performed on planar polished sections of each material. Correlation between carbide microstructure and ductility performance was obtained. The fracture mechanisms were investigated with fractography. A 3D FE‐model was used to simulate the four‐point bending tests and thereby analyse the matrix flow curve. Also the strain at failure was analysed for each material when simulations were performed based on experimental data. SEM‐images of the materials carbide microstructure were used to create 2D FE‐models. The models simulated crack initiation and propagation by removing elements in the steel matrix as the plastic strain reached a critical level. With three variants, simulations of crack initiation and propagation at carbides were investigated. That was carbides with no cohesion to matrix, carbides fixed to the matrix and carbides with internal cracks. Comparison of strains at failure for the 2D and the 3D FE‐models showed good correlation.     

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.     

Casting and rolling in one heat

In regard of the reduction of energy consumption during the production of steel slabs and strips different charging procedures are of high economic interest. Especially hot and direct charging procedures are used in modern ways of strip production like CSP and strip casting. For simulation of these procedures and their related effects on the microstructure and the mechanical properties of the material, the complete process chain of melting, casting and hot rolling in one heat has been realised in a pilot scale. In this concept process parameters such as temperature over time are controlled during the complete process. By variation of the thermomechanical process parameters the microstructure and the mechanical properties gained in different processing routes can effectively be simulated. This new set‐up allows the simulation of the casting and rolling processes not only in a laboratory scale by using conventional compression, tensile and quench tests but now also in a pilot scale. The results will be used as input parameters for FEM simulation. In this paper the set‐up of the laboratory facility is described with the measured and controlled parameters.     

Investigation of spherisation in microstructures of aluminium casting alloys for thixoforging process

Thixoforging combined with low superheat casting (LSC) is a promising shaping process for aluminium casting alloys. LSC process is based on rapid solidification of an alloy which cast with low pouring temperature. With this method, a feedstock material is produced with non-dendritic microstructure that ready for spherisation in reheating sequence of further semi-solid process. Al-Si alloys are still castable even at low temperatures due to their excellent fluidities. This study subjects to present spherisation of A356 and A380 alloy billets cast with LSC process that provides appropriate beginning material with relatively high sphericity. Obtained billet parts were reheated for different times at a semi-solid state temperature. Some of these billets were directly quenched for observing the effects of reheating and the others were thixoforged. With sufficient reheating time, deformation of thixoforging process did not significantly affect on the spherical microstructure. Unnecessarily long reheating period caused excessive grain growth. A356 alloy had higher spherisation tendency than A380 alloy under similar process conditions.     

Metallurgical Phenomena during Processing of Cold Rolled TRIP Steel

Metallurgical phenomena taking place during processing of TRIP Steel are investigated and described with the aim of achieving better understanding of the microstructure development throughout the entire integrated processing routes. Different TRIP steel structure sizes were created by controlling the hot rolling process prior to cold rolling. After that the specimens were intercritically annealed under different conditions to obtain prescribed austenite fractions, and subsequently quenched in salt bath at the bainite transformation temperature. The microstructures had been investigated using light optical microscopy (LOM) and the amount of retained austenite was determined by magnetometry.     

A CrNiCo Alloy as a Potential Tool Material in Semi‐solid Processing of Steels

Semisolid processing of aluminium and magnesium alloys has matured to become a well established manufacturing route for the production of intricate, thin‐walled parts with mechanical properties as good as forged grades. However, this innovative forming technology faces a major challenge in the case of steels. The tool materials must withstand the complex load profile and relatively higher forming temperatures which promote chemical interaction with steel slurries. Thixoforming tools ought to last thousands of forming cycles for industrial application to be attractive. Hot work tool steel dies proved to be entirely inadequate when thixoforming steels. In spite of extensive research on tool materials for the semisolid processing of steels, there is yet no material to fulfil this critical role. The present work was undertaken to explore the potential of a novel CrNiCo alloy as the tooling material in semisolid processing of steel.     

高钛耐磨钢凝固组织中TiC析出相表征

通过一系列不同厚度的模具浇铸得到不同凝固冷却速率的铸坯,采用腐蚀法结合计算机辅助设计(Creo Parametric)技术重构了高钛耐磨钢中液析TiC在基体中的三维形貌,利用OM和SEM对液析TiC的二维和三维形貌进行对比分析,借助TEM和SAXS研究了凝固冷速对TiC析出的影响.结果表明:液析TiC在空间中的三维形貌...     

钢铁半固态成形技术的研究进展和展望

低熔点合金半固态成形技术被专家称为21世纪新兴的金属制造关键技术。该技术已经成功地在商业领域中运行，人们一直在探索将此技术应用于钢铁材料。本文综述了钢铁半固态成形技术的研究开发过程，介绍了钢铁半固态成形的工艺、浆料的制备方法、半固态金属的微观组织演变以及对这种成形工艺的展望。笔者认为电磁搅拌制备浆料方法中的搅拌是无接触搅拌，更适用于高熔点钢铁材料。由于触变成形工艺本身的特点，使其更适宜于钢铁等高熔点材料的半固态成形控制。     

Homogeneity and Equiaxed Grain Structure of Billets

The homogeneity of as‐cast structures is essential to the application of advanced steel materials. The relationship between as‐cast equiaxed grain structure and homogeneity was examined by means of OPA (Original Position Analysis) technology. The results indicate that macrosegregation of the solute elements was obviously suppressed in silicon steel billets with equiaxed grain structure. Furthermore, two methods to achieve an equiaxed grain structure were described. One is to use titanium‐based inoculation technology. TiN particles and TiN/ Ti₂O₃ complex particles can precipitate at the early stage of solidification and then play an important role of heterogeneous nucleation when the solidification condition is carefully controlled. The other method is to maintain a small temperature gradient in the mould to enhance the constitutional undercooling As a result, the as‐cast equiaxed grain structure was well developed in the experimental billets.