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.     

Hot Forging of C45 using PVD (Ti,Al)N/γ‐Al2O3 Coated Dies

The process of hot forging with permanent moulds is a challenge in respect to the very high thermal, mechanical and tribological loads on tools. Ensuring sufficient lifetime application of protective films can be beneficial. Initial screening experiments using PVD coated compression plates show that one of the metastable phases of alumina, the γ‐phase, exhibits high strength and toughness and fulfils the requirements for a protective coating. The next important step in the development towards an industrial application is the implementation on complex tool shapes and verification in real forming experiments. After coating deposition using an industrial coating unit, coated dies were tested in forming experiments under industrial conditions. The forming experiments show an improvement of the wear resistance after 1000 forming cycles for the coated dies compared to the uncoated dies.     

Thermal Fatigue Testing of Tool Materials for Thixoforging of Steels

Semisolid processing, already a well established manufacturing route for the production of intricate, thin‐walled aluminium and magnesium parts with mechanical properties as good as forged grades, faces a major challenge in the case of steels. The tool materials must withstand complex load profiles and relatively higher forming temperatures for thousands of forming cycles for industrial application to be attractive. Since the forming pressures are much lower than those encountered in conventional forging, the principle die failure mechanism in steel thixoforging is expected to be thermal fatigue. Hence, suitable materials able to withstand the steel thixoforming environment for an economically acceptable life, can be best identified with a thermal fatigue test. Such a test is described in the present work. A novel CrNiCo and a nickel‐base superalloy, reported to exhibit superior thermal fatigue resistance in demanding tooling applications, was tested under thermal fatigue conditions encountered in the thixoforming of steels.     

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.     

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.     

Model Tests and Analysis of Corrosion Resistance of Ceramic Tool Dies for Steel Thixoforming

Contact corrosion and cyclic melt corrosion test results of silicon nitride (Si₃N₄) ceramics in contact with semi‐solid and molten steel alloys are analysed in view of steel thixoforming applications. Corrosion effects are identified and model tests are evaluated by comparison of corrosive attack on steel thixoforming dies in application‐relevant forming experiments. The performed corrosion experiments show that a reaction zone of up to 20 μm thickness forms on the ceramic in model tests and forming experiments, respectively. Si₃N₄ ceramics reveal sufficient corrosion resistance in small‐scale forming series. Results show good agreement of contact corrosion tests with thixoforming experiments. Comparability of cyclic melt corrosion tests with thixoforming applications is limited by the casting powder used to prevent oxidation of steel melt.     

Thixoforging of Wrought Aluminum Thin Plates with Microchannels

A direct die-filling thixoforging method is designed to fabricate aluminum thin plates with a pattern of microchannels in a single forming operation. Extruded AA2024 and AA7075 wrought aluminum billets are used. A recrystallization and partial remelting process is used to prepare the semisolid slurries required for the forming process. Under a thixoforging pressure of 70 MPa, AA7075 thin plates are successfully thixoforged in a temperature range of 883 K to 893 K (610 °C to 620 °C), corresponding to liquid fractions of ~30 to 50 pct in the semisolid slurry. AA2024 thin plate requires a thixoforging temperature range of 888 K to 898 K (615 °C to 625 °C), corresponding to the liquid fractions of ~45 to 60 pct. Final microstructures of the thin plates comprise primary α-Al equiaxed globular grains in a matrix of a solidified liquid phase. With increasing thixoforging temperature, the yield strength values continuously decrease. The ultimate tensile strength (UTS) values of the thin plates initially decrease with increasing thixoforging temperature from 883 K to 888 K (610 °C to 615 °C) and from 888 K to 893 K (615 °C to 620 °C) for the AA7075 and AA2024 thin plates, respectively. The UTS values stabilize and slightly enhance when the thixoforging temperatures are further increased to 893 K and 898 K (620 °C and 625 °C) for the AA7075 and AA2024 thin plates, respectively. Very brittle behavior (elongation value of ~1 pct) is observed for the AA7075 thin plates thixoforged at 883 K and 888 K (610 °C and 615 °C). The elongation value increases to 3 pct with increasing the thixoforging temperature to 893 K (620 °C). In contrast, larger elongation values (between 4 and 6 pct) are achieved for the AA2024 thin plates. Increasing the thixoforging pressures from 70 to 100 MPa and then to 150 MPa improves the tensile properties of the thin plates. The tensile properties of the thixoforged thin plates are linked to their microstructural characteristics and processing conditions and are discussed here in detail.     

Forming porous fibrous materials by the pendant drop melt extraction

A method for forming porous material based on metallic fibers, which are obtained by pendant drop melt extraction (PDME) method, is proposed. An installation for forming porous fibrous materials by the PDME method is developed and fabricated. Samples of porous materials made of fibers of heat-resistant and corrosion-resistant steels, nickel alloys, zirconium, etc., are obtained. Mechanisms for forming the bonds between the fibers are investigated and an interaction model between the fibers in a formed material is proposed. The possibility of forming bonds between the fibers in a porous material based on copper and steel fibers at various extraction velocities is evaluated according to the proposed model.     

Manufacturing of Reinforced High Precision Forging Dies

The economical production of near‐net‐shape forging parts with the highest accuracy requires high precision dies with the lowest wear. Compared to metals, ceramics show an outstanding high wear and corrosion resistance accompanied by high thermal durability, which favours their application to hot forging. However, ceramics can not endure tensile stresses. In order to enhance die life, small ceramic inserts can be integrated into the steel die body by brazing, placed within the wear critical areas. In addition to this, alternative methods from the field of surface coating are introduced for reinforcing dies. On the other hand, thin Ti‐containing deposits from the gaseous phase, PVD‐ or PACVD, can increase the wear resistance when applied as a multilayer structure. An added top layer of TiBN or TiB₂ finishes the described batches of TiN‐TiCN‐TiC layers. Results are presented incorporating the expertise of material science, metal forming and production engineering. This includes the development of reinforced forging dies, their design and manufacture by active metal brazing of ceramic inlays or coating processes as well as investigations regarding the grinding of steel‐ceramic composite dies. The latter include parameter studies as well as grinding wheel development by performing thorough investigations of the wear mechanisms which lead to adapted and material‐specific grinding strategies.     

西北有色金属研究院研制的部分钛合金及产业化

西北有色金属研究院是我国重要的钛合金研究与开发的单位之一，近40年来，一直致力于钛合金的研究、开发及产业化，仿制与创新研制出近60种钛合金，一些仿制的钛合金已大批量生产，获得成功的应用，如纯Ti、TC4、半TC4、Ti．Pd、Ti．Mo．Ni、BT20、BT16、HT7M等等；一些创新研制的钛合金也已获得成功的应用和批量生产，如TC21、Ti75、Ti31、CT20、Ti12LC等等，为我国钛合金的发展做出了突出贡献。本文简要综述了西北有色金属研究院仿制与创新研制的部分钛合金及其产业化。     

Prospects of the application of barium-bearing master alloys for the deoxidation and modification of a railroad metal

The prospects of using barium-bearing master alloys for the deoxidation and modification of wheel steel are considered. The oxygen solubility is thermodynamically calculated in carbon-containing wheel steel melts for various deoxidation versions, and the results of laboratory and industrial experiments on the deoxidation and modification of railroad wheel steel are discussed. Industrial experiments performed at OAO Vyksunsk Metallurgical Works show that the use of barium-bearing master alloys for the deoxidation and modification of wheel steel under mass production conditions leads to a decrease in the nonmetallic inclusion contamination of a metal, a decrease in the rejection of solid-rolled railroad wheels, grain refinement in a metal, and an increase in its mechanical properties.     

Ni-based superalloy as a potential tool material for thixoforming of steels

Abstract

Semisolid processing, already a well established manufacturing route for the production of intricate, thin walled aluminium and magnesium parts with mechanical properties as good as forged grades, faces a major challenge in the case of steels. The tool materials must withstand complex load profiles and relatively higher forming temperatures for thousands of forming cycles for this near-net shape process to be attractive for steels on an industrial scale. The potential of a Ni-based superalloy, Inconel 617, reported to exhibit superior thermal fatigue resistance in demanding tooling applications, was investigated. The response to thermal cycling of this alloy at high temperatures was compared with that of X38CrMoV5 hot work tool steel widely used in the manufacture of conventional forging dies. The favourable thermophysical properties of the latter were completely negated by its limited temper resistance, while the Inconel 617 alloy responded to thermal cycling by the usual heat cracking mechanism.     

Influence of Alloying and Heat Treatment on the Abrasive and Impact–Abrasive Wear Resistance of High-Manganese Steel

The basic factors that affect the wear resistance of high-manganese steel are considered. The literature on this topic is reviewed. Conclusions are formulated regarding the materials used in existing studies. Research topics of interest to enterprises that manufacture and employ Hadfield steel are identified. Materials used in the machining of liquid steel are considered. Production technology for experimental high-manganese steel parts is discussed. The composition of the alloy employed as the base is analyzed. The procedure and equipment used to determine the cooling rate of alloys in the mold and to study the wear resistance in conditions of abrasive and impact–abrasive wear are outlined, as well as methods of thermal analysis. Results are presented for the alloying of Hadfield steel by nitrided ferroalloys and other alloys. The coefficients of abrasive and impact–abrasive wear resistance are plotted for different alloying conditions. In addition, the influence of the alloying elements on the wear resistance of high-manganese steel in different wear conditions is studied. The concentrations of the alloying elements corresponding to maximum abrasive and impact–abrasive wear resistance are established. In addition, the results of thermal analysis are presented. The heating of Hadfield steel castings prior to quenching is considered. The temperature ranges corresponding to processes such as excess-phase deposition, the solution of cementite in austenite, and complete solution of phosphide eutectic and metal carbides are established. The temperature limits of oxidation and decarburization of the steel are also determined. On the basis of the results, recommendations are made with a view to increasing the wear resistance of castings made from high-manganese steel for different operating conditions and also to selecting the heat-treatment temperature for such castings.     

Non‐Metallic Inclusions in High‐Manganese‐Alloy Steels

The possibility of applying new high‐strength steels with excellent forming behaviour (TRIP, TWIP and LIP steels) in automotive manufacturing is a significant potential for improvement in the area of reducing weight while simultaneously increasing crash safety. The present work investigates endogenous inclusions in some high‐alloy TRIP and TWIP steels because the most stringent product requirements are tightly related to cleanness. The expected formation of inclusions is discussed based on thermodynamic observations made with ThermoCalc. The solidification conditions were varied in experiments with the so‐called SSCT (submerged split chill tensile) apparatus. Furthermore, different treatment times were set in order to investigate this influence on the inclusions. A catalogue of endogenous inclusions in these new steel grades is currently being created with the help of the automated SEM/EDX inclusion analysis system at voestalpine Stahl GmbH in Linz. Further studies will follow to systematically determine the interactions between steel, slag and refractory materials.     

A small-angle x-ray scattering investigation of the zone formation of “maraging” type alloys

The small angle X-ray scattering from “Maraging” type alloys previously aged at temperatures below 450°C has been studied at room temperature. The behavior, during aging, of two high purity alloys containing Fe Ni Co and Fe Ni Co Mo is compared with an industrial Vascomax 300 type steel. The existence of G.P. zones is pointed out for the two Mo containing alloys. In the case of the quaternary alloy, the evolution of these zones during the aging is clearly defined. For the industrial steel, the interpretation of the observed phenomena is more complex due to the additional small amounts of Ti, Cu, Al and is discussed.     

Physicochemical characteristics,production and application of boron-bearing complex ferroalloys

The expediency of producing and using complex ferroalloys in steelmaking is analyzed in terms the manufacturing technology, the raw materials employed, and the interactions of the ferroalloys with the molten steel. The need to produce complex ferroalloys with boron is established. The fundamental principles for determining the best composition of such alloys are presented. The basic compositions of complex ferroalloys with boron (ferrosilicomanganese with boron, ferrosilicon with boron, ferrosilicomanganese with boron and chromium) are established by studying the physicochemical properties of alloys and their interactions with the steel melt. If the characteristics (melting point, density, melting time of the ferroalloy in liquid steel, etc.) of complex ferroalloys with boron are compared with those of ferroboron, which is widely used, the complex alloys have clear benefits. The composition of the complex ferroalloys with boron includes active elements (Si, Al, Ti) facilitating the binding of oxygen and nitrogen from the steel melt in strong compounds and hence preventing their reaction with boron. The recommended boron content in the ferroalloy is 0.7–2%. That permits increase in the quantity of complex ferroalloys with boron in the steel and hence increase in the reliability and stability of boron assimilation. At elevated temperatures (1430–1570°C), the oxidation of ferrosilicoboron is 4–7 times less than that of ferroboron. Data are presented regarding the industrial production and use of ferrosilicoboron in the steel-smelting shop. The boron assimilation from complex alloys in microalloying of the steel is studied. The use of ferrosilicoboron does not require significant changes in the existing system for reduction by ferrosilicon; the boron assimilation is 77.8–96.3% (mean 86.6%). With a boron concentration of 0.0021–0.0027% in the steel during ladle treatment, its content in the cast metal will be no less than 0.0020%. If boron is introduced in steel by means of ferrosilicomanganese with boron, the boron assimilation is increased by a factor of 1.6 (from 48 to 77%, on average) in comparison with the use of ferroboron.     

Research on the integral hydrobulge forming technology of spheroidal shells

In the present paper an altemative novel process of manufacturing oblate spheroidal vessels is proposed: the integral hydro-bulge forming (IHBF) technology of oblate spheroidal shells. A few mild steel and stainless steel oblate spheroidal shells for industrial use were manufactured using this new technology with one of the major diameters as high as 3 m. There is a critical value of the ratio between the vertical diameter and the horizontal diameter of the shell before forming, which can determine whether the shell will be wrinkled during hydro-bulging. The processes were analysed afterwards, using the explicit finite element code LS-DYNA3D. The numerical results are discussed and compared with the practical processes. Based on the numerical results a few proposals for the improvement of the IHBF technology of the spheroidal shells are presented.     

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).