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.     

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.     

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).     

Combinatorial Alloy Design by Laser Additive Manufacturing

The authors uses laser additive manufacturing (LAM) as a combinatorial method for synthesizing microstructurally and compositionally piecewise graded bulk alloys. Authors fabricate blocks consisting of a sequence of ≈500 μm thick tool steel layers, each with different chemical composition, by laser metal deposition where alloy powders are deposited layer‐wise on a substrate. The reference materials are a Cr–Mo–V hot working tool steel and a Ni‐based maraging steel. The layers between them consist of corresponding blends of the two materials with varying composition from layer to layer (alloy volume fractions 80:20, 60:40, 40:60, and 20:80). The bulk alloy is hot rolled and heat treated. Subsequently each layer is characterized for microstructure, chemical composition and mechanical properties using electron back scatter diffraction, tensile testing, and indentation. The approach is an efficient high‐throughput method enabling rapid probing of novel compositional alloy blends. It can be applied for finding new alloys both, by LAM and for LAM. For the tool steel blends synthesized here, authors observe that the Cr–Mo–V tool steel, when mixed with the Ni‐base maraging steel, can be continuously tuned for a strength‐ductility profile in the range of 800–1650 MPa strength and 15–25% tensile elongation.     

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.     

Response to thermal cycling of tool materials under steel thixoforming conditions

Abstract

The principal failure mechanism of steel thixoforming dies is thermal fatigue owing to forging pressures much lower than those encountered in conventional forging. This makes a properly designed thermal fatigue test the best method to identify suitable tooling materials for the steel thixoforming environment. Samples of X32CrMoV33 hot work tool steel and CrNiCo alloy were cycled thermally between 450 and 750°C, every 60 s for a total of 1500 cycles. While the thermal stresses generated at the surfaces of the two materials were very similar, their responses to thermal cycling were markedly different. The X32CrMoV33 steel was softened by nearly 40% after only 400 cycles, raising serious concerns over its temper resistance under steel thixoforming conditions. The extensive oxidation and subsequent spalling of oxide scales suffered by the X32CrMoV33 hot work tool steel is also a major shortcoming. The performance of the CrNiCo alloy, on the other hand, was judged to be satisfactory with a much thinner heat affected zone and a much better oxidation resistance. Lack of evidence for heat checking in this alloy after 1500 cycles is an encouraging sign.     

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.     

Production of Tailored High Strength Hybrid Sandwich Structures

This project deals with the manufacturing and subsequent treatment of partially strengthened, three layered, symmetrical sandwich structures. Those sandwich materials have the advantages to combine the properties of the used mono‐materials. Additionally, they have a good damping behaviour because of the soft core and good stiffness and a good corrosion resistance if stainless steel sheets are used as cover. The characteristics of such structures are even more improved by the specific local influence of partial reinforcement within the sandwich material. With this type of strengthening, complex material properties can be developed, e.g. high strength with high stiffness, good thermal joining properties at the place of strengthening, as well as an improved strength and strength absorbing ability. The forming behaviour of such composite materials is still unknown. In this research project different sandwich materials ‐ with and without reinforced elements ‐ are analysed in bending tests and in deep‐drawing investigations.     

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.     

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.     

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.     

Thermal Fatigue Testing of Plasma Transfer Arc Stellite Coatings on Hot Work Tool Steels under Steel Thixoforming Conditions

The thermal fatigue performance of Stellite 12 coating deposited on X32CrMoV33 hot work tool steel via the plasma transfer arc (PTA) process was investigated under steel thixoforming conditions. Stellite 12 coating has made a favorable impact on the thermal fatigue performance of the X32CrMoV33 hot work tool steel. The latter survived steel thixoforming conditions lasting much longer, for a total of 5000 cycles, when coated with a PTA Stellite 12 layer. This marked improvement is attributed to the higher resistance to oxidation and to temper softening of the Stellite 12 alloy. The Cr-rich oxides, which form during thermal cycling, provide adequate protection to high-temperature oxidation. In contrast to hot work tool steel, Stellite 12 alloy enjoys hardening upon thermal exposure under steel thixoforming conditions. This increase in the strength of the coating is produced by the formation of carbides and contributes to the superior thermal fatigue resistance of the Stellite 12 alloy. When the crack finally initiates, it propagates via the fracture of hard interdendritic carbides. The transformation of M₇C₃ to M₂₃C₆, which is more voluminous than M₇C₃, promotes crack propagation.     

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.     

An Assessment of Vacuum‐Heat‐Treated H11 Hot‐Work Tool Steel using the KIc/HRc Ratio

Charpy V‐notch (CVN) impact‐test values are widely used in toughness specifications for AISI H11 hot‐work tool steel, even though the fracturing energy is not directly related to the tool design. K_Ic, the plain‐strain stress‐intensity factor at the onset of unstable crack growth, can be related to the tool design; however, K_Ic test values are not widely used in toughness specifications. This is surprising since to the designer K_Ic values are more useful than CVN values because the design calculations for tools and dies of high‐strength steels should take into account the strength and the toughness of materials in order to prevent the possibility of rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture‐toughness testing (ASTM E399‐90), which is difficult to perform reliably for hard materials with a low ductility, could be replaced with a so far non‐standard testing method. A particular problem is that the manufacture of the fatigue crack samples is difficult and expensive, and this has promoted the search for alternative fracture‐toughness testing methods. One of the most promising methods is the use of circumferentially notched and fatigue‐precracked tensile specimens. With this technique the fatigue crack in the specimen is obtained without affecting the fracture toughness of the steel, if it is obtained in soft annealed steel, i.e., prior to the final heat treatment. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell‐C hardness ‐ Fracture toughness K_Ic ‐ Tempering temperature) for some AISI H11 hot‐work tool steel delivered from three steel plants. On the basis of the combined tempering charts the influence of the processing route on the mechanical properties was investigated. In the same way, vacuum‐heat‐treated tool steels were assessed and their properties expressed as a ratio of the fracture toughness to the hardness (K_Ic/HRc).     

Service Life Predictions for Hot Bulk Forming Tools

Hot bulk forming tools are subject to high thermal and mechanical alternating loads which can induce the formation of fatigue cracks in the highly stressed regions of the tool. It this way, premature tool failure occurs with which increased tool costs are associated. It is therefore vitally important to calculate the tool life output during the process design to improve the efficiency. Thermomechanical fatigue tests using the hot‐working tool steel X38CrMoV5‐3 are carried out as the basis for service life predictions in order to characterise the material behaviour subject to cyclic loading. In the tests, the thermal and mechanical loads operating in the tool steel during a forging process are reproduced. In this way, a strain controlled S‐N curve is determined for a specific temperature interval by varying the applied mechanical load. Thus it is possible to consider the damage mechanisms in the material, which operate during the forming process, for computing the service life. Based on the experimentally determined strain controlled S‐N curve, the computation of a fatigue failure is carried out for a practical example with tool fracture. By comparing the material's experimentally determined load carrying capacity with the loading computed by employing the elastic‐plastic material behaviour, the number of forging cycles is ascertained up to incipient cracking. The simulation model introduced here permits an improved prediction of the fatigue crack formation by integrating the cyclic material behaviour subject to similar conditions found in the forging process.     

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.     

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.     

Effects of Hydro‐Impulse Forming of Blanks

This work presents the outcomes of investigations on dynamic effects and parameters of Hydro‐Impulse Forming (HIF) and their influence on the shaping process. Parameters for the blanks exposed to hydro‐impulse forming are defined by FE‐simulations, which use AUTODYN 2D‐3D software. These simulations enable a clear visualization of the processes that occur in the material and the working media. Results are presented for the simulated deep drawing of a semi‐sphere for the aluminium alloy A2024 and the steel QStE340. A larger influence of dynamic forces on the forming process was detected by numerical simulations on the “transmission medium ‐ die ‐ blank” scheme. These outcomes were confirmed by experimental investigations and a classification of the typical malformations during HIF (unusual in conventional forming processes). As shown by the performed experiments and numerical simulations, HIF offers great advantages in comparison with common forming processes, e.g. with regard to the creation of deeper reliefs and low residual springing.     

Assessment of the Primary Structure of Slabs and the Influence on Hot‐ and Cold‐Rolled Strip Structure

The quality of the delivered strips depends ‐ among other things ‐ on the internal structure of the slabs. Solidification phenomena as micro‐ and macro‐segregation are of particular importance. This paper deals with the micro‐ and macro‐segregations in slabs of the steel grades DP and S355. The method for the investigation of segregations based on electron probe microanalysis is described in brief. Characteristic values of segregations are derived from quantitative element distribution images. These results serve as a basis for decisions on further processing steps in the hot and cold rolling mill.     

High-Temperature Sliding Wear Testing of Cathodic Arc Physical Vapor Deposition AlTiN- and AlTiON-Coated Hot Work Tool Steels

Thin hard coatings provide the much needed protection for steel thixoforming tools that must resist wear at high temperatures. The wear resistance of AlTiN- and AlTiON-coated hot work tool steel was investigated at 1023 K (750 °C), measured to be the cavity surface temperature shortly after the steel slurry was forced into the thixoforming die. The wear tests were repeated in exactly the same fashion with uncoated tool steel samples to identify the impact of AlTiN and AlTiON coatings on the high-temperature wear performance of X32CrMoV33 tool steel. The nature, the thickness, and the adherence of the oxide scales impact the tribological behavior. The poor adherence and limited ductility of ferrous oxides promote the failure of the oxide scale impairing the resistance to wear of the hot work tool steel at elevated temperatures. The substantial softening in the X32CrMoV33 hot work tool steel is also critical in the wear volume loss it suffers. AlTiN and AlTiON coatings, on the other hand, form a stable and protective oxide surface layer at high temperatures and therefore provide an enhanced resistance to oxidation. The latter is relatively more resistant to oxidation and is thus the better of the two coatings tested in the present work.