Solid State Phenomena Vols. 116-117

Abstract: The algorithm two-phase flow model, developed in this study, gives die filling patterns, velocity, temperature and solid fraction of rheology material during rheology forging process. To calculate the velocity and temperature fields, the respective governing equations corresponding to the liquid and solid region were adapted. Therefore, respective numerical models considering the solid and liquid phase co-existent within the rheology material have been developed to predict the defects of part manufactured by the rheology forging process. This study has focused on the simulation of the rheology forging process and calculation of the velocity profiles and temperature distribution. And, to predict the liquid segregation in the part, the deviation of velocity between liquid and solid region in the two-phase flow model was analysed.

Abstract: Thixoforming involves shaping metal components in the semi-solid state. Work on the thixoforming of high temperature materials, such as steel, is still at its initial stage; this is mainly due to the high processing temperatures involved and the potential for oxidation. For thixoforming to be possible, it is preferable for an alloy to have an appreciable melting range and before forming the microstructure must ideally consist of solid metal spheroids in a liquid matrix. This paper discusses the thixoforming load versus displacement curves of HP9/4/30 steel semi-solid slugs under compression. The data from the corresponding load-displacement curves is converted into viscosity against shear rate adopting Stefan’s equation for flow between two parallel planes. The viscosity at processing conditions, i.e. at processing temperatures in the range of 1470 to 1480°C and zero to two minutes soak times, showed a rapid decrease initially, which corresponds to a rapid breakdown in the structure, followed by a steady decrease to a near constant value with increasing rate. The work shows pseudoplasticity (or shear-thinning) behaviour of the HP9/4/30 semi-solid slurries. This data would be required for modeling the die fill with these slurries.

Abstract: Thixoforming or Semi-Solid Metal Forming offers many advantages in comparison with casting and conventional forging. The purpose of the present study is to provide the basic microstructure and deformation data for austenitic and ferritic stainless steel under mushy state. As well known, the stainless steels solidify in different modes according to the different chemical compositions. In this paper, microstructural evolution of austenitic stainless steel type 304 which solidifies in FA mode ( L → L +δ → L +δ +γ →δ +γ →γ ),austenitic stainless steel type 310S which solidifies in A mode ( L → L +γ →γ ), and ferritic stainless steel type 430 which solidifies in F mode ( L → L +δ →δ )
are investigated during partial remelting by way of SIMA (Strain Induced Melted Activation). The results show that A and F mode of stainless steels melt directly at the grain boundary without phase transformation during reheating. A banded structure, originating from the primary dendritic segregation of the original ingots, is observed in type 310S steel during further heating. On the other hand, a perfect globular and insegregative two-phase semi-solid structure L +δ can be obtained while heated beyond the banded three-phase L +δ +γ semi-solid state in FA mode austenitic stainless steel type 304. This spheroidization can be attributed to the peritectic reaction occurred in the L +δ +γ semi-solid state. In addition, simple compression tests of these alloys in semi-solid state for varied combination of deformation rate and deformation temperature are conducted to examine the deformation behavior of stainless steel. Flow stress curves exhibit abrupt change in various alloys, even though in the same alloy such as type 304, various flow stresses are observed according to the difference in inner microstructure or morphology. Stress of type 310S steel shows the most reduction as the deformation temperature increasing at the same strain rate condition. The Liquid is centralized to periphery by the compression force in all deformed test pieces. Fracture, observed in all alloys except type 304 steel in globular L +δ semi-solid state, should be resulted from the lack of liquid in L +δ +γ state of type 304 steel and solidification crack in type 310S and type 430 steel. Deformation of solid particles occurs only in L +δ +γ state of type 304 steel. Last in this paper, various deformation mechanisms are proposed for various microstructures.

Abstract: In recent years several attempts were made to transfer the thixoforming technology of steel parts into industrial applicable processes. This paper gives an overview about the progress of a European consortium that established a fully automated thixoforming process for the series production of automotive steel parts. Due to the multi-faceted nature of this technology, problems concerning the development of suitable steel grades and tool materials as well as the development and application of an inductive heating system, a handling unit and of a complex forming tool had to be solved. Besides the development of adapted steel grades and the inductive heating, the handling of the semi solid billets plays a special role because during the manipulation of the parts from the heating station into the tool a loss of heat is unavoidable. Furthermore, scaling of the parts must be prevented. By means of a fully automated process line existing constraints were reduced and the forming process is kept reproducible. Improved silicon nitride composites have been developed as a tool material, which show good mechanical properties in combination with an acceptable chemical stability at the occurring process temperatures as well as in contact with semi solid steel. Basing on the practical experience a comparison of the thixoforming technology to existing processes and an outlook for the future are given.

Abstract: The design and performance of silicon nitride (Si3N4) dies for the semi-solid processing of steels is studied by the example of a punch and a lower swage in upset forging. The observed failure mechanisms and degradation effects are related to short-term effects resulting from mechanical and thermal loads and long-term effects owing to chemical and tribological attack. Results show that well-defined process conditions (cycle time, solidification time in the die) and a ceramic-suitable design are a pre-requisite for reliability and sufficient service life of ceramic dies.

Abstract: A novel ceramic tool concept allows near-isothermal steel thixoextrusion experiments. Thermal shock impacts are successfully eliminated from the load profile of conventional semi-solid processing technologies of high melting point alloys. Thus, the application of thermal shock sensitive oxide ceramics exhibiting excellent corrosion resistance as forming dies is feasible. Extruded steel parts show high shape accuracy at very low extrusion forces.

Abstract: The ESEM (Environmental Scanning Electron Microscope) is an instrument that circumvents a limitation of conventional SEM, in that samples can be examined in a gaseous atmosphere rather than a vacuum. With a heating stage, dynamic processes can be observed in situ at high temperature. In this study, A201 aluminium alloy samples with globular structures have been examined in the semisolid region. In addition, a manipulator has been installed onto the heating stage to allow the probing of semi-solid surfaces. The paper shows the potential for manipulating semi-solid materials in order to better understand thixotropic phenomena.

Abstract: The process of semi solid metal forming with permanent moulds combines the advantages of the process technologies casting and forging. Semi solid metal forming of steel is a challenge with respect to very high thermal, mechanical and tribological stresses of tools. Alumina based ceramics show a very high potential for tool protection. Alumina occurs in different known meta stable phases. First experiments show that the high temperature γ-phase exhibits high strength and toughness. Various methods and technologies have examined γ-phase stabilisation by using different thin film process technologies. By using Physical Vapor Deposition (PVD) it was possible to deposit γ-Alumina in a temperature range, where hot working steel can be utilized. The properties of the by pulsed reactive magnetron sputtering deposited films were analyzed by common thin film techniques revealing hardness, Young´s modulus and coating adhesion. By using the reactive pulsed PVD-process it was possible to stabilize γ-Al2O3 on steel substrates for semi-solid melt protection. The γ-Al2O3 coatings showed an extra ordinary stability for thixoforging of X210CrW12 and 100Cr6 in field tests. The durability of the permanent moulds was increased by using PVD thin film coatings as a tool protection.

Abstract: Some excellent projects have been finished successfully since the last conference on the pre-industrial stage showing the great potential of thixoforming technology for steels [1][2]. Experiments presented in this paper on induction heating, process window and mould filling of three widely-used steel alloys offer an even greater field of application for industrial companies. In the past thin walled cutting tools and complex impellers made of cold working steel X210CrW12 were produced at the Foundry Institute [3][4]. Constitutive on this knowledge the field of applications is enlarged by research on two further steel alloys. Processing of 100Cr6 (roller bearing steel) and 42Cr4 (annealing steel) is challenging due to a decreasing window at a higher temperature level with decreasing carbon content. An exact procedure has been worked out to create new control programs for the induction heating unit. It is based on thermo-chemical calculations and delivers billets with feasible temperature distribution and well defined content of liquid phase. Material characterization is performed in a step-die (seven steps between 25 and 0.5mm), especially developed for demands of semi-solid casting. Mould filling capacity, micro structural evolution and mechanical properties are determined on each of the seven steps. For any reproducible process the knowledge of suitable system parameters is essential. To find the limits of the process window sensitive experimental parameters were changed systematically for each of the three steel alloys. In addition to tool temperature the process windows consist principally of content of liquid phase fs, piston velocity vP and pressure during freezing pfr. Characteristic differences appeared between the different steel grades. The thermal simulation was used to find functional dimensions for the gating system and to shorten process times. Simulation of tools system was used to estimate the additional thermal load induced by higher working temperatures. The abrasive wear at the offsets to the next steps was in the same order of magnitude for all examined steels. These results provide the opportunity for commercial steel grades to cast complex steel parts in high pressure die casting.

Abstract: Thixoforming of steel offers the advantages of casting technology in combination with high mechanical strength that can only be achieved by forging. The progress in establishing this technology in industry depends on the success in the development of suitable steel grades. Recent investigations dealt with the development of steel grades that are especially adapted to the thixoforming process. For this, alloys were developed with a lower solidus temperature and a wider process temperature range compared to classic forging steels. In consequence, the inductive heating process is more tolerant to inaccuracies and for a given liquid fraction the process temperature window is easier to handle. It is desired to obtain great degrees of deformation at rather low forming forces as these parameters determine the size of the needed presses. This behaviour is affected by the present liquid fraction in the slug and the heat transfer between work piece and die. It was detected that variations of the forming force have a direct influence on the quality of the thixoformed parts. In order to make the thixoforming technology of steels competitive versus other forming technologies, the parts must show a favourable microstructure and thus, good in-use properties. In this paper various solutions are compared. The main results obtained in the optimization research, namely, the steel grades adapted to semi-solid forming, the resulting process parameters and the mechanical properties of thixoforming parts will be presented for two exemplary steel grades. By producing a real automotive part, thixoforging of steels with regard to the adapted materials and to the ongoing industrial implementation of this process is proved.