Solid State Phenomena Vols. 141-143

Abstract: Semi-solid 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. This innovative forming technology, however, still remains as a major challenge in the case of steels. One major limitation in the thixoforming of steels has been the lack of suitable tool materials which could withstand the complex load profile and significantly increased forming temperatures which promote chemical interaction between forming dies and liquid steel. For industrial application to be attractive, the lifetime of the thixoforming tools has to be sufficient to ensure the cost effectiveness of this technology. Forming experiments using conventional hot working tool steel as die material have shown massive wear and plastic deformation. Several alternatives including copper alloys, titanium-zirconium-molybdenum based high temperature application alloy TZM, graphite and ceramics with and without suitable ceramic coatings have been tested with limited success. An attempt was made in the present work to investigate the applicability of CrNiCo-based superalloy dies in small scale forming tests in an effort to contribute to the development of semi-solid processing of steels.

Abstract: Rheo-casting of 4045 aluminum alloy bar was operated using an unequal diameter twin roll caster and a cooling slope. The semisolid slurry was made by the cooling slope. The thickness of the bar was 12 mm and width was 100mm. The casting speed was 1m/min. Quantity of the melt poured on the cooling slope was important to make sound bar. The primary crystal became spherical, and its size was about 40μm.

Abstract: A new aluminum alloy (AlSi5Mg0.5Cu0.3Ag) for semisolid die-casting applications was designed, starting from computational thermodynamics calculations by Computherm Database. The goal was to obtain a combination of good castability and proper concentration of hardening elements for strengthening precipitation treatment. The predicted thixotropic properties were verified by measuring the microstructural conventional parameters, such as globule size and shape factor, and the solidification range, by means of differential scanning calorimetry. To complete the characterization of this new alloy and to evaluate its applicability in industrial production, the shear rate and time-dependent flow behavior of the alloy in the semisolid state was studied in a Searle-type rheometer. A future aim of the present research is to try to use rheology testing as the tool to optimize the chemical composition, in order to design an alloy characterized by good mechanical performances and easy processability. Considering the strong influence of the solid fraction content on semisolid alloy viscosity, the rheology tests were interrupted after a certain time and the alloy was deeply freezed using vaporized liquid nitrogen, in order to fix the microstructure and verify the correctness of the thermodynamic simulation.

Abstract: When analysing semi-solid metal alloys or any other suspension in rheometers a major problem is related to wall slip effects which lead to an underestimation of the suspensions viscosity. Using smooth rotating rods in rotational rheometry for analysing suspensions a thin particle free layer of liquid is formed at the surface of the rod due to near-wall segregation. Thus, mainly the viscosity of this layer is measured which is much lower than the one of the bulk phase. Kiljanski [1] proposed a method for rotational rheometers to recalculate such measurements to get viscosity values without slip effects. A device-related change to minimize wall slip is the use of rotating rods with grooved surfaces. Therewith near-wall mixing is produced to destroy the particle free layer. Alternatively, the use of multiple blade vanes is proposed in the literature to avoid wall slip. The differences of the two measuring devices (grooved, vane) are discussed at the example of measurements for a low melting tin-lead alloy in the semi-solid state. It is figured out whether a vane geometry is also suitable to be applied for rheological measurements on metallic suspensions.

Abstract: When SSM material is subjected to a sudden transient the rate of buildup (aging) is negligible compared to the rate of breakdown (shear rejuvenation). While this is generally true, due to prolonged processing or the geometry the local shear rate in some regions may become equal or lower than the critical value, where aging becomes as important as shear rejuvenation. In this work we simulate in detail shear rejuvenation and aging in semisolid slurries. Using a standard thixotropic model used widely for modeling SSM suspensions but a novel computational method we reveal and confirm numerically for the first time shear banding. The phenomenon is found to be time dependent where the material first yields fully and then, after a certain time the yielded front retreats to form two distinct bands -one yielded and one unyielded. This phenomenon must be accounted for in the evaluation of the material constants since the time scale of the process is similar to the time scale of the phenomenon.

Abstract: Thixotropy is essential to semi-solid processing, and because of it the semi-solid material is characterized by ‘shear shinning’. Here, thixotropic strength and thixotropic criteria in semi-solid processing are put forward based on related theories and experiments, and thixotropic mechanism and its influencing factors are also investigated. The results are as follows: 1) the term of thixotropic strength means that with constant shear rate at semi-solid temperature, the semi-solid body begins to flow when the shear stress reach a certain value. This value of shear stress is defined as the thixotropic strength; 2) Thixotropic behavior happens with ‘shear thinning’ because of the deagglomeration of solid particles, while ‘shear thickening’ happens because of the agglomeration at the same time. With increasing shear time, the shear stress increases first and then decreases rapidly to reach a stable value. 3) There are three important factors that influence ‘thixotropic strength’: temperature (hence solid content), initial microstructure (including size, shape factor and uniformity of solid particles) and shear rate.

Abstract: The main goal of this work is the analysis of rheological properties of steel alloy at high temperatures, just below the solidus point, and in the semi-solid state with low liquid phase content. Data obtained from the analysis can form the basis of numerical simulation for designing and optimizing the thixoforming processes. The rheological properties should be known over a wide temperature range so that the simulations could also predict defects such as incomplete die filling. The analysis concerned M2 tool steel alloy. The paper also discusses development of globular microstructure in partially melted steel.

Abstract: The direct analysis and the inverse parameter fitting are methods for determination of unknown material parameters in rheometry. In this paper the efficiency of the both methods is examined on the case of a semi-solid alloy. The difference between the obtained material parameters is studied and the necessity of inverse parameter fitting is discussed.

Abstract: This paper studies the conditions under which strain localisation occurs in partially solid alloys and compares localisation in rheology experiments with features in the industrial processes of Thixomolding® and high pressure die casting (HPDC). To study the fundamentals of localisation, vane rheometry, modified to measure volumetric changes, is used to shear magnesium alloy AZ91 during solidification. Deformation is found to readily localise when the initial microstructure consists of an assembly of crystals in contact. It is shown that such microstructures expand as they are sheared due to Reynolds’ dilatancy, and that localisation takes the form of dilatant shear bands. A study of microstructural features in industrial castings demonstrates that similar dilatant shear bands can form during Thixomoulding® and HPDC.

Abstract: The “Rheo-Characterizer” is an apparatus designed to assess the quality control of the semi-solid material. The working principle is based on the recording and subsequent analysis of the force required to transversally cut the semi-solid slurry at constant speed. Semi-solid slurries produced under different conditions while using the SEED process were analyzed with the “Rheo- Characterizer”. The TiB2 grain refiner was also added to evaluate the impact on the microstructure and the cutting force. The microstructural features were measured and the grain size was analyzed for the different processing conditions. The effect of the cutting temperature on the resulting curve was also investigated. The results show that the “Rheo-Characterizer” is capable of differentiating between the varied microstructural morphologies and the solid fraction of the billet.