Solid State Phenomena Vols. 141-143

Abstract: Magnesium alloys are increasingly used in automotive, aeronautic and electronic applications to produce high performance, light weight parts. In the thixomolding process the semisolid slurry is injected into a mold at controlled temperature such that the melt has specific flow behavior. This allows the fabrication of near net shape components with controlled microstructure and good mechanical properties. The numerical modeling of such applications presents unusual challenges for both the physical modeling and the solution algorithm. This paper presents 3D solutions of the injection molding of semi-solid AZ91 magnesium alloys. The methodology deals with the shear thinning, temperature dependent viscosity behavior and is able to accurately solve the high velocity flows encountered during semi-solid magnesium molding. The approach is applied to the injection of a tensile bar and the results compared with experimental data. The numerical solutions indicate that the material forms a jet at the exit of the gate and a swirling flow forms as the material advances along the first larger diameter section. The wall regions are filled first, leaving a void inside. This agrees very well with the experimental observation.

Abstract: The correlation between the evaluation of the mechanical and of the fatigue behaviour of the rheocast, T5 and T6 heat treated SSM A356 aluminium alloy with respect to the microstructures of the component has been investigated. The study has been carried out on a suspension arm injected in a rheocasting 800 tons plant in Stampal S.p.A. The new rheocasting is a process that allows obtaining the alloys in a semisolid state directly from the liquid state, by controlled cooling of the molten alloys. The resulting microstructures are very fine, free from defects and homogeneous: these characteristics improve the mechanical properties of the alloys and specially the response to cyclic stress, an important issue for a suspension component. After a preliminary tensile test analysis, axial high frequency fatigue tests have been carried out at room temperature on specimen cut out from the suspension arm to determine the Wöhler curve and the number of cycles to failure. The results of this work allow a comparison of the effects of heat treatment process, T5 or T6, on Semi-Solid components for industrial applications in the automotive field. On the basis of these analysis the correlation between microstructure and mechanical performances can be established.

Abstract: Isothermal semi-solid extrusion experiments using steel grade X210CrW12 as work material were performed on an industrial forging machine. An improved and up-scaled tool design was applied, based on the concept of self-heating ceramic dies tested in previous laboratory-scale studies and allowing for die preheating temperatures of up to 1400°C. Steel rods of complex cross sectional geometry were formed at low extrusion forces. Shape accuracy of as-formed rods is accurate and metallurgical examination yields no evidence of liquid phase separation. With a view on the intended industrial implementation construction of ceramic tools has to be improved in order to avoid rupture of ceramic parts due to thermo-mechanically induced stresses and to benefit from the advantages inherent to ceramics in metal forming.

Abstract: Oxide coatings offer great potential for their use in forming operations in the semi-solid state. Advantages of these types of coatings are high resistance against abrasive wear, high hot hardness and low thermal conductivity. Nevertheless deposition by pulsed Magnetron Sputter Ion Plating-PVD for oxide coatings is quite challenging: deposition rates are low and insulating layers on the target surface can cause arcing. On laboratory scale it was possible to deposit γ-Alumina using PVD in a temperature range, where hot working steel can be utilized. The next important step in the development towards an industrial application for larger forming tools is the upscaling process to larger coating units. In this work the process development of oxide coatings on an industrial coating unit for large tools was described. To increase adhesion of oxide top-layer additional bond coats were applied. Different process parameters like oxygen content, total pressure and substrate bias were varied, to improve the performance. The relationship between coating properties and process parameters of the deposited films were characterized by X-Ray-diffraction, Nanoindentation and Scanning Electron Microscopy (SEM). By using reactive pulsed PVD-process it was possible to deposit γ-Al2O3 on large steel tools for semi-solid melt protection. The developed coatings showed for thixoforging processes of X210CrW12 an extraordinary stability in field tests. The lifetime of the permanent moulds was increased by using PVD thin film coatings as a tool protection.

Abstract: This paper presents an analysis of the effect of CO2 laser processing parameters on the surface modification and heat treatment of steels. The CO2 laser and sample movement process parameters are presented. The controlled operation of these in conjunction with each other is required to obtain better surface hardness and structure. H13 tool steel samples were rotated at high speeds to keep exposure times below 0.3s. Laser processed samples were analysed using EDX spectroscopy, optical microscopy, Vickers and Martens micro-hardness testing, and X-ray diffraction (XRD). Results show how the hardness profile through the surface is related to the laser treatment and resultant microstructures. Increased surface hardness was noted due to a complete microstructural transformation to an amorphous state in the glazed samples. The usefulness of such coatings on tool steels, in conjunction with other thermal barriers, for the forming of semi-solid steel alloys is presented.

Abstract: Semisolid forging process requires special movement of the ram, until now only achievable with hydraulic presses. The upper die, placed on the press ram, must move fast during forming of the component in order to reduce segregation phenomena, and, once the part is shaped, the die must squeeze the material so that the contraction of the material is overcome and no shrinkage defects are formed. Traditional mechanical presses are not able to reproduce this kind of cycles, only possible with hydraulic presses. The introduction of servo motors in mechanical presses makes possible the achievement of those requirements, and, therefore, the use of these presses for the semisolid forging processes. The main characteristic of this type of machines is the elimination of the flywheel and the clutch; in the solution proposed by the press builder Fagor, the AC servo motor drives the ram of the press by using a transmission based on gears and a crank. This paper presents some of the results achieved during the semisolid forging of A356 aluminium alloy using a 400 tons Fagor servo motor driven mechanical press installed at the University of Mondragon.

Abstract: The aim of this study was to improve extrudability limit, eliminate welding line and obtain optimum thixoextrusion conditions for manufacturing tubes of 7075 Al wrought alloy. By thixoextrusion, it was possible to improve deformability, control isotropy with extrusion direction, eliminate welding line (seamless) and save cost due to low energy consumption compared with conventional extrusion processes. The welded part was not observed at the welding line area. The grains of thixoextruded tube were homogeneously distributed and equiaxed grains were observed. Therefore, thixoextrusion is the most effective variable for the control of the magnitude of the welding line.

Abstract: Wrought aluminum alloys have wide variety applications in aerospace and automobile industries, due to their superior properties compared to casting aluminum alloys. Rheoforging is a modification of thixoforging, which starts directly from the liquid phase instead of reheating of a billet. In the present process, suitable semi-solid slurries of wrought aluminum alloys are prepared by the LSPSF (low superheat pouring with a shear field) rheocasting process within 25s. The effects of processing conditions on the degree of spherical grain refinement in 2024, 6082 and 7075 alloys are discussed. 2024 and 7075 alloys have been used in order to investigate rheoformability of high performance aluminum alloy. Experimental results show that rheoforging based on LSPSF process can produce relatively homogeneous microstructure throughout the cup-shaped component. However, high solid fraction of semi-solid slurry promotes metal flow and results in solid/liquid segregation. Subsequent optimized heat treatments raise significantly the mechanical properties. Future potentials and challenges to be solved are discussed.

Abstract: This study demonstrates the indirect rheoforging of wrought aluminum alloys fabricated by electromagnetic stirring (EMS). EMS was carried out by varying pouring temperature of theological material and subsequently the rheo-material was forged into the sample which consisted of the direct and indirect forging part, by varying the applied forging pressure. The rheo-material completely filled the die cavity at the applied forging pressure of 150 MPa. To transfer the densification pressure to the end of the part, the applied forging pressure of over 170 MPa was necessary. To control liquid segregation, solid and liquid phases distribute uniformly by deriving the laminar flow. When liquid segregation occurred significantly during rheoforging, the strength revealed as low as 268 MPa. Investigating relationship between microstructural features and mechanical properties of the product, the rheoforged material through EMS revealed the fine and globular microstructure. Microstructure with uniform distribution of solid and liquid phase (no segregation) showed good mechanical property of the rheoforged material whose tensile strength was 341 MPa for Al6061.

Abstract: EN AC-46500 aluminium components are formed by Semi-Solid Rheocasting (SSR) in an industrial plant using a 700 tons high pressure machine. The dies wear was designed by the PLCO model of the ProCast simulation software. The components have had a good structural integrity and the mechanical properties after T6 treatment have been equivalent to that obtained by the same alloy by die cast. The present work describes the SSR forming process, the resulting microstructure as well as the optimization of the ageing heat treatment by hardness evolution. The results of the tensile tests make these clear.