Solid State Phenomena Vols. 141-143

Abstract: Extrusion is a well established technology for the production of complex sections of aluminium alloys. Thixo-extrusion in comparison to traditional hot-extrusion offers several advantages such as lower extrusion and friction forces, higher material fluidity, longer tool life etc. Aluminium alloy 5182 is an important commercial alloy characterized by high strength and ductility, high corrosion resistance and good formability; it is commonly used for the production of wrought automotive components and it is also suitable for semi-solid applications thanks to its wide solidification range. The aim of this paper is to attempt the shaping of 5182 Al-Mg alloy through the thixo-extrusion process using a ceramic tool and evaluating the effect of different routes of making the feedstock on the semisolid microstructure. Particularly, two different methods were investigated: Near-solidus casting and Roll-casting using a cooling slope. All the samples produced were characterized by metallographic analysis in order to measure globule size and shape factor, as the main criteria used for assessing thixo-formability.

Abstract: Inductive re-heating of billets is the state-of-the-art method to enable the desired liquid fraction for thixoforming. In laboratory experiments the temperature might be directly controlled since it is usually possible to apply thermocouples for temperature measurements. Due to technical restrictions temperature measurements especially in the interior of a billet are difficult in industrial heating processes. For this reason, thixoforming billets are heated without temperature monitoring by using proven heating functions. Such functions are usually obtained by trial-and-error or on the basis of FEA simulations. In both cases exhaustive experimental work is usually necessary. Moreover, FEA simulations require thermo-physical material data that is difficult to obtain for industrial relevant heating processes. In this paper an alternative method of experimentally obtaining optimized heating functions by using a fuzzy logic controller is introduced. Under steady-state environmental conditions this method allows a quick and reproducible re-heating. The main advantages of the fuzzy logic based approach are that no specific material data is required, system specific properties such as the efficiency have not be explicitly determined and that the practical implementation could be realized with a minimum of experimental work.

Abstract: The inductive heating of the feedstock material is a very important step in the processing of semi solid metals. On the one hand, the billet has to be heated as fast as possible to the target temperature. On the other hand, it must be guaranteed that the outer area does not begin to melt prematurely. Also, at the end of the heating the billet should have an uniform temperature distribution in order to obtain good forming results. A flatness based control will be presented to calculate the induced power over time trajectory from a desired trajectory for the temperature in the middle of the billet. The temperature trajectory has to be chosen so that the billet has the desired temperature behaviour. Experimental results will be shown for the flatness based inductive heating of X210CrW12 just below the semi solid state.

Abstract: In this work, a numerical model for induction heating is proposed. The heating process considers only interaction of electromagnetic effects and heat transfer, and is represented by an unsteady heat diffusion equation. The numerical simulation of the process is performed using a finite volume method in which the induction heating is represented by a volumetric heat source term. The heat source term is evaluated analytically using principles based on Faraday’s and Biot- Savart laws. The technique is applied to the case of induction heating of a cylindrical A356 aluminum alloy billet. The model predicts magnetic flux density and temperature distribution during the heating process. The effects of process parameters, such as frequency and current density, on the temperature distribution are also studied. The results show that nearly uniform distribution of temperature can be achieved when the billet is heated slowly with low frequency and low supply current density.

Abstract: To enhance the comprehension on the internal rule of microstructure evolvement and quality-controlled relativity, the microstructure during the induction reheating of billets and die casting of parts was systematically investigated. According to general structure observation and aided analysis of some computer simulations, the quantitative relationships between microscopic morphology (including solid fraction and grain shape) and formation state was minutely discussed. The experimental results showed that liquid and solid phase in semi-solid slurry had different filling tendency, fluid velocity of liquid phase in the area of high filling speed was relative quick and easy to flow into far area and corner. Solid particles far from gate possessed fine and round grain. Moreover, the structure configuration of original billets markedly affected forming process; billets with fine and round grain were required for the die-castings of complex shape and thin thickness. In the case of the big change in shape and thickness of die-castings, the design of gating system and the control of plunger speed should be so properly carried out as to avoid unequal distribution of solid and liquid phase in die-casting part as soon as possible.

Abstract: Semi-solid forming offers new potentials for processing of high reactive and hot crack susceptible aluminium-lithium wrought alloys. With the tailored alloy AA1420* (AlLi2.1Mg5.5 +Sc+Zr) a promising material for thixoforming with achievable high strength of up to 500MPa tensile strength and over 400MPa yield strength concomitant with its low density of 2,46g/cm3 is achievable. Due to high solid fractions the effect of solidification shrinkage could be sufficiently decreased with the result of hot-tear-free casting. Simulation supported a critical to cast automotive tie rod was exemplarily manufactured via semisolid- technology with promising results. Furthermore with an improved and advanced heat treatment enhanced mechanical properties, comparable to those of rolled AA1420, were achieved.

Abstract: Rheocasting of alloys A206 and A201 was investigated in this study. Conical bars with different silver contents were produced using CSIR rheoprocess technology, together with high pressure die casting. The results showed that addition of Ag to alloy A206 increased the mechanical properties of the alloy. However, the addition of Ag also resulted in Cu-rich phases to precipitate at the grain boundaries of the as-cast material. The solution treatment used in this study was unable to dissolve all of this phase, especially in the 1.12%Ag-containing alloy. This resulted in slightly decreased mechanical properties compared to the 0.63%Ag-containing alloy. The T6 mechanical properties (strength and elongation) obtained in this study for rheocast A206 and A201 are better than those reported for permanent mould castings of alloy A206 and A201.

Abstract: An improved and self-developed semisolid preparing and rheomoulding device — rotating barrel rheomoulding machine (RBRM) for light alloys in laboratory is introduced in this paper. It mainly consists of a melting furnace, a shearing system with two relative-rotating conical barrels, a central temperature control unit, gas protection system and a die-casting system. Microstructure-processing relationship of A357 aluminum alloy obtained by the RBRM process is investigated by different intensity of turbulence and different shear rate. The experimental results show that the improved RBRM is capable of eliminating coarse dendrites, and producing small and spherical solid particles uniformly distributed in a eutectic matrix. In addition, the process can eliminate entrapped gas and reduce fine shrinkage pores in the specimens as well. Compared with the original self-developed device in our laboratory, the improved equipment has the following advantages: accurate control of stirring temperature; small volume and convenient manipulation; fine and spherical solid particles, chemical and microstructural uniformity throughout the specimens and so on.

Abstract: Rheo-die casting (RDC) based on LSPSF (low superheat pouring with a shear field) rheocasting process has been exploited. In case of secondary die casting aluminum alloy YL112, LSPSF allowed for preparation of sound semi-solid slurry in 15-20s that fully meet the production rate of HPDC, the primary α-Al exhibiting a mean equivalent diameter of 70 μm and shape factor of 0.93, without any entrapped eutectic. Compared to conventional HPDC, RDC improves microstructures in castings. Secondary solidification of semi-solid slurry takes place uniformly throughout the entire cavity, producing an extremely fine and uniform microstructure. The experimental results show the RDC 380 alloy has much improved integrity and mechanical properties, particularly elongation, and heat treatment can be used to enhance the mechanical properties.

Abstract: The CSIR rheo-process was used to prepare the aluminium A356 SSM slurries and thereafter plates (4x80x100 mm3) were cast using a 50 Ton Edgewick HPDC machine. Plates in the as cast, T4 and T6 heat treatment conditions which had passed radiography were then butt laser welded. It was found that the pre-weld as cast, T4 and post-weld T4 heat treated specimens fractured in the base metal. However, the pre-weld T6 heat treated specimens were found to have fractured in the heat affected zone (HAZ).