Papers by Keyword: Solid Fraction

Abstract: Metal direct writing in semi-solid slurry is an innovative technology to realize low-cost printing of load-bearing parts in contrast to laser-based additive manufacturing. However, it is challenging to achieve near net-forming of 3D parts in the current stage because of the out of controlled microstructure and hence the unstable macro extrusion of the used semi-solid slurry. Here, mixed powder remelting (MPR) is introduced to actively design the characteristics of solid phases, i.e., solid fraction, shape factor, and size distribution. Specifically, high-melting-point pure Al powder served as the dispersed solid phases in the liquid phase that transformed from Al-Si alloy powder after remelting, leading to hypoeutectic Al-Si semi-solid slurry. The effectiveness of this approach was experimentally examined and kinetically modelled, to prepare semi-solid slurry with pre-set microstructure. The improved extrusion stability of semi-solid slurry can be anticipated, and it is universal for manufacturing of metal matrix composites slurry.

Abstract: GISS (Gas Induced Superheated Slurry) is one of the more popular processes for the production of commercial semi-solid castings. For the successful production of high-quality castings, it is necessary to understand the impact of processing parameters on the flow behavior of the semi-solid metal. This paper describes the results of laboratory studies to examine the effects of processing conditions on the development of semi-solid feed material during GISS processing for two aluminum casting alloys, ADC12 and A356. Two series of tests were performed. The first involved a simple pouring test along an inclined section of a v-channel, to determine if differences in flow behavior could be identified. The second series of trials examined the effect of processing temperature and time on the consistency and flow behavior of the aluminum alloys. Ladles of molten aluminum alloy were treated using the GISS process at different temperatures for between zero and 10 seconds. After the treatment, the alloy was allowed to further cool in the ladle into the semi-solid temperature range, at which time the flow behavior of was compared. Consistency more suited to semi-solid casting was obtained when the GISS treatment temperature was lower and treatment time longer.

Abstract: The characteristics of the solid phase, namely the volume fraction, particle size, and morphology, are dominant variables that can determine the viscosity of the semi-solid slurry. However, particle size and morphology were always being ignored and the solid fraction was simply determined using the temperature in the conventional power-law viscosity, resulting in a disagreement in the viscosity values in different researches. To make the power-law viscosity model more accurate for predicting the filling process of semi-solid die casting, it is essential to modify this viscosity model based on particle characteristics. Therefore, there is a fundamental demand to prepare semi-solid slurries with different α-Al phase features at first. This is achieved in this study by two kinds of heat history controlling methods: remelting and solidification, which can get slurries with spherical structure and dendric structure, respectively. The semi-solid 357.0 slurries with 0.11-0.43 solid fraction, 137-182μm particle size, and 0.81-0.90 shape factor were prepared in the remelting process, while dendritic structures (shape factor<0.5) with 0.1 and 0.3 solid fractions were obtained by solidification controlling from the full liquid state. Besides, the effect of parameters on the evolution of the α-Al phase has been discussed. These slurries with different solid features will be further used to quantify the influence of primary phase characteristics on rheological behavior and make the power-law viscosity model more accurate for simulation.

Abstract: An internal cooling agent is used in rapid slurry forming (RSF) process to produce a high solid fraction slurry for a short period of time. In the process used in this research, the swarf which is known to be a low enthalpy material was added to the melt as the internal cooling agent. During the process, the swarf started to melt and a semi-solid slurry with a relatively high solid fraction was formed. This slurry was formed by exchanging the enthalpies between the low and high enthalpy materials. A commercial Al-Si-Cu alloy, i.e. AS9U3 Aluminum alloy, was used in this investigation. The microscopic examination showed that the Al-Si eutectic colonies start to melt during the melting process of swarf material resulting in the formation of globular Alpha-Al grains due to the multiplication of secondary dendrites arms. The fracture of dendrites arms and the subsequent spheroidization were suggested to be the origin of non-dendritic globular grains in the final microstructure. The amount of primary globular Alpha-phase was measured by the image analysis software. The results showed that during high pressure die-casting of AS9U3 Aluminum alloy using 4 mm thick samples, around 35 percent solid has been formed at the temperature of 580 oC.

Abstract: Present work was carried out to investigate the measurement accuracy of the sump depth for 7050 billet in direct chill (DC) casting process. Three measuring methods were applied: (i) Al-30wt.% Cu melt was poured into the mold at the steady state of the casting attempting to record the profile of billet sump; (ii) two steel rods were vertically dipped into the centreline and the edge of the billet attempting to measure the distances between the solidus and the free liquid surface in the hot-top; (iii) several ordered thermocouples were pre-arranged onto the starting block and they were solidified into the billet as it was cast, and a series of temperature-time curves was recorded after casting. For the purposes of comparison and evaluation, a numerical model based on the same experiment parameters was also built and the simulation results were compared with measured results by above methods. It indicates that the sump depth measured by thermocouples has the best consistency with numerical results. The most accurate method is temperature recoding by thermocouples. The Al-Cu melt pouring method is more accurate than the rods dipping method, and the sump depths measured by later two methods were the positions about the solid fraction (fs) depths of 0.8 to 0.9.

Abstract: Aluminium semi-solid castings have gained increased attention due to their superior mechanical properties, lower porosity compared to conventional high pressure die cast material. These characteristics suggests that semi-solid casting should be suitable to produce thick-walled structural components, yet most successful applications of semisolid casting have been for thin-walled components. There is a lack of understanding on filling and feeding related defect formation for semi-solid castings with thick-walled cross-sections. In the current study an AlSi7Mg0.3 aluminium alloy was used to produce semi-solid castings with a wall thickness of 10mm using a Vertical High Pressure Die Casting machine. The RheoMetal^TM process was used for slurry preparation. The primary solid α-Al fraction in the slurry was varied together with die temperature. The evaluation of the filling related events was made through interrupted shots, stopping the plunger at different positions. Microscopy of full castings and interrupted test samples were performed identifying the presence of surface segregation layer, shear bands, gas entrapment, shrinkage porosity as well as burst feeding.

Abstract: Computer aided cooling curve analysis (CACCA) is an online prediction tool for the determination of solidification characteristics of metals or alloys. The results of CACCA can be used to accurately determine latent heat and solid fraction needed for modeling of the solidification process. Newtonian and Fourier analysis techniques adopt a data base line fitting technique to the first derivative curve for calculation of the solid fraction and latent heat of solidification. This paper describes the theoretical and experimental procedures involved Newtonian and Fourier analysis techniques with reference to an Al-22% Si alloy. The correlations between the solid fraction and temperature/time for the alloy were determined.

Abstract: In this study, A356 aluminum thin plates (1.2 mm thick) are fabricated using the semi-solid forming process. Using the electromagnetic stirrer, A356-based semi-solid slurry is fabricated. The configuration of the thin die cavity for forging is designed using the fluid analysis of MAGMA software. The dimension of the thin plate is 150 x 150 x 1.2 mm. The semi-solid slurry with 45% solid fraction is created and then injected into the forging die at the 200-ton hydraulic press for compression. Thin plate with semi-solid slurry at 45% of the solid fraction (fs) is fabricated with punch speed of 300 mm/s and punch pressure of 200 MPa for compression the slurry. The formability, mechanical properties and microstructure of a formed thin plate sample are analyzed. As a result, a thin plate with 211.5 MPa of tensile strength and 8.5% of elongation can be formed.

Abstract: Different types of semi-solid processing are used to produce a variety of components. In this context, the use of FE simulations to obtain the filling of the dies and to optimize the semi-solid processing is clearly of a great interest. To carry it out properly in an isothermal case, the semi-solid flow into the die and friction phenomena have to be correctly described. In addition, comparisons between experiments and simulations are needed to assess the reliability of the modeling and to improve the understanding of the processing. In situ visualization of the semi-solid flow during processing is complex since the dies are closed and opaque. One of the main recent work with transparent glass sided dies to film die filling is that by Atkinson and Ward (2006). The purpose of this work is to compare numerical simulations to these experiments. Numerical simulations were performed with the solid mechanics-based software FORGE©. A micromechanical model accounting for the liquid and solid behaviour and their spatial distribution within the material (Favier et al, 2009) was used. The model parameters were identified using rapid compression tests on the A357 aluminium alloy (Favier and Atkinson, 2011). The slurry temperature corresponds to 0.5 solid fraction. Comparisons were focused on the flow behaviour. The impact of the presence of an obstacle and of the shape of the obstacle was investigated. The numerical simulations reproduced quite well the flow behaviour for the case with and without central obstacle. However, the change in flow due to an increase of the ram speed from 250 mm/s to 1000 mm/s is not captured.

Abstract: The A357 Al-Si-Mg cast alloy is commonly used for cast production of automotive components in conventional HPDC and in semi-solid processes. The aim of this work is to study the application of A357 cast alloy in thixoextrusion processes. This technology offers new possibilities of application for cast alloys and several advantages in from of conventional processes of hot-extrusion: lower pressure, minor friction forces, higher material fluidity and longer tool life. The thixoextrusion was experimented in a pilot plant, at different speeds and temperatures, with an induction furnace and a 400 Tn press. With ingots of A357 as cast, a tool with a ratio of 8.5 and with temperatures of extrusion around 560 °C, it is possible to obtain extrusion speeds more than 3 m•s-1. In all cases, when the speed of extrusion increases, the press of extrusion decreases. The effect of heat treatments and extrusion speed in the microstructure of the extruded product is evaluated.