Solid State Phenomena Vols. 116-117

Abstract: In this paper, with a newly self-developed rotating barrel rheomoulding machine(RBRM), microstructures and mechanical properties of rheo-die casting A356 alloy were studied. In order to clearly show the characteristic of rheo-die casting, liquid die casting and semi-solid casting were done too. The experimental results showed that microstructures of rheo-die casting were composed of solid grains, which were finer and rounder, and had fewer pores. In the three technologies, integrated mechanical properties of semi-solid rheo-die casting were the best.

Abstract: The fluid flow of an A356 semi-solid aluminum alloy filling a die consisting of four tensile test bars was investigated. Numerical simulations were carried out by implementing a mathematical model in a finite element software. Additional simulations were also obtained with a physical model in which tomato paste was used as the analogue fluid for the semi-solid aluminum. The modeling results were complemented by a series of experiments where tensile test bars were produced from semi-solid aluminum with a high pressure die casting press. The correspondence observed with the two modeling approaches and the casting experiments is discussed along with the effect the die geometry had on the flow patterns.

Abstract: Rheoforging is a modification of semi-solid forging starting directly from the liquid phase instead of reheating of a billet. In this process semi-solid slurry is prepared by pouring slightly overheated melt over a slope into a holding cup to induce homogenous nucleation. By controlling the casting temperature and holding time in the cup the desired semi-solid slurry with fine, globular microstructure is successfully created. The inline forging trials using this slurry show the principal feasibility of the process.

Abstract: In the present study, the influence of the slurry temperature and the gate velocity on the apparent density of molded products was investigated using a thixomolding machine and a die having a rectangular parallelepiped cavity. Magnesium alloy of AZ91D was injected into the cavity through a rectangular gate. The gate velocity was varied from 0.4 m/s to 6.0 m/s and the slurry temperature was changed between 853K and 903K, respectively. The die temperature was fixed at about 513K. The results were as follows. The apparent density of the molded products at a constant gate velocity was decreased gradually with an increase in the slurry temperature in cases the slurry temperature was lower than 863K, however, it decreased suddenly at around the slurry temperature of 873K, and then it took an almost constant value when the slurry temperature was over 873K. The apparent density of the molded products at a constant slurry temperature was decreased greatly with an increase in the gate velocity in the case the slurry temperature was over 873K, however, it decreases slowly as the gate velocity increased for cases the slurry temperature was under 863K.

Abstract: As part of the foundry technology program, Alcan is developing a liquid-based slurrymaking process known as the SEED technology for semi-solid forming. The technology is presently entering the industrial and commercial stages, and will be used by the first customers in late 2006. The semi-solid process offers many advantages to economically fabricate near-net-shape parts having superior quality. The SEED process helps to overcome problems experienced with thixocasting and especially the high cost of feedstock. The SEED process involves two main steps: 1) heat extraction to achieve a desired liquid/solid mixture, and 2) drainage of an excess liquid to produce a self-supporting semi-solid slug that is formed under pressure. An overview of the industrial SEED technology advantages such as the alloy processing flexibility (A356, AA6061) and large slug dimensions and weights, is presented. The influence of the SEED processing parameters (slurry preparation) on the final mechanical properties and the microstructure evolution is also reported.

Abstract: Semi-solid casting (SSC) techniques have proven useful in the mass production of high integrity castings for the automotive and other industries. Recent research has shown metal matrix nanocomposite (MMNC) materials to have greatly improved properties in comparison to their base metals. However, current methods of MMNC production are costly and time consuming. Thus development of a process that combines the integrity and cost effectiveness of semi-solid casting with the property improvement of MMNCs would have the potential to greatly improve cast part quality available to engineers in a wide variety of industries. This paper presents a method of combining SSC with MMNC in a way that benefits from MMNCs’ tendency to naturally form the globular microstructure necessary for SSC. This method uses ultrasonically dispersed nanoparticles as nucleating agents to achieve globular primary grains such that fluidity is maintained even at high solid fractions. Once particle dispersion is achieved, the material needs no further processing to become a semi-solid slurry of globular primary grains as it cools. This quiescent method of slurry production, while still imposing some constraints on cooling rates, has a large process window making this process capable of industrial rates of throughput. It was found that the key factor to achieving globular microstructure is a sufficiently slow cooling rate at the onset of solidification such that particle-induced nucleation can occur. Once nucleation occurs, continued cooling is virtually unconstrained, with globular microstructure evident in quenched samples as well as samples cooled at rates as slow as 1 °C/min. This method was demonstrated in several material systems using zinc (Zn), aluminum (Al), and magnesium (Mg) alloys and nanoparticles of aluminum oxide (Al2O3), silicon carbide (SiC), and titanium oxide (TiO2). Additionally, several nucleation models are examined for applicability to nanoscale composites.

Abstract: This work describes the processing of different aluminum cast- and wrought alloys in the Cooling-Channel-Process. The investigations take the castability, the microstructural evolution for the SSM-processing and the resulting properties in the test specimen into account. Beside the alloys AlMg5Si2Mn (M59), AlCu4TiMgAg (AF48), AlSi12CuNiMg (SF70), AlSi17Cu4Mg (SF90), AlZnMgCu1,5 (AA7075) the optimization of the alloy AlSi7Mg0,3 (A356) by the addition of magnesium and copper were a main task of the work.

Abstract: Variations of cooling characteristics and solidification microstructure were investigated at various melt pouring temperature, mold temperature and cooling condition for semi-solid slurry of A356 alloy. And metallurgical and mechanical properties were examined with testing samples made by rheocast process based on the obtained cooling conditions. Undercooled state of melt appeared when superheating of melt was below 35
, and it was easy to form equiaxed solidification structure. Forced cooling hardly ever contributed to globular shaping of microstructure although it helped a little to refine microstructure. And slow cooling through insulation and mold heating was effective to form globular shape. Also, the metallurgical and mechanical properties of samples made by this process were better than those of other sample made by conventional processes.

Abstract: In this study effects of secondary cooling rate upon morphology and size of primary particles in Al-7.1wt% Si slurry produced by SSR process were examined. SSR slurries with various solid fractions were poured into a wedge-shaped steel die to produce a range of secondary cooling rates. The extent of primary particle growth during secondary cooling at various cooling rates was estimated at 0.05, 0.1, 0.2, 0.3 solid fractions using a 1/3-power law relationship. The results indicate that for a given cooling rate primary particles in higher solid fraction SSR slurries grow less significantly during quench than those in low solid fraction slurries. The shape factor of primary particles also depends on secondary cooling rate and solid fraction and decreases with increasing the secondary cooling rate and/or the solid fraction.

Abstract: Higher viscosities of semisolid metallic slurries have practically made their gravity casting impossible and resulted in a need for some sort of mechanical force to push such viscous slurries into die cavities. High pressure die casting has been conveniently used for this purpose. However, due to high manufacturing cost of dies, only mass production of semisolid components has been economically justified and practiced so far. In this work, as the first step for exploring the possibility of production of semisolid cast components in expendable molds, fluidity of Al-7wt%Si semisolid slurry in a centrifuging sand mold was examined. This paper reports the preliminary results of the effects of solid fraction, applied force and section thickness on the fluidity of Al-7.1 wt%Si semisolid slurry in the sand mold.