Case Study: Engine Bracket Made by Rheocasting Using the SEED Process

In the context of increasing needs for lightweighting vehicles, semisolid casting of aluminium components is a proven route that can be efficiently applied for automotive parts. Although semisolid forming has not yet reached the market penetration that suits its actual potential, it is currently and efficiently used in many applications around the world on a daily basis. An example of such will be shown. This paper presents a case study on the application of the SEED rheocasting technology for the casting of an engine bracket. The part is made of the widely used AlSi7Mg0.3 alloy and is heat treated in T6 condition to benefit from the enhanced mechanical properties made possible by semi-solid forming. Throughout the development phase, different aspects associated with semisolid casting, such as slurry condition, gate design, mold filling behaviour, lubrication, blistering and others, were addressed successfully. In the final, the combination of the SEED technology with a thorough development process and the specific casting rules for semi-solid forming led to actual commercial production and contributed to weightsaving on the actual part as compared to a former design made from high pressure die casting.

Evaluation of Al-5wt%Si-5wt%Zn as Raw Material for Semisolid Forming

Designing new alloys for semisolid processing is key to the success of semisolid materials technology. While aluminium-silicon and aluminium-zinc alloys have been tested as potential raw materials, ternary aluminium alloys containing silicon and zinc have yet to be tested. As such alloys may exhibit the rheological behaviour required for semisolid forming and the excellent final mechanical properties of Al-Zn alloys, we investigated the thermodynamic aspects of the solid-liquid transition of Al-5.5wt%Si-5wt%Zn alloy, the morphological stability of this alloy in the semisolid temperature range and the corresponding rheological behaviour. Thermo-Calc® simulation software was used to evaluate the solid-to-liquid transition and identify the semisolid temperature range within which the liquid-fraction sensitivity is low and the process is therefore controllable. Based on the results of the simulation, a target temperature of 588 °C was chosen. This is sufficient to produce a liquid fraction of 55 % and a corresponding liquid-fraction sensitivity (dfl/dT) of 0.009 C-1. The Al-5wt%Si-5wt%Zn alloy was prepared by conventional casting in a refrigerated copper mould without grain refining, and the alloy was characterized to determine the stability of the microstructure after heating to 588 °C and holding at this temperature for holding times of 0, 30, 60, 90 and 120 s. The same temperature and holding times were used to evaluate the rheological behaviour in hot compression tests. A grain size of 170 μm, globule size of 100 μm and circularity of 0.6 were achieved, leading to a maximum apparent viscosity of 2 x 105 Pa.s, which rapidly decreased to 3 x 104 Pa.s after a shear rate of 9 s-1 was reached.

Microstructural Evolution during Partial Melting and Semisolid Forming Behaviors of Two Hot-Extruded Magnesium-Rare Earth Alloys

Reheating experiments and semisolid compression tests were conducted on two hot-extruded magnesium-rare earth (Mg-RE) alloys, Mg-8.20Gd-4.48Y-0.36Zr-3.34Zn and Mg-3.75Gd-5.15Y-0.75Zr-3.05Zn by using a multistage hot compression test machine. Dissolution of eutectic compounds, growth of grains, and partial melting took place during the reheating of these Mg-RE alloys and resulted in spherical semisolid slurries at certain temperatures (580 °C for Mg–8.20Gd–4.48Y–3.34Zn–0.36Zr, 560 °C for Mg-3.75Gd-5.15Y-0.75Zr-3.05Zn). Owing to the different alloying element contents of these two Mg-RE alloys, eutectic compounds with different morphologies were found inside them after reheating and rapid cooling processes. The forming characteristics of these two Mg-RE alloys in semisolid state were discussed based on the results of compression tests.