Influence of Mg Content on the Mechanical Properties of Cast Hypereutectic Al-Si Alloys

Résumé L’objectif de ce travail est d’étudier l’influence de la teneur en magnésium (Mg) sur la microstructure et les propriétés mécaniques d’un alliage d’aluminium (Al) – silicium (Si) hypereutectique. Un nouvel alliage hypereutectique à forte teneur en magnésium a été développé en ajoutant 6% et 10% massique de magnésium. Les résultats obtenus montrent qu’en augmentant la teneur en Mg, la phase intermétallique Mg2Si se substitue progressivement à la phase primaire Si. Des calculs thermodynamiques réalisés à l’aide du logiciel FactSageTM ont servi à prévoir la séquence de précipitation des phases au sein des alliages étudiés. Ces calculs ont été réalisés pour des systèmes binaires, ternaires et quaternaires. Pour les alliages auxquels ont été ajoutés de 6% de 10% de Mg, il y a corrélation entre les prédictions thermodynamiques et les séquences de solidification observées, particulièrement pour les deux régions de transition. Dans ces régions de transition, les deux phases primaires Si et Mg2Si précipitent dans un ordre précis, mais différent pour chaque alliage. Pour l’alliage à faible teneur en Mg (6%), le silicium précipite d’abord tandis que pour l’alliage à forte teneur en Mg (10%) l’ordre de précipitation est inversé. Pour les deux alliages, la fraction massique de Mg2Si dans la microstructure est significative.----------Abstract The current study focuses on the influence of the Mg content on the mechanical properties of cast hypereutectic Al-Si alloys. With increasing the Mg content, the formation of the primary Mg2Si tends to substitute for the primary Si progressively in the modified castings with the properties close to the Si crystals, both of which are believed to be important in Al alloys. Other than the Al-Si alloy system has been fully developed, the Al-Mg2Si composites however just start to be concerned recently. A new alloy system is thus established based on the conventional cast hypereutectic Al-Si A390 alloy with increasing Mg content. The formation of the primary Mg2Si is the main characteristic of adding Mg into the alloy A390, tending to substitute for the primary Si progressively. Equilibrium calculations and binary/ ternary/ quaternary phase diagram plots were carried out using the FactSageTM software, predicting the Al-Si alloy system alternatives with the Mg additions. The 6 and 10 wt% Mg alloys were found to be representative of the different decomposition behaviours in two transition regions where both primary phases can be precipitated in a particular order of Si to Mg2Si for the alloy with lower magnesium content and Mg2Si to Si for the second composition. For both alloys, the Mg2Si phase is significant in the aluminum matrix

Characterisation of thrust performance of micro-nozzle machined by micro end-milling

Micro thruster is the power plant of mini-spacecraft. It enables the mini-spacecraft to realize orbit adjustment, station keeping and attitude controlling. Micro nozzle is one of key parts of the micro thruster. The surface roughness of its inner surface significantly influences the thrust performance of the thruster. In this paper, a residual surface model is developed for micro-nozzle obtained by micro machining using a ball end mill and a taper end mill. The residual surface model is then used to investigate the relationship between the surface quality and nozzle thrust performance in nozzle flow field. A thrust measuring apparatus is designed and manufactured to inspect the thrust performance of the machined micro nozzles. Both simulation and experiment results indicate that good machined surface quality is obtained with taper end mill. The nozzle machined with the taper end mill has better thrust performance than that with the ball end mill under the same inlet pressure

An mechatronics coupling design approach for aerostatic bearing spindles

In this paper, a new design approach for aerostatic bearing spindles (ABS) is firstly proposed which takes into account of the interactions between the mechanical and the servo subsystems, including the integration of electromagnetic effects, static pressure characteristics, servo control and mechanical characteristics. According to the air bearing design principle, the geometry of the spindle rotor is designed. The fluid software is used to analyze the influence of the bearing capacity and stiffness on the stability of the spindle. The simulation shows when the air film thickness is 12 μm, the bearing has good load carrying capacity and rigidity. In addition, the influence of motor harmonics on the spindle shaft modes is considered to avoid the resonance of ABS, and to ensure ABS anti-interference capability, proper inertia of ABS is calculated and analyzed. Finally, ABS has a good follow-up effect on the servo control and machining performance through the experimental prototype. The electromechanical coupling design approach for ABS proposed in this paper, can achieve a peak value better than 0.8 μm (surface size: 9 mm × 9 mm) and a surface roughness better than 8 nm in end face turning experiments

Design of ultraprecision machine tools with application to manufacturing of miniature and micro components

Currently the underlying necessities for predictability, producibility and productivity remain big issues in ultraprecision machining of miniature/microproducts. The demand on rapid and economic fabrication of miniature/microproducts with complex shapes has also made new challenges for ultraprecision machine tool design. In this paper the design for an ultraprecision machine tool is introduced by describing its key machine elements and machine tool design procedures. The focus is on the review and assessment of the state-of-the-art ultraprecision machining tools. It also illustrates the application promise of miniature/microproducts. The trends on machine tool development, tooling, workpiece material and machining processes are pointed out

Material removal mode and friction behaviour of RB-SiC ceramics during scratching at elevated temperatures

Thermal assistance is considered a potentially effective approach to improve the machinability of hard and brittle materials. Understanding the material removal and friction behaviour influenced by deliberately introduced heat is crucial to obtain a high-quality machined surface. This paper aims to reveal the material removal and friction behaviours of RB-SiC ceramics scratched by a Vickers indenter at elevated temperatures. The material-removal mode, scratching hardness, critical depth of the ductile–brittle transition, scratching force, and friction are discussed under different penetration depths. The size effect of scratching hardness is used to assess the plastic deformation at elevated temperatures. A modified model is established to predict the critical depth at elevated temperatures by considering the changes in mechanical properties. The results reveal that the material deformation and adhesive behaviour enhanced the ductile-regime material removal and the coefficient of friction at elevated temperatures