Yield behavior of commercial Al-Si alloys in the semisolid state

Abstract

Systematic experimental work and modeling efforts have been conducted to characterize the yield behavior of commercial aluminum alloys in the semisolid state. In this study, extensive compression experiments were performed to measure the yield stress of semisolid aluminum slurries at high solid fractions (0.5 to 1.0), and a cone penetration method was employed to measure yield stress at low solid fractions (Ͻ0.5). A functional relationship between yield stress and temperature/solid fraction has been established for these alloys. The effect of the processing route on the resultant yield stress of the material in the semisolid state was studied by evaluating commercial A356 billets manufactured via magnetohydrodynamic stirring, grain refining, and UBE's new rheocasting (NRC) processes, respectively. Detailed microstructure observations and image analyses reveal that the difference in yield-stress values among the alloys evaluated is intricately related to the semisolid structure. At a given solid fraction, the yield stress of semisolid slurries depends on microstructural indices (i.e., entrapped-liquid content, shape factor of the alpha phase, and the alpha particle size). In addition, numerical simulation results indicate that the finite yield stress of semisolid metals plays a significant role in determining the flow pattern during die filling. Depending on processing conditions, five distinct filling patterns (shell, disk, mound, bubble, and transition) have been identified and confirmed through experimental observations. Recent simulations demonstrate that the finite yield stress is also responsible for flow instabilities encountered in commercial forming operations, such as "toothpaste behavior." Specifically, most flow instabilities can be avoided by properly controlling processing parameters and the initial semisolid microstructure. A stability map that provides a control guide for semisolid processing has been developed and is presented.

Modélisation de l'écoulement de l'aluminium semi-solide dans le moulage sous pression /

“…Le procédé SEED, breveté par Alcan Inc., a été utilisé pour produire l'aluminium semi-solide. figure 1 : Limite élastique vs allongement à la rupture pour diverses méthodes de production, AlSi7Mg0.3 [8] 2-3 figure 2 : Schématisation de la fabrication de lopins de A1SS SEED [17] 2-6 figure 3 : Microstructure typique obtenue par SEED [17] 2-7 figure 4 : Schématisation du procédé du MIT [18] 2-8 figure 5 : Microstructure typique obtenue par MIT [13] 2-9 figure 6 : Schématisation du procédé UBE [19] 2-10 figure 7 : Microstructure typique obtenue par UBE [16] 2-10 figure 8 : Microstructure typique obtenue par MHD [16] 2-11 figure 9 : a) Remplissage partiel de A356 à 7.8 m/s, b) simulation correspondante [23] 2-12 figure 10 : a) Remplissage partiel de A356 à 2.0 m/s, b) simulation correspondante [23] 2-12 figure 11 : Seuil de cisaillement en fonction de la fraction solide pour l'alliage A356 [24] 2-13 figure 12 : Résultats expérimentaux utilisant divers fluides analogues (droite); et simulations correspondantes (gauche) [32] 2-15 figure 13 [40] 3-6 figure 20 : Thixotropie de A356, fs=0.30. Au temps 0, le taux de cisaillement est passé de A) 105 à 1.7 s-1 et B) 1.7 à 105 s-1 [41] 3-7 figure 21 : Viscosité de A356 tel qu'observée par différents expérimentateurs [20,22,40] 3-12 figure 22 : Fraction solide de l'AlSS en fonction de la température (Scheil) Plusieurs de ceux-ci étaient causés par la géométrie du moule et auraient dû être perçus lors des simulations numériques.…”

“…La contrainte limite où la déformation plastique est initiée est appelée «seuil de cisaillement». Le seuil de cisaillement augmente avec la fraction solide pour atteindre sa valeur maximale du matériau lorsque celui-ci est complètement solide [16].…”

Low Cost and Energy Efficient Methods for the Manufacture of Semi-Solid (SSM) Feedstock

“…Specifically, to deal with the discontinuity in the constitutive relation, a regularized model is developed as follows [16]:…”

Characteristics and Effects of Particle Morphology and Tensile Ductility Resulting from FSP of Al–Si–Cu–Ni Casting and Forging Piston

“…Especially these particle phase thermally stable up to 500°C that is beneficial for high temperature and wear application. 8,9) It should be noted that though the matrix could be reinforced with hard particles to acquire higher yield strength, inevitable disadvantage of low ductility will be introduced due to the existence of inadequate morphology of Si particles and other intermetallic fine particles. As the tensile ductility is governed by a number of factors such as processing history, matrix constitution, average grain size, volume fraction and morphology of the reinforcement, many previous reports 1016) have properly applied thermal mechanical process to acquire more homogeneous microstructure to overcome the severe loss of tensile ductility.…”