Continuous Twin Screw Rheo-Extrusion of an AZ91D Magnesium Alloy

Abstract

The twin screw rheo-extrusion (TSRE) is designed to take advantage of the nondendritc microstructure and thixotropic characterization of semisolid-metal slurries and produce simple metal profiles directly from melts. The extrusion equipment consists of a rotor-stator high shear slurry maker, a twin screw extruder, and a die assembly. The process is continuous and has a potential for significantly saving energy, manufacturing cost, and enhancing efficiency. The present investigation was carried out to study the process performance for processing rods of an AZ91D magnesium alloy and the microstructure evolution during processing. The semisolid slurry prepared by the process was characterized by uniformly distributed nondendritic granular primary phase particles. AZ91D rods with uniform and fine microstructures and moderate mechanical properties were produced. For the given slurry making parameters, decreasing extrusion temperature was found to improve microstructures and properties. The mechanisms of particle granulation and refinement and the effect of processing parameters on process performance and thermal management are discussed.

Rheo-Extrusion of an Alloy Specially Developed for Rheo Process Based on Al–Zn–Mg–Cu System

“…in groups 1 and 2, respectively. As mentioned above, Xia and coworkers [19] have demonstrated that 0.5-0.7 solid fractions is a suitable range for rheo-extrusion. So alloy 6 can be the best candidate for rheo-extrusion at F s = 0.6.…”

“…[12][13][14][15][16][17]. Fan et al [18,19] have developed a new SSM processing technology named twin-screw rheoextrusion. Semi-solid rheo-extrusion is a new extrusion process that has several advantages such as low extrusion force, high fluidity of materials, and low friction force between the die and the materials.…”

“…It has also been found that 0.5-0.7 solid fraction is suitable for rheo extrusion, so that the T.S. should be measured at this range of solid fraction [19].…”

Preparation of an Mg–Gd–Zn alloy semisolid slurry by low frequency electro-magnetic stirring

“…is required to enhance the nucleation rate and restrict the growth of primary particles and consequently lead to the formation of slurries with uniform and refined microstructure [9]. Among above fields, while the twin screw shearing [10], gas bubble stirring [11,12] and ultra-sonic vibration [13] are able to produce fine and spherical primary particles, they tend to introduce slags and oxidized films (in particular, Mg melt is highly active and prone to be oxidized [14]) into the melt, which would cause the premature fracture of alloy. On the other hand, electro-magnetic stirring, which would not cause oxidation and subsequent contamination of the melts, appears to be suitable to be used to prepare slurries of Mg alloys.…”

“…T. Campanella and M. Rappaz et al proposed a dendrite fragmentation criterion to study the grain refinement effect by electromagnetic stirring treatment, and proved this theory by experimental study on two kinds of Cu alloys [19]. In addition, previous researches were mainly toward Al alloys and Mg-Al based alloys [10,12,13,[20][21][22], while very little work has been carried out toward the Mg-RE based alloys.…”