Study On Semi-solid Magnesium Alloy And Its Preparation Technique

Semisolid metal processing technology is regarded as one of the near-net shapeforming technologies with wide application in 21 century, and the preparation ofsemi-solid slurry is the key technique. However, research on semi-solid magnesiumalloys is seldom to date, and generally the shear rate of stirring mixer is small. In thispaper, the semi-solid slurry preparation of AZ91D magnesium alloy with twin-screw inhigh shear rate was studied, and the correlation of microstructure and mechanicalproperties of magnesium alloy in semi-solid state was also systematically investigated byrheo-diecasting process. Therefore, this advanced technique opens a new route forgeneralization and application of magnesium alloy products. Effects of semi-solid slurry preparation by single or twin-screw stirring werecomparatively studied for the first time. The results showed that fine round granularcrystals with average diameter of 40μm to 60μm of the prepared slurry could beobtained by twin-screw stirring, and the range of solid fraction could be controlled from15% to 60%. In contrast, because of the weak effect of mixture with low shear rate, thesolidified microstructure of the slurry by single screw was in strip shape or anomalousblock, the range of the primary grain size varied from 80μmto 100μm. The influences of the pouring temperature,the barrel temperature and the shear rateon the quality of the slurry by twin-screw stirring were systematically investigated. Theresults showed that the pertinent processing parameters were as following: a pouringtemperature of the Mg melt of 605℃～625℃; a barrel preheating temperature of 575℃～590℃; and shear rate of 4100s-1～6830s-1. The solid fraction in the slurry increasedand that the grains became finer and rounder with reducing pouring temperature of theMg melt or the barrel temperature. The solid fraction of the semi-solid slurry decreasedwith the increase of the shear rate, and the roundness of the crystals changed slightly. Thereason is that the higher shear rate enhances the shear action on one hand, whilst it IIIshortenes the shear time on the other hand. But the microstructure was distinctlydeteriorated when the shear rate was under 1365s-1. A thin-walled circular plate of 1.1mm thickness was fabricated successfully with a clear contour and good soundness withthis technique, this process has advantages of improvement in the fluidity and feedingcapacity of the semi-solid slurry. An excellent semi-solid feedstock of magnesium alloy was successfully producedby rheo-diecasting or rheo-squeezing process, which was a round stick of 50 mmdiameter and 120 mm height. The average size of grains were 40μm ~60μm and 50μm~70μm, respectively, and the grains made by rheo-diecasting were rounder. These slugfeedstock with non-dendritic primary phase are applicable for thixoforming. The relation of the mechanical properties of semi-solid magnesium alloy with thesolid fraction in slurry and the average size of primary α-phase grains was studied. Theresults showed that the tensile strength and elongation of rheo-diecasting AZ91D alloywere best when the solid fraction varied in 30% to 50%, because of high feedingcapacity of the semi-solid slurry and finer and rounder non-dentritic microstructure. Thetensile strength decreased gradually with the increase the average size of primary α-phase grains. The elongation was almost unchanged when the average size of primaryα-phase grains was varied in 35μm~45μm, but the elongation decreased rapidly whenthe average size of primary α-phase grains was over 50μm. Mathematical model ofmechanical property of semi-solid magnesium alloy sample fabricated by rheo-diecastingprocess was constituted with non-linear regression analysis. The effect of heat treatment on the mechanical property of magnesium alloy wasstudied. The results indicated that the mechanical property of semi-solid magnesiumalloy coul...