1、PDF原文：http:/ 中文3956字Semisolid extrusion molding of Mg-9%Al-1%Zn alloys F. CZERWINSKI Development Engineering, Husky Injection Molding Systems Ltd., Bolton, Ontario, L7E 5S5, Canada A novel technique in manufacturing net-shape components of magnesium alloys, which combines semisolid processing,
2、 extrusion and injection molding, is outlined. For an Mg-9%Al-1%Zn composition, the high-temperature transformations and factors controlling solidification microstructures, are analyzed. C _ 2004 Kluwer Academic Publishers 1. Introduction Extrusion is the plastic deformation process by which a metal
3、 is forced to flow by compression through the die orifice of a smaller cross-sectional area than that of the original billet. Since the material is subjected to compressive forces only, the extrusion is an excellent method for breaking down the cast structure of the billet with little or no cracking
4、 1. Most metals are extruded hot when the billet is preheated to facilitate plastic deformation, but room temperature (cold) extrusion is also exercised. So far, conventional extrusion applications do not utilize preheating materials above the solidus temperature to enter the semisolid range. The ad
5、vantages of processing metallic alloys in a semisolid state are attributed to the globular solid particles which control their thixotropic properties at high temperatures and reduce the content of dendritic forms after subsequent solidification 2. It is well established that the benefits associated
6、with semisolid processing, such as low shrinkage porosity, high tolerances and energy savings, are more evident at high solid fractions. Moreover, the ability to cast at higher solid fractions is of interest in improving billet stability and minimizing material loss during handling. Of all semisolid
7、 technologies, injection molding provides the largest flexibility in terms of the processed solid contents 3. This feature is attributed to the fact that injection molding combines the slurry making and component forming operations into one step, and the slurry is accumulated in a direct vicinity of
8、 the mold gate. So far, these potentials are not explored and commercial applications are limited to liquid-rich slurries, which, for thin-wall sections, may contain solid volumes as low as 510%. As the major obstacle preventing using high solid contents, the premature alloys freezing and incomplete
9、 filling the mold cavity, is reported 4. It was, therefore, anticipated that a drastic increase in solid content, especially above 60%, would transform the flow through the machine nozzle, runners, and mold gate into the extrusion, thus activating interaction between solid particles within the slurr
10、y which would facilitate the mold filling. The verification of such a hypothesis was the objective of this study. 2. Experimental details AZ91D magnesium alloy, used in the present study, had a nominal composition of 8.5% Al, 0.75% Zn, 0.3% Mn, 0.01% Si, 0.01% Cu, 0.001% Ni, 0.001% Fe and an M
11、g-balance. An as-cast ingot was mechanically converted into small chips and processed using a Husky TXM500-M70 prototype system with a clamp force of 500 tons and a 2 m long barrel with a diameter of 70 mm. The component manufactured represented the complex shape with a diameter of 190 mm and a tota
12、l weight, including sprue and runners, of 582 g 3. The mold was preheated to 200C and the slurry was injected at a screw velocity in the range of 0.72.8 m/s. For the gate opening of 221.5 mm2 it converts to the alloys velocity at the molds gate between 12.2 and 48.6 m/s. In order to examine the role
13、 of flow through the gate, the alloy was also injected (purged) into the partly open mold at significantly lower flow velocity at the mold gate. The typical cycle time was approximately 25 s, which corresponds to an average residency time of the alloy within the machine barrel of the order of 100 s.
14、 In some cases, the cycle time was deliberately extended up to 4 times. Metallographic samples of the molded alloy were prepared by grinding with progressively finer SiC paper, mechanical polishing with 1 m diamond paste and colloidal alumina, followed by etching in a 1% solution of nitric acid in e
15、thanol. Stereological analysis was conducted using optical microscopy, equipped with a quantitative image analyzer. 3. Results 3.1. Structural transformations of the alloy during processing Morphologies of an as-chipped alloy are shown in Fig. 1a. According to size determination by the screen method
16、 ASTM E-276-68, the predominant fraction of chips was retained on sieves with openings within the range 0.62 mm, and 75% of them did not pass through the 1.4 mm sieve. As a result of interaction with the chipping tool, the alloy experienced a cold work. The chips deformation is inhomogeneous with an increased strain in an immediate location of the second phase particles (Fig. 1b). (a) (b)
