| Magnesium alloy is the lightest in the nonferrous metals which have been used in the industry. Compared with other nonferrous metals, magnesium alloy has the advantages of high specific strength, predominant resistance for vibration, excellent machining ability, good ability of electromagnetic shielding, electricity transmission and heat transmission; hence it has been to the most suitable material for lightening the structure. Furthermore, magnesium alloy can be reclaimed at the percentage of 100%. So magnesium alloy has a prospect of adopting in auto car, electron and communication industries, and the requirement for magnesium alloy recently has increased rapidly.As the crystal lattice of close-packed hexagonal, magnesium alloy has less slip system, and its plasticity is poor at room temperature. It has difficulties for magnesium alloy to forming in plastic processing. Semi-solid processing (SSP) is a new kind of metal processed method which developed about 30 years ago. SSP of magnesium alloy has the advantages of low cost and higher strength and plasticity of component; hence it has attracted more and more attention from researchers and entrepreneurs. In SSP, the preparation of semi-solid billet (or slurry) is the key problem since it had been developed. As one of the methods of semi-solid billet preparation, the strain induced-melt activation (SIMA) has the prospect of industry application because there is unnecessary for special devices. Direct extrusion and upset, the traditional methods used in SIMA, are difficult to form severe plastic deformation billet, and the size of billet is changed after the SIMA processing. Equal channel angular extrusion (ECAE) technology has attracted more and more attention recently and is famous for the unchanged cross section after extrusion. But, as a kind of inhomogeneity deformation, in the most documents, ECAE was carried out to a billet whose diameter (or length of side) was not more than 20 mm, which makes it impossible to industry application. So, it is inevitable to develop the ECAE technology of large-size magnesium alloy billet in the procedure of industry application. But, once the size of billet increases, if the effect of refining the crystal is still good as well as those whose size under 20 mm, and if the inhomogeneity deformation of billet would affect the semi-solid partial remelting processing. This is the key problem which must be solved in the procedure of promoting the ECAE technology adopting in the semi-solid industry.In this paper, the numerical simulation of ECAE in different die geometry was carried out to study the influence of die geometry on the effective strain inhomogeneity and the suitable die geometry was gained. Then the ECAE experiments of AZ91D magnesium alloy whose diameter was 60 mm were performed at 225℃by different extrusion passes and routes. The material modeling of AZ91D in multi-pass ECAE was built respectively and the multi-pass extrusion in the route of Bc was simulated and the effective strain distribution in different passes, along the axis and in the cross section, was analyzed, the mechanism of inhomogeneity deformation was discussed. The microstructures of different parts in as-ECAE billets were also observed to probe the inhomogeneity of refining crystal. Moreover, the mechanical properties along extrusion direction (ED), normal direction (ND) and transverse direction (TD) were tested and the formed textures by ECAE were measured by X ray diffraction (XRD). Relations of mechanical properties aeolotropy and texture were analyzed. In addition, according to the results above, samples in different part of as-ECAE billet were machined intoφ10×10 cylinder and were partial remelted at 570℃, the evolution mechanisms of semi-solid microstructure and the influence of texture on the semi-solid microstructure were discussed. At last, the feasibility of ECAE adopting in semi-solid industry was probed.The results show that:(1) The origin of inhomogenous behavior during ECAE is the die geometry, namely, the inner angle and the outer angle. For different strain-hardening metals, the suitable die geometry to obtain homogenous effective strain is different. For as-cast AZ91D magnesium alloy, whose diameter was 60 mm, the suitable die geometry is that whose channel angle 90°, outer corner 20°and inner radius 8 mm.(2) In the multi-pass extrusion by route Bc, the end effect becomes inexplicit and extincted finally with the extrusion pass. In the cross section, the distribution of effective strain along the horizontal line is homogeneous, and does not change with the pass. But in the vertical direction, the effective strain appears an inhomogeneous distribution, and the inhomogeneity increases with the extrusion pass. The microstructure shows that the effect of refining crystal of different parts of the as-ECAE billet is different; the effect of refining crystal in middle part is better than that in end part, and the upper part is better than the bottom part. Especially in end part, the microstructure appears streamline. In addition, with the extrusion pass, the grain crystals become more and more tiny and the relationship between their sizes and mechanical properties agrees with the formula of Hall-Petch.(3) In spite of the difference of extrusion pass and route, the mechanical properties are different along the three orthogonal direcntions, TD, ED and ND. In other words, the ECAE process agitates the aeolotropy of mechanical properties formation. After 1st extrusion pass, the index of plane anisotropy (IPA) increased greatly. With the increasing of extrusion pass, the yield strength and ultimate strength increases but the elongation did not change too much. Furthermore, the IPA did not appear apparent, which shows that the factors of affecting the anisotropy are many-sided and complex. The texture analysis shows that the preferred orientation of grain crystal in billet endured one pass and four passes by route Bc is that the basal plane {0001} has a included angle with the extrusion direction of 45°(or 55°). In the billet endured four passes by route A, route Ba and route Bc, the preferred orientation of grain crystal is that the basal plane {0001} is parallel (approximately) to horizontal plane. The texture is the original reason for the formation of aeolotropy.(4) The mechanisms of semi-solid microstructure formation are different with the position in the as-ECAE AZ91D billet. One is the Ostwald ripening mechanism and the other is mechanism of coalescence coarsening. When holding for 20 min at 570℃of the upper-middle part of billet endured ECAE by route Bc, arrange of solid-phase crystal in semi-solid microstructure appears arc-like, which is not observed before in related documents. It can be said that this kind of arrange is another method to reduce the system free energy, not like the agglomeration phenomena. The possible formation mechanism is diffusion of atom and the rotation of solid-phase crystal. Furthermore, the texture formed in the ECAE process does not change too much after holding 5 min at 570℃, which maybe affect the subsequent thixotropy forming, and is one of the main factor should be considered in the establish of thixotropy forming processing. |
PDF Full Text Request