Study Of The Forming Heat Resistant Magnesium Alloy By Continuous Variable Cross-section Direct Extrusion

With the rapid development of both aerospace and automobile industrytowards lightweight, magnesium alloy has a huge potential as the lightest metalmaterials in current applications. Due to magnesium alloy is Hcp and less slipsystems at room temperature result in poor plastic and forming difficultly. Thesevere plastic deformation is one of the best technology to grain refinement,uniform microstructure and improve performance for magnesium alloy has beenconfirmed. However, the traditional severe plastic deformation can achieve thedesired results only by multi-pass reciprocating extrusion, which has somedrawbacks, like complicated process, long circle and folding defects.This paper presents a new extrusion method—continuous variable cross-section direct extrusion (CVCDE). CVCDE is the method which adds someinterim dies between the billet and the die, and a channel with continuousvariable cross-section is composed by the interim dies and the container. Thenthe process of upsetting-drawing-upsetting can be realized when the billet flowsthrough the channel with continuous variable cross-section. Due to intense sheardeformation, microstructure morphology and mechanical properties are changed.The shape and properties can be solved simultaneously by a single extrusion.Firstly, the principle and process characteristics of CVCDE are researched,and based on the principle of CVCDE, a dedicated mold was designed. Theaccumulative strain formula of N interim dies is derived, and the moreaccumulative strain can be achieved under the condition of total extrusion fixed;Secondly, the method of combining numerical simulation and experimentalprocess is used to study heat resistant magnesium alloy ZM6as an example. Compared with traditional extrusion, the extrusion with interim dies has a greatereffect on the grain refinement and improving the tensile strength. At the sametime, the fracture mechanism changes from brittle fracture to toughness fracture.Effect of CVCDE on the grain refinement is found by the study of the differentinterim dies structure and combinations. For example, when the interim die sizeand angle are unreasonable, there are dead zones in the channel with continuousvariable cross-section. At the same time, due to the dead zone defects, the actualaccumulative strain is smaller than the theoretical value.At last, research on the microstructure evolution regularity after solutiontreatment, aging treatment, solution and aging treatment. These researches baseon continuous variable cross-section direct extrusion (CVCDE) of magnesiumalloy with cast and extrusion ZM6. Continuous precipitation phase appeared ingrain internal. The precipitation phase is increasing with aging treatment timeafter aging treatment of extrusion ZM6. Grain size increased obviously aftersolution and aging treatment. A bit of discontinuity precipitation phase appearedin grain boundary with the increasing of time, but the amplification slow down.Above all, continuous variable cross-section direct extrusion (CVCDE)realizes severe plastic deformation used in industrialized. This technologyprovide scientific basis of the research on short process forming technology ofhigh performance magnesium alloy extrusion products.