| Nowadays,countries all over the world are facing the problems of environmental deterioration and resource depletion,which provides unprecedented opportunities for the development of magnesium and its alloys which are rich in resources in China.Magnesium alloy is known as “21 century green engineering material”.In addition to being the lightest metal structure material at present,magnesium alloy also has the advantages of high specific strength,high specific stiffness and good electromagnetic shielding properties.therefore,it has a broad development space and application prospect in aerospace,rail transit,3C and other fields.However,magnesium alloys have close-packed hexagonal crystal structure,and there are few slip systems that are easy to start,resulting in low plasticity and poor formability at room temperature,and the processing process can easily lead to deformation texture of deformed materials,which limits the wide application of magnesium alloys in industry.Hot extrusion is one of the plastic processing methods commonly used in magnesium alloys.The loading method of extrusion is one of the key factors affecting the yield strength and uniformity of the deformed magnesium alloy.The additional shear stress can be generated by adjusting the loading method.The goal of effectively refining the structure and improving the mechanical properties of magnesium alloys.Accumulative back extrusion(ABE)is a kind of deformation process which is suitable for large-scale mass production of large-size billets,which can produce strong shear stress in the deformation process.In this paper,AZ91 D alloy with high alloy content is taken as the research object,ultra-fine grained materials are prepared by ABE process,along with the dynamic precipitation and re-solution behavior of the second phase,which provides a reference for comprehensively improving the comprehensive properties of traditional magnesium alloy materials.The evolution rules of matrix structure,second phase and micro-texture of as-cast magnesium alloy billet deformed by ABE and the evolution of mechanical properties were clarified by means of metallographic microscope,X-ray diffractometer,scanning electron microscope and tensile equipment system.The ABE deformation process experiments under different deformation cycles and temperatures were carried out on a general hydraulic press,and the internal relationship among ABE process parameters,microstructure characteristics and mechanical properties was explored.At the same time,a process path for the preparation of magnesium alloy materials with good properties by ABE deformation was determined.When the fixed deformation temperature is 310 ?,the evolution of grain size and second phase morphology with the increase of cycle times is studied,and the morphology characteristics of matrix structure in the rich and poor regions of the second phase are clarified.The changes of dynamic recrystallization,crystal orientation,basal slip Schmid factor and pyramidal slip Schmid factor are revealed.At the same time,the evolution law of ABE deformation structure with temperature is revealed,and the causes of high temperature deformation twins are explored from two aspects of grain orientation difference and Schmidt factor,and the law of texture evolution,texture composition and formation reasons at different temperatures are expounded.The tensile property test results show that the room temperature ultimate tensile strength and elongation at break of ABE samples deformed at 310 ?/3 cycles are 296 MPa and 21.43% respectively.The high temperature tensile elongation of 340 ?/3 cycles and 380 ?/3 cycles samples are 574.79% and604.81%,respectively.It can be confirmed that ABE wrought magnesium alloy has excellent comprehensive mechanical properties at room temperature and excellent superplastic formability. |
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