Study On Evolution Law Of Internal Void Defects In Rotary Extrusion Of AZ80 Magnesium Alloy

As the lightest metal in engineering structural materials,magnesium alloy plays an important role in the industrial fields such as defense industry,aerospace and electronic communication.However,the casting process of magnesium alloys leads to the unavoidable void defects in the material,which directly destroy the continuity of the metal,and easy to form stress concentration or crack damage in the subsequent plastic forming process,thus shortening the fatigue life of magnesium alloy products and reducing the mechanical properties.Therefore,the development of a reasonable deformation process to promote the closure of void-type defects is of great significance for the production of magnesium alloy products with good service performance.As a new type of plastic forming process,rotary extrusion has the advantages of improving the hydrostatic pressure inside the deformed body and obtaining severe plastic deformation,which creates favorable mechanical conditions for repairing void defects.In this paper,AZ80 magnesium alloy is used as the object to study the evolution law of the internal void defect in the rotary extrusion forming.The factors affecting void closure are discussed.The void evolution model under this deformation mode is established.The validity of the evolution model is verified by experiments.Based on the description of the rotary extrusion deformation process and principle,the stress and strain distribution of each region in the deformation process of AZ80 magnesium alloy are discussed.During the forming process,the deformation zone is in a state of strong compressive shear stress,which can greatly improve the performance of the metal.Due to the rotational motion of the punch,a region of high strain accumulation is formed inside the deformed body,and the deformation strengthening effect is more remarkable.Aiming at the finite element simulation problem of void,the cell method is proposed,and the finite element macro model and the cell model are constructed by Deform-3D finite element analysis software.Determine the initial position of the void,the initial shape of the void and the initial size of the void as the internal factors affecting the void evolution.The control variable method is used to design the experiment to analyze the influence of internal factors on the void evolution.The extrusion speed,rotation speed,deformation temperature and friction coefficient were determined as the external factors affecting the void evolution.The orthogonal experiment was designed to analyze the influence of external factors on the void evolution.The analysis results show that the initial position of the void,the initial shape of the void,the extrusion speed and the rotational speed have a significant impact on the void evolution.The initial void size,deformation temperature,and friction coefficient have little effect on void closure.The analysis of the void closure process reveals that the initial shape of the void is different,and the shape of the void presents different evolution laws.Based on the research method of damage mechanics,the general relationship between void volume and macroscopic mechanical field is derived.It is found that macroscopic equivalent strain and macroscopic stress triaxiality have positive effects on void closure.By analyzing and processing the numerical simulation results,the relationship between the void volume and the macro-mechanical field was clarified,and the void evolution model of AZ80 magnesium alloy in rotary extrusion was obtained by regression.Analysis of the void evolution model,found that high stress triaxiality level and large equivalent strain are conducive to void closure.With the increase of equivalent strain,the influence of stress triaxiality on the void evolution is gradually reduced.The mechanical condition under which the void is closed in this deformation mode is that the equivalent strain is at least 0.75.Rotary extrusion experiments were carried out on the AZ80 magnesium alloy with void using Gleeble 3500 thermal simulator.The results of the analysis showed that the influence of extrusion speed,rotational speed and initial position of the void on the void evolution is consistent with the influence trend obtained by numerical simulation.The relative volume error of the void obtained by the experimental and evolutionary models is small,which verifies the reliability of the numerical simulation and the validity of the void evolution model.