Local Loading Forming Process And Microstructure Evolution Of Large Box Components

National major equipment has been faced with multiple requirements like high mobility and large scale.the lightweight of weapons and equipment has become an important means to improve combat capability.As the lightest metal structure material,magnesium（Mg）and its alloy has a great application value in weapon equipment.However,the development of wrought magnesium alloys has been limited by its hard plastic deformation and the poor formability.Therefore,large component forming technology research effectively promote the development of Mg industry,to achieve the strategic goal of equipment lightweight.In this paper,the forming of large box complex components was studied,finite element numerical simulation and physical experiment was adopted to propose a suitable local loading scheme,which mainly solved the problems of hard forming of large components and high load,and then realized the load reduction forming of large components.The stress field,strain field and velocity field of the box body during local loading were analyzed,and the best forming scheme and deformation parameters were obtained.In addition,by analyzing the deformation condition and microstructure of loading zone and transition zone of the box,their deformation characteristics and microstructure evolution laws were summarized.According to the structural characteristics of components and the forming modes of partial loading,the sequential extrusion and symmetrical partial extrusion were proposed for the target cavity of preforming to meet the requirements of load reduction forming.During the sequential extrusion process,the uneven metal flow resulted in the serious billet accumulation so that the qualified components failed to be prepared.Symmetrical local extrusion had the characteristics of reducing forming load,reducing loading times and controlling metal flow flexibly.In order to avoid the defects such as folding and uneven flow,ensure the forming quality of components and realize the reasonable distribution of load,the single variable method was used to simulate the local loading forming of box components.The mold structure and forming process parameters were optimized by combining the changes of forming load,stress field and strain field.The best matching parameters were concluded: the punch fillet angle was 20 mm,the punch extrusion speed was 0.5mm/s,and the forming temperature was 380℃.According to the different deformation conditions,the box was divided into transition zone and loading zone.The equivalent strain value in the loading zone was high,and the mean value of equivalent strain increased with punch pressing.Because the first and second loading had symmetrical forming characteristics and restricted metal flow,the third extrusion produced equivalent strain on the adjacent formed cavities and the second extrusion without it.In the process of deformation,the equivalent stress acted on the whole billet and diffused from the loaded region to the loaded region and the unloaded region.The mean value decreased with the increase of the pressure distance,and the equivalent stress was generated on the adjacent cavity by subsequent extrusion.Local loading resulted in inconsistent metal flow,and the unformed region shows weak metal flow behavior.Free forging contributed to the billet obtaining the fine microstructure and promoting the precipitation of strengthened phase.The billet provided residual heat for the abnormal grain growth in the billet,and the structure of the member was not uniform.The aggravation of grain refinement behavior was mainly attributed to the increase of equivalent strain value,and the recrystallization content of components decreased from inside region to outside region,leading to the continuous decrease of yield strength.The grains in the transition zone were deflected along transverse direction,and the single orientation of the residual deformed grain resulted in strong texture.The grains in the loading zone were deflected along extrusion direction,and the recrystallization had a constantly weakening texture with random orientation.