Microstructure And Mechanical Properties Of Commercial 7075 Aluminum Alloy Extruded Bar During Semi Solid Thixoforming

Semi-solid forming is a near final forming technology in the temperature range of semi-solid alloy.It uses the excellent deformation ability of non dendritic solid-liquid system and reduces the internal quality problems caused by alloy shrinkage,which is conducive to the preparation of high-performance parts with complex geometric structure.It is a promising material forming method in the 21 st century.7xxx series aluminum alloy is a kind of super high strength wrought aluminum alloy with excellent comprehensive properties.However,it has many processing procedures,long production cycle,and it is difficult to guarantee the strength and accuracy of formed parts.At present,7075 aluminum alloy profiles have been widely used,facing the need of semi-solid forming of large size billets to shorten the semi-solid processing process.In this paper,the microstructure evolution,semi-Solid deformation and mechanical properties of commercial 7075 aluminum alloy extruded bars during recrystallization partial remelting were studied.Through the research,the following results are obtained:The microstructures of commercial 7075 aluminum alloy extruded bar before and after remelting were studied.The results show that:(1)The extruded bar has typical banded grains which are rapidly elongated along the extrusion direction,and fine second phase particles between the banded grains.However,due to the difference of extrusion deformation,the width of grain in the edge of the bar is obviously narrower than that in the center,and the second phase particles are also smaller;(2)DSC shows that the solid-liquid temperature range of the alloy is 526?645?,and non dendritic solid grains can be obtained in the semi-solid temperature range.With the increase of remelting temperature and holding time,the solid grain size increases significantly,and the holding time has more significant effect on grain coarsening;(3)When the measured remelting temperature is 655? and the holding time is 50 min,the size distribution of solid grains at the top and bottom,edge and center of semi-solid billet is more uniform,and the roundness of grains is also ideal.In order to investigate the thixotropic deformation ability of remelted billets,semi-solid billets were extruded.The results show that:(1)The deformation degree of semi-solid alloy increases obviously when the deformation temperature increases from630? to 655? under the same deformation load;(2)During the forming process of semi-solid alloy,it is easy to induce obvious solid-liquid separation,and spherical droplets are found at the far end of the deformed billet;(3)During the deformation of the semi-solid billet at 655?,the microstructure distribution of the semi-solid billet was more uniform,solid-liquid separation was not found,and the coarse solid grains were further refined.In order to investigate the influence of the microstructure of semi-solid forming parts on the properties,the tensile properties of extruded and solid solution aged samples were analyzed.The results showed that:(1)After the remelting temperature of extruded sample was increased from 630? to 655?,the tensile strength of the extruded sample decreased from 364 Mpa to 300 Mpa,while the elongation increased from 15% to 17%;(2)The mechanical properties of the semi-solid formed parts remelted for 50 min at 655? before and after heat treatment show that the tensile strength and elongation of the semi-solid deformed samples after solution and aging treatment are 317 MPa and 17.3%.The tensile strength increases by about 6%,but the elongation does not decrease.(3)The fracture samples were analyzed,the fracture surface semi-solid formed samples has obvious necking,which is characterized by ductile fracture.The results show that commercial semi-solid billet can be used in semi-solid forming directly.Increasing the liquid phase of semi-solid alloy will reduce the mechanical properties.According to the analysis,the shrinkage of the liquid phase between the solid grains will intensify during the semi-solid deformation process,which can cause the formation of intergranular micro-defects,and then affects its mechanical properties.The results of the phased research will be provided for the semi-solid deformation technology.It is of reference value for new theoretical guidance and application.