| At present,with the increasing demands of energy saving,emission reduction and sustainable development,lightweight automobile body has become the frontier and hotspot of automobile technology in the 21st century.AA6111 aluminum alloy has become the preferred material for automobile lightweight because of its high yield ratio,good corrosion resistance,good formability and machinability.However,with the development of lightweight technology,these fields have higher requirements for the properties of aluminum alloys.In this paper,high performance AA6111 aluminum alloy for automobile body is taken as the research object.Nanoparticles reinforced AA6111aluminum alloy is prepared by in-situ reaction synthesis technology.The rheological stress curve of composites material is obtained by thermal simulation experiment,the constitutive equation and hot processing map of composite material are established,thermal deformation behavior is studied.On this basis,the effects of hot extrusion on the structure and properties of composites are studied.In this paper,ZrB2/AA6111 nanoparticles reinforced Al matrix composites were prepared by in-situ reaction.Then,the rheological behavior of in-situ 2 vol.%ZrB2/AA6111 nanoparticles composites at high temperature was studied by Gleeble-3500 thermal simulator.The rheological stress curves of composites were obtained,deformation constitutive equation of composites was constructed and the hot working diagram of composites was drawn.The results show that there are steady rheological characteristics of Al matrix composites during hot deformation at high temperature.With the decrease of strain rate and the increase of temperature,the rheological stress decreases accordingly.Two typical safety windows suitable for processing can be observed from the hot processing map:area B(470-510°C and 0.13-1.1 s-1)is the best hot working window.Microstructure analysis of ZrB2/AA6111 nanoparticles composites after hot extrusion shows that hot extrusion deformation can effectively eliminate casting defects,the distribution of the second phase and reinforcement particles in the composites after hot extrusion deformation is more uniform than that before deformation.Because of dynamic recrystallization behavior,a large number of fine and regular equiaxed grains are produced after deformation.The recrystallization behavior of the composites is significantly affected by different particle content.With the increase of particle volume,the average grain size of the composites decreases gradually,and the number of recrystallized grains increases obviously.However,the elastic modulus and thermal conductivity of hard particles are different from that of aluminum matrix.When the content of particles exceeds a certain value,the probability of cracks and voids caused by stress concentration and heat transfer unevenness increases significantly near the particles,especially the clusters.When the content of particles is 2 vol.%,the grain size is relatively small and less defects.Different hot extrusion temperatures have significant effects on the recrystallization behavior of the composites.With the increase of temperature,the recrystallization degree increases obviously.When the temperature is 500°C,recrystallization occurs completely,but when the temperature reaches 550°C,the crystalline grains will grow abnormally.The properties of in-situ ZrB2/AA6111 nanoparticles composites after hot extrusion show that the strength of the composites increases with the increase of particle content,and the plasticity first increases and then decreases.When the volume fraction of particles is 2%,the comprehensive properties of the composites are the best.The tensile strength and elongation are 362.6 MPa and 24.3%,respectively.At this time,the formability index n value r is the largest,indicating that the formability of composites is relatively good.The yield ratio(?0.2/?b)is slightly higher than that of composites with low particle content,the bending strength is 426.2 MPa,the bending deformation resistance is larger,but the bending elongation is the largest,which is beneficial to bending forming.The strength and plasticity of the composites first increase and then decrease with the increase of hot extrusion temperature.When the hot extrusion temperature is 500°C,the tensile strength and elongation of the composites reach to a high level.At the optimum temperature,365.3 MPa and 24.5%respectively,and at the same temperature,the yield ratio(?0.2/?b)is the smallest and the formability index n value r is the largest,indicating that the forming properties are the best;the bending strength is 425.59 MPa,the bending deformation resistance is larger,but the bending elongation is the largest,which is conducive to bending forming.Considering the mechanical properties and forming properties,when the particle content is 2 vol.%and the extrusion temperature is 500°C,the composite extrusion material has excellent integrated mechanical properties.The analysis of strengthening and toughening mechanism of composites shows that the strengthening mechanisms include Orowan strengthening,fine grain strengthening and dislocation strengthening,among which fine grain strengthening is the most important.In the process of deformation,ZrB2nanoparticles in composites can hinder the movement of dislocations effectively,promote recrystallization,refine grains and enhance the yield strength of composites. |
