Study On The Hot Deformation Behavior And Property Of In-situ?ZrB₂+Al₂O₃? Nanoparticle Reinforced AA6016 Matrix Composites

With the increasing number of vehicles,energy crisis and environmental pollution are becoming more and more serious.Automobile lightweight has become an important way and development strategy of energy saving and emission reduction.AA6016 aluminum alloy,with moderate strength,good formability,good corrosion resistance and other characteristics,has become the first choice of lightweight materials for"replacing steel with aluminum"in automobile lightweight.However,with the rapid development of heavy-duty vehicles,large buses and special vehicles and the application of lightweight technology,the conventional AA6016 aluminum alloy can not meet the demand of its high strength fatigue resistance and impact resistance.The in-situ nano particle reinforced aluminum matrix composite has not only high specific strength and specific stiffness,but also high fatigue resistance and impact resistance,which is an ideal structural material with high strength and toughness.In this paper,nano ZrB₂ and Al₂O₃ particles were introduced into AA6016 aluminum alloy by in-situ synthesis technology to prepare in-situ nano?ZrB₂+Al₂O₃?/AA6016 composite,and the hot deformation behavior,mechanical properties,formability and toughening mechanism were studied.The microstructure of?ZrB₂+Al₂O₃?/AA6016 composite prepared by in-situ synthesis showed that both two kinds of particles were mixed with each other,and the average size of reinforced particles was 65 nm.The high temperature flow behavior of 3 vol.%particles reinforced composite was studied by means of thermal compression simulation,and the flow stress curve was analyzed.It was found that there was a rapid rising stage dominated by work hardening in the early stage of deformation,and then it entered into steady-state rheology,and the flow stress decreased with the increase of temperature and increased with the increase of strain rate,indicating that the composite is a positive stress sensitive material.The constitutive equation of the composite was established by selecting the steady-state rheological value.The optimal processing window of the composite was confirmed by combining the hot process map and the rule of microstructure evolution:Domain B(410?-450?/0.37-1s^-1),which provided the basis for the selection of the following hot extrusion processing parameters.The effect of hot extrusion on the microstructure of?ZrB₂+Al₂O₃?/AA6016 composite showed that hot extrusion can improve the density of the composite,eliminate the casting defects,break the agglomeration and coarse precipitates,improve the structure uniformity of the composite,induce dynamic recrystallization and refine the matrix grains.Further research showed that the content of particle and hot extrusion temperature were the key to dynamic recrystallization.With the increase of particle content,the number of dynamic recrystallization structure increased and the grain size decreased.Among them,the composite with 3 vol.%particles had the most uniform microstructure and the smallest grain,this was because the dislocation energy storage near the particles is high,which can drive the dynamic recrystallization nucleation.The threshold of dynamic recrystallization reduced with the increase of hot extrusion temperature,and the complete dynamic recrystallization took place in the composites with the hot extrusion temperature of 450?.After heat treatment,the dispersed nano Mg₂Si precipitates appeared in the matrix,which made the mechanical properties of the composite increase significantly.The properties of the?ZrB₂+Al₂O₃?/AA6016 composite after hot extrusion showed that the tensile strength and plasticity of the material increased first and then decreased with the increase of the content of reinforcement.The tensile strength and elongation of composites with 3 vol.%particles reached 367MPa and 26.1%,respectively.There was an increase of 29.2%and 10.1%compared with aluminum alloy.It was found that although the yield strength was slightly higher than that of other materials,the formability parameters such as uniform elongation,n value and r value all reached the maximum value,which indicated that the tensile deformation was good.The bending strength of the composite was 721MPa.With the increase of hot extrusion temperature,the tensile strength and plasticity of the material reached the peak value at 450?.The results showed that formability parameters of the composite such as uniform elongation,n value,and r value were the largest at this temperature,and the composite extrusion material had high bending strength and good bending deformation performance compared with other extrusion materials.Considering the mechanical properties,application requirements and formability,the composite with 3 vol.%particles has excellent comprehensive properties after hot extrusion at 450?.The study on the strengthening and toughening mechanism of?ZrB₂+Al₂O₃?/AA6016composite indicated that the improvement of the properties of the composite was the synergistic effect of Orowan strengthening,load transfer strengthening,fine grain strengthening and dislocation strengthening and,and the fine-grained strengthening made the most contribution.This is because in-situ ZrB₂ and Al₂O₃ particles can promote dynamic recrystallization and increase the number of grains and grain boundaries.Moreover,nanoparticles can block dislocation movement,increase dislocation number,and also carry part of the load,reduce the stress on the matrix,so that the strength of the material can be improved.