Research On Microstructure,property And Deformation Mechanism Of Al-Mg-Si Alloys After Severe Plastic Deformation Based On Continuous Extrusion Forming

Al-Mg-Si alloys have attracted much attention in the automotive,aircraft,construction industries and electrical engineering due to their light weight,good corrosion property,excellent strength and high conductivity.Severe plastic deformation(SPD)can effective refine grain size to sub-micrometer or even nanometer scale,thus significantly improving the mechanical properties of Al-Mg-Si alloys.Nowadays numerous SPD techniques were performed on Al-Mg-Si alloys and obtain high strength Al-Mg-Si alloys.But there are still a lot of problems for their limited industrial application,such as small sample size and scale,low production efficiency,low material utilization,high cost,etc.Thus the main objective of this study is to explore a novel continuous SPD technique for industrial application.This work was supported by the National Natural Science Foundation of China(grant number 51671083).Microstructural evolution and mechanical properties of Al-Mg-Si alloys during continuous extrusion forming(Conform),repetitive Conform(R-Conform),high pressure torsion(HPT)and high pressure torsion extrusion(HPTE)process and followed by aging treatment were studied.Systematic studies of the relationship between microstructure,such as grain,dislocation,precipitation,etc.,and mechanical property were carried out.Particularly,the strengthening-toughening mechanisms of processed Al-Mg-Si alloys were to be revealed.The key results were as followed:(1)R-Conform process effectively refined and homogenized the microstructure of Al-Mg-Si alloys,thus improving its strength and ductility.With the increasing of R-Conform pass,due to severe shear deformation grain size gradually decreased and dislocation density gradually increased.Many initial large lath-shaped second phases were severely broken,refined and finally tended to be spherical with the increasing of R-Conform pass.Continuous in-line solution treatment led to continuous formation of supersaturated solid solution.The tensile strength and elongation of Al-Mg-Si alloys simultaneously increased with the increasing of R-Conform pass.After seven R-Conform passes,the tensile strength of Al-Mg-Si alloys was slight higher than that of initial continuous rolling and casting Al-Mg-Si alloys(from 216.2 to 220.3 MPa),but the elongation significantly improved,which was a 74.3%increase(from 18.54 to 32.31%).(2)R-Conform process significantly influenced the precipitation behavior of Al-Mg-Si alloys during subsequent aging treatment,further improving the mechanical property of Al-Mg-Si alloys.After T5 tempering,the?"precipitates in R-Conform deformed Al-Mg-Si alloys were fine and homogeneous distributed.The tensile strength and elongation of Al-Mg-Si alloys after seven R-Conform passes and T5 tempering were 270.9 MPa and 18.83%,respectively.Compared with initial sample without Conform processing and after T5 tempering,an increase of tensile strength of?18.9%and elongation of?23.9%were achieved.While after seven R-Conform passes and T8 tempering,numerous dispersive nano-scale?"precipitates with average length of 2.5 nm were distributed uniformly in the Al-Mg-Si alloys.The length of?"precipitates was much shorter than that of initial sample without Conform processing after T8 tempering.And after seven R-Conform passes and T8tempering,more refined and homogeneous subgrains were obtained with an elongated morphology.There were still high densities of dislocations in the processed Al-Mg-Si alloys.With the combined effects of refined subgrains,high densities of dislocations and large number of nano-scale precipitates,coexisting superior strength(345 MPa)and high elongation(7.6%)were obtained after 7R-Conform passes and T8 tempering.(3)HPTE further vastly improved the hardness of Conform deformed Al-Mg-Si alloys.The hardness of Al-Mg-Si alloys processed after HPTE was gradient distributed,which gradually increased from sample center to edge.And hardness increased with the increasing of HPTE strain,when the strain is 8.1,the hardness reached about 124.5 Hv,which was a 137.0%increase compared with that of Conform deformed sample.The significant hardness improvement was attributed to grain refinement.When the HPTE strain was 8.1,average grain size of Al-Mg-Si alloys was about 206 nm.The relationship between yield strength and grain size of Al-Mg-Si alloys strickly followed the Hall-Petch formula,and the yield strength was given by?(?),where D is grain size.(4)Post-deformation aging has an inverse strengthening phenomenon,strength increment or strength decrement for the severely deformed Al-Mg-Si alloys.It mainly depends on the deformation degree,which can be judged by equivalent strain.When equivalent strain was lower than 5,the strength of Al-Mg-Si alloys can be further improved by low-temperature aging treatment with the grain boundary and precipitation strengthening effect.And with the increasing of equivalent strain,the precipitation strengthening gradually weakened.When the equivalent strain was larger than 5,further aging inversely decreased the strength of Al-Mg-Si alloys,which indicated an intensive plastic deformation induced strengthening saturation phenomenon.The strength degeneration was attributed to the predominant softening effect of grain growth,dislocation annihilation and solute depletion over the strengthening effect of precipitation.When sample was severely deformed at higher temperature,the critical value of equivalent strain increased.(5)Two novel continuous severe plastic deformation methods were proposed using the finite element method,which were called Conform-HPTE and Conform-Twist Extrusion(Conform-TE),respectively.These two novel continuous SPD methods can be realized and used for industrial SPD applications with the advantages of energy saving,high process efficiency,no limitation for billet length and large shear deformation during single pass operation.