| Severe plastic deformation(SPD)can significantly refine the grain size of metals and alloys and significantly improve the strength of materials,while the well-developed grain boundaries of fine grains can improve the coordination deformation ability of materials and enhance plasticity,so the SPD process has become one of the effective methods to prepare metal materials with good overall performance.However,traditional SPD processes generally have the disadvantages of small specimen size,low productivity,and high processing cost,which are difficult to achieve industrial production.Conform continuous extrusion,as an efficient and low-cost process that can introduce intense plastic deformation,has made a major technological breakthrough in the preparation of material size.In this paper,we have developed an extended continuous extrusion intense deformation process by combining the Conform continuous extrusion process and extended extrusion die design,and systematically investigated the evolution of microstructure,mechanical properties and electrical conductivity of Al-Mg-Si alloy before and after the extended continuous extrusion intense deformation process and heat treatment of the alloy.The grain morphology,distribution and dislocation substructure grouping of Al-Mg-Si alloy extended extruded products,and the morphology,distribution and precipitation grouping of precipitation phase of heat treated samples were investigated.The temperature and stress-strain fields of the intense deformation process of extended continuous extrusion were analyzed with Deform-3D finite element simulation software,which revealed the deformation mechanism,grain refinement mechanism,high strength and high conductivity mechanism and inhomogeneous deformation mechanism of extended continuous extrusion,and proposed a new combined deformation process.The results of the paper can provide the theoretical basis and technical support for the commercial application of extended continuous extrusion to produce high-quality aluminum alloy profiles.The main research results of the thesis are as follows:(1)Based on multi-pass bar extended continuous extrusion experiments and microstructure characterization,the metal flow patterns and related mechanisms of Al-Mg-Si alloy bars during severe plastic deformation were investigated.The results show that the small section 6061 aluminum alloy round bar(Φ=10 mm)was continuously extruded through two passes of expansion to prepare the large section round bar(Φ=28 mm).The core grain of the expanded extruded bar(Φ=28 mm)is greatly refined to 5.9 μm in the core region and 7.2 μm in the edge region,and the uniformity of the microstructure is improved.The cooling method(i.e.,water-cooled or air-cooled)has an important effect on the mechanical properties of the extended extruded samples.The air-cooled samples contain a large number of coarse second-phase particles,which significantly reduces the supersaturation solid solution and strength of the alloy matrix.The water-cooled samples possess fine(sub)grains and high density dislocations,resulting in more needle-like β’’ phase precipitation and higher precipitation phase aspect ratio after artificial aging,which increases the tensile strength of the aged alloy samples to 340 MPa,which is nearly 10% stronger and 45 % more plastic than the conventional T6-treated 6061 aluminum alloy round bars.Deform-3D finite element software with The results of the coupled analysis between the Deform-3D finite element software and the cellular automata and recrystallization models show that continuous dynamic recrystalliza tion(CDRX)and geometric dynamic recrystallization(GDRX)promote grain refinement of the alloy.The unique parabolic metal flow along the extrusion direction introduces additional shear strain with a large number of microscopic shear band(MSBs)between its adjacent locations,which promotes CDRX and GDRX,further refining the processed alloy grain size.(2)Based on the single-pass plate extension continuous extrusion experiments and microstructure characterization experiments,the tissue characteristics of Al-Mg-Si alloy continuous casting rod stock and the microstructure distribution pattern of the plate in the width and thickness directions and the related mechanisms were investigated.By introducing the deflector plate structure in the extended die,a large section(width: 140 mm,thickness: 18 mm)of 6013 aluminum alloy continuous casting rod material with small size(Φ=20 mm)was prepared.The results show that the abundant dendrites in the fast-cooled continuous casting rod not only reduce the degree of matrix second-phase segregation,but also lay the foundation for refining the alloy grain size during subsequent deformation.3D-EBSD microstructure results show that parabolic metal flow occurs along the plate width and thickness,and the shear zone formed promotes DRX,which significantly refines the plate grain size(～10 μm).The strength and plasticity of the water-cooled sheets show a non-uniform distribution along the width direction.The presence of a large number of high-density dislocation zones and diffusely distributed nano-scale α(Al Fe Mn Si)particles in the extruded plate tissue,these fine and diffusely distributed nano-phases effectively pinned dislocations and grain boundary movements,thus inducing DRX and refining grain size.the strength of the T5-state treated alloy is lower than that of the T6-state,indicating that the insufficient cooling rate of the bulk plate reduces the matrix supersaturation solid solution and weakens the subsequent age-hardening response.(3)Based on single-pass extended continuous extrusion experiments of conductive rail profiles and microstructure characterization experiments and in-situ resistance experiments,the microstructure distribution pattern of Al-Mg-Si alloy conductive rail sections and the related mechanism were studied.Through the reasonable design of the extension cavity and deflector cavity structure in the extension die,the uniformity of metal flow was improved,and the hydrostatic stress in the narrow position of the cavity was enhanced,and the small diameter(Φ=20 mm)6101 continuous casting rod material was successfully extended and extruded into a large sectional conductive rail profile of 80 mm in width and 38.8 mm in height.The coarse grains(～331 μm)in the 6101 continuous casting aluminum alloy were significantly refined by a large amount of additional shear deformation,and the inhomogeneities in the organization of the cast material were improved.The additional shear deformation introduced by the parabolic metal flow is an important factor to ensure the grain refinement of the conductive rails.The results of the quantitative analysis based on Matthiessen’s criterion show that the main factors affecting the resistivity of the heat-treated alloy are the average length of the precipitated phase,the cross-sectional area,the spacing and the concentration of residual solid solution atoms within the matrix.The hardness of different regions of the conductive rails after aging treatment gradually decreases with increasing temperature and holding time,while t he conductivity gradually increases.multiple groups of heat-treated alloys with T5 treatment achieved high strength and high conductivity(hardness greater than 60 HV and conductivity higher than 58 % IACS),while few T6 treated alloys,indicating that th e solid solution treatment improved the alloy supersaturation and enhanced the subsequent age-hardening response,however,excessive However,too many solid solution atoms back dissolved into the matrix and dislocation annihilation(loss of fast diffusion channels of solid solution atoms)lead to the overall low conductivity of the alloy,which makes it difficult to achieve the high strength and high conductivity index.(4)Based on finite element analysis and microstructure characterization and mechanical property testing,the mechanism of inhomogeneous deformation of the extended continuous extrusion process and its generated macroscopic defects were investigated,and a combination of extended continuous extrusion and drawing process was proposed.The results show that the inhomogeneous distribution of the organization and properties of the extended continuous extrusion products is determined by the cavity structure,and the parabolic metal flow introduced by the extended cavity and the inflow plate not onl y generates additional shear strain γ in the plate width direction,but also shows a heterogeneous distribution of the grain morphology distribution,which affects the strength and plasticity of the alloy.The inhomogeneous flow rate of the metal at the ex it of the extension die and the insufficient hydrostatic stress are the main reasons for the macroscopic defects(cracks)generated in the edge region of the extruded products.The evolution of the grain organization of the alloy prepared by the new cyclic extended continuous extrusion and drawing process indicates that the extended cavity is the key cavity structure for introducing parabolic metal flow.In the extruded specimens,equiaxed grains predominate.In contrast,Elated grains with many cytosolic structures and dislocations were observed in room temperature drawn specimens,and these lattice defects contributed to the promotion of DRX.During extrusion at high temperatures(500 °C),the grain boundary mobility(M)increased significantly,leading to rapid grain growth and enhancing the grain size of the Al-Mg alloy. |
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