| Aluminum matrix composites have the advantages of low density,high specific strength,high specific stiffness,large elastic modulus and so on,and are widely used in aerospace,transportation,microelectronics and other fields.The mechanical properties and thermal stability of the alloy can be further improved by adding hard ceramic particles into the aluminum alloy matrix,and the lightweight effect is almost without loss.Particle reinforced aluminum matrix composites have been widely concerned by scholars at home and abroad,but the related research is mainly focused on the preparation method of the material,and the research on the plastic deformation process of solid billet is less.Extrusion forming is mainly used in continuous profile products with a certain section in the shape.Under the action of the compressive stress from three direction,the internal defects can eliminate in the initial billet,grain structure can be refine,and the mechanical properties and corrosion resistance can be improve,significantly.At present,it is less about the heat flow of particles reinforced aluminum matrix composites change behavior study,so that basic data support is difficult to provide for extrusion forming and other processing technology numerical simulation.Mg element has a great influence on the microstructure and properties of particle reinforced aluminum matrix composites,but the influence of Mg element on extrusion,solution and aging processes is not known.Porthole die extrusion is the most important means to obtain hollow profiles and longitudinal welds are inevitably formed in the process of extrusion.So far,there has been no report on particle reinforced aluminum matrix composites with porthole die extrusion,so it is difficult to judge the feasibility of this kind of materials with porthole die extrusion.To solve above problems,this article took TiB2 particle enhanced 2024 Al alloy composite(TiB2/2024Al)as the research object,through isothermal compression experiment,studied the heat flux change behavior of TiB2/2024Al,constructed the constitutive equation and hot working diagram and contrastively studied under the action of deformation conditions the influence of the microstructure with and without the presence of TiB2 particles.Different Mg content of four kinds of TiB2/2024Al material were designed,the flat die extrusion and solid solution aging heat treatment experiments were carried out and influence law of the TiB2/2024Al organization performance were studied about Mg content on the as-cast,as-extruded and solution-aging states.The porthole die extrusion process was studied by compared TiB2/2024Al with 2024Al,the microstructure of the weld zone and matrix zone of the profile were deeply analyzed and the feasibility of particle reinforced aluminum matrix composites deformed by porthole die extrusion was verified.The main research work of this paper and the conclusions are as follows(1)Based on the isothermal compression experiment,the true stress-strain curve was obtained in the condition of different temperature and strain rate and the heat flow behavior was studied of the TiB2/2024Al composite.Modified arrhenius constitutive model was established with comprehensive consideration of strain rate,deformation temperature and strain compensation and has a high ability to predict the material flow stress.Hot processing diagram was built based on dynamic material model and the parameter selection range(310-340?,0.001-0.03 s-1 and 350-430?,0.1-1 s-1)is better in the hot working production.Compared with 2024Al,the hot compressed TiB2/2024Al sample has smaller grain size,higher misorientation and slightly higher recrystallization degree and the second phase has smaller size and disperses.Under the condition of high strain rate deformation,the deformation grains of TiB2/2024Al contain more abundant subgrains than that of 2024Al,and the lattice defects inside the grains are larger,so the hardness value of TiB2/2024Al is higher than 2024Al.(2)The TiB2/2024Al composites with 3.5wt.%TiB2 particle and 0%,0.8%,1.5%and 2.2%Mg elements were designed,and the extrusion and solution aging heat treatment experiments were carried out under the same process parameters.It was found that the size of precipitates at grain boundaries increased with the increase of Mg content.The presence of a large number of second phase intensifies the grain deformation and torsion,and the storage energy in the grain increases with the increase of Mg content,leading to the abnormal grain growth in the subsequent solution process.When the Mg content is 0%and 0.8%,abnormally large grains exist on the surface of profiles.When the Mg content is 1.5%and 2.2%,all regions of the profiles are occupied by abnormally large grains.A large number of nanoscale Al20Cu2Mn3 phase(T phase)are distributed in the four groups of profiles treated by solution and aging treatment,and the distribution is the most uniform when the Mg content is 1.5%.When no Mg element is added,only ?-Al2Cu strengthened phase is precipitated during aging.After adding Mg,? and S-Al2CuMg phases were precipitated simultaneously,and S phase reached the highest and most uniform distribution when the Mg content was 1.5%.When the Mg content was further increased to 2.2%,locally concentrated S phase variants were generated,and S phase was concentrated along the T phase,and the precipitation near the T phase was reduced,which affected the strengthening effect.With the increase of Mg content,the hardness,ultimate tensile strength and wear resistance of the peak-aged samples gradually increased,and all of them reached the peak value at 1.5%Mg.However,when the Mg content further increased to 2.2%,the mechanical properties decreased.(3)The porthole die extrusion experiment was carried out in 2024 Al and TiB2/2024Al composite with 3.5wt.%TiB2 under the same processing parameters and the extrusion profile has carried on the contrast research.The study found profiles include the fully recrystallized weld zone and the partial recrystallized matrix zone,the grain size of the weld zone and matrix zone was refined for TiB2 particles,and the weld zone width of TiB2/2024Al is bigger than of the 2024 Al.The grain boundary of TiB2/2024Al profile included Al2Cu and Al2CuMg phase at grain boundaries and TiB2 reinforcing particles and the extrusion process refined the grain boundary phase and improved the uniformity of the distribution of second phase.Besides the phase on the grain boundary,the nanoscale Al9Fe0.84Mn2.16Si and TiB2 particles was found in the grain.The TiB2 particles in the weld zone showed hexagonal or equiaxed shape and well combined with the aluminum matrix.In addition,the dislocation density was very low because the grains of weld zone were completely composed of recrystallization.Because of the TiB2 inside the grains of the matrix zone with the low degree of dynamic recrystallization and the serious grain deformation,has an obvious inhibiting effect on dislocation,a large number of dislocations appear in the grains.The ultimate tensile strength of the TiB2/2024Al composite were 255.6 and 263.6 MPa,and the elongation is 18.6%and 19.2%respectively,which indicates that the welding quality of extruded TiB2/2024Al is well controlled,and the mechanical properties are not adversely affected by the addition of particle. |
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