Research On Fabrication Of Ultra-fine Grained Al7075 Alloy Via Cryogenic Temperature Extrusion Machining

The 7 ××× series Al alloy is widely used in aerospace,building materials,medical devices and other fields because of its excellent mechanical and corrosion resistance.7 ××× series Al alloy can be processed into ultra-fine grained materials through SPD methods,and its strength can be further improved to meet the industrial demands.Extrusion machining(EM)is a rising SPD method in recent years,which makes the material experience the double effects of extrusion and shear simultaneously,and finally produces ultra-fine grained strips with regular shape.In the process of room temperature machining,large amounts of heat will be produced,which will anneal the chips,resulting in damage of mechanical properties.In addition,7 ××× series Al alloy is difficult to deform and process at normal temperature.In order to overcome the above defects,a new process,cryogenic temperature extrusion cutting(CT-EM),was proposed in this paper.This article starts with the theory of metal cutting,based on experimental research,combined with numerical simulation technology,and takes 7075 All alloy as the research object.The forming mechanism,grain size prediction,microstructure evolution,mechanical property optimization,aging behavior,and thermal stability of ultra-fine grained chips in CT-EM process are systematically studied.The main research achievements are as follows:The coupled thermal-mechanical finite element model of CT-EM was established,and the microstructure prediction model based on dislocation density constitutive equation was coupled to realize the prediction of chip grain size.The flow net method was used to simnulate the deformation process of the material in the EM process,which was divided into three stages inclding the initial entry,the chip formation and the steady deframation.The influence of compression ratio and tool rake angle on cutting area parameters such as equivalent strain,strain rate,cutting temperature and cutting force was studied.The distribution of grain size in chip was predicted,and the simulation results are in good agreement with the experimental results.It can provide early guidance for the selection of parameters and the optimization of microstructure.The influence mechanism of processing temperature on chip morphology was analyzed from the aspects of macro-morphology and microstructure.The results show that the chips prepared by CT-EM are continuous,the surface morphology is good,and the chip forming is more sufficient,while the chips of RT-EM are discontinuous and segmented,and there are many cracks on the surface.It is proved that the chip forming of 7075 aluminum alloy is temperature dependent.Low temperature treatment can inhibit the dynamic recovery and cluster structure in the material,thus increasing the dislocation accumulation ability,improving the work hardening ability and ductility of the material.The process of optimizing chip structure and mechanical properties by CT-EM combined with artificial aging was put forward.The mechanisms of chip structure evolution and strengthening were systematically studied.The grain size,dislocation density and the second phase transformation of the chip under different cutting parameters were analyzed by various microstructure characterization techniques,and the hardness and strength of the chip were tested by Vickers hardness tester and universal material testing machine respectively.The artificial aging treatment was carried out in order to optimize the microstructure and improve the mechanical properties of ultra-fine grained chips.The results showed that the microstructure of EM chips was greatly refined,the grain size decreased with the increase of compression ratio,the hardness increased with the increase of compression ratio,and the mechanical properties of CT-EM samples were better than that of RT-EM samples.The mechanical properties of CT-EM samples reached the peak aged hardness at 120 ?,20 h.The GP zone and ? ? phase played a major role in strengthening at this stage.Cryogenic treatment can accelerate the precipitation kinetics of second phases;the fracture mode of chip samples is the mixture of ductile fracture and brittle fracture.The thermal stability of CT-EM chips at different annealing temperatures was explored.The CT-EM chips were annealed at 150-400 ? for 1 h to study the grain growth mechanism,the effect of secondary phases on recrystallization,the relationship between recrystallization fraction and hardness.The results showed that with the increase of annealing temperature,the chip grain size increased,and dislocation density decreased,The secondary strengthening phases underwent the process from precipitation to re-melting into the matrix,in addition,the pinning effect of second phases on grain boundry dimished,the recrystallization fraction increased.After annealing at 350 ?,the chip microstructure has completely recrystallized.When the chip was annealed at 400 ?,its grain size jumped from sub-micron level to micron level,and the thermal stability was lost.Therefore,it can be determined that the CT-EM chip has good thermal stability at ? 350 ?.