Study On Plastic Deformation And Foaming Mechanism Of Aluminum Foam Curved Member

Aluminum foam sandwich panel（AFS）has high specific strength,high energy absorption capacity,high gas-liquid permeability and low thermal conductivity,which is widely used in many industrial sectors such as automobile and aerospace fields.In order to solve the problem that the traditional process is difficult to manufacture complex surface shape AFS components,the preparation method of aluminum foam friction stir welding was used in this article.The aluminum plate and Ti H₂ agent were used to prepare AFS precursor by friction stir welding.The final shape of AFS was directly determined by thermal molding and foaming process.Compared with the traditional powder metallurgy and hot pressing/hot rolling diffusion bonding process,aluminum alloy sheet instead of aluminum powder can effectively avoid powder agglomeration and crack defects,and improve the density of the preform.Multi-pass friction stir welding can significantly improve the mechanical properties of the preform.The hot forming process significantly improves the forming performance and forming accuracy of the preform,and forms a complex surface structure at high temperature.After hot forming,the preform foamed in the mold.The combination of hot forming and hot foaming process can solve the problems related to the manufacturing of complex surface AFS components.In this thesis,the effects of welding,forming and foaming on the action of friction stir welding to prepare aluminum foam sandwich plate were explored through relevant tests and microstructure and performance tests.The uneven thermal deformation behavior of AFS preform and its influence on foaming performance were also explored.The microstructure,mechanical properties and forming law of AFS preform,and the foaming and energy absorption characteristics of aluminum foam sandwich plate were comprehensively studied.（1）The 7075 aluminum alloy foam sandwich plate was prepared by friction stir welding.The microstructure morphology and powder distribution uniformity of AFS precursor under different welding parameters were analyzed by metallographic microscope and scanning electron microscope.The results show that the AFS precursor with uniform powder mixing and no obvious defects and sustainable expansion can be obtained when the welding speed is 2000 r/min,the welding speed is 50 mm/min,the welding distance is 3 mm.The welding process showed obvious effects of particle breakage,microstructure refinement and uniform distribution.（2）The forming properties of AFS precursor at different temperatures were studied by uniaxial tensile test and high temperature cupping test.With the increase of deformation temperature,the precursor has the characteristics of brittle-ductile transition from brittle fracture to ductile fracture.In uniaxial tensile test,the elongation of preform is only 6.7%at room temperature.With the increase of temperature,the tensile properties improved,reaching the maximum elongation at 450°C,the true strain reached 0.55,and the elongation reached 73%.The cupping test showed the same results.The precursor had the best formability at 450°C,and the cupping value was close to the test value of 7075-O aluminum alloy rolled sheet.The analysis of fracture scanning,electron backscatter diffraction and transmission electron microscopy showed that the crack initiation around Ti H₂ particles was the main fracture failure mechanism.With the increase of deformation temperature,the plastic deformation ability of precursor was significantly improved under the action of grain boundary sliding plastic deformation mechanism and dislocation combination movement and matrix recrystallization.（3）The foaming law of AFS at high temperature was studied through the foaming test of precursor.The results show that the average pore diameter and porosity of AFS have little fluctuation at different temperatures.When the foaming temperature is680°C and the holding time is 225s,the aluminum alloy foam sandwich plate with uniform pore structure and high quality can be prepared.After pre-deformation,the bubble preferentially nucleates along the deformation direction,and the larger the tensile deformation is,the more obvious the expansion of bubble along the tensile direction is.After solution and aging heat treatment,the hardness of AFS pore wall structure was significantly improved,and the pore wall structure had high strength and foam retention.（4）During the compression test,the stress-strain curve of AFS has three stages:the elastic stage,the plastic yield platform stage and the densification stage with a sharp rise in stress.The absorbed energy per unit volume increases with the increase of strain,and the energy absorption efficiency increases first and then decreases with the increase of strain value.After foaming at 680°C for 225 s,AFS has a strong elastic deformation ability,a smooth yield platform area and a high energy absorption capacity.The fluctuation of energy absorption efficiency is small,and the yield strength reaches 641MPa.Its energy absorption efficiency is 0.4-0.6.