Study On The Formation Of Core Foam In Thin-wall Aluminum Foam Sandwich Structure

Aluminum foam is a new kind of structural and functional engineering material emerging in recent decades.Because of its special porous structure,it has the characteristics of both metal and porous materials.So with a variety of excellent performance,widely used in the automotive industry,rail transit and national defense industry and other related fields.At present,scholars at home and abroad focus on the free foaming behavior of aluminum foam,but there are few researches on the foaming behavior of aluminum foam in closed cavity.The purpose of this study is to deeply study the foaming behavior of aluminum foam in the closed cavity and lay an important theoretical and technical foundation for the preparation of nearly net forming aluminum foam special-shaped parts and the further promotion of the production and application of aluminum foam.In this experiment,pure aluminum based aluminum foam preforms with 0.6%,0.8%and 1%TiH2 content were prepared by powder metallurgy process,and then the preforms were sealed in a cavity with a fixed volume(2,3 and 4 times of their own volume)and foamed to prepare aluminum foam.The foaming behavior of pure aluminum based aluminum foam in closed cavity was studied by the content of titanium hydride.The evolution process of the bubble structure of aluminum foam preform in closed cavity was explored,and the influence of technological parameters such as the content of titanium hydride and holding time on the stability and homogenization of the bubble structure was investigated.The experimental results show that the pure aluminum base aluminum foam preform foams in the closed mold cavity.With the increase of holding time and the content of foaming agent,the foaming agent titanium hydride continues to release gas during the hydrogen release process,and the bubble pores begin to nucleate and grow,and the average number of pores in the bubble body gradually increases.Under the action of extrusion pressure between the bubbles in the bubble body,the roundness of the bubbles increases obviously,the number of the bubbles in the large pore size decreases,the number of the bubbles in the small pore size increases,the bubbles change from irregular to round,the defects of the bubbles decrease obviously,and the foaming formability is improved.However,when the holding time is too long,the average number of bubble holes continues to decrease,the round degree of bubble holes decreases,the melt viscosity decreases,the collapse phenomenon and the increase of solid aluminum layer due to the gravity drainage effect,the bubble hole defects begin to increase,and the foam forming effect is not good.The foaming behavior of aluminum foam preforms and the complete hydrogen release of the foaming agent titanium hydride are favorable with the increase of the volume of the closed cavity and the content of titanium hydride.The bubble body continuously foamed,the size of the bubble pore increased,the distribution of the bubble pore was more uniform,and the shape of the bubble pore was nearly round.As the roundness of the hole increases gradually,the wall of the hole becomes thinner,and the small vesicles merge to form large vesicles.In the closed cavity of 4 times volume,the number of bubbles begins to decrease,and the phenomenon of real aluminum layer appears.Aluminum foam preforms with diameters of 8.5mm and 6.5mm respectively,the smaller the diameter of the preforms,the larger the specific surface area,more heat absorption and faster heat absorption rate,and the preforms have priority in foaming nucleation and growth.However,the foaming effect of aluminum foam preform with diameter of 6.5mm is not as good as that of aluminum foam preform with diameter of 8.5mm.