Gradient Regulation And Impact Properties Of Density-graded Aluminum Foam

In nature,the cancellous bone of the human body can be adjusted adaptively according to the stress field,thus forming the gradient porous structure with the optimal utilization efficiency of the material.In recent years,it has been proposed to develop foam metal with a density gradient to achieve superior performance.Among them,the precise control of the gradient structure of aluminum foam is critical to optimizing its property.In this paper,continuous density-graded aluminum foams were successfully prepared,and the effects of control parameters,i.e.cooling rate,growth rate,holding time,on the eventual relative density gradient were investigated using experiments and numerical simulation methods.By establishing a numerical model coupling the foam melts growth and solidification,the quantitative prediction of the density gradient was realized.The verification test based on the model prediction structure showed that the numerical model obtained accurate prediction results when the foaming agent content was high or the holding time was limited.Experimental research also found that the microstructures of the aluminum foam cell walls varied with the distance away from the cooling end.In addition,the empirical relationship between material parameters and relative density of aluminum foam was obtained through experiments and data fitting methods.Furthermore,the quasi-static compression model of gradient aluminum foam was presented,which can accurately predict the quasi-static compression stress-strain relationship of gradient aluminum foam.In this paper,the dynamic response of density-graded aluminum foam under high-speed impact was studied by means of experiment,theory and finite element simulation.Hopkinson pressure bar test results showed that when the gradient foam rod positively struck the rigid wall at high-speed(133m/s),constant impact stress(9.2MPa,10.9MPa)can be produced by adjusting the relative density distribution.Under the same impact energy,the positive gradient sample has the lowest peak stress and the most stable transmission stress.Moreover,the Taylor impact theory model was modified to accurately predict the contact force between the gradient foam rod and the rigid wall.Meanwhile,the finite element(FE)simulation was used to numerically deduce the effect of various density gradients on the impact stress of aluminum foam and explore the optimal linear relative density distribution of the aluminum foam rod to achieve constant impact stress at an impact velocity of 300m/s.The mechanical responses of equal mass positive gradient foam rod,uniform foam rod and negative gradient foam rod at the same crushing velocity were also compared.The study found that the plastic deformation of the negative gradient sample is the smallest and the energy absorption efficiency is the highest under the same impact energy.The research also indicated that the impact stress evolution is not only related to the initial relative density of the gradient foam rod,but also related to the impact velocity attenuation process and other factors.