Flowing Properties Of Aluminum Melt Foam And Its Influence On The Pore Structure

The closed-cell aluminum foam is light-weight and multifunctional,and has achieved applications in high-tech fields such as high-speed trains,manned aerospace,and defense industries.In the process of fabricating aluminum foam,the aluminum melt foam is the key intermediate state,and its pore structure is conserved through solidification,which influences the properties of the final product greatly.The flow behavior of melt foam affects the evolution of the pore structure,and the pore structure also impacts the rheological properties of melt foam.Therefore,studying the rheological properties of melt foam is of great significance to the development of foam aluminum preparation technology,as well as the pore structure manipulation and material performance optimization.To study the flow characteristics of the melt foam,a rheological testing device was developed.The apparent viscosity and yield stress of the melt foam were measured with the device.The experimental results showed that the apparent viscosity of foam first increased and then decreased with the increase of bulk porosity under low shear rate conditions,and the peak value showed when the porosity was around 0.67.Under high shear rate conditions,the influence of porosity on the apparent viscosity decreased evidently.With the increase of shear rate,the apparent viscosity of the foam decreased,showing a significant shear thinning effect.The foam yield stress increased first and then decreased with the increase of porosity,and the peak value showed when porosity was around 0.70.The pore morphology of the sample obtained by the solidification of the sheared melt foam showed that(1)the shear field in the melt foam was mainly around the surface of the equivalent cylinder formed by the blade rotation;(2)depending on the strain rate and porosity,the shear field may cause bubbles in the melt foam to merge or split.This paper also used the free surface lattice Boltzmann method to establish a numerical model of the melt foam,so as to investigate the growth,yielding,shear flow,and bubble merger/split phenomena of the melt foam.The yield stress and apparent viscosity coefficient predicted by the model were consistent with the measured values.The calculation results showed that:(1)the yield of foam was realized through the rearrangement of pore structure;without the mechanism of film rupture,the yield stress was positively related to the porosity;(2)under the steady shear condition,bubbles near the moving boundary kept rearranging and spontaneously formed a layer-like arrangement,while the pore structure distant from the moving boundary remained almost unchanged and immobile(ie,shear bands appeared);(3)the thin liquid films in the high-porosity foam were prone to rupture when shear loads were applied,and the rupture of a liquid film in the shear filed may cause avalanche-like film ruptures nearby;(4)under high-speed shear conditions,the bubbles of the low-porosity foam near the shear surface could be stretched and even split.The experimental and numerical results all showed that the intervention in the melt foaming stage may realize the adjustment of the size,shape,and orientation of the pore structure,thereby making it possible to optimize the mechanical properties of the aluminum foam through pore structure manipulation.This paper developed a sequential solidification method to progressively lock the growth and flow behavior in the melt foam,so as to obtain unidirectional graded aluminum foam,and the quasi-static and dynamic compression tests were conducted.It was found that the dynamic energy absorption performance of the graded foam was better than that of the equal-weight uniform foam.Utilizing the bionic design idea,this paper designed a radial gradiant in aluminum foam according to the strain distribution characteristics of foam filler in metal tube against compression.By adjusting the cooling boundary conditions of the sequential solidification method,this paper successfully prepared the radial graded aluminum foam and the foam-filled metal tubes.The mechanical test results showed that the radial graded foamfilled tubes were of higher specific energy absorption(SEA)than the uniform counterparts with similar weights.