Study On Foaming Behavior Of Aluminum Foam In Closed Mould

Aluminum foam is a new type of structural material with many excellent properties such as sound absorption,damping,electromagnetic shielding,etc.Meanwhile,it also has specific mechanical properties,such as high strength-to-weight ratio and stiffness-to-weight ratio,especially its unique response to compression loading of which is the ideal filling material for car bumpers.Since domestic and foreign researches focus on the free-foaming behavior of aluminum foam,it is necessary to conduct in-depth research on the foaming behavior in the closed mold in order to manufacture the near-net shaped part.The foamed aluminum precursors was obtained by the process of cold molding+sintering+hot extrusion,and then encapsulated into a closed mold with fixed volume to be foamed to obtain a foamed aluminum shaped piece.By analyzing the pore structure of AlMg₄Si₈ with various percentages?1.0,0.8,0.6,0.4?w.t%titanium hydride and pure aluminum containing 1.0 wt.%titanium hydride foaming in the closed mold with different volume,the influence mechanism of titanium hydride content,holding time,and metallic elements on the pore shape,pore size and pore profile was revealed.The results show that as the increase of the holding time and continuous release of hydrogen from the foaming agent TiH₂,the AlMg₄Si₈-based foamed aluminum precursor,which is foamed in a closed mold with the volume twice as precursor,has higher the average number of pores inside.Under the constant adjustment of pressure,the area of large pores decreased significantly,and the area of small pores increased until the area of all pores became uniform.The pore shape changed from flat strip-like to ellipse or circular.When foaming in the 3,4,and 5 times volume molds respectively,the average number of pores increases continuously and stabilizes in a certain interval.An increase up in both cells size and occurrence rate of large-sized cells.The shape of the holes transfers from flat,oval to Round and Polygon with smooth edges.When pure aluminum-based foamed aluminum just filled with the closed mold twice as the precursor volume,the cell size distribution is very uneven,and the large holes are mostly in the form of flat strips.Then the pressure balance inside and outside the cell makes the large hole area and quantity decrease,but the whole pores area did not change much,and the large pore shape basically maintained a flat-strip shape.At first,the average pore number showed a trend of rising and then decreasing.For the pure aluminum foamed in the 3,4,and5 times volume molds,the large flat holes that appear when the foam just filled up the mold are rapidly merged as the holding time increases,resulting in an increase in the value of maximum pore area.The large-shaped pores shape switched from flat-strip or oval to irregular or polygonal,and the size of the pores is polarized;the average number of pores is slowly decreasing with uneven boundary.Compared with pure aluminum,AlMg₄Si₈ foam has more uniform cell size and distribution,mainly attributed to the following two reasons:1.The melting point of AlMg₄Si₈ alloy is nearer to the starting temperature of hydrogen hydride release,which greatly improves the utilization of titanium hydride.2.Mg can react with Si and Al to form fine Mg₂Si and a small amount of MgAl₂O₄ particles which are beneficial to the uniformity and stability of pore structure.