Study On Powder Metallurgy Foaming Process And Mechanism Of Aluminum Foam

Aluminum foams are kind of new style structural and functional composites,having the advantages of lightweight,high specific stiffness,great energy absorption and good electromagnetic shielding and damping etc.They have a broad application prospect in the automobile manufacturing,aerospace,military defense,construction and transportation industries.Among these preparation methods of aluminum foams,powder metallurgy route has become the hot spot of domestic and foreign scientific research institutions,due to the ability to produce near-net shape aluminum foam components and sandwich panels.At present,domestic and foreign research on preparation of aluminum foams made by powder metallurgy route is not systematically,especially the foaming mechanism is less rarely seen.With the funding of The National Natural Science Foundation of China "an investigation of powder metallurgy foaming mechanism of aluminum foam based on synchrotron radiation-CT."This paper launched a research on the powder metallurgy preparation technology?foam structure evolution and mechanical properties of aluminum foam.In this paper,air-atomized Al powder and titanium hydride(TiH2)powder as the foaming agent were used as the basic materials.Mg powder was used as the metal additive.Al-based foams and AlSi-based sandwich panels were by uniform blending?uni-axial cold compaction or pack rolling process? foaming process.Systematical researches were performed for the preparation process and defect control,successfully reconstructing the image of initial bubble cell evolution by using Micro-CT.Discussing the mechanism of powder metallurgy combine with the theoretical analysis.The valuable results obtained from this paper are as follows.(1).Using the stainless material 1Crl8Ni9Ti as Al-based foams foaming mould,can achieve the rapid heating-up of precursor.The manner of thermal conduction combined thermal conduction with heat radiation.Compared with conventional furnace heating,Foaming time reduce from 15min to 2min,expansion rate can reach 500%.(2).The appropriate technological parameter is summed through a large number of experiments:,pretreatment temperature of TiH2 is 500??heat preservation for 1h;in mixed powder,the addition of TiH2 powder is 1.0wt.%,the addition of Mg powder is 1.0wt.%,the rest is aluminum powder.;Under the condition of the lubricated mould,uni-axial cold compaction with 400MPa pressure can obtain the dense Al-based precursors;Al-based precursors are pre-heating up to 200?,foaming temperature is 770 ?,foaming time is 120s,the high quality aluminum foam material can be obtained under these technological parameters,which cell is uniform and expansion rate is higher.(3).The quasi-static compression deformation of aluminum foam is divided into three stages:linear elastic stage,platform stage,densification stage.Apparent density is the main factor that affects the compression of aluminum foam,the compressive strength and compaction strength increase along with apparent density growth.(4).Foaming process of aluminum foam is divided into four stages:nucleating?growing?merging and fracturing.In the initial sage,cell structure is irregular,cell will grow in 120s.The crack of the cell began to form along the transverse direction.With time increasing,the aluminum melt viscosity is reducing,cell will grow with the longitudinal direction,the shape transform into near circular and polygon.(5).The image reconstruction of pack rolling-powder metallurgy aluminum foam sandwich of the inner cell microstructure is realized by Micro-CT.Acquired the 2D tomography and 3D reconstructed images of cross section and longitudinal section in the initial foaming stage.Through analyzing the distribution of the cell and the porosity,we can found that the growing process of cell is divided into three stages:nucleating?growing and merging.The porosity increase greatly contains two stages,one of them is growing,another one is merging in large range.