Effect Of Heat Treatment Of Precursor On Foaming Characteristics,pore Structure And Mechanical Properties Of Aluminum Foam

Aluminum foam,as a new material integrating structure and function,is widely used in aerospace,rail transit,military and other fields due to its unique structure and functional characteristics.There are many methods for preparing aluminum foam,among which powder metallurgical route is widely used because of its easy control of alloy composition,good repeatability,and the ability to prepare near-net-shaped aluminum foam products and composite components with high quality.However,in powder metallurgical route,the complex method of optimizing pore structure and the narrow process window for the preparation of aluminum foam restrict its development in industrial production.Therefore,a simple and efficient heat treatment process of precursor has been proposed to improve foaming stability,widen the process window for preparing aluminum foam,optimize the cellular structure and improve the mechanical properties of aluminum foam.In this paper,AlSi7Cu4 foamable precursors were prepared by powder metallurgical route.The foaming characteristics of precursors can be improved by precursor heat treatment.The pore structure,microstructure and foaming characteristics of precursors before and after heat treatment were studied.The pore structure and compressive properties of the aluminum foams prepared by precursors heat-treated under different conditions were analyzed and compared.The following research results are obtained:(1)Precursor heat treatment can expand precursors,create pores inside precursors,increase the oxygen content of precursors,and change the decomposition characteristics of TiH2.The pores generated in the heat-treated precursors include open and closed pores,and the proportion of open pores in the later stage of precursor heat treatment exceeds 50%.Oxidation occurs on the surface of open pores during precursor heat treatment.With the prolongation of heat treatment time,the oxygen content in precursors increases gradually and then tends to 0.34 wt.%.During precursor heat treatment,TiH2 decomposes,and a shell layer containing Ti,Al,and Si elements is formed on its surface,which can hinder the decomposition and increase the decomposition temperature of TiH2.(2)Precursor heat treatment can optimize the pore structure of aluminum foams at the initial stage of foaming and improve the foaming stability.In the initial stage of foaming,the decomposition temperature of TiH2 increases after heat treatment to reduce the generation of H2.On the other hand,a large number of open pores in precursors heattreated are prone to overflow of H2.Therefore,the excessive growth of pores and the formation of irregular pores due to excessive hydrogen pressure can be avoided,thereby optimizing the foamed aluminum pore structure at the initial stage of foaming.The decomposition rate of TiH2 decreases after precursor heat treatment,which results in a lower expansion rate of the heat-treated precursors during foaming.In addition,the introduction of appropriate oxides is beneficial to slow down drainage and avoid rapid collapse.Therefore,the heat-treated precursors exhibit more stable foaming characteristics.The excellent pore structure at the initial stage of foaming and stable foaming characteristics of the heat-treated precursors together optimize the pore structure of aluminum foams in the whole foaming process and broaden the process window for preparing aluminum foams with excellent pore structure.(3)The results show that heat treatment at 500? for 6 h is the best heat treatment process for AlSi7Cu4 precursors.The precursors heat-treated at 500? for 6 h have excellent pore structure at the initial stage of foaming and stable foaming characteristics and can produce aluminum foams with a pore diameter of 0.4?1 mm and a sphericity of about 1.1.Precursor heat treatment provides a new process route for preparing aluminum foams with small pores diameter and excellent sphericity.(4)Under the same relative density,compared with as-received precursors,the aluminum foams prepared by the precursors heat-treated at 500? for 6 h and 520? for 1 h have higher compressive strength,which is due to more excellent pore structure and fewer pore wall defects of aluminum foams.When the relative density of aluminum foams is lower than 0.21,the aluminum foams prepared by heat-treated precursors show higher densification strain energy.However,when the relative density of aluminum foams is higher than 0.21,the densification strain energy of aluminum foams is not sensitive to pore structure.