Study On Gradient Pores And Microchannels In Porous Aluminum Prepared By Space Holder Method

Porous aluminum has been used in many fields because of its low relative density,high specific strength and high specific surface area.The method of Na Cl,with high hardness,high melting point,and easily soluble in water,as space holders combined with powder metallurgy,has become a vital fabrication process of porous aluminum due to its simple process and accurate control of pore morphology,size,distribution,and porosity.The functional characteristics of porous materials are the material or energy transferred in the channel connected by pores and contacted with pore wall.The use of gradient pores and micro-channels can improve the transfer and contact efficiency of matter or energy in porous materials.Firstly,the influencing factors of pores and microchannels were studied.According to the material fabrication characteristics,the influence of two sets of technological processes and five kinds of process parameters were studied.The dissolution rate of Na Cl particles was used to characterize the state of micro-channels indirectly and wholly and combined with the traditional direct and regional characterization methods to conduct a comprehensive study.It was found that there were three types of micro-channels in the fabrication process:gap-micro-channel,crack-micro-channel,and reticulation-micro-channel.The three types of micro-channels have significant differences in size and quantity and present various states in different processes.Moreover,the participation of Na Cl in sintering inhibited the formation of reticulation-micro-channels but promoted the formation and development of gap-micro-channels and crack-micro-channels.The Na Cl particle content and size,sintering temperature,and compaction pressure can affect the micro-channels,but the sintering time cannot.Based on these results,the gradient pore was studied,and the macro structure was optimized.When using Na Cl particles as a space holder to prepare gradient porous aluminum,there are some phenomena in the large particle size area,such as uneven pore distribution,low through-hole rate,interlayer crack and so on.The interlayer comparative study found that large Na Cl particles were difficult to coordinate the compaction energy fully and form an excessively concentrated distribution,which further affected the volume expansion in the sintering process.Combining the previous experience and the characteristics of similar structures in biology,the solution and optimization concept of the combination of interlayer gradient and intralayer gradient are proposed for the first time.Porous aluminum with the intralayer gradient can be prepared by adding the small space holder to the large space holder.Many problems caused by large Na Cl particles in the process of compaction and sintering can be solved by this method and its structure.In addition,this structure can also improve the transfer and contact efficiency of material or energy in the large pore area.Then,the effect of alloy reaction on gradient pores and microchannels was studied.It was difficult to get the advantages of the reactive synthesis with space holder method in preparing porous aluminum.Explored a new method that can combine the advantages of both through Ti Al₃-Al porous fabricated by controlling titanium content.Moreover,the composition,structure and mechanical properties of different titanium content in this method were further studied.The reaction intensity of titanium and aluminum was effectively reduced by reducing the titanium content and sintering in the air.The pore wall morphology,the in-situ generated small pores were retained to a certain extent,and the product was only Ti Al₃.A series of complex changes caused by titanium content not only led to morphological changes and matrix transformation but also affected the mechanical properties of porous aluminum.The change in microstructure and mechanical properties in different titanium content stages was very large,which is conducive to enrich the design of structure and function of porous aluminum.Finally,aiming at many problems existing in alloy reaction,the reaction process of alloy elements was controlled to solve them,and the microstructure of porous aluminum was optimized.When Ti Al₃ intermetallics were fabricated by the reactive synthesis of titanium and aluminum powder,the correlation between the intensity and content of the reaction was stable,which is not conducive to the structural design of the material.The magnesium-addition method with the addition of trace magnesium broke this correlation for the first time.Magnesium controlled the reaction process in three stages in different ways:the initial melting and reaction area,the dynamic equilibrium formed by the diffusion rate consistent with the liquidus and the high flowability eutectic liquid phase.The existence of magnesium only affected the process and did not change the products.The diffusion state of magnesium in each stage and the eutectic liquid phase formed with aluminum were the critical factors.By adding magnesium,the reaction process can be changed to adjust the pore structure and increase the designability of the material.