闭孔泡沫铝的孔结构控制

为适应高技术应用中超轻闭孔泡沫铝对孔结构更高的控制要求，对过去十余年中进行的相关研究工作进行了系统整理，发现了胞体尺寸、孔隙率和孔形貌三者之间的内在关系，确定了孔结构控制的关键步骤和相互关系，建立了熔体发泡法影响孔结构控制的工艺技术框架，结合该框架系统论述了3个关键孔结构控制参数（胞体尺寸、孔隙率和孔形貌）与实际应用、制备技术、工艺过程的关系．研究表明，孔结构演变和制备技术工艺中黏度、发泡时间、凝固方式等诸多因素相互影响，互为因果，使得发泡和凝固过程变得十分复杂，给孔结构控制带来困难．要获得高孔隙率泡沫铝，对于纯铝泡沫，不仅需要根据熔体泡沫化时间与孔隙率的对应关系精确控制孔隙率，而且要在孔隙率一时间平台段适时凝固以控制孔径和均匀性，而对于泡沫铝合金，还需要采用多向凝固模式，克服凝固过程中固一液两相区的附加力场引发的收缩问题．对于新型球形孔泡沫铝合金，则需要进一步控制适量的发泡剂（1．0％）和发泡搅拌时间（100s），使平台段降至低孔隙率阶段．面对高技术领域新的需求，提出的二次发泡法较其他技术在制备异型件方面具有更大的优势，并且其延伸发展技术在多功能大型面板开发上具有进一步发展的潜质．

Control of Pore Morphology in Close-Celled Aluminum Foams

Recent high-tech applications of ultralightweight close-celled aluminum foams require a tighter control of their pore morphologies.To this end,the progress made during the past ten years in this field is systematically summarized.Inherent relationships amongst pore size,porosity and pore shape are explored,key steps of pore structure control and their cross-relations are determined,and a general framework of processing steps and techniques affecting pore structure control during the foaming process of molten aluminum is established.Built upon this framework,the relationships of three key control parameters(pore size,porosity,pore shape) with industrial applications,fabrication technologies and processing methods are systematically addressed.It is demonstrated that the evolution of cellular structure is influenced by many processing factors such as viscosity,foaming time and solidification mode.This makes the foaming and solidification processes extremely complicated,leading to difficulties in pore structure control.To obtain pure aluminum foams having high porosity,it needs not only to precisely control the porosity making use of the relationship between porosity and foaming time,but also to solidify in time during the plateau stage on the porosity-time curve to control the pore size and uniformity of pore distribution.To obtain aluminum alloy foams,in addition to the steps mentioned above,multi-directional solidification is also needed in order to overcome the shrinkage caused by the additional force field during cooling.For high strength aluminum alloy foams having spherical pores and relatively low porosities,a suitable amount of vesicant (about 1.0%) and proper stirring time(about 100 seconds) are required in order to drop the plateau stage of porosity-time curve to low porosity regime.Faced with new high-tech needs,the novel method of two steps foaming has more advantages and potentials over other technologies to fabricate unconventionally shaped components and large-sized multifunctional panels.