Effects of tool rotating rate and pass number on pore structure of A6061 porous aluminum fabricated by using friction stir processing

As a new fabrication method of porous aluminum with the advantages of high productivity and low manufacturing cost, the authors proposed a fabrication method called the "FSP route precursor method". FSP (friction stir processing) is a solid-state process involving the generation of friction heat and intense plastic flow simply by inserting a rotating tool and allowing it to traverse through the aluminum alloy matrix. In this method, the precursor is manufactured by mixing a blowing agent powder and a stabilization agent powder into the aluminum alloy matrix using the intense stirring action of FSP. Then, porous aluminum can be obtained by foaming the precursor under suitable conditions. In this study, aluminum alloy 6061 (A6061) plates are used as a starting material and porous aluminum is fabricated by applying the procedure of multipass FSP. The effects of the tool rotating rate and the number of passes on the porosity and pore structure of A6061 porous aluminum are investigated. To obtain porous aluminum with high porosity and high quality (i.e., a uniform pore size distribution and highly spherical pores), the tool rotating rate should be approximately from 1000 to 2200 rpm when the rotating tool traverses the matrix four times and the foaming conditions (holding temperature and holding time when the blowing agent is foamed) are optimized. The porous aluminum obtained has a porosity of 70%, and the average equivalent diameter of pores is approximately from 1.5 to 2 mm.