Foaming of A1050 aluminum precursor by generated frictional heat during friction stir processing of steel plate

The foaming of an A1050 precursor was performed using only the frictional heat generated during the traversing of a friction stir processing (FSP) tool on a steel plate. In this study, the experimental conditions during FSP were optimized for relatively high melting point A1050 precursor. For tool traversing speeds of 10 mm/min and 20 mm/min, the A1050 precursor was successfully foamed, where the precursor gradually foamed during the traversing of the tool. However, too much heat was generated, which resulted in excess heat input, in the case of tool traversing speed of 10 mm/min, whereas there was less excess heat input in the case of tool traversing speed of 20 mm/min. Large pores were observed owing to the coalescence of pores, which may have induced the release of gas from the top surface of the Al foam during foaming, decreasing the porosity of the obtained Al foam. In the case of tool traversing speed of 30 mm/min, the latter half of the Al foam was sufficiently foamed and fine pores were observed, although the first half of the precursor was not foamed. Moreover, it was indicated that a uniform temperature distribution during FSP is necessary to obtain uniform pore structures, which may be achieved by gradually increasing the tool traversing speed. The use of a steel plate with lower thermal conductivity was effective for foaming the precursor, which may introduce too much heat into the precursor with less heat diffusion.