2018 Volume 59 Issue 3 Pages 475-481
Friction stir processing (FSP) was applied for the fabrication of carbon fiber (CF) reinforced magnesium (Mg) alloy, AZ91, metal matrix composite (MMC). The CFs were recycled ones extracted from long CF reinforced plastics. A narrow slit was introduced into AZ91 plate and then filled with the chopped CFs with the length about 1 mm. Subsequently, the rotating tool was plunged and traveled along the slit to disperse CFs into the matrix. Two kinds of FSP tools, namely conventional threaded pin tool and 3-flat pin tool were used. Microstructural observation revealed that chopped CFs were broken up into fine ones with the length less than 20 μm by severe stirring of material, and were dispersed in the stir zone (SZ). The 3-flat pin tool reduced the size and number of defects in the SZ compared with the conventional pin tool. The hardness of the SZ was higher than that of the as-cast material and FSPed one without CFs. Axial fatigue tests were conducted using the MMCs fabricated by the 3-flat pin tool to investigate fatigue properties. Fatigue strengths of the MMCs were comparable with those of the as-cast specimens, but lower than those of the FSPed ones without CFs. Fatigue cracks initiated at the agglomerations of CFs in the MMCs. The lower fatigue strengths of MMCs were attributed to the lower fatigue crack initiation resistance resulting from the inhomogeneous distribution of CFs.
