Microstructure and mechanical properties of cast (Al-Si)/SiCp composites produced by liquid and semisolid double stirring process

A stirring process containing two steps, i.e. liquid and then semisolid sti rring, was used to produce SiC particle reinforced aluminium matrix composi tes. The major advantages of this process are that full wetting of SIC part icles by molten aluminium can be readily achieved at relatively low stirrin g rates, and undesirable Al4C3 is not formed at the Al/SiC interface due to lower processing temperatures, Cast AI-SI matrix composites reinforced wit h 15 and 20 vol.-%SiC particles were produced in the present work. The mech anical properties of the composites were evaluated under the conditions of investment mould casting and heat treatment. For the composites obtained wi thout employing semisolid stirring, the aggregation of SiC particles observ ed in the microstructure of composites resulted in quite poor mechanical pr operties. Observations and analyses indicated that some Al/SiC interfaces w ere very clean, and a reaction product of spinel MgAl2O4 was also found at some Al/SiC interfaces. Silicon dioxide (SiO2) was found to exist on the su rface of as purchased and 250 degrees C dried SiC powders. This SiO2 is inv olved in the spinel reaction at the interface between the SiC particles and the matrix in the present Al/SiC composites.