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The aim of present work is study the effcet of Al-Nb-B _2 O _3 -CuO system reaction products on microstructure and properties of A356 alloy. B _2 O _3 powder, Al powder, CuO powder and Nb powder were used as raw materials to form Al–Nb–B _2 O _3 –CuO reaction system, and A356 alloy was refined by in situ reaction and near-liquidus casting. The size, phase, morphology and distribution of in situ reaction products were observed and analysed by optical microscope, x-ray diffractometer, scanning electron microscope and transmission electron microscope. The products of the Al–Nb–B _2 O _3 –CuO reaction system are mainly Al _3 Nb, NbB _2 and Al _2 O _3 . The ratio of the prefabricated block is 8:1:1:2 . When the prefabricated block was added at 2.5 wt%, the particle size of NbB _2 , Al _3 Nb and Al _2 O _3 is 0.5–1 μ m. The microstructure of A356 alloy is fine and uniform. The average grain size is refined to less than 20 μ m. Compared with untreated A356 alloy, the wear rate is reduced from 2.091% to 0.882%, and the average hardness of the alloy is increased from 41.3HRC to 58.2HRC.