In order to improve the mechanical properties of Al-Cu-Mn alloy,the microstructure and tensile mechanical properties of Al-6.5Cu-0.5Mn-xCr alloy with different Cr contents in as-cast and heat-treated states were investigated by scanning electron microscope,X-ray diffractometer,energy dispersive instrument and electron universal testing machine.The results show that adding Cr can refine the α-Al dendrite structure.After T6 heat treatment,the coarse Al84.6Cr15.4 and retiform θ-Al2Cu phases disappeared.Many fine and dispersed nano meter θ'-Al2Cu phases are precipitated from the matrix,and Cr can promote the precipitation and refinement of θ'-Al2Cu phases.The tensile strength of both as-cast and heat-treated alloys increases first and then decreases with increased Cr content.The tensile strength of the two alloys is the highest when the mass fraction of Cr content is 0.6％,reaching 191.98 MPa and 373.5 MPa,respectively,15.7％and 20.1％higher than the base alloy.Analysis shows that the improvement of mechanical properties of as-cast alloy are mainly due to the refining effect of Cr on α-Al dendrites and grains,while the improvement of mechanical properties of heat-treated alloy are mainly due to the refinement effect of Cr on nanometer θ'-Al2Cu phase and the increase of precipitation amount.
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