The Influence Of Co And Ni Element On Microstructure And High Temperature Tensile Mechanical Properties Of Al-Mg-Si Alloy

High temperature aluminum alloys with lightweight and high strength,are widely used in aerospace and automobile industry.And with its rapid development,more researchs focus on the improvement of its strength and working temperature by precipitation strengthening.In this study,Co Ni was added i nto 6061 aluminum alloy in order to generate a new precipitated phase and further studied the influence of the Co Ni content on the types as well as the form of precipitated phase.Then,the high temperature tensile test was carried out and the high temperature strengthening mechanism was studied.At last,the microstructure and mechanical properties of the alloy was optimized by ultrasonic remelting and introduction of SiC nanowires.For introducing Co Ni elements,the Ni powder and Co powder were ball-milling and briquetted first,then were introduced into molten aluminum alloy at 800°C and solidified in the way of fast cooling.The solution treatment was for 2h at 510°C and aging treatment was for 8h at 190°C.Al9Co₂ precipitation separate out in the alloy,and with the change of the CoNi content,the precipitation phase morphology and species changed.With the improvement of Co Ni content,the bone shape Al5 Fe Si turned into a ball or block Al9Co₂ phase,and then thin short rod organization appeared.As the Co Ni levels continued to increase,the thick needle Al9Co₂ occurred at the same time during the the Al Ni phase appeaered.The atom ratio of Co and Ni had an influence on precipitated phase form.At 300°C and 350°C,the tensile strength of the alloy improved with Co Ni content increased.Al9Co₂ phase in the alloy was stable at high temperature,and had a good interface bonding with matrix.with the higher content,the effect on hindering the dislocation and grain boundary migration was more significant.At 400°C,because of the weaker interface bonding with matrix and softening matrix,it was hard for precipitated phase to hinder the dislocation and grain boundary moving,and the strength was not improved.In the process of fracture,the long rod precipitated phase also have the effect of crack deflection and crack passivation,improving the strength of alloy.Ultrasonic remelting can change the morphology and distribution of the grain and precipitated phase.After ultrasonic remelting,the size of grain decreased and slender rod Al9Co₂ phase in the alloy increased.Because the precipitation phase distribution was more diffuse,the area of interface was the larger which enhanced the interface bonding,and at 400°C the alloy strength was improved.After adding Si C nanowires into the alloy the precipitation phase was in a smaller size.Si C nanowires existed in the matrix and precipitate phase,and had the effect of bridging fiber.The Si C nanowires enhanced the interface strength and the matrix strength.Precipitated phase at high temperature can still have the effect of hindering the dislocation and grain boundary migration.At 300°C the strength of the alloy with 2wt.% Co and 2wt.% Ni was 215 MPa,and it was 124 MPa at 400°C.