| High-strength,lightweight structural metal materials are inevitable requirements for modern industrial development.Thereinto,Al-Si alloys are widely used in the automotive industry,aerospace and other relative fields due to their high-specific strength,excellent casting properties and good corrosion resistance.In recent years,the development of age-hardening Al-Si-Cu alloys based on microalloying and heat treatment optimization has been increasingly researched.However,the heat treatment mostly adopts the "solid solution + water quenching" method,which leads to severe thermal distortion of thin-walled castings and it is difficult to be corrected by downstream processing.In addition,the age-hardening Al-Si-Cu alloys are difficult to nucleate due to the difference in crystal structure between the precipitates and the α-Al matrix,and require a long aging treatment to reach the peak-aged mechanical properties,which is not compatible with the short process and high efficiency required for industrial applications.The use of slow cooling(air cooling,wind cooling)after solid solution can effectively solve the casting deformation problem.However,there is little research on the effect of quenching rates on the microstructures and properties of Al-Si alloys,and further in-depth systematic research is needed.In addition,the effect of microalloyed Sn elements on the age-hardening response of Al-Si-Cu alloys is still unclear,and there is an urgent need to explore the mechanism of Sn elements on the precipitation behavior of Al-Si-Cu alloys.To address the above issues,two age-hardening alloys(Al-10.50Si-1.50Cu-0.20Mn-0.50Mg-0.03B-0.30 Sb,Al-10.50Si-1.50Cu-0.20Mn-0.05Sn)were designed in this study.The effects of different quenching rates after solid solution treatment on the microstructures and properties of Al-10.50Si-1.50Cu-0.20Mn-0.50Mg-0.03B-0.30 Sb alloy are studied.The influence mechanism of trace Sn on the precipitation behavior and microstructures of Al-10.50Si-1.50Cu-0.20 Mn alloy is investigated.The main conclusions are as follows:(1)Microalloying with Mg,B and Sb significantly refines the α-Al primary dendrite size and eutectic Si size of Al-10.50Si-1.50Cu-0.20 Mn alloy.Among them,Mg element can refine the α-Al primary dendrite,eutectic Si size and form Q-phase;B element further refines the α-Al primary dendrite size;Sb element improves the morphology of eutectic Si from long needles to short rods.The composite addition of the three elements reduces the α-Al primary dendrite size from ～1083 μm to ～340 μm and the eutectic Si size from ～34.5μm to ～15.4 μm,resulting in an optimized Al-10.50Si-1.50Cu-0.20Mn-0.50Mg-0.03B-0.30 Sb alloy.(2)The effect of quenching rates on the microstructures of Al-10.50Si-1.50Cu-0.20Mn-0.50Mg-0.03B-0.30 Sb alloy is revealed.In the quenched state,the wind cooled(WC)alloy has a slow quenching rate(～5 °C/s),which reduces the supersaturation of solute atoms and forms a large number of Q’ and β’’phases by atomic diffusion.Meanwhile,the water quenched(WQ)alloy has a fast quenching rate(～200 °C/s),which maintains a high supersaturation of solute atoms and only a small amount of Q’ phase is formed due to the fast quenching rate and the solute atoms do not have time to diffuse.During aging treatment,the WQ alloy has a higher aging drive due to the high supersaturation of solute atoms,which results in a significantly higher number density of precipitates in the peak-aged WQ alloy than in the peak-aged WC alloy.The Q’,θ’ and β’’ phases of the peak-aged WC and peak-aged WQ alloys are significantly coarsened,dissolved and transformed to Q,θ and βequilibrium phases after a 10-minute heat treatment at 300 °C.(3)The effect of quenching rates on the mechanical properties of the Al-10.50Si-1.50Cu-0.20Mn-0.50Mg-0.03B-0.30 Sb alloy is investigated.The hardness of the solid solution WC alloy(～79.8 Hv)is higher than that of the WQ alloy(～70.0 Hv).The room-temperature yield strength of the peak-aged WQ alloy(～324 MPa)is significantly higher than that of the peak-aged WC alloy(～301MPa).The mechanical properties of the two peak-aged alloys decrease significantly with the increase of the tensile temperature.However,the hightemperature properties of the peak-aged WQ alloy are better than those of the peak-aged WC alloy.The quenching rates affects the fatigue properties of the alloy by influencing the number density of precipitates.The fatigue life of the peak-aged WC and peak-aged WQ alloys increase significantly with decreasing stress amplitude,but the fatigue life of the peak-aged WQ alloy is significantly higher than that of the peak-aged WC alloy.The fatigue limit of the peak-aged WQ alloy(～127 MPa)is higher than that of the peak-aged WC alloy(～115 MPa).(4)The influence mechanism of trace Sn on the precipitation behavior of Al-10.50Si-1.50Cu-0.20 Mn alloy is elucidated.Sn can effectively promote the homogeneous precipitation of θ’ phase,which maybe due to the fact that the Sn-rich nano-phase can act as a heterogeneous nucleation site for θ’ phase and promote the non-uniform nucleation of θ’ phase.Thereinto,the activation energies of θ’ phase of Sn-free and Sn-containing alloys are ～128.2 k J/mol and～98.9 k J/mol,respectively.(5)The effect of Sn on the mechanical properties of Al-10.50Si-1.50Cu-0.20 Mn alloy is clarified.The Sn-containing alloy significantly reduce the peak aging time(from ～65 h to ～24 h)and increased the peak hardness(from ～80 Hv to～93 Hv)compared with the Sn-free alloy.The yield strength of the peak-aged Sn-containing alloy and Sn-free alloy is ～183.6 MPa and ～133.5 MPa,respectively.The strengthening mechanism analysis shows that the contribution of the peak-aged strength of the alloy is mainly from the precipitation strengthening,and the difference in yield strength between the two alloys is mainly due to the smaller size and larger number of θ’ phases in the peak-aged Sn-containing alloy and the stronger impediment to dislocations. |
PDF Full Text Request