| Aluminum alloy has the characteristics of high strength,good machinability and excellent corrosion resistance.It is widely used in aerospace,automobile production,building materials and other manufacturing industries.It is the second largest alloy widely used except steel.Some aluminum alloys are machined profiles,and the quality of the ingot directly affects the machining properties of the alloys.However,the ingot microstructure of aluminum alloy has such disadvantages as large grain size,uneven microstructure and large dendrite network.Based on this,the subject took three aluminum alloys,6005A,7N01 and 7A99,as the research objects,and used temperature recorder to record the temperature changes at each point in the process of aluminum alloy casting.Through quantitative analysis of the microstructure and properties of aluminum alloy at different cooling rates,the influence of casting cooling rates on the microstructure and properties of aluminum alloy was studied.Firstly,the solidification process of the alloy is analyzed.From the top of the wedge to the tip,the cooling rates of 6005A,7N01,and 7A99 alloys gradually increased,with increased subcooling and longer platform durations.When the cooling rate increases,the liquidus temperature of 6005A,7N01 and 7A99 aluminum alloy decreases,the generation temperature of Mg2Si phase of 6005A will decrease,and the formation temperature of eutectic structure at low melting point of 7A99 will decrease.Secondly,the effect of cooling rate on lattice distortion is analyzed by XRD.The 6005A aluminum alloy has negative distortion,and the increase of cooling rate will reduce the lattice distortion and the solid solubility of 6005A aluminum alloy.The lattice distortion and solid solubility of 7N01 aluminum alloy increase with the increase of cooling rate.The lattice distortion and solid solubility of 7A99 aluminum alloy decreased with the increase of cooling rate.The effect of cooling rate on the microstructure and properties of the alloy was studied again.The macrostructure of the wedge mold casting aluminum alloy is as follows:bright white columnar crystals perpendicular to the mold wall are grown in the part close to the mold wall,and the length of the columnar crystals from the mold tip to the insulation cap is gradually shortened.In the middle of the sample and the insulation cap are free-growing isoaxial crystals.The increase of cooling rate can significantly reduce the grain size of the alloy,but increase the inhomogeneity of the microstructure.The second phase of cast 6005A aluminum alloy is mainly Mg2Si phase and Al5FeSi phase.With the increase of cooling rate,the number of Mg2Si phase increases significantly,and the Al5FeSi phase changes from long strip to fishbone.The second phase of the as-cast 7N01 aluminum alloy is mainly a small amount of MgZn2 phase.With the increase of cooling rate,the size of the MgZn2 phase decreases obviously.The eutectic phase of as-cast 7A99 aluminum alloy is mainly lamellar MgZn2 phase.With the increase of cooling rate,the phase transition of MgZn2 becomes more slender.The rapid cooling rate in the casting process is beneficial to increase the hardness of the three aluminum alloys,but will reduce the conductivity of the alloys.Finally,the simulation of the low-pressure metal mold casting process of 7A99 was conducted through ProCAST software,and it was found that plan 1 was quick filling of low-temperature casting with small cooling rate,cooling rate of 2℃/s,pouring temperature of 700℃,filling time of 97s,and filling speed of 2 kg/s.The simulation results are as follows:high filling speed,hot crack tendency,and severe shrinkage cavity and porosity tendency.Scheme two is high temperature casting with high cooling rate and slow filling:the cooling rate is 4℃/s,the pouring temperature is 750℃,the filling time is 194 s,and the filling speed is 1 kg/s.The simulation results are as follows:the mold filling is stable,the temperature field distribution is solidified in the order from top to bottom,so there is no tendency of shrinkage cavity and porosity,but the central part of the cylindrical aluminum mold has a severe tendency of thermal crack.By adjusting the simulation parameters,the best solution was obtained:cooling rate was 3℃/s,pouring temperature was 720℃,filling rate was 1.2 kg/s,filling time was 162 s,and the casting temperature was preheated to 260℃,so as to obtain the computer simulation results with no shrinkage cavity and porosity defects and small thermal crack tendency. |
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