Cooling Behavior and Residual Stress with Double Nozzles Spray Quenching for Aluminum Alloy
2xxx series aluminum alloy has been widely used in the aerospace field because of its excellent comprehensive properties.The supersaturated solid solution was obtained by heat solution and quenching treatment,and the finely dispersed precipitation phases were obtained by aging treatment thereafter to meet the demands of ideal microstructure and mechanical properties.In industry fields,the roller hearth furnace is generally used for heat solution and quenching treatment of large-scale aluminum alloy thick plates.In the process of spray quenching,the jet beam impinges on the surface of the high-temperature sample to absorb heat in the form of sensible heat or latent heat.Besides meeting the requirements of mechanical properties through improving the cooling efficiency,it is necessary to pursue the uniformity of cooling behavior.However,due to the complexity of actual working conditions,the cooling rate at different positions of samples may be inconsistent,which will lead to heterogeneous microstructure and properties of samples to a certain extent.On the other hand,the uneven cooling behavior may lead to uneven distribution of residual stress,which will affect the processing and application of samples subsequently.According to the industrial working condition for large scale aluminum alloy thick plates,the ef-fects of nozzle distance and spray flow ratio on cooling behavior and distribution of residual stress at different positions of samples were investigated with double nozzles spraying quenching at laboratory conditions.The spray angle of the full-cone spray nozzle used in the experiments was 76°.Therefore,the spray zone formed by the jet beam on the flat sample surface was a circle with a diameter of 110 mm when the spray height was 70 mm.Because of the axial symmetry of spray zones,the quenching cooling curves and residual stress were measured at two-dimensional(2D)section of the center part along the longitudinal direction.The thermocouples were placed in the normal direction to the spray surface with the distance of 5 mm and distributed with the interval of 10 mm in the longitudinal direction.The transient temperature data was recorded by a temperature monitor device with a frequency of 10 Hz.The standard deviation at 20 s of temperature data and the heat fluxes were calculated by iterative method at different positions to characterize the difference of cool-ing behavior quantitatively.The surface residual stresses of samples were measured by X-ray diffraction method and measured points were distributed on the spray surface along the surface centerline of longitudinal direction with the interval of 10 mm.Firstly,the uni-formity of cooling behavior and residual stress distribution was determined via a single nozzle spray experiment.Then,spray experi-ments with nozzle distance of 80,90,100,110,120,130 and 140 mm were carried out respectively when the spray flow of two noz-zles was identical.Finally,keeping nozzle distance for 100 mm,the double nozzles spray experiments with spray flow ratios of 1∶1,1∶0.75,1∶0.50 and 1∶0.25 were carried out respectively.The results were as follows:within the scope of this study,the even zone radi-us of residual stress was 70 mm with single nozzle spray quenching,which was larger than the even zone radius of cooling rate.For the experiment of spray quenching with double nozzles,when the jets of the two nozzles overlapped more,the cooling capacity of the over-lapping zones was stronger than that of the other jet zones.The heat transfer capacity in the quenching process was related to the area of solid-liquid contact area when volumetric flux and other spray parameters are the same.On one hand,the jet beam was dispersed into a large number of small droplets via a full-cone spray nozzle used in this experiment,which meant the larger solid-liquid contact area in the overlapping zones.On the other hand,the jet upward in the experiment,which would not inhibit the heat transfer efficiency be-cause of the turbulent behavior in the overlapping zones.When the distance between the two nozzles was 100～120 mm,the cooling be-havior at different positions was consistent.When the jets of the two nozzles were far from each other,the cooling capacity of the non-spraying zones was weaker than that of the jet zones.The difference in cooling rate was relatively large because of the complexity of heat transfer behavior near the non-spraying zones.The standard deviation at 20 s and the heat fluxes at different positions showed the same variation tendency.The nozzle distance had little influence on the uniformity of quenching residual stress of samples.When the spray flow of a nozzle decreased,the cooling capacity of the low spray flow side of the sample decreased.Meanwhile,the difference in cooling rate at different positions increased with the increase of spray flow ratio.The distribution of residual stress on the high spray flow side was the same as the distribution with signal nozzle spray quenching.The residual stress on the low spray flow side decreased and turned to uniform distribution through the transition zone when the spray flow ratio was equal to or more than 1∶0.50.
aluminum alloyspray quenchingheat transfer capabilityheat fluxresidual stress
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