Investigation Of Spray Quenching Heat Transfer And As-quenched Flow Stress For 2024 Aluminum Alloy Plate

With the evolution of aircraft design,higher requirements for aluminum alloy and its processing technology have been put forward.And these requirements have a very important influence on the manufacture of aircraft components,e.g.fuselage skin,wing panel.Therefore,it must investigate the control of quenching process,though which a balance between low distribution of residual stress and higher mechanical properties can be obtained.However,the traditional quenching treatment,which is the bottleneck of the aircraft production,often leads to high magnitudes of residual stress and seriously distortion of plates.In recent years,the spray quenching has been become the research hotspot.And this quenching process provides an effective technical way to balance the residual stress and mechanical properties.However,there exist some unsolved problems for spray quenching of aluminum alloy plates,e.g.the heat transfer mechanisms for spray quenching parameters and dissolved salts,the characteristics of quench-induced precipitates and their effects on the flow stress.Therefore,how to systematically and comprehensively study these problems is the key to balance the residual stress and mechanical properties for spray quenching of plates.The aim of the present work is the regulation of residual stress and mechanical properties.Some approaches,e.g.theoretical analysis,experimental research and finite element simulation,are used in this paper.And the material under investigation is aluminum alloy 2024.Finally,the prototype of controlled residual stress and mechanical properties for plates has been built.The main works of this research are as follows:?1?For the heat transfer of spray quenching,some experiments have been performed on the aluminum alloy using the spray quenching platform.The accuracy of experimental data is evaluated by the repeated experiments.Based on B-spline basis function,data noise processing model is established.In comparison with the experimental data,it has a good prediction for true cooling path and can remove the random noise.Then,the heat flux is obtained by the one-dimensional inverse heat transfer method.The evaluation of accuracy shows that the errors of heat flux calculation and heat flux distribution are 1.17% and 5%,respectively,indicating the high resolution precision.?2?The influences of spray parameters and dissolved salts on heat transfer have been sufficiently discussed.The mechanisms of heat transfer are revealed when considering some factors,e.g.the thermal physical parameters of quenching medium,spray volume flux.The results of heat transfer analysis show that there exist three regimes,e.g.transition boiling regime,nucleate boiling regime and singe-phase convection regime when the water temperature is less than 65?.The influences of dissolved salts and spray parameters on heat transfer are not monotonous.Maximum heat transfer ability is found at 3 bar,70? and 70 mm for spray pressure,water temperature and spray distance,respectively.For the dissolved salts,the largest enhancement of heat transfer is shown at the concentration of 0.2 M for solutions of NaCl,CaCl2 and MgSO₄,in contrast to 0.06 M for Na2SO4 solution.?3?For the behavior of flow stress of as-quenched Al-Cu-Mg alloy,the measured flow stress curves are obtained by the isothermal tests.The results show that there exist strain-rate sensitivity in temperature range of 300-450?,and the main softening mechanism is the dynamic recovery.While a strain hardening behavior is found when the temperature is below 300?.In consideration of the homogeneous precipitation,two constitutive models have been established for temperature ranges of 300-400? and 400-450?,respectively.The two models show a good agreement with experimental data.Compared with the ideal variation trend with no consideration of precipitation,the largest difference of activation energy is found at the temperature of 350? which is the nose temperature of 2024 alloy.?4?Considering the influence of Cu-Mg clusters at low temperatures??250??,the flow stress behavior is studied.The radii and volume fractions of precipitates at different cooling rates and temperatures are predicted by a multi-class precipitation kinetics model.The flow stress of as-quenched Al-Cu-Mg alloy is modeled using a dislocation density based model which considers the influences of precipitation,solid solution and forest dislocation.The model predictions are in good agreement with experimental data in terms of the flow stress and volume fraction evolution.The results show that volume fraction of precipitates decreases with the increasing temperature and cooling rate,in contrast to the increasing trend for radius of precipitates.For the large precipitates,the dislocation loops,i.e.geometrically necessary dislocations?GND's?,can influence both the strain hardening rates and magnitudes of flow stress.?5?Considering the plate structure characteristics,the prediction model of residual stress has been established when combined with the spray heat transfer boundary condition and UHARD subroutine definition.The prediction model,through which the evolution of the residual stress can be realized,is a thermal couple model.The prediction results show that the warping occurs due to the existence of compression buckling deformation.There mainly exist compressive stress on the concave,as opposed to the tensile stress on the convex.Based on the interrupted quenching method,the hardness prediction model of T4 temper is established.The analysis results show that the spray quenching has a larger influence on the mechanical properties in air cooling area of samples,in contrast to the minor effect in the spray area.?6?The effects of different spray paths on the residual stress and mechanical properties have been studied,and some factors,e.g.the residual stress,mechanical properties and warping value,are characterized.The results show that the spray distance has the maximum impact on the factors,followed by the dissolved salts.Both the water temperature and spray pressure have the minimum influence on the factors.The optimum performance of spray quenching is found when using the MgSO₄ at concentration of 0.2 M.Meanwhile the spray distance is 80 mm,spray pressure is 3 bar,water temperature is 15?.Compared with the pool quenching,the warp deformation decreases by approximately 46% and the residual stress reduces by about 40%,while the mechanical property has a minor variation.In summary,the prediction models of residual stress and mechanical property have been built based on spray quenching heat transfer and constitutive models.All the above researches will provide theoretical basis and methods for the regulation of residual stress and mechanical properties.