【作者】 张康；

【导师】 陈静青；

【作者基本信息】 西南交通大学 ， 材料科学与工程， 2019， 硕士

【摘要】 A7N01合金属于具有较高比强度的Al-Mg-Zn-Cu合金,其目前已被广泛使用于国内外高速列车行业的车身制造中。由于车体结构采用焊接工艺连接,因此焊接过程对材料性能的影响是一个重要的考虑因素。在焊接条件下,A7N01-T4铝合金焊接接头的组织和性能由于热循环的影响而发生严重劣化,性能劣化最严重的区域通常在铝合金焊接接头的HAZ中,其力学性能降低,同时耐蚀性也变差。劣化区的性能与焊缝的位置距离有很大的关系,因为距离远近影响了热循环峰值温度的大小,同时HAZ的性能在自然时效过程中会随着时间逐渐得到恢复。因此有必要研究热循环中峰值温度和自然时效时间对于HAZ的组织和性能之间的根本关系。本论文以A7N01-T4铝合金板材为研究对象,通过激光-MIG复合焊对铝合金进行单道次焊接,获得合金不同位置的焊接热循环曲线参数,借助Gleeble-3500热物理模拟试验机模拟其不同的热循环曲线,获得HAZ不同峰值温度的微观组织;通过透射电镜（Transmission Electron Microscope,TEM）观察,得到了析出相的化学成分和晶体结构;通过差热分析（Differential Scanning Calorimeter,DSC）分析了析出相的析出规律和热力学特征;通过对原始状态的母材及热模拟试件在不同的自然时效时间进行强度、维氏硬度等力学性能实验,得到了力学性能随自然时效变化的规律;通过对原始状态的母材及热模拟试件在不同的自然时效时间进行电导率、晶间腐蚀和电化学测试,得到了腐蚀性能随自然时效变化的规律;最后通过支持向量机方法对受到两次热循环的试样在自然时效过程中的硬度变化进行回归分析。析出相结构分析结果表眀,A7N01-T4合金中η相（MgZn₂）的活化能为164.17kJ/mol。基于DSC实验推测了高速升温的热循环状态下η’的固溶温度在300℃以下,η的形成和粗化温度在300℃到400℃之间,η的固溶温度在400℃到500℃之间。同时明确了A7N01合金中η析出物与基体的取向关系,并观察到与常规η的取向关系不同的变体。力学性能结果表眀,可以通过对不同峰值温度热循环样品的TEM图片中析出相的分布分析获得定量指标,将这些指标与硬度和强度结合分析可以对热循环样品的力学性能在自然时效过程中的变化作出半定量分析。对于硬度和强度,其在热循环后降低后又随着自然时效时间的增加而明显增大,当曲线趋于稳定时,较高的峰值温度导致较低的硬度和强度。腐蚀性能结果表眀,自然时效第一周后峰值温度为400℃的热模拟样品较300℃和500℃在应力腐蚀（stress corrosion）、晶间腐蚀（Intergranular Corrosion）和电化学腐蚀（electrochemical corrosion）方面的耐蚀性都较好,这可能归因于400℃的样品中在热循环过程中生成了较多粗大的η相,同时随着自然时效时间的增长,各温度样品应力腐蚀、晶间腐蚀和电化学腐蚀的耐腐蚀性都明显提高,恢复或接近于原始T4样品水平。支持向量机（Support Vector Machine,SVM）回归结果表眀,通过交叉验证得到c&g参数并用来训练模型,其回归结果具有较小的均方误差（Mean Square Error,MSE）和平均绝对误差（Mean Absolute Error,MAE）,同时R值较高,这表明预测值和实际值之间的偏差非常小同时相关性很好。同时第7周的硬度预测值与测量值非常一致,表明模型的相关性很好并且不存在过度拟合的迹象。更多还原

【Abstract】 A7N01 alloy is an Al-Mg-Zn-Cu alloy with high specific strength,which has been widely used in the body manufacturing of high-speed train industry at home and abroad.Since the car body structure must be joined using a welding process,the influence of the welding process on the material properties is an important consideration.Because of the influence of welding,the microstructure and properties of A7N01-T4 aluminum alloy welded joints will be seriously deteriorated compared with the base metal.The most serious degraded area is usually the heat affected zone in aluminum alloy welded joints,its mechanical properties and corrosion resistance are reduced.Because the distance between the deteriorated area and the center line of weld is close to the peak temperature of welding thermal cycle,the property of the degraded zone has a great relationship with the distance of weld center line.At the same time,the property of the heat-affected zone is gradually recovered during the natural aging period.It is necessary to study the effect of the peak temperature and natural aging time on the microstructure and property of the heat affected zone.This paper’s study objective is A7N01-T4 aluminum alloy.The welding thermal cycle curve of different distance was tested by single-pass laser-MG hybrid welding method.The welding thermal cycle were simulated by the Gleeble-3500 Thermal Simulation Tester.The thermal cycle curve was used to obtain the microstructure of different peak temperatures of HAZ.The chemical composition and crystal structure of the precipitated phase were obtained by transmission electron microscopy(TEM).The precipitation ruler and thermodynamic behavior of the phase was analyzed by Differential Scanning Calorimeter(DSC);the mechanical properties such as tensile test and Vickers hardness test of the initial T4 and the thermal simulation specimens at different natural aging times were analyzed,and the laws of mechanical properties change with natural aging were obtained.The conductivity test、intergranular corrosion test and electrochemical test were carried out on them,and the corrosion behavior changed with natural aging were obtained.At last,the support vector machine method was used to perform regression analysis on the hardness change of the sample subjected by two thermal cycles during the natural aging period.Microstructure analysis results show that,the activation energy of η in the A7N01 alloy was 164.17 kJ/mol.Based on DSC experiments,it is estimated that the solid solution temperature of η’ in the condition of thermal cycle with high-speed heating rate is below 300 °C,the formation and coarsening temperature of η is between 300 °C and 400 °C,the solution temperature of η is between 400 °C and 500 °C..At the same time,the orientation relationship between the η precipitate and the matrix in the A7N01 alloy was clarified,and a η ariant which orientation relationship is different with the conventional η was observed.Mechanical properties results show that,quantitative data can be obtained by analyzing the distribution of precipitates in the TEM images of different peak temperature samples.Combining these indicators with hardness and strength results can analyze the change of mechanical properties of different thermal cycling samples during natural aging period.The hardness and strength decreases after thermal cycling and then increases with the natural aging time.When the value tends to be stable,the higher peak temperature leads to lower hardness and strength.Corrosion property analysis results show that,the peak temperatue 400 °C samples have better corrosion resistance in stress corrosion,intergranular corrosion and electrochemical corrosion than 300 °C and 500 °C at the first week of natural aging,it may be attributed to 400℃sample with more coarse η phase was formed during the thermal cycle.At the same time,the corrosion resistance of 300℃、400℃ and 500℃sample was significantly improved with the natural aging time increased,recovery to the initial T4 sample.The SVM regression results show that,the c&g parameters are obtained by crossvalidation and used to train the model.The regression results have small MSE and MAE,and the R value is high,which indicates that the deviation between the predicted value and the real value is very small and the correlation is good.At the same time,the hardness predict value with the real value at the 7th week is very consistent,indicating that the model is well correlated and there is no sign of overfitting.更多还原