基于非等温相变行为的AlZnMgCu合金厚板组织性能均匀性的积分回归调控机理

The comprehensive properties and property/microstructure homogeneity of AlZnMgCu alloy thick plate are deteriorated by the through-thickness temperature gradient due to the slow temperature rising/decreasing phenomenon when the plate aged under high temperature and short duration conditions, which also causes difficulty on the regulation of ageing observably. This project aims to stable fabricate high-performance AlZnMgCu alloy thick plate. In view of the non-isothermal phenomenon, the temperature dwelling of retrogression will be abandoned while the heating/cooling ageing technology is intended to adopt. Then the properties and homogeneity of thick plate are considered to be promoted by the comprehensive effect of integral effect of non-isothermal retrogression and the under peak aged treatment before retrogression. The influence rule of the matching of pre-aged microstructure and retrogression integral effect on the non-isothermal retrogression behavior of the second phase will be investigated primarily. The purpose is to obtain the target microstructure after non-isothermal retrogression, which corresponds to the advanced properties and higher homogeneity of thick plate. Based on the target microstructure and the according retrogression degree, the non-isothermal kinetics model will be established drawing lessons from the isothermal LSW theory. Combining kinetics simulation and experimental verifications, the influence rule of the integral retrogression and re-ageing schedule on the strength, corrosion resistance and the through-thickness homogeneity of thick plate will be concluded. The integral retrogression schedule and the relevant mechanism for realizing the target microstructure will be explored. The results are expected to provide essential experimental basis and theoretical guidance for the fabrication of AlZnMgCu alloy thick plate with high strength high corrosion resistance and high homogeneity.

AlZnMgCu合金厚板高温短时时效过程中存在的缓慢升/降温现象产生了板厚向温度梯度，显著增加时效程度的控制难度，降低厚板的综合性能和性能/组织均匀性。本项目以稳定制备高性能AlZnMgCu合金厚板为研究目标，针对非等温现象，拟取消回归保温平台并引入升降温时效技术，通过非等温回归积分效应与回归前的欠时效处理协同提高厚板性能及均匀性。重点研究预时效组织状态与回归积分效应的匹配对第二相非等温回归行为的影响规律，确立有利于厚板性能及均匀性提升的非等温回归态目标组织结构；基于目标组织及其对应的回归程度，在等温LSW理论的基础上建立非等温回归动力学模型；结合动力学模拟及实验验证，总结基于积分效应的积分回归及再时效工艺对厚板强度、耐蚀性及厚向性能均匀性的影响规律，探明实现目标组织结构的积分回归工艺方法及其相关机制。研究结果将为高强耐蚀高均匀性AlZnMgCu合金厚板的制备提供必需的实验依据和理论指导。

在非等温条件下，提高AlZnMgCu厚板的综合性能以及厚板析出组织/性能均匀性的时效调控机理是高品质铝合金厚板热处理中的关键科学技术问题。本项目研究了非等温积分回归及再时效对7055铝合金厚板强度、耐蚀性能及其厚向均匀性的影响。主要结论如下：建立了考虑回归加热速率的回归再时效制度为：105℃,24h+(升温速率3℃/min)190℃,50min+120℃,24h。经升温回归+再时效后，7055铝合金的力学和耐蚀性能分别为：σb =617.5MPa，δ=16.1%，剥落腐蚀等级为EB。7055铝合金的晶界析出相随着回归温度的提高和回归时间的延长而发生Ostwald熟化，粗化规律遵循 LSW 机制，计算得到其粗化激活能为115.2 kJ/mol。三级时效处理后的晶内析出相以η′相和η相为主，晶界相为粗大断续分布的η相。厚板表层晶内析出相相对细小，体积分数高，心层析出相粗化程度稍高，η平衡相增多；表层晶界相相对粗大，断续程度更高。固定回归加热速率(3℃/min)，硬度和电导率的不均匀性都随着回归冷却速率的降低而降低。建立了包含空冷降温处理以补偿7055铝合金厚板厚向组织不均匀性的“积分时效”制度：105℃,24h+(升温速率3℃/min)190℃, 70min(包含加热时间)+ (空冷25min至120℃) 120℃,24h。经“积分时效”处理后，7055铝合金板材的厚向不均匀性由8%下降至4.2%。在此基础上，项目进一步研究了不包含等温回归平台的非等温回归再时效制度。结果表明，回归加热速率和最高回归温度点TA对合金的性能有关键影响。慢回归加热(1℃/min)条件下达标工艺(电导率38%IACS)对应的TA温度为215℃；而快回归加热速率(3℃/min)条件下达标工艺对应的TA温度为225℃。再时效使合金的电导率有所提高。但是硬度的演变与回归加热速率有关。慢回归加热速率(1℃/min)条件下，再时效后合金的硬度下降；快回归加热速率(3℃/min)条件下，再时效后合金的硬度增加。优化后的非等温回归及再时效工艺可以使合金在耐蚀性达到使用要求的同时强度损失较小。经105℃,24h预时效处理后，以3℃/min升温回归至225℃后立即随炉冷却至120℃，并24h保温的再时效处理后，合金的抗拉强度和剥落腐蚀等级分别为616.4MPa和EA，具有近T6态的力学性能和T73态的耐蚀性能。