2014合金的回归再时效

本文采用拉伸实验和透射电镜法,研究了2014铝合金的回归再时效(RRA)处理.结果表明:采用欠时效(100℃(2×2h)与适当回归处理(200×5min)配合的RRA处理,可同时提高2014合金的强度和塑性.适当的回归处理能使欠时效2014合金组织中部分GP区优先长大.随后再进行峰时效处理,不仅能改善晶界析出相的尺寸和分布,而且还能使基体析出相尺寸产生明显的差异.这种尺寸有明显差异的基体析出相协同强化有利于提高2014合金的强韧性.     

时效热处理对7B50超强铝合金组织与性能的影响

通过拉伸试验、维氏硬度测试、电导率测试、晶间腐蚀与剥落腐蚀试验、金相观察及透射电镜分析等,研究新型的4级时效工艺(four-step aging,FSA),即高温短时效—低温长时效—高温短时效—低温时效工艺对Al-Zn-Mg-Cu系7B50超强铝合金组织和性能的影响。结果表明:FSA处理促使7B50铝合金晶界析出相发生球化和细化,晶界析出相的体积分数显著增大并呈非连续分布;与传统的回归再时效RRA工艺相比,经过优化的新型4级时效热处理能明显提高7B50铝合金的力学性能和抗腐蚀性能;经过150℃/5 h→110℃/24 h→150℃/5 h→110℃/12 h的4级时效处理后,合金的室温抗拉强度从582 MPa提高到685 MPa,抗腐蚀性能明显超过回归再时效(RRA)处理的合金。     

三级时效处理对7×××系铝合金组织与性能的影响

采用扫描电镜、透射电镜、硬度测试等手段研究了回归时间对试验铝合金的微观组织以及硬度、电导率、力学性能、断裂韧性等性能的影响.结果表明,三级时效态合金的强度峰值、硬度峰值、导电率和断裂韧性均超过了未回归时效态合金与T6态;回归的过程是晶内析出相GP区部分会先产生回溶,然后不断长大直至转变为η'相,晶界析出相逐渐粗化且不连...     

时效制度对6013铝合金挤压型材屈强比的影响

以挤压态的6013铝合金为研究对象,通过显微硬度测试、单向拉伸实验和组织分析,研究了自然时效、人工时效和回归再时效处理时合金的力学性能变化规律。结果表明:自然时效峰值状态（16 d）的抗拉强度为286 MPa,屈服强度为158 MPa,屈强比为0.54,适合塑性成形;将自然时效峰值状态下的试样进行回归再时效处理（210 °C回归0.5 h+170 °C峰值时效2 h）,抗拉强度为362 MPa,屈服强度为336 MPa,屈强比达到0.92,抗塑性变形能力显著增强。这是因为回归再时效后析出相的尺寸减小,数密度显著增大,析出强化效果显著增强。而析出强化对屈服强度和抗拉强度的影响程度不同,因此可通过时效热处理来调控屈强比,即通过自然峰值时效提高合金的塑性变形性能以成形零件,而在零件成形后采用回归再时效提高其抗变形能力。      

基于厚向组织性能考量的7B50铝合金中厚板回归再时效热处理

为解决T6态高强铝合金强度高而耐蚀性难以满足使用需求，采用三级时效工艺来改善析出强化相特别是晶界析出相的形貌、尺寸、分布等，并通过研究不同回归处理制度对组织、性能的影响而获得适宜7B50铝合金中厚板的三级时效工艺.研究发现提高回归温度或延长回归时间均会使中厚板心部及表层组织的晶内和晶界析出相发生粗化并析出稳定η-MgZn₂相，导致强度下降、电导率上升，其中回归温度对强度和电导率的影响显著.三级时效处理虽使晶内析出相尺寸有所增加，但却使T6态连续分布的晶界析出相呈断续分布，结合心部和表层强度及电导率测量结果认为合适的回归处理制度为165℃/6 h.然而，热轧引起中厚板表层较心部更为严重的变形使表层含有更多的亚晶或亚结构且其分布更均匀，从而使表层更快到达峰时效，进一步的回归再时效处理则使表层析出更多稳定η相，而η相的形成与晶内析出相的粗化长大是造成表层和心部强度差异的关键.虽然淬火/三级时效态表层和心部的晶粒结构存在差异，且局部出现亚晶合并长大，但其对强度的提升效果远低于表层析出稳定η相所引起的强度下降.可见，三级时效工艺并不能缓解7B50铝合金中厚板心部和表层的性能差异，但可使表层和心部的强度、电导率满足某实际工况要求.      

Al-Zn-Mg系铝合金应力腐蚀性能

研究了热处理制度和时效工艺的改变对Al-Zn-Mg系铝合金的组织结构、力学性能和应力腐 蚀性能的影响。研究结果表明:高温预析出可以改变Al-Zn-Mg系铝合金晶界的析出相大小和分 布,从而改善其抗应力腐蚀性能;在T6和T612种人工时效条件下,预析出的合金的抗应力腐蚀 性能均好于无预析出的合金。在自然时效状态下,引入超声波,对无预析出合金的应力腐蚀性能 进行了初步探索,发现超声波可以提高合金的抗应力腐蚀性能,而对合金的硬度无影响。     

高强韧铝合金非等温时效工艺的研究进展

非等温时效工艺作为一种新兴的时效处理方法,能够有效地提高高强韧铝合金的综合性能。通过简要归纳近些年来应用于高强韧铝合金的非等温时效工艺,总结出经不同非等温时效处理后高强韧铝合金析出相的特征、合金力学性能和腐蚀性能的变化情况。非等温时效工艺的效率相较于传统时效工艺有很大提高,并且能够同时调控高强韧铝合金内基体析出相和晶界析出相的种类、尺寸和分布情况,使高强韧铝合金兼具与T6峰值时效态相差不多的力学性能和近T7x过时效态的腐蚀性能。最后,对未来高强韧铝合金非等温时效工艺的研究和应用进行了展望。      

回归再时效对超高强铝合金力学性能及组织的影响

在传统的回归再时效(retrogression and re-aging,RRA)工艺(峰时效)基础上降低预时效或再时效温度,对Fe和Si杂质含量高的超高强Al-Zn-Mg-Cu合金挤压棒材进行RRA处理,通过拉伸性能和疲劳性能测试以及扫描电镜和透射电镜观察,研究RRA工艺对合金力学性能与组织的影响。结果表明:降低预时效或再时效温度都可明显提高该合金的塑性和抗疲劳损伤性能,略微降低合金的抗拉强度。采用峰时效温度(120℃)RRA处理后的合金,晶内的主要析出相为尺寸较大的η′相,不能被位错切割,合金强度较高(674 MPa),但塑性和抗疲劳损伤性能差,伸长率为11.1%,最终应力强度因子幅值ΔK=26.8 MPa·m1/2;降低时效温度可增加析出相中GP区粒子的比例,减小η′相的尺寸,从而提高塑性变形能力以及抗疲劳损伤性能。     

铝合金时效硬化模型的研究进展

相对于众多其他合金,铝合金的时效硬化模型经过近几十年的发展已日趋成熟.利用现有模型可以计算球形、片状和针状析出相的尺寸及体积分数与合金成分、时效时间及时效温度的关系,从而可以研究铝合金的屈服强度在时效过程中的演变规律,对铝合金的设计具有重要的指导意义.该文详细地介绍了铝合金时效硬化模型的发展,并指出了现有模型的不足之处,对模型的未来发展进行了展望.     

7×××系铝合金厚板表层局部"色差"缺陷原因分析

针对7×××系铝合金厚板在低倍检测过程中表层出现的"色差"缺陷问题,采用显微镜、扫描电镜、维氏硬度及电导率等多种检测方法对缺陷进行了定性分析,确定了7×××系铝合金厚板表层"色差"产生的根本原因及其对材料性能的影响机理。结果表明,当7×××系铝合金厚板表层局部淬火冷却不均匀时,合金中的过饱和固溶体将会发生脱溶析出现象,在合金晶界和晶内析出大量的粗大平衡相η。这些平衡相的析出消耗了大量的溶质原子,大大降低了合金固溶体的过饱和度,减少了时效过程中形成时效强化析出相的数量,从而导致合金硬度和强度的下降,电导率升高。可通过均匀控制淬火冷却方式来解决此问题。     

回归再时效7150铝合金力学性能、剥蚀行为及微观组织研究

采用拉伸试验、剥蚀试验、硬度测试及透射电镜（TEM）观察研究了195℃回归时7150铝合金硬度变化及微观组织形祝,以及回归不同时间的RRA工艺对7150铝合金力学性能和剥蚀行为的影响,并与T6及T73进行了比较研究。结果表明,7150-T6铝合金强度高而剥蚀敏感性大;7150-T73铝合金强度降低而耐腐蚀性大幅度提高。195℃回归时,回归时间小于0．5h,7150-RRA铝合金强度高于7150-T6强度,而剥蚀敏感性未有效降低;当回归时间延长至1h,7150-RRA铝合金可保持7150-T6的高强度,而其剥蚀敏感性则大幅度降低。     

A TEM study of microstructural changes during retrogression and reaging in 7075 aluminum

Microstructural changes occurring during retrogression, and during retrogression plus reaging in 7075-T6 aluminum alloy have been investigated by means of transmission electron microscopy, and related to mechanical properties. TEM results indicate that the drop in strength during the initial stage of retrogression was due to the partial dissolution of G.P. zones while the growth of the semi-coherentη ′ was responsible for the rapid recovery of strength. It is suggested that the retrogression and reaging treatment resulted in the increase in volume fraction of G.P. zones and especially η′ precipitates over both the T6 and retrogressed conditions, therefore significantly improving the strength of the alloy.     

Retrogression and reaging and the role of dislocations in the stress corrosion of 7000-type aluminum alloys

The 7000-type aluminum alloys in the T6 temper are known to be highly susceptible to stress-corrosion cracking (SCC). Some years ago, a heat treatment known as retrogression and reaging (RR) was developed by one of the authors (B.C.), providing for enhanced stress-corrosion resistance without any sacrifice of yield or tensile strength in 7075 aluminum alloy. The idea behind the process was based on the suggestion that dislocations developed during quenching from the solution treatment were responsible for susceptibility to stress corrosion. In spite of considerable practical development of the RR process, the above basic hypothesis as to the role of dislocations has never been investigated. In the present work, the effect of the RR treatments on the dislocation structure of 7000-type aluminum alloys was studied using transmission electron microscopy (TEM). A clear relationship has been found between the presence of dislocations adjacent to grain boundaries and the susceptibility to stress corrosion of 7000-type aluminum alloys. The beneficial effect of the RR treatment on the SCC of 7000-type aluminum alloys in the T6 temper is believed to be due to the disappearance of the above dislocations as a result of RR treatment.     

Characterization of retrogression and reaging behavior of 8090 Al-Li-Cu-Mg-Zr alloy

An 8090 Al-Li-Cu-Mg-Zr alloy in the peak-aged (T8) temper was subjected to retrogression treatment at temperatures above and below the δ′ (Al₃Li) solvus line and immediately reaged to various tempers. Retrogression and reaging (RRA) behavior is characterized by hardness testing, tensile testing, transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and electrochemical polarization studies. Retrogression of the T8 temper alloy causes dissolution primarily of δ′ (Al₃Li) precipitates into solid solution that results in a decrease of hardness and tensile strength and an increase of ductility of the alloy. Reaging of the retrogressed state causes reprecipitation of the δ′ precipitates in the matrix resulting in the restoration of strength and ductility properties. Retrogression and reaging to the peak-aged temper, designated at T77 temper, has been found to retain the strength of the conventional T8 temper, but with the gross aging time in the RRA temper almost twice that of the conventional T8 temper, the microstructure of the RRA temper approaches that of the overaged (T7) temper. Thus, RRA treatment contributes to an improvement of stress corrosion cracking (SCC) resistance over the conventional T8 temper while retaining the mechanical properties of T8 temper.     

Stress corrosion cracking of superplastically formed 7475 aluminum alloy

The effects of biaxial superplastic deformation and postforming heat treatment upon the stress corrosion cracking (SCC) of a fine-grained 7475Al alloy plate have been investigated. For all postforming tempered conditions, increasing the extent of superplastic deformation, which created more cavitations, would decrease the mechanical properties, the SCC resistance, and the corrosion resistance. The influence of cavitation on the decay of elongation of the superplastically formed workpieces is larger than that on the decay of its strength. Post-forming tempered by retrogression and reaging (RRA) treatment could effectively improve the SCC resistance of workpieces in postforming T6 temper while not sacrificing the strength. However, the benefit of improving the SCC resistance by means of the postforming RRA temper was decreased with increasing the extent of superplastic deformation, because the SCC susceptibility increased as the extent of superplastic deformation increased for each postforming tempered condition. The cavitation led to more anodic corrosion potential and pitting potential and to an increase in both corrosion current density and passive current density, which would increase the SCC susceptibility.     

Microstructure and properties of high strength aluminium alloys for structural applications

This article discusses the fundamental basis of high strength Al alloy design and describes the role of alloying elements, mechanical processing parameters and heat treatments toward the evolution of microstructure that controls the desired properties i.e. strength, fracture toughness, stress corrosion cracking (SCC) resistance, fatigue crack initiation and propagation resistance, and weldability in 7xxx series Al alloys. The beneficial effects of suitable micro/trace alloying elements, and deleterious effects of certain impurity elements on a variety of properties are further discussed within the present context.     

Optimization of Retrogression Parameters of AA7010 Alloy

The present work has been carried out on an AA7010 aluminium alloy so as to optimize the retrogression and re-aging (RRA) schedule that leads to the optimal combination of mechanical properties and stress corrosion cracking (SCC) resistance. The alloy is heat treated at different retrogression temperatures for varying retrogression time and subsequently the window for optimization of retrogression parameters of RRA schedule is established after re-aging. It is found that retrogression at 473 K for 35 min results into the best combination of the above properties. The enhancement in mechanical properties and SCC resistance is due to the formation of discontinuous and coarse precipitates along the grain boundaries and also the copper enrichment of the precipitates that occur during optimum RRA schedule. It is established that proper control of the process parameters is essential to control the final microstructure and thereby enhance the mechanical properties and SCC resistance of the alloys.     

Rheo-Extrusion of an Alloy Specially Developed for Rheo Process Based on Al–Zn–Mg–Cu System

This study has been directed towards developing 7xxx aluminum alloy for rheo-extrusion. Rheo extrusion was done by a co-rotating twin-screw extruder as a new semi-solid process for production of 7xxx aluminum alloys with high integrity. A super high-strength aluminum alloy was thermodynamically developed with nominal composition of Al–14Zn–9Mg–5.2Cu. The rheo-formability of alloy had been assessed by thermodynamic criteria and mechanical properties. The newly developed alloy showed good rheo-formability in terms of low temperature sensitivity of solid phase and reasonable mechanical properties. The results showed that optimized mechanical properties and microstructure was obtained at 0.6 solid fraction and 450 rpm for screws. The average grain size changed from 300 by conventional casting to 16 µm by rheo-extrusion process, also shape factor changed from 0.3 to 0.9. The mechanical properties of the rheo-extruded samples at these conditions were: UTS of 682, 621 MPa 0.2 % proof stress and 10 % elongation. Furthermore grain coalescence and columnar growth of solid/liquid interface were the main mechanisms that could deteriorate the rheo-formability. Also results showed that increasing of rotation speed could refine grain size and eliminate the grain coalescence but could not overcome instability of interface.