Strengthening-toughening of 7xxx series high strength aluminum alloys by heat treatment

The effects of stepped solution heat treatments on the dissolution of soluble remnant constituents and mechanical properties of 7055 aluminum alloy were investigated. It was shown that a suitable pretreatment at lower temperature can enable complete dissolution of the constituent particles in 7055 alloy without overheating by subsequent high temperature solution treatment. This in turn increased the tensile strength and fracture toughness of 7055 alloy to 805 MPa and 41.5 MPa·m 1/2 respectively, with approximately 9% tensile elongation. The near solvus pre precipitation following after high temperature solution treatment was also studied on 7055 aluminum alloy. The effect of the pre precipitation on the microstructure, age hardening and stress corrosion cracking of 7055 alloy was investigated. The optical and transimission electron microscopy observation show that the near solvus pre precipitation can be limited to grain boundary and enhance the discontinuity of grain boundary precipitates in the subsequent ageing. The stress corrosion cracking resistance of aged 7055 alloy can be improved via the pre precipitation with non deteriorated strength and plasticity.

Impact of cooling rate on exfoliation corrosion resistance of a Li-containing 7xxx aluminum alloy

The impact of cooling rate after solution heat treatment on exfoliation corrosion resistance of a Li-containing 7xxx aluminum alloy was investigated by accelerated immersion and electrochemical impedance spectroscopy test,optical microscope,electron backscatter diffraction and scanning transmission electron microscope.With the decrease of cooling rate from 1700℃/s to 4℃/s,exfoliation corrosion resistance of the aged specimens decreases with rating changing from EA to EC and the maximum corrosion depth increasing from about 169.4μm to 632.1μm.Exfoliation corrosion tends to develop along grain boundaries in the specimens with cooling rates higher than about 31℃/s and along both grain boundaries and sub-grain boundaries in the specimens with lower cooling rates.The reason has been discussed based on the changes of the microstructure and microchemistry at grain boundaries and sub-grain boundaries due to slow cooling.

7000系高强铝合金微观组织结构对应力腐蚀行为和机制影响的研究进展

7000系(Al-Zn-Mg-Cu)高强铝合金是铝合金中强度最高的系列之一,应力腐蚀开裂(SCC)是影响其服役安全稳定性的关键因素。综述了应力腐蚀开裂的阳极溶解机制和氢致开裂(HIC)机制,总结了7000系铝合金中的组织结构对合金SCC行为及机制的影响;从合金化学成分、热处理、预应变、表面处理等方面对SCC敏感性的影响出发,论述了不同组织结构对SCC行为的影响。提出了降低7000系高强铝合金SCC敏感性的重点研究方向:通过优化合金元素、使用新型的热处理工艺和表面处理技术等方法合理调控合金的微观组织结构,进而延长合金的使用寿命,使材料受到更好地保护。

Stress corrosion cracking behaviour of 7xxx aluminum alloys: A literature review

Stress corrosion cracking （SCC） is degradation of mechanical properties under the combined action of stress and corrosive environment of the susceptible material. Out of eight series of aluminium alloys, 2xxx, 5xxx and 7xxx aluminium alloys are susceptible to SCC. Among them, 7xxx series aluminium alloys have specific application in aerospace, military and structural industries due to superior mechanical properties. In these high strength 7xxx aluminium alloys, SCC plays a vital factor of consideration, as these failures are catastrophic during the service. The understanding of SCC behaviour possesses critical challenge for this alloy. The main aim of this review paper is to understand the effect of constituent alloying elements on the response of microstructural variation in various heat-treated conditions on SCC behavior. Further, review was made for improving the SCC resistance using thermomechanical treatments and by surface modifications of 7xxx alloys. Apart from a brief review on SCC of 7xxx alloys, this paper presents the effect of stress and pre-strain, effect of constituent alloying elements in the alloy, and the effect of environments on SCC behaviour. In addition, the SCC behaviours of weldments, 7xxx metal matrix composites and also laser surface modifications were also reviewed.

Effect of three-step homogenization on microstructure and properties of 7N01 aluminum alloys

The effect of different homogenization treatments on the microstructure and properties of the 7N01 aluminum alloy was investigated using hardness measurements, electrical conductivity measurements, tensile and slow strain rate tests, electron probe microanalysis, optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results revealed that three-step homogenization improved the uniformity of Zr distribution by eliminating segregation of the main alloying elements. During the second homogenization step at 350 °C for 10 h, coarse and strip-like equilibrium η phases formed which assisted the nucleation of Al3Zr dispersoids and reduced the width of the precipitate-free zone of A13Zr dispersoids. As a result, coarse recrystallization was greatly reduced after homogenization at 200 °C, 2 h ＋ 350 °C, 10 h ＋ 470 °C, 12 h, which contributed to improving the overall properties of the 7N01 aluminum alloys.

7xxx系铝合金焊接技术研究进展

7xxx系铝合金作为所有系列铝合金强度最高的金属材料,具有优良的综合性能,被广泛应用于各领域中。本文从7xxx系铝合金焊接质量和焊接效率出发,综述了几种常见的7xxx系铝合金焊接技术的发展现状,重点介绍了熔化极惰性气体保护焊(MIG焊)、冷金属过渡焊(CMT)、钨极惰性气体保护焊(TIG焊)等传统电弧焊以及激光焊、电子束焊、搅拌摩擦焊等新型焊接技术的技术特点与应用前景,总结了不同焊接技术相应的优势与不足以及目前各焊接技术急待解决的问题,最后对7xxx系铝合金焊接技术发展前景进行了总结。

高强度7xxx系铝合金热处理调控残余应力研究进展

随着航天装备的不断大型化和轻量化,7xxx系铝合金构件普遍呈尺寸规格大、形状复杂、薄壁弱刚性等结构特点,在加工和使用中易产生强烈的残余应力效应;残余应力的产生会影响加工质量,甚至导致失效。因此,如何有效调控制造过程中的残余应力成为亟待解决的关键难题。总结了航天领域高强度7xxx系铝合金构件残余应力热处理控制技术的研究现状,详细介绍了淬火、冷热循环处理、深冷处理、时效去应力对残余应力的调控效果及作用机制,提出了目前航天高强度7xxx系铝合金残余应力热处理调控的主要问题及发展方向。

Effect of homogenization treatment on microstructure and mechanical properties of DC cast 7X50 aluminum alloy

The evolution of the eutectic structures in the as-cast and homogenized 7X50 aluminum alloys was studied by scanning electron microscopy(SEM), transmission electron microscopy(TEM), energy dispersive spectrometer(EDS), differential scanning calorimetry(DSC), X-ray diffraction(XRD) and tensile test. The results show that the main phases are S(Al2CuMg), T(Al2Mg3Zn3) and Mg Zn2, with a small amount of Al7Cu2 Fe and Al3 Zr in the as-cast 7X50 alloy. The volume fraction of the dendritic-network structure and residual phase decreases gradually during the homogenization. After homogenization at 470 °C for 24 h and then 482 °C for 12 h, the T(Al2Mg3Zn3) phase disappears and minimal S(Al2CuMg) phase remains, while almost no change has happened for Al7Cu2 Fe. There is a strong endothermic peak at 477.8 °C in the DSC curve of as-cast alloy. A new endothermic peak appears at 487.5 °C for the sample homogenized at 470 °C for 1 h. However, this endothermic peak disappears after being homogenized at 482 °C for 24 h. The T(Al2Mg3Zn3) phase cannot be observed by XRD, which is consistent with that T phase is the associated one of S(Al2CuMg) phase and Mg Zn2 phase. The volume fraction of recrystallized grains is substantially less in the plate with pre-homogenization treatment. The strength and fracture toughness of the plate with pre-homogenization treatment are about 15 MPa and 3.3 MPa·m1/2 higher than those of the material with conventional homogenization treatment.

Non-isothermal retrogression kinetics for grain boundary precipitate of 7A55 aluminum alloy

The retrogression kinetics for grain boundary precipitate （GBP） of 7A55 aluminum alloy was investigated by transmission electron microscopy （TEM） observation. The results reveal that the coarsening behavior of GBP obeys “LSW” theory, namely, the cube of GBP average size has a linear dependence relation to retrogression time, and the coarsening rate accelerates at the elevated retrogression temperature. The GBP coarsening activation energy Qo of （115.2±1.3） kJ/mol is obtained subsequently. Taking the retrogression treatment schedule of 190℃, 45 min derived from AA7055 thin plate as reference, the non-isothermal retrogression model for GBP coarsening behavior is established based on “LSW”theory and “iso-kinetics” solution, which includes an Arrhenius form equation. After that, the average size of GBP r（t） is predicted successfully at any non-isothermal process T（t） when the initial size of GBP r0 is given. Finally, the universal characterization method for the microstructure homogeneity along the thickness direction of TA55 aluminum alloy thick plate is also set up.

Rate controlling mechanisms in hot deformation of 7A55 aluminum alloy

The hot deformation behavior of 7A55 aluminum alloy was investigated at the temperature ranging from 300 ℃ to 450 ℃ and strain rate ranging from 0.01 s-1 to 1 s-1 on a Gleeble-3500 simulator. Processing maps were established in order to apprehend the kinetics of hot deformation and the rate controlling mechanism was interpreted by the kinetic rate analysis obeying power-law relation. The results indicated that one significant domain representing dynamic recrystallization （DRX） existed on the processing maps and lying in 410-450 ＆#176;C and 0.05-1 s-1. The conclusions of kinetic analysis correlated well with those obtained from processing maps. The apparent activation energy values calculated in the dynamic recrystallization （DRX） domain and the stability regions except dynamic recrystallization （DRX） domain were 91.2 kJ/mol and 128.8 kJ/mol, respectively, which suggested that grain boundary self-diffusion and cross-slip were the rate controlling mechanisms.

Evolution of secondary phases and properties of 7B04 aluminum alloy during DC homogenization

The effects of the direct current （DC） on the evolutions of hardness and morphology of the secondary phases in 7B04 aluminum alloy homogenized at 380？465 ℃ for 2 h were investigated in detail by electric conductivity measurement, hardness test, X-ray diffraction analysis, field emission scanning electron microscopy and energy dispersive spectrometry. The results show that with increasing temperature from 380 to 465 ℃, the electric conductivity of normal homogenized sample decreases from 34.9%IACS to 28.7%IACS, the hardness increases from HV 96 to HV 146, and the area fraction of secondary phase reduces from 4.5% to 1.89%. While, DC homogenized sample has a higher hardness, a lower electric conductivity and a smaller area fraction of secondary phases at the same temperature. The DC enhances the homogenization process by promoting the diffusibility of the solute atoms and the mobility of vacancy.

Characterization of pitting corrosion of 7A60 aluminum alloy by EN and EIS techniques

The pitting corrosion behaviors of 7A60 aluminum alloy in the retrogression and re-aging （RRA） temper were investigated by electrochemical impedance spectroscopy （EIS） and electrochemical noise （EN） techniques, and the microstructure and the second phase content of the alloy were observed and determined by scanning electron microscopy （SEM） and energy dispersive spectrometer （EDS）. The results show that there exist two different corrosion stages for 7A60 alloy in 3.5%NaCl solution, and the corrosion process can be detected by the appearance of EIS spectrum with two capacitive time constants and the wavelet fractal dimension D extracted from EN. SEM and EDS results also demonstrate that severe pitting corrosion in 7A60 alloy is mainly caused by electrochemical active MgZn2 particles, secondly by Al2MgCu and Mg2Si. Al7Cu2Fe particles make little contribution to the pitting corrosion of 7A60 alloy.

Research on microstructure in as-cast 7A55 aluminum alloy and its evolution during homogenization

The microstructure of the as-cast 7A55 aluminum alloy and its evolution during homogenization were investigated by means of optical microscopy （OM）, scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, X-ray diffraction （XRD）, and differential scanning calorimetry （DSC） analysis. The results indicate that the microstructure of the as-cast 7A55 aluminum alloy mainly consists of the dendritic network of aluminum solid solution, Al/AIZnMgCu eutectic phases, and intermetaUic compounds MgZn2, Al2CuMg, Al7Cu2Fe, and Al23CuFe4. After homogenization at 470℃ for 48 h, Al/AlZnMgCu eutectic phases are dissolved into the matrix, and a small amount of high melting-point secondary phases were formed, which results in an increasing of the starting melting temperature of 7A55 aluminum alloy The high melting-point secondary phases were eliminated mostly when the homogenization time achieved to 72 h. Therefore, the reasonable homogenization heat treatment process for 7A55 aluminum alloy ingots was chosen as 470℃/72 h.

Effect of heat treatment on mechanical properties of thixoformed 7A09 aluminum alloy

The effects of solution-ageing treatment on the mechanical properties of semi-solid metal(SSM)products were studied by the method of orthogonal experiment.The microstructure and tensile properties at different treatment conditions were analyzed.The results show that the effects of solution-ageing treatment were obviously influenced by the original microstructure of the SSM billet. Higher temperature is favorable for the sufficient solution of alloy elements,but the grains will grow up when treated at a higher temperature or soaked for a longer time.There is a contradiction between the maximum tensile strength and elongation percentage. Under the condition of solution treatment at 475℃for 12 h and at 133℃for 20-22 h ageing,better properties including about 540 MPa of tensile strength and more than 6.3%of elongation can be obtained in general.The surplus phases not melted in the substrate and the solution precipitated supersaturated elements are the main factors influencing the properties of the alloys.

Computation of synthetic surface heat transfer coefficient of 7B50 ultra-high-strength aluminum alloy during spray quenching

According to inverse heat transfer theory, the evolutions of synthetic surface heat transfer coefficient（SSHTC） of the quenching surface of 7B50 alloy during water-spray quenching were simulated by the Pro CAST software based on accurate cooling curves measured by the modified Jominy specimen and temperature-dependent thermo-physical properties of 7 B50 alloy calculated using the JMat Pro software. Results show that the average cooling rate at 6 mm from the quenching surface and 420-230 ℃（quench sensitive temperature range） is 45.78℃/s. The peak-value of the SSHTC is 69 kW/（m^2·K） obtained at spray quenching for 0.4 s and the corresponding temperature of the quenching surface is 160 ℃. In the initial stage of spray quenching, the phenomenon called ＂temperature plateau＂ appears on the cooling curve of the quenching surface. The temperature range of this plateau is 160-170℃ with the duration about 3 s. During the temperature plateau, heat transfer mechanism of the quenching surface transforms from nucleate boiling regime to single-phase convective regime.

Effect of connection processes on mechanical properties of 7B04 aluminum alloy structures

The static and fatigue properties of 7B04 aluminum alloy structures connected by riveting and refill friction stir spot welding(refill FSSW)were compared and analyzed.Results show that the static compression load of the typical structure connected by riveting and refill FSSW fluctuated in the range of 117-124 kN,and the shear load was in the range of 89-95 kN.Welds spacing had a small influence on the static load of the structures joined by refill FSSW.However,the fatigue life of riveted structures was lower than that joined by refill FSSW.For the welded structure,the heterogeneous microstructures of the welded joint led to the uneven microhardness,and the hook at the lap interface bent upwards at the same time.These factors made the welded structures during the fatigue test failure along the path of sleeve moving.

Investigation of Stress Corrosion Cracking Initiation of 7A52 Aluminum Alloy

The stress corrosion cracking（SCC） behaviour of 7A52 aluminum alloy in air and in 3.5% NaCl solution was researched by slow strain rate test（SSRT） and SEM-EDS. The SCC susceptibility was estimated with the loss of the reduction in area. The experimental results indicate that the SCC susceptibility of 7A52 aluminum alloy in 3.5% chloride solution is the highest at strain rate of 1×10-6 s-1. The lowest one is under the condition of 1×10-5 s-1. Stress concentration and anode dissolving around Al-Fe-Mn intermetallics initiate micropores which will result in microcracks. The existence of intermetallics in the microstructure may play an important role in understanding the SCC initiation mechanisms of 7A52 aluminum alloy.

Simulation and experimental study of 7A09 aluminum alloy milling under double liquid quenching

To explore the influence of double liquid quenching on the cutting performance of the 7A09 aluminum alloy,quasi-static compression and dynamic impact tests were carried out on the 7A09 aluminum alloy after double liquid quenching using an MTS810.23 universal testing machine and split-Hopkinson pressure bar(SHPB).The experimental data were fitted to obtain the Johnson–Cook constitutive model parameters of the alloy.Simulations of the machining process were carried out using the Deform-3D finite element software.The results showed that the rheological stress increased with the increase in strain rate and the decrease in temperature.The increase in the cutting speed and feed caused the cutting temperature to rise sharply,whereas the influence of the cutting amount on the cutting temperature was weak.Because of the presence of chip nodules,there was extremum in the cutting force vs cutting speed curves.The increase in the feed and cutting depth increased the cutting area Ac,so the cutting force also increased.The simulation results were verified by experiments.The simulation predictions were in good agreement with the test values,and the cutting force and temperature variations with the cutting parameters were the same.Thus,the correctness of the 7A09 aluminum alloy finite element model was verified.

超高强7XXX系铝合金的研究现状及发展趋势

针对7XXX系铝合金从合金化、形变热处理以及近年来兴起的异构组织调控三方面对其研究现状及发展趋势进行总结。综述了合金化、形变热处理(包括大塑性变形)对7XXX系铝合金力学性能的影响,介绍了异构组织的思想、种类及其对7XXX系铝合金力学性能的影响,展望了超高强7XXX系铝合金的发展趋势。旨在为超高强7XXX系铝合金材料产业的持续发展提供帮助。

7X75系列铝合金的发展与展望

7X75系列铝合金作为用途最为广泛的超高强度变形铝合金,凭借其超高的强度,良好的断裂韧性和抗应力腐蚀能力,在固溶处理后良好的塑性,良好的热处理强化效果,在航空航天领域的应用占比很大。本文主要综述了7X75系列铝合金的发展进程、研究现状和典型7X75系列铝合金的特点及应用,介绍了7X75系列铝合金的主要热处理工艺以及各类元素在7X75系列铝合金中的主要作用。