Effects of retrogression and reaging heat treatment on the microstructure,exfoliation corrosion,electrical conductivity,and mechanical properties of AA7050

The effects of different retrogression times and temperatures on the microstructure, electrical conductivity, exfoliation corrosion (EXCO), and hardness of the 7050 aluminium alloy were compared with the T6 and T7451 condition. The EXCO solution was prepared in accordance with ASTM G34‐01 standard and the samples were kept in the solution for 48 hr. Corrosion depths were examined by using scanning electron microscope (SEM) in the cross‐sections microstructure of the samples. The effect of EXCO on the material was investigated using stereo optical microscope images. In microstructure investigations, SEM and energy dispersive X‐ray (EDX) analyses were used to determine the distribution and chemistry of the precipitates at the grain boundaries. The results show that retrogression and reaging (RRA) times and temperatures have significant effects on AA7050 EXCO. Especially after 200°C/30 min RRA process, it has been found that 7050 aluminium alloy has higher mechanical strength than T6 level and high corrosion resistance from T7451 level. According to the results of this study, it has been shown that there is a direct connection between electrical conductivity and EXCO.     

Investigation of the relationship between intergranular corrosion and retrogression and reaging in the AA6063

AA6063 was heat treated with different retrogression temperatures and durations, and the effect of heat treatment conditions on the microstructure, hardness, electrical conductivity, intergranular corrosion (IGC) and electrochemical corrosion behaviours of the AA6063 was determined compared with the T6 condition. The IGC test was applied according to the BS EN ISO 11846: 2008 standard. Moreover, potentiodynamic polarization tests were applied to determine the electrochemical corrosion behaviour of the heat‐treated samples. Electrochemical corrosion tests were carried out by using a Ivium Compactstat potentiostat in 3.5 wt.%. NaCl solution at 24°C with a scanning rate of 0.5 mV/s. The corrosion test cell consisted of the reference electrode (Ag/AgCl), working electrode (test sample) and a reference electrode (platinum). The effect of IGC on the microstructure of AA6063 and corrosion depth values was investigated by using a stereo optical microscope and a light metal microscope, respectively. Corrosion depth examinations were performed on microstructures taken from the cross‐sections of the samples. The chemistry of the precipitates formed at grain boundaries and distribution of the precipitates in the microstructure were investigated by scanning electron microscope, energy dispersive X‐ray and transmission electron microscope analyses. The results showed that retrogression and reaging heat treatment improves both the corrosion resistance and the mechanical properties of AA6063. After 50°C/15 min RRA heat treatment, the highest corrosion resistance and a higher hardness value than the T6 level were obtained.     

The stress corrosion cracking behaviour of heat‐treated Al–Zn–Mg–Cu alloy in modified salt spray fog testing

The stress corrosion cracking behaviour of 7075 (Al–Zn–Mg–Cu) alloy have been studied in a salt spray fog chamber with two vapourised aqueous solutions (0 and 5% NaCl). The paper analyses the stress corrosion resistance of 7075 aluminium alloy with several precipitation‐ageing heat treatments. The results are compared with that obtained in 3.5% NaCl aqueous solution at 20 °C. The salt spray fog testing has permitted a good evaluation of SCC susceptibility in 7075 alloy. All temper conditions studied were susceptible to SCC in the different environments tested. 7075‐T6 temper was the most susceptible, while in all the cases studied 7075‐T73 temper was the least susceptible. Compared to 7075‐T6, 7075‐RRA temper improved the resistance against the SCC process, but the mechanical properties obtained were lower.     

热处理制度对2056铝合金微观组织和抗应力腐蚀性能的影响

采用光学显微分析、扫描电镜及透射电镜观察、慢应变速率拉伸测试等研究2056铝合金在T6、T851及T351热处理状态下的微观组织和抗应力腐蚀性能.结果表明:2056合金在T6态下,晶内析出相主要为粗大的S'相和少量粗大的含锰相,抗拉强度为445.13 MPa,晶界析出相粗大且呈非连续分布,无沉淀析出带(PFz)为0.1～0.2μm,应力腐蚀敏感性最大;T851态下,晶内析出相主要为细小弥散的S'相,合金具有最高的抗拉强度,达到502.01MPa,晶界析出相呈离散状分布,PFZ较窄,约为0.02 μm,抗应力腐蚀性能优于T6态的;T351时效状态下,晶内观察到大量的位错和位错塞积以及少量GPB区,抗拉强度为469.73MPa,介于T6和T851之间,晶界无粗大平衡相析出,无明显PFZ,抗应力腐蚀性能最好.     

Effects of polarisation on mechanical properties and stress corrosion cracking susceptibility of 7050 aluminium alloy

The effects of polarisation (including cathodic and anodic polarisation) on mechanical properties and stress corrosion cracking (SCC) susceptibility (I_SCC) of 7050 aluminium alloy have been investigated by means of polarisation and slow strain rate test. The results of cathodic polarisation experiments showed that the I_SCC increased with shifting negatively the polarisation potential when the cathodic potential E_C≧?1100 mV(SCE), while it decreased with shifting negatively the polarisation potential when the cathodic potential E_CI_SCC increased severely with increasing the polarisation potential. In addition, the extents for the effect of polarisation potential on I_SCC were different among the 7050 aluminium alloy under various aging states. Polarisation was of the biggest effect on the I_SCC of under aged state, the smallest effect of over aged state and the middle effect of peak aged state. The SCC mechanism of aluminium alloy was a combination of anodic dissolution and hydrogen embrittlement, and the effects of hydrogen on SCC increased with increasing the hydrogen concentration.     

高温预析出对Al-Zn-Mg铝合金组织、力学性能和应力腐蚀性能的影响

研究了近固溶度高温预析出工艺对抗应力腐蚀性能较差的中强可焊7A52和仿7039合金的组织、时效硬化和应力腐蚀性能的影响.显微组织观察、力学性能分析及应力腐蚀性能测试结果表明:近固溶度高温预析出处理可使合金在晶界形成不连续的析出相;在保持合金强度、塑性的同时,预析出可提高抗应力腐蚀性能,但预析出温度过低,合金强度呈下降趋势;固溶后于400℃预析出处理在峰值时效状态下可使7A52铝合金强度和抗应力腐蚀性能得到最佳组合,抗应力腐蚀性能达到三级时效后的效果.     

二次回归再时效对Al-Zn-Mg-Cu组织、强度及剥蚀性能的影响(英文)

采用硬度、拉伸和剥落腐蚀测试手段,结合透射电镜和扫描电镜观察以及能谱分析,研究二次回归再时效对组织、力学和剥蚀性能的影响。二次回归再时效处理保持了类似一次回归再时效处理的晶内析出相,得到比T76过时效更粗大且离散的晶界析出相。与T76相比,二次回归再时效处理的晶界析出相具有更高的铜含量和更低的锌含量。因此,二次回归再时效处理保持了一次回归再时效处理的强度,同时得到了比T76过时效处理更高的剥蚀抗力。     

回归冷却速率对7050铝合金力学性能及晶间腐蚀抗力的影响

采用室温拉伸、抗晶间腐蚀性能测试(IGC)、光学显微镜(OM)及透射电镜(TEM)观察等手段,研究3种回归冷却速率(17℃/s,3℃/s,0.02℃/s)对回归再时效(RRA)态7050铝合金力学性能及抗晶间腐蚀性能的影响。结果表明,在快冷(17℃/s)条件下,回归态组织的过饱和固溶体含有较高的空位浓度,有利于再时效析出,再时效态组织的晶界析出相为较粗大的非连续颗粒,并有较宽的无沉淀析出带(PFZ);慢冷(3℃/s或0.02℃/s)条件下,在冷却过程中晶界和晶内均析出了微小的第二相,导致固溶体内空位浓度降低,影响再时效析出,使得再时效态组织的晶界析出相颗粒粗细不均匀,无沉淀析出带变窄。相应地,随冷却速率降低,合金的拉伸强度单调下降,抗晶间腐蚀性能先下降,后略有升高。     

Research on intercrystalline corrosion,exfoliation corrosion,and stress corrosion cracking of Al–Zn–Mg–Sc–Zr alloy

The intercrystalline corrosion, exfoliation corrosion (EXCO), and stress corrosion cracking (SCC) of Al–Zn–Mg–Sc–Zr alloy were investigated by means of constant temperature immersion corrosion method, optical microscopy, transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS). The results show that intercrystalline corrosion, and EXCO susceptibility of Al–Zn–Mg–Sc–Zr alloy decrease gradually with increasing of aging time. Corrosion susceptibility order from low to high is as follows: OA > PA > UA > NA. The SCC susceptibility index of PA temper is more than OA temper at the same strain rate. According to TEM observation, with aging time prolonging, a part of η′ phases transform to η equilibrium phases, which become coarse gradually. The distribution discontinuity of the grain boundary precipitates increases. In addition, for Al–Zn–Mg–Sc–Zr alloy without EXCO, the EIS is comprised by a capacitive impedance arc at high frequency and an inductive impedance arc at low frequency. Once EXCO occurs, the EIS is composed of two capacitive impedance arcs at high frequency and at low frequency, respectively.     

热处理对2519铝合金应力腐蚀开裂敏感性的影响

通过慢应变速率拉伸测试、扫描电镜及透射电镜分析等研究了2519铝合金在T6、T8热处理状态下,在空气和3.5%(质量分数)的NaCl溶液中的应力腐蚀开裂(SCC)行为,研究了应变速率对2519铝合金SCC行为的影响。结果表明:同一热处理状态下,2519铝合金在3.5%的NaCl溶液中应力腐蚀开裂敏感性高于干燥空气中;在应变速率为1.33×10-5s-1时,其SCC敏感性比应变速率为6.66×10-5s-1时的敏感性大;同一应变速率下,T6状态比T8状态SCC敏感性大,T8状态时抗应力腐蚀性能比T6状态时好。     

淬火速度对7085铝合金显微组织和应力腐蚀的影响(英文)

采用力学性能测试、慢应变速率拉伸实验,结合扫描电镜和透射电镜及电化学测试等方法,研究淬火速度对7085铝合金组织和应力腐蚀性能的影响。结果表明,随着淬火速度的降低,合金晶界析出相的尺寸和间距增大,晶界析出相的Cu含量降低;合金的抗应力腐蚀性能随着淬火速度的减少先增强后减弱。晶界析出相的尺寸和分布以及Cu含量是影响合金抗应力腐蚀性能的主要因素。     

Effects of Sn and Ca additions on microstructure,mechanical properties,and corrosion resistance of the as‐cast Mg‐Zn‐Al‐based alloy

Effects of Sn and Ca additions on microstructure, mechanical properties, and corrosion resistance of the as‐cast Mg‐6Zn‐2Al‐based alloy were investigated by SEM, XRD, tensile tests, electrochemical measurements, etc. The higher the Sn content, the higher the yield strength of the Mg‐6Zn‐2Al‐based alloy. Trace Ca addition refined both grains and divorced eutectics in the Mg‐Zn‐Sn‐Al alloys, leading to the best combination of strength and plasticity. Moreover, its influence was more significant for the alloy with a higher Sn content. Furthermore, the combined addition had a beneficial effect on the corrosion resistance improvement of the Mg‐6Zn‐2Al alloy. The Mg‐6Zn‐2Al‐3Sn‐0.2Ca alloy was the most corrosion‐resistant alloy among the nine alloys studied, better than AZ91 and ZA85. It could be ascribed to the decrease and the refinement of divorced eutectics, the higher hydrogen overvoltage of Sn and the Ca grain refinement.     

Effect of intermetallics on susceptibility of Mg alloy welds to liquation cracking

ABSTRACT

The circular-patch welding test was used to study the liquation and liquation cracking of AZ-series Mg alloys. A heat treatment was carried out on the as-received AZ91 alloy to dissolve the γ(Mg₁₇Al₁₂) particles before welding. The circular-patch welding test was then conducted on the heat-treated, as-received AZ91 alloy using AZ61 and AZ91 filler wires. The results showed that the susceptibility of AZ91 alloy to liquation and liquation cracking was significantly reduced by the dissolution of massive γ(Mg₁₇Al₁₂) particles via a heat treatment before welding as the liquation mechanism was changed. Both constitutional liquation and incipient melting occurred in the partially melted zone of the as-received AZ91 welds, while only incipient melting occurred in the heat-treated AZ91 welds.     

Si含量对7050铝合金显微组织和抗应力腐蚀开裂的影响

分别研究Si含量为0．094％、0．134％和0．261％的3种T7651态7050铝合金的组织和应力腐蚀开裂敏感性。结果表明：随着Si含量从0．094％增加到0．261％,Mg2Si相的面积分数从0．16％增加到1．48％,并且尺寸粗化;而其它粗大相（包括Al2CuMg、Mg（Al3Cu,Zn）2和Al7Cu2Fe）的面积分数从2．42％减小到0．78％。合金的电导率随Si含量的增加而增加。合金在空气中进行慢应变速率拉伸时,抗拉强度和伸长率随Si含量的增加而降低;而在3．5％NaCl溶液中进行慢应变速率拉伸时,随Si含量增加,合金应力腐蚀开裂敏感性降低。     

回归制度对7A20铝合金RRA过程中组织性能的影响

系统研究了RRA处理过程回归制度对7A20铝合金组织性能的影响,测试了力学性能和电导率,利用OM和TEM(HRTEM)表征了晶界和晶内的微观组织结构。结果表明:在160 ℃的回归温度下,随回归时间的延长,抗拉强度由630 MPa降低到525 MPa;经120 ℃保温12 h的再时效处理后,抗拉强度由645 MPa升高到710 MPa;回归时间越长,再时效引起的强度增量越大,80 min时达到最大值185 MPa。在200 ℃的回归温度下,回归时间为10 min时,便达到峰值强度715 MPa。随回归温度提高,导电率先升高后降低,与回归时间呈正相关关系。经160、200和240 ℃下峰值强度对应的回归处理后,基体晶粒尺寸分别为55、65和70 μm,相差不大。200 ℃×10 min的回归处理+再时效处理后,晶界处存在不连续的MgZn₂相,尺寸在10~20 nm,与基体呈非共格关系;晶内存在弥散分布的纳米级η′强化析出相,与基体呈共格或者半共格关系;晶界处存在45~55 nm宽度的PFZ(Precipitation free zone)区。     

AZ31镁合金激光焊件的力学性能和应力腐蚀行为

采用Nd-YAG激光对AZ31 HP镁合金进行激光自熔焊接。显微组织分析表明,使用或不使用填料（焊料）AZ61镁合金得到的激光焊接接头的平均晶粒尺寸大约为12μm,显微硬度和拉伸强度与母材相近。然而,慢应变速率拉伸表明,在ASTM D1384溶液中两种焊接接头的抗应力腐蚀性能比母材略差。可观察到应力腐蚀裂纹在焊缝金属萌生并向热影响区（HAZ）扩展。然而,在以AZ61镁合金为填料（焊料）获得的焊接接头中,观察到裂纹起源及扩展出现在热影响区（HAZ）。在慢应变速率拉伸试验中,由于试样表面暴露在腐蚀环境中,在氢氧化镁/氧化镁层形成局部损伤,从而导致应力腐蚀裂纹的生成。     

Al-Zn-Mg合金应力腐蚀断裂的研究进展

Al-Zn-Mg合金的应力腐蚀断裂成为其进一步应用的瓶颈问题。对近年来国内外Al-Zn-Mg合金应力腐蚀断裂的研究进展进行了概括,阐述了主要合金元素和微量元素对合金应力腐蚀断裂的影响,重点分析了高温预析出处理、单级时效、双级时效、三级时效等热处理对析出相和合金应力腐蚀敏感性的作用规律。针对Al-Zn-Mg合金应力腐蚀断裂研究目前存在的问题,提出了未来的研究方向。