Effect of thermo-mechanical processing on filiform corrosion of aluminium alloy AA3005

The effect of laboratory scale thermo-mechanical processing of aluminium alloy AA3005, supplied as 2.9 mm sheet at the hot roll transfer gauge, on filiform corrosion (FFC) susceptibility, electrochemical properties and near surface microstructure was investigated. Corrosion testing showed that the as hot rolled material exhibited poor FFC resistance and that heat treatment of both the as hot rolled and subsequently cold rolled samples resulted in a drastic loss of corrosion resistance. Microstructural characterisation showed that both hot and cold rolling resulted in enhanced surface shear deformation and the formation of a thin layer of metal in the surface with different properties and microstructural characteristics than the underlying bulk material. Enhanced surface deformation resulted in a redistribution of intermetallic particles in this region. Poor FFC properties were correlated with the presence of a higher density of fine intermetallic precipitates in the surface layers than in the underlying bulk. It is suggested that enhanced secondary precipitation of intermetallic particles in the deformed surface layers during high temperature exposure, e.g., during hot rolling or annealing steps, is the main contributing factor to the observed poor FFC resistance.     

Influence of heat treatment and surface conditioning on filiform corrosion of aluminium alloys AA3005 and AA5754

The effect of high-temperature heat treatment combined with different cleaning and pre-treatment practices on filiform corrosion resistance has been investigated for aluminium alloys AA3005 and AA5754. High-temperature heat treatment severely reduces the filiform corrosion resistance of alloy AA3005, while the corrosion properties of alloy 5754 are only moderately affected. The drastic loss of filiform corrosion resistance of alloy AA3005 after high-temperature heat treatment is attributed to preferential microstructural changes in a heavily deformed, micrograined surface layer caused by large surface shear strain during rolling. The enhanced deformation of the near-surface region promotes precipitation of a fine dispersion of intermetallic particles during subsequent heat treatment. The higher density of intermetallic particles combined with lower supersaturated solid solution levels of manganese in the surface layer as compared to the bulk structure makes the heat-treated AA3005 material susceptible to superficial corrosion attacks and results in poor filiform corrosion resistance. Application of a commercial acid cleaning/chromating pre-treatment did not restore the filiform corrosion resistance of heat-treated alloy AA3005. Alloy AA5754, containing lower levels of manganese and iron than AA3005, did not undergo similar preferential microstructural changes during heat treatment. A moderate increase in the amount of filiform corrosion of heat-treated AA5754 samples is attributed to poor protective properties of the thick, magnesium enriched, thermally formed surface oxide on this alloy. Any cleaning/pre-treatment practice that removes the thermally formed oxide on this alloy results in a very high filiform corrosion resistance.     

Effect of heat treatment on electrochemical behaviour of binary aluminium model alloys

In order to understand the role of various trace, impurity and alloying elements on the electrochemical activation of aluminium in chloride solution, binary alloys of type Al-Pb, Al-Bi, Al-In, Al-Sn, Al-Mg and Al-Zn were investigated by use of electrochemical and surface-analytical techniques. Al-Pb and Al-Bi alloys were electrochemically activated as a result of high temperature heat treatment. However, alkaline etching or mechanical polishing significantly reduced the activation. Alloys Al-In and Al-Sn were electrochemically activated regardless of surface condition. The activation of Al-Pb alloys is attributed to enrichment of lead, presumably in metallic form, at the metal-oxide interface by segregation and diffusion along the grain boundaries. In the presence of chloride, lead destabilises the surface oxide, giving rise to unexpected oxidation of the underlying aluminium at potentials well below the pitting potential. It is suggested that the resulting detachment of the Pb from the aluminium surface causes repassivation. Aluminium is not activated by Mg or Zn as a result of high temperature heat treatment.     

热处理对铝合金牺牲阳极电化学性能的影响

针对不同均匀化处理方式会引起合金微观组织和电化学性能变化现象,采用电子探针(EPMA)、动电位极化及恒电流极化等方法研究不同热处理(水淬、空冷、炉冷)方式对Al-Zn-In合金的微观组织和合金在3%NaCl溶液中的开路电位、工作电位、腐蚀形貌和电流效率等电化学性能的影响。结果表明:510℃,10 h的均匀化处理使晶界偏析减少,抑制析氢自腐蚀和晶粒脱落,提高电流效率,但对阳极开路电位、工作电位及溶解行为的影响并不明显,且水淬处理试样的电流效率最高。     

机加工AA7150铝合金的抗丝状腐蚀性能（英文）

采用扫描及透射电子显微技术表征机加工AA7150铝合金近表面变形层的显微组织,并结合动电位极化曲线及丝状腐蚀测试研究机加工后AA7150铝合金的丝状腐蚀行为。研究发现,机加工后AA7150铝合金近表面区域出现晶粒细化、合金元素重新分布以及晶界偏析等特征。机加工过程中强烈的塑性变形导致近表面区域显微组织的改变,从而极大地提高合金近表面变形层的腐蚀倾向。机加工使合金表面粗糙度增大,并导致形成电化学活性明显提高的近表面变形层。在二者共同影响下,机加工后AA7150铝合金的抗丝状腐蚀性能明显下降。     

Filiform corrosion of AA3005 aluminium analogue model alloys

The effect of the metal substrate microstructure on filiform corrosion (FFC) susceptibility was investigated for super purity based model alloys with compositions based on the specifications of AA3005. Variations in alloying levels of the elements iron, silicon and copper were investigated. Alloys with high silicon content were more susceptible to FFC than alloys with low silicon content. The iron content, at the levels investigated, did not strongly affect FFC properties. The apparent detrimental effect of a high silicon content is attributed to the influence of silicon on secondary intermetallic particle precipitation. Given the same thermo-mechanical treatment, alloys with high silicon content underwent more extensive secondary precipitation of manganese containing intermetallic particles than those alloys with a low silicon content. The resulting microstructure is characterised by a higher density of finely dispersed intermetallic particles and a lower content of manganese in the adjacent supersaturated solid solution. These conditions provide a large number of potential corrosion initiation sites on the surface and also enhance microgalvanic coupling between intermetallic particles and the surrounding aluminium rich matrix, thus promoting the propagation of filamental corrosion attacks. Additions of copper had a detrimental effect on the FFC resistance. The role of copper in promoting FFC is attributed to preferential dissolution phenomena during the corrosion process, whereby copper is locally enriched on the corroding surface. This copper enrichment provides additional area for cathodic reaction, thus enhancing the corrosion process.     

Effect of trace elements on electrochemical properties and corrosion of aluminium alloy AA3102

The purpose of this work is to study the effect of heat treatment and chemical processing on the electrochemical behaviour of aluminium alloy AA3102. Aluminium alloy 3102 was electrochemically activated in chloride solution as a result of heat treatment for periods exceeding 10 min at temperatures higher than 400 °C. The electrochemical activation was determined by the presence of deep negative potential transients when exposed to an acidified chloride solution. Furthermore, the anodic current densities became large at a given potential relative to the as-extruded surface as a result of high temperature heat treatment. This activation phenomenon was attributed to enrichment of the surface by lead, which was present in the material as a trace element. Enrichment of lead at the metal-oxide interface was ascertained by GD-OES depth profiling. Chemical and structural changes occurring in the oxide as a result of heat treatment did not have a direct role in the activation process. It was also shown that enrichment of the surface by lead had a sacrificial effect in protecting the surface against pitting corrosion.     

Fatigue behaviour of GMAW welded aluminium alloy AA7020

The aim of this investigation is to evaluate the influence on fatigue behaviour of the finishing of the bulge in a welded aluminium–zinc–magnesium alloy AA7020. It was determined that total or partial elimination of the bulge has very little influence on its behaviour, giving a very similar result on both cases, where one is better than the other by only 3%.     

Effect of solution treatment on the corrosion behaviour of aluminium alloy AA7150: Optimisation for corrosion resistance

Studies for Al-alloys normally employ specimens subject to a fixed solution heat treatment (SHT), and hence the specific role of solutionising is overlooked. It is revealed that SHT has a major role in corrosion of AA7150 as judged by electrochemical, microscopic and profilometry studies. SHT dictates the constituent particle type and population remaining in the alloy for subsequent processing, and is thus not only a principal factor in corrosion, but a factor that can be engineered in developing more damage tolerant AA7150. Effect of an optimised SHT minimising the residual MgZn₂ and Al₂MgCu content on corrosion resistance is demonstrated.     

Effect of post weld heat treatment on properties of variable polarity TIG welded AA2219 aluminium alloy joints

AA2219 aluminium alloy joints were fabricated by variable polarity tungsten inert gas (VPTIG) welding process and the effects of post weld heat treatment (PWHT) on the tensile properties, microstructure and fatigue behaviour of the welded joints were investigated. The VPTIG welding process was adopted because it could meet the need of cathode cleaning and meanwhile it could reduce the deterioration of tungsten electrode furthest. The welded samples were divided into as-welded (AW) sample and PWHT sample. The PWHT method used on the samples was solution treatment (535 °C, 30 min), water quenching and artificial aging (175 °C, 12 h). The experimental results show that, compared with the AW samples, the microstructure characteristics and mechanical properties of the AA2219 joints after PWHT were significantly improved. The improvement of yield strength, ultimate tensile strength, and fatigue strength are 42.6%, 43.1% and 18.4%, respectively.     

The effect of heat treatment on the corrosion behaviour of 319 aluminium alloy

The effect of various heat treatments on the corrosion behaviour of 319 T1 cast aluminium alloy was investigated. From this alloy, specimens were heat treated in T5, T6 and two steps solution heat treatment T6 conditions and afterwards were subjected to electrochemical corrosion in a 0.1 M NaCl solution (pH = 12). From the above treatments, T5 heat treatment did not improve the corrosion resistance of the as‐received alloy in contrast to T6 heat treatment which improved the corrosion resistance of the same alloy. However, two steps solutionizing T6 treatment showed the best corrosion resistance of the aluminium alloy.     

Corrosion behaviour of friction stir welded AA7108 T79 aluminium alloy

The corrosion behaviour of a friction stir welded AA7108 T79 aluminium alloy has been investigated using accelerated testing (ASTM G34 EXCO) and electrochemical measurements. The welded alloy showed the expected zones associated with friction stir welding, namely nugget, thermomechanically affected zone and heat affected zone. Corrosion testing revealed that the edge regions of the thermomechanically affected zone were most susceptible to corrosion. The localized corrosion occurs intergranularly due to the non-uniform distribution of η/η′ (MgZn₂) precipitates within the thermomechanically affected zone.     

焊接方法对AA2219铝合金接头性能的影响(英文)

使用钨电极惰性气体保护焊接、电子束焊接和搅拌摩擦焊接技术制备无填充金属的AA2219铝合金对焊接头。研究三种焊接工艺对材料拉伸、疲劳和腐蚀行为的影响。使用光学和电子显微镜研究显微结构。结果表明,与钨电极惰性气体保护焊和电子束焊接相比,搅拌摩擦焊制备的接头具有较高的拉伸和疲劳性能与较低的耐蚀性能,这主要是由于其中的细化晶粒和均匀分布的强化析出相所引起的。     

Effect of tantalum film on corrosion behaviour of AA6061 aluminium alloy in hydrochloric acid- and chloride-containing solutions

ABSTRACT

The present work is concerned with the corrosion resistance of AA6061 aluminium alloys with tantalum films in hydrochloric acid- and chloride-containing solutions. The tantalum films were produced by magnetron sputtering at different sputtering times (50 and 120 min). The films’ morphologies were observed by metallographic microscope and scanning electron microscope with energy dispersion spectrum. It is shown in this paper that with longer sputtering time, the film’s thickness increased, but it became less dense. The corrosion behaviour was characterised by potentiodynamic polarisation, scanning electrochemical microscopy analysis and an immersion experiment. These investigations revealed that the corrosion resistance of AA6061 aluminium alloy in chloride ions medium and hydrochloric acid was significantly improved after deposition of a tantalum film. Specifically, samples deposited for 50 min exhibited the best corrosion resistance in hydrochloric acid, while samples deposited for 120 min showed best corrosion resistance in sodium chloride solution.     

多级热处理工艺对铝合金AA7049性能的影响(英文)

T6态高强7xxx系列铝合金对应力腐蚀开裂敏感。采用回归和再时效热处理(RRA)可以提高其抗应力腐蚀开裂性能而不降低其强度性能。研究了多级热处理工艺对7049铝合金性能和组织的影响。通过电导率测量、DSC分析和TEM组织观察,考察合金在不同热处理态的组织变化。DSC分析表明,RRA处理会导致合金的显微组织发生显著变化,RRA处理态合金的组织与T6和T73态合金的组织明显不同。RRA处理可以使合金保持在T6态的强度且获得T73态的热力学稳定性能。     

Compression behaviour of semisolid YL112 die casting aluminium alloy following isothermal heat treatment

Abstract

The semisolid compression deformation behaviour of YL112 die casting aluminium alloy with the non-dendritic and dendritic structures has been compared in tests using a Gleeble-1500 thermomechanical simulator. The non-dendritic structure was obtained by isothermal treatment at 570°C for 120 min. In tests up to compressive strains of 0·8, the semisolid compression stress of the alloy with non-dendritic structure initially increases rapidly with increasing strain, then decreases, before reaching a plateau value. Under different deformation temperatures and deformation rates, the maximum compressive stresses are obtained in all cases at strain values of ～0·07. The semisolid deformation stress decreases with increasing deformation temperature and increases with increasing deformation rate. The compression behaviour of the two types of alloy differs. The semisolid compression stress of the alloy with dendritic structure is higher than that of the alloy with non-dendritic structure; and for strains >0·07, the semisolid compression stress increases and decreases with increasing strain for alloys with dendritic and non-dendritic structures respectively.     

热处理对Al-Zn-In-Mg-Ti合金组织与电化学性能的影响

通过对Al-5Zn-0.03In-1Mg-0.07Ti牺牲阳极进行510℃固溶和退火处理,比较该阳极在铸态、固溶及退火态下的组织、电化学性能及腐蚀形貌.结果表明:固溶处理减少了该合金的偏析相,提高了电流效率及实际电容量,腐蚀均匀性有所改善,但工作电位不稳定;退火处理细化了该合金组织,偏析相增多,电流效率达95.9%,实际电容量达2746 A·h·kg-1,且腐蚀均匀;退火处理效果优于固溶处理,有利于该阳极的实际应用.     

A comparative electrochemical study of commercial and model aluminium alloy (AA5050)

A comparative electrochemical study of commercially recycled AA5050 and model AA5050, in their as‐cast state, is carried out to investigate the effect of recycling and furthermore the effect of trace elements like Pb, Cu, etc on electrochemical activation and filiform corrosion (FFC) susceptibility. A systematic combination of surface analytical techniques like scanning Kelvin probe force microscopy (SKPFM), scanning electron microscopy (SEM) and energy dispersive X‐ray (EDX) is used to study the composition and electrochemical behaviour of second phase particles. These results, together with localized breakdown measurements using the electrochemical micro‐cell, elucidate the effect of recycling and the role played by trace elements in influencing the electrochemical behaviour of the alloy.