Aerospace Aluminum Alloy Research Articles

Root cause analysis of pitting corrosion of AA2024 and AA7075 after exposure in salt fog environment

Two grades of anodised aerospace aluminium alloys, AA2024 and AA7075, were analysed to determine root causes of pitting corrosion after the samples were exposed to a salt spray test according to ASTM B177. Methods to investigate pitting corrosion behaviour of the aluminium alloys were divided into two steps: 1) characterisation of pre-exposed (intact) anodised film and stimulated pitting corrosion by polarisation; 2) comparison of samples that passed and failed the salt spray test. The surface properties were characterised by roughness measurements, scanning electron microscope (SEM) and energy dispersion X-ray spectroscopy (EDS). Pitting corrosion was stimulated by potentiodynamic polarisation prior to characterisation of pitting by SEM and EDS to determine pitting initiation point. The results reveal that thickness of the film does not prevent localised corrosion. Cu-bearing intermetallic particles are detrimental to anodic film formation and cause pitting in the film. Potentiodynamic polarisation tests alone cannot determine whether the film will pass or fail the salt spray test for 96 hours.

Reduction in Residual Stress during Plastic Deformation of the Aluminum Alloy 7449

Abstract Rectilinear blocks of the very high-strength aerospace aluminum alloy 7449 were solution treated and cold water quenched. They were then stress relieved immediately by cold compression. The amount of cold compression was systematically varied from 0 to 0.3 %. The amount of plastic strain was limited to well below the industrial standard of ~2 %. The blocks then received a stabilizing aging treatment. The residual stress distribution remaining in the blocks was characterized using neutron diffraction, with the aim of tracking how the through thickness 3-D residual strains and stresses change as cold compression progresses. Stress relief was of the order of 30–50 %, but there was no discernible systematic increase in stress relief as the plastic deformation magnitude was increased. It is suggested that this is mainly due to the differences between the samples being less than the experimental uncertainty of the neutron diffraction strain measurement technique.

INVESTIGATION OF DISTINCTIVE SOLUTIONIZING PROCESS PARAMETERS ON SIGNIFICANTLY INCREASING IN STRENGTH AND DUCTILITY OF As-CAST AZ80 Mg ALLOY

A distinctive solutionizing process parameter was developed and performed to significantly enhance strength and ductility of aged AZ80 Mg alloy. The microstructure-processing-properties relationship was studied to help elucidate the mechanism. Microstructure and mechanical properties were studied by using optical microscopy, Field Emission Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction, hardness and tensile testing. The results show that the solutionization at 380°C/10h followed by 420°C/10h promoted significant dissolution of eutectic Mg17Al12 and intermetallic phases. After standard aging at 200°C/10h, the formation of high-density precipitates Mg17Al12 with more spheroidization precipitates and lesser discontinuous precipitates morphology, which was not observed in conventional heat treatment for aged AZ80 Mg alloy. Fine precipitates Mg17Al12 resulted in a significant increase in ultimate tensile strength (50.3%), yield strength (35%), elongation (106%) and hardness (40%) compared to conventional heat treated cast AZ80 Mg alloy. These properties are comparable to the commonly used aerospace grade cast aluminium alloy A356.

Numerical Modeling the Effects of Chamfer and Hone Cutting Edge Geometries on Burr Formation

A finite element based numerical model to simulate orthogonal machining process and associated burr formation process has been developed in the presented work. To incorporate simultaneous effects of mechanical and thermal loadings in high speed machining processes, Johnson and Cook`s thermo-visco-plastic flow stress model has been adopted in the conceived numerical model. A coupled damage-fracture energy approach has been used to observe damage evolution in workpiece and to serve as chip separation criterion. Simulation results concerning chip morphology, nodal temperatures, cutting forces and end (exit) burr have been recorded. Model has been validated by comparing chip morphology and cutting force results with experimental findings in the published literature. Effects of cutting edge geometries [Hone and Chamfer (T-land)] on burr formation have been investigated thoroughly and discussed in length. To propose optimum tool edge geometries for reduced burr formation in machining of an aerospace grade aluminum alloy AA2024, numerical analyses considering multiple combinations of cutting speed (two variations), feed (two variations) and tool edge geometries [Hone edge (two variations), Chamfer edge (four variations)] have been performed. For chamfer cutting edge, the “chamfer length” has been identified as the most influential macro geometrical parameter in enhancing the burr formation. Conversely, “chamfer angle” variation has been found least effecting the burr generation phenomenon.

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

Aluminum alloys used for aerospace applications provide good strength to weight ratio at a reasonable cost but exhibit only limited corrosion resistance. Therefore, a durable and effective corrosion protection system is required to fulfil structural integrity. Typically, an aerospace corrosion protection system consists of a multi-layered scheme employing an anodic oxide with good barrier properties and a porous surface, a corrosion inhibited organic primer, and an organic topcoat. The present review covers published research on the anodic oxide protection layer principles and requirements for aerospace application, the effect of the anodizing process parameters, as well as the importance of process steps taking place before and after anodizing. Moreover, the challenges of chromic acid anodizing (CAA) substitution are discussed and tartaric-sulfuric acid anodizing (TSA) is especially highlighted among the environmentally friendly alternatives.

Hole-Edge Corrosion Expansion Monitoring Based on Lamb Wave

Corrosion is a critical issue for engineered metallic components in mechanical and aerospace industries. Due to the complexity of aerospace aluminum alloy structure, corrosion is particularly tend to occur and expand in stress concentration areas, such as the edge of a hole, which causes the overall structure to be more likely to fail. In this paper, a Lamb wave-based active sensing method with improved sensors network was used to detect the hole-edge corrosion expansion. A0 wave packet of Lamb wave is extracted from signals, and two damage factors are used as characteristics of the signals. Probabilistic imaging algorithm is used to imaging and quantify the hole-edge corrosion area. Five corrosion extension tests show that the proposed method can effectively locate and quantify the hole-edge corrosion damage expansion of a single-hole structure; furthermore, the normalized amplitude damage index and phase change damage index can be used to predict hole-edge corrosion expansion effectively.

Graphene oxide—Polymethyl methacrylate coatings for Corrosion protection of aerospace aluminium alloy 7075—T651 surfaces

We report a corrosion study on Al 7075—T651 surface. The purpose of the study is to use Graphene oxide—Polymethyl methacrylate (GO—PMMA) coating as corrosion resistant layer on Al 7075—T651, an alloy of aluminium mainly used in aerospace industries. The study is done with and without GO—PMMA coating on Al 7075—T651 respectively, by varying the exposure time of material (24, 48 and 72 h) with HCl in atmospheric condition. The formation of corrosion is resisted by the GO—PMMA coated samples. The effect is studied by material characterization and non-destructive evaluation (NDE) techniques. The formation of aluminium hydroxide in uncoated samples is confirmed by Raman spectroscopy and x-ray diffraction (XRD). Corrosion formation, GO—PMMA coating on plain and coated samples are clearly visible in Scanning Electron Microscope (SEM) study. Shift in current is observed in I–V Characteristics study, due to pitting in plain samples. Material loss is observed, to be low in GO—PMMA coated samples, using pulsed thermography. The work proves that the GO—PMMA coating is very effective in corrosion resistance.

In-situ nanoscopic observations of dealloying-driven local corrosion from surface initiation to in-depth propagation

Dealloying is involved in materials science responsible for fabrication of nanoscale structures beneficially but for corrosion degradations detrimentally. Detailed understanding related to the latter is critical for designing corrosion-resistance alloys and dedicated inhibition systems. Thus, direct nanoscopic observations of nano-structural and compositional evolutions during the process are essential. Here using liquid phase-transmission electron microscopy (LP-TEM), for the first time, we show dynamic evolution of intricate site-specific local corrosion linked to intermetallic particles (IMPs) in aerospace aluminium alloys. To thoroughly probe degradation events, oxidation direction is controlled by purposefully masking thin specimens, allowing for observing top-view surface initiation to cross-sectional depth propagation of local degradations. Real-time capturing validated and supported by post-mortem examinations shows a dealloying-driven process that initiates at IMPs and penetrates into the depth of the alloy, establishing macroscopic corrosion pits. Besides, controversial mechanisms of noble-metal redistribution are finally elucidated.

Dealloying-driven local corrosion by intermetallic constituent particles and dispersoids in aerospace aluminium alloys

Nanoscopic characterization of heterogeneous intermetallic particles (IMPs) which microstructurally and compositionally evolve during local corrosion is crucial in unravelling the mechanisms and sequence of initial and local corrosion events. Herein, we study site-specific initiation events focused on microscopic constituent intermetallic compounds and nanoscopic dispersoids in AA2024-T3 at the nanoscale using a combined quasi in-situ and ex-situ analytical TEM approach. Our findings show a dealloying-driven local corrosion initiation at the studied IMPs that have been considered as cathodic phases traditionally. Besides, local degradation which is a result of galvanic interactions between dealloyed regions of IMPs and their adjacent alloy matrix is largely governed by the intrinsic electrochemical instability of intermetallic compounds.

Stochastic Tensile Failure Analysis on Dissimilar AA6061-T6 with AA7075-T6 Friction Stir Welded Joints and Predictive Modeling

The issue of weld solidification cracking in fusion welding of highly alloyed aerospace aluminum alloys is eliminated in friction stir welding (FSW) because the base materials do not melt during welding. However, in FSW, the weld joint quality characteristics are found with highly sensitive for the variation of process variables. Therefore, this investigation deals with the analysis of the significance of FSW processing conditions, construction of stochastic tensile failure probability models for dissimilar AA6061-T6 with AA7075-T6 aluminum alloy friction stir welded joints, and postulation of their statistical predictive models. The experimental results have shown an effective mixing at the interface of both base alloys attributed to efficient bonding and resulted in the mechanical properties with the weld joints. A lowest tensile strength was achieved for the weld joint produced by the straight cylindrical profiled tool pin, which is 24.51% lower than the UTS of AA6061-T6 unwelded alloy while 59.09% lower than the AA7075-T6 unwelded alloy. The survival probability of the weld joints fabricated at 30 mm/min is 92% for 260 MPa applied stress, but the weld joints fabricated at 20 and 40 mm/min are 70 and 40%, respectively. For 50% reliability of the weld joint, the maximum allowable stress values suggested are 270, 288, and 255 MPa for processed at of 20, 30, and 40 mm/min welding speeds, respectively. The application stresses beyond 290 MPa, and the survival probability of the weld joints fabricated at 40 mm/min is zero. Empirical models postulated for tensile strength, ductility, and microhardness were fund with a reasonable agreement with their experimental measurements statistically. The derived reliability and empirical models can be used to estimate the reliability of the welded joints and predict their mechanical properties to estimate and enhance the functional performance of the welded structures in automobile and aerospace applications.

Dissimilar friction stir welding of AA2198 and AA7475: Effect of solution treatment and aging on the microstructure and mechanical strength

Abstract A post-weld heat treatment consisting of solution treatment and subsequent aging (STA) is widely applied to aluminum joints fabricated by friction stir welding (FSW) to improve the mechanical strength via precipitation hardening. In this study, aerospace aluminum alloys of AA2198 and AA7475 were FSWed in similar and dissimilar states. Differential scanning calorimetry (DSC) was used to trace the precipitation strengthening during the aging of welded specimens. The post-weld aging procedures were designed based on the DSC outputs. Accordingly, welded sheets were solution treated at 480 ℃ and 540 ℃ for 10–90 min, air-cooled and aged at 155 ℃ and 170 ℃ for 2–40 h, respectively. Optical micrographs revealed that due to the faster kinetics of the recrystallization, higher homogenizing temperature led to nucleation of the finer grains from highly stress localized points in the stir zone (SZ) and TMAZ by faster growth rate. Higher time and temperature of the solution-treatment eventuated in accumulation of Cu-enriched intermetallic phases in the grain boundaries at SZ and TMAZ of AA7475, attenuation of the grains adhesion and failure of the sample. Hardness test results showed that the hardness increased in AA7475 alloy while decreasing in AA2198 alloy in the as-welded state. Post weld heat treatment enhanced the hardness in AA2198 and reduced it in AA7475. However, it had no significant effect on the grain size.

The effect of exposure conditions on performance evaluation of post-treated anodic oxides on an aerospace aluminium alloy: Comparison between salt spray and immersion testing

In this work, the effect of the corrosion testing methodology on the degradation of anodized and post-treated AA-2024T3 specimens is considered. Two post-treatments are selected for the anodized layers, namely hydrothermal sealing and cerium-based post treatment. The two post-treatments are selected such that in one case corrosion protection mainly arises from a barrier effect (hydrothermal sealing) and in the other it arises from active inhibition (cerium-based treatment). It is found that salt spray and immersion testing provide similar results for the hydrothermally sealed oxides, but differ substantially for the cerium-treated oxides. The discrepancy is due to the fact that the continuously refreshing electrolyte layer during salt-spray testing promotes film dissolution and hinders precipitation of aluminium hydroxide and cerium compounds, unlike in stagnant bulk electrolyte. Hence, salt-spray testing is likely to be more aggressive than immersion testing for those systems that rely on active inhibition.

Effects of Refill Friction Stir Spot Weld Spacing and Edge Margin on Mechanical Properties of Multi-Spot-Welded Panels

Refill friction stir spot welding (RFSSW) is an emerging technology for joining aerospace aluminum alloys. The aim of the study is to investigate the effects of the refill friction stir spot weld spacing and the edge margin on the mechanical properties of multi-spot-welded AA7075-T6 panels. AA7075-T6 is a baseline aerospace aluminum alloy used in aircraft structures. The study employs an innovative robotic RFSSW system that is designed and developed by Kawasaki Heavy Industries (KHI). The experimental strategy uses Design of Experiments (DoE) to characterize the failure loads of multi-spot-welded panels in terms of the spot weld spacing, edge margin, and heat-affected zone (HAZ) of the spot weld. The RFSSW process leaves behind a thermal “imprint” as HAZ in heat-treatable aluminum alloys. According to the DoE results, larger spot weld spacings with no HAZ overlap produce higher failure loads of multi-spot-welded panels. On the other hand, edge margins that are equal to or less than the spot weld diameter demonstrate abnormal plastic deformations, such as workpiece edge swelling and weld crown dents, during the RFSSW process. The larger edge margins do not demonstrate such abnormal deformations during the welding process.

Challenges and Limitations in Additive Manufacturing of Aluminium Alloys for the Aerospace Industry

Additive manufacturing (AM) processes as the relatively new rapid prototyping technologies for the fabrication of high strength lightweight alloys are one of the most appropriate methods to meet the rising demands in the aerospace industry. In this sense, additively manufactured lightweight aerospace aluminium alloys belong to 2xxx and 7xxx series are extensively used in aircraft structures to achieve the decrease in the weight of aircraft structures. AM processes are the appropriate manufacturing techniques to be used in the aerospace industry to fabricate the aerospace components having complex and unique shapes, which is not possible by means of the conventional manufacturing processes. As a result, AM processes have recently an increased attention in academia and industry. However, the use of AM processes in the aerospace industry is restricted resulting from several factors such as the lack of the globally accepted certifications and standards, fabrication speed, additional surface finishing cost, limitation of the production size, achievable precision and micro-fatigue occurrence regarding to the AM of the aerospace components. In this paper, challenges and limitations in the AM of lightweight aerospace aluminium alloys were investigated. Keywords: Additive manufacturing; 3D printing; Aerospace aluminium alloys; Light weighting; Aerospace. DOI: 10.7176/JSTR/6-05-04

Friction Stir Welding in the Aircraft Production

The paper presents the results of experimental development of the technology of friction stir welding to obtain a nonseparable connection of a special aerospace aluminum, titanium and magnesium alloys, high-temperature steels. Regularities and models of heat balance in the welding zone have been determined, which make it possible to predict the technological possibilities of high-speed friction welding. It is established that high-speed friction welding by mixing allows to obtain a high-quality connection at lower loads on the design of the equipment. On the basis of studies of macro-and microstructure, microhardness, level of residual stresses and strength tests, technological recommendations on the choice of welding conditions and conditions were obtained. Presented of experimental and industrial development of special equipment, high-speed friction welding, design and manufacturing of high hardness tools of complex spatial shapes for welding aircraft materials.

Study on the intelligent model database modeling the laser welding for aerospace aluminum alloy

Abstract With the purpose of improving the modeling efficiency, the model database is established targeting that intelligent modeling of laser welding for aerospace aluminum alloy. The model database consists of material database, heat source model database and weld joint structure database. The aerospace aluminum alloys are classified reasonably in order to establish the material model database which can manage the thermo-physical properties of aluminum alloys. The heat source models of laser welding are summarized to establish model database on the basis of the different energy distribution characteristics. The joint structure database includes butt joint, T-joint and lap joint. Modeling of arbitrary structures can be implemented quickly and efficiently when modelers invoke and assemble this model database. The finite element model is efficiently established and accurately solved by taking the 5A90 and 2219 aluminum alloy laser welding as an example. Finally, it is found that the model database established in this paper can greatly improve the modeling efficiency of laser welding numerical simulation.

T-FSW of Dissimilar Aerospace Grade Aluminium Alloys: Influence of Second Pass on Weld Defects

The restoration of numerous aircraft structures is achievable with effective repair of welded joints. T-joints are often utilized in these structures to provide structural stability, keeping minimal body weight. Multi-pass friction stir welding (FSW) has proved to be useful for improving the quality of aluminium alloy welds employed in the aerospace sector. However, FSW of these alloys in T-configuration has not been sufficiently addressed yet. Even rarer is the discussion of efficacy of second FSW pass, with altered process parameters for improving the weld quality in T-joints. Hence, two commonly used aerospace grade aluminium alloys, namely, AA2024 and AA7075, were friction stir welded in T-configuration, varying three process parameters, i.e., tool rotational speed, welding speed and shoulder diameter. The effect of second FSW pass, performed at an optimum set of parameters, on kissing bond and tunnelling defect was studied in detail. A substantial reduction in the detrimental effect of these weld defects was discussed via tensile testing, micro-hardness and micro-structural observations.

Exploring the use of an AC-DC-AC technique for the accelerated evaluation of anticorrosion performance of anodic films on aluminium alloys

This work explores the use of an AC-DC-AC method for the faster evaluation of the anticorrosion performance provided by anodic films grown on an aerospace aluminium alloy. The test is based on the cyclic application of a cathodic potential step followed by a step of free corrosion and, finally, the measurement of the electrochemical impedance response of the treated surface. During the cathodic polarization step in each cycle, alkalinisation is induced on the specimen surface, which results in attack of the porous anodic oxide. The effects of the magnitude and the duration of the cathodic polarization step are considered, and the impedance responses measured during the various cycles are compared to the responses measured during natural immersion for longer times. The methodology is applied for two types of anodic oxides, which are similar for thicknesses but differ for pore morphology and corrosion behaviour. Qualitative correlation is found between the electrochemical testing method and longer term immersion tests at the free corrosion potential, suggesting that the methodology can provide a faster estimation of anticorrosion performance compared to immersion test.

Effect of Pre-welding Heat Treatment on the Mechanical Properties of Friction Stir Welded Al–4 wt.%Cu Alloys

This research investigates the welding of aluminum alloy Al–4wt.%Cu using friction stir welding (FSW). Besides the traditionally studied FSW process parameters (i.e., pin rotational and transverse speeds), this work addresses the effects of two pre-welding heat treatment conditions (annealing vs. age hardening) on the resulting weld joint. A set of experiments was carried out to assess joint quality under different levels of process parameters with specimens either annealed or age hardened prior to welding. The results revealed that age-hardened specimens prior to welding possessed higher hardness, higher penetration, and less wear rate among joint zones compared to the annealed ones and base metal. Moreover, the best joint quality was found to be obtained at higher welding and rotational speed regardless of pre-welding heat treatments. Findings of interest to industry, it is economically recommended to weld these types of high-strength aerospace aluminum alloys after age hardening.

New Approaches to Friction Stir Welding of Aluminum Light-Alloys

Friction stir welding (FSW) is the most widely used solid-state joining technique for light-weight plate and sheet products. This new joining technique is considered an energy-saving, environment friendly, and relatively versatile technology. FSW has been found to be a reliable joining technique in high-demand technology fields, such as high-strength aerospace aluminum and titanium alloys, and for other metallic alloys that are hard to weld by conventional fusion welding. Several studies accounted for the microstructural modifications induced by solid-state FSW, based on the resulting mechanical properties obtained at the FSW joints, such as tensile, bending, torsion, ductility and fatigue responses. In the last few years with the need and emerging urgency to widen the FSW application fields, broadening the possible alloy systems, and to optimize the resulting mechanical properties, this joining technique was further developed. In this respect, the present contribution focuses on two modified-FSW techniques and approaches applied to aluminum alloys plates. In a first case, an age-hardening AA6082 sheets were double side friction stir welded (DS-FSW). In a second case a non-age-hardening AA5754 sheet was FSW by an innovative approach in which welding pin was forced to slightly deviate away from the joining centreline (defined by authors as RT). In both the cases different pin heights were used, the sheets were subjected to heat treatments (peak hardening T6 for the AA6082, and annealing for the AA5754) and compared to the non-heat treated FSW conditions. Microstructural modifications were characterized by optical microscopy (OM). The mechanical properties were characterized both locally, by nanoindentation techniques, and globally, by tensile (yield, YT; ultimate, UT; and elongation, El) or forming limit curve (FLC) tests. Both the new approaches were directly compared to the conventional FSW techniques in terms of resulting microstructures and mechanical responses.