7xxx Aluminum Alloys Research Articles

Plasma immersion ion implantation and shot peening influence on the fatigue strength of AA 7050-T7451 aluminum alloy

The 7xxx aluminum alloy series is applied in aircraft components that demand high specific mechanical strength and fatigue resistance. The corrosive environment and friction forces imposed on those components reduce the service life. Therefore, anodizing is frequently used to enhance resistance against corrosion and wear. However, anodizing decreases the fatigue life of aluminum alloys. Plasma immersion ion implantation (PIII) is an alternative surface treatment that has been used to improve surface hardness, corrosion resistance and reduce wear damage. The PIII treatment consists of accelerating positive ions towards a sample by applying negative high voltage pulses in plasma. The shot peening (SP) process is a surface treatment that can be used with other surface treatments to improve the fatigue strength. The present work aimed at investigating the fatigue behavior of AA 7050-T451 aluminum alloy treated with SP, PIII, and a combination of both SP and PIII surface treatments. The combination of SP and PIII treatments (SP + PIII) improved the fatigue performance with respect to the base material and was more efficient than PIII in improving the fatigue strength for high-cycle fatigue regime. The fatigue strength improvement is related to the induced compressive residual stresses at the surface layers, which was responsible for delaying the nucleation and early fatigue crack propagation periods. The residual stress at the surface changed from −62 MPa to −130 MPa and −210 MPa for the SP and SP + PIII conditions, respectively. Compared to chromic anodizing, the SP + PIII condition increased the fatigue life in about 4 times at a maximum stress level of 281 MPa. The combination of SP and PIII treatments tested in this work is more advantageous regarding fatigue behavior than chromic anodizing, which reduces the fatigue strength of the material.

Influence of equal-channel angular pressing on the microstructure and corrosion behaviour of a 6xxx aluminium alloy for automotive conductors

An Al-Mg-Si aluminium alloy was shaped by using a two-pass equal-channel angular pressing (ECAP) process. This led to fragmentation of the coarse Fe-rich intermetallics (IMCs), a decrease in grain size and an increase in the high angle grain boundary (HAGB) density, with overconcentration of HAGBs around the IMCs. Corrosion tests in NaCl solution showed that, before and after ECAP, only pitting corrosion occurred. However, for ECAP samples, pits were more numerous due to the fragmentation of the IMCs; they were also larger and less deep, their propagation being strongly influenced by the presence of very small grains around the IMCs.

Numerical simulation of hybrid joining processes: self-piercing riveting combined with adhesive bonding

Reliable simulation of hybrid joining processes using conventional finite element (FE) tools is challenging, because the liquid adhesive must be somehow included in the model. Thus, in this work the viscoelastic properties of the adhesive are substituted with “equivalent” mechanical properties. The complex viscosity of an epoxy-based single-component adhesive was determined at five temperatures between 20‒55 °C and at seven shear rates between 1‒150 s-1 using a rheometer. Flow stresses and strain rates were calculated from the complex viscosities and from the shear rates. For each temperature investigated the relationship between flow stress and strain rate was fitted with a power-law, which enables modeling the actually liquid adhesive as solid with strain rate-dependent flow stress. In order to validate the material model, a defined volume of adhesive was uniaxially compressed. This testing setup was also modelled using the FE software Simufact Forming 15. In the model the Young’s modulus of the adhesive was iteratively adapted until good agreement between the numerical and the experimental force-displacement curves was achieved. The obtained mechanical properties were finally used for modeling the adhesive layer between two 2.0 mm-thick 6xxx aluminum alloy blanks in the hybrid riveting-bonding process. An axisymmetric model including deformable (rivet, upper blank, lower blank, adhesive layer) and rigid (punch, die, blankholder) components was built in Simufact Forming. The cross-section of the hybrid joint obtained from simulation showed very good geometrical agreement with cross-sections obtained from the joining experiments, and just small differences between the calculated and the measured force-displacement curves was observed.

Effect of pre-deformation on quench-induced inhomogeneity of microstructure and hardness in 7050 aluminum alloy

7xxx aluminum alloys usually have high quenching sensitivity, which results in the inhomogeneous microstructure and mechanical properties of large-size components. The effect of pre-deformation on the quench-induced inhomogeneity of the microstructure and properties in a high strength 7050 aluminum alloy was investigated by means of the end-quenching technique, transmission electron microscopy (TEM) and three-dimensional electron tomography (3DET). The size of the η′ precipitates and quench-induced precipitates (QP) and the width of the grain boundary (GB) precipitate-free zones (PFZ) increased with the decrease of the quenching rate, leading to a decrease in hardness along the long axial direction of the end-quenching sample. However, after introducing 20% pre-deformation, the PFZ around the GB and QP in the slow-cooled end of the aged samples was significantly reduced, and dispersive η′ precipitates were formed around the GB and QP instead, which greatly improved the strength of the slow-cooled end. Compared with the sample without pre-deformation, the introduction of 20% pre-deformation increased the hardness of the slow-cooled end by 30% without obviously reducing the hardness of the water-quenched end, which means that the 20% pre-deformation effectively reduced the hardness inhomogeneity of the 7050 aluminum alloy. This work provides a new method for reducing the quench-induced inhomogeneity of aluminum alloys.

Prediction of forming limit diagrams using machine learning

Measuring forming limit diagrams (FLDs) is a time consuming and expensive process. Data-driven and artificial intelligence approaches have been used to facilitate finding relationships between alloy composition, and manufacturing processing conditions with mechanical properties of materials. Machine learning (ML) methods are a promising route to predict FLD of aluminium alloys that are increasingly used in automotive structural applications. In the present work, we developed a machine learning (ML) based tool to establish the relationships between alloy composition / thermomechanical processing route to the material’s FLD, which is a measure of formability. Measured FLDs of various 5XXX and 6XXX aluminium alloys along with their chemical composition and thermomechanical processing parameters like homogenization, hot and cold rolling parameters etc. and the respective mechanical properties like n and r values were manually curated and stored in a database. A two-stage ML model was trained on this database. In the first stage, the minimum and maximum points of the minor strain were predicted using a feature set incorporating chemical composition, thermomechanical processing parameters, and n and r values using support vector regression (SVR). In the second stage, the predicted minor strain in the first stage was used as input in addition to the same feature set to predict the major strain using gradient boost regression (GBR). The trained ML model successfully predicted FLDs with R2 value above 0.93. To study the impact of the difference between a predicted and a measured FLD on stamping simulation results, finite element simulations of a cross die were performed. The difference in draw depth of the cross-die using the predicted FLD and the measured FLD was between 5 to 10%. The current results show that ML can be a viable way for predicting FLDs.

Effect of material structure on trimming and sheared edge stretchability of 6xxx aluminum alloys

Wide variety of 6xxx Aluminum alloys is currently being used for manufacturing of lightweight automotive components. Very often each car manufacturer gets its own variation of the 6xxx alloy. Majority of these alloys have very similar yield and tensile strength as well as the stress-strain curve. Therefore, selection of the best alloy for each technical application might be challenging. In this paper, a comparison of performance of three 6xxx alloys in trimming process and sheared edge stretchability for wide variety of cutting clearances is being studied. This performance is compared with postnecking behavior of the same sheets in a standard tensile test. The ability of material to form visible diffuse neck and the cutting tool indentation prior to initiation of fracture shows some correlation for three studied 6xxx Aluminum alloys. The material with more pronounced diffuse neck shows a stronger tendency to form burrs at larger cutting clearances. However, other parameters of the standard tensile, such as yield stress, tensile strength, uniform elongation and even total elongation of these three alloys are very similar. Sheared edge stretchability measured via tensile testing of sheared samples showed that the materials with larger diffuse necking in the tensile test have higher sensitivity of edge stretchability to the cutting clearance. For large cutting clearances, the crack usually propagates almost vertically and leads to a burr formation. It appeared that the grain aspect ratio also influences the trajectory of the crack: for the materials with substantially elongated grains, the crack typically propagates more vertically. For the materials with nearly equiaxial grains, the cracks show stronger tendency to turn towards the lower cutting edge. The reason for such a behaviour is in crack propagation via grain boundaries. The experimental study of nanohardness measured at grains and grain boundaries for one of the studied alloys indicated that the grain boundaries have about 30% lower nanohardness than the material inside the grains.

Experimental study on the mechanical properties of 7xxx aluminium alloy sheet under different heat treatment conditions

Automotive bodies with high strength aluminium alloys have drawn increasing attentions due to their higher strength and lightweight capability. As one of high strength series of aluminum alloys, peak aged 7xxx series aluminum alloys exhibit superior strength of ∼ 600MPa to the conventional lower or moderate strength alloys. However, in spite of the high strength, the lower ductility has been a technical hurdle to be overcome for the material to be applied to press parts at room temperature, which leads to the development of forming technologies at elevated temperature, or namely hot forming. As an alternative to the hot forming, forming trial at room temperature has been made by performing prior solution heat treatment and quenching. This coupled heat treatment and cold forming increases the formability, while the strength needs to be recovered by additional post treatment. The treatment after forming (or deformation) includes natural aging and paint baking process inducing the artificial aging. The objective of the present study is to experimentally investigate the mechanical properties after different heat treatment conditions, which will be utilized for the future finite element simulations for the investigated forming technology.

Through-Thickness Strain Gradient in a Hot-Rolled Al-Mg Alloy Obtained by Nanoindentation and Glancing Angle X-Ray Diffraction

Combined nanoindentation and glancing angle X-ray diffraction (GAXD) methods were used to study the mechanical properties of near-surface microstructures (NSMs) of a hot-rolled 5xxx aluminum alloy. Nanoindentations with a sharp indenter were carried out with penetration depth ranging from 250 nm to 4500 nm at the near-surface regions as well as the bulk of the hot-rolled alloy. The primary indentation parameter presented here was the hardness, given that plastic strain obtained from GAXD experiment could be correlated with the hardness value. Analysis of nanoindentation hardness results proved that NSMs of this alloy on average were harder than the bulk due to the presence of different surface features discussed here. The GAXD experiments were performed between a minimum incident angle 0.025°, equivalent to 765 nm penetration depth, and a maximum angle 1°, equivalent to 3000 nm penetration depth. The strain-induced broadening in the diffraction peaks was calculated by the Williamson–Hall technique. Both indentation hardness and glancing angle diffraction results confirmed strain gradient as a function of depth in the NSMs of the alloy. Both techniques also showed that the thickness of the subsurface layer (i.e., several micrometers below the surface) of hot-rolled sample was approximately 2.44 μm at the tested areas. Additionally, reported results provided evidence for the ability to use glancing angle X-ray diffraction as a nondestructive tool for the measurement of subsurface layers' thickness as well as microstrain (non-uniform or plastic strain) as a function of depth.

The Role of Grain Refiner in the Nucleation of AlFeSi Intermetallic Phases During Solidification of a 6xxx Aluminum Alloy

Primary grain refinement using inoculant additions and intermetallic compound (IMC) phase selection are critical aspects in the solidification of commercial aluminum alloys, controlling the final mechanical properties in service. Although there have been studies which suggest there are explicit interactions between the two phenomena, they have yet to be fully elucidated. Here, through study of intermetallic phase particles extracted from an inoculated casting, key features relating to the nucleation of different intermetallic phases via eutectic reactions are recognized and explained. In particular, rake-like IMCs are identified as initiation points for the deleterious beta -AlFeSi IMC phase in a model 6xxx series Al alloy. A mechanism is proposed for how {text{TiB}}_{2} inoculant particles, which are commonly used for primary phase refinement, play a role in enhancing the nucleation of intermetallic phases during eutectic reactions at the liquid/alpha -Al interface in the final stages of solidification. The implication of this mechanism is that, after the event of primary grain refinement, any unused {text{TiB}}_{2} inoculant particles could be contributing to IMC formation thereby affecting the overall type, size, and distribution of intermetallic phases in the solidified alloy.

The Effect of Heat Treatments and Nickel Additive on The Microstructure and Hardness of 7075 Aluminum Alloy

The aluminum alloys of the 7xxx series consist of Al with Zn mainly, Mg and Cu. 7xxx aluminum alloys has high mechanical properties making it distinct from other aluminum alloys. In this paper, we examine the effect of adding Nickel and heat treatments on the microstructure and hardness of the 7075 aluminum alloy. Were we added different percentages of nickel [0.1, 0.5, 1] wt% to 7075 Aluminum alloy, and applied various heat treatments (artificial aging T6 and Retrogression and re-aging RRA) on the 7075 alloys that Containing nickel. By applying RRA treatment, we obtained better results than the results obtained by applying T6 treatment, and we obtained the high values of hardness and a smoother microstructure for the studied alloys by the addition of (0.5 wt%) nickel to alloy 7075.

The Effect of Heat Treatments and Nickel Additive on The Microstructure and Hardness of 7075 Aluminum Alloy

The aluminum alloys of the 7xxx series consist of Al with Zn mainly, Mg and Cu. 7xxx aluminum alloys has high mechanical properties making it distinct from other aluminum alloys. In this paper, we examine the effect of adding Nickel and heat treatments on the microstructure and hardness of the 7075 aluminum alloy. Were we added different percentages of nickel [0.1, 0.5, 1] wt% to 7075 Aluminum alloy, and applied various heat treatments (artificial aging T6 and Retrogression and re-aging RRA) on the 7075 alloys that Containing nickel. By applying RRA treatment, we obtained better results than the results obtained by applying T6 treatment, and we obtained the high values of hardness and a smoother microstructure for the studied alloys by the addition of (0.5 wt%) nickel to alloy 7075.

Effect of Copper on the Localized Corrosion and Stress Corrosion Resistance of High Strength Aluminum Alloys

High strength to weight ratio of the 7xxx aluminum alloys offer an exciting alternative to advanced high strength steels in automotive structural applications. Recently, Novelis has developed AdvanzTM 7000 series alloys that not only has higher strength to weight ratio but also improved localized corrosion resistance compared to conventional 7075. In this work, we have studied the effect of copper content on the stress corrosion cracking resistance of the AdvanzTM 7000 in peak-age (T6) conditions. ASTM G110 test showed pitting corrosion morphology in both low and high copper alloys compared to intergranular corrosion in 7075. Electrochemical polarization showed that increase in copper content increased the cathodic activity and pushed the free corrosion potential in noble direction. Slow strain rate tests were carried out according to ASTM G129 in ASTM D1384 solution containing hydrogen peroxide and in silicone oil. The fractured samples were subjected to scanning electron microscopy (SEM) and metallography cross section. These results indicated that the SCC cracks were initiated from the pitting corrosion sites in AdvanzTM 7000, while the intergranular corrosion site acted as crack nucleation site for 7075. The failure mechanism is discussed in terms of shift in corrosion morphology and the role of Al7Cu2Fe particles that had increased with the copper content of the alloys.

Quantification of recrystallization kinetics in industrially processed 5xxx aluminum alloy sheet

The annealing kinetics of two aluminum alloys AA5xxx is investigated. The volume fraction of recrystallized material, Vv, is determined for a series of partly recrystallized samples both by hardness measurements and by electron back scattering diffraction (EBSD). Because of extended recovery overlapping with recrystallization, in particular in AA5182, it is not straightforward to estimate Vv from the hardness measurements. Two methods are suggested: one relying solely on the hardness data, and one in which EBSD data are used to estimate the hardness at the onset of recrystallization. Effects of the choice of method and of parameters on the resulting annealing kinetics are analysed, and the potential and limitations of the two methods are discussed.

Performance evaluation of a novel force measuring device for Friction Stir Welding (FSW) of aluminum alloys

Friction Stir Welding (FSW) has the advantage of generating sound welds on materials that generally present low weldability by traditional fusion welding processes, such as 2xxx and 7xxx aluminum alloys. Force measurement in FSW provides significant information about the process, the machine requirements, the effect of selected parameters, and weld soundness. In this paper, an axial force measuring device was designed, built, and tested to be used in a CNC adapted system. All tests were performed using AA 7075-T6 and a butt joint configuration. The forces obtained by the device match, in profile, key values, and trends, the ones found in literature which allows to conclude that this is a functional and sufficient device for the application.

Some Thoughts on Stress Corrosion Cracking of (7xxx) Aluminum Alloys

Some Thoughts on Stress Corrosion Cracking of (7xxx) Aluminum Alloys

Effect of Main Elements (Zn, Mg, and Cu) on Hot Tearing Susceptibility During Direct-Chill Casting of 7xxx Aluminum Alloys

New 7xxx aluminum alloys with high alloying contents are being designed, which could induce serious hot tearing defects during direct-chill (DC) casting. Among all factors affecting hot tearing of 7xxx alloys, undoubtedly alloying elements play a significant role. In this study, the effect of main alloying elements (Zn, Mg, and Cu) on hot tearing of grain-refined Al-xZn-yMg-zCu alloys was investigated by a dedicated hot tearing rating apparatus simulating the DC-casting process. It was found that the minimum and maximum hot tearing susceptibilities occur for 4 to 6 and 9 wt pct Zn, respectively, indicating the complicated effect of Zn content. The hot tearing resistance of grain-refined Al-9Zn-yMg-zCu alloys is enhanced with increasing Mg content but is deteriorated with increasing Cu content. This can be attributed to the interaction of the thermal stresses, melt feeding, and final eutectics. The observed tendencies of the main alloying elements on hot tearing were also confirmed for four commercial 7xxx alloys. In addition, both the load value at non-equilibrium solidus and the SKK criterion proposed by Suyitno et al. using measured load developments were found to be good indicators in predicting hot tearing susceptibility. This study can provide a beneficial guide in designing 7xxx alloys considering the potential occurrence of hot cracks beforehand.

Evolution of surface characteristics of two industrial 7xxx aluminium alloys exposed to humidity at moderate temperature

This study analyses the evolution of surface characteristics of two industrial high‐strength 7xxx aluminium alloys with a focus on alloy composition and environmental parameters. Based on storage and transport conditions of as‐machined products, the effect of humidity—as liquid and vapour phase—on the natural oxide layer has been studied. The evolution of the natural oxide layer has been analysed by scanning electron microscopy and X‐ray photoelectron spectroscopy. The growth behaviour of the surface layer is dominated by environmental conditions, while microgalvanic activity depends mainly on the alloys' chemical composition and differs significantly for tested alloys. Scanning transmission electron microscopy images demonstrated that the long‐term exposure at moderate temperatures affects the microstructure near the surface, which differs for the analysed alloy compositions. An anomalous precipitation of zinc‐rich particles at the surface and along the precipitate‐free zone is observed for the alloy with higher Zn/Mg ratio and lower Cu content.

Modeling of Microsegregation and Homogenization of 6xxx Al-Alloys Including Precipitation and Strengthening During Homogenization Cooling.

Control of the homogenization process is important in obtaining high extrudability and desirable properties in 6xxx aluminum alloys. Three consecutive steps of the process chain were modeled. Microsegregation arising from solidification was described with the Scheil–Gulliver model. Dissolution of Mg2Si, Si (diamond) and β-AlFeSi (β-Al5FeSi) to α-AlFeSi (α-Al12(FeMn)3Si) transformation during homogenization have been described with a CALPHAD-based multicomponent diffusion Dual-Grain Model (DGM), accounting for grain size inhomogeneity. Mg2Si precipitation and associated strengthening during homogenization cooling were modeled with the Kampmann–Wagner Numerical (KWN) precipitation framework. The DGM model indicated that the fractions of β-AlFeSi and α-AlFeSi exhibit an exact spatial and temporal correspondence during transformation. The predictions are in good agreement with experimental data. The KWN model indicated the development of a bimodal particle size distribution during homogenization cooling, arising from corresponding nucleation events. The associated strengthening, arising from solid solution and precipitation strengthening, was in good agreement with experimental results. The proposed modeling approach is a valuable tool for the prediction of microstructure evolution during the homogenization of 6xxx aluminum alloys, including the often-neglected part of homogenization cooling.

Microstructural characterization of interrupted aging on an AA7050 aluminum alloy

Interrupted aging heat treatments are used to maximize the gain in mechanical properties in 7xxx aluminum alloys through the production of a microstructure with a high density of fine precipitates, which are homogeneously dispersed in the aluminum matrix. In an interrupted aging heat treatment, the first stage, at higher temperature, aims at creating sites for subsequent nucleation of precipitate particles, while the second stage, at lower temperature, aims at the nucleation and growth of precipitate particles. The Interrupted aging heat treatment has been increasingly studied because it confers resistance levels similar to, or even higher than conventional heat treatments (e.g. T6), in addition to producing a more stable microstructure. In this study, a microstructural characterization was carried out using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) in an AA7050 aluminum alloys that was submitted to aging heat treatments T7451, T6 and T6I4-65, seeking a better understanding of the microstructure produced in each heat treatment and the reasons of the gain in mechanical properties. The results indicated that the T6I4-65 interrupted aging heat treatment was the most efficient in creating sites for nucleation of phases in the aluminum matrix, when compared with other conditions, due to the higher density of particles of the phases η′, η and T, in addition to being the only condition to have GPII zones and S phase in its microstructure.

Development of the post-form strength prediction model for a high-strength 6xxx aluminium alloy with pre-existing precipitates and residual dislocations

The applications of lightweight and high strength sheet aluminium alloys are increasing rapidly in the automotive industry due to the expanding global demand in this industrial cluster. Accurate prediction of the post-form strength and the microstructural evolutions of structural components made of Al-alloys has been a challenge, especially when the material undergoes complex processes involving ultra-fast heating and high temperature deformation, followed by multi-stage artificial ageing treatment. In this research, the effects of pre-existing precipitates induced during ultra-fast heating and residual dislocations generated through high temperature deformation on precipitation hardening behaviour have been investigated. A mechanism-based post-form strength (PFS) prediction model, incorporating the flow stress model and age-hardening model, was developed ab-initio to predict strength evolution during the whole process. To model the stress-strain viscoplastic behaviour and represent the evolution of dislocation density of the material in forming process, constitutive models were proposed and the related equations were formulated. The effect of pre-existing precipitates was considered in the age-hardening model via introducing the complex correlations of microstructural variables into the model. In addition, an alternative time-equivalent method was developed to link the different stages of ageing and hence the prediction of precipitation behaviours in multi-stage ageing was performed. Furthermore, forming tests of a U-shaped component were performed to verify the model. It was found that the model is able to accurately predict the post-form strength with excellent agreement with deviation of less than 5% when extensively validated by experimental data. Therefore, the model is considered to be competent for predicting the pre-empting material response as well as a powerful tool for optimising forming parameters to exploit age hardening to its maximum potential in real manufacturing processes.