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Presents an anisotropic ductile damage model based on the fracture mechanisms and physical observations in aluminum alloys. Basis of the damage-plasticity coupling; Mechanisms of the ductile fracture; Analysis of dissipated energy from different irreversible processes.

This work focuses on understanding the mechanical behavior of CMT (cold metal transfer)-welded joints in AA6061-T651 aluminum alloy while establishing a correlation with the microstructural features observed in the weld zones. Furthermore, the mathematical framework of a coupled continuum damage mechanics and anisotropic plasticity model under finite strain formulation is presented. The aim is to accurately describe the mechanical behavior, fracture, and damage evolution of CMT-welded joints in AA 6XXX alloys. The calibration of the proposed model is performed using experimental data obtained from uniaxial tensile tests conducted on various weld zones, including the base material, heat-affected zone, and fusion zone. To capture the overall behavior of the welded joint, a composite framework is employed. This framework incorporates multiple in-series elasto-viscoplastic elements, each representing a specific welding zone. Demonstrating robust predictive capabilities, the model successfully predicts critical mechanical properties, including yield stress, ultimate tensile stress, and strain to fracture, for 6061-T651 aluminum alloy CMT welds and their respective weld zones. Notably, the model achieves a high degree of accuracy, with accuracy of approximately 0.8, 0.98, and 0.9, respectively, compared with experimental values. The experimental results and numerical simulations exhibit a close agreement, validating the accuracy and predictive capability of the proposed model. Additionally, the model demonstrates its ability to capture the evolution of damage throughout the loading process. The proposed modeling approach effectively captures the unique characteristics of each welding zone and provides valuable insights into the fracture and damage evolution of the welds. [ABSTRACT FROM AUTHOR]

This study presents information on the behavior of paint baking (PB) after resistance spot welding of the 5- and 6xxx series aluminum alloys. The weld parameters are optimized, and the weld specimens are baked three times for 20 min at 180 °C to simulate the heat treatments required for paint baking. The mechanical properties of the samples were characterized by using the lap shear test, micro/nanoindentation hardness, and fatigue test. As the mechanical properties of the weld are affected by the characteristics of the heat-affected zone and those of the fusion zone, the microstructure of the cross-sections was also analyzed through optical and electron microscopy. The investigation of the 6xxx series welds showed that the post-processing heat treatment decreased both the strength and the toughness of the weld, which resulted from the reduced hardness of the microstructure. Additionally, the lap shear test indicated that the failure mode for the 6xxx series changed from nugget failure to partial nugget failure after the paint baking process. However, the mechanical properties of the 5xxx welds were not affected as much as the 6xxx series during baking heat treatment. The fatigue test for the 6xxx series showed a different tendency from the lap shear test. Its fatigue properties improved due to an increased elastic modulus after the heat treatment. [ABSTRACT FROM AUTHOR]

Premature fracture of aluminum alloy sheet in blanking process will seriously reduce the forming quality and production efficiency of blanking parts. The purpose of this study is to explore the feasibility of using acoustic emission (AE) technique to detect the crack propagation behavior of metal sheet during blanking. AE analysis results provide effective information about plastic deformation and damage evolution. However, this technique cannot determine the depth of internal damage and the state of crack propagation. The displacement field measured by digital image correlation (DIC) method makes it possible to monitor the evolution of strain field and crack initiation during deformation. Therefore, a coupled real-time tracking method of AE and DIC is proposed to measure the deformation and damage evolution process that occur in blanking sheet. Research findings suggest that this method provides a complete and accurate description for the diverse damage evolution process of blanking aluminum alloy sheet. AE is a promising technique used to characterize and quantify the crack propagation process of metallic sheets in blanking. [ABSTRACT FROM AUTHOR]

A MultiStage Fatigue (MSF) model that admits different hierarchical microstructural features and their stereological information is used to predict the fatigue behavior of 17 different processed aluminum alloys: A000 series (A319, A356, A357, and A380), 2000 series (2024, 2055, 2099, 2198, 2297), 5000 series (5052, 5456), and 7000 series (7050, 7055, 7065, 7075, 7085, 7175). A single set of MSF model constants was validated for all of the aforementioned aluminum alloys, wherein the variation in fatigue life has been captured according to distinct microstructural features (pore size, pore nearest neighbor distance, porosity, particle size, grain size, crystallographic orientation, and misorientation) that differ arising from their native material processing method (casting, rolling, or extrusion). The MSF model's total number of cycles distinguishes two different regimes: crack incubation (Inc) and Microstructurally Small Crack (long cracks are not considered herein). The previous MSF model in the literature had been associated with the pore size, pore nearest neighbor distance, porosity, particle size, and grain size, but a new contribution in this work is the contribution of the grain orientation and misorientation angles. We show that the MSF model now has the necessary and sufficient equations to predict the Process–Structure–Property relationships for aluminum alloys, allowing for expansion of fatigue prediction even beyond the alloys studied herein. [ABSTRACT FROM AUTHOR]

In this work, we investigate a novel testing method for fatigue testing of a thin sheet under fully reversed conditions. Specifically, the fatigue behavior of thin-plate AA6082-T6 was characterized by laminating multiple thin gage specimens together to form a thicker specimen to prevent buckling under low-cycle fatigue testing. In this unique technique, ASTM E606 fatigue specimens were bonded together with a structural adhesive in double and triple laminates, and fatigue behavior was compared against monolithic control specimens. To quantify the fatigue behavior of the laminated AA6082, strain-controlled experiments were conducted, where the fatigue life experimental results exhibited comparable fatigue performance to the published literature. Postmortem analysis of the laminated AA6082 revealed a similar fatigue nucleation and crack growth damage mechanisms compared to the monolithic fatigue specimens. Lastly, a microstructure-sensitive fatigue life model was utilized to elucidate structure–property fatigue mechanism relations of the monolithic, double-, and triple-laminate specimens. The fatigue behavior of laminated specimens exhibits good correlation to wrought AA6082-T6 mechanical properties and thus suggests that laminating sheets together can be used to quantify fully reversed fatigue behavior of thin sheets that would otherwise buckle under compression loading. [ABSTRACT FROM AUTHOR]

A highly anisotropic toughness behavior has been revealed on a forged AA6061 aluminum alloy by toughness tests with CT specimens. The toughness values with specimens loaded on the longitudinal direction are larger than that loaded on the transverse direction due to the anisotropic shape and distribution of coarse precipitates induced by the morphological anisotropy of grains during forging process. Synchrotron radiation computed tomography analysis on as-received material and arrested cracks revealed different fracture modes for the two loading configurations. The damage mechanism has been validated by finite element simulations based on the Gurson–Tvergaard–Needleman micromechanical damage model with different sets of damage parameters for the two loading configurations obtained from quantitative void volume fraction analysis on SRCT data, in situ SEM experiments, and SRCT microstructural analysis. [ABSTRACT FROM AUTHOR]

This paper presents a review on the resistance spot welding (RSW) of Al/Al alloys, Al alloys/steel, Al/Mg alloys, and Al/Ti alloys, with focus on structure, properties, and performance relationships. It also includes weld bonding, effect of welding parameters on joint quality, main metallurgical defects in Al spot welds, and electrode degradation. The high contact resistance, induced by the presence of oxide layer on the surface of Al alloys, and the need for application of high welding current during RSW of Al alloys result in rapid electrode tip wear and inconsistency in weld quality. Studies have shown that cleaning the oxide layer, sliding of a few microns between sheets, enhancing the electrode force, and the application of a low-current pre-heating can significantly reduce the contact resistance and improve joint quality. For Al/steel dissimilar RSW, the technique of resistance element welding, the use of optimized electrode morphology, the technique of RSW with cover plates, and the use of interlayers such as Al-Mg, AlSi12, and AlCu28 alloys were found to suppress the formation of brittle intermetallic compounds (IMC) and improve the joint quality. The employment of pure Ni foil, Au-coated Ni foil, Sn-coated steel, and Zn-coated steel interlayers was also found to restrict the formation of brittle IMCs during RSW of Al/Mg alloys. Furthermore, the techniques of RSW with cover plates and RSW under the influence of electromagnetic stirring effect were found to improve the weldability of Al/Ti dissimilar alloys. [ABSTRACT FROM AUTHOR]

This paper presents a review on resistance spot welding of magnesium alloys, with emphasis on the relationship between microstructure, properties, and performance, under quasi-static and dynamic loading conditions. It also compares the resistance spot welding of magnesium-to-aluminum alloys and the various techniques used to suppress the formation of brittle intermetallic compounds. Resistance spot welding of magnesium-to-steel, weld bonding, the effects of process parameters on joint quality, and the main metallurgical defects in resistance spot welding of magnesium alloys are also deliberated. Studies have shown that the pre-existence of coarse second phase particles in the base metal, the addition of particles, such as titanium powder, and welding under the influence of electromagnetic stirring effect can promote columnar-to-equiaxed transition, microstructure refinement, and improvement in mechanical properties of magnesium alloys resistance spot welds. For magnesium-to-aluminum alloys spot welds, the use of interlayers, such as pure nickel, gold-coated nickel foil, and zinc-coated steel, was found to suppress the formation of brittle intermetallic compounds and thus significantly improve the joint strength. [ABSTRACT FROM AUTHOR]

Stamping lubricant is often applied to sheet metal surface to improve the formability. In this study, the effect of stamping lubricant on the strength of adhesive-bonded 1.0-mm-thick bare aluminum (Novelis X610-T4PD) and 0.9-mm-thick bare aluminum (Novelis X626-T4P) joints was investigated. It was found that while a proper amount of lubricant (~2.21 g/m2, 1.5 μL lubricant on the 25 × 25 mm coupon) applied on the surface of the substrate had little effect on the joint strength, levels more than 2.21 g/m2lubricant significantly decreased the joint strength. When the lubricant amount exceeds the adhesive’s compatibility with the lubricant, the negative effects of pores from lubricant evaporation during curing on the strength overrides the positive effect of increased adhesion energy. Furthermore, the presence of 2.21 g/m2lubricant minimized the reduction of the strength of the joints pre-exposed to neutral salt spray (i.e. a concentration of 50 ± 5 g/L sodium chloride solution). Careful analyses of the results indicated that corrosion of aluminum substrate surfaces of the pre-exposed joints led to the degradation in bond adhesion between the adhesive and substrates, and consequently resulted in the decrease of the joint strength. The hydrophobic lubricant protected the aluminum substrate from electrochemical reaction by damage of the bond adhesion between the adhesive and substrates leading to the lubricated joints having better corrosion resistance than the unlubricated joints. [ABSTRACT FROM AUTHOR]

Abstract: The fatigue behavior of resistance spot welding (RSW) in aluminum 6061-T6 alloy (AlMg1SiCu per International Standard Office nomenclature) was experimentally investigated. Three welding conditions, denoted as “nominal”, “low” and “high”, were studied to determine the microstructure of the weld nuggets. The process optimization included consideration of the forces, currents and times for main weld and post-heating. By successive iterations and “witness samples” collected, the optimum welding parameters were determined. Load control cyclic tests were then conducted on single weld lap-shear joint coupons to study the microstructure and fatigue life properties. These tests were used to characterize the fatigue behavior in spot welded specimens to elucidate the influence of the process parameters. This work revealed that the welding process parameters have a great influence in the microstructure and fatigue life of the 2mm-thick aluminum sheet resistance spot welded joints. Different fatigue failure modes were observed at several load ranges and ratios for a constant frequency and three welding currents. [Copyright &y& Elsevier]

A literature review on methods to consider the mechanical behaviour of cast aluminium alloys in finite element method (FEM) simulations of cast aluminium components has been performed. The mechanical behaviour is related to several microstructural parameters achieved during the casting process. Three different methods to consider these microstructural parameters are introduced. One method predicts the mechanical behaviour of the component using casting process simulation software. The other two methods implement numerical models for the mechanical behaviour of cast aluminium into the FEM simulation. Applications of the methods are shown, including combinations with statistical methods and geometry optimisation methods. The methods are compared, and their different strengths and drawbacks are discussed. [ABSTRACT FROM AUTHOR]

This paper investigates the fracture surface topography of two steel and aluminum alloys subject to bending-torsion fatigue loadings, as well as their susceptibility to fatigue performance and failure mechanisms. Using fracture surface topography data analysis, elements with different geometries were elaborated. A correlation between the fractal dimension, other selected parameters of surface topography such as areal Sx, and fatigue loading conditions was found. Distinctions in particular regions of cracks were also recognized through proving the correctness and universality of the total fracture surface method. The influence of fatigue loading parameters on the surface topography of fatigue fractures was demonstrated. For the analyzed cases, results show that the fractal dimension and standard surface topography parameters represent a correlation between them and loading conditions. As a single parameter, the appropriate loading ratio cannot be outright calculated with fractal dimension, but can be estimated with some approximation, taking into account additional assumptions. [ABSTRACT FROM AUTHOR]

Capacitor discharge welding (CDW) for projection welding provides very high current pulses in extremely short welding times. This requires a quick follow up behaviour of the electrodes during the softening of the projection. The possibilities of experimental process investigations are strongly limited because of the covered contact zone and short process times. The Finite Element Method (FEM) allows highly resoluted analyses in time and space and is therefore a suitable tool for process characterization and optimization. To utilize this mean of optimization, an indirect multiphysical numerical model has been developed in Ansys Mechanical APDL. This model couples the physical environments of thermal–electric with structural analysis. It can master the complexity of large deformations, short current rise times and high temperature gradients. A typical ring projection has been chosen as the joining task. The selected aluminium alloys are EN-AW-6082 (ring projection) and EN-AW-5083 (sheet metal). This paper presents the investigated material data, the model design and the methodology for an indirect coupling of the thermal–electric with the structural physic. The electrical contact resistance is adapted to the measured voltage in the experiment. The limits of the model in Ansys Mechanical APDL are due to large mesh deformation and decreasing element stiffness. Further modelling possibilities, which can handle the limits, are described. [ABSTRACT FROM AUTHOR]

The high-speed deformation behavior of friction stir-welded thin sheets in AA6082-T6 aluminum alloy, under biaxial balanced stretching, was investigated by means of a hemispherical punch test carried out using direct tension-compression Split Hopkinson Bar. The friction stir welding process was performed on thin sheet blanks using a pinless tool; the rotational and welding speeds were kept constant during process. The dynamic tests were carried out, with a punch speed of 4500 mm/s, at different punch stroke values until failure of the friction stir welded sample. It was seen that failure occurs along the welding line at a dome height about 11% higher than that at the onset of necking. Fractographic analysis shows that deformation is localized in the fracture zone. The results were compared with those obtained on friction stir welded blanks deformed under quasi-static condition in order to evaluate the influence of the loading rate on the weld deformation and fracture mechanisms. It was shown that joints deformed under dynamic loading condition are characterized by a dome height at the onset of necking significantly higher than the one measured under quasi-static condition. [ABSTRACT FROM AUTHOR]