Failure of aluminium self-piercing rivets: An experimental and numerical study

The present paper investigates the fracture mechanisms of AA7278-T6 aluminium self-piercing rivets under compression during the riveting process. First, a microstructure investigation was conducted to disclose the grain structure and the particle distribution of the extruded aluminium alloy. Transmission electron micrographs revealed precipitate free zones along grain boundaries. Uniaxial tensile tests in three different directions with respect to the extrusion direction revealed anisotropy of the alloy in strength and ductility and a change in fracture mode with tensile direction. The behaviour of the alloy under compression was studied experimentally using upsetting tests and self-piercing riveting tests. Micrographs of the deformed specimens provided insight into the influence of the microstructure on the deformation and fracture of the alloy under compression. Second, numerical analyses were carried out using a 2-D axisymmetric model in LS–DYNA in an attempt to investigate the role of different physical variables on the final failure of the rivet. The numerical results revealed that constituent particles, precipitate free zones, and friction between the rivet and plates are important for strain localisation and fracture in the rivet.     

Characterization of grain-boundary configuration and fracture surface roughness by fractal geometry and creep-rupture properties of metallic materials

Grain-boundary configuration in heat-treated specimens and fracture surface roughness in creep-ruptured specimens of several kinds of metallic material were quantitatively evaluated on the basis of fractal geometry. Correlations between the fractal dimension of grain boundary, that of fracture surface profile, the creep-rupture properties and the fracture mechanisms of the alloys are discussed. In heat-resistant alloys, the fractal dimension of a nominally serrated grain boundary was always larger than that of a straight grain boundary in the same alloy. The relative importance of the ruggedness of grain boundaries was estimated by the fractal dimension difference between these two grain boundaries. There was a quantitative relationship between the increase of the fractal dimension of the grain boundary and the improvement of rupture ductility and rupture strength owing to grain-boundary serration in the alloy. A similar correlation was also found between the increase in the fractal dimension of the fracture surface profile and the improvement of the creep-rupture properties, since in some cases the fractal dimension of the fracture surface profile was correlated with that of the grain boundary. Both grain boundary and fracture surface profile were assumed to exhibit a fractal nature between one grain boundary length (upper bound) and an interatomic spacing (lower bound). In carbon steels with ferrite-pearlite structure, according to the increase in pearlite volume fraction, the rupture ductility decreased and the fracture mechanism changed from transgranular fracture in pure iron and low-carbon steels to intergranular fracture at ferrite-pearlite grain boundaries in medium-carbon steels, and further to intergranular fracture at pearlite grain boundaries in high-carbon steels. The correspondence between the fractal dimension of the grain boundary and that of the fracture surface was confirmed in ruptured specimens of ferrite-pearlite steels when the grain boundary was the fracture path.     

Tensile deformation behaviour of two aluminium alloys at elevated temperatures

Abstract

The tensile deformation behaviour of two recently developed aluminium alloys in the temperature range 200–550°C is characterized in this paper. The aluminium alloys studied here are an automotive stamping grade Al–Mg–Mn alloy and an Al–Li–Cu alloy. Tensile properties at elevated temperatures were determined under different temperature-strain rate combinations. An analysis of deformation and fracture behaviour at elevated temperatures is also presented. The Al–Mg–Mn alloy and the Al–Li–Cu alloy exhibited extended ductility or mild superplasticity at elevated temperatures. Metallographic and fractographic studies revealed appreciable grain growth and cavitation at elevated temperatures. The fracture elongation of Al–Mg–Mn alloy decreased beyond 430°C. Pronounced apparent strain hardening was observed in the case of the Al–Li–Cu alloy in the temperature range 525–550°C at a very low strain rate. This could be due to dynamic grain growth and/or dislocation structure evolution.     

On the effects of thermomechanical processing on failure mode in precipitation-hardened aluminium alloys

This paper concentrates on the influences of thermomechanical processing on fracture behaviour of Al-Mg-Si-Cu (AA6061) alloys. Important factors are grain boundary structure and extent of matrix- and grain boundary precipitation. Large grain boundary phases in the as-air-cooled alloy, explains its much smaller fracture strain with respect to the water-quenched alloy. With increasing artificial ageing time, the bulk fracture strain of the air-cooled alloy exhibits a minimum. This is due to grain boundary precipitate growth and coarsening affecting the fraction of strain confined to the grain boundary region. For the recrystallized microstructure aged to peak strength, the fraction of intergranular fracture is much larger for the air-cooled alloy. This can be understood on the basis of a much wider precipitate free zone and a smaller grain boundary precipitate volume fraction for the air-cooled alloy, increasing the fraction of strain confined to the grain boundary region by about one order of magnitude with respect to the water-quenched alloy. A much coarser distribution of intermetallic phases in the extruded microstructure is responsible for a larger degree of slip localization. This enhances the tendency for shear- and intergranular fracture, reducing the ductility and thus the fracture strain with respect to that of the recrystallized microstructure.     

Influence of rolling direction on strength and ductility of aluminium and aluminium alloys produced by accumulative roll bonding

Sheets from commercial purity aluminium AA1050 and aluminium alloy AA6016 were processed by accumulative roll bonding to obtain an ultrafine-grained microstructure. The accumulative roll bonded samples showed a significant increase in specific strength paired with high ductility. Despite a strongly elongated grain structure, tensile testing of samples oriented 45° to the rolling direction revealed considerable improvement in elongation to failure compared to the samples oriented parallel to the rolling direction. From hydraulic bulge tests, it was observed that the accumulative roll bonded samples reached higher burst pressures and slightly lower equivalent strains in comparison to the as-received conventionally grain-sized samples. This behaviour reflects the extraordinary mechanical properties of the ultrafine-grained materials and indicates promising metal sheet formability.     

High strain rate superplastic flow stress and post-deformation mechanical properties of mechanically alloyed 2024 aluminium alloy reinforced with SiC particles

Abstract

Composites consisting of 2024 aluminium alloys reinforced with volume fractions of 0, 5, 10, and 15 vol.-% of SiC particles were fabricatedfrom the mechanically alloyed powders by an optimised hot compaction and prestraining process. Fine and equiaxed grain structures with grain sizes of <1 μm were observed within the matrix of each alloy. The composite specimens were compressed at temperatures between 733 and 813 K with a wide strain rate range from 10^?3 to 10 s^?1. Two strain rate regions with different slopes from ～ 5 × 10^?1 s^?1 were found in log (true stress–log (strain rate) curves. In the lower strain rate region of each alloy, the strain rate sensitivity values m were 0.03–0.16. The threshold stress σ_th for each alloy was estimated using an extrapolation procedure. A linear relationship was found between <disp-formula><graphic href="splitsection10-m1.tif"/></disp-formula> and σ_th where V_f is the volume fraction of SiC particles. In the higher strain rate region of each alloy, m values greater than 0.3 were obtained at 773 K, which is very close to the solidus temperature of 775 K for 2024 aluminium alloy. Moreover, the maximum yield strength and elongation for each alloy at room temperature were also obtained in the specimens compressed at 773 K. Thus, it was found that the optimum temperature for the high strain rate superplastic processing of the composites was just below the solidus temperature of the 2024 aluminium alloy. The grain coarsening resulted in the decrease of post-deformation strength and ductility as well as the m value in hot compression above the solidus temperature.     

Rissfortschritts‐ und Ermüdungsverhalten der Aluminiumlegierung EN AW‐6060 nach ECAP und nachgelagerter Wärmebehandlung

Crack growth and high cycle fatigue behaviour of an AA6060 aluminium alloy after ECAP combined with a subsequent heat treatment Crack growth properties of the Al‐Mg‐Si alloy AA6060 as well as the high cycle fatigue behaviour have been investigated after equal‐channel angular pressing (ECAP). In our study, experiments have been conducted on different stages of microstructural breakdown and strain hardening of the material as they were present after different numbers of ECAP passes. A bimodal condition, obtained after two pressings, and a homogeneously ultrafine‐grained condition after eight repetitive pressings have been investigated. Furthermore, optimized conditions with an enhanced ductility, produced by ECAP processing combined with a following short‐time aging treatment were included into the study. Crack growth experiments have been conducted in the near‐threshold regime and the region of stable crack growth, covering a range of load ratios from R = 0.1 up to 0.7. It was found that the lowered fatigue threshold ΔK_th of the as‐extruded material can be enhanced by the combination of ECAP and short‐time aging, owing to the increased ductility and strain hardening capability of this material. By means of SEM investigations and tensile tests, the crack growth properties of the different conditions were related to microstructural and mechanical features. In fatigue tests, load reversals up to failure and the fatigue limit for an as‐extruded condition and an optimized condition after two ECAP‐passes have been compared to the coarse grained initial condition and a remarkable increase in fatigue strength was noted.     

Quasi-brittle fracture during structural impact of AA7075-T651 aluminium plates

The stress–strain behaviour of the aluminium alloy 7075 in T651 temper is characterized by tension and compression tests. The material was delivered as rolled plates of thickness 20 mm. Quasi-static tension tests are carried out in three in-plane directions to characterize the plastic anisotropy of the material, while the quasi-static compression tests are done in the through-thickness direction. Dynamic tensile tests are performed in a split Hopkinson tension bar to evaluate the strain-rate sensitivity of the material. Notched tensile tests are conducted to study the influence of stress triaxiality on the ductility of the material. Based on the material tests, a thermoelastic–thermoviscoplastic constitutive model and a ductile fracture criterion are determined for AA7075-T651. Plate impact tests using 20 mm diameter, 197 g mass hardened steel projectiles with blunt and ogival nose shapes are carried out in a compressed gas-gun to reveal the alloy's resistance to ballistic impact, and both the ballistic limit velocities and the initial versus residual velocity curves are obtained. It is found that the alloy is rather brittle during impact, and severe fragmentation and delamination of the target in the impact zone are detected. All impact tests are analysed using the explicit solver of the non-linear finite element code LS-DYNA. Simulations are run with both axisymmetric and solid elements. The failure modes are seen to be reasonably well captured in the simulations, while some deviations occur between the numerical and experimental ballistic limit velocities. The latter is ascribed to the observed fragmentation and delamination of the target which are difficult to model accurately in the finite element simulations.     

An experimental study on the dynamic fracture of extruded AA6xxx and AA7xxx aluminium alloys

The dynamic fracture behaviour of extruded AA6xxx and AA7xxx aluminium alloys is investigated using an instrumented Charpy test machine and V-notch specimens. The specimens are made from extruded flat profiles with a rectangular cross-section of 10 mm thickness and 83 mm width. The material is in T6 temper, i.e. the peak hardness condition. The alloys have either recrystallized or fibrous grain structure. For each alloy six different Charpy impact tests are carried out in two series. In Series 1, the notch is parallel to the thickness direction of the profile (i.e. through the thickness), while the notch is perpendicular to the thickness direction in Series 2 (i.e. lying in the plane of the flat profile). In each series, the longitudinal direction of the specimen is parallel, 45° or 90° to the extrusion direction. Comprehensive fractographic investigations are carried out for the different tests and alloys. It is found that the dissipated energy is practically invariant to specimen orientation and notch direction for the recrystallized alloy. For the fibrous alloys the dissipated energy is lower when the longitudinal direction of the specimen is 90° to the extrusion direction, i.e. when the notch is parallel with the fibrous grain structure. Further, the energy dissipation is higher for Series 2 than for Series 1 due to substantial delamination and secondary cracking in Series 2. The precipitate-free zones (PFZs) formed adjacent to the grain boundary are weak areas, preferable for crack initiation and growth. This is seen in the fracture surface as facets with high density of small dimples and is more pronounced for specimens with the notch parallel to the fibre direction.     

Failure of AA 6061 and 2099 aluminum alloys under dynamic shock loading

Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200 s⁻¹ and quasi-static compressive loading at a strain rate of 0.0032 s⁻¹. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.     

Microstructure and mechanical behavior of ECAP processed AZ31B over a wide range of loading rates under compression and tension

In this work, a commercial magnesium alloy, AZ31B in hot-rolled condition, has been subjected to severe plastic deformation via four passes of equal channel angular pressing (ECAP) to modify its microstructure. Electron backscatter diffraction (EBSD) was used to characterize the microstructure of the as-received, ECAPed and mechanically loaded specimens. Mechanical properties of the specimens were evaluated under both compression and tension along the rolling/extrusion direction over a wide range of strain rates. The yield strength, ultimate strength and failure strain/elongation under compression and tension were compared in detail to sort out the effects of factors in terms of microstructure and loading conditions. The results show that both the as-received alloy and ECAPed alloy are nearly insensitive to strain rate under compression, and the stress–strain curves exhibit clear sigmoidal shape, pointing to dominance of mechanical twinning responsible for the plastic deformation under compression. All compressive samples fail prematurely via adiabatic shear banding followed by cracking. Significant grain size refinement is identified in the vicinity of the shear crack. Under tension, the yield strength is much higher, with strong rate dependence and much improved tensile ductility in the ECAPed specimens. Tensile ductility is even much larger than the malleability under compression. This supports the operation of 〈c + a〉 dislocations. However, ECAP lowers the yield and flow strengths of the alloy under tension. We attempted to employ a mechanistic model to provide an explanation for the experimental results of plastic deformation and failure, which is in accordance with the physical processes under tension and compression.     

Evaluation of the tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys

The tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys produced by Nuclear Metals, Inc., is determined as a function of temperature. The wrought alloy and the casting alloy are both shown to have a very high stiffness to density ratio compared with common structural materials. The wrought alloy was found to have superior fatigue strength, tensile strength and ductility relative to the casting alloy; it also maintained a greater fraction of its tensile strength as a function of temperature. The stiffness of the materials can be readily explained using standard composite theory, where the material is treated as a discontinuous beryllium-reinforced aluminium matrix composite. The strength of the casting alloy is controlled to a large extent by the strength of its aluminium alloy matrix. In contrast, strengthening increments from both dislocation-based mechanisms and load transfer appear to be operative for the wrought material. Fractographic analysis of tensile specimens showed that preferential failure of the aluminium regions or the beryllium/aluminium interfacial regions occurs under certain circumstances. Fracture analysis of fatigue samples revealed no obvious fracture initiation sites and no evidence of limited/controlled crack growth regions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Einfluss der Al2Cu‐Phase auf die Superplastizität der AlCuMn Legierung

Influence of the Al₂Cu‐phase on the superplasticity of AlCuMn alloy High‐temperature creep‐resistant AlCuMn wrought alloy has been investigated and optimised with respect to their superplastic deformability; a maximal elongation ε of 850 per cent was thus attained at a deformation temperature of 530°C. Prerequisites for superplastic deformation behaviour and for the associated high elongation values of these aluminium alloys are an especially fine‐grained structure as well as a decrease in the amount of Al₂Cu phase and a uniform distribution of this phase in the structure. Superplastic deformation (SPD) results in a pronounced change in the shape of the large particles of the θ‐phase; the particles of this phase thereby form veins along the boundaries of adjacent grains. During deformation, the grains lose their equiaxial shape and elongate in the direction of tension as a result of pronounced intragranular sliding dislocation in the microstructure. Transmission electron micrographs of the deformed structure have revealed a pile‐up of dislocations in the grains of the aluminium alloy. The grain size of commercially available sheets of AlCuMn wrought alloys with a thickness of 1 mm is approximately 30 μm. After optimising, the grain size of the sheets produced by the new method was on 12 μm until 5 μm. The new technique differs only slightly from industrial manufacture.     

High temperature,high strain rate embrittlement of Al-Mg-Mn alloy: evidence of cleavage of an fcc alloy

Abstract

The fracture behaviour in tension of an Al-Mg -Mn alloy has been investigated. At high temperatures and strain rates, intergranular brittle fracture is observed along with cleavage fracture. Intergranular fracture is related to local melting at the grain boundaries. Cleavage occurs in equal proportions on the {100} and {110} crystallographic planes. The area fraction of cleavage facets on the fracture surface has been quantified. Their initiation is shown to be related both to the liquid metal embrittlement of the grain boundaries and to the presence of brittle Mn containing particles at the grain boundaries. Cleavage fracture in an aluminium alloy also requires an inhibition of plastic flow which prevents plastic blunting at the crack tip. It is proposed that this modification of the plastic behaviour is provided by the decrease in stacking fault energy at high temperatures in Al-Mg alloys.     

Effect of cooling rate in overageing and other thermomechanical process parameters on grain refinement in an AA7475 aluminium alloy

Fine-grained AA7475 aluminium alloy sheets were produced in this study by a thermomechanical treatment involving solution anneal, overageing, rolling and recrystallization steps. It has been found that the cooling rate after the intermediate overageing treatment should be fast to obtain the finest grain size. The fast cooling rate ensured the presence of relatively large particles of MgZn₂ and some supersaturation prior to cold rolling. Generally, the final grain structure was heterogeneous, with bands of fine grains lying parallel to the rolling direction. In material rapidly cooled after overageing, bands of fine grains were also observed in the transverse direction and these bands were associated with shear bands formed during rolling. The fine-grained AA7475 alloy sheets with an average grain size of about 9 m showed large tensile elongations of about 800% when deformed at 516 °C and with an initial strain rate of 5×10^–4s^–1.     

Deformation and fracture in Al–Li-base alloys

Abstract

Aluminium–lithium-base alloys are of considerable interest because of their low density and high modulus. However, they have been shown to have low ductility and poor fracture toughness. This has been attributed to a variety of factors, including intense shear band formation, segregation to grain boundaries, and weakened grain boundaries due to precipitation and precipitate-free zones. The authors have investigated the deformation structures observed in binary and more complex commercial alloys. As would be expected, considering the microstructure of the alloys, extensive strain localization and shear band formation occurs in these alloys. However, it is shown that the commercial alloys are less sensitive to strain localization than the model binary alloy systems investigated. The stresss–train behaviour has been investigated. The alloys exhibit jerky flow, which is indicative of negative strain rate sensitivity, and strain rate change tests showed this to be the case. This is consistent with the deformation structures observed. The effect of weakened grain boundaries due to precipitation and precipitate-free zones has been studied by comparing the fracture characteristics of aged and unaged material. It is shown that the mode of failure is identical under appropriate conditions. It is concluded that segregation to grain boundaries is the major cause of the lower ductility and toughness of Al–Li alloys. This possibility has been investigated using in situ fracture surface analysis techniques. Results are presented on grain boundary segregation, and methods of reducing its influence on fracture behaviour are indicated.

MST/570     

Quantification of grain boundary precipitation and the influence of quench rate in 6XXX aluminum alloys

The extent of grain boundary precipitation has a strong influence on the ductility, fracture toughness and formability of Al alloys. An experimental method for quantifying the level of grain boundary precipitation is described and applied to the Al alloy AA6111 subjected to a range of quenching rates. A threshold quenching rate can be identified above which the degree of grain boundary precipitate is insensitive to the quenching rate.     

Fracture mechanisms of aluminium alloy AA7075-T651 under various loading conditions

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.     

Hot workability of an Al-Mg alloy AA5182 with 1 wt% Cu

A comparative study of the hot workability of two aluminium alloys, alloy AA5182 used for automotive applications and a variant modified with 1 wt% copper, has been carried out. Hot torsion tests were performed on both alloys subjected to two different heat treatments: a low temperature preheat to 450 °C and a high temperature preheat at 540 °C. The results from the torsion experiments are interpreted in terms of microstructural features. Both treatments produce the same strength, but the high temperature preheat leads to better ductility. This improvement is related to the homogenization of solute elements in the matrix; and, concerning AA5182 + Cu, also to the dissolution of a non-equilibrium Al-Mg-Cu ternary eutectic present in the as-cast microstructure. The precipitation of (Fe, Mn)Al₆ precipitates in the matrix of both alloys is induced by the high temperature heat treatment. Comparison of the results obtained by hot torsion shows that at low deformation rates AA5182 + Cu has better ductility than the classical alloy, but its ductility is lower at strain rates above 0.6–0.8 s^–1. The null ductility transition temperature is lower compared with that in the classical alloy, restricting the range of hot working temperatures. Inside this range the strength of both alloys is approximately the same, although the strain rate sensitivity coefficient is increased by copper additions. The experimental strength values follow the classical sinus-hyperbolic constitutive equation for hot working.     

Microstructure,tensile properties and fracture behaviour of an Al-Cu-Li-Mg-Zr alloy 8090

The addition of lithium to aluminium alloys has the potential for providing a class of high strength alloys with exceptional properties suitable for aerospace applications. One such candidate is 8090, a precipitation hardenable Al-Li-Cu-Mg alloy. Detailed optical microscopical observations were used to analyse the intrinsic microstructural features of the alloy. It is shown that microstructural characteristics have a pronounced influence on tensile properties and fracture behaviour of the alloy in the peak-aged, maximum strength condition. Tensile test results indicate that the alloy has property combinations comparable with other high strength commercial aluminium alloys. The elongation and reduction in area are higher in the transverse direction of the extruded plate. A change in fracture mode was observed with direction of testing. We rationalize such behaviour based on the grain structure of the material, and the nature, distribution and morphology of the second-phase particles. An attempt is made to discuss the kinetics of the fracture process in terms of several competing mechanistic effects involving intrinsic microstructural features, deformation characteristics of the matrix, brittleness of the grain boundary precipitates and grain boundary failure. The role of stress on particle fracture is highlighted.