Comparison of texture evolution during cold rolling between direct chill and continuous cast aluminium alloy 5052

Abstract

Hot bands of direct chill cast (DC) and continuous cast (CC) aluminium alloy 5052 were cold rolled to different reductions after being annealed at 454 ° C for 4 h. The texture evolution during cold rolling was investigated for both DC and CC AA 5052 by determining the orientation distribution functions of the cold rolled specimens via X-ray diffraction. Texture evolution during cold rolling was predicted by empirical formulas of the variation of the texture components with true strain. The results show that the processing method (DC versus CC) has an effect on the texture of annealed hot bands and the texture evolution during subsequent cold rolling.     

Formability analysis of thin press hardening steel sheets under isothermal and non-isothermal conditions

Aiming at the enhancement of the lightweight potential of press hardening steels, investigations on the formability of thin, boron alloyed, hot dip aluminized steel sheets are carried out. The material formability is described through Forming Limit Diagram (FLD), determined by means of Nakajima formability test of thin 22MnB5 sheets (0.50 mm, 0.80 mm, 1.25 mm) at elevated temperatures. The influence of sheet thickness on forming limits is evaluated under both isothermal and non-isothermal conditions. The effect of different deformation start temperatures is examined. The non-isothermal behavior is further investigated via microstructural analysis and a study on temperature profile during Nakajima test. The results show a significant difference regarding the influence of sheet thickness under isothermal and non-isothermal conditions. Increasing the sheet thickness results, as expected, in higher forming limits for isothermal conditions, whereas for non-isothermal conditions the opposite effect on formability is observed. The obtained Forming Limit Curves (FLCs) are validated through hot stamping simulation and subsequent analysis of different thin components, concluding that in case of thin sheets the isothermal FLC constitutes a more conservative approach, while the non-isothermal one reaches the formability limits with higher accuracy.     

Tensile properties of a few Mg-Li-Zn alloy thin sheets

The tensile properties of experimentally produced Mg-6Li-1Zn, Mg-9.5Li-1Zn and Mg-12Li-1Zn alloy thin sheets at room temperature are investigated in this study. Uniaxial tension tests are carried out for various strain rates between 1.4 × 10^–5 and 8.3 × 10^–2 s^–1, and the microstructural and textural changes during the tests are examined. The Mg-6Li-1Zn sheet is composed mainly of the (hcp) phase and inferior to the other sheets in ductility. The (bcc) phase is dominant in the Mg-9.5Li-1Zn and Mg-12Li-1Zn sheets, and they have a considerable sensitivity to strain rate. It is observed that the grains are elongated with textural change mainly in the phase at low strain rates, and the Mg-9.5Li-1Zn and Mg-12Li-1Zn sheets have sufficiently high ductility at low strain rates. The Mg-9.5Li-1Zn sheet composed of ( + ) two phase is superior to the Mg-12Li-1Zn sheet of single phase in the tensile strength.     

Forming limit diagram prediction of AA5052/polyethylene/AA5052 sandwich sheets

Metal–plastic sandwich sheet has received increasing attention in aeronautical, automotive, marine and civil engineering industries due to its lower density, higher specific flexural stiffness, better dent resistance, better sound and vibration damping characteristics. In the present study, an AA5052/polyethylene/AA5052 sandwich sheet is developed and its formabilities are investigated. A numerical simulation method based on the Gurson–Tvergaard–Needleman (GTN) damage model is used for simulating the forming process of sandwich sheet, in which the interface conditions between skin sheet and core materials are considered by using the cohesive zone model (CZM). The rigid punch dome tests and the Nakazima forming tests are carried out to build the forming limit diagrams (FLDs) of sandwich sheet. A strain history method is applied to determine the limited strain. Comparisons between predictions and experimental results validate the used numerical simulation method. Finally, the influences of polyethylene’s thickness on the formabilities of sandwich sheet are analyzed. Research results show that: AA5052/polyethylene/AA5052 sandwich sheet has a better formability than monolithic AA5052 sheet and the formability of AA5052/polyethylene/AA5052 sandwich sheet increases with increasing the thickness of polyethylene core layer.     

Ductile rupture in thin sheets of two grades of 2024 aluminum alloy

The damage and rupture mechanisms of thin sheets of 2024 aluminum alloy (Al containing Cu, Mn, and Mg elements) are investigated. Two grades are studied: a standard alloy and a high damage tolerance alloy. The microstructure of each material is characterized to obtain the second phase volume content, the dimensions of particles and the initial void volume fraction. The largest particles consist of intermetallics. Mechanical tests are carried out on flat specimens including U-notched (with various notch radii), sharply V-notched and smooth tensile samples. Stable crack growth was studied using “Kahn samples” and pre-cracked large center-cracked tension panels M(T). The macroscopic fracture surface of the different specimens is observed using scanning electron microscopy. Smooth and moderately notched samples exhibit a slant fracture surface, which has an angle of about 45° with respect to the loading direction. With increasing notch severity, the fracture mode changes significantly. Failure initiates at the notch root in a small triangular region perpendicular to the loading direction. Outside this zone, slant fracture is observed. Microscopic observations show two failure micromechanisms. Primary voids are first initiated at intermetallic particles in both cases. In flat regions, i.e. near the notch root of severely notched samples, void growth is promoted and final rupture is caused by “internal necking” between the large cavities. In slanted regions these voids tend to coalesce rapidly according to a “void sheet mechanism” which leads to the formation of smaller secondary voids in the ligaments between the primary voids. These observations can be interpreted using finite element simulations. In particular, it is shown that crack growth occurs under plane strain conditions along the propagation direction.     

Influence of ductile damage on the bending behaviour of aluminium alloy thin sheets

Sheet metal forming involves planar stress states, in the sheet plane, like in tension and simple shear, or stress states characterized by a gradient in the thickness, like in bending. In this latter case, material limit prediction derived from an instability criterion is no longer valid. In this work, a criterion based on a critical void volume fraction, identified from macroscopic tests, is applied to the case of bending of square samples of aluminium alloy AA6016-T4. Mechanical tests are performed at two aging times to quantify its influence on the mechanical behaviour and only the hardening law is modified to take it into account. It is shown that a good correlation is obtained between the critical void volume fraction obtained from tension on notched samples and biaxial expansion, and the onset of crack development in the bent zone. Moreover, macroscopic load recorded during bending is sensitive to ductile damage, which makes this test particularly interesting for damage investigation.     

Formability enhancement of Al 6060 sheets through fiber laser heat treatment

Due to the continuous weight reduction effort in the automotive sector, formability enhancement of aluminum alloys in forming and hydroforming processes is gathering much attention from research institutes and industries. During sheet forming processes, large deformations are desired to obtain complex shapes but these are limited by the appearance of defects such as wrinkling and cracks. To avoid these issues, intermediate annealing heat treatments are often applied as a possible solution. Nevertheless in large components where small details have to be created, local heat treatment through lasers can be cost effective over the furnace treatment of the whole part and it would limit possible geometrical distortion in large components. The following article presents fiber laser process parameters definition on deformed sheets made of Al6060 alloy. Grain structure variation and hardness decrease were studied to correctly select process parameters (laser power, feeding speed and overlapping among subsequent passes) to increase material formability. In addition, a systematic comparison between fiber laser and furnace heat treatment was assessed proving the equivalence of the two methods in terms of achieved mechanical proprieties.     

Welding failure of as-fabricated component of aluminum alloy 5052

This paper describes the failure analysis of the “tray section” made up of aluminum alloy 5052 which is used as a specimen holder in a research reactor. Fracture was observed in the central rod of alloy 5052 before it was taken for service. The fracture had occurred in a brittle mode without any gross plastic deformation at a location where the rod was welded to the stopper plate. Detailed microstructural examination was done using both optical and scanning electron microscopy. The weld fusion zone showed presence of high porosity and eutectic phases mainly along the inter-dendritic regions. These low melting temperature eutectics were rich in Si and Fe and led to weld cracking along the dendritic grains during solidification of the welds. Solidification cracking of alloy 5052 was related to pure aluminum filler wire used for welding that shifted the composition of the welds towards peak cracking sensitivity of 1.5 wt% Mg. The failure of the tray section was concluded to be due to welding defects, e.g. high porosity and solidification cracks. Recommendations to avoid this type of failure are also proposed.     

Formability of Ti14 alloy during semisolid deformation

Abstract

The semisolid formability for Ti14, an α+Ti₂Cu alloy, is compared with the conventional warm formability from the point of forgeability. The forgeability is evaluated by upsetting and die forging tests. The results show that excellent upsettability with the upsetting reduction in height of 70–85% and low upsetting force could be obtained in semisolid state ranging from 1000 to 1100°C, which is better than that in conventional processing. Die forging tests also show excellent workability with a forging ratio of 75% at the temperature range of 1000–1050°C. It can be concluded that the existence of liquid may serve to relax the stress concentrations caused by solid deformation, which causes low deformation resistance and results in improvement of forgeability. Furthermore, dynamic recrystallisation occurred during thixoforging, and the grain refinement was attained, which also results in the improvement of the semisolid formability.     

Elasto-plastic folding of thin sheets

Summary A thin sheet is folded by three methods. An elastic-perfectly plastic moment-curvature relationship is assumed. A plastic hinge occurs whenever the local curvature is larger than a critical value. The problem is solved by both numerical integration and asymptotic expansions. The character of the sheet changes distinctly after plasticity sets in.With 7 Figures     

Mechanical and anisotropic behaviors of Mg–Li–Zn alloy thin sheets

This study examined the mechanical property and formability of the cold-rolled Mg–Li–Zn alloy sheets with two different Li contents. Uniaxial tension and press-forming tests were carried out at room temperature. The tensile properties and formability parameters were correlated with the forming limit diagrams. The test results indicated that the Mg–Li–Zn alloy with a Li content of 6 wt% exhibited reasonable strength levels with moderate fracture elongation and that it did not show good stretchability and drawability at room temperature. The alloy with a Li content of 9 wt% presented excellent ductility even at room temperature and the strength levels were somewhat inferior. From the analysis, it was found that formability of the alloy with a higher Li content of 9 wt% was superior compared to that of the alloy with a Li content of 6wt%. Moreover, the fracture surfaces of the press-formed samples were considered and studied under a scanning electron microscope (SEM). The results showed that the partly ductile and partly brittle fracture pattern was observed in the tension–tension strain condition for both the alloys.     

Prediction of flow curves and forming limit curves of Mg alloy thin sheets using ANN-based models

Multivariable empirical models based on artificial neural networks were developed in order to predict the flow curves and forming limit curves of AZ31 magnesium alloy thin sheets, in warm forming conditions, vs. process parameters and fibre orientation. Experimental tensile and hemispherical punch tests were carried out in order to obtain the experimental data set, in terms of flow curves and forming limit curves, to be used to train the artificial neural networks. A preliminary study, based on the leave one-out-cross validation methodology, has proven the very good predictive capability of the ANN-based models in modelling both flow curves (flow stress level, curve shape and strain at the onset of necking) and forming limit curves (curve shape, major strain values and minor strain limit) under different process conditions and fibre orientations. Then, the generalisation capability of the neural models in capturing the effect of process parameters and fibre orientation on flow curves and formability has been proven by the excellent agreement, in terms of the high correlation coefficients, low relative errors and average absolute relative errors, between predicted and experimental results not investigated in the training set.     

Nonlinear breakup of thin liquid sheets

Summary The breakup of thin planar liquid sheets due to the nonlinear growth of disturbances is determined. Conservation equations are derived using a control volume method and solved using MacCormack's predictor-corrector scheme. It is found that the size and geometry of ligaments, formed during breakup, vary with the weber number. Antisymmetric waves, which spontaneously intensify at higher values of the Weber number, give rise to thin ligaments. At lower values of the Weber number antisymmetric waves formed initially get transformed into symmetric interfaces giving larger ligaments. The magnitude of the initial amplitude of the disturbances is shown to strongly influence the disintegration. The results are compared with experimental results obtained for thin water sheets and good agreement is demonstrated.     

On the tearing of thin sheets

We perform an experimental study to investigate the propagation of one or two cracks in a thin elastic sheet of a brittle material torn in an out-of-plane shear mode. We observe that a single crack always follows a straight path, whereas two cracks propagating simultaneously follow curved paths and merge, forming a tongue-like shape. The present experimental setup allows the understanding of how the energy introduced at a large scale is focused at the crack tip. We find that the geometry of the sheet is determined by the direction of a large scale force, applied to the crack tip, which is perpendicular to cracked surfaces. While the material is deformed at large scales under mode III loading, the geometry of system in the vicinity of the crack tip adapts such that the material is locally broken under a pure mode I loading.     

Friction welding process of 5052 aluminium alloy to 304 stainless steel

Abstract

Type 5052 aluminium alloy was joined to type 304 austenitic stainless steel via a continuous drive friction welding process. The joint strength increased, and then decreased after reaching a maximum value, with increasing friction time. Joint strength depended on the size and shape of the tensile testpiece. Friction weldability could be estimated by electrical resistmetry. The process of friction welding between the aluminium alloy and the stainless steel is proposed to evolve as follows: welding progresses from the outer to the inner region; an unbonded region is retained at the centre of the weld interface with shorter friction time; longer friction time causes the formation of an intermetallic reaction layer at the weld interface; and the reaction layer grows as the friction time increases. When the thickness of the reaction layer increased above a critical value, the joint was brittle and fractured at the weld interface. The joint was sound when there was no unbonded region and a thin reaction layer formed along the entire weld interface.     

Fabrication of ZK60 magnesium alloy thin sheets with improved ductility by cold rolling and annealing treatment

Thin ZK60 magnesium alloy sheets with ultrafine-grain structure were successfully fabricated by continuous cold rolling with proper intermediate annealing treatments at 503–523 K for 30 min. Meanwhile, microstructure uniformity and planar texture anisotropy were strikingly improved by rolling deformation and static recrystallization, resulting in continuous improvement in the strength anisotropy. Excellent ductility more than 30% in fracture elongation was achieved after further annealing treatment at a lower temperature of 473 K. This was primarily attributed to the significant weakening of the {0 0 0 2} pole intensity and grain refinement during the process. It is shown that mechanical properties of the final sheets could be closely controlled by the present process.     

Study on mechanical properties and constitutive model of 5052 aluminium alloy

The stress–strain relationship of 5052 aluminium alloy was investigated via quasi-static tensile tests and split Hopkinson pressure bar tests. The specimens were exposed to various temperatures (25–500°C) and strain rates (10^?4–0.7?×?10^4?s^?1). At strain rates ranging from 0.001 to 3000?s^?1, the material underwent significant work hardening. When the strain rate exceeded 5000?s^?1, the work hardening effect decreased and the flow stress was relatively constant. The Johnson–Cook constitutive model was modified to describe the deformation behaviour of the material subjected to high temperatures and strain rates. The accuracy of the modified model was verified through ballistic impact testing.     

Microstructure and microtextural studies of friction stir welded aluminium alloy 5052

The present investigation presents a composite picture of the microstructural developments in a friction stir welded (FSW) AA5052. Optimized, defect free and chemically homogeneous, FS weld was generalized in four regions – base material (BM), nugget, advancing side (AS) and retreating side (RS), using standard nomenclatures. Each region had its signature of microstructural features. AS had clear indications of shear and of grain fragmentation. The nugget region, on the other hand, had nearly equiaxed grains, with strong in-grain misorientation and presence of grain-interior dislocation structure ruling out contributions from static recrystallization. Equiaxed grains of the nugget region had typical onion ring structure – each ring did approximately correspond to one dominant family of orientation. Microstructural developments, as obtained from relative grain refinement, in-grain misorientation development, relative banding, etc., were most significant in nugget followed by AS and then by RS. Heterogeneous plastic deformation and thermal activation through localized heating/friction were the apparent causes. Most of the friction stir welded specimen fractured away from the nugget and showed ductile mode of failure.     

Plane strain compression of aluminium alloy sheets

The theory of plane strain compression is applied to rolled aluminium alloy sheet. Two contrasting grades of the alloy are tested: naturally aged AC 120 and half-hard HE 30. While AC 120 displays a smooth stress–strain curve under homogenous straining, HE30 shows a serrated stress–strain curve due to its banded plastic strain behaviour. It is shown that, provided the r-values can be established reliably to characterise each sheet’s orthotropy, a flow curve to large strain (≃2) is provided by the plane strain test. Certain modifications to the original test procedure are made to achieve this. Equivalence in flow curves, as required of orthotropic plasticity theory, is examined from plane strain, bulge forming and tension tests conducted at various orientations to the roll. Despite the contrasting limiting strains between the three tests (tension ≃ 0.1, bulge forming ≃ 0.8) an acceptable correlation has been found between their equivalent flow curves across three decades of strain. The dependence of equivalent plastic strain upon equivalent stress for each material conforms to the Hollomon law. The Ramberg–Osgood law allows for the addition of elastic strain.