Pre-strain dependent relaxation behaviour of AA6016 during automotive paint drying processes

Heat treatable aluminium alloys are widely used in modern car bodies to achieve lightweight cars. Compared to steel, aluminium alloys are more sensitive to creep behaviour under high elastic stresses, since their melting point of about 600 °C is much lower. In order to predict permanent deformations of outer shell panels due to thermal stresses in drying processes and ensure highest quality standards regarding appearance to the customer, interdependencies with production process history must be evaluated. In this work, stress relaxation of the aluminium alloy AA6016 in initially T4 temper has been examined within the elastic regime with focus on the dependency of plastic pre-strain during primary creep phase. As a result, pre-strain becomes more important the higher the temperature was set. Hence, the former production process history of aluminium sheet metal parts must be taken into account when viscoelastic deformations are investigated. Based on the test results a pre-strain dependent material model for finite element simulations is pointed out, which can be used for novel deformation analyses of car bodies in automotive paint drying processes that contain aluminium parts out of AA6016.     

Study of the buckling mechanism in laser tube forming with axial preloads

The buckling mechanism of a thin metal tube during laser forming with axial preloads was investigated numerically and experimentally in this study. Metal tubes made of 304 stainless steel were heated by a CO₂ Gaussian laser beam, which induced the buckling phenomenon on the tube surface due to elastic–plastic deformation. This uncoupled thermal–mechanical problem was solved using a 3D finite element method (FEM) and was subsequently satisfactorily verified with axial force measurements. The transient force response and residual stress of the thin metal tube under specific operation conditions were also studied.     

汽车覆盖件弹塑性失稳机理实验模拟研究

利用YBT模型,结合密栅云纹试验,模拟了汽车用钢板的拉伸过程。通过记录、测量、计算和分析,获得试样全部变形过程中的整场连续、瞬时位移、应变和应力信息。结果表明,汽车覆盖件回弹类缺陷是卸载不均匀压缩诱发的局部弹性失稳;起皱类缺陷是加载不均匀导致的塑性失稳。     

Friction stir welding of ribbed panels of aircraft airframes

The results of analysis of ribbed panels used in airframe structures and production experience in the fabrication of airframes by welding ribbed panels to plating by continuous penetration of the plating are used to select the design of experimental ribbed panels of 1561 aluminium alloys for developing production technology. Assembling–welding equipment is constructed and a special tool is used for friction stir welding of T-joints of the ribs with the plating of the panel by continuous penetration of the welding tool into the panel. The welding conditions are determined and experimental full-size ribbed panels are welded.     

On the effect of the orientation of sheet rolling direction in shot peen forming

Peen forming is commonly used in the aerospace industry to shape large and thin panels, such as wing skins. This manufacturing process uses shot peening to introduce unbalanced compressive stresses near the surface of the component. These stresses tend to bend the panel and, when optimized, lead to the desired contour. Sheet materials often exhibit both elastic and plastic anisotropy, which can alter the development of curvatures. Since peen forming relies on compressive stresses to upset equilibrium, resulting curvatures may also be affected by initial stresses in the part. In this work, the influence of the rolling direction orientation with respect to the sample was investigated experimentally and numerically for the first time for aluminium alloy 2024-T3 specimens. Although maximum deflections were only slightly dependent on the rolling direction orientation, it was found that radii of curvature varied by as much as 10% with respect to this parameter. Finite element simulations allowed quantification of the individual effects of non-equibiaxial initial stresses and elastic orthotropy. It was found that these factors can significantly influence curvature development. Comparison of experimental and numerical results suggested that plastic anisotropy should also be taken into account in future studies. The tools developed in this study show promises for the accurate prediction of peen forming process for large scale components.     

Mixed numerical–experimental identification of elastic properties of orthotropic metal plates

This paper compares results of three different methods to determine the in-plane elastic properties of sheet materials. Results obtained with standard resonant beam and tensile tests are used to assess a mixed numerical–experimental technique developed to determine the in-plane elastic properties of orthotropic plates from the resonance frequencies of rectangular plate samples (the so-called ‘Resonalyser’ technique). Test materials were selected on the basis of an expected low degree of elastic anisotropy in order to put the accuracy and sensitivity of the different techniques to assess anisotropic materials to a test. Therefore, aluminium alloy and stainless steel samples were prepared from hot-rolled plates, deliberately avoiding pronounced cold-rolling textures. The differences between the results obtained with the three experimental approaches are critically evaluated.In the case of very thin plates, the existing mixed numerical–experimental Resonalyser procedure succeeded in accurately identifying the elastic material properties. A slightly adapted procedure is proposed to extend the applicability of the Resonalyser procedure to thicker plates.     

Large deformation of thin films and layered flat panels: effects of gravity

A general theory is presented for the large deformation of thin films and layered flat panels in which gravitational forces have a marked influence on the evolution of curvature, shape and instability. Isotropic, linear elastic deformation is considered with small strains and moderate rotations. The thermomechanical properties of the layered material are allowed to vary through the panel thickness so as to derive a general result for multilayers and graded materials. Explicit analytical expressions are derived for the critical curvature and the critical “effective load” at which curvature bifurcation occurs. The analysis considers square, circular and rectangular panels that are simply supported at three points, with the thin film on the panel facing either up or down. A boundary layer analysis is presented for rectangular panels specifically to examine the effect of panel shape on curvature evolution and geometric stability. Computational simulations involving full three-dimensional hyperelastic formulations with large rotations and two-dimensional hyper elastic formulations with moderate rotations were used to assess the validity of the analytical results. Systematic experiments on the large deformation characteristics of flat glass panels with and without silicon nitride thin film deposits were carried out to check the predictive capabilities of the theory. The trends predicted by the theory and its quantitative predictions of bifurcation with and without thin film deposits on the panels were found to be in reasonable agreements with experiments. The limits of the solutions of the present theory for the special case of thin films on substrates with only mismatch strains are shown to converge to prior analytical results. Furthermore, the theory is shown to capture the experimental trends observed during large deformation in thin-film/substrate systems in the absence of gravitational effects.     

Interplay of martensitic phase transformation and plastic slip in polycrystals

We study the interplay between martensitic phase transformation and plastic slip in polycrystalline media. The work is motivated by the phenomenon of superelasticity – the ability of the material to recover strains beyond their apparent elastic limit – observed in shape-memory alloys. Often the recovery is not perfect with residual strain after a deformation and recovery cycle, and the stress–strain curve changes with cycling. We develop a mesoscale model at the single crystal level, and use it to study polycrystals. The model is able to reproduce various observations and provide important insight into the interplay. In particular, we show that transformation and plasticity can occur synergistically, with plasticity providing a mechanism for bridging across poorly oriented and thus non-transforming grains.     

X-ray radiometric inspection of circumferential welded joints in the construction of transmission gas pipelines

ABSTRACT

Friction Stir Spot Welding (FSSW) is a subject that has recently gained interest and been developed, mainly in Titanium alloys, since there are existing studies on alloys of aluminium, magnesium, steels and polymers. Titanium alloys are widely used in the aeronautical industry, mainly as components of undercarriages for commercial planes. This work focuses on the Ti-6Al-4V alloy; this type of alloy has alpha (α) and beta (β) phases and provided better balance in terms of the mechanical properties of hardness and tensile stress. The FSSW process was performed using a high-speed machining centre, using a rotational speed of 500 RPM, a plunge speed of the tool of 2 mm/s and 3 s as dwell time, on plates with a thickness of 1.5 mm and an overlap length of 20 mm. The results showed the effect of the variables of the FSSW welding process on the microstructure of the Ti–6Al–4V alloy caused by the thermal effect and the plastic deformation of the welding process, concluding that the right combination of parameters provides a joint without any defects.     

Exfoliation corrosion testing of aluminium alloys

Abstract

The exfoliation corrosion behaviour of sheet and plate materials of various conventional aluminium and Al–Li alloys has been evaluated using accelerated tests. Results are .compared with atmospheric exposure data published in the literature to assess the applicability of the testing techniques employed. For damage tolerant Al–Li based sheet and plate, the cyclic acidified salt fog (Mastmaasis) test according to ASTM G85, Annex A2 indicated susceptibility to exfoliation corrosion, reproducing the limited outdoor corrosion data for the Al–Li alloys 8090–T81 and 2091–T84 as well as marine exposure results reported for the conventional alloys 2024–T351 and 7075–T7351. Therefore, it appears to be a promising testing technique for predicting the service performance of high strength aluminium alloys. Compared with the ratings determined following the cyclic acidified saltfog tests, the standard Exco test according to ASTM G34 indicated better exfoliation corrosion behaviour of the alloys investigated, except for 8090–T6 sheet and 7075–T7351 plate, which exhibited severe and mild exfoliation respectively. In the modified Exco test suggested by Lee and Lifka, 7075–T7351 panels were susceptible to pitting, whereas the other alloys studied generally suffered more severe exfoliation than in the standard Exco test.     

New technique to control welding buckling distortion and residual stress with non-contact electromagnetic impact

Abstract

A new technique using non-contact electromagnetic forces has been proposed for controlling welding buckling distortion and residual stresses in welded thin plates. The experimental results show that the method can successfully eliminate the buckling distortion and reduce the residual stresses. Three-dimensional finite element modelling has been developed to study the evolution of the stress and strain throughout the welding and electromagnetic impacts. The predicted welding distortion and residual stresses are in good agreement with the experimental results. The numerical analyses show that the reduction in distortion and stress is a result of the change of the plastic strain field in the weld region: electromagnetic impacts reduce longitudinal compressive plastic strain in the local region near the weld, and even produce the tensile plastic strain. Moreover, it is found that the residual stress can promote the changes of the longitudinal plastic strain state under electromagnetic impact.     

Applicability of constant dew point corrosion tests for evaluating atmospheric corrosion of aluminium alloys

Field exposure tests of aluminium alloys were conducted at three sites in Japan. Meteorological data indicated that the dew point of the ambient atmosphere and aluminium panels remained constant for the short-term. Constant dew point corrosion tests were employed to reproduce atmospheric corrosion of aluminium alloys in the laboratory. The corrosion rates, corrosion morphology and corrosion product composition after 7 days of tests in the laboratory were similar to those formed after 3 months of exposure at coastal sites. Not only did the constant dew point corrosion test effectively reproduce the atmospheric corrosion of aluminium alloys, it also accelerated it.     

Influence of a boiling water treatment on the electrochemical properties of a sol–gel film on AA1050

Many studies report that a very simple strategy to improve the corrosion resistance of aluminium is to promote the formation of a pseudo-boehmite (Al₂O₃·H₂O) layer by a hydrothermal process (i.e. immersion in boiling water). In this work the effect on AA1050 panels of different durations of a boiling water treatment was investigated. In particular, the work is focussed on the corrosion protection properties of a sol–gel layer further applied to the treated aluminium panels and its interaction with the underlying pseudo-bohemite layer. Films consisting of 1:1 molar ratio of 3-glycidoxypropyltrime-thoxysilane (GLYMO) and methyltriethoxysilane (MTES) were applied on AA1050 plates. For this purpose, the effect of the different periods of immersion in boiling water was investigated in terms of morphology by means of scanning electron microscope, in terms of surface chemistry by means of FT-IR and in terms of electrochemical activity of the surface by exploiting electrochemical techniques such as polarisation curves.     

Investigation on twisting distortion of thin plate stiffened structure under welding

The welding distortion of thin plate was selected as research subject, which causes loss of dimensional control, structural integrity and increases fabrication costs for straightening. To study the characteristic of the welding distortion on thin plate structures, experiments were conducted. The test model was a thin plate stiffened structure and a large twisting distortion was observed. The welding distortion of the same structure was analyzed as a large deformation problem using a thermal elastic plastic FEM and an elastic FEM based on the concept of inherent deformation. The computed results by both methods showed the twisting distortion which is a typical buckling type deformation and the magnitude of this distortion agreed well with the experimental measurement.     

Gliding at interface during thin film buckling: A coupled atomistic/elastic approach

The buckling of a Cu thin film deposited on a stiff W substrate is studied by means of molecular static simulations using interatomic potentials. The buckling of a preexisting delaminated part of the film is observed under a uniaxial strain without any further decohesion of the interface. A sliding phenomenon is also observed at the edges of the buckle which can be interpreted in terms of glide of misfit dislocations. Integrating this sliding effect in the Föppl-von Kármán theory of thin plates, a modified analytical expression for the deflection of the film has been derived which fits with the deflection obtained from the simulations.     

Prediction of welding distortion during assembly process of thin plate structures

0IntroductionApplication of welding process in fabricating largestructures offers several advantages over mechanical joiningmethods such as improved structural performance,flexibil-ity of design,weight reduction and cost savings etc.Inship and automobile …     

Local flange buckling and its relation to elastic springback in forming of aluminium extrusions

This paper presents a numerical and experimental investigation of interactions between local buckling and elastic springback in forming of thin-walled aluminium extrusions. The cross sectional distortions that are present in typical aluminium sections prior to elastic unloading cannot be disregarded. This is the case even when these beams are bent with relative large radii. The numerical analyses presented herein have shown that both element type and formulation have to be chosen carefully in order to correctly predict elastic recovery. Full integration tri-linear brick elements in combination with the assumed strain formulation have proven to be preferable to linear thick-shell, conventional full integration brick elements as well as reduced integration elements.     

Influence of climatic factors in cyclic accelerated corrosion test towards the development of a reliable and repeatable accelerated corrosion test for the automotive industry

Two designs of experiments made of nine accelerated tests each were used in order to study the influence of a selection of important climatic parameters such as the concentration of NaCl, the drying level, the basic humidity cycle, the frequency of salt spray and the temperature. The accelerated corrosion tests were all performed using automatic chambers. Different automotive materials were selected in order to study the resistance to cosmetic corrosion, perforation corrosion and bi‐metallic corrosion. For cosmetic materials, coated panels of cold rolled steel (CRS), zinc‐coated steel and aluminium alloys were chosen. Perforation corrosion was investigated using crevice panels of CRS, zinc‐coated steel or different grades of aluminium alloys. From the results, the influence of testing conditions on the cosmetic and perforation corrosion of different automotive materials was obtained. As an example, it was shown that an elevation of the temperature from 35 to 45 °C in the cyclic corrosion test increased the scribe creep on painted CRS and aluminium alloys while it has no significant effects on painted hot dip galvanized (HDG) panels. The results were also compared to that obtained after 2 years of exposure at a marine exposure site and on busses driving in area using de‐icing salt. It was shown that one of the tests performed gives a good correlation to field results.     

Black anodic coatings for space applications: Study of the process parameters,characteristics and mechanical properties

Black inorganic anodized aluminium alloys are used for managing passive thermal control on spacecraft and for avoiding stray light in optical equipment. Spalling of these coatings has sometimes been observed after thermal cycling on 2XXX and 7XXX aluminium alloys. This phenomenon could generate particulate contamination in satellites and may affect mission lifetime. In this work, the influences of the four main steps of the process (pretreatments, sulphuric anodizing, colouring and sealing) on the coating characteristics have been studied for a 7175T7351 aluminium alloy. The chemical heterogeneity of the coating has been underlined, and its mechanical behaviour observed through crazing. Scratch-testing, used to evaluate coating adhesion to its substrate, revealed the negative impact of thermal cycling.     

Lamellar eutectic growth with anisotropic interphase boundaries: Experimental study using the rotating directional solidification method

We report on an experimental study of the effects of interphase boundary anisotropy on eutectic microstructures using a new methodology called rotating directional solidification (RDS), which consists of rotating a thin sample with respect to a fixed unidirectional thermal gradient. The systems used are thin, large eutectic grains of the CBr₄–C₂Cl₆ and In–In₂Bi lamellar eutectic alloys. The shape of the observed RDS lamellar trajectories turns out to be a reproducible eutectic-grain-dependent feature, in agreement with the theoretical prediction that these trajectories are approximately homothetic to the Wulff form of the interphase boundary in the sample plane. We show that different modes of lamellar growth, ranging from quasi-isotropic to (crystallographically) locked, exist in different eutectic grains of the two alloys studied. A detailed characterisation of these modes is given, with particular attention to the as-yet poorly understood aspects of locked lamellar growth.