Some studies on wrinkling limit of commercially pure aluminium sheet metals of different grades when drawn through Conical and Tractrix dies

This paper deals with the wrinkling limit diagrams of annealed three different grades of commercially pure sheets namely ISS 19000, ISS 19600 and ISS 19660 having thickness of 2.00 mm when deep drawing into cylindrical cups through Conical and Tractrix dies using a flat bottomed punch. It is well established that, the use of either a Conical die or a Tractrix die can enhance the limiting draw ratio compared with that obtainable in a conventional drawing operation. When a Conical or a Tractrix die is employed the need for a hold down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (dε _r /dε _θ ) or the ratio of strain (ε _r /ε _θ ) reaches a critical value during the drawing process. These values which could be determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above three different grades. An attempt is also made to develop the wrinkling theory that predicts the wrinkling based on results obtained in the form of wrinkling limit diagrams established for the above grades.     

Effect of annealing on the wrinkling behaviour of the commercial pure aluminium grades when drawn through a conical die

Experiments have been carried out to evaluate the onset of wrinkling behaviour in three grades of annealed commercial aluminium sheets namely ISS 19000, ISS 19600 and ISS 19660 having thickness 2.00 mm, while drawing them through a conical die. The deep drawing of circular blanks of different diameters of the aforesaid sheet metals was carried out using flat bottom punch. The analysis of the experimental data has revealed that the onset of wrinkling takes place when the ratio of the plastic strain increments (dε_r/dε_θ) reached a critical value. It was further found that the measured strains followed non-linear paths in the wrinkling limit diagrams. Further, it was observed that the aluminium grade having high strain hardening index value, low yield stress and high normalized hardening rate shows better resistance against wrinkling. Furnace cooled aluminium grades showed better resistance against wrinkling compared with air cooled blanks.     

Fatigue behaviour of commercially pure aluminium processed by rotary swaging

Fatigue strength of ultrafine-grained commercially pure aluminium (Al 1050) produced by severe plastic deformation (rotary swaging) was investigated. Fatigue tests were carried out on smooth and notched specimens. Results show improved static and fatigue strength of the rotary swaging processed material. However, the processed material was highly notch sensitive due to low work hardening capability, low ductility as well as low uniform strain. It was found that post-deformation annealing above recrystallization temperature can additionally enhance the work hardening capability and the ductility of the swaged material, which led to marked reduction in fatigue notch sensitivity. At the same time, this reduction is accompanied with pronounced loss in strength. Fatigue notch sensitivity of commercially pure aluminium can be good correlated to the work hardening capability and ductility. This behaviour was discussed in details based on microstructure and mechanical properties study.     

Studies on void coalescence analysis of nanocrystalline cryorolled commercially pure aluminium formed under different stress conditions

Cryorolling was performed on commercially pure aluminium sheet from an initial thickness of 7 mm to 0.25 mm with a total true strain of around 3.33. Cryorolling was performed in many passes with only 5% reduction in each pass to avoid adiabatic heating during rolling process. Detailed Transmission Electron Microscopic studies showed increased dislocation density and distributed dislocation cell structure. Streaks along with ring pattern in selected area electron diffraction of transmission electron microscopy evidenced the existence of texture component on the surface of rolled sheet along with nanocrystalline sub-structure. Studies on fracture behavior of the cryorolled sheets were performed using specimen of double edge-notch tensile geometry. Compared to the conventionally rolled sheet metal, the strain triaxiality ratio value in the case of cryorolled sheets is insensitive to the void growth analysis due to the presence of nano-sized grains. The ratio of length to the width of voids varies from 1.6 to 2.4 in the case of conventionally rolled sheet. In contrast, length to width ratio is very close to 1.0 in the case of cryorolled sheet. Thus no oblate or prolate voids were observed during cryorolling compared with conventionally rolled sheets and it is observed the formation of equiaxed nanostructured grains. In the case of cryorolled sheets, minimal variation in length to width ratio was observed with the variation in the shear strains, due to the presence of nanostructured grains. Whereas, in the case of conventionally rolled material, wide variation in the length to width ratio with the variations in the shear strains was observed.     

Hydroxyapatite deposition study through polymeric process on commercially pure Ti surfaces modified by laser beam irradiation

Many techniques have been used to coat metallic substrate with bioceramics. The aim of this study was to study the physical-chemical characteristics of polyvinylidene fluoride (α-PVDF)/hydroxyapatite (HA) composite coating, obtained by casting method, on commercially pure titanium (α-CP Ti) substrate surface modified by laser beam irradiation. The preparation of coating was done for mixing α-PVDF pellets shape dissolved in dimethylacetamide (DMA) with HA/DMA emulsion. The mixture was poured onto the α-CP Ti sample and left to dry in an oven. CP Ti plates were coated with α-PVDF/HA composite film, in proportions of 100/00 and 60/40 in weight, and characterized by particle size analysis, scanning electron microscopy, energy dispersive spectroscopy (EDS), X-ray diffractometry, thickness measurement and contact angle. Uniform coating with a small thickness variation along the coated surface was successfully obtained.     

Effect of different yield criteria on prediction of wrinkling initiation of interstitial-free galvanized steel sheet

Thin interstitial-free (IF) galvanized steel sheets have great usage in auto industry due to their high formability and corrosion resistance. However presence of wrinkle waves in deep drawn parts of thin IF steel sheets is a common challenging issue. Achieving a reliable method to predict the wrinkling onset enables the manufacturers to avoid wrinkling by modification of forming parameters. In the present study, sidewall wrinkling of an IF-galvanized steel sheet was both analytically and experimentally examined during a conical cup wrinkling test. Using an energy method approach together with a newly developed deflection function, Hosford and Hill-1948 yield criteria were employed to predict the critical values of stress and cup height at the onset of wrinkling. According to the obtained results, applying Hill-1948 yield criterion resulted in prediction of wrinkling onset closer to the experimental value than Hosford yield criterion. It was also found that the newly presented deflection function has a great suitability for prediction of the real status of wrinkling phenomenon.     

Microstructure and tensile strength of friction stir welded joints between interstitial free steel and commercially pure aluminium

In the present study, the joining of interstitial free steel and commercial pure aluminium was carried out by friction stir welding (FSW) technique using tool rotational speeds of 600, 900, 1200 rpm and traverse speed of 100 mm/min. The microstructure and micro-hardness of the weld interface have been investigated. Optical microscopy was used to characterize the microstructures of different regions of friction stir welding joints. The scanning electron microscopy-back scattered electron (SEM-BSE) images show the existence of the different reaction layers in the welded zone. The Al₃Fe intermetallic compound has been observed in the weld interface and their thickness increase with the increase in tool rotational speed. Tensile strength was also evaluated and maximum tensile strength of ∼123.2 MPa along with ∼4.5% elongation at fracture of the joint have been obtained when processed at 600 rpm tool rotational speed.     

Characterization of microblasted and reactive ion etched surfaces on the commercially pure metals niobium, tantalum and titanium

In surface-roughened metallic implant materials, the topography, chemistry and energy of the surfaces play an important role for the cell and tissue attachment. The highly reactive commercially pure metals niobium, tantalum and titanium were analysed after microblasting (with Al2O3 powder and consecutive shot-peening with ZrSiO2), and after additional reactive ion etching (RIE, with CF4). Scanning electron microscopy in combination with energy-dispersive X-ray analysis and surface roughness measurements showed, for all microblasted surfaces, a heterogeneous roughening (Ra about 0.7 m), and a contamination with blasting particles. RIE resulted in a further roughening (Ra about 1.1 m), and a total cleaning from contaminations, except for traces of aluminium. Determination of surface energy by dynamic contact angle measurements showed an increase in surface energy after microblasting, which further increased after RIE, most pronounced for commercially pure niobium. In conjunction with superior electrochemical properties, this makes niobium and tantalum promising candidates for implant purposes, at least equal to the generally used titanium.     

Correction to: Effect of pulse current on the wear behavior of a cold-rolled commercially pure aluminum sheet

Strength of Materials -     

An experimental study for the effect of different clearances on burr, smooth-sheared and blanking force on aluminium sheet metal

In this study, an experimental study was carried out to examine the effect of different clearances on smooth-sheared depth, burr height and blanking force. Aluminium sheet metals with 0.8, 1, and 1.5 mm thicknesses were used in the experiments. These experiments were carried out in circular blanking dies to 6, 8, 10, and 12 mm diameters. Six different clearances (0.009, 0.064, 0.12, 0.175, 0.231 and 0.287 mm) were used for every material and diameter. The results of the experiment show that burr, smooth-sheared and punch force is strongly related to the clearance value. The results are in agreement with the results of previous theoretical and experimental work in the literature.     

Effect of bulge shape on wrinkling formation and strength of stainless steel thin sheet

Embossing and restoration is a simple method to increase the strength of thin sheet metals by creating concentric pattern of wrinkles onto thin sheet-metal plates. SUS304 stainless steel sheets of 0.4 mm thickness were experimentally bulged using hemispherical punches, and then compressed between two flat platens of a testing machine to produce the wrinkles. The process simulation was carried out using finite element method to investigate the effect of bulge shape and height on the wrinkling formation and sheet strength. Thickening of the plate was observed during the compression step and after the thinning that is occurred during the bulging step. Different restoration patterns were observed as bulge height increases. Also, the number of wrinkles within a pattern and its distribution throughout the plate were depending upon the bulged shape. The pressure bearing capacity of the compressed part rapidly accelerates with the increase of wrinkles inclusion. Virtual deflection tests were carried also out using the finite element method (FEM) to evaluate the effect of the wrinkles pattern on the thin plate rigidity or strength. The results showed that the technique is effective in enhancing the strength, the flexural deflection and reducing the failure risk of thin sheet metals parts. This can contribute in the weight reduction of sheet metal parts. The observed patterns can be utilized in sheet decoration.     

Prediction of solidification behaviour of weld pool through modelling of heat transfer and fluid flow during gas tungsten arc welding of commercial pure aluminium

Abstract

A mathematical model is developed to assess the solidification behaviour of the weld pools. To do so, during gas tungsten arc welding of commercial pure aluminium, equations of conversation of mass, energy and momentum are numerically solved considering three-dimensional steady state heat transfer and fluid flow conditions. The weld pool geometry, weld thermal cycles and various solidification parameters are calculated using temperature and velocity fields acquiring from the utilised model. The solidification behaviour of the weld pool at the weld centreline and the fusion line is then studied using the solidification parameters including temperature gradient G, solidification rate R and the combined forms G/R and GR. In order to verify the predictions, welding experiments are performed and geometry of the weld fusion zone is measured. The calculated geometry of the weld fusion zone is found to be in good agreement with the corresponding experimental result. The predictions show that the cooling rate GR increases toward the centreline while the other solidification parameter G/R shows a different behaviour. In addition, it is found that under the employed welding conditions, as the welding speed increases temperature gradients both at the weld centreline and at the fusion line are reduced.     

Microstructure development and hardening during high pressure torsion of commercially pure aluminium: Strain reversal experiments and a dislocation based model

The effect of strain reversal on hardening due to high pressure torsion (HPT) was investigated using commercially pure aluminium. Hardening is lower for cyclic HPT (c-HPT) as compared to monotonic HPT (m-HPT). When using a cycle consisting of a rotation of 90° per half cycle, there is only a small increase in hardness if the total amount of turns is increased from 1 to 16. Single reversal HPT (sr-HPT) processing involves torsion in one direction followed by a (smaller) torsion in the opposite direction. It is shown that a small reversal of 0.25 turn (90°) reduces hardness drastically, and that decrease is most marked for the centre region. These behaviours and other effects are interpreted in terms of the average density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs). A model is presented that describes the experimental results well. A key element of the model is the assumption that at the very high strains developed in severe plastic deformation processes such as HPT, the dislocation density reaches a saturation value. The model indicates that the strength/hardness is predominantly due to GNDs and SSDs.     

铝合金薄板冲压成形过程中的侧壁起皱及有限元模拟

针对铝合金薄板的侧壁起皱问题,本文通过有限元软件分析工艺参数对成形质量的影响,提出了一种基于数值模拟与智能算法相结合的优化方法。首先,利用最优拉丁超立方抽样进行实验设计,并依据数值模拟获取实验值;其次,基于BP神经网络拟合工艺参数与成形质量之间的关系,预测结果的平均相对误差为2.69%,建立了准确的预测模型;最后,用遗传算法极值寻优获取了一组最优的工艺参数组合,起皱幅值的预测值和仿真值相对误差仅为4.03%,实验结果与仿真分析结果相近,验证了该优化方法的合理性和有效性。研究表明:以料厚、摩擦系数和压边力作为优化变量,以最大起皱幅值最小化为优化目标,建立几何模型,并利用有限元软件Autoform进行仿真分析;依据起皱轮廓线径向位移的实验和数值模拟对比,验证了有限元模型的正确性,表明利用神经网络和遗传算法极值寻优可以有效解决铝合金侧壁起皱缺陷。     

Influence of cold-rolling and annealing conditions on formability of aluminium alloy sheet

Using an Al-Mg-Cu alloy developed for auto body panels, strip sheets are experimentally produced by various cold-rolling and annealing procedures. Tensile and metallographic properties of the sheets and their relations are examined to attain high formability. The elongation is closely related to the grain size, and increases with the final annealing temperature. The rolling texture influences the plastic anisotropy, the Lankford value of the sheets. The comparatively high Lankford values are obtained under the condition that both the intermediate and the final annealing temperatures are higher and the reduction ratio after the intermediate annealing is smaller.     

Effect of salt-water fog on fatigue crack growth behaviour of 7050 aluminium alloy in different orientations

The fatigue crack growth behaviour of 7050 T73651 high strength aluminium alloy that was originally developed for the aircraft industry was investigated in this study. The tests were conducted by using C-T specimens machined in six orientations under the action of constant amplitude sinusoidal load cycles. The tests were first carried out in laboratory air and then repeated in salt-water fog of a 5% NaCl solution to observe the effect of the environment on the fatigue crack growth behaviour. The experimental results showed that the fatigue life, maximum stress intensity range and the fatigue crack growth rate of the specimens were seriously affected by the environment. The severity of the effect, on the other hand, was observed to be dependent on the orientation. The strongest orientation was determined to be L-S, while the weakest was S-L.     

Ultramicrohardness experiments on vapour-deposited films of pure metals and alloys

The ultramicrohardness of thin alloy films is determined by indentation experiments at very low loads. The indentation apparatus is briefly described. We measured the hardness of a number of pure metal films, either vapour deposited or magnetron sputtered. In addition, a detailed study was made of the composition dependence of the hardness of the six binary alloy systems of AgAuCu and AuCoNi. These films were prepared by high vacuum vapour deposition. The thickness of all the films was 1 μm.The hardness data were interpreted with the help of the metastable alloy phase diagram, e.g. as obtained by transmission electron microscopy. The results are discussed in terms of the blocking of the motion of dislocations, solid solution hardening, compound formation, grain size reduction due to alloying, and differences in atomic radii.     

Effects of thickness and texture on mechanical properties anisotropy of commercially pure titanium thin sheets

Simultaneous effects of thickness and texture on the anisotropy of mechanical properties and fracture behaviors of commercially pure titanium thin sheets were studied. The activation of different deformation systems, due to the split distribution of basal texture, led to mechanical properties anisotropy. The crack initiation and propagation energies, when the loading direction was parallel to the initial rolling direction, decreased with increasing thickness ranges from 0.25 to 1 mm. The changes of size, shape and distribution of dimples with increasing thickness confirmed the restriction of deformation systems and the development of triaxial stress state and plane-strain condition at the notch tip. However, in transverse-directed specimens, the energy release rate increased with increasing specimen thickness up to 0.75 mm and then decreased. The fractography of these specimens explained the simultaneous effects of thickness and texture on structural stability and high accommodated plastic deformation at the notch tip.