Yield loci of anisotropic sheet metals

Four sheet metals, having quite different combinations of R-values and strain-hardening behavior were tested in uniaxial tension, uniaxial (through thickness) compression, balanced biaxial tension (bulge test), and plane-strain compression. The results were compared with predictions based upon three different anisotropic yield criteria. Although no single yield criterion proved to describe adequately all of the test results, different criteria provided the best agreement with particular metals. Results from through-thickness compression and bulge tests were nearly identical.

In bulge testing, strains were determined from photographs of printed grids while the radius of curvature at the top of the dome was obtained using a projected fringe technique that involved a holographic grating. Radii measured by this technique were smaller for all materials than those measured by the conventional spherometer.     

Limit strain predictions for strain-rate sensitive anisotropic sheets

The combined effects of material strain-rate sensitivity and anisotropy on necking or “limit” strain predictions are examined for thin sheets with transversely isotropic properties. Various rate dependent constitutive laws based on flow theory and deformation theory of plasticity are considered.The strong effect of material strain-rate sensitivity in increasing the amount of straining prior to localized necking is first emphasized. We then discuss the joint influence of rate dependence and anisotropy on the theoretical limit strains and forming limit curves. Both strain-rate sensitivity and the local shape of the anisotropic yield surface are shown to significantly affect the predicted limit strains.A necking-band bifurcation analysis is also carried out to reveal in an explicit manner the remarkable sensitivity of overall forming limit diagram shapes to the parameters in the anisotropic yield function.     

Yield strength variation in ring-rolled aluminium

Annealed aluminium rings were cold rolled to various degrees of wall reduction. Cylindrical specimens, made from the rolled rings in three mutually perpendicular directions, were loaded in direct compression to determine the change in yield strength due to cold rolling. These results are compared with the values of yield strength that would result if the annealed metal were subjected to equivalent reductions in uniaxial compression. The differences are discussed.     

Finite-element simulation of hole-flanging process of circular sheets of anisotropic materials

A hole-flanging operation on a flat circular sheet with a hole in the center is simulated by an incremental elasto-plastic finite-element method, which incorporates strain-hardening and anisotropy in the direction normal to the sheet, with care taken to describe the boundary conditions of penetration, separation and the alternation of the sliding—sticking state of friction. The simulation clearly demonstrates the processes of generation of deformation shape until unloading. The calculated sheet geometries and the relationship of punch load to punch stroke are in good agreement with the experimental data.The stress at the hole periphery in the flange is assumed to a state of circumferential uniaxial tension, in order to simplify the fracture mode as a simple tension test. By making use of the instability of uniaxial tension, an approximate relationship to determine the onset of necking of the hole periphery in the hole-flanging process is derived and it is found to be influenced by the process geometry and the plastic properties of the material, such as the stress-concentration factor K, strain-hardening n and normal anisotropy R, and the estimated value, being obtained by the derived equation, agrees well with the experimental data.It is noted that the derived relationship for estimating the instability of the hole-flanging process can be combined into the developed finite-element model to simulate the critical condition of the limiting deformation of the hole-flanging process. This combined method could possibly be applied towards improving both the manufacturing process and the design of tools for the hole-flanging operation.     

Analytical and numerical investigation of wrinkling for deep-drawn anisotropic metal sheets

An analysis of the onset of wrinkling is first developed for a doubly curved, elastic–plastic shell element submitted to a biaxial plane stress loading. Plastic yielding is described using a criterion recently proposed for anisotropic sheet metals. The wrinkling limit curves obtained with this analysis are compared with previous results based on different yield criteria. Finite element (FE) simulations of a deep-drawing experiment are also performed using the Abaqus/Explicit code with the aim of comparing the FE results relating to the initiation of wrinkling with the predictions of the analytical model and with experiments from the literature.     

Prediction of forming limits for anisotropic sheets containing prolate ellipsoidal voids

In sheet metal forming operations, the formability of sheet metals is limited by the occurrence of internal damage evolution that eventually yields a localized neck. Thus, designing and optimizing a sheet metal forming process, requires the precise prediction of the forming limits of the sheet materials. Accordingly, the current work attempts to theoretically predict the forming limit diagrams (FLDs) of voided anisotropic sheets using a new version of the Marciniak and Kuczynski (M–K) model. The analysis employs Gologanu–Leblond–Devaux's yield function for materials containing axisymmetric prolate ellipsoidal cavities with random orientations in conjunction with Barlat and Lian's 1989 anisotropic yield criterion. The effect of a void shape parameter on a ductile material under biaxial tensile loading is introduced and examined within the framework of the M–K model, along with the effect of including a first-order strain gradient term in the flow stress. To confirm the validity of the proposed M–K model, the predicted FLDs were compared with experimental results for steel sheets. The predicted forming limits for the voided sheets were found to agree well with the experimental data.     

Cluster identification in AA5754 aluminium sheets using mathematical morphology analysis

Quantitative image analysis of particle distribution in the microstructure of continuous cast (CC) and direct chill cast (DC) AA5754 aluminium alloy sheets have been conducted. This information can be used as an input for modelling mechanical deformation and instability in these materials. The quantitative analysis reveals that there are significant differences in the microstructure of the two materials even though the total content of second‐phase particles is statistically similar. Qualitative observation shows the second‐phase particles to be arranged in the form of streaks parallel to the rolling direction in the CC sheets and in a uniform random manner in the DC sheets. The main difference in the geometric microstructure of the CC and DC material is the spatial arrangement of the second‐phase particles. A new mathematical technique called proximity analysis is developed to identify clusters and group of particles belonging to a cluster. Quantification through proximity analysis reveals that the particle clusters in CC sheet are in the form of long clusters (streaks) parallel to the rolling direction and are significantly longer than those in DC sheets (with the largest cluster in CC being four times larger than DC), and also have anisotropic angular orientation parallel to the rolling direction. The lower value of fracture strain observed in the CC sheets compared to DC sheets is attributed to a combination of large sizes of clusters and their preferential alignment along the rolling direction in the CC microstructure.     

Effect of strain path change on limits to ductility of anisotropic metal sheets

Localized necking in thin metal sheets is analyzed by using the M–K-model approach, and the effect of a number of different non-proportional strain paths prior to the occurrence flow localization are considered. The analyses account for plastic anisotropy, using four different anisotropic plasticity models to fit a set of experimental data for cold-rolled steel sheet. The predicted forming limit diagrams show strong dependence on whether or not the load on the sheet is removed between two load steps on a non-proportional strain path. This dependence is investigated in detail for one of the anisotropic plasticity models, and it is shown that elastic straining plays a large role, as the stresses quickly move from one point of the yield surface to another. When the load is removed between steps, the stress point moves in a different manner, which results in quite different flow localization response.     

Joinability of different thermoplastic polymers with aluminium AA6082 sheets by mechanical clinching

The International Journal of Advanced Manufacturing Technology - The joinability of different rigid thermoplastic polymers with aluminium AA6082-T6 alloy sheets by mechanical clinching is...     

Seam gap bridging of laser based processes for the welding of aluminium sheets for industrial applications

Laser welding has a large potential for the production of tailor welded blanks in the automotive industry, due to the low heat input and deep penetration. However, due to the small laser spot and melt pool, laser-based welding processes in general have a low tolerance for seam gaps. In this paper, five laser-based welding techniques are compared for their gap bridging capabilities: single-spot laser welding, twin-spot laser welding, single-spot laser welding with cold wire feeding, twin-spot laser welding with cold wire feeding and laser/GMA hybrid welding. Welding experiments were performed on 1.1- and 2.1-mm-thick AA5182 aluminium sheets. The resulting welds were evaluated using visual inspection, cross sectional analysis with optical microscopy, tensile tests and Erichsen Cupping tests. The results show that the use of a filler wire is indispensable to increase the gap tolerance. A proper alignment of this wire with the laser spot(s) is crucial. With the single spot laser welding with cold wire feeding, a gap up to 0.6 mm could be bridged as opposed to a maximum allowable gap width of 0.2 mm for single-spot laser welding without filler wire. For 2.1-mm-thick sheets, the laser/GMA hybrid welding process can bridge even gaps up to 1.0 mm. Most welds had a high tensile strength. However, during Erichsen Cupping tests, the deformation of the welds is significantly lower as compared to the base material.     

Influence of sliding direction and sliding speed on the micro-hydrodynamic lubrication component of aluminium mill-finish sheets

Deep drawing involves a micro-hydrodynamic lubrication component of the lubricant which is trapped on very rough plateaus. The use of mill-finish aluminium sheets has shown that sliding speed controls the real/apparent die contact area ratio which conditions friction levels when ridges are perpendicular to the direction of motion. Three-dimensional relocation profilometry can be used to analyse and quantify the behaviour of the lubricant on the same surface area both before and after friction testing (strip drawing with cylindrical dies).     

Yield locus studies of oriented polycarbonate An anisotropic and pressure-dependent solid

Uniaxial and biaxial stress states were employed so as to investigate the yield behavior of oriented polycarbonate. These experimental results are compared with a theoretical yield locus based upon a yield criterion proposed for solids that are both anisotropic and pressure dependent in regard to macroscopic yield behavior. A good correlation between theory and experiment was found.     

Butt autogenous laser welding of AA 2024 aluminium alloy thin sheets with a Yb:YAG disk laser

Higher productivity, lower distortion and better penetration are the main advantages provided by laser welding in comparison with conventional processes. A Trumpf TruDisk 2002 Yb:YAG disk laser is used in this work to increases productivity and quality. Aluminium alloys lead to many technological issues in laser welding, resulting in shallow penetration and defects. In particular, AA 2024 aluminium alloy in a thin sheet is investigated in this paper, being it is used extensively in the automotive and aerospace industries. Bead-on-plate and butt autogenous laser welding tests with continuous wave emission on 1.25 mm thick AA 2024 aluminium alloy sheets were examined morphologically and micro-structurally. The geometric and mechanical features of the welding bead were evaluated via a three-level experimental plan. In addition to the power and speed which are traditionally referred to, beam defocusing was considered as an additional governing factor in a central composite design scheme because it massively affects keyhole conditions. Softening in the fused zone is discussed via Vickers micro-hardness testing and magnesium loss through energy dispersive spectrometry. After properly performing the modelling and optimisation of the fused zone and the cross-section shape factor as the response variables, the laser welding conditions for thin sheets of AA 2024 aluminium alloy are suggested. X-ray and tensile tests were conducted on the specimens obtained with the recommended processing parameters to characterise the AA 2024 disk laser welded beads.     

Experimental methodology to study tribological aspects of deep drawing — application to aluminium alloy sheets and tool coatings

To optimise the contact between aluminium alloy sheets and coated HSS steel tools during deep drawing operation, we present an experimental methodology to decompose an extended slippage by using a strip drawing test with cylindrical tools in multipassing mode. We show in this paper the role of transfer films formed on the tool surface on the evolution of both friction coefficient and surface topography of the metal sheet.     

Drawability of stainless steel sheets

Stainless steels can be classified into two categories, i.e. austenitic and ferritic. These materials are different in crystalline structure: where austenitic stainless steel is face-centered cubic, ferritic stainless steel is body-centered cubic.In general, the quality of stainless steel tends to be assessed by its magnetic properties: it is believed that austentic stainless steel is better than ferritic stainless steel, because it is non-magnetic, but, in fact, the drawability of — magnetic — ferritic stainless steel is somewhat better than that of austenitic stainless steel. Press forming, however, consists of both drawing and stretch forming, and thus in the press forming of stainless steel sheets, it is necessary to have a good knowledge of the metallographic factors governing stretch forming and deep drawing.In this paper, the differences between austenitic and ferritic stainless steel of the metallographic factors which control press forming, are discussed.     

Static crushing of square aluminium extrusions with aluminium foam filler

An experimental investigation was carried out to study the behaviour of square aluminium extrusions filled with aluminium foam under quasi-static loading conditions. Based on the experimental work, simple relations between dimensionless numbers governing the influence of the foam on the characteristics of the crush problem were identified. Furthermore, a simplified set of equations applicable for design of foam-filled components was proposed.     

The folding of heavy fabric sheets

An example of a fabric folding under its own weight is analysed, using bending curve theory to model the fabric cross-section. Reduction of the governing differential equations, and the boundary conditions appropriate to each stage of the motion, to dimensionless form reveals that the parameters which characterize the deformed state depend only on the fabric bending length and feeding height. The form of the function is determined by repeated numerical solution.     

Optimum design for energy absorption of square aluminium columns with aluminium foam filler

The performance in axial compression of square aluminium columns with aluminium foam filler has been assessed based upon existing design formulas for average crush force, maximum force and effective crushing distance. Using an optimisation algorithm, the combination of (1) foam density, (2) column wall thickness, (3) column width, (4) column material strength and (5) total component length giving the component of minimum mass was determined for specific cases. It was found that optimum foam filled columns compared to the traditionally designed non-filled columns showed smaller cross section dimensions in addition to less weight. As a consequence, mass-, length- and volume reductions are possible by utilising foam filler.     

铝合金激光焊接工艺特性

介绍铝合金激光焊接时的工艺特性及焊接难点 ,提出改进措施 ,讨论了焊接过程中裂缝、气孔出现的原因