D. Ravi Kumar

Formability analysis of extra-deep drawing steel

Abstract In the present work, formability of five sheets of aluminum-killed extra-deep drawing (EDD) low carbon steel sheets (with a wide variation in thickness) has been characterized. Forming limit diagrams (FLDs) have been determined experimentally by conducting punch-stretching experiments using suitably designed and fabricated tools. Formability, observed from FLDs, has been correlated with microstructure, mechanical properties and formability parameters like strain hardening coefficient (n) and normal anisotropy ( r ). The strain distribution in the material during punch-stretching under different states of strain has been studied. From the experimental results, it has been found that four of the five sheets have a favorable grain size and mechanical properties to meet the requirements of good formability. The suitability of these sheets for critical deep-drawing applications has been examined in terms of the level and range of critical strains in the FLD. These results correlated well with formability parameters. Earing tendency during drawing is expected to be high for one sheet due to its high planar anisotropy. The level of the FLD increased significantly with the sheet thickness. The effect of n and r on the strain distribution characteristics has been analyzed.

Formability of two aluminium alloys

AbstractThe suitability of two recently developed aluminium alloys (an Al–Mg–Mn alloy and an Al–Li–Cu alloy) for press forming applications has been examined. The characterisation involved the experimental determination of microstructural aspects, tensile properties, and formability parameters such as average plastic strain ratio and planar anisotropy. The forming limit diagram has been experimentally evaluated. A detailed analysis of the strain distribution profiles obtained from punch stretching experiments has been attempted. An attempt has been made to correlate the crystallographic texture with the formability parameters. The fracture surfaces of the punch stretched samples were observed using scanning electron microscopy with a view to obtaining a correlation between fracture behaviour and formability. The alloys, in particular the Al–Mg–Mn alloy, have been found to possess good stretchability but both show very limited drawability. Texture analysis indicated negligible earing during deep drawing. T...

Numerical prediction of limiting draw ratio and thickness variation in hydromechanical deep drawing

Hydromechanical deep drawing is a process for producing cup shaped parts with the assistance of a pressurised fluid. In the present work, a complete experimental set up has been designed and fabricated to conduct hydromechanical deep drawing on a 315 tonne hydraulic press. Limiting draw ratio has been determined by using blanks of various diameters. The process was simulated using explicit finite element code LSDYNA. A comparison was made between simulated and experimental results of conventional and hydromechanical deep drawing using low carbon steel sheets of extra-deep drawing grade of 0.96mm thickness. The simulated results showed reasonable agreement with the experimental data. It was found that by hydromechanical deep drawing higher drawability and more uniform thickness distribution could be obtained when compared to conventional deep drawing.

Cryorolling and warm forming of AA6061 aluminum alloy sheets

ABSTRACT Cryorolling is a severe plastic deformation (SPD) process used to obtain ultrafine-grained aluminum alloy sheets along with higher strength and hardness than in conventional cold rolling, but it results in poor formability. An alternative method to improve both strength and formability of cryorolled sheets by warm forming after cryorolling without any post-heat treatment is proposed in this work. The formability of cryorolled AA6061 Al alloy sheets in the warm working temperature range is characterized in terms of forming limit diagrams (FLDs) and limiting dome height (LDH). Strain distributions and thinning in biaxially stretched samples are studied. Hardness of the formed samples is correlated with ultimate tensile strength to estimate post-forming mechanical properties. The limit strains and LDH have been found to be higher than in the case of the conventional processing route (cold rolled, annealed and formed at room temperature), making this hybrid route capable of producing sheet metal parts of aluminum alloys with high strength and formability. In order to combine the advantages of enhanced formability and better post-forming strength than the conventional cold rolled and annealed sheets, warm forming at 250°C has been found to be suitable for this alloy in the temperature range that has been studied.

A comparison of different neural network training algorithms for hydromechanical deep drawing

In the hydromechanical deep drawing process, a pressure chamber is attached to the drawing die and the cup is drawn into the chamber against the fluid pressure. This process offers several advantages over conventional deep drawing, such as higher drawability, more uniform thickness distribution, better surface finish etc. Optimisation of this process is more difficult because of the large number of variables which interact in a complex way. Artificial Neural Networks (ANN) are being applied to an increasing number of real-world problems of considerable complexity. They offer reliable solutions to a variety of problems (viz. prediction and modelling), where the physical processes are not understood or are highly complex. This paper compares several training algorithms in an attempt to find an ideal artificial neural network-training algorithm to model hydromechanical deep drawing. A comparison was made between ANN trained and experimental results of hydromechanical deep drawing using low carbon extra deep drawing (EDD) grade steel sheets of 0.96mm thickness.

Experimental and numerical investigations on springback in V-bending of tailor-welded blanks of interstitial free steel

Tailor-welded blanks of interstitial free steel are commonly used in complex automotive skin panels. The presence of weld zone, difference in thickness and high anisotropic behaviour affect forming behaviour of tailor-welded blanks significantly. Therefore, incorporation of anisotropy of the sheets and properties of the weld zone in finite element simulations is very important for accurate prediction of springback in bending of tailor-welded blanks. In this study, experimental and finite element simulations of V-bending were carried out on tailor-welded blanks of three thickness combinations, prepared by Nd-YAG laser welding of interstitial free steel sheets of thicknesses 0.8, 1.2 and 1.5 mm. The orientation of the weld line in longitudinally welded blanks was kept at 0°, 45° and 90° with respect to the rolling direction to study the effects of anisotropy on springback in V-bending. The tensile properties of the weld zone in different thickness combinations were determined and incorporated in finite element simulations for prediction of springback. It was observed that springback results were mainly governed by the springback behaviour of the thicker sheet in a particular thickness combination. Weld zone properties affect the springback of tailor-welded blanks more significantly than the anisotropy of the sheets. Accuracy of predicted values of springback in simulations increased when the properties of the weld zone were incorporated in the material model.

Effect of Sheet Thickness and Grain Size on Forming Limit Diagrams of Thin Brass Sheets

Various metals and alloys have huge applications in the form of very thin sheets to manufacture parts such as micro metallic components for electronics and MEMS. In manufacturing of metallic components using very thin sheets, the material behaviour changes significantly with miniaturization due to the size effects—the ‘grain size effect’ and the ‘feature/specimen size effect’. Due to this, formability of very thin sheets depends on sheet thickness to grain size ratio (t/d). But, forming limit diagrams use in FE software meant for prediction of failure are plotted without incorporating the grain size effect and the plane strain condition of Forming Limit Diagram (FLD0) is calculated based on strain hardening exponent (n) and sheet thickness (t). In view of this, the present work is aimed at an investigation of grain size effect (t/d) on FLD0 of very thin brass sheets to achieve an improvement in accuracy of formability prediction in FE Analysis (FEA). In the present work, forming limit diagrams of very thin sheet specimens of CuZn36 brass (50–200 μm) have been experimentally determined.

Effect of temperature and punch speed on forming limit strains of AA5182 alloy in warm forming and improvement in failure prediction in finite element analysis: A case study

Formability of AA5182-O aluminum alloy sheets in the warm working temperature range has been studied. Forming limit strains of sheets of two different thicknesses have been determined experimentally in different modes of deformation (biaxial tension, plane strain and tension–compression) by varying temperature and punch speed. A correlation has been established for plane strain intercept of the forming limit diagram (FLD0) with temperature, punch speed and thickness from the experimental results. This correlation has been used to plot the forming limit diagrams for failure prediction in the finite element analysis of warm deep drawing of cylindrical cups. The effect of strain and strain rate on material flow behavior has been incorporated using a strain rate–sensitive power hardening law in which the strain hardening exponent and strain rate sensitivity index have been experimentally determined. The predictions from simulations have been validated by warm deep drawing experiments. Large improvement in accuracy of failure prediction has been observed using the FLDs plotted based on the developed correlation when compared to the existing method of calculating FLD0 using only strain hardening coefficient and thickness. The results clearly indicate the importance of incorporating temperature and punch speed in failure prediction of Al alloys using FLDs in the warm working temperature range.

Effect of Annealing on Mechanical Properties and Formability of Cold Rolled Thin Sheets of Fe-P P/M Alloys

Phosphorus in steel is known to increase strength and hardness and decrease ductility. Higher phosphorus content (more than 0.05%), however, promotes brittle behavior due to segregation of Fe3P along the grain boundaries which makes further mechanical working of these alloys difficult. In this work, thin sheets of Fe-P alloys (with phosphorus in range of 0.1–0.35%) have been developed through processing by powder metallurgy followed by hot rolling and cold rolling. The effect of phosphorus content and annealing parameters (temperature and time) on microstructure, mechanical properties, formability in biaxial stretching and fracture behavior of the cold rolled and annealed sheets has been studied. A comparison has also been made between the properties of the sheets made through P/M route and the conventional cast route with similar phosphorus content. It has been shown that thin sheets of Fe-P alloys with phosphorous up to 0.35% possessing a good combination of strength and formability can be produced through rolling of billets of these alloys made through powder metallurgy technique without the problem of segregation.

Tooling design and development of set-up for hydro-mechanical deep drawing

Hydro-mechanical deep drawing is a process for producing cup shaped parts with the assistance of a pressurised fluid. The design and development of tooling for hydro-mechanical deep drawing process is discussed in this paper. In the present work, an experimental set-up has been designed and fabricated for drawing cylindrical flat bottom cups. To overcome the limitation of a single action press and to provide variable blank holding pressure during the draw, a hydraulic blank holding system has been designed and developed. Design of the set-up for hydro-mechanical deep drawing requires detailed considerations than that of the forming of components with conventional deep drawing. The different aspects of the development are discussed in this paper.