Deep Drawing Operations Research Articles

Static compensation for elastic tool and press deformations during deep drawing

Although today’s deep drawing tools are thoroughly designed and calculated by means of computer-aided design (CAD), finite element (FE) simulation and computer-aided manufacturing (CAM), the sequence of operations to put a tool into production still encompasses manual and irreproducible labor. In particular, the die spotting is empirical and is almost entirely dependent on the toolmaker’s experience. This fine-tuning of the drawing tool consumes a large amount of time. In minimizing manual die spotting, a large potential to decrease time and costs exists. This article presents error compensation methods to create deep drawing tools, which require less manual die spotting in order to produce sound quality stampings. In FE simulations of deep drawing operations, it is general practice to assume rigid tool and press properties. The fact that die and punch design are based on these simplifications might be one of the main causes that empirical die spotting is still imperative. Therefore, the authors developed a methodology to compensate the tool face for effects of elastic press and tool deformations, which occur under applied process load. The authors demonstrate the static compensation with two examples. The first shows the static compensation for ram tilting caused by an unbalanced load, which can originate from asymmetric part design and/or eccentric mounted tools. The second example describes the compensation for elastic die deformation caused by local and global deflections. In either case, the compensated die face, under applied process load, deformed into the desired die face. This research work shows the potential and limits of a static compensation for effects of elastic tool and press deformations on the final shape of the stamping.

An optimization strategy based on a metamodel applied for the prediction of the initial blank shape in a deep drawing process

The transformation of the sheet into a product without failure and excess of material in a deep drawing operation means that the initial blanks should be correctly designed. Therefore, the initial blank design is a critical step in deep drawing design procedure. Consequently, an easy approach for engineers in predicting the initial blank shape is necessary to reduce wastage in material and to overcome the large time consumed in the classical approaches. Thus, the aim of the present investigation is to propose an automatic procedure for the quick sheet metal forming optimization. In fact, a metamodel will be build based on artificial neural networks which will be coupled then with an optimization procedure in order to predict the initial blank shape in a rectangular cup deep drawing operation. The metamodel is built from the finite element simulations using ABAQUS commercial code. This procedure allows a significant reduce of the CPU time compared to classical optimization one. The results show that the desired shape is in good agreement with the one calculated using the optimized blank shape.

A study on the extent of ironing of EDD steel at elevated temperature

An experimental investigation has been carried out to determine the effect of extent of deformation (ironing) over the cup wall at elevated temperatures. Clearance between punch and die is varied to decrease the amount of deformation over the cup wall. The thickness distribution of the drawn cups at different temperatures and clearance is studied and it is found that even at elevated temperatures there will be a certain limit unto which clearance between punch and die can be decreased. For EDD steel sheet of 2 mm thickness this temperature appears after blue brittle regime in which material exhibits unusual strength. In this region punch load requirements increases which leads to necking at the punch corner and finally fracture in the initial stages of drawing operation. Upon increasing the clearance the quality of cups are similar to that in case of deep drawing operation.

A multi-step analysis for determining admissible blank-holder forces in deep-drawing operations

In present investigation a methodology to determine admissible blank-holder forces in deep-drawing operations was established. According to this methodology, the deep-drawing operation is simulated and the maximum blank-holder forces, for stamping friction stir welded tailored blanks, are established based on the comparison of the numerical principal strains fields, obtained in the numerical simulations, with the limiting strains determined analytically for both base materials. Supporting experiments were performed and its results used to confirm the quality of the numerical predictions.

Analysis of plastic flow localization under strain paths changes and its coupling with finite element simulation in sheet metal forming

Formability of sheet metal is usually assessed by the useful concept of forming limit diagrams (FLD) and forming limit curves (FLC) represent a first safety criterion for deep drawing operations. The level of FLC is strongly strain path dependent as observed by experimental and numerical results and therefore non-proportional strain paths need to be incorporated when analyzing formability of sheet metal components. Simulations using finite element method allow accurate predictions of stress and strain distributions in complex stamped parts. However, the prediction of localized necking is a difficult task and the combination of forming limit diagram analysis with finite element simulations often fail to give the right answer, if complex strain paths are not included in these predictions. In this work a code is presented aimed at formability prediction in sheet metal forming, with a concept and structure which allows the implementation of any hardening law, yield function or constitutive equation without major difficulty. The model incorporates both approaches of the theory of plasticity, namely the phenomenological one and the physical one. Several phenomenological constitutive equations, such as, Swift hardening power law and Voce saturation hardening law, the isotropic von Mises yield criterion, the quadratic Hill yield criterion (Hill’48), the non-quadratic Hill yield criterion (Hill’79) and the Yld’96 Barlat yield criterion as well as a physics-based constitutive model accounting for the texture and strain path induced anisotropy, specifically based on the Van Houtte's anisotropic texture strain-rate plastic potential and Teodosiu and Hu microstructural hardening model, are implemented in the new model. The necking phenomenon is carried out in the framework of heterogeneous materials using the Marciniak–Kuczincki (M–K) analysis coupled with the theory of plasticity. Such code may be used to obtain forming limit curves under linear and complex strain paths as well as being used to be coupled with finite element results, as a post-processing tool, to predict occurrence of necking. Studies are presented to test and validate implemented models including some sensitivity analysis to defined variables. The influence of strain path change is presented through the consideration of several non-proportional loading sequences and theoretical results are compared with experimental ones. Also a selected sheet metal component is considered to test and validate developed code as a post-processing tool for finite element analysis and such results are compared with those obtained experimentally.

Investigation on the effect of curvature on forming limit prediction for aluminium sheet alloys

The investigation of bendability of sheet metal alloys is of great importance for the evaluation of process robustness in production of hemmed sheet metal assemblies. Furthermore, material cracks due to bending can also occur at deep drawing operations. This contribution focuses on the experimental determination of material’s formability at deep drawing, the characterisation of bendability especially for hemming operations and the determination of formability under combined deep drawing and bending loads, respectively.

Quantitative evaluation of oxide impurities in bright annealed stainless steels for deep drawing

Cold rolled SS is suitable for deep drawing operations, however the presence of resilient scales or oxides on the surface can damage the deep drawing punch/die sets and emphasise creep phenomena of the material. These oxides can be formed during the hot rolling process preceding the cold one, or during the bright annealing performed between the steps of thickness reduction of the coil. Then a cleaning of the SS surface before drawing is necessary to have a longer life of the tools. The removal of the surface oxide scale can be performed with electrolytic pickling using neutral electrolytes, but the process parameters can be varied depending on the amount of oxides present on the surface of the starting material. To optimise the process efficiency, then, an univocal scale of the degree of oxidation of the bright annealed SS was determined through microscopical analysis and potentiodynamic tests.

Identification of friction coefficients and hardening parameters using optimization methods coupled with a 3D finite element code

This work deals with the identification of parameters for constitutive models by inverse modeling. This is a topic which has been of increasing interest among researcher within the sheet metal forming community. A method for analytical sensitivity analysis is presented and a non-linear least square technique is used to solve the inverse problem, i.e. minimize the least-square error between a experimental and a model data-set. The constitutive relations are modelled using exponential hardening, isotropic yield conditions and coulomb friction. The experimental data was sampled from a deep drawing operation, and the model data was produced by a corresponding finite element model.

Evolução da textura cristalográfica de chapas de aço inoxidável ferrítico do tipo AISI 430 durante laminação a frio, recozimento e estampagem

A evolução da textura, nos estados "como recebido", laminado a frio, recozido e após a estampagem, e a estampabilidade de aços inoxidáveis ferríticos AISI 430, estabilizados ao nióbio, foram estudadas. Duas corridas de chapas com espessuras de 3,0 e 0,7 mm foram utilizadas. A de maior espessura foi relaminada a frio e recozida. A de menor espessura, de composição química semelhante à primeira, foi laminada a frio, na usina siderúrgica, e, posteriormente, submetida a estampagem. A textura foi avaliada usando DRX em todas as condições. O aço AISI 430, na condição "como recebido", apresentou forte textura {100}<110>, {100}<120> e a fibra g. Após a deformação, a intensidade da fibra g aumentou e apareceu a fibra a. O recozimento causou o desaparecimento da fibra a e o fortalecimento da fibra g, que é uma textura adequada para a estampagem. Embora o aço AISI 430, de espessura 0,7 mm, tivesse apresentado uma forte textura de fibra g, no estado inicial, as propriedades de estampagem não foram boas e o material trincou durante a conformação.

Investigation of fracture behaviour in TRIP steels

In automotive body designing high strength steels are used to achieve weight reduction together with an improvement of crashworthiness. The most important representatives of the AHSS steels are Dual Phase (DP) steels and TRansformation Induced Plasticity (TRIP) steels. The application of high strength steels requires a detailed knowledge about the formability and the forming limits. While the true stress strain curve describes a material’s forming behaviour, the Forming Limit Diagram (FLD) is the common criterion used for a Finite Element based process-layout to analyse failure probability and to prevent the occurrence of factures. But forming of multiphase high-strength steels may lead to the occurrence of cracks in regions of a component, which are save in accordance to the results of Finite Element Analysis (FEA). Mostly areas of the part are involved, in which a combination of tension compression- and biaxial tension-stresses can be observed. These results lead to the assumption that depending on the forming history there might be an unknown forming limit. To generate valid forming limits the identification of critical forming operations becomes necessary. Therefore deep drawing operations of cups with geometrically modified specimens have been carried out. The aim of this modification is to influence the stress and strain condition during forming as well as the corresponding microstructural evolution. Experimental work in combination with numerical investigations shows interdependencies between the forming behaviour of the material, stress condition, strain path and maximum drawing ratio. X-ray, light optical and electron microscopes, thermography, computer tomography and metallography are used to describe the fracture mechanism.

Studying the Effect of Punch Nose Radius on Deep Drawing Operation

Studying the Effect of Punch Nose Radius on Deep Drawing Operation

Adhesive wear in deep drawing of aluminum sheets

This paper focuses on the experimental validation of the common model for adhesive wear initiation by Czichos et al. for conventional aluminum deep drawing operations. A strip drawing test with a 90° bending process is used for the studies. The local contact conditions at the drawing edge are examined by analytical examination and numerical simulation on different degrees of refinement. Contact normal stresses and temperature profiles are investigated and correlated with the experimental observation of adhesive wear.

Comparative study: Mechanical and metallurgical aspects of tailored welded blanks (TWBs)

In order to satisfy the requirements of productivity and quality, in recent years, LASER welding processes of certain grades of steels mainly thin sheets assemblies or tailored welded blanks (TWBs) have given many successful advantages in manufacturing engineering. This welding process of in particular makes it possible very easily to joint the steel parts in thickness and grade identical or different in many industrial applications (car industry, pipe lines, tubes, etc.). Additionally, interstitial free (IF) steels with very low C and N contents have been successfully developed in order to perform specific or complex deep drawing operations in the automotive industry. Major developments of TWB by means of LASER welding in this area have also occurred. Certain grades of IF steel are particularly suitable for deep drawing operations of TWB due to high r values (Lankford ratio) [Grumbach, M., Sanz, G., 1972. Ductile–brittle tests by impact tensile test (ITT). J. Metall. 2, 145–164; Bayraktar, E., Kaplan, D., Buirette, C., Grumbach, M., 2004. Application of impact tensile testing to the welded thin sheets. J. Mater. Process. Technol. 145 (I), 27–39; Bayraktar, E., Kaplan, D., Hugele, D., Jansen, J.P., 2004. Evaluation of pipe line LASER girth weld properties by Charpy and impact tensile tests. J. Mater. Process. Technol. 147 (II), 155–162; Bayraktar, E., Kaplan, D., Grumbach, M., 2004. Application of impact tensile testing to the spot welded sheets. J. Mater. Process. Technol. 153, 80–86]. However, because of the narrowness of the weld bead and important hardness gradient, determination of “toughness” (definition of a criterion of the “ductile/brittle” transition during the impact test) of the welded connection (weld bead and heat affected zone) confirm very delicate in LASER welding. The main objective of this paper was to make a comparative study for microstructural and mechanical-toughness characterisation of the steel assemblies, base metal or welded by LASER. For that a procedure of test specifically adapted was used for welded joints ( t < 4mm). The first part of this study consider different steel grades such as a grade of the FE360BFN steel ( t = 4 mm) and a grade of the interstitial free steel (IF-Ti, t = 0.8 mm) and HR60 ( t = 2.5 mm) welded by LASER that largely used in the automotive industry.

Computer Aided Modeling of Deep-Drawing

Deep-drawing operations are performed widely in industrial applications. It is very important for efficiency to achieve parts with no defects. In this work, a finite element method is developed to simulate deep-drawing operation including wrinkling. A four nodded five degree of freedom shell element is formulated. Isotropic elasto-plastic material model with Von Mises yield criterion is used. By using this shell element, the developed code can predict the bending behavior of workpiece besides membrane behavior. Simulations are carried out with four different element sizes. The thickness strain and nodal displacement values obtained are compared with results of a commercial finite element program and results of previously conducted experiments.

Optimization of the design parameters of modified die in hydro-mechanical deep drawing using LS-DYNA

The use of a modified die enhances the limiting draw ratio compared to that obtainable in a conventional deep drawing operation. Application of these dies, in conventional deep drawing, eliminated the use of blank holder but enhances the tendency of wrinkling in drawn products. In hydro-mechanical deep drawing process, the punch deforms the blank to its final shape by moving against a controlled pressurized fluid. In this paper, a new concept of the application of modified dies in hydro-mechanical deep drawing is presented. The finite element (FE) simulations of a deep-drawing process using modified dies are performed using the 2-D explicit finite element code LS-DYNA, with the aim of optimization of design parameters and the results are compared with the experimental values. The initial design steps in the design of modified die in finite element simulation were taken from the concept of Tractrix die. The use of Tractrix die enhances the draw ratio but simultaneously increases the tendency of wrinkling. In this paper the design parameters of modified Tractrix die for hydro-mechanical deep drawing are optimized for the successful drawing of cups. It is also experimentally verified that by using such modified dies in hydro-mechanical deep drawing, deeper cups are drawn without wrinkling.

Applications of anisotropic yield criteria to porous sheet metal forming simulations

Macroscopic yield criteria for porous ductile sheet metals with anisotropic matrices were proposed recently. The matrix surrounding voids is assumed to follow various planar isotropic yield functions in the current study. Effects of the macroscopic porous yield criterion and the yield function of the corresponding matrix on the sheet metal under a hemispherical punch stretching operation and a cylindrical cup deep drawing operation are then investigated. Mechanisms of the void growth, void nucleation, and void coalescence are simultaneously considered here. The results of the simulation show that the macroscopic yield criterion of the porous sheet metal and the yield function of the corresponding matrix control the strain distribution and the strain localization, but not the punch force, for both operations. Early localization will be induced for the sheet with relatively small initial void volume fraction especially under the cylindrical cup deep drawing operation.

Texture Evolution of Ferritic (AlSl 430) Stainless Steel Strips during Cold Rolling, Annealing and Drawing

The evolution of the crystallographic texture of ferritic stainless steels, starting from the as received (hot rolled) condition from the steel mill, going through cold rolling, annealing and final stamping is analyzed in this paper. Two ferritic stainless steels (Nb stabilized) having a thickness of 3.0 and 0.7mm, have been employed. The thicker one has been cold rolled to 40 and 73% thickness reduction, annealed at 750 and 850°C for 1 hour. The thinner one, with a similar composition, has been 77% cold rolled and annealed at 870°C at the steel plant and subsequently submitted to deep drawing in order to evaluate texture and drawability. Texture has been evaluated using DRX in the as received, cold rolled, annealed and after drawing conditions. Drawability has been evaluated using tensile testing in order to obtain the FLC curves. AISI 430 stainless steel, in the as received condition presented a strong {100} texture in the &lt;110&gt; and &lt;120&gt; directions and the gamma fiber. After cold rolling, the material presented stronger gamma and weaker alpha fibers. Annealing of the cold rolled samples conduced to the vanishing of the alpha and strengthening of the gamma fiber, adequate for deep drawing operations. In spite of the AISI 430 of 0.7mm having presented a strong gamma fiber, other deep drawing properties were not adequate and the material cracked during stamping.

Tribological properties of PVD tool coatings in forming operations of steel sheet

In this work, the tribological properties of tool coatings applied by physical vapour deposition are studied. A first aspect is the resistance to galling and abrasive wear, which are investigated by means of a flat/cylindrical multifrottement test. The observations for the evolution of the friction coefficient discriminate the various coatings as regards the onset of galling. In addition, topographic inspection of the tools and the steel strips yields crucial information about galling scratches and wear tracks supporting a possible quantitative ranking of the different tribological systems. Subsequently, the effect of tool coatings in laboratory deep drawing operations is considered. The modification of the tribological conditions by applying a coating to the tool has a striking influence on the feasibility window to make cups. These observations are described in detail, while also the correlation with the roughness of the coated tools is briefly discussed.

Springback effect evaluation in three-dimensional stamping processes at the varying of blankholder force

The quantitative evaluation of elastic springback in sheet metal stamping processes is still today a very interesting research topic, since this phenomenon causes a manufacturing shape defect. A measurement technique based on the shadow moiré method, already developed by the present authors, has been further upgraded and applied to evaluate the springback phenomenon in the deep drawing of square boxes, the basic three-dimensional stamping process. In particular this technique is able to determine the final lateral surface of the drawn parts, once extracted from the dies, and to compare such surface with the ideal one derived from the die's computer-aided design (CAD) geometry. Proper quantitative coefficients are introduced to describe the investigated shape defect. The proposed technique is applied to a deep drawing operation of aluminium and steel square boxes and the influence of the blankholder force level on the elastic springback is taken into account.

New framework for on-line feedback control of a deep-drawing operation

Various strategies for on-line feedback control for forming operations have been proposed, e.g. expert and database systems, response surface, neural network, etc. The paper presents a new framework for the design of controllers for highly non-linear systems, the framework is based on classical state space control theory combined with a non-linear finite element model and a non-linear optimization scheme for identification of the feed-back gain factors. The process is controlled through distributed blank-holder force and variable total blank-holder force and the objective is to control the flange draw-in.