Deep Drawing Operations Research Articles

Investigation of surface finishing of carbon based coated tools for dry deep drawing of aluminium alloys

Global trends like growing environmental awareness and demand for resource efficiency motivate an abandonment of lubricants in metal forming. However, dry forming evokes increased friction and wear. Especially, dry deep drawing of aluminum alloys leads to intensive interaction between tool and workpiece due to its high adhesion tendency. One approach to improve the tribological behavior is the application of carbon based coatings. These coatings are characterized by high wear resistance. In order to investigate the potential of carbon based coatings for dry deep drawing, friction and wear behavior of different coating compositions are evaluated in strip drawing tests. This setup is used to model the tribological conditions in the flange area of deep drawing operations. The tribological behavior of tetrahedral amorphous (ta-C) and hydrogenated amorphous carbon coatings with and without tungsten modification (a-C:H:W, a-C:H) is investigated. The influence of tool topography is analyzed by applying different surface finishing. The results show reduced friction with decreased roughness for coated tools. Besides tool topography the coating type determines the tribological conditions. Smooth tools with ta-C and a-C:H coatings reveal low friction and prevent adhesive wear. In contrast, smooth a-C:H:W coated tools only lead to slight improvement compared to rough, uncoated specimen.

Effect of microstructure and texture on forming behaviour of AA-6061 aluminium alloy sheet

AA-6061 aluminium alloys are used extensively in automobile and aerospace industries owing to their excellent combination of mechanical and physical properties. This alloy exhibits prominent anisotropy in mechanical properties when produced as cold rolled sheets. In manufacturing, sheet metal anisotropy may have severe consequences for downstream processes such as stamping and deep drawing operations. In the present study, correlations among in-plane anisotropy, strain path and formability of AA-6061 sheet metal are investigated. Using limit dome height tests, forming limit diagrams (FLDs) were constructed for three different sheet directions by applying appropriate strain localization and fracture criteria. For each sheet direction and strain path, microstructure and texture evolution were also observed to identify the origin of in-plane anisotropy and formability of the AA-6061 sheet metal alloy. A detailed analysis of microstructure under different strain paths suggests the direction and texture dependent yield locus to be a significant factor for in-plane anisotropy. Intra-granular crack propagation in a particular sheet direction (transverse direction: TD) may lead to reduced formability of AA-6061. Formability, on the other hand, appears to be highly correlated to the relative fraction of specific texture components. Certain critical texture components such as Cube {001} <100> and Brass {011} <211> influence the forming behaviour of AA-6061 aluminium alloy significantly.

Process Effects in Short Cycle Stretch Forming of Tinplate Can Bodies

Short Cycle Stretch Forming (SCS) is an innovative stretch forming technology developed at the Institute for Metal Forming Technology (IFU) at the University of Stuttgart. The SCS technology combines plane pre-stretching of blank and subsequent deep drawing operations within the same stroke of press ram. The sheet metal thickness is reduced while denting resistance and yield stress increases due to hardening effects.Current research work focuses on applying SCS-technology to rotational-symmetrical bodies. A process simulation for SCS-Cupping process was performed for food cans. Based on these results a tool was manufactured and commissioned. The results showed that the thickness of cup bottoms of two-piece drawn and ironed (D&amp;I) steel cans can be reduced. Therefore, it is possible to save material costs in serial production based on a reduced blank diameter.In this paper the different effects leading to the thinning of steel can bottom and failure types, such as, wrinkling and cracking are observed in a number of experimental series. Based on these results, the tool geometry was optimized and an advanced tool was manufactured. The results of this paper show that SCS-Cupping offers promising potential to save material, as well as outlining the main effects for this technology.

Identification of inelastic parameters based on deep drawing forming operations using a global–local hybrid Particle Swarm approach

Application of optimization techniques to the identification of inelastic material parameters has substantially increased in recent years. The complex stress–strain paths and high nonlinearity, typical of this class of problems, require the development of robust and efficient techniques for inverse problems able to account for an irregular topography of the fitness surface. Within this framework, this work investigates the application of the gradient-based Sequential Quadratic Programming method, of the Nelder–Mead downhill simplex algorithm, of Particle Swarm Optimization (PSO), and of a global–local PSO–Nelder–Mead hybrid scheme to the identification of inelastic parameters based on a deep drawing operation. The hybrid technique has shown to be the best strategy by combining the good PSO performance to approach the global minimum basin of attraction with the efficiency demonstrated by the Nelder–Mead algorithm to obtain the minimum itself.

Study the effect of elevated dies temperature on aluminium and steel round deep drawing

Round deep drawing operation can only be realized by expensive multi-step production processes. To reduce the cost of processes while expecting an acceptable result, round deep drawing can be done at elevated temperature. There are 3 common problems which are fracture, wrinkling and earing of deep drawing a round cup. The main objective is to investigate the effect of dies temperature on aluminium and steel round deep drawing; with a sub-objective of eliminate fracture and reducing wrinkling effect. Experimental method is conducted with 3 different techniques on heating the die. The techniques are heating both upper and lower dies, heating only the upper dies, and heating only the lower dies. 4 different temperatures has been chosen throughout the experiment. The experimental result then will be compared with finite element analysis software. There is a positive result from steel material on heating both upper and lower dies, where the simulation result shows comparable as experimental result. Heating both upper and lower dies will be the best among 3 types of heating techniques.

Failure analysis of an automotive shock absorber cup during manufacturing process

A failure investigation was conducted on an automotive shock absorber cup during the manufacturing process. Due to the complexity of the parts, the manufacturing is carried out through multi deep drawing operations. The occurrence of cracks over the hole-flanged edge of the cups was the major problem in the forming process which causes failure. To determine the causes of failure, a detailed method including microscopic and macroscopic inspections were carried out in this study. The mechanical behavior of test specimens was investigated by means of tensile tests and the material was characterized in terms of Vickers micro-hardness. As a result, it was observed that the quality of the cut surface associated with the material work hardening after each sequence of sheet forming is the principal causes of failure. The damage is governed by the generation of micro defects during the punching process. This damage develops further during the hole-flanging step.

Batch Fluctuations in Short Cycle Stretch Forming of Tinplate can Bodies

Short cycle stretch forming (SCS) is an innovative stretch forming technology, developed by the Institute for Metal Forming Technology (IFU) at the University of Stuttgart. The SCS technology combines plane pre-stretching and deep drawing operations within the same stroke of the press ram. The sheet metal thickness is reduced, and the denting resistance as well as the yield stress are increased due to hardening effects.In this study, the SCS technology is applied to rotational-symmetric bodies. A process simulation of an SCS cupping process was carried out for producing tinplate cans. Based on these results, a tool was produced. First results showed that the metal thickness of cups for two-piece drawn and ironed (D&amp;I) steel can bottoms can be reduced. With this technological goal, it is possible to save the material cost in series production.This paper analyses how batch fluctuations affect the thinning of cup bottoms in SCS cupping. Therefore, preliminary experiments have been conducted, using their results to improve the previously used FE process model. With the aid of this model, an FEA-based parametric study on the variation of material properties is conducted. To examine batch fluctuations, tensile tests have been taken into account, and initial material parameters for simulation, such as friction coefficient, initial blank thickness, Lankford parameter and hardening behaviour, have been varied.The results of this paper show that SCS cupping offers a promising potential for material savings and demonstrate the main effects of batch fluctuations.

Curvature Based Forming Limit Prediction of High-Strength Steel Components with Superimposed Stretching and Bending in the Deep Drawing Process

The state of deformation in deep drawing operations is characterized by superimposed stretching and bending (i.e. stretch-bending). Bending effects, especially for Advanced High Strength Steels (AHSS) are known to influence the material formability. Traditional formability measures such as the Forming Limit Curve (FLC) fail to reliably predict stretch-bending formability. Consequently, to ensure an efficient and economical use of AHSS in the industrial application, current research work is focusing on the reliable numerical prediction of stretch-bending formability of AHSS sheets.Within this work, a phenomenological concept to predict the forming limit (e.g. the onset of necking) in deep drawing processes taking bending effects into account is presented. The proposed concept is based on curvature-dependent (i.e. regarding the principle curvatures κ1 and κ2 of the stretch-bend (convex) sheet surface) forming limit surfaces representing the probability of failure and is calibrated with experimental results from stretch-bending tests and conventional forming test such as a Nakazima test. The results of the phenomenological forming limit criterion are promising and show a more accurate prediction of the drawing depth at failure than the conventional FLC approach. The method contributes also to a probabilistic view on the forming limit of deep drawing parts.

Multi-Stage Cold Deep Drawing of Pure Titanium Square Cup

This paper deals with the formability of pure titanium sheet in square cup deep drawing. Pure titanium has very excellent corrosion resistance. In the metal forming process, pure titanium has very good ductility in cold forming. The normal anisotropy of pure titanium is very high. Therefore, the property is suitable to the sheet metal forming, such as deep drawing process. However, the most important problem is that the occurrence of seizure becomes remarkable in severe forming operations. Many investigations on the effect of processing conditions on the seizure of titanium were carried out. In the present study, the formability of pure titanium sheet in square cup deep drawing was investigated. For the prevention, pure titanium sheets were treated by heat oxide coating. The fresh and clean titanium is not in direct contact with the die during the forming due to the existence of the oxide layer. The material was pure titanium sheets of the JIS grade 2. The initial thickness of the blank was 0.5 mm in thickness. In the deep drawing process, the sheets were employed and a flat sheet blank is formed into a square by a punch. Forming of sheet by multi-stage deep drawing was tried. Various cups were drawn by exchanging the punch and die. The die was taper without a blankholder in the subsequent stages. The effects of the intermediate annealing and tool shape on the occurrence of seizure in square cup deep drawing were also examined. The square cups were successfully drawn by heat oxide coating. The coating of titanium sheet has sufficient ability in preventing the seizure in multi-stage deep drawing operation. The results of the present study revealed that the pure titanium square cups were successfully formed by using heat oxide coating treatment.

Texturing of skin-pass rolls by pulsed laser dispersing

Skin-pass rolling is a substantial process step in the production line of sheet material for automotive applications. Textured rolls are used to emboss an immaterial surface structure on sheets for improving their formability in subsequent deep drawing operations. In this paper, state of the art techniques for texturing skin-pass rolls are discussed in order to assess their ability to meet optimal tribological parameters. Two main problems are identified: firstly, structured rolls are affected by ongoing wear limiting the operating time and necessitates perpetual maintenance. Secondly, state of the art techniques are not able to exploit the theoretically known potential of immaterial surface structures for enabling micro-lubrication mechanisms due to geometrical inadequacies.Based on these findings, a new approach for texturing of skin-pass rolls is introduced. The technique uses a localised dispersing of hard ceramic particles by use of pulsed laser radiation. It is shown in this paper that this technique allows the creation of elevated structures with high hardness on the surface of 1.2379 cold-working steel. The wear behaviour of such textured rolls was investigated in a lubricated twin-disc-test. The experiments show that no significant wear of the laser produced structures occur during the test duration. The observed results lead to the assumption that the proposed technique allows the creation of skin-pass roll structures with significantly higher durability.

알루미늄 판재의 다단계 드로잉에 있어서 원통컵의 치수 정밀도 비교

Deep drawing of cylindrical cups is one of the most fundamental and important processes in sheet metal forming. Circular cups are widely used in industrial fields such as automobile and electronic appliances. Some of these cups are formed by a one-stage process, others such as battery cases and beverage cans are made by a multi-stage process. In the current study the multi-stage deep drawing of aluminum sheet metal is examined. The process consists of two deep drawing operations followed by two ironing operations. The press die, which can be used for the four-stage forming process, was manufactured allowing punch and die components to be easily changed for various experiments. The rolling direction of both the sheet and the drawn cups was always positioned toward the horizontal x-direction on the die face to minimize experimental errors during the progressive forming. The dimensional accuracy of the cylindrical cups formed at each stage and the earing defect due to the anisotropy of sheet were investigated. The influence of anisotropy on the thickness distribution was also examined. Both the thickness and the outer diameter of the cups were measured and compared for each set of experimental conditions. It was found that the dimensional accuracy of cups rapidly improves by employing the ironing process and also by increasing the amount of ironing.

Effect of interface contact conditions on adaptive finite element simulation of sheet forming operations

Interfacial contact can have significant influence on adaptive mesh refinement during finite element simulation of sheet forming process. In this work, effect of contact is examined on the adaptive simulation of deep drawing operations having different friction factor and thickness of sheets. The frictional contact at the die-blank interface in the sheet of various thicknesses is modelled in terms of a constant factor relating frictional stress to shear strength of the material. An h-adaptive technique has been employed for the refinement of mesh. A recovery-type error estimator based on the energy norm is used for guiding the h-refinement. Adaptively refined meshes obtained during simulation of the axisymmetric deep drawing operation using frictional and non-friction contact on different sheet thicknesses are presented and discussed.

A manifold learning-based reduced order model for springback shape characterization and optimization in sheet metal forming

The parameters of a stamping process include the geometry of the tools, the shape of the initial sheet blank, the material constitutive law and the process parameters. When designing the overall process, one has to also take into account the springback effect that appears when the tools are removed and additional surfaces are cut-off. The goal then is to obtain a final shape as close as possible to the desired shape, while satisfying the admissibility constraints on the variable parameters as well as the feasibility constraints frequently expressed in the form of forming limit diagrams. In the present paper we represent the post-springback shape by a level set function. Then, rather than rely on arbitrarily selected case-dependent measurement locations as in the NUMISHEET benchmark problems, we build a reduced order “shape space” where this level set evolves, by extending our recent shape manifold approach to the problem of springback assessment for 3D shapes. Next, we propose an optimization algorithm designed to minimize the gap between the post-springback and the desired final shapes. The required level set functions are generated from a corresponding set of springback shapes predicted by Finite Element simulations. Using our approach, we determine the minimal number of parameters needed in order to uniquely characterize the final formed shape regardless of complexity. Finally, we demonstrate the approach using an industrial test-case: springback assessment of the deep drawing operation of an automotive strut tower.

Improved prediction of strain distribution during mechanical and hydro-mechanical deep drawing processes using microstructure-based dynamic strain hardening and anisotropy

Interstitial free (IF) and drawing quality (DQ) steel sheets were subjected to conventional mechanical deep drawing and hydro-mechanical deep drawing operations. Detailed macroscopic strain measurements were made at different cup depths. These were simulated using PAM-STAMP, a commercial finite element–based software. For continuum-based finite element simulations, the required key material properties are strain hardening exponent (n) and anisotropy index ([Formula: see text]). These were kept either constant or dynamically varied during the simulation. The constant properties were taken from conventional tensile tests of the original material, while the dynamic variation in properties was extrapolated from developments in the crystallographic texture and in-grain misorientation. Considering dynamic properties during simulation provided a superior prediction of macroscopic strains. This study clearly demonstrates the need for considering evolution of critical continuum properties, such as work hardening and anisotropy index, through appropriate microstructural inputs for more realistic macroscopic predictions.

Failure analysis of a vibration isolator used in launch vehicle applications

Vibration isolators are used in launch vehicles to protect sensitive electronic equipments from the rigors of flight structural vibrations during the lift-off and ascent phase. Failure of one such vibration isolator made of AISI 304 is reported in the present study after ground level testing has been completed. The vibration isolator had a crack of length approximately 42mm at the fore end corner. Detailed optical and scanning electron microscopy was conducted on the failed vibration isolator to understand the reasons for cracking. Finally, it was concluded that micro cracks formed during the deep drawing operation (due to large strains at the fore end corner) propagated during vibration testing resulting in the failure of the body of the vibration isolator.

Influence of anisotropy and lubrication on wrinkling of circular plates using bifurcation theory

In this research, wrinkling in cylindrical deep drawing of a circular plate in the flange area is studied and the critical blank holder force in the deep-drawing operations has been achieved. Analytical model based on the Hill’s bifurcation method is used with regard to the inner edge of an annular plate that is subjected to tensile stress. The analysis is formulated in the form of large deformation assumption. The friction between the plate and the die, and the strain-hardening parameters are also investigated. The result of this analysis is the critical blank holder force, which is related to the critical wrinkling condition in the flange area. The Abaqus/Explicit is also used for the finite element simulations. Experimental study in order to validate the results of the analytical and finite element simulations has been performed. Analytical method, experimental tests and finite element method have been investigated to obtain critical blank holder force for four materials: ST12, ST14, copper and brass. These results showed that ST12 and copper plates have the highest and lowest compressive instability, respectively. To study the effect of plastic anisotropy of sheet on the wrinkling, some cold work operations and then heat treatment operations was carried out on three different types of plate parts. The results show that by increasing the plastic anisotropy of plate, wrinkling is partially delayed. The effect of lubricant used in the experimental tests, has been also studied. The effect of three various yield functions on the critical blank holder force, has been studied.

Characterization of Hybrid Components Consisting of SEBM Additive Structures and Sheet Metal of Alloy Ti-6Al-4V

Within this paper the characterization of hybrid components consisting of selective electron beam melting (SEBM) additive structures and sheet metal of alloy Ti-6Al-4V will be presented. Key idea of the new production approach is the combination of the advantages of two different manufacturing processes. On the one hand the very high flexibility of the additive manufacturing process and on the other hand the economic production of conventional geometries by deep drawing operations. Main challenge within this new and innovative process is the identification and quality of the properties of the new hybrid components after the manufacturing process. The necessary evaluation consists of three parts: the analysis of the deep drawing blanks, the additive manufactured structure and finally the connection between both. Whereas standardized testing methods are available for the testing of the blanks and the additive structure, there are hardly scientific publication which deals with the investigation of the connection between them. Therefore, a new testing methods and consequently a new tool design was developed in order analyze the specimens in dependency of different strain- and stress conditions. At the end microstructural investigations were performed to identify the fundamental mechanisms which lead to the different properties on macroscopic scale. The result proofed that in particular the electron beam power has a high influence on the production process and thereby the connection quality.

Scale factor and punch shape effects on the expansion capacities of an aluminum alloy during deep-drawing operations

The effects of punch geometry and sample size on forming limit diagrams in expansion are investigated in the case of a 2024 aluminium alloy. Four configurations were selected: flat punch (Marciniak test) or hemispheric punch and decimetric vs. centimetric tooling dimensions. Both decimetric and centimetric deep-drawing devices are associated with an image correlation tool that allows identifying without any contact the deformation on the surface of planar or non-planar specimens. Strains on the surface of the samples are observed by means of a double numerisation in three dimensions of the sample before and after deformation by using stereoscopic vision and triangulation. Finally, deep-drawing limit of the four configurations are compared in expansion state and with literature. Results mainly show that hemispherical punch allows measuring higher strains and is less sensitive to size effect than Marciniak test.

Formability of Pure Titanium Sheet in Square Cup Deep Drawing

Aiming to expand the use of the product, the formability of pure titanium sheet by square cup deep drawing was investigated. Forming of titanium sheet was tried by multistage deep drawing. In the experiment, the material was pure titanium sheets of the JIS grade 2. The initial thickness of the blank was 0.5mm in thickness. In the deep drawing process, the sheets were employed and a flat sheet blank is formed into a square by a punch. Various cups were drawn by exchanging the punch and die. The die was taper without a blankholder in the subsequent stages. For the prevention, pure titanium sheets were treated by heat oxide coating. The fresh and clean titanium is not in direct contact with the die during the forming due to the existence of the oxide layer. The effect of the plastic anisotropic on the occurrence of seizure in square cup deep drawing was also examined. The square cups were successfully drawn by heat oxide coating. The coating of titanium sheet has sufficient ability in preventing the seizure in multistage deep drawing operation. It was found that the square pure titanium cups were successfully formed by using heat oxide coating treatment.

Deep Drawing with Superimposed Low-frequency Vibrations on Servo-screw Presses

The power train of a modern servo-screw press with low rotational moment of inertia provides higher dynamics and a new kind of flexibility in forming and stamping processes compared to conventional servo presses. In this paper a new technology for deep drawing on servo-screw presses called cushion-ram pulsation is described. It uses superimposed low-frequency vibrations between 10Hz and 50Hz at the cushion and the press ram. For deep drawing operations, the high tensile stresses in the frame of cylindrical cup usually lead to a reduction of material thickness. Thus, and due to the lack of work hardening, fractures frequently occur in the punch radius. The process developed here shifts critical loads to higher drawing ratios by decoupling the drawing operation and the prevention of wrinkles. A high frequency of the cushion-ram pulsation is necessary to allow high productivity. Technological results will be increasingly determined by the machine.