Strip Drawing Test Research Articles

Influence of Hammer-Peened Surface Textures on Friction Behavior

Although the tribological advantages of textured surfaces in sheet metal forming are well known, the texturing of drawing tools is not yet established due to the intensive processing efforts. With the surface treatment technology of machine hammer peening (MHP), deterministic macro- and micro-textures can be machined on free-form surfaces, while simultaneously the time need for tool production is reduced significantly compared to the conventional process chain. Therefore, this paper investigates the influence of hammer-peened surface textures on the friction behavior. First, the general texturing process is optimized and suitable process parameters are identified. Using a strip drawing test, the friction behavior of different geometrical textures, varying ratio of surface textures (area) to the total surface and different lubrication systems are determined. It is shown that MHP with surface textures can reduce friction in sheet metal forming compared to conventionally polished tools.

Wear Analysis of Tool Surfaces Structured by Machine Hammer Peening for Foil-Free Forming of Stainless Steel

Increasing demands concerning the performance of tribological systems for metal forming due to ecological restrictions or technologically increased process loads require the development of innovative tribological systems, especially in forming of stainless steel. It could be shown in preliminary work that surface structures on deep drawing tools manufactured by the incremental forming process machine hammer peening have the potential to reduce friction in strip drawing test by about 58 % in comparison with a ground reference surface. This is explained by the effect of lubricant pockets and a reduced true contact area in the interacting zone. However, due to the effect of a reduced contact area, the wear resistance of these surface structures is of main concern for the effectiveness of their application in deep drawing. Therefore, in this work strip drawing tests are performed over a minimum of 500 repetitions for the evaluation of friction characteristics. Additionally, the coating of the surface structures is investigated to improve the wear resistance of the structures.

Reproducibility of Wear Tests and the Effect of Load on Tool Life in Sheet Metal Forming

The increasing use of advanced high strength steels challenges the forming industry. Data on tool life are not available, thus causing uncertainties in the choice of suitable tribological systems. This paper investigates the reliability of a strip drawing test and the effect of the load level on tool life. Reproducibility of wear tests is barely discussed in literature. This study shows that a wear analysis on the base of a strip drawing test allows reproducible data. However, even small differences of the initial parameters can affect the result. A further investigation shows a distinct relationship between tool life and load level. Furthermore, the predominant wear mechanism is also influenced by the applied load. The findings on wear behaviour and reproducibility can be used for a new approach to estimate tool life in industrial forming processes on the base of a model test.

Experimental and Numerical Investigations on Frictional Behaviour under Consideration of Varying Tribological Conditions

In present time the FE-simulation of body parts in sheet metal drawing processes has to be continuously improved to increase the prediction accuracy. The objective of this paper is to describe a methodology to improve the simulation by mapping the complex tribological conditions of the process in the simulation. This methodology focuses on the dependency of influencing parameters on the friction coefficient. The influence respectively the dependency of these factors has been investigated, both in an experiment of a plane strip drawing test and in a FE-simulation of this test. With the sensitivity analysis in the numerical simulation it is possible to compare the implemented Coulomb-friction-model against the experimental results. The results of the experimental and numerical investigation show the need of further investigations on a more geometrical complex test to prove if it is possible to transfer the knowledge and dependencies that has been found in the experiment with the strip drawing test to a more geometrical complex test.

Friction analysis of alternative tribosystems for a foil free forming of stainless steel using strip drawing test: analysis of physicochemical interactions between coatings and lubricants

Forming of stainless steel sheets with stringent requirements on surface quality is currently realized using protective foils as a separating agent between the tools and the sheet metal. The protective foils are applied with special machines and need to be removed after the forming process or at the end customer. This approach goes along with economic disadvantages. Alternative tribological systems for foil free forming are insufficiently researched and not yet reliably applicable in a production process. The performed research work is based on experimental analyses investigating the physicochemical properties of selected lubricants with regard to the contact angle, the wetting characteristic, the cohesion strength, and intermolecular forces. Additionally, the surface free energy and the wetting envelope of selected coatings and the sheet metal are investigated. The interactions between the tribological properties of the lubricants and the coatings are evaluated performing a strip drawing test. Finally, the performed work discusses and derives basic mechanisms enabling a foil free forming based on friction coefficients from strip drawing.

Coating of Deep Rolled and Hammer Peened Deep Drawing Tools

Mechanical surface treatments like machine hammer peening and deep rolling can substitute an essential part of the manual polishing time in the conventional process chain of die and mold production. However, the increasing use of high strength steels in the automotive industry and the associated wear of deep drawing tools require further wear-protection methods. In this context it is still unknown if hammer peened and deep rolled surfaces can ensure a sufficient adhesive strength of a coating. Therefore, in the present work different coatings are applied on hammer peened and deep rolled surfaces. Finally, the wear behavior is examined in the strip drawing test. The evaluation of the experimental results proves the potential for an industrial application of the mechanically treated and coated tools.

Thermal Influences on Deep Drawing Process of Ferritic and Metastable Austenitic Stainless Steels

Sheet metal forming processes, in particular deep drawing processes, are highly influenced by occurrence of latent and friction heat. Especially when forming metastable austenitic stainless steels, strain-induced martensite formation is suppressed by higher temperatures and therefore influences the material behavior and so called TRIP-effect. This study gives an overview about thermal influences on the deep drawing forming process of metastable austenitic CrNi-steel 1.4301 in comparison with ferritic stainless steels such as 1.4016. Measurements on serial and evaluation tools were carried out to determine occurring temperatures within forming tools. Attention is paid to effects on tribological aspects such as behavior of lubricants at higher temperatures, influence of temperature development on the martensite formation, mechanical properties, forming limit curves as well as heat flow within the forming tools. Lubricants with different temperature stability were compared to each other with determination of friction coefficient in strip drawing tests. Martensite and temperature development during forming of material was measured in non-isothermal tensile tests approving a high dependency of martensite formation on temperature. Forming limit curves for temperatures determined from RT to 140°C for EN 1.4301 are showing high dependency of necking behavior especially under plain strain conditions. Determination of thermal contact conductance coefficients for process and tool relevant material combinations allows interpreting heat flow mechanisms in forming tools and improving forming process to higher robustness. Results of this paper can be used to individually set boundary conditions for thermo-mechanical coupled forming simulation of austenitic stainless steel and process layout of tool temperature control systems.

Friction analysis of thermally sprayed coatings finished by ball burnishing and grinding

Thermally sprayed coatings offer a promising approach as efficient method to increase the wear-resistance of sheet metal forming tools. However, the roughness of thermally sprayed surfaces is quite high. The use of these coatings for deep drawing tools results in poor sheet surface qualities and low drawing ratios. Because it is suspected that high friction is the reason for the low drawability, hard metal coatings (WC–12Co), deposited by high velocity oxygen fuel flame-spraying, were machined by grinding and ball burnishing to improve their friction behavior and the accuracy of the tool shape. The investigation was conducted by plane strip drawing tests. Strips of high strength steel were mated with these novel and effective coatings at different normal contact pressures and drawing velocities. Uncoated friction elements made of C60 steel were considered as reference during the analysis. The results revealed that coated but unmachined friction elements showed high friction values, which led to scratch marks on the sheet surface after drawing. Applying the finishing processes, the friction coefficient could be reduced significantly. Additionally, deep drawing tests were carried out to determine the drawing ratio for coated, unmachined as well as for processed, coated dies. Thermally sprayed and ball burnished as well as thermally sprayed and ground coatings are feasible for deep drawing. Due to the post treatment, the drawing ratio β = 1.8 was increased to 2.0. This is consistent to the results of the friction tests.

Effects of influencing factors on friction coefficient in microsheet forming

The influence of friction on microforming increased dramatically than macroforming with the miniature of the forming parts. The factors which have great impact on friction coefficient in microsheet forming were studied in this paper by strip drawing test. The specimen material was copper C1100, which was thermally treated at 600°C for 12 h in nitrogen atmosphere. The experiments were carried out at room temperature on a universal testing machine under four kinds of punch roughness, sheet roughness, drawing velocity and lubrication conditions respectively. The results showed that the friction coefficient increased with the increasing punch roughness and sheet roughness, but decreased with the increasing drawing velocity. However, the friction coefficient did not change under different lubrication conditions, which is quite different from macroforming. The reason for this phenomenon was also discussed at the end of this paper.

Experimental analysis and modeling of the anisotropic response of titanium alloy Ti-X for quasi-static loading at room temperature

This paper deals with the characterization and modeling of the deformation behavior of titanium alloy Ti-X© under quasi-static conditions at room temperature. The material investigated is a 1.2 mm thick rolled sheet with a strong basal texture. Monotonic tensile, compressive and cyclic tests in various directions relative to the rolling direction were conducted in order to characterize the material behavior. Textural information was obtained from electron backscatter diffraction (EBSD) data. Strip drawing tests were also carried out. The experimental data has been extended to biaxial stress states using the visco-plastic self-consistent texture model (VPSC). The CPB06ex2 model Plunkett et al. (Int J Plast 24:847–866, 2008) was used to describe the yield surface. This model can describe the anisotropy and the tension-compression asymmetry. Distortional hardening was described taking account of the evolution of the model parameters. An extension of the model using the Armstrong and Frederick approach makes it possible to account for the Bauschinger effect during cyclic loading. There was a high level of correlation between the FE simulation and experimental data.

Evaluation of the potential of tool materials for the cold forming of advanced high strength steels

The increasing relevance of lightweight design intensifies the use of advanced high strength steels (AHSS). However, the use of AHSS causes higher stresses on the forming tools, thus reducing tool life and making it impossible to achieve the aspired process reliability.Since there is no reliable information on the wear behavior of tool materials for the forming of AHSS, this study aims to investigate wear and wear development of different tool materials. For this purpose, a strip drawing test with drawbead geometry is used as a model test. Continuous measurements of forces, temperatures of tool and sheet metal, and the roughness of sheet metal allow accurate and detailed analyses of wear.The investigation shows distinct differences in wear resistance of the tested tool materials and reveals characteristic wear development. Furthermore, the results of this study broaden knowledge on dependencies between tool wear, process temperature, and surface changes.

A study of the impact of anti-adhesive coatings on the sheet-titanium forming processes

The paper describes proposition for minimisation of both friction and formation of titanium protrusions on steel tools during sheet-titanium forming. Such a process poses a significant challenge not only because of the limited ability to plastic deformation of the titanium sheets, but also because of the low tribological properties. Titanium sheets have extremely high propensity to form titanium protrusions on steel tools, which aggravates or even makes obtaining the drawn-parts with a smooth surface impossible. In order to reduce or completely eliminate the formation of titanium protrusions on the tools technological lubricants should be used. Unfortunately, due to high surface pressure the lubricant film is interrupted frequently. Thus, the authors propose that the tool surfaces (dies and blank-holders) are covered with anti-adhesive coatings, which protect the tools against creation of the titanium protrusions in case of lubricant film break.The paper presents some results of tribological tests. In the strip-drawing test, results of different frictional pairs are presented based on anti-adhesive coatings deposited on the tool surfaces. The decrease in titanium protrusion formation on the steel tools thanks to the use of anti-adhesive coatings was confirmed in the roughness measurements of the tool surface after the strip-drawing test.

Application of Genetic Algorithm for Optimization of Neural Networks for Selected Tribological Test

In this work was presented the method of determination of the friction coefficient by using multilayer artificial neural networks on the basis of experimental database obtained from the strip drawing test. Using genetic algorithm the optimization of number of input variables of artificial neural networks has been done. As an input parameters for training artificial neural networks following parameters has been used: surface parameters of the sheet and dies, sheet material parameters and clamping force. Some results have pointed out that genetic algorithm has been successfully appled to optimization of training set.

Tribological behaviors of DLC film deposited on female die used in strip drawing

With the miniaturization of parts, dimensions of parts have obvious effects on friction in microforming processes when liquid lubricants are applied. To eliminate size effects of friction, diamond-like carbon (DLC) film was deposited on the surface of female die and blank holder for its superior tribological characteristics. Strip drawing tests were carried out on conditions of different lubricants, and tribological behaviors of DLC film at female die surface were evaluated using punch load. The results reveal that the specimen dimensions and surface topography of specimen have a little effect on tribological behaviors of DLC film than castor oil. DLC film shows lower coefficient of friction (COF) in large deformation and small dimension. With the increasing of test times, wear of DLC film occurs. Raman spectra indicates that the ratio between integrated intensities of D and G peaks (ID/IG) increases with the increasing of tests times, which means that graphitization of DLC film occurs for the rise of temperature induced by the friction at contact surface. The interesting thing is that H content increases with the times of tests especially for high contact load, which is evaluated by the ratio of the slope m of fitted linear background to the intensity of the G peak (m/IG). The increase of H content means that the number of free hydrogen atoms becomes larger for the breakage of CH bonds induced by the friction work. The graphitization and breakage of CH bonds may have an obvious effect on the tribological behaviors of DLC film in microforming.

Influence of Punch Velocity on Spring Back in Micro Forming

In macro forming it is already known, that the punch velocity has an influence on the deep drawing process. This influence is considerably induced by the velocity dependent friction behavior between sample and tool. A further influence is the strain rate dependent forming behavior of the material. In micro range, the influence of punch velocity on the deep drawing process could, due to the size effects, be different from that in macro range, for example the spring back behavior. In this article the influence of punch velocity on the spring back behavior in micro deep drawing is investigated using strip drawing test with two different widths (1 mm and 2 mm). Experiments with aluminum strips with a thickness of 50 µm were performed with punch velocities ranged from 1 mm/s to 1000 mm/s. The strain behavior, which occurs with different punch velocities are investigated on the basis of microsections. The spring back of all samples was measured by an optical measurement system and compared with each other. From the reported work it can be concluded, that with increasing punch velocity the spring back of the complete system is increasing, while the spring back at cup wall stays constant. As reasons can be cited mass inertia effects due to the high velocities and the velocity dependent friction.

Influence of gliding speed and contact pressure on the wear of forming tools

The usage of high-strength instead of mild sheet metals leads to increased wear of forming tools. Especially deep-drawing tools are of high economic relevance. Due to their good corrosion resistance zinc-coated sheet metals are often used in deep drawing operations. In order to enable the design of a sufficiently wear resistant tool which is not unnecessarily overdesigned a prediction of tool wear is desirable. So far, the usage of Archard's law is state of the art. This law does not imply any influence of the contact pressure or the gliding speed on the wear coefficient. The subject of this paper is the influence of different process parameters on the wear of forming tools when zinc-coated sheet metals are processed. Strip drawing tests are used for experimental investigations. In the strip drawing test, the practical conditions of the deep-drawing process can be reproduced while process parameters such as gliding speed, surface pressure and load history can be varied individually. A gray cast iron tool, which is prevalent in automotive industries for forming processes, is used in the tests. For the ascertainment of the impact of the parameter variations on the amount of wear, single parameters are altered under the condition that the cumulative work of friction is kept constant for all tests. The worn surfaces of the tools are analyzed by a confocal white light microscope in order to quantify the wear progress at fixed intervals during the tests. Additionally, EDX-analyses showed the chemical composition of the surface layers. The derived experimental data are used to proof and improve Archard's law. This paper focuses on the influence of gliding speed and contact pressure on the abrasive and adhesive wear of forming tools. It will be shown that the prediction of tool wear under different geometrical and process conditions is possible when finite element simulations and a modified Archard's law are combined.

Tribological behaviour of DLC coatings for sheet metal forming tools

Abstract Sheet metal forming processes usually involve a sliding contact between the blank and tool surfaces, thus requiring the use of lubricants to prevent severe tribological conditions at the tool–workpiece interface. However, improper lubrication policies may have a negative impact on the environment, due to the use of unhealthy degreasing agents to wash the formed parts. In recent years, the introduction of the diamond like carbon (DLC) coatings has represented an attractive solution for the development of dry or nearly-dry forming processes. To be effective, these coatings must strongly bond to the dies substrate in order to grant resistance to the abrasive and adhesive wear. Although many approaches for evaluating the tribological performances of such coatings can be found in the existing literature, they often suffer from limitations in reproducing the interface conditions typical of the industrial processes. The objective of the present research work is to investigate the tribological behaviour of DLC coatings applied to sheet-forming tools under process conditions compared to the ones experienced by the traditional coatings used in industry. Experimental values of the friction coefficient have been determined by means of strip drawing tests and the coatings wear and adhesion to the substrate have been investigated by means of pin-on-disk and scratch tests.

A FE technique to improve the accuracy of drawbead models and verification with channel drawing experiments of a high-strength steel

A drawbead modeling technique is presented to improve the accuracy of finite element simulations in terms of part draw-in and thickness predictions and validated with channel drawing experiments of a high-strength low-alloy steel. The drawing characteristics of 1.5-mm thickness blanks are obtained with strip drawing tests with a round drawbead, and drawbead model parameters are computed for three bead settings. The consequences of bending deformation cycles are determined experimentally on strip draw-in and thickness values, and model limitations of equivalent drawbead elements are also assessed for test conditions in which the drawbead restraint force is lower than the sectional yield limit. The influence of omitted drawbead geometry and overestimated drawbead-exit thickness are described using an analytical model, and a closed form expression is obtained to correct draw-in model error under sectional deformation conditions. Blank thickness and equivalent strain at the drawbead exit are additional drawbead model parameters of the proposed technique. Then, drawing simulations of a variable section, open-ended channel part are performed. The drawbead design, bead settings and tool-blank interface conditions are identical to those in strip drawing tests. Computed draw-in and thickness distributions were compared with measurements on produced channels using an experimental channel draw die. It is concluded that simulation models, based on drawbead force parameters only, overestimate blank thickness at the die entry and bring about relatively high draw-in values along part border lines. The thickness distribution predicted with proposed technique shows an enhanced correlation with on-part thickness measurements, and bead penetration effects on channel border lines are also simulated acceptably.

Textured surfaces for deep drawing tools by rolling

In this paper, macroscopic textured tool surfaces manufactured by rolling are investigated. Focus is on selective adjustment of friction by local texturing of tool areas to influence the material flow during deep drawing operations. Flat strip drawing tests were performed using friction elements with open textures. The texturing influences the friction conditions and affects the material properties of the stripes. The use of these surfaces results in a significant increase in friction, which allows an additional control of the material flow during sheet drawing operations. The main mechanisms for increased drawing forces are elastic deformation near the area of the texture and local plastic deformation on the sheet surface. Using strips made of mild steel, the texturing leads to an increased roughness of the sheet metal surface and, in the case of high surface pressure, to plastic deformations of the strips. Compared to conventional measures like draw beads, rolled-textured surfaces allow to retard the material flow during sheet drawing operation without excessive strain hardening in the sheet material.

Deformation Behavior of Ultra-Thin Metal Foils in Strip Drawing Friction Test

This study focuses on the surface deformation of thin metal foils caused by friction. To clarify the effect of the relative ratio of surface plastic region to the foil thickness on global deformation behavior, strip drawing tests for ultra thin metal foil with 20μm and 100μm thickness were conducted As a result, different surface deformation and elongation behavior under the same friction condition were observed in different thicknesses. Aided by finite element analysis of the friction test, the contribution of the deformation caused by friction to the foil elongation was investigated and the importance of the friction on material deformation in metal foil forming was demonstrated.