Strip Drawing Test Research Articles

Characterization of the Tribological Behavior of Different Tool Coatings and Dry Lubricant for High‐Strength Aluminum Alloys at Elevated Temperatures

The use of high‐strength aluminum components in automotive manufacturing offers the opportunity to reduce vehicle weight significantly and provide new lightweight potentials. In the past, the so‐called hot forming and quench process (HFQ) successfully demonstrates the potential for the production of complex‐shaped components made out of age‐hardenable high‐strength aluminum alloys. Currently, no method permits wear‐free quench forming without the use of lubricants. To fulfill the increasing ecological and economic requirements, it is necessary to identify wear‐reducing techniques to promote this forming technology in the future. This contribution investigates the interaction of lubricant and tool coatings on the tribological performance during quench forming of the high‐strength aluminum alloy AA7075 at elevated temperatures. For this purpose, the tribological behavior is investigated using both, flat strip drawing tests and deep drawing operations. Subsequently, the component quality is compared and discussed. The results demonstrate that tool coatings are effective for the production of high‐strength components in the HFQ process with minimal or even no lubrication and thus provide ecological as well as economic advantages.

Characterization of the interfacial friction properties in the hot stamping process

ABSTRACT In this study, an experimental machine for conducting flat strip drawing tests at high temperature was developed to investigate the interfacial tribological properties and evaluate the coefficient of friction in hot stamping. The strength and rigidity required for the testing machine were examined by finite element analysis and confirmed by actual drawing tests. The consistent and reproducible data further validated the reliable performance of the developed friction test machine. The high temperature tribological characteristics at the interface between the sheet specimen and clamping dies were also investigated. The thermal contact condition between the die and the blank was found to be significantly affected by the geometry of the clamping die, and an optimal clamping die design was then proposed accordingly for generating a nearly uniform contact condition in the experiment. With the developed testing machine, high temperature friction tests were conducted on three kinds of boron steel to evaluate the coefficients of friction under various temperatures and contact pressures. The experimental results revealed that the coefficient of friction at elevated temperatures was more sensitive to temperature than to contact pressure. A lower coefficient of friction was also noted for coated boron steel than for the bare equivalent.

Application of the Finite Element Method to Simulate the Friction Phenomenon in a Strip Drawing Test

Friction is an undesirable phenomenon in the flange area of the drawpiece in sheet metal forming processes, causing a deterioration in surface finish and a decrease in the formability limits of the sheet metal. The aim of this work is numerical analysis using the finite element method of the strip drawing test with two rounded countersamples. This test simulates friction conditions in the flange area of the drawpiece. The results of the experimental research on the influence of surface roughness on the value of the friction coefficient of S235 steel samples were used to verify the numerical results. The relation between the real contact area and the mean roughness Ra of the countersamples was determined. The real contact area increases with the increase of the mean roughness Ra. In sheet metal forming processes, the coefficient of friction depends on the real contact area, and its value increases with the increase of the real contact area.

Assessment of the Tribological Performance of Bio-Based Lubricants Using Analysis of Variance

The purpose of this article is to determine the coefficient of friction of a DC04 steel sheet using a specially designed flat-die strip drawing test. Four different bio-based lubricants, edible (sunflower and rape-seed) and non-edible (karanja and moringa) were used in the study. The experiments were carried out for different contact pressure values. The as‐received specimens were pre‐strained with strains of 7, 14, and 21%. The values of the coefficient of friction as a ratio of the friction force to the normal force were determined. The influence of the viscosity of the lubricant and the contact pressure on the value of the coefficient of friction has been investigated using ANOVA. A tendency to a decrease in the coefficient of friction with increasing contact pressure was observed. Significance results obtained after the ANOVA analysis confirmed the influence of normal pressure and oil viscosity on the value of the coefficient of friction. At the same time, the hypothesis about the influence of the sheet pre-straining on the value of the coefficient of friction was not confirmed by the significant interactions.

Analysis of Tribological Performance of New Stamping Die Composite Inserts Using Strip Drawing Test

This article assesses the tribological performance of new composite tool sets for stamping dies. Four sets of composite countersamples were tested. These consisted of polyurethane resin with mineral filler (base variant) and modified with aluminium powder (10wt%) and roving fabric (5wt%). Strip samples for the strip drawing tests were cut from AMS5599 (Inconel 625) corrosion-resistant nickel alloy, AMS5510 (321) corrosion and heat-resistant steel and AMS6061-T4 heat treatable aluminium alloy sheet metals. The influence of the type of sample material on the coefficient of friction (COF) was observed. The smallest values of the COF over the entire range of clamping force values used on AMS5599 and AMS5510 sheets were observed during tests with countersamples made of the base variant of composite. When testing the AMS6061-T4 aluminium alloy sheet, the countersamples modified with roving fabric provided the lowest value of COF, which stabilised at a value of about 0.197 as pressure was increased.

Process-Integrated Lubrication in Sheet Metal Forming

The deep-drawability of a sheet metal blank is strongly influenced by the tribological conditions prevailing in a deep-drawing process. Therefore, new methods to influence the tribology represent an important research topic. In this work, the application of a process-integrated lubrication in a deep-drawing process is investigated. Most promising geometries of the lubrication channels and outlet openings are first identified by means of numerical simulation at the example of a demonstrator process. Cylindrical test specimens with the specified channel geometries are additively manufactured and installed in a strip drawing test stand. Additive manufacturing enables the possibility of manufacturing complex channel geometries which cannot be manufactured by conventional methods. A hydraulic metering device for conveying lubricant is connected to the cylindrical test specimens. Thus, hydraulically lubricated strip drawing tests are performed. The tests are evaluated according to the force curves and the fluid mechanical buildup of pressure cushion. The performance of process-integrated lubrication is thus analyzed and evaluated. By means of a coupled forming and SPH simulation, the lubrication channels could be optimally designed. From the practical tests, it could be achieved that the drawing force decreases up to 27% with pressure cushion build up. In this research, a hydraulic lubrication in the area of highest contact normal stresses is the most optimal process parameter regarding friction reduction.

The Use of Non-Edible Green Oils to Lubricate DC04 Steel Sheets in Sheet Metal Forming Process

Lubrication is a basic and relatively effective way to reduce friction in sheet metal forming operations. The drive to eliminate synthetic and mineral oils, which are difficult to recycle, from the manufacturing process has opened up opportunities for the use of vegetable-based bio-lubricants. This article presents a comparison of the lubrication performance of two non-edible oils (karanja and moringa) with the most frequently tested edible oils (sunflower and rape-seed). Deep drawing quality low-carbon steel sheets DC04, commonly used in the automotive industry, were used as the test material. Friction tests were carried out under various lubricants and normal pressures in the range between 3 and 12 MPa using the strip drawing test. Furthermore, a study was also made of the effect of a change in the surface topography and the mechanical properties of the sheet metal due to plastic deformation resulting from friction. It was found that under the most favorable lubrication conditions (sample pre-strain 21%, nominal pressure 6 MPa), karanja oil reduced the coefficient of friction by approximately 33%. Both non-edible lubricants provided the best lubrication when testing samples pre-strained at 7% under the whole range of nominal pressures. It was also revealed that in the case of the smallest pre-straining of the specimens (7%), karanja oil was the most effective within nominal pressures of 3–6 MPa, while at higher pressures (9–12 MPa), the moringa oil lowered the value of the coefficient of friction to a greater extent.

A New Test Method to Simulate Deep Drawing Phenomena on the Lab Scale

Expedient simulations and numerical modelling of forming processes depend on a good knowledge of the friction at the tool–sheet metal contact. The absolute value of the friction force or relative changes of this force due to changing conditions are vital input for a reliable numerical model. For that reason, a good selection of a laboratory experiment that can measure the friction under forming conditions is an essential step. This work is part of a large European Union project named ASPECT, which evaluates the friction force evolution in sheet metal forming as a result of changes in contact conditions. Simplified tribological lab-scale techniques showed an temperature dependence of friction opposite to what is measured in state-of-the-art macroscopic flat strip drawing tests. To understand and overcome this contradiction, a new method was developed to integrate the deformation aspect into a friction test; this approach assures closer resemblance to the actual forming conditions. The aim is to develop a method that measures the effect of temperature and speed on the resistance to sliding and deformation, and to provide an efficient prescreening and ranking of forming oils.

Frictional Characteristics of Deep-Drawing Quality Steel Sheets in the Flat Die Strip Drawing Test.

Friction is one of the most important technological phenomena and has a large influence on the flow characteristics of a deformed material. A strip drawing friction test was used to evaluate the friction characteristics of 0.8 mm thick DC04 steel sheets in a sheet forming operation. Friction tests were carried out using a specially designed friction simulator and uniaxial tensile tests were carried out to determine the mechanical properties of the specimens. In addition, measurements of the sheet surface topography were carried out to identify the tribological properties of the specimens. The friction tests were conducted under different pressure and lubrication conditions. A comparative analysis of the results of the friction tests revealed different changes in the surface topography of the test sheets which can be associated with specific friction mechanisms. It was found that the effectiveness of lubrication depends on the lubricant viscosity and nominal pressure. Increasing the nominal pressure intensifies the phenomenon of asperity flattening and reduces the volume of closed pockets of lubricant.

Investigation of the friction behavior between dry/infiltrated glass fiber fabric and metal sheet during deep drawing of fiber metal laminates

During deep drawing processes of fiber metal laminates, such as the newly developed in-situ hybridization process, fibers and metal sheets come into contact while the dry fabric is infiltrated by a reactive matrix system. The viscosity of the matrix increases as polymerization starts during deep-drawing. In the in-situ hybridization process, a dry fiber metal laminate is deep drawn while a thermoplastic matrix system is injected into the glass fiber fabric layer in a resin transfer molding process. During forming of the fiber metal laminate, friction occurs in tangential direction to the metal sheet. The friction plays the main role in preventing the elongation of the sheets in the deep drawing process. Therefore, the measurement of friction coefficients between fibers and metal sheets is essential. In this paper, the friction between sheet metal and dry or infiltrated glass fiber fabric under high contact pressures of 1.67 MPa, as occurring in deep drawing processes, is characterized. A modified strip drawing test setup is used to analyze the coefficient of friction under a constant high contact pressure. Compression tests were performed to show that Coulomb friction can be assumed. Different types of glass fiber fabrics and liquids with defined viscosities are used. It was found that fluids with higher viscosity decrease the friction coefficients in the interface, which is physically explained. For the in-situ hybridization process, it is deduced that with low viscosities, a better infiltration is achieved, while higher viscosities reduce the friction coefficient for better formability.

Assessment of the Tribological Properties of the Steel/Polymer/Steel Sandwich Material LITECOR

The article presents the results of tribological investigations into the steel/polymer/steel sandwich material LITECOR® developed by ThyssenKrupp Steel Europe for applications in the automotive industry. Friction tests were carried out by means of a strip drawing test with the use of a special tribotester mounted on a uniaxial tensile test machine. The influence of sheet deformation on the value of the coefficient of friction (COF) was considered. For this purpose, the samples were subjected to a pre-deformation of 4%, 8% and 12%. Friction tests were carried out with different force values and under different friction conditions, i.e., in dry friction conditions and lubrication of the sheet surface with L-AN 46 machine oil. The highest values of COF were observed for as-received sheets. In contrast, apart from the friction process under the conditions of the lowest force analysed, the lowest value of the COF was observed for pre-strained sheets with a deformation of 12%. The lubrication efficiency of the pre-strained strip specimens with ε = 4% was between 10.5% and 16.3%, with a trend of increasing lubrication efficiency with increasing force. For pre-strained sheets with deformation ε = 12%, there was a trend of decreasing effectiveness from 14.9% to 9.03% with an increase in force.

On the use of LIPSS in single- and multi-scale laser-structured tool surfaces under lubricated conditions

To use water as lubricant, suitable conditions must be created on the process side to compensate its low viscosity. Here, tool surfaces functionalized by micro dimples and laser induced periodic surface structures (LIPSS) in conjunction with high process velocities could play an essential role. This work shows that laser hardened LIPSS wear out in direct water lubricated sliding contact between strip drawing tools and workpiece. Thus, these structures are applied as hierarchical structures to the bottoms of dimples. Though LIPSS' hydrophilicity is preserved, strip drawing tests reveal that the nanostructures do not influence the coefficient of friction as comparisons with dimpled tools without LIPSS show. Instead, the achieved coefficients of friction are in the range of those from tools used in dry condition. Thus, the expected positive influence of functionalized surfaces on lubricated metallic friction couples due to direct as well as in hierarchical structures in water lubricated sliding conditions could not be confirmed.

Tribological Behavior of High-Strength Aluminum Alloys in Combination with Dry Lubricants at High Forming Temperatures

High-strength aluminum alloys provide great potential for weight reduction in vehicle and aircraft production. Nevertheless, forming of these materials is limited at room temperature. Thermally supported forming operations such as hot forming and quenching (HFQ®) allow forming of components without failure and reduced springback. Currently, the adhesive wear and high friction limit the use of HFQ®-operations for high-strength aluminum alloys. Out of this reason, the present paper describes the tribological performance of new developed dry lubricants at elevated temperatures for the alloy AA7075. For this purpose, strip drawing tests were carried out in a modified open tribological system at temperatures between 20 °C and 400 °C. Additionally, the melting behavior of these lubricants, as well as the variation of the lubricant thickness, were investigated. The results show a reduction of the coefficient of friction at elevated temperatures.

Challenges in Tribometry for Warm and Hot Sheet Metal Forming of High Strength Aluminum with Tool Lubrication

For conventional sheet metal forming at room temperature, numerous tribometers were developed in the 20th century. At the present state of the art, unsolved issues for tribometry remain for temperature-supported forming processes of high strength aluminum (e.g. EN AW-7075), in which the sheet is heated to temperatures between 200 and 480 °C. The tribological design of these processes remains a major challenge, which needs to be addressed by investigations with adapted tribometers. In this study, a recently adapted strip drawing test for aluminum warm and hot forming is presented – including a newly developed strip heating unit, a die lubrication system and a die tempering system for efficient tribological testing. The contribution is completed with both, experimental results and a numerical investigation of temperature gradients in the strip drawing test. Finally, it is discussed whether transient process conditions of non-isothermal forming processes with die lubrication should be considered in tribometers for warm and hot sheet metal forming.

Polynomial Multiple Regression Analysis of the Lubrication Effectiveness of Deep Drawing Quality Steel Sheets by Eco-Friendly Vegetable Oils.

Ensuring adequate lubrication is a key task in the sheet metal forming process. The replacement of commonly used synthetic lubricants in metal forming operations by eco-friendly equivalents is a way to introduce sustainable manufacturing. In this paper, six kinds of vegetable oils (linseed, palm, sunflower, cotton seed, soybean and coconut) were used to study the effect of lubricant type on the value of the coefficient of friction (COF) in sheet metal forming. The strip drawing test was used to simulate the friction conditions. The tests were carried out for various lubrication conditions and pressures. The polynomial quadratic regression model was used to determine the relationship between the input variables (test conditions) and the COF. For the range of the nominal pressures considered (2–12 MPa), the following oils provided the highest lubrication efficiency: palm, sunflower and cotton seed. These oils decreased the value of the COF by about 11–16% depending on the nominal pressure. Linseed oil had the most unfavourable properties, reducing the COF by about 7–12%. For the whole range of pressures considered, the increase in the viscosity of the oil caused a reduction in the value of the COF. The effect of oil density on the COF value was similar. The most unfavourable friction conditions occurred when there was low density and low viscosity of the oil at the same time.

Investigation of different lubricant classes for aluminium warm and hot forming based on a strip drawing test

Over the past 15 years, numerous tribological systems for warm and hot aluminium sheet metal forming have been investigated by various researchers. Two previously disregarded aspects will be examined in this paper with an enhanced strip drawing test: the temperature influence on friction and wear performance in an extensive comparison of different lubricant classes in tool lubrication and the bonding performance of different lubricants to the die surface. In depth analyses of the coefficient of friction (COF) and wear performance (wear-free draw-in) for nine different lubricants and additionally several different formulations / mixtures were carried out for sheet temperatures from 20 °C to 425 °C. Furthermore, approaches for future lubricant developments are given.

Tribological behaviour of Ti-6Al-4V titanium alloy sheets measured by a strip drawing test

This article presents the results of friction testing of Ti-6Al-4V titanium alloy 0.5 mm thickness sheets. The strip drawing test is the basic test for assessing the frictional properties of sheets intended for the sheet forming process. The device was mounted on a typical uniaxial tensile testing machine. The tests were carried out under dry friction and lubrication conditions with the use of a wide spectrum of oil types: gear oil 75W-85, engine oil 10W-40, machine oil L-AN 46 and hydraulic oil L-HL 46. Moreover, the use of biodegradable vegetable lubrication is also examined, i.e., rape-seed and palm oils, with the addition of boric acid with a concentration of 5% w/w. Experimental studies have shown that 10W-40 engine oil was the most effective in reducing the coefficient of friction in the pressure range of 10.7 MPa-16.3 MPa (typical pressure range for the deep drawing process).

Assessment of frictional performance of deep drawing quality steel sheets used in automotive industry

This article presents the results of tribological tests of three grades of deep-drawing quality steel sheets used in the automotive industry. The frictional properties of the sheets were anaysed using a strip drawing test. Friction tests were performed under dry friction and lubrication conditions using machine oil L-AN 46. Additionally, friction tests of pre-strained sheets were also carried out. Cylindrical countersamples with different surface roughness were used. Relationship between the surface roughness, pressure force, lubrication conditions and the value of the friction coefficient were evaluated. It was found that increasing the roughness of the countersamples increased the value of the friction coefficient in both friction conditions analysed. The greater the pressure force, the lower efficiency of lubrication. The effectiveness of reducing the value of the friction coefficient ranged from 20-30% and depended on the pressure force, roughness of the countersamples and the degree of specimen pre-straining.

Tribological behaviour of Ti-6Al-4V titanium alloy sheets measured by a strip drawing test

Tribological behaviour of Ti-6Al-4V titanium alloy sheets measured by a strip drawing test

Analysis of the Friction Mechanisms of DC04 Steel Sheets in the Flat Strip Drawing Test

This article presents the use of a specially designed flat strip drawing tester in order to assess the change in surface topography of DC04 steel sheets. The flat strip drawing test simulates friction conditions in the sheet metal-blankholder interface during deep drawing processes. Experimental tests were carried out at various nominal pressures and in dry friction and lubricated conditions. Two widely available gear and engine oils were used in the study. It was found that, in the range of pressures investigated between 3 and 12 MPa, 80W-90 gear oil provided a greater reduction in the value of the coefficient of friction (COF) than 5W-30 engine oil. Gear oil reduced the COF by an average of about 13.4 [%]. The lubrication efficiency depends on the pressure values; the greater the pressure, the lower the lubrication efficiency. A lowering of the value of the main amplitude surface roughness parameters Sa and Sq was generally observed. SEM analysis showed that even under lubrication conditions there was intense flattening of the roughness asperities of the sheet metal.