Sheet-bulk Metal Forming Research Articles

A Constitutive Friction Law for Sheet-Bulk Metal Forming

A Constitutive Friction Law for Sheet-Bulk Metal Forming

Influence of PVD-coating technology and pretreatments on residual stresses for sheet-bulk metal forming tools

Residual stresses in the substrate material are significantly influencing the performance of PVD-coated parts and tools which are exposed to high forces. Especially for forming operations, such as sheet-bulk metal forming, during which normal contact pressures of 1.4 GPa can occur, the reduction of friction and, at the same time, the wear protection by means of thin Cr-based coatings are essential. To ensure a long service life of forming tool and tool coating, each step of the substrate pre-treatment, as well as the magnetron-sputtering process, has to be coordinated and compatible. Therefore, polished as well as nitrided samples consisting of high-speed steel (AISI M3:2) are exposed to a sequence of plasma-based pre-treatments prior to depositing a CrAlN coating. Hardness and Young’s modulus of the substrate and the coating are analysed by means of nanoindentation. To determine the adhesion between coating and substrate, scratch tests are conducted and analysed using a scanning electron microscope. For each step, the residual stresses are determined using sin2ψ measurements, which are correlated to the mechanical properties. A plasma-nitriding process before the CrAlN coating induces high compressive residual stresses into the sample subsurface and at the same time increases the hardness of the surface. This results in higher critical loads during the scratch tests and therefore a better adhesion of the coating on the substrate.

A new test for determining the mechanical and fracture behavior of materials in sheet-bulk metal forming

This paper presents a new experimental test for determining the stress–strain curve and the fracture toughness of sheets to be used in sheet-bulk metal forming (SBMF) applications. The test is based on the utilization of double-notched specimens loaded in shear and combines the plane stress loading conditions of sheet metal forming with the three-dimensional plastic flow conditions of bulk metal forming, which are commonly found in SBMF processes. The methodology to obtain the stress–strain curve involves calculation of the shear stresses and strains along the two symmetric plastic shear zones of the test specimens up to point where cracks start to propagate along the ligaments that connect each pair of opposite notches. The determination of fracture toughness involves characterization of the evolution of load with displacement for a number of test cases performed with specimens having different ligaments between the two symmetric opposite notches. The work is performed on aluminium alloy EN AW 5754 H111 sheets with 5 mm thickness and the results obtained by means of the new proposed test are compared against those from conventional mechanical and fracture characterization tests.

Analysis of Effectiveness of Locally Adapted Tribological Conditions for Improving Product Quality in Sheet-Bulk Metal Forming

Due to current ecological and economic developments there is a growing demand for functional components with complex and closely tolerated geometrical features. Conventional sheet and bulk metal forming operations leads to products which are often limited in their geometrical and functional variety. A promising approach is the process-class sheet-bulk metal forming (SBMF). SBMF is characterised by the application of bulk and sheet forming operations on sheet metals [1]. This combination leads to locally and temporally varying load conditions regarding stress as well as strain states. In order to get high quality parts, controlling the material flow is of major importance. Modified Surfaces, so-called tailored surfaces represent an innovative approach to control the material flow. The objective of the current study is the experimental investigation of the effectiveness of locally adapted tribological conditions using workpiece-and tool-sided tailored surfaces within SBMF processes. The study has shown that the local adaption of workpiece and tool surface increased the heights of functional elements. Thus, using locally adapted tribological conditions leads to an improvement of the quality of the produced gearing components. In a further step the influence of surface modifications on the surface properties of the manufactured components are analysed. Additionally, investigations regarding the wear behaviour of tool-sided surface adaptions lead to the assumption, that the effectiveness of tailored surfaces is reduced during the operating time of the tools.

Superimposed Oscillating and Non-Oscillating Ring Compression Tests for Sheet-Bulk Metal Forming Technology

The new manufacturing technology sheet-bulk metal forming (SBMF) combines the sheet metal forming and bulk metal forming techniques. At the Institute of Forming Technology and Machines (IFUM), a new multistage SBMF process is being developed. In order to reduce the friction and improve the dimensional accuracy of the parts, superimposed oscillation is used within the new SBMF process. SBMF processes allow the manufacturing of solid metal components out of flat steel. To analyse the effect of friction on the superimposed oscillating SBMF process more precisely, superimposed oscillating and non-oscillating ring compression tests at room temperature were carried out. Like the semi-finished products for SBMF process the ring specimens were cut out of a sheet plate by water jet cutting. A new tool system with an integrated hydraulic oscillation system was developed for superimposed oscillating compression of the ring specimens. This tool system enables the absorption of the forming force and displacement stroke of the ring specimen during the ring compression test. After the practical experiments, the force profiles of superimposed oscillating and of non-oscillating process were compared. The influence of the frequency on the surface roughness of ring specimens was investigated. Furthermore, the tribological conditions of the superimposed oscillating ring compression test were analyzed.

Determination of a Constitutive Friction Law Using an Elastic‐Plastic Half‐Space Model

AbstractFriction influences metal forming processes both in economic and technical terms. A precise understanding of friction is inevitable as friction restricts the potential of the product design. Friction depends on the occurring contact loads which is especially true for sheet‐bulk metal forming (SBMF) as the incorporated contact loads appear in a very broad range. Numerical simulations, which are verified experimentally, are carried out to analyse contact interactions which typically appear in SBMF. On the one hand the multi‐scale character of rough surfaces requires a very fine resolution of the contact area, on the other hand a large contact area is necessary to be representative. A half‐space model is chosen for the contact analysis, because it only depends on the two‐dimensional surface boundary which consumes less computing capacity than the finite element method (FEM) with its three‐dimensional volume approach for the same surface resolution and area. The outcome is a constitutive friction law (CFL) consisting of two equations. The law is implemented into the framework of FEM to see the impact in a typical SBMF‐process which aims to form metal sheet into cups with integrated gearing teeth. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of Residual Stress States of Structured Surfaces Manufactured by High-Feed and Micromilling

Abstract The performance of technological surfaces can be optimized via tailored characteristics according to their specific field of usage. These high performance surfaces are needed for the new technology of Sheet-Bulk Metal Forming (SBMF), which is a combination of sheet metal and bulk forming operations. Due to the high surface loads of bulk forming operations, tool surfaces need to be capable to withstand high stress states. Additional to a high wear resistance, the friction coefficient of these surfaces is an important criterion for the material flow of the sheet material. Surface characteristics can be adjusted by using technologies such as high-feed and micromilling processes resulting in different friction coefficients optimizing functional performance of the tools. In dependency of these different manufacturing processes, different residual stresses are induced into the subsurface of the forming tool. Reliability of residual stress measurements via X-ray diffractometry for microstructured surfaces produced through high-feed milling and micromilling is investigated.

Numerical and Experimental Investigations of Multistage Sheet-Bulk Metal Forming Process with Compound Press Tools

Sheet-bulk metal forming is a manufacturing technology, which allows to produce a solid metal component out of a flat sheet. This paper focuses on numerical and experimental investigations of a new multistage forming process with compound press tools. The complete process sequence for the production of this solid metal component consists of three forming stages, which include a total of six production techniques. The first forming stage includes deep drawing, simultaneous cutting and following wall upsetting. In the second forming stage, flange forming combined with cup bottom ironing takes place. In the last stage of the process sequence, the component is sized. To investigate and to improve process parameters such as plastic strain distribution, resulting dimensions and process forces, FEA is performed. Based on these results the developed process is designed.

Determination of Flow Curve and Plastic Anisotropy of Medium-thick Metal Plate: Experiments and Inverse Analysis

Sheet bulk metal forming is widely used for medium-thick metal plate due to its convenience in the manufacture of accurately finished 3D functional components. To obtain precise anisotropy and flow curve of metal plate is a prerequisite for correct simulation of sheet bulk metal forming processes. Inverse analysis of compression test was introduced here to evaluate the sensitivity of different flow curve models and geometric influence of compression test specimen. Besides, a methodology was proposed to compute plastic anisotropic coefficients of Hill quadratic yield criterion, which is based on the ratios of flow curves obtained by inverse analysis of compression tests using specimens cut in six directions on the medium-thick metal plate. The obtained flow curves and anisotropic coefficients were compared with those calculated from tensile tests. Flow curves based on inverse analysis of compression tests cover the curves of the tensile tests well, while the anisotropic coefficients are different, especially for the coefficient related to the RT45 direction. To estimate the effectiveness of the proposed method, the calculated material properties and those based on the traditional tensile tests were applied in a rim-hole process simulation. The simulation results based on the material properties from inverse analysis of compression tests accorded with the tested properties better.

Measuring Systems for Sheet-Bulk Metal Forming

In order to fulfil today’s demands on fast, efficient and sustainable production processes the sheet-bulk metal forming is being developed as a new forming technology within the scope of the SFB/Transregio 73. Characteristically for the sheet bulk metal forming is a three dimensional material flow, which allows for extensive freedom in the design process. To ensure maintaining all the advantages, provided by sheet-bulk metal forming, new inspection concepts for the produced parts as well as for the forming tools have to be developed. For a production-related inspection of produced parts a multi-sensor fringe projection system is under development, which will be employed to detect deviations of features’ form and size. With its sensors of varying measuring range and resolution a feature adapted inspection is possible. Additionally an optical fibre sensor is projected to detect small parts of interest in a very high resolution to enhance the possibilities of the multiscale multi-sensor system. A newly developed endoscopic fringe projection system is used to inspect parts that are out of reach for common optical measuring systems such as the forming tool of the process. This allows for a continuous measurement of tool features and thus the detection of slow growing wear. Challenging for measurement tasks in the sheet-bulk metal forming process are not only the complex geometries but also the harsh environmental conditions and especially for the parts’ inspection, the different surface parameters. In this article the surface parameters of the some sheet-bulk metal formed parts and forming tools will be explained, followed by a description of the different measuring systems. Finally an exemplary evaluation of the influence of the surface properties on an optical measuring system will be shown.

Experimental and Simulative Investigations of Tribology in Sheet-Bulk Metal Forming

Friction has a considerable influence in metal forming both in economic and technical terms. This is especially true for sheet-bulk metal forming (SBMF). The contact pressure that occurs here can be low making Coulomb’s friction law advisable, but also very high so that Tresca’s friction law is preferable. By means of an elasto-plastic half-space model rough surfaces have been investigated, which are deformed in such contact states. The elasto-plastic half-space model has been verified and calibrated experimentally. The result is the development of a constitutive friction law, which can reproduce the frictional interactions for both low and high contact pressures. In addition, the law gives conclusion regarding plastic smoothening of rough surfaces. The law is implemented in the framework of the Finite-Element-Method. However, compared to usual friction relations the tribological interplay presented here comes with the disadvantage of rising numerical effort. In order to minimise this drawback, a model adaptive finite-element-simulation is performed additionally. In this approach, contact regions are identified, where a conventional friction law is applicable, where the newly developed constitutive friction law should be used, or where frictional effects are negligible. The corresponding goal-oriented indicators are derived based on the “dual-weighted-residual” (DWR) method taking into account both the model and the discretisation error. This leads to an efficient simulation that applies the necessary friction law in dependence of contact complexity.

Influence of Surface Modifications on Friction, Using High-Feed Milling and Wear Resistant PVD-Coating for Sheet-Metal Forming Tools

Increasing technological requirements, as well as the demand for an efficient production demands high performance materials and enhanced manufacturing processes. The development of a new manufacturing process, sheet-bulk metal forming (SBMF), is one approach to produce lightweight forming parts with an increased number of functional properties while, at the same time, combining the advantages of sheet and bulk metal forming. For SBMF processes, the specific adjustment of the friction between tool and workpiece for a specifically designed material flow, which is called tailored friction, is of great importance. The reduction of friction is essential in order to ensure a homogeneous forming zone. However, a higher friction can be used to control the material flow to increase the local thickness of the work piece for additional functional integration. This paper shows the development of surface structures for SBMF tools by means of high-feed milling. Process parameters like the tilt angle or the feed are varied to influence the surface parameters of the structures, which results in different tribological properties of the forming tool. The structured surfaces are subsequently coated with a wear resistant CrAlN coating, processed by a magnetron-sputtering process (PVD) to enhance the lifetime and performance of the forming tool. Finally, a ring compressing test is used to investigate the tribological behavior of the coated structures.

Sheet-bulk metal forming – forming of functional components from sheet metals

The paper gives an overview on the application of sheet-bulk metal forming operations in both scientific and industrial environment. Beginning with the need for an innovative forming technology, the definition of this new process class is introduced. The rising challenges of the application of bulk metal forming operations on sheet metals are presented and the demand on a holistic investigation of this topic is motivated. With the help of examples from established production processes, the latest state of technology and the lack on fundamental knowledge is shown. Furthermore, perspectives regarding new research topics within sheet-bulk metal forming are presented. These focus on processing strategies to improve the quality of functional components by the application of process- adapted semi-finished products as well as the local adaption of the tribological system.

Plastic flow and its control in sheet–bulk metal forming of thin-walled functional components

This paper deals with a manufacturing process for sheet metal components with integrated functional elements by sheet–bulk metal forming. In a single forming stage a cup-shaped base body with thin-walled features is deep drawn and its geometry is calibrated by upsetting. The fundamental numerical and experimental investigations include the analysis of the die filling behaviour, part geometry and mechanical properties due to strain hardening with respect to the variation of the blank layout, forming force and material strength. Finally, the process limits are identified and approaches for their enhancement by the application of tailored surfaces are presented.

High-feed milling of tailored surfaces for sheet-bulk metal forming tools

The increasingly investigated and applied production process sheet-bulk metal forming (SBMF) has novel requirements for the forming tools, e.g., the need of an adaptive material flow at different areas of the tool for an adequate form filling. One new method to realize different, defined tribological conditions are tailored surfaces (TS). During the design of forming tools, it is imperative to have profound knowledge about the tribology between the tool and the workpiece. This article introduces structuring with high-feed milling tools as one possibility for influencing the material flow during forming processes and presents a ring-compression test for the quantification of the tribological conditions, which is adapted for SBMF. On the basis of various machined structures, surface parameters are analyzed to identify a correlation with the friction coefficient to gain knowledge about the mechanisms of TS and to be able to choose structures according to the needs of SBMF processes.

Flow Behaviour of 2024 Aluminium Alloy Sheet during Hot Tensile and Compressive Processes

Sheet-bulk metal forming is a new class of sheet forming technology with intended three dimensional material flows as bulk forming process. This new technology usually comprises several traditional forming processes, such as drawing, upsetting, ironing, etc. which exhibits different strain states. The flow behaviours of 2024 aluminium alloy sheet in hot tension and compression were paid attention considering the effect of forming parameters including temperature and initial strain rate in this paper. By regression analysis, the values of deformation activation energy of different processes were calculated. To determined reasonable forming parameters for sheet-bulk metal forming, the power dissipation map and instability map were established based on dynamic materials model theory.

Sheet-Bulk Metal Forming of Symmetric and Asymmetric Parts

The unique process of sheet-bulk metal forming (SBMF) represents a combination of sheet and bulk metal forming by inducing a three-dimensional material flow in sheet metals in a single forming stage. Within this paper two different applications of sheet-bulk metal forming are demonstrated. Hereby two different combined drawing and upsetting processes to realize parts with symmetrically and asymmetrically arranged functional elements are analysed. Finally, this contribution introduces a new machine technique which provides an improvement of the working accuracy of a forming machine and thus has a positive influence on the parts quality. The idea is to use electromagnetic ram guidance to counteract the displacement of the ram due to horizontal forming forces while forming of asymmetric parts.

Influence of Superimposing of Oscillation on Sheet-Bulk Metal Forming

Due to novel processes like sheet-bulk metal forming, the requirements for sheet metal forming are increased. Sheet-bulk metal forming is a new interconnected process in which the part itself is manufactured by deep drawing and the gearing will be produced with bulk forming in a combined process at room temperature. This process is characterized by a triaxial state of stress and a triaxial dimensional change with true strains up to  = 1-2 by using sheet blanks. Within the use of superimposing of oscillation on a sheet-bulk metal forming process the required forming force can be reduced and the accuracy of dimension of the part can be improved. Within this paper the influence of the superimposing of oscillation on the sheet bulk metal forming will be shown on combined ironing and external extrusion process. For the superimposing of oscillation different excitation frequencies will be analysed. Furthermore the die clearance will be varied to increase the requirements on the process. Finally the influence of the different excitation frequencies and the different die clearances will be summarized in cause and effect relationship diagram.

Novel Five-Axis Forming Press for the Incremental Sheet-Bulk Metal Forming

The incremental procedure of sheet-bulk metal forming was classified into two different forming sequences, the discrete and the continuous. Based on these two groups, a movement matrix was developed, which captures required kinematic motions to manufacture a variety of functional components. With the objective of producing near-net-shape workpiece geometries within the Collaborative Research Centre TR73 – sheet-bulk metal forming, the required positioning accuracies of conventional metal forming machines exceed the current state of the art. Therefore, a suitable machine concept was developed and realized. This new machine represents a unique prototype for a flexible application of bulk forming operations to 2 – 3 mm sheets with five motion axes. During continuous forming, such as rolling, and also during simultaneous operations, increased lateral forces prevail. The machine was provided with a high stiffness. That enables a positioning accuracy which, also under load and at rest, correlates the high demands of the sheet-bulk metal forming within a range of ±0.01 mm.

Innovative Tools to Improve Incremental Bulk Forming Processes

Sheet-bulk metal forming is an innovative process with a high potential to generate load-adapted parts with high precision. Bulk forming processes of sheet metals especially require high process forces, resulting in an intense contact pressure and, thus, in a very high abrasive and adhesive wear. As a method to reduce or avoid these common wear phenomena, even hardened or coated tool surfaces are not sufficient. The objective of this paper is to show an improvement of the tool resistance during an incremental forming process by an adapted tool design and the application of structured tool surfaces combined with coatings. For the tool surface the structure of the scarabaeus beetle serves as the basis for a bionic structure. This structure was manufactured by micromilling. Despite the high hardness of the tool material and the complex geometry of the forming tools, very precise patterns were machined successfully using ball-end milling cutters. The combination of bionic structures with coating techniques like physical vapor deposition (PVD) on plasma nitrided tool surfaces is very promising. In this work, the influence of process parameters (workpiece material, lubrication, tool design, stepwise infeed) on the tool resistance during the forming operation was analyzed experimentally. The results of the optimized forming tools were compared to conventional, unstructured, uncoated, and only plasma nitrided forming tools. The different tools were applied to 2 mm thick metal sheets made of aluminum (AlMg3) and steel (non-alloy quality steel DC04). As a result, the process forces could be reduced by a modified shape and surface of the tools. Thus, the lifetime of the tools can be enhanced.