High Temperature Tribological Studies on Surface Engineered Tool Steel and High Strength Boron Steel

The popularity of hot sheet metal forming processes in the recent years has necessitated research efforts to improve tool life and control the friction level during hot forming operations. In this work, the tribological properties of tool steel and ultra high strength boron steel (UHSS) pairs at elevated temperatures have been studied by using a special hot sheet metal forming test rig that closely simulates the conditions prevalent in the real process. This test involves linear unidirectional sliding of a preheated UHSS sheet between two tool steel specimens where new workpiece material is continuously in contact with the tool surface. The study is aimed at investigating different surface treatments/coatings applied on either the tool or sheet surface or on both. The results have shown that it is possible to control the coefficients of friction through surface treatments and coatings of the tool and workpiece materials. The application of a coating onto the sheet material has a greater influence on the friction compared to changing the tool steel surface. After running‐in, the investigated tool steel variants show almost similar frictional behaviour when sliding against the same sheet material. Although coating the UHSS sheet reduces friction, it abrades the tool surface and also results in transfer of the sheet coating material to the tool surface.     

Development of a Robot‐Based Sheet Metal Forming Process

This paper describes a new sheet metal forming process for the production of sheet components for prototypes and small lot sizes. The generation of the shape is based on kinematics and is implemented by means of a new forming machine consisting of two industrial robots. Compared to conventional sheet metal forming machines, this newly developed forming process offers a high geometrical form flexibility, and comparatively small deformation forces enable high deformation degrees. The principle of the procedure is based on flexible shaping by means of a freely programmable path‐synchronous movement of the two robots. The final shape is produced by the incremental infeed of the forming tool in depth direction and its movement along the contour in lateral direction at each level of the depth direction. The supporting tool with its simple geometry is used to support the sheet metal and follows the forming tool at the rear side of the sheet metal. The sheet metal components manufactured in first attempts are of simple geometry like frustum and frustum of pyramids as well as spherical cups. Among other things the forming results are improved by an adjustment of the movement strategy, a variation of individual process parameters and geometric modifications of the tools. In addition to a measurement of the form deviations of the sheet with a Coordinate Measurement Machine, screened and deformed sheets are used for deformation analyses. Furthermore, the incremental forming process is analysed with assistance of the finite element method. In total the results show that a robot‐based sheet metal forming with kinematic shape generation is possible and leads to acceptable forming results. In order to be able to use the potential of this process, a goal‐oriented process design is as necessary as specific process knowledge. In order to achieve process stability and safety, the essential process parameters and the process boundaries have to be determined.     

Incremental Sheet Metal Forming: Numerical Simulation and Rapid Prototyping Process to make an Automobile White‐Body

In order to make an automobile body structure, incremental sheet metal forming is introduced as a rapid prototyping process. Numerical modeling of the process is initially used to predict the deformation of the sheet metal to avoid failure during the incremental forming process using ABAQUS/Explicit finite element code and OYANE's ductile fracture criterion via a VUMAT user material. An automobile CAD model is then designed, and segmented into several parts in order to accommodate the working space of the CNC machine and formability of sheet metal. After that, CAM software is used to generate a tool‐path for making wooden‐dies and all small parts. Finally, a welding process is applied to join all parts which were cut by laser cutting after incremental sheet forming process.     

Material Flow Control and Optimization in High‐Pressure Sheet Metal Forming by Active‐Elastic Tools

High‐pressure forming of metal sheets is an innovative forming technology for the production of complex components and offers high potentials to improve the properties and qualities of sheet metal parts. This report describes investigations of a newly developed active‐elastic tool system referred to as ACTEC system. Unlike the use of a comparable semi‐rigid tool system, the ACTEC system shows improvements with respect to the material flow in the flange area and reduced sheet thinning in critical corner regions of the workpiece. In addition, the clamping forces respectively sealing forces necessary to avoid leakage in the tool system during the forming process can be reduced. Moreover, the specific design of the ACTEC‐system as well as current experimental examinations are presented and discussed.     

Computer Aided Product Optimization of High‐Pressure Sheet Metal Formed Tailor Rolled Blanks

In view of today's increasing economic pressure the industry has to rationalize in order to remain internationally competitive. Achieving higher product quality using less material and machine‐man‐hours is one possibility to reach this goal. The high‐pressure sheet metal forming of tailor rolled blanks (TRB) allows to produce optimized components specially developed for their future function and which cannot be made from conventionally rolled sheet metal. This paper aims at showing that the two processes, i. e. flexible rolling and high‐pressure sheet metal forming (HPSMF), can be well represented in finite element simulations. By linking the finite element models with a combinatory optimizing tool, it is possible to simulate and optimize the entire process chain. Two example components were used to illustrate the principle of the optimization.     

Effects of Anisotropy in Drawing and Extrusion Processes of Bulk Metal Forming

The effects of anisotropy of axisymmetric materials (round bars, tubes) on metal forming processes are discussed. These effects are strongest for thin‐walled hollow materials in metal forming processes when the wall thickness is not predetermined by the die (tube drawing without mandrel, free extrusion of hollow components). Similarly to the normal anisotropy of sheet metal, a high radial anisotropy increases the resistance against a variation of wall thickness in tube drawing. There are also effects in forming solid materials such as forward extrusion of bars whereby the buckling of cross sections is influenced through the variation of radial anisotropy with the distance from the axis. The favourable anisotropy properties depend on the actual priorities. If, for example, for a metal forming process the material anisotropy results in high compressive stresses this may be favourable for increasing the ductility of the material whereas the increase of the load acting on the tool reduces tool life.     

Material Flow Control in High Pressure Sheet Metal Forming of Large Area Parts with Complex Geometry Details

Working media based forming processes show advantages compared to the conventional deep drawing in the range of sheet metal parts with complex geometry details. By High Pressure Sheet Metal Forming (HBU), complex parts can be formed with reduced tool costs, fewer process steps, and improved part properties, particularly by the use of high strength steels. In order to use these advantages to full capacity, the material flow into the area of the geometry details needs to be optimised. The key element for the material flow control is a multi‐point blank holder. In combination with flange draw‐in sensors, a closed loop flange draw‐in control can be built up which guarantees a reproducible material flow and, consequently, defined part properties. Furthermore, a favourable pre‐distribution of sheet metal material can be reached which leads to a widening of the process limits. Considering a large area sheet metal part with a complex door handle element as example, strategies for the material flow control will be discussed in this paper. The conclusions are based on FE‐simulations as well as experimental findings.     

Process Modeling for Precision Forming of Discrete Parts – State of the Technology and Critical Issues

This paper presents an overview on the application of FE simulation as a virtual manufacturing tool in designing manufacturing processes for precision parts. The processes discussed include forging, sheet metal forming and hydroforming. Determination of reliable input parameters to simulate a process is a key element in successful application of process simulation for process design in all the mentioned areas. These issues are discussed in detail. Practical examples of application of FE simulation are presented for improvement of the existing metal forming process and/or designing new metal forming process for manufacturing discrete precision parts in forging, sheet metal forming and hydroforming.     

Tailored Profiles Made of Tailor Rolled Strips by Roll Forming – Part 2 of 2

The main scope of the presented work is to demonstrate the potential of load optimized tubes with a varying thickness distribution in circumferential direction produced by roll forming. As initial material a so called Tailor Rolled Strip (TRS) sheet metal coil produced by Strip Profile Rolling (SPR) method was used instead of plain sheet. The TRS sheet metal is manufactured in a continuously working process by rolling one or more groves in transverse direction into the sheet metal coil. In this paper, the secondary forming of the TRS sheet metal to TRS tubes is investigated by means of FE‐simulations and roll forming experiments. To simulate the manufacturing process of the TRS tube by FEM, an integrated consideration of the process is necessary because of the large local strain hardening in the groves of the initial SPR sheet metal. In experimental roll forming operations welded tubes could be manufactured successfully. The geometrical and material properties of these tubes are analyzed. The reprocessing of TRS tubes by hydroforming is investigated by means of tube bursting tests. It has been found that an additional annealing process is necessary to achieve deformations in the grooved area during the hydroforming process.     

Properties of Large‐Scale Structure Workpieces in High‐Pressure Sheet Metal Forming of Tailor Rolled Blanks

In order to manufacture components optimised in regard to lightweight construction, the use of innovative forming processes like high‐pressure sheet metal forming (HBU) in combination with the use of tailor rolled blanks (TRB) as innovative semi‐finished material is a promising solution. Fundamental investigations on the HBU of TRB have been carried out in a joint research project at the Institute of Forming Technology and Lightweight Construction (IUL), University of Dortmund, and the Institute of Metal Forming (IBF), RWTH Aachen. The experiments performed with cylindrical parts have provided basic knowledge on the sheet material flow and resulting part properties. To achieve sufficient process reliability, a non‐adjustable as well as an adjustable seal system have been tested and proved to be suitable solutions, depending on thickness ratio and thickness gradient within the TRB. In order to demonstrate the lightweight potential of this process chain, a forming tool for an automotive body structure has been designed and tested. The experiments have shown that this large‐scale structure can well be manufactured in the HBU process from a TRB.     

Experimental Analysis on the Frictional Behaviour of Drawbeads in Sheet Metal Forming

In sheet metal forming, drawbeads are commonly used to control uneven material flow, which may cause defects such as wrinkles, fractures, surface distortion and springback. Although friction may not directly change the limiting strain of steel sheets, the tribological conditions in the contact zone between the sheet surface and the tool surface play an important role in determining the limits of the forming process. Friction in the drawbead contact zones affects the flow of the material in the tool and is used deliberately to control the stamping process. Therefore in this study, the frictional behaviour of drawbeads is experimentally investigated by the drawbead friction test. To characterize the effect of processing variables on the friction coefficients, tests are performed for various sheets, lubricants and bead materials suffering different surface treatments. The results obtained from the drawbead friction test show that the friction and drawing characteristics of deforming sheets were strongly influenced by the strength of sheet, viscosity of lubricant and hardness of bead surface.     

薄板冲压成形中摩擦测定装置伺服控制系统

主要介绍了液压伺服系统在汽车板冲压成形中表面摩擦测定装置上的应用 ,分析了系统各部工作原理及动态过程函数的组成。本系统对生产实际冲模中实现冲压过程自动控制具有一定的研究意义     

Investigation on a Deep Drawing and In‐Process Electromagnetic Calibration

Extending forming limits is one of the most important aims of research work in production engineering. One possibility to improve material formability is the application of high strain rates, which can be realized e.g. by means of electromagnetic forming (EMF). A further extension of the forming limits can be achieved by a beneficial combination of EMF and quasistatic forming operations, which allow exploiting the complementary advantages of the different technologies involved. This approach will be described on the basis of a deep drawing and inprocess electromagnetic sheet metal forming calibration in this paper. Thereby, the design as well as the subsequent analysis of the components as well as the combined process plays a distinctive role. Furthermore, the stages of development regarding the integrated tool coil will be presented and the resulting examples discussed. Finally, the setup of the integrated process as well as the feasibility will be shown on an exemplary semi‐industrial workpiece.     

Experimental Study on Springback of Sheet Metal Single Point Incremental Forming

Sheet metal single point incremental forming (SPIF) is a new technology for flexible process.The spring- back phenomenon in single point incremental forming has been discussed.Effects of forming angle and shape of the part are analysed using simple experimental method.Tool diameter, sheet thickness, step size, material parameters and the interaction of them are also analysed by using orthogonal test.The results show that the primary factor af- fecting springback is forming angle.In addition, springback is decreased when the specimen has a larger forming angle.The order of the four factors that influence springback is tool diameter, sheet thickness, step size and materi- al parameters.The forming precision will increase if springabck is decreased by optimizing the forming parameters.     

Modelling of Incremental Bulk and Sheet Metal Forming

In incremental forming operations, forming is accomplished by a number of single, local forming steps with simple, generic tools, and in some cases even by laser or plasma beams. This makes incremental forming processes very flexible. However, complex tool kinematics with a large number of degrees of freedom can cause severe problems for process planning and optimization. In addition, new questions regarding the material behaviour under cyclic plastic deformation arise. This article gives an overview of current problems in the finite element modelling of incremental forming processes, focusing on two examples, hammer forging and incremental CNC sheet metal forming. Based on this, insight is given into current research at the Metal Forming Institute (IBF) regarding adapted simulation methods for incremental forming processes.     

On finite element methods in metal working

In metal forming industries, many products are to be formed in large number and with highly accurate dimensions. To save energy and material it is necessary to understand the behaviour of material and to know the intermediate shapes of the formed parts and the mutual effects between tool and formed part during the forming process. The design of the tool and the manufacturing procedures are increasingly carried out by computer aided design and manufacturing (CAD/CAM). These procedures are normally based on numerical methods which take into account all physical conditions of the deformed material during the process. For this purpose, finite element methods (FEM) have been developed in the past in different ways. This paper highlights some of these methods which are particularly effective in simulating the forming processes.     

Process Modeling and Optimization in Sheet Metal Forming – Selected Applications and Challenges

This paper presents an overview on the application of FE simulation as a virtual manufacturing tool in designing process for manufacturing sheet metal parts. Input parameters to simulate a process are key elements in successful simulation for process design. In this paper several methods to determine input parameters for process simulation are discussed. Practical examples of application of FE simulation are presented for improvement of existing and/or designing new forming processes for manufacturing sheet metal parts.     

Innovative Flexible Metal Forming Processes based on Hybrid Thermo‐Mechanical Interaction

A new approach towards functional gradation of structural parts is presented. This approach is based on the utilization of locally varying thermo‐mechanically coupled effects applied to different initial workpiece geometries. The possible degree of freedom for the gradation of material properties and geometrical shape for sheet metal forming applications as well as for parts produced by bulk metal forming is characterized by the results of metallographic investigations, by mechanical testing and by an indication of the remaining residual stress state. On the basis of experimental results and process simulations, it could be revealed that the ability to exactly control the dynamic microstructural evolution by thermal and mechanical process parameters combined with predefined material design parameters constitutes a key towards the adjustment of flexible material property profiles even for parts with complex three‐dimensional geometry. Beyond that, the integrative aspect of thermal and mechanical treatment already implies the high level of obtainable efficiency resulting from shortening of process chains. However, it is not only the ability to integrate shape generation and property gradation, but also the automatically included positive effect of tailoring process behaviour by a gradation of formability finally allowing to improve process efficiency e.g. by a reduction of forming steps or reduction of (local) tool load.     

1060 Al Electric Hot Incremental Sheet Forming Process: Analysis of Dimensional Accuracy and Temperature

The sheet with 1060 Al often is used to fabricate aluminum parts at room temperature in the incremental sheet forming process due to the fact that the metal has a high ductility in the normal temperature. However, the material can cause a large quantity of springback for parts formed to influence dimensional accuracy. In this paper, the use of the electric hot incremental forming process (EHIF) can effectively improve the dimensional accuracy of parts compared to single-stage forming and double-stage forming at room temperature. The effect of main process parameters, such as tool diameter, feed rate, step size, and current, on temperature is studied in detail using the EHIF. Some target values, namely, the maximum temperature, the average temperature, and the maximum temperature difference, are measured with a cone using 1060 Al. Moreover, the response surface methodology and Box–Behnken design have been employed to analyze results in detail and to establish respectively corresponding models to predict target values. Finally, the evaluating model of temperature uniformity is proposed and verified according to the previous models.