Key Engineering Materials Vols. 504-506

Abstract: Rotary peen forming (RPF) has been developed as a new peen forming process in which the shot is held by a flexible connection and moved on a circular trajectory (see Fig. 1 below). The main advantage compared to a traditional shot peen forming (SPF) processes is that RPF does not need refeed of shot particles. Hence, RPF offers a compact machine design and a flexible use. The RPF process causes localized plastic deformation just as in traditional shot peen forming but involves tangential components which can create shear deformation in the plastic layer. These tangential components depend on the connection used for the setup. Compared to traditional shot peen forming, RPF shows different process characteristics in terms of coverage and the shape of indentations created on the surface of the workpiece. In this paper, a new tool concept for rotary peen forming is evaluated using experiments and numerical investigations of the process. The set-up uses a spring-attenuator system, which leads to more deterministic impact positions than previously used wire-based impactor concepts.

Abstract: In the last decade, a completely new process has been developed using the existing knowledge in machining and in flow forming and spinning processes, so called the Incremental Sheet Forming. In the process, a spherical tool, governed by CNC incrementally forms a sheet metal to form complex and asymmetric parts using minimum tooling. Several works have been published to study the process limits, optimization of paths for the forming of deep walls and to extent the formability of some alloys using temperature as a process variable. Few studies have been also published where the incremental forming of polymers has been studied. In this work, the forming of steel-polymer-steel hybrid material or Laminate Vibration Damping Steels (LVDS) is considered. Spinnability tests are used to study the process limits and the effects of having two independent sheets in the thickness of the initial blank. The maximum forming angle, forming vertical force and maximum strains near the fracture are presented for the sandwich material in comparison to steel with three different thicknesses and same composition.

Abstract: Sheet metal hydroforming has gained increasing interest during last years, especially as application in the manufacturing of some components for: automotive, aerospace and electrical appliances for niche productions. Different studies have been also done to determine the optimal forming parameters making an extensive use of FEA. In the hydroforming process a blank sheet metal is formed through the action of a fluid and a punch. It forces the sheet into a die, which contains a compressed fluid. Many studies have been focused on the analysis of process and geometric parameters influence about the hydroforming process of a single product with main dimensions till to 100 mm. In this paper the authors describe the results of an experimental activity developed on two different large sized products obtained through sheet metal hydroforming. Different geometric and process parameters have been taken into account during the testing phase to study, in particular, the punch radius influence on the process feasibility. An ANOVA analysis has been implemented to study the influence of geometrical and process parameters on the maximum hydroforming depth. Through this work it has been possible to verify that in the hydroforming process of large size products geometry and, in particular, punch radius, are some of the main factors that influences the feasibility of the products. Different considerations can be made about the effects of the blankholder force and the fluid pressure on the maximum hydroforming depth. As further developments, the authors would perform a numerical study in order to enlarge the knowledge of the process design space to other possible values of the punch radius.

Abstract: In present study, spinning process of clad sheets composed of Copper and Stainless steel 304L is investigated experimentally. To achieve the Copper-Steel 304L clad sheet the explosive welding method is used. In order to smooth the surface of clad sheets, a cold roll forming process was carried out. The clad sheets were heat treated to improve the metallurgical bounding and formability. The mechanical properties of copper-steel clad sheet were obtained by experimental measurements. Spinning process was performed on Copper-steel 304L clad sheets with internal layer of copper and external layer of steel. Different experimental tests are carried out to investigate the effects of some influential parameters including the tool path and the tool materials on formability of the clad sheets. Moreover, corresponding numerical simulations were made to verify the experimental values. Finally comparison of thickness strain distribution of a perfect product sample shows a reasonable agreement between numerical and experimental data.

Abstract: Magnesium alloys are light weight and highly recyclable. To use magnesium alloys as parts of products such as automobiles, it is very effective make bosses by using plastic workings because this can reduce material loss and welding for producing parts. The bosses can be produced by a press cylindrical tool with rotation to the surface of the magnesium alloy sheet. To reduce the period of development of the tools in forming condition investigations, numerical simulation with FEM is very useful. FEM technologies have been developed for making bosses with plastic working. Because of the symmetry of the tool, the axisymmetrical FEM is applied as the first step. Bosses were found to increase in height as the friction coefficient increased. To investigate the friction between the tool and the alloy sheets, a ring compression test was conducted. The temperature of the alloy sheets increases as the bosses are formed. Therefore, the compression test was conducted at an elevated temperature. The heat transfer rate also affects the results of the forming simulation. To improve the shape of bosses from the simulation results, 3D simulation was performed. The shape of bosses in the 3D simulation is better than that in the 2D simulation by taking into account the traction at the contact area of the cylindrical tool.

Abstract: Aluminium has become the material of choice for lightweight design. Today medium strength 5xxx and 6xxx-series alloys are widely used in automotive sheet components, substituting conventional steel because of their superior strength to density ratios. The use of these alloys results in a reasonable ratio of cost per weight saving and a good compatibility with existing production techniques in terms of forming and joining. High-strength 7xxx-series (AlZnMgCu) alloys offer the potential for further light weighting, but formability at ambient temperature is severely limited without the employment of pre- and post- forming heat treatment processes. A promising approach to improve the formability of the peak-aged 7xxx aluminium alloys is to utilize warm forming at process temperatures below the material’s recrystallization temperature. Extensive research on formability is required to develop useful components of complex shapes out of this material. This study describes the material behaviour of a high-strength EN AW-7075 T6 aluminium alloy (e.g. AMAG TopForm® UHS) in the temperature range of the warm forming. For the isothermal simulation of a cross die shape part the material parameters such as flow curves, Lankford parameters and forming limit curves were obtained by experimental testing in the relevant temperature range. A comparison of the numerical simulation with the experimental results for the critical drawing depths for the heated cross die tool at three different temperatures shows good agreement. The results presented in this study demonstrate the potential of warm forming for the manufacturing of complex components made of peak-aged 7075 aluminium sheet alloy.

Abstract: The selective control of the frictional behavior (tailored friction) in metal forming processes is of high importance with regard to technical and economic aspects. This applies especially for the sheet-bulk-metal forming process. Milling with intentionally invoked regenerative tool vibrations can be applied in order to generate structured surfaces with tailored friction properties on the forming tool. These structures affect the formation of lubrication pockets during the forming process which determine the local frictional properties exceedingly. The full potential of this emerging technology can, however, only be revealed if the heuristic and design-relevant knowledge is acquired and provided to the tool-designer already in the early phases of process development. One thing the tool-designer has to specify is the local frictional behavior on the tool surface. But, however, he does not know which milling parameters lead to the necessary surface structures because in most cases he has no expert knowledge in milling, tribology and forming tools. In this paper data mining is used to determine the frictional behavior based on these parameters. The potential of this method in the described context is revealed by the application on data derived from simulation results, both from milling simulations and contact simulations. The latter are performed by using a Halfspace model for rough surface contact. Both approaches for these simulations, the data mining process and the results are explained to the reader.

Abstract: In sheet bulk metal forming, locally adapted friction properties of the contact tool/workpiece are an appropriate means for the targeted enhancement of the material flow, enabling an improved form filling and lowered forming forces. However, the implementation of desirable friction conditions is not trivial. And further, friction is inseparably linked to wear and damage of the contacting surfaces. This calls for a methodological approach in order to consider tribology as a whole already in the early phases of process layout, so that tribological measures which allow fulfilling the requirements concerning local friction and wear properties of the tool surfaces, can already be selected during the conceptual design of the forming tools. Thin tribological coatings are an effective way of improving the friction and wear properties of functional surfaces. Metal-modified amorphous carbon coatings, which are still rather new to the field of metal forming, allow tackling friction and wear simultaneously. Unlike many other types of amorphous carbon, they have the mechanical toughness to be used in sheet bulk metal forming, and at the same time their friction properties can be varied over wide ranges by proper choice of the deposition parameters. Based on concrete research results, the mechanical, structural and special tribological properties of tungsten-modified hydrogenated amorphous carbon coatings (a-C:H:W) are presented and discussed against the background of the tribological requirements of a typical sheet bulk metal forming process.

Abstract: Sheet-bulk metal forming is a process used to manufacture load-adapted parts with high precision. However, bulk forming of sheet metals requires high forces, and thus tools applied for the operational demand have to withstand very high contact pressures, which lead to high wear and abrasion. The usage of conventional techniques like hardening and coating in order to reinforce the surface resistance are not sufficient enough in this case. In this paper, the tool resistance is improved by applying filigree bionic structures, especially structures adapted from the Scarabaeus beetle to the tool’s surface. The structures are realized by micromilling. Despite the high hardness of the tool material, very precise patterns are machined successfully using commercially available ball-end milling cutters. The nature-adapted surface patterns are combined with techniques like plasma nitriding and PVD coating, leading to a multilayer coating system. The effect of process parameters on the resistance of the tools is analyzed experimentally and compared to a conventional, unstructured, uncoated, only plasma nitrided forming tool. Therefore, the tools are used for an incremental bulk forming process on 2 mm thick metal sheets made of aluminum. The results show that the developed methodology is feasible to reduce the process forces and to improve the durability of the tools.

Abstract: Combining the loading conditions of two different classes of forming operations, in sheet bulk metal forming processes, contact pressures ranging from a few hundred MPa up to loads exceeding 2,500 MPa are experienced. With the additional need for an enhanced control of the material flow, which is best implemented by locally adapted frictional properties of the contact tool/workpiece, sheet bulk metal forming represents a challenge to tribology. As a consequence, the evaluation of the friction and wear properties of different surface modifications and lubricants within a variety of loading conditions is required. The load-scanning test is a universal tribological model test. Its most distinctive feature is the ability to assess the friction and wear behaviour of a tribological pairing within a whole range of contact loads in a single test run. The simple and quick test method also allows the investigation of plastic contacts. Due to these features, the load-scanning test is of particular interest with regard to the quantitative and qualitative evaluation of the application potentials and limits of use of tribological measures intended for sheet bulk metal forming. Like any model test, the load-scanning test has also specific drawbacks. In some test setups, the stress distribution in the contact area may be non-uniform. Further, the maximum realizable contact pressure may be limited by low yield strengths of the tested materials in combination with the insufficient flow restriction of the contact geometry and/or the moderate machine force. By comparison to other test methods and by giving examples of its application in different scenarios, the present paper discusses the potentials and limitations of the load-scanning test against the background of sheet bulk metal forming.