Key Engineering Materials Vol. 473

Abstract: This paper deals with Incremental Sheet Forming (ISF) a sheet metal forming process that knew a wide development in the last years. A lot of experimental and simulative researches have been conducted in this field with different aims: to study the sheet formability and part feasibility; to define models able to forecast the final sheet thickness; to understand how the sheet deforms and how formability limits can be defined. Another very important issue is related with the tool path optimization. In fact, the process is characterized by high springback which causes dimensional defects. When IF is performed by a robot, the capabilities of the technology is improved in terms of obtainable shapes (it is possible to use the 6 degrees of freedom of the robot), but the shape errors seem to be higher due to the lower robot stiffness in comparison with CNC machine. In this work the comparison between two different ISF configurations, tool mounted on a CNC machine or tool mounted on a robot, is reported. A suitable geometry was investigated working different sheet material types and sheet thicknesses. The results in terms of geometrical accuracy and sheet deformation have been analyzed in order to define advantages and disadvantages of these two techniques. An analysis on the process forces has been carried out too.

Abstract: Single Point Incremental Forming (SPIF) is a modern and flexible alternative to traditional forming techniques. It thanks its flexibility to the fact that it does not require a dedicated tool set to operate. Numerical simulation of the SPIF process requires an accurate FE model. In the past several attempts have been undertaken to use inverse methods for sheet metal SPIF material model identification based on shearing, tensile and indenting tests. The basic idea of this paper is that the results of inverse methods can be improved by using the SPIF process itself as experimental data source. A SPIF experiment dedicated for material identification on a simple geometry using large step sizes is presented and compared with the FE simulation of the forming process based on an initial guess for the material behavior.

Abstract: A research investigation is presented which discusses the practicality of using several image processing and knowledge based techniques for the measurement and classification of cold rolled steel sections. Image analysis techniques can be applied to many different applications and assessing the quality and the accuracy of cold roll formed steel sections is no exception. The operations detailed within this paper are both traditional image processing methods and novel neural network based techniques which are combined together to give a bespoke alternative to the manual processing currently employed to test these sections. The results show the suitability of using image analysis and image processing to aid in the quality control of cold steel roll forming and initial tests have demonstrated great potential for this work.

Abstract: Challenging automotive design in the interaction of modern lightweight strategies to reduce car body weight as well as legislative regulations, impose higher requirements for future car body development. This trend leads to thinner sheet metal blanks and indicates higher requirements for narrow process windows in the entire manufacturing process to ensure the surface quality of outer shell panels. Especially, thermal loads within the coating process might cause local shape deviation in the startup phase of a new product. Further developments in multi-material-design for car body components induce material configurations with a complex deformation behavior due to different thermal expansion characteristics of the materials involved. For these reasons, there is a need to improve the prediction of the surface quality in the early car development process using numerical simulation methods. The influence of process parameters affecting the surface quality is shown and integrated into the process simulation.

Abstract: Electrohydraulic forming of sheet metals is characterized by the usage of large transient hydraulic pressure generated in underwater current discharges. Pulsed power underwater discharges are often categorized as being non-repeatable in terms of pressure map replication. The work described here presents the improvements made in terms of pressure stabilization based on process parameter optimization pertaining to impact electrohydraulic forming. The work consists of a comparative study showing the differences obtained in terms of pressure fields, when discharges initiated by high-voltage breakdown (wireless discharges) and initiated by copper and aluminium wires at otherwise equal test conditions are compared, for a conical discharge chamber. Characteristic pressure maps belonging to the three analyzed discharge conditions are presented and the criteria for a quantitative comparison are set: maximum pressure value, relative pressure scatter and arithmetic mean deviation for a test pressure field. The maximum pressure value characterises the limitation in the sheet material thickness that could be formed at this pressure. For all 3 types of tests the obtained maximum pressure value is nearly equal with a little bit higher level for discharges initiated by aluminium wire. The relative pressure scatter provides information about uniformity of the pressure distribution along loaded area. While wireless tests showed low uniformity with average relative pressure scatter of 33 %, the application of copper and aluminium wire reduced non-uniformity down to 28 and 24 % respectively. The most important effect of the wire introduction has to do with the great increase of stability (repeatability) of pressure fields observed, characterised by a decrease of the arithmetic mean deviation of pressure along a pressure loaded area.

Abstract: Mechanical joining of complex car body components is an essential part of lightweight construction concepts in the field of car body manufacturing. Besides the mechanical behavior of the joints, the influence on the dimensional accuracy is of particular interest, as joining techniques like clinching or self-piercing riveting cause distortion comparable to spot welding. In recent years, a lot of simplified models using the FE-Method to predict the distortion of assemblies caused by welding (weld seams, spot welds) were presented and commercialized. In contrast to thermal joining technologies, there are no such simplified models with practical relevance existing in the mechanical joining technology sector. In this paper, a new method to predict distortion, caused by different mechanical joining technologies, including effects from previous forming processes, and clamping conditions, is presented. The validation of the simplified model takes place due to an extensive design of expe-riments. It can be proved that the distortion of simple as well as of complex specimens can be relia-bly predicted.

Abstract: Laser hardening, compared to other surface hardening processes, presents several advantages: it can be extremely selective and almost without distortion, it achieves higher levels of hardness and thanks to the rapidity of the heating phase it does not require a quenching medium as the material surrounding the heated layer acts as a heat sink. Nevertheless, in case of workpiece with slim geometry, the thermal stress after the treatment induces a sensible deformation on the treatment direction; this deformation can deeply affect the part ability to fulfil its function and/or its reliability. In this paper a solution to recover the workpiece function ability is presented: a second laser treatment (called counter treatment) was executed on the opposite surface of the treated one to reduce the deformation induced by the first treatment. A numerical model was firstly tested and then used to simulate the counter treatment process and to evaluate the optimum process parameters. Based on the numerical analysis, experimental results show that thanks to this technique it is possible to recover up to 80% of the induced deformation.

Abstract: The motivation of this study is to investigate micro warm coining of metals with high strength (e.g. austenitic steels) in the field of fabricating micro functional surface structures. The current micro cold coining technology, which uses punches made out of steel, is limited to soft metals, such as Al and Cu. Conducting micro cold coining on steels, leads to high tool wear and bad form filling. The approach of this study is integrating electric conductive heating into a micro coining system to realize micro warm coining of stainless steel. This paper presents the experimental and numerical analysis of micro warm coining technology using the material stainless steel 1.4401. A coupled thermal-mechanical model in the commercial Finite-Element-code ABAQUS is used to help analyzing the micro warm coining process. The results are summarized as follows: (1) Open die micro warm coining has been realized. The form filling achieved was 56%, which was three times larger than the form filling of cold micro coining on the same material. (2) Both the experiments and simulations showed that faster processing, by increasing the punch velocity, resulted in a slightly improved form filling (5% from 5 mm/s to 100 mm/s). (3) Surface chilling hindered the filling of thin ribs.

Abstract: The request of small features resulting from high productive and good quality processes has been growing in several industrial fields, such as micromechanical, microelectronic and biomedical sectors. In conjunction with this demand from industries, innovative laser sources such as pulsed fiber lasers, are becoming the answer to the machining of innovative and hard to process materials, such as the nanostructured metallic alloys. The pulsed fiber lasers indeed offer a very precise and small dimension tool able to work with high productivity and small thermal alteration, which is of fundamental importance when nanostractured alloys are machined. In this work an experimental study on percussion microdrilling of 0.7 mm thick nanostructured titanium sheets by means of a nanosecond pulsed fiber laser was performed. In particular, the entrance and exit hole quality as well as the shape of the inner hole were investigated as a function of pulse frequency and process time. The drilling time was firstly measured by means of a couple of fast infrared photodiodes. Then, the effect of the process time, immediately before and after the measured drilling time, and pulse frequency on the hole geometry was investigated. Moreover, the evolution of the machined holes as a function of the process time was discussed.

Abstract: Due to size effects new challenges are involved in micro deep drawing compared to macro deep drawing. One of these challenges is that the limit drawing ratio in micro deep drawing becomes smaller than that in macro forming, which limits the application potential of micro deep drawing in an industrial context. In order to extend the application possibilities of micro deep drawing, investigations were carried out on this topic. Own previous work showed that the “tribological effect”, the “global flow behaviour effect” and the “local flow behaviour effect” are responsible for the lower forming limit in the micro range. In this paper, the flow behavior of thin foils is further investigated. Forming limit diagrams of Al99.5 and E-Cu foils with different thicknesses ranging from 20 μm to 100 μm were acquired using an optical measurement system. It was found that the forming limit of thin foils is lower than that of thicker foils. Further analysis indicates that this difference is due to the number of grains in the direction of thickness of the material: more grains give more grain boundaries, which allows more strain of the grains.