Key Engineering Materials Vol. 639

Abstract: Increasing safety requirements for hot stamped structural car body parts are demanding sufficient mechanical properties of integrated welding seams. Especially conventional tailor welded AlSi-coated 22MnB5 sheets are only fulfilling these requirements when ablated before laser welding and thermo-mechanically treated in a correct way. This paper shows a method that evaluates the hot stamping process of tailor welded blanks by press hardening different sheet thicknesses and thickness combinations in a testing tool. Furthermore, appropriate testing methods for the evaluation of mechanical properties of the welding seam are introduced. The results are ultimately compared with a special developed FEM analysis to predict failure cases in future.

Abstract: Tailored press-hardening processes are used to reduce both production costs and component weight. The aim of these development methods is to generate regions zones in the component with both high and low tensile strengths. The B-pillar, for instance, needs high tensile strength in the region of the roof frame to prevent deformation. However, the connection to the body should have lower tensile strength to absorb the energy of a crash.Regarding the production process for tailored welded blanks, the tailored press-hardening processes for monolithic sheets need no joining operation. As an addition to recent publications, this paper presents a modified tailored press-hardening process, with a modified time-temperature process. Starting from the required tailored material properties of the part, with a sheet thickness of 1.5 mm, research has been done on the process window and process design.This contribution concentrates on modifications to the time-temperature profile. After heating the hot-dip galvanized, heat-treatable 22MnB5 steel above its austenitic temperature, the aim is to adjust the material’s mechanical properties within the cooling process.Based on the continuous TTT diagram, the cooling rate has an impact on the material’s mechanical material properties. Different proportions of constituents such as Bainite, Ferrite or Perlite are created by varying the cooling rate. Furthermore, during an intermediate stage in the cooling-down period, the holding temperature has an even stronger effect on the material’s microstructural composition and the corresponding mechanical properties. The rate of the transformation process changes, depending on the intermediate temperature. The third parameter investigated is the holding time at this intermediate temperature. As the holding time is increased a transformation, progressing from austenite to other constituents, can be observed.The results of this parametric study could be transferred to a prototype environment.

Abstract: Forming of near-net-shaped and load-adapted functional components, as it is developed in the Transregional Collaborative Research Centre on Sheet-Bulk Metal Forming SFB/TR 73, causes different problems, which lead to non-optimal manufacturing results. For these high complex processes the prediction of forming effects can only be realized by simulations. A stamping process of pressing eight punches into a circular blank is chosen for the considered investigations. This reference process is designed to reflect the main aspects, which strongly affect the final outcome of forming processes. These are the orthotropic material behaviour, the optimal design of the initial blank and the influences of different contact and friction laws. The aim of this work is to verify the results of finite element computations for the proposed forming process by experiments. Evaluation methods are presented to detect the influence of the anisotropy and also to quantify the optimal blank design, which is determined by inverse form finding. The manufacturing accuracy of the die plate and the corresponding roughness data of the milled surface are analysed, whereas metrological investigations are required. This is accomplished by the help of advanced measurement techniques like a multi-sensor fringe projection system and a white light interferometer. Regarding the geometry of the punches, micromilling of the die plate is also a real challenge, especially due to the hardness of the high-speed steel ASP 2023 (approx. 63 HRC). The surface roughness of the workpiece before and after the forming process is evaluated to gain auxiliary data for enhancing the friction modelling and to characterise the contact behaviour.

Abstract: By applying bulk forming processes on sheet metals, thin-walled functional components with locally restricted wall thickness variations can be manufactured by forming operations. Using tailored blanks with a modified sheet thickness gradient instead of conventional blanks, an efficient controlling of the material flow can be achieved. One possible process to manufacture these semi-finished parts is a flexible rolling process. Based on an established process strategy new results for steels of differing strength and work-hardening behavior are presented in this paper. The influences of each material on the resulting process forces and blank properties regarding the same target geometry are discussed. The tailored blanks are hereby analyzed by their geometrical dimensions, like sheet thickness, and their mechanical properties, e.g. hardness distribution. Additionally, the possibilities of processing these tailored blanks in a deep-drawing and upsetting process are presented with a hereby focus on the residual formability of the tailored blanks.

Abstract: Due to ecological and economic challenges there is a growing demand for lightweight construction by using closely-tolerated complex functional components with variants. Conventional sheet and bulk metal forming operations are often improvident in producing such parts. A promising approach is the process-class “sheet-bulk metal forming” (SBMF). Within SBMF bulk forming operations are applied to sheet metals, often in combination with sheet forming operations [1]. This leads to a significant gradient in load conditions regarding stress and strain states and causes locally varying tribological conditions. Thus, the investigation of the tribological conditions and the provision of suited tribological systems are essential for the successful application of SBMF processes. The objective of the current study is the experimental investigation of the applicability of tribological adaptions by local abrasive blasting on a single-stage process combination of deep drawing and upsetting to produce a component with an external gearing. The manipulation of the local tribological conditions by the use of abrasive blasting leads to a better control of the material flow and in consequence to an improved quality of the components in terms of higher mould filling and cup heights, and a reduced thickening of the sheet in the area of the cup bottom.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: Over these last decades Ti6Al4V has been largely employed for biomedical applications because of its biocompatibility and high strength-to-weight ratio. According to several studies, the implantation success is strongly dependent on the characteristics of its surface, since the roughness and the chemical composition of the surface play an important role in the osseointegration of the implant and in its bioactivity, respectively.In this paper different thermo-electro-chemical treatments were applied to Ti6Al4V sheets to increase their roughness for biomedical applications. The Ti6Al4V sheets underwent three different treatments, namely acid etching, acid etching followed by anodization and etching in a hydrothermal reactor, and finally furnace annealing.After the etching and annealing treatments, the surface morphology of the samples were analysed through Scanning Electron Microscopy, while its chemical composition was identified through the Energy Dispersive Spectroscopy. The surface roughness and topography were evaluated by using a 3D profilometer. Furthermore, the bioactivity of the treated samples was evaluated by diving them in Simulated Body Fluid and then analysed through SEM and EDS analyses.Based on these results, the treatment assuring the best performance in terms of roughness and bioactivity was identified and then applied to tensile samples tested at 700°C and strain rates of 0.1 and 1s^-1 to evaluate the impact of the surface treatment on the material formability and, therefore, to prove the feasibility of applying the hot stamping process to surface modified sheets.

Abstract: Focus of this paper is to model the plastic forming behavior of AA6082, in order to develop the numerical FE analysis of the friction stir welding processes and the simulation of subsequent forming processes. During the friction stir welding process, the temperatures reached are until 500 °C and have a fundamental role for the correct performance of the process so the material data has to show a temperature dependency. Because of the tool rotation a strain rate sensitivity of the material has to be respected as well. In this context, the general material characteristics of AA6082 were first identified for different stress states. For the uniaxial state the standard PuD-Al used in the automotive industry was applied, for the shear state the ASTM B831-05 was used and for biaxial states the ISO 16842 was exploited. To characterize the plastic flow behavior of the AA6082 at elevated temperatures tensile tests were performed according to DIN EN ISO 6892-2 from 25 °C until 500 °C with a strain rate from 0.1 s^-1 up to 6.5 s^-1.