Defect and Diffusion Forum Vol. 414

Abstract: Commonly used lubricants in sheet metal forming usually base on mineral or synthetic oils, water based emulsions, drawing films or hotmelts (waxes). However, these conventional lubricants often contain harmful substances to human health and the environment. For this reason, a novel tribological system for dry metal forming has been developed. Basic principle of this new tribological system comprises the use of volatile media such as CO_{2 (liquid)} and N_{2 (gaseous)}, which are injected into the contact interface under high pressure via injectors integrated into the tool. The volatile media used, lead to a significant reduction in friction and ensure robust friction behavior in deep-drawing processes. The investigations reported about in this contribution particularly focus on the friction behavior occurring in this tribological system at highly loaded radii. The friction investigations were carried out by using a modified stretch-bending-test and showed the effects of the main process-influencing parameters. In the present paper, these results are compared to already published results of the friction behavior investigated by means of flat strip drawing investigations. The findings obtained in this way allow a better understanding and prediction of the tribological system´s properties, thus making it usable for sheet metal forming applications.

Abstract: Materials resistance to edge failure during sheet metal flanging operations is known as stretch-flangeability. It is one of the important concerns in the current automotive sector. Stretch flangeability of sheet metal is estimated by hole expansion test and commonly it is represented by hole expansion ratio (HER). The objective of the present work is to comprehend the hole expansion deformation behavior of DP600 steel. Firstly, finite element analysis was performed to understand the stress state at the edge during the hole expansion test. Thereafter, the effect of stress relaxation was studied by conducting hole expansion tests (HET) in monotonic and interrupted mode. Considerable improvement in the HER was observed. The HER was found to increase with the pre-strain. The combined effect of friction and stress relaxation played a crucial role in delaying the edge failure, resulting in the enhanced HER.

Abstract: Modern lightweight design is often based on multi-material components. For example, structured sheet metals can be equipped with a die-cast light metal insert for structural support. To interlock sheet and insert, structures with undercuts are formed into the sheet in a multi-pass rolling process. In a first pass, structured rolls are used to create a structure of channels and ribs. Undercuts are formed in a consecutive pass by flat rolling those ribs. During die-casting, the melt flows into the channels and forms an interlocking connection once solidified. The joint strength is decisively determined by the undercut geometry. The undercuts formed by material displacement increase with the height reduction in the flat rolling pass. However, after a certain amount of material displacement, the channel side edge starts to fold over the channel bottom and forms an inner notch. Those inner notches can be prone to crack initiation and subsequently lead to component failure. To analyze the surface structure regarding channel depth, undercuts and inner notches as well as finally maximize the joint strength, a combined experimental and numerical study was laid out. The surface of 2.0 mm DC04 was structured with up to 0.5 mm deep channels and then flattened with different height reductions. The results from the 2D explicit FE-model suggest that a process optimum for those surface structures with high undercut width but without inner notch exists at 14% height reduction. However, in the experiments inner notches started to form at approx. 8% height reduction with approx. 20 µm wide undercuts for the given experimental setup. In contrast, maximum undercuts of approx. 50 µm form at 26% height reduction, but also cause inner notches with approx. 60 µm length.

Abstract: For conventional sheet metal forming at room temperature, numerous tribometers were developed in the 20th century. At the present state of the art, unsolved issues for tribometry remain for temperature-supported forming processes of high strength aluminum (e.g. EN AW-7075), in which the sheet is heated to temperatures between 200 and 480 °C. The tribological design of these processes remains a major challenge, which needs to be addressed by investigations with adapted tribometers. In this study, a recently adapted strip drawing test for aluminum warm and hot forming is presented – including a newly developed strip heating unit, a die lubrication system and a die tempering system for efficient tribological testing. The contribution is completed with both, experimental results and a numerical investigation of temperature gradients in the strip drawing test. Finally, it is discussed whether transient process conditions of non-isothermal forming processes with die lubrication should be considered in tribometers for warm and hot sheet metal forming.

Abstract: The damage state in a formed component has a significant influence on the performance of the component in service. Controlling damage evolution during forming through specific modifications of the process parameters will therefore allow an improvement of this performance. The evolution of the stress-strain state during the forming process is the primary influencing factor of the resulting damage state. The stress-strain state is influenced by the friction between tools and workpiece. To investigate the cause-effect relationship between friction and damage evolution in the deep drawing process, Finite Element simulations of the deep drawing of rotationally symmetric cups were performed. Punch velocity and blank holder force were varied. Damage was predicted using a Lemaitre damage model. The damage states predicted using a Coulomb friction law and a model incorporating a dependence on contact normal stress and relative velocity were compared. The parameter-dependent friction model predicted a change in the damage distribution after forming when varying the process parameters, which was not found using the Coulomb friction law.

Abstract: Dual-pressure tube hydroforming (THF) is a tube hydroforming process variant whereby deformation of the tubular specimen is achieved by exerting fluid pressure on both the inside and outside surfaces of the tube. Dual-pressure THF experiments are conducted to study tribological conditions when producing pear-shaped and triangular parts. The pressure-loading paths are designed to exert pressure in oscillatory pattern: I.e., the pressure on the inside was alternated with pressure on the outside causing the tube to expand and contract/buckle as deformation progressed. During tube contraction, the metal-to-metal contact area is substantially reduced. This leads to reduction in friction stress at the tube-die interface, thus increasing formability. Comparing the geometries of the formed parts produced by dual-pressure THF and conventional THF reveals that the former results in a substantial increase in the protrusion height of a pear-shaped specimen.

Abstract: As in all metal rolling processes, Mannesmann cross-roll piercing relies on entrainment by friction between the billet and the rolls. But contrary to other rolling processes, a strong back-push is imposed by the piercing force (Fig. 1). Entrainment of the billet through the mill is therefore a critical problem which can be solved by optimizing the surface state of all tools (rolls, guide rolls, piercing plug). This is why the effect of friction with all the tools on the tube entrainment speed and on the state of stress has been investigated using the 3D Finite Element Method (FEM, ForgeNxT). It has been found that friction between billet and cross rolls is a driving force on the first (upstream) half of the rolls but may become resistant on the downstream part for certain process settings. Friction between piercer plug and hot metal is always resistant. Friction with rotating Diescher guide disks is a driving force in the piercing direction, but works against pierced shell rotation, causing shell torsion (“twist”). If static lateral guide shoes are preferred, their pure sliding friction is resistant in both directions. Friction on the upstream part of rolls must therefore be as high as possible for correct entrainment and process stability, which explains the practice of giving it very high roughness. The surface of the piercer plug must be smoother to minimize its frictional resistance. The surface states of lateral guides and of the downstream half of rolls can be used as process optimization variables. Results suggest how to obtain estimates of friction coefficients from mill measurements.

Abstract: High-strength aluminum alloys provide great potential for weight reduction in vehicle and aircraft production. Nevertheless, forming of these materials is limited at room temperature. Thermally supported forming operations such as hot forming and quenching (HFQ^®) allow forming of components without failure and reduced springback. Currently, the adhesive wear and high friction limit the use of HFQ^®-operations for high-strength aluminum alloys. Out of this reason, the present paper describes the tribological performance of new developed dry lubricants at elevated temperatures for the alloy AA7075. For this purpose, strip drawing tests were carried out in a modified open tribological system at temperatures between 20 °C and 400 °C. Additionally, the melting behavior of these lubricants, as well as the variation of the lubricant thickness, were investigated. The results show a reduction of the coefficient of friction at elevated temperatures.

Abstract: 7000 series aluminum alloy is a high-strength alloy and used in a wide variety of products for the purpose of weight reduction in the field of transportation equipment and aerospace. In particularly, A7075 alloy has the highest strength and is expected to be further applied in a wide range of fields such as aircraft parts and sports equipment. However, it has high deformation resistance and is prone to surface defects called tearing. Tearing affects the productivity because it requires to lower the speed. Tearing is likely to occur under high temperature and high speed conditions. It is thought that the localization of melting of Zn-compounds and additive compounds occur due to heat generated during process leading to tearing. In this study, in order to increase the productivity without tearing, die surface quality was considered to prevent the effect of friction and generated heat at interface between tools and material. In addition, recrystallization was also eliminated by reducing friction because temperature does not increase up to the range of recrystallization temperatures. The AlCrN coating was used to improve die surface quality comparing to Nitriding. It was found that the tearing size and heat generated by using AlCrN coating were small. In addition, grain size was small and observed at the tearing region on the extruded surface. It is thought that the die coating can reduce the effect of friction at interface and prevent the recrystallization of the extruded surface. Furthermore, the localization of melting of Zn-compounds and additive compounds are also decreased. From above results, tearing sensitivity can be decreased by using coating to increase productivity speed of A7075 alloy.

Abstract: In cold forging, the temperature difference between the workpiece and the tool is small and thus deformation analysis is rarely coupled with thermal analysis. However, the friction coefficient of zinc phosphate coating with metal soap has a large temperature dependence. Consideration of the effects of workpiece and tool temperature change on the friction coefficient is thus expected to improve the analytical accuracy of cold forging. Thermally coupled cold forging analysis requires thermal conductivity, specific heat, heat transfer coefficient between the workpiece and the tool, in addition to the temperature dependence of flow stress and friction coefficient. The heat transfer coefficient between workpieces coated with zinc phosphate with metal soap and tools is investigated in this paper. Cold backward extrusion was performed with a 50% reduction of area, and the temperature history in the punch was measured with a thermocouple. The forging speed was 1, 3, and 10 spm. FEM analysis was performed to simulate the experiment by considering the temperature dependence of flow stress and friction coefficient. The heat transfer coefficient was estimated at 20 kW/(m2•°C) by comparing the experimental result and calibration curves.