Key Engineering Materials Vol. 639

Abstract: Large radius air bending has a different loading diagram than conventional bending, which affects the material behavior during the bending process. In order to establish a correct loading diagram, the position of the contact points between the plate and the punch is determinant. The position of the contact points is depending on the evolution of the bending process and the influence of the material is unknown. In this work, the determination of the position of the contact points in large radius air bending has been studied by means of both an experimental campaign and finite element analysis. Experiments were performed on a press-brake with a capacity of 50 metric tons. High-strength steel Weldox 1300 and aluminum alloy AlMg₃, and punches of radii 30, 35 and 40 mm have been used. During the bending process, the punch movement has been monitored and the bending angle has been measured by means of images recorded by a camera system. Based on the obtained results, the relation between the bending angle and the position of the contact points is discussed.

Abstract: In carcass production for flexible pipe systems roll formed profiles are wound around a mandrel forming an interlocking, flexible structure able to withstand collapse from outside water pressure or mechanical crushing. Carcass is often produced in lengths of several kilometres, which implies numerous welds between coils of stainless steel, often duplex grades. The welds are a source of failure, since fracture from time to time occurs here in the winding stage. A simulative test in form of three-point-bending is developed, which shows promising results together with simplified air- and v-bent profiles allowing offline testing of welds for optimisation purposes. Comparative studies are shown possible but discrepancies in boundary conditions cause the maximum strains in the simulative test to differ from those in production. A study of weld failure is done applying the simulative test and tensile tests using GOM ARAMIS 4M system for strain measurements. The results show strain localization at the weld from onset of yielding caused by the soft, heat affected zone next to the weld seam resulting in a local thinning of the strip similar to what is observed in production.

Abstract: As a flexible process, Single Point Incremental Forming has the potential to be widely used as a technique for prototyping, discrete or small batch production. However, its notorious lack of accuracy and formability limit a wide adoption on an industrial scale. While the main factor constraining the feasible batch size is the limited processing speed, recent developments towards High Speed SPIF have moved this restricting boundary considerably. Increased strain rates and friction in HS-SPIF introduce their own specific influence on the achievable accuracy. This paper aims to investigate the effect of HS-SPIF on the bulging of the bottom of a work piece. This explicit type of inaccuracy, also known as the pillowing effect, is mainly located in a region of the sheet which is to be left unprocessed, and is therefore challenging to eradicate. Different papers have focused on bulging of a sheet in conventional SPIF, resulting in knowledge about the inwards bending of the sheet at low wall angles. HS-SPIF, however, tends to show a previously unseen outwards bulge at high wall angles due to its unique thermal behavior.

Abstract: In incremental sheet forming the material properties change dependent on the wall angle. In addition to sheet thinning, material hardening can be observed and for stainless steel the formation of strain induced martensite has been reported. These process characteristics have been extensively examined for the forming of parts in one step. In this study, a first investigation is made to control the material properties independent on the wall angle by a multi-step expanding approach. For this, in the first step a steep wall angle is formed and in the following steps this region is transformed to a shallow wall angle, keeping the material properties. To demonstrate and verify this new approach a small cone frustum with a wall angle of 60° was formed as a preform. This cone frustum was repeatedly expanded in steps of 2° to get finally a cone frustum with 30° wall angle. The stepwise expanding causes a considerably growing of the shape in depth direction, especially at the preformed area. By a heuristic approach the geometric accuracy of the final cone was optimized. The microhardness measurement of the formed cones shows that the final cone frustum has a region, where the material properties are similar to the first formed 60° wall angle. By this new approach the possibility to influence the material properties of the final part purely by tool path is demonstrated. In particular it is possible to form regions with shallow wall angles that have the material properties of regions with steep wall angles.

Abstract: Single Point Incremental Forming (SPIF) is a method of forming sheet metal components that requires only minimal tooling and a standard 3 axis milling machine. The low tooling and setup costs of SPIF make it an ideal method for prototyping and low-volume manufacturing. One of the challenges of SPIF is the development of tool paths that will form parts successfully, without encountering failure modes such as fracture due to wall thinning. To progress beyond a trial and error approach to tool path creation, an accurate and fast method of predicting failure must be developed. Forming limits in SPIF are often characterised by a maximum wall angle, corresponding to thinning limits according to the sine law [1]. While inexpensive computationally, the sine law does not account for secondary strains due to part curvature, and is not applicable to multi-pass forming [2]. A more general method of rapidly predicting wall thinning and strain state of a post-formed part is necessary. One such method is a kinematic model proposed by Bambach (2010) [3] which simulates the displacement of the model during each pass of the tool relying only on geometrical data.In this paper, the kinematic model mentioned above is extended to be applied to multi-pass forming and experimentally tested by comparing model predictions of major and minor strains to experimental measurement. The model is found to accurately predict minor strains during multi-pass forming, while over-predicting major strains, likely due to material property and friction affects unaccounted for in the model. By properly understanding the accuracy and limitations of this model as applied to real forming conditions, toolpath strategies can be generated in future with confidence and with minimal computation time.

Abstract: The aim of this study is to establish general guidelines for minimizing the number of tests required to determine optimum process parameters in terms of formability for laser assisted single point incremental forming (LASPIF). An automotive aluminium alloy (AA5182-O) is selected and the room temperature failure angle of this material is determined experimentally. The straining behaviour as well as sheet thinning of the test part (at its maximum forming angle) is studied using an experimentally validated finite element model. From the thinning rate of the sheet metal and the shape of the contact zone between tool and sheet it is concluded that continuous straining of the sheet on the wall region of the contact area is responsible for extra thinning and failure. Based on the size and position of the contact zone, different laser tool positioning strategies have been used to achieve the highest forming angle. It is concluded that due to an elongated shape of the contact zone in steep wall angle parts and considering a small deviation of the forming robot, the selection of a large spot diameter is necessary in terms of maximum obtainable wall angle. It has been observed that the maximum forming angle is still achievable using a large forward offset. It is concluded that the partial stress-relief annealing of the deformed geometry during the approach of the forming tool, is responsible for this formability enhancement.

Abstract: Punching portions of the sheet are sandwiched between the ceramic billets during rapid resistance heating to prevent hardening of these portions. When the heating temperature is locally lower than that of the austenitic transformation, i.e. below 800 oC, this portion is not hardened without occurrence of martensitic transformation, and thus cold punching of hot-stamped parts becomes easy. The ceramic billets are made of alumina and the heat transfers to the billets. The temperature distribution just after resistance heating, the hardness distribution of the hot-stamped sheet, the cold punching load, the quality of the punched hole, etc. were measured. Hardening of punching portions was successfully prevented by sandwiching between the ceramic billets. The cold punching load for the local prevention of hardening was half of that without local prevention and the delayed fracture was also prevented, whereas the drop in hardness around the sheared edge became larger than that for laser cutting.

Abstract: In virtual design of the hot stamping process, a reliable description of the material flow behaviour is an important input to ensure accurate estimations of the parts feasibility. Currently, to characterise the hot stamping material’s flow behaviour at elevated temperatures, tensile and upsetting tests are available. The measurement of the flow behaviour out of such tests, which is generally temperature and strain rate dependent, still remains a complex task. Therefore traditional methods to measure flow curves out of such measurements are not necessarily precise enough. In this contribution the authors focus on non-isothermal conductive tensile tests of the manganese-boron steel MBW^® 1500 in order to understand its flow behaviour at elevated temperature. Numerical calculations using Finite Element Method (FEM) of the tests itself with correct boundary conditions as well as for all necessary phenomena are used to identify accurately the material’s flow curves by use of inverse optimisation. Finally, for validation purpose the identified flow curves out of the optimisation method were used to simulate the hot stamping of two different parts. Both geometries were chosen such that various strain paths are covered i.e. uniaxial tension to plane strain.

Abstract: The increasing demand for fuel-efficient vehicles has led automotive industry to introduce new alloys in car manufacturing, characterized by a high stiffness and strength-to-weight ratio. Due to their mechanical and chemical properties, aluminium alloys appear potential candidates to replace traditional steels for several parts of the car-body-in-white, even if the limited formability at room temperature, the marked springback and the severe tribological behaviour have often represented important drawbacks in traditional properties. Recently, the use of temperature-assisted processes, such as hot stamping, allows overcoming such limits thanks to substantial increase of the formability and, at the same time, a drastic reduction of springback. However, the choice of proper process parameters, in terms of lubrication at the interfaces between the dies and the blank, and thermal parameters of the dies materials still represent critical points for the feasibility of the process. Recent investigations have proved the limits of assuming constant friction for all the areas of the dies and the steps of the deformations, especially with variable pressures and non-constant temperatures at the interfaces. Such factors, together with the lubricants and the lubricant deposition on the blank, the blank and dies coatings, the surface roughness, the stamping speed are not always considered, despite their considerable influence on the process tribology. In this paper the friction behaviour of commercially available automotive aluminium (AA6016 alloy) is studied. The friction coefficient is measured by means of a new machine for strip draw test at different levels of pressure, temperature, sliding speed and type of lubricant. The results report the investigation of the surface topography of the metal sheets, investigated by Energy-Dispersive X-ray spectroscopy and optical profilometry.

Abstract: Due to oxidation and decarburization during heat treatment prior to hot stamping, various coating products have been developed in the last decades. Press-Hardened Steel (PHS) components for passenger cars are generally coated by aluminized or galvanized (GI) coatings. The aluminized coating presents a good formability at high temperature and permits forming and quenching components in the same press tools with a so called direct hot stamping method. Due to a strong cracking in the base material during direct hot stamping induced by liquid-metal embrittlement (LME), galvanized coated products must be pre-formed at low temperature and undergo heat treatment in separate press units. Moreover, the oxide scale formed on GI parts must be removed by abrasive blasting, whereas aluminized parts can be directly painted after hot forming. However, higher performance in corrosion resistance has been observed for galvanized parts, in particular in cosmetic and cut-edge corrosion. This increase is linked to the sacrificial effect or cathodic protection provided by the layer containing zinc. Daimler AG is investigating the possibility of improving performance of PHS body parts in terms of suitability for direct hot stamping and corrosion protection by developing new coating materials. In the following article, the main particularities and challenges involved in both current coating products will be introduced. The development of specific press tools for this study, as well as the corresponding simulation of hot forming will be presented. Finally, the hot forming behaviour and anticorrosive properties of both current products will be presented.