Key Engineering Materials Vol. 549

Abstract: Complex material models used for the numerical representation of forming processes need in addition to tension and compression tests also shear tests to completely analyse the behaviour of the material under different loading conditions. There are two concepts of shear test one uses symmetrical specimens with two shear zones (according to Miyauchi) the other one specimens with a single shear zone. In both cases, a homogeneous distribution of the strain in the shear zone is essential for the validity of the shear test. Therefore, the length and width of the shear zone in a single shear specimen made from high strength steel according to the ASTM standard geometry were varied. Analysing the resulting strain distribution numerically an optimised sample geometry with a more uniform strain distribution than the ASTM standard was achieved. The numerical results were also validated with experimental shear tests.

Abstract: In the present work, formability of ultra low carbon steel sheets (uncoated interstitial free and galvanized steel sheet) has been characterized and the effectiveness of dry and liquid lubricants on formability of these steel sheets was evaluated by using the standard Erichsen test. Primarily, some important mechanical properties of materials like strain hardening coefficient (n), normal anisotropy (r) were determined using the tensile test and formability of these steel sheets has been correlated with these mechanical properties. Then these steel sheets were subjected to the Erichsen test using four different lubricants. Teflon, PVC, Polyethylene films and mineral oil were used in the experiments. Finite element simulations were done using various friction coefficients. Erichsen tests were conducted at 1 kN blank holder force and 2.4 mm/min punch velocity. Values of the Erichsen index and punch force-displacement curves were determined for each lubricant. PVC film proved to be the most effective lubricant for both materials.

Abstract: Ultra-high strength steels have been widely used in different industrial applications. It is necessary to understand the behavior of these materials in common forming processes such as air bending. It is known that the bendability of ultra-high strength steels is lower than other high-strength steels but what are yet to be discovered are the parameters that define the limits of bendability of these steels. The aim of this study was to investigate the factors affecting the bendability of ultra-high strength steel using optical strain measurements and FEM-modeling of the bending process. By using the true stress-strain relation measured by optical strain measuring system the bendability of ultra-high-strength steel was modeled fairly accurately. As a result, it was noted that the strain distribution at the bend of a steel possessing better uniform strain was more widely distributed and there were no highly localized strains. On the other hand as the failure occurred the strains were considerably smaller than the true failure strain of the material in uniaxial tension. As a conclusion it was stated that the ability to withstand the localization of deformation might describe the bendability of ultra-high-strength steel better than the values of the uniform or true failure strain in uniaxial tension test.

Abstract: High Strength Steels (HSS) and Advanced High Strength Steels (AHSS) are most commonly used nowadays in the automotive industry to achieve a weight reduction of the vehicle while assuring that all the safety requirements are fulfilled. These steels are characterized by very high ultimate stresses, reaching up to 1500MPa with yield strengths of up to 1000MPa. It is mainly because of these exceptional mechanical properties that it is becoming very usual to find these steels in sheets. This study presents a design methodology for integrated security sub-modules (constituting the suspension and steering modules) in the car manufacturing industry. The sub-modules are made up of a steel structure and different anchorage elements (rubber-metal or plastic-metal), which undergo separate surface treatments to avoid corrosion. Afterwards, the elements are traditionally joined by means of adhesives and screws. This process involves a great number of stages, low quality union methods and generation of corrosion areas that shorten its service life. The first step in this study has been the construction of a metallic structure in high strength low alloy steel (HSLA), instead of in traditional steel, in order to benefit from its better safety performance and lower weight at affordable cost, moreover the industrial manufacturing process was simulate with F.E.M in order to reduce the cost in forming manufacturing process. The second step, the improvement proposed is based on the use of laser welding as joining method, a high quality way to reduce the number of anchorage points and to increase the resistance of the sub-module. The last step is the surface treatment on the integrated piece in order provide an unaltered coating. Overcoming the currently need of assembly processes, which usually causes serious damages on the protective surface, this treatment will suppose an increase in the response against corrosion of the pieces. Regarding this aspect, the development of an organic treatment with lower required temperatures (max. 100oC) has been proposed, paying particularly attention to the removal of pollutant elements (Cr (VI), Zn and Ni) involved in traditional methods. This methodology provides automotive suppliers an additional added value and cost reduction, allowing them to increase their competitiveness in a sector that faces up to the transition from the traditional supply chain to a strategic value chain.

Abstract: The present work discusses the effect of the punch radius on the formability of H240LA steel sheets of 1.2mm thickness. A series of hemispherical punch tests (Nakazima tests) and stretch-bending tests with cylindrical punches of different diameters have been carried out in order to characterize the influence of the strain gradient in the sheet failure. The limit strains have been obtained using a recently proposed time-dependent methodology, which is applicable not only in conventional Marciniak and Nakazima tests, but also in situations with a severe strain gradient through the sheet thickness. The results show that formability of H240LA steel sheets increases as the t₀/R ratio decreases.

Abstract: The production of aluminum alloy components through sheet forming processes conducted at elevated temperatures is gaining more and more interest as it gives raise to the possibility of a significant enhancement of the metal formability characteristics, compared to room temperature forming. Aluminum alloy AA5083 blanks, which present a limited formability at room temperature, are usually formed through superplastic forming at elevated temperature: however, this processing route is too slow to be applicable for large batch production, typical for instance of the automotive industry. The paper is aimed at exploring the formability characteristics of the AA5083 when deformed at elevated temperature, but in a range of strain rates higher than those usually applicable in superplastic forming. To this aim, uni-axial tensile tests were carried out, in order to record the material formability characteristics as a function of temperature and strain rate, and to correlate them with the developed microstructural features. It is shown that it is possible to work at higher strain rates, still preserving a significant formability, even without using a conventional fine-grained superplastic alloy.

Abstract: Due to high production rates and the possibility to form complex geometries roll forming has become an increasingly popular forming process for sheet metal. Increasing quantities of high strength steels are used today but can be difficult to form due to their low ductility. One way to partly overcome this problem is to heat the steel in the forming area thus locally increasing the ductility. In the present study partially heated cold rolled high strength AISI 301 type austenitic stainless steel was investigated using electron backscattered diffraction (EBSD), and the results were compared to microhardness measurements. The results show that partial heating will give an almost complete reverse martensite transformation, i.e. martensite (α ́) transforms to austenite (γ), close to the surfaces and grain growth in the middle of the steel sheet. The extension of the heat affected zone can be determined using either microhardness or EBSD measurements. Both these measurements can be used to determine the position of the neutral layer after roll forming. The hardness measurement cannot distinguish between microstructural features but the results are in good agreement with the EBSD results for volume fraction of α ́-martensite. A major advantage of using EBSD is the possibility to characterize and follow the microstructural development when heating and roll forming.

Abstract: ncremental sheet metal forming (ISF) is a suitable process for the production of small batch sizes. Due to the minor tooling effort and low forming forces, ISF enables the production of large components with inexpensive and light machine set-ups. Hence, ISF is an interesting manufacturing technique for aeronautical applications. Sheet metal parts in aircrafts are often made of titanium and its alloys like the high strength alloy Ti Grade5 (Ti6Al4V). The characteristic low formability of Ti6Al4V at room temperature requires forming operations on this material to be carried out at the elevated temperatures. The interaction of heating and deformation cycles results in a microstructure evolution, which is believed to have a high impact on formability and product quality. In the present work, the temperature-dependent microstructural evolution of the as-deformed parts was investigated. Longitudinal pockets with different depths were formed using a laser-assisted ISF process. The microstructural evolution and hardening of the material were analyzed with respect to the local strain in different forming depths and pocket zones. The formability of the material together with the deformation depth and the sheet thickness-reduction were found to be strongly dependent on the applied process temperatures and the activated deformation mechanisms like dislocation glide and dynamic recrystallization.

Abstract: In the last decades, national legislations have become even more restrictive about the application and the disposal of hazardous lubricants in sheet metal forming. As a result, metal forming industry, which traditionally has made large use of synthetic and oil-based lubricants to enhance the production rate and reducing the wear of tools, has been forced to study and develop new solutions to reduce the environmental impact of production processes. The introduction and the continuous improvement of environmental friendly lubrication systems has seen a continuous growth and attention, but today the usage of traditional hazardous lubricants is still significant, requiring expensive cleaning operations and harmful cleaning agents to remove them. The use of solid organic lubricants can reduce or eliminate this drawback, despite their performances may be significantly affected by specific process parameters and the presence of debris. In this paper the performances of a solid organic lubricant have been compared to traditional liquid lubricants applied to sheet metal forming applications. Different surface topographies have been reproduced and the effects in terms of frictional behaviour have been investigated.

Abstract: Axisymmetric die and binder are typically used in the bulge test, where the test specimen is formed by increasing the level of oil pressure (Fig. 1). With this experimental setup a biaxial stress state is induced at the specimen dome, assuming that it is not influenced by friction. The increasing oil pressure in the region of the top of the dome is recorded and the deformation field measured during the forming process. The optical measurement system determines the coordinates, the deformations and the curvature on the outer surface. Based on the forthcoming ISO 16808 these results are directly used for the calculation of the flow curve. In order to determine the flow curve based on the bulge test, an analytical approach is needed for the computation of the stress state at the top of the dome.