Papers by Author: Paolo F. Bariani

Abstract: Hot stamping of quenchenable high strength steels represents the most promising forming technology for the manufacturing of safety and crash relevant car parts. In such process, the manganese-boron steel 22MnB5 is homogeneously austenitized, formed and subsequently quenched in one process step, so that complex geometric structural components can be formed with an ultimate tensile strength up to 1500 MPa. Due to the high temperature, no lubrication is used during the process with consequent high wear and reduced service-life of the dies. Commercial available steel blanks usually present an Al-Si coating that has been specifically developed as a protection from oxidation and decarburization and, at the same time, has proved positive influence also on the decreasing the friction at interface between the blank and the dies during the forming. Although such coating is generally accepted as the only lubrication medium in hot stamping, its performances are considered not appropriate for a good lubrication. The paper presents the comparison of the standard Al-Si coating and a new Zn coating when applied to metal sheets in hot stamping. A novel apparatus to investigate the tribological conditions during sheet metal working processes is presented. In addition to the control of mechanical (i.e. normal pressure) and kinematic parameters (i.e. sliding speed, sliding length), the developed testing machine permits to reproduce the thermal fields and monitor the thermal conditions of the sheet and tool materials. Experiments were carried out on Zn coated 22MnB5 sheets in the range between 700° and 950°C and compared with the performances of the commercial Al-Si coating. The coating performances are investigated for different heating temperatures and soaking times.

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. However, conventional forming processes at elevated temperatures on aluminum alloy sheets are usually carried out under superplastic forming regime conditions, which are too slow to be applicable to mass production typical of the automotive industry. The aim of the present study is to investigate the formability characteristics of AA6016 aluminum alloy sheets 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 to evaluate both the material ductility in terms of true strain at fracture as a function of the temperature and strain rate, and the alloy post-forming characteristics after testing. In such a way, the optimal forming conditions in terms of temperature, strain rate and microstructural features were identified.

Abstract: The constant demand of increasing performances and safety in vehicle industry has led significant innovations in the materials used in sheet metal forming processes. In particular, multiphase steels and lightweight alloys have known higher and higher importance, thanks to the development of new stamping processes at elevated temperatures, which guarantee, at the same time, better formability, lower springback and more accurate micro-structural control in the formed sheets. With respect to these aspects, the correct design and optimization of the new processes cannot prescind of the mechanical characterization of materials in biaxial stress conditions, especially when it strongly varies according to the stress and temperature. In this paper, a novel experimental set-up is presented for determining the in-plane yield locus of sheet metals at elevated temperatures. A cruciform specimen, whose geometry was optimized by numerical simulation, is used for the study of the yield locus in the range of biaxial tensile stresses. The test machine concept is based on punch-wedge mechanism, which uses the vertical movement of the press for the deformation of the specimen along two perpendicular axes. In the first part of the paper, the optimization of the cruciform specimen by thermo-mechanical FE analyses is outlined. Details on the experimental set-up are then given with the description of the apparatus, the measurement of plastic strains and the heating system for tests at elevated temperatures.