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Investigation on thermal, mechanical and microstructural properties of quenchenable high strenght steels in hot stamping operations
Get PDFSheet metal working operations at elevated temperature have gained in the last years even more importance due to the possibility of producing parts characterized by high strength-to-mass ratio. In particular, the hot stamping of ultra high strength quenchenable steels is nowadays widely used in the automotive industry to produce body-in-white structural components with enhanced crash resistance and geometrical accuracy. The optimization of the process, where deformation takes place simultaneously with cooling, and of the final component performances requires the utilization of FE-based codes where the forming and quenching phases have to be represented by fully thermo-mechanical-metallurgical models. The accurate calibration of such models, in terms of material behaviour, tribology, heat transfer, phase transformation kinetics and formability, is therefore a strong requirement to gain reliable results from the numerical simulations and offer noticeable time and cost savings to product and process engineers.
The main target of this PhD thesis is the development of an innovative approach based on the design of integrated experimental procedures and modelling tools in order to accurately investigate and describe both the mechanical and microstructural material properties and the interface phenomena due to the thermal and mechanical events that occur during the industrial press hardening process.
To this aim, a new testing apparatus was developed to evaluate the influence of temperature and strain rate on the sheet metal elasto-plastic properties and to study the influence of applied stress and strain of the material phase transformation kinetics. Furthermore, an innovative experimental setup, based on the Nakazima concept, was designed and developed to evaluate sheet formability at elevated temperature by controlling the thermo-mechanical parameters of the test and reproducing the conditions that govern the microstructural evolution during press hardening. This equipment was utilized both to determine isothermal forming limit curves at high temperature and to perform a physical simulation of hot forming operations. Finally, a thermo-mechanical-metallurgical model was implemented in a commercial FE-code and accurately calibrated to perform fully coupled numerical simulations of the reference process.
The material investigated in this work is the Al-Si pre-coated quenchenable steel 22MnB5, well known with the commercial name of USIBOR 1500P’®, and the developed approach proves to be suitable to proper evaluate high strength steels behaviour in terms of mechanical, thermal and microstructural properties, and to precisely calibrate coupled numerical models when they are applied to this innovative manufacturing technology.
The work presented in this thesis has been carried out at DIMEG labs, University of Padova, Italy, from January 2005 to December 2007 under the supervision of Prof. Paolo F. Bariani
Optimization Of Nakazima Test At Elevated Temperatures
No full textNowadays hot forming of High Strength Steel is gaining the strict requirements of automotive producer: in fact deformation performed simultaneously with quenching assures a fully martensitic microstructure at room temperature and thus high strength properties that allow the thickness reduction of the body-in-white components. Basic aspects of hot stamping are still under investigation and supplementary achievements are expected for a successful application of sheet metal forming technologies at elevated temperatures. Among data needed to settle a numerical model of the process, information about material formability may help in better designing and optimizing hot stamping operations. In the first part of the work, a new experimental apparatus based on Nakazima concept is presented; process parameters are optimized in order to accurately replicate the thermo-mechanical conditions typical of the industrial process, paying particular attention to the thermal and microstructural evolution. On the other hand, as commercial FE codes require the implementation of Forming Limit Diagrams at constant temperature, numerical investigations have been performed in order to determine the proper testing conditions to obtain FLD at nearly constant temperature
Investigation of 22MnB5 mechanical and phase transformation behaviour at high temperature
No full textOne of the main challenges of the automotive industry is represented by the weight reduction of structural components achieved through the great potentiality of enhanced strength to mass ratio of high and ultra-high strength steels. The unacceptable loads on tool and springback phenomena obtained with traditional sheet metal forming operation of such materials at room temperature force industries to shift even more their attention to new forming technologies at elevated temperatures. The hot stamping of quenchable high strength steels offers in fact the possibility to reduce the body-in-white weight maintaining the crash requirements together with increased formability of sheets. However, the optimization of this innovative process chain through reliable FE simulations requires a deep knowledge of material behaviour in terms of microstructural evolution and resistance to plastic deformation in temperature. A new experimental set-up was developed in order to reproduce the thermo-mechanical conditions that material undergoes during the different phases of the industrial process and thus to overcome the absence in the technical- scientific literature of accurate data on material mechanical properties at elevated temperatures. The influence of temperature and strain rate on 22MnB5 elastic and plastic properties was evaluated through uniaxial tensile tests, considering the effective strain after necking by means of an optical strain measurement system. An extensive dilatometric analysis perfomed through a modified axial extensometer, both with and without applied stress, was employed to evaluate the shift of the TTT curves due to stress as well as the transformation plasticity coefficients
Anisotropic and Mechanical Behavior of 22MnB5 in Hot Stamping Operations
No full textThe hot stamping of quenchable High Strength Steels offers the possibility of weight reduction in structural components maintaining the safety requirements together with enhanced accuracy and formability of sheets. The proper design of this technology requires a deep understanding of material behavior during the entire process chain, in terms of microstructural evolution and mechanical properties at elevated temperatures, in order to perform reliable FE simulations and obtain the desired characteristic on final parts. In particular, the analysis of technical-scientific literature shows that accurate data on material rheological behavior are difficult to find; while the lack of knowledge about anisotropic behavior at elevated temperatures is even more evident. To overcome these difficulties, a new experimental set-up was developed to reproduce the thermo-mechanical conditions of the industrial process and evaluate the influence of temperature and strain rate on 22MnB5 flow curves through uniaxial tensile tests; an optical strain measurement system was utilized to evaluate the effective strain after necking. From the same data, plastic anisotropy evolution was determined by means of a specially developed procedure. The influence of different cooling rates was taken into account and the rheological properties were correlated with microstructural changes occurring during deformation, previously evaluated through a dilatometric analysis performed in the same range of temperatures
Investigation of 22MnB5 formability in hot stamping operations
No full textIn the automotive industry, the hot forming of high strength steels offers the possibility to obtain significant reduction of weight without affecting the structural performances of final products. The main reason of its restricted application in the industry lies however in the lack of basic knowledge about mechanical and microstructural characteristics of sheets at elevated temperature, boundary conditions (i.e. friction and heat transfer), sensitivity of the formed component geometrical and mechanical characteristics to all the process parameters. To partially overcome this limitation, this paper presents a novel test based on the Nakazima concept which is able to evaluate the material formability at elevated temperature. Thermal tests with the aid of a dilatometer device were utilised to generate phase transformation data while uni-axial tensile tests, performed simultaneously with quenching, provided flow stress data at different temperatures and strain rates. The effective thermal cycle to apply during the formability test was optimized through numerical simulations of the test itself suitably calibrated through the above data
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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