Modeling of Semisolid Flow

Numerical simulation of semisolid processing can be carried out using one- or two-phase modeling. Various models based from either fluid or solid mechanics formalisms were developed. The most sophisticated one-phase models account for shear thinning, thixotropy, and solidification/melting phenomena. They are of great interest to investigate the flow front and optimize die geometry, temperature of die, and billet. Liquid-solid segregation prediction requires two-phase modeling. Basic equations and constitutive models currently used for numerical simulation of semisolid processing are presented and discussed

42CrMo4 steel flow behavior characterization for high temperature closed dies hot forging in automotive components applications

The application of new forming processes as the high temperature hot forging in closed dies in an industrial environment still requires further investigation due to the lack of flow stress data at these temperatures. To determine the flow behavior of the 42CrMo4 steel at high temperatures hot compression tests have been carried out in a Gleeble® 3800 thermomechanical tester for a temperature range that covers the material behavior from the hot forging until the Nil Ductility Temperature (1250 °C-1375 °C) and for three different orders of magnitudes for the strain rates (0.1 s−1, 1 s−1 and 10 s−1). Then, the Hansel-Spittel model, widely used in automotive commercial software as FORGE®, has been employed to obtain the adequate constants of the constitutive equation for high temperatures. Finally, the newly obtained flow behavior model has been validated by comparison between experimental and simulated compression tests and by the process simulation of a commercial automotive component comparing the results of the simulation with the already made experimental tests in a laboratory cellule of the new technology. Hence, this paper shows the procedure for the determination and the obtention of a new constitutive model for the 42CrMo4 steel flow stress characterization at a temperature range between 1250 °C–1375 °C. This will contribute in the knowledge of material flow stress behavior models at high temperatures and will allow the prediction or simulation of high temperature hot forging in closed dies processes, enhancing the possibility of the application of these technologies from an industrial point of view

Thermal exchange effects on steel thixoforming processes

Steel thixoforging is an innovative semi-solid forming process. It allows the manufacturing of complex parts and minimises the forming load. This work aims to identify and characterise the main feature zones of a thixoforging part. The material flow and the forging load are dependent on the thixoforging speed, the tool temperature and the initial temperature of the slug. The data are obtained for C38 thixoforging steel. A specific extrusion tool was designed that integrates the heating of the tool and the slug. This tool was set up on a high-speed hydraulic press. This work highlights the effects of heat exchange on the microstructure, the internal flow and the mechanical characteristics of thixoforging material. These heat exchanges depend primarily on the working speed and tool temperature. The internal flow is composed of three distinct zones. Among them, only semisolid zone is observed during working. The microstructures of thixoforming C38 steel consist of ferrite, pearlite and bainite

Thermal exchange effects on steel thixoforming processes

Steel thixoforging is an innovative semi-solid forming process. It allows the manufacturing of complex parts and minimises the forming load. This work aims to identify and characterise the main feature zones of a thixoforging part. The material flow and the forging load are dependent on the thixoforging speed, the tool temperature and the initial temperature of the slug. The data are obtained for C38 thixoforging steel. A specific extrusion tool was designed that integrates the heating of the tool and the slug. This tool was set up on a high-speed hydraulic press. This work highlights the effects of heat exchange on the microstructure, the internal flow and the mechanical characteristics of thixoforging material. These heat exchanges depend primarily on the working speed and tool temperature. The internal flow is composed of three distinct zones. Among them, only semisolid zone is observed during working. The microstructures of thixoforming C38 steel consist of ferrite, pearlite and bainite

THERMOMECHANICAL MODELLING AND SIMULATION OF C38 THIXOEXTRUSION STEEL

The present paper focuses the modelling and the simulation of a direct thixoextrusion test achieved on C38 semi-solid steel. To validate the modelling and the simulation, it is important to get various experimental informations during the test and to correlate them with simulated results. In a previous paper (Becker et al, 2008), the macro and micro structure obtained for different process parameters during thixoextrusion of C38 were investigated. In this work, those results are correlated to those obtained by simulations of the processing. The constitutive equation of the material is given by a multi-scale modelling based on micromechanics and homogenization techniques, labelled as micro-macro modelling (Favier et al, 2009). The parameters of the model are determined (i) using literature results and (ii) to match various experimental measurements obtained during the test and described in Becker et al (2008) such as the die temperature during the test and the load-displacement curve. Comparisons between experimental and simulated reveal the presence of complex temperature field and the presence of zones having very low viscosities. These zones contribute actively to the semi-solid material flow

Development of Aluminim/Steel hybrid structures by semisolid forming

Weight reduction is the most cost-effective mean to improve fuel economy and greenhouse gas emissions from the transportation sector. Combining steel and aluminum in a hybrid structure will implement the integrated load-supporting of steel with the advantages of aluminum in weight reduction. However, due to the highly differing physical and mechanical properties, metallurgical bonds between these materials are difficult to achieve with traditional welding methods. These differences in properties lead to the formation of hard and brittle intermetallic compounds along the transition line which will significantly decrease the mechanical properties of the joining area. Production of hybrid components by thixoformig of aluminum on steel is a promising method to manufacture near net shape components with good mechanical properties in a single process step. This process allows joining dissimilar materials in semisolid state at lower temperatures than traditional welding methods, causing a decrease in thickness of the intermetallic layer. In this work, the mount of the front cradle of the car was manufactured in different aluminum alloys and joined to a S355JH2 quality steel tube by thixo transverse forging method. The first part of the dissertation focuses on design and simulation of an automotive hybrid structure and the development of a semi-industrial thixoforming cell. Influence of the main processing parameters such as, solid fraction, mold temperature, compaction time and punch speed are studied. In order to determine the optimum process parameters a thorough mechanical and metallographic analysis of the manufactured components is carried out. The second part of the dissertation deals with fundamental research on the formation and evolution of intermetallic phases when joining of AlSiMg alloys to raw and coated steel, at different joining temperatures in the semisolid range. The results reveal that quality hybrid components can be achieved with this forming method.La reducción de peso es lo más rentable para mejorar la economía de combustible y reducir las emisiones de gases de efecto invernadero procedentes del sector transporte. La unión de ambos materiales permite la creación de nuevas estructuras híbridas que combinan la dureza y resistencia al desgaste de los aceros con la baja densidad de las aleaciones de aluminio. Sin embargo, debido a las propiedades físicas y mecánicas dispares de estos metales, es difícil obtener uniones metalúrgicas con métodos de soldadura tradicionales. Estas diferencias en las propiedades conducen a la formación de compuestos intermetálicos en la intercara que por su naturaleza dura y frágil resultan perjudiciales para su aplicación final. La producción de estructuras híbridas mediante tixoconformado de aluminio sobre acero en un único paso es un proceso prometedor para la fabricación de componentes funcionales con buenas propiedades mecánicas. Este proceso permite unir materiales disimilares en estado semisólido, de forma que la unión de los materiales ocurre a una temperatura inferior que en los métodos tradicionales de soldadura, provocando una disminución del espesor de la capa intermetálica. En el presente trabajo, se ha fabricado un componente real de automoción llamado brazo de la cuna delantera en diferentes aleaciones de aluminio y unido a un tubo de acero de calidad S355JH2. La primera parte de la tesis se centra en el diseño y simulación de una estructura híbrida de automoción y el desarrollo de una célula de tixoconformado semi-industrial. También, se estudia la influencia de los principales parámetros de proceso tales como, fracción sólida, la temperatura del molde, el tiempo de compactación y la velocidad de conformado. Con el fin de determinar los parámetros de proceso óptimos se ha llevado a cabo un exhaustivo análisis mecánico y metalográfico de los componentes fabricados. La segunda parte de la tesis se ocupa de la investigación fundamental de la formación y evolución de las fases intermetálicas que surgen cuando se unen aleaciones AlSiMg con acero en bruto y revestido, a diferentes temperaturas de unión del rango semisólido. Los resultados revelan que es posible obtener componentes híbridos de calidad mediante este proceso de conformado.Ibilgailuen pisua murrizteak eragin nabarmena du erregai-ekonomian eta garraioaren sektoreko berotegi-efektuko emisioak gutxitzean. Bi material horien elkarteak egitura hibrido berriak sortzeko aukera ematen du, zeinak konbinatzen baitituzte altzairuen gogortasuna eta higadurarekiko erresistentzia aluminio-aleazioen dentsitate txikiarekin. Nolanahi ere, konplexua da material desberdinen arteko elkarteak sortzea, ezaugarri desberdinak baitituzte. Ohiko aluminio-altzairu soldaduren kasuan, aluminiotan aberatsak diren metal arteko konposatuak sortzen dira; baina gogorrak eta hauskorrak direnez, kaltegarriak dira azken aplikaziorako. Bestalde, halako elkarte-teknologiek muga handiak dituzte egitura mistoaren geometriari dagokionez. Altzairuaren gaineko aluminiozko egitura hibridoak tixokonformazio bidez urrats bakarrean ekoiztea etorkizun handiko prozesua da propietate mekaniko onak dituzten osagarriak lortzeko. Prozesu honen bidez, material desberdinak elkartu daitezke bestelako soldadura-prozezuetan baino tenperatura baxuagoan eta, horrela, sortutako metalen arteko geruzaren lodiera murrizten da. Lan honetan, aluminiozko aleazio ezberdinen eta 355JH2 kalitatezko altzairuen arteko forja inguruko erdisolidoaren lotura-prozesuaren teknologia garatu da, automobil-industriarako prestazio handiko segurtasun-osagaien ekoizpenean erabili ahal izateko helburuarekin. Tesiaren lehen atalaren ardatza izan da ibilgailuetarako egitura hibrido bat diseinatzea eta simulatzea, eta tixokonformazio erdiindustrialeko zelda bat garatzea. Halaber, prozesuko parametro nagusien eragina aztertu da; besteak beste, hauena: frakzio solidoa, moldearen tenperatura, trinkotze-denbora eta konformazio-abiadura. Prozesuko parametro optimoak zein diren zehazteko, ekoitzitako osagaien analisi mekaniko eta metalografiko sakona egin da. Tesiaren bigarren atalean, bestalde, AlSiMg aleazioak altzairu gordinarekin eta estalduradunarekin elkartzean sortzen diren fase intermetalikoen formazioaren eta eboluzioaren oinarrizko ikerketa egin da. Emaitzek agerian utzi dute posible dela kalitateko osagai hibridoak lortzea konformazio-prozesu honen bitartez

A new route for semi-solid steel forging

Forging in semi-solid state significantly extends the possibilities of classical hot forging. In order to fully exploit its potential, the process requires a specific and demanding environment, penalizing its industrial deployment. In this context, an alternative route is proposed. In the proposed process, semi-solid zones at the heart of the material coexist with surrounding solid zones within the part. The outcome is an optimized process where the benefits of thixoforging are reached at a significant extent within the classical process framework of hot forging. The paper investigates this proposal up to a full-scale proof-of concept in an industrial setting.Thixofranc project (Région Lorraine and Région Champagne Ardennes, TACA project (IRT M2P) and ISEETECH (forging equipment

Inverse finite element modelling and identification of constitutive parameters of UHS steel based on Gleeble tensile tests at high temperature

The authors are grateful to the publisher, Taylor & Francis, for letting the manuscript being archived in this Open Access repository. This is an electronic version of an article that was published in Inverse Problems in Science and Engineering© 2011 Copyright: Taylor & Francis. Inverse Problems in Science and Engineering is available online at: http://www.tandfonline.com/doi/abs/10.1080/17415977.2010.518288International audienceThe rheological behaviour of an ultra high strength (UHS) steel is investigated by Gleeble tensile tests at low-deformation rates and high temperature, from 1200°C to solidus temperature. Results show that large thermal gradients exist in specimens, resulting in heterogeneous deformation, which makes the identification of constitutive parameters difficult from the directly deduced nominal stress-strain curves. The advantages of an inverse identification method - associating a direct finite element model of Gleeble tests and an optimization module - are demonstrated in such conditions. The constitutive parameters identified by this technique have been successfully applied to additional tests, more complex in nature than those used for the identification of parameters. However, such tests combining successive loading and relaxation stages have revealed some limitations of the considered constitutive model

Modeling and validations of control parameters for material extrusion-based additive manufacturing of thixotropic aluminum alloys.

Additive Manufacturing (AM) with metals has been accomplished mainly through powder bed fusion processes. Initial experiments and simulations using Material Extrusion Additive Manufacturing (MEAM) have been performed by various researchers especially using low melting alloys. Recently Stratasys Inc. submitted a patent application for the use of their Material Extrusion technology also called Fused Deposition Modeling (FDM) where they describe the process using thixotropic semi-solid alloys. Currently this process using semi-solid, engineering type alloys such as A356 or THIXALLOY 540 aluminum have not been researched to evaluate the control parameters. This research combines the in-depth knowledge of applying thixotropic semi-solid aluminum alloy processing as used in thixocasting and thixoforming with the MEAM research. Successful implementation of this metal AM process category besides powder bed fusion would result in the gain of certain MEAM process advantages like speed and ease of material handling (filament) for metal AM. In this dissertation thixotropic aluminum alloys have been identified for their applicability for MEAM and optimal pre-processing as well as thermo-mechanical handling in a nozzle has been identified. A review of the optimal heating temperature for an aligned quality of microstructure were completed to provide experimental proof of thixotropic aluminum alloy applicability. As further research aging of such alloys during isothermal holding while pausing or pure movement of a MEAM nozzle will help to derive the required cleaning processes in case the alloy develops an in-adequate microstructure. The research results build the basis for the next phases towards a larger project goal of developing a successful MEAM machine for producing aluminum alloy parts