Elastic behaviour characterisation of TRIP 700 steel by means of loading-unloading tests

The elastic behaviour of TRIP 700 steel under plastic deformation is analysed. The analysis is carried out by means of classical tensile test and loading–unloading cyclic tests. These tests have been performed using high deformation strain gages, which enable an accurate and continuous measurement of strain. An elastic modulus reduction of 20% is observed for 12% plastic deformation. Furthermore, non-linear unloading and loading paths have been found in this work. This is an important difference with respect to other authors and opens new possibilities for the development of new material models to improve the prediction of the post-forming springback of industrial parts, which is an important issue for the automotive industry

Strain path's influence on the elastic behaviour of theTRIP 700 steel

This paper deals with the analysis of thestrain path's influence on the elastic behaviour of TRIP700 steel; it aims to validate the cyclic testing method to characterise inelastic behaviour of advanced high strength steels (AHSS). Different cyclic tests are done, where the strain path is changed from test to test. Large deformation strain gages are used to determine the inelastic behaviour of the specimens at macro-level. At a lower scale, stress measurements are carried out using the XRD technique during an in-situ tensile test: ferrite and austenite phases’stresses are measured before unloading and after loading again to study the strain path's influence. By means of this work it is confirmed that the elastic strain path has no influence on the unloading–loading of this TRIP steel. These results prove that conventional loading–unloading cyclic testing is a valid methodology for a detailed characterisation of the elastic modulus and reliable numerical modelling of springback

High-Speed Material Characterization Using an Instrumented Forging Hammer

Hammer forging is a widely employed manufacturing process to produce parts with excellent mechanical properties. Although the rheological behavior and the microstructural transformation phenomena of metals under hammer forging conditions are of great industrial interest, few materials have been tested in such intermediate strain rates (10–103 s−1) due to the lack of laboratory machines for intermediate speed testing. With the objective of addressing that gap, this paper presents a novel automatic forging simulator comprising an instrumented forging hammer capable of performing intermediate speed deformations, up to 5 m/s. Three data acquisition approaches were evaluated to select the most appropriate approach and obtain valid rheological data from intermediate strain rate tests performed on the developed hammer. First, the data obtained by both a high-speed camera and a load cell was combined to calculate reference flow curves. Then, two additional data monitoring approaches were then analyzed, employing independently first the high-speed camera and then the load cell data. It was concluded that flow curves obtained utilizing only the load cell data offered accurate results without the need for an expensive and complex high-speed camera

Hardening prediction of diverse materials using the Digital Image Correlation technique

In recent years, due to the introduction of higher resistance materials in the automotive sector, sheet metal-forming tool-makers have been forced to deal with more challenging process designs. Therefore, the optimisation of the manufacturing process has become a key factor in obtaining a part which fits the required tolerances, and the finite element method (FEM) is the most widely used technique to speed up that optimisation time. However, to obtain a numerical result as close as possible to those of industrial conditions, the FEM software inputs must be highly accurate. The present work is focused on the hardening extension of the currently available reduced-formability materials, as it is a key factor in the correct prediction of the stress state and hence, of the springback during a sheet metal-forming process. The objective in this work was the selection of the most appropriate hardening model to extend the flow curve beyond the necking limit for a wide variety of material families currently utilised in the industrial environment. To carry out that analysis, a digital image correlation (DIC) technique was utilised during conventional tensile tests to extend the experimental flow curves of the analysed materials. Commonly used hardening models were fitted to the experimental tensile flow curves with the aim of selecting the model that best predicts the hardening behaviour of each analysed material family. The results showed that the DIC technique was valid for the extension of the hardening curve of the analysed materials and for the final selection of the most suitable hardening model for each analysed material family

Comparison of the hardening behaviour of different steel families : from mild and stainless steel to advanced high strength steels

Although steel has been used in vehicles from the automotive industry's inception, different steel gradesare continually being developed in order to satisfy new fuel economy requirements. For example,advanced high strength steel grades (AHSS) are widely used due to their good strength/weight ratio.Because each steel grade has a different microstructure composition, they show different behaviourswhen they are subjected to different strain paths in forming processes. Materials with high yieldstrength tend to be influenced by phenomena of cyclic plasticity such as the Bauschinger Effect, whilelow yield strength materials tend to harden when they are subjected to cyclic loading.Different steel grades are used in different forming processes, which are usually optimised bynumerical tools such as Finite Element Models. This method requires proper hardening rules in order tocorrectly predict the real behaviour of the materials. For instance, AHSS are usually well modelled bymeans of mixed isotropic–kinematic hardening models.The methodology for developing a mixed hardening model to be implemented infinite elementcodes and simulate sheet forming processes requires three steps: (i) an appropriate experimental test toobtain stress–strain curves, (ii) a model able to predict accurately the behaviour of the material and (iii) aparameter identification method. Currently, there are few studies which analyse and model thehardening behaviour of different steel families following the same methodology. In this work, a widerange of steels from low to high yield strengths were characterised and their hardening behaviourmodelled with the same methodology so as to provide comparative data.In particular, the Chaboche and Lemaitre hardening model was successfullyfitted to the experi-mental stress–strain curves obtained from a tension–compression test. The test was performed at lowcyclic deformations (72%) due to the limitation of the test to achieve higher deformations during thecompression without buckling. Therefore, this modelization is useful for low deformation processes suchas the roll levelling process (Silvestre; 2013, Silvestre et al.Steel Res Int; 2012, 1295), in which themaximum deformations achieved are lower than 2%

Characterization of Ti64 forging friction factor using ceramic coatings and different contact conditions

Hot forging processes are highly influenced by the contact conditions between the billet and the dies. A wrong definition of the contact conditions may lead to wrong predictions of the final component geometry, the quantity of material necessary to fill in the cavity, the wear of the tools and the force necessary to manufacture the component. Furthermore, when dealing with titanium alloys, the alpha case formation due to oxidation is critical. For that reason, ceramic coatings are used to prevent billet oxidation during the heating stage and to improve the material flow lowering the friction coefficient between the billet material and tooling. In the present study, Ring Compression tests and T-Shape tests are carried out using ceramic-coated samples and friction behavior of Ti64 in contact with heated tool steel is studied. The final aim of the study has been to analyze the same tribo-system but having different contact pressures, sliding velocities and surface enlargement factors, which could affect the coating behavior. As a result, the friction coefficients are calculated for the above-mentioned tribo-tests by the comparison of the experimental data and numerical simulation results

Simulation of Cold Forging Processes Using a Mixed Isotropic-Kinematik Hardening Model

Cold forging is a manufacturing process where a bar stock is inserted into a die and squeezed with a second closed die. It is one of the most widely used chipless forming processes, often requiring no machining or additional operations to get tight tolerances. Because materials to be formed are increasingly harder and the geometrical complexity is greater, the finite element simulation is becoming an essential tool for process design. This study proposes the use of the Chaboche hardening model for the cold forging simulation of a 42CrMoS4Al material industrial automotive ball pin. The material model has been fitted with experimental data obtained from cyclic torsion tests at different reversal plastic strains as well as monotonic torsion tests at different strain rates. Comparison between the classical isotropic hardening and the new mixed hardening model are presented for the different forging steps

New drawbead tester and numerical analysis of drawbeads closure force

Currently, a great deal of controversy exists regarding the real forces generated in drawbeads during sheet metal forming processes. The present work focuses on the analysis of the uplift force. First, a detailed literature review is carried out to analyse previous experimental procedures used to measure uplift forces. It is found that previous setups do not perfectly replicate the real geometry of industrial drawbeads. In order to obtain reliable forces, an experimental drawbead tester capable of adequately replicating industrial drawbeads is developed. Later, a variety of steels ranging from mild steels to 3rd-generation ultra-highstrength steels are tested and reliable uplift and also restraining force values are obtained. The main purpose of the work is to share with the research community reliable experimental data that allows precise evaluation of the accuracy of current drawbead models and that supports the generation of new numerical and equivalent drawbead models. In parallel to the experimental procedure, a step forward in the understanding of the drawbead closing phenomena is also achieved through a 2D numerical model. The main purpose of the model is to identify the variables that greatly affect uplift force. Going beyond previous studies, in which some variables were analysed, the present work covers, in a holistic manner, the impact that material properties, the geometry of drawbeads and contact behaviour between sheet and drawbead have on the uplift force. It is determined that surprisingly minor geometrical deviations in the drawbead nominal geometry have a large impact on the uplift force

Contact pressure, sliding velocity and viscosity dependent friction behavior of lubricants used in tube hydroforming processes

The final quality of sheet and tube metal formed components strongly depends of the tribology and friction conditions between the tools and the material to be formed. Furthermore, it has been recently demonstrated that friction is the numerical input parameter that has the biggest effect in the numerical models used for feasibility studies and process design. Industrial dedicated software packages have introduced friction laws which are dependent on sliding velocity, contact pressure and sometimes strain suffered by the sheet and currently, temperature dependency is being implemented as it has also major effect on friction. This last dependency on temperature is attributed to the viscosity change of the lubricant with temperature. In this work, three lubricant having different viscosity have been characterized using the tube sliding test. The final aim of the study is to obtain friction laws that are contact pressure and sliding velocity dependent for their use in tube hydroforming modelling. The tests, performed at various contact pressures and velocities, demonstrate that viscosity has a major effect on friction. As shown in the literature, the friction coefficient is also varying with the contact pressure and sliding velocity

Strain path's influence on the elastic behaviour of theTRIP 700 steel

This paper deals with the analysis of thestrain path's influence on the elastic behaviour of TRIP700 steel; it aims to validate the cyclic testing method to characterise inelastic behaviour of advanced high strength steels (AHSS). Different cyclic tests are done, where the strain path is changed from test to test. Large deformation strain gages are used to determine the inelastic behaviour of the specimens at macro-level. At a lower scale, stress measurements are carried out using the XRD technique during an in-situ tensile test: ferrite and austenite phases’stresses are measured before unloading and after loading again to study the strain path's influence. By means of this work it is confirmed that the elastic strain path has no influence on the unloading–loading of this TRIP steel. These results prove that conventional loading–unloading cyclic testing is a valid methodology for a detailed characterisation of the elastic modulus and reliable numerical modelling of springback