Experimental and modelling techniques for hot stamping applications

Hot stamping techniques have been developed for the production of complex-shaped components since the 1970s, increasingly used for the automotive industry. The application of these techniques includes hot stamping of boron steel for critical automobile safety components, and solution heat treatment, forming and cold die quenching (HFQ®) for forming complex-shaped high strength aluminium panels of automobile bodies and chassis structures. The developed forming techniques need dedicated experimental testing methods to be improved for characterising the thermomechanical behaviour of materials at the hot stamping conditions, and advanced materials modelling techniques to be developed for hot stamping applications. In this paper, requirements for thermomechanical tests and difficulties for hot stamping applications are introduced and analysed. The viscoplastic modelling techniques have been developed for hot stamping applications. Improved experimental methods have been proposed and used in order to obtain accurate thermomechanical uniaxial tensile test data and determine forming limits of metallic materials under hot stamping conditions

Determination of heat transfer coefficient for hot stamping process

© 2015 The Authors.The selection of the heat transfer coefficient is one of the most important factors that determine the reliability of FE simulation results of a hot stamping process, in which the formed component is held within cold dies until fully quenched. The quenching process could take up to 10. seconds. In order to maximise the production rate, the optimised quenching parameters should be identified to achieve the highest possible quenching rate and to reduce the quenching time. For this purpose, a novel-testing rig for the Gleeble 3800 thermo- mechanical simulator was designed and manufactured, with an advanced control system for temperature and contact pressure. The effect of contact pressure on the heat transfer coefficient was studied. The findings of this research will provide useful guidelines for the selection of the heat transfer coefficient in simulations of hot stamping processes and useful information for the design of hot stamping processes

Holding fixture for a hot stamping press

A hand held guide for manually positioning a work piece between the anvil rib and tool of a hot die stamping press is described. A groove completed by interchangeable cover plates attached at one end of the guide conforms to a cross sectional dimension common to similar workpieces and, with a force fit, retentively holds each of the workpieces

Tribological Behavior Of Laser Textured Hot Stamping Dies

Hot stamping of high strength steels has been continuously developed in the automotive industry to improve mechanical properties and surface quality of stamped components. One of the main challenges faced by researchers and technicians is to improve stamping dies lifetime by reducing the wear caused by high pressures and temperatures present during the process. This paper analyzes the laser texturing of hot stamping dies and discusses how different surfaces textures influence the lubrication and wear mechanisms. To this purpose, experimental tests and numerical simulation were carried out to define the die region to be texturized and to characterize the textured surface topography before and after hot stamping tests with a 3D surface profilometer and scanning electron microscopy. Results showed that laser texturing influences the lubrication at the interface die-hot sheet and improves die lifetime. In this work, the best texture presented dimples with the highest diameter, depth, and spacing, with the surface topography and dimples morphology practically preserved after the hot stamping tests

New developments of formability evaluation methods for hot stamping

Formability is an essential material property that needs to be considered when selecting materials for hot stamping applications. Due to the difficulties of achieving rapid cooling before deformation and the failure of lubricant systems, however, it is challenging to use conventional Nakajima and Marciniak tests to evaluate the formability of materials under hot stamping conditions. Recently, biaxial test methods have shown great potential to overcome this challenge. In this paper, recent developments of the biaxial test methods for formability evaluation are reviewed, including testing machines, specimen designs, specimen heating methods, testing procedures, and limit strain determination methods. Compared to the Nakajima or the Marciniak tests, the biaxial test method can provide better simulation for hot stamping conditions and it can be a promising method for evaluating the formability of sheet metals under hot stamping conditions. However, more developments such as the standardisation of the specimen designs and the limit strain determination methods, are still needed for the wide use of the biaxial test method

Concept Validation for Selective Heating and Press Hardening of Automotive Safety Components with Tailored Properties

© (2014) Trans Tech Publications, Switzerland.A new strategy termed selective heating and press hardening, for hot stamping of boron steel parts with tailored properties is proposed in this paper. Feasibility studies were carried out through a specially designed experimental programme. The main aim was to validate the strategy and demonstrate its potential for structural optimisation. In the work, a lab-scale demonstrator part was designed, and relevant manufacturing and property-assessment processes were defined. A heating technique and selective-heating rigs were designed to enable certain microstructural distributions in blanks to be obtained. A hot stamping tool set was designed for forming and quenching the parts. Demonstrator parts of full martensite phase, full initial phase, and differentially graded microstructures have been formed with high dimensional quality. Hardness testing and three point bending tests were conducted to assess the microstructure distribution and load bearing performance of the as-formed parts, respectively. The feasibility of the concept has been validated by the testing results

Hot stamping of titanium alloys

Demand for low density and high strength materials in the aviation sector has expanded greatly due to ambitious carbon emission and fuel consumption targets. In order to meet these targets, manufacturers have focused on weight reduction via the use of lightweight materials. In the aerospace sector, high strength structural components are made from titanium alloys. However, the forming of complex-shaped components from titanium alloys is time, energy and cost intensive. One promising solution to overcome these difficulties proposed in the literature is using the hot stamping process to form complex-shaped components from sheet metal with cold dies, and rapidly quenching the workpiece in the dies simultaneously. The hot stamping process promises to reduce the tool wear commonly found in conventional hot forming processes and be an overall more efficient and economical process when compared to conventionally used isothermal hot forming techniques. A novel hot stamping process for titanium alloys using cold forming tools and a hot blank was studied systematically in this thesis. This work aims to investigate the microstructural evolution and flow behavior of a titanium alloy (Ti6Al4V) under hot stamping conditions experimentally, and to model these parameters using the constitutive equations proposed. The material behavior was modelled using mechanism-based viscoplastic constitutive equations to replicate the material response of a two-phase titanium alloy Ti6Al4V under hot stamping conditions. Finally, the developed model's accuracy was validated by comparing to experimental uniaxial tensile tests and microstructural maps of the deformed alloy. Microstructural analysis revealed that the heating and soaking conditions are vital to the microstructure and post-form strength, whereas the plastic deformation during the hot stamping only has a negligible effect on both recrystallization and phase transformation due to the very short deformation time. The developed material model was implemented into the Finite Element (FE) simulation to study the deformation characteristics during the hot stamping process. The verified simulation data were analysed through a novel hot stamping technique with good agreements achieved between the predicted and experimental results. A complex shaped wing stiffener panel component was successfully formed from TC4 titanium alloy, demonstrating the great potential of investigated technology in forming complex shaped titanium alloys components. Finally, Fast light Alloys Stamping Technology (FAST) is proposed for titanium alloys, where fast heating to a twophase titanium alloy sheet with equiaxed microstructure is employed.Open Acces

The comparison of two continuum damage mechanics-based material models for formability prediction of AA6082 under hot stamping conditions

The hot stamping and cold die quenching process has experienced tremendous development in order to obtain shapes of structural components with great complexity in automotive applications. Prediction of the formability of a metal sheet is significant for practical applications of forming components in the automotive industry. Since microstructural evolution in an alloy at elevated temperature has a large effect on formability, continuum damage mechanics (CDM)-based material models can be used to characterise the behaviour of metals when a forming process is conducted at elevated temperatures. In this paper, two sets of unified multi-axial constitutive equations based on material's stress states and strain states, respectively, were calibrated and used to effectively predict the thermo-mechanical response and forming limits of alloys under complex hot stamping conditions. In order to determine and calibrate the two material models, formability tests of AA6082 using a developed novel biaxial testing system were conducted at various temperatures and strain rates under hot stamping conditions. The determined unified constitutive equations from experimental data are presented in this paper. It is found that both of the stress-state based and strain-state based material models can predict the formability of AA6082 under hot stamping conditions

Development of materials characterisation and modelling methods under hot stamping conditions

Hot stamping of sheet steels is used widely in the automotive industry to form safety panel components with complex shapes and high strength. Characterisation and modelling of the formability of materials under hot stamping conditions are essential for the process optimisation and applications. Due to the high austenitic temperature in hot stamping, however, it is difficult to evaluate the formability of the materials under these conditions using existing standard tests such as the Nakajima and Marciniak tests. In this thesis, a novel biaxial testing method has been developed to determine forming limit curves (FLCs) which are the most commonly used tools for evaluating the formability, and fracture forming limit curves (FFLCs). Furthermore, a set of continuum damage mechanics (CDM)-based unified viscoplastic constitutive equations has been formulated to predict both FLCs and FFLCs for the materials under hot stamping conditions. The biaxial testing method has been applied to determine an FLC for AA5754 at room temperature, and the FLC determined has been compared with that obtained using the Nakajima test. The fracture occurred near the centre of the test specimens in all straining conditions used, and the FLCs determined using the two methods were comparable. More importantly, the biaxial testing method has been extended and applied to determine both FLCs and FFLCs for boron steel under hot stamping conditions. The resulting test specimens had a nearly uniform temperature distribution in the gauge area and fractured close to the centre of this area under all test conditions investigated. The CDM-based constitutive equations have been developed from a set of dislocation-based hardening constitutive equations, with two damage variables to model the accumulated damage leading to localised necking and fracture separately. The material constants in the equations have been determined from the curve fitting of the experimental FLCs and FFLCs. Good agreement between the computed and experimental results was observed, indicating the high flexibility of the CDM-based constitutive equations for predicting FLCs and FFLCs under hot stamping conditions.Open Acces

Two-Scale Thermomechanical Simulation of Hot Stamping

Hot stamping is a hot drawing process which takes advantage of the polymorphic steel behavior to produce parts with a good strength-to-weight ratio. For the simulation of the hot stamping process, a nonlinear two-scale thermomechanical model is suggested and implemented into the FE tool ABAQUS. Phase transformation and transformation induced plasticity effects are taken into account. The simulation results regarding the final shape and residual stresses are compared to experimental findings