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

Design optimization of hot stamping tooling produced by additive manufacturing

The design flexibility of Additive Manufacturing (AM) can be utilized to develop innovative and sustainable hot stamping tools with enhanced quenching capability compared to tools manufactured by conventional manufacturing processes. This study proposes a concept for hot stamping tools with integrated lattice structures that selectively substitute a die's solid areas. A lattice structure demonstrates reduced thermal mass and can affect the ability of the tool to absorb heat from the blank and the rate at which the tool is cooled between two consecutive stamping cycles. This study explores the design space of a hot stamping tool with integrated lattice structures. It presents the optimized design for an effective compromise between cooling performance, structural integrity, and several other design parameters shown in the study. The proposed method utilizes a 2D thermo-mechanical finite element analysis model of a single cooling channel combined with Design of Experiments (DoE) to reduce the computational cost. The results show that the integration of lattice structure cannot only deliver improved cooling performance with minimum change in the dimensions of the cooling system but also achieves a faster AM build time since less material is required to be printed

Assessing Mechanical Performance of Dissimilar Steel Systems Made Via Wire-Arc Additive Manufacturing

Hot stamping is part of a specific type of metalworking procedure widely used in the automotive industry. This research seeks to help make hot stamp tooling component production more cost-effective by using large-scale additive manufacturing. Additive manufacturing can produce dissimilar steel components that can be more cost-effective and time-efficient and allow for complex geometries to be made. A dissimilar steel system consisting of 410 martensitic stainless steel and AWS ER70S-6 mild steel is proposed to make hot stamps, making them more cost-efficient. However, the material interface\u27s mechanical behavior in 410SS-mild steel additively manufactured material systems is not well understood. This research seeks to find how these dissimilar hot stamps can potentially fail during service. To assess the mechanical behavior of the material interface, mechanical testing by way of hardness testing, thermal expansion testing, fatigue testing, and microscopic imaging were performed. Samples were heat-treated, and fatigue tests were designed to run for 1200 cycles at a temperature range of 200-600$^{\circ}$C. Fatigue test results show that, as expected, all four samples went through plastic deformation, with hardness test results used to confirm this behavior. Microscopy was done to show the post-test microstructure that shows potential evidence of plastic deformation sites. One of the materials in the dissimilar system did not meet the hardness requirements for hot stamping applications, but recommendations are made to address this

Formability investigations for the hot stamping process

International audienceArcelor Research is developing a numerical tool to support the feasibility analysis and to optimize the design of hot stamped parts made of USIBOR 1500P®. To provide formability data and to feed the development of a fracture criterion, experimental hot stamping tests are carried out at Cemef (Centre for Material Forming). These hot stamping experiments are based on a modified Nakazima-type test. Results reveal that the achievable strain levels depend on process parameters (stroke, velocity, temperature, friction and heat exchange) and blank parameters (initial temperature, thickness and shape). In parallel, a numerical model of these hot stamping tests has been developed with finite element softwares (Forge2®, Forge3® and Abaqus). The numerical simulations confirm the location and the magnitude of the blank thinning. Furthermore, the numerical results are similar to the experimental measurements in terms of punch load, cooling rate and strain distribution. A formability analysis is then performed to study the influence of the blank geometry and the blank temperature on formability

Thermomechanical analysis of additively manufactured bimetallic tools for hot stamping

A comparison between a conventional AISI H13 hot stamping tool and a bimetallic tool consisting of an AISI 1045 core and a laser-deposited AISI H13 coating is performed. In order to analyze the performance of bimetallic tools, the material compatibility and quality of the coating are analyzed. Besides, the mechanical properties are evaluated and compared with those of the conventional tool, obtaining mechanically equivalent results. Nevertheless, the real conductivity of the laser deposited AISI H13 is found to be 16 % lower than the theoretical value. Hence, a thermal model of the hot stamping process is developed, and the performance of various coating thicknesses is evaluated. Results show that, in the present case study, an AISI 1045 tool with a 1 mm AISI H13 coating ensures the mechanical properties and reduces the cycle time by 44.5 % when compared to a conventional AISI H13 tool.e authors gratefully acknowledge the financial support for this study from the European Union, through the H2020-FoF132016 PARADDISE project (contract number 723440) and from the Spanish Ministry of Economy and Competitiveness for the support on the DPI2016-79889-R INTEGRADDI project

Advanced high strength steel in auto industry: An overview

The world’s most common alloy, steel, is the material of choice when it comes to making products as diverse as oil rigs to cars and planes to skyscrapers, simply because of its functionality, adaptability, machine-ability and strength.Newly developed grades of Advanced High Strength Steel (AHSS) significantly outperform competing materials for current and future automotive applications.This is a direct result of steel’s performance flexibility, as well as of its many benefits including low cost, weight reduction capability, safety attributes, reduced greenhouse gas emissions and superior recyclability.To improve crash worthiness and fuel economy, the automotive industry is, increasingly, using AHSS. Today, and in the future, automotive manufacturers must reduce the overall weight of their cars.The most cost-efficient way to do this is with AHSS. However, there are several parameters that decide which of the AHSS types to be used; the most important parameters are derived from the geometrical form of the component and the selection of forming and blanking methods. This paper describes the different types of AHSS, highlights their advantages for use in auto metal stamping s, and discusses about the new challenges faced by stampers, particularly those serving the automotive industry

High-efficiency forming processes for complex thin-walled titanium alloys components: State-of-the-art and Perspectives

Complex thin-walled titanium alloy components play a key role in the aircraft, aerospace and marine industries, offering the advantages of reduced weight and increased thermal resistance. The geometrical complexity, dimensional accuracy and in-service properties are essential to fulfill the high-performance standards required in new transportation systems, which brings new challenges to titanium alloy forming technologies. Traditional forming processes, such as superplastic forming or hot pressing, cannot meet all demands of modern applications due to their limited properties, low productivity and high cost. This has encouraged industry and research groups to develop novel high-efficiency forming processes. Hot Gas Pressure Forming (HGPF) and hot stamping-quenching technologies have been developed for the manufacture of tubular and panel components, and are believed to be the cut-edge processes guaranteeing dimensional accuracy, microstructure and mechanical properties. This article intends to provide a critical review of high-efficiency titanium alloy forming processes, concentrating on latest investigations of controlling dimensional accuracy, microstructure and properties. The advantages and limitations of individual forming process are comprehensively analyzed, through which, future research trends of high-efficiency forming are identified including trends in process integration, processing window design, full cycle and multi-objective optimization. This review aims to provide a guide for researchers and process designers on the manufacture of thin-walled titanium alloy components whilst achieving high dimensional accuracy and satisfying performance properties with high efficiency and low cost

Influence of the connection between forming die and heatpipe on the heat transfer

Hot forming tools are exposed to cyclically changing thermal loads. These conditions are caused by the heat exchange between tool and workpiece during forming followed by spray cooling. This can lead to crack initiation and tool failure. A continuous cooling with heatpipes (HP) inside the active tool components could prevent this. HP use a circular flow of a cooling fluid inside a closed tube, often made of copper. Previous studies showed an influence of the connection by thermal paste between the forming die and the HP, its orientation, as well as its inner surface structure. The use of paste proved essential for closing the contact by filling the microscopic air pockets between the surfaces. Only sintered inner structures can be used for force fit, since others are damaged by deformation and thus lose their efficiency. This research paper deals with the influence of the form and force fit between die and HP. To test the impact, HP were connected with heated model dies on one side and an aluminium block (AB) on the other. Thermocouples were used to monitor the temperature of both, the AB and the model dies. The measured temperature and time difference, the weight and the thermal capacity of the AB were used to calculate the heat flow. Different inner surface structures of HP were varied in addition to their fitting type with the model die. The best heat transfer was achieved by using HP with sintered inner structure and force-fit, resulting in nearly full-surface contact

Tribological behaviour of high thermal conductivity tool steels for hot stamping

In the last years, the use of High Strength Steels (HSS) as structural parts in car manufacturing, has rapidly increased thanks mainly to their favourable strength to weight ratios and stiffness, which allow a reduction of the fuel consumption to accommodate the new restricted regulations for CO2 emissions control, but still preserving or even enhancing the passengers’ safety. However, the formability at room temperature of HSS is poor, and for this reason, complex-shaped HSS components are produced applying the plastic deformation of the sheet metal at high temperature. The use of hot stamping technology, which was developed during the 70’s in Sweden, has become increasingly used for the production of HSS for the car body-in-white. By using this technology, several improvements have been made, if compared with the forming at room temperature, such as the reduction of spring back and the forming forces, the production of more complex shapes, a more accurate microstructure control of the final piece and the achievement of components with high mechanical properties. The hot stamping process of HSS parts consists mainly in heating a metal sheet up to austenitization temperature and then a simultaneous forming and hardening phase in closed dies, water-cooled, to obtain a fully martensitic microstructure on the final components; in this way, ultimate tensile strength passes from 600 MPa up to 1500-1600 MPa. Anyway, several tribological issues arise when the die and metal sheet interact during the forming process at elevated temperatures; the absence of any types of lubricant due to elevate process temperature and in order to preserve the quality of the part for the later stages of the process chain, leads to high friction forces at interface; moreover, and the severe wear mechanisms together with surface damage of forming dies, can alter the quality of the component and can also have an high impact on the process economy due to frequent windows-maintenance or reground of tools. Furthermore, considering that the thermal conductivity of the die material influences the cooling performance, obtained during the quenching phase, and being the quenching time the predominant part of the cycle time, the productivity of the process is influenced too. On this base tool steels play a capital role in this process, as they strongly influence the properties of the obtained final product and have a strong impact to investment and maintenance costs. The survey of the technical and scientific literature shows a large interest in the development of different coatings for the blanks from the traditional Al-Si up to new Zn-based coating and on the analysis of hard PVD, CVD coatings and plasma nitriding, applied on dies. By contrast, fewer investigations have been focused on the development and test of new tools steels grades capable to improve the wear resistance and the thermal properties that are required for the in-die quenching during forming. The research works reported are focused on conventional testing configurations, which are able to achieve fundamental knowledge on friction behaviour, wear mechanisms and heat transfer evaluation, with both a high accuracy for the process parameters and less information about situations that replicate the thermal-mechanical conditions to which the forming dies are subject during the industrial process. Alternatively, the tribological performance have been studied through costly and time-consuming industrial trials but with a lower control on process parameters. Starting from this point of view, the main goal of this PhD thesis is to analyse the tribological performance in terms of wear, friction and heat transfer of two new steel grades for dies, developed for high-temperature applications, characterized by a High Thermal Conductivity with the purpose to decrease the quenching time during the hot stamping process chain and overcome the limits in terms of process speed. Their performances are compared with a common die steel grade for hot stamping applications. To this aim, a novel simulative testing apparatus, based on a pin on disk test, specifically designed to replicate the thermo-mechanical cycles of the hot stamping dies, was used to evaluate the influence of different process parameters on the friction coefficient, wear mechanisms and heat transfer at interface die-metal sheet. Unlike other research works reported in the literature, which individually analyse the friction, the wear mechanisms and thermal aspects, by means of the methodology used in this thesis, the tribological characterization as a whole is obtained by means of a single approach, in order to analyse the simultaneous global evolution of the tribological system

The Bending Magnets for the Proton Transfer Line of CNGS

The project "CERN neutrinos to Gran Sasso (CNGS)", a collaboration between CERN and the INFN (Gran Sasso Laboratory) in Italy, will study neutrino oscillations in a long base-line experiment. High-energy protons will be extracted from the CERN SPS accelerator, transported through a 727 m long transfer line and focused onto a graphite target to produce a beam of pions and kaons and subsequently neutrinos. The transfer line requires a total of 78 dipole magnets. They were produced in the framework of an in-kind contribution of Germany via DESY to the CNGS project. The normal conducting dipoles, built from laminated steel cores and copper coils, have a core length of 6.3 m, a 37 mm gap height and a nominal field range of 1.38 T - 1.91 T at a maximum current of 4950 A. The magnet design was a collaboration between CERN and BINP. The half-core production was subcontracted to EFREMOV Institute; the coil fabrication, magnet assembly and the field measurements were concluded at BINP in June 2004. The main design issues and results of the acceptance tests, including mechanical, electrical and magnetic field measurements, are discussed