Direct Hot Stamping Research Articles

Flexible N-Doped Graphene Electrodes Fabricated via Rapid Direct Hot Stamping for Microsupercapacitors

Flexible N-Doped Graphene Electrodes Fabricated via Rapid Direct Hot Stamping for Microsupercapacitors

Study of the applicability of 22MnB5 sheet metal as protective masks to improve tool life in hot forging process

Low boron steels are the only class of steel capable of forming a fully martensitic microstructure after hot stamping when a cooled tool is used, and the maximum stress can reach 1500 MPa. The purpose of this work is to study the applicability of 22MnB5 sheet metal as protective masks in hot forging dies. Sheets that were 1.3 mm thick were cut in samples of 110 mm diameter and heat treated in conditions similar to direct hot stamping and placed on the hot forging die. Cycles of 25, 50, 75, and 100 forgings were performed, and at each cycle, the metallic mask was replaced for characterization. Microhardness profiles, micrographic analysis by optical and electron microscopy, and profilometry profiles were used to determine the occurrence of surface wear on the masks after each forging cycle. The main types of superficial wear were abrasive and plastic deformation. However, the masks showed high resistance to wear and did not present failures that would make them unusable until the studied condition of a cycle of 100 forgings.

Characterization of tribological conditions within direct hot stamping

Abstract The aim of this study is to create a detailed process understanding regarding the tribological conditions within direct hot stamping. Hot stamping has established as a common technology for manufacturing safety-relevant components in modern car bodies. However, due to the high forming temperatures within hot stamping applications, no suitable lubricants have been developed yet. As a consequence, high friction and severe wear occur during the forming. This affects the tool wear as well as the resulting part quality. For improving the robustness and the efficiency of industrial hot stamping processes, future measures to reduce the tribological loads have to be found. The key to the development of such measures is a detailed process understanding of the tribological conditions. Within this study, friction and wear are analyzed for different workpiece and tool temperatures, relative velocities and contact pressures. Based on the experimental results, detailed knowledge of the critical cause-effect relations between process parameters and tribological behavior is acquired. The friction behavior is analyzed via strip drawing experiments under hot stamping conditions. The amount of wear is determined via confocal microscopy measurements before and after the experiments. The results of this study help to improve the process understanding of the tribological conditions within the hot stamping process and to develop future measures for reducing tool wear.

Numerical modeling of the 22MnB5 formability at high temperature

Abstract It has been recognized in automotive sector that the use of hot stamped parts for the automobile body-in-white allows significant weight saving and reduction of CO2 emissions. The weight reduction is accompanied by high stiffness and increased crashworthiness, which improve the passengers’ safety and allow the development of more complex vehicle’s design. In direct hot stamping, the 22MnB5 sheets are heated above the austenitization temperature, and then, formed and quenched in cooled dies by using a minimum cooling rate of 27°C/s in order to obtain the direct martensitic transformation from the initial austenite. To evaluate part feasibility, minimize production cost and improve process robustness in hot stamping, numerical simulations are becoming increasingly important, where data regarding the formability of the material at high temperature plays a central role in the simulation. A survey of the literature shows that the complex aspects of the process have been extensively investigated from an experimental point of view, but limited studies concerning the numerical modeling of the process can be found. The aim of the present paper is to explore the critical aspects of the numerical modeling in terms of material formability in hot stamping. Numerical simulations of hot Nakajima tests at the critical temperature of 600°C were performed by using explicit numerical method, while experimental tests are carried out in the same condition. The comparison of the numerical results and the experimental data demonstrates the influence that the thermal cycle applied during the experimental tests may have, since it governs the phase transformations that affect the material formability.

Phase Composition and Microstructure of Zn Coated Press Hardening Steels for Direct Hot Stamping

Decarburization and oxidation occur during production of press hardening steels (PHS) used in automotive. Therefore, coatings are applied on the steel surface to prevent this. The most common Zn coatings have not been processable by available direct hot stamping technology ordinarily used to produce the PHS and have been replaced by Al-Si coatings. However, the Al-Si coatings have a negative impact on joining technologies and further processing. Recently, the modification of direct hot stamping, which enables process the PHS with Zn coatings, was introduced. We investigated the microstructure, phase composition and distribution of particular phases (SEM-EDS, XRD), as well as micro-hardness of the Zn coated PHS prepared by the modified direct hot stamping. The PHS revealed the coating with 25 ± 3 μm thickness and continuous interface consisting of α-Fe(Zn) solid solution, Γ and Γ1 intermetallic phases and zinc oxides enriched with manganese. The thorough description of the material is important in development of further processing steps such as joining technologies and corrosion resistance.

Investigation of the influence of process parameters on adhesive wear under hot stamping conditions

Current challenges like increasing safety standards and reducing fuel consumption motivate lightweight construction in modern car bodies. Besides using lightweight workpiece materials like aluminum, hot stamping has been established as a key technology for producing safety relevant components. Producing hot stamped parts out of ultra-high strength steels offers the possibility to improve the crash performance. At the same time the weight of car structure is reduced by using thinner sheet thicknesses. In order to avoid oxide scale formation and ensure corrosion protection, AlSi coatings are commonly deposited on the sheet surfaces used for direct hot stamping. This workpiece coating has a critical impact on the tribological conditions within the forming process and, as a consequence, influences the quality of hot stamped parts as well as tool wear. AlSi coatings have been identified as major reason for adhesive wear, which represents the main wear mechanism in hot stamping. Within this study, the influence of the process parameters on adhesive wear are investigated in dependency of workpiece and tool temperatures, drawing velocities and contact pressures. The tribological behavior is analyzed based on strip drawing experiments under direct hot stamping conditions. The experiments are performed with AlSi coated 22MnB5 in contact with the hot working tool steel 1.2367. For analyzing the amount of adhesion on the friction jaws, the surfaces are characterized by optical measurements. The experiments indicate that higher workpiece temperatures cause severe adhesive wear on the tool surface, while an increase of drawing velocity or contact pressure led to reduced adhesion. The measured friction coefficients decreased with rising amount of adhesion and remained at a constant level after a certain adhesive layer was built up on the tool surface.

Tribological performances of new steel grades for hot stamping tools

In the last years, the use of High Strength Steels (HSS) as structural parts in car body-in-white manufacturing has rapidly increased thanks to their favourable strength-to-weight ratio and stiffness, which allow a reduction of the fuel consumption to accommodate the new restricted regulations for CO2 emissions control. 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 coatings and on the analysis of hard PVD, CVD coatings and plasma nitriding applied on the tools. 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. On this base, the paper deals with the analysis and comparison the tribological performances in terms of wear, friction and heat transfer of new tool steel grades for high-temperature applications, characterized by a higher thermal conductivity than the commonly used tools. Testing equipment, procedures as well as measurements analyses to evaluate the friction coefficient, the wear and heat transfer phenomena are presented. Emphasis is given on the physical simulation techniques that were specifically developed to reproduce the thermal and mechanical cycles on the metal sheets and dies as in the industrial practice. The reference industrial process is the direct hot stamping of the 22MnB5 HSS coated with the common Al-Si coating for automotive applications.

Liquid Metal Embrittlement of Hot Stamped Galvannealed Boron Steel Sheet – Effect of Heating Time on Crack Formation –

Galvannealed (GA) boron steel sheets were subjected to direct hot stamping tests (V-bending) to investigate crack formation behavior. Specimens were heated at 1173 K (900°C) in a combustion gas furnace and hot stamped on a cooled V-shaped die. The heating time was varied to alter the composition, phase, and microstructure of the coating layer. Generally, the reaction zone between the zinc coating layer and metal substrate could not be observed by scanning electron microscopy. For certain specimens, crack penetration occurred along primary austenitic grain boundaries of the substrate, with zinc enrichment observed at the crack sidewalls suggesting the occurrence of liquid metal embrittlement (LME) of the base metal. LME-induced cracking resulted when the coating layer corresponded to a biphasic Fe–Zn ferrite/liquid zinc structure of the binary Fe–Zn phase diagram during heating in the furnace. For cracked specimens, the maximum crack depth increased with increasing total X-ray diffraction intensity of δ and η phases in the coating layer of hot-stamped samples, being positively correlated with the amount of “liquid zinc” in the coating layer.

Description of microstructures and mechanical properties of boron alloy steel in hot stamping process

Hot stamping process has been increasingly applied for producing automotive parts with ultra-high strength property. Hereby, reduction of car body weight and increasing crashworthiness of new generation vehicles could be simultaneously achieved. In this work, a direct hot stamping of the boron alloy steel was investigated by experiment and FE simulation. Temperature evolutions of blank and dies were gathered during the experiments and verified with FE results. Metallographic analysis, hardness measurement and tensile test were performed for different locations of formed samples. Based on time–temperature–transformation (TTT) diagram, local emerged microstructure constituents and corresponding distributed hardness values of the stamped parts were predicted. Additionally, quenching tests under various media were carried out for the examined boron steel. Determined tensile stress–strain curves from each cooling condition were described using Voce hardening equation. Relationships between hardness and Voce model parameters were established. Then, developed constitutive equations were used in combination with the obtained FE results to evaluate local mechanical properties of hot stamped samples. It was found that calculated stress–strain responses for different areas fairly agreed with those from the experiments.

Comparison of tribological and wear performances of AlSi and Zn coatings in hot stamping of boron steel sheets

Since direct hot stamping has become the most relevant technology to manufacture 22MnB5 structural parts for automotive industry, the knowledge of the tribological conditions and the comprehension of the wear mechanisms are crucial for the process competitiveness. AlSi and Zn coatings, which are currently the most used to coat the steel sheets in order to avoid scale formation in the furnace, are supposed to act as lubricants during the forming stage. However, the thermal and mechanical conditions at the blank-dies interface may change significantly within the forming stage, depending on the geometrical features of the dies and the process parameters, so making critical the integrity of the coatings itself.The paper presents the results of experimental investigations on the tribological performances of AlSi and Zn coatings applied to 22MnB5, reproducing the different thermal conditions at the blank-die interfaces. Frictional and wear tests, reproducing the thermo-mechanical conditions typical of hot stamping, were carried out in the temperature range from 800°C down to 600°C. The results show lower friction coefficients for the Zn coating. For both the coatings, the dies wear appears as a combination of adhesion and abrasion, with the latter more relevant at the lower temperatures. Both the coatings show worse performances when the test temperatures are decreased.

Design of Press Tools to Investigate the Hot Stamping Behaviour of Alternative Coatings for Press-Hardened Steel Parts

Due to oxidation and decarburization during heat treatment prior to hot stamping, various coating products have been developed in the last decades. Press-Hardened Steel (PHS) components for passenger cars are generally coated by aluminized or galvanized (GI) coatings. The aluminized coating presents a good formability at high temperature and permits forming and quenching components in the same press tools with a so called direct hot stamping method. Due to a strong cracking in the base material during direct hot stamping induced by liquid-metal embrittlement (LME), galvanized coated products must be pre-formed at low temperature and undergo heat treatment in separate press units. Moreover, the oxide scale formed on GI parts must be removed by abrasive blasting, whereas aluminized parts can be directly painted after hot forming. However, higher performance in corrosion resistance has been observed for galvanized parts, in particular in cosmetic and cut-edge corrosion. This increase is linked to the sacrificial effect or cathodic protection provided by the layer containing zinc. Daimler AG is investigating the possibility of improving performance of PHS body parts in terms of suitability for direct hot stamping and corrosion protection by developing new coating materials. In the following article, the main particularities and challenges involved in both current coating products will be introduced. The development of specific press tools for this study, as well as the corresponding simulation of hot forming will be presented. Finally, the hot forming behaviour and anticorrosive properties of both current products will be presented.

Tribological performances of Zn-based coating in direct hot stamping

In hot stamping of High Strength Steels, the severe tribological conditions make the metal sheet coating one of the most critical choices for the technical and economical success of the process. This paper presents the approach and the results in evaluating a Zinc-based coating applied to boron steel sheets in hot stamping operations. Thermal and physical-simulation experiments were carried out to evaluate the chemical interactions between the coating and the metal sheet and the tribological performances during the process. The coating proved to overcome most of the drawbacks of the currently utilized Al–Si coating.

Investigations on Austenitization Parameters Influencing Wear Behavior Within Hot Stamping

Nowadays, light weight design of the body in white is achieved by manufacturing structural components out of high strength steel grades. Due to the increase of materials tensile strength, a reduction of sheet thickness of the component is possible to attend by a weight reduction of the component with remaining or increasing components strength. Such steel grades are often processed by direct hot stamping, which can be described as forming and quenching in one process step. Due to the high temperature difference and the relative movement between the hot blank and the cold tool surfaces during the forming operation, high thermal and mechanical loads are applied on the tool. In order to avoid decarburization and scaling during austenitization, semi-finished products with an aluminum-silicon pre-coating are used. The pre-coating intensifies the occurrence of wear in terms of adhesive layer build-up on the tool. This leads to a degradation of material flow and workpiece accuracy by increasing number of drawn parts which can result in time consuming and expensive rework to remove the layer build-up and ensure components quality. Within this work, adhesive wear behavior of aluminum-silicon pre-coated boron-manganese steel is investigated under process relevant conditions. Therefore, a wear test rig is used which enables investigations on layer build-up on tools with respect to contact pressure, blank temperature and austenitization parameters following the time-temperature profile within a hot stamping operation. Adhesive wear developing on the tool contact surface is examined in dependency of the used austenitization temperature and dwell time in the furnace by means of topographic measurements of the worn tool.

Phase transformation-based finite element modeling to predict strength and deformation of press-hardened tubular automotive part

In this paper, a new tool design for hot stamping taking advantage of both direct quenching and indirect die quenching is proposed to fabricate the coupled torsion beam axle which is formed from the original tubular-shaped part. This newly proposed hot stamping applies spray of the water before the upper die gets into contact with the tubular part, which significantly enhances the cooling capacity. For other region that cannot be covered by the direct spray, the conventional die quenching method in direct hot stamping process is used. Moreover, for the analysis of the deformation behavior during the proposed hot stamping process, the finite element analysis, which takes the deformation and strengthening induced by the cooling and phase transformation into account, has been carried out. From the analysis, the larger shape change when the cooling time is shorter than 15 s could be explained by the incomplete martensitic phase transformation and phase transformation plastic strain. When the cooling time is short the residual stress after cooling is much higher than those for the longer cooling times.

Isothermal versus non-isothermal hot compression process: A comparative study on phase transformations and structure–property relationships

It is known that the direct hot stamping process in which the specimen is deformed and quenched simultaneously results in a high-strength product without the occurrence of springback. In the current work, the effects of isothermal and non-isothermal thermo-mechanical processes on the phase transformations and the resultant microstructure and mechanical properties of 22MnB5 steel are investigated. For the non-isothermal processing route which is similar to direct hot stamping, the specimens were simultaneously compressed and quenched, while in the isothermal route, the specimens were isothermally deformed and subsequently quenched. The results indicated that higher forming loads as well as Ms and Mf temperatures are the characteristics of the former process over the latter one. Additionally, following the isothermal compression process by quenching resulted in a fully martensitic microstructure.

Determination of tribological conditions within hot stamping

Innovative hot sheet metal forming technologies are gaining an increasing significance in the scope of application of more and more innovative high and ultra high strength steels in the automotive industry. With respect to a numerical process design beside the mechanical and the thermal material characteristics the friction coefficient represents an important input parameter for finite element (FE) simulation. Within the scope of this paper an evaluation method for the determination of the friction coefficient μ for the direct hot stamping process of boron-manganese steels will be presented. Therefore cup deep drawing tests at elevated temperatures following the time-temperature-characteristic of the hot stamping process are carried out. For the calculation of the friction coefficient the approach according to Siebel for the modeling of the maximum drawing force is used.