Hot stamping of boron steel sheets with tailored properties: A review

Recent development trends in metal forming

Hydroforming is a relatively new metal forming process with many advantages over traditional cold forming processes including the ability to create more complicated components with fewer operations. For certain geometries, hydroforming technology permits the creation of parts that are lighter weight, have stiffer properties, are cheaper to produce and can be manufactured from fewer blanks which produces less material waste. This paper provides a detailed survey of the hydroforming literature of both established and emerging processes in a single taxonomy. Recently reported innovations in hydroforming processes (which are incorporated in the taxonomy) are also detailed and classified in terms of “technology readiness level”. The paper concludes with a discussion on the future of hydroforming including the current state of the art techniques, the research directions, and the process advantages to make predictions about emerging hydroforming technologies.

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A state of the art review of hydroforming technology: its applications, research areas, history and future in manufacturing

A custom biaxial testing fixture was used to evaluate new cruciform geometries. Specimens consisting of AA5083, Mg AZ31B, and TWIP steel were quasi-statically deformed to failure at 300oC. We elucidate geometric differences between specimens that accumulate plastic deformation within their gauge areas and those that prematurely fracture. Strain fields are computed with digital image correlation for selected geometries.

Cruciform-shaped specimens for elevated temperature biaxial testing of lightweight materials

Lightweight materials, such as ultra-high strength steel, titanium alloys and aluminum alloys, are used widely in aerospace and automobile industries leading to increasing demands for advanced forming technologies. Recently many innovative sheet- and tube-forming methods have been proposed by researchers and R&D facilities around the word. These methods can greatly improve the formability of materials with low plasticity and can produce complex-shaped parts with good surface qualities. This review aims at providing an insight into the latest developments of particular sheet- or tubular-metal forming process and shows basic applications in modern industry.

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A critical review on special forming processes and associated research for lightweight components based on sheet and tube materials

The numerical simulation of deep drawing processes is state of the art. Beside flow curves and yield locus, friction coefficients for blank-die contact are the most important, but often not exactly known, input parameters. This paper deals with the topic to calculate friction coefficients from Siebel’s formula for calculation of ideal maximum drawing force. Different modifications for special cases e.g. displaced blankholder or big die clearances are taken into account. The modified Siebel formula is used to determine friction coefficients for elevated temperatures up to 600 °C and different dry lubes. The calculated values for the friction coefficients are compared to values identified in numerical simulation.

Determination of friction coefficients in deep drawing by modification of Siebel’s formula for calculation of ideal drawing force

Blank Hydroforming Using Granular Material as Medium-investigations on Leakage

Increasing demands regarding security aspects and light weight construction lead to the application of advanced high strength steels (AHSS) and ultra high strength steels (UHSS) in the automotive sector. Due to high process forces and the reduced formability of these steel grades within cold forming new manufacturing technologies like the hot stamping process are required. Furthermore, crash-performance plays an important role in the automotive industry. Therefore functional optimized components are necessary. Hence, actual research work within the community is focused on manufacturing components with local adjusted mechanical properties. One of the strategies to realize the contradictorily requirements regarding energy absorption and structural integrity is the Tailored tempering process where the cooling rates are adjusted by controlled heating or cooling of different tool zones within the hot stamping process. Thereby knowledge concerning the influence of the different heated tool parts on the heat transfer and the resulting mechanical properties is necessary. Furthermore, the applicability and the accuracy of the calculation approaches used for characteristic values like the heat transfer coefficient in the FE-based simulation have to be analyzed and evaluated. Due to this experiments with a tool which exhibits a heated and a cooled zone were performed according to the Tailored tempering process. During the experiments contact pressures and tool temperatures in the heated tool part were varied and analyzed regarding the influence on the heat transfer. Furthermore, the heat transfer coefficients were calculated and verified by a numerical model built according to the experimental setup and the accuracy of the model was evaluated by the comparison of characteristic values calculated from the experimental and numerical process data.

FE-Simulation of the Heat Transfer by Defined Cooling Conditions during the Hot Stamping Process

The need of lightweight construction for the body in white of modern cars increases due to legal restrictions of CO₂ emissions and the passenger s wishes of safety and low fuel consumption. On e approach of lightweight construction is the use of high and ultra high strength steel, to reduce the weight of the single parts, another approach is the use of modem forming technologies, e.g. hydroforming with the po ssibility to create undercuts, to produce complex parts and thus to reduce the number of parts, the number of joining operations and the weight. The mentioned ultra high strength steels show a poor formability at room temperature and the necessary process forces are high compared to standard deep drawing steels. Warm form ing operations can help to reduce the process forces and to increase form ability [1]. To combine the benefits of warm forming and hydroforming new forming media are necessary to overcome the problem of temperature stability of fluids, which is limited to about 350℃ [2]. Beside gases, which tend to leakage and which are highly compressible, granular material 1ike small ceramic beads can be used as a form ing medium. First results of forming operations using this medium were presented in [3]. The experimental tool used for those tests pressurizes the medium by a punch. To divide the effects caused by temperature and steel grade from the other effects experiments were carried out at room temperature using the well known deep drawing steel DC04. In fluences of the ceramic beads diameter, the number of repetitions, the punch geometry as the punch position, determining the volume of media, on the forming are presented.

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Influences on the Molding in Hydroforming Using Granular Material as a Medium

The need of light weight construction for high efficient vehicles leads to the use of new materials like aluminium and magnesium alloys or high strength and ultra high strength steels. At elevated temperatures the formability of steel increases as the flow stresses decrease. Forming high complex geometries like chassis components or components of the exhaust system of vehicles can be done by hydroforming. The hydroforming process by oils is limited to temperatures of approximately 300 °C and brings disadvantages of possible leakage and fouling. Using granular material like small ceramic beads as medium could be an approach for hydroforming of ultra high strength steels like MS W1200 and CP W800 at temperatures up to 600 °C. The material properties of granular material are in some points similar to solid bodies, in other points similar to liquids. For understanding and simulation of the behaviour of the medium a basic characterisation of ceramic beads with different ball diameters is necessary. Powder mechanics and soil engineering give ideas for experimental setups. For the conversion of these approaches on the one hand the behaviour of the ceramic beads itself has to be characterized, on the other hand the contact between a blank and the beads have to be investigated. For the tests three different kinds of spheres with a diameter between 63 microns and 850 microns are used. In unidirectional compression test compressibility, pressure distribution in compression direction and transversal compression direction and the effect of bead fracture are investigated. The tests are carried out at different compression velocities and for multiple compressions. For determination of friction coefficients between blank and beads and determination of shear stress in bulk under compression a modified Jenike-Shear-Cell for use in universal testing machines with the possibility of hydraulic compression of the beads is built up. The gained data can be used for material modelling in ABAQUS using Mohr-Coulomb or Drucker-Prager model.

Martin Grüner

Papers

Determination of friction coefficients in deep drawing by modification of Siebel’s formula for calculation of ideal drawing force

Blank Hydroforming Using Granular Material as Medium-investigations on Leakage

FE-Simulation of the Heat Transfer by Defined Cooling Conditions during the Hot Stamping Process

Influences on the Molding in Hydroforming Using Granular Material as a Medium

Mechanical Behaviour of Ceramic Beads Used as Medium for Hydroforming at Elevated Temperatures