Papers by Keyword: Hot Stamping

Abstract: The deep drawability and bendability in hot stamping of ultra-high strength steel parts were examined. Although the cold drawability is greatly influenced by the blank shape, the limiting drawing depths for the square and circular blanks were equal for hot stamping because of small flow stress. In hot hat-shaped bending using draw-and form-type tools, the effect of the blankholder force generated with the draw-type tools on the springback was small, and the seizure for the form-type tools was smaller than that of the draw-type tools. Since both edges in contact with the electrodes are not heated for resistance heating, cracks were caused at the edges for resistance heating in the transversal directions in hot stamping of an S-rail with form-type tools, and thus it is required to control deformation of the non-heating zones.

Abstract: Over the last few years, hot stamping has been established as a suitable manufacturing process to produce high-strength structural parts. A tensile strength up to 1500 MPa and a high shape accuracy of the hot stamping parts are achievable. The hot forming tools are thereby stressed by varying thermo-mechanical loads resulting in increased surface wear. In order to reduce expensive and time consuming rework of the forming tools, an analysis of the tribological conditions is required. Purpose of this work is to increase the wear resistance of the tool surface and to investigate the wear behavior. In this regard, a laser alloying process is developed to influence the properties of the base material. Firstly, the alloying elements are selected and the element concentration is determined. Results for the composition of NiCrMo90 are presented, which is added by a wire fed laser alloying process unlike the previously used and already researched methods of powder bed fusion. This wire fed method is engineered to ensure a higher material utilization and to simplify the material feeding. After the alloying process the wear behavior of the alloyed surfaces are examined and compared to a not alloyed control group of pins under similar thermo-mechanical conditions.

Abstract: For the sake of research on the hot stamping property of high strength steel, the stamping forming of USIBOR1500P is simulated by the nonlinear finite element software Dynaform and Ansys/ls-dyna. The initial data simulated on USIBOR1500P is obtained by the hot tensile test. The simulation results show that the martensite weight percentage and Vickers hardness are in inverse proportion to stamping speed and initial die temperature. Otherwise, they are increased when the heat conduction coefficient is rising. Thus they affect the final properties of machinery of U-shape part. The maximum stress appears in the die and punch radius, and the minimum stress appears in the flange and the bottom of the blank. The minimum temperature value appears in the flange and the maximum temperature value appears in the die and punch radius.

Abstract: Present work analyses mathematical modelling to predict the onset of localized necking and rupture by shear in industrial processes of sheet metal forming of aluminium alloy 5083 such as biaxial stretching and deep drawing. Whereas the AA5083 sheet formability at room temperature is moderate, it increases significantly at high temperature. The Forming Limit Curve, FLC, which is an essential material parameter necessary to numerical simulations by FEM, of AA 5083 sheet was assessed experimentally by tensile and Nakajima testing performed at room and 400°C temperatures. Tensile test specimens at 0o, 45o and 90o to the direction of rolling (RD) and Nakazima type specimens at 0o RD of aluminium AA5083 were fabricated. Simple tensile tests at room and 400°C temperatures were performed to obtain the coefficients of plastic anisotropy and material strain and strain rate hardening behavior at different temperatures. Nakazima biaxial tests at room and high temperature, employing spherical punch were carried out to plot the limit strains in the negative and positive quadrant of the Map of Principal Surface Limit Strains, MPLS, of aluminium AA5083 sheet. The “Forming Map of Principal Surface Limit Strains”, MPLS, shows the experimental FLC which is the plot of principal true strains in the sheet metal surface (ε₁,ε₂), occurring at critical points obtained in laboratory formability tests or in the fabrication process of parts. Two types of undesirable rupture mechanisms can occur in sheet metal forming products: localized necking and rupture by induced shear stress. Therefore, two kinds of limit strain curves can be plotted in the forming map: the local necking limit curve FLC-N and the shear stress rupture limit curve FLC-S. Localized necking is theoretically anticipated to occur by two mathematical models: Marciniak-Kuczynski modelling, hereafter M-K approach, and D-Bressan modeling. Prediction of limit strains are presented and compared with the experimental FLC. The shear stress rupture criterion modeling by Bressan and Williams and M-K models are employed to predict the forming limit strain curves of AA5083 aluminium sheet at room and 400°C temperatures. As a result of analysis, a new concept of ductile rupture by shear stress and local necking are proposed. M-K model has good agreement with both D-Bressan models.

Abstract: All sectors of industry experience high demand for shaped products with as good mechanical properties as possible at low costs. Automotive industry, in addition, requires that the parts are of lightweight construction. Consequently, new types of materials and processes have to be combined to design new production chains capable to meet this demand. For instance, there are high-strength low-alloyed steels, whose final properties are attained by advanced heat treating techniques. One of such techniques is the Q&P process which can deliver excellent ultimate strengths exceeding 2000 MPa at a sufficient elongation level of 10 %. When combined with an unconventional forming method, it allows complex-shaped parts with outstanding mechanical properties to be made. One example of such combined procedure is the sequence of internal high pressure forming, hot stamping and Q&P processing. In the present study, thin-walled hollow stock was processed using such a combined procedure. After stepwise optimization of processing parameters, products with martensitic structure and a small amount of bainite were obtained. In all locations of the product which were tested, the ultimate strength exceeded 1950 MPa and elongation reached 15 %

Abstract: Automotive parts made of ultra-high strength steels (UHSS) have been increasingly produced by hot stamping or press hardening of boron alloy steel. In case of novel hot formed components with tailored properties, different heating cycles needed to be applied for different zones, in which varying microstructure characteristics were generated. Mechanical properties of these parts were thus precisely controlled by the microstructure constituents. In this work, stress-strain behaviors of a boron alloy steel undergoing different heat treatment conditions with respect to that modified hot stamping procedure were predicted. Firstly, boron alloy steel sheet specimens were heated up to the austenitization temperature. Afterwards, they were abruptly cooled down to the bainitic temperature range, held for different holding times and finally cooled to room temperature. The microstructures obtained from each condition were characterized by optical microscope (OM) using color tint etching. The stress-strain responses of all generated microstructures were determined by tensile test. By the modeling, flow curves of the individual single phases were described taking into account a dislocation theory based model and their chemical composition. Subsequently, effective flow curves of the heat treated boron alloy steels were calculated by means of the isostrain and non-isostrain method and were finally compared with the experimentally determined curves.

Abstract: The boron steel quenching requirement on hot forming manufacturing processes allows the industry to create tailored parts to improve their mechanical functionality. During the cooling, the microstructure of the material changes depending on the imposed cooling rate. However, an accurate prediction of the cooling ratios is needed in order to correctly design the process. In this work the interfacial heat transfer coefficient (HTC) has been determined at different contact conditions, varying the initial die temperature. Experimental tests have been realized in a SCHMIDT micro servo-press, which is able to compensate the thermal contraction of the blank and tools to precisely keep constant the contact pressure. Temperature evolution of the tools and the blank has been monitored with nine thermocouples. For the determination of the heat transfer coefficient (HTC) an analytical-numerical method has been used leading to a fast and reliable calculation method able to determine the HTC value for each process time. This methodology allows relating the HTC to the blank temperature, difference on temperature on the interface to improve the tailor tempering of boron alloys simulation.

Abstract: Hot forming processes are becoming a successful solution when complex geometrical components with high mechanical properties are desired. In fact, automotive structural components with tensile strengths higher than 1500MPa are being nowadays industrially produced. The technology is based on the forming and quenching of the sheet inside the forming tool using boron steels. Aiming at boosting the advantages of this technology, car manufacturers have started to demand structural components with different mechanical behavior areas in order to improve the impact response of the auto-motive passenger compartment: the so called tailor tempered components. The basic idea is to obtain final parts with different properties like it has been successfully done using tailored welded blanks. Although different solutions exist, one of the most common strategy is to use partially heated tooling, which influences the cooling of the sheet and consequently the local properties. At the present work, a special tooling with independent heated and cooled areas has been developed in order to evaluate the final properties achievable in the tailored tempering process. High and low conductivity alloys have been used to find the process limits and compare them to classical tool steels. Hardness values, Ultimate Tensile Stresses and microstructures are shown for different steels, tool temperatures and contact pressures. In the last part of the paper, the hot spotting results are presented. Different air gap diameters have been used to evaluate the possibility to create soft spots that will enable an easier cutting of geometrically accurate holes and a more suitable and ductile join between different components by using the spot welding.

Abstract: 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.

Abstract: The increasing demand for fuel-efficient vehicles has led automotive industry to introduce new alloys in car manufacturing, characterized by a high stiffness and strength-to-weight ratio. Due to their mechanical and chemical properties, aluminium alloys appear potential candidates to replace traditional steels for several parts of the car-body-in-white, even if the limited formability at room temperature, the marked springback and the severe tribological behaviour have often represented important drawbacks in traditional properties. Recently, the use of temperature-assisted processes, such as hot stamping, allows overcoming such limits thanks to substantial increase of the formability and, at the same time, a drastic reduction of springback. However, the choice of proper process parameters, in terms of lubrication at the interfaces between the dies and the blank, and thermal parameters of the dies materials still represent critical points for the feasibility of the process. Recent investigations have proved the limits of assuming constant friction for all the areas of the dies and the steps of the deformations, especially with variable pressures and non-constant temperatures at the interfaces. Such factors, together with the lubricants and the lubricant deposition on the blank, the blank and dies coatings, the surface roughness, the stamping speed are not always considered, despite their considerable influence on the process tribology. In this paper the friction behaviour of commercially available automotive aluminium (AA6016 alloy) is studied. The friction coefficient is measured by means of a new machine for strip draw test at different levels of pressure, temperature, sliding speed and type of lubricant. The results report the investigation of the surface topography of the metal sheets, investigated by Energy-Dispersive X-ray spectroscopy and optical profilometry.