Hot Press Forming Research Articles

Reviews on the Forming Process of Heat Treatable Aluminium Alloys

Available forming process applied for heat treatable aluminium alloy were reviewed and presented in this paper. The reviews emphasized on the heat treatment, application and contribution of the forming methods. Forming methods discussed includes traditional, warm forming, hydroforming, superplastic forming, hot forming with in-die quench (HFQ) and hot press forming (HPF). The effects of forming processes towards heat treatable aluminium alloy were presented through the understanding on each process strengthening phenomena and the heat treating phenomena. These were to ensure better understanding on heat treatable aluminium alloy composition changes throughout forming process. Finally, concluding remarks the underline challenges of forming heat-treatable aluminium alloy and subsequently highlights the potential work that can be applied in order to ensure a more efficient and sustainable manufacturing agenda for heat treatable aluminium alloy. Available forming process applied for heat treatable aluminium alloy were reviewed and presented in this paper. The reviews emphasized on the heat treatment, application and contribution of the forming methods. Forming methods discussed includes traditional, warm forming, hydroforming, superplastic forming, hot forming with in-die quench (HFQ) and hot press forming (HPF). The effects of forming processes towards heat treatable aluminium alloy were presented through the understanding on each process strengthening phenomena and the heat treating phenomena. These were to ensure better understanding on heat treatable aluminium alloy composition changes throughout forming process. Finally, concluding remarks the underline challenges of forming heat-treatable aluminium alloy and subsequently highlights the potential work that can be applied in order to ensure a more efficient and sustainable manufacturing agenda for heat treatable aluminium alloy.

Influence of surface oxide on mechanical properties of laser-welded hot-press-forming steel with strength of 1.8 GPa

The hot-press-forming (HPF) steel has been proposed to overcome the insufficient formability of the GPa-grade high-strength steel. During the HPF process, a steel sheet is heated over a full austenitization (Ac3) temperature and thick oxidation scales are formed on the surface unless it is protected with an appropriate coating layer. In this study, the laser weldability of 1.8-GPa-grade HPF steel was investigated, and the effect of oxidation scales on the sheets was examined. Bead-on-plate welding and overlap welding were conducted on the 1.8-GPa HPF steel sheets with the oxidized surface layer. The oxide was diluted into the weld metal, which changed the microstructure of the weld metal and had a detrimental effect on the mechanical properties. The oxide layer on the sheet should be removed prior to welding so as to improve the weld strength.

Heuristic design of U-shaped die cooling channel for producing ultra-high strength steel using hot press forming

This paper proposes an optimised cooling channel design for a U-shaped die in the hot sheet metal forming process using a heuristic method. Unlike in previous works, the parameters used in this study for optimising the cooling channel design not only consider the pitch between and diameter of cooling channels but also their distance from the tool surface and wall tool. The cooling channel design is optimised by coupling the heuristic method with finite element thermal and static analyses. The main factors to be considered in the optimisation include the highest heat transfer and the lowest stress that can be achieved by the optimum cooling channel die design. The optimum design was determined by conducting a simulation and was validated by conducting an experiment. The temperature distribution of the FEA model was at most 5% different from the experimental results. The optimum cooling channel design of hot press sheet metal forming can produce ultra-high strength steels which tensile strength and hardness are 51 and 83% greater than those of original boron steels. The heuristic method can also be applied to optimise the cooling channel die design for the production of ultra-high strength steels in the automotive manufacturing industry.

Electrochemical Corrosion and Hydrogen Diffusion Behaviors of Zn and Al Coated Hot-Press Forming Steel Sheets in Chloride Containing Environments

Hot-press forming(HPF) steel can be applied successfully to auto parts because of its superior mechanical properties. However, its resistances to aqueous corrosion and the subsequent hydrogen embrittlement(HE) decrease significantly when the steel is exposed to corrosive environments. Considering that the resistances are greatly dependent on the properties of coating materials formed on the steel surface, the characteristics of the corrosion and hydrogen diffusion behaviors regarding the types of coating material should be clearly understood. Electrochemical polarization and impedance measurements reveal a higher corrosion potential and polarization resistance and a lower corrosion current of the Al-coating compared with Zn-coating. Furthermore, it was expected that the diffusion kinetics of the hydrogen atoms would be much slower in the Al-coating, and this would be due mainly to the much lower diffusion coefficient of hydrogen in the Al-coating with a face-centered cubic structure. The superior surface inhibiting effect of the Al-coating, however, is degraded by the formation of local cracks in the coated layer under severe stress conditions, and therefore further study will be necessary to gain a clearer understanding of the effect of cracks formed on the coated layer on the subsequent corrosion and hydrogen diffusion behaviors.

Fabrication of Hadfield-Cored Multi-layer Steel Sheet by Roll-Bonding with 1.8-GPa-Strength-Grade Hot-Press-Forming Steel

An austenitic Hadfield steel was roll-bonded with a 1.8-GPa-strength-grade martensitic hot-press-forming (HPF) steel to fabricate a multi-layer steel (MLS) sheet. Near the Hadfield/HPF interface, the carburized and decarburized layers were formed by the carbon diffusion from the Hadfield (1.2%C) to HPF (0.35%C) layers, and could be regarded as kinds of very thin multi-layers of 35 μm in thickness. The tensile test and fractographic data indicated that the MLS sheet was fractured abruptly within the elastic range by the intergranular fracture occurred in the carburized layer. This was because C was mainly segregated at prior austenite grain boundaries in the carburized layer, which weakened grain boundaries to induce the intergranular fracture. In order to solve the intergranular facture problem, the MLS sheet was tempered at 200 °C. The stress–strain curve of the tempered MLS sheet lay between those of the HPF and Hadfield sheets, and a rule of mixtures was roughly satisfied. Tensile properties of the MLS sheet were dramatically improved after the tempering, and the intergranular fracture was erased completely. In particular, the yield strength up to 1073 MPa along with the high strain hardening and excellent ductility of 32.4% were outstanding because the yield strength over 1 GPa was hardly achieved in conventional austenitic steels.

High strength steels and aluminium alloys in lightweight body manufacturing

Purpose: of this paper: The main objectives of this paper are to give an overview about the application of various kinds of high strength steels and aluminium alloys in the automotive industry to produce lightweight car body elements to achieve significant reductions in harmful emissions to provide more environmental friendly vehicles which simultaneously fulfils the increased safety requirements, too. In these respects, both high strength automotive steels (e.g. DP, TRIP, TWIP and HPF steels), as well as high strength aluminium alloys (e.g. AA6082, AA7075, etc.) are more and more widely applied in the vehicle manufacturing. Design/methodology/approach: The contradiction between the increased strength and lower formability of these high strength metallic materials is one of the main issues in their application in the automotive industry. Therefore, in this paper primary focus will be placed on the formability properties of these materials, concerning first of all the limits of formability in various cold and hot forming conditions. To fully utilize the potentials of these materials in forming processes the numerical modelling of forming with FEM simulation is of utmost importance. Findings: Recently in the automotive industry the Hot Press Forming of high strength boron-alloyed manganese steels become an industrially established process, while the Hot Forming and Quenching (HFQ) of artificially ageing high strength aluminium alloys now become the focus of scientific research. The paper will analyse the main process parameters and gives comparisons of automotive applications. Research limitations/implications: There are still certain shortages of industrial applications, namely the limits of economic cycle times for economical mass production which needs further research activities in these fields. Practical implications: Since both the materials mentioned above and the forming processes usually applied, furthermore the available benefits are extremely important for the automotive industry these results have significant practical involvement. Originality/value: The applied research methods and the introduced new findings will show the originality of the paper.

Corrosion Assessment for Boron Steel Weldments Prepared by Overlap Welding and Successive Hot Press Forming Processes

Corrosion Assessment for Boron Steel Weldments Prepared by Overlap Welding and Successive Hot Press Forming Processes

Effects of untransformed ferrite on Charpy impact toughness in 1.8-GPa-grade hot-press-forming steel sheets

Hot press forming (HPF) steel sheets are austenitized, press-formed, and rapidly cooled to obtain a martensitic microstructure with an ultra-high strength. When they are insufficiently austenitized, their microstructures might contain a small amount of untransformed ferrite, which can deteriorate impact toughness as well as strength, but its causes and relevant fracture mechanisms have not been clearly verified yet. In this study, thus, 1.8-GPa-grade HPF sheets were austenitized at various temperature and time, and their tensile and Charpy impact test results were analyzed in relation with untransformed ferrite and its effect on fracture mechanisms. In the HPF sheets containing the untransformed ferrite, voids were formed mostly at ferrite/martensite interfaces, and were grown and propagated linearly to form a cleavage crack, whereas deformation bands were well developed without voids or cracks in the non-ferrite-containing sheets. The highly localized strains accommodated in the soft ferrite made ferrite/martensite interfaces or ferrite itself work as fracture initiation sites, which led to the brittle fracture and consequently to the deterioration of impact energy. This result can provide an important idea for optimization of austenitization conditions demanded for ultra-high strength and excellent impact toughness in HPF applications.

1180 MPa급 초고장력강판을 이용한 자동차 상부 시트트랙 개발을 위한 해석적 연구

As emission regulation of vehicles is being reinforced globally, the current requirement of the automobile industry are innovative green technologies that reduce the weight of the vehicle, thereby improving fuel consumption and the amount of exhaust gas emission. The application of ultra-high strength steel (UHSS) for vehicles has specifically been studied for light weight of vehicles. UHSS withstands greater loads than a general steel sheet of the same thickness. The spring-back and formability of the UHSS are also worse than general steel sheet due to their high elasticity and high yield strength. Various methods applied for processing UHSS include roll-forming and hot-press forming. However, these processes have not only high installation cost but also low productivity. This study therefore developed the cold-press forming method to overcome these disadvantages. The objective of this study is to determine the optimum conditions of the cold press required to form the upper seat track using UHSS. Forming analysis predicted the spring-back at each stage of the press forming. The prediction of spring-back was compared with the manufactured upper seat track by try-out, thereby reducing trial and error in the pressing process.

Nanosecond laser ablation of Al-Si coating on boron steel

Al-Si coating is routinely used for hot-press-forming (HPF) steel to prevent oxidation and decarburization under high-temperature processing conditions. However, the Al-Si coating is not suitable for subsequent welding process and has to be removed before laser welding. In this work, the ablation effect of nanosecond pulse laser parameters (i.e., pulse energy, pulse repetition rate, and pulse number) on Al-Si coating was studied. Experimental results show that for the 100ns pulse duration laser ablation, the complete removal of Al-Si coating requires the pulse repetition rate to be larger than 5kHz and a higher pulse repetition rate needs a smaller pulse number. Despite a statistical ablation threshold in long pulse regime, the calculated ablation threshold of fully removed Al-Si coating was obtained at the laser fluence ~4.4J/cm2 at the pulse repetition rate 25kHz. Due to the accumulation effects, increased pulse number and pulse repetition rate will reduce the ablation threshold. The evolution of ablation depth was simulated using Hirschegg model and a gradual increase and then constant ablation depth was found as the pulse number and pulse repetition rate increase.

Hot Forming of Boron Alloyed Manganese Steels

The automotive industry always had a determining role in the whole economy. In sheet metal forming both the material and the technological developments are strongly affected by the development tendencies in car manufacturing. One of the main trends in today’s car manufacturing is to apply light weight construction principles to meet the increased customers’ demand and legal requirements. In this respect, application of high strength steels is regarded as one of the most promising possibilities. Applying high strength steels has a very positive response for many requirements: increasing strength results in the application of thinner sheets leading to significant mass reduction and simultaneously to lower consumption and less harmful emissions however it also leads to formability problems. Hot press forming is one of the most successful solutions to overcome these difficulties. In this paper, a general overview of hot press forming processes will be summarised.

Hardness Prediction of Tailor Rolled Blank in Hot Press Forming Using Quench Factor Analysis

This paper aims to predict the hardness of hot formed part for tailor rolled blank (TRB) by the FE-simulation coupled with quenching factor analysis (QFA). Dilatometry test of boron steel is performed at various range of cooling rates from 0.2 to 100°C/s using the dilatometer with forced air cooling system. The dilatometry test provides a hardness data according to cooling curves which are used to determine the material constants (K1~K5) of QFA and the time‒temperature‒property (TTP) diagram of boron steel. Then, FE‒simulation of hot press forming is conducted to predict the cooling curves of hot formed TRB part with a thickness combination of thicker 1.6mm and thinner 1.2mm which is called as rear side member of automotive component. The cooling curves of FE-simulation are applied to predict the hardness of hot formed rear side member using the QFA. Also, experiment of hot press forming is performed to verify the predicted results and to examine the effect of cooling curves on the hardness.

An investigation of laser cutting quality of 22MnB5 ultra high strength steel using response surface methodology

In hot press forming, changes of mechanical properties in boron steel blanks have been a setback in trimming the final shape components. This paper presents investigation of kerf width and heat affected zone (HAZ) of ultra high strength 22MnB5 steel cutting. Sample cutting was conducted using a 4kW Carbon Dioxide (CO2) laser machine with 10.6µm wavelength with the laser spot size of 0.2mm. A response surface methodology (RSM) using three level Box-Behnken design of experiment was developed with three factors of peak power, cutting speed and duty cycle. The parameters were optimised for minimum kerf width and HAZ formation. Optical evaluation using MITUTOYO TM 505 were conducted to measure the kerf width and HAZ region. From the findings, laser duty cycle was crucial to determine cutting quality of ultra-high strength steel; followed by cutting speed and laser power. Meanwhile, low power intensity with continuous wave contributes the narrowest kerf width formation and least HAZ region.

Eco-efficiency evaluation of binder-free green composites made from bamboo fibres extracted with a machining centre

A natural resource, bamboo, is widely distributed in Asia. Bamboo grows faster than other renewable natural material. Further, bamboo fibre has high specific strength and stiffness. We draw attention to this bamboo as a sustainable and environmentally superior material. We propose a sustainable manufacturing system using bamboo. A self-adhesive fibreboard has been fabricated by end-milling and hot press forming. This paper describes the environmental evaluation of this system using life cycle assessment.

Prediction of part shape and associated material properties in hot-press forming using Unite element analysis

The hot-press forming of a U-channel was conducted on a boron-steel blank. The die consisted of two separate parts in order to perform the partial quenching process. The cold die was initially at 25 °C while the heated die was set to five different temperatures, namely, 25, 120, 220, 320 and 400 °C. The cooling temperature history, Vickers hardness and springback of the channel were measured. A thermo-mechanical-metallurgical model, which accounts for the prior austenite deformation effect, was successfully implemented in the LS-DYNA explicit solver to simulate the hot-press forming process under partial quenching conditions. The predicted and experimental results were compared and found in reasonable agreement.

열유동 해석을 통한 핫프레스 포밍 금형의 냉각 성능 평가

열유동 해석을 통한 핫프레스 포밍 금형의 냉각 성능 평가

핫 프레스 포밍을 위한 고열전도성 금형에 대한 연구

핫 프레스 포밍을 위한 고열전도성 금형에 대한 연구

Morphology of Iron-Zinc Coating for Hot Press Forming Steel

The coating morphology of a hot press forming steel galvanealed by HDPS (hot dipping process simulator) in laboratory was studied in the paper. The results show that, firstly from thermodynamic calculations, the intermetallics increased the melting point of Zn, and it is possible to avoid the melting and volatilization of zinc coating for hot press forming steel strip, when the coating mainly consists of δ phase (~10% Fe) in galvanealing process, and lower re-austenization temperature (below 782°C) in hot press forming process. Secondly, according to the SEM morphology in the paper, the zinc coating formed an alloy predominantly composed of delta (δ) phase, a low iron zeta (ζ) phase formed on the surface, and a high iron gamma (Γ) phase formed at the interface with the steel substrate. The hard and brittle Γ phase failed to grow up on the interface of coating and matrix which results in high shear stress and good coating adherence.

Zn Diffusion and α-Fe(Zn) Layer Growth During Annealing of Zn-Coated B Steel

Direct hot press forming of Zn-coated 22MnB5 steels is impeded by micro-cracks that occur in the substrate due to the presence of Zn during the forming process. A study was therefore undertaken to quantify concentration of Zn across the α-Fe(Zn) coating and on grain boundaries in the α-Fe(Zn) layer and the underlying γ-Fe(Zn) substrate after isothermal annealing of Zn-coated 22MnB5 at 1173 K (900 °C) and to link the Zn distribution to the amount and type of micro-cracks observed in deformed samples. Finite difference model was developed to describe Zn diffusion and the growth of the α-Fe(Zn) layer. The penetration of Zn into the γ-Fe(Zn) substrate after 600 seconds annealing at 1173 K (900 °C) through bulk diffusion is estimated to be 3 μm, and the diffusion depth of Zn on the γ-Fe(Zn) grain boundaries is estimated to be 6 μm, which is significantly shorter than the maximum length (15 to 50 μm) of the micro-cracks formed in the severely stressed conditions, indicating that the Zn diffusion into the γ-Fe(Zn) from the α-Fe(Zn) during annealing is not correlated to the depth of micro-cracks. On the other hand, the maximum amount of Zn present in α-Fe(Zn) layer decreases with annealing time as the layer grows and Zn oxidizes, and the amount of Zn-enriched areas inside the α-Fe(Zn) layer is reduced leading to reduced length of cracking. Solid-Metal-Induced Embrittlement mechanism is proposed to explain the benefit of extended annealing on reduced depth of micro-crack penetration into the γ-Fe(Zn) substrate.

Inverse characterization method for mechanical properties of strain/strain-rate/temperature/temperature-history dependent steel sheets and its application for hot press forming

In order to measure the flow curves of steel sheets at high temperatures, which are dependent on strain and strain rate as well as temperature and temperature history, a tensile test machine and specimens were newly developed in this work. Besides, an indirect method to characterize mechanical properties at high temperatures was developed by combining experiments and its numerical analysis, in which temperature history were also accounted for. Ultimately, a modified Johnson-Cook type hardening law, accounting for the dependence of hardening behavior with deterioration on strain rate as well as temperature, was successfully developed covering both pre- and post-ultimate tensile strength ranges for a hot press forming steel sheet. The calibrated hardening law obtained based on the inverse characterization method was then applied and validated for hot press forming of a 2-D mini-bumper as for distributions of temperature history, thickness and hardness considering the continuous cooling transformation diagram. The results showed reasonably good agreement with experiments