Stamping Tests Research Articles

Numerical modeling and topological analysis of entangled single-wire metal rubber

Metal rubber materials, formed by entangling single metal wires and also known as ESW-MR, have attracted widespread attention due to their unique non-linear load curves and excellent environmental adaptability. The mechanical behavior of these materials, which are created through the weaving or winding of pure metal wires, is influenced by the structural characteristics of semi-flexible fiber cross-linked fibers. This study delves deeply into the microstructure of ESW-MR, with the aim of establishing a numerical model that can accurately simulate and characterize its structure. A method based on arbitrarily defined baselines parametric equations in the global space was adopted, leading to the creation of a parametric model for a continuously wound wire with multiple spiral mechanisms. Finite element simulations based on explicit dynamics were conducted using LS-DYNA and could successfully simulate the cold stamping forming process of ESW-MR blanks. The model was validated through stamping and quasi-static tests, as well as computed tomography (CT) scan tests. The results demonstrate the model accuracy in describing the macroscopic and microscopic characteristics of the material, showing excellent agreement with the actual material. Furthermore, the study successfully extracted the micro-topological features of the material, providing additional quantitative information on the topological characteristics of ESW-MR. This work offers a valuable tool for the study and application of ESW-MR and presents a novel perspective for the numerical simulation of a wide range of entangled materials. It paves the way for gaining a deeper understanding of the microstructure and mechanical behavior of such materials.

Experimental studies of reinforced clay bases of logging roads

The construction and operation of logging roads often takes place in unfavorable engineering and geological conditions. Very often, clayey soils with poor physical, mechanical and strength characteristics lie in the bases of logging roads. Improving these soil characteristics is an urgent task. For this purpose, reinforcement of soil bases with the help of geotextile materials is used. Most often, a horizontal arrangement of reinforcing geosynthetic layers is used. To date, there are a large number of studies of reinforced soil structures of logging roads, but they mainly concern the use of geosynthetics in sandy soils. Research in clayey soils is not enough. Therefore, experimental studies of the operation of reinforced foundations in clayey soils were carried out. The paper presents the results of stamp tests of clay soils reinforced with horizontal reinforcing layers. The studies were carried out in two stages. At the first stage, stamp model studies were performed, which made it possible to establish the optimal designs of reinforced clay bases. Based on the results of this stage, a single-layer horizontal reinforcing layer of geosynthetic material with a tensile rigidity of 2100 kN/m, an outer diameter equal to three stamp diameters, located at a depth equal to 0.25 stamp diameters was selected. The base soil was soft plastic clay. At the second stage, field stamping tests of the reinforced and non-reinforced clay base were performed. To study the stress-strain base, pressure sensors were selected to measure vertical normal stresses and a measuring system for their registration. Vertical displacements at various points of the active zone of soil foundations were evaluated by soil marks having a wire connection with deflection meters. The results of stamp tests showed that the bearing capacity of the reinforced base is 1.5 times higher than the bearing capacity of the unreinforced base. The deformation modulus of the reinforced base is 1.18 times higher than the unreinforced base in the load range from 0 to 100 kPa. The reinforcing layer is included in the work when the precipitation of the stamp is more than 30 mm. The stress-strain state of a reinforced base has significant differences from an unreinforced one. The reinforcing layer significantly reduces vertical stresses in the upper part of the active zone of the base, by an average of 22%/. The experimental stamp tests carried out made it possible to obtain a qualitative and quantitative picture of the operation of reinforced clay bases of logging roads.

Method of Stamping the Progression of a Beverage End Rivet of a Thinner Sheet of AW-5182 Alloy.

This paper presents a new solution for shaping the rivet progression of a beverage end. The classic method uses three operations to press the cylindrical rivet using 0.208 mm and 0.203 mm thick sheets. The increasing demand for aluminium alloys is prompting measures to make more efficient use of this raw material. One possible solution is to produce packaging from ever thinner sheets. This requires the design of new tooling and the preparation of an appropriate technological process. A method has been developed to stamp a hexagonal-shaped rivet from 0.200 mm thick sheet metal, increasing the number of stamping operations to four. The proposed method was verified through a numerical analysis using the PAM STAMP 2022.0 software package. It was found that for appropriately shaped tools, sheet thicknesses of the stamped component could be achieved that were not less than those for the currently used technology, thus eliminating any possible break in the material structure. Suitable tools and experimental stamping tests were carried out for the developed process. In the simulations, the material Al5182_iso_Xmm was adopted from the programme database, while the experiments were performed on a laboratory press using AW-5182-H48 sheets with a thickness of 0.200 mm. The purpose of the study was to determine the validity for the proposed method of forming the rivet of the beverage end.

Influence of subtitle base stabilization with geogrids on the deformability of the earth bed during thawing

In this research papers, several fundamental questions connected with the assessment of the influence of subgrade stabilization using the geogrid reinforcing method on deformability of subgrade during thawing are considered, as also the basic provisions of methods of calculation of the stress-strain state of subgrade reinforced by geogrids during thawing. Several practically significant tasks, such as reduction of general deformability, reduction of uneven distribution of deformations, increase of bearing capacity, and ensuring the stability of earth bed during thawing, can be solved by using modern geomaterials in the construction of earth bed. In order to determine the degree of change in the deformation characteristics of the ground reinforced with planar geomaterials under varying factors of structural, operational, and technological nature, a series of laboratory stamping tests were performed. The test outcomes showed an increase in the total deformation modulus of reinforced soil, depending on the realized variant of reinforcement, under the condition of full inclusion of geomaterial in the work under load, and being the most essential for recompacted soils. The paper proposes a method of accounting for the influence of reinforcing layers of geomaterial on the deformability and bearing capacity of the earth bed during thawing using a conventionally homogeneous equivalent layer with parameters determined by the outcomes of die tests. The proposed method of calculating the stress-strain state of an earth bed reinforced with geogrids during thawing is based on the use of the finite element method and is an effective tool for solving practical tasks of railway construction, including permafrost conditions.

Investigation of deformation mechanics and forming limit of thin-walled metallic bipolar plates

The present study is conducted on forming of the metallic bipolar plates made of 316 stainless steel sheet with a parallel serpentine flow field. The plastic deformation of straight and curved microchannels, forming limit criteria, and deformation mechanics during the process are investigated partially to present a reliable model for estimating fracture initiation. For this purpose, experimental stamping tests are employed to fabricate metallic bipolar plates and the process is simulated by finite element software. The validity of simulation results is examined by comparing thickness distribution and force-displacement curves reflecting 4.76% and 3.85% error rates, respectively. According to experimental observations, fracture starts at a channel depth of 0.610 mm. Hence, for determining the forming limit and predicting the fracture during the process, the deformation mechanic is studied at different points of the microchannels. Results of stress states analysis indicate that the stress state of plane-strain tension up to biaxial tension governs this process. Despite the presence of different loading paths during the process, the critical element in each channel is deformed under plane-strain tension. Therefore, a fracture model is developed based on thinning percentage and equivalent strain to predict the instability of metallic bipolar plates. According to the results, both the equivalent strain and thinning percentage criteria with critical limits of 0.56 and 33.45%, respectively, are considered as an allowable range of plastic deformation during the conventional stamping process of bipolar plates. Results indicate that maximum thinning in all directions is lower than 33.45% by using the modified stamping process.

A Study on Hot Stamping Formability of Continuous Glass Fiber Reinforced Thermoplastic Composites.

In this study, hot stamping tests on continuous glass fiber (GF)-reinforced thermoplastic (PP) composites were conducted under different process parameters using a self-designed hemispherical hot stamping die with a heating system. The effects of parameters such as preheating temperature, stamping depth, and stamping speed on the formability of the fabricated parts were analyzed using optical microscopy and scanning electron microscopy (SEM). The test results show that the suitable stamping depth should be less than 15 mm, the stamping speed should be less than 150 mm/min, and the preheating temperature should be about 200 °C. From the edge of the formed parts to their pole area, a thin-thick-thin characteristic in thickness was observed. Under the same preheating temperature, the influence of stamping depth on the thickness variation of the formed parts was more significant than the stamping speed. The primary defects of the formed parts were cracking, wrinkling, delamination, and fiber exposure. Resin poverty often occurred in the defect area of the formed parts and increased with stamping depth and stamping speed.

Numerical study of fractal analysis of crack propagation in concrete under different strain rates by meso‑scale particle element modeling

In order to explore the influence of concrete multiple cracks development on dynamic improvement mechanism under higher strain rates, a method to quantitatively describe the relationship between stress and multiple cracks development in the procesds of higher strain rates is proposed. By meso‑scale particle element modeling, SHPB stamping tests of concretes under different strain rates (30s−1≤ε˙≤140s−1) are simulated. The simulation results show that the stress-strain relationship and the law of dynamic strength improvement of concrete are consistent with the experimental results. The crack propagation in concrete under different strain rates is recorded and quantitatively described by the fractal box dimension. The crack propagation in concrete under different higher strain rates is divided into three stages, including micro crack initiation and stability stage, connectivity, extension and widening stage and fragmentation stage. This paper establishes and analyzes the relationships between stress, crack fractal dimension and strain rates under different strain rates. The research shows that at low strain rate the development of multiple cracks and failure of aggregates in concrete have a great impact on the dynamic improvement of concrete. As the strain rate increases, the inertia effect has a greater impact on the dynamic improvement of concrete.

An electric-pulse-assisted stamping process towards springback suppression and precision fabrication of micro channels

Micro channel features have been widely used in miniaturized products, such as micro reactor, micro heat exchanger, micro fluidic chip. For example, the metallic bipolar plate (BPP) characterized by dense submillimeter channels is a key component of the proton exchange membrane fuel cell (PEMFC). Since several hundreds of BPPs are assembled in a PEMFC stack, a high precision and uniformity of the formed micro channels is required. The micro/meso stamping process has been attracting tremendous attention due to its unique advantages in efficiency and cost. However, the uneven springback of ultra-thin metallic sheet results in evident nonuniformity in the micro channels arrays. In that regard, an electric-pulse-assisted (EPA) stamping method is developed in this research, in which sheet metals are first stamped and electric pulses are subsequently introduced to reduce the springback of formed micro channels. During a series of 7-channel EPA stamping tests, an evident springback suppression is revealed. In the case of 1 Hz electric pulses with the effective current density of 26.56 A/mm2, 83% of the channel height loss due to the springback is suppressed. Micro hardness tests reveal that the hardening of specimens in the deformation zone is significantly reduced with the introduction of electric pulses. The hardness decreases from 230 to 188 HV due to the accelerated stress relaxation, and the softening effect further leads to lower springback. Based on the analytical calculation and finite element analysis, a combined model is developed to capture the springback suppression effect of electric pulses, which is characterized by the accelerated stress relaxation. The applicability of the EPA stamping is verified and discussed.

Investigation of formability, microstructures and post-forming mechanical properties of heat-treatable aluminum alloys subjected to pre-aged hardening warm forming

Currently, the hot forming of aluminum alloys has garnered significant attention for overcoming poor formability at room temperature (RT). However, the current hot forming technique is facing problems such as time-consuming procedures and high economic cost. To improve forming efficiency, we herein propose a novel forming technique for heat-treatable aluminum alloys, termed pre-aged hardening warm forming (PHF) process. An Al–Zn–Mg–Cu alloy is used to investigate the flow behavior, drawing formability, and post-forming mechanical properties during the aforementioned process. Uniaxial tensile tests and Erichsen tests are conducted to evaluate the plastic flow behavior and drawability, respectively. Stamping tests are performed on a cap beam component to verify the engineering feasibility of the forming technique. Optical microscopy (OM) and transmission electron microscopy (TEM) are performed to characterize the microstructure evolution during the PHF process. The tensile test results show that the pre-aged alloys exhibit true fracture elongation similar to the fracture elongation of the O-temper alloy at RT, and that the Erichsen values of the pre-aged alloys are 5%–16% higher than that of the O-temper alloy at 200 °C. The results of the stamping tests for the cap beam component indicate that the optimal final strength of the stamped component achieves σ/σ0.2 = 566 MPa/500 MPa, exceeding the strength of the AA7075-T6 alloy. OM observations show that the stamping process does not significantly affect the grain sizes of the pre-aged alloys. TEM results show that numerous thermostable GPⅡ zones exist in the pre-aged alloys, and that the stamping procedure transforms the GPⅡ zones to dispersed η′ phases and results in the entanglement of numerous dislocations. The dual effects of phase transformation and plastic deformation during the PHF process contribute to the desirable mechanical properties of the stamped components.

Springback prediction of TC4 titanium alloy V-bending under hot stamping condition

In this paper, the springback of TC4 titanium alloy under hot stamping condition was studied by means of experiment and numerical analysis. Firstly, an analytical model was established to predict the V-shaped springback angle Δα under the stretch-bending conditions. The model took into account of blank holder force, friction, property of the material, thickness of the sheet and the neutral layer shift. Then, the influence of several process parameters on springback was studied by experiment and finite element simulation using a V-shaped stamping tool. In the hot stamping tests, the titanium alloy sheet fractured seriously at room temperature. The titanium alloy has good formability when the initial temperature of the sheet is 750–900 °C. However, the springback angle of formed parts is large and decreases with increasing temperature. The springback angle Aa decreased by 50% from 0.5° to 0.25°, and the angle Δβ decreased by 46.7% from 1.5° to 0.8° when the initial temperature of sheet increased from 750 °C to 900°C. The springback angle of titanium alloy sheet increases gradually with the increase of the punch radius, because of the increase of elastic recovery, the complex distribution of stress, the length of forming region and the decreasing degree of stress. Compared with the simulation results, the analytical model can better predict the springback angle Δα.

Investigation on the deformation behavior and post-formed microstructure/properties of AA7075-T6 alloy under pre-hardened hot forming process

In this study, a refined hot forming process for age-hardened aluminum alloys that is referred to as the pre-hardened hot forming (PHF) process was investigated. The ductility of the AA7075-T6 alloy was significantly improved during this process, which was evaluated through hot tensile tests ranging from 200 °C to 400 °C. Based on these results, hot stamping tests for automotive components were successfully conducted, and mechanical property testing of post-formed parts were carried out. The results indicate that under the forming temperature of 200 °C, the tensile strength of the formed component maintained more than 92% of the original T6 sheet blank tensile strength and more than 93% of the yield strength. Moreover, the post-formed strength did not decrease monotonically with increasing forming temperature. There was an abnormal drop in the strength of the component formed at 300 °C, which is obviously lower than that of the vicinal temperatures (250 °C and 350 °C). Microstructure observations and differential scanning calorimetry (DSC) analysis were conducted to explore the microstructure of the formed components. The results showed that there was no obvious difference among the grain sizes of the formed components under different forming temperatures. The sample formed at 250 °C maintained a large amount of the η′ phase, while the coarse η phase was the dominant precipitate in the microstructure of the sample formed at 300 °C. The contributions of grain boundary strengthening and precipitation strengthening were evaluated based on the Hall-Petch equation and Orowan strengthening theory, respectively. The calculation results showed good agreement with the experimental data. All the results indicated that direct forming of the AA7075-T6 alloy was achieved by the PHF process and that the forming temperature should be limited below 300 °C to obtain satisfactory mechanical properties without a subsequent artificial aging process.

BACKGROUND OF MODERN TECHNIQUES FOR LABORATORY DETERMINATION OF THE ELASTICITY MODULUS OF COHESIVE SOILS AND THEIR DESIGN CHARACTERISTICS IN THE ROAD STRUCTURE (Part 2)

According to the results of laboratory stamping tests, samples (d = 150mm; H=180mm) of bulk with soil compaction having a water saturation factor Sr≥0,85 in the range of values of fluidity index – 0,5≤lℓ≤0,25 , it is concluded that it is possible to determine practically every sample, their modulus of deformability (Епр; Ед), the calculated critical resistance – Ro of soil and its maximum relative deformation λmax. The loading of the samples is carried out by a step load with a proportional (1.3 times) increased pressure, such as at each stage. During the test, the sample is not unloaded but is subject to conditional destruction, that is, a disproportionate increase in the deposits of the die. The duration of each load is three minutes. The graphs of parabolic functions of the type λ=aσb are plotted in logarithmic coordinates in the form of two segments straight on both sides of the fracture. The parameter of the degree of parabola “b” serves for preliminary estimation of the index of deformability: for the modulus of elasticity – b≈1,0 ± 0,2; for the modulus of deformation – b≈2,0 ± 0,5; for the yield modulus – b≥3.0. The work gives a comparison with similar indicators of deformability of other building and structural materials. The elastic modulus (Епр) has only a soil that meets the conditions of the standard seal ρdmax>ρdωρ і ω_o<ωρ; the modulus of deformation (Ед) has a soil when 0<lℓ<0,5, and the modulus of fluidity (Ет) has only a soil lℓ≥0,5. Natural structured soils are characterized by a modulus of structural strength (Est) in the range 0<lℓ< 0, which is determined by compression tests. In order to reduce soil consumption and increase productivity, it is proposed to consider the task of determining soil deformability indices for the calculation of road clothing as a contact and therefore reduce the height of soil samples from 180 mm to 40 mm Keywords: soil, soil engineering structure, compacted soil, design condition, elastic modulus, critical design resistance, maximum relative deformation.

Investigation of Galling Wear Using Acoustic Emission Frequency Characteristics

In the sheet metal stamping process, during sliding contact between the tool and sheet, it is expected that severe events such as tool wear or fracture on the sheet generate acoustic emission (AE) burst waveforms. Attempts have been made in the literature to correlate the AE burst waveform with the wear mechanisms. However, there is a need for additional studies to understand the frequency characteristics of the AE burst waveform due to the severity and progression of the galling wear. This paper will determine the AE frequency characteristics that can be used to monitor galling wear, independent of the experimental process examined. The AE burst waveforms generated during the stamping and scratch tests are analysed in this paper to understand the change in the AE frequency characteristics with the galling severity. These AE burst waveforms were investigated using the Hilbert Huang Transform (HHT) time-frequency technique, band power, and mean-frequency. Subsequently, these AE frequency features are correlated with the wear behaviour observed via high-resolution profilometer images of the stamped parts and scratch surfaces. Initially, the HHT technique is applied to the AE burst waveform to understand the influence of wear severity in the power distribution over the wide AE frequency range. Later, the AE bandpower feature is used to quantitatively analyse the power in each frequency interval during the unworn and worn tool conditions. Finally, the mean-frequency of AE signal is identified to be able to determine the onset of galling wear. The new knowledge defined in this paper is the AE frequency features and wear measurement feature that can be used to indicate the onset of galling wear, irrespective of the processes examined.

BACKGROUND OF MODERN TECHNIQUES FOR LABORATORY DETERMINATION OF THE ELASTICITY MODULUS OF COHESIVE SOILS AND THEIR DESIGN CHARACTERISTICS IN THE ROAD STRUCTURE (Part 1)

According to the results of laboratory stamping tests, samples (d = 150mm; H=180mm) of bulk with soil compaction having a water saturation factor Sr≥0,85 in the range of values of fluidity index – 0,5≤lℓ≤0,25 , it is concluded that it is possible to determine practically every sample, their modulus of deformability (Епр; Ед), the calculated critical resistance – Ro of soil and its maximum relative deformation λmax. The loading of the samples is carried out by a step load with a proportional (1.3 times) increased pressure, such as at each stage. During the test, the sample is not unloaded but is subject to conditional destruction, that is, a disproportionate increase in the deposits of the die. The duration of each load is three minutes. The graphs of parabolic functions of the type λ=aσb are plotted in logarithmic coordinates in the form of two segments straight on both sides of the fracture. The parameter of the degree of parabola “b” serves for preliminary estimation of the index of deformability: for the modulus of elasticity – b≈1,0 ± 0,2; for the modulus of deformation – b≈2,0 ± 0,5; for the yield modulus – b≥3.0. The work gives a comparison with similar indicators of deformability of other building and structural materials. The elastic modulus (Епр) has only a soil that meets the conditions of the standard seal ρdmax>ρdωρ і ωo<ωρ; the modulus of deformation (Ед) has a soil when 0<lℓ<0,5, and the modulus of fluidity (Ет) has only a soil lℓ≥0,5. Natural structured soils are characterized by a modulus of structural strength (Est) in the range 0<lℓ< 0, which is determined by compression tests. In order to reduce soil consumption and increase productivity, it is proposed to consider the task of determining soil deformability indices for the calculation of road clothing as a contact and therefore reduce the height of soil samples from 180 mm to 40 mm Keywords: soil, soil engineering structure, compacted soil, design condition, elastic modulus, critical design resistance, maximum relative deformation.

Influence of process parameters on deep drawing of 2060 Al–Li alloy under hot stamping process

In this paper, the hot stamping formability of 2060 Al–Li alloy was studied by numerical simulation and experimental tests. The thickness of 2060 Al–Li alloy parts was analyzed when the deformation temperature is 400 °C, 450 °C and 480 °C, stamping speed is 15 mm/s, 25 mm/s and 35 mm/s. Simultaneously, the influence of lubrication on the thinning of 2060 Al–Li alloy cup-shaped parts is also analyzed. And Vickers hardness of the forming parts was tested after hot stamping tests. The results show that the deep drawing parts have good thickness uniformity at the deformation temperature ranging from 400 °C to 450 °C. However, the thickness of the cup parts is thinning seriously when the deformation temperature is 480 °C. With the increase of stamping speed, the thinning rate of cup-shaped parts decreases gradually. In this study, the thinning rate of the drawing parts is smaller when stamping speed is 35 mm/s. Compared with stamping parts without lubrication, cup parts with deeper depth can be obtained under better lubrication. With the improvement of lubrication conditions, the crack position of the formed part gradually shifts to the center of the cup. The simulation results and experimental results have good consistence.

Research on Forming Temperature of Metastable Austenitic Stainless Steel Head Based on Strain-Induced Martensitic Transformation

The formation of strain-induced martensite (SIM) is found in metastable austenitic stainless steel (m-ASS) during cold forming, and the presence of SIM may cause reductions in toughness, ductility, and corrosion resistance of m-ASS. These mechanical properties can be restored and improved by proper heat treatment after forming, however, which obviously raises the manufacturing costs. One low-cost way to reduce the SIM amount during m-ASS forming is to maintain the forming temperature at an appropriate level. This paper intends to investigate an approach to determine the optimum forming temperature at which the strain-induced martensitic transformation (SIM-Tr) of m-ASS head during forming can be restrained within a limited intensity. First, static tensile tests were conducted on S30408 conventional cylindrical tensile specimens under different temperatures varying from 20 °C to 180 °C, and then the effect of deformation temperature on SIM was evaluated. Second, according to the stacking fault energy (SFE) calculation method, m-ASS's chemical composition was taken into further consideration to investigate its effect on SIM. Finally, a formula was established based on SIM and chemical composition for optimization of forming temperature. In addition, the results obtained by this formula were compared with those of the experiment by S30408 ASS head stamping tests, and the satisfactory matching is found for the proposed forming temperatures and predicted ferrite number (FN) values (readings of the Ferritescope measurement, as a representation of the amount of martensite in this study). Furthermore, an enhancement in the cryogenic impact properties and a fewer quantity of delta-ferrite in the microstructure of m-ASS heads are observed when warm stamping is performed as compared with the cold stamped head.

Transition of failure mode in hot stamping of AA6082 tailor welded blanks

A novel sheet metal forming process, by manufacturing parts in a single sheet with varying thickness, has been employed in this work. It combines hot forming and cold-die quenching, also known as HFQ®, and the use of aluminium tailor welded blanks (TWBs) into a hybrid process. A series of hot stamping tests on the AA6082 TWBs were performed to investigate the deformation behaviour and failure features. Two failure modes, i.e. circumferential necking and parallel weld necking have been observed in the formed parts depending on the forming speed and thickness ratio of the TWBs. An advanced forming limit prediction model has been developed and further integrated into finite element simulation via a cloud-based multi-objective platform11All the models presented in this paper are available as open access numerical modules at www.smartforming.com, an online knowledge-based cloud platform. to investigate the failure/necking features of AA6082 TWBs. The model incorporates the theories of Hosford yield function, the anisotropic nature of plastic deformation in sheet metals and the Marciniak-Kaczynski (M-K) theory. According to the theories, the incremental work per unit volume ratio (dε¯B∙σ¯B/dε¯A∙σ¯A) between Zone B (thickness imperfect zone) and Zone A (the remainder of the material) is a key parameter determining the formability, by which the complex failure features have been fundamentally studied. The transition of failure mode in a TWB was attributed to the joint effects of temperature, strain rate and loading path changes. Strain rate could accelerate the development of localised necking in the TWBs when the failure mode was in transition from the circumferential mode to parallel mode.

Formability and microstructure evolution mechanisms of Ti6Al4V alloy during a novel hot stamping process

A novel hot stamping process for Ti6Al4V alloy using cold forming tools and a hot blank was presented in this paper. The formability of the material was studied through uniaxial tensile tests at temperatures ranging from 600 to 900°C and strain rates ranging from 0.1 to 5 s−1. An elongation ranging from 30% to 60% could be achieved at temperatures ranging from 750 to 900°C respectively. The main microstructure evolution mechanisms varied with the deformation temperature, including recovery, phase transformation and recrystallization. The hardness of the material after deformation first decreased with the temperature due to recovery, and subsequently increased mainly due to the phase transformation. During the hot stamping tests, qualified parts could be formed successfully at heating temperatures ranging from 750 to 850°C. The forming failed at lower temperatures due to the limited ductility of the material. At temperatures higher than 900 °C, extensive phase transformation of α to β occurred during the heating. During the transfer and forming, the temperature dropped significantly which led to the formation of transformed β, reduction of the formability and subsequent failure. The post-form hardness distribution demonstrated the same tendency as that after uniaxial tensile tests.

Feasibility study of a novel hot stamping process for Ti6Al4V alloy

To investigate the feasibility of a novel hot stamping process for the Ti6Al4V titanium alloy using low temperature forming tools, mechanical properties of the material were studied using hot tensile tests at a temperature range of 600 - 900°C with a constant strain rate of 1s-1. Hot stamping tests were carried out to verify the feasibility of this technology and identify the forming window for the material. Results show that when the deformation temperature was lower than 700°C, the amount of elongation was less than 20%, and it also had little change with the temperature. However, when the temperature was higher than 700°C, a good ductility of the material can be achieved. During the forming tests, parts failed at lower temperatures (600°C) due to the limited formability and also failed at higher temperatures (950°C) due to the phase transformation. The post-form hardness firstly decreased with the temperature increasing due to recovery and then increased due to the phase transformation. Qualified parts were formed successfully between temperatures of 750 - 850°C, which indicates that this new technology has a great potential in forming titanium alloys sheet components.

Study of springback effect of AHS steels using a microstructure based modeling

In sheet metal forming process of automotive parts, springback effect is crucial, in particular, for advanced high strength (AHS) steels, by which local interactions between different phases play an important role on macroscopic response. Otherwise, most structural components with highly complex shapes required multi–step forming procedures. Better understanding and more precise description of plastic deformation behavior and springback occurrence of such formed steel sheets are thus necessary. In this work, the springback of AHS steel grades 780 and 1000 was investigated, for which tension–compression tests were firstly carried out. Then, finite element (FE) modeling was done on both macroscale and microscale using representative volume elements (RVEs) in order to predict the cyclic stress–strain responses. Hereby, the kinematic hardening Yoshida–Uemori (Y–U) model was applied to consider the elastic recovery of material. The material parameters of the Y–U model were identified for the investigated steels as well as their phase constituents. It was verified that the calculated flow stress curves fairly agreed with the experimental ones. Afterwards, hat–shape stamping tests were performed and the results showed that occurred springback angles determined from the experiments and FE simulations were in accordance. Additionally, RVE simulations were conducted for local area of deformed sheet sample. The plastic deformation of various phases in the microstructure of examined steels under load–unloading behavior during the forming process was characterized. This approach can be further used to incorporate the effects of microstructure on the springback effect of AHS steel sheet.