Blanked Edge Research Articles

Efficient net shape forming of high-strength sheet metal parts by Transversal Compression Drawing

In this contribution, a new forming method is presented for high-strength steel sheet metal materials called Transversal Compression Drawing (TCD). For TCD, the blankholder of the conventional deep drawing process is replaced by sliders introducing a vertical force along the blank edge. Compared to conventional deep drawing, compressive stresses are thereby superimposed in the flange area of the sheet metal material, reducing the springback of formed components and the forces required during the process. Proof of this is provided here on the basis of numerical investigations in which conventional deep drawing and TCD are compared using the example of an “S-Rail” component made from DP980. Results of these investigations reveal that with TCD a maximum reduction in springback by more than 5 mm is achieved, leading to an improvement in the component´s overall shape accuracy of more than 55%. Furthermore, frictional work is decreased by − 6.92 kJ/kg, resulting in an overall reduction of maximum forming forces by 67.4%. Besides, TCD enables a trim-free net shape forming of the final part contour in a single operation and an increased material utilization of 4.5% from coil to part. Not least, no sheet thinning occurs with TCD, whereas a maximum thinning of 12% is observed with the conventional deep drawing process.

Partial Thinning Method for Steel Sheets by Bending-Assisted Compression Drawing

Tailored blanks with in-plane differential thickness are required to reduce automobile body weight and satisfy two or more strength requirements in one panel. We developed a method of partial thinning in the width direction of steel sheets by bending-assisted compression drawing. This method reduces shear deformation energy by combining bending deformation and compression deformation, which can lead to a reduction in forming load. In this study, we conducted an experiment to partially reduce the thickness about 10 mm from the width edge of the mild steel sheet by the proposed method. We found that this method enables us to partially reduce the mild steel sheet thickness by 30% on the blank edge side. Furthermore, we combined the proposed method with a continuous stretch-bending method that can partially reduce the thickness in the longitudinal direction (drawing direction). As a result, a tailored blank with in-plane differential thickness was demonstrated experimentally.

Generation mechanism of residual stress at press-blanked and laser-blanking edges of 1.5 GPa ultra-high strength steel sheet

The generation mechanism of the residual stress at press-blanked and laser-blanked edges of a 1.5 GPa ultra-high strength martensite steel sheet was investigated. The residual stress at the press-blanked edges of the 1.5 GPa sheet is highly tensile, whereas the residual stress at the laser-blanked edges is compressive. The high tensile residual stress causes the occurrence of hydrogen-induced delayed cracking and the reduction in fatigue strength. In press blanking of the 1.5 GPa sheet, considerably high compressive stress was generated under the punch corner just before the separation due to a small critical punch stroke, and high tensile residual stress on the fracture surface of the blanked edge was induced by releasing this high compressive stress for the separation. Not only the high strength of the 1.5 GPa sheet but also the low ductility influences the generation of the high tensile residual stress on the fracture surface because of the small critical stroke. On the other hand, the compressive residual stress at the laser-blanked edges of the 1.5 GPa sheet was generated by the volume increase of the martensite transformation at a comparatively low temperature. In addition, an approach for determining the allowable amount of hydrogen for the occurrence of delayed cracking at the edges was proposed, and the relationship between the critical residual stress and the amount of hydrogen for the 1.5 GPa sheet was obtained.

On the mechanics of edge cracking and the reliable determination of edge formability limits

Blanked edge surfaces are rough and hardened. They therefore lead to inhomogeneous deformation on the edge, which can trigger localization within the shear affected zone (up to few mm from the edge). The size and extent of these phenomena are primarily a function of the shearing process and are only marginally coupled to the global/homogeneous deformation behavior of the blank. A direct numerical simulation of such local deformation effects would require a prohibitively high resolution to capture the microgeometry of the edge and thus remains unfeasible in the current industrial practice. A predictive model can therefore only be achieved by determining limit strains on the edge, which are compatible with the homogeneous numerical framework used. The present contribution aims discussing the basic mechanics of edge cracking based on tensile tests with edges blanked with different die clearances. The local and global strain evolutions in the vicinity of the edge are analysed and a new evaluation procedure is proposed for the reliable determination of limit strains. The application of this method in industrial context is also discussed.

Microstructural characterization of blanked surface of C5191 phosphor bronze sheet under ultra-high-speed blanking

The deformation mechanism of C5191 phosphor bronze sheet under ultra-high-speed blanking was investigated. By virtue of a DOBBY-OMEGA F1 ultra-high-speed press, the ultra-high-speed blanking test was conducted on C5191 phosphor bronze sheets with a thickness of 0.12 mm at 3000 strokes per minute. The microstructures of the blanked edges were characterized and analyzed separately by electron back-scatter diffraction (EBSD) and transmission electron microscopy (TEM). The results show that grains in the blanked edges are stretched along the blanking direction. Strong {001}〈100〉 cube textures (maximum pole densities of 9 and 12, respectively) and secondarily strong {011}〈011〉 textures (maximum pole densities of 4 and 7, respectively) are formed in local zones. Additionally, deformation twins are found in the shear zone of the blanked edges which are rotated and coarsened due to the blanking-induced extrusion and local thermal effect which can further form into sub-grains with clear and high-angle boundaries. The C5191 phosphor bronze sheet is subjected to adiabatic shear during ultra-high-speed blanking, accompanied with dynamic recrystallization.

Study of multilayer material deformation conditions by method of finite element analysis

The finite element analysis method of multilayer materials deformation during hot rolling is studied. Physical conditions, on which the model is based, is described. Calculation results showed the multilayer materials center of deformation singularity and difference from condition of blank edge. It is shown that uniform deformation of multilayer material during hot rolling is possible due to formation of layer bonding. Described results are useful for multilayer materials on the base of different metals.

Numerical Simulation on Edge Crack of Advanced High-Strength Steel Considering Blanking Induced Damage

Sheet metal blanking is an indispensible process for stamping process widely used in automotive industry and home appliances. The damage induced by blanking becomes an important factor that may affect the sheet metal formability and cause edge crack. Conventionally, blanking process is simulated by solid element, while forming processes are simulated by shell element, and it is necessary that the damage legacy data transfer from blanking process simulation to forming process simulation so that the forming processes, especially edge crack can be simulated more accurately. In the present work, a new simulation method is proposed to predict edge crack in stamping of advanced high-strength steels considering blanking history. Blanking simulation is first conducted to obtain residual damage values near the blanked edge area, which will be further mapped onto the shell elements near the blanked edge in forming simulations. Several 3D numerical models of hole-expanding and drawing based on shell element are established to investigate crack during stamping and verified by experiments. The results show that the proposed method can efficiently predict the edge crack more accurately.

Numerical Investigation on Key Process Parameters Affecting Blanking of Steel Sheet by Using Finite Element Method

The cutting surface obtained through the blanking process is often characterized by localized plastic deformations followed by crack propagation and ductile failure. The resulting blanked edge is essentially characterized by known areas such as shear zone, fracture zone and also sometimes burr. Their formation depends essentially on various process parameters and cutting material such as punch/die set, punch speed and temperature. The present study was carried out in order to investigate the possibilities for improving the quality of the cut-surface of high-alloy thick sheet steel. It was focused on modeling and analyzing blanking process of steel sheet using finite element method (FEM). The numerical results of the validation simulations were in agreement with the experimental results, thus validating the model parameters used. The model was developed in order to study the effects of process parameters on the blanking of mechanical steel parts, with particular attention to the study of punch-die gap influence.

Experimental Investigation of the Blanked Surface of C5191 Phosphor Bronze Sheet over a Wide Range of Blanking Speeds.

The influence of blanking speed on the blanked surface quality of C5191 bronze phosphorus sheets, with a thickness of 0.12 mm, was systematically studied to demonstrate the mechanism under high speed blanking. The morphology and microstructure of the blanked edge were observed by using a variety of techniques, including optical microscopy (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM). The results revealed that the local temperature and microhardness of the shear zone increased with the increase in blanking speed. Moreover, the quality of blanked edge significantly improved with the increase in blanking speed due to the combined influence of strain rate hardening and thermal softening. In addition, the blanked edge grains were elongated along the blanking direction and formed dislocation cells and sub-grains in some areas. The blanked edge is dominated by {000} <100> cubic texture at higher blanking speeds, and {112} <111> texture at lower blanking speeds. When punched at an ultra-high speed of 3000 strokes per minute (SPM 3000), the local area of the blanked edge exhibited distinct microstructural features, including low dislocation density, nanocrystals with high-angle grain boundaries, and significant differences in grain orientation. Additionally, the selected area electron diffraction (SAED) pattern exhibited a discontinuous ring-like structure, indicating the occurrence of adiabatic shearing with dynamic recrystallization.

Adiabatic blanking of advanced high-strength steels

Adiabatic blanking of advanced high-strength steels with initial flow stresses above 1300 MPa is investigated. The blanked edge exhibits a unique S-shape. Localisation and properties of shear bands are analysed in shear-compression tests. A shear-compression stress state before separation leads to blanked edges without fracture zone, burr, or roll-over. Numerical modelling predicts the characteristic shape of the blanked edge satisfactorily. Physics-based models reveal that the strain rate sensitivity of the workpiece material is the key parameter affecting the width and the surface hardness of the shear band. Stress triaxiality and strain rate sensitivity determine the minimum size of radii in complex parts blanked without failure.

Delayed fracture in cold blanking of ultra-high strength steel sheets

Hydrogen-induced delayed fracture at cold-blanked edges of 1–1.5 GPa ultra-high strength steel sheets was investigated. The blanked edges undergo large shear deformation and tensile residual stress, and thus the risk of delayed fracture is high, especially for the 1.5 GPa sheet. The effects of residual stress, surface quality and hardness of the sheared edge on the occurrence of delayed cracking were examined. Delayed cracking was caused by press blanking, whereas no cracking occurred for laser blanking because of compressive residual stress. For the 1.5 GPa sheet, delayed cracking was prevented by heating above 250 °C and a stain above 0.005.

Numerical quantification of damage accumulation resulting from blanking in multi-phase steel

The high potentials of utilizing high strength steels in the automotive industry have been proved. However, there are still some unsolved challenges. Forming of a component that has been produced by blanking is one of these. Blanking is commonly used in sheet metal forming as the initial cutting process. Yet, it introduces damage into the blanked edges that in subsequent forming steps may lead to crack formation. This problem arises in particular in modern multi-phase steels and can currently not be avoided thoroughly due to a lack of understanding of the relevant influences. In the present work, the effects of the blanking process on an HCT980XD sheet were therefore numerically investigated. To achieve this, the Edge-Fracture-Tensile-Test method was simulated. By using this method, a conventional uniaxial tensile specimen is manufactured with one milled side and one blanked side. This allows to highlight the effect of blanking in a subsequent tensile test. In order to investigate the damage process, the coupled Modified-Bai-Wierzbicki model was applied to simulate first the blanking process and then the tensile test. The results revealed that during the blanking process, the failure initiated from the surface elements near the punch and propagated through the thickness. Meanwhile, another crack initiated from the opposite side of the sheet. The elements of these cracks experienced near pure-shear condition at both damage initiation and fracture moments. At the end of this step, the remaining damage, which is considered as the predamage of the next step, was higher in the middle of thickness. During the subsequent uniaxial tension, the crack in the specimen initiated at a shear cut edge and crossed the width. The possible detailed resolution of loading paths and crack formation shows that the damage mechanics simulation provides researchers with a powerful tool for assessing limit states in forming processes.

Influence of wear on particle emission in aluminum blanking

The pollution of blanked parts with particles emitted by the blanking operation itself can be encountered in different cases of blanking, in particular aluminum blanking. The consequences of this pollution are a degradation of the quality of the parts produced (appearance, function) and, possibly, a degradation of the behavior of the blanking tool. In this context, we studied the effect of the wear of the punch in the case of AW5754 H111 blanking. Press tests were conducted with a complex part shape and with different press test configurations. The results highlighted two types of wear. The first one is called “run-in” (punch surface accommodation) and leads to a reduction of the emission of particles. On the contrary, the second type of wear, called “pick-up” (material transfer from the sheet), leads to increase particle emission. The mechanisms corresponding to the two particle emission modes are studied. The first mechanism is well known and results from the transition from a blanked edge geometry with secondary shear zone to a more conventional geometry without this secondary shear zone. The second mechanism is new, with sheet material being picked up by the punch and then the picked up material being scraped off, eventually leading to particles being deposited on the die and the part. The study also focuses on the effect of lubrication, clearance and punch shape on these mechanisms. The two mechanisms differ by the influence of lubrication: The second mechanism becomes preponderant as soon as lubrication becomes insufficient, and a solution must be found rapidly because of the larger particle emission and the risk of galling of the punch.

Finite Element Analysis of Blanking Operation of Magnesium Alloy (AZ31) Sheet Using Ductile Fracture Criteria and Its Experimental Verification at Various Temperatures

This study focuses on modeling and analyzing sheet blanking process of Magnesium alloy sheet (AZ31) using finite element method at various temperatures. The computational analysis is then compared with the results obtained through the experimental setup. In the finite element model fracture initiation and the shape of the blanked edge are influenced by the value of critical damage criteria (C). The value of critical criteria is determined and compared using the ductile fracture criteria proposed by various researchers by varying various process parameters and temperature. The effect of temperature and process parameters on the geometry of the blanked edge including burr, fracture zone, shear surface and rollover has been investigated. Moreover, numerical predictions were compared with the experimental results.

Influence of Grain Size and Blanking Clearance on Magnetic Properties Deterioration of Non-Oriented Electrical Steel

In motor manufacturing, blanking process of non-oriented electrical steel inevitably induces plastic deformation and residual stress near the blanked edge and thus leads to the deterioration of magnetic properties. However, as for the newly developed non-oriented electrical steel, the scale of grain size is close to both blanking clearance and thickness. Grain size integrated with blanking clearance thus significantly affects the blanked edge quality. In order to reveal the effects of grain size and blanking clearance on magnetic properties deterioration, systematic blanking tests have been conducted using various non-oriented electrical steel sheets (same chemical composition, three thicknesses, and three grain sizes) in a considered clearance range. The blanked edges are characterized by means of optical microscopy, scanning electron microscopy, and micro-hardness tests. Magnetic domain structures are also observed by magnetic force microscopy. The iron losses are measured by single sheet tester method. The burnished ratio decided by clearance and grain size can be taken as the indicator to assess the deterioration of magnetic properties. The blanked edge with larger burnished ratio leads to lower increase of micro-hardness, less degradation of domain structure, and less deterioration of iron loss. In this paper, about 5.8% iron loss can be reduced by optimization of blanking clearance, which is quite pronounced for energy saving. In further study, an approximate linear function of the ratio of optimum clearance to gain size with $D^{-1/2}$ is empirically proposed to demonstrate the grain size dependence on optimum clearance determination. This paper thus provides an in-depth understanding and guidance for reducing the deterioration of magnetic properties caused by the blanking process.

Grain Size Effect on Optimum Clearance Determination in Blanking Non-Oriented Electrical Steel Sheet

Non-oriented electrical steel, as the core magnetic material, is firstly blanked into lamination in motor manufacturing. As for the newly developed steel, there is a general tendency toward thinner and coarser-grained. Due to blanking clearance and thickness are both down to the sub-millimeter scale, grain size becomes an important role in formation of blanked edge quality, which mainly determines the deterioration level of magnetic properties. This paper aims to systemically investigate the influence of blanking clearance and grain size on blanked edge quality. In this research, non-oriented electrical steel sheets of the same chemical composition, 3 thicknesses and 3 grain sizes are prepared for blanking tests over the conventional relative blanking clearance range. The blanking edges are quantitatively examined by means of optical microscopy to visualize the distribution of plastic deformation. The results show that there exists an optimum clearance that leads to a fine blanked edge. In further study, an approximate linear equation of the ratio of clearance/grain size (c/D) vs. D is found for optimizing the blanked edge quality. This research thus provides an in-depth understanding and guidance for optimum blanking clearance determination influenced by size effect.

Investigation of a Modified Hydrodynamic Deep Drawing Assisted by Radial Pressure with Inward Flowing Liquid Process

Hydrodynamic Deep Drawing (HDDRP), the combination of hydroforming and conventional deep drawing, accommodates the advantages of the two processes. A technique, called HDDRP with inward flowing liquid, has been introduced based on the idea of insertion of radial pressure around the blank rim. The radial pressure created on the blank edge, can increase the drawing ratio. Thus, increasing the radial pressure to an amount greater than the cavity pressure, and independent control of these pressures is the basic idea of this research for forming cylindrical parts. To perform the experiments, two independent pumps were used to provide the two pressures independently. The pressure supply system and the die set were designed in a way that provides simultaneous control of the pressures throughout the process. Then, the effects of radial pressure paths on thickness distribution of cylindrical St13 cups were investigated. In addition, a comparison between HDDRP and HDDRP with inward flowing liquid processes has been performed experimentally. Results indicated that using a higher radial pressure than the cavity pressure and controlling their values at any moment of the process enhances the thickness distribution of the formed part in all regions.

Deep Drawing of Circular Blank with Stepped Edge

It is known that a burr of a blank produced by sheet metal shearing falls off during a subsequent process and causes problems. The authors became interested in this problem and carried out deep drawing experiments using a blank with a stepped edge as a substitution for burr. As a result, there was no rupture at the blank edge, but the stepped flange edge wrinkled regularly depending on the design of the step. The authors considered this deformation to have a new industrial value, for example, the structure of the waveform is effective in bonding at an insert molding. Therefore, we decided to examine the fundamental characteristics of this wavy deformation. Furthermore, we thought that the numerical approach to this deformation may provide a hint for elucidating the burr elimination problem in the actual process. Hence, FEM analysis was also carried out to obtain the damage value and maximum principal stress, and the possibility of fracture was discussed.

Преднамеренная фрагментация пластин как специфический прием в технологии начального верхнего палеолита стоянки Кара-бом (Горный Алтай, Россия)

Purpose. Recent investigations have highlighted an Asian variant of the so-called Initial Upper Paleolithic (IUP) broadly comparable in age and material culture to techno-complexes further to the west, but also showing distinct derived features. Several principal common technological features characterize the IUP in East Asia. The main targeted products of flaking were medium or large blades, the latter sometimes of very significant size, and the number of bladelets is also large. Primary flaking is characterized by alternate bidirectional reduction of cores in which spalls, alternately detached from opposing platforms along the long axis of the core, determined the shape of targeted blank: pointed blades. Reduced cores were prismatic, sub-prismatic and flat. Burin-core reduction for bladelet and small blade production was the specific knapping technology employed in the IUP of southern Siberia and Central Asia. Here, we describe and provide corroborating evidence for another distinct technological method employed in the Initial Upper Paleolithic – intentional fragmentation (IF). The most effective means of understanding knapping technology are refitting studies of archaeological collections. This article examines several examples of refitted fragmented cores and blades, as well as debitage as the by-product of blank breakage. Results. Our refitting study includes assemblages from all excavation units and partially divided, relatively homogenous raw material types, representing a diachronic assemblage of Middle and Upper Paleolithic materials, even in very disturbed excavation areas. The present study illustrates the best examples of directional reduction associated with core preparation and tool blank production because of the lack of statistical information for some Upper Paleolithic assemblages from this site. We reconstructed the process of intentional fragmentation for burin-cores and a few large and medium blades. Often, blank breakage produced butterfly-like debitage. Pieces of fragmented blades could have been used as tools. Typical attributes of IF consistently appearing on two transverse edges of blanks and present in the assemblage of artifacts prove the anthropogenic character of these flaking traces. Conclusions. It is probable that intentional fragmentation was used in the Initial Upper Paleolithic assemblage at Kara-Bom because of the influences of mobility and transportation of stone raw material by local settlers. They transported a significant proportion of raw material from primary chert outcrops situated 4–5 km from the Kara-Bom site.

Effect of Weld Line Direction on Tensile Deformation Behavior

The stretch flangeability of tailor-welded blank depends on the angle between weld line and the tangential line of blank edge, which is generally the tensile direction in stretch flanging. This effect was studied by uniaxial tensile testing of tailor-welded high-strength-steel specimens with a weld line at 45, 60, or 90 degrees from the tensile direction. The 60-degree-weld-line specimen provides the highest elongation at the maximum load. This is the angle at which the strain component in the weld-line direction is close to zero. At the other angles, the weld lines, of which hardness sufficiently increases by welding, play a role as a constraint for vicinal base metal. This leads to early strain localization and low elongation. Furthermore, the elongation at the maximum load can, in some cases, exceed the uniform elongation of base metal. This is pronounced by a base metal with a low n-value.