Burnish Zone Research Articles

Evaluating die wear-induced edge quality degradation in trimmed DP980 steel sheets from in situ force response monitoring

Assessing trimmed part quality involves periodic examinations of cut edges and shear affected zones to characterize rollover, burnish, fracture and burr formation. Established techniques are thorough and reliable but inherently time consuming. This study correlated die wear-induced force response evolutions and the observed transitions between shear-induced deformation zones for DP980 (C-0.15 wt%Mn-1.5 wt%Si-0.31 wt%) steel sheets subjected to trimming with AISI D2 steel dies. The findings are useful for peripheral assessment of part quality and tool wear via in situ trimming force/displacement response monitoring. Measurements were collected from a load cell and a non-contact laser displacement transducer embedded within the instrumented trimming press. The sliding wear damage and failure processes induced by die–workpiece interactions were correlated to force/displacement response phases. Trimming tests were conducted for 20,000 cycles to observe and assess transformations between rollover and burnish zones with increasing tool wear after the initial contact and at every subsequent 5000 cycle interval. The mean trimming force and maximum die penetration increased, primarily due to die edge blunting, by 1.33 kN (44.3%) and 194 μm (32.2%), respectively. An enlarged die–workpiece contact area increased the resistance force and delayed the onset of fracture with increased sliding damage between sheared surfaces and nearly doubled burnish zone depths. Quantitative attributes of the trimming force/displacement responses were linked to die impact and sliding contact induced damage features of trimmed parts, demonstrating the potential for straightforward, rapid and detailed in-field monitoring of acceptable limits to sheared edge quality.

A Novel Force Control Strategy for Improved Surface Integrity in Low Plasticity Burnishing

Ball burnishing is a cold work process where a hard ceramic or diamond ball rolls on a metal surface and flattens the roughness peaks under high local pressure. The small deformation created on the surface imposes compressive residual stresses and raises hardness in a shallow sub-surface layer, leading to improved fatigue, corrosion, and foreign object damage performances. Trial-and-error type experimental work to determine the optimum process parameters for a cold-forming process like ball burnishing for acceptable performance is costly. Therefore, the article aims to investigate the effects of various force control strategies in the double-sided low plasticity burnishing (LPB) process to find the effects on deformation and residual stresses on thin Ti6Al4V flat sheets. A 3D static-implicit finite element model was developed with an elastic-rigid plastic flow curve. Simulations were conducted to predict residual stresses and deformationі on the surface. As a result, it was proven that ball burnishing can produce a deterministically controlled surface. An increased vertical force produced higher deformation normal to the surface and, therefore, a deeper pool. As the ball proceeded further, a plowing effect developed such that when a 3.5–4.8 mm deep pool was formed (at a vertical force of 150 N), a peak of 2.8 mm was produced at the front end. Overall, the deformation on the surface and the residual stresses were directly interrelated. Parallel to the deformation on the surface, residual stresses on and beneath the surface also showed some variation. Nevertheless, the predicted residual stress variations were not big. They did not switch to the tensile mode in the burnished zone. Therefore, the whole sheet surface should be burnished to obtain all the compressive residual stresses.

Punching of Ultra-High-Strength Spring Strips: Evolution of Cutting Edge Radius up to 1,000,000 Strokes for Three Punch Materials

Punching of ultra-high-strength spring steel causes critical stresses in the tools. Pronounced wear and even spontaneous failure may occur. Wear of the punches influences the quality of the cutting surfaces of the blanked parts, which is predominantly determined by the cutting edge radius. The radius differs with an increasing number of strokes depending on the punch material. However, there are no studies characterizing the influence of the cutting edge radius on the cutting surface quality on an industrial scale, i.e., considering a very high number of strokes. In the presented study, punches made of high-speed steel, powder metallurgical steel and carbide were used to punch the ultra-high-strength steel 1.4310 (Rm = 1824 MPa) up to 1,000,000 strokes. The experiments were stopped at defined number of strokes, the punches were removed, nondestructively characterized regarding cutting edge radius and wear and reinstalled. It turned out that the radius differed significantly over the number of strokes and, further, varied depending on the punch material. Remarkably, the most low-cost material, precisely the high-speed steel, showed the smallest cutting edge radius of 16 µm and brought the parts with the best cutting surface quality (more than 30% burnish zone) after the maximum number of strokes. The results indicate clearly that the cutting edge radius develops differently for each regarded material and at different number of strokes. Therefore, it is of utmost importance to perform wear tests on different numbers of strokes under industrial conditions. With the knowledge gained, it will be possible to design optimized punches with lower costs and increased lifetime.

Formability of the heat-assisted fine-blanking process for 304 stainless steel plates

To widen the application of the fine-blanking process, the heat-assisted fine-blanking process for 304 stainless steel plate has been conducted. The quality of the sheared surface and the hardness distribution of the deformed part were studied to determine the influence of the forming condition on forming results and the performance of the deformed part. Furthermore, electron backscatter diffraction and X-ray diffraction analyses were conducted to study the microstructure evolution and phase transformation of the deformed part for determining their forming mechanism and underlying reasons for their hardness distribution. The quality of the sheared surface improved with an increase in the forming temperature. When the temperature was increased to 250 °C, a sheared surface with a full burnish zone was obtained for a linear segment. Moreover, the forming temperature not only affected the hardness value of the deformed part, which reduced with an increase in the forming temperature, but also affected the hardness distribution. The experimental results indicated that the “hardness-affected zone” decreased in size with an increase in the forming temperature. Through microstructure and dislocation density analysis, changes in the quality of the sheared surface and hardness of the heat-assisted fine-blanked 304 stainless steel part could be attributed to the variation in the martensite content and dislocation density of deformed part, both of which decreased with increasing forming temperature.

Experimental investigation on electromagnetic punching process of hybrid CFRP/Al stacks under different discharge energies

Abstract Machining holes on composites/metal stacks usually accompanied by burrs, chips and delamination damage. To solve those problems, this paper proposed a high speed punching technology for CFRP/Al stacks: electromagnetic punching (EMP). The machined hole qualities of CFRP/Al stacks (e.g. microscopic morphology, delamination damage and dimensional accuracy) for EMP process with various discharge energies were investigated. Besides, conventional quasi-static punching was performed as a comparing process. Note that the CFRP/Al stacks were fabricated with eight unidirectional CFRP plies and an aluminum alloy sheet, cured in an autoclave. The results showed that burrs of EMP were less than quasi-static punching, the heights of burnish zone under the discharge energies of 5 kJ, 6 kJ and 7 kJ (the corresponding maximum punching speed are 5.46 m/s, 7.63 m/s and 9.50 m/s) were respectively 32.5%, 40.0% and 55% higher than that of quasi-static punching process. Based on the results, the shear mechanism for the punching process was analyzed. The punching process could be divided into four stages, CFRP layers and Al sheet were sheared off sequentially. The dimension measurement results showed that diameter deviation between the hole entrance and exit became smaller under higher discharge energy. While the delamination was aggravated as the discharge energy exceeded 7 kJ, the discharge energy of 6 kJ was the most appropriate to machine holes on the CFRP/Al stacks.

Deformation characteristics and grain size effect of thin silicon steel sheet during shearing

The characteristics of shearing force and shearing surface, working hardening at the shearing edge, and structural evolution of microstructures during the transverse shearing of thin silicon steel sheet (SSS) were explored. On this basis, the influences of the thickness and grain size of the thin SSS and lateral clearance of the tool on shearing deformation processes were further analyzed. The results showed that on the condition that the grain size of the thin SSS increased, the compatibility of deformation was worsened and thus the orientation of grains influenced the deformation process more, consequently leading to the expansion of the plastically deformed zone and therefore increased shearing force. On the condition of having the same lateral clearance of the tool, with the growth of grain size, the rollover zone of shearing surface increased, the burnish zone reduced, burr increased, the plastically deformed zone size increased, the range and degree of work hardening increased, and the quality of shearing surface was impaired. The shearing of the thin SSS with a large grain size was more sensitive to lateral clearance of the tool: various defects were more likely to occur in the shearing fracture surface.

Research on piercing of aluminum sheet assisted with magnetic medium

At present, flexible die medium represented by fluid is mainly used in tube hydropiercing, but it is relatively rare in sheet forming field because of the high requirement of sealing. For this reason, magnetorheological fluid, one of the representatives of intelligent materials, is used as a force transfer medium to provide backpressure for the piercing process of sheet for the first time in this paper, so as to reduce the collapse and improve the quality of fracture surface. The results show that with the increase of internal pressure, the collapse of the area near the cutting edge of punch decreases, the length of burnish zone increases, the aperture is closer to the nominal diameter, and the quality of the hole is significantly improved. The theoretical mechanics model of a single straight chain is established through theoretical analysis, and the magnitude and loading mode of the force exerted on the sheet by magnetic medium are deduced. Through the balanced relationship between punching force and backpressure in the deformed area of sheet, the key parameters affecting the collapse of piercing of sheet are obtained, which has important guiding significance for improving the quality of sheet metal piercing products with magnetic medium.

Mechanisms of die wear and wear-induced damage at the trimmed edge of high strength steel sheets

Die wear during trimming of advanced high strength steel (AHSS) sheets deteriorates the edge quality of trimmed sheets. In this work, the mechanisms of AISI D2 steel trim die wear and their effects on plastic deformation and fracture behaviour of the sheared edge of DP980 steel sheet were examined. A mechanical press equipped with D2 inserts was used to trim DP980 sheets with a clearance of 0.14 mm (10%). Abrasion and microchipping were identified as the wear mechanisms operating at the upper die, with microchipping becoming more dominant after 60,000 trimming cycles. The burr height and the length of the burnish zone of sheared DP980 sheets increased linearly with the chipped die edge percentage. The shear strains in the shear effected zone (SAZ) were estimated using martensite displacements as metallographic markers in the ferrite matrix as a function of the depth beneath the sheared edge. The depth of SAZ, and the plastic strains at a given depth within the SAZ increased with the number of trimming cycles. Correlation of the local stress and strain values generated in the SAZ showed that a saturation flow stress was reached near the sheared edge. The damage in SAZ occurred in the form of crack formation at the martensite/ferrite interfaces and fracture of martensite. Die wear reduced the tensile ductility of the DP980 sheets, and the fracture mode in tension changed from ductile with localized neck to more brittle sheared edge fracture initiating from surface cracks after 60,000 cycles.

Investigations on edge quality and its effect on tensile property and fracture patterns of QP980

Advanced High Strength Steel (AHSS) sheet metals have been widely used in automotive structures due to its safety performance and lightweight potentials, and some AHSS materials have been found more sensitive to edge crack during the stamping process. The tensile property of the sheared edge has great influence on flangability for AHSS. In the present work, the effects of the blanking clearance and shear angle on the quality of the sheared edge and subsequent tensile property for QP980 have been investigated experimentally by microscope and sheared-edge tension test based on digital image correlation (DIC) technology. In addition, the tensile properties of the specimens prepared by wire electrical discharging machining (Wire-EDM) and laser cutting were tested as well. The results show that promising and stable sheared edge quality, and tension with postponed fracture can be achieved if the clearance ratio is set as 7% or 11% for QP980. During blanking with 5° or 10° oblique edge, the ratios of burnish zone and burr zone decrease simultaneously, and the subsequent tensile performance increases significantly. The oblique edge blanking obtains smaller and smoother burr, and the sheared surface benefits from less void damage. In addition, based on the fracture surface morphology and the tracking of the fracture process with DIC technology, two typical fracture patterns (oblique crack and transverse oblique crack) in tension test are illustrated. Oblique crack occurring on the specimens with good edge quality shows obvious necking phenomenon and belongs to the typical ductile fracture. While transverse oblique crack exhibiting with high frequency on the sheared specimens is typical edge fracture and shows mixed characteristics of ductile fracture and brittle fracture.

Influence of cutting tool stiffness on edge formability

For the evaluation of the forming behaviour of cut edges of AHSS the Hole Expansion Test (HET) standardized in the ISO 16630 is generally used. However, the observed Hole Expansion Ratio (HER) is prone to significant scatter. One reason for this scatter is the subjective observation of the through thickness crack by the machine operator. Additionally, the ISO is not very specific in its description of the test setup and material preparation, actually allowing great variations in the cutting process. Although the nominal cutting clearance is specified, a low stiffness of the punching machine or tool can cause a non-uniform clearance along the circumference. Additionally, an oblique position of the punch can cause an angled or eccentric sheared hole, resulting in additional crack initiation sites on the shear cut surface. FE simulations are used to investigate the stiffness effects of a C-frame press design. A comparative round robin analysis based on ISO16630 was performed by cross-testing both cutting and hole expansion setups for a high strength hot rolled steel grade. Significant differences were registered in the HER values, whereby the cutting process was identified to have the largest influence on HER variation. The homogeneity of the burnished zone along the circumference was observed to give valuable information about the cut edge quality and subsequent level of HER values.

Finite element simulation of the punch with inclined edge in the sheet metal blanking process

Finite element (FE) simulation has been performed to investigate the three new punches with inclined edges in the sheet metal blanking process. In this paper, three punches with single inclined edge, double convex inclined edges and double concave inclined edges, respectively, are proposed. The effects of three types of punches and different inclined edge angles on maximum blanking force, die roll width, die roll depth, burnish zone and fracture zone have been investigated and compared by finite element method (FEM). The modified Cockcroft and Latham expression is used as ductile fracture criterion and linked with the simulation. The simulation results indicate the maximum blanking forces of punches with inclined edges is smaller than that of the punch with flat edge, while the blanking process using flat edge punch could obtain better quality of the blanked surface than using the punches with inclined edge.

Experimental study of lancing parameters on stacking strength for laminated sheet

The purpose of this work is to study the effect of lancing parameters on increasing stacking strength in the progressive stamping process for laminated sheet producing motor core parts. Sheet material is silicon steel JIS C2552 with 0.5 mm thickness. Investigating parameters include clearance of cutting and geometry on the lancing punch; straight length of punch bottom and punch angle. It can be concluded that tool with small clearance introduce high burnish zone on the cutting edge thus higher contact area increasing stacking strength. However, tool with large clearance introduce larger size of the blank than its hole thus also increasing stacking strength. The maximum stacking strength obtained from lanced sheet with straight length on the bottom part of 1.5 mm due to proper matching with the size of piercing hole. Small straight length of punch bottom introduced large space to the pierced hole of bottom sheet thus bending of the sheet occurred reducing the joining strength. Excessive straight length of punch bottom caused distortion of sheet under stacking process thus reducing the stacking strength. Large angle on the punch causes high stacking strength because of high compress fit between sheets. However, the small angle of bending punch has also high stacking strength due to larger area of contact.

Effect of technological parameters on microstructure and accuracy of B1500HS steel parts in the hot blanking

Hot blanking process could be used for producing diverse fine blanking parts with comprehensive mechanical properties and to solve the problems in the piercing and trimming of press-hardened parts. To explore the optimal hot blanking process and evaluate the effects of some technological parameters on the phase transformation, fracture quality, and dimensional accuracy of the parts, several different blanking temperatures (450–800 °C) and die clearances (8–25%) were used in the hot blanking of B1500HS steel. The experimental results show that as the blanking temperature increases at a certain die clearance, the dimensional accuracy shows a trend of “negative growth–positive growth–negative growth,” and a higher dimensional accuracy of blanked parts can be attained at blanking temperatures 450–500, 600–650, or 750–800 °C. Besides, the burnish zone width increases, and the burnish and rollover zone widths account for ~ 80% of the steel sheet thickness at 800 °C. At blanking temperatures 650–800 °C, the microstructure of blanked parts is completely martensite, the microhardness of parts is ~ 550 HV, and the perpendicularity of fracture increases as the blanking temperature increases. Parts with a high-dimensional accuracy, better fracture perpendicularity, wide burnish zone, better mechanical properties, and wear resistance can be obtained at blanking temperatures 750–800 °C.

Fracture morphologies of a hot stamped steel and comparisons with several sheet metals

Hot stamping has been widely used in car industry to produce safety components. Most existing researches focused on the stamping and quenching process, but less on the mechanical properties of stamped parts. The fracture behaviors of hot stamped boron steel B1500HS have been studied, and other four commonly used sheet metals with different strengths, including Q235, TRIP780, QP980 and MS1300, were also introduced for comparison. Both uniaxial tests and mechanical trimming tests were performed, and the fracture surfaces under different stress states were observed and discussed. The SEM observations showed that the fracture models are closely related to the stress states, i. e., the tensile surfaces have ductile rupture characters while the trimming surfaces have brittle rupture characters. Compared with other steels, the quenched boron steel has smaller dimple size accompanied by shear planes in the tensile surface, and has smaller burnish zone in the trimming surface, and its cutting surface with ‘S’ like shape is also very different with others. Furthermore, two fitted empirical models were derived to describe the quantitative correlations between the average dimple diameter and the steel strength and between the percentage of burnish zone and the steel strength.

Parameters Design of Discontinuous Dot Indenter in Fine Blanking Process with Different Thickness Workpiece

This paper proposes a new blankholder to fine blanking process. V-ring indenter has been widely applied in fine blanking to produce clean cut parts, however, it is difficult to be manufactured, the machining accuracy of which is hard to ensure and the cost is very high. In this approach, the fine blanking process combined with discontinuous dot indenter was put forward and the parameters design for workpiece with different thickness was studied with the finite element simulation and the orthogonal experiment methods. The larger burnished surface zone can be obtained by optimizing discontinuous dot indenter parameters. In addition, the relationship between the discontinuous dot indenter parameters and the workpiece thickness was got from data processing. Finally, applying this relationship to fine blank workpiece with different thickness, nearly full clean cut surface part was obtained.

Numerical and experimental investigation of the discontinuous dot indenter in the fine-blanking process

The fine-blanking process has been widely applied in industries to produce clean-cut surface parts. The V-ring indenter blank holder, which is an essential part of the fine-blanking tool, is notably important in increasing the cutting surface quality of blanked parts. Because the V-ring indenter is difficult to manufacture, it is difficult to ensure the machining accuracy, and the cost is notably high. In this study, a new discontinuous dot indenter blank holder is presented. The discontinuous dot indenter blank holder can improve the quality of the fine-blanked surface, which is identical to that of the V-ring indenter, but the manufacture process is notably easy, and the cost is notably low. The hydrostatic stress, material flow and quality of the fine-blanked surface in the fine-blanking process were analyzed to reveal the discontinuous dot indenter mechanism. To obtain a larger burnished zone, the finite-element (FE) simulation and ANOVA technique were applied to optimize the discontinuous dot indenter parameters, i.e., dot diameters, fillet radius, distributions, positions and heights. Next, these optimal parameters were applied to fine-blank a spur gear, and the experiment results have a good agreement with the FE simulation results. Moreover, the results also indicate that the discontinuous dot indenter can be used in the fine-blanking process to obtain almost full clean-cut surface parts.

Investigation on Shearing and Local Formability of Hot-Rolled High-Strength Plates

In order to evaluate the shearing quality, the material inhomogeneity through thickness after shearing is introduced by the authors. This study investigates the shearing and local formability of hot-rolled high-strength steel (HSS) plate, which is generally exploited for the manufacturing of the beam of heavy trucks. Various kinds of plates with different thicknesses and strengths are used to figure out the effect of material properties on the shearing quality. Both the shear surface morphology and microhardness distribution of the sheared edge are considered for evaluating the influence of the sheared-edge quality on local formability during the following forming process. Vickers hardness tests are conducted to analyze the microhardness distribution on the shear surface, which is proved to have significant effect on the local formability of the sheared edge. Furthermore, two kinds of bending tests and simulation are employed to study the edge cracking phenomenon, and the results indicate that the junctional zone of burnished zone and fracture zone, which is defined as peak hardness zone (PHZ), has a significant impact on major strain distribution on shear surface in the side bending test and this region is the main cause of edge cracking in normal bending test.

Numerical and experimental investigations on mechanical trimming process for hot stamped ultra-high strength parts

In this paper, the mechanical trimming process for hot stamped ultra-high strength parts was studied, in which the properties of fracture profile were analyzed and the effects of process parameters were evaluated. The research was carried out by FEM simulation with comparison and chosen of damage models, where the critical damage values were obtained through an iterative “predictor-corrector approach”. In addition, an experimental trimming tool was designed to study the effects of variable process parameters and verify the simulation results. By comparison, it was found that the Oyane damage model agreed with the experimental results better than others. The simulation and experimental results showed that the quality of trimmed hot stamped surface was poor with not straight profile and small burnish zone, but it can be improved by choosing proper trimming angle and blade radius. Such phenomena were discussed and explained in terms of stress state. Tensile stress accelerates the damage value while compress stress inhibits it. This paper helps to better understand the mechanical properties of hot stamped ultra-high strength steel.

Fracture Profile and Crack Propagation of Ultra-High Strength Hot-Stamped Boron Steel During Mechanical Trimming Process

Mechanical trimming process for ultra-high strength boron steel after hot stamping was carried out in this study. Shear and tensile tests were designed to analyze the influences of stress state on the fracture mode; trimmed fracture surface and profile were observed and compared to other commonly used steels such as DP980 and Q235 etc.; the crack propagation during trimming process was studied through step-by-step tests. The observation and analysis reveal that the fracture mode of hot-stamped boron steel is highly related to the stress state, it belongs to cleavage fracture on low stress triaxiality but dimple fracture on high stress triaxiality. Such phenomenon is reflected in the trimming process, during which the stress state changes from shear-dominated state to tensile-dominated state. In addition, the burnish zone of trimmed boron steel is much smaller than other high strength steels, and the profile of cutting surface shows an ‘S’-like shape which is destructive to the trimming tool. Moreover, during the trimming process, most martensite laths near the cutting edge are stretched and rotated markedly to the direction of the shear band, and the main crack expands along those grain boundaries, which may penetrate through a few martensite laths and form small crack branches.

Experimental and numerical study on the plane-strain blanking process on an AHSS sheet

The blanking/trimming/cropping process introduces a substantial plastic deformation to the sheet metal and causes premature edge fracture during the subsequent forming process. In an attempt to understand how the blanking process affects edge fracture, an experimental and numerical study was undertaken on the plane-strain blanking process. Blanking tests on a DP780 steel sheet were carried out on a special fixture utilizing a in-situ microscope for the digital mage correlation (DIC) deformation measurement. The DIC method provides a detailed deformation field of the specimen that has not been reported in any other publications before. Interrupted tests were carried out to study the crack formation and propagation during the blanking procedure, while scanning electron microscope was applied to examine the blanked surface quality as well as the edge profile after test. Following the experimental study, a detailed finite element model with mesh size of 0.01 mm in the critical region was established for the numerical investigation. The model features (a) a non-associated Hill 48 flow rule, (b) von-Mises yield condition and (c) modified Mohr–Coulomb fracture model. With material parameters calibrated from the in-plane tests as well as accurate boundary conditions measured in actual tests, the finite element model accurately predicted the blanking process quantitatively. The current study also gave quantitative values of the parameters of interest during the blanking test, such as the global load displacement response and the local strain gradient history. The geometrical features of the blanked edge, i.e. the amount of roll over, the extent of the burnished zone and fracture zone were all accurately predicted by the present simulation.