Cutting, Blanking, Shearing & Trimming

Cutting, Blanking, Shearing & Trimming

 

Advanced High-Strength Steels (AHSS) exhibit high degrees of work hardening, resulting in improved forming capabilities compared to conventional HSLA steels. However, the same high work hardening creates higher strength and hardness in sheared or punched edges, leading to reduced edge ductility. Microstructural features in some AHSS grades contribute to their sheared edge performance.  While laser cutting results in less edge damage than mechanical cutting methods, the heat from laser cutting produces a localized hear treatment, changing the strength and hardness at the edge.  Achieving the best formability for chosen processing path requires generating a consistent good quality edge from the cutting operation.

To avoid unexpected problems during a program launch, use production intent tooling as early in the development as possible. This may be a challenge since blanking dies are usually among the last set of tools completed.  In the interim, many companies choose to use laser cut blanks. Tool, blank, and process development must account for the lower-ductility sheared edges in production blanks.

 

Edge Ductility Measurements

This article describes the impact of cutting and cut-edge quality on edge ductility.  The primary tests which quantify edge ductility are Hole Expansion Testing, 2-D Edge Tension Testing, and Half Specimen Dome Testing.  These links detail the testing procedures.  The Hole Expansion Testing article has additional information pertaining to the effect of burr orientation and punch shape.

 

Cut Edge Quality

Any mechanical cutting operation such as blanking, piercing, shearing, slitting, or trimming reduces edge ductility.  Each of  these processes generate a zone of high work hardening and a reduced n-value. This work hardened zone can extend one-half metal thickness from the cut edge. This is one reason why edges fail at strains lower than that predicted by the forming limit curve for that particular grade (Note that FLCs were developed based on necking failure, and that edge cracking is a different failure mechanism). 

DP and TRIP steels have islands of martensite located throughout the ferritic microstructure, including at the cut edges. These hard particles act as crack initiators and further reduce the allowable edge stretch. Metallurgical changes to the alloy minimize the hardness differences between the phases, resulting in improved edge ductility.  Laser, EDM or water jet cutting approaches minimize work hardening at the edges and the associated n-value reduction, also leading to improved edge ductility.

Putting shear angles into cutting tools is a well-known approach to reduce cutting forces.  Modifying the cutting tool leads to other benefits in terms of edge ductility. Researchers studied the effects of a beveled punch instead of the traditional flat bottom punch.S-9, S-50, S-52 In these studies, the optimized bevel angle was between 3 and 6 degrees, the shear direction was parallel the rolling direction of the coil with a die clearance of 17%.  With the optimal cutting parameters, the hole expansion ratio increased by 60% when compared to conventional flat punching process.  As expected, a reduction in the maximum shearing force occurred – by more than 50% in certain conditions.  Dropping the shearing force helps reduce the snap through reverse tonnage, leading to longer tool and press life.

Multiple studies examine the trimmed edge quality based on various cutting conditions in mechanical shearing operations and other methods to produce a free edge such as milling and cutting using a laser or water jet. Edge quality varies based on parameters like cutting clearances, shear angles, and rake angles on mechanical shearing operations.

A typical mechanically sheared steel edge has 4 main zones – rollover, burnish, fracture, and burr, as shown in Figure 1.

Figure 1: Cross Section of a Punched Hole Showing the Shear Face Components and Shear Affected Zone S-51

Figure 1: Cross Section of a Punched Hole Showing the Shear Face Components and Shear Affected Zone.K-10

 

Parts stamped from conventional mild and HSLA steels have historically relied on burr height as the main measure of edge quality, where the typical practice targeted a burr height below 10% of metal thickness and slightly larger for thicker steel. Finding a burr exceeded this threshold usually led to sharpening or replacing the trim steels, or less likely, adjusting the clearances to minimize the burr.

Greater burr height is associated with additional cold working and creates stress risers that can lead to edge splitting. These splits, however, are global formability related failures where the steel thins significantly at and around the split, independent of the local formability edge fractures associated with AHSS.  A real-world example is shown in Figure 2, which presents a conventional BH210 steel grade liftgate with an excessive burr in the blank that led to global formability edge splitting in the draw die.  The left image in Figure 2 highlights the burr on the underside of the top blank, with the remainder of the lift below it.  The areas next to the split in the right image of Figure 2 shows the characteristic thinning associated with global formability failures.

Figure 2: Excessive burr on the blank led to a global formability split on the formed liftgate.  The root cause was determined to be dull trim steels resulting in excessive work hardening.U-6

Figure 2: Excessive burr on the blank led to a global formability split on the formed liftgate.  The root cause was determined to be dull trim steels resulting in excessive work hardening.U-6

 

Due to their progressively higher yield and tensile strengths, AHSS grades experience less rollover and smaller burrs. They tend to fracture with little rollover or burr. As such, detailed examination of the actual edge condition under various cutting conditions becomes more significant with AHSS as opposed characterizing edge quality by burr height alone. Examination of sheared edges produced under various trimming conditions, including microhardness testing to evaluate work hardening after cold working the sheared edge, provides insight on methods to improve cut edge formability.  The ideal condition to combat local formability edge fractures for AHSS was to have a clearly defined burnish zone with a uniform transition to the fracture zone. The fracture zone should also be smooth with no voids, secondary shear or edge damage (Figure 3).

Figure 3: Ideal sheared edge with a distinct burnish zone and a smooth fracture zone (left) and a cross section of the same edge (right).U-6

Figure 3: Ideal sheared edge with a distinct burnish zone and a smooth fracture zone (left) and a cross section of the same edge (right).U-6

  

If clearances are too small, secondary shear can occur and the potential for voids due to the multiphase microstructure increases, as indicated in Figure 4.  Clearances that are too large create additional problems that include excessive burrs and voids. A nonuniform transition from the burnish zone to the fracture zone is also undesirable. These non-ideal conditions create propagation sites for edge fractures. 

Figure 4: Sheared edge with the trim steel clearance too small (left) and a cross section of the same edge (right) showing a micro crack on the edge. Tight clearance leading to secondary shear increases the likelihood of edge fracture.U-6

Figure 4: Sheared edge with the trim steel clearance too small (left) and a cross section of the same edge (right) showing a micro crack on the edge. Tight clearance leading to secondary shear increases likelihood of edge fracture.U-6

 

There are multiple causes for a poor sheared edge condition, including but not limited to:

  • the die clearance being too large or too small, 
  • a cutting angle that is too small, 
  • worn, chipped, or damaged tooling,
  • improperly ground or sharpened tooling,
  • improper die material, 
  • improperly heat-treated die material, 
  • improper (or non-existent) coating on the tooling, 
  • misaligned die sections, 
  • worn wear plates, and
  • out of level presses or slitting equipment. 

The higher loads required to shear AHSS with increasingly higher tensile strength creates additional deflection of dies and processing equipment. This deflection may alter clearances measured under a static condition once the die, press, or slitting equipment is placed under load. As a large percentage of presses, levelers, straighteners, blankers, and slitting equipment were designed years ago, the significantly higher loads required to process today’s AHSS may exceed equipment beyond their design limits, dramatically altering their performance.

A rocker panel formed from DP980 provides a good example showing the influence of cut edge quality. A master coil was slit into several narrower coils (mults) before being shipped to the stamper.  Only a few mults experienced edge fractures, which all occurred along the slit edge. Understanding that edge condition is critical with respect to multiphase AHSS, the edge condition of the “good” mults and the “bad” mults were examined under magnification. The slit edge from a problem-free lift (Figure 5) has a uniform burnish zone with a uniform transition to the smooth fracture zone. This is in contrast with Figure 6, from the slit edge from a different mult of the same coil in which every blank fractured at the slit edge during forming. This edge exhibits secondary shear as well as a thick burnish zone with a non-uniform transition from the burnish zone to the fracture zone.

Figure 5: Slit edges on a lift of blanks that successfully produced DP980 rocker panels. Note the uniform transition from the burnish zone to the fracture zone with a smooth fracture zone as well.U-6

Figure 5: Slit edges on a lift of blanks that successfully produced DP980 rocker panels. Note the uniform transition from the burnish zone to the fracture zone with a smooth fracture zone as well.U-6

 

Figure 6: Slit edges on a lift of blanks from the same master coil that experienced edge fractures during forming. Note the obvious secondary shear as well as the thicker, nonuniform transition from the burnish to the fracture zone.U-6

Figure 6: Slit edges on a lift of blanks from the same master coil that experienced edge fractures during forming. Note the obvious secondary shear as well as the thicker, nonuniform transition from the burnish to the fracture zone.U-6

 

Cutting Clearances: Burr Height and Tool Wear

Cutting and punching clearances should be increased with increasing sheet material strength. The clearances range from about 6% of the sheet material thickness for mild steel up to 16% or even higher as the sheet metal tensile strength exceeds 1400 MPa.

A study C-2  compared the tool wear and burr height formation associated with punching mild steel and several AHSS grades. In addition to 1.0 mm mild steel (140 MPa yield strength, 270 MPa tensile strength, 38% A80 elongation), AHSS grades tested were 1.0 mm samples of DP 350Y600T (A80=20%), DP 500Y800T (A80=8%), and MS 1150Y1400T (A80 = 3%).  Tests of mild steel used a 6% clearance and W.Nr. 1.2363 / AISI A2 tool steel hardened to 61 HRC.  The AHSS tests used engineered tool steels made from powder metallurgy hardened to 60-62 HRC.  The DP 350/600 tests were run with a TiC CVD coating, and a 6% clearance. Tool clearances were 10% for the MS 1150Y1400T grade and 14% for DP 500Y800T.

In the Tool Wear comparison, the cross-section of the worn punch was measured after 200,000 hits.  Punches used with mild steel lost about 2000 μm2 after 200,000 hits, and is shown in Figure 7 normalized to 1. The relative tool wear of the other AHSS grades are also shown, indicating that using surface treated high quality tool steels results in the same level of wear associated with mild steels punched with conventional tools.

Figure 7: Tool wear associated with punching up to DP 500Y800T using surface treated high quality tool steels is comparable to mild steel punched with conventional tools. C-2

Figure 7: Tool wear associated with punching up to DP 500Y800T using surface treated high quality tool steels is comparable to mild steel punched with conventional tools.C-2

 

Figure 8 shows the burr height test results, which compared burr height from tests using mild steel punched with conventional tool steel and two AHSS grades (DP 500Y800T and MS 1150Y1400T) punched with a PM tool steel. The measured burr height from all AHSS and clearance combinations evaluated were sufficiently similar that they are shown as a single curve.

Figure 8  Burr height comparison for mild steel and two AHSS grades as a function of the number of hits. Results for DP 500Y800T and Mart 1150Y1400T are identical and shown as the AHSS curve.C-2

Figure 8:  Burr height comparison for mild steel and two AHSS grades as a function of the number of hits. Results for DP 500Y800T and Mart 1150Y1400T are identical and shown as the AHSS curve.C-2

 

Testing of mild steel resulted in the expected performance where burr height increases continuously with tool wear and clearance, making burr height a reasonable indicator of when to sharpen punching or cutting tools.  However, for the AHSS grades studied, burr height did not increase with more hits. It is possible that the relatively lower ductility AHSS grades are not capable of reaching greater burr height due to fracturing, where the more formable mild steel continues to generate ever-increasing burr height with more hits and increasing tool wear.

Punching AHSS grades may require a higher-grade tool steel, possibly with a surface treatment, to avoid tool wear, but tool regrinding because of burrs may be less of a problem.  With AHSS, engineered tool steels may provide longer intervals between sharpening, but increasing burr height alone should not be the only criterion to initiate sharpening: cut edge quality as shown in the above figures appears to be a better indicator.  Note that regrinding a surface treated tool steel removes the surface treatment. Be sure to re-treat the tool to achieve targeted performance.

 

Cutting Clearances: General Recommendations

Depending on the source, the recommended die clearance when shearing mild steels is 5% to 10% of metal thickness. For punched holes, these represent per-side values.  Although this may have been satisfactory for mild steels, the clearance should increase as the tensile strength of the sheet metal increases.  

The choice of clearance impacts other aspects of the cutting process.  Small cutting clearances require improved press and die alignment, greater punching forces, and cause greater punch wear from abrasion. As clearance increases, tool wear decreases, but rollover on the cut edge face increases, which in the extreme may lead to a tensile fracture in the rollover zone (Figure 9). Also, a large die clearance when punching high strength materials with a small difference in yield and tensile strength (like martensitic grades) may generate high bending stresses on the punch edge, which increases the risk of chipping.

Figure 9: Large rollover may lead to tensile fracture in the rollover zone.

Figure 9: Large rollover may lead to tensile fracture in the rollover zone.

 

Figure 10 compares cut edge appearance after punching a martensitic steel with 1400 MPa tensile strength using either 6% or 14% clearance.  The larger clearance is associated with greater rollover, but a cleaner cut face.

Figure 10: Cut edge appearance after punching CR 1400T-MS with 6% (left) and 14% (right) die clearance. The bottom images show the edge appearance for the full sheet thickness,  Note using 6% clearance resulted in minimal rollover, but uneven burnish and fracture surfaces.  In contrast, 14% clearance led to noticeable rollover, but a clean burnish and fracture surface. T-20

Figure 10: Cut edge appearance after punching CR 1400T-MS with 6% (left) and 14% (right) die clearance. The bottom images show the edge appearance for the full sheet thickness,  Note using 6% clearance resulted in minimal rollover, but uneven burnish and fracture surfaces.  In contrast, 14% clearance led to noticeable rollover, but a clean burnish and fracture surface.T-20

 

A comparison of the edges of a 2 mm thick complex phase steel with 700 MPa minimum tensile strength produced under different cutting conditions is presented in Figure 11. The left image suggests that either the cutting clearance and/or the shearing angle was too large. The right image shows an optimal edge likely to result in good edge ductility.

Figure 11: Cut edge appearance of 2mm HR 700Y-MC, a complex phase steel. The edge on the right is more likely to result in good edge ductility.T-20

Figure 11: Cut edge appearance of 2 mm HR 700Y-MC, a complex phase steel. The edge on the right is more likely to result in good edge ductility.T-20

 

The recommended clearance is a function of the sheet grade, thickness, and tensile strength.  Figures 12 to 15 represent general recommendations from several sources.

Figure 12:  Recommended Clearance as a Function of Grade and Sheet Thickness. T-23

Figure 12:  Recommended Clearance as a Function of Grade and Sheet Thickness.T-23

 

Figure 13: Recommended Cutting Clearance for Punching.D-15

Figure 13: Recommended Cutting Clearance for Punching.D-15

 

Figure 14: Recommended die clearance for blanking/punching advanced high strength steel. T-20

Figure 14: Recommended die clearance for blanking/punching advanced high strength steel.T-20

 

Figure 15:  Multiply the clearance on the left with the scaling factor in the right to reach the recommended die clearance.D-16

Figure 15:  Multiply the clearance on the left with the scaling factor in the right to reach the recommended die clearance.D-16

 

Figure 16 highlights the effect of cutting clearance on CP1200, and reinforces that the historical rule-of-thumb guidance of 10% clearance does not apply for all grades. In this studyU-3, increasing the clearance from 10% to 15% led to a significant improvement in hole expansion. The HER resulting from a 20% clearance was substantially better than that from a 10% clearance, but not as good as achieved with a 15% clearance. These differences will not be captured when testing only to the requirements of ISO 16630, which specifies the use of 12% clearance.

Figure 16: Effect of hole punching clearance on hole expansion of Complex Phase steel grade CP1200.U-3

Figure 16: Effect of hole punching clearance on hole expansion of Complex Phase steel grade CP1200.U-3

 

Cutting speed influences the cut edge quality, so it also influences the optimal clearance for a given grade. In a study published in 2020G-49, higher speeds resulted in better sheared edge ductility for all parameters evaluated, with those edges having minimal rollover height, smoother sheared surface and negligible burr. Two grades were evaluated: a dual phase steel with 780MPa minimum tensile strength and a 3rd Generation steel with 980 MPa minimum tensile strength.

Metallurgical characteristics of the sheet steel grade also affects hole expansion capabilities. Figure 17 compares the HER of DP780 from six global suppliers. Of course, the machined edge shows the highest HER due to the minimally work-hardened edge. Holes formed with 13% clearance produced greater hole expansion ratios than those formed with 20% clearance, but the magnitude of the improvement was not consistent between the different suppliers.K-56

Figure 17: Cutting clearance affects hole expansion performance in DP780 from 6 global suppliers Citation K-56

Figure 17: Cutting clearance affects hole expansion performance in DP780 from six global suppliers.K-56

 

 

Punch Face Design

Practitioners in the field typically do not cut perpendicular to the sheet surface – angled punches and blades are known to reduce cutting forces.  For example, long shear blades might have a 2 to 3 degree angle on them to minimize peak tonnages.  There are additional benefits to altering the punch profile and impacting angle.

Snap-though or reverse tonnage results in stresses which may damage tooling, dies, and presses. Tools may crack from fatigue.  Perhaps counter to conventional thinking, use of a coated punch increases blanking and punching forces. The coating leads to lower friction between the punch and the sheet surface, which makes crack initiation more difficult without using higher forces. 

Unlike a coated tool, a chamfered punch surface reduces blanking and punching forces.  Figure 18 compares the forces to punch a 5 mm diameter hole in 1 mm thick MS-1400T using different punch shapes. A chamfered punch was the most effective in reducing both the punching force requirements and the snap-through tonnage (the shock waves and negative tonnage readings in Figure 18).  The chamfer should be large enough to initiate the cut before the entire punch face is in contact with the sheet surface.  A larger chamfer increases the risk of plastic deformation of the punch tip.T-20

Figure 16: A chamfered punch reduces peak loads and snap-through tonnage.K-15

Figure 18: A chamfered punch reduces peak loads and snap-through tonnage.K-15

 

A different study P-16 showed more dramatic benefits. Use of a rooftop punch resulted in up to an 80% reduction in punching force requirements compared with a flat punch, with a significant reduction in snap-through tonnage.  Cutting clearance had only minimal effect on the results. (Figure 19)

Figure 17: A rooftop-shaped punch leads to dramatic reductions in punch load requirements and snap-through tonnage.P-16

Figure 19: A rooftop-shaped punch leads to dramatic reductions in punch load requirements and snap-through tonnage.P-16

 

Use of a beveled punch (Figure 20) provides similar benefits.  A study S-52 comparing DP 500/780 and DP 550/980 showed a reduction in the maximum piercing force of more than 50% with the use of a beveling angle between 3 and 6 degrees. The shearing force depends also upon the die clearance during punching, with the optimum performance seen with 17% die clearance. The optimal punching condition results in more than 60% improvement in the hole expansion ratio when compared to conventional flat head punching process.  The optimal bevel cut edge in Figure 21 shows a uniform burnish zone with a uniform transition to the smooth fracture zone – the known conditions to produce a high-ductility edge.

Figure 18: Schematic showing a beveled punch S-52

Figure 20: Schematic showing a beveled punch.S-52

 

Figure 19: A bevel cut edge showing uniform burnish zone with a uniform transition to the smooth fracture zone.S-52

Figure 21: A bevel cut edge showing uniform burnish zone with a uniform transition to the smooth fracture zone.S-52

 

Effect of Edge Preparation Method on Ductility

A flat trim condition where the upper blade and lower blade motions are parallel and there is no shear rake angle is known to produce a trimmed edge with limited edge stretchability (Figure 22, left image).  In addition to split parts, tooling damage and unexpected down time results.  Metal stampers have known that shearing with a rake angle Figure 22, right image) will reduce cutting forces compared with using a flat cut.  With advanced high strength steels, there is an accompanying reduction in forming energy requirements of up to 20% depending on the conditions, which represents a tremendous drop in snap-through or reverse tonnage.  Figure 22 visually describes the upper and lower blade rake angles and the shear rake angle.

Figure 20: Flat trim (left) and shear trim (right) conditions showing rake angle definitions.S-53

Figure 22: Flat trim (left) and shear trim (right) conditions showing rake angle definitions.S-53

   

Researchers have also found that it is possible to increase sheared edge ductility with optimized rake angles. Citation S-53 used 2-D Edge Tension Testing and the Half-Specimen Dome Test to qualify the effects of these rake angles, and determine the optimum settings.  After preparing the trimmed edge with the targeted conditions, the samples were pulled in a tensile test or deformed using a hemispherical punch. The effect of the trimming conditions was seen in the measured elongation values and the strain at failure, respectively.  The results are summarized in Figures 23-25.  Some of the tests also evaluated milled, laser trimmed, and water jet cut samples. Shear Trim 1, 2, and 3 refer to the shear trim angle in degrees. The optimized shear condition also includes a 6-degree rake angle on both the upper and lower blades, as defined in Figure 22.  

Conclusions from this study include:

  • Mechanically shearing the edge cold works the steel and reduces the work hardening exponent (n-value), leading to less edge stretchability. 
  • Samples prepared with processes that avoided cold working the edges, like laser or water jet cutting outperformed mechanically sheared edges.  
  • Optimizing the trim shear conditions or polishing a flat trimmed edge approaches what can be achieved with laser trimming and water jet cutting.
  • Shearing parameters such as clearance, shear angle and rake angle also play a large part in improving edge stretch. 
Figure 21: Effect of edge preparation on stretchability as determined using a tensile test for DP350Y600T (left) and DP550Y980T (right).S-53

Figure 23: Effect of edge preparation on stretchability as determined using a tensile test for DP 350Y600T (left) and DP 550Y980T (right).S-53

 

Figure 22: Effect of edge preparation on stretchability as determined using a dome test for DP350Y600T (left) and DP550Y980T (right).S-53

Figure 24: Effect of edge preparation on stretchability as determined using a dome test for DP 350Y600T (left) and DP 550Y980T (right).S-53

 

Figure 23: Optimizing the trim shear conditions or polishing a flat trimmed edge approaches what is achievable with laser trimming and water jet cutting. Data from dome testing of DP 350Y/600T.S-53

Figure 25: Optimizing the trim shear conditions or polishing a flat trimmed edge approaches what is achievable with laser trimming and water jet cutting. Data from dome testing of DP 350Y/600T.S-53

 

The optimal edge will have no mechanical damage and no microstructural changes as you go further from the edge.  Any process that changes the edge quality from the bulk material can influence performance.  This includes the mechanical damage from shearing operations, which cold works the edge leading to a reduction in ductility.  Laser cutting also changes the edge microstructure, since the associated heat input is sufficient to alter the engineered balance of phases which give AHSS grades their unique properties.  However, the heat from laser cutting is sometimes advantageous, such as in the creation of locally softened zones to improve cut edge ductility in some applications of press hardening steels.

The effects of edge preparation on the shear affected zone is presented in Figure 26.  A flatter profile of the Vickers microhardness reading measured from the as-produced edge into the material indicates the least work-hardening and mechanical damage resulting from the edge preparation method, and therefore should result in the greatest edge ductility.  This is certainly the case for water jet cutting, where a flat hardness profile in Figure 26 correlates with the highest ductility measurements in Figures 22 to 25. Unfortunately, water jet cutting is not always practical, and introduces the risk of rust forming at the newly cut edge.

Figure 24:  Microhardness profile starting at cut edge generated using different methods.  Left image is from S-53, and right image is from C-13

Figure 26:  Microhardness profile starting at cut edge generated using different methods.  Left image is from Citation S-53, and right image is from C-13.

 

Two-stage piercing is another method to reduce edge strain hardening effects. Here, a conventional piercing operation is followed by a shaving operation which removes the work-hardened material created in the first step, as illustrated in Figure 27.P-17 A related studyF-10 evaluated this method with a 4 mm thick complex phase steel with 800 MPa tensile strength.  Using the configuration documented in this reference, single-stage shearing resulted in a hole expansion ratio of only 5%, where the addition of the shaving operation improved the hole expansion ratio to 40%.

Figure 25: Two-stage piercing improves cut edge ductility. Image adapted from P-17

Figure 27: Two-stage piercing improves cut edge ductility. Image adapted from Citation P-17.

 

Figure 28 highlights the benefits of two-stage pre-piercing for specific grades, showing a 2x to 4x improvement in hole expansion ratio for the grades presented.

Figure 27: Pre-piercing improves the hole expansion ratio of AHSS Grades.S-10

Figure 28: Pre-piercing improves the hole expansion ratio of AHSS Grades.S-10

 

Key Points

  • Clearances for punching, blanking, and shearing should increase as the strength of the material increases, but only up to a point. At the highest strengths, reducing clearance improves tool chipping risk.
  • Lower punch/die clearances lead to accelerated tool wear. Higher punch/die clearances generate more rollover/burr.
  • ISO 16630, the global specification for hole expansion testing, specifies the use of 12% punch-to-die clearance. Optimized clearance varies by grade, so additional testing may prove insightful.
  • Recommended clearance as a percentage of sheet thickness increases with thickness, even at the same strength level. 
  • Burr height increases with tool wear and increasing die clearances for shearing mild steel, but AHSS tends to maintain a constant burr height. This means extended intervals between tool sharpening may be possible with AHSS parts, providing edge quality and edge performance remain acceptable.
  • Edge preparation methods like milling, laser trimming, and water-jet cutting minimize cold working at the edges, resulting in the greatest edge ductility,
  • Laser cut blanks used during early tool tryout may not represent normal blanking, shearing, and punching quality, resulting in edge ductility that will not occur in production.  Using production-intent tooling as early as possible in the development stage minimizes this risk.
  • Shear or bevel on punches and trim steel reduces punch forces, minimizes snap-through reverse tonnage, and improves edge ductility.
  • Mild steel punched with conventional tools and AHSS grades punched with surface treated engineered PM tool steels experience comparable wear.
  • Maintenance of key process variables, such as clearance and tool condition, is critical to achieving long-term edge stretchability. 
  • The optimal edge appearance shows a uniform burnish zone with a uniform transition to a smooth fracture zone.

 

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Cutting, Blanking, Shearing & Trimming

Shear Affected Zone

A cut edge has four distinct zones with different characteristics:

  • Rollover – The plastically deformed zone bent as the cutting tools contact the edge of the sheet surface.
  • Burnish – The zone where the cutting tools penetrate into the sheet metal, prior to any fracturing. The sheet metal stresses are such that the surface compresses into the cutting tool, which gives it a flat, smooth, and shiny appearance. The shear zone is another name for this region.
  • Fracture – The zone where the cutting steels fracture the sheet metal, leading to separation from the remainder of the sheet. The surface of this region is rougher than the burnish zone, and is at an angle from the cutting direction.
  • Burr – The metal elongated and pushed out on the trailing edge of the cut.

The cutting process also deforms the metal near the shear face, creating what is known as the shear affected zone (SAZ). The degree and influence of this deformation is a function of the cutting process (shearing, water jet, laser, and so on) as well as the steel microstructure and strength. Many studies conclude plastic deformation within the SAZ is a key contributor to sheared edge stretching failures.

The Shear Affected Zone is the area of work-hardened steel and microstructural damage behind the sheared edge. The deformation pattern in the SAZ includes a large shear-induced rotation of the grains that increases with proximity to the sheared edge. An etched cross-section such as that shown in Figure 1 highlights the grain rotation and gives a visual indication of the size of the zone.

Figure 1: Shear Face Components and Shear Affected Zone.K-1

Figure 1: Shear Face Components and Shear Affected Zone.K-1

 

Work hardening in the SAZ leads to a way to quantify the depth of the SAZ: by creating a hardness profile with readings starting at the edge and progressing deeper into the metal. The end of the SAZ occurs when the hardness readings level off to the bulk hardness.

Figure 2 compares the edge and hardness readings for CP800 and DP780, showing the depth of the SAZ for DP780 is about 41% of the initial sheet thickness of 1.56 mm, and the depth of the CP800 SAZ is 20% of the initial 2.90 mm sheet thickness.P-12

Figure 2: A) Sheared edge and SAZ of CP800; B) Sheared edge and SAZ of DP780; C) CP800 microstructure away from edge; D) DP780 microstructure away from sheared edge; E) hardness profile for the two sheared edges in the rolling direction.P-12

Figure 2: A) Sheared edge and SAZ of CP800; B) Sheared edge and SAZ of DP780; C) CP800 microstructure away from edge; D) DP780 microstructure away from sheared edge; E) hardness profile for the two sheared edges in the rolling direction.P-12

 

Plastic deformation from cutting and subsequent edge expansion forms micro-voids and creates other microstructural damage. The voids grow and combine with neighboring voids to create micro-cracks, which in turn combine with other micro-cracks resulting in the cracks that cause fractures in stampings.

Void nucleation in DP steels occurs through two mechanisms: decohesion of the ferrite-martensite interface or fracture of martensite islands.P-12, A-27, A-28  Figure 3 shows an example of both.

Figure 3: Aligned voids along loading direction. White solid square shows interface damage between martensite and ferrite. White dashed circles show voids formed by cracking of martensite.A-28

Figure 3: Aligned voids along loading direction. White solid square shows interface damage between martensite and ferrite. White dashed circles show voids formed by cracking of martensite.A-28

 

The study in Citation P-12 compared CP800 and DP780. The CP800 microstructure contains ferrite, bainite, and martensite. The DP780 microstructure has more martensite, with a larger strength differential between the phases, which combines to result in a lower nucleation strain and accelerated void nucleation compared with CP800. The DP grade has a larger SAZ, further promoting nucleation, growth, and coalescence of voids. This results in failure at a lower strain and leads to the lower edge stretchability of the DP780 compared to the CP800 alloy.P-12  The ductility of bainite restrains void initiation at high strains, which may play a role in improved sheared edge performance.S-24, S-25

Fracture initiation energy, a measure of fracture toughness correlates with hole expansion and stretch-flangeability as shown in Figure 4.Y-5

Figure 4: Correlation between fracture initiation energy and hole expansion ratio of various metals. Steels A – D are AHSS grades with Tensile strength ranging from 725 – 1000 MPa.Y-5

Figure 4: Correlation between fracture initiation energy and hole expansion ratio of various metals. Steels A – D are AHSS grades with Tensile strength ranging from 725 – 1000 MPa.Y-5

 

In addition to microstructural damage and fracture mechanics, simulation models improve in accuracy when the incorporating the effect of the temperature increase in the localized deformation zone. Simulation of the blanking of AISI 1050 shows a temperature at the cut edge of 440 °C. The edge temperature may be even higher in AHSS grades due to their higher strength.A-29

Figure 5: 1.27mm AISI 1050 steel blanked with 15% clearance. The simulation shows temperature, with the cut face getting as hot as 440 °C.A-29

Figure 5: 1.27mm AISI 1050 steel blanked with 15% clearance. The simulation shows temperature, with the cut face getting as hot as 440 °C.A-29

 

When factoring in the considerations described here, simulation accuracy of hole expansion and sheared edge stretching improves significantly. Citations L-13 and L-14 provide additional background information about the Shear Affected Zone.

Laser Blanking

Laser Blanking

Conventional blanking requires high press forces to shear the sheet metal. As steel strength increases, snap-through tonnage also increases, potentially leading to press and tooling damage. Over time, clearances may change, affecting edge quality. Shearing results in a work-hardened edge, and a shear affected zone know to decrease edge stretchability.

Blanking dies are often among the last dies completed prior to launch. Historically, this led to the use of hand-cut, steel-rule, or laser cut blanks during tryout. When laser cut blanks of AHSS are used during try out, the high hardness in the laser cut edges may damage the blank holder surface. Control the laser cutting parameters to reduce burrs and edge hardness. Deburr the edges of laser cut blanks prior to forming to minimize tool damage, especially when using soft tool materials like kirksite.

Using laser cut blanks during tryout offers product flexibility, since blank dimensions may change as part of the development process. However, upon completion of the blanking die, the dimensions cannot change without costly modifications.

Instead of using a blanking die, some companies are choosing to use laser blanking in production environments. Laser blanking eliminates the cost of blanking die construction and tooling maintenance. A dedicated blanking die for each part is no longer necessary – one laser blanking line can create blanks for multiple parts. Blank shape changes become another way to solve stamping formability problems. This approach also has the flexibility of incorporating blank size optimizations as part of the cost savings efforts targeted through the life of a stamping. Simple programming changes optimize nesting to take advantage of coil width capability improvements made by the sheet metal supplier.

Curved sections on blanking dies present design challenges related to cutting tool steel placement to avoid the creation of a notch at the mating point between two adjacent sections. Maintenance of sharp edges and consistent clearance is another challenge–advanced steel grades damage even the optimal cutting tool steels over time. Laser blanking avoids these concerns.

Multiple heads within the laser blanking system allow for faster processing speeds. According to Citation F-6, typical outer body parts can run 12 to 25 strokes per minute (SPM) on a two-head configuration, while complex body sides tend to run 4 to 6 SPM. Thicker inner structural parts typically run in 6 to 10 SPM area, but configurations with six laser cutting heads increase this rate to 30 to 40 SPM. Daimler achieves the blank production rate of 40 hoods per minute.H-9  On a continuously moving coil, laser heads reach a linear cut speed of 100 meters per minute.S-27  These figures continue to improve with advances in laser technology.

Similar to a shear-affected zone, laser-cut blanks have a heat-affected zone (HAZ) comprising the area from the laser-cut edge into the blank. The typical HAZ distance is under 0.2 mm on steel up to 2 mm thick, which is less than a comparable shear cut edge. In addition, the hardening increase is lower, and edge quality is more consistent than seen in sheared edges.F-6

Note that this discussion relates to the creation of first-operation blanks directly from coil feedstock. For a discussion on Laser Welded Blanks, visit the pages related to Tailor Welded Blanks.

 

Case Study: Die-Free Blanking of Class A Quality & Structural PartsS-114

Production of Class A quality and structural parts without a blanking die is possible, even for high-volume serial production. Laser blanking enables flexible, cost-effective, and sustainable manufacturing, and is capable of reaching 45 parts per minute.

Blanking Lines at a Glance-V2

Figure 1: Laser blanking lines offer additional flexibility over conventional blanking approaches.

 

DynamicFlow Technology (DFT) from Schuler provides highly productive, die-free blanking with lasers—directly from a continuously running steel coil. DFT combines the advantages of flexible laser cutting with the speed of conventional blanking. The video below shows laser blanking with DynamicFlow Technology in action.  

Laser blanking technology addresses market challenges such as frequent die changes, the need to increase capacity, and improving plant floor utilization, material utilization, and downstream processes.

 

Laser Blanking Eliminates Frequent Die Changes

It is important to remember that there are no dies with laser blanking technology, and no dies means no die changes. Overall Equipment Effectiveness (OEE) of up to 80% can be achieved with laser blanking technology. In fact, 4 to 6 million parts per year of various materials are produced with the help of DFT—including mild steel, high-strength steel, and advanced high strength steel. Even processing press hardening steels with an aluminum-silicon coating is possible with laser blanking. Surface and cutting quality can be maintained over this spectrum of steel grades. Laser blanking technology can even achieve effective small batch production of Class A outer body panels and structural parts typically up to 3mm thick.

 

Laser Blanking Increases Plant Output

Competitive high-speed and high-output results can be achieved in multiple ways with laser blanking technology. The above-ground coil-fed line, optimized to achieve short setup time, can handle coils with material widths up to 2,150 mm, weighing up to 30 tons. The material transport is smooth and controlled, simplifying setup, and leading to uninterrupted processing within the laser cell.

There are three highly dynamic, and simultaneously moving, laser cutting heads within the laser cell of these lines. These laser cutting heads cut the programmed blank contour from a continuously moving material coil. Cutting speeds can exceed 100 meters per minute. The material is protected against any process contamination throughout the cutting process by the custom-designed cutting clearance and material transport.

The blanks and the offal (scrap) must be separated once the cutting process is complete. This occurs through the use of automated systems specially adapted to the individual products. Following the separation, the blanks are cleaned, before moving into the automatic stacking system. This ensures outstanding body shell quality.

Figure 2 reveals the high-speed and high-output results for outer body parts. Each part is measured by improved output per minute and hour to achieve an OEE of 80%. Significantly, laser blanking lines can achieve up to 45 parts per minute and reduce costs per blank.

Figure 2: High productivity achieved with laser blanking.

Figure 2: High productivity achieved with laser blanking.

 

Laser Blanking Needs Less Floor Space and Reduces Infrastructure Costs

There is a multitude of ways for manufacturers to increase capacity within new or existing facilities when incorporating laser blanking technology into their production process. The clearest example, indicated within the picture below, is that very little floor space is needed in comparison to conventional blanking lines. All the circled areas in Figure 3 can be eliminated when utilizing laser blanking technology.

Circled areas represent freed floorspace with laser blanking.

Figure 3: Circled areas represent freed floor space with laser blanking.

 

Laser blanking lines are designed to be installed above ground, so they can be used in normal logistics buildings—eliminating the need for special foundations required by press systems.

Capital equipment and operations expenses have drastically increased with supply chain shortages. Laser blanking technology provides cost-effective benefits that can be critical for such challenges.

  For new facilities:

  • No heavy press foundation or loop pits are needed.
  • There are no noise or vibration emissions.
  • It is possible to operate next to press lines.
  • There is very little floor space required.
  • No special infrastructure is needed for die handling.

  For existing facilities:

  • No heavy press foundations or loop pits are needed, and no special infrastructure is needed for die handling—similar to new facilities.
  • There are reduced ground and building costs.
  • Other potential benefits include an optimized tryout process, material cost, and weight savings.

Laser blanking lines require no press foundation, loop or press pits, die cranes, or die storage areas. A crane to handle the coils and a forklift to take the blank stacks where they need to go is all manufacturers need. The building and infrastructure costs of a die-based blanking system normally double the costs for capital equipment investment alone. A laser blanking line eliminates a large portion of these costs.

 

Laser Blanking Improves Material Utilization

Up to 90% of blank costs are determined by the material price. The most significant leverage would be to reduce scrap and save on materials. Schuler conducted research based on the production of 300,000 cars per year, at 350 kg per car and $1,000 USD per ton of steel to provide a realistic inside look at how much cost savings can be achieved with laser blanking. The result was $1 Million USD saved with just 1% of material savings. This is extremely significant as material costs keep increasing.

Laser blanking is the digital way to cut blanks. All that’s needed to create a blanking program is a drawing to be loaded and a material to be selected. The part-specific program can be created offline and modified at any time. It is designed to create optimal combinations of material utilization and output—resulting in a high level of flexibility that significantly reduces development time for optimal blanks while also allowing for need-based production. This makes production planning easier, and it also opens the door to continuous contour optimizations for the forming process. Additionally, laser cutting does not require any gaps between individual parts due to smart nesting capabilities that cannot be achieved in comparison to die nesting or flatbed laser nesting. The combined smart, flexible nesting functions unlock new potential for material savings. Manufacturers can optimize individual blanks and eliminate the separating strip or connection bridges. Scrap savings in the forming process can also be achieved as there are no geometric restrictions due to cutting dies, and manufacturers can continuously optimize or adapt parts.

Figure 4 showcases the comparison of die nesting (the two graphics on the left) versus a laser-optimized blank contour and material savings via smart, laser blanking line nesting (the two images on the right).

Figure 4: Die nesting (left) compared with laser-optimized blank contours highlighting potential material savings (right)

Figure 4: Die nesting (left) compared with laser-optimized blank contours highlighting potential material savings (right)

Laser blanking technology allows for the conversion of rectangular and trapezoidal blanks into contoured blanks. Blank shapes are no longer restricted to straight lines – programmable laser cutting allows for edges with a sinusoidal or arched contour—improving the material utilization rate or forming characteristics. Adaptations of various geometric shapes can be achieved with optimized nesting of laser blank contours. These images in Figure 4 are just an example, with the possibility of more complex shapes and nesting patterns.

Overall, laser blanking lines can have an equivalent throughput to conventional blanking lines, but laser blanking lines can achieve up to 10% greater material utilization.

 

Laser Blanking Improves Downstream Processes

High surface quality can be achieved with laser blanking technology as clean blanks are produced with homogeneous and uniform cutting edges—and no burr cleaning is needed during or after the forming process. There is also a reduction in dripping during the hot forming process—which leads to a dramatically increased lifespan of the hot forming rollers—as the laser melts the dripping layer instead of squeezing it into the cutting edge. Figure 5 provides a comparison of the edges produced via die blanking with a conventional cut finish with that produced by laser blanking with a laser fiber cut finish.

Conventional cut edge condition produced from die blanking (left) and laser fiber cut edge condition produced from laser blanking (right).

Figure 5: Conventional cut edge condition produced from die blanking (left) and laser fiber cut edge condition produced from laser blanking (right).

 

Laser blanking lines are much more cost-effective for the production of blanks when manufacturers keep these five benefits in mind. The costs for personnel, logistics, buildings, dies, storage areas, repairs, and other additional costs can add up to a pricey amount. Laser blanking lines are the smart manufacturing choice as this technology yields a higher OEE in comparison to conventional blanking lines through the elimination of die-related downtime and lengthy time changes. The system only stops for a short time when the coil needs to be changed.

 

Schuler will present laser blanking technology, along with a variety of digital tools that create the “Press Shop of the Future” at FABTECH Chicago 2023 (booth # D41306). Tiago Vasconcellos, Sales Director at Schuler North America, will present “How Smart is Your Press Shop?” during FABTECH’s Educational Conference. The presentation will use The Smart Press Shop, a newly formed joint venture between Porsche and Schuler, as an exemplary case study for smart manufacturing standards. Attendees will discover innovative and practical ways to incorporate digitalization into production and become a state-of-the-art stamping facility directly from Schuler. 

About Schuler Group

Schuler offers customized cutting-edge technology in all areas of forming—from the networked press to press shop planning. In addition to presses, Schuler’s products include automation, dies, process know-how, and service for the entire metalworking industry. Schuler’s Digital Suite brings together solutions for networking forming technology and is continuously being developed to further improve line productivity and availability. Schuler customers include automotive manufacturers and suppliers, as well as companies in the forging, household appliance, and electrical industries. Schuler presses are minting coins for more than 180 countries. Founded in 1839 at the Göppingen, Germany headquarters, Schuler has approximately 5,000 employees at production sites in Europe, China and the Americas, as well as service companies in more than 40 countries. The company is part of the international technology group ANDRITZ.

Schuler’s global portfolio of world-renowned brands include BCN (Bliss Clearing Niagara) Technical Services, Müller Weingarten, Beutler, Umformtechnik Erfurt, SMG Pressen, Hydrap Pressen, Wilkins & Mitchell, Bêché, Spiertz Presses, Farina Presse, Liebergeld, Peltzer & Ehlers, Schleicher, and Sovema Group.

About Schuler North America

Schuler North America (Schuler), headquartered in Canton, Michigan, is the North American subsidiary of Schuler Group. Schuler provides new equipment, spare parts, and a portfolio of lifecycle services for all press systems—including preventative maintenance, press shop design and optimization, turnkey installations, retrofits for existing systems, and localized production and service. Schuler’s best-in-class position in the metalworking and materials industry serves automotive manufacturers and tier suppliers, as well as home appliance, electronics, forging, and other industries.