Advanced Steel Processing Technologies For
Reduced Cost, Reduced Mass and Improved Functional Performance
Laser (Tailor) Welded Blanks
A laser welded blank is two or more sheets of steel seam-welded together into a single blank which is then stamped into a part. Laser welded blank technology allows for the placement of various steel grades and thicknesses within a specific part, placing steel’s attributes where they are most needed for part function, and removing weight that does not contribute to part performance. For example, Figure 1 shows a laser welded body side aperture (outer) with multiple grades and thicknesses. This technology allows for a reduction in panel thickness in non-critical areas, thus contributing to an overall mass reduction of the part.
Figure 1: Body-side outer with exposed laser welds and multi-piece construction.
There are several advantages to a laser welded blank, compared with conventional blanks made from a single grade and part thickness. They include:
- Superior vehicle strength and rigidity
- Consolidation of parts, where one blank can replace several different parts
- Lower vehicle and part weight
- Reduced steel usage
- Improved safety
- Elimination of reinforcement parts
- Elimination of assembly processes
- Reduction in capital spending for stamping and spot-welding equipment
- Reduced inventory costs
- Improved dimensional integrity (fit and finish)
- Achievement of high-performance objectives with lower total costs
- Reduction in Noise, Vibration, and Harshness (NVH)
- Elevated customer-perceived quality
A laser-welded coil (Figure 2) is a continuous coil of steel comprised of individual, separate coils of steel with varying thickness and grades. The basic process takes separate coils, prepares their edges for contiguous joining, and laser welds these together into one master coil. The new strip is then readied for blanking, or to be used as a continuous feed into a transfer press line.
As in laser-welded blanks, the laser-welded coil allows for similar advantages – targeted strength or stiffness where required, while allowing for overall part weight reduction by incorporate thinner materials where possible.
Figure 2: Laser-welded coil process
Potential use of a laser welded coil in an automotive application, using a pro-die-stamping process, includes the following:
- Roof frames
- Roof bows
- Side members
- Seat cross members
- Exhaust systems
This is a manufacturing process of flexible cold strip rolling by varying the gap between two rolls, allowing for different strip thicknesses in the direction of rolling. Figure 3 illustrates the manufacturing principles. The accurate measuring and controlling technology ensure that strip thickness tolerances are maintained. A tailor-rolled coil can be either used for blanking operations (for stamping or tubular blanks) or can be directly fed into a roll-forming line.
Figure 3: The principle of producing a Tailor Rolled Coil.
A comparison of various mechanical sheared edges with water jet, laser and milled edges showed that laser-cut edges achieved elongations that approached that of the ideal milled edge. As a response to increasing AHSS volumes and strength levels, multiple companies have developed laser blanking lines, where a coil is blanked via a laser or series of lasers. These new lines are capable of cutting blanks on a high-volume basis.
There are several advantages to this approach when processing AHSS. Improved edge conditions are less susceptible to edge fracture, and thus is significant. Additional savings can be achieved through the elimination of expensive blank die construction and tooling maintenance costs (no blank die is needed) and thus no expensive trim steels are required (AHSS usually requires more durable and more expensive tool steels). Less floor space is needed because there are no blank dies to store. With no blank dies to remove and replace, faster line transitions occur, which means greater uptime and increased productivity. There is also the opportunity to optimize material utilization through either blank nesting optimization or blanking two or more different blanks out of the same steel strip.
Figure 4 shows an example of optimized laser blank nesting of three different blanks from the same coil. Blank contours can easily be modified after production launch as well. As many AHSS grades are rolling-direction sensitive with respect to edge and shear fracture, alternative nesting can potentially optimize the blank orientation to minimize these types of local formability failures.
Figure 4: Schematic of 3 different blanks to be laser blanked from the same coil to minimize engineered scrap. Not only is engineered scrap minimized, but the superior edge condition significantly reduces the potential for edge fracture.
The decision for laser blanking should be made during development, in order maximize overall process efficiency and avoid building a blank die or other non-essential tooling. Figure 5 shows a typical laser blanking line specifically designed to process AHSS.
Figure 5: Laser blanking line specifically designed to process AHSS. Note the cartridge-based straightener (far left), specifically designed to ensure blanks are flat after processing.
The diverse microstructures and strength levels of AHSS products present some challenges for stamping operations. Cutting and punching clearances are greater for AHSS, and as a general rule, should be increased with increasing sheet material strength. The clearances range from about 6% of the thickness for Mild steel but grow to between 10% and 16% for strength levels of 1 GPa or more.
Two-hole punching studies1 were conducted with Mild steel and AHSS. The first measured tool wear (captured in Figure 1), while the second studied burr height formation (Figure 2).
Wear testing was performed with four 1.0 mm thick sheet steels: Mild 140/270, DP 350/600, DP 500/800, and MS 1150/1400. Tool steels were W.Nr. A2 with a hardness of 61 HRC and a 6% clearance for Mild steel tests. PM tools with a hardness range of 60-62 HRC were used for all AHSS testing. For the DP 350/600, the punch was coated with CVD (TiC) and the clearance was set at 6%. Tool clearances for DP 500/800 were 10%, and for MS 1150/1400 were set at 14%.
Figure 1: Punching up to DP 500/800 with surface treated high quality tool steels can be comparable to Mild steel with conventional tools.1
The studies showed that wear rates for AHSS DP steels punched with surface treated high quality (PM) tool steels were comparable to punching Mild steel with conventional tools. Wear rates for MS were more than twice that of the DP steels. Increasing burr height is often the reason for sharpening trim steels and punches, as burrs can reduce metal formability. For Mild steels the burr height increases with increasing ductility and tool wear.
Figure 2 shows a burr height plateau for AHSS; both materials initially have a burr height related to the material ductility and the sharpness of the tools. AHSS fractures at a maximum possible height that is reached when the maximum local elongation is obtained during punching, after which the burr height does not increase. The Mild steel, which is more formable, will continue to generate higher burr height with increased tool wear.
Figure 2: Burr height comparison for Mild steel and AHSS as a function of the number of hits. Results for DP 500/800 and MS 1150/1400 are identical and shown as the AHSS curve.1
The burr height increased with tool wear and increasing die clearance when punching Mild steel. AHSS may require a higher-grade tool steel or surface treatment to avoid tool wear, but tool regrinding because of burrs should be less of a problem. If the tool has been surface-treated, grinding the tool will remove the surface treatment, so if possible, the tool must be retreated. If burr height is the criterion, high quality tool steels will result in greater intervals between sharpening when punching AHSS, since the burr height does not increase as quickly with tool wear as when punching Mild steel with conventional tool steels.
As there are many different tool steels, tool steel treatments and tool steel coatings, shops are encouraged to identify the dominant mode of tool failure in order to select the tool steel with the properties to counter that failure mode. There are five main types of cold work failure modes involving tool steels: wear, plastic deformation, chipping cracking, and galling. Figure 3 shows examples of these five failure modes.
Figure 3: Five main modes of tooling failure.2
The following case study illustrates the importance of clearly identifying the mode(s) of failure on the part, as well as the mode of failure on the tooling, to improve the selection of counter-measures. A dash reinforcement had been in production for several years, stamped with a 280 MPa yield strength HSLA steel. To improve side impact ratings, the part transitioned to DP 600. Immediately after implementation, stamping scrap rates increased significantly. The failures were all determined to be local formability edge fractures; investigation revealed that the blank for this part was configured and that the blanked edge at the edge fracture was part of the final product edge. Figure 4 shows one of the blanked cut-outs which is then drawn and flanged. The edge condition had burrs and a poor burnish to fracture zone. See Figure 5 for a photo of the edge fracture.
The blank die material was examined and determined to be the same D2 tool steel as was when the steel was HSLA. Further examination determined that no clearance adjustments were made for the higher strength steel, maintained at 10% of metal thickness instead of an optimal setting of 15%. Additionally, the inserts used to make the u-shaped cut-outs were wearing and failing at an alarming rate.
|Figure 4: End of a blank on a dash reinforcement where the blanked cut-out becomes part of the product on the finished part. Close examination shows a poor edge condition.2
||Figure 5: Local formability edge fracture emanating from a blanked edge in a stretch flange operation, and location of metal gainer eventually added to the draw die.
||Figure 6: Worn and broken D2 inserts being used on a DP 600 AHSS steel.
The failures were so frequent that a series of back-up inserts had been constructed so the worn/damaged inserts could be quickly replaced. Figure 6 shows worn and broken inserts. It became clear that the failure modes involved both wear and fractures. An alternative, more durable tool steel (trade name caldie) with increased clearances was inserted in the blanking die. This change enabled the inserts to operate with over 90,000 hits and virtually no insert maintenance required (other than routine cleaning). This change significantly reduced the scrap rate, but sporadic edge fractures were still being experienced. As a result, breakdown panels of the draw, trim and flange operations were examined. It was found that the draw die at the location in question was not forming the part to the final length of line. As DP steels have a very high work hardening rate, stretch flanging a blanked edge significantly increases the potential for edge fracture. To compensate, a small metal gainer was added to the draw die to ensure that the flanging operation deformed the steel via bending and straightening, not additional stretch (see also Figure 5). After these two process changes, scrap rates for edge fractures dropped to virtually zero.
Selecting the proper tool steels for specific grades of AHSS is critically important and will reduce long term maintenance, repair and scrap/rework costs. Some of the more elaborate tool steels, are significantly more expensive than those used with mild steels. As a result, some automakers and steel processors don’t use the more durable (and expensive) tool steels on the entire working surface of the die. They strategically identify high wear and difficult to maintain areas and install tool steels as inserts in those locations. Figures 7 and 8 show caldie inserts installed on blank dies in difficult to maintain locations.
Figure 7: Caldie insert used to address wear and cracking issues on the blank die.2
Figure 8: Caldie insert in a difficult to maintain location on a blank die. 2
1 B. Carlsson, “Choice of Tool Materials for Punching and Forming of Extra- and Ultra High Strength Steel Sheet,” 3rd International Conference and Exhibition on Design and Production of Dies and Molds and 7th International Symposium on Advances in Abrasive Technology, Bursa, Turkey (June 17-19, 2004).
2 Courtesy of Peter Mooney Peter Mooney, 3S-Superior Stamping Solutions, LLC
Die wear is affected by, but not limited to, steel strength and surface friction, contact pressure, sliding velocity, temperature, die surface treatment and lubrication. As the material strength and hardness is increased, an increase in die wear occurs which leads to quality issues in the stamping. In addition, frequent die wear requires more frequent die maintenance or replacement, affecting turnaround times, productivity and cost.
Actions can be taken to prevent excessive wear on die materials when forming Advanced High-Strength Steels. To prevent such wear, new die materials and better coatings have been developed to maintain their hardness without compromising the toughness of the material. Hard material coatings and nitriding have also been used to improve the tribological properties of die surfaces.
Tool and Die Materials
In general, the considerably higher strength levels associated with AHSS grades exerts proportionally increased forces on the die material. AHSS might reach strength levels four to five times higher than mild steels, approaching 2000 MPa. This is based on complex metallurgy that delivers high concentrations of martensite and very high work hardening rates.
The higher forces required to form AHSS require increased attention to tool and die material specifications. Key requirements that need to be specified are the stiffness and toughness of the die substrate, as well as the surface treatments and hardness of the die coating. Life and performance of a draw die is determined by the amount of wear/galling that accumulates during forming, which then defines specific die maintenance intervals. When selecting die materials, important considerations are:
- Sheet metal being processed, characterized by strength, thickness, and surface coating.
- Die construction, machinability, radii sharpness, surface finish, and die hardness, specifically on draw beads and radii.
- Cost per part.
Thickness reduction for weight saving is one primary reason for applications of AHSS. Unfortunately, the reduced thickness of the steel increases the tendency to wrinkle. Higher blankholder loads are required to suppress these wrinkles. Any formation of wrinkles will increase the local load and accelerate the wear effects. Figure 1 shows a draw die with severe die wear due to excessive wrinkling on a DP780 part. It is not uncommon to replace these high wear areas with a more durable tool steel insert to minimize this type of excessive wear condition.
Figure 1: Draw die with significant wear due to excessive wrinkling on a DP780 part.
Tool steel inserts for forming dies must be selected according to the severity of the forming. Surface coatings are recommended for DP 350/600 and higher grades. When coatings are used, it is important that the substrate has sufficient hardness/strength to avoid plastic deformation of the tool surface, even locally. Therefore, a separate surface hardening, such as nitriding, is recommended before the coating is applied. Prior to coating application, it is important to use the die in in pre-production to provide time to adjust forming conditions and establish die performance Surface roughness must be as low as possible before coating. Ra values below 0.2 µm are recommended. Steel inserts with a TiC/TiN coating are recommended for local high-pressure conditions that cause accelerated die wear and zinc flaking.
Table 1, provided by the Auto/Steel Partnership, describes recommended construction materials for stamping dies and components, based on significant research efforts. North American Automotive Metric Standards (NAAMS) are the product of a consortium between Ford, GM, FCA and various North America automotive steel producers.
Table 1 – Recommended Materials for Specific Die Components – NAAMS Standards
Tool steels for cutting, trimming, and punching operations must be similarly selected. For these operations, tensile strength is more important than yield strength. Tool hardness between 58 and 62 HRC is recommended. Coatings may be used to reduce tool wear, but for the highest strength steels (above 1000 MPa tensile strength) coatings may fail due to local deformation of the die material substrate.
To prevent this, hardening of the substrate prior to coating is again strongly recommended. High performance tool steels, such as powder metallurgy (PM) grades, can be expensive but are justified because of their low wear rate and increased life. Ceramic tool inserts have extreme hardness for wear resistance, high heat resistance, and optimum tribological behavior, but have poor machinability and severe brittleness. High costs are offset by reduced maintenance and increased productivity. While not commonly used, the ceramic tool inserts offer a possible solution to high
interface loading and wear.
The following study is a computerized analysis of the energy required to form a cross member with a hat-profile and a bottom embossment at the end of the stroke (Figure 1).
Figure 1: Cross section of a component having a longitudinal embossment to improve stiffness locally.1
Energy requirements increase with punch displacement, as shown in Figure 2, here energy curves are shown for Mild, HSLA 250/350, and DP 350/600 steels. The three dots indicate the start of the embossment formation at a punch depth of 85 mm.
Figure 2: Computerized analysis showing the increase in energy needed to form the component with different steel grades. Forming the embossment begins at 85 mm of punch travel.1
The last increment of punch travel to 98 mm requires significantly higher energy, as shown in Figure 3. Throughout the punch travel however, the two higher strength steels appear to maintain a constant proportional increase over the Mild steel.
Figure 3: A further increase in energy is required to finish embossing.1
Setting Draw Beads
A considerable force is required to set draw beads in AHSS before part formation begins. A nitrogen die cushion may be inadequate to fully set the beads, resulting in insufficient restraining force. Binder separation may occur, resulting in a loss of control for the stamping process and excessive wrinkling of the part or addendum. Based on part geometry, the high impact load on the cushion may occur several inches from the bottom of the press stroke. Since the impact point in the stroke is both a higher velocity point and now at a de-rated press tonnage, shock loads develop and can cause damage to mechanical presses. Additional flywheel energy is dissipated by the high shock loads well above bottom dead center of the stroke. Staggering the heights of the nitrogen cylinders so they do not all engage at the same time is one way to reduce the shock load (see Figure 4).
Figure 4: Staggered nitrogen cylinders designed to reduce the initial shock load when setting draw beads by engaging at different depths in the press stroke. 2
A double-action press will set the draw beads when the outer slide approaches bottom dead center, where the full tonnage rating is available and where the slide velocity is substantially lower. This minimizes any shock loads on die and press with resultant load spikes less likely to exceed the rated press capacity.
1 G. Hartmann, “Das Spektrum Moderner Stahlfeinbleche-Festigkeiten und Auswirkungen auf die Umformung” Verschleißschutztechnik, Schopfheim, Germany (2004).
2 Courtesy of Peter J. Mooney, Managing Director, 3S Superior Stamping Solutions LLC
Header Image: Courtesy of Komatsu
Advanced High-Strength Steel (AHSS) forming challenges can lead to issues with the precision of part formation and stamping line productivity. The stamping industry is developing more advanced die designs as well as advanced manufacturing techniques to help reduce fractures and scrap associated with AHSS stamped from traditional presses. A Servo-driven Press1 is a significant option that has promising results when forming AHSS.
Characteristics of Servo Presses
A Servo Press is a press machine that uses a servomotor as the drive source. The advantage of the servomotor is that it can control both the position and speed of the output shaft compared to a constant cycle speed. In conventional mechanical presses, the press cycles at constant speed and press loads develop slowly, building power to their maximum force at bottom dead center (180-degree crank position), and then they reverse direction. In comparison, the Servo Press uses software to control press speed and position, which is much more flexible.
Servo Presses have a closed-loop feedback system to more accurately control cycle rate and loads, hence delivering a key advantage, which is the application of very high forming loads early in the stamping stroke. New forming techniques have been developed utilizing these features, and automakers are finding that with the use of the Servo Press, more complex part geometries can be achieved while maintaining dimensional precision.
Servo Presses have only been around for about 10 to 15 years, and so are considered “new technology” for the automotive industry. Recent growth in the use of Servo Presses parallels the increased use of AHSS in the structure of new automobiles. A cut-away of a Servo Press is shown below in Figure 1 to give a better understanding on the drive mechanism.
Figure 1: The Servo Press Cut-away 2
Types of Servo Presses
1) Servo motor and ball screw driven press: These Servo Presses employ ball screws to reduce the friction on the screws. Unlike traditional screw presses, this press does not require a clutch/flywheel and the slide velocity can be altered throughout forming. A notable feature of the ball screw type Servo Press is that the maximum force and slide speed are available at any slide position. This can be applied to almost all the forming methods. This capability proves to be very useful for forming with a long working stroke such as an extrusion, and for forming that requires a high-speed motion at the end of the formation.
The torque capacity of the servo motor, the load carrying capacity of the ball screw, and the reduction of the belt drive limit the maximum load. One solution to increase the power of the ball screw driven Servo Press is to increase the number of motors and driving axles (spindles). The only problem with this solution is the cost increases tremendously. Figure 2 shows a mechanism of hybrid Servo Press.
Figure 2: Operating mechanism of a hybrid Servo Press 3
2) Servomotor driven crank press: This press provides a lower-cost alternative compared to ball screw style Servo Presses. Crank presses have a high torque servomotor directly attached to the press drive shaft. A few press manufacturers developed C-Frame presses, which combine the servo driving mechanism with the conventional press structure. These presses are visually similar to mechanical presses, but the servo motor replaces the flywheel, clutch, and main motor. When the main shaft rotates at a constant speed, the stroke-time curve mirrors that of a conventional mechanical press. To increase power (for higher forming loads), crank presses are coupled with two connecting rods.
3) Servomotor driven linkage press: Linkage mechanisms are often used for presses to reduce the slide velocity and increase the load capacity of a given motor torque near the bottom dead center. Servomotor driven linkage presses prove to be advantageous in increasing the approaching and returning speeds by slowing down the slide speed in the working region within the stroke. They also have the advantage of maintaining high load capabilities through a considerably long working stroke.
4) Hybrid Servo Press: The knuckle joint and the linkage mechanisms are used to increase the press power with the crank shaft mechanism, and they can be combined with the ball screw mechanisms for Servo Presses.
5) Servomotor driven hydraulic press: These hydraulic presses have been developed because the high-power servomotors achieve higher slide speed compared with the conventional Servo Presses.
Advantages of Servo Presses
Servo Press technology has many advantages compared to mechanical presses when working with AHSS materials. Press manufacturers and users claim advantages in stroke, speed, energy usage, quality, tool life and uptime; these of course are dependent upon part shape and forming complexity.
1) Adjustable Stroke: The Servo Press has an adjustable stroke; the slide motion can be programmed to exert the required press load for deep draw stampings and then switch to different program routines to allow for shallow part stampings, or even blanking. This makes the Servo Press very versatile.
2) Speed: Compared to a standard mechanical press, Servo Presses manufacturers claim up to 37% increased cycle rates, which translates into better stamping plant productivity. Figure 3 shows cycle rates for comparable stroke heights for both a Servo Press and a traditional mechanical press.
Figure 3: Cycle Rates for Servo and Mechanical Presses 3
3) Energy Savings: The Servo Press has no continuously driven flywheel, cutting the costs of energy consumption. This is especially true in large capacity presses. The installed motor power is greater than that of a mechanical press whose capacity is comparable. However, throughout the stamping operation, the servo-driven motor is used only while the press is moving since the Servo Press has no continuously driven flywheel. Also contributing to nominal energy savings is the dynamic braking operation of the servo driven motor. Through this operation, the braking energy is transferred back into the power system. It is also possible to install an external energy storage feature to make up for energy peaks while reducing the nominal power drawn from the local power supply system (in cases where it is economically justified).
Figure 4 shows a comparison between a Servo Press and a traditional mechanical press with respect to energy use and storage. In a Servo Press, energy from deceleration of the slide is stored in an external device and tapped when the press motion requires more than 235 HP for each motor. The stored energy (maximum of 470 HP) is thus used during peak power requirements, enabling the facility power load to remain nearly constant, around 70 HP.
Figure 4: Energy vs. Displacement for Servo Press 3
4) Quality: Better forming stability translates into fewer part rejections.
5) Longer tool life: Decreasing the tool impact speed while simultaneously reducing the cycle time reduces impact loading, thus maximizing tool life. Increased lubrication effectiveness has been observed and using pulsating or oscillating slide motion can further extend tool life.
6) Uptime: Synchronized clutching and extended brake life allow for less frequent maintenance and better equipment uptime.
We have much more information we can share with you about these presses, more than we can fit into one blog. We encourage you to download your free copy of the AHSS Application Guidelines at worldautosteel.org and have a look at Section 3.C.6. Our next blog will provide a case study on Press Energy.
1 Dr. T. Altan, Professor at Ohio State University
2 Courtesy of AIDA America
3 Courtesy of P. Mooney archives
HEADER IMAGE SOURCE: Press line with ServoDirect Technology, courtesy of Schuler