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.
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.
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.
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.
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.