- Types of Tailored Products
- Benefits of Tailored Products in Automotive Body Construction
- Design, Usage, and Troubleshooting Guidance
Types of Tailored Products
Key materials characteristics for formed parts include strength, thickness, and corrosion protection. Tailored products provide opportunities to place these attributes where they are most needed for part function, and remove weight that does not contribute to part performance.
Tailor Welded Blanks
Tailor welded blanks are a type of tailored product, as are patchwork blanks, tailor welded coils, tailor rolled coils, tailor rolled tubes and tailor welded tubes.
Laser welding is the most common approach to creating these products, but other options like resistance mash seam welding are possible. Mash seam welding requires that the blanks overlap, which adds mass. The heat affected zone (HAZ) for mash seam welds is considerably larger than that of laser welds. Compared with laser welding, the temperature reached when resistance mash seam welding is lower. This may improve formability due to reduced martensite formation, but the larger HAZ and thicker joint may negate this benefit. Non-linear welds are more challenging to produce with the mash seam approach. The following discussion centers on blanks and coils joined by lasers.
Butt-welding two or more flat sheets into a single blank creates a laser welded blank (LWB). Steel grades and thickness may be (and usually are) different in the component sheets. Corrosion protection strategies may be different on each component of the welded blank. “Sub blanks” is another name for these component sheets.
Figure 1 shows a laser welded door ring 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. The lower strength product in the bottom section of the B-Pillar helps dissipate the crash energy in the event of a side impact, playing a key role in crash energy management.
An alternate approach to a laser welded blank involves the creation of what is known as a patchwork blank, consisting of two or more blanks potentially having different thickness or grades. The unique characteristic is that the blanks are placed one on top of the other and (typically) spot welded together while still flat. The patched blank requires only one stamping operation, eliminating the need to join them after forming the components separately. This approach is satisfactory for both cold stamping and hot stamping. The correct number of spot-welds is important in terms of cost and safety. Furthermore, if the components shift relative to one another during the forming operation, the spot welds may shear. There is excellent fit between both parts after forming, allowing for easy application of additional spot welds if needed. Part size, the number and location of reinforcements, and available infrastructure are among the key considerations in deciding between a laser welded or patch blank approach. For example, patchwork blanks may be preferred if the reinforced area is relatively small with complex contours or located within the boundaries of the main blank.
Tailor Welded Coils
Rather than building blanks from individual components, a similar approach is to join entire coils together, edge to edge. These tailor welded coils (Figure 2) are used in coil-fed processes such as blanking, progressive die stamping, transfer press stamping, and roll forming operations. 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 either directly blanked or re-coiled for future blanking or use as feedstock for a continuous coil-fed stamping or roll forming line (Figure 3). Variations in strength, thickness, and coating occur across the width.
Tailor Rolled Coils
Whereas tailor welded coils can vary in properties across the coil width, tailor rolled coils have variable thickness down the length of the coil. Production facilities for tailor rolled coils vary the gap between the rolls used for thickness reduction, allowing for different strip thicknesses in the direction of rolling (Figure 4). Accurate measuring and feedback control technology guarantees the strip thickness tolerances. A significant advantage to this approach results from the transition from one thickness to another with no joints or discontinuities, providing an efficient load path without stress risers. A tailor rolled coil can be either used for blanking operations (for stamping or tubular blanks), or as feedstock into a roll forming line.
After cold rolling to achieve the targeted thickness variation, coils are annealed to reset the mechanical properties. Tensile properties can be uniform in the blank, with only the thickness varying. Tailored properties are generated by adjusting the incoming coil thickness and the associated thickness reductions, leading to different degrees of recrystallization occurring in the annealing step.Z-14 TRBs used in press hardening do not need to be annealed first, and tailored properties can be produced with the appropriate PHS process.
Tailor Rolled Tubes
Tailor rolled coils are the feedstock to produce variable thickness tailor rolled tubes.
Tailor Welded Tubes
Conventional tube production involves roll forming strips to the desired shape and welding the free ends together to create a closed section. A tailor welded tube production process allows the designer to create complex variations in shape, thickness, strength, and coating (Figure 5)
With hydroforming technology, the next step in tubular components is to bring the sheet metal into a shape closer to the design of the final component without losing tailored blank features (Figure 6).
Hydroformed conical tailored tubes offer automotive body engineers an additional approach to crash energy management while achieving a lightweight design. In frontal crash and side impacts the load paths have a key importance on the body design as they have a major bearing on the configuration of the structural members and joints. Figure 7 shows an example of a front-rail hydroformed prototype. The conical tailored tubes for this purpose take advantage of the high work hardening potential of TRIP steel.
Benefits of Tailored Products in Automotive Body Construction
Figure 8 highlights some of the areas within the body structure where companies have considered transitioning to tailor welded blanks. Other tailored products may be suitable in other areas.
Tailored products offer numerous advantages over the conventional approach involving the stamping and assembly of individual monolithic blanks which have a single grade, thickness, and coating, including:
Improved materials utilization
- Certain parts, like door rings (Figure 1), window frames, and door inner panels, have large cutout areas contributing to engineered scrap. Converting these to tailor welded blanks allows for optimized nesting of the individual components. Figure 9 presents an example of optimized nesting associated with body side aperture designs using a tailor welded blank. Reduced blank width requirements may allow for additional suppliers or use of master coils yielding slit mults. In the other extreme, blank dimensions larger than rolling mill capabilities are now feasible.
Elimination of reinforcement parts and reduced manufacturing infrastructure requirements
- In areas needing additional thickness for stiffness or crash performance, conventional approaches require stamping both the primary part and an additional smaller reinforcement and then spot welding the two parts together. The tailored product directly incorporates the required strength and thickness. Compared with a tailored product, the conventional approach requires twice the stamping time and dunnage, creates inventory, and adds the spot welding operation. Tolerance and fit-up issues appear when joining two formed parts, since their individual springback characteristics must be accommodated.
- Similar to the benefits of eliminating reinforcements, tailored products may combine the function of what would otherwise be multiple distinct parts which would need to be joined.
- Conventional approaches to body-in-white construction requires individual parts to have flat weld flanges to facilitate spot welding. Combining multiple parts into a tailored product removes the need for weld flanges, and their associated weight.
Improved NVH, safety, and build quality
- Joining formed parts is more challenging than joining flat blanks first and then stamping. Tailored products have better dimensional integrity. Elimination of spot welds leads to a reduction in Noise, Vibration, and Harshness (NVH). A continuous weld line in tailored products means a more efficient load path.
Enhanced engineering flexibility
- Using tailored products provides the ability to add sectional strength in precise locations to optimize body structure performance.
Easily integrated with advanced manufacturing technologies for additional savings
- Tailored products incorporated into hot stamping or hydroforming applications magnify the advantages described here, and open up additional benefits.
Design, Usage, and Troubleshooting Guidance
Edge Quality of Component Blanks Used to Create the Welded Blank
Minimal burr, maximized sheared edge stretchability, and maximized tool life are characteristics of good sheared edge quality in conventional blanking operations. When creating sub-blanks for assembly into a laser welded blank, there is an additional critical characteristic: squared edges with high straightness are a prerequisite to avoid local gaps between the blank edges causing undercut in the weld bead and the associated loss of mechanical properties (Figure 10). Conventional shearing operations produce the edge seen in Figure 10a. Precision die blanking with tight clearances, as well as laser blanking, produce edges suitable for the blank welding operation (Figure 10b). Another option is to use a conventional shearing approach to produce a slightly oversize blank, and then use precision shears as the first step in the laser welding line. The advantage to this approach minimizes the impact of edge damage which might occur during transportation.
Forming analysis of Welded Blanks
The weld joint and adjacent heat affected zone complicates the forming analysis of welded blanks. There are generally two scenarios that occur: failure perpendicular to the weld and failure parallel to the weld (Figure 10).
Consider the scenario in Figure 11A, where the major strain is perpendicular to the weld line orientation. Failure occurs parallel to the weld in the lower strength, more ductile material. The restraining force provided by the higher strength material results in the weld line shifting towards that side.
With the weld seam moving towards the thicker and stronger metal, deformation occurs in thinner and weaker metal. The movement of the weld seam during stamping requires that a blankholder design with appropriate clearances so the welds can move through the blankholder during the drawing process. The punch-cavity clearances must allow for movement as well – otherwise a pinch-point is created and metal flow stops. These clearances are necessary for movement of the heavier gauge metal and the welds, but can also be a source of wrinkling on the lighter gauge side due to the lack of hold-down force in this area. Also, the potential exists for die wear in the locations in contact with weld seam. To counter the weld line motion to the higher strength side, consider allowing greater material flow from that side of the weld through a less severe draw bead design, reduced blankholder pressure or modified blank outline. These strategies help minimize the risk of this type of split.
Now consider the scenario in Figure 11B, where the major strain is parallel to the weld line orientation. Here, fracture occurs perpendicular to the weld line – typically within the weld itself – due to the poorer mechanical properties of the weld with respect to the base material. To avoid splits in the weld, orient the weld away from the major strain direction and away from areas with high strain concentration.
Use of Advanced High Strength Steels in Laser Welded Products
As sub-blanks are welded together to create the main blank, the beginning or ending of each of the weld lines are not in a steady state condition. As such, the start or stop action of the welding may create a stress riser. After the main blank is built-up, a scalloped cut can remove the affected areas so that only sections of uniform edge quality are formed in subsequent operations.
Laser welding is a high energy density process that focuses the heat in a very localized region as the laser travels at a high speed. This results in a narrow heat affected zone and the rapid quenching leads to the formation of a high-hardness martensite. Increased welding speeds generally require higher energy intensity at the weld which increases peak hardness.
Different welding parameters must be used for advanced high strength steels due to the presence of martensite in the base metal. The microstructure in the heat affected zone is a function of the local time/temperature profile, which changes with distance away from the weld as well as with the composition of the alloy in question. Laser welding some AHSS grades results in tempered martensite in the heat affected zone, leading to a locally softened region. Crash modeling should incorporate the influence of the softened region, which may help dissipate a portion of the crash energy.
However, this softened region creates an area for high strain concentration, leading to premature failure in martensite-containing AHSS grades. Higher strength dual phase steels have more martensite than lower strength dual phase grades, and therefore are more likely to generate HAZ soft zones and reduced formability when used in laser welded blanks. CP and TRIP steels have a lower incidence of HAZ soft zones due to higher alloying content. More information about laser welding is found here.
Blank Stacking, Destacking, and Feeding Problems
Depending on the thickness differences between the component blanks and their relative sizes, embossed dimples on the thinner gauge side can balance the thickness difference and to allow stable coil winding (in the case of tailor welded coil production) or stacking as they are blanked onto a lift. The stack weight and banding of the pallets may contribute to collapsing the dimples, so some companies choose to minimize the stack height.
Automatic destackers and feeders typically require uniform blanks and stacks for trouble-free operation. Dimples and cutouts may interfere with proper operation of overhead vacuum cups or magnetic belts. Moving or deactivating Vacuum cups may be necessary. Dimples should either be outside of the magnetic belt area or have their form away from the belt. Dimples may be a problem for some in-line blank washers and die-lube roll coaters.
Press and Tooling Considerations
When there is a large thickness difference within the component blanks, press tonnage and balance can be a problem. Press tonnage monitors are essential for this type of part, not only for protection of the press equipment, but also for good process control. If press balance is a problem, the tonnage monitors will indicate where the problem is. Supplemental balance devices in the die, such as nitrogen cylinders, may be appropriate, or the stamping may need to move to a larger press.
Tailored products often contain sub-blanks of mixed strength and thickness. The chosen die process must reflect those differences, and account for changes in side-wall curl, springback and drawability characteristics across the part. Mild steel can usually be formed with high strength steel die processes, but high strength steel will not always form satisfactorily with conventional die processes used with mild steels.
- Types of Tailored Products
- Benefits of Tailored Products in Automotive Body Construction
- Design, Usage, and Troubleshooting Guidance