Build quality also should improve. Joining formed parts, each with their own springback and tolerances, is more challenging than joining flat blanks first and then stamping the tailored blank. Also expect improved product performance due to continuous sections rather than relying on fastened joints to transfer loads. Eliminating spot welds or rivets also can reduce noise, vibration and harshness. A continuous weld line used to fabricate tailored products means a more efficient load path and improved dimensional integrity.
Improvements in material utilization may be the strongest factor promoting the use of tailored parts in engineered components. Some parts (such as the door opening panel shown in the accompanying figure) have large cutouts destined to become engineered scrap. For parts such as these, structural requirements necessitate using numerous reinforcements, or making the part from a blank as thick as the thickest section.
In the case of the door opening panel shown, the rocker area closest to the road is at risk of exposure to road salt, and as such body structures must use galvanized steel throughout the entire blank, even though the roof line has minimal risk of corrosion. Converting this part to a LWTB allows for optimized nesting of the individual components, with material strength, thickness and corrosion protection deployed only where the end product benefits from those characteristics.
In addition, reduced width requirements of the sub-blanks can allow stampers to select from a larger array of material suppliers or to use master coils yielding slit mults. At the other extreme, blank dimensions larger than rolling-mill capabilities become feasible, allowing, for example, the stamping of inner side panels of tall SUVs and cargo vans.
Types of Tailored Parts
LWTBs represent just one type of parts where designers can specify material strength, thickness and corrosion protection where these properties are most needed for part function, and remove weight that does not contribute to part performance. Other tailored parts include patchwork blanks, tailor welded and tailor rolled coils, tailor rolled tubes (TRTs) and tailor welded tubes (TWTs). Each of these can be cold formed, and when appropriately designed, may be hot formed as well.
Patchwork blanks consist of a smaller patch blank spot welded to a main blank underneath; the two components may differ in strength and thickness. The spot welds hold the blanks together to prevent shifting during forming operations. One reason to deploy a patchwork blank rather than a LWTB: if the reinforcement is located within the boundaries of the main blank.
Butt welding at the sub-blank edges creates a LWTB; similarly, welds can join entire coils together edge-to-edge, creating tailor welded coils. The new strip either is directly blanked, or recoiled for future blanking or for use as feedstock for continuous coil-fed operations such as progressive-die and transfer-press stamping, and rollforming. Variations in strength, thickness and coating occur across the coil width.
Another option: tailor rolled coils (TRCs), which feature variable thickness and strength down the length of the coil rather than across its width. The gap between rolls in traditional sheet rolling mills decreases in sequential stands to produce progressively thinner sheets. Each roll stand contains structurally supported components to ensure the most uniform thickness possible. In contrast, when producing TRCs, the gap between the rolls used for thickness reduction is intentionally varied smaller and larger in a controlled sequence, allowing for different strip thicknesses in the direction of rolling. TRCs usually require annealing after rolling due to the high degree of cold work. Controlling the amount of thickness reduction in certain areas may allow for local strengthening in the thinner regions after annealing.
TRCs are the feedstock used to produce tailor rolled blanks and variable-thickness TRTs, or as the feedstock for a rollforming line. In the case of TRTs, properties and thickness vary down the length of tube but are consistent radially.
Conventional welded-tube production starts with rollforming monolithic strips to the desired shape and welding the free ends together to create a closed section. The TWT production process allows the designer to create complex variations in shape, thickness, strength and coating as a function of the starting LWTB. These tubes can be hydroformed to further expand design options. MF
Industry-Related Terms: Blank
, Material Utilization
, TransferView Glossary of Metalforming Terms
See also: Engineering Quality Solutions, Inc., 4M Partners, LLC
Technologies: Materials, Welding and Joining