Introducing TWB
Tailor Welded Blank (TWB) consists of metallic sheets with similar or dissimilar thickness, materials, coatings etc. welded in a single plane before forming. TWBs are formed like un-welded blanks to manufacture automotive components. By using this technology, it is possible to produce parts with varying mechanical properties, leading to component optimization (Pallett & Lark, 2001). Among all the welding methods, laser welding is used predominantly to TWBs as the process is quick and produces smaller welds. The main catalyst behind the sudden growth of TWB is to maintain the market share of steel material in the face of competition generated by aluminium alloys towards weight reduction (Pallett & Lark, 2001).
Some of the advantages of using TWBs in the automotive sector are, (1) scrap materials from stamping industries can be reused to have new stamped products, (2) by distributing material thickness and properties, part consolidation is possible resulting in cost reduction and better quality, stiffness and tolerances, (3) provides greater flexibility for component designers, (4) weight reduction of the product can be achieved and hence fuel consumption is lowered, and (5) improved corrosion resistance and product quality. Research shows that a 1% reduction in vehicle weight can result in a reduction of fuel consumption to 0.6 – 1% (Pallett & Lark, 2001). With these advantages, the potential of TWBs was soon recognized by steel industries and a consortium called Ultra Light Steel Auto Body (ULSAB) concept was then developed (WorldAutoSteel, 2020)
Some of the applications of TWB include center pillar, bumper, front door inner, and rear door inner (Irving, 1995; Prange et al., 1994). Nevertheless, the applications are not restricted to automotive sector, as construction industries also show encouraging pathway in using TWB as part of buildings (Pallett & Lark, 2001). Some applications of laser-welded TWBs are listed in Table 2.1(WorldAutoSteel, 2020). In the case of aluminium alloys, 5xxx for automotive inner body panels, and 6xxx for outer body panels are mainly used. Specifically, these include 6111-T4, 5754-O, 6061, 5182, 5052 and 5454. Some of the applications of aluminium TWBs made by laser welding, include deck lids, hoods, floor and door inner panels, side frame rails etc. (Das, 2000). Several applications in European and USA market, variety of welded sheets in the form of three piece blanks, non-linear weld lines, details on noise separation, and introduction of multi-piece blank lines can be noted (Rooks, 2001).
Table 1. | TWB product | Thickness combination (in mm) | Material combination * | Yield strength combination
(in MPa) |
| Body side outer | 1.5/1.0/0.7 | DP/DP/BH | 700/700/250 |
| Wheel inner house | 1.4/0.6/1.1/0.6 | DP/DP/DP/DP | 700/500/700/500 |
| Floor rear | 0.7/0.6/1.1/1.1 | DP/BH/DP/DP | 700/210/350/350 |
| Rear rail | 1.8/1.1 | DP/DP | 700/500 |
| Rocker inner | 1.5/0.7 | DP/DP | 700/700 |
| Reinforcement B-pillar rocker rear | 1.2/1.4 | DP/DP | 700/700 |
| Body side outer | 0.7/1.5/1.0/1.2/0.7 | BH/DP/DP/DP/BH | 260/700/700/700/260 |
| Rail rear outer floor extension | 0.6/1.0 | BH/DP | 210/500 |
| Front door inner front | 1.0/1.2 | MS/MS | 140/140 |
| Rear door inner front | 1.0/1.2 | MS/MS | 140/140 |
[*Material combination: DP – Dual Phase steel; BH – Bake Hardenable steel; MS –Mild Steel]
In laser welding, CO2 and ND:YAG (Neodymium:Yttrium-Aluminum-Garnet) laser welding techniques are widely used to produce TWBs. A typical experimental setup is shown in Figure 1 (a) in which two steel sheets are clamped for CO2 laser welding. Initially, a pointer is displaced at the interface of sheets to check the alignment of sheets with laser source. Figure 1 (b) shows the laser-welded TWB made of steel sheets. Edge quality is crucial and generally, machined edges are preferred for laser based TWB fabrication (Ono et al., 2004). Some of the problems encountered while welding Aluminium sheets are (i) excessive porosity and hot cracking of the fusion zone, (ii) poor coupling during laser welding because of high reflectivity of the metal, (iii) degradation of weld zone properties, and (iv) loss of alloying elements in fusion zone (Das, 2000). Of the two techniques, CO2 based is accepted widely, safer and less expensive. However, it is subjected to high reflectivity of laser beam on the Al solid surface for a particular wavelength. ND:YAG laser welding has the advantage of uniform energy distribution than CO2 laser beam over a wide area. It provides the ability to span over large gaps, while maintaining weld speed and quality.
Figure 1. (a) Laser welding set up for fabricating tailor-welded sheets, (b) Laser welded tailored-sheets deformed until fracture (Narayanan, 2007)
In the following sections, the effect of various laser based TWB parameters on the forming performance are summarized.