１．Overview

Various roads exist worldwide, and SUV vehicles with ladder-frame structures, such as the Toyota Land Cruiser, are essential for daily life (Fig. 1). The ladder frame must excel in “reliability and durability,” “rough road drivability,” “collision safety,” and “rust prevention,” while also being lightweight to comply with fuel consumption regulations (CAFÉ regulations) and carbon neutrality. A common design is the R/F structure (Fig. 2), where reinforcement (R/F) is layered on the main body. However, to reduce weight, a differential thickness structure using linear TWB technology (Tailor Welded Blank，Fig. 3) is employed. We have further advanced weight reduction by developing a curved TWB structure (Fig. 4) with curved welding lines, representing an industry-leading mass production technology. After extensive discussions among relevant departments to ensure welding quality suitable for various road conditions, it took approximately six years to establish this mass production technology.

Fig.1　SUV vehicles with ladder frame structure

２．Mass production technology

The use of curved weld lines raised two major issues: “expanding the range of good welding conditions (robustness)” and “responding to gap changes caused by weld heat distortion”. To ensure strength, the optimal underfill was less than A% of the sheet thickness t on the thin plate side (Fig. 5). Initially, laser welding alone was deemed inadequate for mass production, prompting the development of HLA welding（Hybrid Laser-Arc Welding）, which combines a preceding arc with a following laser. The gap expanded from d1 (first half) to d2 (second half) during welding due to thermal expansion differences between steel plates (Fig. 6). This issue was addressed by melting a wider area with preceding arc welding and stabilizing the process using a twin-spot laser (Fig. 7). The range of good welding conditions was thus broadened by accommodating plate gap tolerances and other variations in mass production.

Takaya Komatsuzaki
Regular Members，Toyota Motor Corporation (471-0826 1, Toyota-cho, Toyota-shi, Aichi, Japan)

Masahiro Onoda
Toyota Motor Corporation (471-0826 1, Toyota-cho, Toyota-shi, Aichi, Japan)

Eita Niisato
Toyota Motor Corporation (471-0826 1, Toyota-cho, Toyota-shi, Aichi, Japan)

Tetsuji Egawa
Toyota Motor Corporation (471-0826 1, Toyota-cho, Toyota-shi, Aichi, Japan)

Hiroshi Uchiyama
Toyota Motor Corporation (471-0826 1, Toyota-cho, Toyota-shi, Aichi, Japan)