Technical Challenges and Die Development in the Application of Laser Welded Blanks

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2021-04-16

There are two forming methods for automotive body parts: separate forming and integral forming.

The separate forming method involves using different presses to shape individual parts separately, which are then welded together to create the final component. While this approach enhances flexibility in material selection, it also leads to higher costs for stamping and machining, increased assembly expenses, and challenges in ensuring precise shape alignment. Additionally, the overlapping of materials during spot welding can inadvertently add weight to the vehicle body.

The integral forming method involves simultaneously shaping multiple parts from a single sheet of material on a single press. From the perspective of vehicle body structure design, each body component has distinct requirements for thickness and corrosion resistance. If a single sheet were used for forming all parts, it would necessitate employing the same material grade, coating type, and thickness across every component—resulting in excessive material selection margins for certain parts. This not only increases the overall weight of the vehicle body but also drives up costs and complicates the forming process.

To reduce vehicle weight, improve body assembly precision, enhance body rigidity, and lower stamping and assembly costs during automotive body manufacturing, it is essential to minimize the number of body components while integrating them into a single, unified structure. As a result, a new production method has emerged—welded blank stamping—which simultaneously addresses the limitations of both traditional separate forming and conventional integrated forming techniques.

The advantages of laser tailor-welded panel technology

Laser welding technology used to produce tailor-welded blanks offers significant advantages—according to research by international scholars, these benefits number as many as 37. The ULSAB (Ultra-Light Steel Auto Body) project produced a prototype vehicle in 1998, featuring 16 tailor-welded blank components that accounted for approximately 45% of the vehicle’s total body weight. Thanks to the adoption of this advanced technology, the number of individual body parts was reduced by about 25%, while torsional rigidity improved by 65%, vibration characteristics were enhanced by 35%, and overall bending stiffness was also strengthened.

The advantages of laser-welded composite panels are primarily evident in the following areas:

a. The reduction in the number of parts, coupled with the resulting simplification of production equipment and manufacturing processes, has significantly boosted productivity while lowering the costs associated with vehicle manufacturing and assembly.

b. Since the product’s various components are joined together via a continuous laser welding process before forming, this enhances product precision and significantly reduces manufacturing and assembly tolerances for the individual parts.

c. By optimizing the components, we’ve reduced weight, which in turn lowers fuel consumption—something that’s especially important in today’s environmentally conscious era. d. Since reinforcing plates are no longer required and there are no overlapping seams, the corrosion resistance of the assembly has been significantly improved. Additionally, eliminating overlaps enhances the corrosion-resistant properties of the components, greatly reducing the need for sealing measures.

e. By rigorously selecting materials based on thickness and quality, we’ve achieved a fundamental leap in component strength and impact resistance. At the same time, the optimized structure ensures that greater energy can be absorbed during an impact, significantly enhancing the crashworthiness of body components and ultimately improving the vehicle’s passive safety performance.

f. The variability in material thickness, combined with its consistent quality, ensures that reinforcement and improvements to critical areas can be carried out seamlessly; enabling the fullest utilization of material properties and achieving the most optimal combination of material performance. For product designers, this also enhances flexibility in product development.

g. By selectively using high-strength, thick materials in specific areas of structural components, material utilization is significantly improved. Additionally, by implementing layout optimization techniques during the blanking process, various steel plates are efficiently combined, dramatically reducing material waste in engineering applications.

h. By employing laser tailor-welding technology, the strength and thickness of the materials are optimally combined, significantly enhancing the structural rigidity and optimizing the vehicle's overall vibration characteristics. This, in turn, leads to reduced vehicle vibrations and noise, ultimately improving the comfort level of the entire car.

i. Enabling the production of wide-body vehicles. Due to limitations in the width of steel mill rolling machines, steel mills can only supply steel plates within certain width constraints. However, as the automotive industry continues to grow, the demand for wider steel plates is becoming increasingly urgent. In this context, laser seam welding emerges as an effective and cost-efficient manufacturing method.

Application of Laser Tailor Welding Technology

Laser Tailor-Welding technology begins by cutting cold-rolled steel sheets—whether of varying or identical thicknesses, strengths, or materials—into precise sizes and shapes. These pieces are then laser-welded together to form a seamless, high-quality integrated component known as the laser tailor-welded panel. Laser welding offers significant advantages, including high efficiency, convenient energy transfer, minimal post-weld distortion and deformation, and exceptionally smooth surface finishes. As a result, this advanced technique is widely used in automotive parts, as illustrated in Figure 1.

Figure 1: Application of Laser Welded Blanks in Automobiles

01 Laser-welded Automotive Door Inner Panel

Traditionally, the interior door panel design requires an additional reinforcement plate to be installed at the front, where the door hinges are mounted, ensuring sufficient strength to support the entire door assembly. This reinforcement plate undergoes processes such as blanking, stamping, sub-assembly welding, and is then integrated into the interior door panel during final assembly.

The newly designed laser-welded door inner panel, based on structural analysis, directly joins steel sheets with varying thicknesses or strengths. By leveraging the distinct material properties of different areas within the part, this approach optimizes the component’s strength and weight compared to using a single sheet.

This design approach directly uses thick, high-strength panels at the door hinge locations, eliminating the need for traditional reinforcement processes. As a result, it reduces production steps, streamlines the manufacturing process, and significantly enhances side-impact resistance—boosting rigidity by approximately 40%. This, in turn, helps minimize interior noise during vehicle operation.

Thanks to the overall stamping of the parts, the trimming process at the edges has been optimized, leading to a significant improvement in material utilization and delivering impressive overall cost benefits. As shown in Figure 2, under the traditional manufacturing process, waste amounts to 0.886 kg, whereas using laser-welded blanking technology reduces waste to just 0.138 kg. If each component can cut down on waste by 0.726 kg, the entire vehicle could achieve a weight reduction of 1.45 kg.

As for reinforcement, traditionally it’s been 2 + 0.8 mm, but by using laser-welded blanks, this can be reduced to 1.8 + 0.8 mm while still achieving the same strength performance. This change allows each component to shed 2.15 kg, resulting in a total weight reduction of 4.3 kg for the entire vehicle. Clearly, laser-welded blanks not only cut down on weight but also help lower production costs.

02 Laser-welded Automotive Pillar

To ensure functional requirements, the pillars must possess adequate rigidity and cushioning performance to guarantee passenger safety during lateral impacts. Traditionally, this would require designing additional reinforcing ribs when using conventional stamping methods. However, by employing tailor-welded technology, two steel plates of different thicknesses can first be joined into a single, unified sheet before being stamped into shape, as shown in Figure 3(a). This approach not only enhances side-impact performance but also eliminates the need for reinforcing ribs, thereby reducing vehicle weight. Additionally, it improves assembly tolerances, simplifies the manufacturing process, and lowers overall production costs.

In laser tailor-welding processes, the position of the weld seam can be adjusted to meet forming requirements, while maintaining strength and hardness that effectively withstand lateral impacts and provide superior bend control. Additionally, by minimizing intermediate steps, material usage can be reduced by up to 14%.

03 Laser-welded Automotive Front Longitudinal Beam

To ensure functional requirements, the front longitudinal beam must possess adequate rigidity and energy-absorbing capabilities, while also maintaining sufficient strength. This ensures that, in a frontal impact, the thinner steel plate first absorbs some of the energy and provides initial cushioning, while the thicker steel plate can withstand the remaining force—resisting deformation or sustaining only minimal distortion—to safeguard passenger safety.

Figure 2: Schematic Illustration of Laser Welded Tailor-Welded Blanks Applied to Automotive Door Inner Panels

Figure 3: Application of laser-welded tailor-welded blanks in automotive pillars (a), front longitudinal beams (b), wheel arches (c), and floor panels (d)

Using traditional stamping methods would require designing additional reinforcing plates, whereas with tailor-welded technology, three steel plates of varying thicknesses can first be precisely welded together into a single, unified sheet before being stamped into shape, as shown in Figure 3(b). This approach not only enhances the shock-resistant performance at the joint but also reduces the number of individual components, lightens the vehicle weight, simplifies the manufacturing process, and ultimately lowers costs.

04 Laser-welded Automotive Wheel Housings

As shown in Figure 3(c), the application of tailor-welded blanks in the wheel arch area also reduces weight and the number of components due to the integrated shock-absorbing structure. This eliminates the need for spot-welded overlaps, spot-welding materials, and spot-welding processes, resulting in tangible cost savings. Additionally, the removal of sealing materials further cuts down on labor costs.

05 Laser-welded Automotive Floor Panels

As shown in Figure 3(d), laser tailor-welded panel technology is also being applied to car floors to enhance crash performance and component stability. This approach not only enables the elimination of lap joints and sealing materials but also helps reduce vehicle weight and facilitates easier recycling. Moreover, laser tailor-welded panels are already widely used in automotive body components such as trunk reinforcement plates, trunk inner panels, shock absorber mounts, bumpers, roofs, chassis structures, and dashboard crossmembers.

Technical Challenges in the Application of Laser Tailor-Welded Blanks

Welded-butt panels also introduce certain challenges to the stamping process. Differences in plate thickness or material properties can lead to uneven deformation of the blank, such as uneven movement or twisting at the weld seam, altered wrinkling conditions, and non-uniform bending of the material. This often results in increased thinning of the part, reduced forming limits at the weld area, accelerated punch wear, and the need for redesigned retaining rings—as well as tighter control over clamping force. Additionally, as the surface area of the sheet metal increases significantly, the required mold surface area grows exponentially, placing higher demands on mold design, manufacturing, and even the supporting equipment. Only by addressing these issues effectively can the widespread adoption of welded-butt panel technology be realized.

01 Design of Stamping Processes

Since laser tailor-welded blanks are preforms made by welding together steel sheets of varying or identical thicknesses, strengths, and materials, the stamping process must account for these differences in material properties, strength, and thickness—and accordingly be adapted. In particular, near the weld seam, the hardened area created by laser welding significantly increases the material's strength, posing a unique challenge during the stamping operation.

Since laser tailor-welded blanks are mostly composed of deep-drawing steel and dual-phase steel, the high-strength steel sheets within them still experience conventional issues such as springback and distortion during stamping—problems that also pose significant challenges in controlling the stamping process.

Studies have shown that the fixed clamping gap method is more effective for sheet metal stamping compared to the constant clamping force approach, as it helps improve formability, reduce weld seam movement, and optimize die loading. The fixed clamping gap method proves to be an efficient clamping technique in tailor-welded blank (TWB) stamping processes. During the stamping of laser-welded blanks, wrinkling typically occurs more easily in the thinner side material.

Tensile tests on laser-welded tailor-welded blanks [7] revealed that the thinner side experiences significant deformation and severe thinning, making it prone to cracking. When the clamping force is too low, wrinkling occurs on the thinner side; conversely, if the clamping force is too high, the thinner side may rupture. Additionally, the weld at the bottom of the component shifts slightly toward the thicker base material, while the welds along the sidewalls and flanges exhibit greater movement toward the thinner base material—this effect becoming more pronounced as you move closer to the edges.

Optimized stamping processes can be employed—such as varying the stamping direction and incorporating process reinforcements—to improve factors like the maximum material thickening rate on thin sides, the highest tensile stress in the sheet metal, and strain at critical points.

Only by adhering to the fundamental principles of stamping deformation can we accurately design the shapes and dimensions of both the blank and semi-finished products, as well as the structural parameters of the die, thereby enhancing the rationality of the stamping process. Moreover, establishing a well-optimized stamping process allows full utilization of the material's deformability, ultimately improving the forming limit.

02 Mold Development

During the stamping process, weld seam flow and material movement in laser-welded tailor-welded blanks are common occurrences. This necessitates accounting for potential weld-seam migration during die design, ensuring the resulting structural design is both functional and efficient.

Weld seam movement, wrinkling on the thinner side, and excessive thinning can be controlled by modifying the tooling and dies—for instance, using stepped dies, segmented retaining rings, or employing multi-cylinder systems for localized edge pressing and stretch-forming ribs—all of which can achieve significant results.

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