1 Brief History and Purpose of Tailor-Welded Blanks

1.1 A Brief History

During the past two decades, government fuel conservation and safety mandates along with environmental concerns have prompted the American automotive industry to design lighter cars for reduced fuel consumption, while improving the overall structure of their vehicles for occupant safety. Corrosion protection was also much improved during this period. These changes added to escalating manufacturing costs at a time when the industry was struggling with a serious threat from global competition.

To reduce weight and costs, alternative materials such as aluminum and composite materials have been proposed and used for body panels, but none has shown the versatility of steel. High strength steels, coated steels, laminated steels and various drawing quality grades give steel the ability to meet most automotive requirements.

One area of concern for product engineering, however, has been the higher density of steel versus alternate materials. The concept of combining various steel options into a welded blank was developed to enable product and manufacturing engineers to “tailor” the blank so that steel’s best properties were located precisely within the part where they were needed. This process not only reduces the weight of the finished part, but also can be used for part integration, thereby eliminating many reinforcements and stiffeners.

Potential Benefits of Tailor Welded Blanks

The potential benefits of the tailor welded blanks are impressive:

• Fewer parts
• Fewer dies
• Fewer spot welds
• Reduced design and development time
• Lower manufacturing costs
• Less material input, better utilization of steel
• Weight reduction
• Improved dimensional accuracy
• Improved structural integrity
• Improved safety
• Increased offal utilization and reduced scrap

1.2 Common Types of Tailored Blanks

Tailor welded blanks are made from prime stock as nested blanks or from collectible offal. Although the tailor welded blank process first was developed as a method for utilizing collectible offal and improving blank nesting possibilities, its greatest potential lies in the area of tailor welded blanks with different thicknesses, coatings and material grades.

This type of tailor welded blank gives the product designer the opportunity to eliminate reinforcements while improving structural and dimensional characteristics.

For example, tailor welded blanks are currently used for body-side frames, door inner panels, motor compartment rails, center pillar inner panels and wheelhouse/shock tower panels. (See Figure 1.2.)

In the near future, tailor welded blanks will be used on an increasing number of automotive body parts due to the reduction in tooling and operating costs.

Figure 1.2: Examples of typical tailor welded blanks. (Click the Image to Enlarge)

1.3 General Design Philosophies

1.3.1 Europe

One of the earliest European applications of tailor welded blanks for automotive applications was by Swedish carmaker Volvo using the resistance mash seam process in 1979. Other European manufacturers (Volkswagen, SEAT) now use the resistance mash seam process for tailor welded blanks and for the most part have concentrated their efforts on structural members such as motor compartment lower frame rails and wheelhouse/shock tower assemblies.

A notable exception is the Audi floor pan, which has been laser welded by Thyssen A.G. in Germany since 1985. This represents the earliest known use of laser-welded blanks in Europe. The blank for this panel is laser welded due to the sheet steel maximum coil width available from the steel mill being less than the required blank size. Also, the maximum resistance mash seam weld length at that time was one meter (40 inches), which did not satisfy the design requirements of the 1960mm (77 inches) weld-joint length.

1.3.2 Japan

In Japan, welded blanks have been produced by laser beam both with and without filler wire by Toyota since 1986. There is no know use of resistance mash seam welding in Japan at this time. Filler wire is used for applications that have an exposed weld in the finished product, such as bodyside frames. (See Figures 1.3.2-A and 1.3.2-B.) Filler wire welds are ground for flush appearance after welding.

Figure 1.3.2-A: Laser welding with a filler wire.

Figure 1.3.2-B: Effects of filler wire on weld hardness. With low carbon content filler wire, it is possible to decrease the hardness of the welded bead. Figure 1.3.2-B shows the relationship between the bead hardness for different filler wire.

Welds that do not require a flush surface for aesthetic or sealing purposes are welded with Toyota’s beam weaving process. Although beam weaving can reduce the concavity of the weld, it doesn’t replace the filler wire process for bodyside frames, as it doesn’t satisfy the requirements for outer surfaces. Both filler wire and beam weaving are used to reduce the need for precision shearing and to allow laser welding of multi-piece, multi-weld blanks in a one fixture set-up. Toyota, with in-house automated welding systems for both two-piece and multi-piece blanks, has been the leading innovator of these processes. (see Bibliography, Azuma, et. al.)

By using die-cut blanks without precision shearing before each weld, Toyota eliminates the repeated process steps of shearing and welding and the inefficient batch processing that is typical for multi-piece, multi-weld blanks being made on other welding systems. Toyota is able to laser butt-weld blanks with a gap tolerance as high as 10 percent of the metal thickness when beam weaving. This allows Toyota to weld die-cut edges. Based on Toyota technical reports, this process appears to have a limit of a 500mm (20 inch) weld length.

1.3.3 North America

The initial use of the tailor welded blanks in North America has been on structural members such as frames, center pillar inners and motor compartment upper rails. Electron beam welds have been used to weld thicker gauge (3.0-5.0mm/0.12-0.20 inch) metal blanks for automotive frame members at A.O. Smith since 1968. More than 100 million welds had been made on tailor welded blanks when production ended in 1993. Use of tailor welded blanks when production ended in 1993. Laser beam welding is used on most of these applications at this time. Resistance mash seam welded blanks have become more available recently, and are increasingly being considered for automotive applications. Current planning by North American and European manufacturers indicates that both resistance mash seam and laser beam welding will be used on future models. When more than one welding process meets the design requirements, availability and cost will be the deciding factors.