IIW White Paper

The trend towards light-weight design is widely established in the automotive industry where steels with very high ultimate tensile strength are thus increasingly applied, ( see Figure 4.2) . This is partly due to the new European regulation which established a fleet-average CO 2 emission target of 130 g/km to be reached by 2015. Modern car bodies contain approximately 50% weight of high strength steels (HSS), which impose new challenges regarding conventional resistance spot welding of such steel grades in similar and dissimilar joints.

Figure 4.2 New steel grades in the automotive industry ( Reproduced courtesy: Arcelo Mittal)

The high strength levels are reached by alloying systems and controlled phase-transformation. During the whole production process very accurate temperature control is needed to obtain these properties. Today, such high strength levels cannot be maintained within welding, especially not for the hot formed materials. Joint properties with only 30% strength during tensile testing compared to the base metal have to be considered when using standard welding technologies and procedures. Therefore a major challenge in the near future will be the adaptation of the construction and the new welding processes ( see Figure 4.3 for cross section of the laser spot welding) to reach acceptable levels of strength, elongation and toughness of the components. Furthermore, the effects that occur when joining dissimilar steel grades have to be investigated. Additional problems appear because of the zinc or other anti-corrosion coatings such as AlSi or MgZn which lead to the formation of pores or intermetallic phases. Also the testing methods for welded components of the new steel grades have to be adapted.

Figure 4.3 Laser spot welded TRIP800 steel sheets of 1.0 mm thickness (Reproduced courtesy: GKSS, M. Kocak)

Aluminium alloys Weldability of components made of aluminium alloys is defined in terms of resistance to hot cracking and porosity. Resistance to hot cracking, which for aluminium includes both solidification cracking and liquation cracking, can vary significantly from one alloy to another. Solidification cracking occurs in the mushy zone trailing the molten weld pool when lowmelting eutectic films are pulled apart at grain boundaries. Liquation cracks form in the heat affected zone as grain boundaries become partially melted. Porosity comes from dissolved hydrogen, picked up from moisture contaminated shielding gas or oil deposits on the weld joint. Due to the little understanding about the formation of these defects at present, future research work has to concentrate on in-depth clarification of such failure phenomena.

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