22
Improving Global Quality of Life
Through Optimum Use and Innovation of Welding and Joining Technologies
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.
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.
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.
Figure 4.2
New steel grades
in the automotive industry
(
Reproduced courtesy:
Arcelo Mittal)
Figure 4.3
Laser spot welded
TRIP800 steel sheets of 1.0 mm
thickness (Reproduced courtesy:
GKSS, M. Kocak)
