Analysis of the mechanical behavior of welded joints using micro-mechanical characteristics

The last decade’s trend of aluminum alloy producers, is to manufacture new aluminum alloys that
are even lighter, have improved mechanical properties as well as to have the ability to be welded.
The structural weight of aluminum alloys can be reduced by adding lithium, since Li is the least
dense metallic element. Third generation Al-Cu-Li alloys are highly promising materials with
improved mechanical properties and damage tolerance behavior compared to other commercially
available 2xxx series Al alloys. These alloys have already been established as structural materials
for aerospace applications mainly because of the reduced density and increased specific strength.
Next to new, lightweight alloys, aerospace industries focused on efficient joining techniques
such as fusion welding, for a further structural weight reduction as well as for a decrease of the
manufacturing costs. Laser beam welding as a joining technique is already established in the
aircraft industry, e. g., for lower fuselage structures, due to higher buckling strength and lower
weight, when compared to conventional riveting. The development of weldable third-generation
Al-Cu-Li alloys, such as AA2198 offers new possibilities for highly complex applications in
aircraft structures. The use of fusion welding such as laser beam welding in Al-Cu-Li alloys
presents a promising method due to the high structural efficiency.
The aim of this Doctoral Thesis is to methodically investigate and solve the weldability
problems of Al-Cu-Li 2198 alloy. Special attention is given to the laser beam welding process
parameters to reduce the structural defects as well as the geometrical imperfections of the welded
joints in order to increase the mechanical properties. The local mechanical properties of the laser
beam welded joints of Al-Cu-Li 2198 alloy were also studied; they were used as an input in a
numerical model in order to predict the evolution of the global flow stress-strain curve of the
welded joint. The final aim of this thesis is to enhance the knowledge and application capabilities
of laser beam welded AA2198 alloy, to make it more usable in the industrial field.
The first part of the thesis focuses on the weldability aspects of the alloy by investigating
different laser beam process parameters with and without the use of appropriate filler material.
The optimal process parameters lead to the reduction of structural defects such as porosity level,
hot-cracking and geometrical imperfections such as underfill and root reinforcement.
Additionally, post welding heat treatment was applied on both autogenous and non-autogenous
joints to investigate the effect of artificial ageing on the mechanical properties of welded sheets.
The second part of the thesis focus on the characterization and prediction of the local
mechanical properties of non-autogenous welded joints of AA2198 alloy with the use of Al-Si
filler wire. For this purpose, micro-flat tensile specimens were extracted from the fusion zone and heat-affected zone and the tensile test results were associated with the respective hardness
measurements. The characterization results showed that the chemical composition of the filler
wire affects the local mechanical properties in the depth of the fusion zone with a decrease in yield
strength from the radiation exposure side to the weld root side.
The experimental results were used as input to a detailed three-dimensional parametric finite
element model of the welded joint to evaluate the effect of geometric parameters like the weld
geometry and geometrical imperfections on the developed stress and strain fields in the welded
joint under axial tensile loading. The proposed model can be used as a powerful tool for the fast
prediction of the tensile mechanical behavior of welded aircraft structures.