Abstract: With the regard to the development of modern car bodies the focus lies on low production
costs, environmental sustainability and high security standards. In order to meet these requirements
the weight of modern car bodies has to be reduced consistently. Amongst other things, this becomes
possible by the use of new high and ultra high strength steels. These materials are characterised by
their high strength, good ductility and a high absorption capacity. In addition they have a lower
density in comparison to other steels. TRIP and TWIP steel belong to these high and ultra high
strength steels as well as iron-manganese steel.
The development of new materials also puts new demands on the joining technologies used for
producing semi finished products and parts of car bodies. Due to its high flexibility, its good
automation and the minor influence on the work piece, laser beam welding is an established
procedure in the automotive series production. The high cooling rates in combination with a carbon
equivalent of the new materials which is usually higher then 0.4% lead to a martensitic
solidification of the weld seam. Martensite is characterized by its low ductility and thus affects the
forming capability as well as the absorption capacity of the welded parts.
In order to avoid this effect a new process has been developed within the scope of the
collaborative research program 362 (SFB 362, 1993-2005) at the Laser Zentrum Hannover. Using
that process the weld seam structure is inductively annealed directly after the welding process.
Experiments with high strength steel like TRIP700 and H320LA have shown that the tempering
leads to an increase of ductility.
The process is suitable for butt joints and overlap joints and is to be transferred into industrial usage
within the scope of the project “Laser Beam Welding of Car Body Parts Made of High and Ultra
High Strength Steel”. Based on the results obtained in the SFB 362 continuous investigations will
be made in order to qualify the process for boron alloyed steel and iron-manganese steel.
Abstract: The availability of lasers with highest beam qualities at laser powers of 1 kW (such as
single-mode fibre laser, which nowadays come close to the theoretical limits) provides a unique tool
to investigate welding process phenomena in a wide range of potential applications from welding
with penetrations of some 50 "m to penetrations of some mm. Thus covering the field of micro
welding as well as of macro welding, scalability of welding processes as well as size effects
associated with the underlying physical phenomena may be of significance.
In this paper, the humping effect will be given a closer look, as this periodic melt pool instability
is an important limitation to possible welding speeds both in the micro and the macro range. Based
on experimental investigations with a single-mode fibre laser (YLR-1000, laser power 1 kW,
BPP < 0.4 mm*mrad), a model based on a modification of Rayleigh’s considerations on the stability
of an inviscid incompressible fluid which is freely suspended in space and maintained only by
surface tension is developed and discussed. It is shown that, within the scope of the investigations,
humping to a large extent can be explained by Rayleigh’s theory, permitting to neglect the influence
of three-dimensional melt flow.
Abstract: Laser butt-welding of AA5083 has been investigated using a high quality CO2 laser in
continuous wave regime. The effect of laser power, welding speed and specimen thickness on
mechanical properties of the welded joints were evaluated by employing a general full factorial
experimental plan design. The experimental results indicate important suggestion to choose proper
combinations of process parameters and achieve high strength butt welded joints.
Abstract: Welding is playing a growing role in transport industry due to relevant advantages it
allows. Friction Stir Welding is considered one of the most promising joining technologies,
especially when it is applied to light alloys. Focusing attention on FSW of T-joints, several
parameters have to be considered, and due to thermo-mechanical features of process, T joints need a
dedicated approach. A set of previously developed experiments has shown that the tilt angle plays a
relevant role in the joint strength. Furthermore it should be observed that T-joints are very often
utilized in aerospace industries since the produced structures are composed of joined skins and
stingers. Numerous data are reported in literature about FSW of butt joints, very few data, to
authors’ knowledge, exists on T joints. In this paper a micro structural and mechanical analysis has
been developed on FSW T-joints of AA 6082 T6 rolled plates, realized setting welding direction
both parallel and perpendicular to rolling direction. The obtained results can be considered as a
further acquired knowledge in the comprehension and the design of FSW processes.
Abstract: Friction stir welding is a newer technology to join materials in the solid phase. Therefore
plenty of problems which appear by melting phase welding technologies for aluminium alloys are
avoided by this process. This is the main chance for friction stir welding to be accepted and
integrated in forthcoming applications, especially for uses out of aluminium alloys. Starting from
former results for friction stir welding of aluminium this article deals with the determination of the
constitutive material properties of friction stir welded aluminium tailored blanks with regard to the
finite-element (FE) simulation of sheet metal forming. The FE simulation of the formability of
welded sheet metal demands the knowledge of the precise mechanical properties of the base
material as well as the characteristic zones of the weld seam, which are affected by friction stir
welding. While ordinary tensile tests can only determine the constitutive behaviour of a simple
tensile specimen, an optical strain measurement can be used to determine flow stresses of the base
and of the welded material with an adapted tensile specimen, respectively. By the usage of the so
called rule of mixture the advantages of this new approach is demonstrated with a comparison of the
tensile forces within FE simulations and experimental validations. For this purpose specimen with
the weld seam oriented perpendicular and parallel to the uniaxial loading direction are utilized.
Abstract: In the paper, a variation of the Friction Stir Spot Welding (FSSW) process has been
considered. In particular, a particular tool path is given after the sinking phase nearby the initial
penetration site. The process mechanics was highlighted and the joint strength was considered at the
varying of the most relevant process parameters. Furthermore macro and micro analyses were
developed in order to highlight the process mechanics and the local material microstructure
evolution. The investigated technology appears a promising joining technique in order to develop
effective spot joints.
Abstract: With the rapidly growing demand for the micro-thin-wall parts, the development of high
accurate forming processes for very thin sheet or foil becomes more and more important. The aim
of this study is to explore the effects of specimen width and grain size on tensile strength of
aluminum alloy 3003 foil in uniaxial tension test. The problem was approached in two ways: firstly,
by reduction of the specimen width, and secondly, by changing the grain size through annealing
crystallization. The uniaxial tension tests were performed on an electronic universal material testing
machine, in which a linear CCD based visual extensometer was used to measure deformation. The
results show that the tensile strength decreases with decreasing specimen width or increasing
surface-to-volume ratio. The tensile strength decreases with decreasing grain size for grain sizes
larger than the specimen thickness.
Abstract: With the increasing trend towards miniaturization and the enhanced demand for small
components, reliable processes for mass production are needed. Today the deep drawing process is
already used to produce large numbers of small parts (diameter < 1 mm) at low costs per part. But a
better understanding of the process in relation to miniaturization is required to improve process
stability, because several aspects of the process change when scaled down. For example, product
accuracy and process parameters can be influenced by changing the ratio of surface to volume or the
ratio of grain size to foil thickness. For the analysis of these effects experiments with geometrically
scaled deep drawing tool sets from 8 mm to 1 mm punch diameter have been carried out, using
CuZn37 foils in different annealed conditions and a foil thickness ranging from 0.3 mm to 0.04 mm.
Additionally, the deep drawing process is simulated via FE-methods to consider influences that
cannot be measured using the available experimental setup, such as temperature conditions resulting
from the heat generated due to plastic dissipation and friction.
Abstract: Mechanical micro deep drawing becomes a more and more industrial relevant process.
But due to size effects new challenges are involved in this process compared to macro deep
drawing. The size effects cause an increase of friction and thus hinder the material flow. The change
of friction in mechanical micro deep drawing is subject of the presented investigations in this paper.
Additionally to this, a new non-mechanical micro deep drawing process is presented, whereby a
laser beam acts as a punch. This new laser deep drawing process is based on a totally different
mechanism compared to thermal laser forming, e.g. forming by laser induced thermal stresses: The
laser produces a pulse with an extremely high power density, which causes plasma generation at the
target and thus a shock wave. The shock wave can be used as in explosive forming, but is smaller
and easier to generate. Recent investigations showed that using this technology laser deep drawing
is possible with a sheet metal out of Al 99.5 and a thickness of 50 'm. The deep drawing process
was carried out with a die diameter of 4 mm and shows promising results.