Light-weight structures produced by laser beam joining

Journal of Materials Processing Technology 115 (2001) 2±8
Light-weight structures produced by laser beam joining for future
applications in automobile and aerospace industry
E. Schubert*, M. Klassen, I. Zerner, C. Walz, G. Sepold
BIAS Bremen Institute of Applied Beam Technology, Klagenfurter St. 22, 28359 Bremen, Germany
Abstract
Light-weight components are of crucial interest for all branches that produce moving masses. The aim to reduce weight has to be
accompanied by high production ef®ciency and component performance. Laser beam joining offers the possibility to manufacture joints of
all light metals and their combinations. This paper will give examples for structures built with aluminium, titanium, magnesium and their
combinations and explain how the requirements of the industry could be ful®lled by applying laser beam joining. For aluminium the
weldability of the so-called non-weldable alloys by use of powder ®ller will be explained for aerospace applications. Another example will
deal with increased process stability during laser beam welding of aluminium. Especially for future car manufacturing applications, Al±
steel and Al±Mg joints will be presented for optimised material use. Stiffened titanium structures for aerospace applications will
demonstrate the potential of a combination of laser welding and straightening for precision manufacturing. # 2001 Elsevier Science B.V.
All rights reserved.
Keywords: Light-weight structure; Joining of material combinations; Aerospace applications; Automotive applications; Aluminium welding; Titanium
welding
1. General considerations on light-weight structures
A crucial factor for the application of new technologies
are the costs. Especially for the substitution of metals by
light-weight alloys beneath all technological properties, the
economical aspects have to be considered. Knowing the
costs for an existing structure with a certain material for
different transport systems it is possible to estimate the cost
savings S over the lifetime for the reduced fuel consumption
due to the lower weight. For a typical car this value S is
approximately 9.4 Euro/kg, for an aeroplane 120 Euro/kg
and for a rocket 8000 Euro/kg. For more details, see [1].
The costs Cconv. for typical structures in conventional
design can be derived from the literature. For cars these
costs are in the range between 14 Euro/kg for steel components and 55 Euro/kg for Al- structures [2±4]. Typical costs
for aeroplanes are between 42 Euro/kg (steel) and 155 Euro/
kg (aluminium) [5], whereas rockets reach ``astronomic''
prices between 6.000 and 100.000 Euro/kg [6].
Using the equation
Csubst : ˆ
S…1
R† ‡ Cconv:
R
*
Corresponding author. Tel.: ‡49-421-218-01; fax: ‡49-421-218-5063.
E-mail address: [email protected] (E. Schubert).
with: S the cost savings over the lifetime due to reduced fuel
consumption for light-weight structures; and: R weight ratio
between conventional and new structure with material substitution.
Depending on the preferential type of loading the structure has to withstand, typical R-values and therefore, typical
allowable costs for economic material substitutions can be
derived, see Table 1.
This clearly shows that even estimating with a very
similar equation a priori considerations of allowable costs
are possible and show good agreements with literature
values, see e.g. [2±4].
As can be seen allowable costs Csubst. for cars are in the
range 29 Euro/kg (substitute steel by aluminium) and
23 Euro/kg (substitute steel by titanium). The economic
window for automotive production can be very small, but
using modern mass production techniques material substitution may be economically reasonable. The very high values
for aircraft and rocket structures give evidence that nearly
every effort for weight reduction is economically worthwhile. The estimated costs are the total costs for the new
structure with material substitution consisting mainly of
tooling, manufacturing and material costs.
The distribution of the costs for typical structures from
steel, aluminium, titanium and magnesium is shown in Fig. 1.
Therefore, knowing the total allowable costs for a new
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 7 5 6 - 7
E. Schubert et al. / Journal of Materials Processing Technology 115 (2001) 2±8
3
Table 1
Allowable costs Csubst. for material substitution in different transport systemsa
Saving S (Euro/kg)
R for preferential loading with
Subst. St/Alwrought T6
Cconv. (Euro/kg)
Csubst. (Euro/kg)
Subst. St/Mg alloy AZ91
Cconv. (Euro/kg)
Csubst. [Euro/kg]
Subst. St/TiAl6V4
Cconv. [Euro/kg]
Csubst. [Euro/kg]
Subst. Alwrought Mg alloy T6/AZ91
Cconv. [Euro/kg]
Csubst. [Euro/kg]
a
Car (saving S ˆ 9.4 Euro/kg)
Aeroplane (120 Euro/kg)
Rocket (8000 Euro/kg)
r
Re
t
r
Re
t
r
Re
t
0.36
14
56
0.24
14
88
0.59
14
30
0.67
55
87
0.2
14
108
0.21
14
102
0.08
14
283
1.04
42
<Kalt
0.61
14
29
0.54
14
34
0.42
14
46
0.88
29
34
±
±
±
±
±
±
0.59
42
155
±
±
±
±
±
±
±
±
±
0.08
42
1905
±
±
±
±
±
±
±
±
±
0.42
42
266
±
±
±
±
±
±
±
±
±
0.59
500
6407
±
±
±
±
±
±
±
±
±
0.08
500
98250
±
±
±
±
±
±
±
±
±
0.42
500
12238
±
±
±
r: density; Re: yield strength; t: stiffness.
structure the maximum manufacturing costs can be estimated to be 25% of the total costs easily from Fig. 1. By
determining, e.g. the costs for the laser joining process it is
possible to decide whether a laser based substitution structure can be economically manufactured.
The following sections give examples for new laser joined
structures using light-weight alloys with better technical
performance.
2. Laser beam welding of aluminium alloys
Aluminium alloys are potential materials for light-weight
constructions especially in the transportation industry due to
their good mechanical properties and low density. An appropriate joining technology is the laser beam welding process,
because of its low localised energy input leading to low
distortion, high strength of the joint and high processing
speeds. This concentrated heat input is based on the deep
penetration effect which is leading to small but deep weld
seams (see Fig. 2).
Despite of the numerous advantages, LBW still suffers
from statistically occurring seam imperfections like notches
or holes in the seam which reduce the mechanical properties
of the joint. The reason for this process instabilities is a
resonant reaction in the system laser-beam, vapour cloud
and keyhole. Experiments [7] show that a part of the laser
Fig. 1. Cost distribution for typical structures in transport systems.
beam is refracted by the vapour plume. This changes the
beam intensity distribution on the workpiece having strong
in¯uence on the keyhole geometry and hence on the geometry and optical properties of the plume and therefore,
again to a changing laser intensity distribution and so on.
This system is able to be put into resonance by itself due to
the low viscosity and thus low damping effect of the molten
material around the keyhole [7]. This last aspect is the reason
for stable processes by welding steel compared to aluminium because of the higher viscosity of steel. Also, the
behaviour of different aluminium alloys depends on their
viscosity. Alloying elements like copper, titanium or iron
increase the viscosity and with it the stability of the welding
process. Elements like silicon or magnesium decrease the
viscosity resulting in more irregularities in the weld seam.
Different methods are appropriate to stabilise the process
by in¯uencing the resonances in the system, like defocusing,
special process gas mixtures, the use of two laser beams,
modulated laser power and the use of ®ller wire (see Fig. 3)
[8].
Using the required process parameters adapted to the
alloy and weld geometry, weld qualities are possible, which
ful®l the requirements of the aircraft industry. In future
Fig. 2. Deep penetration effect by laser beam welding.
4
E. Schubert et al. / Journal of Materials Processing Technology 115 (2001) 2±8
Fig. 3. Stabilising methods by laser beam welding of aluminium alloys.
aircraft bodies the normally riveted skin-stringer joint will
be laser-welded leading to lower costs and a decreased
weight (see Fig. 4). This is possible by using a weldable
AA 6013 alloy.
3. Joining of dissimilar materials
The main problem of thermal joining processes for dissimilar materials is the formation of intermetallic phases.
Typically, these phases are characterised be their extreme
hardness and therefore, brittleness and their existence inside
a joint will decrease the usability of the joint. The key to
overcome this problem is to control the diffusion process
which is the basic for the formation of intermetallic phases.
The developments of BIAS try to reach this aim by using
high power laser beams combined with high joining speeds
to achieve an overall low energy input, resulting in high
temperature gradients for heating and cooling.
An example for this is the technology for joining aluminium to steel. The sheets are overlapped in a small area and
the laser beam heats both sheets. The aluminium melts and
E. Schubert et al. / Journal of Materials Processing Technology 115 (2001) 2±8
5
Fig. 4. Future application of laser beam welded structures in the aircraft
industry.
as a braze wets the steel. During heating and cooling the
diffusion process occurs in a relevant manner, thus the
intermetallic phases grow and form a layer between aluminium and steel. The thickness of that layer determines the
ductility and tensile strength of the joint. Below a thickness
of 10 mm the mechanical properties are acceptable for
technical applications. Fig. 5 shows a cross-section of such
a joint and a 2 m long tailored blank produced by this
process can be seen in Fig. 6.
Earlier studies showed that the process window for this
technology is quite small and opens up with higher beam
power. Therefore, the aim of the actual work is to identify
possibilities to enlarge these windows.
A simple but effective solution is to optimise the contact
between the sheets and thereby improve the heat conductivity thus increasing the process stability and enlarging the
process window. This is done by a special working head
where wheels are pressing the sheets together directly before
and behind the process zone, see Fig. 7. Additionally, the
wheels are guiding the system along the seam without
allowing a displacement of the beam or the sometimes used
®ller wire. Using this head increased the maximum welding
speed up to 40%.
Fig. 5. Cross-section of an aluminium to steel joint.
Fig. 6. Tailored blank with outer parts of steel and an aluminium sheet in
the middle.
Fig. 8 shows different process windows as a function of
beam power and joining speed for sheet thicknesses of
0.9 mm steel and 1.0±1.1 mm aluminium. The smallest
process window bases on theoretical calculations and
experimental results carried out without a special working
head with an aluminium alloy with Mg- and Si-content. The
mean thickness of the intermetallic layer is calculated or
measured. All other windows are a kind of worst case
estimation. Therefore, hundreds of intermetallic layers from
experimental results are measured and a wrapping curve
combines the thickest and therefore, worst layers. This
includes a natural dispersion of the results. Compared with
the mean thickness of a layer this method leads in general to
even smaller process windows. For an optimised process
Fig. 7. Working head for parts of steel and a joining of dissimilar
materials.
6
E. Schubert et al. / Journal of Materials Processing Technology 115 (2001) 2±8
Fig. 8. Process windows for different aluminium alloys joined with and without filler wire compared with an older result based on the set-up without
optimised working head.
Fig. 9. Overview of an aluminium (bottom)±magnesium (top)- joint.
without ®ller wire using pure aluminium the window is
widened caused by the working head and secured by the
described worst case method. Using ®ller wire additionally
enlarges the window, because the Si-content minimises
growing of intermetallic phases. Using an aluminium alloy
with Mg- and Si-content additionally widens the process
window.
Another example is the joining of aluminium to magnesium where again at least two different intermetallic phases
are expected. The joining technology used is identical to the
joining of aluminium to steel except the special working
head which is not used here. The process itself is different
because of the similar melting points of aluminium and
magnesium. Therefore, a melting of both metals cannot be
prevented and the molten metals ¯ow into each other.
Fig. 9 gives an overview of a joint. Fig. 10 shows a crosssection of an aluminium±magnesium- joint. As expected,
Fig. 10. Cross-section of an aluminium±magnesium- joint with diagram of
hardness.
E. Schubert et al. / Journal of Materials Processing Technology 115 (2001) 2±8
the hardness increased in the weld but areas consisting of
intermetallic layers could not be found. Additionally, the
EDX- analysis shows a constantly increasing, respectively,
decreasing content of the metals over the weld without the
expected typical steps. Further research is necessary to
investigate the metallurgical character of the weld.
4. Manufacturing of low distorted structures of
titanium by process combination laser beam
welding and laser beam straightening
Laser beam welding is often applied to prevent high
distortions, but it is not possible to avoid them completely.
7
In many cases a following straightening process is necessary.
One technology is straightening by applying thermal deformation. The laser has been proved as a suitable heat source
for straightening distorted structures [9]. Building aeroplane
structures two boundary conditions have to be considered,
the local deformations and the macro deformations of the
skin. Earlier experiments have shown that low welding
speed causes high local deformations and low angular
distortion. In contrast to that high welding speed leads to
low local deformations and high angular distortion. Fig. 11
shows the local and angular deformation depending on the
welding speed and a principle sketch of the process sequence.
The idea was to minimise the local deformation and to
compensate the higher angular distortion by laser beam
Fig. 11. (a) Local deformation and angular distortion after welding on the outside of the skin [10]; (b) principle sketch of combined laser beam welding with
straightening.
8
E. Schubert et al. / Journal of Materials Processing Technology 115 (2001) 2±8
Fig. 12. Welded polygon (a) and (b) straightened structure.
straightening, because it is not possible to decrease the local
deformation below the required 20 mm, but to eliminate the
high angular distortion. The right sketch shows the sequence
of the two processes. The measured angular distortion is the
base for the straightening parameters.
Fig. 12a shows an example for a stiffened structure built
from 0.8 mm titanium sheet (size 1500 mm 1200 mm)
with 50 welded strings and the resulting polygon distortion.
Fig. 12b illustrates the structure after the straightening
process, with a remaining surface quality due to the local
deformation below 20 mm. These structures may be used for
future wing or fuselage components of aeroplanes.
5. Conclusions
A new generalised concept for evaluating material
substitution for light-weight constructions in transport applications has been developed. An easy equation allows apriori considerations to determine whether a material substitution under given loading conditions is economically
reasonable.
Stating from this point typical examples for new laserbased structures were presented. For aeroplane manufacturing the weldable alloy AA 6013 will substitute a conventional riveting structure. The described example gives details
about the process stabilisation that enables the practical use
of laser beam welding for this high-quality welds.
The second example shows the newest developments in
laser joining of dissimilar materials. By using an improved
working head an increased processing window allows to join
both aluminium±steel and aluminium±magnesium sheets
with higher joining speed for new automotive applications.
The last example deals with a possible new application
also for the aerospace industry. High-precision titanium
structures could be manufactured by a combination of laser
beam welding and straightening, possibly be used for future
wing or fuselage components.
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