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Defect 300
125
EUROPEAN ALUMINIUM ASSOCIATION
11.1. Causes
of deformation
In machine welded structures,
deformation can be caused by:
11.1.1. The direction
of the welds
It is a well known fact that a bead
contracts most at the end of the
weld, which is why the greatest
deformation occurs in the end
zones. So far as possible therefore
it is essential to orient the weld
towards the outside of the workpiece
so as to release as much
stress as possible. Otherwise, with
the weld facing the middle of the
component, the contraction
stresses are “trapped” and deformation
will be greater as a result.
The end of the weld must be finished
to prevent any danger of
cracking on the end crater.
11.1.2. The effect of punching
This is normally due to design
error. If we take the example of a
bulkhead inside a tank, it is
essential that the bulkhead,
which is either deep-drawn or
spun, has a down flanging resting
flat against the body of the
tank and by which the bulkhead
is welded to it. This approach
should prevent the problem of
punching due to the weld contracting
and should minimise
deformation (Figure VIII.4).
Similarly, where the body of a
tank has to be stiffened, it is
essential to place a support plate
between the stiffener and the
skin of the tank to prevent deformation
by the effect of punching
due to weld contraction (Figure
VIII.5). In the absence of a support
plate the tank would
deform under the effect of subsequent
dynamic stresses.
11.2. Solutions
There are a number of solutions
to the above problems:
11.2.1. Use of extrusions
It is worthwhile using extrusions
in the fabrication of chassis as
this can help to:
• position assemblies in less
stressed areas,
• make welds that eliminate
deformation.
11. Design and prevention of deformation
UPSTAND WELDING
FIGURE VIII.4
Upstand
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ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER VIII WELDING 126 | 127
Chassis side members for
instance are usually fabricated
from two extrusions forming the
two flanges of the member and
a sheet for the web.
The assembly shown in Figure
VIII.6 can be automatically MIG
welded with two welding torches
operating simultaneously.
Both methods of butt joining are
possible, i.e. with the side member
positioned vertically or horizontally.
The choice of position
will be dictated chiefly by the
design of the welding bench.
When the side member is horizontal
a support will be needed
to counteract angular deflection.
11.2.2. End stops
These must be positioned so as
to allow free elongation of the
assemblies during welding.
"Compressing" a weld, i.e. preventing
its elongation, greatly
exaggerates contraction and
hence subsequent deformation.
11.2.3. Predeforming
Some weld deformations can be
offset by predeforming the areas
to be welded in such a way that
the assembled components
"come right" after welding.
If the metal is only predeformed
in the elastic zone by clamping,
the results can be very erratic,
and it is therefore advisable to
predeform by bending the metal
in the plastic zone. In this way,
the results will be predictable
and repeatable.
WELDING WITH SUPPORT PLATE
FIGURE VIII.5
FABRICATING A CHASSIS MEMBER
FIGURE VIII.6
Horizontal (flat)
Vertical
(flat)
127
EEUURROOPPEEAANN AALLUUMMIINNIIUUMM AASSSSOOCCIIAATTIIOONN
128
CHAPTER IX
OTHER JOINING
TECHNIQUES
1. ADHESIVE BONDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
1.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
1.2. Advantages and disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
1.3. Types of adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
1.4. Application of adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
1.5. Creep and ageing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
2. SCREWING AND BOLT FASTENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
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