Residual stress and distortion
The solidification and contraction of the weld bead
will induce strain and consequently stresses in the
joint. In the case of the contraction of the solid
metal the stress will be equal to the change in temperature
as the metal cools from its melting point
to its ambient temperature. Under normal circumstances
movement of the weld bead is restricted by
the adjacent body panels in the vehicle structure,
and the stress which could be generated is given by
the Young’s modulus of the material. This stress
level often exceeds the elastic limit of the material
Table 12.6MIG/MAG recommended operating conditions
Metal Wire feed Welding Gas
thickness Shielding Wire dia. Current Voltage speed speed flow
Application (mm) gas (mm) (A) (V) (m/min) (mm/min) (l/min)
Horizontal/vertical fillet 1.0 Argoshield 5 0.8 66 14 3.4 900 12
1.6 Argoshield 5 0.8 130 18 9.7 600 14
Argoshield 5 1.0 160 17 5.9 563 14
3.0 Argoshield TC 1.0 180 20 6.4 391 15
Argoshield TC 1.2 180 17 3.8 430 15
6.0 Argoshield TC 1.2 260 27 7.0 480 16
10.0 Argoshield TC 1.2 290 27 7.4 430 16
Butt weld, flat position 1.0 Argoshield 5 0.8 66 14 3.0 750 12
1.6 Argoshield 5 0.8 84 18 4.0 800 14
Argoshield 5 1.0 125 16 4.0 640 14
3.0 Argoshield TC 1.0 140 19 4.0 350 15
6.0 Argoshield TC 1.2 300 28 7.6 650 16
10.0 Argoshield TC 1.2 300 28 7.6 680 16
320 28 7.6 580 16
Butt weld, overhead 1.0 Argoshield 5 0.8 50 15 4.0 360 12
1.6 Argoshield 5 1.0 150 17 5.9 600 14
3.0 Argoshield TC 1.2 180 19 4.3 520 15
Butt weld, vertical up 1.0 Argoshield 5 0.8 45 15 3.5 350 12
1.6 Argoshield 5 1.0 130 17 4.7 410 14
3.0 Argoshield TC 1.2 135 17 3.1 275 14
336Repair of Vehicle Bodies
and plastic deformation takes place. The stresses
locked in the material, which may reach levels up to
the elastic limit of the material, are called residual
stresses, and the deformation of the material is
known as distortion.
Residual stresses
In a butt weld the weld bead will tend to contract
longitudinally and transversely, and this will induce
tensile stresses in the weld and also balancing compressive
stresses in the sheet material (Figure 12.34).
In the joining processes which rely on heating or
fusion, it is difficult to prevent the formation of residual
stresses. If the heat affected zone is ductile and
defect free (as in thin-sheet panel steel) the presence
of some residual stresses may be acceptable. The
most common technique used to relieve residual
stress in thicker materials is post-weld heat treatment.
This consists of uniform heating of the joint to
a temperature at which the yield stress of the material
is lowered and the residual stresses are relieved
by plastic deformation.
(Figure 12.35). Distortion may result in unacceptable
appearance (buckled body panels), prevention
of subassembly fabrication or the inability of the
structure to perform its intended function (alignment
of body panels after welding).
Figure 12.34Residual stress in a simple butt weld
(BOC Ltd)
Figure 12.35Types of distortion: (a) longitudinal
and transverse shrinkage (b) angular distortion (c)
out-of-plane buckling distortion (BOC Ltd)
Distortion
Distortion may take the form of a change in dimensions
of the joint, transverse or longitudinal shrinkage,
angular movement, or out-of-plane buckling
Gas shielded arc welding 337
The following steps may be taken to minimize
distortion:
1 Use the minimum amount of weld metal. Overwelding
and excessive reinforcement should be
avoided in fillet welds and flat butt welds.
Intermittent or stitch welding may be used.
2 Use square edge on narrow gap procedures to
reduce angular distortion when welding.
3 Use high travel speed and low heat input to
limit heat build-up in the panels to be welded.
4 Use the backstep weld sequence or preset the
joint.
Incorrect weld geometry and
Appearance, defects, loss of
Properties
Overfill
Overfill or excessive reinforcement may be described
as the presence of weld metal which exceeds that
required for the joint (see Figure 12.36). This creates
an unacceptable appearance or surface finish and
would require weld dressing.
Root concavity or suck-back
This may occur if the rear of the weld pool is too hot
or large, and the combined effect of contraction and
surface tension results in a root surface which is
concave (Figure 12.38). The surface profile will
often be smooth and less likely to produce stress than
over-penetration or lack of penetration; however, it
may result in unacceptable loss of cross-section. The
cause of this defect is incorrect parameter selection,
in particular the use of high current or slow travel
speed.
Figure 12.36Excessive reinforcement in (a) butt weld
and (b) fillet weld (BOC Ltd )
Figure 12.37Root convexity: excessive penetration
(BOC Ltd )
Figure 12.38Root concavity: suck-back (BOC Ltd )
(a)
(b)
(c)
Figure 12.39(a) Underfill and (b), (c) undercut
(BOC Ltd )
Root convexity
This results from excessive penetration on the underside
of a full penetration weld. The cause of this
defect is incorrect welding parameter selection
(shielding gas or operating technique) (Figure 12.37).
Underfill and undercut
If insufficient weld metal is deposited in the joint,
the parent material may remain unfused and the joint
may be underfilled (Figure 12.39a). Alternatively,
the weld area may be melted but the combined effect
of the arc force and the flow of the molten metal
may prevent complete joint filling or produce a
depression in the surface at the weld boundaries: this
is known as undercut (Figure 12.39b). Underfill is
usually avoided by careful placement and operating
technique. Undercut normally occurs at high speeds
and high current, and may be avoided by reducing
both current and speed.
338Repair of Vehicle Bodies
Lack of penetration
This is classed as partial penetration and is not acceptable,
particularly with respect to fatigue, loading and
corrosion resistance on the joint, where full penetration
is required. The defect results from poor joint
preparation and unsuitable welding operating parameters,
especially current settings (Figure 12.40).
The source of the gas may be a chemical reaction
such as the formation of carbon monoxide from
carbon and oxygen, or the expulsion of gases
which have been dissolved by the liquid metal but
are relatively insoluble in the solid phase. The
problem is best controlled by eliminating the
source of the pore generating gas. This may be
achieved by effective shielding and the prevention
of leaks, the ingress of air or moisture to gas lines,
and joint surface contamination from oil, grease,
moisture or paint.
Inclusions
Non-metallic inclusions may be trapped in the
joint owing to inadequate cleaning. Inclusions tend
to be more angular than gas pores and can result in
higher stress levels as well as forming a plane of
weakness in the weld. The cause of this defect is
usually insufficient inter-run cleaning, incorrect
welding parameters or incorrect gas shielding.
Solidification cracks (hot cracking)
Solidification cracking occurs when the solidifying
weld pool is exposed to transverse shrinkage
(Figure 12.42). The last part of the weld pool to
solidify is usually rich in low-melting-point impurity
elements, and liquid films are often present at
the centre line of the weld; as a result the ductility
of the weld metal is lowered. The transverse contraction
strain can be significant and, if the lowductility
area along the centre line of the weld is
unable to accommodate this strain, cracking takes
place. The problem is usually avoided by careful
selection of the weld metal composition.
Figure 12.40Lack of penetration (BOC Ltd)
(a)
(b)
Figure 12.41Lack of fusion (a) at sidewall and
(b) inter-run (BOC Ltd)
Figure 12.42Solidification cracks (BOC Ltd)
Lack of fusion
Lack of fusion between successive runs or between
the weld bead and the parent metal can produce serious
crack-like defects. These defects reduce the loadcarrying
area of the joint and produce stresses, and
are therefore unacceptable in welds which are subjected
to fatigue loading or static loading. The cause of
the defect is usually incorrect operating parameters,
high deposition rate, or low heat input (Figure 12.41).
Spatter
Spatter takes the form of small particles of metal
which are rejected from the arc or the weld pool
and adhere to the surface of the metal being joined.
Spatter may be unacceptable for purely cosmetic
reasons: it may also inhibit the application of surface
coatings (as in vehicle painting) and may initiate
corrosion. Spatter usually results from either
excessive voltage or insufficient induction in dip
transfer or argon enriched gas mixtures.
Porosity
Porosity results when bubbles of gas are nucleated
in the weld pool and trapped during solidification.
Gas shielded arc welding 339
12.15 Weld testing and inspection
Weld testing and inspection can be divided into
two types. In non-destructive testing, the test samples
are not destroyed in the process. In destructive
testing, the test samples are destroyed in the
process.
Visible examination
This is normally carried out during and after welding
and prior to any other non-destructive or
destructive test being used. This visual check will
usually determine the following:
Weld size
Profile or weld face shape
Surface defects in weld face
Undercut and overlap
Root defects
Weld penetration
Surface slag inclusions.
Non-destructive testing
Non-destructive testing of weld samples is normally
carried out using the following methods.
Macro examination
This method uses a low-power magnification to
examine weld specimens which have been levelled,
polished and etched to detect the following: lack of
fusion, lack of penetration, porosity, oxide inclusions,
internal cracks.
Crack detection
By using dye penetrant, surface defects in both ferrous
and non-ferrous metals may be detected. A
solution of coloured dye is sprayed on to the weld
and parent metal and allowed to soak. The dye is
then washed off and the surface dried. A liquid
developer is then sprayed on to the weld to give a
uniform dry powder coating which is white in
colour. The coloured dye oozes out of any crack in
the weld into the white coating and can be seen
in normal lighting conditions.
Magnetic particle method
Surface defects in mild steel and low-alloy steels
may be revealed by this method. The test specimen
is connected to a special power source. The magnetic
test liquid is sprayed along the weld. The
magnetic particles then collect along a line of
crack when the current flows through the weld.
Better test results are obtained if the surface of the
weld has first been ground smooth.
As these penetrants and magnetic test liquids
give off harmful vapours, the work must be carried
out under well ventilated conditions.
Ultrasonic inspection
This method uses sound waves which are passed
through the weld. They are transmitted as pulses
by a probe connected to an ultrasonic test set.
Defects reflect these pulses back to the probe
through a flexible cable which is attached to the
ultrasonic test set, where it is displayed on an
oscilloscope screen as deflections of a trace. These
deflections measure the location and size of the
defect. Grease is used between the probe and the
work to improve sound transmission. Also light
grinding of the weld or parent metal surface to
remove spatter may be required before testing.
Radiography
This method uses penetrating X-rays or gamma-rays
which are passed through the weld and recorded on
photographic film held in a light-proof container.
When the film has been processed, any defect in the
weld will show up as shadows in the photographic
image. Only trained and radiation classified personnel
may operate radiography equipment, as the
equipment is extremely dangerous.
Destructive testing
Mechanical testing is a destructive procedure and
therefore cannot be carried out on any component
required for use. Representative test samples produced
under similar conditions to the in-service
components, for example welding procedure tests,
are normally used, and accurate comparisons made.
The tests most frequently used to assess the
properties of welded joints are as follows.
Tensile test
This test can be used to assess the yield point, ultimate
tensile strength and elongation percentage of
the weld specimen. Failure usually occurs in the
parent material; therefore exact measurements are
not usually obtained for the weld itself, although in
this case the ultimate tensile strength (UTS) of the
340Repair of Vehicle Bodies
weld is higher than that of the parent metal. Test
specimens are cut from the designated area of the
weld assembly, the edges are smoothed and the
corners are radiused. If an elongation value is
required, then two centre punch marks often
50 mm apart are applied one on either side of the
weld; this is called a gauge length. The test equipment
can vary, but the basic principles are that two
sets of vice jaws are used to clamp the specimen,
hydraulic power is applied to force the jaws apart,
and a dial calibrated in tonnes or newtons records
the load. As the load increases, the dial registers
the amount of applied load until fracture occurs.
Bend test
Bend tests are carried out on butt welds and are
used to determine the soundness of the weld zone.
The test piece is the full thickness of the material,
with the weld bead reinforcement dressed flush on
both sides. It is common practice to take two specimens
from a test piece: one can be bent against the
face and the other bent against the root. A bend test
machine consists of a roller or former connected to
a ram and operated hydraulically. Unless otherwise
stated, the diameter of the former should be four
times the weld specimen thickness. The specimen
is placed on support rollers and the centre point of
the former is brought into contact with the weld
face or root. Pressure is then applied to bend the
weld specimen through either 90 or 180 degrees.
Nick-break test
In this test procedure the butt joint weld must first
be ground flush. Then two nicks (saw cuts) are
made, one on either end of the joint in the weld
specimen. The weld specimen may be broken during
bending, or alternatively the specimen may be
placed in the jaws of the tensile test machine and
struck with a hammer when tensioned.
Impact test
There are two main types of impact test, the
Charpy V notch and the Izod test. Both tests
employ a swinging pendulum which breaks accurately
prepared specimens to which a notch has
been applied. Both the tests have the same principle,
which is to determine the energy measured in
joules absorbed by the notched test piece at a specified
temperature as it is broken from a single
swinging pendulum. During fracture, energy from
the pendulum will be absorbed by the specimen,
more energy being absorbed by strong materials
than by brittle materials. The distance the pendulum
swings after fracture is measured by a pointer
on a dial calibrated in joules. The lower the value
indicated, the more brittle the specimen; conversely,
the higher the reading, the greater the
toughness.