J O U R N A L O F M A T E R I A L S S C I E N C E 12 ( 1 9 7 7 ) 1 5 3 5 - 1 5 5 1

The use of tensile tests to determine the

optimum conditions for butt welding
polyethylene pipes of different melt
flow index

D. R. deCOURCY, J. R. A T K I N S O N
Department of Metallurgy, University of Leeds, Leeds, Yorkshire, UK

The butt fusion welding of four different grades of polyethylene pipe has been studied
and the feasibility of producing good welds between pipes of different melt flow index
has been examined. Weld quality has been assessedon the basis of tensile tests and micro-
structural studies. The factors influencing the microstructure are discussed; in particular,
the flow of molten polymer within the welds.

1. Introduction by the notch between the welding bead and the

In recent years the use of polyethylene pipes has gauge length. (In practice the welding bead will,
become increasingly widespread in gas distribution of course, always remain.) The quality of a weld
systems. Polyethylene has several advantages over is assessed using two parameters obtained from the
the traditional materials, for example corrosion tensile test results: the percentage elongation to
resistance, lightness and lower cost. A number of failure, which should be similar to the value
different grades of polyethylene have been used as obtained for the unwelded material, and a welding
pipe materials and even if standardization is factorfdefined by Menges and Z6hren [5] as
achieved and only one type of pipe laid throughout
yield strength of welded material
the U.K., it is inevitable that joints between f = yield strength of unwelded material
dissimilar polyethylene pipes will occasionally be
required to repair or extend existing systems. Thus should be equal to or greater than unity. For a
it is essential to know whether these joints can be weld to be deemed satisfactory, tensile specimens
made successfully using existing techniques. cut longitudinally down the pipe and across the
There are two approaches to testing welds in weld should show a large percentage elongation to
polyethylene gas pipes. One can attempt to pre- failure and a welding factor close to unity.
dict the long-term behaviour of the jointed pipe
under service conditions on the basis of stress 2. Experimental
rupture tests [1-4], or the strength of the joint 2.1. Materials
in the short term can be measured and compared The following types of polyethylene pipe have
with that of the unwelded material [5, 6]. In this been studied: (i) Muntz HDPE of outer diameter
paper the second approach has been adopted and 60 mm and wall thickness 5.8 mm (supplied by
tensile tests carried out to observe the effects of Yorkshire Imperial Plastics, Leeds, U.K.), (ii) Muntz
variations in the welding conditions on weld LDPE of outer diameter 60.2 mm and wall thick-
strength. It has been shown [6] that the sensitivity ness 8.3mm (supplied by Yorkshire Imperial
of this technique is greatly increased if the welding Plastics), Off) Rigidex PE of outer diameter 63 mm
bead is removed prior to testing. If the welding and wall thickness 6.0 mm (supplied by Yorkshire
bead is not removed, fracture is always initiated Imperial Plastics), (iv) Aldyl 'A' PE pipes of outer

9 1977 Chapman and Hall Ltd. Printed in Great Britain. 153 5

T A B L E I Properties of materials used
Property Muntz HDPE Rigidex PE Aldyl 'A' PE Muntz LDPE
Density (kg m-3 ) 0.96 0.945 0.94 0.93
Yield stress (MPa) 25.5 23 21 14.5
Elongation (%) 720 640 970 590
Melt flow index* 0.05 0.2 1.5 0.7
*Values stated by suppliers.
diameters 60.2 and 63 mm, and wall thickness pushed together until a welding bead of the correct
6.0mm (supplied by the Dupont Co UK Ltd., size formed. The pressure was maintained during
Derbyshire, U.K.). Some properties of the materials cooling to prevent the formation of shrinkage
are given in Table I. cavities.

2.2. Techniques 2.2.2. Tensile testing

2.2. 1. Welding Tensile test specimens were cut from the welded
A Bielomatik welding machine (Model HV493) pipes using a high speed, air driven router. The
supplied by Bielomatik Lenze Co., West Germany, router was also used to remove the welding beads
was used. The surface temperature of the heating from the specimens. After machining, the gauge
plate was determined accurately using a thermo- length was polished with fine emery paper. Tensile
couple. The welding pressure was calculated from tests were carried out on an Instron testing machine
the welding force, which was shown on the machine, at room temperature. The crosshead speed was
and the area of the pipe end. Prior to welding, the 0.83 mm sec-1 and for a 25.4 mm gauge length this
pipe ends were turned until they were smooth and corresponds to a strain rate of 3.28 x 10 -~ sec -1 .
square and then degreased using an acetone swab.
Butt welds were made using the procedure out- 2.2.3. Etching
lined previously [6]. The pipe ends were heated Specimens to be etched were cut from across the
by contact with the heating plate under slight weld and a smooth surface produced using an
pressure until a small uniform bead had formed M.S.E. Base Sledge Microtome. The etchant [7]
and then for a further period with zero pressure. was a saturated solution of chromic oxide (CrO3)
The heating plate was removed and the pipe ends in water at 340K. The solution with the sub-
25

20

10

i i j | j i i i i
1010 200 300 4(~0 500 600 700 800 900 I000 1100

Strain, "/.
Figure I Typical stress-strain curve for Aldyl 'A'.

1536
merged specimens was contained in "Quickfit" welding temperatures and pressures but always
test tubes maintained at 340 K in an oil bath. An using the same initial heating time of 60 sec. Six
etching time of 10 to 12 h was used. tensile specimens were cut longitudinally across
each weld and from four of these the welding
2.2.4. Transmission optical microscopy beads were removed. The assessment of weld
Sections of thickness 15 ~m were cut from across quality was made using the results from these
the weld using the sledge microtome. The sections four specimens as it has been shown previously
were transferred to microscope slides and cover [6, 7], and confirmed here, that the removal of
slips positioned on top. A few drops of xylene the bead gives greater sensitivity to the effects of
were introduced between the slides and the cover variations in the welding parameters. The weld
slips. The specimens were then viewed using a quality was assessed in terms of the welding
Vickers M55 optical microscope with crossed factor and the percentage elongation at fracture.
polars. In addition the fracture surfaces were examined
in the scanning electron microscope and, where
3. Results possible, studies made of the microstructures
3.1. Results of tensile tests on welds produced in the welds.
between pipes of the same material
All four types of polyethylene were studied in the 3.1.1. A Idyl ",4" welds
same way. Tensile tests were first carried out on Figs. 2 and 3 show the average values of welding
unwelded specimens and typical values of yield factor and percentage elongation plotted against
stress and percentage elongation at fracture ob- welding temperature and welding pressure. It can
tained for each material. A typical stress-strain be seen that a high welding factor and percentage
curve is illustrated in Fig. 1. A series of welds was elongation can be achieved over a fairly wide
then made for each material over a range of temperature range, 453 to 518K, but that the

1000 1.0

800 !. J "
0.99

600 0.98

c-
O
klJ
o~ 400 0.97

2O 0 0.96
f 0

% Elongation
O--
0
423 443 4(53 483 / 503 523

Temperature (K)

Figure 2 Graph of welding factor and percentage elongation versus welding temperature for Aldyl 'A' welds; welding
pressure constant at 0.1 MPa.

!537
 1000 t.0

800 ~.99

600 0.98

f
O~
C
o
MJ
400 0.97
o~

200 0.96

f
~o Elongation (~

= i
O.l 0.2 0.3 0.4 0.5 0.6

Pressure ( M P a )

Figure 3 Graph of welding factor and percentage elongation versus welding pressure for Aldyl 'A' welds; welding tem-
perature constant at 463 K.

welding factor reaches a maximum at about 463 K. which may be ascribed to lack of adhesion. The
Below 433 K, the elongation falls to a very low amount of fibrillation increases as the welding
value although the welding factor decreases only conditions deviate from the optimum. The lack
little. This emphasizes the importance of consider- of adhesion in poor welds is also indicated by
ing both welding factor and percentage elongation etching in chromic acid. At welding temperatures
in determining the weld quality. Up to a welding below 463 K the weld interface is attacked prefer-
temperature of 518 K no adverse effects are noticed entiaUy and this has been stated [6] to be due to
except that the bead is somewhat larger than incomplete adhesion.
desirable. Presumably, if still higher welding tem-
peratures had been used, thermal degradation
would have caused a drop in both welding factor
and percentage elongation. Welding pressures in
the range 0.05 to 0.6MPa are acceptable, the
optimum pressure being 0.1 MPa.
Most specimens gave elongations in excess of
800%. A neck formed and ran through the weld
until fracture occurred at the shoulder at one end
of the gauge length. When the welding bead was
left on, however, fracture was initiated by the
notch between the bead and the gauge length and
the nature of the fracture surface gave an indication Figure4 For specimens welded under near optimum
of the quality of the weld. Fig. 4 shows the conditions no fibrillation was observed when specimens
fracture surface of a good weld, Fig. 5 shows the were tested with the welding bead left on (scanning
fibrillation which characterizes a poor weld and electron micrograph). Bar = 1 mm.
1538
 were examined and in all cases the welding factors
and elongations were very low. Then the effect of
varying the welding pressure at a constant welding
temperature of 473 K was studied. Fig. 7 shows
that a pressure in the region of 0.25 MPa is near
the optimum. Consequently, the variation in weld
quality with temperature was examined at a con-
stant welding pressure of 0.25 MPa. These results
are presented in Fig. 8. It can be seen that the
optimum welding conditions are a welding tem-
perature of 473 K with a welding pressure of
0.25MPa and that even small deviations from
Figure 5 As welding conditions deviate from the optimum these values produce large decreases in weld
the amount of fibrillation visible on fracture surfaces quality.
increases (scanning electron micrograph). Bar = 1 mm.
The stress-strain curves were similar to those
for Aldyl 'A', but most Muntz LDPE specimens
The specimens which necked through the weld failed at the weld after drawing various amounts.
gave stress-strain curves of the type shown in The fracture surfaces always showed some fibril-
Fig. 6. A small drop in the drawing load was lation even when the weld was produced under
observed as the neck passed through the weld. In optimum conditions. When the welding pressures
this region the true stress was around 105 MPa were low, contraction cavities could be seen in the
compared with 94 MPa for the rest of the gauge fracture surfaces and the welds were etched prefer-
length. entially in chromic acid.
It is apparent that satisfactory welds cannot be
3.1.2. Muntz LDPE welds produced in this material. The highest average
Initially, the effect on the quality of the welds welding factor obtained was 0.955 and only welds
of varying the welding temperature whilst keeping with welding factors between 0.99 and 1.0 are
the welding pressure constant at 0.1 MPa was considered satisfactory. In addition there were
examined. Temperatures in the range 463 to 518 K considerable variations in welding factor and

25

20

15

I I i i I I = L i i I
9 100 200 300 400 500 600 700 800 900 ~000 1100

Strain, ",'.

Figure 6 Typical stress-strain curve for a welded Aldyl 'A' specimen that has failed away from the weld.

1539
 1 O00 1.0

800 0.98

600 0.90

.2
g f
o
tkl 4 0 G 0.84

D.92
200

*j[] f ----* ~
[] Z Elongation ~ .

O.' 1 o'.2 0 .' 3 0 '. 4 '

0.5 0 I8

Pressure (MP=)

Figure 7 Graph of welding factor and percentage elongation versus welding pressure for Muntz LDPE welds; welding
temperature constant at 473 K.

tO00 1.0
f 9
Z Elongation

0.98
80(]

0.96

c 60C

0.94

0 f
~9 400
0.92

200
0,90

[] 9

i I I I * i
423 443 463 483 503 523

Temperature ( K )

Figure 8 Graph of welding factor and percentage elongation versus welding temperature for Muntz LDPE welds;welding
pressure constant at 0.25 MPa.

1540
 I000 I.O

800 ),99

/~ 0
600 ).98

_q
i11
40O ).97

200 0.96

O
-_2-
458 418 498 518 538 558

Temperature ( K )

Figure 9 Graph of welding factor and percentage elongation versus welding temperature for Muntz HDPE; welding
pressure constant at 0.1 MPa.

I0o0 1.0

0.99

600 0.98

b,%
r f
C
0
LU
400 10.97

2O0 0.96

i =

0,1 0.2 0.3 0.4 0.5 0.6

Pressure ( M P a )
Figure 10 Graph of welding factor and percentage elongation versus welding pressure for Muntz HDPE; welding tem-
perature constant at 518 K.

1541
elongation between the four specimens from a molten polymer will be much higher than, for
good weld, indicating a variation in quality around example, Aldyl 'A' PE and hence higher welding
the circumference. In view of these observations, pressures will be required to obtain the complete
no welds were made between this material and contact necessary for good adhesion.
any others. When the welding bead was machined off, the
following types of fracture were observed. At low
3. 1.3. Muntz HDPE welds temperatures or pressures the fracture surfaces
Welds were again made over a range of tempera- were fiat and smooth and their appearance
tures and pressures and the results are presented in suggested very limited adhesion. At low welding
Figs. 9 and 10. pressures contraction cavities could be seen at
If the welding pressure is kept constant at 0.1 the interface. For welds made under satisfactory
MPa, the quality of the welds improves with conditions, fracture occurred at the end of the
increasing welding temperature until a tempera- gauge length at elongations greater than 600%.
ture of 518 K is reached. If the welding tempera- The ductile fracture was typical of the unwelded
ture is higher than 528 K, the quality is reduced. material. The stress-strain curves from specimens
The effect of variations in welding pressure using a with good welds showed the usual drop in load as
welding temperature of 518 K is shown in Fig. 10. the neck passed through the weld. The drop was
The optimum welding pressure is 0.1 MPa and larger than that found for Aldyl 'A' and was
there is a sharp drop in weld quality at lower again accompanied by a slight decrease in cross-
welding pressures when the welds become brittle. sectional area. But unlike Aldyl 'A', the true stress
An increase in welding pressure up to 0.6 MPa in this region fell slightly, from 89 to 86MPa. We
has little effect. believe that this is due to frozen-in stresses present
The optimum welding conditions are, therefore, in Muntz HDPE welds. These stresses are relatively
a welding temperature of 518K and a welding large due to the low melt flow index. This will be
pressure of 0.1 MPa. The fall in quality at low discussed further in another publication [8].
pressures is probably due to the very low melt Specimens tested with the welding bead left on
flow index of Muntz HDPE. The viscosity of the failed in the usual way, the fracture being initiated
9 ..... I 1,0
t000,

800 ).99

600 0.98

al

bLJ
400 0.97

0 a
200 0.96
-- a

% Elongation -0

443 463 483 503 523 543

Temperature { K )

Figure 11 Graph of welding factor and percentage elongation versus welding temperature for Rigidex welds; welding
pressure constant at 0.1 MPa.

1542
 j o
10o0 / e ~
J
/
I
9 ~J

3,99

0
600 3.98

.~_

3.97

200 0,96

f _ 9 -

% Elongation ~ 0 "

J , = .... =,,
o 0,1 0.2 0.3 04 0.5 06

Pressure (MPa)

Figure I2 Graph o f welding factor and percentage elongation versus welding pressure for Rigidex welds; welding tem-
perature constant at 493 K.

by the notch between the bead and the gauge less than that for the Muntz material, probably
length. Once again, as the welding conditions because the residual frozen-in stresses are lower in
deviated from the optimum the extent of fibril- Rigidex welds as this material has a higher melt
lation increased. flow index. The fact that less preferential etching
is observed in Rigidex welds supports this view.
3.1.4. Rigidex PE welds The fracture behaviour was similar to that seen in
Welds were made over a range of temperatures and Muntz I-tDPE. If the welding beads were removed,
pressures as for the other materials. First, the vari- fracture occurred away from the welds. If the
ation of weld quality with welding temperature wdding beads remained, fracture occurred at the
was observed using a constant welding pressure of welds and the amount of fibrillation increased as
0.1 MPa. The results are presented graphically in the welding conditions deviated from the opti-
Fig. 11 and show that acceptable welds may be mum.
produced in the temperature range 493 to 523 K,
with the optimum welding temperature being 3.1.5. General comments
about 503 K. Variations in welding pressure were If the results for Aldyl 'A' PE, Muntz HDPE and
then studied at a constant welding temperature of Rigidex PE are compared, the influence of the
493 K. Fig. 12 shows that good results are achieved melt flow index on the welding characteristics
over the pressure range 0.05 to 0.6MPa but that can be seen. As the value of the index decreases,
the optimum welding pressure is probably in the the lower limit of the welding temperature and the
range 0.1 to 0.3 MPa. optimum welding temperature are raised. Hence
The stress-strain curves for specimens which the available temperature range is narrower for
necked through the welds showed the usual trough materials of low MFI (melt flow index) such as
in the drawing load at the weld. In this region the Muntz HDPE. The welding pressure range is also
true stress was calculated to be 80 MPa compared reduced. Muntz HDPE welds deteriorate appreci-
with 83 MPa elsewhere. This difference is much ably at low welding pressures and, to a lesser
1543
extent, if high pressures are used. Materials of Fig. 13 shows that, if the welding pressure
higher MFI require less precise welding conditions remains constant at 0.1 MPa, successful welds
and thus good welds are easier to produce. If the can be made in the temperature range 488 to
MFI is too high, however, this could have a del- 518K, but at temperatures below 488K the
eterious effect on the long term stability and percentage elongation decreases fairly rapidly.
strength of the pipe. Fig. 14 shows the effect of increasing the welding
The results for Muntz LDPE indicate how pressure at a welding temperature of 518K. The
difficult it is to produce even moderate welds with percentage elongation reaches a maximum at a
this material. The welds could not be classified as pressure of 0.1 MPa but only falls slightly even
satisfactory. Butt fusion welding cannot therefore when the pressure reaches 0.6 MPa. However, if
be considered as a possible joining technique. the pressure falls below 0.1 MPa there is a sharp
drop in the percentage elongation. The same
3.2. Results o f t e n s i l e tests o n w e l d s effect was observed for Muntz/Muntz welds but
between dissimilar materials not for welds in Aldyl 'A'.
3.2.1. Welds between Muntz HDPE and A further series of welds was made in which
Aldyl ",4" PE the heating time for one of the pipe materials
A series of welds was made over a range of tem- was varied from the normal time of 60 sec. The
peratures and pressures. The temperature range of
473 to 518K was chosen because the optimum looo
welding temperature for Aldyl 'A' was 473 K and
J
/
~ o

that for the Muntz material 518K. The pressure 8(?0

range chosen was 0.05MPa to 0.6MPa, even
though the optimum welding pressure for both g
~600
materials had previously been determined as O.l =
MPa, as it was felt important to observe the effect ~O 400
of variations in the welding pressure. The heating
times in contact with the hot plate were also
varied in an attempt to reduce the effects of 20o
heating one pipe above and the other below their
optimum welding temperatures. Thus the Aldyl 473 483 493 503 513 523
'A' PE heating .time was reduced from the normal Temperature (K)
value of 60 sec, whilst the heating time for Muntz
Figure 13 Graph of percentage elongation versus welding
HDPE was increased to more than 60 sec. temperature for welds between Muntz HDPE and Aldyl
Six tensile specimens were cut longitudinally 'A'; welding pressure constant at 0.1 MPa.
across each weld and the welding beads removed
from four of these. Tensile tests were then carried
out in the usual way. For those specimens from looo
which the beads had been removed, yielding
always occurr@ in the Aldyl 'A' half of the
specimen. This was expected as the yield stress
for Atdyl 'A' PE is 21 MPa compared with 25.5
MPa for the Muntz material. It means that the
8oo

}6oo
C7

(3
welding factor can no longer be used as a criterion -~4oo
/ 1 ~ 0 .

of weld quality between materials of different

yield stress. The welding factor is found to be 2oc
constant and equal to 1.0 if based on the yield
stress of Aldyl 'A'. It does indicate, however,
that the strength of the weld is at least equal to o 0'1 0'2 0"3 0"4 0"5 0'6
the strength of the weaker component. The per- Pressure (MPm)
centage elongation can still be used as a measure Figure 14 Graph of percentage elongation versus welding
of weld quality, particularly when taken in con- pressure for welds between Muntz HDPE and Aldyl 'A';
junction with microstructural studies of the welds. welding temperature constant at 518 K.
1544
TABLE II The effect of using different initial heating times for the two pipes on the quality of Muntz HDPE/Aldyl
'A' welds
Specimen Welding Welding Heating time for Heating time for Average percentage
Temperature Pressure Muntz HDPE Aldyl 'A' elongations (4 specimens),
(K) (MPa) (sec) (sec) welding beads removed (%)
Ell 518 0.1 70 60 981
El2 518 0.1 80 60 876
E13 518 0.1 90 60 932
El4 518 0.1 60 50 958
El5 518 0.1 60 40 791
El6 498 0.1 80 60 840
El7 498 0.1 90 60 991
El8 473 0.1 90 60 714
El9 473 0.3 90 60 928

details of the welding conditions and the elong- 'A' at 60 sec, and raising the welding pressure
ation values are given in Table II. The results from 0.1 to 0.3 MPa. Overall, there is a clear
here may be summarized as follows. Increasing indication that an increase in the heating time for
the heating time for Muntz HDPE does not appear the Muntz HDPE is beneficial.
to result in any thermal degradation and, whilst Two types of fracture behaviour were observed
the elongation values do not vary significantly, the for those specimens tested with the welding bead
microstructural observations indicate an improve- removed. If the welding temperature or pressure
ment in weld quality. On the other hand, decreasing was too low, fracture occurred at the weld and the
the heating time for Aldyl 'A' (specimen El5) elongation was less than 500%. Initially, yielding
results in a significant decrease in elongation and occurred in the Aldyl 'A' part of the specimen,
therefore in weld quality. The results also show but when the neck reached the weld, failure
that a lower than optimum welding temperature occurred without any yielding of the Muntz
(473 K, specimens El8 and E l 9 ) may be compen- material. For welds made under or near optimum
sated for by heating the Muntz HDPE for a longer conditions, elongations of more than 700% were
time (90 sec), keeping the heating time for Aldyl obtained and the neck again initiated in the Aldyl

20'I~

2 ~5

10

100 200 300 400 500 600 700 800 900 1000
Strain ~
Figure 15 Stress-Strain curve for a specimen taken from a weld between Muntz HDPE and Aldyl 'A'. Failure occurred
away from the weld.
1545
 welding temperatures and at both extremes of
welding pressure, and in these cases the incomplete
adhesion at the interfaces was indicated by the
fibrillar nature of the fracture surfaces (Fig. 16).
Sections from welds were etched in chromic
acid to reveal the microstructures. The Aldyl 'A'
PE etched more rapidly than the Muntz material
and this made it impossible to etch the complete
weld correctly. However, some useful information
was obtained. Fig. 17 is a scanning electron micro-
graph of specimen E4 (welding conditions within
the permissible ranges but no additional heating
Figure 16 At low welding temperatures, when specimens period for the Muntz pipe) etched for 12h in
were tested with the welding bead remaining, a large chromic acid at 340 K. Two features are note-
amount of fibrillation was observed. Bar = 1 mm. worthy: first the large size of the bead on the
'A' and ran through the weld. Fracture finally oc- Aldyl 'A' (upper) half of the weld, and secondly
curred at the end of the gauge length in the Muntz the preferential attack at the interface. Both are
HDPE. Although the yield stress is lower for Aldyl undesirable. The large bead size could impede the
'A' PE, the fracture stress is higher (25.5 MPa com- flow of gas through the pipe and the attack at the
pared with 21 MPa for Muntz HDPE), so final interface implies poor adhesion. Most specimens
fracture occurs in the Muntz material. A typical exhibited microstructures similar to this. However,
stress-strain curve is shown in Fig. 15. The initial a rather better weld is indicated by Fig. 18 for
yielding and subsequent drawing of the Aldyl 'A' specimen El3, where extra heating time before
can be seen, followed by the yielding and drawing welding was given to the Muntz HDPE. The
of the Muntz HDPE. As the neck passes through welding beads are more nearly equal in size and
the weld there is a slight drop in stress which may the attack at the interface is less pronounced.
be associated with the actual weld interface.
Those specimens which were tensile tested with
the welding beads on were found to be less sensitive 3.2.2. Welds between Rigidex PE and
to weld quality and, as in the case of welds between A M y l "A" PE
similar materials, failure was always initiated by As before, a series of welds was made over a range
the notch between the bead and the gauge length. of temperatures and pressures and the duration of
However, the tensile results and the appearances of the heating time was also varied. The results
the fracture surfaces support the conclusions previously obtained for welding each material to
reached for those specimens for which the beads itself provided a good indication of the range in
were removed. Poor welds were produced at low which satisfactory welds could be expected.

Figure 18 Specimen El3 etched in chromic acid. The

Figure 17 Specimen E4 etched in chromic acid. The upper right-hand side of the weld is Muntz HDPE. A longer
half of the weld is Aldyl 'A'. There has been preferential heating time for Muntz HDPE reduces the amount of
attack at the interface and the welding bead is very interracial attack and results in a more even welding
uneven (scanning electron micrograph). Bar = 1 ram. bead (scanning electron micrograph). Bar = 1 mm.
1546
Once again, the Aldyl 'A' PE had the lower yield illustrates the effect of varying the welding press-
stress and the welding factor could not be used as ure. It is apparent that good welds are obtained in
a criterion of weld quality. the temperature range 493 to 523K and in the
Fig. 19 shows the effect on percentage elong- pressure range 0.05 to 0.3 MPa. There is a signifi-
ation of varying the welding temperature using a cant drop in weld quality at low welding tempera-
constant welding pressure of 0.1 MPa, and Fig. 20 tures and at high welding pressures. Table Ill
shows the effect of using different heating times
1000 for the two materials, all other variables remaining
constant. The sizes of the welding beads are made
more nearly equal if either the heating time for
80C the Rigidex PE is increased or the laeating time for
the Aldyl 'A' is reduced; in. both cases high elong-
ation values indicate good welds.
60(
The fracture behaviour of specimens tested
with the beads removed was similar to that of
o 40( specimens from welds between Muntz HDPE and
Aldyl 'A'. If the welding conditions were far
removed from the optimum, fracture always
200 occurred at the weld and the elongations were less
than 464%. Yielding occurred in the Aldyl 'A' half
and the neck extended until it reached the weld.
443 463 483 503 523 The surface showed some fibrillation, indicating
Temperature (K) poor adhesion (Fig. 21). For good welds, with
Figure 19 Graph of percentage elongation versus welding elongations of more than 590%, the neck ran
temperature for welds between Rigidex and Aldyl 'A'; through the weld and the Rigidex PE was also
welding pressure constant at 0.1 MPa. drawn. Failure finally occurred at the end of the
1000
gauge length in the Rigidex material. The stress-

800

600

_o
~ 400
200

O.1 0.2 0.3 0"4 0.5 0.6

Pressure (MPa)
Figure 20 Graph of percentage elongation versus welding Figure 21 For specimens welded away from the optimum
pressure for welds between Rigidex and Aldyl 'A'; welding conditions failure occurred at the weld with some fibril-
temperature constant at 503 K. lation (scanning electron micrograph). Bar = 1 mm.
T A B L E III The effect of using different initial heating times for the two pipes on the quality of Rigidex/Aldyl 'A'
welds
Specimen Welding Welding Heating time for Heating time for Average percentage
Temperature Pressure Rigidex PE Aldyl 'A' PE (S) elongation (4 specimens)
(K) (MPa) (see) (see) welding beads removed
F13 513 0.1 70 60 884
F14 513 0.1 80 60 722
F15 513 0.1 90 60 711
F16 513 0.1 60 50 789
F17 513 0.1 60 40 767

1547
Figure22 Specimen F15 etched in chromic acid. The Figure24 Specimen F9 etched in chromic acid. The
lower half of the weld is Rigidex PE. This specimen was lower half of the weld is Rigidex. High welding pressure
welded under suitable conditions and no attack at the results in a heavily etched zone 3 in Rigidex, some inter-
interface is visible. The welding bead is uniform (scanning facial attack, and a narrow weld region (scanning electron
electron micrograph). Bar = 1 mm. micrograph). Bar = 1 mm.

strain curves obtained from good welds showed As also observed in Aldyl 'A'/Muntz welds, each
the s~ime effects as those seen for the Muntz material exhibits its own characteristic microstruc-
HDPE/Aldyl 'A' system. ture which is not affected by the other.
The results obtained from specimens tested Zone 3 in the Rigidex part of the weld (Fig. 22)
with the welding beads left on were largely insensi- is less heavily etched than usual. This may be be-
tive to variations in the welding conditions. The cause there is less flow than usual in this part of
fracture surfaces showed an increase in fibrillation the weld: the Aldyl 'A' is at a temperature well
and sometimes cavitation if the welding conditions above its normal welding temperature and conse-
were not correct. quently most of the flow probably occurs in this
Etching specimens from Aldyl 'A'/Rigidex material. This is indicated by the fact that the
welds was reasonably successful because both ma- weld region in Aldyl 'A' is slightly thinner than in
terials were etched at about the same rate. An Rigidex. Another possibility is that, because of
etching time between 8 and 12h in concentrated the unusually long heating period, the viscosity of
chromic acid at 340 K produced good results. Fig. the Rigidex melt is correspondingly reduced and
22 shows the result of etching specimen F15. only a small amount of frozen-in stress is produced
There is no preferential attack at the interface and in the weld. We have shown elsewhere [8] that
the welding beads are approximately equal in size frozen-in stress causes preferential etching.
due to the extra heating time for the Rigidex PE. Fig. 23 shows the microstructure o f a specimen
where the welding pressure is too low. If, on the
other hand, the welding pressure is too high (Fig.
24) a pronounced zone 3 is formed in the Rigidex
half due to the excessive flow, and most o f the
molten polymer is squeezed into the beads, re-
sulting in a narrow weld and large beads. Preferen-
tial etching o f the interface also occurs.
Welds between Rigidex PE and Aldyl 'A' could
also be examined using transmission optical micro-
scopy, for both components were yellow and thin
transparent sections could be cut. This allowed the
flow patterns visible in the Rigidex part o f the weld
to be studied. Fig. 25 is a micrograph ofaspecimen
Figure23 Specimen F12 etched in chromic acid. The
lower haft of the weld is Rigidex. Due to the use of a low welded under optimum conditions, i.e. there
welding pressure some etching of the interface has taken is no preferential etching at the interface and all
place (scanning electron micrograph). Bar = 1 mm. the tensile tests gave large elongations. The Aldyl

1548
Figure 25 This shows the appearance of a weld made at Figure 26 This shows the appearance of a weld made
near optimum conditions (transmission optical micro- using a welding pressure which is too low (transmission
graph). • 12. optical micrograph). X 12.

'A' side of the weld is on the left and has its usual though these would be reduced somewhat by the
appearance. The interface is similar to that found extra period of heating which would lower the
previously in Aldyl ' A ' welds, i.e. a chill skin con- melt viscosity. It was not possible to see any flow
taining crystallite nuclei. The Rigidex portion lines in the Muntz HDPE as this was black.
shows the flow patterns discussed elsewhere [5] The reason for the presence of these flow lines
but now there is no "dead-zone". Indeed, the flow near the interface is not clear. It may be that the
lines are densely packed near the interface. This Aldyl 'A' melt, which will flow quite readily at
could explain why the interface appears to be this temperature, tends to drag the Rigidex along
more readily etched than in similar welds in either with it. Alternatively, it may be that the more fluid
component material, particularly at slightly higher Aldyl 'A' is rapidly squeezed from the weld and
pressures. It also may explain why many of the then what deformation occurs in the more viscous
welds between Aldyl 'A' and Muntz HDPE appeared Rigidex melt must be accommodated in a relatively
to be preferentially etched in this way. Muntz small region, resulting in more severe flow than
HDPE having a very low melt flow index is more usual.
likely to contain residual stresses than Rigidex, Fig. 26 is a micrograph o f a specimen made
1549
 using a welding pressure of 0.05 MPa. The Aldyl It has been shown that satisfactory welds can-
'A' side of the weld (on the left) is unchanged but not be made with Muntz LDPE. The highest
now a "dead" zone is present in the Rigidex ma- welding factor obtainable was 0.95 and all the
terial. The relative widths of the two sides of the welds were brittle. Butt fusion welding must there-
weld and the small number of flow lines visible in fore be discounted as a joining technique. These
the Rigidex PE indicate that the amount of flow in results indicate the harmful effects that could re-
the Rigidex melt has been less than that for Aldyl sult if low density fractions are present in poly-
'A'. This specimen was preferentially etched at the ethylene pipes.
interface and though the lack of adhesion cannot Tensile tests can be used to assess the quality of
be detected directly from this micrograph, it can welds between dissimilar materials and to observe
be inferred from the flow pattern in the Rigidex the effect of variations in welding parameters, pro-
which indicates little flow and therefore little riding the welding bead is removed. The welding
pressure. As the welding pressure is increased up to factor is no longer a useful criterion of weld quality
a value of 0.6 MPa the flow lines in the Rigidex because yielding always occurs in the weaker ma-
b e c o m e denser and the weld narrower. terial and the welding factor defined in terms of
The effect of welding temperature on the ap- the weaker material is always unity. However, the
pearance of optical micrographs was very slight fact that fracture occurs away from the weld
until relatively low temperatures were reached and shows that this is at least as strong as the weaker
there was little flow in the Rigidex PE. Then the component. The quality of the weld can be assessed
welds were very similar to that shown in Fig. 26. in terms of the percentage elongation at fracture,
particularly when consideration is also given to the
4. Discussion weld microstructure. In this respect, when the
Using tensile tests, optimum welding conditions weld is etched in chromic acid there should be no
have been found for Muntz, Rigidex and Aldyl preferential etching at the interface as this indicates
'A' polythenes for welds made between pipes of a lack of adhesion.
the same material. The welds have short term Good welds are possible between Muntz HDPE
strengths equal to those of the unwelded materials. and Aldyl 'A' polythene, and between Rigidex and
These conditions are summarized in Table IV and Aldyl 'A'. As the Muntz/Aldyl 'A' system provides
it can be seen that the optimum welding pressures the greatest difference in melt flow indices, it is
are about the same for all these materials but the expected that no difficulty will arise in welding
welding temperatures vary. This is a reflection of other materials within this range. The optimum
the different melt flow indices. It has been found welding pressures are very similar for all three ma-
that welding temperatures below the optimum are terials so the problem is essentially one of deter-
much more harmful than those above, at least in mining optimum welding temperatures and heating
the short term. times.

TAB L E I V Summary of optimum welding conditions

Materials Optimum Welding Optimum Welding PermissibleRange of PermissibleRange
Temperature Pressure Welding Temperature of Welding Pressure
(K) (MPa) (K) (MPa)
Muntz LDPE 473 0.25 Small deviations from the optimum values
produce large decreases in weld quality
Muntz HDPE 518 0.1 508-538 0.1-0.6
.Welding pressures
lower than 0.1 MPa
must be avoided
Rigidex 503 0.1-0.3 493-523 0.05-0.6
Aldyl 'A' 473 0.1 453-518 0.1-0.6
Muntz HDPE/Aldyl 'A' 508 0.1 488-518 0.1-0.3
(An additional 30s Weldingpressures
heating period for the lower than 0.1 MPa
Muntz pipe is beneficial must be avoided
Rigidex/Aldyl 'A' 503 0.1 493-523 0.05-0.3
1550
 The best results (see Table IV) are achieved by References
using welding conditions appropriate for the ma- 1. J. ZIMMERMAN and R. ERNST, Plastverarbeiter 20
terial with the lower melt flow index. The other (1969) 245.
component will then be welded at a temperature 2. G. MENGES and J. EHRBAR,Kunstoffe 53 (1969)
233.
higher than its optimum but this does not appear 3. E. ALF, H. POTENTE and G. MENGES, Paper
to be harmful, at least in the short term. If the presented at the 5th International Conference
material of lower melt flow index is heated for a organized by the Plastics Institute on Designing to
longer time than usual before welding, whilst the Avoid Mechanical Failure, Cranfield, January 1973.
other material is not, the welding bead becomes 4. G. DIEDRICHand E. GANBE,Kunstoffe 60 (1970)
74.
nearly symmetrical. This produces a slightly 5. G. MENGES and J. ZOHREN, Plastverarbeiter 18
larger bead than normal but there are no other (1967) 165.
detrimental effects. 6. P. BARBER and J. R. ATKINSON, 3. Mater. ScL 9
In conclusion, it has been shown that by using (1974) 1456.
the correct conditions it is possible to achieve 7. Idem, ibid 7 (1972) 1131.
8. D. R. deCOURCY and J. R. ATKINSON, "The
welds between polyethylene pipes of widely dif- Microstructures of Fusion Welds in Polyethylene
ferent melt flow index which are, in the short Pipes", to be published.
term at least, as strong as the weaker component.
However, there is a need for more long-term
testing.

Acknowledgment
The authors would like to thank British Gas for a
scholarship for D.R. deCourcy. Received 20 December 1976 and accepted 8 February 1977.

1551