Welding for Challenging Environments. Proceedings of the International Conference on Welding for Challenging Environments, Toronto, Ontario, Canada, 15–17 October 1985

353 Pages·1986·24.043 MB·English

by  Welding Institute of Canada (Auth.)

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WELDING IN THE WORLD
ινΕυιικ<;
FOR CHALLENGING
ENvmoNMEivrs
Proceedings of the International Conference on
Welding for Cliallenging Environments,
Toronto, Ontario, Canada, 15-17 October 1985
Edited by
Welding Institute of C a n a da
0al(ville, Ontario, Canada
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First printing 1986
Library of Congress Cataloging in Publication Data
International Conference on Welding for
Challenging Environments (1985 : Toronto, Ont.)
Welding for challenging environments.
1. Welding-Congresses. I. Welding Institute
of Canada. II. Title.
TS227.A1I56 1985 671.5'2 86-4958
ISBN 0-08-031866-5
In order to make this volume available as economically and
as rapidly as possible, the authors' typescripts have been
reproduced in their original forms. This method
unfortunately has its typographical limitations but it is
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Printed In Great Britain by A. Wheaton & Co. Ltd., Exeter
FOREWORD
These proceedings of the Welding Institute of Canada's Third International
Conference, Welding in Challenging Environments, held in Toronto on
October 14/17, 1985, continues the Institute's commitment to make available,
to the international welding community, the latest advances in specific
technological fields and to provide a forum for international contact and
discussion.
The third Conference follows those held in 1980 on Pipeline and Energy
Plant Piping in Calgary and in 1983 on Welding in Energy Related Projects,
in Toronto. Proceedings of these conferences were also published by
Pergamon Press Inc.
We were pleased to arrange the present conference as an integral part of
the 1985 Metals Congress of the American Society for Metals - the first time
ASM has held their successful Metals Congress outside the USA. I would
like to express our pleasure at the collaboration between the Welding
Institute of Canada and the American Society for Metals that was achieved
in this joint venture.
The contributions which are presented in this publication combine the full
texts of the papers presented with a record of the discussions, edited from
tape recorded proceedings. They form a unique reference to the latest
state of technological development, research and application of welded
fabrications in challenging environments.
The Welding Institute of Canada is proud to make the proceedings of this
conference available on a widely distributed basis and trusts this publica­
tion will be a valued reference.
NORMAN. F. EATON
President
Welding Institute of Canada
ix
RECENT DEVELOPMENTS IK PULSED GAS METAL ARC WELDING
C.J. Allum
GEC Industrial Controls Limited,
Rugby, England.
ABSTRACT
Gas metal arc welding (GMAW) is currently attracting much interest on account of
significant developments in process control over the past few years. These
developments are largely associated with benefits gained from the application of
modern solid state power devices to welding power supplies. In this paper pulsed
current GMAW is considered with emphasis on interactions between pulse parameters,
parameter selection, fusion characteristics and process control.
KEYWORDS
Pulsed current gas metal arc welding, metal transfer, power sources, process
control.
INTRODUCTION
GMAW is arguably the most versatile of all welding processes being capable of
joining in any position a wide range of materials, using manual or mechanised
techniques on thin sheet or sections hundreds of millimeters thick. Process
productivity is potentially high since filler metal is continuously deposited, with
little slag, at high deposition rates (associated with efficient wire melting) and
suitable for use in narrow gap weld preparations. Good toughness with very low
deposited hydrogen levels are achievable meeting the requirements of many demanding
applications. Furthermore GMAW allows close control of plate dilution and finds
applications besides welding in cladding and brazing.
GMAW has however, yet to achieve the potential outlined above. Historically, two
process weaknesses (metal transfer and fusion characteristics) and a number of
equipment related short comings have limited the application of GMAW.
Metal may be transferred in a variety of modes. At very low currents short
circuiting (dip transfer) is required and not all materials are weldable in this
mode. The explosive nature of such transfers gives rise to spatter and
intermittent arcing produces a susceptability to lack of fusion defects. At higher
currents transfer becomes globular and non projected. Further increases in current
result in a spray of small droplets, typically of wire diameter projected across
the arc gap.
2 Welding for Challenging Environments
Synergy is a control technique used in pulsed current MIG welding (Ref 1) where
mean current is determined by wire feed speed such that stable wire melting and
drop transfer occur. The outcome of this technique is simplified process operation
with nominally one knob control. A wide range of methods exist for achieving the
above characteristics but only two basic approaches are considered here (see Ref
8). One technique consists of driving the power supply in response to a wire feed
speed control signal. This might for instance be used to increase pulse frequency
proportionally to wire feed speed demand. Metal transfer can then be controlled
by predetermined unit pulses of current (of specified Ip and Tp) while frequency
control simply changes the time spacing between pulses with the effect of altering
mean current. With this control scheme droplets of uniform size are detached at
every mean current (i.e. W/F in constant) and mean current increases approximately
in proportion to wire feed rate when low background currents are employed. Process
control is then achieved directly from wire feed rate. For this type of control no
arc length self adjustment exists i.e. when the torch is withdrawn from the work
arc length increases with fixed wire extension.
Arc length self adjustment may also be achieved by incorporating voltage control.
Here a voltage error signal is generated (difference between reference voltage and
measured voltage) which in effect modifies the wire feed to pulse frequency ratio
to achieve the desired arc voltage. Features of conventional self adjusting GMAW
are thereby regained. A second so called synergic technique relies entirely on
voltage control to produce frequency modulation without any link between power
supply and wire feed unit. Having set the required voltage and wire feed rate, the
spacing between pre-determined pulses is modulated in self regulating manner i.e.
arc current is self regulating. Process control is again of the one knob type and
when a welding torch is withdrawn from the work both pulse frequency and mean
current then fall in a self regulated manner so as to maintain a given arc voltage.
By reducing process control to nominally one knob a range of further possibilities
are presented. For instance thermal pulsing and backface control of full
penetration. In the first case low frequency modulation of wire feed speed is used
to produce overlapping weld beads which can have beneficial effects on fusion. The
required changes in current changes are then automatically accomodated by the
synergic type control technique. With backface penetration control (as practiced
in TIG welding) a radiation signal from the underbead may be used to modulate top
face heat input thereby controlling penetration (although this technique has yet to
be developed for GMAW).
All of the above techniques rely on steplessly variable control of the current
waveform (especially pulse frequency). This may be achieved electronically with
solid state devices (transistors and a range of thyristors) which are used in power
circuits as current switches or variable resistors. Series regulator circuits use
devices as variable resistors and have inherently high response with low ripple.
However, at low arc voltages most of the process power is dissipated across power
devices and these circuits, although essentially simple, are very inefficient with
high cooling requirements. Switching circuits have much lower losses and air
cooling is often appropriate. Devices are then either on or off and switched at a
frequency characteristic of the circuit design/device capability (typically of
order 20KHz). These circuits generate current ripple and have a slower response
than achievable with series regulators. One praticularly energy efficient and
physically small class of switching circuit are inverters of which there are a
number of basic types. With these switching is on the primary side of the
transformer and transformers can then be made much smaller than for secondary
switched circuits.
Welding for Challenging Environments 3
The first condition is concerned with drop detachment and reflects the observation
that background parameters usually have little influence on the detachment event.
It is often observed (see Fig 1) that a peak detachment can occur at high peak
currents (Ip) of short duration (Tp) or lower peak currents of longer duration such
that (1) is approximately obeyed where D, a detachment parameter, is constant and
influenced by wire composition, wire diameter and shield gas type. Typical values
for D are given in Table 1. In many situations preferred combinations of Ip and Tp
exist and those identified by the writer are also shown in Table 1 where it is
interesting to note that preferred peak currents are typically 1.5 times the so-
called spray transition current (Is).
The second condition is a simple statement of droplet volume (J2i) when one drop per
pulse is detached where W is wire feed rate, F pulse frequency and A is wire cross-
sectional area.
In many situations W = K,I where 1 is mean current and Κ is a constant. Equation 2
then becomes:-
ΐ - Ρ (3)
F K.A
Equations 1 and 3 allow pulse parameters to be specified provided Κ and D are
known. The method is then:-
1. Select droplet volume and calculate I/F. _
2. Select mean current for a given application. I/F then gives required pulse
frequency.
3. Select Ip (at 1.5 Is say) and use D to calculate Tp.
4. Tb is now determined since frequency and pulse duration are known.
5. Background current is also determined and can be evaluated from the expression
for mean current.
In many commercial equipments, rules of this type are incorporated as control
circuitry and the required parameter adjustments are automatically made thereby
resulting in considerable simplifications in process operation (see process
control).
TABLE 1, Detachment Relationships
Parameter Material/Gas
Mild Steel Al S Steel
(Pure) (308)
Argon 5% C02 Argon Argon 2%
dw (mm) 0.8 1.0* 1.2* 1.6* 1.2** 1,2***
D (A S) 160 310 430 640 130 315
F/I (Hz/IOOA) - 60 50-60 40 90 60-65
Ip (A) 200 280 350 400 200 280
Tp (ms) 4.0 4.0 3.5 4.0 3.3 4.0
4 Welding for Challenging Environments
For many materials spray transfer occurs at currents too high to allow managable
weld pool control in any position other than downhand welding. The principal
method of overcoming transfer limitations outlined above was developed over two
decades ago and uses pulses of current to detach small droplets while background
current is held at a level compatible with managable weld pool size in out of
position applications. Only recently with the advent of commercially available
solid state power supplies has the potential of pulsed current welding been
realisable.
Pulse parameters can now be independently and steplessly varied to meet arc
requirements where earlier equipments operated at fixed frequencies (related to
mains frequency). The use of a range of process control techniques has also become
practical (see Section 'Process Control').
Power control developments have undoubtedly removed many of the previous
limitations concerning aspects of metal transfer behaviour and spatter. However,
fusion characteristics appear to be largely determined by mean current and welding
speed, technique and shield gas composition. As such, recent developments in power
control have had relatively little impact on fusion which represents a significant
challenge for future process developments. Indeed, shielding gases used are
largely those previously developed for non-pulsed GMAW applications.
METAL TRANSFER
Current pulsing is applied in GMAW with the central objective of controlling the
transfer of filler material. Using nominally square wave d.c. pulses it has been
found possible to almost eliminate spatter and maintain stable arcs with small
droplet transfers at currents down to about 50 amps (Ref 1). Satisfactory
transfers are achieved by choosing suitable peak and background parameters which
are usually identified by trial and error experimentation (Ref 2 and 3). Often
more than one set of suitable parameters exist. The choice of parameters can in
principal affect a wide range of droplet and arc characteristics including droplet
volume, position of detachment within a pulse, number of droplets per pulse,
droplet acceleration and momentum, arc stiffness and arc stability. In practice
observation suggests that weld beads are tolerant to parameter changes (for a given
mean current) and so general running characteristics (e.g. spatter, stability,
stiffness, arc control) form the main criteria for parameter selection.
An approach widely adopted is to choose parameters giving one droplet per pulse
with detachment near the trailing pulse edge and transfer during the quiet
background period. For given material parameters it is usually observed that the
position within a pulse of drop detachment is determined by peak current (Ip) and
duration (Tp) (Ref 2). Droplet volume is also influenced by background parameters
(Ref 3). Clearly a wide range of parameter combinations exist in pulse current
welding and often it is useful to employ a systematic approach to parameter
selection. A semi-empirical approach to this problem is developed below.
We start by considering conditions related to drop detachment and droplet volume;-
Ip^ Tp = D (1)
y
W A = (2)
Welding for Challenging Environments
* Data from Reference 3
** Data from Reference 4
*** Data from Reference 5
Also see Figures 1 and 2.
WIRE MELTING BEHAVIOUR
Wire melting behaviour has important implications for productivity and this can be
affected by choice of pulse parameters. In pulsed MIG welding high current
excursions have a significant influence on Ohmic heating and produce deposition
rates higher than those achieved at the corresponding steady current. In view of
high current peak excursions a none linear burn-off dependence on current might be
expected. However, this is not generally observed (see Fig 2). To understand why
linear behaviour is often observed consider the mechanisms by which wire melting
occurs. Arc and resistive effects give:-
W = '·< Ϊ + R (I^) ave.
Where ^ is a coefficient related to arc heating and R is related to resistance
offered by the electrically heated wire extension. The value of I averaged over a
square pulse is approximately:-
(I^) ave = (Ip^Tp + Ib^b) F
= Ip^TpF for Ip^Tp»Ib2Tb
= D F
then
W = i< + R D F (4)
I ΐ
The choice of f/F and D can clearly affect deposition rate per amp and maximum
deposition at any mean current is obtained by the use of high frequency, high
detachment parameter conditions (Ref 3). Long electrical extensions and small wire
diameters also enhance deposition but this affect is not unique to pulse c_urrent
welding. It should be noted that linear burn-off curves arc expected when I/F is
constant.
Deposition rate is little influenced by shield gas composition in GMAW. Welding
polarity can however have a very significant effect where 50% higher deposition
rates are observed (Ref 6) in a.c. welding of mild steel (compared with wire
positive deposition rates) although a.c. synergic GMAW sets are not currently
commercially available.
FUSION CHARACTERISTICS
The general characteristics of weld bead and plate fusion shape in GMAW (see Fig 3)
are such that at least eight weld geometry characteristics may be identified which
inturn depend on a variety of process parameters and many of these relationships
have yet to be fully investigated. Emphasise here is therefore largely on
qualitative effects although some quantitative results are discussed.
6 Welding for Challenging Environments
The major process variables influencing plate fusion characteristics are mean
current, welding speed and shield gas composition where the precise choice of pulse
parameters is of secondary importance. For a given mean current and welding speed,
shield gas composition has a pronounced effect on plate fusion shape (see Fig 3).
Plate fusion generally exhibits a finger of penetration with weak secondary fusion
extending towards the bead edges. Secondary fusion is much less pronounced on
relatively low thermal conductivity materials (e.g. steels) than for aluminium.
For steels secondary fusion is enhanced by the addition of C02 or helium to argon
shields. However, non central finger penetration is often observed in helium rich
shields and metal transfer characteristics deteriorate with the addition of high
C02 levels. Significant work remains to identify Ar/He/C02/02/H2 gas mixes which
optimise both transfer and fusion shape and thereby reduce susceptibility to fusion
defects.
Such work is considered particularly important for all-positional manual welding
since dilution is often low (typically 20%) although at high currents/high speeds
satisfactory dilution (30% and above) is achievable.
Dilution (S) is found to be a useful measure of fusion and defined by:-
Ap + Ad
Where Ap and Ad are cross-section areas of plate fusion and deposited bead area
respectively. For thick plates simple modelling considerations have been used to
show that $ is a function of ϊν· (Ref 7, mean current χ welding speed) a result
substantiated b]^ experiment and it is found that ^ behaves as shown in Fig 4. At
high values of Iv, S reaches a saturation value C^m) and plate fusion^area then
increases in proportion to heat input. However, at lower values of Iv, plate
fusion is not a function of heat input. In this region a range of plate fusion
areas corresponding to a given deposition area may exist depending on the structure
of heat input). Such behaviour can be further illustrated by noting that the
assymtotic shape of S = Mlv) becomes almost linear when 1/S is plotted against
1/iv (see Fig 5). Then;
l ie (3)
S ^m Iv
Where C is a constant representing the line slope. Dilution now gives;
Ap = S Ad
1-S
Noting that vJ^d = Ka (where a is wire cross-sectional are a) and substituting for
allows Ap to be expressed as:-
Ap = Κ a (l/v) (6)
' 1 C
+ —
L Sm Iv