Synergic control in MIG welding part 1
M Amin, BSc, BE, is a Senior Research Engineer Technician in the Power Source and Control Section of the Arc Welding Department.
This article is based on Welding Institute Members Report 285/1985, September 1985, 'Synergic control in steady DC open arc and short circuiting arc MIG welding', by M Amin and Naseer Ahmed.
Synergic control was developed originally for pulse MIG welding. This series of articles will describe the derivation of control equations, the development of electronic controllers, the performance of control systems, and the effect of controller response on arc stability.
Synergic control has transformed conventional pulsed current MIG welding, which was impractical because of the difficulties in setting up stable welding conditions, into a usable process. This is because it is capable not only of setting up a stable welding condition automatically with any constant wire feed rate over the operating range, but also provides stable arc operation even when the feed rate is changing, whether gradually or modulated with any waveform. The control was conceived from a parametric relationship first established empirically,
[1,2] namely that pulse frequency or pulse duration, together with background current level, must be increased linearly with wire feed rate. This form, however, is limited in that it expresses only two modes of a more fundamental relationship derived later,
[3-6] which is:
I m (or mW) = I b + I eT pF
The terms included in this equation are defined in Fig. 1. Although the relationship is very simple, it expresses all the possible modes in which the pulse parameters can be varied relevant to the wire feed rate (or mean current).
Fig.1. Schematic pulse current waveform: I p, T p = pulse current amplitude and duration; I b, T b background current level and duration; I m= mean current; T = total cycle duration; I e = I p - I b excess of pulse over background current; F=1/T pulse frequency; W = wire feed rate; m = I/W mean current per unit wire feed rate
In practice, an electronic controller [1,2] or a microcomputer [4-6] computes the relationship in any mode as programmed, according to the wire, feed rate input, and drives a transistor power source, such that commensurable pulse parameters are adjusted automatically providing stable arc operation even for square wave modulated feed rate. All that the operator needs to do is select a suitable wire feed rate for the application. In addition to achieving the prime objective of automatic setting of pulse parameters, synergic control provides several advantages over conventional pulsed MlG operation, which include absence of burnbacks or stubbing-in and arc stability against wire feed rate fluctuations which normally occur with a commercial wire feeder.
Furthermore, the feed rate could be programmed as desired for robotic applications, and also to refine the welding operation. For example, it could be varied to achieve uniform filling of a joint with variable groove configuration. At weld start, the wire approach towards the workplace could be slow to mitigate the impact which could cause buckling of the wire at the feed rolls. At weld stop, feed rate could be reduced gradually to fill the crater, and also avoid wire stubbing into the weldpool where it would freeze and need to be cut. Another important feature of the control is that it enables modulated wire feed rate to be operated for thermal pulsing, at any frequency and with as much difference as desired between low and high feed rate levels of modulation. This technique makes sound welds for the joints with narrow groove preparation. As these advantages have made synergic control attractive to industry, it has already been incorporated in many commercial power sources. [7]
Synergic controls have also been recently developed at The Welding Institute for two other conventional modes of MIG welding - steady DC open arc and short circuiting arc. The approach comprises development of basic relationships, and then design and construction of electronic controllers which execute control equations and regulate it transistor power source. In operation, the control systems initiate and maintain stable arc operation automatically for any wire feed rate, whether constant, variable or modulated for thermal pulsing with any waveform. Therefore, the controls not only simplify setting up the welding conditions but also enable wire feed rate to be programmed as required to refine welding operation and also extend the capability of the process, to accomplish welds with good characteristics.
Description of these controls has been subdivided into four parts, to be reported in this series of four articles. The basic parametric relationships established to characterise steady DC open arc and short circuiting arc operation are described in this article. Articles describing the derivation of control equations, development of electronic controllers, performance of the control systems, and the effect of controller response on arc stability will be published in future issues of the Research Bulletin.
Process requirements for stable operation
Experimental data for establishing parametric relationships were acquired for stable MIG operation, which has two essential requirements. First, any selected wire feed rate must be balanced against wire burnoff rate to maintain a constant arc length - otherwise, an excessive feed rate causes stubbing of the wire into the weldpool, or an excessive burnoff rate results in burnback damaging the contact tube. Second, the metal transfer from the wire end into the weldpool should comprise droplets or 'spray' to achieve a uniform weld deposit. Otherwise, large droplet or 'globular' transfer produces irregular welds having lack of penetration and lack of fusion defects.
With DC open arc welding where a steady current (Fig. 2) [8] is used to operate the arc for a given wire feed rate, the first requirement can be fulfilled over a current range extending from about 50 to 500A. However, the second requisite limits this range to that above a 'critical' level where a transition occurs from the undesired globular to the suitable spray metal transfer. Therefore, the steady DC open arc is unsuitable for operation at low currents, which would be needed to avoid burnthrough in thin sheet materials, and avoid the weldpool becoming excessively large and thus uncontrollable for positional welding.
Fig.2. Typical oscillogram of arc voltage and current using 1.2mm diameter Inconel wire at 225A in argon [8]
For short circuiting arc operation, in which both arc voltage and current fluctuate consistently (Fig. 3), [9] not only can the balance between wire feed rate and burnoff rate be achieved over the entire operating current range, but also the desired small droplet metal transfer is accomplished by means of short circuits which normally occur at a frequency of about 50-200Hz. However, to achieve regular short circuits and consistent metal transfer, avoiding spatter, the rate of rise of current during a short circuit must be controlled. It must neither be excessively fast nor excessively slow. The optimum rate is controlled by inductance in the welding circuit or controlled electronically.
Fig.3. Typical oscillogram of arc voltage and current for short circuiting arc operation using 1mm diameter mild steel wire at 4.2m/min feed rate in CO 2. [9]
Experimental
Equipment
To study basic parametric relationships for both steady DC open arc and short circuiting arc operation, the equipment comprised the welding rig and transistor power source described below.
Welding rig
Tests were carried out on a mechanised rig (Fig. 4) in which the workpiece was traversed uniformly under a stationary welding head, which was mounted vertically such that the contact tube to workpiece distance could be adjusted as desired. The wire electrode was driven through the head by a Welding Institute multigrip wire feeder. [10] This was driven from a reference signal, on the basis that a 0-5V reference input gave 0-15 m/min feed rate output. The calibrated reference signal of the desired waveform was provided by a reference control unit.
Transistor power source
The transistor power source, designed and developed at The Welding Institute, [11] comprised a three phase transformer rectifier unit with the addition of a feedback controlled linear transistor series regulator. The transformer rectifier provided steady DC which was then controlled by the regulator to supply the arc with the desired unipolar current waveform (e.g. smooth DC or modulated waveform). The range of current extended from 0-500A. As the frequency response of the transistor regulator extended from DC to about 10kHz, the power source could provide a square wave modulated current with a range of frequencies up to 1kHz.
The regulator could be operated in either constant current or constant voltage mode. It was driven from an instruction or reference signal, on the basis that 0-5V reference input gave 0-500A regulator output to the arc for the constant current mode and 0-50V output for the constant voltage mode. The calibrated reference signal of the desired level was provided by the associated electronic control unit.
To obtain data for the parametric relationships for open arc operation, the transistor regulator was operated in the constant current mode to maintain accurate control of the current level. As this mode does not provide any self-adjustment, the current had to be carefully adjusted so that the burnoff rate matched the wire feed rate for any required arc length.
For short circuiting operation the regulator was operated in the constant voltage mode. The voltage level and the rate of rise of current during a short circuit were adjusted to achieve consistent short circuit frequency for a given wire feed rate.
Test programme
For open arc operation, the parametric relationships were studied for mild steel wires (BS 2901 : Part I: 1970: A18) of 1.2mm diameter with Ar + 5%CO 2 and Ar + 20%CO 2 shielding gases. For a given wire material/diameter/shielding gas combination, the operating range of wire feed rate and the corresponding ranges of current and voltage were determined for 10, 15 and 20mm electrode extensions, using 5 and 10mm arc lengths for each extension. These tests provided data to establish the basic relationships of welding current against wire feed rate, and are voltage against welding current. In addition, the data were used to determine the effects of electrode extension, arc length, shielding gas and wire diameter on the relationships.
For short circuiting arc operation, the parametric relationships were studied for mild steel wires (BS 2901: Part I: 1970: A18) Of 1.2 and 1mm diameter and Ar + 20%CO 2 and CO 2 shielding gases. For a given wire material/diameter/shielding gas combination, data were obtained on current and voltage for the optimum arc performance over the operating wire feed rate range, so that the current v wire feed rate and arc voltage v current relationships could be established.
Parametric relationship
Steady DC open arc operation
Specific basic relationships
For open arc operation, with a required wire material/diameter/shielding gas combination, the operating variables included wire feed rate, current level and arc voltage, for a given arc length and electrode extension. These variables provided two relationships which could specify the operation entirely; the first, current against wire feed rate (called burnoff), and the second, voltage against current relationship. These relationships, as obtained from the experimental data, are described below.
For the mild steel/1.2mm diameter/Ar + 5%CO 2 combination, using 5mm arc length and 10mm electrode extension (15mm contact tube to workpiece distance), the basic relationships are plotted in Fig. 5. The burnoff relationship (Fig. 5a) was the basis for open arc operation, whereby the required wire feed rate specified a unique current. The relationship is linear such that current increases with feed rate and intercepts the current axis when extrapolated to the feed rate at zero value. The relationship can be expressed by the following equation:
I = 26.4W + 80 . . . [1a]
Where
I is the arc current, A
W is the wire feed rate, m/min
The voltage against current relationship (Fig. 5b) specified a unique voltage for the required current. Over the operating current range, the relationship is linear such that the voltage increases gradually with the current, and makes an intercept on the voltage axis when extrapolated to the current at zero value. The relationship can be expressed by the following equation:
V = 0.028I + 21 . . . [1b]
Where V is the arc voltage, V
Fig.5. Basic relationships for a mild steel wire 1.2mm diameter and Ar + 4%CO 2 shielding at 5mm visible arc length and 10mm electrode extension: a) Burnoff relationship
b) Voltage against current
Effect of process parameters on basic relationships
a) Arc length: For the same welding conditions as above; mild steel/1.2mm diameter/Ar + 5%CO 2 combination, and a 15mm contact tube to workpiece distance but with a 10mm arc length (5mm electrode extension), the basic relationships can be expressed by the following equations:
I = 34.3W + 60 . . . [2a]
V = 0.029I + 23 . . . [2b]
The equations are similar to the corresponding [1a] and [1b] for 5mm are length except that, for the burnoff relationship, the slope is increased from 26.4 to 34.3 A/m/min while the intercept is decreased from 80 to 60A. Whereas, for the voltage against current relationship, the slope is slightly increased from 0.028 to 0.029 V/A, and the intercept is increased from 21 to 23V. These effects are shown in Fig. 6. However, the change in the arc length from 5 to 10mm had no effect on the linearity of either relationship.
Fig.6. Effect of arc length on the basic relationships using 1.2mm diameter mild steel wire in Ar + 5%CO 2: a) Burnoff relationships
b) Voltage against current
b) Electrode extension: The effect of electrode extension from 10 to 15 to 20mm on the basic relationships, at 5mm arc length, is shown in Fig. 7. The two pairs relevant to 15 and 20mm electrode extensions can be expressed by the following equations. For 15mm electrode extension:
I = 24.0W + 80 . . . [3a]
V = 0.028I + 21 . . . [3b]
And for 20mm electrode extension:
I = 20.0W + 80 . . . [4a]
V = 0.029I + 21.5 . . . [4b]
Fig.7 Effect of electrode extension on the basic relationships using 1.2mm diameter mild steel wire in Ar + 5%CO 2: a) Burnoff relationships
b) Voltage against current
The increase it electrode extension from 10 to 15 to 20mm decreased the slope of the burnoff relationship from 26.4 to 24.0 to 20.0 A/m/min, respectively, whereas the intercept remained constant at 80A.
The voltage against current relationships for 15 and 20mm electrode extensions ([3b] and [4b]) are practically identical to those for 10mm electrode extension ([1b]). Hence the change in electrode extension from 10 to 20mm has no significant effect on the voltage against current relationship.
Furthermore, both the relationships for 15 and 20mm electrode extensions are linear, like those for 10mm electrode extension. Therefore, the change in extension from 10 to 20mm has no effect on the linearity of the basic relationships.
c) Shielding gas: For arc operation in Ar + 20%CO 2 shielding gas, at 5mm arc length and 10mm electrode extension, the basic relationships obtained are plotted in Fig. 8, together with those previously obtained for Ar + 5%CO 2. These can be expressed by the following equations:
I = 24.0W + 100 . . . [5a]
V = 0.030I + 24.5 . . . [5b]
Fig.8. Effect of shielding gas on the basic relationships at 5mm visible arc length and 10mm electrode extension using 1.2mm diameter mild steel wire: a) Burnoff relationships
b) Voltage against current
As both the equations are linear, like those for Ar + 5%CO 2, a change of CO 2 content in Ar from 5 to 20% has no effect on the linearity of the basic relationships.
d) Wire diameter: The basic relationships relevant to 1mm diameter wire, at 5mm arc length and 10mm electrode extension in argon + 5%CO 2 can be represented by the following equations:
I = 18.4W + 50 . . . [6a]
V = 0.031I + 22 . . . [6b]
The decrease in wire diameter from 1.2 to 1mm decreased the slope of the burnoff relationship from 26.4 to 18.4 A/m/min, and the intercept from 80 to 50A. For the voltage against current relationship, the slope increased from 0.028 to 0.031 V/A, and the intercept from 21 to 22V. These effects are shown in Fig. 9. The relationships for 1mm diameter wire are linear, like those for 1.2mm diameter.
Fig.9. Effect of wire diameter on the basic relationships at 5mm visible arc length and 10mm electrode extension for mild steel wire in Ar + 5%CO 2: a) Burnoff relationships
b) Voltage against current
Generalised basic relationships
As established above, the specific burnoff and voltage against current relationships were found to be linear over the 5-10mm range of arc lengths, 10-20mm range of electrode extensions, 5-20% range of CO
2 content in the Ar shielding gas and 1-1.2mm range of wire diameters. Therefore, the generalised burnoff relationship over the operating ranges of the parameters can be represented by the following linear equation:
I = m iW + C i . . . [7a]
Where
I is the current, A
W is the wire feed rate, m/min
m i is the slope, A/m/min
C i is the intercept, A
The equation has W as an independent variable representing the required wire feed rate, and I is the corresponding dependent variable. The equation is specified by the slope m i and the intercept C i, for the operation with any required wire material/diameter/shielding gas combination, at a given arc length and electrode extension.
Similarly, the generalised voltage against current relationship can be represented by the following linear equation:
V = m vI + C v . . . [7b]
Where
V is the voltage, V
I is the current, A
m v is the slope, V/A
C v is the intercept, V
The equation has I as independent variable for the required current, and V as the corresponding dependent variable. The equation is specified by the slope in, and the intercept C v for any required wire material/diameter/shielding gas combination, together with a specified arc length and electrode extension.
Short circuiting operation
The burnoff relationships for four wire diameter/shielding gas combinations, comprising 1 and 1.2mm diameters and Ar + 20%CO 2, and CO 2 shielding gases for each diameter with an electrode extension of 12mm, are plotted in Fig. 10. For the operating wire feed rate range of about 2-7 m/min, the corresponding current range is specific for each combination. All the relationships are similar in shape; however, they are not linear but curved.
Fig.10. Burnoff relationships for short circuiting arc using 1 and 1.2mm diameter mild steel wires and CO 2 and Ar + 20%CO 2 shielding, with 12mm electrode extension
For the four combinations, the voltage against current relationships are shown in Fig.11 for the optimum arc operation where the voltage was tuned to obtain regular short circuits and consistent metal transfer. However, each relationship has a working tolerance up to about 2V above and below the voltage for the optimum operation shown, see for example Fig. 12. A lower voltage setting causes progressively prolonged short circuits which result in stubbing arc operation, whereas too high a voltage causes excessive arcing periods and results in an arc operation with inadequate short circuiting frequency and globular metal transfer.
Fig.11. Voltage current relationship for short circuiting arc using 1 and 1.2mm diameter mild steel wires with CO 2 and Ar + 20%CO 2 shielding, and 12mm electrode extension
Fig.12. Voltage range for short circuiting arc using 1.2mm diameter mild steel wire and CO 2 shielding
For the optimum operation, all the relationships are linear and can be expressed by the following equations:
V av = 0.032I av + 16.4 for 1mm diameter/CO 2v combination;
V av = 0.028I av + 16.2 for 1.2mm diameter/CO 2 combination;
V av = 0.027I av + 14.6 for 1mm diameter/Ar + 20%CO 2 combination;
V av = 0.026I av + 14.4 for 1.2mm diameter/Ar + 20%CO 2 combination.
As all the relationships are linear, the generalised voltage against current relationship can be represented by the following linear equation:
V av = M vI av + C v . . . [8]
Where
V av is the average voltage, V
I av is the average current, A
m v is the slope, V/A
C v is the intercept, V
The equation has I av as an independent variable for the required current, and V av as the corresponding dependent variable. The equation is specified by the slope m v and the intercept C v, for operation with any required wire material/ diameter/shielding gas combination, at a specified electrode extension.
Discussion
Steady DC open arc operation has been characterised entirely by the two generalised linear equations. Each equation includes two operating variables, and the slope and intercept to be specified for a given wire material/diameter/shielding gas combination together with the required arc length and electrode extension. These are not valid for the current range below the 'critical' level which is unusable for open arc operation, because the relevant metal transfer is globular and unsuitable for welding. Above this level, where the metal transfer comprises a stream of small droplets or 'spray', the two equations together could specify any required operating point, coordinating wire feed rate, current and voltage, constituting a stable welding condition.
Effects of process parameters
To adapt the generalised equations to a given combination of process parameters, such as wire material, diameter, shielding gas, arc length and electrode extension, the relevant values of the parametric constants need to be specified. Therefore we consider effects of the process parameters on slope and intercept.
Burnoff relationship
In general a change in any of the process parameters alters the slope and intercept of the burnoff relationship. The degree of change has been found to be greater than the experimental error (up to about 5%) for the change from 5 to 10mm arc length, 10 to 20mm electrode extension, 5 to 20%CO 2 in Ar shielding gas or 1 to 1.2mm wire diameter. If two or more parameters are to be changed together, then the effect on the slope and intercept may become annulled or accentuated.
Therefore, to specify the burnoff relationship for a given combination of process parameters, the slope and intercept must be determined. As the relationship is linear, these parameters could easily be derived from two data points, requiring only two arc tests.
Voltage against current relationship
i) Effect on the slope m v: For open arc operation, the values of the slope m v are given in Table 1, for various parametric combinations comprising mild steel wire of 1 and 1.2mm diameter, Ar + 5%CO 2 and Ar + 20%CO 2 shielding gas, 5 and 10mm arc length and 10 and 20mm electrode extension. Over the entire range of combinations, m v varies between 0.025 V/A and 0.031 V/A. As the difference of about ±12% from the maximum and minimum values is greater than the normally acceptable experimental error of ±5%, the overall variation in m v appears to be significant. However, m v is included in the minor term m vI of the V=m vI + C v relationship. The contributions of m vI to the total voltage, obtained by evaluating the equation over the current range 100-500A, are given in Table 2. The variation in m v by ± 12% causes variation in the total voltage of only about ± 3% at 300A, which is significant. Therefore, the slope m v could be considered constant at 0.0284 V/A for all the combinations of the process parameters over the operating ranges.
Table 1: Values of parametric constant m v, using mild steel wire, 1 and 1.2mm diameter, Ar + 5%CO 2 shielding gas, 5 to 20mm electrode extension, and 5 and 10mm arc
| Shielding gas | Contact tube to workpiece distance, mm | Electrode extension, mm | Arc length, mm | Parametric constant, m v, V/A |
| 1.2mm diameter |
| Ar + 5%CO 2 | 15 | 10 | 5 | 0.028 |
| Ar + 5%CO 2 | 15 | 5 | 10 | 0.029 |
| Ar + 5%CO 2 | 20 | 15 | 5 | 0.028 |
| Ar + 5%CO 2 | 25 | 20 | 5 | 0.025 |
| Ar + 20%CO 2 | 15 | 10 | 5 | 0.030 |
| Ar + 20%CO 2 | 15 | 5 | 10 | 0.025 |
| 1mm diameter |
| Ar + 5%CO 2 | 15 | 10 | 5 | 0.031 |
| Ar + 5%CO 2 | 15 | 5 | 10 | 0.030 |
| Ar + 20%CO 2 | 15 | 10 | 5 | 0.030 |
| Ar + 20%CO 2 | 15 | 5 | 10 | 0.028 |
Table 2: Typical contributions made by the minor (m vI) and major (C v) terms * to the total voltage given by the voltage against current equation, V = m vI + C v, for the current range 100-500A
| I,A | m vI,V | Total voltage, V |
| 100 | 2.84 | 27.84 |
| 200 | 5.68 | 30.68 |
| 300 | 8.52 | 33.52 |
| 400 | 11.36 | 36.36 |
| 500 | 14.2 | 39.20 |
ii) Effect on the intercept C v: The values of C v for various combinations of the process parameters are compared in Table 3 for 1 and 1.2mm wire diameters, in Table 4 for 10, 15 and 20mm electrode extensions, in Table 5 for Ar + 5%CO 2 and Ar + 20%CO 2 shielding gases, and in Table 6 for 5 and 10mm arc lengths. It can be seen that no adjustment would be required in the intercept C v for variable wire diameter (table 3) and electrode extension (Table 4), but the value of C v must be adjusted for a change in the shielding gas (Table 5) and arc length (Table 6).
Table 3: Effect of wire diameter on C v for various shielding gas/arc length combinations, using 15mm contact tube to workpiece distance
| Shielding gas | Arc length, mm | Electrode extension, mm | C v,V | % variation relative to mean |
| 1.2mm diameter | 1mm diameter | Mean |
| Ar +5%CO 2 | 5 | 10 | 21.0 | 22.0 | 21.50 | ±2.3 |
| Ar +5%CO 2 | 10 | 5 | 23.0 | 23.0 | 23.00 | ±0.0 |
| Ar +20%CO 2 | 5 | 10 | 24.5 | 25.0 | 24.75 | ±1.0 |
| Ar +20%CO 2 | 10 | 5 | 30.0 | 30.5 | 30.25 | ±0.8 |
Table 4: Effect of electrode extension on C v for constant arc length using 1.2mm diameter mild steel wire
| Contact tube to workpiece distance, mm | Visible arc length, mm | Electrode extension, mm | C v ,V | % variation relative to mean |
| Specific | Mean |
| Ar + 5%CO 2 |
| 15 | 5 | 10 | 21.0 | | |
| 20 | 5 | 15 | 21.0 | 21.2 | ±1.4 |
| 25 | 5 | 20 | 21.5 | | |
| Ar + 20%CO 2 |
| 15 | 5 | 10 | 24.5 | | |
| 20 | 5 | 15 | 25.5 | 25.0 | ±2.0 |
| 25> | 5 | 20 | 25.0 | | |
Table 5: Effect of shielding gas on C v for various wire diameter/arc length combinations, using 15mm contact tube to workpiece distance
| Wire diameter, mm | Arc length, mm | Electrode extension, mm | C v ,V | % variation relative to mean |
| Ar + 5%CO 2 | Ar + 20%CO 2 | Mean |
| 1.2 | 5 | 10 | 21 | 24.5 | 22.75 | ±8 |
| 1.2 | 10 | 5 | 23 | 30.0 | 26.50 | ±13 |
| 1.0 | 5 | 10 | 22 | 25.0 | 23.50 | ±6.4 |
| 1.0 | 10 | 5 | 23 | 30.5 | 26.75 | ±14 |
Table 6: Effect of arc length on C v for various wire diameter/shielding gas combinations, using 15mm contact tube to workpiece distance
| Wire diameter, mm | Shielding gas | Contact tube to workpiece distance, mm | C vV | % variation relative to mean |
| 5mm visible arc length | 10mm visible arc length | Mean |
| 1.2 | Ar + 5%CO 2 | 15 | 21.0 | 23.0 | 22.0 | ±5 |
| 1.2 | Ar + 20%CO 2 | 15 | 24.5 | 30.0 | 27.25 | ±10 |
| 1.0 | Ar + 5%CO 2 | 15 | 22.0 | 23.0 | 22.5 | ±2.2 |
| 1.0 | Ar + 20%CO 2 | 15 | 25.0 | 30.5 | 27.75 | ±10 |
The generalised parametric relationships described have formed a basis for developing synergic control systems which will be described in further articles.
Acknowledgements
Useful discussion with Drs R C Crafer, G R Salter and W Lucas at The Welding Institute and Dr. R A Jarman at the City of London Polytechnic are gratefully acknowledged.
The work was funded jointly by Research Members of The Welding Institute and the Minerals and Metals Division of the UK Department of Trade and Industry.
References
| 1 | Amin M and Watkins P V C | 'Synergic pulse MIG welding'. Welding Institute Members Report 46/1977/P, August 1977. | |
| 2 | Amin | 'Synergic pulse MIG welding'. Metal construction 1981 13 (6) 349-353. | |
| 3 | Amin M | 'Prediction of square wave pulse current parameters for control of metal transfer in MIG welding'. Welding Institute Members Report 83/1978/P, December 1978. | Return to text |
| 4 | Amin M | 'Microcomputer control of synergic pulsed MIG welding'. Welding Institute Members Report 166/1981, December 1981 | Return to text |
| 5 | Amin M | 'Microcomputer control of synergic pulsed MIG welding'. Welding Institute Research Bulletin 1981 22(10) 271-275. | |
| 6 | Amin M | 'Microcomputer control of synergic pulsed MIG welding'. Metal Construction 1986 18 (4) 216-221. | |
| 7 | Middle J E | 'Fabrication methods-up-to-date'. Agricultural Engineer 1984 Summer 43-52. | Return to text |
| 8 | Amin M | 'Pulse current parameters for arc stability and controlled metal transfer in arc welding'. Metal Construction 1983 15 (5) 272-278. | Return to text |
| 9 | Amin M and Naseer-Ahmed | 'Synergic control in steady DC open arc and short circuiting arc MIG welding'. Welding Institute Members Report 285/1985, September 1985. | Return to text |
| 10 | Brown K W | 'Programmed MIG welding'. Metal Construction 1969 1 (6) 286-290. | Return to text |
| 11 | Watkins P V C | 'The transistor controlled DC welding power source'. Welding Institute Members Report 35/10/75, October 1975. | Return to text |
