Fractional Factorial Technique to Predict Welding Current in Gas Metal
Arc Welding

# 

# Masood Aghakhani 

**Abstract**

During research on Gas Metal Arc Welding (GMAW) of aluminum, it was
observed that the welding current was affected by any change in wire
feed rate, arc voltage, nozzle-to-plate distance, torch angle, and
welding speed. To determine accurately the welding current for any set
of these parameters, a mathematical model has been presented. The model
was developed using a two-level fractional factorial design and its
adequacy was tested by the analysis of variance technique. The estimated
and observed values of the welding current have been shown on a scatter
diagram and the interaction effects of different parameters in­volved
have been presented in graphical forms. These results can be utilized
for determining accurately the heat input into the workpiece from which
reliable predictions can be made about the mechanical and the
metallurgical properties of welded joints.

*Keywords*: *Fractional Factorial Technique*; *Welding Current; Gas
Metal Arc Welding; and Aluminum.*

**Introduction**

Gas Metal Arc (GMA) also known as Metal Inert Gas (MIG) is the most
often used welding process for welding aluminum and it's alloys (Ref.
1). In MIG welding, the wire electrode used is thin (0.5-1.6 mm
diameter) and consequently the wire feed speed is high. To achieve
self-adjustment of arc at high-speed rates, it is essential to use power
sources with flat or nearly flat V-I (volt-ampere) characteristics. For
welding with such a power source, the wire feed rate has been given by
Halmoy (Ref. 2) by the following equation:

  -----------------------------------------------------------------------
  ![](media/image1.wmf)

  -----------------------------------------------------------------------

Where

  ----- --------------------- --------------------------------------------
  W     = wire feed rate      

  I     = welding current     

  l     = electrode stick-out 

  a     = area of cross       
        section of the wire   
  ----- --------------------- --------------------------------------------

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Masood Aghakhani, Assistant Professor in Production Engineering (Welding
Technology), College of Engineering, University of Razi,
Kermanshah-67149, Iran.

E-Mail: <aghakhani@razi.ac.ir> & <aghakhanim@yahoo.com>

  ----- -----------------------------------------------------------------
  α     = constant of proportionality for anode or cathode heating

  β     = constant of proportionality for electrical resistance heating
  ----- -----------------------------------------------------------------

It is reported that for aluminum α = 0.75 mm/A-sec and β is negligible
(Ref. 3), thus reducing Halmoy\'s equation for GMAW of aluminum to:

  -----------------------------------------------------------------------
  ![](media/image2.wmf)

  -----------------------------------------------------------------------

or, in other words, the welding current (I) is directly proportional to
the wire feed rate (W). However, during prolonged investigations on GMAW
of aluminum, it was observed that the welding current (I) was not only
affected by the changes made in wire feed rate (W), but arc voltage (V),
nozzle-to-plate distance (N), torch angle (θ), and welding speed (S)
also affected the welding current accordingly. Welding current affects
the metal transfer (pinch effect α $I^{2}$) and the amount of heat input
into the weld pool and consequently it affects the bead shape as well as
the metallurgical and mechanical properties. To avoid any serious
effects on the analysis of results of erroneous assumption that the
welding current is only proportional directly to wire feed rate, it was
found imperative to determine accurately the welding current as affected
by different welding parameters.

This paper reports the development of a mathematical model, based on
practical observations, made during gas metal arc welding of aluminum to
estimate accurately the welding current as affected by welding
parameters.

**Plan of Investigation**

To achieve the desired aim, the planning for investigations were carried
out in the following steps:

  ------ ------------------------------------------------------------------
  1\.    Selection of a mathematical model;

  2\.    Design of experiments;

  3\.    Selection of useful limits of the welding parameters viz.; wire
         feed rate (W), arc voltage (V), nozzle-to-plate distance (N),
         torch angle (θ) and welding speed (S);

  4\.    Developing a design matrix;

  5\.    Conducting experiments as per designed matrix;

  6\.    Estimation of the coefficients of the equation;

  8\.    Checking the adequacy of the model;

  9\.    Testing the significance of the regression coefficients and
         arriving at the final form of the mathematical model;

  10\.   Presenting the estimated and observed data on a scatter diagram;

  11\.   Presenting the significant interactions between different
         parameters in graphical form;

  12\.   Analysis of results; and

  13\.   Conclusions.
  ------ ------------------------------------------------------------------

**Selection of the Mathematical Model**

It was observed from the preliminary experiments that the welding
current (*I*) was affected by wire feed rate, arc voltage,
nozzle-to-plate distance, torch angle, and welding speed within the
limits employed for the study. Under these conditions, the response
function could be expressed as

$$I = \ f\ (W,\ V,\ N,\ \theta,\ S)$$

Assuming a linear relationship and taking into account all possible
two-factor interactions, it could be written as:

  -----------------------------------------------------------------------------------------------------------------------------------------------------------
  $$\begin{array}{r}
  I = b_{0} + b_{1}W + b_{2}V + b_{3}N + b_{4}\theta + b_{5}S + b_{6}\text{WV} + b_{7}\text{WN} + b_{8}W\theta + b_{9}\text{WS} + b_{\text{10}}\text{VN} \\
   + b_{\text{11}}V\theta + b_{\text{12}}\text{VS} + b_{\text{13}}N\theta + b_{\text{14}}\text{NS} + b_{\text{15}}\theta S
  \end{array}$$

  -----------------------------------------------------------------------------------------------------------------------------------------------------------

Where *I* is the estimated welding current and
$b_{1},\ b_{2},\text{.}\text{.}\text{.}\text{.}\text{.}\text{.},b_{\text{15}}$
are the coefficients of the polynomial to be determined.

**Design of Experiments**

The design of experiment, that is, the decision to use different values
of different parameters for conducting a particular experiment was based
on the fractional factorial design, which is a standard statistical tool
to investigate the effects of a number of parameters on the required
response. In addition, interactions between two or more parameters can
be evaluated, which is not possible with the conventional experimental
approach since in that approach all parameters other than being studied
are held constant.

For calculating the main and interaction effects of five variables W, V,
N, θ, S at two levels, a half fractional factorial design was selected.
This design gave ![](media/image3.wmf){width="0.6111111111111112in"
height="0.18055555555555555in"} weld runs to fit an equation and was
sufficient for the evaluation of coefficients.

**Selection of Useful Limits of Welding Parameters**

On the basis of preliminary experiments conducted using 1.6 mm diameter
wire with argon shielding, it was found that for proper bead
configurations the wire feed rate, arc voltage, nozzle-to-plate
distance, torch angle, and welding speed were required to be kept
between 6.1 m/min and 7.6 m/min, 24 V to 29 V, 15 mm to 20 mm, 80 deg to
100 deg and 25 cm/min to 40 cm/min respectively. These limits were
adhered to throughout the investiga­tions reported here.

For the ease of recording and processing of the observed data, upper and
lower levels of welding para­meters were coded as + 1 and -1,
respectively by using the following relationship:

$$\text{Coded}\ \text{Value}\  = \ \frac{\text{Natural}\ \text{Value}\  - \ \text{Average}\ \text{Value}}{\text{Variation}\ \text{Interval}}$$

Two levels of welding parameters, their levels, units, and coded values
are given in Table 1.

**Table 1-**Welding parameters and their limits.

  ------------------ ----------- ----------- -------------- ---------------
  Parameter          Unit        Symbol      Coded Value    Coded Value

                                             Low (-1)       High (+1)

                                                            

  Wire Feed Rate     m/min       W           6.1            7.6

  Arc Voltage        volts       V           24.0           29.0

  Nozzle-to-Plate    mm          N           15.0           20.0
  Distance                                                  

  Torch Angle        deg         θ           80.0           100.0

  Welding Speed      cm/min      S           25.0           40.0
  ------------------ ----------- ----------- -------------- ---------------

**Design Matrix**

The design matrix evolved to conduct sixteen experiments is given in
Table 2 and is based on the method suggested elsewhere (Refs. 4, 5).

**Table 2-**$2^{5 - 1}$Design Matrix

<table style="width:100%;">
<colgroup>
<col style="width: 8%" />
<col style="width: 0%" />
<col style="width: 18%" />
<col style="width: 0%" />
<col style="width: 17%" />
<col style="width: 0%" />
<col style="width: 18%" />
<col style="width: 0%" />
<col style="width: 18%" />
<col style="width: 0%" />
<col style="width: 16%" />
</colgroup>
<tbody>
<tr class="odd">
<td>S. #</td>
<td colspan="2">W</td>
<td colspan="2">V</td>
<td colspan="2">N</td>
<td colspan="2">θ</td>
<td colspan="2"><p>S</p>
<p>(-WVNθ)</p></td>
</tr>
<tr class="even">
<td colspan="2">1</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td>-</td>
</tr>
<tr class="odd">
<td colspan="2">2</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td>+</td>
</tr>
<tr class="even">
<td colspan="2">3</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td>+</td>
</tr>
<tr class="odd">
<td colspan="2">4</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td>-</td>
</tr>
<tr class="even">
<td colspan="2">5</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td>+</td>
</tr>
<tr class="odd">
<td colspan="2">6</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td>-</td>
</tr>
<tr class="even">
<td colspan="2">7</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td>-</td>
</tr>
<tr class="odd">
<td colspan="2">8</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td>+</td>
</tr>
<tr class="even">
<td colspan="2">9</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td>+</td>
</tr>
<tr class="odd">
<td colspan="2">10</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td>-</td>
</tr>
<tr class="even">
<td colspan="2">11</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td>-</td>
</tr>
<tr class="odd">
<td colspan="2">12</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td>+</td>
</tr>
<tr class="even">
<td colspan="2">13</td>
<td colspan="2">+</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td>-</td>
</tr>
<tr class="odd">
<td colspan="2">14</td>
<td colspan="2">-</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td>+</td>
</tr>
<tr class="even">
<td colspan="2">15</td>
<td colspan="2">+</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td>+</td>
</tr>
<tr class="odd">
<td colspan="2">16</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td colspan="2">-</td>
<td>-</td>
</tr>
</tbody>
</table>

**Experimentation**

Employing a mechanized welding system, experiments were conducted as per
the conditions set in the design matrix of Table 2. Welding current
observations were made by making weld runs of 25 cm length on 13 mm
thick plates of aluminum. The power source used was a three-phase
transformer cum full-wave rectifier unit having a flat V-I (volt-ampere)
characteristics with a current capacity of 425 A at 60% duty cycle and
an infinitely variable OCV (open circuit voltage) of 12-48 V in two
ranges. A 1.6 mm diameter NG-6 filler wire (Al-5%Mg) with argon as
shielding gas at a flow rate of 30 l/mm was used.

Sixteen weld runs were made using the conditions set in the design
matrix but to avoid any systematic error in experimentation, the sets of
conditions were chosen at random. The recorded values of welding current
(*I*) for two sets of experiments are given in Table 3.

**Table 3-**Recorded values of welding current

+---+-----+----+----+-----+----+------+----------------------------+
| S | Tr  | W  | V  | N   | θ  | S    | Welding Current            |
| . | ial |    |    |     |    |      |                            |
| # | No. |    |    |     |    |      | \-\-\-\-\-\-\-\-\-\-\      |
|   |     |    |    |     |    |      | -\-\-\-\-\-\-\-\-\-\-\-\-- |
|   |     |    |    |     |    |      |                            |
|   |     |    |    |     |    |      | I~1~ I~2~                  |
+---+-----+----+----+-----+----+------+----------------------------+
| 1 | 1   | \+ | \+ | \+  | \+ | \-   | 260 270                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 2 | 2   | \- | \+ | \+  | \+ | \+   | 240 240                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 3 | 3   | \+ | \- | \+  | \+ | \+   | 260 260                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 4 | 4   | \- | \- | \+  | \+ | \-   | 220 220                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 5 | 9   | \+ | \+ | \+  | \- | \+   | 280 290                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 6 | 10  | \- | \+ | \+  | \- | \-   | 245 245                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 7 | 11  | \+ | \- | \+  | \- | \-   | 270 270                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 8 | 12  | \- | \- | \+  | \- | \+   | 240 240                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 9 | 13  | \+ | \+ | \-  | \- | \-   | 290 290                    |
+---+-----+----+----+-----+----+------+----------------------------+
| 1 | 14  | \- | \+ | \-  | \- | \+   | 240 240                    |
| 0 |     |    |    |     |    |      |                            |
+---+-----+----+----+-----+----+------+----------------------------+
| 1 | 15  | \+ | \- | \-  | \- | \+   | 280 280                    |
| 1 |     |    |    |     |    |      |                            |
+---+-----+----+----+-----+----+------+----------------------------+
| 1 | 16  | \- | \- | \-  | \- | \-   | 240 240                    |
| 2 |     |    |    |     |    |      |                            |
+---+-----+----+----+-----+----+------+----------------------------+
| 1 | 5   | \+ | \+ | \-  | \+ | \+   | 290 290                    |
| 3 |     |    |    |     |    |      |                            |
+---+-----+----+----+-----+----+------+----------------------------+
| 1 | 6   | \- | \+ | \-  | \+ | \-   | 240 250                    |
| 4 |     |    |    |     |    |      |                            |
+---+-----+----+----+-----+----+------+----------------------------+
| 1 | 7   | \+ | \- | \-  | \+ | \-   | 260 260                    |
| 5 |     |    |    |     |    |      |                            |
+---+-----+----+----+-----+----+------+----------------------------+
| 1 | 8   | \- | \- | \-  | \+ | \+   | 245 245                    |
| 6 |     |    |    |     |    |      |                            |
+---+-----+----+----+-----+----+------+----------------------------+

**Evaluation of Coefficients of the Model**

To determine the regression coefficients of the selected model,
following formula (Ref. 4, 5) based on the method of least squares was
used:

  -----------------------------------------------------------------------
  ![](media/image4.wmf){width="2.013888888888889in"
  height="0.6527777777777778in"}

  -----------------------------------------------------------------------

where:

  ----- -----------------------------------------------------------------
  Xji   = value of a factor or interaction in coded form.

  Yi    = average value of the response parameter, that is, the welding
        current in this case

  N     = number of the observations.

  k     = number of the coefficients of the model.
  ----- -----------------------------------------------------------------

A matrix designed to apply this formula to calculate different
coefficients is given in Table 4.

**Table 4-**Design matrix to calculate different coefficients

  ----- ------- ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------- ------- ------- ------- ------- ------- --------
  S.#   Trial   b~0~   b~1~   b~2~   b~3~   b~4~   b~5~   b~6~   b~7~   b~8~   b~9~   b~10~   b~11~   b~12~   b~13~   b~14~   b~15~   Y~avg~
        No.                                                                                                                           

  1     1       \+     \+     \+     \+     \+     \-     \+     \+     \+     \-     \+      \+      \-      \+      \-      \-      265

  2     2       \+     \-     \+     \+     \+     \+     \-     \-     \-     \-     \+      \+      \+      \+      \+      \+      240

  3     3       \+     \+     \-     \+     \+     \+     \-     \+     \+     \+     \-      \-      \-      \+      \+      \+      260

  4     4       \+     \-     \-     \+     \+     \-     \+     \-     \-     \+     \-      \-      \+      \+      \-      \-      220

  5     9       \+     \+     \+     \+     \-     \+     \+     \+     \-     \+     \+      \-      \+      \-      \+      \-      285

  6     10      \+     \-     \+     \+     \-     \-     \-     \-     \+     \+     \+      \-      \-      \-      \-      \+      245

  7     11      \+     \+     \-     \+     \-     \-     \-     \+     \-     \-     \-      \+      \+      \-      \-      \-      270

  8     12      \+     \-     \-     \+     \-     \+     \+     \-     \+     \-     \-      \+      \-      \-      \+      \-      240

  9     13      \+     \+     \+     \-     \-     \-     \+     \-     \-     \-     \-      \-      \-      \+      \+      \+      290

  10    14      \+     \-     \+     \-     \-     \+     \-     \+     \+     \-     \-      \-      \+      \+      \-      \-      240

  11    15      \+     \+     \-     \-     \-     \+     \-     \-     \-     \+     \+      \+      \-      \+      \-      \-      280

  12    16      \+     \-     \-     \-     \-     \-     \+     \+     \+     \+     \+      \+      \+      \+      \+      \+      240

  13    5       \+     \+     \+     \-     \+     \+     \+     \-     \+     \+     \-      \+      \+      \-      \-      \+      290

  14    6       \+     \-     \+     \-     \+     \-     \-     \+     \-     \+     \-      \+      \-      \-      \+      \-      245

  15    7       \+     \+     \-     \-     \+     \-     \-     \-     \+     \-     \+      \-      \+      \-      \+      \-      260

  16    8       \+     \-     \-     \-     \+     \+     \+     \+     \-     \-     \+      \-      \-      \-      \-      \+      245
  ----- ------- ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------- ------- ------- ------- ------- ------- --------

The estimated coefficients of the variables and their interactions
computed using the above relationship are given in Table 5.

**Table 5-**Coefficients of the model

Using these coefficients, the complete model, in coded form, can be
expressed as follows:

  -----------------------------------------------------------------------
  ![](media/image5.wmf)

  -----------------------------------------------------------------------

### Checking the Adequacy of the Model

The adequacy of the model was determined by the analysis of variance
technique. The regression coefficients were determined by the method of
least squares, from which the F-ratio for the polynomial was found. The
F-ratio of the model was compared with the corresponding F-ratio from
the standard tables and it was found that the model was adequate within
95% level of confidence, thus justifying the use of assumed polynomial.
Details of analysis of variance are given in Table 6.

**Table 6-**Details of analysis of variance

+-----+------+-----+-------+------+----+-------+-------------------+
| Re  | Deg  | Va  | Sta   | Vari | \  | \`F\' | Adequacy          |
| spo | rees | ria | ndard | ance | `F |       |                   |
| nse |      | nce |       |      | \' | Ratio | of                |
|     | of   |     | Devi  | of   |    |       |                   |
|     |      | of  | ation |      | R  | Table | Model             |
|     | Fre  |     |       | Adeq | at |       |                   |
|     | edom | Re  | of    | uacy | io |       |                   |
|     |      | spo |       |      |    |       |                   |
|     | for  | nse | Co    |      | M  |       |                   |
|     |      |     | effic |      | od |       |                   |
|     |      |     | ients |      | el |       |                   |
+-----+------+-----+-------+------+----+-------+-------------------+
|     | !    | ![] | !     | ![   | ![ | at    | Whether           |
|     | [](m | (me | [](me | ](me | ]( |       |                   |
|     | edia | dia | dia/i | dia/ | me | (1    | ![](m             |
|     | /ima | /im | mage9 | imag | di | 0,16, | edia/image12.wmf) |
|     | ge6. | age | .wmf) | e10. | a/ | 0.05) |                   |
|     | wmf) | 8.w |       | wmf) | im |       |                   |
|     | !    | mf) |       |      | ag |       |                   |
|     | [](m |     |       |      | e1 |       |                   |
|     | edia |     |       |      | 1. |       |                   |
|     | /ima |     |       |      | wm |       |                   |
|     | ge7. |     |       |      | f) |       |                   |
|     | wmf) |     |       |      |    |       |                   |
+-----+------+-----+-------+------+----+-------+-------------------+
|     |      |     |       |      |    |       |                   |
+-----+------+-----+-------+------+----+-------+-------------------+
| W   | 16   | 9.  | 0.765 | 7.50 | 0. | 2.49  | Yes               |
| eld | 10   | 375 |       |      | 80 |       |                   |
| ing |      |     |       |      |    |       |                   |
| C   |      |     |       |      |    |       |                   |
| urr |      |     |       |      |    |       |                   |
| ent |      |     |       |      |    |       |                   |
+-----+------+-----+-------+------+----+-------+-------------------+

**Checking the Significance of the Coefficients of the Model**

To eliminate the statistical insignificant terms of the model, if any,
it is essential to check the significance of each of the coefficient and
to do that Student\'s \`t\'-test was employed. It was found that the
coefficients of all the main effects and WV, Wθ, Nθ, and θS interactions
were significant. The rest of the coefficients could be dropped
conveniently. Thus the final model with only the statistically
significant coefficients in the coded form is given as:

  ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  $$I = \text{257}\text{.}2 + \text{17}\text{.}8W + 5\text{.}3V - 4\text{.}0N - 4\text{.}0\theta + 2\text{.}8S + 2\text{.}2\text{WV} - 2\text{.}2W\theta - 2\text{.}8N\theta + 2\text{.}8\theta S$$

  ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

**Results**

A large number of observations were taken under different sets of
parameters within the limits investigated and the corresponding
estimated values of welding current were determined from the model. The
observed and the estimated values were then represented in a graphical
form as a scatter diagram shown in Fig. 1.

The significant interactions viz. WV, Wθ, Nθ and θS as deduced from the
model are shown in Figs. 2-5.

**Analysis of Results**

Fig. 1 shows that the estimated and the observed values of the welding
current are scattered close to 45° lines, passing though the origin,
indicating an almost perfect fit of the developed empirical model.
Furthermore, the analysis of results show that the increase in the
levels of W, V, and S resulted in the corresponding increase in welding
current whereas, the increase in the levels of N, and θ proved to be
otherwise.

**Effect of Wire Feed Rate (W)**

Wire feed rate is the most important parameter affecting the welding
current. An increase in welding current from 6.1 to 7.6 m/min resulted
in the increase in welding current by a magnitude of 34.36 amperes. This
increase in welding current with the increase in wire feed rate is to
keep the wire feed rate and burn off rate in equilibrium.

**Effect of Arc Voltage (V)**

The second largest increase in welding current is caused by arc voltage
by a magnitude of 10.6 amperes when it is varied from 24 to 29 volts.
The increase in welding current with the increase in arc voltage
indicates the rising characteristics of the welding arc.

**Effect of Nozzle-to-Plate Distance (N)**

The welding current decreased by approximately eight amperes with the
increase in nozzle-to-plate distance from 15 to 20 mm. This is due to
the fact that as the value of 'N' is increased, the arc length also
increases momentarily thereby increasing the arc voltage but the same is
restored to its original value due to the self-adjustment of the welding
arc. This increases the stick-out and hence the amount of I2R heating.
Increase in the preheating of the stick-out reduces the welding current
slightly so that the equilibrium between wire feed rate and the burn off
rate is maintained.

**Effect of Electrode/Torch Angle (θ)**

The increase in the torch angle from 80° to 100° resulted in the
decrease of welding current by approximately 8 amperes. The reason for
this could be explained due to the fact that as the electrode angle
increases the effective arc length increases, and hence a decrease in
welding current is observed.

**Effect of Welding Speed (S)**

The increase in welding speed from 25 to 40 cm/min resulted in the
increase in welding current by approximately 6 amperes. This could be
due to the fact that at low welding speed the arc is very close to the
deposited metal and since the melting point of the filler wire is quite
low, the current required to maintain the burn off rate equilibrium, is
less and when the welding speed is increased, the heat content of the
arc is reduced and hence to maintain the burn off rate equilibrium, the
welding current requirement increases.

**Effect of Interactions**

Fig. 2 shows that welding current increased with the increase in wire
feed rate. The increase in welding current was 6 and 12 amperes for low
and high values of wire feed rate when, arc voltage was varied from 24
to 29 volts. This shows that the interaction of W&V is highly
significant at higher levels.

Fig. 3 shows that welding current increased with the change in the torch
angle from 80° to 100°, backhand to forehand. The increase in the
welding current was just 3 amperes compared to 12 amperes for low and
high values of wire feed rate.

Fig. 4 shows that welding current decreased with the change in torch
angle from backhand to forehand position. Effect of torch angle on
welding current at low levels of wire feed rate is quite insignificant
compared to the variation of 14 amperes at the high level of
nozzle-to-plate distance.

Fig. 5 shows that welding current decreased with the increase in torch
angle from 80° to 100°. There is no effect of welding speed on the
welding current at low level of torch angle however; it became quite
significant at the high level of torch angle. The figure also shows that
there is hardly any effect of torch angle of 80° on welding current. The
welding current decreased by 12 amperes at 40 cm/min & forehand angle of
100°.

Conclusions

  ----- ------------------------------------------------------------------
  1\.   In GMAW, the welding current is affected not only by the wire feed
        rate but also by the arc voltage, the nozzle-to-plate distance,
        the electrode/torch angle and the welding speed.

  2\.   An empirical model was developed to estimate the welding current
        by correlating these parameters for GMAW of aluminum.

  3\.   At low wire feed rates, the effect of arc voltage on welding
        current was negligible. However, it increased enormously at higher
        feed rates.

  4\.   Increase in nozzle-to-plate distance resulted in the decrease in
        welding current possibly due to the increase in arc length.

  5\.   The increase in arc voltage resulted in the increase in welding
        current indicating a rising volt-ampere characteristic of the
        welding arc.

  6\.   The increase in electrode/torch angle resulted in the decrease in
        welding current.

  7\.   The increase in welding speed resulted in the increase in welding
        current.
  ----- ------------------------------------------------------------------

### Acknowledgement

The author wishes to thank the research vice-chancellor of the
University of Razi, for granting permission to publish this paper.

### References

  ----- --------------------------- ------------------------------------------------------------------
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  3\.                               Trindade, H. 1982. [M.Sc. thesis]{.underline}. School of
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  4\.                               Adler, Yu.P.; Markova, E.V.; and Granovsky, Yu.P. 1975. [The
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  ----- --------------------------- ------------------------------------------------------------------