15 Chapter 5

101

5.1 Introduction

The need to achieve higher productivity and stringent safety requirement have

put growing emphasis on the use of automated welding systems, submerged arc

welding is employed in semiautomatic or automatic mode in industry (Brien, 1978). In

such automated applications, a precise means of selection of the process variables and

control of weld bead shape has become essential because mechanical strength of weld

is influenced not only by the composition of the metal, but also by the weld bead shape

(Hould, 1989). The acceptable weld bead shape depends on factors such as line power

which is the heat energy supplied by an arc to the base plate per unit length of weld,

welding speed, joint preparation, etc. To do these precise relationships between the

process parameters and the bead parameters controlling the bead shape are to be

established. This may be achieved by the development of mathematical expressions,

which can be fed into a computer, relating the weld bead dimensions to the important

process control variables affecting these dimensions. Also, optimization of the process

parameters to control and obtain the required shape and quality of weld beads is

possible with these expressions. A macrophotograph of a real weld bead is shown in

Fig.5.1 (A). Cross section of an ideal weld bead showing the bead geometry is given in

Fig.5.1 (B).

CHAPTER – 5

EFFECT OF WELDING PARAMETERS ON BEAD

GEOMETRY AND FLUX CONSUMPTION

102

(a)

(b)

Fig.5.1 (a) Photograph of a real weld bead (b) Cross-section of an ideal weld bead

Where P: height of penetration (mm); R: height of reinforcement (mm); W: width of the bead

(mm); WPSF: penetration shape factor =W/P; WRFF: reinforcement form factor = W/R.

In the present study, an attempt has been made to investigate the effect of open

circuit voltage, welding current, welding speed and basicity index on bead geometry

and shape relationships (bead width, weld penetration and height of reinforcement,

weld penetration shape factor and weld reinforcement form factor), using developed

fluxes, through experiments based on design matrix. The analysis of variance

(ANOVA) technique has been adopted to check the level and degree of the direct or

interactive effect of welding current, voltage, welding speed and flux basicity index on

features of bead geometry and shape relationship. Response surface methodology has

been applied to derive mathematical models that correspond to the welding phenomena

using developed fluxes. Predictive equations have been used to represent graphically

103

the effects of process parameters on various responses. No work so far has been

performed which considers the four important process parameter used in this study

using fluxes developed from waste flux dust.

5.2 Operating Variables

Control of the operating variables in submerged arc welding is essential if high

production rates and the welds of good quality are to be obtained. The following are

the important variables:

(i) Welding amperage

(ii) Welding voltage

(iii) Welding speed

(iv) Electrode size

(v) Electrode work angle

(vi) Electrode stick-out

(vii) Depth of flux

(viii) Polarity

(ix) Melting rate

(x) Flux basicity index

5.2.1 Welding amperage

Welding current is the most influential parameter because it affects bead shape,

controls the rate at which electrode is melted and therefore also controls the deposition

104

rate, heat affected zone, the depth of penetration, and the amount of base metal melted.

Penetration and reinforcement increase with the increase in welding current.

If the current is too high at a given welding speed, the depth of fusion or

penetration will also be too high so that the resulting weld may tend to melt through

the metal being joined. High current also leads to waste of electrodes in the form of

excessive reinforcement and produces digging arc and undercut. This overwelding

increases weld shrinkage and causes greater distortion. Bead width increases with

welding current until a critical value is reached and then starts decreasing if the

polarity used is DCEP. When DCEN polarity is employed bead width increases with

the increase in current for entire range (McGlone, 1982). For the same flux, heat

affected zone also increases with the increase in welding current (Kaushal and Gupta,

1988). If the current is too low, inadequate penetration or incomplete fusion may

result. Too low current also leads to unstable arc, inadequate penetration and

overlapping.

5.2.2 Welding voltage

Welding voltage varies with the length of the arc between the electrode and

molten weld metal. With the increase in arc length, the arc voltage increases because

lengthening of the arc exposes more of the arc column to the cool boundary of the arc.

Also, the arc column continuously loses the charge carriers by radial migration to the

cool boundary of the arc and therefore, imposing a greater requirement of potential for

105

maintaining appropriate charge carriers between the electrode and weld plate

(Weiman, 1981).

The voltage principally determines the shape of the weld bead cross section and

its external appearance. Increasing the welding voltage with constant current and

welding speed produces flatter, wider, less penetrated weld beads and tends to reduce

the porosity caused by rust or scale on steel. Higher voltage also bridges an excessive

root opening when fit-up is poor. Increase in arc voltage also increases the size of

droplets and hence decreases the number of droplets. The time of the movement of

droplet transfer also increases. Further increase in voltage increases the possibility of

breaking the arc and disrupting the normal welding process. Increase in voltage also

enhances flux consumption which increases pick up or loss of the alloying elements

and therefore affects the mechanical and metallurgical properties of the weld metal

(Gupta and Gupta, 1988; Pandey and Mohan, 2003).

Excessively high voltage produces a wide bead shape that is subject to

cracking, increases undercut and creates difficulty in removing slag. Lowering the

voltage produces stiffer arc, which improves penetration in a deep weld groove and

resists arc blow. An excessively low voltage produces a narrow bead and causes

difficult slag removal along the bead edges.

5.2.3 Welding speed

Welding speed is the linear rate at which an arc is moved along the weld joint.

With any combination of welding voltage and welding current, the effect of changing

106

the welding speed confirms to a general pattern. If the welding speed is increased,

power or heat input per unit length of weld is decreased and less filler metal is applied

per unit length of the weld, resulting in less weld reinforcement. Thus, the weld bead

becomes smaller.

Weld penetration is affected more by welding speed than any variable other

than current. This is true except for excessively slow speeds when the molten weld

pool is beneath the welding electrode. Then the penetrating force of the arc is

cushioned by the molten pool. Excessive speed any cause undercutting, porosity, arc

blow, uneven bead shape, cracking and higher slag inclusion in the weld metal. Higher

welding speed results in less heat affected zone and finer grains (Aksoy et al.1999).

Within limits, welding speed can be adjusted to control weld size and

penetration. Relatively slow welding speed provides time for gases to escape from the

molten metal, thus reducing porosity. An excessive slow speed produces a convex

bead shape which is subject to cracking and excessive arc exposure which is

uncomfortable for the operator. Too low welding speed may also result in a large

molten pool that flows around the arc, resulting in rough bead, slag inclusions and burn

through of the weld plate. Jackson and Shrubsa (1953) reported that the welding speed

did not affect the metal deposition rate significantly.

5.2.4 Electrode size

Electrode size affects the weld bead shape and the depth of penetration at fixed

current. Electrode size also influences the deposition rate. At any given current, a small

diameter electrode will have a higher current density and a higher deposition rate than

107

a larger electrode. However, a larger diameter electrode can carry more current than a

smaller electrode, and produce a higher deposition rate at higher amperage. For the

same values of current, arc voltage and welding speed, an increase in electrode

diameter results in a slight increase in the spread of the bead (Cornu, 1988).

5.2.5 Electrode work angle

The electrode may be held perpendicular to the workpiece or, tilted forward or

backward with respect to the weld pool. As the arc stream tends to align itself along

the axis of the electrode, the weld pool shape is different in each case, and so is the

shape of the weld bead. It is observed that in forehand welding, molten metal flows

under the arc, the depth of penetration and reinforcement are reduced while the width

of the weld increases, whereas in backhand welding the pressure of the arc scoops the

molten metal from beneath the arc, the depth of penetration and height of

reinforcement increases while the width of the weld is reduced (Nadkarni, 1988). The

electrode in perpendicular position results in bead geometry in between those obtained

in the above two cases.

5.2.6 Electrode stick-out and melting rate

The distance between the current pick-up tip and the arc root, called electrode

stick out, has a considerable effect on the weld bead geometry. Normally the distance

between the contact tip and the work is 25-40 mm.

108

The increase in melting rate of the electrode as a result of increase in electrode stick-

out is proportionate to the product of current density and stick-out. The electrode

melting rate in kg/min is given by the relationship,

Electrode melting rate =1

1000 0.35 +

d2

645+ 2.08 × 10−7 ×

IL × 25.4

d2

1.22

(5.1)

Where d, L and I are the diameter of the electrode, electrode stick-out in mm and

current density respectively.

Chandel et al. (1997) reported that the melting rate of the electrode increased

with the increase in the stick out. This effect is particularly more significant with

smaller diameter electrode since electrode heating is caused by the electrode electric

resistance, which increases with the decrease in the electrode diameter. The depth of

penetration decreases with the increase in electrode stick-out. This factor needs to be

given due consideration where deeper penetration is required. Gunaraj and Murugan

(1999) reported that heat affected zone decreased with the increase in stick- out.

Janez (2000) reported that a mutual influence of the arcs was quite strong and

consequently melting rate was high in twin-wire welding. He further reported that arc

energy melted more filler material per wire in twin-wire welding than in single-wire

welding and with the same welding parameters, this required higher wire feed speed in

twin-wire welding.

109

5.2.7 Depth of flux

The depth of the layer of the granular flux influences the appearance and

soundness of the finished weld as well as welding action. If the granular flux layer is

too shallow, the arc will not be entirely submerged in flux. Flashing and spattering

will occur. Apart from injurious to the eyes of the operator, this may lead to poor

appearance of weld and it may also be porous. If the flux layer is too thick, the arc will

be too confined and a rough ropelike appearing weld will result and the weld bead may

be narrow and humped. The gases generated during welding may not be able to escape,

and the surface of the molten weld metal becomes irregularly distorted. Optimum

depth of flux can be established by slowly increasing the flow of flux until the welding

arc is submerged and flashing no longer occurs. The gases will then puff up quietly

around the electrode, sometimes igniting.

5.2.8 Polarity

The amount of heat generated at the electrode and work piece, deposition rate,

bead geometry and mechanical properties are affected by polarity. The change in

polarity from DCEP to DCEN changes the amount of heat generated at electrode and

the work piece and, hence the metal depositing rate, weld bead geometry and

mechanical properties of the weld metal (Robinson, 1983). Little (1976) observed that

the two third of the total heat was generated at the positive electrode and the one third

of the total heat was generated at the negative electrode.

110

It has been reported by Renwick et al. (1976) that DCEN polarity produced higher

deposition rate and reinforcement than with DCEP polarity in submerged arc welding.

Ghosh et al. (1991) observed high yield strength, ultimate tensile strength and hardness

of the weld metal with DCEN polarity as compared to DCEP polarity.

5.2.10 Flux basicity index

Flux basicity index also influences the penetration (Gupta and Gupta, 1988). In

general higher penetration is obtained with the use of low basicity index fluxes due to

high viscosity which enhances the tendency of heat concentration in the narrow zone.

Patchett and Dancy (1980) reported that the penetration increased with the increase in

slag viscosity and surface tension. They also observed that an increase in viscosity, arc

stability and surface tension resulted in deeper penetration.

5.3 Weld Bead Shape

The weld bead shape is an indication of bead geometry which affects the load

carrying capacity of the weldments (Baach et al., 1981., Samiti, 1986) and number of

passes needed to fill the groove of a joint. The bead geometry is specified by bead

width, reinforcement, penetration, penetration shape factor and reinforcement form

factor.

5.3.1 Weld bead width

The weld bead width is the maximum width of the weld metal deposited. It

influences the flux consumption rate and chemistry of the weld metal. Weld bead

111

width is directly proportional to arc current, welding voltage and electrode diameter

and indirectly proportional to the welding speed. The bead width increases with an

increase in electrode diameter (McGlone, 1982). Gupta and Arora (1991) observed that

bead width increased with an increase in current until it reaches a critical value and

then it decreases with an increase in welding current. Yang et al. (1992) investigated

that the bead width was not affected significantly by the types of power source

(constant voltage or constant current) when an acidic fused flux was used. However,

using a basic fused flux with constant current operation showed somewhat larger bead

width than with welds laid using acidic fused flux.

5.3.2 Penetration

Weld bead penetration is the maximum distance between the base plate top

surface and depth to which the fusion has taken place. The more the penetration, the

less is the number of welding passes required to fill the weld joint which consequently

results in higher production rate. It is observed that the penetration is influenced by

welding current, polarity, arc travel speed, electrode stick-out, basicity index and

physical properties of the flux. McGlone (1982) observed that penetration was directly

proportional to welding current. He also observed that the deepest penetration was

achieved when DCEP polarity was used and the least with DCEN polarity. He further

investigated that the penetration was indirectly proportional to welding speed and

electrode diameter. Penetration decreases with the increase in welding speed because

the time during which the arc force is allowed to penetrate into the material‟s surface

112

decreases. The penetration decreases with the increase in electrode diameter due to

decrease in current density (Cornu, 1988). Chandel et al. (1987) reported that the

penetration increased with the decrease in electrode extension and included angle of

the joint. Caddle (1967) reported that the penetration increased with a decrease in

thermal conductivity of the weld metal.

5.3.3 Reinforcement

Reinforcement is the maximum distance between the base metal level and the

top point of the deposited metal. Reinforcement is the crown height of the weld bead

from the base plate. It affects the strength of the weld joint and welding wire

consumption rate. It increases with the increase in welding wire feed rate irrespective

of the welding current and the type of polarity employed (Gunaraj and Murugan 1999).

It is indirectly proportional to welding voltage, welding speed and electrode diameter.

The reinforcement is more with DCEN polarity and less with DCEP polarity. Increase

of reinforcement with an increase of welding filler wire feed rate is mainly due to the

larger amount of metal deposited per unit length. The decrease of reinforcement with

the increase in voltage is due to increase in weld bead width.

5.3.4 Weld penetration shape factor (WPSF) and weld reinforcement form

factor (WRFF)

WPSF and WRFF are also called as coefficients of internal shape and external

shape respectively. The ratio of bead width to penetration and bead width to

113

reinforcement are termed as Weld Penetration Shape Factor (WPSF) and Weld

Reinforcement Form Factor (WRFF) respectively. The smoothness of the weld

increases with the increase in WRFF (Cornu, 1988). Mandotov (1969) and Srihari

(1992) reported that WPSF and WRFF increased with an increase in voltage.

5.4 Flux Consumption

Flux consumption influences the economic aspects of welding and chemical

composition of the weld metal. Flux consumption depends upon the welding

parameters such as welding current, arc voltage, welding speed, polarity and type of

flux. Flux consumption increases with the increase in arc voltage and decrease in

current. The electrode extension has no significant effect on flux consumption (Gupta

and Gupta, 1988). Agglomerated fluxes have low flux consumption as compared to

fused fluxes (Vishvanath, 1982).

5.5 Experimental Procedure

The machine employed for experimentation was Ador TORNADO-800. The

composition of the welding wire (4mm diameter) and base plate are shown in Table 5.1.

The mild steel plates were cleaned chemically and mechanically to remove the oxide

layer and any other source of hydrogen. Bead on plate welds were laid on the plates of

200x75x12 mm size, using the developed fluxes. The ranges of the parameters and their

level have already been reported in Table 3.2 and the parameters were varied as per the

design matrix shown in Table 3.3. The basis for selection of the range and the level of

114

the parameters has already been discussed in Chapter-3. The experiments were

performed in random manner to avoid any systematic error.

After welding, transverse sections of the weld beads were cut from the middle

portions of the plates as specimens. These specimens were prepared by standard

metallurgical polishing methods. The properly polished specimens were etched with a

2% Nital solution for about 30 seconds, which was followed by investigation and

analysis. For each of the bead-on-plate specimens, the important dimensions of the

weld bead geometry were measured. The average response parameters (bead width,

penetration, reinforcement, weld penetration shape factor and weld reinforcement form

factor) and flux consumption were recorded by conducting experiments as per design

matrix (Table 4.2) are shown in Table-5.2. With the help of these observed responses,

models were developed.

Table 5.1 Chemical composition of base plate and electrode wire

Element

(%) C Mn Si S P Ni Cr

Base

Plate 0.23 0.42 0.127 0.039 0.056 0.065 0.113

Electrode

Wire 0.069 1.86 0.1 0.028 0.023 Nil Nil

115

Table 5.2 Observed values of bead parameters and flux consumption

Expt.

Run

No.

Response factors

W

Bead Width

(mm)

P

Penetration

(mm)

R

Reinforcement

(mm)

WPSF

WRFF F

Flux

Consumption

(gms.)

n

(gms)

1 17.76 6.545 3.201 2.71352 5.54827 50

2 17.24 7.349 4.043 2.3459 4.26416 37.02

3 17.81 6.735 3.345 2.64439 5.32436 45.77

4 15.192 6.671 2.955 2.27732 5.14112 40.29

5 16.79 10.455 6.025 1.60593 2.78672 21.88

6 17.48 6.389 2.382 2.73595 7.33837 48.34

7 17.2 6.17 3.245 2.78768 5.30046 43.42

8 17.215 8.56 3.55 2.0111 4.8493 49.53

9 17.54 6.66 2.94 2.63363 5.96599 50.83

10 16.345 7.115 4.58 2.29726 3.56878 40.39

11 16.792 6.4 3.59 2.62375 4.67744 49.08

12 19.475 7.53 2.544 2.58632 7.65527 60.24

13 15.812 8.75 4.392 1.80709 3.60018 32.44

14 16.99 6.525 3.01 2.60383 5.64452 44

15 17.515 11.323 4.191 1.54685 4.17919 26.09

116

Table 5.2 Observed values of bead parameters and flux consumption (Continued)

Expt.

Run

No.

Response factors

W

Bead Width

(mm)

P

Penetration

(mm)

R

Reinforcement

(mm)

WPSF

WRFF F

Flux

Consumption

(gms.)

n

(gms)

16 18.855 7.355 2.899 2.56356 6.50397 52

17 18.985 9.285 3.22 2.0447 5.89596 52.38

18 17.865 7.895 3.135 2.26282 5.69856 39.46

19 16.2 6.6 3.165 2.45455 5.11848 38.28

20 15.295 7.935 4.875 1.92754 3.13744 31.25

21 16.873 6.34 3.192 2.66136 5.28603 54.23

22 19.355 7.835 3.45 2.47033 5.61014 50.93

23 15.18 7.255 4.045 2.09235 3.75278 30.26

24 20.44 6.99 3.565 2.92418 5.73352 65

25 15.56 7.025 3.49 2.21495 4.45845 32.86

26 17.905 11.07 4.906 1.61743 3.64961 37.69

27 17.042 6.7 3.205 2.54358 5.31732 49.79

28 17.365 6.32 3.475 2.74763 4.99712 54.61

29 16.63 8.272 4.005 2.0104 4.15231 35.89

117

5.6 Development of Model

Response surface methodology‟s Box-Bohnken design consisting of twenty

nine experiments was conducted to develop model showing the relationships between

the response Y (bead width, penetration, reinforcement, weld penetration shape factor

and weld reinforcement form factor) and the welding parameters (open circuit voltage

A, welding current B, welding speed C and flux basicity index D) for coded values of -

1 to +1 for each of the welding parameters.

To test the goodness of the fit and validation of the developed models,

adequacy was determined by the analysis of variance technique (ANOVA). The

analysis of variance test was performed to evaluate the statistical significance of the

fitted quadratic models and factors involved therein for response factors W, P, R,

WPSF,WRFF and flux consumption (F). In addition to this, the goodness of fit of the

fitted quadratic model was also evaluated through „lack of fit test‟. The "Prob > F" for

all these tests was found in excess of 0.05, implying that the lack of fit is insignificant.

The results obtained are summarized in Tables-5.3 to 5.8.

All the fitted models are found to be significant, since for all the responses, the

Prob. > F are observed to be less than 0.0001. In other words, there is only a 0.01%

chance that "Model F-Value" larger than those reported in Tables-5.3 to 5.8 could

occur due to noise. The values of "Prob > F" less than 0.05 observed for some factors

involved in model equations, indicate that the contribution of these terms to the model

is significant. On the other hand, the value of "Prob > F" greater than 0.10 indicates

that the impact of model terms are not significant.

118

Table 5.3 ANOVA results for bead width (W)

Source

Sum of

Squares DF

Mean

Square

F

Value Prob. > F Remarks

Model 45.46 14 3.25 29.75 <0.0001 significant

A 12.79 1 12.79 117.19 < 0.0001 significant

B 0.73 1 0.73 6.69 0.0216 significant

C 9.39 1 9.39 86.03 < 0.0001 significant

D 0.18 1 0.18 1.64 0.2213 not

significant

A2 0.25 1 0.25 2.33 0.1493

not

significant

B2 1.86 1 1.86 17.03 0.0010 significant

C2 0.56 1 0.56 5.09 0.0405 significant

D2 3.57 1 3.57 32.66 < 0.0001

signific

ant

AB 0.46 1 0.46 4.24 0.0587 not

significant

AC 1.44 1 1.44 13.21 0.0027 significant

AD 0.27 1 0.27 2.43 0.1410 not

significant

BC 0.34 1 0.34 3.07 0.1016 not

significant

BD 0.065 1 0.065 0.60 0.4522 not

significant

CD 0.019 1 0.019 00.17 0.6851 not

significant

Residual 1.53 14 0.11

Lack of

Fit 0.88 10 0.088 0.54 0.8037

not

significant

Pure

Error 0.65 4 0.16

Cor

Total 46.99 28

119

Table 5.4 ANOVA results for penetration (P)

Source

Sum of

Squares DF

Mean

Square

F


Model 54.66 14 3.90 48.99 < 0.0001 significant

A 0.76 1 0.76 9.57 0.0079 significant

B 7.51 1 7.51 94.26 < 0.0001 significant

C 4.10 1 4.10 51.47 < 0.0001 significant

D 39.50 1 39.50 495.75 < 0.0001 significant

A2 0.68 1 0.68 8.49 0.0113 significant

B2 0.53 1 0.53 6.70 0.0215 significant

C2 0.21 1 0.21 2.62 0.1276

not

significant

D2 5.10 1 5.10 64.02 < 0.0001 significant

AB 2.5E003 1 2.5E003 0.031 0.8619 not

significant

AC 0.16 1 0.16 1.96 0.1835 not

significant

AD 0.054 1 0.05 0.68 0.4244 not

significant

BC 1.34 1 1.34 16.76 0.0011 significant

BD 1.64 1 1.64 20.54 0.0005 significant

CD 1.35 1 1.351243 16.96 0.0010 significant

Residual 1.12 14 0.08

Lack of

Fit 0.88 10 0.088 1.50 0.3692

not

significant

Pure

Error 0.23 4 0.059

Cor

Total 55.77 28

120

Table 5.5 ANOVA results for reinforcement (R)

Source

Sum of

Squares DF

Mean

Square

F


Model 16.67 14 1.19 27.64 < 0.0001 significant

A 6.95 1 6.95 161.30 < 0.0001 significant

B 0.3 1 0.3 7.05 0.0188 significant

C 0.47 1 0.47 10.92 0.0052 significant

D 4.49 1 4.49 104.22 < 0.0001 significant

A2 1.03 1 1.03 23.90 0.0002 significant

B2 0.14 1 0.14 3.21 0.0947

not

significant

C2 0.33 1 0.33 7.57 0.0156 significant

D2 3.17 1 3.17 73.52 < 0.0001 significant

AB 0.71 1 0.71 16.52 0.0012 significant

AC 6.56E003 1 6.56E003 0.15 0.7022 not

significant

AD 1.19 1 1.2 27.71 0.0001 significant

BC 4.556E003 1 4.556E003 0.11 0.7498 not

significant

BD 1.16E004 1 1.16E004 2.693E003 0.9593 not

significant

CD 1.020E003 1 1.020E003 0.024 0.8799 not

significant

Residual 0.60 14 0.043

Lack of

Fit 0.38 10 0.038 0.69 0.7128

not

significant

Pure

Error 0.22 4 0.055

Cor

Total 17.27 28

121

Table 5.6 ANOVA results for weld penetration shape factor (WPSF)

Source

Sum of

Squares DF

Mean

Square

F


Model 4.02 14 0.29 29.30 < 0.0001

significant

A 0.42 1 0.42 42.72 < 0.0001

significant

B 0.31 1 0.31 31.64 < 0.0001

significant

C

6.806E003 1 6.806E003 0.69 0.4185

not

significant

D 2.40 1 2.40 244.91 < 0.0001

significant

A2

0.062 1 0.062 6.35 0.0245 significant

B2

0.17 1 0.17 17.34 0.0010 significant

C2

0.06 1 0.064 6.56 0.0226 significant

D2

0.55 1 0.55 55.77 < 0.0001 significant

AB 4.727E003 1 4.727E003 0.48 0.4988

significant

AC

8.384E004 1 8.384E004 0.086 0.7741

not

significant

AD

0.021 1 0.02 2.18 0.1617

not

significant

BC 0.21 1 0.21 21.31 0.0004

significant

BD

0.027 1 0.027 2.74 0.1200

not

significant

CD 0.051 1 0.05 5.22 0.0384

significant

Residual 0.14 14 9.794E003

Lack of

Fit 0.11 10 0.01 1.36 0.4123

not

significant

Pure

Error 0.031 4 7.803E003

Cor

Total 4.16 28

122

Table 5.7 ANOVA results for weld reinforcement form factor (WRFF)

Source

Sum of

Squares DF

Mean

Square

F


Model 34.29 14 2.45 11.98 < 0.0001

significant

A 16.10 1 16.10 78.72 < 0.0001

significant

B

0.39 1 0.39 1.91 0.1886

not

significant

C

2.737E005 1 2.737E005 1.338E004 0.9909

not

significant

D 4.51 1 4.51 22.07 0.0003

significant

A2

0.49 1 0.49 2.40 0.1436

not

significant

B2

0.01 1 0.01 0.054 0.8193

not

significant

C2

1.49 1 1.49 7.31 0.0171 significant

D2

9.61 1 9.61 46.98 < 0.0001 significant

AB 1.30 1 1.30 6.34 0.0246

significant

AC

0.10 1 0.10 0.49 0.4964

not

significant

AD

0.74 1 0.74 3.64 0.0772

not

significant

BC

3.918E003 1 3.918E003 0.019 0.8919

not

significant

BD

0.037 1 0.037 0.18 0.6754

not

significant

CD

2.482E003 1 2.482E003 0.012 0.9138

not

significant

Residual 2.86 14 0.2

Lack of

Fit 2.03 10 0.2 0.98 0.5583

not

significant

Pure

Error 0.83 4 0.21

Cor

Total 37.15 28

123

Table 5.8 ANOVA results for flux consumption (F)

Source

Sum of

Squares DF

Mean

Square

F


Model 2842.67 14 203.05 27.12 < 0.0001

significant

A 1352.9 1 1352.9 180.7 < 0.0001

significant

B 88.34 1 88.34 11.80 0.0040

significant

C 137.08 1 137.08 18.31 0.0008

significant

D 600.1 1 600.1 80.17 < 0.0001

significant

A2

2.84 1 2.84 0.38 0.5481

not

significant

B2

118.56 1 118.56 15.84 0.0014 significant

C2

2.39 1 2.39 0.32 0.5810

not

significant

D2

175.59 1 175.59 23.46 0.0003 significant

AB

3.26 1 3.26 0.44 0.5201

not

significant

AC

0.10 1 0.10 0.014 0.9084

not

significant

AD

28.50 1 28.50 3.81 0.0714

not

significant

BC

2.36 1 2.36 0.31 0.5836

not

significant

BD 35.62 1 35.62 4.76 0.0467

significant

CD

20.17 1 20.17 2.69 0.1230

not

significant

Residual 104.80 14 7.49

Lack of

Fit 66.72 10 6.67 0.70 0.7054

not

significant

Pure

Error 38.08 4 9.52

Cor

Total 2947.47 28

124

The ANOVA results for bead width (Table5.3) show that A, B, C, B2, C2, D2, AC are

significant model terms. The ANOVA results for penetration that (Table 5.4) reveals

that A, B, C, D, A2, B2, D2, BC, BD, CD are significant model terms. The ANOVA

results for reinforcement (Table 5.5) shows that A, B, C, D, A2, C2, D2, AB, AD are

significant model terms. The ANOVA results for weld penetration shape factor (Table

5.6) shows that A, B, D, A2, B2, C2, D2, BC, CD are significant model terms. The

ANOVA results for weld reinforcement from factor (Table 5.7) reveals that A, D, C2,

D2, AB are significant model terms. The ANOVA results for flux consumption (Table

5.7) reveals that A, B, C, D, B2, D2, BD are significant model terms.

Tables-5.9 to 5.14 show the model summary statistics for all responses. The

coefficients of correlation (R2) for all the models are observed in excess of 0.92 which

inspire confidence in the developed models. The predicted and adjusted R2

values for

all the response models were in reasonable agreement which again validates the fitness

of developed models. The coefficient of variation (C.V.) defined as (S.D./Mean x 100)

of model is measurement of error. The low value of C.V. obtained for all the models

indicates improved precision and reliability of the experiments performed. The

adequate precision values, defined as signal to noise ratio for the fitted value, are

significantly higher than 4 indicating the suitability of models for future prediction.

125

Table 5.9 Model summary statistics for bead width

Std. Dev. 0.33 (R2) 0.967

Mean 17.27 Adjusted (R2) 0.935

C.V.(%) 1.91 Predicted (R2) 0.845

PRESS 7.28 Adequate Precision (AP) 22.143

Table 5.10 Model summary statistics for penetration

Std. Dev. 0.28 (R2) 0.98


C.V.(%) 3.72 Predicted (R2) 0.9087


Table 5.11 Model summary statistics for reinforcement

Std. Dev. 0.21 (R2) 0.965


C.V.(%) 5.75 Predicted (R2) 0.841


Table 5.12 Model summary statistics for weld penetration shape factor

Std. Dev. 0.099 (R2) 0.967


C.V.(%) 4.24 Predicted (R2) 0.829


126

Table 5.13 Model summary statistics for weld reinforcement form factor

Std. Dev. 0.45 (R2) 0.9229


C.V.(%) 9.03 Predicted (R2) 0.6748


Table 5.14 Model summary statistics for flux consumption

Std. Dev. 2.74 (R2) 0.9644


C.V.(%) 6.28 Predicted (R2) 0.8135


To test the accuracy of the models in actual applications, conformity test runs

were conducted by assigning different values for process variables within their

working limits. Specimens were cut from the conformity test plates and their bead

profiles were traced. All bead dimensions were measured. The percentage of errors,

which give the deviation of predicted results of responses from the actual measured

values, were also calculated and presented in Table-5.15. It is found from the table

that the average error for all models is less than 3%.

5.7 Results and Discussion

The developed mathematical models can be employed to predict the geometry

of weld bead and shape relationships for the range of parameters used in the

investigation by substituting their respective values in coded form. The predicted

127

values of response factors W, P, R, WPSF, WRFF and Flux consumption F from

regression equations (5.2) to (5.7) corresponding to different combination of welding

variables reported in Table-3.2 are compared with the corresponding experimental

values. A nice agreement is observed between these values, as evident from Figs.5.2-

5.7.

Table 5.15 Comparison of actual and predicted values of weld bead parameters

% Error = Act. Value − Pred. Value ÷ Pred. Value X100

Trial No.

Predicted values of

bead parameters

Actual values of bead

parameters

% Error

W P R W P R W P R

1 17.83 6.6.48 3.24 17.29 6.54 3.3 -3.02 1 1.85

2 17.28 6.53 3.27 17.63 6.51 3.19

2.02 -0.3 -2.44

3 16.17 7.30 4.66 16.63 7.12 4.62

2.87 -2.46 -3

4 18.89 7.37 2.79 18.87 7.39 2.82

-0.1 0.27 1.07

5 17.45 6.33 2.6 17.57 6.51 2.67

0.68 2.84 2.69

6 18.03 8.23 3.24 18.12 7.99 3.06

0.49 -2.91 3

Av.error(%)

0.49 -0.26 0.52

128

Fig. 5.2 Comparison between measured and predicted value of bead width (W)

Fig. 5.3 Comparison between measured and predicted value of penetration (P)

129

Fig. 5.4 Comparison between measured and predicted value of reinforcement (R)

Fig. 5.5 Comparison between measured and predicted value of WPSF

130

Fig. 5.6 Comparison between measured and predicted value of WRFF

Fig. 5.7 Comparison between measured and predicted value of flux consumption (F)

131

Based on these models, the main and the interaction effects of the process parameters

on the bead geometry were computed and plotted as depicted in Figs.5.8-5.40. The

results show the general trends between the cause and effect. The possible causes for

the effects of different welding variables on bead geometry and shape relationships

were analyzed and discussed below:

5.7.1 Effect of process parameters on bead width

Direct effect

The regression equations obtained for bead width by using multiple regressions

are given below:

Bead Width W = 18.28 + 1.23 ∗ A + 0.29 ∗ B1.06 ∗ C0.10 ∗ D + 0.20 ∗

A2 0.54 ∗ B20.29 ∗ C20.90 ∗ D2

+ 0.34 ∗ A ∗ B0.60 ∗ A ∗ C + 0.19A ∗ D +

0.29 ∗ B ∗ C 0.097 ∗ B ∗ D + 0.052 ∗ C ∗ D (5.2)

Figs.5.8-5.10 show the effect of process parameters on bead width. It is

apparent that bead width increases with the increase in open circuit voltage. As shown

in Fig.5.8, bead width (W) increases from 17.24 to 19.70 mm with the increase in open

circuit voltage from 32 to 38 volts. It can be attributed to the increase in arc length with

the increase in open circuit voltage, which in turn results in spreading of the arc cone at

its base which further results in more melting of work piece instead of penetrating the

plate. This extension in bead width causes corresponding reduction in penetration and

reinforcement. In fact excessive increase in voltage can result in nearly flat bead.

132

Fig. 5.8 Effect of voltage on bead width

Bead width increases from 17.44 to 18.03 mm with increase in welding current from

375 to 475 amperes, as shown in Fig. 5.9. This effect is due to increase in heat input

and the weight of the weld metal deposited (Gunaraj and Murgun, 1999). These factors

contribute to increase in weld pool size and consequently increase the bead width. As

shown in Fig.5.10, weld bead width decreases steadily with the increase in welding

speed. The bead width decreases from 19.04 to 16.92 with increase in welding speed

from 24 to 30 m/hr. This negative effect of speed on W is due to the fact that when

speed increases, the thermal energy transmitted to the base plate from the arc or line

power per unit length of the weld bead decreases and less filler metal is deposited per

unit length of weld bead, resulting in thinner and narrower weld bead. Hence, at lower

travel speeds, the weld bead is larger in mass, whereas at higher travel speeds, it is

lesser in mass. If speed decreases, the bead becomes wider, flatter and smoother

(Olson et al., 1990). It can be explained on the basis of decrease in metal deposition

133

rate and heat input with the increase in welding speed. The effect of basicity index on

bead width is not significant.

Fig. 5.9 Effect of current on bead width

Fig. 5.10 Effect of welding speed on bead width

134

Interaction effect

It is apparent from Figs.5.11 and 5.12, showing interaction of open circuit

voltage and welding speed on bead width (W) that the increase in voltage increases W

for all values of speed. The bead width increases from 17.40 to 21.07 mm and from

16.49 to 17.75 mm with the increase in voltage from 32 to 38 volts, at the welding

speed 24 and 30 m/hr respectively. It shows that the increasing trend of bead width

with the increase in open circuit voltage decreases with the increase in welding speed.

It is due to the fact that open circuit voltage has a positive effect whereas welding

speed has a negative effect on bead width. Therefore, the combined effect of these

parameters causes the decrease in increasing trend of bead width with the increase in

Open circuit voltage.

Fig. 5.11 Interaction effect of voltage and speed on bead width

C = 24m/ hr

C = 30m/hr

135

Fig. 5.12 Response surface due to interaction of voltage and speed on bead width

5.7.2 Effect of process parameters on penetration

Direct effect

The developed model for the penetration is shown below:

Penetration (P) = 6.990.30 * A + 0.95 * B0.70 * C1.53* D+0.32 * A2+ 0.29 *

B2+0.18 * C2+1.07* D20.025* A* B0.20 * A * C + 0.088 * A * D-0.58 * B *

C0.48 * B * D + 0.44 * C * D (5.3)

As shown in Fig.5.13, the penetration (P) increases from 6.33 to 8.22 mm with

the increase in welding current from 375 to 475 amperes. Increase in current gives rise

to enhanced line power per unit length of the weld bead and higher current density,

causing larger volume of the base material to melt and hence, deeper penetration. As

current increases the temperature, the heat content of the droplets also increases, which

results in more heat being transferred to the base material. Increase in current also

136

increases momentum of the droplets, which on striking the weld pool causes a deeper

penetration. An increase in welding current, with other variables remaining constant,

results in increased depth of penetration, increased deposition rate and increased weld

bead size and shape at a given cross-section. It is also attributed to the increase in

digging power of the arc with the increase in welding current. As the current increases,

the intensity of the arc and hence the digging power of the arc and penetration

increases. This is also consistent with the study of MacGlone (1982).

Fig. 5.13 Effect of current on penetration

As depicted in Fig.5.14, the penetration decreases from 7.86 to 6.47 mm with

the increase in welding speed from 24 to 30 m/hr. This could obviously be due to the

reduced line power per unit length of weld bead as speed increases. Also, at higher

welding speeds, the electrode travels faster and covers more distance per unit time. The

combined effects of lesser line power and faster electrode travel speed result in

137

decreased metal deposition rate per unit length of weld bead (Box et al., 1976). It is

also attributed to decrease in heat input, metal deposition rate and digging power of the

arc with the increase in welding speed resulting in decrease in weld metal penetration.

Fig. 5.14 Effect of welding speed on penetration

From Fig. 5.15, it is observed that P decreases from 7.61 to 7.01 when open

circuit voltage increases from 32 to 38 volts. This is obviously due to the fact that the

increase in voltage results in increased arc length and spreading of arc cone at its base

which results in more melting of work piece surface instead of penetrating the plate.

This is consistent with the study conducted by Murugan and Gunaraj (2005). In fact,

excessive increase in voltage can result in nearly flat bead. Flux basicity index also

influences the penetration. It is observed from Fig.5.16, the higher value of

penetration i.e 9.59 mm is obtained with using low basicity index flux (0.6), because

138

low basicity index fluxes have high viscosity which enhances the tendency of heat

concentration in the narrow zone and hence high penetration. This is consistent with

the study conducted by Gupta and Gupta (1988).

Fig. 5.15 Effect of voltage on penetration

Fig. 5.16 Effect of basicity index on penetration

139

Interaction effect

From the Fig.5.17, it is evident that P increases with the increase in welding

current for all values of welding speed. It shows that the weld metal penetration

increases from 6.63 to 9.67 mm and from 6.38 to 7.12, with the increase in current, at

the welding speed of 24 to 30 m/hr respectively. The rate of increase in P with the

increase in current decreases gradually as speed increases. These effects on P are due

to the reasons that current has positive effect but speed has a negative effect on P as

discussed already in the direct effects of current and speed on P. It is found that at

lower values of speed, the positive effect of current on P is stronger but at higher

values of speed, the negative effect of speed on P is stronger. These effects are further

explained with the help of a response surface plot as shown in Fig.5.18. From the

contour surface, it is noted that P is maximum (about 9.67 mm) when current and

speed are at their maximum (+1) and minimum (−1) limits, respectively, and the

lowest value of P (about 6.39 mm) is obtained when current and speed are at their

minimum and maximum limits, respectively.

From Figs. 5.19 and 5.20, it is observed that penetration increases from 8.45 to

11.31 mm and from 6.38 to 7.28 mm, with increase in current, at the basicity index of

0.6 and 1.2 respectively. It is evident form Figs.5.21 and 5.22 that penetration

decreases form 10.91 to 8.63 and from 6.97 to 6.45 with increase in welding speed

from low basicity index to higher value of basicity index. These results can be

explained with the help of effects of welding variables such as welding speed and

basicity index on penetration.

140

Fig.5.17 Interaction effect of current and speed on penetration

Fig. 5.18 Response surface due to interaction of current and speed on

penetration

C = 24m/ hr

C = 30m/hr

141

Fig. 5.19 Interaction effect of current and basicity index on penetration

Fig. 5.20 Response surface due to interaction of current and basicity index on

penetration

D = 1.2

D = 0.6

142

Fig. 5.21 Interaction effect of welding speed and basicity index on penetration

Fig. 5.22 Response surface due to interaction of welding speed and basicity index on

penetration

D = 1.2

D = 0.6

143

5.7.3 Effect of process parameters on reinforcement (R)

Direct effects

The developed model for the reinforcement is shown below:

Reinforcement (R) = 2.94-0.91* A+ 0.19 * B-0.24 * C-0.52* D + 0.40 * A20.15 *

B2+0.22 * C2 + 0.85 * D2 0.42*A*B + 0.041*A*C + 0.41* A* D + 0.034* B * C +

4.071E003 * B * D+0.012*C * D (5.4)

From Figs.5.235.25, it is observed that the reinforcement (R) decreases with

the increase in open circuit voltage and welding speed, it increases with the increase in

welding current. Reinforcement decreases with increase in basicity index due to

similar reasons as described for penetration. It is seen from these graphs that

reinforcement decreases from 4.24 to 2.42 mm with change of voltage from 32 to 38

volts, and decreases from 3.39 to 2.92 mm when welding speed increases from 24 to

30 m/hr. When current changes from 375 to 475 amperes, it changes from 2.59 to 2.97

mm. As evident from Fig.5.26, its value increases from 4.29 to 3.26 mm with increase

in basicity index from 0.6 to 1.2. The reasons for these changes are due to same

reasons as described in preceding section for penetration.

Interaction effects

It is observed from Figs.5.27 and 5.28 that reinforcement decreases with the

increase in voltage, when the current changes from 375 to 475 amperes and it also

decreases with voltage from low basicity index to higher value of basicity index, as

shown in Figs.5.29 and 5.30. These interaction effects can be explained on the basis of

effect of voltage, current and basicity index on reinforcement.

144

Fig. 5.23 Effect of voltage on reinforcement

Fig. 5.24 Effect of welding speed on reinforcement

145

Fig. 5.25 Effect of current on reinforcement

Fig. 5.26 Effect of basicity index on reinforcement

146

Fig. 5.27 Interaction effect of voltage and current on reinforcement

Fig. 5.28 Response surface due to interaction of voltage and current on

reinforcement

B = 375 amp

B =475 amp

147

Fig. 5.29 Interaction effect of voltage and basicity index on reinforcement

Fig. 5.30 Response surface due to interaction of voltage and basicity index on

reinforcement

D = 0.6

D = 1.2

148

5.7.4 Effect of process parameters on weld penetration shape factor (WPSF)

and weld reinforcement form factor (WRFF)

Direct effects

The developed model for the weld penetration shape factor and weld

reinforcement form factor are shown below:

WPSF = 2.62 + 0.22*A0.19 * B+0.028 * C+0.38* D0.098 * A20.16* B20.100*

C20.35* D2+ 0.034 * A* B0.014 * A * C+0.055 * A * D + 0.23* B * C + 0.062 * B

* D0.086* C* D (5.5)

WRFF = 6.27 + 1.38 * A0.22 * B + 1.805E-003 * C + 0.52* D0.28 * A2 + 0.041*

B20.48 * C21.47 * D2 + 0.57 * A * B0.16 * A * C0.33 * A * D + 0.031* B * C

+ 0.073 * B * D + 0.019* C * D (5.6)

Figs.5.31 and 5.32 show the effect of open circuit voltage on both weld

penetration shape factor (WPSF) and weld reinforcement form factor (WRFF). WPSF

increases from 2.29 to 2.74 and with the increase in open circuit voltage from 32 to 38

volts and WRFF increases from 4.61 to 7.38 with increase of open circuit voltage from

32 to 38 volts. This is also in consistent with the studies of Mandotov (1969) and

Srihari (1992).The positive effect of voltage on both the factors is due to the reasons

that bead width (W) increases almost steadily, but penetration (P) and reinforcement

(R) decrease little as voltage increases from −1 to +1 limit as discussed already. Hence,

they increase steadily as voltage increases.

149

WPSF decreases when current increases, but it remains nearly constant from 375 to

400 amperes and it decreases from 2.65 to 2.26 (Fig.5.33) when current changes from

400 to 475 amperes. This could be due to reason that WPSF which is the ratio of W/P,

decreases when current changes from 400 ampere to 475 amperes, because rate of

increase of P is more than that of W with in this range of current. It is also observed in

Fig.5.34 that WPSF increases 1.89 to 2.64 with increase of basicity index from 0.6 to

1.2. It is seen from Fig.5.34 that WRFF decreases from 4.28 to 5.31 with increase of

basicity index from 0.6 to 1.2. These results can be explained with the help of effects

of welding variables on bead width, penetration and reinforcement respectively.

Fig. 5.31 Effect of voltage on WPSF

150

Fig. 5.32 Effect of voltage on WRFF

Fig. 5.33 Effect of current on WPSF

151

Fig. 5.34 Effect of basicity index on WPSF

Fig. 5.35 Effect of basicity index on WRFF

152

Interaction effect

Figs.5.36 and 5.37 show the interaction effects of voltage and current on

WRFF. It is evident from these graphs that WRFF increases for all values of current

when voltage increases from 32 to 38 volts. But the increasing trend of WRFF is more

at higher value of current i.e. at 475 amperes than that at 375 amperes. This is due to

fact that WRFF= W/R, W increases with increase of voltage and nearly remains

constant with change of current whereas R decreases with increasing voltage and

increases with the increase of current. Thus, voltage has a positive effect on WRFF

whereas current has a negative effect on WRFF. This increasing trend of WRFF at

higher current is due to more positive effect of voltage on WRFF. This can also be

explained on the basis of effects of voltage and current on bead width and

reinforcement.

The interaction effect of current and speed on WPSF is shown in Fig.5.38.

Fig.5.39 shows the effect of welding speed on WPSF with change in basicity index.

These can be explained on the basis of effects of welding variables on the bead width

and penetration respectively.

153

Fig. 5.36 Interaction effect of voltage and current on WRFF

Fig. 5.37 Response surface due to interaction of voltage and current on WRFF

B: Current

B = 375 amp

B =475 amp

154

Fig. 5.38 Response surface due to interaction of current and speed on WPSF

Fig. 5.39 Response Surface due to interaction of welding speed and basicity

index on WPSF

155

5.7.5 Effect of process parameters on flux consumption

Direct effects

The developed model for the flux consumption is shown below:

Flux Consmption (F) = 48.11 + 12.69 * A3.24 * B4.04 * C + 5.97 * D 0.66 *

A24.28 * B2 + 0.61* C26.30 * D2 + 0.90 * A * B0.16 * A * C2.02 * A * D+0.77

* B * C + 2.26 * B *D1.70*C*D (5.7)

As shown in Fig. 5.40, flux consumption increases from 34.757 to 60.139 gms

with the increase in open circuit voltage from 32 to 38 volts. This is due to the

positive effect of voltage on bead width. This is also consistent with the findings of

Gunaraj and Murgun (1999).

In Fig. 5.41, the effect of change in welding speed on flux consumption can be

explained on the basis of effect of speed on bead width. At higher welding speed there

is less spread of bead, and hence less consumption of flux compared to that at lower

speed.

Fig. 5.42 shows the effect of basicity index on consumption of flux. Flux

consumption increases with the increase in basicity index. This is due to the fact that

because low basicity index fluxes have high viscosity which enhances the tendency of

heat concentration in the narrow zone and hence high penetration and lower bead

width. This is consistent with the study conducted by Gupta (1988). The Flux

consumption decreases with the increases in current, as shown in Fig.5.43. This can be

156

explained on the basis of individual effect of welding current on bead width,

penetration and reinforcement.

Interaction effect

Response surface due to interaction of current and basicity index on flux

consumption is shown in Fig.5.44. These effects can be explained on the basis of

individual effects of current and basicity index on bead width and consumption of flux.

Fig.5.40 Effect of voltage on flux consumption

157

Fig.5.41 Effect of welding speed on flux consumption

Fig.5.42 Effect of basicity index on flux consumption

158

Fig.5.43 Effect of current on flux consumption

Fig. 5.44 Response surface due to interaction of current and basicity index on flux

consumption