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International Journal of Performability Engineering, Vol. 12, No. 2, March 2016, pp. 155-172.

© Totem Publisher, Inc., 4625 Stargazer Dr., Plano, Texas 75024, U.S.A

*Corresponding author’s email:[email protected] 155

Orbital TIG Welding Process Parameter Optimization using Design of Experiment for Satellite Application

M. KARTHIKEYAN1*, VALLAYIL N. A. NAIKAN2, R. NARAYAN1 and D.

P. SUDHAKAR1

1Liquid Propulsion System Centre (LPSC), Indian Space Research Organisation (ISRO),

Bangalore –560008, India. 2Reliability Engineering Centre (REC), Indian Institute of Technology (IIT), Kharagpur–

721302, India

(Received on October 26, 2015, revised on January 29, 2016)

Abstract: This paper highlights the optimization of Orbital Tungsten Inert Gas (OTIG)

welding process parameters by Design of Experiment (DOE) using Taguchi method

forwelding of stainless steel of 6mm diameter and 0.7 mm thickness for satellite

propulsion feed system. This proposed methodology identifies the optimum parameters

for welding and brings out the significance of the individual parameter, combination of

any of the two parameters (interaction effect) using Taguchi method by linear model

analysis of Signal to Noise (SN) ratio and means verses input parameters. Detailed

experiments were carried out and optimum parameters are arrived. Further these are tested

by different methods to evaluate the strength required for intended application. This

ensures sound and reliable weld joint. The optimum levels of these parameters thus

developed are being followed and no call for any rework is reported thereafter. By varying

the input parameters (current, RPM, gap between electrode and the job), the weld

penetration level or weld quality has been studied in several 6mm diameter tube

specimens andthe significance were studied and discussed in the test results.The Factorial

Factor design is followed for minimum of 27 (three parameters and three levels=3×3=27)

samples for this experiment. By further fine tuning the experiment, the optimized values

achieved are current 18.35 Amps, electrode rotation 10 rpm and gap between electrode

and job 0.8 mm. The weld specimen quality was verified in accordance with the user’s

quality standards and found satisfactory. This approach is easy to develop and easy to use

that assures the best combination of parameters required for Orbital TIG welding which

yield strong and defect free weld joint for Satellite application.

Keywords: OTIG, Power supply, failure effects, propulsion, satellite integration, optimal

current etc.

1. Introduction

The propulsion system provides the reaction control capability for attitude control and

orbit positioning in Polar & Geosynchronous orbits. The propulsion configuration consists

of many components and includes 6mm & 10mm outer diameter plumb lines (Propellant

feed lines) end connection with thickness of 0.7mm requiring Orbital TIG welding during

final integration of Satellites. The factors affecting the weld joints are precise control of

weld current, voltage input, gap between work pieces (plumb lines with propulsion

components), gap between electrode and work pieces, selection of electrode geometry,

bandwidth, electrode travel speed (RPM), flow rate of shielding gas used etc.

mailto:[email protected]

M. Karthikeyan, Vallayil N. A. Naikan, R. Narayan and D. P. Sudhakar

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Figure 1: Feed Lines (Propulsion Schematic)

The variation of any of these parameters will certainly affect quality of weld in the

propulsion system resulting in weld defects such as undercut, lack of penetration, non-

uniformity, cracks, excessive weld bead width, excessive weld-puddle overlap etc. Hence

it becomes necessary to optimize the parameters based on experts’ opinion. Here control

Orbital TIG Welding Process Parameter Optimization using Design of Experiment for

Satellite Application

157

of weld current, RPM and gap between electrode and tube are taken as parameters for

study purpose. The initial setting of tubes to be welded is carried out by an experienced

operator to ensure the quality and rest of the welding operation is carried out by machine.

The Orbital TIG Welding power supply used for this welding provides consistent and

repeatable weld results. This welding study was done to evaluate the impact of individual

parameter and combination of parameters on the quality and reliability of weldments for

Spacecraft Propulsion system. This analysis involves selection of different matrices as

shown in the Table4 to determine the quality of each weld joint. Further to this analysis,

optimized values/ measures were suggested to avoid defects leading to critical failure of

weld joints. Therefore the analysis has been performed based on parameters given in the

table during integration phase of propulsion system to achieve defect free welding and

ensure high reliability.

The propulsion system is configured as Gas module (gas bottle, pyro valves, filter,

pressure regulators, check valve, latch valve, fill and drain valves etc.), Liquid module

(propellant tanks, pyro valves, filters, fill and drain valves) and Thruster module as shown

in the schematic (Figure 1). There are about 350 numbers of Orbital TIG weld joints to

interconnect propulsion components using 6mm and 10 mm plumb lines.

As the feed lines carry the propellants to the propulsive device, any leak in the welds

shall lead to catastrophic mission failure and the losses in terms of effort and money will

be substantial. Even a minor leakage will lead to disaster. In view of this, it is essential

that all the joints in the propellant feed lines need to be totally leak proof and this

demands the effective implementation of orbital TIG welding. In order to ensure this,

various reliability analysis tools such as FMECA, CED, FTA and PA are applied for this

process to guarantee improvements and effectiveness. This paper discusses the optimum

set of parameters for OTIG Welding, significance of each vital parameter and the

interaction behaviour using Taguchi Methodology by linear model analysis of Signal to

Noise (SN) ratio and Mean versus input parameters

2. Literature survey

Orbital TIG welding for feed lines of a propulsion system is a specialized application and

hence not many literatures are available in public domain. However, the related welding

investigations carried out by others are given here.

Sarafin [1]stressed the importance of high quality, integral weldments for space

applications and outlined the merits of orbital welding towards this. Devereaux and

Franscois [2] covered the details of MMH/NTO bipropellant propulsion system involving

propellant feed system that helps for transfer orbit manoeuvres and station keeping

operations. Synder [3] highlighted a complex ‘bang-bang’ xenon feed system and

suggested that a more simplified feed system is desirable in future. Selding [4] highlighted

the importance of leak proof propellant storage and feed systems and outlined the failure

of a communication satellite launched by Ariane 5 rocket due to leak in feed system. A

small leak of propellant can lead to a failure of a big mission with huge cost and enormous

efforts. Cristiano [5] highlighted the water hammer effect in the propellant feed line. He

also cautioned about the adiabatic compression detonation due to vapour or propellant. He

suggested the limit for pressure surge. Lin [6] cautioned about water hammer effect in

propellant feed system due to rapid propellant/gas entry during pressurization and

propellant feeding. Leca [7] analysed the water hammer effect in satellites due to sudden



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pyro valve opening by experimental and software computations. Claude [8] has enlisted

spacecraft failures and the reasons there of since 1957. Chang[9] clearly enumerated the

huge losses (in millions of dollars) due to launch failures and analyzing the failure

between 1957 and 1999 indicated that out of 4378 launches, 390 failed and major causes

pointed to propulsion system failure due to fuel leakage among other reasons, this point to

the importance of defect free strong welding. Peasura [11] studied the shielding gas effect

on properties of HAZ and fusion zone on GTAW of Al – alloy AA5083 evaluating

hardness. Ding [12] outlined the importance of failure analysis to improve reliability. The

author used Fish bone analysis to bring out the relationship between failure modes and

causes. Samardžić [12] emphasized pipe welding for steam boiler using metal arc

welding, and the application of TIG welding and automatic orbital welding process.

Deshmukh and Sorte [14] have shown that the welding input parameters play a very

significant role in determining the quality of a weld joint with minimum distortion.Kumar

[15] studied TIG parameters and pitting corrosion of Al – alloys using ANOVA,

regression analysis and mathematical models. He found that Peak current (Ip) and

frequency have direct and base current (Ib) and pulse on time have inverse relation to

pitting. Kumara and Sundarrajan [15] have presented the results of extensive research on

improving the mechanical properties of AA 5456 aluminium alloy welds by pulsed TIG

deploying Taguchi method, regression models and analysis of variance. They also studied

the effect of planishing on properties. Palani [17] studying TIG parameters of Al-65032

using Taguchi and mathematical models found that welding speed impacts weld strength

and percentage elongation more than other parameters. Padmanabhan [18] studied

optimization of pulse TIG parameters on tensile strength of AZ31B Mg alloy and found

that the parameters peak current 210A, base current 80A, pulse frequency 6 Hz gave the

maximum tensile strength of 188 Mpa. Mahadevi and Manikandan [19] worked on TIG

welding parameters of AZ61 magnesium alloy by using response surface methodology

and analysis of variance and studied the effect of current, speed, voltage, electrode stick

out on tensile strength and percentage elongation. Sudhakar [20]analyzed the brazing

parameters for joining stainless steels by Ni braze alloy for rocket chamber application by

employing L9 Taguchi orthogonal array and AVOM and ANOVA. The influence of

parameters on strength and the interaction effects were studied. Joshi [20] studied the

MIG and TIG weldings. The authors have used the Design of Experiment method for

conducting the experiments and data analysis. Welding current, gas flow and wire feed

rate were the input parameters and the tensile strength of the weld was the output

parameter. Full factorial experimental design was used for this. Arya [22] utilized grey

relational analysis and Taguchi method to optimize the process parameters of MIG

welding such as weld current, arc voltage, welding speed, gas flow and wire diameter with

respect to bead geometry. Lothongkum [23]studied pulsed TIG parameters on Delta –

ferrite, shape factor and bead quality in orbital welding of AISI 316L. Trivedi [24]

reported that the shape and dimension of the weld bead affects strength, metallurgical

aspects and weld cracking effects. Karadeniz [25] studied the influence of weld

parameters on weld penetration in ERD emir 6842 steel of 2.5 mm thickness by robotic

GMAW and found that increasing weld current increased the penetration. Kumar and

Jindal [25] investigated the optimization of weld parameters such as current, voltage and

gas flow for GMAW using L9 Taguchi experimental design. Kumar etal. [26] studying

pulse TIG parameters using L27 Taguchi array found weld speed and input current

influenced weld strength more than other parameters such as voltage, stand-off distance,

pulse on –off times and gas flow. Gracia [28] researched on these welding programs for



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orbital TIG welding i.e., pulsed current & increasing speed, constant current, pulsed

current & decreasing speed and suggested that constant current provided better results

based on mathematical tests. Ratnayake and Vik [29] suggested a methodology to

recognize the most frequently appearing imperfect and defective welds by grouping based

on welding procedure specifications (WPSs) that contribute to the highest level of quality

deterioration. It is also emphasized that the kind of training that should additionally be

provided to the welders.

From the above literature reviews, it has been observed that there is need for

improving quality and reliability of the thin propellant feed tube welding for satellite

applications. These literatures provide that the penetration level of stainless steel is either

more or less depending on certain parameters during normal welding process of orbital

Pulsed-TIG welding process. Hence it is essential to increase the weld strength or

efficiency by optimizing the process parameters like current, RPM, gap between tube and

electrode to improve weld quality. The DOE approach appears to be appropriate to

address the quality issues on OTIG welding and to develop a robust process capable of

providing the required strength and quality of the propellant feed system for satellite

applications. As large numbers of reworks are reported in real practice, it is also felt the

need for optimizing weld parameters on stainless steel 304L tube by Taguchi analysis to

yield strong and reliable joints for satellite application.

3. Plan of investigation

In this work, the OTIG welding process is analyzed by experimental methods. Good

quality welding is influenced by several parameters (variables and constants) such as

current, RPM, standoff distance, gap between tubes where they butt each other, thickness

of the tube, purge/shield gas, electrode geometry, and voltage input etc. These are

enumerated in the Table 1. The cleanliness of tubes is another factor to be considered

prior to welding. This study is focused on only those parameters that have direct and

significant impact on the soundness of weld joints. The following steps are involved in

carrying out this experiment.

Table 1: Variables and Constants (Parameters)

Parameters (their units) Range Type V/C*

Current (Amps) 17.8 to 18.9 V

RPM (Nos) 9 to 10.5 V

Gap: Electrode & Tubes (mm) 0.7 to 0.9 V

Gap between Tubes (mm) 0.05 to 0.5 V

Thickness of Tubes (mm) 0.7 C

Purge Gas flow (Psi) 3 to 4 V

Electrode Geometry (mm) Sharp/Wider V

Voltage input (Volt) 240 C

* V-Variable, C-Constant

3.1 Identification of parameters

The ranges in which the parameters are being used for performing the welding

experiments are given in Table 2. Only three parameters that have direct significant


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impact in the soundness of weld joints such as current, RPM and stand-off distance (gap

between electrode and tubes) are chosen for this experiment. These parameters are chosen

based on the previous experience in the welding process and the supporting literature as

discussed in the earlier section.

Table 2: Identification of Process Parameters

Factors Low (L) Medium(M) High(H)

Current(C) (Amps) 17.8 (C1) 18.35(C2) 18.9(C3)

RPM(R) (Nos.) 9.5(R1) 10(R2) 10.5(R3)

Gap(G) (mm) 0.7(G1) 0.8(G2) 0.9(G3)

3.2 Selection of Parameter ranges

In this paper, Swagelok welding machine is used for experimentation. The bead on trials

is made in 6mm Stainless Steel tube of wall thickness 0.7mm. The welding parameters

were selected on the basis of various trial runs by checking their effect on depth of

penetration. Though several parameters influencing welding are listed in Table 1, after

analyzing the direct significance, only three parameters at three levels were considered for

this experiment. The trials were taken for low, medium and high levels of the selected

three process parameters as shown in Table 2. Further, the Design of Experiment (DOE) is

carried out as shown in Table 4.

3.3 Material Properties

Propulsion system employs both stainless steel tubes and titanium tubes and we consider

here stainless steel 304L tubes. The parent metal, its Chemical Composition and

Mechanical properties are furnished in Table 3.

Table 3: Properties of SS 304 L

Chemical composition

percent by weight Mechanical properties

C 0.03 Tensile strength Ultimate 590 MPa

Cr 18-20 Yield Strength 250 MPa

Ni 8-12 Comp. Strength 250 MPa

Mn 2 Density 7890 kg/m3

P 0.045 Poisson ratio 0.29

S 0.03 Melting Point 1399 - 1454 °C

Si 0.75 Percent Elongation 35-50%

Al 0.1 Hardness 80(Rc)

Fe Balance Young Modulus 200 GPa

3.4 Testing Methods

After performing welding, normally the following tests can be done to validate the

weldment to ensure whether it meets the satellite requirements.

Liquid penetrant test

Pressure Decay by pressure transducer

Pressure Decay by pressure gauge



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Hydraulic Test

X-Ray Radiography

Grey method

Tear down method and UT test

Though above tests are possible in specimen level, liquid penetrant test and

hydraulic test are not recommended to do in the spacecraft level as those are involved

with specific liquids to perform the test which are not advisable after the final stage of

integration of tubes in the spacecraft. Tear down method and UT test are not possible

after integration. However remaining tests are recommended in system level.

3.5 Design of Experiments (DOE)

Taguchi methods are statistical methods developed to improve the quality of

manufactured goods and more recently applied to other domains such as engineering,

biotechnology, marketing etc. Professional statisticians appreciated the improvements

brought out by Taguchi’s development of designs for studying variation leading to

innovations in the design of experiments. The different reasons that cause variation in

design parameters and in the manufacturing process are termed as noises. The optimum

and most efficient way to solve these problems of variation is to make the design &

process insensitive to the effect of noises which are the causes of variation. This

underlying principle of robust design is the back bone of Taguchi analysis. It is a

scientifically disciplined mechanism for evaluating and implementing improvements in a

process with parametric optimization for the objective function, minimizing the effects of

undesirable noises. The purpose is to determine the optimum level for each process

parameter and to establish their relative significance.

Analysis of variance (ANOVA) is similar to regression in that it is used to investigate

and model the relationship between a response variable and one or more independent

variables. In this study general linear model is used to determine the influence of welding

speed, current and gas flow rate on ultimate tensile strength and % Elongation. After

conducting the experiments, the data taken are analyzed to determine the effects of

various parameters. The analysis of variance (ANOVA) and Taguchi analysis are used to

interpret the data. The statistical software MINITAB 14 is used for design of experiment.

4. Experimentation

OTIG welding experiments were initially carried out on specimens and then on tubes of

diameter 6mm and thickness 0.7mm as per the L27 matrix given in Table 4. This array has

three columns for process parameters and twenty seven rows of the combination of

parameters used for conducting the experiments and study three levels of parameters. The

welded specimens were tensile tested after radiography and bend tested subsequently.

4.1 Edge Preparation of Tubes

Extreme care has been taken for the edge preparation as a part of contamination control.

The required plump lines to be welded are cut from the source using chip-less cutter.

Further tube end connections are squared to 90o to match for butt welding as shown in

Fig. 2, de-burring by standard tool, cleaning by Isopropyl alcohol, drying by Nitrogen gas

and ensuring cleanliness through magnifying glass are standard procedure followed during


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edge preparation. The specimen are prepared in the same method and tested as per

standard.

Figure 2: Edge Preparation Prior to Weld

4.2 Experimental trials

Orbital TIG welding has been carried out using Swagelok welding machine for stainless

steel specimens.

Figure 3: Experimental Set Up for OTIG Welding

Figure 3shows the welding experiment set up. After getting satisfied with the results

of specimen, trials were made on 6mm tube used for propellant feed lines as per

experimental design as shown on Fig.5. As explained in section 3.2, the three important

welding parameters were experimented at three levels i.e. low, medium, and high of each.

Totally 27 experiments were conducted as per L27 array of experiments. The current

sequence and the current levels are given in Fig. 4.



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Figure 4: Current Sequence

4.3 Testing

The ultimate tensile strength of the trial pieces welded was tested in the calibrated

universal tensile testing machine. The tensile tests were carried out according to the

ASTM standards. Due to pulling effect of machine, tube undergoes deformation. Twenty

seven specimens were tested and values obtained are given in Table.4. The specimen after

the test is shown in Fig. 6 and the welded tubes as seen in X-ray film is given in Fig.

7.The Input matrix of Taguchi design is given in Table 4 as shown below.

Figure 5: L-27 Matrix Welded Tubes Figure 6: Specimen after the Test


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Figure 7: Welded Tubes in X-Ray Film

Table 4: Experimental Layout Using L27 Array and Responses

Sl.No Current(A) RPM Gap(mm) Max.

Load(N)

UTS

(N/mm2)

1 C1 17.8 R1 9.5 G1 0.7 5010 407.10

2 C1 17.8 R1 9.5 G2 0.8 6910 447.39

3 C1 17.8 R1 9.5 G3 0.9 6930 470.47

4 C1 17.8 R2 10 G1 0.7 7010 485.98

5 C1 17.8 R2 10 G2 0.8 7020 501.01

6 C1 17.8 R2 10 G3 0.9 7030 535.54

7 C1 17.8 R3 10.5 G1 0.7 7070 541.16

8 C1 17.8 R3 10.5 G2 0.8 7090 547.72

9 C1 17.8 R3 10.5 G3 0.9 7120 551.45

10 C2 18.35 R1 9.5 G1 0.7 7140 566.84

11 C2 18.35 R1 9.5 G2 0.8 7240 562.70

12 C2 18.35 R1 9.5 G3 0.9 7370 578.33

13 C2 18.35 R2 10 G1 0.7 7390 585.55

14 C2 18.35 R2 10 G2 0.8 7420 591.60

15 C2 18.35 R2 10 G3 0.9 7380 582.78

16 C2 18.35 R3 10.5 G1 0.7 7320 563.31

17 C2 18.35 R3 10.5 G2 0.8 7170 557.92

18 C2 18.35 R3 10.5 G3 0.9 7140 556.03

19 C3 18.9 R1 9.5 G1 0.7 7130 547.96

20 C3 18.9 R1 9.5 G2 0.8 7120 546.41

21 C3 18.9 R1 9.5 G3 0.9 7110 539.40

22 C3 18.9 R2 10 G1 0.7 7090 512.03

23 C3 18.9 R2 10 G2 0.8 7060 499.16

24 C3 18.9 R2 10 G3 0.9 7040 472.71

25 C3 18.9 R3 10.5 G1 0.7 7010 460.77

26 C3 18.9 R3 10.5 G2 0.8 6920 436.17

27 C3 18.9 R3 10.5 G3 0.9 4320 337.25



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5. Analysis of data and recording the responses (signal to noise ratio)

The S/N ratio is the ratio of size of signal factor effect to the size of error factor effect.

The S/N ratio consolidates several repeated output responses into a single value which

reflects the amount of variation present. The S/N ratio measures the sensitivity of quality

characteristic to external noise factor which is not under control. The highest S/N ratio

implies the least sensitivity of output response to noise factors. On the basis of

characteristic, three S/N ratios are available namely lower the better, higher the better and

nominal the better. In this paper higher-the-better is used for maximizing tensile strength

of the weld.

Table 5: Results of Signal to Noise Ratio Analysis

CURRENT RPM GAP UTS N/mm2 SNR

17.8 9.5 0.7 407.1 52.194

17.8 9.5 0.8 447.39 53.0137

17.8 9.5 0.9 470.47 53.4506

17.8 10 0.7 485.98 53.7324

17.8 10 0.8 501.01 53.9969

17.8 10 0.9 535.54 54.5758

17.8 10.5 0.7 541.16 54.6665

17.8 10.5 0.8 547.72 54.7712

17.8 10.5 0.9 551.45 54.8301

18.35 9.5 0.7 566.84 55.0692

18.35 9.5 0.8 562.7 55.0055

18.35 9.5 0.9 578.33 55.2435

18.35 10 0.7 585.55 55.3513

18.35 10 0.8 591.6 55.4406

18.35 10 0.9 582.78 55.3101

18.35 10.5 0.7 563.31 55.0149

18.35 10.5 0.8 557.92 54.9314

18.35 10.5 0.9 556.03 54.902

18.9 9.5 0.7 547.96 54.775

18.9 9.5 0.8 546.41 54.7504

18.9 9.5 0.9 539.4 54.6382

18.9 10 0.7 512.03 54.1859

18.9 10 0.8 499.16 53.9648

18.9 10 0.9 472.71 53.4919

18.9 10.5 0.7 460.77 53.2697

18.9 10.5 0.8 436.17 52.7931

18.9 10.5 0.9 337.25 50.559

The higher-the-better performance characteristic is expressed

S/NHB= -10 log {(1/y12+1/y22…1/yn2)/n} (1)

Where n is the number of repetition of output response in the same trial and y is the

response. Table.5 shows the result of Signal to Noise Ratio (SNR).

The above table shows the result obtained from the orthogonal array and signal-to-

noise ratio. Here there is a need to increase the tensile strength of the tube, hence higher-

the-better is selected and the main effects for the Signal to noise ratio is plotted.


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6. Data Analysis using Taguchi techniques

Results of the ANOVA are presented in this section

Figure 8: Linear Model Analysis: SN Ratio

Versus Current, RPM and Gap

Figure 9: Linear Model Analysis: Men Versus

Current, RPM and Gap



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7. Experimental Results and Discussion

This section presents the results of the experiments such as effects of each parameter and

their combinations and analysis of residual plots.

7.1 Effect of Parameter Analysis

Analysis of Variance (ANOVA) is used to investigate and model the relationship between

a response variable and one or more independent variables. In this study, Taguchi notation

tells the number of runs, factors, and levels for each factor in the design. In this example,

the notation L27 (3x3) means a Taguchi orthogonal array with 3 factors with 3 levels

each. The experiment includes a signal factor which is ultimate tensile strength.

A dynamic response experiment, in which the goal is to optimize the relationship

between the input and the output of the system, includes a signal factor. Taguchi linear

model is used to study the influence of welding current, RPM, gap between tube as input

and electrode on ultimate tensile strength as output. In this experiment, the influence of

individual parameter and combination of parameters are also studied. The following are

the effects or results of parameters. The MINITAB software is used for this analysis. The

analysis shows Current is the most influencing parameter having rank 1, then the RPM

stands second followed by the Gap between tube and electrode in the third place.

For the S/N ratios, Current (p=0.000) and RPM (p=0.023) are significant at the 0.05

α-level.The remaining factor gap is not significant to the response.

If combination is considered for the S/N ratios, current* RPM (p=0.000), and current

*gap (p=0.017) are significant at the 0.05 α-level. The remaining RPM*gap is not

significant to the response.

R-Sq = 96.9% R-Sq (adj) = 89.8% shows that the process desired signal is more

Factors Current, RPM and gap between tube and electrode could explain the

variability of response of UTS with 89.8%

By this analysis, the effect or significance and rank of individual parameters were

obtained from the Linear Model Analysis of SN ratios. In addition, the individual

parameters contribution is also obtained from the Linear Model Analysis of Mean. i.e the

influence or the contribution by current, RPM and gap between tube and electrode are

88.1, 23.8 and 7.3 respectively.

In the experiment, pin hole or burn through is observed due to high current. Due to

that, the tensile strength obtained is the lowest among all values. This also apparently

appeared in the analysis as Unusual Observations for SN ratios as 27th element. R denotes

an observation with a large standardized residual. In Response Table for Signal to Noise

Ratios, higher is better was chosen as higher the ultimate strength, it is always better.

7.2 Main and Interaction Effects

The main and the interaction effects on SN ratio are shown in the figures 10 and 11

respectively. Figure 10 clearly shows that the mean of SN ratio increases with increase of

current till 18.35Amps and decreases with increase of current further. The mean of SN

ratio also increases with increase of RPM till 10 and decreases with increase of RPM

further. However, this effect is not appreciable when compared to that with respect to the


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variation in the current. The mean of SN ratio does not significantly change with increase

or decrease of the gap. Similar observations are seen for mean also in the main effect plot

(Fig 11).

The mean of SN ratio increases with the combination of increase of current and RPM

till 18.35Amps and RPM of 10 and decreases with increase of current and RPM further

(Fig. 12). The figure also shows that the mean of SN ratio increases with the combination

of current and RPM. It can also be seen in the figure that the mean of SN ratio does not

appreciably increase with the combination of RPM and gap for low and medium level and

further decreases for higher gap. Similar observations can also be seen for mean in the

main effect plot (Fig. 13).

Figure 10: Main Effect Plot for SN ratio Figure 11: Main Effect Plot for Means

7.3 Residual plots

In residual plots, two outliers are found and remaining data fall within the confidence

limits of 95%. This is also understood from frequency curve. Residual verses the fitted

value shows that data are scattered. Fig. 14 shows the Residual plot for SN ratio and Fig.

15 shows the residual plot for means. Unusual observation is noticed in 27th data as it has

reduced the UTS value (Fig. 14). Similar observations are seen in residual plots for mean

also.

Figure 12: Interaction Plot for SN ratio Figure 13: Interaction Plot for Means

Me

an

of

SN

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Main Effects Plot (data means) for SN ratios

Signal-to-noise: Larger is better

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Signal-to-noise: Larger is better

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CURRENT

18.90

17.80

18.35

RPM

10.5

9.5

10.0

GAP

0.9

0.7

0.8

Interaction Plot (data means) for Means



169

Figure 14: Interaction Plot for Means Figure 15: Residual Plot for SN ratio

8. Conclusions

This paper has presented the optimization of process parameters of OTIG welding of 6mm

propellant feed tubes made of 304L Stainless Steel material for satellite system using

statistical optimization techniques. Design of experiments as per Taguchi’s L27 array was

used for the study. Taguchi linear model analysis of Signal to Noise (SN) ratio and means

versus input parameters were carried out. Analysis of variance, interaction effects and

residual plots were also made. Based on experimental results and confirmation tests, the

following conclusions can be drawn.

The optimum values obtained from the selected factors for welding of 6mm

SS304L tube are Current 18.5 A, RPM 10 and Gap between tube and electrode

0.8mm.

The most important parameter affecting the responses have been identified as

Current and this is followed by the RPM. Therefore, keeping good control over

the weld current is the key action which decides the weld strength and its quality.

Validation of weld strength is determined by tensile test. Also it was found that

there is good improvement in tensile strength values after optimizing the

parameters. Hence a good quality weld is obtained from optimized current, RPM

and gap chosen in the experiment.

By this study, the optimized process parameters would definitely improve the

reliability of propellant feed system for satellite propulsion which is highly

essential for the successful space missions.

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Acknowledgment

Authors wish to thank Shri S. Ravi, Shri V. N. Misale, Shri C. Paulmany and team, and

Dr. Heeralal Gargama for their useful contributions in this work. Authors wish to express

their gratitude to Shri G. Narayanan, Deputy Director, SRQA, ShriS . Somanath, Director

LPSC and ASC Committee for their constant encouragements and kind permission to

submit this paper in any technical journal.

M. Karthikeyan is a Deputy Division Head for Satellite control components in System

Reliability and Spacecraft Propulsion and Sensors Group, LPSC (Liquid Propulsion

System Centre), ISRO (Indian Space Research Organisation), Bangalore. He took his

Master degree in Quality Management (MS) from Birla Institute of Technology and

Science, Pilani. He has wide experience in handling Chemical propellants like Cryogenic,

Bi and Mono propellants for Rockets and Satellites. He has been responsible for filling

propellants to Rocket stage and Spacecraft at launch Pad like Sriharikota, India and

KOUROU, French Guyana. He is responsible for quality surveillance which involves

OTIG welding for satellite integration. He is recipient of Team excellence award for

GSAT-12 (2011). He worked as Project Manager for several Indian satellite missions in

propulsion aspects. [email protected]



172

Vallayil N.A.Naikan is a Professor and Head in Reliability Engineering Centre at Indian

Institute of Technology (IIT), Kharagpur, India. He took his Masters and Doctorate degree

in Industrial Engineering and Management with specialization in Quality and Reliability

Engineering from IIT, Kharagpur, India. He has worked with Union Carbide (India),

Indian Institute of Management, Ahmedabad, and Indian Space Research Organization,

Ahmedabad, India. He has visited the Chinese University of Hong Kong for doing

research and has worked and visiting professor in the University of Maryland USA. He

has published his research works in many international journals, and presented papers in

many conferences. He is also actively involved in industrial consultancy and sponsored

projects. His research interests include condition monitoring, mechanical system

reliability, probabilistic risk and system assessment, quality planning and management,

reliability of engineering systems, and simulation. [email protected].

R. Narayan is Group Director in System Reliability and Spacecraft Propulsion and

Sensors Group, LPSC, ISRO, Bangalore. He joined in ISRO and served in

Thiruvananthapuram and presently in Bangalore. He took his Master degree in Quality

Management from Birla Institute of Technology and Science, Pilani. He has got

experience in filling propellant to Rocket stages and Space crafts at launch Pad like

Sriharikota, India and KOUROU French Guyana. He has been chief of Quality Assurance

for several Indian satellites in propulsion aspects. He has been responsible for effecting

essential improvements in propulsion components. He is the chief consultant for

resolving any satellite problem during launch. He has published many international

journals, and presented papers in conferences. [email protected]

D. P. Sudhakar is a Deputy General Manager in LPSC, ISRO responsible for delivering

all Liquid stages for PSLV and GSLV of Indian launch vehicles. He has completed his

Master degree in Anna University Chennai and his Doctorate degree in Kerala University

India. He has published in many international journals, and presented papers in

conferences. He is a manufacturing specialist and trained in Russia in Cryogenic Rocket

technology. He worked as Quality Control chief for Liquid propulsion rocket systems. He

is a recipient of best alumina award in school and team excellence award in ISRO.

[email protected]




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