1. Introduction
Gas metal arc welding is a welding process which has various industrial applications. In GMAW an electric arc produced between electrode & work piece metals, this causes electrode & work piece metal to melt and join1). The joining area is called as weld4). GMAW process parameters affecting the quality of welding & productivity. In auto ancillary fabrication industry length of penetration is one of the most important factors which should achieve during welding. This Length of penetration is measured at depth of T/20 and minimum requirement of which is 0.8xT so as to get optimum weld strength where T is thickness of metal to be welded. The input parameters which are mainly responsible for welding penetration is welding current, welding voltage, welding speed & these parameters plays an important role in achieving the desired penetration in welded joints. In this research work, EN 10025 S 235 Grade mild steel material has been used with specimen size of 100 × 80 × 3mm. Design of experiments based on Taguchi technique is used to evaluate the data. An orthogonal array, signal to noise (S/N) ratio and analysis of variance (ANOVA) has been used to find the best welding penetration in mild steel materials by using different range of input parameters.
We have used Taguchi DOE method for the experimentation as the method is an efficient method for optimization of process parameters. The method was developed by Dr. Genichi Taguchi, who was a Japanese quality management consultant; the method is based on orthogonal array experiments, which gives reduced optimum setting of process control parameters. The method was selected for the experimentation because the method is cost effective, less time consuming, greater accuracy, and robust design for large scale application. By using Taguchi method for process parameters optimization Design of Experiments (DOE) were selected. For the experimentation 3 factors & 3 level Taguchi design model was used, Based on this model we have generated L9 orthogonal array & total 9 runs were taken for the experiment. Minitab 15.0 software was used to generate the orthogonal array through Taguchi Design.
2. Experimental Procedure
2.1 Sample preparation
Table 1
EN 10025 S 235 Grade Material Properties
Table 2
Filler Wire Material Properties
We have used Gas Metal Arc Welding process for the experimentation, as the process has high weld deposition efficiency & better weld quality with low operation cost. Experiments were carried out by using semiautomatic controlled GMAW processAK 600AMP welding machine. Copper coated steel wire of ER70 S6 grade with 0.8 mm diameter in the coil form was used for the welding. Shielding gas Ar-C02 used for shielding purpose with percentage of Ar: 82% & CO2:18%2).
Mild Steel plate EN10025 S 235 Grade used as base material with specimen size of 100 × 80 × 3mm. The welding is carried out in 2F welding position & the joint taken for welding was Tee joint. The welding is done by the qualified welder and is done as per the guidelines of EN ISO 15609-13).
2.2 Experimental Procedure.
2.2.1 Selection of important process parameters
The controllable process parameters6) were selected conduct the experiments are Current, Voltage &Wire speed. The objective of this study as discussed earlier is to optimize the welding process parameters for enhancing weld penetration. A total of the three welding process parameters were chosen as the controlling factors.
2.2.2 To decide the upper & lower limits process parameters
The selection of upper & lower limit of process parameters has been done on the basis of machine manufacturer ESAB guideline & standards which is based on filler wire diameter, shielding gas percentage & type of weld joint.
2.2.3 Development of orthogonal array
A matrix experiments consists of a set of experiments where the settings of the various products or process parameters are changed that which are to be studied from one experiment to another. After conducting a matrix experiment the data from all experiments in the set taken together are analyzed to determine the effects of several parameters. Conducting matrix experiments using special matrices, called Orthogonal Array7), allows the effects of several parameters to be determined efficiently and is an important technique in Parameter Design.
L9 orthogonal array5) was developed by using 3 factors & 3 levels Taguchi design.
Experimentation for the welding process as per the parameters mentioned in orthogonal array8)
2.2.4 Preparation of samples & experimental trials
Total 09 Nos. of trials conducted as defined in orthogonal array sampling plan, test piece used for
the trials as per the defined size (100×80×3 mm)
2.2.5 Penetration inspection
After preparation of all 09 Nos. of samples, length of penetration is measured by taking cut section of all samples & using semi automatic micro graphical microscope (Rapid-I designed by Customised techonologies Bangalore, India).
Microscope with embedded camera can be controlled and programmed using computer. For the ease of operation already saved program can be used and real time reports are generated in generic format.
2.2.6 Analyze Taguchi design
To analyze the Taguchi Design, Signal to Noise(S/N) ratio calculated5). As we required maximum length of penetration as a result, larger is better option was selected. After experimentation, S/N ratio calculated through the penetration reading.
Graph for the response table for SN Ratios & Response table for Means can be plotted as follows
As it can be seen from main effects plot for SN ratio and main effects plot for means; favorable outcome i.e. optimal (maximum) length of penetration will be at a combination of A-1, B-2, C-3. If seen at combinations obtained through L-9 orthogonal array; there is no such combination exists as stated above, so for verification new sample is prepared with parameter combination as A-1 current 160 A, B-2 voltage 19 V and C-3 wire speed 12 m/min. And after doing routine procedure for penetration inspection; the length of penetration is observed as 7.2334 mm which is higher than the lengths of penetration observed earlier.
Thus from this we can say that optimal set of parameters to get optimal(maximum) length of penetration for EN 10025 S 235 grade material is 160 A current, 19V voltage and 12 m/min wire speed.
Table 4
Sampling plan for process parameters as per taguchi method
Table 5
S/N Ratio calculation
2.2.7 Adequacy of the model
Table 6
Response for S/N ratios; larger is better
| Level | A | B | C |
|---|---|---|---|
| 1 | 14.86 | 12.89 | 12.23 |
| 2 | 13.39 | 14.29 | 13.69 |
| 3 | 8.19 | 12.99 | 14.43 |
| Delta | 6.67 | 1.40 | 2.20 |
| Rank | 1 | 3 | 2 |
Table 7
Response table for means
| Level | A | B | C |
|---|---|---|---|
| 1 | 5.59 | 4.38 | 4.09 |
| 2 | 4.75 | 5.29 | 4.80 |
| 3 | 3.92 | 4.58 | 5.35 |
| Delta | 1.67 | 0.91 | 1.26 |
| Rank | 1 | 3 | 2 |
The model checked for the adequacy by using ANOVA i.e. Analysis of Variance Method.
As per ANOVA, the value of P should be less than 0.05 for every parameter for the desired adequacy & it has been confirmed that the model is adequate from the below results.
General Linear Model: SNRA1 versus A, B, C.
2.2.8 Optimum process parameters
Regardless of the category of the quality characteristic, a greater S/N ratio corresponds to better quality characteristics. Therefore, the optimal level with the greatest S/N ratio. The optimal levels of process are:
From the graph of S/N Ratio & mean values
Current - A 1 = 160 A
Voltage - B 2 = 19 V
Wire Speed - C 3 = 12 m/min.
3. Conclusion
The present study is carried to discuss an application of the Taguchi Method for obtaining the maximum length of penetration. We have conducted various trials for the optimization of process parameters & reach to following conclusion.
1) Optimization of process parameters can be effectively done by using 3 levels, 3 factors & L9 orthogonal array
2) Better penetration observed is 7.2334 mm at Current 160 A, Voltage 19 V, Wire Speed 12 M/min
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