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This project work focuses on the reduction of weld undercuts using the Taguchi method. The phenomenon of weld undercuts constitutes a major problem for the welding industry. When undercuts occur, and particularly when such cuts are deep, it has a negative impact on the weld as it lowers the integrity and quality of the weldment. Therefore, efforts are made globally to reduce the depth of such weld undercuts to the barest minimum. Several optimization methods have been adopted; however, in this study, the Taguchi method is applied. “The smaller the better components” of the Taguchi method is applied. From the results obtained from applying this Taguchi method, the optimum process parameters obtained are A
_{2}-B
_{1}-C
_{2}, which are a voltage of 20 V, a current of 180 A, and a welding speed of 130 mm/s, required to form an undercut of 0.03 mm. Whereas the existing process parameters used by the company are A
_{1}-B
_{3}-C, which make an undercut to a depth of 0.09 mm. It is concluded that the use of Taguchi method has been able to reduce the depth of undercut as shown in this study. A step-by-step approach is presented in the study.

Weld bead geometry is severely negatively affected by the occurrence of the undercut phenomenon. Undercuts can be described as the presence of grooves along the edges of the weldment, usually observed in the welding of unskilled welders. These welders are often satisfied with their metal material joints simply being held together by the solidified molten weld metal upon cooling, without considering whether or not there was an adequate molten weld metal penetration in the parent metal’s joints gaps. Petershagen [

This study attempts to find the most economic solutions of applying models, tested in other areas, to weld technology. This is all in the bid to optimize gas metal arc welding process parameters required to reduce the formation of undercuts to the barest minimum. The integrity of the weldment could be improved by tweaking the welding process and removing practices which undermine the process. The top and undersides of a weldment are expected to meet specific standards. The top side which constitutes the bead is expected to have optimized bead geometry, where the bead height and width are preferred to be smaller in size. The undersides, where undercuts predominantly occur, usually appear in between parent metal joints at the point where the weldment ought to have leveled out with the parent metals.

When there is insufficient molten weld metal penetration and the point where the weld metal does not level out with the parent metal, the difference between the parent metal and the weld metal determines the size of the weld undercut. Garg et al. [

The heat affected zone (HAZ) includes the part of the parent metal which is intensively affected by the arc heat. Parts of this parent metal may melt and the melt would either flow into a lower part of the parent metal or vaporize into the atmosphere. This movement of melt could eventually cause the cross-sectional area of the parent metal to be reduced.

Alloying elements vaporization must also be taken into consideration. Alloying elements like zinc and magnesium are very volatile when exposed to intense heat. Therefore, if these alloying elements are exposed to prolonged heat application during welding, they have the tendency to vaporize. Garg et al. [

The authors also said that insufficient deposition of filler metal during welding could also cause weld undercut. This insufficient filler metal deposition could be as a result of very low voltage and current. These very low levels of voltage and current may not generate enough arc heat to melt the filler metal. Xu et al. [

In this study, “the smaller the better part” of the Taguchi method is applied to reduce the formation of weld undercuts to the barest minimum.

Five weld joints were made on a 60 mm × 40 mm × 10 mm mild steel plate. This process was repeated nine times producing a total of forty five weld joints. For each application of the nine process parameters, the five undercuts made in each weld joint were measured and the average recorded. In all, a total of nine undercuts were recorded as presented in

Spadea and Frank [

Experiment runs | voltage | current | Welding speed | Undercut sizes, mm |
---|---|---|---|---|

1 | 1 | 1 | 1 | 0.08 |

2 | 1 | 2 | 2 | 0.05 |

3 | 1 | 3 | 3 | 0.12 |

4 | 2 | 1 | 3 | 0.03 |

5 | 2 | 2 | 1 | 0.15 |

6 | 2 | 3 | 2 | 0.09 |

7 | 3 | 1 | 2 | 0.11 |

8 | 3 | 2 | 3 | 0.07 |

9 | 3 | 3 | 1 | 0.06 |

The methods adopted in conducting this study are as follows:

An L_{9} Taguchi orthogonal matrix layout, shown in

A welding operation was done by utilizing the range of process parameters as shown in

Undercuts in the weldments were measured and recorded;

The Taguchi optimization process relating to the smaller the better was applied using Equation (1);

The signal to noise ratio generated was clustered and arranged into their respective positions indicating their optimum process parameters as shown in

Signal to noise ratio vs process parameters graphs were drawn to show the effect of S/N ratios on the process parameters in relation to the optimum process parameters as shown in Figures 3-5;

Factors | Levels | ||
---|---|---|---|

1 | 2 | 3 | |

A. Voltage, V | 18 | 20 | 22 |

B. Current, A | 180 | 200 | 260 |

C. Welding speed, mm/s | 85 | 105 | 130 |

A Level | A Voltage | B Current | C Welding Speed | Total Average of S/N Ratio | |||
---|---|---|---|---|---|---|---|

Sum of S/N ratio | Average of S/N ratio | Sum of S/N ratio | Average of S/N ratio | Sum of S/N ratio | Average of S/N ratio | 22.3259 | |

1 | 66.3752 | 22.1251 | 71.5680 | 23.8560^{*} | 62.8534 | 20.9511 | |

2 | 67.8510 | 22.6170^{٭} | 65.5968 | 21.8656 | 66.1080 | 22.0360 | |

3 | 66.7072 | 22.2357 | 63.7686 | 21.2562 | 71.9720 | 23.9907^{٭} |

^{*}signifies the optimum level based on the smaller-the-better criterion.

The analysis of variance of the parameters shown in

A confirmation test as shown in

Applying the smaller the better methodology by using Equation (1).

Signal to Noise ratio for the smaller the better function is

where, n is the sample size and y is the mean weld undercut in mm.

The S/N ratio for the individual control factors as extracted from

For voltage

Parameter | Process Parameter | Degree of Freedom | Sum of Squares | Variance | F-ratio | Contribution (%) |
---|---|---|---|---|---|---|

A | Voltage, V | 2 | 0.1332 | 0.0666 | 0.00096 | 0.09 |

B | Current, A | 2 | 3.6973 | 1.8487 | 0.0268 | 2.53 |

C | Welding Speed, mm/s | 2 | 4.7457 | 2.3729 | 0.0344 | 3.24 |

Error | 2 | 137.8180 | 68.9090 | - | 94.14 | |

Total | 8 | 146.3942 | - | - | 100 |

Existing Process Parameters | Optimum Process Parameters | Improvement in S/N ratio | |
---|---|---|---|

Process Parameters | A_{1}-B_{3}-C_{1} | A_{2}-B_{1}-C_{3} | |

Undercut Measurement (mm) | 0.09 | 0.03 | 0.06 |

S/N dB | 23.4759 | 25.8119 | 2.336 |

Average S/N ratio for voltage

For current,

Average S/N ratio for current

For welding speed,

Average S/N ratio for welding speed

The sum and average S/N ratios for the process parameters are shown in

Athreya and Venkatesh [_{2}-B_{1}-C_{3}.

Figures 3-5 show the S/N ratio graph where the dashed line is the value of the total mean of the S/N ratios. The Figures clearly show the interactions between the levels of each process parameters.

Analysis of Variance (ANOVA)

Sum of square,

where n is the number of test conducted

% Contribution =

F-ratio =

Confirmation Test

Using

where

The optimal parameters are A_{2}-B_{1}-C_{3} and the corresponding S/N ratios are 22.6170, 23.8560 and 23.9907 respectively.

The total mean of S/N ratio,

Therefore,

The existing process parameters in use for welding processes are A_{3}-B_{1}-C_{2}.

Its S/N ratio is calculated as follows:

This paper tends to use the Taguchi optimization tool relating to the smaller the better technique in reducing the depth of weld undercut made during the welding operations of weldments. The orthogonal matrix layout in

From _{2}-B_{1}-C_{3}. This indicates that a welding process parameter consisting of 20 V, a current of 180 A and a welding speed of 130 min/s are required to make an undercut in the weldment to a value of 0.03 mm. The existing process parameter utilized by the company is A_{1}-B_{3}-C_{1} which made an undercut with an average depth of 0.09 mm. By using the optimum welding process parameters, an improvement of an S/N ratio of 2.336 and 0.06 mm depth of undercut were made over the S/N ratio and depth of cut made by applying the existing process parameters.

The effects of the S/N ratios on the process parameters were also investigated and are expressed in Figures 3-5.

However, the extent of these process parameters consisting of voltage, current and welding speed in reducing the depth of the weld undercut were measured by determining the contribution made by each process parameter. The contribution made by these process parameters were determined by applying the analysis of variance approach as presented in

From

_{1}- B_{3}-C_{1}, utilized by the company has an S/N ratio of 23.4759 db which produced a weldment with an average undercut measurement of 0.09 mm. The optimum process parameters of A_{2}-B_{1}-C_{3} obtained by applying the Taguchi method has an S/N ratio of 25.8119 db which produced a weldment with an average undercut of 0.03mm, from the above, it can be deduced that by using the optimum process parameters there is a reduction of 0.06mm of undercut from 0.09 mm obtained in the weldment made by using the existing process parameters. This also gave an improvement in S/N ratio of 2.3360 db over that made by using the existing process parameters.

Weld undercuts are a very important factor considered in assessing the integrity of weldments. The higher the depth of the undercut, the lower the integrity of the weldment. Therefore, this study is designed to determine optimum process parameters that can reduce this undercut to the barest minimum. The application of the wrong process parameters can greatly affect the integrity of the weldment.

Taguchi orthogonal array, L_{9}, was used to design the various compositions or combinations of the process parameters. These process parameters were used to make weldments. The undercuts formed in these weldments were measured by using a Wiki Scan as a laser based tool, which does a 3D scan of joints and weld information on size, porosity and undercut.

The S/N ratios of the measured undercuts were determined and the optimum process parameters were obtained by applying “the smaller the better technique” of the Taguchi method. Also the contributions of each of the process parameters were determined to find out the extent to which each of the process parameters affected the formation of undercut in the corresponding weldment. It was found that welding speed has the greatest input in the reduction of weld undercuts.