^{1}

^{2}

^{3}

Poor combination of input process parameters has resulted in an innumerable amount of weld failure due to its negative influence on the microstructural and mechanical properties of the welded joints. To improve the welded joint, it is imperative that the material toughness be optimized. The aim of this study is to predict and enhance the toughness of mild steel welded joint using Response Surface Methodology (RSM). 10 mm mild steel plate was cut into 200 piece coupons measuring 27.5 × 10 × 10 mm for the experiment, after welding of the piece, 100 specimens of 55 × 10 × 10 mm were produced and the experiment was performed 20 times. Charpy impact tester was employed to measure the degree of toughness of the material, and results were analyzed using RSM. The results produced an optimum impact test of 275.514 joules at a desirability value of 95.6%. This optimum impact test was achieved through the use of current of 120.00 amp, voltage of 20.00 volt and gas flow rate of 12.00 L/min. The weld current was found to have a greater influence on the impact strength of the weldment as compared to voltage and gas flow rate at a moderate level.

Toughness is usually regarded as the ability of materials to absorb energy before fracture, although, failure in welded materials can also arise from resonance (Etin-Osa and Achebo 2017) [

This research work was conducted at the Department of Welding and fabrication technology, Petroleum Training Institute (PTI), Warri, Delta State, Nigeria. The Tungsten Inert Gas (TIG) welding method was adopted, thereafter, the samples from the welding process were subjected to impact test.

The 10 mm mild steel plate was cut into 200 piecescouponsmeasuring 27.5 × 10 × 10 mm for the experiment, after welding of the piece, 100 specimens of 55 × 10 × 10 mm were produced, the experiment was performed twenty (20) times as presented in

The study produced twenty experimental runs, each experimental run comprising the current, voltage and gas flow rate, used to join two pieces of mild steel plates measuring 55 mm × 10 mm × 10 mm. The impact test strength, was measured. The responses are shown in

According to literature, the difference between the predicted and adjusted R^{2} must be less than 0.2. In this research, a difference of 0.131 which is less than 0.2 was obtained in

Process parameters | Unit | Symbol | Low (−) | High (+) |
---|---|---|---|---|

Welding Current | Amp | I | 120 | 170 |

Welding Voltage | Volts | V | 20 | 25 |

Gas Flow Rate | Lit/mill | F | 12 | 14 |

Run | A: Welding Current | B: Welding Voltage | C: Gas Flow Rate | Impact Test |
---|---|---|---|---|

Amp | Volts | Lit/mill | J | |

1 | 145 | 22.5 | 13 | 235.144 |

2 | 145 | 22.5 | 13 | 227.136 |

3 | 187.045 | 22.5 | 13 | 259.168 |

4 | 145 | 22.5 | 11.3182 | 257.712 |

5 | 170 | 20 | 12 | 234.416 |

6 | 145 | 18.2955 | 13 | 230.048 |

7 | 170 | 25 | 14 | 265.502 |

8 | 120 | 20 | 14 | 235.872 |

9 | 170 | 25 | 12 | 243.152 |

10 | 120 | 25 | 12 | 270.088 |

11 | 120 | 20 | 12 | 272.272 |

12 | 102.955 | 22.5 | 13 | 278.096 |

13 | 170 | 20 | 14 | 219.128 |

14 | 145 | 22.5 | 14.6818 | 232.232 |

15 | 145 | 22.5 | 13 | 230.776 |

16 | 145 | 22.5 | 13 | 234.072 |

17 | 145 | 26.7045 | 13 | 249.704 |

18 | 145 | 22.5 | 13 | 238.784 |

19 | 120 | 25 | 14 | 261.352 |

20 | 145 | 22.5 | 13 | 219.128 |

Std. Dev. | 6.35 | R^{2} | 0.9354 |
---|---|---|---|

Mean | 244.69 | Adjusted R^{2} | 0.8773 |

C.V. % | 2.59 | Predicted R^{2} | 0.7463 |

Adeq Precision | 12.8046 |

^{2} and C^{2} obtained a P-val less than 0.05. This would increase the accuracy of our mathematical model in predicting the responses.

Source | Sum of Squares | df | Mean Square | F-value | p-value | |
---|---|---|---|---|---|---|

Model | 5836.6 | 9 | 648.51 | 16.1 | <0.0001 | significant |

A-Welding Current | 873.47 | 1 | 873.47 | 21.68 | 0.0009 | |

B-Welding Voltage | 909.73 | 1 | 909.73 | 22.58 | 0.0008 | |

C-Gas Flow Rate | 479.55 | 1 | 479.55 | 11.9 | 0.0062 | |

AB | 126.51 | 1 | 126.51 | 3.14 | 0.1068 | |

AC | 340.57 | 1 | 340.57 | 8.45 | 0.0156 | |

BC | 533.04 | 1 | 533.04 | 13.23 | 0.0046 | |

A² | 2399.52 | 1 | 2399.52 | 59.56 | <0.0001 | |

B² | 107.94 | 1 | 107.94 | 2.68 | 0.1327 | |

C² | 296.85 | 1 | 296.85 | 7.37 | 0.0218 | |

Residual | 402.85 | 10 | 40.28 | |||

Lack of Fit | 159.88 | 5 | 31.98 | 0.658 | 0.6714 | not significant |

Pure Error | 242.97 | 5 | 48.59 | |||

Cor Total | 6239.45 | 19 |

Based on the P-value obtained in

IT = 230.91 − 8.00 A − 5.93 B − 5.93 C + 3.98 AB + 6.52 AC + 8.16 BC + 12.90 A 2 + 2.74 B 2 + 4.54 C 2 (1)

where, A = voltage, B = current, C = gas flow rate.

The plot in

The 3D surface plot was employed to examine the effect of the welding voltage and current on the impact absorption of mild steel specimen. At a gas flow rate of 13 L/min, the current and voltage could be varied to obtain the 3D surface plot architect presented in

The interphase in

Finally, from the optimal solution, the contour plots showing the impact response variable of voltage and current at a gas flow rate of 12 L/min, against the optimized value of the input variable is presented in

Number | Welding Current | Welding Voltage | Gas Flow Rate | Impact Test | Desirability | |
---|---|---|---|---|---|---|

1 | 120.000 | 20.000 | 12.000 | 275.514 | 0.956 | Selected |

2 | 120.000 | 20.036 | 12.000 | 275.378 | 0.954 | |

3 | 120.010 | 20.000 | 12.007 | 275.285 | 0.952 | |

4 | 120.177 | 20.000 | 12.000 | 275.201 | 0.951 | |

5 | 120.000 | 20.119 | 12.000 | 275.069 | 0.949 |

The Fit Statistics for the Impact Test in ^{2} value of 0.9354, Predicted R^{2} of 0.7463 and an Adjusted R^{2} of 0.8773 were obtained. The difference between the predicted and adjusted R^{2} of less than 0.2 was recorded, indicating a significant model. With an adequate Precision of 12.805, demonstrating a significant model. It meant that the mathematical model in Equation (1), can be employed to navigate the design space. In

In this study a mathematical model for predicting impact test in Equation (1) has been developed with and an optimum impact test of 275.514 joules at a desirability value of 95.6%. This optimum impact test was achieved through the use of current of 120.00 amp, voltage of 20.00 volt and gas flow rate of 12.00 L/min. The weld current was found to have a great influence on the impact strength of the weldment as compared to voltage and gas flow rate at a moderate level.

The authors declare no conflicts of interest.

Ebhota, L.M., Ogbeide, O.O. and Abhulimen, I.U. (2021) Prediction and Selection of the Best Process Parameters to Improve Toughness of Mild Steel Welded Joints. Open Access Library Journal, 8: e7743. https://doi.org/10.4236/oalib.1107743