# Optimization of welding parameters on pores migration in Laser-GMAW of 5083 aluminum alloy based on response surface methodology

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## Abstract

The mathematical model to reveal the relationship between process parameters and pores migration distance of 5083 aluminum alloy Laser-GMAW was established based on the center composed design of response surface methodology. Three of the welding parameters were chosen as the factors: The welding current, welding speed, and the laser-arc distance. The distance between pores and weld bottom was calculated as the response value. The analysis of variance was used to test the significance of the model. According to the result of experiments, the low welding speed and high current were beneficial to decrease the solidification rate of the molten pool, the pores could overflow from the weld in time. When welding speed is increasing, it is necessary to appropriately increase the welding current and the Laser-arc distance, the volume of the molten pool would be expanded, pores were difficult to be caught by solidifying wall. The optimized parameters of the 30 mm aluminum alloy Laser-MIG hybrid welding were calculated based on the established model. The maximum pores migration distance can be obtained in 135 A/0.6 m/min/1.22 mm (welding current/welding speed/laser-arc distance).

## Keywords

Laser-arc welding Pores migration Response surface methodology## 1 Introduction

The molten aluminum alloy has a small viscosity. During the Laser-GMAW of aluminum alloys, the keyhole is unstable, It easily collapses to cause pores-defects. Katayama [1] found that the high welding current is conducive to reduce the pores-defects, but further increasing the thickness of the workpiece (more than 4 mm), the pores-defects were still difficult to control. Marsunawa [2] used an X-ray high-speed imaging system to observe the keyhole stability and molten pool behavior during laser welding. They found the keyhole collapsing is a major factor of the pores generation, and Zhao [3] got a similar conclusion with him. Zhang et al. [4] studied the relationship between the molten pool characteristics and the porosity of 6082 aluminum alloy in Laser hybrid welding. He pointed out that arc could expand the volume of the molten pool, and the larger volume of the molten pool, the easier escaping for the pores migration. However, most researchers used the workpieces thickness less than 8 mm, And there is less study on optimizing process parameters of the large-thickness aluminum alloy (more than 8 mm) in the Laser-GMAW. Haboudou [5] pointed out that the content of hydrogen and the content of Mg/Zn alloy are important factors leading to the collapsing of the keyhole. Zhang [6] believed that Laser-CMT hybrid welding can improve the stability of the laser welding and reduce the porosity in the weld. Ola and Doern [7] pointed out that the laser-arc distance could affect the pores defects. Nielsen [8] believed that laser hybrid welding was suitable for thick workpieces welding, compared with Laser welding, it has lower requirements for welding assemblage. Bunaziv [9] found that the addition of helium gas can effectively reduce the pores defects, but it has no obvious effect on the welding stability. However, it is difficult to completely solve the pores defects in aluminum alloy laser welding. It is necessary to optimize the process parameters of the aluminum alloy Laser-GMAW, and the pores in the aluminum alloy welding can float out of the weld as much as possible.

There are interactions between various parameters, so it is not possible to design a single factor test to establish the mathematical model. Response surface methodology (RSM) can effectively evaluate the interaction between various parameters. Many models were built to optimize the welding parameters in the welding area by using RSM. Cai [10] used RSM to optimize the ternary shielding gas composition in the tandem narrow gap welding of Q235, When the shielding gas composition was at 10% CO_{2}, 79% Ar and 11% He, the welding solidifying wall penetration was increased, and the solidifying wall defects was avoided. Karthikeyan and Balasubramanian [11] used RSM to predict the optimized friction spot welding process parameters for joining AA2024 aluminum alloy. Olabi [12] used the response surface methodology to optimize the CO_{2} laser welding process parameters of dissimilar materials. Benyounis [13] used RSM to optimize the laser-welding parameters of medium carbon steel. Elatharasan and Kumar [14] used the Response Surface Method to optimize the process parameters of 6061 aluminum alloy friction stir welding, and they also established a model to reveal the relationship between the process parameters and the mechanical properties.

In this study, Based on the response surface methodology, A mathematical model was established to reveal the relationship between Laser-welding parameters and the pores migration distance. An optimized parameter was found, so the pores will overflow from the weld during the 5083 aluminum alloy thick workpiece Laser-GMAW. In our previous work, Welding current, welding speed and Laser-arc distance were chosen as the factors. Welding current ranges in 100–150 A, When the welding current is higher than 150 A or lower than 100 A, the welding is unstable. Welding speed ranges of 0.50–1.00 m/min, and the distance between the laser and arc ranges of 0.00–6.00 mm.

## 2 Experimental set-ups

Chemical composition of base metal and filler metal (wt%)

Element | Al | Si | Cu | Mg | Zn | Mn | Ti | Cr | Fe |
---|---|---|---|---|---|---|---|---|---|

Base metal | Bal. | ≤ 0.40 | ≤ 0.10 | 4.0–4.9 | ≤ 0.25 | 0.40–1.0 | ≤ 0.15 | 0.05–0.25 | 0.00–0.40 |

Filler metal | Bal. | 0.40 | 0.10 | 4.3–5.2 | 0.25 | 0.50–1.00 | 0.05–0.20 | 0.05–0.25 | 0.40 |

The sections were cut off in the middle part of the weld, Five specimens were cut from each weld, there were 3–5 mm interval between each specimen. Both sections of the specimens were polished by sandpaper, the distance between the pores and the bottom of the weld was measured by polarizing microscope (Leica DM4000). The pores migration distances of five specimens were calculated and averaged to reduce the measurement error. The average value was given as the final response value.

- (a)
Selecting variables and a range of variables.

- (b)
Designing test plans with software design-expert 7.0.

- (c)
Performing welding experiments according to test plan.

- (d)
Processing specimens, observing and recording experiment results.

- (e)
Entering the experimental results, and using software calculations to establish the fitting equations and models.

- (f)
Testing the accuracy of the fitted model.

- (g)
Analysis of results.

Based on the mathematical model, an optimized parameter was given to get the largest pores migration distance, then compare the fact-value and theoretical value.

## 3 Create a fitting model

_{i}—coded value of Variable X; X

_{max}—The maximum value that the variable X can reach; X

_{min}—The minimum value that the variable X can reach.

In this study, the welding current (I), welding speed (V) and Laser-arc distance (L) were selected as the experiment factors. When the aluminum alloy is laser-welded, Huang [15] found the keyhole collapsing often occur at the bottom of the weld. The distance between the weld bottom to the pores can be considered as the pores migration distance. The distance from the pores to the bottom of the weld (S) is chosen as the response value to reveal the parameters’ effect on the pores migration.

The correspondence between the encoded values and the actual values

Factors | Unit | Coded value | ||||
---|---|---|---|---|---|---|

− 1 | 1 | 0 | − 1.68 | 1.68 | ||

I | A | 110 | 140 | 125 | 100 | 150 |

V | m/min | 0.60 | 0.90 | 0.75 | 0.50 | 1.00 |

L | mm | 1.22 | 4.78 | 3.00 | 0.00 | 6.00 |

the experimental protocol

Run | Current | Welding speed | Laser arc distance | The distance of pores floating (mm) |
---|---|---|---|---|

1 | 0 | 0 | 0 | 13.524 |

2 | 0 | 0 | 0 | 13.300 |

3 | 1 | − 1 | 1 | 16.400 |

4 | 0 | 0 | 0 | 15.090 |

5 | 0 | 0 | 0 | 13.415 |

6 | 1 | 1 | 1 | 16.880 |

7 | − 1 | 1 | − 1 | 11.120 |

8 | 0 | − 1.68 | 0 | 17.020 |

9 | 1 | 1 | − 1 | 12.270 |

10 | − 1 | 1 | 1 | 10.650 |

11 | − 1.68 | 0 | 0 | 6.930 |

12 | − 1 | − 1 | 1 | 10.740 |

13 | 0 | 0 | 1.68 | 14.020 |

14 | − 1 | − 1 | − 1 | 16.800 |

15 | 0 | 0 | − 1.68 | 13.150 |

16 | 0 | 1.68 | 0 | 13.640 |

17 | 0 | 0 | 0 | 13.900 |

18 | 0 | 0 | 0 | 11.140 |

19 | 1.68 | 0 | 0 | 12.763 |

20 | 1 | − 1 | − 1 | 18.045 |

*P*value is larger than the F value) is 0.3264 (more than 0.05), which means that the lack of fit is not significant. The mathematical model is convergent.

the F-test data of the fitted equations

Source | Sum of squares | Df | Mean square | F value |
| |
---|---|---|---|---|---|---|

Model | 125.30 | 9 | 13.92 | 12.43 | 0.0002 | Significant |

I | 42.51 | 1 | 42.51 | 37.95 | 0.0001 | |

V | 20.54 | 1 | 20.54 | 18.34 | 0.0016 | |

L | 0.32 | 1 | 0.32 | 0.29 | 0.6028 | |

I * V | 0.028 | 1 | 0.028 | 0.025 | 0.8771 | |

I * L | 11.27 | 1 | 11.27 | 10.06 | 0.0100 | |

V * L | 17.54 | 1 | 17.54 | 15.66 | 0.0027 | |

I * I | 10.91 | 1 | 10.91 | 9.74 | 0.0109 | |

V * V | 16.46 | 1 | 16.46 | 14.69 | 0.0033 | |

L * L | 2.94 | 1 | 2.94 | 2.62 | 0.1363 | |

Residual | 11.20 | 10 | 1.12 | – | – | |

Lack of fit | 6.77 | 5 | 1.35 | 1.53 | 0.3264 | Not significant |

Pure error | 4.43 | 5 | 0.89 | – | – | |

Cor total | 136.50 | 19 | – | – | – |

## 4 Effect of process parameters on pores migration distance

Based on the mathematical model, the parameters’ effects on the pores migration distance were analyzed by the Design Expert 8.0. The interaction of each two parameters were given in the 3D-Response surface. When the other parameter ranged from low level to high level, The 3D-Response surface also changed.

### 4.1 Effect of welding speed and laser-arc distance on pores migration

### 4.2 Effect of welding current and laser-arc distance on pores migration

### 4.3 Effect of welding speed and welding current on pores migration

In a low laser-arc distance, the volume of molten pool is small, the molten pool solidification rate is mainly determined by the laser power and welding speed. Laser power has been selected as 10 kW to ensure the max penetration, the welding speed becomes the main factor of solidification rate. When the Laser-arc distance increased, the volume of the molten pool increased, and the shape of the molten pool changes. The cooperation between the arc and laser decreased due to the large laser-arc distance. The welding current determines the volume and the cooling rate of the molten pool.

### 4.4 Result analysis

Welded arc pit size

Number | Current (A) | Welding speed (m/min) | Laser-arc distance (mm) | Crater pit lateral length (mm) | Crater pit longitudinal length (mm) | Pores migration distance (mm) |
---|---|---|---|---|---|---|

9 | 140 | 0.90 | 1.22 | 18.94 | 10.18 | 12.27 |

6 | 140 | 0.90 | 4.78 | 20.50 | 10.60 | 16.88 |

20 | 140 | 0.60 | 1.22 | 18.44 | 10.40 | 18.05 |

3 | 140 | 0.60 | 4.78 | 20.60 | 11.70 | 16.40 |

### 4.5 Optimized process parameters

Predicted and actual values of the pores migration distance

Welding current (A) | Welding speed (m/min) | Laser-arc distance (mm) | Pores migration distance (mm) | Error (%) | |
---|---|---|---|---|---|

Predictive value | 135 | 0.6 | 1.22 | 17.33 | – |

Actual value | 135 | 0.6 | 1.22 | 16.70 | 3.6 |

## 5 Conclusions

- 1.
Based on the CCD method of response surface methodology, a model was established to figure out the effects of welding current, welding speed, and laser-arc distance on the distance of pores migration during the laser hybrid welding of 30 mm 5083 aluminum alloy.

- 2.
When the welding speed is at a low level, the larger welding current is benefit for pores migration, but the laser-arc distance has no obvious effect on pores escaping, the molten pool cool rate is slow, and larger pores migration distance can be obtained. When the welding speed is at a high level, the molten pool cooling rate is faster, and it is necessary to increase the welding speed and the Laser-arc distance at the same time to increase the volume of the molten pool, so the pores can not be captured by the side wall of molten pool.

- 3.
Based on the mathematical model, the optimal parameters can be obtain: 135 A/0.6 m/min/1.22 mm (welding current/welding speed/laser-arc distance). The error between predict value and actual value is less than 5%, the mathematical model based on the CCD is correct.

## Notes

### Acknowledgements

I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. This study has no founding support.

### Compliance with ethical standards

### Conflict of interest

No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.

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