# The Influence of WEDM Parameters Setup on the Occurrence of Defects When Machining Hardox 400 Steel

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Setup and Material

#### 2.1. Experimental Material

#### 2.2. WEDM Machine Setup

_{off}), gap voltage (U), discharge current (I), wire feed (v) pulse on time (T

_{on}) and their limit values (Table 1). The limit values or each parameter setup were based on vast previous tests as well as the recommendations of the machine manufacturer.

_{c}when programming the machine but the speed is based on the setting of individual machine parameters. The cutting speed and the number of wire electrode breaking (the number was recorded on each separate sample, with the cut length of 3 mm each, as shown in Figure 1), make the resulting total machining time. The electrode breakage happens because of the inappropriate machine parameters setup. The rewiring of the wire electrode lasts 1 min, so a frequent breaking is not desirable and efficient for the machining process. The WEDM cutter used allowed a direct speed measurement during the machining process.

^{−1}, however, there was a single wire electrode breakage from all the machined samples. For Sample 31 that was cut at the second highest speed of 4 mm·min

^{−1}, no breakage occurred, so it can be said that the fastest machining will be achieved with the machine parameter setting U = 50 V, T

_{on}= 10 µs, T

_{off}= 30 µs, v = 14 m·min

^{−1}, and I = 35 A. In the following chapter, the optimization of machine setting parameters was made in order to maximize cutting speed in order to increase machining efficiency in the form of reduced machining time.

## 3. Statistical Evaluation of the Cutting Speed

_{c}using statistically significant parameters. The significance level chosen was 5%, with the model being developed using the “stepwise” method to select statistically significant parameters.

_{2}= 93.31%, thus, a model for the cutting speed expresses 93.31% of the variability of the measured cutting speed v

_{c}. The significant factors were pulse off time, discharge current, pulse on time and pulse off time wire feed interaction. Due to the model hierarchy, the wire feed factor has not been removed because has a significant interaction with the pulse off time.

## 4. Analysis of the Machined Surface and Discussion

#### 4.1. Experimental Methods

#### 4.2. Surface Topography Analysis

_{on}= 6 µs, T

_{off}= 30 µs, h = 14 m·min

^{−1}, and I = 35 A. This sample had a Ra value of 2.08 µm, which is significantly lower than that of Atlug research [21], but with different heat treatments for this steel. In contrast, the worst surface quality was evaluated for Sample 25, where all parameters except for parameter Sz reached the highest values. From this point of view, the parameter Sz, which can be defined as the sum of the highest value of the projection height and the highest depth value of the depression in the delimited area, appears to be a parameter reflecting the eventual occurrence of isolated protrusions resulting from the adherence of the discharged material particles on the machined surface.

#### 4.3. Surface Area Analysis

#### 4.4. The Subsurface Analysis

#### 4.5. The Analysis of TEM Lamella

## 5. Conclusions

- Sample 31 was machined at the highest speed of 4 mm·min
^{−1}without breaking of the wire electrode with machine setting parameters: U = 50 V, T_{on}= 10 µs, T_{off}= 30 µs, v = 14 m·min^{−1}and I = 35 A, - using a regression analysis, a mathematical model was created to calculate the cutting speed, with a positive effect of the factor T
_{on}, I and the negative effect of T_{off}on the cutting speed, based on the Main effect plot, - the lowest roughness parameter values were obtained for Sample 30, i.e., only Ra 2.08 µm, (by machine parameters setup: U = 70 V, T
_{on}= 6 µs, T_{off}= 30 µs, v = 14 m·min^{−1}, and I = 35 A), and this pattern also had a visibly smoother 3D relief surface, - the surface morphology of Sample 30 with the lowest roughness was covered by a noticeably smaller amount of recast layer than all other samples, and there was also significantly less diffusion of elements from the wire electrode,
- the subsurface analysis revealed a large number of cracks up to 22 µm in length and burned cavities in all samples produced, except for Sample 30, whose surface was covered with only a small amount of small cavities,
- the recast layer thickness for all samples examined was below 20 µm and covered the entire surface of each sample except for Sample 30, which was only covered by the recast layer by 20%, and the thickness did not exceed 5 µm,
- the analysis of TEM lamellae showed a homogeneous distribution of elements in the base material, with the recast layer as the only area with different concentration of elements.

_{on}= 6 µs, T

_{off}= 30 µs, v = 14 m·min

^{−1}and I = 35 A. Using this setting, it is possible to use a machine on a surface with a roughness of 2.08 µm and only a small amount of burned cavities. This setting will extend the life of the part and ensure that it functions properly.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 4.**The evaluated parameters of the basic profile, profile and planar parameters of individual experimental samples.

**Figure 5.**(

**a**) the evaluated surface topography parameters of Sample 25, including color−filtered relief of its surface; (

**b**) evaluated surface topography parameters of Sample 30, including a color−filtered relief of its surface; (

**c**) 3D relief of Sample 30 from a given area obtained by AFM.

**Figure 6.**The sample surface morphology including magnified SEM (BSE) detail (

**a**) Sample 30 machined with parameters: T

_{off}= 30 µs, U = 70 V, T

_{on}= 6 µs, I = 35 A, v = 14 m·min

^{−1}; (

**b**) Sample 25 machined with parameters: T

_{off}= 40 µs, U = 60 V, T

_{on}= 8 µs, I = 30 A, v = 12 m·min

^{−1}.

**Figure 7.**The sample surface morphology including magnified SEM (BSE) detail (

**a**) Sample 25 machined with parameters: T

_{off}= 40 µs, U = 60 V, I = 30 A, T

_{on}= 8 µs, v = 12 m·min

^{−1}; (

**b**) Sample 30 machined with parameters: T

_{off}= 30 µs, U = 70 V, I = 35 A, T

_{on}= 6 µs, v = 14 m·min

^{−1}.

**Figure 8.**The cross sectional view showing subsurface SEM (BSE) defects (

**a**) Sample 25 machined with parameters: T

_{off}= 40 µs, U = 60 V, T

_{on}= 8 µs, I = 30 A, v = 12 m·min

^{−1}; (

**b**) Sample 1 machined with parameters: T

_{off}= 40 µs, U = 70 V, T

_{on}= 8 µs, I = 30 A, v = 12 m·min

^{−1}; (

**c, d**) Sample 20 machined with parameters: T

_{off}= 50 µs, U = 70 V, T

_{on}= 6 µs, I = 25 A, v = 14 m·min

^{−1}; (

**e, f**) Sample 13 machined with parameters: T

_{off}= 50 µs, U = 70 V, T

_{on}= 10 µs, I = 25 A, v = 10 m·min

^{−1}.

**Figure 9.**The cross section of Sample 30 machined with parameters: U = 70 V, T

_{on}= 6 µs, T

_{off}= 30 µs, v = 14 m·min

^{−1}and I = 35 A, SEM (BSE), magnified at 1000×.

**Figure 10.**(

**a**) TEM lamella; (

**b**) maps of the distribution of individual elements in the examined area of the lamella.

Parameter | Discharge Current (A) | Pulse off Time (µs) | Pulse on Time (µs) | Gap Voltage (V) | Wire Feed (m·min ^{−1}) |
---|---|---|---|---|---|

Level 1 | 25 | 50 | 6 | 50 | 10 |

Level 2 | 30 | 40 | 8 | 60 | 12 |

Level 3 | 35 | 30 | 10 | 70 | 14 |

Number of Sample | Discharge Current (A) | Gap voltage (V) | Pulse off Time (µs) | Pulse on Time (µs) | Wire Feed (m·min^{−1}) | Number of Sample | Discharge Current (A) | Gap Voltage (V) | Pulse off Time (µs) | Pulse on Time (µs) | Wire Feed (m·min^{−1}) |
---|---|---|---|---|---|---|---|---|---|---|---|

1 | 30 | 70 | 40 | 8 | 12 | 18 | 30 | 60 | 40 | 8 | 12 |

2 | 30 | 60 | 30 | 8 | 12 | 19 | 30 | 60 | 40 | 8 | 12 |

3 | 25 | 60 | 40 | 8 | 12 | 20 | 25 | 70 | 50 | 6 | 14 |

4 | 30 | 60 | 40 | 10 | 12 | 21 | 25 | 50 | 30 | 6 | 14 |

5 | 30 | 50 | 40 | 8 | 12 | 22 | 30 | 60 | 40 | 8 | 12 |

6 | 30 | 60 | 50 | 8 | 12 | 23 | 25 | 70 | 30 | 10 | 14 |

7 | 30 | 60 | 40 | 6 | 12 | 24 | 25 | 50 | 50 | 6 | 10 |

8 | 35 | 60 | 40 | 8 | 12 | 25 | 30 | 60 | 40 | 8 | 12 |

9 | 30 | 60 | 40 | 8 | 10 | 26 | 25 | 50 | 50 | 10 | 14 |

10 | 30 | 60 | 40 | 8 | 14 | 27 | 25 | 50 | 30 | 10 | 10 |

11 | 30 | 60 | 40 | 8 | 12 | 28 | 35 | 50 | 50 | 6 | 14 |

12 | 35 | 50 | 30 | 6 | 10 | 29 | 35 | 50 | 50 | 10 | 10 |

13 | 25 | 70 | 50 | 10 | 10 | 30 | 35 | 70 | 30 | 6 | 14 |

14 | 35 | 70 | 30 | 10 | 10 | 31 | 35 | 50 | 30 | 10 | 14 |

15 | 30 | 60 | 40 | 8 | 12 | 32 | 30 | 60 | 40 | 8 | 12 |

16 | 35 | 70 | 50 | 6 | 10 | 33 | 25 | 70 | 30 | 6 | 10 |

17 | 35 | 70 | 50 | 10 | 14 | – | – | – | – | – | – |

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**MDPI and ACS Style**

Mouralova, K.; Prokes, T.; Benes, L.; Bednar, J. The Influence of WEDM Parameters Setup on the Occurrence of Defects When Machining Hardox 400 Steel. *Materials* **2019**, *12*, 3758.
https://doi.org/10.3390/ma12223758

**AMA Style**

Mouralova K, Prokes T, Benes L, Bednar J. The Influence of WEDM Parameters Setup on the Occurrence of Defects When Machining Hardox 400 Steel. *Materials*. 2019; 12(22):3758.
https://doi.org/10.3390/ma12223758

**Chicago/Turabian Style**

Mouralova, Katerina, Tomas Prokes, Libor Benes, and Josef Bednar. 2019. "The Influence of WEDM Parameters Setup on the Occurrence of Defects When Machining Hardox 400 Steel" *Materials* 12, no. 22: 3758.
https://doi.org/10.3390/ma12223758