# Influence of Aluminum Laser Ablation on Interfacial Thermal Transfer and Joint Quality of Laser Welded Aluminum–Polyamide Assemblies

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

_{1}, m

_{1}, k

_{1}are properties of material 1, and σ

_{2}, m

_{2}, k

_{2}are properties of material 2, σ = (σ

_{1}

^{2}+ σ

_{2}

^{2})

^{0.5}is the effective root mean square roughness (RMS) so that σ

_{1,2}= $\sqrt{\frac{1}{L}{{\displaystyle \int}}_{0}^{L}{y}^{2}\left(x\right)\mathrm{d}x}$, where L is the profile traced length; m = (m

_{1}

^{2}+ m

_{2}

^{2})

^{0.5}is the effective mean asperity slope, and m

_{1,2}= $\frac{1}{L}{{\displaystyle \int}}_{0}^{L}\left|\frac{\mathrm{d}y\left(x\right)}{\mathrm{d}x}\right|\mathrm{d}x$ is the mean absolute asperity slope. H is the microhardness of the softer material, P is the applied pressure, and k

_{h}is the harmonic mean thermal conductivity at the interface where k

_{h}= 2k

_{1}k

_{2}/(k

_{1}+ k

_{2}). It is clear from Equation (1), which can be utilized in describing the TCR between metal and polymer in their solid state, prior to the laser joining process [24], that TCR has a proportional relation to the ratio of the root mean square roughness σ to the asperities slope, m.

## 2. Experimental Method

#### 2.1. Materials

#### 2.2. Laser Ablation

_{p}), beam guidance speed (V), lines, focal position, Al rolling direction, ablation hatching orientation, and laser beam power percentage, only f

_{p}and V had a significant influence on laser welded Al–PA joints’ resistance to shear failure. This research focuses on further evaluating the effects of those significant laser-ablation conditions, as shown in Table 1, on Al surface properties, thermal transfer across the joining partners, and corresponding joint quality. Based on previous investigations [5], six ablation conditions (see Table 1) were chosen for this research, as they resulted in a wide range of joint quality. Since ablation was performed with a q-switched laser, increasing pulse frequency results in decreasing the peak pulse power and fluence of the laser beam. However, overlap ratio between consecutive laser pulses (see Figure 1) depends on both pulse frequency and beam guidance speed, as described by Equation (2).

#### 2.3. Laser-Beam Welding

#### 2.4. Joint-Area Assessment

#### 2.5. X-Ray Photoelectron Spectroscopy (XPS)

^{2}area were ablated on an Al sample, each region with different ablation condition (see Table 1). Six points per ablated region were investigated by measuring a survey spectrum (3 scans, 200 eV energy pass) and high-resolution spectra for the regions of Al 2p, O 1s, and C 1s (20 scans, 20 eV pass energy) atoms.

#### 2.6. Scanning Electron Microscope (SEM)

^{2}area were ablated on an Al sample. The sample conductivity was enhanced by depositing a fine layer of conductive lacquer in contact with the untreated aluminum part of the sample. The area that was coated by this lacquer was not observed.

#### 2.7. Energy-Dispersive X-Ray Spectroscopy (EDX)

^{−4}Pa (water vapor) and an accelerating voltage of 15 kV. A 0.15 × 0.13 mm

^{2}area was scanned and an average spectrum was obtained on the whole area. The elemental composition is calculated from this spectrum, assuming the sample is only composed of aluminum, oxygen, and carbon. This assumption is made after observing the whole EDX spectra and identifying the peaks exhibiting a significant height.

#### 2.8. Surface Topography

^{2}area were ablated on an Al sample. Four measurements were performed on each region; two along the axis of applied pulling forces during shear testing, and two perpendicular to it. The roughness profile was calculated with a cut-off length of 0.25 mm. Roughness parameters Rq (average quadratic height or “root-mean-square” roughness) and Rdq (average quadratic slope) were calculated following ISO 4287 [26], and their average value is reported.

#### 2.9. Laser Flash Analysis (LFA)

^{2}squared geometry, using an Accutom 50 dicing tool from Struers (Ballerup, Denmark). Then, ablated samples were arranged in layered configuration, together with a 1 mm thick polished aluminum sample with the same dimensions as the ablated ones, as shown in Figure 4. Samples were coated on both external faces using Graphit 33 spray from Kontakt Chemie (Iffezheim, Germany) containing 1–5 w/w % of graphite powder in order to have a consistent absorbance to the laser beam and consistent emissivity to the IR detector.

## 3. Results

#### 3.1. Joint Strength

^{2}) of 0.98 illustrates very low variability in the calculated strength, as indicated by a slope of 34.69 MPa. In addition, results show equal and matching joint area on corresponding Al and PA samples, demonstrating prominence of cohesive failure mode for the ablated aluminum and indicating that the reported variation in joint quality is less likely to be a result of variations in interfacial chemical-bonding behavior. Results confirm that laser-ablation parameters have a significant influence on the joint quality, manifested in the joint area as shown in Figure 6, but no influence on the joint strength. Table 2 shows the effect of laser-ablation parameters on joint resistance to shear load.

#### 3.2. X-Ray Photoelectron Spectroscopy (XPS)

#### 3.3. Scanning Electron Microscope (SEM)

#### 3.4. Energy-Dispersive X-Ray Spectroscopy (EDX)

#### 3.5. Surface Topography

#### 3.6. Laser Flash Analysis

## 4. Discussion

^{2}/s) is lower than pure aluminum (94 mm

^{2}/s) at 300 K [31], this means that a thicker aluminum oxide layer would act as a larger thermal insulator for the transmission of heat to PA during welding. Thus, an increase in oxygen concentration would result in a decrease in volume of molten polymer during welding, leading to a smaller joint area and a decline in the joint’s resistance to failure.

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Schematic drawing illustrating six laser pulses irradiating Al surface during laser-ablation process and demonstrating laser pulses overlap ratio (O) between two consecutive pulses.

**Figure 3.**Schematic of hypothesized interfacial thermal-energy transfer during laser-welding process across (

**a**) ideally flat conforming surfaces and (

**b**) ablated aluminum surfaces in contact with smooth polyamide.

**Figure 6.**Stitched microscopic images of PA joint area after failure (dark areas), illustrating effect of laser-ablation parameters on joint quality.

Parameter | Frequency (kHz) | Speed (mm/s) | Peak Pulse Power (kW) | Fluence (J/cm^{2}) | Overlap Ratio (%) |
---|---|---|---|---|---|

P1 | 85 | 250 | 11 | 15.2 | 93 |

P2 | 40 | 1000 | 35 | 28.6 | 44 |

P3 | 70 | 1000 | 15 | 17.9 | 68 |

P4 | 85 | 1750 | 11 | 15.2 | 54 |

P5 | 120 | 1750 | 5 | 9.12 | 68 |

P5-95% | Same as Al_5 but with 95% power | 4.75 | 8.66 | 68 |

Ablation Parameters | P1 | P2 | P3 | P4 | P5 | P5-95% |
---|---|---|---|---|---|---|

Average shear load (N) | 580 ± 41 | 800 ± 65 | 1222 ± 143 | 1341 ± 172 | 1415 ± 113 | 1465 ± 65 |

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**MDPI and ACS Style**

Al-Sayyad, A.; Bardon, J.; Hirchenhahn, P.; Vaudémont, R.; Houssiau, L.; Plapper, P.
Influence of Aluminum Laser Ablation on Interfacial Thermal Transfer and Joint Quality of Laser Welded Aluminum–Polyamide Assemblies. *Coatings* **2019**, *9*, 768.
https://doi.org/10.3390/coatings9110768

**AMA Style**

Al-Sayyad A, Bardon J, Hirchenhahn P, Vaudémont R, Houssiau L, Plapper P.
Influence of Aluminum Laser Ablation on Interfacial Thermal Transfer and Joint Quality of Laser Welded Aluminum–Polyamide Assemblies. *Coatings*. 2019; 9(11):768.
https://doi.org/10.3390/coatings9110768

**Chicago/Turabian Style**

Al-Sayyad, Adham, Julien Bardon, Pierre Hirchenhahn, Regis Vaudémont, Laurent Houssiau, and Peter Plapper.
2019. "Influence of Aluminum Laser Ablation on Interfacial Thermal Transfer and Joint Quality of Laser Welded Aluminum–Polyamide Assemblies" *Coatings* 9, no. 11: 768.
https://doi.org/10.3390/coatings9110768