Subject Area

Mechanical Engineering

Abstract

Laser induced plasma-plume and condensed particles attenuate the laser beam energy, resulting in keyhole instability and lacking penetration. There are various airflow devices that are currently used in both industrial and laboratory laser welding settings to suppress the detrimental effect of plasma plume and hot spatters. A systematic experimental and numerical study is presented in this dissertation that investigates different external airflow effects on the remote laser welding process used in automotive manufacturing applications.

The vertical airflow impact on the remote laser welding was first studied experimentally. The airflow velocity profiles of a coaxial vertical flow device were measured using unique tracers and Particle Image Velocimetry (PIV). The weld penetration depth was measured at different locations from beam-on-plate tests. A low-velocity zone was found near the centerline on the surface of the laser focal plane. A strong linear inverse correlation was established between the velocity drop and the penetration depth. The experimental work was then expanded to study the horizontal airflow impact on the remote laser welding. Aided by computational fluid dynamics (CFD) modeling, a laminar airflow fan that can precisely control the horizontal flow direction and velocity was designed and fabricated. The plasma-plume dynamics was recorded by a high-speed digital imaging system. The flow direction variance was found to correlate with the probability density function of the plasma distributions. The effects of horizontal airflow direction and velocity along with laser power and scanning speed on the weld penetration depth and surface width were identified. Finally, the horizontal airflow impact on remote laser welding of galvanized steel was investigated. The keyhole entrance-opening and plasma-plume dynamics were recorded by two synchronized high-speed cameras recorded. The keyhole behaviors under different flow directions were analyzed in both time and frequency domains. It is found that the plume bending attenuates the laser beam and reduces the keyhole opening size. A novel airflow fixture design was proposed to minimize the horizontal airflow impact.

To overcome the limitation of measurements and observation, 3D computational fluid dynamics simulations were performed to investigate the vertical coaxial flow field and the interaction between the horizontal flow and the laser-induced plasma plume in greater details. The coaxial vertical airflow was modeled by coupling two turbulence models for the internal and external domains while the horizontal airflow was modeled by a single turbulence model. The gas mixture consisting of more than one component was modeled by the mass fractions of its multi-component species. The dynamic movement of the laser beam and the plume outgassing process was simulated by a moving keyhole volume. The airflow velocity distribution and a low-velocity region from the CFD simulations were found to be consistent with those as observed in the vertical airflow experiments. The effects of horizontal airflow velocity and direction on the plume bending and the temperature distribution on the metal plate surface were clearly established by CFD simulations. For the experimental condition considered, the plume height was found to stabilize at several millimeters away from the start point.

Degree Date

Winter 2022

Document Type

Thesis

Degree Name

Ph.D.

Department

Mechanical Engineering

Advisor

Wei Tong

Number of Pages

170

Format

.pdf

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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