Most of the processes that utilize the desired effects of ultrasonic vibrations during solidifying melt exhibit relatively slow cooling and solidification rates. The applica-tion of ultrasonic vibraapplica-tions to rapidly solidifying melt is not well investigated. As
rapid melting/solidification is encountered during several laser manufacturing pro-cesses such as surface modification (laser melting, alloying, cladding, and composite surfacing), forming (laser welding/joining), and material removal (laser machining) processes, the application of ultrasonic vibrations during laser processing presents a great potential for improving the microstructure, metallurgical quality, and material removal rates of the processed materials. Recently, Zheng and Huang reported ultra-sonic vibrations-assisted femtosecond machining of microholes in Nitinol substrates with an improvement in hole wall surface quality and higher hole aspect ratio [49].
They used femtosecond pulsed laser (Ti-Sapphire) in combination with ultrasonic vibrations (frequency: 40 kHz; amplitude: 2.5 µm) and reported that ultrasonic vi-brations facilitates the removal of ablated particles by enhancing the heat transfer of the particles (i.e. better cooling of the particles, and hence, reduced tendency of the particles to bond to the hole wall and substrate surface) [49]. They also mentioned that the depth of the hole increased from 1.65 mm to 1.95 mm while the ultrasonic vibration was applied to the femtosecond laser drilling. Fig. 1.14 shows the schematic of the drilled holes and SEM images of the wall of the drilled holes with and without application of the ultrasonic vibration. They also reported that the aspect ratio of the drilled micro-holes was increased from 50 to 56 where the ultrasonic vibration is applied.
Chiu et al. [50] also investigated the ultrasonic vibration-assisted (frequency: 20 kHz; amplitude: 10 µm) pulsed excimer laser (KrF laser) machining and cleaning of the PZT materials. In these studies on ultrasonic vibration-assisted laser machining, pulsed lasers were used and the dominant material removal mechanism was ablation [48,50]. The pulsed lasers such as pulsed KrF and Ti:Sapphire lasers are traditionally used for drilling, and the application of ultrasonic vibrations in these studies was to facilitate the removal of ablated debris and particles from the surface to improve surface quality and machining rate. Lau et al. [51] and Yue et al. [52] also reported
Figure 1.14: Schematic of the drilled holes and SEM images of the wall of the drilled holes with and without application of the ultrasonic vibrations [49]
that the simultaneous application of ultrasonic vibrations during pulsed laser drilling improve the material removal and quality of laser drilled holes. With this approach of ultrasonic vibrations-aided pulsed laser drilling, about 20% increase in hole depth and about 30% reduction in heat affected zone were reported by for aluminum matrix composites. However, even with these advances, laser drilling of large aspect-ratio holes with acceptable surface quality and reproducibility is still a challenge in adopting the technology for wider applications. Kang et al. [53] also investigated the application of ultrasonic vibration (frequency: ∼30 kHz; longitudinal amplitude: 3 µm) on the ns-laser (pulse width: 200 ns; max. pulse energy: 1mJ) machining. Furthermore they reported reaching a higher surface quality, reduced re-solidified layer thickness and inhibition of on oxide layer formation.
1.4.1 Enhancement of heat transfer by ultrasonic vibrations
Application of the ultrasonic vibrations agitates the fluid (e.g. air, water, molten metal) around/near the ultrasonic tip. The enhancement of surface convection, and consequent loss of energy, is expected due to the induced fluid flow during application of ultrasonic vibrations. Significant efforts have been made to study the effect of ultrasound on the heat transfer enhancement. It has been reported that ultrasonic vibration can enhance the convection heat transfer coefficient (h) up to 25 times. For example, Nomura et al. [54] used a 600 W ultrasonic generator with a frequency of 40 kHz to investigate the heat transfer enhancement on the narrow surface. They reported a 10-fold increase in the heat transfer coefficient in presence of ultrasound.
In another study, Wong et al. [55] studied the natural convection enhancement with Pt wire in different liquid mediums (frequency: 20-300 kHz). They reported that applying ultrasonic vibration increases the heat transfer coefficient by a factor of 8. In one of the first studies on the effect of ultrasonic vibration on heat transfer, Fairbanks [56] reported an increase 4 times greater in heat convection from the surface of heated steel (up to 973 K) to the air. The influence of ultrasonic vibration parameters on enhancement of the convection heat transfer coefficient is summarized and presented in Fig. 1.15 [57].
Figure 1.15: The influence of ultrasonic vibration parameters on enhancement of the convection heat transfer coefficient [57]
CHAPTER 2
HYPOTHESIS AND OBJECTIVES
While the conventional laser drilling is primarily performed using pulsed lasers, the continuous wave lasers are used in surface engineering applications. Continuous wave (CW) laser surface melting, with steady state melting conditions reached at suffi-ciently high laser scanning velocities, results in the formation of well-defined resolid-ified bead on the surface. A continuous laser irradiation at a spot (i.e. stationary irradiation without scanning) on the surface also forms a well-defined melt pool, al-beit with some surface rippling in some cases. These characteristics are very useful for surface modification of materials, and hence, continuous wave lasers are most appropriate for laser surface engineering applications as described in [59]. The con-tinuous wave lasers are also used in cutting applications where the assist gases expel the melt from the bottom of the cutting front/kerf. However, the use of continuous wave lasers, even with the presence of assist gases, in material drilling applications is limited.
Significant efforts have been made in the past to use the desirable effects associated with the application of ultrasonic vibrations during conventional manufacturing, re-sulting in the emergence of a new field of ultrasonic vibration-assisted manufacturing.
Most of the processes that utilize the desired effects of ultrasonic vibrations during solidifying melt exhibit relatively slow cooling and solidification rates. The applica-tion of ultrasonic vibraapplica-tions to rapidly solidifying melt is not well investigated. As rapid melting/solidification is encountered during several laser manufacturing pro-cesses such as surface modification (laser melting, alloying, cladding, and composite
surfacing), forming (laser welding/joining), and material removal (laser machining) processes, the application of ultrasonic vibrations during laser processing presents a great potential for improving the microstructure, metallurgical quality, and material removal rates of the processed materials. The proposed ultrasonic vibration-assisted continuous wave laser surface processing extends the energy-efficient laser melting (no drilling) regime for laser drilling applications and enables continuous (instead of discontinuous pulsed) drilling, atomization, and physical texturing of materials.
With the widespread use of continuous wave CO2 lasers in industry, the proposed laser surface processing approach is likely to expand the applications of these lasers for flexible manufacturing.
2.1 Global Hypothesis
Major hypothesis of this work is that high power ultrasonic vibrations with medium to high longitudinal amplitudes (displacements) (5-75 µm) are expected to facilitate the melt expulsion in the continuous wave (CW) laser surface melting process and extend the laser melting regime of the CW laser-material interactions for machin-ing of materials and expand the applications of these widely used lasers for flexible manufacturing.