This PDF file contains the front matter associated with SPIE Proceedings Volume 11679

Welcome and Introduction to SPIE Photonics West LASE conference 11679: High-Power Laser Materials Processing: Applications, Diagnostics, and Systems X

Not only surface treatment has established itself as a field of application for the Direct Metal Depostion process, direct "printing" or the production of near net shape structures is an essential field of application for metal powder build-up welding. Although the nozzle technology has been continually improved, the powder efficiency is below 100%. It is far away from process boundary conditions to take the decision whether metal powder or wire is the best solution for a specific application. In the recent years the use of wire was not part of the discussion e.g. when talking about a multi-directional welding path or building 3D structures because of the missing tool. The new processing head CoaxPrinter is a promising solution to increase the use of wire in LMD. The compact and easy to use device has shown excellent build up results with repeatable quality and homogenous inner structure of the melted wire, supported by OCT sensor technology.

Structural monitoring during laser processing has recently been achieved with optical coherence tomography (OCT). Although a couple of industrial OCT monitoring systems have appeared, most are based on spectral-domain OCT, which uses an 800-nm band light source. In some applications, a longer wavelength light is preferable because of its transmission property. Hence, we constructed a swept-source OCT system using an industrial wavelength-swept light source that outputs a 1.3-μm band light. Since the light source employs an electro-optic crystal of potassium tantalate niobate (KTa_1-xNb_xO₃, KTN) as a driver for sweeping the output wavelength, it has no mechanically moving components and can stably be used in industrial applications that require quantitative analyses. It produces a wavelength-swept light of 6 mW in average power, 1310 nm in central wavelength, and 80 nm in bandwidth with a repetition rate of 20 kHz. The light is derived to an interferometer module, which is composed of fiber components, a reference reflector, and a balanced photo-detector. The module is connected to a probe head, which includes a beam scanner for observing the profile around the laser processing point. The beam scanner is controlled by a voltage waveform that arises from a multifunctional board, which also converts the interference signal to digital data. The obtained data are numerically processed in real time and converted to tomographic images. As an example, we applied our system to the in-process monitor of laser plastic welding, where plates of acrylonitrile butadiene styrene were used as samples.

Laser beam welding in vacuum (LaVa) is a process modification of laser beam welding that combines the advantages of laser and electron beam. Due to the reduced pressure, the evaporation temperature of Aluminum and Iron decreases. by approx. 1000 K. As a result, the penetration depth can be significantly increased compared to conventional laser beam welding, while maintaining the same beam power. State of the art research shows that previously unattainable penetrations depths of up to 80 mm can be achieved in steel. Up to now, laser beam welding in vacuum has been a stationary process and was bound to vacuum chambers. However, since high penetration depths usually involve large components, the great potential in the thick plate metal sector could not be exploited. Therefore, laser beam welding with mobile vacuum was developed, where a small vacuum chamber is moved alongside the welding process so that the vacuum is generated locally around the weld bath. A dynamic seal is installed between workpiece and mobile vacuum chamber, so that a pressure of approx. 10 - 20 mbar can be generated, which is sufficient for the process. Current investigations show that steel plates with a thickness of 40 mm can be joined in one layer with only 12 kW beam power. Furthermore, the process shows a high tolerance for edge offsets and gaps. In the latter case, filler material is used to bridge gaps larger than 0.5 mm.

Multi-kilowatt Laser Beam Welding (LBW) processes must take up three challenges to keep improving its performance: handling high power, shaping the output beam and reducing focus shift. This will lead to a higher quality and speed as well as the capability to weld thicker parts. We describe here a beam shaper compatible with industry standard equipment (collimation and focusing modules, arm robot and laser) handling up to 16kW average power delivering a mm-wide annular shape and reducing the focus shift. The LBW processes improvements on different materials are described.

We have demonstrated high-precision cutting and drilling of CFRP using a 3-kW CW single-mode fiber laser and a galvanometer scanner. A 3-mm-thick thermoset CFRP was successfully cut with 100 scans at the scanning speed of 13 m/s, which corresponds to effective cutting speed of 7.8 m/min. We employed the multi-pass method, in which a laser is scanned on the same position at high speed, and the optimized scanning position shift to realize short-time cutting. The time interval between scans was less than 20 ms, which was much shorter than general time interval in multi-pass method. Even with such short interval, the width of heat-affected zone (HAZ) was controlled to 97 μm on average. Drilling demonstration was carried out by using the same setup as the cutting experiment. Holes with a diameter of 6.4 mm have been successfully drilled on a 2-mm-thick thermoset CFRP. The processing time was 2.7 seconds, which was equivalent to that in mechanical processing. Since a HAZ tends to expand in the direction of the carbon fibers due to their high heat conductivity, it is difficult to reduce the width of HAZ in all directions around the processed area. To overcome this challenge, we optimized the gas injection condition as well as the scanning condition. As a result, the widths of HAZs were successfully controlled to about 100 μm in all directions.

High productive laser drilling processes usually employ an on-the-fly single-pulse drilling process. This process achieves a productivity of up to several hundred holes per second but is usually limited to hole diameters in the order of magnitude of ~100 μm and hole depths below 1 mm. Moreover, the geometrical and metallographic hole quality is limited. Larger or deeper holes can be drilled by means of percussion drilling processes which also results in an increased hole quality. However, the productivity for percussion drilling is only in the range of a few holes per second at best due to the positioning time of the optic for each hole. In this paper we present the development of a new drilling process which combines the advantages of an on-the-fly process with the achievable size and quality of a percussion drilling process. Instead of a single pulse, an elaborately designed short pulse burst is emitted to drill the hole during the relative movement between the drilling optic and workpiece. The impact of each pulse of the pulse burst on the final shape of the hole is evaluated by a systematic variation of the process parameters. A drilling process to achieve a hole diameter of Ø500 μm in 2 mm thick aluminum was designed and a drilling speed of 15 holes per second has been demonstrated with a relative standard deviation of less than 5% for the entry and exit diameter.

In this work, we experimentally study the features of laser drilling of composite highly porous ceramics consisting of aluminosilicates, glass and a small addition of polystyrene (EPS). It was found that the formation of a liquid phase can contribute to the strengthening of ceramics. A list of factors that determine the dynamics of hole formation is proposed. These include not only the processes of heating, melting, ablation, and ejection, well known previously for metals and dense ceramics, but also a number of other specific processes: (a) boiling of the component of the mixture with the lowest temperature of thermal decomposition and its escape from the affected zone; (b) a melt of less volatile components; (c) sealing the ceramics adjacent to the walls due to the mechanical sealing effect of ablative pressure; (c) glass transition of the liquid phase upon cooling after the end of irradiation.

Ultra-short pulse laser machining has been applied to the polishing of polycrystalline diamond (PCD) wafers in order to generate a smooth surface finish and reduce mechanical polishing time. Past studies were first carried out with a 5W laser highlighting the difference in ablation rates between PCD grades and the possible graphitization of diamond on the surface of micrometric PCD grades over a fluence threshold. Some upscaling work was undertaken at 80W with a 3-pulse burst reducing the Sa of a micrometric PCD grade lapped surface by 50% with a volume removal rate double that of the conventional mechanical polishing technique. From these previous base investigations, an ultra-short pulse laser delivering an average power of 1kW at 500fs via state-of-the-art thin disk multi-pass amplification is implemented here to achieve a higher ablation rate for high throughput processing. This is the first time that such an average power is applied on polycrystalline diamond in the ultra-short pulse regime. A burst mode is also implemented which is demonstrated to reduce the Sa by 10% and 55% on fine and coarse grade surfaces respectively compared to single pulse processing. From 80W to 1kW, the ablation rate is increased by a factor of 70 on micrometric PCD grades while the Sa of the initial lapped surface is reduced by 14% without any graphitization of the diamond structure. However, no improvement of the Sa is performed on the initial surface of coarser grades due to the formation of cavities (~5μm wide) potentially caused by the spallation of diamond grains.

Laser shock peening is a new and important surface treatment technique that can enhance the mechanical properties of metal materials. Normally, the nanosecond laser with pulse-width between 5 ns and 20 ns is used to induce a high-pressure shock wave that can generate plastic deformation in the top layer of metals. The femtosecond laser shock peening in the air has been studied recently, which can induce higher pressure shock wave than that of traditional nanosecond laser shock peening in a very short time. The NiTi alloy is processed by femtosecond laser shock peening, then a nanoindentation device is used to measure its surface hardness and residual stress. The hardness results of NiTi alloy before and after treatment show that the femtosecond laser shock peening can increase the hardness of NiTi alloy, which also shows that the femtosecond laser can be used to perform laser shock peening on NiTi alloy without coating.

We have developed an optical model capable of designing and evaluating lenses for high power lasers. The model is used to design two types of optical lenses suited for high power laser applications. A three element objective lens with a focal length of 150mm and NA of 0.2 is capable of achieving diffraction limited performance. However, the performance is greatly compromised when conventional fused silica is used. Both the spot size and waist location are too sensitive to lens temperature variation. To overcome the issue, an athermal lens with both fused silica and CaF₂ elements can perform well over a wide range of temperature variation from 20 °C to 300 °C. Similarly, a f-theta lens with a focal length of 200mm, scanning angle of ±18° is designed. Both thermal lensing caused by material as well as environment temperature are analyzed. Overall, the f-theta lens is not as sensitive as the objective due to its small NA. Nevertheless, thermal lensing can still affect the process and needs to be addressed. With a similar approach, an athermal f-theta lens with a combination of fused silica and CaF₂ elements can perform well over a wide range of temperature variation from 20 °C to 300 °C.