Siu-Chung Tam

A technical review of the laser drilling of aerospace materials

Abstract Drilling cooling holes in aerospace materials with a neodymium-doped: yttrium aluminum garnet (Nd:YAG) laser is an established technique used in the aerospace industry. However, there are new challenges as the industry begins to use materials that can witstand higher temperatures and pressures. In this paper, the mechanisms of some laser-drilling processes, the types of laser used, the quality characteristics of a laser-drilled hole, the effects of drilling parameters, and the advantages and limitations of the laser hole-drilling operation are reviewed.

Ball burnishing of 316L stainless steel

The process of ball burnishing AISI 316L stainless steel was studied. Taguchi techniques for the statistical design of experiments were employed to establish the optimum parameters for achieving good surface finish (response) on flat specimens. An L16 (45) orthogonal array was chosen to study the effects of five burnishing parameters (control factors). The roughness of the pre-burnished surface was included as a noise factor. It was found that the type of ball material, the depth of penetration, the burnishing speed and the type of lubricant significantly affect, at a 99% level of confidence, the surface finish of the burnished specimens. Feed was significant at a 95% level of confidence. The minimum roughness could be achieved using ZrO2 as the ball material, kerosene as lubricant, a depth of penetration of 20 μm, a feed of 110 μm and a burnishing speed of 300 mm/min. A confirmation test yielded an average surface finish of 1.017 μm (Rtm), which fell within a 90% confidence interval constructed about the predicted optimum. Using this set of parameters, a 3D curved surface was similarly burnished. The curved surface was generated using the solid modeler CATIA running on an IBM 4381 P13 mainframe. Pre-burnishing milling and ball burnishing were performed in a single setting on a LeBlond Makino FNC40 vertical machining centre. Surface roughness values varying between 0.6 and 0.9 μm (Rtm) were obtained for the curved surface.

Optimization of laser deep-hole drilling of Inconel 718 using the Taguchi method

Abstract The use of a Nd:YAG laser to drill deep cooling holes in aerospace components is a stringent operation. Various characteristics pertaining to the input beam, focussing lens, and assist gas have to be optimized in order to produce a hole economically and of sufficiently good quality. This paper reports the use of the Taguchi technique of experimental design in optimizing the process parameters for drilling deep-holes in nickel-based superalloy, Inconel 718. The thickness of the material is 25.0 mm. Oxygen is the assist gas andthe focal length of the focussing lens is 300 mm. The effects of five process parameters — pulse energy, pulse duration, pulse shape, focal position, and assist gas pressure — have been explored. The various parameters are assigned to an L18 orthogonal array. The primary response under study is the drilling time. It is predicted that a minimum drilling time of 31.51 s is needed to drill a hole with a pulse energy of 30.0 J, a pulse duration of 1.8 ms, a “treble” pulse shape, a focal position of 0.0 mm into the material, and an oxygen pressure of 0.35 MPa. Confirmatory experiments have produced results that lay within the 95% confidence interval.

A review of the Nd: YAG laser marking of plastic and ceramic IC packages

Abstract The marking of integrated circuit (IC) packages with neodymium-doped yttrium aluminium garnet (Nd: YAG) lasers has become a widely accepted technique in the microelectronics industry. This paper presents a literature survey and discussion of the various methods and mechanisms of Nd: YAG laser marking of plastic and ceramic IC packages. The effects of the material properties (e.g. the absorptivity and the melting point) and the marking parameters (e.g. power density, the focal position and the marking speed) on the mark legibility characteristics (e.g. the mark contrast and the mark width) are reviewed. Economic advantages and technological limitations of thr marking of IC packages with Nd: YAG lasers are discussed also.

Influence of the thermal effect on the TEM 00 mode output power of a laser-diode side-pumped solid-state laser

A fraction of pump power has been converted to TEM00 mode laser power for a side-pumped solid-state laser by use of a space-dependent rate equation. We investigated the pump-to-mode (TEM00) ratio when scaling laser-diode side-pumped solid-state lasers to high-power levels by including the thermal effect in the space-dependent rate equation. Based on the assumption that Gaussian pump power is the same at any cross section of a laser rod, we resolved the output power with a space-dependent rate equation; temperature distribution in the laser rod was obtained; the optical path difference distribution was derived, and we estimated the diffraction losses that result from thermally induced spherical aberration by use of the Strehl intensity ratio. We determined that thermally induced diffraction losses are strongly dependent on pump power and on the pump-to-mode ratio.

A review of the laser processing of aircraft components

Abstract Composite materials and some exotic alloys have been used in the fabrication of both structural and non-structural members of the air-frames and engines of aircrafts. Different machining processes, such as the use of traditional solid tools with modified cutters, ultrasonic cutting and drilling, water-jet cutting, abrasive water-jet cutting, electron beam welding, and various laser processes have been applied successfully to the processing of these materials. In this paper, the physical mechanisms, the advantages as well as the limitations, and examples of application of the laser processes that are used to machine aircraft components are reviewed.

Mechanical alloying of a TiCTiN ceramic system

Abstract Reported here is a preliminary study of the mechanical alloying (MA) process of an all-ceramic-phases component, i.e., TiC + TiN. The respective ceramic powders were mixed in weight proportions of 50:50 and 70:30 and milled in a planetary ball mill at a ball-to-powder weight ratio of 20:1. High angle XRD peaks were used to calculate the lattice parameters before and after milling for different milling times. The lattice parameter measurements suggested that Ti(C,N) solid solution was formed during the MA process. Inter-particle necking was observed. The rate of the solid solution reaction seems to be independent of the compositional change in the TiCTiN mixture. Particle size refinement is achieved mostly during the first few hours of milling.

Modeling pulse shape of Q-switched lasers

A general approach describing the pulse shape and temporal width of Q-switched lasers has been derived. Based on a normalized intermediate variable describing the ratio of population inversion density at the beginning of Q-switch to that at threshold, a generalized characteristic equation that governs passively and actively Q-switched lasers is derived. From the characteristic equation, the pulse symmetry property and pulse width can be described and easily calculated for any given operating parameters, and the waveform of the laser pulse can be reconstructed without solving the laser rate equations. An approach is also given for the case of intracavity frequency-doubled Q-switched lasers. Theoretical results show agreement with experiments for a diode pumped Nd:YAG laser system that is passively Q-switched by a Cr/sup 4+/:YAG absorber.

Analysis of a dynamical procedure on diode-end-pumped solid-state lasers

For solid-state lasers, we have determined that the overlap integrals V/sub th/, related to the threshold pump power, and V/sub slope/, related to the laser slope efficiency, dynamically change with pump power. The increase of diffraction loss is also caused by the dynamical change of the laser mode size, which alters the Fresnel number of the cavity. Subsequently, the threshold pump power and the laser slope efficiency also change dynamically. The dramatic increase of the laser mode radius at the laser crystal and the sudden decrease of the laser mode radius at the output mirror when operating near the stability limit make the laser output suddenly drop. A longer cavity will have a larger threshold pump power and a smaller total output power.

Thermal lensing of diode side-pumped solid-state lasers

The pump energy distribution in a diode side-pumped solid-state laser, is an overlap of propagating Gaussian beams. A simple model has been developed to calculate the thermal focal length of a diode side-pumped solid-state laser, which is based on a thermal model with a Gaussian heat density in any cross section of a laser rod. It can be seen that as the waists of pump beams increase, the energy distribution tends to be uniform and the thermal focal length tends to be long, which means a smaller thermal focusing.