Outstanding mechanical and physical properties like high thermal resistance, high hardness and chemical stability have encouraged use of structural ceramics in several applications. The brittle and hard nature of these ceramics makes them difficult to machine using conventional techniques and damage caused to the surface while machining affects efficiency of components. Laser machining has recently emerged as a potential technique for attaining high material removal rates. This review paper aims at presenting the state of the art in the field of laser machining of structural ceramics and emphasizes on experimental and computational approaches in understanding physical nature of the complex phenomena.

Laser machining of structural ceramics—A review

Simple formulas are derived for the degradation in the beam-quality factor, M(2), of an arbitrary laser beam caused by quartic phase distortions such as those that might occur in a spherically aberrated optical component, a thermally aberrated laser output window, or a divergent beam emerging from a high-index dielectric medium as in a wide-stripe, unstable-resonator diode laser. A new formula for the defocus correction that is needed to collimate optimally a beam with quartic phase aberration is also derived. Analytical results and numerical examples are given for both radially aberrated and one-dimensionaltransversely aberrated cases, and a simple experimental measurement of the beam-quality degradation produced by a thin plano-convex lens is shown to be in good agreement with the theory.

Analysis of laser beam quality degradation caused by quartic phase aberrations.

A numerical model combining the methods of enthalpy, effective-viscosity and volume-of-fluid is developed to simulate the metal transfer process in gas metal arc welding. The model describes not only the influence on droplet profile and transfer frequency of electromagnetic force, surface tension, and gravity, but it can also model the nonisothermal phenomena such as heat transfer and phase change. The model has been used to study the shape of the melting interface on the welding wire, the droplet oscillation at wire tip, the characteristics of relevant physical variables and their roles in metal transfer. We find that the taper formation in spray transfer is closely related to the heat input on the unmelted portion of the welding wire, and the taper formation affects the globular–spray transition by decelerating the transfer process. The formation of satellite drops during the metal transfer process is also considered. High-speed photography, laser-shadow imaging, and metallographic analysis validate the numerical model, and recommendations are made on the topics that require further consideration for a more accurate metal transfer model.

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Modelling and analysis of metal transfer in gas metal arc welding

Residual Stresses at Laser Surface Remelting and Additive Manufacturing

In laser material removal using a continuous wave or long-pulsed laser, the primary material removal mechanism melts with molten metal often ejected by an assisting oxygen jet. Interactions between heat transfer, oxygen diffusion, and gas dy- namics have been studied. In laser machining using a Q-switched solid state laser with pulse width on the order of nanoseconds, the primary material removal mechanism is ablation, but substantial melting is still present if a metallic material is concerned. Two mechanisms, ablation and melting, are modeled together and the property discontinuity and boundary conditions at the liquid/vapor interface are provided. When the laser pulse width further reduces to the order of picoseconds or femtoseconds, as in a Ti:sapphire laser, the dominant material removal mechanism changes yet again, with little thermal damage and wavelength independence observed. Thermal aspects are examined in the context of the short timescales of these pulses compared with the timescale of photon-electron-lattice interactions. Research needs in laser ma- terial removal in each of the three time regimes are discussed.

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Time scale effects in laser material removal: a review

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Dynamic Behavior in Laser Gas Cutting of Mild Steel(Welding Physics, Processes & Instruments)

Mechanism of laser beam absorption was analyzed in key hole welding of thin plate and deep penetration welding of thick plate on the basis of the two-dimensional cavity model where beam is absorbed by cavity wall and ionized metal vapor. The beam absorptivity of the cavity wall A and the absorption coefficient of the plasma u are approximately 50 % and 1- 1.5 cm-1, respectively. In key hole welding of thin plate, laser beam is absorbed predominantly by the liquid cavity wall and the cavity shape is self-controlled so as to absorb the energy required for bead formation. In deep penetration welding of thick plate, laser power is absorbed mainly by plasma where the self-controlling is less effective, and hence excess energy absorbed at low welding speeds is w'sted to widen the bead with little increase in penetration depth. In vacuum of 10-3-10-4 torr, u decreases down to approximately 0.5 cm-1, providing large penetration depth.

Beam Absorption Mechanism In Laser Welding

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High Power CO_2 Laser Welding of Thick Plate : Multipass Weding with Filler Wire(Welding Physics, Process & Instrument)

Change of focus length due to CO2 laser beam absorption in ZnSe lens was analyzed
by the thermal conduction theory and the physical optics. Temperature distribution
was calculated in ZnSe lens where the thermal conductivity varies with temperature.
Transient focus position was determined within an accuracy of by a diagnostic
system where laser beam passing through a rotating slit probe moving along the beam
axis is detected by a pyroelectric detector.
Focus length was found to be shortened quickly in the initial 10-20 sec, reaching
steady state in 60-90 sec in ZnSe lens of 2 inch-diameter. At absorbed power density
wab<5 W/cm2, the focus shift is proportional to wab, and increases more rapidly at
wab>S W/cm2 with accompanying aberration. The measured focus shift was found to
agree well with calculated values. Effect of lens-to-holder contact on the transient
focus shift are also discussed.

Analysis of thermally induced optical distortion in lens during focusing high-power CO2 laser beam

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Hiroshi Maruo

Papers

Dynamic Behavior in Laser Gas Cutting of Mild Steel(Welding Physics, Processes & Instruments)

Beam Absorption Mechanism In Laser Welding

High Power CO_2 Laser Welding of Thick Plate : Multipass Weding with Filler Wire(Welding Physics, Process & Instrument)

Analysis of thermally induced optical distortion in lens during focusing high-power CO2 laser beam

Quality in Laser-Gas-Cutting Stainless Steel and its Improvement