Search

Pulsed lasers are used in a variety of materials processing applications that range from heating for metallurgical transformation to scribing vehicle identification numbers on anodized aluminum strips. These lasers are commonly configured to deliver a large quantity of heat energy in very short time intervals and over very small areas due to the manner in which radiant energy is stored within, and then released from, the laser resonator. At the present time, little is known about the effect of pulse duration on thermomechanical distortion during heating without phase change. To explore this issue, a boundary element method was developed to calculate temperature, displacement, and thermal stress fields in a layer that is rigidly bonded to an inert semi-space. The layer absorbs thermal energy from a repetitively pulsed laser in the plane of its free surface. The effects of two pulse durations, which differ by four-orders-of-magnitude, were examined in this work. The temporal profiles of ultrafast pulses of the order of ten picoseconds (such as those emitted by a mode-locked laser), and pulses of the order of tens-of-nanoseconds (such as those emitted by a Q-switched Nd:YAG laser) were mathematically modeled using a rectified sine function. The spatial profile of each pulse was shaped to approximate a Gaussian strip source. The equations of coupled thermoelasticity, wherein the speed of mechanical distortion due to material expansion during heat absorption is finite, but the speed of heat propagation within the layer is infinite, were solved for both pulse durations. The resulting temperature and stress fields were compared with those predicted in the limit of no thermomechanical coupling. [DOI: 10.1115/1.1365155]

A theoretical model that predicts the time and position of gap nucleation along the metal-shell interface during solidification of a pure metal on a sinusoidal mold surface is presented. The ratio of the mold surface amplitude to its wavelength is assumed to be much less than one and hence it is used as a perturbation parameter in the analysis. The molten metal perfectly wets the mold surface prior to the beginning of solidification, and this leads to a corresponding undulation of the metal shell thickness. A nonuniform distortion develops in the shell due to the lateral temperature gradient induced by the modest spatial variation of the mold surface. This causes a variation in the contact pressure so that the growing shell pushes harder on the mom in some places, but in other places it starts to pull away from the mold. Gap nucleation is assumed to occur when the contact pressure falls to zero. The conditions for gap nucleation in the surface troughs are examined since a corresponding increase in pressure at the crests signals the possibility of a growth instability in the shell at later stages of the process. A series expansion for the contact pressure is presented which is appropriate for early solidification times. This reveals how the contact pressure varies with the mold surface wavelength. This solution is compared with a numerical solution for the contact pressure that is not limited to early solidification times. Gap nucleation times are calculated for pure aluminum and iron shells for selected mold surface wavelengths. The associated mean shell thicknesses are calculated as a function of wavelength at selected mean molten metal pressures. [S0021-8936(00)02901-9]

In this second part, we examine the contact pressure ratio, [P.sup.tr], at the lowest points of the upper mold surface troughs in a directional solidification process using the theoretical methodology developed in Part I. Since there is ample experimental evidence that the mold surface topography affects gap nucleation at the mold-shell interface and the uniformity of the shell we explore how the wavelength of the upper mold surface impacts the evolution of [P.sup.tr] for specific material combinations and process parameters. For this purpose, the mold-shell materials are assumed to be combinations of four pure materials, viz., aluminum, copper, iron and lead: these materials offer a wide range of thermal and mechanical properties. Critical wavelengths, for which [P.sup.tr] and its time derivative simultaneously equal zero, are predicted for all mold-shell material combinations. The theoretical model also predicts the existence of wavelength bands which are delimited by upper and lower critical wavelengths. All wavelengths that lie within the bands lead to gap nucleation, whereas all wavelengths that lie outside of the bands do not. The effects of distortivity ratio, which is a measure of the extent to which the mold-shell interface deforms under a given thermal loading, and selected process parameters (such as the mean mold thickness, contact resistance, and pressure) on bandwidth size, are considered in detail. Extensions of the present work to more sophisticated models that might lead to rudimentary mold topography design criteria are considered. [S0021-8936(00)03301-8]

A theoretical model of the gap nucleation process during pure metal solidification on a deformable mold of finite thickness is presented. Both surfaces of the mold follow a sinusoidal lay for which the ratio of the amplitude to the wavelength, or aspect ratio, is much less than one. This makes the aspect ratio a convenient perturbation parameter for the thermal and mechanical problems since it is indicative of the spatial variation in the surfaces. The thermal and mechanical fields are coupled along the upper surface of the mold through a pressure-dependent thermal contact resistance. The main goal of the model is to develop a means for examining the contact pressure along the mold-shell interface and how variation of the mold surface wavelength affects the time and location of gap nucleation. Gaps, which result from irregular distortion of the shell due to the modest variation of the mold surface geometry, are assumed to nucleate when the contact pressure locally falls to zero. The model leads to two coupled differential equations for the shell thickness and contact pressure perturbations which are solved with a numerical scheme. Using a series solution methodology, it is shown that the contact pressure perturbation predicted by the present model reduces to that for a rigid, perfectly conducting mold (which was considered in another work) in the limit of zero mold thickness. In the companion paper, we specifically examine various combinations of pure materials acting either as the shell or the mold material. The concept of a critical wavelength, which separates those wavelengths that lead to gap nucleation at the crests, from those that lead to gap nucleation at the troughs, is introduced. The potential for development of design criteria for mold surface topographies using the present theoretical model as a limiting solution for finite element models of more complex casting processes is discussed. [S0021-8936(00)03201-3]

A study determines the laser heat induced thermal stress field on the surface of an elastic half-space, for mixed-mode structure beams. The two lowest order modes from a cylindrical laser resonator undergo superposition leading to a spatial heat energy distribution on the material surface. The shapes of temporal pulses from the laser depend on the temporal pulse profile. Tensile radial and circumferential stresses near the axis depend on the subsurface shear stresses and a compressive normal stress. A laser pulse with temporal profile, and radial intensity distribution, applies heat on the material.

The article discusses the problems of boundary frictionless thermoelastic contact for a layer. Equations to formulate contact problems are derived.

Abstract: We explore the formation of a LaFe5H n hydride by means of density functional theory. Enthalpies of formation ΔH with respect to the elemental metals and H2 are calculated for various hydrogen configurations in four prototype crystal structures. We find ΔH <0 in many cases, suggesting the existence of LaFe5H n , as does Miedema''s semi-empirical model. ΔH is a minimum for the LaFe5H7 stoichiometry with hydrogen occupying the 4e, 8g, and 16m sites in the orthorhombic Cccm structure. Phonon dispersion relations and elastic constants computed for that structure exhibit no anomalies, demonstrating vibrational stability. Similar results for LaFe5 indicate the possible formation of that compound under pressure. [Copyright &y& Elsevier]

Abstract: Systematic theoretical results for La(TM)5H n materials with TM one of the magnetic transition metals Fe, Co, or Ni are discussed. For TM=Co, Ni the antecedent LaCo5 and LaNi5 intermetallics and hydrides are well known. While no La–Fe compounds exist, LaFe5 hydrides are predicted to form by both Miedema''s phenomenological model as well as by first-principles density functional theory (DFT) for several prototype crystal structures. Enthalpies of formation and saturation hydrogen contents derived from Miedema''s model and DFT are compared. Our phonon calculations for LaCo5 and LaCo5H4 yield new information on their crystal structures. [Copyright &y& Elsevier]

We have investigated the energetics of hydrogen site occupation in LaNi5Hn (hexagonal P63mc structure) and LaCo5Hn (orthorhombic Cmmm structure) via calculation of the site-dependent enthalpies of hydride formation ΔH. For each structure ΔH was determined for a broad variety of hydrogen configurations. In LaNi5Hn (LaCo5Hn) we find that the minimum ΔH occurs for hydrogen filling of the 2b6c16c2 (4e4h) sites, precisely those identified by neutron diffraction. Hydrogen-richer hydrides are predicted for both structures, in qualitative agreement with experiments performed at higher pressures. [ABSTRACT FROM AUTHOR]