Danny O'Neill MacCallum

CALCULATIONS OF MOTT SCATTERING CROSS SECTION

Calculations of Mott elastic scattering cross section of electrons for most elements of the periodic table up to element number 94 in the energy range 20 eV–20 keV have been performed. The Dirac equation transformed to a first‐order differential equation was solved numerically. The influence of the choice of atomic potential on the scattering factor was studied in comparison to a simple muffin‐tin approximation of the atomic potential in solids. The application of calculated cross sections to a conventional Monte Carlo model for electron scattering using modified Bethe equation is described and results concerning the electron backscattering for different atomic potentials are compared.

Effect of surface tension on melt pool dynamics during laser pulse interaction

A previously developed model for simulation of recoil-pressure induced melt displacement during laser pulse interaction has been upgraded to include the restraining effect of surface tension. The results of numerical simulations of melt displacement/ejection during laser welding and drilling using this enhanced model are presented. In particular, the dependences of the threshold pulse energies for melt displacement and melt ejection as functions of laser pulse duration, beam radius and beam intensity distribution are computed and analysed.

On the possibility of microwelding with laser beams

We define microwelds as having fusion zone dimensions of <100 μm. At such small dimensions the required laser irradiance to produce melting is at or above conventional estimates of the irradiance required to produce drilling. The question thus arises if such small microwelds can be made via laser processes. A theoretical criterion defining the threshold pulse energy and beam intensity required for melt displacement (necessary for penetration-mode welding or drilling) is proposed. The results of numerical simulation present dependences of the threshold pulse energy and beam intensity as functions of laser pulse duration and beam radius. Experimental verification of the proposed criterion is described and a comparison of theoretical predictions and measurements is presented.

Four-color imaging pyrometer for mapping temperatures of laser-based metal processes

A 4-color imaging pyrometer was developed to investigate the thermal behavior of laser-based metal processes, specifically laser welding and laser additive manufacturing of stainless steel. The new instrument, coined a 2x pyrometer, consists of four, high-sensitivity silicon CMOS cameras configured as two independent 2-color pyrometers combined in a common hardware assembly. This coupling of pyrometers permitted low and high temperature regions to be targeted within the silicon response curve, thereby broadening the useable temperature range of the instrument. Also, by utilizing the high dynamic range features of the CMOS cameras, the response gap between the two wavelength bands can be bridged. Together these hardware and software enhancements are predicted to expand the real-time (60 fps) temperature response of the 2x pyrometer from 600 °C to 3500 °C. Initial results from a calibrated tungsten lamp confirm this increased response, thus making it attractive for measuring absolute temperatures of steel forming processes.

CO2 laser welding fused silica.

The feasibility of laser welding of fused silica (aka quartz) has been demonstrated recently by others[1,2]. An application requiring hermetic sealing of a thin, pressure-bearing quartz diaphragm to a thicker frame led us to explore this technique. We found that laser welding techniques normally used for metallic parts caused scorching and uneven melting. Contrary to “standard” practices (near-focus, high travel speed, high power density), successful welds in fused silica required a broad heat source applied over a large area under a slow rotation to gradually heat the glass through the annealing, softening and finally working temperatures. Furthermore, good mechanical contact between the parts to be joined played an even more important role in this process than in typical metallic joints.Temperature profiles are measured and discussed as related to laser power, rpm’s, number of passes, joint geometry and beam orientation; and compared to an FEM thermal model.We will discuss the effects of laser power, travel speed, number of passes, joint geometry and part thicknesses on achieving hermeticity and cosmetically-acceptable joints. Finally, we will examine the differences between standard and antithetical laser welding techniques used.The feasibility of laser welding of fused silica (aka quartz) has been demonstrated recently by others[1,2]. An application requiring hermetic sealing of a thin, pressure-bearing quartz diaphragm to a thicker frame led us to explore this technique. We found that laser welding techniques normally used for metallic parts caused scorching and uneven melting. Contrary to “standard” practices (near-focus, high travel speed, high power density), successful welds in fused silica required a broad heat source applied over a large area under a slow rotation to gradually heat the glass through the annealing, softening and finally working temperatures. Furthermore, good mechanical contact between the parts to be joined played an even more important role in this process than in typical metallic joints.Temperature profiles are measured and discussed as related to laser power, rpm’s, number of passes, joint geometry and beam orientation; and compared to an FEM thermal model.We will discuss the effects of laser power, tr...

Unified characterization of surfaces and gases in MEMS devices

Chemical and physical materials-aging processes can significantly degrade the long-term performance reliability of dormant microsystems. This degradation results from materials interactions with the evolving microenvironment by changing both bulk and interfacial properties (e.g., mechanical and fatigue strength, interfacial friction and stiction, electrical resistance). Eventually, device function is clearly threatened and as such, these aging processes are considered to have the potential for high (negative) consequences. Sandia National Laboratories is developing analytical characterization methodologies for identifying the chemical constituents of packaged microsystem environments, and test structures for proving these analytical techniques. To accomplish this, we are developing a MEMS test device containing structures expected to exhibit dormancy/analytical challenges, extending the range of detection for a series of analytical techniques, merging data from these separate techniques for greater information return, and developing methods for characterizing the internal atmosphere/gases. Surface analyses and data extraction have been demonstrated on surfaces of various geometries with different SAMS coatings, and gas analyses on devices with internal free volumes of 3 microliters have also been demonstrated.

Recoil force measurements during pulsed ND:YAG laser spot welds (1008).

Recently, the evaporative recoil pressure effect induced by high intensity laser irradiation on molten zone motion in welds has become increasingly appreciated. Theory indicates that so-called conduction mode welds are in fact rarely encountered. Given that shapes and sizes of fusion zones are so dependent upon recoil force, the ability to model fusion zone behavior requires correct implementation of the physics involved, particularly as size scales decrease and surface energy effects increase in relative magnitude. Our presentation discusses validation experiments supporting such model development. Two techniques are discussed, a calibration method using sensitive piezoelectric force gauges, and a more general tool using a microphonic method. Each technique has advantages and disadvantages, which will be discussed. For example, while the piezo force gauge technique is readily understandable, it requires a very lightweight sample in order to avoid smearing of the force signal. However, when the sample size becomes very small, other phenomena begin to affect the gauge, giving apparently negative force measurements! The microphonic technique can be applied to actual welds, but needs careful consideration as well to eliminate comb-filtering, echoes and sample ringing. Measurements on 304L will be presented and discussed relative to contemporary theories.Recently, the evaporative recoil pressure effect induced by high intensity laser irradiation on molten zone motion in welds has become increasingly appreciated. Theory indicates that so-called conduction mode welds are in fact rarely encountered. Given that shapes and sizes of fusion zones are so dependent upon recoil force, the ability to model fusion zone behavior requires correct implementation of the physics involved, particularly as size scales decrease and surface energy effects increase in relative magnitude. Our presentation discusses validation experiments supporting such model development. Two techniques are discussed, a calibration method using sensitive piezoelectric force gauges, and a more general tool using a microphonic method. Each technique has advantages and disadvantages, which will be discussed. For example, while the piezo force gauge technique is readily understandable, it requires a very lightweight sample in order to avoid smearing of the force signal. However, when the sample siz...

Application of melt ejection criterion in simulation of micromachining with laser

The theoretical criterion defining the threshold pulse energy and beam intensity required for melt ejection is proposed. The results of numerical simulation present dependencies of the threshold pulse energy and beam intensity as functions of laser pulse duration and beam radius. The experimental verification of proposed criterion is described and the comparison of theoretical predictions and measurements is presented. The criterion is applied for simulation of laser drilling metal foil with thickness in the range 25 μm - 125 μm using laser beam with 12 μm beam radius and pulse durations 10 ns and 100 ns. The computational results are used to interpret the results of experimental study of laser drilling of 125 μm aluminum foil using a single mode beam of a XeCl laser performed at the Nederlands Centrum voor Laser Research (NCLR) and the University of Twente. Additional results on Nd:YAG spot welds in pure Ni are also presented.

Analysis of Laser Weld Induced Stress in a Hermetic Seal

Laser welding of glass-to-metal electrical connectors is a common manufacturing method for creating a hermetically sealed device. The materials in these connectors, in particular the organic glass, are sensitive to thermal induced residual stress and localized heating. An analytical laser weld model is developed that provides simulation and analysis of both thermal and mechanical effects of the welding process. Experimental studies were conducted to measure the temperature at various locations on the connector. The laser weld is modeled using both surface and volumetric heating directed along the weld path to capture the thermal and mechanical response. The weld region is modeled using an elastic-plastic weld material model, which allows for compliance before welding and stiffening after the weld cools. Results from a finite element model of the glass-to-metal seal are presented and compared with experimental results. The residual compressive stress in the glass is reduced due to the welding process but hermeticity is maintained.

Measurement of Laser Weld Temperatures for 3D Model Input

Laser welding is a key joining process used extensively in the manufacture and assembly of critical components for several weapons systems. Sandia National Laboratories advances the understanding of the laser welding process through coupled experimentation and modeling. This report summarizes the experimental portion of the research program, which focused on measuring temperatures and thermal history of laser welds on steel plates. To increase confidence in measurement accuracy, researchers utilized multiple complementary techniques to acquire temperatures during laser welding. This data serves as input to and validation of 3D laser welding models aimed at predicting microstructure and the formation of defects and their impact on weld-joint reliability, a crucial step in rapid prototyping of weapons components.