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.
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.
It has been shown that thermal energy imparted to a metallic substrate by laser heating induces a transient temperature gradient through the thickness of the sample. In favorable conditions of laser fluence and absorptivity, the resulting inhomogeneous thermal strain leads to a measurable permanent deflection. This project established parameters for laser micro forming of thin materials that are relevant to MESA generation weapon system components and confirmed methods for producing micrometer displacements with repeatable bend direction and magnitude. Precise micro forming vectors were realized through computational finite element analysis (FEA) of laser-induced transient heating that indicated the optimal combination of laser heat input relative to the material being heated and its thermal mass. Precise laser micro forming was demonstrated in two practical manufacturing operations of importance to the DOE complex: micrometer gap adjustments of precious metal alloy contacts and forming of meso scale cones.
A 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 the 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 a laser beam with 12 μam beam radius and pulse durations 10 ns and 100 ns. The computational results are used to interpret the results of an 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.
A non-contaminating, non-contact method to open glass-cap type MEMS (Micro-electromechanical systems) packages by separating the silicon substrate from the glass cover using a CO2 laser is presented. Current methods for opening these packages are cumbersome, can lead to sample contamination and are not easily done under vacuum. The package is placed in an evacuated chamber connected to gas-sampling equipment and processed through a ZnSe (transparent to 10.6 μm laser radiation) window. Laser-induced heating promotes initiation and propagation of cracks in the cover glass or at the glass/Si interface resulting in separation of the cover from the substrate. Two techniques are discussed. First, local perimeter heating of the package creates a compressive stress zone, surrounded by a tensile stress zone. Tensile zone motion relative to natural or artificially induced flaws promotes selective crack growth and propagation leading to complete separation. Second, overall heating of the package creates a coefficient of thermal expansion (CTE) difference. In both techniques the sudden release of stored residual stresses may be sufficient to "flip" the lid off the substrate. Careful tuning of the process (temperature rise and energy density) is necessary to minimize or eliminate chip debris and avoid package degassing which confuses gas analysis.
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.
Careful characterization of laser beams used in materials processing such as welding and drilling is necessary to obtain robust, reproducible processes and products. Recently, equipment and techniques have become available which make it possible to rapidly and conveniently characterize the size, shape, mode structure, beam quality (Mz), and intensity of a laser beam (incident power/unit area) as a function of distance along the beam path. This facilitates obtaining a desired focused spot size and also locating its position. However, for a given position along the beam axis, these devices typically measure where the beam intensity level has been reduced to I/ez of maximum intensity at that position to determine the beam size. While giving an intuitive indication of the beam shape since the maximum intensity of the beam varies greatly, the contour so determined is not an iso-contour of any parameter related to the beam intensity or power. In this work we shall discuss an alternative beam shape formulation where the same measured information is plotted as contour intervals of intensity.