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.