Laser welding has many advantages over traditional joining methods, yet remains underutilized. NIST has undertaken an ambitious initiative to improve predictions of weldability, reliability, and performance of laser welds. This study investigates butt welding of galvanized and ungalvanized dual-phase automotive sheet steels (DP 590) using a 10 kW commercial fiber laser system. Parameter development work, hardness profiles, microstructural characterization, and optical profilometry results are presented. Sound welding was accomplished in a laser power range of 2.0 kW to 4.5 kW and travel speed of 2000 mm/min to 5000 mm/min. Vickers hardness ranged from approximately 2 GPa to 4 GPa across the welds, with limited evidence of heat affected zone softening. Decreased hardness across the heat affected zone directly correlated to the appearance of ferrite. A technique was developed to non-destructively evaluate weld quality based on geometrical criteria. Weld face profilometry data were compared between light optical, metallographic sample, and frequency-modulated continuous-wave laser detection and ranging (FMCW LADAR) methods.
To meet the semiconductor industry’s demands for accurate measurements on excimer lasers, we have developed a system using the correlation method to measure the nonlinear response of pulse energy detectors of excimer laser at 193 nm. The response of the detector under test to incident laser pulse energy is compared to the corresponding response of a linear monitor detector. This method solves the difficulties caused by large pulse-to-pulse instability of the excimer laser and delivers measurement results with an expanded uncertainty (k=2) of 0.8 %.
We have constructed a prototype calorimeter that will serve as a primary standard for measurements of 157 nm excimer laser power and energy. The construction and performance of the prototype will be discussed. In addition, we have performed a series of thermal characterization measurements on the prototype. From these measurements, we deduce that the uncertainty in the prototype's electrical calibration factor is less than 0.2 percent. This number is less than or comparable to the uncertainty of the NIST primary standards for use with 193 and 248 nm excimer lasers. The 157 nm standards are part of a beamsplitter-based measurement system for laser power and energy calibrations. To control and determine the ambient measurement conditions, we have constructed a nitrogen-purged enclosure for this system. We are able to achieve O2 concentrations of less than 3 parts per million (ppm) inside the enclosure.
We determined the damage thresholds and lifetimes of several materials using 157- and 193-nm excimer lasers and a beam profile technique similar to that described in ISO 11254-2. We made these measurements to select an appropriate absorbing material for use in our primary standard laser calorimeter for 157-nm excimer laser energy measurements. The materials we tested were nickel-plated sapphire, chemically-vapor-deposited silicon carbide (CVD SiC), nickel-plated copper, and polished copper. Applied pulse energy densities (or dose) ranged from 80 to 840 mJ/cm<SUP>2</SUP>. We determined the applied dose from a series of laser beam profile measurements. Silicon carbide had the highest damage threshold: 730 mJ/cm<SUP>2</SUP> per pulse. For this reason, and because of its high thermal and electrical conductivities, we have chosen silicon carbide as the absorber material for the 157-nm calorimeter. We also conducted long-term exposure studies in cooperation with MIT Lincoln Laboratory at a pulse energy density of 5 mJ/cm<SUP>2</SUP> to simulate typical calorimeter operating conditions. No aging effects or other surface changes were observed at these dose levels after 500 million pulses, corresponding to a projected calorimeter lifetime of 50 years.