Laser welding has significantly improved since its first commercial deployment in 1966, and is the preferred joining method when high weld quality, high production speed, and low thermal distortion are required. Laser welding (LW) encompasses a wide range of similar and dissimilar materials as is often the case in the automotive, aerospace, and medical industries. However this technology has not been implemented in many structurally critical applications, i.e. automotive chassis components, despite 50 years of development work and documentation in literature. NIST has undertaken a multi-part initiative to support extending LW to structurally critical components, with the current work focusing on nano-scale characterization using electron microscopy, diffraction, and atom probe tomography. In the automotive sector, much of the fundamental LW literature relies on evaluation of macroscopic weld quality coupled with mechanical properties assessment. However, more rigorous and fundamental examination of the phase transformations are needed to fully optimize laser processing for complex microstructures such as HSLA, dual-phase, TRIP, and TWIP steels such as is often performed during heat treatment studies . Subtle differences in sub-micron scale local chemistry and/or microstructure morphology can dramatically affect mechanical behavior, thus the need to reliably characterize these differences using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Furthermore, atom probe tomography (APT) provides a tool for direct measurement of local bulk chemistry and boundary segregation. Atomic mobilities and kinetic diffusion rates and can be inferred more accurately with APT measurements, providing critical parameters for predictive modeling.
Microscopy and Microanalysis
steel, laser welding, welding, laser materials processing, atom probe tomography, electron microscopy