Laser Welding of Steels

ABSTRACT Fundamentals of high-power laser welding are reviewed and unique features relative to other welding processes are noted. A brief description is given of the preferred characteristics of laser, focusing and ancillary equipment suitable for high-power production applications. Specific welding performance is noted for a range of steel compositions and thicknesses and process limitations are identified. INTRODUCTION The potential of high-power laser systems for production welding applications has long been recognized. Seam-welding procedures employing pulsed laser systems were initially developed shortly after the first operation of a laser in 1960. Although such procedures were well-suited for precision welding of delicate and/or complex assemblies, however, it was found that welding speeds and penetration capabilities for pulsed systems were inadequate for large-scale production applications. Development of high-power, industrially-suited, continuously-operating, C02M laser systems in the late 1960's significantly enhanced the laser's capability for welding. Within the past fifteen years, the pace of laser welding development has quickened at an increasing rate. Welding performance has been demonstrated in stainless, low-carbon and alloy steels, titanium alloys, nickel-base alloys and in some aluminum alloys. A maximum single-pass weld penetration of 50 mm has been achieved in alloy steel and welding speeds to approximately 1000 mm/sec have been demonstrated in 0.2 mm thick material. The influence of process parameters on welding performance has been identified and the range of current applicability of laser welding has been delineated. Within the past five years, the most significant advances in laser welding have come in the area of reduction to routine production practice. An ever increasing number of multi kilowatt, carbon-dioxide laser systems are demonstrating their capability for reliable operation under severe production conditions. In the following, current laser welding technology is identified with specific emphasis on laser welding performance in steels. PROCESS FUNDAMENTALS From a welding viewpoint, the laser may be considered simply as a radiant energy source. The individual photons which comprise the laser beam exhibit an energy corresponding to the laser transition. For the carbon-dioxide laser, which is currently the only system suitable for multikilowatt production use, the photon energy is 0.12 eV and the corresponding wavelength is 10.6 micron. Since this wavelength in the infrared portion of the electromagnetic spectrum does not transmit through ordinary optical materials (glass, quartz, etc.), special materials such as zinc selenide must be employed. Extensive use of front surface reflective optics also characterizes carbon-dioxide laser applications. Because the energy in a laser beam is highly ordered, the beam can be focused to provide extremely high power densities. From basic principles, it is known that the minimum spot diameter to which the beam can be focused is of the order of the beam wavelength. This provides the capability for attaining power densities of the order of 106 W/cm2 with relatively modest power levels of a few kilowatts.