Holographic diffractive optical elements allow improvements in conduction laser welding of steels

Abstract

Computer generated holographic elements have been successfully used as a novel beam delivery method in conduction limited laser welding 316L stainless steel using a 1.2kW CO2 laser. An ideal weld zone profile was thermally modelled to produce the desired thermal contours in the substrate. This was then utilised to fabricate a diffractive optical element (DOE), which returned the desired flux distribution specific to this operation. A number of completely bespoke beam irradiance distributions have been produced in this way. When compared to an equivalent Gaussian beam distribution, focussed using conventional lenses, significant improvements in weld quality are seen. Material migration, for example, is considerably reduced, as diffractives afford more control over the thermal gradient on the substrate surface. This means less material is drawn to the outer periphery of the weld, from the viscous melt pool. One of the most noteworthy advantage to using diffractive optics is the huge depth of field they provide, when compared to conventional lenses and Gaussian optics. The equivalent lens focal distance would be 460mm, giving a large depth of field regardless; and the resultant beam is composed of an array of diffracting planes, meaning an ever greater flexibility in usable focal range.The weld microstructures have been investigated using EBSD (Electron Back Scatter Diffraction) and EDS (Energy Dispersive Spectroscopy); and in 316L stainless steel, show substantial differences between Gaussian and diffractive welds. Gaussian beam weld microstructures in 316L stainless steel, tend to have much larger average grain sizes (up to approx. 2500% of substrate); moreover, the grains have a comparably smaller misorientation angle, meaning untreated Gaussian welds would be more susceptible to mechanical failure. The microstructural advantage to using diffractive optics to shape the laser beam are that the grain structure is on average, smaller (up to approx 1500% of substrate) and equiaxed, like that of the substrate, leading to better mechanical properties without the need for heat treatment.Computer generated holographic elements have been successfully used as a novel beam delivery method in conduction limited laser welding 316L stainless steel using a 1.2kW CO2 laser. An ideal weld zone profile was thermally modelled to produce the desired thermal contours in the substrate. This was then utilised to fabricate a diffractive optical element (DOE), which returned the desired flux distribution specific to this operation. A number of completely bespoke beam irradiance distributions have been produced in this way. When compared to an equivalent Gaussian beam distribution, focussed using conventional lenses, significant improvements in weld quality are seen. Material migration, for example, is considerably reduced, as diffractives afford more control over the thermal gradient on the substrate surface. This means less material is drawn to the outer periphery of the weld, from the viscous melt pool. One of the most noteworthy advantage to using diffractive optics is the huge depth of field they prov...