Conduction laser welding involves initiating a melt pool by exposure to high power laser induced light and controlled thermal conduction. Existing welding techniques generally provide enough energy to join the component but have no real control over the melt pool. This process can invariably lead to overheating in adjacent areas or even the melt pool itself, often causing unavoidable effects, such as ‘burn through’. The present work presents a procedure in which a desired melt pool shape is conceived, and a bespoke beam irradiance distribution is designed to match. The beam is shaped not by conventional lenses but by a diffractive holographic optical element (DHOE). The DHOE utilises holography to wholly create highly complex three-dimensional energy distributions through constructive and destructive interference. This technique allows novel beam irradiance distributions to be applied to conduction mode laser welding, with the melt pool transverse profile being shaped to a specific design. Holographic conduction laser welding has been shown to be successful and represents a significant step forward in the industry, as demonstrated in this case in both mild and stainless steels. The fusion zone is shown to be particularly influenced by the shape of the illuminating laser beam profile, and many of the welds demonstrate a highly novel weld profile because of this. The use of a bespoke beam irradiance distribution allows control of the heat flow to the workpiece, and this allows greater control over material migration due to surface tension effects. Many of the welds demonstrate unique surface solidification patterns directly linked to the beam profile used. The DHOE also presents a number of additional advantages, such as an increased usable depth of field, allowing for less stringent set-up tolerances. Comprehensive metallography has been performed on samples of these welds through the use of optical microscopy, electron microscopy, electron backscatter diffraction and energy dispersive (X-ray) spectroscopy. These techniques offer in depth analysis of crystal size, shape, orientation and phase. By incorporating DHOEs into a laser welding process, not only does the melt pool shape become controllable, but also the crystal growth is highly influenced. Many of the undesirable attributes of a conventional laser weld are reduced by using a beam distribution created by a DHOE, bringing the microstructure of the weld pool closer to that of the parent material.

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.

Conventional laser cladding systems use a focussing nozzle to direct the beam to the work piece, and by using either blown or pre placed powder, they are able to fuse the powder to the substrate. The shortfall of this process is that, once sectioned, a parabolic transverse profile is seen. This is due to the Gaussian irradiance distribution of the beam and no conventional optics can remedy the problem. Diffractive optical elements, however, can be designed for each process. Models of thermal conduction in the powder and the substrate can be utilised to produce a diffractive optical element specific to this operation. This enables a rectangular spot with a semi-idealised intensity cross section to be delivered to the work piece, reorganising 90% of the 1.2kW CO2 laser beam. When the diffractive optical element is applied to the laser cladding process, the resultant transverse clad profile is uniformly rectangular with a lack of ‘shoulder rounding’.The laser cladding operation produced complex microstructures, which could not be satisfactorily categorised with optical microscopy. This lead to the simplification of the process by removing the powder. Autogenous bead on plate laser welds have therefore been manufactured, using both conventional optics and diffractive optical elements, in order to determine the solidification microstructure of 316L stainless steel. Optical microscopy, electron back-scatter diffraction (EBSD), energy dispersive spectroscopy (EDS), have been employed to assist in the characterisation of the complex microstructures produced.