Product Code: ICAL08_109

The Use of Holographically Customised Beam Profiles to Control the Overall Physical Cross Section of Direct Metal Depositions
Authors:
John Tyrer, Loughborough University; Loighborough Great Britain
Matthew Gibson, Loughborough University; Loughborough Great Britain
Rebecca Higginson, Loughborough University; Loughborough Great Britain
Presented at ICALEO 2008

This work considers the variation in flux density used during deposition and its role in controlling the overall cross sectional profile of the final track. Circular beams of various intensity profiles typically generate little variation in track cross section. When a moving laser beam interacts with a surface there is a temperature gradient induced in the material. The nature of the gradient will depend on a number of factors, but is heavily driven by beam intensity distribution and beam shape. Circular beams produce temperature gradients in the weld driving a doming of the molten weld pool and the resulting deposition. This necessitates the overlapping of adjacent tracks, reducing the time efficiency of the process and changes to the properties of the deposition as overlapping tracks are reheated. It will also have detrimental effects on the build-up of multiple layers, as subsequent layers are built from an uneven surface. The ability to change to a non circular profile is demonstrated. Rectangular beam shapes of asymmetric flux distributions are shown to provide control of the weld pool temperature distributions. Control of the weld pool temperature gradients allows manipulation of the weld pool flow, allowing customisation of the deposition profile. A novel beam shaping method has been developed at Loughborough University in the UK. The Diffractive Optical Elements (DOE) work as a hologram to reconstruct the beam shape and intensity into any defined profile, with a typical 90% efficiency. These DOE have been utilised to set up improved temperature gradients in the depositions, therefore controlling and even reversing the directions of the forces shaping the weld profile. By controlling the beam intensity profile, and therefore driving weld pool flows, depositions with an improved profile have been produced. These welds show a flattened top surface and a reduction in contact angle. This reduces the overlap required between tracks and provides a flatter foundation for subsequent layers to build from. These affects lead to an increase in build rate and the reduction or removal of post weld machining.