Product Code: ICAL09_1505

The Control of Grain Size during Welding and Direct Metal Deposition using Three Dimensional Time-temperature Control Provided by Holographic Beam Shaping
Authors:
Matthew Gibson, Loughborough University; Loughborough Great Britain
Rebecca Higginson, Loughborough University; Loughborough Great Britain
John Tyrer, Loughborough University; Loughborough Great Britain
Matthew Blackmur, -; -
Presented at ICALEO 2009

When the laser beam is scanned a temperature profile is induced in the workpiece. The temperature distribution will be a function of the beam shape and intensity profile. The use of a circular beam leads to an uneven temperature distribution even when a top hat profile is used. This is caused by the material on the centreline of the beam getting irradiated for longer than an area towards the edge. The uneven temperature distribution will lead to a difference across the weld or deposition in terms of the temperature reached and the rate of cooling. Typically the weld will see a slower cooling rate and maximum temperature along the centreline. This leads to large columnar crystal growth, similar to a cast microstructure with an associated reduction in mechanical performance. A novel method for controlling time-temperature distribution in the material is through the use of a Holographic Optical Element (HOE). The HOE is a computer generated kinoform diffraction pattern that redistributes the beam intensity and shape into any customer designed profile. Through the use of a predictive non-linear thermal model the desired temperature distribution throughout the interaction region was input, and then propagated to the surface. This surface temperature profile was then converted to be the output of the kinoform beam shaping algorithm. This beam shape forms a complex and non intuitive beam profile allowing a controlled time-temperature profile, that can only be achieved in this way. Depositions have been made using the redesigned beam distribution and analysed using optical microscopy and Electron Back Scatter Diffraction (EBSD) to show a better than 50% reduction and grain size, an improvement in grain aspect ratio and a reduction and reshaping of the heat affected zone (HAZ)