The way the temperature is distributed inside the material is of prime importance in laser materials processing. Optimisation of different laser processes requires control over the temperature distribution in order to control microstructure and residual stressesb y manipulating heating/cooling rates and thermal gradients. Currently most of the laser material processing is carried out by either circular or rectangular beam geometry with variation in temperature distribution caused by either the variation of laser power, spot size or scanning speed. Variations in these parameters are often limited by other processing conditions, therefore the possibility of modifying the microstructure and residual stresses are limited. If any other parameter can be identified, such that variation of this parameter alone (i. e. without changing the laser power or scanning speed) can alter the temperature distribution, then it will provide added flexibility to the process control. One possible method of varying the temperature distribution, and hence the heating /cooling rates and thermal gradients is to modify the geometry of laser beams. The effect of laser beam geometry, particularly of non-conventional laser beam geometries on laser processing of materials has received very little attention. This thesis presents a detailed numerical investigation of the effects of non-conventional laser beam geometries on laser surface heating, laser transformation hardening, laser tube bending and laser melting of metallic materials. The numerical models have been validated by experiments using a diode laser. The temperature distributions, heating/cooling rates, thermal gradients, stress distributions and distortions were evaluated. The work presented in this thesis highlights the different attributes of conventional and non-conventional laser beam geometries. Laser beam geometries were found to influence heating and cooling rates as well as residual and normal stress distributions in materials processing. In particular, the donut beam results in transient bi-axial stress state which does not exist for any other solid beam geometry. These attributes can be utilised as an advantage not only in the investigated processes but could also be utilised for improving various other laser material processes
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:545860 |
| Date | January 2007 |
| Creators | Shakeel, Safdar |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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