Applications of pulsed lasers for phase transformation and welding

Abstract: This work deals with the interaction of pulsed laser radiation with solid, metallic materials at moderate and very high power densities. The interaction mechanism depends upon the properties of both the laser and the target material. The wavelength, pulse energy and pulse duration of a laser can be chosen to suit a particular application whilst the physical properties of the target material are usually fixed. The reflectivity, absorptivity, thermal diffusivity and conductivity, density, specific heat, melting point, boiling point, latent heat of melting, latent heat of vaporisation, ionisation energy, plasma opacity, plasma density, etc. of the target all influence the interaction with the laser beam. In addition, the mechanical and metallurgical properties of the target can be important in certain situations, for example high pressure shock waves in some materials can lead to work hardening. This process is known as shock hardening. At very high power densities or fluxes, 109 - 1014 Wcm-2 [Jcm-2s-1], the explosion of a superheated plasma causes a high-pressure shock wave, which, in turn causes micro structural changes in certain target materials (papers I-III). Investigations into this phenomenon were performed with TEA C02, Q-switched Ruby, Nd:YAG and Iodine photo dissociation lasers. At moderate power densities or fluxes, 104 - 107 [Jcm-2s-1], the focused pulse only melts a limited volume of the metal parts involved. This is the interaction mechanism found in laser welding (papers IV-VI). These investigations were performed with an Nd:YAG laser and studied the interaction mechanism with pairs of materials usually difficult to join in industrial applications: tungsten alloys to cobalt and iron-base alloys, tungsten carbide to tool steel, aluminium to copper, etc. In some cases, the addition of filler metals was found useful to produce sound welds, forming ternary phases or a 'dispersed ensemble' during the process.