EFFECT OF BEAM FOCUSING DEPTH ON THE QUALITY OF HOLE PRODUCED BY LASER BEAM

Abstract

Laser drilling (LO) is a heat based multivariate-machining process in which a high power pulsed laser beam having very high irradiance is used. As Laser beam comes out from focusing optics, it attains a certain minimum diameter at focal plane of optics called "beam waist". After the beam waist the beam diameter increases with the distance. Thus use of a defocused beam gives larger spot size of laser beam on the workpiece surface thus reduces the energy density at that place. The spot size is not possible to measure directly due to high irradiance associated with laser beam. An indirect thermal imaging technique was therefore used for estimation of the beam diameter variation. A 400W pulsed Nd:YAG laser was used to drill holes in nickel base superalloy test specimen by focusing at different levels (in range of 0 to 6.35mm). The objective of this dissertation work is to examine the effect of beam defocusing at the surface on the quality of holes. The holes that have been drilled were characterized by using video-imaging technique, which is a quick method for hole diameter measurement based on laser focusing optics. The quality of drilled hole was assessed by the amount of spatter, circularity of entry diameter and taper angle. The experimental results indicate that minimum taper is obtained when beam is made to focus slightly below the surface of workpiece. However, to obtain holes with smallest diameter and maximum circularity use of fine focused beam on the surface of work-material is necessary. II