Quality Evaluation of Micro Drilled Hole Using Pulsed Nd:YAG Laser: a Case Study on AISI 316

Abstract

Laser drilling is increasingly becoming a suitable choice for performing micro-drilling on advanced engineering and bio-compatible materials. In laser drilling operation, two types of major defects like spatter deposition and taper are generally encountered. In order to reduce the drilling defects, understanding the influence of process parameters on quality of hole is highly desirable. The influence of process parameters such as laser energy, pulse repetition rate, pulse width and gas pressure on the quality characteristics viz. circularity at entry and exit, taper and spatter area is studied in this work using Taguchi’s L27 orthogonal array. Analysis of variance (ANOVA) indicates that pulse width is most influencing parameter having percentage contribution of 43.981, 51.93, 88.28 and 39.94 for circularity at entry, circularity at exit, taper and spatter area respectively. It is observed that spatter area and heat affected zone (HAZ) increases with increase in laser energy and pulse width. This is due to the fact that heat input increases causing more material to melt but sufficient time is not allowed to completely flush away the molten material. As a result, heat is not dissipated adequately resulting in spatter formation and heat affected zone. The analysis also reveals that increase in pulse repetition rate and gas pressure leads to decrease in spatter area. Increase in pulse repetition rate causes increase in laser power leading to formation of laser supported absorption (LSA) wave which may cause the blockage of input energy. Hence, material ejection becomes less and scope for formation of spatter turns out to be less. Micrographs of drilled surface are analyzed using scanning electron microscope (SEM). The micrograph study suggests that increase in laser energy leads to increase in HAZ and circularity. The study also reveals that increase in pulse repetition rate leads to increase in burr formation in the hole periphery. Empirical models have been proposed to predict the quality characteristics of drilled hole. Mean relative error within 10% between the predicted value from empirical relations and experimental value for quality characteristics of hole indicates the validity of the proposed models. In order to obtain optimum parametric setting for all the quality characteristics, a well-known evolutionary approach known as genetic algorithm (GA) is used. The suggested best parametric conditions are definitely useful for the practitioners to avoid trial-and-error process of setting the process parameters for obtaining laser drilled holes of desired quality.