Statistical analysis of recast formation in laser drilled acute blind holes in CMSX-4 nickel superalloy

The present study investigates the effects of four controllable variables on the metallurgy and drilling velocity of laser percussion drilled acute (30A degrees to surface) blind holes in 2 mm thick flat pieces of nickel superalloy-CMSX-4 material. Design of experiment and statistical modelling were used in the experimental study to understand parameter interactions. Pulse energy, pulse width, pulse frequency and gas pressure were chosen as the independent process variables. The response surface method was used to develop the models for each of the responses: physical characteristics of wall and bottom recast layer and drilling velocity (function of material removal rate). A central composite design was chosen as it offered the most economical number of experiments for the required information. The significant process factors in each model were identified based on the analysis of variance and the models were checked by complete residual analysis. It was found that high pulse energy and short pulse width gave the maximum drilling velocity and lowest recast layer thickness at the hole bottom, whereas low pulse energy and short pulse width minimised the wall recast layer thickness.