Application of the central composite design in optimization of laser transformation hardening parameters of commercially pure titanium using Nd:YAG laser

Abstract

In this study, the application of response surface methodology (RSM) and central composite design for modeling, optimization, and an analysis of the influences of dominant laser-processing parameters namely: laser power (LP), scanning speed (SS), and focused position (FP) on heat input (HI) and hardened bead geometries such as hardened bead width (HBW), hardened depth (HD), angle of entry of hardened bead profile (AEHB) of laser transformation hardened surface quality of commercially pure titanium sheet of 1.6 mm in thickness using continuous wave (CW) 2-kW Nd:YAG laser is discussed. Experimental bead on trials of laser transformation hardening process were conducted on commercially pure titanium sheet. The design of experiments concept has been used to optimize the experimental conditions. The experimental data were collected based on a three-factor-five-level full central composite design. The multiple regression analysis using RSM was conducted to establish input–output relationships of the process. The mathematical models were developed and tested for adequacy using analysis of variance and other adequacy measures. The results show that models explain the heat input and hardened bead surface geometry successfully and satisfactorily. Using the developed models, the main and interaction effect of the laser process input variables on the heat input and hardened bead profile responses were investigated. It is found that all the investigated laser parameters are affecting the performance of the HI and hardened bead profile parameters significantly. The predicted results are compared with the experimental results and are good agreement with heat input and hardened bead profile parameters. The optimized laser process parameters obtained by numerical optimization using RSM, those ensuring the minimum HI = 181.33J/cm, maximum HBW = 2.292 mm, minimum HD = 0.162 mm, and minimum AEHB = 17.94° are LP = 760.08 W, SS = 2,460.3 mm/min and FP = −30 mm.