Multi-objective optimisation and multi-criteria decision making in SLS using evolutionary approaches

Purpose - The goal of this study is to carry out multi-objective optimization by considering minimization of surface roughness (Ra) and build time (T) in selective laser sintering (SLS) process, which are functions of "build orientation". Evolutionary algorithms are applied for this purpose. The performance comparison of the optimizers is done based on statistical measures. In order to find truly optimal solutions, local search is proposed. An important task of decision making, i.e. the selection of one solution in the presence of multiple trade-off solutions, is also addressed. Analysis of optimal solutions is done to gain insight into the problem behavior. Design/methodology/approach - The minimization of Ra and T is done using two popular optimizers - multi-objective genetic algorithm (non-dominated sorting genetic algorithm (NSGA-II)) and multi-objective particle swarm optimizers (MOPSO). Standard measures from evolutionary computation - "hypervolume measure" and "attainment surface approximator" have been borrowed to compare the optimizers. Decision-making schemes are proposed in this paper based on decision theory. Findings - The objects are categorized into groups, which bear similarity in optimal solutions. NSGA-II outperforms MOPSO. The similarity of spread and convergence patterns of NSGA-II and MOPSO ensures that obtained solutions are (or are close to) Pareto-optimal set. This is validated by local search. Based on the analysis of obtained solutions, general trends for optimal orientations (depending on the geometrical features) are found. Research limitations/implications - A novel and systematic way to address multi-objective optimization decision-making post-optimal analysis is shown. Simulations utilize experimentally derived models for roughness and build time. A further step could be the experimental verification of findings provided in this study. Practical implications - This study provides a thorough methodology to find optimal build orientations in SLS process. A route to decipher valuable problem information through post-optimal analysis is shown. The principles adopted in this study are general and can be extended to other rapid prototyping (RP) processes and expected to find wide applicability. Originality/value - This paper is a distinct departure from past studies in RP and demonstrates the concepts of multi-objective optimization, decision-making and related issues.