Multi-objective optimization of laser powder bed fused titanium considering strength and ductility: A new framework based on explainable stacking ensemble learning and NSGA-II

Abstract

Achieving the simultaneous enhancement of strength and ductility in laser powder bed fused (LPBF-ed) titanium (Ti) is challenging due to the complex, high-dimensional parameter space and interactions between parameters and powders. Herein, a hybrid intelligent framework for process parameter optimization of LPBF-ed Ti with improved ultimate tensile strength (UTS) and elongation (EL) was proposed. It combines the data augmentation method (AVG ± EC × SD), the multi-model fusion stacking ensemble learning model (GBDT-BPNN-XGBoost), the interpretable machine learning method and the non-dominated ranking genetic algorithm (NSGA-Ⅱ). The GBDT-BPNN-XGBoost outperforms single models in predicting UTS and EL across the accuracy, generalization ability and stability. The SHAP analysis reveals that laser power (P) is the most important feature affecting both UTS and EL, and it has a positive impact on them when P < 220 W. The UTS and EL of samples fabricated by the optimal process parameters were 718 ± 5 MPa and 27.9% ± 0.1%, respectively. The outstanding strength-ductility balance is attributable to the forward stresses in hard α’-martensite and back stresses in soft α_m’-martensite induced by the strain gradients of hetero-microstructure. The back stresses strengthen the soft α_m’-martensite, improving the overall UTS. The forward stresses stimulate the activation of dislocations in hard α’-martensite and the generation of <c+a> dislocations, allowing the plastic strain to occur in hard regions and enhancing the overall ductility. This work provides a feasible strategy for multi-objective optimization and valuable insights into tailoring the microstructure for improving mechanical properties.