Effect of design optimization and material selection on the fatigue performance and fracture mechanism of a crank arm

The nature of additive manufacturing techniques enables them to fabricate complex designs with intricate features that are not feasible to produce via traditional ways. However, such changes in the original designs can alter the stress flow throughout the components and their mechanical performance; such issues can impose limitations against the feasibility of design optimization via additive manufacturing and must be considered doing so. This study investigates the design optimization of a high-performance bicycle crank arm fabricated by additive manufacturing using the laser powder-bed fusion technique (L-PBF). The main aim of the optimization was to decrease the overall weight of the component while maintaining its fatigue performance. Further, increasing the strength-to-weight ratio of the crank arm by decreasing the unit length of its material was considered in this research by changing the L-PBF’s raw material from stainless steel 316L to lighter alloys, i.e., Al5X1 aluminum alloy and Ti64 titanium alloy. Finally, each alloy’s defect distribution (including porosities and lack of fusions), microstructure, and fractography analysis were considered to establish a correlation between each material’s characteristics and its performance as an engineering component under cyclic loads. The results indicated successful design optimization using 316L and Ti64 alloys as raw materials for additive manufacturing; however, components made of the aluminum alloy had comparably inferior fatigue performance. Such discrepancies in fatigue performance can be attributed to their different defect distributions and surface quality features.