Fracture Analysis of R14 Aluminum Alloy Shifting Fork

Fracture occurred to a R14 aluminum alloy shifting fork casting in the truck during service. Chemical composition, microstructure, fracture morphology and micro-area composition analysis of the fractured shifting fork casting were analyzed by means of photoelectric direct reading spectrometer, metallographic microscopy, SEM and SDD EDS to investigate the fracture cause. The analysis results indicate that the chemical composition, mechanical properties and hardness of the fractured shifting fork casting were all qualified. However, the near-surface pinhole grade and R angle of abrupt change area of the cross-section did not meet the design requirement. Due to porosity defects at the abrupt change area of the cross-section and stress concentration, cracks initiated from the porosity defects while the failed part bore tensile stress and propagated rapidly under the circulating tensile stress, which led to the final fracture. To avoid such failure, some measures were proposed and applied, such as performing pinhole testing after shifting fork casting's compression molding and increasing R angle of abrupt change area of the cross-section.