Residual Stress Characterization for Marine Gear Cases in As-Cast and T5 Heat Treated Conditions with Application of Neutron Diffraction

Mercury Marine has recently implemented an advanced aluminum-silicon alloy (Mercalloy™ 362) into a redesigned, lightweight gear case for use in high powered marine engines. The company is currently in the process of tuning the manufacturing process to reduce the evolution of residual stress during the casting process. Preliminary work was performed by the authors on non-machined, as-cast and T5 heat-treated gear case castings. The primary results from that study indicate relatively high residual stresses (~120 MPa) were present in the radial and hoop orientation of the as-cast gear case (Sediako in Thermec, 2018 [1]). With the expectation that the machining process would likely increase the stress throughout the gear cases, it was necessary to perform an additional study on machined gear cases in both the as-cast and T5 state. This study presents the residual stress and strain characterization of machined, as-cast and T5 heat-treated marine gear cases using neutron diffraction (ND) at the Canadian Nuclear Laboratories. A comparison of the results from this study to the preliminary results, revealed that the machining process increases the stress by approximately 45%. However, the ND results also indicate that heat treatment was successful in alleviating approximately 50% of the residual stress in the machined and non-machine gear cases. The results from this study provide the company with valuable information for further optimizing the design and manufacturing technologies for the highly advanced lightweight marine gear case.