Residual Stress Analysis of A362 Aluminum Alloy Gear Case Using Neutron Diffraction

Abstract

Mercury Marine has used a new alloy, Mercalloy A362, for the manufacturing of a re-designed lower unit transmission gearcase. The enhanced strength of the alloy allowed for a substantial weight reduction in the new design. The purpose of this study was to examine and determine why cracking may develop in the gear casing during in service testing. Two types of material states, (i) as cast and (ii) heat treated were compared. Metallography and neutron diffraction analysis was carried out at locations identified as being areas of high stress by Magma software – which was performed in a separate study. Microstructural characterization at these locations revealed microstructural and the compositional differences. Differences in the porosity, eutectic phase, and volume fraction of the precipitates were observed at various locations of interest in each material state. The residual stress analysis was performed with application of neutron diffraction and revealed that the stresses in the as-cast component reached a maximum value of 120 MPa, which is below the yield strength of the alloy. The heat treatment applied to the castings reduced the stress by approximately 50 MPa. Based on the microstructure and neutron diffraction results, it is likely that performing a heat treatment process extends the lifetime of the component, however, it may not completely eliminate the cracking problem. Farther studies are currently nearing completion, targeting the mass production of the redesigned gearcase.

Residual Stress in Automotive Powertrains: Methods and Analyses

“…Conversely, the high penetration of neutrons can allow for precise measurements of strain in the bulk of the material and in both thick and thin walled sections. Although this method can be expensive and time consuming, ND permits the measurement of all orientations of strain at each location of interest which allows for the additional generation of 3D residual stress profiles throughout the component [14], [15]. Although each method has been shown to be sufficient for specific applications, the literature comparing the two methods is sparse.…”