Quality improvement of an aluminum gearbox housing by implementing additive manufacturing

The production of small to medium-size aluminum gearbox housings at large quantities and high rates is commonly carried out by die casting. The manufactured parts are characterized by a very good surface finish and dimensional accuracy. However, a certain amount of gas porosity and shrinkage holes are to be expected. Since defects affect the mechanical properties of the housing, their reduction is also a main goal in the enhancement of the process. Improved material properties lead to higher strength and better gear performance. Despite die casting being cheap in comparison to sand casting, the costs of the casting equipment, whose main components are two hardened steel dies with the desired shape, are very high. Based on one particular gearbox housing, the objective of the present development project was to improve the quality of the gearbox housing by reducing its casting defects, while achieving simultaneously an overall improvement of the casting process. To this end, a new kind of die with integrated cooling channels was designed and evaluated using additive manufacturing technologies. First, the usual defects like cold casting sprue, sticking due to overheating, and solidification cavities of the cast part as well as the cooling behavior of the standard die were analyzed. Thus, the areas to be cooled down in a controlled way were identified and the corresponding cooling system was developed. Afterwards, an FEM analysis was carried out to check the die’s integrity during the casting process. The model was built according to given operating conditions. The stress analysis was efficiently studied on a variety of cooling channel designs. The results led to new and improved knowledge concerning the die’s stability. 3D metal printing was suitable for the manufacturing, because a clear additional benefit compared to the standard die was expected. Selective laser melting (SLM) was chosen because this technology allows the cooling system to be printed without a supporting structure. The material chosen was the tool steel 1.2709 (X3NiCoMoTi18-9-5). The die was printed and machined to its final dimensions using this material. Finally, the die casting mold was assembled and 15,000 casting rounds were executed by varying the temperature of the cooling oil. The type, number, and distribution of failures of the cast parts as well as the cycle duration were analyzed. As a result of the present work, the quality of the gearbox housing was improved concerning porosity and shrinkage holes. Furthermore, due to the targeted solidification, the casting process was shortened by about 11% per casting cycle.