Process development for forging lightweight multi-material gears

The work presented in this thesis discusses the development of an innovative method for manufacturing tailored gears, specifically bi-metallic lightweight gears through the use of the forging process. Utilising this method, gears can be constructed from multiple metals, where high strength, high density materials are located in regions of high stress concentration, such as the tooth flank, tooth root and regions in contact with shaft attachment mechanisms. On the other hand, lower strength, lower density materials can be located at less critical regions, such as the central region of the gear, hence reducing weight. A patent has been filed for the production process of forging multi-material lightweight gears. To investigate this process, bi-metallic gear construction was studied, where a high strength outer ring; and low strength cylindrical or annular core placed within the confines of the ring, allowed for the production of high strength teeth. Experimental and simulation work was conducted to better understand the material flow which occurs during the forging process, and hence its implications on the structural integrity of the gear. To allow for experimental trials, a tool set was designed and manufactured, and used in conjunction with a forming press to forge gears of a spur gear profile. Gears were produced under both cold and hot forging conditions using model materials (lead and copper) and engineering alloys (aluminium and mild steel) respectively. This construction was evaluated for a range of ring thicknesses. A simplified Finite Element (FE) model was established to analyse the material flow and ring thickness distribution during the cold forging operation. Data for the materials commercially pure lead, copper (C101), aluminium alloy (Al 6082), mild steel (230M07) and gear steel (16MnCr5) were obtained through compressive experiments undertaken on Instron and Gleeble testing machines. Constitutive equations were calibrated to a unified constitutive equation model incorporating the physical parameters of stress, plastic strain rate, isotropic hardening, and dislocation density to model the behaviour of aluminium alloy, mild steel and gear steel allowing for the creation of a FE model representing the hot forging process. Furthermore, three locking mechanisms between the two materials were examined: macro-mechanical locking, micro-mechanical locking and diffusion bonding; which when coupled together may prevent disengaging during operation. In addition, the root and contact stresses experienced by bi-metallic gears were also compared to a single material steel gear through an FE model to identify performance differences. Finally, recommendations and future research directions are presented.Open Acces