Forging lightweight bimetal gears

Abstract

This thesis is concerned with the development of the concept of forged lightweight multi-metal gears. By manufacturing gears comprising of high performance gear steel in highly stressed regions, and lightweight aluminium alloy in low stress regions, large reductions in transmission weight and rotational inertia are achieved. Utilising the forging process to manufacture such bi-metal gears lends the advantage of fast processing time, as well as providing methods of joining between the constituent metals. In order to develop the forging process, and to investigate joining between the metals, an in depth understanding of the forging process is necessary. To achieve this a three dimensional simulation of the hot forging process was developed, incorporating representative definitions for material data, frictional interaction, and heat transfer. The data produced by the simulations gives information regarding: (i) material flow; (ii) overall punch load; and (iii) interfacial conditions, such as contact pressure, slip, and temperature. An understanding of material properties is fundamental for the accurate simulation of the gear forging process. Optimum materials for the construction of a forged lightweight multi-metal gear were selected using an objective optimisation process, and the plastic flow stress properties of the materials were determined via compressive trials, at a number of temperatures and strain rates. This flow stress is characterised utilising constitutive visco-plastic equations, the constants of which were determined using a newly developed optimisation system. Prototype gears were forged investigating a number of initial ring thicknesses, varying temperatures of billet, and the use of different materials. The gear forging experiments demonstrate the macro-mechanical axial and rotational locking which is inherent to the forging of multi-metal gears. It was also shown that bonding between the aluminium core and steel ring typically occurs along the tooth root and flank, whereas bonding is absent from the tooth tip. Data from simulations indicates that this was due to significantly lower surface slip at the tooth tip region in comparison to the rest of the gear, which is insufficient to disrupt the oxide layer on the steel ring. Microscopy of gear segments revealed the presence of iron oxide as the primary barrier to metallurgical joining. Further experimental trials were conducted to investigate the formation of a metallurgical join between the steel and aluminium alloys of the ring and the core. A test rig was developed to establish rapid and cost-effective trials which replicate the essential interface conditions determined from bi-metal gear forging simulations. The trials established that oxide is primarily formed on the steel component during heating, and thus methods to improve metallurgical joining should reduce or eliminate the growth of oxide during the heating process. The utilisation of a silver interlayer in conjunction with macroscopic texturing of the steel was demonstrated to provide an effective barrier to oxidation, and to enable a strong join between the two metals.