The Influence of Surface Extrusion Densification on the Microstructure and Mechanical Properties of Iron-Based Powder Metallurgy Gears

In this study, the surface extrusion densification process is used to improve the surface density, hardness, and mechanical strength of powder metallurgy gears. A mixture of pre-alloyed powders, 0.3 wt.% graphite, and 0.4 wt.% Lube HD lubricant was used as experimental raw materials. These powders were compacted into experimental gears at a pressure of 1,600 MPa and then sintered at 1,120°C for 30 minutes. The sintered gears achieved surface densification by passing through extrusion dies under pressure at a 1 mm/sec speed. The influence of different extrusion amounts (∆W = 0, 0.046, 0.116, 0.186, and 0.246 mm) on the microstructure and mechanical properties of iron-based powder metallurgy gears was investigated (∆W is defined as the reduction in the cross- bar distance between two teeth in the extrusion die plate). The results show that surface densification by extrusion can simultaneously apply normal stress and shear stress, resulting in a reduction of porosity on the gear surface, which in turn forms a densified layer on the surface. The thickness of the densified layer increases with the amount of extrusion. In addition, the surface densification by extrusion improves the surface microhardness and crushing strength of the gears. In particular, the gears with ∆W = 0.246 mm have the highest surface microhardness and fracture toughness. The porous model in DEFORM was used to simulate the surface extrusion densification process. The simulation results showed trends in the relative density distribution consistent with the experimental results, with a higher relative density at the gear surface, followed by a decrease as the distance from the surface increased and the densified region expanded with increasing extrusion amounts. In addition, there was a high degree of correlation between the simulated and experimental results in terms of densification layer thickness.