IMPROVING THE MECHANICAL AND PHYSICAL PROPERTIES OF AN ALUMINUM POWDER METALLURGY METAL MATRIX COMPOSITE VIA HOT UPSET FORGING

Abstract

Aluminum powder metallurgy (APM) represents an advanced metal forming technology premised on a three-stage production sequence of die compaction, liquid phase sintering, and post-sinter sizing. It is principally utilized within the automotive sector to produce lightweight structural components including camshaft bearing caps, heat sinks, planetary gear carriers, and transmission retainer plates. While the list of commercialized products continues to grow steadily, certain opportunities remain inaccessible as the physical and mechanical properties of APM products often fall below their wrought counterparts. This restraint is underpinned by two key microstructural traits - residual porosity and a thermodynamically stable 3D-interconnected network of oxide phases. These limit the load bearing capacity of a component while providing an intergranular failure path. Ferrous powder metallurgy has seen notable metallurgical benefits and ensuing commercial proliferation when carefully designed, sintered preforms are forged to full density. The aim of this study was to employ a comparable sinter-forge methodology to APM alloys and then ascertain its effectiveness through detailed characterization of the microstructure and physical/mechanical properties of the forged product. The alloy system used was a commercial 2XXX series APM alloy coupled with one of two aluminum nitride (AlN) ceramic powders to create a metal-matrix composite (MMC). Constitutive analysis of the deformation behaviour was completed using a Zener-Holloman analysis and 2D process mapping. For several compositions, a domain of stable flow characterized by dynamic recovery was observed from 400°C to 500°C and strain rates 0.05 s-1 to 5 s-1. Near-theoretical densities (99.5%) were obtained for a variety of MMC compositions, although up to 100.0% densities were also achieved in select instances. Shear strain elongated and flattened residual pores, markedly reduced the level of porosity at Al-AlN interfaces and disrupted the continuous network of fine MgO crystallites into a semi-continuous network without invoking fracture or decohesion of AlN particulate. Such transitions led to demonstrable gains in mechanical properties with the extent dependant on the type and concentration of AlN incorporated. In the case of tensile properties, UTS increased by 48 to 91 MPa, elastic modulus improved by up to 9GPa while tensile ductility saw up to a five-fold rise. Gains in fatigue performance were particularly astute with a 98 MPa (57%) increase in bending fatigue strength observed.