Ultrasound-assisted pressureless infiltration of molten aluminium into alumina capillaries

Abstract

The wettability between molten metal and reinforcement significantly affects the characteristics and the productivity of metal-matrix composites (MMCs) fabricated by pressure casting, which is one of the promising methods to produce MMCs. In wetting composite systems, full infiltration into fibre or particulate preforms can be achieved without an external applied pressure, and non-infiltrated defects are then inhibited from appearing at contact points of reinforcement. However, most composite systems strengthened with ceramic reinforcements are unfortunately known to be non-wetted. A variety of methods to improve the wettability have been studied vigorously: addition of appropriate alloying elements to molten metal matrix [1, 2], surface treatment on reinforcements [3, 4] and the optimum selection of processing parameters such as a molten metal temperature [5]. These are chemical approaches that reduce the contact angle by changing the balance of interfacial energies between the molten metal, reinforcement and atmosphere. In contrast, a water droplet on the nylon substrate vibrated with 200-800 Hz was found to reduce its contact angle compared with that on the stationary substrate [6]. In order to realize full infiltration with an extremely low applied pressure in non-wetted composite systems, it was aimed to improve the apparent wettability between molten metal and ceramic reinforcement by ultrasonic vibration as a physical approach. The ultrasonic vibration system consisted of a Langevin-type Pb(ZrxTil_x)O3 oscillator and a titanium step-horn. The resonant frequency (f) of 20.5 kHz was measured by an impedance analyser with the step-horn end immersed in molten aluminium. Ultrasonic vibration was transmitted into molten aluminium through a direct contact of the step-horn and melt. TiB2 was coated on the tip of the step-horn by a spark coating method, so as to avoid the dissolution of titanium into molten aluminium. The longitudinal amplitude (bu), which was measured at the end-surface of step-horn by an optical displacement sensor, was affected by the ultrasonic power (Iu). The f and I~ reported below are the values measured in an immersed state, unless otherwise stated. oi-A120 3 capillaries with inside diameter 0.80 mm and length 15 mm were supplied for the infiltration