Semi-solid Processing of Engineering Alloys by a Twin-screw Rheomolding Process

摘要

A twin-screw rheomolding process has been developed for near net-shape production of engineering components, in which liquid alloy is melted and fed into a twin-screw extruder, where the liquid alloy is intensively sheared and cooled to a temperature in the interval of liquids and solidus, and the sheared semi-solid slurry is then transferred to a shot sleeve and subsequently injected into a mold cavity for component shaping. The fluid flow in the twin-screw rheomolding process is characterized by high shear rate, high intensity of turbulence and cyclic variation of shear rate. The experimental results of rheomolded alloys have demonstrated that the developed rheomolding process is capable of producing fine and spherical primary particles uniformly distributed in the matrix. Solidification under high shear rate and high intensity of turbulence can result in substantial grain refinement due to the increased effective nucleation rate caused by the increased temperature and composition uniformity. It is also found that fine and spherical particles formed during the secondary solidification are promoted by high shear rate, high degree of turbulence, longer shearing time and higher solid fraction during the the primary solidification under forced convection.