Papers by Author: Hong Yang Zhao

Abstract: Twin-Roll-Casting process is a rapid solidification process combining with hot rolling deformation. The liquid metal will be squeezed out but not sucked in from the mush zone caused by rolling deformation. This is different from conventional continuous casting process and will greatly affect the microstructure morphology and surface cracks of as-cast strip. In this paper, a 2D FEM model was employed to simulate the thermal mechanical behavior during twin-roll casting process of Mg alloy AZ31. The FEM results were compared with experimental results and show the effect of rolling deformation on the microstructure morphology and surface cracks of as-cast sheet.

Abstract: Twin- roll casting of magnesium alloys is a rapidly solidifying process, which can directly produce thin strips from molten metal. In this paper, based on a pilot vertical type twin-roll caster, 3D models of twin-roll casting process were used for simulation of thermal flow and solidification on depending of casting speed, molten pool level and pouring temperature. The effect of side dams on thermal flow behavior was also considered. Based on the analyzed results, a proper concave slot nozzle was designed to compensate temperature variation across the width of strip. Comparing with parallel type slot nozzle, it can adjust temperature distribution along the roll width continuously by adopting proper concave curves, which is more flexible than rectification boards delivery system. Strips of magnesium alloy AZ31 were produced in the pilot caster with the concave slot nozzle. The experiments show that surface quality of as cast strip was improved by using the new type of nozzle.

Abstract: Stress intensity factor K is analyzed by measuring distribution of sum of the principal stresses around slit tip by an infrared stress measuring device. Under the biaxial tensile stress, both K-values of the isotropic materials with slit angle 0° and 45° show there is no influence of the slit angle at Mode I. The tolerance of the measured K-values of the test specimens which are compared with measured K-values by photoelastic method are around 10-20
and these increase as applied cyclic load increases. These tolerance can be reduced up to about 8% by modifying measurements. The effectiveness of the thermoelastic stress measurement method to KI value analyzing under biaxial tensile stress was confirmed.

Abstract: Strip casting is a promising route to directly produce magnesium alloy sheet by twin roll casting method. As-cast strips with proper microstructure and good surface quality are requested to serve as next rolling feedstock. In order to restrain the coarse dendrite growth and get uniform grain, a new method of in-line heat treatment was proposed. The as-cast strip was dipped into an oil tank after exiting the rolls and then was quenched with oil at various temperatures. The effect of oil temperature, casting speed and pouring temperature on microstructure and mechanical properties of cast strip was investigated.