Study On The Solidification Front Control And Defect Formation Mechanism Of Magnesium Alloys Strip Processed By Twin-Roll Casting

Twin-roll casting (TRC) is a near-net-shape process which combines rapidsolidification with hot rolling by one-step method to produce thin strips directly frommolten metal. It is believed that strip casting-annealing is a promising route toproduce magnesium alloy strip with low cost and high productivity. Position andconfiguration of solidification front in set-back play an important role on thesolidification and deformation behaviors during twin-roll casting processing ofmagnesium alloys strip. Furthermore, this effects influence the defect-formation ofstrips. On the basis of brief review of the history and the evaluation of previousresearch works, the controlling of solidification front, melt distribution anddefect-formation mechanism of magnesium alloys strip processed by twin-rollcasting was investigated experimentally and theoretically.In the first part, the effect of parameters on the position of the solidificationfront and surface defects of strip was analyzed regardless of the difference of moltenmetal temperature distribution in set-back. Experimental results show the roll speedhas a prominent effect on the solidification front position and surface-defectsforming. Optimum zone of kiss-point of solidification layer was achieved from thetheoretically derivation and experimental results. A kiss-point model which considersthe strip thickness, set-back length and roll speed was established to optimizeprocess and enhance surface quality of magnesium alloys strip. Results showed thatthe model of kiss-point of solidification layer effectively optimized the twin-rollcasting processing and enhanced the surface quality of magnesium alloys strip.In the second part, the effect of configuration of feed tip nozzle on cavitytemperature field and location/shape of the solidification front was investigated by themethod which combined numerical simulation with experiments; furthermore, hot lineforming mechanism and correlated controlling methods was exploited.Temperature field in magnesium twin roll casting under regular configuration was simulated utilizing the SOLA-VOF finite difference method, simulation resultsshowed magnesium melts in the set-back presented extraordinary non-uniformtemperature distribution in the width direction. Experiments results indicated thatunder regular configuration, with temperature difference up to25℃，hot line defectsof various degree was observed on the strip. Therefore, the regular configuration offeed tip nozzle was unable to achieve the qualified strip.During twin-roll casting processing of magnesium alloys, if local temperatureswere higher than the non-equilibrium solidification solid line temperature after thedeparture of the strip from the rollers, and hot line defects were thus generated. Meltdistribution had a prominent influence on the temperature distribution uniformity, thusdetermined the location/shape of the fully solidified line, which resulted in the oddsand severance degree of hot line defects in twin-roll casting process of magnesiumalloy.The optimized configuration of feed tip nozzle was achieved by numericalsimulation which was validated by the experiments. Experimental results indicatedthat under the optimized configuration, the melts temperature difference in the tipfront zones can be decreased to less than10℃. The simulation results were validatedwith experimental results, and the hotline-free magnesium strips have achieved bytwin-roll casting though the optimized configuration of feed tip nozzle.The third part of this thesis was focused on the forming mechanism of surfacebleeds and centerline segregations were studied.Results showed that surface bleeds were compromised by solute richintermetalics. The experimental investigations were mainly conducted on so-calledstop-samples, which are produced by abruptly interrupting casting. By analyzing theheat, mass and momentum transfer and the deformation characters in the set-back ofmagnesium alloys strips, the character, forming mechanism and measurement ofsurface bleeds were discussed. In practical processing, surface segregation can bealleviated by shortening the length of solidification zone, controlling the melts height and lower the solute percentage to the lower limits.Centerline segregation of magnesium alloys strip was compromised by soluterich intermetalics. Solidification and deformation behavior of the twin-roll castinghave a major influence over the centerline segregation. Deformation in the twin-rollcasting squeezed the solute rich melts in the central of strips, and melts moved againstthe roll direction, which changes the shape of the solidification interface. Bycontrolling the solidification front in a proper location, the overheated melts suppliedby feed tip could washouts the solidification front, which can cause the re-melt of thesolidification front and the forced convection. It was beneficial to the solute diffusion,and the center line segregation could be decreased or eliminated. When otherconditions determined, by controlling the cast velocity according to the modelsinduction, the solidification front could be controlled to the proper domain, andcenterline segregation could be decreased or eliminated.Through the guidance of the research results, twin roll casting of magnesiumalloys could be stabilized and optimized, and the defects-free magnesium alloys stripshad been achieved by twin-roll casting.