Papers by Author: Xi Tao Wang

Abstract: A novel thermomechanical process to manufacture hot-rolled transformation-induced plasticity (TRIP) steels was developed based on dynamic transformation of undercooled austenite (DTUA). Between DTUA and the isothermal bainitic treatment, only one-step controlled-cooling was required. The microstructure evolution of hot-rolled C-Mn-Si and C-Mn-Al-Si TRIP steels based on DTUA was investigated by hot uniaxial compression tests using a Gleeble1500 simulation test machine. The results indicated that during DTUA, the kinetics of ferrite formation was fast, the volume fraction of ferrite formed was determined by applied strain. In comparison with the process based on static transformation of austenite, a more uniform multiphase microstructure with fine ferrite grains was formed, the bainite packets were small and had relatively random orientations, the retained austenite distributed uniformly and had relatively high volume fraction. Hot-rolled TRIP steels based on DTUA demonstrated better mechanical properties, especially for C-Mn-Al-Si TRIP steel.

Abstract: The rheo-diecasting (RDC) process, a novel semi-solid processing technology, was used to produce cast components with high integrity, fine and uniform microstructure, and therefore enhanced performance. AZ91D samples with 1-3 mass%Gd were solidified by RDC process. It was found that under intensive forced convection, the primary α-Mg phase produced inside the twin-screw slurry maker had fine particle size, spherical morphology and uniform distribution throughout the samples. Microstructure observations showed that Gd addition resulted in the formation of Al2Gd which dispersed in the α-Mg matrix. The size and amount of β-Mg17Al12 phase was reduced and its continuity was broken, which was the main reason for improving mechanical properties of the AZ91D alloy at high temperatures. The amount of Al2Gd particles increased with increasing Gd addition. From EPMA quantitative analysis, almost all Gd reacted with Al, leading to the low concentration of Gd in the α-Mg matrix. The Vickers hardness increased monotonously from HV=50.4 to HV=67.3 with increasing amount of Gd. This improvement was attributed to the consummation of aluminum in melt by precipitation of the Al2Gd phases.