modeling of hot tearing in aluminium alloy /

Abstract

Hot tearing is one of the "universal" defects in many aluminium casting products. The formation of hot tearing involves complex solidification phenomena and processing conditions. During the DC casting process, it often initiates at the surface and propagates towards centre. In the present work, a coupled solidification model (microstructure) and hot tearing model has been developed in order to predict the hot tearing susceptibility of aluminium alloys and thus to better control its occurrence.The microstructure modeling in this project was progressed from two-dimensional simulation of Al-4.5%Cu binary alloy solidification, and the modeling results were compared with the experimental data. The information provided by the microstructural model, such as temperature field, evolution of solid fraction, the final grain size and the microstructure morphology, is essential for the further study of hot tearing.In order to predict the hot tearing susceptibility during solidification, two hot tearing criteria with different aspects have been implemented into the microstructural model. The Lahaie Bouchard (LB) criterion is based on the mechanical response of the semi-solid body, while the Rappaz Drezet Grenaud (RDG) criterion considers the pressure drop from both thermal deformation and solidification shrinkage through the mushy zone.In the microstructure -hot tearing model using LB criterion, by combining the constitutive law that reflected the viscous behaviour and the critical fracture based on the capillary force, a critical strain can be calculated to identify when and where hot tearing 11 could occur. The critical strain is influenced by cooling condition, strain rate, solid fraction and alloy composition. The modeling results give an overall indication of the appearance of hot tearing, and can be used to investigate the susceptibility of hot tearing tendency influencing by the solidification parameters and the thermal properties of the Al-Cu alloys.To valid the coupled microstructure-hot tearing model, some experiments have been carried out on the Direct Chill Ingot Surface Simulator (DCSS). The displacements under applied tensile loads were measured and the occurrence of the hot tearing was observed.The results from modeling and experiments were compared and discussed. The complete microstructure -hot tearing model (with the LB criterion), corresponds with the facture stress range found in the experimental data. By further correction based on a recently modified constitutive model, the critical strain range between the modeling and experiment can give better agreement to identify the appropriate condition for the susceptibility of hot tearing.The modeling results from the RDG criterion showed that it is more suitable for the central type of hot tearing. It needs to be studied further to know whether this criterion is applicable for the surface hot tearing as in the case of DC cast sheet ingots. Ill ACKNOWLEDGEMENTSIt is with great pleasure that I acknowledge all those who were involved directly or indirectl...