چكيده به لاتين
The casting industry requires the development of new and cost effective processes that can help to cast a wider range of industrial alloys, including wrought grades of aluminum alloys. The main obstacle against the shape-casting of these alloys is their high sensitivity to hot teaing or solidification cracking. In this research, we tried to investigate the reason of the sensitivity of the aluminum alloy AA7068 to the defect by examining the mechanism of hot tearing formation (initiation and propagation), and then, by comparing the solidification behavior of the alloy during controlled diffusion solidification (CDS) and conventional solidification to declare how the CDS process interferes with mechanism. For this purpose, by modifying the Navikov mold, a Constrained Rod Casting (CRC) metal mold was constructed and one of its branches was equipped with a force and temperature sensor. Casting in this mold, in constant preheating (400 oC), and in three processing modes, with 50oC (Conv.), with 5oC (NLC) superheating, and CDS mixture, the effect of solidification processing on hot tearing susceptibility was estimated both qualitatively and quantitatively. For hot tearing, a slight stress equivalent to 0.52 MPa is required, which shows the high tearing severity of the alloy. The tear surface was studied by scanning electron microscopy (FESEM) and there observed evidences supporting that eutectic solidification has been progressing at the tearing instance. In parts of the tear surface, the freezing of the eutectic was completed and the weak intermediate phase between the σ phase in the eutectic and the primary phase was identified as one of the factors affecting the sensitivity of the alloy to the hot tearing. With the determination of the role of eutectic solidification, a thorough analysis of the history of solidification of the alloy was recorded by recording the cooling curve and thermal analysis. A phenomenal feature was observed on the first derivative curve just before the peak of the final eutectic. This feature was associated with a stop in the growth of the primary phase due to local equilibrium at the S/L interface leading to a transition from the non-faceted to the faceted interface. The length of this feature in the CDS mode is 60% shorter than that of the conventional mode, indicating a delay in the growth stoppage (non-faceted-faceted transition) with the CDS process. The chemical composition of the intermetallic and final eutectic phases was analyzed by EDS analysis and its granulation structure by EBSD analysis. After the stoppage at around TPE= 510 °C, the remaining liquid and supersaturated from aluminum becomes supercooled. In this case, the σ phase deposition begins through surface absorption and then continues to concentrate on the emergent screw dislocation existing on the primary phase surface. Upon reaching the critical radius, the σ phase nucleates at TE=481 °C and, in accordance with the lemaignan mechanism, extends to surface dendrites on the primary phase. The temperature difference between the TPE temperature and TE is the brittleness range of the alloy. In this range, the surface absorption of the σ phase is performed and, therefore, the probability of forming solid eutectic bridges between adjacent grains is very high. With the formation of solid eutectic bridges, the melt is captured, and the activation of the self- or solid-feeding mechanism through the deformation of the surrounding solid, breaks solid eutectic bridges thereby initiating the tearing. With the failure of the bridges, the tensile strength of the material is merely due to the presence of liquid bridges. The application of macroscopic strains on such a material, by exceeding a critical strain, leads to the breakdown of molten bridges and tear propagation. The CDS processing method delays the stoppage and through this, postpones the formation of solid eutectic bridges and activation of self-feeding mechanism and thus preventing the occurrence of a hot tearing by interfering with the initiation stage.
