Investigating the Effect of Cooling Rate and Nucleation on the Microstructure and Tensile Strength of Alloy 6061
摘要
In this research, two casting methods, CO2 sand casting and permanent mold casting, were employed to investigate the effect of cooling rate on the grain refinement of 6061 aluminum alloy in the presence of an Al-5Ti-1B grain refiner. A reduction in grain size was observed in both casting methods. By adding a grain refiner, the grain size decreased from 153 to 94 μm, while the intensity of grain size reduction by changing the mold material was greater than adding a nucleating agent. Through the use of a grain refiner, the grain size within the permanent mold was lowered from 65 to 42 μm. EDS analysis confirmed the presence of Titanium (Ti) and Boron (B) elements at the grain boundaries, which led to an increase in nucleation sites in both casting conditions through the formation of intermetallic compounds. With the increase in the number of buds and the presence of many grain boundaries, which act as a factor against dislocations and surface plastic changes, an increase in hardness was observed in samples with grain refiner, and the hardness increased from 31 to 45 HB in sand samples, and in metal molds, the hardness reached from HB 56 to HB 67 with the addition of the grain refiner. The tensile strengths obtained for the sand-cast, sand-cast with refiner, permanent mold, and permanent mold with refiner samples were 101 MPa, 123 MPa, 144 MPa, and 175 MPa, respectively, indicating the highest strength for the inoculated permanent mold sample. The presence of microvoids in the fracture images indicated ductile fracture of the samples with a grain refiner. Examination of the fracture surfaces of the sand-cast samples revealed the influence of casting defects, such as gas and shrinkage porosity, contributing to premature fracture and reduced mechanical properties.