Effect of Melt Shearing Time by Vertical Spiral Stirrer on Hot Tearing ‎Behavior of A206 Alloy

In casting, one way to reduce hot tearing is by modifying the microstructure. In this study, the microstructure of A206 alloy was modified using a melt shearing process at various times, and the effect of melt shearing time on hot tearing behaviour was analyzed. A steel chamber with two baffles and a spiral stirrer was used to apply shear stress to the melt. The sheared melt was then cast in a graphite mold in the form of a constrained rod, and optical and SEM microscopes were utilized to investigate grain size. To evaluate the susceptibility to hot tearing, the quantity of crack length density was used as a criterion. The results revealed that melt shearing process reduces the size of shrinkage pores, making it harder to initiate hot tearing, and increases the alloy's resistance against hot tearing. Also, due to decreasing the size of shrinkage pores, the ultimate strength is increased up to 57%. As the melt shearing time increased up to 3 minutes, the as-cast microstructure became less uniform; however, after 3 minutes, the microstructure became significantly more uniform. Additionally, the grain size decreased up to 37%, and the rate of this decrease accelerated after 3 minutes, leading to an increase in resistance to hot tearing. The decrease in grain size due to melt shearing caused an increase in the number of grain boundaries and a reduction in the thickness of the grain boundary precipitation layer, resulting in a softer behaviour observed on the fracture surface.