جستجوی مقالات مرتبط با کلیدواژه « hot tearing » در نشریات گروه « مواد و متالورژی »

In this study the effect of grain refinement was studied on the hot tearing behavior of 3 wt. % Ce-modified 2024 Al alloy. Towards this end, different amounts of Ti (0.01, 0.02, and 0.05 wt. %) were added to the molten alloy through Al-5Ti-1B master alloy. According to the results, adding Ti marginally improved the fluidity length, relatively reduced the size shrinkage micropores, dispersed the micropores, reduced the grains size, and decreased the alloy hot tearing susceptibility (HTS). The addition of 0.01, 0.02, and 0.05 wt. % Ti decreased the alloy porosity content, grain size, and the HTS by 5, 20, 31%, 20, 75, 80%, and 9, 24, 34%, respectively. Based on the microstructural characterization and hot tear subsurfaces examination results, the relative improvement of hot tearing behavior can be explained by the refinement of grains and its positive consequences such as decreasing the liquid film thickness between the grains (which increases the capillary force and improves inter-grain feeding), increasing the alloy strength, decreasing the linear contraction, and decreasing the stress applied on grain boundaries as easy growth locations for the hot tear cracks.

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.

This study was conducted to investigate the effect of Si addition (1, 2, 3, and 5 wt. %) on the microstructure and castability of hypoeutectic Al-4.5Cu-Si alloys. According to the results, Si addition increased the average size and volume fraction of eutectic Si platelets in the microstructure. Adding Si also improved the fluidity and decreased the porosity content of the alloy, and enhanced its resistance against hot tearing. According to the constrained rod test results, the hot tearing sensitivity index (HTS) of the alloys containing 1, 3, and 5 wt. % Si, was decreased by 48, 78, and 88%, respectively. In agreement with hot tearing testing results, the extensive existence of dendrites and shrinkage micropores on the hot torn surface of the alloy without Si implies on its low potential in feeding the solidification shrinkages and healing hot cracks. Adding Si decreased the amount of shrinkage micropores on the hot torn surface and due to improving the fluidity as well as increasing the amount of Al-Cu-Si ternary eutectic at the last stage of solidification, improved the feeding characteristics of alloy. Therefore, the amount of free dendrite arms was significantly decreased on the fracture surface. However, adding 5 wt. % Si increased the amount of eutectic Si on the hot torn surface giving rises to a more brittle fracture mode.

The effect of Mn addition (2 and 4 wt%) on the microstructure and castability of hypoeutectic Al-2Ni alloy was investigated. According to the results, Mn promotes the formation of Mn(Ni)-rich intermetallics in the microstructure. In the case of Al-2Ni-2Mn alloy the intermetallic compounds are interdendritic type whilst in Al-2Ni-4Mn alloy in addition to interdendritic intermetallics, the large and primary Mn-rich compounds with platelets, polyhedral and dendritic morphologies are present. The fluidity results show that the addition of 2 and 4 wt% Mn enhances the mushy solidification of Al-2Ni alloy leading to a fluidity reduction of 7 and 30%, respectively. Based on the microstructural observations and thermal analysis results, this reduction can be attributed to the formation of primary Mn-rich compounds in the molten alloy. Manganese addition, also, exerts negative impact on the hot tearing resistance of Al-2Ni alloy. The hot tearing susceptibility index (HTS) of Al-2Ni-4Mn alloy is 5 and 12 times higher when compared to those of Al-2Ni-2Mn and Al-2Ni alloys, respectively. SEM investigation of hot teared microcracks and the presence of free dendrites and primary Mn-rich compounds on the fractured surfaces imply on the critical role of primary compounds on the formation of hot tear microcracks.

The effect of Fe addition (0.5, 1, 1.5 and 2 wt %) and Mn modification (Mn/Fe=0.5) on hot tearing behavior of F332 Al alloys was investigated. The results show that due to the formation of fine interdendritic -Al5FeSi platelets, Fe addition up to 0.5 wt% improves high temperature tensile properties and promotes the formation of equiaxed grains thereby increases the hot tearing resistance of the alloy by about 25%. Further addition of Fe up to 2 wt%, however, increases the size and volume fraction of -platelets, decreases the tensile properties, reduces the fluidity and interdendritic feeding and consequently substantially increases the hot tearing susceptibility (HTS). Mn addition, however, was shown that changes the morphology of -platelets to less harmful Chinese script, polyhedral or star-like α-Al15(Fe,Mn)3Si2 whereby improves the tensile properties and interdendritic feeding characteristic of the alloy. Therefore, the HTS value is decreased to zero for 0.5 and 1 wt% Fe alloys, but increased by 50 and 40 %, respectively in the case of 1.5 and 2 wt% Fe containing alloys.

The effect of iron (0.2 to 2 wt%) on the hot tearing susceptibility of Al-5Si and Al-9Si alloys was investigated. According to the results, addition of iron up to the 2 wt% improved hot tearing resistance of Al-5Si alloy and decreased its HTS by about 100%. This improvement was assumed to be mainly due to the fine and interdendritic distribution of Fe-rich intermetallic compounds. In the case of Al-9Si alloy however, despite of the lower solidification range and higher fluidity, addition of iron up to 2 wt% deteriorate hot tearing resistance. The HTS index was increased continuously by three times. Microscopic examination of the hot tear cracks reveals the precipitation of higher amounts of -Al5FeSi in interdendritic regions which adversely affect the tensile strength and elongation as well as blockage the interdendritic feeding channels thereby decrease the hot tearing resistance.