Rapid Solidification Of Undercooled Low-melting Babbit Journal Bearing Alloys

The containerless rapid solidification of undercooled metallic melts is of importance in research of materials science in space. The present dissertation carried out systematical investigations on the containerless rapid solidification of undercooled babbit alloys of Pb-Sb, Sn-Sb and Pb-Sn-Sb systems by means of fluxing technique by using silicone oil as denucleating agent and drop tube processing. The heat transfer model was introduced to analyze the rapid solidification behaviors of Sn-Sb alloys. The influence of rapid solidification conditions on the structure formation and frictional behaviors of Pb-Sn-Sb alloys was investigated. By means of fluxing technique, Pb-11.2wt%Sb eutectic alloys, Pb-15wt%Sb and Pb-18wt%Sb hypereutectic alloys were undercooled to 33.1K(0.13TL), 73K(0.25TL) and 67.9K (0.23 TL) respectively. The asymmetric extended coupled zone of Pb-Sb eutectic alloy system was achieved by theoretical calculation. It is indicated that the deviation of undercooled Pb-11.2wt%Sb eutectic alloys from coupled zone results in the formation of incomplete eutectic structure. With the increase of undercooling, the growth morphology of primary phase (Sb) experiences a transition from blocky shape to dendritic shape and the structure macrosegregation reduces in Pb-15wt%Sb and Pb-18wt%Sb hypereutectic alloys; under high undercooling condition, both alloys enter extended coupled zone and complete eutectic structure occurs. Under the conditions of containerless processing, the microstructures of Pb-11.2wt%Sb eutectic alloys transform from lamellar eutectic to anomalous eutectic; Pb-15wt%Sb and Pb-18wt%Sb hypereutectic alloys exhibit notably refined grain; Spherical anomalous eutectic grains form in Pb-15wt%Sb hyperereutectic alloy.The rapid solidification behaviors of undercooled Sn-16wt%Sb hyperperitectic alloys were studied. Rapidly-solidified Sn-16wt%Sb hyperperitectic alloys exhibit notably refined grain and highly dispersed primary intermetallic SnSb phase, indicating that the device has achieved grain refinement effects as well as those in high vacuum drop tube at home and abroad. The calculation results indicate that the cooling rate of a 0.1 mm size droplet is 105K/s. Every stage during the cooling procedure of the droplets is closely related with the diameter and nucleation undercooling of the droplets.The microstructure evolution of undercooled Pb-Sn-Sb ternary eutectic alloys wasinvestigated. Under the conditions of containerless processing, Pb-4wt%Sn-12wt%Sb ternary eutectic alloys exhibit a transition from lamellasome structure to granular structure; the macrosegregation of Pb-15wt%Sn-15wt%Sb ternary alloys was inhibited, the growth morphology of primary phase SnSb experiences a transition from blocky shape to dendritic shape and spherical anomalous eutectic grains occur in the alloys. The friction and wear experiments results show that sticking wear takes place during the friction procedure, which results in the poor wear-resisting property of ternary eutectic alloys, due to the low quantity of hard particle of primary phase SnSb; the wear-resisting property of rapidly-solidified Pb-15wt%Sn-15wt%Sb ternary hypereutectic alloys is better than that of as cast ternary hypereutectic alloys. It is because that the growth morphology of primary phase SnSb experiences a transition from blocky shape to uniformly distributed granular shape or dendritic shape, which relieves the stress concentration of matrix caused by the tip of hard particle.