Coupling Effects of Melt Treatment and Ultrasonic Treatment on Solidifying Microstructure and Mechanical Performance of
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SINCE the 1970s, due to high refinement efficiency, environmental protection, and extensive adaptation, ultrasonic treatment has attracted increasing attention in the biomedical, chemical and chemical engineering, and metallurgical industries.[1,2] So far, the ultrasonic treatment has been widely applied to light metal and alloys, which implies great potential prospects in microstructure controlling and performance optimization, including grain refinement, degassing, dendrite modification, and strength reinforcement.[3–5] However, there are tremendous controversies and conflicts regarding the mechanism and application of ultrasonic treatment due to the lack of visualized observations and experimental verification. So far, two common recognized mechanisms have been proposed to explain the
ZHENG DESHUANG, CHEN RUIRUN, MA TENGFEI, DING HONGSHENG, SU YANQING, GUO JINGJIE, and FU HENGZHI are with the School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P.R. China. Contact e-mail: [email protected] Manuscript submitted January 10, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
ultrasonic effects, which can be divided into two groups: cavitation-enhanced nucleation and cavitation-induced fragmentation.[6,7] However, the specific theoretical foundations and applicable conditions are still chaotic and ambiguous.[3,8] First, what is the precise mechanism of cavitation-enhanced nucleation: cavitation supercooling or inclusions activation? Second, what is the exact approach for cavitation-induced fragmentation: shock waves arising from the cavitation collapse or flow shear stress resulting from the acoustic streaming? Third, what is the role of solutes in the grain refinement and are solutes required for the ultrasonic treatment? Last, what is the influence of solidification conditions on the ultrasonic effects? Puga et al.[9] studied the effects of melt temperature on ultrasonic treatment effects in Al-Si-Cu alloy; the ultrasonic treatment is able to refine coarse grains above the melt liquidus temperature, and the too-high melt temperature will suppress ultrasonic treatment effects. However, Liu et al.[10] and Chen et al.[11] revealed that ultrasonic treatment is effective just within the melt crystallization temperature and the ultrasonic treatment effects are mainly dependent on ultrasonic cavitation. Besides, melt treatment is also of great importance in the TiAl-based alloys solidifying microstructure. Yang
Table I. Solidification Conditions Heating power (kW) Cooling power (kW) Ultrasonic treatment time (s) Specimen length (mm)
Experimental Parameters and Solidification Conditions Superheat Degree
8[S1] 0 0/30 30
7[S2] 0 0/30 30
6[S3] 0 0/30 30
et al.[12] researched the influences of high-temperature solution treatment on b-solidifying TiAl alloy, and the results show that the high-temperature solution treatment could mostly eliminate element segregation and alleviate casting stress, obtaining ultrafine homogenous grain structure. Previous research by the authors has reported the application
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