The improved microstructures and properties of 7075 alloys produced by a water-cooling centrifugal casting method
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INTRODUCTION
GRAINREFINING has been regarded as an effective means to upgrade materials properties t~,2,3] as it gives rise to better mechanical properties of metals, such as strength, toughness, ductility, fatigue strength, and stress corrosion resistance. The grain-refining methods which are carried out in the melting and casting stage are generally more direct and efficient in obtaining a fine-grained product than those being applied in the subsequent processing on ingots or billets. This is due to a reduction in the homogenization time and savings in the thermomechanical or heat-treatment efforts for refining the grain structure. Additionally, refining of second-phase particles and microsegregation are concomitant with the grain refining,t3] This is especially true for those inclusions formed during solidification. The principles of grain-refining casting methodst4] are as follows: (1) an increase in cooling rates; (2) an addition of grain refiners; and (3) application of external forces. Increasing the cooling rate can accelerate the nucleation rates and thus lead to a small grain sizes as well as small dendrite arm spacings. Chill block, metal mold, or cooling bath may be employed on the basis of this principle. A grain-refining agent can induce a number of crystal nuclei near the solidliquid interface and thus prohibit the growth of columnar grain. Aluminum alloys are, for example, often inoculated by A1-Ti-B series refinerst5,6] to give smaller equiaxed grains. Turbulent flow and cavitation can be generated by the application of a mechanical or electromagnetic forces JIEN-WEI YEH, Professor, is with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 30043, Taiwan, Republic of China. SHANG-HAW JONG, formerly Graduate Student with the Department of Materials Science and Engineering, National Tsing Hua University, is Engineer, New Sun Metal Industry Co., Ltd., Kaoshung, 825, Taiwan, Republic of China. WEN-PIN LIU, Associate Professor, is with the Department of Mechanical Engineering, Chengshiu Junior College of Technology and Commerce, Kaoshung, 83305, Taiwan, Republic of China. Manuscript submitted November 8, 1993. METALLURGICALAND MATERIALSTRANSACTIONSA
in the melt. Turbulent flow can, as a result, enhance crystal multiplicationt7.s.9] by breaking and remelting the dendrite. Cavitation can also undercool the local liquid and develop nucleation by the high negative pressure which originated from a cavity collapseY ~ Typical methods include stirring, supersonic/mechanical vibration, rotating mold, rotary magnetic field, etc. Despite the fact that many techniques based on these principles have been developed and applied in the foundry field for a long time, they have, at best, only achieved grain sizes of approximately 100 /zm.t2.3J Hence, this grain size currently is a bottleneck for further improving properties since the progress of the wrought process and technique seems to have reached its limit of capability.tll] In other words, if the grain size of the ingots i
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