Fabrication Of Shape Memory Alloy Pipe Joints By The Upward Heated Mold Continuous Casting Process

In order to overcome the defects of horizontal heated mold continuous casting process, Upward heated mold continuous casting (Uhmcc) equipment was fabricated in this study. The melt in the graphite crucible was led by a horizontal runner to a vertical upward equipped heated mold, from which the melt was drawn upward by a dummy tube and solidified directionally into a tube. A water cooler was equipped above the mold to cool the being cast tube. In order to prevent water inside the cooler from flowing down along the tube to enter the mold, a vacuum chamber was mounted on the bottom of the cooler. When the vacuum chamber was connected to the vacuum pump, air outside the chamber will flow into the chamber along the circular gap around the tube, bringing the water flowing down along the tube to the pump. Three centripetal cramps were mounted along the tube to prevent the tube from swaging, which otherwise will cause the tube to bend.CuAlBe alloy is prone to be oxidized at melting state. The thick oxidation film covered on the melt surface will block the contact of the melt level detector and the melt. In addition, Be-bearing melt intends to strongly adhere on the detector. When the melt level descends, the melt still adheres on the detector until the melt level further descends to some extent. In both these cases, the detector cannot indicate the melt level correctly. In this study, a thin piece of graphite was put flowing on the melt surface and located just beneath the detector. The detector contacts the melt via graphite piece, so that the melt level can be indicted promptly.CuAlBe tubes dia.10mm were cast with Uhmcc process. Casting parameters including melt head, mold temperature, cooling distance and casting speed were tested. The results show that with melt head of 2mm, mold temperature of 1050℃, cooling distance of 5mm, casting speed of 40mm/min, perfect tubes with straight and sound surface can be obtained. Lower melt head cannot provide enough fluidity, resulting in partial empty of the tubes, whereas higher melt head will lead to breakout of the melt. Shorter cooling distance and lower casting speed will cause surface cracking, whereas longer cooling distance and higher casting speed will lead to breakout.CuAlBe pipe joints were fabricated on the base of CuAlBe tubes. Performances of the pipe joints were examined. The CuAlBe tubes were cut into 15mm long pieces and then heated to 850℃ held for 30min and subsequent water chilling to eliminate 72 phase. The inner holes were reamed to about 7.5mm and expanded to 8.04mm,8.26mm and 8.56mm respectively at temperatures of-30℃(<Ms),20℃(Ms～As) and 80℃(>Af). The results show that the maximum expanding rate reaches 11.3%at temperature lower than Ms, with recovery rate of 92.75%. With increase of temperature to MS～AS, both expansion rate and recovery rate decrease to 9.98%and 91.8%respectively. As temperature increases higher than As, both expansion rate and recovery rate decrease to 7.97%and 87.27%respectively. With the increase of expansion rate, both As and Af temperature increase greatly, while Ms varies not much, and Mf decreases slightly. Maximum transformation hysteresis of 74℃ occurs at maximum expansion rate of 11.3%.Connections of tubes were conducted. A thin layer of epoxy was applied on the surface of the inner hole of the pipe joint. Two copper tubes of OD 8mm were inserted into two ends of the joint. Slightly turn the joint around to ensure the epoxy smear uniformly on the copper tubes. Heat the pipe joint to around 200"C with a blower for 1 minute. Sealing performances include pressurized seal in which 3.0MPa nitrogen gas was applied for 5min and hydrostatic pressure test in which 18.6MPa water was applied. Both tests were successfully passed.