Study On The Downward Ohno Continuous Casting Process Of Memory Alloy Pipe Joint

Flow resisters were mounted just before the entrance of the mold to reduce the static liquid metal pressure to prevent the breakout of liquid metal during downward heated mold continuous casting process.Five types of resister were tried,including ceramic ball type,ceramic foams,vertical partition type,narrow slits and small holes.The flow resistance test results showed that the small hole type baffle mainly used surface tension to achieve flow resistance.When the diameter of the small hole was 0.5mm,the residual head could reach 8mm,which could significantly reduce the pressure head of the liquid metal upstream of the mold,thus preventing breakout during downward heated mold continuous casting.The flow with a ceramic ball baffle was not easy to control,while in vertical partition baffle copper wire leadings were needed,which was inconvenient for operation.With ceramic foam baffle,because of the larger aperture,the residual head was too low,seldom blocking effect appeared.The slit-type resister mainly used frictional resistance to achieve choking,even if the bridging length re aches 50 mm and the slit width was 0.3mm,the residual head was only 2~3mm,and the choke effect was not significant.An innovated downward continuous casting equipment was designed in which a small hole type baffle with aperture of 0.5 mm was mounted upstream of the mold.The equipment consisted of remelting system,mold system,drawing system and cooling system.The process parameters were set as follows: melting temperature of 1120 ?,horizontal tube temperature of 1130?,mold temperature of 1060?,seeping head of 55 mm,residual head pressure of 8 mm.The cooling distance was set to 10 mm,the continuous casting speed was set to 80mm/min,the cooling water flow rate was set to 1L/min,the nitrogen flow rate to the graphite crucible was set to 3L/min,and the nitrogen flow rate to the exit of the casting mold was set to 0.6L/min.With these above process parameters,CuAlBe shape memory alloy tubes with smooth surface was continuously cast.The alloy tubes were then cut into sections with a length of 10 mm.After 850 ? 30 min solution treatment and mechanical processing,these sections were expanded at room temperature and-50? respectively,with expansion rates of from 4% to 12%,respectively.The recovery rates were calculated according to the dimensions measured after recovery on heating.The results showed that when expanding at room temperature,cracks occurred in the pipe joint specimens when the expansion rate reached 6%.No cracking was observed in specimens expanded below-50°C.These results were consistent with the appearance of the peaks on the expansion curves both at room temperature and low temperature.With an expansion rate of 6% at low temperature,the recovery rate reached 100%.As the expansion rate increased,the recovery rate decreased to 87% at an expansion rate of 12%.The optical microscopic observation showed that some cracks originated from the intersection point of three grains,while some from the area of inclusion or shrinkage,indicating that the initiation of microcracks was related to grain boundary defects and casting defects.Scanning electronic microscopy observation showed t hat the origin of the microcracks in CuAlBe memory alloy joint cracks was generally at grain boundaries and casting defects,such as inclusions,shrinkage holes,etc.When the crack extended,the cross-sectional area of the sample was reduced,so that the stress increased,causing the crack to rapidly pass through the grain and form a transgranular fracture.To sum up,the use of an innovated type of downward heated mold continuous casting equipment and a baffle device could significantly reduce the metal head,prevent liquid metal breakout,and lead to a higher casting speed and smooth surface of the CuAlBe tubes.Expansion of pipe joint at low temperature could prevent the occurrence of cracking due to the lower strength of martensite.