Metal Casting Technologies : MCT-3RDQRT-2017
34 www.metals.rala.com.au METAL Casting Technologies 3rd Quarter 2017 35 and applying pressure after pouring were not effective, since before developing full pressure inside the vessel, the casting solidified partially. The problem was overcome by arranging a liquid metal basin at the top of the vessel having attachment of stopper rod to control pouring. After developing full pressure, stopper rod was lifted to fill liquid metal in the LFC. In an earlier work, it was reported that the porosity, elongation and high cycle fatigue strength of sand casting are 1.0%, 3.0% and 55.2 MPa, 0.1%, 5% and 82.7 MPa for permanent mould castings and 0.01%, 8% and 110.3 MPa for LFC under pressure . In another work on LFC under pressure, wedge shaped castings in A356, A319 and A206 aluminium alloy castings were produced under 1 atm. and 10 atm. pressure. Open cavity castings were also produced for comparison. The castings were heat treated and their porosity and tensile properties were evaluated. Pore size and frequency decreased substantially for all these three alloys with the application of pressure. Elongation of A356, A319 and A206 castings increased by 50-130%, 13-45% and 110-750% respectively in LFC with the application of pressure. Comparing the LFC and open cavity castings in A206 alloy, the elongation are 110-750% and 3-560% respectively under pressure. Higher elongation % in LFC compared to open cavity casting proves that the pressure is more accessible to the surface all over the castings in LFC through loose sand, whereas in open cavity casting, the pressure is active only over the liquid metal surface in riser. Plasma Treatment Casting The Plasma Treatment Casting (PTC) process has been developed applicable to LFC with the objective of reducing porosity and refining the microstructure of a casting . The PTC process uses a DC power source, servo motor and PTC electrode to generate a moving electric plasma arc on top of the liquid metal in riser of a LFC casting. The moving electric plasma in riser induces changing magnetic field and hence intensive stirring occurs in molten metal in the casting. The effect of this stirring is finer microstructure with less DAS and reduction in porosity size and porosity ratio. Experiments have been conducted to produce cylinder head castings in aluminium by LFC under plasma treatment. The PTC (+) electrodes was connected at the top risers and (-) connection was provided at the bottom or desired placed in the casting. Aluminium alloy was poured in the LFC moulds and optimized current and voltage were applied for predetermined timing repeated four times during the entire solidification time. Optical microscopy showed lower DAS by 40%, however, still finer microstructure was observed near (-) connection. Porosity size reduced from 0.4 – 0.9mm for conventional LFC to 0.1 – 0.3mm for PTC castings. Similarly porosity ratio decreased from 0.8 -1.2% in conventional LFC castings to 0.2 – 0.3% for PTC castings. Conclusion LFC is an economical process for production of complex castings in Al and iron, where a number of cores are required. However higher porosity and fold/lustrous carbon defects are formed due to formation of gases by decomposition of the pattern material during pouring. In Replicast process, these defects are eliminated by removing the pattern material by firing before pouring. However the time required to produce the ceramic shell around EPS pattern is longer and additional cost of firing the shell is involved. Other techniques such as vacuum assisted filling, solidification under pressure and plasma treatment casting reduces porosity and improves mechanical properties of castings. Hence cost, productivity, safety and convenience of each technique needs to be evaluated in terms of improvement in mechanical properties for its commercial application in LFC. n TECHNICAL FEATURE TECHNICAL FEATURE REFERENCES 1) H. F. Shroyer, US Patent No. 2,830,343 (Apr.15, 1958) 2) H. S. Lee, AFS Casting Metals Research Journal, Vol.9 (1973) p.112. 3) H. Littleton, B. Miller, D. Sheldon and C. Bates, Foundry Management and Technology, Vol.124 (1996) pp.37-40. 4) M. J. Lessiter, Modern Casting, Nov. (2000), pp.54-55. 5) S.I. Bakhtiyarov and R. A. Overfelt, ASME, Rheology and Fluid Mechanics of Non-linear Materials, Vol. 252 (2000) pp. 71-77. 6) M. C. Ashton, S. G. Sharman and A. J. Brookes, Materials and Design Vol.5, Issue -2 (1984), pp.66-75. 7) J. P. Brown, Foundry Trade Journal, Vol. 156, (1984) pp. 71-74. 8) L. Bichler, A. Elsayed, K. Lee and C. Ravindran, AFS Transactions, (2007) pp.733-745. 9) H. E. Littleton and A. P. Druschitz, AFS Transactions (2010) pp.493-501. 10) Q. Li, W. Su, W. Dai and Q. Han, AFS Transactions (2013) pp. 557-563. 11) P. C. Maity, Foundry Management and Technology, Nov. (2005) pp.21-22. 12) H. E. Littleton and J. Griffin, Project Final Report on “Manufacturing Advanced Engineered Components using Lost Foam Casting Technology”, July (2011) pp. 54-76. 13) G. R. Muginstein, E. Kiperwasser, R. Rosen and D. Yardeni, AFS Transactions (2010) pp. 487-491. the foam, put in mould boxes and surrounded with refractory sand [6, 7]. The sand is compacted on a vibrating table before a partial vacuum is applied to the sand to create a firm support for the shell, after which molten metal is poured in. In conventional LFC, burning of EPS during filling of liquid metal causes entrapment of pattern pyrolysis products and results in fold and porosity defects in castings. Using inert ceramic shells by removing traces of EPS pattern by firing reduces the possibility of porosity defects in Replicast process castings. The process can be used to make high integrity castings, with improved machinability and an excellent surface finish, weighing from a few grams, up to 3.5 tonnes. LFC under vacuum During filling of a LFC, pattern pyrolysis products causes fold and porosity defects in castings. These defects reduce the mechanical properties of castings and surface quality of castings is also impaired when pyrolysis products are deposited on surface. Fold defect on the surface of aluminium alloy castings are known as ‘alligator skin’ and similar defect on iron castings is known as ‘lustrous carbon’ due to their appearance. The root cause of fold defect is merging liquid metal fronts during filling of LFC. Residual pyrolysis products of EPS and aluminium oxide in aluminium alloy castings are entrapped in the merging liquid metal fronts resulting in fold defects. The occurrence of merging liquid metal fronts is more severe in LFC compared to conventional sand castings due to chaotic filling in LFC resulted from burning of EPS pattern. Lower metal temperature at the last filling regions of a casting is another factor for fold defect. Increasing the pouring temperature reduces the fold defect. However the solubility of hydrogen gas in liquid aluminium increases with temperature. Hence during solidification more porosity would be formed in aluminium alloy castings when poured at higher temperature. Vacuum assisted filling of liquid metal is one measure to reduce fold and porosity defects in LFCs [8-10]. To apply vacuum in LFC, the coated EPS pattern assembly is placed in a flask similar to that used in V-process, in which four vertical walls are hollow and perforations are provided at the inner surfaces of the vertical walls. The pattern assembly is embedded in loose sand and a plastic film is placed at the top of the flask except the pouring cup area to seal for creating vacuum. Aluminium alloy castings have been produced with this set up. Pyrolysis products on the surface of the castings and porosity were less when cast under vacuum, as the gaseous products of decomposition of EPS were extracted out more effectively compared to conventional LFC during filling of liquid metal. In conventional LFC, high gas pressure is generated in between liquid metal front and residual EPS during filling. This effect increases the pouring time. When poured under vacuum, gases are extracted fast through coating and therefore the pouring time is reduced. Therefore castings should be filled completely with lower metal temperature when the pouring time is less. In actual production lower pouring temperature could be used to fill the castings completely. Therefore porosity in vacuum assisted pouring was further reduced due to lower pouring temperature, since less dissolved hydrogen is present in liquid metal at lower temperature. Tensile test of the aluminium alloy castings poured under vacuum showed improved mechanical properties. The ultimate tensile strength and elongation % of the castings improved when poured under vacuum and at lower temperature. These improvements can be attributed to the reduced porosity % and pore size in the castings produced under vacuum. LFC under pressure Attempts have been made to reduce porosity and improve mechanical properties of castings by applying pressure during solidification. There are reports on applying pressure over riser to reduce porosity in aluminium alloy castings . However the pressure is not accessible to all regions of a complex casting depending upon the progress of solidification. Application of pressure during solidification is effective to reduce gas porosity by suppressing its nucleation and pressure feeding minimizes shrinkage porosity. Earlier works reported that porosity in castings solidified under pressure reduced significantly, but mechanical properties were not improved unless gassy metal was used. On the other hand, works in the recent past on the effect of pressure during solidification of LFC of complex shapes reported both reductions in porosity as well as improvements in mechanical properties . To apply pressure during solidification of a LFC, the coated pattern assembly is placed inside a pressure vessel and compacted with unbounded sand. Attempts to close the vessel PYROLYSIS PRODUCTS ON THE SURFACE OF THE CASTINGS AND POROSITY WERE LESS WHEN CAST UNDER VACUUM, AS THE GASEOUS PRODUCTS OF DECOMPOSITION OF EPS WERE EXTRACTED OUT MORE EFFECTIVELY COMPARED TO CONVENTIONAL LFC DURING FILLING OF LIQUID METAL. IN CONVENTIONAL LFC, HIGH GAS PRESSURE IS GENERATED IN BETWEEN LIQUID METAL FRONT AND RESIDUAL EPS DURING FILLING. LFC IS AN ECONOMICAL PROCESS FOR PRODUCTION OF COMPLEX CASTINGS IN AL AND IRON, WHERE A NUMBER OF CORES ARE REQUIRED. HOWEVER HIGHER POROSITY AND FOLD/LUSTROUS CARBON DEFECTS ARE FORMED DUE TO FORMATION OF GASES BY DECOMPOSITION OF THE PATTERN MATERIAL DURING POURING.
MCT DEC 2017 (4TH QRT)