Metal Casting Technologies : MCT SEP 2018 (3RD QRT) WHOS WHO OF METALS
METAL Casting Technologies 3rd Quarter 2018 33 the interface is developed by immersing preheated Ni-Resist insert into the Al-fin bath containing 5-13% Si at 720-730°C . With increase in immersion time, the thickness of the bond layer increased up to 400 sec. and then it remained almost constant. On the other hand, the maximum bond strength was found at bond layer thickness of about 23μm. Microstructure and interfacial bonding have been compared in an Al alloy hollow cylinder with steel insert at the inner periphery under different surface treatment conditions: hot dip aluminizing, surface modifier with ammonium chloride solution at 80°C for 600 S and surface modifier + aluminizing . The aluminizing method and surface modifier, when applied separately, improves the bond strength, but the maximum improvement in bonding was observed when both surface modifier and aluminizing method were used, as measured by the push-out test. zz Heatztreatmentzofzcastingzwithzinsert A casting having insert is subjected to heat treatment to undergo diffusion at the insert – casting interface and hence to improve the bond strength. In a typical heat treatment schedule , an Al alloy piston with S. G . iron insert was heat treated i) at 503K for 7 hrs and quenched in air and ii) at 773K for 9 hrs., quenched in water and then tempered at 473K for 2 hrs. Although the water- quenched piston exhibited higher bond strength, as confirmed by the measurement of shear strength of the interface, cracks were observed near the interface of this piston. Hence the air- quenched pistons exhibited better performance. EDS analysis of the interface shows reaction layers in Al-Si alloy casting with iron/steel inserts. Various intermetallic compounds such as Fe2Al5, FeAl3, Al8Fe2Si and Al2Fe3Si have been found at the reaction layers, where Fe2Al5 is the major phase. Recycling of Al alloy casting with insert Aluminium alloy castings containing iron or steel insert, when used as remelting scrap, can cause contamination of the melt with iron. In case of batch melting, the contamination is less, whereas in holding furnace such as for die casting, Fe content would be considerably high. Fe present in Al alloy form brittle platelet β-Fe-rich phases which lowers the mechanical properties of aluminum alloys. Hence the Al alloy castings containing iron or steel inserts should be segregated with the help of suitable designations on such castings. These can be melted separately by using two crucibles, one over another. Crucible at the top should have small holes at the bottom so that molten Al can pass through these holes and get collected at the crucible at the bottom. The inserts are supposed to get collected at the crucible at the top that can be used in iron and steel melting. Another possibility is that the detrimental effect of Fe can be neutralized by use of high cooling rate, solution heat treatment, and addition of elements such as Mn, Cr, Be, Co, Mo, Ni, V, W, Cu, Sr which modify the platelet Fe-rich phases in aluminum alloys. Conclusion The drawback of low strength and wear resistance of light weight Al alloys can be overcome by providing suitable inserts at critical locations of castings. Sufficient bond strength between the insert and casting should be ensured by available methods for effective utilization of the higher strength and wear resistance of the inserts. The concept of insert is already being used in various automobile components made of Al alloy and there is endless scope to redesign the existing Al alloy components for improved efficiency in service. n 32 www.metals.rala.com.au TECHNICAL FEATURE Introduction luminum alloys have been used extensively in automobile and aerospace castings due to its light weight providing higher fuel efficiency. However the strength and wear resistance of these alloys are less than iron and steel, which results in difficulty under certain circumstances during its application. For example, threads in Al alloy are damaged quickly due to their low shear strength and wear resistance. Similarly the dynamic loading applied in service at some locations of an Al alloy component may be very high that makes it susceptible to cracking and failure. Therefore an insert made of cast iron or steel are provided at such critical locations of an Al alloy component, e.g. a surface subjected to wear, threaded areas and a section that is highly stressed [1-8]. All major types of casting process can be followed to produce Al alloy castings with inserts. Sand casting and die casting are common for production of castings with insert, although special casting processes such as lost foam casting has been used successfully. Cast iron and steel inserts are common in Al alloy castings due to their higher strength, hardness, wear resistance and lower cost compared to Al alloys. Copper based alloys such as brass and bronze can be used as insert in Al alloy castings, but higher cost and higher density (than iron and steel) of these alloys are the unfavorable factors that restrict their commercial application. Placing of inserts Inserts are to be placed in a desired location inside a mould/die in such a manner that it fulfills the requirement of higher strength, wear resistance in that particular region of the casting. To fix the insert inside the mould/die cavity, projections similar to core print are to be provided in the insert and corresponding pockets in the mould/die are to be made so that the turbulence of liquid Al during filling of mould/die cavity do not dislodge the insert. Additional care needs to be taken in case of pressure die casting, where the gate velocity of liquid Al is very high. The prints are positioned keeping in view of closing direction of mould/die. Bonding between insert and casting Bonding between insert and casting is vital to transfer load from Al alloy section to higher strength insert during service. Higher strength of the insert would be useful only when the bonding between insert and casting is perfect and strong. Shear strength of the bond is generally more important for effective load transfer. The quality of the bond is evaluated by push-out test, in which the insert is pushed out by pressure on it from the Al alloy casting. The load is applied in such a direction that the interfacial bond is broken along the plane of the insert – casting interface. In case of casting with insert, the bonding of casting and insert depends on several factors such as volume ratio of the two parts, pouring and freezing conditions and physical and chemical factors of the bonding surface. Volume ratio of the two parts is decided by the design of the component that cannot be varied. Pouring and freezing conditions are also limited that cannot be controlled to favour bonding of casting and insert. Therefore majority of the attempts have been made to modify the surface characteristics of the insert to improve the bond strength as discussed below: zz Zn-platingzofzinsert Introducing a third metal in between the casting and insert is one of the methods to improve bonding between them. Zn-plating over the surface of steel insert has been found to improve the bonding between casting and insert . Zn-plating protects the surface of the insert from rusting and hence improves the interfacial bonding. The interface of an Al alloy casting with steel insert produced by Lost Foam Casting (LFC) process showed intermetallic compounds of Al, Fe and Si, whereas Zn was totally absent at the interface. Probably Zn dissolved in the Al of the casting material. Small amount of carbon was also present at the interface that is believed to be from the residue of burning of pattern material of LFC process. The presence of the intermetallic compounds at the interface is likely to improve the bond strength between the casting and insert. zz Aluminizingzofzinsert In aluminizing process, the insert is dipped in a bath of liquid Al alloy at around 780°C for 3-5 minutes, which forms an interlayer containing both Al and iron . Other alloying elements such as Si present in the Al alloy may also be present in this interlayer. The insert is taken out of the liquid Al bath and placed immediately in the mould/die followed by pouring. In this case the bonding takes place between the interlayer and the Al alloy rather than iron/steel insert and Al alloy. Al-fin bond is one type of bond formed by aluminizing process at the interface of Al alloy piston and piston ring carrier made of Ni-Resist, a high alloy cast iron . In alfinization, Al coating at A Aluminium alloy castings with cast iron and steel inserts By Dr P.C. Maity, Metal casting and materials Engineer E: email@example.com Induction Melting Systems Induction OEM for: SHAPING THE FUTURE OF I NDUCTION Global Sales & Service www.AjaxTocco.com World Headquarters 1745 Overland Avenue Warren, Ohio USA 44483 +1-330 -372-8511 +1-330 -372-8608 Fax 24 / 7 Customer Service: 800-547-1527 z Coreless & Channel Furnaces z Power Supplies z Ferrous & Non-ferrous z Melting, Holding & Duplexing z Retrofits & Rebuilds z Coil Repair z Parts & Service z Installation & Construction Service, Support & Manufacturing REFERENCES 1) G. Durrant, M. Gallerneault and B. Cantor, J. Mater. Sci., (1996) Vol.31, pp.589-602. 2) J. Pan, M. Yoshida, G. Sasaki, H. Fukunaga, H. Fujimura and M. Matsuuara, Scr. Mater. (2000) Vol.43, p.155. 3) V. C . Viala, M. Peronnet, F. Bosselet and J. Bouix, Composites :Part A (2002), Vol.33, pp.1417-1420. 4) K. Bouche, F. Barbier and A. Coulet, Mater. Sci. Eng. A 249 (1998) pp.167-175. 5) S. Kobayashi and T. Yakou, Mater. Sci. Eng. 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