CHAPTER 3 SPECIAL CASTING PROCESSES

1. ME 333 PRODUCTION PROCESSES II CHAPTER 3 SPECIAL CASTING PROCESSES 3.1INTRODUCTION • The process used for making a casting depends on; • the quantity to be produced, • the metal to be cast, • the complexity of the part. • Sand molds are single-purpose molds, and are completely destroyed after the metal has solidified. • Quite obvious the use of a permanent mold effect considerable saving in labor cost. • A summary of the various special casting methods, which will be discussed in this chapter, is as follows: • Casting in metallic molds • Centrifugal casting • Precision or investment casting • Continuous casting • Shell molding CHAPTER 3 SPECIAL CASTING PROCESSES

2. ME 333 PRODUCTION PROCESSES II 3.2METAL MOLD CASTING PROCESSES Permanent molds must be made of metals capable of withstanding high temperatures. Because of their high cost they are recommended only when many casting are to be produced. Although permanent molds are impractical for large castings and alloys of high melting temperatures, they can be used advantageously for small and medium-sized castings that are manufactured in large quantities. Die Casting: Die casting is a process in which molten metal is forced by pressure into a metal mold known as a ‘die’. The usual pressure is 100 to 125 atm. It is the most widely used of any of the permanent mold processes. There are two methods employed: • Cold-chamber Die Casting • Hot-chamber Die Casting CHAPTER 3 SPECIAL CASTING PROCESSES

3. ME 333 PRODUCTION PROCESSES II • Cold-chamber Die Casting: • Material to be cast is molten outside the machine. • Used for materials having high melting temperature Tm> 550°C, i.e. brass, aluminum, and magnesium. • (2) Hot-chamber Die Casting: • Materials to be cast is molten inside the machine. • Used for materials having low melting temperature Tm< 550°C, i.e. zinc, tin, and lead. • (The principal distinction between the two is determined by the location of the melting pot. • In the hot-chamber pot is included in the machine; and • In the cold-chamber pot is separate from the machine, • metal is introduced to injection cylinder by other means.) CHAPTER 3 SPECIAL CASTING PROCESSES

4. ME 333 PRODUCTION PROCESSES II Fig. 3.1 Types of Die Casting CHAPTER 3 SPECIAL CASTING PROCESSES

5. Schematics of hot-chamber die-casting CHAPTER 3 SPECIAL CASTING PROCESSES

6. Schematics of cold-chamber die-casting CHAPTER 3 SPECIAL CASTING PROCESSES

7. Die Casting Cavities Die Casting Products CHAPTER 3 SPECIAL CASTING PROCESSES

8. ME 333 PRODUCTION PROCESSES II • Advantages of die casting: • The process is rapid (since both dies and cores are permanent) • The surface quality is very good. (The smooth surface improves appearance and reduces work required for other operations.) • Dimensional tolerances are very good comparing to sand casting. (The size is so accurately controlled that little or no machining is necessary.) • The scrap loss is low since sprue, runners, and gates can be remelted. CHAPTER 3 SPECIAL CASTING PROCESSES

9. ME 333 PRODUCTION PROCESSES II • The optimum production quantity ranges from 1,000 to 200,000 pieces. • Max. Weight of brass die casting is about 2 kg, but aluminum die castings of over 50 kg are common. • Small to medium size castings can be made at a cycle rate of 100 to 800 die fillings per hour. • The life of dies depends on the metal cast and may range from 10,000 fillings for brass to several million if zinc is used. • If the part is big and complex, a single-cavity mold is used. If the parts are small and quantity is large, multiple-cavity die can be used. • Optimum no. of parts: 1000-200,000 • Weight of part: Brass about 2 kg • Aluminum about 50 kg • Die life: about 10,000 filling for Brass • Several millions for filling Zinc • Other methods of metal mold castings:low-pressure permanent mold casting, gravity permanent mold casting, slush casting and pressed casting. CHAPTER 3 SPECIAL CASTING PROCESSES

10. ME 333 PRODUCTION PROCESSES II 3.3 DIE-CASTING ALLOYS • A relatively wide range of nonferrous alloys can be die-cast. The principal base metals used are ZINC, ALUMUNIUM, MAGNESIUM, COPPER, LEAD and TIN. They are classified in two groups: • Low-temperature alloys (casting temp. below 5500C), • High-temperature alloys • Zinc-Base Alloys : Over 75% of die castings are produced from zinc-base alloys. Melting point is around 4000C. So, they are cast by Hot-chamber die casting. Aluminum improves mechanical properties. Copper improves tensile strength and ductility. Magnesium makes casting stable in microstructure. Zinc alloys are widely used in: automotive industry, washing machines, refrigerators, business machines, etc. CHAPTER 3 SPECIAL CASTING PROCESSES

11. Aluminum-Base Alloys : They are used due to their lightness in mass and resistance to corrosion. Compared to zinc alloys they are more difficult to cast (melting point around 5500C). Since molten aluminum will attack steel if kept in continuous contact with it, the cold-chamber process generally is used. Principal elements used as alloys with aluminum are SILICON, COPPER, and MAGNESIUM. Silicon increases hardness and corrosion resisting properties, copper increases mechanical properties, magnesium increases lightness and resistance to impact. They are generally used in aerospace industry and production of pistons. CHAPTER 3 SPECIAL CASTING PROCESSES

12. ME 333 PRODUCTION PROCESSES II Copper-Base Alloys : Die casting of brass and bronze have presented a greater problem due to their high casting temperature. Temperatures are around 870 to 10400C, which need heat resisting die material. Copper alloys are cold chamber die-cast. Copper-base alloys have extensive use in miscellaneous hardware, electric-machinery parts, small gears, marine, automotive and aircraft fittings, chemical apparatus, and numerous other small parts. CHAPTER 3 SPECIAL CASTING PROCESSES

13. Magnesium-Base Alloys : It is alloyed principally with ALUMINUM, but may contain small amounts of SILICON, MANGANESE, ZINC, COPPER, and NICKEL. They have the lowest density. Their casting temperatures are around 670-7000C, so, cold chamber die casting is suitable. Copper parts Zinc parts Aluminium parts Magnesium parts CHAPTER 3 SPECIAL CASTING PROCESSES

14. CHAPTER 3 SPECIAL CASTING PROCESSES

15. Die Materials: • Hot-worked tools steels • Mold steels • Maraging Steels • Refractory Metals Properties: • High hot strength • High temperature wear resistance CHAPTER 3 SPECIAL CASTING PROCESSES

16. ME 333 PRODUCTION PROCESSES II 3. 4. CENTRIFUGAL CASTING (Savurma Döküm) Centrifugal casting is the process of rotating a mold while the metal solidifies so as to utilize centrifugal force to position the metal in the mold. Castings of symmetrical shape lend themselves particularly to this method, although many other types of casting can be produced. Fig. 3.2 Centrifugal casting machine for casting steel or cast iron pipe CHAPTER 3 SPECIAL CASTING PROCESSES

17. ME 333 PRODUCTION PROCESSES II Setup for true horizontal centrifugal casting CHAPTER 3 SPECIAL CASTING PROCESSES

18. Centrifugal casting is often more economical than other methods. Cores in cylindrical shapes and risers or feed-heads are both eliminated. The castings have a dense metal structure with all impurities forced back to the centre where frequently they can be machined out. Piston rings weighing 50-100 grams to paper mill rolls weighing over 42 tons have been cast in this manner. Aluminum engine block uses centrifugally cast iron liners. In some alloys, the heavier elements tend to be separated from the base metal, known as “gravity segregation”. The metal is forced against the walls on the mold with a centrifugal force of approximately 70 g, which is a force 70 times that of the force of gravity alone on the casting. Forces as high as 150 g have been used but are seldom found necessary unless very thick-walled cylinders are being cast. CHAPTER 3 SPECIAL CASTING PROCESSES

19. ME 333 PRODUCTION PROCESSES II Semi-centrifugal casting - In this method, centrifugal force is used to produce solid castings rather than tubular parts. - Density ofthe metal in the final casting is greater in the outer sections than at the center of rotation. - The process isused on parts in which the center of the casting is machined away, such as wheels and pulleys. CHAPTER 3 SPECIAL CASTING PROCESSES

20. 3.5.PRECISION OR INVESTMENT CASTING (Hassas Döküm) • Precision or investment casting employed techniques that enable very smooth highly accurate castings to be made from both ferrous and non-ferrous alloys. • The process is useful in casting unmachinable alloys and radioactive metals. • There are a number of processes employed, but all incorporate a sand, ceramic, plaster (alçı), or plastic shell made from an accurate pattern into which metal is poured. • Advantages of investment techniques are; • intricate forms with undercuts can be cast; • a very smooth surface is obtained with no parting line; • dimensional accuracy is good; • unmachinable parts can be cast; • may replace die casting for short runs. • Main disadvantage is that it is expensive and not suitable for big parts. CHAPTER 3 SPECIAL CASTING PROCESSES

21. Replica Split pattern Wax pattern Wax Tree Plaster Coated Patterns are produced from wax (mum) or plastics which are subsequently melted from the mold, leaving a cavity having all the details of the original pattern. Present practice requires that a replica of the part to be cast made from steel or brass. From this replica a bismuth or lead-alloy split mold is made. After wax is poured into the mold and solidification takes place, the mold is opened and the wax pattern removed. Fig. 3.3 Processes of Precision or Investment Casting CHAPTER 3 SPECIAL CASTING PROCESSES

22. Replica Split pattern Wax pattern Wax Tree Plaster Coated Several patterns are usually assembled (Wax Tree) with necessary gates and risers by heating the contact surfaces (wax welding) with a hot wire. This cluster is molded by silica sand, plaster or ceramic slurries. After the mold material gets strength, the mold is placed upside down and heated in an oven for several hours to melt out the wax and to dry the mold. The casting can be produced by gravity, vacuum, pressure, or centrifugal casting. When the solidification finished, mold is broken away and gates and risers are cut-off. Fig. 3.3 Processes of Precision or Investment Casting CHAPTER 3 SPECIAL CASTING PROCESSES

23. Figure Schematic illustration of investment casting, (lostwax process). Castings by this method can be made with very fine detail and from a variety of metals. CHAPTER 3 SPECIAL CASTING PROCESSES

24. Examples • Investment casting of an integrally cast rotor for a gas turbine. • Wax pattern assembly. • Ceramic shell around wax pattern. • Wax is melted out and the mold is filled, under a vacuum,with molten superalloy. • The cast rotor, produced to net or near-net shape. Crosssection and microstructure of two rotors: (top) investment-cast; (bottom) conventionally cast. CHAPTER 3 SPECIAL CASTING PROCESSES

25. ME 333 PRODUCTION PROCESSES II 3. 6. CONTINUOUS CASTING) Continuous casting consists of pouring molten metal into one end of a metal mold open at both ends, cooling rapidly, and extracting the solid product in a continuous length from the other end. This is done with copper, brass, bronze, aluminum, and to a growing extent, cast iron and steel. Fig. 3.4 A continuous casting rolling process CHAPTER 3 SPECIAL CASTING PROCESSES

26. Continuous casting is done for a number of purposes. It is suitable for any shapes of uniform cross-section: round, square, rectangular, hexagonal, fluted, gear toothed, and many other forms; solid or hollow. A growing use is to produce blooms, billets, and slabs for rolling structural shapes. This is cheaper than rolling from ingots. A bloom has a square cross section with a minimum size of 15 by 15 cm. A billet is smaller than a bloom and may have any square section from 4 cm up to the size of a bloom. Slabs may be rolled from either an ingot or a bloom. They have a rectangular cross-sectional area with a minimum width of 25 cm and a minimum thickness of 4 cm. The width is always three or more times the thicknesses, which may be as much as 40 cm. Plates, skelp and thin strips are rolled from slab. CHAPTER 3 SPECIAL CASTING PROCESSES

27. Continuous casting offers several advantages: • It yields 10% or more over rolling from ingots. Ingots have an appreciable amount of porous end, which returns back to furnace. This waste is eliminated in continuous casting. • 2. A hollow center occurs from shrinkage in continuous casting but it is welded shut after four rolling passes. Continuous cast structure is more uniform and dense. • 3. Physical properties and surface finishes are comparable to those obtained in other metal mold processes. • Continuous casting is essentially automatic, and unit labor cost is low. Dies or molds are made of copper or graphite. CHAPTER 3 SPECIAL CASTING PROCESSES

28. ME 333 PRODUCTION PROCESSES II 3. 7. SHELL MOLDING Shell moulding is a casting process in which the mold is a thin shell (typically 9mm) made of sand held together by a thermosetting resin binder. Steps in shell moulding: (1) a match-plate or cope-and-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin; (2) Box is inverted so that sand and resin fall onto the hot pattern, causinga layer of the mixture to partially cure on the surface to form a hard shell; (3) Box is repositioned so that loose, uncured particles drop away; (4) Sand and shell is heated in oven for several minutes to complete curing (5) Shell mold is stripped from the pattern (6) Two halves of the shell mold are assembled, supported by sand or metal shot in a box, (7) The finished casting is removed CHAPTER 3 SPECIAL CASTING PROCESSES

29. Advantages: • Good surface finish (up to 2.5 mm) • Good dimensional accuracy (±0.25 mm) • Suitable for mass production • Disadvantages: • Expensive metal pattern Area of application: Mass production of steel casting of less than 10 kg Two halves of a shell mold pattern CHAPTER 3 SPECIAL CASTING PROCESSES

30. 3. 8. Expandable-pattern casting (Lost foam Process) This process uses a polystyrene pattern, which evaporates upon contact with molten metal to form a cavity for the casting; this process is also known as lost-foam casting. It has become one important casting process for ferrous and nonferrous metals, particularly for the automative industry. CHAPTER 3 SPECIAL CASTING PROCESSES

31. 3. 9. Casting Techniques for single-crystal components Methods of casting turbine blades: (a) directional solidification; (b) method to producea single-crystal blade; and (c) a single-crystal blade with the constriction portion still attached. CHAPTER 3 SPECIAL CASTING PROCESSES

32. THE END CHAPTER 3 SPECIAL CASTING PROCESSES