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1.0: IntroductionCasting is the most direct fabrication method for converting liquid metal into a shaped component as it is a process of pouring liquid metal into a mould where it cools and solidifies. It is one of the methods of shaping metals. There are many casting processes are available in the market which are shell mould casting, investment casting, permanent mould casting, die casting and ingot casting.Casting process is extensively used in the manufacturing because complex shape can be made easily with the casting process and it can cast any ferrous and non-ferrous materials.
Besides, the tools required are very simple and inexpensive as well as it able to cast any size and weight even up to 200 tons. However, the disadvantages of casting are it will have many casting defects appear on the cast product and the sand cast products are having poor dimensional accuracy and surface finish. Furthermore, design of gating system as well as risering system are very important in casting process for the sake of producing sound casting.
In modern metal casting, steel and aluminium are two competitors for the top choice. Thousands of manufacturers around the world rely on both metals to produce strong, light and corrosion-resistant metal products.In this assignment, the production and metallurgy of ductile cast iron versus aluminium alloy casting will be studied.2.0 : Production and Metallurgy of Ductile Cast Iron CastingCast iron is an alloy of iron that contains 2 to 4% carbon, along with varying amounts of silicon and manganese and traces of impurities such as sulphur and phosphorus.
It is made by reducing iron ore in a blast furnace. CITATION The18 l 17417 (The Editors of Encyclopaedia Britannica, 2018) Since cast iron is having low melting point, thus, it is easy to be cast as it can be melted easily and has lower shrinkage properties. However, it has low impact resistance and ductility. Although cast iron is brittle, but it is strong. Currently, there are 4 types of cast iron which are white cast iron, gray cast iron, ductile cast iron and malleable cast iron. In cast iron, the graphite and carbon exist in the form of squiggly lines where it also known as flakes. These flakes structures are eventually causing the cast iron to have lower mechanical properties as they act as points of stress concentration within a structure. Hence, flake graphite cast irons are brittle. Irons of high ductility known as malleable iron can be made by giving irons a prolonged heat treatment which can be up to 120 hours at 900C. This heat treatment produces a structure containing spheroidal modules of graphite but is an expensive process. The discovery of a process for producing nodular irons by direct casting process gave a more economic method of obtaining high strength as well as high ductility castings. In fact, there is a process called inoculation. Inoculation is an important process in the production of cast iron. This is done by the introduction of fine nuclei of desired element that influences the structural formation of the casting in a specific way. CITATION Qua18 l 17417 (Qualitas Exports, 2018) During this process, the microstructure of the undesired graphite or cementite can be modified into a more desired form. The inoculation process can be done during the pouring or transfer ladles and in-steam before the casting process. Inoculants such as 0.01% cerium or 0.05% magnesium are added to the liquid metal to prevent the formation of eutectic carbides and to provide sufficient nucleation sites for graphite formation, hence, significantly increased the number of graphite nodules.Figure SEQ Figure * ARABIC 1: Un-inoculated microstructure.Figure SEQ Figure * ARABIC 2: Inoculated microstructure.Based on Figure 1 and 2, it can be observed that the inoculated specimen shows better mechanical properties compared to un-inoculated one. Thus, inoculation process is necessary in order to enhance the mechanical properties of the cast iron. Figure 3 shows the microstructure of ductile cast iron. Ductile cast iron is a type of cast iron that contains 3 to 4% carbon and 1.8 to 2.8% silicon and low impurities with spherical graphite structures. It has good fluidity, machinability, strength, wear resistance, toughness and hardenability. Generally, there are four types of ductile irons which are ferritic, pearlitic ductile irons, martensitic ductile irons and austenitic ductile irons. CITATION SAT14 l 17417 (SATYENDRA, 2014)Figure SEQ Figure * ARABIC 3: Ductile cast iron (x50). CITATION Web07 l 17417 (Weborchard, 2007)Malleable cast iron is seldom use as the ductile cast iron can be produced with a faster, more easy and economical process. Nodular irons produced by direct casting have similar ductility and impact strengths to malleable irons but have higher tensile strengths. Hence, ductile cast iron is more preferred to be used in casting.In order to produce a sound casting by using ductile cast iron, the gating and risering system are the main factors that affecting the quality of casting. Since ductile cast iron is a ferrous material, thus, pressurised gating system is used. In this system, the ingate area is the smallest, thus, maintaining a back pressure throughout the gating system. Due to the back pressure, the metal is more turbulent and generally flows full and thereby can minimise the air aspiration even when a straight sprue is used. This system generally provides a higher casting yield as there is less metal left in gating system due to smaller volume. Figure SEQ Figure * ARABIC 4: (a) Pressurised gating system. CITATION Pra15 l 17417 (Devanpalli, 2015)Atmospheric pressure is important in the risering of casting. In ferrous casting, uphill feeding is possible. This is because the skin strength develops relatively rapidly and able to feed to superior heights. Besides, downhill feeding also can be used for ferrous casting. Other than feeding system, the moulding method also important as to produce sound casting. For ductile cast iron, carbon dioxide moulding is used in which sodium silicate is used as binder. In this method, the carbon dioxide is expected to form a weak acid which hydrolyzes the sodium silicates resulting in amorphous silica which forms the bond.Last but not least, the slag trap system that suitable to be used for ductile cast iron casting is cope-runner drag-ingate system. It is a general practice to cut the runner in the cope and the ingate in the drag to help in the trapping of slag which are light than the molten metal such as ferrous metals. 3.0 : Production and Metallurgy of Aluminium Alloy Casting Aluminium alloys are grouped according to the major alloying elements they contain. The 4XXX group is alloyed with silicon for ease of casting. Silicon is good in metallic alloys used for casting. This is because it increases the fluidity of the melt, reduces the melting temperature, decreases the contraction associated with solidification and is very cheap as a raw material. Aluminium-silicon alloys form a eutectic at 11.7 wt% silicon, the eutectic temperature being 577C. This represents a typical composition for a casting alloy because it has the lowest possible melting temperature. Al-12-Si wt% alloys are therefore common. CITATION RCo18 l 17417 (R. Cornell and H. K. D. H. Bhadeshia, 2018)Aluminium silicon casting alloy shows a microstructure as shown in Figure 5. The microstructure consists of grey plates of silicon in a white matrix which is rich in aluminium. Although the alloy is slightly hypoeutectoid in composition, there is evidence that solidification started with primary aluminium dendrites. Since silicon particles have poor bonding in the aluminium matrix, large plates of silicon are detrimental to the mechanical properties.Figure SEQ Figure * ARABIC 5: Microstructure of unmodified Al-Si alloy.In hypoeutectic alloys, it is common to refine the size of the aluminium crystals by grain refinement for applications requiring good mechanical properties. In Al-Si alloys, a treatment called eutectic modification is often used to change the eutectic silicon from coarse plate-like into fine fibrous morphology, which is less harmful to mechanical performance. CITATION SOt11 l 17417 (S.Otarawanna, A.K.Dahle, 2011) The properties of aluminium cast alloy is non-heat treatable, moderate strength, moderate ductility, good wear resistance, very good cast properties as well as good corrosion resistance. In order to produce sound casting, non-pressurised gating system is used for non-ferrous casting. A non-pressurised gating system having choke at the bottom of the sprue base and with the total runner area and ingate area higher than the sprue area. In this system, there is no pressure existing in the metal flow system and thus it helps to reduce turbulence.Figure SEQ Figure * ARABIC 6: Non-pressurised gating system. CITATION Pra15 l 17417 (Devanpalli, 2015)In this system, it has tapered sprue, sprue base wall and pouring basins. When the metal is to enter the mould cavity through multiple ingates, the cross section of the runner should be reduced at each of the runner break up to allow for equal distributions of metal through all the ingates. In contrast with pressurised gating system, non-pressurised system is having a lower metal velocity and turbulence in the gating system. In this gating system, the casting yield will be lower as the gating system requires large runners and gates.For non-ferrous casting, downhill feeding is more suitable but not uphill feeding as non-ferrous casting is having a slow development of adequate skin strength. In non-ferrous casting, the only way to prevent atmospheric dishing or puncturing at casting hot spots is to use risers which are higher than the casting itself. In addition, for the sake of producing aluminium casting, sand casting, permanent mould casting, die casting as well as investment casting can be used. Permanent mould casting is a process where molten metal is poured into permanent metal mould while die casting is a process where the molten metal is forced into the moulds under pressure.Last but not least, the slag trap system that suitable to be used for aluminium alloy casting is drag-runner cope-ingate system. It is recommended that the runners must be placed in the drag and the ingates in the cope so that the dross which is heavier compared to aluminium is restricted. 4.0: Compare and Contrast between Production and Metallurgy of Ductile Cast Iron versus Aluminium Alloy Casting Ductile Cast Iron Aluminium AlloySimilaritiesBoth are alloys.Both can use sand casting method.Both can be cast by using sand casting.Both have good fluidity and good wear resistance.DifferencesFerrous Ferrous or non-ferrous Non-ferrousIron, carbon, silicon Alloy element SiliconLower Shrinkage properties Higher (4-8%)Good castability and machinability. High strength, toughness, ductility, and hardenability. Properties Non-heat-treatable, moderate strength and ductility, very good cast properties, good corrosion resistance.Heavier Weight LighterHigh (1150 to 1200C) Melting point Low (550-600C)Yes Subject to corrosion NoSpherical nodules of graphite Microstructure Dark plates of silicon, aluminium dendritesCarbon dioxide moulding method Casting method Sand casting, Permanent mould casting, Die casting, Investment castingPressurised Gating system Non-pressurised Uphill or downhill feeding Risering system Downhill feedingDrag Position of ingate CopeCope Position of runner Drag5.0: RecommendationIn order to produce a casting with maximise casting yield, the gating system should be designed as small as possible but still fulfil the requirement as big gating system will have lower casting yield. Nonetheless, small riser will have higher casting yield compare to big riser as it consumes minimum metal. Grouping of several casting around a single riser are actually helps in increasing the casting yield since the same riser will be able to feed more than one casting. Since the riser is surrounded by hot casting, the riser will not be solidified so fast and it dies not required a large riser volume. Hence, smaller riser can be used, resulting in a higher casting yield.For pressurise gating system that used in ductile cast iron casting, the relatively high metal velocities in the flow channels may cause severe turbulence to occur at junctions and corners. In order to solve the problems mentioned above, proper and careful streamlining must be employed as shown in Figure 7. For non-pressurised gating system, drag, runners and cope gate aid in maintaining a full runner. However, careful streamlining also is a must to eliminate the separation effects and consequent air aspiration. Figure SEQ Figure * ARABIC 7: A fully streamlined gating system.For the sake of improving mechanical properties of ductile cast iron, inoculation process has to carry out while for aluminium alloy, modification must be done to refine the grain.6.0: ConclusionIn a nutshell, both ductile cast iron and aluminium alloy casting have many similarities and also differences. They are the most common materials that use for casting due to their good castability properties. From this assignment, the production and metallurgy of ductile cast iron and aluminium alloy casting were studied.For ferrous material like ductile cast iron, pressurised gating system can be used while for non-ferrous material, non-pressurised gating system is used. Furthermore, uphill feeding only can be used for ferrous materials due to the adequate skin strength generated while for non-ferrous material, downhill feedings only can be used as the non-ferrous material is too slow to generate adequate skin strength.For the slag trap system, ductile cast iron casting is using cope-runner drag-ingate system while the aluminium alloy casting is drag-runner cope-ingate system. Lastly, the ductile cast iron casting can be produced by using carbon dioxide moulding while for aluminium alloy casting, sand casting, permanent mould casting, die casting as well as investment casting can be used.
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