PDT 111 Manufacturing Process CHAPTER 2 1 Concept
- Slides: 64
PDT 111 Manufacturing Process CHAPTER 2. 1 : Concept and Methodologies of Solidification Processes Powerpoint Templates Page 1
Course Outcome 2 Ability to describe and analyze the concept & methodologies of solidification processes. Powerpoint Templates Page 2
Metal Casting-Fundamental 1. Overview of Casting Technology 2. Heating and Pouring 3. Solidification and Cooling Powerpoint Templates Page 3
Solidification Processes • Starting work material is either a liquid or is in a highly plastic condition, and a part is created through solidification of the material. • Solidification processes can be classified according to engineering material processed: – Metals – Ceramics, specifically glasses – Polymers and polymer matrix composites (PMCs) Powerpoint Templates Page 4
Figure 10. 1 Classification of solidification processes. Powerpoint Templates Page 5
Casting • • • Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity. The term casting also applies to the part made in the process. Steps in casting seem simple: 1. Melt the metal 2. Pour it into a mold 3. Let it freeze Powerpoint Templates Page 6
Capabilities and Advantages of Casting • Can create complex part geometries. • Can create both external and internal shapes. • Some casting processes are net shape; others are near net shape. • Can produce very large parts. • Some casting methods are suited to mass production. Powerpoint Templates Page 7
Disadvantages of Casting • Different disadvantages for different casting processes: – Limitations on mechanical properties. – Poor dimensional accuracy and surface finish for some processes; e. g. , sand casting. – Safety hazards to workers due to hot molten metals. – Environmental problems. Powerpoint Templates Page 8
Parts Made by Casting • Big parts: – Engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, bells, big statues, pump housings. • Small parts: – Dental crowns, jewelery, small statues, frying pans. • All varieties of metals can be cast, ferrous and nonferrous. Powerpoint Templates Page 9
Overview of Casting Technology • Casting is usually performed in a foundry. Foundry = factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished casting. • Workers who perform casting are called foundrymen. Powerpoint Templates Page 10
The Mold in Casting • Contains cavity whose geometry determines part shape. – Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling. – Molds are made of a variety of materials, including sand, plaster, ceramic, and metal. Powerpoint Templates Page 11
Open Molds and Closed Molds Figure 10. 2 Two forms of mold: (a) open mold, simply a container in the shape of the desired part; and (b) closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity. Powerpoint Templates Page 12
Two Categories of Casting Processes 1. Expendable mold processes - mold is sacrificed to remove part. – Advantage: more complex shapes possible. – Disadvantage: production rates often limited by time to make mold rather than casting itself. 2. Permanent mold processes - mold is made of metal and can be used to make many castings. – Advantage: higher production rates. – Disadvantage: geometries limited by need to open mold. Powerpoint Templates Page 13
Advantages and Disadvantages • More intricate geometries are possible with expendable mold processes. • Part shapes in permanent mold processes are limited by the need to open the mold. • Permanent mold processes are more economic in high production operations. Powerpoint Templates Page 14
Sand Technology • The properties of a moulding sand are a function of the sand characteristics and the binder used to bond the sand grains together to form the mould. • The sand directly affects: – – – – refractoriness permeability expansion of the moulding sand binder demand moisture demand ( in clay bonded sands) flowability of the moulding sand the quality of casting surface finish ( a function of sand grain size and distribution) Powerpoint Templates Page 15
The Properties and Characteristics of Sands Used for Moulding • Refractoriness – The ability of the sand to withstand the pouring temperature of the metal without melting. – The presence of impurities will impair refractoriness. – Pure silica melts at 1710°C but natural moulding sand melts as low as 1350°C; suitable for Al, Cu base alloy, cast iron but not for steel. – zircon (Zr. O 2 Si. O 2) has melting point of 2420°C. Powerpoint Templates Page 16
The Properties and Characteristics of Sands Used for Moulding (Cont) • Permeability – Refers to the ease with which gases will permeate through a bonded sand mould. – When metal enters the mould cavity it must displace the air in order to replace it and replicate the mould shape. Failure to do so will results in a short run casting. – Large grains provide large pores which allow air to pass through easily whilst small grains have small pores which create more difficult route for the gas to escape. – Large grain is more preferable but at the expense of surface finish. Therefore mixture of grain sizes is employed which may results in infilling of the voids between larger grains by smaller ones thus reducing the overall permeability. Powerpoint Templates Page 17
The Properties and Characteristics of Sands Used for Moulding (Cont) Water conversion to steam or binder decomposition escaping from sand grains The effect of grain size on permeability, showing the increased friction associated with passage of the gas through sands of small grain size Powerpoint Templates Page 18
The Properties and Characteristics of Sands Used for Moulding (Cont) • Strength – For a given amount of binder a sand consisting of large grains would provide higher strength than a sand consisting of small grains because larger grains would present a smaller surface area and would therefore thicker coating binder. – To achieve the same strength, finer sand with larger area would require a higher binder demand. – Comparatively high binder demand is required when mixture of grain sizes is employed due to increase in area/points of contact between grains. Powerpoint coated by the binder. Templates Page 19
The Properties and Characteristics of Sands Used for Moulding (Cont) – Grain shape influence; • The binder coats the grain and how the individual grains contact one another, • Rounded grains have an even coating of binder and present good contact area to their neighbour, but angular grains have uneven coatings and poor areas of contact. • Moulding sands must be compacted to maximise their density and improve their strength at the points of contact. • The sands should flow readily and this is a function of shape in that rounded grains flow more readily than angular grains which tend to interlock. Powerpoint Templates Page 20
The Properties and Characteristics of Sands Used for Moulding (Cont) Additional points of bonding contact provided by infilling of smaller grains between larger ones. The effect of grain shape on clay coating and the uneven clay coating associated with angular grains, together with the poor contact area provided by points and points to flats. Powerpoint Templates Page 21
The Properties and Characteristics of Sands Used for Moulding (Cont) • Surface Finish – Factors influencing surface finish • Factors which improve permeability impair surface finish. The use of small grains for moulding provide a good surface finish. • Refractoriness • Metal pouring condition such as temperature and velocity. • High metal pouring temperature and velocity are likely to encourage penetration of the metal between the sand grains. Powerpoint Templates Page 22
Sand Casting Mold Figure 10. 2 (b) Sand casting mold. Powerpoint Templates Page 23
The Green Sand • Sand Mixing – A typical green sand for high pressure moulding contain; • • • 5 to 7% clay 2. 5 to 3. 5% water 0. 5% starch up to 3% coal dust the remainder is silica sand – Such a sand would develop a green compression strength around 140 kpa and have a compactibility value between 35 and 45%. Powerpoint Templates Page 24
The Green Sand Moulding The sand used for green sand moulding must fulfil a number of requirements: i. iii. iv. v. vi. Must pack tightly around the pattern, which means that it must have flowability. Capable of being deformed slightly without cracking, so that the pattern can be withdrawn i. e it must exhibit plastic deformation. Have sufficient strength to strip from the pattern and support its own weight without deforming, and to withstand pressure of molten metal when the mould is cast (green strength). Must be permeable, so that gases and steam can escape from the mould during casting. must have dry strength, to prevent erosion of the mould surface by liquid metal during pouring as the surface of the mould cavity dries out. Possess refractoriness, to withstand the high temperature involved in pouring without melting or Powerpoint fusing to the casting. Templates Page 25
Sand Casting Mold Terms • Mold consists of two halves: – Cope = upper half of mold – Drag = bottom half. • Mold halves are contained in a box, called a flask. • The two halves separate at the parting line. Powerpoint Templates Page 26
Forming the Mold Cavity • Mold cavity is formed by packing sand around a pattern, which has the shape of the part. • When the pattern is removed, the remaining cavity of the packed sand has desired shape of cast part. • The pattern is usually oversized to allow for shrinkage of metal during solidification and cooling. • Sand for the mold is moist and contains a binder to maintain its shape. Powerpoint Templates Page 27
Use of a Core in the Mold Cavity • The mold cavity provides the external surfaces of the cast part. • In addition, a casting may have internal surfaces, determined by a core, placed inside the mold cavity to define the interior geometry of part. • In sand casting, cores are generally made of sand. Powerpoint Templates Page 28
Gating System • Channel through which molten metal flows into cavity from outside of mold. • Consists of a downsprue, through which metal enters a runner leading to the main cavity. • At the top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprue. Powerpoint Templates Page 29
Riser • Reservoir in the mold which is a source of liquid metal to compensate for shrinkage of the part during solidification. • The riser must be designed to remain molten until after the main casting solidifies in order to satisfy its function. Powerpoint Templates Page 30
Heating the Metal • Heating furnaces are used to heat the metal to molten temperature sufficient for casting. • The heat required is the sum of: 1. Heat to raise temperature to melting point. 2. Heat of fusion to convert from solid to liquid. 3. Heat to raise molten metal to desired temperature for pouring. Powerpoint Templates Page 31
Pouring the Molten Metal • For this step to be successful, metal must flow into all regions of the mold, most importantly the main cavity, before solidifying. • Factors that determine success: – Pouring temperature – Pouring rate – Turbulence Powerpoint Templates Page 32
Pouring the Molten Metal • Pouring temperature is the temperature of the molten metal as it is introduced into the mold. • Pouring rate refers to the volumetric rate at which the molten metal is poured into the mold. If the rate is too slow, the metal will chill and freeze before filling the cavity. • Turbulence in fluid flow is characterized by erratic variations in the magnitude and direction of the velocity throughout the fluid. The turbulences can cause metal oxides entrapped, wearing away the mold surface and erosion. Powerpoint Templates Page 33
Solidification of Metals • Transformation of molten metal back into solid state. • Solidification differs depending on whether the metal is: – A pure element or – An alloy Powerpoint Templates Page 34
Solidification of Metals Solidification requires 2 stages; nucleation and growth: • Nucleation occurs when a small piece of solid forms from liquid. The solid must achieve a certain minimum critical size before it is stable. • Growth of the solid occurs as atoms from the liquid are attached to the tiny solid until no liquid remains. Powerpoint Templates Page 35
Cooling Curve for a Pure Metal • A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point) Figure 10. 4 Cooling curve for a pure metal during casting. Powerpoint Templates Page 36
Solidification of Pure Metals • Due to chilling action of mold wall, a thin skin of solid metal is formed at the interface immediately after pouring. • Skin thickness increases to form a shell around the molten metal as solidification progresses. • Rate of freezing depends on heat transfer into mold, as well as thermal properties of the metal. Powerpoint Templates Page 37
Figure 10. 5 Characteristic grain structure in a casting of a pure metal, showing randomly oriented grains of small size near the mold wall, and large columnar grains oriented toward the center of the casting. Powerpoint Templates Page 38
Solidification of Alloys • Most alloys freeze over a temperature range rather than at a single temperature. Figure 10. 6 (a) Phase diagram for a copper‑nickel alloy system and (b) associated cooling curve for a 50%Ni‑ 50%Cu composition during Powerpoint Templates casting. Page 39
Figure 10. 7 Characteristic grain structure in an alloy casting, showing segregation of alloying components in center of casting. Powerpoint Templates Page 40
Solidification Time • Solidification takes time. • Total solidification time TTS = time required for casting to solidify after pouring. • TTS depends on size and shape of casting by relationship known as Chvorinov's Rule: Where; TTS = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent with typical value = 2; and Cm is mold constant. Powerpoint Templates Page 41
Mold Constant in Chvorinov's Rule • Mold constant Cm depends on: – Mold material. – Thermal properties of casting metal. – Pouring temperature relative to melting point. • Value of Cm for a given casting operation can be based on experimental data from previous operations carried out using same mold material, metal, and pouring temperature, even though the shape of the part may be quite different. Powerpoint Templates Page 42
What Chvorinov's Rule Tells Us • A casting with a higher volume‑to‑surface area ratio cools and solidifies more slowly than one with a lower ratio. – To feed molten metal to main cavity, TTS for riser must greater than TTS for main casting. • Since mold constants of riser and casting will be equal, design the riser to have a larger volume‑to‑area ratio so that the main casting solidifies first – This minimizes the effects of shrinkage Powerpoint Templates Page 43
Shrinkage in Solidification and Cooling Figure 10. 8 Shrinkage of a cylindrical casting during solidification and cooling: (0) starting level of molten metal immediately after pouring; (1) reduction in level caused by liquid contraction during cooling (dimensional reductions are exaggerated for clarity). Powerpoint Templates Page 44
Shrinkage in Solidification and Cooling Figure 10. 8 (2) reduction in height and formation of shrinkage cavity caused by solidification shrinkage; (3) further reduction in height and diameter due to thermal contraction during cooling of solid metal (dimensional reductions are exaggerated for clarity). Powerpoint Templates Page 45
Solidification Shrinkage • Occurs in nearly all metals because the solid phase has a higher density than the liquid phase. • Thus, solidification causes a reduction in volume per unit weight of metal. • Exception: cast iron with high C content – Graphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change. Powerpoint Templates Page 46
Shrinkage Allowance • Patternmakers account for solidification shrinkage and thermal contraction by making mold cavity oversized. • Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance. • Casting dimensions are expressed linearly, so allowances are applied accordingly. Powerpoint Templates Page 47
Directional Solidification • To minimize damaging effects of shrinkage, it is desirable for regions of the casting most distant from the liquid metal supply to freeze first and for solidification to progress from these remote regions toward the riser(s). – Thus, molten metal is continually available from risers to prevent shrinkage voids. – The term directional solidification describes this aspect of freezing and methods by which it is controlled. Powerpoint Templates Page 48
Achieving Directional Solidification • Desired directional solidification is achieved using Chvorinov's Rule to design the casting itself, its orientation in the mold, and the riser system that feeds it. • Locate sections of the casting with lower V/A ratios away from riser, so freezing occurs first in these regions, and the liquid metal supply for the rest of the casting remains open. • Chills ‑ internal or external heat sinks that cause rapid freezing in certain regions of the casting. Powerpoint Templates Page 49
External Chills Figure 10. 9 (a) External chill to encourage rapid freezing of the molten metal in a thin section of the casting; and (b) the likely result if the external chill were not used. Powerpoint Templates Page 50
Riser Design • Riser is waste metal that is separated from the casting and remelted to make more castings. • To minimize waste in the unit operation, it is desirable for the volume of metal in the riser to be a minimum. • Since the geometry of the riser is normally selected to maximize the V/A ratio, this allows riser volume to be reduced to the minimum possible value. Powerpoint Templates Page 51
Example of riser design A cylindrical riser must be designed for a sand-casting mold. The casting itself is a steel rectangular plate with dimensions 7. 5 cm x 12. 5 cm x 2 cm. Previous observations have indicated that the total solidification time (TTS) for this casting =1. 6 min. The cylinder for the riser will have a diameter-to-height ratio= 1. 0. Determine the dimensions of the riser so that its TTS=2. 0 min. Powerpoint Templates Page 52
Solution First determine the V/A ratio for the plate. Its volume V= 7. 5 x 12. 5 x 2. 0 = 187. 5 cm³, and its surface area, A= 2 [(7. 5 x 12. 5) + (7. 5 x 2. 0) + (12. 5 x 2. 0)] = 267. 5 cm². Given that TTS = 1. 6 min, we can determine the mold constant Cm using a value of n = 2. Powerpoint Templates Page 53
Solution Next we must design the riser so that its total solidification time is 2. 0 min, using the same value of the mold constant. The volume of the riser is given by: And the surface area is given by: Powerpoint Templates Page 54
Solution Since we are using a D/H ratio = 1. 0. Substituting D for H in the volume and area formulas, we get: and, Thus the V/A ratio = D/6, Using this ratio in Chvorinov’s equation, we have: Powerpoint Templates Since D=4. 7 cm, H= 4. 7 cm for the riser. Page 55
Solidification Defects Shrinkage: • Almost all materials are more dense in the solid state than in the liquid state. During solidification, the material contracts, or shrinks as much as 7%. • Bulk of the shrinkage occurs as cavities and if solidification begins at all surfaces of the casting, or as pipes. • A common technique to solve is through placement of a riser. Powerpoint Templates Page 56
Solidification Defects Shrinkage: Material Aluminium Copper Magnesium Zinc Iron Lead Galium Water Powerpoint Templates Shrinkage (%) 7. 0 5. 1 4. 0 3. 7 3. 4 2. 7 +3. 2 +8. 3 Page 57
Solidification Defects Several types of macroshrinkage; unidirectional, cavity, pipe Sections through an aluminium casting a) no rise is used, concentrated shrinkage is present in the thick section of the casting. b) shrinkage is contained in the riser, thus producing a Powerpoint Templates sound casting. Page 58
Solidification Defects 1. Interdendritic shrinkage: Found when extensive dendritic growth occurs where liquid metal is unable to flow from a riser through the fine dendritic network to the solidifying metal. Thus small shrinkage pores are produced throughout the casting. It can be reduced through fast cooling; dendrites may be shorter, permitting liquid to flow through the dendritic network. Powerpoint Templates Page 59
Solidification Defects (con’t) Shrinkage can occur between the dendrite arms Small secondary dendrite arm spacing result in smaller, more evenly distributed shrinkage porosity Short primary arms help avoid Powerpoint Templates shrinkage Interdendritic shrinkage in an aluminium alloy (80 X) Page 60
Solidification Defects 2. Gas Porosity • Many metals dissolve a large quantity of gas when they are liquid. • During solidification e. g aluminium solid metal retains a small fraction of the hydrogen. • Excess hydrogen forms bubbles that may be trapped in the solid metal, producing gas porosity. • The porosity may be spread uniformly throughout the casting and may be trapped between dendrite arms. Powerpoint Templates Page 61
Solidification Defects • Minimisation of gas porosity can be done by keeping the liquid temperature low, by adding materials to the liquid to combine with the gas and form solid, or by assuring the partial pressure of the gas remains low. • This can be achieved by placing the molten metal in a vacuum chamber or bubbling inert gas through the metal. Powerpoint Templates Page 62
Assignment In 5 pages by hand-writing, discuss the following solidification defects; inclusion, hot tearing, misrun, penetration and cold shuts in term of: 1. Source of the defects problem. 2. Effect to the castings. 3. Methods to prevent the defects. Submit before 19 April 2013 at 5 pm. Powerpoint Templates Page 63
The End. . Any Questions? Powerpoint Templates Page 64
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