CASTING Chapters 10, 11, 12 Group 7 Members William Waller James Ball William Ramos Christmas Trieu Perry Barrow 2/22/06
What is Casting? Casting is a process where molten metal is poured into a patterned mold. The metal is allowed to solidify into a part and is removed from the mold to be manufactured. Three Important considerations: 1. How the metal cools and solidifies in the mold 2. How the molten metal flows into the mold cavity 3. How the mold influences the metal
Solidification Of Metals Pure Metals Vs Alloys Pure Metals have very defined melting/freezing points and solidifies at a specific temperature. Pure Metals have a more uniform “columnar grain” growth.
Solidification of Metals (cont. ) a. Columnar grains of a pure metal b. cast structure of solidsolution alloy c. Non-columnar structure
Solidification Of Metals (Cont. ) Alloy solidification begins at some temperature TL and is not completed until it reaches a temperature TS. In between TL and TS, coined the “Mushy Zone, ” both liquid and solid exists. Here is where columnar dendrites may form which contributes to detrimental factors. Effects of cooling rates on Alloys varies. The faster the alloy cools the more columnar dendrites may form. The slower the rate, the less columnar dendrites.
Solidification of Metals (Cont. ) Illustration of alloy solidification and temperature distribution.
Fluid Flow Fluid flow of molten metal is extremely important in casting. Note how the liquid metal has to flow through the system.
Fluid Flow (continued) Bernoulli’s Theorem Explains how the fluid behaves by relating pressure, velocity and elevation of the fluid at any location within the system. Mass Continuity Explains that for an incompressible liquid in a mold with impermeable walls the volume rate of flow is the same at one location as it is at another location.
Fluid Flow (continued) Sprue Design A tapered design or a choke is used to insure that the fluid flow is sufficiently slowed to prevent aspiration. Flow Characteristics An important consideration of flow through a grate is how much turbulence the fluid experiences. Turbulence creates the formation of dross. The more that turbulence is eliminated and laminar flow increases the less dross is created.
Fluid Flow (continued) Turbulent flow Laminar flow
Fluidity of Molten Metal ØViscosity: The higher the value of viscosity the lower the fluidity ØSurface Tension: higher the value of surface The tension of the liquid metal reduces the fluidity. ØInclusions: adverse affects on fluidity. An analogy has is like oil with sand particles in it. This would reduce the liquids fluidity. ØSolidification Pattern of the alloy: The manner in which solidification takes place influences fluidity.
ØMold Design: influences how the fluid travels through the mold. ØMold Material and Its Surface Characteristics: How the mold conducts heat and how rough its surface is influences fluidity. ØDegree of Superheat: defined as the increment of temperature of an ally above its melting point, Superheat improves fluidity by delaying solidification. ØRate of Pouring: slower the rate of pouring The the lower the fluidity due to higher cooling rates when pouring is slow.
Heat Transfer Solidification Time explains how much time is required for an object to solidify depending on its volume and surface area. Depending on the solidification time, the cavities of the mold begin to form solidified metallic skin on it interior walls. Thus, the interior of the mold becomes smaller. Solidified skin on a steel casting.
Heat Transfer (continued) Shrinkage Due to thermal expansion, when a metal cools it tend to shrink in size. Three events of this phenomena can cause dimensional problems as well as cracking. 1. 2. 3. Contraction of the molten metal as it cools prior ro its solidification. Contraction of the metal during phase change from liquid to solid. Contraction of the solidified metal as its temperature drops to ambient temperature.
10. 6 Defects A- Metallic Projections: fins, flash, or projections B- Cavities- consist of rounded/ rough internal/ external cavities including blowholes, pinholes, and porosity C- Discontinuities- cracks, cold or hot tearing and cold shuts. D- Defective surface- surface folds, laps, and adhering sand layers.
10. 6 Defects E- Incomplete casting: premature solidification, insufficient volume of metal poured, pouring metal too slowly or molten being at too low a temperature. F- Incorrect dimensions/shapes: improper shrinkage allowance, pattern mounting error, deformed pattern or warping G- Inclusions- generally nonmetallic. Occurs during melting, solidification, and molding. Regarded harmful b/c they act as stress raisers which reduces the strength in casting. Inclusions form during melting when molten metal reacts with oxygen or mold material. Chemical reactions in molten metal may also produce inclusions.
10. 6. 1 Porosity -Caused by shrinkage or gas or both. -Thin sections in casting solidify quicker than thicker regions RESULT: Molten metal flows into thicker regions before proper solidification. Porous regions may develop at center b/c of contraction as the surfaces of thicker regions begin to solidify first. (Think of a funnel in the middle of casting) (cavities) Micro Porosity- when liquid solidifies and shrinks between dendrites and dendrite branches.
Chapter 11 Metal Casting Processes n n n Expendable & Permanent mold processes Applications, advantages, and limitations of common casting Casting of single crystals Inspection techniques Foundries and their Automation
11. 1 Introduction Two trends on casting industry: -Mechanization & Automation: led to significant changes in equipment and labor. Advanced machine & automated process-control systems replace traditional casting methods. -Increasing demand for HIGH QUALITY CASTINGS with close dimensional tolerances.
11. 1 Introduction 1. Major Categories of Casting Practices: Expendable Molds: After casting solidifies, mold broken up to remove casting 2. Permanent Molds: Used repeatedly and designed so casting can be removed easily and mold used for next casting. 3. Composite molds: Have expendable & permanent portions. Used in various casting processes: improve strength, cooling, and economics of casting process
11. 2 Expendable-Mold Casting Processes 11. 2. 1 Sand Casting consists of: A) Placing a pattern in sand to make imprint B) Incorporate gating system C) Removing pattern and filling mold cavity with molten metal D) Allow metal to solidify E) Break away sand mold F) Remove casting
Types of Sand Molds 1) Green-Sand Molding- mix of sand, clay & water. “Green” refers to moistness and dampness of mold while metal is being poured. LEAST EXPENSIVE and sand easily recycled Cold Setting Processes- bonding of mold w/o heat 2) Cold-Box Mold- Binders blended into sand to chemically strengthen grains. More Accurate but more Expensive than Green-Mold. 3) No Bake Mold- liquid resin mixed w/ sand mixture hardens.
Patterns -used to mold sand mixture into shape of casting -b/c patterns are used repeatedly, strength & durability of materials must reflect # of castings that mold will produce. Types of patterns: -One-piece: simpler shapes and low quantity production; inexpensive -Split: two piece, complicated shapes -Match-plate: picture
Patterns Rapid Prototyping Machine- can fabricate a pattern at a fraction of time and cost of machining a pattern. *Pattern design is a critical aspect of the total casting operation. Design must provide for Shrinkage, Ease of Removal from mold by taper or draft, & proper metal flow into mold cavity.
Sand Molding Machines Vertical Flaskless Molding. Sandslingers- Fill under high pressure steam. Used to fill large flasks & operated by machine. High speeds cause sand to be placed & rammed appropriately Impact Molding- compacted by controlled explosion or instantaneous release of compressed gas. Molds contain uniform strength and good permeability Vaccum Molding
Sand Casting Operation Removal of Casting: 1)castings cleaned by blasting w/ steel shot 2)risers & gates cut off 3)Castings cleaned electrochemically to remove surface oxides, may even be heat treated Finishing Operations: involve machining, straightening, forging with dies to obtain final dimensions Final Step: Inspection to meet all design & quality control requirements
11. 2. 2 Shell Molding -Produces many types of castings w/ close dimensional tolerances & good surface finish @ low cost. -Unless molds are properly vented, trapped air & gas can cause serious problems in ferrous castings. *High Quality of finished casting reduces cleaning, machining, and other finishing costs significantly. Complex shapes - produced with less labor, and easily automated
Precision Casting 11. 2. 3 Plaster-mold casting 11. 2. 4 Ceramic- mold casting 11. 2. 6 Investment Casting High Dimensional Accuracy & Good Surface Finish 11. 2. 3 Plaster-Molding process 1) Made of Plaster of Paris (gypsum or calcium), talc, & silica flour 2) Mix w/ water: Mixture poured over pattern. 3) Plaster sets and is removed, the mold dried at high temps to remove moisture. 4) Mold halves assembled, preheated and molten metal is poured. 5) *Gases evolved during solidification cannot escape, so molten metal is poured under a vacuum or under pressure. 6) Low Thermal Conductivity allows casting to cool slowly, allowing a more uniform grain structure is obtained w/ less warpage
11. 2. 4 Ceramic Mold casting -Similar to plaster mold process EXCEPT it uses refractory metal materials suitable for HIGH TEMPS. High-temperature resistance of refractory molding materials allows for casting of high temp alloys, stainless steel, and tool steels. *Expensive Process but castings have good dimensional accuracy and surface finish
11. 2. 5 Evaporative Pattern Casting (lost-foam) -Mold produced for every casting (polystyrene pattern evaporates upon contact with molten metal) *Polymers require considerable energy to degrade, so large thermal gradients are present. The degradation products are vented into the surrounding sand. Advantages: 1) Simple process: Design flexibility 2) Inexpensive flasks are satisfactory 3) Plastic is inexpensive; easily processed into complex shapes, sizes, and fine surface detail 4) Casting requires minimal finshing and cleaning
11. 2. 6 Investment Casting (lost-wax process) -Pattern made of wax then dipped repeatedly into a slurry of refractory material and allowed to dry. -Mold heated and held UPSIDE-DOWN to allow melt out of wax. *the wax can be recovered and reused Molten metal then is then poured. TREE- A number of patterns joined to make one mold
11. 2. 6 Investment Casting (lost-wax process) -Mold materials and labor costly -Suitable for high-melting point alloys with good surface finishes and close dimensional tolerances; few or no finishing operations cut costs of casting
Ceramic-Shell Investment Casting -Uses same type of wax pattern. The pattern is dipped into coarsergrained silica to build up coatings and develop proper thickness so pattern can withstand thermal shock due to pouring -rest of process similar to investment casting -Used for precision casting of steels and high-temperature alloys. Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly. (b) Ceramic shell around wax pattern. (c) Wax is melted out and the mold is filled, under a vacuum, with molten superalloy. (d) The cast rotor, produced to net or near-net shape.
Permanent-Mold Casting v. Two halves of a mold are made from materials with high resistance to erosion and thermal fatigue (ex iron, steel, graphite) v. Part weight can range from. 1 kg to 300 kg but typically weigh less than 25 kg v. Medium quality surface finish, porosity, shape complexity v. High dimensional accuracy v. Good to average cost for equipment and labor v. Can produce 5 -50 parts/mold hour v. Typically requires 1000 parts before being used
Permanent-Mold Casting Bronze mold dated about 1400 B. C.
Permanent-Mold Casting Vacuum Casting v Used for thin walled (0. 75 mm) complex shapes with uniform properties ex gas-turbine components from superalloys. Process uses slight vacuum (2/3 atm) also reduces porosity. Metal only about 55 C above liquidus temperature. Automated, and production costs similar to green-sand casting. v CLA – metal melted in air, parts are made at a high volume and low cost v CLV - Metal melted in vacuum, involve reactive metals such as aluminum, titanium, zirconium, and hafnium
Permanent-Mold Casting v v Slush casting – used for small production runs for ornamental and decorative items and toys from low melting point metals (lead, zinc, and tin alloys) Similar operation used in making hollow chocolate shapes. Pressure casting - uses pressurized gas to force metal up into the mold, pressure is maintained until metal has solidified
Permanent-Mold Casting Die Casting v Product weight <0. 01 to 50 kg v Good surface finish, low porosity, capable of high complexity parts v Excellent dimensional accuracy +/- 0. 001 tolerance v Takes several weeks before first product output v Produces 2 -200 parts/mold hour v Requires approximately 10, 000 parts for use
Permanent-Mold Casting Die Casting v Hot chamber process - Uses a piston to trap molten metal and forces it into die cavity thru gooseneck and nozzle. Used for low melting point alloys (zinc, magnesium, tin, and lead) v Cold-chamber process – Same as hot chamber, except chamber not heated, used for high melting point alloys. (aluminum, magnesium, and copper) v Die to part weight is about 1000 to 1 … i. e. a 2 kg part requires a 2000 kg die
Permanent-Mold Casting Die Casting
Permanent-Mold Casting Centrifugal Casting v v Used for hollow cylindrical parts like pipes, gun barrels, posts Due to differences in density, lighter elements collect in center, therefore properties of casting vary through its thickness. Results in forces upward of 150 g for thick - walled parts Can produce parts over 5000 kg, but takes months for first part is produced.
Permanent-Mold Casting Squeeze Casting v v Results in parts made to near-net shape and fine surface detail for both ferrous and non ferrous material. Particularly good for overcoming feeding difficulties that arise from casting metals with a long freezing range.
Permanent-Mold Casting Semi-solid Metal forming v v Working at extremely high temperatures can reduce die life. When the metal enters the die it is a combination of both solid and liquid. As the metal is agitated, it’s viscosity is lowered, this process is called thixoforming. Advantages v Parts are homogenous with high strength v Thin and thick parts are produced v Parts can subsequently be heat treated Disadvantage v Higher costs than die casting
Quality Assurance Inspections v v Controlling each stage during casting is important in maintaining good quality. Nondestructive testing – visual and optical inspection of outer surface for defects, can also involve radiographic testing for vital components. Destructive testing - various specimens are removed from various sections to test for strength. , durability, porosity, and any other defects. Pressure testing - usually performed on cast components such as pumps, valves, and pipes to test for seal leakage.
The melting practice is important because it affects the quality of the castings. When melting your metal stock in a furnace, you would add flux and other slagforming constituents to refine your metal. The type of furnace you use also affects the quality of castings as well.
• Electric arc: found in many foundries for it’s high rate of melting, low pollution and ability to hold the metal for alloying purposes • Induction: (coreless induction): great for mixing characteristics for alloying and adding a new charge of metal or (core or channel furnace): great for superheating, holding and duplexing • Crucible: most commonly used furnace because it can be heated using various fuels such as commercial gases, fuel oil, fossil fuels or electricity. Can be stationary, tilting or movable. • Cupolas: Furnaces that can be operated continuously. They have high melting rates so produce large amounts of metal. They are currently starting to be replaced by induction furnaces because of the high costs involved. • Levitation melting: Involves magnetic suspension of the molten metal where an induction coil simultaneously heats, stirs and confines the melt eliminating the need for a crucible. This leaves the castings free of refractory inclusions and of gas porosity, and gives it a uniform fine grain.
Melting Practice and Furnaces Electric Arc Furnace Induction Furnace
Melting Practice and Furnaces Crucible Furnace Cupolas Furnace
Levitation melting furnace Video clip of an electric arc furnace: http: //www. matter. org. uk/steelmatt er/steelmaking/eaf. htm Multi-melt inductor furnace
Foundries (from the Latin word fundere meaning melting and pouring) are where all casting operations are carried out. Foundries required a lot of manual labor in the past, but at present many of the operations are now automated. Automation varies from foundry to foundry depending on what and how much is needed to be produced. If a foundry requires production in the hundreds of thousands then most work is automated. But, if a foundry only has short production runs then less automation is required. http: //www. grede. com/foundry_terms/foundry_t erms_frameset. shtml good site for terms about foundries Inside of a foundry
This chapter describes general design considerations and guidelines for metal casting, ways of avoiding defects in metal casting, characteristics of alloys commonly cast together, and outlines the economics involved in casting operations.
METAL CASTING (CH 12) Design, Materials, and Economics n Design Considerations n Guidelines for successful casting n Characteristics and Applications of metals n Economic considerations
Design Considerations n n n Corners, angles and section thickness Flat areas Shrinkage
Design Considerations n n Draft Dimensional Tolerances Lettering and Markings Finishing Operations
Casting Alloys Non Ferrous n n n Aluminum Magnesium Copper Zinc Tin Lead
Casting Alloys Ferrous n Cast Irons Largest quantity of metals n Wear resistance, hardness and good machinability n n Cast Steels High cast temperature (1650°C, 3000 °F) n Applied to railroads, mining, chemical plants, oil fields and heavy construction n n Stainless steels n Casting similar to normal steels
Economics of Casting n Cost depends on several factors n n n Materials-materials with higher melting points require expensive dies and machinery Equipment-the more sophisticated the equipment the more the cost Labor-single more costly factor in manufacturing
References n n n http: //en. wikipedia. org/wiki/Image: Bronze_spearhead_mold. J PG http: //www. emachineshop. com/machines-molding/images/diecast-machine. jpg http: //www. diecasting. org/faq/ http: //www. diecasting. org/design/case 2/images/Vacuum. Metal Pour. jpg http: //www. offshoresolutions. com/products/metal/images/casti ngs/centrifugal. Casting. Diagram. jpg http: //www. empirecastings. com/images/Proces 7. gif http: //www. me. gatech. edu/jonathan. colton/me 4210/ccdc 3. gif http: //www. stahl-online. de/images/2 -1 -1_Bild 5_30862. jpg http: //www. inductothermindia. com/photos/furnace. jpg http: //www. nbm-houston. com/images/jpg/holding-furnace. SMALL. jpg http: //www. dueker. de/ENG/bilder/kupol. jpg
References(cont) n http: //www. bhrgroup. co. uk/graphics/pwteeturbulent. jpg http: //i 7. ebayimg. com/04/i/04/b 4/60/35_1_b. JPG http: //www. me. gatech. edu/jonathan. colton/me 4210/mfgvide os. html http: //www. nd. edu/~manufact/index 3. htm http: //www. norcanhydro. com/ images/nor_index 2. jpg n http: //www. kitchen. com/. . . / images/scanpan-fp. jpg n http: //www. greg-j. com/ files/pipes. jpg n http: //www. cs. berkeley. edu/. . . / IMGS/Crankshaft. jpg n http: //www. mech. northwestern. edu n http: //www. thaimetalcasting. com/ Product/product. html n http: //www. eere. energy. gov/. . . /profile. html n n
References(cont) http: //www. ima-eu. org/ en/textfoundry. htm http: //www. tth. com/ Metal%20 casting. htm https: //webmail. unm. edu/Redirect/ifcln 1. ifc. org/ifcext/envi ro. nsf/Attachments. By. Title/gui_foundries_WB/$FILE/fou ndries_PPAH. pdf https: //webmail. unm. edu/Redirect/www. grede. com/foundr y_terms/foundry_terms_frameset. shtml
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