Ch 5&6.pdf

Study guide Basic process of casting: Make a hole, melt metal and pour it into the hole, solidify, remove and clean. -molten metal flows by gravity or other force into a mold where it solidifies into shape of the mold cavity - casting includes both making ingots and shapes...

Study guide Basic process of casting: Make a hole, melt metal and pour it into the hole, solidify, remove and clean. -molten metal flows by gravity or other force into a mold where it solidifies into shape of the mold cavity - casting includes both making ingots and shapes exactshape Netshape furtherprocesses need no needtobeapplied -there are disadvantages associated with different methods of casting, such as limitations on mechanical properties, porosity, poor dimensional accuracy, surface finish, safety hazards - foundry- a factory equipped for making molds, melting and handling metal in molten form, casting etc; workers called foundrymen Casting: Mold: determines the shape of the the cast part; must be slightly oversized to allow for shrinkage that occurs in metal during solidification and cooling; consists of two parts cope- upper half of mold, drag- bottom half (which are both contained in a box called the flask. open -made of various materials mold liquidmetalispoureduntilits fillsopencavity T.is tf P s g8eway Metal is first heated to temp to transform to liquid, poured in cavity of mold (open mold) or into a gating system wihich permits the metal to flow from outside the mold into the cavity As soon as the metal enters the mold, it begins to cool and start solidification. Two main casting processes: expendable-mold: mold in which metal solidifies must be destroyed; permanent mold: one that can be used over and over to produce many castings expendable madeoutofsand plasteretc permenanttypicallymadeoutofmetal -Solidification: Nucleation phenomena (growth of single crystal/grain). Diecasting isan stagewheretinyclustersofnucleiformwithinliquidor solution initial acting effemastating -Crystal nucleation is the process that begins in a liquid or solution phaseasand leads to the poinffff formation of molecular proto-aggregates (nuclei, embryos) that may then evolve into macroscopic crystals by the further process of crystal growth -Solidification involves the transformation of the molten metal back into solid state. It differs depending on if the metal is a pure element or an alloy. Pure metal solidifies at a constant temp equal to its freezing point, which is also the melting point -Solidification: Chill zone, columnar zone, (pure metals) equiaxed zone (alloys). Equiaxed structures (uniform nucleation). Chill zone: a thin skin of solid metal that forms at the interface immediately after pouring Columnar zone(pure metals): a region where the grain structure is elongated in the direction of the solidification equiaxed zone (alloys): where grains are equal in all direction, form when cooling is more uniform and less directional Equiaxed structure (uniform nucleation): crystal grains that have the same size and shape in all directions LOOK A Bottom FOR EQUIAXED STUFF -Solidification: Dendrite growth and compositionà “mushy” zone due to gap from Ts and Tl. Coherence. Dendrite growth: grain formation that occur in direction away from heat transfer; grows inwardly as needles of solid metal; happens in pure metals and alloys Mushy zone: solid liquid region that has a soft consistency; the temperature difference between the liquidus (Ti- the temperature where freezing Begins) and the solidus (Ts- the temperature where freezing is completed) that causes the nature of dendritic growth is such that an advancing zone is formed in which both liquid and solid metal exist. -Solidification: Short vs. long freezing range. (Freezing is from liquid to solid Feeding. Short: metals with short freezing range solidify over a narrow temperature span, good for controlling casting, pure metals tend to have this Long: solidify over a broader temperature span, different parts of alloys solidify at different temperatures, shrinkage can also occur -Solidification: Solidification speed &Grain size & Properties (note on grain refinement). -Solidification: Microsegregation, macrosegregation (example graphite). Micro: non-uniform distribution of solute elements on a very small scale within the solid; different elements in the alloys don’t mix properly, causing elements to be more concentrated than others; can cause parts of the metal to be stronger or weaker than others Macro: occurs on a much larger scale than micro; can lead to big differences in material properties -Solidification: Gas solubility and precipitation. When metals start to solidify, solubility of gases decreases, which means the metal can no longer hold as much gas in the solution, so it tends tends to form gas bubbles within the metal, which creates holes Precipitation: solid particles mature during solidification, can make strengthen the materials -Solidification: Time and Chvorinov’s rule. Local solidification time: the metal’s latent heat of fusion is released into the mold Total solidification time: is the time between pouring and complete solidification -Solidification: Dimensional changes upon L-L, L-S, S-S shrinkage. -Shrinkage porosity: Sources and remedies. Directional solidification, end effect, chills and risers. Sources and remedies: occurs in all metals because the solid phase has a higher density than the liquid phase. Can remedies by the use of risers in sand casting and use of molten metal at low pressure for die casting Directional solidification: regions of the casting that is most distant from the Liquid Metal supply to freeze first and for solidification to progress form these remote regions toward the risers. In this way, molten metal will continually be available from the risers to prevent shrinkage voids during freezing How are visers filled -achieved by observing Chvorinov’s rule: by locating sections of the casting with lower V/A - feed from thick to thin ratios away from the riser, freezing will occur first in these regions and supply of Liquid Metal for the rest of casting will remain open until bulkier sections solidify End Effect: allows the casting to solidify faster at the end increasing the distance form the end that will be sound free from shrinkage Chills: adding a metallic block at the end of the casting allows it to extract more heat from the end, increasing the end effect. They help reduce shrinkage by promoting more uniform cooling Risers: reservoir of molten metal that supplies extra metal to fill in the shrinking areas -Solidification: Feeding: Riser rules: Solidify after casting (size and modulus and feeding aids), enough metal to feed, head pressure, open feeding path. Risers must be the last part to solidify (by having modulus or aids) must have enough Liquid Metal to feed casting shrinkage must be placed on the heavy isolated sections (hot spots- the last volume to solidify) must have open path to the hot spot must have positive pressure to feed casting (must be higher than casting or pressurized) -Gating: -Function to fill mold cavity with the cleanest metal possible. molten metal flows into cavity from outside the mold -Gating system components: Pouring cup/basin: minimizes splash and turbulence sprue: how metal enters the runner runner: leads into main cavity gate: controls the flow molt,en metal from the runner to the mold cavity mold cavity: hole shaped to the desired part Other: risers: reservoir in the mold that serves as a source of Liquid Metal for the casting to compensate for shrinkage during solidification cores: create internal cavities or complex features that cannot be achieved through the mold alone -Gating: Bernoulli’s theorem: h+p/rho*g+v^2/2g=constant. -used to calculate flow of a liquid metal through the gating system and into the mold -h= head (cm), p= pressure on the liquid (N/cm^2), rho= density (g/cm^3), v= flow velocity (cm/ s), g= gravitational constant (cm/s^2); the equation ignore friction losses -Law of continuity: Q=v*AàNote aspiration in sprue and tapered sprues. (h=1/2gt^2) Q= volumetric flow rate, v= velocity, A= cross sectional area of liquid states that the volume rate of flow remains constant throughout the liquid Tapered vs Straigth Sprue: if a straight sprue is used, negative pressure at the bottom of the sprue will cause the aspiration (enters airway) in the into the metal stream contaminating it, to avoid  helpsvolumeflowratetobeequal fair this tapered sprues, which create positive pressure, are used taperedsprue entering mold I attop bottomofsprue -Turbulence. What is it. Problems. Solutions. Chokes. -Turbulence: chaotic and irregular flow of molten metal, can come in contact with oxygen and form oxides and films, or lead to formations of frozen pellets of metal; can all creating casting problems -Best practice is to fill the mold smoothly -Other solutions: low height pour, pouring basins, tapered sprue, bottom well/sprue well, properly sized system, filters I Chokes: narrow sections or constrictions in the gating system mold that restrict flow of molten metal as it enters the mold cavity -Fluidity of molten metal (“Fillability”): Temperature and superheat affect viscosity of liquid and amount of heat to be removed, mold design-V/SA ratios and, mold material (sand vs. metal) affect heat transfer rates, pouring rate. Spiral test. Pouring temperature: The fluidity of molten metal increases with the pouring temperature. Mold material: The conductivity of the mold material can affect the fluidity of molten metal. A mold with high conductivity will extract heat from the liquid metal faster, which can reduce the time the liquid is mobile and lower its fluidity The fluidity of molten metal decreases as its viscosity increases Spiral Test- designed to quantify how far the metal will travel prior to solidifying. A spiral mold with uniform cross section is made, metal is poured and the distance the metal has traveled is measured -Defects: Hot tear, mold erosion, misrun/cold shut (nonfill), scab (cope surface pops), incorrect dimensions. Hot tear: occurs when casting is restrained from contraction by unyielding mold during the final stages of solidification or eatery stages of cooling Mold erosion: sharp transitions that cause high and low pressure areas, mold erosion can occur, sand particles end up as defects in the casting Misruns: castings that solidify before completely filling in the mold cavity Scabs (cope surface pops): rough areas on the surface of the casting due to encrustations of sand and metal, caused by portions of the mold surface flaking off during solidification and becoming embedded in the casting surface Incorrect dimensions: can lead to parts not fitting or functioning as expected Physical Change: material is melted and then solidified in the desired shape such as casting Consolidation: processes where the starting material is a fine powder where the particles are bonded together such as powder metal Deformation: are where forces are applied to a material to get to change shape under pressure such as forging Material removal: where material is progressively taken off from a larger piece of material such as machining Property change only: processes that only change the properties but not the geometry such as heat treatment Classification also possible by material METAL CASTING PROCESSES- CHAPTER 6 -Process families. -Flowchart sand casting process. Sandrastingmost widely usedcastingprocess expendablemoldisoanew moldmustbemadeeachtime mold container for t he -Sand casting: Basic mold structure. Flask, cope and drag, and the rest previously seen. -pouring molten metal into a sand mold, allowing the metal to solidify, and then breaking up the mold to remove casting than.MYtgeacsfor typicallybigger -cavity in the sand mold is formed by packing sand around a pattern (duplicate of the part of be cast) and then removing the pattern by spreading the mold into two halves. The mold contains gating and a rising system. If the casting is to have internal surfaces, a core must be included in the mold Flask: a container that holds both patterns for the cope and the drag Cope and drag: spilt pattern halves are attached to separate plate, so that the two sections of the mold can be fabricated independently. Cope is top and drag is the bottom half icimilarto matchplate -Green sand: Sand, clay and water.; most widely used mold type, packed down together, said to be green because of its moisture content aposses goodstrength collapsbilitypermeabilityreusability -Chemically bonded sand (CBS): Sand and chemical binder (adhesive) leastexpensive Stronger than green sand and used to make cores and sometimes molds Types: liquid resin + liquid catalyst mixed with sand and they harden as the react, liquid resin + gas catalyst, resin premixed with sand typically sand glued together -Patterns for sand process: Materials; one piece, split, matchplates; draft; shrink rule (contraction allowance); core prints and core boxes. -Materials used to make patterns: wood, plastics, and metal -One piece: solid pattern same geometry as the casting, adjusted in size for shrinkage and machining -Split: consists of two pieces, dividing the part along a plane coinciding with the parting line of the mold  match plates: two pieces of the split pattern are attached to opposites sides of a wood or metal plate Core prints/boxes: specific feature in the mold used to support and position cores within the mold cavity surfaces intothemoldprior to pouringTypicallymadeout Core full scale model ofinterior inserted Shrink rule: ofsandcompacted M.irn g iEm n gtr -Sand molding equipment: Squeeze, slingers, impact (pneumatic), vertical flaskless. Squeeze: squeezingthesand by aroundapattern pneumaticpressure Slingers: sand grains are impacted against the pattern at a high speed Impact pneumatic: squeezing the sand around the pattern by air pressure Vertical flask less: green molding sand is blown into the mold and the sand is compressed by having two match plates squeeze the sand into a block with pressure on both sides the pattern is then removed from the sand block and cores are added. The next block is closed and compete the mold. Note that each sand block contains one half of two molds, so each cycle produces a full mold. This is the most productive method of sand casting. flask in amechanizedsystemofmoldproductionEachsandmondisproduced using Ye fiftieth -Shell mold: Process and pattern description. Process in which the mold is a thin shell made of sand held together by thermosetting resin binder typicallymatchplate The shell sand process uses a precoated sand with a chemical binder. A preheated pattern made of metal (a) is covered with sand (b). As the heat from the pattern permeates into the sand, the binder in the sand next to the pattern heats up, activates and binds together. Then the pattern is flipped again and the unbonded sand falls off from the pattern (c). Then the pattern and mold will be placed in an oven (not shown) to finish off the binder reaction. Then the mold, which resembles a shell, is ejected from the pattern (d), glued to another half (e) and prepared for pouring. Not all shell knolds are buried in a flask. -Lost foamhave better process: surfacefinishthen Process, patterns. green sand goodtolerances In lost foam casting foam patterns of the final shape are produced and assembled into a cluster. This cluster is then dipped into a ceramic slurry which contains a binder, water and finely ground ceramic, such that a thin layer is formed on the surface of the parts. The cluster is then removed and the slurry allowed to dry. Then the cluster is placed in a flask and unbonded sand is placed around it and compacted with vibration. Metal is then poured into the pattern, which causes the foam to vaporize and the metal takes its place. Then the flask is dumped and the castings retrieved and finished. Expanded polystyrene casting process: (1) pattern of polystyrene is coated with refractory compound; (2) foam pattern is placed in mold box, and sand is compacted around the pattern; and (3) molten metal is poured into the portion of the pattern that forms the pouring cup and sprue. As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus allowing the resulting mold cavity to be filled newpatternis needed  I g -Investment casting: Process. The investment casting process begins with a wax injection die. Here the liquid or semisolid wax is injected under pressure (a). Once cooled and solidified, the pattern is ejected from the die (b). Then several patterns will be attached to a cluster called a tree (c). This tree assembly will provide the gating and risering system to the castings. Once the tree is assembled it is dipped into a ceramic slurry composed of water, binder and ceramic flours (d). Immediately, the tree is coated with a ceramic material called stucco which is similar in consistency to sand (e). Then the assembly is alled to dry so the coating develops strength. This slurry/stucco coating is repeated several times (typically 6 to 12 cycles depending on the part). Once the mold is completed (f) the wax is melted out (g), the mold is preheated to about 2/3rds the melting temperature of the metal, and then it is poured (h). Having the mold preheat strengthens it and also keeps the metal liquid longer so thinner sections can be cast. Lastly the mold is removed (i) and castings cut off and cleaned j. Note that the wax pattern is an exact replica of the casting, just slightly oversized. capableofmakingcastingsofhighaccuracy intricate i finish waxcanberecovered goodsurface for -Permanent mold casting reuse In permanent mold processes the main advantage is that the mold is not destroyed by the process so it can be reused many times. The permanent molds are mostly made of steel or cast iron and are preheated prior to casting. Mold coatings are also used in order to minimize mold damage and to slow down the cooling rate to prevent premature freezing of the casting. One problem is that with multiple thermal cycles, the mold expands and contracts and develops fatigue cracks. The metals that can be poured into permanent molds are largely limited to low temperature metals such as aluminum and zinc. Copper based alloys can be done with limitations. Since the metallic mold cools down the metal faster, it produces fine grains in the metal which can lead to improved mechanical properties. also hasmorerapidsolidificationcaused themetalmold i -Die casting: Hot/cold chamber by Cold: In cold chamber die casting the permanent mold is mounted on two platens that open and close it. The liquid metal is poured into a pipe called the shot sleeve, then the plunger rod pushes the metal under high pressure into the mold cavity. Once the mold is full, the metal solidifies, the mold opens and an ejection system pushes the casting out. The mold then closes and the cycle repeats. Because of the automated nature of die casting, the molds tend to be substantially more expensive than permanent molds. However, because of filling under pressure it is possible to produce thinner and more complex castings than permanent mold. Hot: In hot chamber die casting, the plunging mechanism is incorporated into the melting furnace. The tooling and cycle are similar to the cold chamber process however, instead of pouring the metal into the injection mechanism, this is immersed in the molten metal bath. Thus, the shot sleeve fills automatically with liquid metal, and when ready it is pushed into the mold through a special conduit called a gooseneck. Once the metal is injected into the mold, it cools and solidifies, the mold opens, and the casting is ejected. The main advantage of hot chamber die casting is that a separate furnace is not needed and extra metal handling is avoided. Hot chamber is limited to lower temperature melting metals than aluminum such as zinc. Thus, all aluminum die casting is done in the cold chamber process and zinc die casting in the hot chamber process. -Centrifugal casting. The centrifugal casting process uses permanent molds in order to produce radial components such as pipes. The mold is placed on top of two rollers and begins to spin. Once the mold is spinning at the proper rpm molten metal is poured into the mold through a special spout. The spinning mold causes the metal to "coat" the interior of the mold and it is maintained spinning until the metal solidifies. Once the metal has solidified, the mold is stopped, the casting pulled out, and the process repeated. The reason the casting can be pulled out from a straight molds is that the mold expands with the heat of the molten metal and the casting contracts as it cools down, creating a gap between casting and mold. For more complex shapes sand molds or assembled molds can be used, but they must be properly rotationally balanced -Squeeze casting. Sequence of operations in the squeeze-casting process. The metal is melted, then it is poured into an open die (metallic mold). Then the die is closed with pressure and the pressure is maintained during the solidification process. This minimizes the formation of microshrinkage and also produces very fine grains. This in turn produces parts with excellent mechanical properties. In a sense this process combines the advantages of casting and forging. The tooling is relatively expensive as it needs to be built to withstand very high forces and it is limited to much simpler shapes than other casting processes. -Vacuum/pressure assisted processes. vacuumisused todrawthemoltenmetalintothe i In the V-process, sand with no binder is used. Rather the sand is held together pretty much like a vacuum packed package of coffee holds its shape. The process begins with a pattern (1). Then, a film of plastic is heated above it (2), then stretched on top of it (3). The pattern has many tiny holes so a vacuum can pull the soft heated plastic on top of it. Then a special flask is placed on top of the pattern (4), filled with sand (5), and another piece of plastic is placed on the top and a vacuum through the sand is used to pull both pieces of plastic together holding the sand in place (6). Then the mold is stripped from the pattern (7), joined with the other half of the mold (8), and poured (9). The main advantage of the process is the lack of a binder which reduces fumes and gasses and related defects. One disadvantage is that it is necessary to maintain a vacuum on the mold at all times and if the vacuum fails, then the mold will collapse and be lost. This method is usually used for castings in the hundreds of pounds. -Directional solidification: equiaxed- equal dimensions, directional solidification, single crystal.: -Molten metal practices: Melting: Fuel, arc (steel), induction. -Molten metal practices: Alloying, inoculation, degassing, fluxing. Alloying: metals that make a alloy Inoculation: fast cooling of the cast iron production to enhance its qualities fluxing: go from solid to liquid state degassing: removal of dissolved gases from liquids -Design issues: -Directional solidification/hot spots: sections should be uniform to avoid shrinkage cavities -Patternmakers shrinkage: occurs after the solidification process when the casting is cooled to room temperature. This phenomenon is due to thermal contraction. A shrinkage allowance must therefore be factored into the design at the start of the process. -Parting line considerations -where the mold splits into two halves creative parting lines eliminates the use of cores and improve design -flat and one sided parting lines are the best for ease of manufacture elimination of undercuts eliminates the need for a core eliminate use of a crew and can improve performance - avoiding uniform sections, undercuts, square corners, and isolated heavy sections is desirable -Draft- a taper that prevents the pattern from dragging on the surface of the mold damaging it ithe amount ofthickness thatis reduced -Core issues: cores should be used discriminately to add value through internal features, party consolidation, or more component function -Dimensional Tolerances: there are significant differences -Machining allowance: additional material left on the casting for where machining surfaces is necessary -Residual stresses: the internal stress distribution locked into a material. These stresses are present even after all external loading forces have been removed -Economics: Tooling cost (including ancillary equipment), process cost (including labor), production rates. columnar fixed