制造工程及技术(热加工)10章

1、CHAPTER 10Fundamentals of Metal Casting Introduction Solidification of Metals Fluid Flow Fluidity of Metals Heat Transfer Defects Casting uses the idea that a liquid metal can take the shape of any vessel containing it. When the metal cools it has taken the shape of its container Casting is one of t

2、he most versatile manufacturing processes Low cost and quick Easier to manufacture Can produce intricate shapes and internal openings Can produce parts in one piece Best suited for composite componentsImportant Considerations Flow of Molten Metal Solidification and Cooling Type of Mold Material Invo

3、lves liquid metal turning back in to solid metal The process is different for Pure metals and alloys Can be divided into two steps:Formation of stable nucleiGrowth of crystalsPure Metals Have a clearly defined melting point Temperature remains constant during freezing Solidifies from the walls of th

4、e mold toward the center of the partGrain Structure for Pure MetalsTwo types of grains are formed for a pure metal Fine equiaxed grains ColumnarRapid cooling at the walls produces fine equiaxed grainsColumnar grains grow opposite of the heat transfer throughout the mold following the chill zoneEquia

5、xed GrainsIf crystals can grow approximately equally in all directions equiaxed grains will grow.Large amounts of under cooling is needed near the wall of the mold.Solidification in alloys begins when the temperature drops below the liquidus TL and is complete when it reaches the solidus, TS.Within

6、the TL and TS Temperature range, the alloy is like a slushy with columnar dendritesSlow cool rates results in course grain structures (102 K/s)Faster cooling rates produce finer grain structures (104 K/s)For even faster cooling rates, the structures are amorphous (106 108 K/s)Grain size influences s

7、trength of a materialSmaller grains have higher ductility and strengthSmaller grains help prevent hot tearing and/or cracks in the castingBasic casting system: Fluid is pored though a pouring basin Flows though the gating system into the mold cavitySchematic of typical riser-gated casting.Fig : Sche

8、matic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification. Sprue is a vertical channel though which the molten metal flows downward in the moldRunners channels that carry the molten metal from the sprue

9、to the mold cavityGate is the portion of the runner though which the molten metal enters the mold cavityRisers serve as reservoirs to supply the molten metal necessary to prevent shrinkage.Principles of fluid flow Bernoullis Theorem ContinuityFlow Characteristics: turbulence is an important consider

10、ation in gating systems.Reynolds Number is used to quantify this aspect 0 Re laminar flow 2000 Re mixture of laminar and turbulent flow Re 20 000 = severe turbulenceTechniques for minimizing turbulence Avoid sudden changes in flow direction Dross or slag can be eliminated by vacuum casting Use of fi

11、lters eliminates turbulent flow in the runner systemFluidity of Molten Metal : The capability of molten metal to fill mold cavities is called fluidity.The following influence fluidity Characteristics of molten metal Viscosity Surface tension Inclusions Solidification pattern of the alloy Casting par

12、ameters Mold design Mold material and its surface characteristics Degree of superheat Rate of pouring Heat transferNote: Castability describes the case with which a metal can be cast to obtain a part with good quality. Fig : A test method for fluidity using a spiral mold. The fluidity index is the l

13、ength of the solidified metal in the spiral passage. The greater the length of the solidified metal, the greater the length of the solidified metal, the greater is its fluidity.Important consideration in casting Heat flow in the system Complex Depends of flow characteristicsSolidification Time A fun

14、ction of the volume of a casting and its surface area Solidification time = C volume 2 surface area Effects on solidification time Mold Geometry Skin thicknessShrinkage causes dimensional changes and, sometimes cracking, is the result of the following: Contraction prior to solidification Contraction

15、 during phase changes Contraction as temperature drops to ambient temperatureFig : Solidified skin on a steel casting. The remaining molten metal is poured out at times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which based on this p

16、rincipleDefectsMetallic projectionsFinsFlashMassive projectionsSwellsRough surfacesCavitiesInternal or external Blow holes Pin holes Shrinkage cavitiesDiscontinuitiesCracksCold or hot tearingCold shuntsFig : Examples of hot tears in castings. These defects occur because the casting cannot shrink fre

17、ely during cooling, owing to constraints in various portions of the molds and cores. Exothermic (heat-producing) compounds may be used (as exothermic padding) to control cooling at critical sections to avoid hot tearing.Slags and other foreign material entrapped in the metal can become inclusions to

18、o.Defective surface(i) Folds Laps Scars (iv) Adhering sand layersOxide scaleIncomplete casting(i) Misruns (ii) Insufficient volume(iii) Runout due to loss of metal from mold(iv) Temperature too low when metal is poured(v) Metal is poured to slowIncorrect Dimensions or ShapeImproper shrinkage allowancePattern mounting error Irregular contraction(iv) Deformed patternWarped castingInclusions form after melting, solidificatio