1IE 337: Materials & Manufacturing Processes IE 337 Lecture 4: Metal Casting 1IE 337: Materials & Manufacturing ProcessesLectures 7&8: Introduction to Metal CastingChapters 10 & 6S.V. Atre
2Assignment HW 2 due today CH 21,22 and 24 Problems In Assignments folder
3This Time Basic Process Casting Terminology Heating Pouring Phase diagramsSolidification
4Common process attributes: CastingCommon process attributes:Flow of Molten Liquid Requires HeatingHeat Transfer of Liquid in Mold Cavity During and After PouringSolidification into Component
5Metal Casting Processes Categories Expendable mold processes - mold is sacrificed to remove partAdvantage: more complex shapes possibleDisadvantage: production rates often limited by time to make mold rather than casting itselfPermanent mold processes - mold is made of metal and can be used to make many castingsAdvantage: higher production ratesDisadvantage: geometries limited by need to open mold
6Overview of Sand Casting Most widely used casting process, accounting for a significant majority of total tonnage castNearly all alloys can be sand cast, including metals with high melting temperatures, such as steel, nickel, and titaniumParts ranging in size from small to very largeProduction quantities from one to millions
8Gating SystemChannel through which molten metal flows into cavity from outside of moldConsists of a downsprue, through which metal enters a runner leading to the main cavityAt top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprue
9RiserReservoir in the mold which is a source of liquid metal to compensate for shrinkage during solidificationThe riser must be designed to freeze after the main casting in order to satisfy its function
10Heating the MetalHeating furnaces are used to heat the metal to molten temperature sufficient for castingThe heat required is the sum of:Heat to raise temperature to melting pointHeat of fusion to convert from solid to liquidHeat to raise molten metal to desired temperature for pouring
11Heating the Metal Heat to melting point: rVCs(Tm-To) Heat of fusion to convert from solid to liquid: rVHfHeat molten metal to pouring temp.: rVCl(Tp-Tm)r = density, V = volume, Cs = specific heat,Hf = latent heat of fusion, T = temperatureProperties vary with temperature and phaseMelting may occur over a temperature rangeHeat loss to the ambient
12Pouring the Molten Metal For this step to be successful, metal must flow into all regions of the mold, most importantly the main cavity, before solidifyingFactors that determine success:Pouring temperaturePouring rateTurbulence
14Pouring Calculations 2 Volumetric flow rate remains constant Q =v1A1 = v2A2Q: volume rate of flow cm3/sv: velocity of the liquid metalA: cross-sectional area, cm2CONTINUITY EQUATIONIncrease in area results in decrease in velocity
15Pouring Calculations 3Flow of liquid metal through gating system & moldv = (2gh)1/2v: velocity of the liquid metal at base of sprueg = gravitational acceleration, 981 cm/s2h = height of sprue, cm
16Example 1The downsprue leading into the runner of a certain mold has a length = 175 mm.The cross-sectional area at the base of the sprue is 400 mm2.The mold cavity has a volume = m3.Determine: (a) the velocity of the molten metal flowing through the base of the downsprue, (b) the volumetric flow rate, and (c) the time required to fill the mold cavity.
17Example 1: Solution (a) Velocity v = (2gh)0.5 = (2 x 9810 x 175)0.5 = 1853 mm/s(b) Volume flow rate Q = vA= 1853 x 400= 741,200 mm3/s(c) Time to fill cavity MFT = V/Q
18Solidification of Pure Metals A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point)
19Solidification and Cooling Figure 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).
20Microstructure: ICharacteristic 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
21Solidification of Alloys Most alloys freeze over a temperature range rather than at a single temperature
22Microstructure: II Micro-segregation Characteristic grain structure in an alloy casting, showing segregation of alloying components in center of casting
23Microstructure: II Macro-segregation Characteristic grain structure in an alloy casting, showing segregation of alloying components in center of casting
24Phase DiagramsA graphical means of representing the phases of a metal alloy system as a function of composition and temperatureA phase diagram for an alloy system consisting of two elements at atmospheric pressure is called a binary phase diagramOther forms of phase diagrams are discussed in texts on metallurgy and materials science
25Phase DiagramsComposition is plotted on the horizontal axis and temperature on the vertical axisAny point in the diagram indicates the overall composition and the phase or phases present at the given temperature under equilibrium conditions
26Copper-Nickel Phase Diagram Figure 6.2 Phase diagram for the copper‑nickel alloy system.
27Copper‑Nickel (Cu-Ni) Alloy System Solid solution alloy throughout entire range of compositions below the solidusNo intermediate solid phases in this alloy systemHowever, there is a mixture of phases (solid + liquid) in the region bounded by the solidus and liquidus
28Chemical Compositions of Phases The overall composition of the alloy is given by its position along the horizontal axisHowever, the compositions of liquid and solid phases are not the sameThese compositions can be found by drawing a horizontal line at the temperature of interestWhere the line intersects the solidus and liquidus indicates the compositions of solid and liquid phases, respectively
29ExampleDetermine compositions of liquid and solid phases in the Cu-Ni system at an aggregate composition of 50% nickel and a temperature of 1316oC (2400oF)
30Inverse Lever Rule – Step 1 The phase diagram can be used to determine the amounts of each phase present at a given temperatureUsing the horizontal line that indicates overall composition at a given temperature, measure the distances between the aggregate composition and the intersection points with the liquidus and solidus, identifying the distances as CL and CS, respectively
31Inverse Lever Rule – Step 2 The proportion of liquid phase present is given byL phase proportion =And the proportion of solid phase present is given byS phase proportion =
32Applications of the Inverse Lever Rule Methods for determining chemical composition of phases and amounts of each phase are applicable to the solid region of the phase diagram as well as the liquidus‑solidus regionWherever there are regions in which two phases are present, these methods can be utilizedWhen only one phase is present, the composition of the phase is its aggregate composition under equilibrium conditions, and the inverse lever rule does not apply
33Tin-Lead Phase Diagram Figure 6.3 Phase diagram for the tin‑lead alloy system.
34Effect of Solidification Rate Mechanical properties of 2 identical composition samples with different cooling rates
35Solidification Time Solidification takes time Total solidification time TST = time required for casting to solidify after pouringTST depends on size and shape of casting by relationship known as Chvorinov's Rule
36Chvorinov's Rule where TST = total solidification time; V = volume of the casting;A = surface area of casting;n = exponent usually taken to have a value = 2; andCm is mold constant
37Mold Constant in Chvorinov's Rule Cm depends on mold material, thermal properties of casting metal, and pouring temperature relative to melting pointValue 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
38Example 1In casting experiments performed using a titanium alloy and a zircon sand mold, it took s for a cube-shaped casting to solidify. The cube was 50 mm on a side.If the same alloy and mold type were used, find the total solidification time for a cylindrical casting in which the diameter = 30 mm and length = 50 mm.
39Example 1: Solution Cube Volume V Cylinder Volume V Cube Area A = (50)3= 125,000 mm3Cube Area A= 6 x (50)2= 15,000 mm2Cube (V/A)= 125,000/15,000= 8.33 mmCm = TST/(V/A)2= 155/(8.33)2= 2.23 s/mm2Cylinder Volume V= pD2L/4 = p(30)2(50)/4= 35,343 mm3Cylinder Area A= 2pD2/4 + pDL= p(30)2/2 + p(30)(50)= 6126 mm2Cylinder (V/A)= 35,343/6126= 5.77 mmTST = Cm(V/A)2= 2.23 (5.77)2 = 74.3 s
40What 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 ratioSince riser and casting mold constants will be equal, design the riser to have a larger volume‑to‑area ratio so that the main casting solidifies first
41Shrinkage in Solidification and Cooling Figure 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).
42Shrinkage in Solidification and Cooling Figure (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).
43Solidification Shrinkage Occurs in nearly all metals because the solid phase has a higher density than the liquid phaseThus, solidification causes a reduction in volume per unit weight of metalException: cast iron with high C contentGraphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change
44Shrinkage AllowancePatternmakers account for solidification shrinkage and thermal contraction by making mold cavity oversized (See Table 10.1)Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance
45IE 337 Lecture 4: Metal Casting 1 Example 2A mold cavity has the shape of a cube, 100 mm on a side. Determine the volume and dimensions of the final cube after cooling to room temperature if the cast metal is copper.Assume that the mold is full at the start of solidification and that shrinkage occurs uniformly in all directions.For copper, solidification shrinkage is 4.9%, solid contraction during cooling is 7.5%.S.V. Atre
46Example 2: Solution Volume of cavity V = (100)3 = 106 mm3 Volume of casting V= 106( )( )= 879,675 mm3Dimension on each side of cube= (879,675)0.333= mm
47You should have learned Basic ProcessSand Casting TerminologyHeatingPouringPhase diagramsSolidification