Published on February 16, 2014
Lecture 3 Metal Casting
TABLE 11.1 Process Advantages Limitations Sand Some finishing required; somewhat coarse finish; wide tolerances. Shell mold Good dimensional accuracy and surface finish; high production rate. Part size limited; expensive patterns and equipment required. Expendable pattern Summary of Casting Processes Almost any metal cast; no limit to size, shape or weight; low tooling cost. Most metals cast with no limit to size; complex shapes Patterns have low strength and can be costly for low quantities Plaster mold Intricate shapes; good dimensional accu- racy and finish; low porosity. Limited to nonferrous metals; limited size and volume of production; mold making time relatively long. Ceramic mold Intricate shapes; close tolerance parts; good surface finish. Limited size. Investment Intricate shapes; excellent surface finish and accuracy; almost any metal cast. Part size limited; expensive patterns, molds, and labor. Permanent mold Good surface finish and dimensional accuracy; low porosity; high production rate. High mold cost; limited shape and intricacy; not suitable for high-melting-point metals. Die Excellent dimensional accuracy and surface finish; high production rate. Die cost is high; part size limited; usually limited to nonferrous metals; long lead time. Centrifugal Large cylindrical parts with good quality; high production rate. Equipment is expensive; part shape limited.
Basic Features Pattern and Mould ◦ A pattern is made of wood or metal, is a replica of the final product and is used for preparing mould cavity ◦ Mould cavity which contains molten metal is essentially a negative of the final product ◦ Mould material should posses refractory characteristics and with stand the pouring temperature ◦ When the mold is used for single casting, it made of sand and known as expendable mold ◦ When the mold is used repeatedly for number of castings and is made of metal or graphite are called permanent mould ◦ For making holes or hollow cavities inside a casting, cores made of either sand or metal are used.
Melting and Pouring ◦ Several types of furnaces are available for melting metals and their selection depends on the type of metal, the maximum temperature required and the rate and the mode of molten metal delivery. ◦ Before pouring provisions are made for the escape of dissolved gases. The gating system should be designed to minimize the turbulent flow and erosion of mould cavity.The other important factors are the pouring temperature and the pouring rate.
Solidification and Cooling ◦ The properties of the casting significantly depends on the solidification time cooing rate. ◦ Shrinkage of casting, during cooling of solidified metal should not be restrained by the mould material, otherwise internal stresses may develop and form cracks in casting. ◦ Proper care should be taken at the design stage of casting so that shrinkage can occur without casting defects.
Removal, Cleaning, Finishing and Inspection ◦ After the casting is removed from the mould it is thoroughly cleaned and the excess material usually along the parting line and the place where the molten metal was poured, is removed using a potable grinder. ◦ White light inspection, pressure test, magnetic particle inspection, radiographic test, ultrasonic inspection etc. are used
Classification of casting processes
Open and Closed Mould
Sand Casting (Expandable-mould, Permanentpattern Casting)
Use of chaplets to avoid shifting of cores Possible chaplet design and casting with core
Production steps in sand casting including pattern making and mold making
Patterns Variety of patters are used in casting and the choice depends on the configuration of casting and number of casting required ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ Single-piece pattern Split pattern Follow board pattern Cope and drag pattern Match plate pattern Loose-piece pattern Sweep pattern Skeleton pattern
(a)Split pattern (b) Follow-board (c) Match Plate (d) Loose-piece (e) Sweep (f) Skeleton pattern
Pattern allowances Shrinkage allowance Draft allowance Machining allowance Distortion allowance
Moulding Materials Major part of Moulding material in sand casting are 1. 2. 3. 70-85% silica sand (SiO2) 10-12% bonding material e.g., clay cereal etc. 3-6% water Requirements of molding sand are: (a) (b) (c) (d) Refractoriness Cohesiveness Permeability Collapsibility The performance of mould depends on following factors: (a) Permeability (b) Green strength (c) Dry strength
Effect of moisture, grain size and shape on mould quality
Melting and Pouring The quality of casting depends on the method of melting. The melting technique should provide molten metal at required temperature, but should also provide the material of good quality and in the required quantity. Pouring vessels
Molten metal is prevented from oxidation by covering the molten metal with fluxes or by carrying out melting and pouring in vacuum Ladles which pour the molten metal from beneath the surface are used The two main consideration during pouring are the temperature and pouring rate Fluidity of molten metal is more at higher temperature but it results into more amount of dissolved gases and high temperature also damage the mould walls and results into poor surface quality of the casting To control the amount of dissolved gases low, the temperature should not be in superheated range In ferrous metals, the dissolved hydrogen and nitrogen are removed by passing CO. In non-ferrous metals, Cl, He, or Ar gases are used. Therefore, fluidity and gas solubility are two conflicting requirements. The optimum pouring temp. is therefore decided on the basis of fluidity requirements. The temp. should be able to fill the whole cavity at the same time it should enter inside the voids between the sand particles.
Cooling rate depends on casting material and configuration. It also depends on volume and surface area of the casting also. The pouring rate should be such that solidification does not start and the cavity is completely filled without eroding mould surface and undue turbulence.
The Gating System 1. 2. 3. 4. Minimize turbulent flow so that absorption of gases, oxidation of metal and erosion of mould surfaces are less Regulate the entry of molten metal into the mould cavity Ensure complete filling of mould cavity, and Promote a temperature gradient within the casting so that all sections irrespective of size and shape could solidify properly
The Gating System A: pouring basin B: Weir C: Sprue D: Sprue well E: Runner F: Ingates G: Runner break up H: Blind J: Riser
Use of chills
Cooling and Solidification Pure metal Alloy
Mechanism of Solidification Pure metals solidifies at a constant temp. equal to its freezing point, which same as its melting point. The change form liquid to solid does not occur all at once. The process of solidification starts with nucleation, the formation of stable solid particles within the liquid metal. Nuclei of solid phase, generally a few hundred atom in size, start appearing at a temperature below the freezing temperature. The temp. around this goes down and is called supercooling or undercooling. In pure metals supercooling is around 20% of the freezing temp. A nuclease, more than a certain critical size grows, and causes solidification.
By adding, certain foreign materials (nucleating agents) the undercooling temp. is reduced which causes enhanced nucleation. In case of pure metals fine equi-axed grains are formed near the wall of the mold and columnar grain growth takes place upto the centre of the ingot. In typical solid-solution alloy, the columnar grains do not extend upto the center of casting but are interrupted by an inner zone of equiaxed graines. My adding typical nucleating agents like sodium, magnesium or bismuth the inner zone of equiaxed grained can be extended in whole casting.
Solidification Time Once the material cools down to freezing temperature, the solidification process for the pure metals does not require a decrease in temperature and a plateau is obtained in the cooling curves, called thermal arrest. The solidification time is total time required for the liquid metal to solidify. Solidification time has been found to be directly proportional to volume and inversely proportional to surface area.
Location of Risers and Open and Closed Risers •Top riser has the advantage of additional pressure head and smaller feeding distance over the side riser. •Blind risers are generally bigger in size because of additional area of heat conduction.
Why Riser? The shrinkage occurs in three stages, 1. When temperature of liquid metal drops from pouring to zero temperature 2. When the metal changes from liquid to solid state, and 3. When the temperature of solid phase drops from freezing to room temperature The shrinkage for stage 3 is compensated by providing shrinkage allowance on pattern, while the shrinkage during stages 1 and 2 are compensated by providing risers. The riser should solidify in the last otherwise liquid metal will start flowing from casting to riser. It should promote directional solidification. The shape, size and location of the risers are important considerations in casting design
Cleaning and Finishing 1. 2. 3. 4. 5. Casting is taken out of the mould by shaking and the Moulding sand is recycled often with suitable additions. The remaining sand, some of which may be embedded in the casting, is removed by means of Shot blasting. The excess material in the form of sprue, runners, gates etc., along with the flashes formed due to flow of molten metal into the gaps is broken manuaaly in case of brittle casting or removed by sawing and grinding in case of ductile grinding. The entire casting is then cleaned by either shot blasting or chemical pickling. Sometimes castings are heat treated to achieve better mechanical properties.
Casting Defects Defects may occur due to one or more of the following reasons: ◦ ◦ ◦ ◦ ◦ ◦ Fault in design of casting pattern Fault in design on mold and core Fault in design of gating system and riser Improper choice of moulding sand Improper metal composition Inadequate melting temperature and rate of pouring
Classification of casting defects Surface Defect Blow Scar Blister Drop Scab Penetration Buckle Casting defects Internal Defect Blow holes Porosity Pin holes Inclusions Dross Visible defects Wash Rat tail Swell Misrun Cold shut Hot tear Shrinkage/Shift
Surface Defects These are due to poor design and quality of sand molds and general cause is poor ramming Blow is relatively large cavity produced by gases which displace molten metal from convex surface. Scar is shallow blow generally occurring on a flat surface. A scar covered with a thin layer of metal is called blister. These are due to improper permeability or venting. Sometimes excessive gas forming constituents in moulding
Drop is an irregularly-shaped projection on the cope surface caused by dropping of sand. A scab when an up heaved sand gets separated from the mould surface and the molten metal flows between the displaced sand and the mold. Penetration occurs when the molten metal flows between the sand particles in the mould. These defects are due to inadequate strength of the mold and high temperature of the molten metal adds on it. Buckle is a vee-shaped depression on the surface of a flat casting caused by expansion of a thin layer of sand at the mould face. A proper amount of volatile additives in moulding material could eliminate this defect by providing room for expansion.
Internal Defects The internal defects found in the castings are mainly due to trapped gases and dirty metal. Gases get trapped due to hard ramming or improper venting. These defects also occur when excessive moisture or excessive gas forming materials are used for mould making. Blow holes are large spherical shaped gas bubbles, while porosity indicates a large number of uniformly distributed tiny holes. Pin holes are tiny blow holes appearing just below the casting surface. Inclusions are the non-metallic particles in the metal matrix, Lighter impurities appearing the casting surface are dross.
Insufficient mould strength, insufficient metal, low pouring temperature, and bad design of casting are some of the common causes. Wash is a low projection near the gate caused by erosion of sand by the flowing metal. Rat tail is a long, shallow, angular depression caused by expansion of the sand. Swell is the deformation of vertical mould surface due to hydrostatic pressure caused by moisture in the sand. Misrun and cold shut are caused by insufficient superheat provided to the liquid metal. Hot tear is the crack in the casting caused by high residual stresses. Shrinkage is essentially solidification contraction and occurs due to improper use of Riser. Shift is due to misalignment of two parts of the mould or incorrect core location.
Casting with expendable mould: Investment Casting
Advantages and Limitations Parts of greater complexity and intricacy can be cast Close dimensional control 0.075mm Good surface finish The lost wax can be reused Additional machining is not required in normal course Preferred for casting weight less than 5 kg, maximum dimension less than 300 mm, Thickness is usually restricted to 15mm Al, Cu, Ni, Carbon and alloy steels, tool steels etc. are the common materials
Permanent mould casting: Die casting Graphite+oil
General Configuration of a Die Casting Machine
In Die casting the molten metal is forced to flow into a permanent metallic mold under moderate to high pressures, and held under pressure during solidification This high pressure forces the metal into intricate details, produces smooth surface and excellent dimensional accuracy High pressure causes turbulence and air entrapment. In order to minimize this larger ingates are used and in the beginning pressure is kept low and is increased gradually
Cycle in Hot Chamber Casting
Cycle in Cold Chamber Casting
Centrifugal Casting •A permanent mold made of metal or ceramic is rotated at high speed (300 to 3000 rpm). The molten metal is then poured into the mold cavity and due to centrifugal action the molten metal conform to the cavity provided in the mould. •Castings are known for their higher densities in the outer most regions. •The process gives good surface finish •Applications: pipes, bushings, gears, flywheels etc.
Comparison of Casting Processes
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