METALLIC MATERIALS PROCESSED THROUGH CASTING

In the construction of machines the weight of cast-metal parts very important -

The physical principle underlying the casting process is:- any liquid takes the form of the dish if it is put on -

Advantages: by casting you can get parts of complex shapes, with hollow,

impossible or very difficult to achieve through other means some materials - grey and malleable cast iron, brass with high

content of Zn, bronze with high content of Sn, some of Al-alloys can not be processed through plastic deformation by casting can get good quality parts, compact, with superior physical and technological properties;

can get a good dimensional accuracy for results there is no need of further processing, which means a significant savings in material, workmanship and manufacturing cost less;

the possibility to reuse the metal waste immediately through re-melting and casting other parts;

much more productive compared to other manufacturing processes.

Pattern = element used in molding with configuration and dimensions close to those of finished part used to obtain theconfiguration of outer surfaces of part.

Materials used to make patterns: - wood; - metal (aluminum alloy, bronze, cast iron); - plaster, plastics, cement.

To protect them from moisture, the patterns are painted in conventional color for identification:

- pieces of cast iron - red; - pieces of steel - blue; - non ferrous parts - yellow.

Cores = elements used to obtain the interior configuration of part. - mounted and fixed in cavity by means of extensions known like core grids;

CASTING PROCESSES

1. GREEN MOLD CASTING

Forming

- hand molding for unique pieces or small series:- in forming frames;- in soil foundry;

- machining molding

Hand-molding work in forming frames;- depending on the number of boundary surfaces is used two or more frames

Machining molding - for mass production of small and medium sized parts. T

For molding on the molding plates are bonded metal patterns.

Forms - use in green state the loam is unique there is in a silo above the machine.

Advantages: a better packing; more precise form

a smaller machining addition; high productivity; labor economy.

STRUCTURE OF A GREEN SAND MOLD: 1'-finished part; 2'- part bore; 1-the mold cavity; 2- the core; 3- marks for the core; 4- runner; 5, 6- vents; 7- lower frame; 8- top frame; 9- pattern

sand; 10, 11- green sand; 12- upper mold half; XX - Separating surface.

Basic steps involved in making a casting green sand mold

IN PERMANENT MOLD CASTING, sometimes referred to as gravity die casting, a metal mold consisting of two or more parts is repeatedly used for the production of many castings of the same form. The liquid metal enters into the mold by gravity. The cores are usually made of sand or plaster.

Permanent mold casting is particularly suitable for the high-volume production of castings with fairly uniform wall thickness and limited undercuts or intricate internal coring. The process can also be used to produce complex castings, but production quantities should be high enough to justify the cost of the molds.

. Compared to sand casting, permanent mold casting permits the production of more uniform parts, with closer dimensional tolerances, superior surface finish, and improved mechanical properties.

2. PERMANENT MOLD CASTING

Permanent mold casting has the following limitations:

Not all alloys are suitable for permanent mold casting Because of relatively high tooling costs, the process can be prohibitively

expensive for low production quantities Some shapes cannot be made using permanent mold casting, because

of parting line location, undercuts, or difficulties in removing the casted part from the mold

Coatings are required to protect the mold from attack by the molten metal

Metals that can be cast in permanent molds include the aluminum, magnesium, zinc, and copper alloys and hypereutectic gray iron. Practical sizes of permanent mold castings differ according to materials cast, part configuration, and number of parts needed.

1 cavity2 - pouring funnel3 - foot casting4 feeder channel5 aeration channel

Before the casting, metal shapes has to be preheated at 200 - 3000C -to reduce the thermic shock and the cooling speed of the first part

3. CASTING IN FORMS USING EASILY FUSIBLE PATTERNS

Specific absence of separation plan - no need mold release (model is extracted from the cavity by liquefaction, so the

process is called casting with "lost patterns").

Parts made by this process:- mechanics parts for typewriters and sewing;- mechanisms for weapons;- medical instruments;- dental pieces;- parts for computers;- turbine blades; waveguides;- cams, pawls, gears, levers.

The method has great accessibility, because it does not requirespecific equipment.

Steps to be taken:

- execution of the easy fusible pattern; - assembly of the patterns on a metal rod to make a cluster; - obtaining of the shell mold; - mold release:

- it will be extract the metal rod;- the form is heated with steam or hot air at 60 .. 120 C - to melt the pattern; - drying forms - by a progressive heating and its maintaining form between 50 and 250 C; - forms calcinations - by heating the shells in the oven in order to strengthen them; - packaging forms in order to pour; - preheating forms in order to cast the liquid metal.

4. CENTRIFUGAL CASTINGS

The centrifugal casting process uses rotating molds to feed molten metal uniformly into the mold cavity. Directional solidification provides for clean, dense castings with physical properties that are often superior to those of the static casting processes.

Centrifugal castings are produced by pouring molten metal into a rotating or spinning mold. The centrifugal force of the rotating mold forces the molten metal against the interior cavity (or cavities) of the mold under constant pressure until the molten metal has solidified.

Cylindrical castings are generally preferred for the centrifugal casting process. Tubular castings produced in permanent molds by centrifugal casting usually have higher yields and higher mechanical properties than castings produced by the static casting process.

Centrifugal casting is the most economical method of producing a superior-quality tubular or cylindrical casting with regard to casting yield, cleaning room cost, and mold cost.

Centrifugal castings can be best described as isotropic, that is, having equal properties in all directions. By utilizing the outstanding advantage created by the centrifugal force of rotating molds, castings of high quality and integrity can be produced because of their high density and freedom from oxides, gases, and other nonmetallic inclusions

An economic advantage of centrifugal castings is the elimination or minimization of gates and risers.

All metals that can be cast by static casting can be cast by the centrifugal casting process, including carbon and alloy steels, high-alloy corrosion- and heat-resistant steels, gray iron, ductile and nodular iron, high-alloy irons, stainless steels, nickel steels, aluminum alloys, copper alloys, magnesium alloys, nickel- and cobalt-base alloys, and titanium alloys.

Nonmetals can also be cast by centrifugal casting, including ceramics, glasses, plastics, and virtually any material that can be made liquid or pourable

The technological process for a part made by centrifugal castings comprises:

- alloy elaboration; - mold preparation in order to casting (cleaning, applying refractory paint inside, preheating); - fitting the mold lid; - closing defenders of protection; - positioning the trough; - pouring the alloy in the same time with starting rotation movement; - alloy solidification into the mold under the influence of centrifugal force; - stopping the mold rotation;- extracting the part using the pusher actuated by a pneumatic cylinder.

Horizontal centrifugal casting - machines are generally used to make pipe, tubes, bushings, cylinder sleeves (liners), and cylindrical or tubular castings that are simple in shape

1 - trough casting; 2 - casting mold

Materials

Any material that can be statically cast can also be centrifugally cast.Materials that are currently processed by horizontal centrifugal casting: high-strength low-alloy steels - are important in offshore structures

that must withstand extreme climatic conditions; for example, offshore drilling equipment in the North Sea - must be weldable with ductile-to-brittle transition temperatures below -40 C. Likely candidate materials for use under such conditions are manganese-molybdenum steels with micro-alloying additions of V, Ni, or Nb. Ex: Heavy-wall weldable pipes with good mechanical properties.

duplex stainless steels, and chromium-molybdenum alloy steels -resistant to thermal fatigue and wear and have good toughness; steel tubes can be produced by horizontal centrifugal casting in any section size. Castings retain strength at temperatures to 600 C, have good ductility, and are weldable without special precautions.

bimetallic tubes with metallurgical (rather than mechanical) bonds. They are most commonly produced by successively casting one alloy inside the other. Bimetallic tubes are used for two primary reasons: to reduce cost by using an exotic material bonded to a less expensive backing material, and to obtain combinations of properties that could not be achieved by other methods. there are no general rules regarding what materials can be combined

in centrifugally cast bimetallic tubes, although it may be beneficial to cast the inner layer of such tubes from a material that is more fusible than the outer material. In addition, relatively thin inner layers should be manufactured from alloys with coefficients of thermal expansion smaller than that of the outer alloy. In this way, the thin inner layer is put into compression, making it more resistant to cracking.

Three applications for centrifugally cast parts in the iron and steel industry.

(a) Continuous casting roller; (b) Winding spool. (c) Annealing furnace rollers.

Miscellaneous applications for centrifugal castings.

(a) Hydraulic cylinders. (b) Float glass roller. (c) Exterior columns of Beaubour

Museum, Paris.

1 - Casting mold;

2 - hot brick;

3 - ladle;

4 - liquid metal;

5 finished part

The range of application of VERTICAL CENTRIFUGAL CASTINGmachines is considerably wider. Castings that are not cylindrical, or even symmetrical, can be made using vertical centrifugal casting.

5. DIE CAST

Parts that can be obtained: -carburetors;- tools; - toys

Weight parts: - from 90 g to 25 kg

Materials: lead alloys, zinc pressure can reach 35 MPa cadence can be reached at 900 cycles per hour

Steel castings

- have lower molding properties compared to gray cast iron: - high melting temperature; - low flow; - high linear shrinkage - between 1.6 and 2%; - great tendency of segregation.

additional measures against conventional casting technology:forming mixtures with higher refractivity and high mechanical strength (compression); for small and medium pieces casting in green shapes; for large pieces casting in dry shapes;cavities control which appear into the thermal nodes of the wall piece; building the pouring gate to allow liquid material flow quietly, without eddies; parts annealing and normalization;

for parts of alloyed steels hardening and tempering; casting temperature of the liquid metal - the greater as the part walls are thinner.

Non-alloyed steel for casting parts have:

- 0,2...0,4 %C;- 0,4...0,8 %Mn;- 0,25...0,6 %Si;- max. 0,04...0,05 %S, P.

Application:- wheels for cables and chains;

- gears;

- reductor housing;

- barrel of pump;

- crankshaft;

- flange;

- crosshead (temperature between 10 and 250 C).

Low alloy steel - frequently with Cr, Ni, Mn, Co - more difficult to cast - used for their better mechanical properties and for the possibility to improve these properties by subsequent heat treatment.

Alloy steel - for parts which require high strength, good corrosion resistance at high temperatures or wear (e.g.. steel alloy casting "maraging" type Ni-Cr-Mo containing Cr, Cu and Mo).

Steel castings: = coarse grained + casting defects (micro-blisters and goals) mechanical strength, plasticity and toughness lower than of the rolled steel;

- are easy wieldable.

pouring turnare heat treatment tratament termic

mold formdimensional tolerances

tolerandimensional

sand molds form de nisip surface finish finisare

permanent mold form permanentmechanical properties

proprieti mecanice

die matri gating system sistem de alimentare

foundry turntorie centrifugal casting turnare centrifugal

core miez pipes evi

wax cear bushings buce

flow curgere sleeve manon

pattern model

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