Metal Casting Processes

– Considered to be the sixth largest industry in the USA

– copper smelting technique around 3000 BC

– the ancient Egyptians invented the ‘lost-wax’ molding process

– the Chinese developed certain bronze alloys

– in 1340 - cast iron

– in 1826 - malleable iron

– in 1948 - nodular cast iron

– It is among the oldest methods of net-shape and near net-shape manufacturing

– The important factors are:

• solidification and accompanying shrinkage

• flow of the molten metal into the mold cavity

• heat transfer during solidification and cooling of the metal in the mold

• influence of the type of mold material

• Solidification of metals

• Casting Alloys– Ferrous alloys

• cast irons: wear resistance hardness, and good machinability• a family of alloys: gray cast iron (gray iron), nodular (ductile or

spheroidal) iron, white cast iron, malleable iron, and compacted-graphite iron

• magnesium base alloys - good corrosion resistance and moderate strength

• cast steels - high temperatures required up to 1650 degree C• cast stainless steels - have a long freezing range and high

melting temperatures, high heat and corrosion resistance– Nonferrous alloys

• aluminum base alloys• copper base alloys• zinc base alloys• high temperature alloys

• Cast irons– This is a family of ferrous alloys composed of iron, carbon (from

2.11% to 4.5%), and silicon (up to 3.5%). They are classified according to their solidification morphology as:

– Cast iron:

• gray, white, ductile (nodular), and malleable 2.25% to 4.4%C and 1.15% to 3% Si

• used as structural material (structures and frames of machine tools, presses, and rolling mills, the housings of water turbines and of large diesel engines

– Ingot casting and continuous casting

• shaping of the molten metal into a solid form - an ingot - for further processing by rolling it into shapes, casting it into semifinished forms, or forging

• ingots may be square, rectangular, or round in cross-section, and their weight ranges from a few hundred pounds to 300 tons

• Ferrous alloy ingots:

– certain reactions take place during solidification

» significant amounts of oxygen and other gases can dissolve in the molten metal during steelmaking

» much of these gasses are rejected during solidification of the metal

» the rejected oxygen combines with carbon, forming carbon monoxide, which causes porosity in the solidifies ingot

» depending on the amount of gases evolved during solidification, three types of steel ingots can be produced: killed, semi-killed, and rimmed

– Liquid metals have much greater solubility for gases than do solids. Gases either accumulate in regions of existing porosity, such as interdendritic areas, or they cause microporosity in the casting, particularly in cast iron, aluminum, and copper. Dissolved gases may be removed from the molten metal by flushing or pouring with an inert gas or by melting and pouring the metal in vacuum.

– Ingots

• 10-40 tons for rolling

• up to 300 tons for open die forging

• oxygen, hydrogen, nitrogen are dissolved in molten steel

• depending on the measure to deoxidize the steel, different kinds of steel are produced: the steel, different kinds of steel are produced: killed, semikilled, capped, or rimmed steel

• The amount of oxygen dissolved in molten steel increases with the decreasing %C

• in the low carbon steels deoxidizing elements are: Al, Mg, Si, they are rimmed or capped

• steels with C>3% are produced as killed or semikilled

• segregation - different components of steel in different parts of the ingot purer metal solidifies first

• killed steels are the least segregated

• rimmed steels with 0.06 - 0.15%C

• 0.15 - 0.3%C semikilled steels

• >0.3%C - fully killed steels

– Vacuum degassing to eliminate O2, N, H

– Vacuum is soft, 0.1 - 0.2 mmHg

– the surface area of the droplets is larger than their volume

• Continuous casting– conceived in the 1860s

– major improvements in efficiency and productivity and significant reductions in cost

– the molten metal in the ladle is cleaned and equalized in temperature by blowing nitrogen gas through it for 5 to 10 min. The metal is then poured into a refractory lined intermediate pouring vessel (tundish) where impurities are skimmed off. The molten metal travels through water cooled copper molds and begins to solidify as it travels downward along a path supported by rollers (pinch rolls)

• Cast structures– depend on

• the composition of the particular alloy

• the rate of heat transfer

• the flow of the liquid metal

Melting Practice and Furnaces

– Furnaces are charged with melting stock consisting of liquid and/or solid metal, alloying elements, and various other materials such as flux and slag forming constituents.

– Fluxes have several functions, e.g. for aluminum alloys:

• cover fluxes

• cleaning fluxes

• drossing fluxes

• refining fluxes

• wall cleaning fluxes

– To protect the surface of the molten metal against atmospheric reaction and contamination the pour must be insulated either by covering the surface of mixing the melt with compounds that form a slag.

• Melting Furnaces• electric arc• induction• crucible• cupolas

– Electric arc Furnaces: high rate of melting, much less pollution, and the ability to hold the molten metal for any length of time for alloying purposes.

– Induction furnaces: used in smaller foundries, produce composition controlled smaller melts.

• the coreless induction furnace (a crucible completely surrounded with a water cooled copper coil, high frequency current, a strong magnetic stirring action during induction heating)

• a core or channel furnace (low frequency - 60 Hz, used in nonferrous foundries, suitable for superheating, holding, and duplexing)

– Crucible furnaces: heated with commercial gases, fuel oil, fossil fuel, electricity. They may be stationary, tilting, or movable. Used for ferrous and nonferrous metals.

– Cupolas: are basically refractory lines vertical steel vessels that are charge with alternating layers of metal, coke, and flux. They operate continuously, have high melting rates, and produce large amounts of molten metal.

– Levitating melting: magnetic suspension of the molten metal. An induction coil simultaneously heats a solid billit and stirs and confines the metal.

• Foundries and foundry automation:– the casting operations are usually carried out in foundries

– foundry operations initially involve two separate activities:

• pattern and mold making (CAD, CAM, and RP)

• melting the metals while controlling their composition and impurities

– the rest of operations, such as pouring into molds carried along conveyors, shakeout, cleaning, heat treatment, and inspection, are also automated

• a die casting facility can afford automation

• a jobbing foundry producing short production runs may not be automated

– The properties of the cast metal may be improved after casting:

• high temperature isostatic pressing (HIP) - argon is used to pressurize the casting (P = 200 MPa, T = 2000C)

• applied for superalloy and Ti casting

• eliminates porosity and improves toughness and fatigue strength

• steel and iron castings may be quenched and tempered

• Al and Ti castings - subjected to solid solution or precipitation hardening treatments

• annealing - for homogenization of the micro and macrosegregation

• stress relief - heat treatment

– It is necessary to consider

• the fluidity of the metal

• pressure and velocity distribution in the casting system

• heat extraction

• the propagation of the solidification front

• use of advanced computer programs

– Fluidity - the ability to fill the various details of the mold cavity

• it is affected by the modes of the solidification front

• by surface tension

• oxide films

• the thermal permeability of the mold material

• it improves by the temperature of the molten metal and the mold (slower cooling, coarser grains)

• dendrites clog the channels

– Heat transfer

• from pouring to solidification and cooling to room temperature

• it depends on many factors related to the casting material and the mold and process parameters

• Shrinkage– Metals shink (contract) during solidification and cooling.

Shrinkage, which causes dimensional changes - and sometimes cracking - is the result of:

• contraction of the molten metal as it cools prior to its solidification;

• contraction of the metal during phase change from liquid to solid (latent heat of fusion);

• contraction of the solidified metal (the casting) as its temperature drops to ambient temperature

– The largest amount of shrinkage occurs during cooling of the casting. The amount of contraction for various metals during solidification is shown in Table 5.1. Not that gray cast iron expands. The reason is that graphite has a relatively high specific volume, and when it precipitate as graphite flakes during solidification,k it causes a net expansion of the metal. Silicon has the same effect in aluminum alloys.

• Basic requirements of casting processes– mold cavity

• single use molds

• multiple use molds

– melting process

– pouring technique

– solidification process

– mold removal

– cleaning, finishing, and inspection

• Casting terminology– construction of a pattern

– construction of a core

• the mold cavity

– riser - provides a reservoir of material that can flow into the mold cavity to compensate for any shrinkage

– vents may be included to provide an escape of the gases

– gating system - to deliver the molten metal to the mold cavity

• Defects– Depending on casting design and method, several defects can develop in

castings. Because different names have been used to describe the same defect, the International Committee of Foundry Technical Associations has developed standardized nomenclature consisting of seven basic categories of casting defects:

• metallic projections, consisting of fins, flash, or massive projections such as swells and rough surfaces

• cavities, consisting of rounded or rough internal or exposed cavities, including blowholes, pinholes, and shrinkage cavities

• discontinuities such as cracks, cold or hot tearing, and cold shuts. If the solidifying metal is constrained form shrinking freely, cracking and tearing can occur. Although many factors are involved in tearing, coarse grain size and the presence of low melting segregates along the grain boundaries increase the tendency for hot tearing. Incomplete castings result from the molten metal being at too low a temperature or pouring the metal too slowly. Cold shut is an interface in a casting that lack complete fusion because of the meeting of two streams of partially solidified metal.

– defective surface, such as surface folds, laps, scars, adhering sand layers, and oxide scale

– incomplete casting, such as misruns (due to premature solidification), insufficient volume of metal poured, and runout (due to loss of metal from mold after pouring)

– incorrect dimensions or shape, owing to factors such as improper shrinkage allowance, pattern mounting error, irregular contraction, deformed pattern, or warped casting

– inclusions, which form during melting, solidification, and molding. Generally nonmetallic, they are regarded as harmful because they act like stress raisers and reduce the strength of the casting

• Porosity– caused by shrinkage or trapped gases, or both

– porosity is detrimental to the ductility of a casting and its surface finish

– porosity caused by shrinkage can be reduced or eliminated by various means

• adequate liquid metal feeding

• external and internal chills

– The rate of heat dissipation affects the formation of shrinkage cavities

– Hot tears are casting defects caused by tensile stresses as a result of restraining a part of the casting.

– Cast metals are generally weaker in tension in comparison with their compressive strengths

– casting process allows to distribute the masses of a section

– distribute masses in order to lower the magnitude of tensile stresses in highly loaded areas of the cross section and to reduce material in lightly loaded areas.

– It is recommended to make the small projection separate and attach it to the large casting by an appropriate joining method.

– Machining should be performed only on areas where it is absolutely necessary.

• The ribs should be as thin as possible

• Parabolic ribs are better than straight ribs in terms of economy and uniformity of stress.

• Safety in foundries– As in all other manufacturing operations, safety is an important

consideration, particularly because of the following factors:

• dust from sand and other compounds used in casting, thus requiring proper ventilation and safety equipment for the workers

• fumes from molten metals and lubricants, as well as splashing of the molten metal during the transfer or pouring

• the presence of fuels for furnace, the control of their pressure, the proper operation of valves, etc.

• the presence of water and moisture in crucibles, molds, and other locations, since it rapidly converts to steam, creating severe danger of explosion

• improper handling of fluxes, which are hygroscopic, thus absorbing moisture and creating a danger

• inspection of crucibles, tools, and other equipment for wear, cracks, etc.