Casting Metallurgy

8/14/2019 Casting Metallurgy

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UNIT II

Casting Metallurgy


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Shrinkage

Total reduction in volume of a casting due to

partial reductions at each stage of solidification.

Reduction in volume at each stage of

solidification of a casting.


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Volume change (contraction) that occurs when

molten metal solidifies after being cast into a

pattern mold. It is compensated for in three ways:by using the indicated water:powder ratio for the

refractory investment to gain the maximal setting

expansion of which that investment is capable; by

exposing the investment to moisture as the

refractory investment sets, causing some

hydroscopic expansion; and by properly heating the

mold to achieve thermal expansion. The totalexpansion must equal the contraction of the metal

being cast.


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There are three types of shrinkage: shrinkage ofthe liquid, solidification shrinkage and

patternmaker's shrinkage.

The shrinkage of the liquid is rarely a problembecause more material is flowing into the moldbehind it.

Solidification shrinkage occurs because metalsare less dense as a liquid than a solid, so duringsolidification the metal density dramaticallyincreases.

Patternmaker's shrinkage refers to the shrinkagethat occurs when the material is cooled from thesolidification temperature to room temperature,which occurs due to thermal contraction.


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Progressive & Directional Solidification

Directional solidification and progressivesolidification describe types of solidificationwithin castings.

Directional solidification describes solidificationthat occurs from farthest end of the casting andworks its way towards the sprue.

Progressive solidification, also known as parallel

solidification,is solidification that starts at thewalls of the casting and progressesperpendicularly from that surface.


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Most metals and alloys shrink as the material

changes from a liquid state to a solid state. Therefore, if liquid material is not available to

compensate for this shrinkage a shrinkage

defect forms. When progressive solidification dominates

over directional solidification a shrinkage

defect will form.


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The geometrical shape of the mold cavity has direct effect onprogressive and directional solidification. At the end of tunnel typegeometries divergent heat flow occurs, which causes that area of thecasting to cool faster than surrounding areas; this is called an endeffect. Large cavities do not cool as quickly as surrounding areasbecause there is less heat flow; this is called a riser effect. Also note

that corners can create divergent or convergent (also known as hotspots) heat flow areas. In order to induce directional solidificationchills, risers, insulating sleeves, control of pouring rate, and pouringtemperature can be utilized.


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Directional solidification can be used as a purificationprocess. Since most impurities will be more soluble inthe liquid than in the solid phase during solidification,

impurities will be "pushed" by the solidification front,causing much of the finished casting to have a lowerconcentration of impurities than the feedstockmaterial, while the last solidified metal will be enrichedwith impurities.

This last part of the metal can be scrapped or recycled.The suitability of directional solidification in removing aspecific impurity from a certain metal depends on thepartition coefficient of the impurity in the metal inquestion, as described by the Scheil equation.

Directional solidification is frequently employed as apurification step in the production of multicrystallinesilicon for solar cells.


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Degassing

Degassing is the most effective way of

reducing porosity. Degassing involves

bubbling argon and/or other gases throughthe melt to absorb hydrogen and other

impurities.


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There are various methods of degassing media helium sparging, warming, and subsequent filteringand vacuum degassing are the most popular.

The method suggested in the USP is to heat the mediato 45C then filter it through a 0.45m filter undervacuum and stirred for about 5 minutes before beingplaced directly into the dissolution vessel (the

paddles/baskets should be switched off until theanalysis is ready to start). At no time must thetemperature be allowed to drop below 37C.

This method of degassing has been shown to reduce

the level of dissolved gases by about 85% which isenough to ensure that the air will not affect thedissolution results.


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Helium Sparging

Helium sparging can be effective but is costly to use forlarge volumes, as it requires a constant supply ofhelium gas to continually bubble through the media.

It degasses the liquid by absorbing the gases that are

dissolved in the media into the helium bubbles andcarrying them out of solution.

One of the major problems with this method is that themedia can become saturated with helium which causes

similar problems to being saturated with air and it isdifficult to measure the amount of helium in the liquid.


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Warming & Filtering

Heating and filtering the media is fairlyreliable and is the method described in USP 23(it actually specifies heating to 45 C, followed

by filtration through a 0.45m filtermembrane).

This will remove about 85% of the dissolvedoxygen, although the media then has to be

cooled before the dissolution test which givesit time to reaerate.


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Vacuum Degassing

Vacuum degassing can remove more than 95% ofthe dissolved gas and if the media is held undervacuum (as it is in the Dosaprep) then it will notbe able to reaerate before it is placed in thedissolution vessel.

Other common laboratory methods of degassingsuch as sonication or membrane degassing are

not practical for degassing the large volumesrequired for dissolution testing and are moresuited for HPLC.


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Steel Degassing


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Aluminium Degassing

In the case of aluminium alloys, a degassing step is usually necessary toreduce the amount of hydrogen dissolved in the liquid metal.

If the hydrogen concentration in the melt is too high, the resulting castingwill be porous as the hydrogen comes out of solution as the aluminiumcools and solidifies. Porosity often seriously deteriorates the mechanicalproperties of the metal.

An efficient way of removing hydrogen from the melt is to bubble argon ornitrogen through the melt.

To do that, several different types of equipment are used by foundries.When the bubbles go up in the melt, they catch the dissolved hydrogenand bring it to the top surface.

There are various types of equipment which measure the amount of

hydrogen present in it. Alternatively, the density of the aluminium sampleis calculated to check amount of hydrogen dissolved in it.

In cases where porosity still remains present after the degassing process,porosity sealing can be accomplished through a process called metalimpregnating.


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Casting Metallurgy