23-07-2015 10:05

Figure : Typical sand casting process

23-07-2015 10:05

23-07-2015 10:05

Homogenous nucleation

Heterogenous

23-07-2015 10:05

23-07-2015 10:05

Chill Zone

Columnar Zone

Equiaxed grains

23-07-2015 10:05

Gases are almost universally more soluble in liquid metalsthan in solid metals.

As metals cool, and particularly as they freeze, dissolvedgases tend to be expelled from solution to form gas holes.

23-07-2015 10:05

The gas holes may be in the form of large blowholes, finerpin holes, or still finer micro porosity, depending on theamount of gas present and the manner in which the metalor alloy freezes.

Gases dissolve in metals not as molecules such as H2, CO2,etc. but as elements such as H, O, C etc.

On solidification, the elements may recombine to formmolecular gases, and in turn form gas holes.

Hydrogen is the most common of the element, it is solublein all metals to a greater or lesser extent.

As the temperature decreases, the gas solubility alsodecreases until the solidification temperature is reached.

23-07-2015 10:05

23-07-2015 10:05

Gases are absorbed in molten metal under following conditions

1. In the furnace from the atmosphere and from wet charges.2. In the furnace or ladles from incompletely dried linings or

wet tools

3. In passing through air from furnace to ladle or from ladle tomold.

4. In the mold from moist atmosphere or excess moisture orvolatile hydrocarbons in mold materials

23-07-2015 10:05

In order to prevent gas absorption some precautions mustbe taken care -

1. Melt under slag’s or protective atmospheres or in vacuum.

2. Reduce chances of absorption of gas by keeping all itemsthat must come in contact with the liquid metal.

3. Melt and pour at temperatures as low as possible tominimize gas solubility

4. Handle the metal as little as possible.

It is usually impractical to prevent gas absorption entirely.

23-07-2015 10:05

When it is not possible to prevent gas absorption, the hazardsof dissolved gases may be reduced by

Removing the gas or gases, or

Removing or isolating one or more of the components ofthe gas.

In case of diatomic gases such as H2 O2 and N2 the amount ofgas dissolved at any particular temperature may be expressedby the equation

V = K √PWhere, V = volume of diatomic gas dissolved

K = constant and

P = pressure of gas above and around the metal.

23-07-2015 10:05

This eq. shows that, if hydrogen iscompletely removed from theatmosphere around a melt, anyhydrogen gas will tend to escape.

Thus melting and pouring under avacuum is one method, but a veryexpensive one for obtaining gas-free castings.

Vacuum degassing is an attemptto combine the advantages ofvacuum melting with the economyof air melting.

Here the metal is melted in air andthen placed in an evacuatedchamber (for degassing) beforepouring; pouring takes place in theopen atmosphere.

23-07-2015 10:05

Flushing with an inert gas isthe most common method ofremoving gas employing theprinciple described earlier.

In this method, the inert gasis bubbled directly throughthe metal; the pressure of thedissolved gas (usuallyhydrogen) within each inertgas bubble is zero, and anydissolved gas migrates readilyto the bubble where itcollects and is flushed fromthe metal baths.

23-07-2015 10:05

FLUIDITY

Fluidity is defined as the ability of a liquid of a metalflow to flow readily, as measured by the length of a standardspiral casing.

When the mold is filled through a gating system, or whenthe mold has thin sections, it is often found that the metalfails to fill the mold completely, owing to solidification innarrow channels in the gating system or in the mold. Thisdifficulty is attributed to lack of fluidity.

23-07-2015 10:05

Fluidity Test Fluidity spiral is widely used for cast steel.

The fluidity of alloy is rated as the distance, in inches that the metal runs in the spiral channel

23-07-2015 10:05

Factors Affecting the Fluidity1. The intrinsic fluid properties of the molten metal viscosity, surface tension, the character of the surface oxide film, inclusion content and manner in which the particular alloy solidifies

2. Casting conditions part configurations liquid-static pressure drop casting head and velocities mold material mold surface characteristics heat flux rate of pouring and degree of superheat

23-07-2015 10:05

Hot Tearing

Hot tearing takes place in alloys that have an extendedfreezing range.

When solid and liquid coexist over a wide range oftemperatures, contraction of the solid around the rigidmold surface can initiate rupture in partially solidifiedregions.

The methods used to avoid hot tearing◦ reducing the pouring temperature,

◦ reducing the die temperature,

◦ increasing the pressurization time and increasing the draft angles onthe casting.

23-07-2015 10:05

Improper feeding leads to internal hot tears.

Internal hot tears are radially disposed discontinuities insidecastings, more commonly steel castings, which can bedisclosed by radiography.

The discontinuities resemble external hot tears, except thatthey are radial rather roughly parallel.

The tears emanate from a low density area, giving theradiograph an octopus – like appearance.

The cure for internal hot tears is improved feeding.

23-07-2015 10:05

Hot Tears in castings

23-07-2015 10:05

Shrinkage of Liquids and Metals Cast metals shrink in three distinctly different steps.

They undergo liquid contraction as they cool from thepouring temperature to the solidification temperature,solidification contraction as they freeze, and solidcontraction as the solid casting cool to room temperature.

23-07-2015 10:05

The last contraction results in a casting slightly smaller thanthe mold cavity, and so patterns are made slightly larger insize than the desired casting. This is the so-called “patternmaker’s shrinkage”.

Liquid state Solid state [FCC]

23-07-2015 10:05

Shrinkage rate

23-07-2015 10:05

Defects in Casting

Hot tears

Metallic Projections

Cavities

Discontinuities

Defective surface

Incomplete Casting

Incorrect dimensions or shape

Inclusions due to shrinkage

23-07-2015 10:05

Cast Frame

Design of Castings

23-07-2015 10:05

Successful Casting process…………

Flow of the molten metal in the mold cavities,

gating systems

rate of cooling and

the gases evolved would influence the quality of a casting

Two types of design issues in casting:

1. Geometric features and tolerances in the part

2. Mold features needed to produce the desired casting

Design of Castings

23-07-2015 10:05

Corners, angles, and section thickness

Flat areas

Shrinkage

Draft

Dimensional tolerances

Lettering and markings

Finishing operations

Design of Castings

23-07-2015 10:05

Parting line is the line or plane separating the upper (cope) and lower (drag) halves of molds

Parting line should be along a flat plane rather than be contoured

Location is important as it influences mold design, ease of molding, number and shape of cores required, method of support and the gating system

Parting line should be low for less dense metals and located at mid-height for denser metals

Design of Castings

23-07-2015 10:05

a) Design for minimum casting stresses

b) Design for directional solidification

c) Design of metal flow

d) Design for minimum costing

e) Safety aspects

Design of Castings

23-07-2015 10:05

i. Corner radius

External corners with radii = 10 – 20 % of section thickness

By rounding the corners, the resistance of ductile metals to fatigue rupture, static strength and deflection of grey cast iron are increased.

Design of Castings

23-07-2015 10:05

ii. Joining of sections

0.1t, the resistance to fatigue stress is limited.

1t, there is 40 to 50% more stress endurance,

4t, there is 120% more stress endurance than that with 0.1t radius.

The best way to change section thickness when necessary is to do it gradually.

Design of Castings

23-07-2015 10:05

iii. Replace sharp angles and corners with suitable radii.

Inside corner of the castings has higher temperature

Due to this overheating, shrinkage defects occurs and

Thermal stress concentration may induce a heat check or hot tear

23-07-2015 10:05

Design of Castings

iv. Use tubular and reinforced C sections rather than standard I, H and channel sections to obtain improved load bearing capabilities under complex loads.

23-07-2015 10:05

Design of Castings

v. Stiffness structure

Simplify, streamline and stagger complex sections to obtain improved stiffness.

If possible, stiffness should be obtained with corrugated sections rather than ribs.

23-07-2015 10:05

Design of Castings

Complexity of Cast structures

23-07-2015 10:05

Design of Castings

vi. Ribs

Ribs should neither be too shallownor too widely spaced

Ribs thickness should approximately0.8 casting thickness

Ribs should solidify before thecasting section they adjoin.

Ribs should not be provided on boththe sides of the casting; thisincreases casting costs and defects.

Cross coupled rib preferred to ribs onboth sides of a casting

Eliminate ribs from tension stressedcastings with U orcorrugated(Shaped into alternatingparallel grooves and ridges) sections.

23-07-2015 10:05

Design of Castings

i. Section selection

Bring the minimum number ofsections together

Avoid ‘T’ junctions to avoid ‘Hotspot’

Cast welding is the alternative

23-07-2015 10:05

Design of Castings

Measures to eliminate hot spot in a T section

23-07-2015 10:05

Design of Castings

ii. Heavy sections should not be fed through light sectionsbecause light sections will solidify first and causeshrinkage defects in heavy sections (a).

iii. Risers should be placed suitably to ensure soundness ofcasting (b).

23-07-2015 10:05

Design of Castings

iv. Horizontal flat surfaces should be avoided

It is difficult to prevent center line shrinkage as temperature gradients are not favorable

Slag, dross etc. may collect on upper flat surfaces

Flat surfaces in castings should be made either vertical or inclined.

v. Avoid isolated hot spots. Since cooling rate is isolated inareas such as small cores surrounded by metal, this regionis likely to shrink or tear in the absence of chilling orspecial feeding.

23-07-2015 10:05

Design of Castings

23-07-2015 10:05

Design of Castings

vi. Parting-line placement

Keep parting line as even as possible so that pattern andmold making is simplified and the casting is produced withminimum no. of fins.

Locate heavier sections at or near the parting line wherethey can be easily fed by riser.

23-07-2015 10:05

Design of Castings

vii. Bosses on casting surfaces

A boss which is an isolated enlarged mass of metal is difficult tocast solid without a shrinkage defect.

Shrinkage defect may be removed by◦ Feeding the boss with a separate riser.

◦ Casting the boss with a hole is for its functioning

23-07-2015 10:05

Design of Castings

Minimum section thickness which permits to molt flow

= f (Metal composition, Fluidity of the molt, molt temperature)

Thin sections develop defects such as misrun or cold shut.

Approximate recommended minimum thickness for sand casting of different metals and alloys are given below:

Gray iron, Al, Mg and Cu alloy

Thin castings ……. Not less than 3mm

Large castings ……. Not less than 6mm

Steel castings

Small …… Not less than 6mm

Large …… Not less than 12mm

23-07-2015 10:05

Design of Castings

A good casting design will incorporate saving wherever possible.

modify the casting design in order to Simplify patternconstruction and Withdraw the pattern from the sand moldeasily.

Reduce the number of operations required to make a mold andcores and to assemble them for making a casting.

Place the parting line properly.

Deep pockets in the casting increase molding cost.

Dry sand cores add considerably to the cost of a casting.

If it is difficult to produce one piece casting, the cast-weldconstruction may prove to be a good alternative.

Substantial economy may be achieved by incorporating morethan one finished part into a single casting

23-07-2015 10:05

Design of Castings

Preventive maintenance cost/ cost of casting

Corrective maintenance/ cost of subsequent machining

23-07-2015 10:05

Design of Castings

A good casting design aims at reducing the weight of thecasting especially when it is a component of aircraft, spacecraftor of any automobile

Weight of a casting can be reduced by

◦ Using light materials having high strength to weight ratio; this ofcourse results in increased cost.

◦ Reducing existing casting dimensions through stress analysis etc.

◦ Using high strength ferrous material in place of low or mediumstrength ferrous material. This reduces physical size and weight ofthe casting.

23-07-2015 10:05

Design of Castings

Physical hazards

Radiation

Noise

Fire and explosion

Respiratory hazards

Vision hazards

Electrical hazards, etc

23-07-2015 10:05

Design of Castings

23-07-2015 10:05

Design of Castings

Safety First!

Hazardous waste management

Hearing Protection

Protection of head, eye, leg and hand

Handling dust problem

Automation in foundry

Wear leather work boots, preferably with steel toes.

Wear trousers, not shorts and long sleeve shirt or welder's vest.

Wear loose-fitting leather clothes, or at least clothes made of a material, whichis fairly flame resistant.

Safety glasses are an absolute minimum.

23-07-2015 10:05

Design of Castings

23-07-2015 10:05

Design of Castings