solidification shrinkage

8/6/2019 Solidification Shrinkage

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Solidification Shrinkage

There are three distinct stages of shrinkage as molten metal alloys solidify:

y liquid shrinkagey liquid-to-solid shrinkagey

solid shrinkage

Liquid Shrinkage is the contraction of the liquid before solidification begins. While important to metalcasters,

it is not an important design consideration.

Liquid-to-Solid Shrinkage is the shrinkage of the metal as it goes from the liquid's disconnected atoms andmolecules to the formation of crystals of atoms and chemical compounds, the building blocks of solid metal.The amount of solidification shrinkage varies a great deal from alloy to alloy. Figure 1 provides a guide to theliquid-to-solid shrinkage of the most common ferrous and nonferrous alloys. As shown, shrinkage can varyfrom very little to high shrinkage volumes. Alloys can be further classified into three groups based on their

solidification range:

y directionaly

eutecticy equiaxed

Failure to recognize the impact of liquid-to-solid (solidification) shrinkage is one of the worst errors that "rule

of thumb" design handbooks make.

Liquid-to-Solid shrinkage is an extremely important consideration for the design engineer. In some alloys,disregard for this type of shrinkage results in voids in the casting. Both the design and foundry engineer havethe tools to combat this problem, but the designer has the most cost-effective tool, that is geometry.


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Geometry can be found that meets structural needs and solidification shrinkage needs. For some alloys,

finding that geometry can be very simple. For other alloys, finding that geometry is the real essence of goodcasting design. Should that geometry not be found for difficult alloys, the foundry engineer must resort to

"thermal trickery" to create fluid flow and heat transfer patterns that the geometry fails to provide.

"Thermal trickery" is creative stuff, a major weapon in the expert foundry engineer's arsenal, but it isexpensive. Eliminating thermal trickery with good design makes castings that cost less to produce and cost

less to process and assemble.

Solid Shrinkage (often called patternmaker's shrink) occurs after the metal has completely solidified and is

cooling to ambient temperature. Solid shrinkage changes the dimension of the casting from those in the moldto those dictated by the rate of solid shrinkage for the alloy.

In other words, as the solid casting shrinks away from the mold walls, it assumes final dimensions that mustbe predicted by the patternmaker. This variability of patternmaker's shrink is a very important designconsideration.

This uncertainty about patternmaker's shrink is why foundrymen normally recommend producing a firstarticle (sometimes called a sample casting) to establish what dimensions really are before going intoproduction. There is high risk in assuming that the solid shrinkage predictions built into patterns/dies and

coreboxes will result in final dimensions that are "close enough" to prediction to fit within allowabletolerance.

Despite all the good planning, the nature of patternmaker's shrink is unpredictable enough and important

enough that adjustments will probably be necessary on the pattern to achieve the final production

dimensions and tolerances.

Slag / Dross Formation

According to the dictionary, slag and dross are synonyms meaning: "refuse from melting of metals ".Obviously, no one wants "refuse" in castings.

Among foundrymen, slag and dross have slightly different meanings. Slag is usually is associated with thehigher melting point metals (ferrous metals) and is composed of liquid nonmetallic compounds (usuallyfluxed refractories), products of alloying and products of oxidation in air. Dross, on the other hand, usually is

associated with lower melting point metals (non-ferrous alloys) and often means the nonmetallic compoundsproduced primarily by the molten metal reacting with air.

Some molten metal alloys are much more sensitive to slag/dross formation than others. Castings made fromthese alloys are much more prone to contain nonmetallic inclusions. There are casting processes, qualitycontrol techniques and design considerations that can dramatically reduce the likelihood of nonmetallic

inclusions in casting. Design geometry guidelines to minimize the possibility of nonmetallic inclusionsaffecting the surface quality of castings ...

Pouring Temperature

Metal castings are produced in molds that must withstand the extremely high temperature of liquid metals.

Interestingly, there really are not many choices of refractors to do the job. As a result, high molten metaltemperatures are very important to casting geometry as well as what casting process should be used.

The following is a summary of common foundry alloys and their pouring temperatures:

Pouring Temperature Chart

Alloy F C


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Solder ~450 ~230

Tin ~600 ~300

Lead ~650 ~345

Zinc Alloys 650-850 345- 455

Aluminum Alloys 1150-1350 620- 735

Magnesium Alloys 1150-1350 620- 735

Copper-base Alloys 1650-2150 900-1180

Cast Irons; Gray, Ductile 2450-2700 1340-1480

Monel (70 Ni, 30 Cu) 2500-2800 1370-1540

Nickel-based Superalloys 2600-2800 1430-1540

High Alloy Steels 2700-2900 1480-1600

High Alloy Irons 2800-3000 1540-1650

Carbon & Low Alloy Steels 2850-3100 1565-1700

Titanium Alloys 3100-3300 1700-1820

Zirconium Alloys 3350-3450 1845-1900

For practical purposes, sand and ceramic materials with their refractory limits of 3,000 - 3,330F (1650-1820C) are the most common mold materials used today.

As the temperature of the molten metal alloy increases, design consideration must be given to heat transferproblems and thermal abuse of the mold itself.

Metal molds, such as those used in diecasting and permanent molding, also have temperature limitations. In

fact, most of the alloys on the list are beyond the refractory capability of metal molds (except for special thingeometry designs, alloys from the copper-base group and up require sand or ceramic molds).

Carbon and low alloy steels approach the limit of sand and ceramic refractories and titanium and zirconium

alloys go beyond it, creating special situations. So, it is easy to see the abuse that sand and ceramic moldsare subjected to when pouring temperatures approach the refractory limits. The same holds true for lowertemperature molten metal alloys that approach the refractory limit of plaster or metal molds.

Again there are design considerations that will compensate for thermal abuse and hot-spot problems in themold. These are covered in more detail later in this section.

Modulus of Elasticity

The measure of stiffness of a metal itself (without regard to material shape) is known as the modulus ofelasticity or Young's Modulus. In the case of metals, it is a function of metallurgy, and it is a mechanicalproperty of the metal alloy.

Although this is a parameter discussed at length in engineering books on material science, it is a commonmeasurement in foundries; in most foundries, the modulus of elasticity is a parameter measured virtually

every day. The modulus of elasticity is similar to the elastic (straight-line) portion of the stress/strain diagramcreated whenever a test bar is pulled on a tensile test machine.

more needs to be add here Figure 4 and Figure 5 must be included somehow.

Another important design engineering fact about modulus of elasticity is that it is independent of metalshape, that is, casting geometry.

Section Modulus


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Another measure of stiffness is section modulus, which is stiffness from shape or geometry; unlike modulusof elasticity, it has nothing to do with the material.

Actually section modulus is an aspect of moment of inertia which is a function of a shape's cross-sectionalarea in combination with its height.

Two important conclusions can be drawn from the mathematics:

y The only factors in the equations are shape!y The final equation gives the engineer clues about how the shape of the I-beam could be varied to

maximize moment of inertia and therefore the section modulus while minimizing the amount o f

material in the beam.

Stiffness from geometry of section modulus is a very powerful engineering tool. The knowledge of sectionmodulus enables the engineer to create metal shapes that are much stiffer than the material itself could everbe.

The most significant observation that can be made about stiffness from geometry is that there is no

other method besides metal casting that can offer so much geometry in the design and manufacture ofcomponent shapes.

Another significant observation is that design stress in a structural part is directly related to sectionmodulus. In fact, it is a direct, inverse relationship in which increasing section modulus decreasesstress.

We now see an important synergism between modulus of elasticity and section modulus. Modulus ofelasticity determines how much stress a metal can safety carry before it begins to deform permanently andsection modulus enables the engineer to use geometry to keep the stress within safe bounds. As we havelearned, creative use of section modulus enables relatively weaker metals to do the work of stronger ones.

The development of engineering computer hardware and software for making and analyzing solid models hasenabled a quantum leap in the use of section modulus to increase the stiffness of structural components andreduce the stress within them. In fact, these tools are making the power of metalcasting geometry much moreaccessible to design engineers because they enhance so significantly the ability to visualize in three

dimensions.

solidification shrinkage

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