Here we present a beginners’ guide for metal manufacturing method Casting and the term castability. The sections to cover the subject is as follows:
- What is Casting?
- What is Sand Casting?
- What is Die-Casting?
- The definition of Castability.
- Material Properties Which Affect Castability.
What is Casting?
Casting Process: Casting is usually the first step in manufacturing. In casting, a material in liquid form is poured into a mold where it is allowed to solidify by cooling (metals) or by reaction (plastics). The mold can be filled by gravitational forces or under pressure. The mold cavity is carefully prepared so that it has the desired shape and properties. The cavity is usually made oversize to compensate the metal contraction as it cools down to room temperature. This is achieved by making the pattern oversize. After solidification, the part is removed from the mold. By using casting method, big and complex parts can be produced.
What is Sand Casting?
Sand casting is used to make large parts (typically Iron, but also Bronze, Brass, Aluminum). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic). The cavity in the sand is formed by using a pattern (an approximate duplicate of the real part), which are typically made out of wood, sometimes metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Cores are placed in the cavity to form holes of the desired shapes. A riser is an extra void created in the mold to contain excessive molten material. In a two-part mold, which is typical of sand castings, the upper half, including the top half of the pattern, flask, and core is called cope and the lower half is called drag. The parting line or the parting surface is line or surface that separates the cope and drag. Sand castings generally have a rough surface sometimes with surface impurities, and surface variations.
What is Die-casting?
In Die-casting the metal is injected into the mold under high pressure. This results in a more uniform part, generally good surface finish and good dimensional accuracy,. For many parts, post-machining can be totally eliminated, or very light machining may be required to bring dimensions to size. Die casting molds (called dies in the industry) tend to be expensive as they are made from hardened steel or other high refractory materials-also the cycle time for building these tend to be long. Therefore die-casting is a good choice for high quantities (mass production), whereas it rises the expenses too much for low quantities. Furthermore the stronger and harder metals such as iron and steel cannot be die- cast. Materials with relatively low melting points such as Aluminum , Zinc and Copper alloys are the materials predominantly (mainly) used in die-casting. Die casting is limited to smaller parts up to 25 kg.
What is Castability?
Castability: Castability is a term, which reflects the ease with which a metal can be poured into a mold to obtain a casting without defects. Castability depends on part design and material properties. Here we shall only concentrate on material properties, which affect castability.
Material Properties Which Affect Castability:
a) Melting temperature (or temperature range):
Melting temperature is an important material property for castability. In casting, generally low melting points are desired, because low melting points require less energy to melt the material. Casting temperature has to be higher than the melting temperature. Casting temperature must also be adjusted according to casting technique and the complexity of the casting. Casting temperature also determines the materials fluidity. For a high fluidity, we have to choose a higher casting temperature. In addition the melting point also influences the selection of the mold material. If the melting temperature is too high, the mold material has to be more refractory and probably expensive. Low melting point is also important for long service life of molds. Pure metals and eutectic alloys melt and solidify at constant temperature. Alloys mostly have a solidification range and also amorphous solids (including many polymers) do not have a sharp melting point. For good castability of a metallic alloy, the solidification range has to be small. If the temperature range where the liquid and solid phases are both present very high, microsegregation and microporosity will occur. This is the reason why eutectic alloys (solidification at constant temperature) are preferred for casting alloys.
Melting Temperatures of Some Metals and Alloys
It is a measure of how well the liquid will flow and fill a mold cavity. Complex shaped castings cavities require the best fluidity. The same applies also to the casting process, which uses molds that include rapid cooling rates, like permanent metal mold process. Poor fluidity is less concern when the metal is cast by plaster or investment casting processes (slower cooling!) Fluidity is not only a material property, it is also affected by casting temperature, mold type, mold temperature etc. There are special technological tests to determine the fluidity under certain conditions.
c) Latent Heat of Fusion:
Latent Heat of Fusion is the heat required per unit mass to change a materials state to another state i.e. from a solid to liquid. For pure metals this heat is absorbed at constant temperature. When the transition from one state to another happens over a temperature range, it is not appropriate to define a latent heat of fusion.
d) Specific Heat:
Specific heat (c) is the energy amount which is used to rise temperature of 1 kg material by 1 °C (K). In casting process generally low specific heat is desired because low specific heat results in a low energy requirement to reach the melting temperature. Specific heat also affects the difference between melting temperature and casting temperature. When materials have high specific heat, difference between melting temperature and casting temperature can be less, because materials with high specific heat do not cool very easily as the amount of energy to be removed for cooling is high.
e) Thermal Conductivity:
Thermal conductivity coefficient affects the cooling rate. It also determines the temperature gradient and the internal stresses due to temperature differences. Because during solidification if some parts of the material cool rapidly and other parts of the material stay hot, there will be differences in the shrinkage and as a result internal stresses or some cracks may develop in the material. The cooling rate may also affect the phase transformation and the microstructure of a material (eg. martensite transformation in steel)
f) Thermal Diffusivity :
The heat transfer in a solidified casting is not steady-state. So it is realistic to consider the diffusivity rather than the conductivity. It is a measure of the rate at which a temperature disturbance at one point in a body travels to another point.
g) Coefficient of Expansion:
Metals expand when heated and contract when cooled. As a result of this, the dimensions of the material change during solidification and cooling in the mold cavity. We have to design the mold cavity by considering the expansion coefficient. Generally the cavity has bigger dimensions than the desired part.
h) Resistance to Hot Cracking:
During solidification, the hot metal has a very low strength, but it has to shrink as it cools. Due to temperature differences there will be strain mismatches in the cooling part. The modulus of elasticity determines the stress level of internal stresses which develop during cooling. The ductility determines whether a failure will occur due to these strain mismatches. If stresses are produced because of some factor that restrains the free contraction of the metal, the metal may not be able to resist this stress and cracks, also known as hot tears, will occur. Hot tearing is likely to be more problematic in permanent metal molds than in sand molds which are weak enough so that they can collapse as the casting shrinks.
Most metals expand when heated and contract when cooled. During the solidification the volume of the material will decrease. If no measures are taken this shrinking will result in casting defects like “lunker” and porosity. Shrinkage allowance is therefore one of the basic considerations during the dimensioning of the patterns. The amount of shrinkage is characteristic for each material.
j) Pressure Tightness:
The solidification shrinkage in some alloys creates a significant number of quite small internal voids. In some cases these voids, called porosity, permit gases to pass through the wall of the casting. Pressure Tightness is the ability to hinder gases to pass through.
k) Metallurgical Purity:
Metallurgical purity is important factor for castability. Impurities can also cause local stresses when the material solidifies so as result of this situation hot tearing or hot cracking increase. For example in steels sulfur layers are weak points for hot tearing.
l) Chemical Affinity :
For a good castability, material should not go into reaction with its environment, which is the mold and the atmosphere. If the chemical affinity is high oxidation may occur, in some cases the casting process has to be made under controlled atmosphere. Otherwise the casting quality will be badly affected in terms of dimensional stability and internal integrity.
m) Gas solubility:
The solubility in a material will drop during solidification and cooling. If gases are present in the melt and if they cannot escape, they will cause porosity in the casting.
n) Vapor Pressure:
During melting and pouring of the liquid metal alloy some elements can evaporate from the melt and the alloy’s chemical composition can change if their vapor pressure is too low (eg. zinc in brass).
Source: MANUFACTURING PROPERTIES of ENGINEERING MATERIALS Lecture Notes by Prof. Ahmet Aran http://www2.isikun.edu.tr/personel/ahmet.aran/mfgprop.pdf