Alloy Physical Characteristics

The performance of any given alloy is affected by its physical and mechanical characteristics. All of these characteristics vary significantly amongst different properties of different alloys.

Physical characteristics:

1. Capability for filling the cavity of the mold

It is also known as the fluid life of the alloy. This helps in determining the flowing capability of the metal via tapered canals forming thin sections.

This fluid life of the alloy is mainly affected by the temperature levels of the molten material. The relation between the temperature and fluid life of the alloy is not directly proportional. That means, extremely high temperature of the material doesn't increase the fluid life. Also, fluid life doesn't necessarily increase proportionately to the temperature as there are other factors affecting the fluid life as chemical, physical, which determines the elastic behaviors of the liquid.

Understanding the fluid life of the material helps in distinguishing more details which further helps in designing of the cast. They are:

  • Minimum width of the material can be determined
  • Lengthwise measurement of the thin section can also be determined

Apart from the above, it helps in attaining other finest details of the material, which are not easily available otherwise. A shorter fluid life never binds the cost of the design. In fact, a poor fluid life determines following features of the casting as flexible shape formation, improved aspects of the base part of the manufactured casting, bigger inscription option, thinner and finer sections in the mold etc.

Some molds are dehydrated during casting development. But this development procedure reduces the convection effects, which is the heat transfer process causing reduced fluid life.

When molten metal allows solidification, they pass through three stages:

  • Liquid shrinkage
  • Liquid-to-solid shrinkage
  • Solid shrinkage

Liquid Shrinkage - It is the process of retrenchment of liquid beginning before the solidification process starts. Though it is important to metal casters, it doesn't play an important role in the designing process.

Liquid-to-Solid Shrinkage- When the molecules and atoms of the molten metal starts converting to solid chemical composite it is known as Liquid to solid or solidification shrinkage. Though, this amount of shrinkage varies depending upon the metal properties of the alloy and can lie between extreme high and extreme low shrinkage volumes. Shrinkage volumes of the metal helps in categorizing under three groups:

  • Directional
  • Eutectic
  • Equiaxed

However, it becomes very important to understand the affect of Liquid-to-Solid Shrinkage from the designing point of view with the help of drafting geometry of the design to achieve the configuration of the casting and its solidification shrinkage requirements. An alternate solution in the absence of geometry formation for high tech alloys is known as "Thermal Trickery". Since, it's an expensive process, most of the casting designs avoid its use.

Solid shrinkage- it is the last stage of the molten metal when the metal is cooled to solidify. This is also well known as patternmaker's shrink. During this stage, facets of the design are entirely changed as per the shrinkage rate of the alloy. This changeability of solid shrinkage plays a crucial role in the design consideration.

2 Slag & Dross Formation

xSlag is generally associated with ferrous metals having high melting point, and they are the composition of liquid nonmetallic composites such as fluxed refractors, alloying products, and corrosion products. Dross, alternatively, is coupled with non-ferrous metals having lower point of melting metal, and is generally composed of the nonmetallic composites produced mostly by the reactions of molten metal with the air.

3. Pouring Temperature

The molds are required to withstand enormously high temperature of molted metals. With the increase in the temperature of the molten metal alloy, design considerations are required for heat flow issues of the mold.

Table 1.1 - Chart of Pouring Temperature for different alloys:

Alloy °F °C
Solder ~450 ~230
Tin ~600 ~300
Lead ~650 ~345
Zinc Alloys 650-850 345- 455
Aluminum Alloys 1150-1350 620- 735
Copper based Alloys 1650-2150 900-1180
Gray, Cast Irons Ductile 2450-2700 1340-1480
High Irons Alloys 2800-3000 1540-1650
Titanium Cast Alloys 3100-3300 1700-1820
Zirconium Cast Alloys 3350-3450 1845-1900

Apart from the physical characteristics, mechanical characteristics are also crucial in metal casting design. Two important characteristics are:
Mechanical Characteristics:
  • Modulus of elasticity - This helps in determining the stiffness of the given casting design.
  • Section Modulus- This determined the stiffness of the casting geometry of the alloy.
Together, these physical and metallurgical features determine the designing, production and utilization of metal castings.
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