Skip to main content

Metals for Casting

Metals for Casting

Most commercial castings are made of alloys rather than pure metals. Alloys are easier to cast and properties of the resulting product are better compared to pure metals.
Casting alloys can be classified as
  1. Ferrous or
  2. Nonferrous.

1. Ferrous Casting alloys:

The ferrous casting alloys is subdivided into two categories
  1. Cast iron and
  2. Cast steel.
1.1 Ferrous Casting Alloys: Cast iron
Cast iron is the most important of all casting alloys. The tonnage of cast iron castings is several times that of all other metals combined.
There several types of cast iron are listed under:
  1. Gray cast iron,
  2. Nodular iron,
  3. White cast iron,
  4. Malleable iron, and
  5. Alloy cast irons.
Typical pouring temperatures for cast iron is around 1400°C, depending on composition.
1.2 Ferrous Casting Alloys: Steel
The mechanical properties of steel make it an attractive engineering material and the capability to create complex geometries makes casting an appealing process. However, great difficulties are faced by the foundry specializing in steel. First, the melting point of steel is considerably higher than for most other metals that are commonly cast. The solidification range for low carbon steels begins at just under 1540°C (2804°F). This means that the pouring temperature required for steel is very high about 1650°C (3002°F). At these high temperatures, steel is chemically very reactive. It readily oxidizes, so special procedures must be used during melting and pouring to isolate the molten metal from the air. Also, molten steel has relatively poor fluidity, and this limits the design of thin sections in components cast out of steel. Several characteristics of steel castings make it worth the effort to solve these problems. Tensile strength is higher than for most other casting metals, ranging upward from about 410 Mpa (59,465 lb/in2). Steel castings have better toughness than most other casting alloys. The properties of steel castings are isotropic; strength is virtually the same in all directions.By contrast, mechanically formed parts (e.g., rolling, forging) exhibit directionality in their properties. Depending on the requirements of the product, isotropic behavior of the material may be desirable. Another advantage of steel castings is ease of welding. They can be readily welded without significant loss of strength, to repair the casting or to fabricate structures with other steel components.

2. Nonferrous Casting Alloys

Nonferrous casting metals include alloys of aluminum, magnesium, copper, tin, zinc, nickel, and titanium.
Aluminum alloys are generally considered to be very castable. The melting point of pure aluminum is 660°C (1112°F), so pouring temperatures for aluminum casting alloys are low compared to cast iron and steel. Their properties make them attractive for castings: light weight, a wide range of strength properties attainable through heat treatment, and ease of machining. Magnesium alloys are the lightest of all casting metals. Other properties include corrosion resistance, as well as high strength-to-weight and stiffness-to-weight ratios.
Copper alloys include bronze, brass, and aluminum bronze. Properties that make them attractive include corrosion resistance, attractive appearance, and good bearing qualities. The high cost of copper is a limitation on the use of its alloys. Applications include pipe fittings, marine propeller blades, pump components, and ornamental jewelry.
Tin has the lowest melting point of the casting metals. Tin-based alloys are easy to cast. They have good corrosion resistant but poor mechanical strength, which limits their applications to pewter mugs and similar products not requiring high strength.
Zinc alloys are commonly used in die casting. Zinc has a low melting point and good fluidity, making it highly castable. Its major weakness is low creep strength, so its castings cannot be subjected to prolonged high stresses.
Nickel alloys have the good hot strength and corrosion resistance, which make them suited to high-temperature applications such as jet engine and rocket components, heat shields, and similar components. Nickel alloys also have a high melting point and are not easy to cast.
Titanium alloys for casting are corrosion resistant and possess high strength-to-weight ratios. However, titanium has a high melting point, low fluidity, and a propensity to oxidize at high temperatures. These properties make it and its alloys difficult to cast.

Comments

Post a Comment

Popular posts from this blog

Grashof’s Law

Grashof’s Law The Grashof’s law states that for a four-bar linkage system, the sum of the shortest and longest link of a planar quadrilateral linkage is less than or equal to the sum of the remaining two links, then the shortest link can rotate fully with respect to a neighboring link. Consider a four-bar-linkage. Denote the smallest link by S, the longest link by L and the & other two links by P and Q. If the Grashof’s Law condition is satisfied i.e S+L ≤ P+Q, then depending on whether shortest link ‘S’ is connected to the ground by one end, two ends, or no end there are 3 possible mechanisms. They are: Double crank mechanism Double-rocker mechanism  and Crank and Rocker Mechanism 1. Double crank mechanism In double crank mechanism, the shortest link ‘S’ is a ground link. Both input crank and output crank rotate at 360°. Grashof’s condition for double crank mechanism: s+l > p+ q Let:  ‘s’  = length of shortest link, ‘l’  = length...
3 comments
Read more

Fundamental Concepts and Terms in Vibration

Fundamental Concepts and Terms in Vibration Vibration is a mechanical phenomenon, It is a movement first in one direction and then back again in the reverse direction. e.g: the motion of a swinging pendulum, the motion of a tuning fork. Any simple vibration is described by three factors: its amplitude; its frequency and rate of oscillation. Some of the general terms you will come across while studying on vibration topic are Oscillatory motion, Simple Harmonic Motion, Periodic Motion. now we will see the above mentioned terms in brief. Oscillatory motion is described as motion that repeats itself in a regular intervals of time. for example a sine wave or cos wave or pendulum. The time taken for an oscillation to occur is often referred to as the oscillatory period. Simple Harmonic Motion: Simple Harmonic Motion is periodic motion in which the restoring force is directly proportional to the displacement. F = -k*x A simple harmonic motion of a pendulum is an example of m...
1 comment
Read more

Gruebler’s Equation

Gruebler’s Equation Degrees of freedom for planar linkages joined with common joints can be calculated through Gruebler’s equation. Gruebler’s equation is given by the formula: where, n = total number of links in the mechanism j p  = total number of primary joints (pins or sliding joints) j h  = total number of higher-order joints (cam or gear joints) Mechanisms and structures with varying mobility for Figure (a), (b) and (c) Most linkages used in machines have a single degree of freedom. An example of single degree-of-freedom linkage is shown in figure (a). Linkages with zero or negative degrees of freedom are termed locked mechanisms. Locked mechanisms are unable to move and form a structure. A truss is a structure composed of simple links and connected with pin joints and zero degrees of freedom. An example of locked mechanism is shown in figure (b). Linkages with multiple degrees of freedom need more than one dri...
1 comment
Read more