Non-ferrous Metals Casting

Ferrous metals are extensively used in the engineering industry because of their superiority, range of mechanical properties and lower costs. Still, non-ferrous metals are also used in various applications for their specific properties compared to ferrous alloys in spite of their generally high cost. Desired mechanical properties can be obtained in these alloys by work hardening, age hardening, etc, but not through normal heat treatment processes used for ferrous alloys. Some of the principal non-ferrous materials of interest are aluminium, copper, zinc, and magnesium

1. Aluminium

Of all non-ferrous alloys, aluminium and its alloys are the most important because of their excellent properties. Some of the properties of pure aluminium for which it is used in the engineering industry are:

• 1) Excellent thermal conductivity(0.53 cal/cm/C)
• 2) Excellent electrical conductivity (376 600/ohm/cm)
• 3) Low mass density (2.7 g/cm)
• 4) Low melting point (658C)
• 5) Excellent corrosion resistance
• 6) It is nontoxic.
• 7) It has one of the highest reflectivities (85 to 95%)and very low emissivity (4 to 5%)
• 8) It is very soft and ductile as a result of which it has very good manufacturing properties.

Some of the applications where pure aluminium is generally used are in electrical conductors, radiators fin materials, air conditioning units, optical and light reflectors, and foil and packaging materials. 

In spite of the above useful applications, pure aluminium is not widely used because of the following problems:

• 1) It has low tensile strength(65 MPa) and hardness (20 BHN)
• 2. It is very difficult to weld or solder.

The mechanical properties of aluminium can be substantially improved by alloying. The principal alloying elements used are copper, manganese, silicon, nickel and zinc.

Aluminium and copper form the chemical compound CuAl2. Above a temperature of 548 C it dissolves completely in liquid aluminium. When this is quenched and artificially aged (prolonged holding at 100 - 150C), a hardened alloy is obtained. The CuAl2, which is not aged does not have time to precipitate from the solid solution of aluminium and copper and thus is in an unstable position(super-saturated at room tempera ture). The ageing process precipitates very fine particles of CuAl2, which causes the strengthening of the alloy. This process is called solution hardening.

The other alloying elements used are up to 7% magnesium, up to 1. 5% manganese, up to 13% silicon, up to 2% nickel, up to 5% zinc and up to 1.5% iron. Besides these, titanium, chromium and columbium may also be added in small percentages. The composition of some typical aluminium alloys used in permanent moulding and die casting is given in Table 2. 10 with their applications. The mechanical properties expected of these materials after these are cast using permanent moulds or pressure die casting is shown in Table 2.1

2. Copper

Similar to aluminium, pure copper also finds wide application because of its following properties

• 1) The electrical conductivity of pure copper is high(5.8 x 105 /ohm/cm) in its purest form. Any small impurity brings down the conductivity drastically. For example, 0. 1% phosphorous reduces the conductivity by 40%.
• 2) It has a very high thermal conductivity (0. 92 cal/cm/C)
• 3) It is a heavy metal (specific gravity 8.93)
• 4) It can readily be joined together by brazing
• 5) It resists corrosion,
• 6) It has a pleasing colour.

Pure copper is used in the manufacture of electrical wire, bus bars, transmission cables, refrigerator tubing and piping.

The mechanical properties of copper in its purest state are not very good. It is soft and relatively weak. It can be alloyed profitably to improve the mechanical properties. The main alloying elements used are zinc, tin, lead and phosphorous.

The alloys of copper and zinc are called brasses. With a zinc content up to 39%, copper forms a single phase (α-phase) structure. Such alloys have high ductility. The colour of the alloy remains red up to a zinc content of 20%, but beyond that it becomes yellow. A second structural component called β-phase appears between 39 to 46% of zinc. It is actually the inter-metallic compound CuZn which is responsible for the increased hardness. The strength of brass gets further increased when small amounts of manganese and nickel are added.

The alloys of copper with tin are called bronzes. The hardness and strength of bronze increase with an crease in tin content. The ductility is also reduced with the increase in tin percentage above 5. When aluminium is also added(4 to 11%), the resulting alloy is termed aluminium bronze, which has a considerably higher corrosion resistance. Bronzes are comparatively costly compared to brasses due to the presence of tin which is an expensive metal.

3. Other Non-Ferrous Metals

Zinc

Zinc is principally used in engineering because of its low melting temperature (419.4 C) and higher corrosion resistance, which increases with the purity of zinc. The corrosion resistance is caused by the formation of a protective oxide coating on the surface. Principal applications of zinc are in galvanizing to protect steel from corrosion, in printing industry and for die casting.

The disadvantages of zinc are the strong anisotropy exhibited under deformed conditions, lack of dimensional stability under ageing conditions, a reduction in impact strength at lower temperatures and the susceptibility to inter-granular corrosion. It cannot be used for service above a temperature of 95.C because it will cause substantial reduction in tensile strength and hardness.

Its widespread use in die castings is because it requires lower pressure, which results in higher die life compared to other die casting alloys. Further, it has very good machinability. The finish obtained by zinc diecasting is often adequate to warrant any further processing, except for the removal of the flash present in the parting plane.

Magnesium

Because of their light weight and good mechanical strength, magnesium alloys are used in very high speeds. For the same stiffness, magnesium alloys require only 37. 2% of the weight of C25 steel thus saving in weight. The two principal alloying elements used are aluminium and zinc. Magnesium alloys can be sand cast, permanent mould cast or die cast. The properties of sand-cast magnesium alloy components  are comparable with those of the permanent mould cast or die-cast components. The die-casting alloys generally ally have high copper content so as to allow them to be made from the secondary metals to reduce costs. They are used for making automobile wheels, crank cases, etc. The higher the content, the higher the mechanical strength of magnesium-wrought alloys such as rolled and forged components. Magnesium alloys can be readily welded by most of the traditional welding processes. A very useful property of magnesium alloys is their high machinability. They only require about 15% of power for machining compared to low carbon steel.