Aluminum and Its Alloys

Introduction

The non-ferrous metals are those which contain a metal other than iron as their chief constituent. The non-ferrous metals are usually employed in industry due to the following characteristics:

  • Easy to fabricate i.e., casting, rolling, forging, welding and machining,
  • High resistance to corrosion,
  • Very good electrical and thermal conductivity,
  • Low weight and attractive appearance.

The various non-ferrous metals used in engineering practice are aluminium, copper, lead, tin. Zinc, nickel etc, and their alloys.

Aluminium

The chief source of Aluminium is a clayey mineral called ‘Bauxite’, which is a hydrated aluminium oxide (AI2O3.2H2O). In India, it is found in some districts of Bihar, Karnataka, Madhaya Pradesh, Maharashtra and Tamil Nadu.

Properties and Uses. It is a silvery white, light metal having specific gravity 2.7 and melting point 657°C. Its tensile strength varies from 90 MPa to 150 MPa(mega Pascal). In its pure state, the metal would be weak and soft for most purposes. But when mixed with small amounts of other alloys, it becomes hard and rigid. So, it may be blanked, formed, drawn, turned, cast, forged and die cast.

Its good electrical conductivity is an important property and is widely used for overhead cables. The high resistance to corrosion and its non-toxicity makes it a useful metal for cooking utensils under ordinary conditions. It is extensively used in aircraft and automobile components where saving of weight is an advantage.

Automobile Components
Fig.1 Automobile Components

Aluminium Alloys

The Aluminium may be alloyed with one or more other elements like Copper, Magnesium, Manganese, Silicon and Nickel. The addition of small quantities of alloying elements converts the weak metal into hard and strong metal, while still retaining its light weight. The main aluminium alloys are discussed below:

Super Dural

It is improved type of Duralumin having a modified composition. If annealed at 500ºC, the strength is 16.5 tons per square inch. But after 72 hours, it increases to 26.5 tons per square inch.

Alclad

It is an alloy, which is in sheet form, is protected against corrosion by having a layer of pure Aluminium on either side. Dural sheets are coated with (5% of its own thickness) pure Aluminium on both the sides and are known as Alclad.  The Alclad resists corrosion of Dural by preventing the corroding media coming in contact with the core.

Alclad Sheet
Fig 2 Alclad Sheet

Aldural

Aldural is an aluminium-dural alloy coated with another aluminium alloy containing Magnesium and Silicon on both sides which resist corrosion.

R.R. Alloy (Hiduminium)

This is developed by Rolles Royce Company and hence this alloy name is covered in D T D specification132, 133 and the specification for the whole range is:

Cu0.05 to 5.0%Iron0.6 to 1.5%
Ni0.2   to 1.5%Titanium0.0 to 0.5%
Mg0.1   to 0.5%Silicon0.2 to 0.5%

Within these limits both castings and forging alloys have been developed. One alloy of R.R. 56/D.T.D. 130 has tensile strength of 28.32 tons per   square inch, which an elongation of 10 to 20% on 2” bars. This is also sometimes called Super Dural.

Y –alloy

It is also called copper-aluminium alloy. The addition of Copper to pure aluminium increases its strength and machinability. The composition of this alloy is as follows,

Copper = 3.5-4.5%; Manganese = 1.2-1.7%; Nickel = 1.8-2.3%; Silicon, Magnesium, Iron = 0.6% each and the remainder is Aluminium.

This alloy is heat treated and age hardened like Duralumin. The ageing process is carried out at room temperature for about five days. It is mainly used for cast purposes, but it can also be used for forged components like Duralumin.

Uses

Since Y-alloy, has better strength (than duralumin) at high temperature, therefore, it is much used in aircraft engines, cylinder heads and pistons.

Cylinder Head
Fig 3 Cylinder Head

Alpax

It has a base of Aluminium consisting up to 10% Silicon. Its specific gravity is lower than that of Aluminium but its strength is about double the same. It is used for casting and has good resistance to corrosion.

Mg.7

It has a base of Aluminium containing Magnesium and a small percentage of Manganese. It is as light as Aluminium, but its strength is about double that of Aluminium. It has high resistance to inter-crystalline corrosion. No heat treatment is necessary to produce strength. It can also be welded without destroying the strength of surrounding material. It can be forged and cast.

Lautal

Lautal is an alloy of German origin. It contains about 39% Aluminium the remaining is Copper and Silicon. Small amount of iron also will be present. It has great resistance to sea-water corrosion and other corrosive atmospheric influence. Tensile strength is about 24 to 27 tons per square inch and is fairly ductile resembling mild steel in its properties.  As with Duralumin, it ages. It can easily be machined, forged and drawn. Lautal is used for marine purposes and   also for sea going aircraft fittings.

Electron

Electron is a Magnesium base alloy containing 10% Aluminium, 1.5% Zinc and 1% Manganese and is available in the form of rods, bars, sheets, tubes and casting. It is approximately 50% lighter in weights than other Aluminium alloys. Special protective treatment on the surface is necessary as it is very susceptible to corrosion.

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