Page 20 - Welding of Aluminium and its Alloys

12 The welding of aluminium and its alloys

(a) (b) (c)
2.2 The three crystalline forms of metals: (a) body-centred cubic; (b)
face-centred cubic; (c) close-packed hexagonal. (From John Lancaster,
Metallurgy of Welding, 6th edn, 1999.)

electrical conductivity and the ability to deform extensively before fractur-
ing by a process known as slip, where one plane of atoms slides over its
neighbours.
In metals the atoms are arranged in a regular three-dimensional pattern
repeated over a long distance on what is termed a space lattice. Conven-
tionally, these atoms are visualised as solid spheres. The smallest atomic
arrangement is the unit cell, the least complicated unit cell being the simple
cube with an atom at each corner of the cube. In metals the three most
common arrangements are body-centred cubic (BCC), face-centred cubic
(FCC) and hexagonal close packed (HCP). Schematic views of the three
structures are given in Fig. 2.2.
Each crystal structure confers certain physical properties on the metal.
The face-centred cubic metals, of which aluminium is one, are ductile,
formable and have high toughness at low temperatures. Although single
crystals can be obtained it is more common for metals to be polycrystalline,
that is, made up of a very large number of small grains. Each grain is a
crystal with a regular array of atoms but at the boundaries between the
grains there is a mismatch, a loss of order, in the orientation of these arrays.
Both the grain boundaries and the size of the grains can have a marked
effect on the properties of the metal.

2.2.2 Grain size control
Grain size is not generally used to control strength in the aluminium alloys,
although it is used extensively in reducing the risk of hot cracking and in
controlling both strength and notch toughness in C/Mn and low-alloy steels.
In general terms, as grain size increases, the yield and ultimate tensile
strengths of a metal are reduced.The yield strength s y, is related to the grain
size by the Hall–Petch equation:
s y = s I + kd -12
y