Page 26 - Welding of Aluminium and its Alloys

18 The welding of aluminium and its alloys

Table 2.1 Summary of mechanical properties for
some aluminium alloys

Alloy Condition Proof UTS Elongation
2
2
(Nmm ) (Nmm ) (%)
1060 O 28 68 43
1060 H18 121 130 6
5083 O 155 260 14
5083 H34 255 325 5
6063 O 48 89 32
6063 TB(T4) 100 155 15
6063 TF(T6) 180 200 8
2024 O 75 186 20
2024 TB(T4) 323 468 20

UTS: ultimate tensile strength

2.2.6 Summary

This chapter is only the briefest of introductions to the science of metals,
how crystal structures affect the properties and how the fundamental mech-
anisms of alloying, hardening and heat treatment, etc., are common to all
metals. Table 2.1 gives the effects of solid solution strengthening, cold
working and age hardening. It illustrates how by adding an alloying element
such as magnesium, the strength can be improved by solid solution alloy-
2
ing from a proof strength of 28N/mm in an almost pure alloy, 1060, to 115
2
N/mm in an alloy with 4.5% magnesium, the 5083 alloy. Similarly, the
effects of work hardening and age hardening can be seen in the increases
in strength in the alloys listed when their condition is altered from the
annealed (O) condition. Note, however, the effect that this increase in
strength has on the ductility of the alloys.

2.3 Aluminium weldability problems

2.3.1 Porosity in aluminium and its alloys

Porosity is a problem confined to the weld metal. It arises from gas dis-
solved in the molten weld metal becoming trapped as it solidifies, thus
forming bubbles in the solidified weld (Fig. 2.7).
Porosity can range from being extremely fine micro-porosity, to coarse
pores 3 or 4mm in diameter. The culprit in the case of aluminium is hydro-
gen, which has high solubility in molten aluminium but very low solubility
in the solid, as illustrated in Fig. 2.8. This shows a decrease of solubility to
the order of 20 times as solidification takes place, a drop in solubility so