Weldability of materials
Aluminium alloys
Aluminium and its alloys are used in fabrications because of their low
weight, good corrosion resistance and weldability. Although normally low
strength, some of the more complex alloys can have mechanical properties
equivalent to steels. The various types of aluminium alloy are identified and
guidance is given on fabricating components without impairing corrosion and
mechanical properties of the material or introducing imperfections into the
weld.
Material types
As pure aluminium is relatively soft, small amounts of alloying elements are
added to produce a range of mechanical properties. The alloys are grouped
according to the principal alloying elements, Specific commercial alloys have a
four-digit designation according to the international specifications for wrought
alloys or the ISO alpha - numeric system.
The alloys can be further classified according to the means by which the
alloying elements develop mechanical properties, non-heat-treatable or
heat-treatable alloys.
Non-heat-treatable alloys
Material strength depends on the effect of work hardening and solid solution
hardening of alloy elements such as magnesium, and manganese; the alloying
elements are mainly found in the 1xxx, 3xxx and 5xxx series of alloys. When
welded, these alloys may lose the effects of work hardening which results in
softening of the HAZ adjacent to the weld.
Heat-treatable alloys
Material hardness and strength depend on alloy composition and heat treatment
(solution heat treatment and quenching followed by either natural or artificial
ageing produces a fine dispersion of the alloying constituents). Principal
alloying elements are found in the 2xxx, 6xxx, 7xxx and 8xxx series. Fusion
welding redistributes the hardening constituents in the HAZ which locally
reduces material strength.
Most of the wrought grades in the 1xxx, 3xxx, 5xxx, 6xxx and medium strength
7xxx (e.g. 7020) series can be fusion welded using TIG, MIG and oxyfuel
processes. The 5xxx series alloys, in particular, have excellent weldability.
High strength alloys (e.g. 7010 and 7050) and most of the 2xxx series are not
recommended for fusion welding because they are prone to liquation and
solidification cracking.
Filler alloys
Filler metal composition is determined by:
- weldability of the parent metal
- minimum mechanical properties of the weld metal
- corrosion resistance
- anodic coating requirements
Nominally matching filler metals are often employed for non-heat-treatable
alloys. However, for alloy-lean materials and heat-treatable alloys,
non-matching fillers are used to prevent solidification cracking.
The choice of filler metal composition for the various weldable alloys is
specified in BS 3019 Pt 1 for TIG and BS 3571 Pt 1 for MIG welding; recommended
filler metal compositions for the more commonly used alloys are given in the
Table.
| Designation |
ISO |
Classification |
Filler |
Application |
| 1080A |
A1998 |
NHT |
1080A |
Chemical plant |
| 3103 |
A1-Mn1 |
NHT |
4043A |
Buildings, heat exchangers |
| 4043A |
A1-Si5 |
- |
- |
Filler wire/rod |
| 5083 |
A1-Mg4.5Mn |
NHT |
5556A |
Ships, rail wagons, bridges |
| 5251 |
Al-Mg2 |
NHT |
5356 |
Road vehicles, marine |
| 5356 |
Al-Mg5 |
- |
- |
Filler wire/rod |
| 5556A |
AlMg5Mn |
- |
- |
Filler wire/rod |
|
|
|
|
|
| 6061 |
Al-Mg1SiCn |
HT |
4043A/5356 |
Structural, pipes |
| 7020 |
Al-Zn,4. 5Mg1Mn |
HT |
5556A |
Structural, transport |
| HT = Heat Treatment NHT = Non heat treatable
|
Imperfections in welds
Aluminium and its alloys can be readily welded providing appropriate
precautions are taken. The most likely imperfections in fusion welds are:
- porosity
- cracking
- poor weld bead profile
Porosity
Porosity is often regarded as an inherent feature of MIG welds; typical
appearance of finely distributed porosity in a TIG weld is shown in the
photograph. The main cause of porosity is absorption of hydrogen in the weld
pool which forms discrete pores in the solidifying weld metal. The most common
sources of hydrogen are hydrocarbons and moisture from contaminants on the
parent material and filler wire surfaces, and water vapour from the shielding
gas atmosphere. Even trace levels of hydrogen may exceed the threshold
concentration required to nucleate bubbles in the weld pool, aluminium being one
of the metals most susceptible to porosity.
To minimise the risk, rigorous cleaning of material surface and filler wire
should be carried out. Three cleaning techniques are suitable; mechanical
cleaning, solvent degreasing and chemical etch cleaning.
Mechanical cleaning
Wire brushing (stainless steel bristles), scraping or filing can be used to
remove surface oxide and contaminants. Degreasing should be carried out before
mechanical cleaning.
Solvents
Dipping, spraying or wiping with organic solvents can be used to remove
grease, oil, dirt and loose particles.
Chemical etching
A solution of 5% sodium hydroxide can be used for batch cleaning but this
should be followed by rinsing in HNO3 and water to remove reaction products on
the surface.
In gas shielded welding, air entrainment should be avoided by making sure
there is an efficient gas shield and the arc is protected from draughts.
Precautions should also be taken to avoid water vapour pickup from gas lines and
welding equipment; it is recommended that the welding system is purged for about
an hour before use.
Solidification cracks
Cracking occurs in aluminium alloys because of high stresses generated across
the weld due to the high thermal expansion ( twice that of steel) and the
substantial contraction on solidification - typically 5 % more than in
equivalent steel welds.
Solidification cracks form in the centre of the weld,, usually extending
along the centreline during solidification. Solidification cracks also occur in
the weld crater at the end of the welding operation. The main causes of
solidification cracks are as follows:
- incorrect filler wire/parent metal combination
- incorrect weld geometry
- welding under high restraint conditions
The cracking risk can be reduced by using a non-matching, crack-resistant
filler (usually from the 4xxx and 5xxx series alloys). The disadvantage is that
the resulting weld metal may have a lower strength than the parent metal and not
respond to a subsequent heat treatment. The weld bead must be thick enough to
withstand contraction stresses. Also, the degree of restraint on the weld can be
minimised by using correct edge preparation, accurate joint set up and correct
weld sequence.
Liquation cracking
Liquation cracking occurs in the HAZ, when low melting point films are formed
at the grain boundaries. These cannot withstand the contraction stresses
generated when the weld metal solidifies and cools. Heat treatable alloys, 6xxx,
7xxx and 8xxx series alloys, are more susceptible to this type of cracking.
The risk can be reduced by using a filler metal with a lower melting
temperature than the parent metal, for example the 6xxx series alloys are welded
with a 4xxx filler metal. However, 4xxx filler metal should not be used to weld
high magnesium alloys (such as 5083) as excessive magnesium-silicide may form at
the fusion boundary decreasing ductility and increasing crack sensitivity.
Poor weld bead profile
Incorrect welding parameter settings or poor welder technique can introduce
weld profile imperfections such as lack of fusion, lack of penetration and
undercut. The high thermal conductivity of aluminium and the rapidly solidifying
weld pool make these alloys particularly susceptible to profile imperfections.
Copyright by TWI, 1999
About the IIW /
Mumbai Branch /
Other Branches /
Coming Events /
Technical Lectures
Mumbai Weldnet /
Trends in welding /
Related Websites /
IIW Forum /
Feedback /
Home
REPRODUCED - COURTESY TWI-UK