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What you should know about welding aluminum
In recent years, the use of aluminum in manufacturing has become more
prevalent because of its light weight and other attributes that make it
an attractive alternative to steel. In fact, the aluminum welding market
is expected to grow at a rate of 5.5 percent annually based primarily
on the assumption that the automotive industry will continue to increase
its use of aluminum.
But, those experienced in the welding of steel will find aluminum to be
a different breed – the normal welding characteristics of steel don’t
always apply to aluminum. For example, aluminum’s high thermal conductivity
and low melting point can easily lead to burnthrough and warpage problems
if proper procedures are not followed.
In this article, we will first take a look at various alloying elements
and how they affect aluminum; then we will turn our attention to welding
procedures and the parameters that will create the best quality weld.
Lastly, we will examine some new technology breakthroughs that make welding
aluminum a little easier.
Alloying Elements
To understand aluminum, you must first understand some basics about aluminum
metallurgy. Aluminum can be alloyed with a number of different elements,
both primary and secondary, to provide improved strength, corrosion resistance
and/or general weldability.
The primary elements that alloy with aluminum are copper, silicon, manganese,
magnesium and zinc. But, before we examine them in detail and what they
bring to aluminum, it is important to note that these alloys fall into
two classes: heat-treatable or nonheat-treatable.
Heat-Treatable vs. Nonheat-Treatable Alloys
Heat-treatable alloys are those that can be heated after welding
to regain strength lost during the welding process. To heat-treat an alloy
means heating it at a high temperature, putting the alloying elements
into solid solution and then cooling it at a rate which will produce a
supersaturated solution. The next step in the process is to maintain it
at a lower temperature long enough to allow a controlled amount of precipitation
of the alloying elements.
With the nonheat-treatable alloys it is possible to increase strength
through cold working or strain hardening. To do this, a mechanical deformation
must occur in the metal structure, resulting in increased resistance to
strain, producing higher strength and lower ductility.
Further
Distinctions
To further designate aluminum alloys, they can also be classified by a
temper designation which are as follows: F = As fabricated, O = Annealed,
H = Strain hardened; W = Solution heat-treated and T = Thermally treated,
which can designated heat treatment, or cold working aging. For example
an alloy may carry the designation of 2014 T6. This means that it is alloyed
with copper (2XXX series) and the T6 refers to the fact that it is solution
heat-treated and artificially aged.
For purposes of this article, we will discuss wrought alloys, which are
those aluminum alloys that are rolled from ingot or extruded with customer
specified shapes. But please note that alloys can also be divided into
cast alloys. Cast alloys are those used to manufacture parts from molten
alloys of aluminum poured into molds. Cast alloys are precipitation hardenable
but never strain hardenable. The weldability of these alloys is affected
by casting type – permanent mold, die cast, and sand – since the casting
surface is critical to welding success. A three-digit number, plus one
decimal i.e. 2xx.x designates the cast alloys. Weldable grades of aluminum
castings are 319.0, 355.0, 356.0, 443.0, 444.0, 520.0, 535.0, 710.0 and
712.0.
Alloying Elements
Now, that you understand some of the terminology, let’s take a look at
the different alloying elements:
Copper (which carries a wrought alloy designation of 2XXX series)
provides high strength to aluminum. This series is heat-treatable and
mainly used in aircraft engine parts, rivets and screw products. Most
2XXX series alloys are considered poor for arc welding because of their
sensitivity to hot cracking. These alloys are generally welded with 4043
or 4145 series filler electrodes, which have low melting points to reduce
the probability of hot cracking. Exceptions to this are alloys 2014, 2219
and 2519, which are easily welded with a 2319 filler wire.
Manganese (3XXX series) added to aluminum yields a nonheat-treatable
series used for general-purpose fabrication and build-up. Moderate in
strength, the 3XXX series is used for forming applications including utility
and van trailer sheet. It is improved through strain hardening to provide
good ductility and improved corrosion properties. Typically welded with
4043 or 5356 electrode, the 3XXX series is excellent for welding and not
prone to hot cracking. Its moderate strengths do prevent this series from
being used in structural applications.
Silicon (4XXX series) reduces the melting point of aluminum and
improves fluidity. Its principle use is as filler metal. The 4XXX series
has good weldability and is considered a nonheat-treatable alloy. Alloy
4047 is becoming the alloy of choice in the automotive industry, as it
is very fluid and good for brazing and welding.
Magnesium (5XXX series), when added to aluminum, has excellent
weldability with a minimal loss of strength and is basically not prone
to hot cracking. In fact, the 5XXX series has the highest strength of
the nonheat-treatable aluminum alloys. It is used for chemical storage
tanks and pressure vessels at elevated temperatures as well as structural
applications, railway cars, dump trucks and bridges because of its corrosion
resistance. It looses ductility when welded with 4XXX series fillers due
to formation of Mg2Si.
Silicon and Magnesium (6XXX series) combine to serve as alloying
elements for this medium-strength, heat-treatable series. It is principally
used in automotive, pipe, railings, structural and extruding applications.
The 6XXX series is somewhat prone to hot cracking, but this problem can
be overcome by the correct choice of joint and filler metal. This series
can be welded with either 5XXX or 4XXX series without cracking – adequate
dilution of the base alloys with selected filler alloy is essential. A
4043 electrode is the most common for use with this series.
Zinc
(7XXX series) added to aluminum with magnesium and copper produces the
highest strength heat-treatable aluminum alloy. It is primarily used in
the aircraft industry. The weldability of the 7XXX series is compromised
in higher copper grades, as many of these grades are crack sensitive (due
to wide melting ranges and low solidus melting temperatures.) Grades 7005
and 7039 are weldable with 5XXX fillers.
Other elements (8XXX series) that are alloyed with aluminum (i.e.
lithium) all fall under this series. Most of these alloys are not commonly
welded, though they offer very good rigidity and are principally used
in the aerospace industry. Filler metal selection for these heat-treatable
alloys include the 4XXX series.
Pure Aluminum (1XXX series), though not an alloying element, is
considered nonheat-treatable and is used primarily in chemical tanks and
piping because of its superior corrosion resistance. This series is also
used in electrical bus conductors because of its excellent electrical
conductivity. 1XXX series are easily welded with 1100 and 4043 alloys.
In addition to the primary aluminum alloying elements, there is a number
of secondary elements, which include chromium, iron, zirconium, vanadium,
bismuth, nickel and titanium. These elements combine with aluminum to
provide improved corrosion resistance, increased strength and better heat
treatability.
Physical Properties
Now that you have a basic background on aluminum metallurgy, we will move
into the physical properties of base metal aluminum and how it compares
to other metals, primarily steel.
The reason
why aluminum is becoming specified for so many jobs is its physical properties.
For instance, aluminum is three times lighter than steel and yet offers
higher strength when alloyed with the right elements. It can conduct electricity
six times better than steel and nearly 30 times better than stainless
steel. This high electrical conductivity makes the effect of electrical
stick-out in GMAW (Gas Metal Arc Welding) less significant when compared
to steel (we will cover this concept in more detail later in this article.)
In addition,
aluminum provides excellent corrosion resistance, is easy to shape and
join, and also is non-toxic for food applications. Since it is non-magnetic,
arc blow is not a problem during welding. With a thermal conductivity
rate that is five times higher than steel and being less viscous, aluminum
can easily be welded out-of-position. Aluminum does have its drawbacks,
though, since its high thermal conductivity tends to act as a heat sink
making fusion and penetration more difficult.
Since aluminum has a low melting point 1,200 degrees F (half that of steel)
for the same wire size, the transition current for aluminum is much lower
than it is for steel. Also, for the same welding current, the burn-off
rate is about twice that of steel.
Chemical Properties
In terms of chemical composition, aluminum has a high maximum solubility
for hydrogen atoms in the liquid form and a low solubility at the solidification
point. This means that even a small amount of hydrogen dissolved in the
liquid weld metal will tend to escape as the aluminum solidifies and porosity
is likely to occur – a great cause of concern during the welding process.
Also, aluminum combines with oxygen to form an aluminum oxide layer instantaneously
as it is machined. This layer is very porous and can easily trap moisture,
oil, grease and other materials. The oxide provides excellent corrosion
resistance, but must be taken off before welding as it prevents fusion
due to its high melting point (3700 degrees F). Mechanical cleaning, solvents,
chemical etching and purging are used to take off the oxide layer.
Mechanical
Properties
Mechanical properties such as tensile strength, yield and elongation are
affected by the choice of aluminum base and filler alloys. For groove
welds, the Heat Affected Zone (HAZ) dictates the strength of the joint.
In nonheat-treatable aluminum alloys, the HAZ will be completely annealed
and the HAZ will be the weakest point. Heat-treatable alloys require much
longer periods at annealing temperatures combined with slow cooling to
completely anneal them so that weld strength is less affected. Such items
as preheating, lack of interpass cooling, and excessive heat input from
slow, weaving weld passes all increase peak temperature and time at temperature,
which means minimum strength levels might not be met.
For fillet welds, strength is dependent on the composition of the filler
alloy used to weld the joint. In structural applications, the selection
of 5XXX instead of 4XXX series filler can provide twice the strength
The nonheat-treatable
alloys offer excellent ductility when using matching fillers, though lower
ductility results from welds made with 4XXX series. Heat-treatable alloys
do not exhibit high ductility, and post-weld heat treatments generally
reduce ductility.
Conclusion
Aluminum has many attractive attributes that make it the material of choice
for a host of applications, although it can be different to weld. But,
with a good understanding of metallurgy and the latest tools and technology
on the market, aluminum can be dealt with successfully.
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