Aluminum is very easy to weld, contrary to popular
opinion. Aluminum melts at a fairly low temperature, 1218°F. Like
copper, aluminum transmits heat very readily. This makes it difficult to
get a small, specific part of the piece hot enough to weld. Welders
experience problems in heavy sections because they dont put enough
preheat into the part. (Remember, the heat is continually being
dissipated.) Thin or small pieces of aluminum will heat up very easily
and, in fact, can very quickly reach the melting point and end up as
molten metal on the floor.
The proper way to preheat aluminum is to use a heat indicator that will
show you when to start welding. Two simple devices are temperature
indicating crayons or soot from the oxy-acetylene flame. When soot is on
the welding surface and is heated, it will disappear when the base metal
reaches 550°F. For Torch and TIG Welding, preheat to about 550°F before
welding. For Arc Welding, preheat to about 350°F. Remember, thinner
sections need less preheat and heavier sections need higher preheat To
better understand the makeup of aluminum alloys, the following is a list
of the various grades of aluminum. The Aluminum Association has set a
designation system as follows:
1XXX Series: These are aluminums of 99% or higher purity. They
are used primarily in the electrical and chemical industries. These
grades are easily welded.
2XXX Series: Copper is the principal alloy in this group. This
group provides extremely high strength when properly heat treated. These
alloys do not produce as good corrosion resistance and are often clad
with pure aluminum or special-alloy aluminum. These alloys are used in
the aircraft industry. They are usually
difficult to weld.
3XXX Series: Manganese is the major alloying element in this
group. These alloys are non-heat-treatable. Manganese is limited to
about 1.5%. These alloys have moderate strength and are easily worked
and welded.
4XXX Series: Silicon is the major alloying element in this group.
It can be added in sufficient quantities to substantially reduce
melting point and is used for brazing alloys and welding electrodes.
Most of the alloys in this group are non-heat-treatable. These grades
are easily welded.
5XXX Series: Magnesium is the major alloying element of this group. These alloys are of medium strength. They possess good welding characteristics, good resistance to corrosion, but the amount of cold work should be limited. These grades are easily welded.
6XXX Series: Alloys in this group contain silicon and magnesium
which make them heat treatable. These alloys possess medium strength and
good corrosion resistance and have good welding properties.
7XXX Series: Zinc is the major alloying element in this group.
Magnesium is also included in most of these alloys. Together they result
in a heat-treatable alloy of very high strength. This series is used for
aircraft frames. This grade is often heat treated to get
higher tensile strengths. They are usually unweldable.
The casting grades of aluminum have a two or three digit identification
number, the more common grades being 122, 355 and 356. Most casting
grades of aluminum are high in silicon. Silicon makes aluminum thin flow
when it is molten so that it will get into all parts of the mold during
the casting operation. Silicon enhances weldability, hence aluminum
castings are usually easily welded.
As with any casting, care must be taken when large heavy parts are
preheated to higher temperatures. Because metal usually gets weaker as
it gets hotter, heavy casting can collapse. Very often, steel braces or
supports must be placed in the casting to keep it from distorting or
collapsing while welding.Occasionally, parts that appear to be aluminum
are actually magnesium. Magnesium has the same color and is light in
weight like aluminum. To identify the base metal, scrape an area about
the size of a penny with a knife blade. Put a drop of soldering or
battery acid on the cleaned spot. If the drop starts to bubble and turn
black, the part is magnesium. If not, it is aluminum.
Most aluminums, whether they are cast, sheet, or wrought, are easily
welded using MG 400. This versatile alloy is used as an arc rod (DC
Reverse Polarity) in heavier sections, and as a brazing alloy on thin
sections. It is a thin flowing alloy for good fit-up and a bead forming
alloy for poor fit-up. If the flux is removed and the core wire cleaned,
it can be used as a 4043 type TIG wire.
For aluminum parts that must be brazed with high strength at the lowest
possible bonding temperature, use MG 410. This alloy should be used only
where good fit-up is the rule. Extremely dirty material is easily brazed
using MG 420. This product is a flux cored brazing alloy with an
aggressive flux to make easy joints on the dirtiest aluminum. Soldering
is easily accomplished using the MG 460 kit. The joint area should be
mechanically cleaned by scraping or sanding. Squeeze enough flux onto
the surface to cover the entire joint area. For best results, the parts
should be preheated with a torch, then a soldering iron should be used
to apply the MG 460 alloy to the joint. If a torch is used, it is best
to preplace a piece of MG 460 alloy into the fluxed area and heat the
area uniformly until the flux starts to bubble; remove the torch and
wait until the flame goes out. Let it cool slowly. Rinse in water to
remove the flux residue.
