Welding Aluminum Alloys
The unique combination of light weight and relatively high strength
makes aluminum the second most popular metal that is welded. Aluminum
is not difficult to join but aluminum welding is different from welding
steels.
Aluminum possesses a number of properties that make welding it different
than the welding of steels. These are:
- Aluminum oxide surface coating.
- High thermal conductivity.
- High thermal expansion coefficient.
- Low melting temperature.
- The absence of color change as temperature approaches the melting point.
The normal metallurgical factors that apply to other metals apply to aluminum as well.
Aluminum is an active metal and it reacts with oxygen in the air to
produce a thin hard film of aluminum oxide on the surface. The melting
point of aluminum oxide is approximately 1926oC, which is almost three
times the melting point of pure aluminum, 660oC. In addition, this
aluminum oxide film, particularly as it becomes thicker, will absorb moisture from the air.
Moisture is a source of hydrogen which is the cause of porosity in aluminum
welds. Hydrogen may also come from oil, paint, and dirt in the weld area.
It also comes from the oxide and foreign materials on the electrode or
filler wire, as well as from the base metal. Hydrogen will enter the
weld pool and is soluble in molten aluminum. As the aluminum solidifies
it will retain much less hydrogen and the hydrogen is rejected during
solidification. With a rapid cooling rate free hydrogen is retained
within the weld and will cause porosity. Porosity will decrease weld
strength and ductility depending on the amount.
The aluminum oxide film must be removed prior to welding. If it is not
all removed small particles of un-melted oxide will be entrapped in the
weld pool and will cause a reduction in ductility, lack of fusion, and
may cause weld cracking.
Other reasons that aluminum welding is different are due to its high
thermal conductivity and low melting temperature. Aluminum conducts
heat from three to five times as fast as steel depending on the specific
alloy. This means that more heat must be put into the aluminum even
though the melting temperature of aluminum is less than half that of
steel.
Because of the high thermal conductivity, preheat is often used for
welding thicker sections. If the temperature is too high or the period
of time is too long it can be detrimental to weld joint strength in
both heat-treated and work-hardened alloys. The preheat for aluminum
should not exceed 204oC, and the parts should not be held at that
temperature longer than necessary. Because of the high heat conductivity
procedures should utilize higher speed welding processes using high heat
input. Both the gas tungsten arc and the gas metal arc processes supply
this requirement.
The high heat conductivity of aluminum can also be helpful since if heat
is conducted away from the weld extremely fast the weld will solidify
very quickly. This with surface tension helps hold the weld metal in
position and makes all-position welding with gas tungsten arc and gas
metal arc welding practical.
The thermal expansion of aluminum is twice that of steel. In addition,
aluminum welds decrease about 6% in volume when solidifying from the
molten state. This change in dimension or attempt to change in dimension
may cause distortion and cracking.
The final reason why aluminum is different to weld from steels is that
it does not exhibit color as it approaches its melting temperature.
Welding Copper-Base Alloys
Copper and copper-base alloys have specific properties which make them
widely used. Their high electrical conductivity makes them widely used
in the electrical industries and corrosion resistance of certain alloys
makes them very useful in the process industries. Copper alloys are also
widely used for friction or bearing applications.
Copper shares some of the characteristics of aluminum. Attention should
be given to its properties that make the welding of copper and copper
alloys different from the welding of carbon steels.
Copper alloys possess properties that require special attention
when welding. These are:
- High thermal conductivity.
- High thermal expansion coefficient.
- Relatively low melting point.
- It is hot short, i.e., brittle at elevated temperatures.
- The molten metal is very fluid.
- It has high electrical conductivity.
- It owes much of its strength to cold working.
Copper has the highest thermal conductivity of all commercial metals
and the comments made concerning thermal conductivity of aluminum
apply to copper, to an even greater degree.
Copper has a relatively high coefficient of thermal expansion,
approximately 50% higher than carbon steel, but lower than aluminum.
One of the problems associated with copper alloys is the fact that
some of them, such as aluminum bronze, have a coefficient of
expansion over 50% greater than that of copper. This creates
problems when making generalized statements about the different
copper-based alloys.
The melting point of the different copper alloys varies over a relatively
wide range, but is at least 538oC lower than carbon steel. Some of the
copper alloys are hot short. This means that they become brittle at high
temperatures. This is because some of the alloying elements form oxides
and other compounds at the grain boundaries, embrittling the material.
Copper does not exhibit heat colors like steel and when it melts it
is relatively fluid. This is essentially the result of the high
preheat normally used for heavier sections. Copper has the highest
electrical conductivity of any of the commercial metals and this
is a definite problem in the resistance welding processes.
All of the copper alloys derive their strength from cold working. The heat
of welding will anneal the copper in the heat-affected area adjacent to
the weld and reduce the strength provided by cold working. This must be
considered when welding high-strength joints.
There is one other problem associated with the copper alloys that contain
zinc. Zinc has a relatively low boiling temperature, and under the heat
of an arc will tend to vaporize and escape from the weld. For this reason
the arc processes are not recommended for the alloys containing zinc.
Welding Magnesium - Base Alloys
Magnesium is the lightest structural metal. It is approximately
two-thirds as heavy as aluminum and one-fourth as heavy as steel. Magnesium
alloys containing small amounts of aluminum, manganese, zinc, zirconium,
etc., have strengths equaling that of mild steels. They can be rolled
into plate, shapes, and strip.
Magnesium can be cast, forged, fabricated, and machined. As a structural
metal it is used in aircraft. It is used by the materials-moving industry
for parts of machinery and for hand-power tools due to its strength to
weight ratio.
Magnesium can be welded by many of the arc and resistance welding
processes, as well as by the oxy-fuel gas welding process, and it
can be brazed. Magnesium possesses properties that make welding it
different than the welding of steels. Many of these are the same as
for aluminum. These are:
- Magnesium oxide surface coating
- High thermal conductivity
- Relatively high thermal expansion coefficient
- Relatively low melting temperature
- The absence of color change as temperature approaches the melting point.
The normal metallurgical factors that apply to other metals apply to
magnesium as well. Magnesium is a very active metal and the rate of
oxidation increases as the temperature is increased. The melting point
of magnesium is very close to that of aluminum, but the melting point
of the oxide is very high. In view of this, the oxide coating must be
removed.
Magnesium has high thermal heat conductivity and a high coefficient
of thermal expansion. The thermal conductivity is not as high as
aluminum but the coefficient of thermal expansion is very nearly
the same. The absence of color change is not too important with
respect to the arc welding processes.
Welding Nickel - Base Alloys
Nickel and the high-nickel alloys are commonly used when corrosion
resistance is required. They are used in the chemical industry and the
food industry. Nickel and nickel alloys are also widely used as filler
metals for joining dissimilar materials and cast iron.
When welding, the nickel alloys can be treated much in the same manner
as austenitic stainless steels with a few exceptions. These exceptions are:
- The nickel alloys will acquire a surface oxide coating which melts
at a temperature approximately 538oC above the meting point of the base
metal.
- The nickel alloys are susceptible to emrittlement at welding
temperatures by lead, sulphur, phosphorus, and some low temperature
metals and alloys.
- Weld penetration is less then expected with other metals.
When compensation is made for these three factors the welding procedures
used for the nickel alloys can be the same as those used for stainless
steel. This is because the melting point, the coefficient of thermal
expansion, and the thermal conductivity are similar to austenitic
stainless steel.
It is necessary that each of these precautions be considered. The surface
oxide should be completely removed from the joint area by grinding,
abrasive blasting, machining, or by chemical means. When chemical
etches are used they must be completely removed by rinsing prior to
welding. The oxide which melts at temperatures above the melting point
of the base metal may enter the weld as a foreign material, or impurity,
and will greatly reduce the strength and ductility of the weld.
The problem of embrittlement at welding temperatures also means that the
welding surface must be absolutely clean. Paints, marking crayons, grease,
oil, machining lubricants, cutting oils may all contain the ingredients
which will cause embrittlement. They must be completely removed from
the weld area to avoid embrittlement.
Finally, with respect to the minimum penetration, it is necessary to
increase the opening of groove angles and to provide adequate root
openings when full-penetration welds are used. The bevel or groove
angles should be increased to approximately 40% over those used for carbon.
Almost all the welding processes can be used for welding the nickel
alloys. In addition, they can be joined by brazing and soldering.
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