Aluminum is a very sought after material due to its many desirable properties. Some of these properties are: lightweight, excellent strength to weight ratio, easy machinability, formability, and weldability. Although pure aluminium has limited usability, it has vast usage when it is alloyed with few alloys. Fabrication using aluminium involves welding. And welding aluminum requires knowing its characteristic behaviors.
In this article we shall discuss the characteristic properties of aluminum that distinguish it from other metals. Later on we shall discuss a few aspects of welding aluminum.
Commercially produced aluminium alloys can be categorized into two:
- Wrought alloys: These are the alloys designed for mills. The final form is obtained by performing mechanical work on the metal such as forming, etc.
- Casting alloys: As the name suggests, these are the alloys in which the final form is attained by casting the molten metal in a mould.
Uses Of Aluminum
Aluminum finds usage as a deoxidizing element in the manufacture of steel. Steel containing small amounts of aluminium is called killed steel. Aluminum finds extensive usage in aviation industry due to it’s good strength to weight ratio.
Due to its’ good machinability, formability and weldability, it is also used in making kitchen utensils, railway cars, transmission lines, etc.
Properties Of Aluminum
For pure aluminium:
Tensile strength: 6000 – 16000 psi (41,370 to 110,320 kPa),
Brinell Hardness Number: 17 – 27
Specific Gravity: 2.7
Melting Point: 660°C
For alloyed aluminum:
Tensile strength: 30,000 to 75,000 psi (206,850 to 517,125 kPa)
Brinell Hardness Number: 100-130
As can be seen, the strength increases manifold when aluminum is alloyed with a few elements. It’s electrical conductivity is very high, although not as high as copper (it is around 60% of copper). Also, as mentioned repeatedly above, it’s strength to weight ratio is high as well.
Tests To Determine Aluminum
There are several easy tests that can be done to determine whether a metal is aluminum or not, starting with its appearance. Here are a few of the tests that can be done to determine aluminum:
Aluminum is light greyish to silver color in appearance. It has a very bright surface when polished. When exposed to atmosphere for long time, its top surface reacts with oxygen in the air, and forms aluminum oxide.
This oxide sits on the aluminum surface in the form of a thin layer, tenacious in character, and dull in appearance. Weight wise, aluminum is lighter than a similarly sized steel or iron. Aluminum has an uncanny resemblance to magnesium.
It can be distinguished from magnesium by applying a drop of silver nitrate on its surface solution. The silver nitrate solution leaves a deposit of black colored silver on the surface of magnesium, while it leaves no such thing on the aluminum surface.
Unlike steel or iron, aluminum does not turn red before melting. It retains its shape till it is melted, then collapses all of a sudden. This phenomenon is called hot shorting. As soon as the metal has melted, a thick film of greyish aluminum oxide covers the molten metal.
Its combination of lightweight and high strength makes aluminum a popular metal. It is weldable, by use of correct welding procedure and right welding technique. Various arc welding methods can be used for welding aluminum such as TIG welding, MIG welding, etc.
Using arc welding processes such as TIG welding is better for welding aluminum, because the arc can be manipulated, heat input can be controlled, and problems such as distortion can be avoided.
A large number of aluminum alloys are found in the market. It is important to identify correctly the alloy that is being welded. A correct identification helps us in choosing the right filler rod, and deploy right welding technique.
Aluminum Association Inc have developed a four digit system to designate the different kinds of aluminum alloys, as shown in the table below:
|Designation||Major Alloying Element|
|1XXX||99% aluminum and over|
|6XXX||Magnesium and silicon|
Let us see about each type in a little detail.
1XXX series: These are mostly pure aluminum. These are used in the electrical industry for high conductivity of aluminum.
2XXX series: In this group, copper is the main alloying element. When given the right heat treatment, copper alloyed aluminum has very high strength. However, these alloys do not offer much corrosion-resistance , and are therefore often cladded with either pure aluminum or an alloy. The 2XXX series finds usage in the aircraft industry.
3XXX series: In this series of aluminum alloys, manganese is the chief alloying element. The percentage of manganese is kept to below 1.5%. The strength of these alloys is moderate, and these lend themselves easily to cold working.
4XXX series: In this group, silicon is the major alloying element. It is added in substantial percentages. Addition of silicon reduces the melting point of the alloy. These alloys are generally non-heat treatable.
5XXX series: In this series of aluminum alloys, magnesium is the main alloying addition. The strength of these alloys is medium. The weldability of these alloys is fairly good, and they offer good corrosion resistance. However, they aren’t too amenable to cold working.
6XXX series: These alloys are made by adding magnesium and silicon to aluminum. Usually heat treatable, they have moderate strength and offer good corrosion resistance.
7XXX series: In this group of alloys, zinc is the main alloying addition, with small additions of magnesium. The resulting alloys are heat treatable, possess high strength, and therefore used in manufacture of aircraft body.
Welding Aluminum Alloys
Welding of aluminum and its’ alloys is not the same as welding of steel. The characteristic features of aluminum such as it’s high thermal conductivity, high coefficient of thermal expansion, presence of aluminum oxide on the surface, etc. render it’s weldabilty quite different from steel.
In order to produce a good weld, it is important to take into consideration the type of alloy that is being welded, and understand it’s metallurgical properties.
The aluminum is a highly reactive metal. It reacts with the air, and forms a thin hard layer of aluminum oxide on it’s surface. The melting point of this oxide layer is 1982° (3600°F). This is almost thrice of the melting point of pure aluminum. Any strategy for obtaining a good aluminum weld must take into account this oxide layer.
The aluminum oxide also absorbs moisture from the air. Moisture is a source of hydrogen. This hydrogen forms the cause behind porosity in aluminum welds. Besides, this moisture, hydrogen can also get added to the weldment from its usual sources such as oil/grease on the weld surface. Pre-existing paint on the weld surface also contains organic compounds that can act as potential sources of hydrogen.
Besides the above, dirt and foreign particles on the filler rod surface can also introduce hydrogen to the weld.
Similar to steel, hydrogen has more solubility in molten aluminum, where it is present in the nascent form. As the metal cools down, nascent hydrogen turns into molecular hydrogen. It then finds that it is no longer welcome in the microstructure.
That is, the solubility of hydrogen is far lesser in solidified aluminum than in liquid aluminum. So, the hydrogen tries to escape from the metal. If the cooling rate of the metal is low enough, hydrogen gets adequate time to make good its’ escape. However, if the cooling rate is rapid, the hydrogen does not get enough time to escape out harmlessly and gets trapped.
If some trapped hydrogen remains inside the metal after solidification, it either appears as a crack, or as porosity. Hence, cleaning of the base metal surface is of high importance to eliminate sources of hydrogen.
Aluminum and aluminum alloys should not be cleaned with caustic soda or cleaners with a pH above 10. This is because aluminum is quite reactive, and it may react with the cleaning agent chemically.
How To Clean The Aluminum Oxide Film Before Welding?
Aluminum oxide is unique feature of aluminum. Removing it from the surface of base metal before welding is necessary to obtain a good weld. If the oxide layer is not removed properly, small particles of aluminum oxide fuse into the weld metal during welding, and get trapped inside the weld metal. These trapped oxides reduce the ductility of the weldment, and can potentially promote cracking.
Removal of the aluminum oxide film can be achieved through several means: chemical, mechanical and electrical. Chemical and mechanical methods are more common, while the electrical method is used in a limited number of applications.
The mechanical method is the simplest one. It simply involves rubbing the metal surface with a wire brush (which must be made of stainless steel), and/or, sandpaper, or any other suitable means.
Chemical cleaning is done in two ways:
One is through use of chemical cleaning agents. These are of two types: etching type and non-etching type. Commonly, non-etching type is used, as it is easy and safer to handle. It is useful when handling parts that are relatively already clean.
Etching type agents can also be used. However, they have a higher amount of acid content in them. Hence it is important to handle them with necessary safety precautions. Using these solutions achieves better cleaning, but it is a less safe option.
Another way is to dip the part into hot and cold rinsing solutions.
A less common method of removing the oxide film is through use of aluminum welding fluxes. Separately supplied flux is used when doing gas welding. In stick welding, the covering flux on the stick electrode contains the necessary ingredients for cleaning action.
When aluminum flux is used, care must be taken to remove any remnants of it from the metal surface so that it does not interfere with the welding. Likewise, when etching solutions are used for cleaning, the surface must be made clean of them after their use, lest they be cause for corrosion in the future.
Another means of cleaning is the electrical means.
This method uses the principle of cathodic bombardment. During GTAW welding of aluminum with alternating current, this happens automatically. When the electrode turns positive during the AC cycle, cathodic bombardment of the metal surface occurs, which cleans the surface of the oxide film.
Due to this reason, TIG welding of aluminum with the AC current is commonplace. However, aluminum is a chemically reactive metal. The oxide layer starts to immediately reform. The rate of re-formation of oxide film is not too rapid. It is recommended that the welding of the part be completed within 8 hours after the cleaning of the surface has been done.
As this time increases, the prospects of achieving a quality weld decrease.
How Is Aluminum Welding Different From Welding Of Steel?
The thermal conductivity of aluminum is high. It is almost thrice that of steel. The heat supplied by arc is quickly evacuated to distant regions of the metal. Hence a high heat input is required while welding aluminum even though it’s melting point is less than half of that of steel. (m.p. of Al – 660°C, m.p. of steel – 1550°C).
For higher thicknesses, preheat is employed to reduce the rate of cooling of the metal occurring due to the metal’s high thermal conductivity.
However, the temperature of preheat does not exceed about 400°C (204°C). Also, preheat is only maintained for as long it is absolutely necessary. Prolonged preheat and high temperature decreases the tensile strength of both heat-treated and non-heat treated aluminum alloys.
High heat input in welding produces low rate of cooling, while low heat input is characterised by high rate of cooling. In order to counteract high thermal conductivity of aluminum, welding procedures using high heat input are used so that rate of cooling can be slowed down to some extent.
High thermal conductivity also means that the molten metal solidifies quickly. So this property is helpful in out of position welding. Becuase of quick solidifying property, all-position welding of aluminum with TIG and MIG welding processes becomes practical.
The coefficient of thermal expansion of aluminum is also high in comparison to steel, it is almost twice that of steel. Due to this, the change in volume during heating and cooling is higher. This means that the problem of distortion and warpage is also more severe in welding of aluminum.
Another way in which aluminum welding is different from welding of steel is it’s color. Unline steel, aluminum does no become red when approaching its melting point. Only after melting, it becomes a dull red.
Also, aluminum flux is used for cleaning of aluminum oxide film when soldering and brazing of aluminum using a torch. This practice is not found in welding of steel.
Resistance Welding Aluminum
The role of resistance welding processes such as spot welding, seam welding, etc. is important in the welding of aluminum, especially in the welding of high strength heat treatable aluminum alloys. These alloys are difficult to weld with the conventional arc welding methods.
Like arc welding, it is necessary to remove the oxide film in resistance welding too. This is because the electrical resistance offered by the oxide film is high, and is not consistent too. So, to obtain resistance welds of high strength – it is necessary to remove the oxide film.
Spot welding Aluminum
The strength and aesthetic appearance of a spot welding depend upon the amount of resistance between the two parts being joined. If proper cleaning has been done, this resistance is low. The surface cleaning removes oil, grease, dirt and other contamination from the metal surface before start of welding. The spot welds are generally designed to carry shear loads.
Sometimes tension loads and a combination of tension and shear are also expected. In such cases, the spot welds should be qualified by way of procedure qualification tests to ascertain the strength of the spot weld and determine whether it meets the expectation of the application.
Seam welding Aluminum
Similar to seam welding of steel, seam welding of aluminum uses the same principles. The resistance welding electrode of the spot welding is replaced with a wheel that rolls along the length of the work-piece, and creates a continuous weld between the two mating sheets.
Multiple seams can be overlapped together to obtain a uniformly spread joining of the two sheets.
Seam welding can be improvised to produce a series of spot welds in succession. In this, the setup remains same. Only the supply of current is fed in pulses, so that instead of getting a continuous weld, we get evenly spaced intermittent spot welds. This helps us in joining longer lengths of sheet at a faster rate than the spot welding set up.
Electron Beam Welding Aluminum
Electron beam welding is a process in which a beam of electrons (hence the name electron beam welding) is directed on the work-piece at a high speed. The kinetic energy of electrons converts to heat on striking the work-piece. This heat melts the metal. This action happens in a vacuum chamber which is quite small in size. The size of this vacuum chamber forms the limiting factor for size of the weld.
The intensity of heat in electron beam welding is very high in comparison to conventional arc welding methods. The arc welding methods and oxyfuel welding only melt the metal adjacent to the arc or flame. The size of molten pool and depth of penetration depends upon the size of electrode, and is a little more in SAW process.
In comparison, electron beam welding heat is so intense that it is enough to vaporize a hole through the entire thickness of the work-piece in an instant.
The side walls of this hole have molten metal on them. As the beam progresses ahead, it melts more metal at the advancing edge of the beam. This molten metal fills up the hole on the rear side of the beam. As the beam progresses, such filling keeps progressing, and the weld is thus made.
Electron beam welding is suitable for aking edge joints, lap joints, and butt joints.
Filler metal is not generally used in this process.
Stud Welding Aluminum
Aluminum joining is also possible through stud welding. The conventional stud welding equipment can be used using drawn arc capacitor discharge technique. Aluminum studs of diameter 3/16 in. (4.7 mm) to ¾ in. (19 mm) can be welded with this technique.
The ordinary stud welding gun has slight modifications done to it to facilitate welding of aluminum studs. One of these modifications includes a provision for flow of high purity argon shielding gas. The polarity used for this process is DCEP. The stud is given the positive polarity while the work-piece is attached to the negative terminal.
The aluminum studs come with a small pointed projection at the welding end. This projection helps in striking the arc, and establish the long arc length required for aluminum.
Studs of smaller diameter [1/16 in. to ¼ in. (1.6 mm to 6.4 mm) diameter] can be welded without using shielding gas, with either capacitor discharge or drawn arc capacitor discharge methods.
For this welding, an electrostatic storage system that supplies low voltage just the instant of welding is used. This system has high capacitance. The arcing occurs for a very short instant during which the tip of the stud melts. Following this, the inbuilt spring in the equipment pushes the stud with high force against the work-piece. This achieves the weld between stud and work-piece.
A small tip or projection is used at the end of the stud for the purpose of initiating the arc. Stud welding can be done without the use of such a tip as well. However having a tip gives better results.