Welding

Aluminium alloys are arc welded, in an inert atmosphere (argon, helium or a mixture of the two) and there are two techniques:

1_ARC WELDING IN AN INERT ATMOSPHERE WITH A REFRACTORY ELECTRODE OR USING THE TIG (Tungsten Inert Gas) PROCEDURE. | In this process, an electric arc is created between a refractory tungsten electrode and the piece to be welded, while a jet of inert gas, usually argon, surrounding the electrode, protects the melting bath against oxidation. A hand-held rod feeds the melting bath. This procedure uses a HF-stabilised alternating current source specifically designed for the welding of aluminium alloys. It is used in thicknesses between 1 and 6 mm and can be robotised.

2_ARC WELDING IN AN INERT ATMOSPHERE WITH A CONSUMABLE ELECTRODE OR USING THE MIG (Metal Inert Gas) PROCEDURE. | In this welding process, the aluminium or the aluminium alloy serves both as an electrode and as a filler metal. Wire is supplied from a pre-wound spool, which is automatically unrolled to the welding tool, a gun, as it is consumed. The energy for welding is supplied by a direct current source. The connection is made with reverse polarity (-) on the part to ensure both stripping and fusion of the electrode wire. This procedure, which can be used for products with a thickness of more than 2.5 mm, can also be automated. The manual version of MIG is commonly called semi-automatic welding. For some years, welding material manufacturers have offered pulsed current sources. This equipment makes it possible to weld fine thicknesses of 1.5 to 4 mm very easily. For medium and large thicknesses, its advantage in relation to traditional sources has not been proved.

In MIG welding, filler materials are always used, while in TIG welding they may or may not be used. The best properties of the welded joint in terms of strength, corrosion and absence of cracks are obtained when using filler materials according to the table shown below.

There is no general rule for choosing the filler materials due to the type of use and the most relevant parameter in each case. Those with a high magnesium content AlMg5 (EN AW 5356-5556) give greater strength, while that of AlSi5 (EN AW 4043) is more resistant to cracking and provides better metal flow during the fusion process of treatable alloys. These types of alloy (AlCu - AlMgSi - AlZn), must not be welded with filler material from the same alloy group due to the cracking process. If the material is going to be anodised after welding, the AlSi5 filler material will be avoided because it will take on a very dark colour in the welded area. In order to reduce the danger of stress corrosion and increase strength, Cu has been added to AlZnMg alloys. Doing this also worsens weldability. Various investigations indicate that a maximum of 0.2% Cu can be added, before the danger of hot cracking increases considerably. In this case, AlSi5 (EN AW 4043) is chosen.

Cleaning before welding is essential to achieve good results. Dirt, oil, traces of grease, moisture and oxides must be removed beforehand, either by mechanical or chemical means. For normal workshop work the following procedure can be chosen: 
_1 Dirt removal and cold degreasing with alcohol or acetone. 
_2 Washing with water and drying immediately to avoid the risk of oxidation. 
_3 Mechanical removal by: 
- Brushing with a stainless steel rotating brush. 
- Scraping with abrasive sandpaper or a file. 
- By blasting.

When there are stricter requirements in terms of preparation, chemical cleaning can be carried out according to the following process: 

_1 Dirt removal. 
_2 Degreasing with perchloroethylene at 121°C. 
_3 Washing with water and drying immediately. 
_4 Removal of aluminium oxide in the following way: 
- Alkaline cleaning with e.g. NaOH. 
- Acid cleaning with e.g. HNO3 + HCl + HF. 
- Washing with water and drying immediately. 
- Neutralisation with HNO3 (after treatment with NaOH). 
- Bath in deionised water. 
- Immediate drying with hot air. Chemical methods require expensive equipment for surface treatment and cannot always be used for this reason. However, removal of the oxide or degreasing in the welding area must never be omitted.

The inert gases Argon and Helium are always used as protective gases for MIG and TIG welding. During welding, the inert gas cools the welding nozzle and, at the same time, protects the electrode and the melting bath. The gas also participates in the electrical process in the arc. The commercial gases that are generally used are the following:

_Argon, 99.95% pure. 
_Argon + Helium (30/70, 50/50) for MIG welding, gives a wider and hotter melting bath. 
_Helium for direct current. In TIG welding it provides hotter melting and higher welding speed, but it is more expensive and requires higher consumption.

Pure Argon is the gas that is most often used and should be used in normal workshop welding because it is much cheaper and requires less gas flow. Helium is only used when greater penetration is required, for example, in welding at an angle or when welding a very thick material.

When aluminium is welded, different types of smoke and gases are produced; as is the case in steel welding. In order to avoid propagation of this contamination it is advisable to install smoke and gas extractors. The intensity of the arc is much greater than in steel welding and under no circumstances should you look at the arc without a suitable protection mask. Intense ultraviolet (UV) radiation can damage eyes and skin, and therefore the aluminium welder should wear protective clothing that covers the entire body. The amount of gas depends on the welding method, filler material and type of alloy. TIG welding produces a considerably lower amount of smoke than MIG welding, due to the lower energy content in the arc. In MIG welding the highest amounts of smoke are produced by welding alloys of AlZnMg with AlMg5 (EN AW 5356-5556) as filler material. This is why good general ventilation is necessary in combination with individual measures such as fresh air masks or local (in situ) smoke extraction devices.

The electrical process in the arc is of the utmost importance for understanding what happens in the welding of aluminium. In principle welding can take place with direct current (DC) or with alternating current (AC). If we look first of all at DC, we can choose between two cases of different polarities, negative polarity and positive polarity.

Negative polarity transfers most of its energy to the workpiece, 70%, so that we obtain a deep melting bath, with good penetration. The load on the electrode is reduced, which is an advantage in TIG welding. A great disadvantage when using this polarity is that the arc breaks the oxide film, so that pre-treatment of the material is required, such as careful preparation of edges, very careful cleaning and bevelled edges.

In combination with a pulsed arc, it is possible to weld fine plates from 0.06 mm. Welding with direct current and positive polarity (reversed polarity) is used for MIG welding. In terms of the distribution of heat, typically 70% corresponds to the electrode. The melting bath is relatively wide and quite shallow, resulting in little penetration.

The determining advantage for the use of positive polarity consists in the breaking effect of the oxide film of the arc, with such efficiency, that said film is no longer an obstacle to achieving good quality in the welding. The mechanism for this effect of breaking the oxide film is not completely known, but one explanation is that it is due to the bombardment of the surface with positive metal ions similar to the cleaning of surfaces by blasting.

Although the arc has this property, the removal of the oxide should not be omitted before welding. The arc is not capable of breaking the thick oxide films formed during hot rolling of plates, but only the thin layers that are formed after cleaning. Welding with alternating current (AC) implies that the polarity is changed approximately 100 times per second and, therefore, the properties of AC welding can be considered to be the average between the two cases in welding with direct current. The heat distribution is almost the same between the electrode and the workpiece; the penetration and the width of the melting bath fall between the values ​​that apply for the two previous cases. The arc still has a breaking effect on the oxide film. Alternating current is used in normal TIG welding with argon as a protective gas. The current absorbed by the equipment is altered due to the rectifying action of the arc; for this reason, a TIG welding machine has been designed to compensate for this effect.

For MIG welding of aluminium, the same equipment can be used as for welding steel with CO₂. The capacity of the energy source is chosen according to the expected production For welding thicknesses of up to 10 mm, this is generally 250-300 A. The feeding system should preferably be of the "push-pull" type, i.e. a combination of push-pull effect, but push-only types can also be used for short wire guides and wire with a diameter of 1.6 mm. Due to its lower strength, aluminium allows pushing in short lengths. However, alloys such as those of the AlMg5 group (EN AW 5356-5556) are much harder than those of the AlSi5 group (EN AW 4043) and the pure aluminium alloy Al 99.5 (EN AW 1050) allows pushing in longer lengths. In any case, the length of the wire duct should always be as short as possible, and in its path, the curve radiuses should be wide, thus avoiding loops and pronounced contours.

The state of the nozzles and ducts should be periodically monitored by cleaning them of shavings and traces of material deposited on them.

MIG welding is always carried out with direct current (DC), with pure argon as a protective gas and is suitable for all welding positions, even on roofs. Welding in a vertical position is always done in an upwards direction. The quality of the welding is usually high but the risk of porosity is always greater than in TIG welding, since due to the fact that the arc is self-regulating, it can temporarily become unstable, which can cause interference in the supply of material. This method is highly suitable for both manual and mechanised welding, in thicknesses of 3 mm or more. Expert welders can weld even thinner material. If the welding quality requirements are low, even thinner materials can be welded, but in this case the arc does not work in the pure “spray” area, due to the low voltage, which induces a tendency to a short arc. The welding speed for manual welding is from 0.3 to 0.75 m/min. and for robotised welding from 2 to 3 m/min. This relatively high speed makes the method more productive than TIG welding and in combination with the high energy density in the arc, a heat affected zone (HAZ) is obtained that is narrower than in TIG welding. This is a favourable factor since the deformation due to the welding decreases as the contribution of heat energy is reduced. The MIG method has many fields of application, which has led to the development and refinement of the welding technique.

The common way of joining two plates in an overlapped junction is by resistance spot welding. However, this method requires heavy investment in machinery and is limited to thicknesses of up to 4 mm. As an alternative method, the MIG spot welding method can be used, which can be carried out with some of the standard MIG equipment, supplemented by a time relay and a gas nozzle. The welding is checked by pressing the gun against the upper plate. The welding time is adjusted by a time relay, which achieves good reproducibility. Penetration can be controlled so that the molten part penetrates between the two plates. The preferred method depends on the thickness of the lower plate. The advantages from the point of view of construction are based on the fact that large differences in thickness can be accepted between the upper and lower plates. When it comes to large thicknesses, in the upper plate the welding can be facilitated by making a drill hole.

By superposing a current with a frequency of 16 - 100 Hz over the normal current, a short duration pulse can be obtained, with properties such that material with thicknesses of less than 3 mm can be welded. On each maximum pulse, a drop of filler material is released. The advantages of this method are as follows:

• Thinner metal can be welded, 1.5 mm
• Different thicknesses can be welded more easily
• It is easier to weld with variable openings
• Thicker filler materials can be used

The recent use of thick aluminium, particularly alloy AlMg4.5Mn (EN AW 5083), has led to the development of a technique specially adapted for these purposes, based on the MIG method. In this respect we can mention the Sciaky NARROW GAP method which, with supports placed obliquely one behind the other, allows butt welding without preparation of edges and with an opening of 6-9 mm. for thick materials. In Japan, the NHA method (NARROW GAP HORIZONTAL welding process for aluminium) has been developed for horizontal openings. A double-wrapped torch with oscillating movement is automatically guided along the joint. The advantages of these two variants on the MIG method consist of the better use of the heat and the lower volume of the joint, which leads to an increase in productivity.

Table of recommended amperes for MIG welding

In the welding of aluminium with tungsten arc and inert protective gas (TIG), a 50 Hz alternating current source, a superimposed high frequency (AC) current, is used. The electrode is non-fusible pure tungsten or tungsten alloyed with zirconium. This welding method can be used in all positions and, when used correctly, allows high quality welding to be achieved. The danger of porosity is lower than in MIG welding. The arc breaks the oxide film and therefore, for automatic welding, wire is used on spools. As a rule, TIG welding is used for thicknesses of 0.7 to 10 mm. but there is not really a maximum limit. The welding speed is lower than in the SOLIM.

The edges should be prepared carefully so that there are no openings between the plates, since it is much easier to control the melting bath if the separation is minimal. In thicknesses greater than 5 mm. the edges of the joints to be welded must be bevelled. When it comes to welding thin plates, it is more advantageous to use a template to avoid distortions due to the heat of the welding and separations between edges due to these distortions.

Many energy sources for TIG have the capacity for pulse welding. For others, an additional unit can be easily connected. The principle is similar to that described in MIG-pulse welding, with the only difference being that TIG welding is carried out at a much lower frequency, approximately 10 Hz. This means that the pulses are visible which, ultimately, may be a source of imitation for the welder. The technique can be applied to both AC and DC welding. It works with two current levels. The lowest one is chosen so that the arc does not switch off. The highest level is generally higher than in normal TIG welding. The periods of the different levels may vary. The advantage is that a perfect weld can be achieved with a lower average current intensity than in normal welding. The contribution of heat is lower and thinner material can be welded: 0.3 to 0.4 mm. With combinations of DC and pulse, thicknesses of around 0.05 mm can be welded.