SMENCO is proud to say that when it comes to welding aluminium, we have always been leading the way. In this area you simply cannot find a better combination of expertise, equipment, consumables – all supported locally.
We have compiled this overview of welding Aluminium. We’ll discuss the properties of aluminium, it’s weldability, some of the complexities and issues you’re likely to encounter when welding aluminium and the technologies that can be used to get the best results.
Welding aluminium is fundamentally different from welding steel. The melting point of steel is around 1,500 C, aluminium is around 600 C and aluminium alloys are around 570-660 C.
Comparing steel to aluminium we see:
- Al 99.5658 – 659 C (almost at fusion point) – The pores cannot degas in time.
- AlMg 4.5Mn575 – 640 C (longer solidification range) – The longer time period enables the pores to degas better.
- The thermal conductivity is four times higher, necessitating high thermal input during welding.
- The thermal expansion is around twice as large, increased tension and distortion occur in the weldment.
On top of the issues listed about another problem that must be taken into account is the high-melting point of the oxide layer (fusion temperature of around 2040 C) which envelops the weldment and impedes welding.
Aluminium cannot become brittle or age-hardened in the heat-affected zone. On the contrary – a loss of strength may be expected on strain-hardened alloys.
|Pure Aluminium (Al 99.9, al 99.5 etc)||Good Weldability|
|Naturally hard allows (AlMg and AlSi alloys)||Good Weldability|
|Age-hardened alloys (AlMgSi and AlZnMg)||Good Weldability|
|AlCu (approx. 6% Cu and Zr), AlCuMg and AlZnMgCu (approx 1.4-3% Cu hot cracking)||Limited Weldability|
Casting alloys are basically weldable, although this will be affected by the presence and nature of any casting defects, except in the case of die-casting.
The complexities and challenges of aluminium welding
The main complexity of welding Aluminium is that it’s covered in a thin oxide layer that melts at 2040 C, conducts heat very well and is prone to large deformations when heated. A large amount of heat needs to be applied to get past the oxide layer but that heat cannot be sustained or it will deform the weldment.
Pure aluminium has poor mechanical properties so you’ll almost always be working with an alloy. There are a wide range of alloys for different purposes and an equally wide range of suitable fillers.
Welding dissimilar metals adds another layer of complexity. The successful joining of dissimilar materials requires precise knowledge of the properties of each material.
For the reasons stated above aluminium is highly regarded however it’s strength and low cost makes steel indispensable in many areas of industry. While not common, it is to be expected that fabricators will be called on to join aluminium and steel.
Joining Steel and Aluminium
When joining steel and aluminium under the influence of heat, what is known as an ‘intermetallic phase’ is created at the interface between the two materials. The more heat that is applied, the more extensive the intermetallic phase and the poorer the mechanical properties of the join will be. Also the different thermal expansion coefficients of the two materials creates a stress field around the join. There is also a possibility for corrosion to form as a result of the large electrochemical potential difference of steel compared with aluminium.
The past few years have seen massive improvements in MIG welding technology. Fundamental improvements were made to power sources and welding processes, with completely new standards being reached in some areas. These innovative advances have been triggered by new materials and applications and by the increased use of mature microelectronics and digital technology.
In MIG welding, the consumable metal electrode is both the filler material and the arc carrier. Filler wire is fed via two or four drive rollers into the welding torch, where the current is transferred via the contact tube. The free wire end is surrounded by a gas nozzle. The shielding gas flows out preventing chemical reactions between the hot workpiece surface and the surrounding air ensuring the strength and durability of the weld metal. Inert and active gases can be used as shielding gases. This is why we refer to metal inert gas (MIG) welding and metal active gas (MAG) welding.
CMT may be the king of aluminium welding but because MIG welding is more familiar and more portable than CMT, pulsed MIG welding is a more common solution. Like CMT pulsed MIG welding maintains a low current arc and superimposes short periodic bursts of high current in order to detach and transfer drops of molten metal (aluminium alloy electrode wire in this case) into the weld pool at a rate of one drop per pulse. The average current is low, but the metal transfer still occurs when the high current is pulsed allowing good welds with materials like aluminium that are particularly vulnerable to heat deformation. The result is less spatter, less deformation and less post-treatment.
Out of the mains voltage, inverter power sources produce a pulsed voltage with high frequency. This voltage arrives at the welding transformer which, thanks to the high frequency, can be designed in a much lighter, compact and efficient way than stepped power sources. Inverter power sources also have a rectifier. Fully digital inverter power sources with signal processor generate a continuously adjustable output current that is constantly measured and kept within the ideal range. This ensures practically spatter-free welding, and the output choke is no longer required. Wirefeeding is provided by either an integrated wire drive inside the power source housing, or an external wire-feed unit. The manual and machine welding torches are available in gas-cooled and water-cooled versions. Gas-cooled welding torches are cooled by the shielding gas which flows through, while water-cooled welding torches also have effective liquid cooling using a pump and heat exchanger. For welding currents of 300 A and above, water-cooled welding torches are standard.
Application and Advantages
If it can be said that to begin with, the MIG process proved itself highly useful for rationalised welding of unalloyed and low-alloy structural steels, today it can be best put to use for aluminium alloys and high-quality structural steels, thanks to the pulsed arc technique. Characteristic of the pulsed arc technique is the controlled material transfer. In the ground current phase, the energy supply is reduced to such an extent that the arc is still only just stable and the surface of the workpiece is preheated. The main current phase uses a precise current pulse for targeted droplet detachment. An unwanted short circuit with simultaneous droplet explosion is ruled out, as is uncontrolled welding spatter.
Regardless of the type of arc, MIG displays significant advantages over other welding processes. These include good deposition rate, deeper fusion penetration, simple handling and total mechanisation, in addition to high productivity.
Most recently, the TIG welding process has been facing greater and greater competition from the ever-perfected MIG/MAG process and its related processes. These processes drastically increase productivity without concessions to quality. Despite slower welding speed and lower deposition rate, the TIG process still is the best guarantee for the highest quality results. Last but not least, innovations in the power source sector ensure a sustained future for TIG welding.
The core of a TIG welding torch is a non-consumable, temperature-resistant tungsten electrode. The arc that proceeds from it heats and melts the material. As required, a filler wire is fed in manually or with a wire-feed unit. In many cases, a narrow gap needs no filler material at all when being welded. Ignition of the electrode normally takes place without the tungsten electrode touching the workpiece. This requires a high-voltage source that temporarily switches on during ignition. For the majority of metals, welding itself takes place using direct current. However to prevent heat deformation aluminium is usually welded using alternate current.
The nozzle for shielding gas is fitted around the tungsten electrode. The gas that flows out protects the heated material from chemical reactions with the surrounding air, thereby ensuring the required strength and durability of the weld metal. Inert gases such as argon, helium or their compounds are used as shielding gases.
Argon is the most-used shielding gas for TIG welding. It optimises the ignition properties, as well as the stability of the arc, and helps obtain a better cleaning zone than helium.
Modern inverter power sources offer the advantage of a faster reaction to changes in the welding process. A pulsed voltage with a very much higher frequency, rather than the mains voltage, arrives at the transformer. Due to the high frequency, this has a much lighter, compact and efficient design than the thyristor power sources. The low current ripple of the transformer output current means a substantially more compact design, or no need for the output choke. The rectifier simply consists of uncontrolled diodes.
For generating an alternate current (AC) for aluminium welding, AC-compatible power sources have an inverter downstream from the rectifier. Many power sources allow the user to set a sinusoidal or rectangular alternate current, as well as a combination of the two. A sinusoidal welding current has an unstable, if very soft arc. With a rectangular welding current, the current stabilises the arc. The audibly louder operating noise however requires the user to work with ear protection. A combination of sinusoidal and rectangular welding current is very stable and extremely soft at the same time.
TIG welding torches are available in gas-cooled and water-cooled versions. Gas-cooled welding torches are cooled by the shielding gas which flows through, while water-cooled welding torches also have effective liquid cooling using a pump and heat exchanger. There are also TIG welding torches with an integral device for mechanised wirefeeding.
Application and Advantages
TIG welding is a versatile process that can be used for all weldable materials and applications. The main application area is stainless steels, aluminium and nickel alloys. The concentrated, stable arc provides high weld metal quality and an even seam, with no spatter or slag. For applications with the highest demands on quality, for example pipelines in reactor construction, this process is the first choice. In addition, the use of filler metal is unnecessary. For sheet thicknesses of less than 4 mm, mechanised wire feeding produces economical welding speeds. Only the welding of thicker sheets means limited cost effectiveness, whereby only welding the root pass is recommended. Welding the filling runs is better with powerful processes such as MIG/MAG or submerged arc welding.
For many applications, a pulsed welding current is helpful for preventing overly intensive melting of the base metal and associated weld drop-through. For light-gauge sheets especially, the weld build-up is easier to achieve, as the base metal only melts in sections, and then solidifies again.
Wherever aluminium is exposed to the air, an oxide layer forms immediately on the surface. The layer has a melting point of 2015 °C. Aluminium itself however melts at 650 °C. If the oxide layer remains solid, the molten aluminium on the oxide layer would run off, and a weld joint would be impossible. The oxide layer must therefore be removed, by positive polarity of the electrode for example. One disadvantage however would be a deterioration of the welding properties, as the tungsten electrode must be negatively poled in TIG welding. The solution is to weld with alternate current. During the positive half-wave, the oxide layer breaks open. The negative half-wave increases the fusion penetration and generates the required welding power.
Cold Metal Transfer (CMT) is a modern joining method that satisfies the increasingly stringent demands of process stability, reproducibility and cost-effectiveness.
Where CMT really shines is in it’s ability to weld very thin materials, 0.3mm and up. Aluminium to aluminium, stainless to stainless are the most common applications however steel to aluminium welding is what CMT was initially developed to do.
The CMT® process evolved from the continuous adaptation of the MIG process to resolve the problems posed by the joining of steel and aluminium. All the technologies that have been used to join steel and aluminium in the past have only been able to deal with certain geometries or have required extensive control inputs. Although the perceived wisdom among many metallurgists was that steel and aluminium could not be welded together, extensive research in the field of MIG welding indicated that arc welding was indeed a potential way of joining the two materials.
CMT® is a controlled process that allows the material transfer to take place with barely any flow of current. The aluminium base material melts together with the aluminium filler material, with the melt wetting the galvanised steel. The filler wire is constantly retracted at very short intervals. The precisely defined retraction of the wire facilitates controlled droplet detachment to give a clean, spatter-free material transfer. The wire moves at a very high frequency and requires a quick-response, gearless wire drive directly on the torch. The main wirefeeder is not be able to keep up with these movements so the wirefeeding hose is provided with a wire buffer that compensates for the high speed forward and backward movement of the wire.
CMT® welding uses digital inverter power sources. The welding system uses the same latest state-of-the-art hardware as a MIG system, while taking the specific requirements of aluminium welding or aluminium/steel welding into account.
A highly-dynamic wirefeeder is mounted directly on the welding torch allowing the power source to detect a short circuit, then the welding current drops and the filler wire starts to retract. Exactly one droplet is detached, with no spatter. The filler wire then moves forwards again and the cycle is repeated.
High frequency and extreme precision are required for controlled material transfer. The wire drive on the welding torch is designed for speed, not high tractive forces. As noted above the main wirefeeder is too slow so the wire buffer on the wirefeeding hose is used to convert the superimposed, high-frequency wire movement into a linear wirefeed.
Application and Advantages
The strength of the CMT® process is without doubt its ability to join steel and aluminium. Although the steel base material is only wetted during this brazing process and does not melt, extensive trials always resulted in a break in the aluminium base material, not in the weld seam.
Light-gauge aluminium welding (0.3 – 0.8 mm) sheets is perfectly feasible. The low level of heat insertion in the CMT® process means that a weld pool is not required – there is no risk of weld drop-through.