Aluminium is a popular material choice for use in aerospace and other transport industries owing to its high strength to weight ratio and the impact this has on fuel economy. Although titanium and composite materials are becoming ever popular, around 70% of commercial airframes are still made from aluminium.

This article is for the welding of harden-able and heat treatable aluminium alloys that are not cast (Al-Si), information on welding cast aluminium can be obtained by contacting our team at www.aabcowelding.com.

Is aluminium welding difficult?

Aluminium is not considered difficult to weld in that it can be welded with good results consistently. A well written welding procedure should be applied to the process to ensure equipment configurations, machine settings, consumables and weld preparation are correct to produce the required results.

GTAW (TIG) and GMAW (MIG) are the most common welding techniques used on aluminium, with the use of laser and friction stir gaining traction (more details of laser and friction stir welding processes can be obtained upon request from Aabco Welding Services Sdn Bhd).

Aluminium requires different equipment configurations to those we would use for welding of steels or other materials. For GTAW welding of aluminium a machine with alternating current (AC) is used. Additional machine features such as wave balance and wave form control are desired. In certain scenarios welding aluminium with direct current (DC) can achieve good results, though for repeated performance it is generally better to use AC. 

For GMAW welding of aluminium we may utilise the use of a push/pull spool gun to provide smooth wire feeding, preventing the aluminium filler wire getting caught on the torch lead inner lining.

For both GTAW and GMAW welding, use of a water cooled torch paired with a high duty cycle welding power source is advised if the welding is prolonged due the high heat input required.

There are specific issues that can arise from welding aluminium that need to be considered:
  • Distortion - high thermal conductivity causing extreme heat dissipation, this in-turn requires additional heat input and often leads to distortion of the aluminium.
  • Loss of strength - due the high thermal conductivity requiring more heat input, the size of the heat affected zone can be very large worsening the problem. 
  • Porosity - hydrogen is soluble in molten aluminium and so becomes entrapped in the solidifying weld pool resulting in porosity.
  • Lack of fusion - this is caused by the layer of oxide on the surface of the aluminium. This complicates the welding as this oxide has a higher melting temperature than the aluminium itself (1926 and 660°C respectively). This oxide layer can lead to lack of fusion weld defects if not correctly removed, potentially resulting in failure of the weld joint.
  • Hot cracking - this occurs when there is an imbalance of solutes in the weld pool.
  • Welder skill - aluminium during welding has different characteristics to other materials re-training if welders who are familiar with other materials is critical.

These are all issues that are specific to aluminium and do not arise in the welding of other materials in the same way.

All of these issues can be avoided with a carefully designed welding procedure. Aabco Welding Services are specialist welding engineers who provide welding consultancy and training & qualification for all manual welding processes and materials. For any welding related enquires contact us Contact - Aabco Welding Services Enquiry

Let's take a look at the above common aluminium welding defects and the respective preventative measures in more detail,

Distortion

We can see from the below material properties why distortion is likely to incur more so in aluminium than with other common materials:

Co-efficient of thermal expansion

High value - greater local expansion and yielding – higher level of stress on cooling.

Carbon steel - 1

Stainless steel - 1.4

Aluminium - 1.7

Thermal conductivity

Low value – higher heat retained in welded zone – higher level of stress on cooling.

Carbon steel - 1

Stainless steel - 0.3

Aluminium - 4.2

Yield strength

Higher yield = higher residual stress.

Carbon steel - 1

Stainless steel - 1.2

Aluminium - 0.5

Modes of elasticity

Measure of stiffness, greater stiffness resists distortion.

Carbon steel - 1

Stainless steel - 0.95

Aluminium - 0.3

We see that the co-efficient of thermal expansion is almost double that of steel whilst the yield strength is half. These two mechanical properties combined with the very high rate of thermal conductivity and very low elasticity create a perfect recipe for excessive distortion.

Avoiding distortion on aluminium is not an easy task, though there are many methods we can use to reduce the distortion;

How to avoid distortion:
  • Back step welding, this is a method of welding whereby we weld in one direction starting an inch from the end of the joint and weld towards the end, then the next weld is an ich from the start of the prior weld and so on. This controls the weld length and temperature of the material aiding to reduce distortion compared to weld the whole joint in one movement.

Back step welding info graphic

  • Simply allowing the part to cool, if there are multiple parts to be welded spread the weld across multiple parts, coming back to the first part after 10-15 minutes.
  • Designing to keep welding to a minimum, there should be only sufficient weld to fulfil the component service requirements. Over welding (excessive weld length and throat thickness) only adds additional heat input and stresses to the material, not to mention the wasted time and consumables costs.
  • If the parts being welded are high volume we may opt to use fixtures that incorporate forced cooling.  Since the crystal structure of aluminium does not change the  with temperate there are little to no metallurgical implications of rapid colling post welding.
  • High welding speed should be used where practical to reduce the overall heat input value.

Loss of Strength

Many aluminium alloys are cold worked to increase the strength of the material. Cold working is where the material grains are elongated by processes such as rolling to increase the strength and ductility of the material.  When welding the heat from the arc removes these property gains by invertedly annealing the heat affected zone. As we learned from the explanation of distortion, the heat affected zone is much larger in aluminium than other most other materials this the problem is magnified.

How to avoid loss of strength:

It is not possible to recover the loss of strength in solid solution strengthened alloys and so the weld joint softening needs to be accounted for at design stage. This can be achieved by increasing the material thickness or by adding gussets to achieve the desired strength values.

In the case of age hardened alloys if the material is of small size it is possible to recover some of the lost strength by natural ageing though this will only work where the weld composition is the same as the parent material.

Porosity

Hydrogen is the cause of porosity in aluminium welds. Hydrogen being the most abundant gas on earth means that contamination from all sources can result in hydrogen porosity. Hydrogen is very small and thus is not always noticeable by the eye. Aluminium welds should be checked using a dye penetrant testing method. This a fast cost effective non-destructive test that can be conducted anywhere.

Potential sources of hydrogen contamination are;

  • Oils and grease on the material surface
    • Cutting fluid
    • Cleaning compounds
    • Finger prints/sweat
    • Dirty gloves etc.
  • Moisture
  • Paint
  • Contamination of shielding gas
  • Permeable gas lines
How to avoid porosity:

The parent material and filler wire should first be mechanically cleaned and then chemically cleaned. An acceptable example is mechanically cleaning with a stainless steel wire brush (dedicated to be used on aluminium only) and then cleaned with acetone wiped on with a lint free cloth and left to dry naturally. Since Cleaning should take place immediately before welding, ensuring all material is dry before welding. Using AC and adjusting the balance correctly will also aid to remove the oxide layer during welding. On a modern inverter welding machine the AC balance can be adjusted to provide either more cathodic cleaning action (more time spent in the AC region) or more penetration (time spent in the DC region). This should be used in conjunction with the above methods not as a replacement as there adjustments will have further implications on weld quality according to the specific scenario. As an example, more penetration may be required on thicker aluminium and more cleaning action may be required for repair of a in service item.

Lack of Fusion

The corrosion resistance of aluminium comes from the layer of oxide that is on the surface, the oxide layer is not visible to the eye, highly durable and self-healing. This oxide layer melts at around 1926°C , some 1400°C higher than the melting point of the aluminium itself. If not removed this oxide layer negatively impacts weld quality in  number of ways;

  • Porosity
    • The oxide layer can be a barrier that retains moisture and grease, as described previously, this is a source of hydrogen that is readily absorbed in the weld pool but rejected upon solidification to result in weld porosity.
  • Lack of fusion
    • The oxide can break down into small solid particulates that remain as solid on the molten weld pool, remaining in the finished weld. This causes lack of fusion weld defects, resulting in reduced strength and ductility.
How to avoid lack of fusion:

as with porosity the key is to remove the oxide layer, correct mechanical and chemical cleaning of parent matwerial and filler wire is essential. use of AC welding with a correctly set AC balance and waveform will provide further protection. 

Hot Cracking

In contradiction to other materials, hot cracking of aluminium is not caused by a build-up of impurities in the weld. With regards to aluminium alloys, hot cracking is a result of main alloying elements in the parent material and filler wire. In most cases the hot cracks will occur at the end of the weld as this is where the shrinkage stresses peak whilst liquid films remain.

Fortunately we are able to use a phase diagram to portray alloys that are prone to hot cracking from high to low risk. This information is readily available in published articles and your filler wire consumables supplier will be able to advise you.

How to avoid hot cracking:

The key to the avoidance of hot cracking is matching the filler wire alloying elements to the alloying elements in the parent material. Either a lot or a little eutectic liquid is required to prevent cracking.

When there is little eutectic liquid the liquid films are small in length and low volume, this prevents any minute cracks from propagating.

When there is a lot of eutectic liquid there is enough liquid to back fill any cracks that may have appeared prior to full solidification of the joint.

Hot cracking occurs when there is an intermediate volume of eutectic liquids on the weld pool. Dilution of the weld joint can be a factor in the composition of the allying elements on the weld pool, therefore as a general rule aluminium should not be welded autonomously.

A reputable consumable supplier should be able to provide you with a table to aid in the selection of the correct filler wire composition for aluminium, you may also contact a consumables manufacture for advice.

Welder Skill

During welding of most materials, the colour change is a good reliable indicator to the welder of the temperature of the material for a quality weld to be produced. Aluminium does not change colour when heat is applied and so the welder need to the different behaviour of aluminium to be successful. Retraining of welders is critical to be able to produce satisfactory weld quality and is recognised by all welding standards in the need to qualify aluminium welders separately to other materials.

Aabco Welding Services Sdn Bhd

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We offer a range of services to meet the welding needs of all industries, from aerospace, nuclear, oil and gas, pharmaceutical to construction and more.

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