How To Weld Aluminuim Well

How to weld aluminum? You may be wondering about this, which is really a very difficult project for newcomers to welding, this article will be an ultimate guide to help learn more about that how to weld aluminum material. this is the outline of the article.

• Introduction
• Characteristics of aluminuim
• Aluminuim alloys designation
• Aluminuim weldability problems
• Selection of Welding filler materials
• Preparation for welding
• Welding processes used in Aluminuim welding
• TIG Welding
• MIG welding
• Pulsed MIG

Introduction

Aluminuim and aluminuim alloys belong to the non-ferrous materials and are characterized by their light weight and high corrosion resistance that is why these alloys are used in many important applications:

1. Airoplanes: where aluminuim represents around 80% of the weight of a typical civilian airoplanes (fig. 1)
2. Bulk carrier and ships super-structures (fig. 2)
3. Railway rolling stock, roadside furniture, pipelines and pressure vessels, civil and military large structures like bridges and in manufacturing foil which is used in packaging industry..
4. Military armoured vehicles where a combination of light weight and ballistic performance makes it the ideal material for fast Military vehicles (fig. 3).

Fig. 1 Boeing SUGAR Volt concept aircraft ‐ Wikimedia Commons

Fig. 2 ships built mainly by using aluminuim

Fig. 3 Armoured army vehicle M113 armored personnel carrier.

So, due to the diversity of modern uses of aluminuim alloys, the welding engineer should be aware of the different types of aluminuim alloys and how to weld them without any degradation of properties that is why in this article, we will go through the different alumiuim alloys and the proper welding procedures that are to be followed and the modern welding techniques used in welding such type of alloys.

Characteristics of aluminuim

There are some characteristics of aluminuim and aluminuim alloys that should be taken into consideration:

1. The high difference between between the melting point of aluminuim and aluminuim oxide where the melting point of aluminuim is a round 1400 degree celsius and that of aluminuim oxide is 2060 degree celsius, so as aluminuim oxide is formed during welding, it becomes very difficult to have sound weld due to the lack of fusion of aluminuim oxide due to its higher melting point, so you have to use a technique to break this oxide layer continuously during welding.
2. The oxide film on aluminium is durable, highly tenacious and selfhealing and are formed in normal temperature that is why aluminuim alloys have excellent corrosion resistance without any additional protection and consequently they become suitable for exposed applications like aeroplanes.
3. Aluminuim welding includes high probability of distortion relative to steel as the coefficient of thermal expansion of aluminium is approximately twice that of steel.
4. High dissipation of welding heat during welding relative to steel due to the high thermal conductivity of aluminuim that reach 6 times that of steel in addition to the the high specific heat of aluminuim which reach 2 times that of steel. That is why welding of aluminuim requires highly concentrated source of heat by using pulsed processes to get complete fusion.
5. Resistance spot welding of aluminuim is very difficult due to the high electrical conductivity of aluminuim which reach three-quarters that of copper but six times that of steel.
6. Aluminium does not change colour as its temperature rises, unlike steel. This can make it difficult for the welder to judge when melting is about to occur, making it imperative that adequate retraining of the welder takes place when converting from steel to aluminium welding.
7. Aluminium is non-magnetic which means that arc blow is eliminated as a welding problem.
8. The fact that aluminium has a face-centred cubic crystal structure (see Fig. 4) means that it does not suffer from a loss of notch toughness as the temperature is reduced. That is why, aluminuim is used in cryogenic applications to a temperature -200 degree celsius.
9. Aluminium has the same crystal structure on heating and cooling cycles unlike steel which undergoes crystal transformations, so rapid cooling has no effect on aluminuim alloys.

Fig. 4 Face Centered Cubic Unit cell

Aluminuim alloys designation 

There is a numeric method adopted by the European Committee for Standardisation (CEN) will be used as standard. This system uses four digits to identify the wrought alloys and five digits to identify the cast alloys.

Table 2: Cast aluminuim alloys classification.                   

In the European system the prefixes are used:

•  ‘AB’ denotes ingots for remelting.
•  ‘AC’ denotes a cast product.
• ‘AM’ a cast master alloy.
•  The prefix ‘AW’ a wrought product. 

Aluminuim weldability problems

Before going deep in the welding processes and techniques used in welding, we have to know first the common chalenges that face you as a welding engineer, welding supervisor, and welder during welding aluminuim and aluminuim alloys:

Fig. 5 Finely distributed porosity in TIG plate butt weld 6mm thickness. Courtesy of TWI Ltd

Gases that cause porosity comes from the moisture on the joints or from the used shielding gases. The sudden change in gases solubility like H2 is the main reason of porosity (see in Fig. 6) that is why, it is extremely difficult to produce a porosity-free weld in aluminuim.

Fig. 6 Solubility of hydrogen in aluminuim.

In MIG welding, there is a phenomenon known as cathodic cleaning to break the formed oxide layer where the electrode is connected to the positive pole of the power source in which the electrones flow from the work piece to the electrode and ions flow in the opposite direction resulting in the break of oxide layer.

But there is one important point to be explained: why we are using AC and DCEP current during MIG and AC only for TIG welding?

Fig. 7 Arc physics for MIG welding process.

Fig. 8 Arc physics for TIG welding process.

Fig. 9 Arc physics with changing current polarity

from figures 7, 8, and 9,  it is apparent that the behaviour with using electrode negative and electrode positive is not the same as the nature of electrodes used are not the same where in MIG welding the filler metal is the electrode itself which is aluminum having the same number of electrons on the atomic scale where aluminum have  13 electrons so the emition power of  electrons is the same like base metal  that is why electrode positive in MIG is very suitable to have deep penetration as the surface area of base metal is very large relative to electrode resulting in stable arc and cathodic cleaning.

On the other hand in TIG welding, the arc is initiated using non-consumable electrodes like tungsten which have more electrons on the atomic scale where each atom have 74 electrons that is why TIG with negative electrode is more suitable as it achieve more penetration as shown in figures above due to the high emition rate of electrons during electrode negative process assembly but this process is not efficient for welding aluminum as it will not help in cathodic cleaning that is why AC welding by TIG is recommended for aluminum.

From the name of the problem, it is clear that this issue occurs during welding when the weld metal is still hot and this issue is very common during welding some aluminuim alloys due to the presence of low melting point components along the grain boundaries of solidified matrix.

Fig. 10 Solidification cracking in a 3mm thick A6082 plate using 4043 filler metal TIG weld. Courtesy of TWI Ltd.

Fig. 11 Effect of solute atom concentration on hot cracking sensitivity.

As shown in Fig. 11, the hot cracking sensitivity depends mainly on the concentration of alloying element in the aluminuim alloy. 

In summary, if hot cracking is encountered, there are some factors to be used to eliminate such issue:

• Using filler wires that can produce small grain size welds and this can happen by the addition of titanium, zirconium or scandium will act as nuclei for the formation of a very fine grain during solidification.
• Proper preparation of weld joints by controlling edge preparation and joint spacing to permit sufficient filler metal to be added to the joint to control the weld metal composition to be out of hot short range.
•  Shortening time in the hot short range by using high welding speed.

Selection of Welding filler materials 

Aluminuim filler metals are selected based on the issue that the welding engineer is concerned about such as crack resistance, strength, colour, and corrosion resistance unlike the selection of steel filler metals which are selected based on chemical and mechanical properties.

AWS A5.10 “ Specification for bare aluminuim and aluminuim alloy welding electrodes and rods” includes 15 separate filler metal compositions, comprising alloys in the 1XXX, 2XXX, 4XXX and 5XXX series. 

 Table 4:  Guide to filler material selection                                      

There are a number of specific points to be made to amplify the guidance given in Tables 4:

• When you are welding alloys containing more than 2% Magnesium, avoid using filler metals containing silicon to prevent the formation of intermetallic compounds of magnesium silicide (Mg3Si) which cause joint embrittelement.
• When using 5XXX filler metals with more than 5% Mg like 5654, the service temperature shall be considered not to exceed 65 degree celsius to avoid the formation of Al2Mg which makes the alloy susceptible to stree corrosion. In this case you should use filler metals with lower Mg content like 5454 or 5554 which contain 3% Mg.
• High-purity 5654 is preferred for the welding of high-purity aluminium in hydrogen peroxide service.
• 4643 should be used with 6XXX alloys as the small content of magnesium will improve the response to solution treatment.
• The pure aluminium 1XXX alloys are very soft and ductile so, the wire feeding problems can be experienced.
• Low magnesium (<2%) 5XXX alloys such as 5251 may become susceptible to hot cracking issues during welding when using filler metals with matching chemical composition that is why it is recommended to use Al-Mg5 type instead.
• It is recommended to use 5039 filler wire when welding 7XXX alloys to give good results in low-dilution applications.
•  Take care when welding 6XXX alloys as they exhibit solidification cracking if welded autogenously.
• Alloying elements Titanium and zirconium are sometimes added to filler metals to reduce the risk of weld metal hot cracking by means of grain refinement.

Preparation for welding 

Weld preparation include cutting, chamfering, and fine adjusting processes that commonly use whether mechanical tools like the traditional ones that are used in all traditional workshops like cutting and grinding light tools or preparation can be done by using plasma-arc cutting tool.

Mechanical cutting tools are the most cost-effective methods that are commonly used in workshops due to their lower capital rather than laser and plasma cutting systems.

Edge preparation for welding can be done by mechanical methods using many tools including high-speed milling machines, edge planers, routers and various types of saws in addition to air-powered equipment in which you have to take care that the supplied air is dry and clean not cause any porosity during welding.

Plasma-arc can be used whether for welding or cutting and it is considered the most common thermal process in cutting aluminuim alloys which can be operated in manual, mechanised or fully automated modes.

Fig. 12 Schematic illustrating the principles of plasma-jet cutting. Courtesy of TWI Ltd.

As shown in Fig. 12, the plasma cutting torch uses tungsten electrode which is surrounded by gas flow of plasma gas which can be air, argon, argon-hydrogen, nitrogen, carbon dioxide.

There are two types of plasma-arc including transferred-arc and nontransferred-arc. In the transferred-arc, the arc is struck between the tungsten electrode and workpiece but in case of nontransferred arc, the arc is struck inside the plasma torch between the tungsten electrode and the annulus. In cutting, transferred arc is usually used.

The composition of the gas for plasma cutting depends on the required quality of the cut, the thickness of the metal to be cut and the cost of the gas (Table 3). Air is the cheapest option and single gas systems utilising air and a hafnium electrode have been developed for the cutting of materials up to approximately 6mm in thickness (Fig. 13).

Fig. 13 Air plasma cutting. Courtesy of TWI Ltd.

Table 3 Suggested parameters for plasma-jet cutting

For TIG welding of aluminuim, there are a suggested welding preparations as per BS 3019 as shown in the below Table 4:

Thickness (mm)	Edge preparation	Remarks
20swg = Flanging should 0.9mm and be used only 16swg = where square 1.6mm		Flanged edge preparations are recommended when square edge preparations are impractical.
3.8 mm		When backing bar cannot be used, welding from both sides is recommended.
4.8mm
6.4mm		If no backing bar is used, back chipping to the sound weld metal is done and sealing run is added
9.5mm		If no backing bar is used, back chipping to the sound weld metal is done and sealing run is added. When welding from the other side, chip first the back side to the sound weld metal then start welding from the back side.
12.7mm		Preheating may be necessary
4.8-6.4mm Over 6.4mm-12.7mm

 Table 4 suggested edge preparation for TIG welding according to BS 3019.

For MIG welding, there are suggested edge preparations as per BS 3019 as per the below table 5:

Material Thickness (mm)	Edge preparation	Remarks
1.6-4.8mm		Backing bar gives a greater control on penetration
6.4-9.5mm		Weld from both sides
4.8-12.7mm		Suitable for welding from both sides
6.4-12.7mm		Back chipping is a must before welding from the other side
6.4-19.1mm		Suitable for welding from both sides but back chipping is a must in addition to preheating.
12.7-25.4mm		Back chipping is a must before welding from the other side

 Table 5 suggested edge preparations for MIG welding.

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Welding processes used in Aluminuim welding

TIG Welding

TIG stands for tungsten inert gas welding and also called GTAW which stands for gas tungsten arc welding as per the USA standard but as per european standard, the process takes a numerical abbreviation which is 144 for TIG welding.

As shown in Fig. 14, a non-consumable tungsten electrode is used to create the welding arc which represent the source of welding heat and this arc is protected by inert gas which commonly is argon to protect the electrode, arc column and weld pool.

Fig. 14 Schematic of the TIG welding process.

TIG welding of aluminuim usually takes place by using AC current to achieve the cleaning effect by removing the oxide layer and this takes place on the electrode positive half cycle in which electrones flow from the workpiece to the non-consumable electrode and ions flow in the opposite direction but in the other half of AC cycle in which the electrode is negative, the electrode cool down and deep penetration is achieved.

In the AC welding, the arc is extinguished and reignited every half cycle as the arc current passes through zero,that is why a high-frequency power source is usually used to achieve instant arc reignition a high-frequency (HF), enabling the welding arc to reignite with a minimum delay. 

As shown in Fig. 15, the modern equipment uses a solid state circuitry which change the AC current from the normal sine wave form to the squared AC wave form. These power sources can be adjusted as required whether by changing the frequency or changing the time of the positive or negative halves of the cycle.

The latest inverter-based units provide a high degree of control with the electrode negative duration time can be adjusted from 50% to 90% of the cycle resulting in more penetration. Increasing the frequency results in a more focused arc, increasing penetration, enabling faster travel speeds to be usedand reducing distortion.

Fig.15 AC squared waves.

There are recommended welding parameters for aluminuim welding as shown in Table 6:

Table 6 suggested welding parameters-argon gas shielding in flat position.

You will notice that there is no any recommendations for TIG welding parameters for welding aluminuim above 10 mm thickness due to economic considerations where TIG welding is rarely used.

The AC/DC TIG 200P welder is a powerful aluminum TIG welding machine that offers welding with TIG AC Square/Triangular/Pulse Square/Pulse Triangular/DC Pulse/DC/STICK methods. It can be used for welding various metals, including aluminum, steel, stainless steel, carbon steel, and copper. The patented DASH-ARC technology improves arc striking speed, welding efficiency, and results, while the pulse, square, and triangular waveform functions prevent material burning, minimize distortion, and improve bead appearance. The digital control system makes the machine reliable and stable, and the large LED display allows for easy parameter adjustment. 

Fig. 16 The Multi process AC/DC TIG 200P welder

MIG welding

MIG stands for metal inert gas welding in which welding consumables are in the form of spools not electrodes like in SMAW or TIG and wire is fed to the welding torch through wire feed unit and shielding for the molten wire is done by using inert gas which is usually argon or mix of argon and helium or helium.

The MIG arc requires a power source that will provide direct current and with a suitable relationship established between welding current and voltage, this relationship being known as the power source dynamic characteristic. As mentioned above the MIG process uses a continuous wire feed and for the majority of welding operations it is important that the rate at which the wire burns off in the arc is matched by the wire feed speed. Failure to do this can result in an unstable arc and variable weld quality.

Recent power source developments have been successful in enabling the MIG process to be also used with AC. Most of the heat developed in the arc is generated at the positive pole, in the case of MIG welding the electrode, resulting in high wire burn-off rates and an efficient transfer of this heat into the weld pool by means of the filler wire. 

In MIG welding. The molten filler wire transfer to the joint through different types of transfer modes that mainly depend on the current and shielding gas used and can be related to wire diameter in case of aluminuim welding as in Table 7.

Table 7 Metal transfer mode and wire diameter.

When MIG welding aluminium the low melting point of the aluminium results in spray transfer down to relatively low welding currents, giving a spatter-free joint.

In the below Table 8, the recommended welding parameters for aluminuim welding using MIG welding:

Table 8 Suggested welding parameters of aluminuim using MIG welding.

Fig. 16 ARCCAPTAIN MIG200 200 Amps Multi-Process MIG Welder, 6 in 1 Multi Welding Machine (MIG, Spot, TIG, MMA...) - ARCCAPTAIN MIG200 | Unboxing and Test

One of the good welders that is used in aluminuim welding is ARCCAPTAIN MIG200 which uses dual voltage multi-process 6 in 1 mig welder , which is very suitable for diy, fabrication, repair projects, etc.,  Whether you are a beginner or a professional welder, you can easily use it. The MIG200's multi purpose design make it an excellent investment for anyone looking for a reliable and efficient mig welder.

The MIG 200 integrates the functions of stick welder, mig welder, aluminum welder and lift tig welder, which is really a all purpose welder. Given its performance and affordable price, this welder is a real outstanding mig welder for beginner and professional welder. It’s ideal for the typical DIY project because it can weld practically anything a homeowner could need to. You can adjust the current easily adjusted by the large LED display. It's compact size makes it light and ultra-portable. This machine weighs only 28.2 lbs, you can easily take it anywhere you need to weld.

Pulsed MIG

The pulsed MIG process uses a low ‘background’ current, sufficient to maintain the arc but not high enough to cause the wire to melt off. On this background current a high-current, ‘peak’ pulse is superimposed. Under optimum conditions this causes a single droplet of molten filler wire to be projected across the arc into the weld pool by spray transfer. It is thus possible to achieve spray transfer and a stable arc at low average welding currents. This enables very thin metals to be welded with large diameter wires where previously very thin wires, difficult to feed in soft aluminium, needed to be used.The lower currents also reduce penetration, useful when welding

thin materials and also enable slower welding speeds to be used, making it easier for the welder to manipulate the torch in difficult access conditions or when welding positionally.

Fig. 17 DC Pulsed Current,Miller Welding 

As shown in Fig. 17, the change in frequency leeds to a change in the number of pulses for the same portion of time and also you can control the time period at the pulse current and background current.

Summary:

The article provides an overview of aluminum welding, including the characteristics of aluminum, aluminum alloy designation, aluminum weldability problems, selection of welding filler materials, preparation for welding, and welding processes used in aluminum welding. It highlights the importance of understanding different aluminum alloys and proper welding procedures to maintain the integrity of welded joints.

Aluminum is widely used in various applications due to its lightweight and high corrosion resistance. It is used in airplanes, ships, railway rolling stock, bridges, and military vehicles. However, welding aluminum presents challenges such as oxide film formation, porosity, hot cracking, and distortion.

To overcome these challenges, various welding processes are used, including TIG (tungsten inert gas) welding and MIG (metal inert gas) welding. TIG welding uses a non-consumable tungsten electrode and AC current to break the oxide layer and achieve deep penetration. MIG welding uses a continuous wire feed and shielding gas to protect the weld pool.

The article also provides recommended welding parameters for TIG and MIG welding of aluminum based on thickness, joint type, current, voltage, filler diameter, and travel speed. It emphasizes the importance of proper edge preparation and selection of filler materials based on the desired properties of the welded joint.

The article is a good guide for welding engineers, supervisors, and welders in understanding the fundamentals of aluminum welding and implementing effective welding procedures for aluminum alloys.