Aluminum Welding: A Practical Guide

Aluminum is a lightweight, corrosion-resistant, and versatile metal that is widely used in various industries, such as automotive, aerospace, construction, and manufacturing. However, welding aluminum can be challenging due to its unique properties, such as high thermal conductivity, low melting point, and oxide formation. In this blog, we will provide some basic information about aluminum welding, including the common methods, filler metal selection, preparation, and applications.

Aluminum Alloy and Temper Designations

Before we dive into the welding techniques, it is important to understand the different types of aluminum alloys and their designations. Aluminum alloys are classified into two groups: wrought and cast. Wrought alloys are formed by mechanical processes, such as rolling, extruding, or forging, while cast alloys are formed by pouring molten metal into molds.

Wrought alloys are further divided into eight series, based on their main alloying elements. The most common series are the 1xxx, 3xxx, 5xxx, and 6xxx series, which contain aluminum, manganese, magnesium, and magnesium-silicon, respectively. Each series has different characteristics and applications, depending on the alloy composition and heat treatment. For example, the 1xxx series has high electrical and thermal conductivity, but low strength, while the 6xxx series has moderate strength and good formability, but lower corrosion resistance.

The temper designation indicates the mechanical properties and the condition of the alloy, such as whether it has been annealed, cold-worked, or heat-treated. The temper designation consists of a letter followed by one or more digits. The most common tempers are O (annealed), H (strain-hardened), T (thermally treated), and F (as-fabricated). For example, 6061-T6 is a wrought alloy of the 6xxx series that has been solution heat-treated and artificially aged to achieve a high level of strength.

Cast alloys are designated by a four-digit number, followed by a decimal point and a temper designation. The first digit indicates the major alloying element, the second digit indicates the alloy modification, and the last two digits identify the specific alloy. For example, 356.0 is a cast alloy that contains mainly aluminum, silicon, and magnesium, and has an as-cast temper.

Filler Metal Selection

The choice of filler metal for aluminum welding depends on the base metal composition, the desired weld properties, and the welding process. The filler metal should have a similar melting range and chemical compatibility with the base metal, as well as adequate strength, ductility, and corrosion resistance. The filler metal should also minimize the formation of defects, such as porosity, cracking, and lack of fusion.

The most common filler metals for aluminum welding are the 4xxx and 5xxx series, which contain silicon and magnesium, respectively. Silicon is added to lower the melting point and improve the fluidity of the filler metal, while magnesium is added to increase the strength and corrosion resistance of the weld. The 4xxx series is suitable for welding cast alloys, while the 5xxx series is suitable for welding wrought alloys.

The filler metal selection also depends on the welding process, as different processes have different requirements for the filler metal form, size, and feedability. For example, gas tungsten arc welding (GTAW) uses a filler rod that is manually fed into the weld pool, while gas metal arc welding (GMAW) uses a filler wire that is continuously fed by a wire feeder. The filler rod or wire should have a diameter that matches the thickness of the base metal and the current level.

The following table shows a general guide for selecting the filler metal for aluminum welding, based on the base metal alloy and the welding process. However, this table is not exhaustive and does not cover all possible combinations and conditions. Therefore, it is advisable to consult the filler metal manufacturer or the welding code for specific recommendations.

Preparation for Welding

To achieve a high-quality weld, it is essential to prepare the base metal and the filler metal properly before welding. The preparation steps include cleaning, cutting, joint design, and preheating.

Cleaning

Cleaning the base metal and the filler metal is necessary to remove any contaminants that may affect the weld quality, such as dirt, oil, grease, oxide, or moisture. Contaminants can cause defects, such as porosity, lack of fusion, or cracking, as well as reduce the strength and corrosion resistance of the weld.

The cleaning methods depend on the type and degree of contamination, as well as the welding process. Some of the common cleaning methods are:

• Mechanical cleaning: This method involves using a stainless steel wire brush, a sanding disc, or a grinding wheel to remove the surface oxide layer and any loose particles. Mechanical cleaning should be done in the direction of the weld and only on the area to be welded. The cleaning tool should be used only for aluminum and not for other metals, to avoid cross-contamination.
• Chemical cleaning: This method involves using a solvent, an acid, or an alkaline solution to dissolve or loosen the oxide layer and any organic residues. Chemical cleaning should be done with proper safety precautions and followed by rinsing and drying. The cleaning solution should be compatible with the aluminum alloy and the filler metal, and should not leave any harmful residues.
• Degreasing: This method involves using a degreaser, such as acetone, alcohol, or trichloroethylene, to remove any oil or grease from the surface. Degreasing should be done with a clean cloth or a spray, and followed by wiping or air-drying. The degreaser should not contain any chlorinated hydrocarbons, as they can cause hydrogen embrittlement and cracking.

Cleaning should be done as close as possible to the welding time, as aluminum tends to form a thin oxide layer quickly when exposed to air. The oxide layer has a higher melting point than the base metal and can interfere with the weld penetration and fusion. Therefore, it is recommended to weld within a few hours after cleaning, or to use a shielding gas or a flux to protect the weld area from oxidation.

Cutting

Cutting the base metal is necessary to create the desired shape and size of the workpiece, as well as to prepare the joint edges for welding. The cutting method should produce a smooth, clean, and square edge, without excessive distortion, burrs, or slag.

Some of the common cutting methods for aluminum are:

• Shearing: This method involves using a shear machine to cut the metal with a blade or a punch. Shearing is suitable for thin sheets and simple shapes, but it can cause distortion and edge hardening.
• Sawing: This method involves using a circular saw, a band saw, or a hacksaw to cut the metal with a toothed blade. Sawing is suitable for thick plates and complex shapes, but it can cause noise, dust, and heat.
• Plasma cutting: This method involves using a plasma torch to cut the metal with a jet of ionized gas. Plasma cutting is suitable for any thickness and shape, but it can cause dross, slag, and heat-affected zone.
• Laser cutting: This method involves using a laser beam to cut the metal with a focused beam of light. Laser cutting is suitable for any thickness and shape, but it can cause heat-affected zone and high cost.

The cutting method should be selected based on the material thickness, the desired accuracy, the available equipment, and the cost. The cutting speed and the feed rate should be adjusted according to the manufacturer’s recommendations, to avoid overheating, warping, or cracking. The cutting edge should be inspected for any defects or irregularities, and cleaned if necessary.

Joint Design

Joint design is the process of selecting and arranging the joint type, the joint geometry, the joint fit-up, and the joint gap for welding. The joint design should provide adequate strength, alignment, and accessibility for welding, as well as minimize the distortion, stress, and cracking.

The joint type is the configuration of the joint, such as butt, corner, lap, tee, or edge. The joint type should be selected based on the material thickness, the load direction, the weld position, and the welding process. For example, a butt joint is suitable for joining two plates of the same thickness, while a lap joint is suitable for joining two plates of different thicknesses.

The joint geometry is the shape and angle of the joint edges, such as square, bevel, V, U, J, or double-V. The joint geometry should be selected based on the material thickness, the weld penetration, and the welding

process. For example, a square edge is suitable for thin plates, while a bevel edge is suitable for thick plates.

The joint fit-up is the alignment and positioning of the joint edges, such as flush, offset, or mismatch. The joint fit-up should be selected based on the material thickness, the weld size, and the welding process. For example, a flush fit-up is suitable for thin plates, while an offset fit-up is suitable for thick plates.

The joint gap is the distance between the joint edges, which affects the weld penetration and fusion. The joint gap should be selected based on the material thickness, the filler metal, and the welding process. For example, a small gap is suitable for thin plates, while a large gap is suitable for thick plates.

The following table shows a general guide for selecting the joint design for aluminum welding, based on the material thickness and the welding process. However, this table is not exhaustive and does not cover all possible combinations and conditions. Therefore, it is advisable to consult the welding code or the welding engineer for specific recommendations.

Preheating

Preheating the base metal is the process of applying heat to the metal before welding, to raise its temperature to a certain range. Preheating is necessary for some aluminum alloys, especially the heat-treatable alloys, such as the 2xxx, 6xxx, and 7xxx series, to prevent cracking and distortion.

Preheating can provide the following benefits for aluminum welding:

• Reduce the thermal gradient and the thermal shock, which can cause cracking and distortion.
• Increase the solubility and the diffusion of hydrogen, which can cause porosity.
• Reduce the hardness and the strength of the base metal, which can improve the weldability and the ductility.
• Reduce the shrinkage and the residual stress, which can cause distortion and cracking.

The preheating temperature and time depend on the base metal alloy, the material thickness, the joint design, and the welding process. The preheating temperature should be high enough to achieve the desired effects, but low enough to avoid overheating, melting, or burning the metal. The preheating time should be long enough to ensure a uniform temperature distribution, but short enough to avoid oxidation, degradation, or aging of the metal.

The following table shows a general guide for selecting the preheating temperature and time for aluminum welding, based on the base metal alloy and the material thickness. However, this table is not exhaustive and does not cover all possible combinations and conditions. Therefore, it is advisable to consult the welding code or the welding engineer for specific recommendations.

The preheating method can be done by using a gas torch, an electric heater, an induction coil, or an oven. The preheating method should be selected based on the material size, the joint location, and the available equipment. The preheating method should ensure a uniform and controlled heating, without overheating, melting, or burning the metal.

The preheating temperature and time should be monitored and verified by using a thermometer, a pyrometer, a thermocouple, or a temperature-indicating crayon. The preheating temperature and time should be maintained until the welding is completed, to avoid thermal fluctuations and cracking.

Welding Processes

There are various welding processes that can be used for aluminum welding, such as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), flux-cored arc welding (FCAW), plasma arc welding (PAW), and laser beam welding (LBW). Each welding process has its own advantages and disadvantages, depending on the material thickness, the joint design, the weld position, and the weld quality.

The following table shows a general comparison of the welding processes for aluminum welding, based on the material thickness, the welding speed, the weld appearance, the weld penetration, and the weld defects. However, this table is not exhaustive and does not cover all possible combinations and conditions. Therefore, it is advisable to consult the welding code or the welding engineer for specific recommendations.

Gas Tungsten Arc Welding (GTAW)

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is a welding process that uses a non-consumable tungsten electrode to create an arc between the electrode and the workpiece, and a filler rod to add metal to the weld pool. The arc and the weld pool are protected by a shielding gas, such as argon or helium, to prevent oxidation and contamination.

GTAW is suitable for welding thin to thick aluminum plates, as it provides excellent weld appearance, good weld penetration, and low weld defects. GTAW also allows precise control over the heat input, the arc length, and the filler metal addition, which can improve the weld quality and reduce the distortion. However, GTAW is a slow and complex welding process, which requires high skill and experience, as well as special equipment and accessories.

The following are some of the key factors that affect the GTAW performance and quality for aluminum welding:

• Electrode selection: The electrode should be made of pure tungsten or tungsten alloyed with thorium, cerium, or lanthanum, to improve the arc stability and the electrode life. The electrode should have a diameter that matches the current level and the material thickness, and a tip shape that matches the arc characteristics and the weld penetration. For example, a pointed tip is suitable for low current and deep penetration, while a spherical tip is suitable for high current and shallow penetration.
• Electrode polarity: The electrode polarity should be alternating current (AC), to achieve a balance between the cleaning and the penetration effects. The cleaning effect is the removal of the oxide layer from the base metal by the electrode positive (EP) cycle, while the penetration effect is the melting of the base metal by the electrode negative (EN) cycle. The balance between the cleaning and the penetration effects can be adjusted by changing the AC frequency, the AC balance, and the waveform. For example, a higher frequency, a lower balance, and a square waveform can increase the penetration effect, while a lower frequency, a higher balance, and a sine waveform can increase the cleaning effect.
• Shielding gas selection: The shielding gas should be pure argon or argon mixed with helium, to protect the arc and the weld pool from oxidation and contamination. The shielding gas should have a flow rate that matches the nozzle size and the current level, and a pressure that matches the ambient conditions and the weld position. For example, a higher flow rate and a higher pressure are suitable for windy or overhead welding, while a lower flow rate and a lower pressure are suitable for calm or flat welding.
• Filler metal selection: The filler metal should be compatible with the base metal, as discussed in the previous section. The filler metal should have a diameter that matches the material thickness and the current level, and a length that matches the joint length and the weld position. The filler metal should be clean and dry, and stored in a sealed container to prevent contamination and moisture absorption. The filler metal should be manually fed into the weld pool at a suitable angle and speed, to avoid overheating, melting, or freezing.
• Welding technique: The welding technique should provide a smooth and consistent weld bead, with adequate fusion, penetration, and reinforcement. The welding technique should also minimize the heat input, the distortion, and the defects. The welding technique depends on the material thickness, the joint design, the weld position, and the welder’s skill and preference. Some of the common welding techniques are:
  • Forehand technique: This technique involves moving the torch and the filler rod in the same direction, from left to right or from right to left, depending on the welder’s handedness. The torch and the filler rod should form an angle of 10 to 20 degrees with the workpiece, and the arc length should be 1 to 2 mm. The torch and the filler rod should move in a straight or a slightly oscillating motion, to create a uniform and narrow weld bead. The forehand technique is suitable for thin to medium plates, as it provides fast welding speed, good weld appearance, and low heat input.
  • Backhand technique: This technique involves moving the torch and the filler rod in the opposite direction, from right to left or from left to right, depending on the welder’s handedness. The torch and the filler rod should form an angle of 20 to 30 degrees with the workpiece, and the arc length should be 2 to 3 mm. The torch and the filler rod should move in a circular or a triangular motion, to create a wide and deep weld bead. The backhand technique is suitable for medium to thick plates, as it provides slow welding speed, good weld penetration, and high heat input.

Gas Metal Arc Welding (GMAW)

Gas metal arc welding (GMAW), also known as metal inert gas (MIG) welding, is a welding process that uses a consumable wire electrode to create an arc between the electrode and the workpiece, and to add metal to the weld pool. The arc and the weld pool are protected by a shielding gas, such as argon or argon mixed with oxygen, carbon dioxide, or helium, to prevent oxidation and contamination.

GMAW is suitable for welding thin to thick aluminum plates, as it provides fast welding speed, good weld penetration, and low weld defects. GMAW also allows automatic or semi-automatic control over the wire feed, the current level, and the arc length, which can improve the weld quality and reduce the operator fatigue. However, GMAW is a complex and sensitive welding process, which requires special equipment and accessories, as well as careful adjustment and maintenance.

The following are some of the key factors that affect the GMAW performance and quality for aluminum welding:

• Wire electrode selection: The wire electrode should be compatible with the base metal, as discussed in the previous section. The wire electrode should have a diameter that matches the material thickness and the current level, and a length that matches the joint length and the weld position. The wire electrode should be clean and dry, and stored in a sealed container to prevent contamination and moisture absorption. The wire electrode should be continuously fed by a wire feeder at a suitable speed and tension, to avoid tangling, jamming, or breaking.
• Wire polarity: The wire polarity should be direct current electrode positive (DCEP), to achieve a stable arc and a good weld penetration. The wire polarity should be matched with the power source and the wire feeder, to avoid reverse polarity, which can cause arc instability, spatter, and lack of fusion.
• Shielding gas selection: The shielding gas should be pure argon or argon mixed with oxygen, carbon dioxide, or helium,to protect the arc and the weld pool from oxidation and contamination. The shielding gas should have a flow rate that matches the nozzle size and the current level, and a pressure that matches the ambient conditions and the weld position. The shielding gas should also have a composition that matches the wire electrode and the weld properties. For example, argon is suitable for most wire electrodes, as it provides a stable arc and a good weld appearance, while argon mixed with oxygen or carbon dioxide can improve the arc stability and the weld penetration for some wire electrodes, but may cause more spatter and porosity, while argon mixed with helium can increase the heat input and the weld penetration for some wire electrodes, but may cause more arc instability and distortion.
• Welding technique: The welding technique should provide a smooth and consistent weld bead, with adequate fusion, penetration, and reinforcement. The welding technique should also minimize the heat input, the distortion, and the defects. The welding technique depends on the material thickness, the joint design, the weld position, and the welder’s skill and preference. Some of the common welding techniques are:
  • Short-circuiting transfer: This technique involves using a low voltage and a high wire feed speed, to create a series of short circuits between the wire electrode and the workpiece, which melt the wire electrode and transfer it to the weld pool. The short-circuiting transfer is suitable for thin plates, as it provides low heat input, low spatter, and low distortion, but it may cause low weld penetration and lack of fusion.
  • Globular transfer: This technique involves using a medium voltage and a medium wire feed speed, to create large droplets of molten metal at the tip of the wire electrode, which detach and fall into the weld pool by gravity. The globular transfer is suitable for medium to thick plates, as it provides high heat input, high weld penetration, and high deposition rate, but it may cause high spatter, high distortion, and porosity.
  • Spray transfer: This technique involves using a high voltage and a high wire feed speed, to create small droplets of molten metal at the tip of the wire electrode, which are propelled into the weld pool by the arc force. The spray transfer is suitable for thick plates, as it provides high heat input, high weld penetration, and high deposition rate, but it may cause high spatter, high distortion, and porosity.
  • Pulsed-spray transfer: This technique involves using a pulsed current, which alternates between a high peak current and a low background current, to create a spray transfer during the peak current and a short-circuiting transfer during the background current. The pulsed-spray transfer is suitable for any thickness, as it provides a balance between the heat input, the weld penetration, and the weld appearance, and it can also reduce the spatter, the distortion, and the porosity.

Applications of Aluminum Welding

Aluminum welding has a wide range of applications in various industries, such as automotive, aerospace, construction, and manufacturing. Aluminum welding can provide the following benefits for these industries:

• • • Lightweight: Aluminum is a lightweight metal, which can reduce the weight and the fuel consumption of the vehicles, aircraft, and structures, as well as improve the performance and the efficiency.
    • Corrosion-resistant: Aluminum is a corrosion-resistant metal, which can withstand the exposure to the weather, the chemicals, and the saltwater, as well as extend the service life and the durability of the vehicles, aircraft, and structures.
    • Versatile: Aluminum is a versatile metal, which can be formed into various shapes and sizes, as well as joined with various methods, such as welding, brazing, soldering, or adhesive bonding, to create complex and customized designs and products.

Some of the examples of the applications of aluminum welding are:

• • • Automotive: Aluminum welding is used to join the aluminum components of the vehicles, such as the engine, the transmission, the chassis, the body, and the wheels, to reduce the weight and the emissions, as well as improve the performance and the safety.

      

    • Aerospace: Aluminum welding is used to join the aluminum components of the aircraft, such as the fuselage, the wings, the tail, and the landing gear, to reduce the weight and the fuel consumption, as well as improve the performance and the reliability.
    • Construction: Aluminum welding is used to join the aluminum components of the structures, such as the bridges, the buildings, the towers, and the pipelines, to reduce the weight and the maintenance, as well as improve the strength and the stability.
    • Manufacturing: Aluminum welding is used to join the aluminum components of the products, such as the furniture, the appliances, the tools, and the equipment, to reduce the cost and the waste, as well as improve the quality and the functionality.

Conclusion

Aluminum welding is a challenging but rewarding skill that can create strong and durable joints for various applications. Aluminum welding requires a good understanding of the aluminum alloys and their designations, the filler metal selection, the preparation for welding, and the welding processes. Aluminum welding also requires a proper equipment and accessories, as well as a careful adjustment and maintenance. Aluminum welding can provide a lightweight, corrosion-resistant, and versatile solution for various industries, such as automotive, aerospace, construction, and manufacturing.