Aluminum welding can be a challenging process due to the metal’s unique properties, but with the right techniques and expert insights, it can be mastered. In this comprehensive guide, we will explore the various methods of aluminum welding, including TIG, MIG, and plasma arc welding, and provide tips and tricks for achieving high-quality welds.
One of the key factors in successful aluminum welding is proper preparation. This includes cleaning the metal surface to remove any contaminants, such as oil or dirt, that can interfere with the welding process. Additionally, selecting the appropriate filler metal and shielding gas is crucial for achieving strong, defect-free welds. Our guide will provide in-depth information on these topics, as well as expert insights on heat control, joint design, and post-weld treatment. By following the techniques outlined in this guide, you can improve your aluminum welding skills and produce high-quality welds for a variety of applications.
Aluminum Welding
Aluminum Welding Processes
Two are the main welding processes used for aluminum alloys:
- GTAW, commonly called TIG (tungsten inert gas);
- GMAW, commonly called MIG (metal inert gas).
Both processes use a protective shield of Inert Gas Generatorinert gas to prevent oxidation of the weld zone. Both of them offer fast, high quality welds in all grades of material, thicknesses and positions.
When the correct filler material is used, properly, the welded joints have virtually the same corrosion resistance to the parent material.
Other welding processes include:
- Flux shielded processes: FCAW (flux core arc welding) and SMAW (shielded metal arc welding). Due to corrosion from flux and overall poor weldability, common steel fabrication processes are not practical and for this reason they are not recommended for marine structural applications.
- Friction Stir Weld (FSW).
- Plasma Arc Weld (PAW).
GTAW (TIG)
The TIG welding process was first used in 1941 for aircraft production. It uses a non-consumable tungsten electrode protected by a ceramic cone to form an arc, which is shielded by inert gas. The torch needs current and inert gas to weld. The following alternatives are available with this process:
- filler rod may or may not be used;
- alternate or direct current (AC or DC);
- inert gas may be argon or helium gas used;
- air or water-cooled.
Water-cooled torches are preferred as these better cool and protect torch at higher weld currents.
Ceramic cups are not durable with GTAW as high heat shortens their useful life. Small cups are used in tight joints due to larger size of water-cooled torches.
Figure 1 is a sketch showing a typical TIG torch.
Figure 2 is the photograph of a TIG welding.
The advantages of GTAW (TIG) process are:
- clean welds with no flux;
- possibility to use in all positions;
- high visibility;
- absence of spatter;
- possibility to weld all weldable metals;
- narrow beads and low distortion;
- best process for aluminum and magnesium;
- most suitable process for thinner metal.
The disadvantages of GTAW (TIG) process are:
- process is slower and less productive than other processes;
- welding is more complicated and requires greater welder’s skill;
- high Frequency interference may occur (radios, TV’s, etc.).
Alternate current GTAW (TIG) welding is very common. It provides:
- cleaning action;
- shallower penetration;
- breaks oxide layer (reverse polarity action);
- high frequency arc initiation avoids “scratch starts” and keep arc going through the alternate current cycle;
- more arc stability using pure tungsten electrode.
Direct current GTAW with electrode negative polarity:
- does not provide a cleaning action like alternate current;
- requires smaller tungsten electrode characteristics;
- provides alloyed not pure, but 2 % thoriated;
- provides deeper penetration due to concentrated arc;
- originates less weld pool;
- is stiffer harder to control arc;
- requires more skill;
- requires more interpass cleaning.
Figure 3 shows a sketch of typical direct current (negative electrode) GTAW process.
GMAW (MIG)
The GMAW (MIG) process has been introduced in 1948 and is now the most widely used process, being used for approximately over 90 % of all structural fabrication.
With this process, wire electrode is continuously fed into weld pool and thickness of welded parts ranges from 2 mm to unlimited thickness.
Figure 4 shows a GMAW welding.
Shielding is achieved by use of argon or helium as inert gas. The advantages and disadvantages deriving from the use of either of these gases are as follows:
1) ARGON
- concentration is required to be at least 25 % when mixed with helium;
- is derived by separation process from air;
- is heavier than air and blankets weld well;
- provides clean welding of aluminum and magnesium;
- performs better with alternate current.
2) HELIUM
- concentration should be 5 % to 75 % mixed with Argon;
- requires greater arc voltage;
- arc runs hotter;
- allows deeper penetration;
- speed is faster;
- distortion is less;
- more weld spatter may result as compared to argon.
Figure 5 and 6 show the welding equipment for GMAW (MIG).
In particular, Figure 7 shows shielding gas supply equipment.
Figure 8 shows a spool gun wire feeder.
Figure 9 shows a power supply equipment.
Finally, Figure 10 shows typical power supply equipment.
Friction Stir Weld (FSW)
Friction stir welding is a solid state welding process in which metals join without melting, mainly used today in aerospace industry. It makes possible dissimilar metal combinations.
FSW is used in limited application for ship production and is restricted to flat panels and plate to stiffener attachments.
Figure 11 shows the macros showing the friction stir weld aspect.
Figure 12 shows the procedures used for friction stir welding.
Figure 13 shows the tools used for friction stir welding.
Aluminum Welding Preparation
Weld preparation is generally machined edges by hand held routers or milling machines.
Plasma arc is acceptable for cutting & beveling. However, oxy fuel cut like steel should never be used.
Weld prepared surfaces must be cleaned just prior to weld by solvent wash or wipe to degrease. Solvents must completely be removed to avoid contamination.
It is important to remove surface oxide layer. Grind, file or scrape should be done just prior to weld.
Figure 14 shows the typical bevel preparations.
Material Cleaning Techniques
The material cleaning techniques are:
- mechanical cleaning;
- solvent degreasing;
- chemical etch cleaning.
A) MECHANICAL CLEANING
Wire brushing (stainless steel bristles), scraping or filing can be used to remove surface oxide and contaminants.
Degreasing should be carried out before mechanical cleaning.
B) SOLVENTS
Dipping, spraying or wiping with organic solvents can be used to remove grease, oil, dirt and loose particles.
C) CHEMICAL ETCHING
A solution of 5 % sodium hydroxide can be used for batch cleaning, but this should be followed by rinsing in HNO3 and water to remove reaction products on the surface.
Weld Procedure Specification (WPS) and Welding Workmanship
Aluminum is parameter sensitive and more variables exist than for steel. The following precautions and recommendations should be followed:
- Amperage and voltage are key values for MIG (GMAW) welds.
- Parameters change as welding position changes.
- Arc should be held to back fill the weld pool and help prevent crater cracks at the end of any weld bead.
- Arc Voltage is critical for a sooth bead, if Arc Voltage is not correct, the followings consequences are possible:
- ARC VOLTAGE TOO LOW → Splatter.
- ARC VOLTAGE TOO HIGH → Wide flat bead.
- Electronic crater fill should be used (if available) to help prevent crater cracks.
- Shielding gas post flow should be used to cool tungsten electrode (GTAW) and weld to avoid oxidation.
- Air entrainment should be avoided by making sure that the gas shielding and the arc is protected from drafts.
- Avoid water vapor pickup from gas lines and welding equipment.
- Welding system should be purged for about an hour before use.
Preheating and Post-Weld Heat Treatment (PWT)
Preheating is generally NOT recommended. It can be required only if ambient temp is low (4 °C) or for thick sections.
In case preheating is required, preheat is not to exceed:
- 93 °C for 5 000 series alloys;
- 176 °C for other alloys.
Whenever an aluminum ally is preheated prior to welding:
- time at preheating temperature must be held to a minimum;
- local overheating is to be avoided (damage above 204 °C are possible);
- pyrometers or temperature indicator are to be used to verify the temperature, as aluminum does not change color as it heats.
Postweld heat treatment of aluminum is seldom practical or recommended.
Welding Aspects of Aluminum
Figure 15 shows how aluminum welding appears, under increasing magnification.
Miscellaneous
A) OXIDE LAYER
Aluminum and oxygen have very strong chemical affinity. They instantly forms transparent, tenacious refractory layer. This layer has high melting point like a ceramic film.
This oxide layer can originate arc-starting problems, which might lead to the following welding defects:
- dross (oxide looks like black pepper in weld);
- lack of fusion, weld fluidity problems.
B) HIGH THERMAL CONDUCTIVITY
Aluminum is an excellent conductor of heat, therefore welding is different than steel. This creates the following advantages and disadvantages:
- Higher welding currents (more Amps = Hotter).
- Welding speeds faster (hot and fast).
- Stringer beads (narrow welds not wide weaves).
- Lack of fusion / penetration problems.
- Starts cold and stops tend to form crater cracks.
C) NO COLOR CHANGE
Aluminum alloys when heated do not show any visible color change. That means that there is no way to tell if welding is overheating.
Overheating results in:
- changes in material properties;
- loss of cold work (lower strength);
- change in temperature (lower strength);
- easy to burn the welder, as there is no way to tell how hot a welded area is. Blisters may happen before the welders know it.
D) HIGH COEFFICIENT OF THERMAL EXPANSION
Aluminum alloys have thermal expansion twice as greater than steel for the same change in temperature. This means that if fit-up is too tight, joints will bind and distort. This also results in:
- higher shrink on cooling;
- overall distortion different than steel.
