07-10-2019, 09:15 AM
Aluminum alloys are lightweight and corrosion-resistant in marine environments, making the superior material for use in the shipbuilding industry.
There are two aspects that make general aluminum welding different from welding steel in the shipbuilding industry, and both are related to the fact that much of the welding is performed outdoors:
Both aspects will be discussed in more detail on the next few pages.
Know Which Aluminum Alloy You Are Welding
It isn’t enough to know that you are welding aluminum. Welding professionals have to know which alloy is about to be welded, and they have to know the correct filler metal to use. The most common alloys used in shipbuilding are the Al-Mg 5XXX sheet alloys and the Al-Mg-Si 6XXX extrusion alloys, primarily because of their excellent resistance to corrosion.
Most aluminum alloys are easily arc weldable. Virtually all of the alloys in the 1XXX, 3XXX, 4XXX, 5XXX and 6XXX series can be arc welded using GTAW and GMAW. However, most of the alloys in the 2XXX and 7XXX series cannot be arc welded. The only arc weldable alloys in the 2XXX series are 2219 and 2519. Almost all of the other 2XXX alloys will crack severely when welded. Similarly, the only alloys in the 7XXX family that can be arc welded are 7003, 7004, 7005 and 7039.
Specifically, be particularly wary of 2024 and 7075. It is tempting to buy one of these alloys for repair welding because they are strong and commonly available, but they are crack-sensitive to arc welding.
Welding an “unweldable" alloy or using the wrong filler alloy can result in immediate cracking or a premature failure in service at a later time.
When Should You Preheat?
Some people believe that all aluminum welds, both GTAW and GMAW, should be preheated. This isn’t true. In fact, if you have the correct equipment, preheat is rarely necessary.
Additionally, excessive preheat can severely degrade the mechanical properties. The last step in the heat treatment is aging, which is conducted at temperatures between 325 and 400°F. If you preheat to or above the aging temperature, you will ruin the alloy’s mechanical properties.
This is not to say that all preheating is bad. For example, if outside temperatures and humidity are very high, it is acceptable to preheat to 200°F to drive off moisture. Just don’t get carried away.
Storage of Aluminum Sheet, Plate and Extrusions
For most industries, the standard recommendation is to store aluminum inside prior to fabrication. While that is a good recommendation, it probably is unrealistic in the shipbuilding industry, where most welding is performed outside. There are more realistic storage recommendations for shipbuilding.
Make Sure You Clean the Parts Adequately Before Welding
Cleaning aluminum before welding consists of two separate and distinct operations. First, all oils, lubricants, machining coolants and other hydrocarbons used in manufacturing must be removed. Any hydrocarbons remaining on the material will get into the welding arc and release hydrogen gas, which causes weld porosity.
Removing hydrocarbons can be done in one of two ways. The most common is to wipe the plate with a clean rag saturated with a good degreasing solvent, such as acetone, toluene, MEK or carburetion cleaner. (Alcohols are not good degreasers and should not be used to clean aluminum.) Alternatively, but less common, the aluminum can be dipped into a tank containing a mild alkaline solution, then rinsed and dried.
Secondly, remove any heavy oxides from the aluminum before welding. This is normally done by hand or mechanically using a stainless steel wire brush. In cases where the materials are stored outside and develop a heavy gray oxide, it may be necessary to remove the oxide using a sanding or grinding disk.
Use the Correct Equipment
Before beginning each shift, welders should make sure the welding equipment is correctly set for aluminum. This is especially important if the equipment is interchangeably used to weld different materials.
Specifically for GMAW welding:
Make sure the spool brake is not set too tight. Excessive tension will cause wire feeding problems. The brake should just be tight enough so that the spool doesn’t freewheel when the wire is stopped.
Make sure the drive rolls are made for aluminum and have the correct diameter. Using V groove drive rolls, which are made to weld steel, on aluminum will deform the aluminum wire and cause feeding difficulties.
Set the drive roll tension only tight enough so that the drive rolls don’t slip on the wire. Excessive tension will cause the filler wire to deform.
Make sure the correct plastic inlet and outlet guide bushings are in place. Use of the steel parts used to feed steel wire will cause feeding problems.
Make sure the gun liner is the correct plastic material used for aluminum. Use of the helical steel gun liner typically used to feed steel wire will take small shavings off the soft aluminum wire and cause the liner to clog up.
Make sure the contact tip is the proper size for the aluminum wire being fed. Some people believe it is a good idea to use an oversized contact tip – for example, a 1/16-inch contact tip to feed 3/64-inch wire. DO NOT DO THIS. The tip must be of a size to freely pass the wire through but still tight enough to provide uniform electrical contact between the contact tip and wire. An oversized contact tip will not allow uniform current transfer, resulting in an excessive number of burnbacks.
There are two aspects that make general aluminum welding different from welding steel in the shipbuilding industry, and both are related to the fact that much of the welding is performed outdoors:
- Storage of aluminum plates and extrusions is critical to avoid oxidation
- Extra care must be taken to avoid breezes in the welding area
Both aspects will be discussed in more detail on the next few pages.
Know Which Aluminum Alloy You Are Welding
It isn’t enough to know that you are welding aluminum. Welding professionals have to know which alloy is about to be welded, and they have to know the correct filler metal to use. The most common alloys used in shipbuilding are the Al-Mg 5XXX sheet alloys and the Al-Mg-Si 6XXX extrusion alloys, primarily because of their excellent resistance to corrosion.
Most aluminum alloys are easily arc weldable. Virtually all of the alloys in the 1XXX, 3XXX, 4XXX, 5XXX and 6XXX series can be arc welded using GTAW and GMAW. However, most of the alloys in the 2XXX and 7XXX series cannot be arc welded. The only arc weldable alloys in the 2XXX series are 2219 and 2519. Almost all of the other 2XXX alloys will crack severely when welded. Similarly, the only alloys in the 7XXX family that can be arc welded are 7003, 7004, 7005 and 7039.
Specifically, be particularly wary of 2024 and 7075. It is tempting to buy one of these alloys for repair welding because they are strong and commonly available, but they are crack-sensitive to arc welding.
Welding an “unweldable" alloy or using the wrong filler alloy can result in immediate cracking or a premature failure in service at a later time.
When Should You Preheat?
Some people believe that all aluminum welds, both GTAW and GMAW, should be preheated. This isn’t true. In fact, if you have the correct equipment, preheat is rarely necessary.
Additionally, excessive preheat can severely degrade the mechanical properties. The last step in the heat treatment is aging, which is conducted at temperatures between 325 and 400°F. If you preheat to or above the aging temperature, you will ruin the alloy’s mechanical properties.
This is not to say that all preheating is bad. For example, if outside temperatures and humidity are very high, it is acceptable to preheat to 200°F to drive off moisture. Just don’t get carried away.
Storage of Aluminum Sheet, Plate and Extrusions
For most industries, the standard recommendation is to store aluminum inside prior to fabrication. While that is a good recommendation, it probably is unrealistic in the shipbuilding industry, where most welding is performed outside. There are more realistic storage recommendations for shipbuilding.
Make Sure You Clean the Parts Adequately Before Welding
Cleaning aluminum before welding consists of two separate and distinct operations. First, all oils, lubricants, machining coolants and other hydrocarbons used in manufacturing must be removed. Any hydrocarbons remaining on the material will get into the welding arc and release hydrogen gas, which causes weld porosity.
Removing hydrocarbons can be done in one of two ways. The most common is to wipe the plate with a clean rag saturated with a good degreasing solvent, such as acetone, toluene, MEK or carburetion cleaner. (Alcohols are not good degreasers and should not be used to clean aluminum.) Alternatively, but less common, the aluminum can be dipped into a tank containing a mild alkaline solution, then rinsed and dried.
Secondly, remove any heavy oxides from the aluminum before welding. This is normally done by hand or mechanically using a stainless steel wire brush. In cases where the materials are stored outside and develop a heavy gray oxide, it may be necessary to remove the oxide using a sanding or grinding disk.
Use the Correct Equipment
Before beginning each shift, welders should make sure the welding equipment is correctly set for aluminum. This is especially important if the equipment is interchangeably used to weld different materials.
Specifically for GMAW welding:
Make sure the spool brake is not set too tight. Excessive tension will cause wire feeding problems. The brake should just be tight enough so that the spool doesn’t freewheel when the wire is stopped.
Make sure the drive rolls are made for aluminum and have the correct diameter. Using V groove drive rolls, which are made to weld steel, on aluminum will deform the aluminum wire and cause feeding difficulties.
Set the drive roll tension only tight enough so that the drive rolls don’t slip on the wire. Excessive tension will cause the filler wire to deform.
Make sure the correct plastic inlet and outlet guide bushings are in place. Use of the steel parts used to feed steel wire will cause feeding problems.
Make sure the gun liner is the correct plastic material used for aluminum. Use of the helical steel gun liner typically used to feed steel wire will take small shavings off the soft aluminum wire and cause the liner to clog up.
Make sure the contact tip is the proper size for the aluminum wire being fed. Some people believe it is a good idea to use an oversized contact tip – for example, a 1/16-inch contact tip to feed 3/64-inch wire. DO NOT DO THIS. The tip must be of a size to freely pass the wire through but still tight enough to provide uniform electrical contact between the contact tip and wire. An oversized contact tip will not allow uniform current transfer, resulting in an excessive number of burnbacks.