Alongside steel, aluminum is the most widely used construction material. Its biggest advantage is that it is just as strong but only around half as heavy. The use of aluminum is therefore preferred for lightweight constructions.

INTERESTING FACTS ABOUT ALUMINUM

Pure aluminum (Al99.5) is not very strong. However, it is used as the basis for alloys with a strength that matches up to steel.

Welding aluminum is particularly difficult because the material is surrounded by an oxide layer. This only melts at around 2015 degrees Celsius, while aluminum itself melts at around 650 degrees depending on the alloy. If the oxide layer were to be melted in the conventional manner then the aluminum would run away, and welding would be impossible. It is therefore necessary to destroy or displace the oxide where possible.

WHAT ALLOYS OF ALUMINUM ARE THERE?

The properties of pure aluminum can be changed using alloys.  For example, adding magnesium considerably increases the strength of the material. 

The most important components of aluminum alloys and their properties are:

• Magnesium (Mg)
  0.3–7% higher strength, finer grain size
• Manganese (Mn)
  0.3–1.2% better corrosion resistance, higher strength
• Copper (Cu)
  roughly 5% higher strength, lower corrosion resistance, important for hardening capacity
• Silicon (Si)
  12% for casting, reduces the melting point to 577°C, however with coarse grains

Another challenge of welding aluminum involves the formation of oxide film on the work surface. The melting point of aluminum oxide is approximately 3x the melting point of pure aluminum, which can result in particles of aluminum oxide contaminating the weld and leading to porosity issues. In most cases, oxide film must be removed either by mechanical or chemical means prior to welding. Aluminum oxide can affect laser welding: oxide films can change the reflectivity of the parts surface, which negatively impacts the amount of laser energy making it to the base metal.

The amount of pre-weld preparation is largely dependent on the condition of the aluminum parts to be welded, and that is generally dependent on the storage conditions and the cleanliness of the machine procedures used to make the part thus far.

To avoid oxide films and hydrocarbon contamination, aluminum to be laser welded must be thoroughly cleaned. This is often achieved mechanically, using stainless steel wire brushes, grinding, filing or scraping to remove any oxides. Alternatively, there are chemical cleaning methods utilizing immersions in caustic solutions and water that are effective at removing aluminum oxide.

Hydrocarbon residue on aluminum parts can generally be removed using acetone or alcohol based solvents. Avoid using chlorinated solvents in the welding area because they may form toxic gases when heated. Hydrocarbon contamination must be removed before abrading the surface to remove aluminum oxide.

Special care in machining and assembly must be taken because aluminum is softer than most metals. Contaminates can easily be transferred to a part and then pushed under the surface of the joint.

• Machining methods that leave a ground or smeared surface should be avoided.  For example, a band saw will leave metal smeared along the path of the blade.  This problem can be overcome by filing or machining the joint edge post cutting.
• Avoid grinding process if possible. If grinding cannot be avoided, use a course disk.
• When cleaning a surface with solvents, use clean cloth such as cheese cloth or paper towels.  Do not use shop rags that may be contaminated with oil residue.
• Avoid using compressed shop air to blow off debris from the area of the joint. Compressed air contains moisture and oil contaminates. If a part must be blown off, use a bottled gas such as nitrogen or argon.
• Use a stainless steel wire brush to clean a joint only after solvent cleaning.  Wire brushing prior may imbed hydrocarbons and other contaminates in the aluminum.
• Always use a new or recently cleaned stainless steel brushes to clean a joint.  Older brushes sitting around a work bench may contain oils and other contaminates.  Do not use brushes that have been used on other metals as metal flakes can be carried on the brush bristles, then imbedded under the surface of the aluminum during brushing.
• Stainless steel wire brush any metal surface that has been etched.  By-product residuals from etching can alter the chemical composition of the weld pool.
• Clean all wire brushes and cutting tools frequently.

Laser welding requires a fairly precise joint in order to maintain permissible gap and mismatch. Good weld fixturing is necessary so that the laser beam can be placed accurately. Laser welding and cutting are thus inherently machine guided processes.