Introduction to Laser Welding of Aluminum Alloys
Aluminum (Al) (or aluminium for many of our friends around the world) and its alloys are the most widely used non-ferrous metals today. You will find aluminum and its alloys in industries as varied as aerospace and sporting goods and electronics and consumer packaging. The wide and growing use of aluminum alloys is a result of its defining properties which include:
Density – aluminum is the lightest of all ordinary metals.
Electrical and thermal conductivity.
Diversity of alloys (see Table below for a summary of the main aluminum alloys).
Ease of fabrication.
Ability to be recycled indefinitely without losing any of its intrinsic qualities.
Table: Summary of main aluminum alloys
|Alloy designation||Description/typical applications||Main laser welding issues|
|1xxx Series (pure Al)||Electrical, thermal conductors|
|2xxx Series (Al-Cu)||Aerospace||Cracking, porosity|
|3xxx Series (Al-Mn)||Cooking utensils, chemical containers||Some porosity|
|5xxx Series (Al-Mg)||Automotive||Porosity; some cracking depending upon Mg content|
|6xxx Series (Al-Mg-Si)||Aerospace and automotive||Porosity, cracking|
|7xxx Series (Al-Zn)||Automotive||Mainly porosity due to Zn|
|8xxx Series (Al-Li)||Aerospace||Mainly porosity|
Laser welding is widely used to join aluminum and its alloys. However, aluminum poses difficulty in welding because of three main properties: (1) thermal conductivity, (2) surface reflectivity to infrared and near infrared laser radiation, and (3) characteristics of the molten alloy including low viscosity and volatility of low boiling point alloying elements (e.g. Mg, Zn). These and other material related difficulties can lead to issues with weld and heat affected zone cracking, degradation in the mechanical properties, and inconsistent welding performance.
|Crack free and porosity free butt weld of 2 mm thick Al6061 and Al5052. The slight top bead undercut (concavity) can be eliminated by welding with an appropriate filler material such as Al4043 or Al4047 wire.|
Of course, this by no means implies that laser welding is not an excellent choice for joining these materials. Instead, it means that welding processes must be designed with these factors in mind. So, how do we ensure a reliable, robust laser process when welding aluminum alloys?
With appropriate welding procedures it is possible to produce reliable, robust laser processes for welding a range of aluminum alloys. The main tenets of these procedures are summarized below.
- Stable keyhole welding – high power lasers with good beam quality produce a stable keyhole during welding.
- Clean the surface before welding – the main cause of porosity is the evolution of hydrogen gas during weld metal solidification. Hydrogen can originate from lubricants, moisture in the atmosphere and surface oxides, or the presence of hydrogen in the parent material. Goods quality welds can be achieved for the most alloys by cleaning the surfaces prior to welding and by providing adequate inert gas shielding of the weld pool.
- Addition of an appropriate filler wire reduces cracking in crack sensitive alloys such as the 2xxx and 6xxx series alloys. The improvement is due to the filler material, which provides reinforcements to the weld zone by eliminating undercut from the top or root bead. The filler material also replaces lost alloying elements and improves tolerance to the joint fit up.
- Modulated output from continuous wave lasers can also help with cracking and porosity . Modulation controls heating and cooling of the weld metal which reduces the freezing range of the weld metal and minimizes the tendency for solidification cracking.