Application of Laser Cleaning Prior to Laser Welding of Aluminum Alloys
Aluminum alloys are typically used in automotive and aviation industry due to their light weight, corrosion resistance and flexibility. Aside from aluminum which is the major ingredient, other elements such as magnesium, manganese, silicon, tin, zinc and copper can also be found in aluminum alloys. The parts for aluminum alloys are joined together using several processes such as mechanical fastening, arc welding and laser welding. For the case of laser welding normally disk lasers are used. Disk lasers are characterized by a thin active medium that is surrounded by a heat sink on one side and laser output on the other side. Filler wires used for typical welding operations are also used in case of laser welding of aluminum alloys. Figure 1 shows the process of laser welding with filler wire.
Figure 1: Laser welding with filler wire. Picture taken from Laser Focus World, Industrial Laser Solutions.
Although laser welding is a fast process as compared to other similar techniques, there is a problem that arises while doing it. A huge amount of porosity appears in the surface oxide layer of the alloy that gives rise to a reduction in joint strength as compared to the parent material by a factor of 2 or more. Hydrogen atoms trapped on the surface as a contaminant, get released due to heat and move inside the surface and after the heat of the laser welding is removed and the surface solidifies, they give rise to porosity of the surface. Figure 2 shows the porosity of the surface due trapping of hydrogen bubbles.
Figure 2: Appearance of Porosity due to hydrogen bubbles in aluminum alloys. Picture taken from ESAB welding and cutting products
The molten pool of aluminum that is created by the laser welding process, traps hydrogen molecules and releases them as porosity when the material solidifies. Other organic surface contaminants such as oil and grease can also contribute to trapping of hydrogen molecules. The hydrogen bubbles typically move slower than speed of liquid solidification and hence they get trapped. The bubbles speed which depend on several properties such as the bubble radius, density, viscosity of the liquid, etc. can be calculated. The diameter of the trapped hydrogen bubbles is typically of the order of tens of microns.
Aside from the trapping of hydrogen molecules in the surface oxide layer of the alloy, when the laser fluence (power per unit area) exceeds a certain limit (of the order of megawatts per square centimeter), in a process known as keyhole welding, the porosity could penetrate in the bulk of the material. As the high energy density laser beam evaporates the aluminum alloy, it generates a high recoil pressure and penetrates deep into the material. If the material surface tension is bigger than the vapor recoil pressure or the material solidifies quickly, the bubbles get trapped in the heat zone and form large porosities of the order of 0.5 micron diameter. Figure 3 shows the keyhole welding and keyhole porosity.
In order to fix this problem, laser cleaning of the aluminum alloy surface can be done prior to laser welding. The laser cleaning mechanism would clean the surface and remove all or most of the contaminants from the surface and therefore they can not contribute to production of porosity after being exposed to the heat of laser welding. Based on a study of laser cleaning of an aluminum alloy which contained small concentrations of copper, fluorine and manganese (Reference 1) , it was found that depending on the laser power, there can be a 10-90% porosity level produced. However by performing laser cleaning prior to laser welding the porosity level was reduced to less than 1% in all cases.
Figure 4 shows the laser cleaning of aluminum-silicon alloy using a pulsed laser.
Figure 4: Pulsed laser cleaning of cast aluminum-silicone alloy. The picture has been taken from Youtube.