Aluminum alloy is generally used in automobile, aerospace, aviation and sports industry because of its light weight, easy forming / processing and good strength.Mechanical riveting, arc welding, brazing, friction stir welding, laser welding, hybrid laser / arc welding are commonly used to connect aluminum alloy panels and sheets.The laser welding speed is very fast and the thermal deformation is small.However, laser welding of aluminum alloy usually results in a large number of pores, so the strength of the solder joint is very low - usually 50% - 75% of the base metal.There are many reasons for porosity in laser welding aluminum alloy: surface pollution, including hydrogen in the oxide layer.Hydrogen is usually trapped in an oxide layer on the surface of an aluminum alloy.During laser welding, hydrogen will be released and dissolved in the melt pool. The solubility of hydrogen in liquid aluminum is much higher than that in solid aluminum.In the liquid state, the solubility of hydrogen is: s is the solubility of hydrogen at 273 ° K and 760t, T is the temperature (k), P is the partial pressure of hydrogen (1t = 0.133mbar). It can be seen from the above that the solubility of hydrogen in liquid aluminum is 20 times that in solid aluminum.

Therefore, aluminum alloy will absorb a lot of hydrogen in the molten pool and release it in the form of pores during solidification.Other surface organic pollutants such as oil and grease can also trap hydrogen in the welding area.The pores produced by hydrogen are usually small (less than 0.6mm in diameter). The pores in the bath move slowly.For the pores with a diameter of 60 μ m in aluminum alloy, the moving speed is 4-6mm / s, while the solidification speed of liquid is usually 20-50mm / s, so the pores are difficult to be discharged.This usually occurs in deep penetration welding, where the laser power density exceeds 120W / cm2.For the relatively thick aluminum alloy (more than 3.5mm), keyhole welding can achieve deep penetration.The keyhole is formed because of the high pressure steam / plasma generated in the laser welding process, and the strong recoil pressure generated when the steam leaves the molten pool will generate a deep steam hole.Compared with the laser welding with the lower power density, keyhole can make laser beam absorbed on deeper surface of the material.If the keyhole is not firm (once the surface tension of the material exceeds the steam recoil pressure, the keyhole will collapse) or the material solidifies too fast, the steam will be trapped in the melting area and form large pores (generally greater than 0.5mm).

There are many ways to reduce the porosity of laser welding light alloy.For example, control gas injection, swing beam, double beam welding, and surface cleaning. The use of low laser power conduction limited laser welding usually produces fewer pores, but the processing efficiency is also lower.

Shanghai runqia has made a case study on the use of laser cleaning technology to reduce the porosity produced in laser welding. The objects are 6000 series alloy (aa6014) or commonly known ac-170px for car body, which contains 0.5-0.7% aluminum, 0.35% (maximum) fluorine, 0.2% copper, and 0.05-0.2% manganese.In the process of forming aluminum alloy plate, titanium and zirconium (4mg / m2) and solid lubricant are coated on the surface of aluminum alloy plate, which will remain on the surface of material after forming. Welding work is done by filler wire and 5.3kw disc laser.

The influence of laser power (2-5.3kw), welding speed and sheet gap on the formation of welding porosity when using aa4043 (5.3% silicon) welding wire for corner welding.Under different welding conditions, the linear energy (energy per unit length) is similar, but the higher the laser power (the faster the speed), the more porosity.However, the number of pores is still large. The diameter of these pores is usually less than 250 μ M.

Laser cleaning uses nanosecond pulse Nd:YAG laser (200W) and grating scanning (200m M / s) were used to remove the surface layer of about 25 μ M.Laser cleaning greatly reduces the formation of pores, and reduces the porosity to less than 1%.The above laser cleaning / welding technology has been applied in automobile manufacturing.The scientific results provide more confidence and help for introducing laser welding technology into aluminum alloy welding.

The above research of Shanghai runqia Electronic Technology Co., Ltd. has clearly demonstrated the benefits of applying laser cleaning to laser welding aluminum alloy for improving quality (reducing porosity), and the technology has been applied to industrial production.It is worth noting that although laser cleaning plays a key role in reducing weld porosity when welding thin aluminum alloy, the effect will not be so obvious when welding thick aluminum alloy, because the main reason of pore formation is the effect of keyhole, rather than surface oxides and pollutants.