Laser Cleaning Technology Before Aluminum Alloy Welding-ShangHai RunQia Electronics Technology Co.,Ltd.

Laser Cleaning Technology Before Aluminum Alloy Welding

Aluminum alloys are widely used in automotive, aerospace and sports industries due to their light weight, excellent formability and machinability, as well as good mechanical strength. Common joining methods for aluminum alloy panels and thin sheets include mechanical riveting, arc welding, brazing, friction stir welding, laser welding, and hybrid laser-arc welding.

Laser welding features high welding speed and low thermal deformation. However, laser welding of aluminum alloys tends to generate a large number of pores, resulting in extremely low weld strength — usually only 50%–75% of that of the base metal. There are multiple causes for pore formation during laser welding of aluminum alloys, among which surface contamination, especially hydrogen contained in the surface oxide layer, is the primary factor.

Hydrogen is normally trapped inside the natural oxide film on aluminum alloy surfaces. During laser welding, hydrogen is released and dissolved into the molten pool. The solubility of hydrogen in liquid aluminum is far higher than in solid aluminum.Solubility formula definition:S refers to the hydrogen solubility under conditions of 273 K and 760 Torr;T stands for temperature (K);P represents the partial pressure of hydrogen (1 Torr = 0.133 mbar).

It can be seen that the solubility of hydrogen in liquid aluminum is approximately 20 times that in solid aluminum. Consequently, the aluminum alloy molten pool absorbs a large amount of hydrogen, which is released in the form of pores during solidification. Other organic surface contaminants such as oil and grease also cause hydrogen entrapment in the welding zone.

Pores induced by hydrogen are usually small, with a diameter below 0.6 mm. Pores in the molten pool move slowly. For pores of 60 μm in diameter in aluminum alloys, the moving speed is only 4–6 mm/s, while the solidification rate of liquid metal generally reaches 20–50 mm/s, making it difficult for pores to escape. This phenomenon commonly occurs in deep penetration welding when the laser power density exceeds 120 W/cm².

For welding relatively thick aluminum alloys (over 3.5 mm), keyhole welding can realize deep penetration. A keyhole forms owing to the high-pressure vapor and plasma generated during laser welding. The intense recoil pressure produced when vapor escapes from the molten pool creates a deep vapor cavity. Compared with heat-conduction laser welding at lower power density, the keyhole allows the laser beam to be absorbed deeper inside the material.

If the keyhole is unstable — it collapses once the material surface tension exceeds the vapor recoil pressure — or if the material solidifies too rapidly, vapor will be confined within the molten zone and form large pores, generally larger than 0.5 mm in diameter.

Various methods are available to reduce porosity during laser welding of light alloys, including controlled gas shielding, beam oscillation, dual-beam welding, and surface cleaning. Low-power conduction-mode laser welding usually produces fewer pores but suffers from lower processing efficiency.

Shanghai Runqia conducted a case study on applying laser cleaning technology to reduce porosity in laser welding. The research object was 6000-series automotive aluminum alloy AA6014, also commonly known as AC-170PX. The material contains 0.5%–0.7% aluminum, maximum 0.35% fluorine, 0.2% copper, and 0.05%–0.2% manganese.

During sheet forming, the aluminum alloy surface is coated with titanium–zirconium treatment (4 mg/m²) and solid lubricant, which remain on the surface after forming. Welding experiments were carried out using filler wire and a 5.3 kW disk laser.

The study investigated the influences of laser power (2–5.3 kW), welding speed and sheet gap on pore formation during fillet welding with AA4043 filler wire (5.3% silicon). Under similar linear energy input per unit length, higher laser power accompanied by faster welding speed resulted in greater porosity. Leaving a gap between thin sheets can significantly reduce pores, though porosity still remained at a relatively high level, with pore diameters mostly below 250 μm.

Laser cleaning adopted a 200 W nanosecond pulsed Nd:YAG laser with grating scanning at 200 mm/s to remove the surface layer of approximately 25 μm in thickness. Laser cleaning drastically suppressed pore formation, lowering the overall porosity to below 1%. The above laser cleaning and integrated welding technology has been practically applied in automobile manufacturing, providing reliable technical support and confidence for the widespread application of laser welding in aluminum alloy joining.

The research conducted by Shanghai Runqia Electronic Technology Co., Ltd. clearly demonstrates the benefits of applying laser cleaning prior to laser welding of aluminum alloys in improving welding quality and reducing porosity. This technology has already been put into industrial mass production.

It is worth noting that although laser cleaning plays a critical role in reducing welding porosity for thin aluminum alloy sheets, its effect is less obvious when welding thick aluminum alloy profiles. This is because porosity formation in thick profiles is mainly dominated by keyhole instability rather than surface oxides and contaminants.

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