Aluminum alloy has the advantages of low density, high strength and good corrosion resistance, so it is widely used in automotive industry, new energy, aerospace and construction industry.
At present, laser welding has been widely used in the production of aluminum alloy products.
Compared with traditional welding methods, laser welding can provide higher production efficiency, better weld quality, realize high precision welding and automation of complex structures.
Laser welding is a technology which radiates high-intensity laser to metal surface and makes metal melt and recrystallize to form weld by thermal coupling between laser and metal.
According to the thermal mechanism of laser welding, it can be divided into thermal conduction welding and deep penetration welding.
Thermal conductive welding is mainly used in package welding or micro nano welding of precision parts;
Laser deep penetration welding is mainly used in the welding of materials that need complete penetration. Its welding process will make the material vaporize and keyhole phenomenon appears in the molten pool. It is the most widely used laser welding method at present, and it is also the preferred method of aluminum alloy welding.
Characteristics of laser welding:
- Swing laser welding is used for laser welding of aluminum alloy without adding auxiliary materials;
- Welding equipment is simple, no consumables, easy to automation;
- The requirement of welding gap is high, otherwise the weld is easy to collapse;
- Welding starting point and end point are easy to produce key points;
- The welding process is stable and easy to produce welding defects;
Case: building decoration industry – 5 series aluminum alloy door frame welding
Due to the inherent physical properties of aluminum alloy, such as low laser absorptivity, low boiling point of alloy elements, high thermal conductivity, high thermal expansion coefficient, relatively wide solidification temperature range, high solidification shrinkage, low viscosity and high hydrogen absorption in liquid, it is easy to produce defects such as pores and hot cracks in the process of laser welding.
Among them, porosity is the most common defect form in the process of laser welding of aluminum alloy. It will damage the compactness of weld metal, weaken the effective cross-sectional area of weld, reduce the mechanical properties and corrosion resistance of weld. Therefore, effective measures must be taken to prevent porosity and improve the internal quality of weld.
The way of restraining gas hole in laser welding
1. Inhibition of welding porosity by surface treatment before welding
Surface treatment before welding is an effective method to control metallurgical pores in laser welding of aluminum alloy. Generally, surface treatment methods include physical mechanical cleaning and chemical cleaning.
Porosity of weld after different surface treatment methods
After comparison, the best process is chemical treatment (metal cleaning agent cleaning – water washing – alkali washing – water washing – acid washing – water washing – drying).
25% NaOH aqueous solution was used to remove the surface thickness of the material in alkali washing, and 20% HNO3 + 2% HF aqueous solution was used to neutralize the residual alkali in acid washing.
After the surface treatment, the test plate shall be welded within 24 hours. When the test plate stays for a long time after treatment, the assembly shall be wiped with absolute alcohol before welding.
2. Controlling welding porosity by welding process parameters
The formation of weld porosity is not only related to the surface treatment quality of the weldment, but also related to the welding process parameters.
The influence of welding parameters on weld porosity is mainly reflected in weld penetration, that is, the influence of weld back width ratio on weld porosity.
Effect of weld penetration on weld porosity
The results show that when the back width ratio r > 0.6, the concentration distribution of chain pores in the weld can be effectively improved; when the back width ratio r > 0.8, the existence of large pores in the weld can be effectively improved, and the residual pores in the weld can be largely eliminated.
3. Controlling welding porosity by choosing proper shielding gas and flow rate
The selection of shielding gas directly affects the quality, efficiency and cost of welding.
In the process of laser welding, the correct blowing of shielding gas can effectively reduce the weld porosity.
Effect of different shielding gas types on weld porosity
As shown in the figure above, Ar (argon) and He (helium) are used to protect the weld surface
In the process of laser welding of aluminum alloy, the different ionization degree of Ar and He to laser results in different weld formation.
The results show that the porosity of the weld with Ar as shielding gas is less than that with He as shielding gas.
At the same time, we should also pay attention to the fact that a large amount of plasma produced by welding can not be blown away when the gas flow rate is too small (< 10L / min), which makes the welding pool unstable and increases the probability of pore formation.
The gas flow rate is moderate (about 15L / min), the plasma is effectively controlled, the protective gas has a good anti-oxidation effect on the solution pool, and the porosity is the least.
Too much gas flow is accompanied by too much gas pressure, which makes part of the protective gas mixed into the inside of the solution pool and makes the porosity increase.
It is always a difficult problem to restrain the porosity defect in laser welding aluminum alloy
Affected by the properties of aluminum alloy itself, the phenomenon of no porosity can not be completely avoided in the welding process, only the porosity can be reduced.
Through the process optimization before and after welding and experimental summary, we put forward three ways to inhibit porosity, which can be referred by colleagues in the industry.