Aluminum and aluminum alloy with its low density, good corrosion resistance, good thermal conductivity, decorative characteristics, are widely used in transportation, construction and electronic communication and other fields. Aluminum and aluminum alloy in the industry is an important metal after steel, but also the most widely used a non-ferrous metal.
Laser welding is a modern welding technology with high precision and high efficiency, which has been widely used in industrial production. The characteristics of aluminum alloy itself make its related welding technology face some urgent problems: surface refractory oxide film, joint softening, easy to produce porosity, easy to thermal deformation and thermal conductivity is too large, which seriously affect the welding quality, performance and beauty.
Below, we will analyze several aluminum and aluminum products in the welding process often encountered problems, causes and corresponding solutions.
1. Stomata problem
Different types of aluminum alloys produce different types of pores. It is generally believed that aluminum alloys produce the following types of pores during the welding process.
A. Hydrogen pores. After the aluminum alloy is melted in a hydrogen environment, its internal hydrogen content can reach more than 0.69ml/100g. However, after solidification, its equilibrium hydrogen dissolving capacity is only 0.036ml/100g at most, which is nearly 20 times the difference between the two. Therefore, in the process of transformation from liquid to solid, the excess hydrogen in liquid aluminum must be precipitated. If the precipitated hydrogen can not float smoothly, it will converge and integrate bubbles remaining in the solid aluminum alloy to become pores.
B. Protect the gas generated by the pores. In the process of high energy laser welding aluminum alloy, due to the strong evaporation of metal at the front of the small hole at the bottom of the molten pool, the protective gas is sucked into the molten pool to form bubbles, which become pores when the bubbles cannot escape and remain in the solid aluminum alloy.
C. Pores caused by the collapse of small holes. In the process of laser welding, when the surface tension is greater than the vapor pressure, the small hole will not be stable and collapse, and the metal will not be able to fill the hole.
There are also many practical measures to reduce or avoid stomatal defects in aluminum alloy laser welding, such as adjusting the laser power waveform, reducing the unstable collapse of small holes, changing the beam focus height and oblique irradiation, applying electromagnetic field during the welding process, and welding in a vacuum. In recent years, there has been the use of wire filling or preset alloy powder, composite heat source and dual focus technology to reduce the porosity of the process, with good results.
2. Crack problem
Aluminum alloy is a typical eutectic alloy, which is more prone to hot cracking under rapid solidification by laser welding. The formation of low melting point eutectic such as AL-Si or Mg-Si at the boundary of columnar crystals during the crystallization of weld metal is the reason for the cracking.
In order to reduce thermal cracking, laser welding can be performed by filling wire or presetting alloy powder. In the use of YAG lasers, by adjusting the laser waveform, control the heat during the welding process, fully preheat the welding parts before welding, and adjust the protective gas, crystallization cracks can also be reduced.
3. Black weld
Laser welding aluminum blackening is mainly caused by insufficient laser power, the defocusing amount is not adjusted to the appropriate value, or the laser lens is damaged. In this case, the energy of the laser fails to reach the melting threshold of the oxide layer on the surface of the aluminum, causing the aluminum to mix with air and impurities, resulting in blackening of the welding site.
In order to better obtain the quality and beauty of welding, it is recommended to first select the appropriate power and set the correct defocusing amount in the process of aluminum welding. Secondly, you can also check whether the protective lens is damaged.
4. The bite edge is not fused
Reasons for bite edge not fused:
A. The welding speed is too fast, and the liquid metal in the weld will not be redistributed on the back of the small hole, forming A biting edge on both sides of the weld.
B. The joint assembly gap is too large, the molten metal in the joint filling is reduced, and it is also easy to bite.
C. At the end of laser welding, if the energy decline time is too fast, the small hole is easy to collapse, and it will also cause local edge biting.
The solution:
A. Control the processing power and speed matching of laser welding machine to avoid edge biting.
B. The edge of the weld found in the inspection can be polished, cleaned and repaired to make it meet the requirements of the acceptance standard.
C. We can reduce the root gap of welding parts; Secondly, adjust the focus of the laser beam to determine the correct spot position; In addition, we can also ensure the welding quality of aluminum after melting by increasing the laser power and adjusting the focus position of the defocus amount.
Improving the stability of laser welding process and weld forming and improving welding quality are the goals pursued by everyone in the industry. Therefore, new technologies such as laser-arc composite process, wire-filled laser welding, preset powder laser welding, dual focus technology and beam shaping will be further improved and developed.
