How to solve the problems of aluminum alloy laser welding
DemarkChina Laser
LASER HANDHELD & AUTOMATION EQUIPMENT SOLUTIONS | Laser cutting, Welding, Cleaning Cladding, Press brake, ODM, OEM
At present, as the complexity of automobile pipelines increases, there are more and more welding points, which inevitably brings many flame welding problems. Of course, each welding method has its own advantages and disadvantages. This article analyzes the feasibility of laser welding air conditioning pipelines.
1. How to solve the problems of aluminum alloy laser welding
Today, laser welding is widely used in the machining industry. In addition, laser technology also has the characteristics of small welding heat input, small impact on the welding heating area, and not easy to deform, so it has received special attention in the field of aluminum alloy welding.
On the other hand, due to the processing characteristics of aluminum alloys, there are some welding difficulties in laser welding of aluminum alloys. How to solve these problems?
Question 1: Aluminum alloy has low laser absorption rate.
This problem is mainly due to the problem of aluminum alloy materials. Due to the high initial reflectivity and high thermal conductivity of the aluminum alloy to the laser beam, the aluminum alloy has a low absorption rate of the laser beam before melting. Aluminum alloy has a strong reflection effect on laser light. This is due to the high density of free electrons inside the aluminum alloy in the solid state, which easily interact with photons in the beam and reflect away the energy. Research shows that the reflectivity of aluminum alloy to gas CO2 laser is as high as 90%, and the reflectivity to solid laser is also close to 80%. At the same time, aluminum alloy has a strong thermal conductivity, resulting in a very low absorption rate of aluminum alloy for laser light. Therefore, appropriate measures must be taken to improve the laser absorption rate of aluminum alloys.
To address this problem, solutions mainly include the following aspects:
1. Surface pretreatment of aluminum alloy materials. Aluminum alloys have high laser response. Perform appropriate pre-treatment on the aluminum alloy surface, such as anodizing, electrolytic polishing, sand blasting, sandblasting, etc. It can significantly improve the surface's absorption of radiant energy. Studies have shown that the crystallization tendency of aluminum alloys after removing the oxide film is higher than that of the original aluminum alloys. In order not to damage the surface finish of the aluminum alloy and simplify the laser welding process, the welding process can be used to increase the surface temperature of the workpiece and increase the laser absorption rate of the material.
2. Reduce the spot size and increase the laser power density. The laser absorption of aluminum alloy is improved by increasing the laser power density. The increase in laser power density will produce a pinhole effect in the welding molten pool, which can greatly increase the laser absorption rate of the material.
3. Change the welding structure to make the laser beam reflect multiple times in the gap to facilitate aluminum alloy laser welding. The form of the joint will affect the absorption of laser light. V-shaped grooves and square grooves are more conducive to the formation of keyholes than joints without grooves, which increase the laser power density and increase the laser absorption rate of aluminum alloys.
Problem 2: It is easy to produce pores and hot cracks. Aluminum alloy laser welding process is prone to pores and hot cracks.
Porosity is the most common and most important type of defect in laser welding of aluminum alloys. Stomatal types can be divided into 2 categories.
One is due to the sharp decrease in hydrogen solubility during the cooling process of aluminum alloy laser welding. The hydrogen content of the molten aluminum alloy can reach 0.69mL/100g. The hydrogen content of the aluminum alloy after cooling and solidification is 0.036mL/100g, which is supersaturated. The hydrogen precipitates to form hydrogen pores. In addition, there is an oxide film on the surface of the aluminum alloy. During welding, the crystal water on the surface of the aluminum alloy, the moisture in the air and the protective gas are directly decomposed into hydrogen. These hydrogen pores do not have time to escape during the rapid cooling process of aluminum alloy laser welding, but remain in the weld to form hydrogen pores.
The other type is the hole formed because the keyhole generated during the laser welding process is unstable and collapses, and the liquid metal has no time to fill it. Excessive pores will reduce the density of the weld, reduce the load-bearing capacity of the joint, and reduce the strength and plasticity of the joint to varying degrees.
There are many measures to reduce pore defects in aluminum alloy laser welding, such as changing the trajectory of the laser beam, using beam oscillation to stir the molten pool, increasing the possibility of pores escaping from the surface, using wire filling or alloy powder, and Measures such as dual-spot technology and laser hybrid welding can achieve the effect of reducing pores, but they are difficult to fundamentally eliminate. Aluminum has relatively good thermal conductivity, and the laser power waveform can be adjusted during the welding process according to the material, thickness and surface condition of the aluminum alloy. As shown in the figure, welding can be carried out using the front tip wave type, or welding can be carried out using the wave type with front preheating and then heat preservation, both of which play a certain role in reducing explosion points and pores. It can reduce the unstable collapse of pores, change the irradiation angle of the laser beam, and apply a magnetic field during welding. It can also effectively control the pores generated during the welding process.
The cause of hot cracks in aluminum alloy laser welding is mainly related to its own characteristics and welding process. When aluminum alloy solidifies, the shrinkage rate is large (up to 5%), the welding stress and deformation are large, and the weld metal will produce a low melting point eutectic structure along the grain boundaries during crystallization, which weakens the grain boundary bonding force and plays a role in the tensile stress. Thermal cracks form below
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.The tendency of hot cracks can be reduced by filling wire or alloy powder. The tendency of hot cracks can also be reduced by adjusting the welding process parameters to control the speed of heating and cooling. When using YAG lasers, the heat input can be controlled by adjusting the pulse waveform to reduce crystal cracks.
Problem 3: Decreased mechanical properties of welded links - softening
The combustion loss of alloy elements during the welding process reduces the mechanical properties of aluminum alloy welded connections.
"Softening" is the reduction in strength and hardness of a welded joint. When laser welding aluminum alloy joints are used, the weld structure and heat-affected zone of the welded joint also have softening problems. A large number of studies have shown that the softening phenomenon of aluminum alloy welding is difficult to fundamentally eliminate. However, compared with gas shielded welding, laser welding reduces the heat input and makes the softening zone of the weld narrower. Compared with gas metal arc welding of aluminum alloy laser welding, the degree of "softening" of laser welded joints is lower, and the tensile strength increases with the increase of welding speed. The impact of plasma on the welding process The ionization energy of the aluminum element is low, and it is easier to form metal plasma during laser welding. The plasma causes refraction and deflection of the laser, thereby changing the focus position of the laser beam, reducing the weld penetration ratio, and affecting Welded joint quality. The method of pre-positioning powder on the surface of the workpiece is used to weaken the expansion jump of the plasma in the height direction, so that the plasma can maintain a relatively stable jump amplitude on the surface of the workpiece.
Unstable pores during the welding process of aluminum alloys lead to a decrease in the mechanical properties of the welded joints. Aluminum alloys mainly include Zn, Mg and Al. During welding, aluminum has a higher boiling point than the other two elements. Therefore, some low-boiling point alloy elements can be added when welding aluminum alloy components, which is beneficial to the formation of small holes and the firmness of the welding.
2. Aluminum alloy laser welding technology
1 Aluminum alloy laser self-fusion welding
Laser autogenous welding refers to a welding method that uses a high-energy-density laser beam as a heat source to impact the surface of the base material, causing the base material to melt itself to form a welded joint. For aluminum alloy laser welding, the surface of the aluminum alloy has a high reflectivity of laser, which requires a large laser power during welding; the diameter of the laser spot is small, the precision requirements for the welding tooling are high, and the tolerance for parts gaps is low, which usually requires parts The gap value is below 0.2mm; the heating and cooling speed are fast during the welding process, there are many welding pore defects, the laser energy density is concentrated, and the keyhole effect can easily lead to concave and undercut welds. Therefore, the welding process parameters have a higher requirements. Laser autogenous welding has the advantages of good welding quality, fast welding speed and easy automation in aluminum alloy welding, and is widely used in the automotive industry. In the electric vehicle industry, aluminum alloy laser self-fusion welding is mainly used to seal the power battery casing. In the aluminum body of a domestic new energy vehicle company, the welding of door assemblies and side panels also uses aluminum alloy laser self-fusion welding.
2 Aluminum alloy laser filler wire welding
In laser filler wire welding, the laser still serves as the main heat source to melt the metal to be welded, but an automatic wire feeding device is used to continuously feed filler metal into the molten pool to achieve the metallurgical connection process. Compared with laser autogenous welding, laser wire filling welding relaxes the requirements for gap accuracy in the welding process. By filling welding wires with different components, it improves the metallurgical properties of the weld, prevents welding hot cracks and pores, and improves the stability of the welding process. properties and joint mechanical properties
.Aluminum alloy laser wire filler welding has the characteristics of good appearance quality and looser process gap accuracy than laser autogenous welding. It is usually used on the exterior surfaces of the car body, such as between the top cover and side panels, and between the upper and lower outer panels of the trunk lid. There are also some models that use laser wire filler welding to weld aluminum alloy doors in order to obtain higher welding quality.
3 Aluminum alloy laser-arc hybrid welding
Laser-arc hybrid welding combines two heat sources, laser and arc, with completely different physical properties and energy transmission mechanisms, and works together on the workpiece to be welded. It not only gives full play to the respective advantages of the two heat sources, but also complements each other. shortcomings. In laser-arc hybrid welding of aluminum alloys, the arc can guide the laser heat source, improve the laser absorption capacity of the aluminum alloy and the energy utilization during the welding process, and the surface formability of the weld is better than that of laser autogenous welding. In addition, the introduction of arc can greatly reduce the clamping accuracy of the welding workpiece. At the same time, the arc has a diluting effect on the plasma of laser welding, which can reduce the shielding effect of the plasma on the laser. Laser plays an important role in the stability of the arc, so that the arc can stably act on the joint during high-speed welding, which can improve the welding quality of the joint and increase the welding speed.
in conclusion
The energy density of the aluminum alloy laser welding beam can reach 109W/cm2. It also has the advantages of concentrated heating, small thermal damage, large weld depth-to-width ratio, and small welding deformation. The welding process is easy to integrate, automate, and flexible, and can achieve high speed and high speed. Precision welding, and the welding process does not require a vacuum environment and does not produce X-ray. It is especially suitable for high-precision welding of complex structures. The most attractive feature of aluminum alloy laser welding is its high efficiency. To give full play to this high efficiency, it must be applied to large-thickness deep welding. Therefore, the research and application of high-power laser for large-thickness deep-penetration welding will be an inevitable trend in future development. Large-thickness deep-penetration welding highlights the pinhole phenomenon and its impact on weld porosity. Therefore, the formation mechanism and control of pinholes have become more and more common and will become a hot issue of widespread concern and research in the industry.
Improving the stability, welding seam formation and welding quality of the laser welding process is the goal pursued by people. Therefore, new technologies such as laser arc composite process, filler wire laser welding, preset-free powder laser welding, dual focus technology, beam shaping and other new technologies will be further improved and developed.