At present, with the increase in the complexity of the automotive pipeline, more and more welding points, inevitably brings a lot of flame welding problems, of course, each welding method is to have their own advantages and disadvantages. This article to analyze the feasibility of laser welding air conditioning pipeline.
A how to solve the problem 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 welding heat area influence, not easy to deform, etc. Therefore, it has received special attention in the field of aluminum alloy welding.
On the other hand, due to the processing characteristics of aluminum alloy, there are some welding difficulties in aluminum alloy laser welding. How to solve these problems?
Problem 1:Aluminum alloy has low absorption rate of laser.
This problem is mainly due to the aluminum alloy material. Due to the high initial reflectivity and high thermal conductivity of the aluminum alloy to the laser beam, the aluminum alloy has low absorption of the laser beam before melting. Aluminum alloys have a strong reflection effect on laser light due to the high density of free electrons within the aluminum alloy in the solid state condition, which tends to interact with the photons in the beam and reflect the energy away. Studies have shown that the reflectivity of aluminum alloys is as high as 90% for gaseous CO2 lasers and close to 80% for solid lasers. At the same time, aluminum alloys have strong thermal conductivity, resulting in low absorption of laser light by aluminum alloys. Therefore, appropriate measures must be taken to improve the absorption of laser light by aluminum alloys.
For this problem, the solution mainly includes the following aspects:.
1. Surface pretreatment of aluminum alloy materials. Aluminum alloy has high laser response. Appropriate pretreatment of aluminum alloy surface, such as anodic oxidation, electrolytic polishing, sand blasting, sand blasting, etc. can significantly improve the absorption of radiant energy on the surface. Studies have shown that the crystallization tendency of aluminum alloy after removal of the oxide film is higher than that of the original aluminum alloy. In order not to destroy the surface finish of the aluminum alloy, simplify the laser welding process, you can use the welding process to increase the surface temperature of the workpiece to improve the material's absorption of the laser.
2. Reduce spot size and increase laser power density. By increasing the laser power density to improve the absorption of aluminum alloy to the laser. Increased laser power density will make the welding molten pool to produce a small hole effect, which can greatly enhance the material to the laser absorption rate.
3. Change the welding structure, so that the laser beam is reflected many times in the gap to facilitate the laser welding of aluminum alloy. The form of joint will affect the absorption of laser. v-bevel and square bevel are more conducive to the formation of keyhole than no-bevel joints, so that the laser power density increases, and the aluminum alloy's absorption of laser increases.
Problem 2:Easy to produce porosity and thermal cracks, aluminum alloy laser welding process is prone to porosity and thermal cracks.
Porosity is the most frequent and the most important type of defects in aluminum alloy laser welding. The types of porosity can be divided into 2 categories.
A class is due to aluminum alloy laser welding in the cooling process of hydrogen solubility drops sharply, molten state aluminum alloy hydrogen content up to 0.69mL/100g, cooling solidification of aluminum alloy hydrogen content of 0.036mL/100g, oversaturated hydrogen precipitation and the formation of hydrogen pores. In addition, there is a layer of oxide film on the surface of the aluminum alloy, and the crystalline water on the surface of the aluminum alloy, air and moisture in the protective gas directly decompose into hydrogen during welding. These hydrogen pores in the rapid cooling process of aluminum alloy laser welding to escape, and stay in the weld to form hydrogen pores.
Another category is due to the laser welding process produced by the keyhole instability and collapse, the liquid metal is too late to fill the holes formed. Excessive porosity will reduce the denseness of the weld, reduce the load-bearing capacity of the joint, and will make the strength and plasticity of the joint have different degrees of reduction.
Reduce the aluminum alloy laser welding in the porosity defects in a number of measures, such as changing the walking track of the laser beam, using beam oscillation to the molten pool for stirring, increase the possibility of porosity escaped from the surface, the use of filler wire or filler alloy powder, as well as the use of dual-spot technology and laser composite welding and other measures can be achieved to reduce the effect of the porosity, but it is difficult to be eliminated from the root. Aluminum's thermal conductivity is relatively good, according to the material of aluminum alloy, thickness and surface condition in the welding process to adjust the laser power waveform. As shown in the figure before the tip of the waveform for welding, can also be used before the preheating after the insulation waveform for welding, are to reduce the blowing point and porosity play a certain role. It can reduce the unstable collapse of the pores, change the angle of irradiation of the laser beam, and apply a magnetic field in the welding, but also can effectively control the pores produced during the welding process.
The reason for thermal cracking in aluminum alloy laser welding is mainly related to its own characteristics and welding process. Aluminum alloy solidification shrinkage (up to 5%), welding stress and deformation, and the weld metal in the crystallization along the grain boundaries will produce a low melting point eutectic organization, so that the grain boundaries of the bonding force weakened in the tensile stress under the action of the formation of hot cracks.
Adopting the method of filling wire or alloy powder can reduce the tendency of hot cracking, and controlling the heating and cooling speed by adjusting the welding process parameters can also reduce the tendency of hot cracking. When using a YAG laser, the heat input can be controlled by adjusting the pulse waveform to minimize crystal cracking.
Problem 3:Decrease in mechanical properties of welded links - softening
Burning loss of alloying elements during the welding process reduces the mechanical properties of aluminum alloy welded links.
"Softening" is the phenomenon of reduced strength and hardness of welded joints. When laser welding aluminum alloy joints is used, the weld tissue and heat-affected zone of the welded joints have the same softening problem. A large number of studies have shown that the softening phenomenon of aluminum alloy welding is difficult to eliminate fundamentally, but compared with gas shielded welding, laser welding due to reduced heat input, so that the weld softening zone is narrower. Aluminum alloy laser welding and melting electrode gas shielded welding compared to laser welded joints, "softening" degree is lower, and the tensile strength with the increase in welding speed and increase. Plasma on the welding process of the impact of the ionization energy of the aluminum element is low, laser welding is more likely to form a metal plasma, plasma caused by laser refraction, deflection, thus changing the focal point of the laser beam position, so that the weld depth ratio is reduced, affecting the quality of welded joints. Adopt the method of pre-positioned powder on the surface of the workpiece to attenuate the expansion of the plasma in the height direction of the jump, so that the plasma on the surface of the workpiece can maintain the relative stability of the jump amplitude.
Unstable pores in the aluminum alloy welding process lead to a decrease in the mechanical properties of the welded joint. Aluminum alloy mainly includes Zn, Mg and Al. In the welding process, the boiling point of aluminum is higher than the other two elements. Therefore, some alloying elements with low boiling points can be added when welding aluminum alloy elements, which is conducive to the formation of small holes and the firmness of welding.
Two aluminum alloy laser welding technology
1 aluminum alloy laser self-melting welding
Laser self-melting welding refers to the high energy density laser beam as a heat source, impact to the surface of the base material, so that the base material itself melts, the formation of welded joints welding method. For aluminum alloy laser welding, aluminum alloy surface of the laser reflectivity is high, welding requires a larger laser power; laser spot diameter is small, the precision requirements of the welding tool is high, the tolerance value of the parts gap is low, usually requires the parts gap value of 0.2mm the following; the welding process of heating and cooling speed, the welding of the porosity defects, the concentration of the laser energy density, the keyhole effect is easy to lead to weld concave and the phenomenon of biting edges, therefore, for the welding process, the laser beam is a high energy density laser beam heat source. The phenomenon of biting edge, therefore, the welding process parameters have high requirements. Laser self-melting welding in aluminum alloy welding reflects the advantages of good welding quality, fast welding speed and easy automation, and is widely used in the automotive industry. In the electric vehicle industry, the sealing of the power battery shell is mainly used in aluminum alloy laser self-melt welding. A new energy vehicle enterprises in the aluminum body, the door assembly and the side of the structural components of the welding is also used in aluminum alloy laser fusion welding.
2 Aluminum alloy laser filler wire welding
Laser filler wire welding in the laser is still as the main heat source to melt the welded metal, but the use of automatic wire feeding device to the molten pool continuously fed into the filler metal to achieve the metallurgical connection process. Compared with the laser self-melting welding, laser filler wire welding relaxes the requirements of the welding process gap accuracy, by filling the wire of different compositions, to improve the metallurgical properties of the weld, to prevent the generation of weld thermal cracks and porosity, and to improve the stability of the welding process and the mechanical properties of the joints.
Aluminum alloy laser filler wire welding has the characteristics of good appearance quality, process gap precision is looser than laser self-melt welding, etc. It is usually applied in the appearance surface of the body, such as between the top cover and the side enclosure, and between the upper and lower panels of the outer plate of the luggage compartment cover. There are also some models in order to get a higher welding quality and the use of laser filler wire welding to weld aluminum alloy doors.
3 aluminum alloy laser - arc composite welding
Laser - arc composite welding is the laser and arc 2 kinds of physical properties, energy transfer mechanism is very different from the heat source composite together, and together in the role of the welded workpiece, not only give full play to the 2 kinds of heat source of their respective advantages, but also make up for each other's shortcomings. In the aluminum alloy laser - arc composite welding, the arc can guide the laser heat source, improve the aluminum alloy on the laser absorption ability and welding process energy utilization, and weld surface shaping than laser self-melt welding. In addition, the introduction of the arc can greatly reduce the mounting accuracy of the welded workpiece, while the arc has a dilution effect on the laser welding plasma, which can reduce the shielding effect of the plasma on the laser. The laser plays an important role in the stabilization of the arc, so that the arc can be stabilized in high-speed welding on the joint, which can improve the welding quality of the joint and increase the welding speed.
Conclusion
Aluminum alloy laser welding beam energy density up to 109W/cm2, at the same time has the advantages of concentrated heating, thermal damage, weld depth and width ratio, welding deformation, etc., the welding process is easy to integrate, automation, flexibilization, high-speed and high-precision welding can be achieved, and the welding process does not require a vacuum environment, does not produce X-ray, especially suitable for high-precision welding of complex structures. The most attractive feature of aluminum laser welding is its high efficiency, and to give full play to this high efficiency, it is necessary to apply it to the large thickness of the deep fusion welding. Therefore, research and application of high-power laser for large thickness deep fusion welding will be the inevitable trend of future development. Large thickness deep fusion welding highlights the pinhole phenomenon and its effect on weld porosity, so the formation mechanism and control of pinholes become more and more popular and will become a hot issue of general concern and research in the industry.
Improving the stability of the laser welding process, weld formation and weld quality are the pursued goals. Therefore, new technologies such as laser arc composite process, filler wire laser welding, non-preset powder laser welding, dual-focus technology, beam shaping, etc. will be further improved and developed.
