Compared with traditional welding, laser welding has the advantages of small heat input and thermal influence, large aspect ratio, and automatic welding process.
Aluminum alloy has light weight, good toughness, high yield ratio and easy processing and forming. It is widely used in welding structure products such as containers, machinery, electric power, chemical industry, aviation and aerospace.
The use of aluminum alloy instead of steel plate welding can greatly reduce structural quality.
Aluminum is a more active metal with low ionization energy and high thermal conductivity. It is easy to form a refractory Al2O3 film on the surface, and it is easy to form defects such as unfused, pores, inclusions, and thermal cracks in the weld, which reduces the mechanical properties of the welded joint.
Compared with argon tungsten arc welding or melting argon arc welding, laser welding has narrow weld seams, small heat-affected zones, reduced overlap joints, precise controllable welding process, and automation.
At present, laser welding is mainly used for thin-walled electronic components, structural parts, aerospace parts, etc. Research on 10,000-watt fiber lasers for deep penetration welding of large and thick plates is the future development trend.
1. Aluminum alloy classification and weldability
Aluminum and aluminum alloys can be divided into:
- 1000 series (industrial pure aluminum)
- 2000 series (Al-Cu series)
- 3000 series (Al-Mn series)
- 4000 series (Al-Si)
- 5000 series (Al-Mg)
- 6000 series (Al-Mg-Si)
- 7000 series (Al-Zn-Mg-Cu)
According to the process characteristics, aluminum alloy can be divided into deformed aluminum alloy and cast aluminum alloy.
Among them, the deformed aluminum alloy is divided into two categories: non-heat-treated strengthened aluminum alloy and heat-treated strengthened aluminum alloy.
Different aluminum alloys have different welding properties. For example, non-heat-treated aluminum and aluminum alloy 1000 series, 3000 series and 5000 series have good weldability. 4000 series alloys have very low crack sensitivity.
For 5000 series alloys, when ω (Mg) = 2%, the alloy cracks. As the magnesium content increases, the welding performance is improved, but the ductility and corrosion resistance become worse.
The 2000 series, 6000 series and 7000 series alloys have a higher tendency of hot cracking, poor welding seam formation, and significantly reduced aging hardness after welding.
To sum up, it is necessary to adopt appropriate technological measures for aluminum alloy welding, and correctly select welding methods and filler materials to obtain good welded joints.
Surface treatment of the material before welding, use organic solvents to remove oily dirt, and then immerse in NaOH solution, rinse the surface with running water and then perform the photochemical treatment.
The processed weldments were subjected to welding process experiments within 24 hours.
2. The main problems existing in aluminum alloy laser welding
Laser welding uses the laser as a high-energy density light source, which has the characteristics of fast heating and instantaneous solidification, and the aspect ratio is as high as 12:1.
However, due to the high reflectivity and good thermal conductivity of aluminum alloy and the shielding effect of plasma, some defects will inevitably occur during welding. The two most important defects are pores and thermal cracks.
Due to the strong reflection of aluminum alloy to laser, the first problem encountered in aluminum alloy laser welding is how to effectively improve the material’s absorption of laser light.
Based on some characteristics of the aluminum alloy itself, the laser welding process is more complicated, and it is urgent to improve and perfect.
2.1 Laser absorption rate
The higher the absorption rate of the material to the laser, or the smaller the heat transfer coefficient and temperature conductivity coefficient, the easier the laser energy is absorbed by the surface of the material, the surface temperature rises rapidly, and the material melts or evaporates.
The reflectivity of various metals to lasers of different wavelengths is shown in Table 1.
Table 1 The reflectivity of metals to lasers of different wavelengths at room temperature (%)
The reflectivity of various metals decreases as the wavelength becomes shorter, and the reflectivity of Ag, Al, Cu to laser light is as high as 90% or more.
This undoubtedly increases the difficulty of laser processing.
At room temperature, the absorption rate of CO2 laser by aluminum alloy is extremely low, 98% of the laser energy will be reflected by the aluminum alloy surface, and the reflectivity of Nd:YAG laser is also up to 80%.
It can be seen that aluminum alloy has the characteristics of high reflectivity to laser light and low absorption rate.
This is because of the high density of free electrons in aluminum alloys.
Under the strong vibration of light electromagnetic waves, strong reflected waves and weaker transmitted waves are generated. The reflected waves are not easily absorbed by the aluminum alloy surface, so the aluminum alloy surface has a higher reflectivity to the laser at room temperature.
2.2 Induction and stabilization of “small holes”
In the laser welding process, when the laser energy density is greater than 3.5*10^6W/cm2, ions will be generated.
The welding method is carried out by deep penetration welding.
The principle is mainly the “small hole” effect.
The appearance of “small holes” can greatly increase the absorption rate of the material to the laser, and the weldment is fused at high energy density to obtain a good welding effect.
The primary problem in laser welding of aluminum alloys is the difficulty in inducing and maintaining the stability of small holes, which are caused by the material properties of the aluminum alloy itself and the optical properties of the laser beam.
As mentioned earlier, Al at room temperature can reflect 80% of the energy. Coupled with its good thermal conductivity, a large laser energy density threshold is required to produce “small holes”.
Such a threshold exists in different aluminum alloy laser welding processing.
Once the input power is greater than this value, the transmission of laser energy into the material is no longer limited by heat conduction, and the welding is carried out by deep penetration welding.
The laser radiation will cause the base metal to evaporate strongly and form an evaporation groove. The laser beam penetrates into the material through the evaporation groove, and the weld depth and welding efficiency also increase sharply.
For highly reflective materials, such as aluminum alloys and copper alloys, it is necessary to provide a very large power density during welding.
This has certain requirements for the selection of welding models and collimating and focusing lenses.
2.3 Mechanical properties of welds
Refinement strengthening, solid solution strengthening, and aging precipitation strengthening are several strengthening mechanisms of aluminum alloys.
Even with these strengthening mechanisms, the large amount of evaporation of low melting point alloy elements such as Mg and Zn during laser welding will cause the weld to sink and reduce the hardness and strength.
During the instantaneous solidification process, after the fine-grained strengthened structure is transformed into the as-cast structure, its hardness and strength will decrease.
In addition, the presence of cracks and pores in the weld leads to a decrease in tensile strength.
In short, the problem of joint softening is another problem in laser welding of aluminum alloys.
There are two main types of pores in the laser welding process of aluminum alloy: hydrogen gas holes and pores caused by keyhole bursting.
(1) Hydrogen hole.
Aluminum alloy is easy to form oxide film on the surface at high temperature, and the oxide film is easy to absorb moisture in the environment.
When heated by laser, water is decomposed to produce hydrogen, and the solubility of hydrogen in liquid aluminum is about 20 times that of solid aluminum.
During the instantaneous solidification of the alloy, the solubility of hydrogen decreases sharply when it changes from liquid aluminum to a solid state. If the excess hydrogen in the liquid aluminum cannot smoothly rise and overflow, it will form hydrogen pores.
Such pores are generally regular in shape, larger in size than dendrites, and solidification patterns of dendrites can be seen on the inner surface.
(2) Keyhole collapsed.
The welding hole is in equilibrium with its own gravity and atmospheric pressure. Once the balance is broken, the liquid metal in the molten pool cannot flow over and fill in time, and irregular holes will be formed.
Studies have found that the magnesium content of the inner wall of the hole is about 4 times that of the vicinity of the weld.
Because the cooling rate of laser welding is too fast, the problem of hydrogen gas holes is more serious, and there are more holes caused by the collapse of small holes in laser welding.
2.5 Thermal cracking
Aluminum alloy is a typical eutectic alloy, and it is prone to hot cracks during welding, including weld crystallization cracks and HAZ liquefaction cracks.
Usually, crystal cracks appear in the weld zone, and liquefaction cracks appear in the near-joint zone.
Among aluminum alloys, the 6000 series Al-Mg-Si alloys are especially sensitive to cracks.
The base metal has undergone rapid heating and cooling. During the instantaneous solidification and crystallization process, due to the large degree of undercooling, the crystal grains grow along the direction perpendicular to the center of the weld, forming Al-Si or Mg-Si, Al at the columnar grain boundary -Mg2Si and other low-melting eutectic compounds, weaken the bonding force of the crystal plane, easy to produce crystal cracks under the action of thermal stress.
In the aluminum alloy welding process, some low-boiling elements (Mg, Zn, Mn, Si, etc.) are easy to evaporate and burn. The slower the welding speed, the more serious the burning, which changes the chemical composition of the weld metal.
Due to the segregation of components in the weld zone, eutectic segregation will occur and grain boundary melting will occur, and liquefaction cracks will form at the grain boundary under stress, which will reduce the performance of the welded joint.
3. Aluminum alloy laser welding process
In order to achieve laser welding of aluminum alloys and solve the above-mentioned problems, it is mainly solved from the following aspects.
3.1 Gas protection device
The most important factor influencing the loss of low melting point elements in the aluminum alloy is the pressure when the gas is sprayed from the nozzle. By reducing the nozzle diameter, increasing the gas pressure and flow rate can reduce the burning loss of Mg, Zn, etc. during the welding process, and it can also Increase penetration.
There are two blowing methods, direct blowing and side blowing, and you can also blow up and down the weldment at the same time.
Choose the blowing method according to the actual situation during welding.
3.2 Surface treatment
Aluminum alloy has a high reaction to laser. Proper surface pretreatment of aluminum alloy, such as anodic oxidation, electrolytic polishing, sandblasting, sandblasting, etc., can significantly improve the absorption of beam energy on the surface.
Studies have shown that the tendency of aluminum alloys to crystallize cracks after removing the oxide film is greater than that of the original aluminum alloys.
In order not to damage the surface state of the aluminum alloy, but also to simplify the laser welding engineering process, the surface temperature of the workpiece can be increased by pre-welding to increase the material’s absorption rate of the laser.
3.3 Laser parameters
Welding lasers are divided into pulsed lasers and continuous lasers. When the wavelength of pulsed lasers is 1064nm, the beam is particularly concentrated, and the pulse single point energy is larger than that of continuous lasers.
However, the energy of pulsed lasers generally does not exceed, so thin-wall weldments are generally suitable.
3.3.1 Pulse mode welding
When laser welding, the appropriate welding waveform should be selected. Common pulse waveforms include square wave, spike wave, double peak wave, etc.
Usually the time of a pulse wave is in milliseconds.
During a laser pulse, the reflectivity of the metal changes greatly.
The reflectivity of the aluminum alloy surface to light is too high. When a high-intensity laser beam hits the material surface, 60%-98% of the laser energy on the metal surface will be lost due to reflection, and the reflectivity changes with the surface temperature.
Therefore, the best choice for welding aluminum alloy is sharp wave (see Figure 1) and double peak wave.
The rising phase of the waveform is to provide greater energy to melt the aluminum alloy.
Once the “small hole” in the workpiece is formed, when the deep penetration welding starts, the absorption rate of the liquid metal to the laser increases rapidly after the metal is melted. At this time, the laser energy should be quickly reduced, and the welding should be performed at a low power to avoid splashing.
The slow-down part of the welding waveform has a longer pulse width, which can effectively reduce the occurrence of pores and cracks.
Using this waveform, the weld is melted and solidified repeatedly to reduce the solidification rate of the molten pool.
This waveform can be adjusted appropriately when welding samples of different types.
Figure 1 Pulse waveform of welding aluminum alloy
Choosing the right amount of defocus can also reduce the generation of pores.
The change of defocus has a great influence on the surface formation and penetration of the weld.
Using negative defocus can increase penetration, while in pulse welding, positive defocus will make the weld surface smoother and more beautiful.
Due to the high reflectivity of aluminum alloy to laser, in order to prevent the vertical reflection of the laser beam from perpendicular incidence and damage the laser focusing lens, the welding head is usually deflected to a certain angle during the welding process.
The diameter of the solder joint and the effective bonding surface increase with the increase of the laser tilt angle.
When the laser tilt angle is 40°, the largest solder joint and effective bonding surface are obtained.
The welding point penetration and effective penetration decrease with the laser tilt angle. When it is greater than 60°, the effective welding penetration decreases to zero.
Therefore, tilting the welding head to a certain angle can appropriately increase the weld penetration depth and penetration width.
In addition, in laser welding of aluminum alloy, the faster the welding speed, the more likely it is to crack.
Because the welding speed is too fast and the degree of undercooling is large, the grains in the weld zone are refined, and a large number of “beam crystals” growing in the same direction are formed, which is beneficial to the generation of cracks on the crystal plane between the beam crystals.
If the welding speed is too fast, the penetration depth of the weldment becomes relatively small.
3.3.2 Continuous mode welding
Embrittlement or even cracks occur when using traditional laser welding.
The use of continuous laser welding because the heating process is not like the sudden cooling and heating of the pulse machine, the crack tendency is not obvious during welding, and the fiber laser welding most aluminum alloys will not be brittle and has certain toughness after welding, which has obvious advantages.
Industrial pure aluminum can be welded well with pulsed laser welding, and generally there will be no cracks after welding.
However, in some industries, the surface needs to be polished after welding, and there will be dents after laser pulse welding, and the amount of polishing will increase, which increases the processing cycle and production costs. Continuous lasers can solve these problems.
Figure 2 shows the comparison of the welding seam of the battery shell after pulse laser welding and continuous laser welding.
It can be seen from Figure 2 that the impulse solder joints are uneven, undercut, the surface is dented, there are many spatters, and the strength after welding is not high.
In order to improve the quality of the weld seam, continuous laser welding is used. The weld seam surface is smooth and uniform, free of spatter and defect, and no cracks are found in the weld seam.
Figure 2 Pulse and continuous welding of Al-Mn alloy
Arc craters are prone to appear during argon arc welding, and laser welding is the same.
Small craters are prone to appear at the end, which can be improved by gradual exit during welding, that is, a slow rise and slow fall stage is set in the waveform;
In addition, the welding speed can be appropriately increased during welding to avoid small pits.
In the welding of aluminum alloy, continuous laser has obvious advantages.
Compared with the traditional welding method, the production efficiency is high, and no wire filling is required;
Compared with pulse laser welding, it can solve the defects generated after welding, such as cracks, pores, spatter, etc., and ensure that the aluminum alloy has good mechanical properties after welding;
There will be no dents after welding, and the amount of polishing and grinding after welding is reduced, saving production costs.
However, because the CW laser has a relatively small spot, the assembly accuracy of the workpiece is high.
3.4 Introducing alloying elements
Preventing thermal cracks is one of the key technologies for laser welding of aluminum alloys.
6000 series alloys are very sensitive to cracks. When ω(Mg2Si) =1%, hot cracks will appear. It can be improved by adding suitable alloying elements to adjust the chemical composition of the molten pool, such as adding Al-Si or Al-Mg -Si powder has certain advantages in reducing cracks.
In addition, the welding effect can be improved by wire feeding, and a uniform weld seam can be obtained, and the weld seam hardness has also been improved.
The content of Mg and Si in the dendrite in the fusion zone increases due to the introduction of the filler material, and the β” solid solution strengthening effect will increase the strength of the joint.
Generally, 6063 and 6082 aluminum alloys are filled with Al-5Si and Al-7Si welding wires, 6013 and 6056 plates are welded with CO2 and Nd: YAG lasers, respectively, and Al-12Si welding wires are filled.
3.5 Other process methods
Aiming at the stability of the aluminum alloy laser welding process and the quality of the weld.
At present, the research hotspot of aluminum alloy laser welding is the use of a composite process, that is, the high energy density of the laser and the larger heating range of the arc are coupled, giving full play to the advantages of the two heat sources, and combining the characteristics of high energy density beam quality and stable arc , Complement each other.
For high-reflective materials such as aluminum alloy, laser hybrid welding can preheat or melt the surface of the material by arc energy, which greatly improves the absorption of laser energy by aluminum alloy.
Shida et al. used a 10 kW CO2 laser combined with TIG and MIG arcs to weld aluminum alloys. The introduction of arcs greatly improved the laser energy utilization rate, and the weld penetration ratio also increased by 5%-20%.
At the same time, the weld surface is smooth and well-formed.
Laser hybrid welding increases the geometric size of the molten pool through the coupling of the laser beam and the arc, and changes the flow conditions of the material in the molten state, which is beneficial to the elimination of pores.
Dual-beam welding of aluminum alloy is also a way to eliminate air holes. A 6 kW continuous fiber laser was used to perform dual-beam butt welding of 5052 aluminum alloy. The two-beam parallel and serial welding modes and welding at different welding speeds were studied. Seam morphology and organization.
Research has found that there are large holes in the welds welded in parallel with dual beams, and welding aluminum alloys in series can achieve good weld formation without pores.
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