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.
Main high power lasers in laser welding applications
|Performance/type||Carbon dioxide laser||NdYAG|
|Wavelength / um||10.6||1.06||1.06||1-2|
|Electric conversion efficiency/%||12-15||2-6||20||8-10|
|Beam output||Optical lens||Optical fiber||Optical fiber||Optical fiber|
|Laser brightness(103W/mm2steradian)(4kW/h)||—||7.5(focal length:200;|
CO2 gas laser
The working medium is CO2 gas, with an output of 10.6 μm wavelength laser can be divided into cross flow and axial flow according to the laser excitation structure.
Although the output power of cross flow CO2 laser has reached 150KW, the beam quality is poor and is not suitable for welding;
Axial flow CO2 laser has good beam quality and can be used for welding aluminum alloy with high laser reflectivity.
The working medium is ruby, neodymium glass and neodymium doped yttrium aluminum garnet, and the output wavelength is 1.06 μm laser.
YAG laser is easier to be absorbed by metal than CO2 laser, and is less affected by plasma.
It is optical fiber transmission, flexible welding operation and good accessibility of weld position.
At present, YAG laser is the main laser for aluminum alloy structure welding.
YLR fiber laser
It is a new laser developed after 2002.
It takes optical fiber as matrix material, doped with different rare earth ions, and the output wavelength range is 1.08 μm, which is also optical fiber transmission.
The fiber laser adopts the double clad fiber structure revolutionarily, which increases the pump length and improves the pump efficiency, so as to greatly improve the output power of the fiber laser.
Compared with YAG laser, YLR fiber laser appears later, but it has the advantages of small volume, low operation cost, high beam quality and high laser power.
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.
Application of laser welding structure of the aluminum alloy
Since the 1990s, with the development of science and technology and the emergence of high-power and high brightness lasers, the integrated, intelligent, flexible and diversified development of laser welding technology has become more and more mature.
More attention has been paid to the application of laser welding in aluminum alloy structures in various fields at home and abroad.
At present, some automobile manufacturers in China have adopted laser welding technology in some new models.
With the development of laser welding technology for aluminum alloy thick plates, laser welding will be applied to the structure of armored vehicles in the future.
In order to realize lightweight manufacturing, the application and research of laser welding of aluminum alloy sandwich structure is a research hotspot in the structural manufacturing of ships and high-speed trains.
Aluminum alloy is an important metal structure material for aerospace structure.
Therefore, in developed countries such as Japan, the United States, Britain and Germany, great attention is paid to the research of aluminum alloy laser welding technology.
With the development of fiber laser welding technology, fiber laser welding and laser arc hybrid welding technology have been listed as the focus of aluminum alloy welding technology in the aviation manufacturing field of advanced countries, especially thick plate welding and dissimilar metal welding.
For example, the American NALI project is carrying out research on fiber laser welding and laser arc hybrid welding technology for the combustion chamber structure of civil aircraft and JSF aircraft engines.
Characteristics of laser welding of aluminum alloy
Compared with conventional fusion welding, aluminum alloy laser welding has concentrated heating, large weld depth width ratio and small welding structure deformation, but there are also some deficiencies, which can be summarized as follows:
1) The small diameter of the laser focus spot leads to high requirements for workpiece welding and assembly accuracy.
Generally, the assembly gap and misalignment amount need to be less than 0.1mm or 10% of the plate thickness, which increases the difficulty of implementing complex three-dimensional weld structure;
2) Because the reflectivity of aluminum alloy to laser is up to 90% at room temperature, laser deep penetration welding of aluminum alloy requires high power.
The research on laser welding of aluminum alloy sheet shows that:
The laser deep penetration welding of aluminum alloy depends on the double threshold of laser power density and linear energy.
The laser power density and linear energy jointly restrict the molten pool behavior in the welding process and are finally reflected in the weld forming characteristics.
The process optimization of full penetration weld can be evaluated by the back width ratio of weld forming characteristic parameters;
3) Aluminum alloy has low melting point and good fluidity of liquid metal. It produces strong metal vaporization under the action of high-power laser.
The metal vapor / photoinduced plasma cloud formed with keyhole effect affects the absorption of laser energy by aluminum alloy during welding, resulting in instability in deep penetration welding process, and the weld is prone to defects such as porosity, surface collapse and undercut;
4) Laser welding has fast heating and cooling speed, and the weld hardness is higher than that of arc.
However, due to the burning loss of alloy elements in aluminum alloy laser welding, which affects the alloy strengthening effect, the aluminum alloy weld still has the problem of softening, so as to reduce the strength of aluminum alloy welded joints.
Therefore, the main problem of aluminum alloy laser welding is to control weld defects and improve the properties of welded joints.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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.
Defect control technology of laser welding of aluminum alloy
Under the action of high-power laser, the main defects of laser deep penetration weld of aluminum alloy are porosity, surface collapse and undercut.
The surface collapse and undercut defects can be improved by laser wire filling welding or laser arc hybrid welding;
However, it is difficult to control weld porosity defects.
The existing research results show that there are two kinds of characteristic pores in laser deep penetration welding of aluminum alloy.
One is metallurgical pores, which are caused by material pollution or air intrusion in the welding process, just like arc fusion welding;
The other is process porosity, which is caused by the unstable fluctuation of small holes inherent in the process of laser deep penetration welding.
In the process of laser deep penetration welding, the small hole often lags behind the movement of the beam due to the viscosity of liquid metal, and its diameter and depth fluctuate under the influence of plasma/metal vapor.
With the movement of the beam and the flow of molten pool metal, the incomplete deep penetration welding is closed due to the flow of molten pool metal, and bubbles appear at the tip of the small hole.
In full penetration deep penetration welding, bubbles appear at the waist in the middle of the small hole.
Bubbles migrate and roll with the flow of liquid metal, or escape from the molten pool surface, or are pushed back to the small hole.
When the bubbles are solidified by the molten pool and captured by the metal front, they become weld pores.
Obviously, metallurgical pores are mainly controlled by pre welding surface treatment control and reasonable gas protection in the welding process, and the key of process pores is to ensure the stability of pores in the process of laser deep penetration welding.
According to the research of domestic laser welding technology, the air hole control of aluminum alloy laser deep penetration welding should comprehensively consider all links before welding, welding process and post-welding treatment, which can be summarized as the following new processes and technologies.
Pre welding treatment method
Pre welding surface treatment is an effective method to control the metallurgical pores of aluminum alloy laser weld.
Generally, the surface treatment methods include physical mechanical cleaning and chemical cleaning.
In recent years, laser impact cleaning has also appeared, which will further improve the automation of laser welding.
Parameter stability optimization control
The process parameters of aluminum alloy laser welding usually include laser power, defocus, welding speed, composition and flow of gas protection.
These parameters not only affect the protection effect of welding area, but also affect the stability of laser deep penetration welding process, so as to affect the weld porosity.
Through laser deep penetration welding of aluminum alloy sheet, it is found that the stability of small hole penetration affects the stability of molten pool, and then affects the weld forming, resulting in weld porosity defects.
Moreover, the stability of laser deep penetration welding is related to the matching of laser power density and linear quantity.
Therefore, determining reasonable process parameters for stable weld forming is an effective measure to effectively control the porosity of aluminum alloy laser weld.
The research results of full penetration stable weld forming characteristics show that:
The ratio of weld back width to weld surface width (weld back width ratio) is used to evaluate the weld forming and stability of aluminum alloy sheet.
When the laser power density of thin plate laser welding is reasonably matched with the line energy, a certain weld back width ratio can be ensured, and the weld porosity can be effectively controlled.
Double spot laser welding
Double spot laser welding refers to the welding process in which two focused laser beams act on the same weld pool at the same time.
In the process of laser deep penetration welding, one of the main reasons for the formation of weld porosity is to close the gas in the small hole in the weld pool.
When double spot laser welding is used, the opening of the small hole is large due to the action of two light sources, which is conducive to the escape of internal metal vapor and the stability of the small hole, so as to reduce the weld porosity.
The research on laser welding of A356, AA5083, 2024 and 5A90 aluminum alloys shows that double spot laser welding can significantly reduce weld porosity.
Laser arc hybrid welding
Laser arc hybrid welding is a welding method in which laser and arc act on the same molten pool.
Generally, laser is the main heat source, and the interaction between laser and arc is used to improve the penetration and welding speed of laser welding and reduce the welding assembly accuracy.
Using filler wire to control the microstructure and properties of welded joints, and using the auxiliary effect of arc to improve the stability of laser welding holes, which is conducive to reducing weld porosity.
In the process of laser arc hybrid welding, the arc affects the metal vapor/plasma cloud induced by the laser process, which is conducive to the absorption of laser energy and the stability of small holes.
The results of laser arc hybrid welding of aluminum alloy also confirm its effect.
Fiber laser welding
The keyhole effect in laser deep penetration welding is due to the strong vaporization of metal under the action of laser.
The vapor force of metal vaporization is closely related to laser power density and beam quality, which not only affects the penetration of laser welding, but also affects the stability of keyholes.
Seiji et al. studied SUS304 stainless steel high power fiber laser and showed that:
During high-speed welding, the molten pool is elongated, the spatter is restrained, the small hole fluctuation is stable, and there is no bubble at the tip of the small hole.
When the fiber laser is used for high-speed welding of titanium alloy and aluminum alloy, the weld without pores can also be obtained.
Research on shielding gas control technology for titanium alloy fiber laser welding by Allen et al. shows that:
By controlling the position of welding shielding gas, it can prevent the involvement of gas, reduce the small hole closing time, stabilize the welding small hole, and change the solidification behavior of the molten pool, so as to reduce the weld porosity.
Pulsed laser welding
Compared with continuous laser welding, the pulse mode of laser output can promote the periodic and stable flow of molten pool, which is conducive to the escape of bubbles in molten pool and reduce weld porosity.
T Y Kuo and S L Jeng studied the effect of laser power output mode of YAG laser welding on the porosity and properties of welds of SUS 304L stainless steel and inconel 690 superalloy.
The results show that for square wave pulse laser welding, when the base power is 1700W, the weld porosity decreases with the increase of pulse amplitude ΔP, in which the porosity of stainless steel decreases from 2.1% to 0.5%, and that of superalloy decreases from 7.1% to 0.5%.
Post weld composite treatment technology
In practical engineering application, even if strict surface treatment is carried out before welding and the welding process is stable, weld porosity will inevitably occur in aluminum alloy laser welding.
Therefore, the method of eliminating porosity by post weld treatment is very important.
At present, this method is mainly modified welding.
Hot isostatic pressing technology is one of the methods to eliminate internal porosity and shrinkage porosity of aluminum alloy castings.
It is combined with stress heat treatment after aluminum alloy laser welding to form a composite process of hot isostatic pressing and heat treatment of aluminum alloy laser welding components, which can not only eliminate weld porosity, but also improve joint performance.
Due to the characteristics of aluminum alloy, there are still many problems to be further studied in the application of high-power laser welding.
The main problem is to control weld porosity defects and improve welding quality.
The engineering control of porosity in aluminum alloy laser weld should comprehensively consider all links before welding, welding process and post-welding treatment, so as to improve the stability of the welding process.
Many new technologies and processes have been derived, such as laser cleaning before welding, back width ratio control optimization of welding process parameters, double beam laser welding, laser arc hybrid welding, pulse laser welding and optical fiber laser welding.