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Aluminum Laser Welding Basics

Aluminum-Laser-Welding

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Aluminum alloy has lightweight, has good toughness, has a high yield ratio, and is easy to procedure and type. It is extensively used in welding structure products such as containers, machinery, electric power, chemical market, aviation, and aerospace.

Making use of aluminum alloy rather than steel plate welding can significantly minimize structural quality.

Aluminum is a more active metal with low ionization energy and high thermal conductivity. It is simple to form a refractory Al2O3 film on the surface, and it is easy to form problems such as unfused pores, additions, and thermal fractures in the weld, which minimizes the mechanical homes of the welded joint.

Compared with argon tungsten arc welding or melting argon arc welding, laser welding has narrow weld joints, small heat-affected zones, minimized overlap joints, an exact, manageable welding procedure, and automation.

At present, laser welding is mainly utilized for thin-walled electronic elements, structural parts, aerospace parts, etc. Research on 10,000-watt fiber lasers for deep penetration welding of big and thick plates is the future advancement trend.

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 qualities, aluminum alloy can be divided into the deformed and cast aluminum alloy.

The deformed aluminum alloy is divided into non-heat-treated strengthened aluminum alloy and heat-treated strengthened aluminum alloy.

Different aluminum alloys have various welding properties. Non-heat-treated aluminum and aluminum alloy 1000 series, 3000 series, and 5000 series have good weldability. 4000 series alloys have extremely low crack sensitivity.

For 5000 series alloys, when ω (Mg) = 2%, the alloy cracks. As the magnesium content increases, the welding performance is improved. However, the flexibility and deterioration resistance worsens.

The 2000 series, 6000 series, and 7000 series alloys have a greater propensity of hot breaking, poor welding joint formation, and substantially reduced aging hardness after welding.

To sum up, it is essential to embrace proper technological measures for aluminum alloy welding and properly select welding methods and filler materials to acquire excellent bonded joints.

Before welding, surface area treatment of the product, usage organic solvents to eliminate oily dirt, immerse in NaOH option, wash the surface area with running water, and carry out the photochemical treatment.

The processed weldments underwent welding procedure experiments within 24 hours.

The Main Issues Exist In Aluminum Alloy Laser Welding

Laser welding utilizes the laser as a high-energy density light source, quickly heating and rapid solidification characteristics, and the element ratio is as high as 12:1.

Due to the high reflectivity and great thermal conductivity of aluminum alloy and the protecting impact of plasma, some defects will undoubtedly occur throughout welding. The two crucial defects are pores and thermal fractures.

Due to the strong reflection of aluminum alloy to laser, the first issue encountered in aluminum alloy laser welding is effectively improving the product’s absorption of laser light.

Based on some of the aluminum alloy’s characteristics, the laser welding process is more complex and in urgent need of improvement and perfection.

Laser Absorption Rate

The higher the absorption rate of the product to the laser, or the smaller the heat transfer coefficient and temperature level conductivity coefficient, the easier the laser energy is taken in by the surface of the material, the surface temperature level increases rapidly, and the material evaporates or melts.

The reflectivity of numerous metals to lasers of different wavelengths is shown in the Table.

λ/μmAgAlCuCrNiSteel
0.7957782566858
1.06978091587563
10.6999898939593
The reflectivity of metals to lasers of various wavelengths at room temperature level (%)

The reflectivity of numerous metals reduces with much shorter wavelengths, with Ag, Al, and Cu showing up to 90% or more of the laser light.

This circumstance certainly increases the problem of laser processing.

At space temperature, the absorption rate of a CO2 laser by aluminum alloy is incredibly low. The aluminum alloy surface will show 98% of the laser energy, and the reflectivity of Nd: YAG laser is likewise approximately 80%.

The aluminum alloy has high reflectivity and low absorption of laser light because of the high density of complementary electrons in aluminum alloys.

Under the strong vibration of light, electromagnetic waves strongly showed waves, and weaker transmitted waves were produced. The aluminum alloy surface area does not easily soak up the shown waves, so the aluminum alloy surface has a greater reflectivity than the laser at space temperature.

Induction And Stabilization Of “Little Holes”

In the laser welding procedure, ions will be produced when the laser energy density surpasses 3.5 * 10 ^ 6W/cm2.

The welding technique is performed by deep penetration welding.

The principle is primarily the “small hole” effect.

The appearance of “little holes” can greatly increase the absorption rate of the product to the laser, and the weldment is merged at high energy density to obtain an excellent welding effect.

The main issue in laser welding of aluminum alloys is the trouble in preserving the stability and causing small holes triggered by the product residential or commercial properties of the aluminum alloy itself and the optical homes of the laser beam.

As pointed out previously, Al at space temperature level can reflect 80% of the energy. Due to its good thermal conductivity, a large laser energy density threshold is needed to produce “small holes”.

Such a limit exists in different aluminum alloy laser welding processing.

Once the input power is greater than this value, laser energy transmission into the product is no longer restricted by heat conduction. The welding is performed by deep penetration welding.

The laser radiation will cause the base metal to vaporize highly and form an evaporation groove. The laser beam permeates the material through the evaporation groove, and the weld depth and welding effectiveness also increase sharply.

For extremely reflective products, such as aluminum alloys and copper alloys, it is needed to offer an extremely large power density during welding.

This has particular requirements for the selection of welding designs and collimating and focusing lenses.

Mechanical Homes Of Welds

Refinement fortifying, solid option conditioning, and aging precipitation fortifying are numerous enhancing mechanisms of aluminum alloys.

Even with these enhancing systems, the large amount of evaporation of low melting point alloy components such as Mg and Zn during laser welding will trigger the weld to minimize the hardness, sink, and strength.

Its firmness and strength will reduce throughout the instantaneous solidification process after the fine-grained strengthened structure is transformed into the as-cast structure.

In addition, the presence of fractures and pores in the weld results in a reduction in tensile strength.

In brief, the problem of joint softening is another issue in laser welding of aluminum alloys.

Hole In Aluminum Laser Welding

There are two primary types of pores in the laser welding process of aluminum alloy: hydrogen gas holes and pores brought on by keyhole bursting.

Hydrogen Hole

Aluminum alloy is simple to form an oxide film on the surface at high temperatures, and the oxide movie is simple to soak up moisture in the environment.

When warmed by a laser, water is disintegrated to produce hydrogen, and the solubility of hydrogen in liquid aluminum is about 20 times that of strong aluminum.

During the rapid solidification of the alloy, the solubility of hydrogen reduces dramatically when it changes from liquid aluminum to a solid state. Hydrogen pores will form if the excess hydrogen in the liquid aluminum can not efficiently increase and overflow.

Such pores are normally regular in shape, bigger in size than dendrites, and solidification patterns of dendrites can be seen on the inner surface.

Keyhole Collapsed

The welding hole remains in equilibrium with its gravity and air pressure. Once the balance is broken, the liquid metal in the molten swimming pool can not flow over and complete time, forming irregular holes.

The magnesium content of the inner wall of the hole was approximately four times higher than that near the weld.

Because the cooling rate of laser welding is too fast, the issue of hydrogen gas holes is more major, and there are more holes triggered by the collapse of small holes in laser welding.

Thermal Splitting

Aluminum alloy is a normal eutectic alloy, and it is susceptible to hot fractures throughout welding, including weld formation fractures and HAZ liquefaction fractures.

Normally, crystal cracks appear in the weld zone, and liquefaction fractures appear in the near-joint zone.

Amongst aluminum alloys, the 6000 series Al-Mg-Si alloys are especially sensitive to cracks.

The base metal has gone through rapid heating & cooling. During the instant solidification and crystallization procedure, the crystal grains grow along the direction perpendicular to the center of the weld due to the large degree of undercooling. Forming Al-Si or Mg-Si, Al at the columnar grain limit -Mg2Si and other low-melting eutectic substances, compromise the binding force of the crystal airplane, easy to produce crystal cracks under the action of thermal stress.

In the aluminum alloy welding process, some low-boiling components (Mg, Zn, Mn, Si, and so on) are easy to burn and evaporate. The slower the welding speed, the more severe the burning, which alters the chemical composition of the weld metal.

In the aluminum alloy welding process, some low-boiling components (Mg, Zn, Mn, Si, and so on) are easy to vaporize and burn. The slower the welding speed, the more severe the burning, which changes the chemical structure of the weld metal.

Due to the partition of parts in the weld zone, the eutectic partition will happen, grain border melting will happen. Liquefaction cracks will form at the grain limit under stress, minimizing the bonded joint’s efficiency.

Aluminum Alloy Laser Welding Process

It is generally solved from the following aspects to accomplish laser welding of aluminum alloys and resolve the abovementioned issues.

Gas Security Device

The most crucial factor influencing the loss of low melting point components 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, and throughout the welding process and increase penetration.

There are two blowing approaches, direct blowing, and side blowing, and you can likewise blow up and down the weldment all at once.

Pick the blowing approach according to the real situation during welding.

Surface Treatment

Aluminum alloy has a high reaction to the laser. Correct surface pretreatment of aluminum alloy, such as anodic oxidation, electrolytic polishing, sandblasting, etc., can significantly improve the absorption of beam energy on the surface.

Studies have shown that the propensity of aluminum alloys to take shape cracks after removing the oxide film is higher than that of the initial aluminum alloys.

In order not to harm the surface state of the aluminum alloy and simplify the laser welding engineering procedure, the surface temperature of the workpiece can be increased by pre-welding to increase the material’s absorption rate of the laser.

Laser Specifications

Welding lasers are divided into pulsed lasers and constant lasers. When the wavelength of pulsed lasers is 1064nm, the beam is particularly focused, and the pulse single point energy is larger than that of constant lasers.

Nevertheless, the energy of pulsed lasers normally does not surpass, so thin-wall weldments are normally suitable.

Pulse Mode Welding

Laser welding should choose the appropriate welding waveform. Common pulse waveforms consist of a square wave, spike wave, double peak wave, etc.

. Normally, the time of a pulse wave is in milliseconds.

Throughout a laser pulse, the reflectivity of the metal changes greatly.

The reflectivity of the aluminum alloy surface area to light is too expensive. When a high-intensity laser beam hits the product surface area, 60% -98% of the laser energy on the metal surface will be lost due to reflection, and the reflectivity changes with the surface area temperature level.

The finest choice for welding aluminum alloy is sharp wave (see Figure 1) and double peak wave.

The rising phase of the waveform is to offers greater energy to melt the aluminum alloy.

As soon as the “small hole” in the workpiece is formed, when the deep penetration welding begins, the absorption rate of the liquid metal to the laser increases quickly after the metal is melted. In this case, the laser energy needs to be quickly reduced, and the welding needs to be performed at low power to prevent sputtering.

The slow-down part of the welding waveform has a longer pulse width, successfully reducing pores and cracks.

This waveform allows the weld to be consistently melted and strengthened to decrease the solidification rate of the molten pool.

When welding samples of different types, this waveform can be adjusted properly.

DM 20210923114046 001
Figure 1. Pulse Waveform Of Welding Aluminum Alloy.

Choosing the ideal quantity of defocus can likewise decrease the generation of pores.

The change of defocus has an excellent impact on the surface development and penetration of the weld.

Utilizing unfavorable defocus can increase penetration, while favorable defocus will make the weld surface smoother and more beautiful in pulse welding.

Due to the high reflectivity of aluminum alloy to laser, to prevent the laser beam’s vertical reflection from perpendicular incidence and damage the laser focusing lens, the welding head is typically deflected to a particular angle during the welding procedure.

The size of the solder joint and the effective bonding surface boost with the increase of the laser tilt angle.

When the laser tilt angle is 40 °, the largest solder joint and effective bonding surface area are acquired.

The penetration and reliable penetration of the weld spot decreases with the laser tilt angle. When it is higher than 60 °, the efficient 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.

Since the welding speed is too fast and the degree of undercooling is large, the grains in the weld zone are improved, and a great deal of “beam crystals” growing in the very same instructions are formed, which is advantageous to the generation of fractures on the crystal aircraft between the beam crystals.

If the welding speed is too quick, the penetration depth of the weldment ends up being fairly small.

Continuous Mode Welding

When using conventional laser welding, Embrittlement or even fractures happen.

Using continuous laser welding because the heating process is not like the abrupt cooling and heating of the pulse maker, the crack propensity is not obvious during welding. The fiber laser welding, many aluminum alloys will not be brittle and has certain strength after welding, which has obvious benefits.

Industrial pure aluminum can be welded well with pulsed laser welding, and typically, there will be no cracks after welding.

The surface area must be polished after welding in some markets, and there will be dents after laser pulse welding. The quantity of polishing will increase, which will increase the processing cycle and production expenses. Constant lasers can solve these problems.

Figure 2 shows the contrast 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 area is dented, there are many spatters, and the strength after welding is not high.

To improve the quality of solder joints, constant laser welding is utilized. The weld seam surface is smooth and uniform, free of spatter and defect, and no fractures are found in the weld joint.

DM 20210923114046 002
Figure 2. Pulse And Constant Welding Of Al-Mn Alloy.

Arc craters are prone to appear during argon arc welding, and laser welding is the very same.

Little craters are susceptible to appear at the end, which can be enhanced by progressive exit during welding; that is, a slow rise and slow fall phase is embedded in the waveform.

In addition, the welding speed can be properly increased throughout welding to avoid small pits.

In the welding of aluminum alloy, the continuous laser has obvious advantages.

Compared to the traditional welding technique, the production performance is high, and no wire filling is needed.

Compared to pulse laser welding, it can fix the problems generated after welding, such as fractures, pores, spatter, etc., and ensure that the aluminum alloy has excellent mechanical residential or commercial properties after welding.

There will be no dents after welding, and the amount of grinding and polishing after welding is reduced, conserving production costs.

However, since the CW laser has a reasonably small area, the assembly precision of the workpiece is high.

Presenting Alloying Aspects

Preventing thermal fractures is one of the key technologies for laser welding of aluminum alloys.

6000 series alloys are conscious fractures. When ω( Mg2Si) =1%, hot fractures appear, it can be enhanced by adding appropriate alloying components to adjust the chemical structure of the molten pool, such as adding Al-Si or Al-Mg -Si powder has particular benefits in reducing fractures.

In addition, the welding result can be improved by wire feeding, a consistent weld joint can be obtained, and the weld seam hardness has also been enhanced.

The material of Mg and Si in the dendrite in the combination zone boosts due to the introduction of the filler material, and the β” solid option fortifying result will increase the strength of the joint.

Normally, 6063 and 6082 aluminum alloys are filled with Al-5Si, and Al-7Si welding wires, 6013 and 6056 plates, respectively, are welded with CO2 and Nd: YAG lasers Al-12Si welding wires are filled.

Other Procedure Techniques

We focus on the stability of the aluminum alloy laser welding procedure and the quality of the weld.

At present, the research study hotspot of aluminum alloy laser welding is making use of a composite process; that is, the high energy density of the laser and the larger heating variety of the arc are combined, giving complete play to the benefits of the 2 heat sources, and integrating the qualities of high energy density beam quality and steady arc, Complement each other.

For high-reflective products such as aluminum alloy, laser hybrid welding can preheat or melt the product’s surface area by arc energy, which greatly improves the absorption of laser energy by aluminum alloy.

Shida et al. used a 10 kW CO2 laser integrated with TIG and MIG arcs to weld aluminum alloys. The introduction of arcs considerably 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. It alters the flow conditions of the product in the molten state, which is advantageous to the elimination of pores.

Dual-beam welding of aluminum alloy is also a method to eliminate air holes. A 6 kW constant fiber laser was utilized to carry out 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 company.

Research study has found large holes in the welds bonded in parallel with dual beams, and welding aluminum alloys in series can achieve good weld development without pores.

Various aluminum alloys have various welding residential or commercial properties. Laser welding needs to choose the proper welding waveform. The surface needs to be polished after welding in some markets, and there will be damage after laser pulse welding. A 6 kW constant fiber laser was used to carry out dual-beam butt welding of 5052 aluminum alloy. The two-beam parallel and serial welding modes and welding at various welding speeds were studied.

Conclusion

The increasing importance of laser welding in aluminum alloy processing has made the selection of a laser welder a challenge. See our other article: How to choose a laser welder

If you need to welding and think laser welding is a good fit for you, contact our experts to discuss your requirements.

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