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What Are the Latest Technologies of Aluminum Alloy Laser Welding?

1. Preface

Aluminum alloy has a high specific strength, good corrosion resistance and a wide range of materials.

It is an excellent lightweight structural material. It is widely used in automobile, rail transit, aerospace, shipbuilding and other industries.

In recent years, as a high efficiency, low heat input, high flexibility and high quality connection technology, laser welding has attracted more and more attention and application in the domestic market.

The application and development of aluminum alloy laser welding technology are mainly affected by three factors:

(1) The development of aluminum alloy materials

The weldability of materials and the improvement of properties such as strong plasticity and corrosion resistance meeting application conditions;

(2) The maturity of laser welding process research and welding quality evaluation;

(3) Laser welding equipment, including the rapid development of the laser generator, and the diversification of laser beam output form and regulation mode.

Based on the above, according to the application needs of different industries and scenarios, the laser welding system that can be selected and matched has more flexibility and applicability in cost and process.

2. Difficulties in laser welding of aluminum alloy

There are many difficulties in laser welding of aluminum alloy affected by the properties of the matrix material.

For example, the low surface tension of liquid aluminum causes the collapse of small holes in laser deep penetration welding and the formation of pores;

The low melting point alloying elements contained in the aluminum alloy are burned in the process of laser welding, resulting in the weakening of weld performance;

The element composition of eutectic alloy with low melting point is easy to form cracks during cooling solidification;

The heat-affected zone softens under the action of welding thermal cycle;

The surface tension of liquid aluminum is low, the thermal conductivity of solid aluminum is high, and Al, Mg and other elements are easy to oxidize, resulting in poor weld surface forming.

In addition, in order to reduce the requirements of aluminum alloy laser welding on assembly clearance and inhibit the occurrence of weld cracks, laser wire filling welding or laser arc hybrid welding technology is usually used.

Since the aluminum alloy welding wire is relatively soft, it is necessary to ensure the directivity of the welding wire from the wire feeding nozzle to avoid problems such as unmelted welding wire and deviation of filled weld;

At the same time, the oxide film on the surface of the original aluminum alloy and the newly generated oxide layer of molten aluminum will affect the spreading effect of filler wire.

Fig. 1 shows some process problems existing in laser welding of aluminum alloy.

Rough surface

a) Rough surface

Serious oxidation

b) Serious oxidation

Internal pores

c) Internal pores

Longitudinal crack

d) Longitudinal crack

Poor spreading

e) Poor spreading

Unmelted welding wire

f) Unmelted welding wire

Forming bias

g) Forming bias

Fig.1 Problems in laser welding of aluminum alloy

3 . Aluminum alloy laser welding equipment

As early as the 1970s, there were reports on the research of aluminum alloy laser welding. The laser light sources used experienced CO2 laser and Nd: YAG laser.

At present, fiber laser, disk laser and semiconductor laser occupy the dominant position in the application market.

In addition to improving the laser beam quality, operation and maintenance cost, power output stability, equipment cost and lightness, according to the actual welding process requirements, laser sources with different forms of output beams such as point ring spot and energy adjustable spot have also been developed.

For example, the YLS-AMB series beam mode adjustable fiber laser of IPG company.

According to the data on its official website, the laser power distribution of the laser is shown in Fig. 2.

As can be seen from Fig. 2, the laser beam is composed of a central spot and an annular spot. The central spot has high energy density and can achieve relatively large penetration;

The annular spot has low energy density and can stabilize the molten pool and reduce spatter.

Laser products with similar functions include CSM-ARM adjustable ring mode fiber laser of coherent laser company and “bull eye” spot laser of Feibo laser company.

At the same time, there are lasers that can output compound beams of different wavelengths, such as KOMA LHYTE, which can compound fiber lasers and diode lasers;

Chuangxin laser introduced HMB multi-wavelength composite laser, which has the ability of multi-wavelength and point ring spot output.

The development and changes of the laser output beam enhance the application ability of laser welding, including inhibiting the porosity of aluminum alloy laser welding and improving the weld formation.

Center beam

a) Center beam (50μmcore diameter: up to 9kw; 100 μmcore diameter: up to 12KW)

Annular beam

b) Annular beam (outer diameter 300μmor 600μm)

Center + ring beam

c) Center + ring beam (up to 25kW)

Fig. 2 Schematic diagram of IPG YLS-AMB output beam mode

In addition, after the laser beam is transmitted to the user terminal through the optical fiber, many new changes have taken place in the way of welding with the laser beam.

In addition to the traditional ways such as double beam, laser and arc combination, there are also beam controllable laser welding joints represented by scanning galvanometer and beam swing function.

The emergence of such products makes the conventional single-beam laser form a new direction completely different from the traditional process in the laser welding process, which greatly expands the research direction and application scope of aluminum alloy laser welding.

The research on swing beam laser welding shows that the beam swing will increase the volume fraction of equiaxed crystals in the weld area of 6-series aluminum alloy, so as to enhance the toughness of 6061 aluminum alloy butt joint  and reduce the crack sensitivity of 6016 aluminum alloy lap joint.

By selecting appropriate swing beam welding parameters, the generation of pores in the welds of 5-series aluminum alloy butt joint and lap joint can be eliminated.

In addition to thin plate aluminum alloy, swing laser is used in the research of thick plate aluminum alloy laser wire filling welding to realize 130mm thick 5A06 aluminum alloy single pass 45 layer welding.

The average porosity of longitudinal weld is 1%, and there are no welding defects such as incomplete fusion and cracks.

The development of lasers, laser heads and other products has played a key role in solving the inherent problems of aluminum alloy laser welding and promoting the application of aluminum alloy laser welding technology.

4. Application and development of laser welding of aluminum alloy

Aluminum alloy laser welding is widely used in automobile, aerospace and other advanced manufacturing industries in Europe and America, such as aluminum alloy roof and side wall laser brazing, aluminum alloy door laser fusion welding, aluminum alloy T-shaped structure laser filler welding of Airbus fuselage lower wall plate and other typical applications.

Compared with the traditional aluminum alloy riveting method, laser welding has been proved to be an effective technical method in improving production efficiency, reducing production cost and reducing structural weight.

In China, with the rapid development of new energy vehicles and rail transit high-speed trains, the implementation of domestic large aircraft projects, and the mature application of laser equipment integration technology and laser welding supporting sensing and detection technology, aluminum alloy laser welding has become an application state dominated by new energy vehicle industry.

In terms of laser welding of aluminum alloy train body and aircraft wall panel, it is mainly in the stages of technology research and development, verification test and product trial production, which is still a certain distance from large-scale application.

4.1 Laser welding of aluminum alloy battery shell of new energy vehicle

In the new energy vehicle industry, due to the increased weight of battery pack, there is a higher demand for structural lightweight.

Therefore, compared with carbon fiber reinforced composites with higher cost and high-strength steel with higher density, aluminum and aluminum alloy have undoubtedly become the first materials for all kinds of battery shell structures, from cell shell and lug, module and connector, to battery tray, aluminum alloy plate profiles and cast aluminum alloys have been widely used.

Square shell cell is the most popular product for laser welding of aluminum alloy, including shell sealing, explosion-proof valve, pole column, liquid injection hole and soft connection.

The materials used include pure aluminum and 3-Series aluminum alloy, with good weldability. Especially under the use of swing laser welding process, it forms a welding joint with almost no defects and meets the sealing conditions.

The above process adopts the welding joint of conventional fiber laser and scanning galvanometer to realize high-quality and high-efficiency laser welding.

At present, complete customized laser welding production line equipment has been formed in the market.

The battery modules and battery trays of new energy vehicles are highly personalized.

They mainly use 6-series aluminum alloy with high strength, and some use 5-series aluminum alloy.

At present, MIG welding process and friction stir welding technology are mainly used.

According to different requirements and design characteristics of products, there are roughly three types.

The first is the non-load bearing module battery shell, which is characterized by the existence of aluminum alloy plates with plate thickness ≤ 1.5mm and no sealing requirements for the overall structure.

The welding is carried out in the form of lap penetration welding, butt welding and lap fillet welding.

The penetration depth and penetration width can be met by single laser or swing laser.

The requirements of such products are relatively simple, so the process is not difficult, and has been applied in production.

The technical scheme is mainly provided by laser head manufacturers and laser system integrators.

However, due to the high requirements for product assembly clearance due to single laser welding, the consistency of welding quality is greatly affected by the dimensional accuracy of incoming materials and clamping process.

The second is that the product has sealing requirements. Some requirements must bear the pressure holding conditions for a certain time.

The thickness of the plate is usually 3 ~ 5mm. It is assembled with aluminum alloy profiles, involving butt joint, corner joint, lap joint and other forms.

Because the product size is smaller than the battery tray and the service conditions are relatively low, both the manufacturer and the user intend to upgrade the welding process from MIG welding to laser welding.

At present, it is in the exploration and testing stage of the laser welding process, which is mainly implemented by scientific research institutes, laser suppliers and parts manufacturers.

The third kind is the battery tray for the product to bear external force load.

At present, it is mainly composed of the bottom plate and profile frame spliced by aluminum alloy profiles.

The profile wall thickness is about 2mm, the bottom plate splicing thickness is 5 ~ 8mm, the bottom plate and frame are MIG welded, and some products use cast aluminum alloy to obtain the integrated structure of the bottom plate and frame.

Due to the low efficiency and large deformation of MIG welding and friction stir welding, as well as the consumables cost investment of mixing head, manufacturers hope to introduce high-efficiency and high-quality laser welding technology.

However, the structure of the battery tray is relatively complex, the product design does not consider the characteristics of the laser welding process, and the bottom plate butt welding has high requirements for joint strength.

Many factors limit the application of laser welding technology.

At present, the development of relevant aluminum alloy laser welding technology is mainly carried out in scientific research institutes and some product design manufacturers.

For the second and third products, due to the large sheet thickness and the crack tendency of 6-series aluminum alloy materials, laser wire filling welding or laser arc hybrid welding process can be adopted.

In addition to the problems faced by the laser welding process itself, the detection and evaluation methods and standards for the joint quality of laser welding and the quality of laser welded products need to be developed simultaneously.

4.2 New energy vehicle body aluminum alloy laser welding

The most mature application of aluminum alloy laser welding technology in automobile body is laser brazing and door laser fusion welding.

Aluminum alloy laser brazing is mainly used for the welding of aluminum alloy roof and side wall and aluminum alloy trunk lid and is applied to joint venture brand Cadillac CT6, independent brand Weilai ES8 and other models.

Laser brazing has high requirements for the function of the laser head.

In addition to the directivity of the welding wire, it also needs to find the position during the welding process, adjust the focus and welding wire position according to the fluctuation of the sample, and monitor the surface quality of the weld after welding, so as to meet the demand for high surface quality of appearance parts such as roof and trunk lid.

Laser fusion welding of aluminum alloy doors is widely used. Scanning galvanometer welding joints are usually used, including lap penetration welding and lap fillet welding.

Due to the low surface tension of liquid aluminum and the relatively thin sheet thickness (about 1.2 mm), it is easy to have poor forming problems such as welding penetration and pricking in practical application, as shown in Fig. 3.

Based on this, improving the welding quality in mass production is an application factor to be considered in aluminum alloy laser welding.

Poor forming problems such as back collapse and surface prick of laser fusion welding weld of a certain door

Fig. Poor forming problems such as back collapse and surface prick of laser fusion welding weld of a certain door

4.3 Laser welding of aluminum alloy car body of rail transit train

In recent years, China’s rail transit manufacturing industry has developed rapidly.

With the development of high-speed trains, the materials for train body are developing towards lightweight and maintenance-free.

At present, they mainly include carbon steel, stainless steel, aluminum alloy, etc.

Among them, stainless steel laminated laser welding technology has been applied to subway production and manufacturing to replace resistance spot welding.

With regard to laser welding of carbon steel, CRRC Tangshan Locomotive cooperates with Shanghai Institute of Optics and precision machinery, Chinese Academy of Sciences to develop laser welding technology of carbon steel.

At present, technological breakthrough has been achieved in laser welding of equal thickness, unequal thickness and T-shaped joints, and trial production of side wall structural parts has been completed.

Friction stir welding technology is mainly adopted for the aluminum alloy material of car body, and the material is mainly 6-series aluminum profile.

In terms of aluminum alloy laser welding, CRRC SiFang locomotive has carried out the research and development of laser arc hybrid welding technology and sample trial production for the three major components of floor, roof and side wall in high-speed maglev long and thin-walled aluminum alloy body, as well as the composition of sandwich end plate.

The pioneering application of laser arc hybrid welding technology in 600km/h high-speed maglev train is realized.

The use of laser welding significantly improves the manufacturing accuracy of the car body, improves the production efficiency, and reduces the production cost of subsequent processing and maintenance.

It has a broad prospect of popularization and application.

4.4 Laser welding of aircraft aluminum alloy wall panel structure

Aircraft lightweight plays an important role in reducing fuel consumption, improving endurance mileage and prolonging aircraft life.

Compared with titanium alloy and carbon fiber composites, the cost of aluminum alloy is relatively low.

Therefore, aluminum alloy accounts for a large proportion in aircraft fuselage manufacturing, mainly 7-series, 6-series and 2-Series aluminum alloys.

In the connection of fuselage panel skin and stringer, the traditional method uses riveting technology, and the skin and stringer adopt overlapping structure.

Due to the extra weight generated by the overlapping edge of rivet and stringer and low production efficiency, the stringer and skin are changed into T-shaped structure, and laser wire filling welding is carried out simultaneously on the left and right sides to replace the overlapping edge and rivet, which has an obvious effect on reducing the body weight, improving the connection efficiency and reducing the manufacturing cost.

For example, the 8 wall panels of Airbus A380 are manufactured by bilateral laser synchronous welding technology, reducing the weight of the fuselage by 10%.

In the current production and application, the material for laser welding is mainly 6-series aluminum alloy.

In the application research of aluminum alloy T-shaped structure in China, the research on double-sided laser welding process of aluminum-lithium alloy with great application prospect has been focused, and the sample trial production has been carried out.

However, there are some key problems affecting the performance of Al-Li alloy laser welding, such as joint softening, corrosion, weld porosity and cracks, which need to be solved.

5. Conclusion

The application and development of aluminum alloy laser welding technology depend on the innovation of aluminum alloy materials, laser welding process and welding equipment.

It is usually necessary to carry out special process R & D, performance evaluation and equipment construction according to the actual application characteristics, especially for large-scale structural parts such as rail trains and aircraft.

It takes years or even more from technical research to production and application.

At present, aluminum alloy laser welding technology is mainly used in the case of relatively good weldability and relatively simple service conditions.

Laser welding of new Al-Li alloy and high-strength 7-series aluminum alloy for aviation is facing more complex welding metallurgical problems.

Laser welding of aluminum alloy thick plate structure in shipbuilding, pressure vessel and other industries needs to solve the problems of process and equipment.

How to realize high-performance aluminum alloy laser welding with relatively poor weldability and high-efficiency laser welding of aluminum alloy thick plate complex structure.

It is the development direction of aluminum alloy laser welding technology.

The localization of welding equipment, the stability of products and its adaptability in welding application are another development direction of aluminum alloy laser welding technology.

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