Aluminum alloys possess a high specific strength and good corrosion resistance, making them a versatile material for various applications. They are well known for their lightweight properties, making them an ideal choice for the structural components in several industries, including automobiles, rail transit, aerospace, and shipbuilding.
Recently, laser welding has gained significant attention and usage in the domestic market for its high efficiency, low heat input, high flexibility, and high-quality connection technology.
The application and development of aluminum alloy laser welding technology are mainly affected by three factors:
(1) Development of Aluminum Alloy Materials:
The weldability of materials has improved, leading to stronger plasticity and enhanced corrosion resistance to meet application requirements.
(2) Maturity of Laser Welding Process Research and Welding Quality Evaluation:
Research into laser welding processes and welding quality evaluations have become more advanced.
(3) Laser Welding Equipment:
The rapid advancement in laser generator technology, as well as the diversity of laser beam output forms and regulation modes, has resulted in a more flexible and applicable laser welding system, allowing for a greater selection and matching to the specific needs of various industries and scenarios in terms of cost and process.
2. Difficulties in laser welding of aluminum alloy
Laser welding of aluminum alloy poses many challenges due to the properties of the matrix material.
For instance, the low surface tension of liquid aluminum causes small holes to collapse during laser deep penetration welding and forms pores. Additionally, the low melting point alloying elements in the aluminum alloy tend to burn during laser welding, weakening the weld performance.
Moreover, the element composition of eutectic alloy with a low melting point can result in crack formation during cooling solidification, and the heat-affected zone softens due to the welding thermal cycle.
Furthermore, the low surface tension of liquid aluminum, high thermal conductivity of solid aluminum, and ease of oxidation of elements such as Al, Mg can result in poor weld surface forming.
To reduce the requirements of aluminum alloy laser welding on assembly clearance and prevent the occurrence of weld cracks, laser wire filling welding or laser arc hybrid welding technology is commonly used.
However, as the aluminum alloy welding wire is relatively soft, it is crucial to ensure the welding wire’s directivity from the wire feeding nozzle to prevent problems like unmelted welding wire and deviation of filled weld.
Simultaneously, the oxide film on the surface of the original aluminum alloy and the newly generated oxide layer of molten aluminum can affect the spreading effect of filler wire.
Figure 1 illustrates some of the process problems associated with laser welding of aluminum alloy.
a) Rough surface
b) Serious oxidation
c) Internal pores
d) Longitudinal crack
e) Poor spreading
f) Unmelted welding wire
g) Forming bias
Fig.1 Problems in laser welding of aluminum alloy
3 . Aluminum alloy laser welding equipment
In the 1970s, research reports on aluminum alloy laser welding were already available, with CO2 and Nd: YAG lasers being used as light sources.
Today, fiber lasers, disk lasers, and semiconductor lasers are dominating the market. To meet the actual welding process requirements, laser sources with different forms of output beams such as point ring spot and energy adjustable spot have been developed, along with improvements in laser beam quality, operation and maintenance cost, power output stability, equipment cost, and portability.
For example, IPG company’s YLS-AMB series beam mode adjustable fiber laser features a laser power distribution shown in Fig. 2 on its official website. As shown, the laser beam comprises a central spot and an annular spot. The central spot has high energy density and can achieve relatively large penetration, while the annular spot has low energy density and can stabilize the molten pool and reduce spatter.
Coherent Laser Company’s CSM-ARM adjustable ring mode fiber laser and Feibo Laser Company’s “bull eye” spot laser are among laser products with similar functions.
Some lasers can output compound beams of different wavelengths, such as KOMA LHYTE, which can compound fiber lasers and diode lasers, and Chuangxin Laser’s HMB multi-wavelength composite laser, which has the ability of multi-wavelength and point ring spot output.
The development and changes in laser output beam have enhanced the application ability of laser welding, including inhibiting the porosity of aluminum alloy laser welding and improving the weld formation.
a) Center beam (50μmcore diameter: up to 9kw; 100 μmcore diameter: up to 12KW)
b) Annular beam (outer diameter 300μmor 600μm)
c) Center + ring beam (up to 25kW)
Fig. 2 Schematic diagram of IPG YLS-AMB output beam mode
Moreover, several new changes have occurred in the way of welding with laser beams after they are transmitted to the user terminal via optical fibers.
Alongside the traditional methods like double-beam and laser and arc combination, beam-controllable laser welding joints have emerged, represented by scanning galvanometer and beam swing function.
The introduction of such products has completely altered the direction of conventional single-beam laser welding, creating a novel approach to the process. This expansion of research direction and application scope has greatly advanced aluminum alloy laser welding.
Research on swing-beam laser welding has demonstrated that the beam swing increases the volume fraction of equiaxed crystals in the weld area of 6-series aluminum alloy, enhancing the toughness of 6061 aluminum alloy butt joints and reducing the crack sensitivity of 6016 aluminum alloy lap joints.
By selecting appropriate swing-beam welding parameters, it is possible to eliminate the generation of pores in the welds of 5-series aluminum alloy butt and lap joints.
Moreover, swing laser is employed in the study of thick plate aluminum alloy laser wire filling welding, achieving 130mm thick 5A06 aluminum alloy single-pass 45-layer welding.
The average porosity of the longitudinal weld is 1%, and there are no welding defects such as incomplete fusion and cracks.
The development of lasers, laser heads, and other related products has played a pivotal role in resolving the inherent issues of aluminum alloy laser welding and facilitating the application of this technology.
4. Application and development of laser welding of aluminum alloy
Aluminum alloy laser welding is widely used in advanced manufacturing industries such as automobiles and aerospace in Europe and America. Typical applications include aluminum alloy roof and side wall laser brazing, aluminum alloy door laser fusion welding, and aluminum alloy T-shaped structure laser filler welding of Airbus fuselage lower wall plate.
Compared to the traditional aluminum alloy riveting method, laser welding has proven to be an effective technique in improving production efficiency, reducing production costs, and decreasing structural weight.
In China, aluminum alloy laser welding has become an application state dominated by the new energy vehicle industry due to the rapid development of new energy vehicles, rail transit high-speed trains, and the implementation of domestic large aircraft projects. This is due to the mature application of laser equipment integration technology and laser welding supporting sensing and detection technology.
Regarding the laser welding of aluminum alloy train bodies and aircraft wall panels, it is currently in the stages of technology research and development, verification testing, and product trial production. It is still some distance away from large-scale application.
4.1 Laser welding of aluminum alloy battery shell of new energy vehicle
In the new energy vehicle industry, the increased weight of battery packs has resulted in a higher demand for lightweight structures. As a result, aluminum and aluminum alloys have become the preferred materials for all types of battery shell structures, from cell shells and lugs, to modules, connectors, and battery trays. Carbon fiber reinforced composites are costlier, and high-strength steel is denser, making aluminum a more viable option.
The most popular product for laser welding of aluminum alloy is the square shell cell, which includes shell sealing, explosion-proof valves, pole columns, liquid injection holes, and soft connections. The materials used for this process are pure aluminum and 3-Series aluminum alloy, which have excellent weldability. The swing laser welding process ensures that the welding joint is almost free of defects and meets sealing conditions.
To achieve high-quality and high-efficiency laser welding, this process uses the welding joint of conventional fiber lasers and scanning galvanometers.
Currently, a complete range of custom laser welding production line equipment is available in the market. The battery modules and battery trays used in new energy vehicles are highly personalized, mostly employing 6-series aluminum alloy for its high strength, while some use 5-series aluminum alloy. The most commonly used welding processes are MIG welding and friction stir welding technology. There are roughly three types of products, depending on their requirements and design characteristics.
The first type is the non-load bearing module battery shell. It is characterized by the use of aluminum alloy plates with a thickness ≤ 1.5mm, and the overall structure does not require sealing. The welding process used includes lap penetration welding, butt welding, and lap fillet welding.
The depth and width of penetration can be achieved using either a single laser or a swing laser. The requirements for such products are relatively simple, and therefore the process is not difficult to implement. This technique has been applied in production.
The technical scheme is primarily provided by laser head manufacturers and laser system integrators. However, the use of a single laser welding process can result in inconsistencies in the welding quality due to high requirements for product assembly clearance, which in turn greatly affects the dimensional accuracy of the incoming materials and clamping process.
Secondly, the product has sealing requirements, and some of these requirements must withstand pressure-holding conditions for a certain period. The plate’s thickness is typically between 3 to 5mm, and it is assembled with aluminum alloy profiles, involving butt joint, corner joint, lap joint, and other forms.
Since the product size is smaller than the battery tray, and the service conditions are relatively low, both the manufacturer and the user are planning to upgrade the welding process from MIG welding to laser welding.
Currently, the laser welding process is being explored and tested, primarily by scientific research institutes, laser suppliers, and parts manufacturers.
The third type of component is the battery tray, which is designed to bear external force loads.
Currently, it is mainly composed of an aluminum alloy profile frame spliced together with a bottom plate. The profile wall thickness is about 2mm, and the bottom plate splicing thickness ranges from 5 to 8mm. The bottom plate and frame are connected by MIG welding, and some products use cast aluminum alloy to achieve an integrated structure of the bottom plate and frame.
However, MIG welding and friction stir welding have low efficiency, cause large deformations, and require costly consumables, which is why manufacturers are looking to introduce high-efficiency and high-quality laser welding technology.
Despite this, the battery tray structure is relatively complex, and the product design did not consider the characteristics of the laser welding process. The butt welding of the bottom plate requires a high joint strength, which poses a significant challenge.
Many factors limit the application of laser welding technology in this context.
Currently, the development of aluminum alloy laser welding technology is primarily taking place in scientific research institutes and some product design manufacturers.
For products two and three, due to the thick sheet and 6-series aluminum alloy materials’ tendency to crack, laser wire filling welding or laser arc hybrid welding processes can be utilized.
Along with addressing the challenges of the laser welding process itself, it is also essential to concurrently develop methods for detecting and evaluating joint quality, as well as establishing standards for the quality of laser welded products.
4.2 New energy vehicle body aluminum alloy laser welding
The most advanced application of aluminum alloy laser welding technology in the automobile body is laser brazing and door laser fusion welding.
Aluminum alloy laser brazing is mainly used for welding aluminum alloy roofs, side walls, and trunks. This technique has been applied to models such as the joint venture brand Cadillac CT6, independent brand Weilai ES8, and others.
Laser brazing has strict requirements for the laser head’s function. In addition to the welding wire’s directivity, it also needs to locate the position during the welding process, adjust the focus and welding wire position based on the sample’s fluctuations, and monitor the surface quality of the weld after welding. All these measures are necessary to meet the high surface quality demands for appearance parts, such as roofs and trunk lids.
Laser fusion welding of aluminum alloy doors is widely employed. Scanning galvanometer welding joints are typically used, including lap penetration welding and lap fillet welding.
Due to the low surface tension of liquid aluminum and relatively thin sheet thickness (about 1.2 mm), welding penetration and pricking may occur during practical application, as shown in Figure 3.
Thus, improving the welding quality in mass production is an essential consideration in the aluminum alloy laser welding application.
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 undergone rapid development. The advancement of high-speed trains has led to the use of lightweight and maintenance-free materials in train body construction. Currently, the primary materials used are carbon steel, stainless steel, and aluminum alloy.
Stainless steel laminated laser welding technology has replaced resistance spot welding in subway production and manufacturing. CRRC Tangshan Locomotive is working with the Shanghai Institute of Optics and Precision Machinery, Chinese Academy of Sciences to develop laser welding technology for carbon steel. They have already achieved technological breakthroughs in laser welding for equal thickness, unequal thickness, and T-shaped joints. Trial production of side wall structural parts has also been completed.
Friction stir welding technology is mainly used for the aluminum alloy material of car bodies, specifically 6-series aluminum profiles. CRRC SiFang locomotive is researching and developing laser arc hybrid welding technology for the three primary components of floor, roof, and side walls in high-speed maglev trains’ long and thin-walled aluminum alloy bodies. They have also researched and developed the composition of sandwich end plates.
The use of laser arc hybrid welding technology in 600km/h high-speed maglev trains is a pioneering application. The use of laser welding significantly improves the manufacturing accuracy of the car body, production efficiency, and reduces the cost of subsequent processing and maintenance. Laser welding has broad prospects for popularization and application.
4.4 Laser welding of aircraft aluminum alloy wall panel structure
Lightweight aircraft play a crucial role in reducing fuel consumption, improving endurance mileage, and extending the life of aircraft. Compared to titanium alloy and carbon fiber composites, aluminum alloy is relatively inexpensive.
Therefore, aluminum alloy accounts for a significant proportion of aircraft fuselage manufacturing, particularly the 7-series, 6-series, and 2-series aluminum alloys.
Traditionally, riveting technology is used to connect fuselage panel skin and stringer, and the skin and stringer adopt an overlapping structure. However, this method generates additional weight due to the overlapping edge of the rivet and stringer and has low production efficiency.
To address this, the T-shaped structure is adopted for the stringer and skin, and laser wire filling welding is carried out simultaneously on the left and right sides to replace the overlapping edge and rivet. This method has an apparent effect on reducing body weight, improving connection efficiency, and reducing manufacturing costs.
For instance, the eight wall panels of Airbus A380 are manufactured using bilateral laser synchronous welding technology, which reduces the fuselage’s weight by 10%. Currently, the material used for laser welding is mainly the 6-series aluminum alloy.
In China, research on the application of aluminum alloy T-shaped structure has focused on the double-sided laser welding process of aluminum-lithium alloy, which has great application prospects, and sample trial production has been carried out. However, some key problems affect the performance of Al-Li alloy laser welding, such as joint softening, corrosion, weld porosity, and cracks, which need to be addressed.
The application and development of aluminum alloy laser welding technology rely on innovation in aluminum alloy materials, laser welding processes, and welding equipment.
Special process R&D, performance evaluation, and equipment construction are usually necessary, particularly for large-scale structural parts such as rail trains and aircraft, according to their actual application characteristics.
It can take years or even longer to go from technical research to production and application.
Currently, aluminum alloy laser welding technology is primarily used in cases where weldability is relatively good and service conditions are relatively simple.
However, laser welding of new Al-Li alloy and high-strength 7-series aluminum alloy for aviation is facing more complex welding metallurgical problems.
Furthermore, laser welding of aluminum alloy thick plate structures in shipbuilding, pressure vessels, and other industries needs to address process and equipment issues.
Developing high-performance aluminum alloy laser welding with relatively poor weldability and high-efficiency laser welding of aluminum alloy thick plate complex structures is the direction of the technology’s development.
Moreover, localizing welding equipment, improving product stability, and enhancing its adaptability in welding applications are other development directions of aluminum alloy laser welding technology.