Laser welding of aluminum and steel: breaking through the "industrial forbidden zone" of dissimilar material connections

In the field of industrial manufacturing, the connection between aluminum and steel was once considered a "forbidden zone". The physical and chemical properties of these two metals differ greatly - the melting point of aluminum is only about 660 ℃, while that of steel is as high as about 1538 ℃; The thermal expansion coefficient of aluminum is nearly twice that of steel; More importantly, aluminum and steel are prone to forming hard and brittle intermetallic compounds during the welding process, leading to a significant decrease in joint strength.

However, with the development of laser technology, this "industrial forbidden zone" is gradually being conquered. This article will take you to a deeper understanding of the technical principles, challenges, and innovative solutions of laser welding between aluminum and steel

Technical Challenge: Why are aluminum steel welding so difficult?

The welding of aluminum and steel is essentially a technological competition against the differences in material properties. When aluminum and steel meet in a molten pool, aluminum atoms and iron atoms diffuse into each other, forming a series of Fe Al intermetallic compounds such as FeAl, Fe3Al, FeAl2, etc.

These compounds have extremely high hardness, some even reaching HV 810, but they are also very brittle. When these brittle phases form a continuous layer at the joint interface, the toughness and strength of the joint will sharply decrease.

More complicated is that there is a huge difference in thermal conductivity between aluminum and steel. The thermal conductivity of aluminum is as high as 237 W/m · K, while that of steel is only around 50 W/m · K. This means that during the welding process, heat will be unevenly dispersed, leading to difficulties in forming the weld seam.

In addition, the dense oxide film (Al2O3) on the surface of aluminum has a melting point as high as 2050 ℃, far higher than the melting point of aluminum itself, which can lead to defects such as lack of fusion and porosity during the welding process.

01 Laser technology, innovative solution for dissimilar metal welding

Faced with the challenges of aluminum steel welding, traditional welding methods are often inadequate, while laser technology, with its high energy density, precise control capability, and low heat input characteristics, provides various innovative solutions.

Laser fusion brazing technology is one of the effective methods. This technology causes the aluminum side to melt, while the steel side does not melt. The melted aluminum brazing material wets and spreads on the surface of the steel, forming a metallurgical bond.

By precisely controlling the heat input, the thickness of the intermetallic compound layer can be controlled within the range of 1.5-4 μ m. Studies have shown that the compound layer within this thickness range has little effect on the joint performance.

Composite heat source welding is another breakthrough solution. Combining laser with pulsed MIG arc, utilizing the synergistic effect of laser stabilized arc and arc enhanced absorption, to achieve high-quality welding.

The maximum speed of welding aluminum/coated steel with a large spot Nd: YAG laser pulsed MIG composite heat source can reach 5m/min, and the tensile strength of the joint can reach 72% -75.7% of the strength of the aluminum base material.

Laser deep melting point welding achieves deep melting welding through keyhole effect. Orthogonal experimental optimization shows that laser power has the greatest impact on the mechanical properties of the joint, and the optimal parameters can achieve a tensile load of 1470 N.

02 Process Innovation, from Microscopic Texture to Intelligent Control

In recent years, there have been numerous technological innovations in the field of aluminum steel laser welding, from material surface treatment to intelligent control systems, and multiple technological breakthroughs have jointly promoted the development of this field.

Surface microtexture technology enhances the spreading ability of molten aluminum by constructing microstructures on the steel surface, reducing the wetting angle to within 15 ° and increasing the wetting area to 2.8 times that of traditional processes.

The application of intermediate layer materials is an effective strategy for controlling intermetallic compounds. Researchers used the KAlF4+Sn+Zn formula flux to increase the joint strength to 83.6% of the aluminum base material, which is equivalent to the strength of conventional arc welded joints.

Adding intermediate layers such as Ni foil can also alter the interfacial reaction process and suppress the formation of brittle phases.

Optimizing process parameters is equally crucial. Taking the research of Shanghai University of Engineering and Technology as an example, they optimized the parameters to achieve a tensile shear strength of 98.86 MPa for the joint between 316L stainless steel and 6061 aluminum alloy.

The precise adjustment of laser power, pulse width, frequency, and welding speed has a decisive impact on the quality of the weld seam.

The intelligent real-time control system represents the future direction. The integration of machine learning algorithms to dynamically adjust welding parameters and achieve precise control of intermetallic compound thickness has become a focus of cutting-edge research.

03 Technology Application, Moving from Laboratory to Industrialization

Aluminum steel laser welding technology is rapidly moving from laboratory research to industrial applications, especially in the field of new energy vehicles, demonstrating enormous market potential.

The laser welding technology for aluminum silicon coated hot formed steel developed by Shanghai Institute of Optics and Fine Mechanics has developed a low-cost high-strength steel welding wire through innovative dilution rate control alloying strategy, and achieved a one-step direct laser wire filling welding process.

This technology has been tested in steel mills such as Baowu Group and Ansteel, and the first demonstration production line is planned to be built in the Yangtze River Delta region by the end of 2025.

The aluminum/steel heterogeneous joint technology developed by the Zhengzhou Research Institute of Harbin Institute of Technology has achieved a joint strength of over 90% of the base material, and has been applied to components such as Jiangling Motors door panels, Kunshan Baojin laser welded aluminum/steel heterogeneous joint panels, and new energy vehicle front windshield energy absorbing boxes.

Laser bonding technology combines the advantages of laser welding and bonding, and lap welds 1.4mm thick galvanized steel and 1.2mm thick 6016 aluminum alloy to obtain high-quality joints with a shear strength of 41.45 MPa.

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