Can titanium be welded to stainless steel?

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In the field of metal welding, the welding of titanium and stainless steel has always been a challenging problem. However, after extensive practice and research, we have found that titanium can be welded onto stainless steel. It's just that this process requires a special technique to overcome the many challenges brought about by the differences in physical and chemical properties between the two metals. The following is a specific analysis of this issue and a detailed elaboration of some feasible welding methods:

I. Welding Difficulties

Formation of intermetallic compounds

When titanium is welded to stainless steel, the parts where they come into contact will undergo mutual diffusion and reaction of elements. The elements such as iron (Fe), chromium (Cr), and nickel (Ni) contained in stainless steel can undergo chemical reactions with titanium, easily forming brittle compounds, such as FeTi, Fe₂Ti, and Cr₂Ti. The formation of these brittle compounds will have an extremely adverse effect on the performance of the welded joint, significantly reducing the strength and toughness of the joint, making the welded area prone to fracture when subjected to external forces, and seriously affecting the welding quality.

Physical performance differences

In terms of physical properties, there are obvious differences between titanium and stainless steel. The melting point of titanium is relatively high, approximately 1668℃, while that of stainless steel is between about 1400 and 1450℃. This difference in melting points makes it difficult to control the heat input for both to reach the appropriate melting state simultaneously during the welding process. In addition, titanium has a relatively low thermal conductivity, which means that during welding, the heat conduction speed on the titanium side is slower, easily leading to uneven heat input. This uneven heat input will generate residual stress at the welding site and also cause the workpiece to deform, further affecting the accuracy and quality of the welding.

Oxidation sensitivity

Titanium, as a metal, is highly sensitive to oxidation in high-temperature environments. It is prone to chemical reactions with elements such as oxygen and nitrogen in the air, generating oxides and nitrides. The formation of these oxides and nitrides not only alters the chemical composition and properties of titanium, but also has a negative impact on the quality of welded joints. Therefore, when welding titanium with stainless steel, it is essential to strictly protect the welding area to prevent titanium from reacting with external elements at high temperatures.

Ii. Feasible welding methods

Diffusion welding

Diffusion welding is a welding method in which atoms at the welding interface diffuse under the action of high temperature and pressure to achieve connection. For the welding of titanium and stainless steel, there are strict requirements for the process parameters. Generally speaking, the temperature needs to be controlled at around 900±50℃, the duration should be maintained at 30 to 60 minutes, and the pressure should be approximately 5MPa. Under such process parameters, the thickness of interfacial pores and brittle phases can be effectively reduced, thereby improving the quality of welded joints.

To further improve the welding effect, intermediate layer materials are usually used. For instance, Ni, Cu, Ag, etc. are selected as intermediate layers. These intermediate layer materials can prevent the direct contact between iron (Fe) and titanium (Ti), thus avoiding the formation of excessive brittle compounds. Meanwhile, they can form phases with good toughness with titanium and stainless steel respectively, such as Ni-Ti or Cu-Ti phases. Taking silver (Ag) as the intermediate layer as an example, in the actual welding test, the shear strength of the welded joint using this intermediate layer can reach 410MPa, demonstrating a good welding effect.

Electron beam welding

Electron beam welding is a welding method carried out in a vacuum environment. It precisely heats the welding area through a high-energy-density electron beam and can well control the state of the molten pool. When welding titanium and stainless steel, the combined use of the Cu-V composite intermediate layer can effectively suppress the formation of brittle phases and improve the strength and toughness of the joint. The advantage of this welding method lies in its concentrated energy, fast welding speed, and the ability to reduce some adverse effects during the welding process. However, it also has relatively high requirements for equipment and technology.

Laser welding

Laser welding is characterized by fast heating and cooling rates. This feature enables it to reduce the formation of brittle phases when welding titanium and stainless steel. In practical operation, direct welding can be achieved by adopting the beam bias technique or using the Cu intermediate layer. However, it should be noted that the strength of the joint welded by laser varies relatively greatly, which may be related to factors such as the energy distribution of the laser and the stability of the welding parameters. Therefore, when using laser welding to weld titanium and stainless steel, it is necessary to precisely control and optimize the welding parameters.

Vacuum brazing

Vacuum brazing is a welding method that achieves connection by filling the weld seam with filler metal in a vacuum environment. For the welding of titanium and stainless steel, titanium-zirconium-copper-nickel or silver-based brazing filler metals are usually used. Welding in a vacuum environment can prevent titanium from undergoing oxidation reactions with elements such as oxygen in the air at high temperatures, ensuring the quality of the welded area. This method is particularly suitable for the welding of thin-walled or precision components. For example, when welding with Ag-Cu-Zn brazing filler metal, the shear strength of the joint can reach 100-254MPa, meeting the requirements of many practical applications.

Explosive welding

Explosive welding is a welding method that uses the impact force generated by the explosion of explosives to achieve the connection of metals. It is applicable to the manufacturing of large-area composite panels, such as titanium-steel composite panels. During the process of explosive welding, by reasonably controlling the amount of explosives and the explosion conditions, titanium and stainless steel can be closely combined under the instantaneous high pressure. However, this method also has certain limitations. It is restricted by the shape of the workpiece and may not be very applicable to some workpieces with complex shapes. Moreover, after explosive welding, subsequent heat treatment is usually required to further enhance the performance of the welded joint. During the heat treatment process, it is necessary to strictly control the temperature between 500 and 550℃ to prevent the occurrence of de-welding due to excessive temperature.

Iii. Key Control Measures

Intermediate layer selection

In the welding of titanium and stainless steel, the selection of the intermediate layer is of vital importance. Intermediate layer materials such as Ni, Cu and Ag can effectively improve the interfacial reaction conditions and reduce the proportion of brittle phases. Different intermediate layer materials have their own characteristics and application scopes, and a reasonable selection should be made based on specific welding requirements and material properties. For example, in some occasions with high requirements for corrosion resistance, the Ni intermediate layer may be given priority. In some occasions where conductivity is required, the Ag intermediate layer may be a better choice.

Protective gas

To prevent titanium from undergoing oxidation reactions at high temperatures, high-purity protective gases need to be used. Generally speaking, high-purity argon (99.99%) is a commonly used protective gas. During the welding process, by introducing high-purity argon gas into the welding area, it can isolate the welding area from the air, preventing titanium from coming into contact with elements such as oxygen and nitrogen, thereby ensuring the quality of the welding part.

Parameter optimization

Controlling the heat input and cooling rate is the key in the welding process of titanium and stainless steel. An appropriate heat input can ensure that both metals reach a good melting state without generating excessive heat, thus avoiding deformation of the workpiece and an increase in residual stress. A reasonable cooling rate can reduce the formation of brittle phases and improve the performance of welded joints. Therefore, in the actual welding process, it is necessary to optimize and adjust the welding parameters based on factors such as the welding method, material thickness, and workpiece shape.

Iv. Application Fields

This type of welding technology has extensive applications in actual production and life. In the field of chemical equipment, due to titanium's excellent corrosion resistance and stainless steel's good mechanical properties and economy, welding the two together can fully leverage their respective advantages, enhancing the service life and performance of chemical equipment. In aerospace structures, the welding of titanium and stainless steel can be used to manufacture some key components, meeting the requirements of the aerospace field for high strength, lightweight and corrosion resistance of materials. In fields such as nuclear industry components and shipbuilding, this welding technology also plays a significant role, providing strong support for the development of related industries.

To sum up

The welding of titanium and stainless steel requires the selection of appropriate welding methods based on specific application scenarios, and the problem of interfacial brittleness can be solved by reasonably choosing the intermediate layer and optimizing process parameters. In actual production practice, diffusion welding and vacuum brazing are relatively widely applied due to their high controllability. However, no matter which welding method is adopted, it is necessary to strictly follow the relevant process requirements and operating procedures to ensure the welding quality.

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