I. Problems with welding dissimilar metals
Problems inherent to welding dissimilar metals have impeded its development, such as the constitution and performance of the fusion zone in dissimilar metals.
Damage to the structure from welding dissimilar metals often occurs in the fusion zone, as the characteristics of the welding crystal differ along the segments near the fusion zone, leading to the formation of a transition layer with poor performance and compositional changes.
Furthermore, due to prolonged exposure to high temperatures, the diffusion layer in this region will expand, further increasing the heterogeneity of the metal.
Furthermore, during or after welding dissimilar metals, or after heat treatment or high-temperature operation, it is common to observe the phenomenon of carbon from the low-alloy side “migrating” across the weld boundary into the high-alloy weld, forming layers of decarburization and carburization on both sides of the fusion line.
This results in a layer of decarburization on the original low-alloy material and a layer of carburization on the high-alloy weld side.
The impediment and prevention of the use and development of dissimilar metallic structures are mainly manifested in the following areas:
1. At room temperature, the mechanical properties of the dissimilar metal welded joint area (such as tensile, impact, bending, etc.) generally surpass the performance of the original welded material.
However, after prolonged operation at high temperature, the performance of the joint area is inferior to that of the original material.
2. There is a martensitic transition zone between the austenitic weld and the pearlitic parent material.
This zone has lower toughness and is a brittle layer of high hardness, which is a weak area that leads to component failure. Reduces the reliability of the welded structure.
3. Post-weld heat treatment or carbon migration during high-temperature operation may result in the formation of a carburized layer and a decarburized layer on both sides of the fusion line.
It is typically believed that the decarburized layer, due to carbon reduction, causes significant changes to the structure and properties of the area (usually degradation), making it prone to premature failure during service.
The failure sites of many high-temperature pipelines in service or under test are concentrated in the decarburized layer.
4. Failure is related to conditions such as time, temperature and cyclical stress.
5. Post-welding heat treatment cannot eliminate the residual stress distribution in the joint area.
6. Inhomogeneity of chemical composition.
When welding dissimilar metals, the metals on both sides of the weld seam and the alloy composition of the weld seam differ significantly.
During the welding process, both the original material and the welding material will melt and mix.
This level of mixture uniformity changes with the welding process, and the degree of mixture uniformity can vary greatly at different locations of the welded joint, leading to a lack of homogeneity in the chemical composition of the welded joint.
7. Inhomogeneity of the metallographic structure.
Due to the discontinuity of the chemical composition of the welded joint and the experience of the welding thermal cycle, different structures appear in different areas of the welded joint, often resulting in extremely complex structural formations in some areas.
8. Discontinuous performance.
The chemical composition and metallurgical structure of welded joints cause variations in their mechanical properties.
The strength, hardness, plasticity, toughness, impact resistance, high temperature creep and lasting performance differ significantly in various regions of the welded joint.
This substantial inconsistency causes different behaviors in different regions of the joint under identical conditions, manifesting as areas of weakening and strengthening.
Particularly under high temperature conditions, welded joints of dissimilar metals often fail prematurely during service.
II. Features of welding different metals using different methods
Most welding methods can be applied to welding dissimilar metals, but when choosing a welding method and establishing process measurements, the characteristics of welding dissimilar metals must be taken into account.
Based on the requirements of the base material and the welded joint, fusion welding, pressure welding and other methods have found applications in welding different metals, each with its own advantages and disadvantages.
1. Fusion welding
Fusion welding is widely used in welding dissimilar metals.
Common fusion welding methods include coated electrode welding, submerged arc welding, gas shielded arc welding, electroslag welding, plasma arc welding, electron beam welding, and laser welding.
To reduce dilution, slow down the melting rate, or control the amount of melting of different base metals, methods with higher energy density of the heat source, such as electron beam welding, laser welding, or plasma arc welding, are normally chosen.
To minimize the melting depth, technological measures such as indirect arc, oscillating welding wire, band electrode and additional non-electrified welding wire can be adopted.
However, in any case, as long as it is fusion welding, part of the base material will always melt at the weld seam, causing dilution.
In addition, it will also form intermetallic compounds, eutectic structures, etc.
To alleviate these adverse effects, it is imperative to control and reduce the residence time of metals in the liquid or solid state at high temperatures.
However, despite continuous improvements and advancements in fusion welding methods and procedures, it remains a challenge to solve all the problems associated with welding different types of metals.
Given the diversity of metals and the wide range of performance requirements, along with varying joint styles, in many cases, pressure welding or other welding methods must be employed to solve specific welding problems related to different metal joints.
2. Pressure welding
Most pressure welding methods only heat the metals to be welded to a plastic state or not at all, characterized mainly by the application of a certain pressure.
Compared with fusion welding, pressure welding has certain advantages when welding dissimilar metal joints, as long as the shape of the joint allows it and the welding quality meets the requirements, pressure welding is often a more reasonable choice.
During pressure welding, the joining surface of different metals may be melted or remain solid, but due to the effect of pressure, even if there is molten metal on the surface, it will be squeezed out (such as in flash welding and friction welding).
Only in some cases does the metal that was molten remain after welding under pressure (as in spot welding).
Pressure welding, due to lack of heat or low heating temperature, can mitigate or avoid the adverse effects of thermal cycling on the properties of the parent metal and prevent the formation of brittle intermetallic compounds.
Some forms of pressure welding can even squeeze out intermetallic compounds that have formed in the joint.
Furthermore, no dilution-related changes in weld metal properties occur during pressure welding.
However, most pressure welding methods have certain requirements for joint shapes.
For example, spot welding, seam welding and ultrasonic welding must use lap joints; at least one part must have a rotational cross-section in friction welding; explosive welding is only applicable to larger area connections.
Pressure welding equipment is also not yet widespread. These factors undoubtedly limit the application range of pressure welding.
3. Other methods
In addition to fusion welding and pressure welding, there are several other methods for welding dissimilar metals. For example, brazing is a method that uses a filler metal to join different base metals together.
However, the focus here is on a special type of brazing method.
One such technique is known as fusion brazing, where the material with a lower melting point in a different metal joint is subjected to fusion welding and the material with a higher melting point is subjected to brazing. The filler metal normally corresponds to the base metal with a low melting point.
As such, the process between the filler metal and the low-melting base metal is essentially a fusion welding process of the same metal and does not present any unique challenges.
The interaction between the filler metal and the high-melting base metal is a brazing process. The base metal does not melt or crystallize, avoiding many welding-related problems.
However, this requires the filler metal to wet the base metal effectively.
Another technique is known as eutectic brazing or eutectic diffusion brazing. This method involves heating the contact surface of dissimilar metals to a certain temperature which forms a low melting point eutectic at the point of contact.
This low-melting eutectic becomes liquid at this temperature, essentially creating a brazing method that requires no additional filler metal.
Of course, this requires that the two metals form a low-melting eutectic.
During diffusion welding of dissimilar metals, an intermediate layer is introduced and under low pressure, the intermediate layer melts or forms a low melting point eutectic upon contact with the metals to be welded.
This thin layer of liquid, after a specific period of heat preservation, allows the interlayer to diffuse entirely into the base metal and become uniform, resulting in a different metal joint without an interlayer.
These methods usually involve a small amount of liquid metal during the welding process, which is why they are also called liquid phase transition welding. Their common feature is the absence of fused structures at the joint.
III. Considerations for Welding Dissimilar Metals
1. Consider the physical, mechanical properties and chemical composition of welds.
The. From the perspective of equal strength, select welding rods that satisfy the strength of the base metal.
Alternatively, considering the weldability of the base metal, choose welding rods that do not have the same strength, but that offer good weldability.
However, the weld structure must be considered to meet the requirements of equal strength and equal stiffness.
B. Make sure the alloy composition matches or approaches that of the base metal.
w. When the base metal contains a greater amount of harmful impurities such as carbon (C), sulfur (S) and phosphorus (P), choose welding rods with superior crack resistance and porosity resistance. It is suggested to use titanium-calcium type welding rods. If this does not solve the problem, low hydrogen sodium type welding rods can be used.
2. Consider working conditions and welding performance requirements.
The. When subjected to dynamic loads and impact loads, in addition to ensuring resistance, high impact toughness and elongation are required.
In this case, choose welding rods with a low hydrogen, titanium-calcium and iron oxide content.
B. If weldments are in contact with corrosive media, it is necessary to select an appropriate stainless steel welding rod based on the type, concentration and operating temperature of the medium, as well as whether it is general corrosion or intergranular corrosion.
w. During operating conditions that involve wear, differentiate whether it is general wear or impact wear, and whether wear occurs at room temperature or high temperature.
d. For operations under extreme temperature conditions, choose welding rods that guarantee low or high temperature mechanical performance.
3. Consider the complexity of the assembly form, the rigidity level, the preparation state of the welding opening and the welding position.
The. For welding parts with complex shapes or high thickness, the weld metal experiences significant shrinkage stresses during cooling, which can cause cracking.
It is essential to choose welding rods with high resistance to cracking, such as low hydrogen rods, high toughness rods or ferric oxide rods.
B. For welding parts that cannot be reversed due to certain conditions, it is necessary to select welding rods capable of performing welding in all positions.
w. For welding parts where the welding area is difficult to clean, choose rods that are highly oxidative and insensitive to oxidized skin and grease, to avoid the occurrence of defects such as air holes.
4. Consider welding site equipment.
In locations without DC welding machines, it is not appropriate to choose welding rods that operate only on DC power. Instead, rods that can use both AC and DC power should be selected.
Certain steel materials, such as heat-resistant pearlitic steel, require post-weld stress relief.
However, if equipment conditions or inherent structural limitations prevent heat treatment, it is recommended to use rods made of basic non-metallic materials, such as austenitic stainless steel, which do not require post-welding heat treatment.
5. Consider improving welding techniques and protecting workers’ health.
In places where both acidic and alkaline welding rods meet the requirements, acidic rods should be preferred.
6. Consider labor productivity and economic rationality.
When the performance is the same, lower-priced acidic welding rods should be selected instead of alkaline ones.
Among acidic welding rods, titanium and titanium-calcium types are more expensive.
Considering the situation of our country's mineral resources, we should strongly promote the use of titanium-iron coated bars.
3. Consider the complexity of the assembly form, the rigidity level, the preparation state of the welding opening and the welding position.
The. For welding parts with complex shapes or high thickness, the weld metal experiences significant shrinkage stresses during cooling, which can cause cracking. It is essential to choose welding rods with high resistance to cracking, such as low hydrogen rods, high toughness rods or ferric oxide rods.
B. For welding parts that cannot be reversed due to certain conditions, it is necessary to select welding rods capable of performing welding in all positions.
w. For welding parts where the welding area is difficult to clean, choose rods that are highly oxidative and insensitive to oxidized skin and grease, to avoid the occurrence of defects such as air holes.
