A lot of people believe that bronze cannot be welded because it is too brittle. This is untrue. Bronze can be fused in a number of ways and the grain melts at a lower temperature so it does not cool and create large areas of inclusions like cast iron does.
Bronze can be melted together to create seams without putting it through a restretching process. The seam welds can be as high as 20 MPa of tensile strength with no cracking or porosity in the welds. Bronze can be melted in other metals like tin or coal to create new alloys and introduce desirable properties.
While Mercury is liquid metal that is a non-solid that cannot be easily can welded, however, during one time period would have been very common in certain trades. One prominent use of mercury that would have made it easy to find small amounts is in old thermometers. However, mercury can be harmful to the body and should not be used even if it could be welded.
Dissimilar Metal Welding
Many industries require joining unlike materials as parts of machines, tools, and more. When working with dissimilar material welding, a welder must consider several factors to ensure a strong, crack-free joint. Not only do the types of metal matter, but so do the weld material and temperature. These factors are only a handful of the considerations a welder must make when choosing a method for joining dissimilar materials. With expert knowledge and the right tools, a welder can create successful joints between many unalike metals.
Can Dissimilar Metals Be Welded?
The need for dissimilar materials in one metal piece is common. These unalike metals may require a welded joint to connect them. As long as the completed weld offers adequate strength to avoid cracking or failure, the joint is a success. However, the required strength of a weld or its ductility may change depending on the use of the welded part.
So, it is possible to weld different types of metals, but with qualifications. The skill and knowledge of a welder greatly determine the success of the job.
What Is Dissimilar Welding?
Dissimilar welding refers to the process of connecting materials with different alloys through welding. The filler material and both metals need to be evaluated before choosing the best way to connect the metals. While fusion welding is a popular method, it does not work well for some combinations of metals. Other methods may provide a more durable hold, especially for uses in high-stress environments.
What Factors Do You Need to Consider in the Welding Process?
When a welder joins two dissimilar materials, they must consider several factors before deciding the best welding method and tools to use. These factors depend on the composition of the metals because even alloys of the same metal fall under the category of dissimilar metals. For example, carbon steel and stainless steel have different properties and require just as much planning as welding together unalike metals, such as copper and aluminum.
Physical and chemical properties are two of the many considerations a welder must account for when planning a welding task with dissimilar metals. Find out more about these factors and how a welder can overcome metal differences by changing aspects of the project.
1. MELTING POINTS OF THE METALS
Because fusion welding typically melts some of both bases, the melting points of both metals play important parts in the type of welding done and the filler material used. The welder needs to use a temperature that melts both metals. High heat and fast welding can mitigate serious problems with the integrity of the completed joint. Using techniques that do not melt the metals could also overcome difficulties from varying melting points.
2. COEFFICIENTS OF THERMAL EXPANSION OF BOTH METALS
The coefficient of thermal expansion describes the way an object’s size changes as that object changes temperature. When welding dissimilar metals, each metal’s coefficients of thermal expansion should not differ greatly to avoid problems with the finished weld.
When the metals have significant differences in thermal expansion coefficients, changes in temperature around the welded joint can cause excessive strain on the weld. Specifically, the stresses will focus on the intermetallic zone, where the two metals blend together with the filler material. The welded joint has a greater chance of thermal fatigue in this intermetallic zone, especially in applications with a high number of temperature cycles.
Welders who connect metals with dissimilar thermal expansion coefficients can complete the job with some changes. To reduce the chances of thermal expansion differences from causing thermal fatigue in the joint, welders can use methods that join the metals while minimizing the intermetallic zone or that do not require the melting of both metals.
3. ELECTROCHEMICAL DIFFERENCE
Differences in metal electrochemistry relate to the possibility of corrosion in the intermetallic zone. Metals that sit closer on the electrochemical scale to each other provide a simpler welding process than those that are far apart. Corrosion strikes the intermetallic area when electrochemical differences are large. Finding ways to minimize the intermetallic zone may lessen joint problems due to electrochemical differences.
4. SOLUBILITY OF EACH METAL
One of the most important factors for a strong weld between different materials is the solubility of each metal. The metals should ideally be interpolable with each other. However, some metals do not have compatibility in this way. In those instances, using a third metal that is soluble with both can help create the weld.
For example, using nickel as an intermediary metal on both surfaces may work to create the desired strength. Treating steel and copper with a layer of nickel before welding them together creates a weld joint that connects nickel to nickel. Since nickel is a soluble metal for both copper and steel, it adheres well to each side of the joint.
This use of nickel is one example of a composite insert used between different metals. Non-fusion welding methods typically adhere to the insert material onto both surfaces. When welding the pieces together, the composite on both edges of the base metals allows for like-to-like welding.
5. FINAL USE OF THE WELDED COMPONENT
Lastly, applications for the welded component should factor into the choice of welding methods for dissimilar materials. For example, a welded part used in a boiler may undergo multiple thermal cycles that will test the strength of the weld. Cold temperature cycles cause more stress on a welded joint than hot cycles.
Additionally, welded joints used in contact with electrolyte liquids need a filler material with a corrosion resistance that exceeds both base metals. Choosing a filler material that has a lower resistance to corrosion could quicken the rate of galvanic corrosion.
Another instance of application affecting the components used for a welded joint includes adding abrasion-resistant plates to heavy construction equipment. The filler metal needs to reduce cracking in the heat-affected zone while reducing stress on the joint.
The final use is also important when one base metal will not experience the same temperature changes as another. In a boiler, one base metal may experience thermal cycling while the other does not. To prevent damage in this instance, a filler material that both protects from thermal fatigue and provides stress relief is necessary.
How Do You Weld Dissimilar Metals?
Welding dissimilar metals does not come with a simple, standardized process that applies to all situations. Using nonstandard welding methods or adding transition materials to the surfaces could be the steps a welder must take for a successful weld.
With so many factors at play, welders must show a knowledge of the metals they work with when joining dissimilar metals. To avoid problems from thermal fatigue, cracking at the weld or other signs of a poor junction, always consult a professional welder when completing projects that require joining dissimilar metals. An expert welder can choose the best welding method, materials and adaptations to meet the project needs and metal properties needs.
Which Type of Welding Is Usually Used to Join Dissimilar Materials?
The type of welding typically used for dissimilar metals is fusion welding, which welders also use for joining similar metals. In some instances, a welder may need to determine another method for joining dissimilar metals. For all these types of joining, the welder will need to consider:
- The composition of the intermetallic layer
- The unmixed zone (UMZ) of base materials that did not combine into the intermetallic layer
- The heat-affected zone (HAZ)
To ensure a proper bond between dissimilar metals, the welder must minimize the impact the material differences have on the HAZ, UMZ and intermetallic layer. For certain jobs, reducing these impacts can improve the strength of the joint.
1. FUSION WELDS
Fusion welding includes traditional welding methods such as gas tungsten arc and gas metal arc. Gas tungsten arc welding is also known as tungsten inert gas (TIG) welding, and gas metal arc welding is also known as metal inert gas (MIG) welding. These processes use a filler to create the weld. Compared to other joining methods, fusion welds add more filler.
When using fusion welds, choose a filler that matches the strength of the weaker metal, which reduces cracking probability. If using a filler that has a significantly greater tensile strength than the weaker metals, excessive strain on the weaker metal could cause weld failure.
Factoring in preheating requirements of both metals will help ensure a more successful fusion weld. For instance, if one metal requires a preheat temperature of 600 degrees Fahrenheit and the other only needs 225 degrees of preheating, you may need to make adjustments. If, like this scenario, the higher temperature is greatly different from the lower one, the metal with a lower preheat requirement could sustain damage. To avoid serious damage when welding general use products, you could compromise preheat temperatures halfway between the two temperatures.
For some metals, buttering the joint edges with a metal that is soluble with the dissimilar metals and the filler could help the process of fusion welding. For example, depositing nickel alloy onto the weld surface of a piece of stainless steel and doing the same on the joint location of a piece of copper creates a surface that will adhere to both the copper and steel and the connecting nickel filler. To weld the two surfaces, use a nickel filler to connect them through fusion welding.
2. LOW-DILUTION WELDING
When fusion welding will not suffice, low-dilution welding could work as an alternative. These methods include using a laser weld, pulsed arc weld or electron beam to join materials without the need for a filler. The two welded surfaces do not melt as much compared to other methods. For special applications or high-production processes, low-dilution welding is used more often than fusion welding for dissimilar metals.
Electron beam welding joins dissimilar pieces well because the high energy used avoids problems of unalike thermal conductivities. This method works by producing very little in the intermetallic layer and using no filler. However, it requires speed from the welder to execute successfully.
3. NON-FUSION JOINING OF DISSIMILAR MATERIALS
Non-fusion joining includes processes such as friction welding, diffusion bonding, soldering and explosion welding. Friction welding only melts a small bit from one of the base surfaces. Because the welder discards the melted portion, mixed metals and the intermetallic area are reduced. By heating only one side, the HAZ also remains as small as possible.
This type of non-fusion welding works well when joining the following pairs of metals:
- Steel and aluminum
- Steel and copper alloys
- Stainless steel and nickel alloys
Explosion welding uses less heat to eliminate the HAZ and intermetallic layers. With this method, the welded portion will not exceed the strength of the weaker metal.
What Are Metals That Cannot Be Welded?
When it comes to what metals can be welded together and what cannot, several factors come into play. Similar metals pose the fewest problems when welding. Dissimilar materials often require some changes to the process or additional materials to ensure success. A welder with knowledge of metals and their physical and chemical properties can select a filler material to bridge unalike materials properly. Other binding methods for different metals may require the welder to use a different process.
Extreme differences in the melting points or electrochemistry of metals can make them more difficult to join using standard methods. For example, fusion welding does not work well with the following pairs:
- Titanium and steel
- Aluminum and copper
- Aluminum and stainless steel
- Aluminum and carbon steel
Alternative joining methods for these metals may include laser welding, diffusion bonding, friction welding, mechanically fastening, adhesive or solid-state welding. Experts can distinguish the best among these alternatives when connecting metals that have very dissimilar properties.