Welding of Dissimilar Metals

There tend to be several applications in which weldments are made from metals involving different compositions. The same is true of a mechanical wear problem, a high-temperature condition, as well as various other types of conditions wherein different properties will be required from different parts of the same weldment.

This results in the requirement for joining dissimilar metals. A successful weld between dissimilar metals is one which is just as strong as the weaker of the two metals being joined, that is, possessing a sufficient amount of tensile strength together with ductility in order that the joint won't fail in the weld. These kinds of joints are generally obtained in a variety of distinctive metals and using a number of welding processes.

Why are dissimilar metals welded together?

Ashas become recognizable by now, all metals possess distinctive properties, including two metals with the same name, for instance, austenitic steel, can have different properties too. Dissimilar metals are welded together in order to maximise around the advantages that each metal provides, at the same time minimising the drawbacks.

For illustration, aluminium together with steel are a widely used combination. Steel is a strong, economical and easy-to-work with metal, consequently is frequently the go-to solution for numerous industrial sectors for instance the automotive sector. Aluminium, in contrast, isn’t as low-cost or as strong, and is particularly more complicated to work with, however , is substantially lighter as compared to steel. In addition to this, its resistant to corrosion and rust. Consequently, a combination of both of these metals is an excellent solution to maximise upon these advantages.

Another widely used combination is welding stainless steel and copper as a consequence of high level of electrical conductivity which is provided.

Factors To Be Considered

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It is critical to take into consideration various factors before endeavoring to join two different metals, since this would determine not only precisely how successful the particular welding would be, but additionally for how long the newly joined components will probably last.

Solubility

Firstly, the solubility, which is the chemical properties associated with a substance which determines its ability to dissolve within a solvent, of the two dissimilar metals is required to be common. In case the metals cannot be dissolved together, then the welding process definitely will fail.

Inter-metallic compounds

Sufficient research is required to be performed relating to the intermetallic compounds which will form inside the transition zone between the two metals, examining factors like the crack sensitivity, exactly how susceptible they are to corrosion as well as its ductile ability. This range of factors is the reason why it is typically necessary to work with a ‘buttering layer’ which is readily soluble together with the two dissimilar metals.

Weldability

The standard of weldability, incorporating the solubility and intermetallic compounds, represents the ability of the two metals under consideration to be successfully welded without ultimately causing cracks or some other form of unfavorable conditions. This will range from metal to metal. Subsequent to identifying the weldability, appropriate filling metals or buttering layers are indentified to use to get a consistent transition.

Part of the calculating process is accomplished by determining the carbon equivalency (CE) of the dissimilar metals, and that can be achieved by making use of the following formula:

CE = C + (Mn + Si) / 6 + (Cr + Mo + V) / 5 + (N i+ Cu) / 15

Knowing the CE facilitates the determination of the temperatures to utilise before, during and after the welding process, in addition to how susceptible the newly welded metals will be to cracking. Understanding this enables the suitable treatments, such as using a reduced degree of heat during the pre-heat and interpass stages, or even by selecting low-hydrogen filler metals.

In addition, the information of the chemical make-up of the two dissimilar metals being utilized will be essential, considering that it is normally more than likely that these will be different (nevertheless this may not necessarily be the case). This can be identified making use of one of the following standards:

• ASTM – American Society of the International Association for Testing and Materials
• ASME – American Society of Mechanical Engineers
• AISI – American Iron and Steel Institute
• SAE – Society of Automotive Engineers

There are numerous standards, and many stipulate the chemical requirements, others consentrate on the mechanical requirements, and some consider both.

Thermal expansion

Another aspect to consider is the coefficient of thermal expansion of the two metals involved, which pertains to precisely how the metals will change shape in response to a change in temperature. If these are typically too distinctive then the internal residual stresses of the two metals, which are the stresses that happen to be present after all the external forces have been completely removed, is going to be substantially greater, which will decrease the operating life of the newly welded metals.

Melting rates

Just for the reason that two dissimilar metals might have different thermal expansion rates, they might likewise have different melting rates, which will contribute to an immediate problem between the two metals. If this is the case, then assuming that a high heat input welding process is used, in that case both metals should melt and weld rapidly enough for this to not become a issue.

Corrosion

Corrosion can take place in between the transition area of the two metals. In case the two metals are on significantly different sections of the electrochemical scale, in that case this signifies an increased level of susceptibility to corrosion, which is, as expected, a damaging problem in the new weld.

End-service conditions

Finally, in addition to adequately investigating and analyzing the different chemical and mechanical make-ups of the dissimilar metals that are being used, together with how effectively they can be welded together, it is essential to take into account the end-service conditions that the dissimilar metals are going to be subject to.

As previously outlined, the differing levels of temperature expansion is required to be factored in, irrespective of whether it's during the welding process or subsequent to, nevertheless the abrasiveness of the end-service conditions that the new weld is going to be operating in requires be considered .

Some industrial sectors, for instance the construction industry, commonly safeguard their heavy equipment using welded-on abrasive-resistant plates for their machinery. However, this kind of same process cannot always be utilised in the identical manner with metals associated with a lower tensile-strength since this increases the probability of cracking in addition to a shorter fatigue life. Using a small fillet weld and a cracking-resistant filler metal will help solve this challenge by reducing the heat input and residual stress levels on the abrasive-resistant plate.

Appropriately considering the heat in addition to abrasiveness that the welds are going to be subject to within their end-service can assist to prepare by way of selecting the most appropriate filler metals and joint designs to extend the life of the new weld.

Dissimilar Metal Welding

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The types of procedures principally described for obtaining quality welds are generally between identical metals or at the least of metals of equivalent composition and properties. There are several applications, on the other hand, wherein weldments are produced from metals of different compositions. This is especially valid of a mechanical wear problem, a high-temperature situation, as well as other conditions wherein different properties are required from different parts of the same weldment.

This necessitates the need for joining dissimilar metals. Welding dissimilar metals is being undertaken on a regular basis and therefore the information for effectively joining several of the more prevalent metal combinations is important.

A effective weld between dissimilar metals is engineered so it is as strong as the weaker of the two metals being joined, i.e., possessing an adequate amount of tensile strength and ductility in order that the joint won't fail inside weld. Such joints are usually achieved in several different metals and using a variety of welding processes.

The problem of fabricating welds between dissimilar metals pertains to the transition zone between the metals along with the intermetallic compounds formed within this transition zone. For the fusion type welding processes, it is essential to examine the phase diagram of the two metals involved. Should there be mutual solubility of the two metals, the dissimilar joints are generally developed successfully. When there is minimal solubility between the two metals being joined, the weld joint is definately not successful.

The intermetallic compounds which are formed, between the dissimilar metals, will have to be examined to find out their crack sensitivity, ductility, susceptibility to corrosion, etc. The microstructure of this intermetallic compound is extremely important. In some instances, it will be significant to utilize a third metal which is soluble with each metal in order to produce a successful joint.

Another factor associated with predicting a successful service life for a dissimilar metals joint pertains to the coefficient of thermal expansion of both materials. If these are broadly different, there will be internal stresses created within the intermetallic zone during any temperature change of the weldment. In the event the intermetallic zone is exceedingly brittle, service failure will probably shortly occur.

The difference in melting temperatures of the two metals which are being joined also needs to be considered. It's of primary interest whenever a welding process utilising heat is involved considering the fact that one metal is going to be molten long before the other when subjected to the same heat source. When metals of different melting temperatures and thermal expansion rates need to be joined the welding process which includes a high heat input designed to make the weld quickly offers an advantage.

The difference of the metals on the electrochemical scale is an indication of their susceptibility to corrosion at the intermetallic zone. Whenever they are far apart on the scale, corrosion is definitely a serious problem.

In certain situations, the only method to create a successful joint is by using a transition material between the two dissimilar metals. An illustration of this is the effort to weld copper to steel. The two metals are certainly not mutually soluble, but nickel is soluble with both of them. Therefore, by utilizing nickel as an intermediary metal, the joint can be made. Two methods are widely-used:

• use a piece of nickel, or
• deposit several layers of nickel alloy on the steel, i.e., buttering or surfacing the steel with a nickel weld metal deposit.The nickel or nickel deposit can be welded to the copper alloy utilizing a nickel filler metal. Such a joint will provide satisfactory properties and will also be successful.

Another technique of joining dissimilar metals is the utilization of a composite insert between the two metals at the weld joint. The composite insert consists of a transition joint between dissimilar metals produced by a welding process that doesn't involve heating. By selecting the proper materials for the composite insert, like metals which can be welded to like metals in making the fusion weld joint.

Welding Processes for Composite Inserts

A brief description of a portion of the welding processes which can be used for composite inserts which include transition joints and therefore do not employ filler metals.

Explosion welding is utilized to join several so-called incompatible metals. In explosion welding, the joint properties are going to be equal to those of the weaker of the two base materials. Since minimum heat is introduced, there is absolutely no melting, no heat-affected zone, and no thermal compounds are formed.

The characteristic sine wave pattern of the interface considerably increases the interface area. This process is utilized for cladding, but is in addition utilized to produce composite transition inserts utilized for the fusion welding of dissimilar metals. Composites containing a transition joint are commercially available between aluminium and steel, aluminium and stainless steel, aluminium and copper, and other materials.

Cold welding is useful for making dissimilar metal transition joints. Cold welding process doesn't use heat, consequently eliminates the heat-affected zone along with the intermetallic fusion alloy. Minimal mixing of the base metals happens. It's regularly used to join aluminium to copper.

Ultrasonic welding is additionally utilized for welding dissimilar metals considering the fact that minimal heat is developed at the weld joint. Ultrasonic welding can be utilised only for extremely thin materials or small parts.

Friction welding is in addition utilized for joining dissimilar metals and additionally for making composite transition inserts. Diverse dissimilar combinations of metal have been welded, including steel to copper base alloys, steel to aluminium, stainless to nickel base alloys, etc. In friction welding, only a minuscule amount of the base metal is heated and that which is melted is thrown from the joint, consequently, the intermetallic material is maintained to a very minimum. The heat-affected zone is additionally negligible.

High-frequency resistance welding process is additionally widely used for dissimilar metal welding. In high-frequency resistance welding, the heat is concentrated on the very surface of the parts being joined and pressure applied is sufficient to produce welds of many dissimilar materials. It is usually utilized for joining copper to steel at very high speeds.

Diffusion welding is widely used for aerospace applications of dissimilar metals welding.

Percussion welding is also made use of but this process is restricted to wires or small parts. The laser beam welding process has additionally been used nevertheless it is restricted, currently, to very thin materials.

Electron beam welding process has already established wide application for joining dissimilar metals. Electron beam welding process makes use of high-density energy together with fast welding speed. It seems to overcome the difference of thermal conductivity when welding metals together possessing wide variation of thermal conductivity. Additionally, the weld zone is extremely small and filler metal isn't introduced. Since there is such a small amount of intermetallic compound formed, electron beam offers an advantage for numerous dissimilar combinations.

Flash butt welding process tends to make high-quality welds between copper and aluminium. Utilizing proper controls. all or most of the molten metal is forced out of the joint and the weld is complete as a solid state process. Flash butt welds are produced in rods, wires, bars, and tubes.

Arc Welding. There are numerous welding requirements to join dissimilar metals wherein the above processes can't be used. In such cases, the three popular arc welding processes are generally utilised. These are the shielded metal arc welding process, the gas tungsten arc welding process, and the gas metal arc welding process.

Welding Aluminium to Various Metals

There is a wide difference of melting temperatures, for aluminium, approximately 650°C, and for steel, approximately 1538°C. Aluminium will melt and flow away well before the steel has melted.

The aluminium iron phase diagram demonstrates that a number of complex brittle intermetallics are formed. It is noticed that iron aluminium alloys that contains in excess of 12% iron have little or no ductility. Furthermore, there exists a wide difference in the coefficient of linear expansion, in thermal conductivity, and in specific heats of aluminium and steel. This would introduce thermal stresses of considerable magnitude.

The most successful method is to use an aluminium-steel transition insert with each metal welded to its own base metal utilising any of the three arc welding processes.

The other way is to coat the steel surface with a metal compatible with aluminium. The success of this form of joint is dependent upon the metal utilized to coat the steel, the thickness of the coating, and the bond between the coating and the steel surface. A coating of zinc on steel can be utilized and the aluminium welded to it with the gas tungsten arc welding process. A high-silicon aluminium filler wire needs to be used. Directing the arc toward the aluminium; pulsing will assist the welder.

For welding aluminium to stainless steel transition inserts are available. It is additionally possible to utilise the coating technique. A coating for the stainless steel is pure aluminium coating, which can be applied by dipping clean stainless steel into molten aluminium. Another method to obtain a compatible coating is by tinning the stainless steel using a high-silicon aluminium alloy. The aluminium surface can then be gas tungsten arc welded to the aluminium. The arc needs to be directed toward the aluminium; pulsing will assist the welder.

The welding of aluminium to copper is accomplished by using a copper-aluminium transition insert piece.

Welding Copper to Various Metals

Copper and copper-base alloys can be welded to mild and low-alloy steels and to stainless steels. For thinner sections, within the gauge metal thickness, the gas tungsten arc welding process can be utilised with a high-copper-alloy filler rod. The pulsed mode will make it easier to obtain a good quality weld. The arc need to be directed to the copper section to minimise pickup of iron.

In the heavier thicknesses, first overlay or butter the steel using the same filler metal thereafter weld the overlayed surface to the copper. It is recommended to avoid excessive penetration into the steel portion of the joint considering the fact that iron pickup in copper alloys generates a brittle material. The copper is required to be preheated.

Another method is to overlay the copper with a nickel-base electrode. A second overlay or layer is usually recommended on thicker materials. When making the overlay welds on thick copper, the copper needs to be preheated to ~540°C (1000°F).

The overlay or buttered surface of the copper part needs to be smoothed to provide a uniform joint preparation. Effort need to be made to minimise dilution of the copper with the nickel electrode. The shielded metal arc process, the gas tungsten arc or gas metal arc welding processes can all be used. The selection would be determined by equipment available and the thickness of the material being joined.

Copper can also be joined to stainless steel, and brass can be joined to mild and low-alloy steels.

Welding Nickel-Base Alloys to Steels

Nickel-base alloys which include Monel and Inconel can be successfully welded to low-alloy steel utilizing the Monel analysis of filler material when working with any of the arc welding processes. In the case of Inconel to mild or low-alloy steel the Inconel base electrode will be used. A similar situation also is applicable to the welding of Inconel or Monel to stainless steels. In this case the Inconel or Monel type electrode is utilized.

Welding Dissimilar Metals with Dissimilar Strengths

Above types of procedures demonstrate to weld dissimilar metals together, especially if they have varying levels of thermal expansion that can cause internal residual stress. On the other hand, it is also possible that residual stress may be resulting from dissimilar strength levels, and for that reason we examine methods to successfully weld metals with dissimilar strength levels.

Although the two metals by nature are dissimilar, attempt is required to be made to appropriately match them together. This is accomplished by making certain the tensile strength of the filler metal and the metal with the lower level of strength are as comparable as is possible. An exact match cannot be found, but by keeping the distance between those two figures as small as possible, it minimizes the probability of the weld cracking.

As an illustration, attempting to weld A514 low-alloy steel, with a minimum tensile strength of 110-KSI, with A36 steel that has a minimum tensile strength of 58-KSI, then then the selection is a filler metal which has similar KSI levels to the A36 steel, including a metal with a 70-KSI.

On occasion, the filler metal may have a lower tensile strength as compared to both the higher and lower strength of metals. As an illustration, two metals with strengths of 100-KSI and 130-KSI could theoretically be welded using a 70-KSI filler metal. Nonetheless, each metal is different, and the welding specifications should be verified first. It is best to avoid possibly overmatching the filler metal since this could result in a high level of stress, as a consequence lessening the usage life of the weld.

Applications Of Dissimilar Metal Welding

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Despite the fact of using two dissimilar metals together, they effectively develop into one following the welding process, and for that reason can withstand an incredibly high level of strain and stress. Consequently, dissimilar metal welding is utilized in numerous industries and has a variety of unique applications.

The automotive and aerospace industries

It is commonly utilised in high-volume industries including the automotive industry or the aircraft industry, where two different parts will have been welded together and will require to handle incredible pressures to provide a high level of safety and security. By way of example, it is employed to weld together two separate parts of an aeroplane fuselage, which need high levels of strength at high altitudes.

The battery and electronics industry

The battery industry and the electronics industry are closely intertwined, and the other would be unable to operate without the presense of other. It's forecasted that the average household owns at a minimum 24 electronic products or devices, and this number is only set to increase.

This basically couldn’t be possible without the use of dissimilar metal welding in the manufacturing of batteries and electronic products. As the use of electronics and batteries is constantly on the rise, dissimilar metal welding will certainly continue thriving being a vital process too.

Lithium-ion batteries are merely one type of battery constructed with the laser welding technique, however , are most likely the most important battery to have been invented. Using strong rechargeable properties, they're the battery that you will find inside our everyday consumer electronics.

They are commonly manufatured from mixtures of cobalt, nickel, lithium and manganese, as:

• The metals cobalt and nickel provide stability
• Manganese supplies a low-cost substitution for cobalt that's only used sparingly in this process considering that it is toxic on a large scale. Manganese in addition has a high thermal threshold too

As is visible, a combination of these metals is precisely what provides the greatest number of benefits for a lithium-ion battery, and this also simply wouldn’t be achievable without the dissimilar metal welding technique.

Another primary reason that has witnessed a greater requirement for such type of laser welding is with the rise of electric cars, experiencing the battery and electronics industries developing a much closer link with the automotive industry. As companies search for a growing number of strategies to bring affordable electric cars to the market, it’s sure that this particular link will only continue to raise.

As well as being utilized for manufacturing batteries, in addition, it finds use with regard to fine wires, fuel cells, and in many cases medical devices in the electronics industry.

Other uses

Dissimilar metal welding is going to be utilised in common power plant, chemical plant and food processing applications since it could join ferritic low alloy steel with austenitic stainless steel, a metal which is widely used in these industries.

Finally, in addition, it has applications in numerous other industrial applications for the purpose of fittings, forgings, and tubes, typically found in heat exchangers, liquid metal reactors, and boilers.