Dissimilar Metal Welding
Dissimilar metal welding, as compared to traditional laser welding, is the joining of two separate metals which wouldn’t ordinarily weld together as they have different chemical and mechanical qualities, and are from different alloy systems.
It’s important to note that two metals that appear the same and may even have the same name can be joined together. If their core properties are different but they share the same name, they are still dissimilar in nature.
This has benefits for many different industries, such as the construction, automotive and electronics industry, whereby there is often a necessity to weld together different parts and components to save on material costs or to use the best possible metals at all times for the perfect end product.
In this white paper, we explore dissimilar metal welding in more depth, looking at the process, factors to consider and which industries you can find it in, as well as how SPI Laser’s Fiber Laser range can be used effectively and efficiently not just for laser welding, but for dissimilar laser welding as well.
What factors need to be considered?
It is important to consider various factors before attempting to join two different metals, as this will determine not only how successful the welding will be, but also how long your newly joined components are likely to last.
Firstly, the solubility that is the chemical properties of a substance which determines its ability to dissolve in a solvent, of the two dissimilar metals must be mutual. If the metals cannot be dissolved together, then the welding process will fail.
Sufficient research must be conducted on the intermetallic compounds that will form within the transition zone between the two metals, investigating things such as the crack sensitivity, how susceptible they are to corrosion and its ductile ability. This range of factors is why it can be useful to use a ‘buttering layer’ that is easily soluble with the two dissimilar metals. We explore buttering layers in more depth below.
The level of weldability, incorporating the solubility and intermetallic compounds, refers to the ability of the two metals in question to be successfully welded without resulting in cracks or any other kind of negativity. This will vary from metal to metal. Once you have determined the weldability, you can also appropriately select any other filling metals or buttering layers you’ll use for a smooth transition.
Part of the calculating process is done by determining the carbon equivalency (CE) of the dissimilar metals, and can be done using the following formula:
- CE = C + (Mn + Si) / 6 + (Cr + Mo + V) / 5 + (N i+ Cu) / 15
Knowing the CE will allow you to determine the temperatures to use before, during and after the welding process, as well as how susceptible your new welded metals will be to cracking. Knowing this will allow you to prepare appropriately, such as by using a lower level of heat during the pre-heat and interpass stages, or by choosing low-hydrogen filler metals.
You will, of course, also need to know the chemical make-up of the two dissimilar metals that you will be using, as it is more than likely that these will be different (although this is not always the case). This can be determined using 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 various standards, and some spell out the chemical requirements, others focus on the mechanical requirements, and some look at both.
You must also consider the coefficient of thermal expansion of the two metals involved, which relates to how your metals will change shape in response to a change in temperature. If these are too different then the internal residual stresses of the two metals, which are the stresses that are present after all the external forces have been removed, will be much greater, which can greatly reduce the operating life of your new welded metals.
Just as the two dissimilar metals may have different thermal expansion rates, they may also have different melting rates, which will cause an immediate problem between the two metals. If this is the case, then as long as a high heat input welding process is used, then both metals should melt and weld quickly enough for this to not be a problem.
Corrosion can occur in between the transition area of the two metals. If the two metals are on wildly different sections of the electro chemical scale, then this suggests a high level of susceptibility to corrosion, which is, of course, a damaging problem to the new weld.
Finally, as well as properly researching and evaluating the different chemical and mechanical make-ups of the dissimilar metals that you are using, and how well they can be welded together, it is important to consider the end-service conditions that your dissimilar metals will be subject to.
Above we have already explained that the differing levels of temperature expansion must be factored in, whether this is during the welding process or after, but you must also factor in the abrasiveness of the end-service conditions that your new weld will be operating in.
Some industries, such as the construction industry, typically protect their heavy equipment using welded-on abrasive-resistant plates to their machinery. However, this same process cannot always be used in the same way with metals of a lower tensile-strength as this increases the chances of cracking and a shorter fatigue life. Using a small fillet weld and a cracking-resistant filler metal can help to solve this issue by reducing the heat input and residual stress levels on the abrasive-resistant plate.
Appropriately considering the heat and abrasiveness that your welds will be subject to in their end-service will allow you to prepare by selecting the right filler metals and joint designs to prolong the life of your new weld.
The welding process
Once you have thoroughly investigated and considered the above factors, you can begin the welding process. Dissimilar metal welding is, for the most part, extremely similar to the welding of two similar metals. From smartphones to submarines, objects of all different sizes and from a variety of industries incorporate welding into their manufacturing process. This is done by physically melting the two metals together until they form one strong, connected joint. This fusion of metals is completed by using a high level of heat from the laser beam to cause the melting.
The complication with this type of welding is that often, two very distinct, very different metals may be getting welded together, which means it is not always as easy as simply melting the two parts together to form a bond. The problems arise in the transition zone between the two metals, where the intermetallic compounds are formed.
A buttering layer, as mentioned earlier, is often used in between the two parent metals and the weld metal to make the joining process smoother and help to provide an easy transition, solving many of the factors that need to be considered above. The success of the buttering layer depends on which metal coating is used, how thick the coating is, and how successfully the buttering layer has bonded with the metal that it is coated on. The following example demonstrates the use of a buttering layer within a dissimilar metal welding process, the differing qualities of the metals used, and the order in which they are aligned:
|Component||Material||Yield Strength (Megapascal) at 300°C||Tensile Strength (Megapascal) at 300°C|
|Buttering Layer||309L / 308L||333||441|
|Carbon or Low Alloy Steel||A508||463||640|
For a successful dissimilar metal weld to have taken place, the new forming joint needs to be as strong as the metal with the weaker tensile strength, so that you know the joint will be able to withstand any stresses that it faces; in the example shown above this is the stainless steel. Copper and steel are two other metals which are often welded together, but both possess very different properties and are not mutually soluble. However, nickel is soluble with both of them and so can be used as the buttering layer, either as a whole piece of nickel or as smaller nickel deposits on the steel surface.
Below is a useful video on the properties of stainless steel:
Residual stresses in a dissimilar metal weld
Residual stress in both dissimilar or similar metal welds is caused by the thermal contraction of the weld metal and the heated metal that is opposite, meaning that the residual stress distribution is extremely similar between the two different types of metal welding processes.
Residual stress levels can be both good and bad, as it can either positively or negatively affect fatigue strength, fatigue life, crack propagation, and resistance to environmental factors such as corrosion. Whether it is good or bad depends on the levels of residual stress, and it is recommended to measure what the resulting stress levels will be in your dissimilar metal weld, either by physically testing it or calculating it on a computational welding system.
Some new welds undergo post-weld heat treatment to ensure that the strength of the two newly joined metals is maintained. However, this can cause a new set of residual stresses to be formed as you will be heating up the two metals again with potentially different thermal expansion levels.
These residual stresses will be:
- Those parallel to the welding direction will be of a higher tensile nature in the metal that has the higher level of expansion, while it will be compressive within the metal that has a lower level. There will be a discontinuity in the stress levels at the joining
- There will be stress at the joining, particularly at the intersection with the surface, which could contribute to cracking
- There may be some localised transverse residual stress located at the surface near the interface
As previously stated, residual stress can be both a positive or a negative factor; sufficient preparation and investigation should leave you prepared for these eventualities. It is worth noting that residual stress levels will be relieved when the weld is heated up, such as during post-weld heat treatment, but just bear in mind that residual stress levels could occur after this; be sure to factor this into any predictions and simulations.
Other solutions, such as shot peening, attempt to manage residual stress levels by bombarding the metal with streams of metal shots. The aim of a process like this is to cause beneficial residual stress levels to increase its fatigue life.
As even the smallest change in residual stress levels can have a huge, and potentially damaging, effect on your components, it is vitally important to know the residual stress state at all times.
Welding dissimilar metals with dissimilar strength levels
Above we looked at how to weld dissimilar metals together, particularly if they have differing levels of thermal expansion which could cause internal residual stress. However, it is possible that residual stress could also be caused by dissimilar strength levels, and so below we look at how you can successfully weld metals with dissimilar strength levels.
Although the two metals by nature are dissimilar, you must do your best to appropriately match them together. This is done by making sure the tensile strength of the filler metal and the metal with the lower level of strength are as similar as possible. You won’t be able to find an exact match, but by keeping the distance between those two figures as small as possible, you’ll be reducing the chances of the weld cracking.
As an example, if you are 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 you want to choose a filler metal that has similar KSI levels to the A36 steel, such as a metal with a 70-KSI.
On occasion, the filler metal can have a lower tensile strength than both the higher and lower strength of metals. For example, two metals with strengths of 100-KSI and 130-KSI could theoretically be welded with a 70-KSI filler metal. However, each metal is different, and you’ll need to check the welding specifications first. You should avoid ever overmatching the filler metal as this can result in a high level of stress, thereby reducing the usage life of your new weld.
Why are dissimilar metals welded together?
As has been highlighted in this article, all metals have different properties, and even two metals with the same name, such as austenitic steel, can have different properties too. Dissimilar metals are welded together in order to maximise on the benefits that each metal produces, while minimising the drawbacks.
For example, aluminium and steel are a popular combination. Steel is a strong, cheap and easy-to-work with metal, so is often the go-to choice for many industries such as the automotive sector. Aluminium, on the other hand, isn’t as cheap or as strong, and is more complicated to work with, but is much lighter than steel. Alongside this, it is resistant to corrosion and rust. So, a combination of these two metals is a great way to maximise on these benefits.
Another popular combination is welding stainless steel and copper due to the high level of electrical conductivity that is provided. If you want to know more on how to weld these various metals together, please read the sections below.
The welding of aluminium to various metals
If you’re looking to weld aluminium to other various metals, then, considering the factors above, the following steps must be taken.
Welding aluminium to steel
Aluminium and steel have very different melting points, with the former melting at around 650°C, while the latter melts at around 1538°C. Alongside this, the two dissimilar metals have very different levels of thermal conductivity and expansion. To successfully weld aluminium and steel together, the steel surface can have a buttering layer applied that is compatible with the aluminium metal. A coating of zinc is the most commonly used metal.
Welding aluminium to stainless steel
Welding aluminium to stainless steel is also possible by using the buttering layer technique, and this is usually a pure aluminium coating which is applied to the stainless steel. This is applied by dipping the clean stainless steel into molten aluminium.
Welding aluminium to copper
To weld aluminium and copper together, you should use a copper-aluminium transition insert piece.
Aluminium alloys are an extremely popular and useful metal, and consist of a mix of aluminium and another metal. An aluminium alloy is typically 85% aluminium, and then the remaining percentage is comprised of a metal such as copper, silicon, tin, magnesium or zinc.
Alloys are used instead of pure aluminium because the mix of metals used maximises the benefits of the aluminium alloy. The metal itself offers a range of advantages such as its ductile and corrosion resistant nature, as well as a high level of electrical conductivity. But it is alloyed with other metals in order to provide it with a higher strength too.
Given these properties that aluminium alloys can offer to so many industries, there has always been great interest in joining the alloy with metals such as steel, titanium, magnesium and copper.
Other common forms of alloys are:
- Amalgam, with the main metal of mercury, typically used for dental fillings
- Brass, with the main metals of copper and zinc, typically used for electrical plugs or hinges
- Solder, with the main metals of lead and tin, typically used for joining metals
The welding of copper to various metals
Welding copper to steel and stainless steel
Copper and copper-based alloys can be successfully welded to the low-alloy and mild-alloy steels and stainless steels. This can be done using a high-copper-alloy filler for areas where the metal is thinner, and for thicker sections, the steel should have a buttering layer of the high-copper-alloy filler, and then welded to the copper.
The copper must be pre-heated, as it has a melting point of approximately 1,085°C. It must also be noted that the steel metal should not be excessively penetrated in to, as any iron pickup within the copper will create a brittle insert that is more susceptible to cracking.
Another approach is to cover the copper with a nickel-based electrode, and if the metal is particularly thick, then a second layer is usually recommended. To complete this process for thicker coppers, it should be pre-heated to 540°C.
Welding copper to aluminium
In this YouTube video, Dr. Marshall Jones, one of the pioneers of dissimilar metal welding, talks of how he discovered the technique of welding copper to aluminium:
What applications does it have?
Despite the fact that you have used two dissimilar metals together, they have essentially become one after the welding process, and so can withstand an incredibly high level of strain and stress. Because of this, dissimilar metal welding is used in many industries and has a variety of different applications.
The automotive and aerospace industries
It is commonly used in high-volume industries like the automotive industry or the aircraft industry, where two different parts will have been welded together and will need to handle incredible pressures to provide a high level of safety and security. For example, it is used to weld together two separate parts of an airplane fuselage, which need high levels of strength at such high altitudes.
The battery and electronics industry
The battery industry and the electronics industry are closely intertwined, and one would be unable to operate without the other. It is predicted that the average household owns at least 24 electronic products or devices, and this number is only set to rise.
This quite simply couldn’t be possible without the use of dissimilar metal welding in the production of batteries and electronic products. As the use of electronics and batteries continues to rise, dissimilar metal welding is sure to continue growing as a vital process too.
Lithium-ion batteries are just one type of battery created using the laser welding technique, but are arguably the most important battery to have been invented. With strong rechargeable properties, they are the battery that you will find in our everyday consumer electronics.
They are commonly constructed from mixtures of cobalt, nickel, lithium and manganese, as:
- The metals cobalt and nickel provide stability
- Manganese provides a low-cost substitution for cobalt which is only used sparingly in this process as it is toxic on a large scale. Manganese also has a high thermal threshold too
As can be seen, a combination of these metals is what provides the greatest number of benefits for a lithium-ion battery, and this simply wouldn’t be possible without the dissimilar metal welding technique.
Another key reason that has seen a greater need for this type of laser welding is with the rise of electric cars, seeing the battery and electronics industries gaining an even closer link with the automotive industry. As companies look for more and more ways to bring affordable electric cars to the market, it’s sure that this link will only continue to grow.
As well as being used for manufacturing batteries, it also finds use for fine wires, fuel cells, and even medical devices in the electronics industry.
You’ll see dissimilar metal welding being used in common power plant, chemical plant and food processing applications as it can join ferritic low alloy steel with austenitic stainless steel, a metal that is commonly used in these industries.
Finally, it also has uses in many other industrial applications for fittings, forgings, and tubes, commonly found in heat exchangers, liquid metal reactors, and boilers.
Using fiber lasers for dissimilar metal welding
Dissimilar laser welding can be conducted in a variety of different ways, and one of the preferred methods is fiber lasers usage. Fiber lasers are designed to be maintenance free with ‘fit and forget technology’, making them a reliable, efficient and effective solution to many other technologies.
Regardless of the thickness, or how different the chemical and mechanical properties of two dissimilar metals are, successful welding can be achieved using fiber lasers. Lasers from the redPOWER range have been designed for power and control during the welding process, and can work with all of the key metals including aluminium, brass, copper, and various types of steel. A continuous wave range of laser offering strengths of 200W to 1kW, can weld everything from thin steel to thick carbon steel and stainless steel.
This makes them the perfect solution for dissimilar metal welding whether this is for medical devices, fine wires, or other larger industrial solutions found in the automotive or aerospace industries.
The use of Pulsed Lasers such as the redENERGY G4 represents another direction for dissimilar welding, offering both commercial and technical benefits. For further information on dissimilar welding using Pulsed Fiber Lasers, please visit our Webinar – Welding with ns Pulsed Fiber Lasers.
Contact us here at SPI Lasers
To learn more about how our fiber lasers can effectively weld dissimilar metals together, as well as perform a range of other tasks such as laser cutting, marking and engraving, please complete our contact form, contact us on +44(0)1489 779 696 or by email at firstname.lastname@example.org and one of our expert team will be in touch.
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