What Is Diffusion Welding
Welding is a process where objects get joined together. Different types of welding are available, and diffusion welding is one of the most crucial types for particular industries.
What is diffusion welding? It is a solid-state welding process in which you can bond the same or different metals together. You can also do diffusion welding of thermoplastic materials.
In this write up, we will break down the basics of diffusion welding, its process, types, applications, and more. Read on to learn about diffusion welding in detail.
What Is Diffusion Welding and Its Principle?
Diffusion welding, also known as diffusion bonding, is a technique of joining two metals. The joining is widely known as the solid-state joining process. In the diffusion welding process, two surfaces of materials merge (act of coalescence) together due to the application of high pressure and temperature within a particular time frame.
Time, pressure, and temperature transform the bonding line of the surfaces of workpieces into a continued bonded face of the metals. In diffusion bonding, no filler materials or additional additives are used.
The diffusion welding process can join the same or different metals.
The Working Principle of Diffusion Welding
The diffusion welding process works on the principle of atomic diffusion. In the diffusion bonding process, two metal surfaces are pressurized at 50 to 90% of their melting temperature.
As a result, the atoms of the metals' surfaces move due to high pressure and temperature. Pressure helps distribute the atoms all over the surfaces of the welding metals, and heat ensures the diffusion bonding of metal to the level of continuity surface (bonded single surface) of the materials.
History of Diffusion Welding
The idea of diffusion welding came almost 100 years ago, most likely in the 1920s. This was not direct diffusion welding but rather a primitive form of diffusion bonding.
In particular, the welding process was first used to join gold and copper to make jewellery. Goldsmiths used to hammer solid gold and make it thin gold plates.
Then, they used a copper substrate and weighed it down on the thin gold film. After placing them together, they were usually placed in a hot oven, and hot pressure was used to form the bond. This process was known as “hot pressure welding/HPW.”
However, exact diffusion welding came in the 1950s. The modern diffusion bonding method is developed by ancient Soviet scientist N.F. Kazakov in 1953. Since then, diffusion bonding technology is being developed day by day, and precise toolings have been designed and developed for different methods.
In particular, viable solid-state diffusion welding has been becoming popular and has been used to join high-precision metals like titanium and superalloys since the 1960s.
Diffusion Bonding Equipment List
In general, diffusion bonding equipment is not universal. Diffusion welding product manufacturers need to customize their diffusion bonding equipment.
Here, you will get a general idea about diffusion bonding machines and equipment.
Polymer Copper and Aluminum Diffusion Welding Machine: Diffusion welding product manufacturers use these diffusion welding machines to merge and create soft connections between copper and aluminum.
Hot Press Vacuum Furnaces Diffusion Welding Machine: This machine is used to join metals with vacuum hot pressing. Graphite tooling is used as the carrier of the materials in the vacuum furnace diffusion machine that is to be joined.
Gas Pressure Diffusion Welding Machine: Gas is used to create pressure and heat to join metals.
Other Equipment
Chamber for Diffusion Welding: The chamber where the materials are put for a controlled welding environment.
Pressing Unit: The pressing unit applies pressure on the materials that are to be welded.
Fixturing and Tooling: Used for holding and aligning the welding materials during the diffusion bonding process.
Cleaning Equipment of the Materials: Tools like grit blasting or ultrasonic cleaners are used to clean the surface of the materials before starting the diffusion welding.
Inert Gas Supply: Gas like argon gas is used to protect the welding atmosphere in gas pressure diffusion welding.
Types of Diffusion Welding and Diffusion Welding Process
Diffusion bonding is generally considered of two types: solid-state diffusion bonding and TLP diffusion welding.
Solid State Diffusion Welding
Solid-state diffusion welding is the process of joining two flat surfaces of materials. It uses optimal pressure, 50% to 90% of the materials' melting temperature, and a precise time frame.
Based on the materials, the time required for metal diffusion bonding can vary from minutes to hours. For your convenience, here we are showing you the melting temperature of different metals.
Please note these are the absolute melting temperatures. But for diffusion welding, reaching 50% to 90% of melting temperature is required regarding the metals.
Please note these are probably melting temperatures. These can be changed based on the welding atmosphere and pressure applied. The pressure required for diffusion welding of different materials can vary from 350 to 700 kg/cm2.
2 Stages of Solid State Diffusion Bonding Process Completion
Solid-state diffusion welding is completed in two stages. Here, we have discussed how these happen.
First Stage
The focus of the first stage of solid-state diffusion bonding is to achieve a bond between the surfaces of the metal workpieces that are going to be bonded. This happens due to the application of required pressure on the metals.
The applied pressure removes the roughness and contaminants of the surfaces and disperses the surfaces evenly. As a result, the surface metal contact is established to be bonded in the second stage.
Second Stage
The second stage mainly includes the actual diffusion and grain growth on the bonding surfaces. This happens as the very close stage of completed welding.
Heat is applied in this stage up to 90% of the melting temperature of the metals. The applied heat increases the bonding rate.
The bonding line of the surfaces is eliminated, and the final diffusion bonding is established.
Finally, the very strong metallic diffusion bonding is completed across the contacted areas of the surfaces.
2 Methods to Perform Solid-State Diffusion Welding
You will see different methods are used for solid-state diffusion bonding. Here, we have discussed the 2 most used methods of solid-state diffusion bonding process.
Gas Pressure Diffusion Bonding
Basically, non-ferrous metals are welded through the gas pressure diffusion welding method. For example, copper, nickel, titanium, aluminum, etc.
The process involves the following.
The workpieces are placed together with proper alignment and put on the gas pressure welding machine chamber.
Inert gas like argon is used, and heat is applied up to 90% of the melting temperature of the metal. (More than 815°C for most materials, and more than 500°C for aluminum).
The gas pressure is created during the heating application and dispersed across the bonding surfaces of the workpieces.
Vacuum Pressure Diffusion Bonding
The vacuum pressure diffusion bonding method is used to create diffusion bonding of ferrous metals like stainless steel and its alloys. The process involves the following.
Unlike gas pressure, mechanical or hydraulic pressure is used for vacuum pressure diffusion bonding through graphite tooling.
The workpieces are put in the vacuum chamber, and heat is applied like gas pressure. In vacuum pressure diffusion, the pressure applied is higher than the pressure applied in gas pressure bonding.
In general, you need to apply a temperature of around 1150°C, and in terms of pressure, it is around 700 kg per square cm.
TLP Diffusion Welding
TLP, or transient liquid phase diffusion bonding, is a little different from traditional diffusion welding. Unlike traditional diffusion bonding, a thin metal interlayer is used between the base materials to be joined.
The interlayer creates a liquid phase, and the liquid phase solidifies the materials to be joined. The liquid phase is temporary, and that is why the type of diffusion welding is called transient liquid phase diffusion welding.
It is a low-thermal diffusion welding process, generally done by the Eutectic bonding phase.
Eutectic Diffusion Bonding Phase
TLP diffusion welding takes place through the Eutectic bonding phase as follows.
You have to put a thin interlayer between the workpieces that are to be bonded.
The heat will melt the interlayer at a lower temperature, and the melted interlayer will infuse the bonding surfaces of the base materials.
The temperature will remain constant and lower than the melting temperature of the base materials.
The constant temperature of the liquid phase decreases the required pressure of the base materials, solidifies the surfaces, and makes the bonding. This is because the liquefied phase increases the dissolution rate of the oxide layer of the bonding surfaces with constant heat.
The interlayer materials that are used in TLP diffusion bonding, including-
Copper for gold, zirconium alloys, titanium alloys, etc.
Boron for Ni alloys
Silver
Titanium
Nickel, etc.
Materials That Can Be Diffusion Welded
Both metals and plastics can be used for diffusion welding. Here is a list of different materials that can be diffusion welded.
Metals
Similar metals can be diffusion welded, including the following.
Diffusion-bonding copper and diffusion-bonding aluminum are the most common ones for electric and battery industries.
Diffusion bonding titanium alloys and nickel alloys are used for diffusion welding.
Diffusion bonding stainless steel, nickel, tungsten, etc.
Dissimilar metals can also be diffusion bonded in the following manners:
Copper and aluminum
Stainless steel and nickel
Titanium and Inconel, etc.
Plastics
Diffusion bonding plastic materials can also be done, including the following ones:
Diffusion bonding acrylic
Polycarbonate (PC)
Polyetherimide (PEI)
Polypropylene (PP)
Polyvinylidene fluoride (PVDF), etc.
Application of Diffusion Welding
Diffusion welding applications are vast in different sectors. The main use of diffusion-bonded products is compact heat exchangers in different industries. These are some components made from diffusion bonding.
Copper and aluminum busbars
Wire harness
Hard/rigid connectors
Flexible/soft connectors
Metal braided wires
Heat exchangers
Complex aerospace components
Here are some of the most popular sectors where diffusion welding products are produced.
Electric Industry
Diffusion bonding is used in the electric industry to make copper and aluminum power transformer busbars, wire harnesses, soft and rigid connectors, etc., as well as heavy-duty conductors.
They conduct high currents. Copper and aluminum diffused products are energy-saving, and they improve overall current transformations.
Battery Technology
Small, flexible metallic strips join Individual battery cells within a battery pack. These interconnects can be joined to the battery cells using diffusion bonding, which establishes a robust, low-resistance electrical channel for effective current flow. This is essential to maximize battery performance.
New Energy Storage
The positive and negative electrolytes can be separated from one another in the battery stack by using diffusion bonding to form leak-proof connectors in the flow plates.
Electric Vehicles
Battery Enclosure: In NEVs, aluminum sheets for the battery enclosure can be joined via diffusion bonding. This results in an airtight, robust, and lightweight shell.
Components of Electric Motors: Diffusion bonding enables the combining of different metals in the rotor and stator of electric motors.
Aerospace Industry
The aerospace industry manufactures lightweight and high-strength titanium components through diffusion welding. The bonding components are used in turbine blades, helicopter rotor hubs, honeycomb fabrication, rocket engines, etc.
Diffusion Bonding Advantages and Disadvantages
Diffusion bonding has both advantages and some limitations. But there are many advantages. Let’s look at both advantages and limitations.
Advantages
Strong and Hermetic Joints: Diffusion bonding produces a bond that is almost as strong as the parent material through atomic diffusion. Additionally, the joint is hermetic, which prevents gas or liquid leaks.
Surface Joining: Unlike generic welding, where a single line or corners are joined, diffusion welding allows the joining of entire surfaces.
No Fillers: In diffusion bonding, no filler materials are used, and the surfaces are bonded directly to each other. The binding is clean, and it forms the workpieces as base materials. Please note, using interlayers is not the same as the fillers.
Welding Dissimilar Materials: Dissimilar materials are very hard to bond, and some materials are impossible to weld. However, diffusion welding allows dissimilar materials to be bonded with ease.
Small to No Material Deformation: Since the same base material is joined with up to 90% melting temperature, there is very small to almost no deformation. This helps to maintain the properties and structure of the materials.
Multiple Bonding: In diffusion welding, multiple bonding can be done at once, saving time and money.
Automated: Since the process is automated, no manual labor force is required, and there is no chance of impurities.
Disadvantages
Despite having a lot of advantages, diffusion bonding has some limitations, as follows.
High Set-Up Cost: Diffusion bonding equipment is costly to set up initially.
Not Available for General Usage: Since the cost is higher, diffusion bonding machines are not for generic people. These are only used by bulk product manufacturers commercially.
Not for Mass production: The diffusion welding process requires close attention, and the machines require additional care. This is not a mass-production technology at all.
Dependability: Diffusion bonding is highly dependent on several parameters. We have discussed the parameter impact in the later section.
Parameters That Impact the Process of Diffusion Welding
There are some parameters that impact the entire diffusion bonding process and its outcome. These are very crucial, and you must ensure a balanced implementation of the following parameters.
Pressure
Pressure is among the most crucial parts of the diffusion bonding process. Adequate pressure is required for the initial melting of the base materials and the development of even contact points between the surfaces.
Consistent and evenly distributed pressure ensures proper initial contact of workpiece surfaces and proper bond formation.
Maximum pressure level regarding the materials ensures more adequate joint formation. By the way, to initiate more pressure, you have to spend more money on diffusion welding equipment.
Temperature
Temperature is the most crucial parameter of the diffusion welding process. Temperature must be controlled and well implemented regarding the base materials you use.
For example, you can see the table we have made before showing the melting temperatures of different metals.
Though initially, adequate pressure on the materials impacts the most, temperature plays the most crucial role during the entire bonding process. Precise temperature increases the rate of atomic diffusion.
Pressure and temperature are correlated. The more the temperature and pressure, the less time you need to finish the bonding process cycle. As we have mentioned earlier, you must not exceed the temperature of 50 to 95% of melting temperature of any materials for diffusion.
Time
Time is crucial for diffusion bonding, but it is experimental. For example, as we mentioned before, more pressure and heat decrease the time requirement.
On the other hand, less pressure and heat increase the process time. The longer the time, the less efficient the bonding. So, the time must be set with experimental results so that the entire process doesn’t require more time.
Surface Preparation
The workpieces' surfaces need to be thoroughly cleaned and correctly machined or ground. Clean surfaces reduce the requirements for pressure and temperature because they guarantee appropriate contact between the materials to be bonded.
Oxides, chemically mixed films, or organic, aqueous, or gaseous films are what you should use for cleaning and preparing the surfaces.
Use of Inter-Layers
For TLP diffusion bonding, selecting the right interlayers is crucial, impacting the bonding strength. For example, for diffusion bonding of gold, copper oxide interlayer is most suitable.
Interlayers also solve the compatibility issue of bonding dissimilar materials because they contain required diffusive elements.
With minimum deformation, the base materials are protected from unwanted deformation during the diffusion bonding process. On the other hand, improper interlayers can make the bonding unstable or create strength issues.
Conclusion
So, what is diffusion welding? It is a solid-state diffusion bonding process that joins similar and dissimilar materials. The products produced through this bonding process are used in different industries like aerospace, electric, energy, etc.
Metals and thermoplastics can be diffusion welded. It is a highly sophisticated process and not suitable for general production. Only top-level manufacturers can provide diffusion bonding services with precise and highly sophisticated equipment.
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