In this article I will particularly focus on titanium, as it is the easiest of all common engineering materials to join by diffusion bonding, due to its ability to dissolve its own oxide at bonding temperatures. So, let’s see in detail how diffusion bonding works, why it is a preferred joining method for Titanium (Ti) and Titanium alloys, and which heat cycle is required for the diffusion bonding of titanium in vacuum furnaces. In addition, I’ll give you a few examples of diffusion bonding applications.
How the diffusion bonding process works
Diffusion bonding or diffusion welding is a solid state joining process. This bonding technique is based on the atomic diffusion of elements at the joining interface. Diffusion bonding is a very attractive process for the strong bonding of dissimilar engineering materials in order to form engineering devices and structures. The process has been used most extensively in the aerospace industries for joining materials and shapes that otherwise could not be made (for example, multiple-finned channels and honeycomb construction).
Joining of dissimilar materials with different thermo-physical characteristics, which is not possible by other processes, may be achieved by diffusion bonding. Metals, alloys, ceramics and powder metallurgy products can be joined by diffusion bonding process with minimum macroscopic deformation. High precision components with intricate shapes or cross sections can be manufactured without subsequent machining. This means that good dimensional tolerances for the products can be attained. By diffusion bonding process the chemical heterogeneities can be minimized. Furthermore, usual defects such as crack, distortion and segregation can be avoided with this technique.
To produce a metallurgical joint between dissimilar metals, a faster diffusion rate accomplished by higher bonding temperature and longer holding time between the materials is necessary. Nowadays, the majority of bonding operations are performed in vacuum furnaces. Diffusion bonding relies on temperature, pressure, time, and (ultra low) vacuum levels to facilitate atomic exchange across the interface between the materials.
Why diffusion bonding is adopted for Titanium
Titanium (Ti) is an excellent material widely used in industrial applications because of its high specific strength, good erosion resistance and favorable high temperature properties. It is 30% stronger than steel and yet is 40% lighter, and while it is 60% heavier than aluminium it is twice as strong. Moreover, titanium is used in combination with Aluminium (Al), Manganese (Mn), Iron (Fe), Molybdenum (Mo) and other metals in order to further enhance its considerable strength, particularly at high temperatures (to rocket engine fuels), and its anti-corrosive properties.
In the aerospace industry titanium is used in manufacturing the structural components of wings as well as skins for hydraulics systems in aircraft, various components of aircraft engines and the cabins of spacecraft, where its qualities are irreplaceable. Its exceptional characteristics have many uses in marine environments, for propellers on boats and ships or other parts subject to corrosion, as well as for submarine equipment. In the military sector Ti and Ti alloys are used in the production of rockets, missiles and other equipment. In medical sector titanium is use to make hip and knee replacements, pace-makers, bone-plates and screws and cranial plates for skull fractures. Demand for titanium is increasing in the petrochemical industry and for oil platforms at sea as well as for the production of motorcycles.
With the increased use of titanium and its alloys, the joining process of Ti and its alloys is of great interest. Unfortunately, welding of titanium and titanium alloys is difficult as they are highly chemically reactive at high temperatures and tend to oxidize at very low partial pressures of oxygen. During the welding process, titanium alloys pick up oxygen and nitrogen from the atmosphere very easily. As a result, vacuum diffusion bonding is a preferred joining method for titanium and titanium alloys.
Let’s now take a look at vacuum diffusion bonding regarding the heat cycle required for diffusion bonding of titanium. In addition, I’ll give you a few examples of diffusion bonding applications.
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How vacuum furnaces work for diffusion bonding of titanium
Regarding the heat cycle required for the diffusion bonding of titanium, the vacuum furnace must operate at high temperatures and with highly pressurized argon gas. The vacuum is required to eliminate even the smallest traces of hydrogen, in particular, but also other gases or vapors, including oxygen, nitrogen and water vapor. The vacuum also plays a key role regarding the cleanliness of the parts, a crucial requirement for ensuring successful treatment, in that it makes it possible to eliminate oil or solvent vapors and traces of moisture at low temperatures and can provide an indication about whether to interrupt the cycle owing to the evaporation of pollutants before ruining the heat. The vacuum is maintained until attaining the bonding temperature and only when this temperature is reached does the gas pressure reach the process set. Given that these facilities are usually large, a significant quantity of argon is necessary, and this method allows a reduction in the amount of argon required by using the temperature to help increase pressure.
High temperatures and high pressure are not typical characteristics of traditional vacuum furnaces for heat treatments, which have a water-cooled vacuum chamber and a heat chamber which isolates the hot zone from the cold wall of the vessel. The pressurized gas tends to neutralize the isolation capacity of the material used for the heat chamber and the greater the gas permeability of the material, the more pronounced this effect will be. In vacuum furnaces operating at extremely high temperatures (2000 °C) and with extremely high pressure (hundreds of bars), shields are used which are independent from the vessels, to protect them, in order to intercept the heat flow using a water-cooled circuit installed specifically for this purpose. Since the vessel is very thick in order to deal with the high pressure, it would not benefit from a cooling jacket so as not to exceed the maximum temperature on the internal surface. There would be a risk of the vessel exploding!
In furnaces for the diffusion bonding of titanium, the temperatures involved reach around 1000 °C with pressures of tens of bars, meaning it still possible to use a graphite board to isolate the hot zone. However, the temperature stratification introduced by the convection currents must be offset by ensuring the design of the heat chamber is vertically asymmetrical, both in terms of heat isolation (non-uniform thickness) and the resistor. This configuration is completely different from the usual design of vacuum furnaces, in which uniform irradiation is obtained through the highest possible symmetry of all conditions, and requires more experience on the part of the manufacturer.
Where diffusion bonding better applies
Nowadays diffusion bonding can be used to produce turbine blades by welding the two lateral elements of the blade with another titanium shape in the middle. The uncovered surfaces of the internal shape are covered with a layer of ceramic dust. Once the welding treatment has been completed, pressure is used to blow out the sides and raise the edges of the intermediary metal. This solution is an alternative to the honeycomb structure. The part is then given the twist typical of an aerofoil blade through hot pressing in a die. The use of blades produced using this method improves engine performance. We believe it is because of greater form drag at high temperatures.
Another application relates to the production of titanium heat exchangers for use in marine environments and in contact with sea water. The same technique described above is used and in a similar furnace. In this case a layer of ceramic dust is also inserted between the elements, which sets out the areas where diffusion cannot take place. Once the various elements of the exchanger have been bonded, pressurized gas is introduced, which separates the non-welded surfaces, creating the internal pathway of the liquid through the exchanger. In general these products are very large, and therefore the benefits of this material do not only relate to the capacity to resist corrosion but also the issue of weight, which becomes important for the type of setting in which it will be used.
Finally, diffusion bonding is used in vacuum furnaces to produce structural elements for cars. This application solves problems associated with conventional TIG-bonding. Joints produced through TIG-bonding do not provide the same guarantees as those in diffusion bonding. The seam left by TIG-bonding is discontinuous and results in porosity and it is therefore hard to achieve a good finish.
So, what do you think of diffusion bonding? Do you have an additional application to share with us?
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