Titanium (Ti) has the ability to dissolve its own oxide at bonding temperatures, making it the easiest of all common engineering materials to join by diffusion bonding.
This article particularly focuses on this material and describes how diffusion bonding works, why it is the ideal joining technique for titanium and its alloys, and which heat cycle is needed for diffusion bonding of titanium in vacuum furnaces. The article also provides some examples of diffusion bonding applications.
How the Diffusion Bonding Process Works
Diffusion bonding, also known as diffusion welding, is a solid state joining process that is based on the atomic diffusion of elements at the joining interface.
This bonding technique provides an attractive option for the robust bonding of dissimilar engineering materials to form engineering structures and devices.
The process has been used extensively in the aerospace industries to join shapes and materials that otherwise could not be produced - for instance, honeycomb construction and multiple-finned channels.
Diffusion bonding can be used to join dissimilar materials with different thermo-physical characteristics, which is not possible by other processes. The dffusion bonding process can be used to join ceramics, metals, alloys, and powder metallurgy products, with minimum macroscopic deformation.
High precision components with complex cross sections or shapes can be produced without the need for subsequent machining, making it possible to achieve good dimensional tolerances for the products. Chemical heterogeneities can also be minimized with the diffusion bonding process. Additionally, this technique can be used to prevent typical defects such as segregation, crack, and distortion.
In order to produce a metallurgical joint between dissimilar metals, a faster diffusion rate achieved by longer holding time and higher bonding temperature between the materials is essential. Today, most of the bonding operations are carried out in vacuum furnaces. Diffusion bonding depends on time, pressure, temperature, and (ultra low) vacuum levels to enable atomic exchange across the interface between the materials.
Why Diffusion Bonding is Adopted for Titanium
Titanium is an excellent material, used extensively in industrial applications due to its high specific strength, favorable high temperature properties, and good erosion resistance. It is 60% heavier than aluminum (Al) and yet is twice as strong, and while it is 40% lighter than steel but 30% stronger.
Titanium is also used in combination with other metals such as Iron (Fe), aluminum, manganese (Mn), molybdenum (Mo), etc. to further improve its anti-corrosive properties and its considerable strength, especially at high temperatures (to rocket engine fuels).
Titanium is used in the aerospace industry to produce structural components of wings as well as various components of aircraft engines, skins for hydraulics systems in aircrafts, and cabins of spacecraft, where its qualities are irreplaceable. Titanium has exceptional characteristics that make it suitable for use in submarine equipment as well as in marine environments for propellers on ships, boats, or other parts that are subject to corrosion.
In the medical sector, titanium is used to produce pace-makers, hip and knee replacements, cranial plates for skull fractures, and bone-plates and screws. In the military sector, titanium and titanium alloys are used to make missiles, rockets, and other equipment. The demand for titanium is also growing for the production of motorcycles, in the petrochemical sector, and for oil platforms at sea.
Due to the increased use of titanium and titanium alloys, the joining process of these materials is of major interest. However, it is not easy to weld titanium and titanium alloys because they tend to oxidize at low partial pressures of oxygen and are highly chemically reactive at high temperatures.
During the welding process, titanium alloys easily pick up nitrogen and oxygen from the atmosphere, so the preferred joining method for titanium and titanium alloys is vacuum diffusion.
The article further reviews vacuum diffusion bonding regarding the heat cycle needed for diffusion bonding of titanium. It also provides several examples of diffusion bonding applications.
How Vacuum Furnaces Work for Diffusion Bonding of Titanium
With regard to the heat cycle required for the diffusion bonding of titanium, it is important to ensure that the vacuum furnace operates at high temperatures and with highly pressurized argon gas. The vacuum can remove even the smallest traces of hydrogen as well as other vapors or gases such as nitrogen, oxygen, and water vapor.
The vacuum also has a key role regarding the cleanliness of the parts. which is important to ensure successful treatment, because this makes it possible to remove oil or solvent vapors and traces of moisture at low temperatures, and it gives an indication about whether to interrupt the cycle due to the evaporation of pollutants before ruining the heat.
The vacuum is maintained until the bonding temperature is achieved, and only once this temperature is reached does the gas pressure reach the process set. As these facilities are often large, a significant amount of argon is required, and this method enables a reduction in the amount of argon needed by using the temperature to help increase pressure.
High pressure and high temperatures are not the standard characteristics of traditional vacuum furnaces for heat treatments. These furnaces 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 if the gas permeability of the material is greater, the effect will be more pronounced.
Shields are used in vacuum furnaces that operate with very high pressure (hundreds of bars) and very high temperatures (2000 °C). These shields are independent from the vessels and are used to protect them, so that the heat flow is intercepted by using a water-cooled circuit specifically installed for this purpose.
As the vessel is very thick in order to tackle the high pressure, it would not benefit from a cooling jacket so as not to surpass the maximum temperature on the internal surface. There would be a risk of the vessel exploding.
In furnaces used for the diffusion bonding of titanium, the temperatures can reach approximately 1000 °C with pressures of tens of bars. This means, the hot zone can still be isolated with a graphite board but convection currents introduce temperature stratification that must be offset by making sure that the design of the heat chamber is vertically asymmetrical, in terms of both the resistor and heat isolation (non-uniform thickness).
This configuration is entirely different from the typical design of vacuum furnaces, where uniform irradiation can be achieved through the highest possible symmetry of all conditions, and needs more experience from the manufacturer.
Where Diffusion Bonding Better Applies
Currently, diffusion bonding can be used to make turbine blades by joining the two lateral elements of the blade with another titanium shape in the center. A layer of ceramic dust covers the exposed surfaces of the internal shape. Once welding treatment is complete, pressure is used to blow out the sides and elevate the edges of the intermediary metal; this is an alternative solution to the honeycomb structure.
Following this, the part is given the twist typical of an aerofoil blade through hot pressing in a die. Engine performance is improved using blades produced through this method. This may be due to greater form drag at high temperatures.
Another application of diffusion bonding relates to the production of titanium heat exchangers for use in marine environments and in contact with sea water. The same method mentioned above is used in a similar furnace.
In this example, a layer of ceramic dust is inserted between the elements, which define the areas where diffusion cannot occur. After joining the various elements of the exchanger, pressurized gas is introduced that separates the non-welded surfaces and creates the internal pathway of the liquid via the exchanger.
These products are generally large, and hence the material’s benefits relate both to the capacity to resist corrosion and the issue of weight, which becomes crucial for the type of setting in which it is to be used.
Diffusion bonding is also used in vacuum furnaces to make structural elements for cars. This application overcomes the issues related to traditional TIG-bonding. Joints produced through TIG-bonding do not provide the same guarantees as those produced through diffusion bonding. The seam left by TIG-bonding is discontinuous and leads to porosity, making it difficult to obtain a good finish.
This information has been sourced, reviewed and adapted from materials provided by TAV Vacuum Furnaces.
For more information on this source, please visit TAV Vacuum Furnaces.