The technology of joining materials is vital to the growth of various industries where particularly demanding requirements and sophisticated materials are involved. These industries include aerospace, automotive, shipbuilding, oil, petrochemical and process engineering.

Demanding joining applications have led to increased attention being paid to diffusion bonding. This method is widely used for production of thin-metal components and parts with very complex shapes. Joints made by diffusion bonding meet the requirements for most critical structures in terms of strength, toughness, tightness and resistance to heat and corrosion. Since the process is conducted under a vacuum, a diffusion-bonded joint has minimal impurity content, even in the case of highly reactive metals. Hence, diffusion bonding has found significant application for the fabrication of complex titanium-alloy components.

Key Features of Diffusion Bonding

Diffusion bonding is an attractive joining technology for advanced engineering components, especially when conventional fusion-welding processes degrade the properties of the materials in the heat-affected zone. This bonding technique offers the unique potential to join different metals while avoiding the formation of brittle intermetallic phases, which tend to form in the melt pool during cooling. Diffusion bonding, at times called diffusion welding, is a process by which a joint can be made between different components through the action of diffusion of atoms across the interface.

Diffusion bonding operates on the principle of solid-state diffusion, where the atoms of two solid surfaces intersperse themselves over time. This is usually implemented by applying high pressure, in conjunction with necessarily high temperature, to the materials to be bonded. The process is typically accomplished in vacuum chambers. With properly selected process variables (temperature, pressure and time), the material at and adjacent to the joint will have the same strength and plasticity as the bulk of the parent materials. It is the only process known to preserve the properties inherent in monolithic materials, in both metal-to-metal and nonmetal joints.

This technology has the ability to produce high-quality joints showing a homogeneous structure and high strength of nearly base-material values. When the process is conducted in vacuum, the mating surfaces are not only protected against further contamination (such as oxidation), they are also cleaned because the oxides dissociate, sublime or dissolve and diffuse into the bulk material. As a result, neither metallurgical discontinuities nor porosity exist across the interface.

A key feature of diffusion bonding is that it does not require the use of filler material. A diffusion-bonded product does not gain in weight as happens with ordinarily welded or brazed components. Moreover, there is no need for subsequent machining, so there’s no associated loss of valuable metal. A further advantage is that it can join any components no matter how complicated their shape or cross section may be. As a matter of fact, the process has been used most extensively in the aerospace industries for joining shapes that otherwise could not be made (e.g., honeycomb construction and multiple-finned channels).

Applications of Diffusion Bonding

Diffusion bonding is becoming increasingly critical for transportation industries as vehicles that range from cars and trucks to aircraft are made lighter to reduce fuel consumption and save on ever-higher costs of fuel. Cutting the weight out of cars, trucks and aircraft becomes possible by reducing the gauges of the wall sections for parts that are used to build them. Diffusion bonding is a critical technology for high-efficiency reactors – heat exchangers and fuel cells – and is well known for producing microstructured components deployed in telecommunication, mechanical engineering, medicine and biotechnology.

Diffusion bonding is used instead of precision brazing for end-use applications in which the use of a filler alloy might compromise intricate features and hermeticity. This particular bonding technique is commonly used on accelerators and minicoolers where braze joints and braze fillets can shift the resonant frequency of a cavity or add a very thin thermal shunt layer, which is avoided in diffusion bonding. It is often the joining method of choice in the production of shim assemblies for mini- or micro-channel devices that are used for manifolds, biomedical implants, nozzles, mixers and other precision assemblies. It is also appropriate in situations where the temperatures of the end-use application are very high and there is a risk of the alloy materials softening and the joints weakening in service.

A wide range of components can be joined by diffusion bonding with minimal macroscopic deformation. This means that good dimensional tolerances for products can be attained. Its versatility for unusual material combinations and its relative ease of use for titanium alloys have resulted in more and more industrial users taking advantage of this process.

Diffusion Bonding for Titanium (Ti)

Strength, lightweight, corrosion resistance, biocompatibility: This is the essence of titanium, a metal that is lighter than normal steel but much heavier than aluminum. It is, however, twice as strong as aluminum itself. Pure titanium (Ti), as with most elemental metals, is rarely used commercially, but alloys of Ti are frequently used for a wide variety of purposes.

Titanium alloys have been widely used in the aerospace industry due to their ability to maintain high strength at elevated temperature and high resistance to corrosion. In some modern aircraft, titanium has been used in everything from the outer “skin” and the landing gear to the hydraulic pipes and the innermost parts of the jet engines.

Marine industries have taken a special interest in titanium as well since materials that are in consistent contact with salt water demand a higher resistance to corrosion. But because titanium resists corrosion, is biocompatible and has an innate ability to join with human bone, it has become a staple of the medical field as well. From surgical instruments to orthopedic rods, pins and plates, medical and dental titanium has truly become the fundamental material used in medicine. Parts made from titanium and its alloys are also being used increasingly in the chemical-processing, automotive and nuclear industries.

Due to the increased use of titanium and titanium alloys, the joining process of these materials is of major interest. It is not easy to weld titanium and its alloys, however, because they tend to oxidize at low partial pressures of oxygen and are highly chemically reactive at high temperatures. One of the critical things to remember about welding titanium is that contamination also causes embrittlement, and embrittlement is the leading cause of weld failure. Contamination can come from oil on fingers, lubricants, cutting fluids, paints, dirt or other substances. As a result, the preferred bonding method for titanium and titanium alloys is vacuum diffusion bonding.

Understanding Vacuum Diffusion Bonding

Titanium is one of the most challenging materials to weld because it has an affinity with oxygen, so it needs to be shielded really well. Titanium welding is typically done in a vacuum chamber. Vacuum diffusion bonding relies on temperature, pressure, time and vacuum levels to facilitate atomic exchange across the interface between the materials.

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 can remove even the smallest traces of hydrogen as well as other vapors or gases such as nitrogen, oxygen and water vapor.

Vacuum also plays a key role regarding the cleanliness of the parts, which is important to ensure successful treatment. Vacuum makes it possible to remove oil and traces of moisture from a product at low temperatures, and it 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 the bonding temperature is achieved, and only once this temperature is reached does the gas pressure reach the process setpoint. Because these systems are often large, a significant amount of argon is required. This method enables a reduction in the amount of argon needed by using the temperature to help increase pressure.

High temperatures and high pressure are not typical characteristics of traditional vacuum furnaces for heat treatment. 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.

The temperatures involved in furnaces for the diffusion bonding of titanium reach around 1000°C (1832°F) 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 (nonuniform thickness). This configuration is completely different from the usual design of vacuum furnaces, where uniform irradiation is obtained through the highest possible symmetry of all conditions, and requires more experience from the manufacturer.

For more information: Contact Giuseppe Tonini, CEO & president, TAV Vacuum Furnaces SPA, Via dell’industria 11- 24043, Caravaggio, Italy; tel: +39 0363 355711; e-mail: info@tav-vacuumfurnaces.com; web: www.tav-vacuumfurnaces.com