**5. Discussion and outlook**

of one or more, a barrel-shaped profile results, accompanied by a high percentaged deforma‐ tion. Flat parts, however, possess a low deformation at the same conditions. For example, for disks of 160 mm in diameter, a deformation of 10% was obtained for a height of 10 mm for *T* = 1075°C, *t* = 4 h, *p* = 25 MPa. For a height of 150 mm, however, the deformation was more than

The number of layers affects the deformation obtained at the same conditions since the roughness of more surfaces must be levelled. For example, a conical sample consisting of 51 layers had a deformation more than 30% higher than the same sample geometry consisting of

**Figure 21.** Conical samples made of 1.4301, T = 1000°C, t = 4 h, F = 17.55 kN, corresponding to 15–25 MPa. Left: Before diffusion welding. Middle: Five segments, deformation: 5.41 and 5.11%, respectively. Right: Sample made of 51 layers;

Diffusion bonding can be carried out using hot isostatic pressing (HIP) at a high isostatic pressure applied by argon of up to 2.500 bar or using a heated press with uniaxial load. For HIP, the parts must be placed inside a steel shield container which is evacuated before sealing.

Additionally, sticking of the parts to the container must be prevented, e.g., by rock wool layers in between or boron nitride spray, or the container has to be machined off afterwards. When using fibrous materials, desorption from a high specific surface area at high temperatures has to be considered. However, also parts with an irregular bonding plane can be welded by HIP, since homogeneous pressure is applied. HIP is widespread and offered by service providers,

Diffusion bonding using uniaxial heated presses is performed under a protective inert gas atmosphere or high vacuum. Only a few companies supply equipment for diffusion welding,

This makes the handling of the parts and the process itself rather expensive.

33% [22].

212 Joining Technologies

five segments only (**Figure 21**).

deformation: 8.34% [6].

e.g., ABRA Fluid AG [23].

**4. Equipment for diffusion welding**

Diffusion welding is the only welding process allowing for full cross-sectional welding, mostly without any liquid phase formation. Since the whole part is subjected to a heat treatment, attention must be paid to undesired material changes. Any cold work hardening effect disappears and the grain size will be larger than before.

With reasonable efforts, high-melting metals, e.g., tungsten or tantalum, cannot be welded.

The equipment is expensive. The process mostly runs batch-wise. Depending on the machinery and the geometry of the parts to be joined, the output is relatively low. Mostly, costs are high.

**References**

Grundstoffindustrie, 1972, p. 446.

978–3–446–42064–9, 2010, p. 70.

7th ed. ISBN 978-1-305-07676-1, 1989, 153 pages.

Munich, ISBN 978-3-446-41194-4, 2008, p. 92.

nanofoil/ [Accessed: 2016–06–23].

*Engineering Materials,* 2014, vol. 16, issue 11, pp. 1381–1390.

Springer, 2011, p. 55.

er-Vieweg, 2012, p. 6.

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[6] T. Gietzelt, V. Toth, A. Hüll, F. Messerschmidt, R. Dittmeyer: "Systematic investigation of the diffusion welding behavior of the austenitic stainless steel 304 (1.4301)", *Advanced*

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The process has to be optimised with respect to temperature, bearing pressure and time, taking into account the composition of an alloy and the mechanical history of the semi-finished product. This makes it an interesting field of research for materials scientists. High tempera‐ tures and long bonding times are favourable as long as grain growth is not important.

Overall, the result of diffusion welding is difficult to control and depends on many other geometrical factors as well. Therefore, it is used mainly for special applications or in the aerospace industry where cost pressure is lower.

The design of a part must be adapted to diffusion welding, e.g., in terms of a constant distri‐ bution of the bonding net cross-section across the part to prevent sink marks. High vacuum tightness is a necessary but not a sufficient criterion for diffusion welding of apparatuses.

To obtain good welding results, a certain deformation always must be accepted. It depends on, e.g., the aspect ratio of the parts and the number of layers to be joined. Obviously, microchannels inside a part will affect the amount of deformation additionally. For multi-layered parts, a higher deformation is required to achieve high vacuum tightness since more surfaces have to be levelled. In consequence, it needs a lot of experience to define appropriate param‐ eters, especially for the bearing pressure, to ensure a sufficient deformation related to the number of layers. Hence, it is not possible to give an exact value of deformation necessary to obtain high vacuum tightness for a material itself.

Since a long bonding time makes it more difficult to control the deformation at a constant bearing pressure, a short increase of bearing pressure for approaching the surfaces may be helpful. Time should be given in between for closure of remaining pores at a reduced constant bearing pressure without a steady strain rate.

As shown for different types of steel, also the material properties and surface passivation layers may have high impact on the behaviour during diffusion welding. Not all materials of the same class can be welded at the same temperature since passivation layers may possess different thermal stability. Often an increased temperature is required to achieve grain growth across the bonding planes, depending on the alloying elements and its content.
