**1. Introduction**

Diffusion welding is the only welding technique by means of which full cross-sectional welds, also of internal structures, can be obtained. Normally, there is no liquid phase and the monolithic compound is formed completely under solid-state conditions.

**2. Micro-structure of metals and the impact of lattice defects on diffusion**

Diffusion Bonding: Influence of Process Parameters and Material Microstructure

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To minimise the energy of a system, isolated metal atoms tend to arrange in a regular lattice at positions according to the annihilation of attractive and repulsive forces (**Figure 1**). The positions are well-defined and specific of each metal. Hence, they can be used, e.g., for determining the composition of an alloy by means of WDX (wavelength dispersive X-ray). When forming a compound, atoms split up into positively charged atomic nuclei, while valence electrons are transferred to the so-called electron gas and can move freely within the

Depending on the thermal energy of the whole system, the positively charged atomic nuclei oscillate around their position, leading to a thermal expansion (**Figure 2**). According to *Grüneisen's rule*, linear expansion is in the range of 2% and volumetric expansion is 6–7% up to the melting point of a metal [1]. Hence, the melting point can be used to estimate the thermal coefficient of expansion. Below the melting temperature, the oscillation amplitude is about

lattice. Consequently, metals are good conductors of heat and electricity.

**Figure 1.** Equilibrium of attractive and repulsive forces in the metallic lattice [1].

**welding**

**2.1. Atoms in the lattice of metals**

*2.1.1. Thermal expansion*

12% of the lattice constant [2].

**Figure 2.** Thermal oscillation of atoms.

For the conditions to be appropriate, mechanical properties across the joined part are compa‐ rable to the bulk material. Due to heating of the whole parts, no distinct heat-affected zone (HAZ) is formed. However, properties are changed compared to the as-delivered conditions of the material. This may cause problems in some cases.

For diffusion welding, special and expensive equipment is required: the parts have to be mated at high temperatures by applying high forces depending on the size and cross-sec‐ tion to be welded under a vacuum or inert gas environment. Equipment and parts are heat‐ ed mostly indirectly by radiation. To limit thermal stress, the heating rates are restricted to some 10 K/min.

The welding process takes place in vacuum and cannot be performed on site. Mating surfa‐ ces must be free of any impurities and have a low surface roughness without deep scratch‐ es. Joining of multiple layers is possible in one step.

Diffusion welding is always accompanied by a certain deformation of the parts. This defor‐ mation depends mainly on bonding temperature, bonding time and bearing pressure. Unfortunately, influences of temperature and bearing pressure are non-linear, making it difficult to predict the deformation of a new design. Additionally, secondary impacts on deformation and the quality of joining may be due to specific geometric parameters, e.g., the aspect ratio, the number of layers, the micro-structure of the material itself and surface layers.

Recently, thin coatings of other metals, forming a temporary liquid phase (TLP) by passing a eutectic composition, or multiple layers of different metals of nanometre thickness exploiting the enormous interfacial energy of such compounds were investigated.

In contrast to conventional welding techniques, such processes are highly complex. The process has to be optimised for each material and even for different compositions of alloys depending on the geometry. For this reason, application of diffusion welding is limited to the aerospace industry or special applications where other welding techniques fail. For ex‐ ample, large- and thin-walled titanium sheets are joined to reinforcing structures and inter‐ nal cooling channels for injection moulding tools and nozzles of rocket engines.

Unfortunately, not all the information necessary for reproducing the results, e.g., material, procedure of sample preparation and process parameters, is given in the literature.

For joining micro-structured components, additional aspects must be taken into account.

The aim of this chapter is to summarise knowledge on diffusion welding in conjunction with the fundamental processes taking place inside of the micro-structure of a material. For this, lattice defects are discussed according to their dimensionality.
