Diffusion welding is a solid joining technique allowing for full cross-section welding. There is no heat-affected zone, but the whole part is subjected to a heat treatment. By diffusion of atoms across the bonding planes, a monolithic compound is generated.The process takes place in a vacuum or inert gas atmosphere at about 80% of the melting temperature and is run batch-wisely. Hence, it is rarely used despite its advantages to achieve holohedral joints and is widespread in the aerospace sector only.The quality of a diffusion-welded joint is determined by the three main parameters bonding temperature, time, and bearing pressure. The difficulty tailoring the process is that they are interconnected in a strong nonlinear way.Several additional factors may influence the result or may change the material, e.g. surface roughness and passivation layers, all kinds of lattice defects, polymorphic behaviour, and formation of precipitations at grain boundaries, design of the parts to be welded and its aspect ratio as well as mechanical issues of the welding equipment. Hence, experiments are necessary for almost each special part.In this chapter, an overview about the experience of diffusion welding is given. Influences are discussed in detail and conclusions are derived.

The deformation behavior of SS 304 is investigated at three diffusion welding temperatures and different durations. Samples 100 mm in height consisting of five pieces and manufactured from pultrusion round stock are used for studying the diffusion across the bonding planes. The joining crosssection varies from top to bottom covering bonding pressures from 15 to 25 MPa. From the results, a regression equation is derived allowing the calculation of expected deformation for any other temperature and duration within the investigated range. The approach is validated by an experiment with arbitrary temperature and welding duration. To study the influence of the grain size, orientation and passivation layers, a sample made of 51 sheets, 2 mm in thickness each, is diffusion welded and the results are compared.[*] T.

For uniaxial diffusion welding, deformation depends also on the diameter and height of the parts due to friction between sample and stamp applying the load. Also, the number of surfaces leveled during the welding process affects the deformation: for multilayered parts it is higher than for parts consisting of two halves only. Here, samples of different diameters and heights made of SS 304 are investigated at T ¼ 1 075 C, p ¼ 25 MPa, and t ¼ 4 h. These values are chosen to achieve reasonable deformation at large diameters and for flat samples as well. A regression is derived to precalculate the deformation. It is verified by experiments for arbitrary diameters and heights. Samples made of sheet material are diffusion-welded to investigate the influence of the number of layers.

Solid state diffusion bonding is used to produce monolithic parts exhibiting mechanical properties comparable to those of the bulk material. This requires diffusion of atoms across mating surfaces at high temperatures, accompanied by grain growth. In case of steel, polymorphy helps to limit the grain size, since the microstructure is transformed twice. The diffusion coefficient differs extremely for ferritic and austenitic phases. Alloying elements may shift or suppress phase transformation until the melting range. In this paper, diffusion bonding experiments are reported for austenitic, ferritic, and martensitic stainless steels possessing varying alloying elements and contents. Passivation layers of different compositions are formed, thus affecting the local diffusion coefficient and impeding diffusion across faying surfaces. As a consequence, different bonding temperatures are needed to obtain good bonding results, making it difficult to control the deformation of parts, since strong nonlinearities exist between temperature, bonding time, and bearing pressure. For martensitic stainless steel, it is shown that it is very easy to obtain good bonding results at low deformation, whereas ferritic and austenitic stainless steels require much more extreme bonding parameters.

Process parameters for diffusion bonding are temperature, dwell time, and contact pressure. Temperature and contact pressure have opposite effects on deformation. The effect of temperature on deformation was investigated in steps of 20 K from 1015 to 1135 °C. Contact pressure and dwell time were 16 MPa and 4 h, respectively. The deformation increase steadily with temperature. Yield strength and tensile strength decrease slightly with temperature, which is attributed to grain growth. The elongation‐at‐fracture values are 100–105%. For 925 to 995 °C, values for elongation at fracture decrease. It was investigated if comparable mechanical properties can be obtained at a temperature of 850 °C only. Experiments with higher constant contact pressures were supplemented by tests with superimposed short load peaks. Similar and higher values for the yield strength were achieved. A correlation of yield strength, tensile strengths and elongation‐at‐fracture values with contact pressure and contact pressure regime was found. The values for elongation at fracture are significantly lower than those for higher temperatures. This even applies to parameter sets at different temperatures, leading to almost identical deformations. Reduced elongation‐at‐fracture values at 850 °C are attributed to microscopically small defects in the bonding plane and to notch effects.