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High-strain-rate deformation in ultrasonic additive manufacturing was analyzed by performing microstructural characterization via electron microscopy. The micro-asperities on the top tape surface, which were formed by contact with the sonotrode surface, underwent cyclic deformation in the shear direction at high strain rates during welding with an additional tape. This caused plastic flow and crushing of the micro-asperities, and a flattened interface was formed between the upper and lower tapes. Further, surface oxide films were fractured and dispersed by ultrasonic vibration, and metallurgical welding was achieved.

Dissimilar welds between Al alloy and stainless steel were produced with an ultrasonic welding technique. The weld strength increased with the welding energy. The welds produced with sufficiently high energy exhibited nugget pull-out failure of the Al alloy during the lap shear strength test. The welds with weld energies of more than 1.05 kJ fractured in the base metal and were severely deformed by the ultrasonic vibration, and recrystallization occurred around the weld interface owing to the shear deformation and heating during the ultrasonic welding.

Friction stir welding was applied to a zirconium alloy, Zircaloy-4, using a Co-based alloy tool. Zircaloy-4 was successfully welded, and a shiny, smooth surface was obtained. FSW produced a defect-free weld with a fine equiaxed grain structure in the stir zone, which caused an increase in hardness.

The electron backscattering diffraction technique was employed to study the crystallography of the transformed β microstructure developed in a friction stir welded Ti–6Al–4V alloy. Direct orientation measurements in the α and β phases together with analysis of the misorientation distribution in the α phase showed that β-to-α phase transformation was governed by Burgers orientation relationship.

The electron backscattered diffraction technique was employed to examine the microstructural evolution in the high-temperature β-phase field during friction stir welding of a Ti–6Al–4V alloy. Reconstruction of the β-grain structure demonstrated that development of the grain structure is presumably governed by grain elongation and transverse grain subdivision. Orientation measurements in the retained β-phase showed that the material flow may be described in terms of a simple-shear deformation arising from the {1 1 0}〈1 1 1〉 slip.

The effective strain rate during friction stir welding (FSW) of Al alloy 1050 was estimated experimentally by simulating the recrystallized grains of the stir zone through a combination of the plane-strain compression at various strain rates and the subsequent cooling tracing the cooling cycle of FSW. With the plane-strain compression test, it was possible to simulate the recrystallized grain structure of the friction stir welds, and the effective strain rate was estimated to be about 2–3 s −1 .

Corrosion properties were evaluated in a friction stir welded 304 stainless steel. The degree of the sensitization was small in the heat affected zone, but the advancing side of the stir zone was corroded significantly because of the formation of the sigma phase.

A dissimilar friction stir weld of Al alloy 1050 and Mg alloy AZ31 was produced. The weld had a large volume of intermetallic compound Al 12 Mg 17 and significantly higher hardness in the weld center. The present study suggests that constitutional liquation resulted in the intermetallic compound Al 12 Mg 17 in the weld center.

A local region in the preferentially etched line at the root part of a friction stir welded Al alloy was directly observed by TEM. TEM clarified that the preferentially etched line consisted of zigzag region having a high density of amorphous Al2O3 particles which originated from the initial butt surface oxide layer.

Friction stir welding (FSW) was applied to an accumulative roll-bonded (ARBed) Al alloy 1100. FSW resulted in reproduction of fine grains in the stir zone and small growth of the ultrafine grains of the ARBed material just outside the stir zone. Consequently, FSW effectively prevented the softening in the ARBed alloy.

Rapid formation of sigma phase occurred in 304 stainless steel during friction stir welding. A possibility was suggested that the sigma formation can be accelerated by the emergence of delta-ferrite at high temperature and the subsequent decomposition of the ferrite under the high strain and recrystallization induced by friction stirring.

An analytical transmission electron microscopic study of a twin-induced grain boundary engineered 304 austenitic stainless steel demonstrated that the chromium depletion at a low-energy boundary segment introduced by twin-emission into a random boundary was smaller than that of the original random boundary after sensitization.

Microstructural evolution of magnesium alloy AZ61 during friction stir welding and details of the relationship between the microstructure and mechanical properties are discussed. Tensile properties of the weld of Mg alloy AZ61 were strongly influenced by crystallographic orientation distribution as well as by grain size and dislocation density.

The effect of Zr on reduction of hardness and microstructure in an FS weld of equal channel angular-pressed Al alloy was investigated. Zr addition to Al suppressed dynamic recovery in the thermomechanically affected zone and enabled retention of the high hardness of the ECA-pressed material throughout the weld.

Duplex stainless steels are characterized by favorable combination of mechanical and corrosion properties, consisting roughly of equal parts of austenite ({gamma}) and ferrite ({alpha}). But exposure to elevated temperatures brings partial decomposition of ferrite to austenite and sigma phase, which deteriorates their properties. Sigma phase forms often at ferrite/austenite ({alpha}/{gamma}) interfaces through nucleation process. The heterogeneous nucleation of sigma phase at an {alpha}/{gamma} interface depends on the chemical driving force and the interfacial energy. Many studies have examined the effect of chemical driving force on sigma phase formation in duplex and austenitic stainless steel weld metals with different chemical compositions, but no detailed report has described the influence of {alpha}/{gamma} interfacial energy on sigma phase nucleation. The Kurdjumov-Sachs (K-S) orientation relationship is accepted to bring a coherent and low energy {alpha}/{gamma} interface in duplex stainless steels. The coherency of {alpha}/{gamma} interface can affect the sigma phase formation. The present study has examined the effect of crystallographic orientation relationship at {alpha}/{gamma} interface on sigma phase formation in a duplex stainless steel weld metal where the chemical element distribution is relatively uniform because of rapid cooling during weld thermal cycle.