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Friction-stir welding (FSW), considered to be the most significant development in metal joining in a decade, has been the subject of much scientific interest and industrial application. Developed and patented in 1991 at The Welding Institute (TWI) in the UK, [1] the FSW joining process is based on extreme plastic deformation in the solid state and no associated bulk melting is involved. [2] Its application has been guided by a pressing industrial need, successfully applied to the joining of Al alloys, [3][4][5][6][7] and it appears to be a promising process for joining dissimilar alloys.In recent years, transportation systems have increasingly used light-weight materials, such as high strength steels, Al alloys and Mg alloys, to save energy and reduce pollution. The need to combine such different materials into a hybrid material system, e.g. car bodies and aircraft, is the force driving the development and improvement of welding techniques including FSW. Different concepts for tool materials [8][9][10][11] have lead to increasing interest in FSW of steels as well as of steel-based dissimilar welds.Compared to fusion welding techniques, solid-state welding processes such as FSW have particular advantages for joining dissimilar materials with very different melting intervals and large differences in physical and mechanical properties. [3,12] It has been shown that mild steels, [13] stainless steels [14][15][16] and high strength steels [17] can be friction-stir welded to Al alloys and that the comparatively low heat input characteristic of the process reduces or even prevents the formation of brittle intermetallic phases. [18][19][20] In FSW, the joint is created by frictional heating and associated stirring or forging action as the tool moves along the joint interface. Hence, tool geometry, welding parameters, joint design and the materials to be welded exert significant effects on the material flow pattern and temperature distribution. An understanding of the mechanical and thermal processes during FSW is needed for optimising process parameters and controlling the microstructure and properties of the joints.In particular, material flow and operating mechanisms during FSW are not fully characterised. These factors are, however, crucial to the optimisation of tool geometry and welding procedure that is necessary to obtain high structural efficiency welds. Thus, an increase in the number of approaches to investigating material flow during FSW has been observed in the literature. Reynolds et al. [21][22][23] used a marker insert technique to provide a qualitative characterisation of material flow metallographically. They described FSW as an in-situ extrusion process where the tool shoulder, the pin, the backing plate, and the cold base material form an extrusion die, where the hot material is extruded around both sides of the pin (leading and trailing) into the cavity being vacated by the pin as it moves forward. Complex material movement has also been analysed in dissimilar joints [24][25][26][27] and has been desc...

Friction stir welding (FSW) of dissimilar joints offers serveral advantages over conventional fusion joining techniques since no associated melting is involved. In this study, FSW was applied to join AA6181‐T4 aluminium alloy to HC340LA high strength steel and a systematic investigation of the microstructure formation, the development of FexAly intermetallic compounds and their influences on the mechanical behaviour is presented. Based on fractography analyses, a possible failure scenario for the joint during shear test is also proposed.

The deformation behavior of the Mg-RE alloys ME21 and WE54 was investigated. Although both alloys contain rare earth elements, which alter and weaken the texture, the flow curves of the alloys deviate significantly, especially in uniaxial compression test. Apart from the higher strength of the WE54 alloy, the compression flow curve does not exhibit the typical sigmoidal shape, which is associated with tension twinning. However, optical microscopy, X-ray texture measurements, and EBSD analysis reveal the activity of tension twinning. The combination of in situ energy-dispersive X-ray synchrotron diffraction and EPSC modeling was used to analyze these differences. The investigation reveals that twin propagation is decelerated in the WE54 alloy, which requires a change of the twinning scheme from the 'finite initial fraction' to the 'continuity' assumption. Furthermore, an enhanced activity of the hc+ai pyramidal slip system was observed in case of the WE54 alloy.

Due to their applicability for manufacturing dense, hard and stable coatings, Physical Vapor Deposition (PVD) techniques, such as High Power Impulse Magnetron Sputtering (HiPIMS), are currently used to deposit transition metal nitrides for tribological applications. Cr-Al-N is one of the most promising ceramic coating systems owing to its remarkable mechanical and tribological properties along with excellent corrosion resistance and high-temperature stability. This work explores the possibility of further improving Cr-Al-N coatings by modulation of its microstructure. Multilayer-like Cr1−xAlxN single films were manufactured using the angular oscillation of the substrate surface during HiPIMS. The sputtering process was accomplished using pulse frequencies ranging from 200 to 500 Hz and the resulting films were evaluated with respect to their hardness, Young’s modulus, residual stresses, deposition rate, crystallite size, crystallographic texture, coating morphology, chemical composition, and surface roughness. The multilayer-like structure, with periodicities ranging from 250 to 550 nm, were found associated with misorientation gradients and small-angle grain boundaries along the columnar grains, rather than mesoscopic chemical modulation of the microstructure. This minute modification of microstructure along with associated compressive residual stresses are concluded to explain the increased hardness ranging from 25 to 30 GPa, which is at least 20% over that expected for a film of the same chemical composition grown by a conventional PVD processing route.

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