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Friction Stir Welding (FSW) is a solid-state welding technique that uses the heat generated from friction to assemble a wide variety of materials. Irrespective of having a lower heat input as compared to conventional welding techniques, friction stir welds are still prone to significant thermal-induced stresses and distortions owing to the uneven heating and cooling cycles that a weld goes through. Surprisingly, not several reviews have addressed both the residual stresses and distortions of friction stir welds despite their crucial impact on the weld performance. Therefore, the current paper reviews their development, their correlation with process parameters, and ways to reduce them. Moreover, it explains the current status of process modeling and research gaps in the area of interest.

Friction stir welding (FSW) has matured considerably since its introduction in 1991. Over the last decades, it has indeed branched and been applied in different fields, such as automotive, aerospace, railway, and shipbuilding. This article aims to survey the basic knowledge related to the conventional FSW of aluminum alloys in order to provide a tool for understanding the friction stir processes. The review covers the five basic process parameters: rotational speed, welding speed, tool geometry, tilt angle, and plunge depth. Furthermore, it presents the related equations and recommended ranges of those parameters to facilitate the process design step for industrial implementation. A sample of 30 published articles was drawn for that purpose. The current article also discusses the main five properties most researchers are interested in, namely, microstructure, microhardness, tensile strength, residual stresses, and distortion.

In the present research, a coupled Eulerian–Lagrangian model is developed to predict the forces and defects produced from AA6063 friction stir welding process. Furthermore, the obtained results from the developed model were validated experimentally by performing the welding process at different rotational and welding speed values. The results revealed an appearance of tunnel defect at all welded joints with surface flash. The current model successfully predicted both defects’ evolution. Additionally, it also generated force measurements comparable to those obtained experimentally. In addition, the developed Coupled Eulerian–Lagrangian model successfully predicted the generated force and the defects during friction stir welding process with maximum error of 14% and 18%. However, it failed in predicting the tunnel positions. In terms of the tensile strength, the largest tensile strength value was at 1250rpm and 50 mm/min, whereas the lowest one was at 1000 rpm and 25 mm/min. Moreover, the fracture locations were at the advancing side of all the welded joints.

Lignin is the second most abundant natural polymer after cellulose. It has high molecular weight and poor dispersity, which lowers its compatibility with other polymeric materials. Accordingly, it is hard to integrate lignin into polymer-based applications in its native form. Recently, lignin valorization, which aims to boost lignin value and reactivity with other materials, has captured the interest of many researchers. The volatility of oil and gas prices is one strong incentive for them to consider lignin as a potential replacement for many petroleum-based materials. In this chapter, lignin valorization processes, namely hydrogenolysis, pyrolysis, hydro-thermal liquefaction, and hydro-thermal carbonization, are discussed in brief. The chapter also discusses the synthesis of lignin-based epoxy resin as an already existing example of a lignin-based product.

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