Friction stir welding (FSW) and friction stir spot welding (FSSW) techniques are becoming widely popular joining techniques because of their increasing potential applications in automotive, aerospace, and other structural industries. These techniques have not only successfully joined similar and dissimilar metal and polymer parts but have also successfully developed polymer-metallic hybrid joints. This study classifies the literature available on the FSW and FSSW of thermoplastic polymers and polymer composites on the basis of joining materials (similar or dissimilar), joint configurations, tooling conditions, medium conditions, and study types. It provides a state-of-the-art and detailed review of the experimental studies available on the FSW and FSSW between similar thermoplastics. The mechanical properties of FSW (butt- and lap-joint configurations) and FSSW weld joints depend on various factors. These factors include the welding process parameters (tool rotational speed, tool traverse speed, tool tilt angle, etc.), base material, tool geometry (pin and shoulder size, pin profile, etc.) and tool material, and medium conditions (submerged, non-submerged, heat-assisted tooling, cooling-assisted tooling). Because of the dependence on many factors, it is difficult to optimize the welding conditions to obtain a high-quality weld joint with superior mechanical properties. The general guidelines are established by reviewing the available literature. These guidelines, if followed, will help to achieve high-quality weld joints with least defects and superior mechanical properties. Apart from parametric-based studies, the statistical-based studies (e.g., analysis of variance (ANOVA)-based studies) are covered, which helps with the determination of the influential parameters that affect the FSW and FSSW weld joint strength. Also, the optimal ranges of the most influential process parameters for different thermoplastic materials are established. The current work on the development of general guidelines and determination of influential parameters and their operating ranges from published literature can help with designing smart future experimental studies for obtaining the global optimum welding conditions. The gaps in the available literature and recommendations for future studies are also discussed.

Friction stir welding (FSW) and friction stir spot welding (FSSW) of thermoplastics is becoming popular because of its many industrial applications. This paper specifically reviews the experimental studies available on the FSW and FSSW of high density polyethylene (HDPE), as it is a high demand thermoplastic material used in many industrial applications. The effect of process parameters (tool rotational speed, traverse speed, plunge depth, etc.), tool material and geometry (profile and size), and medium conditions (open-air, submerged, heat-assisted, cooling-assisted, etc.) on the weld joint mechanical properties are discussed. These studies help with narrowing the ranges of all the parameters involved, to optimize welding conditions to achieve highest joint strength. ANOVA (analysis of variance)-based studies are also discussed which help in determining the relative significance of the parameters affecting the joint strength. The thermal behavior of the weld joints, through differential scanning calorimetry (DSC) studies, has also been discussed. Finally, a few recommendations for future studies are presented.

Vertical take-off and landing (VTOL) is a futuristic mode of transport that can potentially revolutionize the way we commute. However, despite the technical leads offered by VTOL, the feasibility of this state-of-the-art urban commute needs to be established from various aspects. This study assesses the temporal and economic benefits offered by VTOL technology over the conventional road taxi service. The mode ‘taxi’ is chosen as it is thought to compete, as a chief mode of urban transport, with the VTOL transport in future. The comparison is based on two decisive factors: the peak hour travel-time and fare by taxi and VTOL on 44 routes in five metropolitan cities: New York, Los Angeles, London, Munich and Shanghai. Analysis of Variance (ANOVA) is performed on the data set and conclusion is drawn on the statistical significance of the difference in the response variables (time and fare) by mode choice (VTOL versus Taxi). VTOL is found to offer significant advantages over the surface transport (taxi) in terms of time and fare. The findings of this study are expected to assist in determining the practicality of VTOL systems. Moreover, the methodology used here is reproducible and provides a framework for testing the practicality of other futuristic transport modes (such as hyperloop).