Friction stir welding (FSW) is a green, pollution-free, low-energy technology with high manoeuvrability. Thermoplastic plastics have extensive applications in the present industry because they offer excellent physical and corrosion properties, high degree freedom of processing and design. In this paper, the current state of FSW/P in plastics industry, including tool improvement, welding methods, process parameters optimisation, metal and polymer joining as well as composites fabrication, has been addressed. Although it presents a major challenge, FSW/P has a great potential to produce defect-free joint and fabricate composites in polymer materials.

A 55Mn2SiCr steel is developed by a novel multiple-step process, which involves austenitizing at 900 C for 30 min, rapid quenching to 210 C, then holding at 170 C for 5 min, and isothermally holding at 250 C for different times, and finally cooling in air. The mixed microstructure consists of lenticular prior martensite (PM), fine needle bainitic ferrite (BF), and filmy retained austenite (RA). The results show that the highest tensile strength of 2030 MPa with a bending strength of 4000 MPa is achieved at 250 C for 120 min. This is attributed to a synergistic multiphase strengthening effect. The presence of martensite formed during the quenching process prior to the isothermal treatment, accelerates the kinetics of subsequent nano-scaled super bainitic transformation by bainitic laths nucleating quickly at the martensite-austenite interfaces. The product of austenite fraction and its carbon content is found to be another important factor for controlling the strength. In addition, the phase evolution as well as carbon partitioning mechanism during isothermal treatment is discussed.

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A multiple phase structure characterised by a mixture of lenticular prior martensite (PM), fine needle bainitic ferrite and film retained austenite (RA) of an unalloyed ductile iron is developed. The designed austempering consists of initial rapid quenching to 210, 200 and 180°C respectively and finally austempering at 220°C for 240 min. The optimum mechanical properties, with a tensile strength of 1330 MPa, an elongation of 3.1% and 422HB, can be achieved by controlling the volume fraction of PM to 12.3% and the RA content to 18.1%. This is mainly attributed to PM that can accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies.

In this article, the feasibility of preparing high-density polyethylene (HDPE)/copper (Cu) composites by submerged friction stir processing was investigated. The results showed that the Cu powders were dispersed with the flow of matrix in the action of stir. The surface layer of the composites was more uniform than core, and the advancing side was more homogenous than retreating side. The crystalline content of HDPE/Cu composites was higher than the processing materials without Cu powders. Compared with the matrix, the tensile strength of composites was lower, however, higher than that processing samples under the same processing parameters. The microhardness of composites was higher than either the matrix of the sample without Cu powders.