Date of Award

2016

Document Type

Open Access Thesis

Department

Mechanical Engineering

Sub-Department

Aerospace Engineering

First Advisor

Michael Van Tooren

Abstract

The joining of thermoplastics through welding, a specific form of fusion bonding, offers numerous advantages over mechanical joining. It eliminates the use of costly fasteners and has only a limited effect on the strength of the parts being joined since it does not require the introduction of holes and loading pins, and it does not create significant stress concentrations. A specific form of welding, Friction Stir Welding, was investigated for the creation of butt joints of unreinforced polyphenylene sulfide (PPS) and short carbon fiber reinforced polyetheretherketone (PEEK) plates. Friction stir welding requires a rotating pin, a shoulder arrangement, relative movement between the tool and the weld piece and a clamping mechanism to hold the weld piece in place. Analytical models and experimental results show that the heat generated by the FSW tool is insufficient to produce the heat required to weld thermoplastic materials which makes FSW of polymers different from FSW of metals. A second heat source is required for preheating the thermoplastic parts prior to welding. A resistance type surface heater was placed at the bottom of two identical weld pieces for the experiments. Two types of shoulder design i.e. a rotating shoulder and a stationary shoulder were developed. Taguchi’s Design of Experiment method was utilized to investigate the welding process, where duration of heating, process temperature, tool rotational speed and tool traverse speed were used as the welding parameters. The quality of the welding process was assumed to be indicated by the weld strength. DoE revealed that one of the process parameters, tool traverse speed, had significant influence on the tensile strength of PPS samples. While PPS sample showed relatively lower tensile strength with higher traverse speed, short carbon fiber reinforced PEEK samples had higher tensile strength with higher traverse speeds.

In addition to tensile tests on dog bone shaped specimen, fracture toughness tests were performed for both PPS and PEEK samples to identify the fracture toughness of these materials. Presence of un-welded section in the welded specimen due to the setup of the experiments yielded notched tensile strengths during the tensile test process. With the help of fracture toughness values of these materials, notched tensile strengths of the welded samples were compared with the notched tensile strengths or residual tensile strengths of the base materials. In this study, residual joint efficiency of PEEK samples was found higher than that of PPS samples. Additionally, notched tensile strengths of the welded samples were compared with un-notched tensile strengths of the materials. The notched tensile strengths of PPS and PEEK were found about 80% and 75% of the respective base materials. Micrographs of PEEK samples showed the presence of more voids and cracks in the weld line compared to the un-welded samples.

In this study, continuous friction stir welding process has been developed for butt joining of unreinforced PPS and short carbon fiber reinforced PEEK. The process parameters and the experimental setup can be utilized to investigate the weldability of different types of thermoplastic composites and various types of joint configurations.

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