Controlling the pressure force to obtain a better quality of aluminum 6061 friction stir welded joint
DOI:
https://doi.org/10.15587/1729-4061.2019.159286Keywords:
Friction stir welding, normal force, friction force, coefficient of friction, tensile strengthAbstract
Friction stir welding (FSW) is one of the mechanical joint methods that need no filler metal. The heat is obtained from the friction force between the base metal and friction tool. The FSW is proposed by The Welding Institute – TWI in 1991 which is hoped to overcome problems to weld metals with low weldability. Parameters of FSW determine the quality of the joint and in this paper the normal force as an independent parameter varied at 13,000, 14,000 and 15,000 N. The rotation and feeding speeds are controlled at 1,092 RPM and 200 mm/min respectively. From the experimental results, it is found that 15,000 N provided the strongest joint (specimen 1). The highest heat input was claimed to cause a better joint. This high heat input provides enough weld nugget and time for the aluminum to grow its grains, which in turn increase the tensile strength of the FSW joint.
However, from the average values and the trend line it cannot be assumed that the greater pressure force always produces higher tensile strength. The tensile strengths were almost at the same level (115.44±1.56 MPa) meanwhile the calculated heat input is linearly dependent on the pressure force: 73.59, 79.25 and 84.91 J/mm for pressure forces equal to 13,000, 14,000 and 15,000 respectively. The recorded temperature, which indicates heat input showed the same trend with tensile strength, it was on the same level, those are: 491.55±2.22 °C. There must be some aspects which were not taken into account in the heat input mathematical model. Or maybe, the assumption of higher heat input is not always accomplished by the higher pressure force. Based on the literature, it is found that the yield strength is mitigated at a higher temperature. This lower yield strength decreases the friction force for a constant normal force which in turn causes declining heat input which contradicts the previous assumption.
Another literature that inspected the coefficient of friction as a function of temperature showed that the coefficient is altered due to temperature.
And finally, the higher normal force does not mean a stronger joint can be produced by means of Friction Stir Welding
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