The Effect of Water Cooling on the Mechanical Properties of FSW Butt Joints Made of Aluminium Alloy AA7075-T651

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Abstract
The article presents test results concerning the mechanical properties of friction stir welded joints (FSW) and underwater friction stir welded joints (UWFSW) made of aluminium alloy AA7075-T651. The analysis of microhardness distribution revealed two positive effects of water cooling, i.e. the reduction of the heat affected zone (HAZ) and an increase in the microhardness of the low hardness zone by approximately 15 HV0.1. Static tensile test results revealed that water cooling led to an increase in the yield point of the FSW joint by approximately 18 % (58 MPa) and tensile strength by approximately 9 % (43 MPa). Under low-cycle fatigue conditions, the UWFSW joints were characterised by higher stress amplitude, lower plastic strain amplitude and a lower number of cycles preceding the failure (of the UWFSW joints) than that preceding the failure of the “classical” FSW joints.