STUDY OF MECHANICAL PROPERTIES AND MICROSTRUCTURE OF FRICTION STIR WELDING OF DISSIMILAR ALLOY

Abstract

Tungsten inert gas (TIG) welding is the most commonly used joining process for aluminum alloy for AA6061 and AA7075 which are highly demanded in the aerospace engineering and automobile sector, but there are some defects occurs during TIG welding like micro-crack, coarse grain structure, and porosity. To improve these defects, the TIG welded joint is processed using friction stir processing (FSP). In this work, the FSP tool pin rotates on an already welded joint by TIG welding to improve the lower the welding load and weld quality by adjusting the processing parameters of friction stir processing and analyzed the mechanical properties of TIG, FSW and TIG+FSP welded joint and then computational fluid dynamics based numerical model was developed to predict the temperature distribution and material flow during FSW of dissimilar aluminum alloys of AA6061 and AA7075 by ANSYS fluent software. The tensile strength, micro-hardness and residual stress of friction stir welded joint was observed 221.3 MPa, 97 HV and 29 MPa respectively at tool rotational speed of 1300 rpm, traverse speed of 30 mm/min with tilt angle 20 and the tensile strength of TIG welded joints with filler ER4043 and ER5356 was observed 158.6 MPa and 176.2 MPa respectively. After friction stir processing on TIG welded joint, the maximum tensile strength (255 MPa), microhardness (105 HV) and minimum residual stress (28.3 MPa) for TIG+FSP welded joints were observed at tool rotational speed of 1300 rpm, traverse speed of 45 mm/min with tilt angle 1 with filler ER4043, whereas the maximum tensile strength (281.1 MPa), microhardness (107.1 HV) and minimum residual stress (18.3 MPa) for TIG+FSP welded joints were observed at tool rotational speed of 1300 rpm, traverse speed of 30 mm/min with tilt angle 2 with filler ER5356. The empirical relationships were developed to analyze the tensile strength, % strain, residual stress and microhardness of TIG+FSP welded joint of AA6061 and AA7075 at 95% confidence level. The optimized value of tensile stress, percentage strain, microhardness at nugget zone and residual stress at nugget zone are 218.82 MPa, 24.15, 90.21 HV and 45.19 MPa respectively, whereas the optimized processing parameters i.e. tool rotational speed, feed rate and tilt angle are 1076.24 rpm, 37.76 mm/min and 1.730 respectively for filler ER 4043 and the optimized value of tensile stress, percentage strain, microhardness at nugget zone and residual stress at nugget iv

zone are 266.66 MPa, 29.74, 103.19 HV and 21.66 MPa respectively, whereas the optimized processing parameters i.e. tool rotational speed, feed rate and tilt angle are 1278.12 rpm, 35.95 mm/min and 1.750 respectively for filler ER 5356. The predicted peak values of temperature at the weld region were calculated by the ANSYS software and found the maximum temperature about 515ºC at tool rotation of 1300 rpm. To increases the tool rotational speed and decrease the feed rate, leads to increases in heat input in TIG+FSP welded joint. The residual stress of TIG welded joints with filler ER4043 and ER5356 was observed 77 MPa and 63 MPa respectively. The grains in the nugget zone in TIG+FSP welded joint was observed much finer than the TIG welded joint at fusion zone. The large dimples and quasi cleavage with a sharp edge and various depths were found on the fractured tensile specimen surface of low tool rotational speed whereas fine dimples were found at high tool rotational speed of TIG+FSP welded joints.