Joints between two different grades of aluminium alloys are need of the hour in many light weight military structures.In this investigation,an attempt has been made to join the heat treatable(AA 6061) and non-heat treatable(AA 5086) aluminium alloys by friction stir welding(FSW)process using three different tool pin profiles like straight cylindrical,taper cylindrical and threaded cylindrical.The microstructures of various regions were observed and analyzed by means of optical and scanning electron microscope.The tensile properties and microhardness were evaluated for the welded joint.From this investigation it is founded that the use of threaded pin profile of tool contributes to better flow of materials between two alloys and the generation of defect free stir zone.It also resulted in higher hardness values of 83 HV in the stir zone and higher tensile strength of 169 MPa compared to other two profiles.The increase in hardness is attributed to the formation of fine grains and intermetallics in the stir zone,and in addition,the reduced size of weaker regions,such as TMAZ and HAZ regions,results in higher tensile properties. Joints between two different grades of aluminum alloys are need of the hour in many light weight military structures. This investigation, an attempt has been made to join the heat treatable (AA 6061) and non-heat treatable (AA 5086) aluminum alloys by friction stir welding (FSW) process using three different tool pin profiles like straight cylindrical, taper cylindrical and threaded cylindrical. microstructures of various regions were observed and analyzed by means of optical and scanning electron microscope. tensile properties and microhardness were evaluated for the welded joint. Fv this investigation it is founded that the use of threaded pin profile of tool contributes to better flow of materials between two alloys and the generation of defect free stir zone. It also resulted in higher hardness values ​​of 83 HV in the stir zone and higher tensile strength of 169 MPa compared to other two profiles. The increase in hardness is attributed to the formation of fine grains and intermet allics in the stir zone, and in addition, the reduced size of weaker regions, such as TMAZ and HAZ regions, results in higher tensile properties.