Investigation of geometric and physical properties of bushing structure in thermal friction drilling process

Friction drilling is a method that in which the evacuated material softened and flowed by the effect of heat generated in the cross-sectional area of the workpiece and rotating conical tool. The purpose of friction drilling is to increase the length of the connection in thin-walled materials with the help of the bushing formation at the bottom of the hole. In this study, A7075-T651 alloy, using extensively in aeronautics and automotive industries and St37 steel sheets with 2 mm, 4 mm, 6 mm, 8 mm ve 10 mm thicknesses were frictional drilled Ø5 mm, Ø10 mm, Ø15 mm ve Ø20 mm in diameters. Moreover, 1120 rpm spindle speed, 25 mm/rev feed rate, High Speed Steel (HSS) and Tungsten Carbide (WC) conical tool with 36o tool conical angle and 20 mm in length of cylindrical zone were selected. The effect of the selected parameters, such as material thickness and hole diameter, on the height (ha) and density (ρ) of bushing, also the volume of the material evacuated from the drilled material and then forming the bushing height, microstructure and microhardness of the bushing were investigated. In A7075-T651 sheets, bushing height increased with increasing both tool diameter and material thickness. Therefore, the maximum bushing height was measured as 13.40 mm at Ø20 mm in hole diameter and 10 mm in drilled materials sheet thickness, but the minimum bushing height was recorded as 3.43 mm at Ø5 mm in hole diameter and drilled material sheet thickness. Furthermore, the greatest bushing wall thickness was recorded as 4.25 mm at 6 mm material thickness and Ø20 mm in hole diameter, but 0.965 mm at 2 mm in material thickness and Ø5 mm in hole diameter. The density of the bushing increased in direct proportion with the increasing hole diameter. The maximum density values were recorded at 6 mm material sheet thickness for all selected hole diameters. The volume of the bushing increased linearly with increasing both material thickness and hole diameter. Thus, the maximum density values were obtained at 6 mm in material thickness with a hole diameter of Ø20 mm. The maximum micro hardness value of A7075-T651 aluminium alloy is 82.3 HV, maximum micro hardness value for St37 steel material, where there is a big change in micro structure, grain boundaries and structures of St37 steel material, where voids occur in micro structure of the bushing formed as a result of the process and 183 HV. It has been found that the wall thickness of the friction drilled material with tapered tool diameter has a significant effect on the bushing height, bushing wall thickness, microstructure and micro hardness. The results show that the material walls thickness of the most suitable material for hole diameters Ø10, Ø15 and Ø20 mm is 6 mm, but for the hole diameter of Ø5 mm, material wall thicknesses of 4 mm and smaller are suitable.