The friction drilling process utilizes frictional heat to increase the thickness of sheet metal parts by making holes with bushes using a non-traditional drilling tool. Unfortunately, cracks are found in the formed bushings during friction drilling of brittle materials. The purpose of the present study is to improve the quality of friction-induced bushes in brittle A356 aluminum sheets via a performance of a pre-drilling with different diameters (2, 2.5, and 3 mm). The pre-drilling is aimed to reduce cracks and petal formation as compared with the solid sheets. The friction drilling processes were carried out by a K100 conical tool, with 10 mm diameter and 58° cone angle. The friction drilling parameters involve different rotational speeds (2000, 3000, and 4000 rpm) as well as different feed rates (40, 60, and 80 mm/min). The characteristics of the thermally induced bushes (shape, dimensions, and surface roughness) were investigated in solid and pre-drilled sheets. Furthermore, we studied the microstructure evolution and hardness distribution in the thermo-mechanical affected zone and the heat-affected zone surrounding the bush. It was found that, the formed bushings in the pre-drilled sheets did not have cracks or petal formation in comparison with the other formed bushes in the solid sheets. The pre-drilled specimens showed an average bushing height increase from 5 to 6 ± 0.8 mm with the increase of the tool rotational speed from 2000 to 4000 rpm. At rotational speeds of 2000 and 4000 rpm, the pre-drilled specimens with a diameter of 2.5 mm had a bushing height which is 0.5 ± 0.2 mm greater than the other pre-drilled specimens. In addition, the surface roughness of the pre-drilled sheets decreased with the increase of the rotational speed and/or reduction of the feed rate. Moreover, the microstructure near the drilling zone exhibited a fine Si particles distribution. However, the hardness values near the drilled surfaces were 15 ± 8 HV lower than those of the base metal in both solid and pre-drilled sheets. |
