The machinability of three commercial samples of the 6351 aluminum alloy with different silicon content was investigated in this work. Several parameters were used to evaluate the machinability in turning process, including the quality of the machined surface and cutting force. A design of experiments with three levels was used focusing on low values of feed rate (0.10, 0.15 and 0.2 mm/rev). The other parameters involved were: depth of cut (1.0, 1.5 and 2.0 mm), the silicon content (1.1, 1.2 and 1.3%) and two sets of cutting speed, one in the build up edge region (80, 100 and 120 m/min) and the other in a built up edge free region (200, 600 and 1000 m/min). The surface roughness parameter evaluated was Rq. A second design of experiment with three levels using higher values of feed rate (0.2, 0.35 and 0.5 mm/rev) and depth of cut of 2.0 mm was used to evaluate the influence of the silicon content in the cutting force. The effect of cutting fluid (dry machining, minimum quantity of fluid and over head cooling) was also analyzed. The results show that the silicon content has influence on the surface roughness. The statistical model in the build up edge region explains 79.95% of the total variation of roughness and 99% for cutting forces, for the other region this value is 81.99% for surface roughness and 98.96% for cutting force. The diameter of the workpiece has an influence on the results because the variation of hardness.

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