A study of chip formation, surface defects, and burr formation in orthogonal machining of A356 cast aluminum

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University of New Brunswick
Cast A356 aluminum is commonly employed in the automotive industry due to its high strength to weight ratio and high resistance to corrosion. This relatively inexpensive raw material contains a high silicon content which plays a key role in the machining performance. To understand how the material microstructure is linked to chip formation and surface defects, a series of orthogonal turning tests were performed on A356-F and A356-T6. Experimental findings demonstrated that the chip formation mechanism and resulting surface quality are directly related to the material microstructure. The stronger, more homogeneous structure with hard, silicon phase spheroids allow A356-T6 to produce more uniform chips and a superior surface finish. To examine the tool exit conditions and the burr formation mechanism, a series of orthogonal grooving tests were performed using an orthogonal machining quick-stop on samples of A356-F, A356-T5, and A356-T6. For all tests, a negative burr, or break out, was observed. The mechanism of break out was consistent for all materials but the influence of the material microstructure was found to alter its geometry. Three different exit angles, relative to the cutting direction were tested; 90°, 60°, and 30°. When exiting the workpiece at a 90° angle, a moment was found to act on the chip root eventually causing tensile failure along a break out plane and a uniform chip foot. Exiting at any other angle resulted in a second moment acting upon the chip root, producing a stepped break out.