Abstract
This thesis presents the results and conclusion of an experimental investigation to demonstrate an optimised machining strategy for single point turning to enhance the surface integrity of a machined Ti-6Al-4V ELI component with the goal to improve the fatigue performance. To this end a single point turning operation was optimized with specific emphasis on the cutting speed to obtain the most beneficial compressive residual stress field for an enhanced fatigue life.
The literature showed that a compressive residual stress field is typically created during conventional machining of Ti6-Al-4V titanium alloy and that its magnitude may be significantly affected by cutting speed. Conventional single point turning was therefore conducted to manufacture the specimen for fatigue testing with cutting speeds varying between 40 and 110 m/min. The residual stress results (2nd principal) showed that the machining induced a compressive residual stress varying between ± 340 MPa at 40 m/min to ±480 MPa at 110 m/min demonstrating a clear increase at the higher cutting speed (110 m/min). Initially the low cycle fatigue life was assessed for different cutting speeds by in plane bending fatigue testing.
The low cycle fatigue results subsequently displayed a significant increase (±75%) in fatigue life as a function of cutting speed between 40 m/min and 110 m/min. The fatigue results did however display some uncertainty especially at the highest cutting speed and it was not immediately obvious what effect the apparent surface of the specimen had on the results. It was therefore deemed necessary to repeat the fatigue testing with rotating bending fatigue testing.
New fatigue specimen was therefore machined on a different machine tool that produced specimen with consistent surface roughness. A bespoke rotating bending fatigue testing machine was also designed and constructed that allowed for additional sensory feedback during testing. Similar residual stress results as those obtained for the in plane bending fatigue testing were found with the 2nd principal stress varying from ±380 MPa at 50 m/min to ±450 MPa at 100 m/min. The subsequent rotating bending fatigue testing demonstrated very similar results. An improvement in fatigue life of nearly 90% was demonstrated at the higher cutting speed of 100 m/min when compared to 50 m/min. This correlates with the increase in the compressive residual stress.
The research clearly demonstrated that fatigue life enhancement is possible by optimizing the cutting speed within a relatively narrow band when machining Ti-6Al-4V ELI titanium alloy. This is a direct consequence of indirectly controlling the residual stress field by controlling the basic cutting parameter of cutting speed. It has been successfully demonstrated for a medium to low cycle stress state but there is no apparent reason why this will not extent to the high cycle fatigue domain where crack formation plays the most significant role.
Keywords: Ti-6Al-4V ELI; Residual stress measurement; In-plane bending fatigue; Rotating-bending fatigue.