Cutting force, Fc (N) ' Cutting speed (m/min)

Transcription

1 159 Rake face Adhesion Rake face Nose wear Tool life = 50.2 min Nose wear Tool life = 30.9 min Figure Nose wear at the cutting edge of T4 coated carbide insert after machining Ti- 6Al-4V alloy with a coolant pressure of 20.3 MPa at a speed of 110 m min -1 and 120 m min -1. Rake face Crater wear Rake face Nose wear Nose wear Flank face Tool life = 12.7 min Tool life = 4.3 min Figure Wear at the cutting edge of T4 coated carbide insert after machining Ti-6Al-4V alloy in argon enriched environment at a speed of 100 m min -1 and 120 m min Component forces when machining with various carbide insert grades Figures 4.21 and 4.22 show variations in cutting and feed forces, respectively, when machining Ti-6Al-4V alloy with different grades of carbide tools under various cutting speeds and machining environments. The component forces were recorded at the beginning of cut when the cutting edge has not undergone pronounced wear. Figure 4.21 suggests that the cutting forces generally decreased with increase in coolant pressure when machining with T1

2 160 and T4 insert grades and generally increase with increase in coolant pressure when machining with T2 and T3 insert grades. Figure 4.21 also shows that in general, cutting forces marginally increased with increasing speed when machining with T1 and T2 in all cutting environments tested, unlike T3 and T4 insert grades. Cutting forces recorded with T2 tool grade were generally higher than those recorded with T3 tool in all conditions tested. High cutting forces were generated when machining with T1 and T4 tool grades in argon environment relative to conventional coolant flow. 360 T1 (CCF) T1 (11MPa) T1 (20.3 MPa) T2 (CCF) T2 (11MPa) T2 (20.3 MPa) T3 (CCF) T3 (11MPa) T3 (20.3 MPa) T4 (CCF) T4 (11MPa) T4 (20.3 MPa) T1 (7 MPa) T1 (Argon) T4 (Argon) Cutting force, Fc (N) ' Cutting speed (m/min) Figure Cutting forces (Fc) recorded at the beginning of cut when machining Ti-6Al-4V alloy with different cemented carbide grades under various cutting conditions. Recorded feed forces generally increase marginally with an increase in cutting speed (Figures 4.22). This figure also shows that feed forces increased with an increase in coolant pressure when machining with T1, T2 and T3 tool grades, unlike T4 tool. It can be seen from Figure 4.22 that the highest feed forces were generated when machining with T1 tool grade under a coolant pressure of 7 MPa. Machining with T4 tool grade in the presence of argon provided the lowest feed forces in all conditions tested.

3 T1 (CCF) T1 (11MPa) T1 (20.3 MPa) T2 (CCF) T2 (11MPa) T2 (20.3 MPa) T3 (CCF) T3 (11MPa) T3 (20.3 MPa) T4 (CCF) T4 (11MPa) T4 (20.3 MPa) T1 (7 MPa) T1 (Argon) T4 (Argon) Feed force, Ff (N) Cutting speed (m/min) Figure Feed forces (F f ) recorded at the beginning of cut when machining Ti-6Al-4V alloy with different cemented carbide grades under various cutting conditions Surfaces roughness and runout deviation when machining with various carbide insert grades Figure 4.23 and 4.24 show the surface roughness and circular runout deviation values, respectively, recorded when machining Ti-6Al-4V alloy with different grades of carbides at various cutting speeds and under various cutting environments. It can be seen from Figure 4.23 that the surface roughness values recorded in all the conditions investigated varied between 0.3 and 1.0 µm, well below the stipulated rejection criterion of 1.6 µm. This figure, however, shows evidence of deterioration of the surface finish when machining at higher speed conditions.

4 162 T1 (CCF) T1 (11MPa) T1 (20.3 MPa) T2 (CCF) T2 (11MPa) T2 (20.3 MPa) T3 (CCF) T4 (CCF) T3 (11MPa) T4 (11MPa) T3 (20.3 MPa) T4 (20.3 MPa) T1 (7 MPa) T1 (Argon) T4 (Argon) 1.2 Surface roughness Ra ( m) Cutting speed (m/min) Figure Surface roughness values recorded at the beginning of cut when machining Ti- 6Al-4V alloy with different cemented carbide grades under various cutting conditions. Runout tolerances are used to control the functional relationship of one feature to another or a feature to a datum axis. This tolerance is applicable to rotating parts where the composite surface criterion is based on the part function and design requirements. When dealing with three-dimensional objects, circular runout is defined as the amount that is allowed to deviate from the central axis at one cross section (POLLACK, 1988). Figure 4.24 shows that runout values recorded in all the conditions investigated varies between 2 and 14 µm. These values are far lower than the stipulated rejection criterion value of 100 µm. Machining with T4 tool grade provided lower runout values in all conditions investigated compared to other tool grades. Runout values recorded increased with increasing cutting speed when machining with T2 and T3 insert grades under conventional coolant flow while no variation was observed when machining with T2 and T3 inserts under high coolant pressures.

5 163 T1 (CCF) T1 (11MPa) T1 (20.3 MPa) T2 (CCF) T2 (11MPa) T2 (20.3 MPa) T3 (CCF) T3 (11MPa) T3 (20.3 MPa) T4 (CCF) T4 (11MPa) T4 (20.3 MPa) T1 (7 MPa) T1 (Argon) T4 (Argon) Runout ( m) Cutting speed (m/min) Figure 4.24 Runout variation recorded at the end of cut when machining Ti-6Al-4V alloy with different cemented carbide grades under various cutting conditions Surfaces generated after machining with various carbide insert grades Figures show micrographs of surfaces generated when machining with uncoated carbide T1 tool grade under various machining environments and at different cutting speeds. Surfaces generated consist of well-defined and uniform feed marks running perpendicular to the direction of relative work-tool motion with no evidence of plastic flow. There is evidence of localised incipient melting of the machined surfaces when machining at a higher cutting speed of 130 m min -1 under conventional coolant flow (Figure 4.25 ). Figure Surfaces generated after machining with uncoated carbide T1 tool with conventional coolant supply at cutting speeds of 110 m min -1 and 130 m min -1.

6 164 Figure Surfaces generated after machining with uncoated carbide T1 tool with a coolant pressure of 7 MPa at cutting speeds of 100 m min -1 and 130 m min -1. Figure Surfaces generated after machining with uncoated carbide T1 tool with a coolant pressure of 11 MPa at cutting speeds of 110 m min -1 and 120 m min -1. Figure Surfaces generated after machining with uncoated carbide T1 tool with a coolant pressure of 20.3 MPa at cutting speeds of 120 m min -1 and 130 m min -1.

7 165 Figure Surfaces generated after machining with uncoated carbide T1 tool in an argon enriched environment at cutting speeds of 110 m min -1 and 120 m min -1. Figures 4.30 and 4.31 show micrographs of surfaces generated when machining with uncoated carbide (T2) and PVD coated carbide (T3) tool grades under high pressure coolant supplies of 11MPa and 20.3 MPa at a cutting speed of 110 m min -1. There are well-defined uniform feed marks running perpendicular to the direction of relative work-tool motion. Figure Surfaces generated after machining with uncoated carbide T2 tool with coolant pressures of 11 MPa and 20.3 MPa at a cutting speed of 110 m min -1.

8 166 Figure Surfaces generated after machining with coated carbide T3 tool with coolant pressures of 11 MPa and 20.3 MPa at a cutting speed of 110 m min -1. Typical surfaces generated when machining with PVD coated carbide (T4) tool grade under various machining environments at a cutting speed of 120 m min -1 as shown in Figure Well-defined uniform feed marks running perpendicular to the direction of relative work-tool motion with no evidence of plastic flow can be seen. No surface tears and chatter marks were observed after machining Ti-6Al-4V alloy with various cemented carbide tool grades. Generally machining with all the carbide grades under high pressure coolant supplies generated acceptable machined surfaces, conforming to the standard specification established for machined aerospace components (Rolls-Royce CME 5043).

9 167 (c) (d) Figure Surfaces generated after machining with coated carbide T4 tool with conventional coolant supply, in argon enriched environment, (c) coolant pressure of 11 MPa and (d) 20.3 MPa at a cutting speed of 120 m min Surface hardness after machining with various carbide tool grades Figures are plots of the variations of microhardness values recorded from the top of the machined surface up to about 1.5 mm below the machined surface. Note that the range of measured values on the graphs is demarcated by confidence interval (C.I.), represented by the minimum (Min.) and maximum (Max.) Vickers hardness values recorded for Ti-6Al-4V alloy bars prior to machining. This is because the hardness of alloys varies within a range of values. The range (C.I.) is used here instead of the average hardness because it gives a realistic assessment of the material hardness.

10 168 Figures are plots of microhardness values of machined surfaces after machining with uncoated carbide T1 tool grade under various machining environments. The plots show relatively low variation in hardness when machining with T1 tool grade with conventional coolant flow at cutting speeds up to 110 m min -1 (Figure 4.33). There is, however, evidence of surface hardening beyond the bulk hardness of material at the top surface when machining at speeds in excess of 110 m min -1. Surface hardening up to about 0.4 mm below the machined surfaces of Ti-6Al-4V alloy was observed after machining with uncoated carbide T1 tool grade at a cutting speed of 120 m min -1. Softening of the machined surfaces also occurred, especially, at the higher speed of 130 m min -1. Figure 4.34 shows evidence of softening at the top surface up to about 0.15 below the machined surfaces after machining with T1 tool with a coolant pressure of 7 MPa at speeds of 100 m min -1 and 120 m min -1. This figure also shows evidence of surface hardening up to about 0.3 mm below the machined surfaces when machining at a cutting speed of 110 m min -1. Figure 4.35 shows the evidence of softening of machined surfaces after machining with T1 tool with a coolant pressure of 11 MPa at speeds up to 120 m min -1. However, evidence of hardening of the machined surfaces up to about 0.4 mm depth was observed when machining at highest cutting speed of 130 m min -1. Machining with T1 tool grade under the highest coolant pressure of 20.3 MPa gave minimum hardness variation with a uniform distribution of hardness values within the confidence interval of hardness values prior to machining (Figure 4.36). This figure, generally, shows softening of the machined surfaces when machining at all the cutting speeds investigated. Figure 4.37 shows microhardness values of machined surfaces after machining with T1 tool grade in the presence of argon. It can be seen that hardness values are generally distributed within the confidence interval of the hardness values.