26 Optimization of Kerf Quality During CO2 Laser Cutting of Titanium Alloy Sheet Ti-6Al-4V and Pure Titanium Ti 217 According to Fig. 26.4, kerf width size variation demonstrates the opposite behavior to that for the laser output cutting speed. In fact, increasing laser output power increases the kerf width size, which is more pronounced for the titanium alloy. This is due to the high temperature exothermic reactions, which provide excess energy in the cutting section. Since the assisting gas used is nitrogen, the presence of the oxygen remains low in the cutting section. However, high oxygen affinity of titanium alloy gives rise to the oxidation reactions taking place locally in the cutting section. Therefore, localized thermal erosion is resulted at the kerf surface while causing the deep striation formations. In addition, this gives rise to the formation of varying kerf width size along the cut edges. Consequently, kerf width size increases with increasing output Laser power. 26.5 Kerf Width Optimization Taguchi method of robust parameter design is an offline statistical quality control technique in which the level of controllable factors or input process parameters are so chosen to nullify the variation in responses due to uncontrollable or noise factors. In Taguchi method, the experimental values of quality characteristics are used to compute the quality loss values for each quality characteristic in all experimental runs. In the present case the smaller value of kerf width is desired, therefore the S/N ratio for smaller-the-better case will be used which is given below in equation form [18]. S N =−10log 1 n y 2 (26.1) Where n is the number of observations and y the observed data or each type of the characteristics. S/N ratios obtained from this equation are given in Table 26.4. As shown in Table 26.4 and according to the Taguchi method, the optimum cutting conditions is found as S/N equal to −53.53 and −53.47 for kerf width Kwof Titanium alloy Ti-6Al-4V and Kwof pure Titanium Ti respectively. Thus, the optimum cutting conditions which were the laser power of 2 kW, the cutting speed of 2400 mm/min and the gas pressure of 8 bars. The interpretations can be made according to the level values of Pu, V, and p factors obtained for kerf width Kwof Ti6Al-4V given in Table 26.5 and for Kwof pure Titanium Ti given in Table 26.6. The different values of S/N ratio between maximum and minimum are (main effect) also shown in these tables. Figures 26.5 and 26.6 show the graphic of the level values given in Tables 26.5 and 26.6 respectively. The cutting speed and the gas pressure are two factors that have the highest difference between values 1.94 and 0.94 respectively for Titanium alloy and 0.91 and 0.81 respectively for pure Titanium Ti. Based on the Taguchi prediction the Table 26.4 S/N Ratios values EXP. No Pu (kW) V (mm/min) p (bar) [Ti6Al4V] S/N ratios [Ti] S/N ratios 1 2 480 2 −55.89 −55.19 2 2 480 8 −55.36 −54.27 3 2 480 14 −55.67 −54.99 4 2 1440 2 −55.30 −54.69 5 2 1440 8 −54.56 −53.69 6 2 1440 14 −55.14 −54.53 7 2 2400 2 −54.73 −53.75 8 2 2400 8 −53.53 −53.47 9 2 2400 14 −53.68 −54.32 10 3 480 2 −56.32 −55.55 11 3 480 8 −55.44 −54.88 12 3 480 14 −57.43 −56.01 13 3 1440 2 −55.78 −55.14 14 3 1440 8 −54.90 −54.75 15 3 1440 14 −55.36 −55.37 16 3 2400 2 −54.96 −54.19 17 3 2400 8 −53.57 −54.49 18 3 2400 14 −53.98 −55.20
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