Figure 3 shows the characterization results of the interlamellar spacing and microhardness of the materials and their relationships. It can be observed that the FC300Mo + RG presented the smallest interlamellar perlite spacing, on average 0.29 µm. Its microhardness, on the other hand, was the highest among all gray cast irons and the lowest relative to the FV450 vermicular cast iron. This is an indication that both materials should present an increase in ultimate tensile strength and abrasiveness.
Table 5 represents the statistical percentage difference of the interlamellar spacings and microhardness average of the materials depicted in Fig. 4. The positive values indicate that there was an increase in the analyzed parameter, while negative values correspond to a reduction. It can be observed that there was no significant statistical difference between the interlamellar spacings of the materials used in this work. On the other hand, the comparisons (FC250 vs FV450), (FC300Mo vs FV450) and (FC300Mo + RG vs FV450) showed a significant statistical difference in microhardness.
Table 5
Statistical percentage difference of interlamellar spacings and microhardness
Comparison
|
Interlamelar spacings
|
Microhardness
|
Diference
|
p-value
|
Diference
|
p-value
|
FC250 vs FC300Mo
|
– 6.06%
|
0.4359
|
+ 4.67%
|
0.2873
|
FC250 vs FC300Mo + RG
|
– 12.12%
|
0.1912
|
+ 12.58%
|
0.0633
|
FC250 vs FV450
|
– 3.03%
|
0.6213
|
+ 30.93%
|
0.0009
|
FC300Mo vs FC300Mo + RG
|
– 6.45%
|
0.5449
|
+ 16.29%
|
0.2339
|
FC300Mo vs FV450
|
+ 3.22%
|
0.7081
|
+ 25.08%
|
0.0040
|
FC300Mo + RG vs FV450
|
+ 10.34%
|
0.3296
|
+ 16.29%
|
0.0276
|
Figure 4 shows the behavior of the current signal measured throughout the machining of the cast iron material FC250. The current average is used to calculate the cutting power for each test.
Figures 5 and 6 represent the cutting power, derived from the deduction of the power measured during milling, already with the due subtractions of the power acquired in the empty mode. In all, one test and two repetitions were performed for each set of cutting parameters.
It is noted that the cutting power tends to increase with the increase of the cutting speed. This is also accompanied by a higher generation of heat and, consequently, by a reduction of the material strength, which, in turn, further facilitates the machining and dismantling of the material. This is typical of ductile materials like steels, but not so common for brittle materials like cast irons. As for the latter, the chip is discontinuous, so that higher cutting speeds increases the thickness of the lamellae and, consequently, intensifies the force, rather than diminishing it [Trent et al., 2000].
For Gabaldo et al., [2009], cutting power has an important influence on the milling process of cast irons, especially those used in the manufacture of engine blocks. With higher wear on the cutting edges, the contact area between workpiece and tool increase chip deformation. Consequently, the power consumption required to cut cast iron also increases.
Another important factor to be considered in the long run is the machine stability during servicing, which depends on the quality and state of maintenance. It can be seen that the cast iron FV450, Fig. 7, was the only that caused the highest power consumption. This alloy has also the highest ultimate tensile strength and hardness among all studied materials. On the other hand, the graphs show that the material presenting the lowest cutting power was the FC250, followed by the FC300Mo.
Diniz et al., [2006] state that the increase in power is not only linked with flank wear, but also to other types of wear and failures that can, as in the case of crater wear, inversely decrease the power consumption as the effective angle of exit increase.
According to Bagetti et al., [2009], the high strength and hardness of vermicular cast iron promotes higher cutting forces than conventional grey cast irons, thus, requiring about 20–30% more power for simple machining operations, as well as more robust clamping systems.
Tables 6 and 7 represent Figs. 5e 6 as for the average and statistical percentage difference in the shear power. The positive values indicate that there was an increase in cutting power, whereas the negative indicate a reduction.
Table 6
Statistical percentage difference of the cutting power (v = 230 m/min; fz = 0.1 mm/tooth)
|
|
Concordant
|
Discordant
|
Tool
|
Comparison
|
Diference
|
p-value
|
Diference
|
p-value
|
A
|
FC250 vs FC300Mo
|
+ 39.85%
|
0.0013
|
+ 1.28%
|
0.7699
|
FC250 vs FC300Mo + RG
|
+ 46.50%
|
0.0008
|
+ 18.18%
|
0.0152
|
FC250 vs FV450
|
+ 64.12%
|
0.0003
|
+ 40.71%
|
0.0012
|
FC300Mo vs FC300Mo + RG
|
+ 4.75%
|
0.3187
|
+ 16.68%
|
0.0197
|
FC300Mo vs FV450
|
+ 17.35%
|
0.0176
|
+ 38.92%
|
0.0014
|
FC300Mo + RG vs FV450
|
+ 12.02%
|
0.0501
|
+ 19.05%
|
0.0132
|
B
|
FC250 vs FC300Mo
|
+ 8.61%
|
0.1133
|
+ 6.67%
|
0.1887
|
FC250 vs FC300Mo + RG
|
+ 22.58%
|
0.0078
|
+ 71.63%
|
0.0002
|
FC250 vs FV450
|
+ 53.01%
|
0.0006
|
+ 41.05%
|
0.0012
|
FC300Mo vs FC300Mo + RG
|
+ 12.86%
|
0.0418
|
+ 60.89%
|
0.0004
|
FC300Mo vs FV450
|
+ 40.86%
|
0.0012
|
+ 32.21%
|
0.0025
|
FC300Mo + RG vs FV450
|
+ 24.80%
|
0.0058
|
– 17.82%
|
0.0088
|
Table 7
Statistical percentage difference of cutting power (v = 350 m/min; fz = 0.2 mm/tooth)
|
|
Concordant
|
Discordant
|
Tool
|
Comparison
|
Diference
|
p-value
|
Diference
|
p-value
|
A
|
FC250 vs FC300Mo
|
+ 24.33%
|
0.0062
|
+ 3.62%
|
0.4323
|
FC250 vs FC300Mo + RG
|
+ 30.91%
|
0.0029
|
+ 22.63%
|
0.0077
|
FC250 vs FV450
|
+ 76.83%
|
0.0002
|
+ 63.02%
|
0.0003
|
FC300Mo vs FC300Mo + RG
|
+ 5.29%
|
0.2751
|
+ 18.34%
|
0.0148
|
FC300Mo vs FV450
|
+ 42.22%
|
0.0011
|
+ 57.32%
|
0.0004
|
FC300Mo + RG vs FV450
|
+ 35.07%
|
0.0019
|
+ 32.93%
|
0.0024
|
B
|
FC250 vs FC300Mo
|
– 0.41%
|
0.9236
|
+ 9.47%
|
0.0913
|
FC250 vs FC300Mo + RG
|
+ 15.31%
|
0.0255
|
+ 23.52%
|
0.0068
|
FC250 vs FV450
|
+ 39.21%
|
0.0014
|
+ 38.97%
|
0.0014
|
FC300Mo vs FC300Mo + RG
|
+ 15.79%
|
0.0233
|
+ 12.83%
|
0.0420
|
FC300Mo vs FV450
|
+ 39.79%
|
0.0013
|
+ 26.94%
|
0.0045
|
FC300Mo + RG vs FV450
|
+ 20.72%
|
0.0102
|
+ 12.50%
|
0.0451
|
It is observed an increase in cutting power with the increase in mechanical strength for the cast irons tested: FC250, FC300Mo, FC300Mo + RG and FV450. It can also be seen that concordant milling has a greater influence on the cutting power difference, when comparing to the tested cast iron alloys.
Once analyzing the power consumed, it can be seen that the FV450 presents the highest ultimate tensile strength and hardness of all studied materials, consequently, being the toughest to cut. On the other hand, the FC250 presented the lowest cutting power at various conditions, whereas the cast irons FC300Mo and FC300Mo + RG depicted intermediate values.
Not only has the FC250 alloy the lowest ultimate tensile strength but also a graphite morphology that facilitates its machining, when compared to the other studied alloys. Many other variables combined play important roles in the tool´s life span. As the tool wears down, discrepancies occur in the machining process, as for instance, cause the temperature to rise along with the cutting force or power and change the surface, worsening the final finish [Naves, 2009].
Table 8 shows the analysis of variance of the cutting parameters with a 95% reliability, as indicated in Table 3. It is observed that the variables tolling type, cutting speed, cutting feed and material type have significantly influenced the process. On the other hand, the variable cutting type did not present any influence on the results at all.
Table 8
Analysis of variance for cutting power
|
Univariate Tests of Significance ANOVA for machining time: Sigma-restricted parameterization and effective hypothesis decomposition
|
Effect
|
SS
|
DoF
|
MS
|
F
|
p-value
|
Intercept
|
17118856
|
1
|
17118856
|
3175.810
|
0.000001
|
Tooling type
|
129362
|
1
|
129362
|
23.999
|
0.000048
|
Cutting type
|
5202
|
1
|
5202
|
0,965
|
0.335330
|
Cutting speed
|
333744
|
1
|
333744
|
61.915
|
0.000001
|
Cutting feed
|
302254
|
1
|
312284
|
59.365
|
0.000001
|
Material type
|
449516
|
3
|
149839
|
27.797
|
0.000001
|
Standard Error
|
134760
|
12
|
5390
|
|
|
Figure 7 represents the correlations between cutting power and interlamellar spacings at various cutting conditions. Taking into account the graphitic phase stress concentration on the surface of the material and its interactions with the pearlitic matrix, it is evident the overlapping effects of the phenomena on the integrity of the shear in the cutting zone, caused by the presence of this second phase.