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0007-8506/$ – see front matter 2010 CIRP. doi:10.1016/j.cirp.2010.03.095
102 S. Basturk et al. / CIRP Annals - Manufacturing Technology 59 (2010) 101–104
Fig. 1. (a) Plasma boronizing reactor, (b) plasma generation, and (c) schematic overview of the setup.
induced secondary electron emission are released. This process keeps going on continuously and these processes make the glow discharge a self-sustaining plasma.
2.2. Plasma boronizing
Plasma boronizing is a thermo-chemical surface modification and diffusion process in which boron atoms diffuse into the surface of the tool to produce hard boride zone. The plasma boronizing setup consists of a boron releasing gas supplied into a reactor where boron ions are formed in a glow discharge. Under suitable conditions excited boron particles are generated in the glow discharge.
Plasma boronizing of tungsten carbide tools was performed in a newly developed setup (Fig. 1) with a gas mixture of 10% BF3, 40% Argon, and 50% H2 at different gas temperatures and durations. Table 1 shows temperatures and duration of the plasma boronizing of identical tungsten carbide inserts. BF3 is dissociated in the plasma and the boron is deposited on the WC insert surface and diffuses into the insert material to form boride phase. This phase is expected to improve machining performances by increasing wear resistance of the tools.
Plasma boronizing of inserts was performed at various temperatures and durations (Table 1). Four inserts were plasma boronized for each insert set given in Table 1. When the scanning electron microscope (SEM) images were analyzed it was observed that plasma boronizing process caused boron to penetrate under the surface of WC inserts significantly. SEM images of nonbor-onized and plasma boronized insert from insert set 4 are shown in Fig. 2. It illustrated that approximately 185 mm deep boron
Table 1
Plasma boronizing parameters.
Insert set Reactor temperature (8C) Duration of plasma
boronizing (h)
1 Nonboronized WC tool –
2 600 6
3 700 4
4 800 1
5 850 1
Fig. 2. SEM images of inserts: (a) cross-section of nonboronized insert 1, (b) cross-section of plasma boronized insert 4 and illustration of 185 mm plasma boronized zone under the surface.
penetrated zone was generated (Fig. 2b). Distribution of the boron through the cross-section of the insert was scanned along the line represented in Fig. 3. Results have shown that boron amount decreases rapidly beyond the boron penetration zone. Microhard-ness values were measured as 1448 HV for nonboronized insert set 1 and as 3100 HV in the boron penetrated zone for insert set 4.
Cutting forces were measured in orthogonal machining and oblique face milling tests on Ti–6Al–4V to compare the perfor-mances of the inserts. It was found that the certain sets of WC tools, depending on plasma boronizing parameters, show lower resul-tant cutting forces and much better wear resistance than nonboronized inserts.
3. Experimental machining tests
Four sets, each set including four cutting inserts, were plasma boronized using the conditions listed in Table 1. These plasma boronized inserts and four nonboronized WC inserts (all having 68 rake angle and 88 clearance angle) were used in orthogonal machining and in oblique face milling experiments for perfor-mance comparisons. In all these tests, cutting forces were measured using a three-axis Kistler dynamometer (model 9257B). For the comparisons of wear, nonboronized insert and four sets of identical plasma boronized inserts (Table 1) were used in slot face milling tests. In all the experiments, workpiece material was Ti–6Al–4V grade 5.