Tool wear in dry helical milling for hole-making in AISI H13 hardened steel

Helical milling is a hole-making process which can be applied to achieve a high quality finished boreholes in hardened steels. Due to the drilling process limitations, which are intensified when applied in hardened steels, the helical milling process can be applied on hole-making tasks in moulds and dies industry, since milling have been widely applied in moulds and dies machining to replace high-cost operations like grinding and electrical discharge machining. However, to succeed in achieving high-quality boreholes in hardened parts, which presents high added value due to previous operations, tool wear in the helical milling of hardened steels should be more investigated. In the present study dry helical milling tool life tests were conducted in AISI H13 hardened steel parts, varying the cutting velocity. The flank wear on frontal cutting edges was progressively measured through optical microscopy, and SEM/EDS was performed in frontal and peripheral worn cutting edges. The wear occurred progressively in the flank of the frontal cutting edges with adhesion and oxidation as main wear mechanisms. In the peripheral edges, it was observed coating loss, adhesion of workpiece material in the tool clearance surface, besides fracture in the tool nose flank with the highest cutting velocity. A nested ANOVA was performed to evaluate the burr height in the borehole exit. The tool life stage was statistically significant in the burr height.


Introduction
Helical milling is a hole-making process which can be applied to attain boreholes in hardened steels with high quality, feasible productivity and with a low wear rate. In helical milling, the mill tool follows a helical path with concomitant rotational movement around its own axis [1]. The helical milling allies continuous and discontinuous cut, with frontal and peripheral cutting edges [2], respectively, guaranteeing good chip formation, evacuation and breaking [3], low cutting forces [4], besides good fluid conditions [5]. As a consequence of the process kinematics, the borehole diameter can be defined by adjusting the helical diameter without changing the tool, reducing tool inventory, and reducing setups [6]. In hardened steels, generally applied in the moulds and dies industry, the application of the conventional drilling process presents several problems, from unpredictable catastrophic failure of the drill cutting edges to high thrust forces levels, besides difficult chip evacuation, hampering to achieve reliability due to the high added value of parts which will generally undergo hole-making operations in the last stages of manufacturing [7]. Consequently, the helical milling process may be applied in hardened steels [3,8] and other difficult to cut materials [9][10][11] to achieve finished boreholes in just one operation, guaranteeing low cutting forces, good borehole quality in terms of dimensional, geometrical and microgeometrical tolerances [12], besides the opportunity of monitoring the tool wear progression [13].
Some recent studies addressed the tool wear in the helical milling operation. Li et al. [14] studied the tool wear of carbide end mills with TiAlN coating in the helical milling of the Ti-6Al-4V alloy. The main wear mechanisms were chipping/fracture, diffusion, and oxidation on frontal cutting edges. Burr levels on the exit of boreholes were correlated with average flank wear on frontal cutting edges. Qin et al. [15] also studied the tool wear in the helical milling of Ti-6Al-4V comparing tungsten carbide (WC) tools coated with TiAlN and coated with diamond. The observed tool wear mechanisms were adhesion, oxidation, coating flaking, and chipping. Considering the flank wear of frontal cutting edges, the TiAlN-coated tool presented higher life than the diamond-coated tool during helical milling of the Ti-6Al-4V alloy. Tool wear in the helical milling of CFRP was investigated by Wang et al. [16]. Abrasion, adhesion, coating flanking were the main wear mechanisms. The delamination level in the entry of boreholes increased along with the progression of the tool wear. Zhao et al. [17] studied the tool wear in helical milling and drilling of Ti-6Al-4V. While in drilling the main failure modes were due to fracture, non-uniform flank wear and micro-chipping, in helical milling the identified wear mechanisms were crater, adhesion, chipping, and flaking. With helical milling, it was obtained more than double of boreholes when compared with drilling.
Iyer et al. [3] studied the tool wear in drilling and helical milling of AISI hardened D2 steel (60 HRC). Testing four drill types, three twist drills presented severe fracture after making in the first hole, while one of the twist drills presented flank wear higher than 0.3 mm also after the first borehole. In helical milling, it was obtained 10 boreholes with a TiCN-coated indexable carbide insert tool and 16 boreholes with a TiAlN-coated ball nose carbide end mill. In helical milling, the wear occurred progressively with attrition and microchipping in the flank of the frontal cutting edges. The authors used aircooling to assist in chip removal.
The hardened steels are susceptible to quenching heat treatment to achieve high hardness levels, from 52 to 62 HRC [18]. The high hardness of the hardened steels results in high cutting forces and severe friction in the tool-chip contact area [19]. In the moulds and dies industry, the process of milling after the heat treatment has been applied to save cycle times, to avoid setups and high-cost machining operations such as grinding and electrical discharge machining [20,21]. Consequently, to overcome the challenges of hard milling is mandatory. More specifically, the helical milling of these hard-to-cut materials can also present better results and lower costs when compared to drilling.
Since the tool wear in the helical milling of hardened steels still have been little researched, the present work is on tool wear and life in dry helical milling for hole-making of AISI H13 hardened steel. This study has novelty in the tool wear mechanisms investigation through SEM/EDS, besides the study of the burr height evolution in the exit hole with regard to the tool wear evolution. The tool wear curves were also provided, and the Taylor tool life Equation was calculated to give a notion of the tool wear resistance when applied to the helical milling in the hard-to-cut material AISI H13 hardened steel.

Experimental procedure
Helical milling tool life tests were carried out at the machining and tribology research group (MACTRIB) laboratory at University of Aveiro. The tests were performed in a machining centre model VCE500 from Mikron®, with 11kW of power and maximum spindle speed of 7500 RPM. To measure the wear progression in the tool during helical milling hole-making operation it was used an optical microscope TM 510 from Mitutoyo® with camera Moticam 2 and image acquisition software Images Plus 2.0 ML both from Motic®, all owned by MACTRIB. The experimental setup is exposed in Figure 1. in [mm]. It is important to mention that fzt is calculated with respect to the hole diameter.
The tool material, which is indicated for hard milling, is GC 1610 Sandvik® designation,   Sandvik®, associated with the EDS spectra in Figure 3(b). As observed in Figure 3(b) the composition of the carbide tool is WC-Co. A small quantity of O was omitted of the EDS analysis.
The AISI H13 hardened steel workpieces were cooled in a vacuum oven and supplied by Ramada Aços®. The chemical composition of the material provided by Ramada Aços® is summarized in Table 2. The AISI H13, DIN X40 CrMoV5-1 designation, is recommended for applications in aluminium extrusion dies, and moulds for thermoplastics. These applications present several hole-making necessities which makes feasible the study of an efficient hole-making operation such as helical milling.    Figure 5 shows the hardness profile of the AISI H13 hardened steel workpieces with correspondent data in Table 3. The hardness profile was obtained considering two workpieces which were measured in four positions equidistant of 3 mm starting from the centre, resulting in four measurements positions. Since the workpieces diameter is 25.4 mm, in a radius of 12.7 mm, it was possible to measure in only four positions. As it can be seen in the Figure 5, it was collected one measurement in the centre of each workpiece and three measurements in each one of the other radial positions. The workpieces presented lower levels of hardness nearby the centre with more than 56 HRC from 6 mm from the centre to the periphery.   Distance from centre [mm] with eccentricity between tool centre point and borehole centre point, e = Dh /2 = 4 mm. Figure 6 shows the helical milling tool life tests in AISI H13 hardened steel schematic drawing, showing the workpiece dimensions, borehole, tool, and helical diameters. A Form Talysurf Intra from Taylor Hobson® from the Metrology laboratory of UFSJ was used to measure the burr height of the boreholes. The burr measurement setup is exposed in Figure 7. However, as in this study the tool life tests were performed considering the helical milling process, the recommendations of the standard were not applied.
In the helical milling tool life tests, the average flank wear of the frontal cutting edges were measured progressively. The end of tool life criteria was VBB = 0.15 mm.
Two tools were worn until the end of the life with distinct cutting velocities, according to the Table 4. The cutting velocity adopted in the condition 1 was vc = 60/min to respect the tool manufacturer's recommendation bounds. The condition 2 was set to test an extreme condition defined through preliminary tests as a borderline cutting velocity in the helical milling of AISI H13 steel with the selected cutting tool. It is known that the cutting velocity is the cutting condition more influent on tool wear since its increase accelerates the wear mechanisms related with friction and temperature. As it was observed, the burr formation with the wear progression, the burr height was measured in the workpieces machined in the beginning, middle and end of the tool life, three workpieces for each tool life stage considering the two conditions in Table 4.
For each workpiece, the burr height was measured in three positions angularly equidistant from 120º, as illustrated in Figure 8. The cap generated at the borehole exit were collected and the thickness values were measured using a micrometer from Mitutoyo® with resolution of 0.01 mm.
The statistical analyses with the assumptions tests were conducted considering a significance level α = 0.05 using the software Minitab® 17.     Figure 11 presents images obtained by SEM, Figure 11(a), and by the optical microscope, Figure 11 Figure 9(a). The EDS spectrum of the region (iv) is at Figure 12(b). This tool was worn with vc = 60 m/min.

Results and discussion
The atoms with the associated percentage in mass and atomic are summarized in Table 6. and low hardness resulting from the loss of carbides, as explained by Qin et al. [15].
However, some amount of the oxidation may have occurred after the tool life tests, in the time interval between tests and measurements by SEM/EDS.   Figure 13 presents the magnification obtained by SEM of the area (ii) from Figure   9. EDS was conducted for the areas (v) and (vi) with spectra obtained exposed in the     Table 8 presents the percentage of the elements found in the region (vii) associated with Figure 15    is magnified in Figure 16(b), to show in detail the flank face of the nose of the end mill.
The colour of the nose flank is similar to the one observed in the flank of the frontal cutting edges, Figure 11(a). It was highlighted two areas in Figure 16 Table 9 presents the percentage of elements in the regions (xi) and (xii) of Figure   16(b) observed through EDS. The region (xi), which is darker than the region (xii), presented the tool substrate elements W and C, the presence of Fe, indicating adhesion, and O, indicating oxides formation with these metallic elements. The region (xii), when compared with the region (xi), presented the higher percentage of the substrate elements W and C, however with a fewer quantity of Fe and C, due to the small dark spots observed.
The presence of Co characterizes the bonding face of the carbide, ensuring toughness and plasticity, as explained by Mao et al. [23] and Wang et al. [24].   The EDS spectra of the regions (xiv) and (xvi) of the Figures 18(a) and 18(b), respectively, are in Figure 19, with the percentage of the identified elements through EDS in Table 10. For the region (xiv) it can be observed the presence of the elements of the tool coating Ti, Al and N. Besides these elements, which add up to almost 50% in mass, it may be observed Fe and O aplenty, characterising the adhesion of Fe followed by oxidation, forming the compound FeO. In the region (xvi) it was observed the presence in abundance of the WC hard phase compound, besides Co of the bounding phase, and Fe in feel quantity due to adhesion of the workpiece material.    Therefore, the tool life stage within each vc level presented a significant effect in the burr height in the helical milling of AISI hardened steel H13.
The factorial ANOVA is not ideal in this situation since the levels of the factor stage are different when changing the vc level. Then, the explanation of the interaction effect would be difficult and controversial, since the nested factor is not independent.
However, it is important to explain that the nested factor presents the sum of squares     Figure   23 presents the histogram for the cap thickness measurements with confidence intervals for mean and standard deviation, besides normality test with no evidence to reject the null hypothesis of normality of thickness of the caps.