Machinability Assessment of Aluminium Alloy EN AW-7075 T651 Under Varying Machining Conditions

Aluminium alloys are widely used in the aerospace and automotive industries, among others [1,2]. This is primarily owing to their low density, high corrosion resistance and good strength properties. However, the peculiarities of these industries require high quality of manufactured components, which implies a need for continuous improvement of machining processes. The selection of appropriate cutting conditions is therefore crucial for ensuring optimum machining results, and hence research must be conducted in this area [3–5].

The machining result depends on numerous factors such as the machine tool, cutting tool and cutting parameters. The dynamics of this process, which is significantly related to the cutting force, is of importance, too. The selection of optimum machining conditions has been of interest to researchers from all over the world. By varying technological parameters, it is possible to exert a significant impact on the cutting process and its effects. The machining of alloy EN-AW 7075 conducted with variable cutting parameters was investigated in [6]. The study showed that the spindle speed and feed rate had the primary impact on surface roughness. Increasing the values of these parameters resulted in improved surface quality. The depth of cut, on the other hand, showed no specific effect on surface condition. Similar conclusions were included in the paper [7], also concerning EN-AW 7075 alloy. It was shown that cutting speed had an approx. 85% effect on surface roughness and chip segmentation. However, this applied to high speed machining, because when standard cutting speeds were used, the significance of cutting speed decreased, while the influence of feed and depth of cut increased. A study [8] investigated the milling process of pure EN-AW 7075 alloy, EN-AW 7075 alloy with Sc and Li additives, as well as of EN-AW 7075 after different heat treatments. The applied feed rate was found to have the greatest effect on cutting forces. A reduction in the feed value had a positive effect on the cutting force. This observation was especially important when machining high-hardness alloys, for which it was recommended using a cutting fluid in order to reduce the cutting force. It was also more advantageous to use higher cutting speeds. A study investigating the machining of EN-AW 7050 conducted by Ping et al. [9] showed that the cutting force components remained constant despite an increase in the cutting speed and a higher cutting temperature. The depth of cut was found to have the greatest effect on the cutting force, as increasing the depth of cut value resulted in a several-fold increase in the components of this force. The effect of tool geometry was also investigated. It was observed that as the rake angle was increased, the cutting force components and the cutting temperature decreased. In contrast, an increase in the tool tip radius produced the opposite effect. This implies that the cutting tool geometry, too, has impact on the machining process.

The effect of rake angle and clearance angle was investigated in [10] for the EN-AW 7075 alloy. It was shown that the “sharper” cutting edge caused the cutting temperature to decrease and the cutting force to increase at the same time. Other tools caused the cutting temperature to increase, which probably led to plasticization of the material and a decrease in the cutting force. The study [11] focused on rake angle and nose radius. Choosing the right rake angle proved to be a difficult task, as increasing the rake angle results in a reduction of the infeed force, while at the same time the crossfeed and thrust forces increase intensively. However, by increasing the nose radius, the cutting forces were reduced by up to several times. A study [12] showed that the cutting tool’s helix angle had a significant impact on the cutting process. The cutting temperature was reduced when using a small helix angle tool. This angle had a greater effect on the cutting temperature than the spindle speed and feed. The use of a large helix angle, on the other hand, had a positive effect on surface roughness and was more significant than a variable feed. Ikhries et al. [13] compared the results of a machining process conducted with a flat and a ball tool. The use of the flat tool resulted in enhanced surface quality and a higher material removal rate. In the machining process conducted with the flat tool, the feed rate had the dominant effect on surface roughness, while for the ball tool – the depth of cut had the greatest effect on surface roughness.

Not only does the overall geometry of the tool matter, but the cutting edge microgeometry is of significance too, as demonstrated in [14]. The results showed the margin width to be of the greatest significance, as any increase in this parameter led to reduced surface quality. The corner radius was found to have a nonlinear impact, as the best effect was obtained using intermediate radius values. In contrast, the relief angle did not have any significant effect on surface roughness. Schönecker et al. [15] developed HSS end mills with structure elements on the flank face. The research showed that the proposed tool geometry reduced the cutting force and the susceptibility to chatter vibration, which, in turn, allowed the depth of cut to be increased. This effect became more powerful with increasing the size of structure elements.

The material of a cutting tool is of vital importance, too. Kucz-maszewski et al. [16] conducted a comparative analysis of the results of an HSM (High Speed Milling) process for EN-AW 2024 that was conducted using a carbide and a PCD tool. Lower values of the roughness parameter Ra over the entire length of the machined surface were obtained with the PCD tool, regardless of the rolling direction. However, the differences were relatively small. The use of the diamond tool resulted in lower deflection along the longitudinal rolling direction. A comparison of carbide and PCD tools was also undertaken in [17]. The use of the PCD tool for milling EN-AW 5754 and EN-AW 6082 alloys resulted in several-fold lower values of the surface roughness parameter Ra. The tool was also found to be less sensitive to changes in cutting parameters. The dependence of high surface quality on the use of diamond tools was also confirmed in [18]. The best results were obtained using low cutting speeds, feeds and cutting depths. The diamond tool also had a positive effect on the shape of formed chips, which promoted improved surface quality. O'Toole et al. [19] compared the use of a PCD tool and a CBN (Cubic Boron Nitride) tool in the micromilling of pure aluminium. The PCD tool produced better surface quality and several-fold lower cutting force components. The tool also exhibited lower wear. Similar results were obtained in the simulation of a milling process for the EN-AW 7075-0 alloy [20]. The use of the PCD tool resulted in lower cutting forces and feed forces than those obtained in the milling process conducted with a tool made of carbide K10. This also meant lower cutting temperatures. The favourable properties of polycrystalline diamond were also found to promote reduced burr formation [21]. The benefits of using diamond tools were confirmed not only by research on milling, but also by studies devoted to turning [22–26].

The machining effect can also be enhanced via tool coatings. The benefits of using coatings in micromilling were confirmed in a study [27]. It was demonstrated that compared to an uncoated tool, the use of DLC (Diamond-Like Carbon) and NCD (Nano-Crystalline Diamond) coatings reduced the cutting forces by up to 20-30%. The use of the DLC-coated tool also produced the best surface quality. Similar observations were made in a study [28] which investigated the machining process of EN-AW 6082-T6. The use of the DLC-coated tool produced significantly better results than when using uncoated and AlCrN-coated tools. The obtained surfaces had higher quality and better properties. The DLC-coated tool also generated lower vibration. It should be emphasized that the coating material selection depends on the specific application, as the use of a wrong coating can even deteriorate the machining process and its results [29].

The selection of a suitable machining strategy is also important. In [30], conventional and plunge milling strategies were considered. Plunge milling was shown to improve productivity and reduce tool wear, but at the same time generated more vibration and poorer surface quality. This machining strategy can therefore be an interesting option for rough machining. The type of milling can also affect the machining effect. Burhanudin et al. [31] compared climb and conventional milling using an HSS end mill. The results showed that climb milling produced better surface quality, with significantly greater differences obtained for EW-AW 5052 than for EN-AW 7075. The study also demonstrated that increasing the cutting allowance promoted improved surface roughness. A similar investigation of the EN-AW 7075-T6 alloy was made in [32]. In this study, however, no clear differences were observed between the two types of treatment. This could be due to the fact that the carbide tool had a different geometry or that the aluminium alloy was subjected to a different heat treatment condition.

Sivalingam et al. [33] showed that the machining effects for aluminium alloy EN-AW 7075 can also be improved by using lubricants. They investigated several cooling methods: MQL (Minimum Quantity Lubrication), cryogenic, and a combination of MQL and cryogenic. The use of lubrication led to reduced friction, which resulted in lower cutting forces and tool wear. Surface quality was also improved, with fewer defects occurring. The most benefits were achieved using a combination of MQL and cryogenic. The positive effect of cooling on surface roughness was also confirmed in a study [34]. The use of cooling led to a more than half reduction in the surface roughness parameter Ra. Nevertheless, the spindle speed was found to have a much greater effect on surface roughness. Yapan et al. [35] conducted a study on EN-AW 6082 using MQL with graphene nanoparticles. The use of nanofluid resulted in significant improvements in cutting force, surface roughness, temperature and carbon emission. A study [36] compared dry machining and SQL (Small Quantity Lubrication) with sunflower oil. The application of SQL reduced the cutting force by up to 35% and the surface roughness by even up to 45%. The effect of using SQL in milling was much better than that achieved in turning. Innovative solutions were also proposed for cutting force reduction, such as vibration assisted milling [37,38], which could additionally be implemented over helical paths [39].

Based on the literature review, it can be seen that the milling process depends on many factors. In order to obtain an optimal machining effect, it is necessary to determine the influence of individual machining conditions. The objective of this study is to determine the effect of variable cutting parameters and different cutting tool types on surface quality and cutting force. For improving sur-face quality, one of the first steps to be taken is to change the cut-ting parameters, as this does not require additional tooling and equipment. Therefore, an attempt is made to determine the extent to which varying the machining conditions will affect the milling ef-fect. The impact of the cutting tool is also considered, since the end mills used in this study are made of different materials and have different geometries.

The novelty of the research consists in a comprehensive anal-ysis of the influence of various cutting parameters on the machina-bility of the EN AW-7075 T651 aluminum alloy, considering two dif-ferent cutting tools. This provides new information on the optimiza-tion of the milling process of this aluminum alloy, which can contrib-ute to improving the quality and efficiency of manufactured parts. A wide range of cutting parameters corresponding to conventional and High Speed Cutting allows for better representation of real manufacturing conditions.

The study involved investigating the stability and effects of a milling process for aluminium alloy EN AW-7075 T651. Owing to its high strength and corrosion resistance combined with low specific weight, this material is widely used in the aerospace and automo-tive industries. Test specimens came in the form of cuboids with the dimensions of 160x70x33 mm. The milling process was performed on an AVIA VMC 800HS milling centre – Figure 1.

Fig. 1.

Experimental setup: a) schematic diagram; b) experimental procedure

The milling process was conducted using variable cutting parameters, as shown in Table 1. The variable parameters were the cutting speed v_c and the feed per tooth f_z, while the constant parameters were the depth of cut equal to 1 mm and the width of cut equal to 12 mm.

The selection of technological parameters was based on the recommendations of the cutting tool manufacturers, as well as the authors' experience in machining aluminium alloys. The tests were carried out on the basis of one pass for one configuration of technological parameters for two tools.The milling process was carried out using two 12 mm diameter milling cutters: –

a cutter with three PCD inserts (Bryk D10.1210), each having a length of 25 mm, with an overall length of 82 mm and a tooth angle of 0°;

During the tests, no significant wear on the cutting tools was found to affect the results.

Cutting forces were measured during the milling process using a Kistler 9257B piezoelectric force gauge connected to a 5070A amplifier, from which the signal was sent to a DAQ 5697A module. The maximum values of the cutting force components F_x, F_y and F_z were analysed.

The study also included an evaluation of surface roughness parameters Ra, Rz and RSm. The measurements were made using a Hommel Tester T1000 contact profilometer in accordance with the PN-EN ISO 4287 standard.

Based on the obtained results, the following conclusions were drawn: –

The use of a diamond tool led to a significant improvement in surface quality. Although the greatest differences were observed for the roughness parameter Ra, the results of the parameters Rz and RSm clearly differed as well.

In future works, it is planned to extend the research conducted so far with additional indicators, such as: tool wear (especially at high speed cutting), vibrations, cutting temperature or tool deflection. These are phenomena that also have a significant impact on the course of machining and its effects. Analysis of additional parameters will allow for a better understanding of the processes occurring during milling of this group of materials. The research will also be continued for other aluminum alloys.