1 Introduction

7xxx series aluminum alloys stand out with their superior mechanical strength compared to other alloy compositions (Ref 1). These alloys are classified as a precipitation-hardened alloy type by an obstacle to the dislocation movements of small and homogeneously dispersed precipitates (Ref 2). A heat-treatment process reconfigures the crystal structure to strengthen the mechanical properties of alloys. Generally, the heat treatments applied to AA7075 are T4, T6, and T7 by their usage area. T4 refers to natural aging, while T6 and T7 refer to artificial aging. On the other hand, SCC sensitivity may increase when these alloys are tempered, especially to the highest strength. SCC is a dangerous type of corrosion and can cause sudden damage even under mechanical loading conditions that are well below the tensile strength of the materials (Ref 3). AA7075 (Al-Zn-Mg-Cu) is used in aircraft, aerospace, automobile, and transportation industries to make durable and lightweight structural components (Ref 4,5,6). Surface integrity is critical to fatigue failure, especially in an application in the aerospace industry (Ref 7), as drilling causes micro-cracks on the hole surface that weaken the material against fatigue damage. The main properties of metallic materials which affect machinability are hardness, ductileness, and toughness. Zn and Mg alloying elements present in the AA7075 produce the MgZn2 intermetallic phase resulting in an aging treatment, which causes an increase in the strength. Aluminum alloys are comparatively soft materials having specific problems in drilling (Ref 5, 8).

In the drilling process, some hard deposits in the aluminum alloy and the sticking tendency due to the soft character of the matrix material adversely affect the tool life more than expected (Ref 9). The soft and ductile properties of aluminum alloy could lead to thermal softening on the material during the operation which causes built-up edge formation that accumulated at the tool edge and material adhesion on the drilled surface (Ref 10, 11). The effects of heat treatment on machinability performance as a parametric variable have been revealed by some researchers. Tash et al. investigated the machinability performance of heat treated 356 and 319 aluminum alloys and stated that the heat treatment would increase the hardness and decrease the built-up edge BUE formation (Ref 12). Yarar and Karabay investigated the machinability of AA6061 for T6 and T1 conditions. The results showed that T6 heat treatment significantly reduced the formation of BUE. Similarly, lower cutting forces and lower surface roughness values were found in the test specimens to which T6 was applied (Ref 13). Ashok et al. investigated the effects of heat treatment on 7075 aluminum alloy by measuring the burr height. Experimental results showed that hardened aluminum alloy exhibited the lowest burr height compared to as-cast and annealed aluminum alloy (Ref 14). However, these studies are very limited and there are very few studies examining the effects of heat treatment on machining especially for 7075 aluminum alloy. According to the parameters discussed in previous studies on aluminum drilling, the main parameters affecting surface quality in drilling are spindle speed, feed rate, tool inclination with the drilled surface, and feed direction (Ref 15). Machining studies without statistical methods are unsuccessful in determining which parameter has a significant impact on performance characteristics, and it requires a lot of experimentation and data (Ref 16). Therefore, there is a lot of literature on cutting parameters optimization in machining. For example, Sofuoğlu et al. applied multi-objective optimization based on input parameters using hybrid decision-making for machining (Ref 17). In another study, Arapoğlu et al. used artificial neural networks for surface roughness prediction (Ref 18), and in another study, Orak et al. used an ANN-based decision-making method to determine the optimum processing parameters (Ref 19). Similarly, Gök et al. investigated the effect of the cutting tool material on chatter vibrations in the machining of AA7075 using regression analysis (Ref 20). Çakıroğlu and Acır investigated the drill bit temperature in the drilling of Al 7075 alloys using Taguchi Design Method and found that the most influencing factor on the drill bit temperature is the feed rate (Ref 21). Rajeswari and Amirthagadeswaran studied multi-response optimization of end milling in aluminum composites using Response Surface Methodology (RSM) based grey relational analysis (Ref 22). Their findings indicate that spindle speed and weight percentage of SiC are the most prominent factors affecting the machinability of hybrid composites. Nripen Mondal et al. performed regression analysis with satisfactory accuracy for finding optimal conditions in an aluminum alloy drilling (Ref 23). According to their study, burr height increases with increasing feed rate. In these studies, the target material properties were kept constant, and it was aimed to optimize the machining process by changing the cutting parameters or cutting tool properties. Therefore, unlike previous studies, the present investigation deals with a microstructure configuration suitable in various industrial applications using heat treatments. In this study, heat treatments commonly used in industrial applications were performed on the material with a view to presenting the relationship between microstructure and machinability. The properties of drilling parameters on chip formation were investigated when drilling AA7075 test samples. RSM is one of the powerful optimization tools frequently used in experimental research in recent years for statistical evaluation. This study reports optimizing the multiple performance characteristics for different temper conditions in the dry drilling of AA7075. Heat generation detrimentally affects hole quality. Thus, the temperature and the surface roughness were measured and analyzed during drilling for each alloy condition. This article provides an overview of industrial process planning applications using AA7075 materials, providing information for deciding the process step sequence of the drilling process. The findings presented in the research allow manufacturing engineers to evaluate their decision on whether to drill before or after heat treatment. This study also includes finite element analyses (FEA) to verify and validate the research data.

2 Material and Method

2.1 Heat-Treatment and Microstructural Characterization

This study reports the effect of various heat treatment treatments on the machinability properties of AA7075 series aluminum alloy. For this purpose, different heat treatments were applied to the extruded AA7075 specimens with a dimension of Ø25 mm. Table 1 shows the chemical composition of the AA7075 used in this study. Table 2 presents the different heat treatments applied to the test specimens. Table 3 shows the mechanical properties of AA7075 for different temper conditions.

Table 1 The chemical composition of the investigated AA7075 (wt.%)
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Table 2 Different heat treatment processes applied to specimens in the study
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Table 3 The mechanical properties of different AA7075 [24, 25]
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Microstructural investigations were performed using an optical microscope (OM) and a Jeol JSM 6060 scanning electron microscope (SEM) with energy-dispersive x-ray spectroscopy (EDX) attachment to observe the changes in microstructure properties obtained after heat treatments. For this purpose, specimens were polished and etched with Graff Sergeant’s solution for 20 s to reveal the microstructure.

Aging conditions have a significant effect on the size, quantity, and distribution of precipitates. During aging treatment, the size and quantity of precipitates increase, initial precipitates are generally coherent, then becoming semi-coherent, and finally incoherent with the matrix, respectively. The size, distribution, and consistency of precipitates with the matrix have a significant effect on the properties of Al-Zn-Mg-Cu alloys (Ref 26,27,28). Figure 1 shows the microstructure of the heat-treated specimens in different conditions. Figure 1(a) shows that the microstructure with O conditions is very coarse and contains intermetallic precipitates. Figure 1(b) and (d) present more homogeneous and finely distributed MgZn2 (η) precipitates in the microstructure of F and T6 heat treatment specimens. These precipitates become coarser and non-uniform distributed with T7 heat treatment as seen in Fig. 1(e). Moreover, the precipitation content of the T4 heat-treated samples was lower than that of the F, T6 and T7 heat-treated samples. The chemical compositions of the precipitates observed in the microstructural investigations were determined in the EDX analysis. According to EDX analysis results given in Fig. 1, while small-sized precipitates observed in grain and grain boundaries contain Mg and Zn, coarse precipitates contain Cu, Fe, Mn, and Si. MgZn2 is the primary precipitate that increases the hardness in AA7075 series aluminum alloy. Coarse Al2CuMg, AlMnFeSi, and Al7Cu2Fe precipitates are formed during solidification and cannot be dissolved by solution treatment (Ref 29,30,31).

Fig. 1
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Microstructures of the heat-treated specimens

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Hardness tests were applied using a Future-Tech Vickers hardness tester under a 3 kg load for 10 s. Five measurements were carried out for each specimen to have an average value. Hardness values of the heat-treated test specimens in different conditions are given in Table 2. High strength and hardness are obtained by reaching the optimum distribution of the fine-sized and high amounts of precipitates. Fine and uniform MgZn2 (η) precipitates coherent with matrix lead to an increase in the hardness and strength of 7075 alloys (Ref 31). Hence, the T6 and F heat-treated specimens show the highest material hardness. The lowest material hardness was obtained in the 0-condition test specimen as expected. Also, it was seen that T4 conditions have a lower hardness value when compared with T6 heat-treated test specimens. This situation shows that more aging time is required to achieve better properties under T4 heat-treatment conditions. Due to over-aging with T7 heat treatment, the precipitates coarsened, and the hardness of the test specimens decreased.

2.2 Experimental Set Up, Optimization and Validation Procedure

A Toss United TU5032B vertical drill machine was used in the drilling operations. The experimental setup is provided as a schematic overview in Fig. 2. A Kistler 9272 piezoelectric dynamometer and a model 5070A amplifier were instrumented to collect thrust and torque data. A total of 3 samples were prepared for each temper condition, and each sample thickness was 10 mm. All drill bits had a nominal diameter of 6 mm. No cutting fluid was used during drilling and all experiments were repeated 3 times. The average surface roughness (Ra) is used to evaluate the surface roughness (Ref 15, 32). The surface roughness was measured using 2D Mitutoyo SJ-301 from three different points. The geometric characteristics of each hole have been evaluated with SEM and stereo microscope images. Moreover, chip formations were characterized using thermal images obtained from the infrared camera. Drilling temperatures were recorded with a FLIR-A325sc model infrared camera. The infrared cameras utilize infrared radiation to determine the surface temperature of objects without any physical contact, which is very useful for applications like drilling. The utilization of infrared cameras in drilling operations has been employed, as in early research (Ref 32). The temperature measuring points were the 5 mm upper surface of the drill bit and 5 mm lower from the upper surface of the drilled hole. The distance between the thermal camera and temperature measuring points was 50 and 52.5 cm for the drill bit and hole surface, respectively. The various aging treatments of 7075 aluminum alloys in drilling were considered and optimized by conducting drilling experiments using Ø6 mm HSS-G high-performance ground standard twist drill, three spindle speeds, three feed rates, as seen in Table 4.

Fig. 2
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Experimental setup of the drilling operations

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Table 4 Selected factors and levels for the DoE model
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The full factorial experimental design was employed with the selected parameters (Ref 13, 33). RSM is used to statistically evaluate the effect, significance, and interactions of the independent variables, and the optimal dry drilling parameter settings. The purpose of creating a response surface is to estimate the optimum point of this region, which provides particular properties in a design plane consisting of many parameters that are effective on the result in an experimental study. Although the function obtained is representative of the experimental data, it is crucial that the model fit is high, which is necessary to identify the most significant factors with the number of potential influences on one or more responses. The RSM approaches save time, resources and energy by defining cause-and-effect relationships in mathematical models. The model was validated based on analysis of variance (ANOVA), and regression equations were obtained using the RSM. The relationship between the independent variables and the responses in RSM is defined by a second-order polynomial model given in Eq 1.

$$y = \beta_{0} + \sum\limits_{i = 1}^{k} {\beta_{i} x_{i} } + \sum\limits_{i = 1}^{k} {\beta_{ii} x^{2} } + \sum\limits_{i = 1}^{k - 1} {\sum\limits_{j = 2}^{k} {\beta_{ij} X_{i} } X_{j} } + \varepsilon$$
(1)

where y is the predicted response, β0 is constant, βi, βii, and βij represent coefficients of linear, quadratic, and interaction terms, respectively. While X shows the coded variables, ε indicates the error. ANOVA tables, contour, and main effect plots were used in the analysis of the experimental results. ANOVA tables are given in supplementary materials. Test results in the 95% confidence interval for the P values were considered in all variance analyses (Ref 13, 34,35,36,37).

3 Results

3.1 Evaluation of Thrust Force and Torque

The measurement of forces in the drilling system is essential to investigate the effects of workpiece properties. Figure 3 and 4 show the main effect and contour plots for thrust force and torque, respectively. From the main effect plots, torque (Mz) and thrust force (Fz) both similarly increased by about 93%, with the increased feed rate from 0.1 to 0.3. The opposite this, the spindle speed increase resulted in a slight decrease of Mz and Fz. Data from this study showed that increasing the feed rate from 0.1 to 0.3 mm/rev contributed 49.3% to the thrust force. A previous study found that increasing the feed rate from 0.05 to 0.15 mm/rev contributed 92% to the thrust force in drilling AA7075 alloy (Ref 38). Giasin et al. studied the cutting forces in drilling Al2024 (Ref 48). They found that the percentage contribution of feed rate on cutting forces was 48.36%. Therefore, it can be inferred that an increase in feed rate in aluminum drilling tends to induce a dramatic increment in the value of the thrust force. Among the heat treatment types, the differences in Fz obtained varied by up to 10%. Drilling T6 heat-treated samples resulted in the lowest thrust force value around the mean of 100 N.

Fig. 3
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Main effect plots for thrust force, torque, material, and drill temperature

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Fig. 4
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Contour plots of (a) thrust force, (b) torque, (c) material temperature, and (d) drill temperature for different cutting parameters

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Contour graphs obtained depending on the cutting parameters are given in Fig. 4. Increasing the feed rate for each temper condition causes a progressive increase in the thrust force. On the other hand, spindle speed increase shows a pretty weak effect, which decreases Fz in the T6 condition and increases in the T7 condition. Mz values increased with increasing feed rate for all heat-treatment conditions. Alternatively, the weak effect of the spindle speed increase in T6 and T7 conditions was also observed for Mz values. A relatively small difference in hardness values of T6 and T7 treated samples results in weak influence in Fz and Mz. As a result, it was seen that the distributions obtained for the cutting torques and forces were compatible with each other.

3.2 Temperature Analyses during the Drilling Operation

The temperature during the drilling operation is a limiting factor of machining because the tool life decreases with higher temperatures (Ref 39). In addition, since the temperature increase affects the main mechanical properties of the material due to the complex thermo-mechanical interaction between the tools and the workpiece, such as elasticity and plasticity, the hole surface quality is also indirectly affected (Ref 40, 41). A part of research on thermal aspects of the drilling operations has concentrated on measuring the temperature of obtaining the mean temperatures in the cutting tool and the workpiece surface. These measurements provide information about the temperatures on the shear plane and the tool-chip interface, and the intensity and distribution of heat sources in drilling operations. There is always heat conduction occurring within the workpiece, the chip, and the tool. The thermo-mechanical effect occurs during material removal processes as plastic deformation of the workpiece and the friction along with the tool-chip interface. Therefore, it is substantial to understand how drilling temperatures are affected by the drilling parameters. It is necessary to know the amount of heat generated in both the drill bit and the workpiece during the drilling of the samples, as well as the temperature level that occurs accordingly (Ref 32, 42, 43). The temperatures arising in the drilling process of the tool and workpiece are examined in Fig. 3 and 4, depending on the different heat treatments applied to AA7075 samples. There is an insignificant change of mean 3 °C in terms of the effects of feed rate variables on tool temperature. Spindle speed is a more effective parameter according to the analysis data on the tool temperature. The difference in spindle speed parameter selection causes an increase of up to 10 °C on the tool temperature. While the increase in feed rate from 0.1 to 0.3 mm/rev raised the tool temperature by 4.3%, the increase in spindle speed from 715 to 3030 rev/min elevated it by 14%. Özcelik and Bağcı drilled the AA7075-T651 parts at f = 0.1, 0.15, 0.2 mm/rev and 1910, 2547, 3184 rpm (Ref 44). Their findings are in line with the data of this study. The presented data indicated that increasing the feed rate raised the drill bit temperature by 2.12% at high spindle speed while increasing the spindle speed elevated it by 10.54%. Therefore, the effect of feed rate increment on tool temperature in drilling AA7075 is restricted, whereas spindle speed increment is more prominent. Among the heat-treatment types, samples in the T7 condition have a significant unfavorable effect on tool temperature. Other types of heat treatment, F, T4, and T6, give negligibly different results. As demonstrated by the thermal camera images taken during the drilling operation in Fig. 5, generated heat dissipates by the workpiece, the tool, and chips to the environment. The high thermal conductivity of the AA7075 material contributes to the rapid cooling of the specimen by providing heat dissipation.

Fig. 5
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Heat generation after drilling with 0.1 feed rate and 1520 spindle speed

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Figure 3 and 4 show the occurred temperatures in the workpiece at different heat treatment conditions during the drilling process, depending on the process parameters. As a result of the increase in parametric level, both feed rate and spindle speed factors, there is a slight decrease in the temperature of the workpiece during operation. The occurred temperature in the workpiece for the AA7075-T7 heat treatment condition is higher than the other heat-treatment conditions. T4 and T6 heat treatments gave similar results on workpiece temperature. However, the specimen in the F condition showed the lowest heating behavior during the drilling process. As a result, temperatures of the hole surface and drill bit are limitedly affected by the difference in heat-treatment type.

In Fig. 4(c) and (d), the workpiece and drill bit showed a maximum effect of 7 °C (39-46 °C) and 10 °C (60-76 °C) on the temperature measurement results, respectively. The heat generated in the deformation zone flows into the drill bit & the chip, and hence it should be included in the total temperature rise of the chip. In conventional drilling, most of the heat flows into the workpiece. Part of this heat is then transferred to the chip along the shear plane through convection from the workpiece. The drilling time calculations give us the actual heat transfer time in tested cases. Machining time and cutting speed for drilling are calculated with the formulations below.

Machining time for drilling, T = L/(N.f) (T: time [minutes], L: dept of hole [mm], N: drill speed [rpm], f: feed rate [mm/rev]).

Cutting speed for drilling, Cs = (π D N)/1000 (Cs: cutting speed [m/min], D hole diameter [mm], N: drill speed [rpm]).

Considering drilling operations as two parametric limit values, higher and lower. Nominated conditions are as higher for 3030 rpm—0.3 mm/rev, and as lower for 715 rpm—0.1 mm/rev. Calculated the drilling times are 0.011 and 0.14 min for higher and lower drilling conditions, respectively. Besides, cutting speed values are 57 m/min and 13.5 m/min for higher and lower drilling conditions, respectively. These calculations show that most of the heat conducts into the chip, as there is inadequate time for the generated heat to flow into the workpiece. Another point of view is to consider the effect of the heat generated during the drilling operation on the workpiece material. Considering the hot deformation temperature at 350 °C and the semi-solid forging temperature at 500 °C for Al 7075, the drilling operation around 40 °C was interpreted as a negligible effect (Ref 23).

To summarize, the heat dissipation during drilling is transferred to the chips rather than the workpiece, since the drilling times are short depending on the test conditions. This is also illustrated by the thermal images given in Fig. 5. It shows that in all 4 different cases, the drilling temperatures are taken by the chips formed during drilling. This observation will also be discussed further with the drilling model proposed in section 3.5.

3.3 Chip Formation and Hole Surface Quality Assessment

The very ductile and soft properties of nonferrous metals make them hard to drill because of the prolonged period of contact with the sharp cutting edges of the tool. In machining operations, aluminum alloys have a tendency to stick to the cutting tool, resulting in BUE and cutting tool wear (Ref 13, 45). The BUE is detrimental to surface roughness and dimensional accuracy. According to the temperature investigation, we have presented in the previous section, there isn’t any correlation between the generated heat and thrust force or torque shown in Fig. 3. Therefore, chip formation and hole surface are related primarily to the microstructure and properties of the materials compared to generated heat. In this study, since the aluminum alloy composition does not change, two opposite phenomena can dominate the machinability due to the structural differences caused by different heat treatments. The first fact is that the amount and size of the hard secondary phase in the structure are effective on the abrasive character of the alloy during processing. The second fact is that stacked cutting edge created by adhering to the workpiece makes chip formation and evacuation difficult that resulting in a detrimental effect on hole geometry and roughness. Which of these two phenomena will be prominent will vary depending on whether the structure originating from heat treatment has a hard phase or a soft adhesive phase. Intermetallic particles as secondary hard phase can be set in solid solution, grain structure, and dislocations. Forming of the particles as fine strengthening, dispersoid, and constituent role in AA7075 are all changed by the heat-treatment types (Ref 31).

Figure 6 shows OM images along the drilled holes and SEM images of the hole entry & exit, and a set of images presenting after operation condition of the drill bits for each heat-treatment condition. Figure 6(a) shows the hole inner surface and tool wear for the 7075-O condition. On the drilled hole entrance and exit examination, the hole gets wider along with the drilling continues. The reason for hole geometry deterioration and the dense deposits of metal plastered on the tool is the adhesion behavior of the soft character of the 7075-O structure during processing. The hole geometry is quite distorted, and undesirable compared to other conditions. Moreover, the intermetallics dissolved in the microstructure deformed the inner surface of the hole during cutting due to its brittle and fragile structure. This case indicates that condition O is not suitable for drilling processes. Furthermore, the thrust force was approximately 102 N in this condition. This value is about %25 higher than other temper conditions. Figure 6(b) shows deep incision marks in the F condition sample, even at the entrance 0.1 mm/rev. These deep traces have reached half the length of the hole. The entry diameter of the drilled hole is larger than the exit diameter. It decreases continuously up to the exit and reaches the minimum. The high hardness and brittle character of F condition sample is the most likely cause of these defects. The images for the T4 condition in Fig. 6(c) show a significant improvement in hole geometry and BUE compared to the findings for the O condition. We can list these improvements as follows; reduction in deep groove marks at the entry of the hole from OM image, reduction of dimensional difference in hole entry & exit, reduction of the amount of plastered residue on the drill bit cutting edge according to SEM images. The listed improvements are each even more pronounced in T6 heat-treatment sample results in Fig. 6(d). In Fig. 6(e), entry grooves and accumulation of workpiece material against the rake face in the post-processing images of the T7 heat-treated samples were more damaging than the T6 sample. AA7075 has four types of intermetallic compounds as secondary phases. SEM images in Fig. 1 show the secondary phase distribution for each specimen. The T6 condition exhibits the finer granular η-phases than the T7 sample. Besides, the microstructures of all heat-treatment conditions contain relatively fine AlMnFeSi and coarse Al2CuMg, Al7Cu2Fe precipitates originating from the residual Cu, Fe, and Si element. Heat treatment variables such as soaking time and annealing temperature are the main factors in microstructure formation. Reportedly, annealing applications reduce yield strength and improve tensile elongation compared to as-received AA7075 materials. According to the knowledge obtained from previous studies, fine secondary particles do not show deformation in the tensile test and have limited influences on the overall structure in terms of deformation mechanism. It has also been reported that lattice turns are limited nearby the thin secondary phases. In contrast, coarse secondary particles deformed and showed a fracture (Ref 30, 46). Grains with coarse secondary particles would exhibit an easy slip system and transformed strain through the slip bands, which promote deep incision marks and sticky BUE formation in the F and T6 condition samples. During the cutting process, the coarse secondary particles underwent tear and separation.

Fig. 6
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A set of selected micrographs for evaluating the BUE and hole formations

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When the SEM images for tool wear are examined, in all the wear micrographs except the drill bit wear given in Fig. 6(d), an intense BUE is noticeable, especially on the cutting edges. When the wear mechanism in Fig. 6(d) is examined in more detail, it is seen that the BUE is concentrated towards the drill bit rather than the cutting edges. Moreover, for the F temper condition given in Fig. 6(b), it is understood that the BUE density is similar to each other at the cutting edge and the drill bit. In Fig. 6(a), (c) and (e), there is intense adhesive wear on both the drill bit and the cutting edges. According to these observations, it turns out that the cutting quality of the tested aluminum alloys in the drilling processes is related to the applied heat treatments. Adhesive wear is more dominant for other heat treatments that show more ductile characteristics than T6 temper properties. This result is also compatible with the thrust force results.

In Fig. 7, different chip formations obtained in experiments conducted under the same cutting parameters (0.1 mm/rev, 1520 rpm) are presented. While deformed chip morphology occurs in Fig. 7(a), there are relatively fewer differences among other chip structures. The lower strength, hardness, and high ductility properties of the AA7075-O sample made the chip flow irregular during drilling. It tends to stick to the drill instead of being ejected, and the chip structure deteriorates rapidly. In addition, coarse intermetallics in the microstructure provided fragility during cutting and negatively affected the plastic deformation mechanism in chip formation. As a result, the uneven chip flow deforms the hole's inner surface and increases the thrust forces. Besides, the chip formations that occur in Fig. 7(b) and (d) are in continuous form and spiral structure. Continuous chip structure occurs at lower feed rates and in the processing of relatively ductile metals such as aluminum. This chip structure adheres to the tool and causes BUE formation (Ref 15). BUE formation is observed for each temper condition in Fig. 6. Among the chip structures, the F and T6 conditions have the most desirable chip form. It is thought that MgZn2 precipitates, which display a finer and more homogeneous distribution in the microstructure, provide an advantage during cutting. In Fig. 7(c), it is seen that chip formation begins in a new flat helical form before the conical spiral chip formation ends (Ref 13). In T4 heat treatment, the chip structure can be formed in a different form, as it cannot reach the sufficient precipitate density and therefore the maximum hardness. In Fig. 7(e), (a) relatively damaged continuous spiral chip formation is seen. Because the grain structure, which became coarse due to over-aging, decreased the hardness, increased the efficiency of the secondary phases in the microstructure, and deformed the chip structure.

Fig. 7
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Chip formations for various heat-treatment conditions (0.1 mm/rev, 1520 rpm)

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Figure 8 shows that the change in feed rate affects average surface roughness. Surface micro-cracks caused by the cutting mechanism are the main reason affecting the in-hole roughness and fatigue life (Ref 47). The surface roughness measurement was separately performed at the hole entry and exit. For each temper condition, the roughness level at the hole entry is higher than the hole exits. This is visible in the images of the inner surface of the hole in Fig. 6. The helical traces observed in the figure increased the roughness at the hole's entrance. These tracks diminished towards the hole exit because of adhesive wear at the cutting edges of the drill bit. As a result, this situation caused narrower hole exit geometry. It is also determined that the T6 temper condition has the lowest average roughness. The high strength of T6 has facilitated chip formation and flow. Thus, higher surface quality occurred. The chip structure given in Fig. 7(d) for T6 has the desired helical and undeformed structure for drilling processes. Continuous contact of the drill bit with the inner surface of the hole heats the drill and the workpiece, which increases the ductility and deformations of the drilled hole, resulting in higher surface roughness (Ref 48, 49). The maximum cutting temperatures reached for the T7 temper condition, as shown in Fig. 3. The chip structure in Fig. 7(e) also explains occurring the roughest surface in the T7 temper condition. In general, the drilled holes become smoother with the increase of the feed rate. Considerably, feed rate in 0.3 mm/rev gives better surface roughness. The average Ra values were 50.2% higher in the entrance zone than in the exit zone for all heat treatment conditions at f = 0.1 mm/rev. While at f = 0.3 mm/rev, the average Ra values deviated by only 3.3% in the entrance zone compared to the exit zone for all heat treatment conditions. The present study identifies that low feed rate in drilling AA7075 results in variable roughness throughout the length of the hole. Aamir et al. stated that Ra generally decreases with increasing cutting speed (Ref 35). However, Yaşar presented a contradicting finding that Ra increased with higher cutting speed (Ref 38). Yaşar argued that the contradiction is due to the relatively soft character of the AA7075 workpiece compared to other metals, causing it to be prone to BUE formation. Giasin et al. reported that while drilling Al2024, increasing the cutting speed would compromise the surface quality of the hole (Ref 48). The discrepancy in previous studies may be due to the lack of consideration of the different surface characteristics in the entry and exit zones in AA7075 drilling. There are conflicting interpretations of surface roughness in the literature. This study clarified the origin of these inconsistencies by evaluating the surface roughness of the entry and exit regions in separate analyses.

Fig. 8
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Effects of the feed rate on the surface roughness (1520 rpm)

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A high thrust force is expected to have a negative effect on surface roughness. However, the high ductility properties of 7075 revealed a different result than expected in this study. The increasing feed rates caused trimmings of more aluminum to adhere to the inner surface of the hole during drilling. While this increased the thrust force, it caused a decrease in the surface roughness. The roughness measurements at the hole entrance and exit separately were also made to reveal this observation. As a result, the increase in adhesive effects decreases the surface roughness.

3.4 Regression Equations

The regression equations are given in Table 5 for thrust force, torque, material, and drill temperature were experimentally derived with the use of regression analysis and ANOVA. Here, the coefficients of linear, interaction, and quadratic terms are presented separately for each response. The ability of the regression equations to estimate the test results is expressed by the R2. Predicted R2 shows the ability to predict possible new observations with the obtained regression equation (Ref 50). R2 and R2 (pred.) values obtained for each response are given in Table 5. Accordingly, the statistical model has a high enough ability to predict the simulation results.

Table 5 Empirical models of drilling operation responses
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3.5 3D FE Model of the Drilling Process

In this section, the drilling process of 7075 aluminum alloy is modeled using Deform 3D software. 3D computational modeling is utilized to predict chip formation, thrust force, and temperature. Based on the experimental results, the drilling process was modeled for 7075-T6 alloy and feed rate of 0.1 mm/rev and 1520 rpm, considering that T6 alloy gives the most optimum and effective results. There are modeling studies for 7075-T6 alloy using different Johnson–Cook material models in the literature (Ref 51, 52). The study employed table data that included larger-scale strain, strain rate and temperature measurements recorded individually. Tabular data format in the Deform 3D library was selected to define the mechanical and thermal properties of AA7075-T6.

$$\overline{\sigma } = \left( {\overline{\varepsilon },\;\mathop {\overline{\varepsilon }}\limits^{ \cdot } ,\;T} \right)$$
(2)

where \(\overline{\sigma }\) is flow stress, \(\overline{\varepsilon }\) is effective plastic strain, \(\dot{\overline{\varepsilon }}\) is effective strain rate, and T is temperature. This model is suitable for the drilling process as it defines the flow behavior as a function of strain, strain rate, and temperature. Other mechanical and thermal properties used in modeling are given in Table 6. The logarithmic interpolation method was chosen as the interpolation method. The implicit Lagrangian computational routine was used to simulate the drilling process and the distorted elements of the mesh with continuous adaptive remeshing (Ref 53). The workpiece and drill were defined as a plastic and rigid body, respectively (Ref 54). The frictional condition was defined as the constant shear friction factor, m = 0.7, between the aluminum workpiece-drill interface (Ref 52). The workpiece has meshed into nearly 1.5 × 105 tetrahedron elements with a minimum element size of 0.03 mm and the high-density mesh was placed at the drilling zone. The tetrahedron element type can be efficiently used in plastic deformation simulations (Ref 55). Similarly, the drill bit was discretized into 1 × 104 tetrahedron elements with higher mesh density in the drilling contact zone. The 3D FE model of the drilling process is shown in Fig. 9.

Table 6 Mechanical and thermal properties of workpiece used in FE simulations
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Fig. 9
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3D FE model of drilling procedure and thrust force graph

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The thrust force graph obtained after the simulation is also given in Fig. 9. Accordingly, thrust force was found at about 71.62 N. This value predicts the experimental thrust force data under the same parameters with an error of 4.62%, which is an insignificant difference. Consequently, it was observed that the developed 3D model for the drilling process is consistent with the experimental results.

Figure 10 shows the temperature, effective stress and effective strain distributions occurring on the workpiece during drilling. Accordingly, the maximum effective strain and stress values were found to be ~ 16 (mm/mm) and ~ 790 MPa, respectively. After the regular chip flow started, the maximum stress and strain values have remained at these values throughout the simulation until the chip formation reached the conical structure. It was also found that the temperature in the contact zone was about 40-45 °C. Figure 11 presents the temperature evolution during the simulation at point P1 selected from the center of the workpiece. According to the results, the temperature increases at point P1 continued until approximately 50-55 °C and then started to decrease. It is seen that the heat generated during cutting is removed from the workpiece by chip. In addition, it is seen that the predicted temperature values are in accordance with those given in Fig. 5(c).

Fig. 10
figure 10

Predicted temperature, strain and stress distributions in the chip and workpiece (AA7075-T6, 0.1 mm/rev, 1520 rpm)

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Fig. 11
figure 11

Predicted temperature distribution for the P1 point

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The thrust force and temperature distribution obtained in the simulation so far are compatible with the experimental data. These data are decisive for parameters affecting chip formation. This conformity is expected to be in harmony with the chip structure during drilling. In Fig. 12, chip structures obtained both in experimental and simulation are given. At first glance, it is striking that the chip structure formed in the simulation is almost the same as that obtained in the experiment. One of the parameters used to compare chip formations is chip thickness (Ref 52, 56). The chip thicknesses obtained in the experiment were measured using an OM. On the chip thicknesses comparing, the experimental results are in perfect agreement with the simulation results. As a result, the 3D model developed for AA7075-T6 is consistent in terms of temperature, force, and chip structures.

Fig. 12
figure 12

Comparison of chip formation obtained in simulation and experiment (AA7075-T6, 0.1 mm/rev, 1520 rpm)

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4 Conclusions

This study investigated the effect of heat treatment properties on the drilling behavior and chip formation mechanisms of AA7075.The following conclusions can be drawn:

  • Because of the high thermal conductivity of AA7075 material, the temper conditions have a negligible effect on the workpiece temperature during the drilling process. The change in feed rate increased the tool temperature by an average of 3 °C, while the changes in spindle speed increased it by 10 °C. The maximum temperature increase in the workpiece was 7 °C. Chip formation and hole surface quality are related to the microstructure rather than heat generation. Since the elemental composition did not change, the abrasive effect during machining varied according to the amount and size of the secondary phase caused by structural differences due to heat treatments. Another factor, the stacked cutting edge, had a detrimental effect on the hole geometry and roughness due to the difficulty of chip formation and evacuation. The feed rate of 0.3 mm/rev in all heat treatments reduced the Ra value by around 50% and provided good surface roughness.

  • The F and T6 condition samples resulted in deep incision marks up to half the hole length. The T7 temper condition revealed entry grooves and accumulation against the rake face and exhibited more damaging character than the T6 samples. One finding was that the T6 temper condition had the lowest thrust value 100 N on average. The difference in Fz value between the heat treatment types varied up to 10%.

  • Chip formation of O condition sample tends to stick to the drill and deformed form. The T7 sample results in a damaged continuous spiral form, while the F and T6 conditions have the most desirable chip formation. It was concluded that MgZn2 precipitates showing a relatively fine and homogeneous distribution in the microstructure of F and T6 conditions provide an advantage during cutting.

  • There is good agreement between the experimental and computational analysis for chip formation, thrust force, and temperature distribution. Chip formations are very similar to each other. The numerical analysis predicted the experimental thrust data under the same parameters with an error of 4.62%. The numerical analysis also estimated the chip edge thickness data based on the average of three discrete point values under the same parameters with an error of 0.59%.