1 Introduction

Considering machinability as a system property allows us to define machinability as an interaction phenomenon between the workpiece, cutting tool (tool material and geometry), and cutting medium (wet or dry cutting) for different removal sequences such as turning, drilling, tapping, milling, and sawing under different cutting conditions which are cutting speed, feed rate, and depth of cut [1,2,3,4,5,6]. Many research investigations were undertaken in order to analyze the role of the tool geometry on the machinability of different alloys under different cutting conditions for different machining processes. In the case of turning, Fang et al. [7] introduced a new model to predict chip formation in chamfered and honed tools for aluminum alloys and concluded that the ratio between thrust force and cutting force varies according to the thickness of the uncut chip. In addition, the thrust force can exceed the cutting force if the uncut chip thickness is less than the critical thickness, which can be determined by the cutting speed and tool geometry [8,9,10]. On the other hand, with respect to drilling, Tash [9] reported that in casting alloys with highly abrasive materials, it is preferable to use solid carbide drills with twist drills to make the edges more resistant to wear at higher speeds [10, 11]. Soares et al. [12] investigated chip breaker systems in different cemented carbide and PCD diamond cutting tools, in order to evaluate their quality in the machining of aluminum alloys. They reported that inserts with big grooves, and a high angle of entrance in the chip breaker, showed good results in power consumption, surface roughness, and chip control for roughing operations (f > 0.14 mm/rev) in high silicon-containing aluminum alloys [13].

Tool wear is the main factor that controls tool life. It takes place as a gradual process because of continuous interaction between the cutting edge and workpiece under specific cutting conditions, until the failure of the tool. The wear process depends on tool material and geometry, workpiece material, and the cutting parameters and medium [12, 14]. Material properties such as strength, ductility, and hardness also affect different machinability aspects as cutting force, chip formation, and surface integrity [15]. Hamed [16] has summarized the above findings schematically as shown in Fig. 1.

Fig. 1
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Schematic diagram illustrating the machining environment [16]

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The present work was undertaken to optimize the alloy composition and heat treatment conditions, and to compare its performance with the widely used Al-Si based 319 and 356 commercial alloys [17]. The study was extended to also investigate the machinability behavior of the HT200 alloy castings under the different heat treatment conditions, to include this important aspect of the production process. This article presents the results on drilling and tapping of HT200 alloy castings and compares them with those of the 319 and 356 reference alloys. It should be mentioned here that the drilling and tapping parameters using a CNC machining center were decided in consultation with senior designing engineers from General Motors–USA.

2 Experimental procedure

The HT200 base alloy was supplied by Nemak. The composition of the alloy shown in Table 1 is proprietary to Nemak. The alloy is an Al-Cu alloy containing higher Si than 206 and 201 alloys, but a much lower concentration of silicon in comparison to the Al-Si-based B319 and A356 alloys which were used as reference alloys for comparison with the HT200 alloys. The 319 and 356 alloys were grain refined and modified using Al-5%Ti-1%B and Al-10%Sr master alloys, respectively. In the context of these alloys, it is important to mention here that in a previous study carried out by Zedan [15], the effect of metallurgical parameters on the machining behavior (drilling and tapping) of 356 and 319 alloys was studied using a Makino A88E machine for which extensive machinability data was collected. In the present case, a Huron K2X8five CNC machine was employed and the same 319 and 356 alloys were used to confirm the data obtained. It is in this sense, therefore, that these alloys are considered as reference alloys for comparison purposes with the HT200 alloy in the present study.

Table 1 Chemical analysis of the studied alloys
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A nomination system was used to identify the alloys. Alloy A was used to indicate HT200 alloy in the as-cast condition, alloy B was used to identify HT200 alloy subjected to T5 heat treatment, while alloy C was used to indicate HT200 alloy subjected to T7 heat treatment, and the reference alloys B319 and A356 were termed alloy D and alloy E, respectively. The alloys used in this study were provided in the form of ingots, cut into smaller pieces, dried and melted in a SiC crucible of 120-kg capacity at a temperature of 750° ± 5 °C, using an electric resistance furnace. Measured amounts of additives were calculated and added to the melt using a perforated graphite bell, plunged deep into the melt to ensure homogeneous distribution.

The melt was degassed by injecting dry argon gas into the melt at a constant rate for 15–20 min by means of a degassing impeller rotating at a speed 120–130 rpm. Following degassing, the melt surface was carefully skimmed to remove the dross and oxide inclusions. Then, the melt was poured at 740 °C into the waffle-plate graphite-coated metallic mold shown in Fig. 2 to produce the castings for conducting the drilling tests, and an ASTM B-108 permanent mold for preparing tensile test bars. The machinability test casting blocks had initial dimensions of 300-mm length, 200-mm width, and 30-mm thickness with five ribs with an average width of 25.4 mm. Seventy castings were produced for the drilling tests. A summary of the heat treatments used is presented in Table 2. It should be mentioned here that heat treatment of Al-Cu alloys was recommended by Nemak-Europe whereas heat treatment of 319 and 356 alloys were based on the authors 30 years of experience.

Fig. 2
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a Mold used for preparing castings for drilling tests, b casting obtained

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Table 2 Heat treatment procedure
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Tensile test bars (5 bars per alloy in the form of a bundle) from the different alloys were prepared using the ASTM B-108 mold, and heat treated following the same procedures as in the case of the blocks. Each bundle consists of five test bars with standard dimensions (50-mm length and 12.7-mm cross-sectional diameter) in order to evaluate the average tensile properties (UTS, YS, and %El) over the five bars tested in each case. The tests were carried out at ambient temperature (25 °C). A Servohydraulic MTS Mechanical Testing machine was used to carry out the tests, using a strain rate of 4 × 10−4 s−1. A strain gauge extensometer (with a 50.8-mm range) attached to the gauge length of the test bar was used to measure the elongation. The data was recorded by the data acquisition system of the machine and analyzed using the Test Works 4 software associated with the system to provide the yield strength at 0.2% offset strain, ultimate tensile strength, and the percentage elongation values.

The drilling process was carried out at a cutting speed of 240 m/min (15,000 rpm) and 0.2 mm/rev feed rate, using a Guhring 16101256 M6 drilling tool from Carbide steel, as shown in Fig. 3. The coolant used while drilling was Hocut 4549, normally used with high-speed processes. The machining conditions are summarized in Table 3. Prior to drilling, the machine was calibrated by the specialist from the Aeronautical Technology Center where the work took place.

Fig. 3
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a Shape and dimensions of the used drilling tool. b Shape and dimensions of the tapping tool used in the present work

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Table 3 Machining conditions for drilling
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The drilling characteristics of the HT200 and reference alloys were evaluated by means of cutting force measurements, tool life evaluation, built up edge (BUE) measurements, and chip formation characteristics. Tapping of the drilled holes was carried out using Guhring 971 H6 M6 6HX-Carbide taps (Fig. 3b). These tap series are classified as carbide tools, with a significant concentration of cobalt. This concentration enhances the wear resistance of the carbide taps and, more importantly, results in a longer than average tool life.

For determining the cutting forces, a specific configuration was employed to carry out the test, in which four 6-component piezoelectric quartz crystal dynamometers were installed on the base plate which accommodated the workpiece. These dynamometers transform the acting forces into proportional electric charges, which can then be transformed in terms of newton according to the charge intensity. Eight charges are generated by these four dynamometers in different directions, which are Fx12, Fx34, Fy14, Fy23, Fz1, Fz2, Fz3, Fz4, at a sampling rate of 10 MHz, which means that eight readings are recorded every 0.0000001 s during the whole machining process. Charge conversion into forces takes place by passing the charges through amplifiers in order to magnify the signal, then converting them by analog to digital circuit, then representing the data and recording it through a special interface on LabView. The data is then used to calculate the major force components and moment components.

A specific plan was followed to evaluate built up edge and tool wear, by measuring these periodically after drilling a certain number of holes for the five alloys. A Keyence 2000 type digital microscope with × 100 magnification was used to facilitate clear examination and measurement of the wear and built up accumulation. In addition to tool wear and built up edge evaluation, a chip characterization test was also done by measuring the number of chips obtained for a specific mass of removed material for the five alloys, using a calibrated weight scale.

The data collected from the drilling process were imported in the form of mdt files to be processed on MATLAB. A sophisticated code was written in order to read, represent, and process the data with the aid of specialized libraries such as signal process toolbox and curve fitting library. The raw data were the forces recorded between every two sensors for the horizontal forces, and the recorded values from each sensor in vertical forces, in a high frequency rate of recording (10 MHz).

3 Results and discussion

3.1 Microstructure and tensile properties

Figure 4 shows the optical microstructures of the three alloys—A, D, and E—sectioned from their as-cast tensile bars. The large amount of Cu in the alloy A reflected in the precipitation of coarse Al2Cu phase throughout the entire matrix along with a few α-Al15(Fe,Mn)3Si2 phase particles as presented in Fig. 4a. As reported earlier [18], modification with Sr would lead to a divorced eutectic reaction where (Al-Si) eutectic has been observed separated from the (Al-Al2Cu) one-alloy D (Fig. 4b). In addition, the α-Fe phase particles are also rejected in front of the advancing (Al-Si) eutectic (Fig. 4c). Pucella et al. [14] reported on the inverse precipitation of α-Fe in Sr-modified alloys. In this case, the α-Fe phase precipitates within the α-Al during solidification of the alloy (Fig. 4d). In other words, the α-Fe precipitates prior to the formation of the α-Al network. The importance of this reaction is to harden the soft α-Al leading to more-or-less uniform strength over the entire alloy. Figure 4e is an enlarged micrograph of alloy E revealing partial transformation of β-Al5FeSi phase to π-Al8Mg3FeSi phase at ~ 560 °C followed by the precipitation of Mg2Si phase at about 545 °C (Fig. 4f) [19].

Fig. 4
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Microstructure of as-cast: a alloy A; b, c alloy D; d–f alloy E

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Table 4 lists the tensile properties of the five used alloys following the heat treatments described in Table 2. The as-cast HT200 alloy itself shows relatively good characteristics, with almost 96% of the yield strength of the 319 alloy after T7 treatment, but with a significantly low ultimate tensile strength and elongation.

Table 4 Tensile properties of the studied alloys
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The T5 heat treatment of alloy B for 5 h increased its elongation by 1.2%, coupled with a slight improvement in its ultimate tensile strength by about 5% and better improvement in the alloy yield strength by about 12%, from the original as-cast value (alloy A). In comparison, alloy C (in the T7-treated condition) showed real improvement in all three properties: the elongation increased by about 3% and both the yield strength and the ultimate tensile strength increased by about 17% above those obtained from alloy A.

In terms of comparison between alloy HT200 and the commercial alloys, it can be noticed that the T7 heat-treated HT200 alloy (coded alloy C) reveals a comparable performance to the 319 alloy for the T7 treatment in terms of yield strength but with a higher elongation. On the other hand, alloy C exhibits lower yield strength compared to the 356 alloy (T6 heat-treated) despite its comparable ultimate tensile strength and ductility values. Figure 5 demonstrates the effect of heat treatment on the size and distribution of Al2Cu phase particles in HT200 alloys, as confirmed from the associated EDS spectrum in Fig. 5d. Figure 6 reveals dense precipitation of hard eutectic Si particles in the matrix during the solidification process (Fig. 6a, 1000 VHN) and Mg2Si phase particles in alloy E in the T6 condition. In a previous study, the authors determined that the Si particle density is approximately 41,500 particles/mm2 [18]. Considering the cross section area of the drill used (approximately 36 mm2), the tool is instantaneously passing through ~ 2 million hard Si particles.

Fig. 5
figure 5

Backscattered electron images showing precipitation in HT200 alloys: a alloy A, b alloy B, c alloy C, d EDS spectrum obtained from c

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Fig. 6
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a Backscattered electron image showing precipitation of Si particles following solutionizing treatment, b a high-magnification image of a-note the marking of the surface of the Si particles, c fracture of Si particles under tensile load, d ultra-fine Mg2Si particles in alloy E in the T6 condition, e corresponding EDS spectrum

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3.2 Drilling characteristics

3.2.1 Cutting forces

A restricted criterion was designed for evaluating tool life for the drilling process. The criterion is to use one tool for each alloy to machine 2700 holes. Three conditions were considered to indicate tool failure: if the full margin width is worn in the outer corner, or flank wear achieves 0.375 mm, or the tool fails or gets broken [6]. Figure 7 shows a schematic presentation of a drilled block whereas Fig. 8 reveals the wearing of a new tool (Fig. 8a) after drilling 2700 holes (Fig. 8b) using alloys A and E as an example. An important point noted in regard to the cutting tool surface of alloy E (Fig. 8c) was that an initial deterioration and notches appeared on the cutting tool surface with alloy E but not with the HT200 alloys. In alloy E, this notch appears after 900 holes, which indicates fast deterioration of the tool edge, which is to be expected, in keeping with the effect of high silicon content on machinability in the Al-Si alloys.

Fig. 7
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A schematic diagram showing a drilled block mounted on the drilling stage—180 holes drilled per block (dimensions are in mm)

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Fig. 8
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a New drill, b same tool after drilling 2700 holes showing signs of wear—alloy A, c after 900 holes—alloy E

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Initial results for forces were obtained by applying the algorithm methodology over recorded data during the drilling process, as illustrated in Fig. 9. The initial data included 12,500 holes drilled for the five alloys; the data was filtered digitally on 1000 Hz low pass filter to obtain the participating effect of rotation on the axial and resultant force, whereas the rotation frequency is almost 250 Hz. The graph indicates the raw data of axial force and resultant force through 2500 holes for each alloy. Due to the large amount of data in Fig. 9, which may mask the actual variation in the drilling forces (Fz), the results were replotted vs number of blocks in Fig. 10, where each spot represents the average of 180 holes drilled per block.

Fig. 9
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Axial cutting forces through different alloys vs number of the drilled holes

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Fig. 10
figure 10

Average axial cutting forces through different alloys vs number of the drilled block

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The main observation to be noted from Fig. 10 is that alloy E shows the highest cutting forces in both axial and resultant components, while alloy A shows the minimal cutting forces. In addition, the differences in average cutting forces between alloys B, C, and D are small, although there is a wide difference in their mechanical properties. It can also be noticed that T5 heat treatment of the HT200 alloy, as is the case for alloy B, reduces the necessary cutting forces in the drilling process somewhat, in comparison to the as-cast condition (alloy A). In addition, alloy B performed much better with respect to cutting forces compared to alloy E.

A slight tendency toward increase in the cutting forces with the aging treatment can be noted in the different alloys. This tendency can be noticed by the slight increase in the average force measured for each block for each alloy and may be interpreted in terms of tool deterioration with the number of holes drilled. The average cutting forces are presented in Table 5. These average values were obtained over 180 holes drilled per block × 14 blocks drilled for each alloy.

Table 5 Average cutting forces for studied alloys
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3.2.2 Built up edge—height and width

BUE is one of the major factors that affect the quality of the machining process, specifically in terms of surface roughness and hardness of the machined surface [20, 21]. Built up edge generally takes place because of heat generated due to friction, as some hardened particles from the flowing metal over the tool surface are welded to the tool edge because of the localized heat to form a new non-regular cutting edge. These chips start to accumulate over the cutting edge until it achieves the critical size to break. Although the mechanism is almost the same for all materials, the built up edges formed in different alloys and cutting conditions vary widely in terms of size and shape [22]. The effect of heat built up edge on the machining process is significant with respect to different variables of the material removal process such as chip size, tool life, surface finish, and dimensional control [10, 15]. In order to control the built up edge building process, various considerations are used in designing the machining process such as increasing the cutting speed and increasing the rake angle of the tool. Oliaei and Karpat [23] reported that the extent of the built up edge and the surface roughness decrease with increasing cutting speed, and that increase in the rake angle decreases side flow and the size of the built up edge.

Figure 11 shows the height of the BUE after taking the measurements through the whole drilling process for the five alloys. As mentioned in the experimental section, coolant showers were used during drilling. Thus, the observed peak in Fig. 11 (alloy B) may have resulted from the soldering of chips as shown in the inset micrograph. Their removal would have taken place because of applied showers of coolant that would have separated the chip from the tool. The BUE over one side did not exceed 543 μm in height—as shown by the broken arrow in Fig. 12b, whereas the highest average was almost 400 μm; both cases were noted for alloy B. In terms of height, alloy B showed a higher tendency to accumulate BUE during drilling, compared to the other alloys, which showed an average BUE within the range of 150 μm. The photograph of the cutting tool in Fig. 12c reveals that separation of BUE starts to occur when its height is around 250 μm on one face. With respect to the built up edge width, it can be noted from Fig. 13 that alloy C had the highest effect on tool BUE accumulation. The average width for alloy C was about 600 μm in comparison to the other alloys with an average of about 300 μm. Alloy B also showed relatively higher average BUE width measurements than the rest of the alloys, but still lower than alloy C.

Fig. 11
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Built up height during drilling

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Fig. 12
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Changes in the thickness of BUE in with the increase in number of drilled holes: a fresh tool, and after b 1260 holes (alloy B), c 1980 holes (alloy A), d 2700 holes (alloy A)

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Fig. 13
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Built up width in drilling

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3.3 Tapping characteristics

3.3.1 Tapping forces

Figure 14a and b compare the obtained tapping forces. For similar tapping parameters, alloys D and E generate the highest cutting forces (~ 120 N) since these alloys have high silicon content (5–7%) compared to tapping forces for the HT200-based alloys (~ 75 N). Note also that for alloy E, the breakage of a tap occurred after 1260 holes, which could indicate that the material is more difficult to machine. A second tapping test was performed to validate this result and it was possible to tap the same number of plates before tool failure. Apparently, the presence of 3.5% Cu in 319 alloy improved the tool life to about 2160 holes, which is approximately halfway between the 356 alloy and alloy A. In other words, the presence of copper in 319 alloy balanced to some extent the tool wearing behavior caused by the eutectic Si particles.

Fig. 14
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Effect of alloy type and heat treatment on a number of holes before tool breakage, b tapping forces in the Z-direction

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3.3.2 Tap wearing

During the microscope inspection of the tapping tools, no wear could be detected. Figure 15 shows no trace of wear or material transfer presence that could stick on the tools. For this reason, no graph of wear or accumulation of build-up material could be produced. Figure 15a and b illustrate, respectively, wearing of alloys A and E at the end of the tapping process.

Fig. 15
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The state of the tool used to tap the alloy: a A, b E. Note the absence of wear in a

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4 Conclusions

Based on the results obtained from this work, the following conclusions may be drawn.

  1. 1-

    Drilling

Tool life in drilling tests on Al-Cu alloys regardless the heat treatment can last up to 2700 holes with no sign of failure compared to 900 holes obtained from 356 alloy before wearing. Analysis of the cutting forces showed that alloy E revealed the highest cutting force whereas Al-Cu alloys showed close values. Accumulation of built up edge showed significant increase in both height and width in alloys B and C, in comparison to alloys A, D, and E. Although all chips had a conical shape, the surfaces of HT200 chips were less shiny compared to those obtained from alloy E.

  1. 2-

    Tapping

The T200-based alloys revealed excellent machinability with no sign of tap wearing after 2700 holes. In contrast, the tool failed after 1600 holes in the case of 356 alloy and after 2160 holes for 319 alloy. Thus, it is concluded that the presence of 3.5% Cu in the 319 alloy may have helped in reducing the severity of wearing due to eutectic Si particles. The tapping forces reached 120 N prior to failure (2160 holes) compared to about 75 N in the case of HT200-based alloys (~ 2700 holes).