Analysis of the Microstructure, Properties and Machinability of Al-Cu-Si Alloys

As part of the studies conducted in the field of broadly understood casting of non-ferrous metals, selected results on the impact of variable additions of copper and silicon in aluminium were presented. A series of melts was carried out with copper content kept constant at a level of 2% (1st stage) and 4% (2nd stage) and variable contents of silicon introduced into aluminium. The crystallization characteristics of the examined alloys and the percentage of structural constituents at ambient temperature were obtained by modelling the thermodynamic parameters of individual phases with the CALPHAD method. The microstructure of the obtained alloys was examined and microhardness was measured by the Vickers-Hanemann method. The alloy properties were assessed based on the results of mechanical tests, including ultimate tensile strength (UTS), hardness (BHN) and elongation (E). The machinability of the tested alloys was analyzed in a machinability test carried out by the Keep-Bauer method, which consisted in drilling with a constant feed force. The obtained results clearly indicate changes in the images of microstructure, such as the reduction in grain size, solution hardening and precipitation hardening. The changes in the microstructure are also reflected in the results of mechanical properties testing, causing an increase in strength and hardness, and plasticity variations in the range of 4 ÷ 16%, mainly due to the introduced additions of copper and silicon. The process of alloy strengthening is also visible in the results of machinability tests. The plotted curves showing the depth of the hole as a function of time and the images of chips produced during the test indicate an improvement in the wear resistance obtained for the tested group of aluminium alloys with the additions of copper and silicon.


Introduction
Non-ferrous metals and alloys play a very important role in technology and it would be difficult to imagine the development of this technology without the contribution of modern casting materials based on non-ferrous metals.The continuous progress of science and technology has resulted in the introduction to the market of a large number of non-ferrous alloys with very diverse compositions and properties, used mainly in such areas as electrical engineering, and automotive, aviation, rocket, transport and construction industries.In practice, aluminium alloys are most often used.Owing to the introduction of alloying elements, they are characterized by very good mechanical properties [1], combined with excellent castability, low density and good corrosion resistance [2][3][4][5].
Efforts are continuously made to improve their usability thanks to such procedures as the selection and optimization of chemical composition, refining and modification processes, the selection of casting technology and heat treatment parameters [6][7][8][9][10][11][12][13][14].All these treatments allow obtaining materials with very good mechanical properties, high placticity, and satisfactory hardness,which in turn gives products characterized by good, resistance to the effect of high temperature and corrosive environment.Aluminium-silicon alloys are widely used in the automotive industry [15][16][17][18], aviation [19][20][21][22][23][24][25][26].Copper controls the strength and hardness of casting aluminium alloys.Aluminium alloys, including AlCu alloys, are the subject of studies of the effect of additives on the nucleation process and crystal growth [27][28].Ternary alloys from the Al-Cu-Si system are gaining more and more popularity.They are lightweight and superior to Al-Si alloys in terms of strength and to Al-Cu alloys in terms of corrosion resistance.These features explain the growing interest in Al-Cu-Si alloys in the automotive and aviation industries [29][30][31][32].

Materials and Methods
The aim of this study was to analyze the influence of the variable content of silicon additions on the properties of an Al -Cu alloy.In alloys made of pure components, special attention was paid to the mechanical properties of samples cast into metal moulds.Chemical and microstructural analyses were performed during the tests, and properties such as ultimate tensile strength (UTS), elongation (E) and machinability were determined.Hardness (HBN) was measured by the Brinell method and microhardness (µHV) by the Vickers-Hanemann method.
In the first stage of research work, it was planned to prepare the melts and produce samples of Al alloys with a constant copper content of 2% and variable silicon contents, introduced into aluminium in the amounts of 0.5%, 1.5%, 3%.In the second stage, the constant copper content was 4%, while silicon was introduced in the amounts of 0.1%, 2% and 4% (Table 1)   The equilibrium crystallization analysis was carried out by the CALPHAD (CALculation of PHAse Diagrams) method using a Thermo-Calc package with the TCS Al-based Alloys Database [40][41][42].
Melts were prepared in a pilot foundry of the AGH Faculty of Foundry Engineering.A medium frequency induction furnace was used for metal melting.The charge was melted in a chamottegraphite crucible.The following charge materials were used: electrolytic aluminium A199.99 (designated as AR1) according to PN-EN 1676:2020-09, crystalline silicon and cathode electrolytic copper in the form of plates with the minimum copper content of 99.99% according to PN-EN 1982:2017-10.As part of preliminary tests, the primary AlCu50 alloy was prepared.During casting, the pouring temperature was controlled and kept in the range of 730-750ºC, the metal mould temperature was 150-170ºC.
The quality of the performed melts was checked by the analysis of chemical composition.For this purpose, a SPECTROMAXx emission spectrometer was used.The results of the analysis are presented in Table 2.They confirm that melting was carried out in a proper way, since the obtained samples of AlCuSi alloys contain only small amounts of impurities, mainly iron and zinc.Microstructure was examined by the light microscopy (LM) using an Eclipse LV 150 microscope.The microhardness (µHV) of structural phases was tested by the Vickers-Hanemann method.The ultimate tensile strength (UTS) and the elongation (E) of the alloys were assessed on an INSTRON device, model 1115.
Hardness was measured by the Brinell method (BHN).The assessment of machinability was done by the Keep-Bauer method measuring the cutting force.The machinability test consisted in drilling a 6 mm diameter hole in the prepared samples using drill [43].

Thermodynamic predictions
Modelling of alloys by the CALPHAD method was carried out for the experimental chemical composition shown in Table 2, using a Thermo-Calc software.The program analyzes the three main elements, i.e.Al, Cu and Si, and also elements such as zinc and iron, present in the alloy and treated as impurities.The alloy cooling process was simulated allowing for the phase transformations under equilibrium conditions.
As a result of modelling, graphs of theoretical crystallization were plotted for the examined alloys.The graphs show the effect of individual elements on the crystallization characteristics of individual phases.A sample simulation graph of the AlCuSi alloy cooling process is shown in Figures 1-4.In modelled cases of crystallization of the examined alloys, at a temperature of 20 o C, four phases occur in the alloy structure and their proportional content depends on the chemical composition.The three phases are typical phases present in AlCu and AlCuSi alloys, i.e.FCC_L12 phase (green colour in the graphs and tables) the α phase treated as aluminium, AL2CU_C16 phase (grey colour)the intermetallic θ phase -Al 2 Cu phase, and SI-DIAMOND_A4 phase (blue colour), which corresponds to the crystal structure of diamond (A4) and in this case is pure silicon.The fourth disclosed AL9FE2SI2 phase (brown colour) is a multicomponent phase associated with the presence of Fe in the tested alloys containing Al, Fe and Si, respectively.A small amount of zinc revealed in the composition of the tested alloys is part of the FCC_L12 phase.The content of individual elements in the phases that occur during alloy crystallization is summarized in Table 3. Table 3.The content of individual elements in the phases occurring during crystallization of the examined AlCuSi alloys Tables 4 and 5 show the results of Thermo-Calc modelling of the tested AlCuSi alloys.The increasing content of copper and silicon additions directly affects the drop of the crystallization start temperature T 1 and the remaining temperatures T 2 -T 7 determined during modelling of the tested alloys crystallization process.Alloys 1.1 and 2.1 are AlCu alloys where the α phase (FCC_L12) and the precipitates of the Al 2 Cu phase (AL2CU_C16) are observed to occur at ambient temperature.The presence of the remaining phases, i.e.SI-DIAMOND_A4 and AL9FE2SI2, results from the trace content of silicon and iron in the chemical composition of the alloy, determined by the Thermo-Calc program at a level of 0.05-0.07%.The Al 7 Cu 2 Fe and Al 13 Fe 4 phases present during crystallization are unstable and do not occur at ambient temperature.In the temperature range of 550-559 o C, the introduction of silicon in an amount exceeding 1.6% results in more silicon appearing in the form of the SI-DIAMOND_A4 phase, which at ambient temperature (20 o C) reaches the percent content of an alloying constituent.The Al 2 Cu phase crystallizes in the solid state and its crystallization temperature depends on the copper content (430-513 o C).In all cases, at 20 o C, the content of the Al 9 Fe 2 Si 2 phase is at a low level of 0.07 -0.13%.

Microstructural characterization
In the next stage of the studies, the impact of the introduced variable additions of silicon and copper on changes in the images of the microstructure of the examined alloys was evaluated.A 10% aqueous NaOH solution was used for etching of the metallographic specimens.Figures 5-9 show microstructure of the examined alloys with different content of copper and silicon.The drawings show images of the microstructure of the tested samples with a variable copper content of 2% and 4%, and a variable addition of silicon.Based on the phase diagram of the Al-Cu-Si alloy, calculated by the Calphad method, it is possible to determine individual constituents revealed in the microstructure of the examined alloys.The images of microstructure show the contribution of individual phases at ambient temperature, modelled with the Thermo-Calc program.
In the images of microstructure, the dominant constituent is the solid solution of copper in α aluminium, accompanied by numerous precipitates of the Al 2 Cu phase crystallites.With the increasing percent content of silicon, the presence of characteristic silicon crystallites in the form of large, irregular precipitates is also observed.
The precipitates of the Al 2 Cu phase, occurring in the microstructure of the examined alloys, are characterized by a favourable dispersion distribution, which increases both hardness and strength.However, a very high proportional contribution of the Al 2 Cu phase, present especially at the grain boundaries, can weaken the alloy and even prevent making a sound, crack-free casting.

Mechanical properties
A comparison of the results of the strength, hardness and microhardness testing is presented in Table 6 and graphically in Figure 10.Analyzing the results of testing the tensile strength R m of alloys with 2% and 4% Cu, it can be concluded that alloys with a higher copper content can reach higher R m values than alloys with 2% Cu.The increase in R m with variable silicon additions is due to the solution hardening of this alloy.The solid solution of copper in aluminium present in the microstructure, silicon atoms and the Al 2 Cu intermetallic phase nucleating at the dislocations and crystallizing at the solution grain boundaries lead to an increase in mechanical properties.Careful analysis of the obtained data shows that silicon addition to the base alloys causes a significant increase in the elongation E, regardless of copper content in the alloy.The elongation was increasing with the increasing addition of silicon.In the remaining cases (samples Nos.2.1, 2.2 and 2.3), a decrease in elongation was noticed, probably related to the formation of Al 2 Cu phases, which contributed to alloy hardening but at the cost of slightly higher brittleness.
Additionally, the samples with 2% Cu content showed a much higher elongation E, i.e. maximum 16.5%, compared to samples with 4% Cu content, which reached the maximum elongation of 7.6%.
Analyzing the results of hardness measurements, it can be concluded that silicon addition has a positive effect on the hardness of individual samples.In all cases, the hardness has increased with respect to the base alloy.
The hardness of the alloy with 2% Cu and 2% Si is 47.8 HB and it increases to 55.2 HB.On the other hand, the hardness of the alloy with 4% Cu and 2% Si is 61.7 HB and it increases to 68.1 HB.The increase in the hardness of the samples is related to the presence of silicon atoms in the crystal structure.The difference in the diameters of Al and Si atoms causes high stresses and elastic deformation of the crystal lattice, which in turn result in the alloy hardening.Additionally, the alloy hardening process can be enhanced by the large precipitates of the Al 2 Cu phase.
Microhardness tests carried out on the examined group of alloys allowed determining whether the variable amounts of additives introduced to AlCuSi alloys can cause hardening of these alloys through precipitation of the intermetallic phases, or whether they can also have some impact on the matrix of the examined samples.
The results of microhardness measurements carried out by the Vickers-Hanemann method are presented in Table 6.
The results of microhardness measurements lead to the conclusion that silicon addition has a positive effect on the microhardness of the AlCuSi alloy matrix examined in individual samples of the alloy.In all cases, the microhardness of the aluminium matrix has increased compared to the base alloy.
The microhardness of the alloy with 2% Cu and 2% Si is 55.7 μHV and it increases to 64.8 μHV.On the other hand, the microhardness of the alloy with 4% Cu and 2% Si is 74.6 μHV and it increases to 79.5 μHV.

Machinability analysis
Machinability was tested by the method which consists in drilling a hole with a constant feed force [6].The collected results in the form of curves plotted in the hole depth -time system clearly show the machinability of the examined material.
The results of the machinability tests carried out on the examined alloys are shown in Figures 11 -12  The method used for the determination and comparison of the machinability of the examined aluminium alloys containing the additions of Cu and Si with samples made from the reference material, i.e. pure aluminium in the form of ingots, and with the AG10 (AlMg10) alloy (described in the literature as machinable and polishable) allows for the following conclusions: − the shape of the curves on the cutting path -time graph changes, and the changes are related to the cutting process itself.In the initial period of drilling, the drill sinks faster into the material, then chips are formed and their removal increases the cutting resistance and changes the slope of the curves plotted for the examined samples.According to the collected data, the best machinability was obtained in sample No. 2.1 (AlCu4,3Si), while other alloys showed comparable values of the machinability.In pure aluminium ingots, the nature of the cutting was changing in a very clear manner with the elapsing time of testing.In the initial stage, cutting was proceeding very quickly, mainly due to the low hardness of the alloy.Then, chips were formed, which significantly increased the cutting resistance and changed the slope of the curve in the graph illustrating the cutting process, − the AG10 (AlMg10) alloy used for the comparative analysis was characterized by the machinability slightly inferior to the examined AlCuSi alloys.The AG10 alloy cutting curve indicates the lack of cutting resistance due to chip evacuation nearly throughout the entire drilling range.The appearance of chips generated in the cutting process (Fig. 12) confirms their significant impact on the speed of drilling a hole with a constant feed force.

Conclusions
Testing of primary aluminium alloys with variable additions of copper and silicon has shown a relatively high impact of the additives used.
The results of metallographic tests confirm changes in the grain size, clearly marked by precipitates at the grain boundaries of a solid solution of copper in aluminium.Changes in the number of phase precipitates at the grain boundaries are also observed.Silicon is mainly released in the form of primary precipitates and as an Al 9 Fe 2 Si 2 compound in phases with aluminium and iron in amounts depending on the other constituents.Copper introduced in the form of an AlCu50 master alloy occurs as an Al 2 Cu intermetallic phase.Zinc, being in a negligible amount treated as an impurity (up to 0.1%), is transferred into the α solution.
The method of modelling the crystallization process fully reflects what is depicted in the images of the microstructure of the examined alloys, facilitating at the same time an identification of the obtained results.Detailed SEM analysis would confirm the convergence of the obtained results.
Changes in the microstructure are also very clearly visible in the results obtained during testing of the mechanical properties.Owing to the presence of copper and silicon, both strength and hardness increase, while plasticity undergoes changes in the range of 4 to 16%.The alloy with a 4% addition of copper (2.1-2.3)achieves higher tensile strength and hardness.Compared to the alloy with 2% copper content, this is mainly the result of a higher proportional content of the Al 2 Cu dispersion precipitates and of the presence of a few primary silicon precipitates, although the latter ones deteriorate the alloy plastic properties.
The hardening of the alloy is also very clearly visible in the results of the machinability tests carried out by the Keep-Bauer method.Both the recorded curves showing the hole depth as a function of time and the images of chips generated during the test indicate an improvement in the wear resistance of the examined aluminium alloys with the additions of copper and silicon.The measurement of machinability gives information not only on the alloy susceptibility to machining by cutting, but also on the wear resistance of the alloys tested.
Increasing the wear resistance of the tested alloys is associated with many stages of casting production, including proper selection of the best chemical composition and of the crystallization and solidification rate.Moreover, the increase in wear resistance depends on the properties of the microstructural constituents, the alloy matrix in particular, and also on the amount and form of the precipitates of the intermetallic phases.

Fig. 10 .
Fig. 10.A comparison of the results of the strength, elongation and hardness testing .

Fig. 11 .
Fig. 11.Averaged machinability results obtained for the examined samples

Fig. 12 .
Fig. 12.Comparison of the images of chips produced during machinability testing

Table 2 .
The results of the chemical analysis of the tested samplesThe content of elements in the alloy (wt%)

Table 4 .
The results of Thermo-Calc modelling of characteristic phase transformation temperatures in the tested AlCuSi alloys (1.1-1.3)

Table 5 .
The results of Thermo-Calc modelling of characteristic phase transformation temperatures in the tested AlCuSi alloys (2.1-2.3)