Investigation of microstructure, machinability, and mechanical properties of new-generation hybrid lead-free brass alloys

1 Introduction

Brass, whose main alloys are made of copper and zinc, is the preferred non-ferrous material in the industry due to its good mechanical properties, relatively high corrosion resistance, and excellent machinability.

Lead is another alloying element added to brass alloys to improve machinability and is particularly found in the brass alloys which are generally used in the production of fittings for the transportation of sanitary water [1]. Furthermore, it acts as a chip breaker and leads to decrease in tool wear by virtue of lubricative properties [2]. Since lead is one of the very hazardous heavy metals having toxic properties,the presence of it in the composition of some metallic alloys has to be avoided even at very low trace amounts [3]. The contain of lead in metal alloys has been restricted by community service organization such as Restriction of Hazardous Substances 2011/55/EU [RoHS] Directive up to max 0.1%wt. and Safe Drinking Water Act, USA Directive 2000/53/EC [ELV] up to max 0.25%wt. Thus, the development of new lead-free brass alloy to comply with new legislation is required [4]. Brass, whose main alloys are copper and zinc, is the preferred non-ferrous material in the industry due to good mechanical properties, relatively high corrosion resistance and excellent machinability. Lead is non-soluble in brass matrix and generally precipitates with globular form with a diameter of 1–10 µm at grain boundaries [4,5,6].

There are several alternatives in the substitution of lead such as bismuth, silicon, and selenium to enhance the machinability of brass [7,8,9].

Gr particle is one of the most prominent candidates because they are cheap and environmentally friendly and acts as solid lubricant; however, they are hardly distributed in brass matrix by casting method due to high density differences [10]. Bismuth which is another alternative element that can replace lead has similar properties, but it is considered a rare element and is more expensive than lead; therefore, manufacture of brass with bismuth is very costly. Moreover, it segregates to the grain boundaries, which results into the brittleness of the part.

Titanium (Ti) has been used as reinforcement to produce graphite–brass composites. Thus, it has been observed that the interface bonding between Gr and brass matrix is improved by interfacial reaction of Ti with Cu and Gr to form intermetallic compounds, and also increasing addition of nano titanium has enhanced the mechanical properties of brass. It has been determined that although Gr added to the brass alloy matrix with Ti improves the cutting performance, it deteriorates the mechanical properties considerably [11].

Silicon improves machinability of brass by promoting formation of secondary hard phases in the microstructure. On the other hand, it raises the cost of machinability by accelerating tool wear rate [12]. Manufacturing process with selenium of brass is very complex and cost-producibility is very high; therefore, it is not appropriate for mass production with selenium [6].

There are many studies about silicon (Si) and molybdenum disulfide (MoS₂) as less toxic additives than lead, but no study has considered the production of brass by combining them. Each alternative has some negative aspects. The aim of this study was to develop a new generation of lead-free brass alloys through the hybridization of Si and MoS₂. Both Si and MoS₂ are widely available commercially. The new-generation brass alloy was produced with lower-toxicity elements, which could lead to the enhancement of machinability. The low-cost production and easily available additives are other positive aspects. The new alloy could be used in faucet system components and shows similar mechanical properties to other alternative lead-free alloys used in the industry.

2 Experimental process

In order to prepare the hybrid brass alloy, Cu–Si master alloys which had been produced by casting method were placed in a Gr crucible and then commercial pure Cu, and Zn was added into the preheated Gr crucible in an appropriate order. The chemical composition of the main alloying elements supplied for brass production is shown in Table 1. The molten process was carried out at about 1,120°C temperature by induction furnace, and followed by addition of different amount of MoS₂ to the molten brass. The crucible was drilled on the edge to measure the desirable temperature with K-type thermocouple. Borax is used as a commercial flux to insulate from the atmosphere and trace amount of high purity aluminum was added into molten brass to minimize zinc evaporation. The molten alloys were poured into ingot mold which made by hot steel tool according to ISODISC w303. The dimensions of the ingot mold are 20 mm in diameter and 150 mm in length.

The chemical composition of the master alloys and the prepared sample are shown in Table 2. As-received Ecobrass and C3600 leaded brass were used as reference samples.

Components produced for faucet system by casting method are directly used after machining process. In order to provide the comparison conditions for the same level, reference samples taken commercially were reproduced by casting method under the same conditions. In this article, C3600 leaded brass is named free cutting brass and will be abbreviated as F.C.B from now on.

The lower parts of casting alloys were cut into slices with 10 mm thickness and then ground by the range of 120 and 2,500 grind paper for metallographic examination, successively. It was polished by solid lard to obtain a flat surface. So as to reveal the microstructure of each sample, electrolytic etching was performed at 2 V for 20 s with an aqueous solution containing 60% phosphoric acid. General microstructure was examined using an optical microscope. The morphology of the prepared samples was determined by scanning electron microscopy (SEM, FEI Sirion XL 30). Phase composition was determined by X-Ray Diffraction (XRD) techniques (Bruker D8 Advanced). The chemical composition of prepared samples was identified by the energy-dispersive X-ray spectrometer (EDS) connected to the SEM instrument, and X-ray fluorescence spectroscopy equipment (Thermo Scientific Niton XL2 Gold).

The machinability of the prepared and reference samples was evaluated in terms of surface roughness and chip formation. Tool wear, which is one of the determining parameters of machinability, could not be examined due to the need of too many samples and taking too much time. Machinability parameters include very different combinations. While choosing these parameters, the previous article about brass alloys were taken as a basis [13]. Machinability test was performed using commercial lathe instrument which has 2 kW spindle power and highest spindle speed of the machine was 2,500 rpm. Machining operation condition is presented in Table 3. In order to eliminate the different structures and defects on the surface of the prepared samples during casting, machining process was performed from the surface prior to main machining operation.

The samples were formed in accordance with the EN-ISO 6892 standard to carry out a tensile test with a Devotrans DVT GP universal testing machine. Hardness test of transverse cross-section of casting samples was carried out with HMV-2T, Shimadzu microhardness tester instrument under 0.02 kgf for 15 s at room temperature. Measurement was performed five times starting from side of sample to the center of sample.

Most alloying elements exist as a solid solution in the brass matrix. Therefore, the copper-zinc diagram partially loses its validity. In order to determine the phases in the phase diagram of the brass alloy, the zinc equivalent was formulated by equation (1).

This equation can just estimate the phases on the copper-zinc diagram by basing on fictitious zinc equivalent. The formula is described with X% symbol as follows [14]:

where C _Cu is the Cu content in brass, C _Zn is the Zn content in brass, C _i is the sum of additives content in brass, and K _i is the fictitious zinc equivalent value of additives.

The fictitious zinc equivalent value of Si, Al, and Sn elements significantly influences the reaction on binary Cu–Zn diagram and existence of phases on microstructure are 10, 6, and 2, respectively [15]. In this study, zinc equivalent calculations were taken into account while determining the chemical components and phase distributions.

3 Results and discussion

3.1 XRD analysis

Figure 1 shows the result of XRD analysis for the prepared samples. All samples consist of alpha and beta phases, including reference samples. The alpha phase exists as a solid solution in the beta prime phase, ordered intermetallic compound, which transforms from beta phase below 450°C. The peaks at 42.36, 49.64, and 72.24° detected on the diffraction pattern of both the MoS₂ samples and reference samples was associated with alpha phase. In addition, the peaks appearing at 43.28, 62.87, and 79.39° on diffraction pattern was corresponded to beta prime phase. The lower peaks detected at 31.37, 36.34, and 62.30° on the diffraction pattern of F.C.B brass was associated with lead element. Undefined peaks were detected at 39.97 and 45.59°, which are considered to belong to к phase by the result of diffraction pattern of Ecobrass. In the study conducted with the same composition, the presence of silicon-rich kappa and gamma phases dispersed in the matrix was detected in differential thermal analysis analysis [16]. In another study with the same composition, it was suggested that this peak, which was not identified because of the analysis performed with a different XRD source, belonged to the kappa phase [17].

Figure 1

XRD pattern results of samples on spectrum.

When the diffraction patterns of MoS₂ samples were compared, the peak intensities of the beta prime phase increase whereas the peak intensities of the alpha prime phase decrease with the increase in the silicon contents. This case indicated that the increase in the silicon content in microstructure promotes the beta prime phase formation. This result is in agreement with the results of Scanning Electron Microscopy-Energy Dispersive Spectroscopy.

Furthermore, volume fractions of beta prime phase and alpha phase were calculated by using equation (2), where Vα is the volume fraction of alpha phase [18].

Using the same equation, the volume fraction of the beta phase is calculated. Volume fraction of beta prime phase and alpha phase for all the samples are presented in Table 4. The increase in the volume fraction of the beta prime phase in the microstructures of MoS₂ samples can be attributed to zinc equivalent value. Most of the additives except nickel additive promote the ratio of beta phase formation [19].

Table 4

Volume fraction of alpha and beta prime phases for all samples

Samples	Alpha phase (vol%)	Beta prime phase (vol%)
F.C.B	0.625425	0.384575
Ecobrass	0.538664	0.461336
MoS2-1	0.808646	0.191354
MoS2-2	0.305895	0.694105
MoS2-3	0.182758	0.817242

Figure 2 shows from the result of XRD analysis scanned in a narrow angle on diffraction pattern of F.C.B and Ecobrass samples. The presence of silicon atoms, which is a substitutional atom in unit cell in brass matrix, brings about micro-strain due to different radii of silicon and zinc atoms [13]. Since the radius of the silicon atom (1.11 Å) is smaller than the radius of the zinc atom (1.34 Å), it causes the formation of tensile stress on the lattice. The peaks shifted to the right in the result of XRD analysis because the measured d-spacing is smaller than normal due to the tensile sites. This is related to the Poisson ratio.

Figure 2

XRD patterns comparison of samples F.C.B and Ecobrass with narrow scan.

3.2 Microstructure examination

The microstructure of the prepared and reference samples obtained from optic microscope is shown in Figure 3. In general, there are significant difference appearing in the samples of microstructure after casting process. The microstructure of Ecobrass is a typical casting structure consists of dendritic structure with coarse grains. Microstructure of F.C.B has fine grains with agglomerated lead particles which are located at alfa–beta interfaces due to the insolubility of lead in alfa and beta prime phases. Dark spots appear on the microstructure of F.C.B (Figure 3a), which was already reported in pervious study [20]. Figure 3c shows Widmanstätten structures in MoS₂-1 sample which are separated from each other by grain boundaries. During solidification, the alpha phase preferentially precipitates in the entire beta matrix to form the Widmanstätten structure that is compatible with the hyper-peritectic (L + α → β) composition in the Cu–Zn phase diagram [21]. The formation mechanisms of Widmanstätten structure have been reported in studies with similar composition [20]. The Widmanstätten structure precipitated at the beta phase grain boundary regions as small needle like shapes is shown in Figure 3d. Grains appeared in the microstructure of MoS₂-3 sample due to the increase in the addition of silicon amount, thus the dominant phase has become beta prime phase in the matrix. This case has been confirmed by the result of XRD analysis (Figure 1).

Figure 3

Microstructure on longitudinal cross-section of samples taken by optical microscopy (a) F.C.B; (b) Ecobrass; (c) MoS₂-1; (d) MoS₂-2; and (e) MoS₂-3.

The size of the grain in the microstructure of MoS₂-3 is smaller than that in the microstructure of MoS₂-2 because of the high content of silicon, which results in grain refinement. During solidification, many additions, including alloying elements, form a nuclei to form a new grain at the liquid–solid interface [23]. Furthermore, the alloying element causes inhibition of the driving force of α-phase formation [24].

The longitudinal cross-sectional SEM images of all samples are presented in Figure 4. From SEM and EDS studies, it was found that the microstructure of F.C.B. alloy contains a typical alpha-beta dual phase. Lead particles appear in bright contrast in the brass matrix and are homogenously distributed at grain boundaries (Figure 5a). Since the lead element has low melting point, it maintains the liquid form during solidification and thus it can be seen in Figure 4a as lead particles have agglomerated form at the alpha–beta interface. The SEM micrograph image of the Ecobrass reference sample shows alpha and beta phases with dominant beta phase. The SEM images of MoS₂-1 have two Widmanstätten morphologies, one is needle-like formation, while the other is plate-like formation. The plate formation of Widmanstätten α phase is related to Zn atoms constantly diffusing out from the Widmanstätten phase due to the fast mobile regions like grain boundaries. On the other hand, slow mobile Zn atoms pave the way for the formation of needle-like Widmanstätten structure due to the hindering diffusivity from alloy elements. As can be seen from Figure 4c, the island formation is the same morphology as Widmanstätten structure but its orientation is different. The reason for different appearance of Widmanstätten structure is random orientation during solidification and the information about this appearance of the Widmanstätten structure is given in detail in the previous study [25].

Figure 4

Micrographs of SEM under secondary electron images mode: (a) F.C.B; (b) Ecobrass; (c) MoS₂-1; (d) MoS₂-2; and (e) MoS₂-3.

Figure 5

The results of EDS analysis from regions labeled (a–c) in the SEM image of F.C.B.

Figure 5 shows the EDS analysis results of the F.C.B sample. EDS analysis was performed to determine the presence of alpha, beta, and secondary particles and their amounts. In the EDS analysis, a selected areas mode was adopted. As shown in Figure 5a, the intensity of lead particles is high from the EDS taken from aggregated white particles, thus it is understood that they have represented lead-rich particles. Since the alpha phase is rich in copper, the EDS analysis in Figure 5b represents the alpha phase. With the increasing amount of zinc in the alloy, the microstructure turns from alpha phase to beta phase and the crystal structure transforms from face-centered cubic to a body-centered cubic crystal system. The EDS analysis in Figure 6c represents the beta phase because the intensity of zinc has been augmented.

Figure 6

The results of EDS analysis from regions labeled (a–c) in the SEM image of MoS₂-3.

Figure 6 shows the results of EDS analysis taken from two different selected areas and one-point on the microstructure of the MoS₂-3 sample. From the result of the EDS analysis of MoS₂-3 sample, the peaks of Si and Sn elements on spectrum in Figure 6a have high intensity. Considering the zinc equivalent, this region probably represents the beta phase. The intensities of the peaks of the silicon and tin were augmented, and the existence of the sulfur peak was detected in Figure 6b. This was probably due to the complex intermetallic compounds occurring at grain boundaries.

With the increase of Si content, the morphology has turned into beta grains with large size and Widmanstätten α phase precipitated in acicular form at mostly grain boundaries as shown in (Figure 5d). The microstructure of the MoS₂-3 sample consists entirely of beta grains and intermetalic compounds (IMC) particles located at the grain boundaries of beta grains. When the SEM image was examined more closely, it was seen that snowflake-shaped complex compounds precipitated at grain boundaries and their surroundings. These complex compounds were formed with the increase in the amount of MoS₂. From the result of the EDS analysis shown in Figure 6c, it is quite difficult to give information about the composition of the IMC in this region. MoS₂ can decompose and react with the other alloying elements. In previous studies, it was emphasized that the addition of MoS₂ could thermodynamically turn into Cu₂S and ZnS compounds at high temperatures, but no information was given about the kinetics of the process [26,27]. From the results of EDS analysis, it is thought that complex compounds with partial sulfides and partial oxides are formed due to the presence of oxygen in the environment and high temperature effect.

3.3 Machinability

3.3.1 Chip formation and morphology

The chip formation and morphology are the fundamentals of machinability characterization. Chip formation types are categorized into two main groups, continuous and discontinuous. Since all of the samples have good chip breakage features, they form discontinuous chip shapes during the machining process.

The obtained chips after machining process were collected and to evaluate their formation and dimensions, as shown in Figure 7. The chip formation of all samples was according to the ISO 6385-G1 standard using photographs taken after the machining process. Disposable chips are light and have small chip dimension for all samples. According to the reports of Copper Development Association Incorporation, the brass material has both higher machinability and results in lower tool-tip wear. In addition, it has superior surface finish quality in terms of aesthetics. Therefore, the power and robustness of the latest high-speed machine tools and shops have the capacity to increase the rates of extracting brass workpiece materials in milling, drilling, and turning operations.

Figure 7

Macrographs of chip formation for (a) F.C.B; (b) Ecobrass; (c) MoS₂-1; (d) MoS₂-2; and (e) MoS₂-3.

The chip formation in F.C.B alloy is of elemental type, as shown in Figure 8a. A research report presented by the Copper Development Association Incorporation also confirmed previously the elemental type chip formation. Lead causes a low coefficient of friction and significantly affects the chip form during machining [28]. This case tends to form chips in smaller form compared to other brass alloys; however, it is not to be preferred in industrial environments because one of the main alloying elements is lead which has toxic properties. In addition, fine chip particles scatter around during the chip removal process. Scattering of chips into the various machine components, especially electrical ones, causes dysfunction of equipment over time, such as electrical short circuits. The chips formation of Ecobrass, MoS₂-2, and MoS₂-3 samples correspond to the arc chip with loose form because of the presence of dominant beta phase. The chips formation of MoS₂-1 sample correspond to the short tubular chip form due to alpha Widmanstätten structure. MoS₂-1 sample exhibits lower chip breakage performance amongst other samples due to the high percentage of ductile alpha phase [6]. Microstructural changes, phase transformations, and the presence of alloying elements can significantly affect the chip formation and size during the process [29]. Both the phase dispersion and presence of intermetallics in the brass matrix have enormous effect on chip size and shape [30].

Figure 8

Optical microscopy images of chip formation for (a) F.C.B; (b) Ecobrass; (c) MoS₂-1; (d) MoS₂-2; and (e) MoS₂.

Chip morphology substantially affects chip size and formation. Thus, optical and SEM images were used to examine the chip morphology, as shown in Figures 7 and 8. The occurrence and mechanism of chip formation of the samples are different from each other. The chips of FCB have good chip breakage features, and exhibits fragmented form. Often in the literature, this appearance of the sawdust is called “Sawtooth” or “closed-split sawtooth.” The reason for this appearance is that a great deal of fragmentation appears in the FCB chips due to lower plastic deformation in the cutting region. Lead considerably reduces the friction and chip thickness ratio in cutting zone leading to a split saw-tooth type appearance of MoS_2-3’s chip shape [31]. The addition of additives has paved the way for an inhomogeneous brass microstructure due to the precipitation of ultrafine IMCs around the grain boundaries. Thus, it can facilitate to be fragmented chips by virtue of existence of shear/slip bands. The more separated each tooth is in the serration, the highest chip fragmentation takes place eventually. Thus, the serrated chips turned into the fragmented chips. Additives with lubricating properties in the brass matrix cause low friction forces and low tool temperatures at the contact length between the tool and the chip, so the chip forms preferentially result in much less serrated formation than saw tooth formation. In addition, the tool life of cutting tools substantially improves due to the low thermal deformation [32].

The chip formation of Ecobrass is completely compatible with saw-tooth formation and not separated like F.C.B and MoS₂-3 samples. The chip formation of Ecobrass mainly due to the beta phase, which has higher Si content in the microstructure [14]. The chip formation for MoS₂-1 and MoS₂-2 samples has a serrated or toothlike appearance. Plastic deformation is more difficult in the cutting region compared to the chip formation of FCB and MoS₂-3 samples.

SEM micrographs of broken chips were investigated to determine the mechanism of chip morphology. The chip morphology of Ecobrass sample, which resulted from inhomogeneous deformation due to existence of dominant beta prime phase, has many cracks and traces of crack propagation on the contact surface. In materials prone to brittleness, it is possible to have such cracks on chip surfaces under tension. As shown in Figure 9b, the chip shapes have a serrated appearance in separate segments with a thickness of about 10 µm in the F.C.B. A flat smooth surface which takes place by shear motion of two neighbor fractured surface on chip surface during machining process are noticeable, and related to the ductile fracture surface features. It appears that serrated components appear distinctively on a fragmented chip form for MoS₂-3 sample, as shown in Figure 9c, while slip surface and undeformed surface appear on chip surface formed with lamella formation. On the other hand, high-density cracks are abundant on the chip surface of both F.C.B and Ecobrass, bringing about traces of concaveness and a cavity, hence increasing the surface roughness values. Lower plasticity resulted in smooth regions on the shear surface and fracture surface during the machining process. The presence of secondary phases, inclusions, and other irregularities (many interfaces in matrix) induces stress concentration in the microstructure [33].

Figure 9

SEM images of chip surface morphology: (a) Ecobrass, (b) F.C.B, and (c) MoS₂-3.

3.3.2 Surface roughness

Surface quality is a measurement of surface roughness, which is classified differently in various industries. It is also a desirable characteristic for aesthetics. For this reason, surface roughness is an important criterion for machinability. The outer layers of brass samples called the “chill zone”, were removed at a certain thickness to obtain a relatively homogeneous microstructure. Since this “chill zone” does not reflect the real mechanical behavior of the samples, the part of this zone was removed as chip by turning operation and then machinability test was performed. Among the average surface roughness values of the samples shown in Figure 10, the MoS₂-3 sample, had an average surface value of 0.63 µm, which was the lowest value. The F.C.B sample had the highest value with an average surface roughness value of 2.25 μm. However, it has been determined that the lubricating effect of the additives significantly affects the surface roughness. Ecobrass and F.C.B samples contain different ratios of alpha and beta phases and have their Ra values close to each other. The dispersion of Pb within the brass matrix is not homogenous. The cause of which is the preferential accumulation of Pb in globular form at alfa/beta interface [31]. The surface roughness value of MoS₂-1 sample is 1.36 μm, showing a better value than the reference samples. In previous studies, the ductility of materials, the presence of secondary phases, and strain hardening significantly affect the surface roughness after machining. Surface irregularities differ depending on the conditions during processing [34]. Compared to the reference samples, the surface roughness values were quite low due to the lubricating property of the MoS₂ addition. Its lubricating property is based on the fact that it is structurally composed of a multi-layered structure [35]. In addition, the increased Si alloy element content in brass caused a higher beta volume fraction in the microstructure and caused a decrease in the surface roughness due to the structure passing from the dual phase to the mono phase system.

Figure 10

Average surface roughness values of the samples.

3.4 Mechanical properties

3.4.1 Hardness

Figure 11, shows the average values of the hardness samples measured from five different regions of the microstructure. The hardness value of Ecobrass, F.C.B, and MoS₂-3 samples exhibited nearly the same behavior, while MoS₂-3 and MoS₂-1 samples have lower hardness values. The high Si amount in the Ecobrass alloy promotes the formation of the beta prime phase, which has higher hardness than the alpha phase. Fine grains in the FCB sample and the existence of IMCs in MoS₂-3, which act as obstacles against dislocation movement, are also factors that cause an increase in the hardness [36]. The high hardness values of the MoS₂-3 sample can be attributed to oxide precipitates at the grain boundaries. It has been reported that oxidized intermetallic precipitates formed by chemical reaction increase crack formation and locally increase the microhardness values of these precipitated regions [37].

Figure 11

Average hardness values of the samples.

3.4.2 Ultimate tensile strength

Figure 12 shows both the ultimate tensile strength and elongation of all tested samples. The sample of F.C.B exhibits moderately high tensile strength and excellent elongation properties since microstructure of F.C.B consists of fine grains. Ecobrass sample has the highest ultimate tensile strength due to the presence of dominant beta prime phase. MoS₂-1 sample has a moderate percentage elongation and the lowest tensile strength because it has its Widmanstätten structure in the brass matrix. In previous studies, it has been reported that the microstructure wholly consisting of intergranular Widmanstätten structure can promote the formation of cracks, so the microstructure becomes brittle and decreases the mechanical properties [38,39]. The MoS₂-3 sample had the lowest percentage of elongation since it had IMCs that precipitated at grain boundaries in the microstructure. Since the diameter of Sn and Zn atoms are larger than the diameter of a Cu atom, it is attracted by the edge dislocations that occur during tensile stress which reduces the elastic energy [40]. Thus, the material gradually begins to lose its ductility. The promising results of the MoS₂-2 alloy show desirable mechanical properties due to the small number of acicular Widmanstätten structures that precipitated at the grain boundaries. A small number of Widmanstätten structures in the brass matrix significantly reduces the deterioration of mechanical properties.

Figure 12

Comparison of the ultimate tensile strength and elongation of the samples.

4 Conclusion

In this study, the microstructure, machinability, and mechanical properties of hybridized new generation brass alloys were investigated. The major findings of this study were as follows:

Characterization:

1. The increase in the content of Si and MoS₂ additives promoted beta phase formation; besides, resulting in variation in morphology formations in brass alloys.

2. The microstructure and XRD analysis revealed that peritectic reaction has been shifted to lower zinc content by addition of Si in Cu–Zn phase diagram. Thus, the microstructure was changed from duplex (α + β) phases to a single beta phase with the increase in the Si and Sn contents in the brass matrix.

3. Peaks shifted on the XRD pattern when Sn and Si elements located as substitutional atoms in the crystal lattice induced stress rising fields in the brass matrix.

4. SEM images revealed that due to the lower atomic mobility during solidification, Widmanstätten α precipitated and grew with acicular morphology inside the grains and at grain boundaries in the microstructure of MoS₂-2.

Machinability:

1. The presence of hard and brittle phases in the brass matrix affected the formation, distribution, and morphology of chips.

2. The chip fracture mechanism was determined by the presence of hard brittle phases, while the distribution and size of IMC’s influenced the shear band formation and micro voids.

3. The samples with MoS₂ additives had the lowest surface roughness values due to its lubricant effect.

4. One of the chip formations had type “C” shapes (loose), which are generally found in samples with high beta fraction in the microstructure.

Mechanical properties:

1. The addition of additives promotes the formation of beta prime phase; thus, bringing about increased hardness values of samples.

2. Intergranular Widmanstätten structures deteriorate the mechanical properties by promoting crack initiation. On the other hand, Widmanstätten structure with needle-like shape that mostly formed at grain boundaries enhanced the mechanical properties by hindering slip that plane movement.

3. MoS₂-1 sample had low hardness, low mechanical properties, and continuous chip form compared to other samples due to Widmanstätten morphology in the whole microstructure. Variation in the chip size and shape depending on the microstructure, as well as the mechanical and hardness values were greatly affected by the microstructure morphology. In some cases, the presence of soft and hard phases in the microstructure had more effect on the machinability rather than mechanical properties.

Future studies should consider investigating the machinability performance after heat treatment, as well as the corrosion behavior of new-generation hybridized brass alloys.