Study of the Effect of Tin Addition in Aluminum–Copper Alloys Obtained from Elemental Powders

Abstract

Powder metallurgy enables the production of composite materials, which are of great interest to different branches of the automotive, aerospace, and medical industries. This work investigated the sintering of an Al-xCu and Al-xCu-0.1Sn alloy, with copper concentration between 3.5 and 4.5% and tin added in the range of 0.1%. Compressibility curves were drawn, and the samples were sintered in a high-purity nitrogen-controlled atmosphere furnace. The composites were subjected to subsequent solubilization heat treatment, with cooling in low concentration polymer solutions and artificial aging (T6). The samples were studied using optical, scanning electron, Vickers microhardness, and X-ray diffraction techniques. The results indicated the effectiveness of cooling the samples after solubilization in polymer solutions, the influence of the addition of tin on the aging time, and the mechanical properties of the alloys as a function of the T6 cycles applied.

1. Introduction

The powder metallurgy process is an attractive manufacturing technique for obtaining mechanical components with a final shape, or very close to this condition, serialized with high mechanical performance and dimensional control [1,2,3,4,5,6,7]. The process is considered environmentally friendly as it has a lower environmental impact than other conventional manufacturing processes due to various factors, such as lower costs, lower energy consumption, and shorter processing times [1]. It is possible to manufacture parts with unique characteristics like controlled porosity, nominal density, excellent surface finish and wear resistance, through the developing of special alloys that involves the combination of materials that would be impossible to obtain by melting or casting (example: metals with very different melting points, or metal–ceramic composites). Compared to casting or hot forging, the alloys obtained by powder metallurgy are generated in the solid state, and low temperatures are required to obtain antagonistic [4]. Another advantage includes the possibility of incorporating several reinforcement phases into the same metal matrix and combining a higher fraction of reinforcement particles compared to processes involving casting [1,2]. In modern Engineering, aluminum alloys have a major impact on structural fabrication due to the combination of lightweight, weldability, corrosion resistance, and sustainability [8]. Aluminum metal matrix composites can improve their mechanical properties by using non-metallic reinforcements or adding dissimilar alloying elements with different physical and chemical properties.

The addition of alloying elements such as Cu, In, Bi, Sb, Pb, and Sn influences the sintering of aluminum alloys [4]. Copper is one of the main alloying elements used alongside Aluminum, largely due to the substantial age-hardening response of the Al-Cu alloy [3,9,10,11,12,13,14,15,16].

In the sintering process, copper has two ideal characteristics: a single liquid phase in which Aluminum is continuously soluble, and the maximum solubility of copper in Aluminum is 5.65% at 548 °C [3,11,16]. However, Copper’s melting point (1085 °C) is nearly 2 times that of Aluminum. The liquid phase is formed due to a eutectic between Al and Al₂Cu at 548 °C. The major problem lies in the diffusivity of copper in Aluminum, which is 5000 times faster than that of Aluminum in copper. This difference in the diffusion of copper in aluminum increases the homogenization rate but causes expansion through the Kirkendall effect [12]. Therefore, it can be assumed that the sintering of the Al-Cu alloy is strongly dependent on process variables (in particular, the size of the copper particles and the heating rate) [12]. Perhaps, Al-Sn is one of the only alloys to exhibit all the characteristics of an ideal liquid phase sintering system [12]. Tin’s melting point (232 °C) is much lower than that of Aluminum; therefore, no formation of intermetallic phases occurs. Tin has moderate solubility in solid Aluminum, while Aluminum is completely soluble in liquid tin and does not form immiscible liquids [4]. The diffusivity of Al in the Sn liquid phase is faster than that of tin. The tin, in particular, has shown beneficial effects on the sintering of Al-Cu-Mg alloys when nitrogen is used as a protective atmosphere and mixtures are obtained using elemental powders [3,5,7]. In addition, reduced tin additions make precipitation processes in Al-Cu alloys more effective, increasing peak hardness by around 20% and reducing aging times from 12 h to just two h in some alloys [4,5,6,7]. Obtaining different alloys in powder metallurgy can contribute to obtaining antagonistic mechanical properties typical of composite materials [17,18,19,20,21,22,23]. This effect can occur during sintering or heat treatment. The main heat treatment applied to increase the mechanical properties of aluminum alloys is solubilization with cooling in water and subsequent artificial aging [24]. Although cooling aluminum alloys in water is not a highly critical element, it, like other alloys, is susceptible to forming the vapor phase for subsequent boiling and convection [24]. The use of small concentrations of polymers such as polyvinylpyrrolidone (PVP) in aqueous solutions contributes to forming an instantaneous surface film on the surface, helping the samples pass straight through to boiling and convection [24,25]. There are few studies on these conditions in aluminum alloys.

In this sense, this research aimed to compare the effect of adding tin to the Al-xCu binary alloy, giving rise to an Al-xCu-0.1%Sn ternary alloy from elemental powders [4,5,6,7]. The main objective is to evaluate the sintering response of the mixture and its relationship with the microstructure and mechanical properties (microhardness) after solubilization heat treatment with cooling in a polymer solution containing 5% PVP in water and artificial aging (T6).

2. Materials and Methods

This study used commercial elemental powders of aluminum, copper, and tin with controlled granulometric characteristics and chemical composition to obtain the lowest possible dispersion. Maxepoxi Industry and Commerce Ltda (Sao Paulo, Brazil) supplied the aluminum powders, Cromato Chemicals Ltda (Sao Paulo, Brazil) supplied the copper powders, and Multicel Pigments Industry and Commerce Ltda (Sao Paulo, Brazil) supplied the tin powders. The powders were atomized with water.

The previous characterization of the metal powders was carried out to guarantee predictable properties and repeatability of results. This characterization was carried out using scanning electron microscopy (SEM, JEOL Ltd., Tokyo, Japan) to observe the powders’ morphology, average grain size, and grain size distribution. X-ray diffraction analysis (XRD) was performed on each powder sample to investigate its composition. After the initial characterization, the constituents were weighed and mechanically mixed in a Y-type mixer at 45 RPM for 60 min for each formulation. A Bel precision balance, model Mark L 5202 (BEL Engineering s.r.l, Milan, Italy), with a resolution of 0.01 g, was used for this.

Table 1. Proposed chemical composition of the specimens.

ALLOY	AL %	CU %	SN %
Al-3.5Cu	96.50	3.50	-
Al-4.0Cu	96.00	4.00	-
Al-4.5Cu	95.50	4.50	-
Al-3.5Cu-0.1Sn	96.40	3.50	0.10
Al-4.0Cu-0.1Sn	95.90	4.00	0.10
Al-4.5Cu-0.1Sn	95.40	4.50	0.10

shows the chemical compositions of the mixtures studied. The aluminum powder has a purity content of 99.7%. Ethylene bis (stearamide; EBS), a conventional lubricant for powder metallurgy, was applied for compaction and was subsequently removed by the thermal process (Dewaxing).

Equation (1) shows how the theoretical density of the four mixtures was determined using the rule of mixtures.

$${\rho }_T={\displaystyle \frac{m_{Al}+m_{Cu}+m_{Sn}}{\frac{m_{Al}}{{\rho }_{TAl}}+\frac{m_{Cu}}{{\rho }_{TCu}}+\frac{m_{Sn}}{{\rho }_{TSn}}}}$$

(1)

where ${\rho }_T$ is the theoretical density of the alloy, $m_{Al}$ is the mass of Aluminum, $m_{Cu}$ is the mass of copper, $m_{Sn}$ is the mass of Tin, ${\rho }_{TAl}$ is the theoretical density of Aluminum, ${\rho }_{TCu}$ is the theoretical density of copper, and ${\rho }_{TSn}$ is the theoretical density of Tin.

Three samples were then taken from each mixture for the compressibility test. A double-acting uniaxial compression die made from AISI D6 steel was used to compact the samples in a hydraulic press with a capacity of 15 tons. Cylindrical specimens with a diameter of 18.88 mm and a height of 6.5 mm were generated. The compaction procedure was carried out following the recommendations of ASTM B925-15, which standardizes the procedure for compacting powder metallurgy samples [4]. The compressibility test was applied, consisting of the application of increasing loads and unloading up to a previously determined starting point, with the displacements recorded by a Zaas comparator clock, Model 03, 0001 (ZAAS Tech, Karachi, Pakistan), with a graduation of 0.01 mm and a maximum displacement of 10 mm [5]. The geometric method was used to determine the green density, considering the mass, radius, and height of the pellets obtained after compaction. Measurements were made using a Digimess model 100.170 digital caliper, with a measurement resolution of 0.01 mm; a Mitutoyo analog micrometer with a resolution of 0.01 mm; and a Bel precision scale, model Mark L 5202, with a resolution of 0.01 g. After checking the green density, the samples were sent to the sintering process in an EDG Inox Line series furnace with a heating capacity of up to 1200 °C in a nitrogen atmosphere. The sintering cycle was determined based on literature reviews dealing with the sintering of class 2XXX alloys, i.e., those with Cu as the main alloying element [11,12]. Figure 1a shows the stages of the sintering cycle: (i) heating at a rate of 10 °C/min from room temperature to a temperature of 350 °C; (ii) followed by a plateau of 30 min for the process of volatilization of the lubricant, a process called “Dewaxing”; (iii) heating at a rate of 10 °C/min to a temperature of 590 °C, maintained at this temperature for 90 min for sintering, (iv) cooling in the furnace to room temperature. Figure 1b indicates the stages of the artificial aging cycle (T6): solubilized at 540 °C for 120 min, followed by cooling in a polyvinylpyrrolidone (PVP) solution with a 5% concentration in 95% water with subsequent artificial aging at temperatures of 160 °C and 200 °C, with microhardness measurements every 3 h, until 12 h. The heat treatment was carried out to verify the effective increase in microhardness in the samples and to determine the correlation between microhardness and the types of precipitates generated during artificial aging [2,7,11,12]. Microhardness tests by Vickers microindentation were carried out on the specimens after sintering to evaluate the sintering response and the increase in microhardness in a Shimadzu microhardness tester, model HMV-2T (Shimadzu Corporation, Kyoto, Japan). The procedure followed ASTM B933: Standard Test Method for Microindentation Hardness of Powder Metallurgy (PM) [12]. For each sample, measurements were taken at 10 different points, randomly distributed to avoid regions very close to pores, using a load of 100 g HV 0.1 and 10 s of loading.

After sintering, new density measurements were taken to assess the variation in densification during the sintering process. Based on the density values, it was possible to calculate the final porosity of the pieces and compare them with the values obtained in the green strength compacts [4,12,13,14,15,16]. To determine the final density after sintering, procedures were used following ASTM B962 Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle, using a Bel precision balance, model Mark L5202, with a resolution of 0.01 g [12]. The samples underwent metallographic preparation for the morphological analysis of the microstructures of the sintered specimens. The optical microscopy analyses were conducted after metallographic preparation on an optical microscope with reflected light and an image analysis system—Olympus GX 51S (Olympus Corporation, Tokyo, Japan) with a digital image acquisition system. The samples were then subjected to scanning electron microscopy microstructural analysis at higher magnification using JEOL JSM - 6610LV equipment. X-ray diffractometry (XRD) analyses were carried out using Bruker D8 Advance equipment to characterize and compare the phases formed in the samples after sintering. The following test parameters were used: voltage of 40 kV, current of 40 mA, copper tube, wavelength of 1.5418 Å, scan angle between 30° and 120° with a step of 0.05° [16,17,18]. The data obtained were compared with the NIST crystallographic chart code 03-065-2869, which shows the crystallographic “peaks” of ferrous and non-ferrous alloys. The samples were subjected to the T6 heat treatment in the last stage.

3. Results and Discussions

3.1. Sample Compressibility Curves

Figure 2 compares the compressibility curves concerning the density achieved in g/cm³ for the analyzed mixtures. The state-of-the-art review found that all the curves had a typical shape and values. Comparing the compressibility curves of the six mixtures, it can be seen that adding more alloying elements did not considerably affect the compressibility properties of the mixtures. This phenomenon is due to the high ductility of pure aluminum powder [6].

The curves for all the mixtures show the high compressibility of the aluminum mixtures, given that for relatively low stresses, in the order of 200 MPa, theoretical density ratios in the 90% range are already achieved [2,6,7,8,9]. It is also possible to observe that for tensions above 400 MPa, the compressibility curves plotted take on values close to 95% of the theoretical density, and, from this point on, no significant increase in densification is obtained, making it possible to admit optimization in the compaction pressure applied to the mixtures [10].

3.2. Particle Size and Morphological Characterization of Elementary Powders

Figure 3 shows the SEM micrograph results for the aluminum powder. The vast majority of the particles have a morphology tending towards irregular spherical shapes. Analyzing the sample using Image J software made it possible to obtain particle data ranging from 149 to 4 μm, with an average of around 16 μm.

Figure 4 shows the SEM micrograph results for the copper powder added to the Aluminum. By analyzing the sample using Image J software, particle data ranged from 136 μm to 4 μm, with an average particle size of 19 μm. The morphology of the copper particles is more irregular than that of the Aluminum, as shown in Figure 4.

Figure 5 shows the SEM micrograph results for the tin powder added as an alloying element. Analyzing the sample using Image J software made it possible to obtain particle data ranging from 28 μm to 0.2 μm, with an average particle size of 3 μm. Therefore, the particle size of tin powder is much smaller than aluminum and copper’s. The morphology of the tin particles is spherical and homogeneous, as in the studies by [7].

3.3. Density Analysis Before and After Sintering

The average density values of the samples can be seen in

Table 2. Average density of samples, theoretical, before (green) and after sintering (True).

Alloy	Theoretical		Green		True
	Average	StDev	Average	StDev	Average	StDev
Al-3.5Cu	2.7676	0.0038	2.349	0.0230	2.5095	0.0035
Al-4.0Cu	2.7775	0.022	2.3468	0.0120	2.5122	0.0027
Al-4.5Cu	2.7875	0.0049	2.3374	0.0167	2.5238	0.0049
Al-3.5Cu-0.1Sn	2.7694	0.037	2.2879	0.0250	2.5133	0.0026
Al-4.0Cu-0.1Sn	2.7793	0.019	2.3149	0.0144	2.5249	0.0052
Al-4.5Cu-0.1Sn	2.7893	0.0012	2.3162	0.0137	2.5256	0.0012
Average	2.7784	0.0146	2.3254	0.0175	2.5182	0.0033

.

The compaction process showed high reproducibility, resulting in low standard deviation values in the green samples. After sintering, there was a significant reduction in density and an increase in standard deviation. These values indicate the sintering process’s effectiveness in forming the alloy [2,5,9,15]. The nitrogen-rich atmosphere used in sintering can break down the alumina layer of the powders, resulting in exothermic reactions that release heat and facilitate the formation of aluminum nitrides [12,15]. Statistical analysis results indicated a week-long influence of the composition on the theoretical, green, and true density of the Al-xCu and Al-xCu-0.1Sn materials.

3.4. Artificial Aging Curves

The average microhardness value of the sintered Al-xCu and Al-xCu-0.1Sn samples was 42.067 HV_0.1. The determination of the solubilization and aging temperature was based on studies highlighted in the literature on powder metallurgy applied to aluminum alloys. Figure 6 shows curves relating the core microhardness to the 3, 6, 9, and 12 h of artificial aging at 160 °C and 200 °C. In these T6 heat treatment conditions, the increase in microhardness compared to the sintering condition is evident in all the alloys. The Al-4.5Cu-0.1Sn alloy showed the highest microhardness value when artificially aged at 200 °C for 6 h. The second-highest hardness peak was in the Al-4.5Cu alloy artificially aged for 12 h. Note that the microhardness result achieved in Al-4.5Cu-0.1Sn with 6 h of artificial aging corresponded to 93.6 HV, while the Al4.5Cu alloy showed a microhardness of 91 HV for 12 h of artificial aging. In this way, it was possible to relate the trace addition of tin to the shift of the point of maximum microhardness to the left, i.e., it is possible to obtain maximum microhardness by adding tin in a shorter T6 heat treatment time [17].

The artificial aging curves of Al-4.5Cu and Al-4.5Cu-0.1Sn alloys are relevant for the microhardness results and the behavior of the curves. The fundamentals of precipitation hardening are related to the solubility limit in solids, which decreases with temperature [3,8,16]. After the end of solubilization, rapid cooling causes the separation of the θ phase so that the alloy remains in an unstable supersaturated state at low temperatures. If, however, the alloy is left to age for a period of time after quenching, the second phase precipitates [3,8,16]. This precipitation occurs by a process of nucleation and growth. However, the size of the precipitates becomes finer as the temperature at which the precipitation occurs is reduced, and the hardening of the alloy is associated with a critical dispersion of the precipitate [3,8,16]. If, at any temperature, aging is carried out for long times, coarsening of the particles occurs (i.e., the small ones tend to redissolve, and the large ones to grow), and the numerous finely dispersed small particles are gradually replaced by a smaller number of more widely dispersed and coarser particles. In this condition, the mechanical strength of the alloy is reduced by overaging [3,8,16]. Therefore, the SEM and DRX analyses are conducted only in these two groups of samples (Al-4.5Cu and Al-4.5Cu-0.1Sn).

3.5. Microstructures of Sintered Samples and After the T6 Cycle

Figure 7 shows the microstructure of the Al-4.5Cu and Al-4.5Cu-0.1Sn alloys under optical microscopy. Grain contours and micropores are heterogeneously distributed in the aluminum matrix. No segregation or microstructural banding was detected, indicating a homogeneous distribution and substitutional diffusion of copper in the aluminum matrix [7,16]. The microstructures were revealed using Keller’s reagent.

Studies indicate that adding Sn significantly affects the densification behavior when the powders are compacted at low pressures [7,12]. The effect of adding Sn is minimal when the powders are compacted at high pressure (400 MPa). It is believed that due to the low melting temperature of Sn (237 °C), the molten Sn fills the gaps between the particles of the compacted powder and therefore causes lower porosity values. In addition, Sn diffuses into the liquid phase during sintering and reduces its surface tension, thus providing better wettability [7,12]. Sn is therefore an extremely important element in lowering porosity at the pressures applied to the samples. The information generated in the analysis of the samples by scanning electron microscopy using secondary electron detection and backscattered electron detection techniques can be seen in Figure 8 and Figure 9. In the backscattered electron technique, the image contrast is formed as a function of the atomic number of the chemical elements present in the sample, i.e., it is possible to identify the variation in the chemical composition of the elements contained in the micrograph by the contrast [4,5,6]. It was possible to determine the presence of well-defined aluminum grains in shades of grey in both the solubilization and artificial aging heat treatment conditions (T6) [7,9,10]. New grains were also formed due to the sintering process, which showed heteroepitaxy linked to the condition of the elementary powders [11,12,13]. The intergranular and intragranular porosity in darker tones, with irregular morphology and much smaller dimensions when compared to the grains, has its main nucleation factor from compaction [14,15]. The formation of sintering necks on contiguous grains showed that sintering was successful [15,16,17,18,19,20,21,22]. Structures formed by small, lighter colored elements, identified mainly in micrographs with backscattered electron detection, were identified as Al₂Cu intermetallics. Grain contours with different morphologies have been found near the aluminum powder grains [16,17,18,19,20,21,22].

3.6. X-Ray Diffraction of T6 Samples

Figure 10a shows the diffractograms for the Al-4.5Cu sample, in the diffractogram of the sintered samples. The diffractograms of the samples after solubilization at 520 °C and artificial aging at 160 °C are highlighted in red. The diffractogram highlighted in blue was obtained for the samples that underwent heat treatment with solubilization at 520 °C and artificial aging at 200 °C. The peaks are characteristic of Al₂Cu and Al₂O₃. This factor reveals greater oxidation due to the higher temperature during the artificial aging process at 200 °C. Figure 10b shows the diffractograms for the sample with the chemical composition Al-4.5Cu-0.1Sn.

The main hardening mechanism in Al-Cu alloys is related to copper precipitation [13]. Although some precipitates appeared after sintering, they were less intense than in the samples with T6 heat treatment. After solubilization and artificial aging, the copper peaks rearranged themselves, generating Al-Cu peaks with greater intensity during aging at 160 °C and 200 °C in the form of precipitates [14].

4. Conclusions

Based on this work, it was possible to evaluate the behavior of the Al-4.5Cu and Al-4.5Cu-0.1Sn mixtures obtained via powder metallurgy and carry out comparative analyses after the heat treatments of solubilization and artificial aging (T6). Also, the information generated in the study of the samples by scanning electron microscopy using secondary electron detection and backscattered electron detection techniques is decisive for decision-making [23]. The conclusion is that

• All the mixtures under study show similar compressibility trends. Specimens compacted at a tension of 200 MPa resulted in average values of 83% of the theoretical density for each alloy.
• For all the mixtures, an increase in density was observed when comparing the specimens compacted in green with those after sintering.
• Intergranular pores smaller than the grains and well distributed in the matrix were observed in the samples without and with added tin.
• The samples with added tin showed a greater response to hardening when subjected to solubilization at 540 °C for 2 h with cooling in a 5% PVP polymer solution and artificial aging at 200 °C for 6 h. It was evident that adding 0.1% tin by mass increased the response to heat treatment. Under this condition, the aging time was shorter for the Al-4.5Cu-0.1Sn samples.
• XRD analysis showed that the Al₂Cu phase was formed in all samples with and without the addition of tin.

New advances in powder metallurgy have contributed to the safe use of composite materials in various applications. For future research, there is interest in studies related to adding other percentages of Cu and Sn, verifying their effect on the T6 heat treatment. The effects of higher concentrations of polymer solutions on cooling after solubilization can also be studied. The article’s limitations include the lack of isostatic compaction at high pressures, the use of different lubricants and the possibility of hot compaction. These points will be used as a reference in future research.