Effect of sintering temperature on the microstructure and properties of Al–7.5Cu–1Si alloy

With increasing requirements for environmental protection and energy conservation, Al–Cu powder alloys play an important role in machine weight reduction. During the preparation of the Al–Cu powder alloy, an appropriate sintering temperature helps improve the comprehensive performance of the alloys. In this study, powder metallurgy was used to prepare Al–7.5Cu–1Si alloys, and the effects of different sintering temperatures on their relative sintering density, hardness, and tensile strength were studied. The microstructure and phase composition of the alloy were observed and analyzed by scanning electron microscopy and x-ray diffraction, and the mechanical properties of the alloy were studied using a Brinell hardness tester and a universal material testing machine. By analyzing the microstructural changes of the alloys at different sintering temperatures, the optimal sintering temperature of the alloy was determined to obtain the best properties. The results demonstrated that the sintering temperature significantly affected the diffusion and migration of the metal particles inside the alloy. When the temperature was low, numerous pores were present in the alloy, and the degree of bonding between the metal particles was poor. As the temperature increased, the number of pores in the alloy decreased, and good metallurgical bonding occurred between the particles. The relative densities and mechanical properties of the alloys were significantly affected by the sintering temperature. When the temperature was below 570 °C, the relative density and mechanical properties were low. At 590 °C, the alloy underwent deformation and was in an over-sintered state. The best sintering temperature was 570 °C. This is because at this temperature, the relative sintering density, hardness, and tensile strength of the alloy all reached maximum values, which were 98.6%, 71 HB, and 153 MPa, respectively.


Introduction
Owing to their excellent advantages, such as low density, good thermal conductivity, high specific strength, excellent corrosion resistance, and low cost, aluminum alloys are widely used in aerospace, transportation, automotive, marine, and construction [1,2].Al-Cu alloys are aluminum-based alloys with copper as the main alloying element and are widely used in the manufacture of aircrafts, automotive bodies, and other components [3,4].The powder metallurgy technology can prepare parts with precise dimensions and smooth surfaces, and it is difficult for the ordinary casting process to process the complex shape of the parts, which has the advantages of saving processing time and reducing cutting processes [5].However, aluminum is significantly active and easily reacts with oxygen to form a dense oxide film that covers the surface of the powder.During sintering, this oxide film hinders the diffusion and migration of metal atoms and metallurgical bonding between the metal particles, which is not conducive to the sintering process [6,7].To promote the sintering of alloys and enhance their properties, researchers have added alloying elements and controlled the sintering temperature to promote metallurgical bonding between metal particles and accelerate the densification of alloys [8][9][10][11][12][13].Liu et al [8] found that when the sintering temperature was 555 °C, local melting of the B 4 C-AlSi eutectic alloys destroyed the oxide film between particles, and good metallurgical bonding occurred between the particles.The plasticity and compressive strength of this alloy were the highest.An et al [9] studied the effects of sintering temperature on the strength and microstructure of powder metallurgical Al-Cu-bearing alloys.They found that when the sintering temperature was low, the oxide film on the surface of the powder particles was not easily destroyed, and there was no significant metallurgical bonding between the metal particles, resulting in low alloy strength.When the sintering temperature exceeded 560 °C, the alloy underwent over-sintering.At 550 °C, good metallurgical bonding occurred between the powder particles, resulting in a good microstructure and the best strength.Dang et al [10] found that the AlCuMgSi alloy sintered in a nitrogen atmosphere had the densest microstructure and the best mechanical properties.At 590 °C, the generated Al 2 Cu liquid phase fully filled the pores between the aluminum particles, resulting in a dense microstructure with good metallurgical bonding between the particles.The tensile strength of the alloy was the highest.Sercombe et al [11] found that adding a small amount of Sn element to Al-Cu alloys and increasing the sintering temperature from 590 to 620 °C, improved the density and mechanical properties of the alloy.Xing et al [12] found that adding Fe to Al-Cu alloys, and when the Fe content is 1.5%, sintering temperature is 570 °C, and sintering time is 40 min, a large amount of liquid phase is generated in the alloy, which rapidly fills the pores under capillary action.The density and mechanical properties of the alloy were satisfactory.Xie et al [13] found that adding magnesium to Al-Cu alloys can generate spinelstructured magnesium-aluminum oxides that disrupt the oxide layer, thereby promoting mutual diffusion between aluminum and copper.When the magnesium content is 1% and the sintering temperature is 620 °C, the liquid phase in the alloy increases, fully filling the pores, thereby reducing the size and number of pores.The densities and mechanical properties of the alloys reached their maximum values.Meng et al [14] found that the addition of Si to Al-Cu alloys can significantly refine the grains and improve the tensile strength, compressive strength, and hardness of the alloy.To sum up, aluminum alloys exhibit different microstructures and properties at different sintering temperatures.Therefore, it is necessary to study the sintering mechanisms of aluminum alloys.
Currently, there is no report on the effect of sintering temperature on the microstructure and properties of powder metallurgy Al-Cu-Si alloy.The sintering mechanism of Al-Cu-Si alloys remains unclear.Therefore, in this study, a powder metallurgy method was used to prepare Al-7.5Cu-1Si alloys by adding Si in the form of an Al-Si alloy powder, and the effects of different sintering temperatures on the microstructure and properties of the alloy were investigated.Exploring the sintering mechanism of Al-Cu-Si alloys provides a basis for their applications.

Experimental
Experimental materials: Ar atomized aluminum powder: (<25 μm, purity 99.90%),electrolytic copper powder: (<35 μm, purity 99.90%), Al-Si powder: (<48 μm, mass fraction of Si is 10%, purity 99.90%).All powders were obtained from Benyu Metal Materials Co., Ltd in Qinghe County.Powder mixtures with a chemical composition (mass fraction, %) of Al-7.5Cu-1Si were mixed for 3 h in a tank mill (GMS) with a 304 stainless steel ball, and the powder-to-ball ratio was maintained at 1:3.The mixed powder was pressed at 470 MPa for 3 min to produce green compacts with a size of f20 mm × 10 mm.The green compacts were sintered in a tube furnace (OTF-1200X) under flowing nitrogen (purity 99.999%).The sintering process was as follows: First, the green compacts were sintered to 400 °C at a heating rate of 10 °C min −1 , and the holding time was 30 min.Thereafter, the compacts were further heated at a rate of 5 °C min −1 to the sintering temperature, and the holding time was 60 min.Finally, the compacts were cooled to room temperature in a furnace.A schematic of the sintering process for the Al-7.5Cu-1Sialloy is shown in figure 1.
The densities of the sintered samples were determined using the Archimedes' method.The samples were polished and etched using Keller's (comprising 95 ml of H 2 O, 2.5 ml of HNO 3 , 1.5 ml of HCl, and 1.0 ml of HF) reagent.The microstructure of the alloy was observed using scanning electron microscopy (SEM, JEOL JSM 7200F).The phase composition of the alloy was determined using x-ray diffraction (XRD, Mini Flex 600) at a scanning rate of 4°min -1 .The hardness of the alloy was measured using a digital Brinell hardness tester (310HBS-3000) with a hard alloy steel ball diameter of 2.5 mm, and at a load of 635 N for 15 s, and the average of three measurements was obtained.Tensile tests were performed using a universal material testing machine (HY-0580) at a displacement speed of 0.5 mm min -1 .

Results and discussion
3.1.Phase composition and microstructure of the alloy Figure 2 shows a sample of the Al-7.5Cu-1Sialloy sintered at different temperatures.When the sintering temperature reached 590 °C, the sample began to deform; thus, in this study, the highest sintering temperature was 570 °C.
Figure 3 shows the XRD patterns of the Al-7.5Cu-1Sialloy sintered at different temperatures.The alloys sintered at different temperatures comprised α-Al, Al 2 Cu, and Si phases.As shown in figure 4, the number of Ω phases was small; thus, it was not detected.
Figure 4 shows the SEM images of the Al-7.5Cu-1Sialloys sintered at different temperatures, and table 1 summarizes the EDS results for the different phases in the Al-7.5Cu-1Sialloy with different sintering temperatures.At 490 °C, as shown in figure 4(a), there are several big pores between the metal particles, and the size and distribution of the pores are not uniform.The alloy comprised the α-Al matrix phase, granular Si phase (distributed in a few metal particles), small flake-like Al 2 Cu phase, and pores.At 510 °C, as shown in figure 4(b), the Si phase in the metal particles decreased, but the size of some pores between metal particles increased.According to the Al-Cu-Si ternary eutectic phase diagram [15], when the composition was Al-13.6Cu-6Si, at a temperature of approximately 520 °C, there was a ternary eutectic transformation: L → α(Al) + β(Si) + θ(CuAl 2 ).Therefore, during the sintering process, when the mass ratio of Al, Cu, and Si reached this proportion in a localized area, the eutectic reaction produced a small amount of liquid phase.This liquid phase flowed along the solid particle surfaces, leaving voids at the original aluminum particle positions, causing the sintered sample to swell and increasing the size of the pores in the localized areas of the alloy.At 530 °C, as shown in figures 4(c) and (c'), the size of the pores in the alloy decreased, the gaps between particles decreased, and Si particles gradually diffused to the particle boundaries.the Ω phase is a sheet-like material with a chemical composition of Al 2 Cu.According to the EDS results, the chemical composition of the Ω phase was 77.8 at.%Al and 22.2 at.%Cu, which was consistent with the results in reference [16].In addition, some Al 2 Cu phases began to aggregate and grow.At 550 °C, as shown in figures 4(d) and (d'), the pores in the alloy clearly decreased, the gaps between particles further reduced, and Si particles inside the metal particles disappeared.A small number of Si particle phases were distributed at the particle boundaries.The Ω phase in the alloy increased.At 570 °C, as shown in figures 4(e) and (e'), there were almost no pores and gaps between particles in the alloy, and good metallurgical bonding occurred between the particles.A large number of Ω phases began to precipitate and were distributed within the alloy.The Al 2 Cu phase aggregated and grew at the particle boundaries, forming large flake-like structures.A small amount of the granular Si phase was uniformly distributed at the boundaries of the particles.

Properties of the alloy
Figure 5 shows the relative sintering density of Al-7.5Cu-1Si alloys.At 490 °C, the relative sintering density was lower than the green density.As the temperature increased, the relative sintering density of the alloy decreased first and thereafter increased.When the temperature reached 570 °C, the relative sintering density of the alloy reached 98.6%, nearly fully dense.At 490 °C, the bonding between the metal particles is not strong, and Cu diffuses into the Al particles, leaving voids at original Cu positions; therefore, the sintering density is low [17].At 510 °C, owing to the occurrence of a eutectic reaction, a liquid phase was generated locally in the alloy, leading to an increment in the pores of the alloy, and swelling occurred.Thus, the relative sintering density of the alloy was the lowest.With the temperature gradually increasing, the movement of atoms increased, the sintering neck formed and grew in the contact area between particles, the interconnected pores between the particles contract into nearly spherical closed pores, the volume of pores decreased, and the distance between particles decreased [18].With temperature further increasing, a large number of liquid phases formed in the alloy, and liquid phases filled the pores of the alloy.Thus, the relative density increased [19].
Figure 6 shows the hardness values of the alloys at different temperatures.The hardness trend was consistent with that of the relative density.As the temperature increased, the hardness initially decreased and thereafter increased.When the temperature reached 570 °C, the hardness of the alloy reached a maximum of 71 HB.At lower sintering temperatures, there were a large number of pores between the metal particles, resulting in lower hardness.At 510 °C, because the size of the pore in the alloy increased and the relative density decreased, the hardness of the alloy decreased.As the temperature gradually increased, the number of pores decreased and the relative density increased, thereby increasing the hardness.Additionally, because the Ω phase dispersed in the matrix increased which is the strengthening phase of the alloy [20], the hardness of the alloy further increased.
Figure 7 shows the tensile strength (ultimate tensile strength) of the alloys sintered at different temperatures.As the temperature increased, the tensile strength of the alloy gradually increased, followed by a leveling-off trend.At 570 °C, the tensile strength reached the highest, approximately 153 MPa.As shown in to figure 3, at lower temperatures, the alloy has more pores and a lower sintering density, and the diffusion of elements is insufficient, leading to a lower strength.At 510 °C, with further diffusion of elements in the alloy, the silicon gradually diffuses outward, resulting in increased strength of the alloy.As the temperature increased, further diffusion of silicon results in uniform distribution of silicon within the matrix, and the precipitation of the Ω phase within the alloy increased gradually.Thus, the strength of the alloy increased [ 15,21].When the sintering temperature was higher than 550 °C, the strength of the alloy increased slowly owing to the aggregation of Al 2 Cu and the growth of matrix grains (as shown in figure 4(e)).Further, the strengthening effect was weakened.

Table 1 .
EDS results for the different phases in the Al-7.5Cu-1Sialloy with different sintering temperatures.