An Improved Process for Solving the Sintering Problem of Al-Si Alloy Powder Metallurgy

Abstract

The Al-Si alloy prepared by the traditional powder metallurgy method has the problems of difficult sintering and low mechanical properties. In this paper, rotary extrusion combined with rapid solidification/powder metallurgy technology (RS/PM) is proposed and designed to produce an Al-10Si alloy. In subsequent sintering at different temperatures, better metallurgical bonding between particles can be achieved to fulfil the aim of improving the mechanical properties of the material. Additionally, the continuous preparation of materials can be achieved. Finally, it is verified that the density of the Al-10Si alloy sintered at 525 °C is 2.61 g/cm³, reaching 97.0% of the theoretical density, and the tensile strength reaches 336 MPa, which is much higher than that of the alloy prepared by the traditional casting method and powder metallurgy method.

1. Introduction

The Al-Si alloy has the characteristics of low density, a low thermal expansion coefficient, high hardness, wear resistance, good heat resistance and high specific strength. It has broad application prospects in aerospace, automobiles, electronics and other fields, especially for the piston blades of automobile engines [1,2,3,4]. The Si phase and alloy structure of Al-Si alloy prepared by the conventional casting method are coarse, and its mechanical properties are poor, which limits its application in high-performance parts [5,6]. When aluminum alloy is subjected to a certain stress, it will cause concentrated stress to cause cracks inside the part. Reducing the sharp angle of the primary silicon in hypereutectic Al-Si alloy and refining the grains can improve the comprehensive mechanical properties of Al-Si materials. To improve the microstructure and properties of cast Al-Si alloy, at present, some special preparation techniques can effectively reduce the size of the primary silicon, mainly including semi-solid casting technology [7], electromagnetic stirring technology [8], high-gravity technology [9], modification treatment [10], mechanical vibration [11] and ultrasonic melt treatment technology [12].

Among them, modification is a relatively simple grain refinement method. By adding a modifier close to the critical nucleation size of the grain, the fine nuclei are provided during the solidification of the alloy, and the effect of grain refinement is achieved. This method improves the mechanical properties of Al-Si alloy to a certain extent. However, in these methods, most of them use Al-Si alloys prepared by the casting method, which inevitably produce casting defects such as shrinkage, porosity and macrosegregation in the alloy, which will greatly affect the comprehensive mechanical properties of the alloy. At present, the properties of materials obtained by this method cannot meet the high performance requirements for scientific and technological progress. Additionally, as a non-heat-treatment-strengthened aluminum alloy, the deformation treatment of Al-Si alloy can also influence its microstructure and increase its properties [13]. However, with the development of lightweight automobiles, people have established higher requirements for the performance of materials. Although traditional plastic-processing technology is widely used, due to the small strain generated during the deformation process, the refining effect on high-performance aluminum alloy grains is very limited, which makes it difficult for some alloy components with strict performance requirements to meet the corresponding performance requirements after general plastic deformation.

The rapid solidification/powder metallurgy (RS/PM) technique combines rapid solidification technology with powder metallurgy technology to achieve the fine-tuning of the matrix structure while significantly reducing the size of the silicon phase [14,15,16]. This method is a near-net-shape-forming technology that causes no casting defects. Therefore, the performance of aluminum–silicon alloy prepared by rapid solidification/powder metallurgy technology is higher than that of alloys prepared using the traditional casting method. Furthermore, in RS/PM, the primary silicon is refined from the as-cast polygonal block to micron-sized particles, which are dispersed and evenly distributed in the matrix, thereby strengthening the dispersion distribution of the second phase. At the same time, powder metallurgy aluminum alloy materials have the advantages of low density, high specific strength and good corrosion resistance, which could meet the application requirements of lightweight automobiles in the future. However, the powder metallurgy of aluminum alloy has the problems of difficult sintering and poor mechanical properties [17,18,19]. Therefore, it is of great significance to develop a new technique or method to solve the sintering problem of powder aluminum alloy for its further application. In our previous study, a new process of green extrusion combined with sintering was presented. The combination properties of Al-Si-Cu alloy were greatly improved by squeezing the oxide film on the surface of aluminum alloy powder and sintering to achieve metallurgical bonding [20].

Based on our previous work, a new process of rotary extrusion combined with sintering is proposed and designed in this study. At the same time as positive extrusion, the material produces a certain pure shear deformation through the rotation of the die. During the operation of the equipment, a variable-diameter concave die is driven to rotate by a transmission shaft, and at the same time, the upper bottom plate of the hydraulic press drives the convex die downward and performs positive extrusion inside the concave die, so that the alloy powder is extruded and formed through the variable-diameter concave die, completing the rotary extrusion process. During the experiment, the rotation speed of the variable-diameter concave die and the downward speed of the convex die can also be adjusted by adjusting the frequency converter box and hydraulic operating system. The superposition of deformation can not only refine the structure, but also break the oxide film on the powdered aluminum alloy’s surface. Metallurgical bonding between particles can be achieved during subsequent sintering. In this paper, the effect of sintering temperature on the microstructure and properties of the rapid solidification rotary extrusion green alloy was systematically studied.

2. Experimental Section

2.1. Sample Preparation

The pre-alloyed Al-10wt. % Si powder was prepared by the rapid solidification atomization method; the chemical composition of the powder is shown

Table 1. Chemical composition table of Al-Si powder (wt. %).

Alloy	Al	Si	Lubricant
Content	89.46	9.94	0.6

. In order to reduce the friction between the powder particles and between the powder and the mold, polyethylene glycol was selected as the lubricant. Firstly, Al-Si pre-alloyed powder and 0.6wt. % polyethylene glycol were mixed in a mixer and stirred for 4 h. Then, the mixed powder was poured into the extrusion die. Then, the mixed powder was poured into the pressing die, and a cylinder with a diameter of 10 mm was prepared under 800 MPa pressure. The cylinder was put into the rotary extrusion die, and the rotary extrusion die was used. The rotary extrusion with the extrusion ratio of 4.9 was then carried out to prepare of Al-Si alloy rods. The rotating extrusion bar of the Al-Si alloy was obtained with a forward extrusion speed of 2 mm/s and rotation speed of 56 r/min. In this process, a mixture of graphite, PEG, and gasoline is used as a lubricant inside the mold.

2.2. Sintering Process

To obtain the final Al-Si alloy samples, the above alloy rods were sintered in a tube furnace at 495 °C, 505 °C, 515 °C, 525 °C, 535 °C and 545 °C, respectively, for 1.5 h with the Ar protection atmosphere to obtain the final Al-Si alloy samples. The matching samples were designated with A1, A2, A3, A4, A5 and A6, respectively. The heating rate was 5 °C/min. At the end of sintering, the temperature was cooled with the furnace.

2.3. Characterization Methods

The particle size and distribution of Al-Si powder particles were detected by a laser particle size analyzer (LS13~320, Beckman, Brea, CA, USA). The Rigaku-D/max-A X-ray diffractometer (XRD) using Cu-Kα radiation was employed to evaluate the phase composition of samples. Accelerating voltage, tube current, scanning range, and scanning speed were set as 40 kV, 100 mA, 10–90° and 1°/min, An optical metallographic microscope (OM) and a scanning electron microscope (SEM) were used to examine the morphology of the materials. In this experiment, 2% hydrofluoric acid was used to corrode the samples for SEM detection. The JMD electron density (specific gravity) was used to measure the alloy’s density. A Vickers hardness tester (YZHV-1000P, Shanghai ZK Optical Instrument Co., Ltd., Shanghai, China) and electronic universal testing machine (CMT-5305, MTS Industrial Systems (China) Co., Ltd., Shanghai, China) were used to test the materials’ tensile and hardness properties, respectively. Furthermore, based on a commercial Finite Element Method (FEM) software, Deform 11.0, the finite element analysis of Al-Si alloy powder rotary extrusion forming was performed.

3. Results and Discussion

Figure 1a depicts the number distribution curve of alloy powders with various particle sizes. Al-Si powder has a mean particle size of 15.37 μm, and with the increase in powder size, the number becomes increasingly lower. The particle size distribution of powder conforms to normal distribution. The SEM images of Al-Si alloy powder are displayed in Figure 1b. As can be observed, the majority of the powdered Al-Si particles are made using quick solidification technology resembling spheres. The powder particles have a homogeneous distribution and distinct sizes that align with the findings illustrated in Figure 1a. The powder particles are of different sizes and are evenly distributed. However, the surface of the powder is rough with small droplets. There are small particle defects around the large-sized particles, which are smooth on the surface of the small particles. It can be inferred that the powder morphology may be related to the particle size. It is found that the degree of concavity and convexity on the surface of the powder increases with the increase in the particle size, so the small particles can fill the concavity inside the large particles during the pressing process, making the powder easier to be compacted and the internal pores of the alloy sample smaller [20].

In the gas atomization process, the shape of the powder is influenced by the relative durations of the solidification time and the spheroidization time of the gas-atomized alloy droplets. If the alloy droplets are completely solidified before sufficient spheroidization occurs, the resulting powder shape will be elongated or irregular. Spheroidization refers to a process in which a raw material undergoes physical or chemical reactions at high temperatures and under specific atmospheric conditions to form spherical or near-spherical particles. When the solidification time is shorter than the spheroidization time, the powder particles tend to be elongated or irregular. In contrast, when the solidification time exceeds the spheroidization time, the alloy droplets achieve a high degree of spheroidization before complete solidification, resulting in short, strip-like spherical particles with a smoother surface. The spheroidization time, τ_sph, of the aerosolized alloy droplets can be calculated using Equation (1),

$${\tau }_{sph}={\displaystyle \frac{3{\pi }^2{\mu }_L}{64V\sigma }}(r_1^4-r_2^4)$$

(1)

where $\sigma$ denotes the surface tension of the liquid metal, ${\mu }_L$ represents the viscosity of the liquid metal, V is the volume of the atomized droplet, and $r_1$ and $r_2$ denote the diameter of the spheroidized droplet and the minimum diameter of the droplet before spheroidization, respectively. Given that $r_1$ is ten times greater than $r_2$, $r_1$ can be neglected. By combining Equations (1) and (2), it was evident from Equation (2) that the droplet diameter, viscosity, and surface tension collectively determine the spheroidization time of the droplet.

$$V={\displaystyle \frac{4}{3}}\pi r_1^3$$

(2)

$${\tau }_{sph}={\displaystyle \frac{9\pi {\mu }_L}{256\sigma }}{r}_1$$

(3)

For Figure 1c, the alloy powder was pressed into a cylindrical ingot using a mold, and then extruded into a cuboid aluminum alloy rod through the rotary extrusion process. Figure 2a presents the X-ray diffraction (XRD) pattern of samples A1–A6. As shown in the figure, all diffraction peaks of the Al-Si alloy correspond to the diffraction peaks of the Al and Si phases. The diffraction peaks of the Al phase are observed at 38.472°, 44.738°, 65.133°, 78.227°, and 82.435°, which are attributed to the (111), (200), (220), (311), and (222) crystal planes, respectively. The diffraction peaks of the Si phase are observed at approximately 28.442°, 47.302°, 56.121°, 69.130°, 76.377°, and 88.026°, corresponding to the (111), (220), (311), (400), (331), and (422) crystal planes, respectively. The sintering temperature does not significantly alter the phase composition of the alloy. However, as the temperature increases, the full width at half maximum (FWHM) of the Si phase diffraction peaks decreases slightly, indicating gradual growth of the Si phase particles. This can be explained by the fact that the grain size of the Al-Si alloy is small due to the large plastic deformation introduced during the rotary extrusion process. As the temperature rises, the grains first recrystallize, and with further heating, the grains begin to grow. Moreover, compared to the unsintered samples, the relative intensity of the diffraction peaks of the Si phase increases, and this intensity continues to increase slightly with higher sintering temperatures, suggesting that the precipitation ratio of the Si phase increases as the temperature rises. Figure 2b shows the XRD patterns within the 2θ range of 44–45.5 degrees. As the sintering temperature increases, the positions and intensities of the diffraction peaks of both the Al and Si phases shift significantly. This change reveals the microscopic mechanism by which Si gradually dissolves into the Al matrix. High-temperature sintering promotes the diffusion of Si atoms into the Al lattice, allowing Si atoms to progressively incorporate into the Al lattice structure, thereby forming a solid solution. Since the atomic radius of Si is smaller than that of Al, its incorporation into the Al lattice causes slight lattice expansion, increasing the lattice spacing. As the sintering temperature increases from 495 °C to 545 °C, the diffraction peaks in the XRD patterns, particularly the Al phase peak near 2θ ≈ 45 degrees, gradually shift toward lower angles. This shift indicates an increase in lattice spacing, which can be attributed to the greater solubility of Si at higher temperatures, enabling more Si atoms to dissolve into the Al matrix. This leads to lattice expansion and a rearrangement of the microstructure. Additionally, the dissolution of Si may affect the defect density within the Al matrix, altering the stress distribution in the lattice, which further intensifies the lattice expansion. As Si atoms dissolve into the Al matrix during high-temperature sintering, an Al-Si solid solution is formed, resulting in an increase in lattice spacing, as evidenced by the low-angle shift of the diffraction peaks in the XRD patterns.

For aluminum alloy powder metallurgy, the presence of an oxide film on the surface of the aluminum powder complicates achieving perfect metallurgical bonding (i.e., the disappearance of grain boundaries). The physical bonding between particles, which results in a large number of grain boundaries, significantly reduces the comprehensive mechanical properties of the alloy. To assess the feasibility of the process, microscopic tests using optical microscopy (OM) and scanning electron microscopy (SEM) were performed on all samples. Figure 3a–e show the OM images of samples A1–A5. The rotating extruded samples exhibit a distinct metallurgical bonding effect, characterized by small primary silicon particles and high compactness. This indicates that the oxide film on the surface of the aluminum powder was broken, and grain refinement occurred due to the rotary extrusion process. During subsequent sintering, the grain boundaries disappeared, leading to metallurgical bonding. The figures show that the silicon particles slightly increased in size as the temperature rose, which can be attributed to the growth of the silicon phase with increasing temperature. The Al-Si alloy produced via the RS/PM method exhibited a fine microstructure without any shrinkage, porosity, or other defects, as evidenced by the silicon particles, which had an average size of ~5 μm. The black voids in the metallographic images are due to defects caused by overburning (not specified). Therefore, to ensure that the alloy undergoes powder metallurgy with metallurgical bonding, an optimal sintering temperature is required. This temperature must be high enough to allow diffusion and bonding of the alloy particles, but must not exceed the overburning temperature of the material. The SEM images of the Al-Si alloy sintered at various temperatures are shown in Figure 3f–j. As the temperature increased, the corners and edges of the eutectic Si grains became progressively smoother and more rounded, and they were evenly distributed throughout the Al matrix. This behavior aligns with the results of the optical melting analysis. It should be noted that the samples were heavily corroded by hydrofluoric acid (HF) during this experiment, which caused the Al surface to be eroded, leaving holes. These holes could be mistaken for sintering-related porosity or gas overflow, which could affect the observed degree of densification in the SEM images.

To investigate the effects of different sintering temperatures on the properties of the alloy, the density, microhardness, and tensile properties of the samples were primarily evaluated, and the results are presented in Figure 4. In powder metallurgy, the degree of densification of the alloy is a key factor in ensuring its strength [20,21,22,23]. Therefore, this study employed a self-designed large plastic deformation process involving both forward extrusion and rotary extrusion to enhance the density of the Al-Si alloy. The sintering process for spherical particles generally follows these stages: (I) initial particle contact before sintering, (II) sintering neck growth during the early sintering stage, and (III) pore spheroidization during the later sintering stage. Sintering is the final critical step in the powder metallurgy process [24,25,26], playing a decisive role in the final product’s properties, as defects caused during sintering cannot be rectified in subsequent processes. Hence, sintering effectively acts as a "gatekeeper" for product quality. In this experiment, to assess the impacts of different sintering temperatures on the properties of powder metallurgy Al-Si alloys, the density, microhardness, and mechanical properties of the alloy samples sintered at various temperatures were studied. The density and microhardness curves of the alloys sintered at different temperatures are shown in Figure 4a and Figure 4b, respectively.

Table 2. Comparison of density, hardness, elongation and tensile strength of Al-Si alloy sintered at different temperatures.

Samples (ID)	Density (g/cm3)	Microhardness (HV)	Tensile Strength (MPa)	Elongation
A1	2.612	52.4	280	6.5
A2	2.606	51.5	300	7.8
A3	2.600	50.5	312	7.5
A4	2.598	50.0	336	8.1
A5	2.580	47.0	275	7.5

summarizes the comparison of density, hardness, and tensile strength for Al-Si alloys sintered at different temperatures. According to the observed trends in density, the microhardness of the alloy decreased as the sintering temperature increased. Notably, the maximal work hardening occurred primarily in the sample that was extruded but not sintered, yielding a density of 2.623 g/cm³ and a microhardness of 56 HV. Al-Si alloy rods prepared by the rotary extrusion process exhibited a density of 98.3% of the theoretical density, even without sintering. However, residual lubricants remained in the alloy, and due to the expansion of compressed gases during high-temperature extrusion, some porosity inevitably developed in the alloy structure after extrusion, explaining the decline in density after sintering. Additionally, excessively high temperatures led to the overburning of the aluminum alloy, resulting in reduced density. As the temperature increased, grain growth occurred, and the processing effect diminished. The decrease in density at temperatures below 525 °C is likely due to the expansion of compressed gas during high-temperature extrusion, which then escapes through exit holes during sintering. At temperatures above 535 °C, the alloy overburned, leading to a rapid decrease in density. The driving force of the sintering process is to reduce the free energy of the system by lowering the surface curvature and area, with sintering ultimately occurring through shrinkage.

Tensile strength is a critical performance index for evaluating the service behavior of aluminum alloys and serves as the primary criterion for material performance assessment. The tensile strengths of the alloy samples at different sintering temperatures are shown in Figure 4c. From the figure, it is apparent that the tensile strengths of samples A1, A2, A3, A4, and A5 are 280 MPa, 300 MPa, 312 MPa, 336 MPa, and 275 MPa, respectively. Clearly, the tensile strength of the alloy exhibits a trend of initially increasing and then decreasing with temperature. The Al-Si alloy sintered at 525 °C shows the best performance compared to other temperature-sintered samples, with a tensile strength of 336 MPa. This increase in tensile strength with temperature can be attributed to the enhanced metallurgical bonding of the alloy. However, at higher sintering temperatures, the growth of primary Si particles occurs, transforming the primary Si phase into a hard and brittle phase within the Al-Si alloy. The Al phase, on the other hand, is a softer, tougher phase that contains numerous hard particles dispersed in the soft matrix. Therefore, the morphology, size, and distribution of Si particles significantly influence the alloy’s mechanical properties. Finer powder particles lead to a larger surface area and a higher packing density, which reduces the diffusion distance between metal atoms during sintering. This results in a higher density product. Consequently, in general, the density and strength of the final product are inversely related to the particle size of the powder. During this process, the powder particles undergo severe plastic deformation, which promotes their rearrangement and results in an increase in the density of the alloy.

Previous studies have shown that the properties of Al-Si alloys are strongly influenced by the morphology, size, and distribution of their silicon particles. Specifically, as the particle size increases, the stress threshold required for crack initiation in the silicon particles also increases. Consequently, smaller and finer silicon particles are more susceptible to fracture. It has been observed that crack initiation is more likely to occur at regions where the silicon particles exhibit a high aspect ratio [27]. In this study, the Al-Si alloy is primarily processed by powder metallurgy techniques, including powder pressing and extrusion. The silicon particles in the alloy are very fine, and after severe plastic deformation, the Si phase is not only small but also exhibits an approximately spherical morphology. This ensures that the alloy components maintain integrity during service, preventing particle detachment and thereby enhancing both strength and service life.

Figure 3k–o displays the fracture morphology of the alloys at various temperatures. It is evident that all alloys exhibited several tiny dimples, indicative of strong ductile fracture. The samples sintered at 525 °C showed larger and finer dimples, resulting in higher strength and maximum elongation. In this case, the fracture morphology of the alloy exhibited a trend from brittleness to toughness and then back to brittleness as the temperature increased. When the temperature was lower than 525 °C, the fractures of the alloy showed both dimples and cleavage fracture morphology, suggesting that at lower sintering temperatures, there were instances of poor metallurgical bonding between the alloy particles, which significantly affected the alloy’s strength. When the temperature exceeded 525 °C, the dimples on the alloy fracture surfaces grew, primarily due to grain growth as the temperature increased. At higher temperatures, the alloy experienced overburning, contributing to brittle fracture. However, compared to the fracture morphology of traditional casting and other powder metallurgy methods, the ductile fracture observed in this study was more prominent, with smaller and more uniform dimples. The technique employed in this study is clearly superior to conventional methods for improving the properties of Al-Si alloys. The strength of traditional cast Al-Si alloys is approximately 200 MPa [28,29,30,31]. This improvement can be attributed to the dual deformation process of rotary and forward extrusion, which significantly refines the microstructure of the Al-Si alloy, breaks the oxide coating on the aluminum powder surface, and facilitates metallurgical bonding during sintering. The resulting small grains and homogeneous microstructure of the powder metallurgy alloys contribute to their improved mechanical properties. The mechanism is illustrated in Figure 4d. After severe plastic deformation of the Al alloy powder through forward and rotary extrusion, the Al₂O₃ film on the surface was broken, exposing a new interface [20]. Excellent metallurgical bonding was achieved after subsequent sintering. The broken fine Al₂O₃ particles acted as a reinforcing phase, strengthening the alloy and enhancing its mechanical properties. The temperature field of the extrusion process was simulated using the commercial software Deform from the American Scientific Forming Technology Company (Columbus, OH, USA), as shown in Figure 4e. The extrusion outlet temperature was controlled below 500 °C. According to the simulation, the outlet temperature during the preparation of the Al-Si alloy remained within a reasonable range, well below the alloy’s melting point, which was key to ensuring that the alloy did not burn out and maintained excellent performance. As the outlet distance increased, the alloy temperature decreased, indicating that the simulation results were in good agreement with the actual experimental findings.

In summary, the Al-Si alloy prepared by our method outperforms other Al-Si alloys prepared by the same proportion method. This is primarily due to the large plastic deformation induced by forward and rotary extrusion, which breaks the alumina film on the surface of the aluminum alloy powder. As a result, the particles undergo deformation and refinement, exposing new interfaces, and achieving metallurgical bonding after sintering. The density of the alloy reaches more than 98% of the theoretical density, effectively addressing the challenging issue of powder metallurgy sintering. Additionally, the Si particles in the Al-Si alloy prepared by powder metallurgy are small, less than 2 μm, which is significantly smaller than those in alloys produced by casting and forging. The particles also exhibit a nearly spherical shape, preventing separation from the matrix under stress, a marked improvement over the shape of Si particles in cast alloys. This process not only plays a crucial role in advancing high-performance Al-Si alloys, but also holds substantial industrial value for the broader field of aluminum alloy powder metallurgy, contributing to the advancement of lightweight automotive technologies.

4. Conclusions

The Al-Si alloy produced by the traditional powder metallurgy method faces challenges such as difficult sintering and low mechanical properties. In this study, a novel rotary extrusion and sintering (RS/PM) process was proposed. The method was applied to an Al-10wt. %Si alloy, and the effects of different sintering temperatures on the microstructure and properties of the alloy were investigated. The results show that the microstructure of the alloy was significantly refined. The size of the Si particles in the Al-Si alloy prepared by this process was small, measuring less than 2 μm, with a nearly spherical shape. The grain boundaries were almost eliminated, and good metallurgical bonding between the particles was achieved. The density of the alloy reached 98.6% of the theoretical density, while the strength reached 336 MPa, both of which were significantly higher than those of Al-Si alloys prepared by traditional methods. As the sintering temperature increased, the hardness and strength of the alloy initially decreased and then increased. This material has potential applications as a major candidate for automotive engines, aircraft parts, and aerospace vehicles. The rotary extrusion process not only proved effective for Al-Si alloy powders, but also holds significant value as a reference for the powder metallurgy of other aluminum alloys, as well as for improving the microstructure refinement of other materials.