SiCp reinforced magnesium-based composite material casting and casting method thereof

Abstract

The invention provides a SiCp reinforced magnesium-based composite material casting and a casting method thereof.The casting method comprises the following steps that magnesium alloy and magnesium intermediate alloy are mixed and heated to be completely melted, and alloy melt is obtained; wherein the magnesium intermediate alloy contains rare earth elements; adding a preheated SiCp reinforced phase into the alloy melt by adopting a method of combining semi-solid stirring and liquid ultrasonic assistance, and stirring and mixing to obtain a magnesium-based composite material melt; after the magnesium-based composite material melt is heated to the target temperature, the magnesium-based composite material melt is rapidly poured into a preheated casting mold, after casting is completed, the cooling speed is controlled till the melt is cooled, and the SiCp reinforced magnesium-based composite material is obtained; and the SiCp reinforced magnesium-based composite material is put into a hot isostatic pressing process heating furnace to be treated, and the SiCp reinforced magnesium-based composite material casting is obtained. According to the casting method, the mechanical property of the magnesium-based composite material casting can be improved on the basis that the casting defects of the SiCp reinforced magnesium-based composite material casting can be remarkably reduced.

Description

Technical Field

[0001] This invention relates to the field of metal composite materials technology, and in particular to a SiCp-reinforced magnesium-based composite casting and its casting method. Background Technology

[0002] In recent years, my country has actively researched lightweight, high-performance materials in fields such as communications, automobiles, and aerospace. Magnesium alloys, with their excellent physical and mechanical properties, have become one of the most widely used metallic materials, and my country possesses the world's largest magnesium reserves. Magnesium has a density of only 1.739 g / cm³. 3 Magnesium alloys have attracted much attention due to their unique advantages, such as low density, light weight, high specific strength and stiffness, and dimensional stability. Studies have shown that magnesium-based composite materials prepared by adding reinforcements such as SiC and Al2O3 can greatly improve the limitations of magnesium alloys and expand their application scenarios.

[0003] However, existing research largely focuses on wrought magnesium matrix composites, while systematic studies on cast magnesium matrix composites remain relatively lacking. In fact, casting processes offer significant advantages in adapting to the manufacture of complex structures, reducing production costs, and achieving large-scale production. They are particularly suitable for the one-piece molding of thin-walled, curved, and internally complex components, avoiding multi-stage deformation processing and reliance on high-precision equipment, thus making them more competitive in terms of energy consumption and economy. Despite the good formability and cost-effectiveness of cast magnesium matrix composites, their application still faces a series of inherent challenges, mainly including defects such as shrinkage porosity and shrinkage cavities that are easily generated during casting, as well as insufficient strength due to grain coarsening.

[0004] Therefore, there is an urgent need to provide a SiCp-reinforced magnesium matrix composite casting and its casting method. Summary of the Invention

[0005] This invention provides a SiCp-reinforced magnesium matrix composite casting and its casting method, which can solve the problems of shrinkage porosity, shrinkage cavities and insufficient strength caused by grain coarsening that are easily generated in traditional cast magnesium matrix composites during the casting process.

[0006] In a first aspect, the present invention provides a casting method for SiCp-reinforced magnesium-based composite material castings, the casting method comprising the following steps: (1) The magnesium alloy and the magnesium master alloy are mixed and heated until completely melted to obtain an alloy melt; wherein the magnesium master alloy contains rare earth elements; (2) A method combining semi-solid stirring and liquid ultrasonic assistance is used to add preheated SiCp reinforcing phase to the alloy melt, and after stirring and mixing, magnesium-based composite material melt is obtained; (3) After heating the magnesium-based composite material melt to the target temperature, it is quickly poured into the preheated casting mold. After casting, the cooling rate is controlled until the melt is cooled to obtain SiCp reinforced magnesium-based composite material. (4) The SiCp reinforced magnesium matrix composite material is placed in a hot isostatic pressing furnace for processing to obtain the SiCp reinforced magnesium matrix composite casting.

[0007] Preferably, in step (1), the magnesium alloy is a Mg-Al alloy, and the magnesium master alloy is at least one of Mg-Gd alloy, Mg-Y alloy or Mg-Sc alloy.

[0008] More preferably, the mass percentage of rare earth elements in the magnesium master alloy in the alloy melt is 0.1~2.7wt%.

[0009] Preferably, the method of combining semi-solid stirring with liquid ultrasonic assistance to add a SiCp reinforcing phase to the alloy melt, followed by stirring and mixing to obtain a magnesium-based composite material melt, includes: (21) Under a protective atmosphere, the temperature of the alloy melt is reduced to the semi-solid temperature. After removing the oxide layer on the surface of the melt with a slag remover, the stirring paddle is extended into the crucible so that the entire alloy melt can be stirred. (22) Start the stirring device to mechanically stir the alloy melt into a stable vortex state, then gradually add the preheated SiCp reinforcing phase, while stirring without vortex and increasing the furnace temperature until the melt appears obviously liquid, end the semi-solid stirring process, and remove the stirring paddle. (23) Add an ultrasonic treatment tool rod and insert it into the alloy melt. Perform ultrasonic treatment to make the SiCp reinforcing phase uniformly dispersed in the alloy melt to obtain a magnesium-based composite material melt.

[0010] Preferably, in step (21), the semi-solid temperature is 580~600℃, and the stirring paddle is 5-10mm away from the bottom of the crucible.

[0011] Preferably, in step (22), the mechanical stirring speed is 400~500 r / min and the stirring time is 10~15 min.

[0012] Preferably, in step (23), the ultrasonic treatment tool rod extends into the alloy melt by 2-3 mm, the ultrasonic temperature is 700-750℃, and the ultrasonic time is 10-20 min.

[0013] Preferably, the temperature of the SiCp reinforcing phase after preheating is 550~600℃, and the protective gas atmosphere is a mixture of CO2 and SF6 with a volume ratio of 40:1.

[0014] Preferably, the protective atmosphere is a mixture of CO2 and SF6 with a volume ratio of 40:1.

[0015] Preferably, the particle size of the SiCp reinforcing phase is 5~20μm, and the volume fraction of the SiCp reinforcing phase in the magnesium-based composite melt is 10~20%.

[0016] Preferably, in step (3), the target temperature is 700~750℃, the temperature of the preheated casting mold is 200~300℃, and the cooling rate is 20~50℃ / s.

[0017] Preferably, in step (3), the casting mold is coated with a coating agent; wherein the coating agent is preferably zinc oxide or graphite oil.

[0018] Preferably, in step (4), the temperature of the hot isostatic pressing is 400~430℃, the pressure is 120~150MPa, and the time is 2~3h.

[0019] Preferably, in step (4), after the hot isostatic pressing process is completed, the process further includes a step of slowly cooling to a preset temperature under constant pressure, and then releasing the pressure to obtain a SiCp-reinforced magnesium matrix composite casting; wherein the preset temperature is 190~210℃.

[0020] In a second aspect, the present invention provides a SiCp-reinforced magnesium-based composite casting obtained by the casting method described in the first aspect.

[0021] Compared with the prior art, the present invention has at least the following beneficial effects: (1) In this invention, magnesium alloy is selected as raw material and mixed and melted with magnesium master alloy containing rare earth elements. The rare earth elements can generate rare earth phase, which directly improves the mechanical properties (such as strength and hardness) of the composite material and improves the casting properties such as the fluidity of the melt. In addition, the endogenous rare earth phase has a relatively high elastic modulus, which, together with the added SiCp reinforcing phase, can further improve the yield strength and elastic modulus of magnesium-based composite material. Furthermore, with the synergistic control of various process designs such as semi-solid stirring and liquid ultrasonic assistance, cooling rate control during casting process and hot isostatic pressing, the casting defects of SiCp reinforced magnesium-based composite castings are significantly reduced, and the mechanical properties of magnesium-based composite castings are comprehensively improved. (2) In this invention, by synergistically adding the exogenous phase (SiCp reinforcing phase) and the endogenous phase (rare earth elements), and by integrating and synergistically controlling four key processes, the casting defect problem of SiCp reinforced magnesium matrix composite castings was successfully solved, and its mechanical properties were comprehensively improved: First, by using semi-solid stirring combined with liquid-assisted high-energy ultrasonic technology, SiCp particles were strongly dispersed, effectively eliminating the particle agglomeration phenomenon generated during stirring. This directly prevented the formation of shrinkage porosity and shrinkage cavity defects due to SiCp agglomerates hindering melt feeding during solidification. High-energy ultrasonic treatment also significantly improved the wettability of molten magnesium alloy to SiCp particles, enhanced the interfacial bonding strength, and avoided the problem of weakened interfacial bonding that may be caused by rapid cooling. Second, by strictly controlling the cooling rate of the casting, on the one hand, the microstructure was refined and the SiCp distribution was more uniform; on the other hand, even if a small amount of shrinkage porosity and shrinkage cavity defects were generated, the size was controlled within a small range, laying the foundation for subsequent processing. Finally, using hot isostatic pressing (HIP) technology, the material undergoes creep flow under high temperature and pressure. As a hard phase, SiCp's surrounding micro-shrinkage and porosity defects (especially those not completely avoided in step 2 or caused by solidification shrinkage, and those controlled in step 3) are effectively compacted and eliminated during this process. This not only achieves final densification but also optimizes the matrix structure and interface state, thereby comprehensively improving the mechanical properties of the casting. In this way, the goals of zero-order SiCp agglomeration, near-zero defects, and simultaneous improvement of casting strength and plasticity are achieved. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is an optical microstructure diagram of a SiCp-reinforced magnesium-based composite material provided in Embodiment 2 of the present invention; Figure 2 This is an optical microstructure diagram of a SiCp-reinforced magnesium-based composite material provided in Embodiment 3 of the present invention; Figure 3 The matrix alloy hot cracking performance test time-temperature diagram of a SiCp reinforced magnesium matrix composite material provided in Example 4 of this invention; Figure 4 The hot cracking performance test time-temperature diagram of a SiCp-reinforced magnesium matrix composite material provided in Comparative Example 3 of this invention; Figure 5These are physical images and scanning electron microscope images of a SiCp-reinforced magnesium matrix composite material provided in Embodiments 1 and 5 of the present invention; wherein, (a) and (b) are physical images, (a) and (c) correspond to Embodiment 1, and (b) and (d) correspond to Embodiment 5; Figure 6 These are scanning electron microscope images of a SiCp-reinforced magnesium-based composite material provided in Example 1 and Comparative Example 1 of the present invention; wherein, (a) corresponds to Comparative Example 1 and (b) corresponds to Example 1; Figure 7 These are three-dimensional X-ray images of a SiCp-reinforced magnesium matrix composite material provided in Embodiment 1 and Comparative Example 1 of the present invention; wherein, (a) corresponds to Comparative Example 1 and (b) corresponds to Embodiment 1; Figure 8 This is a composite image of a SiCp-reinforced magnesium matrix composite material obtained by SEM (scanning electron microscopy) and EDS (energy dispersive spectroscopy) in Example 1 of this invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] Silicon carbide (SiCp) particles are widely used to strengthen cast magnesium matrix composites due to their low cost, high hardness, good wear resistance, and relatively good thermal stability and wettability in molten magnesium. However, several key technical problems still exist in the practical application of cast magnesium matrix composites with SiCp reinforcement: First, the interfacial compatibility between SiCp and the magnesium matrix is ​​not ideal, easily forming a brittle reaction layer at the interface, reducing the toughness and load-bearing capacity of the composite material; second, SiCp has a higher density than molten magnesium, and is prone to sedimentation or agglomeration during solidification, leading to uneven distribution of the reinforcing phase and affecting the consistency of material properties; in addition, the addition of SiCp further increases the melt viscosity and reduces fluidity, making it more difficult to control casting defects (such as porosity and inclusions); finally, large SiCp particles may become microcrack initiations, especially under the action of solidification shrinkage stress, exacerbating early material failure. Therefore, how to overcome the above problems through process optimization and interface control while maintaining the economic and performance advantages of SiCp has become an important direction for current research on cast magnesium matrix composites.

[0026] To address one or more of the aforementioned problems, embodiments of the present invention provide a casting method for SiCp-reinforced magnesium-based composite material castings, the casting method comprising the following steps: (1) The magnesium alloy and the magnesium master alloy are mixed and heated until completely melted to obtain an alloy melt; wherein the magnesium master alloy contains rare earth elements; (2) A method combining semi-solid stirring and liquid ultrasonic assistance is used to add preheated SiCp reinforcing phase to the alloy melt, and after stirring and mixing, magnesium-based composite material melt is obtained; (3) After heating the magnesium-based composite material melt to the target temperature, it is quickly poured into the preheated casting mold. After casting, the cooling rate is controlled until the melt is cooled to obtain SiCp reinforced magnesium-based composite material. (4) The SiCp reinforced magnesium matrix composite material is placed in a hot isostatic pressing furnace for processing to obtain the SiCp reinforced magnesium matrix composite casting.

[0027] In this embodiment of the invention, magnesium alloy is selected as the raw material and mixed and melted with a magnesium master alloy containing rare earth elements. The rare earth elements can generate rare earth phases, which directly improve the mechanical properties (such as strength and hardness) of the composite material and also improve the casting properties such as melt flowability. Furthermore, the endogenous rare earth phase has a relatively high elastic modulus, and its combined action with the added SiCp reinforcing phase can further improve the yield strength and elastic modulus of the magnesium-based composite material. By synergistically adding the added phase (SiCp reinforcing phase) and the endogenous phase (rare earth elements), and by integrating and synergistically controlling four key processes, the casting defect problem of SiCp reinforced magnesium-based composite castings has been successfully solved, and its mechanical properties have been comprehensively improved: First, a semi-solid stirring combined with liquid-assisted high-energy ultrasonic technology is used to strongly disperse SiCp particles, effectively eliminating particle agglomeration during stirring. This directly prevents shrinkage porosity and shrinkage defects caused by SiCp agglomerates hindering melt feeding during solidification. High-energy ultrasonic treatment significantly improved the wettability of molten magnesium alloy to SiCp particles, enhanced interfacial bonding strength, and avoided the interfacial weakening problem that might occur due to rapid cooling. Secondly, by strictly controlling the cooling rate of the casting, on the one hand, it promoted microstructure refinement and a more uniform SiCp distribution; on the other hand, it kept the size of even a small number of shrinkage cavities and defects within a small range, laying the foundation for subsequent processing. Finally, using hot isostatic pressing (HIP) technology, creep flow was induced in the material under high temperature and pressure. As a hard phase, SiCp effectively compacted and eliminated the surrounding micro-shrinkage cavities and defects (especially those not completely avoided in step 2 or caused by solidification shrinkage, and those whose size was controlled in step 3). This not only achieved final densification but also optimized the matrix structure and interfacial state, thereby comprehensively improving the mechanical properties of the casting. Thus, the goals of zero-order SiCp agglomeration, near-zero defects, and simultaneous improvement of casting strength and plasticity were achieved.

[0028] According to some preferred embodiments, in step (1), the magnesium alloy is a Mg-Al alloy, and the magnesium master alloy is at least one of Mg-Gd alloy, Mg-Y alloy or Mg-Sc alloy; in the alloy melt, the mass percentage of rare earth elements in the magnesium master alloy in the alloy melt is 0.1~2.7wt% (for example, it can be 0.1wt%, 0.6wt%, 1wt%, 1.6wt%, 1.8wt%, 2wt%, 2.5wt% or 2.7wt%), and when there are two or more magnesium master alloys, the content of each rare earth element in the alloy melt is 0.1~0.9wt% (for example, it can be 0.1wt%, 0.3wt%, 0.5wt%, 0.8wt% or 0.9wt%).

[0029] In this embodiment of the invention, using a Mg-Al alloy as the matrix, the presence of a certain amount of rare earth elements, through the introduction of Mg-Gd, Mg-Y, or Mg-Sc master alloys, can significantly improve the overall performance of magnesium-based composite castings. Specifically, the rare earth elements Gd / Y preferentially consume oxygen and hydrogen in the alloy melt (inhibiting the formation of harmful MgO / MgH2 phases), effectively purifying the melt and reducing viscosity, thereby offsetting the deterioration of fluidity caused by the addition of SiCp, improving casting performance, and reducing the tendency for hot cracking. Simultaneously, the in-situ generated high-modulus rare earth phases such as Al2Gd and Al2Y not only act as heterogeneous nucleation sites to promote grain nucleation, refine grains, and pin dislocations to improve the strength of composite castings, but also work with the added SiCp reinforcing phase to improve the elastic modulus and yield strength of the composite castings. Furthermore, the rare earth phases can transform the SiCp / Mg interface from a weak mechanical bond to a strong metallurgical bond, enhancing load transfer efficiency and inhibiting the formation of brittle phases such as Mg2Si, forming a strong and toughened interface structure. This ultimately achieves a simultaneous improvement in strength, modulus, and plasticity, particularly alleviating the problem of a sharp decrease in plasticity caused by the addition of SiCp. However, if the content of rare earth elements added to the alloy melt is too high, it will lead to a decrease in the plasticity of magnesium-based composite castings.

[0030] According to some preferred embodiments, the method of combining semi-solid stirring with liquid ultrasonic assistance to add a SiCp reinforcing phase to the alloy melt, and then stirring and mixing to obtain a magnesium-based composite material melt, includes: (21) Under a protective atmosphere, the temperature of the alloy melt is reduced to the semi-solid temperature. After removing the oxide layer on the surface of the melt with a slag remover, the stirring paddle is extended into the crucible (5-10 mm from the bottom of the crucible) so that the entire alloy melt can be stirred. (22) Start the stirring device to mechanically stir the alloy melt into a stable vortex state, and then gradually add the preheated SiCp reinforcing phase. While stirring without vortex, increase the furnace temperature until the melt appears obviously liquid. During the addition process, the temperature of the alloy melt should be continuously increased so that the SiCp reinforcing phase can be added effectively. After adding the SiCp reinforcing phase, continue stirring for 5-10 minutes, and then while stirring without vortex, increase the furnace temperature until the melt appears relatively obviously liquid. End the semi-solid stirring process and remove the stirring paddle. (23) Add an ultrasonic treatment tool rod and insert it into the alloy melt for 2-3 mm. Perform ultrasonic treatment to make the SiCp reinforcing phase uniformly dispersed in the alloy melt. Then remove the ultrasonic treatment tool rod to obtain the magnesium-based composite material melt. The ultrasonic treatment tool rod needs to be preheated before the ultrasonic switch can be turned on, or it can be suspended above the alloy melt for a period of time to raise its temperature before being inserted into the alloy melt.

[0031] It should be noted that, in this embodiment of the invention, before mixing the magnesium alloy and the magnesium master alloy, a step of removing the oxide scale from the surface of the magnesium alloy and the magnesium master alloy is included to ensure the smooth progress of subsequent steps, thereby ensuring the quality of the composite material casting.

[0032] According to some preferred embodiments, in step (21), the semi-solid temperature is 580~600℃ (e.g., 580℃, 590℃ or 600℃); in step (22), the mechanical stirring speed is 400~500r / min (e.g., 400 r / min, 420 r / min, 450 r / min, 480 r / min or 500 r / min), and the stirring time is 10~15min (e.g., 10min, 11min, 12min, 13min or 15min); in step (23), the ultrasonic temperature is 700~750℃ (e.g., 700℃, 710℃, 720℃, 730℃, 740℃ or 750℃), and the ultrasonic time is 10~20min (e.g., 10min, 15min or 20min).

[0033] In this embodiment of the invention, the alloy melt is first cooled to a semi-solid temperature and stirred to initially disperse the SiCp reinforcing phase. Considering that simple mechanical stirring is insufficient to completely break up the SiCp reinforcing phase agglomerates, resulting in uneven distribution and stress concentration in the composite material, this embodiment further employs high-energy ultrasonic treatment after the semi-solid stirring pre-dispersion of the SiCp reinforcing phase. This not only generates instantaneous high-pressure microjets in the alloy melt, directly breaking up the SiCp agglomerates and forming strong convection to promote the dispersion of the reinforcing phase and prevent sedimentation, but also the cavitation microjets remove the SiO2 oxide layer on the surface of the SiCp reinforcing phase, exposing a fresh surface and improving wettability, thereby further enhancing the interfacial bonding strength between the SiCp reinforcing phase and the matrix. Furthermore, the enhanced interfacial bonding can offset the tendency for interfacial weakening caused by subsequent rapid cooling.

[0034] According to some preferred embodiments, the temperature of the SiCp reinforcing phase after preheating is 550~600℃ (for example, it can be 550℃, 580℃ or 600℃); the protective gas atmosphere is a mixture of CO2 and SF6 with a volume ratio of 40:1.

[0035] In this embodiment of the invention, before adding the SiCp reinforcing phase, the SiCp reinforcing phase is first preheated at the above-mentioned temperature, which is within the semi-solid temperature range. This avoids the preheating temperature being too high, which indirectly leads to a decrease in the solid volume fraction during semi-solid stirring, and also avoids the preheating temperature being too low, which leads to an increase in the solid volume fraction during semi-solid stirring and makes stirring difficult.

[0036] According to some preferred embodiments, the particle size of the SiCp reinforcing phase is 5~20μm (e.g., it can be 5μm, 10μm, 15μm or 20μm), and the volume fraction of the SiCp reinforcing phase in the magnesium-based composite melt is 10~20% (e.g., it can be 10%, 12%, 15%, 18% or 20%).

[0037] In this embodiment of the invention, by adding a certain amount and a certain particle size of SiCp reinforcing phase, the appropriate size and volume fraction of SiCp reinforcing phase can fully fill the magnesium alloy matrix and avoid the generation of a large number of particle agglomerations. The experiments of this invention have confirmed that if the particle size of the SiCp reinforcing phase is too small, agglomeration is very likely to occur in the casting production, resulting in poor dispersion and excessively high viscosity of the alloy melt, poor fluidity, and difficulty in filling the casting cavity. On the other hand, if the particle size of the SiCp reinforcing phase is too large, the SiCp reinforcing phase is prone to sedimentation during the casting process, and the large particle size is prone to microcracks, reducing its hot cracking tendency and significantly reducing the mechanical properties of the composite casting.

[0038] According to some preferred embodiments, in step (3), the target temperature is 700~750℃ (e.g., 700℃, 720℃, or 750℃), and the temperature of the preheated casting mold is 200~300℃ (e.g., 200℃, 220℃, 250℃, 280℃, or 300℃). According to some preferred embodiments, the cooling rate is 20~50℃ / s (e.g., 20℃ / s, 30℃ / s, 40℃ / s, or 50℃ / s); the casting mold is coated with a coating agent; wherein, the coating agent is preferably zinc oxide or graphite oil.

[0039] In this embodiment of the invention, the temperature of the magnesium-based composite material melt is first raised to the target casting temperature. After the temperature stabilizes, the oxide layer on the surface of the melt is removed, and then the melt is quickly poured into a preheated mold. After casting, the cooling rate is controlled to room temperature to obtain a magnesium-based composite material ingot. By first reasonably controlling the preheating temperature of the casting mold, it is possible to avoid excessive temperature causing grain growth and more shrinkage cavities. Secondly, the cooling rate during the solidification process of the melt is precisely controlled at 20~50℃ / s, which ensures that the SiCp reinforcing phase is uniformly distributed and the grain size is small, while avoiding the problem of interface debonding caused by excessively fast cooling rate, and ensuring that it has sufficient fluidity to fill the cavity, so that the endogenous rare earth phase has sufficient incubation time.

[0040] If the cooling rate is too fast, although it can quickly "freeze" the distribution of the SiCp reinforcing phase, it will cause the following problems: ① The excessively fast cooling rate causes the melt viscosity to rise sharply and the fluidity to decrease, resulting in the inability to fill the mold cavity; ② Rapid cooling easily leads to the formation of micropores and microcracks around SiCp, resulting in interface debonding; ③ High-modulus phases such as Al2Gd and Al2Y require a certain incubation time, and excessively fast cooling rate easily leads to insufficient nucleation rate, which prevents them from exerting the advantages of rare earth phases. If the cooling rate is too slow, irreversible structural degradation will occur during the solidification process. On the one hand, the slow cooling rate leads to excessively long solidification time, which easily causes coarse grains; on the other hand, the "pushing" of the solid-liquid interface causes SiCp to agglomerate at the grain boundaries, exacerbating the uneven distribution of SiCp in the matrix. Furthermore, due to insufficient feeding of the molten metal, shrinkage pores are easily formed in the area near the agglomerated SiCp. Meanwhile, excessively slow cooling rates can easily lead to the formation of coarse dendritic structures, and the melt feeding channels between dendrites are easily blocked, forming isolated liquid phase regions, resulting in micropores due to shrinkage during final solidification. In summary, in this embodiment of the invention, the cooling rate is controlled at 20~50℃ / s, and by dynamically balancing grain refinement, reinforcement distribution, and interfacial bonding strength, the microstructure and properties of composite material castings are synergistically optimized.

[0041] According to some preferred embodiments, in step (4), the temperature of the hot isostatic pressing is 400~430℃ (for example, it can be 400℃, 410℃, 420℃ or 430℃), the pressure is 120~150MPa (for example, it can be 120 MPa, 130 MPa, 140 MPa or 150 MPa), and the time is 2~3h (for example, it can be 2h, 2.5h or 3h).

[0042] In this embodiment of the invention, the final densification treatment of the cast SiCp-reinforced magnesium matrix composite material after cooling rate regulation is carried out by hot isostatic pressing (HIP). This further repairs defects such as micropores or microcracks generated near SiCp in the cast SiCp-reinforced magnesium matrix composite material, improves the interfacial bonding strength between SiCp and the magnesium matrix, and since the shrinkage cavities formed in step (3) of this embodiment are relatively small, the HIP process of this invention can ensure a certain strength while eliminating micropores. Furthermore, the high temperature and high pressure of HIP can further make the SiCp distribution more uniform, refine the grain size, and dissolve the brittle phase at the grain boundaries, thereby further improving the mechanical properties of the composite material. Thus, a triple optimization of defect repair, microstructure homogenization, and mechanical property improvement of the composite casting is achieved.

[0043] According to some preferred embodiments, in step (4), after the hot isostatic pressing process is completed, the process further includes a step of slowly cooling to a preset temperature under constant pressure, and then releasing the pressure to obtain a SiCp reinforced magnesium matrix composite casting; wherein the preset temperature is 190~210℃ (for example, it can be 190℃, 200℃ or 210℃).

[0044] Unlike traditional methods that involve heat treatment after hot isostatic pressing, in this embodiment of the invention, after hot isostatic pressing, the casting is slowly cooled while maintaining constant pressure. This helps to promote the precipitation of the precipitated phase while avoiding cracking during cooling. Furthermore, the preferred cooling rate is 2~5℃ / min, which ensures the precipitation of the second phase, eliminates residual stress, improves interfacial strength, and solves the problem of cracking during heat treatment of castings with high residual stress. At the same time, it eliminates the heat treatment process, reduces energy consumption and the risk of casting cracking, and ultimately achieves the integration of hot isostatic pressing and heat treatment processes, thereby improving the mechanical properties of magnesium-based composite castings.

[0045] In summary, in the embodiments of the present invention, in terms of raw material selection, the synergistic addition of external SiCp reinforcing phase and endogenous rare earth phase is adopted, and four process designs are combined: semi-solid stirring and liquid ultrasonic assistance, controlled cooling rate, and hot isostatic pressing. Through the synergistic control of raw materials and process parameters, the mechanical properties of magnesium matrix composite castings are comprehensively improved (mechanical strength is increased by 15%~35%) while reducing casting defects of SiCp reinforced magnesium matrix composite castings.

[0046] This invention also provides a SiCp-reinforced magnesium-based composite casting, which is obtained by casting using any of the casting methods described above.

[0047] The SiCp-reinforced magnesium-based composite material prepared in the embodiments of the present invention achieves simultaneous improvement in SiCp zero-order agglomeration, near-zero defects, and strength and plasticity.

[0048] To more clearly illustrate the technical solution and advantages of the present invention, the following describes in detail a SiCp-reinforced magnesium-based composite casting and its casting method through several embodiments.

[0049] Example 1: (1) After removing the oxide scale from the selected magnesium alloy (Mg-Al alloy) and magnesium master alloy (Mg-Gd alloy and Mg-Y alloy), weigh them according to the proportion, mix them, and heat them until completely melted to obtain the alloy melt; wherein, the mass percentage of rare earth element Gd in the alloy melt is 1wt%, and the mass percentage of rare earth element Y in the alloy melt is 0.5wt%; (2) Under the protective atmosphere of a mixture of CO2 and SF6 with a volume ratio of 40:1, the temperature of the alloy melt is reduced to the semi-solid temperature of 585°C. After removing the oxide layer on the surface of the melt with a slag remover, the stirring paddle is extended into the crucible (5 mm from the bottom of the crucible) so that the entire alloy melt can be stirred. Mechanical stirring was initiated using a stirring device (stirring speed 450 r / min) to bring the alloy melt into a stable vortex state. Then, preheated SiCp reinforcing phase (preheated temperature 580℃, particle size 10μm) was gradually added while stirring without vortexes until the melt exhibited a distinctly liquid state. During the addition process, the temperature of the alloy melt was continuously increased to ensure effective addition of the SiCp reinforcing phase. After adding the SiCp reinforcing phase, stirring continued for 10 minutes. Then, while stirring without vortexes, the furnace temperature was increased until the melt exhibited a more distinctly liquid state, ending the semi-solid stirring process and removing the stirring paddle. The volume content of the SiCp reinforcing phase in the alloy melt was 10%. After the ultrasonic treatment tool rod is preheated, the ultrasonic switch can be turned on. The ultrasonic treatment tool rod is inserted into the alloy melt by 3 mm and ultrasonically treated at 750℃ for 15 min to make the SiCp reinforcing phase uniformly dispersed in the alloy melt, thus obtaining the magnesium-based composite material melt. (3) Before casting, preheat the casting mold to 250°C, then raise the magnesium-based composite material melt to the target temperature of 720°C and quickly pour it into the preheated casting mold. After casting, control the cooling rate to 30°C / s. After the melt cools to solidification, take it out to obtain SiCp reinforced magnesium-based composite material. (4) The SiCp reinforced magnesium matrix composite material is placed in a hot isostatic pressing furnace, and the temperature is controlled at 415℃, the pressure at 120MPa, and the time at 3h. After the hot isostatic pressing process is completed, the pressure is kept constant and the cooling rate is controlled at 5℃ / min to slowly cool to 200℃. Then the pressure is released and the air is vented to obtain the SiCp reinforced magnesium matrix composite material casting.

[0050] Example 2: Example 2 is basically the same as Example 1, except that in step (2), the particle size of the SiCp reinforcing phase is 20 μm.

[0051] Example 3: Example 3 is basically the same as Example 1, except that in step (2), the particle size of the SiCp reinforcing phase is 50 μm.

[0052] Example 4: Example 4 is basically the same as Example 1, except that in step (3), the magnesium-based composite material melt is raised to the target temperature of 750°C and quickly poured into the hot cracking performance test mold. After casting, the cooling rate is controlled at 30°C / s. After the melt cools to solidification, it is taken out to obtain SiCp reinforced magnesium-based composite material.

[0053] Example 5: Example 5 is basically the same as Example 1, except that in step (3), the cooling rate is 100℃ / s.

[0054] Comparative Example 1: Comparative Example 1 is basically the same as Example 1, except that step (4) is omitted, i.e., hot isostatic pressing is not performed.

[0055] Comparative Example 2: Comparative Example 2 is basically the same as Example 1, except that in step (1), no magnesium intermediate alloy is added.

[0056] Comparative Example 3: (1) After removing the oxide scale from the selected magnesium alloy (Mg-Al alloy) and magnesium master alloy (Mg-Gd alloy and Mg-Y alloy), weigh them according to the proportion, mix them, and heat them until completely melted to obtain the alloy melt; wherein, the mass percentage of rare earth element Gd in the alloy melt is 1wt%, and the mass percentage of rare earth element Y in the alloy melt is 0.5wt%; (2) Before casting, preheat the casting mold to 250°C, then pour the alloy melt into the mold, raise the temperature to the target temperature of 750°C, and quickly pour it into the hot cracking performance test mold. After casting, control the cooling rate to 30°C / s. After the melt cools to solidification, take it out to obtain the magnesium-based composite material.

[0057] The SiCp-reinforced magnesium matrix composite castings from the above embodiments and comparative examples were subjected to performance tests, and the test results are as follows: Figures 1 to 7 As shown.

[0058] The SiCp-reinforced magnesium matrix composites in Examples 2 and 3 were subjected to optical (OM) microstructure observation, as follows: Figure 1 and Figure 2 As shown, when the particle size of the SiCp reinforcing phase is 20 μm, the SiCp in the SiCp-reinforced magnesium matrix composite is uniformly distributed and free of agglomeration. Figure 1 When the particle size of the SiCp reinforcing phase is 50 μm, SiCp in the SiCp-reinforced magnesium matrix composite material undergoes sedimentation and microcracks are present. Figure 2By comparing Example 1 and Comparative Example 3, it can be found that compared with the alloy melt without SiCp reinforcement, adding SiCp reinforcement with an appropriate particle size to the alloy melt makes the composite material less prone to cracking during rapid cooling and has no impact on its casting properties (e.g., ...). Figure 3 and Figure 4 As shown). Figure 5 As shown, a suitable cooling rate can ensure complete filling of the composite casting (Example 1), while an excessively fast cooling rate (Example 5) results in incomplete filling. Furthermore, the faster the cooling rate, the less pronounced the necklace-like distribution of SiCp. Further, before hot isostatic pressing (… Figure 6 a, Comparative Example 1) and after hot isostatic pressing ( Figure 6 b. SEM analysis of the magnesium-based composite casting in Example 1 shows that hot isostatic pressing (HIP) can further eliminate shrinkage cavities in the magnesium-based composite casting. Furthermore, three-dimensional X-ray analysis of the composite casting before and after HIP yields the following results: Figure 7 As shown, after hot isostatic pressing ( Figure 7 b) The composite material has almost no porosity. Meanwhile, SEM analysis was performed on the composite casting after hot isostatic pressing and slow cooling in Example 1, as shown... Figure 8 As shown, the precipitation of high-modulus phases such as Gd and Y in the intermediate alloy is beneficial to improving the strength of the composite casting. Compared with Comparative Example 2, the mechanical strength of the composite casting in Example 1 is increased by about 20%, and the interface defects of SiCp reinforced magnesium matrix composite are significantly improved.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A casting method for SiCp-reinforced magnesium matrix composite castings, characterized in that, The casting method includes the following steps: (1) The magnesium alloy and the magnesium master alloy are mixed and heated until completely melted to obtain an alloy melt; wherein the magnesium master alloy contains rare earth elements; (2) A method combining semi-solid stirring and liquid ultrasonic assistance is used to add preheated SiCp reinforcing phase to the alloy melt, and after stirring and mixing, magnesium-based composite material melt is obtained; (3) After heating the magnesium-based composite material melt to the target temperature, it is quickly poured into the preheated casting mold. After casting, the cooling rate is controlled until the melt is cooled to obtain SiCp reinforced magnesium-based composite material. (4) The SiCp reinforced magnesium matrix composite material is placed in a hot isostatic pressing furnace for processing to obtain the SiCp reinforced magnesium matrix composite casting.

2. The casting method according to claim 1, characterized in that, Step (1), the magnesium alloy is a Mg-Al alloy, and the magnesium master alloy is at least one of a Mg-Gd alloy, a Mg-Y alloy, or a Mg-Sc alloy; and / or The mass percentage of rare earth elements in the magnesium master alloy is 0.1~2.7wt%.

3. The casting method according to claim 1, characterized in that, The method employing a combination of semi-solid stirring and liquid ultrasonic assistance involves adding a SiCp reinforcing phase to the alloy melt, followed by stirring and mixing to obtain a magnesium-based composite material melt, comprising: (21) Under a protective atmosphere, the temperature of the alloy melt is reduced to the semi-solid temperature. After removing the oxide layer on the surface of the melt with a slag remover, the stirring paddle is extended into the crucible so that the entire alloy melt can be stirred. (22) Start the stirring device to mechanically stir the alloy melt into a stable vortex state, then gradually add the preheated SiCp reinforcing phase, while stirring without vortex and increasing the furnace temperature until the melt appears obviously liquid, end the semi-solid stirring process, and remove the stirring paddle. (23) Add an ultrasonic treatment tool rod and insert it into the alloy melt. Perform ultrasonic treatment to make the SiCp reinforcing phase uniformly dispersed in the alloy melt to obtain a magnesium-based composite material melt.

4. The casting method according to claim 3, characterized in that, In step (21), the semi-solid temperature is 580~600℃, and the stirring paddle is 5-10mm away from the bottom of the crucible; In step (22), the mechanical stirring speed is 400~500 r / min, and the stirring time is 10~15 min; and / or In step (23), the ultrasonic treatment tool rod extends into the alloy melt by 2-3 mm, the ultrasonic temperature is 700-750℃, and the ultrasonic time is 10-20 min.

5. The casting method according to claim 3, characterized in that, The temperature of the SiCp reinforcing phase after preheating is 550~600℃; and / or The protective atmosphere is a mixture of CO2 and SF6 in a volume ratio of 40:

1.

6. The casting method according to claim 1 or 3, characterized in that, The particle size of the SiCp reinforcing phase is 5~20μm, and the volume fraction of the SiCp reinforcing phase in the magnesium-based composite melt is 10~20%.

7. The casting method according to claim 1, characterized in that, In step (3), the target temperature is 700~750℃, and the temperature of the preheated casting mold is 200~300℃; the cooling rate is 20~50℃ / s; and / or The casting mold is coated with a coating agent; wherein the coating agent is preferably zinc oxide or graphite oil.

8. The casting method according to claim 1, characterized in that, In step (4), the temperature of the hot isostatic pressing is 400~430℃, the pressure is 120~150MPa, and the time is 2~3h.

9. The casting method according to claim 1, characterized in that, In step (4), after the hot isostatic pressing process is completed, the process further includes a step of slowly cooling to a preset temperature under constant pressure, and then releasing the pressure to obtain a SiCp-reinforced magnesium matrix composite casting; wherein the preset temperature is 190~210℃.

10. A SiCp-reinforced magnesium-based composite casting, characterized in that, It is cast using the casting method described in any one of claims 1 to 9.

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