Method for obtaining ternary composite phase reinforced Mg-Al series magnesium alloy-based composite material in situ by adding nano silicon nitride and preparing casting with complex structure

Abstract

The invention discloses a ternary composite phase reinforced Mg-Al series magnesium alloy-based composite material obtained in situ by adding nano silicon nitride, which comprises the following raw material components in percentage by weight: 2-10% of nano silicon nitride particles and the balance of Mg-Al series magnesium alloy particles, and the sum of the weight percentages of the components is 100%. A complex-structure casting prepared from the material by using a semi-solid injection molding process has high elastic modulus, high rigidity, high temperature resistance and certain elongation. The invention further discloses a method for preparing the casting with the complex structure through the semi-solid injection molding technology. The casting prepared through the method has the advantages of being high in elasticity modulus, high in rigidity, resistant to high temperature and certain in elongation.

Description

Technical Field

[0001] The present invention belongs to the technical field of manufacturing metal matrix composites, and specifically relates to a ternary composite phase reinforced Mg-Al based magnesium alloy matrix composite obtained by in-situ adding nano-silicon nitride, and also relates to a method for preparing complex structure castings by a semi-solid injection molding process. Background Art

[0002] In recent years, with the replacement demand for green lightweight materials, more and more magnesium alloy materials have been used to replace structural parts such as aluminum alloys, steels, and cast irons. However, the mechanical properties of magnesium and magnesium alloys are relatively low, and the elastic modulus is only about 45 GPa, showing a large gap compared with these materials. Although in recent years, there have been great progress in the technologies for improving the mechanical properties of magnesium alloys and new materials, many scholars and enterprises have developed some series of high-strength rare-earth magnesium alloys for mechanical properties and achieved relatively ideal practical application effects. However, the currently developed high-strength and tough magnesium alloys are close to or meet the requirements of the mechanical property indexes of high-strength aluminum alloys in terms of strength indexes, but their physical property - elastic modulus still cannot meet the index requirements of aluminum alloys or other metal materials.

[0003] As is well known, the elastic modulus of a metal material is an inherent property of the material itself and is an important performance parameter of engineering materials. From a macroscopic perspective, the elastic modulus is a measure of the size of the material's ability to resist elastic deformation; from a microscopic perspective, it is a reflection of the bonding strength between atoms, ions, or molecules. The larger its value, the greater the resistance of the material to a certain amount of elastic deformation, that is, the greater the stiffness of the material, that is, under a certain stress, the amount of elastic deformation is smaller. Usually, the elastic modulus E refers to the stress required for the material to produce unit elastic deformation under an external force, and is an index reflecting the material's ability to resist elastic deformation, equivalent to the stiffness in an ordinary spring. This is a measure of the material's ability to resist elastic deformation, that is, it characterizes the ease or difficulty of the material to resist elastic deformation when subjected to an external force. The low elastic modulus of magnesium alloys means that under a given external force condition, the material produces a larger elastic deformation, cannot maintain good shape stability, and is prone to excessive deformation. The elastic modulus is closely related to the binding force between atoms. The stronger the binding force, the stronger the ability of atoms to resist relative displacement, and the higher the elastic modulus of the material. The size of the elastic modulus reflects the strength of the binding force between atoms inside the material from another aspect. From a microscopic perspective, the elastic deformation of the material results from the change in the distance between atoms. Thus, it can be seen that all factors affecting the bonding strength can affect the elastic modulus of the material, such as the bonding mode, crystal structure, chemical composition, microstructure, temperature, etc. Magnesium alloys have an HCP structure, and their c-axis is much larger than the a-axis, which is an inherent property determined by their crystal structure.

[0004] However, due to differences in alloy composition, heat treatment state, cold plastic deformation, etc., the elastic modulus E value of metal materials only fluctuates by 5%. Generally speaking, the elastic modulus of metal materials is a mechanical property index that is insensitive to the microstructure. Alloying, heat treatment (fiber structure), cold plastic deformation, etc. have little effect on the elastic modulus E. At the same time, external factors such as environmental temperature and loading rate also have little impact on it. Therefore, in general engineering applications, the elastic modulus E is regarded as a constant. The elastic modulus and thermophysical properties of common materials are compared in Table 1 as follows.

[0005]

[0006] In practical applications, a quantitative relationship between stress and strain, namely Hooke's law, can be established based on the magnitude of the elastic modulus E. Within the elastic limit, stress is proportional to strain, that is, σ = Eε (where σ is stress, E is the elastic modulus, and ε is strain). This relationship provides an important basis for the analysis and calculation of the mechanical properties of materials, enabling engineers to accurately predict the deformation of materials under different loading conditions, and thus conduct reasonable structural design. That is, the deformation of structural components under the action of loads can be calculated based on parameters such as the elastic modulus of the material, so as to ensure the safety and reliability of structural components or equipment. It can be seen that in the process of material research and development and selection, the elastic modulus is an important reference index, which can be used to classify and identify materials and understand the basic performance characteristics of materials. For example, in the aerospace field, in order to reduce the weight of aircraft while ensuring structural strength, carbon fiber composite materials with high elastic modulus and low density will be preferentially selected.

[0007] Therefore, so far, even though many process measures have been taken in the preparation of magnesium alloys, and even if the tensile property UTS≥450MPa meets the requirements, the magnesium structural materials still cannot fully meet the requirements for the elastic modulus like aluminum alloy structural components, such as E≥60GPa, or even exceed 70GPa. This has become the biggest obstacle to replacing aluminum with magnesium. However, with the increasingly prominent demand for lightweight at present, for the lightweight application of magnesium alloy structural components, it is expected to replace steel, copper, aluminum, etc. with magnesium. In particular, it is hoped that magnesium alloys can comprehensively approach the performance indexes of aluminum alloys, such as tensile mechanical strength, hardness, elastic modulus E, etc. In addition, since the melting point of magnesium is only 649°C, this also limits the mechanical properties of magnesium alloys at a certain temperature. It can be seen that how to solve the deficiencies in strength, elastic modulus, and heat resistance at a certain temperature is still a thorny problem facing scientific and technological workers.

[0008] According to the Mg-Al binary phase diagram, the solid solubility of Al in Mg can reach up to 12.7 wt%, which has a strong solid solution strengthening effect and provides the possibility for precipitation strengthening. Therefore, Mg-Al series alloys have become the earliest designed cast magnesium alloys and are also the most widely used magnesium alloy system at present. However, most Mg-Al alloys need to add other alloying elements to achieve the corresponding effects, and relatively effective alloying elements are Zn, Mn, Si, and RE elements. Among them, the AZ series alloys are the most widely used magnesium alloy series at present, and the most representative grades are AZ31, AZ80, and AZ91D alloys. In addition, alloys such as AM60B and ZM5 are also widely used in the field of die-cast magnesium alloys. The second phase precipitated during the solidification and aging of Mg-Al alloys is the β-Mg 17 Al 12 phase, whose crystal structure is a bcc structure, the space group is 143m, and the lattice constant is 1.06 nm. The discontinuous precipitation of precipitates in Mg-Al alloys occurs at the grain boundaries of the alloy, and the orientation relationship is (110) β ∥(0001) α ,

[111] β ∥[1-210] α , satisfying the Burgers relationship. The β-Mg 17 Al 12 precipitates in AZ91 alloy are classified as follows: ① The β-Mg 17 Al 12 phase in the form of lamellae, which maintains the Burgers relationship with the matrix. This type of precipitate can be judged from the morphology to account for the majority of the precipitates in Mg-Al alloys; ② The prismatic β-Mg 17 Al 12 phase perpendicular to the basal plane of the magnesium alloy, and the expression of the orientation relationship is (110) β ∥(0001) α , [1-10] β ∥[0-110] α , that is, satisfying the Crawley system; ③ The prismatic γ-Mg 17 Al 12 phase with the long axis at an angle of 15° to the c-axis of the magnesium matrix, and the orientation relationship is (11-5) β ∥(0001) α ,

[110] ∥[10-10] α , that is, satisfying the Porter relationship. It can be seen that the β-Mg 17 Al 12The eutectic melting point is relatively low, resulting in difficulty in pinning grain boundaries at higher temperatures and poor creep resistance. The second and third types of precipitates in the grains are perpendicular to the basal plane of the magnesium alloy, which can effectively hinder the basal plane slip of the magnesium alloy, so they can have a strong strengthening effect. However, the first type of precipitate is parallel to the basal plane of the magnesium alloy, and its strengthening effect is relatively poor. Moreover, there are no G.P. zones and other transition phases in this type of precipitate, which is an important reason for the poor age hardening effect of Mg-Al alloys.

[0009] At present, various ceramic particles, graphene, and carbon nanotubes have begun to be applied in magnesium alloys in order to obtain magnesium matrix composites. The chemically stable Si3N4 particles at the nanoscale can be used as the reinforcement of the composite material. Since the strengthening effect obtained by the composite material depends to a large extent on the ability to transfer stress from the matrix to the stronger reinforcement phase, it is crucial to obtain a strong interface bond between the reinforcement phase and the matrix.

[0010] Patent "SW-CNTs and N-SiC p Reinforced Magnesium Alloy Workpiece and Method" (Application No.: CN201911303080.7, Authorization Publication No.: CN111057972B, Authorization Publication Date: August 06, 2021) discloses SW-CNTs and N-SiC p reinforced magnesium alloy workpieces, which have a significant improvement in elastic modulus. "GR / N-SiC p Composite Reinforced Magnesium Matrix Composite Material and Its Preparation Method" (Application No.: CN201911304582.1, Authorization Publication No.: CN111057923B, Authorization Publication Date: June 15, 2021) discloses GR / N-SiC P composite reinforced magnesium matrix composite materials. This preparation method solves the agglomeration problem of N-SiC P and has a certain effect on improving the elastic modulus. However, all of the above patents use composite material workpieces or wires prepared by plastic deformation based on rapidly solidified magnesium powder or magnesium alloy powder. However, due to the use of rapid solidification technology for powder making, the preparation process is cumbersome and the manufacturing cost is relatively high. At the same time, due to the use of plastic deformation processes such as extrusion, the obtained composite materials may be simple structural shapes such as profiles, plates, and wires, and it is difficult to realize complex structural parts for industrial applications. Therefore, the production processes of these magnesium-based or magnesium alloy-based composite materials are complex, the comprehensive cost is relatively high, and the actual forming of complex structure castings cannot be achieved.

[0011] All kinds of metal matrix composites have developed rapidly, and their application fields are also expanding, which puts forward higher requirements for the properties of composites. However, at present, the basic research on the composite mechanism and interface strengthening mechanism of magnesium matrix composites is still insufficient. Especially due to the active characteristics of magnesium, chemical reactions may occur, resulting in the decomposition and change of the particulate reinforcement phase. For example, the patent "Preparation Method of Mg2Si Reinforced Magnesium Alloy" (Application No.: CN 200910254577.4, Authorization Publication No.: CN101781720A, Authorization Publication Date: May 4, 2011) discloses a preparation method of Mg2Si reinforced magnesium alloy by in-situ self-generation through the common melting of SiO2 powder or particles and magnesium ingots. The patent "In-situ Self-generated Aluminum Nitride and Magnesium Disilicide Reinforced Magnesium Matrix Composite and Its Preparation Method" (Application No.: CN 200710047944.4, Authorization Publication No.: CN 100491566C, Authorization Publication Date: May 27, 2009) discloses a preparation method of magnesium matrix composite by pressing Si3N4 particles wrapped in aluminum foil into a molten magnesium-aluminum solution, in-situ self-generating AlN and Mg2Si, and casting and solidifying in a metal mold. The patent "Preparation Method of an In-situ Self-generated High Volume Fraction Mg2Si Reinforced Mg-Al Matrix Composite" (Application No.: CN 201410414061.2, Authorization Publication No.: CN104131190A, Authorization Publication Date: August 24, 2016) discloses a method of obtaining an in-situ self-generated Mg2Si reinforced Mg-Al matrix composite by melting pure magnesium, pure aluminum and pure silicon powder according to the component ratio, melting the master alloy ingot under argon protection by high-frequency induction heating, and then remelting the master alloy ingot to a certain temperature, holding for a period of time, and solidifying under the action of a pulsed magnetic field. The common feature of these patents is to enhance magnesium and magnesium alloys by in-situ self-generation of Mg2Si or AlN and Mg2Si in a molten magnesium or magnesium alloy solution. Its stability and preparation process need to be improved and perfected urgently, and the stability, distribution uniformity of the reinforcement phase, and the mechanical properties and corrosion resistance of the matrix also need to be further improved.

[0012] At present, according to the data comparison in Table 1, there are various problems in simply using one or two particulate phases to strengthen magnesium alloys to obtain magnesium matrix composites. The use of nano N-Si3N4 particles and even some composite strengthening particle means has emerged, and various processes are also emerging in an endless stream. So far, the casting method of composites is used the most, but the biggest and most obvious problem of this method is that the distribution and stability of the reinforcement phase, the agglomeration of reinforcement phase particles, interface problems, and defects such as pores cannot be stably controlled.

[0013] With the development of aerospace, 3C and military technologies and the increasingly urgent market demand for green lightweighting, it has become imminent to meet the needs of some specific fields through magnesium matrix composites. Only in this way can higher specific strength, modulus, hardness, dimensional stability, as well as excellent wear resistance, corrosion resistance, vibration damping performance and high-temperature performance be achieved, and better match the performance requirements. In recent years, due to the increasingly wide and mature application of the magnesium alloy semi-solid injection molding process (Thixomolding), common raw materials such as AZ80, AZ91D, AM60B, ZM5 magnesium alloys prepared by special mechanical cutting equipment, or other particles of Mg-Al series magnesium alloys, etc., are in the shape of long strips. It can be seen that complex structure castings similar to those of ordinary high-pressure die casting can be prepared by the semi-solid injection molding process. However, there are very few reports on the preparation of new magnesium-based or magnesium alloy-based processes using this new process of semi-solid injection molding (Thixomolding) in the field of magnesium alloys, and it is even rarer to prepare magnesium alloy-based materials with high elastic modulus. Summary of the Invention

[0014] The first object of the present invention is to provide a ternary composite phase-reinforced Mg-Al series magnesium alloy matrix composite material obtained by in-situ addition of nano-silicon nitride. The complex structure castings prepared from this material by the semi-solid injection molding process have high elastic modulus, high stiffness, high temperature resistance and a certain elongation rate.

[0015] The second object of the present invention is to provide a method for preparing complex structure castings by the semi-solid injection molding process. The castings prepared by this method have high elastic modulus, high stiffness, high temperature resistance and a certain elongation rate.

[0016] The first technical solution adopted by the present invention is to obtain a ternary composite phase-reinforced Mg-Al series magnesium alloy matrix composite material by in-situ addition of nano-silicon nitride, which is composed of the following raw material components by weight percentage: 2-10% of nano-silicon nitride particles, and the balance is Mg-Al series magnesium alloy particles, and the sum of the weight percentages of the above components is 100%.

[0017] The present invention is further characterized in that:

[0018] The size of the nano-silicon nitride particles is 100nm-500nm, and the nano-silicon nitride is α-type nano-silicon nitride particles; the alloy composition of the Mg-Al series magnesium alloy particles is AZ80, AZ91D, AM60B, ZM5 magnesium alloy or Mg-6%Al-1%Y alloy.

[0019] The second technical solution adopted by the present invention is a method for preparing complex structure castings by the semi-solid injection molding process, which includes the following steps:

[0020] Step 1, high-energy ball milling and powder mixing:

[0021] Weigh the following raw materials by weight percentage respectively: 2-10% of nano-silicon nitride particles, and the balance is Mg-Al series magnesium alloy particles. The sum of the above component weight percentages is 100%;

[0022] Put the weighed mixture of nano-silicon nitride particles and Mg-Al series magnesium alloy particles into a ball mill, add grinding balls, and carry out powder mixing under argon protection;

[0023] Step 2, semi-solid injection molding;

[0024] Use the powder mixture obtained in Step 1 to produce (α-Si3N4 + Mg2Si + AlN) / Mg-Al series magnesium matrix composite complex structure castings through the semi-solid injection molding production process.

[0025] The features of the present invention also lie in:

[0026] In Step 1, the size of the nano-silicon nitride particles is 100nm - 500nm; the alloy components of the Mg-Al series magnesium alloy particles are AZ80, AZ91D, AM60B, ZM5 magnesium alloy or Mg-6%Al-1%Y alloy;

[0027] The grinding balls used are zirconia balls with a diameter of Φ5mm.

[0028] In Step 1, the powder mixing time in the ball mill is 8 hours - 20 hours.

[0029] In Step 1, the weight ratio of the grinding balls to the materials is 1:(5 - 10), and the rotational speed of the ball mill is 40rpm - 80rpm.

[0030] In Step 2, the equipment used in the semi-solid injection molding production process is a semi-solid injection molding device, which specifically includes a hopper with the mixed powder material placed inside; a feeder is arranged below the hopper, and the hopper is connected to the feeder; a barrel is arranged below the feeder, and the feeder is connected to the side wall of the barrel. A nozzle is arranged at the front end of the barrel, and a screw drive unit and a high-speed injection system are sequentially connected to the tail end of the barrel. A screw shear propulsion rod is arranged inside the barrel and is driven by the screw drive unit. A heating coil is arranged on the outer wall of the barrel; a nozzle spout is also arranged at the front end of the nozzle; one end of the nozzle spout is sequentially connected to a moving mold and a stationary mold, and the inside of the moving mold and the stationary mold is connected to form a casting cavity for the complex structure casting;

[0031] In Step 2, the process of using the semi-solid injection molding device to prepare the composite complex structure casting is as follows:

[0032] The material after mixing powders in Step 1 is directly placed into the hopper of the semi-solid injection molding device. Through the feeder, the material after mixing powders directly enters the spiral shear propeller, and the material after mixing powders is spirally propelled by the rotary drive. The material after mixing powders is gradually heated by the heating coil until part of the material after mixing powders melts or the β-Mg 17 Al 12 phase melts to form a semi-solid slurry. Through the semi-solid injection molding process, the semi-solid slurry is propelled by the spiral shear propeller through the high-speed injection pressure system and is successively injected into the special mold cavity composed of the moving mold and the stationary mold at high speed through the nozzle and the nozzle orifice. The semi-solid slurry crystallizes, nucleates, and grows under high pressure in the special mold to realize the solidification process, and a complex structure casting of (α-Si3N4+Mg2Si+AlN) / Mg-Al series magnesium matrix composite is prepared.

[0033] In Step 2, the heating coil is composed of a preheating section I heating coil, a preheating section II heating coil, a heating section I heating coil, a heating section II heating coil, and a nozzle section heating coil. Among them, the preheating set temperatures of the two preheating section heating coils, namely the preheating section I heating coil and the preheating section II heating coil, are both 450°C - 550°C; the heating set temperatures of the two heating section heating coils, namely the heating section I heating coil and the heating section II heating coil, are both 550°C - 620°C, and the heating set temperature of the nozzle section heating coil is set at 580°C - 630°C at the nozzle section.

[0034] The beneficial effects of the present invention are as follows:

[0035] (1) In the material structure of the present invention, the ternary composite particle phase (α-Si3N4+Mg2Si+AlN) reinforces the Mg-Al series magnesium alloy matrix composite material. First of all, this composite material avoids the deficiencies of a single particle reinforcing phase. According to the in-situ reaction: 4[Al]+6[Mg]+Si3N4 = 4AlN+3Mg2Si, it can be seen that the ratio of AlN:Mg2Si obtained by the in-situ reaction is 4:3, and the content of the remaining unreacted or incompletely reacted N-(α-Si3N4) depends on the process of semi-solid injection molding.

[0036] (2) In the material structure of the present invention, both Mg2Si and AlN in the ternary composite particle phase are obtained by in-situ reaction. At the same time, the AlN phase and the matrix have the same hcp lattice structure. Although the Mg2Si phase has an fcc structure and is obtained by the in-situ reaction of [Mg] and [Si], the wettability between (Mg2Si + AlN) and the Mg-Al alloy semi-solid slurry system is extremely good. It can be seen that the present invention solves the interfacial problem between (Mg2Si + AlN) and the α-Mg grains of the magnesium alloy matrix, avoiding the problem of microcracks at the interface between the two. Secondly, during the semi-solid injection molding process, not only does mechanical powder mixing promote the uniform distribution of N-Si3N4, but also the rotation and propulsion of the spiral push rod cause the flipping, flow, and secondary distribution of the N-Si3N4 particle reinforcement phase that may agglomerate, which is also conducive to promoting its uniform distribution. In addition, the reaction between N-Si3N4 and [Mg] and [Al] in the semi-solid slurry also solves the agglomeration problem of the added N-Si3N4 particles, thus avoiding the problem of microscopic defects in the composite material caused by the agglomeration of N-Si3N4 particles. It can be seen that the preparation method of the present invention solves the interfacial bonding problem of the ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) and the agglomeration problem of the traditional process.

[0037] (3) In the composite material of the present invention, due to the strengthening effect of the ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) on the matrix, the elastic moduli of the three are 280 - 320 GPa, 120 GPa, and 350 GPa respectively. Therefore, the composite material of the present invention has a high elastic modulus and high stiffness, which increase with the increase in the content of the particle phase. At the same time, the melting points of the three are 1900 °C (decomposition), 1358 °C, and 2300 °C respectively. These strengthening particles can play a pinning role when the matrix bears stress and strain, and can inhibit the high-temperature deformation of the composite material. Therefore, the composite material of the present invention has relatively high high-temperature performance.

[0038] (4) The method of the present invention effectively avoids the problems of agglomeration and interfacial defects of the ternary composite particle phase (α-Si3N4 + Mg2Si + AlN), avoids the problem of microcracks at the interface between the ternary composite particle phase and the matrix, and can ensure that the composite material has a high elastic modulus and high stiffness. At the same time, since the added Si3N4 is a nano-scale particle and the residence time of the Mg-Al-based magnesium alloy semi-solid slurry in the semi-solid injection molding process is very limited, usually only 0.5 minutes to 1.5 minutes, the in-situ generated Mg2Si + AlN also does not have sufficient growth time, so it must also be of nano-scale. Thus, when the semi-solid injection slurry containing the nano-scale ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) enters the metal mold with a complex structure and under high pressure, the matrix of the Mg-Al-based magnesium alloy semi-solid slurry begins to nucleate and grow, and finally complete solidification is achieved. And these high-melting-point nano-scale ternary composite particle phases (α-Si3N4 + Mg2Si + AlN) can serve as heterogeneous nucleation cores, not only effectively reducing or eliminating the lamellar β-Mg 17 Al 12 phase in the Mg-Al-based alloy matrix, but also effectively refining the α-Mg matrix. Such a result has a very beneficial effect on the strain of the composite material. Therefore, the composite material not only has high strength but also has a certain elongation rate.

[0039] (5) The preparation method of the present invention avoids the defect problems existing in the prior art, that is, the problems of N-Si3N4 dispersion, agglomeration, wettability and density in the preparation of magnesium matrix composites by melting casting method, self-propagating high-temperature synthesis method, exothermic reaction method, direct reaction synthesis method, mechanical alloying method, powder metallurgy method and conventional reaction in-situ reinforcement, as well as a series of problems such as combustion and explosion that may occur in magnesium alloy powder. In particular, it effectively solves the interfacial problem of the ternary composite particle phase (α-Si3N4 + Mg2Si + AlN), avoids the problem of interfacial microcracks, there are no obvious interfacial bonding defects between the ternary composite particle phase and magnesium grains in the microstructure, and there are no obvious cracks; at the same time, in particular, it solves the agglomeration problem of N-Si3N4 particles, gives full play to the strengthening effect of N-Si3N4 particles on the matrix, and in-situ generates nano-scale (Mg2Si + AlN). The microstructure is very uniform, there are no large particle defects formed by obvious agglomeration of N-Si3N4 particles in the microstructure, and the N-Si3N4 particles are evenly distributed, avoiding the microdefects caused by the agglomeration of N-Si3N4 particles. These give full play to the composite strengthening effect of the ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) on the comprehensive mechanical properties and physical and chemical properties of the Mg-Al-based magnesium alloy matrix composite material.

[0040] (6) The microstructure of the (α-Si3N4 + Mg2Si + AlN) / Mg-Al-based magnesium alloy matrix composite structure prepared by the method of the present invention is fine, the grain size is only 0.5 - 20 μm, and the mechanical properties are excellent, reaching a tensile strength of ≥ 300 MPa, an elongation rate of ≥ 6%, and a density of ≤ 2.0 g / cm 3 , and the elastic modulus reaches about 55 - 75 GPa. This expands the application field of magnesium matrix composites, especially meeting the requirements of magnesium alloy matrix composites for high strength and toughness, high elastic modulus, high stiffness, and high temperature performance in some special occasions. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a schematic flow chart of the method of the present invention;

[0042] Figure 2 is a schematic structural diagram of the semi-solid injection molding device of the method of the present invention.

[0043] In the figure, 1. Composite material complex structure casting, 2. Nozzle spout, 3. Semi-solid slurry, 4. Heating zone, 5. Feeder, 6. Material after mixing powder, 7. Hopper, 8. High-speed injection molding system, 9. Rotary drive, 10. Heating coil, 10-1. Preheating section I heating coil, 10-2. Preheating section II heating coil, 10-3. Heating section I heating coil, 10-4. Heating section II heating coil, 10-5. Nozzle section heating coil, 11. Screw shear propeller, 12. Nozzle, 13. Moving mold, 14. Static mold, 15. Barrel. DETAILED DESCRIPTION OF THE INVENTION

[0044] The present invention will be described in detail below with reference to the drawings and specific embodiments.

[0045] The present invention provides a ternary composite-reinforced Mg-Al-based magnesium alloy matrix composite obtained by in-situ addition of nano-silicon nitride, which is composed of the following raw material components by weight percentage: 2 - 10% of nano-silicon nitride particles with a size of 100 - 500 nm, and the balance is Mg-Al-based magnesium alloy particles, and the sum of the weight percentages of the above components is 100%. During the semi-solid injection molding manufacturing process, part of Mg2Si and AlN are synthesized by in-situ self-generation reaction, so as to realize the ternary composite-reinforced (α-Si3N4 + Mg2Si + AlN) / Mg-Al-based magnesium alloy matrix composite. The nano-silicon nitride is the original nano-scale particles directly added, and Mg2Si + AlN are synthesized by in-situ self-generation reaction during the semi-solid injection molding manufacturing process: 4[Al] + 6[Mg] + Si3N4 = 4AlN + 3Mg2Si. Their sizes are all nano-scale particles, and the ternary composite-reinforced particles (α-Si3N4 + Mg2Si + AlN) are all nano-scale.

[0046] The size of the nano-silicon nitride particles is 100nm - 500nm, and the nano-silicon nitride is α-type nano-silicon nitride particles; the alloy composition of the Mg-Al series magnesium alloy particles is AZ80, AZ91D, AM60B, ZM5 magnesium alloy or Mg-6%Al-1%Y alloy. The Mg-Al series magnesium alloy particles are mechanically machined particles of the special Mg-Al series magnesium alloy for semi-solid injection molding.

[0047] In the ternary composite reinforcement particles, N-Si3N4 is the directly added and original particles, Mg2Si + AlN are the in-situ generated particles, and the ternary composite reinforcement particles (α-Si3N4 + Mg2Si + AlN) are all at the nano-scale. In the microstructure of the (α-Si3N4 + Mg2Si + AlN) / Mg-Al series magnesium alloy matrix composite material, there is no obvious agglomeration phenomenon of N-Si3N4 particles, and there are no obvious microcracks; Mg2Si + AlN are in-situ generated nano-particles, which have good wettability with the Mg-Al series magnesium alloy matrix, and the interface is tightly bonded without obvious microcracks.

[0048] The present invention also provides a method for preparing a composite material complex structure casting by semi-solid injection molding, which is a method for in-situ generating Mg2Si + AlN by using semi-solid injection molding (Thixomolding) technology with the addition of nano-scale N-Si3N4, and preparing a complex structure casting of (α-Si3N4 + Mg2Si + AlN) / Mg-Al series magnesium matrix composite material, as Figure 1 shown, specifically including the following steps:

[0049] Step 1, high-energy ball milling and powder mixing:

[0050] Weigh the following raw materials according to weight percentages respectively: 2 - 10% of nano-silicon nitride particles, and the balance is Mg-Al series magnesium alloy particles, and the sum of the above component weight percentages is 100%;

[0051] Put the weighed nano-silicon nitride particles and the Mg-Al series magnesium alloy particle mixture into a ball mill, and add grinding balls to carry out powder mixing under argon protection to promote the uniform coating of the nano-silicon nitride particles on the surface of the Mg-Al series magnesium alloy particles;

[0052] Step 2, semi-solid injection molding (Thixomolding);

[0053] The particles after powder mixing in Step 1, that is, the Mg-Al series magnesium alloy particles coated with nano-silicon nitride particles, are used to obtain a complex structure casting of (α-Si3N4 + Mg2Si + AlN) / Mg-Al series magnesium matrix composite material through the semi-solid injection molding production process.

[0054] In Step 1, the size of the nano-silicon nitride particles is 100 nm - 500 nm; the alloy composition of the Mg-Al series magnesium alloy particles is AZ80, AZ91D, AM60B, ZM5 magnesium alloy or Mg-6%Al-1%Y alloy;

[0055] The grinding balls used are zirconia balls with a diameter of Φ5 mm.

[0056] In Step 1, before ball milling, the nano-silicon nitride particles, Mg-Al series magnesium alloy particles and grinding balls weighed according to the component weight percentages are loaded into a special ball milling container; before loading into the special ball milling container, an argon gas pipe is inserted deep into the bottom of the container, and argon gas is introduced to exhaust the air in the container as much as possible. After loading the mixed materials, argon gas is continuously introduced for 5 minutes, and finally the argon gas pipe is taken out and the lid is covered; the container containing N-Si3N4 particles, Mg-Al series magnesium alloy particles and zirconia balls is placed in a ball mill, and the materials are mixed in the ball mill for 8 hours - 20 hours under argon protection to promote the uniform coating of N-Si3N4 particles on the surface of Mg-Al series magnesium alloy particles.

[0057] In Step 1, the weight ratio of the grinding balls to the materials (zirconia balls: the mixture of nano-silicon nitride particles and Mg-Al series magnesium alloy particles) is 1:(5 - 10), and the rotation speed of the ball mill is 40 rpm - 80 rpm.

[0058] In Step 2, the equipment used in the semi-solid injection molding production process is a semi-solid injection molding device, such as Figure 2 shown, specifically including a hopper 7, in which the mixed powder material 6 (the mixed powder material 6 is the mixed particles of nano-silicon nitride particles and Mg-Al series magnesium alloy particles after ball milling in Step 1) is placed; a feeder 5 is arranged below the hopper 7, and the hopper 7 is communicated with the feeder 5; a barrel 15 is arranged below the feeder 5, the feeder 5 is communicated with the side wall of the barrel 10, a nozzle 12 is arranged at the front end of the barrel 15, and a screw drive unit 9 and a high-speed injection system 8 are successively connected to the tail end of the barrel 15. A screw shear propulsion rod 11 is arranged in the barrel 15, and the screw shear propulsion rod 11 is driven by the screw drive unit 9. A heating coil 10 is arranged on the outer wall of the barrel 15; a nozzle spout 2 is also arranged at the front end of the nozzle 12; one end of the nozzle spout 2 is successively connected to a moving mold 13 and a stationary mold 14, and the inside of the moving mold 13 and the stationary mold 14 is communicated to form a casting cavity;

[0059] In Step 2, the process of preparing the composite material complex structure casting by the semi-solid injection molding device is as follows:

[0060] The material 6 after mixing powders in Step 1 is directly placed into the hopper 7 of the semi-solid injection molding device. Through the feeder 5, the material 6 after mixing powders directly enters the screw shear propeller 11, and the material 6 after mixing powders is spirally propelled by the rotary driver 9. The material 6 after mixing powders is gradually heated through the heating coil 10 until the material 6 after mixing powders is partially melted or the β-Mg 17 Al 12 phase melts to form a semi-solid slurry 3. Through the semi-solid injection molding process, the semi-solid slurry 3 is propelled by the screw shear propeller 11 through the high-speed injection system 8 and is successively injected into the special mold cavity composed of the moving mold 13 and the stationary mold 14 at high speed through the nozzle 12 and the nozzle orifice 2. The semi-solid slurry 3 crystallizes, nucleates, and grows under high pressure in the special mold to realize the solidification process, and a (α-Si3N4+Mg2Si+AlN) / Mg-Al series magnesium matrix composite complex structure casting 1 is prepared.

[0061] In Step 2, after the material 6 after mixing powders enters the heating zone, as the temperature gradually rises, during the gradual melting process of the β-Mg 17 Al 12 phase distributed at the grain boundaries in the microstructure, and after the semi-solid slurry 3 has been formed, the [Mg] and [Al] atoms contained in the semi-solid slurry 3 will react in-situ with the nano-scale α-Si3N4: 4[Al]+6[Mg]+Si3N4 = 4AlN+3Mg2Si, so that AlN and Mg2Si particles are in-situ generated in the semi-solid slurry 3 and have good wettability with the semi-solid slurry 3.

[0062] In Step 2, the heating coil 10 is composed of a preheating section I heating coil 10-1, a preheating section II heating coil 10-2, a heating section I heating coil 10-3, a heating section II heating coil 10-4, and a nozzle section heating coil 10-5. The heating area of the heating coil 10 is the heating zone 4. Among them, the preheating set temperatures of the two preheating section heating coils, namely the preheating section I heating coil 10-1 and the preheating section II heating coil 10-2, are both 450°C - 550°C; for the two heating section heating coils, namely the heating section I heating coil 10-3 and the heating section II heating coil 10-4, the heating set temperatures of the heating coils are both 550°C - 620°C, and the heating set temperature of the nozzle section heating coil 10-5 is set at 580°C - 630°C at the nozzle section.

[0063] In Step 2, the material 6 after mixing powders in Step 1 is in the material barrel 15. As the screw rotates, the granular material is propelled by the screw towards the nozzle. Meanwhile, two preheating section heating coils (preheating set temperature 450°C - 550°C) and two heating section heating coils (heating set temperature 550°C - 620°C) are arranged outside the material barrel 15, and another heating coil is provided in the nozzle section of the barrel (heating set temperature 580°C - 630°C) to provide heat sources for the mixed materials in the screw shear propeller 11, and the metal particles in the semi-solid slurry at the final nozzle section are in a semi-molten or molten state.

[0064] In Step 2, α-Si3N4 is particles at the nanoscale, namely N-(α-Si3N4), which is more conducive to the in-situ reaction between N-Si3N4 particles and [Al]+[Mg] in the semi-solid slurry 3 of the Mg-Al alloy system to form AlN phase and Mg2Si phase, and the finer the particle size, the more conducive to the progress of the in-situ reaction. At the same time, the AlN phase and Mg2Si phase formed by the in-situ reaction are also at the nanoscale, that is, the ternary composite particle phase (Si3N4 + Mg2Si + AlN) in the obtained composite material structure are all nano-particle strengthening phases.

[0065] In Step 2, according to the basic principles of thermodynamics, for the in-situ reaction 4[Al]+6[Mg]+Si3N4 = 4AlN+3Mg2Si, the reaction Gibbs free energy ΔG < 0 and the reaction enthalpy of formation ΔH < 0. It can be seen that the above reaction is feasible thermodynamically. From another aspect, the higher the temperature of the semi-solid slurry 3 during the semi-solid injection molding process, that is, the higher the set temperature of the nozzle section heating coil 10-5 set at the nozzle section, the more conducive to the progress of the in-situ reaction 4[Al]+6[Mg]+Si3N4 = 4AlN+3Mg2Si. That is to say, indirectly, the content of each component of the ternary composite particle phase (Si3N4 + Mg2Si + AlN) in the structure can be controlled by controlling the temperature of the semi-solid slurry at the nozzle, and further, the control of the composite strengthening effect of the composite material can be realized.

[0066] In Step 2, according to the in-situ reaction: 4[Al]+6[Mg]+Si3N4 = 4AlN+3Mg2Si, it can be known that the ratio of AlN:Mg2Si obtained by the in-situ reaction is 4:3, and the content of the remaining unreacted or incompletely reacted α-Si3N4 depends on the process of semi-solid injection molding.

[0067] During the heating and screw propulsion of the Mg-Al based magnesium alloy particles, due to the presence of lamellar β-Mg with a low melting point that has a Burgers relationship with the matrix in the matrix structure 17 Al 12Phase. During the heating process, as the temperature of the mixed material in the spiral shear propeller continuously rises, the mixed material is propelled forward by the spiral shear propeller, and the temperature of the mixed material also continuously increases. The first to melt is the low-melting-point lamellar β-Mg 17 Al 12 Phase. At this time, [Al] and [Mg] atoms must exist in the semi-molten metal liquid. The added Si3N4 may undergo the following seven reactions in the heating process and in the semi-solid slurry of the Mg-Al series magnesium alloy:

[0068] 4Al(l)+ Si3N4(s)=4AlN(s)+3Si(s) (1)

[0069] 6Mg(l)+ Si3N4(s)=2Mg3N2(s)+3Si(s) (2)

[0070] 2Al(l)+Mg3N2(s)=2AlN(s)+3Mg(l) (3)

[0071] 6Mg(l)+ Si3N4(s)=2Mg3N2(s)+3Si(l) (4)

[0072] 4Al(l)+ Si3N4(s)=4AlN(s)+3Si(l) (5)

[0073] 2Mg(l)+Si(s)=Mg2Si(s) (6)

[0074] 2Mg(l)+Si(l)=Mg2Si(s) (7)

[0075] According to the basic principles of thermodynamics, combined with relevant data, the reaction Gibbs free energy ΔG of the above seven chemical reaction equations can be calculated to be less than 0, indicating that thermodynamically, the above reactions can all occur; the reaction enthalpy of formation ΔH is less than 0, indicating that these reaction processes are all exothermic processes. Therefore, according to thermodynamic calculations, it is thermodynamically feasible to synthesize AlN reinforced phase particles by reacting Si3N4 with semi-solid slurry of Mg-Al series magnesium alloy. At the same time, the released [Si] can react with [Mg] to synthesize Mg2Si reinforced phase particles. Overall, that is, the reaction equation: 4[Al] + 6[Mg] + Si3N4 = 4AlN + 3Mg2Si can occur. At the same time, increasing the Al content in the Mg-Al series alloy can promote the formation of AlN phase + Mg2Si phase. In addition, Mg has an hcp structure (lattice constants a = 0.3202 nm, c = 0.5199 nm), and the AlN phase also has an hcp structure (lattice constants a = 0.3113 nm, c = 0.4981 nm); the Mg2Si phase has an fcc crystal structure (lattice constant a = 0.635 nm). It can be seen from the literature retrieval that with the increase of the contents of alloying elements Mn and Al, the wettability of the AlN phase + Mg2Si phase and the semi-solid slurry system of Mg alloy increases.

[0076] In step 2, Si3N4 has two crystal forms, α-Si3N4 (lattice constants a = 0.515 nm, c = 1.154 nm) and β-Si3N4 (lattice constants a = 0.515 nm, c = 0.604 nm). Their lattice constants are different, which indicates that in the c-axis direction, the atomic arrangement of the α-Si3N4 phase is more loose, which is more conducive to the progress of the reaction equation: 4[Al] + 6[Mg] + Si3N4 = 4AlN + 3Mg2Si, while the β-Si3N4 phase is more compact. Therefore, α-Si3N4 is selected in the present invention.

[0077] At the same time, it is well known that when the Si content in the magnesium alloy is 0.5wt%, Mg2Si is irregular Chinese character-shaped; when the Si content is 1.0wt%, in addition to the irregular Chinese character-shaped Mg2Si phase, polygonal block-shaped Mg2Si will also appear; when the Si content is 2.0wt%, in addition to the irregular Chinese character-shaped Mg2Si phase and polygonal block-shaped Mg2Si, a dendritic Mg2Si phase will also appear. Therefore, it is generally avoided that the Si content in magnesium alloy exceeds 1.0wt%, otherwise measures must be taken to modify and refine the Mg2Si phase. At the same time, since the temperature of the semi-solid injection forming process is lower than that of general melting and casting, even in ordinary melting and casting, α-Si3N4 particles cannot completely react with [Al] + [Mg] in the semi-solid slurry of Mg-Al alloy. There is no doubt that the addition of nano-scale silicon nitride particles is more conducive to the in-situ reaction between N-Si3N4 particles and [Al]+[Mg] in the Mg-Al alloy semi-solid slurry to form AlN phase and Mg2Si phase, and the smaller the particle size, the more conducive to the reaction. It can be seen from another aspect that the higher the temperature of the semi-solid slurry during the semi-solid injection molding process, the more conducive to the above-mentioned in-situ reaction 4[Al]+6[Mg]+Si3N4=4AlN+3Mg2Si, that is, the content of each component of the ternary composite particle phase (α-Si3N4+Mg2Si+AlN) in the organization can be indirectly controlled by controlling the temperature of the semi-solid slurry, thereby achieving a composite strengthening effect on the composite material.

[0078] The method of the present invention introduces N-(α-Si3N4) and in-situ generates a ternary composite particle phase (α-Si3N4+Mg2Si+AlN) composite-reinforced Mg-Al series magnesium alloy matrix composite material during the semi-solid injection molding process. The microstructure is fine, and the ternary reinforcing phase (α-Si3N4+Mg2Si+AlN) particles are all nanoscale. The grain size of the α-Mg matrix of the composite material is only 0.5-20 μm. It can be seen that the strain of the composite material has a very beneficial effect. This composite material avoids the deficiencies of a single particle reinforcing phase. The (Mg2Si+AlN) obtained by in-situ reaction has excellent wettability, solves the interface problem between (Mg2Si+AlN) and the α-Mg grains of the magnesium alloy matrix, avoids the problem of microcracks at the interface between the two, and solves the uniform distribution of the ternary reinforcing phase (α-Si3N4+Mg2Si+AlN), avoiding the microscopic defect problem caused by agglomeration; in addition, it is feasible to in-situ generate AlN and Mg2Si phases using N-Si3N4. Since the AlN and Mg2Si phases are in-situ generated, they have good interfacial compatibility with the matrix. The sizes of the AlN and Mg2Si phases are also nanoscale, and the AlN and Mg2Si phase particles are evenly distributed, effectively avoiding problems such as limited size of the reinforcing phase particles, poor bonding between the reinforcing phase and the matrix, and uneven distribution of the reinforcing phase when preparing composite materials by the traditional external addition method. Therefore, the prepared magnesium matrix composite material has good mechanical and physical properties. Thus, a heat-resistant Mg-Al series magnesium matrix composite material with a high elastic modulus of ternary composite reinforcement (α-Si3N4+Mg2Si+AlN) is realized. At the same time, the problem of internal pores is solved, and the occurrence of pore defects inside the casting is avoided.

[0079] Due to the extremely high elastic modulus and melting point of the ternary reinforcing phase (α-Si3N4+Mg2Si+AlN), the comprehensive mechanical properties and physical and chemical properties of the Mg-Al series magnesium alloy matrix composite material with the composite strengthening effect of the ternary composite particle phase (α-Si3N4+Mg2Si+AlN) are fully exerted. The grain size is 0.5-20 μm, and the mechanical properties of the composite structure casting are excellent, reaching a tensile strength ≥ 300 MPa, an elongation ≥ 6%, and a density ≤ 2.0 g / cm 3 , and the elastic modulus reaches 55 GPa-75 GPa. The composite material has a high elastic modulus and high stiffness, and has relatively high high-temperature performance. This expands the application field of magnesium matrix composite materials, especially meeting the requirements of magnesium alloy matrix composite materials with high strength and toughness, high elastic modulus, high stiffness, and high-temperature performance in some special occasions.

[0080] Example 1

[0081] Weigh the following raw materials by weight percentage respectively: 2% of N-(α-Si3N4) powder, and the balance is AZ80 magnesium alloy particles dedicated to semi-solid injection pressing. The sum of the weight percentages of the above components is 100%; among them, the nominal particle size of the N-(α-Si3N4) particles is 100 nm. The ball-to-material weight ratio (zirconia balls: magnesium alloy mixture) is 1:5; the diameter of the grinding balls used is Φ5 mm, and the grinding balls are ZrO2 balls. The high-energy ball mill rotates at 40 rpm, and the powder is mixed by high-energy ball milling for 8 hours under argon protection.

[0082] Put the mixed powder of AZ80 magnesium alloy particles wrapped with N-(α-Si3N4) particles into a semi-solid injection pressing production line (the semi-solid injection pressing production line uses a semi-solid injection molding device). The preheating temperatures set for both preheating sections are 450 °C, the temperatures set for both heating sections are 550 °C, and the temperature set for the nozzle section is 580 °C.

[0083] The obtained complex structure casting of the AZ80 magnesium alloy matrix composite material with composite strengthening of ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) has fine microstructure, the grain size is 0.5 - 20 μm, excellent mechanical properties, the tensile strength at room temperature is 315 MPa, the yield strength is 295 MPa, the elongation is 9.5%, and the density is 1.82 g / cm 3 , and the elastic modulus is 55 GPa. The tensile strength at 150 °C is 280 MPa, the yield strength is 272 MPa, and the elongation is 12.5%.

[0084] Example 2

[0085] Weigh the following raw materials by weight percentage respectively: 10% of N-(α-Si3N4) powder, and the balance is AZ91D magnesium alloy particles dedicated to semi-solid injection pressing. The sum of the weight percentages of the above components is 100%; among them, the nominal particle size of the N-(α-Si3N4) particles is 500 nm. The ball-to-material weight ratio (zirconia balls: magnesium alloy mixture) is 1:6; the diameter of the grinding balls used is Φ5 mm, and the grinding balls are ZrO2 balls. The high-energy ball mill rotates at 50 rpm, and the powder is mixed by high-energy ball milling for 20 hours under argon protection.

[0086] Put the mixed powder of AZ90 magnesium alloy particles wrapped with N-(α-Si3N4) particles into a semi-solid injection pressing production line (the semi-solid injection pressing production line uses a semi-solid injection molding device). The preheating temperatures set for both preheating sections are 550 °C, the temperatures set for both heating sections are 620 °C, and the temperature set for the nozzle section is 630 °C.

[0087] The obtained composite strengthened AZ91D magnesium alloy matrix composite with a ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) has a fine structure for complex structure castings, with a grain size of 0.5 - 20 μm, excellent mechanical properties, a tensile strength of 342 MPa at room temperature, a yield strength of 325 MPa, an elongation of 6%, and a density of 1.96 g / cm 3 , and an elastic modulus of 75 GPa. The tensile strength at 150 °C is 331 MPa, the yield strength is 302 MPa, and the elongation is 7.2%.

[0088] Example 3

[0089] Weigh the following raw materials by weight percentage respectively: 5% of N-(α-Si3N4) powder, and the balance is AM60B magnesium alloy particles for semi-solid injection molding. The sum of the weight percentages of the above components is 100%; among them, the nominal particle size of the N-(α-Si3N4) particles is 200 nm. The ball-to-material weight ratio (zirconia balls: magnesium alloy mixture) is 1:7; the diameter of the grinding balls used is Φ5 mm, and the grinding balls are ZrO2 balls. The high-energy ball mill rotates at 60 rpm, and the powder is mixed by high-energy ball milling for 12 hours under argon protection.

[0090] Put the mixed powder of AM60B magnesium alloy particles coated with N-(α-Si3N4) particles into a semi-solid injection production line (the semi-solid injection production line uses a semi-solid injection molding device). The preheating temperature of both preheating sections is set at 520 °C, the temperature of both heating sections is set at 600 °C, and the temperature of the nozzle section is set at 610 °C.

[0091] The obtained composite strengthened AM60B magnesium alloy matrix composite with a ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) has a fine structure for complex structure castings, with a grain size of 0.5 - 20 μm, excellent mechanical properties, a tensile strength of 336 MPa at room temperature, a yield strength of 315 MPa, an elongation of 8.2%, and a density of 1.90 g / cm 3 , and an elastic modulus of 64 GPa. The tensile strength at 150 °C is 308 MPa, the yield strength is 292 MPa, and the elongation is 9.5%.

[0092] Example 4

[0093] Weigh the following raw materials by weight percentage respectively: 8% of N-(α-Si3N4) powder, and the balance is ZM5 magnesium alloy particles for semi-solid injection molding. The sum of the weight percentages of the above components is 100%; among them, the nominal particle size of the N-(α-Si3N4) particles is 300 nm. The ball-to-material weight ratio (zirconia balls: magnesium alloy mixture) is 1:8; the diameter of the grinding balls used is Φ5 mm, and the grinding balls are ZrO2 balls. The high-energy ball mill rotates at 70 rpm, and the powder is mixed by high-energy ball milling for 16 hours under argon protection.

[0094] The mixed powder of ZM5 magnesium alloy particles coated with N-(α-Si3N4) particles is placed into a semi-solid injection molding production line (the semi-solid injection molding production line uses a semi-solid injection molding device). The preheating temperatures of the two preheating sections are both set at 530 °C, the temperatures of the two heating sections are both set at 610 °C, and the temperature of the nozzle section is set at 620 °C.

[0095] The obtained complex structure casting of the composite strengthened ZM5 magnesium alloy matrix composite material with a ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) has fine microstructure, the grain size is 0.5 - 20 μm, excellent mechanical properties, the tensile strength at room temperature is 343 MPa, the yield strength is 321 MPa, the elongation is 6.5%, and the density is 1.92 g / cm 3 , and the elastic modulus is 70 GPa. The tensile strength at 150 °C is 319 MPa, the yield strength is 292 MPa, and the elongation is 8.3%.

[0096] Example 5

[0097] Weigh the following raw materials by weight percentage respectively: 6% of N-(α-Si3N4) powder, and the balance is AM60B magnesium alloy particles dedicated for semi-solid injection molding. The sum of the weight percentages of the above components is 100%; among them, the nominal particle size of the N-(α-Si3N4) particles is 400 nm. The weight ratio of balls to materials (zirconia balls: magnesium alloy mixture) is 1:9; the diameter of the grinding balls used is Φ5 mm, and the grinding balls are ZrO2 balls. The rotational speed of the high-energy ball mill is 80 rpm, and the high-energy ball milling and powder mixing are carried out for 18 hours under argon protection.

[0098] The mixed powder of AM60B magnesium alloy particles coated with N-(α-Si3N4) particles is placed into a semi-solid injection molding production line (the semi-solid injection molding production line uses a semi-solid injection molding device). The preheating temperatures of the two preheating sections are both set at 540 °C, the temperatures of the two heating sections are both set at 610 °C, and the temperature of the nozzle section is set at 610 °C.

[0099] The obtained complex structure casting of the composite strengthened AM60B magnesium alloy matrix composite material with a ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) has fine microstructure, the grain size is 0.5 - 20 μm, excellent mechanical properties, the tensile strength at room temperature is 338 MPa, the yield strength is 319 MPa, the elongation is 8.0%, and the density is 1.90 g / cm 3 , and the elastic modulus is 66 GPa. The tensile strength at 150 °C is 304 MPa, the yield strength is 292 MPa, and the elongation is 9.3%.

[0100] Example 6

[0101] Weigh the following raw materials by weight percentage respectively: 8% of N-(α-Si3N4) powder, and the balance is Mg-6%Al-1%Y magnesium alloy particles for semi-solid injection pressing. The sum of the weight percentages of the above components is 100%; among them, the nominal particle size of the N-(α-Si3N4) particles is 200 nm. The ball-to-material weight ratio (zirconia balls: magnesium alloy mixture) is 1:10; the diameter of the grinding balls used is Φ5 mm, and the grinding balls are ZrO2 balls. The high-energy ball mill rotates at 80 rpm, and the powders are mixed by high-energy ball milling for 16 hours under argon protection.

[0102] Put the mixed powder of Mg-6%Al-1%Y magnesium alloy particles coated with N-(α-Si3N4) particles into a semi-solid injection pressing production line (the semi-solid injection pressing production line uses a semi-solid injection molding device). The preheating temperature of both preheating sections is set at 560 °C, the temperature of both heating sections is set at 620 °C, and the temperature of the nozzle section is set at 630 °C.

[0103] The obtained complex structure casting of the composite strengthened Mg-6%Al-1%Y magnesium alloy matrix composite material with the ternary composite particle phase (α-Si3N4 + Mg2Si + AlN) has a fine microstructure, the grain size is 0.5 - 20 μm, excellent mechanical properties, the tensile strength at room temperature is 365 MPa, the yield strength is 332 MPa, the elongation is 6.0%, and the density is 1.94 g / cm 3 , and the elastic modulus is 68 GPa. The tensile strength at 150 °C is 334 MPa, the yield strength is 312 MPa, and the elongation is 8.2%.

Claims

1. Add nano silicon nitride in situ to obtain a ternary composite phase reinforced Mg-Al magnesium alloy matrix composite material, characterized in that: The raw material components are composed of the following raw material components by weight percentage: 2-10% of nano silicon nitride particles and the balance of Mg-Al series magnesium alloy particles. The sum of the weight percentages of the above components is 100%.

2. The ternary composite phase reinforced Mg-Al magnesium alloy matrix composite material obtained by adding nano silicon nitride in situ according to claim 1, characterized in that: The size of the nano silicon nitride particles is 100nm-500nm, and the nano silicon nitride is α-type nano silicon nitride particles; the alloy composition of the Mg-Al series magnesium alloy particles is AZ80, AZ91D, AM60B, ZM5 magnesium alloy or Mg-6%Al-1%Y alloy.

3. A method for preparing a complex structure casting by a semi-solid injection molding process, characterized in that: The following steps are involved: Step 1, high energy ball milling powder mixing: Weigh the following raw materials according to weight percentage: 2-10% nano silicon nitride particles, the balance Mg-Al magnesium alloy particles, the sum of the weight percentages of the above components is 100%; The weighed mixture of nano silicon nitride particles and Mg-Al magnesium alloy particles is placed in a ball mill, and grinding balls are added to mix the powders under argon protection; Step 2, semi-solid injection molding; The particles obtained by mixing the powders in step 1 are subjected to a semi-solid injection molding production process to obtain a complex structure casting of a (α-Si3N4+Mg2Si+AlN) / Mg-Al magnesium-based composite material.

4. The method for preparing a complex structure casting by semi-solid injection molding process according to claim 3, characterized in that: In step 1, the size of the nano silicon nitride particles is 100nm-500nm; the alloy composition of the Mg-Al series magnesium alloy particles is AZ80, AZ91D, AM60B or ZM5 magnesium alloy; The grinding balls used are Φ5mm zirconia balls.

5. The method for preparing a complex structure casting by semi-solid injection molding process according to claim 3, characterized in that: In step 1, the mixing time in the ball mill is 8 hours to 20 hours.

6. The method for preparing a complex structure casting by semi-solid injection molding process according to claim 3, characterized in that: In step 1, the ball-to-material weight ratio of ball milling is 1:5-10, and the ball mill speed is 40rpm-80rpm.

7. The method for preparing a complex structure casting by semi-solid injection molding process according to claim 3, characterized in that: In step 2, the equipment used in the semi-solid injection molding production process is a semi-solid injection molding device, which specifically includes a hopper (7), in which the mixed powder material (6) is placed; a feeder (5) is arranged below the hopper (7), and the hopper (7) is connected to the feeder (5); a barrel (15) is arranged below the feeder (5), and the feeder (5) is connected to the side wall of the barrel (15), a nozzle (12) is arranged at the front end of the barrel (15), and a nozzle (12) is arranged at the rear end of the barrel (15) connected in sequence. A spiral drive unit (9) and a high-speed injection molding system (8); a spiral shearing push rod (11) is arranged in a barrel (15), the spiral shearing push rod (11) is driven by the spiral drive unit (9), and a heating ring (10) is arranged on the outer wall of the barrel (15); a nozzle nozzle (2) is also arranged at the front end of the nozzle (12); one end of the nozzle nozzle (2) is connected to a movable mold (13) and a static mold (14) in sequence, and the movable mold (13) and the static mold (14) are internally connected to form a casting mold cavity of a complex structure casting; In step 2, the process of preparing a composite material complex structure casting by a semi-solid injection molding device is as follows: The mixed material (6) in step 1 is directly placed in a hopper (7) of a semi-solid injection molding device, and the mixed material (6) directly enters a spiral shear propeller (11) through a feeder (5), and the mixed material (6) is spirally propelled through a rotary driver (9), and the mixed material (6) is gradually heated through a heating ring (10) until the mixed material (6) is partially melted or the β-Mg 17 Al 12 Phase melting forms a semi-solid slurry (3); through a semi-solid injection molding process, the semi-solid slurry (3) is pushed by a spiral shear propeller (11) through a high-speed injection system (8), and is sequentially injected and pressed at high speed into a special mold cavity composed of a dynamic mold (13) and a static mold (14) through a nozzle (12) and a nozzle orifice (2); the semi-solid slurry (3) crystallizes, nucleates and grows under high pressure in the special mold to realize the solidification process, and prepares a (Si3N4+Mg2Si+AlN) / Mg-Al series magnesium-based composite material complex structure casting (1).

8. The method for preparing a complex structure casting by semi-solid injection molding process according to claim 7, characterized in that: In step 2, the heating coil (10) is composed of a preheating section I heating coil (10-1), a preheating section II heating coil (10-2), a heating section I heating coil (10-3), a heating section II heating coil (10-4) and a nozzle section heating coil (10-5), and the area heated by the heating coil (10) is the heating zone (4); wherein the preheating setting temperatures of the preheating section I heating coil (10-1) and the preheating section II heating coil (10-2) are both 450°C-550°C; the heating setting temperatures of the heating section I heating coil (10-3) and the heating section II heating coil (10-4) are both 550°C-620°C, and the heating setting temperature of the nozzle section heating coil (10-5) is set at 580°C-630°C in the nozzle section.

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