Method for producing a multi-scale precipitation-strengthened magnesium alloy based on semi-solid casting

Abstract

The application is a method for preparing a multi-scale precipitation strengthening magnesium alloy based on semi-solid casting. The method comprises the following steps: (1) slurry preparation and mixing: selecting two or more magnesium alloys with different precipitation strengthening behaviors, heating and melting the magnesium alloy with strong aging precipitation into liquid state, and cooling in the furnace to form a semi-solid slurry between solid-liquid phase lines; pouring the semi-solid slurry into a screw stirring device; adding the magnesium alloy with weak aging precipitation into the screw stirring device in the form of chips, and stirring to obtain a mixed slurry; (2) rheological extrusion: pouring the mixed slurry into a preheated injection sleeve, and pushing the plunger to obtain a magnesium alloy profile; (3) solid solution and aging treatment of alloy elements for multi-scale precipitation. Through semi-solid casting, the application utilizes multi-scale precipitation of alloy elements to obtain soft and hard phases with different precipitation strengthening degrees, and in the deformation process, significant heterogeneous deformation-induced strengthening and additional work hardening are formed, thereby obtaining a high-strength and high-toughness heterogeneous magnesium alloy material.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of material preparation, and particularly relates to a method for preparing a multi-scale precipitation strengthening magnesium alloy based on semi-solid casting. BACKGROUND

[0002] Lightweight is the mainstream trend of the current manufacturing industry. Magnesium alloy gradually highlights its advantages in the fields of consumer electronics, transportation, aerospace, etc. due to its light weight, high specific strength and specific stiffness, good damping and shock absorption performance, good electromagnetic shielding performance, excellent cutting performance and green environmental protection. Compared with light metal materials such as aluminum alloy and titanium alloy, the challenge faced by the engineering application of magnesium alloy material is low absolute strength and poor plasticity. Rare earth alloying is an important way to strengthen and toughen magnesium alloy. Rare earth elements and magnesium are both hexagonal close-packed crystal structures, the atomic radius of most rare earth elements differs from magnesium by less than 15%, and the electronegativity varies greatly, so the solid solubility of rare earth elements in magnesium is high, which has the effects of solid solution strengthening and precipitation strengthening in magnesium alloy; but the aging precipitation strengthening will reduce the plasticity of magnesium alloy, thereby limiting the application and development of magnesium alloy, so how to obtain aging strengthening in magnesium alloy while considering its high plasticity and high toughness is the current hotspot in the field of magnesium alloy materials.

[0003] Y.T. Zhu et al. in the article "Heterostructured materials: superior properties from hetero-zone interaction" published in Materials Research Letters, 2021, 9: 1-31, mentioned that heterostructured materials have high strength and high toughness excellent mechanical properties that traditional homogeneous materials cannot match. This is because heterostructured materials have soft and hard zones, back stress is generated in the soft zone during deformation, front stress is generated in the hard zone, and the two zones deform cooperatively to produce hetero-zone-induced (HDI) strengthening and additional work hardening, thereby improving the yield strength; and the deformation of the hard zone to the soft zone during deformation has a constraint effect, which delays the formation of stress concentration areas such as shear bands, which helps to maintain the ductility of the material, thereby achieving coordination of strength and ductility. Therefore, heterostructured design and regulation of magnesium alloy is expected to obtain high-strength and high-toughness magnesium alloy materials.

[0004] Chinese invention patent CN109868380A "Preparation method of multi-scale precipitation strengthened magnesium alloy material", introduces the preparation of two or more different effective hardening behavior of magnesium alloy by selecting the chip, combined with plastic deformation and heat treatment, forming precipitation phase in a variety of chips, so as to obtain multi-scale precipitation strengthened magnesium alloy material, the advantages of this method are: (1) the prepared material can obtain high strength and high toughness mechanical properties, (2) large size extruded rod and plate can be prepared, which can be suitable for industrial production; but this technology also has the following disadvantages (1) in the process of chip preparation, compaction and other processes, the surface area of the chip is large, and the interface oxidation is difficult to control, (2) multi-pass plastic deformation, complex process, time-consuming and labor-consuming, and easy to appear hole and other defects.

[0005] S.S.Liu et al. in "Materials Science & Engineering A", Materials Science & Engineering A, 2021, 812: 141094, published a paper "Enhanced strength and ductility AZ91 alloy with heterogeneous lamella structure prepared by pre-aging and low-temperature extrusion", a magnesium alloy with coarse and fine crystal non-uniform lamella is prepared by pre-aging treatment and low-temperature extrusion. The synergistic effect between coarse / fine crystal lamella is beneficial to strain transmission, relieving local stress concentration and promoting the improvement of mechanical properties. The advantages of this technology are: (1) there is no interface oxidation inclusion; (2) only one plastic deformation is needed, the process flow is simple and easy to operate. However, the material prepared by this method has single organization and limited microstructure regulation, which causes limited mechanical property regulation range. SUMMARY

[0006] The purpose of the present application is to provide a method for preparing a heterogeneous magnesium alloy by introducing heterogeneous metal chips in the semi-solid slurry through solid-liquid mixed semi-solid casting, combined with specific rheological extrusion and heat treatment. Through solid-liquid mixed semi-solid casting, combined with rheological extrusion, a magnesium alloy material with multi-scale precipitation of alloying elements is prepared, the interface between soft and hard phases is continuous and complete, and there is no oxidation inclusion; through heterogeneous heat treatment, soft and hard phases with different precipitation effects can be obtained, in the deformation process, significant back stress strengthening and heterogeneous induced strengthening are formed, so as to obtain high strength and high toughness heterogeneous magnesium alloy.

[0007] The technical solution for achieving the purpose of the present application is: a method for preparing a multi-scale precipitation strengthened magnesium alloy based on semi-solid casting, comprising the following steps:

[0008] Step (1): pulping and mixing: two or more magnesium alloys with different precipitation strengthening behaviors are selected, the magnesium alloy with strong aging precipitation is put into a resistance furnace under inert gas protection, heated and melted into liquid state, and then cooled in the furnace to form semi-solid slurry between solid-liquid phase line; the semi-solid slurry is poured into a screw stirring device; the magnesium alloy with weak aging precipitation is made into chips and added into the screw stirring device for stirring, and then the slurry is stabilized after static state to obtain mixed slurry;

[0009] Step (2): rheological extrusion: the mixed slurry prepared in step (1) is poured into a preheated injection sleeve with a plunger under inert gas protection, the plunger is pushed, and the slurry passes through the mold cavity and the graphite support cavity with a cooling system in sequence to obtain a magnesium alloy profile;

[0010] Step (3): multi-scale precipitation of alloying elements: the magnesium alloy profile formed in step (2) is subjected to solid solution treatment and heterogeneous aging treatment to obtain a heterogeneous magnesium alloy material with multi-scale precipitation of alloying elements.

[0011] Further, the magnesium alloy with strong aging precipitation is any one or a mixture of any number of Mg-Gd and Mg-Y series alloys; the magnesium alloy with weak aging precipitation is AZ31, AM50, or ZK60.

[0012] Further, the mass ratio of the semi-solid slurry formed by the magnesium alloy with strong aging precipitation to the chips made of the magnesium alloy with weak aging precipitation in step (1) is 1:1 to 5:3.

[0013] Further, the temperature for heating in the resistance furnace in step (1) is 760℃±5℃, and the holding time is 30-60 min.

[0014] The stirring speed is 100-500 r / min, and the stirring time is 1-10 min.

[0015] Further, the size of the chips in each direction is not greater than 1 mm.

[0016] Further, the preheating temperature of the injection sleeve with a plunger in step (2) is 100-250℃, and the pushing speed of the plunger is 2-6 cm / s.

[0017] Further, the solid solution treatment in step (3) is specifically: the solid solution temperature is 400-550℃, and the solid solution time is 1-24 hours.

[0018] Further, the heterogeneous aging treatment in step (3) is specifically: the aging temperature is 100-250℃, and the aging time is 1-200 h.

[0019] A multi-scale precipitation strengthened magnesium alloy is prepared by the above method.

[0020] Compared with the prior art, the present application has the following advantages:

[0021] (1) The present application can flexibly select alloy types and adjust solid-liquid mixing ratios according to requirements, and has strong controllability of material structure and a large range of mechanical property control.

[0022] (2) The heterogeneous magnesium alloy of the present application has good heat resistance and strength and toughness, and the microstructure of the multi-scale precipitation of alloying elements has no obvious directionality, and is suitable for application on multi-directional force components.

[0023] (3) The present application can prepare large-size plate, block, rod and other heterogeneous magnesium alloy materials, and has simple operation and can meet the needs of industrial production.

[0024] (4) The present application can use industrial waste to make the scraps, and has low production cost and energy saving and environmental protection. BRIEF DESCRIPTION OF DRAWINGS

[0025] Fig. 1 It is a schematic diagram of pulp preparation and mixed pulp forming in the examples.

[0026] Fig. 2 It is a schematic diagram of rheological extrusion in the examples.

[0027] Fig. 3 It is a schematic diagram of solid solution treatment in the examples.

[0028] Fig. 4 It is a schematic diagram of aging treatment in the examples.

[0029] Fig. 5 It is a schematic diagram before and after precipitation; wherein (a) is a schematic diagram before multi-scale three-dimensional precipitation of alloying elements in the examples, and (b) is a schematic diagram after multi-scale precipitation of alloying elements in the examples.

[0030] Explanation of reference signs:

[0031] 1 - strong aging precipitation magnesium alloy ingot, 2 - crucible, 3 - resistance furnace, 4 - screw stirring device, 5 - weak aging precipitation magnesium alloy scrap, 6 - vacuum glove box, 7 - injection sleeve, 8 - vacuum furnace, 9 - oil bath furnace. DETAILED DESCRIPTION

[0032] The present application will be further described in detail below with reference to the accompanying drawings.

[0033] As shown in the examples below, different magnesium alloys with different precipitation strengthening effects are selected, and the following examples include three processes, including: pulp preparation and mixed pulp, rheological extrusion, and multi-scale precipitation of alloying elements, wherein the specific steps are as follows: Figs. 1-5

[0034] ​The first step is to prepare the slurry and mix the slurry: select two or more magnesium alloys with different alloying elements and precipitation strengthening effects; under the protection of oxygen and inert gas, the magnesium alloy with strong aging precipitation is heated and melted into a liquid state in a resistance furnace, and then slowly cooled to the solid-liquid phase line to form a semi-solid slurry, which is poured into a spiral stirring device; the magnesium alloy with weak aging precipitation is made into chips and added to the spiral stirring device for stirring, and then the slurry is stabilized by static state to obtain a mixed slurry;

[0035] The second step is rheological extrusion: under the protection of oxygen and inert gas, the mixed slurry is poured into a preheated injection sleeve with a plunger, the plunger is pushed at a certain speed, and the slurry passes through the mold cavity and the graphite support cavity with a cooling system in sequence to obtain a magnesium alloy profile;

[0036] The third step is to precipitate alloying elements in multiple scales: the formed magnesium alloy profile is subjected to solid solution treatment and heterogeneous aging treatment to obtain a heterogeneous magnesium alloy material with multiple scales of alloying element precipitation.

[0037] The slurry preparation is carried out under the protection of oxygen and inert gas, the magnesium alloy with strong aging precipitation is heated and melted in a resistance furnace, the resistance furnace temperature is set to 760℃±5℃, and the temperature is maintained for 30-60min to stabilize the melt, and then the melt is slowly cooled in the furnace to form a semi-solid slurry between the solid-liquid phase line, which is poured into a spiral stirring device; the magnesium alloy with weak aging precipitation is made into chips, the size of the chips in each direction is not greater than 1mm, and the chips are added to the spiral stirring device, the stirring speed is 100-500r / min, the stirring time is 1min-10min, and the slurry is stabilized by static state after stirring to obtain a mixed slurry.

[0038] The mass ratio of semi-solid slurry and chips during stirring is 1:1-5:3.

[0039] The rheological extrusion is carried out under the protection of oxygen and inert gas, the mixed slurry is poured into a preheated injection sleeve, the preheating temperature is 100-250℃, the plunger pushes the slurry at a certain speed, the pushing speed is 2cm / s-6cm / s, and the slurry passes through the mold cavity and the graphite support cavity with a cooling system in sequence to obtain a magnesium alloy profile.

[0040] The magnesium alloy profile after forming is subjected to multiple scale precipitation of alloying elements; first, the magnesium alloy profile is subjected to vacuum solid solution treatment to eliminate defects and dynamic precipitation during rheological extrusion, the solid solution temperature is selected between 400-550℃, and the solid solution time is 1-24 hours; the sample is subjected to heterogeneous aging treatment in an oil bath, the aging temperature is 100-250℃, and the aging time is 1-200h.

[0041] The strong aging precipitation magnesium alloy has a large number of precipitated phase structures after heteromorphic aging treatment, and the weak aging precipitation magnesium alloy has no precipitated phase structure or a small amount of precipitated phase structures, so that a magnesium alloy material with multi-scale alloy element precipitation is obtained.

[0042] Example 1

[0043] A magnesium alloy Mg-10Gd ingot 1 with a mass of about 400 g is placed in a crucible 2, and the whole is placed in a resistance furnace 3 under the protection of an argon atmosphere for heating and melting, the temperature of the resistance furnace is set to 755 DEG C, and the melt is kept for 40 min to stabilize, and then slowly cooled in the furnace to form a semi-solid slurry between the solid-liquid phase lines, and the slurry is poured into a spiral stirring device 4. About 100 g of AZ31 scraps 5 are added to the spiral stirring device, the thickness of the scraps is 0.5 mm to 1 mm, and the length is 0.5 mm to 1 mm, and the stirring is carried out under the protection of an argon atmosphere, the stirring speed is 500 r / min, the stirring time is 5 min, and the slurry is stabilized after the stirring is completed, and a mixed slurry is obtained.

[0044] Under the protection of an argon atmosphere in a vacuum glove box 6, the mixed slurry is poured into a preheated injection sleeve 7 with a plunger, the preheating temperature is 250 DEG C, the plunger is pushed at a certain speed, the injection pushing speed is 3 cm / s, the slurry is sequentially passed through a mold cavity and a graphite support cavity with a cooling system, and a magnesium alloy material is obtained.

[0045] A vacuum furnace 8 is used for vacuum solid solution treatment of the magnesium alloy material to eliminate deformation defects and dynamic precipitation in the deformation process, the solid solution temperature is selected to be 500 DEG C, and the solid solution time is 12 hours; an oil bath furnace 9 is used for aging treatment of the sample, the aging temperature is 200 DEG C, and the aging time is 100 h.

[0046] Example 2

[0047] (1) A magnesium alloy Mg-8Y ingot 1 with a mass of about 400 g is placed in a crucible 2, and the whole is placed in a resistance furnace 3 under the protection of an argon atmosphere for heating and melting, the temperature of the resistance furnace is set to 755 DEG C, and the melt is kept for 40 min to stabilize, and then slowly cooled in the furnace to form a semi-solid slurry between the solid-liquid phase lines, and the slurry is poured into a spiral stirring device 4. About 100 g of AM50 scraps 5 are added to the spiral stirring device, the thickness of the scraps is 0.5 mm to 1 mm, and the length is 0.5 mm to 1 mm, and the stirring is carried out under the protection of an argon atmosphere, the stirring speed is 500 r / min, the stirring time is 5 min, and the slurry is stabilized after the stirring is completed, and a mixed slurry is obtained.

[0048] (2) Under the argon atmosphere protection of the vacuum glove box 6, pour the mixed slurry into the preheated injection sleeve 7 with plunger, the preheating temperature is 250℃, the plunger is pushed at a certain speed, the injection pushing speed is 3cm / s, the slurry passes through the mold cavity and the graphite support cavity with cooling system in turn, and the magnesium alloy material is obtained.

[0049] (3) The magnesium alloy material is subjected to vacuum solid solution treatment in the vacuum furnace 8 to eliminate deformation defects and dynamic precipitation in the deformation process, the solid solution temperature is selected as 500℃, and the solid solution time is 12 hours; the sample is subjected to aging treatment in the oil bath furnace 9, the aging temperature is 200℃, and the aging time is 100h.

[0050] Example 3

[0051] (1) Put the magnesium alloy Mg-Gd and Mg-Y mixed ingot 1 with a mass of about 400g into the crucible 2, and put the whole into the electric resistance furnace 3 under the protection of argon atmosphere, heat and melt, the electric resistance furnace temperature is set to 760℃, and the temperature is kept for 60min to stabilize the melt, and then the furnace is cooled to the solid-liquid phase line to form a semi-solid slurry, pour the slurry into the spiral stirring device 4. Add about 100g of ZK60 scrap 5 into the spiral stirring device, the scrap is 0.5mm-1mm thick and 0.5mm-1mm long, stir under the protection of argon atmosphere, the stirring speed is 500r / min, the stirring time is 5min, and after stirring, the slurry is stabilized to obtain the mixed slurry.

[0052] (2) Under the argon atmosphere protection of the vacuum glove box 6, pour the mixed slurry into the preheated injection sleeve 7 with plunger, the preheating temperature is 250℃, the plunger is pushed at a certain speed, the injection pushing speed is 3cm / s, the slurry passes through the mold cavity and the graphite support cavity with cooling system in turn, and the magnesium alloy material is obtained.

[0053] (3) The magnesium alloy material is subjected to vacuum solid solution treatment in the vacuum furnace 8 to eliminate deformation defects and dynamic precipitation in the deformation process, the solid solution temperature is selected as 500℃, and the solid solution time is 12 hours; the sample is subjected to aging treatment in the oil bath furnace 9, the aging temperature is 200℃, and the aging time is 100h.

Claims

1. A method for producing a multi-scale precipitation strengthened magnesium alloy based on semi-solid casting, characterized in that, The method comprises the following steps: Step (1): slurry preparation: selecting two or more magnesium alloys with different precipitation behaviors, heating and melting the magnesium alloy with strong aging precipitation into liquid state under inert gas protection, and then cooling to semi-solid slurry between solid-liquid phase line; pouring the semi-solid slurry into a screw stirring device; crushing the magnesium alloy with weak aging precipitation and adding it into the screw stirring device for stirring, and then standing to stabilize the slurry to obtain mixed slurry; Step (2): rheological extrusion: pouring the mixed slurry prepared in step (1) into a preheated injection sleeve with a plunger under inert gas protection, pushing the plunger, and then making the slurry pass through a mold cavity and a graphite support cavity with a cooling system in sequence to obtain a magnesium alloy profile; Step (3): multi-scale precipitation of alloying elements: performing solid solution treatment and heterogeneous aging treatment on the magnesium alloy profile formed in step (2) to obtain a heterogeneous magnesium alloy material with multi-scale precipitation of alloying elements.

2. The method of claim 1, wherein, The magnesium alloy with strong aging precipitation is any one or a mixture of any several of Mg-Gd and Mg-Y series alloys; the magnesium alloy with weak aging precipitation is AZ31, AM50 or ZK60.

3. The method of claim 2, wherein, The mass ratio of the semi-solid slurry formed by the magnesium alloy with strong aging precipitation to the crushed pieces of the magnesium alloy with weak aging precipitation in step (1) is 1:1-5:

3.

4. The method of claim 3, wherein, The heating temperature in the resistance furnace in step (1) is 760℃±5℃, and the holding time is 30-60 min. The stirring speed is 100-500 r / min, and the stirring time is 1-10 min.

5. The method of claim 4, wherein, The size of the crushed pieces in each direction is not greater than 1 mm.

6. The method of claim 5, wherein, The preheating temperature of the injection sleeve with a plunger in step (2) is 100-250℃, and the pushing speed of the plunger is 2-6 cm / s.

7. The method of claim 6, wherein, The solid solution treatment in step (3) is specifically as follows: the solid solution temperature is 400-550℃, and the solid solution time is 1-24 h.

8. The method of claim 7, wherein, The heterogeneous aging treatment in step (3) is specifically as follows: the aging temperature is 100-250℃, and the aging time is 1-200 h.

9. A multi-scale precipitation-strengthened magnesium alloy, characterized by, The method is prepared by any one of claims 1-8.

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