Preparation method of high-toughness wrought magnesium alloy

Abstract

According to the preparation method of the high-strength and high-toughness wrought magnesium alloy, the high-strength and high-toughness wrought magnesium alloy is prepared through casting, heat treatment and thermal deformation by taking a grain boundary and an intragranular second phase of an as-cast alloy blank as a mechanical property regulation and control basis of a thermal deformation alloy. According to the method, the grain boundary and the intracrystalline second phase of the Mg-(3-9) Al-(0-9) Zn-(2-5) Ca-(0-1) Mn as-cast alloy blank serve as the mechanical property regulation and control basis of the wrought alloy, the high-toughness wrought magnesium alloy with the large strength and plasticity matching range is prepared through a traditional thermal deformation technology, and the performance of the wrought magnesium alloy can be regulated and controlled by controlling the grain boundary and the intracrystalline second phase; therefore, the method can be applied to different service scenes. The preparation process utilizes traditional casting, heat treatment and thermal deformation processes, is simple and easy to operate and low in cost, and broadens the commercial application of the wrought magnesium alloy.

Description

Technical Field

[0001] The invention relates to a preparation method of a high-strength and toughness deformable magnesium alloy, and particularly belongs to the technical field of magnesium alloy heat treatment and deformation processing. Background Art

[0002] Magnesium alloys are the lightest metal structural materials. However, due to their hexagonal close-packed crystal structure, they exhibit a poor balance between strength and ductility. Widely used commercial AZ-based wrought magnesium alloys generally have a yield strength of no more than 180 MPa, a tensile strength of no more than 300 MPa, and an elongation of approximately 12%. Another commercially available class of ZK-based magnesium alloys, such as ZK60 extruded bar in the T5 temper, has a yield strength of 280 MPa, a tensile strength of 350 MPa, and an elongation of 11%. These commercial magnesium alloys exhibit relatively low yield strengths. In recent years, to address the application of magnesium alloys in industries such as aviation and automotive that urgently require lightweighting, a large number of magnesium alloys with excellent strength-ductility balance have been developed. However, these alloys are primarily based on rare earth elements, significantly increasing cost and alloy density, while diminishing the intended lightweighting effect. To further expand the commercial application of magnesium alloys, it is necessary to achieve a good balance between the mechanical properties of rare earth-free magnesium alloys across a wider range of strength and ductility using traditional deformation processes to accommodate diverse service scenarios.

[0003] This invention utilizes a magnesium alloy billet with a controlled second phase to produce a high-strength and toughness wrought magnesium alloy with a wide range of strength-ductility matching using low-cost conventional processes. The billet's components by mass percentage are: Al 3-9 wt%, Ca 2-5 wt%, Zn 0-9%, Mn 0-1 wt%, with the remainder being Mg. Heat treatment is used to manipulate the billet's microstructure, resulting in a highly thermally stable second phase, the Mg-Al-Ca Laves phase, both at the grain boundaries and within the grains. The billet is then subjected to thermal deformation to produce a high-strength and toughness wrought magnesium alloy. By varying the heat treatment conditions, the size and distribution of the second phase can be manipulated, enabling the yield strength and elongation of the wrought magnesium alloy to be adjusted between 250 MPa and 450 MPa, respectively, and 2% to 30%, respectively. This allows for diverse service scenarios and expands the commercial application value of magnesium alloys. Summary of the Invention

[0004] In response to the above situation, the present invention proposes a method for preparing a high-strength and toughness wrought magnesium alloy. The method of the present invention uses the grain boundaries and intragranular second phases of the cast alloy billet as the basis for regulating the mechanical properties of the hot-deformed alloy. The method prepares the high-strength and toughness wrought magnesium alloy through casting, heat treatment, and hot deformation. The method specifically includes the following steps: Step 1: Ingredients Mg, Al, Zn, Mg-Ca master alloy and Mg-Mn master alloy are used as raw materials for batching; the mass percentages of Al, Ca, Mn and Zn are 3 wt% to 9 wt%, 2 wt% to 5 wt%, <1 wt% and < 9% respectively, and the rest is Mg; Step 2: Casting Mg, Mg-Ca master alloy and Mg-Mn master alloy are placed in a crucible of a melting furnace protected by a mixture of CO2 and SF6, heated to 720-740°C, and melted. Al and Zn preheated to 300°C are then added to the melt at the same time. After the melt is kept warm for 20-30 minutes, a cast alloy billet is cast by a melting method to obtain an alloy ingot. Step 3: Heat Treatment The alloy ingot is heat treated at 300-450°C for 10-100 hours to adjust the microstructure of the billet so that a second phase with high thermal stability is simultaneously formed at the grain boundaries and within the grains. After heat treatment, a heat-treated alloy is obtained. The volume fraction of the second phase at the grain boundaries ranges from 1 vol% to 20 vol%, and the volume fraction of the second phase within the grains ranges from 0.2 vol% to 4 vol%. Step 4: Heat Deformation The heat-treated alloy is thermally deformed at a processing temperature of 200-400°C to obtain a hot-deformed alloy; when the processing temperature is 350-400°C, 300-350°C and 200-300°C, the strain rate range of the hot deformation is 1-10s -1 , 0.01~0.1s -1 and 0.001~0.01s -1 ; Step 5: Cooling The hot-deformed alloy is air-cooled or quenched to room temperature to obtain a high-strength and tough deformed magnesium alloy with an ultrafine grain volume fraction of more than 10%, a yield strength of 250MPa~450MPa, and an elongation of 2%~30%.

[0005] The grain boundary second phases are the highly thermally stable C14-Mg2Ca second phase, the C36-(Mg, Al)2Ca second phase, the C15-Al2Ca Mg-Al-Ca Laves phase, and the Ca2Mg6Zn3 phase, and the intracrystalline second phase is the highly thermally stable C15-Al2Ca second phase.

[0006] The grain size of the ultrafine crystals is less than 0.5 μm.

[0007] The smelting method is metal mold casting or semi-continuous casting.

[0008] The thermal deformation is performed by extrusion, rolling or forging.

[0009] The quenching medium is water or oil.

[0010] The Mg-Ca master alloy is a Mg-20wt%Ca master alloy.

[0011] The Mg-Mn master alloy is a Mg-5 wt% Mn master alloy.

[0012] The beneficial effects of the present invention are as follows: The present invention creatively utilizes the grain boundaries and intragranular second phases of the Mg-(3-9)Al-(0-9)Zn-(2-5)Ca-(0-1)Mn cast alloy billet as the basis for regulating the mechanical properties of the deformed alloy. This invention utilizes conventional hot deformation processes to produce a high-strength and tough deformed magnesium alloy with a wide range of strength-plasticity matching. By controlling the grain boundaries and intragranular second phases, the properties of the deformed magnesium alloy can be regulated to achieve application in various service scenarios. The present invention's preparation process utilizes conventional casting, heat treatment, and hot deformation processes, making it simple, easy to operate, and low-cost, broadening the commercial application of deformed magnesium alloys. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 : Heat-treated alloy Mg-3Al-3Ca prepared in Example 1 of the present invention under low magnification SEM image; Figure 2 : Heat-treated alloy Mg-3Al-3Ca prepared in Example 1 of the present invention under high magnification SEM image; Figure 3 : XRD spectrum of heat-treated alloy Mg-3Al-3Ca prepared in Example 1 of the present invention; Figure 4 : TEM high-resolution image of the second phase in the grain of heat-treated alloy Mg-3Al-3Ca prepared in Example 1 of the present invention; Figure 5 : Example 1 of the present invention prepared high strength and toughness wrought magnesium alloy Mg-3Al-3Ca physical diagram; Figure 6 : OM image of high strength and toughness wrought magnesium alloy Mg-3Al-3Ca prepared in Example 1 of the present invention; Figure 7 : Room temperature tensile mechanical properties of high strength and toughness wrought magnesium alloy Mg-3Al-3Ca prepared in Example 1 of the present invention; Figure 8 : Heat-treated alloy Mg-3Al-5Zn-3Ca-0.4Mn prepared in Example 2 of the present invention at low magnification SEM image; Figure 9 : Heat-treated alloy Mg-3Al-5Zn-3Ca-0.4Mn prepared in Example 2 of the present invention at high magnification SEM image; Figure 10: Room temperature tensile mechanical properties of the high strength and toughness wrought magnesium alloy Mg-3Al-5Zn-3Ca-0.4Mn prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0014] Example 1 The preparation steps of the high-strength and toughness wrought magnesium alloy Mg-3Al-3Ca (Mg-3wt%Al-3wt%Ca) are as follows: Step 1. Ingredients: 410 g of industrial pure Mg ingot, 15 g of industrial pure Al ingot and 75 g of intermediate alloy Mg-20 wt% Ca were weighed and mixed as raw materials, totaling 500 g.

[0015] Step 2, casting: Place the prepared pure Mg ingot and Mg-20wt%Ca master alloy into a crucible of a melting furnace protected by a mixture of CO2 and SF6, heat it to 720-740°C, and after melting, add pure Al preheated to 300°C into the melt. After keeping the melt warm for 20 minutes, pour it into a metal mold to obtain a Mg-3Al-3Ca alloy ingot.

[0016] Step 3, heat treatment: The Mg-3Al-3Ca alloy ingot was held at 350°C for 40 hours to obtain a heat-treated alloy Mg-3Al-3Ca. The grain boundary secondary phases were C14-Mg2Ca and C36-(Mg, Al)2Ca, and the intragranular secondary phase was C15-Al2Ca. The grain boundary secondary phase content was approximately 10 vol%, and the intragranular secondary phase content was approximately 2 vol%.

[0017] Figure 1 and Figure 2 SEM images of heat-treated alloy Mg-3Al-3Ca at different magnifications; Figure 3 is the XRD pattern of heat-treated alloy Mg-3Al-3Ca; Figure 4 This is a high-resolution TEM image of the second phase in the grain of heat-treated Mg-3Al-3Ca alloy.

[0018] Step 4, thermal deformation: The heat-treated alloy Mg-3Al-3Ca was cut into cylinders with a diameter of 40 mm and a height of 29 mm, and the cylinders were heated at 350 °C and a strain rate of 0.1 s -1 The hot-deformed alloy Mg-3Al-3Ca rods with a length of 270 mm and a diameter of 10 mm were obtained by extrusion deformation.

[0019] Step 5, cooling: After hot deformation, quenching to room temperature, the quenching medium is water, and an extruded rod of a high-strength and tough wrought magnesium alloy Mg-3Al-3Ca is obtained, with an ultrafine crystal volume fraction of more than 10%, a yield strength of 422 MPa, a tensile strength of 434 MPa, and an elongation of 2.78%. The grain size of the ultrafine crystal is less than 0.5 μm, and the second phase at the grain boundary and the second phase within the crystal are broken into fine particles of 10 nanometers to 2 microns.

[0020] Figure 5 This is a physical picture of the high-strength and toughness wrought magnesium alloy Mg-3Al-3Ca; Figure 6 This is the OM image of the high-strength and toughness wrought magnesium alloy Mg-3Al-3Ca, where the ultrafine grains account for about 16 vol%; Figure 7 Room temperature tensile mechanical properties of high strength and toughness wrought magnesium alloy Mg-3Al-3Ca.

[0021] Example 2 The preparation steps of the high-strength and toughness wrought magnesium alloy Mg-3Al-5Zn-3Ca-0.4Mn (Mg-3Al-5Zn-3Ca-0.4Mn) are as follows: Step 1. Ingredients: 345g of industrial pure Mg ingot, 15g of industrial pure Al ingot, 25g of industrial pure Zn ingot, 75g of master alloy Mg-20wt%Ca and 40g of Mg-5wt%Mn are weighed and prepared as raw materials, totaling 500g.

[0022] Step 2, casting: Place the prepared pure Mg ingot, Mg-20wt%Ca and Mg-5wt%Mn master alloy into a crucible of a melting furnace protected by a mixture of CO2 and SF6, heat to 720-740°C, and after melting, add pure Al and pure Zn preheated to 300°C into the melt. After keeping the melt warm for 20 minutes, pour the melt into a metal mold to obtain a Mg-3Al-5Zn-3Ca-0.4Mn alloy ingot.

[0023] Step 3, heat treatment: The Mg-3Al-5Zn-3Ca-0.4Mn alloy ingot was held at 350°C for 10 hours to obtain a heat-treated alloy Mg-3Al-5Zn-3Ca-0.4Mn. The secondary phases at the grain boundaries were C15-Al2Ca and Ca2Mg6Zn3, and the secondary phase within the grains was C15-Al2Ca. The content of the secondary phase at the grain boundaries was approximately 10 vol%, and the secondary phase within the grains was approximately 0.5 vol%.

[0024] Figure 8 and Figure 9 SEM images of heat-treated alloy Mg-3Al-5Zn-3Ca-0.4Mn at different magnifications.

[0025] Step 4, thermal deformation: The heat-treated alloy Mg-3Al-5Zn-3Ca-0.4Mn billet obtained in the previous step was cut into a cylinder with a diameter of 40 mm and a height of 29 mm. The cylinder was heated at 350 °C and a strain rate of 0.1 s -1 The hot-deformed alloy Mg-3Al-5Zn-3Ca-0.4Mn rods with a length of 270 mm and a diameter of 10 mm were obtained by extrusion deformation.

[0026] Step 5, cooling: After hot deformation, quenching to room temperature, the quenching medium is water, and an extruded rod of a high-strength and tough wrought magnesium alloy Mg-3Al-5Zn-3Ca-0.4Mn is obtained, with an ultrafine crystal volume fraction of about 5%, a yield strength of 234 MPa, a tensile strength of 318 MPa, and an elongation of 14.2%. The grain size of the ultrafine crystal is less than 0.5 μm, and the second phase at the grain boundary and the second phase within the crystal are broken into fine particles of 10 nanometers to 3 microns.

[0027] Figure 10 Room temperature tensile mechanical properties of high strength and toughness wrought magnesium alloy Mg-3Al-5Zn-3Ca-0.4Mn.

Claims

1. A method for preparing a high-strength and high-toughness wrought magnesium alloy, characterized by: The method uses the grain boundaries and intragranular second phases of the as-cast alloy billet as the basis for controlling the mechanical properties of the hot-deformed alloy, and prepares a high-strength and toughness wrought magnesium alloy through casting, heat treatment, and hot deformation. The method specifically includes the following steps: Step 1: Ingredients Mg, Al, Zn, Mg-Ca master alloy and Mg-Mn master alloy are used as raw materials for preparation; the mass percentages of Al, Ca, Mn and Zn are 3 wt% to 9 wt%, 2 wt% to 5 wt%, <1 wt% and < 9% respectively, and the rest is Mg; Step 2: Casting Mg, Mg-Ca master alloy and Mg-Mn master alloy are placed in a crucible of a melting furnace protected by a mixture of CO2 and SF6, heated to 720-740°C, and melted. Al and Zn preheated to 300°C are then added to the melt at the same time. After the melt is kept warm for 20-30 minutes, a cast alloy billet is cast by a melting method to obtain an alloy ingot. Step 3: Heat Treatment The alloy ingot is heat treated at 300-450°C for 10-100 hours to adjust the microstructure of the billet so that a second phase with high thermal stability is simultaneously formed at the grain boundaries and within the grains. After heat treatment, a heat-treated alloy is obtained. The volume fraction of the second phase at the grain boundaries ranges from 1 vol% to 20 vol%, and the volume fraction of the second phase within the grains ranges from 0.2 vol% to 4 vol%. Step 4: Heat Deformation The heat-treated alloy is thermally deformed at a processing temperature of 200-400°C to obtain a hot-deformed alloy; when the processing temperature is 350-400°C, 300-350°C and 200-300°C, the strain rate range of the hot deformation is 1-10s -1 , 0.01~0.1s -1 and 0.001~0.01s -1 ; Step 5: Cooling The hot-deformed alloy is air-cooled or quenched to room temperature to obtain a high-strength and tough deformed magnesium alloy with an ultrafine grain volume fraction of more than 10%, a yield strength of 250MPa~450MPa, and an elongation of 2%~30%.

2. The method for preparing a high-strength and toughness wrought magnesium alloy according to claim 1, wherein: The grain boundary second phases are the highly thermally stable C14-Mg2Ca second phase, the C36-(Mg, Al)2Ca second phase, the C15-Al2Ca Mg-Al-Ca Laves phase, and the Ca2Mg6Zn3 phase, and the intracrystalline second phase is the highly thermally stable C15-Al2Ca second phase.

3. The method for preparing a high-strength and toughness wrought magnesium alloy according to claim 1, wherein: The grain size of the ultrafine crystals is less than 0.5 μm.

4. The method for preparing a high-strength and toughness wrought magnesium alloy according to claim 1, wherein: The smelting method is metal mold casting or semi-continuous casting.

5. The method for preparing a high-strength and toughness wrought magnesium alloy according to claim 1, wherein: The thermal deformation is performed by extrusion, rolling or forging.

6. The method for preparing a high-strength and toughness wrought magnesium alloy according to claim 1, wherein: The quenching medium is water or oil.

7. The method for preparing a high-strength and toughness wrought magnesium alloy according to claim 1, wherein: The Mg-Ca master alloy is a Mg-20wt%Ca master alloy.

8. The method for preparing a high-strength and toughness wrought magnesium alloy according to claim 1, wherein: The Mg-Mn master alloy is a Mg-5 wt% Mn master alloy.

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