High strength cast magnesium alloy and preparation method thereof

Abstract

A high strength cast magnesium alloy, relating to the technical field of magnesium alloy material preparation. The composition and mass percentage of cast magnesium alloy are: Zn 7.0%, Al 3.0%˜5.0%, Mn 0.3%˜0.5%, RE 0.5%˜1%, the total amount of unavoidable impurities is less than or equal to 0.04%, and the allowance is Mg, wherein the RE includes La and Ce, La and Ce account for 35% and 65% of the total amount of RE, respectively. Among them, Mn, La and Ce are added in the form of Mg-5 wt. % Mn, Mg-30 wt. % La and Mg-30 wt. % Ce intermediate alloys respectively. Then it is prepared through battering, melting, melt purification, pouring and heat treatment. By adding RE, the alloy melt can be purified, and the corrosion resistance and casting performance of the alloy can be increased. The tensile strength of the alloy is 300 MPa˜314 MPa, the elongation is 7%-13%, and the light rare earth content is low, the raw material and processing cost is low, and it is easy to realize mass production.

Claims

1. A high strength cast magnesium alloy, is characterized in that the composition and mass percentage of the cast magnesium alloy are: Zn 7.0%, Al 3.0%˜5.0%, Mn 0.3%˜0.5, RE 0.5%˜1%, the total amount of inevitable impurities is less than or equal to 0.04%, and the balance is Mg, wherein the RE include La and Ce, which accounte for 35% and 65% of the total added RE, respectively, among them, Mn, La and Ce are added in the form of Mg-5 wt. % Mn, Mg-30 wt. % La and Mg-30 wt. % Ce intermediate alloys respectively.

2. The method for preparing a high strength cast magnesium alloy according to claim 1, comprising the steps of: {circle around (1)} Batching: Zn, Al, Mg-5 wt. % Mn, Mg-30 wt. % La, Mg-30 wt. % Ce and Mg are batched according to mass percentage and then dried; {circle around (2)} Smelting: firstly putting the Al, Mg-5 wt. % Mn and Mg into a iron crucible, then melted using a resistance furnace, the melting temperature is controlled between 740° C. and 760° C. and a gas mixture is used as protective gas during melting, after all the metal in the crucible has been melted, Zn, Mg-30 wt. % La and Mg-30 wt. % Ce are finally added thereto, the initial alloy melt is obtained after 15˜20 mins of smelting; {circle around (3)} Melt purification: RJ-6 refining agent is used to refine the alloy melt obtained in step {circle around (2)} in which the amount of RJ-6 refining agent is 1˜2% of the weight of the total furnace charge, during the refining process, it needs to be stirred for 3˜5 min, after refining, the alloy melt is stirred and slagged, and then the temperature is adjusted to 720˜740° C. and the heat is kept for 20˜30 min; {circle around (4)} Casting: the alloy melt obtained in step {circle around (3)} is poured into the mould and is demoulded after 5˜10 mins of pouring, and is cooled to room temperature in air to obtain the aloy ingots; {circle around (5)} Heat treatment: the alloy ingots obtained in step {circle around (4)} are subjected to primary solid solution treatment at 350° C. for 40 h; secondary solid solution treatment at 370° C. for 8 h and finally quenched in water at a temperature of 10˜20° C.; after the solid solution treatment, the high strength cast magnesium alloy is obtained by preaging at for 24 h, then at 175° C. for 2 h, and finally quenching in water at a temperature of 10˜20° C.

3. The method of claim 2, wherein Zn, Al, Mg-5 wt. % Mn, Mg-30 wt. % La, Mg-30 wt. % Ce and Mg should be polished in advance in step {circle around (1)}.

4. The method of claim 2, wherein the gas mixture in step {circle around (2)} is CO.sub.2 and SF.sub.6 in a volume ratio of 99%:1%.

5. The method of claim 2, wherein the RJ-6 refining agent in step {circle around (3)} needs to be dried at 200˜250° C. for 30 mins before using.

6. The method of claim 2, wherein said mould in step {circle around (4)} is a permanent metal mould and needs to be preheated at 200° C. for 30˜60 min.

Description

ILLUSTRATIONS

[0028] FIG. 1 is a comparison diagram of tensile strength between embodiments of the invention and proportions.

[0029] FIG. 2 shows the relationship between phase transition and temperature in embodiment 1 during solidification.

[0030] FIG. 3 shows the relationship between phase transition and temperature in embodiment 2 during solidification.

[0031] FIG. 4 shows the relationship between phase transition and temperature in embodiment 3 during solidification.

[0032] FIG. 5 shows the solidification temperature variation in the fragile region of magnesium alloy in embodiments 1-3.

[0033] FIG. 6 is a partial enlargement of FIG. 5.

SPECIFIC IMPLEMENTATION METHODS

[0034] In order to make the technical means, creation features, purpose and function of the invention more clear and easy to understand, the invention is further elaborated in combination with the specific implementation mode:

Embodiment 1

[0035] {circle around (1)} Batching: preparing Zn 7.0%, Al 3.0%, Mn 0.4%, RE 1.0%, total impurities less than 0.04%, and the balance is Mg, in which RE includes La and Ce, La and Ce account for 35% and 65% of the total RE addition, respectively. La and Ce are added in the form of Mg-30 wt. % La and Mg-30 wt. % Ce intermediate alloys. The oxide film on the surface of the raw material is sanded off and then dried. [0036] {circle around (2)} Al, Mg-5 wt. % Mn and Mg are first put into a clean iron crucible of the resistance furnace, the melting temperature is controlled at 740° C., and the volume ratio of 99%:1% CO.sub.2 and SF.sub.6 are used as protective gases. After all the metals in the crucible were melted, Zn, Mg-30 wt. % La and Mg-30 wt. % Ce were added at last. An initial alloy melt is obtained after 20 minutes of melting. [0037] {circle around (3)} Melt purification: After obtaining preliminary alloy melt, RJ-6 refining agent is used to purify and refine the alloy melt. The amount of refining agent is 1% of the weight of the total charge, and the refining agent needs to be dried at 200° C. for 30 min. During refining, the alloy melt is evenly stirred with a slotted spoon to ensure fully contact between the refining agent and the alloy melt. The stirring time is 5 mins. After refining, the alloy melt was treated by stirring and scraping, and then the temperature was set to 720° C. and kept warm for 30 min before pouring. [0038] {circle around (4)} Casting: The mold is permanent metal mold. In order to improve the fluidity of melt and ensure the filling ability, the mold needs to be preheated at 200° C. for 60 min. After 10 min of pouring, the mold is demoulded and cooled to room temperature in the air to get the final alloy ingot. [0039] {circle around (5)} Heat treatment: The alloy obtained in step is treated at 350° C. for 40 h for primary solid solution, 370° C. for 8 h for secondary solid solution, and quenched in water with a water temperature of about 10° C. After the completion of solid solution treatment, pre-aging at 75° C. for 24 h, and finally quenching at 175° C. for 2 h in water with a water temperature of about 10° C. High strength cast magnesium alloy is obtained.

Embodiment 2

[0040] {circle around (1)} Batching: preparing Zn 7.0%, Al 4.0%, Mn 0.3%, RE 0.8%, total impurities less than 0.04%, and the balance is Mg, in which RE includes La and Ce, La and Ce account for 35% and 65% of the total RE addition, respectively. La and Ce are added in the form of Mg-30 wt. % La and Mg-30 wt. % Ce intermediate alloys. The oxide film on the surface of the raw material is sanded off and then dried. [0041] {circle around (2)} Al, Mg-5 wt. % Mn and Mg are first put into a clean iron crucible of the resistance furnace, the melting temperature is controlled at 750° C., and the volume ratio of 99%:1% CO.sub.2 and SF.sub.6 are used as protective gases; After all the metals in the crucible were melted, Zn, Mg-30 wt. % La and Mg-30 wt. % Ce were added at last. An initial alloy melt is obtained after 18 minutes of melting. [0042] {circle around (3)} Melt purification: After obtaining preliminary alloy melt, RJ-6 refining agent is used to purify and refine the alloy melt. The amount of refining agent is 1.5% of the weight of the total charge, and the refining agent needs to be dried at 230° C. for 30 min. During refining, the alloy melt is evenly stirred with a slotted spoon to ensure full contact between the refining agent and the alloy melt. The stirring time is 4 min. After refining, the alloy melt was treated by stirring and scraping, and then the temperature was set to 730° C. and kept warm for 25 min before pouring. [0043] {circle around (4)} Casting: The mold is permanent metal mold. In order to improve the fluidity of melt and ensure the filling ability, the mold needs to be preheated at 200° C. for 40 min. After 7 min of pouring, the mold is demoulded and cooled to room temperature in the air to get the final alloy ingot. [0044] {circle around (5)} Heat treatment: The alloy obtained in step {circle around (4)} is treated at 350° C. for 40 h for primary solid solution, 370° C. for 8 h for secondary solid solution, and quenched in water with a water temperature of about 15° C. After the completion of solid solution treatment, pre-aging at 75° C. for 24 h, and finally quenching at 175° C. for 2 h in water with a water temperature of about 15° C. High strength cast magnesium alloy is obtained.

Embodiment 3

[0045] {circle around (1)} Batching: preparing Zn 7.0%, Al 5.0%, Mn 0.5%, RE 0.5%, total impurities less than 0.04%, and the balance is Mg, in which RE includes La and Ce, La and Ce account for 35% and 65% of the total RE addition, respectively. La and Ce are added in the form of Mg-30 wt. % La and Mg-30 wt. % Ce intermediate alloys. The oxide film on the surface of the raw material is sanded off and then dried. [0046] {circle around (2)} Al, Mg-5 wt. % Mn and Mg are first put into a clean iron crucible of the resistance furnace, the melting temperature is controlled at 760° C., and the volume ratio of 99%:1% CO.sub.2 and SF.sub.6 are used as protective gases; After all the metals in the crucible were melted, Zn, Mg-30 wt. % La and Mg-30 wt. % Ce were added at last. An initial alloy melt is obtained after 15 minutes of melting. [0047] {circle around (3)} Melt purification: After obtaining preliminary alloy melt, RJ-6 refining agent is used to purify and refine the alloy melt. The amount of refining agent is 2% of the weight of the total charge, and the refining agent needs to be dried at 250° C. for 30 min. During refining, the alloy melt is evenly stirred with a slotted spoon to ensure full contact between the refining agent and the alloy melt. The stirring time is 3 min. After refining, the alloy melt was treated by stirring and scraping, and then the temperature was set to 740° C. and kept warm for 20 min before pouring. [0048] {circle around (4)} Casting: The mold is permanent metal mold. In order to improve the fluidity of melt and ensure the filling ability, the mold needs to be preheated at 200° C. for 30 min. After 5 min of pouring, the mold is demoulded and cooled to room temperature in the air to get the final alloy ingot. [0049] {circle around (5)} Heat treatment: The alloy obtained in step {circle around (4)} is treated at 350° C. for 40 h for primary solid solution, 370° C. for 8 h for secondary solid solution, and quenched in water with a water temperature of about 20° C. After the completion of solid solution treatment, pre-aging at 75° C. for 24 h, and finally quenching at 175° C. for 2 h in water with a water temperature of about 20° C. High strength cast magnesium alloy is obtained.

Comparative Example 1

[0050] {circle around (1)} Batching: preparing Zn 7.0%, Al 3.0%, Mn 0.4%, total impurities less than 0.04%, and the balance is Mg. The oxide film on the surface of the raw material is sanded off and then dried. [0051] {circle around (2)} Al, Mg-5 wt. % Mn and Mg are first put into a clean iron crucible of the resistance furnace, the melting temperature is controlled at 740° C., and the volume ratio of 99%:1% CO.sub.2 and SF.sub.6 are used as protective gases; After all the metals in the crucible were melted, Zn was added at last. After 20 min of melting, the initial alloy melt was obtained. [0052] {circle around (3)} Melt purification: After obtaining preliminary alloy melt, RJ-6 refining agent is used to purify and refine the alloy melt. The amount of refining agent is 1% of the weight of the total charge, and the refining agent needs to be dried at 200° C. for 30 min. During refining, the alloy melt is evenly stirred with a slotted spoon to ensure full contact between the refining agent and the alloy melt. The stirring time is 5 min. After refining, the alloy melt was treated by stirring and scraping, and then the temperature was set to 720° C. and kept warm for 30 min before pouring. [0053] {circle around (4)} Casting: The mold is permanent metal mold. In order to improve the fluidity of melt and ensure the filling ability, the mold needs to be preheated at 200° C. for 60 min. After 10 min of pouring, the mold is demoulded and cooled to room temperature in the air to get the final alloy ingot. [0054] {circle around (5)} Heat treatment: The alloy obtained in step {circle around (4)} is treated at 350° C. for 40 h for primary solid solution, 370° C. for 8 h for secondary solid solution, and quenched in water with a water temperature of about 10° C. After the completion of solid solution treatment, pre-aging at 75° C. for 24 h, and finally quenching at 175° C. for 2 h in water with a water temperature of about 10° C. High strength cast magnesium alloy is obtained.

Comparative Example 2

[0055] {circle around (1)} Batching: preparing Zn 7.0%, Al 5.0%, Mn 0.5%, total impurities less than 0.04%, and the balance is Mg. The oxide film on the surface of the raw material is sanded off and then dried. [0056] {circle around (2)} Al, Mg-5 wt. % Mn and Mg are first put into a clean iron crucible of the resistance furnace, the melting temperature is controlled at 760° C., and the volume ratio of 99%:1% CO.sub.2 and SF.sub.6 are used as protective gases; After all the metals in the crucible were melted, Zn was added at last. After 15 min of melting, the initial alloy melt was obtained. [0057] {circle around (3)} Melt purification: After obtaining preliminary alloy melt, RJ-6 refining agent is used to purify and refine the alloy melt. The amount of refining agent is 2% of the weight of the total charge, and the refining agent needs to be dried at 250° C. for 30 min. During refining, the alloy melt is evenly stirred with a slotted spoon to ensure full contact between the refining agent and the alloy melt. The stirring time is 3 min. After refining, the alloy melt was treated by stirring and scraping, and then the temperature was set to 740° C. and kept warm for 20 min before pouring. [0058] {circle around (4)} Casting: The mold is permanent metal mold. In order to improve the fluidity of melt and ensure the filling ability, the mold needs to be preheated at 200° C. for 30 min. After 5 min of pouring, the mold is demoulded and cooled to room temperature in the air to get the final alloy ingot. [0059] {circle around (5)} Heat treatment: The alloy obtained in step {circle around (4)} is treated at 350° C. for 40 h for primary solid solution, 370° C. for 8 h for secondary solid solution, and quenched in water with a water temperature of about 20° C. After the completion of solid solution treatment, pre-aging at 75° C. for 24 h, and finally quenching at 175° C. for 2 h in water with a water temperature of about 20° C. High strength cast magnesium alloy is obtained.

[0060] The tensile strength and elongation of the alloy obtained by each embodiment and ratio are shown in the table 1 below:

TABLE-US-00001 Age-state performance Elongaiton Solution treatment Aging treatment UTS (MPa) (%) Embodiment 1 350° C./40 h + 370° C./8 h 75° C./24 h + 175° C./2 h 308 ± 3 MPa 10 ± 0.5% Embodiment 2 350° C./40 h + 370° C./8 h 75° C./24 h + 175° C./2 h 314 ± 3 MPa 13 ± 0.5% Embodiment 3 350° C./40 h + 370° C./8 h 75° C./24 h + 175° C./2 h 312 ± 3 MPa 12 ± 0.5% Comparative 350° C./40 h + 370° C./8 h 75° C./24 h + 175° C./2 h 290 ± 3 MPa  9 ± 0.5% example 1 Comparative 350° C./40 h + 370° C./8 h 75° C./24 h + 175° C./2 h 298 ± 3 MPa  7 ± 0.5% example 2

[0061] It can be seen from FIG. 1 and the above table in Example 1 and Comparative example 1 and Example 3 and Comparative example 2 that the cast grains of magnesium alloy are obviously refined by mixing and adding two light rare earth elements, La and Ce. Moreover, the addition of rare earth elements can combine with Al element in the alloy to form Al—Re strengthening phase, which can improve the creep property of the alloy. The growth of other second phases is inhibited, and the second phase is distributed in fine dispersion state, which plays the role of dispersion strengthening, and finally improves the strength and plasticity of magnesium alloy.

[0062] As shown in FIG. 2-4, the relationship between the phase transition and temperature during solidification of the alloy in Embodiment 1-3 is clearly demonstrated. It can be seen from the figure that the formed phases mainly include Hcp, Al.sub.4Mn, AlMgZn-phi, RE.sub.3Al.sub.11, Ce.sub.2Mg.sub.53Zn.sub.45 and La.sub.5Mg.sub.42Zn.sub.53 phases in the late solidification period. It can be found that after adding RE, in addition to Hcp, Al4Mn and AlMgZn-phi phases, intermetallic compound phases containing rare earth elements, such as RE.sub.3Al11, are formed in the alloy. According to the analysis, the precipitation of rare earth phase can effectively inhibit the growth of grain, resulting in the effect of fine crystal strengthening.

[0063] The fraction of AlMgZn-phi phase increases with the increase of Al content. Moreover, it can be observed from the phase diagram that the second phase containing rare earth is formed before the ternary AlMgZn-phi with larger volume fraction, which is conducive to preventing the growth of the ternary AlMgZn-phi and continuous precipitation, thus refining the ternary AlMgZn-phi, improving the alloy strength and improving the plasticity of the alloy.

[0064] At the same time, the hot cracking tendency of magnesium alloys can be changed by adding rare earth elements La and Ce. As shown in FIGS. 5-6, Pandat software is used to calculate the solidification curve of the alloy of the present invention, and commercial magnesium alloy AZ91 is taken as a reference object. The results are shown in the table 2 below:

TABLE-US-00002 susceptible Temperature difference of freezing range susceptible freezing range alloys (f.sub.L: 0.1-0.01) (ΔTc/° C.) AZ91 426.4-396.sup.  30.4 Embodiment 1 351.6-336.7 14.9 Embodiment 2 357.4-339.3 18.1 Embodiment 3 362.2-339.4 22.8

[0065] As can be seen from the above table 2, the temperature difference (ΔTc) in the susceptible freezing range of embodiments 1-3 is smaller than AZ91 alloy, so the hot cracking tendency of the magnesium alloy of the present invention is smaller.

[0066] In addition to the composition factors of magnesium alloys, solidification conditions are also an important factor affecting the hot cracking tendency of alloys. Solidification conditions include casting temperature, mold temperature, cooling rate and other parameters. In the present invention, the solidification conditions are optimized through literature research and experiment, so as to effectively reduce the hot cracking tendency of magnesium alloy in the casting process.

[0067] Finally, the above embodiments are used only to illustrate the technical scheme of the invention and not to restrict it. Although the invention is described in detail with reference to the better embodiments, ordinary technicians in the field should understand that the technical scheme of the invention may be modified or equivalent replaced without deviating from the purpose and scope of the technical scheme of the invention. They shall be covered by the claims of the invention.