KR20080069380A - Mg alloy for casting - Google Patents

Abstract

An Mg alloy for casting is provided to distribute fine crystal grains whose shapes are controlled so as to limit crystal grain boundary fracture and to improve thermal features. An Mg alloy for casting includes aluminum of 3 to 11 wt%, silicon of 1 to 4 wt%, tin of 1 to 9 wt%, 0.1 to 1 wt% of at least one of antimony, phosphor, and calcium, the remaining magnesium, and unavoidable impurities. Heat resisting features of the Mg alloy for casting is improved at 100 to 150°C.

Description

Magnesium alloy for casting {Mg alloy for casting}

1 is a fluid test specimen of magnesium molten metal according to the aluminum content,

  (A) is a specimen of Mg-3wt% Al-0.3wt% Zn alloy,

  (B) is a specimen of Mg-5wt% Al-0.3wt% Zn alloy,

  (C) is a specimen of Mg-7wt% Al-0.3wt% Zn alloy,

  (D) is a specimen of Mg-9wt% Al-0.3wt% Zn alloy,

  (E) is a specimen of Mg-11wt% Al-0.3wt% Zn alloy,

Figure 2 shows the magnesium alloy structure according to the aluminum content,

  (A) is a structure photograph of Mg-3wt% Al-0.3wt% Zn alloy,

  (B) is a structure photograph of Mg-5wt% Al-0.3wt% Zn alloy,

  (C) is a structure photograph of the Mg-7wt% Al-0.3wt% Zn alloy,

  (D) is a structure photograph of the Mg-9wt% Al-0.3wt% Zn alloy,

  (E) is a structure photograph of the Mg-11wt% Al-0.3wt% Zn alloy.

Figure 3 shows the behavior of the alloying elements added to the Mg ₂ Si phase crystallized in the structure of the present invention magnesium alloy,

  (A) is a structure photograph of Mg-5wt% Al-2wt% Si alloy,

  (B) is a structure photograph of Mg-5wt% Al-2wt% Si-0.2wt% Ca alloy,

  (C) is a structure photograph of the Mg-5wt% Al-2wt% Si-0.2wt% P alloy,

  (D) is a structure photograph of Mg-5wt% Al-2wt% Si-0.2wt% Sb alloy,

  (E) is a photograph of the structure of Mg-5wt% Al-2wt% Si-0.2wt% Sr alloy.

Figure 4 shows the present invention magnesium alloy structure,

  (A) is a photo of Mg-5wt% Al-2wt% Si-3wt% Sn-0.2wt% Sr alloy

  (B) is a photo of Mg-5wt% Al-2wt% Si-6wt% Sn-0.2wt% Sr alloy.

Figure 5 as showing the present invention magnesium alloy structure

  (A) is an image of Mg-5wt% Al-2wt% Si-6wt% Sn-0.2wt% Sr alloy,

  (B) is Energy Dispersive Spectroscopy (EDS) spectrum.

6 shows the present invention magnesium alloy structure

  (A) is a photograph of the casting structure of Mg-5wt% Al-2wt% Si-3wt% Sn-0.2wt% Sr alloy,

  (B) is a photograph of the casting structure of Mg-5wt% Al-2wt% Si-6wt% Sn-0.2wt% Sr alloy.

7 shows the present invention magnesium alloy structure

  (A) is a structure photograph after T4 heat treatment of Mg-5wt% Al-2wt% Si-3wt% Sn-0.2wt% Sr alloy,

  (B) is a structure photograph after T4 heat treatment of Mg-5wt% Al-2wt% Si-6wt% Sn-0.2wt% Sr alloy.

6 is a strain-time graph of magnesium alloys under conditions of 150 ° C. and 65 MPa.

7 is a strain-time graph of magnesium alloys under conditions of 150 ° C. and 30 MPa.

7 is a strain-time graph of magnesium alloys under the conditions of 120 ° C. and 65 MPa.

8 is a strain-time graph of magnesium alloys under the conditions of 120 ° C. and 65 MPa.

According to the present invention, aluminum, silicon, tin, strontium, antimony, phosphorus, calcium, and the like are appropriately added to magnesium, thereby strengthening the magnesium matrix and retarding the breakdown of grain boundaries, thereby improving the thermal properties and high temperature strength of the magnesium alloy. It will tub on a quiet magnesium alloy.

Crude oil, which has recently risen sharply in price, is expected to continue to rise in the future, and as interest in energy saving is increasing, the automotive sector is no exception.

In addition, due to the environmental pollution problem caused by automobile exhaust gas, various countries around the world are making efforts to strengthen environmental regulations.

In particular, the exhaust gas emitted from more than 500 million automobiles operating around the world has a serious environmental pollution problem. Cars have developed a lot as a means of transportation, which is the most basic element of modern civilization. Modern civilizations without cars cannot exist, but emissions from cars have a profound impact on global warming, and are one of the main causes of air pollution, and regulations are being implemented or prepared to reduce vehicle emissions. .

Accordingly, car makers are developing automobiles using electricity, fuel cells, hydrogen, etc. as energy sources, or hybrid vehicles using fossil fuels and electricity in parallel, and some of them are currently on the market.

However, as described above, a vehicle with no pollution or a very small amount of vehicles still needs much time to become common. The most effective method as an intermediate step is the use efficiency of fuel consumed in driving with the reduction of carbon dioxide emission. It can be said that the weight of the car to increase the weight.

In other words, various studies have been attempted to improve the fuel efficiency of automobiles and to prepare for exhaust gas regulations.

In general, the weight reduction of automotive parts includes a method using low-weight lightweight materials and a volume reduction with high strength or high modulus modulus materials. In the former case, aluminum alloys, magnesium alloys, engineering plastics, etc. Although it is being used, the research on magnesium alloys has been actively conducted since magnesium alloys, which are not only lighter than aluminum alloys, but also easier to recycle than plastics, are recognized as future automotive parts and materials.

Magnesium alloys, a specific gravity of a comparison of the light re-aluminum 2/3 level of about 1.74g / cm ^3, even superior nasal, and has a cutting machinability, and vibration absorption capability, an attractive metal to the free sea water shortage in resources in terms of raw material to be.

In addition, because the resources can be recycled, magnesium alloys are attracting attention as an environmentally friendly ultra-lightweight material that can replace steel and aluminum alloys in the automotive industry, and they are rapidly used in aircraft and the automobile industry, which are expected to improve fuel efficiency through light weight. It is increasing.

Magnesium as described above has been steadily increasing in production. In recent years, about 70% of magnesium has been used as a magnesium alloy as a material for automobile parts, and magnesium alloy has replaced aluminum or iron-based parts. Although it is used in various industrial fields, in particular, it contributes to fuel efficiency improvement and environmental pollutant reduction effect through light weight of transportation equipment such as automobile and aviation industry.

Magnesium alloys are applied to automotive parts such as engine cylinder head covers, instrumental panel substrates, seat frames, manual transmission housings, steering column components, and intake manifolds. Folds, cross bar beams, and the like.

Meanwhile, after the 1997 Kyoto Conference on the Prevention of Global Warming, it was decided to reduce the CO ₂ emissions of the developed countries by more than 5% from 1990 to 2008. The next generation of vehicles with gas capabilities is being developed.

In particular, in various countries around the world, various projects can be carried out by self or national cooperation, for example, ULSAB (Ultra Light Steel Auto Body) project, PNGV (Partnership for a New Generation of Vehicles) project in Germany, heat-resistant Mg composite material in Germany Eco-friendly car weight reduction programs such as the project are being carried out. The projects are mainly focused on replacing magnesium alloys with power system components, which account for about 25% of the vehicle's total weight.

However, the power system components of automobiles are AZ 91 (Mg-9wt% Al-1wt% Zn) and AM 50 (Mg-5wt), which are general commercial magnesium alloys, because most of them are given high rotational power while being operated at a high temperature due to their driving characteristics. Mg-Al-based magnesium alloys such as% Al-max. 1 wt% Mn) and AM 60 (Mg-6 wt% Al-max. 1 wt% Mn) are restricted in use.

In other words, the above limitation is that although most commercial magnesium alloys have excellent castability and room temperature mechanical properties, they have lower absolute strength and toughness than other commercial structural alloys, that is, aluminum alloys, and especially creep at high temperatures. This is due to the problem that the characteristics are rapidly deteriorated, and there is a significant limitation in replacing the aluminum alloy, which is used as a material for the power system component operating at a high temperature, with the magnesium alloy.

In addition, Mg-Al- or Mg-Al-Zn-based, AZ-based magnesium alloy, Mg ₁₇ Al ₁₂ phase as the main strengthening phase produced when aluminum is added is thermally unstable at 100 ℃ or more, magnesium alloy base In order to exhibit the solid-solution inside, mechanical properties are remarkably lowered in the temperature range of 100 ° C or higher, and are limited to alloys for power system components of transportation equipment requiring high temperature properties.

However, in order to improve the castability of the magnesium alloy having a low latent heat of solidification, it is necessary to add aluminum.

In the case of the recently developed heat-resistant magnesium alloy system, research is made to improve high temperature creep characteristics by adding a low cost calcium or silicon, or rare earth metals such as expensive scandium or yttrium to form a stable compound at high temperature. This is going on.

However, in the former case, since die sticking or hot cracking is caused, it can be manufactured only by die casting. In the latter case, the manufacturing cost of using expensive rare earth metal It is difficult to commercialize due to the rise and poor castability.

The present invention was devised to solve all the problems of the conventional casting magnesium alloy, which is restricted in the use of automotive power system components, and improves the high temperature creep characteristic of 80 ° C. or higher without compromising the castability of the magnesium alloy. In addition, the amount of aluminum is controlled so that new alloying elements which suppress the formation of the main strengthening image are added, and the fine grains of which the shape is controlled are dispersed so that the thermal properties are improved while the fracture through the grain strengthening and the grains is suppressed. It is an object of the present invention to provide a casting magnesium alloy.

The above object of the present invention is achieved by silicon, tin, strontium, antimony, phosphorus, calcium and the like.

The present invention suppresses the formation of the Mg ₁₇ Al ₁₂ phase, which is the main strengthening phase of the magnesium alloy, by adding a new alloy element capable of forming a stable compound at a high temperature while ensuring the castability of the magnesium alloy. The control is related to the magnesium alloy that improves the creep characteristics of the magnesium alloy, first of all, as follows.

The suppression of the formation of the Mg ₁₇ Al ₁₂ phase is achieved by controlling the amount of aluminum added, and the improvement of creep properties is achieved by thermally in the grains in order to form a thermally stable secondary phase at grain boundaries and to suppress deformation due to dislocation shifting. By a method of dispersing a stable secondary phase.

In other words, a method of forming a thermally stable secondary phase at a grain boundary is a method for suppressing grain boundary sliding, which is a major creep deformation mechanism of magnesium alloy, to form coarse secondary phases of continuous form at grain boundaries. In this case, the grain boundary sliding can be effectively suppressed, but it can cause a sharp decrease in the elongation.

Therefore, in order to prevent a sudden drop in elongation, a thermally stable secondary phase should be formed in grain boundaries discontinuously or semi-continuously, thereby preventing the decrease in elongation and controlling the state of formation of the secondary phase so that grain boundary sliding can be effectively suppressed. .

In addition, the method of dispersing a thermally stable secondary phase in the crystal grains is caused by the additional deformation that is made through dislocation diffusion in the matrix, that is, by movement or generation of dislocations such as dislocation climb or dislocation creep. As a method for effectively suppressing deformation, by controlling the microstructure so that a thermally stable and fine secondary phase is formed in the crystal grains, it is possible to prevent the movement or diffusion of dislocations caused by the secondary phase.

Casting magnesium alloy of the present invention implementing the above methods, 3 to 11wt% aluminum; 1 to 4 wt% silicon; 1-9 wt% tin; At least one of strontium, antimony, phosphorus and calcium, and 0.1 to 1wt% of the balance and the balance of magnesium and other unavoidable impurities, the reason for limiting the content of each component is as follows.

Aluminum is an element that improves the mechanical properties and castability of magnesium alloys, but because it forms a thermally unstable crystallographic phase called Mg ₁₇ Al _{12 in} the microstructure of magnesium alloys, aluminum is a cause of lowering the heat resistance characteristics of magnesium alloys. If the aluminum content is less than 3wt%, castability is greatly reduced and casting is impossible, and when more than 11wt% is added, the amount of crystallization of Mg ₁₇ Al ₁₂ phase becomes too high, leading to a decrease in ductility due to brittleness. Caused.

Silicon is a component for further improving the heat resistance and mechanical properties of the magnesium alloy, and chemically reacts with magnesium to form Mg ₂ Si crystallized phase. The silicon has a process composition of 1.37wt% and a process composition. In the following, a crystallized image, which is also called a Chinese script shape, which has a rough shape, is generated, and since it is formed into a polygon or a sphere above process composition, its shape control is required.

That is, in the case of controlling the shape of the Mg ₂ Si crystallized phase, since the dispersion can be uniformly dispersed in the crystal grains, the matrix is strengthened and there is an effect of suppressing the breaking phenomenon through the crystal grains.

In addition, since the melting point is about 10 ° C. or more, it is a thermally stable phase, so that the thermal instability of the Mg ₁₇ Al ₁₂ phase can be sufficiently overcome, but when the content is less than 1wt%, the amount of thermally stable Mg ₂ Si crystallized phase is increased. At least, it is difficult to improve the heat resistance characteristics of the magnesium alloy, and if it exceeds 4wt%, the amount of coarse crystallographic phases is increased more than necessary, thereby lowering the tensile characteristics of the magnesium alloy.

Tin is basically an element that serves to improve the heat resistance characteristics of magnesium alloy, and simultaneously crystallizes a nano-sized Mg ₂ Sn phase thermally stable to a matrix structure of magnesium alloy and a plate-shaped Mg ₂ Sn phase around grain boundaries. It not only reinforces the matrix at the same time, but also delays the fracture of grain boundaries, and improves the thermal properties of the magnesium alloy.

That is, the nano-sized Mg ₂ Sn phase interferes with dislocation movement in the grain to improve the room temperature and high temperature strength of the material, and the plate-shaped Mg ₂ Sn phase in the grain boundary suppresses the movement of the grain boundary generated at high temperature deformation, thereby suppressing the high temperature. If the content is less than 1wt%, the amount of thermally stable Mg ₂ Sn crystallization phase is too small, and there is little effect of improving the heat resistance characteristics of magnesium alloy, and the amount of addition exceeds 9wt%. There is no effect of further improving the heat resistance.

Strontium, antimony, phosphorus, calcium, and the like are elements added to control the shape of the coarse Mg ₂ Si crystallized phase. Since each has an independent effect, one or more of them are added depending on the composition of the magnesium alloy. do.

The strontium, antimony, phosphorus, calcium, etc., to control the shape of the crystallized phase by providing a non-uniform nucleation site of the crystallization phase or by controlling the surface tension energy, the number of non-uniform nucleation sites when the total content added is less than 0.1wt% It is extremely small, making shape control almost impossible, and when added in excess of 1wt%, it does not affect shape control by reacting with other alloying elements aluminum, tin, and magnesium, a base alloy element, to form an intermetallic compound.

The heat resistance characteristics of the magnesium alloy for casting of the present invention, which is configured as described above, will be described as follows.

Example

In order to examine the effect of aluminum on the flowability of the molten magnesium alloy, the fluidity test of the Mg-Al-1wt% Zn alloy with 3wt%, 5wt%, 7wt%, 9wt% and 11wt% aluminum, respectively, was performed. As shown in the figure, it can be seen that as the amount of aluminum increases, the flowability of the molten metal becomes better.

As the amount of aluminum increases, as shown in FIG. 2, not only the amount of Mg ₁₇ Al ₁₂ crystal phase in the magnesium matrix increases, but also the grain size decreases, thereby improving mechanical properties.

In addition, as shown in FIG. 3, as the addition of silicon, a crude Chinese script-shaped Mg ₂ Si phase is crystallized ('ga' in FIG. 3), and the shape control elements calcium, antimony, phosphorus and strontium It can be seen that the shape of the Mg ₂ Si phase can be controlled, in part or in whole, by the addition of the back and the like.

In addition, (a) and (b) of FIG. 4 show Mg-5wt% Al-2wt% Si-3wt% Sn-0.2wt% Sr and Mg-5wt% Al-2wt% Si-6wt% Sn-0.2, respectively. As a photograph of a wt% Sr alloy, the addition of tin to a magnesium alloy causes the fine compounds (described as "unknown fine particles") in the grains to be not entirely uniform but to be crystallized as a whole, while the crystallographic phases present at the grain boundaries are discontinuous. However, it can be seen that it has a semi-continuous form.

On the other hand, the fine intermetallic compound in the crystal grains, according to the transmission electron microscope structure photograph shown in Figure 5, it can be confirmed that the nano-sized Mg ₂ Sn phase.

And, to investigate the effect of Sn on the microstructure of Mg-5wt% Al-2wt% Si-0.2wt% Sr alloy, 3wt% and 6wt% of Sn were added to prepare the alloy by gravity mainly Was observed.

6A and 6B show Mg-5wt% Al-2wt% Si-3wt% Sn-0.2wt% Sr and Mg-5wt% Al-2wt% Si-6wt% Sn-0.2wt% Sr alloys, respectively. Microscopic observation of the microstructure of Mg ₁₇ Al ₁₂ , polygonal shape (polygonal shape), Mg ₂ Sn phase, etc. were determined in the grain boundary and crystal grains, Mg-5wt% Al-2wt in the grains It can be seen that a fine intermetallic compound was produced which could not be observed in the% Si-0.2wt% Sr alloy.

7A and 7B are Mg-5wt% Al-2wt% Si-3wt% Sn-0.2wt% Sr and Mg-5wt% Al-2wt% Si-6wt% Sn-0.2wt% Sr alloys, respectively. The microstructure of T4 after heat treatment is that the thermally unstable Mg ₁₇ Al ₁₂ phase is employed in all the matrix structures. Unlike the Mg ₁₇ Al ₁₂ phase, the Mg ₂ Al phase is not dissolved in the matrix structure. In addition to the Mg ₂ Si phase it can be confirmed that the phase can improve the creep characteristics.

In this case, grain sizes of the Mg-5wt% Al-2wt% Si-3wt% Sn-0.2wt% Sr and Mg-5wt% Al-2wt% Si-6wt% Sn-0.2wt% Sr alloys are shown in Table 1 below. same.

           division        Particle size (㎛)   Mg-5wt% Al-2wt% Si-3wt% Sn-0.2wt% Sr         29.3   Mg-5wt% Al-2wt% Si-6wt% Sn-0.2wt% Sr         27.5

8 to 11 show the strain-time graph evaluating the high temperature characteristics of the magnesium alloy of the present invention, it can be seen that the amount of deformation is completely reduced by the addition of silicon and tin, which improves the heat resistance characteristics of the magnesium alloy It means.

In addition, the AZ 91 magnesium alloy (Mg-9wt% Al-1wt% Zn), which is a widely used commercial magnesium alloy, and the magnesium alloy of the present invention were subjected to high temperature characteristic tests at 150 and 120, respectively. Strain was measured and shown in Tables 1 and 2 below.

division      30 Mpa      65 MPa Comparative material  AZ 91 8.86 × 10 ^-7 5.84 × 10 ^-5 Invention  Mg-5% Al-2% Si-3% Sn-0.2% Sr 2.47 × 10 ^-8 3.15 × 10 ^-6  Mg-5% Al-2% Si-6% Sn-0.2% Sr 4.12 × 10 ^-9 6.34 × 10 ^-7  Mg-5% Al-2% Si-9% Sn-0.2% Sr 2.32 × 10 ^-9 9.11 × 10 ^-8

division      30 Mpa      65 MPa Comparative material  AZ 91 7.58 × 10 ^-8 4.73 × 10 ^-6 Invention  Mg-5% Al-2% Si-3% Sn-0.2% Sr 9.81 × 10 ^-10 1.23 × 10 ^-7  Mg-5% Al-2% Si-6% Sn-0.2% Sr 1.58 × 10 ^-10 2.38 × 10 ^-8  Mg-5% Al-2% Si-9% Sn-0.2% Sr 9.12 × 10 ^-11 4.12 × 10 ^-9

As can be seen from Tables 1 and 2, the inventive material according to the present invention has a minimum creep strain of at least 10 ^-3 or more than that of AZ 91 magnesium alloy, which is a commercial magnesium alloy. It means that the heat resistance of the magnesium alloy can be further improved.

On the other hand, since the minimum creep strain of the invention material shown in Table 1 shows less results than the minimum creep strain of the comparative material shown in Table 2, the excellent high temperature characteristic of the invention material can be confirmed once again.

In addition, in the case of aluminum, which is widely used as a material for dynamometer parts of transportation equipment, the high temperature characteristics of 150 ° C. and 35 MPa atmosphere have a creep strain of about 0.25%, and the high temperature characteristics of 150 ° C. and 50 MPa atmosphere have a creep strain. As about 0.1%, the creep strain of the invention according to the present invention is also almost similar to 0.1%, it is considered that it can sufficiently replace the conventional aluminum alloy material.

In addition, the use temperature of the magnesium alloy of the present invention based on the high temperature properties identified in the present invention is 100 ~ 150 ℃ similar to the aluminum alloy material.

As described above, the magnesium alloy of the present invention can be used as a material of automotive power system parts because the heat resistance at 100 ℃ or more significantly increased, and thus, can contribute significantly to the weight reduction of automobiles in the future. It is expected to be.

Claims (1)

In the casting magnesium alloy containing aluminum, 3-11 wt% aluminum; 1 to 4 wt% silicon; 1-9 wt% tin; A casting magnesium alloy, characterized in that it is composed of at least 0.1 to 1 wt% of strontium, antimony, phosphorus, calcium, and the balance of magnesium and other unavoidable impurities.

Images