WO2016137210A1

WO2016137210A1 - Plastic deformation magnesium alloy having excellent thermal conductivity and flame retardancy, and preparation method therefor

Info

Publication number: WO2016137210A1
Application number: PCT/KR2016/001770
Authority: WO
Inventors: 이인영; 김강형; 전현석
Original assignee: 이인영
Priority date: 2015-02-25
Filing date: 2016-02-24
Publication date: 2016-09-01
Also published as: US20190112693A1

Abstract

The present invention relates to a magnesium alloy having excellent thermal conductivity and flame retardancy and facilitating plastic deformation, and a preparation method therefor, the magnesium alloy containing 0.5-5 wt% of zinc (Zn) and, as a high-melting-point-oxide-film-forming-element, 0.6-3.5 wt% of tin (Sn) in magnesium, wherein the total amount of alloying elements is 2.5-6.3 wt% by selectively adding 1.5 wt% or less of one or more of calcium (Ca), silicon (Si), manganese (Mn) and mischmetal, when necessary. The method for preparing the alloy comprises adding high-melting-point alloying elements in the form of a master alloy to a molten alloy of magnesium and zinc, melting the same, casting the same, removing the chill zone of a cast material, carrying out diffusion annealing, and then carrying out molding through a tempering process such as rolling, extrusion or forging. According to the present invention, there are effects of: improving ductility by the action of alloying elements inhibiting the formation of plate-like precipitates in a magnesium base texture; enabling extrusion even with a pressure of 1,000 kgf/cm² or less since the plastic deformability of a magnesium alloy is improved; allowing wide application thereof as a radiation plate or a structural material of portable electric home appliances, automobile and aircraft parts, and the like since the present invention is suitable for a use requiring fire safety by having a thermal conductivity of 100 W/m·K or more and a flame retardancy satisfying the requirements of aircraft material usage; and contributing to weight reduction.

Description

Magnesium alloy for plastic working with excellent thermal conductivity and flame retardancy and its manufacturing method

The present invention imparts plastic workability by adding 0.5-5% by weight of zinc to the magnesium (hereinafter,% by weight) to solid solution, and contains 0.6-3.5% by weight of tin (Sn) as a high melting point oxide film forming element, if necessary. Thermal conductivity and flame retardancy of 2.5 to 6.3% by weight of the total alloying elements are added by selectively adding at least 1.5% by weight or less of calcium (Ca), silicon (Si), manganese (Mn) and mischmetal (Mischmetal). The present invention relates to a magnesium alloy having excellent plasticity and easy to process.

As is well known, magnesium alloy is currently the lightest metal among the commercially available metal materials, and the use of magnesium alloy is rapidly expanding as a material of various parts in place of aluminum in the metal material usage to achieve further weight reduction. The demand for fuel economy and the application to mobile electronics are increasing rapidly.

Magnesium alloys have the lightest density of 1.74 g / cc among commercially available structural alloys, which is two-thirds the density of aluminum. In addition, magnesium alloy has excellent machinability, high vibration damping property, excellent absorbency against vibration and shock, excellent electromagnetic shielding function, and the like. In addition, the reason for the rapid expansion of magnesium alloy into computers, mobile phones, automobile parts, etc. in recent years is that it has excellent light weight and reproducibility, shielding against electromagnetic waves, and is capable of forming as a thin shape such as castability superior to aluminum. Because.

However, magnesium has a dense hexagonal lattice structure with a small slip system, which is essential for plastic deformation, and is mainly formed by casting due to poor extrudability and formability. However, sand casting has many limitations in shape, and molding by die casting causes many problems in subsequent surface treatment processes because the cast structure is porous in its characteristics. Meanwhile, materials such as AZ31 and AM20 alloyed with aluminum and zinc or manganese were developed to enable plastic processing using the ductility of single-phase solid solution. However, they have developed an anisotropy in one direction due to the development of the bottom surface tissue after annealing and strong anisotropy and easy formation of tensile twins. .

That is, when extruded at a temperature of 400 ℃ or more, due to the frictional heat with the die is extruded in the temperature zone where the low-melting process liquid and alpha magnesium solid solution coexist, there was a problem that the appearance of fine cracks on the surface appear wrinkle-like defects. These fine surface defects should be removed because they lower the fatigue strength, but they are not easy to remove due to cost, environment, and safety issues caused by dust ignition. Accordingly, in order to obtain a clean product without surface defects, extrusion is performed at a temperature of 350 ° C. or lower, which causes the pressure to be increased at least five times higher than the aluminum extrusion pressure of 1000 kgf / cm 2.

In addition, the AZ-based and AM-based magnesium alloys, which have been used for many times for annealing, had problems of not guaranteeing flame retardancy due to the low melting point process.

These AZ-based and AM-based magnesium alloys are formed of an initial precipitate of high-melting point iron-based impurities (Fe, Ni) or copper (Cu) having low solid solubility during solidification, and then beta Mg ₁₇ Al ₁₂ Since the compounds make coarse plate-like precipitates, and these precipitates are linked to interfere with heat transfer, there is a problem that the thermal conductivity is greatly lowered even if the amount is about 3-4%. (Ed. GL Song, Corrosion of Magnesium Alloys, 2011, pp. 137-146)

Therefore, when flame is applied to the material, the thermal conductivity is low, so the temperature rises easily at the heating part in the structure, and when dissolved, it reacts easily with oxygen in the air to ignite, and even if the flame is extinguished, the temperature of the material decreases due to late thermal diffusion. There is a serious problem that it is difficult to keep burning because it is difficult to extinguish quickly. Concerns about fire acted not only in automobiles but throughout the industry, significantly delaying the application of magnesium materials.

For this reason, attempts have been made to add rare earths to impart flame retardancy to magnesium.

Existing materials that satisfy this are alloys such as WE43, ZE41, ZE10 or Elektron 21, which are yttrium, niobium (Nb), samarium (Sm), ytterbium (Yb), gadolium (Gd), neodymium (Nd) and zirconium It is an alloy containing a rare earth element such as (Zr). These alloys are excellent in flame retardancy due to the strong oxide film of rare earth elements, but they do not satisfy the demands of the market because they require large amounts of expensive elements or poor plasticity. However, in the case of rare earth elements, containing 4% or more has the adverse effect of greatly reducing the malleability, and in general, the thermal conductivity decreases as the amount of the alloying element is increased. In particular, zirconium is an element that refines the grain size and improves flame retardancy, but has a very low thermal conductivity and plastic workability. As a result, about 1% of magnesium is added to reduce the thermal conductivity by 50 to 70%.

WE43 with zirconium added has 51 ~ 54W / m-K and ZE41 has 24W / m-K thermal conductivity and low ductility, so it is mainly used as casting material rather than plastic processing. Elektron 21 contains about 4% of rare earth elements and 0.5% or less of zinc, which is excellent in thermal conductivity of 116W / m-K and excellent in suppressing ignition but very low in elongation of 2%. ZE10 is also difficult to use for plastic processing in the industry because it is formed by special method such as ECAP because of low thermal conductivity and plastic processability by addition of zirconium.

In addition, Korean Patent Registration No. 10-1367892 introduces a high temperature magnesium alloy magnesium alloy and a manufacturing method, which combines aluminum and calcium as the calcium oxide is reduced by adding 0.5 to 3.8% by weight of calcium oxide (CaO) to the magnesium alloy molten metal. To impart flame retardancy. However, this alloy also has a disadvantage that the plastic workability is significantly reduced.

In addition, a method of improving thermal conductivity by mixing silicon carbide (SiC) or fibrous alumina with magnesium has been attempted, but it is not suitable for annealing because it degrades plastic workability. (A. Rudajevova et al., On the Thermal Characteristics of Mg Based Composites, Kompozyty, 4, 10, 2004)

As such, it was considered to be a difficult problem to obtain flame retardancy and plastic workability in magnesium alloy.

As an example of improving the plastic workability by adding rare earth, Korean Patent No. 10-0509648 introduces a method of manufacturing a magnesium alloy sheet having excellent plastic workability by adding zinc and yttrium to magnesium. In the present invention, however, the molten metal containing 0.5-5.0% of zinc and 0.2-2.0% of yttrium was cast in a plate shape of 35 mm thickness and rolled to 1.0 mm plate through annealing to improve the plastic workability of the rolled plate material. As a result, no solution has been provided for the segregation of the core and the specific gravity segregation of zinc, which is increased when 3% or more of zinc is added. In addition, in this invention, the method of improving the flame retardancy of the material was not considered at all.

In order to improve the thermal conductivity and high temperature stability of the material, a method of controlling the shape by adding strontium (Sr) or calcium, which is an earth metal, to beta Mg ₁₇ Al ₁₂ is proposed. (A. Kielbus et al., The Thermal Diffusivity of Mg-Al-Sr and Mg-Al-Ca-Sr and Casting Magnesium Alloys, Defect and Diffusion Forum, Vol. 326-328, 2012, pp.249-254) Strontium or Calcium By increasing the surface tension of the beta-phase precipitates, it is possible to suppress the precipitation of the precipitates into the lamellar phase at the grain boundaries, to improve the thermal conductivity by reducing the size of the precipitates, and to suppress the ignition by forming a dense oxide film on the surface when the molten metal is exposed to the flame. . However, this material is 6 ~ 9% aluminum and 0.8 ~ 2% strontium or 1.5 ~ 2.2% calcium, and the alloying elements total 8 ~ 11%, so its malleability is low. Not only are they unsuitable, but the thermal conductivity of this alloy is improved by 75% compared to that of AZ91 and is only 87 W / mK, similar to AZ31, which is less than the thermal conductivity of 100 W / mK expected in the present invention.

In Korean Patent No. 10-1276665, a magnesium alloy capable of plastic processing solved flame retardancy with an alloy based on magnesium (Sn) 4-10% and calcium (Ca) 0.05-1.0%. However, in the present invention, due to the problem of maintaining the melt temperature at 850 ~ 900 ℃ to dissolve high melting point elements such as calcium, manganese, yttrium, erbium unnecessarily increases the gas concentration and the oxide in the melt to increase the impurity concentration In addition to the problem, there is a problem that the high probability of ignition of the molten metal inhibits the operational stability.

As another alternative, Korean Patent Registration No. 10-1406111 introduces an alloy in which 6.5-7.5% of tin, 1% of zinc and aluminum are added to magnesium.

These alloys have improved flame retardancy, but because they contain a large amount of tin with high precipitation hardening, they have low plastic workability and low thermal conductivity, so they need to be heat treated for a long time at a high temperature of 480 ~ 500 ℃ to extrude the billet. At a pressure of 1000 kgf / cm 2 or less obtained in a conventional extruder for aluminum in the industry, there is a disadvantage in that plastic working is difficult.

In addition, Korean Patent No. 10-0519721 discloses a high-strength magnesium alloy in which magnesium is 6% zinc as a basic composition and 0.4-3% of manganese, aluminum, silicon, and calcium are additionally added. However, when the alloy is manufactured as a commercial billet, a large amount of zinc may cause specific gravity segregation, which may cause the billet to break during extrusion or deteriorate in plastic workability.In the detailed description of the present invention, only the high strength and plastic workability are mentioned. There was no basis for predicting gender performance.

As such, magnesium alloys were initially developed to be inclined to plastic processability or to be flame retardant. However, for commercialization, there is a demand for satisfying both thermal conductivity and flame retardancy and securing plastic processability. This is because structural materials are not simply satisfied with strength and formability in order to commercialize, and safety can be ensured by preventing the spread of fire only by suppressing the ignition property of magnesium material when a fire occurs.

In order to facilitate development in the face of such delays in technology development, the FAA revised the standard for flame retardancy testing of magnesium alloys for aircraft seat structures.

According to this 2014 amendment (DOT / FAA / TC-13 / 52), fire must not ignite within two minutes of exposure to an oil burner flame for four minutes (240 seconds), and within three minutes after the burner is turned off. A total of 7 minutes of flame retardancy tests to be extinguished shall meet the specification if no more than 10% of the initial weight is lost.

Figure 1 shows a schematic of the aircraft magnesium alloy flame retardancy test apparatus and specimen approved by FAA.

The present invention was created to eliminate the above problems, and contains 0.5 to 5% by weight of zinc (Zn) and 0.6 to 3.5% by weight of tin (Sn) as a high melting point oxide forming element, but calcium (Ca) if necessary ), Silicon (Si), manganese (Mn) and mischmetal (Mischmetal) by selectively adding 1.5% by weight or less to manage the total amount of alloying elements from 2.5 to 6.3% by weight, excellent thermal conductivity and flame retardancy While the plastic workability is improved, extrusion is possible even at a pressure of 1000 kgf / cm 2 or less, and an object of the present invention is to provide a magnesium alloy for annealing and a method for producing the alloy having excellent thermal conductivity and flame retardancy of 100 W / mK or more.

In order to achieve the above object, the present invention contains 0.5 to 5% by weight of zinc and 0.6 to 3.5% by weight of tin (Sn) as a high-melting-point oxide film-forming element, but if necessary, calcium (Ca), silicon (Si), and manganese. Selectively add one or more of Mn and Mischmetal to 1.5 wt% or less to manage the total amount of alloying elements at 2.5 to 6.3 wt% to obtain a magnesium alloy having excellent thermal conductivity and flame retardancy and improved plastic workability. Get In the magnesium alloy of the present invention, high melting point elements other than zinc and stocks are added in the form of a mother alloy, and mechanical stirring is performed to remove component segregation and to cast. Thereafter, after removing the surface coating structure of the casting material and performing diffusion annealing, it is then molded into a predetermined shape through an annealing process such as rolling, extrusion or forging.

According to the present invention, the high malleability of the magnesium alloy can be plasticized without surface defects even at an extrusion pressure of 1000 kgf / cm 2 or less, and has excellent flame retardancy and thermal conductivity, and thus requires heat conduction required in portable home appliances, automobiles, aircraft parts, and the like. By satisfactory performance and flame retardancy, there is an effect of providing a magnesium alloy extruded material which can be usefully used at a low cost.

1 is a schematic view of a flame retardant test apparatus and specimen.

Figure 2 is a schematic diagram showing the stirring process during mold cooling.

3 is a high temperature stable phase precipitation state diagram of the alloy 1 comparative example.

4 is a high temperature stable phase precipitation state diagram of an alloy 2 comparative example.

5 is a high temperature stable phase precipitation state diagram of the alloy 3 comparative example.

6 is a high temperature stable phase precipitation state diagram of the alloy 4 comparative example.

7 is a high temperature stable phase precipitation state diagram of the alloy 5 Comparative Example.

8 is a high temperature stable phase precipitation state diagram of Alloy 6.

9 is a high temperature stable phase precipitation state diagram of Alloy 7.

10 is a high temperature stable phase precipitated state diagram of alloy 8. FIG.

11 is a high temperature stable phase precipitation state diagram of Alloy 9.

12 is a high temperature stable phase precipitation state diagram of Alloy 10.

13 is a high temperature stable phase precipitation state diagram of Alloy 11. FIG.

14 is a high temperature stable phase precipitation state diagram of Alloy 12. FIG.

15 is a high temperature stable phase precipitation state diagram of the alloy 13 Comparative Example.

16 is a high temperature stable phase precipitation state diagram of the alloy 14 comparative example.

17 is a high temperature stable phase precipitation state diagram of an alloy 15 comparative example.

18 is a casting structure of alloy 7.

Fig. 19 shows a casting structure of alloy 10.

20 is an electron micrograph of the alloy 7 casting structure.

Fig. 21 is an extruded material structure of alloy 6.

22 is an extruded material structure of alloy 7.

23 is an extruded material structure of alloy 10.

24 is a thermal conductivity of Alloy 7.

25 is a thermal conductivity of Alloy 10.

26 is a flame retardant test scene.

The present invention contains 0.5 to 5% by weight of zinc in magnesium and 0.6 to 3.5% by weight of tin (Sn) as a high melting point oxide forming element, but if necessary, calcium (Ca), silicon (Si), manganese (Mn) and misc By selectively adding at least 1.5% by weight or less of the metal (Mischmetal) to manage the total amount of alloying elements at 2.5 to 6.3% by weight, not only has thermal conductivity and flame retardancy, but also high melting point alloying elements are added as a master alloy to provide 720 It can melt | dissolve at the temperature below ° C, and there exists an effect which suppresses oxide impurities. It is a magnesium alloy having excellent malleability and easy plastic working by reducing the segregation of the alloying elements by performing mechanical stirring during solidification of the molten metal. In particular, by lowering the alloying amount of expensive elements, it is possible to provide a magnesium alloy that suppresses the melting point and thermal conductivity drop and satisfies both cost reduction and flame retardancy.

The reason why the addition range of the alloying elements in the present invention is limited as set forth above will be described.

Zinc is dissolved in Mg to change the c / a axis ratio, thereby inhibiting the development of the bottom aggregate tissue, and uniform solid working is possible due to the work hardening effect of MgZn ₂ and MgZn ₅ precipitates in solid solution. However, less than 0.5% is less effective, so it is difficult to expect the work hardening and malleability required for plastic processing materials, and when it exceeds 5%, the bottom aggregate structure, which inhibits formability after annealing, is strengthened. It is lowered, and the segregation tendency increases in the molten magnesium, and the precipitated phases such as Mg ₂ Zn ₃ and Mg ₁₂ Zn ₁₃ are laminated with the alpha phase to form a low melting point process phase around 340 ° C., thus limiting flame retardancy. . Preferably 1.0 to 4.0% is suitable.

Tin improves high-temperature plasticity by making Mg ₂ Sn precipitates, which are stable at high temperatures above 560 ℃ and by appropriately distributing them, and when exposed to flames in the air, contributes to the improvement of flame retardancy by forming SnO ₂ oxide films with melting points above 1600 ℃. However, in the present invention, when the addition amount is less than 0.6%, it is difficult to expect the effect. If the addition amount is more than 3.5%, the melting point is lowered and the ignition point may be lowered below 500 ° C., thereby impairing the flame retardancy, and the excessive amount of Mg ₂ Sn precipitates. Since not only the malleability falls but also the manufacturing cost rises, 1.0 to 3.0% is preferable.

Mischmetal is a rare earth alloy containing 65% to 78% of cerium (Ce) and lanthanum (La), and the rest is neodymium (Nd), praseodymium (Pr) and indispensable impurities. Mischmetal is used as a substitute for lanthanum for cost savings because it is cheaper and has the same effect as lanthanum (La) or cerium (Ce), which have a lanthanum effect in magnesium alloy. Cerium and neodymium also exhibit flame retardancy by forming oxide films such as CeO ₂ and Nd ₂ O _{3 with} melting points of 2400 ° C and 2200 ° C, respectively, when exposed to flames in the atmosphere. Due to the high temperature stable phase, the high temperature strength is kept high, and the property of suppressing warpage, sagging or premature melt bar-separation at high temperatures by the flame is strong.

However, since cerium in the mismetal has a high solubility limit for magnesium, 0.5% is the maximum solubility limit, and an excessive amount of the atomic metal of the mismetal constituents tends to form coarse precipitates, so the present invention is limited to 1.5% or less. . In the present invention, the reason why the constituent elements in the rare earth metal are expressed by the mismetal is that the cost increases when the constituent elements are added as a single metal. Therefore, the addition of the constituent elements alone does not depart from the scope of the present invention.

Lanthanum is used as a substitute for misch metal in the present invention.As a representative metal in rare earth metals, the high solubility limit of magnesium is 12.4%, and it is combined with MgZn ₂ precipitate to form a long period stacking order structure of hexagonal system. Precipitates are formed to form a lamellar lamellar process phase at the grain boundaries. Precipitates are transformed into homogeneous intermetallic compounds through spinodal decomposition during homogenization heat treatment after casting, and dispersed during annealing to contribute to strengthening dispersion. When exposed to a flame in the air, a La ₂ O ₃ oxide film having a melting point of 2300 ° C. or more is formed to exhibit flame retardancy. In addition, due to the high temperature stable phase, the high temperature strength is maintained, and the property of suppressing warpage, sag or premature melt bar-separation at high temperatures due to the flame is strong. In the present invention, however, the precipitate tends to be coarse with increasing amount, so it is limited to 1.5% or less.

Calcium is a group 2 alkaline earth metal such as magnesium. Mg ₂ Ca, which has a melting point of 715 ° C, which is a secondary coagulation phase, is formed between dendritic tissues or is employed in a matrix with zinc, causing recrystallization in a disordered direction during heating. It has the effect of inhibiting development and miniaturizing grains, and improves flame retardancy by forming a CaO oxide film having a melting point of 2600 ° C. or more when exposed to flame in the air. In addition, due to the high temperature stable phase, the high temperature strength is maintained, and the property of suppressing warpage, sag or premature melt bar-separation at high temperatures due to the flame is strong. However, in the present invention, the amount of Mg ₂ Ca particles is excessively increased and the malleability may decrease, so the amount is limited to 1.5% or less.

When silicon is added, a high temperature stable phase such as Mg ₂ Si is formed to refine the grain size and exert a precipitation strengthening effect. The precipitate is easy to coarse, but the addition of calcium can adjust the size or shape of the precipitate. When exposed to a flame in the atmosphere, a SiO ₂ oxide film having a melting point of 1600 ° C. or more is formed to exhibit flame retardancy. Due to the high temperature stable phase, the high temperature strength is kept high, and the property of suppressing warpage, sagging or premature melt bar-separation at high temperatures by the flame is strong. However, the present invention is limited to 1.5% or less because the precipitate begins to coarsen with increasing amount.

Manganese has a maximum solubility of 2.2% in magnesium. Magnesium alloys have a crystallization reaction in which the alpha phase of manganese precipitates and the excitation reaction in which the delta phase precipitates at 650 ° C., thereby minimizing grain size and improving corrosion resistance. In particular, when exposed to a flame in the atmosphere has an effect of improving the flame retardancy by forming a MnO oxide film having a melting point of more than 1900 ℃. In the present invention, it is effective to refine the coarse plate-shaped Mg ₁₇ Al ₁₂ precipitate formed when aluminum is present as an impurity and to refine the MgZn ₂ precipitate during recrystallization during annealing. However, the present invention is limited to 1.5% or less because the effect is saturated and the malleability decreases as the amount added increases.

In the present invention, 0.5 to 5% by weight of zinc (Zn) and 0.6 to 3.5% by weight of tin (Sn) as the high melting point oxide film forming element, but if necessary, calcium (Ca), silicon (Si), manganese (Mn) and misch When molten metal is prepared by selectively adding one or more kinds of metals (Mischmetal) to 1.5 wt% or less and undergoing mechanical stirring during the solidification process, the malleability is ensured to be extruded at a pressure of 1000 kgf / cm2 or less and exposed to flame. In this case, a high melting point oxide film is formed on the surface of the material to exhibit flame retardancy, satisfies thermal conductivity at the same time, thereby preventing the occurrence of premature melting due to local heating and shortening the self-extinguishing time of the melted material. A detailed description is given below.

The reason for managing the total amount of alloying elements in the present invention at 2.5 to 6.3% is less effective at improving the malleability and flame retardancy at less than 2.5%, and when the alloying elements are added in excess of 6%, compounds and precipitates are excessively high in thermal conductivity. It lowers, and since it has low malleability and melting point, it limits.

In the prior art, when a large amount of rare earth or alkaline earth metal is added, the ignition is suppressed and the flame retardancy is improved. However, the melting point of the material is lowered and the thermal diffusion rate is also lowered as the amount is increased. In the present invention, since the local heating at the site exposed to the flame at the time of fire causes the melt down and the time of the molten metal increases, the total amount of the alloy is suppressed to 6.3% or less.

In addition, in the present invention, even if the alloying element is managed in a total amount of 6% or less, when the aluminum or zirconium is excessively present as an impurity, the thermal conductivity is lowered. Aluminum contains a large amount of coarse plate-shaped Mg ₁₇ Al ₁₂ which contains only about 3%, which lowers thermal conductivity and consumes rare earth or alkaline earth elements and impairs flame retardancy. Therefore, aluminum is limited to 1% or less as impurities. . In addition, since zirconium combines with other rare earth elements in the dendritic structure at the grain boundary to form lamellae, the zirconium is limited to 0.5% or less because it significantly lowers the thermal conductivity and increases the brittleness, thereby deteriorating the malleability.

In the present invention, the thermal conductivity of 100 W / mK or more is exhibited by the above-described method, and tin (Sn), calcium (Ca), silicon (Si), manganese (Mn) and mischmetal (Mischmetal) as high melting point oxide film forming elements. It is characterized by satisfying the flame retardancy of more than 120 seconds of ignition time, less than 180 seconds of specimen extinguishing time after burner extinguishing, weight loss of 10% or less in the burner ignition test.

In addition, in the present invention, in order to reduce segregation of alloy elements in the molten metal and to improve plastic workability and physical properties, high melting point alloy elements excluding calcium and tin, which are low melting points in alloys (calcium, silicon, manganese, misc metal, lanthanum) They are added to the master alloy and characterized in that the molten metal is mechanically stirred.

Magnesium alloys often have a large difference in specific gravity between magnesium and other alloying elements. For example, zinc and tin tend to segregate in the center or under the mold, and coarse development of dendritic crystals leads to uneven macroscopic composition of billets. It causes a decrease in extrusion performance. In the case of severe segregation, hot tearing occurs in the center of billet, which causes deformation, cracking, and fine wrinkles during the extrusion process. And reliability.

In the present invention, by adding a high melting point alloying element as a mother alloy, it is possible to dissolve while managing the temperature of the melt at 720 ° C. or lower, thereby lowering the possibility of ignition of the melt, and the coagulation nucleus already formed in the mash zone in the melt through mechanical stirring. By dispersing it, it is possible to promote uniform solidification in the molten metal, to reduce segregation and to refine grains. In particular, magnesium alloy does not have unpaired hole electrons, so the magnetization power is weak, so that it is difficult to obtain a sufficient magnetic stirring effect, and mechanical stirring is effective.

Specific methods for this are further described in detail in the Examples.

The method of manufacturing the master alloy lowers the melting point of the master alloy by forming a lump or granule, the main component of which is to be alloyed in the molten magnesium, in a shielding gas atmosphere shielding from the atmosphere, and by mechanically stirring the composition near the process composition. .

According to the experimental results of the present inventors, the total component segregation difference between the upper and lower parts of the casting material was less than 1% when mechanical stirring was performed during gravity casting of 30 kg magnesium alloy billet containing 4% zinc. There was more than 8% segregation difference. Therefore, it can be seen that mechanical stirring is effective for removing segregation. This segregation causes the billet to break at the segregation boundary during extrusion, or remains after extrusion, causing visual defects and uneven physical properties.

In the present invention, a stirring method using a motor and an impeller will be described as a method of mechanical stirring. However, the use of attraction or other mechanical agitation does not detract from the effects sought in the scope of the present invention. In the present invention, the casting material (for example, billet) subjected to mechanical agitation when the molten metal is stirred while the solidification process proceeds inside, disperses the high temperature stable phase and the high temperature precipitates formed as solids in the melt zone in the molten metal, thereby acting as a solidification core. The structure of the casting material is uniform, segregation is eliminated, and the grains are effective.

As shown in FIG. 2, in the present invention, the molten alloy is blocked from the atmosphere by the shielding gas injected through the cover 3 and the shielding gas pipe 4 combined with the crucible or the mold 1. While the billet is solidifying, the alloy composition is made uniform by inserting and stirring a small impeller 22 into a molten stainless steel lever 21 which is operated by a motor M through a through hole provided in the cover. By moving up and down to the marsh zone 8, the coagulation nuclei can be dispersed to refine the crystals. Impeller 22 used in the present invention is also included in the scope of the present invention using other metals, ceramic materials, composite materials, or depositing, plating, penetrating or spray coating other materials on the impeller. The diameter of the impeller is 1/5 to 2/3 of the diameter of the billet. If the diameter of the impeller is larger than this, it is limited because it requires a large motor due to the load. In addition, if less than 1/5 of the billet diameter, the stirring effect is insignificant, which is not suitable for segregation prevention.

2 is an example of gravity casting in which the crucible is extracted and solidified after melting in the crucible, and the right figure is an example of continuous casting.

EMBODIMENT OF THE INVENTION Below, this invention is demonstrated concretely by the Example which melt | dissolves in a crucible furnace.

In the present invention, when producing the magnesium alloy molten magnesium is first dissolved in the crucible and the temperature is maintained at 680 ~ 720 ℃. The crucible was made of stainless steel, and the melting atmosphere shields contact with the atmosphere by flowing a gas mixed with 0.25 to 0.3% SF ₆ in carbon dioxide gas. Then, zinc and / or tin are added as alloying elements, and other high melting point alloying elements (mischmetal, lanthanum, calcium, silicon, manganese) are added in the form of a master alloy close to the process composition. Following the addition of these alloying elements in the composition of Table 1, after stirring and stabilizing the molten metal, the crucible is extracted and charged into a cooling bath. In this process, the refrigerant is injected or charged into a tank filled with a refrigerant, such as water of 30 ° C. or lower, to cool the crucible to promote solidification of the molten metal.

When cooling in a bath, the crucible is cooled at a rate of about 70-200 ° C / min depending on the capacity of the bath and the temperature of the refrigerant. If cooling by injecting the refrigerant, the cooling rate is higher, and when the crucible is sprayed and cooled in a water tank, it is cooled at a cooling rate of about 200 ~ 600 ℃ / min. In continuous casting where the solidification speed is to be increased, the mold and billet are increased by increasing the injection pressure. Intense cooling can result in cooling at a rate of about 400-900 ° C / min. However, if the cooling rate exceeds 900 ℃ / min, there is a problem that the crack in the center due to the heat shrinkage stress due to the cooling rate difference inside and outside the billet.

While the magnesium alloy molten metal is solidified, an impeller made of stainless steel is inserted into a crucible to mechanically stir the molten metal two or three times to disperse the coagulation nuclei formed in the cast material, thereby obtaining a cast material having a low segregation and a fine structure. By machining the cast material, the surface chill (Chill) structure was removed to obtain a billet having a diameter of 74 ~ 75mm, and was subjected to diffusion annealing at 380 ℃ for 2 hours and cooled to room temperature.

Then, the diffusion-unfolded billet was preheated at 380 ° C. for 1.5 hours, and the alloys of Table 1 were extruded from an extrusion die to form a sheet having a width of 50 mm and a thickness of 8 mm.

Example alloys of Table 1 were mostly extruded at 750-900 kgf / cm 2 pressure. However, the alloys of Comparative Examples 1 and 15 were extruded, but due to the low melting point process, microcracks exist on the surface of the plate. In Comparative Example 14, the extrusion temperature was lowered to around 340 ° C. and the extrusion pressure was increased to 1500 kgf / cm 2. Surface microcracks could be prevented, but after the burner was removed in the flame retardancy test, the natural digestion time was exceeded, and the weight loss was excessive. In Table 1, the evaluation of extrusion firing was performed when O, when a clean and smooth surface was obtained, and the extrusion speed satisfies the speed of 1.5 ~ 2m / min, when some fine wrinkles occurred on the surface or the extrusion speed was about 1m / min. If not satisfactory, the reason is described.

In Comparative Examples 2, 3, 4, 5, 13, and 16, the extruding pressure was increased during extrusion, and the cylinder was stopped or the billet was broken.

In Comparative Example 15, the extrusion temperature was lowered to around 300 ° C. and extruded at a high pressure of 5300 kgf / cm 2, but side cracks occurred due to excessive precipitation of beta Mg ₁₇ Al _12, but it was removed by processing to obtain a flame-retardant specimen. Nevertheless, the low flash point did not meet the criteria for both initial ignition time, natural digestion time and weight loss in the flame retardant test.

The plate-like sample thus obtained was diffused and unannealed at 380 ° C. for 1 hour and processed into a diameter of 12.7 mm and a thickness of 2 mm, and thermal conductivity was measured by a laser flash method according to ASTM E4161. The alloys of the present invention were 125 W / mK at a high temperature of 100 ° C. or higher. It was confirmed to exhibit the above thermal conductivity.

The ignition point of the processed chip obtained from the sample was measured by thermogravimetric analysis (TGA) using a differential scanning calorimeter (DSC), and a flame retardancy test was performed using a burner by processing a specimen having a width of 38.1 mm, a thickness of 6.4 mm, and a length of 508 mm. Each was carried out twice.

As shown in Table 1, the alloys of the present invention have a ignition point of more than 550 ° C., a ignition time of 120 seconds or more, a burner digestion within 180 seconds of spontaneous fire extinguishing, satisfactory flame retardant conditions of less than 10% weight loss, plastic workability, and thermal conductivity. Doing.

These results show that in the alloys of the comparative example, the malleability is lowered when the total amount of the alloying elements is 6.5 to 9.45%, so that the extrusion stress increases or the thermal conductivity tends to decrease.

On the other hand, the example alloys of the present invention were produced in 4.4 ~ 6.0% range showed excellent thermal conductivity and flame retardancy, yet easy plastic processing.

3 to 17 is a high temperature stable phase precipitation state diagram of the alloys and comparative alloys according to the present invention, the high temperature stable phase begins to appear at least 430 ℃ or more in the state diagram of the comparative example is insufficient or excessively excessive precipitation of the high temperature stable phase It can be seen that. 18 to 23 show the structure of the alloy of the present invention. Looking at the precipitates of the cast structure in Figure 18 to 20 was suppressed the formation of coarse lamellae in the grain boundary, it can be seen that the precipitates are finely dispersed during the extrusion process in Figure 21 ~ Figure 23 of the extruded material.

24 to 25 is a thermal conductivity measurement graph of the present invention, as shown in Table 1, the alloy of the present invention has a thermal conductivity of 115W / mK or more 51 ~ 54W / of WE43 used as a conventional AZ-based alloy or currently flame retardant alloy Compared with the 24W / mK of mK and ZE41, it shows higher thermal conductivity and satisfies the flame retardancy test. Figure 26 is a flame retardant test scene of the present invention, the alloy of the present invention has the effect of improving the flame retardancy due to the small effect of shortening the melt time by the local heating when it touches the flame due to the excellent thermal conductivity.

[Description of the code]

1: mold 2: billet casting material

3: cover 4: shielding gas piping

5: injection hole 6: continuous casting billet support

7: Coolant injection nozzle 8: Mashed zone

21: lever 22: impeller

41: induction coil M: motor

Claims

Zinc: 0.5 to 5% by weight, tin: 0.6 to 3.5%, which is a high melting point oxide forming element, remainder: consisting of inevitable impurities and magnesium, among which the aluminum and zirconium are managed to be 0.5% or less. Magnesium alloy with excellent thermal conductivity and flame retardancy and easy plastic processing.
The method of claim 1,

The magnesium alloy is added at least 1.5% by weight of at least one selected from calcium, silicon, manganese, and mischmetal, and the total amount of the alloying elements is 2.5 to 6.3%, and the plastic processing is excellent in thermal conductivity and flame retardancy. This easy magnesium alloy.
Magnesium now is put into a melting furnace in a state of being cut off from the atmosphere to dissolve the molten magnesium to prepare a molten magnesium and maintain the temperature at 680 ~ 720 ° C.

Adding zinc to the molten magnesium and dissolving it to prepare a magnesium-zinc alloy molten metal;

Preparing a magnesium alloy molten metal by adding at least one selected from tin, yttrium, mismetal, calcium, silicon, and manganese as a master alloy to the magnesium-zinc molten metal and performing mechanical stirring;

Cooling the mold containing the magnesium alloy molten metal to produce a casting material, characterized in that the excellent thermal conductivity and flame retardancy, the plastic process is easy to manufacture a magnesium alloy manufacturing method.
The method of claim 3,

The cooling of the molten metal is cooled by charging a mold into a tank filled with a refrigerant or by spraying a refrigerant on the side of the mold, while mechanically agitating the molten metal during cooling to disperse the coagulation nuclei in the mash zone while providing excellent thermal conductivity and flame retardancy. Method for producing magnesium alloy easy plastic processing.
The method of claim 4, wherein

Stirring during cooling of the molten metal is excellent in thermal conductivity and flame retardancy, and plastic processing, characterized in that the impeller having a diameter of 1/5 to 2/3 of the billet diameter is inserted into the mold to mechanically stir to disperse the solidified core of the mash zone. Method for producing this easy magnesium alloy.
The method of claim 3,

While cooling the mold in the cooling step at a cooling rate of 70-200 ° C./min in the water tank in the cooling step, while mechanically stirring the molten metal in the mold during cooling, the thermal conductivity and flame retardancy are excellent, Method for producing magnesium alloy easy plastic processing.
The method of claim 3,

In the cooling step, the magnesium alloy molten metal is injected into the mold of the continuous casting apparatus, and the refrigerant is injected to the mold side and the billet surface, and cooled at a cooling rate of 200 to 900 ° C / min, during which the impeller is inserted into the mold and mechanically stirred. A method of producing a magnesium alloy having excellent thermal conductivity and flame retardancy, and easy plastic working, characterized in that the coagulation nuclei in the mash zone are dispersed.