Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C23/00—Alloys based on magnesium
  • C22C23/02—Alloys based on magnesium with aluminium as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C23/00—Alloys based on magnesium
  • C22C23/04—Alloys based on magnesium with zinc or cadmium as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/06—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of magnesium or alloys based thereon

Definitions

• the present invention relates to high strength magnesium alloys and a method for preparing the same, and more specifically to magnesium alloys having improved mechanical properties including strength, hardness, and elongation while having an improved formability, high strength and elongation and their economical processing method, by adding specific alloying elements or changing processing conditions including specific heat treatments.
• Mg-Zn-based alloys exhibit a superior age hardening behavior. These alloys exhibit relatively high strength and ductility while having advantages in that it is easily processible and weldable.
• Mg-Zn-based alloys have also disadvantages since it is difficult for them to be applied to a casting process such as die-casting because the addition of Zn to Mg tends to increase the formation of micro- pores during casting.
• Mg-Zn-based alloys have a limitation in improvement in strength because it is not easy to refine the microstructure by the addition of alloying elements or an over-heating heat treatment, compared to other magnesium alloys. This limitation has restricted their commercial applications.
• research efforts have been made to add alloying elements to an Mg-Zn binary alloy. Examples of these research efforts are as follows.
• J. P. Doan and G. Ansel have proposed a method in which Zr is added to an Mg-Zn-based alloy to refine the grain size of the alloy, thereby improving the strength of the alloy (J. P. Doan and G. Ansel, Trans. AIME, vol. 171 (1947), pp. 286-295).
• this method there is a difficulty in adding Zr to magnesium melts because Zr has a high melting point.
• the addition of rare earth elements such as La, Ce or Nd, or Th to Mg-Zn alloy has been known. This method is known to have advantages in that it is possible to reduce micro-pore formation and improve strength at high temperatures and weldability.
• this method has significant cost disadvantages due to expensive alloying elements, compared to other magnesium alloys.
• Table 1 shows tensile properties of commercial cast alloys and wrought alloys.
• US Patent No. 4,997,622 discloses properties of magnesium alloys prepared by a rapid solidification processing method. According to This Patent, magnesium alloys prepared by a rapid solidification processing method exhibit improved yield strength, tensile strength and elongation. However, the results of research made up to the present show that those alloys have high processing costs and limited applications, compared to the existing commercial alloys.
• An object of the invention is to provide high strength magnesium alloys exhibiting improvements in refining of the microstructure and precipitation behavior, enhancements in mechanical properties such as hardness, strength, and elongation, and an improvement in formability in accordance with an addition of alloying elements, less expensive than those conventionally used, to an Mg-Zn-based alloy.
• Another object of the invention is to provide a method for preparing high strength magnesium alloys exhibiting a definitely superior elongation, compared to strength, using an optimum heat treatment condition, and to provide an economical processing condition for the preparation method.
• the present invention provides high strength magnesium alloys consisting essentially of 3 ⁇ 10 wt.% Zn, 0.25 ⁇ 3.0 wt.% Mn, and the balance of Mg and inevitable impurities.
• the magnesium alloy may further contain 1 ⁇ 6 wt.% Al.
• the magnesium alloy may further contain 0.1 ⁇ 4.0 wt.% Si, or a combination of 0.1 - 4.0 wt.% Si and 0.1 ⁇ 2.0 wt.% Ca.
• the content of Al is not more than the content of Zn.
• the content of Zn is 5.0 ⁇ 7.0 wt.%
• the content of Mn is 0.75 ⁇ 2.0 wt.%
• the content of Si is 1.5 ⁇ 3.0 wt.%
• the content of Ca is 0.3 - 1.0 wt.%.
• the important feature of the present invention is to add Al, as an alloying element, to an Mg-Zn-based alloy so as to achieve a decrease in yield strength resulting in an improvement in formability and an enhancement in work hardening ability, thereby to provide a high strength magnesium alloy having a high strength and a high elongation.
• the present invention provides a method for preparing the high strength magnesium alloy, in which an addition of Mn to a magnesium melt is achieved by adding a Zn-Mn mother alloy to the magnesium melt.
• the high strength magnesium alloy is prepared in the form of a cast ingot by adding a Zn-Mn mother alloy having an Mn content of
• the high strength magnesium alloy may be preferably prepared in the form of a cast ingot by adding a Zn-10 - 20 wt.% Mn mother alloy to the magnesium melt in a temperature range of 670 to 720°C, adding an Mg-Si mother alloy to the magnesium melt, and adding Zn or Zn along with Al and/or Ca to the magnesium melt.
• the cast ingot may be subsequently subjected to a homogenization process in a temperature range of 340 to 410°C for 6 to 12 hours, so that it is formed into a billet.
• the billet may be mechanically worked after being preheated in a temperature range of 150 to 400°C for 30 minutes to 2 hours.
• the worked or wrought body may be subjected to a primary aging process in a temperature range of 70 to 100°C for 24 to 96 hours, and then to a secondary aging process in a temperature range of 150 to 180°C for 48 hours or more.
• a solution heat treatment may be carried out in a temperature range of 340 to 410°C for 6 to 12 hours.
• a stretching of 3 to 7% may be carried out prior to the double aging process.
• composition range of each alloying element used in accordance with the present invention is limited to the above mentioned range is as follows:
• the maximum solid solubility limit of Zn in an Mg matrix is 6.2 wt.% at 340°C. Where Zn is added in an amount of 3.0 wt.% or more to Mg matrix, it forms an acicular precipitation phase when it is subjected to a heat treatment, thereby exhibiting an age hardening behavior. Generally, the content of Zn is determined, based on the solid solubility limit thereof. When Zn is added in an amount of about 5.0 ⁇ 7.0 wt.% approximating to the maximum solid solubility limit thereof, it is possible to obtain a maximized age hardening behavior.
• the content of Zn is limited to a range of 3 ⁇ 10 wt.%, preferably a range of 5.0 ⁇ 7.0 wt.%, in accordance with the present invention.
• Mn Manganese
• Mg Manganese
• Mn contributes to an improvement in corrosion resistance when it is added in an amount of 0.1 wt.% or more.
• Mg is added for purposes other than the improvement in corrosion resistance, for example, a strengthening purpose, it may contribute to an improvement in the strength of the alloy product at its content of 0.25 ⁇ 2.0 wt.% even though the effect may be varied, depending on the matrix alloy of the alloy product.
• Mn existing in the wrought body serves to refine the precipitation phase of the Mg-Zn binary alloy when the wrought body is subjected to an aging process following a solution heat treatment, thereby providing effects of an improvement in strength and an improvement in elongation. Based on this fact, addition of Mn is made to strengthen the alloy in accordance with the present invention.
• the minimum content of Mn is determined to be 0.25 wt.% in accordance with the present invention. Taking into consideration the maximum solid solubility limit of Mn and the processing method used, it is difficult to add a large amount of Mn using a general melting process. Where Mn is added in an amount of 3.0 wt.% or more, it is mainly present in the form of ⁇ -Mn in the matrix. Thus, the surplus amount of Mn does not contribute to the improvement in the properties of the alloy, but results in an undesirable result in terms of the preparation costs. Accordingly, the content of Mn is limited to a range of 0.25 ⁇ 3.0 wt.%, preferably a range of 0.75 ⁇ 2.0 wt.%, in accordance with the present invention.
• Al exhibits a maximum solid solubility limit of about 12 wt.% at 437°C. It is known that an Mg ⁇ AI- ⁇ 2 precipitation phase is formed in Mg-AI binary alloys in accordance with a heat treatment used. However, addition of Al according to the present invention is irrespective of the formation of such Mg- t AI 12 precipitation phase. In accordance with the present invention, the addition of Al is adapted to improve the major strengthening phase in an Mg-Zn-Mn-based ternary alloy, that is, the Mg-Zn- based acicular precipitation phase.
• the content of Al is determined within a range involving limited formation of Mg-AI-based precipitation phases, taking into consideration the temperature range of heat treatment, such as the aging temperature range, and the content of Zn added as a major alloying element.
• the lower content limit of Al is determined to be 1.0 wt.% because the solid solubility limit of Al in the Mg matrix corresponds to about 1 wt.% in the aging temperature range.
• the upper content limit of Al is determined to be 6.0 wt.%.
• the possibility of the formation of the Mg-AI-based precipitation phase is greatly increased.
• Such a precipitation phase may be coarsely precipitated at grain boundaries, and even interior of grains at a certain heat treatment temperature. Since this precipitation phase is very brittle in terms of strength, it provides a fracture path when the alloy is subjected to a fracture, thereby resulting in a degradation in strength. For this reason, it is desirable for the content of Al to be less than the content of Zn.
• Si is hardly soluble in an Mg matrix
• Si forms an Mg 2 Si phase, when it is added to the Mg matrix as an alloying element.
• Such a compound may provide a dispersion strengthening effect when its morphology and/or size is modified in the preparation and heat treatment procedures of the wrought body.
• the inventors experimentally found that a desired dispersion strengthening effect, as mentioned above, is obtained when Si is added to an Mg-Zn-AI-Mn-based quaternary alloy.
• the content of Si is less than 0.1 wt.%, the intended effect of the Si addition can hardly be expected.
• the content of Si is limited to a range of 0.1 ⁇ 4.0 wt.%, preferably a range of 1.5 ⁇ 3.0 wt.%, in accordance with the present invention.
• Ca 0.1 ⁇ 2.0 wt.%
• Ca is added to an Si- containing Mg-Zn-AI-Mn alloy in accordance with the present invention.
• a Ca content of less than 0.1 wt.% it is hardly expected to observe the effect of improving the Mg 2 Si phase.
• Ca is added in an amount of 2.0 wt.% exceeding considerably the maximum solid solubility limit of Ca in the Mg matrix at 516°C, that is, 1.34 wt.%, an Mg 2 Ca precipitation phase is formed.
• the inventors experimentally found that it is possible to more effectively control the morphology of the Mg 2 Si phase formed in the Mg-Zn-AI-Mn alloy at a Ca content of 0.3 - 1.0 wt.%, and thus to obtain improvements in strength and elongation.
• the content of Ca is limited to a range of 0.1 - 2.0 wt.%, and preferably 0.3 - 1.0 wt.%, in accordance with the present invention.
• the principal impurities of the Mg alloy should be appropriately limited because they mainly have fatally adverse affects on the corrosion resistance of the alloy, rather than on the mechanical properties of the alloy.
• Impurities generally known include Fe, Ni, and Cu. Although Cu has adverse effects on corrosion resistance in Mg-AI-based alloys widely used, it has no significant effect in the Mg-Zn-based alloy according to the present invention.
• Fe and Ni are regarded as impurities to be limited in their contents. Typically, these impurities are conservatively limited to a maximum content of 0.005 wt.%. Adverse effects resulting from Fe may be eliminated by an addition of Mn.
• Mg alloys adverse effects of Fe can be minimized by the reduction of the content ratio between Fe and Mn, Fe/Mn, to 0.032 or less. Since Mn is basically added in accordance with the present invention, it is possible to effectively eliminate adverse effects of Fe on corrosion resistance in so far as the content of Fe is less than the conservative limit. In the case of Mg alloys, the remaining impurities including Fe, Ni, and Cu are typically limited to a maximum content of 0.3 wt.% based on the total content thereof.
• the preparation method of the present invention which uses a fluxless melting method in addition to the above mentioned specific composition, has an important feature in that Mn is added in the form of a Zn-Mn mother alloy, taking into consideration of the fact that it is impossible to add Mn to molten magnesium using a method of directly melt Mn into the molten magnesium, because Mn has a very high melting point.
• a method was used in which Mn is added in the form of a flux. Since molten magnesium involves a danger of burning when it is exposed to air, a flux is used which serves to shield the molten magnesium from air, thereby inhibiting the danger of burning.
• an Mn-added flux is conventionally used to achieve a desired addition of Mn.
• Mn is penetrated into the melt by diffusion.
• this method there is a limitation on the amount of Mn added.
• this method involves many difficulties associated with the preparation of an intended alloy.
• Mn is added to molten magnesium heated to a temperature, at which Mn melts directly, in a protective gas atmosphere capable of preventing the molten magnesium to burn.
• an Mg-Mn mother alloy is separately prepared.
• a desired amount of Mn is added using the prepared Mg-Mn mother alloy.
• this method requires an expensive melting device configured to control the given atmosphere.
• a large amount of magnesium may be lost during the preparation of the mother alloy, because magnesium exhibits a high vapor pressure at a high temperature.
• the method using the Mg-Mn mother alloy involves an increase in the processing costs.
• the temperature of the magnesium melt, to which the Zn- Mn mother alloy is added is limited to a range of 670 to 720°C, taking into consideration of the fact that although the melting point of magnesium is about 650°C, the magnesium melt secures a sufficient fluidity at a temperature of at least 670°C, and the fact that there is an increased possibility of burning at a temperature of the magnesium melt exceeding 720°C.
• the Zn-Mn mother alloy preferably has a Zn-10 - 20 wt.% Mn composition having an Mn content of 10 to 20 wt.% so that it is sufficiently melted in the above mentioned temperature range of the magnesium melt.
• a stirring process is carried out during the addition of the Zn-Mn mother alloy to the magnesium melt.
• Si is added in the form of an Mg-Si mother alloy.
• the temperature, at which the addition of Si is carried out is limited to a range of 700 to 720°C, taking into consideration of a high melting point of the mother alloy and a desired inhibition of burning at the surface of the magnesium melt. In this case, it is more preferable to conduct a stirring process during the addition of the mother alloy.
• Zn is added alone or along with Al.
• Ca may be selectively added.
• the addition of Zn is carried out after a furnace cooling process conducted for the magnesium melt, in order to reduce the loss of Zn exhibiting a high vapor pressure at the alloy preparation temperature used.
• the furnace cooling may be carried out to a temperature of about 670°C, taking into consideration of the fluidity of the magnesium melt. It is more preferable to conduct a stirring process during the addition of those elements.
• the resultant Mg melt is then cast to form an ingot.
• the casting is conducted after the Mg melt is furnace-cooled to a temperature of 660 to 670°C, in order to inhibit the generation of heat from the Mg melt as much as possible.
• the cast alloy ingot prepared in accordance with the above mentioned method is subjected to a homogenization treatment in order to eliminate segregation of alloying elements possibly generated during the casting process, and non-uniformity of the wrought body, in terms of properties, resulting from the segregation.
• the homogenization treatment is carried out at a temperature of 340 to 410°C for 6 to 12 hours, taking into consideration of conditions capable of allowing precipitation phases resulting from the major alloying element, that is, Zn, to be sufficiently dissolved, and a desired thermal stability of the alloy.
• the cast ingot is formed into a billet to be extruded.
• the billet is preheated at a temperature of 150 to 400°C for 30 minutes to 2 hours, and then subjected to mechanical working processes including extrusion, rolling, forging, swaging, and drawing, in the same temperature range.
• mechanical working processes including extrusion, rolling, forging, swaging, and drawing, in the same temperature range.
• magnesium alloys do not have a desired workability at ambient temperature.
• the magnesium alloy is subjected to hot mechanical working processes.
• the working temperature is experimentally determined to be within a range capable of securing the soundness of the wrought body.
• the wrought body is primarily aged at a temperature of 70 to 100°C for 24 to 96 hours, and secondarily aged, just after the primary aging process, at a temperature of 150 to 180°C for 48 hours or more.
• Such a double aging process is adapted to maximize the effect of the precipitation phase contributing to an improvement in strength by conducting the primary aging process at a temperature not higher than the G.
• P. zone solvus temperature of the predominant precipitation phase of Mg-Zn-based alloys, that is, the ⁇ i' phase and then conducting the secondary aging process at a temperature higher than the primary aging temperature.
• the primary aging temperature is limited to a range of 70 to 100°C slightly lower than the generally-known G.
• P. zone solvus temperature of the ⁇ i' phase, and the primary aging time is determined to be a period of time enough to expect an improvement in hardness to a desired level.
• the second aging temperature is limited to a range of 150 to 180°C. At a second aging temperature of less than 150°C, there is a problem associated with the execution of the aging process because a lot of time is required until a maximum hardness is obtained. At a second aging temperature exceeding 180°C, the maximum hardness obtained cannot reach a desired level even though it is rapidly obtained.
• the wrought body is subjected, prior to the double aging process, to a solution heat treatment for 6 to 12 hours at a temperature of 340 to 410°C corresponding to a temperature range, in which precipitation phases possibly generated during the working process can be present in the form of solid solutions, in order to maximize the effect of the precipitation phase contributing to an improvement in strength.
• the temperature range and period of the solution heat treatment are determined, based on the phase diagram of the Mg-Zn binary system, taking into consideration of conditions capable of allowing precipitation phases resulting from the major alloying element, that is, Zn, to be sufficiently dissolved, and a desired thermal stability of the alloy.
• the amount of stretching in a working process involving a heat treatment to strengthen a subject alloy is limited to a range from an elastic limit to a maximum strength limit, based on a strain measured by a tensile test for the alloy conducted before the heat treatment. In accordance with the present invention, therefore, the amount of stretching is limited to a range of 3 to 7 %.
• the magnesium alloy of the present invention can exhibit an elongation improved by two times or more over the maximum strength one of the existing commercial extruded alloys described in Table 1 , that is, a ZC71 alloy, while maintaining a strength level similar to that of the ZC71 alloy.
• Mn is added in the form of a Zn-Mn mother alloy, compared to conventional methods in which Mn is added in the form of an Mg-Mn mother alloy.
• Fig. 1a is a photomicrograph of an Mg-Zn-based binary alloy extruded product (Z6);
• Figs. 1b and 1c are photomicrographs of respective extruded products of an Mg-Zn-based alloy added with Mn and an Mg-Zn-based alloy added with Al and Mn (ZM61 and ZAM621);
• Figs. 1d and 1e are photomicrographs of respective extruded products of an Mg-Zn-based alloy added with Al, Mn and Si, and an Mg-Zn- based alloy added with Al, Mn, Si and Ca (ZAM631 + 2.5Si, and ZAM631 +2.5Si + 0.4Ca);
• Fig. 2 is a graph depicting the age hardening behavior of the Mg-Zn- based binary alloy extruded product (Z6) exhibited during an aging process;
• Fig. 3 is a graph depicting a comparison of respective age hardening behaviors of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product
• Fig. 4 is a graph depicting a comparison of respective age hardening behaviors of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited during a double aging process following a solution heat treatment with the age hardening behavior of the Mg-Zn-based binary alloy extruded product (Z6) exhibited when the same treatment is conducted;
• Fig. 5 is a graph depicting respective age hardening behaviors of the Al + Mn + Si-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si) and Al + Mn + Si + Ca-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si + 0.4Ca) exhibited during a double aging process;
• Fig. 6 is a graph depicting a comparison of respective tensile properties of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited at ambient temperature with the tensile properties of the Mg-Zn-based binary alloy extruded product (Z6) exhibited at ambient temperature;
• Fig. 7 is a graph depicting respective tensile properties of the Al + Mn + Si-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si) and Al + Mn + Si + Ca-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si + 0.4Ca) exhibited at ambient temperature;
• Fig. 8 is a graph depicting a comparison of respective tensile properties of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited at ambient temperature after a double aging process with the tensile properties of the Mg-Zn-based binary alloy extruded product (Z6) exhibited at ambient temperature after the double aging process;
• Fig. 9 is a graph depicting respective tensile properties of the Al + Mn + Si-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si) and Al + Mn + Si + Ca-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si + 0.4Ca) exhibited at ambient temperature after a double aging process;
• Fig. 10 is a graph depicting a comparison of respective tensile properties of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited at ambient temperature after a double aging process following a solution heat treatment with the tensile properties of the Mg-Zn- based binary alloy extruded product (Z6) exhibited at ambient temperature when the same treatment is conducted; and Fig.
• FIG. 11 is a graph depicting a comparison of respective tensile properties of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited at ambient temperature after a double aging process following a 5% stretching process with the tensile properties of the Mg-Zn- based binary alloy extruded product (Z6) exhibited at ambient temperature when the same treatment is conducted.
• alloy cast products were prepared which have rated compositions described in the following Table 2, respectively.
• melting of a raw alloy and alloying elements was achieved using a fluxless melting method in which a mixture gas of CO2 + 0.5 % SF6 is sprayed over the surface of the melt at a flow rate of 2 ⁇ /minute.
• a steel crucible was also used in the melting process.
• Mn was added in the form of a Zn-15 wt.% Mn mother alloy to the melt at a temperature of 700°C. Thereafter, the melt was stirred for 5 minutes using a stirrer, and then cooled to 670°C in a furnace cooling fashion.
• the cooled melt was added with Zn alone or along with Al, and then stirred for 2 minutes.
• this Si was added in the form of an Mg-10 wt.% Si mother alloy to the melt.
• the resultant melt was then stirred at 720°C for 10 minutes.
• the melt was cooled to 670°C in a furnace cooling fashion.
• the cooled melt was added with Zn alone or along with Al and/or Ca, and then stirred for 2 minutes. Thereafter, the melt was furnace- cooled to 660°C. Finally, the crucible was completely dipped in water maintained at ambient temperature. Thus, an alloy cast product was prepared.
• each alloy cast product prepared as above, was subjected to a homogenization process at a temperature of 340 to 410°C for 12 hours. The alloy cast product was then formed into a billet, which was, in turn, preheated at a temperature of 320 to 360°C for 30 minutes. The billet was then extruded by an extrusion machine, in which the temperature of the billet container and die was set to a temperature of 320 to 360°C. Thus, an extruded alloy product was prepared.
• Figs. 1a, 1b, and 1c are photomicrographs of Z6, ZM61 , and ZAM621 alloy extruded products prepared as mentioned above, respectively.
• Figs. 1d and 1e are photomicrographs of ZAM631 + 2.5Si and ZAM631 + 2.5Si + 0.4Ca alloy extruded products, respectively.
• the Z6 alloy which is a conventional magnesium alloy, has a grain size of about 22 ⁇ m
• the ZM61 and ZAM621 alloys according to the present invention have grain sizes of about 12 ⁇ m and about 8 ⁇ m, respectively.
• the ZAM631 + 2.5Si and ZAM631 + 2.5Si + 0.4Ca alloys have grain sizes of about 12 ⁇ m and about 6 ⁇ m, respectively.
• the microstructural grain size of the alloy is reduced by about 1/2.
• the conventional Mg- Zn alloy is added with 1 wt.% Mn and 2 wt.% Al, its grain size is reduced by about 1/3.
• the ZAM631 + 2.5Si + 0.4Ca alloy that is, where 0.4 wt.% Ca is added to the ZAM631 + 2.5Si alloy, it has a grain size of about 6 ⁇ m corresponding to about 2/3 the grain size of the ZAM621 alloy. Consequently, it can be found that in the alloys according to the present invention, a grain size reduction is obtained by about 2/3 for those added with Mn and Al and about 3/4 for those added with Si and Ca along with Mn and Al.
• Fig. 2 is a graph depicting the age hardening behavior of the Mg-Zn- based binary alloy extruded product (Z6) exhibited during an aging process.
• Z6 alloy extruded product For the Z6 alloy extruded product, a single aging process and a double aging process were carried out to obtain maximized improvements in hardness and strength, respectively.
• the Z6 alloy extruded product was primarily aged at 90°C for 48 hours, and then secondarily aged at 180°C for 384 hours under the condition in which a variation in age hardening behavior was periodically measured within the secondary aging period.
• the measured age hardening behavior variation is depicted in Fig. 2. Referring to Fig. 2, it can be found that the alloy subjected to the double aging treatment exhibits an increase in maximum hardness and a reduction in the time taken to obtain a maximum hardness, compared to the alloy subjected to the single aging treatment.
• Fig. 3 is a graph depicting a comparison of respective age hardening behaviors of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited during a double aging process with the age hardening behavior of the Mg-Zn-based binary alloy extruded product (Z6) exhibited during the double aging process.
• ZM61 Mn-added Mg-Zn-based alloy extruded product
• ZAM621 Al + Mn-added Mg-Zn-based alloy extruded product
• the measured age hardening behavior variation is depicted in Fig. 3.
• Fig. 3 it can be found that the ZAM621 alloy prepared by adding 2 wt.% Al and 1 wt.% Mn to the Z6 alloy exhibits an improvement in hardness by about 35 % in an extruded state, and by about 20 % in a state subjected to a maximum double age treatment, compared to the Z6 alloy.
• the ZM61 alloy prepared by adding only Mn to the Z6 alloy exhibited no or little hardening effect during the aging period even though it exhibited a high hardness in an extruded state.
• the maximum hardness of the ZM61 alloy was lower than that of the Z6 alloy.
• FIG. 4 is a graph depicting a comparison of respective age hardening behaviors of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited during a double aging process following a solution heat treatment with the age hardening behavior of the Mg-Zn-based binary alloy extruded product (Z6) exhibited when the same treatment is conducted.
• ZM61 Mn-added Mg-Zn-based alloy extruded product
• ZAM621 Al + Mn-added Mg-Zn-based alloy extruded product
• each alloy was subjected to a double aging treatment.
• the age hardening behavior of each alloy resulting from the double aging treatment is depicted in Fig. 4.
• Fig. 4 it can be found that where the Z6 alloy is added with Mn alone or along with Al, it substantially exhibits an improvement in hardness during the age hardening process.
• the addition of the alloying element or elements contributed to an improvement in hardness by 10 % or more, based on the maximum hardness.
• the age hardening behavior of the ZM61 alloy prepared only by an addition of Mn was considerably different from the age hardening behavior exhibited under the heat treatment condition involving no solution heat treatment followed by the double aging treatment in that a remarkable improvement in hardness was obtained.
• the maximum hardness of the ZM61 alloy was similar to the ZAM621 alloy prepared by adding both Al and Mn.
• Fig. 5 is a graph depicting respective age hardening behaviors of the Al + Mn + Si-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si) and Al + Mn + Si + Ca-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si + 0.4Ca) exhibited during a double aging process.
• each of the ZAM631 + 2.5Si and ZAM631 + 2.5Si + 0.4Ca alloys was primarily aged at 70°C for 48 hours, and then secondarily aged at 150°C for a given period of time under the condition in which a variation in age hardening behavior was periodically measured within the secondary aging period.
• Fig. 6 is a graph depicting a comparison of respective tensile properties of the cast products of the Mn-added Mg-Zn-based alloy, extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited at ambient temperature with the tensile properties of the Mg-Zn-based binary alloy extruded product (Z6) exhibited at ambient temperature.
• ZM61 extruded product
• ZAM621 Al + Mn-added Mg-Zn-based alloy extruded product
• Z6 binary alloy extruded product
• Fig. 7 is a graph depicting respective tensile properties of the Al + Mn + Si-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si) and Al + Mn + Si + Ca-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si + 0.4Ca) exhibited at ambient temperature.
• ZAM631 + 2.5Si Al + Mn + Si + Ca-added Mg-Zn-based alloy extruded product
• Fig. 7 it can be found that the addition of 2.5 wt.% Si and 0.4 wt.% Ca to the ZAM631 alloy results in an increase in the maximum tensile strength exhibited in an extruded state. In a wrought state using an extrusion process, the alloy exhibited a superior elongation of 16 % or more. Detailed results are described in the above Table 3.
• Fig. 8 is a graph depicting a comparison of respective tensile properties of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited at ambient temperature after a double aging process with the tensile properties of the Mg-Zn-based binary alloy extruded product (Z6) exhibited at ambient temperature after the double aging process.
• ZM61 Mn-added Mg-Zn-based alloy extruded product
• ZAM621 Al + Mn-added Mg-Zn-based alloy extruded product
• Z6 Mg-Zn-based binary alloy extruded product
• Fig. 8 The tensile properties of each alloy, exhibited after the double aging process, are depicted in Fig. 8. Referring to Fig. 8, it can be found that the case involving the double aging treatment exhibits an increase in the yield strength and maximum tensile strength of each alloy, compared to the case involving no double aging treatment. It can also be found that both cases exhibit similar elongations, respectively.
• the tensile properties of each alloy, exhibited after a tensile test conducted following the double aging treatment, are described in the above Table 4.
• Fig. 9 is a graph depicting respective tensile properties of the Al + Mn + Si-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si) and Al + Mn + Si + Ca-added Mg-Zn-based alloy extruded product (ZAM631 + 2.5Si + 0.4Ca) exhibited at ambient temperature after a double aging process.
• Each extruded alloy product was primarily aged at 70°C for 48 hours, and then secondarily aged at 150°C for 24 hours.
• the tensile properties of each alloy, exhibited after the double aging process, are depicted in Fig. 9. Referring to Fig. 9.
• the ZM61 alloy prepared by adding Mn to the Z6 alloy exhibits a slight increase in tensile properties by virtue of the double aging treatment, compared to the Z6 alloy.
• the ZAM621 alloy prepared by adding both Al and Mn to the Z6 alloy exhibited a superior strength over the Z6 alloy, by virtue of the double aging treatment.
• the ZAM621 alloy exhibits a remarkable increase in maximum tensile strength. All alloys exhibit a superior elongation even after the double aging treatment.
• Fig. 10 is a graph depicting a comparison of respective tensile properties of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited at ambient temperature after a double aging process following a solution heat treatment with the tensile properties of the Mg-Zn- based binary alloy extruded product (Z6) exhibited at ambient temperature when the same treatment is conducted.
• ZM61 Mn-added Mg-Zn-based alloy extruded product
• ZAM621 Al + Mn-added Mg-Zn-based alloy extruded product
• each alloy extruded product was primarily aged at 70°C for 48 hours, and then secondarily aged at 150°C for 96 hours.
• the tensile properties of each alloy, exhibited after the double aging process, are depicted in Fig. 10. Referring to Fig. 10, it can be found that when the solution heat treatment is carried out prior to the double aging treatment, the ZM61 alloy exhibits a considerable increase in yield strength and maximum tensile strength while exhibiting an elongation similar to that of the Z6 alloy. Although the ZAM621 alloy exhibited a reduced yield strength, compared to the ZM61 alloy, it was similar to the ZM61 alloy in terms of the maximum tensile strength. In particular, the ZMA621 alloy exhibited a considerable increase in elongation.
• Table 5 The tensile properties of each alloy exhibited at ambient temperature after the double aging treatment following the solution heat treatment are described in the following Table 5.
• Fig. 11 is a graph depicting a comparison of respective tensile properties of the cast products of the Mn-added Mg-Zn-based alloy extruded product (ZM61) and Al + Mn-added Mg-Zn-based alloy extruded product (ZAM621) exhibited at ambient temperature after a double aging process following a 5% stretching process with the tensile properties of the Mg-Zn- based binary alloy extruded product (Z6) exhibited at ambient temperature when the same treatment is conducted.
• ZM61 Mn-added Mg-Zn-based alloy extruded product
• ZAM621 binary alloy extruded product
• each extruded alloy product was primarily aged at 70°C for 48 hours, and then secondarily aged at 150°C for 96 hours.
• the tensile properties of each alloy, exhibited after the double aging process, are depicted in Fig. 11. Referring to Fig. 11 , it can be found that the ZAM621 alloy exhibits an improvement in strength, compared to the case involving no stretching process.
• the ZAM621 alloy also exhibited an elongation of 20 % or more. All alloys substantially exhibited an improvement in strength by virtue of the stretching process followed by the double aging process.
• the present invention provides a magnesium alloy having improvements in the hardness and strength at ambient temperature and an enhancement in elongation by adding Mn alone or along with Al to an Mg-Zn binary alloy while simultaneously adding Si alone or along with Ca to the Mg-Zn binary alloy to prepare a wrought body having a reduced grain size, and conducting a heat treatment or a working process involving a heat treatment for the wrought body.

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