Abstract:
The invention belongs to magnesium alloy design field, and relates to a low-cost high-plasticity wrought magnesium alloy. The magnesium alloy is made from the raw materials with components as follows: between 0.10% and 1.00% by mass of tin, between 0.10% and 3.00% by mass of aluminum, between 0.10% and 1.00% by mass of manganese, and commercially pure magnesium and inevitable impurities in balance. The magnesium alloy is prepared by the steps of: melting magnesium and aluminum, adding tin and then adding microalloyed element manganese, stirring, refining, casting to form ingots followed by homogenized heat treatment, and extruding to obtain a corresponding profile; or directly extruding to obtain a corresponding profile without homogenization. The invention is characterized by controlling the content of the high-cost raw material tin through using the raw material aluminum that is low in cost and low in melting point to obtain a low-cost high-plasticity wrought magnesium alloy.

Description:
[0001]    This application claims priority to Chinese Patent Application No. 201310258233.7 filed Jun. 26, 2013, which is incorporated by reference herein. 
       TECHNICAL FIELD 
       [0002]    The invention relates to metal material field, in particular, a low-cost high-plasticity wrought magnesium alloy with both good strength and toughness and its preparation method. 
       BACKGROUND ART 
       [0003]    Magnesium alloy has the merits of low density, high specific strength, and excellent electromagnetic shielding performance as well as good damping property and the like, and it has abundant magnesium resources in China. Nowadays, energy source becomes less and less, and people have an urgent desire to use magnesium alloy having a lower density in a large scale instead of structural materials having a higher density, so as to achieve energy saving and exhaust reduction. Hence, magnesium alloy has become the focus of research owing to the pursuit of light weight. However, there are not many types of existing developed commercial magnesium alloy. The solid solubility of tin in magnesium at the eutectic temperature of 561° C. is 14.48%, and it is obtained that the solid solubility at room temperature is only less than 1.00%, providing a wide variation range of solid solubility. It is possible to utilize the variation of solid solubility along with temperature to generate a precipitate which led to good dispersion strengthening contributions, so magnesium-tin systems have gained attention of many researchers in recent years. 
         [0004]    WANG Huiyuan, et al. discloses a magnesium-tin-aluminum-strontium-manganese multi-component wrought magnesium alloy in a Chinese patent (publication No.: CN101985714a) entitled “a high-plasticity magnesium ally and its preparation method”, In this patent, a high-plasticity magnesium-tin-aluminum-manganese-strontium wrought magnesium alloy is prepared by the process of cast rolling or conventional casting followed by deformation, and comprises 0.10% to 3.00% by mass of tin, 0.10% to 6.00% by mass of aluminum, 0.01% to 2.00% by mass of manganese and 0.001% to 2.00% by mass strontium. With respect to the above alloy, the added amount of tin is relatively large, and the price of tin is higher than conventional alloying elements such as aluminum, leading to increase in cost of the alloy. In this project, the addition of strontium has no evident influence on the property of the alloy, but would increase the cost. In addition, strontium element is highly active and very combustible in air, thus addition of strontium to the magnesium alloy easily causes burning loss of the alloy during melting, going against control on the components. 
       DESCRIPTION OF THE INVENTION 
       [0005]    The invention provides a wrought magnesium alloy and its preparation method, for the purpose of decreasing alloy cost and preventing burning loss during melting while ensuring high plasticity thereof. 
         [0006]    The raw materials of the low-cost wrought magnesium alloy involved in the present invention comprise between 0.10% and 1.00% by mass of tin, between 0.10% and 3.00% by mass of aluminum, between 0.10% and 1.00% by mass of manganese, and commercially pure magnesium and inevitable impurities in balance, wherein the commercially pure magnesium, aluminum and tin all have a purity of 99.00% or more; and manganese is added in the form of 4% magnesium-manganese intermediate alloy. 
         [0007]    Preferably, the aluminum is in an amount of 1.00% by mass, the tin is in an amount of 1.00% by mass, and the manganese is in an amount of 0.30% by mass. 
         [0008]    The magnesium alloy is prepared by the steps of: 
         [0009]    (1) smelting ingots: weighing out the raw materials according to the components, smelting pure magnesium and pure aluminum at a temperature between 720° C. and 740° C. under protection of a protective gas, increasing the temperature to 740° C. after all components are molten, adding tin that has been preheated to 150° C. and magnesium-manganese intermediate alloy that has been preheated to 300° C. to 400° C. after the temperature is stable, adding a refining agent and fully stirring for 3 min to 6 min, standing at 720° C. for 10 min to 20 min, removing the dross on the surface of the melting metal liquid, and casting at a temperature of 660° C. to iron mold preheated at 250-350° C.; 
         [0010]    (2) homogenizing: covering the magnesium ingots obtained from the step (1) with graphite, homogenizing at 410° C. to 500° C. for 24 h, and subsequently water quenching to obtain homogenized samples; 
         [0011]    (3) extruding: preheating the magnesium ingots homogenized in the step (2) at 250° C. to 350° C. for 2 h after turning of outer cylinder of the as-homogenized ingots, coating a magnesium alloy lubricant thereon, and extruding at 250° C. to 350° C. with an extrusion ratio of 20:1 to 80:1 at an extrusion speed of 0.50 to 3.00 m/min to obtain profiles; or preheating the magnesium ingots from the step (1) at 250° C. to 350° C. for 2 h after turning the outer cylinder, coating a magnesium alloy lubricant, and extruding at 250° C. to 350° C. (preferably controlled at 300° C.) with an extrusion ratio of 20:1 to 80:1 at an extrusion speed of 0.50 to 3.00 m/min. 
         [0012]    The invention optimizes the alloy components based on the magnesium-tin-aluminum-manganese-strontium multi-component wrought magnesium alloy, and reduces the content of the alloying element tin that is more expensive without reducing the strength and plasticity of the alloy, so as to decrease the cost. The addition amount of tin is not more than 1.00% in the present invention, because the inventors found, by a great deal of investigation and magnesium-tin-aluminum ternary phase diagrams, the solid solubility of tin in magnesium at about 250° C. is almost zero, and when tin is added in an amount of less than 1.00%, a large quantity of small and dispersed Mg 2 Sn could be precipitated through extrusion at 250° C. as a second phase which could improve the strength of the alloy. Since the second phase of Mg 2 Sn generated by precipitation of tin and magnesium is precipitated from the wrought magnesium alloy in parallel to the base plane, so addition of excessive tin does not make much contribution to the strength. Moreover, it was found in combination with a great deal of experiments that when the addition amount of tin is more than 1.00%, the yield strength does not change evidently with the increase in the content of tin (e.g. example 6). In addition, for cast magnesium alloy, the strontium added has good effect on purifying melt and refining grains. However, strontium does not play an important role in refining the grains in wrought magnesium alloy, and the wrought magnesium alloy is mainly an alloy with fine grains obtained by the processing means such as extruding and rolling. On the other hand, since the content of tin in the system is not very high, and the Mg 2 Sn precipitated after deformation exhibits a small and dispersed morphology, addition of strontium as an alloying element would not have a large effect on improving the morphology of the precipitated phase. Therefore, addition of strontium in wrought magnesium alloy has no remarkable influence on the properties of the alloy, but increases the cost. Furthermore, it was found by comparison (example 7) that strontium-containing alloy tends to combust during smelting, thereby inducing deterioration of melt quality and finally causing decrease in the elongation. Consequently, the present invention achieves refinement of grains by extrusion at a relatively low temperature (such as 250-300° C., particularly 300° C.) and prepares a low-cost high-plasticity magnesium-tin-aluminum-manganese wrought magnesium alloy, without using strontium element that easily leads to burning loss of melt under the premise of not influencing the properties of the alloy. 
     
    
     PREFERRED EMBODIMENT OF THE INVENTION 
     EXAMPLES 
     Example 1 
       [0013]    (1) The following components were weighed out according to weight percentage: tin 1.00%; aluminum 1.00%; manganese 0.30%; and magnesium in balance. 
         [0014]    (2) Pure magnesium and pure aluminum were smelted at a temperature between 720° C. and 740° C. under protection of a protective gas, the temperature was increased to 740° C. after all components were molten, after the temperature was stable, tin that had been preheated to 150° C. and magnesium-manganese intermediate alloy that had been preheated to 300° C. were added. Subsequently, hexachloroethane was added as a refining agent, and the mixture was fully stirred for 3 min to 6 min. The resultant melt was allowed to stand at 720° C. for 20 min, the dross on the surface was removed, and the melt at a temperature of 660° C. was casted to an iron mold that had been preheated to 350° C. to obtain ingots. 
         [0015]    (3) The magnesium ingots obtained were covered with graphite, homogenized at 420° C. for 24 h, and then water quenched to obtain homogenized samples. 
         [0016]    (4) The magnesium ingots homogenized were preheated at 250° C. for 2 h after skin layer removing, coated with a magnesium alloy lubricant, and extruded at 250° C. with an extrusion ratio of 25:1 at an extrusion speed of 0.90 to 1.20 m/min to obtain rods. The obtained alloy has a tensile strength of 289 MPa, a yield strength of 255 MPa and an elongation of 21.0%. 
       Example 2 
       [0017]    (1) The following components were weighed out according to weight percentage: tin 1.00%; aluminum 1.00%; manganese 0.30%; and magnesium in balance. 
         [0018]    (2) Pure magnesium and pure aluminum were smelted at a temperature between 720° C. and 740° C. under protection of a protective gas, the temperature was increased to 740° C. after all components were molten, after the temperature was stable, tin that had been preheated to 150° C. and magnesium-manganese intermediate alloy that had been preheated to 400° C. were added. Subsequently, hexachloroethane was added as a refining agent, and the mixture was fully stirred for 3 min to 6 min. The resultant melt was allowed to stand at 720° C. for 10 min, the dross on the surface was removed, and the melt at a temperature of 660° C. was casted to an iron mold that had been preheated to 250° C. to obtain ingots. 
         [0019]    (3) The magnesium ingots obtained were covered with graphite, homogenized at 420° C. for 24 h, and then water quenched to obtain homogenized samples. 
         [0020]    (4) The magnesium ingots homogenized were preheated at 300° C. for 2 h after skin layer removing, coated with a magnesium alloy lubricant, and extruded at 300° C. with an extrusion ratio of 25:1 at an extrusion speed of 0.90 to 1.20 m/min to obtain rods. The obtained alloy has a tensile strength of 293 MPa, a yield strength of 260 MPa and an elongation of 21.0%. 
       Example 3 
       [0021]    (1) The following components were weighed out according to weight percentage: tin 1.00%; aluminum 1.00%; manganese 0.30%; and magnesium in balance. 
         [0022]    (2) Pure magnesium and pure aluminum were smelted at a temperature between 720° C. and 740° C. under protection of a protective gas, the temperature was increased to 740° C. after all components were molten, after the temperature was stable, tin that had been preheated to 150° C. and magnesium-manganese intermediate alloy that had been preheated to 350° C. were added. Subsequently, hexachloroethane was added as a refining agent, and the mixture was fully stirred for 3 min to 6 min. The resultant melt was allowed to stand at 720° C. for 15 min, the dross on the surface was removed, and the melt at a temperature of 660° C. was casted to an iron mold that had been preheated to 300° C. to obtain ingots. 
         [0023]    (3) The cast magnesium ingots were preheated at 300° C. for 2 h after skin layer removing, coated with a magnesium alloy lubricant, and extruded at 300° C. with an extrusion ratio of 25:1 at an extrusion speed of 0.90 to 1.20 m/min to obtain rods. The obtained alloy has a tensile strength of 290 MPa, a yield strength of 262 MPa and an elongation of 20.0%. 
       Example 4   
       [0024]    (1) The following components were weighed out according to weight percentage: tin 0.75%; aluminum 1.00%; manganese 0.30%; and magnesium in balance. 
         [0025]    (2) Pure magnesium and pure aluminum were smelted at a temperature between 720° C. and 740° C. under protection of a protective gas, the temperature was increased to 740° C. after all components were molten, after the temperature was stable, tin that had been preheated to 150° C. and magnesium-manganese intermediate alloy that had been preheated to 350° C. were added. Subsequently, hexachloroethane was added as a refining agent, and the mixture was fully stirred for 3 min to 6 min. The resultant melt was allowed to stand at 720° C. for 18 min, the dross on the surface was removed, and the melt at a temperature of 660° C. was casted to an iron mold that had been preheated to 300° C. to obtain ingots. 
         [0026]    (3) The magnesium ingots obtained were covered with graphite, homogenized at 420° C. for 24 h, and then water quenched to obtain homogenized samples. 
         [0027]    (4) The magnesium ingots homogenized were preheated at 300° C. for 2 h after skin layer removing, coated with a magnesium alloy lubricant, and extruded at 300° C. with an extrusion ratio of 25:1 at an extrusion speed of 0.90 to 1.20 m/min to obtain rods. The obtained alloy has a tensile strength of 283 MPa, a yield strength of 230 MPa and an elongation of 20.0%. 
       Example 5 
       [0028]    (1) The following components were weighed out according to weight percentage: tin 1.00%; aluminum 2.00%; manganese 0.30%; and magnesium in balance. 
         [0029]    (2) Pure magnesium and pure aluminum were smelted at a temperature between 720° C. and 740° C. under protection of a protective gas, the temperature was increased to 740° C. after all components were molten, after the temperature was stable, tin that had been preheated to 150° C. and magnesium-manganese intermediate alloy that had been preheated to 350° C. were added. Subsequently, hexachloroethane was added as a refining agent, and the mixture was fully stirred for 3 min to 6 min. The resultant melt was allowed to stand at 720° C. for 20 min, the dross on the surface was removed, and the melt at a temperature of 660° C. was casted to an iron mold that had been preheated to 300° C. to obtain ingots. 
         [0030]    (3) The magnesium ingots obtained were covered with graphite, homogenized at 420° C. for 24 h, and then water quenched to obtain homogenized samples. 
         [0031]    (4) The magnesium ingots homogenized were preheated at 300° C. for 2 h after skin layer removing, coated with a magnesium alloy lubricant, and extruded at 300° C. with an extrusion ratio of 25:1 at an extrusion speed of 0.90 to 1.20 m/min to obtain rods. The obtained alloy has a tensile strength of 280 MPa, a yield strength of 210 MPa and an elongation of 21.6%. 
       Comparative Example 6 
       [0032]    (1) The following components were weighed out according to weight percentage: tin 3.00%; aluminum 1.00%; manganese 0.30%; and magnesium in balance. 
         [0033]    (2) Pure magnesium and pure aluminum were smelted at a temperature between 720° C. and 740° C. under protection of a protective gas, the temperature was increased to 740° C. after all components were molten, after the temperature was stable, tin that had been preheated to 150° C. and magnesium-manganese intermediate alloy that had been preheated to 300° C. to 400° C. were added. Subsequently, hexachloroethane was added as a refining agent, and the mixture was fully stirred for 3 min to 6 min. The resultant melt was allowed to stand at 720° C. for 10 min to 20 min, the dross on the surface was removed, and the melt at a temperature of 660° C. was casted to an iron mold that had been preheated to 250° C. to 350° C. to obtain ingots. 
         [0034]    (3) The magnesium ingots obtained were covered with graphite, homogenized at 420° C. for 24 h, and then water quenched to obtain homogenized samples. 
         [0035]    (4) The magnesium ingots homogenized were preheated at 300° C. for 2 h after skin layer removing, coated with a magnesium alloy lubricant, and extruded at 300° C. with an extrusion ratio of 25:1 at an extrusion speed of 0.90 to 1.20 m/min to obtain rods. The obtained alloy has a tensile strength of 288 MPa, a yield strength of 253 MPa and an elongation of 20.0%. 
       Comparative Example 7 
       [0036]    (1) The following components were weighed out according to weight percentage: tin 1.00%; aluminum 3.00%; manganese 0.30%; strontium 0.30% and magnesium in balance. 
         [0037]    (2) Pure magnesium and pure aluminum were smelted at a temperature between 720° C. and 740° C. under protection of a protective gas, the temperature was increased to 740° C. after all components were molten, after the temperature was stable, tin that had been preheated to 150° C. as well as magnesium-manganese intermediate alloy and magnesium-strontium intermediate alloy that had been preheated to 300° C. to 400° C. were added. Subsequently, hexachloroethane was added as a refining agent, and the mixture was fully stirred for 3 min to 6 min. The resultant melt was allowed to stand at 720° C. for 10 min to 20 min, the dross on the surface was removed, and the melt at a temperature of 660° C. was casted to an iron mold that had been preheated to 250° C. to 350° C. to obtain ingots. 
         [0038]    (3) The magnesium ingots obtained were covered with graphite, homogenized at 420° C. for 24 h, and then water quenched to obtain homogenized samples. 
         [0039]    (4) The magnesium ingots homogenized were preheated at 300° C. for 2 h after skin layer removing, coated with a magnesium alloy lubricant, and extruded at 300° C. with an extrusion ratio of 25:1 at an extrusion speed of 0.90 to 1.20 m/min to obtain rods. The obtained alloy has a tensile strength of 295 MPa, a yield strength of 205 MPa and an elongation of 17.5%. 
         [0040]    In each of the above examples, the protective gas is a mixture of sulfur hexafluoride and carbon dioxide, which comprises carbon dioxide supplemented with 0.5%-1.5% of sulfur hexafluoride.