Abstract:
A process for producing a lithium-containing alloy material is described. The process supplies a light alloy material applicable to the design of lightweight structural components. The process includes first melting alloy materials at a required ratio into a homogeneous alloy melt, then pouring the alloy melt into a ladle protected with an inert gas and pre-filled with a lithium material, where the lithium material is vigorously flushed and mixed with a hot stream of the alloy melt, and diffused into the alloy melt, and then after uniformly mixing, pouring the lithium-containing alloy melt into a mold to form an ingot and produce a lithium alloy. The process solves the fundamental problems of both contamination and uncontrolled component caused by longtime overheat in traditional melting techniques, and is a novel, safe, economic, and efficient manufacture process.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an alloy material, and particularly to a process for producing a lithium-containing alloy material. 
         [0003]    2. Related Art 
         [0004]    Lithium material is a metal having the lowest density (the density of lithium is 0.534 g/cm 3 ), which makes the density of its alloys relatively decreased, and it is thus a most preferable candidate in the production and design of lightweight structural components. However, lithium is a very active element, has a low melting point (the melting point of lithium is 180.54° C.), and is very easily oxidized and volatilized when being heated, but the melting points of alloy elements added in lithium alloys are much higher than that of lithium, so that the melting of lithium alloys is extremely difficult. 
         [0005]    The traditional melting process of lithium alloys uses a vacuum induction melting (VIM) technique, which can be generally divided into two methods, forward feed and backward feed. The melting technique with forward feed includes the steps of (1-1) evacuating the induction melting furnace (10 −1  to 10 −5  Pa) to clean the chamber in the induction melting furnace; (1-2) placing a lithium lump into a crucible which is placed in the chamber of the induction melting furnace, induction heating at low power, and continuously evacuating to completely remove the gas in the chamber; (1-3) charging argon into the chamber, and increasing the heating power, to melt the lithium lump at such a high temperature that can melt other alloy material, for example, the melting temperature of metals such as titanium, aluminum, manganese and magnesium; and (1-4) then continuously adding the alloy material to the molten lithium material for fusion, thereby forming a lithium alloy melt. The alloy material directly settles to the bottom of the crucible due to having a higher specific gravity than that of the lithium material, at this time, the impurities attached to the surface of the alloy material such as moisture and oil stain will enter into the molten lithium material jointly, such that contaminants such as oxygen, hydrogen or carbon are present in the molten lithium material, causing the formation of the compounds such as lithium oxide or lithium carbide in the lithium alloy melt. 
         [0006]    It is particularly emphasized here that lithium oxide or lithium carbide has a specific gravity similar to that of the lithium alloy melt, and is mixed into the lithium alloy melt, so that lithium oxide and lithium carbide cannot be gravitationally separated from the lithium alloy melt; moreover, when hydrogen is solid-dissolved into it, the lithium-containing alloy melt will become further stable, so once the lithium alloy melt is contaminated, it will be very difficult to completely remove the contaminants; in (1-5), the only way is raising the temperature by further increasing the power while extending the time, in order to achieve complete fusion of the alloy materials settled to the bottom of the crucible and the molten lithium, however, this can not only cause more lithium to volatilize, but also cause the molten lithium material to absorb more contaminants, leading to a failure of uncontrolled component; in (1-6), after completely fusion (actually, it is difficult to perceive non-molten solid lump that settles to the bottom of the crucible), the lithium alloy melt is allowed to stand for a while and poured into a mold where it is allowed to cool to form an ingot, thereby finishing the production of a lithium alloy. 
         [0007]    The melting technique with backward feed includes the steps of: (2-1) evacuating the induction melting furnace first to clean the chamber as possible; (2-2) continuously evacuating, placing high-melting point alloy material into a crucible, heating at low and then high power to melt it, and maintaining at a quite high temperature, until the alloy material is completely melted; (2-3) charging argon, and then placing a lithium material into the crucible in which the lithium material is melted due to intense heat, and floats to the uppermost layer where contamination very easily occurs, however, due to the vast difference in specific gravity between the alloy material and the lithium material (the density ratio is about 3-20), a more longer fusion time is required, so that the problems of contamination and component volatilization caused by too longer fusion time and overheat cannot be avoided at all; and (2-4) standing for a while and then pouring into a mold to form an ingot after complete fusion of the lithium alloy melt. 
         [0008]    In summary, in the conventional lithium alloy melting techniques, the vacuum induction melting technique requires that the lithium material must be heated to the melting temperature of the alloy material first, and maintained at such a temperature for a long time to ensure complete dissolution of the alloy material added later. Due to the vast difference in melting point between the lithium material and the alloy material, the lithium material is volatilized in such high temperature environment, causing the content of lithium material in the lithium alloy unable to be controlled efficiently, and the loss of lithium material. Moreover, in the conventional technique, direct addition of the alloy material into the molten lithium material can easily cause the molten lithium material to be contaminated by the impurities attached to the surface of the alloy material, therefore, there exists a problem that contaminants are present in the lithium alloy melt, thereby causing uncontrolled component to make a failure in the lithium alloy production. 
         [0009]    Furthermore, in the melting technique with backward feed, because the difference in specific gravity between the lithium material and the alloy material is very high, in the addition of the lithium material into the alloy melt, the molten lithium material will float to the uppermost layer of the alloy melt, and the risk of contamination of the lithium material increases since the lithium material is unable to be wrapped by the alloy melt and thus exposed out of the surface of the alloy melt. Moreover, fully uniformly mixing between the lithium material and the alloy melt cannot be achieved in a short time because the lithium material is unable to settle into the alloy melt, and a uniformly mixed lithium alloy can only be obtained by extending the fusion time between them. 
         [0010]    Therefore, all the conventional lithium alloy melting techniques cannot avoid the problems of contamination and uncontrolled component caused by long overheat, to make the whole process uncertain, resulting in the quality of the lithium alloy unable to be improved efficiently. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is directed to a process for producing a lithium-containing alloy material, to improve the problems occurred in the conventional manufacture process of lithium alloys that the volatilization of lithium material in high temperature environment leads to the loss of lithium material, and uncontrolled component content of lithium material in the produced lithium alloy, as well as the problem that when being fused with the alloy material, the lithium material is easily contaminated with impurities and thus contaminants are present in the lithium alloy resulting in a failure in lithium alloy production, and also improve the inefficient mixing between the lithium material and the alloy material due to the difference in specific gravity, and the problem that a uniformly fused lithium alloy can be obtained only after long time fusion. 
         [0012]    The present invention discloses a process for producing a lithium-containing alloy material, including (1) placing at least one alloy element into a crucible in a vacuum induction melting furnace; (2) melting the alloy element into an alloy melt by induction heating in the vacuum induction melting furnace; (3) pouring the alloy melt into a ladle protected with an inert gas and pre-filled with a lithium material; (4) shaking the ladle, to vigorously flush and mix the lithium material with the alloy melt, thus forming a molten lithium alloy; and (5) pouring the molten lithium alloy into a mold to form an ingot, thereby forming a lithium alloy. 
         [0013]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the step (2) includes evacuating the vacuum induction melting furnace, to induction heat the alloy element in a vacuum environment. 
         [0014]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the step (2) includes induction heating the alloy element in atmospheric environment. 
         [0015]    In the process for producing the lithium-containing alloy material disclosed in the present invention, a pre-heating step of the ladle is further included before the step (3). 
         [0016]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the step (4) includes strengthening the stirring of the alloy melt and the lithium material by means of an agitating apparatus. 
         [0017]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the agitating apparatus is a vibrator. 
         [0018]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the agitating apparatus is an induction coil. 
         [0019]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the alloy element is one selected from the group consisting of aluminum, magnesium, manganese, zirconium, zinc, titanium, scandium, yttrium, copper, silver, and silicon, or a mixture thereof. 
         [0020]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the alloy element is magnesium, to produce a lithium-magnesium alloy by melting. 
         [0021]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the alloy element is aluminum, to produce a lithium-aluminum alloy by melting. 
         [0022]    The process for producing the lithium-containing alloy material disclosed in the present invention includes placing the alloy element into a vacuum induction melting furnace and melting it by heating to form an alloy melt, by which the contaminants attached to the surface of the alloy element can be completely removed, so as to avoid the contamination of the lithium material by the contaminants. Then, the alloy melt is poured into a ladle protected with an inert gas and pre-filled with the lithium material, where the lithium material is flushed and mixed with, and wrapped by, the alloy melt, such that the lithium material is diffused into the alloy melt while being melted. Therefore, in addition to the avoidance of the contamination of the lithium material by protection with the alloy melt, uniformly mixing of the lithium material with the alloy melt and significant reduction of the mixing time are also achieved, thereby a high quality lithium alloy material is obtained. 
         [0023]    The description on the content of the present invention above and the description on the embodiments below are used to exemplify and explain the principle of the present invention and provide further explanation on the claims of the present invention. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [0024]    No drawings. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    In order to make the content of the present invention more comprehensible, the embodiments of the present invention are described below. 
       [Embodiment 1] Melting of Lithium-Magnesium Alloy 
       [0026]    Magnesium is a very important light metal (1.74 g/cm 3 ), and can maintain the low density property of the lithium alloy if it can be added into the lithium alloy in mass, in view of this, the present invention attempts to use magnesium as the main alloy element in mixing with lithium, to produce a lithium-magnesium alloy with unprecedented performances. The process for producing the lithium-containing alloy material disclosed in the present invention includes mixing the alloy elements having high melting point such as magnesium (Mg), aluminum (Al), zinc (Zn), zirconium (Zr), scandium (Sc), and yttrium (Y) at a required weight ratio, or selecting magnesium as a single alloy element to form a magnesium-lithium alloy with lithium. In this embodiment, a plurality of alloy elements are used for the production of the lithium alloy, and a mixture of the alloy elements obtained by formulation at a required weight ratio has a higher specific gravity and melting point than those of the lithium. 
         [0027]    The alloy elements mixed at a required weight ratio are placed into a crucible in a vacuum induction melting furnace, and the induction melting furnace is evacuated to a high vacuum level (10 −1  to 10 −5  Pa), to make the alloy material in a vacuum environment. The weight proportions of each alloy materials are as shown in table 1, expressed in weight percent (wt %). 
         [0028]    The alloy material is pre-heated by induction heating at low power, so as to assist in degassing and de-fouling of the alloy material, to remove the contaminants attached to the surface of the alloy material and thus avoid the contamination to the lithium material caused by these contaminants in later steps. Then argon is introduced into the induction melting furnace, and the heating power is increased slowly, to melt the alloy material by induction heating into an alloy melt. The temperature is kept at 700° C. to 850° C. for a suitable period of time (depending on the species and amounts of the alloy elements), until the alloy elements are completely melted into the alloy melt. 
         [0029]    Then, the completely melted alloy melt is poured into a particularly sized ladle (such that the depth of the alloy melt is equivalent to the diameter of the ladle), and the ladle is pre-filled with a required amount of lithium lump. When the alloy melt is poured into the ladle, the lithium lump is vigorously flushed and mixed with a hot stream of the alloy melt, and wrapped by the alloy melt, such that the lithium material is melted and diffused into the alloy melt. At this time, the lithium material has a high diffusion property in the alloy melt due to having a lower specific gravity than that of the alloy material, so that the purpose of uniform mixing of the lithium material with the alloy melt is achieved. In such a step, the relationship among the amount of the alloy melt, temperature and the amount of lithium material should be considered in order to improve the success rate of melting of the lithium alloy. Moreover, pre-heating of the ladle can be carried out before pouring the alloy melt into the ladle, and the ladle is disposed on a vibrator or stirring is strengthened with an agitating apparatus such as electromagnetic induction coil, thereby increasing the mixing efficiency between the lithium material and the alloy melt. 
         [0030]    The lithium alloy melt formed by uniformly mixing the lithium material with the alloy melt is poured into a mold few minutes after the alloy melt is poured into the ladle, and a lithium-magnesium alloy ingot can be taken out of the mold after cooling to below 100° C. 
         [0031]    By the process for producing the lithium-containing alloy material disclosed in the present invention, the problems of contamination and uncontrolled component of the lithium material caused by longtime overheat in conventional meting technique are avoided, and a series of high-quality super-light lithium-magnesium alloys as shown in table 1 below are successfully produced, and the ingots have an outer diameter of 205 mm, a length of 500 mm, and a weight of 25 Kg. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Element 
               
             
          
           
               
                   
                 Lithium 
                 Magnesium 
                 Aluminum 
                 Manganese 
                 Zinc 
                 Zirconium 
                 Scandium 
                 Yttrium 
               
               
                 Alloy 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
               
               
                   
               
             
          
           
               
                 A 
                 5.5 
                 93.5 
                 — 
                 — 
                 1   
                 — 
                 — 
                 0.05 
               
               
                 B 
                 8 
                 91 
                 1 
                 — 
                 — 
                 — 
                 0.02 
                 — 
               
               
                 C 
                 10 
                 89 
                 — 
                 0.3 
                 0.5 
                 0.2 
                 — 
                 — 
               
               
                 D 
                 15 
                 84 
                 — 
                 — 
                 1 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
         [0032]    No micro-bubble is found when examining the appearance and the section exposed by cutting off the casting head for these ingots. Then these lithium-magnesium alloys are directly extruded between 180° C. and 250° C. into plates of 3 mm thick, and then examined for cold roll process. It is found that these alloys can have a rolling percent of above 50% due to their very excellent and stable forming property, and successfully rolled into thin plates (0.15 to 1.0 mm), in which the inter-annealing temperature used is 220° C. Comparison results of the mechanical physical properties between the lithium-magnesium alloys of the present invention and the light aluminum and titanium materials are summarized in table 2. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Physical 
               
               
                   
                 Property 
               
             
          
           
               
                   
                 Den- 
                 Elastic 
                 Tensile 
                   
                   
                   
               
               
                   
                 sity 
                 Modulus 
                 Strength 
                   
                   
                 Specific 
               
               
                   
                 (ρ) 
                 (E) 
                 (σ) 
                 Elongation 
                 Damping 
                 Strength 
               
               
                 Material 
                 g/cm 3   
                 GPa 
                 MPa 
                 (ε) % 
                 Capacity 
                 E/ρ 
               
               
                   
               
             
          
           
               
                 A 
                 1.58 
                 45 
                 160 
                 25 
                 — 
                 29 
               
               
                 B 
                 1.50 
                 44 
                 140 
                 40 
                 0.05 
                 29 
               
               
                 C 
                 1.43 
                 43 
                 120 
                 55 
                 0.01 
                 30 
               
               
                 D 
                 1.35 
                 43 
                 90 
                 70 
                 0.01 
                 32 
               
               
                 Aluminum 
                 2.71 
                 70 
                 9 
                 45 
                 0.002 
                 26 
               
               
                 (1100-O) 
               
               
                 Titanium 
                 4.51 
                 100 
                 500 
                 25 
                 0.002 
                 22 
               
               
                 (α-Ti) 
               
               
                   
               
             
          
         
       
     
         [0033]    All the lithium-magnesium alloys above are very applicable in loudspeaker membranes, since they have physical properties such as low density, high specific stiffness, high damping capacity, and high formability, i.e. three-high and one-low properties. As an attempt, alloy plate C of 0.15 mm thick is selected, pressed into a loudspeaker membrane, and assembled into a loudspeaker unit, which is then tested for changes in sound pressure level (SPL) curve at different frequencies in an anechoic chamber by standard test method for evaluation of loudspeakers, and compared with those of an aluminum loudspeaker of the same type. It is found that sound pressure levels at below 500 Hz are of little difference; in a bandwidth from 500 to 7000 Hz, however, the sound pressure level of the lithium-magnesium loudspeaker is more stable, and the harmonic distortion is smaller, indicating the application potential in this aspect. 
       [Embodiment 2] Melting of Lithium-Aluminum Alloy 
       [0034]    Aluminum is also a light metal (2.71 g/cm 3 ), and also a preferable additive for the lithium-containing alloy, so a lithium-aluminum alloy is also selected in the present invention for comparison, and it has the following melting steps. Alloy materials such as aluminum (Al), magnesium (Mg), manganese (Mn), copper (Cu), titanium (Ti), zirconium (Zr), silver (Ag), zinc (Zn), and silicon (Si) are weighed out at the proportions in table 3, melted into an alloy melt at 800° C. following the step in Embodiment 1, then poured into a ladle and uniformly mixed with lithium, and poured into a mold to form an ingot, in this way, the problems of contamination and uncontrolled component caused by longtime overheat are also avoided. The ingots have an outer diameter of 205 mm, a length of about 500 mm, and a weight of about 40 Kg. Again, no micro-bubble is found when examining the appearance and the section exposed by cutting off the casting head of these ingots, and they are directly extruded at 400° C. into pipes and plates, which have an elongation of above 15% under extrusion, have cold rollability and better warm rollability, and are applicable to the design of lightweight structural materials, for example, sports equipments such as bicycle. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Element 
               
             
          
           
               
                   
                 Lithium 
                 Aluminum 
                 Copper 
                 Magnesium 
                 Manganese 
                 Zirconium 
                 Titanium 
                 Silver 
                 Zinc 
                 Silicon 
                 Scandium 
               
               
                 Alloy 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
                 wt % 
               
               
                   
               
               
                 E 
                 2.5 
                 94 
                 2.5 
                 0.3 
                 0.1 
                 0.15 
                 0.1 
                 — 
                 — 
                 — 
                 0.1 
               
               
                 F 
                 2.5 
                 93 
                 1.5 
                 1.0 
                 0.1 
                 0.15 
                 0.1 
                 — 
                 0.2 
                 0.2 
                 — 
               
               
                 G 
                 1.5 
                 92 
                 5.5 
                 0.4 
                 — 
                 0.15 
                 — 
                 0.4 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
       [Embodiment 3] Semi-Atmospheric Lithium Alloy Melting 
       [0035]    In order to further decrease the production cost of the lithium alloy, attempts are made for simplification of the melting process. The magnesium alloy melt in Embodiment 1 is melted in atmosphere by flux covering in stead, and the aluminum alloy melt in Embodiment 2 is alternatively melted in atmosphere. After the degassing (hydrogen) step is completed, both of them are transferred into a compartment protected with an inert gas, poured into a ladle pre-filled with a lithium material, uniformly mixed and then poured into a mold to form an ingot. In this way, the problems of contamination and uncontrolled component caused by longtime overheat are also avoided, and the resulting ingots have comparable quality to that of the ingots above, suggesting that the semi-atmospheric lithium alloy melting has also achieved unprecedented success. 
         [0036]    In the process for producing the lithium-containing alloy material disclosed in the present invention, the contaminants attached to the surface of the alloy elements are completely removed since the alloy elements are first placed into the vacuum induction melting furnace and melted by heating, therefore the contamination of the lithium alloy in the manufacture process is avoided. By flushing and mixing of the lithium material with the alloy melt, and diffusion of the lithium material in the alloy melt, the lithium material is melted by the heat from the alloy melt, by which the loss of the lithium material caused by volatilization of the lithium material can be prevented, and the lithium material can be further protected from contamination, and at the same time, the purposes of uniformly mixing the lithium material and the alloy melt, significantly lowering the mixing time, and producing a high-quality lithium alloy material also can be achieved.