Patent Publication Number: US-2017368607-A1

Title: Functionally graded w-cu composite

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority from pending U.S. Provisional Patent Application Ser. No. 62/342,952, filed on May 29, 2016, and entitled “A FABRICATION METHOD FOR FUNCTIONALLY GRADED W—CU COMPOSITE IN LOW TEMPERATURE,” which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present application generally relates to functionally graded composites, particularly to functionally graded tungsten-copper composites, and more particularly to methods for fabrication of functionally graded tungsten-copper composites. 
     BACKGROUND 
     Tungsten-copper (W—Cu) system has been extensively investigated, due to the attractive combination of low thermal expansion and high thermal stability of tungsten, and the high thermal conductivity of copper. W and Cu have no mutual solubility, therefore their mixture forms a W—Cu composite that may provide excellent resistance to electric discharges, low thermal expansion coefficient, high hardness, high melting point, low vapor pressure, and high arc erosion resistance of tungsten with high electrical and thermal conductivity of copper. Therefore, the combination of W and Cu as a metal matrix composite (MMC) or as functionally graded composites (FGC) has gained significant importance for commercial use. 
     Tungsten and copper have remarkably different thermal expansion coefficients (i.e., tungsten: 4.5×10-6, copper: 16.6×10-6). In case of fabricating W—Cu composites by pure bonding of the two metals and forming a W—Cu double layer composite, different thermal expansion coefficients of W and Cu may result in stress being exerted at the bonding line of the tungsten-copper double layer. The stress can reduce the service life, and even cause failure of the joints that are especially prone during cool-down from the initial joining temperature or in the course of heat cycles during service. Fabricating a functionally graded W—Cu composite may be a suitable approach to overcome such problems. 
     SUMMARY 
     In one exemplary embodiment, a method is disclosed herein for fabricating a functionally graded tungsten-copper composite (W—Cu FGC). The disclosed method may include the following steps: a binder alloy powder may be prepared that may include mechanically alloyed metal powders of nickel (Ni), copper (Cu), and manganese (Mn); the binder alloy powder may be mixed with a pure tungsten (W) powder to obtain a modified W powder; a plurality of W—Cu composite powders may be prepared by mixing the modified W powder with pure copper powder with different ratios; the plurality of W—Cu composite powders may then be stacked inside a die; the stacked plurality of W—Cu composite powders may be pressed inside the die to obtain a W—Cu compact; and the W—Cu compact may be sintered to obtain a W—Cu FGC. 
     According to some exemplary embodiments, the binder alloy powder may include 20 to 40 wt % of Ni, 10 to 80 wt % of Mn, and a corresponding weight percent of copper. The preparation of the binder alloy powder may involve ball milling pure metal powders of Ni, Cu, and Mn. According to an exemplary embodiment, the ball milling may be carried out at 300-400 rpm with a ball to powder ratio of 20 to 1 for 10 to 20 hours. 
     According to other exemplary embodiments, the modified W powder comprises 10 to 30 wt % of the binder alloy powder. According to another exemplary embodiment, the content of the modified W powder in the plurality of W—Cu composite powders varies between 100 vol % to 0 vol %. 
     According to an exemplary embodiment, stacking the plurality of W—Cu composite powders inside a die may involve filling the die with the W—Cu composite powders layer by layer, such that each layer may have a different composition. 
     According to another exemplary embodiment, pressing the stacked plurality of W—Cu composite powders inside the die may include exerting a pressure of 500-1000 MPa on the stacked plurality of W—Cu composite powders. 
     According to yet another exemplary embodiment, sintering the W—Cu compact includes heating the W—Cu compact with a specific heating rate in a sintering process at a temperature of at most 1000° C. According to some exemplary embodiments, the specific heating rate may be about 10° C./min. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a abrication method for a functionally graded tungsten-copper composite, consistent with one or more exemplary embodiments of the present disclosure. 
         FIG. 2  shows a ternary compositional range of Ni, Cu, and Mn in a binder alloy powder, consistent with one or more exemplary embodiments of the present disclosure. 
         FIG. 3  illustrates an exemplary stacking process in a cylindrical die, consistent with one or more exemplary embodiments of the present disclosure. 
         FIG. 4  illustrates a scanning electron microscope (SEM) image of functionally graded W—Cu composite prepared as described in detail in connection with example 1 of the present disclosure. 
         FIG. 5  shows SEM microstructures of four exemplary layers of functionally graded W—Cu composite prepared as described in detail in connection with example 1 of the present disclosure. 
         FIG. 6  shows SEM image of the interface between pure Cu layer and its adjacent W—Cu composite layer in the functionally graded W—Cu composite prepared as described in detail in connection with example 1 of the present disclosure. 
         FIG. 7  illustrates the microstructure of W-rich layer of the functionally graded W—Cu composite prepared as described in detail in connection with example 1 of the present disclosure. 
         FIG. 8  illustrates the contiguity of W—W particles for layers of functionally grade W—Cu composite prepared as described in detail in connection with example 1. 
         FIG. 9  shows relative densities of different layers of functionally grade W—Cu composite prepared as described in detail in connection with example 1. 
         FIG. 10  shows hardness of different layers of functionally grade W—Cu composite prepared as described in detail in connection with example 1. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is an exemplary method for fabricating a functionally graded W—Cu composite. A functionally graded W—Cu composite may contain a number of stacked layers, each layer containing different amounts of W and Cu. The composition of layers may start from a pure Cu layer from one side of the functionally graded composite and gradually the Cu content may be decreased and the W content of the layers may be increased, such that a last layer may be a W-rich layer. In order to fabricate a functionally graded W—Cu composite according to aspects of the present disclosure, first a modified W powder may be prepared by mixing pure W powder with a binder alloy powder and then this modified W powder may be mixed with pure Cu powder to prepare different W—Cu composite powders with different compositions. Then these W—Cu composite powders may be stacked in a die and then they may be pressed and sintered in order to fabricate a multi-layer functionally graded W—Cu composite. The modification of the W powder with a binder alloy powder may make it possible to prepare the functionally graded W—Cu composite at lower sintering temperatures. 
       FIG. 1  illustrates an exemplary fabrication method  100  for a functionally graded tungsten-copper (hereinafter W—Cu) composite, consistent with one or more exemplary embodiments of the present disclosure. Method  100  may include steps of: preparing a binder alloy powder (step  101 ); preparing a modified Tungsten (hereinafter W) powder (step  102 ); preparing a number of W—Cu composite powders with different compositions (step  103 ); stacking the W—Cu composite powders in a die (step  104 ); cold pressing the stacked W—Cu composite powders in the die to form a W—Cu compact (step  105 ); and sintering the W—Cu compact in order to form a functionally graded W—Cu composite (step  106 ). 
     Referring to  FIG. 1 , step  101  may involve preparing a binder alloy powder by mechanical alloying of pure metal powders of nickel (Ni), copper (Cu), and manganese (Mn) with specific weight ratios.  FIG. 2  illustrates the ternary compositional range of Ni, Cu, and Mn in the binder alloy powder as shaded area  200  on triangular graph  201 . Referring to  FIG. 2 , according to some exemplary embodiments, Ni may have a content between 20 to 40 wt %, Mn may have a content between 10 to 80 wt %, and Cu is always present in the composition with a content that may be calculated by Equation (1) below. 
       Cu content=100−Ni content−Mn content  Equation (1)
 
     The mechanical alloying of pure metal powders of nickel (Ni), copper (Cu), and manganese (Mn) with specific weight ratios may involve milling pure metal powders of nickel (Ni), copper (Cu), and manganese (Mn) using ball milling. In some exemplary embodiments, pure metal powders of nickel (Ni), copper (Cu), and manganese (Mn) may be milled using planetary ball milling. 
     According to some exemplary embodiments, ball milling may be carried out at 300-400 rpm with a ball to powder ratio of 20 to 1 for 10 to 20 hours. A process control agent (PCA), for example ethanol or a mixture of ethanol and stearic acid may be used in the milling process. 
     In step  102 , prepared binder alloy powder may be mixed with a pure tungsten (W) powder in order to obtain a modified W powder. In an exemplary embodiment, the binder alloy powder may be mixed with the pure W powder with a ratio of 1 to 9 using ball milling. According to some exemplary embodiments, the binder alloy powder may have a content between 10 to 30 wt % in the mixture. According to some exemplary embodiments, the binder alloy powder and the pure W powder may be milled at 250 rpm with a ball to powder ratio of 1 to 1 for 45 minutes. 
     In step  103 , preparing W—Cu composite powders may involve mixing the modified W powder with pure copper powders with different ratios to prepare a number of W—Cu composite powders with different compositions. The compositions may vary between a pure modified W powder to a pure copper powder. The content of modified W powder in different compositions may vary between 100 vol % to 0 vol %. 
     In step  104 , stacking the W—Cu composite powders in a die may involve filling the die with the W—Cu composite powders layer by layer, such that each layer may have a different composition. In an exemplary embodiment, at least two layers of the W—Cu composite powders may be poured into the die layer by layer. After pouring each layer into the die, the surface of the layer may be flattened. Each layer may be poured onto the flatten surface of the previous layer in order to form a multi-layer W—Cu stack. 
     Referring to  FIG. 3 , in some exemplary embodiments, W—Cu composite powders  301  with different compositions may be poured into a steel die  300  layer by layer to form a multi-layer sample that may include a number of layers  302  with different compositions stacked on one another inside the die  300 . 
     Table 1 reports different W—Cu composite powders with different compositions. Referring to Table 1, at least two layers with a composition selected from Table 1 may be stacked inside the die. In an exemplary embodiment, 11 layers of W—Cu composite powders may be stacked inside the die with compositions as set forth in Table 1. According to some exemplary embodiments, the bottom layer in the die may be a pure Cu layer corresponding to W—Cu composite powder labeled as 11 in Table 1 and the top layer in the die may be a modified W layer corresponding to W—Cu composite powder labeled as 1 in Table 1. According to other exemplary embodiments, the content of copper in the W—Cu composite powders may be incrementally reduced to zero layer by layer, moving up the layers in the stack from the bottom layer (examplary layer 11) to the top layer (examplary layer 1), according to Table 1. According to some exemplary embodiments, W—Cu composite powders are stacked inside the die as layers with similar thicknesses. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 W—Cu composite powders with different compositions. 
               
            
           
           
               
               
               
            
               
                 W—Cu 
                 Composition (volume %) 
                   
               
            
           
           
               
               
               
            
               
                 Composite 
                 Modified W 
                 Pure Cu 
               
               
                 Powder 
                 Powder 
                 Powder 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 100 
                 0 
               
               
                 2 
                 90 
                 10 
               
               
                 3 
                 80 
                 20 
               
               
                 4 
                 70 
                 30 
               
               
                 5 
                 60 
                 40 
               
               
                 6 
                 50 
                 50 
               
               
                 7 
                 40 
                 60 
               
               
                 8 
                 30 
                 70 
               
               
                 9 
                 20 
                 80 
               
               
                 10 
                 10 
                 90 
               
               
                 11 
                 0 
                 100 
               
               
                   
               
            
           
         
       
     
     With reference to  FIG. 1 , once the layers of W—Cu composite powders are stacked inside the die according to step  104 , then method  100  may proceed to step  105  where the stacked W—Cu composite layers inside the die may be pressed by for example by a hydraulic press with a pressure of for example 500-1000 MPa in order to form a W—Cu compact. 
     In step  106 , the W—Cu compact may be removed from the die and it may be heated with a specific heating rate in a sintering process at a temperature of for example about 1000° C. for a predetermined amount of time in order to obtain a functionally graded W—Cu composite. According to some exemplary embodiments, the W—Cu compact may be heated at a rate of about 10° C./min and solid phase sintering may be carried out for about 3 hours at a temperature of about 1000° C. 
     Example 1: Fabricating a Functionally Graded W—Cu Composite 
     In this example, a functionally graded W—Cu composite having 11 layers is fabricated by a powder metallurgy method, consistent with exemplary embodiments of the present disclosure. In this exemplary embodiment, commercial high-purity W powder with particle size of 7.5-8.5 μm, high purity Ni powder with a mean particle size of 10 μm, high purity Mn powder with a mean particle size of 63 μm and high purity Cu powder with a mean particle size of 30 μm were used as the starting materials. Ni, Cu and Mn metal powders were mechanically alloyed, in order to prepare a binder alloy (BA). 
     Table 2 summarizes the composition and the operating condition for BA preparation. Referring to Table 2, a mixture of Ni (40 wt %), Cu (26.67 wt %), and Mn (33.33 wt %) powders were mechanically alloyed using ball milling at 300 rpm for 10 hours with a ball to powder ratio of 20 to 1. A process control agent (PCA) containing methanol and stearic acid was utilized in the ball milling process. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Composition and operating conditions for 
               
               
                 preparation of binder alloy powder. 
               
            
           
           
               
               
            
               
                   
                 Ball milling conditions 
               
            
           
           
               
               
               
               
               
               
            
               
                 Element 
                 Content (wt %) 
                 RPM 
                 Time 
                 B:P 
                 PCA 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Ni 
                 40 
                 300 
                 10 hr 
                 20:1 
                 Ethanol + stearic acid 
               
               
                 Cu 
                 26.67 
               
               
                 Mn 
                 33.33 
               
               
                   
               
            
           
         
       
     
     The prepared BA powder was mixed with W and Cu powders in 10 different compositions, to prepare 10 different W—Cu composite powders. The compositions of the layers are presented in Table 3. In this exemplary embodiment, the ratio of Tungsten to Binder was 9:1 in all 10 composite powders. The mixing was carried out by ball milling at 250 rpm with a ball to powder ratio of 1:1 for 45 minutes. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Composition of W—Cu composite powders in different layers. 
               
            
           
           
               
               
               
               
               
            
               
                 Layer 
                 Mn (wt %) 
                 Ni (wt %) 
                 Cu (wt %) 
                 W (wt %) 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 3.333 
                 4 
                 2.666 
                 90 
               
               
                 2 
                 3.15 
                 3.781 
                 8.059 
                 85.07 
               
               
                 3 
                 2.948 
                 3.538 
                 13.9 
                 79.62 
               
               
                 4 
                 2.724 
                 3.27 
                 20.45 
                 73.56 
               
               
                 5 
                 2.473 
                 2.968 
                 27.78 
                 66.78 
               
               
                 6 
                 2.19 
                 2.629 
                 36.04 
                 59.15 
               
               
                 7 
                 1.87 
                 2.244 
                 45.4 
                 50.49 
               
               
                 8 
                 1.503 
                 1.804 
                 56.1 
                 40.59 
               
               
                 9 
                 1.08 
                 1.296 
                 68.47 
                 29.16 
               
               
                 10 
                 0.585 
                 0.702 
                 82.91 
                 15.8 
               
               
                 11 
                 0 
                 0 
                 100 
                 0 
               
               
                   
               
            
           
         
       
     
     W—Cu composite powders with compositions as set forth in Table 3 were carefully poured into the steel die layer by layer in the order reported in Table 3, i.e., first layer number 11 was poured into the die and its surface was flattened, then layers 10 to 1 were poured on top of one another respectively. Once stacking the powders inside the die was completed, a 1000 MPa pressure was applied on the stacked layers to obtain a W—Cu compact. Afterwards, the W—Cu compact was removed from the die smoothly. The sintering step of the W—Cu compact consisted of heating the compact at a rate of 10° C./min and solid phase sintering for 3 hours under vacuum at 1000° C. in order to form a functionally graded W—Cu composite. 
     The sintered functionally graded W—Cu composite was then cut in half and was prepared by mechanical grinding and polishing followed by a slight etching with a solution of 20 ml NH 4 OH, 10 ml H 2 O 2  and 20 ml H 2 O. The microstructure of the functionally graded W—Cu composite was observed by a scanning electron microscope (SEM). The amount of W and Cu materials, as well as porosities were measured using standard image analysis tools. 
     The contiguity measurements were carried out using a fine square grid on the SEM images. For this purpose, a few micrographs with similar magnifications (×250) were selected. The number of intersections of grid lines with the phase boundaries in microstructure was recorded as Nsl (for W—Cu interfaces) and Nss (for W—W interfaces). The contiguity, Css, was calculated using Equation (2) below: 
         C   ss =2 N   ss /(2 N   ss   +N   sl )  Equation(2)
 
     The hardness of layers were observed by using Vickers hardness tester under indentation load of 10 kg according to ASTM standard E384, for 10 seconds per test with six times for each layer of functionally graded W—Cu composite. 
     Example 2: Microstructure Characterization 
       FIG. 4  illustrates an SEM micrograph  400  of functionally graded W—Cu composite prepared as was described in detail in connection with example 1. The functionally graded W—Cu composite contains 11 layers  401 - 411 . A well-graded compositional transition may be observed in  FIG. 4 . Dark areas represent copper and lighter areas are indicative of Tungsten-rich areas. The top layer  401  is the W-rich layer and the bottom layer  411  is the Cu-rich layer. The copper content of the layers gradually decreases from the bottom layer  411  toward the top layer  401 . 
       FIG. 5  shows SEM microstructures of layers 3-5 and 7-9 (as labeled in Table 3) at ×250 magnification. SEM images of layers 3, 4, and 5 are designated by reference numerals  501 ,  502 , and  503 , respectively and SEM images of layers 7, 8, and 9 are designated by reference numerals  504 ,  505 , and  506 , respectively. Layers with higher contents of Cu show more homogeneity. Homogeneity may enhance the density, hardness and electrical conductivity of W—Cu composite. 
       FIG. 6  shows SEM image of the interface between pure Cu layer (labeled as layer 11 in Table 3 and layer  411  in  FIG. 4 ) and its adjacent W—Cu composite layer (labeled as layer 10 in Table 3 and layer  410  in  FIG. 4 ). As illustrated in  FIG. 6 , the interface between these two layers is continuous without any obvious cracks. 
       FIG. 7  illustrates the microstructure of W-rich layer  401  of  FIG. 4 . As illustrated in  FIG. 7 , the W particles are efficiently sintered. The microstructure consists of W particles bonded to one another with an interpenetrating binder alloy matrix. Without being bound by any particular theory, it seems that the addition of Mn to the binder alloy may result in a less morphological change of W particles and it may decrease the solubility of W in the binder alloy matrix phase. 
       FIG. 8  illustrates the contiguity of W—W particles for layers of functionally grade W—Cu composite. The contiguity of W—W particles is calculated by Equation (2). Referring to  FIG. 8 , the contiguity decreases linearly from layer 2 (W-rich) to layer 10 (Cu-rich). In other words, lower contiguity is obtained for layers where Cu content is high. 
     For quantitative analysis, SEM micrographs of each layer of functionally grade W—Cu composite, with the same magnifications were analyzed by an image analysis software to obtain statistical data on the relative densities and the composition of each phase.  FIG. 9  shows the relative density of each layer (layers are labeled based on Table 3). There was a significant increase in the relative density for the first four layers and the relative density rose by 2.27%. Relative density was at its highest value at layer No. 4 and then fell at a much slower pace from layer 4 to 11. Generally, all the layers had a very high relative density (96.96-99.23%) that may indicate neck growth and diffusion during sintering. 
     Example 3: Mechanical Properties 
     Hardness measurements were conducted according to ASTM E384 standard test method. A 10 kg force was applied for 10 seconds. The results of hardness testing are reported in  FIG. 10 , which may indicate that the hardness of the functionally graded specimen increased from 57.6 (layer 11, pure Cu) to 208 HV (layer 1, W-rich). Since the hardness of composite may depend on the volume fraction of the harder phase, the hardness of composite decreased as the amount of harder W phase decreased. Meanwhile, the hardness value for layers 3, 4, and 5 were superior to that of commercial W—Cu composites. 
     Six samples were prepared using six compositions (No. 1, 3, 5, 7, 9 and 11) reported in Table 3 by the same method that was described for preparing the functionally graded W—Cu composite, in order to prepare tensile specimens for mechanical properties measurement. Sub-size tensile test specimens were cut out from the six samples using an electrical discharge machining process. The tensile tests were performed at room temperature. 
     The mechanical properties of the six sample layers are summarized in Table 4. Not bound by any particular theory, there may be some factors that may determine the mechanical behavior including composition, microstructure, powders, processing techniques, post-sintering deformation and porosity. In the case of functionally grade W—Cu composite prepared as described in connection with example 1, there was no post-sintering deformation, as well as the fact that the condition of starting powders and processing were the same for all layers of the functionally grade W—Cu composite. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Mechanical properties of different layers. 
               
            
           
           
               
               
               
               
            
               
                   
                 Elastic Modulus 
                 Ultimate tensile 
                 Fracture Toughness 
               
               
                 Layer 
                 (GPa) 
                 strength (MPa) 
                 (MPa · m 0.5 ) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 289.5 
                 447.3 
                 4.52 
               
               
                 3 
                 254.3 
                 424.7 
                 8.84 
               
               
                 5 
                 224.3 
                 372.8 
                 9.6  
               
               
                 7 
                 195.8 
                 310.2 
                 — 
               
               
                 9 
                 124.4 
                 240.1 
                 — 
               
               
                 11 
                 112.7 
                 188.9 
                 — 
               
               
                   
               
            
           
         
       
     
     Referring to Table 4, the elastic modulus ranges from 112 GPa for layer 11 (Pure Cu) to 289.5 GPa for layer 1 (90 wt % W). The ultimate tensile strength UTS continuously varied from 188.9 MPa for layer 11 to 447.3 MPa for layer 1. The degree of hardening and strengthening due to second phase particles (here W particles) depends on the distribution of particles in the ductile matrix (Cu phase) as well as its shape. When the content of hard W particles increases, the mean inter-particle spacing may decrease and therefore the hardness and strength may improve. The variation of fracture toughness (which determines the resistance of composites to crack propagation) for layers 1, 3 and 5 are reported in the Table 4. The fracture toughness was observed to decrease with increasing the W content. 
     As-produced W—Cu composites may be used in a variety of applications, such as heavy duty contact materials (W 70-85 Wt. %), resistant and arc welding electrodes (RMW classes 10-11-12), heat sink materials (W 75-90 Wt. %), electro discharge machining electrodes (W 55-80 Wt. %), electrodes and nozzles for plasma spraying gun, radiation shielding materials and power packaging for micro-electronics applications. 
     Moreover, functionally graded W—Cu composites may be used as plasma facing components (PFCs) in fusion reactors, since resistance to erosion and high temperature, resistance to collisions of high energy ions and neutrons and high thermal conductivity are crucial criteria of plasma facing materials in fusion reactors.