Met. Mater. Int., Vol. 20, No. 6 (2014), pp. 1145~1150 doi: 10.1007/s12540-014-6019-1
Fabrication of W-Cu Alloy via Combustion Synthesis Infiltration Under an Ultra-Gravity Field 1,2, *
Yuepeng Song
1,3
4
4
4
, Qian Li , Jiangtao Li , Gang He , Yixiang Chen , and Hyoung Seop Kim
2, *
1
Shandong Agricultural University, Mechanical and Electronic Engineering College, Shandong Provincial Key Laboratory of Horticultural Machineries and Equipments, Tai’an 271018, China 2 Pohang University of Science and Technology, Department of Materials Science and Engineering, Pohang 790-784, Korea 3 Shandong Transport Vocational College Taishan, Shandong, Tai’an, 271018, China 4 Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Beijing 100190, China (received date: 20 December 2013 / accepted date: 24 February 2014) Tungsten copper alloy with a tungsten concentrate of 70 vol% was prepared by self-propagating hightemperature synthesis in an ultra-gravity field. The phase structures and components of the W-Cu alloy fabricated via this approach were the same as those via traditional sintering methods. The temperature and stress distributions during this process were simulated using a new scheme of the finite element method. The results indicated that nonequilibrium crystallization conditions can be created for combustion synthesis infiltration in an ultra-gravity field by the rapid infiltration of the liquid copper product into the tungsten compact at high temperature and low viscosity. The cooling rate can be above 100,000 K/s and high stresses in tungsten (~5 GPa) and copper (~2.6 GPa) were developed, which passivates the tungsten particle surface, resulting in easy sintering and densifying the W-Cu alloy. The reliability of the simulation was verified through temperature measurement and investigation of the microstructure. The W-Cu composite-formation mechanism was also analyzed and discussed with the simulation results. Keywords: tungsten copper alloy, combustion synthesis infiltration technique, microstructure, simulation
1. INTRODUCTION Tungsten copper (W-Cu) composites are widely used for engineering components in defense, aerospace, and nuclear power technologies, as well as in electronic devices, and energy systems; due to their attractive combination of the high thermal conductivity of copper and the low thermal expansion and high thermal stability of tungsten [1-5]. Joining techniques for W and Cu strongly needs to consider the huge differences in their densities and melting temperatures. Various methods based on sintering theory of formation of the W-Cu composites, either with fixed compositions or as graded structures, have been widely investigated [4-8]. It has been pointed out that high sintering temperature, long time, and low density induce serious problems in the coexistence of these two phases [4-8]. Although an infiltration technique [9] is generally known to be suitable for overcoming these serious shortcomings, and it enables production of high den*Corresponding author:
[email protected],
[email protected] KIM and Springer
sity W-Cu composites, infiltration processing is often expensive and imposes a limit to the maximum Cu content of 30 wt% [1,10,11]. Recently, fabrication of new materials using combustion syntheses under the ultra-gravity field (UGF) has become a subject of special interest [12]. In this method, the mixture products of liquid phases (ceramics and metallic) in aluminothermic systems are separated under UGF owing to their dierent densities. The final products after compaction exhibit excellent properties because of their pore-free high density in the UGF field. Many function materials, such as ceramics, cermet, refractory alloy etc., have been prepared through this combustion method [13-23]. In this paper, a novel fabrication technique, combustion synthesis infiltration under an ultra-gravity field (CSIUGF) [18], has been explored for the production of W-Cu composites for various applications. The main steps in this process are: i) thermit compact of CuO/Al/W is ignited in the ultragravity field, ii) ceramic/metal mixtures are segregated under the ultra-gravity field, and iii) the liquid metal product at high temperature infiltrates the compact, sintering and cooling to
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form W-Cu composites. Via the CSIUFG method, it is possible to realize rapid melting of the W-Cu phase, to obtain high density W-Cu composites in ultra-gravity fields, and to ensure rapid solidification before melt-volatilization and reduction. Our primary aim for the present study is to demonstrate the feasibility of producing W-Cu composites by the CSIUGF technique, and to investigate the product of this method in terms of microstructural analysis. The second goal is to demonstrate the phase transformation mechanism by simulation of the temperature and stress distributions during the CSIUGF process, using the finite element method (FEM).
2. EXPERIMENTAL PROCEDURE 2.1. Materials and process The powders used for fabricating W-Cu composites were commercially pure W, Al, and CuO (purities are all more than 99.2%). The Fisher particle size of the W powder and Al powder were < 3 m, and that of the Cu-powder was < 47 m. Previous research indicated that adding W particles less than 16wt% to the CuO/Al thermit could reduce the production of the Cu gas-phase fraction and keep the same system adiabatic temperature (2846 K, the boiling temperature of Cu) [19,20]. The chemical equation for the system is: 2Al + 3CuO + W = Al2O3 + 3Cu + W.
(1)
Pure W powders were cold-pressed under 10 MPa to a compact size of 40 mm 6 mm. According to Eq. (1), the CuO/Al thermit with 16 wt% W added, was cold-pressed at 10 MPa to a compact size of 40 mm 30 mm. The compact of pure W particles was put on the bottom of the graphite in a super-gravity combustion-synthesis device, and the compact of the CuO/Al/W thermit with 16 wt% W, was placed on the pure bottom layer of W. The gravity coefficient was 800 g. Current was applied to the W wire in order to ignite the briquettes, and the current was controlled within 20 A. A schematic and the equipment of CSIUGF are shown in Fig. 1.
2.2. Material testing and characterization The microstructure of the specimens was examined using a scanning electron microscope (SEM, HITACHIS-4300, Hitachi, Japan) equipped with a Noran Vantage EDS-thermo system designed for chemical analysis, and an X-ray diffracto-meter (D/max, Rigaku, Japan). After using the etchant (FeCl3 + hydrochloric acid solution) on the metallographic specimen, the microstructure was observed using a metallurgical microscope (XJP-6/6A, Alltion, China). The hardness of the samples was determined on a cross-section using a TH-5 hardness tester with a 100 g load for up to 15 s. The temperature was measured using an SPT1200 handheld infrared thermometer.
3. RESULTS AND DISCUSSION 3.1. Microstructure and microhardness of W-Cu composites The formation of the W-Cu alloy composite was made using CSIUGF. Firstly, the thermit compact of CuO/Al/W was ignited in the ultra-gravity field of 800 g. The reaction can generate a large amount of heat to melt products into liquid, which were separated in the high gravity eld due to their difference in density. Next, the high temperature Cu liquid product (2486 K) with low viscosity was rapidly inltrated into the W powders compact. The W particles in the compact of the CuO/Al/W thermit were clustered on top of the compact of pure W particles; with a characteristic structure of chains and clumps in the ultragravity field. Finally, the W/Cu metal phases on the bottom were sintered and then compacted as they cooled in UGF. The integrated material with Cu of the top position jointing the W/Cu of bottom position was obtained. An analysis of the microstructure was performed to verify the possibility of manufacturing W/Cu alloy via this approach. The results of the microstructure inspection indicated that the metal products and Al2O3 can be naturally separated in the ultra-gravity field. The phase structures and components of the W-Cu alloy fabricated on the bottom via this approach, are the same as those via the traditional sintering methods
Fig. 1. Schematic (a) Schematic illustration of CSIUGF and the equipment of CSIUGF (b) Equipment of CSIUGF.
Fabrication of W-Cu Alloy via Combustion Synthesis Infiltration Under an Ultra-Gravity Field
Fig. 2. Microstructure of W particles of original and after combustion synthesis, in ultra-gravity field: (a) initial microstructure of W particles and (b) microstructure of W particles after combustion synthesis in ultra-gravity field.
given in literatures [1-3]: finer microstructure, lower impurity content, and fewer air holes. W-Cu alloy with a W content of 70 vol%, measured by image analysis of the polished crosssection, was deposited on graphite substrates [21]. Fig. 2 shows images of the W powders before and after the reaction. It was demonstrated that by taking advantage of self-propagating high-temperature synthesis in the ultra-gravity field, the W-Cu alloy presented apparent sintering morphology, i.e. multiple W particles melted together with a significant increase in size. For a comparison of the W morphology, the surface before the reaction is shown in Fig. 2(a) and the surface of a W particle passivated after reaction is shown in Fig. 2(b). Comparing the tungsten particles surface before reaction and after the sintering, remarkable characteristic differences can be displayed with irregular sharp edges for the raw particles and with smooth shapes of the sintered W particles, as can be seen in Figs. 2(a) and 2(b), respectively. There must be a large thermal stress in the surface during synthesis because of high tensile strength of W material (approximately 1.92 GPa)[24], which can induce the outside edge breakage into the smooth surface of the particles. Clear cracks in the microstructure of the W-Cu alloy can be observed in the circular region of Figs. 3(a) and 2(b). It
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Fig. 3. (a) SEM photograph exhibiting cracks in the W/Cu material and (b) enlarged view of the left circle zone.
should be noted that cracks mainly appeared in the Cu material between the W particles. This characteristic can be attributed to the great thermal-stress generated during the cooling process. Some cracks were found between W particles as shown in Fig. 3(b). Fig. 3(b) clearly shows the combination at the W/Cu interface. It is possible to identify the phenomenon of W particle passivation after self-propagating, high-temperature synthesis. In addition, Fig. 3(b) also shows that some cracks were generated in the angle position of W particles due to stress concentration. Furthermore, Fig. 3(b) shows the Cu material around W particles appearing fine grained microstructure. The reason for the formation of the microstructural characteristics needs further investigation. In comparison to the hardness of pure Cu material, the hardness of this microstructure significantly increased (250-270 HB); however, the hardness of the W-30%Cu composites produced by traditional sintering methods was 180-200 HB. The results prove that by using the CSIUGF method a material with finer grains and greater hardness can be fabricated. 3.2. Metallography The phase composition of the samples was determined using an X-ray (D/max, Rigaku, Japan) diffractometer. It was
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Fig. 4. XRD and EDS inspection of the W/Cu alloy prepared via combustion synthesis in an ultra-gravity field: (a) XRD inspection of W/Cu alloy and (b) EDS inspection of W/Cu alloy.
observed that the material consisted of two kinds of metals, W and Cu, without any other ingredients or diffra-ction peaks of the phase structure, as shown in Fig. 4(a). Fig. 4(b) shows SEM photographs of the microstructures of these samples. It was demonstrated that the microstructure of the W-Cu alloy prepared by CSIUGF was a single phase structure of W and Cu, and that the phase structure and components of the WCu alloy fabricated via this approach, was the same as those via traditional sintering methods. It is possible to use this cost-effective manufacturing technique for the preparation of W-Cu composites. 3.3. Finite element modeling and analysis As described in the previous chapter, there are remarkable characteristic differences of tungsten particles surface before reaction and after sintering. Hence, large thermal stresses in the surface will occur during the formation process of W/Cu
materials, which can also results in the cracks shown in Figs. 2(b) and 3. Moreover, the non-equilibrium condition of CSIUGF, e.g. high temperature, combustion wave speed during the synthesis, rapid inltration, and superfast cooling speed, will develop a non-equilibrium crystallographic process during the materials formation [25]. It is very difficult to measure the stress and cooling rate during the process. Based on the fabrication process for W-Cu composites via combustion synthesis infiltration with CuO/Al/W thermit in an ultra-gravity field, a new simulation method using ANSYS code was developed with the modeling sequence: first micro unit, next whole unit, and then micro unit [26]. The temperature and stress distributions during the CSIUGF process were simulated. The simulation steps were listed as follows: 1) When building a micro unit, the irregular pores were simplified (W particles accounted for 70% of the area) to simulate the process of the liquid Cu product at high-temperature and low-viscosity rapid infiltrating the W compact; and equilibrium temperature was obtained. The temperature of W particles was set to room temperature (298 K) and the high temperature of the liquid Cu was set to 2846 K. 2) W-Cu alloy with a W content of 70 vol% was modeled. The temperature distribution from equilibrium temperature to room temperature was simulated, and the variation of heat flux over time was obtained for the nodes in different parts. The environmental temperature was set to 293 K, and the initial temperature of the W/Cu alloy was set by the micro-unit simulated results. The graphite temperature was set to 298 K, and an 800 g gravity field was applied in the Y-direction. The coefficient of convection heat transfer between graphite and air was set at 65 W/(m2.K). 3) Finally, building a micro-unit of different parts again, the corresponding nodes were applied with simulation heat flux to make a finite element analysis. The model is shown in Fig. 5. The most important result in this qualitative analysis is that the non-equilibrium conditions (high cooling rate and compaction due to the movement of the atoms under the ultragravity field) can be created for CSIUGF. The liquid Cu product at high-temperature (2,846 K) and low-viscosity rapidly infiltrate the W compact; then dropped to 1203 K during an extremely short period (0.02 s) with a cooling velocity above 100,000 K/s. In the simulation, the von Mises stress of the Cu material exceeded the tensile strength (210 MPa) in the process of the equilibrium temperature (1,203 K) dropping to room temperature. The liquid Cu at high temperature that touched W particle surfaces caused a rapid increase in surface temperature, and a large von Mises stress that instantaneously exceeded 5 GPa. This far exceeded the tensile strength (1.92 GPa) of the W material. The von Mises stress in the center of the W particles can exceed 1.8 GPa as well, and the stress-distributions in the central and outer areas were extremely inhomogeneous, resulting in perfoliate cracks of the W particles, as clearly shown in Fig. 3(b). Moreover, the stress values
Fabrication of W-Cu Alloy via Combustion Synthesis Infiltration Under an Ultra-Gravity Field
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Fig. 5. Simulation modeling (whole and micro unit model).
CSIUGF. The rapid infiltration of the liquid Cu product, at high-temperature and low-viscosity into the W compact, results in high cooling velocity and high stress. This results in penetrative cracks in the Cu material. The temperature of W particle surfaces rose rapidly and a large von Mises stress emerged instantaneously, which burst and passivated the particle surface. Moreover, the cracks thus formed between the W particles were infiltrated by liquid Cu at high temperature, which improved the combination state of the W at the Cu interface, thereby benefitting the sintering of the W-Cu alloy.
4. CONCLUSIONS Fig. 6. Variations of stress over time of different tungsten nodes in a micro unit (center and surface).
of W and Cu decreased with declining temperature. Stress variation of the W nodes over time is shown in Fig. 6. The reliability of the simulations was verified through temperature measurement and investigation of the microstructure. It was found that the temperature results of the simulations agree well with those of tests via SPT1200 handheld infrared thermometer. After cooling for 700 s, the W/Cu alloy for the region A in Fig. 5, the experimental temperature 27 °C was in good agreement with the FEM simulation temperature 25.64 °C. These results of the simulations are valid for the investigation of the microstructure of W-Cu alloy via
Self-propagating, high-temperature synthesis in an ultragravity field was found to be a very good method to produce W-Cu composites which exhibit the same aspects of phase structure and components of W-Cu alloy, as those fabricated via traditional sintering methods. Furthermore, the microstructure fabricated via this approach created a product that was much harder (250-270 HB). From the temperature and stress results of the FEM-analyses, it was determined that the liquid Cu product, at high temperature and low viscosity, rapidly infiltrates the W compact and results in high cooling rate (>100,000 K/s); inducing high stress. Also, as the temperature of the W particle surface rose rapidly, a large von Mises stress emerged instantaneously. The extent and quality of the W/Cu sintering is improved because of the bursting and passivation of the particle surface.
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ACKNOWLEDGEMENTS This work was supported by the financial support of the China National Natural Science Foundation under grant no. 51001111, 51372255, 51201173 and Chinese National Fusion Project for ITER (NO. 2013GB110005), ITPA (2010GB106003), Graduate Innovative program of Shandong Province (SDYY11079). Program of Agricultural Data Industry Technology Innovation Strategic Alliance(75007).
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