On The Preparation Of W-Cu Alloy By Hydrothermal Synthesis Combination And Co-reduction Method

Because of the excellent mechanical and physical properties, good arc erosion resistance, remarkable thermal and electrical conductivity and strong ability of microwave absorption, etc., W-Cu alloy has been widely used in many fields, such as electrical contact materials, heat sink materials, electric spark machining material, arc resistant electrode materials, heavy electrical contact materials, the radiator material in high density areas. However, W-Cu alloy is composed by the body-centered cubic structure of tungsten and face-centered cubic structure of copper; meanwhile, the W-Cu system exhibits either mutual insolubility or negligible solubility, so W-Cu is called pseudo alloy. Therefore, W-Cu powder compacts show very bad sinterability. It is difficult for W-Cu alloy to make the relative density of sintered products higher than98%even when using liquid phase sintering above the melting point of the Cu phase. So some of W-Cu alloy can not meet the requirements of high technology applications, as limited its applications. In recent years, the methods of preparation of W-Cu alloy turn to the preparing process of fine grain W-Cu composite powders. Some studies have reported that the powders with small particle size and uniform distribution played an important roles in W-Cu system based on particle rearrangement for dominant mechanism of sintering densification largely, helping to improve the sintering densification rate and density, finally W-Cu alloy with high density and excellent physical properties was got.In this present, a new technology of W-20wt.%Cu alloy preparation by hydrothermal synthesis and co-reduction are put forward, in order to prepare the composite powders in which both of W and Cu elements are evenly diffuse distribution and retain mixed state on molecular level in the solution. Meanwhile, the powder size of W-Cu alloy can reach to nanoscale and meet the requirements of improving the sintering properties and preparing the high density W-Cu alloy with remarkable thermal and electrical conductivity. Ammonium metatungstate (sodium tungstate) and copper nitrate as the precursor materials have been used to prepare W-Cu oxide products by hydrothermal synthesis method, and co-reduction and Spark Plasma sintering (SPS) have been used to prepared W-Cu alloy. The microstructures and physical properties of the prepared powders and alloy are characterized by XRD, SEM and HTEM. The main research results are as follows.1. Ammonium metatungstate and copper nitrate are used as the precursor materials to prepare the W-Cu oxide composite powder by hydrothermal synthesis method. The results show that the grain size of the oxide composite powders are very fine, but the products are easily agglomeration. Meanwhile, it was found that the recovery of the copper was low with about20%-30%. Lowering pH(pH=4~5) can improve the particle size and uniformity of the powder, but the copper content decreased. Enhancing pH (pH=4~5) can increase the copper content, but the phenomenon of agglomeration is intensified. So this precursor materials can not constitute the perfect reaction system.2. W-Cu composite powders have bee successfully prepared by hydrothermal synthesis method using sodium tungstate and copper nitrate as the precursor materials. It was found that the copper content achieved20wt.%and the requirements of the testing was met. Adjusting the mixed solution of the precursor to be alkaline, controlling the pH value was at9, the hydrothermal products are mainly formed by CuWO4·2H2O and Cu2WO4(OH)2. Subsequently, the hydrothermal products could be completely decomposed into the oxide composite powders composed of WO3, CuO and CuWO4-x after calcinating at550℃. In the polycrystalline mixture, WO3phases acted as the core, wrapped by CuO and CuWO4-x phase. The morphology of the oxide composite powders exhibited as spheres with the size about2~5μm, and particles are distributed uniformly. W-Cu composite powders can be produced after the oxide composite powders is reduced at the temperature of700~900℃for2h. The morphology of the W-Cu powders show the typical structure of W being packaged by Cu with the size around70nm. For the microstructure of the W packaged by Cu, the core is W phase with the thin layer of Cu phase. The particle of W-Cu composite powders presented as spheres which expressed two phases one and retain mixed state on molecular level in the solution.3. SPS technology have been successfully used to prepare W-Cu alloy. With the increase of sintering temperature and sintering time, both of W and Cu phase are distributed more uniformly, and hardness, density and conductivity of the alloy are subsequently increased. Under the sintering process of950℃for5min, the excellent physical properties of the W-Cu alloy can be achieved with the98.9%density,222.8HV vickers hardness and21.7MS/m conductivity.