Research On Slag Used In The Preparation Of CuCr Alloy Through SHS-Metallurgy Method

CuCr alloy is a contact material which is widely used in the high-voltage vacuum switchs.The main industrial production methods of the CuCr alloy are powder metallurgy method,infiltration method and vacuum arc melting method.The traditional production methods have some defects such as high production costs,process complexity,low yield and requires vacuum device.Therefor,a new method to prepare CuCr alloy using oxide as raw materials was proposed:Self-propagation High-temperature Synthesis(SHS)-metallurgy method.This method contains three key steps:in the first step,miscible high-temperature melts are obtained through Al-thermite reaction;in the second step,the oxide inclusions are removed through slag refining;and in the final step,the CuCr alloy ingots which meet the requirements are obtained through rapid cooling.Both in the Al-thermite reaction process and slag refining process,the properties of smelting slag have crucial impacts on the removal of inclusions and separation of alloy melts and slag.In the preset work,the smelting slag used in the preparation of CuCr Alloy through SHS-metallurgy method has been systematically studied.Currently,the research on Al2O3 based slags focuses on slags used in the steelmaking process and ESR process,and the research on high Al2O3 content slags which were used in the thermite reduction processes is not sufficient.The effects of different additives on the thermo-physical properties of high Al2O3 content slag were studied in the present work.The results show that:When CaO:Al2O3 = 1:2,the increase of CaO content in the slag results in the decrease of viscosities and densities,and the decrease at first and increase subsequently of solidification temperatures and surface tensions,the extreme points are at 30%and 10%,respectively.The increase of MgO content in the slag results in the decrease at first and increase subsequently of viscosities,densities,solidification temperatures and surface tensions,the extreme points are at 3%.The increase of CaF2 content in the slag results in the decrease of viscosities,densities,solidification temperatures and surface tensions.The increase of SiO2 content in the slag results in the increase of viscosities,densities,solidification temperatures and surface tensions.The increase of CaF2 content in the slag results in the decrease of viscosities and surface tensions,and the increase of densities and solidification temperatures.The Cr2O3 and Al2O3 inclusions in the alloy,which were generated in the thermal reduction process,should be absorbed by slag in the refining process.Therefore,the effects of Al2O3 and Cr2O3 on the thermo-physical properties of molten slag were measured,and the dissolving process of oxide inclusions into slag were investigated.The results show that:the dissolution of Al2O3 and Cr2O3 into the molten slag could results in the increase of viscosities,densities,solidification temperatures and surface tensions of the slag,which leads to the deterioration in its metallurgical properties.The influence of the chemical composition of slag on the dissolution rate of Al2O3 inclusions into the CaF2-CaO-MgO-Al2O3-SiO2,CaO-Al2O3-CaF2 refining slag and CaO-Al2O3 based smelting slag were investigated.The dissolution rate of Al2O3 into the molten slag increases with the increasing temperature and rotation speed,and the control step is diffusion of Al2O3 in the slag matrix.Two high melting point intermediate compounds,CaO · 2Al2O3(CA2)and CaO · 6Al2O3(CA6),exist in the concentration boundary layer.In the dissolution process,Al2O3 successively went through CA6 layer and CA2 layer,and dissolved into the slag matrix at the interface between CA2 layer and molten slag.The measurement of thermo-physical properties of molten slags is difficult and expensive.Consequently,the estimation of the various properties of slags from their chemical compositions has great significance.In the present work,the KTH viscosity model was modified to make it suitable for aluminum thermal reduction slag system,the estimated values change trend consistently and in good agreement with the values measured in the present work.A new model to estimate electrical conductivities of molten slag system was established and applied to the Al thermal reduction slag system,estimated values are in good agreement with previous measured values.Al-thermal reduction process is one of the key steps in the preparation of CuCr alloy through the SHS-metallurgy method.Referring to the study of physicochemical properties of the CaO-Al2O3 slag system,a new slag for the preparation of CuCr alloys was obtained.Then,the influences of slag composition and Cr2O3 combination amount on the aluminum thermal reduction process of CuCr alloy were investigated.The results show that:the selected slag has good metallurgical properties,and it can effectively remove pores and inclusions in the CuCr alloy.Increasing Cr2O3 amount in raw material could regulate the CuCr alloy composition and effectively improve the Cr content,but would cause the increased losses of Cr,which were due to the presence of unreduced Cr oxides in slag.The chromium-rich phase particles have an average diameter DA of 12～40pm,and the increasing Cr2O3 amount could cause the increase of chromium-rich phase particle size and the content of large size chromium-rich phase particle.Slag refining is the key step to remove pores and inclusions in the CuCr alloys prepared by SHS-metallurgy method.The refining processes of CuCr alloy were researched in the present work,and the results show that:the remelting process can effectively remove pores and inclusions in CuCr alloy,but cannot reduce the Al content of the alloy,and also lead to serious loss of Cr due to the gravity segregation during the heating process.Direct refining process can refine the grain,remove pores and large size inclusions,and reduce the residual Al inthe alloy.Compared with the batchwise feeding,continuous feeding methods have better refining effect.The chromium-rich phase particles of CuCr alloy prepared by Aluminum thermal reduction-direct refining method have an average diameter DA of 6.7～21.35μm,and large size chromium-rich phase particles(larger than 50μm)rarely occurs;Using Aluminum thermal reduction-direct refining method,CuCr25 and CuCr40 alloys with the Cr content meeting the national standards were prepared,wherein the remaining A1 elements were less than 0.2%,and the average diameter of the Cr-rich phase particles were 16.44μm and 19.58μm,respectively.