Effect of interactions between bubbles and graphite particles in copper alloy melts on microstructure formed during centrifugal casting: Part II. Experiments

During centrifugal casting of copper alloys containing graphite particles, both particles and bubbles move under the influence of centrifugal forces and influence the final microstructure, including porosity and the distribution of graphite. The movement of graphite particles and bubbles in the melts of copper alloys, originally containing 7 and 13 vol pct graphite particles and centrifugally cast at 800 and 1900 rpm in horizontal rotating molds, has been examined. Microstructural observations of sections of these centrifugal castings show that the graphite particles are segregated near the inner periphery and the amount of porosity in the graphite-rich zone is higher than the porosity in the graphite-free and transition zones. The intimate association of porosity with graphite particles in the graphite-rich zone was explained on the basis of attachment of graphite particles to bubbles in the melt and the viscosity of the melt, which increases with increasing concentration of graphite particles near the inner periphery of the castings. It was found that the amount of the porosity in the graphite-rich zone increases with volume fraction of graphite particles used in this study; the size of the porosity in the graphite-rich zone also increases with increasing rotational speed of the mold. This suggests that the graphite particles and bubbles were attached to each other in the melt and they did not get separated during centrifugal casting conditions of the present study. The present experiments qualitatively confirm theoretical computations.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part I. Theoretical analysis of nucleation behavior

The selection of the primary solidifying phase in undercooled stainless steel melts is theoretically analyzed in terms of nucleation theory. Nucleation phenomena are considered using different models for the solid-liquid interface energy. The classical nucleation theory for sharp interfaces and an improved modification, the diffuse interface theory, are applied. The influence of deviations of the nucleus composition from the overall alloy composition is also revealed. A preferred nucleation of the metastable bcc phase in fcc equilibrium solidification-type alloys is predicted. The critical undercooling of metastable crystallization as a function of alloy composition is calculated for an isoplethal section at 69 at. pct Fe of Fe₆₉Cr_31-x Ni _x alloys. The results are summarized in a phase selection diagram predicting the primary solidification mode as a function of undercooling and melt composition.     

Study and analysis of the structural constituents of billets hardened by fine-grained particles and formed by centrifugal casting

The structural and phase constituents of metal samples cut from hollow tube billets hardened by fine tungsten and silicon carbides and formed by centrifugal casting are studied.     

Redistribution of particles during casting of composite melts: Effects of buoyancy and particle pushing

When a melt containing a dispersion of second phase particles is solidified, the initial distribution of the particles can change due to three phenomena, namely, buoyant motion of the particles, pushing of the particles by the moving solidification front, and by convection currents in the melt. This paper presents a computer simulation model using which, the net redistribution due to the combined effect of the first two phenomena can be predicted. the existing theory for calculating the critical velocity for particle pushing is extended to include the effect of the buoyancy force and a numerical correlation is developed for easy calculation of the critical velocity. This correlation is incorporated into a computer programme which tracks the position, velocity and direction of the solidification front as well as the position of each particle in the melt as a function of time. The final positions of the particles describe the distribution of the particles in the solidified material. Predicted distributions for various heat extraction rates and particle sizes are presented for a system of silicon carbide particles in a pure aluminium melt solidifying unidirectionally as well as multidirectionally in cylindrical moulds. It is shown that for any heat extraction rate there is an optimum particle size which gives the maximum uniformity of distribution in the solidified material.     

The microstructure and recrystallization of flow-formed oxide-dispersion-strengthened ferritic alloy: Part I. Deformation structure

Oxide-dispersion-strengthened (ODS) alloy tubes with a nominal composition of Fe-19.5 pct Cr-5.5 pct Al-0.5 pct Y₂O₃ have been cold flow formed to deformation levels between 72 and 92 pct. The deformation structure of the tubes has been studied using a variety of techniques including transmission electron microscopy (TEM). The deformation cells produced by flow forming are elongated in both the hoop and axial directions, especially at deformation levels above 80 pct. In this case, most deformation cells can be regarded as ribbons, lying roughly along the “rolling direction,” which for flow forming is a helix around the tube surface. No obvious particle alignment was found in the tubes. Although the initial grain size is submicron, transition bands composed of parallel-sided long deformation cells similar to those in deformed single-crystal specimens have been observed in the transverse section of the tubes and a macroscopic shear band has been observed in the longitudinal section of the 92 pct deformed tube.     

Relationship between fracture toughness, fracture path, and microstructure of 7050 aluminum alloy: Part I. Quantitative characterization

The fracture toughness of Al-Zn-Mg-Cu-based 7XXX aluminum alloys decreases with an increase in the extent of recrystallization. In this contribution, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the 7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging condition. The fracture path is quantitatively correlated to fracture toughness, and the bulk microstructural attributes estimated via stereological analysis. In the companion article, these quantitative data are used to develop and verify a multiple-fracture micromechanism-based model that relates the fracture toughness to a number of microstructural parameters of the partially recrystallized alloy plate.     

On the flow criteria for suspending solid particles in inductively stirred melts: Part I. newtonian behavior

A mathematical formulation has been developed to represent the behavior of both buoyant and nonbuoyant particles in a fluid which is undergoing turbulent recirculating flow. In the formulation the fluid velocity field is represented by the turbulent Navier-Stokes equations, in conjunction with thek-ɛ model for the turbulent viscosity. Regarding the fluid-particle interactions in calculating the drag, allowance has been made for both the time smoothed and the fluctuating velocity components. The principal findings of the work are that turbulence plays a key role in suspending the particles in the melt, and that for identical density differences, it is much easier to prevent the flotation of a buoyant particle than the sedimentation of a particle which is heavier than the fluid.     

On the flow criteria for suspending solid particles in inductively stirred melts: Part I. newtonian behavior

A mathematical formulation has been developed to represent the behavior of both buoyant and nonbuoyant particles in a fluid which is undergoing turbulent recirculating flow. In the formulation the fluid velocity field is represented by the turbulent Navier-Stokes equations, in conjunction with thek-ɛ model for the turbulent viscosity. Regarding the fluid-particle interactions in calculating the drag, allowance has been made for both the time smoothed and the fluctuating velocity components. The principal findings of the work are that turbulence plays a key role in suspending the particles in the melt, and that for identical density differences, it is much easier to prevent the flotation of a buoyant particle than the sedimentation of a particle which is heavier than the fluid.     

Transformations in a Ti-24AI-15Nb alloy: Part I. Phase equilibria and microstructure

A variety of heat treatments have been employed to explore phase equilibria and the development of microstructure in a Ti-24Al-15Nb alloy. These include solution treatments both above and below the β-transus, followed by controlled cooling and aging at temperatures high enough to preclude ω-phase formation. The phase fieldsβ, β _o, α₂ + β_o, α₂ +β _o + O, andβ _o + O have been identified in the alloy, and schematic time-temperature-transformation (TTT) curves are proposed for continuous cooling transformations from theβ phase. The composition of the α₂ andβ _o phases in the α₂ +β _o region and of the α₂,β _o and O phases in the ternary phase field have been obtained by analytical electron microscopy.     

Influence of capillary pressure in bubbles on their attachment to particles during froth flotation: Part two

The calculation of the energy possibility of the transfer of spherical bubble A into adhered bubble M or the A → M transition (AMT) shows how large the influence of the capillary pressure (P _c) on the results of calculation is and how, as bubbles A and M diminish and P _c in them increases, the range of the possible variation of P _cM in bubble M abruptly narrows. The influence of P _cM on the energy barrier on the path to the adherence of the bubble to a hydrophilic surface drops almost to zero, but the preference to the adherence of the bubbles to a hydrophobic surface is retained, although the difference noticeably decreases compared with large bubbles. The data of the practice of the first processes of froth flotation by microbubbles are the confirming experimental base of the results of this precision calculation. A decrease in wetting angle θ _M to 0.02° and the beginning increase in the spreading coefficient of the adhered bubble over the substrate particle apparently also promotes the adherence process in practice.     

Microstructural evolution during liquid phase sintering: Part I. Development of microstructure

During liquid phase sintering, numerous solid-solid particle contacts can be generated by particle motion within the fluid. It is shown that, somewhat surprisingly, Brownian motion can produce such contacts. If such contacts are accompanied by particle adherence, the particles can then subsequently fuse into one (i.e., coalesce) by the liquid state analog of the evaporation-condensation mechanism of sintering. An isolated microstructure will develop if the time for particle coalescence is much less than the time between contacts. A highly skeletal arrangement of particles will form under the converse condition. Using these principles, a “microstructure map” is calculated in which the expected morphology of microstructure (i.e., skeletal or isolated) is related to the solid particle volume fraction, the kinetic and thermodynamic parameters affecting particle coalescence, and the frequency of particle contacts by Brownian motion. Some discussion of the thermodynamic and morphological factors affecting the probability of particle adherence after contact is presented.     

铜粉对MIM钨铜合金组织和性能的影响

采用金属注射成形（MIM）技术制备了钨铜合金,定量表征了铜粉的粉末粒度和粒形,重点研究了铜粉粒度和粒形对MIM钨铜合金组织与性能的影响。通过对比铜粉的粒径、粒度分布宽度、长宽比、粗糙度、赘生物指数和钝度等特征参数,破碎铜粉与水雾化铜粉颗粒呈枝晶状,粒径远小于还原铜粉,但破碎铜粉粒度分布宽,微观结构上的规则度、表面光滑程度以及分散程度最佳。破碎铜粉混合钨粉为原料,通过MIM技术制备钨铜注射生坯致密度高、缺陷少,烧结后钨铜合金的组织与性能最优,致密度为96.2%,硬度为235HV,抗弯强度为1 200 MPa,热导率为128 W/(m·K),电导率为30%IACS。     

铜含量对Al-Zn-Mg-Cu-Zr-Cr-Yb超强铝合金组织与性能的影响

采用铸锭冶金法制备Al-Zn-Mg-xCu-Zr-Cr-Yb超强铝合金,通过金相显微镜和扫描电镜观察以及晶间腐蚀、剥落腐蚀与应力腐蚀等实验,研究Cu含量(0.8%~2.2%,质量分数)对该合金组织与性能的影响。结果表明:随Cu含量降低,合金的残余结晶相数量减少,力学性能提高,抗腐蚀性能先升高后降低。当Cu含量为1.3%时,Al-Zn-Mg-Cu-Zr-Cr-Yb合金的综合性能最佳,抗拉强度、屈服强度和伸长率分别为720.7 MPa,704.2 MPa和9.1%;应力腐蚀指数ISSRT=0.031。     

烧结温度对石墨/铜复合材料微观组织与性能的影响

通过超声波分散结合机械球磨湿磨法对铜粉和石墨粉进行混料,利用放电等离子烧结（SPS）技术制备石墨/铜复合材料。运用扫描电镜（SEM）、X射线衍射仪（XRD）和电导仪等表征手段,研究了烧结温度对石墨/铜复合材料的微观组织与性能的影响。结果表明:超声波分散和机械球磨湿磨法可使石墨均匀分散于铜基体,并与铜基体形成良好的界面结合。当烧结温度从700 ℃升高到750 ℃,石墨/铜复合材料的相对密度、维氏硬度、抗压强度和电导率分别提高了1.6%,6.7%,11.3%和5.3%;当烧结温度从750 ℃升高至900 ℃时,其相对密度、维氏硬度、抗压强度和电导率均呈现下降趋势。当烧结温度为750 ℃时,石墨/铜复合材料组织均匀致密,平均晶粒尺寸约为6.4 μm,相对密度为96.3%,维氏硬度（HV_0.5）为60.7,抗压强度为422 MPa、电导率为86.7%IACS,综合性能较优。      

Effect of calcium addition on microstructure,casting fluidity and mechanical properties of Mg-Zn-Ce-Zr magnesium alloy

The influence of Ca addition on the as-cast microstructure, casting fluidity and mechanical properties of the Mg-4.2Zn-1.7Ce-0.5Zr(wt.%) alloy was investigated. The results showed that the as-cast alloys consisted of α-Mg matrix, Ca-contained T-phase and Mg51Zn20 phase. Addition of 0.2 wt.%–0.6 wt.% Ca led to effective grain refinement and enhanced the fluidity of the alloys. When the content of Ca was 0.2 wt.%, the alloy exhibited the finest grain size of 35.9 μm, and the filling length was increased by approximately 55.4% compared with the quaternary alloy. The improvement of the fluidity was attributed to the grain refinement, less energy dissipation and the oxidation resistance of Ce and Ca. With an increase in Ca content, the yield strength increased gradually, whereas the ultimate tensile strength and elongation showed a decreasing tendency. Moreover, the fracture surface mode was quasi-cleavage fracture.     

Effect of copper on microstructure and strength and ductility of low alloy wear resistance steel

The effect of copper on the microstructure and strength and ductility of low alloy wear resistance steels without copper and copper bearing was studied. The CCT curve was calculated by JmatPro software. The microstructure was analyzed by OM and TEM and the mechanical properties and ductility were tested by universal tensile testing machine and impact testing machine. The results show that the element of copper increases the stability of austenite and the transformation of ferrite and pearlite is postponed for the copper bearing steel. The microstructure is composed of matensite and lower bainite for the experimental steels and the content of martensite of the steel bearing copper is higher than the steel without copper. There are nano- size precipitations of (Nb,Ti)C and (Nb,Ti,Mo)C in the matrix of the two steels. The yield strength and the impact energy at -60?? of the steel with 0. 49 mass% copper is higher than that of the steel without copper. The element of copper is benefit to improve strength and low temperature ductility for the low alloy resistant steel.     

Fundamental studies of copper anode passivation during electrorefining: Part I. development of techniques

Anode passivation is an extremely important phenomenon in commercial copper electrorefining operations. Due to the electrochemical nature of the refining process, various electrochemical techniques have been applied to examine anode passivation behavior. In this investigation, cyclic voltammetry, linear potential sweep, and chronopotentiometry measurements were carried out on several commercial copper anodes, as well as on pure electrowon (EW) copper cathode, in synthetic electrolytes simulating the primary constituents for industrial operations. The important parameters associated with anode passivation were identified and compared for the three electrochemical techniques. Chronopotentiometry is an extremely sensitive technique and proved to be the most suitable for analyzing and characterizing copper anode passivation.     

间接挤压铸造工艺参数对铝合金中Si偏析的影响

研究间接挤压铸造工艺条件下,浇注温度、挤压压力、挤压速度、冷却速度及参数间的交互作用对6066铝合金中Si元素的偏析影响规律.以凝固后零件热节位置硅的质量分数与合金初始硅的质量分数的差值定量表征偏析程度,采用考虑一级交互作用的四因素两水平正交设计,研究间接挤压条件下硅的偏析现象.结果发现:浇注温度、挤压压力、挤压速度和冷却速度对硅偏析都有影响,其中浇注温度是影响最显著的因素.随着浇注温度的升高,铝合金中Si偏析程度减小.挤压压力和挤压速度对硅偏析的影响次之,但两者的影响趋势相反;模具冷却能力的影响程度与挤压压力和挤压速度的交互作用的影响程度相似,铜模套(高冷速)比钢模套(低冷速)的硅偏析程度要轻.间接挤压铸造条件下,工件热节位置可以出现硅的负偏析.     

Effect of the size distribution of alpha particles on microstructure evolution during heat treatment of an alpha/beta titanium alloy

A model was developed to describe the diffusional growth of a size distribution of alpha particles in Ti-6Al-4V during cooldown following alpha/beta solution treatment. Model results for initial microstructures with the same initial volume fraction of alpha but comprising either a distribution of particle sizes or a uniform particle size (equal to the average size of the distribution) yielded almost identical results in terms of final volume fraction and average particle size. The simulations quantified the narrowing of the size distribution associated with the different growth rates of small and large particles.     

A mathematical model for the solution mining of primary copper ore: Part I. leaching by oxygen-saturated solution containing no gas bubbles

An unsteady-state, one-dimensional model which simulates the solution mining of a rubblized copper ore body deeply buried below the water table has been developed. Leaching is accomplished by pumping water saturated with oxygen into the bottom of the flooded rubble chimney. The physical processes incorporated in the present model include the axial convective transport of mass and heat, axial dispersion of mass, mass transfer between the bulk fluid and solid surface, and pore diffusion within the ore fragments. The solution withdrawn from the top of the chimney is recycled through the bottom of the chimney, and the temperature of the chimney is allowed to build up by means of the heat generated during leaching. The coupled model equations are solved numerically by an implicit finite-difference method. Model calculations for the leaching process are made for two different modes of operation: (1) constant flow-rate and (2) variable flow-rate of the leach solution during leaching. The calculated results from both modes of operation indicate that thefractional recovery of copper increases with decreasing ore particle size, ore grade, pyrite/chalcopyrite molar ratio, and shape factor. Copper recovery is rather insensitive to the chimney porosity under typical operating conditions.