Crystallographic texture change during grain growth

This article outlines the importance of anisotropic interfacial properties for microstructure evolution. The anisotropic properties of interfaces profoundly affect the development of microstructure during thin film deposition, sintering, grain growth, and recrystallization, to name but a few processes. The properties of interfaces vary from mildly anisotropic, as for the energy of the solid-liquid interface, to strongly anisotropic, as in the case of diffusion rates along grain boundaries. As a companion to this set of articles on interfacial anisotropy, this article demonstrates the connection between grain boundary anisotropy, primarily in mobility, and texture development during grain growth. A Monte Carlo model is used to investigate the evolution of the so-called cube texture component during grain growth of a polycrystal in which the texture is the result of prior deformation. For more information, contact A.D. Rollett, Carnegie Mellon University, Materials Science & Engineering Department, Pittsburgh, PA 15213; (412) 268-3177; fax (412) 268-7596; e-mail rollett@andrew.cmu.edu.     

Chemical-mechanical planarization of Cu and Ta

As device dimensions continue to shrink, multilevel (>8) interconnects are required to efficiently implement complex logic device designs in a single silicon chip. When the number of metal interconnect levels increases, the available depth-of-focus budget of lithographic tools imposes stringent planarity requirements that can only be met currently by chemical-mechanical planarization (CMP). Improved speed and performance are extracted from such devices by switching to copper from Al/Cu as the interconnect metal and to lower dielectric constant inner layers. Use of copper also requires the simultaneous introduction of diffusion-barrier/adhesion-promotion layers of tantalum or TaN. This paper reviews some of the recent advances in the fundamental understanding of the interplay between the mechanical and chemical components of the material-removal process during CMP of copper and tantalum films. The emphasis will be on the role of different process variables in slurries containing silica or alumina abrasives in hydrogen peroxide/glycine solutions. For more information, contact S.V. Babu, Clarkson University, Center for Advanced Materials Processing & Department of Chemical Engineering, Clarkson University Box 5705, Potsdam, New York 13699-5705; (315) 268-3999; fax (315) 268-6654; e-mail babu@clarkson.edu.     

Response surface methodology for optimization of plasma spraying

Response surface methodology was used to describe empirical relationships among three principal independent variables that control the plasma spraying process. The torch-substrate distance, the amount of hydrogen in the primary gas (argon), and the powder feed rate were studied. A number of dependent variables (responses) were determined, including the deposited layer roughness, density, hardness, chemical composition, and erosion rate. The technique facilitates mapping of the responses within a limited experimental region without much prior knowledge of the process mechanisms. The maps allow process optimization and selection of operating conditions to achieve the desired specifications of the plasma sprayed coating. To illustrate the approach, a simple system of WC-12%Co was deposited on a mild steel substrate. The resulting response surfaces were used to define optimum, or “robust,” deposition parameters.     

Processing maps: A status report

In the last two decades, processing maps have been developed on a wide variety of materials including metals and alloys, metal matrix composites, and aluminides, and applied to optimizing hot workability of materials and for process design in bulk metal working. Processing maps consist of a superimposition of efficiency of power dissipation and the instability maps, the former revealing the “safe” domain for processing and the latter setting the limits for avoiding undesirable microstructures. The dynamic materials model, which forms the basis for processing maps, is discussed in relation to other materials models. The application of dynamical systems principles to understanding of deterministic chaos in the system will help in achieving a greater degree of microstructural control during processing. The patterns in the hot working behavior as revealed by the processing maps of several classes of alloys relevant to technology are reviewed briefly. Processing maps have also been applied to analyze several industrial problems including process optimization, product property control, and defect avoidance, and a few examples are listed. With the processing maps reaching a matured stage as an effective tool for optimizing materials workability, expert systems and artificial neural network models are being developed to aid and prompt novice engineers to design and optimize metal processing without the immediate availability of a domain expert, and the directions of research in this area are outlined.     

Assessing Process and Coating Reliability Through Monitoring of Process and Design Relevant Coating Properties

Thermal spray in general, plasma spray in particular, is a highly complex process with numerous interacting variables associated with generation of the spray stream, deposit formation dynamics, and the resultant property linkages. Compounding this variability further are both the spatial (different booths and different locations) and temporal (process start-stops, hardware degradation, operator etc.) effects. As such, an understanding of process and coating consistency and variability offers significant challenges. Recent scientific advances as well as measurement tools have enabled elucidation of the intrinsic variabilities associated with each of the process sub-steps; however, integrated understanding of the system level reliability is still lacking. This article seeks an integrated assessment of process and coating reliability through systematic measurements of variabilities during each stage of the process subjected to different operating parameters. Through critical examination of first-order process maps, the influence of process parameters on particle state is reviewed for repeated spray runs with a single parameter effect as well as across a spectrum of process parameters. In addition, influence of these changes on design-relevant coating properties were obtained for plasma-sprayed zirconia through recourse to novel in situ and ex situ substrate curvature measurements. Finally, the implications of such integrated reliability studies have been explored through collaborative experiments conducted in the industrial sites.     

Extrusion of an aluminum alloy prepared from rapidly solidified powder

A study on powder extrusion is attempted to extend the understanding of the extrusion process and the interrelationship between process variables and forces involved. The extrusion characteristics of a promising Al-Si alloy produced from rapidly solidified powder, and the effects of such variables as temperature, reduction ratio, and die geometry on extrusion pressure and friction force have been investigated. Results show that extrusion starts prior to the attainment of a peak pressure. The friction force between the billet and lubricated container occupies only about 4.5% of the peak pressure. This friction is therefore not principally responsible for the pressure drop of about 50% from a peak value to a steady state occurring during extrusion—a finding which is in conflict with the traditional understanding of extrusion. It is also found that temperature-compensated strain rate Z can satisfactorily reflect the combined effects of the process variables on the pressure requirement. The relationship between the Z parameter and the pressure required for the extrusion of the powdered metal (PM) alloy has been formulated, and thus together with the consideration of press capacity and product quality, the extrusion process can be well controlled.     

Siemens and siemens-like processes for producing photovoltaics: Energy payback time and lifetime carbon emissions

Polysilicon photovoltaics will play a significant role in meeting the world’s shortfall in electrical energy this century. The photovoltaic industry relies on high-purity silicon produced in the Siemens process. New Siemens-like processes (which convert metallurgical silicon to trichlorosilane and deposit purified silicon through the decomposition of silane) and metallurgical processes for producing solar silicon are under development. Their energy payback time and lifetime carbon emissions are reviewed. The history of development of Siemens and Siemens-like processes is summarized.     

3D welding and milling: Part I–a direct approach for freeform fabrication of metallic prototypes

Solid Freeform Fabrication (SFF) gives engineers a new freedom to build parts that have thus far proved difficult to manufacture using conventional machining. However, the surface finish and accuracy of SFF parts are lower than those of conventionally machined parts. A process combination of additive and subtractive techniques is currently being developed in order to overcome this problem. A novel hybrid approach of our group called ‘3D welding and milling’ uses gas metal arc welding as an additive and milling as a subtractive technique, thereby exploiting the advantages of both processes. Compared to other deposition processes, gas metal arc welding is the most economic way of depositing metals. In this paper, the initial results of the process development and the characterization of the parts fabricated by this process are reported.     

Metal solid freeform fabrication using semi-solid slurries

A new process for the direct solid freeform fabrication (SFF) of metallic prototypes and components offers a significant advantage over most other metal-SFF processes: it does not involve the use of powders, thus minimizing porosity and shrinkage distortion. This process utilizes the unique rheological and thermophysical properties of semi-solid-metal (SSM) slurries to build a near-netshape metallic component in one step, without the need of sintering, molds, roughmachining, or post-processing operations. A stream of semi-solid is deposited over a moving substrate that follows a three-dimensional pattern. The high viscosity of semi-solid slurries and their particular rheology allows the stream to be deposited over previous layers in a controlled fashion, without traces of an interface. Because the rate of deposition is an order of magnitude faster than in other SFF processes, manufacturing is also faster. In addition, distortion problems characteristic of other processes involving fully molten metal are significantly reduced because the material deposited is already partially solid. In this paper, the first implementation of this technology is presented in detail. Eventually, this process could be useful in the production of a small series of large metallic components that would otherwise be produced by casting or machining. Those processes cost more and result in lower-quality components. For more information, contact P.F. Mendez, Massachusetts Institute of Technology, Department of Materials Science and Engineering, Room 4-133, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139; (617) 253-3204; fax (801) 729-6929; e-mail pat@mit.edu.     

纳米WC—Co复合粉的烧结特征

对纳米 WC— Co复合粉的烧结特征以及怎样充分利用这一特征来控制合金晶粒长大进行了介绍。指出 ,由喷雾转化法生产的纳米 WC— Co复合粉是一种预合金粉 ,它的烧结应该属于超固相线液相烧结范畴。与传统的液相烧结相比 ,超固相线液相烧结时 ,合金的微观结构和性能对烧结温度、工艺参数及合金成分更为敏感 ,因此必须对这些参数进行严格控制     

Sensing, modeling and control for laser-based additive manufacturing

Laser-Based Additive Manufacturing (LBAM) is a promising manufacturing technology that can be widely applied to part preparation, surface modification, and Solid Freeform Fabrication (SFF). A large number of parameters govern the LBAM process. These parameters are sensitive to the environmental variations, and they also influence each other. This paper introduces the research work in RCAM on improving the performance of the LBAM process. Metal powder delivery real-time sensing and control is studied to achieve a controllable powder delivery for fabrication of functionally graded material. A closed-loop control system based on infrared image sensing is built for control of the heat input and size of the molten pool in the LBAM process. The closed-loop control results show a great improvement in the geometrical accuracy of the built features. A three-dimensional finite element model is also established to explore the thermal behavior of the molten pool in the closed-loop controlled LBAM process.     

磨损机制图的研究现状及发展趋势

磨损机制图作为一种研究方法,能更加系统和直观地表征磨损磨损机制。从磨损机制图可以得到磨损机制的分类、磨损机制间的转变关系及转变条件、磨损的控制因素及类型、磨损率、一定工况下材料的耐磨性等多种信息,从而估算磨损率、分析给定条件下的磨损机理、预测磨损机制随条件变化的趋势。为了促进磨损机制图研究方法的完善及其应用的发展,本文对国内外磨损机制图的研究现状了综述,重点讨论了磨损机制图研究的最新成果及发展动向     

Processing maps for optimizing gas atomization and spray deposition

A validated mathematical model was used to generate processing maps for producing near-net shapes of high-performance products by gas atomization and spray deposition. The processing maps are useful to optimize and control the manufacturing process on the shop floor for producing quality products. The number of trial runs and cost of production can be significantly reduced with the help of processing maps. Using a dispersion-strengthened aluminum alloy as an example, this article examines how processing maps can be used to choose a set of process variables for producing the end product of desired quality.     

Control of thermal spray processes by means of process maps and process windows

A general method to map and control thermal spray processes, ensuring predefined levels of selected final coating properties, is presented. The method relies on monitoring and individually controlling particle velocity and particle temperature through selected spray gun parameters. Mapping of the process results in process maps describing the individual effect of particle velocity and particle temperature on each selected coating property of concern; in this case, different features of the microstructure and deposition efficiency. From the information provided by the process maps, a process window is constructed. This process window provides the limits within which particle velocity and particle temperature are allowed to vary to fulfill a predefined coating specification. To verify the method, two predefined thermal barrier top coatings—one porous and one dense—were produced by air plasma spray with satisfactory results.     

New applications for tantalum and tantalum alloys

High-strength tantalum alloy usage has been limited since the cancellation of the manned space-power program in 1969. Yet, because of its unique combination of mechanical properties, fabricability, and high melting temperature, certain tantalum alloys are still used in applications where no other material is suitable, primarily for applications in the electronics industry. For more information, contact R.W. Buckman, Jr., Refractory Metals Technology, P.O. Box 10055, Pittsburgh, Pennsylvania 15236; (412) 653-0940; fax (412) 653-0940; e-mail rmtbuckman@juno.com. Author’s Note: All compositions are in weight percent unless otherwise indicated.     

Utilization of gas-atomized titanium and titanium-aluminide powder

A gas-atomization process has been developed producing clean, high-quality, prealloyed spherical titanium and titanium-aluminide powder. The powder is being used to manufacture hot-isostatically pressed consolidated shapes for aerospace and nonaerospace allocations. These include gamma titanium-aluminide sheet and orthorhombic titanium-aluminide wire as well as niche markets, such as x-ray drift standards and sputtering targets. The powder is also being used in specialized processes, including metal-matrix composites, laser forming, and metal-injection molding. For more information, contact J.H. Moll, Crucible Research, 6003 Campbells Run Road, Pittsburgh, Pennsylvania 15205-1022; (412) 923-2955; fax (412) 788-4665; e-mail jmoll@crucibleresearch.com.     

Sensors for Intelligent Processing of Materials

To implement new process control strategies, including intelligent processing of materials, advanced sensors are required to nonintrusively evaluate process and microstructure variables. This overview examines the nature and characteristics of emerging sensors based upon nondestructive evaluation technologies and other new measurement methods. Also featured are various aspects of sensors emerging from recent R&D efforts and the trade-offs between sensor needs and process understanding. In general, sophisticated sensors can reduce the dependence upon quantitative process models for process control and vice-versa.     

Experimental investigations into rapid prototyping of composites by novel hybrid deposition process

Solid Freeform Fabrication (SFF) gives engineers a new freedom to build parts that have been impossible to manufacture using conventional techniques. However, the surface finish and accuracy of SFF parts remain lower than those of parts that have been machined in conventional methods such as milling. A process combination of additive and subtractive techniques is currently being developed by our group at KIST to overcome this problem. The hybrid approach called “3D welding and milling” uses welding as an additive, and conventional milling as a subtractive technique. As part of this process development, two different building strategies have been developed to build multi-material parts directly. The results prove the applicability of the 3D welding and milling process for rapid prototyping of bimetallic parts. A significant potential application is for the rapid prototyping of injection mold inserts.     

Process Control and Characterization of NiCr Coatings by HVOF-DJ2700 System: A Process Map Approach

The concept of ‘process maps’ has been utilized to study the fundamentals of process-structure-property relationships in high velocity oxygen fuel (HVOF) sprayed coatings. Ni-20%Cr was chosen as a representative material for metallic alloys. In this paper, integrated experiments including diagnostic studies, splat collection, coating deposition, and property characterization were carried out in an effort to investigate the effects of fuel gas chemistry (fuel/oxygen ratio), total gas flow, and energy input on particle states: particle temperature (T) and velocity (V), coating formation dynamics, and properties. Coatings were deposited on an in situ curvature sensor to study residual stress evolution. The results were reconciled within the framework of process maps linking torch parameters with particle states (1st order map) and relating particle state with deposit properties (2nd order map). A strong influence of particle velocity on induced compressive stresses through peening effect is discussed. The complete tracking of the coating buildup history including particle state, residual stress evolution and deposition temperature, in addition to single splat analysis, allows the interpretation of resultant coating microstructures and properties and enables coating design with desired properties.     

Recent advances in the deformation processing of titanium alloys

Titanium (Ti) alloys are special-purpose materials used for several critical applications in aerospace as well as non-aerospace industries, and extensive deformation processing is necessary to shape-form these materials, which poses many challenges due to the microstructural complexities. Some of the recent developments in the deformation processing of Ti alloys and usefulness of integrating the material behavior information with simulation schemes while designing and optimizing manufacturing process schedules are discussed in this paper. Discussions are primarily focused on the most important alloy, Ti-6Al-4V and on developing a clear understanding on the influence of key parameters (e.g., oxygen content, starting microstructure, temperature, and strain rate) on the deformation behavior during hot working. These studies are very useful not only for obtaining controlled microstructures but also to design complex multi-step processing sequences to produce defect-free components. Strain-induced porosity (SIP) has been a serious problem during titanium alloy processing, and improved scientific understanding helps in seeking elegant solutions to avoid SIP. A novel high-speed processing technique for microstructural conversion in titanium has been described, which provides several benefits over the conventional slow-speed practices. The hot working behavior of some of the affordable α+β and β titanium alloys being developed recently—namely, Ti-5.5Al-1Fe, Ti-10V-2Fe-3Al, Ti-6.8Mo-4.5Fe-1.5Al, and Ti-10V-4.5Fe-1.5Al—has been analyzed, and the usefulness of the processing maps in optimizing the process parameters and design of hot working schedules in these alloys is demonstrated. Titanium alloys modified with small additions of boron are emerging as potential candidates for replacing structural components requiring high specific strength and stiffness. Efforts to understand the microstructural mechanisms during deformation processing of Ti-B alloys and the issues associated with their processing are discussed.