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--> 7 Consolidation Processes Consolidation processes consist of the assembly of smaller objects into a single product in order to achieve a desired geometry, structure, or property. These processes rely on the application of mechanical, chemical, or thermal energy to effect consolidation and achieve bonding between objects. Interaction between the material and the energy that produces the consolidation is a key feature of the process. This interaction can be either beneficial or detrimental to the final product. In some cases, the consolidation energy enhances the structure or properties of the material and is an integral part of the process. For example, in the forging of powder preforms, the mechanical energy not only consolidates the powder but also imparts macroscopic geometry to the part while improving the microstructure of the material. In other cases, the energy used to effect consolidation is detrimental to the structure or properties of the product. For example, in fusion welding, the heat of melting achieves bonding between the objects but also can create an undesired microstructure in the heat-affected zone of the joint, causing distortion and detrimental residual stresses. Consolidation processes are employed throughout the manufacturing sequence, from the initial production of the raw material to modification of the final assembly. One group of consolidation processes involves the production of parts from particulate or powders of metals, ceramics, or composite mixtures. These consolidated products are typically semifinished and require final thermal or machining processes. In some material systems, consolidation of powders produces feedstock billets for extensive processing into continuous mill products of bar, rod, wire, plate, or sheet. Other consolidation processes produce composites, with either polymer, graphite, metal, or ceramic matrices. Welding and joining processes, a unique group of consolidation processes, are used to combine subcomponents, often of dissimilar materials, into permanent assemblies. The performance of the final component is often governed by the quality of the joining process. This chapter presents an overview of the research needs and
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--> opportunities in powder processing, consolidation of polymeric composites, and welding/joining unit processes. Powder Processing Among the various manufacturing technologies, powder processing is the most diverse because of its ability to economically fabricate high-quality, complex components to close tolerances from almost all materials. Powder processing starts with particles having specific attributes of size, shape, packing, and composition and converts them into a strong, precise, high-performance component. Key process steps include the shaping or compaction of the particles and thermal bonding of the particles using sintering. The two steps can be combined into a single operation, as in vacuum hot pressing, or more typically, performed in sequence. In production, these processes effectively use automated operations with relatively low energy consumption, high material utilization, and low capital costs. Further, the process sequence is inherently flexible, since it can be applied to a wide range of materials (German, 1994; Jenkins and Wood, 1991; Klar, 1983, 1984; Lenel, 1980). Powder processing uses a different approach than traditional component fabrication. Not only are the chemistry, heat treatment, and microstructure variable, but the distribution of phases and microconstituents, including porosity and reinforcing phases, is controllable. Powder is a finely divided solid—typically smaller than one millimeter in size—of a controlled composition and can be combined with other materials, such as polymers, to ease forming or create composites. An important characteristic of powder is its high surface area to volume ratio, which leads to behavior that lies between that of a solid and that of a fluid. Powders will flow under gravity to fill containers or die cavities, so in this sense they behave like liquids. They are compressible like a gas, but the compression of a powder is essentially irreversible, like the plastic deformation of a solid. Powder processing is widely used and growing. Powder-based components are often selected for their low costs, but there are recognized advantages in improved quality, homogeneity, and performance properties. A few examples illustrate the established diversity of products: lamp filaments, dental restorations, oil-less bearings, spark plugs, aircraft brakes, connecting rods, timing gears, lightweight armor, electrical contacts, nuclear fuel elements, orthopedic implants, business machine parts, high-temperature filters, sporting equipment, horseshoes, and jet-engine disks. The specialization of R&D in the powder processing field is a reflection of the manufacturing segmentation by production schemes and materials. For
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--> example, a typical research concentration might be on metallic powders or ionic materials. Furthermore, the fabrication approach tends to further subdivide the industry, based on specific production routes. Because of this segmentation, the powder processing industry largely performs application-specific research with most of the fundamental process improvements coming from suppliers. Much of the powder processing industry does not have a strong research history and often lags in technological applications. Fundamental research is performed at universities, with some recent, successful industry-university partnerships in atomization, spray forming, powder injection molding, and advanced ceramics. The transfer of this technology to industry has been effective through industrial hiring of the university graduates. The important technology areas in powder processing are based on key aspects of the fabrication sequence: development of powder alloys, production of powders, compaction, sintering, densification, process control. A necessary focus for U.S. research is on emerging powder-processing technologies to sustain industrial growth. Current opportunities for such growth include magnetic materials (especially the high-performance, rapidly solidified iron-neodymium-boron magnets), microelectronic components (such as tungsten-copper parts formed through powder injection molding), functionally gradient composites (for example, metal-ceramic acoustical energy absorbers), electromagnetic materials, and ultra-small biomechanical components. Advanced Powder Alloys The iron-neodymium-boron alloy system offers exceptional hard magnetic properties. Since this system exhibits segregation in casting, powder-based techniques using rapid solidification are the mainstay fabrication approaches. Research directed at moving these materials into widespread production is needed, especially in the context of a high-productivity route such as injection molding. This development would benefit several other high-performance powder products, including alloys and composites based on tungsten, beryllium, magnesium, and aluminum. Several ceramic powders, namely nitrides, carbides, and borides are also viable candidates for this process. The number of materials being successfully processed by powder injection molding techniques continues to increase. The current list includes most common engineering materials. The future promises to add several composite materials, including tungsten-copper, molybdenum-copper, silicon carbide-aluminum, and aluminum nitride-aluminum for applications in microelectronics. Although considerable research activity is occurring in the area of composites, more attention is needed to establish a balance between cost, microstructure, properties,
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--> and performance. Powder processing techniques play a critical role in the fabrication of new metal-matrix composites; however, ceramic-matrix composites appear to offer major property benefits. Functionally gradient materials are another area needing more research. One emerging application is in the area of acoustic materials, where elastic modulus and density can be tailored to give passive sound attenuation. Such materials are useful in forming lightweight soundproofing. Powder Production The advent of total quality programs in the powder processing industry has resulted in considerable evidence that subtle variations in the powder production process reflect changes in powder characteristics. This realization suggests the importance of producing uniform powders characterized by high purity and compositional homogeneity. This requires in situ diagnosis of product quality during powder production. Further, there is increased need for smaller powders for applications such as injection molding and microelectronics. As powders become smaller, handling problems increase, often leading to contaminated powder and high scrap generation. Thus, research on powder production must be closely coupled with studies on properties, processing, and handling concerns. Plasma processing, reactive synthesis, mechanical alloying, chemical precipitation, and gas atomization techniques are being applied to the generation of novel powders, especially fine powders produced from reactive or refractory materials. The use of plasma techniques for the generation of uniform micrometer-sized powders is an exciting development. Mechanical alloying is recognized for its key role in the fabrication of dispersion-strengthened alloys and amorphous alloys. Powders fabricated using atomization techniques include most of the high-performance alloys (e.g., superalloys and titanium alloys). Chemical precipitation techniques have demonstrated unique abilities to form small, uniform powders, but much effort is needed to scale-up these approaches to viable production quantities. Near-term research efforts need to focus on production of large quantities of small powders to identify potential scale-up difficulties. Considerable recent research attention has been directed to the generation of coated powders. Coatings are formed by electrolysis, chemical vapor, physical vapor, fluid bed, spouted bed, and mechanical deposition techniques. This technology represents engineering at the individual particle level, forming coated particulates. These coated powders create opportunities at the microlevel to engineer improved processing and homogeneity. These efforts should to be focused on establishing processing benefits using fine powders. Research is
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--> needed to determine the conditions for minimizing segregation of constituents during handling and compaction. Shaping Die compaction is currently the mainstay of powder shaping. Equipment that integrates the latest sensors, control logic, and defect detection schemes, together with artificial intelligence for process monitoring and control, can substantially advance these processes. Also, tool design and press operation in die compaction can be improved by applying process simulation and modeling. With progressive improvements coupled to developing powders for uniform die filling and compaction, die pressing can be applied to larger and more-complex components. The field of powder injection molding has attracted considerable manufacturing attention. While the technical concepts are in hand, the technology for automating and producing large quantities of inexpensive parts is immature. A standardized process is required to establish a protocol for improved quality. Research aimed at the implementation of in situ sensors, vision systems, and defect detection schemes is necessary before widespread adoption will be practical. Further links are needed between the molding operation and rational process specification in terms of the powders, binders, and tooling for successful molding. Computer simulation software exists to optimize several aspects of plastic injection molding. Similar research can be conducted for powder injection molding, taking into account the differing rheology, thermal conductivity, momentum, and density. Considerable research is needed to develop processing technologies to fabricate larger components, with greater shape complexity and closer precision. The fabrication of high-performance materials, such as titanium and silicon nitride, by powder injection molding is widely discussed, but improved processing control to move them into commercial reality is lacking. Thus, several aspects of this exciting manufacturing technology must be addressed by future research, as the technology promises enormous potential benefits. Sintering A major effort is needed to integrate the fundamental knowledge of sintering into production practice. As an example, considerable progress has been made in modeling fundamental mass transport events during sintering. However,
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--> this information has not been linked to prediction of performance attributes, so it is largely ignored by the industrial community. Consistent with the trend toward improved performance, there is continued interest in better sintering practices, especially those using liquid phases. The widespread use of liquid-phase sintering reflects the benefits of faster atomic transport. Further, rate-controlled sintering is a concept wherein the cycle time remains short while the time-temperature program is varied to suit the microstructure or other product goals. As new sensors and control and analysis techniques become available, there is an urgent need to apply these to sintering. Effective application will require assessment and integration of in situ inspection and processing into the furnaces to move into a true real-time controlled mode of operation. Additionally, much effort is needed to upgrade the models of sintering, such as the need to consider the furnace as a dynamic thermal processing system. These developments will allow the production of sintered products of high dimensional uniformity and specific final attributes. Densification Most products formed by powder processing suffer property decrements in the finished component from residual porosity. The exceptions are distended materials for energy absorption and filters in which the void spaces provide the desired property. Several post-sintering densification steps can be applied to remove pores, and there are batch processes in which shaping and sintering are performed simultaneously. Examples include spray forming, semisolid forging, powder forging, hot forming using pseudo-isostatic conditions, gas forging, and hot isostatic compaction. Most of these processes are expensive and poorly utilized by industry. In each case, there are prospective components for application, but the application development is guided by trial and error and needs to be better supported by contemporary process understanding. Fundamental research is needed to bring together a unified view of densification that includes consideration of stress state, strain rate, powder characteristics, temperature, and time to allow rational analysis of complex geometries. Instrumentation And Control The powder processing industries require research to develop, integrate, and adapt modem tools to improve production processes. The application of microcomputers is progressing as the requirements for consistent products
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--> become widespread. Further, there is increased use of computational techniques in support of die compaction, hot isostatic compaction, injection molding, and sintering. There is a need for research on advanced instrumentation and control of production equipment, including compaction presses, sintering furnaces, and injection molding machines. In the latter example, die-cavity instrumentation can be used to make instantaneous machine corrections to ensure uniform quality compacts. Although the concepts are known, the types of sensors and signals and their interpretation and utilization need thorough research. The coupling of such instruments to expert systems eventually will allow intelligent processing of materials. Efforts are in progress to implement such a system for control of powder atomization using in situ particle size analysis and of hot isostatic compaction using in situ density measurements. These concepts need to be extended to shaping and sintering. For example, vision systems need to be developed for both die compaction presses and powder injection molding machines. Such systems would allow automated inspection of the tooling after each cycle to identify damage from an improperly ejected compact and excessive tooling wear. Another potential advance is in diagnostic thermal imaging of high-temperature processes (e.g., sintering, atomization, and plasma spraying), which would allow real-time analysis and process control. Polymeric Composites A composite material consists of two or more discrete materials whose combination results in enhanced properties. In its simplest form, it consists of a reinforcement phase, usually of high modulus and strength, surrounded by a matrix phase. The properties of the reinforcement, its arrangement and volume fraction, define the mechanical properties of a composite material. The matrix performs the important functions of acting as a stress transfer medium for the fibers. Continuous fiber-reinforced materials offer the highest specific strengths and moduli among engineering materials.1 A carbon fiber-epoxy structural part, for 1 In this section, only aligned continuous fiber composites will be considered, since these are the materials with the highest properties of interest. Short fiber composites, such as injection-molded glass or carbon fiber-reinforced polymers, will not be considered here, since the manufacturing processes for such materials are mature and are direct extensions of processes used in the plastics industry. Metal-matrix composites are addressed in Chapter 4, ''Phase-Change Processes.''
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--> example, has only about 20 percent of the weight of a steel structure of equal stiffness. In addition, the weight savings obtained by substituting composite materials in one component often results in the achievement of additional weight savings in other components of the system, due to lowered inertia, increased fundamental structural frequencies, and other factors. Composites also have the advantage of good corrosion resistance and the potential for integration of component piece parts, such as molded-in rib stiffeners, without the need for subsequent assembly operations and fasteners, which are often required for metallic structures. Consolidation in composites can be considered to occur at two levels: the fibers are infiltrated with the matrix to form a lamina or ply, and the individual laminae are consolidated together to form the final structure. In the prepreg process, these two levels are distinctly separated, since the fiber/matrix consolidation process forms the prepreg, which is then laid-up to form the laminate or final component (NRC, in press). In other processes, such as resin transfer molding (RTM), fiber/matrix infiltration and the consolidation of the final part are done in a single stage. Single-stage consolidation processes are attractive, because they eliminate the additional cost associated with prepreg production; however, two-stage consolidation processes have major advantages that often outweigh the benefits of single-stage consolidation. These include flexibility in part geometry, high fiber content, excellent fiber wet-out, and better control of fiber volume fraction distribution. Because of these advantages, prepreg processing is firmly entrenched in high-value products, such as aerospace applications, in spite of its high cost. The high fiber volume fractions (50-75 percent) in aligned continuous-fiber composites require the matrix material to have low viscosity to allow infiltration into fiber-to-fiber spacings that are a small fraction of the fiber diameter. Alternatively, high pressures can be used with high-viscosity matrices for successful infiltration. The latter approach is used in polymer processing of filled thermoplastics; however, this approach is only possible for short-fiber composites. In continuous-fiber polymer composites in which fiber integrity and orientation must be maintained, the approach is to use low-viscosity resins such as epoxies or polyesters, which are cross-linked once infiltration is completed. In prepreg manufacture, the matrix is partially crosslinked after infiltration. Consolidation of prepreg plies in the final component is done during the curing process. The prepreg process has the advantage of being able to accommodate a wide variety of part geometries and fiber orientations with tight resin-content control. However, the high cost of composites structures manufactured using prepreg processes is due in part to the high cost of prepreg production, handling, cutting, and manual assembly of the final composite part.
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--> In resin transfer molding, catalyzed resin is injected into a closed mold that contains the dry fiber reinforcement (preform). After the mold is filled (i.e., after fully infiltrating the fibers), curing of the resin results in completion of the consolidation process. In structural reaction injection molding (S-RIM), reacting resin streams are mixed and injected under pressure into a closed mold containing the fiber preform. S-RIM processes are relatively rapid (cycle times of 1-10 minutes), while cycle times for resin transfer molding range from 10 to 100 minutes. The resin transfer molding process can accommodate a higher fiber content than S-RIM with concomitant higher mechanical properties. Pultrusion is a mature technology but is presently restricted to the use of polyester resins. Pultrusion has the advantage of being a continuous process; however, it has the disadvantage of having poor transverse strength (transverse to the pultrusion axis). This problem has been addressed through the incorporation of felts, mats, and woven fabrics that provide transverse reinforcement. However, there is still the problem of low delamination strength in the direction perpendicular to the section profile. The pultrusion of closed-hollow sections, such as square and round tubing, is still difficult and uncommon. Hollow cross-sections are difficult to grip and have high surface-friction forces in the die, which necessitate high pull loads that can cause fiber failure during the process. Braided over-wrapped plies have been introduced in pultrusion to provide off-axis reinforcement. However, the slow speed of the braiding process severely restricts this approach. Tape wrapping could provide the same benefit at much higher production rates. Current research in composites processing has followed three basic avenues: Prepreg processing. Improving production rates by automating prepreg handling and ply placement techniques has been investigated with mixed success. Part of the problem is the long cure times required for prepreg resin systems. In addition, handling thin tacky sheets is a difficult task to automate. Much work has been devoted to tape laying machines, cutting plies to shape, and layup geometries. The results have been useful in identifying the limits of prepreg layup processes; however, the basic drawbacks of prepregs—long cure times, low-temperature storage, and secondary waste generation—have not been adequately addressed. The lack of success in prepreg automation is illustrated by the fact that the largest user of carbon fiber prepregs, the sporting goods industry, has largely concentrated its production in countries where labor can be obtained at a low cost, since almost entirely manual processes are used. Process control in resin transfer molding and S-RIM. Current R&D efforts are devoted to improving process control through the incorporation of sensors and microprocessor control of injection rates, temperature, pressure, and time. The aim is to automate these processes further. In addition, several efforts
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--> are under way to automate the manufacture of preform and reinforcement mats. While resin transfer molding can produce parts with high fiber volume-fraction, the cycle times are long; efforts to speed cycle time and develop models for improving mold and gate design, as well as process design, are necessary in order to increase the production rate of this promising process. Composite sheet forming. Several research programs have been conducted on the mechanics and modeling of composite sheet forming. Production rates are still low, and there are difficulties inherent in forming sheets in which compressive strain fields, which cause fiber microbuckling, are unacceptable, because the strength of the part is severely compromised. In addition, this process, unlike resin transfer molding, requires high pressures, which call for expensive tooling. Low-cost tooling concepts for this process need to be developed to bring unit cost down, especially for the low production volumes inherent in many composite products. Welding And Joining Processes Evolution of welding and joining technologies can be traced by process, materials, or application. Historically, these processes have developed empirically and have been rapidly applied to technological problems, driven by the tremendous commercial benefits derived from improvements in joining processes in manufacturing. For example, lasers were used within less than five years of their discovery as a source of controlled energy delivery for repair welding of vacuum tubes. Today, virtually any new source of heat energy is immediately evaluated as a candidate joining technology. In most cases, the emerging technology has preceded a fundamental understanding of the process. The need for welding and joining is ubiquitous, as most manufactured products rely on welding or joining in some form. Indeed, only monolithic parts can be made without joining. Unfortunately, joining methods are generally imperfect, either in properties or in affordability, and there is a constant search for improved processes. In addition, welding and joining progress has become a prerequisite for applications in advanced materials. The ultimate quality and reliability of manufactured products is often determined by the quality of the joints (Eagar, 1990; David and Vitek, 1993). The perfect joint is one that is indistinguishable from the material surrounding it. Some processes, such as diffusion bonding, come very close to this ideal; however, such processes seem to be either cost-intensive or restricted to a narrow group of materials. Experience indicates that there is no universal joining process that will perform adequately on all materials in all geometries.
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--> As a result, an engineer must be able to select wisely the best process for a particular material in a given application. Given the great economic and technical importance of welding and joining to any manufactured product, one might ask why a fundamental science has not developed around this field. There are several answers to this question. First, welding and joining are not scientific or engineering disciplines but are processes. As with any process, scientific principles from many different disciplines are involved (e.g., physics, chemistry, mechanics, electronics, and materials). In this sense, there is a science base for welding and joining already in existence, and it is up to the engineer to gather the available information and to apply it judiciously to the problems of welding and joining. Second, welding and joining are material- and application-driven technologies. The revolution that has occurred in materials science and engineering over the past two decades has not been matched by improvements in joining science and technology. It is becoming increasingly apparent that the usefulness of many new materials is limited by the ability to manufacture products made from these materials in an economic, reliable, and rapid manner. As designs utilizing new materials require higher performance standards, the number of acceptable joining technologies becomes more restricted. In contrast, as the functionality of materials becomes more specific, the number of joints and the number of dissimilar material combinations increases. The result is a manufactured product with increased cost and decreased reliability. Welds are often the weakest part of the structure and are generally located at highly stressed locations. In addition, joining often comes near the end of the manufacturing process, when the cost of scrap is high. This is particularly, critical because designers are specifying an increasing diversity of materials in their products, which increases the number of joints. As the materials become more specialized, they are used closer to their performance limits, and, hence, greater requirements are placed on the joints. The result is an increased number of joint failures in spite of improving joining technologies and quality control processes. A number of new materials and processes are emerging, though what can be considered "emerging" depends on the industry as well as the type of material being considered. A novel material for one industry may be a traditional material in another industry; therefore, the challenges facing use of an emerging material are specific to an industry. Since few, if any, novel materials will be used in a monolithic form, they must be integrated into the structure or product using joining technology. Unfortunately, joining technologies do not exist for many of these new materials. On the other hand, industry has a significant need for joining traditional materials more economically, at high productivity and with high quality. This may or may not require new joining processes, but it certainly
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--> will require new resources. In summary, new joining technologies are needed to provide both effective, reliable processes for joining novel materials and improved or novel processes for joining traditional materials. Although there are many nontraditional processes that offer significant advantages in specialized applications, many of the traditional welding processes have unique advantages, which will continue to make them the processes of choice in a number of applications. For example, in the fabrication of heavy structures, arc welding will continue to dominate due to the flexibility and economy of this process. Resistance spot welding will continue to be used in the automotive industry, because it is relatively inexpensive compared with more expensive and complex laser and electron beam processes (see Figure 7-1). Nonetheless, these newer processes will continue to grow in importance and need to be studied in increasing detail. With regard to novel materials, productivity is not the major challenge in joining such materials. Rather, the question is whether some of these materials can be joined at any cost. As materials designers use more-complex structures, which are tailored to specific applications, the use of the material is pushed to its limits of performance. This creates severe problems for the performance and integrity of the welded joint. Whereas design engineers traditionally designed with available materials, they now design materials for specific applications. As an example, consider the production of aircraft structures. Fifty years ago, designers selected from available materials such as wood, canvas, and aluminum. Today, design engineers dream of a hypersonic passenger airplane (known as the Orient Express), which must endure surface temperatures of 1500 °C, in addition to being lightweight, high strength, and resistant to hydrogen degradation. These materials—advanced intermetallics and composites—must be developed to meet the design rather than the design being tailored to the properties of available materials. Unfortunately, joinability is rarely factored into the design of these new materials, creating great difficulties when an attempt is made to utilize the materials in a real structure. The cost of many new materials is so high and their properties are so specialized that they will only be used where they are essential. As a result, products will contain more joints, and a greater fraction of these will be between dissimilar materials. This will only compound problems of quality and reliability in the final product. The common design rule of eliminating all possible joints is being violated at an increasing rate. Due to a desire to use the minimum amount of costly, high-function materials, joints are being placed in more aggressive environments. The properties of joints are being pushed to the limit. One challenge for joining engineers is failure at the joints. It is no longer possible to select the joint configuration or joining process as an afterthought of the design. Joining technology must become an integral part of the product design.
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--> Figure 7-1 Production costs for commercial welding processes (adapted from Eagar, 1990). Thus, some of the challenges in joining emerging materials include faster new product development cycles; fabrication of smaller components, especially in the electronics industry; and more dissimilar material combinations. The development of new materials should include parallel development of the necessary manufacturing processes, such as joining, as a critical priority, especially considering the specialty applications with small numbers of parts. Otherwise commercial use of these materials will be severely retarded. With regard to traditional materials whose properties and joining methods are well known, incremental improvements will continue to be made in the quality and economy of the product.
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--> Research Opportunities Powder Processing There is a clear need for production technologies geared to homogeneous, clean, reproducible, small powders, typically from the higher-performance materials. Future applications and processing developments are largely contingent on the availability of these powders. Research on scale-up of production processes for small, high-quality powders at low cost is an urgent need. There is considerable potential for improved powder products that are based on developing coating techniques for forming composite particles. Such research must focus on producing uniform, homogeneous materials in large quantities. Consolidation technology upgrades depend on improved modeling and analysis capabilities, coupled with implementation of a new production control logic. Shaping technologies have similar needs for uniformity, dimensional control, and productivity. Die compaction is the dominant shaping technology and needs research support to better design and control the shaping operation. This research should include attention to more-intelligent presses and better tooling design strategies. Powder injection molding has enormous potential. Research is needed on sensors and analysis techniques to isolate defect sources and institute defect detection schemes within the molding operation. Interactions between the powders, binders, molding process, and component geometry must be better understood, and this knowledge must be incorporated in control system software. For sintering there is a need to link the atomistic scale understanding to the macroscopic level in terms of furnace instrumentation, control, and optimization. Intelligent processing schemes are needed to examine the atmosphere, component size and shape, thermal goals, and furnace conditions to ensure proper sintering. Research on densification is needed to allow rational process analysis. A generic analysis framework that includes a materials stress state, strain rate, powder characteristics, and temperature during sintering should be developed.
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--> Consolidation Of Polymeric Composites Thermosetting prepreg materials with more-robust shelf lives and the ability to be stored at room temperature are needed. Also, improved thermoplastic prepreg materials are needed that address the materials' high cost, relatively high processing temperatures, and lower resistance to solvents. Innovative equipment designs are needed that allow for the flexible introduction of hybrid construction (i.e., multiple families of fiber types) and woven reinforcement configurations. Such equipment can fully utilize the unique properties of prepreg materials. Optimization of fiber coatings to provide good adhesion to the resin and ease of composite processing is required. Good wetting fiber preforms may go a great way in extending the process limits and increasing processing speeds, particularly in resin transfer molding. For example, it is well known that sizing (coating) applied to glass fibers greatly improves the fibers coupling to an epoxy matrix. Developments in automated layup and vacuum bagging that enable these processes to be accomplished without the need for secondary materials (e.g., vacuum bags, bleeder, and sealant) are important research needs. Such developments will overcome environmental concerns due to the amount of waste attributed to the secondary materials inherent in present prepreg production methods. Equipment technology borrowed from the textile and paper industries may prove useful in its application to prepreg processing in which nontacky prepregs are used. The integration of sensors and heating elements in an intelligent mold is needed for production of high-quality composite structures containing variable thickness features and hybrid materials. Models for RTM and S-RIM are needed that adequately address the reaction kinetics and the time- and temperature-dependent dynamics of reactive fluid flow during processing. Further, sensors and real-time control schemes must incorporated in present equipment to improve quality control, surface finish, and the production of hybrid composites. Low-viscosity resin systems are needed for RTM and S-RIM processes to provide sufficient rigidity to the part, so that it can be ejected from the mold in a short time and subsequently post-cured to reduce in-mold cycle times. The construction of accurate fiber preforms is essential to the production of high-quality pans by resin transfer modeling or S-RIM techniques. Innovative equipment design is needed for automated handling of dry fiber and creating preforms with the required local fiber orientations and fiber distribution densities. The application of textile technology, including stitching, braiding, and
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--> weaving, should be further studied in the context of handling high modulus fiber preforms. The delamination strength of pultruded products must be improved if this process is to gain wide use. Investigation of bulked rovings and scrim fabrics are potentially useful research directions. Innovative approaches to reduce pull forces, improve surface finish, and incorporate higher-temperature resin systems are crucial for the development of pultrusion to make closed hollow sections. Reinforced polymer composites have poor surface finish due to high fiber volume fraction and resin shrinkage during cure. Research is needed on improving surface finish through the use of in-mold coatings or, in the case of pultrusion, through co-extrusion of an unreinforced polymer. Welding And Joining Processes Joining of novel metals, composites, and ceramics will require validation of existing processes, as well as invention of new technologies. Characterization of the properties and performance of the joints are required before designers will have the confidence to specify use of these materials in new products. Innovative processes that have great potential for achieving superior joints in advanced materials include high-energy density processes (e.g., laser and electron beam), solid-state joining processes (e.g., diffusion bonding and friction welding), and advanced arc processes that provide independent control of base-plate heating and filler material melting rate. Lower-cost, more-reliable sensor technologies are required for the automation of joining processes that could lead to significant enhancements in productivity. The challenge is particularly great to develop sensors for fusion welding, where the high temperatures and large thermal gradients make process measurement extremely difficult. Research is needed to develop approaches to eliminate or control the residual stresses and distortion that result from localized heating and nonuniform cooling of many joining processes, so that parts can be manufactured with precision. The greatest limitation to improving the quality and reliability of joints is improved understanding of the process physics; therefore, this should be a high priority for future R&D. More-powerful simulation models are needed, and they should be verified by thoughtful experimentation.
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--> Most welding processes are extremely complex from the viewpoint of conventional control theory. The process physics must be understood, and control algorithms developed that can handle the nonlinear behavior of welding systems.
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--> References Eagar, T.W. 1990. Challenges in joining emerging materials. Proceedings of the International Conference on Advances in Joining Newer Structural Materials held 23-25 July 1990 in Montreal, Canada. Oxford: Pergamon Press. David, S.A., and J.M. Vitek, eds. 1993. International Trends in Welding Science and Technology. Materials Park, Ohio: ASM International. German, R.M. 1994. Powder Metallurgy Science. Princeton, New Jersey: Metal Powder Industries Federation. Jenkins, I., and J.V. Wood, eds. 1991. Powder Metallurgy: An Overview. London: The Institute of Metals. Klar, E., ed. 1984. Powder Metallurgy. Vol. 7, Ninth edition of Metals Handbook. Materials Park, Ohio: ASM International. Klar, E., ed. 1983. Powder Metallurgy: Applications, Advantages, and Limitations. Metals Park, Ohio: ASM International. Lenel, F. V. 1980. Powder Metallurgy Principles and Applications. Princeton, New Jersey: Metal Powder Industries Federation.
Representative terms from entire chapter: