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Use of Lightweight Materials in 21st Century Army Trucks Chapter Two New Materials and Processing Opportunities INTRODUCTION Any discussion of new materials and processing opportunities can be wide-ranging, so to structure the discussion here, the problem is divided by application and time frame. For the purposes of this report, only structural elements are considered; the power generation and electrical parts of trucks are not considered. The structural elements of a truck are divided into the following three categories: Frame: The primary structural element in all current military trucks is a steel frame that runs the length of the vehicle; the engine, drivetrain, suspension, and truck bed are all attached to the frame; Secondary structural elements: The secondary structural elements are the parts of the truck that carry passengers and cargo—for example, the cab and the cargo bed. Although these elements may account for a significant portion of the vehicle’s weight, they do not provide the essential strength or stiffness of the truck; and Structural drivetrain: This category includes driveshafts, the suspension, the steering mechanism, and braking components. These elements may contribute significantly to vehicle weight and are critical to the vehicle’s safe and reliable functioning. With respect to time frames for the maturation of research and development and the insertion of new technologies, chronological time is not appropriate—there are Army trucks in operation using 30-year-old designs that may very well remain in the field for the next 30 years. At the same time, new systems will most likely be developed that will enable the use of new materials, processes, and designs. Time frames are therefore operationally defined as follows: Short term: This category refers to improved materials choices that can be substituted for existing materials in existing truck systems. This
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Use of Lightweight Materials in 21st Century Army Trucks category does not include any fundamental redesign of the truck or its subsystems. The substitution of higher-strength or coated materials are examples of short-term opportunities. Care must be exercised in making such changes because any materials substitution can alter the vehicle’s response to terrain changes and affect its ability to perform certain functions. Medium term: A new design or significant rebuilding of a proven truck platform represents an opportunity for more aggressive materials substitutions. Such an instance might involve the use of a modestly different architecture or different joining methods. For example, the replacement of a truck’s steel frame rails with hydroformed tubes would require changes in several other design aspects and would thereby open up opportunities for materials substitutions. Long term: The present truck paradigm consists of a power plant burning a single fossil fuel and providing power to the vehicle’s wheels through driveshafts. It is unclear how long this paradigm will remain the dominant one. Prototypes of hybrid electric vehicles have been produced.1 In the future, truck architecture may become modular, with power plants providing electric power to driven axle or bed modules. This would eliminate the need for driveshafts and fundamentally change frame configurations. Such changes in the basic truck paradigm would enable the use of radically different materials. Selection of New Materials and Processes Component Shape In specifying a material for the final design of a vehicle or component, a number of characteristics must be considered. Materials properties play an important role in the performance of a component, affecting: strength, density, fracture toughness, fatigue resistance, cost, availability, available forms, formability, joinability, and corrosion or environmental resistance. The suitability of any mechanical system for meeting a performance objective is only partly governed by materials, however. The configuration, or shape, of the component also has a large effect. For example, an I-beam shape is much stronger and stiffer with respect to cantilever-bending loads than a simple cylindrical shape of the same mass per length. Other characteristics, 1 Nimmer, S. Oshkosh Truck Corporation. Presentation to the committee, August 2002.
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Use of Lightweight Materials in 21st Century Army Trucks such as section stiffness, force for permanent deformation, energy absorption, and fracture load, can be affected by the shape of the component as well as by the material from which it is made. A process has been developed for selecting the optimal materials for an application on the basis of objective functions such as physical properties, cost, and the type of loading to which the material will be subjected.2 This approach is not a finite element analysis package, but rather a materials selection process that includes a database on materials, properties, costs, and advantages, with the data accessible in many ways. The approach has been extended to include the effect of component shape, and many features have been incorporated into commercial software.3 This type of design tool can be of great service in making short-term design and materials selection decisions. The design of Army trucks should include a minimum-weight study as the final step in the design process. As is often done with commercial vehicles, a minimum-weight, optimal-shape design study should be performed on a vehicle after the preliminary designs, including the selection of lightweight materials, are complete. The minimum weight-design would lead to structural shapes (e.g, cross-sectional shapes of truck frameworks) that correspond to a significant reduction in vehicle weight. A weight reduction of at least 5 percent should be expected. It is, of course, essential that the performance of the truck not be degraded by the reduction of weight. As part of the minimum-weight process, constraints should include those involving structural integrity, noise, vibrations, armor cladding, on-road requirements, survivability, crashworthiness, stealthiness, load capacity, and load flexibility. Satisfaction of constraints in these areas will require a complex computer program that cycles among various analyses corresponding to the constraints. Shape selection of the actual minimum-weight structural member is a reasonably efficient process. Typically, a geometric description of the cross section of various members is fed into an optimization routine that requires shape-sensitivity functions. Finite element analyses of member cross sections are introduced. The optimization program shapes the cross section with minimum weight as the objective function.4 2 M. Ashby. 1992. Materials Selection in Mechanical Design. Oxford, U.K.: Pergamon Press. 3 For example, <http://www.grantadesign.com>. Accessed March 2003. 4 W.D. Pilkey. 2002. Analysis and Design of Elastic Beams: Computational Methods. New York, N.Y.: Wiley.
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Use of Lightweight Materials in 21st Century Army Trucks Low-Cost Processing The issue of materials and component shape selection is further complicated by the realities of available material forms, manufacturing methods, cost, joining methods, and production volumes. In selecting a processing method, engineering judgment is required, and formal design methods are of limited value. The difficulty of shaping a material is directly related to the characteristics of the material. Although machining can always be used to create a shape, it is prohibitively expensive for many applications, such as frames for trucks. Net shape processes such as casting or stamping are low-cost methods of fabricating complex geometries, but these techniques cannot be used with all materials. In general, lower-strength materials are more easily fabricated by casting or sheet forming. Advanced modeling and simulation tools could be used cost-effectively to evaluate various alternative materials for a given application or to optimize vehicle designs.5 However, the data needed to design components to meet specific performance requirements, e.g., fatigue life, is often unavailable for the specific material being evaluated. The lack of adequate databases needed for accurate finite element modeling (FEM) exacerbates the design community’s existing lack of familiarity with lightweight materials. In addition, appropriate models for processing lightweight materials and for predicting the performance of components manufactured using these materials must be developed. Very good models currently exist for casting, forging, rolling, and extrusion. More such work would greatly help transition new materials into appropriate applications in new designs. The cost of transforming materials into desired shapes is dependent on a number of things, including production volume. Sand, lost wax, and lost foam castings can be done by hand at low fixed tooling costs. These processes are amenable to the low production volumes commonly associated with military tactical trucks. Higher-quality parts can be formed by means of die casting or squeeze casting, but the costs of dies and tooling are significant and can run into the tens of thousands of dollars. Amortizing these costs over the number of units to be made is difficult. In sheet metal forming, the matched tools used for mass production again typically cost tens of thousands of dollars. However, in aerospace production, much-less-expensive 5 National Research Council. 2003. Materials Research for 21st Century Defense Needs. Washington D.C.: National Academy Press.
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Use of Lightweight Materials in 21st Century Army Trucks single-sided dies are used in conjunction with processes such as rubber-pad hydroforming. The marginal cost per part is much higher than in mass-production stamping, but this higher cost is offset by lower tooling and setup costs. Low production volumes can enable the use of more expensive materials provided that tooling and fabrication costs are minimized. For example, for Freightliner trucks and Panoz roadsters, the use of superplastic aluminum alloys is favored over the less expensive (and higher strength) cold-formable alloys, largely because the one-sided tooling used in superplastic forming is relatively inexpensive. When varied designs are being manufactured, many factors must be considered in order to find a solution that approaches the optimal. Because of the low production volumes typical for Army truck manufacturers, it may not be possible to take advantage of all of the low-cost processing experience of automobile manufacturing. Some technologies that may become viable at low production volumes include the following: Superplastic forming, especially for cases in which commercial superplastic alloys, such as 5083, exist; Compression molding of thermosetting polymer composites; Rubber-pad hydroforming of sheet components; Tube hydroforming; Electromagnetic forming of tubes and sheets; Explosive forming of tubes and sheets; Thin-walled castings; Other agile shaping methods for thermosetting polymer composites; and Interfacing with joining technology. The increased use of new lightweight materials depends on the development of robust joining processes that produce acceptable joint properties at costs and assembly times comparable to those for resistance spot welding of steel. The joining of these new materials to themselves and to other materials presents technical challenges. Improved joint designs must also be developed in order to take full advantage of the benefits that these materials can provide. Significant work must be done to develop joint designs, methodologies for dealing with galvanic effects, mechanical fastener technologies, nonfusion joining, and hybrid joints.
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Use of Lightweight Materials in 21st Century Army Trucks Repair and Disposal of New Materials The use of new materials in Army trucks will have consequences with respect to vehicle assembly, repair, and disposal. When assembling new vehicles or recapitalizing older vehicles, it will be necessary to ensure that galvanic isolation exists between parts made from different materials in order to avoid galvanic coupling effects that can lead to corrosion. Galvanic isolation will have to be maintained during inspection, maintenance, and repair. Welding, bonding, brazing, and other repair and replacement procedures will therefore become more complicated. Composites are a relatively new class of materials now being used selectively on Army vehicles. Composites are very attractive because they can be designed with specific properties. However, a number of factors must be considered before a decision is made to use composite materials in an Army vehicle. Composite repair procedures are very different from those for metallic materials and are generally not known within the Army’s repair facilities. The insertion of composite materials into Army vehicles must therefore be accompanied by new repair manuals and the training of Army personnel in these new procedures. Careful handling and storage of composites is also necessary, because their properties are sensitive to the presence of surface flaws. Other issues with these materials include the decrease in strength that results from absorption of water in places where the surface protective coating is penetrated. Many composite properties are also temperature-sensitive, so the use of composites in extreme climates must be carefully monitored. Careful handling and storage can eliminate many potential problems for this class of materials. A coding system might be used to differentiate material types and to facilitate proper assembly, repair, and disposal. In addition, training will be required for the proper maintenance of vehicles that have been manufactured with new materials or that have had parts changed in recapitalization programs. Maintenance and repair manuals should be continuously updated. However, it will be difficult to keep issuing and delivering repair-manual updates to the necessary field and depot repair facilities. One solution would be for vehicle original equipment manufacturers (OEM) to maintain repair and maintenance manuals on a Web site, along with information on the symptoms, possible causes, and remedies for known truck
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Use of Lightweight Materials in 21st Century Army Trucks problems. Repeated failure of any one part could be communicated to the OEM by e-mail so that corrective action could be taken. Finally, end-of-life disposal and recycling issues will become more complex as new and different materials are introduced. Many disposal yards are equipped to separate different classes of materials for recycling. When older Army trucks are sold for disposal, the contracts should specify which materials classes must be separated and how the disposal facility is held accountable for appropriate recycling. The Vehicle Recycling Partnership, formed between the three large domestic automobile manufacturers, proved that dismantling a vehicle was too labor-intensive and time-consuming to be cost-effective. By comparison, a full vehicle can be shredded in approximately 40 seconds, and the subsequent separation and sorting of metals are both fully automated. The sequence of fluff removal, magnetic separation, heavy/light media flotation and separation, eddy current separation, color separation, and, finally, laser-induced breakdown spectroscopy is now well established. All of the common metal groups can be separated in an industrial process, and in fact a specialty steel company in Detroit is presently operating plants in both the United States and Europe to do this separation. SHORT-TERM OPPORTUNITIES The short-term problem as defined in the introduction to this chapter is constrained by the absence of changes in present truck designs and by the use of commercially available materials and forms. Under these constraints, the approach pioneered by Ashby and described above in the subsection "Component Shape" can be used to identify appropriate materials options.6 The basis of the Ashby materials selection process is the use of quantitative performance indices, or mathematical functions of service requirements, geometric parameters, and materials properties. The higher the performance index, the better suited a material is for a particular job, with the part weight needed to reach a given level of performance typically inversely related to its performance index. Table 2-1 illustrates some relevant performance indices. 6 M. Ashby. 1992. Materials Selection in Mechanical Design. Oxford, U.K.: Pergamon Press.
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Use of Lightweight Materials in 21st Century Army Trucks TABLE 2-1 (a) Performance indices for Minimum Weight (Cost, Energy) Design: Stiffness and Strength Component Shape and Loading Stiffness Design: Maximize Strength Design: Maximize Tie (tensile strut) load, stiffness, length specified, section area free E/ρ σf/ρ Torsion bar or tube torque, stiffness, length specified, section area free G1/2/ρ σf2/3/ρ Beam loaded externally or by self-weight in bending; stiffness, length specified, section area free E1/2/ρ σf2/3/ρ Column (compression strut) failure by elastic buckling or plastic compression; collapse load and length specified, section area free E1/2/ρ σf/ρ Plate loaded externally or by self weight in bending; stiffness, length, width specified, thickness free E1/3/ρ σf1/2/ρ Plate loaded in-plane; failure by elastic buckling or plastic compression; collapse load, length and width specified, thickness free E1/3/ρ σf/ρ Rotating disks, flywheels energy storage specified — ρf/ρ Cylinder with internal pressure elastic distortion, pressure and radius specified; wall thickness free E/ρ σf/ρ Spherical shell with internal pressure elastic distortion, pressure and radius specified, wall thickness free E/(1 - v)ρ σf/ρ NOTES: To minimize cost, use the above criteria for minimum weight, replacing density ρ by Cρ, where C is the cost per kilogram. To minimize energy content, use the above criteria for minimum weight, replacing density ρ by qρ, where q is the energy content per kilogram. KEY: E = Young's modulus; G = shear modulus; σf = failure strength; ρ = density. SOURCE: Reprinted from Materials Selection and Design, M. Ashby, Table 5-1, Copyright 1992, Oxford, U.K.: Pergamon Press, with permission from Elsevier Science. This approach provides a good quantitative basis for making the first step in materials selection decisions. It also provides a basis for selecting short-term candidate materials for use in Army truck applications. Several of these candidate materials are discussed below.
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Use of Lightweight Materials in 21st Century Army Trucks TABLE 2-1 (b) Performance indices for Minimum Weight (Cost, Energy) Design: Crack Length Component Shape and Loading Crack Length Fixed: Maximize Crack Length ≈ Min Section: Maximize Tie (tensile strut) load, length specified, section area free KIC/ρ KIC4/3/ρ Torsion bar or tube torque, length specified, section area free KIC 2/3/ρ KIC4/5/ρ Beam loaded externally or by self-weight in bending; stiffness, length specified, section area free KIC2/3/ρ KIC4/5/ρ Column (compression strut) failure by elastic buckling or plastic compression; collapse load and length specified, section area free KIC2/3/ρ KIC4/5/ρ Plate loaded externally or by self weight in bending; stiffness, length, width specified, thickness free KIC1/2/ρ KIC2/3/ρ Plate loaded in-plane in tension; collapse load, length and width specified, thickness free KIC/ρ KIC2/ρ Rotating disks, flywheels energy storage specified KIC/ρ KIC/ρ Cylinder with internal pressure elastic distortion, pressure and radius specified; wall thickness free KIC/ρ KIC2/ρ Spherical shell with internal pressure elastic distortion, pressure and radius specified, wall thickness free KIC /(1-v)ρ KIC2/(1-v)ρ NOTES: To minimize cost, use the above criteria for minimum weight, replacing density ρ by Cρ, where C is the cost per kilogram. To minimize energy content, use the above criteria for minimum weight, replacing density ρ by qρ, where q is the energy content per kilogram. KEY: KIC = fracture toughness; ρ = density. SOURCE: Reprinted from Materials Selection and Aluminum and Magnesium Alloys Aluminum and magnesium alloys are candidates for the replacement of steel in Army truck applications. The commercial aircraft industry is based on aluminum, with the empty weight of a typical commercial airplane being
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Use of Lightweight Materials in 21st Century Army Trucks TABLE 2-1 (c) Elastic Design Component and Design Goal Maximize Springs specified energy storage, volume to be minimized σf2/E Springs specified energy storage, mass to be minimized σf2/Eρ Elastic hinges radius of bend to be minimized σf/E Knife edges, pivots minimum contact area σf3/E2 and E Compression seals and gaskets maximum contact area with specified maximum contact pressure σf/E and 1/σf Diaphragms maximum deflection under specified pressure or force σf3/2/E Rotating devices, centrifuges maximum angular velocity, radius specified, wall thickness free σf/ρ Ties, columns maximum longitudinal vibration frequencies E/ρ Beams maximum flexural vibration frequencies E1/2/ρ Plates maximum flexural vibration frequencies E1/3/ρ Ties, columns, beams, plates maximum self damping η NOTES: To minimize cost, use the above criteria for minimum weight, replacing density ρ by C ρ, where C is the cost per kilogram. To minimize energy content, use the above criteria for minimum weight, replacing density ρ by q ρ where q is the energy content per kilogram. KEY: E = Young's modulus; σf = failure strength; ρ = density; η = loss coefficient. SOURCE: Reprinted from Materials Selection and Design, M. Ashby, Table 5-1, Copyright 1992, Oxford, U.K.: Pergamon Press, with permission from Elsevier Science. composed of approximately 70 to 75 percent aluminum. However, the total substitution of aluminum for steel in Army truck applications is unlikely for several reasons, including the higher cost of aluminum and the need for ballistic protection in some trucks. Table 2-2 compares the properties of several ferrous, aluminum, and magnesium alloys, including typical values for each alloy class. These values
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Use of Lightweight Materials in 21st Century Army Trucks TABLE 2-2 Comparison of Properties of Some Steels, Aluminum Alloys, and Magnesium Alloys Materials Density (g/cm3) Young’s Modulus (GPa) Strength Range (MPa) Steels (typical) 7.85 200 200-850 1018 cold finished 7.87 205 370 1040 cold finished 7.84 200 550 302 stainless, 10% cold work 7.86 193 635 410 tempered at 540 ºC 7.8 200 1,005 Aluminum alloys (typical) 2.7 70 100-400 6061 T651 2.7 69 310 2024 T6 2.77 72 345 5083-H32 2.66 71 206 7075 T6 2.81 72 503 Magnesium alloys 1.8 45 100-300 Extruded AZ10A-F 1.76 45 155 Extruded AZ1B 1.77 45 165 SOURCE: MatWeb Material Property Data. Available at <http://www.matweb.com>. Accessed March 2003. indicate that density and modulus vary only modestly over the full alloy family. However, strength, ductility, fracture toughness, and fatigue resistance can vary substantially depending on the alloy and its processing history. Low-density materials such as aluminum and magnesium have definite design advantages in terms of elastic properties (see Table 2-3), even when the specific strength or stiffness remains the same. These materials have significant performance advantages when loaded in torsion or bending because the greater section thickness at fixed weight gives greater resistance to bending or dents. In pure tensile loading, performance indices scale with E/ρ. In these applications, steel and aluminum and magnesium alloys perform similarly. Magnesium alloys cannot be immediately considered for truck applications because of their low ductility and limited processibility. In addition, stress corrosion cracking caused by the presence of in-service residual stresses has limited the use of magnesium alloys in commercial vehicle applications. However, potential applications in the near future include castings for transmission casings or transfer cases, and magnesium
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Use of Lightweight Materials in 21st Century Army Trucks High-Strength Steel Alloys Steel remains the lowest-cost structural material for trucks, and important advances have been made in grain refinement, corrosion resistance, processing, and innovative designs for crashworthiness. These advances are the result of industry collaborations over the past decade. In the commercial automotive and truck industries, there has been a proliferation of a wide variety of high-strength martensitic steel alloys and fabrication methods that are strong candidates for application in future Army trucks. Through the use of dual-phase steel alloys (such as DP600 for frame rails, cross members, suspension components, and wheels) and advanced fabrication methods (such as hydroforming, tailor welded blanks, and laser welding) the Improved Materials and Powertrain Architectures for Trucks (IMPACT) program has demonstrated a mass savings of about 1,310 lb (a 25 percent weight reduction) and an improvement in fuel economy of 8 miles per gallon in a Ford F150 light truck.17 These results are highly applicable to Army light trucks replacing the C/K class. Certain unresolved issues remain with the use of high-strength steel alloys (i.e., TRIP, martensitic, and dual-phase), such as design optimization, material scrap, tooling investment, and overall formability and springback (i.e., the tendency of a sheet to relax when the forming loads are removed). These factors must be addressed jointly by industry. Ultrahigh-carbon Steels Ultrahigh-carbon steels (UHCSs) contain between 1.0 and 2.1 percent carbon and are hypereutectoid steels. These steels have been processed to become superplastic at high temperatures, and strong and ductile at room temperature. The higher the amount of carbon, the higher the strength of the steel. Although these steels have been in development since the mid-1970s through collaborations between industry and research institutions, they are not yet off-the-shelf items. This is principally due to the fact that steel companies will not proceed with production until a large order is guaranteed.18 17 ULSAB (UltraLight Steel Auto Body). Presentation on Advanced Vehicle Concepts to the U.S. Army Tank-automotive and Armaments Command, April 30, 2002. 18 J. Sandelin. 2000. Patenting and licensing university research results: The challenges of disruptive technologies. R&D Enterprise-Asia Pacific 3(1-2):24-32.
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Use of Lightweight Materials in 21st Century Army Trucks UHCSs have the potential to reduce weight in trucks’ structural components to meet future Army requirements. Potential applications of these steels include their use in fabricating vehicle components that require high strength—such as side impact beams, bumpers, and wires in tires. The desired UHCS structure is the pearlitic state. Pearlite, which consists of alternating layers of iron carbide (cementite) and iron (ferrite), is an in situ, self-laminated, nanoscale composite. This structure can be created directly from primary mechanical processing operations without any additional heat treatment. A typical tensile strength of an as-extruded UHCS bar is 1,000 Mpa, with an elongation of 10 percent. A pearlitic UHCS can lead to higher wire strengths than those of conventional eutectoid composition steels.19 Aluminum Alloys Aluminum offers the greatest potential for weight reduction in truck bodies, but it also requires the use of different construction techniques. Aluminum space frames have been the subject of much research and development in the past decade, owing to the need to improve strain rate sensitivity (i.e., crash performance) and to enhance the metal’s capability to support major vehicle body and fatigue loads. Strain rate sensitivity also affects formability. The space frame sections can be joined by welding or adhesive bonding. All-aluminum truck cabs and wheels are already in use in several classes of commercial and Army trucks. For example, the MTVR truck cabs have stamped aluminum components that are adhesively bonded. These applications of aluminum have resulted in significant maintenance cost savings. Aluminum alloy 2519 (Al-Cu-Mg) is a high-performance alloy that can be used to meet the strength, weight, and mobility requirements of future Army trucks using armor plate, forgings, and extrusions (Fisher et al., 2002). Aluminum alloy 2519 was developed by Alcoa and the Army as a weldable material with ballistic penetration resistance superior to that of Al-Mg (5xxx) alloys and without the susceptibility to stress-corrosion cracking. This material is also being considered by the Marine Corps for fabrication of the Advanced Amphibious Assault Vehicle (AAAV). However, the lack of design data for extrusions and forgings from aluminum alloy 2519 has resulted in 19 E.M. Taleff, J.J. Lewandowski, and B. Pourladian. 2002. Microstructure-property relationships in pearlitic eutectoid and hypereutectoid carbon steels. Journal of the Minerals, Metals, and Materials Society 57 (7): 25-30.
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Use of Lightweight Materials in 21st Century Army Trucks the alloy's not being more widely used. Research in hot workability testing and manufacturing technology developments is currently under way at the National Center for Excellence in Metalworking Technology. Corrosion for such aluminum alloys is largely in the form of pitting, and it may increase the O&S costs of the vehicle system, whereas stress-corrosion cracking can lead to catastrophic failure. These alloys can, however, be clad with pure aluminum for added corrosion resistance. Specific improvements in aluminum fabrication technology and cost reduction that would accelerate the medium-term use of aluminum alloys in future Army trucks include the following: Reduction in sheet raw material prices (e.g., by way of continuous casting); Design optimization for crew cabs (such as that done for the steel unibody) and optimization of space frames; and Improved sheet formability, castability (such as ultralarge castings), and joining technologies (such as friction stir welding) for higher-strength aluminum alloys such as the 5000 and 6000 series to enable the fabrication of larger, more integral structures. Friction stir welding produces joints that are much less susceptible to galvanic corrosion. Because the metal is not melted, the galvanic corrosion precursor precipitates in the grains, and boundaries are not formed. Alloys that have traditionally been difficult to weld can now be joined using this technology. Magnesium Alloys The relative value of lightweight materials such as magnesium is just now being demonstrated for passenger vehicles. Therefore, magnesium is a good candidate for consideration in the medium term for newer, lighter Army vehicles. Although magnesium is not currently used in Army trucks, it has significant potential as a replacement for steel because it is one of the lightest structural metals and because of its high specific strength, damping capacity, and dent resistance. Because magnesium also has potential for replacing steel in automobiles, it is likely that applications developed by the automotive industry (such as closures and instrument panels) can be
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Use of Lightweight Materials in 21st Century Army Trucks leveraged for Army Class 2B and 6 trucks. In order to provide adequate corrosion protection, magnesium components must be coated. The challenges to implementing magnesium structures in Army trucks include these: technical performance modeling, supplier die-casting and sheet-forming infrastructure, material feedstocks and price stability, vehicle design and development experience, reliability, safety, serviceability, and closed-loop recyclability. Considerable potential exists for collaboration between the Army, DOE laboratories, and the automotive industry for jointly addressing these challenges. Provided that significant improvements can be achieved in feedstock quality, die-casting processing and handling, cost reduction, and structural quality, die-cast and wrought magnesium alloys (e.g., ZK60 and AZ31) have a number of potential component applications in military trucks, including the following: Body and closure components for door and hood inner panels, support modules, A and B pillars, and roof-opening panels; Powertrain components for transmission, transfer case and cover, and engine block (brackets, mountings, housings, oil pan, covers); Road wheels and spare wheels; and Interior components for instrument panel cross-vehicle beam, seats, front and rear backs, and steering components (pump housings, brackets, steering wheel). Metal Matrix Composites Metal matrix composites have been of military, as well as commercial, interest for nearly three decades, and the market for aluminum MMCs is projected to grow at a 14 percent overall rate to $173 million by 2004. Currently, aluminum MMCs are used primarily in low-volume, specialized applications in the aerospace, automotive, defense, and electronics packaging industries (ALMMCC, 2002). Although these industries have prototyped a number of component applications, only a few have reached production. The majority of these have been limited, niche, applications. Two of the few high-volume, commercialized applications are MMC brake drums and rotors, dominated by Duralcan and Alcan, and MMC pistons for gasoline engines, commercialized by Toyota and Mitsubishi for use in selected small-model vehicle engines. European piston suppliers have developed the squeeze-casting process for making MMC pistons.
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Use of Lightweight Materials in 21st Century Army Trucks The Army has investigated the development of MMCs for application to tank tread shoes, using selective silicon carbide (SiC) whisker reinforcements in high-wear locations.20 USCAR and DOE—as well as several European and Japanese OEMs—are investigating applications of aluminum MMCs using the powder metallurgy route for low-cost transmission gears and connecting rods. Early commercial successes include the castable-aluminum MMCs, marketed under the trade name of Duralcan, using SiC particulate reinforcement, and wrought products that use alumina (Al2O3) particulate reinforcement. Both intermediate raw materials are available in billet form and are increasingly used in fabricating brake drums and rotors. These materials replace the current gray cast iron, with its weight-related problems and performance shortcomings. The A359 aluminum MMC with SiC particulate (20 percent volume fraction or higher) is a good, general-purpose MMC for structural applications because of its superior heat conduction and reduced storage capacity. For cast-iron brakes, temperature spikes to 700 ºC are common. Brake components of this new material would have a lower operating temperature, which would result in a friendlier environment for the lining, rotor, or drum materials.21 This improvement would also allow for greater use of embedded sensors that could improve structural health monitoring and maintenance applications.22 The potential of MMC technology remains largely untapped, however, for a number of reasons. First, there are technical issues that need to be resolved, including fabrication costs, materials handling, and machinability. In the case of cast and powder metal aluminum MMC products, nondestructive evaluation (NDE) technologies must be integrated into manufacturing processes to ensure that consistent component density and minimum variation and discontinuity in properties are achieved. Second, the supplier base is small and fragmented. There is a wide range of potential composite systems, and the cost of materials development and testing has been prohibitively high for individual suppliers. High-volume end uses have 20 D. Ostberg. TACOM. U.S. Army Materials Research and Development Activities. Presentation to the committee, April 2001. 21 M.J. Denholm. 2001. Application and manufacture of Al MMC components in light vehicles. Composites in Manufacturing Quarterly. 17(2):1-5. 22 R.M. Hathaway, Oshkosh Truck Corporation. Presentation to the committee, May 9, 2002.
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Use of Lightweight Materials in 21st Century Army Trucks therefore not been pursued. Finally, the Army depots have limited experience in remanufacturing processes. The Army, as a major stakeholder and potential beneficiary, should partner with defense contractors and commercial aluminum MMC suppliers to develop and demonstrate new applications of aluminum MMCs in lightweighting (e.g., powertrain, brake, and suspension components). Research topics outlined in the aluminum MMC technology roadmap include these:23 Development of new aluminum MMC materials, critical processes, and design databases; Modeling of engineered materials and product performance, processing, and costs; Rapid prototyping and short-run production for cost-effective applications; and Improved machinability and joinability (e.g., maintaining clamp loads). The establishment of a joint commercial-military MMC user resource center might be a valuable way of disseminating knowledge and guidance to military users and component manufacturers. Currently, there is no trade or industry organization that serves the MMC field specifically with respect to technology transfer. A resource center could direct users to sources of information on materials properties, uses, and characteristics—for example, to military handbook data and the educational modules being put together by The Minerals, Metals, and Materials Society (TMS). In addition, it could serve as the focal point for information exchange between end users, suppliers, the government, and the scientific community. Processing and machinability data, for example, could be made available to everyone. Polymer Matrix Composites Polymer matrix composites with fiberglass or carbon filaments have already been demonstrated and applied in limited production volumes on Army trucks. Thus far, the application of PMCs has had mixed results. For 23 Aluminum Metal Matrix Composites Consortium (ALMMCC). 2002. Aluminum Metal Matrix Composites Technology Roadmap. Ann Arbor, Mi: ALMMCC. Available at <http://www.almmc.com>. Accessed January 2003.
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Use of Lightweight Materials in 21st Century Army Trucks example, the use of sheet molding compound (SMC) chopped-fiberglass-reinforced polyester hoods for the Humvee reportedly delivered increased payload, reduced weight, and improved corrosion resistance. However, this application developed maintenance problems due to material delamination, cracking, and susceptibility to impact damage. These problems resulted in high failure rates and increased maintenance and replacement costs. On the other hand, the use of PMC truck hoods for the MTVR is considered a success in lightweighting and production.24 Mixed results such as these often occur when a new technology is introduced. The successful application of composites in MTVR hoods indicates that the problems with the Humvee hoods should be investigated to determine a solution. The Composite Armored Vehicle (CAV) requires lightweight hull and turret structures of composite and ceramic armor able to withstand ballistics for production capability at cost-competitive rates. Vacuum-assisted resin transfer molding (VARTM) has emerged in recent years as a commercially viable method for the low-cost production of high-performance composite structures. The use of a single-sided, polyester molding tool at low pressure and a glass- or carbon-fiber preform in the VARTM process has been demonstrated in the boating industry as well as on Dodge Viper body panels. Applications of thermoset polyurethane and epoxy resin systems have already achieved commercial success. Newer, more recyclable engineering and structural thermoplastic resins, such as cyclic thermoplastic polyesters, are strong candidates for component applications in the medium to long term. The Army should leverage the experience of the commercial automotive industry and other military services in the development, application, and demonstration of PMC and lightweight armor materials. A collaboration in the mid-1990s between USCAR’s Automotive Composites Consortium, DOE, and NIST-ATP, combined with the U.S. Air Force Materials Laboratory, resulted in new, full-scale process capability demonstrations of the programmable powder preforming process (P4) technology from Europe. The P4 technology has led to several successful commercial applications of fiberglass and low-cost carbon fiber for automobile components, such as composite pickup-truck boxes, body side panels, and other structural, crash-resistant closure components on current niche passenger vehicles. Some of the relevant 24 S. Nimmer, Oshkosh Truck Corporation. Presentation to the committee, August 2002.
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Use of Lightweight Materials in 21st Century Army Trucks strategies and areas needing research investments include those of optimizing molding and preforming cycle times, reducing reinforcement prices (especially for carbon fiber), and developing new vehicle architectures and design for manufacturability. Improved Casting Technologies The automotive sector is the largest user of nonferrous (i.e., aluminum, zinc, and magnesium) castings, accounting for nearly 60 percent of total shipments. Several competing, economically viable processing routes, such as lost foam casting, die casting, semisolid casting, and rapid prototyping, have been demonstrated and implemented in the automotive industry. Ferrous and nonferrous alloy castings are already extensively used in the construction of Army vehicle bodies, as well as for most naval ship and submarine hull structures, machinery, suspension, and powertrain components. Care must be taken in processing, because high-strength steel castings are sensitive to hydrogen embrittlement, stress-relief embrittlement, and stress-corrosion cracking. Other cast stainless steels (including austenitic, dual-phase, and precipitation hardening types) have high potential for use where corrosion resistance is required and corrosion protection by coatings cannot be provided. Nickel-based, titanium, and aluminum castings, as well as advanced PMCs, also have strong growth potential in Army trucks in a limited number of structural lightweighting applications, as the technologies and fabrication processes are closer to achieving robustness and economies of scale. Research and development (R&D) efforts are under way at suppliers such as Oshkosh Truck Corporation and Stewart and Stevenson and at other Army research facilities. LONG-TERM OPPORTUNITIES Long-term opportunities are defined at the beginning of this chapter as those that would result from changes in the basic truck paradigm and would thereby enable the use of radically different materials. Titanium Alloys The use of titanium in production automobiles and trucks is essentially unknown, although it is being studied for future use when costs can be controlled. The use of titanium in components for military vehicles appears
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Use of Lightweight Materials in 21st Century Army Trucks poised for significant growth in the long term, when different vehicle configurations become more common. Titanium alloys are of interest to designers of Army ground vehicles because of their unique combination of ballistic-survivable, corrosion-resistant, and mechanical properties. These alloys have high specific-yield strength, fracture toughness, and fatigue resistance. When reinforced or blended with suitable particulate materials, such as SiC, the strength and performance of titanium are dramatically enhanced. The use of titanium alloys has been limited by their high cost relative to that of steels and aluminum alloys, the high rate of waste in production, and difficulty in machining and welding. Recent developments in processing technologies, such as single-melt cold hearth electron beam melting and plasma-arc melting, however, have reduced the cost of titanium feedstock significantly. Combining the use of single-melt Ti-6Al-4V with near-net-shape processing and compositing technologies such as casting, forging, and powder metallurgy can reduce fabrication costs even further. An ambitious, collaborative R&D program is presently under way between the U.S. Army Tank-Automotive Research, Development, and Engineering Center (TARDEC) and titanium suppliers to develop a single-melt, plasma-arc, cold hearth melting process for casting Ti-6Al-4V slabs that can be directly rolled into armor plate. The development of such a process would reduce the cost of producing titanium plate. The reduction of weight resulting from the use of titanium would help enhance vehicle and armament deployment and performance. Programs are also under way at USCAR and various DOD and DOE agencies. These programs are examining the issues of purity and processibility of titanium powder from competing low-cost feedstock processes that have recently been demonstrated for extracting titanium from ore and solution. When low-cost processing of titanium is realized and combined with its demonstrated property and performance attributes, there is a strong likelihood that titanium could replace aluminum as well as steel in a variety of armor applications. In addition, titanium springs could be developed as both medium- and long-term applications. Net Shape Manufacturing Additive metal-processing technologies, such as laser-engineered net shaping, are being pioneered by companies such as Optomec, Laserfare, and
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Use of Lightweight Materials in 21st Century Army Trucks the POM Group. These technologies, which have the advantage of producing a net shape in situ, are being demonstrated and evaluated for use in repair and remanufacturing operations for structural components, as well as for die repairs and modifications. These processes hold great promise for helping the Army meet future needs for products and spare parts on demand, whether in short-run production or as replacement spares in the field. Self-Repair, Self-Maintenance Condition-based maintenance and self-monitoring structure technologies are essential for the success of FCS and FTTS. The anticipated future growth of MMC components and less harsh operating environments are expected to result in the greater use of sensors integrated in vehicle components to serve this function. Combined with these advances, there is a need to bring Army depots abreast of new maintenance technologies (NDE, repair, and manufacturing) and to further leverage product and process developments being conducted in private industry, government, and nongovernmental organizations, including the DOE national laboratories, the U.S. Air Force, the U.S. Navy, and the National Center for Manufacturing Sciences.
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Representative terms from entire chapter: