Open-die forging of structurally porous sandwich panels
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NTRODUCTION
LIGHTWEIGHT sandwich panels consisting of honeycomb or porous cores bonded between stiff face sheets have high structural efficiency and are used in many applications where high bending and buckling resistance are needed. These include the wing skins and floor panels of aircrafts, the hulls of some ships, and high-performance skis.[1] Numerous materials have been used to construct structurally efficient sandwich panels. The stiff, strong face sheets have often been constructed from fiber-reinforced polymer matrix composites or aluminum, while the low density cores have been made from aluminum or paper-resin honeycombs, inexpensive polymeric foams, or even balsa wood.[1] Titanium sandwich panel construction for high-temperature sandwich structures has exploited diffusion bonding and superplastic forming methods.[2] However, the high cost of manufacturing these systems has limited their application and has stimulated an interest in the development of lower cost alternatives. One approach to titanium sandwich panel manufacture is based on an entrapped gas processing method.[3] In this approach, titanium powder is sealed inside a titanium canister under a significant (several atmosphere) pressure of an inert (relatively insoluble) gas such as argon. This canister is then consolidated by hot isostatic pressing to a moderate relative density (85 to 98 pct). The high pressure within the remaining gas-filled pores inhibits full pore closure and the attainment of theoretical density. These pores remain during the subsequent thermomechanical forming of a panel by either hot forging or rolling. When this structure is finally annealed, the pores expand, creating a component with fully dense face sheets (derived from the original can) and a core with a relative density approaching 50 pct.[4] The subsequent mechanical performance of these lowdensity core (LDC) panels depends significantly on the D.M. ELZEY, Assistant Professor, and H.N.G. WADLEY, Edgar Starke Professor and Associate Dean of Research, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. Manuscript submitted June 29, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
core’s density, the relative thickness of the core and face sheet, and the quality of the face sheet/core interface. The core density and the core and face sheet thicknesses are a complicated function of the initial can thickness and gas pressure, the initial powder size distribution, the material properties of the can and powder, and the processing conditions used for consolidation, subsequent hot forming, and the final annealing treatment. As efforts continue to deduce the best process sequence,[5] interest is growing in the development of predictive models that relate the core density to process conditions. These models are needed to understand the conflicting dependencies of key product attributes such as porosity content on the method and conditions used for processing. The modeling is complicated by the high levels of porosity
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