7.1.2 MANUFACTURING PROCESSES 7.1.2.1 Continuously Reinforced MMC Vacuum hot pressing(VHP)is the most common method of manufacturing con- tinuous fiber-reinforced MMC.The starting material(precursor)is made either by drum winding a set of parallel fibers on a thin matrix foil with the fibers held in place by a polymeric binder or by plasma spraying the matrix material onto a layer of parallel fibers that have been previously wound around a drum (Figure 7.8). Sheets are then cut from these preformed tapes by shears or dies and stacked into a layered structure.The fiber orientation in each layer can be controlled as desired. The layup is inserted into a vacuum bag,which is then placed between the preheated platens of a hydraulic press.After the vacuum level and the temperature inside the vacuum bag reach the preset values,the platen pressure Matrix foil with a fugitive binder Fiber wrapped around spool the drum (a) Plasma spray Fiber gum spool (b) Heated To vacuum pump platens Vacuum bag (c) FIGURE 7.8 Schematic of drum winding and vacuum hot pressing for manufacturing continuous-fiber MMC(a)drum winding,(b)cut and stack,and (c)vacuum hot pressing. 2007 by Taylor Francis Group.LLC
7.1.2 MANUFACTURING PROCESSES 7.1.2.1 Continuously Reinforced MMC Vacuum hot pressing (VHP) is the most common method of manufacturing continuous fiber-reinforced MMC. The starting material (precursor) is made either by drum winding a set of parallel fibers on a thin matrix foil with the fibers held in place by a polymeric binder or by plasma spraying the matrix material onto a layer of parallel fibers that have been previously wound around a drum (Figure 7.8). Sheets are then cut from these preformed tapes by shears or dies and stacked into a layered structure. The fiber orientation in each layer can be controlled as desired. The layup is inserted into a vacuum bag, which is then placed between the preheated platens of a hydraulic press. After the vacuum level and the temperature inside the vacuum bag reach the preset values, the platen pressure Fiber spool Fiber spool Plasma spray gum Matrix foil with a fugitive binder wrapped around the drum Heated platens To vacuum pump Vacuum bag (a) (b) (c) FIGURE 7.8 Schematic of drum winding and vacuum hot pressing for manufacturing continuous-fiber MMC (a) drum winding, (b) cut and stack, and (c) vacuum hot pressing. 2007 by Taylor & Francis Group, LLC
is raised and held for a length of time to complete the consolidation of layers into the final thickness.Typically,the temperature is 50%-90%of the solidus temperature of the matrix,and the pressure ranges from 10 to 120 MPa (1.45 17.4 ksi).Afterward,the cooling is carried out at a controlled rate and under pressure to reduce residual stresses as well as to prevent warping. The consolidation of layers in VHP takes place through diffusion bonding, plastic deformation,and creep of the metal matrix [16].The basic process param- eters that control the quality of consolidation are temperature,pressure,and holding time.Another important parameter is the cleanliness of the layers to be joined.Polymeric binder residues or oxide scales can cause poor bonding between the fibers and the matrix as well as between the matrices in various layers.There- fore,careful surface preparation before stacking the layers can be very beneficial. 7.1.2.2 Discontinuously Reinforced MMC 7.1.2.2.1 Powder Metallurgy In this process,atomized metal powders (typically with a mean diameter of 15 um or -325 mesh size)are blended with deagglomerated whiskers or particulates.Deagglomeration may involve agitation of a liquid suspension of the reinforcement in an ultrasonic bath.The blended mixture is cold- compacted in a graphite die,outgassed,and vacuum hot-pressed to form a cylindrical billet.The pressure is applied in the hot-pressing stage only after the temperature is raised above the solidus temperature of the matrix alloy. Currently available SiCp-reinforced aluminum billets range from 15 cm (6 in.)to 44.5 cm (17.5 in.)in diameter.Most billets are extruded to rods or rectangular bars using lubricated conical or streamline dies.Various structural shapes can be fabricated from these rods and bars using hot extrusion,while sheets and plates can be produced by hot rolling on conventional rolling mills. Other metalworking processes,such as forging and shear spinning,can also be used for shaping them into many other useable forms. Typical tensile properties of SiCw-and SiCp-reinforced aluminum alloys are given in Table 7.2.All of these composites were manufactured by powder metallurgy and then extruded to rods or bars.Sheets were formed by hot rolling the bars.Following observations can be made from the table: 1.The secondary processing of extrusion and rolling can create significant difference in tensile properties in the longitudinal and transverse direc- tions,especially in whisker-reinforced composites. 2.Ultimate tensile strength,yield strength,and modulus of discontinuously reinforced aluminum alloys increase with increasing reinforcement content,but elongation to failure(which is related to ductility)decreases. 3.For the same reinforcement content,whiskers produce stronger and stiffer composites than particulates. 2007 by Taylor Francis Group,LLC
is raised and held for a length of time to complete the consolidation of layers into the final thickness. Typically, the temperature is 50%–90% of the solidus temperature of the matrix, and the pressure ranges from 10 to 120 MPa (1.45– 17.4 ksi). Afterward, the cooling is carried out at a controlled rate and under pressure to reduce residual stresses as well as to prevent warping. The consolidation of layers in VHP takes place through diffusion bonding, plastic deformation, and creep of the metal matrix [16]. The basic process parameters that control the quality of consolidation are temperature, pressure, and holding time. Another important parameter is the cleanliness of the layers to be joined. Polymeric binder residues or oxide scales can cause poor bonding between the fibers and the matrix as well as between the matrices in various layers. Therefore, careful surface preparation before stacking the layers can be very beneficial. 7.1.2.2 Discontinuously Reinforced MMC 7.1.2.2.1 Powder Metallurgy In this process, atomized metal powders (typically with a mean diameter of 15 mm or 325 mesh size) are blended with deagglomerated whiskers or particulates. Deagglomeration may involve agitation of a liquid suspension of the reinforcement in an ultrasonic bath. The blended mixture is coldcompacted in a graphite die, outgassed, and vacuum hot-pressed to form a cylindrical billet. The pressure is applied in the hot-pressing stage only after the temperature is raised above the solidus temperature of the matrix alloy. Currently available SiCp-reinforced aluminum billets range from 15 cm (6 in.) to 44.5 cm (17.5 in.) in diameter. Most billets are extruded to rods or rectangular bars using lubricated conical or streamline dies. Various structural shapes can be fabricated from these rods and bars using hot extrusion, while sheets and plates can be produced by hot rolling on conventional rolling mills. Other metalworking processes, such as forging and shear spinning, can also be used for shaping them into many other useable forms. Typical tensile properties of SiCw- and SiCp-reinforced aluminum alloys are given in Table 7.2. All of these composites were manufactured by powder metallurgy and then extruded to rods or bars. Sheets were formed by hot rolling the bars. Following observations can be made from the table: 1. The secondary processing of extrusion and rolling can create significant difference in tensile properties in the longitudinal and transverse directions, especially in whisker-reinforced composites. 2. Ultimate tensile strength, yield strength, and modulus of discontinuously reinforced aluminum alloys increase with increasing reinforcement content, but elongation to failure (which is related to ductility) decreases. 3. For the same reinforcement content, whiskers produce stronger and stiffer composites than particulates. 2007 by Taylor & Francis Group, LLC