Discontinuously reinforced aluminum

Hardness. A limited number of investigations have evaluated FSW of discontinuously reinforced aluminum alloys (Ref 25, 96, 97). In the work of Nakata et ak, hardness and mechanical properties were established for 6061-T6 A1... 

J.E. Spowart, Z.-Y. Ma, and R.S. Mishra, The Effect of Eriction Stir Processing (PSP) on the Spatial Heterogeneity of Discontinuously-Reinforced Aluminum (DRA) Microstructures, Friction Stir... 

Discontinuous fiber MMCs are isotropic and are superior in terms of specific strength, elastic modulus, toughness, and thermal conductivity compared to conventional alloys. They are used for brackets and fittings that are subject to multiaxial loads. The ability to engineer the CTEs of discontinuously reinforced aluminum (DRA) composites makes it possible to save substantial weight by replacing Invar or Kovar in critical design situations where a low GTE material is needed. 

Pig, I The molerial cost versus performance of various oluminurrHTiatrix composites. DRA, discontinuous /reinforced aluminum... 

M. Mahoney, W. Harrigan, and J. Wert, Friction Stir Welding SiC Discontinu-ously Reinforced Aluminum, INALCO 98, Seventh Int. Conf. Joints in Aluminum (Cambridge, U.K.)... 

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are used as matrix materials. The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers concentrations normally range between 10 and 60 vol%. Continuous-fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. However, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, or particulates of silicon carbide and aluminum oxide. In a sense, the cermets (Section 16.2) fall within this MMC scheme. Table 16.9 presents the properties of several common metal-matrix, continuous and aligned fiber-reinforced composites. 

Polymer matrix composites (PMCs), or fiber-reinforced plastics (FRPs). provide a wide range of properties and behavior. Materials with discontinuous fibers are slightly stiffer than conventional unreinforced plastics, whereas the fully aligned continuous fiber systems can record exceptionally high specific properties (property divided by density), exeeeding those of competing materials such as steel and aluminum. There are a virtually infinite number of materials, and material formats that can be combined to form a composite material, as shown in Table 1. 

SiC whiskers were initially developed in the early 1960s. However, their use was originally to be applied to the reinforcement of metal matrices, such as aluminum. These metal matrix composites were only a small commercial success, mainly because of the high cost of the whiskers. The first application of whisker reinforcement to ceramics, and in particular alumina, did not occur until the 1980 s. For the purposes of this article, SiC whiskers will be defined to be acicular or needle-like shaped, discontinuous, nearly single crystals. 

Corrosion Proporlios. Marine corrosion of silicon carbide/aluminum composites is much less severe than that observed on graphite/aluminum MMCs. Discontinuous silicon carbide/aluminum MMCs, however, are susceptible to localized corrosion. Mild-to-moderate pitting has been reported on SiC whisker- and particulate-reinforced composites containiirg 6061 and 5000 series aluminum matrices exposed for a maximum of 42 months in splash/spray and marine atmospheric environments. The d ree of corrosion present on the composites is slightly accelerated compared to that on unreinforced alutrtittum alloys. 

---