On the nature of the Fe-bearing particles influencing hard anodizing behavior of AA 7075 extrusion products
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I.
INTRODUCTION
NUMEROUS studies have shown that gains in toughness in wrought Al–based alloys can be obtained by the reduction of Fe, which is the dominant impurity in all grades of Al.[1–5] Given the restricted solid solubility of Fe in Al (i.e., about 0.04 wt pct at the eutectic temperature of 655 7C and negligible at ≤427 7C[6]) and the fact that the solid solubility of Fe in Al is reduced by alloying elements,[6] constituent particles containing Fe form early during the solidification process. These particles are rich in Al, so that even relatively small amounts of Fe can produce significant amounts of these particles.[6] Depending on the alloy compositions and the thermal exposures involved, these particles may contain varying amounts of Cu, Si, Mn, and Cr.[2,3,6,7] Due to their insoluble nature, the volume fraction of these particles after ingot preheating cannot be modified by dissolution.[3] These particles, on the other hand, breakup during working, and appear in a uniaxially worked wrought product as rows of particles or ‘‘stringers.’’ As a result, when fracture toughness is measured in transverse directions, toughness is greatly reduced because stresses are then normal to the aligned weak particles.[8] Recent studies have demonstrated that the Fe-bearing constituent particles are also detrimental to the nature and growth of the hard anodic oxide coating formed on commercial wrought Al alloys.[9] Using Al-Zn-Mg-Cu–based AA 7075 alloy extrusions, it has been shown that the Febearing particles, which survive the hard anodizing treatment (i.e., those which do not undergo dissolution during A.K. MUKHOPADHYAY, Scientist, is with the Defence Metallurgical Research Laboratory, Hyderabad-500 058, India. Manuscript submitted April 24, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
anodizing), result in a steep rise in bath voltage with time, thus, not allowing the desired coating thickness to be obtained in an uninterrupted, single step. The presence of such particles causes a nonuniform growth of the anodic oxide film and gives rise to anisotropy in the hard anodizing behavior of these alloys, in that a greater damage to the growth of the anodic film is caused by the alignment of these particles in a direction normal to the growth direction of the anodic film. These studies[9] further indicated that Febearing particles of only certain compositions, involving relatively higher Si and Mn contents in the alloy, are harmful to the hard anodizing behavior of these materials. It is, therefore, important to identify different Fe-bearing particles formed due to the variations in the impurity levels and to correlate such results to the hard anodizing response of the materials. Such information would provide a basis for the Al alloy designers as to the extent to which the Si, Mn, and Fe impurities are to be controlled in order to obtain a suitable response to hard anodizing for the commercial wrought Al–based alloys. In this article, using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmi
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