Effect of vanadium and zirconium on the formation of CrAI 7 primary crystals in 7075 aluminum alloy
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DONALD J. BEERNTSEN P r i m a r y crystals of CrA17 which form during the semicontinuous casting of 7075 aluminum alloy can, u n d e r some casting conditions, grow l a r g e enough to constitute significant defects in the final wrought products. The formation of such l a r g e primary crystals requires an alloy composition in the liquid + CrAfT phase field, plus quiescent growth sites in the molten alloy which are at temperatures just above the p e r i tectic. Control of the chromium content to values near the minimum required in 7075 alloy (0.18 wt pct) is not usually sufficient to avoid the formation of primary crystals, because other elements present in the commercial alloy also contribute to t h e i r f o r m a tion. Steele and Collinst derived a quantitative " c h r o mium equivalence" equation to describe the contribution of other elements to the formation of such crystals in a 7075-type alloy. From equilibrated alloys they quantitatively extracted and analyzed CrA17 crystals, and calculated the Cr, Ti, Mn and F e contents of the liquid with which the crystals had been in equilibrium. Using a l e a s t squares procedure to minimize e r r o r in the phase boundary value they expressed their results as follows (in wt. pct): Cr + 0.797 T i + 0.090 Mn + 0.142 F e = 0.244. With our own data we simply calculated c r y s t a l / liquid distribution coefficients and averaged and normalized them. A boundary value was then obtained by applying the resulting element coefficients to the liquid analyses and averaging. (See below.) When this procedure was applied t o Steele and Collins' raw data (using the most reliable 13 of t h e i r 14 alloys) the following was obtained: Cr + 0.66 T i + 0.21 Mn + 0.073 F e = 0.246. This type of analysis does not give a boundary value of minimum e r r o r . However, at equilibrium the distribution coefficient is a direct reflection of the free energy of the system and as such accurately reflects the relative preference of the individual elements for the respective phases. This is preferable from an operating standpoint because it is partly through m a n i pulation of these secondary elements that the potential for formation of primary crystals can be minimized. The boundary value is determined primarily by the zinc, magnesium, and copper contents. While these have little direct effect on the formation of CrA17 primary crystals, they are the strongest determinants of the peritectic temperature, that point at which a DONALD J. BEERNTSEN is Staff Research Metallurgist with the Kaiser Aluminum & Chemical Corp. Centerfor Technology, Pleasanton, CA 94566. Manuscript submitted March 4, 1977. METALLURGICAL TRANSACTIONS B
aluminum f i r s t appears. Therefore, inherent in the boundary value is a statement of the temperature coefficient of chromium solubility. Qualitative microprobe analyses in our own l a b o r a t o r i e s had shown that vanadium also had a strong influence on CrAI7 formation. Vanadium has become a common impurity in reduction cell metal, so the current experiments were run t
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