The effect of ingot processing treatments on the grain size and properties of Al alloy 7075
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T H E major shortcomings of commercial high strength wrought 7000 series aluminum alloys are low ductility, low toughness and poor s t r e s s corrosion resistance, especially in the short t r a n s v e r s e direction. Frankford Arsenal has been conducting studies aimed at eliminating these deficiencies and improving the strength of these alloys through the use of improved processing techniques. The early work in this area x-4 showed that elimination of second phase constituents induced substantlal Improvements in ductility and toughness and some improvement in fatigue resistance at equivalent strength levels when compared to c o m m e r c i a l alloys. These improvements were achieved by the use of high purity materials, by controlled solidification techniques to achieve a small dendrite arm spacing and by optimum homogenization treatments. Other work directed towards improving the p r o p e r ties of 7000 s e r i e s alloys, carried out at Instituto Spertmentale dei Metallt Leggeri (ISML) under a US/Italy cooperative r e s e a r c h program, has been reported by DiRusso et a l . 5'6 In that work, a new technique termed final thermal mechanical treatment (FTMT) was developed. This technique involves the application of plastic deformation between an Initial and a final artificial aging step. With FTMT the strength of the 7000 series aUoys can be increased by 20 to 25 pct with only a minimal loss of ductility and toughness. In addition to the property improvements achieved by better solidification techniques and by advanced t h e r mal mechanical treatments (FTMT), it was considered that improvements could also be achieved by controlling the grain morphology. Although there is much information on the effect of grain morphology in pure metals and solid solution alloys, little data are available r e garding htgh strength aluminum alloys. Therefore, the US/Italy cooperative r e s e a r c h p r o g r a m c a r r i e d out at ISML also included investigations on grain morphology effects. The results of those studies 7-~ showed that in 7075 alloy the properties related to ductility, such as elongation, reduction in area, and toughness were improved by the use of an intermediate thermal mechanical treatment (ITMT) which was designed to produce J. WALDMANand H. SULINSKIare Metallurgists and H. MARKUS is Chief, Materials Engineering Division, Pitman Dunn Laboratory, Frankford Arsenal, Philadelphia, Pa. 19137. Manuscript submitted May 29, 1973. METALLURGICAL TRANSACTIONS
a wrought product with grains that are finer than those obtained by conventional processing. Preliminary work also indicated that forged 7075 plate processed by ITMT exhibited improved s t r e s s corrosion resistance, x~ ITMT involves a new concept in ingot processing in that the original cast grain boundaries are eliminated by a recrystallization step prior to conventionally working the material into the final wrought products. In the ITMT p r o c e s s reported by DiRusso et al, 7-x~ i . e . , ISML-ITMT, the 7075 ingots are partially homogenized, worked
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