Microstructural Evolution during Ingot Preheat in 7xxx Aluminium Alloys for Thick Semiproduct Applications

Abstract

Recent 7xxx aluminum alloys have been designed for the finite use of thick semiproduct with contolled amount of constituent phases which mostly evolve during ingot preheat. In this study, the effects of constitutional change and preheat conditions of 7175 and 7050 type alloys on the evolution of constituent phases [M-, T-, S-phase and dispersoid] are presented. The constiuents evolve depending on the constitutional effect, primarily the change of Zn:Mg ratio, preheat condition comprising temperature and cooling rate following preheat. T- and M-phase are reprecipitated during cooling after preheat, depending on the alloy constitutions. S-phase is evolved depending on the constitution and preheat temperature, rather than preheat cooling rate. Prominent precipitation temperature interval of constituents are discussed in view of quaternary phase evolutions. In addition, evolutions of dispersoids together with M-phase are discussed. Specific alloy designs and preheat conditions could provide controlled microstructures for the thick 7xxx semiproducts.