Development of generic grain refiner alloys for cast and wrought Al-alloys containing silicon and zirconium

Abstract

Due to recent legislation aimed at reducing carbon emissions into the environment through weight reduction, the automotive and aerospace industries are using light alloys such as aluminium silicon (Al–Si) and aluminium zirconium (Al–Zr) instead of steel due to their excellent mechanical properties and low weight to strength ratio. In order to further improve mechanical and metallurgical properties in these alloys, grain refinement is usually used in industry. However, the current and most widely used grain refiner Al–5Ti–B is unable to refine Al–Si alloys with silicon content greater than 3 wt.%., and Al–Zr alloys due to poisoning of the refiner by silicon and zirconium. The Al–5Ti–B refiner also contains larger Al3Ti particles and agglomerates of TiB2 which affect its efficiency and suitability in industrial applications where thin sheets are required. In this study, a new technique which improves the microstructure and efficiency of the Al–5Ti–B refiner has been developed. This involves the reaction of potassium tetrafluoroborate (KBF4) and potassium hexafluorotitanate (K2TiF6) salts at shorter reaction time before ultrasonic processing during solidification. This leads to the formation of a new Al3Ti morphology and de-agglomeration of TiB2 particles which enhances its grain refinement efficiency by 20%. Secondly, through phase diagram analysis of Al grain refining systems and crystallography studies, it was observed that Al3Ti and Al3Nb display similar lattice parameters with atomic misfit of 4.2% and would undergo a peritectic reaction with α-Al at low contact angles. Based on this, and using the duplex nucleation theory and poisoning by Si and Zr, a new quaternary grain refiner containing aluminium, titanium, niobium and boron (Al–4Ti–Nb–B) has been developed. This novel grain refiner has been found to be efficient in Al–Si alloys and Al–Zr, both at laboratory and industrial scales, and to improve the mechanical properties of the alloys despite the presence of Ti in the alloy. It was observed that the addition of Nb to an Al–Ti–B system leads to the formation of solid solution phases of Al3Ti1-xNbx, Al3Nb1-xTx, and (Ti1-xNbx)B2 which prevents poisoning by Si and Zr. Experimental simulations showed that Al3Nb1-xSix rather than Ti(Al1-xSix)3 are formed in Al–Si alloys, and Al3(Ti1-xNbx) and (Al3Ti1-xNbx)B2 phases are formed in Al–Zr alloys rather than Al3(Zrx,Ti1-x), B2(Zrx,Ti1-x) or ZrB2 phases. A new grain refining mechanism, ‘The Quad Nucleation Theory’ based on four nucleation events in Al–4Ti–Nb–B has been proposed. Other newly developed quaternary and ternary novel grain refiners capable of refining aluminium silicon alloys are also presented in this thesis. This includes a novel method of refining Al–Si alloys using phosphorus and niobium.