Influence of Al-Nb-B grain refiner addition on the microstructure of aluminium alloys

Abstract

The rising demand for lightweight automobiles is driving the development of high-performance aluminium alloys in a sustainable manner. Grain refinement via chemical inoculation is crucial for microstructure control during the production of aluminium alloys and is fundamental to its integrity and performance. Currently, the predominant refiner system adopted in industry is the titanium-based Al-5Ti-B grain refiner; however, its application is not without its drawbacks, primarily silicon poisoning in cast alloys and zirconium poisoning in zirconium-containing wrought alloys which negatively impacts its effectiveness. Alternative refiner systems have been investigated in a bid to combat these deleterious phenomena, one of which is the niobium based (Al-Nb-B) grain refiner system. Intensive research in this field over the last ten years has shown that the Al-Nb-B grain refiner systems are as effective, or even better, in the refinement of silicon-containing alloys than its Ti-based counterpart and more importantly, offers more resistance to silicon poisoning. The resilience of this grain refiner system in cast alloys especially over sustained melt hold periods bodes well for the industry. However, most of the studies in the field of niobium grain refiner development have been conducted using grain refiner systems synthesised in the laboratory. In a collaborative research study with industry, the influence of industrially produced Al-Nb-B grain refiner systems on the refinement of commercial alloys was systematically explored and the results presented for the first time. It is shown in this work that Al-Nb-B grain refiners are indeed effective in the refinement of industry alloys although refiner efficiency is dependent on alloy composition. Consequently, addition practice has to be tailored and optimised for individual alloy systems. The fading phenomena in conventional grain refiners is a difficult problem to unravel as it is still unclear if fading is a result of particle settling or solute interactions with nucleating substrates. A study of this phenomena using an Al-Nb-B grain refiner containing substrates with certain size ranges is presented. It is demonstrated that although the rapid settling of large particles is partially responsible for the decrease in refiner effectiveness, solute interaction with nucleants over time is probably a more critical factor. This solute-substrate chemical interaction is also observed between zirconium and the Al3Nb nucleating phase. The importance of titanium in restricting the growth of nucleated grains especially in low solute alloys cannot be overstated; however, studies in growth restriction effects have either used no refiner systems or only titanium-based grain refiners. To this end, a systematic study on the role of solutal Ti on the effectiveness of Al-Nb-B grain refiner system was conducted and outcomes presented. It is observed that while Al-Nb-B grain refiner does not achieve optimum effectiveness in Ti-free cast alloy systems and requires a minimum amount of solutal Ti to achieve optimum efficiency, the growth restriction effects of titanium by itself is ineffective in a multicomponent alloy, in the absence of nucleating substrates. A critical aspect of aluminium component production is the soundness and integrity of the final cast part. Fundamental to this is the alloy composition and processing conditions involved. How these two factors are affected by Al-Nb-B grain refiner addition to silicon-containing alloys is explored in this work. The uniform distribution of second phase particulates, as a direct result of grain refinement, is observed to reduce their size, which in turn contributes to an improvement in the mechanical properties of certain alloys. Qualitative assessment of fluidity characteristics suggest that alloy systems benefit from the addition of Al-Nb-B grain refiners, although improvements observed from quantitative data lie within experimental error. The other key qualitative assessment of volumetric shrinkage behaviour of Al-Nb-B inoculated castings indicate a clear shift from conventionally known shrinkage behaviour to one which offers greater integrity to the final cast parts.