For over half a century, grain refinement of aluminium alloys has been achieved by chemical inoculation; current grain-refinement practice involves the addition of master alloys (e.g. Al – Ti – B, Al – Ti – C) before casting, introducing inoculant particles to the melt. These particles act as nucleation points for α-Al grains, resulting in a uniformly fine, equiaxed as cast microstructure. Despite the ubiquity of this process, its underpinning science was not fully understood, hindering development of the area. From the 1950s onwards, the phase responsible for nucleation in alloys refined by Al – Ti – B was fiercely disputed. The debate focused so closely on this issue that other important factors were frequently ignored. During the 1990s, this debate was resolved through careful thermodynamic reasoning and novel experiments that derived their inspiration far from the foundry. This review focuses on subsequent experimental work and modelling: the expansion of the current understanding of grain ref...

Understanding mechanisms of grain refinement of aluminium alloys by inoculation

Tensile behavior of dissimilar friction stir welded joints of aluminium alloys

The second-generation aluminum-magnesium-scandium (Al-Mg-Sc) alloy, which is often referred to as Scalmalloy®, has been developed as a high-strength aluminum alloy for selective laser melting (SLM). The high-cooling rates of melt pools during SLM establishes the thermodynamic conditions for a fine-grained crack-free aluminum structure saturated with fine precipitates of the ceramic phase Al₃-Sc. The precipitation allows tensile and fatigue strength of Scalmalloy® to exceed those of AlSi10Mg by ~70%. Knowledge about properties of other additive manufacturing processes with slower cooling rates is currently not available. In this study, two batches of Scalmalloy® processed by SLM and laser metal deposition (LMD) are compared regarding microstructure-induced properties. Microstructural strengthening mechanisms behind enhanced strength and ductility are investigated by scanning electron microscopy (SEM). Fatigue damage mechanisms in low-cycle (LCF) to high-cycle fatigue (HCF) are a subject of study in a combined strategy of experimental and statistical modeling for calculation of Woehler curves in the respective regimes. Modeling efforts are supported by non-destructive defect characterization in an X-ray computed tomography (µ-CT) platform. The investigations show that Scalmalloy® specimens produced by LMD are prone to extensive porosity, contrary to SLM specimens, which is translated to ~30% lower fatigue strength.

Comparison of Microstructure and Mechanical Properties of Scalmalloy® Produced by Selective Laser Melting and Laser Metal Deposition.

The influences of erbium (Er) on the microstructure and mechanical properties of Al–Mg–Mn–Zr alloys have been investigated. It has been found that about 0.2 wt.% Er can be dissolved in the matrix, excess Er atoms segregate at grain boundaries to form primary Al 3 Er. Addition of 0.4 wt.% Er refines the grain size of the as-cast alloy due to the formation of primary Al 3 Er. The solid solution decomposes to form a dispersion of secondary Al 3 Er, with facets parallel to {1 0 0} and {1 1 0} planes, during homogenization at 470 °C. The secondary Al 3 Er precipitates improve strength, especially the elevated temperature strength. The yield strength, at 150 °C, of the alloy with 0.2 wt.% Er is 50% higher than that of the Er-free alloy. The recrystallization temperature of the alloy with 0.4 wt.% Er is about 25 °C higher than that of the alloy without addition of Er.

The effect of erbium on the microstructure and mechanical properties of Al–Mg–Mn–Zr alloy

Scandium increases the strength of aluminium alloys via three mechanism: 1) solid solution strengthening, 2) precipitation hardening, 3) grain structure control. Despite the well documented benefits, scandium has found very limited use in commercial grade aluminium alloys due to its high cost. However, new efficient extraction technologies promise an ensured supply of scandium and a significant drop in cost. Development of the next generation of aerospace Al alloys will come from innovation in alloy chemistry. One such innovation could be the addition of scandium in combination with Zirconium which increases the specific strength and stiffness of aluminium alloys through the precipitation of the L12 Al3(Sc, Zr) dispersoid. However, very little is known about the interactions of the Al3(Sc, Zr) dispersoid and the θ′-phase. Here, the effects of Sc and Zr additions to a model Al-Cu alloy were examined. The precipitates were investigated through TEM and APT. EBSD was used to characterise the texture of the studied alloys. Finally, the ageing response of the alloys was monitored through tensile testing. The refinement of the Cu precipitates accounted for an increase of up to 120 MPa of the peak aged strength and the core/shell dispersoids accounted for up to 40 MPa.

P.G Lloyd

Papers

The effect of scandium on the microstructure, mechanical properties and weldability of a cast Al–Mg alloy