Ultrasound Assisted Casting of an AM60 Based Metal Matrix Nanocomposite, Its Properties, and Recyclability

Abstract

An AM60 magnesium alloy nanocomposite reinforced with 1 wt % of AlN nanoparticles was prepared using an ultrasound (US) assisted permanent-mould indirect-chill casting process. Ultrasonically generated cavitation and acoustic streaming promoted de-agglomeration of particle clusters and distributed the particles throughout the melt. Significant grain refinement due to nucleation on the AlN nanoparticles was accompanied by an exceptional improvement in properties: yield strength increased by 103%, ultimate tensile strength by 115%, and ductility by 140%.Although good grain refinement was observed, the large nucleation undercooling of 14 K limits further refinement because nucleation is prevented by the formation of a nucleation-free zone around each grain. To assess the industrial applicability and recyclability of the nanocomposite material in various casting processes, tests were performed to determine the effect of remelting on the microstructure. With each remelting, a small percentage of effective AlN nanoparticles was lost, and some grain growth was observed. However, even after the third remelting, excellent strength and ductility was retained. According to strengthening models, enhanced yield strength is mainly attributed to Hall-Petch strengthening caused by the refined grain size. A small additional contribution to strengthening is attributed to Orowan strengthening.

Advanced Metal Matrix Nanocomposites

“…As a result, mechanical properties of the metal matrix nanocomposite are enhanced [8,21]. In the nanocomposites processed by ultrasonic melt treatment, metal matrices are largely focused on aluminum [116,117,[123][124][125][126][127][128], magnesium alloys [6,[129][130][131], and transition metal alloys [8]. Typical nanoparticle reinforcers [8,[132][133][134] include SiC, Al 2 O 3 , and various carbon polymorphs (e.g., graphene, carbon nanotubes, and diamond nanoparticles).…”

“…The grain refining effect of the introduced nanoparticles is very clearly illustrated when, as a part of the EU project EXOMET, a die-casting alloy AM60 was reinforced with 1 wt% AlN nanoparticles [127]. The AM60 is a commercial alloy used in the automotive industry from which, for example, steering wheels, seat frames, or housings are die-cast.…”

Recent Developments in the Application of the Interdependence Model of Grain Formation and Refinement

“…In a broader study of the AM60-AlN system, a Hall-Petch relationship with grain size was found despite the very large number density of nanoparticles [12].…”

“…(a) (b) Figure 11. Microstructure and grain size of (a) AM60, 1277 ± 300 m and b) AM60+AlN, 85 ± 6.2 m [12].…”

“…In this example the former approach is taken where UST is applied above the liquidus temperature to ensure well-wetted nanoparticles and their uniform distribution throughout the melt [12]. One weight percent of AlN powder was added to the AM60 melt, stirred, UST applied for 5 minutes, and then the melt was poured into a 100 mm diameter cylinder which was lowered into a water bath for directional solidification from the bottom of the cylinder.…”