Effect of ultrasonic treatment on grain refinement and mechanical properties of Al-2Mg alloy

ABSTRACT In the present study, to achieve grain refinement and enhance the mechanical properties of Al-2Mg alloy, ultrasonic treatment (UT) has been applied to the melt. The effects of this treatment on the microstructure, porosity alleviation, and mechanical properties of Al-2Mg have been thoroughly investigated. A microstructure study of the Al-2Mg UT alloy has revealed a noteworthy enhancement in grain refinement, resulting in a 3-to-4-fold reduction in grain size as compared to a non-UT Al-2Mg alloy. After UT, the yield tensile strength, the ultimate tensile strength, and the microhardness of Al-2Mg alloy are increased by 21%, 34%, and 36% respectively, with a large improvement of 143% in the ductility.


Introduction
In modern structural applications, Al-Mg alloys have emerged as a potential material for making components with moderate strength, corrosion resistance, and formability [1].The alloy is, however, susceptible to solidification defects like hot tearing and microporosity.Grain refinement has been used in the processing of aluminium and its alloys for many years to reduce segregation and casting defects while also improving mechanical properties.The addition of grain refiners and growth-restricting elements, and the application of external forces, are examples of techniques adopted for grain refinement [2].Some demerits of grain refiners are higher cost of alloying elements, low efficiency under conventional casting set-ups, process complexities (for example hydrogen enrichment of the Al melt), and the hazardous nature of alloying substances.Thus, physical grain-refinement methods have the potential to be appealing to the industry [3].Ultrasonic treatment (UT) is one such successful approach for creating a fine-grained cast microstructure and enhancing homogeneity via cavitation-generated nucleation [4].Although many studies have achieved grain refinement by UT during solidification [5], very few studies show that UT of the melt also results in grain refinement.Previous researches have shown that short UT in the liquid state generates a finegrained and homogeneous microstructure with excellent performances in hypoeutectic Al-Si alloys, TiAl alloy [6], and Al-Mg-Sc alloy [7].However, the influence of UT on the microstructural attributes of the Al-2Mg alloy system along with the refinement achieved in the microstructure is yet to be comprehensively investigated.In this regard, to expand the prospects of understanding this correlation in a binary alloy system, which limits the role of solute particles as a potential nucleation site for grains, the examination of ultrasonically treated Al-Mg has been performed.The current study investigates the effects of UT on the microstructural evolution of Al-Mg alloy in terms of grain size, second-phase distribution and mechanical properties.

Materials and methods
In an electric resistance furnace, commercially pure aluminium (99.85% purity) and magnesium (99.85% purity) were melted at 750°C together to make Al-2Mg alloy.In a preheated 400 °C rectangular cast-iron mould (diameter 22 mm and length 250 mm), half of the prepared melt was poured.In the remaining melt, UT was performed for 5 min with a preheated (at 750 °C) ultrasonic radiator (Ti-6Al-4 V) at 20.1 kHz frequency and 1.75 kW power before the pour.The alloy chemical composition was determined from the samples machined from the middle portion of the casting for both conditions, i.e.UT and non-UT.Spark emission spectroscopy was employed and the results are compiled in Table 1.For metallographic investigation and hardness testing, samples with dimensions 1 × 1 × 1 cm were prepared.Using Electric Discharge Machining, flat tensile specimens of ASTM standard E8M-04 were machined from both the castings, namely non-UT Al-2Mg and UT Al-2Mg alloys.For microstructural analysis, samples were electrolytically etched with H 3 BF 4 solution and analysed under a polarised light microscope (LM).The average grain sizes were calculated using the line intercept method.For this purpose, 50 LM images were analysed with ImageJ software.Scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) was used to identify the second-phase particles present in the alloys.To determine mechanical properties, Vickers microhardness tests at 300-g load, 15 s dwell time and tensile tests on a universal testing machine at a strain rate of

Results
The results shown in Table 1 depict the chemical compositions of the major alloying elements in the non-UT and the UT Al-2Mg alloys.The UT alloy has a higher Ti content than the non-UT alloy because of Ti pickup from the ultrasonic probe.Figure 1(a and b) show optical micrographs of the non-UT and UT Al-2Mg alloy, indicating a higher degree of microporosity in the non-UT alloy than in the UT alloy.Employing Archimedes principle, the densities of the non-UT alloy and UT alloy were measured to be 2.569 and 2.6206, respectively.Based on density measurements, the porosities of the non-UT alloy and UT alloy are estimated to be 4.1% and 2.2%, respectively (Figure 1f).Thus, the UT alloy has a porosity level significantly lowered by 2.1%.
Figure 1(c and d) depict polarised optical microstructures of the non-UT and UT alloys, respectively.Surprisingly, after UT application for 5 min, the coarse dendritic grains in the non-UT Al-2Mg alloy with an average grain size of 831 ± 84 μm transform to nearly non-dendritic equiaxed grains with an average grain size of 230 ± 12 μm (Figure 1e).Thus, a 3-4-fold reduction in the alloy grain size is achieved by UT.
SEM micrographs of the non-UT and UT Al-2Mg alloys depicted in Figure 2 indicate the presence of a second phase.In the non-UT Al-2Mg alloy, the second phase exhibits a large, elongated structure (Figure 2a and e), whereas in the UT Al-2Mg alloy, the second phase is much refined in size and shape (Figure 2c and f).EDS analysis of the phases shown in Figures 2(b and d) confirm the phase to consist of Al 2 O 3 particles.Figure 2g shows the XRD analysis of both alloys.All the peaks observed in the diffractogram correspond to the major phase of α-Al.The second phase of Al 2 O 3 particles is present only in minor quantity in the material formed and hence was not detectable in the XRD pattern.
The graphs in Figure 3(a and b) depict 0.2% offset tensile yield strength (σ 0.5% ) and the Vickers microhardness variations.After the application of UT, the yield strength, the ultimate tensile strength (σ UTS ), the % strain and the hardness values of the Al-2Mg alloy increased by 21%, 34% (125 MPa to 167 MPa), 143% (13.7% to 33.3%) and 36% (46 HV to 57 HV), respectively.To determine the cause and type of material failure, fractographs were taken.Fracture surfaces of non-UT and UT Al-2Mg alloys are shown in Figure 3(c  and d).

Discussion
Porosity is typically developed during the casting process in Al-Mg alloy for to two main reasons: (a) dissolved gas (hydrogen) and (b) the development of a complex Al-Mg oxide [8].Hydrogen is dissolved in atomic form in molten aluminium and must be converted to a molecular form before being removed from the melt.Ultrasonic waves from UT causes ultrasonic cavitation forming tiny bubbles in the liquid melt, which expand and implode on account of cyclic rarefactions and compressions.Owing to the presence of local pressure difference between the bubble and the liquid melt, the diffusion of dissolved hydrogen is aided by the tiny bubbles formed during the early stages of cavitation [9].As time passes, cavitation bubbles continue growing and rising to the surface of the melts.These floating bubbles burst and release hydrogen to the atmosphere, reducing the hydrogen content in the melt.Furthermore, cavitation implosion can produce shock waves in a specific areaa secondary effect known as 'acoustic streaming'.Shock waves create large stress generation produced by cavitation implosion that can also break complex Al-Mg oxide films into smaller and simpler ones [10].Different mechanisms can account for ultrasound-induced grain refinement.Ultrasonic cavitation can generate a large number of microhotspots and shock waves, with transient temperatures reaching 5000 K and regional pressures reaching 5 GPa [6].According to the Clausius-Clapeyron equation, high pressure can boost heterogeneous nucleation by increasing melt supercooling [5].Furthermore, segregating the elements causes structural undercooling in front of the interface, which aids nucleation of α-Al grains.Al 2 O 3 or impurity particles could be the nucleating particles in this case.As a result, the nucleation rate increases, resulting in significant grain refinement.Alternatively, cavitation can increase the wetting of inclusions and cause endothermic vaporisation of liquid inside the expanded bubble.Cavitation bubbles can clean the surface of inclusions by breaking the gaseous films, allowing the poorly wetted inclusions to be sufficiently wetted by the melt to act as solidification nuclei [11].One or more of these mechanisms could be responsible for the ultrasonic grain refinement in Al-2Mg alloy in the current study.
Ti also has an effect on grain refinement owing to its segregating and growth restriction factor (Q) in Al alloys [12].Low Ti addition (∼ 0.1%), on the other hand, is ineffective in commercial alloys, particularly those containing Si and Mg [13].Hence, Ti pick-up from the sonotrode in the UT Al-2Mg alloy had negligible effect on the grain refinement.
The superior mechanical properties of UT Al-2Mg alloys are primarily attributed to their uniform and fine microstructure, as well as their low porosity.Fine grains, as per the fine-grain strengthening theory, can improve material strength by following the Hall-Petch equation [14].Furthermore, the elimination of micro-segregation and the creation of a homogeneous microstructure will aid in the improvement of strength.Other strength improvement mechanisms such as solution annealing, dislocation creation and grain boundary strengthening as discussed by Xiaoru Zhuo et al. require further processing of the alloy [15].
The introduction of ultrasonic agitation in cast melt leads not only to the refinement of the grains but also results in the shattering and subsequent distribution of second-phase particles (Al 2 O 3 in this case, as shown in Figure 2).The finer grain structure not only improves resistance to the dislocation slip movement at the grain boundaries but also increases the probability of higher slip planes favourably aligned to the direction of deformation.The smaller fragmented and de-clustered second-phase particles lower the particle cracking rate thereby reducing the possibility of microcrack generation.The reduction of porosity and bi-films also decreases the possibility of microvoid coalescence, crack initiation and propagation [16][17][18].Hence, on account of all these factors, improvement of ductility is observed in the alloy after UT.
For the non-UT Al-2Mg alloy, Figure 3c shows that its fracture surfaces have microvoids, dimples, and some shrinkage porosity, with ductile fracture features dominating.In the UT Al-2Mg alloy (Figure 3d), in some areas, microcracks can be seen along the grain boundary, indicating that intergranular fracture has occurred.In other areas, however, there are no microcracks along the grain boundary, and transgranular fracture is observed.UT Al-2Mg alloys also have tear ridges, a few fine dimples, and cleavage facets.Thus, ultrasonically treated alloys show mixed mode failure.

Conclusions
The effects of UT on the microstructure and mechanical properties of Al-2Mg alloy have been investigated.Along with porosity mitigation, grain refinement has been achieved with grain size reduced from 831 ± 84 μm to 230 ± 12 μm, and the grain morphology is changed from a coarse dendritic structure to fine non-dendritic equiaxed grains.σ 0.5% , σ UTS , ductility and microhardness of the alloy are significantly improved by the application of UT.Mechanisms that seem to be operational in the grain refinement process have been discussed.UT shifted the fracture mode from ductile to mixed.

Table 1 .
Chemical composition (wt.%) of Al-2Mg alloy (non-UT and UT).0.01 mm/min were performed.Mean values for hardness and tensile tests for both the alloys were taken to maintain result reproducibility.After the tensile test, fractographs were taken by SEM.