Effects of sonication amplitude on the microstructure and mechanical properties of AZ91E magnesium alloy

P.Emadi,B.Andilab,C.Ravindran

Centre for Near-net-shape Processing of Materials,Ryerson University,350 Victoria Street,Toronto,ON M5B 2K3 Canada

Abstract Light metals are gaining increased attention due to ecological sustainability concerns and strict emission regulations.Magnesium(Mg)is one such metal that has the potential to replace high density components,which can reduce emissions through lightweighting.However,the mechanical properties of Mg alloys must be improved for them to become viable candidates for structural applications.To this end,the current study examines the effect of sonication vibrational amplitude on the microstructure and mechanical properties of AZ91E Mg alloy.The molten alloys were subjected to ultrasonic treatment at a frequency of 20 kHz,180 s of processing time and vibrational amplitudes ranging from 1.25 to 15μm.The resultant castings were characterized using optical microscopy,scanning electron microscopy and tensile testing.It was found that sonication with amplitudes up to 7.5μm was able to effectively refin the secondary phases of the alloy.Similar trends were observed for grain size and yield strength.The refinemen in microstructure was likely caused by the fine grain size and cavitation induced undercooling of the liquid metal.In addition,it was also noted that even the lowest level of amplitude(1.25μm)was able to increase the density,improve the ultimate tensile strength and ductility of the castings.The tensile strength and ductility were thought to have been enhanced by ultrasonic degassing and refinemen in microstructure,while the yield strength was improved through the Hall-Petch effect.The results from this study provided a basis for optimizing the sonication process and promoting its use in industry.As a result,Mg alloys improved through ultrasonic processing have the potential to replace higher density components,with consequent energy efficien y and environmental and ecological benefits? 2022 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Magnesium;Casting and solidification Ultrasonic processing;Vibrational amplitude;Grain refinement Mechanical properties.

1.Introduction

Magnesium(Mg)alloys are excellent candidates for replacing higher density metals such as iron(Fe)and aluminum(Al).Magnesium is abundant and possesses a high strengthto-weight ratio,making it a critical material where weight reduction is a priority[1,2].Lightweighting of electric vehicles,internal combustion vehicles and electronics is critical for emission reduction and ecological sustainability.Because of these factors,Mg has been gaining increasing attention throughout the past decades.The percentage of components made from Mg alloys is increasing year after year.However in order to further increase the use of Mg alloys in industry,there is a need to enhance its mechanical properties.

Presently,most Mg alloys have poor tensile strength and ductility relative to Al alloys.Due to the low strength of Mg relative to other structural metals,its usage is limited to nonstructural components.Several methods have been proposed to improve the properties of cast Mg alloys.Some examples are The Elfina Process[3],Carbon inoculation[4]melt superheating[5]or sintering[6].Additionally,grain refinemen is also known to be a highly effective means for enhancing the strength of cast alloys[7,8].The grain refinemen process refers to the formation of fin grains which can improve strength and ductility through hindering dislocation motion at grain boundaries.Currently,zirconium(Zr)addition has been identifie as an effective means for the grain refinemen of Al-free Mg alloys[9].However,Zr addition is not effective for the more popular Al-bearing Mg alloys such as AZ91.This is in part due to reactions between Zr and Al available in the metal,which eliminates the grain refinin potential of Zr[2].

The current methods for improving the properties of Mg alloys are inadequate.Zirconium addition fails to refin both Mg-Al and Al-free alloys and it is not cost effective.The Elfina process is only effective for Mg-Al alloys that contain manganese(Mn)and reduces alloy corrosion resistance due to Fe addition.Superheating is expensive,time consuming and increases the rate of oxidation as well as dissolved gasses.Carbon addition is mostly carried out through the addition of C2Cl6,but it leads to the emission of harmful gasses[10].As a result,an alternative method for the refinemen of Mg is required.Ideally,an effective refinin technique should be suitable for both Al-free and Al-bearing Mg alloys.

Ultrasonic treatment(UST)has recently emerged as an effective means for the refinemen of metals and alloys[11-15].The process entails the application of high-intensity ultrasonic vibration to the liquid metal using a sonotrode.Sonication requires low-cost equipment and processing times are relatively short.Furthermore,it can be applied to both Al-free and Mg-Al alloys.However,the sonication process is poorly understood.Significan research and development are required in order to elevate the technology to a level for industrial application,especially for processing molten Mg alloys.

While several studies in the fiel of ultrasonic processing of light metals are available,few have been completed with industrial application in mind[16].For example,a number of studies examine the effects of sonication during solidificatio[17-20].This is not effective for castings with complicated shapes since the sound waves cannot reach all areas of a complex mold.Other studies use lengthy refinin times,which is unacceptable from an efficien y perspective,since it leads to time lost during the casting process[21-23].In addition,several studies sonicate at high power levels,several times higher than the reported threshold for cavitation to occur[24-26].This leads to a higher level of energy consumption as well as more expensive equipment.Hence,further research is required to optimize sonication processing parameters,such as ultrasonic power,to improve its useability in industrial settings.

The purpose of this research is to examine the effects of vibrational amplitude on the microstructure and mechanical properties of AZ91E Mg alloy,which was not extensively investigated in prior studies.Consequently,its effect on the properties and microstructure of Mg are not well understood.In this study the experimental procedure consisted of systematic increases in amplitude both above and below the threshold for cavitation to occur.This was done to clearly understand the impact of sonication amplitude on the resultant tensile properties and secondary phases of the alloy.The results from this study are critical for ultrasonic treatment process optimization in an industrial setting,especially since maintaining high levels of amplitude is more energy intensive.Therefore,Mg alloys improved through optimized sonication processes can lead to their increased use in the automotive,aerospace and electronics industries.

Table 1Average chemical composition of AZ91E ingots.

2.Materials and methods

Ingots of AZ91E alloy with chemical composition detailed in Table 1 were used in this study.An electric resistance furnace was used for melting of the ingots.Prior to melting,the ingots were placed in a low-carbon steel crucible and preheated to 200 °C for 30 min.Subsequently,the ingots were heated to 740 °C for melting with an atmosphere of 4.7 L/min of CO2and 0.5 vol.% SF6to prevent oxidation.An ASTM standard B108-6 tensile mold was used[27].The mold was preheated to 350 °C prior to pouring to ensure adequate fillin and a controlled solidificatio rate.The liquid metal was poured at 720°C to match industrial practices.Two identical round tensile test coupons were produced from the same casting mold.

Ultrasonic treatment was performed using a Sonic Systems UK ultrasonic vibration device(model L500).The frequency of vibration was fi ed at 20±1 kHz.The amplitude of vibration was varied from 1.25 to 15μm and applied to the liquid metal using a titanium alloy sonotrode.Sonication was carried out at a melt temperature of 740 °C for 180 s,with the sonotrode immersed 10 mm below the melt surface.After treatment,the surface of the liquid metal was skimmed of dross and poured.Two trials with a total of four samples per condition were evaluated to ensure repeatability of the results.

Tensile testing was done using a United Universal Testing Machine(Model STM-50 kN)at a nominal pull rate of 12.5 mm/min.The %Elongation was measured with an extensometer attached to the gage section.Testing was done at ambient temperature.After testing was completed,sections for metallography were extracted 10 mm away from the fracture surface.Specimens were ground using SiC paper and polished using 9,3 and 1μm diamond compound with an ethanol-based dispersing agent.

For grain size analysis,samples were solution heat treated at 413 °C for 24 hr,according to ASTM B661-12[28].To reveal the grain boundaries,the samples were etched using a 95% water and 5% citric acid solution,for 15 s with minor agitation.A Nikon Eclipse metallurgical microscope(Model MA200)was used to capture images for measuring grain size.For each sample,20 images were captured at 100X,and measurement was performed using Clemex Vision PE image analysis software with due consideration to ASTM E112-13[29].The microstructural analysis was carried out using a JEOL scanning electron microscope(SEM)(Model 6380LV),and the phases were identifie using EDX.

Fig.1.Scanning electron micrographs of AZ91E sonicated with a vibrational amplitude of a)0μm(base alloy),b)1.25μm,c)3.75μm,d)7.5μm,e)11.25μm,and f)15μm.

3.Results

3.1.Microstructural analysis

Fig.1a displays a backscattered electron micrograph of the base alloy.The microstructure consisted of theα-Mg matrix,β-Mg17Al12eutectic phase and Mn-Al intermetallics.In the base condition,massiveβ-Mg17Al12were observed,which precipitated in coarse networks.Theβ-phase is the last phase to form,so it typically precipitates from grain boundaries[30].The phase consists of three morphologies,lamellar,partially divorced and fully divorced.With the application of UST at 1.25μm,a noticeable change in the microstructure was not observed(Fig.1b).However,as observed in Fig.1c,increasing the amplitude to 3.75μm led to minor refinemen in theβ-phase.The previously massive networks appeared to be smaller in size and shorter in length.Further increasing the ultrasonic amplitude of vibration to 7.5μm,led to a marked refinemen in theβ-phase(Fig.1d).The massive eutectic morphology was completely absent,and the microstructure was dominated by short and fin networks.Moreover,the lamellar eutectics were more pronounced due to the absence of the largeβ-phase.Increasing the amplitude of vibration to 11.25 and 15μm did not appear to further enhance the eutectic phase(Fig.1e and f).

In contrast to theβ-phase,the Mn-Al intermetallics are the firs phase to form during solidificatio[30].The intermetallics have been reported to act as nucleation sites forα-Mg[31].These phases usually present themselves in either needles or clusters of round particles,as observed in Fig.1a for the base condition.With the application of UST at 1.25μm,no change was observed in the shape and distribution of the Mn-Al phase(Fig.1b).However,as seen in Fig.1c,at a vibrational amplitude of 3.75μm,both the needle-like morphology and the clustering were not seen in the microstructure.Further increasing the amplitude to 7.5μm led to comparable results but with the phase appearing visually smaller(Fig.1d).No further changes were observed by increasing the amplitude beyond 7.5μm(Fig.1e and f).

Fig.2.Optical micrographs of AZ91E sonicated with a vibrational amplitude of a)0μm(base alloy),b)1.25μm,c)3.75μm,d)7.5μm,e)11.25μm,and f)15μm.

3.2.Grain size analysis and density

Fig.2a displays an optical micrograph of the base alloy captured after solution heat treatment according to ASTM B661-12[28]and etching with citric acid.The microstructure of the base alloy primarily consisted of coarse grains with irregular grain boundaries.With the application of UST for 3 min at an amplitude of 1.25μm,a clear change in the microstructure was seen(Fig.2b).The grain boundaries became more regular,and the overall structure appeared to transition to a more equiaxed state.Moreover,a combination of smaller and larger grains was observed.A similar grain shape and size distribution was seen for the samples that underwent UST at 3.75μm vibrational amplitude,which can be seen in Fig.2c.However,upon increasing the amplitude of vibration to 7.5μm,the grain size was significantl fine compared to the base alloy,and the grain size distribution was more homogeneous(Fig.2d).Further increasing the vibrational amplitude to 11.25 and 15μm(Fig.2e and f)only serves to produce a more homogenous grain size distribution with no significan changes to the grain size.Effectively no change was observed by increasing the amplitude from 11.25 to 15μm.

Fig.3.Average grain diameter of the AZ91E base alloy and the sonicated alloys,with error bars representing one standard deviation about the sample mean.

The change in average grain size with varying UST vibrational amplitude can be seen in Fig.3.The grain size of the base alloy was 214±27μm.With the application of UST at 1.25μm amplitude,the grain size decreased to 145±30μm.Further increasing the vibrational amplitude to 3.75 decreased the grain size to 116±30μm.It must be noted that the error in average grain size was relatively high for the base alloy and the alloys processed at 1.25μm and 3.75μm amplitudes.This was due to the microstructure containing a combination of fin and coarse grains which was apparent in Fig.2a,b and c.The grain size of the alloy processed with a UST amplitude of 7.5μm was 73±7μm.The error in grain size at this level was smaller,which was in line with the optical microstructure observed in Fig.2d.The fines grain size,60±3μm,was observed for the samples that underwent UST at 11.25μm amplitude(72% decrease).Moreover,further increasing the amplitude to 15μm did not lead to a fine grain size.In effect,the grain sizes of the alloys processed 7.5,11.25 and 15μm amplitudes were similar.

Fig.4.Density of the AZ91E base alloy and the sonicated alloys,with error bars representing one standard deviation about the sample mean.

Fig.4 summarizes the density of the cast samples that was measured using the Archimedes principle.Measurements were taken from the same samples that were used for microstructure analysis.The density of the base alloy was measured as 1.785 g/cm3.This was significantl lower than the reported density of AZ91E,1.81 g/cm3[30].Such an outcome was expected since the casting procedure did not use a flu agent or degassing technique such as bubbling with inert gas.With the application of UST at 1.25μm amplitude,the density increased to 1.815 g/cm3.However,further increasing the vibrational amplitude did not appear to increase the density.Therefore,applying a range of vibrational amplitude from 1.25 to 15μm did not produce different outcomes.

3.3.Mechanical properties

The mechanical properties of the base and refine alloys measured by tensile testing at ambient temperature are summarized in Fig.5.The UTS of the base alloy was determined to be 138.7±1.9 MPa(Fig.5b).Upon applying ultrasonic treatment with a vibrational amplitude of 1.25μm,the UTS increased to 169.6±4.3 MPa.This translated to an increase in strength of approximately 22%.Further increasing the amplitude of vibration to 3.75,7.5,11.25 or 15μm did not improve the UTS.Fig.5c displays the ductility of the samples expressed as%Elongation(%EL).The ductility of the base alloy was 1.3±0.1%.After UST at 1.25μm amplitude,the%El increased to 3.0±0.3%(131%increase).Similar to the UTS,the ductility of the samples did not improve by increasing the vibrational amplitude beyond 1.25μm.

For the base condition,the yield strength was 98.1±2.9 MPa.The YS improved with increasing vibrational amplitude up to 7.5μm.The YS for the 1.25μm vibrational amplitude was 99.6±0.1 MPa.Further increasing the amplitude to 3.75,increased the YS to 105.1±0.6%.The highest YS was achieved at the 7.5μm amplitude which was 110.0±1.2 MPa.In effect,the YS improved by approximately 12%.Increasing the amplitude to 11.25 or 15 did not further improve the YS.Upon examining Fig.5a,it is clear that the trend for YS differed from that of UTS and%EL.The tensile strength and ductility drastically improved with only 1.25μm vibrational amplitude and remained stable thereafter.In contrast,the yield strength of the alloys showed steady signs of improvement up to 7.5μm vibrational amplitude and did not increase for 11.25 and 15μm.

Table 2Summary of parameters used for calculation of ultrasonic intensity.

3.4.Ultrasonic intensity

The Intensity of the sonication process is of critical importance during irradiation.It can be described as the ultrasonic energy that is transferred to the liquid metal through the sonotrode(Horn)[32].Sonication intensity(I)has been define by Eskin as[32]:

Where,ρis the density of the liquid metal,c is the speed of sound in the liquid metal,fis the frequency of the sonication process and A is the vibrational amplitude.Quested et al.[33]reported the density of liquid AZ91 as 1.66 g/cm3.In our study,the frequency was 20 kHz and the vibrational amplitude ranged from 1.25 to 15μm.In an earlier study,the speed of sound in the liquid metal was set as 1500 m/s[34].It must be noted that the speed of sound in liquid AZ91E is not available in literature.As a result,researchers typically assume 1300 m/s as an approximation for low melting point metals[35].However,a logical assumption that has been utilized in past research is to assume the speed to be near that of the speed of sound in water at room temperature[34].Such an assumption is acceptable since liquid Mg has a density and viscosity that is near that of water.A summary of the parameters used is provide in Table 2.

Using Eq.(1),the intensity of the sonication was plotted against the change in vibrational amplitude.The result is summarized in Fig.6.Researchers have reported that the minimum intensity needed to produce fully developed cavitation in Mg alloys is between 80 and 100 W/cm2[32,36].This level of intensity corresponds to amplitudes between 6.25 and 7.25μm.Furthermore,using vibrational amplitudes of 11.25 or 15μm would lead to intensities that are 2.4 and 4.2 times the minimum threshold for cavitation to occur,respectively.

Fig.5.Tensile properties of the AZ91E base alloy and the sonicated alloys,a)%Elongation,b)Ultimate tensile strength and c)Yield strength,with error bars representing one standard deviation about the sample mean.

Fig.6.Ultrasonic intensity calculated using Eq.(1).

4.Discussion

It was found that the base casting had a lower density compared to that of theoretical AZ91E alloy.That is,the density of the base casting was determined as 1.785 g/cm3while that of AZ91E is 1.81 g/cm3[30].It is likely that the lower density of the base casting was caused by porosity due to dissolved gasses.Especially,since no degassing agent was used as part of the experimental procedure.After the application of UST,the density of the samples was near that of the reported value for AZ91E.It must be noted that even sonicating with the minimum available amplitude of the system,1.25μm,led to an improvement in density.The improvement remained consistent with increasing vibrational amplitude up to 15μm.This result can be explained by the degassing ability of the sonication process.

Research has shown that the cavitation process is highly effective in removing dissolved hydrogen from liquid metal[37,38].The degassing process operates through the accumulation of dissolved gas in pulsating bubbles through diffusion.This occurs during the rarefaction stage of bubble oscillation[32].Moreover,the cavitation bubbles then grow,coalesce to form larger bubbles,floa to the surface of the melt,and escape to the atmosphere.It should be mentioned that the effectiveness of ultrasonic degassing increases with the number of foreign particles or inclusions within the liquid metal[32].This is especially important for Mg alloys,since they have a high affinit for oxygen,which leads to the melt containing a large number of oxides relative to Al[39].With the considerable number of foreign particles in the melt,they act as substrates for the formation of cavitation bubbles,thereby effectively reducing the cavitation threshold[32].This can explain the outcome that even a small vibrational amplitude,far lower than the reported minimum needed for fully developed cavitation,can effectively degas the AZ91E alloy melt.

Grain size reductions of up to 72% were achieved in this study.This was a result of UST at 11.25μm amplitude.However,it must be said that the range of amplitudes from 7.5 to 15μm produced comparable results.This is significan since the amplitude threshold for fully developed cavitation is approximately 7μm.As a result,once the cavitation is fully developed,increasing the amplitude of vibration any further will not lead to further improvements in grain size.

The main mechanism of grain refinemen in this case was likely the wetting of fin oxides in the melt[40,41].The oxides in turn can act as heterogenous nucleation sites for the nucleation of Mg grains.This occurs through the cavitation phenomenon where the pulsating bubbles of gas undergo stages of expansion and compression until their eventual collapse[32].The collapse of gas bubbles can produce high intensity shock waves throughout the melt.The shockwaves have been reported to aid the liquid metal in reaching all areas of inclusions such as cracks and crevices that were previously fille with gas[32].It is likely that once cavitation is fully developed at approximately 7μm,a sufficien number of oxides are activated in the melt.It is known that only a fraction of nucleation sites or agents can contribute to the solidificatio process[42].As a result,although increasing the sonication intensity may in fact wet more inclusions,since only a fraction take part in solidification the newly activated substrates do not lead to a fine grain size.

The secondary phases in this study were also considerably affected by the sonication process.The results showed that apart from the 1.25μm amplitude,all the sonicated alloys displayed signs of eutecticβ-Mg17Al12refinement In particular,with increasing sonication amplitude up to 7.5μm,theβ-phase became shorter in length and smaller in size.Such an outcome has been reported for grain refine castings of Mg[43].This is due to theβ-phase predominantly precipitating on grain boundaries.Moreover,since the grain size of the castings decreased with increasing amplitude,theβ-phase became more refined However,since the grain size did not change between 7.5 and 15μm,it would follow that theβphase would not be refine further.

In addition to theβ-Mg17Al12phase,the Mn-Al based intermetallics were refine as well.In the base condition,the intermetallics were observed to be in either needle-like morphologies or clusters of round particles.However,as the sonication amplitude increased,both the shape and the distribution of the Mn-Al intermetallics were improved.Namely,the needle-like morphology was transformed to a more round shape.As well,the particle clustering was eliminated,and the phase became more evenly distributed throughout the Mg matrix.Such a transformation of the Mn-Al phase has been reported in the literature and is likely a result of the ultrasonic induced melt pressure changes[17,44].

During the sonication process,if the intensity is sufficien to prompt cavitation,localized pressure changes can occur in the liquid metal.If the pressure change is high enough,certain areas in the melt can become undercooled,thereby triggering the formation of precipitates.This is especially important for the Mn-Al intermetallic,since it is the firs phase to form during solidification Alternatively,similar to theβphase,the refinemen of the Mn-Al phase was not observed after sonication with 1.25μm amplitude and only became apparent between 3.75μm and 15μm.This was to be expected since cavitation was likely not fully developed at lower amplitudes.As a result,the pressure changes would not have been sufficien to prompt nucleation.

Both the tensile strength and the ductility of the castings showed improvements with the application of UST.It was observed that the improvements were similar for all UST amplitudes from 1.25 to 15μm.This was likely a result of the change in porosity of the castings.Porosity is known to have a detrimental effect on both UTS and ductility[45,46].Moreover,the trend observed for the change in density was almost identical to the changes in UST and ductility.This result suggested that the three parameters are likely linked.

In contrast,the same trend as UTS and ductility was not observed for YS.This is likely due to the change in grain size of the alloys with UST.According to the classical Hall-Petch relation,the YS of metals and alloys typically improve with decreasing grain size.In this study,the grain size of the alloys decreased with increasing vibrational amplitude up to 7.5μm and was stable thereafter.Similarly,the YS of the casting followed the same trend.

The refinemen of secondary phases would also have a positive influenc on the tensile properties of the sonicated alloys.As mentioned previously,theβ-Mg17Al12eutectic phase predominantly precipitates on grain boundaries in the form of continuous networks.The phase is deleterious to mechanical properties due to its brittle nature[30].Therefore,refinemen of theβ-phase would likely bring about noticeable improvements in mechanical properties.In addition,refine ment of the Mn-Al phase likely contributed to improving mechanical properties as well.As observed in a previous study,spheroidization of these brittle phases and their homogenous distribution can reduce stress concentrations in the microstructure and prevent premature failure of the castings[13].

It is clear ultrasonic treatment is an effective method to improve the microstructure and mechanical properties of permanent mold cast AZ91E alloy.Even a minor vibrational amplitude was shown to improve the UTS,YS,ductility,and density of the castings.However,higher amplitudes up to 7.5μm were required to produce optimal grain size and YS.It was also observed that once the cavitation was theoretically determined to be fully developed at 7.5μm,increasing the vibrational amplitude had little to no effect on the resultant mechanical properties and microstructural features.Ultrasonic treatment is significan in its capacity to refin cast alloys,since it can do so without altering the chemistry of the alloy.This is critical since in many applications chemical compositions cannot be altered.

This study presents an important analysis pertaining to the effects of vibrational amplitude on the effectiveness of UST,which can provide a step towards improving the sonication process and promoting its industrial application.It must be noted that future work is required to fully optimize UST.In particular,the effects of varying sonication time at different vibrational amplitudes as well as the role of sonotrode depth must be investigated.As a result,cast Mg alloys refine with UST have the potential to replace high density Al and Fe based alloys,leading to lightweighting and improved efficien y.

5.Conclusion

This study examined the effects of UST on the microstructure and mechanical properties of AZ91E Mg alloy.Permanent mold cast samples were prepared using a fi ed sonication frequency and sonotrode depth with vibrational amplitudes varying from 2.5 to 15μm.The castings were evaluated based on grain size,secondary phase characteristics and tensile properties.Small vibrational amplitudes resulted in significan changes to grain size and secondary phases.Similar trends were seen for the strength properties and the density.The results of this study enabled a comprehensive understanding of the relationships between microstructural features,mechanical properties,and UST vibrational amplitude.

The following conclusions can be drawn from this study:

1 Ultrasonic treatment led to the refinemen of the alloy grain size.Specificall,the grain size decreased steadily up to 7.5μm vibrational amplitude but,only slight changes were observed by increasing the amplitude to 11.25 and 15μm.It was suggested that the grain size was enhanced through heterogenous nucleation from newly wetted fin oxides present in the liquid metal.

2 A similar trend was observed for secondary phases.Gradual improvements were observed up to 7.5μm vibrational amplitude with no change thereafter.The refinemen inβ-Mg17Al12phase was thought to be a result of the change in grain size.Alternatively,the change in Mn-Al intermetallics were suggested to have been caused by cavitation induced undercooling of the liquid metal.

3 The YS of the samples also increased up to 7.5μm amplitude and remained stable thereafter.The increase in YS was thought to be a result of fine grain size through Hall-Petch strengthening.

4 The density of the sonicated castings showed a sudden improvement with application of UST at 1.25μm amplitude.Increasing the amplitude up to 15μm did not change the density further.The change in density was suggested to have occurred through ultrasonic degassing of the melt.

5 The UTS and ductility also improved with the application of 1.25μm vibrational amplitude with no changes thereafter.This was thought to be a result of the change in density due to sonication,which followed an identical trend.

Declaration of Competing Interest

None.

Acknowledgements

The authors recognize the kind support of the Natural Sciences and Engineering Research Council(NSERC)for the award of Canada Graduate Scholarships(CGSD3-535728-2019 and CGSD3-559982-2021).The authors would like to thank Mr.Alan Machin and Mr.Adam Belcastro for their support with this research.