Recent Developments On UltRasonic Cavitation BaseD .
Recent Developments on Ultrasonic Cavitation Based SolidificationProcessing of Bulk Magnesium NanocompositesG. Cao, H. Konishi and X. LiUniversity of Wisconsin-Madison, Madison, WI, USACopyright 2008 American Foundry SocietyAbstractThis paper presents the results from our recent developmentin cast bulk Mg nanocomposites. SiC nanoparticlesreinforced magnesium and magnesium alloys includingpure magnesium, and Mg-(2, 4)Al-1Si and Mg-4Zn weresuccessfully fabricated by ultrasonic cavitation baseddispersion of SiC nanoparticles in magnesium melt. Ascompared to un-reinforced magnesium alloy matrix, themechanical properties including tensile strength and yieldstrength were improved significantly while the ductilitywas retained or even improved. In the microstructure, thegrain size was refined considerably by SiC nanoparticles.While some micro SiC clusters still exist in the magnesiummatrix, ultrasonic cavitation based processing is veryeffective in dispersing SiC nanoparticles. A SEM studyshowed that SiC nanoparticles were dispersed quite wellin the areas outside micro SiC clusters. A TEM study onthe interface between SiC nanoparticles and magnesiumalloy matrix indicates that SiC nanoparticles bondedwell with Mg matrices without forming an intermediatephase.Keywords: magnesium, nanocomposites, ultrasoniccavitation and dispersion.Introductionhas an excellent role in strengthening the magnesium. Thestructure of magnesium was refined. The tensile strength andelongation of the composite were also improved. Ma et al[6] studied the TiB2-TiC particles reinforced AZ91D alloy.The results showed that the hardness and wear resistance ofthe composites were higher than those of the unreinforcedAZ91 alloy. Pahutova et al [7] studied the creep resistance ofsqueeze cast AZ91 and QE22 magnesium alloys reinforcedby 20 vol% Al2O3 short fibers and showed that the creepresistance of reinforced materials was considerably improvedcompared to the monolithic alloys. Ye et al [8] reviewedthe recent progress in magnesium matrix composites. Theconventional methods such as stir casting, squeeze casting,and powder metallurgy were mostly used in fabricating themagnesium matrix composites reinforced by powders/fibers/whiskers. Some other processing techniques such as in-situsynthesis, mechanical alloying, pressureless infiltration, gasinjection, and spray forming were also sometimes used. Formagnesium matrix composites, the mechanical propertiessuch as tensile strength, creep resistance and fatigue resistancewere improved, but at the cost of other properties, typicallyductility. The poor ductility of magnesium matrix compositeslimits the widespread application of Mg based MMCs.Magnesium based metal matrix composites (MMCs)have been extensively studied as an attractive choice forautomotive and aerospace applications due to their lowdensity and superior specific properties including strength,stiffness and creep resistance. Xi et al [1] studied the Ti6Al-4V particulate (TAp) reinforced magnesium matrixcomposite which is fabricated by powder metallurgy route.The tensile strength of the composite was markedly higherthan that of the unreinforced magnesium alloy. Xi et al [2]also studied the SiC whiskers-reinforced MB15 magnesiummatrix composites by powder metallurgy and found thatthe mechanical properties of SiCw/MB15 composite weresignificantly influenced by powder-mixing methods of PM.Li et al [3] studied the hot deformation behavior of SiCwhiskers reinforced AZ91 magnesium matrix compositesin compression. It was found that the microstructureevolutions involved the movement of SiC whiskers and thechanges of the matrix, and the rotation and the broken SiCwhiskers tended to be obvious with the increasing strain,and also high density of dislocation was observed in theAZ91 matrix at the initial stage of compression (1%). Jianget al [4] studied that magnesium metal matrix composites(MMCs) reinforced with B4C particulates fabricated bypowder metallurgy. The hardness and wear resistance of thecomposites were higher than those of as-cast Mg ingot andincreased with increasing amount of B4C particulates from10 to 20 vol.%. Zhang et al [5] studied that carbon nanotubereinforced magnesium metal matrix composite by stirringthe carbon nanotube into magnesium melts. It was foundthat carbon nanotube, especially chemical nickel-plated one,International Journal of Metalcasting/Winter 08Nanoparticle reinforcements can significantly increase themechanical strength of the matrix by more effectively promotingparticle hardening mechanisms than micron size particles. Afine and uniform dispersion of nanoparticles provides a goodbalance between the strengthener (non-deforming particles,such as SiC nanoparticles) and inter-particle spacing effectsto maximize the yield strength and creep resistance. It is57
expected that the properties of metals reinforced by ceramicnanoparticles (less than 100 nm), that is, metal matrix nanocomposites (MMNCs), would be enhanced considerably (e.g.superior strength and creep property at elevated temperatures,higher fatigue life, and better machinability) while the ductilityof the magnesium matrix is retained.melt for about 32 mm. The melt temperature for ultrasonicprocessing was controlled at about 700oC (1292oF). Nb is ahigh temperature element and does not react with Mg at themelt temperature. Nano-sized β-SiC particles were fed intothe Mg melt through a steel tube. The average size of the SiCparticles used in this study was about 50 nm.Solidification processing such as stir casting that utilizedmechanical stirring was a widely used technique to producemagnesium matrix composites that were reinforced bymicro ceramic particles. A combination of good distributionand dispersion of micro particles can be achieved bymechanical stirring. However, to produce magnesiummatrix nanocomposites, it is extremely challenging for theconventional mechanical stirring method to distribute anddisperse nanoparticles uniformly in metal melts because ofthe much higher specific surface areas in nanoparticles. Inorder to achieve a uniform dispersion and distribution ofnanoparticles in magnesium matrix nanocomposites, Lanet al [9], Li et al [10], and Yang et al [11] developed anew technique that combined solidification processes withultrasonic cavitation based dispersion of nanoparticles inmetal melts. It was reported [13] that ultrasonic cavitationcan produce transient (in the order of nanoseconds) micro“hot spots” that can have temperatures of about 5000ºC (9032oF), pressures above 1000 atms, and heating and cooling ratesabove 1010 K/s . The strong impact coupling with local hightemperatures can potentially break nanoparticle clusters andclean the particle surface. Since nanoparticle clusters areloosely packed together, air could be trapped inside the voidsin the clusters, which will serve as nuclei for cavitations.The size of the clusters ranges from nano- to micro-metersdue to the attraction force among nanoparticles and the poorwettability between nanoparticles and metal melts.Figure 1. Experimental set-up for fabricating Mg-SiCnanocomposites.In this paper, SiC nanoparticles reinforced pure magnesium,Mg-4Zn and Mg-(2, 4)Al-1Si were fabricated by ultrasoniccavitation based solidification processing. The microstructureand overall mechanical properties of nanocomposites werestudied to understand the effect of nanoparticles in as castMg alloys.ExperimentalFigure 1 shows the schematic experimental setup for theultrasonic cavitation based fabrication of nano-sized SiCreinforced magnesium matrix nanocomposites. It mainlyconsists of a resistance heating furnace for melting magnesium,nanoparticle feeding mechanism, protection gas system, andultrasonic processing system. A mild steel crucible, which is114 mm in inside diameter (ID) and 127 mm in height, wasused for melting and ultrasonic processing. A Permendurpower ultrasonic probe, made of C-103 niobium alloy was usedto generate a 17.5KHz and maximum 4.0 kW power outputfor melt processing. The niobium probe is 35 mm in diameterand 39 cm in length. The ultrasonic probe was dipped into the58About 800 to 900g of pure (99.8%) magnesium or magnesiumalloys were used. The magnesium melt pool was protectedby CO2 0.75% SF6. Pure Mg and pure (99.99%) Zn wereused for making Mg-4Zn alloy matrix. After pure Mg wasmelted, pure Zn was added into the Mg melt. Pure Mg, pureAl and Al-50%Si master alloy were used for making Mg-2Al1Si and Mg-4Al-1Si matrices. Similar to melting of Mg-4Znmatrix alloy, pure Al and Al-50%Si master alloy were addedinto the magnesium melt after Mg was melted. It should benoted that no Zr element was added. Zr is an effective grainrefiner in Mg alloys that contain no Al or Mn. It is expectedthat SiC nanoparticles can refine the grain size of Mg alloyssignificantly. After ultrasonic processing, the magnesium meltwas cast into a steel permanent mold that was preheated toabout 400oC (752 oF). The mold was designed and fabricatedaccording to ASTM B 108-03a. In each casting experiment,two standard tensile specimens as shown in Fig.2 can beobtained. An additional graphite pouring cup was used toguide the melt and serve as housing for the SiC ceramic filter,which has a dimension of 55 mm x 55 mm x 12 mm and anaverage pore size of 2.3 mm to 2.9 mm.International Journal of Metalcasting/Winter 08
Figure 2. Permanent mold casting samplesfor tensile testing.To obtain mechanical properties, specimens with a diameterof 0.375” (9.5 mm) and a gage length of 1.75” (44.5 mm)were tested in a MTS tensile testing machine. In order toget the precise value of yield strength, an extensometer witha 1’’ (25.4 mm) gage length was clamped to each tensilesample. The cross head speed is set to be 5 mm/min. Whenthe strain reaches 1%, the tensile testing machine will pausetemporally so that the extensometer can be taken off. At thefirst stage of tensile testing, the strain data comes from theextensometer. After the extensometer is taken off, the tensiletesting continues and the strain data comes only from thedisplacement reading of the tensile testing machine.The microstructure of the samples was studied by opticalmicroscopy, scanning electron microscopy (SEM) andtransmission electron microscopy (TEM). The magnesiumnanocomposites samples for optical microscopy and SEMwere cut, mounted, mechanically ground and polished. TEMsamples were prepared using ion milling.Results and DiscussionFigure 3a through 3d show the strengthening effects of SiCnanoparticles on the mechanical properties of pure magnesium,Mg-4Zn and Mg-(2, 4)Al-1Si. The yield strength of Mg/2%SiCInternational Journal of Metalcasting/Winter 08increased 80%, as compared to that of pure Mg. The good ductilityof pure magnesium is also retained in Mg/2%SiC. The mechanicalproperties of nanocomposites are very different from those ofcomposites materials reinforced by micropowders, in whichductility decreased while the yield strength and ultimate tensilestrength were improved [9]. . In Mg-4Zn/1.5%SiC, the yieldstrength and ultimate tensile strength increased 73% and 90%respectively, and the ductility was also improved significantly. Inpure Mg-4Zn, the ductility was about 9%, but in Mg-4Zn/1.5%SiC,the ductility reached over 20%, which was more than twice thatof pure Mg-4Zn alloy matrix. This possibly could be attributedto, at least, to the following three aspects: Firstly, although theseSiC clusters appeared as micro clusters, most of the nanoparticlesin the micro-clusters are still separated to single nanoparticles orsintered nano-clusters. Secondly, the negative effects of some SiCmicro-clusters were balanced by the positive effects of the grainrefining effects and strengthening effects of the well dispersedSiC nanoparticles. Thirdly, the improvement of ductility in Mg4Zn/1.5%SiC nanocomposites can also be partly attributed tothe decreasing of porosity and crack-like shrinkage areas in Mg4Zn. Because of the wide freezing temperature range of Mg-4Zn,some porosity and crack-like shrinkage areas formed at the endof solidification. These porosity and crack-like shrinkage areasare detrimental to the ductility. However, this is the first time inour study to show this significant ductility-enhancing effect foras-cast Mg matrix nanocomposites. Normally, the ductility ofas cast magnesium nanocomposites (e.g. pure Mg reinforced bySiC nanoparticles) was only retained. While the mechanism forthis significant ductility enhancement is not well understood, itcertainly warrants further study.In Mg-2Al-1Si/2%SiC and Mg-4Al-1Si/2%SiC nanocomposites, the yield strength and ultimate tensile strengthincreased significantly while the good ductility of Mg-2Al-1Siand Mg-4Al-1Si was also retained. From the tensile testing ofMg/2%SiC, Mg-4Zn/1.5%SiC and Mg-(2, 4)Al-1Si/2%SiC,the yield strength and ultimate tensile strength of all threenanocomposites materials were improved significantly andthe good ductility of the unreinforced matrix was retained oreven improved. The combination of higher strength and goodductility of magnesium matrix nanocomposites is promisingfor structural applications in various industries.According to a recent published analytical model [14] forpredicting the yield strength of particulate reinforced metalmatrix nanocomposites, the strengthening of nanoparticlescome from the Orowan strengthening effects, enhanceddislocation density and load bearing effects. It is pointedthat Orowan strengthening is not significant in metal matrixcomposites enhanced by microparticles and it becomesmore favorable in metal matrix nanocomposites due to thatthe Orowan bowing is necessary for dislocations to bypassthe nanoparticles. A small volume fraction of nanoparticlescan significantly improve the yield strength of metal matrixnanocomposites. This is also a clear contrast to the magnesiummatrix composites reinforced by micro particles or fibers,which contains 10 to 30 vol% reinforcement phases.59
(a)(b)(c)(d)Figure 3. Mechanical properties of (a) pure Mg and Mg/2%SiC, (b) Mg-4Zn and Mg-4Zn/1.5%SiC, (c) Mg-2Al-1Si and Mg2Al-1Si/2%SiC, (d) Mg-4Al-1Si and Mg-4Al-1Si/2%SiC.Figure 4 shows the optical microstructures of puremagnesium, Mg/2%SiC, Mg-4Zn, and Mg-4Zn/1.5% SiC.The grain size in Mg/2%SiC is much finer than that inpure Mg, as shown in Fig 4a and b. The grain refinementcan also be seen from SEM images (as shown in Figure5a and b) of the fracture surfaces of pure Mg and Mg/2%SiC samples after they are tensile tested. Similarly,the microstructure of Mg-4Zn/1.5%SiC is also refined bythe addition of SiC nanoparticles. In as cast Mg-4Zn, theaverage grain size is about 150 µm while in as cast Mg4Zn/1.5% SiC nanocomposits, the average grain size isreduced to about 60 µm. According to the classic Hall-60Petch equation: σy σ0 Kyd-1/2, where σy is the yieldstrength, σ0 and Ky are material constants, and d is themean grain size. The value of Ky is dependent on thenumber of slip systems. It is higher for HCP metals thanfor FCC and BCC metals [9]. Since Mg is an HCP metal,the grain size affects the yield strength more significantly.It is also well known that grain size has a strong effect onthe ductility and toughness of materials. The significantimprovement of yield strength can be attributed tothe grain refining effects and strengthening effects ofnanoparticles including Orowan strengthening effects,enhanced dislocation density and load bearing effects.International Journal of Metalcasting/Winter 08
(a)(b)(c)(d)Figure 4. Optical microstructure of (a) pure Mg, (b) Mg/2%SiC, (c) Mg-4Zn and (d) Mg-4Zn/1.5%SiC.(a)(b)Figure 5. Fracture surface of (a) pure Mg and (b) Mg/2%SiC.International Journal of Metalcasting/Winter 0861
In the microstructure of Mg-4Zn, as shown in Fig.4c, someshrinkage porosity was observed. This could be due to a verywide solidification range for Mg-4Zn when compared to othermagnesium alloys such as AZ91D and Mg-Al alloys. Thewide freezing range also results in poor castability. It was alsoworth mentioning that the castability of Mg-4Zn/1.5% SiCwas also improved considerably as compared to pure Mg4Zn alloy. Little hot tearing existed in as cast Mg-4Zn/1.5%SiC nanocomposites. The fluidity of Mg-4Zn/1.5%SiCnanocomposites melt is also significantly higher than that ofMg-4Zn melt. In casting pure Mg-4Zn alloys, a temporarypause of Mg-4Zn melt can be seen immediately after the meltis poured into the pouring cup, while no temporary pausein the case of Mg-4Zn/1.5%SiC nanocomposites can benoticed. The considerable improvement in castability can beattributed to a finer grain size in Mg-4Zn/1.5% SiC castingsand a higher fluidity of Mg-4Zn/1.5%SiC melt. Generally,a finer grain size can improve melt feeding characteristics,increase hot tearing resistance, and minimize shrinkage andporosity.(a)(b)(c)Figure 6. SEM images of (a) Mg/2%SiC, (b) Mg-2Al-1Si/2%SiC and (c) Mg-4Zn/1.5%SiC.62International Journal of Metalcasting/Winter 08
Figure 6a shows a low magnification SEM image of Mg/2%SiCnanocomposites. The white areas are SiC clusters. These clusterswere distributed uniformly. Although ultrasonic cavitation isquite effective in dispersing SiC nanoparticles, the processingparameters were still not optimized. Further improvement onthe dispersion of nanoparticles is needed. Similar to Mg/2%SiCnanocomposites, some SiC clusters still existed in the Mg-2Al1Si/2%SiC and Mg-4Zn/1.5%SiC nanocomposites and theyalso distributed uniformly as shown in Fig. 6b and 6c.Figure 7 shows a higher magnification SEM image outside thearea of SiC clusters in the Mg/2%SiC sample. It is clear thatSiC nanoparticles were dispersed very well. The significantimprovement of mechanical properties would mostly beattributed to the well dispersed SiC nanoparticles. In Fig.7,it seems that some of the nanoparticles were even sintered.From the low magnification SEM images, one would expectthe ductility of Mg/SiC nanocomposites would be reduceddue to the clusters. However, the tensile testing showed thatthe ductility of Mg/SiC nanocomposites was still high. Thispossibly can be explained from the following two aspects:Firstly, although these SiC clusters appeared as microclusters, they are still separated by magnesium between thenanoparticles inside the small clusters and they are differentfrom the SiC micro powders in conventional MMCs. Secondly,the negative effects of SiC clusters were balanced by thepositive effects of the grain refining effects and strengtheningeffects of the well dispersed SiC nanoparticles.Figure 7. Higher magnification SEM image of Mg/2%SiC.(a)(b)Figure 8. SEM images of (a) Mg-2Al-1Si/2% SiC nanocomposites and (b) Mg-4Zn/1.5% SiC.International Journal of Metalcasting/Winter 0863
Similarly, from the SEM images of Mg-2Al-1Si/2%SiCand Mg-4Zn/1.5%SiC nanocomposites as shown in Fig. 8aand 8b, SiC nanoparticles were also dispersed quite well.From the above SEM images, it suggests that ultrasoniccavitation processing was very effective in dispersing SiCnanoparticles. This good dispersion was very difficult toobtain via traditional mechanical stirring.Fig. 9a and 9b show TEM images of the interface betweenSiC nanoparticles and the magnesium or Mg-4Zn matrix. Nointermediate phase occurs, which suggests that there is littlereaction between SiC nanoparticle and magnesium or Mg4Zn matrix. In Fig. 9a, strain contrast was observed near theinterface, indicating that SiC are bonded to magnesium verywell. Fig. 10 showed the TEM image of the interface betweenof SiC nanoparticle and Mg-4Al-1Si matrix. It showed agood bonding between SiC and Mg-4Al-1Si matrix, and nointermediate phase was detected.(a)(b)Fig.9. TEM image of the interface of (a) SiC and Mg in Mg/2%SiC, (b) SiC and Mg-4Zn in Mg-4Zn/1.5%SiC.Figure 10. Interface between SiC and Mg-4Al-1Si alloy matrix. SiC grain shows Moire fringes. Mg-4Al-1SiC alloy matrixonly shows (101) fringe, suggesting that SiC bond to Mg-4Al-1Si alloy matrix without forming an intermed
ultrasonic processing system. A mild steel crucible, which is 114 mm in inside diameter (ID) and 127 mm in height, was used for melting and ultrasonic processing. A Permendur power ultrasonic probe, made of C-103 niobium alloy was used to generate a 17.5KHz and maximum 4.0 kW power output for melt processing.
cavitation did not change very much when the cavitation coefficient changed. It was also found that the front edge location of sheet cavitation was not influenced very much by the unsteady fluctuation of cavitation on the blade. The front edge location of sheet cavitation was measured based on the image, and it was 7.8 % of the chord length
It is standard practice in cavitation testing laboratories to include sketches or photographs of cavitation patterns in test reports. Descrip-tive terms are used to identify the various types of cavitation observed during tests, typified below, in Figure 1. Figure 1. Cavitation types Description of cavitation appearances
several areas. Cavitation is primarily due to poor pump system design and a lack of awareness about how cavitation is caused. 1.2.1 CAVITATION Cavitation can have a serious negative impact on pump operation and lifespan. It can affect many aspects of a pump, but it is often the pump impeller that is most severely impacted.
Main effect to achieve contamination removal in ultrasonic cleaning: energy driven cavitation ultrasonic Within the fluid medium , each point along the wave oscillates with pressure ranging between maximum and a minimum, where cavitation bubbles grow. In almost all cleaning applications, it is important to control the cavitation energy.
ultrasonic as significantly more aggressive ultrasonic cavitation within the ultrasonic tank, resulting in faster and more efficient cleaning in reduced time. (It should be noted that the vast majority of the ultrasonics produced by the various competitive ultrasonic manufacturers utilize the older non-sweep ultrasonic technology which can be seen
Prediction of Control Valve Cavitation Using Machine Learning Technique DOI: 10.9790/1676-1505012635 www.iosrjournals.org 27 Page Figure 1: Pressure distribution across a control valve and cavitation points (the arrows on the valve indicate the flow direction of a typical valve) II. Parameters Of Cavitation And Numerical Calculations .
HOW TO PRODUCE CAVITATION 2.1. Cavitation Cavitation is a hydrodynamic phenomenon from the liquid phase to the gas phase produced by increasing the flow velocity to reduce the pressure. Ber- noulli's equation is 1 2 ρ2 Lvp const. (1) where, v, ρ Land pare the flow velocity, density of the liquid and pressure.
5 SUGGESTED READINGS Smith, G.M. 1971. Cryptogamic Botny. Vol.I Algae & Fungi. Tata McGraw Hill Publishing Co., New Delhi. Sharma, O.P. 1992.
