Solidification of aluminium alloys under ultrasonic irradiation using water-cooled resonator

7/29/2019 Solidification of aluminium alloys under ultrasonic irradiation using water-cooled resonator

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September 1998

.Materials Letters 37 1998 2734

Solidification of aluminium alloys under ultrasonic irradiationusing water-cooled resonator

V. Abramov a,), O. Abramov b, V. Bulgakov b, F. Sommer a

aMax-Planck-Institut fur Werkstoffwissenschaft, Seestr. 75, D-70174 Stuttgart, Germany

bInstitute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky prospect 31, 117907 Moscow, Russian Federation

Accepted 23 February 1998

Abstract

The present investigation attempts to evaluate the effect of ultrasonic treatment using water-cooled resonator on the

microstructure and properties of different commercial Al-based alloys. The effect of ultrasonic treatment on the as-cast alloy

microstructure can be summarized as follows: reduction of mean grain size, variation of phase distribution and better

material homogeneity and segregation control. Ultrasonically treated samples have elongation values much higher than those

obtainable with control processes. Our investigations confirm great advantages of ultrasonically treated ingots of hypereutec-

tic AlSi alloys upon deformation in semisolid state. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: Aluminium alloy; Ultrasonic irradiation; Water-cooled resonator; Solidification

1. Introduction

The final quality of a cast is broadly dependent

upon many factors which will have an effect on the

solidification of the metal. Any structural defect

occurring in the cast products may be transferred to

the final product. Thus, any process which would

reduce defects and improve the metal structure of the

cast product could clearly be of benefit to the foundry

industry. The major problem associated with alu-

minium alloy casting is an inability to completelyeliminate the various production defects in their

structure, such as dendrite structure, gas cavities,

segregation and rough intermetallic inclusions. The

)

Corresponding author.

improvement in the structure of aluminium alloys .can be produced mainly by two routes which are: i

.mechanical or magneto-hydrodynamic stirring, ii

intensive ultrasonic treatment during solidificationw x1,2 .

Introduction of high-power ultrasonic vibration

into a liquid alloy leads to cavitation and acoustic

streaming. Cavitation involves the formation, growth,

pulsating and collapsing of tiny bubbles in the melt.

The compression rate of these unsteady states can be

so high that their collapses generate hydraulic shockwaves. Primary crystallites are broken up by hy-

draulic shock waves, thus producing artificial sources

of nuclei. The propagation of high-intensity ultra-

sonic wave also involves the initiation of steady-state

acoustic streamings in the melt. The total effect of

various kinds of streams is to vigorously mix and so

homogenize the melt.

00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. .P I I : S 0 1 6 7 - 5 7 7 X 9 8 0 0 0 6 4 - 0


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( )V. Abramo et al.rMaterials Letters 37 1998 27 3428

In the present work, we studied the possibility of

using a water-cooled resonator for ultrasonic treat-

ment of different types of aluminium alloys. It should

be noted that the water-cooled resonator placed in

the melt acts as cooler and has also an effect on the

solidification kinetics. Varying the water rate, res-

onator shape and immersion, and the ultrasound

intensity, one can better control the temperature dis-

tribution in the melt and thus the microstructure of

the material. In this case the most effective crystal-

lization will take place not at the walls of the cru-

cible, but in the neighbourhood of the cooling ele-

ment. Thus, the region of intensive crystallization

will be positioned in the cavitation region and the

region of the most intensive mixing. Therefore, hy-

draulic shock waves generated by the collapse of

cavitation bubbles effectively break particles of the

already solidified alloys and acoustic streams gener-

ated by ultrasonic treatment will homogeneously dis-tribute these fine solid particles. When ultrasonic

vibrations are coupled to the solidifying metal, struc-

tural changes occur including grain refinement, sup-

pression of columnar grain structure, increased ho-

mogeneity and reduced segregation.

2. Calculation and design of waveguide system for

treatment of aluminium alloys

The transmission of ultrasonic vibration to thesolidifying melt is not an easy problem because a

resonator can rapidly fail under the effect of temper-

ature and cyclic stresses. For these purposes, special

ultrasonic equipment and technologies were devel-

oped. Ultrasonic units as a rule include at least five .blocks Fig. 1 , namely, the electric ultrasonic gener-

Fig. 1. Schematic diagram of the ultrasonic unit. UG s the electric

ultrasonic generator, MT s magnetostrictive transducer, WGSs

the waveguide system, TL stechnological load, CSscontrol sys-

tem.

. .Fig. 2. Ultrasonic oscillations a in rod waveguide system and b

in MTS.

. .ator UG , magnetostrictive transducer EMT , the .waveguide system WGS for transmission the ultra-

.sonic vibration from EMT to technological load TL . .technological block TB and control system CS .

Fig. 3. Pattern of deformation during the ultrasonic vibration with . .frequency of 25 kHz for MTS with c plane top and d spherical

top.


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( )V. Abramo et al.rMaterials Letters 37 1998 27 34 29

.Fig. 4. Schematic diagram of experimental set-up: 1 water-cooled

. .MTS, 2 magnetostrictive transducer, 3 ultrasonic generator.

To achieve a high level of technological efficiency

of ultrasonic units, two conditions must be fulfilledw x .1,3 : 1 The system UG EMTWGS is operated at

or in the vicinity of their electric and mechanicalresonance the frequency of electric current of the

UG s frequency of the natural resonance of the

EMT s frequency of the natural resonance of the. .WGS . 2 The matching of EMT and WSG must

Table 1 .Chemical composition of the AlSi alloys wt.%

Alloy Si Cu Mn Al

AlSi9 9.5 0.9 0.17 balance

AlSi12 12.9 3.1 0.3 balance

AlSi17 17.1 4.5 0.55 balance

Table 2 .Chemical composition of wrought alloy wt.%

Zn Mg Cu Zr Al

8.54 2.73 2.35 1.5 balance

ensure an effective transmission of the vibrationsthrough the contact area.

In order to improve the matching effect using a

larger radiating surface, it has been recently pro-w xposed 4 to convert the longitudinal vibrations of

EMT into vibration emitted radially from IR. In this

case not only the top of the resonator but all its

surface will effectively irradiate ultrasonic energy.

Therefore, for increasing the efficiency of the sys-

tem, longitudinal vibrations shall be converted to

radial ones. For transmission of radial vibration into

a load, the geometric size of IR must be calculated insuch way that the resonance of longitudinal and

radial modes of vibrations have been correctly ad- .justed. This Mode Transformation System MTS

w xhas been developed and recently filed as a patent 4 .

MTS is a tubular waveguide system which transfers

the longitudinal mode oscillations to those directed .perpendicular to the surface Fig. 2 of the tube and

irradiated to the liquid metal.

Experimental search of dimensions of waveguide

system is complicated and expensive. The use of

analytical methods of calculation of MTS is very

restricted because of complexity of the entire task.They can be used only for obtaining a first guess of

searching parameters to solve the entire problem

numerically. In order to simplify and speed up the

procedure of determination of waveguide system pa-

rameters together with or instead of experimental

work, computation of these parameters was carried

out.

The waveguide system model is always three-di-

mensional. In this case, the appropriate accuracy in

the frequency analysis can be usually achieved if the

large number of finite elements is used. Numerical

Table 3 .Chemical composition of the antifriction aluminium alloy wt.%

Sn Sb Cu Si Fe Al

7.1 -0.3 2.6 1.2 -0.15 balance


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. .Fig. 5. AlSi17 alloy cooling curves: a control, b solidified with ultrasonic treatment.

treatment of this problem is complicated because

only a restricted number of the lowest resonance

frequencies and corresponding modes can be com-

puted whichever numerical approach is employedw x5 . The finite element package COSMOS together

with graphical visualisation was used for the calcula-

tions. In order to design the most effective wave- .guide system, we examined two possibilities: a the

.tube with plane top; b the tube with spherical top.

Distribution of radial amplitude at 25 kHz for both

possibilities according to finite element calculation is

shown in Fig. 3. Our calculations show that in

comparison with the pipe with spherical top in case

of plane top, a large contribution into vibrationenergy is made by the top itself and vibration of the

top strongly dominates over the vibration of the pipe.

And what is more, in case of the plane top, it is

possible that vibration is exhibited only by the top .with small displacement of the pipe with frequency

close to 25 kHz. Therefore, for construction of our

waveguide system, we used the tube with spherical

top. The scheme of the ultrasonic system provided

with a water-cooled resonator is shown in Fig. 4.

The problem of construction and the choice of

materials for the waveguide system, which could run

during a long time in an aluminium melt, is also veryw ximportant and actual. Dobatkin and Eskin 3 used

ultrasound intensities in the range of 720 W cmy2

and found that carbon steel resonators were dis-

solved quickly in liquid aluminium whereas 18 wt.%

Cr9 wt.% Ti steel resonators have a live time ofonly 12 min. Niobium alloys proved to be more

w xresistant 6 . Laboratory tests were based on the use

of special benches, where ultrasonic vibrations were

. .Fig. 6. Microstructure of AlSi9 alloys: a control, b solidified with ultrasonic treatment.


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fed to the solidifying mold-held melt by top trans-

mission method. A 10 kW ultrasonic generator and

magnetostictive transducer were used in those tests.

3. Experimental procedure

The ultrasonic treatment was carried out in a

cylindrical ceramic crucible of approximate dimen-

sions 80 mm diameter and 160 mm height. The

thermocouple was positioned to record temperature

of the melt at a position between centre of the charge

and crucible wall, i.e., 30 mm from the crucible wall.

In our experiments we used hypoeutectical, eutecti-

cal and hypereutectical AlSi alloys, high-strength

wrought AlZnMgCu alloy, antifriction AlSn

alloy and technically pure aluminium. The chemical

compositions of the alloys are shown in Tables 13.

The antifriction behaviour of the AlSn basedalloys was tested using dry friction testing without

lubrication. These experiments are easier to perform

than the experiments with lubrication. Antifriction

testing was carried out using a friction machine type

SNTs-2 with axle-insert arrangement, sliding veloc-

i t y o f 2 m sy1 and pressure of 1.4 MPa. The

specimens used for testing the antifriction properties

were manufactured in the form of a disk with a

diameter of 45 mm, on which five grooves for

antifriction properties were made. During the tests

the roller slides on the surface of the groove. Theroller with diameter of 35 mm was made from 0.45%

carbon steel not heat-treated and having the hardness

HB 196200. The loss in weight of the sample and

the roller during the test is a measure of the wear

loss.

To study the thixotropic behaviour, control and

treated specimens were upset in electro-hydraulic

press under semisolid conditions. The specimens

measuring 20 mm=20 mm=10 mm were de-

formed at 5808C after heating and holding for 15

min. During the deformation, the height of the speci-

mens was twice decreased to 5 mm. The variation of

the deformation load during the upsetting was mea-

sured.

Tensile tests were done with an Instrone machine,

at a loading rate of 0.5 mm miny1. Tensile speci-

mens had working length-to-diameter ratio equal to

10.

4. Results and discussion

It was revealed that the use of the water-cooled

waveguide system for ultrasonic treatment increases

twice the solidification rate in comparison with the

control process. As an example, the cooling curves

of AlSi7 alloys are shown in Fig. 5. The structure

changes in solidifying metal are generally due to the

processes in the melt and the two-phase liquidsolid

zone, i.e., crystal nucleation, dispersion and mixing.

These processes depend on cavitation and streaming

as well as process factors and material properties.

The shock waves appeared during the collapse of thecavitation bubbles near the surface of the water-

cooled waveguide system, causing some crystals at

the solidification front to break down and move



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Table 4

Effect of cast structure on mechanical properties of specimens of

AlSi alloys

. .Alloy Casting technique UTS MPa El % HB

AlSi9 Conventional casting 200 2 91

AlSi9 Casting with UST 220 3 90

AlSi12 Conventional casting 190 0.5 97

AlSi12 Casting with UST 230 2.0 94AlSi17 Conventional casting 160 0.5 102

AlSi17 Casting with UST 180 1.8 98

toward the liquid bulk. In addition to breaking down

growing crystals, the ultrasound has also an effect on

the nucleation rate. There are two mechanisms of

cavitation effect on the nucleation rate. Cavitationactivates insoluble particles e.g., oxides, ultrafine

.particles of some intermetallics that exist in the

melt, and turns them into solidification sites. Frag-ments of destroyed dendrites also act as solidifica-

tion sites. Another mechanism is described by Ka-w xpustina 7 . During the expansion half-period, the

bubble rapidly increases in size, and the liquid evap-

orates inside the bubble. The evaporation and expan-

sion tend to reduce the bubble temperature. A de-

crease of bubble temperature below equilibrium tem-

perature results in melt undercooling at the bubble

surface, and hence in the probability that a nucleus

will be formed on a bubble. As a result of all these

effects, ultrasonic treatment of the melt reduced liq-

uidus undercooling from 2.18C to 0.98C. This changes

in the cooling curves shows that nucleation takes

place more easily.

Hypoeutectical, eutectical alloys prepared at con-

ventional casting have dendritic structure and silicon

Fig. 9. Variation of deformation load upon upsetting of AlSi17 . .alloy specimens at 5808C: a conventional casting, b casting

with ultrasonic treatment.

inclusions crystallise as hexagonal plates joined to-

gether at a centre into star-shaped pattern as they .appear in cross-section. Figs. 6 and 7 . By contrast,

ultrasonic irradiation refined the silicon crystals and

distribute them uniformly over the section. Most of

the silicon plates were dissembled and broken during

the ultrasonic treatment and forming spheroidized

crystals. The ultrasonically induced refinement of

silicon inclusions and dendrite-to-subdendrite struc-

ture change improves the mechanical properties.

Table 4 lists some mechanical characteristics as

strength, ductility and hardness of the alloys in both

conditions.

Various microstructure of hypereutectic Al17

wt.% Si alloys are shown in Fig. 8. It is clear that

prior to ultrasonic treatment, the primary Si crystals

within the alloy have the faceted morphologies.

However, ultrasonic treatment results in morphologi-



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. .Fig. 10. Microstructure of AlZn Mg Cu alloys: a control, b solidified with ultrasonic treatment.

cal changes of primary Si crystals from faceted to

spherical. Fragmentation of large primary crystals

followed by aggregation of the fragmented Si is

considered to be responsible for spheroidization of

primary Si crystals.

As shown in Table 4 as-cast hypereutectic AlSi

alloys demonstrated modest increase of strength and

decrease of hardness. Typically, the ultrasonic treat-

ment results also in an increase of the plasticity 3.5

times. The increase in ductility makes hot cracking

during casting less probable. Changes in microstruc-

ture of AlSi17 alloy facilitated the deformation of

such material in semisolid conditions. The variation

of deformation load during the upsetting of speci-

mens by 50% at temperature 5808C is shown in Fig.

9. The maximum stress of upsetting is 30% lower forultrasonically treated alloys. Probably, in this case

similar to the sliding of ultrafine grains during super-

plastic deformation in the solid state, one can ob-

serve the thixotropic sliding of nondendritic grains in

the semisolid conditions. Therefore, ultrasonic treat-

ment is promising for the preparation of thixocastible

hypereutectic AlSi alloys.

For high-strength wrought AlZnMgCu alloys,

microstructure analysis suggests that ultrasound

firstly increases the nucleation rate of solidification

sites and secondly, suppress segregation and ho-

mogenise chemical composition of as-cast material.

Ultrasonic treatment during the casting results in .dendrite-free grain structure Fig. 10 . This type of

the structure can only arise if the grain size is

smaller than or equal to the dendrite cell size pro-

duced at a given solidification velocity. Simultane-

ously, ultrasonic treatment leads to refinement of

inclusions of the Mg Zn and MgAlCu phases. As-2cast high-strength wrought AlZnMgCu alloyshows an increase of strength from 220 MPa to 250

MPa after ultrasonic treatment. Typically, the ultra-

sonic treatment results also in an increase of the

plasticity from 12 to 16%.

. .Fig. 11. Microstructure of pure aluminium: a control, b solidified with ultrasonic treatment.


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Pure metals have poorer ultrasonic tractabilityw xthan alloys. According to Ref. 1 pure aluminium

cannot be effectively treated using rod-type not wa-

ter-cooled waveguide system. But also here the struc-

ture changes during the solidification in an ultrasonic

field. Fig. 11 demonstrates that ultrasonic treatment

of high-purity Al substantially refine the microstruc-

ture, which results in better mechanical properties.

The tensile strength improves by 35% from 52 MPa

to 72 MPa in ultrasonically treated material and the

hardness increases by 14% from HB 17.2 to HB

19.7. The hardening is paralleled by an increase of

the elongation from 48 to 52%.

5. Conclusions

Our experiments demonstrated that practically all

the aluminium alloys of major commercial interestcan be ultrasonically treated using water-cooled

waveguide system. In conclusion, as regards the

ductility characteristics, it must be outlined that ul-

trasonically treated samples have elongation values

much higher than those obtainable with control pro-

cesses. Our investigations confirm great advantages

of ultrasonically treated ingots of hypereutectic AlSi

alloys upon deformation in semisolid state.

Acknowledgements

The authors wish to thank the VW Foundation .Contract No. Ir70871 and the INCO-COPERNI-

.CUS Programme Contract No. IC15-CT96-0740

for financial support.

References

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.High-Intensity Ultrasound on the Phase Interface in Metals ,

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Solidification of aluminium alloys under ultrasonic irradiation using water-cooled resonator

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