Niatrep Lanruoj Eht Ot Ylppa Taht Sremialcsid

8/14/2019 .Niatrep Lanruoj Eht Ot Ylppa Taht Sremialcsid

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Accepted Manuscript

I n f l u e n c e o f v i b r a t i o n o n t h e s o l i d i f i c a t i o n b e h a v i o u r a n d t e n s i l e p r o p e r t i e s o f

a n A l - 1 8 w t % S i a l l o y

G . C h i r i t a , , I . S t e f a n e s c u , D . S o a r e s , F . S . S i l v a

P I I : S 0 2 6 1 - 3 0 6 9 ( 0 8 ) 0 0 3 8 4 - 1

D O I : 1 0 . 1 0 1 6 / j . m a t d e s . 2 0 0 8 . 0 7 . 0 4 5

R e f e r e n c e : J M A D 2 0 6 1

T o a p p e a r i n : M a t e r i a l s a n d D e s i g n

R e c e i v e d D a t e : 2 2 M a y 2 0 0 8

R e v i s e d D a t e : 1 8 J u l y 2 0 0 8

A c c e p t e d D a t e : 2 2 J u l y 2 0 0 8

P l e a s e c i t e t h i s a r t i c l e a s : C h i r i t a , , G . , S t e f a n e s c u , I . , S o a r e s , D . , S i l v a , F . S . , I n f l u e n c e o f v i b r a t i o n o n t h e

s o l i d i f i c a t i o n b e h a v i o u r a n d t e n s i l e p r o p e r t i e s o f a n A l - 1 8 w t % S i a l l o y , M a t e r i a l s a n d D e s i g n ( 2 0 0 8 ) , d o i : 1 0 . 1 0 1 6 /

j . m a t d e s . 2 0 0 8 . 0 7 . 0 4 5

T h i s i s a P D F f i l e o f a n u n e d i t e d m a n u s c r i p t t h a t h a s b e e n a c c e p t e d f o r p u b l i c a t i o n . A s a s e r v i c e t o o u r c u s t o m e r s

w e a r e p r o v i d i n g t h i s e a r l y v e r s i o n o f t h e m a n u s c r i p t . T h e m a n u s c r i p t w i l l u n d e r g o c o p y e d i t i n g , t y p e s e t t i n g , a n d

r e v i e w o f t h e r e s u l t i n g p r o o f b e f o r e i t i s p u b l i s h e d i n i t s f i n a l f o r m . P l e a s e n o t e t h a t d u r i n g t h e p r o d u c t i o n p r o c e s s

e r r o r s m a y b e d i s c o v e r e d w h i c h c o u l d a f f e c t t h e c o n t e n t , a n d a l l l e g a l d i s c l a i m e r s t h a t a p p l y t o t h e j o u r n a l p e r t a i n .
http://dx.doi.org/10.1016/j.matdes.2008.07.045http://dx.doi.org/10.1016/j.matdes.2008.07.045http://dx.doi.org/10.1016/j.matdes.2008.07.045


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INFLUENCE OF VIBRATION ON THE SOLIDIFICATION BEHAVIOUR AND TENSILE

PROPERTIES OF AN Al-18wt%Si ALLOY

Chirita, G.1, Stefanescu, I

2, Soares, D.

1, Silva, F.S.

1

1 Mechanical Engineering Department; School of Engineering, Minho University, PORTUGAL2Faculty of Mechanical Engineering, Dunarea de Jos University Galati, ROMANIA

________________________________________________________________________________

Abstract. This paper is concerned with the influence of vibration on mechanical properties ofcastings. The main vibration effects include: promotion of nucleation and thus reducing as-cast grain

size; reduction of shrinkage porosities due to improved metal feeding; and production of a more

homogenous metal structure. In the present study, mechanical mold vibration was applied to an AlSi hypereutectic alloy at fixed amplitude and different frequencies. Tensile tests were done on

specimens obtained with the different vibrating frequency levels. Experimental results show thatmechanical properties were influenced by the level of applied frequency. The tensile strength was

improved for low vibration frequencies but decreased for high frequencies, as compared with gravity

castings without vibration. A microstructure analysis along with a solidification behavior study wasperformed in order to understand the mechanism responsible for the previous behavior. A heat-

transfer mechanism, that is acceleration dependent, seems to be the responsible for the shift inmechanical properties response to the vibration effect.

Keywords: vibration; acceleration; mechanisms.

________________________________________________________________________________

1. INTRODUCTION

Due to many advantages such as good thermal conductivity, excellent castability, high strength-to-weight ratio, wear and corrosion resistance, pressure tightness and good weldability, aluminium-

silicon alloys are considered one of the most commonly used foundry alloys. Controlling the

microstructure that results from the casting process is considered one of the main challenges faced

by todays foundry industry. Fine equiaxed microstructures generally exhibit favourable mechanicalproperties of strength and ductility with low susceptibility to microporosity and cracks.The use of mechanical, sonic or ultrasonic vibration may have the advantage of promoting grain

refinement, increased density, degassing, low shrinkage porosities, and changes of the shape, size

and distribution of the second phase [1-9].Regarding the vibration effect in microstructure it is documented [1] that applying mechanical

vibration to a mould during solidification may have an effect on mechanical properties of the

casting. The responsible is the microstructure where the lamellar spacing tends to reduce and silicon

morphology becomes fibrous with the increasing of the vibration amplitude as compared to gravity

casting. However, it is also reported that exceeding a critical value of vibration amplitude, thesilicon tends to coarsen [1]. Fragmented primary dendrites with thicker dendrite arm thickness and

reduced solidification time were obtained on Al8% Si with rectilinear vibration by transforming

rotary motion of a DC motor, 100 cycles/min ( 2 Hz). The same level of vibration was applied toAl12% Si and it was reported a reduction of the eutectic cell size from 5 to 1.6 mm and a tendency

of coarsening of eutectic Si [2]. Significant reduction in gas content was obtained with lowfrequency melt agitation in Al20Si [3]. With an applied vibration at a constant frequency of 100

Hz and different amplitudes from 18 to 199 m an increase between 19 and 68 %, in percentelongation was reported while ultimate stress had a slight change, about 3% [1]. The increase in

elongation was correlated with the increase in the amount of eutectic volume fraction compared to

the non-vibrated case [1]. In another study [9] the amount and size of pores were increased in LM25


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[AlSi 7,15%] and LM6 [AlSi 12,30%] alloys with increasing frequencies between 15 and 41.7 Hz

and amplitudes between 0,125 and 0,5 mm.

Thus, it is clear that vibration may promote changes in microstructure and consequently in

mechanical properties, either increasing or decreasing it. However the mechanisms under which

those changes occur are still unclear. This work proposes a mechanism that is able to explain thereason for the shift on metallurgical and mechanical properties with vibration acceleration.

2. EXPERIMENTAL METHODS AND MATERIALS

Materials

The material used for castings is a commercial AlSi18 alloy with the following chemical

composition (wt%): 18Si22; Fe0,75; 1,5Cu3,0; Zn0,2; Mg0,1; Mn0,3; Ni0,5; Pb0,1;Sn0,05; Ti0,2.

Methods

The material was melt at 8000C and poured into a permanent mould which was preheated at 130

0C. A high frequency induction furnace (Titancast 700 mP Vac, from Linn High Term, Germany),

equipped with a vacuum chamber, was used for melting. A charge of approx. 240g of material wasused in each melting, always performed under vacuum. After melting in the induction furnace thematerial was poured into the mould which was attached on the system that provides the mechanical

vibration due to the eccentric of the shafts (Fig.1). The vibration was linearly applied with 0,5 mm

amplitude for all castings and different frequencies, namely 0 Hz, 8 Hz and 24 Hz.The obtained castings were heat treated with a temper during 8h at 200C.

For the analysis of the solidification behavior two thermocouples type K were attached to the mouldin order to acquire the temperature during the pouring and solidification of the material. Two holes

were done in the wall of the mould in which thermocouples were inserted at a distance of 2 mm

from the inside wall surfaces. The positions of the inserting points of the thermocouples are shown

in figures 2 and 3.

Specimens for mechanical tests were cut from each casting in three slices (each slice indicates the

position in the mould) in order to compare the properties of the aluminum alloy not only betweendifferent frequencies but also in different places of the ingot. (Fig.4).

Tensile tests were done in a Dartec tensile testing machine at room temperature. After doing thetensile tests the specimens were cut close to the fracture area and polished. An optical measurement

method was used to do the phase quantification and microstructure analysis. The microstructureexamination was made on the middle part of specimen slice 1 of each casting.

3. RESULTS

Rupture strength and rupture strain results are shown on Figs. 5 and 6 respectively. It is clear on Fig.5 that rupture strength increases in about 31% for the casting with 8Hz in any position, when

compared to simple gravity castings, and that rupture strength decreases in about 13% for castingswith 24Hz of vibration, as compared to simple gravity castings, again in all three positions. It is alsoobserved that there is a tendency of properties to increase from position 3 to position 1 in the case of

vibrated gravity with 8 Hz and 0 Hz but with no tendency on vibrated gravity with 24Hz.

Regarding rupture strain results, Fig. 6 shows that rupture strain is much higher (95%) for thevibrated gravity process with 8Hz in any position then vibrated gravity with 8Hz and 0Hz. It is also

clear a tendency of increasing properties from position 3 to position 1 in all the casting processes.


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Thus mechanical properties increase with vibration till a certain level frequency (acceleration) but

decrease for higher levels of vibration frequencies.

Figs 7 and 8 show the temperature readings during solidification for the three frequencies used. It is

clear that the maximum temperatures reached in the mould, in both positions (down and front) are

obtained on the vibration level of 8 Hz. For 0Hz and 24 Hz results are closer and different in thetwo different positions (down and front).

On table 1 (also represented on fig. 7) are quantified both the solidification starting point and thesolidification intervals (difference between starting and ending solidification points) for the threevibration levels in position down (results were quantified in this position because the starting and

ending solidification points are more clearly identified. It is shown on fig. 7 the starting and endsolidification points. The starting solidification point is the point when there is an increase in

temperature resulting from latent heat of fusion of the first phase to solidify. The ending

solidification point corresponds to the last inflection temperature point indicating the end of thelatent heat of fusion release of the eutectic constituent.

It is observed that for 8Hz the solidification starts first and last less. 0Hz and 24 Hz have closer

results for both the solidification starting point and interval.

Table 1 Solidification characteristics for the three vibration frequencies:

solidification starting time and solidification interval.Reading position: Down

Solidification frequency (Hz) 0 8 24

Starting Solidification Point (s) 9,6 5,0 8.5

Solidification Interval (s) 9,5 9,3 10.5

In Fig 9 is observed that, considering the 0 Hz frequencies as a comparison value, the amount ofeutectic increases for the 8Hz test in about 11% and decreases in about 8% for the 24 Hz test. This

happens for the three tested positions. The volume fraction of-Al phase shows the opposite effect

since the alloy has essentially these two phases (Fig. 9).The presence of isolated alpha phase in the microstructure can be attributed to a high undercooling

level associated with the alloy and casting process characteristics [10].

Fig 10 provides the silicon lamellae thickness. It is observed that the thinner values are observed for

the 8Hz test and the thicker ones are for the 24Hz, in all three positions.

Fig. 11 provides the microstructure pictures for the different vibration levels (0Hz, 8Hz and 24Hzfrequency). Basically, a thinner structure is found for 8Hz and courser structure for 24 Hz. It can

also be observed that the morphology of eutectic silicon was modified from a dispersed coral likeform (0Hz) to a more finely and nested coral like form (8Hz) and to a coarse acicular plate-like form

with less coral like eutectic form for 24Hz.

4. DISCUSSION

Mechanical results depend on metallurgical features which itself depend on solidification behavior.

Starting from the obtained solidification curves it is clear that the 8Hz test is the one that starts andcompletely solidify first (and also has the smaller solidification interval)(table 1). Thus it is

expected a thinner microstructure in these castings (Figs. 10 and 11), a higher amount of eutectic

(fig. 10)[8] and, as a consequence, better mechanical properties (both rupture strength and rupturestrain)(Figs. 5 and 6). Regarding the comparison between 0Hz and 24 Hz, the results provide the

same reasoning: 24 Hz is the one that takes longer to solidify (table 1); has the coarsestmicrostructure (Figs 10 and 11); has the lowest amount of eutectic (Fig. 10); and consequently has

the poorer mechanical results, in particular tensile strength (figs 5 and 6). Thus, there is a perfect

correlation between solidification behavior, microstructure, and mechanical properties.


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These results are basically in accordance with refs [1-2], and [8]. Furthermore it is also observed a

peak shift in mechanical properties, after a certain frequency (acceleration), as reported in [1].

The differences between position 1 and 3 are explained by the distance to the mould wall and

consequent solidification behavior. The material in position 1 solidifies first and then has the

thinner microstructure (Fig. 10), the higher amount of eutectic phase (Fig. 9), and consequently thebetter mechanical properties (Figs. 5 and 6), among all positions. Position 3 obviously has the lower

properties.The main aspect is therefore to find an explanation that may provide a physical explanation for thephenomena of the mechanical properties shifting point with vibration level.

5. PROPOSED MECHANISM

These results may be explained based on heat transfer mechanisms both in the liquid phase and inthe mould wall interface. The proposed mechanism is schematically provided in Fig. 12, and is the

following:

In the case of 0Hz casting, there is a normal contact between liquid metal and mould. The heattransfer is normal and the solidification rate has a certain value giving rise to a determinedmicrostructure;

In the case of 8 Hz the vibration induces a higher heat transfer to the mould (higher cooling rate)due to the alternated movement of the liquid (see Figs. 7 and 8 with higher initial solidification

temperatures in the mould). Furthermore this movement may also provide displacement of thegermen solidification sites providing a higher solidification rate. The contact between liquid

metal and mould is about the same as for the 0Hz case. However as the heat convection in the

liquid phase is improved by the vibration movements the liquid temperature near the interface is

higher what explains the higher temperatures in the mould obtained in the cooling curves for

these experiments (fig. 7 and 8). The lower cooling times (table 1) resulted from the faster liquidcooling rate and the improved nucleation characteristics by germen distribution in the liquid.

The consequence is that the microstructure is thinner (see Figs. 10 and 11) and the amount of

eutectic is higher (see Fig. 9). Consequently rupture strength and rupture strain increase (seeFigs. 5 and 6);

In the case of 24 Hz the vibration should induce even a higher heat transfer inside the liquid tothe mould interface due to the quicker alternated movement of the liquid. However, the relative

acceleration between liquid metal and mould seems to be high enough in order to create a lossof contact (low pressure zones in the interface liquid metal-mould)(in front position) and even

eventually low pressure bubbles as those originated in cavitation. This situation occurs when the

surface tension (liquid mould wall) is not low enough in order to keep the surface contactwhen high acceleration rates (frequency*amplitude) exist. A loss of contact means to switch the

heat transfer mode from totally conductive to a conductive + convective transfer in the interface.Because the convective heat transfer is much lower than the conductive heat transfer the heat

transfer and consequent solidification rate substantially decreases (see mainly Fig. 8 with lowerinitial solidification temperature in the mould). The consequence is a courser microstructure (seeFigs. 10 and 11) and a lower eutectic content (see Fig. 9). Mechanical properties should then

decrease (this is particularly evident for rupture strength)(Fig. 5 and 6).Although with the same consequence, in the down position a different mechanism may occur.

There is a loss of contact due to the tangential movement of the liquid (in front position there is

a normal contact see figs. 1 and 3 with vibration direction and temperature readings/walls

positions and detail on Fig. 12). This loss of contact is due to the liquid movement and

roughness effect (see detail on Fig. 12).


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This means that vibration can be beneficial as well as detrimental depending on the acceleration

level between liquid metal and microstructure and surface tension of the liquid metal and mouldmaterial. This may be the reason why some papers attribute to vibration beneficial effects and others

detrimental effects on properties.

5. CONCLUSIONS

Solidification behavior as obtained by solidification curves, microstructure analysis and mechanical

results seem to point out that:

Vibration affects the solidification rates and its characteristics; Vibration has an influence on mechanical properties; Its influence seem to be due to heat transfer related aspects; Vibration increases heat transfer in the liquid; Vibration may substantially reduce heat transfer in the interface metal-wall due to the loss of

contact under mainly two hypothetic mechanisms: high surface tension and wall roughness

effect.

6. ACKNOWLEDGEMENT

The research presented here was carried out in Materials Testing Laboratory of the Mechanical

Engineering Department of University of Minho, and was supported by Fundao para a Cincia e

Tecnologia (Portugal)through the PhD grantwith the reference SFRH / BD / 19618 / 2004.

7. REFERENCES

[1] Abu-Dheir Numan, Marwan Khraisheh, Kozo Saito, Alan Male, Silicon morphology

modification in the eutectic Al-Si alloy using mechanical mold vibration, Materials Science andEngineering A393 pp. 109-117, (2004).[2] T.P. Fisher, Effects of vibrational energy on the solidification of aluminium alloys, Br.

Foundryman 66 (3), 7183, (1973).

[3] A.A. Ivanov, G.G. Krushenka, Preparation of AlSi alloying composition by means ofvibration, Liteinoe Proizvod (3) (1993) 78 (Russian); Met. Abs. 46-0019,(1992).

[4] X. Jian, T.T. Meek, Q. Han, Refinement of eutectic silicon phase of aluminum A356 alloyusing high-intensity ultrasonic vibration, Scripta Materialia 54, 893896, (2006).

[5] F.C. Robles Hernandez, J.H. Sokolowski, Comparison among chemical and electromagnetic

stirring and vibration melt treatments for AlSi hypereutectic alloys, Journal of Alloys andCompounds 426, 205212, (2006).

[6] M.T.Alonso Rasgado, K. Davey, The effect of vibration on surface finish for semisolid and cast

components,Journal of Materials Processing Technology, Vol. 125-126, 543-548, (2002).[7] M.T. Alonso Rasgado, K. Davey, Vibration and casting surface finish, Journal of MaterialsProcessing Technology 153154, 875880, (2004).

[8] Chirita, G.; Stefanescu, I; Barbosa, J.; Puga, N.; Soares, D., Silva, F.S., On the Assessment OfPrecessing Variables In a Vertical Centrifugal Casting Technique, submitted to International

Journal of Cast Metals Research

[9] Kadir Kocatepe, Effect of low frequency vibration on porosity of LM25 and LM6 alloys,Materials and Design, Vol. 28 Issue 6, 1767-1775, (2006)


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[10] H.S. Kanga, W.Y. Yoon, K.H. Kimb, M.H. Kimc, Y.P. Yoon, Microstructure selections in the

undercooled hypereutectic AlSi alloys, Materials Science and Engineering A 404, 117123,

(2005)


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Fig. 1 - Mechanical vibrating device. e - excentricity

e

Vibration direction


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Fig. 2 Thermocouples positions on the mould


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Fig. 3 Temperature reading positions in walls of the mould: 1 Down; 2 - Front


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Fig. 4 - Position of the specimen slices in castings. Positions 1, 2, and 3.

1

2

3

30 mm

30 mm

90 mm

pouring

direction


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0

20

40

60

80

100

120

140

160

180

200

1 2 3

Stress

[MPa]

Position

Rupture strength Vibration 8HzVibration 24Hz

Vibration 0Hz

Fig. 5 - Rupture strength for 0Hz, 8Hz and 24Hz frequencies for the three sample positions.


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0.0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3

Strain[%

]

Position

Rupture strain Vibration 8HzVibration 24Hz

Vibration 0Hz

Fig. 6 - Rupture strain for 0Hz, 8Hz and 24Hz frequencies for the three sample positions.


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100

150

200

250

300

350

400

0 5 10 15 20 25 30 35 40 45 50

Temperature

[C]

Time [s]

DOWN

Gravity

8Hz

24 Hz

Fig. 7 - Solidification curves for the different vibration levels on the down position of the mould

Temperature

acquisition point

on the mould


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200

220

240

260

280

300

320

340

0 5 10 15 20 25 30 35 40 45 50

Temperature

[C]

Time [s]

FRONTGravity

8Hz

24 Hz

Fig. 8 - Solidification curves for the different vibration levels on the front position of the mould

Temperature

acquisition point

on the mould


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0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4

Volumefraction[%]

Position

Al18Si

(Al) phase-Vibration 24HzEutectic-Vibration 24Hz(Al) phase-Vibration 8HzEutectic-Vibration 8Hz(Al) phase-Vibrat ion 0HzEutectic-Vibration 0Hz

Fig. 9 Phases volume fraction for the three different vibration levels and tested positions.


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0

0.5

1

1.5

2

2.5

3

0 1 2 3 4

Thicknes

s

[mm]

Position

Gravity

Vibrated gravity 8Hz

Vibrated gravity 24Hz

Fig. 10 Eutectic silicon lamellas thickness for the three different vibration levels and tested positions.


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0Hz 8Hz

24Hz

Fig. 11 Microstructure of castings with vibration at different frequencies (500x)


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Fig. 12 Heat transfer variation with acceleration and mechanism representation.

Front position

Down position

Liquid movement

direction - tangential

Liquid movement

direction - normal

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Niatrep Lanruoj Eht Ot Ylppa Taht Sremialcsid