A R C H I V E S o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310)Volume 10

Issue 3/2010

175 – 178

35/3

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0 , I s s u e 3 / 2 0 1 0 , 1 7 5 - 1 7 8 175

The effect of casting porosities on cavitational erosion

of intermetallic alloy FeAl36

R. Jasionowski a,*, W. Przetakiewicz a, D. Zasada b, a Institute of Basic Technical Sciences, Maritime University of Szczecin, Szczecin, Poland

b Department of Metallurgy and Material Technology, Military University of Technology, Warszawa, Poland *Corresponding author: r.jasionowski@am.szczecin.pl

Received 30.04.2010; accepted in revised form 01.07.2010

Abstract

The machinery and equipment elements operating in a turbulent fluid flow, are exposed to destruction as a result of the impact of the cavitation, corrosion and abrasion processes, among which are hardest to minimize the imploding cavitation bubbles. Repeated cavitation implosions of bubbles give rise to cracks, material loss, resulting in increased flow resistance and reduction of the efficiency of the device, or even its destruction. In order to prevent or mitigate the cavitation phenomenon and its harmful effects, two basic methods are applied. The first of these is the selection of geometrical parameters and hydraulic machinery and the relevant elements of the streamlined shape and flow channels. The second solution is the selection of engineering plastics with greater resistance to cavitation. In case of materials manufactured with the casting method, a very important role is being played by the quality of manufactured casting having the smallest number of casting defects. The aim of the present study was to examine the effect of casting porosities of an intermetallic alloy FeAl36 on cavitational erosion. Keywords: cavitation erosion, cavitation, intermetallic alloys

1. Introduction Microstreams of liquid developed during the implosion of

cavitational bubbles as well as the action of pressure waves from disappearing bubbles are the main causes of destructions on swilled surfaces leading to a loss of material, i.e. to cavitational erosion. Recurrent implosions of cavitational bubbles induce development of a non-uniform stress state which causes the strengthening of surface layer and modification of the microgeometry of surface and cracks leading to a detachment of material particles. Progressive cavitational erosion on the swilled surfaces of machine and installation elements, i.e. water turbines, steam turbines, impeller pumps, screw propellers, cylinder liners of water-cooled engines, induces the destruction of these machine

elements, increase of the flow resistances, and reduction of the efficiency of the aforementioned installations [1-3].

The process of cavitational erosion induces destruction of the material, which consists in plastic strain, material losses, microstructure changes and surface micro- and macrogeometry changes. Diversity of this process causes the destruction of solid body in result of cavitation to be hardly predicted. The Apart from the intensity of cavitational phenomenon, the course of cavitational destruction depends also on properties of the material itself. The carried out analyses of many materials showed that cavitational resistance does not depends one strength parameter only. The process of cavitational destruction, having a fatigue character, causes that materials with larger resistance to cavitational erosion are first of all characterised by high hardness

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and micro-hardness as well as fine-grained one-phase structure having internal compressive stresses [4-6].

When comparing the cavitational resistance of one-phase and multi-phase materials, the former have considerably higher strength. Differences in the elastic and plastic properties of grains adjacent to each other have an unfavourable effect on multi-phase materials. The resistance of multi-phase material depends largely on the behaviour of a weakest phase from among those forming the material. Most frequently, places of the initiation of cavitational destruction in multi-phase materials are inter-phase and grain boundaries, i.e. the places with the largest concentration of internal stresses. An additional factor causing an increase in the rate of cavitational erosion is non-metallic inclusions, impurities, and material discontinuities hidden under the surface in the form of shrinkage porosities, pores and gas bubbles. Among one-phase materials, the structure of material (grain size and crystallographic texture) and the quality of manufactured casting (quantity and type of casting defects) have a fundamental effect on their resistance to cavitational erosion [7, 8].

The aim of the present study was to examine the effect of casting porosities of an intermetallic alloy FeAl36 on cavitational erosion.

2. Research methods

The study covered four samples with the FeAl36 intermetallic alloy with molybdenum, boron, zirconium and carbon microadditions. The chemical composition of the examined material and the selected mechanical properties are presented in Table 1.

Table 1. FeAl36 intermetallic alloys subjected to cavitation erosion

FeAl36 Element

Al Mo Zr B C Fe Density [kg/m3]

Hardness HV0,1

36,0 0,22 0,10 0,01 0,13 63,54 6255 297,34

The intermetallic FeAl36 alloy is in the cast state and had the α-state one-phase structure of the solid aluminium solution in iron (Fig. 1).

Fig. 1. Typical microstructure of FeAl36 alloy after cast

Element %Wt %At Al 21,56 36,35

Total 100,00 100,00

Zr 0,81 0,40 Fe 77,63 63,25

FeAl36

Fig. 2. EDAX analysis results on the FeAl36

The examination of cavitational erosion was carried out on a streaming-blowing apparatus. Samples for the examination were of the cylindrical shape, 20 mm in diameter and 6 ± 0,5 mm height. Sample surface roughness, measured by means of PGM-1C profilographometer, ranged 0,010÷0,015 μm. The samples were mounted vertically in rotor arms, parallel to the axis of water stream pumped continuously at 0,06 MPa through a nozzle with a 10 mm diameter, 1,6 mm away from the sample edge. The rotating samples stroke against the water stream. Water flow intensity was constant and amounted to 1,55 m3/h. The samples were examined for the period of 30 minutes, took out from the fixtures, degreased in an ultrasonic washer for 10 minutes at 30°C, dried in a laboratory drier for 15 minutes at 120°C and weighed, than mounted again in the rotor arms, maintaining the initial position in relation to the water stream. The analyses included four samples, examined for the priod of 3000 minutes.

3. Study results and their analysis The course of cavitational erosion in the examined FeAl36

alloys is very similar to each other. In the initial period, the water stream action on the surface of samples induces the strengthening of surface layer and the increase of micro-hardness. Also the plastic strain was observed on the surface of samples, manifested in the uplift and collapse of the adjoining grains. In the initial stage of analysis, the following effects were observed on the surface of examined samples: - cavities induced by the implosion of cavitational bubbles to be found near the sample surface or by the blow of very small impurities to be found in the water stream, - cracks on the material surface along the grain boundaries, - first single losses of material on the grain boundaries (Fig. 3.)

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a)

b)

Fig. 3. Effects of cavitational erosion of intermetallic FeAl36 alloy: a) uplift and collapse of the grain, b) mass decrement in

triple point and edgewise of the grain

Further exposure of the material surface to cavitational loading leads to development of cracks on the material surface. As a result of grain edge crushing, craters form along the cracks (Fig. 4). The erosion of sample surface increases, but the loss of material is still very small. After 1200 minutes of exposure, it merely amounts to about 1-2 mg. Figure 5 shows the state of intermetallic FeAl36 sample surface.

Fig. 4. Effects of cavitational erosion of intermetallic FeAl36 alloy - craters edgewise of the grain

Fig. 5. Effects of cavitational erosion of intermetallic FeAl36 alloys after 1200 min.

Further action of water stream on the surface of FeAl36 alloy samples induces its faster and faster destruction; the surface of samples is being covered with small-depth pitting. In addition, places with single pitting already occurring there enlarge their depth and area as affected by cavitational loading, where grain boundaries are harder and harder to be detected.

Large divergence in the kinetics of destruction of FeAl36 samples is observable after an 1800-minute test (Fig. 6).

mass loss Δm [mg]

time [min]

0 1000 2000 3000

0

20

40

60

80

100

Fig. 6. Results of cavitational erosion intermetalli alloy FeAl36

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This was induced first of all by casting defects hidden under the surface of samples (Fig. 7). The effect of porosity induced an increase in the rate of mass loss through detachment of whole grain agglomerates and development of cavitational erosion inside the material, causing deep pitting on the material surface (Fig. 8). The mass loss in FeAl36 alloy samples having hidden casting defects under the surface is 8 times larger when compared to those without them.

a)

b)

Fig. 7. Casting defects hidden under the surface of samples FeAl36

4. Conclusion

For decades, interesting materials of the new generation are the alloys based on ordered intermetallic FeAl. They have several unique properties such as high strength, resistance to oxidation and corrosion in an aggressive environment, the structural stability in a wide temperature range, low smelting, cost, high abrasion resistance, high elasticity modulus, high melting point and confirmed in laboratory studies 8-10 times higher resistance to cavitation erosion of materials currently used in flow-through devices.

Alloys with the intermetallic phase matrix owe an increase in the resistance to cavitational erosion first of all to their one-phase structure and large hardness. A factor responsible for such alarge resistance is also the high quality of manufactured casting having the smallest number of casting defects. In order to avoid casting defects in cast FeAl alloys, it is necessary to control the technological process (proper selection of temperatures) or to apply other casting method, e.g. the centrifugal method.

Literature [1] C.E. Brennen, Cavitation and Buble Dynamics, Oxford

University Press, 1995. [2] L.J. Briggs, The Limiting Negative Pressure of Water, Journal

of Applied Physics, Vol. 21 (1970) 721-722. [3] D.H. Trevena, Cavitation and tension in liquids,

IOP Publishing Ltd, 1987. [4] M. Głowacka J., Hucińska, Stan badań nad niszczeniem

kawitacyjnym stopów metali i ich ochroną przed tym procesem, Inżynieria Materiałowa, 2(2001)79.

[5] K. Steller, O mechanizmie niszczenia materiałów podczas kawitacji, Zeszyty Naukowe Instytutu Maszyn Przepływowych PAN, Gdańsk 1983, Nr 175/1107/83.

[6] R. Jasionowski, W. Przetakiewicz, D. Zasada, J. Grabian, Wpływ nadtapiania laserowego na zużycie kawitacyjne wybranych stopów, VI Konferencja Naukowa „Obróbka Powierzchniowa 2005”, Kule, 20-23.09.2005.

[7] A. Karimi, J.L. Martin, Cavitation erosion of materials, International Metals Reviews, Vol. 31, No. 1 (1986), 1-26.

[8] J. Steller, International Cavitation Erosion Test and quantitative assessment of material resistance to cavitation, Wear, Vol. 233-235, p. 51-64.

Fig. 8. Deep pitting on the material surface of samples FeAl36