322 composite material technology iv · figure 2: schematic diagram of a stand for section...

Aluminium - SiC ceramic particles

composites. The technology of shape

composites production.

J. Gawroriski, M. Cholewa, J. Szajnar

Foundry Institute, Silesian Technical University,

44 100 Gliwice, Towarowa 7, Poland

ABSTRACT

In a paper some problems concerning section aluminium -ceramic particlescomposites production are discussed in a case of AlMg2-SiC. Silicon carbidesurface activator has been used in production technology and two stages ofstirring: first mechanic stage with an agitator inside a furnace for meltingcomposite base with reinforcing material, and second inside a casting mouldby means of rotating reversing magnetic field (RRMF). Stirring assisted withRRMF enables to obtain any shape of composite casting, which is illustratedby an example of cruciform cast, where whirlpool movement of liquidcomposite in cross arms is limited, nevertheless achieved SiC particlesdistribution homogeneity is good.

INTRODUCTION

Composites of aluminium alloys and ceramic particles make constructionmaterials of very wide range of applications, e.g.: abrasion wear resistantelements, sliding parts, wearing quality elements and workable castings [4].Such composites base could be typical casting alloys as silumin or workablealloys. Workable alloys, as for instance AlMg2 features bad technologiccasting properties (low runnability, big solidifying contraction), but presenceof Mg causes increased wettability of ceramic particles by liquid ground massdue to high chemical affinity of Mg to SiC as well as due to surface activationof SiC caused by Mg. That is why there is a sense to design composites of atype workable aluminium alloys-ceramic particles (SiC) not only because ofground mass properties (good mechanic properties, high corrosion resistance,good weldability, deep drawing ability), but also because of high abrasive wearresistance [2]. In that way composite features a set of properties which isimpossible to obtain in standard technologies such as heat treatment forinstance.

Transactions on Engineering Sciences vol 4, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533

322 Composite Material Technology IV

OWN RESEARCHES CARRIED OUT

Own researches of AlMg2-SiC composite production were carried outaccording to data in Table 1.

Table 1: Scope of researches of AlMg2 - SiC composite

Variability of technologic parameters Mechanic properties of composite

- ground mass temperature- SiC quantity- SiC grain size- quantity of surface activator- magnetic field intensity

- compressive strength- hardness, HB- impact strength- abrasive wear

Table 2 contains a list of technologic parameters of composite productionprocess changeability and changes ranges.

Table 2: Composite production process technologic parameters variability

Sample No.

0

01

1

2

3

4

5

6

7

8

9

10

11

M

i[%]

0

0

1

3

1

3

1

3

2

2

2

2

2

3

T[°C]

720

740

740

700

700

740

720

720

720

720

700

740

720

720

Zfom]

0

0

<40

<40

100

100

63

63

<40

100

63

63

63

various

Comments

magnetic field on

magnetic field off

magnetic field on

as above

as above

as above

as above

as above

as above

as above

as above

as above

as above

as above

i - quantity of SiC, M - mixture of various granulations of SiC,T - mixing and pouring temperature, Z - granulation of SiC,


Composite Material Technology IV 323

Description of own researches courseAlMg2 composite ground mass alloy was melted in medium frequencyinduction furnace using protecting and refining slag. Stirring of the ceramicparticles into molten metal was carried out inside of a furnace at overheattemperature for AlMg2 alloy (see Tabl. 2) by means of a frame stirrer, withrotation speed 1000 rpm (stage I till ceramic particles introduction andsinking), and 250 rpm (stage II after sinking of ceramic particles). Total timeof stirring of composite inside a furnace was about 3 minutes.

SiC particles with required granulation were thermally activated at 1200 °C,cooled, then coated with a surface activator: water solution of sodium andbarium oxides. Surface activator was prepared from chemically purecomponents: boric acid and borax as 4% water solution of barium and sodiumoxides.Composite was cast into shell moulds. Shape of section castings is presented

in Fig. 1.

4-

CNJ

2*

Figure 1: Geometric form of section composite of "X" and "T" type.

Shell mould was situated on a stand for casting inside reverse vortex magneticfield and was cast with composite in magnetic field of constant induction Bvalue = 50 mT, as measured on a surface of the casting (Fig. 2).Section castings production technology of composites assisted with in-mouldstirring is an original solution of The Foundry Institute of the SilesianTechnical University [5J.

Microscopic, mechanical and wear-out researches resultsTo check distribution and wettability of SiC particles by ground mass, 0 and



01 specimens were used as standards without any ceramic particles, whichwere cast with and without magnetic field. Specimens 2, 4 and 7 were used ascomposites with variable number of particles and sizes (variable granulation)cast in variable pouring temperature.

Figure 2: Schematic diagram of a stand for section composite casting;1) Medium frequency induction furnace, 2) SiC dispenser, 3) Casting mouldwith a cruciform cavity, 4) Rotational reversing magnetic field generator.

In Fig. 3, 4 and 5 microstructures of composites are presented. Excellentwettability can be observed of SiC with AlMg2 alloy ground mass. Quantityof surface activator is somewhat too high, it is visible in some microsectionsnot only close to SiC but inside ground mass as well. It undoubtedly influencessection composites mechanic properties. In Fig. 6 SiC dispersion particlesdistribution is shown inside cruciform section casting - in cross arms and in itscenter, where vortex axis of magnetic field was. The aim of this research wasto confirm a fact known earlier [1] of effective stirring of molten metal insidecasting mould by means of reverse and vortex magnetic field in areas of a



mould which are situated outside of the vortex axis - inside of distant from axiscast walls as well as in the center of the cast where vortex axis is situated.Such statement has profound significance in production process of sectioncastings which are not bodies of revolution.Carried out researches of hardness (Fig. 7) and abrasive wear resistance

made by means of Skoda Savine (Fig. 8) machine also confirm favourabledistribution of SiC in castings.

Fig. 3: AlMg2 alloy structure -cruciform section casting withoutmagnetic field influence. Magn.20 x.

J t"-v;\<A « *-, ^ r _^ y .":*f.t' $kt * ''

Fig. 4: AlMg2 alloy structure- cruciform section castinginfluenced with magnetic field- refinement of structure canbe seen. Magn. 20 x.

Fig. 5: AlMg2-SiC compositestructure, cruciform casting -good wettability of SiC byground mass shows. Magnif.120x.

Beside basic stirring inside a furnace or crucible additional stirring carried outinside a casting mould, where forced movement of liquid suspension groundmass - dispersion addition stops, allows - especially inside thicker walls ofsection composite - favourably and homogeneously distribute dispersiveparticles reinforcing ground mass, even when pouring temperature is



Figure 6: Section composite casting cross-section along horizontal plane (inhalf-height) with marked places where samples for polished sections weretaken. Homogenous distribution of SiC can be seen. Magn. 20x.

comparatively high, conductive to separation. So in-mould stirring protectsseparation of even very well wetted SiC particles by a ground mass.Such phenomenon was acknowledged in case of other composites: aluminium-Cgraphite, aluminium-SiC, aluminium-titanium carbides, and so on.Course of researches presented in table 2 was carried out for low quantity

of dispersive addition introduced to ground mass, not exceeding 3% of SiC.



7 6 (

Place¥ *

T «4 »

».._.+._+_i

HI5/25C

,

1 2# # A

3t 4

t *

/ '

3)/30

Pla

1

39.3

6

37.:

ce

HB5/250/30

)

2

38.6

7

40.,

1

39.6

11

2

3 4 5

37.1 35.7 34.3

HD.v = 37.65

2 3 4 5

38.0 37.4 37.2 39.2

IB_. = 38.4

Figure 7: Distribution of hardness in section composites (casting X and T type).

Sample 4

c i cSample 5

Place

RRK

0.1

1

796.47

0.1

2

720.23

0.1

3

760.51

Place

R%

1

1.58

2

1.61

3

1.55

= 0.47

R, R% - coefficient of wear resistance [I/mm*](Rk with addition SiC - composite)

Figure 8: Abrasive wear resistance measurement in horizontal cross-section ofcruciform composite.

It is neither a limit of technologic possibilities, nor caution against segregationof SiC occurrence. Earlier check-out researches for 10% addition of SiCshowed high homogeneity of dispersion particles distribution. But mechanic



workout of composites above 3-4% of SiC content is almost impossible,especially cutting. So further increase of disperse addition up to 10% of SiCis possible only if composite product is plastic worked for removal of porosityof composite and reinforcement ordering [2].

Section composites cast have imperceptible gas caused porosity becauseduring ground mass melting slag refining and degassing is carried out.Magnetic field aided stirring during pouring and inside a mould does not caseany significant changes in gas content.

OBSERVATIONS AND CONCLUSIONS

1) Introductory as well as basic researches confirmed usefulness of boron andsodium oxides as surface activators for SiC particles, facilitating theirwettability by section composite alloy ground mass. High usefulness of surfaceactivator was also confirmed in AlSi-Cgraphite composite researches [3].2) Production of section composites shaped as utility castings are is possiblewhen ground mass is stirred with dispersive particles by means of standardmechanic mixer before pouring mould as well as inside a mould by means ofmagnetic field.3) Mechanic properties - especially hardness of composite - does not differmuch from ground mass hardness without SiC disperse particles added,because small quantity of dispersive addition causes that during hardnessmeasurements test ball "squeezes" disperse particles in soft ground mass.4) Wear-off tests show that even small increase of SiC quantity from 1 % to3% causes two times increase of abrasive wear resistance of a composite. Itsuggests that wear resistance can be controlled with SiC addition over a widerange.5) Utility section composites obtained ready from a mould create possibility tominimize necessary mechanic fettling work of castings, what in case of"hardening" dispersive additions have big industrial significance.

REFERENCES

1. Gawroriski, J. and Szajnar, J. Postapy Technologii Maszyn i UrzQdzeri,Polish Academy of Sciences periodical, Vol. 3 - 4, pp.21 - 32, 19872. Sledziona, J/Aluminium Composites-Ceramic Particles - Production andProperties', Proceedings I Polish Conference on Composite Metal Materials,Krakow, Poland, 1992.3. Gawronski, J. and Cholewa, M. 'Aluminium-graphite Dispersive Compositewith Favourable Tribologic Properties', Proceedings I Polish Conference onComposite Metal Materials, Krakow, Poland, 1992.4. Majmudar, B.S. et al., Mater. Sc. Eng. 68, 1984, pp. 85 - 925. Gawronski, J. and Cholewa M., Patent PL No. 157721, 1993


322 composite material technology iv · figure 2: schematic diagram of a stand for section...

Documents