Journal of Materials Processing Technology 162–163 (2005) 127–130

The structure and properties of sinters produced from compositepowders Al–Al2O3–Al3Fe–Al3Ti

JozefSleziona∗, Maciej Dyzia, Jerzy Myalski, Jakub WieczorekSilesian University of Technology, ul. Krasinskiego 8, PL40-019 Katowice, Poland

Abstract

The paper presents some results of the research in scope of received composite powders by using liquid metal method. Controlled conditionof reaction between ilmenite and liquid aluminium provide to obtained the composite powder consist of intermetallics (Al–Ti, Al–Fe system)and alumina.© 2005 Elsevier B.V. All rights reserved.

Keywords:Aluminothermal reaction; Ilmenite; Composite powder

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. Introduction

More and more important in the range of heterophaseomposites production technologies are the powder metal-urgy technologies. Mixtures of powders for compressionre prepared by different methods which ensure a homoge-eous structure with a ceramic particle reinforced intermetal-

ic phase matrix. One of powders production method is SHSollowed by grinding and hot compression. There is a varietyf technological solutions which lead to obtaining a compos-

te of predefined properties. One of them is hot compressionn a vacuum, another - impulse compression[1].

The current great interest in the possibility of using theluminothermal reaction in the production of composite pow-ers is related to the use of ilmenite. The product of theeaction between ilmenite and aluminium are intermetal-ic phases from the Al–Ti and Al–Fe systems, and aluminaarticles. In the classical metallurgical technology, this re-ction is used for the production of ferrotitanium contain-

ng up to 30% of Al [2]. One of the solutions developedhen conducting the research on composites based on an

present paper presents some results of the researchscope.

2. Materials for the research and method ofcomposite powder production

FeO·TiO2 ilmenite of Norwegian production, grain sibelow 70�m, and technically pure, A0 grade aluminium pduced by ZM Skawina (Poland), were used for the fabricaof composites. The chemical composition of the ilmenitecording to Titania Company (Norway), is as follows: 44.3TiO2, 2.81% SiO2, 3.80% MgO and 0.2% V2O5; the rest35.94% Fe and total: 0.167% (P,S) – impurities.

The technological concept of the production of a cposite powder of a predefined phase composition consin making use of the aluminothermal reaction which taplace between aluminium and ilmenite. The reaction cawritten down in the following way:

FeO· TiO2 + (8+ x)Al → Al2O3+Al3Fe+ Al3Ti + xAl

ntermetallic phase from the Al–Fe–Ti[3] system was ob-aining a composite powder, from which, after treatmentsoaking, grinding, separation), sinters were produced. The

(1)

Reaction (1) is thermally activated; it starts at a temperatureof 680◦C[4] and proceeds very rapidly. Excess of aluminiumis supposed to ensure that ilmenite is completely reacted andt arei and

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∗ Corresponding author.E-mail address:sleziona@pols.katowice.pl (J.Sleziona).

924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserveoi:10.1016/j.jmatprotec.2005.02.183

o slow down the reaction. The product of this reactionntermetallic phases from the Al–Fe and Al–Ti systems

128 J. Sleziona et al. / Journal of Materials Processing Technology 162–163 (2005) 127–130

Fig. 1. Phase composition of composite powders after producing at 720◦C and size reduction in ball mill.

alumina. The composite powders were fabricated in an in situreaction of liquid aluminium with ilmenite powder of particlesize 40–80�m incorporated into it using stir casting. To en-sure control over the aluminothermal reaction, the minimumtemperature of molten aluminium during the incorporationof the ilmenite powder did not exceed 720◦C. The ilmenitepowder incorporated into liquid aluminium was priorly driedand heated to a temperature of 500◦C. The rate of stirringthe liquid metal amounted to 280 rpm. The process tempera-ture was predefined based on a differential thermal analysis(DTA) [4]. The reaction led to the production of powder of aphase composition (Al2O3, Al3Fe and Al3Ti) and size below1 mm. The powder’s phase composition was determined byX-ray radiography (Fig. 1).

Predominating components of this powder are aluminium,alumina and intermetallic phases Al3Ti and Al13Fe4. Thepowder production process was conducted until the liquidaluminium ran low, however, the quantity of the ilmenite in-corporated into it was always constant and amounted 20-wt.%in relation to the aluminium applied. The obtained compositepowder was subjected to separation at further stages in orderto separate large aluminium particles which had not beencompletely reacted. The large particles’ share in relation tothe primary aluminium volume did not exceed 5%. Next, theso prepared powder was subjected to grinding in a crusherand ball mill, after which screening took place and fractionsb em fortp

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eci-m

- cold compaction followed by sintering,- hot compaction and sintering in a Degussa press.

Powders 25–45�m and 45–80�m in size were used forthe investigation. Cold pressing was conducted by means ofa two-sidedZywiec press of a maximum load equal 600 MN.Roll-shaped compacts, 30 mm in diameter and 15 mm high,were obtained. Changeable compaction pressure was applied:150, 300 or 500 MPa. The sintering process proceeded in anargon atmosphere, at a temperature of 1000◦C/2 h. The sin-tering method applied turned out to be unsatisfactory. Notonly did oxidation of the specimens occurred, but also de-formation with partial foaming. This method of compositesproduction should be considered inappropriate. It is sinteringof composites at a temperature below 1000◦C that allowsobtaining a good quality material. The composite hardness,however, is comparable to cast composites and it does not ex-ceed 200 HB. The structure of the composites produced aftercold compaction and sintering at 800◦C is shown inFig. 2.

In the composite structure, fragments of not completelyreacted ilmenite particles are visible. In order to avoid the ox-idation of compacts during sintering, a reducing atmosphereis necessary, whereas in order to avoid the foaming of com-pacts, sintering must be conducted under pressure. Such con-ditions are met when compacting and sintering in a Degussapress. Hot compacting and sintering using the Degussa pressw car-b n oft om-

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elow 40 or 40–80�m and above 80�m were obtained. Thaximum size of the composite powder particles used

he fabrication of the specimens did not exceed 120�m. Theowder grain composition after grinding is shown inTable 1.

. Technological process and structure of compositeinters

Two methods were applied for the production of spens (sinters) from composite powder:

as carried out in graphite moulds coated with boronide. Lack of such coating led to an additional reactio

he composite powder with carbon. A few variants of c

able 1rain composition of composite powder

article size (�m) Particle weight fraction (%

20 1.55–45 8.55–80 20.50–125 17.525–200 23.5200 28.5

J. Sleziona et al. / Journal of Materials Processing Technology 162–163 (2005) 127–130 129

Fig. 2. Structure of composite obtained by cold compaction of powders (compaction pressure 300 MPa), (a) after sintering at 800◦C/1 h and (b) visible particleof ilmenite.

Fig. 3. Structure composite obtained by compaction of powders and sintering at 800◦C (Degussa pressure 15 MPa/15 min).

pacting and sintering were carried out, for each of which dif-ferent compacting temperature was chosen. The compactionwas conducted at 800, 1000 or 1100◦C, each time applyingpressure of 15 MPa and compaction time of 15 min. The testsmade have shown that compacting at temperatures of 800 and1000◦C allows obtaining a sinter whose hardness, however,is low compared to a cast composite.

The working pressure each time led to the removal of aliquid phase from the sinter. The structure of such a materialis shown inFig. 3.

The application of a compacting temperature of 1100◦C,in turn, initiated an additional synthesis of the compos-ite, which induced a rapid temperature and pressure in-crease in the die. Maintaining the compacting pressure of15 MPa at that temperature led each time to tearing themould. This was probably caused by intensive precipita-tion of oxygen as well as a rapid temperature growth.To avoid such effects, it is necessary that powders af-ter their fabrication are roasted at a temperature above1000◦C.

compo

Fig. 4. Phase composition of composite after producing from site powders on Degussa pressure in vacuum at 1100◦C/10 MPa/15 min.

130 J. Sleziona et al. / Journal of Materials Processing Technology 162–163 (2005) 127–130

Fig. 5. Structure of composite obtained by compaction of powders and sintering at 1100◦C (Degussa pressure 800◦C/15 MPa/15 min, unloading, holding at1100◦C, additional pressing after cooling down to 1000◦C and pressing at 10 MPa/15 min).

Roasting of a composite powder after its production (be-fore size reduction) at a temperature above 1000◦C leads tothe completion of the phase synthesis process and obtaininga state of equilibrium (Fig. 4).

Sinters obtained from powder after cold compaction andsintering are porous (at compaction pressure of 300 MPa, theporosity can even exceed 20%) and hardly sinterable, even at atemperature of 1000◦C. Only hot sintering in a vacuum (min.800◦C) under pressure of at least 10 MPa ensures obtaininga product of very low porosity (below 5%) and very goodquality. Some examples of the structure of sinters after hotcompaction are presented inFig. 5. The hardness measure-ments made have shown that as the compacting temperatureincreases, the sinter’s hardness grows as well. For a materialobtained after compacting at 1000◦C under 10 MPa, at timeof 15 min (Degussa press), hardness considerably exceeds1000 HV.

An optimum variant of the process that allows obtaininga composite of hardness above 1000 HV without extraroasting of the composite powder after its fabrication isas follows: heating the composite powder up to 1000◦C,compacting under pressure of 15 MPa, stress relief ofthe compact heating of the prethickened compact up to1100◦C (an exothermic reaction occurs), cooling thecompact to 1000◦C and compacting under 10 MPa pressurefor 15 min. Thus obtained composite has a heterophases eso fa itionsd ty of3

4. Conclusions

The applied procedure of composites production frompowders obtained in situ from the Al–ilmenite system showspromising results. It requires, however, refining the produc-tion process to obtain a composite powder of a stable phasecomposition. This is linked to the problem of conducting an insitu reaction between liquid aluminium and ilmenite powderincorporated into it. Ilmenite must be completely reacted sothat in the process of powder pressing and sintering, only re-actions connected with achieving an equilibrium state of theintermetallic phases could take place. Both the structure andphase composition of the composites obtained from com-posite powders of a complex phase composition show thatthe compaction and sintering parameters were selected cor-rectly. The composites produced are characterized by highhardness, exceeding 1000 HV, low porosity, below 2.5% andlow density of 3.5 g/cm3.

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