INVESTIGATIONS OF CARBIDE FORMATION PROCESSES UPON CAVITATION INFLUENCE ON THE AL-TI MELTS WITH GRAPHITE ACTIVATOR FOR THE PURPOSE OF CAST COMPOSITE PRODUCTION

E.A.Pastukhov, N.A.Vatolin, E.A.Popova, and L.E.Bodrova

Institute of Metallurgy UD RAS, Ekaterinburg

The carbide formation processes taking place upon cavitation influence on the Al-Ti melts for the purpose of producing cast composite materials by synthesis of a hardening phase inside an aluminum matrix were studied. The measurements were carried out with the equipment [1] functioning like an acoustic resonator, allowing one to create an advanced cavitation mode in a few seconds with the help of low-frequency elastic oscillations delivered into the melt through a graphite oscillator.

The carbide formation processes were studied in two aluminum melts differ in content of titanium and impurity elements. Chemically pure granular aluminum and aluminum-titanium ligature with 3.57 mass % of Ti were used as the originals. Cavitation influenced on the melts in argon atmosphere during increasing an overheating temperature from 70-100 to 370-400 degrees above liquidus in each 100 degrees within five minutes at each point. Total treatment time was 20 minutes. After each action stage ingots were flooded into a graphite mould, from which the samples for further investigations were cut. Under the same time-temperature conditions but without any cavitation treatment check test pieces were smelted. Specimens were tested by chemical, metallographic, X-ray phase, and X-ray microspectrum analyses. A microhardness of phase components was determined and integral structure parameters were estimated with the use of the SIAMS-600 image analysis system.

Metallographic section analysis revealed the carbides start forming during cavitation influence on the melt from 200 degrees overheating above liquidus and achieve a maximum of content at 350-400 degrees overheating and after 20 minutes of treatment. The structures of the Al-0.6%TiC1-x and Al-2.8%TiC1-x composites obtained are presented in Figs. 1a and 2a, respectively.

Fig.1. Microstructure of the Al-0.6%TiC1-x composite (a) and the Al-0.72%Ti alloy (b). Magnif. 70.

Titanium carbides approx. 10μm sized and their clusters of up to 100-150μm are uniformly distributed in the aluminum matrix. In the check test pieces the structure of the Al-0.72%Ti alloy is presented by the α-phase, titanium oversaturated solid solution in aluminum and small individual intermetallides (Fig.1b), and for the Al-2.9%Ti alloy there are the Al3Ti intermetallic compounds of different size in the α-phase (Fig.2b). Data of chemical and X-ray phase analyses of the samples treated (400° overheating, 20 min.) are introduced in the Table.

Fig.2. Microstructure of the Al-2.8%TiC1-x composite (a) and the Al-2.9%Ti alloy (b). Magnif. 70.

The amount of titanium carbides formed increases with increasing the titanium content on the original melt. For the initial melt with 3.57 % of Ti, the whole graphite hitting into the melt from the oscillator during the cavitation influence is going into TiC1-x without forming undesirable Al4C3. These results agree with a thermodynamic modeling for carbide formation processes upon inflaming the Al3Ti + C mixture in the SHS-process [2].

According to the X-ray phase analysis of the Al-2.8%TiC1-x composite the unit cell parameters of the titanium carbide is equal to 0.43194(4) nm, which corresponds to 0.67 atomic ratio of C/Ti [3].

By microspectrum analysis it was revealed the clusters, visible on the section, consist of titanium carbides, surrounded by oxycarbide and oxide impurities (Al4O4C, Al2O3, SiC etc.). Being wetted with aluminum they tower differently over the matrix and thus produce an effect of significant volumetric percentage as compared with a mass percent defined by X-ray analysis or calculated according to the results of chemical analysis for titanium and carbon.

The low values of microhardness for the clusters are perhaps caused by their multi-phase make-up. The microhardness data for phases involved in composites and alloys obtained are shown in the Table.

It is evident, the microhardness of the α-phase increases with an increase of titanium content. The differences between the values of microhardness for the α-phase in alloys with and without cavitation treatment are insignificant. The microhardness of a hardening phase in the second composite is twice to the first one.

Note, after remelting the Al-06%TiC1-x composite and 30 minutes exposure at 750°C its structure remains unchanged (Fig.3). Besides, composite equalization takes place, causing an increase of microhardness both of the matrix and the carbide containing phase.

Fig.3. Microstructure of the Al-0.6%TiC1-x composite after remelting and 30 minutes exposure at 1023K. Magnif. 70.

Tabel. Results of chemical and X-ray phase analyses of the investigated samples and microhardness measured on the phases involved

Alloy composition, duration of cavitation treatment / Content, mass % (chemical analysis) / Content of hardening phases,
mass % (Xray analysis) / Microhardness, МPa
Ti / С / TiC / А14С3 / А14О4С / А12O3 / А13Тi / hardening phase / matrix
Al-0.72%Ti,
without treatment / 0.72 / 0.11 / Not revealed / - / 274
Al-0.6%TiC1-x,
20min. treatment / 0.70 / 0.47 / 0.6 / 0.6 / 0.2 / 0.7 / - / 600 / 291
Al-0.6TiC1-x,
after remelting and 30min. exposure at 750°С / 0.70 / 0.47 / 0.6 / 0.6 / 0.2 / 0.7 / - / 846 / 323
Al-2.9%Ti(*)
without treatment / 2.9 / 0.13 / Not revealed / - / 439
Al2.8%TiC1x(**),
20min. treatment / 3.1 / 0.74 / 2.8 / 0.4 / 0.1 / 1390 / 404
(*) – contains 0.345% Fe and 0.161% Si;
(**) – contains 0.316% Fe and 0.218% Si.

Thus, during cavitation influence on the Al-Ti melts upon 350-400 degrees overheating above the liquidus an interaction between titanium and graphite hitting into the melt from the oscillator takes place. The resulting carbides and oxycarbides are distributed uniformly in the melt volume. The higher titanium content, the more complete process of TiC1-x formation takes place and less amount of Al4C3 is formed. An exposure of the composite in the melting state leads to equalizing the compositions of both the matrix and the carbide phase.

The experiment carried out revealed a possibility to produce Al-based cast composites, hardened with carbide containing phase, with the help of low frequency elastic oscillations in cavitation mode acting on the aluminum melts, containing carbide-forming element, by a graphite oscillator.

The investigations were carried out with the financial support of Russian Foundation for Basic Research, Project No.02-03-32313a.

REFERENCES

  1. E.A.Pastukhov, E.A.Popova, L.E.Bodrova, N.A.Vatolin. Rasplavy, 1998, No.3, p.7-13 (in Russ.).
  2. A.G.Makarenko, V.I Nikitin, E.G.Kandalova. Liteinoe proizvodstvo, 1999, No.1, p.38-39 (in Russ.).
  3. G.V.Samsonov, G.Sh.Upadhaia, V.S.Neshpor. Physical Material Authority of Carbides. Kiev, "Naukova dumka", 1974, 456pp (in Russ.).

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