Short CommunicationStudy of Al/Cu rich phases formed in A356 alloy by inserting Cu wire in pattern in LFC process
Introduction
Lost foam casting (LFC) has been used as a production process for more than 50 years. In this process the embedded polystyrene pattern in the sand mould is decomposed by molten metal. So the metal replaces the polystyrene pattern and duplicates all features of the pattern [1]. Currently, many casting facilities are dedicated strictly to the lost foam process because of its interesting and numerous advantages such as no mould parting line, no cores, more accurate dimensions, no environmental pollutants, ability to produce complex pieces and also cost reduction [2], [3], [4].
It has been shown that if one inserts metallic wire or fiber in the polystyrene pattern before pouring the melt it will be possible to study the interaction of the inserted core with the poured melt. The slow motion of molten metal within the mould, in the full mould process, is an advantage for such studies. Researches performed in this field have shown the possibility to investigate the effects of different inserts such as copper, plain steel, stainless steel, other metallic wires and also some non-metallic fibers on pure aluminum or aluminum alloys matrix. The results have shown that many phases and intermetallic compounds of Al/Fe and Al/Cu may form and concentrate on a small area around the insert if the matrix is Al alloy [5], [6], [7], [8]. Similar researches have shown interesting features like the changes of graphite types and morphology when the alloy selected as the matrix was grey iron [9]. Such studies have shown the potential of the performed technique for making composite and also detailed study of the various phases which may form in the vicinity of the insert [7], [8].
This investigation makes use of the above mentioned technique (named here as inserted in-polystyrene pattern wire, IIPPW) to study the reaction of a molten aluminum alloy and copper wires having various diameters, to investigate which type of phases and compounds may form during the reaction of the inserted wire with the molten alloy, while the melt is slowly filling the mould. Copper was used because it is one of the few elements that its phase diagram with aluminum has been studied extensively and forms different phases and compounds [10], [11], [12], [13], [14], [15], [16], [17], [18]. Fig. 1 shows an image taken from Al2Cu/Al(Cu) eutectic in aluminum–copper matrix with its interesting features [10].
According to the binary phase diagram of Al/Cu, shown in Fig. 2 and Table 1 [14], various phases and intermetallic, may form depending on chemical composition and temperature. The Al rich corner of this phase diagram reveals that Cu dissolves in small quantities (0.25 at% Cu) at room temperature. At quantities less than 33 at% Cu one can see a eutectic transformation in a broad region which contains CuAl2 (θ) and solid solution of Cu in Al. Two different phases with the chemical composition of Al2Cu and AlCu (η) can be seen in the range of 33–50 at% Cu. At 50–54 at% Cu, AlCu (η) phase forms and at 54–64 at% Cu different phases such as η–ξ ξ–δ and δ–γ can be seen in the structure. At quantities higher than 64 at% Cu, actually at Cu rich corner, a eutectoid transformation and solid solution of Al in Cu can be formed [12], [13], [14], [15].
Section snippets
Experimental procedure
Four test bars were cut and prepared from polystyrene with nominal density of 20 Kg/m3. Pure copper wires with various diameters namely 0.4, 0.8 and 1.2 mm and length of 150 mm were cut, washed in alcohol and inserted in polystyrene patterns, as shown in Fig. 3a. One of the test bars had no insert inside in order to be compared with other test bars. The patterns were assembled as shown in Fig. 3b.
CO2–sodium silicate sand process was used for moulding (Note that the moulding process was not an
Results and discussion
As the aluminum melt is poured into the mould, the polystyrene pattern burns and the melt embraces the copper wire in test bars that contain the insert. Fig. 4 shows a general view of this phenomenon. Fig. 5 presents an optical image, taken from a test bar with no Cu insert inside (reference sample), showing a conventional A356 aluminum alloy microstructure.
Based on microstructures, shown in Fig. 6, Fig. 8, samples fall into three categories after cooling to the room temperature. These three
Conclusion
The following conclusions can be drawn from the discussion of the experimental results:
- (1)
Three different cases were observed when an inserted in pattern wire was surrounded by a melt. These were: (a) complete melting, (b) partial melting, and (c) no melting of the wire.
- (2)
A wire affected zone formed around the inserted wire. The wire affected zone itself was divided into two different zones. These were (a) composition affected zone, and (b) cooling affected zone.
- (3)
The composition affected zone was a
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