급속응고법으로 제조한 과공정 Al-17Si-5Fe 합금 압출재의 미세조직 및 기계적 특성

Mechanical Characteristics and Microstructures of Hypereutectic Al-17Si-5Fe Extruded Alloys Prepared by Rapid Solidification Process

Abstract

In this study, the mechanical characteristics and microstructure of hypereutectic Al-17Si-5Fe extruded alloys prepared by a rapid solidification process (RSP) were investigated. The hypereutectic Al alloy was fabricated by means of RSP and permanent casting. For RSP, the Al alloy melted at $920^{\circ}C$, cooling the specimens at a rate of $10^6^{\circ}C/s$ when the RSP was used, thus allowing the refining of primary Si particles more than when using permanent casting, at a rate of about 91%. We tested an extrusion RSP billet and a permanent-cast billet. Before the hot-extrusion process, heating to $450^{\circ}C$ took place for one hour. The samples were then hotextruded with a condition of extrusion ratio of 27 and a ram speed of 0.5 mm/s. Microstructural analyses of the extruded RSP method and the permanent casting method were carried out with OM and SEM-EDS mapping. The mechanical properties in both cases were evaluated by Vickers micro-hardness, wear resistance and tensile tests. It was found that when hypereutectic Al-17Si-5Fe alloys were fabricated by a rapid solidification method, it becomes possible to refine Si and intermetallic compounds. During the preparation of the hypereutectic Al-17Si-5Fe alloy by the rapid solidification method, the pressure of the melting crucible was low, and at faster drum speeds, smaller grain alloy flakes could be produced. Hot extrusion of the hypereutectic Al-17Si-5Fe alloy during the rapid solidification method required higher pressure levels than hot extrusion of the permanent mold-casted alloy. However, it was possible to produce an extruded material with a better surface than that of the hot extruded material processed by permanent mold casting.

Fig. 1. Schematic diagram of Rapid Solidification Process.

Fig. 2. Optical microstructures of hypereutectic Al-17Si-5Fe alloy by permanent mold casting.

Fig. 3. Morphologies and microstructures hypereutectic Al-Si-Fe Al alloy flakes fabricated by rapid solidification process (by experimental order in table 4).

Fig. 4. Comparison on primary Si phase size of hypereutectic Al-Si-Fe Al alloy ingot and flakes.

Fig. 5. SEM-EDS mapping analysis of hypereutectic Al-Si-Fe Al alloy flakes produced by rapid solidification process.

Fig. 6. Surface of as-extruded Al-Si-Fe alloy ; (a) PMC and (b) RSP.

Fig. 7. Optical Microstructures of (a) Permanent Method Extrusion rod and (b) RSP Method Extrusion rod.

Fig. 8. Comparison on tensile strength of as-extruded alloy bars (PMC, RSP).

Fig. 9. Vicker's hardness of as-extruded Al alloy bars about different temperature : (A) Transverse Direction and (B) Logitudinal Direction.

Table 1. Chemical Compositions of hypereutectic Al-17Si-5Fe alloy.

Table 2. Rapidly solidification process conditions for hypereutectic Al-17Si-5Fe alloy flakes.

Table 3. Experimental order for Al alloy flakes production.

Table 4. Results of wear resistance test (weight reduction rate) measurement.