Materials Science-Poland, Vol. 26, No. 3, 2008

Sinker electrical discharge machining of aluminium matrix composites

P. CICHOSZ*, P. KAROLCZAK Institute of Production Engineering and Automatization, Wrocław University of Technology

Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland

Principal features of aluminium matrix composites are put forward. Emphasis is put on temperatures of melting and coefficients of heat conduction of matrix and reinforcement. The paper presents results of electrical discharge machining of aluminium matrix composites with particular attention given to thick-ness of the defected layer after machining. Influence of various machining parameters on the behaviour of saffil fibres and matrix material in the affected zone is presented. Scanning micrographs and roughness measurements are used to analyse surface finish following machining. The influence of used current parameters on the quality of surface layer after electrical discharge machining is discussed. The results for composite materials are compared with those for aluminum alloys.

Key words: aluminium matrix composites; machining; spark erosion

1. Introduction

Metal matrix composites (MMC) are increasingly popular as materials of choice for fabricating high-strength parts in such areas as aviation, aerospace, automotive industry, electrotechnology, sports and recreation [1]. At present, a large variety of composites are commercially available. The matrix can be metal, polymer or ceramics, and reinforcement can be of similar types in form of fibres, particles or powders. The investigation described in this paper was carried out on aluminium composite materials reinforced with ceramic Al2O3 fibres of the saffil type manufactured by ICI SAFFIL. Cast AlSi9Mg (AK9) alloy constituted the matrix The compositions and properties of the materials under under study are given in Tables 1–3. The presented method of fabricating parts with suitably arranged fibres worked out at the Institute of Production Engineering and Automation of the Wro-cław University of Technology [2] consisted in preparing a fluid mixture of saffil fibres and silica cohesive agent. The solution was then filtered off, formed into a desired shape, dried and fired at a high temperature. The saffil brick was then placed at a given place of a mold, poured with a molten metal and squeeze cast. The casting was further subject to electrical discharge (ED) machining. __________

*Corresponding author, e-mail: piotr.cichosz@pwr.wroc.pl

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Machining of composites is difficult [3–5]. Hard reinforcement induces high wear in cutting tools. Application of wear-resistant materials such as polycrystalline dia-mond may be limited by their prohibitive cost. Intricate shapes of workpieces may also render traditional material removal operations inapplicable. That is why EDM applied to shaping MMC parts is increasingly attracting attention of manufacturers and investigators [6–9].

Table 1. Chemical composition and properties of Al2O3 saffil fibres

Chemical composition [%] Properties

Al2O3 SiO2 Fibre

diameter [μm] Ρ

[kg⋅m–3] Rm

[MPa] E

[GPa] Θ

[deg] λ

[Ω/μΚ] 96 4 3 3300 2000 300 2320 29-30

Table 2. Approximate chemical composition of AlSi9Mg alloy [wt. %]

Si Cu Mg Mn Fe Ti 9.5 < 0.05 0.35 < 0.1 < 0.18 0.15

Table 3. Minimum properties of AlSi9Mg alloy

YS0.2 [MPa]

UTS [MPa]

A50mm [%] HB Θ

[deg] λ

[W/(m⋅K)] 190 230 2 75 600 150–170

EDM in MMCs poses many challenges due to the presence of two phases with

completely different properties. A ductile matrix has a low melting point and a high thermal conductivity while the brittle reinforcement is characterized by a high melting point and low thermal conductivity. High thermal resistance of composites has an adverse effect on the efficiency of ED machining processes [6] and is one of those aspects that require a lot of investigative effort. The aim of the presented paper is to investigate applicability of sinker ED machining to shaping aluminium composites reinforced with saffil fibres.

2. Experimental

The experiments were conducted on an EDM 16 machine with an M1E electro-lytic copper electrode and glifer dielectric fluid. The surfaces to be machined were ground prior to tests to ensure uniform initial conditions. The following three sets of current parameters were used: a) U = 80 V, Iz = 0.8 A, Ir = 3 A, b) U = 80 V, Iz = 1 A, Ir = 5 A, c) U = 80V, Iz = 4 A, Ir = 40 A, where Iz is the intensity of ignition, Ir – work-ing intensity and U – voltage.

For the sake of comparison, the same conditions were used in EDM of reference AlSi9Mg alloy without saffil reinforcement. The ED machined surfaces were exam-ined with optical and scanning microscopy. Surface roughness was evaluated using

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a Taylor Hobson Form Talysurf 120L profilographometer. Mesohardness was meas-ured using a LECO tester.

3. Sinker ED machining of pure Al alloy

Figure 1 shows a micrograph of the defect-affected layer in pure Al alloy pro-duced by the sinker ED machining operation adopted in this study. Clearly visible are the heat-affected zone and a crater left by spark discharge. Microstructure of the alloy has been recast and fragmented. Finer microstructure resulted in higher values of mesohardness. In the areas of strong thermal influence it reached 130–147 HV and was by 65% higher than that of the bulk material. Depth of the said layer was strongly dependent upon machining parameters and varied from 20 to 120 μm. Surface rough-ness as determined by Ra was found to be approximately equal to 6 μm.

Fig. 1. Optical micrograph across the defect -affected layer of AlSi9Mg workpiece following the

sinker EDM operation. Sinker EDM parameters: U = 80 V, Iz = 1 A, I r = 5A, machining area size:

120 mm2, h ≈ 60 μm, Ra = 6.17 μm

4. Sinker ED machining of the composite

ED machining of the composite was done using three sets of current parameters listed in Sect. 2. The first set (a) with low currents is shown in action in Fig. 2.

Fig. 2. Optical micrograph across the defect-affected layer of composite workpiece following the sinker EDM operation. Sinker EDM parame-

ters: U = 80 V, Iz = 0.8 A, I r = 3A, machining area size: 120 mm2, h ≈ 30 μm

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Fig. 3. Surface roughness profiles for a composite specimen following the sinker EDM operation:

a) Ra = 3.55 μm, b) Ra = 5.1 μm, c) Ra = 17 μm

Depth of the heat-affected zone was equal to about 30 μm. Only mild metallurgi-cal changes could be observed within that zone. The matrix was slightly recast and made finer. Ceramic fibres were generally left undamaged. Those directly affected by

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spark discharges were degraded. Some fibres were found to stick out a few μm from the surface. Hardness of the zone (148–157 HV) was higher than that of the bulk ma-terial. Surface roughness was found to be Ra = 3.5–4 μm (Fig. 3).

Fig. 4. Scanning micrograph of the ED machined composite surface; U = 80 V, Iz = 0.8A, Ir = 3 A

Fig. 5. Scanning micrograph of the ED machined composite surface; U = 80 V, Iz = 0.8A, Ir = 3 A

Fig. 6. Scanning micrograph of the ED machined composite surface; U = 80 V, Iz = 0.8 A, Ir = 3 A

Fig. 7. Scanning micrograph of the ED machined composite surface; U = 80 V, Iz = 0.8 A, Ir = 3 A

Figure 4 shows the scanning micrograph of the composite surface machined with the parameters (a). Clearly visible are craters produced by spark discharges. Both the matrix and reinforcement are heavily recast. Few fibres were found undamaged (de-noted by 1). At a place denoted by 2 there was a fibre-now molten up completely. Its pres-ence was established by spectrometer examination of the surface targeted at oxygen.

Figure 5 shows another area of the surface machined with the parameters (a). A centrally placed object resembles a fibre. Chemical analysis determined that there is no oxygen at this place so it must be a peculiarity of solidification.

Figure 6 shows a rare case where saffil fibres did remain on the machined surface. There are three fibres visible with analytically confirmed high oxygen content. Numerous craters with smooth contours are the evidence that the area was melted to a high depth and the resulting long time of cooling helped the craters to be smoothed out.

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A high-magnification scanning micrograph in Fig. 7 shows a single crater with an unmelted fragment of a thick (~5 μm) fibre. It was probably this large fibre volume that saved the fibre from being melted. By using higher current parameters (set (b)) a higher intensity of thermal action was achieved. The heat-affected zone has now a depth of about 70 μm (Fig. 8). Undamaged fibres no longer protrude from the sur-face. The overall surface finish has deteriorated and Ra has now risen to 5–6 μm (Fig. 3).

Fig. 8. Optical micrograph across

the defect-affected layer of composite workpiece following the sinker EDM operation; sinker EDM

parameters: U = 80 V, Iz = 1 A, I r = 5 A, machining area size: 120 mm2, h ≈ 70 μm

Fig. 9. Optical micrograph of the ED machined composite surface; Sinker EDM parameters:

U = 80 V, Iz = 4A, I r = 40 A, machining area size: 120 mm2, h ≈ 120 μm

Fig. 10. Scanning micrograph of surface layer of the ED machined composite:

U = 80 V, Iz = 4 A, Ir = 40 A

Fig. 11. Scanning micrograph of surface layer of the ED machined composite;

U = 80 V, Iz = 4 A, Ir = 40 A

The third current parameters set (c) caused the biggest changes in the composite top layer. The depth of melting was now h = 120 μm. Numerous deep craters were noticed (Fig. 9). A new feature were blisters located just beneath the surface. They

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frequently contained saffil fibres. The top composite layer crystallized in the direction perpendicular to the surface. Surface roughness was the worst of the three cases (Ra = 17 μm, Fig. 3). Optical examination of the recast layer produced by the set (c) current parameters revealed no undamaged fibres. Spectrometer probe examination detected areas with increased oxygen content attesting to the fibres being dissolved in the matrix and forming new Al–O compounds.

Fig. 12. Scanning micrograph of surface layer of the ED machined composite; U = 80V,

Iz = 4 A, Ir = 40 A

Mesohardness of the recast layer was low: 113–130 HV. In some places the hard-ness reached 280–320 HV (area 1 in Fig. 10). These fibres were initially assumed to be molten fibres but the assumption turned out false as no higher oxygen content was detected. Those anomalies in hardness remain to be further investigated. Special atten-tion was paid to fibres located just beneath the recast layer produced by set (c) (Fig. 11). The fibres are fragmented and broken, they must have been damaged by high energy of the pulses. The extent of damage decreases with increasing depth (Fig. 12).

4. Final remarks

The investigation showed that ED machining process parameters affect the condi-tion of surface layer in machined aluminium MMCs. Low current parameters resulted in a thin layer with a recast structure of increased hardness. Reinforcing fibres were generally left undamaged, some of them protruding from the surface. Some fibres were melted down and then resolidified as featureless blobs.

Higher process parameters resulted in severe detrimental changes to the surface layer structure. Higher material removal rates produced a very rough finish with poor surface integrity (numerous craters and sub-surface blisters).

There is a need for working out optimized patterns of current density and fre-quency of sparks that would eliminate or reduce the extent of finishing operations necessary for removing the recast layer.

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References

[1] SOBCZAK J., WOJCIECHOWSKI S., , Kompozyty (Composites), 2 (2002), 25 (in Polish). [2] NAPLOCHA K., Optimization of process variables in manufacturing the AK9 type materials rein-

forced with Al2O3 fibres, Institute of Production Engineering and Automation, Wrocław University of Technology, Doctor Thesis, Wrocław 1999 (in Polish).

[3] JANKOWIAK M., KAWALEC M., KRÓL G., Machining ability of cutting edges in turning Al2O3-particle reinforced Al alloys, ZN Politechniki Rzeszowskiej, Mechanika 44, 1995, 133 (in Polish).

[4] CRONJAGER L., MEISTER D., Machinability of fibre and particle-reinforced aluminium, Annals of CIRP, 41 (1992), 63.

[5] TETI R., Machining of composite materials, Annals of CIRP, 51 (2002), 611. [6] PEROŃCZYK J., BIAŁO D., Kompozyty (Composites) 1 (2001), 211 (in Polish). [7] PEROŃCZYK J., BIAŁO D., Kompozyty (Composites) 3(2003), 366 (in Polish). [8] TRZASKA M., PEROŃCZYK J., BIAŁO D., Kompozyty (Composites) 5 (2005), 51 (in Polish). [9] WANG CH., YAN B., J. Mat. Proc. Techn., 12 (2000), 90.

Received 13 June 2007 Revised 15 October 2007