For many metal alloys, the process of metal cutting is accompanied by extensive plastic deformation and fracture. To study this process, quick stop sectional samples of hypoeutectic Al-Cu alloy chip formation, either as conventionally cast alloy or as “semi solid metal” are used. The type of chip formation is classified according to crack formation mechanism and propagation. During cutting, in all specimens used, quasi-continuous chips with built-up edge (BUE) are obtained. The formation of BUE is undesirable since it is a highly deformed body with a semi stable top which periodically breaks away giving rise to poor workpiece surface quality.Za mnoge metalne legure, proces rezanja je praćen intenzivnom plastičnom deformacijom i lomom. Za istraživanje tog procesa u radu je korištena metoda brzog zaustavljanja procesa rezanja. Materijal uzorka je podeutektička Al-Cu legura, lijevana konvencionalno i kao „polu čvrsti metal“ (semi solid metal-SSM). Vrsta strugotine koja je obrazovana klasifi cirana je prema mehanizmu nastajanja i širenja pukotina. Tokom rezanja, u svim uzorcima nastajala je, kvazi-kontinuirano strugotina sa naslagama (BUE). Stvaranje BUE je nepoželjan proces, jer je to jako deformirani volumen sa polu stabilnim oblikom, koji povremeno raste i odvaja se, što dovodi do lošeg kvaliteta obrađene površine

B. Savković

D. Rajnović

J. Wannasin

L. Sidjanin

P. Kovač

Za mnoge metalne legure, proces rezanja je praćen intenzivnom plastičnom deformacijom i lomom. Za istraživanje tog procesa u radu je korištena metoda brzog zaustavljanja procesa rezanja. Materijal uzorka je podeutektička Al-Cu legura, lijevana konvencionalno i kao „polu čvrsti metal“ (semi solid metal-SSM). Vrsta strugotine koja je obrazovana klasifi cirana je prema mehanizmu nastajanja i širenja pukotina. Tokom rezanja, u svim uzorcima nastajala je, kvazi-kontinuirano strugotina sa naslagama (BUE). Stvaranje BUE je nepoželjan proces, jer je to jako deformirani volumen sa polu stabilnim oblikom, koji povremeno raste i odvaja se, što dovodi do lošeg kvaliteta obrađene površine.For many metal alloys, the process of metal cutting is accompanied by extensive plastic deformation and fracture. To study this process, quick stop sectional samples of hypoeutectic Al-Cu alloy chip formation, either as conventionally cast alloy or as “semi solid metal” are used. The type of chip formation is classified according to crack formation mechanism and propagation. During cutting, in all specimens used, quasi-continuous chips with built-up edge (BUE) are obtained. The formation of BUE is undesirable since it is a highly deformed body with a semi stable top which periodically breaks away giving rise to poor workpiece surface quality

Wannasin, J.

Savković, B.

Rajnović, D.

Sidjanin, L.

Kovač, P.

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34  METALURGIJA 51 (2012) 1, 34-38P. KOVAČ, L. SIDJANIN, D. RAJNOVIĆ, B. SAVKOVIĆ, J. WANNASIN THE MICROSTRUCTURE INFLUENCE ON THE CHIP FORMATION PROCESS OF AL-CU ALLOY CAST CONVENTIONALLY AND IN SEMI SOLID STATERecevied – Prispjelo: 2011-01-20Acctepted – Prihvaćeno: 2011-03-25Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 51(1) 34-38 (2012)UDC – UDK 669.715:621.96:621.773:620.18=111For many metal alloys, the process of metal cutting is accompanied by extensive plastic deformation and fracture. To study this process, quick stop sectional samples of hypoeutectic Al-Cu alloy chip formation, either as convention-ally cast alloy or as “semi solid metal” are used. The type of chip formation is classifi ed according to crack formation mechanism and propagation. During cutting, in all specimens used, quasi-continuous chips with built-up edge (BUE) are obtained. The formation of BUE is undesirable since it is a highly deformed body with a semi stable top which periodically breaks away giving rise to poor workpiece surface quality.Key words: aluminium alloys, cutting process, chip formation, microstructure, semi solid metal Utjecaj mikrostrukture na proces nastajanja strugotine za konvencionalnu i polutekuću lijevanu Al-Cu leguru. Za mnoge metalne legure, proces rezanja je praćen intenzivnom plastičnom deformacijom i lomom. Za istraživanje tog procesa u radu je korištena metoda brzog zaustavljanja procesa rezanja. Materijal uzorka je podeutektička Al-Cu legura, lijevana konvencionalno i kao „polu čvrsti metal“ (semi solid metal-SSM). Vrsta strugotine koja je obrazovana klasifi cirana je prema mehanizmu nastajanja i širenja pukotina. Tokom rezanja, u svim uzorcima nastajala je, kvazi-kon-tinuirano strugotina sa naslagama (BUE). Stvaranje BUE je nepoželjan proces, jer je to jako deformirani volumen sa polu stabilnim oblikom, koji povremeno raste i odvaja se, što dovodi do lošeg kvaliteta obrađene površine.Ključne riječi: legure aluminija, proces rezanja, nastajanje strugotine, mikrostruktura, polu čvrsti metalINTRODUCTIONAluminium-copper alloys offer both high strength and excellent ductility, where those values are approach-ing some grades of ductile iron and they are signifi cant-ly higher than these of Al-Si-Mg family. They have a number of potential applications in automotive industry to reduce vehicle weight, including automotive suspen-sion knuckles, vehicle control arms, and differential carrier parts, aerospace and military castings [1]. How-ever, the alloy is very diffi cult to cast because of its ten-dency for hot tearing. Hot tearing is a severe casting defect that gen erally occurs when casting solidifi es and con tracts under conditions that hinder the free con-traction of casting parts. To overcome this problem, a semi-solid gravity casting can be used. The material produced by this technology is referred to as “Semi Solid Metal” (SSM). Rheocast produced semi-solid metal which reduces hot tearing tendency since the slur-ry possesses fi ne-grained and non-dendritic structure. In addition, partially solidifi ed metals result in less solidifi -cation contraction [2, 3]. The SSM characteris tics and potential benefi ts are presented in Table 1 [4].P. Kovač, L. Sidjanin, D. Rajnović, B. Savković, Faculty of Technical Science, University of Novi Sad, Serbia.J. Wanasin, Faculty of Engineering, Prince of Songkla Univer-sity, Thailand.Generally, machining by material removal process is the most common process in the pro duction of machine parts. Cutting process is suitable for machining of al-most all materials (ferrous and non-ferrous, soft and hard, ductile and brittle, etc.). Machinability of materi-als is one very important characteristic of materials and expresses capability of machining with the most eco-nomic methods [5, 6]. The characteristics of machine parts (as duration, fa-tigue, wear, etc.) depend not only on the construction shape, but also on surface layer state after metal cutting. The parameters and phenomena, like state and quality of surface layer, cutting temperature, cutting forces, etc., are strictly connected with chip geometry and shape [6]. The chip forming process theoretically might be ex-plained by different models. One of the most known is Brix model [7], which represent chip forming like suc-cessive lamellas forming process, Figure 1. Although, this model is very sophisticated, it only takes into count behavior from post factum observation like the veloci-ties, the cutting forces and the geometry around the tool and ignores material microstructure [8]. It was the rea-son that in this study an attempt has been made to cor-relate the chip formation process and the material microstructure of Al-Cu alloys. 35METALURGIJA 51 (2012) 1, 34-38P. KOVAČ et al.: THE MICROSTRUCTURE INFLUENCE ON THE CHIP FORMATION PROCESS OF AL-CU ALLOY CAST...It is known, that the machinability of con ventionally cast Al-Cu alloys depends primarily on the shape, size and distribution of the eutec tic and Al2Cu precipitation present in the micro structure [9, 10]. As mentioned be-fore, in “Semi Solid Metal” production of Al-Cu alloy, the mi cro structure morphology is changed. This change may consequently infl uence the ma chinability; there-fore, the study of SSM machinability is of great inter-est.In this paper, chip root specimens of hypo eutectic Al-Cu alloy obtained by a quick stop method are stud-ied. The main goal is to study the microstructure infl u-ence of conventional and semi solid state produced Al-Cu alloy micro structure, on the chip formation process. The paper is part of continual research pro gram of material composition and microstruc ture infl uence on chip formation process at the Faculty of Technical Sci-ences, Department of Production Engineering and also the subject matter of cooperation with Prince of Song-kla University, Faculty of Engineering in Thailand.MATERIAL AND EXPERIMENTAL PROCEDURE The infl uence of microstructure on chip for mation process during aluminium alloy cutting (Al-4,4 %Cu-0,17 %Fe), was studied. Two types of specimens were used. The fi rst one was cast conventionally while the second was produced by semi-solid gravity casting (SSM). To pro duce the SSM specimens, a new tech-nique was used. This novel process uses gas bubbles as the medium to agitate molten metal during initial stages of solidifi cation [2, 3]. The confi guration uses porous graphite rod as gas diffuser to pro vide the fi ne argon gas bubbles. The diffuser was connected to Argon gas cylin-der equipped with gas fl ow meter, Figure 2. The alloy was melted and then injected with Argon gas through the graphite rod at the temperature of approx. 650 °C for about 10 sec. The slurry was than casted in a mold. After casting, specimens of dimension 10×10×50 mm were prepared and machined on vertical 12 kW milling machine, by means of a milling head with hard metals inserts. Machining was carried out without cutting liq-uid. To obtained chip root specimens, quick stop method for freezing of cutting process was used. This method is originally developed by the authors from the Faculty of Technical Sciences, Novi Sad and it can be used in all cutting process technologies [11].The specimens for microstructure analysis were pre-pared by standard metallographic tech nique and were examined on Leitz-Orthoplan light microscope and JEOL JSM6460LV scan ning electron microscope equipped with an energy dispersive (EDX) spectrome-ter, INCA X-sight Oxford Instruments, operated at 20 kV.Table 1 Characteristics of SSM for exploitation [4]Characteristics Potential benefi ts or applicationsLower heat content than liquid metalHigher speed of part forming Higher speed continuous castingLower mold erosionFerrous part formingForming of other high-melting point materialsForming of reactive metalsSolid present of time of mould fi llingLess shrinkage voidsLess feeding requiredLess macro segregationFine grain structureViscosity higher than in liquid metals, and con-trollableLess entrapped mould gasesReduced oxides-improved machinabilityLess mold attackImproved surface fi nishAutomationNew processesFlow stress lower than for solid metalsForming of Intricate partsHigh speed part formingLower cost part formingHigh speed forming of continuous shapesNew processesAbility to incorpo-rate other materi-alsCompositesAbility to separate liquid and solidPurifi cationFigure 1  Brix model with lamellas formation during chip forming process [7]Figure 2  Schematic of the apparatus used in the study (not to scale) [3]36  METALURGIJA 51 (2012) 1, 34-38P. KOVAČ et al.: THE MICROSTRUCTURE INFLUENCE ON THE CHIP FORMATION PROCESS OF AL-CU ALLOY CAST...RESULTS AND DISCUSSIONThe microstructure of conventional casting samples and semi solid metal (SSM) samples are presented in Figures 3 to 6, respectively. In both types of samples, microstructure consists of α solid solution and Al2Cu phase. However, the grain size and distribution of Al2Cu are dif ferent. Conventional cast samples have coarse α solid solution the grains and precipitation of Al2Cu par-ticles inside the grain and small Al2Cu particles in the eutectic (α+Al2Cu) which is as a thin fi lm along grain boundaries (Figures 3 and 4). The semi-quantitative chemical analyses of eutectic and Al2Cu particles are shown in Table 2, from which it can be seen that Al2Cu phase might be with or without Fe. The microstructure of SSM consists of fi ner grains of α solid solu tion and eutectic (α+Al2Cu) without Al-2Cu pre cipitation inside grains, Figure 5. The detail of SSM microstructure is shown in Figure 6, while chemi-cal composition is presented in Table 3. The grain re-fi nement was obtained by dendrite braking due to intro-duced argon gas bubbles during solidifi cation. Further-more, the amount of eutectic was increased, Figures 5 and 6.Table 3 EDX analyses of SSMSpectrum Al Fe CuSpectrum 1 98,69 - 1,31Spectrum 2 66,68 - 33,32Spectrum 3 81,60 1,26 17,15The cross sections of chip root samples ob tained during milling are shown in Figures 7 and 8. The cutting conditions were: speed v=27,7 m/min, feed s1=0,562 mm/tooth and depth of cut 3 mm.It can be seen that cutting process consists of inten-sive plastic deformation in primary and secondary cut-ting zone and fracture. The line of the grains texture is approximately parallel in the whole width of chip, Fig-ures 7 and 8. The angle of grain texture changes accord-ing to cut ting conditions and material properties. Only in the small area which is in contact with tool sur face, the line of grain texture is bended (secon dary de-Figure 3 Microstructure of conventionally cast alloyFigure 4  Positions of EDX analyses for conventionally cast alloyTable 2 EDX analyses of conventionally cast alloySpectrum Al Fe Ni Cu ZnSpectrum 1 65,04 1,05 0,45 33,46 -Spectrum 2 89,79 - - 9,46 0,75Spectrum 3 54,64 11,33 0,60 33,43 -Figure 5 Microstructure of SSMFigure 6 Positions of EDX analyses for SSM37METALURGIJA 51 (2012) 1, 34-38P. KOVAČ et al.: THE MICROSTRUCTURE INFLUENCE ON THE CHIP FORMATION PROCESS OF AL-CU ALLOY CAST...formation zone). Actually, grain texture bend ing is re-sulting from contact friction and pressure of the tool rake surface during chip forming process. The angle of texture ψ is higher than the angle of shear plane φ. The lamellas represented in the Brix model (see Figure 1) are actually deformed grains and they are nearly of equal thickness (see Figures 7 and 8). In all specimens, the chip is quasi-continu ous with lamellas and easy breaking. The SSM has smaller grains; consequently the lamellas are thinner than lamellas in conventionally casted alloy where grains are larger. Moreover, in the most cases, lamellas in conventionally alloy are con-sisted of one grain, while in the case of SSM when grains are smaller, lamella are often made of a few.The outside of chip is rough (like saw tooth) as the deformation of grains on this surface is not limited and during cutting, the material is squeezed and fl ows out. The size of “saw tooth” depends on the size of lamellas i.e. on the size of grains; when the grains are smaller, “saw tooth” on the opposite side of the chip are smaller and vice verse. The presence of the eutectic in micro structure in-creases the appearance of cracks during the cutting process. Actually, the micro voids are formed on Al2Cu eutectic particles as they are less deformable than the matrix, Figure 9. Because of this, during shearing, dis-locations are required on the interface. These disloca-tions pile up until their combined energy exceeds the internal energy and then crack or decohesion occurs [11, 12]. The higher amount of the eutectic, the higher number of nucleated voids. Furthermore, coale scence of micro-voids leads to micro cracks appearance. This enables easier process of chip formation and breaking, Fig-ure 10.It can be noticed that besides microstructure refi ne-ment by SSM, some argon gas bubbles might be cap-tured in metal. These bubbles gen erate conditions for appearance of microcracks. Although those microcracks enable easier proc ess of chip forming and braking they might generate the porosity as an undesirable defect in cast.During machining of all tested samples by cutting conditions used, built up edge (BUE) is continually formed which is typical for alu minium, Figure 11. The identifi ed BUE is semi stable high plas tic deformed part of material and it is partially connected with chip and machined surface. The SSM samples have more stable and continual BUE on chip root. Figure 7 Chip root on conventionally cast alloyFigure 8 Chip root on SSMFigure 9 Microvoids in eutectic (SSM)Figure 10 Microcracks appearance trough eutectic in SSM 38  METALURGIJA 51 (2012) 1, 34-38P. KOVAČ et al.: THE MICROSTRUCTURE INFLUENCE ON THE CHIP FORMATION PROCESS OF AL-CU ALLOY CAST...According to all above stated, it might be concluded that SSM has better machinability than conventionally cast Al-Cu.CONCLUSIONSAccording to results obtained a following conclu-sion can be suggested:- During milling, in all specimens used, quasi-contin-uous chips with built-up edge (BUE) were ob-tained.- The chip type depends primarily on the amount and morphology of eutectic in Al-Cu alloy. - Higher amount of eutectic in SSM stimulates micro-voids nucleation and coalescence and further ap-pearance of microcracks. This infl uences easier machinability of SSM than conventionally cast Al-Cu alloy.- The surface fi nish is of poor quality for both types of specimens as the constant BUE is present. Further-Figure 11  Built up edge on the chip root (conventionally cast specimen)more, in the case of SSM some argon gas bubbles are present on the surface contributing to even lower surface fi nish quality.REFERENCES[1] J. Wannasin, D. Schwam, J.A. Yurko, C. Rohloff, G. Woycik, Metal Processing, Solid State Phenomena, 116-117 (2006) 76-79.[2] J. Wannasin, R.A. Martinez, M.C. Flemings, Metal Proces-sing, Solid State Phenomena, 116-117 (2006) 366-369.[3] R.A. Martinez, Formation and Processing of Rheocast Mi-crostructures, Ph.D. Thesis, MIT, Cambridge, MA, June 2004.[4] Z. Fan, International Materials Reviews, 47 (2002) 2, 49-85.[5] E.M. Trent, Metal Cutting, Butherworth-Heinemann, Oxford, UK, 3rd edition, 1991, 188-241.[6] P. Kovac, L. Sidjanin, Tribology in Industry, XVII (1995) 1, 12-16.[7] M. Kronenberg, Machining Science and Application, Per-gamon Press, 1966,  410.[8] M. Sekulić, Z. Jurković, M. HadžiStević, M. Gostimirović, Metalurgija, 49 (2010) 4, 339-342.[9] E.M. Elgallad, F.H. Samuel, A.M. Samuel, H.W. Doty, Journal of Materials Processing Technology, 210 (2010) 13, 1754-1766.[10] M. Tash, F.H. Samuel, F. Mucciardi, H.W. Doty, S. Valtier-ra, Materials Science and Engineering A, 434 (2006) 207–217.[11] L. Sidjanin, P. Kovac, Materials Science and Technology, 13 (1997) 439-444.[12] G. Avramovic-Cingara, Ch.A.R. Saleh, M.K. Jain, D.S. Wilkison, Metallurgical and Materials Transactions A, 40 (2009) 3117-3127.Note:  The English language proofreading was done by Professional lecturer - Sworn Court Translator, Gordana Rašković, Novi Sad, Serbia

The microstructure infl uence on the chip formation process of Al-Cu alloy cast conventionally and in semi solid state

Utjecaj mikrostrukture na proces nastajanja strugotine za konvencionalnu i polutekuću lijevanu Al-Cu leguru

Utjecaj mikrostrukture na proces nastajanja strugotine za konvencionalnu i polutekuću lijevanu Al-Cu leguru

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