THE FRACTOGRAPHY STUDY OF FATIGUE FRACTURE SURFACES IN Al-Si CAST ALLOYS

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1 Powder Metllurgy Progress, Vol.15 (2015), specil issue 138 THE FRACTOGRAPHY STUDY OF FATIGUE FRACTURE SURFACES IN Al-Si CAST ALLOYS L. Hurtlová, E. Tillová, M. Chlupová Astrct The contriution descries frctogrphy study of the Al-Si frcture surfce chrcteristics. The chrcteristics of n Al-Si frcture surfce depend on structurl prmeters morphology nd porosity. The morphology of structurl prmeters nd porosity depends on used technologies of Al-Si cst founding. Recycled (secondry) luminium cst lloys contin more dditions tht form vrious intermetllic phses in microstructure. It is necessry to study the ftigue frcture surfces in order to oserve the influence of structurl prmeters of different size, mount, dispositions, morphology, etc.. The ftigue frcture surfces were oserved using scnning electron microscopy SEM fter the ftigue test. The results showed tht the existence of csting defects nd different morphology of structurl prmeters (especilly eutectic Si prticles nd Fe-rich intermetllic phses) hs considerle influence on ftigue frcture surfces in oth types of experimentl mterils. The ftigue re of stle crck propgtion region consists of trnscrystlline ftigue frcture of α-phse with smooth res (especilly Fe-rich phses) (Stge II). Ftigue stritions were oserved very spordiclly. The finl phse (Stge III) of ftigue frcture consists of trnscrystlline ductile frcture of Al mtrix (α-phse - with plstic strin rnges) nd Cu-rich intermetllic phses; nd with trnscrystlline clevge frcture of Si prticles nd Fe-rich phses. Keywords: Ftigue of Al-Si lloys, ftigue frcture surfces, frctogrphy nlysis of Al-Si lloys INTRODUCTION The most widely used technologies of founding Al csting re snd csting nd die csting (grvity csting; high pressure die csting; low pressure die csting; vcuum die csting; squeeze csting or squeeze forming) [1]. Melting nd csting led to formtion of disturing textures, such s csting disturnces nd disturnces risen y het tretment. Lower cooling rte setting y csting into snd mould (snd csting) cuses grnulr structure nd lower vlues of properties. Higher cooling rte setting y csting into metllic mould (chill csting) cuses fine-grined structure nd higher vlues of properties. The lower mechnicl properties nd reliility of luminium cst lloys cn e principlly cused y the presence of defects nd inhomogeneities, which could e preferentil ftigue initition sites [2]. Ftigue is prolem tht cn ffect ny prt or component tht moves. Automoiles on rods, ircrft wings nd fuselges, ships t se, nucler rectors, jet engines, nd lnd-sed turines re ll suject to ftigue filures [3]. The Al lloys ftigue lifetime study is importnt ecuse of their ppliction in utomotive nd erospce Lenk Hurtlová, Ev Tillová, Mári Chlupová, University of Žilin, Fculty of Mechnicl Engineering, Deprtment of Mterils Engineering, Univerzitná 8215/1, Žilin, Slovk Repulic

2 Powder Metllurgy Progress, Vol.15 (2015), specil issue 139 industries [4]. The verstility of luminium mkes it the most widely used metl second to steel [5]. B. Zhng et l. [6] found tht ftigue crcks initited from porosity in the luminium mteril solidified t slow cooling rtes, while, s cooling rte incresed, the ftigue crcks initited from ner surfce eutectic microconstituents. The process of ftigue generlly consists of three stges (Fig.1) [7]: crck initition (stge I); progressive ( stle ) crck growth cross the component (stge II); nd finl sudden frcture of the remining cross section of the component (stge III). Stges I nd II re lso clled ftigue region. Ftigue frcture or filure of mteril under stress is significntly lower thn yield stress. Frcture occurs fter continuous ccumultion of dmge due to repeted mechnicl, therml or mechnic-therml loding up to filure. In the cse of ftigue filure there is no mcroscopic deformtion of the solid. The whole process occurs in microvolume t the crck tip or t locl site on the surfce during the cretion of ftigue crck nucleus [8]. Fig.1. Three stges of ftigue frcture [3] The present study is prt of lrger reserch project, which ws conducted to investigte nd provide etter understnding of the influence of different csting methods, which chnge the structurl prmeter on the chrcteristics of the frcture surfce in recycled (secondry) AlSi9Cu3 cst lloy. In prticulr, the ftigue frcture surfces of cst luminium-silicon-copper lloy hve een investigted. EXPERIMENTAL MATERIAL AND PROCEDURES The experimentl mterils re cst AlSi9Cu3 lloy (prepred from recycled of luminium scrp) delivered from the compnies Uneko, spol.s.r.o. (snd csting) nd Confl.s. (chill csting). AlSi9Cu3 cst lloy hs lower corrosion resistnce nd is suitle for high temperture pplictions (dynmic exposed csts without extensive requirements on mechnicl properties) - it mens to mx. 250 C. The smples for ftigue tests were extrcted (lthe-turning nd milling opertion) from the delivered circulr rs (snd csting - ø 20 x 300 mm; chill csting - ø 18 x 150 mm). The experimentl mterils were not modified or grin refined. Chemicl composition of oth experimentl mterils ws checked out y using rc sprk spectroscopy nd is shown in Tle 1. T.1. Chemicl composition of experimentl mterils AlSi9Cu3 - snd csting - Uneko, spol.s.r.o. Element Si Cu Mn Zn Mg Fe Ni Ti Sn P Al [wt.%] reminder AlSi9Cu3- chill csting - Confl.s. Element Si Cu Mn Zn Mg Fe Ni Ti Sn P Al [wt.%] reminder

3 Powder Metllurgy Progress, Vol.15 (2015), specil issue 140 Ftigue tests of snd chill smples were crried out on rotting ending ftigue mchine - ROTOFLEX operting t 30 Hz, lod rtio R = -1 nd t room temperture of 20 ± 5 C. In this type of mchine, smple with round cross section is sujected to delweight lod while erings ensure rottion. A given point on the circulr test section surfce is sujected to sinusoidl stress mplitude from tension on the top to compression on the ottom with ech rottion [7]. The ftigue tests of chill csted smples were crried out using Virophores Amsler HFP 5100 testing mchine with symmetricl pushpull lod nd t room temperture of 20 ± 5 C. The ftigue frcture surfces were exmined using scnning electron microscope VEGA LMU II fter testing in order to find the fetures responsile for crck initition. RESULTS AND DISSCUSIION The three stges of ftigue frcture were identified in oth types of experimentl mteril. Within the ftigue test oundries we estlished tht high stress mplitude cuses smll ftigue region nd lrge region of finl sttic rupture (Figs.2, 3). The decresing stress mplitude leds to incresing ftigue region of stle crcks propgtion (Figs.2, 3). Fig.2. Typicl ftigue frcture surfces (Stge I, Stge II, Stge III) for experimentl smples csted into snd mould, SEM ) σ = 88 MP, N f = cycles; ) σ = 54 MP, N f = cycles Fig.3. Typicl ftigue frcture surfces (Stge I, Stge II, Stge III) for experimentl smples csted into metllic mould, SEM ) σ = 85 MP, N f = 2, cycles; ) σ = 50 MP, N f = cycles

4 Powder Metllurgy Progress, Vol.15 (2015), specil issue 141 Stge I initition plce Ftigue frcture initites t the mximl stress concentrtion on the surfce of cyclic loded solids [8]. Previous work hs shown tht the most importnt metllurgicl prmeters ffecting the Al-lloy s resistnce to ftigue re the mount nd size of csting defects (micro-porosities, oxide inclusions nd shrinkge porosities), secondry dendrite rm spcing (SDAS), α-mtrix, Si-prticles nd Fe-rich phses [9-11]. The studies shown tht defects such s pores, microshrinkge nd voids were identified s crck initition plce for oth experimentl mterils. The occurrence of these cst defects is cused smll soluility of hydrogen during solidifiction of Al-Si lloys. It ws confirmed tht if there re mrked cst defects in the structure, they ehve preferentilly s initition plces of ftigue frcture. Cst defects were detected ner the surfce of the test ftigue smples. Detils of initition sites for oth types of experimentl mterils show Fig.4. Fig.4. Frctogrphy nlysis of the initition sites (Stge I), SEM ) snd mould cst lloy; ) chill mould cst lloy. The snd cst smples hve more initition sites compred to the chill cst smples tht hd lwys only one initition site during the experiment. The initition sites for smples cst into snd moulds were more concentrted t one point with the decresing of stress mplitude. Stge II progressive stle crck propgtion cross the smples The ftigue frcture surfce ws influenced very significntly y structurl components (eutectic silicon, intermetllic phses) nd their distriution in the cross section. The stle growth region of smples cst into snd moulds is chrcterised y trnscrystlline ftigue frcture of α-phse with smooth res (Fig.5). The smooth res were identified s Fe-rich phses nd Si prticles y using SEM (Fig.5). The typicl spect of ftigue - stritions - were oserved only in few isolted cses in n re etween stge II nd stge III of ftigue frcture (Fig.5c). The re of stle crck propgtion for mterils cst into metllic mould (stge II) is chrcterised y trnscrystlline ftigue frcture of α-phse which forming fine relieves; intercrystlline ftigue frcture of iron intermetllic phses nd trnscrystlline ftigue frcture of eutectic silicon nd Fe-rich phses, too (Fig.6). The smooth res were identified s Fe-rich phses nd Si prticles y using SEM (Fig.6). The typicl spect of ftigue - stritions - were oserved only in few isolted cses (Fig.6c).

5 Powder Metllurgy Progress, Vol.15 (2015), specil issue 142 c Fig.5 Frctogrphy nlysis of the stle crck propgtion in smples csted into snd mould (Stge II), SEM ) Stge II; ) smooth res Fe-rich phses nd Si prticles; c) re etween Stge II nd Stge III c Fig.6 Frctogrphy nlysis of the stle crck propgtion in smples csted into metllic mould (Stge II), SEM ) Stge II; ) smooth res Fe-rich phses nd Si prticles; c) re etween Stge II nd Stge III.

6 Powder Metllurgy Progress, Vol.15 (2015), specil issue 143 Stge III finl rupture c e Fig.7 The finl rupture frctogrphy nlysis of oth experimentl mterils, SEM ) Stge III in snd mould smples; ) Stge III in metllic mould smples; c) frcture of Si prticles; d) frcture of Fe-rich phses; e) frcture of Cu-rich phses. The lst stge for oth types of experimentl mterils (stge III - Fig.7) consisted of trnscrystlline ductile frcture with plstic strin rnges of Al mtrix (α-phse - Fig.7,) nd trnscrystlline clevge frctures of Si prticles (Fig.7c) nd lso rittle iron

7 Powder Metllurgy Progress, Vol.15 (2015), specil issue 144 intermetllic phses (Fig.7d). The trnscrystlline ductile frcture relted with the Cu-rich intermetllic phses, too (Fig.7e). CONCLUSSIONS Different csting method cused chnges in the morphology of structurl components (eutectic silicon, intermetllic phses sed on Fe). These morphologies significntly ffect the ftigue frcture surfce of experimentl mteril. The pores (csting defects) initited for ftigue crck propgtion in oth types of smples. The hrd nd rittle eutectic Si prticles nd Fe-rich intermetllic phse cuse trnscrystlline nd intercrystlline ftigue frcture nd trnscrystlline clevge frcture; mtrix with Cu-rich intermetllic phses cuse trnscrystlline ftigue frcture nd trncrystlline ductile frcture. The frcture surfce of the finl rupture is formed from trnscrystlline clevge nd ductile frcture. Trnscrystlline clevge frcture ws dominnt on the frcture surfce. Trnscrystlline clevge frcture is relted to the presence of lrge hexgonl plte-si prticles in the structure nd lso rittle iron intermetllic phses. The trnscrystlline ductile frcture of Al mtrix (α-phse) is oserved in the smller surfce. Copper intermetllic phses led to trnscrystlline ductile frcture, too. Acknowledgement The uthors cknowledge the finncil support of the projects VEGA No. 1/0533/15, KEGA No. 044ŽU-4/2014 nd Europen Union - the Project ITMS REFERENCES [1] ville on line [2] Avlle, M., Belingrdi, G., Cvtort, MP.: Proceedings of the Institution of Mechnicl Engineers, Prt L: Journl of Mterils Design nd Applictions, vol. 216, 2002, p. 25 [3] Ftigue Chpter 14. ASM Interntionl. Elements of metllurgy nd engineering lloys, 2008, p. 243 [4] Beln, J.: Key Engineering Mterils, vol. 635, 2014, p. 9 [5] Uhríčik, M., Plček, P., Sovirová, A., Snopiński, P.: Mnufcturing technology, vol. 14, 2014, no. 3, p. 467 [6] Zhng, B., Chen, W., Poirier, R.: Ftigue Frcture Engineering Mteril Structure, vol. 23, 2000, no. 5, p. 417 [7] Bokůvk, O., Nicoletto, G., Guglino, M., Kunz, L., Plček, P., Nový, F., Chlupová, M. In: Ftigue of Mterils t low nd high frequency loding. EDIS, 2014, p. 146 ISBN [8] Plček, P., Chlupová, M., Nicoletto, G., Bokůvk, O.: Eduction Aid for multimedi lectures. CETRA, 2003, p. 177 ISBN [9] Moreir, MF., Fuoco, R.: AFS Trnsctions, vol. 2, 2006, p. 1 [10] Go, YX., Yi, JZ., Lee, PD., Lindley, TC.: Act Mterili, vol. 52, 2004, no. 19, p [11] Bonollo, F., Tovo, R.: Ftigue in Al csting lloys: metllurgicl spects. TALAT Lecture 1254 EAA Europn Aluminium Associte, 1999