UDK 669.715:620.178.3 Pregledniznanstveničlanek ISSN 1580-2949 MTAEC 9, 36(1-2)11(2002) FATIGUE PROPERTIES OF HIGH-STRENGTH Al-Zn-Mg-Cu ALLOYS WITH DIFFERENT LEVELS OF PURITY UTRUJENOSTNE LASTNOSTI VISOKO TRDNIH ZLITIN Al-Zn-Mg-Cu Z RAZLIČNO ČISTOSTJO Maja Vratnica1, Jelisav Čurović2, Zijah Burzić2 1Faculty of Metallurgy and Technology, 81000 Podgorica, Cetinjski put bb, Yugoslavia 2Technical Institute of the Yugoslav Army, 11000 Belgrade, Katani}eva 15, Yugoslavia majaŽcg.ac.yu Prejem rokopisa - received: 2001-03-12; sprejem za objavo - accepted for publication: 2001-10-10 We have investigated the effect of iron and silicon impurities on the endurance limit (Sf) of wrought Al-Zn-Mg-Cu alloys. For four alloys with a similar base chemical composition and different iron and silicon contents we performed standard fatigue tests in the minimum and maximum loading range of R = -1. The fatigue fracture surface was investigated in the region of the crack initiation. We have shown that the greater the iron and silicon levels the lower the endurance limit of the alloys at ambient temperature. Key words: high-strength aluminium alloys, fatigue properties, Fe impurity, Si impurity, intermetallic particles Raziskan je bil vpliv vsebnosti nečistot (železo in silicij) na časovno nihajno trdnost. Standardni preizkusi utrujenosti so bili opravljeni po metodi največje in najmanjše napetosti z R = 1 za štiri zlitine s podobno osnovno kemijsko sestavo in različno vsebnostjo železa in silicija. Prelomna površina je bila preiskana z vrstičnim elektronskim mikroskopom s poudarkom na področju začetka razpoke. PrivečjivsebnostiFe+Sije bila nižja časovna nihajna trdnost. Ključne besede: visoko trdne Al-zlitine, utrujenostne lastnosti, nečistoče (Fe+Si), intermetalne spojine 1 INTRODUCTION High-strength aluminium alloys belong to the Al-Zn-Mg-Cu system. Zinc and magnesium in an aluminium solid solution increase the strength of worked and cast alloys 1,2. These alloys have good casting properties; however, low ductility, lack of toughness and poor fatigue strength prevent their more widespread use. Barely soluble elements, such as iron and silicon, even though only present in small quantities have a predominantly negative influence on the properties and are, therefore, considered as impurities. It was proved experimentally that the higher the iron and silicon content, the greater the quantity of insoluble intermetallic phases in the microstructure 1-6. These phases increase the inhomogeneity of the microstructure and so decrease the fracture toughness 1,6-9, ductility 1,2,6, and, to a lesser degree, the mechanical properties in general 1,8,10. Since these alloys are also used for structural elements that support alternating and impact loads, the aim of this investigation was to determine the influence of the presence of iron and silicon on the fatigue strength. 2 EXPERIMENTAL WORK Four Al-Zn-Mg-Cu alloys with different levels of purity and similar amounts of alloying elements were investigated. Their compositions are shown in table 1. The alloys were prepared in standard industrial conditions. The materials were cast, homogenised, pressed, forged and heat treated. Each casting was heat treated according to the procedure T6: solution annealing, quenching and artificial ageing, with the aim of obtaining the optimum mechanical properties. The test specimens had a circular cross-section and a diameter of 10 mm. The tests were performed on a resonant pulsator that produced stresses by the oscillation of unbalanced masses. The test, as prescribed in the ASTM E 486 standard 11, for the endurance limit (Sf) was conducted for a number of cycles, typically 107 220 -200-i ". 180-|. I-120-1 D ,c- I A al] oy L 3 alloy 'a.. « M > i —i al] m oy I yvr-* A alloj J- o4 io5 io6 107 108 Number of cycles, N Figure 1: S-N curves of the investigated alloys /12/ Slika 1: Odvisnosti S-N za raziskane zlitine MATERIALI IN TEHNOLOGIJE 36 (2002) 1-2 11 M. VRATNICA ET AL.: FATIGUE PROPERTIES OF HIGH-STRENGTH Al-Zn-Mg-Cu ALLOYS Table 1: Chemical composition of the investigated alloys Tabela 1: Kemična sestava zlitin Alloy name Element, mas. % Fe SiTi Cu Zn V Cr Mn Mg B Zr L1 0,16 0,09 0,015 1,55 7,30 0,007 0,18 0,29 2,26 0,003 0,13 L2 0,19 0,07 0,010 1,70 7,78 0,012 0,18 0,24 2,52 0,009 0,120 L3 0,24 0,08 0,020 1,57 7,35 0,004 0,17 0,26 2,31 0,002 0,120 L4 0,29 0,13 0,028 1,35 7,31 0,007 0,19 0,33 2,52 0,003 0,008 Table 2: Parameters of the statistical analysis of the S-N curves Tabela 2: Parametri statistične analize odvisnosti S-N Alloy Mathematical function Log C m Variant Standard deviation Dispersion L1 Sf= C.Nm 2,3923 - 0,0192 1,0721 1,4925 1 : 2,79 L2 Sf= C.Nm 2,4465 - 0,0325 1,0900 1,5613 1 : 3,13 L3 Sf=C.Nm 2,4403 - 0,0355 1,0267 1,2796 1 : 1,88 L4 Sf=C.Nm 2,3798 - 0,0348 1,0359 1,3300 1 : 2,08 or 108. Altogether, 71 round specimens were investigated for all four alloys. To better understand the behaviour of these alloys under conditions of variable loads, the fracture surface was examined in the scanning electron microscope and the different intermetallic phases were identified using EDAX analysis with special attention paid to the fracture surface near the crack initiation point. 3 RESULTS The dependence of stress amplitude on the number of cycles for the four investigated alloys is shown in figure 1. ¦¦Č . |j"*i fa* \ . ¦ > * ¦ -'- ' rČČr..\ ¦¦¦ \ ,rv Figure 2: Fracture initiation surface and EDS spectrum in the crack-initiation area of the L1 alloy Slika 2: Področje začetka preloma in spekter EDS. Zlitina L1 Figure 3: Fracture initiation surface, intermetallic particles and EDS spectrum in the crack-initiation area of the L2 alloy Slika 3: Področje začetka preloma, zrna intermetalne spojine in spekter EDS. Zlitina L2 12 MATERIALI IN TEHNOLOGIJE 36 (2002) 1-2 M. VRATNICA ET AL.: FATIGUE PROPERTIES OF HIGH-STRENGTH Al-Zn-Mg-Cu ALLOYS Figure 4: Fracture initiation surface, intermetallic particles (probably Mg2Si) and EDS spectrum in the crack-initiation area of the L3 alloy Slika 4: Področje začetka preloma, zrna intermetalne spojine (verjetno Mg2Si) in spekter EDS. Zlitina L3 In table 2 the data relating to the statistical analysis of the curves in figure 1 are shown. An SEM picture and EDAX spectra are shown in figures 2 to 5. 4 DISCUSSION The greatest endurance limit (Sf) at 107 cycles of 178 MPa was obtained for the L1 alloy with the lowest content of Fe and Si, whereas the lowest fatigue strength endurance limit of 132 MPa was found for the L4 alloy with the highest content of Fe and Si. Since the amount of alloying elements is similar and they are found in the alloy in a form that could not directly affect the fatigue behaviour, we conclude that the reduction in the endurance limit is caused by the higher impurity content, in other words, the increased amount of insoluble intermetallic phases containing iron and silicon. Figure 5: Fracture surface and EDS spectrum in the crack initiation area of the L4 alloy Slika 5: Področje začetka preloma in spekter EDS. Zlitina L4 It was reported that the share of insoluble particles of intermetallic phases increases in proportion to the amount of iron and silicon 1-6. The greater the number of such particles in the plastically deformed zone the more the local plasticity and fracture resistance are affected and so the crack propagation rate increases 1,8,10,13-15. Consequently, the heterogeneity introduced to the microstructure by the intermetallic particles affects, to a great extent, the fatigue behaviour of the alloys. The analysis of the fatigue fracture surfaces did not show conclusive results, since in the crack-initiation area of the alloys L1 and L4 no particles of intermetallic phases were found using the SEM. Such particles were found on the initial fracture area of the alloys L2 and L3 and identified as (CuFeMn)Al6 and Mg2Si. It seems, therefore, very probable that similar intermetallic particles were lost from the fracture of specimens L1 and L4 or that they remained on the sister fracture surface of the specimen. Qualitative microstructural analysis can be used for a reliable analysis of the composition and a determination of the stoichiometry of intermetallic phases. In this case we calculated it on the basis of the intensity of the EDS spectra 1-4, considering the chemical composition of the alloys and relevant references 1,2,4,16 on phases in alloys of the same or similar type and composition. As a result, the phases are defined as the most probable. 5 CONCLUSION The results of this investigation show that the greater the iron and silicon content in an Al-Zn-Mg-Cu alloy the MATERIALI IN TEHNOLOGIJE 36 (2002) 1-2 13 M. VRATNICA ET AL.: FATIGUE PROPERTIES OF HIGH-STRENGTH Al-Zn-Mg-Cu ALLOYS lower is the endurance limit. On two of the fracture surfaces examined we found particles of intermetallic phases with SEM analyses at the crack-initiation point. We can conclude that the decrease in the endurance limit is due to these phases, which decrease the local ductility and strength of the alloys. 6 REFERENCES 1 A. F. Belov, Promišljenie Aluminievie Splavi, Spravočnik, Metallur-gija, Moskva, 1984, 121-154 2 L. F. Mondolfo, Aluminijum Alloys, Structure and Properties, Butterworths, London, 1976, 844-864 3 Z. Cvijovi}, Doctoral dissertation, TMF, Belgrade, 1987 4 R. Ayr, J.Y. Koo, J.W. Steeds, B.K. Park, Metallurgical Transactions, (1985) 1925 5 R. Ratzi, F. Jeglitsch, F. 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