Fatigue strength of Al-Cu alloy was examined by a repeated tensile mode when the specimens were aged and reversion annealed. The specimen quenched from 723K and aged fully around room temperature showed fatigue strength which depended on the existence of soft layer, while the specimen quenched from 723K and aged in the same way showed fatigue strength independent of the existence of the soft layer. Fatigue strength of the specimen, of which the soft surface layer was removed, was the same for either quenching temperature. Fatigue strength became higher when the soft surface layer was thickened with reversion annealing for 600s at 323K after aging. The soft surface layer was thought to increase fatigue strength of the aged Al-Cu alloy as well as Al-Zn alloy

Kanadani, Teruto

Sakakibara, Akira

Okayama University Digital Information Repository

Effect of the Soft Surface Layer on Fatigue Strength of LowTemperature Aged Al-2mass % Cu Alloy

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering ,Okayama University,VoI.30, No.2, pp. 9 -13, March 1996Effect of the Soft Surface Layer on Fatigue Strength of LowTemperature Aged AI-2mass%Cu AlloyAkira SAKAKIBARA* and Teruto KANADANI**(Received February 7 , 1996)Fatigue strength of Al-Cu alloy was examined by a repeated tensile mode when thespecimens were aged and reversion annealed. The specimen quenched from 723K andaged fully around room temperature showed fatigue strength which depended on theexistence of soft layer, while the specimen quenched from 723K and aged in the same wayshowed fatigue strength independent of the existence of the soft layer. Fatigue strength ofthe specimen, of which the soft surface layer was removed, was the same for eitherquenching temperature. Fatigue strength became higher when the soft surface layer wasthickened with reversion annealing for 600s at 323K after aging. The soft surface layerwas thought to increase fatigue strength of the aged Al-Cu alloy as well as Al-Zn alloy.1. INTRODUCTIONA little softer surface layer has often been found in Al-Zn alloys when they are aged after quenching because ofthe difference of GP zone growth depending on the part of the specimen(1-5l. Fatigue strength, which is known tobe sensitive to the surface condition, was studied on the alloy when various heat treatments were given and variousthird elements were added, and it was revealed that the soft surface layer in the Al-Zn alloy increased fatiguestrength, at least in the repeated tensile mode(6-8).Existence of soft surface layer has also been observed in Al-Cu alloy(9). This alloy, as Al-Zn, is one of thetypical age-hardenable alloys, but its low temperature age hardening is caused by plate shaped GP zones(lO,ll) whilespherical GP zones are -responsible to the age hardening in Al-Zn alloy. Relation of the precipitationmicrostructure in Al-Cu alloy to fatigue strength was examined by Suzuki et al. Difference in fatigue strength wasfound between the alloy which had a microstructure of GP zones and the one which had a microstructure of e'phases (12).This paper reports the effect of the formation of soft surface layer on the fatigue strength in repeated tensilemode when GP[I] zones are produced in the Al·2mass%Cu alloy during the aging around room temperature.2. EXPERIMENTAL PROCEDURES2.1 SpecimenAl-2.0mass%Cu (nominal composition) alloy was made by melting in atmosphere pure metals, 99.996%Al and99.999%Cu, in a high alumina crucible. Ingot, 15mm in diameter and about 150 mm in length, was homogenizedfor 180ks at 753K, hot forged at 753K to a plate of 5mm in thickness. Strips of 1.1mm thick and of O.7mm thickwere made by cold rolling the plate with several intermediate annealings at 753K. Specimens for hardness test,7mm in width and 60mm in length, were cut out from the strip of 1.1mm thick and strain annealed for coarseningthe grain to about 3mm in diameter. Specimens for fatigue test, 4mm in width and 13mm in length, were cut outfrom the strip of O.7mm thickand polished mechanically and electrolytically to the thickness of 0.6mm.Quenching was done by drawing by hand the specimen which had been placed for 3.6ks in the slit betweenaluminum blocks maintained at a prescribed temperature (quenching temperature, TQ) and immersing it into icedwater quickly. Aging was done in an ethanol bath at an aging temperature, TA, and reversion annealing in ansilicon oil bath at a reversion temperature, TR, both to the precision of :to.5K.Electrolytic polishing was given to the specimens after aging/reversion in a perchloric acid-ethanol (1:4)solution at 276K to remove surface layer, the voltage and current density being 20V and 500Alm2•2.2 Hardness TestHardness was measured at room temperature in the same way as reported previously(l-3), with a Vickersmicrohardness tester for indentation loads between 0.25N and 2.9N, and with an ultra microhardness tester(Akashi MZT-1) for indentation loads below O.lON.* Department of Mechanical engineering** Okayama University of Science910 Akira SAKAKIBARA and Teruto KANADANI2.3 Fatigue TestThe specimens which had just been heat treated and the ones whose surface layer, 20 to 30f.UD., was removedafter the heat treatment was attached to a fatigue tester (Shimadzu UF15) and number of cycles before the failurewas measured under the repeated tensile loads at 30Hz in the stress ratio of 1.3. Results and DiscussionFig.l Isothennal curve in resistance and hardness of the Ai-2mass%Cu alloy during aging at 293K after quenchingfrom 823K.3.23.0 c:E2Jh,2fj a:2AFigure 1 shows variation of electric resistivity and hardness with aging time (tA) when Al-2%Cu alloy was agedat 293K after quenching from 823K. Resistivity and hardness, both increase with tA at first and saturate at about30ks of tAo Only GP[I] zones are thought to be formed in such an alloy as is aged around room temperature to thestate where resistivity and hardness no longer change<lO,ll). Thickness of the soft surface layer was estimated bymeasuring hardness in the central region of grains (apart from grain boundary) with various indentation loads.Figure 2 shows variation of hardness with load for the specimen just aged for 600ks at 293K and the ones aged inthe same way and electropolished by several tens f.UD.. Hardness of the specimen as-aged is constant at the loadlarger than 0.49N but becomes lower at O.25N, which indicates that the surface layer is somewhat softer than theinterior and that hardness of the interior is uniform.. Electropolishing by 1Of.UD. increases hardness at evry loadbut the hardness at 0.25N is still lower than that at 0.49N indicating soft surface layer remaining. Furtherpolishing by 1Of.UD. removes the difference in hardness with load, showing absence of soft surface layer. Thisuniform hardness was the same as that obtained whenstill further electropolishing by 30f.UD. was done.Thickness of the soft surface layer (th) is, therefore,considered to be larger than 1Of.UD. and smaller than20f.UD.. Similar results are shown in Fig.3 for TQ =723Kbut th in this case is 20f.UD. or larger, a little thicker thanthat for TQ =823K.Figure 4 shows relation of the number of stresscycles before failure (N) to the stress amplitude (a) for theAl-2%Cu specimen heat treated in the same way as theone in Fig.2. Symbols with rightward arrow mean thatthe specimens did not fail at that number of cycles. a-Ncurve for the specimens whose surface layer about 20f.UD.was removed by electropolishing coincides with that forthe specimens not electropolished, that is, any differenceis not observed in fatigue strength between them. a-Ncurve is also shown in Fig.5 in the case of lowerquenching temperature, TQ = 723K, where the th wasthicker than 20f.UD... There is observed an increase infatigue strength of the specimens not electropolishedrelative to that of the specimens elctropolished but the60 60TQ =823 KTQ =723 KTA=293 K. tA=600ksTA =293 K. tA=600 ks=.... = ",,=JIli >> ~ :I::I: 5050 I- ~ O} ~ O}--&- 20-&-10-0--- 30 fJm • Polished--(}- 20 \-1m• Polished~50 ...I;}- 5040 I I 40 101100 101 10-1 10°10-1 Load I NLoad / NFig.2 Variation ofdependence ofhardness on the indentationload when various thickness of surface layer was removedfrom the specimen aged for 600ks at 293K after quenchingfrom 823K.Fig.3 Variation of dependence of hardness on theindentation load when various thickness of surface layerwas removed from the specimen aged for 600ks at 293Kafter quenching from 723K.Effect of Soft Surface Layer on Fatigue Strength of Aged Al-Cu Alloy 11TQ=723 KTA =293 K • tA=600ks105 106N / cycles60~~ 40..........'0200104107TQ =823 KTA = 293 K • tA= 600 ks105 106N / cycles60~~ 40'"'0 200104Fig.4 a-N plot for the specimens aged for 600ks at 293Kafter quenching from 823K. l':,: surface removed, 0:surface not removed.Fig.5 a-N plot for the specimens aged for 600ks at 293Kafter quenching from 723K. l':,: surface removed, 0:surface not removed.difference is not so distinguished as in the case of AI-Zn alloy(S-S) where th, more than 50IJlll, is remarkably thick.Fatigue strength of the AI-Cu specimen was examined when the soft surface layer was thickened by givingreversion heat treatment after aging. Fig.6 shows change of dependence of hardness on the indentation loadwhen isochronal annealing for 600s at every ten degrees was given to the AI-2%Cu alloy specimen aged andelectropolished. Additional formation of soft surface layer is observed in the early stage of the annealing. AI-2%Cu alloy specimen was annealed for 600s at 323K after aging in the same way as in Fig.4 to form soft surfacelayer 40lJlll thick. Only the surface layer was softened and hardness of the interior remained the same, Hv =52,as was before this reversion annealing. a-N curve for these specimens in Fig.7 shows considerable increase infatigue strength compared with that for the specimens as-aged.Increase in fatigue strength with reversion might be explained, not by the increase in the soft surface layer,but by the change in the microstructure of the alloy because some GP[I] zones should be coarsened while somezones are resolved during annealing at such a low temperature as is fairly low relative to the solvus temperature of2%Cu alloy, 393K(lO). Moreover changes during repeated loading should be taken into account(l3.14). To examinethese effects fatigue test was carried out for the specimens whose soft surface layer, about 30lJlll in thickness, waslargely removed by electropolishing after the same aging and annealing as in Fig.7. The a-N curve for thespecimenselectropolished, in Fig.8, is clearly lower than that before electropolishing and almost coincident withthe one obtained for the specimens as-aged. Similar result to Fig.8 was shown in Fig.9 for the same experimentwith the specimens quenched form lower temperature, 723K. It is reasonable, therefore, to consider that the80 -Ta =823 KTA=293 K tA=180ks TR/K TQ =823K. TA =293K. tA =600 ks~8-i-e- -0-293 60 - TR=323K. tR=600s.-;:/'0- -.- -{)"-30350 o /-- -0- 313 &. '~"7 e A-A- -V- 323 ....... a.// -e- 353 ~ 40 ........ , . 0->........... ~:c -A- 363 ......../ .-.--.- 373 b...............~ ./-y- 383 ""'----y __:f"_.---,............. -0- 393 20 -40 ...?----o_O- -£\.-403~::..--&--£\.--£\.-I I I00.1 1 10 104 105 106 107Load /N/ cyclesNFig.6 Variation ofdependence ofhardness on the indentationload during annealing isochronally the specimen aged for180ks at 293K after quenching from 823K.Fig.7 a-N plot for the specimens annealed for 600s at 323Kafter aging in the same way as in Fig.4. Dashed curverepresents a-N plot for the specimens as-aged.12 Akira SAKAKIBARA and Teruto KANADANI105 106 107N / cyclesTQ=723K. TA=293K. tA=600ks" TR=323K. tR=600s,""......_-60TQ=823 K. TA=293K, tA=600ks 60"TR=323 K. tR=600 s8!."rf!. " ~,~ ....... ,,4040 ......_-"---'0'b ~20 200 0104 105 106 107N / cyclesFig.8 o-N curve for the specimens whose surface layer about301lffi in thickness was removed by electropolishing afteraging and annealing in the same way as in Fig.7. Dashedcurve represents o-N curve for the specimens notelectropolished.Fig.9 o-N curve for the specimens whose surface layerabout 301lffi in thickness was removed by electropolishingafter aging and annealing in the same way as in Fig.7except for TQ=723K. Dashed curve is for the specimens notelectropolished.increase in fatigue strength with reversion annealing is caused by the existence of the surface layer rather thancaused by the change in microstructure.It has been reported that the microstructure of GP zones obtained in the AI-Zn and AI-eu alloy after full agingdepends much on the quenching temperature(15,16). However, (J-N curves for the specimens quenched from 823Kand for the ones quenched from 723K, shown in Fig.8 and Fig.9 respectively, coincide well. This support theexplanation that existence of soft surface layer is more responsible to the increase of fatigue strength than the GPzone microstructure.Figures 10 and 11 shows variation of hardness with depth from the surface for the specimens before and afterfatigue test. For the specimens with soft surface layer, a little hardening is observed near the surface after thefatigue test, (J = 17MPa and N = 105cycles, but no hardening in the interior beyond certain depth from the surface.Even after a large number ofcycles of repeated loading the surface layer remained softer than the interior. On theother hand dependence of hardness on the depth from surface, either before or after the fatigue test, was notobserved at all for the specimens which was electropolished by 30!Jlll, thus they contained no soft surface layer.Hardness near the grain boundary was also examined before and after the fatigue test and the results obtained,Figs.12 and 13, is similar to Figs.10 and 11. Hardening of the soft surface layer during repeated loading cycles,therefore, is considered to be due to the growth of GP zones promoted by vacancies generated during the plasticdeformation as well as the strain hardening. Such a change as this in the soft surface layer may act as a kind of60 TQ=723 KTA=293 K. t A= 600 ks>:J:50r:f = 21 MPao N= 0 } cycles6. N=1054010-1 10° 101Load/N60 TQ=723 KTA =293 K • tA=600 ks30 ~m Electro-polished> A oA oA "'"n..""17<:1::c ~ '=' '='50()= 27 MPao N= 0 } cycles6. N=10540I10-1 10° 10'Load/ NFig. I0 Dependence of hardness on the indentation loadbefore and after fatigue test for the specimen which hadbeen aged for 600ks at 293K after quenching from 723K,thus had a thick soft surface layer.Fig.11 Dependence of hardness on the indentation load beforeand after fatigue test for the specimen which had beenelectro-polished after aging in the same way as in Fig.lO,thus had no soft surface layer.Effect of Soft Surface Layer on Fatigue Strength of Aged Al-Cu Alloy. 1360 TQ =723 KTA=293 K • tA=600 ks60 TQ=723 KTA=293K. tA =600ks40fJ-m Electro-polished>:::c504010-1(j =27 MPao N = 0 }CYcles6. N =10 5100Load / N>:::c50(f=27 MPao N= 0 }CYcieS6. N =1 05100l..oo.d /NFig. 12 Dependence ofhardness near the grain boundary on theindentation load before and after fatigue test for thespecimen which had been aged for 600ks at 293K afterquenching from 723K, thus had a thick soft surface layer.Fig.13 Dependence of hardness near the grain boundary onthe indentation load before and after fatigue test for thespecimen which had been electro-polished after aging in thesame way as in Fig.10, thus had no soft surface layer.buffer against the nucleation and propagation of fatigue cracks under the repeated tensile loading, but details arenot clear at present.4. CONCLUSION. Age hardening and reversion of AI-2mass%Cu alloy was studied with microhardness test and fatigue test inrepeated tensile mode and the following results were obtained:(1) Fatigue strength does not depend on the existence of soft surface layer when the specimen is quenched from823K and aged around room temperature. When quenched from 723K and aged, where thicker soft surface layeris formed, specimens with a soft surface layer show higher fatigue strength than the ones without a soft surfacelayer. Fatigue strength of the specimens without a soft surface layer is identical irrespective of quenchingtemperature.(2) Fatigue strength of the age-hardened specimens increases when the thickness of the soft surface layer isincreased by annealing for 600s at 323K, where reversion occurs.(3) In the AI-Cu alloy the existence of soft surface layer increases fatigue strength in repeated tensile mode as inthe AI-Zn alloy.ACKNOWLEDGMENTAuthors thank to Mr. Tatsuhisa Yamane and Mr. Takayuki Sasakura for their collaboration in hardness test.REFERENCES(1) M.Ohta, H.Yamada, T.Kanadani, ASakakibara and M.Yamada: J.Japan Inst. Light Metals, 33(1983), 212.(2) M.Ohta, T.Kanadani, ASakakibara, H.Yamada and M.Yamada: Phys. Stat. Sol.(a), 77(1983), K49.(3) M.Ohta, T.Kanadani, ASakakibara, H.Yamada and M.Yamada: Phys. Stat. 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Effect of the Soft Surface Layer on Fatigue Strength of LowTemperature Aged Al-2mass % Cu Alloy

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