Presented work discusses research results concerning the effect of the T6 heat treatment process, including soaking of the alloy near the solidus temperature, holding in this temperature and next cooling in cold water (20 oC), as well as exposing to the artificial ageing to check the change in HB hardness and microstructure of the EN AC-AlSi12Cu-NiMg (EN AC-48000) alloy modified with strontium and cast into metal moulds. The temperature range of solutioning and ageing treatments was selected on the basis of crystallization curves recorded with the use of thermal-derivative method. Performed investigations enabled to determine the optimal parameters (temperature and time) of solutioning and ageing heat treatments and their effect on the change in alloy’s hardness

J. Pezda

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63METALURGIJA 53 (2014) 1, 63-66J. PEZDATHE EFFECT OF THE T6 HEAT TREATMENT ON HARDNESS AND MICROSTRUCTURE OF THE EN AC-AlSi12CuNiMg ALLOYReceived – Prispjelo: 2013-03-18Receiver – Prihvaćeno: 2013-05-25 Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 53(1) 63-66 (2014)UDC – UDK 669.715:621.35.001.43:620.17=111J. Pezda, ATH Bielsko-Biała, Faculty of Chipless Forming Technology, Bielsko-Biała, PolandPresented work discusses research results concerning the eff ect of the T6 heat treatment process, including soaking of the alloy near the solidus temperature, holding in this temperature and next cooling in cold water (20 oC), as well as exposing to the artifi cial ageing to check the change in HB hardness and microstructure of the EN AC-AlSi12Cu-NiMg (EN AC-48000) alloy modifi ed with strontium and cast into metal moulds. The temperature range of solution-ing and ageing treatments was selected on the basis of crystallization curves recorded with the use of thermal-de-rivative method. Performed investigations enabled to determine the optimal parameters (temperature and time) of solutioning and ageing heat treatments and their eff ect on the change in alloy’s hardness.Key words: aluminum alloys, modifi cation, thermal analysis, heat treatment, hardnessINTRODUCTIONThe Al-Si alloys, commonly named as silumins, per-tain to the most popular aluminum casting alloys. Me-chanical and technological properties of castings made from these alloys depend on the correctly performed process of melting and pouring, the structure of the casting and the mould, as well as their heat treatment. Current investigations connected with the improve-ment of mechanical properties of aluminum alloys per-tain mostly to the advancement in selection of alloy addi-tions methods, using contemporary acquired knowledge of the synthesis of alloys, the improvement in selection of complex modiﬁ ers, the reduction of quantities of the hy-drogen and gaseous porosity, the reduction of non-metal-lic impurities, the development and the implementation of modern heat treatment technology [1-5].A standard heat treatment consists in solutioning (holding the alloy in temperature below temperature of the eutectic reaction in order to dissolve the precipita-tions of Mg2Si, homogenize the chemical elements con-centration on the cross-section of dendrites of the α phase and the change in the silicon con precipitations morphology), and ageing (soaking of the supersaturated alloy to separate strengthening phases from the super-saturated solid solution – in case of the silumins com-prising magnesium or copper only, the precipitation strengthening is obtained as a result of the phases pre-cipitation of Mg2Si, Al2CuMg and Al2Cu) [6-8]. Hold-ing the castings in constant temperatures during prede-termined period of time results in the improvement of the mechanical properties such as: Rm tensile strength and HB hardness; and in simultaneous worsening of plasticity (A5, KCV). Due to the fact that the growth of alloy’s strength after heat treatment is very often ac-companied with the reduction of the plasticity, their op-timal relation should be selected depending on a given application of the alloy.A remedy in such case could be the implementation of new methods of heat treatment, like for instance the thermo-cyclic heat treatment (TCO) consisting in mul-tiple heating and cooling of products [9], the silicon spheroidization treatment [10] or the usage of optimiza-tion methods to select the process’ parameters (temper-ature and duration of solutioning and ageing treatments), basing on the analysis of their effect on the change of mechanical and technological properties of the alloys [11-14]. Another very important issue connected with heat treatment of the silumins is the selection of tem-perature and the duration of solutioning and ageing treatments. It is connected with the necessity to obtain the optimal results, for instance the improvement of me-chanical properties of processed alloy, as well as with the economical aspect of heat treatment operations.EXPERIMENTAL METHODIn the course of investigations the eutectic EN AC-48000 (AlSi12CuNiMg) alloy was used, which was melted in an electric resistance furnace. After melting the alloy was reﬁ ned with Rafal 1 preparation in quan-tity of 0,4 % of the mass charge, in temperature 730 oC and next, after 60 minutes, the oxides and the slag from the metal-level were removed and the modiﬁ cation treatment with strontium was performed, using AlSr10 master alloy in quantity of 0,4 % of the mass charge (0,04 % Sr).64  METALURGIJA 53 (2014) 1, 63-66J. PEZDA: THE EFFECT OF THE T6 HEAT TREATMENT ON HARDNESS AND MICROSTRUCTURE OF THE EN...The process of solidiﬁ cation and soaking (melting) was recorded with the use of the thermal derivative method (ATD method), using the automatic Crystaldi-mat analyzer. Figure 1 shows the recorded curves from the ATD method with temperature range of solutioning and ageing treatments marked.Test pieces used in the hardness test were cast in metal mould heated to the temperature of 250 oC. Cast test pieces from the investigated material underwent so-lutioning and artiﬁ cial ageing treatments. After per-forming the heat treatment, hardness of the alloy was measured with the use Brinell hardness tester of the PRL 82 type, with ø 10 steel ball, at 9800 N load sus-tained for 30 seconds.The chemical composition of the investigated alloy is shown in Table 1. Table 1 Chemical composition /mas. %Chemical composition / mass %Si Cu Zn Fe Mg11,0 1,15 0,15 0,50 1,10Ti Mn Ni Cr Al0,05 0,35 1,35 0,09 restThe analysis of the chemical composition was per-formed with the use of the emission spectroscopy (an emission spectrometer type GDS 850A with glow-dis-charge).The temperature range of solutioning heat treat-ments of the investigated alloy (500 - 530 oC) and the range of ageing temperatures (180 - 310 oC) are marked on the thermal curve (Figure 1). Solutioning time amounted from 0,5 to 3 hours, while ageing time was within the interval between 2 to 8 hours.To determine temperatures and durations of solu-tioning and ageing treatments aimed at improvement of HB hardness of the investigated alloy, one implemented three-stage fraction plan of the investigations with four variables, three blocks and 27 systems.Photos of metallographic structures were made with the use of Neophot 32 microscope and the computer picture analysis system - MultiScan.RESULTSHardness of the alloy after reﬁ nement amounted to 75 HB 10/1000/30. The modiﬁ cation did not result in any visible growth of hardness of the alloy, which amounted to 77 HB 10/1000/30. After performing heat treatment, obtained hardness of the alloy amounted to HB 10/1000/30, and was in the range from 59 to 143. Figure 2 shows the average values of HB hardness of the EN AB-48000 alloy after heat treatment in rela-tion to the alloy without heat treatment for 27 systems of assumed testing plan (with symbol “M” is marked the test piece from modiﬁ ed alloy, without heat treat-ment).The comparison of the obtained average parameters taken from the investigations of the alloy after heat treatment and without heat treatment, shows the highest growth of HB hardness in case of the system no. 13 (so-lutioning temperature - 520 oC; solutioning time - 1,5 hour, ageing temperature - 180 oC; ageing time - 5 hours) and system no. 19 (solutioning temperature - 535 oC; solutioning time - 0,5 hour; ageing temperature - 180 oC; ageing time - 5 hours) (143 HB 10/1000/30).The lowest hardness was obtained in case of the sys-tems 7, 9, 18, 24, which were characterized by the high temperature of ageing (310 oC) in the full range of age-ing times. Its value was within the range of 59 - 65 HB 10/1000/30, and it was decreased in comparison to the reﬁ ned and the modiﬁ ed alloy.Figure 3 shows a spatial diagram of the effect of temperatures and solutioning time on HB hardness 10/1000/30 for the investigated alloy with ﬁ xed value of the ageing temperature - 180 oC and the time of age-ing – 2 hours.The effect of temperature and time of ageing treat-ment on HB hardness 10/1000/30 at constant tempera-ture (535 oC) and solutioning time (0,5 hour) is present-ed in Figure 4.Figure 5 shows structures of the alloy after reﬁ ne-ment and modiﬁ cation.Figure 1 Curves of the ATD methodFigure 2  The HB hardness change of the EN AB-48000 alloy for individual systems of the testing plan65METALURGIJA 53 (2014) 1, 63-66J. PEZDA: THE EFFECT OF THE T6 HEAT TREATMENT ON HARDNESS AND MICROSTRUCTURE OF THE EN...)1(02,003,001,002,018,04,014,294,023,11,10158,403,1252434232412124432221xxxxxxxxxxxxxxxxHB−−−+−−−+−++−=where: x1 – solutioning temperature, x2 – solutioning time, x3 – ageing temperature, x4 – ageing time. Correla-tion coefﬁ cient R2=0,98.Figure 7 shows the observed (real) and the forecast values of the HB hardness of the investigated alloy.Three ranges of HB hardness (60 - 70; 90 - 110; 120 - 150) shown in Figure 7 are directly related with tem-perature of ageing treatment amounted respectively to (310, 235 and 180 oC).Alloy structures after performing heat treatment op-erations in case of the test pieces taken from the system characterized by the highest value of hardness (system no. 13 - solutioning temperature - 520 oC; solutioning time - 1,5 hour; ageing temperature - 180 oC; ageing time - 5 hour) are shown in Figure 6.As it can be seen in Figure 6, the morphology of the microstructure changed after the T6 heat treatment. The irregular eutectic phase was converted into ﬁ ne sphe-roidized Si particles uniformly distributed in the Al ma-trix, what signiﬁ cantly improves the mechanical prop-erties.Based on the results of the performed investigations one described, with mathematical dependence (1) in form of second-degree polynomial, an effect of heat treatment parameters on the change of HB hardness of the alloy.Figure 3  The eff ect of temperature and time of solutioning on HB hardness of the EN AB-48000 alloy (ageing temperature - 180 oC; ageing time - 2 hours)Figure 4  The eff ect of temperature and time of ageing treatment on HB hardness of the EN AB-48000 alloy (solutioning temperature - 535 oC; solutioning time - 0,5 hour)Figure 5  Microstructure of the EN AC-48000 alloy: a) after refi nement, b) after modifi cationFigure 6  Microstructure of the EN AC-48000 alloy after heat treatment66  METALURGIJA 53 (2014) 1, 63-66J. PEZDA: THE EFFECT OF THE T6 HEAT TREATMENT ON HARDNESS AND MICROSTRUCTURE OF THE EN...CONCLUSIONSThe improvement of mechanical properties of the EN AB-42000 silumin in the aspect of its hardness change due to the performance of the T6 heat treatment is possible only in case of the selection of suitable solu-tioning and ageing treatment parameters. The implementation of the ATD method enabled ini-tial determination of the temperature range of solution-ing and ageing treatments of the investigated alloy.Obtaining the highest hardness determines the adop-tion of:- ageing temperatures up to 180 oC,- ageing times from 2 to 5 hours,-  solutioning temperatures in the range of 520 - 535 oC,- solutioning times from 0,5 to 1,5 hour.The usage of increased temperatures of ageing treat-ment has an adverse effect on the change of HB hard-ness of the investigated alloy, causing its decrease.REFERENCES[1] J. Sobczak, Podstawy syntezy stopów, Instytut Odlewnic-twa, Kraków, 1997, pp. 98-127.[2] L. 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Starink, Precipitates and intermetallic phases in precipitation hardening Al–Cu–Mg–(Li) based al-loys, International Materials Reviews, 50(2005), 193-215.[9] V. K. Fediukin, Termotsiklicheskaia Obrabotka Metallov i Detalei Mashin, Mashinostroenie, Lenigrad (1989).[10] E. Orgis, A. Wahlen, H. Luchinger, P. J. Uggowitzer, On the silicon spheroidiza- tion in Al–Si alloys, Journal of Light Metals, 2(2002), 263-269.[11] W. Orłowicz, M. Mróz, M. Tupaj: Selecting of heat treat-ment parameters for AlSi7Mg0,3 alloy, Archives of Found-ry, 6(2006), 350-357.[12] J. Pezda, Predicting of mechanical properties of EN AB-46000 alloy subjected to dispersion hardening, Archives of Foundry Engineering, 11(2011), 103-108.[13] A. Manente, G. Timelli, Optimizing the Heat Treatment Pro-cess of Cast Aluminium Alloys, Recent Trends in Processing and Degradation of Aluminium Alloys, 9(2011), 197-220.[14] J. Pezda, Effect of selected heat treatment  parameters of AK9 silumin on its tensile strength, Archives of Foundry Engineering, 8(2008), 81-84.Note:  The responsible translator for English language is Rafał Mazur, Bielsko-Biała, PolandFigure 7  The diagram of the forecast and the observed values of the HB 10/1000/30 hardness

The effect of the T6 heat treatment on hardness and microstructure of the en AC-AlSi12CuNiMg alloy

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