The effect of the heat treatment on mechanical properties of the A356 alloy was verified in course of tensile tests. Keeping of the alloy during 0,5 to 3 hours in temperature 520 - 550 °C, and subsequent ageing in temperature 165 °C during 5 ÷ 8 hours, enables obtainment of maximal tensile strength Rm. In case of the elongation A5, ageing is of the main importance, and should be performed in temperature 325 °C during 2 ÷ 5 hours

J. Pezda

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221METALURGIJA 53 (2014) 2, 221-224J. PEZDAINFLUENCE OF HEAT TREATMENT PARAMETERS ON THE MECHANICAL PROPERTIES OF HYPOEUTECTIC Al-Si-Mg ALLOYReceived – Prispjelo: 2013-05-21Accepted – Prihvaćeno: 2013-10-10Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 53(2) 221-224 (2014)UDC – UDK 669.715:621.35.001.43:620.17=111J. Pezda, ATH Bielsko-Biała, Faculty of Chipless Forming Technology, Bielsko-Biała, Poland The eff ect of the heat treatment on mechanical properties of the A356 alloy was veriĀ ed in course of tensile tests. Keeping of the alloy during 0,5 to 3 hours in temperature 520 - 550 oC, and subsequent ageing in temperature 165 oC during 5 ÷ 8 hours, enables obtainment of maximal tensile strength Rm. In case of the elongation A5, ageing is of the main importance, and should be performed in temperature 325 oC during 2 ÷ 5 hours.Key words: Al-Si casting alloys, modiĀ cation, thermal analysis, heat treatment, mechanical propertiesINTRODUCTIONThe aging-hardenable cast aluminum alloys, such as A356 alloy, are being increasingly used in automotive industry systems due to their high strength to weight ratio, excellent casting characteristics, and good me-chanical properties including wear resistance, low coef-fi cient of thermal expansion, high thermal conductivity, high corrosion resistance and low cost, providing af-fordable improvements in fuel effi ciency. The alloys are mainly used in the automotive industry (e.g., produc-tion of water-cooled cylinder blocks, suspension arms, and wheels), aerospace (e.g. aircraft pump parts, aircraft fi tting and control parts) and engineering applications, where are used heavy duty structural parts requiring high strength [1-7]. Solidifi cation after casting process predetermines technological and mechanical properties of cast compo-nents via the resulting microstructure. To commonly used methods aimed at improvement of mechanical properties of the Al-Si castings belong grain refi nement, modifi cation and heat treatment.Grain refi nement is done by adding of Ti-B or Ti-C based grain refi ners into the melt to refi ne the grain size of the casting into fi ne-equiaxed structure [8-9].The modifi cation most often resolves itself into metal batch of such elements like Na, Sr , Sb etc. in order to  change the silicon morphology from acicular fl ake to fi -brous, resulting in improved ductility and toughness [10-11]. Of these, only Sr , Na, and Sb produce signifi  cant modifi cation at low levels of addition [12-14]. Thus, they are the only elements used widely in industry.Further improvement of mechanical properties en-ables is possible owing to use of heat treatment. The most used heat treatment for the Al-Si-Mg alloy is T6 treatment, which consists in solution heat treatment and natural or artifi cial ageing [15].The precipitation hardening through  heat treatment will precipitate the alloying elements in the form of fi ne coherent particles of Mg2Si and Al2Cu inside the grains during the aging stage to harden the alloy.Reduction of manufacturing costs of castings en-forces implementation of a procedures ensuring optima (short) time of performer heat treatment. Objective of the present work is determination of an effect and selection of optimal parameters of T6 heat treatment for the A356 alloy in aspect of obtainment of improvement of its mechanical properties.EXPERIMENTAL METHODTo the investigations was used hypo-eutectic A356 (EN AC-AlSi7Mg0,3) alloy, having chemical composi-tion as shown in Table 1.Table 1 Chemical composition /mas. %Si Cu Zn Fe Mg Ti Mn Ni Pb Al7,2 0,25 0,3 0,6 0,3 0,05 0,27 0,04 0,02 restInvestigated alloy was melted in an electric resist-ance furnace. After melting the alloy was refi  ned with Rafal 1 preparation in quantity of 0,4 % of the mass charge, in temperature 730 oC and next, after 60 min-utes, 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).To defi ne temperature ranges of solutioning and age-ing treatments, one performer registration of heating (melting) of the alloy with use of thermal-derivative analysis method (ATD).In the Figure 1 are presented curves of the ATD method of the investigated alloy.222  METALURGIJA 53 (2014) 2, 221-224J. PEZDA: INFLUENCE OF HEAT TREATMENT PARAMETERS ON THE MECHANICAL PROPERTIES OF HYPOEUTECTIC...Implemented ATD method consists in continuous registration of temperature of the alloy in course of its crystallization, resulting in generation of a thermal curve (t = f(τ)), depicting proceeded thermal processes, and simultaneously plotted its derivative (dt/dτ), which emphasizes less distinct transformations on the thermal curve. It enables quick and accurate assessment of qual-ity of liquid metal.Modifi ed alloys were poured into permanent mould to production of standardized specimens of castings to the strength tests. Permanent mould was heated to tem-perature of 250 oC. The heat treatment was performed for the refi ned and modifi ed alloy. Poured specimens had under gone dispersion hardening with holding in temperature near to temperature of solidus. The treat-ment consisted in heating of poured specimens to tem-perature of solutioning, holding in such temperature, and subsequent cooling down in cold water (20 oC), and next artifi cial ageing.The temperature range of solutioning heat treat-ments of the investigated alloy (465 - 550 oC) and the range of ageing temperatures (165 - 325 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. After performed heat treatments, top determine ob-tained mechanical properties of the investigated alloy , one performed tensile tests on the ZD-20 testing ma-chine according to the EN ISO 6892-1:2010 standard.To determine temperatures and durations of solu-tioning and ageing treatments, aimed at improvement of mechanical properties of the investigated alloy, one im-plemented three-stage fraction plan of the investigations with four variables, three blocks and 27 systems.RESULTSThe tensile strength Rm of the alloy after refi nement and modifi cation amounted to 196 - 203 MPa. After performed heat treatment, obtained tensile strength Rm was included within the range from 154 to 335 MPa. In the Figure 2 are presented values of the tensile strength Rm of the investigated alloy for individual 27 systems of assumed tree-stage plan of the testing with four variables (tensile strength of initial alloy prior the heat treatment was marked with symbol “M”).The highest growth of the tensile strength in case of the system no. 25 (solutioning temperature - 550 oC; so-lutioning time - 3 hours, ageing temperature - 165 oC; ageing time - 8 hours) and system no. 19 (solutioning temperature - 550 oC; solutioning time - 0,5 hour; age-ing temperature - 165 oC; ageing time - 5 hours), respec-tively 335 and 315 MPa. Slightly lower tensile strength within limits of 300 MPa was obtained for the systems no. 4, no. 10 and no. 13, being characterized by, like in case of the systems no. 19 and no. 25, low ageing tem-perature (165 oC) during 5 to 8 hours.The lowest tensile strength was obtained in case of the systems 3, 9, 12, 21, 27, which were characterized by high temperature of the ageing (325 oC) in the full range of ageing times. Obtained tensile strength for these systems, amounting to 158 - 163 MPa shows a distinct drop, comparing with the alloy without the heat treatment.Effect of temperature and time of performed solu-tioning and ageing heat treatments on change of the ten-sile strength Rm is presented in the spatial diagrams (Figure 3). In case of temperature and time of ageing treatment (Figure 3a) were taken constant values of ageing parameters - ageing temperature - 180 oC and time of the ageing – 8 hours. For temperature and time of the ageing (Figure 3b), one applied temperature of the solutioning equal to 550 oC and solutioning time equal to 0,5 hour.Obtaining the highest tensile strength determines adoption of solutioning temperatures in the range of 520 - 550 oC, solutioning times from 0,5 to 3 hours, age-ing temperatures up to 165 oC and ageing times from 5 to 8 hours.In case of the elongation A5, its value for the alloy after refi nement and modifi cation amounted to 6 - 7 %. After performed heat treatment, obtained elongation A5 was included within range from 3,7 to 15,8 %.Figure 1 Curves of the ATD method Figure 2  The tensile strength Rm change of the A356 alloy for individual systems of the testing plan223METALURGIJA 53 (2014) 2, 221-224J. PEZDA: INFLUENCE OF HEAT TREATMENT PARAMETERS ON THE MECHANICAL PROPERTIES OF HYPOEUTECTIC...In the Figure 4 are presented, in the same way like in case of the tensile strength Rm, changes of the elonga-tion for individual systems of the testing plan.The highest elongation A5, amounting to 15,8 % was obtained for the system no. 21 (Figure 4), which was characterized by the following parameters of the heat treatment: solutioning temperature - 550 oC; solutioning time - 3 hours ageing temperature - 165 oC; ageing time - 8 hours. Equally high elongation (15,3 and 14,9 %) was obtained for the system no. 12 (solutioning tem-perature - 520 oC; solutioning time - 0,5 hour , ageing temperature - 325 oC; ageing time - 2 hours), system no. 27 (solutioning temperature - 550 oC; solutioning time - 3 hours, ageing temperature - 325 oC; ageing time - 2 hours) and system no. 24 (solutioning temperature - 550 oC; solutioning time - 1,5 hour , ageing temperature - 325 oC; ageing time - 5 hours).The lowest elongation amounting from 3,8 to 5,7 %  was obtained for the systems no. 4, 7, 10, 14, 20, which  were characterized by the low temperature of the ageing  Figure 3  The eff ect of temperature and time of heat treatment on tensile strength of the A356 alloy: a) for temperature and time of solutioning, b) for temperature and time of ageing treatmentFigure 4  The elongation A5 change of the A356 alloy for individual systems of the testing plan(165 - 235 oC) and time of ageing 5 - 8 hours. Hence, ob-tained elongation A5 for these systems constitutes a drop  with respect to initial alloy (without heat treatment).In the Figure 5 is presented an ef fect of temperature  and time of solutioning treatments on change of the elon-gation A5 of the investigated alloy at constant parameters of ageing treatment (Figure 5a) amounting to 180 oC and 2 hours, and constant parameters of the solutioning (Fig-ure 5b) amounting to 535 oC and 0,5 hour.Figure 5  The eff ect of temperature and time of heat treatment on elongation A5 of the A356 alloy: a) for temperature and time of solutioning, b) for temperature and time of ageing treatment224  METALURGIJA 53 (2014) 2, 221-224J. PEZDA: INFLUENCE OF HEAT TREATMENT PARAMETERS ON THE MECHANICAL PROPERTIES OF HYPOEUTECTIC...Obtaining the highest tensile strength determines  adoption of solutioning temperatures in the range of 520  - 550 oC, solutioning times from 0,5 to 3 hours, ageing  temperatures 325 oC and ageing times from 2 to 5 hours.Limitation of tendency to rapid drop of the elonga-tion A, due to ageing in low temperatures, is connected with shortening of ageing duration to 2 hours (system no. 1, 16 and no. 22 - Figures 2 and 4), what enables obtainment of the tensile strength within limits of 280 MPa and elongation A5 from 8 - 11 %.CONCLUSIONSSelection of suitable parameters of performed T6 heat treatment is a prerequisite to obtain required efects in form of changed mechanical properties ( Rm and A5) of the investigated alloy.Ageing heat treatment is of the main importance in process of the heat treatment, because temperature and time of the ageing have fundamental efect on growth of the tensile strength Rm and obtained elongation A5. Low ageing temperatures (165 oC) ef fect advanta-geously on growth of the tensile strength, being simul-taneously a factor limiting (reducing) the elongation. Whereas, increased temperatures of the ageing (325 oC), having effect on improvement (growth) of the elonga-tion, result in distinct drop of the tensile strength.REFERENCES[1] J. R. Davis (Ed.), Aluminum and Aluminum Alloys, ASM International, Material Park, Ohio, 1993, pp. 623.[2] W.B. Lee, Y.M. Yeon, S.B. Jung, The im- provement of mechanical properties of friction-stir-welded A356 Al alloy, Materials Science and Engineering A, 355(2003), 154-159.[3] P. D. Lee, A. Chirazi, D. See, Modeling microporosity in aluminum-silicon alloys: a review , Journal of Light Me-tals, 1(2001), 15-30.[4] Z. Górny, Odlewnicze stopy metali nieżelaznych, WNT, Warszawa, 1992, pp. 188-190.[5] Q.G. Wang, Plastic Deformation Behavior of Aluminum Casting Alloys A356/357, Metallur gical and Materials Transactions A, 35( 2004), 2707-2718.[6] M. Merlin, G. 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Pezda, Effect of modifying process on mechanical pro-perties of EN AB-42000 silumin cast into sand moulds, Archives of Foundry Engineering, 9(2009), 187-190.[12] S. Hegde, K. N. Prabhu, Modifi cation of eutectic silicon in Al–Si alloys, Journal of Materials Science, 43(2008), 3009-3027.[13] R. Francis, J. Sokolowski, Prediction of the aluminum sili-con modifi cation level in the AlSiCu alloys using artifi cial neural networks, Metalur gija - Journal of Metallur gy, 14(2008), 3-15.[14] J. F . Major , Porosity Control and Fatigue Behavior in A356-T61 Aluminum Alloy, AFS Transactions, 105(1997), 901-906.[15] D. Apelian, S. Shivkumar , G. Sigworth, Fundamental Aspects of Heat Treatment of Cast Al-Si-Mg Alloys, AFS Transactions, 97(1989), 727-742.Note:  The responsible translator for English language is Andrzej Kapłon, Bielsko-Biała, Poland

Influence of heat treatment parameters on the mechanical properties of hypoeutectic Al-Si-Mg alloy

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