Predicting of mechanical properties of EN AB alloy subjected to dispersion hardening

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1 A R C H I V E S of F O U N D R Y E N G I N E E R I N G Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences ISSN ( ) Volume 11 Issue 4/ /4 Predicting of mechanical properties of EN AB alloy subjected to dispersion hardening J. Pezda Institute of Chipless Technology, ATH Bielsko-Biała, Willowa, Bielsko - Biała Corresponding author. address: jpezda@ath.bielsko.pl Received ; accepted in revised form Abstract Improvement of silumin properties in range of classic methods involves change of morphology of silicon precipitation through: modification treatment of the alloy, maintaining suitable temperature when superheating and pouring into moulds, as well as perfection of heat treatment processes. Dispersion hardening with holding of the alloy in temperature near to temperature of solidus, consisting in heating of poured specimens up to temperature of solutioning, holding the specimens in such temperature, and next cooling down in cold water (0 0 C) and next artificial ageing, what have effect on change of mechanical properties of EN AB alloy, while selection of suitable parameters of dispersion hardening treatment is a condition of obtainment of positive effects in form of improved mechanical properties. Obtained dependencies enable determination of mechanical properties of the investigated alloy before commencing of solutioning and ageing treatments. Keywords: Modification, ATD, Heat treatment, Mechanical properties 1. Introduction New methods of melting, pouring into moulds and heat treating of casting alloys, enabling reduction of materials consumption through improvement of their mechanical and technological properties were extorted by modern technology of production. The most common casting alloys produced on base of aluminum are silumins, i.e alloys of Al-Si system. It is connected with wide series of operational and technological advantages of this group of alloys [1-4]. Silumins belong to alloys which are characteristic of low specific gravity, relatively low melting temperature, good thermal conductivity and corrosion resistance, satisfactory strength parameters in ambient and increased temperature, as well as excellent technological parameters (good machinability, good castability, low shrinkage). To important disadvantages of the silumins which restrict their application as heavy duty machinery components is susceptibility to generation of coarse grain structure and low (comparing with e.g. cast iron) level of plasticity [1-]. Precipitations of silumin present in Al-Si alloys in form of compact primary precipitations and ramified lamellas in α-al+si eutectic mixture, constitute nearly pure wall-form crystals of this elementary substance [3, 5-8], creating areas of stress concentration and initiation of micro-cracks. Important role in elimination of these harmful phenomena is played by spheroidization of eutectic silumin in process of modification. Molecules of silicon, present in normal conditions of cooling operation as thick and acicular crystals, act as cracking inhibitors, reducing mechanical properties. Modification with strontium changes morphology of silicon from acicular to fibrous one, what results in considerable reduction of Si molecules size, and A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 4 / 0 1 1,

2 corresponding to it growth of quantity of molecules on unitary surface [9]. Obtained structure of a casting has direct effect on mechanical and technological properties of machinery parts; and therefore suitable implementation of knowledge on crystallization to control of kinetics of the crystallization of produced alloys, in order to optimization of obtained structure and introduction of modern methods of heat treatment, enabling significant improvement of strength parameters of alloys, belong to significant factor which leads to improvement of castings quality. Crystallization processes of near-eutectoid and hypo-eutectoid silumins, as well hyper-eutectoid ones, are broadly described in the references [1-4, 10-1]. Methods of continuous selection of alloying additives (with use of attainments of synthesis of alloys) [, 4, 13-16], modification of silumin structure [1-3, 13, 16-0] or modern technologies of their heat treatment [-3, 1-3] belong to the main areas of investigations aimed at improvement of mechanical and technological properties of silumins. Heat treatment of aluminum alloys, aimed mainly at growth of their strength consists in dispersion hardening (precipitation hardening), i.e. on successive solutioning treatment of solid solution and ageing. The main condition which constitutes basis to precipitation hardening is reduction of limiting solubility of alloying constituents in solid state together with reduction of temperature [, 3]. Generally known methods of heat treatment, connected with holding of the castings in constant temperature during predetermined period of time result in improvement of mechanical properties such as: tensile strength R m and hardness HB, with simultaneous worsening of plasticity (A 5, KCV). Due to fact that the most often growth of strength of alloy after heat treatment is accompanied by reduction of plasticity, optimal composition of the alloy should be selected depending on a given application of the alloy. Solution in such case could be implementation of new methods of heat treatment like for instance thermo-cyclic heat treatment (TCO) consisting in multiple heating and cooling of a products [4], Silicon Spheroidization Treatment - SST) [5] or implementation of optimization methods to select parameters of the process (temperature and duration of solutioning and ageing treatments) based on analysis of their effect on change of mechanical and technological properties of alloys [6-8].. Methodology of the research To the research work one used the EN AB (AlSi9Cu3(Fe)) alloy. To prepare specimens to the testing, the alloy was melted in electric resistance furnace in temperature of about 70 C. The successive treatment consisted in refining. To the refining one used Rafal 1 preparation on quantity of 0,4% of mass of charge. Refined alloy, after removal of oxides and slug from metal-level, was modified with AlSr10 master alloy in quantity of 0,5% of mass of charge (0,05% Sr). Modified alloys were poured into permanent mould to production of standardized specimens of castings to strength tests. Permanent mould was heated to temperature of 50 o C. Poured specimens had underwent dispersion hardening with holding in temperature near to temperature of solidus. The treatment consisted in heating of poured specimens to temperature of solutioning, holding in such temperature, and next cooling down in cold water (0 o C), and next artificial ageing. Temperature ranges of solutioning and ageing treatments were selected on base of recorded curves from ATD method. The ATD method has been used to registration of crystallization of alloys and metals for many years, both in research work and to control of alloys quality within industrial environment [, 11, 1, 9-31]. Process of alloy solidification and melting was recorded with use of fully automated Crystaldimat analyzer The ATD analysis was performed for raw alloys, refined alloys, and the alloys refined and modified with strontium. Diagrams showing course of crystallization of refined and modified alloy with plotted ranges of temperature of solutioning and ageing treatments are presented in the Fig. 1. t, o C τ, s Fig. 1. Curves of the ATD method for EN AB alloy In the Table 1 are listed parameters of heat treatment operations for three stage plan of the testing with four variables, on base of this plan there was determined an effect of temperatures, durations of solutioning and ageing treatments on tensile strength R m, elongation A 5, impact strength KCV and hardness HB of the investigated alloys. Temperatures of solutioning and ageing treatments were selected on base of points values from ATD curves (Fig. 1). Table 1. Heat treatment parameters of EN AB alloy Solutioning temperature t p [ o C] Solutioning duration p [min.] Ageing temperature t s [ o C] Ageing duration s [min.] t p t s t p t s t p t s After accomplished heat treatments the specimens to strength tests were prepared according to PN-88/H-8800 standards, whereas static strength tests were performed with use of ZD-0 tester. Chemical composition of the investigated alloy is presented in the Table. Analysis of the chemical composition was performed with use of spectrometry method (emission spectrometer with arc excitation of GDS 850A type) 104 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 4 / 0 1 1,

3 Table. Chemical composition of EN AB alloy EN AB Si Fe Cu Zn Ti Mn Ni Sr Pb Cr Mg Al [%] [%] [%] [%] [%] [%] [%] [%] [%] [%] [%] [%] From pig sow 8, 0,6 4 0,06 0,17 0,4 0,03-0,06 0,04 0,4 rest Refined 8,3 0,75 3,9 0,65 0,18 0,50 0,05-0,05 0,08 0,37 rest Modified 8,5 0,65 4, 0,65 0,17 0,4 0,03 0,035 0,05 0,07 0,3 rest In the Figure is presented scheme of the testing. 3. Description of obtained results Tensile strength R m Tensile strength obtained in case of raw alloy (from pig sows) amounted from 13 to 3 MPa. After refining there was observed a slight change of tensile strength R m (35-40 MPa). Performed treatment of modification of the investigated alloy has enabled obtainment of tensile strength R m in limits of 50 MPa. After performed heat treatment, tensile strength amounted from 188 to 37 MPa. The equation () describes dependency between input values and values of tensile strength R m of the alloy after performed heat treatment. Fig.. Scheme of the tests The next stage of the investigations consisted in strength tests of the alloy, which have given the following scope of R m and A 5 parameter values (table. 3). Table. 3. Mechanical properties after heat treatment R m 46 11,33x 5x 8,3x 3,16x 98,5x 1,5 x 4,11x 7,5x () Coefficients of Regression: R = 0,85; Corrected R = 0,73; Residual MS = 61,9. The Fig. 3 depicts a system showing measured and approximated values for the variable R m. R m A 5 [MPa] [%] ,7-3,7 On base of the plan one assumed functions of the testing object in form of second degree polynomial (1) Z b (1) 0 b1 x1 b11x1 b x bx b3 x3 b33x3 b4 x4 b44x4 where: z- resultant factor (R m, A 5 ), b 0, b 1, b 11,..., b 44 estimators of regression, x 1 solutioning temperature, x solutioning duration, x 3 ageing temperature, x 4 ageing duration. Fig. 3. Diagram of anticipated and observed values for the variable R m In the Figs. 4-5 are shown spatial diagrams illustrating effect of temperature and duration of solutioning and ageing treatments on change of tensile strength R m of the investigated alloy. A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 4 / 0 1 1,

4 - solutioning duration (1 to hour), - ageing temperature (below 40 o C), - ageing time (up to hour). Elongation A 5 Performed treatment of modification of the investigated alloy has enabled obtainment of the elongation A 5 within interval of 1,1 1,6 %. performed heat treatment of the alloy one has obtained elongation A 5 in range from 0,7 to 3,7 %. Making comparison of the obtained average values of the parameters from the test for the alloy after heat treatment, and the alloy without heat treatment, one ascertained more than twice growth of the elongation A 5. The equation (3) illustrates dependency between assumed input values and value of the elongation A 5 of EN AB alloy after performed heat treatment. Fig. 4. Effect of temperature and duration of solutioning on tensile strength R m of EN AB alloy (t s =180 o C, s =60 minutes) A 1,81 0,07x 0,05x 0,3x 0,18x 0,86x 0,56x 0,6x 0,8x (3) Coefficients of Regression: R = 0,7; Corrected R = 0,65; Residual MS = 0,8 Effect of individual input variables on change of A 5 value is shown in the spatial diagrams (Fig. 6). Fig. 5. Effect of temperature and duration of ageing on tensile strength R m of EN AB alloy (t p =500 o C, p =60 minutes) Making analysis of an effect of individual variables on change of the R m one confirmed: - reduction of ageing temperature to value below 40 o C is advantageous for growth of the R m, - time of ageing to obtain maximal value of the R m should amount to up to hour. - solutioning temperature should have value at maximal 500 o C. Temperature below 510 o C is connected with reduction of obtained values of the R m. Longer time of the solutioning doesn t have any significant effect on change of the R m. The highest values of the tensile strength R m were obtained for the following parameters: - solutioning temperature ( o C), Fig. 6. Diagram of anticipated and measured values for variable A 5 In the Figs. 7-8 are shown spatial diagrams illustrating effect of temperature and duration of solutioning and ageing treatments on change of the elongation A 5 of the investigated alloy. 106 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 4 / 0 1 1,

5 - ageing temperature (up to 80 o C), - ageing time (up to hour). 4. Conclusions Fig. 7. Effect of temperature and duration of solutioning on elongation A 5 of EN AB alloy (t s =300 o C, s =180 minutes) The essence of the performed investigations concerned determination of an effect of temperature and time of performed solutioning and hypo-eutectoid ageing treatments of EN AB alloy in aspect of the best possible to be obtained mechanical properties. Performed heat treatment, with holding of the alloy in temperature near to solidus and next cooling down in cool water (0 0 C) with artificial ageing and cooling in air, resulted in precipitation hardening of the investigated alloy and effected in additional growth of mechanical properties. Holding of the alloy in temperature near to solidus effects not only in growth of concentration of elementary substances (e.g. Cu and/or Mg) in solid solution, constituting potential source of precipitation processes, but also advantageous change of morphology of eutectic crystals of silicon, which take form similar to spherical. Selection of suitable parameters of solutioning and ageing treatments becomes indispensable condition to have improvement of mechanical properties of the alloy. For EN AB alloy, growth of temperature and time of ageing results in drop of tensile strength R m and hardness HB, and growth of elongation A 5 and impact strength KCV of the alloy. Obtained dependencies enable determination of mechanical properties of investigated initial alloy in aspect of possibility of their improvement after performed heat treatment. References Fig. 8. Effect of temperature and duration of ageing on elongation A 5 of EN AB alloy(t p =510 o C, s =60 minutes) Temperature and time variables have an effect on change of the elongation A 5 in the following way: - ageing temperature with value above 80 o C is advantageous for growth of elongation, - ageing time should exceed hour (maintaining time of solutioning to hour), - solutioning temperature should have value above 500 o C, - solutioning time should have value from 0 to 80 minutes. The highest values of the elongation A 5 were obtained for the following parameters: - solutioning temperature ( o C), - solutioning duration (0 80 minutes), [1] P. Wasilewski, Silumins modification and its effect on structure and properties, PAN Krzepnięcie metali i stopów, Zeszyt 1, Monografia, Katowice (1993). [] S. Pietrowski, Silumins, Wydawnictwo Politechniki Łódzkiej, Łódź (001). [3] Z. Poniewierski, Crystallization, structure and properties of silumins, WNT, Warszawa (1989). [4] Z. Górny, Casting alloys of non-ferrous metals, WNT, Warszawa (199). [5] M. Warmuzek, E. Zemlak, Usage of fractography analysis to determination of an effect of structural state on a selected properties of casting alloys, Praca Naukowo Badawcza Instytutu Odlewnictwa, Kraków (1997). [6] J.R. Davis, Aluminium and aluminium alloys, ASM Speciality Handbook, (1993). [7] L. F. Mondolfo, Aluminium alloys, structure and properties, Butterworths, London-Boston, (1976). [8] Shu-Zu Lu, A. Hellawel, Modification of Al-Si alloys: microstructure, thermal analysis and mechanics, IOM, vol. 47, No. (1995). [9] S. Shivkumar, S. Ricci Jr., D. Apelian, in: B. Clossest (Ed.), Production and Electrolysis of Light Metals, Pergamon Press, New York, NY, (1989) A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 4 / 0 1 1,

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