Assessment of T6 Heat Treatment Effectiveness of Hypo-eutectic Silumins with Limited Parameters of Solutioning Treatment

Heat treatment processes, due to qualitative requirements for the cast machinery components and restrictions on energy consumption resulting on the one hand from environmental concerns, and on the other hand from a requirements coming from minimization of manufacturing costs, are resulting in searching after a technologies enabling obtainment of satisfactory results, in form of improved mechanical properties mainly, while minimizing (limiting) parameters of successive operations of the heat treatment. Heat treatment of the T6 type presented in this paper consists in operations of heating of investigated alloys to suitably selected temperature (range of this temperature was evaluated on the base of the ATD method), holding at such temperature for a short time, and next rapid cooling in water (20 o C) followed by artificial ageing, could be such technology in term s of above mentioned understanding of this issue. Performed T6 heat treatment with limited parameters of solutioning operation resulted in visible increase in tensile strength R m of AlSi7Mg, AlSi7Cu3Mg and AlSi9Cu3(Fe) alloys.


Introduction
The Al-Si alloys (silumins) belong to the most numerous groups of casting alloys based on aluminum [1][2][3].The alloys, because of their mechanical properties, relatively good fluidity and low specific gravity and corrosion resistance, are used mainly in metal and automotive industries [4][5][6].To production of automotive structural elements are used mainly the alloys, denominated according to the Aluminium Association system as from 3xx.x series (Al-Si-Mg, Al-Si-Cu, Al-Si-Cu-Mg).Most of powertrain components (80-85%) are cast from aluminum alloys, others are produced in process of rolling, extrusion and forging mainly.Among components received from casting process the biggest share belongs to cylinder heads, casings of transmission gears, cylinder blocks and various engine components (cylinder head covers, oil pans, water and oil pump housings, collectors, housings and rotors of turbo-compressors).Wheel rims, cylinder linings, slide bearings and pistons are also produced as a castings.The silumins are characterized by a soft metallic matrix and hard precipitations of silicon.Main role in shaping properties of such alloys belongs to volume fraction and morphology of microstructural components [1][2][3]7], and to a possible heat treatment, as it occurs in case of alloys with additives of Mg and Cu [8][9][10].
Requirements for the mechanical properties, course of casting process and chemical composition, as well as structure of the alloy, all of them affect directly on parameters of the heat treatment process (time and temperature of solutioning and ageing).The main condition, being basis of the precipitation hardening of the alloys, is changing solubility of alloying components in solid state, growing as temperature increases [11][12][13].Heat treatment of the T6 type in case of the alloys from 3xx.x series comprises three stages: -solution heat treatment having activated diffusion mechanisms and resulting in change in morphology of eutectic silicon [1-2, 14-17], rapid cooling allowing for entrapping of existing vacancies and atoms of alloying components in supersaturated solid solution [1,18,19] by placing the element in water, in brine solutions, in fluidized beds, in aqueous solutions of polymer, or cooling with compressed air, or cooling in hot (60-90 o C) water [20][21][22].-natural ageing (at room temperature), or artificial ageing (at elevated temperature) [10,12,13]; to precipitation from the solution of finely-dispersed and hardened phase.To obtain homogenous supersaturated structure of the alloy, heating operation is performed at temperature which is lower with 20-30°C than eutectic temperature, due to existing risk of generation of surface defects and possible partial melting of eutectic mixture.Time required to the homogenization is determined by solutioning temperature and spacing of dendrite arms (DAS), or spacing of arms of secondary dendrites (SDAS) [23][24][25].In case of heat treated aluminum alloys, hardening alloy-forming elements like Cu and Mg do not show any sufficient solubility in solid state; however such solubility decreases as temperature decreases.
The next aspect connected with the solutioning is spheroidizing of precipitations of Si [2,15,16], i.e. change of its morphology affecting on obtained mechanical properties [2,14,26].In result of heating operation of the silumins, not only increase of concentration in solid solution of elements being potential source of precipitation processes (Cu and/or Mg) can occur, but also change of morphology of eutectic silicon crystals can occurtheir coalescence and spheroidizing [2].Owing to this, the material as a whole does not show any worsening of plastic properties, in spite of hardening of solid solution due to later ageing of the castings [2,27,28].
The present study concerns assessment of an effectiveness of performed T6 heat treatment, characterized by suitably selected ranges of solutioning operation parameters in aspect of obtained tensile strength R m of same selected hypo-eutectoid silumins.

Experimental methods
Owing to addition of silicon, the investigated casting alloys (AlSi7Mg, AlSi7Cu3Mg and AlSi9Cu3(Fe)) feature good casting ability and abrasion resistance, as the silicon has effect on their improvement.These alloys can be poured both into sand moulds and permanent moulds, as well as can be used in pressure die casting, and in a new technologies like thixocasting and squeeze casting [29][30][31].These alloys are characterized by a high strength, are heat treatable (dispersion hardening) and easily welded.Among disadvantages of such alloys are: red brittleness, low plasticity and decreased corrosion resistance.Such alloys are used, among others, to production of cylinder head castings, engine crankcases, water cooled cylinder blocks, components of aviation pumps, car wheel rims, etc.
Investigated alloys were melted in electric furnace at temperature 720-760 °C, and next, refined with Rafal 1 preparation (0,4% mass of metallic charge) and modified with master alloy AlSr10 (0,5-0,6 % mass of the charge).Compliance of chemical constitution of the investigated alloys with the PN EN 1760 standard was confirmed with use of spectrometer of GDS 850A type.Metallic moulds (Fig. 1) were poured with the alloys to obtain the castings, which in the next succession were used (after the treatment) to production of the test pieces.The moulds, prior to the pouring, were heated to 220-250 °C, and next, such temperature was kept during process of pouring into moulds.

Fig. 1. Metal mould to pouring of the test pieces
To evaluate temperature ranges of the solutioning and ageing treatments of the investigated alloy, it has been implemented the Thermal-Derivative Analysis (TDA), such method is used to record and analyze of course of crystallization process [32][33][34][35].
In the Figures 2-4 hereinafter are presented registered diagrams from the TDA method with ranges of solutioning temperature of the investigated alloys.
Use of the ATD method has allowed to evaluate maximal temperatures of solutioning treatment not resulting in partial melting of the alloy (point B -Fig. 2-4) and minimal temperatures connected with thermal effects generated by low alloyed phases rich in Cu and Mg (point A -Fig. 2-4).In the table 1 are presented ranges of the heat treatment parameters of the investigated alloys.The samples were solutioned in cold water (20 o C).After performed artificial ageing, the test pieces were prepared according to PN-EN ISO 6892-1:2010P standard (measuring length 50 mm, diameter 10 mm).Static tensile strength test was performed on the ZD-20 testing machine.Obtained results were processed with use of StatSoft Statistica ver.13 software package.

AlSi7Mg alloy
The alloy without the heat treatment was characterized by tensile strength R m at the level 200 MPa, whereas the heat treatment resulted in change of the tensile strength, which was included within range 154 -335 MPa.In the Figure 5 is presented, in form of spatial diagrams, effect of temperature and time of the solutioning and ageing operations on the tensile strength R m , at assumed constant values of ageing temperature 165 o C and ageing time 5 hours (for the solutioning Fig. 5a) and solutioning temperature 520 o C and ageing time 1 hour (for the ageing -Fig.5b).

AlSi7Cu3Mg alloy
The alloy without the heat treatment was characterized by the tensile strength at the level 194-205 MPa.
The heat treatment has allowed for obtainment of the R m in broad range from 158 to 368 MPa.
In the Figure 6 is depicted how the tensile strength R m is changing depending on temperature and time of solutioning and ageing operations, with fixed at constant level temperature of ageing (175 o C) and ageing time of 5 hours (for the solutioning -Fig.6a), and solutioning temperature of 500 o C and solutioning time of 1 hour (for the ageing -Fig.6b).
The highest increase in the tensile strength R m (368 MPa) has been obtained after solutioning at temperature

Conclusions
Obtained results of the study are pointing at existing possibility of limitation of the solutioning parameters, maintaining high values of mechanical properties of hypo-eutectoid alloys from 3xx.x series.Solutioning of the AlSi7Mg alloy at temperatures 520 and 550 o C for 30 and 90 minutes has enabled to obtain maximal values of the R m .In case of these alloys (AlSi7Cu3Mg and AlSi9Cu3(Fe) brands), these temperaturesdue to a higher content of Cu -were lower and were equal to 485 and 510 o C when solutioning time amounted to 90 minutes.
In this case, additive of Cu at the level of 3% enables to obtain, after the heat treatment, maximal values of the R m higher with 10-30%.
However, it must be very clearly underlined that it is important to take the process into considerations as a whole, i.e. also considering parameters of ageing operation, having effect on obtained results.

Fig. 5 .
Fig. 5. Change of the R m strength of the AlSi7Mg alloy in function of time and temperature a) solutioning, b) ageing 485 o C for 90 minutes (ageing temperature 175 o C; ageing time 8 hours).Prolongation of solutioning time to 3 hours (ageing temperature 175 o C; ageing time 5 hours) enabled to obtain R m = 365 MPa.Grosselle et.al. [40] applying solutioning temperature lower with 5 o C and solutioning time shorter with 1 hour had obtained R m at the level of 289-309 MPa; after ageing at 220 o C of the alloy used typically for treatment of engine blocks [41].However, initial globular structure of the alloy obtained from rheocast process enabled reduction of solutioning temperature to 470 o C [42] with maintained tensile strength R m at the level of 350 MPa.The same strength was obtained in course of the investigations, making solutioning treatment of the alloy at temperature 510 o C in time of 1,5 hour, and next, ageing at temperature 175 o C for 5 hours.

Fig. 6 .Fig. 7 .
Fig. 6.Change in the R m strength of the AlSi7Cu3Mg alloy in function of temperature and time: a) solutioning, b) ageing