SLEDOVÁNÍ DILATA NÍCH ZM N A MIKROSTRUKTURY ODLITK Z PÍSTOVÉ HLINÍKOVÉ SLITINY

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1 SLEDOVÁNÍ DILATA NÍCH ZM N A MIKROSTRUKTURY ODLITK Z PÍSTOVÉ HLINÍKOVÉ SLITINY OBSERVING THE DILATATION CHANGES AND MICROSTRUCTURE OF CASTINGS FROM PISTON ALUMINIUM ALLOY Ji í MORÁVEK a, Iva NOVÁ b a Technická univerzita v Liberci, Studentská 2, Liberec, eská republika, jiri.moravek@tul.cz b Technická univerzita v Liberci, Studentská 2, Liberec, eská republika, iva.nova@tul.cz Abstrakt Tento lánek se zabývá sledováním sou initele teplotní smr tivosti a mikrostruktury pístové hliníkové slitiny a vlivu pou itých slévárenských forem a r zných licích teplot na tyto vlastnosti. Toto bylo d láno proto, aby bylo mo né porovnat dilata ní vlastností slitin v souvislosti s mikrostrukturou odlitk. Koeficienty teplotní smr tivosti byly sledovány u hliníkových slitin pou ívaných pro výrobu píst spalovacích motor p i tuhnutí a chladnutí odlitk ze slitiny AlSi12CuNiMg (eutektické, AW 48000). Odlitky byly odlévány do trvalých a netrvalých forem a byl pozorován vliv formy materiálu na sledované vlastnosti. Materiály pou itých slévárenských forem byly: Ocel, písková formovací sm s s bentonitovým pojivem a formovací sm s s vodním sklem vytvrzovaná CO 2 pro netrvalé formy (pro simulaci vlivu jader na dilata ní zm ny). Dal ími prom nnými byly také teploty lití a tlou ky st ny odlitk. P ed odléváním byla tavenina metalurgicky o et ena rafina ní solí. Dilatace byly nam eny ve speciálním za ízení pou ívajícím formu, která má dutinu s tvaru písmene I. Za ízení se skládá ze dilatometru z polské provenience CRYSTALDIAGRAPH PC-4T2L, induk ního sníma e pro m ení dilata ních zm n a termo lánku NiCr-NiAl (typ K) pro m ení teploty. Z dilata ních k ivek byly vypo teny hodnoty sou initele teplotní smr tivosti a z odlitk byly odd leny vzorky a jejich mikrostruktury byly pozorovány na optickém mikroskopu. Klí ová slova: Pístová slitina, dilatace, koeficient teplotní smr tivosti Abstract This article deals with observing of coefficient of thermal contraction and microstructure of piston aluminium alloy and influence of used casting moulds and various casting temperatures on them. This was done to make it possible to compare dilatation properties of alloys in components with microstructure of castings. Coefficients of thermal contraction were observed in aluminium alloys used for piston making of gas engines during solidification and cooling of castings from piston alloy AlSi12CuNiMg (eutectic, AW 48000). Castings were cast into the permanent and non-permanent moulds and the influence of mould material was observed. Materials of moulds were: Steel (permanent moulds), sand mixture with bentonite binder and sand mixture with water glass hardened by CO 2 for non-permanent moulds (for simulation influence of cores on dilatation changes behavior). The conditions varied were also temperatures of casting and wall thickness of castings. Before cast melt was metallurgical treated by refining salt. Dilatations were measured in special equipment uses the mould which has a cavity with shape of letter I. Equipment consist from dilatometer of polish provenience CRYSTALDIAGRAPH PC-4T2L, inductive sensor for measuring the dilatation changes and thermocouple NiCr-NiAl (type K) for temperature measurement. From dilatation curves were calculated values of coefficient of thermal contraction and form castings were separated the specimens and their microstructure were observed on optical microscope. Keywords: Piston alloy, dilatation, coefficient of thermal contraction

2 1. INTRODUCTION In automotive field have the high use castings from foundry aluminum alloys. The most used are alloys of aluminum and silicon, called silumins. More automobile parts are produced from this alloys e.g.: pistons of gas engines, cylinder and gear boxes blocks, cylinder heads, wheel discs, etc. Most of these parts are one of the most important components and in the same time the most stress parts of automobile. Stresses of these components are usually at high temperatures and dilatation properties of these alloys are little explored. Especially during their solidification and cooling and in the case of casting alloys it is necessary to observe the linear and volume changes especially during solidification of Al-Si alloys, where the eutectic silicon crystallization is important. On our workplace - Department of Engineering Technology, Technical University of Liberec, we are focused on observing the proportions changes during solidification of castings. For these measurements we used zinc and aluminum alloys. Observed alloy was AlSi12CuNiMg which is used for gravity casting of pistons of gas engines. These measurements were executed on equipment which was designed and constructed on our department. This equipment uses the mould which has a cavity with shape of letter I and serves for observing the proportions changes, let us say dilatation during solidification and cooling casts into mould from different materials: steel, sand mixture or CT mixture, etc. 2. CONTRACTION OF CASTINGS The melts has a bigger volume than the solid state of castings. Melt during solidification going through three different contraction intervals. The first one is when melt is in liquid state, above temperature of liquidus, and melt level fall down. Second stage is during solidification of melt between the solidus and liquidus temperatures and it is accompanied by shrinking and volume changes during solidification give rise to centered shrink hole, during solidification of pure metals, and porosity, during solidification of alloys, in castings. This contraction occurs at the freezing point, because, in general, solid has greater density as compared to the liquid and this causes several problems. These include requirement for feeding, that is defined here as a process for compensation of solidification contraction. In case of aluminium-silicon alloys the volume growth caused by silicon solidification is not able to equalize the volume which was lost at aluminium crystallization and therefore it is necessary to make feeders on castings from aluminium alloys, especially during gravity casting. Table 1 shows density and volume changes of some metals. The volume changes, volumetric shrinkage, could be describes by this formula: 1 V γ = (1) V T 0 where is: coefficient of the volumetric shrinkage for temperature interval [K -1 ]; V 0 initial temperature volume [m 3 ]; V volume alternation [m 3 ]; T temperature alternation [K].

3 Tab. 1 Hodnoty zm n hustot a objem vybraných kov Table 1 Values of density and volume changes of select metals Material Melting temperature [ C] Density [kg.m -3 ]during room temperature Density [kg.m -3 ] during melting temperature Volume alternation [%] AI ,1 Bi ,32 Cd Co ,26 Cu ,3 Fe ,16 Mg ,1 Mn ,4 Ni ,11 Pb ,22 Si Zn ,08 The third stage of contraction is during cooling of casting which is in solid state, under the temperature of solidus, means from freezing temperature to room temperature. During this process starts the dimensional changes of castings and it follows that the contraction in solid state is a very important process for production of the pattern. Contraction in solid state is dependent on the used casting method. The way of casting solidification, reflections of its internal pressure ratios and external volume changes is possible to observe with the help of dilatation measuring. 3. EXPERIMENTAL OBSERVING OF DILATATION CHANGES The dilatation changes were observed with help of special measuring equipment which has a cavity with shape of letter I. This cavity is designed and assembled from a frame with two clamps and replaceable blocks from moulding material. The first clamp is fixed and the second clamp is flexible. The flexible clamp is connected to an inductive sensor by a silicon pipe. A dilatometer of polish provenience CRYSTALDIAGRAPH PC-4T2L records dilatation changes of solidification and cooling casting. At the same time the equipment records temperature of solidifying casting in heat axis of casting. A thermocouple NiCr- NiAl (type K) was used for temperature measurement. Scheme of measuring equipment is in fig. 1. Obr. 1 Schéma m ícího za ízení, 1- pevná elist, 2- vym nitelná p ílo ka, 3- dutina, 4- pohyblivá elist, 5- táhlo, 6- základní deska, 7- magnet, 8- induk ní sníma Fig. 1 Scheme of measuring equipment, 1- fix clamp, 2- replaceable block, 3- cavity, 4- flexible clamp, 5- silicone pipe, 6- base plate, 7- magnet, 8- inductive sensor The melt from aluminium alloy AlSi12CuNiMg was prepared in electrical resistance furnace and melted down in graphite pot. Before cast melt was protected with refining salt T3. Experiments were made into the moulds

4 from steel (permanent mould, chill casting), sand mixture with bentonite binder and sand mixture with water glass hardened by CO 2 for non-permanent moulds (for simulation influence of cores on dilatation changes behavior). The conditions varied were also temperatures of casting which were 675, 700 and 725 C and wall thickness of castings 7 and 27mm x 157mm, fig. 2 shows measuring equipment during measuring process. Specimens for optical microscope analysis were taken from the heat axis in the middle of each casting. Specimens were embedded into the plastic mould and ground and polished and their structure was observed on optical microscope, fig 4. Obr. 2 Pohled na m ící za ízení b hem m ení Fig. 2 Measuring equipment during measuring process From realized experiments has been obtained 70 dilatation and cooling curves, for each casting conditions was made 5 experiments. Values of coefficient of temperature contraction were computed from dilatation curves, examples of obtained curves are in fig 3, in temperature intervals 100 to 200 C and 200 to 300 C from this formula and average values of are mentioned in table 2: 2 T1 ) 1 [ ] l α = K (2) l *( T o where is: coefficient of temperature contraction [K -1 ], l casting contraction [mm], l 0 original lenght of casting [mm], (T 2 -T 1 ) difference of temperatures [ C].

5 , Brno, Czech Republic, EU Obr. 3 a) Časová závislost dilatace u odlitku do kovové formy, slitina AlSi12CuNiMg, licí teplota 725 C, tloušťka stěny odlitku 7mm, b) Časová závislost dilatace u odlitku do formy z CT směsi, slitina AlSi12CuNiMg, licí teplota 700 C, tloušťka stěny odlitku 27mm, c) Časová závislost dilatace u odlitku do formy z pískové formovací směsi, slitina AlSi12CuNiMg, licí teplota 675 C, tloušťka stěny odlitku 27mm. Fig. 3 a) Time dependence of dilatation of casting in steel mould, alloy AlSi12CuNiMg, casting temperature 725 C, wall thickness 7mm, b) Time dependence of dilatation of casting in CT mould, alloy AlSi12CuNiMg, temperature 700 C, wall thickness 27mm c) Time dependence of dilatation of casting in sand mould, temperature 675 C, wall thickness 27mm. A B C Obr. 4 a) Mikrostruktura odlitku ze slitiny AlSi12CuNiMg litého do kovové formy, licí teplota 725 C, tloušťka stěny odlitku 7mm, b) Mikrostruktura odlitku ze slitiny AlSi12CuNiMg litého do formy z CT směsi, licí teplota 700 C, tloušťka stěny odlitku 27mm, c) Mikrostruktura odlitku ze slitiny AlSi12CuNiMg litého do formy z pískové formovací směsi, licí teplota 675 C, tloušťka stěny odlitku 27mm. Fig. 4 a) Microstructure of casting from AlSi12CuNiMg cast into the steel mould, casting temperature 725 C, wall thickness 7mm, b) Microstructure of casting from AlSi12CuNiMg cast into the CT mould, casting temperature 700 C, wall thickness 27mm, c) Microstructure of casting from AlSi12CuNiMg cast into the sand mould, casting temperature 675 C, wall thickness 27mm.

6 Tab. 2 Vypo ítané hodnoty sou initele teplotní smr tivosti Table 2 Computed values of coefficient of thermal contraction Casting conditions: Mould type, Wall thickness [mm], Casting temperature [ C] Coefficient of thermal contraction [K -1 ] in temperature interval [ C] 300 to to 100 Steel, 27, 675 0, , Steel, 27, 700 0, , Steel, 27, 725 0, , Sand, 27, 675 0, , Sand, 27, 700 0, , Sand, 27, 725 0, , CT, 27, 675 0, , CT, 27, 700 0, , CT, 27, 725 0, , Steel, 7, 725 0, , Sand, 7, 700 0, , Sand, 7, 725 0, , CT, 7, 700 0, , CT, 7, 725 0, , CONCLUSION From obtained dilatation curves were calculated values of coefficient of thermal contraction in temperature interval from 100 to 300 C which is maximum working temperature, normal working temperature is around 200 C, of piston in engine. Average value of in each interval is 4,009x10-5 [K -1 ] in interval C and 6,045x10-5 [K -1 ] in interval C which are the same values which are mentioned in literature. The microstructure and phases in the piston alloy were, eutectic phase which contains phase and silicon needles and high ratio of different kinds of intermetallics, which are usually presented on grain boundaries. In steel moulds there were also quite big particles of unsolved silicon with irregular geometric shape. The ratio of intermetallic phases, was higher during lower temperatures (675 and 700 C), in sand and CO 2 and in bigger wall thickness, but the kind of intermetallics were the same in all moulds, temperatures and wall thicknesses. The article was prepared within the score of project MSM and on the base of financial support of project SGS 2822 on TUL in frame of support of specific academic research REFERENCES [1] CAMPBELL, J.: Casting. Elsevier Butterworth-Heinemann, [2] MICHNA,. a kol. Encyclopedia of Aluminium. 1. vyd. Pre ov, 2005.