G. Kugler, P. Mrvar, M. Petrič, A. Križman, M. Terčelj
Univerza v Ljubljani, Naravoslovnotehnična fakulteta v Ljubljani, Oddelek za materiale in metalurgijo, Slovenija / University of Ljubljana; Faculty of Natural Sciences and Engineering, Department of Materials and Metallurgy, Slovenia
Izboljšava mehanskih lastnosti
Improving the Mechanical Properties of AM60 Foundry Alloy
Povzetek
Zlitina MgAl6Mn se običajno uporablja za litje, izboljšanje njenih mehanskih lastnosti pa se doseže z različnimi toplotnimi obdelavami po litju. Ta študija se osredotoča na možnost izboljšanja mehanskih lastnosti zlitin MgAl6Mn z majhnimi deformacijami v vročem na izbranem območju ulitka po končanem strjevanju. Uliti material se je strdil s hitrostjo hitrosti ohlajanja 2, 7 in 50 K/min. Primerni pogoji za toplotno obdelavo so bili določeni s tlačnim preizkusom v temperaturnem razponu 200-450 °C, razponu hitrosti reformacije 0,001-10s-1 in raztezku 0,8 vsakega ulitka. Na vzorce, izdelane iz ulitih blokov, so delovali majhne deformacije v vročem in dejanski raztezek v razponu 0,1-0,4 z uporabo vnaprej ocenjenih primernih temperatur in razponov hitrosti deformacije, mehanske lastnosti pa so bile določene na podlagi nateznih preizkusov in Charpyjevega preizkusa. Trikratno povečanje natezne trdnosti je bilo izmerjeno pri vzorcih z majhnimi deformacijami v vročem in pri hitrosti ohlajanja za strjevanje 7 K/min in več.
Abstract
MgAl6Mn alloy is usually used for casting and improving of its mechanical properties. It is usually achieved by various heat treatment procedures after casting. This study was focused on the possibility to improve the mechanical properties of MgAl6Mn alloy by imposing the small hot deformation on a selected area of casting after completed solidification. This cast material solidified with cooling rates of 2, 7 and 50 K/min, respectively. Then appropriate conditions for hot working were found using compression tests in the temperature range between 200-450°C, strain rates range 0.001-10s-1 and strain of 0.8 for each as-cast state. On samples made from cast blocks small hot deformations of true strain in the range between 0.1 - 0.4 using previously assessed appropriate temperature and strain rate ranges were imposed and mechanical properties so by tensile as well as by Charpy tests were determined. Three times increase of tensile strength was obtained for samples with imposed small hot deformation and cooling rates at solidification of 7K/min and above.
1 Uvod
V zadnjih letih je zmanjševanje teže ter porabe energije postalo zelo pomembna naloga avtomobilske in prometne industrije. Posledično je v porastu uporaba magnezijevih zlitin, saj imajo v primerjavi s preostalimi kovinami zelo nizko težo,
1 Introduction
In recent years reducing of weight as well as of energy consumption are very important tasks induced by the automotive and transportation industries. This favors the application of magnesium alloys since they beside the very low weight in comparison
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prav tako pa omogočajo odlično strojno obdelavo, reciklirnost, dimenzijsko stabilnost in imajo izvrstne mehanske lastnosti, zato konkurirajo celo aluminijevim zlitinam, v nekaterih aplikacijah pa celo jeklu. Magnezijeve zlitine so lahko zato pogosto primeren kandidat za zamenjavo aluminijevih in jeklenih konstrukcijskih delov, vendar pa je zaradi slabše preoblikovalnosti v vročem, lezenja in odpornosti na korozijo obseg njihove uporabe omejen [1-10].
Po drugi strani pa je dobro znano, da termodinamično deformirane zlitine prinašajo niz prednosti v primerjavi z litimi ustrezniki, zaradi slabše preoblikovalnosti magnezijevih zlitin pa se litje v primerjavi z deformacijami v vročem za proizvodnjo konstrukcijskih delov uporablja pogosteje. Omejena preoblikovalnost magnezijevih zlitin je posledica pomanjkanja primernega sistema zdrsa, pri temperaturah nad 250 °C, pri katerih je mogoče aktivirati nebazalni sistem zdrsa, pa je mogoče zagotoviti izboljšano preoblikovalnost.
Ena izmed najpriljubljenejših zlitin z ogromnim potencialom za najrazličnejše aplikacije je MgAl6Mn, iz katere se običajno proizvajajo konstrukcijski deli, včasih celo z deformacijo v vročem [11-17]. Običajno je treba pred deformacijo v vročem kot tudi po njej izvesti postopek homogenizacije za izboljšanje mehanskih lastnosti ulitkov. Prav tako je znano, da postopek homogenizacije magnezijevih zlitin zahteva ogromno energije in časa, saj lahko traja več ur, s čimer pa se posledično povečuje izhajanje toplogrednih plinov [3, 17-22]. Uporaba takšne magnezijeve zlitine bi lahko postala pogostejša, če bi bilo mogoče izboljšati njene mehanske lastnosti ter znižati proizvodne stroške. Zato je iskanje novih načinov za zmanjševanje emisij toplogrednih plinov, proizvodnih stroškov ter izboljšanje mehanskih lastnosti magnezijevih zlitin več kot smiselno.
to other metal exhibits also have good machinability, recyclability, dimensional stability as well as mechanical properties, and can therefore compete with aluminum alloys and in some applications even with steels. Mg alloys can thus be in many cases very promising candidate for replacement of aluminum and steel structural parts but their lower hot workability, creep strength and corrosion resistance limit their broader use [1-10].
On the other hand it is well known that thermal-mechanically deformed alloys possess several advantages over their cast counterparts, but due to low workability of magnesium alloys casting route is more frequently applied in comparison to hot deformation for production of structural parts. Limited workability of Mg alloy is namely a consequence of lack of sufficient slip systems, but at temperatures above 250°C at which the non-basal slip systems can be activated improved hot workability can be obtained.
One of the most popular magnesium alloys with great potential for many applicationsisMgAl6Mnfromwhichstructural parts are usually produced by casting and in some cases also by hot deformation [11-17]. Usually homogenization process is required for improving of mechanical properties of castings prior and also after hot deformation. Furthermore, it is also known that the process of homogenization of magnesium alloys is energy and time consuming since this can take several hours that consequently also increases emission of greenhouse gases [3, 17-22]. Application of this Mg alloy could be increased if the mechanical properties could be improved or production costs reduced. Thus it is reasonably to search for a new way for reducing the emission of greenhouse gases, production costs as well as increasing of mechanical properties at Mg alloys.
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Namen te študije je raziskati strjevanje ter možnosti toplotne obdelave zlitine MgAl6Mn brez predhodnih homogenizacijskih postopkov. Strjevanje gravitacijsko lite magnezijeve zlitine MgAl6Mn je bilo že raziskano »in situ«, in sicer s pomočjo preproste toplotne analize. Rezultati poskusov so bili nadgrajeni z rezultati metalografske analize. Prav tako je bilo deformacijsko vedenje ulitkov zlitin MgAl6Mn v temperaturnem razponu 200450 °C in pri razponu hitrosti deformacije 0,001-10 s-1 določeno s pomočjo metodologije procesnih map.
2 Ulitki - poskusni postopki in
rezultati
Preiskovana zlitina je bila pripravljena s ponovnim taljenjem standardne zlitine MgAl6Mn v grafitnem lončku znotraj indukcijske peči. Kemijska sestava uporabljene zlitine MgAl6Mn je prikazana v Preglednici 1. Taljenje je bilo izvedeno v zaščitnem argonskem ozračju. Ko je bila dosežena temperatura 720 °C, je bila talina iz grafitnega lončka ulita v merilno celico, prikazano na Sliki 1, nato pa so bile izmerjene krivulje ohlajanja.
Toplotna analiza je bila izvedena na podlagi preprostega postopka, ki se imenuje ETA1.2 (oglejte si Sl. 1).
Preprosta toplotna analiza »in situ« (ETA) je metoda tako za raziskave kot preverjanja po zaključenih procesih strjevanja in ohlajanja. Tako je mogoče na posreden način opredeliti mikrostrukturo in lastnosti. Povezava med binarnimi faznimi
The aim of this research was to study the solidification and the possibility of hot working of MgAl6Mn foundry alloy without prior homogenization process. The solidification of gravity cast magnesium alloy MgAl6Mn has been investigated with the "in situ" simple thermal analysis. The experimental results were upgraded with the results of metallographic analysis. Furthermore the deformation behavior of as-cast MgAl6Mn alloy in the temperature range 200-450°C and in the strain rate range 0.001-10 s-1 were determined using processing maps methodology.
2 Casting - Experimental Procedure
and Results
The investigated alloy was prepared by the re-melting of the standard MgAl6Mn alloy, which was done in the graphite crucible of the inductive furnace. The chemical composition of the employed MgAl6Mn alloy is presented in Table 1. The melting was done under the protective atmosphere of argon. When the temperature of 720°C was achieved the melt from the graphite crucible was cast into measuring cells as shown in Figure 1, and the cooling curves were measured.
Thermal analysis was conducted in the simple thermal analyses called ETA1.2 (see Figure 1).
An "In situ" simple thermal analysis (ETA) is both an investigating and controlling method for following the solidification and cooling process of alloys. This way the microstructure and properties could
Preglednica 1: Kemijska sestava zlitine MgAl6Mn v [wt. %] Table 1: Chemical composition of MgAl6Mn alloy in [wt.%].
Al	Zn	Mn	Cu	Si	Fe	Ni	Mg
6,28	0,1267	0,3056	0,005	0,0294	0,0032	0,0008	preostanek / rest
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Slika 1: Oprema z merilno celico za toplotne analize »in situ« a) in Predstavitev merilne celice za toplotne analize »in situ« s strani prodajnega zastopnika ter vzorci za nadaljnje preiskave b).
Figure 1: Equipment with the measuring cell for »in situ« thermal analyses (a) and detailer presentation of the measuring cell for "in situ" simple thermal analyses and samples for further investigations (b).
diagrami, krivuljo ohlajanja in iz preiskanega vzorca pridobljeno mikrostrukturo je prikazana na Slikah 2a-(c):
Po litju tekoče kovine v merilno celico je bila izmerjena najvišja temperatura 700 °C (oglejte si Sl. 2b). Nato sledita hlajenje in krčenje v tekočem stanju, dokler ni dosežena temperatura likvidusa.
Proces strjevanja se je začel in končal pri temperaturi solidusa. Izmerjena hitrost ohlajanja v središču merilne celice je znašala 7 K/s. Mikrostrukturne komponente sestavljajo v prvi vrsti precipitirani kristali magnezija aMg. Ob robovih kristalnih zrn se je evtektična zlitina aMg+ Al12Mg17 strdila, saj je prišlo do lokalnega povečanja aluminijeve frakcije v talini kot posledica aluminijeve izceje, zaradi česar je prišlo do evtektičnega strjevanja. Vzrok se skriva v dejstvu, da primarni strjeni kristali aMg nimajo nazivne kompozicije zlitine XAl, ampak je koncentracija Al povečana in pomaknjena v levo proti evtektični točki, kot je prikazano na Sliki 2. Nastali heterogeni evtektik je značilen za nizke hitrosti
be determined indirectly. The connection between binary phase diagram, cooling curve and obtained microstructure of the investigated sample are shown in Figures 2a-c.
After the pouring of molten metal into measuring cell, the maximal temperature that was detected was 700°C (see Figure 2b). Then the cooling and contraction in liquid state follows as long as a liquid temperature is reached. Further the solidification process occurred and finished at solidus temperature. Measured cooling rate in the centre of the measuring cell was 7 K/s. Microstructure components are composed of primary precipitated crystals of aMg. Along the boundaries of the crystal grains the eutectic aMg+ Al12Mg17 has been solidified, where the local increasing of the aluminum fraction was taking places in the melt due to the aluminum segregation and therefore the eutectic solidification occurred. The reason for that is that the primary solidified crystals of aMg don't have nominal composition of alloy, XAl, but
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Slika 2: Strjevanje zlitine MgAl6Mn pri nizki hitrosti ohlajanja: binarni fazni diagram (a), krivulja ohlajanja (b) in pridobljena mikrostruktura ulitka pri hitrosti strjevanja 7 K/min (c).
Figure 2: Solidification of MgAl6Mn alloy at low cooling rate: binary phase diagram (a), cooling curve (b), and obtained as-cast microstructure at solidification rate of 7 K/min (c).
ohlajanja. Na podlagi tipičnih parametrov krivulje ohlajanja je bilo mogoče sklepati in oceniti kakovost zlitine.
3 Toplotna obdelava -
poskusni postopki in rezultati
Za tlačno preizkušanje je bil uporabljen računalniško nadzorovan simulator metalurških stanj Gleeble 1500DA (oglejte
instead the concentration of Al increased and shifted left towards eutectic point as shown in Figure 2. Formed heterogeneous eutectic is characteristically for the low cooling rates. From typical parameters of a cooling curve it was possible to conclude and estimate the quality of alloy.
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si Sliko 3). Za zmanjšanje trenja med valjastim vzorcem in orodjem ter v izogib medsebojni zvaritvi je bilo uporabljeno grafitno mazivo. Valjasti vzorci vrste Rastegew z dimenzijami $=10 mm x 15 mm so bili izrezani iz gravitacijsko litih blokov velikosti 30^40x40 mm. Med preizkušanjem je napetostni modul kontrolnega sistema naprave Gleeble 1500D izračunal površino trenutnega preseka vzorca na podlagi izmerjene L-deformacije ter nato še dejansko napetost in deformacijo. Preizkušanje je bilo izvedeno v temperaturnem razponu 200-450°C in šestih različnih hitrostih deformacij, tj. 0,001, 0,01, 0,1, 1,0, 5,0 in 10,0 s-1. Hitrost segrevanja je znašala 3,0 °C/s, čas pregrevanja za deformacijsko temperaturo pa je znašal 15 s. Po deformaciji so bili vzorci hitro pogašeni z vodo, da bi se ohranila mikrostruktura za metalografske preiskave.
Procesna mapa je bila določena na podlagi dinamičnega modela materiala, ki ga je razvila in ga pogosto uporablja skupina Y. V. R. K. Prasad [23-25]. Procesne mape materiala je mogoče opisati z eksplicitno predstavitvijo odzivov na uvedene procesne parametre. Gre za prekrivanje učinkovitosti izgube moči in mape nestabilnosti.
3 Hot Working - Experimental
Procedure and Results
A computer controlled simulator of metallurgical states Gleeble 1500D was used for compression testing (see Figure 3). For reduction of friction between the cylindrical specimen and the tool and to avoid their mutual welding, graphite lubricant was used. Cylindrical specimens of Rastegew type with dimensions $=10 mm x 15 mm were cut from gravity cast block of dimensions 30x40x40 mm. During testing the stress module in the Gleeble 1500D control system calculated the instantaneous cross-sectional area of the specimen from the L-strain measurement and computed the true stress and true strains. Testing was performed in the temperature range of 200 - 450°C, at six different strain rates, i.e. 0.001, 0.01, 0.1, 1.0, 5.0 and 10.0s-1. The heating rate was 3.0°C/s, and the soaking time on deformation temperature was 15 s. After deformation the specimens were rapidly quenched with water to preserve the microstructure for metallographic investigations.
The processing map has been determined on the basis of a dynamic material model, which has been developed
Slika 3: Shematski prikaz sistema za tlačno preizkušanje Gleeble 1500D z vzorci in tlačnimi čeljustmi.
Figure 3: Schematic representation of the Gleeble 1500D compression testing arrangement with sample and compression jaws.
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Obdelovanec pod deformacijskimi pogoji v vročem tega modela deluje kot razsipnik energije. Konstitutivna enačba opisuje način, na katerega se energija P v katerem koli trenutku pretvori v dve obliki, toplotno energijo G, zaradi česar naraste temperatura, mikrostrukturne spremembe povzroči transformacija metalurške dinamike J, stanja pa ni mogoče povrniti. V glavnem so izgube posledica prenehanje naraščanja temperature, le majhen del energije pa se izgubi kot posledica mikrostrukturnih sprememb. Delitev moči med G in J kontrolira konstitutivni pretok materiala, določa pa jo občutljivost na hitrost deformacije m meje tečenja, podane z [23-25]
dj êda ¿crdlog(cr) A log(cr) dG adê êad\og(ê) ~ A log(f)
= m (1)
V primeru idealnega razsipnika je mogoče prikazati enaki količini J in G, kar posledično pomeni M = 1 in J = Jmax, učinkovitost izgube moči pa je ponazorjena z enačbo
(2)
Razlike vrednosti n ter temperatura T in hitrost deformacije ¿predstavljajo relativno vrednost izgube energije med mikrostrukturnimi spremembami. Mikrostrukturne spremembe so lahko stabilne, npr. takšne, ki vključujejo dinamično obnovo in dinamično rekristalizacijo, ali nestabilne, ki vključujejo: klinaste razpoke, nastanek praznin ob trdnih delcih, staranje dinamične deformacije in makrostrukturne razpoke.Kermednestabilnimispremembami nastajajo nove površine, je potrebne več energije, medtem ko stabilne spremembe vedno potekajo ob dislokaciji roba zrna. Mapa nestabilnosti je opredeljena kot merilo stabilnosti dinamičnega materiala, diferencialni koeficient funkcije izgube
and widely used by the group of Y. V. R. K. Prasad [23-25]. The processing map of material can be described as an explicit representation of its response on the imposed process parameters. It is a superimposition of the efficiency of power dissipation and an instability map.
The workpiece under hot deformation conditions of this model works as an energy dissipater. The constituent equation describes the manner in which energy, P, is converted at any instant into two forms, thermal energy, G, making temperature increase and microstructural change caused by transform of metallurgical dynamics, J, which are not recoverable. In general, most of the dissipation is due to a temperature rise and only a small amount of the energy dissipates through microstructural changes. The power partitioning between G and J is controlled by the constitutive flow behavior of the material and is decided by the strain rate sensitivity, m, of flow stress given by [23-25]
dj èda èad\og(a) A log(cr) dG adê êad\og(ê) ~ A log(f)
= m (1)
For an ideal dissipator it can be shown that both quantities J and G are equal, which consequently means that m = 1 and J = Jmax whereas the efficiency of power dissipation is given by:
(2)
The variation of temperature, T, and strain rate, e , represents the relative value of energy dissipation occurring through microstructural changes. Microstructural changes can be stable, which includes a dynamic recovery and dynamic recrystallization, and instable which includes: wedge cracking, void formation at hard particles, dynamic strain ageing and macrostructural cracking. Since
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pa mora zadostiti pogoju neenakosti, podanemu z naslednjo formulo
Na Sliki 4 je prikazana procesna mapa z učinkovitostjo izgube moči in mapa nestabilnosti kontur pri temperaturnem razponu 200 °C do 450 °C ter hitrosti deformacije 0,001s-1 do 10 s-1 in pri dejanskem raztezku 0,4. Kot je razvidno, se nestabilna cona pojavi v temperaturnih razponih med 200-250 °C in 400-450 °C, tako pri nižji kot višji hitrosti deformacije. Pri nižjih vrednosti učinkovitosti izgube moči pri omenjenih temperaturnih razponih ter celoten razpon hitrosti deformacije nakazuje, da lahko kot posledica mikrostrukturnih sprememb nastanejo razpoke.
Slika 4: Procesna mapa, tj. prekrivanje mape izgube moči in mape nestabilnosti pri dejanskem raztezku 0,4 in pri temperaturnem razponu med 200-450 °C in hitrostih deformacije 0,001-10 s-1 s prikazanimi položaji stanj za vzorce in mikrostrukture, ki so podani na samostojnih slikah.
Območja z nižjimi vrednostmi učinkovitosti izgube moči ležijo tako v zgornjih kot spodnjih območjih vseh
new surfaces are formed during instable changes, more energy is required, while stable changes always take place by grain boundary migration. The instability map is defined by a stability criterion for a dynamic material, where the differential quotient of its dissipative function has to satisfy an inequality condition, given by the following expression:
Figure 4 shows the processing map with efficiency of power dissipation and instability contour map for temperature range from 200°C to 450°C and strain rates 0.001s-1 to 10s-1 at true strain of 0.4. As can been seen the instable zone appears in the temperature ranges between 200 - 250°C and 400 - 450°C, at both higher and lower strain rates. Also lower values for efficiency of power dissipation for mentioned temperature regions and entire interval of strain rates indicate the microstructural changes that can result in cracking.
Areas with lower values of efficiency of power dissipation lie on the lower and upper regions of all the tested strain rates. Mentioned lower values for efficiency of
Slika 4: Procesna mapa, tj. prekrivanje mape izgube moči in mape nestabilnosti pri dejanskem raztezku 0,4 in pri temperaturnem razponu med 200-450 °C in hitrostih deformacije 0,001-10 s-1 s prikazanimi položaji stanj za vzorce in mikrostrukture, ki so podani na samostojnih slikah.
Figure 4: Processing map, i.e. superimposition of power dissipation map and instability map at true strain of 0.4 for temperature range from 200 - 450°C, and strain rates 0.001 - 10s-1 with depicted positions of conditions for samples and microstructures which are given on separate figures.
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preizkušenih hitrosti strjevanja. Omenjene nižje vrednosti učinkovitosti izgube moči označujejo, da se bistven delež energije prenese v obliki toplote in manjši del v mikrostrukturne spremembe, kar lahko nakazuje nižjo stopnjo preoblikovalnosti. Slika 5a prikazuje makroskopski pogled nastanka razpok na površini vzorca, preoblikovanega pri hitrosti deformacije 10 s-1 in temperaturi 450° C. Nastale razpoke so posledica precipitiranega evtektika aMg + Al12Mg17 na robovih zrn, kot je prikazano na Sliki 5b. Ker je ta evtektik pri 450 °C v
power dissipation indicate that larger part of energy is transferred in heat and smaller part in microstructure changes, which could indicate lower deformability. On Figure 5a the macroscopic view of occurrence of cracks on the surface of the specimen deformed at strain rate of 10s-1 and temperature of 450°C is presented. Observed cracks are the consequence of precipitated aMg + Al12Mg17 eutectic on grain boundaries as shown in Figure 5b. Since the mentioned eutectic is in liquid state at 450°C and due to relative motion on grain boundaries
Slika 5: Deformacijski pogoji pri hitrost deformacije 10 s-1 in temperaturo 450 °C: makroskopski pogled na deformiran vzorec (a) in razpoke na robovih zrn (b); deformacijski pogoji pri hitrosti deformacije 10 s-1 in temperaturi 450 °C: makroskopski pogled na vzorec (c) in dinamična rekristalizacija (d).
Figure 5: Deformation conditions strain rate 10s-1 and temperature 450°C: macroscopic view of deformed specimen (a), and grain boundary cracking (b); deformation conditions strain rate 10s-1 and temperature 450°C: macroscopic view of specimen (c), and dynamic recrystallization (d).
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tekočem stanju in zaradi relativnega gibanja na robovih zrn med toplim preoblikovanje, nastajajo mikrorazpoke. Pri stiskanju pri 450 °C nastajajo razpoke pri vseh hitrostih deformacije, razen pri najnižji, tj. 0,001 s-1 (oglejte si Slike 5c-d). Prisotnost dinamične rekristalizacije, ki je prikazana na Sliki 5d, zavira večanje razpok, ki so posledica evtektika aMg + Al12Mg17 na robovih zrn.
Pri stiskanju pri temperaturi 400 °C in hitrostih deformacije 5 in 10 s-1 so prav tako nastajale razpoke (Slike 6a-b). Takšne razpoke so posledica precipitacije evtektika
during hot forming process it results in micro-cracking. At compression at 450°C the cracks occur at all strain rates, with exception of lowest one, i.e. 0.001s-1 (see Figures 5c-d). The presence of dynamic recrystallization shown in Figure 5d, hinder the cracks growth as a consequence of aMg + Al12Mg17 eutectic on grain boundaries.
The compression on temperature 400°C and strain rates 5 and 10s-1 also results in cracking (Figure 6a-b). These are a consequence of precipitation of mentioned
Slika 6: Deformacijski pogoji pri hitrosti deformacije 5 s-1 in temperaturi 400 °C: makroskopski pogled na deformiran vzorec (a) in razpoke v mikrostrukturi (b); deformacijski pogoji pri hitrosti deformacije 1,0 s-1 in temperaturi 400 °C: makroskopski pogled na vzorec (c) in dinamična rekristalizacija (d).
Figure 6: Deformation conditions strain rate 5s-1 and temperature 400°C: macroscopic view of deformed specimen (a), and cracks in microstructure (b); deformation conditions strain rate 1.0s-1 and temperature 400°C: macroscopic view of specimen (c), and dynamic recrystallization (d).
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aMg + Al12Mg17 in intermetalne faze na robovih zrn.
Pri nižjih hitrostih deformacije večanje razpokzaviraprocesdinamičnekristalizacije. Na Sliki 6c je prikazan makro-pogled deformiranega vzorca pri 400 °C in hitrosti deformacije 1 s-1, Slika 6d pa prikazuje mikrostrukturo tega vzorca, na kateri je jasno vidna dinamična rekristalizacija. Pri nižjih temperaturah, npr. 200-250 °C, toplotno preoblikovanje ni mogoče ne glede
aMg + Al12Mg17 eutectic and Al12Mg17 intermetallic phase on grain boundaries
At lower strain rates process crack growth is hindered by the process of dynamic recrystallization. Figure 6c shows the macro-view of deformed specimen at 400°C and strain rate of 1s-1, and Figure 6d shows the microstructure of this sample where the presence of dynamic recrystallization is clearly visible. In the lower temperature region, i.e. 200 - 250°C
Slika 7: Deformacijski pogoji pri hitrosti deformacije 0,001 s-1 in temperaturi 250 °C: makroskopski pogled na deformiran vzorec (a) in razpoke v mikrostrukturi (b); deformacijski pogoji pri hitrosti deformacije 1,0 s-1 in temperaturi 300 °C: makroskopski pogled na vzorec (c) in mikrostruktura brez razpok (d).
Figure 7: Deformation conditions strain rate 0.001s-1 and temperature 250°C: macroscopic view of deformed specimen (a), and cracks in microstructure (b); deformation conditions strain rate 1.0s-1 and temperature 300°C: macroscopic view of specimen (c), and cracks-free microstructure (d).
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na hitrost deformacije. Običajni videz vzorca in deformirana mikrostruktura znotraj tega območja sta prikazana na Slikah 7a-b.
Slika 7c prikazuje makroskopski pogled deformiranega vzorca pri 300 °C in hitrosti deformacije 1 s-1. Njegova površina in mikrostruktura sta prikazani na Sliki 7d in sta brez razpok. Rezultat je bil enak tudi pri temperaturi 300 °C in višjih hitrostih deformacije, tj. 5s-1 in 10s-1. Območja z nižjimi vrednostmi učinkovitosti izgube moči, tj. n<0,2, prikazana na Sliki 4, so povsem skladna s podatki o vedenju zlitine med stiskanjem v vročem. Pomnite, da je na Sliki 4 območje, v katerem je toplo preoblikovanje mogoče, označeno s črtkanimi črtami.
4 Sklepi
Strjevanje zlitin Mg-Al je bilo raziskano na podlagi krivulj strjevanja. Zaradi prisotnosti mangana v proučevani zlitini MgAl6Mn nastaja intermetalična spojina Al4Mn pri nižjih koncentracijah raztopljenega aluminija v staljeni kovini, evtektik pa nastaja pri nižjih hitrostih ohlajanja. Deformacija gravitacijsko litih vzorcev v temperaturnih razponih 200-450 °C in v razponu hitrosti ohlajanja 0,001-10s-1 je bila proučena s pomočjo procesnih map. Na podlagi naših raziskav smo dognali naslednje:
•	za zlitino sta značilna mehčalna mehanizma dinamična obnova in dinamična rekristalizacija med plastično deformacijo v temperaturnem razponu 300-400 °C in pri nižjih hitrostih ohlajanja 0,001-1s-1. To pomeni, da je v teh temperaturnih območjih in pri takšnih hitrostih ohlajanja mogoče izvajati postopek toplega preoblikovanja.
•	Pri temperaturah nad 430 °C pa je za material zaradi taljenja evtektika aMg + Al12Mg17 na robovih zrn značilna manjša voljnost.
hot forming is not possible at any strain rate. Typical appearance of specimen and its microstructure deformed within this region are shown in Figures 7a-b.
In Figure 7c a macroscopic view of deformed specimen at 300°C and strain rate of 1s-1 is shown. Its surface and its microstructure shown in Figure 7d are cracks free. The same results were also obtained at temperature 300°C for higher strain rates, i.e. 5s-1 and 10s-1. Areas with lower values of efficiency of power dissipation, i.e. n<0.2 shown in Figure 4 are in very good agreement with obtained behavior of alloy during hot compression. Note that in Figure 4 the area, where safe hot forming is possible, is hatched.
4 Conclusions
From the cooling curves the solidification of the Mg-Al alloys was studied. Due to the presence of manganese in the studied MgAl6Mn alloy the formation of the inter-metallic compound Al4Mn is taking place with decreasing concentration of the dissolved aluminum in the molten metal, but the formation of eutectic occurred at low cooling rate. The deformation characteristic of gravity cast samples in the temperature range 200 - 450°C and in the strain rate range 0.001 - 10s-1 has been studied using processing maps. The following conclusions can be drawn from our investigation:
•	The alloy exhibit dynamic recovery and dynamic recrystallization softening mechanism during plastic deformation in the medium temperature range 300
-	400°C and at lower strain rates 0.001
-	1s-1. Thus hot deformation can be performed in these temperatures and strain rates regions.
•	At temperatures higher than 430°C the material exhibit lower ductility due
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Pri temperaturah 250 °C in manj je prišlo do nastanka razpok na robovih zrn zaradi precipitacije intermetalične faze Al12Mg17 in evtektika aMg + Al12Mg17.
S tehnološkega in proizvodnega vidika je torej za toplo preoblikovanje optimalna temperatura 300 °C, uporabiti pa je mogoče tudi višje hitrosti deformacije, tj. 5,0-10.0s-1.
Trikratno povečanje natezne trdnosti, tj. 318 MPa, je bilo zagotovljeno v toplo preoblikovanem vzorcu pri temperaturi 300 °C in hitrosti deformacije 1,0 s-1 in hitrostjo ohlajanja med strjevanjem 7 K/ min in več.
to melting of aMg + Al12Mg17 eutectic which is located on a grain boundaries. At temperatures 250°C and lower, grain boundary cracking due to precipitates of Al12Mg17 intermetallic phase and aMg + Al12Mg17 eutectic has been observed.
From technological and production point of view the optimal temperature for hot forming is 300°C where also higher strain rates, i.e. 5.0 - 10.0s-1, could be applied.
Three times increase of tensile strength, i.e. 318 MPa, was obtained for samples with imposed small hot deformation at a temperature of 300°C and strain rate of 1.0s-1 at cooling rates during solidification of 7K/min and above.
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