Fabrication of AM50 magnesium matrix composite with titanium particles by stir casting method

Inżynieria Materiałowa 3 (211) (2016) 115 119 DOI 10.15199/28.2016.3.4 Copyright SIGMA-NOT MATERIALS ENGINEERING Fabrication of AM50 magnesium matrix composite with titanium particles by stir casting method Katarzyna Natalia Braszczyńska-Malik *, Elżbieta Przełożyńska Institute of Materials Engineering, Czestochowa University of Technology, Czestochowa, Poland, * kacha@wip.pcz.pl The paper focuses on the experimental magnesium matrix composite reinforced with Ti particles fabricated by the stir casting method. The main objective of the study was to develop a new type of composites with metallic particles fabricated by a simple and inexpensive casting method. For this purpose, one of the cheapest and most widely used alloys, AM50 (Mg Al Mn system), was selected as the matrix alloy. The investigated material was prepared on the basis of the AM50 commercial magnesium alloy with 30 wt % spherical Ti particles. The experimental composite was obtained by introducing Ti particles to the mechanical mixing of the molten magnesium alloy under a protective atmosphere. The prepared composite suspension was gravity cast into a metal mould. Analyses of the AM50-Ti p composite microstructure were carried out by light microscopy, scanning electron microscopy (SEM + EDS) and X-ray diffraction (XRD). Brinell hardness of the examined material was also measured. Additionally, the weight fraction of the Ti particles was verified by determining their volume fraction using the linear method. The obtained composite exhibited uniform distribution of the Ti particles within the magnesium matrix alloy. According to the presented results of the investigation, no new phases were revealed by the microstructure observations and XRD techniques. The phase composition of the composite was typical for the used component. The matrix alloy was composed of an α-mg, α + γ eutectic and Al 8 intermetallic phase. Key words: magnesium matrix composites, Ti particles, stir casting, microstructure. 1. INTRODUCTION Metal matrix composites (MMCs) reinforced with different fibers or particles are among various composites of the most recent structural construction materials. Obtaining the designed properties of these composite materials depends, however, on achieving the desired microstructure, which is the effect of numerous factors, like: shape, distribution, size, volume fraction of the reinforcement, type of matrix alloy, type and parameters of the fabrication process and component bonding type [1 4]. Nevertheless, previous studies [5 8] clearly proved that brittle ceramic reinforcement (like SiC, C gr, TiC, Al 2 O 3 etc.) causes a strong decrease in the ductility of the final material. Therefore, different materials are sought as a reinforcement component for metal matrix composites. In comparison with ceramic particles, metallic reinforcements have better wettability with molten matrix alloys, greater ductility and higher thermal and mechanical compatibility with the metallic matrix [9 10]. It should be noted that the main factor which classifies the metallic phase as a classical composite reinforcement is the deficiency of (or very low) mutual solubility between the metal matrix and the metallic reinforcement phase. Among various MMCs, magnesium matrix composites reinforced with particles deserve special consideration, due to their unique combination of properties such as low density, high specific strength and stiffness, exception dimensional stability and high damping capacity [1 6]. In lightweight magnesium alloys, aluminium constitutes the main alloying element, chiefly because of its low price, high availability, low density and advantageous effect on corrosion and strength properties. The most commonly used magnesium alloys are the AZ or AM series [11, 12]. Recently, some studies have also described the effect of different metallic reinforcements (with high melting points and very low solubility in magnesium) such as Ni, Cu, Ti or Ti6Al4V particles on the microstructure and properties of magnesium composites [13 15]. Of particular note are Ti particles because the solubility of Ti in solid Mg is practically negligible and titanium (and its alloys) characterised by a high Young s modulus, hardness and sufficient elongation compared to magnesium alloys [16 29]. It was reported in [16] that an addition of 5.6 wt % Ti particles to pure magnesium improved both the strength and ductility of the final materials. Additionally, it should be noted that compared to conventional ceramics used as reinforcements of magnesium matrix composites, the wettability of titanium by molten magnesium is very good [14]. Recently, many studies on magnesium based composites with titanium have been conducted, but most of them are based on the production of experimental materials by powder metallurgy techniques [15, 19 27]. Compared with other methods like powder metallurgy or mechanical alloying, infiltration or self-propagation high-temperature synthesis, stir casting is the most economical (costs as little as one-third to one-tenth for mass production) and an easily adopted method [3, 30 32]. In the present paper, the AM50 magnesium matrix alloy reinforced by Ti particles is presented. The main objective of the study was to develop a new type of composites with metallic particles fabricated by a simple and inexpensive casting method. For this purpose, one of the cheapest and most widely used alloys, AM50, was selected as the matrix alloy. 2. EXPERIMENTAL PROCEDURES Commercial ingots of AM50 magnesium alloy with a nominal chemical composition given in Table 1 were used in this study. Titanium powder in the form of spherical particles, presented in Figure 1, with the nominal composition given in Table 2, was chosen as the reinforcement. The fraction of the Ti particles was below 50 µm. The AM50 Ti p composite was obtained by introducing 30 wt % Ti particles to the mechanical mixing of the molten AM50 matrix alloy in a steel crucible under a protective atmosphere. The prepared composite suspension was gravity cast into a steel mould (diameter 20 mm). The fabrication process parameters such as mixing and casting temperature, time and rate of suspension stirring, mould temperature etc. were chosen experimentally. The specimens for microstructure investigations were prepared by standard metallographic procedures including wet prepolishing and polishing with different diamond pastes without contact with water. To reveal the microstructure, the samples were etched NR 3/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 115

Table 1. Chemical composition of AM50 alloy according to standard ATM B93-94, wt % Tabela 1. Skład chemiczny stopu AM50 zgodnie z normą ASTM B93-94, % mas. Alloy AM50 Al Mn Zn Si Fe a) Cu 4.5 5.3 0.28 0.5 max. 0.02 max. 0.05 max. 0.004 max. 0.008 Mg balance Table 2. Chemical composition of Ti particles according to standard ASTM B-348, grade 1, wt % Tabela 2. Skład chemiczny cząstek tytanu zgodnie z normą ASTM B-348, % mas. C O N 0.02 0.10 0.18 0.02 H Fe Others each 0.01 max. 0.004 max. 0.1 Ti balance b) c) Fig. 1. Micrograph of used Ti particles; SEM Rys. 1. Mikrofotografia zastosowanych cząstek Ti; SEM in a 1% solution of HNO3 in C2H5OH for about 60 s. The microstructures were observed with an Olympus GX51 light microscope and a JSM-6610LV scanning electron microscope. Additionally, the weight fraction of the Ti particles was verified by determining their volume fraction using the linear method. The phase compositions of the investigated alloys were analyzed by X-ray diffraction (XRD) using a Brucker D8 Abvence diffractometer. CuKα X-ray radiation was used. Reflexes from particular phases were identified according to ICDD PDF cards. Brinell hardness of the AM50 Tip composite was measured using WPM Leipzig testing machine. The measurements were done using a carbide ball of 2.5 mm diameter and load of 612.9 N. d) 3. RESULTS AND DISSCUSION Figure 2 shows the microstructure of the gravity cast AM50 Tip composite. The fabricated composites are characterized primarily by uniform distribution of the Ti particles within the matrix. Neither clusters of the Ti particles nor any consequences of floating or sedimentation of the reinforcing phase, frequently occurring in gravity cast composites, are observed, which was especially visible on the unetched cross-sections (Fig. 2a). Such a uniform distribution of Ti particles was possible owing to the good wettability of the titanium particles by the molten matrix alloy and the easy creation of a bond between the metal metal components. Additionally, due to the relatively large size of the Ti particles and relatively high solidification velocity of the composite in the metal mould, the phenomenon of 116 Fig. 2. Microstructure of AM50 Tip composite: a) unetched, b) d) etched sample Rys. 2. Mikrostruktura kompozytu AM50 Tip: a) próbka nietrawiona, b) d) trawiona INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

pushing the reinforced particles by the growing matrix dendrites did not occur. The real (verified) volume fraction of Ti particles in the composite was equal to 13.59±1.43 vol. % (i.e. 28.65 wt %), which corresponds to the assigned mass fraction, demonstraing the precision of the frabrication process of the designed composite. It should also be noted that both metals (Mg and Ti) have a hexagonal closed packed (hcp) structure, which can suggest heterogeneous nucleation of the α-mg solid solution on the Ti particles. In the presented study, the process of suspension preparation and casting were conducted below the temperature of allotropic transformation of titanium, which is equal to 1155 K. Additionally, the lattice incoherence (expressed as the quotient of the difference of the lattice parameters) between Mg and Ti is less than 0.1, which also suggests the possibility of a nucleation process of magnesium on titanium. On the other hand, due to the relatively large size of the spherical Ti particles, this influence of reinforcement is still ambiguous. Figure 2b shows the distinct distribution of the Ti particles against the background of the magnesium matrix dendrites. The location of some Ti particles inside the α-mg dendrites suggested the nucleation of Mg on Ti, but this influence in different areas of the observed microstructure is not distinct. The microstructure of the gravity cast matrix alloy has a dendritic structure with a visibly very strong segregation of the alloying elements. The microstructure of the matrix alloy consisted of an α-mg solid solution dendrite and α + γ eutectic, typical for cast AM50 magnesium alloy. In Mg Al type alloys, the γ-phase is an intermetallic compound with a stoichiometric composition of Mg17Al12 (at 43.95 wt % Al) and an α-mn-type cubic unit cell [11]. Non-equilibrium solidification conditions caused non-uniform distribution of aluminium in the alloy microstructure. During soa) lidification, large crystals of the α-mg solid solution depleted in aluminium (in comparison to the equilibrium system) were created from the liquid. Local fluctuation of the chemical composition of the alloy caused formation of the α + γ eutectic at the final stage of solidification. The α + γ eutectic (fully or partially divorced depending on the Al mass fraction and alloy solidification rate) is also observed in all cast commercial Mg Al alloys. It should be noted that the α + γ eutectic was also observed near the Ti particles, but rather sporadically (Fig. 2d and 3). Additionally, a small blocky manganese-type phase (Al8Mn5) was also revealed (Fig. 3a). This phase is observed in commercial AM and AZ series alloys, due to the presence of manganese in the alloy chemical compositions [11]. Due to the low weight percentage of manganese in the matrix alloy chemical composition, an Al8Mn5 intermetallic phase was created under a limited volume fraction. Figure 4 presents a secondary electron image with point analyses of the investigated material. Analyses of the alloying elements in the AM50 Tip composite confirmed the presence of the α-mg solid solution (spectrum 1 in Figure 4) and the α + γ eutectic (spectrum 4 in Figure 4). Additionally, the boundaries between the components were devoid of any new phases (spectrum 3 in Figure 4). It should be noted that the creation of new phases (as products of potential reactions between the matrix alloying elements and reinforcement, i.e. Al Ti-type) were not revealed. The presented results are in contradiction to those described for Mg Al Ti materials prepared by the powder metallurgy or reactive sintered method [9, 24, 25] where the Al3Ti intermetallic phase was formed, while the AM50 Tip composite microstructure observations did not reveal new structural constitutions which can be created due to potential reactions between the components. b) Fig. 3. Microstructure of AM50 Tip composite; SEM Rys. 3. Mikrostruktura kompozytu AM50 Tip; SEM Fig. 4. Microstructure of AM50 Tip composite with chemical analysis of selected areas; SEM + EDS Rys. 4. Mikrostruktura kompozytu AM50 Tip wraz z analizą chemiczną w wybranych mikroobszarach; SEM + EDS NR 3/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 117

The X-ray diffraction pattern for the AM50 Ti p composite is presented in Figure 5. It also confirmed that the fabricated material is composed of the Ti α, an α-mg solid solution, the γ phase and Al 8 intermetallic phase, although the volume fraction of this Al Mn-type phase is practically on the boundary of the XRD detection limit. It should also be noted that no reflexes from different potential phases were revealed by this method. The Brinell hardness of the fabricated composite was equal to 70±5 HB, while the hardness of the matrix alloy cast in the same conditions was about 57±5 HB. The above results clearly prove that the used Ti particles can play an important role in strengthening the composite. The obtained experimental results also correspond to values calculated from the simple and well-known Rule of Mixture, which can be expressed as: HB = f HB + ( 1 f ) HB (1) comp p p p m where HB comp, HB p, HB m the Brinell hardness for the composite, particles and matrix, respectively, f p volume fraction of the particles. The theoretical value obtained from the above equation is equal to 72 HB (for verified volume fraction of Ti particles equal to 13.6 vol. % and literature data for Ti hardness equal to 170 HB according to [33]). The difference between the experimental and calculated values of composite hardness is about 3%, which is in the range of experimental error. 4. CONCLUSIONS The fabricated cast AM50 magnesium matrix composite with Ti particles is characterized by a uniform arrangement of the reinforcement phase within the matrix. 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Inżynieria Materiałowa 3 (211) (2016) 115 119 DOI 10.15199/28.2016.3.4 Copyright SIGMA-NOT MATERIALS ENGINEERING Wytwarzanie metodą odlewniczą kompozytu na osnowie stopu magnezu AM50 z cząstkami tytanu Katarzyna Natalia Braszczyńska-Malik *, Elżbieta Przełożyńska Instytut Inżynierii Materiałowej, Politechnika Częstochowska, * kacha@wip.pcz.pl Słowa kluczowe: kompozyty magnezowe, cząstki Ti, metoda odlewnicza, mikrostruktura. 1. CEL PRACY Głównym celem pracy było wytworzenie materiału kompozytowego na bazie magnezu z cząstkami metalowymi za pomocą taniej i prostej metody odlewania grawitacyjnego. Na osnowę kompozytu wybrano komercyjny stop magnezu AM50 (typu Mg Al Mn). Jako fazę zbrojącą zastosowano sferoidalne cząstki tytanu. Zakres badań obejmował analizę mikrostruktury wytworzonego materiału metodami mikroskopii świetlnej, skaningowej mikroskopii elektronowej (SEM + EDS), rentgenowskiej analizy fazowej (XRD) oraz metalografii ilościowej. Przeprowadzono również pomiary twardości kompozytu sposobem Brinella. 2. MATERIAŁ I METODYKA BADAŃ Materiał wyjściowy stanowiły wlewki komercyjnego stopu magnezu AM50 (typu Mg Al Mn) o składzie chemicznym zamieszczonym w Tabeli 1. Jako fazę zbrojącą zastosowano proszek tytanu w postaci cząstek o sferoidalnym kształcie (rys. 1) i wielkości <50 µm, którego skład chemiczny zamieszczono w Tabeli 2. Materiał kompozytowy wytworzono na drodze odlewniczej poprzez wprowadzenie 30% mas. cząstek Ti do mechanicznie mieszanego ciekłego stopu magnezu. Proces prowadzono w atmosferze ochronnej. Uzyskaną suspensję kompozytową odlano grawitacyjnie do metalowej formy. Parametry procesu wytwarzania, tj. temperaturę odlewania, czas i prędkość mieszania, temperaturę formy itd. dobrano eksperymentalnie. Próbki do badań mikrostruktury przygotowano standardową metodą obejmującą szlifowanie na papierach i polerowanie na suknach z użyciem past diamentowych. W celu ujawnienia szczegółów mikrostruktury zgłady metalograficzne trawiono 1% roztworem kwasu HNO 3 w C 2 H 5 OH przez ok. 60 s. Obserwacje mikrostruktury przeprowadzono za pomocą mikroskopu świetlnego Olympus GX51 oraz skaningowego mikroskopu elektronowego JSM- -6610LV (SEM + EDS). Skład fazowy badanego materiału analizowano metodą rentgenowskiej analizy fazowej (XRD) za pomocą dyfraktometru typu Brucker D8 Advance. Do wytworzenia promieni X użyto lampę miedzianą. Identyfikacji faz dokonano na podstawie aktualnej bazy danych ICDD PDF. Pomiary twardości sposobem Brinella przeprowadzono za pomocą twardościomierza WPM Leipzig, stosując kulkę z węglików spiekanych o średnicy 2,5 mm oraz siłę nacisku 612,9 N. Ponadto dokonano weryfikacji udziału objętościowego cząstek Ti w kompozycie metodą liniową. 3. WYNIKI I ICH DYSKUSJA Rysunek 2 przedstawia mikrostrukturę materiału kompozytowego typu AM50 Ti p wytworzonego metodą odlewniczą. Uzyskany kompozyt odznaczał się równomiernym rozmieszczeniem cząstek Ti w osnowie magnezowej. Nie zaobserwowano konsekwencji flotacji czy sedymentacji zbrojenia, które często występują podczas odlewania grawitacyjnego kompozytów zbrojonych cząstkami, co jest szczególnie widoczne na rysunku 2a. Równomierne rozmieszczenie cząstek w kompozycie wynika z dobrej zwilżalności cząstek tytanu przez ciekły stop magnezu oraz łatwego tworzenia połączenia pomiędzy komponentami metal metal, a przede wszystkim poprawnego mieszania suspensji i warunków jej krystalizacji. Zweryfikowany metodą metalografii ilościowej udział cząstek Ti w kompozycie wynosił 13,59±1,43% obj. (tj. 28,65% mas.), co odpowiada ilości fazy zbrojącej wprowadzonej do osnowy magnezowej podczas przygotowywania suspensji. Uzyskane dane wskazują na dokładność procesu wytwarzania zaprojektowanego kompozytu. Warto zaznaczyć, że zarówno Mg, jak i Ti mają heksagonalną budowę krystaliczną, co sugeruje heterogeniczne zarodkowanie roztworu stałego α-mg na cząstkach Ti. Mikrostrukturę odlanego grawitacyjnie stopu zastosowanego na osnowę kompozytu charakteryzuje budowa dendrytyczna z widoczną silną segregacją pierwiastków stopowych. Składnikami fazowymi są roztwór stały α-mg oraz eutektyka α + γ, co jest typowe dla zastosowanego stopu AM50. Warto zaznaczyć, że występowanie eutektyki α + γ zauważono również w pobliżu cząstek Ti, ale zjawisko to występuje raczej sporadycznie (rys. 2d i 3). Ponadto w strukturze zaobserwowano związek międzymetaliczny Al 8, co jest konsekwencją niewielkiej zawartości Mn w stopie bazowym (rys. 3a). Rysunek 4 przedstawia mikrostrukturę otrzymanego materiału kompozytowego wraz z analizą EDS składu chemicznego. Wykazała ona występowanie obszarów o składzie odpowiadającym fazom α-mg (spectrum 1 na rysunku 4) oraz eutektyki α + γ (spectrum 4 na rysunku 4). Rentgenowska analiza fazowa (rys. 5) potwierdziła obecność tych faz. Zaobserwowano refleksy pochodzące od roztworu stałego α-mg, eutektyki α + γ oraz fazy międzymetalicznej Al 8, występujących w osnowie i Ti α. Nie ujawniono żadnych nowych faz z układu Ti Al, które mogłyby powstać w wyniku reakcji pomiędzy zastosowanymi komponentami. Twardość wytworzonego kompozytu AM50 Ti p wynosiła 70±5 HB, podczas gdy twardość stopu zastosowanego na osnowę odlanego w tych samych warunkach 57±5 HB. Uzyskane wyniki dowodzą, że zastosowane cząstki Ti odgrywają ważną rolę w umocnieniu kompozytu. 4. PODSUMOWANIE Wytworzony kompozyt na osnowie stopu magnezu AM50 zbrojony cząstkami tytanu charakteryzował się równomiernym rozmieszczeniem cząstek fazy zbrojącej. Umożliwiły to dobra zwilżalność cząstek Ti przez ciekły stop magnezu oraz odpowiednio dobrane parametry procedury przygotowania suspensji, jak i jej odlewania. Wprowadzenie cząstek Ti do stopu AM50 nie spowodowało zmian w składzie fazowym stopu osnowy. NR 3/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 119