NR 3/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 115

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

Katarzyna Natalia Braszczyńska-Malik*, Elżbieta PrzełożyńskaInstitute 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-Tip composite microstructure were carried out by light microscopy, scanning electron microscopy (SEM + EDS) and X-ray diffrac-tion (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 Al8Mn5 intermetallic phase.

Key words: magnesium matrix composites, Ti particles, stir casting, microstructure.

Inżynieria Materiałowa 3 (211) (2016) 115÷119DOI 10.15199/28.2016.3.4© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

Metal matrix composites (MMCs) reinforced with different fib-ers or particles are among various composites of the most recent structural construction materials. Obtaining the designed proper-ties 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, Cgr, TiC, Al2O3 etc.) causes a strong decrease in the ductility of the final material. Therefore, different materials are sought as a rein-forcement component for metal matrix composites. In comparison with ceramic particles, metallic reinforcements have better wetta-bility with molten matrix alloys, greater ductility and higher ther-mal 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 metal-lic reinforcement phase.

Among various MMCs, magnesium matrix composites rein-forced with particles deserve special consideration, due to their unique combination of properties such as low density, high spe-cific 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 mi-crostructure 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) character-ised 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 ex-perimental materials by powder metallurgy techniques [15, 19÷27]. Compared with other methods like powder metallurgy or mechani-cal alloying, infiltration or self-propagation high-temperature syn-thesis, 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 rein-forced 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. Tita-nium 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–Tip composite was obtained by introducing 30 wt % Ti particles to the mechanical mixing of the molten AM50 matrix al-loy 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 prepolish-ing and polishing with different diamond pastes without contact with water. To reveal the microstructure, the samples were etched

116 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

in a 1% solution of HNO3 in C2H5OH for about 60 s. The micro-structures 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 composi-tions of the investigated alloys were analyzed by X-ray diffraction (XRD) using a Brucker D8 Abvence diffractometer. CuKα X-ray ra-diation 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.

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 sedi-mentation 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 rela-tively large size of the Ti particles and relatively high solidification velocity of the composite in the metal mould, the phenomenon of

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 Al Mn Zn Si Fe Cu

AM504.5÷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 H Fe Others each

0.02 0.10÷0.18 0.02 0.01 max. 0.004 max. 0.1

Ti balance

Fig. 1. Micrograph of used Ti particles; SEMRys. 1. Mikrofotografia zastosowanych cząstek Ti; SEM

Fig. 2. Microstructure of AM50–Tip composite: a) unetched, b)÷d) etched sampleRys. 2. Mikrostruktura kompozytu AM50–Tip: a) próbka nietrawiona, b)÷d) trawiona

a)

b)

c)

d)

NR 3/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 117

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 hexago-nal 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 in-coherence (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 spheri-cal 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 nucle-ation of Mg on Ti, but this influence in different areas of the ob-served microstructure is not distinct.

The microstructure of the gravity cast matrix alloy has a den-dritic structure with a visibly very strong segregation of the al-loying 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 so-

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 de-pending 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 analy-ses 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 (spec-trum 4 in Figure 4). Additionally, the boundaries between the com-ponents 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 re-inforcement, i.e. Al–Ti-type) were not revealed. The presented re-sults are in contradiction to those described for Mg–Al–Ti materi-als 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 re-veal new structural constitutions which can be created due to poten-tial reactions between the components.

Fig. 3. Microstructure of AM50–Tip composite; SEMRys. 3. Mikrostruktura kompozytu AM50–Tip; SEM

Fig. 4. Microstructure of AM50–Tip composite with chemical analysis of selected areas; SEM + EDSRys. 4. Mikrostruktura kompozytu AM50–Tip wraz z analizą chemiczną w wybranych mikroobszarach; SEM + EDS

a) b)

118 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

The X-ray diffraction pattern for the AM50–Tip composite is presented in Figure 5. It also confirmed that the fabricated mate-rial is composed of the Tiα, an α-Mg solid solution, the γ phase and Al8Mn5 intermetallic phase, although the volume fraction of this Al–Mn-type phase is practically on the boundary of the XRD de-tection 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 Mix-ture”, which can be expressed as:

HB HB HBcomp p p p m= ⋅ + − ⋅f f( )1 (1)

where HBcomp, HBp, HBm – the Brinell hardness for the composite, particles and matrix, respectively, fp – 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 ac-cording to [33]). The difference between the experimental and cal-culated 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 reinforce-ment phase within the matrix. Due to the good wettability of the Ti particles by the molten magnesium alloy, both uniform distribution of the reinforcement and an easy fabrication process are possible. The introduction of Ti particles into the AM50 alloy does not cause changes in the phase composition of the magnesium matrix.

REFERENCES

[1] Braszczyńska K. N.: Contribution of SiC particles to the formation of the struc-ture of Mg–3 wt % RE cast composites. Z. Metallkde. 94 (2) (2003) 144÷148.

[2] Braszczyńska K. N., Lityńska L., Zyska A., Baliga W.: TEM analyses of interfaces between components in magnesium matrix composites rein-forced with SiC particles. Mater. Chem. Phys. 81 (2003) 326÷328.

[3] Braszcztńska-Malik K. N.: Structure analyses of Mg–Zn–Zr matrix alloy composites reinforced with SiC particles. Kompozyty 7 (1) (2007) 51÷55.

[4] Ye H. Z., Liu X. Y.: Review of recent studies in magnesium matrix com-posites. J. Mater. Sci. 39 (2004) 6153÷6171.

[5] Chen L., Yao Y.: Processing, microstructures, and mechanical properties of magnesium matrix composites: A review. Acta Metall. Sin. (Engl. Lett.) 27 (5) (2014) 762÷774.

[6] Braszczyńska K. N., Bochenek A.: The Young’s modulus (E) and fracture toughness (JIC) of MMCs reinforced with SiCp. Sci. Eng. Compos. Mater. 9 (3) (2000) 149÷158.

[7] Zhang X., Wang H., Liao L., Teng X., Ma N.: The mechanical properties of magnesium matrix composites reinforced with (TiB2 + TiC) ceramic particles. Mater. Lett. 59 (2005) 2105÷2109.

[8] Fan J., Zhang H., Dong H., Xu B., Zhang Z., Shi L.: Effects of process-ing technologies on mechanical properties of SiC particulate reinforced magnesium matrix composites. J. Wuhan Univ. Technol. Mater. Sci. Ed. 29 (4) (2014) 769÷772.

[9] Sankaranarayanan S., Jayalakshmi S., Gupta M.: Effect of addition of mutually soluble and insoluble metallic elements on the microstructure, tensile and compressive properties of pure magnesium. Mater. Sci. Eng. A530 (2011) 149÷160.

[10] Seetharaman S., Subramanian J., Gupta M., Hamuda A. S: Influence of micron-Ti and nano-Cu additions on the microstructure and mechanical properties of pure magnesium. Metals 2 (2012) 274÷291.

[11] Braszczyńska-Malik K. N.: Discontinuous and continuous precipitates in magnesium–aluminium type alloys. J. Alloys Compd. 447 (2009) 870÷876.

[12] Kulekci M. K.: Magnesium and its alloys applications in automotive in-dustry. Int. J. Adv. Manuf. Technol. 39 (2008) 851÷865.

[13] Dudina D. V., Georgarakis K., Li Y., Aljerf M., LeMoulec A., Yavari A. R., Inou A.: A magnesium alloy matrix composite reinforced with metallic glass. Compos. Sci. Technol. 69 (2009) 2734÷2736.

[14] Kondoh K., Kawakami M., Imai H., Umeda J., Fujii H.: Wettability of pure Ti by molten pure Mg droplets. Acta Mater. 58 (2010) 606÷614.

[15] Sankaranarayana S., Jayalakshmi S., Gupta M.: Effect of individual and combined addition of micro/nano-sized metallic elements on the micro-structure and mechanical properties of pure Mg. Mater. Des. 37 (2012) 274÷284.

[16] Hassan S. F., Gupta M.: Development of ductile magnesium composite materials using titanium as reinforcement. J. Alloys and Compd. 345 (2002) 246÷251.

[17] Ye H. Z., Liu X. Y.: Microstructure and tensile properties of Ti6Al4V/AM60B magnesium matrix composite. J. Alloys and Compd. 402 (2005) 162÷169.

[18] Raghunath B. K., Karthikeyan R., Ganesan G., Gupta M.: An investigation of hot deformation response of particulate-reinforced magnesium +9 % ti-tanium composite. Mater. Des. 29 (2008) 622÷627.

[19] Pérez P., Garcés G., Adeva P.: Mechanical properties of a Mg–10 (vol.%)Ti composite. Compos. Sci. Technol. 64 (2004) 145÷151.

[20] Lu L., Lai M. O, Froyen L.: Effects of mechanical milling on the proper-ties of Mg–10.3% Ti and Mg–5% Al–10.3% Ti metal–metal composite. J. Alloys and Compd. 387 (2005) 260÷264.

[21] Xi Y. L., Chai D. L., Zhang W. X., Zhou J. E.: Ti–6Al–4V particle rein-forced magnesium matrix composite by powder metallurgy. Mater. Lett. 59 (2005) 1831÷1835.

[22] Xi Y. L., Chai D. L., Zhang W. X., Zhou J. E.: Titanium alloy reinforced magnesium matrix composite with improved mechanical properties. Scripta Mater. 54 (2006) 19÷23.

[23] Yang Z. R., Wang S. Q., Gao M. J., Zhao Y. T., Chen K. M., Cui X. H.: A new-developed magnesium matrix composite by reactive sintering. Composites Part A39 (2008) 1427÷1432.

[24] Umeda J., Kawakami M., Kondoh K., Ayman EL-S., Imai H.: Microstruc-tural and mechanical properties of titanium particulate reinforced magne-sium composite materials. Mater. Chem. Phys. 123 (2010) 649÷657.

[25] Olszówka-Myalska A., Przeliorz R., Rzychoń T., Misiowiec M.: Micro-structure of Mg–Ti–Al composite hot pressed at different temperature. Solid State Phenom. 191 (2012) 199÷207.

[26] Olszówka-Myalska A.: Evoluation of titanium particles microstructure in aluminium matrix composite obtained by powder metallurgy. Inżynieria Materiałowa 3 (4) (2007) 200÷204.

[27] Esen Z., Dikici B., Duygulu O., Dericioglu A. F.: Titanium–magnesium based composites: mechanical properties and in-vitro corrosion response in Ringer’s solution. Mater. Sci. Eng. A573 (2013) 119÷126.

[28] Kumruoglu L. C.: Production of Mg–3Al based composites reinforced with Ti6Al4V particles. Acta Phys. Pol. A125 (2) (2014) 432÷434.

[29] Meenashisundaram G. K., Gupta M.: Low volume fraction nano-titanium particulates for improving the mechanical response of pure magnesium. J. Alloys and Compd. 593 (2014) 176÷183.

[30] Braszczyńska-Malik K. N., Przełożyńska E.: Microstructure of AZ91–Ti6Al4V metal–metal composite in as-cast conditions and after heat treat-ment. Compos. Theory Pract. 14 (4) (2014) 224÷228.

[31] Braszczyńska-Malik K. N., Przełożyńska E.: Metal–metal cast compos-ites. Arch. Foundry Eng. 14 (3) (2014) 110÷113.

[32] Przełożyńska E., Braszczyńska-Malik K. N.: Possibilities of fabricating Mg–Al–RE cast magnesium matrix composites reinforced with Ti par-ticles. Arch. Foundry Eng. 15 (3) (2015) 73÷76.

[33] Donachie M. J.: Titanium: A technical guide. 2nd ed., ASM International, Materials Park, Ohio 44073-0002 (2000).

Fig. 5. X-ray diffraction pattern of AM50–Tip compositeRys. 5. Rentgenogram kompozytu AM50–Tip

NR 3/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 119

Wytwarzanie metodą odlewniczą kompozytu na osnowie stopu magnezu AM50 z cząstkami tytanu

Katarzyna Natalia Braszczyńska-Malik*, Elżbieta PrzełożyńskaInstytut Inżynierii Materiałowej, Politechnika Częstochowska, *kacha@wip.pcz.pl

Inżynieria Materiałowa 3 (211) (2016) 115÷119DOI 10.15199/28.2016.3.4© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: kompozyty magnezowe, cząstki Ti, metoda odlewnicza, mikrostruktura.

1. CEL PRACYGłównym celem pracy było wytworzenie materiału kompozytowe-go 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 mikrosko-pii elektronowej (SEM + EDS), rentgenowskiej analizy fazowej (XRD) oraz metalografii ilościowej. Przeprowadzono również po-miary twardości kompozytu sposobem Brinella.

2. MATERIAŁ I METODYKA BADAŃ

Materiał wyjściowy stanowiły wlewki komercyjnego stopu ma-gnezu AM50 (typu Mg–Al–Mn) o składzie chemicznym zamiesz-czonym w Tabeli 1. Jako fazę zbrojącą zastosowano proszek tyta-nu w postaci cząstek o sferoidalnym kształcie (rys. 1) i wielkości