Materials and Design 49 (2013) 1009–1015

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Materia ls and Design

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Effect of Sn addition on microstructure, mechanical and casting properties of AZ91 alloy

Yunus Turen ⇑Department of Metals, Karabuk University, 78200 Karabuk, Turkey

a r t i c l e i n f o

Article history:Received 3 January 2013 Accepted 12 February 2013 Available online 5 March 2013

Keywords:Magnesium alloy FluidityHot tearing Mechanical properties

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⇑ Tel.: +90 370 4338200; fax: +90 370 4338204.E-mail address: yturen@karabuk.edu.tr

a b s t r a c t

In this study, effect of Sn addition on microstructure, mechanical and the casting properties of AZ91 mag- nesium alloy have been studied. Results from the microstructural analysis showed that refinement of Mg17Al12 phase took place and new Mg 2Sn phase was formed as Sn was added into the AZ91 alloy. Flu- idity increased with 0.5 wt.% Sn content then decreases rapidly as the Sn content exceeded above 0.5 wt.%. Hot tear susceptibility (HTS) decreased with 0.5 wt.% Sn addition to AZ91 magnes ium alloy above which it increased considerably. Tensile strength and elongation increased by addition of up to 0.5 wt.% Sn above which it decreased with increasing Sn content. The increase of mechanical properties was attributed to transformat ion of lamellar to fully divorced eutectic b phases by 0.5 wt.% Sn addition existed in grain boundaries of the magnesium matrix. The reduction in the mechanical properties above 0.5 wt.% Sn was attrib uted to formation of clustered Mg 2Sn phase.

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1. Introduction

As the lightest metallic structura l material, high specificstrength, specific stiffness and machinability, magnesiu m and its alloys are increasingly used in many engineeri ng areas including portable microelectroni cs, telecommunicati on, aerospac e and automobile industrie s due to their low density [1]. Especially, alloy Mg–9Al–0.8Zn–0.2Mn (AZ91) is the most favoured magnesium al- loy, being used in approximately 90% of all magnesium cast prod- ucts [2]. Additions of minor alloying elements such as Sb, Ca, Bi, Pb and rare earth (RE) to AZ91 alloy have been studied to improve casting, microstructure stability or creep propertie s of the alloy [3–8]. Effects of rare-earth, alkaline earth, and silicon additions to magnesium alloys have been reviewed [3]. Balasubramani et al. [4] studied the influence of Pb and Sb additions on the precip- itation sequence and ageing kinetics of AZ91 alloy. Guangyin et al.[5,6] reported that small amount of Sb or Bi additions to the AZ91 alloy resulted in significant increases in yield strength and creep resistance at elevated temperature s up to 200 �C, but slight de- creases of ductility. Zang et al. [7] examine d the microstructural features, tensile propertie s and creep resistances of AZ and ZA ser- ies alloys having small additions of Ca or Sr. Wenwen et al. [8]investigated the effects of Ca and lanthanum-r ich misch metal additions to AZ91 alloy on the microstructure and mechanical properties. They [8] reported that small amounts of Ca addition to AZ91 did not cause the formation of any new phases in the

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microstru cture, but refined the as-cast microstructur e and in- creased the thermal stability of the b phase so that the yield strength and creep resistance of the alloy were significantly im- proved. Additions of lanthanum-r ich misch metal resulted in the formatio n of needle-shap ed particles, which showed high thermal stability.

However , in the literature, few works [9,10] have been reported on the effect of Sn addition on the mechanical and microstructural behaviou r of AZ91 alloy. Li et al. [9] have been investiga ted the ef- fect of Sn on the microstru cture and compressive deformat ion behaviou r of the AZ91D ageing alloy. Their results showed that in- creased Sn content in the AZ91D alloy resulted in discontinuo us precipita tion along the grain boundari es in the form of lamellar precipita tes of the Mg 17Al12 phases. The AZ91D alloy had a higher ultimate fracture strength than the base 2.0 wt.% Sn alloy due to the large numbers of grains containing the cellular discontinuo us precipita tion forming nearby the grain boundaries. On the other hand, Jihua et al. [10] have investigated effects of Sn addition on microstru cture and mechanical properties of ZA (Mg–Zn–Al) alloy.Their work showed that minor addition of Sn to ZA alloy resulted in suppressi on of the eutectic transformat ion and the refinement of divorced eutectics which improved ambient and elevated-tem per- ature strength.

Literatur e review revealed that the effect of Sn (as a minor alloying addition) to AZ series Mg alloys, especially AZ91 alloy in as cast condition, is scarce. Therefore, the aim of this work was to investigate effects of various Sn additions on the microstru cture and mechanical propertie s of AZ91 cast alloy as well as its casting propertie s (i.e. fluidity and hot tearing).

Table 1Chemical composition of the alloys.

Alloy Elements (wt.%)

Al Zn Mn Sn Mg

AZ91 9.13 0.78 0.18 – Bal.AZ91 + 0.5Sn 8.80 0.87 0.22 0.53 Bal.AZ91 + 1Sn 9.95 0.78 0.14 1.11 Bal.AZ91 + 2Sn 9.20 0.83 0.19 2.34 Bal.

1010 Y. Turen / Materials and Design 49 (2013) 1009–1015

2. Experimental procedur e

Mg, Al, Zn, Sn ingots with a minimum purity of 99.9% were pur- chased from Sakarya Metal Co., Turkey. The alloys were prepared by melting pure Mg together with Al alloying additions in a graph- ite crucible under Ar gas atmosph ere at 750 �C and then held for 20 min before pouring. Zn and Sn additions were carried out 1 min. before casting to avoid losses of Zn and Sn due to vaporisa- tion. The molten alloys were then cast into cast iron moulds (pre-heated to 250 �C) for fluidity, hot tearing and mechanical tests under protective SF 6 gas. Detailed information about the casting process can be found in Ref. [11]. Hot-tearing resistance was eval- uated by using a Constrained Rod Casting (CRC) as shown in Fig. 1a.The molten metal fluidity tests were employed by using spiral mould. The schematic representat ion of the spiral mould is shown in Fig. 1b. The amount of Sn content in AZ91 alloy has been se- lected as 0.5, 1.0 and 2.0 wt.%. The chemical compositions of the studied alloys are shown in Table 1.

The alloy specimens were used in as-cast form in the present study. The hardness values were determined by Vickers hardness testing with a load of 50 N. At least 10 successive hardness mea- surements were carried out on each sample. Tensile specimens with a diameter of 16 mm were cast in a metal mould then ma- chined by turning down to a diameter of 8 mm and a length of 40 mm. The tensile tests were performed (ASTM: E 8M-99) with a crosshead speed of 0.5 mm/min at room temperature . Each data represents the average of at least five samples tested. X-ray Diffrac- tion (XRD) analysis (Analytical Empyrean) was carried out under Cu Ka radiation with the incidence beam angle of 2�. The diffrac- tion angle range was between 20 � and 100 � with a step incremen tof 0.02 � and a count time of 1 s.

Microstruct ure evaluations were carried out by using scanning electron microscopy (SEM). Samples having 10 mm in length and 25 mm in diameter were machined then subsequent ly ground from 220 to 1000 grit emery papers followed by polishing with 1 lm diamond paste for microstructure evaluations .

3. Results and discussion

3.1. Microstructu re

Fig. 2a–d shows microstructur e of AZ91 and AZ91 + xSn alloys respectively . The microstru ctural analysis revealed that a network

Fig. 1. Schematic representation of (a) fluidity spiral an

of the b phase around the grain boundaries had been formed in AZ91 alloys. Visually, from SEM image the b phase can be distin- guished from the magnesium matrix as appearing brighter around the surrounding darker matrix. The XRD analysis indicated AZ91 alloy consisted of a-Mg solid solution, b phase being the compound of intermetallic Mg 17Al12 phase whereas the AZ91 + xSn alloys con- sisted of additional Mg 2Sn phases alongside with a-Mg and bphases (Fig. 3). Published literature [10,12–15] revealed that com- monly known primary a-Mg, intermetalli c b and a + b eutecticphases are present in AZ series alloys in accord with the present work. It was reported that formation of b phase was due to changes in the solidification behaviour of the melt by Zn addition. Although the solid solubility of Al in Mg is 12.6 wt.%, the presence of b and/ora + b eutectic phases observed with AZ91 alloy was reported to be due to the higher segregation tendency of Zn and the degree of constituti onal undercoolin g ahead of the solid–liquid interface during the early stages of cooling [12,16]. This reduces the size of the interdendr itic space, which in turn favours the a + b eutectic[17].

Fig. 4a–c shows higher magnification of SEM micrographs of AZ91 alloy with and without Sn added alloys. Evidently, addition of Sn to AZ91 alloy transformed lamellar eutectics into fully di- vorced b eutectics. It has been previously reported [16,18,19 ] thatfully divorced, partially divorced and lamellar eutectic morpholo- gies can be formed in magnesiu m alloys depending on alloying additions and/or solidification conditions. Detailed information on the eutectic morphologies of magnesium casting alloys and the variation of these morphologies can be found in Refs. [16,18].Systemat ic study on the effect of Sn on eutectic morphology of AZ series Mg alloys especially as cast condition, seem not to be re- ported previously. Work of Li et al. [9] showed that increased Sn content in the AZ91D alloy resulted in discontinuous precipita tion along the grain boundari es in the form of lamellar precipitates of the Mg 17Al12 phases. However, it should be noted that their

d (b) hot tearing moulds (as proposed in Ref. [27]).

Fig. 2. Effect of Sn additions on the microstructure of AZ91 alloy: (a) AZ91, (b) AZ91 + 0.5Sn, (c) AZ91 + 1Sn and (d) AZ91 + 2Sn.

Fig. 3. XRD patterns of AZ91 and AZ91 + xSn alloys.

Y. Turen / Materials and Design 49 (2013) 1009–1015 1011

resulting microstructur e was in the form of aged condition rather than as cast condition. On the other hand, work of Jihua et al.[10] showed that minor addition of Sn to ZA alloy resulted in sup- pression of the partially eutectic transformation to fully divorced

eutectics in agreement with the present work. Our previous work [20] showed that the presence of Pb in solid solution might have inhibited the diffusion of Al and Mg atoms. Since Sn has the similar characteri stics as Pb, it can be inferred that Sn has the same effect

(a) (b)

Lamellar eutectics

Fully divorced eutectics

(c)

Element Weight% Atomic%

Mg K 73.01 82.69

Al K 14.01 14.30

Sn L 12.98 3.01

Totals 100.00

Clustered Mg2Sn

1 +

Fig. 4. Higher magnification of SEM micrographs of AZ91 alloy with and without Sn added alloys: (a) AZ91, (b) AZ91 + 0.5Sn and (c) AZ91 + 2Sn.

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on the diffusion kinetics of Al and Mg atoms. Addition of Pb [20]and Ti [16] resulted in refining of b eutectics by transformi ng from lamellar to fully divorced b eutectics which increased the mechan- ical and corrosion properties of AZ91 cast alloys considerably.

A different phase in the fully divorced b eutectics have been ap- peared in 2 wt.% Sn containing alloys as shown in (Fig. 4c). EDS analysis showed that this phase contains Sn as well as Mg and Al (Fig. 4c) being Mg 2Sn phase. Presence of Mg 2Sn phase in Sn con- taining AZ91 alloy is evident as shown in Fig. 3.

3.2. Fluidity

Fluidity is, in casting terminology , the distance to which ametal, when cast at a given temperature, will flow in a test mould

before it is stopped by solidification [21]. Therefore, fluidity is alength measured in millimetres or metres. Fig. 5 shows the influ-ence of Sn content on the fluidity of AZ91 magnesium alloys. For AZ91 magnesium alloy, its filling length increases with 0.5 wt.%Sn content, but decrease s rapidly as the Sn content exceeds above 0.5 wt.%. The solidification gap, formatio n of oxide and surface ten- sion are the most important paramete rs which affect the fluidity of a molten metal [22]. It is expected that, Sn as a surface active ele- ment decreases the surface tension of molten magnesium leading to an increase in the fluidity [23]. There seem to be no report on effect of Sn on fluidity and/or surface tension of AZ series magne- sium alloys. It may be postulate d that Sn is an element which has lower evaporati on temperature [24] may cause the disruption of the MgO film which forms on the surface of molten metal during

0,0 0,5 1,0 1,5 2,0100

120

140

160

180

200

220

240

260

280

300F

low

leng

th, m

m

Sn content. wt.%

Fig. 5. Fluidity of AZ91 alloy as a function of its Sn content.

AZ91 AZ91+0.5Sn AZ91+1Sn AZ91+2Sn0

2

4

6

8

10

12

14

16

18

20

HT

S

ALLOY

Present work G.Cao et al.

Fig. 6. Hot tearing susceptibility (HTS) of AZ91 alloy as a function of its Sn content.

Y. Turen / Materials and Design 49 (2013) 1009–1015 1013

filling of the mould promoting the fluidity. On the other hand,however, the sharp decrease with the increase in Sn content above 0.5 wt.%, may be explained as severe disruption of the oxide filmwhich may lead to increased thermal conductivity (i.e. faster cool- ing rate) of the melt owing to weakened or fragmented oxide filmat the surface and the better wettability [25] between mould and the melt. It was noted that higher Sn containing samples presented more shiny surfaces as compared to AZ91 alloy. However, this needs further investigatio n.

0

50

100

150

200

250

300

350

400

450

500

AZ91+2 SnAZ91+1 SnAZ91 AZ91+0,5 Sn

UTS YS EL,%

Sn content, wt. %

Str

engt

h, M

Pa

0

2

4

6

8

10

12

14

16

18

20

Elongation, E

L, %

Fig. 7. Ultimate tensile strength (UTS) and yield strength (YS) of AZ91 alloy as afunction of its Sn content.

3.3. Hot tearing

Hot tear is called as; irreversible cracks (tears) that formed in the mushy zone during solidification of the castings. In the mushy zone, porosity is generally formed during late stages of solidifica-tion due to shrinkage of the melt. Zheng et al. [26] reported that the hot tearing susceptibility (HTS) of Mg–Al binary alloys declines with an increase in the amount of added Al to Mg. Wang et al. [27]investigated the hot tear of Al containing Mg alloys as a function of their Zn content. They reported [27] that the quantity of phase with low melting point in grain boundaries was increased, its melt- ing point was decreased and the hot-tearing susceptibility was in- creased with Zn additions. The segregati on of Zn element in grain boundaries was the main contribution to the high hot-tearing sus- ceptibility of Mg–9Al–xZn alloys. Cao and Kou [28] evaluated HTS of binary Mg–Al alloys as follows:

HTS ¼ RðfcrackflengthflocationÞ ð1Þ

In the above equation the values of the crack width factor fcrack

were 1 for short hair line, 2 for full hair line, 3 for crack and 4 for half broken rod. The values of the crack length factor flength were4 for the longest rod, 8 for the second longest rod, 16 for the third longest rod and 32 for the shortest rod. The values of the crack location factor flocation were 1 for cracking at the sprue end, 2 at the ball end and 3 in the middle of the rod. Fig. 6 shows HTS of AZ91 alloy as a function its Sn content. Fig. 6 also illustrates HTS results of Mg–Al binary alloy reported by Cao and Kou [28] along-side with the present work. Their results are reasonably in agree- ment with the present work. It is evident that HTS decreases with 0.5 wt.% Sn addition to AZ91 alloy. However, HTS increases at higher concentratio ns (>0.5 wt.%) of Sn. The decrease of HTS with 0.5 wt.% Sn addition is attributed to suppression of lamellar eutectics at a certain level. However, increase in HTS with Sn addi- tion above 0.5 wt.%, may be due to the increased Al content near the fully divorced b eutectics. Wang et al. [27] reported that Zn addition to Mg–Al binary alloy promoted both formatio n of fully

divorced eutectics and segregation of Al near the b eutectics. They [27] discussed that segregation of Al near the grain boundaries by Zn addition, increases the quantity of eutectic in grain boundaries and decreasing the end-solidify ing temperat ure of the alloys.Increasin g the quantity of low melting-poi nt liquid in grain bound- aries must decrease the adhesiveness among the grains (den-drites), and make it easier to be separated by external solidification stress [27]. The discussion by Wang et al. [27] alsomay apply to the present work since Sn is a low melting-p oint me- tal, its excessive addition may lead to hot tearing of AZ91 alloys during solidification. This is also confirmed by Fig. 4c that Sn had been formed in the fully divorced b eutectics as clustered Mg 2Snphase.

3.4. Mechanica l properties

Fig. 7 shows the dependence of the ultimate tensile strength (UTS), yield strength (YS), elongation (EL) and hardness (HV) of AZ91 alloy depending on its Sn content, respectively. Evidently,UTS and YS increase with addition of Sn to AZ91 alloy. It was noted that the UTS and YS of AZ91 alloy increased from 144 to �195 (i.e. a 35% increase) and from 95 to �140 MPa (i.e. a 47% in- crease) by 0.5 wt.% Sn respectivel y above which both UTS and YS decrease d with increasing Sn content up to 2.0 wt.%. Elongation,EL, was also increased considerably with addition of up to

1014 Y. Turen / Materials and Design 49 (2013) 1009–1015

0.5 wt.% Sn then decreased with increasing Sn content up to 2.0 wt.% (Fig. 7). Hardness, HV, was not altered notably by Sn addition as shown in Fig. 8. These results also are in accord with our previous works [16,20] that addition of Pb and Ti increased the mechanical properties of AZ91 alloy. As explained in the works of other researchers [12–17], the presence of b intermet al- lics means that there is a continuous easy crack path of brittle phase along the a-Mg grain boundary. The alloy is expected to ex- hibit lower ductility because the net-like brittle intermetalli cphase at the a-Mg grain boundary easily breaks up and causes cracking during plastic deformat ion. As a consequence, there is aresulting low elongation and concomitan t low strength for the AZ91 alloy. The results are consistent with the previous works [16,20] and the work of Wu et al. [29] who reported UTS and EL of cast AZ91 alloy as 145 MPa and 1.5% respectively . However,Zhao et al. [30] reported 126 MPa UTS and 3.3% EL for AZ91 alloy.The deviations from the reported results [29,30] may be due to the production conditions such as casting temperature , solidifica-tion condition and compositi on of the alloy which may affect the morphology of the intermetalli cs.

Apparently, lamellar b eutectics have more surface area as com- pared to fully divorced b eutectics, thus more crack sites. The in- crease in the UTS, YS and EL with Sn addition is attributed to transformat ion of lamellar eutectics to fully divorced eutectics as well as presence of Mg 2Sn precipitates . Jihua et al. [10] reportedthat the addition of Sn to ZA magnesiu m alloy resulted in the sup- pression of the lamellar eutectic transformat ion and the refine-ment of divorced eutectics in accord with the present work. It is believed that Sn addition may cause precipitatio n effect by finerMg2Sn precipitates distribut ed throughout the a-matrix, which could block the movement of dislocations by Orowan looping. It was reported [10] that a minor addition of Sn contributed to the formation of the dispersed short rod-like Mg 2Sn particles and the improvement in the ambient and elevated- temperature strength of ZA magnesium alloy, however, excessive amount of Sn (>0.5 wt.%) in their work, reduced the strength of the alloy. In the present work, higher concentration of Sn content (i.e. >0.5 wt.%),also had resulted in the reduction of the UTS, YS and EL. This was believed to be due to coarsening effect of Mg 2Sn precipita tes. In- deed, a different particle containing Sn (Mg2Sn) in the b eutecticshas been observed and crack at the interface between b eutecticsand Mg 2Sn is evident as shown in Fig. 4c. Therefore, excessive Sn addition leads to the coarsening of Mg 2Sn particles as well as in- creased hot tearing as discussed earlier, and thus results in the de- cline of strength and plasticity of the alloy entirely consisten t with the work of Jihua et al. [10].

0,0 0,5 1,0 1,5 2,060

65

70

75

80

85

90

Sn content, wt. %

Har

dnes

s, H

V

Fig. 8. Hardness (HV) of AZ91 alloy as a function of its Sn content.

4. Conclusion s

The following conclusion can be drawn from the present work;

� Addition of Sn to AZ91 magnesium alloy transformed lamellar eutectics into fully divorced b eutectics.� A clustered phase in the fully divorced b eutectics have

appeared in 2 wt.% Sn containing alloys being Mg 2Sn phase.� Fluidity of molten AZ91 magnesiu m alloy increases with

0.5 wt.% Sn content, but decrease s rapidly as the Sn content exceeds above 0.5 wt.% attributed disruption of the MgO filmon the surface of molten metal. The sharp decrease with the increase in Sn content above 0.5 wt.%, was explained as severe disruption of the oxide film.� Hot tear susceptibility, HTS, decreases with 0.5 wt.% Sn addition

to AZ91 alloy above which increased the HTS considerabl yattributed to presence of low melting-p oint Sn at the grain boundari es.� UTS, YS and EL were increased considerably with addition of up

to 0.5 wt.% Sn then decrease d with increasing Sn content up to 2.0 wt.% which attributed to transformat ion of lamellar eutec- tics to fully divorced eutectics as well as presence of Mg 2Snprecipita tes.

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