Effect of friction stir processing on tensile and fracture behaviour of AZ91 alloy

Accepted Manuscript Title: Effect of friction stir processing on tensile and fracture behaviour of AZ91 alloy Authors: A. Raja, V. Pancholi PII: DOI: Reference:

S0924-0136(17)30166-8 http://dx.doi.org/doi:10.1016/j.jmatprotec.2017.05.001 PROTEC 15209

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

2-11-2016 23-3-2017 1-5-2017

Please cite this article as: Raja, A., Pancholi, V., Effect of friction stir processing on tensile and fracture behaviour of AZ91 alloy.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2017.05.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of friction stir processing on tensile and fracture behaviour of AZ91 alloy A. Raja1, V. Pancholi1* 1

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Uttarakhand, 247667 (India) [email protected] , [email protected] *

Corresponding author address: Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Uttarakhand, 247667 (India) Email address: [email protected] (Vivek Pancholi) Abstract Multipass friction stir processing (FSP) was performed on as-cast (AC) AZ91 magnesium alloy with different tool probe lengths to introduce layered microstructure through the thickness. Three microstructural variations were developed. They were, half thickness fine grain microstructure (HFG), surface modified-fine grain microstructure (SFG) and full thickness fine grain microstructure (FFG). FSP was performed at tool rotation rate of 720 rpm and transverse speed of 150 mm/min. The coarse α-Mg dendrites of 100 μm were refined to approximately 2 μm. Network of β-Mg17Al12 interdendritic particles were broken and distributed uniformly after multipass FSP. Tensile test and notch fracture toughness test were conducted to understand the effect of layered microstructure on mechanical properties. The tensile properties, namely, yield strength, tensile strength and percentage elongation of AC material were found to be 92 MPa, 100 MPa and 0.8% respectively and the corresponding values for FFG were improved to 242 MPa, 327 MPa and 4.7%. For HFG and SFG, these values were found to follow the rule of mixture. Similarly, apparent fracture toughness (KQ) values of single edge notch bend (SENB) specimen without precrack were compared and the results showed improvement from 6.2 MPa√m in AC to 12.3 MPa√m in FFG. Keywords: Microstructure variation; Multipass FSP; AZ91 alloy; Tensile properties; Apparent fracture toughness. Introduction

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The cast AZ91 Mg alloys, containing about 9 wt. % Al and 1 wt. % Zn, is the widely used cast Mg alloy due to combination of excellent castability, good corrosion resistance and reasonable mechanical properties at the room temperature. However, due to the presence of coarse α-Mg dendrites and large plate like β-Mg17Al12 phase in the inter-dendritic region, AZ91 alloy exhibit poor strength and ductility which restricts its application. Researchers tried various methods for microstructural refinement in order to improve its mechanical properties. Grain refinement can be achieved through conventional forming processes like rolling, forging and extrusion. These processes are performed at elevated temperatures since room temperature formability of Mg alloys is very poor due its hexagonally closed pack (HCP) crystal structure and lesser than five independent slip systems. Another approach for grain refinement is use of severe plastic deformation (SPD) methods namely, equal channel angular pressing (ECAP), high pressure torsion (HPT), accumulative roll bonding (ARB), friction stir processing (FSP) etc. SPD can be performed as secondary manufacturing process, in other words SPD can be done directly on cast billets as well as on the wrought alloys. Out of all the SPD processes listed, FSP is the most promising process due to the flexibility it offers. For example, the process can be easily scaled as bulk processing, it can be performed at ambient temperature and depth of grain refinement can be controlled by the selection of different tool pin lengths. FSP is a solid state processing technique in which homogeneous and refined microstructure can be obtained as a result of dynamic recrystallization. Mishra and Ma, (2005) reviewed FSP technique and brought out that it can be used as an effective process to improve properties of cast alloys. Sato et al., (2005) used multipass FSP to improve the formability of cast AZ91 Mg alloy. Feng and Ma, (2007) worked on the same alloy using FSP and found enhancement in tensile properties. Cavaliere and De Marco, (2007) reported improvement in fatigue properties due to grain refinement and elimination of casting defects and later Ni et al., (2009) brought out effect of fine precipitates on fatigue properties of AZ91 alloy after FSP. del Valle et al., (2015) used FSP with 2

liquid nitrogen cooling system to obtain ultra-fine grain (UFG) and found improvement in room temperature and high temperature tensile properties. In all these works on cast AZ91 alloy, enhancement in mechanical properties were attributed to grain refinement, dissolution and break up of hard secondary phase particles and elimination of porosity and other defects. Caceres et al., (2002) brought out that the strength of AZ91 alloys was determined by the combined effect of grain size, solid solutions and precipitates strengthening. Hutchinson et al., (2005) in their work on modelling of precipitate effects on strengthening mechanism mentioned that strengthening due to dislocation-precipitates interactions in AZ91 alloys could be improved by increasing the precipitate number density. Hence, the microstructural changes after FSP of AZ91 alloy are favourable in enhancing its mechanical properties. However, there is hardly any work done to evaluate fracture toughness of the FSPed magnesium alloys. Somekawa and Mukai, (2005) observed improvement in fracture toughness due to increase in plastic zone size under the crack tip due to grain refinement in pure Mg and Xia et al., (2014) also reported improvement in fracture toughness of wrought AZ31 Mg alloy after grain refinement by ECAP. Mansoor and Ghosh, (2012) studied effect of multi-pass FSP on extruded ZK60 Mg plate. They found improvement in mechanical properties of processed alloy, which was attributed to layered microstructure with grain size of 2-5 μm and 100 μm. Similarly, Witkin, (2003) and Oskooie et al., (2015) worked on Al alloys through cryomilling and high energy planetary ball milling methods respectively to fabricate mixed coarse and fine grain structures and obtained optimum strength and ductility. Moreover, Wang et al., (2002) achieved increased strength and ductility in Cu after thermomechanical treatment with a mixed grain size distribution of micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300nm) grains. The matrix grains impart high strength whereas the coarse grains facilitate strain hardening mechanisms to give high tensile ductility. Perron et al., (2008) performed molecular dynamic simulation on polycrystalline Al samples and found a linear relationship between grain size and 3

surface roughness to an extent of applied strain, while, Liu et al., (2011) reported that surface modification by shot peening decreases surface roughness due to grain refinement and becomes reason for improvement in mechanical properties in Mg alloy. Pancholi and Kashyap, (2007) obtained better bulge profile during superplastic bulge forming of AA8090 Al-Li alloy which contain three distinct microstructural layers along the thickness. Later, Pradeep and Pancholi, (2014) deliberately fabricated layered microstructure containing coarse and fine grains in Al 5082 alloy using FSP and found that the inhomogeneous microstructures possess superior superplastic formability than the homogeneously grain sized materials. It is clear that non-uniform or layered microstructure sometimes give superior properties than uniform microstructure. FSP is a potential technique to introduce these kind of microstructures. In the present work, FSP of AZ91 alloy was performed; i) to break cast structure and refine the microstructure and, ii) to get combination of coarse and fine grain microstructures. Effect of microstructural development was then studied in terms of tensile properties and fracture toughness. Experimental Method The cast AZ91 alloy billets were received from Hindustan Aeronautics Limited (HAL). The billets were machined in to plates of size; 10 mm thick, 100 mm wide and 110 mm length. These plates were subjected to multipass FSP at a tool rotational speed of 720 rpm and a traverse speed of 150 mm/min with a tool tilt angle of 3 degrees. During FSP, copper backing plate was used to avoid tunnelling defect by effective transfer of heat generated due to friction between the tool shoulder and the sample. Three tools with same shoulder diameter of 28 mm and threaded conical pin of diameter 8 mm at shoulder and 4mm at tip but with different pin lengths were used. Approximately, 70 mm of 100 mm width of the plate was processed, which required 30 to 35 passes with 50% overlap towards the retreating side. After multipass FSP the 10 mm plates were machined to 6 mm. The experimental details are presented in Table 1.

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Table 1: Experimental conditions used to obtain microstructural variation in through thickness direction. Processing configurations

FSPed surface FSP tool pin height Material removed after FSP Final condition of 6 mm thick plate

HFG

SFG

FFG

(half thickness fine grained microstructure)

(surface modified fine grained microstructure)

(full thickness fine grained microstructure)

Processed on one surface 5 mm

Processed on both surfaces. 4 mm

Processed on one surface 7 mm

2 mm from both the surfaces

2 mm from both the surfaces

1 mm from top and 3 mm from bottom surface.

Top 3 mm FSPed and bottom 3 mm as-cast (AC).

Top and bottom 2 mm FSPed. Middle 2 mm AC

Full Thickness, 6 mm, FSPed

Tensile specimens with gauge dimensions of 25 mm×6 mm×6mm were machined parallel to the processing direction according to the standard ASTM B557M – 15. The cross head speed of 1.5 mm/min was used to conduct tensile test at a strain rate of 1×10-3 s-1. For fracture toughness the SENB specimen or 3 point bend specimen were prepared with thickness 6 mm, width 24 mm and span length of 90 mm with notch parallel to the process direction. The SENB sample and its notch details are shown in the Fig.1. The notches were made using CNC wire-cut electro – discharge machining (EDM). Fracture toughness tests were performed without fatigue precrack as variation in microstructure would create vast difference in the precrack length in through thickness. The cross head speed was maintained at 1.5 mm/min in order to correlate the fracture characteristics with the tensile failure. All these tests were performed on Instron 8800 servo-hydraulic testing system at ambient temperature. Serrated hydraulic grips were used for tensile test whereas three-point bend fixture was used for fracture toughness test.

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Fig. 1: (a) Schematic of multipass FSPed plate with location of tensile sample and (c) SENB specimen with notch details taken from processed zone Microstructural characterization was carried out by optical microscope (Leica DMI 5000). Field emission scanning electron microscope (FESEM - FEI Quant 200F) was used for microstructural characterization, energy-dispersive spectroscopy (EDS) as well as for fracture surface characterization. Rigaku Smart Lab X-ray diffractometer was used for phase identification in AZ91 alloy before and after FSP. Analysis of diffraction peaks was accomplished using X’pert Highscore Plus software. The specimens for optical microscopy and scanning electron microscopy (SEM) were prepared by mechanical polishing using SiC papers with grit size in the sequence of 320, 800, 1200, 1500 and 2000 and water as lubricant. Subsequent polishing was performed using diamond paste of 6μm, 3μm and 1μm and ethanol as lubricant. After mechanical polishing, etching was done by immersing the sample in acetic-picral solution (6 g picric acid, 5 ml acetic acid, 100 ml ethanol, 10 ml water) for 15 - 20 seconds. For electron backscattered diffacrtion (EBSD) the sample 6

surface was polished in Buhler Ecomet 250 grinder/polisher using chemomet cloth and 0.02 µm particle sized colloidal silica using methanol as lubricant. The EBSD scan was obtained using 20 kV accelerating voltage with a step size of 0.08 µm. Results Microstructure Macrostructure of all the four configurations i.e. AC, HFG, SFG and FFG are shown in a three dimensional montage in Fig.2. The unprocessed AC material shows macroscopically uniform microstructure with coarse dendrites, as shown in Fig. 2(a). The HFG material having fine grained (FG) microstructure in the processed zone followed by a thin transition region and as-cast microstructure in the unprocessed zone is shown in Fig. 2(b). Material processed from both the surfaces (SFG), shown in Fig. 2(c) contain 2 mm of FG microstructure from both the surfaces and the middle 2 mm has as-cast microstructure with transition region on either side of as-cast region. Fig. 2(d) shows full thickness fine grained material (FFG) having FG microstructure throughout the 6 mm thickness. Macrostructures in Fig. 2(b) and Fig. 2(d) exhibit non-uniform microstructure having semi-circular rings. This particular feature is visible for the processing conditions in which tool with longer pin was used.

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Fig. 2: Macrostructure of a) AC, b) HFG, c) SFG and d) FFG samples. As-cast microstructure regions, fine grained regions (FG), transition regions and semi-circular ring patterns are mentioned. Arrows x, y and z represent process direction, transverse direction and thickness direction respectively. The optical microstructures of the different regions shown in the macrostructure (see Fig. 2) are shown in Fig.3. As-cast microstructure, shown in Fig. 3(a), contain dendritic region of 100 μm average size and interdendritic region with network of plate like precipitates. The secondary electron image of as-cast microstructure is shown as inset in Fig. 3(a) and elemental analysis using EDS in Table 2. Fig. 3 in conjunction with Table 2 brings out that dendritic regions (A) showed elemental composition of 5.16 wt.% Al whereas, interdendritic region (B) showed 34.41 wt.% Al and 2.46 wt.% Zn.. FSPed region with fine grained microstructure and broken, uniformly distributed precipitates is shown in Fig. 3(b). In the inset of Fig. 3(b), the SEM image of FSPed region have two regions C and D which contain 10.28 wt.% Al and 22.59 wt. % Al respectively. The elemental analysis confirms that regions A and C are α – Mg matrix and regions B and D probably contain β-Mg17Al12 precipitates. Fig. 3(c) shows the transition region which contain elongated grains. The magnified image of the transition region is shown in Fig. 3(d). Non uniformity in grain size in the form of periodic semi-circular ring patterns observed in FFG is shown in the Fig. 3(e) and its magnified image in Fig. 3(f). Detailed phase analysis of the as-cast 8

and FFG samples was done by XRD peak identification to find out dissolution or re-precipitation of any new phase. Fig. 4 shows XRD pattern from these two samples. In both the samples, the peaks either belonged to α – Mg or β-Mg17Al12. However, the intensities of two set of peaks are quite different. Intensities of peaks belonged to β-Mg17Al12 reduced in FFG samples which suggested precipitate dissolution. Since no new peak emerged after dissolution one can say that there was no reprecipitation.

Fig. 3: Four distinct microstructure regions observed from macrostructures along the transverse plane a) as cast microstructure and magnified SEM image insert, b) completely fine grained microstructure with an insert of magnified SEM image; c) microstructure of transition region; d) transition region in higher magnification; e) semi-circular ring patterns and f) semi-circular ring pattern at higher magnification Table 2: Elemental distribution obtained using EDS, at different regions marked in Fig. 3. Mg

Al

Zn

Wt.%

Wt.%

Wt.%

A

94.84

5.16

-

B

63.13

34.41

2.46

C

89.72

10.28

-

D

77.51

22.59

-

Region

9

Fig. 4: Phase identification in as-cast and FSPed (normal surface of FFG) material by XRD peaks. Disappearance and reduction in intensity of β-peaks indicate dissolution of second phase in to the α-matrix EBSD measurement EBSD analysis of multipass FSPed sample was carried out to understand microstructural evolution after processing. The EBSD map was obtained from plane perpendicular to transverse direction (X-Z plane) of the FFG material. Fig. 5(a) shows grain size map. It shows that grain size varies from less than 1 µm to 5 µm with an average grain size of 2 µm. Fig. 5(b) shows grain orientation spread (GOS) map. GOS value indicates amount of substructure in the microstructure. High GOS value represents sub-grains and high dislocation density. On the other hand GOS < 1 indicates recrystallized grains. The inverse pole figure map in Fig. 5(c) and the {0001} and {101̅0} pole figures are shown in Fig. 5(d) which denotes non-basal texture in the transverse direction (Y – direction) and basal texture with slight tilt in the process direction (X-direction) respectively. Jain et al., (2013) in their work reported yield anisotrpy with better mechanical properties in transverse direction when compared to process direction in FSPed AZ91 alloy due to such strong basal texture in process direction. Hence in this work all the mechanical tests were carried out in the process direction as a safe design criterion.

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Figure 5: (a) Grain size map, (b) GOS map, (c) IPF map and (d) Pole figures obained at processed region from transverse plane of HFG sample. Mechanical properties Tensile properties The true stress - true strain curves of specimens with different microstructural regions are presented in Fig. 6. The as-cast (AC) material exhibited lowest strength and elongation values whereas, FFG sample showed highest strength and elongation values. On comparing FFG sample with AC, the yield strength (YS) increased from 92.53MPa to 242.28MPa, ultimate tensile strength (UTS) from 100.51 MPa to 327.67 MPa and ductility from 0.8 % to 4.7%. The YS, UTS and ductility of HFG were 158.14 MPa, 174.64 MPa and 1.2 % respectively. Although, surface modified – fine grained microstructure (SFG) showed yield strength (203.18 MPa) and tensile strength (229.91 MPa) comparable to FFG, improvement in elongation (2. 0 %) was insignificant as work hardening region was observed only in the FFG specimen.

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Fig. 6: True stress versus True strain curve of samples with microstructural variation through the thickness Fig. 7 shows fractograph of tensile samples with fracture surfaces perpendicular to processing direction (tensile direction).Typical features of brittle, cleavage fracture were observed in the as-cast (AC) sample (Fig. 7(a)). The fracture surface in HFG sample exhibited two distinct regions in two halves as shown in the Fig. 7(b).One half was as-cast region characterized by cleavage surface and another half was processed region characterized by intergranular fracture. Fracture surface contain flow lines very similar to the one observed in the microstructure after FSP. Similarly, SFG also had two distinct regions in three parts as shown in Fig. 7(c). Material up to 2 mm depth from the surface has intergranular fracture and the middle region has characteristic cleavage fracture of cast material. The FFG sample has intergranular fracture morphology as shown in the Fig. 7(d). Thus, the fractured surfaces clearly differentiate the processed and unprocessed region with intergranular fracture and cleavage fracture morphology respectively. The magnified images of the cleavage fracture and intergranular fracture are shown in the Fig. 7(e) and Fig. 7(f) respectively.

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Fig. 7: Fractograph of uniaxial tensile tested samples with the fracture surfaces in process plane; (a) AC, (b) HFG, (c) SFG, (d) FFG (e) transcrystalline fracture in as cast region and (f) intergranular fracture in FSPed regions To bring out the effect of microstructural variation on fracture behaviour, fracture profile in through thickness were obtained using optical microscopy, as shown in Fig. 8. The analysis was carried out to understand crack propagation path. AC sample in Fig. 8(a) shows zigzag fracture path whereas, the processed region of HFG in Fig. 8(b) and SFG in (Fig. 8(c)) has smooth and curved profile. Fig. 8(d) shows irregular fracture profile of FFG material. The zigzag fracture profile of AC material is due to crack propagated along interdendritic particle, as shown in scanning electron image in Fig. 8(e). The high magnification SEM image of HFG and SFG in Fig. 8(f) reveal that the 13

smooth fracture profile was due to microstructural refinement. However, irregular fracture profile observed in FFG is due to non-uniform microstructure at semi-circular ring patterns as shown in Fig. 8(g).

Fig. 8: Optical images of tensile sub-surfaces of (a) AC, (b) HFG, (c) SFG and (d) FFG along the transverse plane which is parallel to the loading axis and FE-SEM images of fracture profile of (e) as-cast region, (f) FSPed region without inhomogeneities and (g) FSPed region with inhomogeneities Fracture Toughness The apparent notch fracture toughness values (notch KQ) of single edge notched bend (SENB) specimens, without pre crack, were also observed to follow a trend similar to the tensile test. The plot of load against crack opening displacement (COD) is shown in the Fig. 9(a). The AC sample did not show any sharp peak at maximum, whereas pop-in had occurred in the SFG sample. The HFG and FFG samples too exhibited sharp maximum load. Fig. 9(b) shows notch KQ values of different samples which indicate improvement in fracture toughness from AC to FFG sample. The AC specimen showed lowest KQ value of 6.2 MPa √m and FFG had highest KQ value of 12.3 MPa 14

√m with HFG and SFG showing KQ values of 10.9 MPa √m and 12.1 MPa √m respectively. The values mentioned are apparent notch fracture toughness (KQ). These values may not indicate the actual material property but are consistent enough to compare the fracture toughness with different processing conditions.

Fig. 9: Notch fracture toughness test results of samples with different microstructural configurations (a) load vs. crack opening displacement (COD) curve and (b) comparison of apparent notch fracture toughness The fractograph of fracture toughness tested samples are shown in Fig.10. Similar to the fractograph of tensile samples, the fracture toughness samples too have clear distinction between processed and unprocessed regions. In as-cast (AC) sample there is a void formation at the centre of the notch, marked as box in Fig. 10(a) which is identified as crack initiation site. Similarly in SFG sample, void formation was observed at the notch in as-cast region (Fig. 10 (c)) though the void formation in SFG is not as prominent as in AC sample. Secondary cracks were observed on both processed and unprocessed region around the notch of HFG. The spot with secondary crack on processed region was considered as crack initiation site (Fig. 10 (b)) as there is no indication of void formation in the as-cast region. The fractograph of FFG with the crack initiation site was shown in 15

the Fig. 10 (d). The magnified images of crack initiation sites of AC, HFG, SFG and FFG are shown in the Fig. 10 (e), (f), (g) and (h) respectively.

Fig. 10: Fractograph of SENB samples; (a) AC, (b) HFG, (c) SFG and (d) FFG with their crack origins marked inside the box and respective magnified images in (e), (f), (g) and (h). Discussion The multipass FSPed AZ91 alloy is characterized by very fine matrix grains (~2 μm) and β-particles distributed uniformly in the microstructure with some non-uniformity in the grain size observed in HFG and FFG. The non-uniform microstructure contains bands of as-cast microstructure and processed microstructure. Interestingly the distance between bands was approximately equal to the pitch (traverse speed/rotational speed = 208 μm) of the FSP process. It is also worth noting that in both HFG and SFG conditions tool pin length was more than 5 mm whereas, in SFSP condition it was less than 5 mm. Swaminathan et al., (2010) studied peak temperature during FSP of NiAl bronze alloy and reported that temperature at depths below 4 mm from the shoulder will be 50 – 100oC lower than that at the surface. Therefore, it appears that the non-uniform microstructure is due to improper mixing of the material during FSP. Ma et al., (2008) observed similar β-phase particles rich bands during FSP of cast AZ91 and AZ80 magnesium alloys which was attributed to the lower strain rate/strain and temperature conditions which in turn 16

depends on tool rotational and traverse speeds. In the location where uniform microstructure was observed, the eutectic β-Mg17Al12 network in the as-cast AZ91 alloy disappeared after multipass FSP due to mechanical shearing and dissolution by frictional heat. Based on the Mg–Al phase diagram and the differential scanning calorimetry (DSC) analysis by Feng and Ma, (2007), the heating temperature of 437ᵒC causes the dissolution of the eutectic β-Mg17Al12 phase into the magnesium matrix in AZ91 alloy. The EDS analysis indicated that FSPed region had 10.28 wt.% Al, i.e. approximately equal to the aluminum concentration in AZ91alloy. This value is much higher than that (5.16 wt.% Al) in the as cast alloy, indicating significant dissolution of the eutectic βMg17Al12 phase into magnesium matrix during FSP. Celotto and Bastow, (2001) had reported that the maximum solid solubility of aluminum in magnesium was as high as 12.9 wt.% at the eutectic temperature and about 1.5 wt.% at room temperature. It implies that matrix grains of FSPed AZ91 alloy were in supersaturated solid solution condition. The XRD pattern in Fig.4 supports EDS data, ensuring the existence of α-Mg and β-Mg17Al12 phases in both as-cast and FSPed AZ91 samples. Intensity attenuation and disappearance of certain diffraction peaks of β-phase confirms dissolution of β - phase and enrichment of magnesium matrix after FSP. Hence, it is clear that FSP has resulted in breaking of dendritic microstructure accompanied by refinement and uniform distribution of alloying elements. In the present work, different microstructural configuration and proportions namely, AC, HFG, SFG and FFG were tried to identify optimum condition for better mechanical properties. FFG with the yield and tensile strength of 242 MPa and 327 MPa respectively and with 2.1% elongation had shown superior tensile properties. Improvement of 162 % in yield strength and 226 % in tensile strength was observed over as cast material. On the other hand, HFG and SFG had shown 71 % and 120 % improvement in yield strength and, 74 % and 129 % improvement in tensile strength, respectively. The results represent that the tensile properties of HFG and SFG were in between the AC and FFG samples. Hence, we can say that the properties were only dependent on proportion of 17

processed microstructure and not on the microstructural configuration. Therefore, the rule of mixture can be applied to predict properties, as shown in Fig. 10. In general, any property following rule of mixture should fall between lower bound and upper bound values whose expressions for ultimate tensile strength (UTS) is given in the following equation

(𝜎

𝑓

𝑈𝑇𝑆,𝑓

+

−1

1−𝑓 𝜎𝑈𝑇𝑆,𝑚

)

≤ 𝜎𝑈𝑇𝑆,𝑐 ≤ (𝑓𝜎𝑈𝑇𝑆,𝑓 + (1 − 𝑓)𝜎𝑈𝑇𝑆,𝑚 )

(1)

In the above equation f is the fraction of processed region.𝜎𝑈𝑇𝑆,𝑓 , 𝜎𝑈𝑇𝑆,𝑚 and 𝜎𝑈𝑇𝑆,𝑐 are the UTS of fully processed microstructure, as-cast microstructure and composite microstructure respectively. Yield strength (YS) of composite microstructures were also calculated in similar fashion by using YS of as cast and processed microstructure in place of corresponding UTS values. However, there is a deviation in yield strength from rule of mixture at SFG, as its value was more than the upper bound, as shown in Fig. 11(a). Unlike yield strength the tensile strengths of AC (0% fine grained region), HFG (50% fine grained region), SFG (67% fine grained region) and FFG (100% fine grained region) in Fig. 11(b) are well within the bounds. The deviation from rule of mixture in yield strength can be attributed to ambiguity in measurement by using 0.2 % strain offset. On the other hand, maximum load at failure can be found without any ambiguity, hence tensile strength is found to follow rule of mixture within the bounds.

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Fig. 11: Comparison of experimental result with the rule of mixture for (a) yield strength and (b) tensile strength. At 0%, 50%, 67% and 100% of FSPed region are indicated as AC, HFG, SFG and FFG samples respectively. The grain size contribution to the yield strength is evaluated using the familiar Hall–Petch relation (equation (2)). 𝜎𝑔 = 𝜎0 +

𝑘 √𝑑

(2)

In the equation σ0 was the friction resistance for dislocation movement within the polycrystalline grains while k was a measure of the local stress required to transmit plastic flow at a grain boundary The constant k depends on the type of material, microstructure and process. For the cast AZ91 alloy, σ0 and k were taken as 11 MPa and 370 MPa √μm respectively (Caceres et al, 2002) and for fully FSPed AZ91 alloy, these values were 10 MPa and 160 MPa √μm respectively (Wang et al, 2006). The equation estimates that increase in strength due to grain size effect from 48 MPa in as-cast condition to 123 MPa after FSP. Caceres and Rovera, (2001) investigated solid solution strengthening of Mg by Al in a series of polycrystalline Mg-Al alloys. The solid solution strengthening contribution was evaluated as, 𝜎𝑠 = 𝐶𝑋

2⁄ 3,

where X is the atomic fraction of solute (Al), which was 4.67 % and 9.35 % before and

after processing respectively and, C = 197 (MPa X - 2/3). Due to processing, the strengthening contribution from solid solution improved from 26 MPa to 43 MPa. Both grain size and solid 19

solution strengthening made sure that processed region possess superior tensile properties when compared to the as-cast region. Hence in accordance with rule of mixture the strength of the material increases with increasing proportion of processed fine grain microstructure. Like yield strength and tensile strength, elongation was also maximum for FFG. The percentage elongation of as-cast microstructure was 0.8 % whereas it was 4.7 % for FFG. The HFG and SFG had shown elongation of 1.2 % and 2.0 % respectively. From the stress-strain curve in Fig. 6 it can be observed that except FFG all other conditions did not show uniform plastic deformation. Stress-strain curve of FFG however showed considerable uniform plastic deformation but without significant strain hardening. Figure 10 clearly shows that crack initiated either in as-cast microstructure (AC, SFG) or in non-uniform microstructure which contain bands of processed and as-cast microstructures (HFG, FFG). Comparison of Figs. 7 and 8 on the other hand brings out the crack propagation path. In as-cast microstructure, crack path is through precipitate/matrix interface which accounts for the transgranular fracture surface. In processed material, the crack propagation is through grain boundary precipitate/matrix interface and grain boundaries, which gave it typical intergranular fracture surface. One of the method to improve ductility of any material is through strain hardening. In as-cast material ductility was poor due to solute segregation in the interdendritic region. These regions were brittle in nature and acted as preferential sites for crack nucleation and propagation through interconnected network of interdendritic region (see Fig. 3a). Since crack nucleation and propagation occurred at an early stage of deformation, the as-cast material did not get an opportunity for strain hardening. On the other hand, in the processed material the solute content was redistributed and interdendritic region was broken and uniformly distributed (see Fig. 3b). Since the β phase was not continuous hence, grains underwent deformation before separation at the α/β interface. Both these microstructural changes allow material to exhibit significant strain hardening. It is evident in the stress-strain curves of the as-cast and processed materials. The fracture morphology also shows significant difference between the two materials. The as-cast 20

material exhibited transgranular morphology wherein failure is primarily through cleavage of the β phase. The processed material on the other hand exhibited intergranular morphology The values of the apparent notch fracture toughness of as-cast, HFG, SFG and FFG materials are 6.2 MPa√m, 10.9 MPa√m, 12.1 MPa√m and 12.3 MPa√m respectively. It follows the same trend as tensile strength that AC possess lowest notch KQ value and FFG possess highest, whereas HFG and SFG have notch KQ values between that of AC and FFG materials. Thus, the fracture toughness of the material was enhanced completely at full thickness grain refinement. Somekawa and Mukai, (2005) reported that fracture toughness of the material depends on tensile properties which in turn depend on the grain size, according to Hall-Petch effect. The effect of solid solution on Mg-Zn binary alloy were studied by Somekawa et al., (2006) and reported that fracture toughness increases with the increase in solute concentration. Precipitate shape effect on the fracture toughness of ZK60 alloy was studied by Somekawa et al., (2007) and they reported that the change in shape of precipitates from rod to spherical increases the fracture toughness of that alloy. In the present work also, after FSP, there is significant grain refinement from 100 μm to 2 μm and solute concentration increases from 5.16 wt% Al to 10.28 wt% Al. The uniform distribution of broken secondary phase particles as shown in Fig. 3(b) is responsible for enhancement in fracture toughness of the material. Load vs. COD plot of as-cast material in Fig. 9(a), showed gradual drop in load after reaching its maximum. This behaviour coupled with void formation and cleavage fracture observed in the fractograph (Fig. 10(a)) is the characteristics fracture behaviour of AC material. Sharp peak load was attained in HFG and FFG with crack initiation observed at the bands of inhomogeneous microstructure as apparent from the fractograph in Fig. 10(b) and Fig. 10(d) respectively. SFG had a pop-in phenomenon observed from Fig. 9(a). Crack initiation in as-cast microstructure region of SFG can be observed from its fractograph (Fig. 10(c)). It was reported by Willoughby, (1986) that pop-in could occur in a heterogeneous material such as a weldment due to cleavage crack initiation 21

in brittle region which could be arrested by the tougher matrix. In SFSP, cleavage initiated from ascast, brittle, microstructure region was arrested by tougher FSPed region, albeit not enough to increase its toughness further as the maximum load after pop-in was lower than the initial peak load. Thus any small amount of cast microstructure in FSPed region is detrimental in enhancing the tensile properties as well as crack resistance behaviour. Conclusions The layered microstructure with variation in grain size through the thickness was processed using multipass friction stir processing. The important conclusions are: • Fully refined microstructure (FFG) exhibits superior tensile and fracture behaviour in comparison to as cast, half and surface modified fine grained materials. • The strength of HFG and SFG, which contain different proportions of fine grain size microstructure was observed to follow the rule of mixture. • Grain refinement with dissolved and broken up precipitates were primary reasons for improvement in tensile and fracture characteristics of multipass FSPed AZ91 alloy and any proportion of as-cast microstructure is detrimental to mechanical properties.

Acknowledgement Authors acknowledge Professor B.K. Mishra for his valuable guidance in conducting fracture toughness experiments. Authors also acknowledge S.P. Madhavan, Manager, Foundry and Forging Division, Hindustan Aeronautics Limited, Bangalore, India, for generously providing AZ91 alloys for this research. The authors also wish to thank the laboratory staff of the Department of Metallurgical and Materials Engineering, IIT Roorkee, for maintaining the experimental facilities. References Caceres, C.H., Davidson, C.J., Griffiths, J.R., Newton, C.L., 2002. Effects of solidification rate and ageing on the microstructure and mechanical properties of AZ91 alloy. Mater. Sci. Eng. A 325, 344–355. 22

Caceres, C. H. and Rovera, D. M. 2001. Solid solution strengthening in concentrated Mg-Al alloys. J. Light Met. 1, 151–156. Cavaliere, P., De Marco, P.P., 2007. Fatigue behaviour of friction stir processed AZ91 magnesium alloy produced by high pressure die casting. Mater. Charact. 58, 226–232. Celotto, S., Bastow, T.J., 2001. Study of precipitation in aged binary Mg-Al and ternary Mg-Al-Zn alloys using 27Al NMR spectroscopy. Acta Mater. 49, 41–51. del Valle, J. A., Rey, P., Gesto, D., Verdera, D., Jiménez, J. A., Ruano, O. A., 2015. Mechanical properties of ultra-fine grained AZ91 magnesium alloy processed by friction stir processing. Mater. Sci. Eng. A 628, 198–206. Feng, A. H., Ma, Z.Y., 2007. Enhanced mechanical properties of Mg–Al–Zn cast alloy via friction stir processing. Scr. Mater. 56, 397–400. Hutchinson, C.R., Nie, J.F., Gorsse, S., 2005. Modeling the precipitation processes and strengthening mechanisms in a Mg-Al-(Zn) AZ91 alloy. Metall. Mater. Trans. A 36, 2093– 2105. Jain. V., Mishra R.S., Gupta A.K., Gouthama, 2013. Study of β-precipitates and their effect on the directional yield asymmetry of friction stir processed and aged AZ91C alloy. Mater. Sci. Eng. A 560, 500–509. Liu, W.C., Dong, J., Zhang, P., Korsunsky, A.M., Song, X., Ding, W.J., 2011. Improvement of fatigue properties by shot peening for Mg-10Gd-3Y alloys under different conditions. Mater. Sci. Eng. A 528, 5935–5944. Ma, Z.Y., Pilchak, A. L., Juhas, M.C., Williams, J.C., 2008. Microstructural refinement and property enhancement of cast light alloys via friction stir processing. Scr. Mater. 58, 361–366. Mansoor, B., Ghosh, A. K., 2012. Microstructure and tensile behavior of a friction stir processed magnesium alloy. Acta Mater. 60, 5079–5088. Mishra, R.S., Ma, Z.Y., 2005. Friction stir welding and processing. Mater. Sci. Eng. R Reports 50, 1–78. 23

Ni, D.R., Wang, D., Feng, A. H., Yao, G., Ma, Z.Y., 2009. Enhancing the high-cycle fatigue strength of Mg–9Al–1Zn casting by friction stir processing. Scr. Mater. 61, 568–571. Oskooie, M.S., Asgharzadeh, H., Kim, H.S., 2015. Microstructure , plastic deformation and strengthening mechanisms of an Al-Mg-Si alloy with a bimodal grain structure. J. Alloys Compd. 632, 540-548. Pancholi, V., Kashyap, B.P., 2007. Effect of layered microstructure on superplastic forming property of AA8090 Al-Li alloy. J. Mater. Process. Technol. 186, 214–220. Perron, A., Politano, O., Vignal, V., 2008. Grain size , stress and surface roughness. Surf. Interface Anal. 40, 518–521. Pradeep, S., Pancholi, V., 2014. Superplastic Forming of Multipass Friction Stir Processed Aluminum-Magnesium Alloy. Metall. Mater. Trans. A 45, 6207–6216. Sato, Y.S., Park, S.H.C., Matsunaga, a., Honda, a., Kokawa, H., 2005. Novel production for highly formable Mg alloy plate. J. Mater. Sci. 40, 637–642. Somekawa, H., Mukai, T., 2005. Effect of grain refinement on fracture toughness in extruded pure magnesium. Scr. Mater. 53, 1059–1064. Somekawa, H., Osawa, Y., Mukai, T., 2006. Effect of solid-solution strengthening on fracture toughness in extruded Mg – Zn alloys. Scr. Mater. 55, 593–596. Somekawa, H., Singh, A., Mukai, T., 2007. Effect of precipitate shapes on fracture toughness in extruded Mg – Zn – Zr magnesium alloys. J. Mater. Res. 22, 965–973. Swaminathan, S., Oh-Ishi, K., Zhilyaev, A.P., Fuller, C.B., London, B., Mahoney, M.W., McNelley, T.R., 2010. Peak stir zone temperatures during friction stir processing. Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 41, 631–640. Wang, Y., Chen, M., Zhou, F., Ma, E., 2002. High tensile ductility in a nanostructured metal. Nature, 419, 912-914. Wang, Y. N., Chang, C. I., Lee, C. J., Lin, H. K., Huang, J. C., 2006. Texture and weak grain size dependence in friction stir processed Mg–Al–Zn alloy. Scr. Mater. 55, 637–640. 24

Willoughby, A.A., 1986. Significance of Pop-In fracture toughness testing. Int. J. Fract. 30, 3–6. Witkin, D., 2003. Al – Mg alloy engineered with bimodal grain size for high strength and increased ductility. Scr. Mater. 49, 297–302. Xia, Y., Yulong, L., Li, L., 2014. Effect of grain refinement on fracture toughness and fracture mechanism in AZ31 magnesium alloy. Procedia Materials Science 3, 1780–1785.

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