Strength enhancement of magnesium alloy through equal channel angular pressing and laser shock peening

Journal Pre-proofs Full Length Article Strength enhancement of magnesium alloy through equal channel angular pressing and laser shock peening T.R. Praveen, H. Shivananda Nayaka, S. Swaroop, K.R. Gopi PII: DOI: Reference:

S0169-4332(20)30511-0 https://doi.org/10.1016/j.apsusc.2020.145755 APSUSC 145755

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

1 November 2019 7 January 2020 11 February 2020

Please cite this article as: T.R. Praveen, H. Shivananda Nayaka, S. Swaroop, K.R. Gopi, Strength enhancement of magnesium alloy through equal channel angular pressing and laser shock peening, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145755

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Strength enhancement of magnesium alloy through equal channel angular pressing and laser shock peening T. R Praveen1,a,*, H. Shivananda Nayaka1,b, S. Swaroop2,c, Gopi K. R3,d 1Department

of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal, India-575025

2Surface

Modification Laboratory, School of Advanced Sciences, Vellore Institute of Technology, Vellore, India-632014

3Department

of Mechanical Engineering, Malnad College of Engineering, Karnataka, Hassan, India573202

[email protected], [email protected], [email protected], [email protected]

*Author

for correspondence

Abstract: AM80 magnesium alloy was processed by Equal Channel Angular Pressing up to 2 passes under route BC and C, to study the effect of change in microstructure. Microstructures were examined under optical microscope. Samples processed by route BC showed heterogeneous grain structure with good tensile strength compared to that processed by route C. Tensile tests of 2-pass equal channel angular pressed samples showed high tensile strength under route BC. Laser shock peening without coating was carried out on route BC sample for further grain refinement on the surface. Laser pulses with power density of 8 GWcm-2, under different percentages of cover, were used to peen the surface at high strain rate. Microstructures were analysed through scanning and transmission electron microscope, and fine grains of less than 100 nm were observed. Tensile tests indicated that the laser shock peened samples had increased strength and ductility. Fracture details from tensile tested specimens, were examined under SEM. Mixed mode of brittle and ductile fractures was observed in ECAP processed samples. Fracture surface of Laser shock peening without coating treated on equal channel angular pressed samples showed small dimples near the peened surface and intensity of dimples increased with increase in percentage of cover. Wear test was carried out on before and after Laser shock peening without coating processed samples on pin-on-disc wear test machine. Increase in friction coefficients and wear rate was observed due to roughness induced by peening effect and it decreased after increase in sliding distance due to increase in hardness. Nano indentation experiments were carried out to examine the mechanical characterization at nano level, and it expose the effect of LSPwC in terms of increase in hardness at peened region. Key words: Magnesium, AM80, ECAP, LSPwC, SPD, Grain Refinement, Wear. 1. Introduction Current trend of optimizing technology is driving towards employing light weight structural materials to increase the vehicle efficiency and material recyclability. Magnesium (Mg) and its alloys have drawn significant interest due to excellent specific strength (strength to density ratio) and recyclability, Mg is the seventh most available element in earth crust and it can be extracted from sea water also [1–4]. Mg and its alloys are adopted in electronic devices, locomotives, automobiles, aerospace, power tools and many more applications to reduce weight [5]. Kulekci [6]reported the

advantages of mg by increasing its volume in automotive industries, which decrease the emissions by optimizing vehicle weight. Mg and its alloys have modulus of elasticity in the range of 10-40 GPa, which is nearly equal to human bone and hence they have attracted interest in orthopaedic applications. Presently, titanium and steel-based implants are used for bio-medical treatments and it can be replaced by bio-degradable Mg implants. It has some major limitation with respect to elastic modulus, cold working, toughness, creep, corrosion and exhibits high directional anisotropy. MgAluminium (Al) alloys are relatively easier to manufacture compared to other mg alloys, and hence they are cheaper and can be cast into thin and long sections. Also the addition of Manganese (Mn) enhances the ductility and increases their corrosion resistance [7]. Hexagonal close packed (HCP) crystal structure resists ductility and formability of Mg at room temperature. Grain refinement enhances ductility and strength and it can achieved by enhancing cooling rate during solidification, adding grain refiner to molten metal and using severe plastic deformation (SPD) techniques. Manufacturing process such as extrusion, drawing also causes refinement of grains during process, but there is change in dimensions from original. Bagherpour et al. [8] gave a broad review of SPD techniques used for grain refinement. Equal channel angular pressing (ECAP) is a simple method of SPD to improve the mechanical properties. Factors which influence ECAP to achieve ultrafine grain refinement are temperature, deformation rate, processing routes, internal angle and outer radius [9]. Ono et al. [10] addressed the behaviour of polycrystalline Mg in a wide range of temperature (77-523 K) and grain size (43-172 µm). Yield strength (σy) decreased with increase in temperature and grain size (d), as predicted by Hall-Petch equation [11], σy = σ0 + kyd-1/2

(1)

where σy is the yield stress of the material composed of frictional stress (σ0) which influenced by temperature and characteristic locking parameter (ky), ky depend on impurities, orientation of slip system, critical resolved shear stress and distance from nearest dislocation source in adjacent grain, and change in grain size (d) directly affects the yield stress. Laser shock peening without coating (LSP) is an effective surface modification technique to introduce compressive residual stress at surface region. LSP enhances fatigue life, wear resistance and corrosion resistance[12,13]. In some cases grain refinement with LSP, resulting in further strengthening of the material was also observed [14,15]. Recently Kalentics et al. [16] executed LSP on 316 L stainless steel, and noticed transformation of fine grains near peened region. Pulsed laser beam is used to generate discontinuous laser shocks in the form of energy packets for a particular interval of time based on required power density. These laser shocks are converted into stress waves and induce plastic deformation on surface and plastic deformation can be controlled by changing various parameters of the process [17]. Recently Wu et al. [18] investigated effect of LSP on pure titanium under different temperatures. Microhardness increased with grain refinement and significant improvement in tensile strength and plasticity was also observed. Effect of LSP depends on peak pressure exerted by shock wave, duration pulse and number of pulses. Moving mechanism possesses, sliding of contact surfaces, which slide against hard materials and leads to wear of the material surface. Tribological performance of sliding system depends on surroundings such as temperature, humidity and sliding media under applied conditions like load, velocity of slide, sliding distance and roughness of surface [19]. Mg is a soft material and requires potential resistance for wear related applications [20,21]. Gopi et al. [22] studied effect of ECAP under route BC on AM80 alloy. ECAP 2-Pass sample showed better hardness and wear resistance

increased at different loads and different sliding distances. Increase in wear resistance is due to change in hardness occurred by grain refinement through ECAP. In the past decade, considerable work was carried on magnesium alloy to increase its mechanical properties thorough grain refinement [23–25]. Most of the work was carried out by bulk grain refining of material, which doesn’t give an enough control to tailor the grains. LSP were responsible only for near surface effect, up to certain depth from peened surface. As LSP induces compressive stresses at peened surface, counterpart of induced (tensile) stresses stabilized inside the material. If the material possesses critical thickness, then the tensile stresses were generated at opposite side of peened surface. Hence thickness of the substrate is an important criteria in LSP [26]. Executing LSP on ECAP processed sample gives an option for further enhancement of strength by refining of grains at near surface. LSP may induce compressive residual stresses at surface, which enhances the tensile and fatigue strength. Mechanical characterization of AM80 upto four pass with route BC has been done in our previous work [27], in which 2-pass ECAP processed AM80 alloy possessed a grain size of 45 µm reduced from 100 µm of as-cast condition with increased tensile strength by 200 %. In continuation of this investigation, due the advantages of LSP outlined above, LSP was done on Mg alloy to increase the surface properties [28,29]. Hence, the scope of this article is to analyze the microstructure and mechanical properties of Mg alloy through ECAP and further processed by LSPwC. ECAP enhances the strength of the base material by refining the coarse grains and LSP further refines the ECAP processed grains at surface. The tailored material will have different material properties at surface and core of the material, such as hardness and elasticity. At surface, there is a resistance to nucleation and propagation of fatigue cracks, and increase in strength of material, which leads to increases in reliability of component [25]. Magnesium alloys are potentially identified as bio degradable material for creating implants, which dissolve in body itself without any harmful effects. But it is highly corrosive; hence it dissolves before the expected life. If the material is tailor made with ECAP and LSP, there is a increases in resistance to corrosion at base material and surface, hence materials starts corrode at different rate from LSP treated surface [30] to ECAP processed core [31], and corrosion can also be controlled as per the requirement. 2. Material and ECAP preparation Cast AM80 alloy was received in the form of circular rod of diameter 15.8 mm and length 96.0 mm from Venuka Engineering, Hyderabad, India. Nominal composition of the alloy is shown in Table 1. These billets were homogenized in a closed furnace for 24 hours under 350 °C. Homogenized samples were subjected to ECAP at 275 °C using the split die, with circular channel angle (ϕ) 110° and outer radius (φ) 20°, as shown in Fig.1. Limitation of slip systems in HCP were compensated by activating twins by heating the material for 275 °C, which leads to deformation of crystal structure along c-axis till the prismatic and pyramidal slip systems get activated [32]. Molybdenum disulphide (MoS2) is used as lubricant and pressing was carried out using 400 kN universal testing machine (UTM) under constant deformation rate of 0.5 mms-1. After one pass, samples were processed under route BC (rotated by 90°) and route C (rotated by 180°) for ECAP 2-pass.ϕ is an important factor to control the effective strain at each pass, materials which have low ductility (Mg) are difficult deform when ϕ is lesser than 90°. Influence of working temperature is a key factor for achieving optimal result, which enhances the necessity slip system to deform homogeneously while pressing. Due to high working temperature, and low deformation rate, there is a possiblity of dynamic recrystallization during ECAP process, which favours the grain growth. Hence possible low temperature was preferred to achieve fine grains.

Table 1. Nominal Composition of AM80 Mg alloy (Wt.%). Elements Al Mn Fe Wt.% 8.23 0.45 0.48

Cu 0.67

Zn 0.21

Mg Balance

Fig.1.Schematic representation of (a) Sectional view of ECAP sample pressed by ram (b) ECAP pass with route BC and route C 2.1. Laser Shock Peening without Coating ECAP samples were further processed by laser shock peening without coating (LSPwC) process. Samples processed after 2 pass ECAP (route BC) alone were used for LSPwC for reasons to be explained in the section 3.2. LSPwC was carried out using Nd:YAG laser operated at a wavelength of 1064 nm, and pulse duration of 10 ns. Dichromic mirror was used to deflect the beam followed by focusing with a convex lens of focal length of 758 mm as showed in Fig. 2(a). Resulting beam had a spot diameter (D) of 0.8 mm. Samples were placed on CNC controlled workstation to achieve required X, Y motion (translation for plane samples) and X, R1 (translation and rotation for ASTM E8 Samples) motion. Continuous water flow was used during the operation to achieve thin transparent layer (1.0 mm) on peening surface [33] as showed in Fig. 2(b). Surface layers which exposed to LSPwC vaporises instantaneously to very high temperature (~ 10,000 K), and followed by formation of high pressure plasma blast. This generates shock waves and travels into substrate, by creating plastic deformation at surface. Water was used as the dielectric media to absorb part of laser energy which causes surface damage [34]. Peening area was overlapped with 70 % by pulses (Δx, Δy, ΔR1 = D*0.3 as shown in Fig. 2(c)) and peening was repeated for 100, 200 and 300 % of coverage at 8 GW/cm2.

Fig. 2 (a) Schematic view of LSPwC setup, (b) CNC worktable with laser spot with transparent layer created by water, and (c) Laser pulse spot of diameter 0.8 mm with overlapping parameter. 2.2. Microstructures and Tensile test Cross-sections of ECAP and ECAP+LSPwC processed samples, normal to pressing direction are extracted for microstructural investigation. Polishing was done with abrasive papers followed by diamond paste (0.25µm), and etched with 5.0 g picric acid, 10 ml of acetic acid, 70 ml of ethanol and 10 ml of distilled water [35]. Microstructures were analysed with optical microscope as per ASTME-112 [36] for ECAP processed samples. SEM and TEM analysis were carried on ECAP and LSPwC processed samples. SEM (SEM-JEOL JSM 6380LA) requires sample preparation similar to optical microscope and same procedure is carried out. TEM samples require specialized preparation technique, as sample preparation was started with cutting the required portion (5 mm) of sample under slow cutting machine, further polishing has done to decrease the material thickness to 100 µm at very slow rate with silicon carbide paper of 1000 to 2000 grade. LSP peened sample possess higher surface roughness at treated region, hence slow polishing has done to achieve the flat surface (approximately 50 µm depth removed). After achieving the thickness of 100 µm and flatness on both sides, samples were punched for diameter 3.0 mm, and further material were removed by creating dimple at centre of specimen using dimpling machine, and effective depth of 5 to 10 µm achieved. As magnesium is highly reactive with water, hence ethanol with diamond paste of 0.25 µm is used during dimpling. Final small perforation was created by ion beam milling (Gatan-PIPS 691) at centre. TEM (TEMJOEL-JEM-2100) was used to extract images and observe ultrafine grains, dislocations and selected area electron diffraction (SAED). Tensile tests were conducted according to ASTM-E8 [37] using Shimadzu AG-X PlusTM universal testing machine.

2.3. Wear Test Wear tests were conducted using pin-on-disc wear test setup and samples were prepared as per ASTMG99-05 (Fig. 3). Samples were machined to diameter 8 mm and length 28 mm. SiC papers upto 2000 grit was used for polishing ECAP processed samples. LSPwC processed samples were tested with the surface (high roughness) finished by peening. Samples were placed against a steel (EN31, HRC 65 Hardness) disc of diameter 120 mm. Wear test was conducted with DU-COM-TR-20LEPHM 400-CHM-600 wear test machine at room temperature with sliding velocity 1 m/s and load of 40 N for 2500 m sliding distance. Samples were analysed with wear rate and friction coefficients. Worn surfaces were investigated with SEM.

Fig. 3. (a) Wear specimen according to ASTMG99-05, and (b) Schematic diagram of pin-on-disc setup. 2.4. Nano indentation Nano indentation experiment was carried out using Agilet G200 to estimate the hardness at surface level. As the specimen was processed with ECAP and LSPwC, it is important to examine the properties along the depth. TB15269 diamond tip is used for the indentation with penetration rate of 45 Hz with step size of 2 nm. Fig. 4 shows the location of experiment performed, and table 1 show the data points selected for investigation.

Fig. 4 (a) locations of Nano indentation carried out on ECAP and ECAP+LSP processed sample, and (b) zoomed view of data points

Table 1: Data points for measurement of hardness Sl. No.

1

2

3

4

5

6

7

8

9

10

11

12

Data points

a

a-b

b-c

c-d

d-e

e-f

f-g

g-h

h-i

i-j

j-k

k-l

Distance (µm)

50

50

100

100

100

100

100

100

100

100

100

100

3. Results and Discussion 3.1. Microstructural Analysis

Fig. 5. Microstructures of (a) As-cast, (b) homogenized (c) ECAP1-Pass, (d) ECAP 2-Pass with route BC and (e) ECAP 2-Pass with route C. Microstructures of as-received, homogenized, ECAP 1-Pass, ECAP 2-Pass using route BC, and ECAP 2-Passusing route C is shown in Fig. 5. Solidification of molten metal initiated with nucleation of Mg between 600-650°C and formation of eutectic phases at 437°C with Mg-Mg17Al12. Fig. 5(a) shows microstructure of AM80 at as received condition. α-Mg dendrites having secondary arms (A) with six fold symmetry. Eutectic of Mg17Al12 (B) and aluminium rich solid solution (C) is distributed in between secondary arms were observed [7]. After homogenized for 24 hours at 350 °C, change in microstructure was observed due to static recrystallization from dendritic to equiaxed grains, having

average grain size of 100 µm (Fig. 5(b)) [38]. Homogenized samples were inserted in ECAP die and pressed using UTM at 275°C, at deformation rate of 0.5 mms-1 and this operating condition is same for all ECAP passes. Microstructure was extracted normal to extrusion direction of 1-Pass sample. Coarse grains are divided into smaller grains near grain boundaries of large grains with an average grain size 60 µm as shown in Fig. 5(c). ECAP 1-Pass sample is re-inserted into ECAP die for 2-pass (using route BC) by rotating sample by 90° and pressed against the die. Microstructures were extracted and further grain refinement is achieved compared to ECAP 1-Pass condition. Average grain size observed is 38 µm and still heterogeneous grain structure is visible (Fig. 5(d)) and this leads to the formation of bimodal grain structure [39]. Route Bc leads to inhomogeneous uniform strain, along the longitudinal direction, and it called as circumferential twisting [40]. ECAP 1-pass sample was again subjected to ECAP using route C by rotating the sample by180° and the obtained microstructures are shown in Fig. 5(e). Grains are refined with average grain size of 45 µm and distributed well across the cross-section. Route C is more homogeneous compared to route BC with finer grain size due to shearing action along ϕ, and there is a reverse of shear deformation at every pass after first pass, but the texture doesn’t reverse. Which results in reproduction shear texture by strain reversal [41]. 3.2. Tensile Strength and Fracture morphology

Fig. 6 Tensile characteristic of AM80 at different stages of processing. Tensile tests was conducted according to ASTM-E8 [37] and the obtained stress-strain curve is shown in Fig. 6 for as-cast, homogenized, ECAP 1-Pass, ECAP 2-Pass with route BC and ECAP 2-Pass with route C samples. Ultimate tensile strength (UTS) was slightly increased from as-cast to homogenized with increase in ductility from 6 to 8%. It is due to dissolved intermetallic particles, increase in workability and reduced inter-granular cracking due to static recrystallization [42]. UTS increased by 70% for ECAP 1-Pass sample, 115% for ECAP 2-Pass sample of route BC and 82% for ECAP 2-Pass sample of route C compared to homogenized sample. Percentage elongation also increased from 6 to 26% from homogenized to ECAP 1-Pass sample. UTS increased with increase in number of ECAP passes due to grain refinement and also depends on crystallographic texture and Schmid factor[27]. Magnesium having HCP system possess limited slip system for plastic deformation, hence it revealed a brittle fracture and cleavage as the principle fracture mode[43,44]. Fig. 7(a) shows the fracture

surface of as-cast sample. There are some undesired features as secondary cracks and micro-voids, that are exposed and small cracks are converged to main crack and forms tearing ridges. River flow pattern is observed on the surface which confirms trans-granular fracture indicating poor plastic deformation. Fracture is characterized by cleavage facets, steps and rivers. Dimples are also visible in some regions, the combination of cleavage and dimples are observed along with undesired cracks. Fracture is a combination of brittle and ductile fracture and more dominated by brittle nature [35]. Fig. 7(b) shows fracture surface of ECAP processed 2-Pass sample under route BC and fracture surface exposed combination of ductile and brittle fracture[45]. Cleavage steps were also found followed by small trans-granular cracks and direction of crack growth is decided based on coalescence of small cracks to main crack. As shown in microstructure(Fig.5(d)), 2-pass sample under route BC showed bimodal structure, where small grains are surrounded by large grains and these leads to heterogeneous fracture surface having different sizes of dimples. Small dimple causes more elongation compared to large dimples. This fracture surface consists of cleavage facets, steps, small dimples and large dimples with some secondary cracks and influences combination of ductile and brittle nature (Fig. 6). Fig. 7(c) shows the fracture surface of ECAP processed 2-Pass sample under route C condition with larger grain size and homogeneous compared to route BC condition (Fig. 5(e)). Fracture surface contains cleavage facets, steps, different sizes of dimples and no significant secondary cracks were present due to processing by route C (180° rotation of sample). Fracture behaviour is similar to route BC, having combination of ductile and brittle fracture with increase in percentage of elongation. This kind of fracture observed by Li et al [46], where alternate extrusion was executed on AZ31. Ductile fracture had higher fracture strength compared to brittle fracture. Based on the foregoing discussion it is clear that the 2 pass ECAP samples processed by route BC have desirable tensile behaviour and hence they were used for further processing with LSPwC.

Fig. 7. SEM Image of fracture surface for (a) as-cast, (b) 2-Pass Route BC, (c) 2-Pass Route C and (d) near surface zoomed view of Route BC.

3.3. Microstructure analysis of ECAP+LSPwC samples

Fig. 8. SEM Image of (a) ECAP 2-pass at route BC, (b) ECAP 2P+LSPwC 100 % of coverage, (c) ECAP 2P+LSPwC 200 % of coverage, and (d) ECAP 2P+LSPwC 300 % of coverage. Fig. 8(a) shows the SEM image with EDAX data of 2-pass ECAP processed sample, due to shearing action at intersection of ECAP channels, grains were ruptured due to severe plastic deformation. Deformation is assisted by multiple mechanisms operated together, which includes different slip systems, such as basal, prismatic and twinning. Temperature plays key role while formations of grain refinement during ECAP, some of the (pyramidal) slip system get assisted by temperature in Mg, but relapse from smaller grains to bigger grain occurred due to dynamic recrystallization. SEM image shows rupture of grains in-homogeneously across the sample. EDX data shows the chemical composition data of material at surface, which involves majorly Mg, alloying elements Al, and Mn and some impurities as Fe, Zn and Cu. Fig. 8(b) show microstructure of LSPwC at 8 GW/cm2 with 100 % coverage carried out on 2-Pass ECAP processed samples. Images were extracted perpendicular to peened surface for investigation of grain refinement. Next level grain refinement was clearly visible near the peened region, and grains having size of 5 to 2 µm were observed through SEM. Further increase of peening effect by repeating for 200 and 300 % of coverage, grains were ruptured more and more due to severe plastic deformation at peened surface as shown in Error! Reference source not found.. LSPwC with 200 % of coverage forms a microstructure in the form of cluster of flower petals by Mg17Al12, and 300 % of coverage reveals the more damage on grains in sub microns level. Investigation of microstructure with SEM, conforms the grain refinement took at different processing stage. At this stage, bulk refinement happened at ECAP processing, and grain size was inversely proportional to number of passes [47]. Due to LSP, depends and percentage of coverage, plastic deformation occurs at high strain rate at peening surface, and effective grain refinement was observed at surface level.

ECAP induces permanent deformation in material due to shearing action, where arrangement of atoms change their surroundings based on degree of strain induced during deformation. Fig. 9 (a) shows the TEM image of ECAP 2-pass processed AM80 with SAED pattern, grains were refined and dislocation lines were observed. Full grains were not visible here due large grains of average size of 45 µm (Fig. 5), and the formations of precipitated phases were observed. Plastic deformation were occurred due to movement of dislocations and caused by slip and twinning in a crystal [48]. Formation of twins and twin boundaries slowdowns the nucleation of dislocations and formation dislocation pile-up, this influences the increase in strength and ductility of crystal [49]. Fig. 9 (b) shows the TEM images of AM80 alloy processed by 2-Pass ECAP, and LSP by 8 Gwcm-2 by 100 % of coverage (1 time). Sample was extracted from approximate 50 µm from the surface due to surface roughness raised during peening. Banded structure was observed due to LSP at surface level, which is formed due to plastic deformation, Fig. 8 (b-1) shows high dislocation density with shear bands induced by LSP [50]. Precipitates were clearly visible and more compared to ECAP processed sample, and dislocation cells were dominated. Fig. 8 (b-2) shows the nano-grains of average size of 70 nm formed during LSP. LSP were repeated once again on LSP processed sample with ECAP to achieve 200 % of coverage. This causes repeated plastic deformation at surface due to repetitive peening, which leads to more distorted surface damage. Fig. 9 (c) shows the TEM image of ECAP 2-pass processed under route Bc and LSP processed with 8 Gwcm-2 by 200 % of coverage, with SAED pattern, There was an intersection of bands in Fig. 9 (c-1) were observed due to change in peening location during repetitive peening, and laser pulses may overlapped at different percentage (not controlled) during second time. As mentioned more deformation at surface, more fine grains compared to 100 % coverage of LSP. The average grain size observed with 200 % of coverage is 20 nm (Fig. 9 (c-2)).

Fig. 9. TEM image of (a) ECAP 2-pass processed under route Bc with SAED pattern, (b) ECAP 2P+LSPwC 100 % of coverage, (c) ECAP 2P+LSPwC 200 % of coverage and (d) ECAP 2P+LSPwC 300 % of coverage As the LSP process repeated one more (third) time with same energy density (8 Gwcm-2) on 200 % coverage sample, 300 % coverage was achieved. Fig. 9 (d) shows the TEM image of ECAP 2-pass processed under route Bc and LSP processed with 8 Gwcm-2 by 300 % of coverage, with SAED

pattern. Repetitive and over lapped banded structures were visible in Fig. 9 (d-1), needle like, elongated grains were observed. Highly distorted grains were observed in Fig. 9 (d-2) (b). SAED pattern reveals the formation of sub grains in the order of nano meters, due to repetitive severe plastic deformation occurred during LSP at surface level. 3.4. Tensile Strength and Fracture morphology of ECAP+LSPwC

Fig. 10. Tensile stress-strain behaviour of (a) ECAP processed AM80 with LSPwC at different percentage of coverage and (b) Enlarged view of tensile behaviour for ultimate tensile strength. Fig. 10 shows stress-strain behaviour of LSPwC processed AM80 samples, processed with ECAP 2Pass under route BC. LSPwC energy density kept constant as 8 GW/Cm2, and repeated multiple times based on percentage of cover. which enhances the tensile strength by small percentage [18] and there is considerable increase in tensile strength and ductility from ECAP to ECAP+LSPwC processed samples with respect to percentage of coverage. Improvements in tensile strength due to further grain refinement occurred by plastic deformation caused due to higher strain rate, which evolves twins and dislocations [43,44]. Grain refinement enhances the grain boundaries, facilitates to dislocation accumulation capacity, leads to strengthening of material during tensile straining [51]. The difference in slope within elastic limit represents the change in stiffness. LSPwC processed samples possesses low stiffness compared to ECAP processed sample due to change in hardness at peened surface [52].

Fig. 11. SEM Image of fracture surface for LSPwC with 100 % coverage of 2-Pass ECAP processed route Bc samples. Fig. 11 shows the SEM images of fractured surface of ECAP processed 2-Pass sample, subjected to SMAT with LSPwC of energy density 8 GW/Cm2.This surface treatment creates dents due to plastic

deformation produced by shock waves [53]. Fig. 7(d) shows the fracture surface near the machining surface without LSP processing and nominal surface roughness was observed before LSPwC. Fracture on regions other than LSP region is similar to ECAP processed 2-Pass sample under route BC (Fig . 7(a)). Change in hardness near peened surface, delays crack initiation and fracture occur due to overloading in tensile mode. Slight increase in tensile strength was observed in Fig. 10(b). SEM image (Fig.11) shows the existence of small micro-crack, cleavage facets by crossing the grain boundaries, small dimples were created on cleavage facets due to peening. This evidence provides change in secondary phases due to LSPwC processing. Small dimples indicate the ductile behaviour and enhancement of ductility [54]. Fig. 12 shows the fracture surface for LSPwC processed sample with 200% coverage on ECAP processed 2-Pass sample under route BC with clear indication of formation of small dimples. Ductile fracture propagates with load and it can withstand limited load near to LSPwC processed surface. Further increase in loads evolves cleavage steps by coalescence of dimples, as observed in Fig. 12(a). Once the crack propagate to inside, cleavage will dominate (as the effect of LSPwC ceases here) the fracture mode and leads to catastrophic failure as mixed mode of ductile and brittle fracture. As percentage of dimples increase, more elongation is observed (Fig. 12(b)) in stress-strain graph on LSP processed with 200% coverage sample.

Fig. 12. SEM Image of fracture surface for LSPwC processedwith 200% coverage of ECAP 2-Pass processed route BC samples.

Fig. 13. SEM Image of fracture surface for LSPwC processed with 300 % coverage of 2-Pass ECAP processed route BC samples. Fracture surface of LSPwC processed sample for 300% cover on ECAP processed 2- Pass sample under route BC is shown in Fig. 13. Separation of peened layer is observed, as a result of peening and

further more plastic deformation and some over-peened regions are also observed in Fig.13 (a). Many small sized dimples were observed near the peening surface and voids are nucleated at inclusions which are grown by higher loads. Coalescence of these voids induces cracks and leads to separation of material [54]. Percentage of dimples near the LSPwC processed surface is high when compared with 100 and 200% samples and hence more ductility is shown in Fig. 10 (b). 3.5. Wear property Fig. 14(a) shows the relation between friction coefficients and sliding distance at a load of 40 N. ECAP processed 2-pass sample shows no change in coefficient of friction with sliding distance. Grain refinement took throughout the sample and hardness increased all over the sample and it possess same high hardness compared to other ECAP passes [22]. Laser shock peening creates plastic deformation at surface in the form of dents occurred at high strain rate (SPD) and causes further grain refinement at surface [29,55]. This leads to increase in hardness near the surface [56]. Coefficient of friction of LSPwC processed samples were slightly higher than ECAP processed sample at surface and starts decreasing with increase in sliding distance due to worn-out of material. LSPwC was done at different percentage of coverage and 300% coverage sample showed better resistance to wear. Fig. 14(b) shows the behaviour of wear rate with respect to sliding distance. ECAP sample showed better resistance to wear rate at early stage compared to LSPwC processed sample. Wear rate of LSPwC sample increases with percentage of cover at early stage due to increase in surface roughness (loss of contact surface) and starts decreases with increase in percentage of cover. Samples with 300% coverage possess high surface roughness due to repeated peening and deep dents cause non uniform contact against steel disc. Wear rate start decreasing as surface become uniform as material starts to wear. Once wear surface turn to uniform surface, wear rate became minimal compared to other samples due to high hardness.

Fig. 14. (a) Friction coefficients and (b) wear rate of ECAP processed 2-Pass sample under route BC additionally surface treated with LSPwC AM80 alloy.

Fig. 15. SEM images of worn surfaces of (a)ECAP processed 2-Pass sample under route BC(b) ECAP processed with LSPwC 100% coverage (c) 200% coverage (d) 300% coverage and (e) EDS image of worn surface with elemental mapping. SEM images of worn surfaces of 2-Pass ECAP processed sample and LSPwC processed samples with ECAP at different percentage of cover are shown in Fig. 15. Worn surfaces consists of groove in the direction of sliding, caused by plastic deformation occurred by abrasive wear mechanism. Surface roughness increases with abrasive grooves and causes increase of friction coefficients [57] and width of abrasive groove marks depends on size of debris. Delamination occurs due to detachment of material caused by cracks perpendicular to sliding direction. Fig. 15(a) shows worn surface of ECAP sample having abrasive wear with delamination, deep plough marks indicates increase in surface roughness and softness of material compared to LSPwC processed samples. Fig. 15(b, c & d) shows worn surfaces of ECAP+LSPwC processed sample with 100, 200 and 300% of coverage. The surface became smoother after wear, compared to ECAP processed sample due to increase in hardness by LSPwC. Fig. 15(e) shows the elemental mapping of energy dispersive X-Ray spectrometer near delamination region and presence of oxygen was found on worn surface. Oxidation will be one of the dominating wear mechanism in Mg based alloys and causes easy detachment of oxidised fragments [58]. These fragments are responsible for abrasive wear and also found in all samples. 3.6. Nano indentation

Fig. 16. shows the data extracted from nano indentation experiment for ECAP 2-pass processed AM80 sample. (a) Load V/s displacement curve, (b) Hardness along the depth (c) modulus along the depth, and (d) nano indenter marks after executing the tests on 2-pass ECAP processed sample Nano indentation experiments were carried out approximately till the depth of 2000 nm, at strain rate of 0.05 s-1, tip TB15269. Fig. 16 shows the data extracted from Nano indenter. The penetration of indenter tip associated with trapezoidal load function, enforces the long duration of maximum load and fast unloading, which assist to minimize the viscoelastic effects, and enhances the precision of experiments. Fig. 16 (a) shows the load v/s displacement of nano indenter along the depth. 2000 nm depth were kept as the limit for the penetration. Experiments were conducted using high precision load cell with capacitor based transducer for measurement of displacement. At each second indenter were penetrated 45 times in the step size of 2 nm. Fig. 16 (b & c)) shows the hardness and modulus profile along the depth, and experiments were repeated for five trials at each case to avoid surface uncertainty. Fig. 16 (d) show the indentation mark produced by tip during indentation.

Fig. 17. (a) Load V/S displacement plot at different processing stage extracted at surface and (b) Hardness measurement of AM80 sample at different processing stage along the depth Typical load-displacement curve of indentation was shown in fig. 17 (a), which contains the data measured at surface level of peening, as-cast and ECAP processed samples. The loading curve of ascast sample showed least slope compared to other samples. ECAP 2-pass processed sample showed capable of further load bearing capacity and increase in slope proves the increase in stiffness compared to as-cast sample. ECAP+LSP 100 % coverage sample shows the enhancement of strength and ductility through change in slope of loading curve and capable of receiving more displacement compared to other samples. ECAP+LSP 200 and 300 % coverage sample shows the increase of strength and loss of ductility, which were observed lower displacement into surface. The area under the loading and unloading curve represents the energy dissipated during plastic deformation (Wp) and the area under the unloading curve (hatched region) gives the energy recovered due to relapsing of elastic deformation. The total work done was occurred in two parts, as mentioned below [59] Wtotal = Welastic+Wplastic (1) Hardness is a resistance faced for plastic deformation caused by standard indenter. Fig. 17 (b) show the plot of hardness measured against the depth of peening at different processing stage. Berkovich diamond indenter where used for extracting data and hardness (H) as H = Pmax/A

(2)

Where Pmax is the maximum load on indenter, and A is the area deformed under load Pmax. As cast sample showed 0.92 Gpa of hardness and it measured at one surface. Further processed ECAP sample showed hardness of 1.36 Gpa and little variation was observed, and minimum was observed at 1.28 Gpa. Samples were further processed with LSP for 100, 200 and 300 % of coverage, and significantly hardness also increased with percentage of coverage. As the LSP is surface treatment process, hardness value also decreased along the depth, which shows the effect of LSP on AM80 alloy. Here the samples were targeted to decrease the grain size through two different plastic deformation methods, which implies in increase in grain boundaries. As cast sample showed average grain size of 100 µm, which further reduced into average grain size of 45 µm. LSP on ECAP processed sample showed further grain refinement at peened surface shows elongated banded type of grains (TEM Images) with a average width of 100 nm. The effect of grain sizes on indentations was shown in fig. 18Error! Reference source not found..

Fig. 18. Effect of Nano indentation on grain refinement The ratio between average grain size versus dimension of indenter tip decreases with increase in grain refinement. When the load penetrates into sample, it starts deforming in its elastic limit, then further transfers into plastic limit. Before plastic deformation, plastic zone evolves near the tip region, and it

starts growing further away from tip as the load increases [60]. The plastic zone evolves in less number of grains or a single grain in large grain material, where number of grains increased in plastic zone in small grain material. As the number of grains increases in plastic zone, the percentage of grain boundaries also increases (Mg12Al17), which causes more resistance for plastic deformation. Hence penetration to the material decreases with indentation for small grain materials and hardness increases. Mechanism of strengthening: there was a clear distinguish of microstructures with respect each pass of ECAP, which ensure the refinement of grains due to shearing action during ECAP. Metals under medium to high strain, introduces significant changes in microstructures and texture. There was formation of new grain boundaries, such as geometrically necessary boundaries, which formed by separation of crystallites from selected slip system and/or strain amplitude. Identical dislocation boundaries are developed by trapping the movement of dislocations at shear plane. The dislocations boundaries composed with large strain have wide range of misorientation [61]. This alters the schmid factor of the crystal and influences the critical resolved shear stress of the slip systems. Hence strength got affected and found increases in present case. LSPwC is another method of SPD, which affects only till certain depth, but there is also formation of new grain boundaries on ECAP processed surface (peened) region. TEM images of LSPwC+ECAP shows the banded structure caused by large strain, and processed sample is made up of different kind grain sizes tailored by ECAP and LSPwC shown in figure 19, and obtained different properties based on grain and grain boundary characteristics. This combined effect of ECAP+LSPwC enhances the hardness of material due to change in grain size and orientation, and it reflects on tensile and wear cauterization of material.

Fig. 19 schematic representation of (a) unprocessed grains and (b) ECAP+LSPwC processed grains 4. Conclusions AM80 Mg alloy was subjected to ECAP under routes BC and C upto 2 passes, followed by LSPwC. Following are conclusions made during the study:  

Dendritic structure was observed in as-cast material with characteristic 6-fold symmetry and equiaxied poly-crystals were observed after homogenization at 350°C for 24 hours. Grain refinement is observed in the samples processed by ECAP and the grain size reduced from 100 to 60 µm during 1st pass. Bimodal grain structure was observed after 2nd pass with route BC. Homogeneity is observed in route C compared to route BC under 2nd pass. Highest tensile strength was observed with route BC.

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Fracture surface of ECAP processed samples having cleavage and dimples, which signifies mixed mode of fracture. Percentage of ductility depends on dimples present in fracture surface caused by grain refinement. Further refinement of grains were observed near peened surface after treatment of LSPwC, SEM images revels the formations of sub-grains, and grains similar to cluster of flower petals were observed in ECAP+LSPwC 200 % of coverage. Banded structures were observed at near surface of peened region, nano grains in the order of 50 nm is observed after LSPwC treatment. Increase in strength and ductility were observed with LSPwC processed sample on route BCsample, due to further grain refinement. Fracture surface of LSPwC processed samples retrieve details of small dimples present near to peened region and these dimples slightly improves load carrying capacity of ECAP processed samples. Material separation is observed at 300% covered samples, which indicates the occurrence of over peening. Further peening may decrease the strength due to reduction cross section area or induces crack on surface. ECAP+LSPwC processed samples showed better wear resistance compared with ECAP processed sample and wear resistance improved with multiple peening. Worn surface showed delamination of oxidised fragments, which causes abrasive marks on wear surface. LSPwC enhances surface hardness at peened region, as the percentage of coverage of peening increases, increase in hardness also noticed. Hence the energy absorbing capacity of the material during failure also increases.

This investigation reveals a detailed study of LSPwC on ECAP processed AM80 alloy with increase in tensile strength and ductility were observed due to SPD occurred on surface.

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Credit author statement: Praveen T. R.: Conceptualization, Methodology and Experiment, writing,H. Shivanada nayaka: reviewing and Experimental support, Satya Swaroop: Reviewing, Editing, Methodology and Experimental support, Gopi K. R.: Experimental support

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Praveen T R Corresponding author

Highlights: Tailoring of grains under ECAP and LSPwC two different severe plastic deformation techniques. Strengthening of low strength magnesium alloy by grain refinement at different processing level. Refinement of grains achieved from 100 µm to 50 nm.