A Novel Turning-Induced-Deformation Based Technique to Process Magnesium Alloys

metals

Article A Novel Turning-Induced-Deformation Based Technique to Process Magnesium Alloys

Sravya Tekumalla 1, Manasa Ajjarapu 2 and Manoj Gupta 1,* 1 Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore 2 Department of Mechanical Engineering, Sastra Deemed University, Tamilnadu 613401, India * Correspondence: [email protected]; Tel.: +65-6516-6358

Received: 9 July 2019; Accepted: 26 July 2019; Published: 29 July 2019

Abstract: A magnesium alloy was fabricated through the consolidation of chips accumulated during the turning process, followed by cold compaction and hot extrusion. A variation in the depths of cut was done during turning to understand the effect of deformation imparted during primary processing on the mechanical properties of an AZ91 alloy (Mg–9 wt.% Al–1 wt.% Zn–0.3 wt.% Mn). The results revealed a significant improvement in compressive strengths (up to 75%) with increased depth of cut, without compromising ductility through the development of fine-grained structures and prior plastic strain induction. This approach resulted in superior materials vis-a-vis conventional deformation techniques and promotes cost and energy efficiency through recycling industrial metal swarf, which is a significant environmental and economic concern.

Keywords: magnesium; turning; deformation; processing; strength

1. Introduction For magnesium to replace the relatively heavier aluminum, titanium, and steels in structural applications, there are still several challenges that are persistent such as the inferior strength, ductility, corrosion resistance, etc. of magnesium [1]. Among the magnesium alloys, the most widely used commercial alloy AZ91 exhibits an excellent combination of mechanical properties, corrosion resistance, and cast ability [2]. However, improved strength is still needed to extend its applications. There have been several attempts made previously to improve its mechanical properties through altering the processing or modifying the alloy chemistry. The effectiveness of severe plastic deformation (SPD) techniques such as equal channel angular pressing (ECAP) to improve the mechanical properties of AZ91 alloy has been widely studied [3–5]. While this is a hot deformation technique, it was demonstrated that cold severe plastic deformation processes such as HPT (high pressure torsion) can render the AZ91 alloy superplastic [6]. Apart from these SPD techniques, effects of torsion deformation and ageing have also been studied [7] on age hardenable alloys such as the AZ91 alloy. It has been reported that this method was effective in reducing the yield asymmetry of the alloy, and deformation by torsion enhanced the effect of age-hardening that led to continuous precipitation, hence improving the mechanical properties of the alloy. Thus, significant enhancement of the properties can be achieved based on the extent of deformation introduced in the material. A majority of these deformation techniques are wrought techniques where the material undergoes secondary processing such as extrusion, rolling, or forging. In this work, the concept of deformation was used as an inspiration and applied as an extension to the primary processing to synthesize materials with better mechanical properties. In this regard, a novel approach of producing deformed material from chips/turnings obtained during the turning (machining) process followed by the conventional powder metallurgy of the turnings is proposed, as given in Figure1. Turning of a metal involves the removal of material in the form of chips/turnings through localized deformation as stated in [8]. This approach of inducing

Metals 2019, 9, 841; doi:10.3390/met9080841 www.mdpi.com/journal/metals Metals 2019, 9, x FOR PEER REVIEW 2 of 9 Metals 2019, 9, 841 2 of 9

stated in [8]. This approach of inducing deformation is deemed to be an efficient form as it uses chips deformationthat are usually is deemed scrap formed to be an during efficient the form removal as it of uses material chips thatto obtain are usually the desired scrap geometry. formed during In the theUS, removal machining of materialand other to related obtain operations the desired accounts geometry. for an In expenditure the US, machining higher than and hundred other related billion operationsdollars ($100 accounts billion) for annually an expenditure [9]. Most higher machine than hundredtools (>80%) billion typically dollars used ($100 in billion) the manufacturing annually [9]. Mostindustry machine are metal tools (cutting>80%) typicallyin nature, used leading in the to manufacturinglarge volumes of industry metal swarf. are metal Recycling cutting these in nature, metal leadingwastes tocould large enhance volumes economic of metal swarf.profit and Recycling reduce these the environmental metal wastes could impact enhance of manufacturing economic profit (ore andmining reduce and the metal environmental refining to meet impact the of metal manufacturing stocks of global (ore mining demand and annu metalally) refining [10]. Furthermore, to meet the metaldue to stocks the excellent of global machinability demand annually) of most [magnes10]. Furthermore,ium based materials, due to the the excellent machining machinability of magnesium of mostcan be magnesium done via dry based cutting materials, where the the machining need for cutting of magnesium fluids or can lubricants be done is via eliminated dry cutting before where the therecycling need for operation, cutting fluids thus ormaking lubricants it an iseconomical eliminated process before the[11]. recycling Thus, this operation, work proposed thus making a novel, it anefficient, economical and processeconomic [11 approach]. Thus, this of workimproving proposed the mechanical a novel, effi cient,properties and economicof a magnesium approach alloy of improvingthrough a theturning-induced-deformation mechanical properties of a technique. magnesium Th alloye as-cast through ingots a turning-induced-deformation used in this work for turning technique.and the generation The as-cast of ingots chips used as several in this workdie cast for turningmagnesium and thealloys generation were machined of chips asto several obtain diethe castrequired magnesium dimensions alloys and were surface machined finish, to and obtain the themagnesium required swarf dimensions was generated and surface in this finish, process and [12]. the magnesiumAlthough previous swarf was works generated have been in this undertaken process [12 on]. Although the consolidation previous of works chips have (solid been state undertaken recycling) onof themagnesium, consolidation aluminum, of chips and (solid copper state [13,14], recycling) there of has magnesium, been no systematic aluminum, study, and copperto the authors’ [13,14], therebest hasknowledge, been no systematicon the properties study, to theof magnesium authors’ best alloy-based knowledge, chips, on the especially properties ofin magnesiumlight of the alloy-baseddeformation chips, imparted especially to inthe light material of the deformationbased on the imparted variation to the of material the machining based onthe parameters. variation ofFurthermore, the machining this parameters. work aimed Furthermore, to give direction this work to the aimed solid-state to give recycling direction toof thechips solid-state generated recycling not just ofthrough chips generated turning, but not through just through all machining turning, but processes through in all general. machining processes in general.

Figure 1. Mechanism of this approach vs. ECAP (equal channel angular pressing). Figure 1. Mechanism of this approach vs. ECAP (equal channel angular pressing). 2. Materials and Methods 2. Materials and Methods 2.1. Processing 2.1. Processing In this study, AZ91 alloys, supplied by Tokyo Magnesium Co. Ltd. (Yokohama, Japan), were used asIn thethis basestudy, materials. AZ91 alloys, The AZ91supplied ingots by wereTokyo subjected Magnesium to a Co. turning Ltd. operation(Yokohama, performed Japan), were on aused lathe as machine the base at materials. 395 rpm andThe aAZ91 feed ingots rate of were 62 mm subjected/min. In to this a turning operation, operation a piece performed of material on in a thelathe form machine of chips at/ turnings395 rpm and was a removed feed rate from of 62 the mm/min. surfaces In of this the operation, work-piece a /pieceingot of using material a carbide in the insertform (TNMGof chips/turnings 160404-HQ; was Grade removed CA515, from Hare the surfaces & Forbes of machinerythe work-piece/ingot house, Sydney, using Australia)a carbide insert in a turning(TNMG tool 160404-HQ; holder (Model Grade No. CA515, WTJNR-2020-K16, Hare & Forbes Hare machinery & Forbes house, machinery Sydney, house, Australia) Sydney, in Australia) a turning withtool aholder tool height (Model of No. 20 mm. WTJNR-2 The parameter020-K16, Hare that varied & Forbes in the machinery turning operationhouse, Sydney, was the Australia) depth of with cut (DOC)a tool height where of the 20 ingots mm. The were parameter given three that di variedﬀerent in DOCs: the turning 0.5 mm operation DOC, 1.0was mm the DOC,depth andof cut 1.5 (DOC) mm DOC,where while the ingots all of were the other given parameters three different were DO keptCs: constant. 0.5 mm DOC, Compaction 1.0 mm of DOC, the collectedand 1.5 mm turnings DOC, waswhile done all of uniaxial the other at roomparameters temperature were kept in a consta hydraulicnt. Compaction press for a durationof the collected of 1 min turnings at a pressure was done of uniaxial at room temperature in a hydraulic press for a duration of 1 min at a pressure of 1000 psi.

Metals 2019, 9, 841 3 of 9

1000 psi. The resultant compacted billets were then retrieved and soaked at 250 ◦C for 1 h before undergoing secondary thermo-mechanical processing. A 150-ton hydraulic press was used for the thermo-mechanical processing (hot extrusion) at a die temperature of 250 ◦C, resulting in rods of 8 mm in diameter. During extrusion, colloidal graphite aided as a lubricating agent. These rods, obtained from billets of 0.5 mm DOC turnings, 1 mm DOC turnings, and 1.5 mm DOC turnings were nomenclated as AZ91_0.5DOC, AZ91_1DOC, and AZ91_1.5DOC, respectively, in this study. Furthermore, as a benchmark, the AZ91 ingot was also extruded directly, without turning, at the same parameters and henceforth termed as AZ91_AR (where AR stands for As Received) and served as the comparative base material/benchmark.

2.2. Density and Porosity Measurements The density of the samples was measured using the Archimedes’ principle. The weights of the samples were measured using an ER-182A electronic balance (A&D Engineering, Thebarton, Australia) with an uncertainty of 0.0001 g. The densities were also measured using a gas pycnometer where an ± inert gas (helium) was used as the displacement medium and the values were cross checked. Porosity of the samples was measured using the theoretical and experimental densities under the assumption that the discrepancies between the theoretical and experimental densities were due to the porosity of the material.

2.3. Microstructure and X-Ray Diﬀraction Grain characteristics were computed using the line intercept method following ASTM E112–13 standards on images obtained from the scanning electron microscope (SEM). Microstructural characterization was performed through SEM (JEOL-JSM-6010, Jeol USA Inc., Peabody, MA, USA) with an attached EDS (energy dispersive spectrometry) analysis. Furthermore, secondary phase analysis was done on an automated Shimadzu-LAB XRD series 6000 using a Cu Kα radiation of wavelength 1.54 A◦ at a scan speed of 2◦/min.

2.4. Mechanical Characterization Micro-hardness was measured with the aid of a digital micro-hardness tester (Shimadzu (Asia Paciﬁc) Pte Ltd, Singapore, Singapore) in accordance with ASTM E384-11e1 (245.5 mN of load; 15 s of dwell time). Compressive loading using a servo-hydraulic fully automated mechanical testing machine (Eden Prairie, MN, USA), MTS 810, was done uniaxial following the ASTM E9-09 standard at a quasi-static strain rate of 1.6 10 4 s 1. × − − 3. Results and Discussion

3.1. Plastic Deformation Induced during Turning When the as-received ingots were subjected to turning, the work-piece/metal underwent a material removal process by a sharp cutting tool. This cutting action involved two main steps: (i) formation of a chip, which happens through a localized shear process in a narrow region where the metal is compressed; and then (ii) the metal is made to flow on the face of the tool. Chip formation involves the work material to be deformed by shear, and as chips are removed, new surfaces are exposed to the tool. Figure2a gives a realistic view of chip formation, showing two distinct shear deformation zones i.e., a high shear strain zone at the tool-work-piece interaction, resulting in high plastic deformation and a low shear strain zone due to the tool-chip friction resulting in a low plastic deformation of the chip. Figure2b validates this by demonstrating the presence of shear bands on the chips at the two zones marked as low and high plastic deformation zones. The image was taken from a section of the chips randomly. It may be noted that the materials with increased depths of cut appeared to have more discontinuous chips. With an increment in the depths of cut from 0.5 mm to 1.5 mm progressively, the material removal rate (which is directly proportional to the depth of cut) was increased, implying the Metals 2019, 9, 841 4 of 9 increaseMetals 2019 in, the 9, x extentFOR PEER of REVIEW deformation in the materials. Thus, it can be noted that materials with 1.54 mmof 9 DOC have chips that imparted a higher degree of shear deformation when compared to the material withnoted 0.5 that mm materials and 1 mm with DOCs 1.5 mm [9]. DOC have chips that imparted a higher degree of shear deformation whenThese compared turnings to the were material then consolidatedwith 0.5 mm and and 1 subjectedmm DOCs to [9]. cold compaction, like the powder These turnings were then consolidated and subjected to cold compaction, like the powder metallurgy route, to obtain a billet (Figure2c) that was then soaked and hot extruded. During soaking, metallurgy route, to obtain a billet (Figure 2c) that was then soaked and hot extruded. During the mechanically bonded turnings in the billet softened due to the raised temperature of soaking soaking, the mechanically bonded turnings in the billet softened due to the raised temperature of (250 ◦C) and fused together to form bonds under the influence of high temperature. This process soaking (250 °C) and fused together to form bonds under the influence of high temperature. This is thought to be like the process of sintering, where green compacts are heated below the melting process is thought to be like the process of sintering, where green compacts are heated below the temperature of the metal to fuse the particles together. Although, in the current scenario, a soaking melting temperature of the metal to fuse the particles together. Although, in the current scenario, a temperature of 250 C would not entirely suffice to completely fuse the turnings together, hot extrusion soaking temperature◦ of 250 °C would not entirely suffice to completely fuse the turnings together, (a process that involves high temperatures as well as pressure) helps accelerate this process to obtain a hot extrusion (a process that involves high temperatures as well as pressure) helps accelerate this well fused bulk metal product. process to obtain a well fused bulk metal product.

(a) (b)

(c)

FigureFigure 2. 2.(a ()a Chip) Chip formation formation during during turning; turning;( b(b))chips chips/turnings/turnings (Depth(Depth ofof cutcut DOCDOC of 1.5 mm) with apparentapparent plastic plastic deformation deformation zones; zones; ( c()c) turnings turnings compacted compacted into into a a billet. billet.

3.2.3.2. E ﬀEffectect of of Deformation Deformation on on Microstructural Microstructural Features Features TheThe results results of of the the density density and and porosity porosity measurementsmeasurements ofof thethe extrudedextruded materialsmaterials are given in TableTable1. 1. The The theoretical theoretical density density of of the the AZ91 AZ91 alloy alloy was was 1.835 1.835 g g/cc./cc. TheThe AZ91_ARAZ91_AR material i.e., the extruded-asextruded-as received received material material had had an an experimental experimental density density of 1.816of 1.816 g/cc g/cc with with a porosity a porosity of 1.06%. of 1.06%. It was It observedwas observed that the that DOC the materials DOC materials also exhibited also exhibite a similard a rangesimilar of range porosity of withporosity almost with no almost deviation no deviation when compared to the AZ91_AR material. This shows that the consolidation of the chips and hot extrusion helped in the elimination of porosity in the final material. Thus, the effects of porosity or voids in the materials on the properties can be considered negligible.

Metals 2019, 9, 841 5 of 9 when compared to the AZ91_AR material. This shows that the consolidation of the chips and hot extrusion helped in the elimination of porosity in the ﬁnal material. Thus, the eﬀects of porosity or voidsMetals in2019 the, 9, materialsx FOR PEER on REVIEW the properties can be considered negligible. 5 of 9

Table 1.1. Density and porosity results. Composition Experimental Density (g/cm3) Theoretical Density (g/cm3) Porosity (%) Composition Experimental Density (g/cm3) Theoretical Density (g/cm3) Porosity (%) AZ91_AR 1.816 1.835 1.06 AZ91_AR 1.816 1.835 1.06 AZ91_0.5 DOC 1.815 1.835 1.10 AZ91_0.5 DOC 1.815 1.835 1.10 AZ91_1 DOC 1.812 1.812 1.835 1.835 1.27 1.27 AZ91_1.5 DOCDOC 1.817 1.817 1.835 1.835 1.02 1.02

The microstructures (along the longitudinal section) revealed very fine recrystallized grains for The microstructures (along the longitudinal section) revealed very ﬁne recrystallized grains for all of the materials (Figure 3a–d), particularly the ones with prior induced deformation (turning). all of the materials (Figure3a–d), particularly the ones with prior induced deformation (turning). However, all materials exhibited a bimodal grain structure i.e., bands of dynamically recrystallized However, all materials exhibited a bimodal grain structure i.e., bands of dynamically recrystallized (DRXed) and elongated worked grains. Figure 3 indicates that with an increase in deformation in the (DRXed) and elongated worked grains. Figure3 indicates that with an increase in deformation in the material during turning (DOC), the average DRXed grain size decreased progressively. materialAZ91_1.5DOC during exhibited turning (DOC), the lowest the average DRXed DRXedgrain size grain (1.27 size µm), decreased which progressively.was 60% lower AZ91_1.5DOC than that of µ exhibitedthe AZ91_AR. the lowest DRXed grain size (1.27 m), which was 60% lower than that of the AZ91_AR.

(a) (b)

FigureFigure 3. 3. MicrostructureMicrostructure ofof ((aa)) AZ91_AR;AZ91_AR; (b) AZ91_0.5DOC; ( (cc)) AZ91_1DOC; AZ91_1DOC; and and (d (d) )AZ91_1.5DOC AZ91_1.5DOC alloysalloys with with insetsinsets ofof averageaverage DRXedDRXed values.values.

TheThe individualindividual chipschips shownshown in Figure 21bb werewere subjectedsubjected toto XRDXRD toto identifyidentify the presence of phasesphases prior prior to to compactioncompaction andand extrusion.extrusion. The resu resultslts in Figure Figure 44aa show that the materials materials comprised comprised ofofα α-Mg-Mg andand MgMg1717Al12 phasesphases in in the the as-received as-received condition condition as as well well as as in in the the 0.5, 0.5, 1, 1, and and 1.5 1.5 DOC DOC turningsturnings/chips./chips. ComparedCompared toto thethe as-received material in the the ingot ingot condition, condition, the the normalized normalized intensity intensity ofof Mg Mg1717AlAl1212 peakspeaks in in the the turnings turnings was was much much lower, lower, particularly particularly at atan an angle angle of of~36°. ~36 This◦. This is believed is believed to tobe be due due to to the the reduction reduction in in the the size size of of the the phases, phases, thereby thereby changing changing their their distribution distribution due due to to the the processingprocessing that that led led to to the the drop drop in in the the intensity intensity from from the the corresponding corresponding phase phase when when subjected subjected to to x-ray x- ray diffraction. Furthermore, the XRD of AZ91 in the as-received condition revealed the lowest intensity at 34° (corresponding to the (0001) basal plane), indicating a strong basal texture. However, it is to be noted that the turnings did not exhibit any dominant texture as seen from their intensities due to the random selection of differently oriented turnings for XRD. Figure 4b also provides a qualitative idea about the texture of the materials. All the materials exhibited a strong texture with

Metals 2019, 9, 841 6 of 9 diﬀraction. Furthermore, the XRD of AZ91 in the as-received condition revealed the lowest intensity at 34◦ (corresponding to the (0001) basal plane), indicating a strong basal texture. However, it is to be noted that the turnings did not exhibit any dominant texture as seen from their intensities due to the random selection of diﬀerently oriented turnings for XRD. Figure4b also provides a qualitative Metals 2019, 9, x FOR PEER REVIEW 6 of 9 idea about the texture of the materials. All the materials exhibited a strong texture with the intensity atthe 2 Theta intensity of 34 at◦ corresponding2 Theta of 34° corresponding to the basal plane, to th beinge basal the plane, lowest being in the the XRD lowest of thein the cross XRD section of the ofcross the samples. section of Although the samples. there Although could be there a variation could be in a the variation texture in strengths, the texture it was strengths, noted thatit was all noted the materialsthat all the exhibited materials a strong exhibited extrusion a strong texture. extrusion texture.

(a) (b)

FigureFigure 4. 4.XRD XRD of of (a ()a base) base materials materials i.e., i.e., turnings turnings/chips/chips before before extrusion, extrusion, and and (b ()b extruded) extruded materials. materials.

Furthermore,Furthermore, SEM SEM images images (Figure (Figure3a–d) 3a–d) and XRD and (Figure XRD 4(Figure) revealed 4) thatrevealed the materials that the contained materials α-Mgcontained+ Mg 17αAl-Mg12 phases + Mg17 distributedAl12 phases across distributed the microstructure. across the micr For recrystallizationostructure. For torecrystallization occur, adequate to prioroccur, deformation adequate prior to provide deformation nuclei and to provide sufficient nucl storedei and energy sufficient to drive stored their energy growth to is necessary.drive their Increasinggrowth is thenecessary. deformation, Increasing i.e., bythe increasing deformation, the depthi.e., by of increasing cut in this the case, depth increases of cut thein this rate case, of nucleationincreases fasterthe rate than ofit nucleation increases the faster rate than of growth it incr [eases15]. Therefore, the rate of the growth final DRXed [15]. Therefore, grain size reducesthe final withDRXed increased grain deformation,size reduces with as was increased the case deformation, for AZ91_1.5DOC. as was However, the case for the AZ91_1.5DOC. presence of phases However, also afftheects presence the final of grain phases size. also Increased affects the presence final grai of continuousn size. Increased bands presence of precipitates of continuous was observed bands in of theprecipitates AZ91_DOC was alloy observed compared in tothe AZ91_AR. AZ91_DOC This a suppressedlloy compared the completeto AZ91_AR. nucleation This suppressed of new grains the duringcomplete extrusion nucleation in the of deformed new grains alloys, during resulting extrusion in the in the presence deformed of high alloys, amounts resulting of worked in the presence grains, whichof high was amounts also expressed of worked previously grains, which in [7]. was also expressed previously in [7].

3.3.3.3. E Effectﬀect of of Deformation Deformation on on Mechanical Mechanical Properties Properties TheThe micro-hardness micro-hardness and and compressive compressive properties properties are are given given in in Figure Figure5a. 5a. A A general general observation observation waswas made made that that the the micro-hardness micro-hardness of of those those materials material withs with induced induced deformation deformation was was higher higher than than that that ofof the the material material that that was was not not imparted imparted with with any any prior prio deformation.r deformation. Furthermore, Furthermore, with with increase increase in in the the deformationdeformation (i.e., (i.e., increased increased DOC), DOC), the the micro-hardness micro-hardness increased increased signiﬁcantly significantly to to as as high high as as 185 185 HV HV in in AZ91_1.5DOC.AZ91_1.5DOC. The The compressive compressive yield yield strength strength followed followed a similar a similar trend trend as that as ofthat micro-hardness, of micro-hardness, i.e., thei.e., compressive the compressive yield yield strength strength progressively progressively increased increased with with increments increments in DOC in DOC as seen as seen in Figure in Figure5b. 5b. AZ91_1.5DOC exhibited the highest compressive yield strength of about 364 MPa, which was 75% higher than that of AZ91_AR. This shows the significance of this method in imparting strength to the material. Furthermore, the strains to failure were in the same range as that of the as received condition, with all the materials exhibiting a reasonable failure strain of >15%, indicating no compromise in the ductility of the materials under compressive loading. The remarkable increment in the compressive yield strength in the DOC materials as against the as received (AZ91_AR) was due to a few critical factors. One such factor is the development of fine-

Metals 2019, 9, x FOR PEER REVIEW 7 of 9 recrystallized grained structures in DOC materials compared to AR, which helps in improving the yield strength following the Hall–Petch equation [16,17]. This was substantiated by AZ91_1.5DOC exhibiting a grain size 60% smaller than that of the AR alloy and a yield strength 75% higher than the AR alloy. Furthermore, the prior deformation in the turnings resulted in strain hardened materials when compared to the as received material. Strain hardening, therefore, is another dominating strengthening mechanism that was responsible for AZ91_1.5DOC exhibiting a high compressive yield strength. Thus, this significant enhancement in the mechanical properties of the materials can Metalsbe attributed2019, 9, 841 to their grain structure, which is also correlated to the extent of prior deformation7 of 9 during turning (in the form of increasing DOCs). Another possible mechanism is the dispersion AZ91_1.5DOCstrengthening of exhibited magnesium the highestby the oxides compressive [18] stemming yield strength from the of about surface 364 of MPa, the chips. which However, was 75% higherthis needs than further that of study AZ91_AR. and understanding This shows the before signiﬁcance conclusive of this remarks method can in imparting be made strengthas to whether to the material.the size of Furthermore, the oxides plays the strains a crucial to failure role in were the instrengthening the same range mechanism. as that of the Furthermore, as received condition,the oxide withsize also all the affects materials the ductility. exhibiting Hence, a reasonable it is the failure future strain scope of > of15%, this indicating work to noidentify compromise the isolated in the ductilitycontribution of the of materialssuch a mechanism. under compressive loading.

(a) (b)

Figure 5.5. (a(a)) Mechanical Mechanical properties properties of materials;of materials; and and (b) representative (b) representative engineering engineering stress-strain stress-strain curves ofcurves the materials. of the materials.

3.4. MechanismThe remarkable and Comparison increment with in theConventional compressive Deformation yield strength Processes in the DOC materials as against the as received (AZ91_AR) was due to a few critical factors. One such factor is the development of fine-recrystallizedThis turning grained induced structures deformation in DOC approach materials comparedwas compared to AR, whichto the helps conventional in improving cold the yielddeformation strength techniques following thesuch Hall–Petch as torsion equation induced [ 16deformation,,17]. This was which substantiated was undertaken by AZ91_1.5DOC by [7], to exhibitingunderstand a grainthe effectiveness size 60% smaller of this than approach. that of the Figure AR alloy 6 gives and athe yield mechanical strength 75%properties higher thanof the the same AR alloy.AZ91 Furthermore,alloy processed the through prior deformation this approach in the and turnings the torsion resulted induced in strain deformation hardened approach. materials In when the comparedtorsion-induced to the asdeformation received material. approach, Strain the hardening, materials therefore,were solution is another treated dominating for 3 h at strengthening 420 °C and mechanismextruded at that400 was°C and responsible then subjected for AZ91_1.5DOC to free end torsion exhibiting deformation. a high compressive Although yield the processing strength. Thus, and thisthe process significant parameters enhancement differed in from the mechanical this work, th propertiese two types of theof deformation materials can were be attributeddone on the to same their grainmaterial structure, (i.e., AZ91) which and is alsowere correlated compared to in the terms extent of their of prior compressive deformation properties. during turning It was evident (in the form that ofthe increasing yield strength DOCs). of AZ91_1.5DOC Another possible was about mechanism 45% higher is the than dispersion that of the strengthening torsion deformed of magnesium and aged byAZ91, the oxidesindicating [18] stemmingthe effectiveness from the of surface this technique of the chips. in However,improving this strength. needs further Furthermore, study and a understandingmechanism of beforethis approach conclusive was remarks established can be al madeongside as toa comparison whether the to size the of conventional the oxides plays SPD a crucialtechniques role such in the as strengtheningECAP (Figure mechanism.1). ECAP usually Furthermore, results in a the uniform oxide sizedeforma alsotion affects of bulk the ductility.material Hence,after multiple it is the passes future scope[3], with of thisa high work amount to identify of strain the isolated imparted contribution to the material. of such This a mechanism. results in a tremendous improvement in the properties of the material that is processed by ECAP. While this 3.4.technique Mechanism initially and Comparisonimparts a non-uniform with Conventional deformation Deformation (localized Processes primary and secondary plastic deformation zone) in each chip, however, this non-uniform deformation is overcome by the This turning induced deformation approach was compared to the conventional cold deformation consolidation of randomly oriented (Figure 2c) and deformed chips during compaction, which leads techniques such as torsion induced deformation, which was undertaken by [7], to understand to a rather uniform arrangement of chips; in other words, a near uniform deformation of the bulk the effectiveness of this approach. Figure6 gives the mechanical properties of the same AZ91 alloy material similar to that of ECAP occurs. It must be noted that the amount of strain imparted in ECAP processed through this approach and the torsion induced deformation approach. In the torsion-induced is much higher than the current technique and was used only as a means of comparison. Thus, this deformation approach, the materials were solution treated for 3 h at 420 ◦C and extruded at 400 ◦C and then subjected to free end torsion deformation. Although the processing and the process parameters differed from this work, the two types of deformation were done on the same material (i.e., AZ91) and were compared in terms of their compressive properties. It was evident that the yield strength of AZ91_1.5DOC was about 45% higher than that of the torsion deformed and aged AZ91, indicating the effectiveness of this technique in improving strength. Furthermore, a mechanism of this approach was established alongside a comparison to the conventional SPD techniques such as ECAP (Figure1). ECAP usually results in a uniform deformation of bulk material after multiple passes [3], with a high amount Metals 2019, 9, 841 8 of 9 of strain imparted to the material. This results in a tremendous improvement in the properties of the material that is processed by ECAP. While this technique initially imparts a non-uniform deformation (localized primary and secondary plastic deformation zone) in each chip, however, this non-uniform deformation is overcome by the consolidation of randomly oriented (Figure2c) and deformed chips during compaction, which leads to a rather uniform arrangement of chips; in other words, a near uniformMetals 2019 deformation, 9, x FOR PEER ofREVIEW the bulk material similar to that of ECAP occurs. It must be noted that8 of the 9 amount of strain imparted in ECAP is much higher than the current technique and was used only as a meanstechnique of comparison. is highly effective Thus, this in techniqueimparting isnear highly-uniform effective deformation in imparting and near-uniform strength to deformationmagnesium andalloys. strength However, to magnesium one limitation alloys. with However, this technique one limitation is that the with total this volume technique of isthe that resultant the total extruded volume ofmaterial the resultant is lower extruded when compared material isto loweran extrudate when compared of a cast billet to an because extrudate of of the a castair pockets billet because between of the airchips pockets in the between consolidated the chips billet. in theFurthermore, consolidated other billet. secondary Furthermore, deformation other secondary methods deformation like rolling methodsor swaging like could rolling also or swaginglead to similar could alsoimprovements lead to similar in the improvements mechanical inproperties, the mechanical which properties,would be whichan interesting would bedirection an interesting of research direction to recycle of research magnesium to recycle alloys magnesium and composites. alloys and composites.

Figure 6. Comparison between turning-induced and torsion-induced deformation. 4. Conclusions 4. Conclusions AZ91 alloys were successfully fabricated through the consolidation of chips formed during turning,AZ91 followed alloys bywere cold successfully compaction fabricated and hot extrusion. through This the workconsolidation revealed thatof chips the increase formed in during depth ofturning, cut during followed turning by impartedcold compaction a higher and amount hot extrusion. of deformation This work in the revealed chips, resulting that the in increase improved in hardnessdepth of andcut during compressive turning strengths imparted of AZ91a higher alloy. amou A maximumnt of deformation compressive in the yield chips, strength resulting of 364 in MPaimproved wasobserved hardness in and the AZ91_1.5DOCcompressive strengths alloy, which of AZ91 was 75% alloy. higher A maximum than the AZ91_AR compressive alloy. yield This wasstrength due toof the364 development MPa was observed of fine-grained in the AZ91_1.5DOC structures in the alloy, AZ91_1.5DOC which wasalloy, 75% whichhigher werethan 60%the smallerAZ91_AR than alloy. that This of the was AZ91_AR due to alloythe development as well as the of plastic fine-grained deformation structures induced in inthe the AZ91_1.5DOC chips during turningalloy, which that resultedwere 60% in strainsmaller hardening. than that This of the turning-induced-deformation AZ91_AR alloy as well as technique,the plastic in deformation addition to conventionalinduced in the deformation chips during routes turning (such as that torsion), result ised a viablein strain route hardening. for the strengthening This turning-induced- of Mg alloys. deformation technique, in addition to conventional deformation routes (such as torsion), is a viable Authorroute for Contributions: the strengtheningConceptualization of Mg alloys. and design of experiments, S.T. and M.G.; Performed the experiments and analyzed the data, S.T. and M.A.; Writing—original draft preparation, S.T.; Writing—review and editing, S.T. and M.G.; Project administration, S.T.; Funding acquisition, M.G. Author Contributions: Conceptualization and design of experiments, S.T. and M.G.; Performed the experiments Funding: and analyzedThis the research data, wasS.T. fundedand M.A by.; theWriting—original Ministry of Education, draft preparation, Singapore, S.T.; grant Writing—review number R 265 000 and 622 editing, 112. Acknowledgments:S.T. and M.G.; ProjectThe administration authors would, S.T.; like Funding to thank acquisition, Juraimi Bin M.G. Madon for his assistance in extruding all of the materials. Funding: This research was funded by the Ministry of Education, Singapore, grant number R 265 000 622 112. Conflicts of Interest: The authors declare no conflicts of interest. Acknowledgments: The authors would like to thank Juraimi Bin Madon for his assistance in extruding all of the materials.

Conflicts of Interest: The authors declare no conflicts of interest.

References

1. Tekumalla, S.; Nandigam, Y.; Bibhanshu, N.; Rajashekara, S.; Yang, C.; Suwas, S.; Gupta, M. A strong and deformable in-situ magnesium nanocomposite igniting above 1000 °C. Sci. Rep. 2018, 8, 7038. 2. Chung, C.W.; Ding, R.G.; Chiu, Y.L.; Gao, W. Effect of ECAP on microstructure and mechanical properties of cast AZ91 magnesium alloy. J. Phys. Conf. Ser. 2010, 241, 012101.

Metals 2019, 9, 841 9 of 9

References

1. Tekumalla, S.; Nandigam, Y.; Bibhanshu, N.; Rajashekara, S.; Yang, C.; Suwas, S.; Gupta, M. A strong and

deformable in-situ magnesium nanocomposite igniting above 1000 ◦C. Sci. Rep. 2018, 8, 7038. [CrossRef] [PubMed] 2. Chung, C.W.; Ding, R.G.; Chiu, Y.L.; Gao, W. Effect of ECAP on microstructure and mechanical properties of cast AZ91 magnesium alloy. J. Phys. Conf. Ser. 2010, 241, 012101. [CrossRef] 3. Bryła, K.; Dutkiewicz, J.; Litynska-Dobrzynska, L.; Rokhlin, L.L.; Kurtyka, P. Influence of number of ECAP passes on microstructure and mechanical properties of AZ31 magnesium alloy. Arch. Metall. Mater. 2012, 57, 711–717. [CrossRef] 4. Sun, J.; Yang, Z.; Han, J.; Liu, H.; Song, D.; Jiang, J.; Ma, A. High strength and ductility AZ91 magnesium alloy with multi-heterogenous microstructures prepared by high-temperature ECAP and short-time aging. Mater. Sci. Eng. A 2018, 734, 485–490. [CrossRef] 5. Yuan, Y.; Ma, A.; Jiang, J.; Lu, F.; Jian, W.; Song, D.; Zhu, Y.T. Optimizing the strength and ductility of AZ91 mg alloy by ECAP and subsequent aging. Mater. Sci. Eng. A 2013, 588, 329–334. [CrossRef] 6. Al-Zubaydi, A.S.J.; Zhilyaev, A.P.; Wang, S.C.; Reed, P.A.S. Superplastic behaviour of AZ91 magnesium alloy processed by high-pressure torsion. Mater. Sci. Eng. A 2015, 637, 1–11. [CrossRef] 7. Song, B.; Wang, C.; Guo, N.; Pan, H.; Xin, R. Improving tensile and compressive properties of an extruded AZ91 rod by the combined use of torsion deformation and aging treatment. Materials 2017, 10, 280. [CrossRef] [PubMed] 8. Ghadbeigi, H.; Bradbury, S.R.; Pinna, C.; Yates, J.R. Determination of micro-scale plastic strain caused by orthogonal cutting. Int. J. Mach. Tools Manuf. 2008, 48, 228–235. [CrossRef] 9. Sinha, N. Ta202a: Introduction to Manufacturing Processes; Department of Mechanical Engineering, IIT Kanpur: Kanpur, India, 2017. 10. Xiong, B.; Zhang, X.; Li, F.; Hu, H.; Liu, C. Recycling of aluminum A380 machining chips. In Light Metals 2015; Hyland, M., Ed.; Springer International Publishing: Cham, Switzerland, 2016; pp. 1011–1015. 11. Guo, Y.B.; Salahshoor, M. Process mechanics and surface integrity by high-speed dry milling of biodegradable magnesium–calcium implant alloys. CIRP Ann. 2010, 59, 151–154. [CrossRef] 12. ASM International. Machining of magnesium and magnesium alloys. In Machining; ASM International: Materials Park, OH, USA, 1989; Volume 16. 13. Wan, B.; Chen, W.; Lu, T.; Liu, F.; Jiang, Z.; Mao, M. Review of solid state recycling of aluminum chips. Resour. Conserv. Recycl. 2017, 125, 37–47. [CrossRef] 14. Hu, M.-L.; Ji, Z.-S.; Chen, X.-Y.; Wang, Q.-D.; Ding, W.-J. Solid-state recycling of AZ91D magnesium alloy chips. Trans. Nonferrous Met. Soc. China 2012, 22, s68–s73. [CrossRef] 15. Rios, P.R.; Siciliano, F., Jr.; Sandim, H.R.Z.; Plaut, R.L.; Padilha, A.F. Nucleation and growth during recrystallization. Mater. Res. 2005, 8, 225–238. [CrossRef] 16. Yang, W.; Tekumalla, S.; Gupta, M. Cumulative effect of strength enhancer—lanthanum and ductility enhancer—cerium on mechanical response of magnesium. Metals 2017, 7, 241. [CrossRef] 17. Chen, Y.; Tekumalla, S.; Guo, Y.B.; Shabadi, R.; Shim, V.P.W.; Gupta, M. The dynamic compressive response of a high-strength magnesium alloy and its nanocomposite. Mater. Sci. Eng. A 2017, 702, 65–72. [CrossRef] 18. Tekumalla, S.; Shabadi, R.; Yang, C.; Seetharaman, S.; Gupta, M. Strengthening due to the in-situ evolution of

ß10 Mg-Zn rich phase in a ZnO nanoparticles introduced Mg-Y alloy. Scr. Mater. 2017, 133, 29–32. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).