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Procedia Engineering 207 (2017) 1719–1724

International Conference on the Technology of Plasticity, ICTP 2017, 17-22 September 2017, International Conference on theCambridge, Technology United of Plasticity, Kingdom ICTP 2017, 17-22 September 2017, Cambridge, United Kingdom Shear of 304L stainless steel – microstructural aspects Shear forming of 304L stainless steel – microstructural aspects M. Guillota,*, T. McCormackb, M. Tuffsb, A. Rosochowskia, S. Hallidayb, M. Guillota,*, T. McCormackP.b, M.Blackwell Tuffsb,a A. Rosochowskia, S. Hallidayb, a aUniversity of Strathclyde, 75P. Montrose Blackwell Street, Glasgow G1 1XJ, United-Kingdom bRolls-Royce, PO Box 31, Derby DE24 8BJ, United-Kingdom aUniversity of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, United-Kingdom bRolls-Royce, PO Box 31, Derby DE24 8BJ, United-Kingdom

Abstract Abstract Shear forming is an incremental cold forming process. It transforms 2D plates into 3D structures commonly consisting of conicalShear geometry. forming isRoller(s) an increme pushntal the coldblank forming onto a coneprocess.-shaped It transformsmandrel, resulting 2D plate ins ainto decrease 3D structures of the initial commonly thickness. consisting The shear of conicalforming geometry. process has Roller(s) diverse push advantages the blank, such onto as a coneimproved-shaped material mandrel, utilisation, resulting enhancedin a decrease product of the characteristics, initial thickness. good The surface shear finisformingh, consistent process hasgeometric diverse control advantages and ,reduced such as production improved materialcosts. Shear utilisation, forming enhanced has potential product application characteristics,s in a widegood range surface of finisconicalh, consistent geometries geometric used within control advanced and reduced aerospace production structures, costs. which Shear are formingcurrently has manufactured potential application from bulks forgingsin a wide with range high of associatedconical geometries used costs. within advanced aerospace structures, which are currently manufactured from bulk with high associatedResearch machining findings costs.related to shear forming have been published over the past two decades, however, important remaining questionsResearch have findings still to berelated answered to shear, with forming this paper have addressing been published one such over gap the associated past two with decades, the material however, deformation important mechanism remaining. questionsSeveral studies have stillhave to demonstrated be answered , thewith impact this paper of key addressing process variablesone such ongap the associated final geometry with the and material surface deformation roughness, mechanismsuch as the. feeds,Several roller studies nose have radius demonstrated and mandrel/roller the impact offset of key. Althoughprocess variables the material on the outputs final geometryare essential and, surfaceas they roughness,link directly such with as the feeds,mechanical roller properties nose radius of theand final mandrel/roller components, offset the .microstructure Although the and material texture outputs of the materialare essential after ,shear as they forming link directlyhave rarely with been the mechanicalstudied. Achieving properties a greaterof the finalunderstanding components, in thisthe microstructurearea could reduce and the texture reliance of the upon material mechanical after shear testing forming to validate have therarely process been andstudied. ease theAchieving exploitation a greater route understanding of the technology in this into area advanced could reduce aerospace the relianceapplications. upon mechanical testing to validate the process andFirst easely the, this exploitation paper presents route the of printhe cipletechnology of shear into formi advancedng and itsaerospace related terminologyapplications.. Then, a brief overview of the shear forming processFirst lyincluding, this paper its presentshistory and the prinoriginciple is givenof shear. The formi areasng and of focusits related are a terminology selection of. theThen, main a brief variables overview encountered of the shear within forming this process includingwhich could its historyimpact andthe finalorigin properties. is given. TheThe areasgeneration of focus of localare a stressesselection due of theto deviationsmain variables from encounteredthe sine law, within the angle this processvariations, which and couldforces impactrequired the during final properties.the forming Theoperation generation are also of localconsidered. stresses This due isto explored deviations in fromthe context the sine of law,forming the angle304L variations,stainless steel and plate forcess. required during the forming operation are also considered. This is explored in the context of forming 304L stainless steel plates.

* Corresponding author. E-mail address: [email protected] * Corresponding author. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Plasticity. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity.

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. 10.1016/j.proeng.2017.10.928

10.1016/j.proeng.2017.10.928 1877-7058 17202 M. GuillotM. Guillot et al. /et Procedia al. / Procedia Engineering Engineering 00 (201 2077) (2017)000–000 1719 –1724 2 M. Guillot et al. / Procedia Engineering 00 (2017) 000–000 This paper is written based upon experiments which observed a 52% thickness reduction using one roller. Scanning Electron MicroscopyThis paper (SEM), is written Elect basedron BackScatter upon experiments Diffraction which (EBSD) observed and ahardness 52% thickness testing reductionwere utilised using to onecharacterise roller. Scanning the shear Electron formed Microscopymaterial. The (SEM), initial Elect heterogeneousron BackScatter microstructure Diffraction with (EBSD) relatively and hardnessequiaxed testinggrains wereevolved utilised into ato microstructure characterise the with shear elongated formed material.grains along The with initial shear heterogeneous bands. A microstructuretexture analysis with revealed relatively a simple equiaxed shear grains mechanism evolved ofinto deformation a microstructure, and thewithoretical elongated and grainsexperimental along shearwith sshear were bands.found to Amatch. texture analysis revealed a simple shear mechanism of deformation, and theoretical and experimentalIdentifying shear the mechanisms were found of todeformation match. was the first step to a better understanding of the effect imparted by the shear forming processIdentifying on the thematerial. mechanism Further of deformationstudies will look was theat the first impact step to of a the better key understanding process variables of the on effect both theimparted microstructure by the shear and forming texture processas well ason heat the material.treatment Furtheroptimisation. studies will look at the impact of the key process variables on both the microstructure and texture as well as heat treatment optimisation. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. ©Peer 201-review7 The underAuthors. responsibility Published by of Elsevier Ltd. Peer-review under responsibility of thethe scientific scientific committee committee of the International of the International Conference Conferenceon the Technology on the of TechnologyPlasticity. of PeerPlasticity-review. under responsibility of the scientific committee of the International Conference on the Technology of PlasticityKeywords: incremental. forming; shear forming; stainless steel; simple shear. Keywords: incremental forming; shear forming; stainless steel; simple shear.

1. Introduction 1. Introduction Shear forming is also known as flow forming, shear spinning, shear turning, power spinning, or spin . It is an iShearncremental forming cold is formingalso known process as flow which forming, manufactures shear spinning, rotationally shear symmetrical turning, power conical, spinning concave,, or spin or convex forging hollow. It is anprofile incrementald engineering cold forming components process. M whichinimal manufactureswaste and a dimens rotationallyional accuracy symmetrical similar conical, to that concave, of machined or convex components hollow profileare obtainedd engineering using today’s components CNC .flow Minimal formers waste. Shear and a forming dimens ionalshould accuracy not be similarmistaken to forthat conventional of machined componentsspinning, as arethe obtainedlatter combines using today’s tension CNC and compressionflow formers .of Shear the material forming without should intentionalnot be mistaken thickness for conventional reduction of spinningthe starting, as theblank. latter C onversely,combines tensionshear forming and compression involves only of the compression material without forces intentionaland reduces thickness the initial reduction material ofthickness the starting [1]. blank.Although C onversely,spinning methods shear forming were born involves with thonlye art compression of potting clayforces in ancientand reduces Egypt the [2 ],initial the firstmaterial papers thickness mentioning [1]. Althoughshear forming spinning were methodspublished were 60 years born agowith and the its art expansion of potting has clay mainly in ancient occurred Egypt over [2 the], the past first two papers decades. mentioning shearFig forming. 1 describes were shearpublished forming’s 60 years three ago operation and its expansional steps: 1. has a flat mainly circula occurredr blank overis clamped the past against two decades. a mandrel by a tailstock;Fig. 1 describes2. the mandrel shear isforming’s rotated; 3.three one operation or more alrollers steps: then 1. a force flat circula the blankr blank to incrementally is clamped against take the a mandrel shape of by the a tailstock;mandrel by 2. thefollowing mandrel a programedis rotated; 3. path one using or more a controlled rollers then feed force rate. the Generally,blank to incrementally only a single take pass the is shape required of the to mandrelproduce theby finalfollowing shape a but programed multiple passespath using can bea controlledundertaken feed to enhance rate. Generally, dimensional only control a single. pass is required to produceThe final the finalthickness shape of but the multiple part, S1, passesis determined can be undertakenfrom the sine to lawenhance (Equation dimensional 1), where control α is the. angle of the mandrel andThe S0 the final initial thickness blank ofthickness. the part, The S1, iskey determined impact of fromthe sine the lawsine is law that (Equation a small angle 1), where α will α produce is the angle a smaller of the thickness mandrel andS1, requiring S0 the initial larger blank levels thickness. of deformation. The key impact As such, of ththee sineminimum law is anglethat a ofsmall the anglemandrel α will possible produce to sheara smaller form thickness is in the Srange1, requiring 10 to 18°, larger depende levelsnt of on deformation. the material As being such, formed; the minimum smaller angleangles of as the low mandrel as 3 to possible 4°, however, to shear can form be a ischieved in the rangeusing multiple10 to 18°, passes depende [1].nt Deviationson the material from being the sine formed; law are smaller sometimes angles observed as low as and 3 to lead 4°, tohowever, failure orcan defects be achieved in the usingpart, which multiple are passesdue to induced[1]. Deviations stress, a phenomenonfrom the sine directly law are noticeable sometimes on observed the flange and of leadthe workpiece to failure .or Material defects buildin the- partup is, whichone of are the due main to causesinduced of s tresdeviations, a phenomenon and can be directly prevented noticeable by using on more the flange rollers of and/or the workpiece multiple .passes Material [3]. build - up is one of the main causes of deviation and can be prevented by using more rollers and/or multiple passes [3]. (1) (1) 𝑆𝑆1 = 𝑆𝑆0 . 𝑠𝑠𝑠𝑠𝑠𝑠 (𝛼𝛼) 𝑆𝑆1 = 𝑆𝑆0 . 𝑠𝑠𝑠𝑠𝑠𝑠 (𝛼𝛼)

Fig. 1. Shear forming principle: (a) Set-up; (b) In operation; (c) Part completed. Fig. 1. Shear forming principle: (a) Set-up; (b) In operation; (c) Part completed. During shear forming, axial, radial, and tangential forces are generated between the blank and the roller, out of whichDuring the tangentialshear forming, force axial,is usually radial, found and totangential be the smallest forces are. M generatedultiple researchers between thehave blank attempted and the to roller,calculate out the of whichlatter by the considering tangential forceshear isdeformation usually found or a tocombination be the smallest of shear. M ultipledeformation researchers and bending have attempted strain. In to 2001, calculate Chen the et latter by considering shear deformation or a combination of shear deformation and bending strain. In 2001, Chen et M. Guillot et al. / Procedia Engineering 207 (2017) 1719–1724 1721 M. Guillot et al. / Procedia Engineering 00 (2017) 000–000 3 al. [4] linked these forces to the process variables and in 2003, Kim et al. [5] obtained convergence of experimental and theoretical tangential forces by considering a pure shear model. Understanding the forces required to shear form a blank is essential to define the limitations of such a process and this involves understanding the material behavior as well as the effect of process parameters. Although Zhan et al. [6] and Radović et al. [7] studied the microstructure and texture of shear formed components made of Al and Al-Mg alloys, the material deformation mechanism was not identified. Common materials currently industrially shear formed are steel, copper, aluminum, and nickel, plus a much wider range of materials have been successfully shear formed in laboratory trials [1]. Potential future applications include a wide spectrum of conical structures used in gas turbine engines, aerospace structures and military equipment.

2. Experimental set-up

The material used in this study was a 6 mm thick 304L stainless steel sheet that had been hot rolled, solution annealed, and pickled in accordance with the standard EN 10088-2. Its chemical composition and mechanical properties are given by Tables 1 and 2 respectively. Prior to shear forming, the blanks were water jet cut with an external diameter of 430 mm and an internal diameter of 150 mm.

Table 1. Chemical composition (% by mass) of the as-received sheet. C Cr Mn N Ni P S Si 0.018 18.050 1.773 0.067 8.035 0.031 0.002 0.449

Table 2. Mechanical properties of the as-received sheet. Yield strength at Elongation after fracture Ultimate tensile strength Vickers hardness 0.2% 1% A50 A5 635 N/mm² 348 N/mm² 391 N/mm² 52 % 53 % 186 HV1

Shear forming experiments were carried out at the Advanced Forming Research Centre (AFRC) – University of Strathclyde, using a WF Maschinenbau 3-roller-flow forming and spinning machine of the type STR 600 3/6 pictured in Fig. 2 (a). One 3 mm nose radius roller was utilised to shear form the blanks onto a mandrel with a 31.5° angle, which according to the sine law corresponds to a thickness reduction of 52%. The resultant geometrical transformation is shown by Fig. 2 (b). During the process, the surface speed and feed rate were held constant at 480 m/min and 1.0 mm/rev respectively. An oil-based coolant, Houghton Hocut 795B, was employed at a flow rate of 400 L/min. Five specific areas of interest taken from a cross-sectional slice of the final parts, identified by Fig. 2 (c), were then analysed using Scanning Electron Microscopy (SEM) and Electron BackScattered Diffraction (EBSD), with a microscope type FEI Quanta 250 FEG. Hardness tests were performed using a micro-hardness tester of type Zwick- Roell Indentec ZHμ, at 1 kg force. The measurements were made in accordance with the ISO 6507-1 standard. The level of confidence in the results was of 95%. The values taken into account for this study are an average of the three indents across the thickness of the sample.

Fig. 2. (a) Flow former; (b) Blank and shear formed part; (c) Areas of metallurgical interest.

1722 M. Guillot et al. / Procedia Engineering 207 (2017) 1719–1724 4 M. Guillot et al. / Procedia Engineering 00 (2017) 000–000

Micrographs were taken in BackScatter (BS) mode. The coordinate systems adopted for reporting the microstructural data of area M1 were the direction (RD), transverse direction (TD) and normal direction (ND) whereas the cylindrical coordinates (R, θ, Z) were used for the remaining areas. The texture was analysed by EBSD with a step size of 1.0 and 0.3 µm for the as-received and shear formed areas respectively. Pole Figures (PFs) and Orientation Distribution Functions (ODFs) were displayed using the settings given by Tables 3 and 4.

Table 3. Settings employed to generate the PFs. Half Cluster Coordinate Projection Projection Planes Colours Contour lines width size system type hemisphere Step width of 1.5 10° 5° R, θ, Z Stereographic Upper {100}, {110}, {111} Max. intensity at 6 Max. at 6

Table 4. Settings employed to generate the ODFs. Cluster Coordinate Sample Half width Colours Contour lines Method Resolution Calculation size system symmetry Gaussian Based on full map Gaussian, 5° 5° R, θ, Z Rainbow At 2, 4, 6, 8, and 10 Triclinic 128x32x32 estimation

3. Results and discussion

Fig. 3 shows the micrographs of the part in the different areas of interest. From EBSD analysis it was estimated that the as-received material, represented by area M1, had approximately equiaxed grains with an average grain size of 11.3 µm (ASTM 9.5-10) with 70% of the grains being under or equal to 10.0 µm. The initial material was heterogeneous and had a relatively weak rolling texture.

Fig. 3. Micrographs along the shear formed part.

The shear formed areas, M2 to M5, presented elongated grains and only slight microstructural differences were noticed. Shear bands representative of high local deformation during plastic deformation were observed. They presented primary orientations along the θ axis of the cylindrical coordinates and slightly tilted from the shear forming direction. These aligned shear bands were ascertained with a principal spacing around 1 µm. Simple shear, a specific M. Guillot et al. / Procedia Engineering 207 (2017) 1719–1724 1723 M. Guillot et al. / Procedia Engineering 00 (2017) 000–000 5 mechanism of deformation, is detected when one direction of the initial material which is free to rotate on both ends remains constant and everything else rotates relative to it. Fig. 4 (a) illustrates this phenomenon with the transformation of a square to a rhombus along with the rotation of the ellipsoid axes from their original position, which is another characteristic of simple shear. This mechanism was identified on the BS images of Fig. 3 (areas M2 to M5) where elongation of the grains and shear bands along the main ellipsoid axis are shown. Although the actual stress- strain state in the material can be significantly triaxial and complex, the model of the process can be simplified assuming that at each contact between the roller and material only elementary simple shear takes place. In this case, the intensity of this shear, γ, can be estimated as α tangent. Fig. 4 (b) is the plot of the shear as a function of the angle of the mandrel. The shear for the current geometry was about γ=1.6. An EBSD map was acquired in the middle of the thickness of area M4 and an austenite to martensite transformation was observed. The PFs are given by Fig. 5. According to the Kurdjumov-Sachs orientation relationship, the plane {111}fcc is parallel to {110}bcc. This can be seen in these specific PFs as they show the same elements in identical areas, which is characteristic of shear transformation. This transformation of austenite to martensite was also proved by the part becoming magnetic. By comparing the PF {111}fcc with the PFs presented by Barnett et al. [8], an equivalent strain of about 1.7 was identified. Few of the main torsion texture components, which are defined in Fig. 6, were noticed on the PFs {111}. To look at this deformation in more detail, ODFs were calculated. In the case of simple shear deformation, the specific components B and B̅ are identified on ODFs displayed at Ф=55° and Ф2=45°. They are generally observed for these settings at Ф1=0, 120, 240° and Ф1=60, 180, 300° for B and B̅ respectively [9]. The ODFs of both fcc and bcc phases are given by Fig. 5. In both cases, the components B and B̅ were identified whereas the components A1*, A2* and C only appeared clearly for the fcc phase. According to these results, the mechanism of deformation was simple shear.

Fig. 4. Simple shear: (a) Illustration of the principle; (b) Plot of the shear as a function of the angle of the mandrel.

Fig. 5. Texture representations for the middle of the thickness of area M4 (top: PFs; bottom: ODFs). 1724 M. Guillot et al. / Procedia Engineering 207 (2017) 1719–1724 6 M. Guillot et al. / Procedia Engineering 00 (2017) 000–000

Fig. 6. Main torsion texture components: (a) Miller indices; (b) Representation on ODFs.

Some low-force hardness tests were performed to see if the hardness changed across the part. The results for area M1, considered as the as-received material, met the data given by the material certificate in Table 2. The outcomes showed a variation within the part of 5HV1 in the shear formed areas, which suggests that the microstructure is reasonably homogeneous in this area, as is borne out in Fig. 3. The highest hardness values were recorded at the transition of as-received and shear formed areas with 370HV1.

4. Conclusions

The initial material with relatively equiaxed grains evolved to a structure with elongated grains after being shear formed. Its hardness doubled during the process to reach values around 370HV1. High local deformation was observed in the form of shear bands along the θ axis of the cylindrical coordinates and slightly titled from the shear forming direction. Micrographs and hardness analysis showed a homogeneous microstructure along the shear formed length of the part. A transformation of austenite to martensite was observed, which was proved in practice by the part becoming magnetic. The main mechanism of deformation was identified as simple shear thanks to experimental and theoretical shears and identification of the main torsion texture components on the PFs and ODFs for both fcc and bcc phases of the material. Shear forming has a clear effect on the microstructure, hardness and texture of the material. The effect of process parameters on the resulting material characteristics of the formed component is of great relevance to this technology and further studies should explore this area. Also planned are detailed assessments across the thickness of shear formed parts to establish the existence of any gradient. These are important considerations which should be included at the early design stage of any engineering component shaped by shear forming.

Acknowledgements

Special thanks to Rolls-Royce and the Engineering and Physical Sciences Research Council (EPSRC) for their financial support. A particular mention to the University of Strathclyde, especially the AFRC, whose co-operation and technical assistance were greatly appreciated.

References

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