Influence of Process Fluctuations on Residual Stress Evolution in Rotary

materials

Article Inﬂuence of Process Fluctuations on Residual Stress Evolution in Rotary Swaging of Steel Tubes

Svetlana Ishkina 1,* , Dhia Charni 2, Marius Herrmann 1, Yang Liu 1,2,Jérémy Epp 2,3 , Christian Schenck 1,3, Bernd Kuhfuss 1,3 and Hans-Werner Zoch 1,2,3

1 Bremen Institute for Mechanical Engineering-bime, University of Bremen, Badgasteiner Str. 1, 28359 Bremen, Germany; [email protected] (M.H.); [email protected] (Y.L.); [email protected] (C.S.); [email protected] (B.K.); [email protected] (H.-W.Z.) 2 Leibniz Institute for Materials Engineering-IWT, Badgasteiner Str. 3, 28359 Bremen, Germany; [email protected] (D.C.); [email protected] (J.E.) 3 MAPEX Center for Materials and Processing, University of Bremen, Bibliothekstr. 1, 28359 Bremen, Germany * Correspondence: [email protected]

Received: 8 February 2019; Accepted: 8 March 2019; Published: 14 March 2019

Abstract: Infeed rotary swaging is a cold forming production technique to reduce the diameter of axisymmetric components. The forming is achieved discontinuously by a series of radial strokes that are spread over the shell of the part. Due to tolerances within the rotary swaging machine, these strokes perform individually and the resulting stroke pattern is not homogeneous with regards to circumferential and longitudinal distribution. Nevertheless, in combination with the high number of performed strokes and the large contact area between the dies and the part, the external part properties, such as diameter, roundness and surface roughness, show even values along the finished part. In contrast, strength-defining internal part properties, like microstructure and residual stress components, are more sensitive to the actual pattern and temporal sequence of the individual strokes, which is investigated in this study. The impact of process fluctuations during the conventional process, which are induced by the tolerances of machine tool components, was verified by numerical simulations, physical tests and measurements of residual stress distributions at the surface and at depth. Furthermore, a method is introduced to maintain the stroke following angle ∆ϕ at zero by flat dies, and thus the actual pattern and temporal sequence of the strokes was homogenized. The results show that the residual stress fluctuations at the surface could be controlled and reduced. Furthermore, it is demonstrated that the depth profile of the residual stresses at a distance of 300 µm from the surface developed independently from the process fluctuations.

Keywords: ﬁnite element method; incremental bulk forming; cold forging; X-ray diffraction

1. Introduction Rotary swaging is an incremental cold forming process for reducing and proﬁling the cross section of axisymmetric components, such as bars and tubes. It is established in the automotive and aerospace manufacturing industries [1]. This production technique provides several advantages such as high static and dynamic strength due to strain hardening [2]. Additionally, by the use of hollow shafts and by the advantageous development of the wall thickness during rotary swaging, cost-effective production of lightweight components is possible due to the optimal use of material resources [3]. In infeed rotary swaging, the part is axially fed into the swaging head with the feed velocity vf (see Figure1). The diameter is reduced incrementally by a revolving and radially oscillating motion of (four) dies. This motion is induced by the rotating swaging axle that entrains the base jaws with cams on the top surface. These cams pass the cylinder rollers and push the tools inwards. The swaging head

Materials 2019, 12, 855; doi:10.3390/ma12060855 www.mdpi.com/journal/materials Materials 2019, 12, x FOR PEER REVIEW 2 of 13 Materials 2019, 12, 855 2 of 13 of (four) dies. This motion is induced by the rotating swaging axle that entrains the base jaws with cams on the top surface. These cams pass the cylinder rollers and push the tools inwards. The swaging exhibitshead exhibits 12 cylinder 12 cylinder rollers. rollers. The oscillation The oscillation of the of dies the is dies initiated is initiated and controlled and controlled by the dimensionsby the anddimensions proportions and proportions of machine toolof machine components tool components in the swaging in the head swaging that constitute head that the constitute pressure the column betweenpressure thecolumn part between and outer the ring part of and the machineouter ring frame. of the Thesemachine components frame. These include components the dies, include the spacers, the basedies, jawsthe spacers, with cams the and base the jaws cylinder with cams rollers. and All the of thesecylinder machine rollers. parts All of feature these technicalmachine tolerances.parts Hence,feature thetechnical geometrical tolerances. deviation Hence, of thethe pressuregeometrical columns deviation that reassembleof the pressure with everycolumns single that stroke generatereassemble a scatterwith every within single the stroke impact generate pattern aon sca thetter surfacewithin the of theimpact part. pattern on the surface of the part.

FigureFigure 1. 1. IllustrationIllustration of ofthe the swaging swaging head head for forinfeed infeed rotary rotary swaging. swaging.

During the the closing closing of of the the swaging swaging dies, dies, the the part part may may be shifted be shifted backwards backwards due dueto the to die the angle die angle in the the reduction reduction zone zone that that is too is too steep steep to ensure to ensure self-locking. self-locking. When Whenthe plas thetic plasticdeformation deformation starts, the starts, thematerial material flows flows mainly mainly in the feed in the direction feed direction (positive (positivematerial flow), material but also flow), in the but negative also in direction the negative direction[4]. The resulting [4]. The sum resulting of the backward sum of theshift backward of the part shiftand the of thenegative part material and the flow negative constitutes material the flow constitutesaxial back pushing. the axial This back back pushing. pushing can This be backmeasured pushing during can the be process measured [5]. It duringis strictly the related process to [5]. the actual tribological conditions in the contact area between the part and dies. Consequently, the It is strictly related to the actual tribological conditions in the contact area between the part and impact pattern is further distorted. Despite this inhomogeneity and the discontinuous nature of the dies. Consequently, the impact pattern is further distorted. Despite this inhomogeneity and the process, the combination of the axial feed per stroke value lst of the part and the circumferential shift discontinuous nature of the process, the combination of the axial feed per stroke value l of the part of the dies between consecutive strokes allows for producing parts with constant external properties,st and the circumferential shift of the dies between consecutive strokes allows for producing parts with like final diameter, length, roundness and surface roughness, over the entire formed length [6]. constantDuring the external process, properties, every spot likeof the final finished diameter, part experiences length, roundness about 100 and effective surface strokes. roughness, over the entireSeveral formed studies length have [6 ].been During carried the out process, to establish every optimal spot process of the conditions finished part concerning experiences surface about 100quality effective and geometry, strokes. e.g., by analyzing the influence of dry process conditions [5], feed per stroke valuesSeveral [6], radial studies feed have per been stroke carried values out during to establish plunge optimal rotary process swaging conditions [7] or true concerning strain [8]. surface qualityFurthermore, and geometry, the internal e.g., properties by analyzing of the the part, influence such as ofits drymicrostructure, process conditions hardness [5 ],and feed residual per stroke valuesstress components, [6], radial feed which per strokeare important values during for its stre plungength, rotary have swagingbeen investigated: [7] or true Lim strain found [8]. Furthermore,that the thehardness internal at propertiesthe outer surface of the part,increased such aswith its microstructure,increasing diameter hardness reduction and residual of steel stresstubes components,[9] and whichKocich arestated important that rotary for its swaging strength, induces have been a strong investigated: fiber texture Lim foundin tungsten that the heavy hardness alloys at [10]. the outer surfaceSimulations increased are used with to investigate increasing the diameter process reduction and the material of steel behavior tubes [9 ]during and Kocich rotary statedswaging. that For rotary swagingexample, inducesthe material a strong flow fiber [4], plastic texture strain in tungsten [11] and heavy strain alloysrates [12] [10]. have Simulations been investigated. are used toThe investigate von theMises process equivalent and thestress material was examined behavior by during Ameli rotaryet al., swaging.who found For it to example, be directly the related material to flowthe [4], plastic strain [11] and strain rates [12] have been investigated. The von Mises equivalent stress was examined by Ameli et al., who found it to be directly related to the residual stresses [13]. They found that the residual stresses were mostly affected by the feed per stroke. Furthermore, Ghaei found, in his Materials 2019, 12, 855 3 of 13

finite element rotary swaging simulation, that the axial stresses of steel tubes could be controlled by the die geometry. Additionally, he showed that tubes featured higher tensile residual stress at the outer surface after rotary swaging without mandrel compared to rotary swaging with mandrel, in both the axial and tangential direction [14]. In general, the residual stresses are strongly related to the plastic strain during rotary swaging and exhibit much larger scattering than the produced geometry. In [15], the evolution of the surface residual stress after rotary swaging reflected the great fluctuations of lubricated as well as dry formed parts. The investigations showed that this inhomogeneity occurred due to the complexity of both the interaction between dies and part as well as the material flow history. The results were also influenced by the feed velocity vf. Another study of the swaged tubes showed that the strain was very high at the outer surface of the tube and decreased strongly towards the inner wall of the tube [16]. Furthermore, rotary swaging could introduce material inhomogeneity, which could be influenced by the process parameters [17]. The main question in this work is whether intrinsic process fluctuations that are generated by technical tolerances of the machine components influence the scattering of the resulting part properties, in particular, the imprinted residual stress components. The employed method to analyze the influence of spreading of the strokes on the part surface was to set the stroke following angle ∆ϕ = 0◦. In this case, the part rotates with the same angular velocity as the swaging axle. In consequence, the same pressure column assemblies hit the same side of the part in a short sequence [18]. Simulations of the rotary swaging process with constant and with varying stroke heights were carried out. In experimental tests, the actual process parameters were monitored during rotary swaging of tubes and the resulting residual stresses of the produced parts were measured afterwards.

2. Methods

2.1. Modelling and Simulation For this study, a 2D axisymmetric model was built in the general finite element software ABAQUS (version 6.14-1, Dassault Systèmes, Vélizy-Villacoublay, France). 2D models are often used due to their short calculation time compared to 3D models. The chosen assumptions were: the material model was based on combined hardening (kinematic and isotropic); the friction coefficient was based on coulomb’s law [19] (lubricated contact pair steel-steel µ = 0.1 [20]) with penalty friction formulation and was assumed to be constant over the complete contact area and during the whole forming; the process was adiabatic, thus the thermal issues including plastic heat generation, heat conduction and heat exchange were neglected; the die was modelled as rigid and the tube as a deformable body. The plastic properties of the part (steel E355) were experimentally determined by tensile tests. Further parameters were d0 = 20 mm, s0 = 3 mm, d1 = 15 mm (see Figure2). In the calculation, a CAX4RT mesh was applied to the part. The mesh size was set to 0.1 mm in the radial direction and to one tenth of the feed per stroke value lst in the axial direction. The feed velocity was set to vf = 2000 mm/min (lst = 0.92 mm). The feeding of the part as well as the stroke of the dies were controlled by a time series (displacement tables). The motion of the dies was designed by the shape of the cam and the diameter of the cylinder rollers. The stroke height (hT) was principally set to 1 mm. To ensure that the material experienced the whole deformation process, the feed length (z) was set to 100 mm. At the end of the dynamic explicit analysis, the dies were removed and the simulation was proceeded by a static general step to release the springback [21] in order to obtain the residual stress components. To investigate the influence of small changes in the process, the simulation was performed with fluctuating die strokes (SIMStroke), as well as without fluctuations (SIMIdeal). A change in the stroke height hT was realized in such a way that every third stroke was decreased by 10 µm, which was the tolerance of the cylinder roller diameter of the rotary swaging machine that was used for experiments. The axial stress component Szz was analyzed at the outer surface of the tubes in the region zanalyzed = 28 mm between the points z1 to z2 without load. Materials 2019, 12, 855 4 of 13 Materials 2019, 12, x FOR PEER REVIEW 4 of 13 Materials 2019, 12, x FOR PEER REVIEW 4 of 13

Figure 2. Principle of the 2D axisymmetric model with the qualitative stress result of SIMIdeal. FigureFigure 2. Principle 2. Principle of the of the2D 2Daxisymmetric axisymmetric model model with with the thequalitative qualitative stress stress result result of SIM of SIMIdeal.Ideal . 2.2. Rotary Swaging Setup 2.2.2.2. Rotary Rotary Swaging Swaging Setup Setup Rotary swaging experiments were carried out with E355 steel tubes (Mannesmann precision RotaryRotary swaging swaging experiments experiments were were carried carried out out with with E355 E355 steel steel tubes tubes (Mannesmann (Mannesmann precision precision tubes,tubes, Helmond, Helmond, Netherlands) Netherlands) with with an aninitial initial length length of l of0 = l3000 = 300± 0.85± mm,0.85 mm,an initial an initial diameter diameter of d0 = of tubes, Helmond, Netherlands) with an initial length of l0 = 300 ± 0.85 mm, an initial diameter of d0 = 20d ±0 0.30= 20 mm± 0.30 and mm an initialand an wall initial thickness wall thicknessof s0 = 3 mm of (as s0 =specified 3 mm (asby the speciﬁed manufacturer). by the manufacturer). These tubes 20 ± 0.30 mm and an initial wall thickness of s0 = 3 mm (as specified by the manufacturer). These tubes These tubes were produced by cold drawing. To eliminate existing residual stresses and ensure a werewere produced produced by by cold cold drawing. drawing. To To eliminate eliminate existing existing residual residual stresses stresses and and ensure ensure a ahomogeneous homogeneous homogeneous microstructure, a normalization heat treatment was carried out in a vacuum furnace microstructure,microstructure, a anormalization normalization heat heat treatment treatment was was carried carried out out in in a avacuum vacuum furnace furnace before before rotary rotary swaging.before rotary The tubes swaging. were Thepurg tubesed with were Nitrogen purged gas, with N Nitrogen2, (2 bars) gas, in a Nvacuum2, (2 bars) furnace in a vacuum (Ipsen, Kleve, furnace swaging. The tubes were purged with Nitrogen gas, N2, (2 bars) in a vacuum furnace (Ipsen, Kleve, (Ipsen, Kleve, Germany) before establishing the vacuum environment. The samples were then Germany)Germany) before before establishing establishing the the vacuum vacuum environm environment.ent. The The samples samples were were then then gradually gradually heated heated to to gradually heated to 890 ◦C and maintained at this temperature for 5 h. After that, they were cooled to 890890 °C °C and and maintained maintained at at this this temperature temperature for for 5 h.5 h. After After that, that, they they were were cooled cooled to to 600 600 °C °C at at 1.5 1.5 K/min K/min 600 ◦C at 1.5 K/min for 2 h, then to 220 ◦C at 5 K/min, and ﬁnally slowly cooled to room temperature. forfor 2 2h, h, then then to to 220 220 °C °C at at 5 5K/min, K/min, and and finally finally sl owlyslowly cooled cooled to to room room temperature. temperature. The The yield yield strength strength The yield strength of the heat treated material was 372 MPa and the tensile strength was 536 MPa. ofof the the heat heat treated treated material material was was 372 372 MPa MPa and and the the te tensilensile strength strength was was 536 536 MPa. MPa. Round Round dies, dies, see see Figure Figure ® ® 3a,Round made dies,by powder-metallurgical see Figure3a, made by ASP powder-metallurgical2023® steel were used ASP for the2023 swaging steel were process. used forThe the die swaging angle 3a, made by powder-metallurgical ASP 2023 steel were◦ used for the swaging process. The die angle ofprocess. the reduction The die zone angle was of α the = 10° reduction and the zone length was of αthe= calibration 10 and the zone length was of l thecal = calibration20 mm. The zone reduction was lcal of the reduction zone was α = 10° and the length of the calibration zone was lcal = 20 mm. The reduction = 20 mm. The reduction zone featured a spray-coated tungsten carbide layer to increase the effective zonezone featured featured a spray-coateda spray-coated tungsten tungsten carbide carbide layer layer to to increase increase the the effective effective friction friction between between the the dies dies friction between the dies and the part in order to reduce the axial reaction force during the process. andand the the part part in in order order to to reduce reduce the the axial axial reaction reaction force force during during the the process. process.

FigureFigure 3. 3.DieDie geometry geometry and and die die assembly assembly of ofthe the closed closed (a ()a round) round dies; dies; (b ()b flat) flat dies. dies. Figure 3. Die geometry and die assembly of the closed (a) round dies; (b) flat dies. The experiments with the stroke following angle ∆ϕ = 0◦ were realized by using flat swaging The experiments with the stroke following angle ∆φ = 0° were realized by using flat swaging dies,The see Figureexperiments3b, made with of the 1.2379 stroke steel. following These dies angle featured ∆φ = 0° a flatwere surface realized in theby using reduction flat swaging as well dies, see Figure 3b, made of 1.2379 steel. These dies featured a flat surface in the reduction as well as asdies, in see the Figure calibration 3b, made zones. of The1.2379 die steel. angle These was dies also featuredα = 10◦. a The flat closedsurface dies in the formed reduction a square as well with as inin the the calibration calibration zones. zones. The The die die angle angle was was also also α α= =10°. 10°. The The closed closed dies dies formed formed a asquare square with with a a dimetera dimeter of the of inner the inner circle circle equal equal to 15 tomm. 15 All mm. experiments All experiments were carried were carriedout with out lubrication, with lubrication, using usingdimeter mineral of the oilinner Condocut circle equal KNR to 22. 15 Duringmm. All swaging, experiments the parts were were carried fixed out in with a hydraulic lubrication, clamping using mineralmineral oil oil Condocut Condocut KNR KNR 22 22 (company, (company, city, city, country). country). During During swaging, swaging, the the parts parts were were fixed fixed in in a a hydraulicdevice. After clamping axial feedingdevice. After of z = ax 130ial mm feeding into theof z swaging = 130 mm unit, into the the swaging swaging axle unit, rotation the swaging was stopped axle tohydraulic prevent clamping additional device. strokes After from ax affectingial feeding the of surface z = 130 andmm theinto part the wasswaging pulled unit, out the of swaging the swaging axle rotationrotation was was stopped stopped to to prevent prevent additional additional strokes strokes from from affecting affecting the the surface surface and and the the part part was was pulled pulled head. The rotary swaging experiments were performed to reduce the initial diameter d0 = 20 mm to outout of of the the swaging swaging head. head. The The rotary rotary swaging swaging expe experimentsriments were were performed performed to to reduce reduce the the initial initial a final diameter/edge length of d1 = 15 mm. In addition to the die geometry, the feed velocity was diameter d0 = 20 mm to a final diameter/edge length of d1 = 15 mm. In addition to the die geometry, diameter d0 = 20 mm to a final diameter/edge length of d1 = 15 mm. In addition to the die geometry, thevaried. feed velocity The main was experimental varied. The settingsmain experimental are given in settings Table1. are Only given one in parameter Table 1. Only was changedone parameter in each experiment.the feed velocity For eachwas varied. set of parameters, The main experimental five samples settings were swaged. are given During in Table the process,1. Only one the rotationparameter of waswas changed changed in in each each experiment. experiment. For For each each set set of of parameters, parameters, five five samples samples were were swaged. swaged. During During the the process,the tubes the formed rotation by of round the tubes dies was formed recorded by roun fromd thedies first was contact recorded of the from tubes the with first the contact dies untilof the the deformationprocess, the rotation was completed. of the tubes formed by round dies was recorded from the first contact of the tubestubes with with the the dies dies until until the the deformation deformation was was completed. completed.

MaterialsMaterials2019, 12 2019, 855, 12, x FOR PEER REVIEW 5 of 135 of 13

Table 1. Design of experiments. Table 1. Design of experiments. Feed Velocity Die Feed per Stroke Stroke Following Experiment Feed Velocity Feed per Stroke Stroke Following Experiment vf (mm/min)Die GeometryGeometry lst (mm) Angle ∆φ vf (mm/min) lst (mm) Angle ∆ϕ R0500 500 round 0.23 arbitrary R0500 500 round 0.23 arbitrary R1000 1000 round 0.46 arbitrary R1000 1000 round 0.46 arbitrary R2000R2000 20002000 roundround 0.920.92 arbitrary arbitrary ◦ F0500 500 flat 0.23 0 F0500 500 flat 0.23 ◦ 0° F1000 1000 flat 0.46 0 F1000 1000 flat 0.46 ◦ 0° F2000 2000 flat 0.92 0 F2000 2000 flat 0.92 0° 2.3. Residual Stress Measurement 2.3. Residual Stress Measurement Residual stress distributions near the surface are important for the fatigue life [22]. The residual stresses wereResidual therefore stress extensivelydistributions measured near the surface at different are important samples for by the X-Ray fatigue diffraction life [22]. The (XRD). residual The measurementsstresses were therefore were performed extensively using measured a GE inspection at different technologies samples (GEby X-Ray inspection diffraction technologies, (XRD). The Ahrensburg,measurements Germany) were diffractometer performed using Type a ETA GE 3003inspecti equippedon technologies with a 2D-detector. (GE inspection A Vanadium technologies, filteredAhrensburg, Cr-Kα radiation Germany) was diffractometer used with a beam Type diameter ETA 3003 of equipped 1 mm. The with resulting a 2D-detector. {211} diffraction A Vanadium peakfiltered of α-iron Cr-K wasα radiation evaluated was using used the with sin 2aψ beam-method diameter [23]. of Axial 1 mm. and The tangential resulting residual {211} diffraction stresses peak wereof measured α-iron was along evaluated a path ofusing 40 mm the onsin the2ψ-method outer surface [23]. Axial of the and tubes tangen withtial astep residual size ofstresses 1 mm were startingmeasured 30 mm along away a from path theof 40 calibration mm on the zone outer (see surface Figure of4 ).the To tubes obtain with additional a step size information of 1 mm starting on the30 microstructure, mm away from the the full calibration width at halfzone maximum (see Figure (FWHM) 4). To ofobtain the diffractionadditional peaksinformation was also on the evaluated.microstructure, In cold forming the full applications width at half of maximum non-hardened (FWH steel,M) of the the FWHM diffraction is strongly peaks was influenced also evaluated. by the coldIn cold working forming and correlatesapplications directly of non-hardened with the hardness steel, (highthe FWHM FWHM is value strongly reflects influenced high hardness), by the cold whichworking allows aand qualitative correlates evaluation directly with of its the distribution hardness (high [24]. TheFWHM residual value stresses reflects and high the hardness), FWHM of which the diffractionallows a qualitative peaks were evaluation measured of on its the distribution selected samples [24]. The from residual each test stresses run. Inand addition the FWHM to the of the surfacediffraction measurements, peaks were analyses measured of residual on the stresses selected and sample FWHMs from were each also performedtest run. In inaddition depth. to For the this, surface measurements, analyses of residual stresses and FWHM were also performed in depth. For this, the the material removal was done by electrochemical etching using a solution of H3PO4 (phosphoric acid) material removal was done by electrochemical etching using a solution of H3PO4 (phosphoric acid) and H2SO4 (sulfuric acid) with a volume ratio of 4:1. and H2SO4 (sulfuric acid) with a volume ratio of 4:1.

Figure 4. Sketch of the residual stress measurement positions (mm) at the outer surface of swaged Figure 4. Sketch of the residual stress measurement positions (mm) at the outer surface of swaged tubes (z—axial residual stresses; x—tangential residual stresses). tubes (z—axial residual stresses; x—tangential residual stresses). 3. Results 3. Results 3.1. Simulation 3.1. Simulation The simulation results reflect the behavior of the material flow during the process. The ratio of kinetic andThe simulation internal energy results gained reflect a the maximum behavior value of the of material 4.5% at theflow very during first the contact process. of the The part ratio of and die,kinetic which and is internal lower thanenergy the gained generally a maximum accepted value limit of of 4.5% 10%. at The the axialvery first stress contact component of the Spartzz, and whichdie, accords which with is lower the axial than residual the generally stress component accepted limit along of the 10%. outer The surface axial stress of the component tube, is shown Szz, in which Figureaccords5. In the with SIM theIdeal axialwith residual constant stress feed component per stroke andalong constant the outer stroke surface height of values,the tube, Szz isat shown the in outerFigure surface 5. fluctuated In the SIM onlyIdeal marginally. with constant However, feed per small stroke changes and inconstant the stroke stroke height height of 10 values,µm already Szz at the led toouter valley surface peaks influctuated the axial only stress marginally. component However, Szz distribution small withchanges a difference in the stroke of about height 20% of (see 10 µm zz SIMStrokealready). led to valley peaks in the axial stress component S distribution with a difference of about 20% (see SIMStroke).

Materials 2019, 12, 855 6 of 13 Materials 2019, 12, x FOR PEER REVIEW 6 of 13

Materials 2019, 12, x FOR PEER REVIEW 6 of 13

Figure 5. Distribution of axial stress component Szz calculated by FEM on the outer surface of the tube from Figure 5. Distribution of axial stress component Szz calculated by FEM on the outer surface of the tube z1 to z2 without (SIMIdeal) and with (SIMStroke) variations of the stroke height values. from z1 to z2 without (SIMIdeal) and with (SIMStroke) variations of the stroke height values. Residual stress components are sensitive to the process parameters and their fluctuations. Such fluctuationsResidualFigure 5. stressDistribution are likely components inof axialrotary stress swaging are component sensitive due toS tozz thecalculated the diameter process by FEM tolerance parameters on the ofou terthe andsurface cylinder their of the rollers fluctuations. tube fromof ± 5 Suchµm fluctuationsz and1 to z the2 without tolerances are (SIM likelyIdeal of )the in and rotary initial with (SIMpart swagingStroke geometry.) variations due to Additionally, the of the diameter stroke vibrations height tolerance values. of of the the machine cylinder can rollers further of ± 5 disturbµmResidual and the the stroke stress tolerances heightcomponents ofduring the initialare rotary sensitive part swaging. geometry. to the process Additionally, parameters vibrations and their fluctuations. of the machine Such can furtherfluctuations disturb are the likely stroke in height rotary duringswaging rotary due to swaging. the diameter tolerance of the cylinder rollers of ± 5 µm3.2. and Process the tolerances Analysis of the initial part geometry. Additionally, vibrations of the machine can further 3.2. Process Analysis disturbBy the using stroke flat height dies, duringthe stroke rotary following swaging. angle ∆φ was forced to be 0°, hence the sample rotated withBy using the same flat velocity dies, the as stroke the swaging following axle and angle the∆ tϕools.was In forced this case, to bethe 0rota◦, hencetion N the over sample the complete rotated 3.2. Process Analysis withforming the same processes velocity was as the equal swaging to 78.8 axle revolutions and the tools.for vf In= 500 this mm/min, case, the 39.4 rotation revolutions N over thefor v completef = 1000 formingmm/minBy processes using and flat 19.7 was dies, revolutions equal the stroke to 78.8 for following revolutionsvf = 2000 angle mm/min. for ∆φ vf was =The 500 forced mean mm/min, tovalue be 0°, 39.4and hence revolutionsthe deviationthe sample for of rotated vthef = part 1000 rotation by swaging with round dies are illustrated in Figure 6. This rotation of the samples in spite mm/minwith the and same 19.7 velocity revolutions as the swaging for vf = axle 2000 and mm/min. the tools. The In this mean case, value the rota andtion the N deviation over the complete of the part rotationformingof its by fixation processes swaging in the withwas feeding equal round deviceto dies 78.8 are wasrevolutions illustrated caused byfor in th v Figureef =rotating 5006 mm/min,. This dies. rotation The 39.4 part revolutions of rotated the samples less for with v inf = spite higher1000 of itsmm/min fixationfeed per inand stroke the 19.7 feeding values revolutions l devicest due for to was thevf = causedreduced 2000 mm/min. by number the rotating The of strokesmean dies. value that The actedand part the on rotated deviationthe tube. less Aof with regressionthe higherpart rotationline for by the swaging part rotation with roundcan be diescalculated are illustrated by equation in Figure 1: 6. This rotation of the samples in spite feed per stroke values lst due to the reduced number of strokes that acted on the tube. A regression of its fixation in the feeding device was caused by the rotating dies. The part rotated less with higher line for the part rotation can be calculated by Equation (1): feed per stroke values lst due to the reduced𝑁 number of strokes that acted on the tube. A regression(1) ° line for the part rotation can be calculated by equationz × φpart 1: The angle φpart is the part rotation, whichN is= induced by◦ closed swaging dies during the rotation of(1) lst × 360 the axle. However, the value of the part rotation deviated from the reference line shown in Figure 6. 𝑁 ° (1) TheThis angle canϕ partbe explainedis the part by rotation, the contact which time isof inducedthe pa rt and by closedthe dies. swaging With higher dies feed during per thestroke rotation values, of theThe axle.the angle part However, isφ partfed is deeperthe the part value into rotation, of the the dies partwhich between rotation is induced consecutive deviated by closed from strokes swaging the and reference φdiespart isduring line higher shown the due rotation in to Figure longer of 6 . Thisthecontact can axle. be However,during explained the the byprocess. value the contact of the part time rotation of the part dev andiated the from dies. the With reference higher line feed shown per strokein Figure values, 6. theThis part can is be fed explained deeper intoby the the contact dies betweentime of the consecutive part and the strokes dies. With and higherϕpart feedis higher per stroke due tovalues, longer contactthe part during is fed the deeper process. into the dies between consecutive strokes and φpart is higher due to longer contact during the process.

Figure 6. Influence of the feed velocity vf on the part rotation N over the complete forming processes

R0500, R1000 and R2000.

FigureFigure 6. 6.Inﬂuence Influence of of the the feed feed velocity velocity vvff on the part rotation rotation N N over over the the complete complete forming forming processes processes R0500R0500,R, R10001000 andand R2000. .

Materials 2019, 12, x FOR PEER REVIEW 7 of 13

3.3. Residual Stress Analysis For most samples that were rotary swaged by round dies, a fluctuating compressive stress was observed along the axial position with few points in the tensile stress area. Residual stress measurements of the tube sample 1 of R0500 showed a periodic fluctuation in both axial and tangential directions,Materials 2019 see, 12, Figure 855 7. The FWHM exhibited small variations, but with nearly the same periodicity7 of 13 as the residual stress values. Most of the samples processed with round dies showed comparable distributions3.3. Residual Stressand all Analysis samples exhibited similar residual stress tendencies in both axial and tangential directions. Therefore, only axial residual stresses were further considered in this work. TheFor mostmaterial samples in its initial that were state rotarywas exposed swaged to by resi rounddual stress dies, arelaxation ﬂuctuating heat compressive treatment, thus stress it canwas be observed assumed along that the the observed axial position residual with stress few fluctuations points in the result tensile from stress the rotary area. swaging Residual process stress asmeasurements postulated in of the the previous tube sample section. 1 of R Furtherm0500 showedore, a additional periodic ﬂuctuation experiments in both were axial performed and tangential using diesdirections, with a seediamond-like Figure7. The carbon FWHM (DLC) exhibited coating small [25] and variations, without but the with rough nearly tungsten the same carbide periodicity coating. as the residual stress values. Most of the samples processed with round dies showed comparable The DLC coating had a thickness of 2.2 µm and an indentation hardness HIT of 18 GPa. Residual stress anddistributions FWHM were and allevaluated samples and exhibited comparable similar fluctuatio residualns stress were tendencies also observed in both at axialthe surface. and tangential Hence, thatdirections. coating Therefore, was not the only cause axial of residual these fluctuations. stresses were further considered in this work.

Figure 7. Axial and tangential residual stresses and FWHM measured along the outer surface of R Figure 7. Axial and tangential residual stresses and FWHM measured along the outer surface of R05000500 sample 1 (the full width at half maximum, FWHM). The material in its initial state was exposed to residual stress relaxation heat treatment, thus it To further investigate these fluctuations, measurements were carried out at several positions can be assumed that the observed residual stress fluctuations result from the rotary swaging process below the surface of a different R0500 sample, sample 2, which showed similar fluctuating residual as postulated in the previous section. Furthermore, additional experiments were performed using stresses as observed in the R0500 sample 1 with different magnitudes (see Figure 8a). In particular, a dies with a diamond-like carbon (DLC) coating [25] and without the rough tungsten carbide coating. highly pronounced residual stress peak between the axial positions of 20 mm and 30 mm shifted the The DLC coating had a thickness of 2.2 µm and an indentation hardness H of 18 GPa. Residual stress residual stress values to tensile. This is however not a singular effect, ITsince the residual stresses and FWHM were evaluated and comparable fluctuations were also observed at the surface. Hence, showed periodic distribution with variable amplitude over the entire analyzed region. This that coating was not the cause of these fluctuations. periodicity was observed in all samples processed with this strategy (see Figures 7–9) and was To further investigate these fluctuations, measurements were carried out at several positions induced by the fluctuation of local deformation during the process. This means that the fluctuations below the surface of a different R sample, sample 2, which showed similar fluctuating residual are a signature of the rotary swaging0500 process in the employed setup. Within few tens of micrometers stresses as observed in the R sample 1 with different magnitudes (see Figure8a). In particular, below the surface, the fluctuation0500 of residual stress values were still persistant but progressively a highly pronounced residual stress peak between the axial positions of 20 mm and 30 mm shifted decreased and the values shifted to tension. In the same way, the FWHM (see Figure 8b) decreased the residual stress values to tensile. This is however not a singular effect, since the residual stresses at 10 µm below the surface and the variations were also progressively reduced between 10 µm and showed periodic distribution with variable amplitude over the entire analyzed region. This periodicity 40 µm in depth. was observed in all samples processed with this strategy (see Figures7–9) and was induced by the fluctuation of local deformation during the process. This means that the fluctuations are a signature of the rotary swaging process in the employed setup. Within few tens of micrometers below the surface, the fluctuation of residual stress values were still persistant but progressively decreased and the values shifted to tension. In the same way, the FWHM (see Figure8b) decreased at 10 µm below the surface and the variations were also progressively reduced between 10 µm and 40 µm in depth.

Materials 2019, 12, x FOR PEER REVIEW 8 of 13

Materials 2019, 12, 855 8 of 13 Materials 2019, 12, x FOR PEER REVIEW 8 of 13

(a) (b) Figure 8. Distributions of the axial residual stresses and FWHM along 40 mm axial paths in different depths

from the outer surface of R0500 sample 2: (a) axial residual stresses; (b) FWHM. Additionally, several samples showed strong scattering and no clear trend of the residual stresses like both samples of condition R2000, which were produced with exactly the same process parameters (compare sample 1 and sample 2 in Figure 9). On the one hand, some of the measured points of both samples exhibited tensile stresses (from 10 mm to 17 mm) and showed large differences between both samples of 276 MPa to −396 MPa. On the other hand, some positions showed comparable values of compressive stresses for both samples (from 30 mm to 40 mm). From these results it can be stated that the process reproducibility in terms of residual stress generation is not given. This low reproducibility of the internal properties could be caused not only by the individuality of the strokes, which is also shown by the simulation results with the change of the stroke height values (SIMStroke), but also possibly by the sensitvity of the process to external factors, such as the initial sample axial deviation or tube wall eccentricity that are common issues in mandrel- free rotary swaging. In addition,(a) the sampling resolution of the X-ray diffraction(b) method (XRD) was too low Figure(about 8. 1Distributions mm) to reflect of the changes axial residual of the stresses residual and FWHMstresses along in the 40 mm range axial of paths the analyzedin different feed depths per Figure 8. Distributions of the axial residual stresses and FWHM along 40 mm axial paths in different stroke valuesfrom the (l stouter < 1 mm).surface Thus, of R0500 the sample fluctuations 2: (a) axial with residual respect stresses; to the(b) FWHM.fluctuations of the process could depths from the outer surface of R sample 2: (a) axial residual stresses; (b) FWHM. be seen only conditionally. 0500 Additionally, several samples showed strong scattering and no clear trend of the residual stresses like both samples of condition R2000, which were produced with exactly the same process parameters (compare sample 1 and sample 2 in Figure 9). On the one hand, some of the measured points of both samples exhibited tensile stresses (from 10 mm to 17 mm) and showed large differences between both samples of 276 MPa to −396 MPa. On the other hand, some positions showed comparable values of compressive stresses for both samples (from 30 mm to 40 mm). From these results it can be stated that the process reproducibility in terms of residual stress generation is not given. This low reproducibility of the internal properties could be caused not only by the individuality of the strokes, which is also shown by the simulation results with the change of the stroke height values (SIMStroke), but also possibly by the sensitvity of the process to external factors, such as the initial sample axial deviation or tube wall eccentricity that are common issues in mandrel- free rotary swaging. In addition, the sampling resolution of the X-ray diffraction method (XRD) was too low (about 1 mm) to reflect changes of the residual stresses in the range of the analyzed feed per stroke values (lst < 1 mm). Thus, the fluctuations with respect to the fluctuations of the process could be seen only conditionally. Figure 9.9. Axial residual stresses and FWHM distribution measured along the outer surface for two

samples (sample 11 andand samplesample 2)2) ofof RR20002000. .

Additionally, several samples showed strong scattering and no clear trend of the residual stresses like both samples of condition R2000, which were produced with exactly the same process parameters (compare sample 1 and sample 2 in Figure9). On the one hand, some of the measured points of both samples exhibited tensile stresses (from 10 mm to 17 mm) and showed large differences between both samples of 276 MPa to −396 MPa. On the other hand, some positions showed comparable values of compressive stresses for both samples (from 30 mm to 40 mm). From these results it can be stated that the process reproducibility in terms of residual stress generation is not given. This low reproducibility of the internal properties could be caused not only by the individuality of the strokes, which is also shown by the simulation results with the change of the stroke height values (SIMStroke), but also possibly by the sensitvity of the process to external factors, such as the initial sample axial deviationFigure or tube 9. Axial wall residual eccentricity stresses that and are FWHM common distribution issues inmeasured mandrel-free along the rotary outer swaging. surface for In two addition, the samplingsamples resolution (sample 1 and of thesample X-ray 2) of diffraction R2000. method (XRD) was too low (about 1 mm) to reflect changes of the residual stresses in the range of the analyzed feed per stroke values (lst < 1 mm). Thus, the fluctuations with respect to the fluctuations of the process could be seen only conditionally. Materials 2019, 12, 855 9 of 13 Materials 2019, 12, x FOR PEER REVIEW 9 of 13

MaterialsInIn order order 2019 toto, 12 determinedetermine, x FOR PEER the REVIEW cause cause of of the the residual residual stress stress fluctuations, fluctuations, process process variations variations using using flat9 of flat13 diesdies were were performed. performed. Contrarily to to the the round round bars bars from from the the previous previous investigations, investigations, the the samples samples In order to determine the cause of the residual stress fluctuations, process variations using flat producedproduced using using thethe flatflat diedie setupsetup exhibited square square sections sections due due to to ∆φ∆ ϕ= 0°,= 0 with◦, with quite quite homogenous homogenous dies were performed. Contrarily to the round bars from the previous investigations, the samples distributionsdistributions ofof residualresidual stresses at at the the outer outer surface, surface, with with values values between between 400 400 MPa MPa and and 500 500MPa MPa produced using the flat die setup exhibited square sections due to ∆φ = 0°, with quite homogenous tensile stress (see Figure 10). This homogeneity could also be observed by the FWHM distribution tensiledistributions stress (see of Figure residual 10). stresses This homogeneity at the outer surface, could also with be values observed between by the 400 FWHM MPa and distribution 500 MPa with small variations around 2.35°◦. For the higher feeding velocity (see Figure 10b), slightly higher withtensile small variationsstress (see Figure around 10). 2.35 This. For homogeneity the higher could feeding also velocity be observed (see Figureby the FWHM10b), slightly distribution higher variations resulted but they were still low compared to the case of using round dies and arbitrary ∆φ. variationswith small resulted variations but they around were 2.35° still low. For comparedthe higher tofeeding the case velocity of using (see round Figure dies 10b), and slightly arbitrary higher∆ϕ . variations resulted but they were still low compared to the case of using round dies and arbitrary ∆φ.

(a) (b) (a) (b) FigureFigure 10. 10. AxialAxial and tangential residual residual stress stresseses and and FWHM FWHM distribution distribution measured measured along along the surface: the surface: (a) F0500 sample 2; (b) F2000 sample 1. (a)F0500Figuresample 10. Axial 2; (b and)F2000 tangentialsample residual 1. stresses and FWHM distribution measured along the surface: (a) F0500 sample 2; (b) F2000 sample 1. FromFrom thethe previousprevious results, it it could could be be observed observed that that rotary rotary swaging swaging using using flat ﬂat dies dies and and a stroke a stroke followingfollowingFrom angleangle the ∆φ∆ previousϕ == 0° 0 ◦ledled results, to tomuch much it could higher higher be homogeneity observed homogeneity that of rotary surface of surfaceswaging residual residual using stresses flat stresses dies than and with than a strokethe with theprevious previousfollowing round roundangle die ∆φ die setup. = setup.0° led Additionally, to Additionally, much higher the thehomogeneity reproducibility reproducibility of surface of the of theresidual process process stresses was was evaluated than evaluated with by the by investigatinginvestigatingprevious three roundthree samples sa diemples setup. processed processed Additionally, with with the the the same same reproducibility process process parameters parameters of the as as process with with the the was ﬂat flat evaluated dies. dies. The The axial by axial residual stress values of three samples of the experiment F0500 are shown in Figure 11. It can be residualinvestigating stress values three of sa threemples samples processed of thewith experiment the same process F0500 are parameters shown in as Figure with the11. flat It can dies. be The seen thatseen theaxial that residual residual the residual stresses stress stressesvalues were of all were three in a all closesamples in a range close of the around range experiment around 450 MPa F 0500 450 to are MPa 600 shown MPa, to 600 in while Figure MPa, the while11. differences It can the be differences in-between one sample was very low and did not exceed 100 MPa. in-betweenseen that one the sample residual was stresses very low were and all did in not a close exceed range 100 around MPa. 450 MPa to 600 MPa, while the differences in-between one sample was very low and did not exceed 100 MPa.

Figure 11. Axial residual stress distribution measured along the surface of three different samples of F0500. FigureFigure 11. Axial11. Axial residual residual stress stress distributiondistribution measured measured along along the surface the surface of three of different three different samplessamples of F0500. of F0500.

Materials 2019, 12, 855 10 of 13

MaterialsMaterials 20192019,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 1010 ofof 1313 Complementary to the surface measurements, residual stress depth profiles were measured at Complementary to the surface measurements, residual stress depth profiles were measured at differentComplementary square section to the workpieces surface measurements, and are presented residual in Figure stress 12depth. The profiles residual were stresses measured in both at different square section workpieces and are presented in Figure 12. The residual stresses in both differentdirections, square and the section FWHM, workpieces changed and significantly are presented in the in Figure first 30 1µ2m. The under residual the surface stresses and in both then directions, and the FWHM, changed significantly in the first 30 μm under the surface and then directionsstabilized, for and deeper the FWHM positions., chang Thise correlatesd significantly well with in theresidual first stress 30 μm depth under profiles the surface at round and samples then stabilized for deeper positions. This correlates well with residual stress depth profiles at round stabilizedand metallographic for deeper examinations positions. This in a correlates previous wellwork with [15] that residual showed stress a layer depth of profiles highly elongated at round samples and metallographic examinations in a previous work [15] that showed a layer of highly samplesgrains at and the surfacemetallographic within a layer examinations of 30 µm inthickness. a previous Comparable work [15 microstructure] that showed wasa layer also of found highly for elongated grains at the surface within a layer of 30 μm thickness. Comparable microstructure was elongatedthe round grains and the at flat the rotary surface swaging within setup a layer in theof 30 present μm thickness study, see. Comparable Figure 13. microstructure was alsoalso foundfound forfor thethe roundround andand thethe flatflat rotaryrotary swagswaginging setupsetup inin thethe presentpresent studystudy,, seesee FigureFigure 1313..

((aa)) ((bb))

Figure 12. Residual stresses and FWHM depth profiles of square section samples: (a) F0500 sample 3; FigureFigure 12. RResidualesidual stress stresseses and FWHM depth profiles proﬁles of square sectionsection samples:samples: ((aa)F) F05000500 samplesample 3 3;; (b) F2000 sample 1. ((bb))F F20002000 samplesample 1. 1.

((aa)) ((bb)) Figure 13. Microstructure of the swaged parts with: (a) round dies with arbitrary ∆φ; (b) flat dies with FigureFigure 13. Microstructure ofof thethe swaged swaged parts parts with: with: (a )(a round) round dies dies with with arbitrary arbitrary∆ϕ ;(∆φb); flat(b) diesflat dies with with ∆φ = 0°. ∆∆φϕ == 0° 0.◦ . AA final final comparison comparison of of axial axial residual residual stresses stresses resulting resulting from from the the different different process process variations variations A final comparison of axial residual stresses resulting from the different process variations investigatedinvestigated inin thisthis studystudy isis shownshown inin FigureFigure 1414 forfor thethe surfacesurface valuesvalues andand thethe valuevalue 300300 μmμm belowbelow investigated in this study is shown in Figure 14 for the surface values and the value 300 µm below thethe surface.surface. TheThe samplessamples thatthat werewere coldcold formedformed withwith thethe roundround diedie setupsetup predominantlypredominantly showedshowed the surface. The samples that were cold formed with the round die setup predominantly showed compressivecompressive axialaxial residualresidual stressesstresses withwith highhigh scattering,scattering, whilewhile thethe samplessamples thatthat werewere coldcold formedformed compressive axial residual stresses with high scattering, while the samples that were cold formed withwith thethe flflatat diedie setupsetup featuredfeatured homogenoushomogenous tensiletensile axialaxial residualresidual stressesstresses atat thethe surface.surface. TheThe averageaverage with the flat die setup featured homogenous tensile axial residual stresses at the surface. The average ofof axialaxial residualresidual stressesstresses waswas aboutabout 300300 MPaMPa forfor allall samplessamples 300300 μmμm belowbelow thethe surfacesurface.. BasedBased onon thesethese of axial residual stresses was about 300 MPa for all samples 300 µm below the surface. Based on results,results, itit seemsseems thatthat inin thethe investigatedinvestigated parameterparameter range,range, tthehe residualresidual stressstress distributiondistribution atat surfacesurface regionsregions is is strongly strongly influenced influenced by by the the process process parameter parameter,, whilewhile the the evolution evolution atat depthdepth is is mostly mostly

Materials 2019, 12, 855 11 of 13 these results, it seems that in the investigated parameter range, the residual stress distribution at Materials 2019, 12, x FOR PEER REVIEW 11 of 13 surface regions is strongly inﬂuenced by the process parameter, while the evolution at depth is mostly independent.independent. These These results results support support the the conclusion conclusionss of of previous previous investigations investigations on on S235 S235 steel, steel, where where a a sisimilarmilar observat observationion was made [15 [15].

Figure 14. Average axial residual stress values at the surface and 300 µm below for Figure 14. Average axial residual stress values at the surface and 300 μm below for different experiments. different experiments.

44.. Conclusions Conclusions InIn this this study, study, infeed infeed rotary rotary swaging swaging experiments experiments with with E355 E355 steel steel tubes tubes were were carried carried out. The out. influenceThe inﬂuence of slightly of slightly fluctuating ﬂuctuating process process parameters parameters on onresidual residual stress stress distribution distribution was was investigated. investigated. From the results, the following conclusions cancan be drawn:

 zz • InIn simulations, the the axial stress component S zz dependsdepends on on the the individual individual stroke stroke during during rotary rotary swaging. FluctuationsFluctuations in in the the stroke stroke height height of a of few a microns, few microns, which which are in the are range in the of range the tolerance of the toleranceof the components of the components of the swaging of the head, swaging lead to head, strong lead fluctuations to strong influctuations the simulated in the residual simulated stress residualdistribution stres ats d theistribution surface. at the surface. • The recorded part rotation N over t thehe complete forming process depends on the set parameter feed per stroke lst. N decreases with a decreasing number of strokes (higher feed velocity vf) but feed per stroke lst. N decreases with a decreasing number of strokes (higher feed velocity vf) but increasesincreases with an increased feed per stroke value duedue toto thethe longerlonger contact.contact.  The residual stress distribution at the surface and in the first few tens of microns is influenced • The residual stress distribution at the surface and in the first few tens of microns is influenced by by each stroke in the process. The fluctuations observed after swaging with round dies (arbitrary each stroke in the process. The fluctuations observed after swaging with round dies (arbitrary ∆φ) can be explained by the slight differences between the round and flat dies due to tolerances ∆ϕ) can be explained by the slight differences between the round and flat dies due to tolerances of the machine components, as seen in the simulation. It can be assumed, that the local plastic of the machine components, as seen in the simulation. It can be assumed, that the local plastic deformation history resulting from the series of individually performed strokes produces local deformation history resulting from the series of individually performed strokes produces local individual residual stress values, at least at the very surface. individual residual stress values, at least at the very surface.  The residual stresses at the surface of the round parts (arbitrary ∆φ) show strong fluctuations • The residual stresses at the surface of the round parts (arbitrary ∆ϕ) show strong fluctuations both in the positive and negative region. They shift with variations in tensile stress under the both in the positive and negative region. They shift with variations in tensile stress under the surface and tend to become homogeneous. surface and tend to become homogeneous.  At the surface, the fluctuation can significantly be reduced at the square parts by controlling the • At the surface, the fluctuation can significantly be reduced at the square parts by controlling the stroke following angle ∆φ to be zero. This measure leads experimentally to a constant stroke stroke following angle ∆ϕ to be zero. This measure leads experimentally to a constant stroke pattern that consists of a sequence of periodic impacts. The use of flat dies with a stroke pattern that consists of a sequence of periodic impacts. The use of flat dies with a stroke following following angle of 0° leads to homogeneous and reproducible residual stresses at the surface. angle of 0◦ leads to homogeneous and reproducible residual stresses at the surface.  A comparison of the global results show that the process parameters (die, stroke following angle • A comparison of the global results show that the process parameters (die, stroke following and feeding velocity) strongly influence the surface residual stress distribution, while the angle and feeding velocity) strongly influence the surface residual stress distribution, while the evolution in depth (300 μm) is almost unchanged for all investigated process variations within evolution in depth (300 µm) is almost unchanged for all investigated process variations within this study. this study. The findings confirm the demand of controlling the process fluctuations in rotary swaging of tubes if beneath geometric features, also internal material properties like residual stresses are pursued. These fluctuations are unavoidable and cumulate in a stroke pattern scattering over the surface of the part. In future work, this scattering should be addressed by controlling the stroke

Materials 2019, 12, 855 12 of 13

The findings confirm the demand of controlling the process fluctuations in rotary swaging of tubes if beneath geometric features, also internal material properties like residual stresses are pursued. These fluctuations are unavoidable and cumulate in a stroke pattern scattering over the surface of the part. In future work, this scattering should be addressed by controlling the stroke following angle ∆ϕ and the feed per stroke value lst. Controlling ∆ϕ is restricted to demands concerning the part geometry. Setting the stroke following angle to zero was a convenient method to demonstrate the feasibility of controlling the residual stresses, but is not suitable for a common rotary swaging process. To yield parts with compressive residual stress and good geometrical properties like roundness, strategies must be developed to satisfy both gains of internal and external part property demands. Hence, the evaluation of residual stress distribution in the complete volume are currently ongoing using neutron diffraction to evaluate the effect of process parameters at a larger scale.

Author Contributions: Conceptualization, M.H., S.I., D.C., C.S. and J.E.; methodology, S.I., L.Y. and M.H.; software, L.Y., S.I. and M.H.; validation, S.I. and D.C.; formal analysis, S.I., D.C. and M.H.; investigation, S.I., D.C., M.H., C.S. and J.E.; resources, B.K. and H.-W.Z.; data curation, S.I. and D.C.; writing—original draft preparation, S.I. and D.C.; writing—review and editing, M.H., C.S., J.E., B.K. and H.-W.Z.; visualization, S.I.; supervision, M.H., C.S. and J.E.; project administration, B.K. and H.-W.Z.; funding acquisition, B.K. and H.-W.Z. Funding: This research was funded by the German Research Foundation (DFG Deutsche Forschungsgemeinschaft) within the sub-project (ZO 140-23-1 and KU 1389/16-1) “Control of component properties in rotary swaging process” of the priority program SPP 2013 “The utilization of residual stresses induced by metal forming”. Acknowledgments: The authors appreciate the support of the China Scholarship Council (CSC). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Altan, T.; Oh, S.I.; Geogel, H.L. Metal Forming: Fundamentals and Applications; American Society for Metals: Materials Park, OH, USA, 1983; p. 17. ISBN 087-170-1677. 2. Rauschnabel, E.; Schmidt, V. Modern applications of radial forging and swaging in the automotive industry. J. Mater. Process. Technol. 1992, 35, 371–383. [CrossRef] 3. Piwek, V.; Kuhfuss, B.; Moumi, E.; Hork, M. Light weight design of rotary swaged components and optimization of the swaging process. Int. J. Mater. Form. 2010, 3, 845–848. [CrossRef] 4. Moumi, E.; Ishkina, S.; Kuhfuss, B.; Hochrainer, T.; Struss, A.; Hunkel, M. 2D-Simulation of Material Flow during Infeed Rotary Swaging Using Finite Element Method. Procedia Eng. 2014, 81, 2342–2347. [CrossRef] 5. Herrmann, M.; Kuhfuss, B.; Schenck, C. Dry Rotary Swaging—Tube Forming. Key Eng. Mater. 2015, 651–653, 1042–1047. [CrossRef] 6. Lim, S.J.; Choi, H.J.; Na, K.H.; Lee, C.H. Dimensional Characteristics of Products Using Rotary Swaging Machine with Four-Dies. Solid State Phenom. 2007, 124–126, 1645–1648. [CrossRef] 7. Kuhfuss, B.; Piwek, V.; Moumi, E. Manufacturing of micro components by means of plunge rotary swaging. In Proceedings of the 9th International Euspen Conference, San Sebastian, Spain, 2–5 June 2009. 8. Kuhfuss, B.; Moumi, E.; Piwek, V. Inﬂuence of the Feed rate on Work Quality in Micro Rotary Swaging. In Proceedings of the 3rd International Conference on Micromanufacturing (ICOMM 2008), Pittsburgh, PA, USA, 9–11 September 2008; pp. 86–91. 9. Lim, S.J.; Choi, H.J.; Lee, C.H. Forming characteristics of tubular product through the rotary swaging process. J. Mater. Process. Technol. 2009, 209, 283–288. [CrossRef] 10. Kocich, R.; Kunˇcická, L.; Dohnalík, D.; Macháˇcková, A.; Šofer, M. Cold rotary swaging of a tungsten heavy alloy: Numerical and experimental investigation. Int. J. Refract. Met. Hard Mater. 2016, 61, 264–272. [CrossRef] 11. Zhang, Q.; Jin, K.; Mu, D.; Ma, P.; Tian, J. Rotary swaging forming process of tube workpieces. Procedia Eng. 2014, 81, 2336–2341. [CrossRef] 12. Liu, Y.; Herrmann, M.; Schenck, C.; Kuhfuss, B. Plastic deformation history in infeed rotary swaging process. In Proceedings of the 20th International ESAFORM Conference on Material Forming (ESAFORM 2017), Dublin, Ireland, 26–28 April 2017. [CrossRef] Materials 2019, 12, 855 13 of 13

13. Ameli, A.; Movahhedy, M.R. A parametric study on residual stresses and forging load in cold radial forging process. Int. J. Adv. Manuf. Technol. 2007, 33, 7–17. [CrossRef] 14. Ghaei, A.; Movahhedy, M.R.; Taheri, A.K. Finite element modelling simulation of radial forging of tubes without mandrel. Mater. Des. 2008, 29, 867–872. [CrossRef] 15. Charni, D.; Ishkina, S.; Epp, J.; Herrmann, M.; Schenck, C.; Zoch, H.W.; Kuhfuss, B. Residual stress generation in rotary swaging. In Proceedings of the 5th International Conference on New Forming Technology (ICNFT 2018), Bremen, Germany, 18–21 September 2018. [CrossRef] 16. Liu, Y.; Herrmann, M.; Schenck, C.; Kuhfuss, B. Plastic Deformation Components in Mandrel Free Infeed Rotary Swaging of Tubes. Procedia Manuf. 2019, 27, 33–38. [CrossRef] 17. Abdulstaar, M.A.; El-Danaf, E.A.; Waluyo, N.S.; Wagner, L. Severe plastic deformation of commercial purity aluminum by rotary swaging: Microstructure evolution and mechanical properties. Mater. Sci. Eng. A 2013, 565, 351–358. [CrossRef] 18. Ishkina, S.; Kuhfuss, B.; Schenck, C. Inﬂuence of the relative rotational speed on component features in micro rotary swaging. Mater. Sci. Eng. Chem. 2015, 21, 1–7. [CrossRef] 19. Tan, X. Comparisons of friction models in bulk metal forming. Tribol. Int. 2002, 35, 385–393. [CrossRef] 20. Herrmann, M.; Schenck, C.; Kuhfuss, B. Graded Structured Tools for Dry Rotary Swaging. Dry Met. Form. Open Access J. 2018, 4, 18–24. 21. Song, F.; Yang, H.; Li, H.; Zhan, M.; Li, G. Springback prediction of thick-walled high-strength titanium tube bending. Chin. J. Aeronaut. 2013, 26, 1336–1345. [CrossRef] 22. Bomas, H.; Schleicher, M. Application of the weakest-link concept to the endurance limit of notched and multiaxially loaded specimens of carburized steel 16MnCrS5. Fatigue Fract. Eng. Mater. Struct. 2005, 28, 983–995. [CrossRef] 23. Epp, J.; Huebschen, G.; Altpeter, I.; Tschuncky, R.; Herrmann, H.-G. X-ray diffraction techniques for material characterization. In Materials Characterization Using Nondestructive Evaluation (NDE) Methods; Woodhead Publishing: Sawston, UK, 2016; pp. 81–124. ISBN 978-0-08-100040-3. 24. Epp, J.; Surm, H.; Hirsch, T.; Hoffmann, F. Residual stress relaxation during heating of bearing rings produced in two different manufacturing chains. J. Mater. Process. Technol. 2011, 211, 637–643. [CrossRef] 25. Hasselbruch, H.; Herrmann, M.; Mehner, A.; Zoch, H.-W.; Kuhfuss, B. Development, characterization and testing of tungsten doped DLC coatings for dry rotary swaging. In Proceedings of the 4th International Conference on New Forming Technology (ICNFT 2015), Glasgow, UK, 6–9 August 2015. [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/).