Microstructural Investigation of a Friction-Welded 316L Stainless Steel with Ultraﬁne-Grained Structure Obtained by Hydrostatic Extrusion

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Article Microstructural Investigation of a Friction-Welded 316L Stainless Steel with Ultraﬁne-Grained Structure Obtained by Hydrostatic Extrusion

Beata Skowro ´nska 1,* , Tomasz Chmielewski 1 , Mariusz Kulczyk 2, Jacek Skiba 2 and Sylwia Przybysz 2

1 Faculty of Production Engineering, Warsaw University of Technology, Narbutta 85, 02-524 Warsaw, Poland; [email protected] 2 Institute of High Pressure Physics, Polish Academy of Sciences (Unipress), Sokołowska 29, 01-142 Warsaw, Poland; [email protected] (M.K.); [email protected] (J.S.); [email protected] (S.P.) * Correspondence: [email protected]

Abstract: The paper presents the microstructural investigation of a friction-welded joint made of 316L stainless steel with an ultrafine-grained structure obtained by hydrostatic extrusion (HE). Such a plastically deformed material is characterized by a metastable state of energy equilibrium, increasing, among others, its sensitivity to high temperatures. This feature makes it difficult to weld ultrafine-grained metals without losing their high mechanical properties. The use of high-speed friction welding and a friction time of <1 s reduced the scale of the weakening of the friction joint in relation to result obtained in conventional rotary friction welding. The study of changes in the microstructure of individual zones of the friction joint was carried out on an optical microscope (OM), scanning Citation: Skowrońska,B.; electron microscope (SEM), transmission electron microscope (TEM) and electron backscattered Chmielewski, T.; Kulczyk, M.; Skiba, diffraction (EBSD) analysis system. The correlation between the microstructure and hardness of the J.; Przybysz, S. Microstructural friction joint is also presented. The heat released during the high-speed friction welding initiated Investigation of a Friction-Welded the process of dynamic recrystallization (DRX) of single grains in the heat-affected zone (HAZ). The 316L Stainless Steel with additional occurrence of strong plastic deformations (in HAZ) during flash formation and internal Ultrafine-Grained Structure Obtained friction (in the friction weld and high-temperature HAZ) contributed to the formation of a highly by Hydrostatic Extrusion. Materials deformed microstructure with numerous sub-grains. The zones with a microstructure other than the 2021, 14, 1537. https://doi.org/ base material were characterized by lower hardness. Due to the complexity of the microstructure 10.3390/ma14061537 and its multifactorial impact on the properties of the friction-welded joint, strength should be the

Academic Editor: Daniel Casellas criterion for assessing the properties of the joint.

Received: 24 February 2021 Keywords: 316L stainless steel; hydrostatic extrusion; rotary friction welding; microstructure; Accepted: 17 March 2021 ultraﬁne-grained metal; recrystallization Published: 21 March 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- The basic criteria determining the suitability of a material for a particular construction iations. are, among others, its rigidity (represented by an appropriate modulus of elasticity) and strength (interpreted in different ways depending on the type of material). It has been well known that strength and toughness of polycrystalline metals are improved with decreasing the grain size. Concerning the strength, Hall-Petch relationship was empirically described Copyright: © 2021 by the authors. in the formula (1). (−1/2) Licensee MDPI, Basel, Switzerland. σy = σ0 + kd (1) This article is an open access article distributed under the terms and where: σy is the yield strength of the polycrystalline material, σ0 the friction stress, k the conditions of the Creative Commons Hall-Petch constant, and d the average grain size [1]. Attribution (CC BY) license (https:// Significant refinement of the microstructure can be achieved through, among others creativecommons.org/licenses/by/ adding more alloying elements and/or applying complex thermomechanical treatments [2]. 4.0/). However, they are costly methods and often their potential for improvement is limited.

Materials 2021, 14, 1537. https://doi.org/10.3390/ma14061537 https://www.mdpi.com/journal/materials Materials 2021, 14, 1537 2 of 19

The solution to the limitations of classic methods of metal strengthening is, e.g., cold plastic processing of metal or more effective metal forming processes with severe plastic deformation (SPD) [3] In these processes, the increase in mechanical properties is achieved by controlled grain refinement at low temperatures [4,5]. The SPD process allows for obtaining high grain refinement to a nanometric level (where the average grain size is below 100 nm). Also, the refinement of grains above 100 nm but below 1 µm (ultrafine- grained materials) causes a significant increase in mechanical properties [6,7]. And the use of such materials as construction materials allows to significantly reduce the weight of the construction or reduce the quantity of expensive elements used as alloying additives. One of the methods that enables strong grain refinement is the hydrostatic extrusion— a plastic deformation process that differs from conventional direct extrusion by the presence of a liquid medium (usually oil) [8]. The results of Sillekens’ research [9] comparing, conventional extrusion with hydrostatic has shown that HE eliminates the disadvantages of the conventional method such as heterogeneity in the degree of deformation and properties of the extruded elements, as well as the formation of a texture gradient (due to process conditions). It has also been observed that the frictional forces occurring during HE are incomparably lower, which significantly lowers the pressures needed for plastic working and there is a smaller temperature increase during hydroextrusion [10,11]. It is i.a. the effect of the three-axis stress state created during the HE process by the medium surrounding the extruded material in the working chamber. Depending on the type of material and the obtained degree of microstructure refinement, the hydrostatic extrusion process is carried out once or several times (resulting in high cumulative deformation). The application of HE to materials, i.e., Al, Al-Si, Ag and Cu [12], silicon bronze (C65500) [13], Ti [14], Ni [15] resulted in, in all cases, an increase in strength of properties. The influence of the hydrostatic extrusion process on changes in the microstructure, mechanical and physical properties in 316L austenitic steel is described in [16,17]. In order to determine the sensitivity to welding (high temperature thermal cycle) of materials with fine grained microstructure obtained by means of plastic deformation, knowledge of the chemical composition and basic mechanical properties is not sufficient. Because the course of the process, i.e., thermomechanical rolling or SPD, also affects the internal energy of the deformed material [18,19]. During cold plastic deformation of polycrystalline materials, some of the energy not converted into heat is stored inside the material (defragmented grains). It mainly accumulates in regions around lattice defects (i.e., dislocations, twins). The increase in free energy over the energy corresponding to the equilibrium state of the deformed material (annealed and slowly cooled) causes that it remains in the metastable equilibrium state (stable at ambient temperature). Therefore, the stored energy of the material significantly affects its thermal stability and sensitivity to welding. Since materials with a greater deviation from equilibrium according to the laws of thermodynamics, they will need a smaller stimulus, e.g., in the form of a thermal pulse at a lower temperature/energy, to bring them back to equilibrium. Regardless of the fusion welding method used, energy is supplied to the material in the form of heat to create a joint. Stored strain energy is also a driving force for processes such as recovery and recrystallization [20]. They cause structural changes that bring the material closer to the state of thermodynamic equilibrium, and the energy stored is gradually released. Although, conventionally recrystallization phenomenon happening during annealing after plastic deformation is the major way of increasing the grain size of metallic materials [21], in the case of semi-finished products with the e.g., Ultrafine- Grained (UFG) structure it is an unfavorable phenomenon. It has been observed that in materials subjected to thermomechanical treatments or deformed by means of severe plastic deformation process, the phenomenon of recrystallization causes the loss of high mechanical properties (refinement of the UFG structure) [22]. Therefore, welding of such materials without losing their high properties, especially in the heat-affected zone, is much more difficult. For joining thermomechanically processed steels with a thickness of 10 mm, the use of hybrid welding methods, i.e., laser + metal active gas (MAG) [23] or plasma + Materials 2021, 14, 1537 3 of 19

MAG [24,25], reduced the amount of heat supplied to the material and limited the range of recrystallization. Also, the weld formed by melting and primer crystallization of the parent material during laser welding was characterized by lower mechanical properties (i.e., hardness and impact toughness) than, for example, a MAG weld (in which, together with the additional material, elements increasing the mechanical properties of the weld are supplied) [26]. Positive effect of arc welding of thermomechanical steels in a water environment on reducing structural degradation in HAZ was also observed [27,28]. The way to reduce the degradation of the microstructure of the ultrafine-grained material can also be the use of one of the methods of the solid-state joining process, such as: rotary friction welding (RFW), linear friction welding (LFW), friction stir welding (FSW) [29,30], and diffusion bonding [31]. These methods are often used to join materials that are difficult to welding [32,33] or have significantly different physical and mechanical properties [34–36]. The undoubted advantage of the solid-state joining process in terms of joining highly deformed materials is that during friction welding of the same materials, the process takes place below its melting point. Despite the advantages of using materials with a fine microstructure and solid-state joining methods, a few publications describe friction welding of UFG and most of them concern joining metals and their alloys with quite high plasticity [37,38]. However, the described results confirm the possibility of obtaining (with appropriately selected parameters) limiting the degradation of the microstructure and the loss of increased properties of the native material in the heat-affected zone. Joining process at weldability of UFG 316L steel after hydrostatic extrusion by means of classic rotary friction welding was not widely used and reported in literature (because of high risk of degradation in HAZ), but the high-speed friction welding (HSFW) method was used in [39,40]. The main advantage of that approach is the possibility of shortening time of welding cycle and narrowing the HAZ. Due to these parameters, during HSFW a high temperature gradient is obtained. Additionally, occurring of high temperature is on friction surface mainly and risk of overheating in HAZ is relatively low. This paper presents an analysis of changes in the microstructure of individual zones of joints caused by the high-speed friction welding process. Additionally, these changes were correlated with the results of hardness measurements in welded joints.

2. Materials and Methods The material subjected to high-speed friction welding was 316L austenitic steel after the hydrostatic extrusion process. The principle of the HE method is presented in Figure1. The billet (1) is placed in a working chamber (2), the remaining chamber space is filled with the pressure transmitting medium (3). The moving piston (4) compresses the medium, increasing the hydrostatic pressure acting on the billet. After obtaining a critical pressure value (characteristic for the extruded material) and after overcoming the frictional resistance of the charge-die, the process of plastic deformation begins. The material is pressed through the hole in the die (5) sliding on the layer made of the working liquid and the lubricant applied to the billet (before the HE process). The extruded product (8) is cooled just behind the die. Depending on the die used, different shapes of profiles and pipes cross-sections, as well as a degree of reduction ratio (R) and true strain (ε) are obtained. True strain in the HE process depends on the ratio of the cross-section before and after the extrusion process and is calculated according to formula [41]: ! ! A d2 = = o = o ε lnR ln ln 2 (2) A f d f

where: R is the degree of reduction, Ao and do—cross-section or diameter of material before extrusion, Af and df—cross-section or diameter of the material after extrusion. Materials 2021, 14, x FOR PEER REVIEW 4 of 19

pipes cross-sections, as well as a degree of reduction ratio (R) and true strain (ε) are obtained. True strain in the HE process depends on the ratio of the cross-section before and after the extrusion process and is calculated according to formula [41]: 2 퐴표 푑표 휀 = 푙푛푅 = 푙푛 ( ) = 푙푛 ( 2) (2) 퐴푓 푑푓 Materials 2021, 14, 1537 4 of 19 where: R is the degree of reduction, Ao and do—cross-section or diameter of material before extrusion, Af and df—cross-section or diameter of the material after extrusion.

Figure 1. Scheme of the process of hydrostatic extrusion. In paper [17], the hydrostatic extrusion of 316L austenitic steel was characterized with In paper [17], the hydrostatic extrusion of 316L austenitic steel was characterized the use of the reduction ratio in the range of 1.6 < R <4.85, the corresponding true strain of with the use of the reduction ratio in the range of 1.6 < R <4.85, the corresponding true 0.47 < ε <1.58 and their impact on changes in the microstructure and mechanical properties strain of 0.47 < ε <1.58 and their impact on changes in the microstructure and mechanical of the material. The friction-welded rods were hydrostatically extruded in one pass with properties of the material. The friction-welded rods were hydrostatically extruded in one R = 3.42, which corresponds to ε = 1.23. As a result, a rod with a diameter of 6 mm was pass with R = 3.42, which corresponds to ε = 1.23. As a result, a rod with a diameter of 6 obtained, its chemical composition and mechanical properties are presented in Table1. mm was obtained, its chemical composition and mechanical properties are presented in Table 1. Chemical compositionTable (wt.%) 1. of the examined stainless steel (manufacturer speciﬁcation) and mechanical properties before and after HE. Table 1. Chemical composition (wt.%) of the examined stainless steel (manufacturer specification) and mechanical propertiesC before Si and after Mn HE. P S N Cr Mo Ni Cu Co 316L 0.017C 0.36Si 1.82Mn 0.30P 0.026S 0.077N 16.88Cr Mo 2.04 10.14Ni 0.38Cu 0.10Co 316L 0.017 0.36 Ultimate1.82 0.30 0.026 Yield0.077 16.88 Elongation2.04 10.14 0.38 0.10 Hardness TensileUltimate Strength YieldStress Elongationto Fracture ε Hardness TensileUTS (MPa) Strength YSStress (MPa) to Fracturef (%) HV0.2 316L UTS 610 (MPa) YS (MPa) 285 εf 65(%) HV0.2 205 316L 316L 1250610 285 1180 11.965 205 355 after316L HE 1250 1180 11.9 355 UFG 316L after HE 1080 1017 0.63 - afterUFG HSFW 316L 1080 1017 0.63 ‒ after HSFW The use of HE resulted in the strong modiﬁcation of the typical 316L rod microstructure afterThe metallurgical use of HE processes—the resulted in the microstructure strong modification with equiaxed of the typical grains 316L (and rod annealing micro- twins)structure and after the formation metallurgical of a strongly processes band—the microstructure. microstructure Figure with2 shows equiaxed microstructure grains (and ofannealing the native twins) material and (after the formation HE), observed of a instrongly the direction band ofmicrostructure. rod extrusion, Figure because 2 furthershows analysismicrostructure of welded of the joints native had material been realized (after HE), in the observed same plane. in the Thedirect applicationion of rod ofextru- the Nomarsion, because System—Differential further analysis Interference of welded joints Contrast had (DIC)been realized revealed in the the topography same plane. of The the bandapplication microstructure of the Nomar (Figure System2b). —Differential Interference Contrast (DIC) revealed the topography of the band microstructure (Figure 2b).

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Figure 2.2. MicrographMicrograph of thethe microstructuremicrostructure of thethe basebase materialmaterial afterafter one-passone-pass HE,HE, longitudinallongitudinal cross-section,cross-section, (a)) opticaloptical observation (b)) withwith Nomar—DifferentialNomar—Differential InterferenceInterference ContrastContrast (DIC).(DIC).

The preparation of the 316L316L steel bars for welding after the HE, consisted ofof cutting into sections with a length of 26 mm, in order to avoid too much heat causing changes in the microstructure of the material, the cutting was made on a band saw with the use of cooling and emulsion lubrication. FrictionFriction-welded-welded joints made of 316L austenitic steel after hydrostatic extrusion were made on the welding machine Harms Wende HWM RSM200 achieving rotational speeds above 6000 rpm. In In order order to select the best set of parameters,parameters, numerous quick friction welding tests were carried out. For fricfriction-weldedtion-welded joints, it is most desirable that the heatheat-affected-affected zone (HAZ) has an even distribution (the same width) along the resulting friction weld. weld. From From the the friction friction welding welding tests tests carried carried out out with with various various sets sets of parameters, of parameters, the the best joint shape and the highest strength at the level of UTS = 1080 MPa and YS = 1017 best joint shape and the highest strength at the level of UTS = 1080 MPa and YS = 1017 MPa MPa (Table1) showed the joint made with the parameters presented in Table2. Therefore, (Table 1) showed the joint made with the parameters presented in Table 2. Therefore, this this friction-welded joint was selected for detailed microstructure tests. friction-welded joint was selected for detailed microstructure tests.

Table 2. Parameters of high-speed friction welding of 316L steel after HE. Table 2. Parameters of high-speed friction welding of 316L steel after HE.

RotationalRotational speed speed set in theset frictionin the friction phase phase 8000 [rpm]8000 [rpm] Friction phase duration 60 [ms] Pressure on the front ofFriction the samples phase in duration the friction phase 255 [MPa]60 [ms] PressurePressure on joint on surfacethe front of theof the samples samples in the in upset the phasefriction phase 318 [MPa]255 [MPa] Pressure on joint surface of the samples in the upset phase 318 [MPa] Frictionally welded joints were tested on the cross-section (along the axis of the Frictionally welded joints were tested on the cross-section (along the axis of the bars). For this purpose, the samples were ﬁrst cut on wire electrical discharge machining bars). For this purpose, the samples were first cut on wire electrical discharge machining (WEDM)—the cutting line was 0.5 mm away from the rod axis to leave an allowance for (WEDM)—the cutting line was 0.5 mm away from the rod axis to leave an allowance for grinding and polishing operations. Samples for OM, SEM and hardness measurements after grindinginclusion and in epoxy polishing resin operations. were wet grinding Samples with for theOM, use SEM of discsand withhardness a grain measurements size of 80 to after2500. inclusion After obtaining in epoxy the resin axis ofwere the bars,wet grinding polishing with was the performed use of discs on a polishingwith a grain cloth size with of 80 to 2500. After obtaining the axis of the bars, polishing was performed on a polishing Al2O3 suspension. Finally, the samples were etched with Mi16Fe (10 mL HNO3 + 30 mL cloth with Al2O3 suspension. Finally, the samples were etched with Mi16Fe (10 mL HNO3 C3H8O3 + 20 mL HF) reagent. Samples for TEM-investigation were taken in the axis of +the 30 rods mL inC3H a cross-section8O3 + 20 mL HF) parallel reagent. to the Samples axis and for perpendicular TEM-investigation to the friction were taken weld. in Thin the axisfoils of were the prepared rods in a from cross disc-shaped-section parallel samples to 3the mm axis in diameter and perpendicular and about 0.15to the mm friction thick. weld.The discs Thin were foils pre-groundwere prepared with from 1000 disc grit-shaped paper and samples electrolytically 3 mm in diameter polished and using about the 0.15TenuPol—5STRUERS mm thick. The discs machine. were pre Preparation-ground with of samples 1000 grit for paper EBSD-investigation and electrolytically included pol- ishedgrinding using of samples the on TenuPol silicon— carbide5STRUERS abrasive machine. papers of decreasing Preparation granulation of samples (from 400 for EBSDto 4000).-investigation The last stage included was polishing grinding with of samples the use ofon a silicon diamond carbide suspension abrasive with papers a grain of

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decreasing granulation (from 400 to 4000). The last stage was polishing with the use of a Materials 2021 14 , , 1537 diamond suspension with a grain size of 3 and 0.25 µm and a SiO2 suspension with6 of 19 a grain size of 0.1 µm. The metallographic observations of the friction-welded joint were made under optical microscope Olympus BX51M (equipped with differential interference contrast), coupled with a digital camera and a computer with Olympus Stream Es- size of 3 and 0.25 µm and a SiO2 suspension with a grain size of 0.1 µm. The metallographic observationssentials software, of the with friction-welded magnification joint from were 25× made to 500 under×. The opticalmicrostructure microscope was Olympus observed BX51Mon a JEOL (equipped JSM-7600F with scanning differential electron interference microscope contrast), (SEM) coupled equipped with with a digital Schottky camera field andemission a computer gun (FEG), with Olympusa JEOL JEM Stream 1200 Essentials EX transmission software, electron with magniﬁcation microscope from(TEM) 25 and× to a 500Quanta×. The 3D microstructure FEG high-resolution was observed SEM equipped on a JEOL with JSM-7600F e.g., electron scanning backscattered electron microscope diffrac- (SEM)tion (EBSD) equipped analysis with Schottkysystem. ﬁeld emission gun (FEG), a JEOL JEM 1200 EX transmission electronMeasurements microscope (TEM) of the and hardness a Quanta distribution 3D FEG high-resolution across the friction SEM- equippedwelded joint with were e.g., electroncarried out backscattered using the Vickers diffraction method (EBSD) on analysis a LEITZ system. MINILOAD 8375 hardness tester at a loadMeasurements of 100 g and a 15 of s the test hardness period. The distribution method can across be used the friction-weldedfor both hard and joint soft were ma- carriedterials. out using the Vickers method on a LEITZ MINILOAD 8375 hardness tester at a load of 100 g and a 15 s test period. The method can be used for both hard and soft materials. 3. Results 3.3.1. Results The Optical Metallography of Friction-Welded Joint 3.1. The Optical Metallography of Friction-Welded Joint Macroscopic observations of the analyzed friction-welded joint (Figure 3) showed the previouslyMacroscopic mentioned observations uniform of the distribution analyzed friction-welded of HAZ along joint the (Figure friction3) weld showed and the no previouslydefects at mentioned the macro uniform level. The distribution friction-welded of HAZ joint along was the friction characterized weld and by no a defects narrow atheat the-affected macro level. zone Thewith friction-welded a width of approx. joint 0.6 was mm characterized and a friction by weld a narrow width heat-affected of less than zone0.2 mm. with a width of approx. 0.6 mm and a friction weld width of less than 0.2 mm.

FigureFigure 3.3. MacrostructureMacrostructure ofof thethe friction-weldedfriction-welded jointjoint fromfrom 316L316L steel steel after after HE. HE.

bending of its base material bands in the radial direction is visible. The high-temperature Materials 2021, 14, x FOR PEER REVIEW 7 of 19

Microstructure studies have revealed that for UFG 316L steel friction-welded joints, the heat-affected zone (HAZ) has a dual character of microstructure. Therefore, in Figure 4, Materials 2021, 14, 1537 7 of 19 an additional division of the HAZ was made into high- and low-temperature HAZ. The low-temperature HAZ is a zone of partial plasticized material, where the bending of its base material bands in the radial direction is visible. The high-temperature HAZ is characterizedHAZ is characterized by a vertical by a verticalarrangement arrangement of the strands of the strands along the along friction the friction weld, in weld, this zonein this the zone material the materialis fully plasticized. is fully plasticized. The weld is The the weld area iswhere the areathe streak where microstruc- the streak turemicrostructure of the base material of the base has materialbeen compl hasetely been lost. completely The observations lost. The of observations the microstructure of the ofmicrostructure the joint with of DIC the showed joint with that DIC the showed weld zone, that in the comparison weld zone, with in comparison the others, with has the mostothers, homogeneous has the most character. homogeneous character.

Figure 4. MicrostructureMicrostructure of of the the friction friction joint joint from from 316L 316L steel steel after after HE HE with with Nomarski Nomarski—Differential—Differential Interference Interference Contrast Con- (DIC)trast (DIC)..

AssumingAssuming that that the the phenomena phenomena occurring occurring during during rotary rotary friction friction welding welding are symmetri- are sym- metrical.cal. In the In case the ofcase homonymous of homonymous joints onjoints both on sides both of sides the weld, of the the weld, same the effect same of effect changes of changesin the microstructure in the microstructure is obtained is (Figure obtained4), therefore(Figure 4), further therefore characteristics further characteristics of the observed of thezones observed of the friction-welded zones of the friction joint have-welded been joint presented have been for one presented side of thefor jointone side only. of the joint only. 3.2. SEM Investigation of Welded Joints 3.2. SEMThe Investigation SEM-investigation of Welded using Joints the lower electron image (LEI) detector was carried out to more accurately illustrate the changes in the microstructure of the material after The SEM-investigation using the lower electron image (LEI) detector was carried out hydrostatic extrusion caused by the high-speed friction welding process. Figure5a to more accurately illustrate the changes in the microstructure of the material after hy- shows a cross-section (at the axis of the rods) of a fragment of friction-welded joint, drostatic extrusion caused by the high-speed friction welding process. Figure 5a shows a while Figure5b–d show the microstructures of individual zones made at 10 times cross-section (at the axis of the rods) of a fragment of friction-welded joint, while Figure 5b–d magnification than in Figure5a, with magnification 2500 ×. showThe the HAZ, microstructures in which the of lower individual temperature zones made occurs, at is 10 characterized times magnification by strongly than bent in Figurestrands 5a, in with the radial magnification direction 2500 (up× to. the ﬂash). Figure5d also shows a strong difference in topographyThe HAZ, ofin thewhich individual the lower strands. temperature A different occurs, conﬁguration is characterized of the by microstructure strongly bent strandswas shown in the by radial the high-temperature direction (up to HAZ the flash). (Figure Figure5c). The 5d friction also shows welding a strong process difference did not incause topography the loss ofof thethe bands, individual but a strands. change inA theirdifferent arrangement configuration from of horizontal the microstr to vertical,ucture wasalong shown the frictional by the high weld.-temperature Additionally, HAZ the vertical(Figure strands5c). The are friction disturbed welding by the process lines thatdid notare mostcause likely the loss the of grain the boundaries. bands, but a In change the friction in their weld, arrangement the streaked from character horizontal of the to vertical,base material along wasthe frictional completely weld. lost Additionally, (Figure5a). In the the vertical presented strands friction-welded are disturbed joint, by it the is linesnot possible that are tomost distinguish likely the interface. grain boundaries. The weld showsIn the friction a homogeneous weld, the microstructure streaked character with ofevenly the base distributed material micro-voids. was completely lost (Figure 5a). In the presented friction-welded joint, it is not possible to distinguish interface. The weld shows a homogeneous microstructure with evenly distributed micro-voids.

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Figure 5. Microstructure of the transverse cross-sectioncross-section of friction-weldedfriction-welded jointjoint ofof UFGUFG 316L steel. SEM-observationSEM-observation in the LEI mode: (a) mag. ×250,250, (b (b––dd) )mag. mag. ××2500,2500, of of friction friction wel weld,d, high high-- and and low low-temperature-temperature HAZ, HAZ, respectively respectively..

The SEM-investigationSEM-investigation alsoalso showedshowed a a different different shape shape and and frequency frequency of microof micro discon- dis- continuitiestinuities in individualin individual zones zones of of the the friction-welded friction-welded joint joint (Figure (Figure5a). 5a). Horizontal Horizontal micro micro discontinuities were were observed observed in in the the base base material, material, their their distribution distribution was was random. random. In the In HAZthe HAZ they they change change their their position position according according to to the the arrangement arrangement of of the the bent bent or vertical material strands. strands. Due Due to to the the friction friction occurring occurring on theon frontthe front surfaces surfaces of the of rods the to rods be joined, to be joined,the micro the discontinuities micro discontinuitie in thes friction in the friction weld have weld a completelyhave a completely different different character character than in thanother in areas other of areas the friction of the friction joint. joint. 3.3. TEM Investigation of Friction-Welded Joint 3.3. TEM Investigation of Friction-Welded Joint The thin foils for TEM investigation were prepared from a transverse cross-section of The thin foils for TEM investigation were prepared from a transverse cross-section the friction-welded joint. Perforations were made in two areas, in the HAZ between the of the friction-welded joint. Perforations were made in two areas, in the HAZ between high- and low-temperature zones and between the friction weld and the high-temperature theHAZ. high A sample- and for low testing-temperature the base zones material and after between hydrostatic the frictionextrusion weld was prepared and the highseparately.-temperature Figure6 HAZ. shows A the sample TEM for images testing in a the bright base ﬁeld material with theafter corresponding hydrostatic extrusion selected wasarea prepared electron diffraction separately. (SAED) Figure patterns6 shows obtained the TEM for images each areain a ofbright a friction-welded field with the joint. corresponding selected area electron diffraction (SAED) patterns obtained for each area of a friction-welded joint. The use of hydrostatic extrusion to plastically-deformed the 316L steel resulted in the formation of a strands structure in the extrusion direction. The width of individual strands is below 500 nm (Figure 6a), while it is not possible to observe the boundaries of individual grains.

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(a) base material (b) low-temperature HAZ

Figure 6. 6. BrightBright field ﬁeld TEM TEM images images with with corresponding corresponding SAED SAED patterns patterns obtained obtained for for(a) base (a) base material material in longitudinal in longitudinal sec- tionsection to HE, to HE, (b) (lowb) low-temperature-temperature HAZ, HAZ, (c) high (c) high-temperature-temperature HAZ, HAZ, (d) friction (d) friction weld weld in transverse in transverse cross cross-section-section of the of fric- the tion-welded joint. friction-welded joint.

InThe the use heat of- hydrostaticaffected zone extrusion (both low to- plastically-deformed and high-temperature, the Figure 316Lsteel 6b,c) resulted, it can be in observedthe formation that the of a conditions strands structure occurring in theduring extrusion high-speed direction. friction The welding, width of despite individual the frictionstrands isphase below of 500 only nm 60 (Figure ms, caused6a), while the it pishenomenon not possible of to dynamicobserve the recrystallization. boundaries of Amongindividual the grains.highly damaged microstructure with numerous dislocation weaves, in which it is impossibleIn the heat-affected to distinguish zone the (both grains low- and and sometimes high-temperature, also the sub Figure-grains,6b,c), the it bounda- can be riesobserved of single that recrystallized the conditions grains occurring can during also be high-speed observed. Frictionfriction welding, weld (Figure despite 6d) the is characterizedfriction phase ofby only strongly 60 ms, defective caused the grains phenomenon full of dislocation of dynamic tangles recrystallization. both close Among to the boundariesthe highly damagedand inside microstructure the grains themselves. with numerous dislocation weaves, in which it is impossible to distinguish the grains and sometimes also the sub-grains, the boundaries of single recrystallized grains can also be observed. Friction weld (Figure6d) is characterized

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by strongly defective grains full of dislocation tangles both close to the boundaries and inside the grains themselves. 3.4. EBSD Investigation of Friction-Welded Joint 3.4. EBSDThe microstructure Investigation of of Friction-Welded the characteristic Joint zones of the friction-welded joint (marked in FigureThe microstructure 4) was also tested of theby EBSD. characteristic The tests zones were performed of the friction-welded at the same height joint (marked—at a indistance Figure4 of) was 1/4 of also the tested radius by from EBSD. the Theedge tests of the were rods, performed without taking at the sameinto account height—at the a distanceheight of of the 1/4 resultin of theg radius flash. fromThe maps the edge from of the the EBSD rods, analysis without showing taking intothe following account the heightmicroscopic of the information resulting ﬂash. are presented The maps below: from grain the EBSD orientation analysis map showing and grains the bound- following microscopicary map. For information the grain areorientation presented maps below: (Figure grain 7), orientation the inverse map pole and figure grains (IPF) boundary was map.used, For showing the grain the three orientation main crystallographic maps (Figure7 directions), the inverse in grains pole distinguished ﬁgure (IPF) was by dif- used, showingferent colors; the three red for main (001), crystallographic green for (101) directions and blue for in grains(111). Figure distinguished 8 shows bythe differentmaps colors;for the red distribution for (001), of green grain for boundaries (101) and in blue the formicrostructures (111). Figure of8 showsthe studied the maps areas. for The the distributionlow-angle gra ofin grain boundaries boundaries with in misorientation the microstructures degree ofof the2°–5° studied were highlighted areas. Thelow-angle in red, grainthe low boundaries-angle grain with boundaries misorientation (LAGBs) degree with of5°– 215°◦–5 in◦ weregreen highlighted and the high in-angle red, thegrain low- angleboundaries grain boundaries(HAGBs) with (LAGBs) misorientation with 5◦–15 degree◦ in greenlarger andthan the15° high-anglewere marked grain in blue. boundaries The (HAGBs)distributions with ofmisorientation grain size and degree misorientation larger than 15 angles◦ were in marked individual in blue. areas The of distributions the fric- oftion grain-welded size and joint, misorientation along with the anglesaverage in values, individual are summarized areas of the and friction-welded presented in Figures joint, along 9 and 10, respectively. with the average values, are summarized and presented in Figures9 and 10, respectively.

(a) base material (b) low-temperature HAZ

FigureFigure 7. 7.Inverse Inverse pole pole ﬁgure figure (IPF)(IPF) mapsmaps for differentdifferent zones zones of of the the friction friction-welded-welded joint joint..

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(a) base material (b) low-temperature HAZ

Figure 8. TheThe micropatterns micropatterns of of the the misorientation misorientation grain grain boundaries boundaries for for different different areas areas of the of theweld weld (corresponding (corresponding to IPF to IPFmaps) maps)..

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Figure 9. Grain size distribution in characteristic zones of friction-weldedfriction-welded joints.joints.

Grain orientation and grains boundary maps conﬁrmed the different nature of changes in the microstructure in the various zones of the friction-welded joint. It was observed that the base material (316L steel after hydrostatic extrusion) is characterized by significantly plastically deformed crystalline microstructure of band character with numerous dislocation tangles and sub-grains (Figure7a), and the fraction of HAGBs was 72% (Figure10a). The average grain size was 1.7 µm, with the largest share of grains below 1 µm (25.5%) and numerous grains below 100 nm (Figure9a). In the heat-affected zone, not only a significant change in microstructure can be observed compared to the base material, but also large differences in the size and arrangement of grains in the separated low- and high-temperature HAZ. In the low-temperature HAZ (Figure7b), single recrystallized grains can be observed among the deformed grains arranged in the direction of rod extrusion and bending of the strands. The average grain size was 5.8 µm (Figure9b). In this zone, the length of the grain boundaries with low- and low-angle misorientation was comparable to the high-angle ones (Figure8b). In

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the high-temperature HAZ (Figure7c), and also in the friction weld (Figure7d), grains partially stretched in the vertical direction, surrounded by numerous equiaxial fine grains, were observed. The average grain size was 2.7 µm in the HAZ (Figure9c) and 2 µm in the friction weld (Figure9d), with almost 50% of the surface of the grains in the friction weld being in the range of 1–2 µm. The friction weld is characterized by the Materials 2021, 14, x FOR PEER REVIEW 13 of 19 most uniform microstructure. In both cases, most of the grain boundaries are HAGBs (Figure8c,d and Figure10c,d).

Figure 10. Distribution of misorientation angles in individual areas of the friction joint (corresponding to the IPF maps in Figure 77))..

GrainFor all orientation tested zones and of the grains friction-welded boundary joint, maps in confirmed the misorientation the different angles nature distribu- of ◦ changestions there in the is a microstructure high peak for thein the angle various of 3 zones(Figure of 10 the), whichfriction is-welded typical joint. for the material afterIt SPD was treatment observed [ 42 that]. However, the base the material misorientation (316L steel angle after size hydrostatic for all of them extrusion) is of the is ◦ characterizedhigh angle type by (above significantly 15 ). The plastically base material deformed and friction crystalline weld are microstructure characterized of by bandgrain ◦ characterboundaries with with numerous the highest dislocation mean value tangles of the and disorientation sub-grains (Figure angle (approx. 7a), and33 the). fraction of HAGBs was 72% (Figure 10a). The average grain size was 1.7 µm, with the largest 3.5. Hardness Measurements share of grains below 1 µm (25.5%) and numerous grains below 100 nm (Figure 9a). In the heat-affectedDue to the zone, results not of only microscopic a significant observations change and in microstructurethe differences observed can be observed between comparedthe analyzed to the zones base of thematerial, friction-welded but also large joint, differences measurements in the of size microhardness and arrangement across the of grainsjoint were in the also separated made. The low measurement- and high-temperature points, apart HAZ. from the In previouslythe low-temperature tested zones HAZ (i.e., (Figurethe weld, 7b), high- single and recrystallized low-temperature grains HAZ can and be observed native material), among werethe deformed also planned grains in thear- ranged in the direction of rod extrusion and bending of the strands. The average grain size was 5.8 µm (Figure 9b). In this zone, the length of the grain boundaries with low- and low-angle misorientation was comparable to the high-angle ones (Figure 8b). In the high-temperature HAZ (Figure 7c), and also in the friction weld (Figure 7d), grains partially stretched in the vertical direction, surrounded by numerous equiaxial fine grains, were observed. The average grain size was 2.7 µm in the HAZ (Figure 9c) and 2 µm in the friction weld (Figure 9d), with almost 50% of the surface of the grains in the friction weld

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being in the range of 1–2 µm. The friction weld is characterized by the most uniform microstructure. In both cases, most of the grain boundaries are HAGBs (Figures 8c,d and 10c,d). For all tested zones of the friction-welded joint, in the misorientation angles distributions there is a high peak for the angle of 3° (Figure 10), which is typical for the material after SPD treatment [42]. However, the misorientation angle size for all of them is of the high angle type (above 15°). The base material and friction weld are characterized by grain boundaries with the highest mean value of the disorientation angle (approx. 33°).

3.5. Hardness Measurements Due to the results of microscopic observations and the differences observed between the analyzed zones of the friction-welded joint, measurements of microhardness across the joint were also made. The measurement points, apart from the previously tested zones (i.e., the weld, high- and low-temperature HAZ and native material), were also planned in the areas in the transition areas between these zones. The measurement was taken near the longitudinal axis of the rod/joint. For the results to be statistically reliable, the measurements were made in four measurement lines. However, due to the small width of the individual zones of the joint, Materials 2021 14 , , 1537 each of the measurement lines was divided into two separate lines in order to avoid14 ofthe 19 impact of the prints with each other. The course of hardness measurements (for 2 measuring lines) is shown in Figure 11. Figure 12 shows the obtained hardness distribution withareas the in marked the transition standard areas deviation. between these zones. The measurement was taken near the longitudinalThe results axis of of the the hardnessrod/joint. measurements showed that the change in the micro- structureFor thecaused results by tothe be friction statistically welding reliable, process the (including measurements the shape were and made orientation in four mea- of thesurement grains lines. and the However, disorientation due to the angle small of width the boundaries of the individual between zones them) of the reduced joint, each the hardness.of the measurement Compared lines to the was base divided material into twowith separate hardness lines of approx.in order 430to avoid HV0.1, the the impact re- mainingof the prints part withof the each friction other. joint The has course a hardness of hardness of approx. measurements 330 HV0.1 (for and 2 measuring small standard lines) deviationis shown inbars Figure indicate 11. Figure a more 12 homogeneous shows the obtained nature hardness of their microstructure. distribution with the marked standard deviation.

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Figure 11. Image showing the course of hardness measurements. Figure 11. Image showing the course of hardness measurements.

Figure 12. Hardness distribution in the friction-welded joint made of 316L steel after hydrostatic extrusion. Figure 12. Hardness distribution in the friction-welded joint made of 316L steel after hydrostatic extrusion. The results of the hardness measurements showed that the change in the microstruc- ture4. Discussion caused by and the Conclusions friction welding process (including the shape and orientation of the grains4.1. Base and Material the disorientation angle of the boundaries between them) reduced the hardness. ComparedPerforming to the basehydrostatic material extrusion with hardness (in one of pass) approx. of 430316L HV0.1, steel resultedthe remaining in obtaining part of therods friction with high joint mechanical has a hardness properties of approx. (Table 330 1). HV0.1 As a andresult small of subjecting standard deviationthe material bars to indicatehigh deformation, a more homogeneous it is characterized nature by of a their limited microstructure. possibility for further plastic processing and an increased sensitivity to heat [43]. Less thermal stability is caused by the strong deformation and grain refinement, the accumulation of additional energy in the material and limitation of the mobility of grain boundaries [44]. Microscopic tests have shown that in the case of 316L steel, the hydrostatic extrusion process causes a strong deformation of the equiaxial grains and the obtaining of a band microstructure. In the case of other materials subjected to this process, such microstructure was not obtained. The HE process caused such a strong deformation of the grains in 316L steel that it was not possible to fully determine their size even during TEM research. Nevertheless, it was observed that the width of individual bands is less than 500 nm, the clear boundaries between them are the slip planes, and inside the strands, areas with different dislocation density and sub-grains can be distinguished. The EBSD studies showed that HAGBs accounted for 72% of the base material, while among grain boundaries with a misorientation angle <15° as much as 22% accounted for the misorientation angle 3°. The average grain size was 1.7 µm, with 25.5% of grains below 1 µm. Numerous equiaxial grains with an area below 100 nm were observed around the deformed grains. The base material has a hardness of 430 HV0.1, but it is also characterized by considerable heterogeneity, as evidenced by the values of the standard deviation marked in the chart.

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4. Discussion and Conclusions 4.1. Base Material Performing hydrostatic extrusion (in one pass) of 316L steel resulted in obtaining rods with high mechanical properties (Table1). As a result of subjecting the material to high deformation, it is characterized by a limited possibility for further plastic processing and an increased sensitivity to heat [43]. Less thermal stability is caused by the strong deformation and grain reﬁnement, the accumulation of additional energy in the material and limitation of the mobility of grain boundaries [44]. Microscopic tests have shown that in the case of 316L steel, the hydrostatic extrusion process causes a strong deformation of the equiaxial grains and the obtaining of a band microstructure. In the case of other materials subjected to this process, such microstructure was not obtained. The HE process caused such a strong deformation of the grains in 316L steel that it was not possible to fully determine their size even during TEM research. Nevertheless, it was observed that the width of individual bands is less than 500 nm, the clear boundaries between them are the slip planes, and inside the strands, areas with different dislocation density and sub-grains can be distinguished. The EBSD studies showed that HAGBs accounted for 72% of the base material, while among grain boundaries with a misorientation angle <15◦ as much as 22% accounted for the misorientation angle 3◦. The average grain size was 1.7 µm, with 25.5% of grains below 1 µm. Numerous equiaxial grains with an area below 100 nm were observed around the deformed grains. The base material has a hardness of 430 HV0.1, but it is also characterized by considerable heterogeneity, as evidenced by the values of the standard deviation marked in the chart.

4.2. Heat-Affected Zone In the HAZ, the dual nature of the microstructure was observed. The formation of two microstructures is primarily caused by a different amount of heat supplied to a given area during the friction phase of the joining process, therefore an additional division was made into low-temperature and high-temperature HAZ. In the literature [45] occur other divisions of the HAZ friction joint, e.g., the author divided the joint into the following zones: contact zone, fully plasticized zone, partly deformed zone, undeformed zone, thermomechanically treated [46]. In the case of the low-temperature heat-affected zone, bending of the base material strands was observed. The deformation of the bands from the horizontal to the vertical direction is primarily caused by the second stage of the friction welding process—the upsetting phase, in which the set pressure (upsetting pressure) increases, causing the plasticized material to be squeezed out and a flash formed. The supplied heat allowed for deformation of the material in this zone, but also resulted in the loss of the effect of the lamellar microstructure observed during the TEM- investigation of the base material. Among the highly deformed microstructure of full dislocation tangles, it was possible to observe randomly spaced sharp boundaries of partially recrystallized grains. In the case of a high-temperature HAZ, the microstructure is still streaked—strands arranged along the weld. During SEM tests it was observed that they are cut by grain boundaries. TEM investigation reveals partially disappearance of sub-grains. The EBSD studies confirmed the presence of recrystallized grains among the strongly deformed (elongated) grains. The low-temperature HAZ was characterized by the highest average grain size of 5.8 µm, with a grain size share below 1 µm at the same level of 15% as for the high-temperature HAZ, with an average grain size of 2.7 µm. In both HAZ regions, the average grain boundary misorientation angle was below 30◦, and the only zone in the friction joint with a comparable proportion of LAGBs and HAGBs boundaries is the low-temperature HAZ. However, in the entire HAZ, the share of boundaries with a misorientation angle of 3◦ remained above 20%. According to Humphrey’s theory on the influence of the nature of grain boundaries, LAGB subgrain structures are inherently unstable, and structures with medium and high misorientation angles may show greater Materials 2021, 14, 1537 16 of 19

stability [47]. Therefore, the nature of the grain boundaries also inﬂuences the thermal stability of UFG materials. Regardless of the nature of changes in the observed microstructure, the heat generated in the friction welding process softened the material. In the HAZ zone, the hardness decreased by an average of 100 HV0.1. However, the HAZ zone has a more homogeneous microstructure than the base material (lower standard deviation bars of mean hardness value).

4.3. Friction Weld As a result of the mutual friction of the faces of the welded bars and the generated heat, the friction weld has a completely different microstructure from the other areas of the friction-welded joints. The occurrence of dynamic recrystallization and mutual shear of the newly formed grains (due to the frictional moment) resulted in the formation of a homogeneous, ﬁne-grained but also highly damaged (numerous dislocations inside and at the grain boundaries) microstructure. The averageUSV grain Symbol size Macro(s)was 2 µm, and the grain Description size distribution assumed the shape of the Gaussian distribution, where 47% of the grains 0241 Ɂ \textglotstopvari LATIN CAPITAL LETTER GLOTTAL STOP have a surface area of 1–2 µm. The mean angle of misorientation was 32.5◦, the value 0242 ɂ \textglotstopvarii LATIN SMALL LETTER GLOTTAL STOP results, among others, from a large share of HAGBs at the level of 70% and a reduced to 0243 Ƀ \textbarcapitalb LATIN CAPITAL LETTER B WITH STROKE 16% share of grain boundaries with a misorientation angle of 3◦. Despite the fact that such 0244 Ʉ \textbarcapitalu LATIN CAPITAL LETTER U BAR a microstructure was obtained, the friction weld did not have a different average hardness 0245 Ʌ \textturnedcapitalv LATIN CAPITAL LETTER TURNED V than the HAZ. 0246 Ɇ \textstrokecapitale LATIN CAPITAL LETTER E WITH STROKE 0247 ɇ \textstrokee LATIN SMALL LETTER E WITH STROKE 4.4. Micro Discontinuities of the Friction Weld Joint 0248 Ɉ \textbarcapitalj LATIN CAPITAL LETTER J WITH STROKE During the SEM-investigation, micro discontinuities0249 ɉ of a different\textbarj nature were ob- LATIN SMALL LETTER J WITH STROKE

served in individual zones of the friction joint, but024A it is difficultɊ to\texthtcapitalq clearly define the cause LATIN CAPITAL LETTER SMALL Q WITH HOOK TAIL of their occurrence. 024B ɋ \texthtq LATIN SMALL LETTER Q WITH HOOK TAIL According to Erbel [48], in the case of materials024C subjectedɌ \textbarcapitalr to the SPD process, the LATIN CAPITAL LETTER R WITH STROKE dislocation movement (as the basic mechanism of024D plastic deformation)ɍ \textbarr does not disturb the LATIN SMALL LETTER R WITH STROKE integrity of the material. Only the hindrance of the024E dislocationɎ movement\textbarcapitaly by, for example, LATIN CAPITAL LETTER Y WITH STROKE some grain boundaries or foreign inclusions create024F the riskɏ of material\textbary discontinuity. Tests LATIN SMALL LETTER Y WITH STROKE carried out by Erbel, consisting in twisting (in0250 a specialɐ device)\textturna with arbitrarily large LATIN SMALL LETTER TURNED A deformation of aluminum (99.5%), Armco iron and0251 M63 brassɑ confirmed\textscripta that applying a LATIN SMALL LETTER ALPHA sufficiently high 3-axis uniform compression prevents0252 theɒ formation\textturnscripta of dislocation gaps LATIN SMALL LETTER TURNED ALPHA inside the grains. On the other hand, discontinuities0253 occurredɓ in impurities\m{b} clustered at the LATIN SMALL LETTER B WITH HOOK \m{b} grain boundaries or crystals of the less plastic phase (in the case of\texthtb M63). Loss of cohesion of plastic materials, deformed with the participation\textbhook of hydrostatic pressure (according to Erbel) takes place in the following0254 stages:ɔ 1.\m{o} It begins with cracking LATIN SMALL LETTER OPEN O \textopeno (brittle nature) of less plastic components, with relatively small deformations.\textoopen 2. The spread of discontinuities is related to the sliding along the0255 grain boundaries,ɕ \textctc taking place at much LATIN SMALL LETTER C WITH CURL

greater deformations. The less plastic components,0256 withoutɖ deforming,\M{d} are an obstacle LATIN SMALL LETTER D WITH TAIL to displacement until they crumble. In this way, fractures of relatively\textrtaild large dimensions are formed along the grain boundaries, corresponding to the grain\textdtail dimensions of the less 0257 ɗ \m{d} LATIN SMALL LETTER D WITH HOOK plastic components. \texthtd At large deformations, when the texture that creates the grain\textdhook boundaries coincide with the operating direction τmax (as in torsion or0258 hydrostaticɘ extrusion),\textreve the resulting gap LATIN SMALL LETTER REVERSED E systems will be in parallel planes. 0259 ə \schwa LATIN SMALL LETTER SCHWA Similar material discontinuities were observed by Klassek et\textschwa al. [49] in 316LVM steel 025A ɚ \m{\schwa} LATIN SMALL LETTER SCHWA WITH HOOK after hydrostatic extrusion. During the tests of the charge material,\texthookabove{\schwa} despite the high purity of the steel, multi-phase inclusions, i.e., Al2O3, MnS and other\textrhookschwa Si-rich inclusions, were demonstrated. The process of hydrostatic extrusion025B (with ɛkum =\m{e} 1.84) caused a change in LATIN SMALL LETTER OPEN E the shape of inclusions—they were elongated in the extrusion direction\textepsilon and decreased in \texteopen the cross-section. \textniepsilon According to the theory and results quoted,025C the applicationɜ \textrevepsilon of the friction welding LATIN SMALL LETTER REVERSED OPEN E process caused further deformation of the micro025D discontinuities,ɝ \texthookabove{\textrevepsilon} but their ﬁnal shape in a LATIN SMALL LETTER REVERSED OPEN E WITH HOOK \textrhookrevepsilon 025E ɞ \textcloserevepsilon LATIN SMALL LETTER CLOSED REVERSED OPEN E 025F ɟ \B{j} LATIN SMALL LETTER DOTLESS J WITH STROKE \textbardotlessj \textObardotlessj 0260 ɠ \texthookabove{g} LATIN SMALL LETTER G WITH HOOK \texthtg 0261 ɡ \textscriptg LATIN SMALL LETTER SCRIPT G

0262 ɢ \textscg LATIN LETTER SMALL CAPITAL G 0263 ɣ \m{g} LATIN SMALL LETTER GAMMA \textbabygamma \textgammalatinsmall \textipagamma 0264 ɤ \textramshorns LATIN SMALL LETTER RAMS HORN

0265 ɥ \textturnh LATIN SMALL LETTER TURNED H

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given area of the friction-welded joint depended on the conditions prevailing in this area. Unambiguous determination of the causes of the observed micro discontinuities requires additional research. Conducting the process of high-speed friction welding of 316L steel after hydrostatic extrusion with a short friction time (60 ms) and rotational speed of 8000 rpm caused the release of heat, which initiated the process of recrystallization of single grains in heat-affected zone. The additional occurrence of strong plastic deformations (in HAZ) during flash formation and internal friction (in the friction weld and high-temperature HAZ) contributed to the formation of a highly deformed microstructure with numerous sub-grains. A number of phenomena determining the properties of the joint, but interacting in the opposite way, have been observed. The friction-welded joint is characterized by a gradient of many properties, including structure directivity, degree of recrystallization, average grain size, and hardness. The sub-grain structure significantly hinders the absolute measurement of grain size. And the obtained results are also relative in nature related to the magnification used for the observation. The zones with a microstructure other than the base material were characterized by lower hardness. However, regardless of the nature of the changes in the microstructure, the level of hardness reduction in all zones was similar with a mild course of changes. Due to the complexity of the microstructure and its multifactorial impact on the properties of the friction-welded joint, strength should be the criterion for assessing the properties of the joint.

Author Contributions: Conceptualization, B.S. and T.C.; methodology, J.S. and M.K.; formal analysis, S.P.; investigation, B.S. and T.C.; resources, M.K.; writing—original draft preparation, B.S. and T.C., writing—review and editing, T.C. and B.S.; visualization, J.S. and S.P.; supervision, B.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Experimental methods and results are available from the authors. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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