Study on the Effect of Energy-Input on the Joint Mechanical Properties of Rotary Friction-Welding

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Article Study on the Effect of Energy-Input on the Joint Mechanical Properties of Rotary Friction-Welding

Guilong Wang 1,2, Jinglong Li 1, Weilong Wang 1, Jiangtao Xiong 1 and Fusheng Zhang 1,*

1 Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi’an 710072, China; [email protected] (G.W.); [email protected] (J.L.); [email protected] (W.W.); [email protected] (J.X.) 2 State Key Laboratory of Solidiﬁcation Processing, Northwestern Polytechnical University, Xi’an 710072, China * Correspondence: [email protected]; Tel.: +86-029-8849-1426

Received: 17 October 2018; Accepted: 3 November 2018; Published: 6 November 2018

Abstract: The objective of the present study is to investigate the effect of energy-input on the mechanical properties of a 304 stainless-steel joint welded by continuous-drive rotary friction-welding (RFW). RFW experiments were conducted over a wide range of welding parameters (welding pressure: 25–200 MPa, rotation speed: 500–2300 rpm, welding time: 4–20 s, and forging pressure: 100–200 MPa). The results show that the energy-input has a significant effect on the tensile strength of RFW joints. With the increase of energy-input, the tensile strength rapidly increases until reaching the maximum value and then slightly decreases. An empirical model for energy-input was established based on RFW experiments that cover a wide range of welding parameters. The accuracy of the model was verified by extra RFW experiments. In addition, the model for optimal energy-input of different forging pressures was obtained. To verify the accuracy of the model, the optimal energy-input of a 170 MPa forging pressure was calculated. Three RFW experiments in which energy-input was equal to the calculated value were made. The joints’ tensile strength coefficients were 90%, 93%, and 96% respectively, which proved that the model is accurate.

Keywords: rotary friction-welding; energy-input; joint mechanical properties

1. Introduction Rotary friction-welding (RFW) is a method of manufacturing that has been used extensively in recent times due to its advantages such as low heat input, production time, ease of manufacture, and environment friendliness [1,2]. The main parameters of rotary friction-welding are welding pressure, welding rotational speed, welding time, and forging pressure [3,4]. The influence of welding parameters on joint performance has always been a research focus of rotary friction-welding, whether in terms of numerical models or experiments. According to previous literature, it has been found that the RFW joints’ tensile strength increases with the increase of friction time until it reaches a maximum point, after which the tensile strength decreases [5]. The welding time determines the amount of heat flux in the joint and the width of the heat-affect zone (HAZ), which increases as the welding time increases [6]. Regarding welding pressure, it was found that friction pressure has a significant effect on tensile strength and the width of HAZ, both of which increased along with the increase in friction pressure until reaching a certain value [7]. In addition, friction pressure also affects the temperature gradient, the required weld power, and the burn-off rate during the RFW process [2]. For the rotational speed, numerical analyses found that an increase in rotational speed causes the weld interface to reach a quasi-stable temperature quicker, which starts material extrusion earlier and increases the upsetting rate remarkably [8]. The rotational speed also affects the heat input of the joint; thus, it has a significant

Metals 2018, 8, 908; doi:10.3390/met8110908 www.mdpi.com/journal/metals Metals 2018, 8, 908 2 of 14 Metals 2018, 8, x FOR PEER REVIEW 2 of 14 effectjoint; on thus, the notch it hastensile a significant strength effect and on impact the notch toughness tensile[ strength9]. Moreover, and impact the overall toughness width [9]. of the Moreover, weld zonethe reduces overall withwidth the of increasethe weld of zone rotational reduces speed with [ 10the]. increase of rotational speed [10]. AsAs discussed discussed above, above, the the parameters parameters of weldingof welding time, time, welding welding pressure, pressure, and and rotational rotational speed speed all all havehave a significanta significant effect effect on on joint joint performance performance when when considered considered separately. separately. However, However, in in practical practical applicationsapplications of of RFW, RFW, it isit oftenis often necessary necessary to adjustto adjust several several welding welding parameters parameters simultaneously simultaneously accordingaccording to to the the actual actual condition, condition, such such as as the the welding welding material material characteristics characteristics and and the the welding welding machine’smachine’s condition. condition. Therefore, Therefore, it it isis necessarynecessary to evaluate the the significance significance of of each each parameter parameter on on joint jointperformance performance when when considering considering all all parameters parameters synt synthetically.hetically. SuchSuch anan understanding understanding will will help help to to improveimprove the the weld weld quality, quality, to eliminate to eliminate weld weld defects defec andts and transfer transfer knowledge knowledge of welding of welding procedures procedures to newto conditionsnew conditions (e.g., for (e.g new., for geometry new geometry or new materials).or new materials). It is generally It is knowngenerally that known welding that pressure welding andpressure rotation and speed rotation determine speed the determine characteristics the characteristics of the power of inputthe power during input the during RFW process, the RFW and process, the integraland the of powerintegral for of weldingpower for time welding is the energy-input.time is the energy-input. For the RFW For method, the RFW heat method, flux is generated heat flux is bygenerated the conversion by the of conversion mechanical of energy mechanical into thermal energy energyinto thermal at the energy interface at ofthe the interface work pieces of the [ 11work]. Inpieces addition, [11]. the In amountaddition, of the energy-input amount of energy-inpu dictates a successfult dictates a welding successful process, welding the process, quality the of joint,quality theof shape joint, of the flash, shape the of micro-structure, flash, the micr aso-structure, well as the as residual well as stressthe residual [12,13]. stress Therefore, [12,13]. it isTherefore, reasonable it is to usereasonable the energy-input to use asthe an indexenergy-input to characterize as an the jointindex performance to characterize comprehensively. the joint Inperformance this work, thecomprehensively. effect of the energy-input In this work, on the the joint’s effect mechanicalof the energy-input property on was the investigated; joint’s mechanical an energy-input property was modelinvestigated; as a function an energy-input of welding pressure,model as rotationala function speed,of welding and pressure, welding timerotational was obtained, speed, and and welding the accuracytime was of theobtained, model and was the verified accuracy by experimentalof the model wa data.s verified The model by experimental that describes data. the The variation model of that optimaldescribes energy-input the variation with of forging optimal pressure energy-input also was with established, forging pressure and an excellent also was performance established, jointand an wasexcellent obtained performance based on the joint calculated was obtained optimal based energy-input on the calculated value. optimal energy-input value.

2. Experimental2. Experimental Procedures Procedures TheThe specimen specimen used used in the in present the present investigation investigation was 304 stainlesswas 304 steelstainless (304SS) steel rods (304SS) with diameters rods with of 25diameters mm. The of specimens 25 mm. The were specimens welded by were the C320 welded (Friction by the Welding C320 (Friction Eng. & Tech. Welding Co. Ltd., Eng. Hanzhong, & Tech. Co. Shaanxi,Ltd., Hanzhong, China) continuous-drive Shaanxi, China) rotary continuous-drive friction-welding rotary machine friction-welding (maximum machine of 45 kW (maximum and 320 kN of 45 forgingkW and load). 320 AccordingkN forging to load). our preliminary According to experiments our preliminary and the experiments literature onand 304SS the literature RFW [5, 6on,9, 14304SS], theRFW welding [5,6,9,14], experiments the welding were conductedexperiments using were welding conducted parameters using welding in which parameters the welding in pressure which the variedwelding from pressure 40 MPa tovaried 200 MPa, from rotational40 MPa to speed 200 MPa, varied rotational from 500 speed rpm varied to 2300 from rpm, 500 and rpm welding to 2300 time rpm, variedand welding from 4 s time to 20 varied s, all while from under4 s to 20 forging s, all whil pressurese under of forging 100 MPa, pressures 120 MPa, of 100 140 MPa, MPa, 120 160 MPa, MPa, 140 180MPa, MPa, 160 and MPa, 200 MPa,180 MPa, respectively. and 200 DuringMPa, respective the RFWly. process, During the the welding RFW process, power was the recordedwelding bypower a computerwas recorded through by ana computer A/D converter through with an A/D a sampling converte timer with of 0.01a sampling s, and thetime surface of 0.01 temperature s, and the surface of thetemperature joint was measured of the by joint an infrared was measured thermal imaging by an instrument infrared (Tecthermal VarioCAM@hr, imaging instrument head-HS Infra, (Tec Dresden,VarioCAM@hr, Germany) head-HS with a samplingInfra, Dresden, time of Germany) 0.02 s. The with data a collectedsampling bytime the of infrared 0.02 s. The thermography data collected instrumentby the infrared had been thermography calibrated using instrument the data that had thermocouple been calibrated collected, using andthe thedata emissivity that thermocouple of 304SS wascollected, determined and to the 0.8. emissivity Figure1 shows of 304SS the location was determin of the experimentaled to 0.8. Figure equipment 1 shows and the the location diagram of of the theexperimental temperature-detected equipment area. and the diagram of the temperature-detected area.

(a) (b)

temperature detected area

FigureFigure 1. The1. The (a) ( imagea) image of experimentalof experimental equipment equipment and and (b) ( diagramb) diagram of temperatureof temperature detected detected area. area.

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AfterAfter welding, welding, the the the axial axial axial cross-section cross-section cross-section of of of the the the joint joint joint was was was obtained obtained obtained for for for microstructural microstructural microstructural examination. examination. examination. TheThe test test test specimens specimens specimens for for for Optical Optical Optical Microscope MicroscopeMicroscope examinat examinationexaminationion were were mounted mounted for for for polishing, polishing, polishing, then then then etched etched etched by by aby areagent areagent reagent of of 5 of 5g 5gof gof FeCl ofFeCl FeCl3 dissolved3 dissolved3 dissolved in in dilute indilute dilute hydrochloric hydrochloric hydrochloric acid acid acid(15 (15 mL (15 mL HCl mL HCl HCl+ +60 60 +mL 60mL H mL 2HO).2O). H The2 O).The microstructureThemicrostructure microstructure of of the ofthe thespecimens specimens specimens was was was observed observed observed us us usinginging a apolarizing polarizingpolarizing microscope microscopemicroscope (Olympus(Olympus (Olympus PMG3, PMG3, PMG3, Missouri,Missouri, TX, TX, USA), USA), and and the the microhardness microhardness was was meas measured measuredured on on the the cross-section cross-section perpendicular perpendicular to to the the frictionfrictionfriction interface interface interface using using using a a microhardnessa microhardness microhardness tester testertester (Shimadzu (Shimadzu(Shimadzu HMV-2T, HMV-2T,HMV-2T, Dallas, Dallas,Dallas, TX, TX, USA) USA) USA) under under under a loada a load load of of 2of 2N. 2N. N.The The The microhardness microhardness microhardness test test area area includes includesincludes all allall of ofof the thethe weldingwelding welding zone. zone. zone. Tensile TensileTensile test test test specimens specimens specimens were were preparedprepared according according to to GB/T228.1-2010. GB/T228.1-2010. Figure FigureFigure 22 2shshows showsows the the geometry geometry of of the the tensile tensile test test specimen.specimen. specimen. ThreeThree sets sets of of tensile tensile testing testing were were conducted conducted on on a tensile-testing a tensile-testing machine machine (Instron (Instron 3382, 3382, loading loading range range range 0~1000~100 kN) kN) kN) using using using a a crossheaa crosshead crosshead dspeed speed speed of of of 1mm/min 1mm/min 1mm/min at at atroom room room temperatur temperature, temperature, e,and and and its its its mean mean mean value value value was was was taken. taken. taken. FigureFigure 33 displays displays3 displays aa typicaltypicala typical engineering engineering stress-strainstress-strain stress-strain curvecurve curve ofof of thethe the 304SS304SS 304SS RFWRFW RFW joint. joint. After After tensile tensile testing, testing, thethethe fracturefracture fracture specimens specimens specimens were were were etched etched etched by the by by same the the reagentsame same reagent asreagent described as as described above,described and above, theabove, macro-morphology and and the the macro- macro- morphologyofmorphology fracture surface of of fracture fracture were surface observed surface were were by SEMobserv observ (JSM–6390A,eded by by SEM SEM (JSM–6390A, JEOL, (JSM–6390A, Tokyo, JEOL, Japan). JEOL, Tokyo, Tokyo, Japan). Japan).

60mm60mm 32mm32mm

2.5mm2.5mm 6mm 6mm 10mm 10mm R=12mmR=12mm

FigureFigure 2. 2.TheThe The schematic schematic schematic diagram diagram diagram of oftensile tensile specimen. specimen.

800800

700700

600600

500500

400400

300300 Tensile stress (MPa) stress Tensile 200(MPa) stress Tensile 200

100100

0 0 00 5 5 10 10 15 15 20 20 25 25 StrainStrain (%) (%) FigureFigure 3. 3.TypicalTypical Typical engineering engineering stress-strain stress-strain stress-strain curve curve curve of of304 304 stainless stainless steel steel (SS) (SS) rotary rotary friction-welding friction-welding (RFW)(RFW) joint joint joint (welding (welding (welding pressure pressure pressure is is is40 40 40 MPa, MPa, rotation rotation rotation speed speed is is is 500 500 rpm, rpm, welding welding time time time is is is9 9 s,9 s,and and forging forging pressurepressure is is 200 200 MPa). MPa). 3. Results and Discussion 3. 3.Results Results and and Discussion Discussion 3.1. The Relationship between Energy-Input and Tensile Strength 3.1.3.1. The The Relationship Relationship between between Energy-Input Energy-Input and and Tensile Tensile Strength Strength Figure4 shows the variation trend of the joint’s tensile strength with energy-input under different Figure 4 shows the variation trend of the joint’s tensile strength with energy-input under forgingFigure pressures. 4 shows It can the be seenvariation that the trend tensile of strengththe joint’s has tensile the same strength variation with trend energy-input with the change under different forging pressures. It can be seen that the tensile strength has the same variation trend with ofdifferent the energy-input, forging pressures. i.e., with It an can energy-input be seen that increase, the tensile the strength tensile strength has the increasessame variation until ittrend reaches with the change of the energy-input, i.e., with an energy-input increase, the tensile strength increases until athe maximum change andof the then energy-input, decreases. i.e., In addition, with an energy an optimum-input increase, energy-input the tensile exists strength for different increases forging until it reaches a maximum and then decreases. In addition, an optimum energy-input exists for different pressuresit reaches that a maximum makes the and joint then have decreases. the highest In additi tensileon, an strength. optimum (The energy-input variation characteristicsexists for different of forging pressures that makes the joint have the highest tensile strength. (The variation characteristics optimumforging pressures energy-input thatcorresponding makes the joint to have forging the high pressureest tensile will be strength. discussed (The in Sectionvariation4). characteristics Four group of optimum energy-input corresponding to forging pressure will be discussed in Section 4). Four experimentsof optimum under energy-input a forging corresponding pressure of 200 to MPa forging were pressure selected towill study be discussed the inﬂuence in Section mechanism 4). Four of group experiments under a forging pressure of 200 MPa were selected to study the influence energy-inputgroup experiments on the joint under strength. a forging They pressure are numbered of 200 fromMPa No.1 were to selected No.4, as to indicated study the in Figureinfluence4. mechanism of energy-input on the joint strength. They are numbered from No.1 to No.4, as indicated Tablemechanism1 lists the of welding energy-input parameters, on the energy-input joint strength. value, They and are tensile numbered strength from of No.1 these to four No.4, experiments. as indicated inin Figure Figure 4. 4.Table Table 1 lists1 lists the the welding welding parameters, parameters, energy-input energy-input value, value, and and tensile tensile strength strength of of these these fourfour experiments. experiments.

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(a) (b)

(e) (f)

Figure 4.4. TheThe variation variation trend trend of theof the 304SS 304SS RFW RFW joint’s joint’ tensiles tensile strength strength with energy-input with energy-input under forging under pressuresforging pressures of (a) 100 of MPa, (a) 100 (b) MPa, 120 MPa, (b) 120 (c)140 MPa, MPa, (c)140 (d) MPa, 160 MPa, (d) 160 (e) 180MPa, MPa, (e) 180 and MPa, (f) 200 and MPa. (f) 200 The MPa. bars associatedThe bars associated with the tensile with the strength tensile (the strength mean (the value mean of three value tensile of three strength tensile measurements) strength measurements) correspond tocorrespond standard deviations.to standard deviations.

Table 1. Experiment parameters and the joint’s tensile strength under a forging pressure of 200 MPa. Table 1. Experiment parameters and the joint’s tensile strength under a forging pressure of 200 MPa. Friction Pressure Rotation Energy-Input Average Tensile No. Friction Rotation Speed FrictionFriction Time (s) Energy- Average Tensile No. (MPa) Speed (rpm) (kJ) Strength (MPa) No.1Pressure (MPa) 40 (rpm) 1200 Time 4 (s) Input 25 (kJ) Strength 603 (MPa) No.1 No.240 40 1200 500 4 9 25 35 698 603 No.3 120 800 7 55 635 No.2 No.440 160 500 1400 9 19 35 120 592 698 No.3 120 800 7 55 635 3.2.No.4 The Thermal160 Cycle of Different Joints 1400 19 120 592

3.2. TheFigure Thermal5 shows Cycle the of Different evolution Joints curves of the joint’s temperature of No.1 to No.4 specimens. As shown in Figure5a, the highest temperature of No.1 to No.4 specimens during the RFW process are 1100 Figure◦C, 939 5◦ C,shows 1046 the◦C, evolution and 1017 curves◦C, respectively. of the joint’ Ons temperature the other hand, of No.1 the energy-input to No.4 specimens. of No.1 As to shown in Figure 5a, the highest temperature of No.1 to No.4 specimens during the RFW process are 1100 °C, 939 °C, 1046 °C, and 1017 °C, respectively. On the other hand, the energy-input of No.1 to

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No.4No.4No.4 is isis 25 2525 kJ, kJ,kJ, 35 35 kJ, kJ, 55 55 kJ, kJ, and and 120 120 kJ, kJ, respective respectively.respectively.ly. We WeWe can cancan see seesee that thatthat the thethe maximum maximummaximum temperature temperaturetemperature has hashas no nono obviousobviousobvious variation variationvariation regularity regularity with with the the change change of of ener energyenergygy input, input, i.e., i.e., with with the the increase increase of of energy-input, energy-input, thethethe maximum maximummaximum temperature temperaturetemperature has has has no no no obvi obvious obviousous increase increase or or decrease decrease trend.trend. trend. InIn In fact,fact, fact, thethe the joint’sjoint’s joint’s temperature temperature temperature is ismainlyis mainly mainly inﬂuenced influenced influencedby by by thethe the rotationalrotational rotational speed speed and and welding welding pressure pressure pressure [11,15]. [ 11[11,15].,15]. However,However, However, the thethe energy-input energy-inputenergy-input determinesdeterminesdetermines the thethe thermal thermalthermal cycle cycle cycle time time time of of joint, joint, i.e., i.e., the the timetime time fromfrom from thethe the weldweld weldbeginning beginning beginning to to to the the the joint joint joint cooling cooling cooling to toroomto room room temperature. temperature. temperature. As As seen seen seen in inin Figure FigureFigure 5b,5 5b,b, when whenwhen the the energy-input energy-input energy-input increases increases increases from from from 25 25 25kJ kJ tokJ to 120 to120 120kJ, kJ, the kJ,the thermalthethermal thermal cycle cycle time cycle time increases timeincreases increases from from 478 478 from s sto to 1157 478 1157 ss. s. to Th Th 1157e ereason reason s. Thefor for this reasonthis is is that that for the the this more more is thatthe the energy-input, theenergy-input, more the theenergy-input,the wider wider the the area area the of wider of joint joint thethat that area is is heated heated of joint during during that is th heatedthe ewelding welding during process, process, the welding which which results process, results in in whicha alonger longer results cooling cooling in a timelongertime after after cooling welding. welding. time after welding.

(a) (a) (b(b) )

FigureFigureFigure 5. 5.5. The The evolution evolution of of (a ()a temperature)) temperature during duringduring RFW RFWRFW process processprocess and andand (b ((b)b )cycle) cyclecycle time timetime with withwith energy-input. energy-input.energy-input. 3.3. Variation of Microstructure with Energy-Input 3.3.3.3. Variation Variation of of Microstr Microstructureucture with with Energy-Input Energy-Input Figure6 shows the microstructure of base metal (BM). The BM employed in this study is 304SS FigureFigure 6 6shows shows the the microstructure microstructure of of base base metal metal (BM). (BM). The The BM BM employed employed in in this this study study is is 304SS 304SS rods, of which the supply state is annealing treatment after rolling; thus, the initial microstructure rods,rods, of of which which the the supply supply state state is is annealing annealing treatmen treatment tafter after rolling; rolling; thus, thus, th the einitial initial microstructure microstructure consists of uniform equiaxed grains with an average size of 26 µm. consistsconsists of of uniform uniform equiaxed equiaxed grains grains with with an an average average size size of of 26 26 μ μm.m.

FigureFigure 6. 6.6. Microstructure Microstructure of ofof the thethe 304SS 304SS304SS base basebase metal. metal.metal.

FigureFigureFigure 77 7 showsshows shows the the the typical typical typical microstructure microstructure microstructure morpho morphology morphologylogy of of of a a awelded welded welded joint. joint. joint. Three Three Three zones zones zones consisting consisting consisting ofofof weld weldweld zonezone zone (WZ),(WZ), (WZ), thermo-mechanically thermo-mechanically thermo-mechanically affected affected affected zone zone zone (TMAZ), (TMAZ), (TMAZ), and baseand and metalbase base (BZ)metal metal can (BZ) (BZ) be observed.can can be be observed.observed.The WZ consists of equiaxed grains. The reason is that the WZ undergoes the most severe plastic deformationTheThe WZ WZ consists andconsists the of highestof equiaxed equiaxed temperature grains. grains. The The thermal reason reason is cycle is that that comparedthe the WZ WZ undergoes undergoes to other the zones, the most most which severe severe results plastic plastic in deformationsufﬁcientdeformation dynamic and and the the recrystallization highest highest temperature temperature (DRX) occurringthermal thermal cy incycle thecle compared compared WZ. On other to to other other hand, zones, zones, partial which which DRX results or results no DRX in in sufficientoccurssufficient in dynamic thedynamic TMAZ, recrystallization recrystallization and a large degree (DRX) (DRX) of occurring bending occurring deformation in in the the WZ. WZ. occursOn On other other in thehand, hand, TMAZ partial partial that DRX resultsDRX or or inno no a DRXclearDRX occurs radialoccurs in ﬂow in the the directionTMAZ, TMAZ, and existingand a alarge large in degree thedegree microstructure. of of bending bending deformation deformation occurs occurs in in the the TMAZ TMAZ that that results results inin a aclear clear radial radial flow flow direction direction existing existing in in the the microstructure. microstructure.

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FigureFigure 7. Typical7. Typical microstructure microstructure cataloguingcataloguing of of 304SS 304SS joint joint welded welded by RFW. by RFW. Figure 7. Typical microstructure cataloguing of 304SS joint welded by RFW. Figure 8 shows the TMAZ microstructure of specimens No.1–No.4. The TMAZ grain sizes of Figure8 shows the TMAZ microstructure of specimens No.1–No.4. The TMAZ grain sizes of specimensFigure No.1–No.4 8 shows are the 50 TMAZ μm, 30 microstructure μm, 32 μm and of 40 specimens μm, respectively. No.1–No.4. It should The TMAZ be noted grain that sizes the of specimens No.1–No.4 are 50 µm, 30 µm, 32 µm and 40 µm, respectively. It should be noted that the TMAZspecimens grain sizeNo.1–No.4 of specimen are 50 No.1 μm, shows 30 μm, obvious 32 μm coandarsening 40 μm, (nearly respectively. two times It should that of be the noted BM). that This the TMAZis due grainTMAZ to sizethe grain energy-input of size specimen of specimen of No.1 No.1, No.1 shows which shows obviousis insufficient obvious coarsening co toarsening induce (nearly partial(nearly two DRX two times timesin the that thatTMAZ. ofof thethe As BM). Figure This This is due to5 shows, theis due energy-input tospecimen the energy-input No.1 of No.1,experiences of whichNo.1, whichthe is insufﬁcientshortest is insufficient thermal to induce tocycle. induce Thus, partial partial the DRX TMAZDRX in in the grainsthe TMAZ.TMAZ. of specimen As As Figure Figure 5 shows,No.15 specimen shows, mainly specimen No.1undergo experiences No.1 torsion experiences and the elongation, shortest the shortest thermal whic thhermal cycle.cause cycle. grain Thus, Thus, coarsening, the TMAZthe TMAZ as grains Figure grains of specimen8aof shows.specimen No.1 mainlyHowever,No.1 undergo mainly as torsionFigure undergo 8b and shows, torsion elongation, with and theelongation, which energy-in cause whicput grain hincreasing, cause coarsening, grain the coarsening, average as Figure TMAZ as8 aFigure shows.grain 8a size However,shows. of as FigurespecimenHowever,8b shows, No.2 as decreases withFigure the 8b to energy-inputshows, 30 μm. with The themain increasing, energy-in reason for theput the averageincreasing, grain si TMAZze the reduction average grain sizeisTMAZ that of partial specimengrain DRXsize No.2 of decreasesoccursspecimen to in 30the µ No.2TMAZm. Thedecreases under main the to reason effect 30 μm. of for Thewelding the main grain thermal reason size andfor reduction theplastic grain deformation. si isze that reduction partial As theis DRX that energy-input partial occurs DRX in the TMAZfurtheroccurs under increases, in the the effectTMAZ the of underthermal welding the cycle effect thermal grows of welding longer and thermal plastic, and the deformation.and grains plastic of deformation. the As TMAZ the energy-input Asgrow the energy-inputunder the further thermalfurther effect. increases, the thermal cycle grows longer, and the grains of the TMAZ grow under the increases, the thermal cycle grows longer, and the grains of the TMAZ grow under the thermal effect. thermal effect. (a) (b) (a) (b)

Figure 8. The thermo-mechanically affected zone (TMAZ) microstructure of specimens (a) No.1,

(b) No.2, (c) No.3, and (d) No.4. Metals 2018, 8, x FOR PEER REVIEW 7 of 14

MetalsFigure2018, 8, 9088. The thermo-mechanically affected zone (TMAZ) microstructure of specimens (a) No.1, (b7) of 14 No.2, (c) No.3, and (d) No.4.

FigureFigure9 9 presents presents thethe WZWZ microstructuremicrostructure ofof differentdifferent specimens. The The average average WZ WZ grain grain size size of ofspecimens specimens No.1–No.4 No.1–No.4 is 19 is μ 19m,µ m,17 μ 17m,µ 25m, μ 25m,µ andm, and40 μ 40m,µ respectively.m, respectively. The WZ The undergoes WZ undergoes the most the mostsevere severe plastic plastic deformation deformation and andthe thehighest highest temp temperatureerature thermal thermal cycle cycle compared compared to to other other zones, whichwhich resultsresults inin adequateadequate DRXDRX occurringoccurring inin thisthis zonezone [[16].16]. ForFor specimensspecimens No.1No.1 andand No.2,No.2, thethe DRXDRX resultsresults inin thethe ﬁnefine graingrain inin thethe WZ.WZ. However,However, whenwhen thethe energy-inputenergy-input increases,increases, thethe sizesize ofof WZWZ grainsgrains increaseincrease duedue toto thethe longerlonger thermalthermal cycle,cycle, whichwhich eventuallyeventually resultsresults inin thethe largerlarger grainsgrains beingbeing generatedgenerated inin thethe WZWZ ofof specimenspecimen No.4.No.4.

(a) (b)

FigureFigure 9.9. The weld zone (WZ)(WZ) microstructuremicrostructure ofof specimensspecimens ((aa)) No.1,No.1, ((bb)) No.2,No.2, ((cc)) No.3,No.3, andand ((dd)) No.4.No.4.

3.4.3.4. HardnessHardness DistributionDistribution TheThe microhardnessmicrohardness distributiondistribution throughoutthroughout thethe weldingwelding surfacesurface fromfrom weldweld zonezone toto basebase materialmaterial waswas measured.measured. FigureFigure 1010aa illustratesillustrates thethe directiondirection ofof microhardnessmicrohardness measurement,measurement, thethe hardnesshardness ofof thethe WZ,WZ, TMAZ,TMAZ, HAZ,HAZ, andand BM,BM, whichwhich werewere measured,measured, respectively,respectively, forfor eacheach joint.joint. FigureFigure7 7b–eb–e shows shows thethe microhardnessmicrohardness distributiondistribution ofof differentdifferent joints.joints. TheThe resultsresults showshow thatthat thethe averageaverage hardnesshardness valuevalue ofof thethe BMBM isis 213213 Hv,Hv, andand thethe averageaverage WZWZ hardnesshardness valuevalue ofof specimensspecimens No.1–No.4 were 252, 242, 230, andand 218218 Hv, Hv, respectively. respectively. It It can can be be observed observed that that the th WZe WZ hardness hardness of all of four all four specimens specimens was higherwas higher than thatthan of that the of BM. the This BM. is This due tois thedue strain to the hardening strain hardening effect during effect the during welding the processwelding [ 17process]. In addition, [17]. In theaddition, WZ of the sample WZ of No.1 sample is the No.1 area is with the thearea highest with the hardness highest of hardness the whole of joint;the whole the same joint; result the same also existsresult inalso sample exists No.2.in sample However, No.2. However, with the energy-input with the energy-input increasing, increasing, the highest the hardness highest hardness value of specimensvalue of specimens No.3 and No.4No.3 occursand No.4 in the occurs TMAZ. in Thisthe TMAZ. is due to This the grainis due size to ofth thee grain WZ becomingsize of the large WZ asbecoming the energy-input large as increases,the energy-input as Figure increases,9 shows; moreover, as Figure the9 shows; coarsening moreover, grains resultsthe coarsening in the decrease grains inresults hardness in the [18 decrease,19]. When in hardness the energy-input [18,19]. When increases the energy-input to 120 kJ, i.e., increases specimen to No.4,120 kJ, the i.e., hardening specimen ofNo.4, the WZthe hardening caused by strainof the isWZ almost caused offset by bystrain the softeningis almost causedoffset by by the grain softening growth caused [20]. Moreover, by grain thegrowth plastic [20]. deformation Moreover, inthe the plastic TMAZ deformation turns severely in the as the TMAZ energy-input turns severely increases, as the which energy-input results in aincreases, more severe which deformable results in a hardening. more severe It shoulddeformabl bee noted hardening. that an It obviousshould be softening noted that phenomenon an obvious appearedsoftening inphenomenon the TMAZ ofappeared specimen in No.1.the TMAZ The lowest of specimen TMAZ hardnessNo.1. The of lowest specimen TMAZ No.1 hardness is 198 Hv, of whichspecimen decreases No.1 is 7% 198 compared Hv, which with decreases that of 7% the comp BM. Asared discussed with that above, of the theBM. average As discussed TMAZ above, grain sizethe average of specimen TMAZ No.1 grain is nearlysize of twospecimen times No.1 of that is ofnearly the BM,two whichtimes of leads that toof athe decrease BM, which in hardness. leads to Thea decrease low TMAZ in hardness. hardness The will low make TMAZ this hardness zone a weak will areamake of this the zone joint, a which weak wearea will of the discuss joint, in which next section.we will discuss In addition, in next the section. TMAZ In hardness addition, of the specimen TMAZ hardness No.2 is slightly of specimen higher No.2 than is that slightly of the higher BM. It also can be observed from Figure 10c that the hardness distribution of specimen No.2 is smooth

Metals 2018, 8, x FOR PEER REVIEW 8 of 14 Metals 2018, 8, 908 8 of 14 than that of the BM. It also can be observed from Figure 10c that the hardness distribution of specimen andNo.2 has is no smooth catastrophe and point,has no which catastrophe corresponds point, to the which uniformity corresponds of the microstructure, to the uniformity as discussed of the inmicrostructure, Section 3.2. as discussed in Section 3.2.

(a)

(b)

(c)

(d)

(e)

Figure 10. A diagram of the (a) microhardness test point and (b–e) microhardness distribution of Figure 10. A diagram of the (a) microhardness test point and (b–e) microhardness distribution of specimens No.1–No.4. specimens No.1–No.4. 3.5. The Relationship of Tensile Specimen Fracture Zone and Energy-Input 3.5. The Relationship of Tensile Specimen Fracture Zone and Energy-Input Figure 11 represents the morphology of the fracture tensile specimen and the microstructure near theFigure fracture 11 represents zone. From the Figure morphology 11a, it can of bethe observed fracture thattensile the specimen microstructure and the near microstructure the fracture zonenear ofthe specimen fracture No.1zone. isFrom the elongated Figure 11a, coarse it can grain. be observed By comparing that the this microstructure microstructure near to the the fracture TMAZ microstructurezone of specimen of specimen No.1 is the No.1, elongated shown bycoarse Figure grai8a,n. it By is clear comparing that the this No.1 micr tensileostructure specimen to the fractured TMAZ atmicrostructure the TMAZ. As of discussed specimen above, No.1, the shown TMAZ by average Figure grain8a, it sizeis clear of specimen that the No.1No.1 istensile 50 µm, specimen and the hardnessfractured of atthe the TMAZTMAZ. is As 29% discussed lower than above, that the of TM BM.AZ The average coarse grain grain size and of low specimen hardness No.1 lead is 50 to theμm, TMAZand the becoming hardness theof the weak TMAZ zone is of29% joint lower No.1. than When that of the BM. energy-input The coarse increases,grain and nolow obvious hardness grain lead to the TMAZ becoming the weak zone of joint No.1. When the energy-input increases, no obvious

Metals 2018, 8, 908 9 of 14

Metals 2018, 8, x FOR PEER REVIEW 9 of 14 coarsening area and softening zone occurs in joint No.2, which gives the joint its excellent mechanical properties.grain coarsening Though area the and tensile softening specimen zone of No.2occurs also in fracturedjoint No.2, at which the TMAZ, gives the the fracture joint its surface excellent of No.2mechanical is a curvilinear properties. form, Though and the the tensile tensile specimen strength of No.2 joint also No.2 fractured is 698 MPa, at the which TMAZ, is 94%the fracture of that ofsurface the BM. of No.2 As Figure is a curvilinear9c,d shows, form, the and WZ the grain tensile size strength of joints of No.3 joint and No.2 No.4 is 698 is obviouslyMPa, which coarser is 94% thanof that that of the of theBM. BM, As Figure which 9c,d results shows, in thethe No.3WZ grai andn size No.4 of tensile joints No.3 specimens and No.4 fracturing is obviously at the coarser WZ, asthan Figure that 11 ofc,d the shows. BM, which results in the No.3 and No.4 tensile specimens fracturing at the WZ, as Figure 11c,d shows.

(a) No.1

(b) No.2

(d) No.4

FigureFigure 11.11. PhotographPhotograph ofof macroscopicmacroscopic andand microstructuremicrostructure ofof fracturefracture tensiletensile specimen.specimen.

4. Modeling of the Energy-Input To establish a model for energy-input with welding parameters as variables, we must first fit the energy-input curve as a quadratic function, i.e.,

Metals 2018, 8, 908 10 of 14

4. Modeling of the Energy-Input To establish a model for energy-input with welding parameters as variables, we must ﬁrst ﬁt the energy-inputMetals 2018, 8, x curve FOR PEER as a REVIEW quadratic function, i.e., 10 of 14

2 Q = C ti = C + C t + C t2 (1) 𝑄=𝐶∑ i 𝑡 =𝐶0 +𝐶1 𝑡+𝐶1 𝑡 (1) i=0 inin which whichQ Qis is energy-input, energy-input,t ist is welding welding time, time, and andC 0C, 0C, 1C,1 and, andC C2 2are are fitting fitting coefficients. coefficients. FigureFigure 12 12 shows shows the the comparison comparison of of the the calculated calculated curves curves and and experimentally experimentally measured measured curves curves ofof energy-input. energy-input. The The fitting fitting equation equation described described by by Equation Equation (1) (1) can can be be seen seen to to fit fit very very well well with with the the experimentalexperimental data data of of energy-input. energy-input. Table Table2 lists2 lists the the values values of of fitting fitting coefficients coefficientsC 0C, 0C, 1C,1 and, andC 2Cfor2 for differentdifferent welding welding parameters. parameters.

(a) (b)

(e)

FigureFigure 12. 12.Comparison Comparison between between calculated calculated and and experimental experimental values values of of energy-input energy-input under under different different rotationalrotational speeds speeds when when weldingwelding pressure pressure was was constant constant at at (a) ( a25) 25MPa, MPa, (b) 40 (b) MPa, 40 MPa, (c) 80 ( cMPa,) 80 MPa,(d) 120 (dMPa,) 120 MPa,and (e and) 160 (e MPa.) 160 MPa.

Metals 2018, 8, 908 11 of 14

Table 2. The coefﬁcient of different welding experiments.

P (MPa) N (rpm) C0 C1 C2 500 −3969.91 7720.94 −160.97 800 1792.98 6995.59 −144.81 1100 1594.45 6956.77 −160.58 25 1400 4350.03 6335.06 −137.91 1700 4423.71 6233.98 −131.23 2000 4319.92 5931.05 −121.72 500 −4249.60 7053.29 −128.14 1100 2637.38 6802.22 −157.76 40 1700 4130.76 6353.79 −149.41 2300 3590.85 6437.81 −163.03 500 −3599.73 8111.83 −121.67 800 2395.92 7738.01 −158.53 1100 3134.80 7545.50 −156.31 80 1400 3832.45 6973.78 −158.59 1700 4268.07 6899.27 −131.95 2300 4324.83 6509.14 −124.84 500 −4163.32 9006.37 −131.23 800 −2473.33 8864.62 −174.24 1100 1636.11 8242.70 −147.53 120 1400 1745.32 7831.08 −133.24 1700 3049.07 7313.38 −100.93 2300 3238.76 3238.76 −73.20 1100 825.70 8365.38 −90.80 1400 897.32 8165.46 −80.14 160 1700 1452.71 7813.79 −74.77 2300 1436.52 6701.22 −15.79 1100 359.98 9759.89 −94.88 200 1400 140.55 7950.32 −16.90

The equation of C0, C1, and C2, in which the variables are welding pressure and rotation speed, can be obtained by regression analysis of the data in Table2, i.e.,

C0 = −2305 − 13.20P + 3.69N (2)

C1 = 8237 + 12.31P − 1.58N (3)

C2 = −219 + 0.49P + 0.04N (4) Taking Equations (2)–(4) into account in Equation (1), the energy-input can be expressed as a formula of welding pressure, welding rotational speed, and welding time, i.e.,

Q = (−2305 − 13.20P + 3.69N ) + (8237 + 12.31P − 1.58N)t + (−219 + 0.49P + 0.04N)t2 (5)

To verify the accuracy of the model, extra welds were made with new parameters. Figure 13 shows the comparison of the calculated and experimental curves. As the results show, the model closely predicts the actual energy-input data. As the weld experiments of Figure 13 were made with parameters that are outside of those used in creating the model, they demonstrate that the model accurately predicts the energy-input beyond the initially investigated parameters. Metals 2018, 8, 908 12 of 14 Metals 2018, 8, x FOR PEER REVIEW 12 of 14

(a) (b)

FigureFigure 13. 13.Comparison Comparison between between calculated calculated and and experimental experimental values values of energy-inputof energy-input under under welding welding parametersparameters of of (a )(ap) p= = 200200 MPa,MPa, N == 1400 1400 rpm; rpm; (b ()b p) =p 80= 80MPa, MPa, N = N1200= 1200rpm; rpm;(c) p = ( c100) p MPa,= 100 N MPa, = 1500 N =rpm; 1500 and rpm; (d) and p = (140d) p MPa,= 140 N MPa, = 1500N rpm.= 1500 rpm.

AsAs discussed discussed in in Section Section 3.1 3.1,, there there is anis an optimal optimal energy-input energy-input value value that that causes causes the the joints joints to to have have thethe maximum maximum tensile tensile strength strength under under different different forging forgin pressures.g pressures. Figure Figure 14 14shows shows the the variation variation trend trend ofof optimal optimal energy-input energy-input for for different different forging forging pressures. pressures. The The results results indicate indicate that that the the larger larger the the forging forging pressure,pressure, the the less less the the optimal optimal energy-input. energy-input. The The optimal optimal energy-input energy-input decreases decreases from from 105 105 kJ kJ to to 35 35 kJ kJ whenwhen the the forging forging pressure pressure rises rises from from 100 100 MPa MPa to to 200 200 MPa. MPa. The The deformation deformation is ais necessarya necessary factor factor for for formingforming a RFW a RFW joint joint [21]. [21]. When When the forging the forging pressure pressure is large, is thelarge, joint the is easilyjoint is deformed, easily deformed, which means which a smallmeans amount a small of amount energy-input of energy-input could obtain could a stableobtain jointa stable [14, joint22]. As[14,22]. seen As in Figureseen in 14 Figure, a three-order 14, a three- functionorder function ﬁts the experimental fits the experimental data very data well, very which well, is which in the followingis in the following form: form:

2 −4 3 Q 𝑄= 690545 = 690545− 10642.84 − 10642.84p f + 𝑝 58.96+ 58.96p f 𝑝− 1106− 1106× 10  10p f 𝑝 (6)

in which pf is the forging pressure. in which p is the forging pressure. Extraf experiments under a forging pressure of 170 MPa, which are not included in those used in Extra experiments under a forging pressure of 170 MPa, which are not included in those used establishing the model, were made to verify the accuracy of Equation (6). Firstly, the optimal energy- in establishing the model, were made to verify the accuracy of Equation (6). Firstly, the optimal input for a forging pressure of 170 MPa was calculated based on Equation (6), which was 41 kJ. energy-input for a forging pressure of 170 MPa was calculated based on Equation (6), which was Subsequently, two welding parameters (welding pressure, and speed and time) were randomly set, 41 kJ. Subsequently, two welding parameters (welding pressure, and speed and time) were randomly and the other parameter can be calculated according to Equation (5). In this study, three group set, and the other parameter can be calculated according to Equation (5). In this study, three group experiments were made to verify the calculated optimal energy-input accuracy or otherwise disprove experiments were made to verify the calculated optimal energy-input accuracy or otherwise disprove it. it. The welding pressure and rotational speed of first group was 100 MPa and 1300 rpm, respectively, The welding pressure and rotational speed of ﬁrst group was 100 MPa and 1300 rpm, respectively, and the calculated welding time was 6.1 s. The welding pressure and welding time of the second and the calculated welding time was 6.1 s. The welding pressure and welding time of the second group group was 40 MPa and 8 s, respectively, and the calculated rotational speed was 2000 rpm. The was 40 MPa and 8 s, respectively, and the calculated rotational speed was 2000 rpm. The rotational rotational speed and welding time of third group was 800 rpm and 5 s, respectively, and the speed and welding time of third group was 800 rpm and 5 s, respectively, and the calculated welding calculated welding pressure was 160 MPa. Table 3 shows the welding parameters and tensile strength pressure was 160 MPa. Table3 shows the welding parameters and tensile strength of these three of these three verified experiments. As shown in Table 3, the strength of all the three joints was over veriﬁed experiments. As shown in Table3, the strength of all the three joints was over 90% that of the 90% that of the BM. It is proven that the optimal energy-input model (Equation (6)) is accurate. BM. It is proven that the optimal energy-input model (Equation (6)) is accurate.

Metals 2018, 8, 908 13 of 14 Metals 2018, 8, x FOR PEER REVIEW 13 of 14

FigureFigure 14.14.Variation Variation trend trend of of optimum optimum energy-input energy-input with with forging forging pressure. pressure.

Table 3. Welding parameters and tensile strength of verification experiments. Table 3. Welding parameters and tensile strength of verification experiments. No. Friction Pressure (MPa) Rotation Speed (rpm) Friction Time (s) Tensile Strength (MPa) Friction Pressure Rotation Speed Friction Time Tensile Strength No. No.5(MPa) 100 (rpm) 1300 (s) 6 (MPa) 93% No.6 40 2000 8 90% No.5No.7 100 160 1300 800 6 5 93% 96% No.6 40 2000 8 90% 5. ConclusionsNo.7 160 800 5 96% In this study, the effect of energy-input on the mechanical properties of RFW 304SS joints was 5. Conclusions investigated. RFW experiments were conducted over a wide range of welding parameters. Based on the resultsIn this and study, discussion the effect above, of energy-input the conclusions on arethe asmechanical follows: properties of RFW 304SS joints was investigated. RFW experiments were conducted over a wide range of welding parameters. Based on (1)the resultsWhen and forging discussion pressure above, is kept the conclusions constant, the are tensile as follows: strength increases with the increase of energy-input until reaching a maximum and then decreases. (2)(1) ThoughWhen forging the maximum pressure temperature is kept constant, has no the obvious tensile variationstrength regularityincreases with with the the increase change ofof energy-input,energy-input until the thermal reaching cycle a maximum has a positive and then correlation decreases. with the increase of energy-input, i.e., (2) asThough energy-input the maximum increases, temperature the duration has time no from obvious the commencement variation regularity of welding with to the joint change cooling of toenergy-input, room temperature the thermal increases. cycle has a positive correlation with the increase of energy-input, i.e., (3) Anas energy-input empirical model increases, was established the duration that time describes from the energy-input commencement as aof function welding of to the joint welding cooling parameters.to room temperature The accuracy increases. of the model was verified by extra RFW experiments. (3) An empirical model was established that describes energy-input as a function of the welding (4) An empirical model for the optimal energy-input of different forging pressures was obtained. parameters. The accuracy of the model was verified by extra RFW experiments. Then, the optimal energy-input for 170 MPa forging pressure was calculated. Three group (4) An empirical model for the optimal energy-input of different forging pressures was obtained. experiments were made based on the calculated energy-input value. The joints’ tensile strength Then, the optimal energy-input for 170 MPa forging pressure was calculated. Three group coefficients of these three experiments were 93%, 90%, and 96%, respectively. The results proved experiments were made based on the calculated energy-input value. The joints’ tensile strength the accuracy of the model for optimal energy-input. coefficients of these three experiments were 93%, 90%, and 96%, respectively. The results proved Authorthe Contributions: accuracy of theJ.L., model F.Z. and for G.W. optimal conceived energy-input. and designed the experiments. G.W. and W.W. carried out the experiments. J.X. and G.W. analyzed the experimental data. G.W. wrote the manuscript. J.L., F.Z. and J.X. reviewedAuthors theContributions: manuscript. J.L., F.Z. and G.W. conceived and designed the experiments. G.W. and W.W. carried Funding:out the experiments.This work is J.X. supported and G.W. by theanalyzed National the Natural experimental Science data. Foundation G.W. wrote of China the (Grandmanuscript. No: 51475376 J.L., F.Z. andand 51575451).J.X. reviewed the manuscript.

ConﬂictsFunding: of This Interest: work Theis supported authors declare by the noNational conﬂict Natural of interest. Science Foundation of China (Grand No: 51475376 and 51575451). References Conflicts of Interest: The authors declare no conflict of interest. 1. Bouarroudj, E.; Chikh, S.; Abdi, S.; Miroud, D. Thermal analysis during a rotational friction welding. ReferencesAppl. Therm. Eng. 2017, 110, 1543–1553. [CrossRef] 2. Li, W.Y.; Vairis, A.; Preuss, M.; Ma, T.J. Linear and rotary friction welding review. Int. Mater. Rev. 2016, 1. Bouarroudj, E.; Chikh, S.; Abdi, S.; Miroud, D. Thermal analysis during a rotational friction welding. Appl. 61, 71–100. [CrossRef] Therm. Eng. 2017, 110, 1543–1553, doi:10.1016/j.applthermaleng.2016.09.067. 2. Li, W.Y.; Vairis, A.; Preuss, M.; Ma, T.J. Linear and rotary friction welding review. Int. Mater. Rev. 2016, 61, 71–100, doi:10.1080/09506608.2015.1109214.

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