Effect of Trajectory Curvature on the Microstructure and Properties of Surfacing Wall Formed with the Process of Wire Arc Additive Manufacturing

coatings

Article Eﬀect of Trajectory Curvature on the Microstructure and Properties of Surfacing Wall Formed with the Process of Wire Arc Additive Manufacturing

Tao Feng 1, Lishi Wang 1,*, Zhongmin Tang 2, Shanwen Yu 1, Zhixiang Bu 1, Xinbin Hu 1 and Yihang Cheng 1 1 Hubei Provincial Key Laboratory of Green Materials of Light Industry, College of Materials and Chemical Engineering, Hubei University of Technology, Wuhan 430068, China; [email protected] (T.F.); [email protected] (S.Y.); [email protected] (Z.B.); [email protected] (X.H.); [email protected] (Y.C.) 2 Hubei Sanjiang Aerospace Hongyang Electromechanical CO. LTD, Xiaogan 432100, China; [email protected] * Correspondence: [email protected]; Tel.: +86-27-59750482

Received: 4 November 2019; Accepted: 6 December 2019; Published: 11 December 2019

Abstract: Curvature effects are typically present in the process of additive manufacturing (AM), particularly for wire arc additive manufacturing. In this paper, stainless-steel wire was adopted to deposit thin-walled samples with different curvatures. Optical microscopy,SEM, EDS and micro-hardness was used to analyse the microstructure, composition and properties of the samples. The result shows that the bottom region of the thin-walled sample had a mainly planar and cellular crystal microstructure. For the middle region, the microstructure revealed mainly dendrites, and the top layer has equiaxed dendrite morphology. The microhardness value of the bottom was greater than that of the middle, and the microhardness value of the middle was greater than that of the top. Moreover, the grain size of the inner part (direct to curvature radius) was larger than that of the outer part, and the micro-hardness value exhibited an increasing tendency from the inner to the outer side. With enlarging curvature, the degree of grain size differences and micro-hardness variants decreased. Finally, an investigation with a low carbon steel wire showed that it had a similar curvature effect for its AM specimen.

Keywords: wire arc additive manufacturing; curvature eﬀect; microstructure and properties

1. Introduction Additive manufacturing (AM), also known as rapid forming technology, was proposed by Charles W. Hul in the 1980s [1]. “AM as a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies” [2]. Compared with traditional machining operations and other manufacturing technologies, additive manufacturing technology can quickly and accurately manufacture complex parts with high forming efficiency, which can save design and manufacturing costs, to a certain extent [3–5]. The heat sources used in additive metal manufacturing include lasers, electron beams, arcs, etc. [6–12]. Frequently used AM materials are Polymers, ceramics and metals [13,14]. Recently, AM has been widely used in various industries, such as aerospace. The AM process has been used to produce the central flange of the Chinese-made aircraft C919. The selective laser melting process is also used in the renovation and coating of precision parts [15]. The AM process plays an important role in Industry 4.0, because it saves time and has lower costs [16]. Compared to laser and electron beam AM, wire arc additive manufacturing (WAAM) equipment, with its low cost and high forming efficiency, is suitable for manufacturing large components [17–19].

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In the process of arc additive manufacturing, it is complicated by the thermal effect of the arc pre-deposited adjacent region being reheated with subsequent welding passes [20,21]. Besides, the heat accumulation during the forming process has different effects on the microstructure and properties of formed parts. Therefore, in industrialized applications of WAAM technology, it is valuable to understand the influence of the arc thermal effect on the microstructure and performance of different zones of the formed parts. Liu et al. [22] prepared single-layer multilayer samples at different Coatings 2019, 9, x FOR PEER REVIEW 2 of 13 deposition rates. The results show that the mechanical properties could be much improved with a suitable depositionIn the process rate, of because arc additive it hasmanufacturing, a finer and it is more complicated uniform by the microstructure. thermal effect of Zhouthe arc et al. [23], studied thepre-deposited effect of anadjacent external region magnetic being reheated field with on subsequent the microstructure welding passes and [20,21]. mechanical Besides, the properties of shapedheat specimens. accumulation They during found the thatforming the process microstructure has different was effects more on uniform,the microstructure and the and mechanical properties of formed parts. Therefore, in industrialized applications of WAAM technology, it is propertiesvaluable were superiorto understand than the wheninfluence utilizing of the arc an thermal external effect magneticon the microstructure field. Ge and et performance al. [24] studied the thermal history,of different microstructure zones of the formed and propertiesparts. Liu et ofal. formed[22] prepared parts single-layer in different multilayer locations, samples showing at that the preheatingdifferent eff depositionect from rates. previously The results built show layers that the can mechanical be effectively properties used could to be reduce much improved residual stresses. The coolingwith rate a suitable firstly deposition decreased rate, rapidly,because it andhas a thenfiner and kept more stable uniform near microstructure. the top layer. Zhou The et al. parts can [23], studied the effect of an external magnetic field on the microstructure and mechanical properties be a straightof shaped wall specimens. or a block, They and found they that can the also micros betructure curved. was Thus, more uniform, it is usually and the inevitable mechanical to have a curved travelingproperties path were for superior additive than manufacturing.when utilizing an ex Theternal presence magnetic of field. a curve Ge et mayal. [24] result studied in the a di fference in performancethermal betweenhistory, microstructure the inside andand properties outside ofof formed the curve. parts Forin different example, locations, under showing aerospace that or high temperaturethe preheating and high-pressure effect from previously conditions, built it layers may becan necessarybe effectively to used carry to reduce out some residual intensive stresses. treatment The cooling rate firstly decreased rapidly, and then kept stable near the top layer. The parts can be a measures on the side with poor performance. Specific and in-depth analysis of the influence of straight wall or a block, and they can also be curved. Thus, it is usually inevitable to have a curved the curvaturetraveling of thepath pathfor additive on the manufacturing. microstructure The andpresence properties of a curve of may fabricated result in a parts, difference is still in lacking. Stainless-steelperformance components between withthe inside a complex and outside structure of the curve. are widelyFor example, used under in the aerospace industry. or high Meanwhile, WAAM providestemperature a noveland high-pressure processing conditions, method it optionmay be necessary for them. to carry This out paper some intensive investigates treatment the effect of trajectorymeasures curvature on onthe side the microstructurewith poor performance. and mechanicalSpecific and in-depth properties analysis of of thin-walled the influence samples of the with a curvature of the path on the microstructure and properties of fabricated parts, is still lacking. 304 stainless-steelStainless-steel wire components arc additive with manufacturing a complex struct process.ure are widely Moreover, used in lowthe industry. carbon steelMeanwhile, wire was used to verify theWAAM effect provides of curvature a novel processing on WAAM method structure option and for them. performance. This paper investigates the effect of trajectory curvature on the microstructure and mechanical properties of thin-walled samples with a 2. Experimental304 stainless-steel wire arc additive manufacturing process. Moreover, low carbon steel wire was used to verify the effect of curvature on WAAM structure and performance. The stainless-steel wire used in the experiment was ER304. The wire diameter was 1.0 mm, and the substrate2. Experimental material was Q235 steel plate. The composition of the welding wire and substrate material is shown in TableThe stainless-steel1. The oxide wire film used on in the the surface experiment of thewas substrateER304. The platewire diameter was cleaned was 1.0 withmm, and mechanical methods,the and substrate then wiped material with was acetone Q235 steel before plate. welding. The composition The additive of the welding manufacturing wire and systemsubstrateused here consistedmaterial mainly is of shown the Megmid in Table 1. MIG500 The oxide welder film on the (Shenzhen, surface of the China) substrate and plate the was ABB1410 cleaned robotwith (Zurich, mechanical methods, and then wiped with acetone before welding. The additive manufacturing Switzerland),system as used shown here in consisted Figure1 .mainly The sample of the Megm was reciprocallyid MIG500 welder deposited (Shenzhen, without China) pre-heating and the before welding, andABB1410 each robot pass (Zurich, had a Switzerland), fixed curvature. as shown in Figure 1. The sample was reciprocally deposited without pre-heating before welding, and each pass had a fixed curvature.

FigureFigure 1. The 1. systemThe system in sitein site used used forfor wire wire arc arc additive additive manufacturing. manufacturing.

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Coatings 2019, 9, x FOR PEER REVIEW 3 of 13 The path curvature of the experimental parts is shown in Figure2. The sample was cut with a wire-cutThe electricalpath curvature discharge of the machine experimental (DK7750, parts Star peak,is shown Taizhou, in Figure China). 2. The The sample metallographic was cut etching with a solutionwire-cut forelectrical stainless-steel discharge consisted machine of nitric(DK7750, acid, hydrofluoricStar peak, Taizhou, acid and China). glycerine, The and metallographic their volume ratioetching was solution 1:2:1. The for stainless-steel sample was etched consisted by swabbing of nitric acid, with hydrofluoric the solution acid until and the glycerine, boundary and between their thevolume layers ratio was was clearly 1:2:1. displayed. The sample Next, was the etched transverse by swabbing hardness with measurement the solution was until performed the boundary on the first,between fifth the and layers tenth was layers, clearly from displayed. the left (inside) Next, th sidee transverse to the right hardness (outside), measurement using a Vickers was performed hardness testeron the (THVS-1MDX-AXY, first, fifth and tenth Qifeng, layers, Chengdu,from the left China). (inside) The side conditions to the right of the (outside), hardness using test had a Vickers a point intervalhardness of tester 0.03 mm,(THVS-1MDX-AXY, a load of 200 g, andQifeng, a dwell Chengdu, time of China). 10 s. Finally, The conditions the sample of wasthe polishedhardness andtest etchedhad a point again interval by swabbing, of 0.03 in mm, order a load to observe of 200 theg, and metallographic a dwell time structured of 10 s. Finally, with a the metallographic sample was microscopepolished and (Olympus etched again GX51, by Tokyo, swabbing, Japan). in order to observe the metallographic structured with a metallographic microscope (Olympus GX51, Tokyo, Japan).

Figure 2. Traveling paths for three-types of curvature samples. Figure 2. Traveling paths for three-types of curvature samples. The traveling path of low carbon steel (ER50-6, the composition of the welding wire is The traveling path of low carbon steel (ER50-6, the composition of the welding wire is shown in shown in Table1) was the same as the stainless-steel welding wire, in terms of deposited layers. Table 1) was the same as the stainless-steel welding wire, in terms of deposited layers. The The metallographic etching solution for low carbon steel contained 4% (v/v) nitric acid alcohol. metallographic etching solution for low carbon steel contained 4% (v/v) nitric acid alcohol. Table 1. Composition of welding wire and substrate material. Table 1. Composition of welding wire and substrate material. Wire Type Substrate Material Chemical Composition (wt %) Wire Type Substrate Material Chemical Composition (wt %) ER304 ER50-6 Q235 ER304 ER50-6 Q235 C 0.10 0.06–0.15 0.12–0.20 C 0.10 0.06-0.15 0.12–0.20 Si 0.65 0.80–1.15 0.30 Si 0.65 0.80-1.15 ≤≤0.30 Mn 1.72 1.40–1.85 0.30–0.65 Mn 1.72 1.40-1.85 0.30–0.65 Cr 17.5 0.15 ≤ − NiCr 17.5 9.3 ≤0.150.15 − ≤ − PNi 9.3 ≤0.150.025 −0.045 − ≤ ≤ S P − ≤0.0250.025 ≤0.0450.045 S −− ≤ ≤0.025 ≤≤0.045

At the first stage, the investigation was carried out with a large number of basic technological At the first stage, the investigation was carried out with a large number of basic technological experiments, such as using single-pass single-layer and single-pass multi-layer. We compared and experiments, such as using single-pass single-layer and single-pass multi-layer. We compared and analysed the morphology (weld height, weld width, etc.) of the single-pass single-layer, and the analysed the morphology (weld height, weld width, etc.) of the single-pass single-layer, and the depositing effect and the dimensional accuracy of the single-pass multi-layer forming test, under depositing effect and the dimensional accuracy of the single-pass multi-layer forming test, under different technological parameters. Some single-layer single-pass weld shapes with various process different technological parameters. Some single-layer single-pass weld shapes with various process parameters are shown in Figure3. The process parameters were gradually adjusted to get a stable parameters are shown in Figure 3. The process parameters were gradually adjusted to get a stable geometric weld for ER304 stainless-steel welding wire, and ER50-6 low carbon steel welding wire in geometric weld for ER304 stainless-steel welding wire, and ER50-6 low carbon steel welding wire in this experiment, as shown in Table2. this experiment, as shown in Table 2.

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Figure 3. Single-layerSingle-layer single-pass single-pass weld shapes with various process parameters.

Table 2. Arc additive manufacturing process parameters. Wire Type Process Parameters Wire Type Process Parameters ER304 ER50-6 ER304 ER50-6 Welding voltageWelding voltage 23 V 23 V 20 V 20 V Welding current 125 A 110 A Welding current 125 A 110 A Welding speed 5 mm/s 5 mm/s Welding speed 5 mm/s 5 mm/s Protective gas type and composition 95% Ar + 5% CO2 100% CO2 Protective gas type andProtective composition gas ﬂow 95% Ar + 5% 18 L CO/min2 16 L/100%min CO2 Protective gasWire flow diameter 18 L/min 10 mm 10 mm 16 L/min Wire diameter 10 mm 10 mm 3. Results and Discussion 3. Results and Discussion 3.1. Macroscopic Morphology and Microstructure of Stainless-steel Specimens 3.1. Macroscopic Morphology and Microstructure of Stainless-steel Specimens Figure4 shows a deposited sample produced by ER304 stainless-steel wire. It can be seen that the beadFigure surface 4 wasshows smooth, a deposited and the sample overlap produced fusion between by ER304 the stainless-steel welds was sound, wire. without It can be collapse. seen that the bead surface was smooth, and the overlap fusion between the welds was sound, without collapse.

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(a) (b) (a) (b) Figure 4. Wire arc additive manufacturing (WAAM) stainless-steel wall. (a) The three-types of FigureFigure 4. Wire 4. Wire arc additive arc additive manufacturing manufacturing (WAAM) (WAAM) stainless-steel stainless-steel wall. wall. (a) The (a) three-typesThe three-types of curvature of curvature of the samples; (b) examination cross-section (A: middle of the first layer; B: middle of the of thecurvature samples; of ( bthe) examination samples; (b) examination cross-section cross-section (A: middle (A: of middle the first of layer;the first B: layer; middle B: middle of the of fifth the layer; fifth layer; C: middle of the tenth layer; D: inside area of the fifth layer; E: outside area of the fifth C: middlefifth layer; of the C: tenth middle layer; of the D: tenth inside layer; area D: of inside the fifth area layer; of the E: fifth outside layer; area E: outside of the area fifth of layer). the fifth layer).layer). Figure5a–c show the metallographic microstructures of the A, B and C regions shown in Figure4, FigureFigure 5a–c 5a– showc show the the metallographic metallographic microstructuremicrostructuress of of the the A, A, B B and and C C regions regions shown shown in inFigure Figure of the curvature 0 sample, respectively. The white part in the figures is austenite, and the black part is 4, of4, ofthe the curvature curvature 0 0sample, sample, respectively. respectively. TheThe white partpart inin the the figures figures is is austenite, austenite, and and the the black black part part ferrite.is ferrite.is ferrite. Meanwhile, Meanwhile, Meanwhile, from from from the crystalthe the crystal crystal morphology, morphology,morphology, region regionregion A was AA was was mainly mainly mainly fine fine fine cell cell cell crystal. crystal. crystal. RegionRegion Region B B wasB mainlywaswas mainly coarse mainly dendritescoarse coarse dendrites dendrites and had and secondaryand hadhad secondarysecondary dendrites. dendrites.dendrites. Besides, Besides, itsBesides, dendrite its its dendrite spacingdendrite wasspacing spacing significantly was was largersignificantlysignificantly than that larger oflarger region than than A. that that Region ofof regionregion C is mainly A.A. RegiRegion composedon CC isis mainlymainly of free-form composedcomposed equiaxial of of free-form free-form crystals. equiaxial equiaxial crystals.crystals.

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

Figure 5. Cont. Coatings 2019, 9, 848 6 of 13 Coatings 2019, 9, x FOR PEER REVIEW 6 of 13 Coatings 2019, 9, x FOR PEER REVIEW 6 of 13

(e(e) ) (f()f )

FigureFigure 5. 5.Metallographic Metallographic structures structures ofof thethe curvaturecurvature 0 sample. (( aa)) metallographic metallographic microscope microscope optical optical imageimage atat the atthe bottomthe bottom bottom of theof of sample; thethe sample;sample; (b) SEM ((bb micrograph)) SEMSEM micrograph at the bottom atat thethe of thebottombottom sample; of of (the cthe) metallographicsample; sample; (c )( c) microscopemetallographicmetallographic optical microscope microscope image at theoptical optical middle imageimage of the atat sample; thethe middle (d) SEM ofof thethe micrograph sample;sample; ( d( atd) )SEM theSEM middle micrograph micrograph of the at sample; atthe the (middlee)middle metallographic of ofthe the sample; sample; microscope (e ()e )metallographic metallographic optical image microscopemicroscope at the top optical of the sample; imageimage at at ( fthe )the SEM top top micrographof of the the sample; sample; at ( thef )( SEMf) top SEM of themicrographmicrograph sample. at atthe the top top of of the the sample. sample.

The ratio of the temperature gradient (G) to the growth rate (R) determines the mode of The ratio of the temperature gradient (G) to the growth rate (R) determinesdetermines the mode of solidification, and the microstructure after solidification [25]. R = νcosα, and α is related to the solidification,solidification, andand the the microstructure microstructure after after solidification solidification [25]. [25].R = νRcos = αν,cos andα,α andis related α is torelated the distance to the distance to the centreline, here ν denotes the welding speed. In Figure 5, the positions of the bottom, todistance the centreline, to the centreline, here ν denotes here ν the denotes welding the speed. welding In Figure speed.5 ,In the Figure positions 5, the of positions the bottom, of middlethe bottom, and middle and top metallographic structures are located on a vertical line, and the distances from the topmiddle metallographic and top metallographic structures are structures located on are a verticallocated line,on a andvertical the distancesline, and fromthe distances the centre from line the are centre line are basically the same. It can be considered that the growth rates (R) of the three parts are basicallycentre line the are same. basically It can the be same. considered It can that be considered the growth that rates the (R )growth of the three rates parts(R) of are the similar, three parts and that are thesimilar, value ofandG /thatR is the approximately value of G/R is depended approximately on G .depended Due to the on heat G. Due dissipation to the heat of dissipation the substrate, of the the similar,substrate, and thatthe temperature the value of gradientG/R is approximately G at the bottom depended of the additive on G. Due parts to was the heatmaximized, dissipation and ofthe the temperature gradient G at the bottom of the additive parts was maximized, and the G/R was also the substrate,G/R was the also temperature the largest. Hence, gradient it was G at a mainlythe bottom planar of structure the additive and composedparts was of maximized, cellular crystal and at the largest. Hence, it was a mainly planar structure and composed of cellular crystal at the bottom, as G/Rthe was bottom, also the as largest.shown Hence,in Figure it was5a,b. a mainlyAs the planarheight structureof the deposited and composed layer was of cellularincreased, crystal the at shown in Figure5a,b. As the height of the deposited layer was increased, the temperature gradient thetemperature bottom, as gradient shown Gin in Figure the middle 5a,b. was As smaller the height than thatof thein the deposited bottom, and layer the wasG/R inincreased, the middle the G in the middle was smaller than that in the bottom, and the G/R in the middle was also smaller temperaturewas also smaller gradient than G that in the of themiddle bottom, was due smaller to the than preheating that in theeffect bottom, of the previouslyand the G/ Rformed in the layer. middle thanwasThe also that middle smaller of the was bottom, than mainly that due ofdendrites, tothe the bottom, preheating as shown due to einff theect Figure preheating of the 5c,d. previously Due effect to ofthe formed the heat previously layer.accumulation The formed middle of layer.the was mainlyTheformed middle dendrites, portion, was mainly asboth shown the dendrites, intemperature Figure as5c,d. shown gradient Due in to G theFigure and heat the5c,d. accumulation G /DueR ratio to ofthe ofthe heat the top formed accumulation portion portion, were ofthe both the theformedsmallest. temperature portion, In addition, gradientboth thethe Gheattemperatureand dissipation the G/R gradientratio direction of theG ofand topthe the portionmolten G/R pool wereratio at theofthe the smallest.top topwas portionchanged, In addition, were so the the heatsmallest.top dissipation portion In addition, was direction mainly the heatequiaxed of the dissipation molten dendrite, pool direction as at shown the of top inthe wasFigure molten changed, 5e,f. pool soat the top was portion changed, was mainly so the equiaxedtop portionFigure dendrite, was 6a,b mainly are as the shown equiaxed inside in and Figure dendrite, outside5e,f. metallographi as shown in Figurec structures 5e,f. of the fifth layer of the curvature 1 sampleFigure6 6a,b(positiona,b are are the the D inside insideand E and andin outsideFigure outside 4b). metallographic metallographi Figure 6c,d c structuresare structures the inside of of the theand fifth fifth outside layer layer ofmetallographic of the the curvature curvature 1 sample1 samplestructures (position (position of the D fifth andD and Elayer in FigureE of in the Figure4 curvatureb). Figure 4b). 62Figurec,d sample. are 6c,d the It insidecouldare the be and seeninside outside that and the metallographic outside dendrite metallographic spacing structures and ofstructures the fifthsecondary layerof the of dendritefifth the layer curvature spacing of the 2 in sample.curvature the middle It could2 sample.inside be site seen It werecould that larger thebe dendriteseen than that thatspacing the of dendritethe andmiddle the spacing outside. secondary and dendritethe secondary spacing dendrite in the middlespacinginside in the sitemiddle were inside larger site than were that larger of the than middle that outside. of the middle outside.

(a) (b)

Figure 6. Cont.

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Figure 6. Microscope morphologymorphology ofof thethe twotwo sidessides inin the the ﬁfth fifth layer layer for for the the curvature curvature 1 1 and and curvature curvature 2 (c) (d) samples.2 samples. (a ()a the) the inside inside for for the the curvature curvature 1 1 sample; sample; ( b(b)) the the outside outside forfor thethe curvaturecurvature 11 sample;sample; ((cc)) thethe insideFigure forfor 6. thetheMicroscope curvaturecurvature morphology 22 sample;sample; ((dd of)) the thethe outside outsidetwo sides forfor in thethe the curvaturecurvature fifth layer 22 sample.forsample. the curvature 1 and curvature 2 samples. (a) the inside for the curvature 1 sample; (b) the outside for the curvature 1 sample; (c) the Forinside curved for the passes, curvature the radius 2 sample; at the (d) theinside outside and for outs outside theide curvature sites is is different di2 sample.ﬀerent [26], [26], as as shown shown in in Figure Figure 77.. According to the testtest conditions,conditions, the width of the single-passsingle-pass multi-layer additive was about 6mm, and theFor welding curved speedpasses, of the the radius experimentexperiment at the inside waswas 55 and mmmm/s. /outss. Thus,ide sites from is thethedifferent formulaformula [26], as shown in Figure 7. According to the test conditions, the width of the single-pass multi-layer additive was about 6mm, 𝑣 v= (𝑅 + 3), 𝑣 v= (𝑅 − 3) (1) vo = (R + 3), v = (R 3) (1) and the welding speed of the experimentR was 5 mm/s.i Thus,R − from the formula vo: actual welding speed of outside; vi: actual welding speed of inside; v: actual welding speed. 𝑣 = (𝑅 + 3), 𝑣 = (𝑅 − 3) (1) vito was: actual concluded welding that speed for of 𝑣 outside;, the curvaturevi: actual 1 weldingsample was speed equal of inside; to 5.3 vmm/s,: actual and welding 𝑣 was speed. equal Itto was 4.7 concludedmm /s.v o𝑣: actual that forweldingvo, the speed curvature of outside; 1 sample vi: actual was equalwelding to 5.3speed mm of/s, inside; and vi vwas: actual equal welding to 4.7 mmspeed./s. vito ofwas the concluded curvature that 2 sample for 𝑣 was, the equal curvature to 5.5 1 mm sample/s, and wasvi wasequal equal to 5.3 to mm/s, 4.5 mm and/s. 𝑣 was equal to 4.7 mm /s. 𝑣

Figure 7. Schematic diagram of the weld path for a specific curvature.

Therefore, theFigure actualFigure 7. welding7.Schematic Schematic traveling diagram diagram speed of of the the weldon weld the path path outside for for a a specific specificwas higher curvature. curvature. than that of the inside. Under the same welding parameters, higher welding speeds on the outside made the heat input relativelyTherefore,Therefore, small, the theand actual actual the cooling welding welding rate traveling traveling relatively speed speed high, on on theresulting theoutside outside inwas awas smallhigher higher pitch thanthan of that dendritesthat ofof thethe on inside.inside. the Underoutside,Under the theand samesame a finer weldingwelding grain size. parameters,parameters, Meanwhile, higherhigher it can weldingwelding be seen speedsspeeds in Figure onon the 6the that outsideoutside the difference mademade thethe in heat heatgrain input input size relativelybetweenrelatively the small, small, inside andand and the the the cooling cooling outside rate rate of the relatively relatively curvature high, hi gh,2 sample resultingresulting is not inin aapparent,a small small pitch pitch compared of of dendrites dendrites to that on ofon the the outside,curvatureoutside, and and 1 sample. a a finer finer grain Theregrain size. size.were Meanwhile, Meanwhile,two reasons itit for cancan this bebe seenphenomenon.seen inin FigureFigure6 Firstly, 6that that the the in di thedifferencefference process in in of grain grain forming size size betweenabetween curved the paththe inside inside for arc and and additive the the outside outside manufacturing, of of the the curvature curvature a ma 2 gnetic2 sample sample field is is not notwould apparent, apparent, be generated compared compared in to theto that that adjacent of of the the curvaturearea.curvature The 1magnetic 1 sample. sample. Therefield There density were were two twoin reasonsthe reasons inside for for is this thislarger phenomenon. phenomenon. than that Firstly,of Firstly, the inoutside, in the the process processwhich of causes of forming forming the a directiona curved ofpath arc for thrust arc additiveto be directed manufacturing, to the outside a ma gnetic[27]. Thefield asymmetry would be generatedof this force in thecauses adjacent the asymmetryarea. The magnetic of the droplet field [28],density which in theresults inside in a is transport larger than of heat that input of the to outside,the outside which [29]. causes Whereas the direction of arc thrust to be directed to the outside [27]. The asymmetry of this force causes the asymmetry of the droplet [28], which results in a transport of heat input to the outside [29]. Whereas Coatings 2019, 9, 848 8 of 13 curved path for arc additive manufacturing, a magnetic field would be generated in the adjacent area. The magnetic field density in the inside is larger than that of the outside, which causes the direction of Coatings 2019, 9, x FOR PEER REVIEW 8 of 13 arc thrust to be directed to the outside [27]. The asymmetry of this force causes the asymmetry of the dropletthe effect [28 of], heat which input, results caused in a transportby the difference of heat inputin speed to thebetween outside the [29 inside]. Whereas and outside, the eff ectwas of offset heat input,by the causedmagnetic by thearc diblow.fference Secondly, in speed a betweenhigher effect the inside of magnetic and outside, arc blow was could offset bybe theobtained magnetic by the arc blow.larger Secondly,curvature. a higher effect of magnetic arc blow could be obtained by the larger curvature.

3.2. Microhardness Distribution of Stainless-steel Samples 3.2. Microhardness Distribution of Stainless-steel Samples Figure8 shows the microhardness distribution result. The hardness values of the first and fifth Figure 8 shows the microhardness distribution result. The hardness values of the first and fifth layers were mostly distributed above 200 HV, which were higher than that of the 304 stainless-steel layers were mostly distributed above 200 HV, which were higher than that of the 304 stainless-steel substrate. This indicates that the microstructure of the first and fifth layers was denser and more substrate. This indicates that the microstructure of the first and fifth layers was denser and more uniform than that of the 304 stainless-steel substrate. Meanwhile, the hardness of the tenth layer uniform than that of the 304 stainless-steel substrate. Meanwhile, the hardness of the tenth layer was was rather less than 200 HV, which was caused by its slightly lower compactness, than that of 304 rather less than 200 HV, which was caused by its slightly lower compactness, than that of 304 stainless-steel. From the overall observation and analysis, the hardness of the first layer was greater stainless-steel. From the overall observation and analysis, the hardness of the first layer was greater than that of the fifth and tenth layers. Moreover, the fifth layer was slightly harder than that of the than that of the fifth and tenth layers. Moreover, the fifth layer was slightly harder than that of the tenth layer. Generally, hardness is related to the grain size, carbide particle size and their distribution. tenth layer. Generally, hardness is related to the grain size, carbide particle size and their distribution. Due to the conventional heat dissipation of the substrate, the bottom layer of weld metal had the Due to the conventional heat dissipation of the substrate, the bottom layer of weld metal had the fastest cooling rate, the smallest weld grain, and the highest micro-hardness. Figures9 and 10 show fastest cooling rate, the smallest weld grain, and the highest micro-hardness. Figures 9 and 10 show the elemental distribution mapping of the middle layer and top layer with EDS analysis. It can be seen the elemental distribution mapping of the middle layer and top layer with EDS analysis. It can be that the concentration of Cr and C in the grain boundary position was slightly higher than that in the seen that the concentration of Cr and C in the grain boundary position was slightly higher than that innerin the regioninner region of the grain.of the grain. The trend The oftrend the of top the layer top waslayer more was obviousmore obvious than that than of that the of middle the middle layer. Thislayer. elemental This elemental distribution distribution originates originates from thefrom relatively the relatively slow coolingslow cooling rate ofra thete of top the layer. top layer. It had It suhadffi cientsufficient time time to precipitate to precipitate more more carbides carbides at the at grain the grain boundaries, boundaries, and cause and cause the carbon the carbon content content in the innerin the regioninner region of the grainsof the grains to be relatively to be relatively small, whichsmall, which slightly slightly decreased decreased the micro-hardness the micro-hardness value. However,value. However, in the middle in the layer,middle the layer, cooling the rate cooling was slightlyrate was faster, slightly causing faster, more causing carbide more particles carbide to remainparticles within to remain the grains, within and the less grains, precipitation and less toprecipitation the grain boundaries. to the grain Hence, boundaries. the hardness Hence, of the middlehardness layer of the was middle slightly layer higher was than slightly that higher of the topthan layer. that of the top layer.

Figure 8. Transverse microhardness distribution of top, middle and bottom of the curvature 0 sample. Figure 8. Transverse microhardness distribution of top, middle and bottom of the curvature 0 sample.

Coatings 2019, 9, 848 9 of 13 Coatings 2019, 9, x FOR PEER REVIEW 9 of 13

Figure 9. ElementalElemental distribution distribution of of the the top top layer layer mapped by EDS.

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FigureFigure 10. 10. ElementalElemental distribution distribution of of the the middle middle layer layer mapped mapped by by EDS. EDS.

FigureFigure 1111 showsshows the the lateral lateral hardness hardness distribution from from inside to outside. outside. The The curvature curvature 0 0 sample sample was relatively even in terms of hardness distribution, while while both both the the curvature curvature 1 1 and and curvature curvature 2 2 samplessamples hadhad aa rising rising trend trend from from inside inside to outside. to outside. Besides, Besides, it can it be can seen be that seen the that trend the of thetrend curvature of the curvature2 sample tended 2 sample to be tended gentler to than be thatgentler of the than curvature that of 1the sample, curvature as the 1 di sample,ﬀerence as in hardnessthe difference between in hardnessthe two sides between of the the curvature two sides 2 sampleof the curvature was smaller 2 sa thanmple that was of smaller the curvature than that 1 sample. of the curvature This result 1 sample.also corresponds This result to also the metallographiccorresponds to the microstructure metallographic shown microstructure in Figure6. shown in Figure 6.

Coatings 2019, 9, 848 11 of 13 Coatings 2019, 9, x FOR PEER REVIEW 11 of 13

Coatings 2019, 9, x FOR PEER REVIEW 11 of 13

FigureFigure 11. 11. TransverseTransverse microhardness distribu distributiontion in the middle of the curvaturecurvature 0, 1, and 2 samples. Figure 11. Transverse microhardness distribution in the middle of the curvature 0, 1, and 2 samples. 3.3. Microstructure of the Low Carbon Steel Sample 3.3. Microstructure of the Low Carbon Steel Sample 3.3.The Microstructure metallographic of the structure Low Carbon of Steel the Sample WAAM low carbon steel sample is shown in Figure 12. The metallographic structure of the WAAM low carbon steel sample is shown in Figure 12. When theThe curvature metallographic was small, structure the diofff theerence WAAM of grain low carbon size between steel sample inside is andshown outside in Figure was 12. larger. When the curvature was small, the difference of grain size between inside and outside was larger. WhenWhen the curvaturethe curvature increased, was small, the the di differencefference ofof grain size size between between inside inside and and outside outside was decreased,larger. When the curvature increased, the difference of grain size between inside and outside decreased, but but theWhen grain the sizecurvature inside increased, was still the larger difference than that of gr foundain size in between the outside. inside Theand outside reason decreased, that the structure but the grain size inside was still larger than that found in the outside. The reason that the structure and and propertiesthe grain size of inside additive was manufacturing still larger than that parts foun wered in a theffected outside. by curvature, The reason wasthat consideredthe structure toand be the properties of additive manufacturing parts were affected by curvature, was considered to be the differenceproperties in actual of additive welding manufactur travelinging speed parts betweenwere affected theinside by curvature, and outside. was considered With the increasingto be the of differencedifference in actual in actual welding welding traveling traveling speed speed betweenbetween thethe insideinside and and outside. outside. With With the the increasing increasing of of curvature, the enhanced effect of the magnetic arc blow reduced the difference in grain size between curvature,curvature, the theenhanced enhanced effect effect of of the the magnetic magnetic ararcc blow reducedreduced the the difference difference in grainin grain size size between between the inside and outside, owing to a difference in velocity. the insidethe inside and and outside, outside, owing owing to to a adifference difference in in velocity.velocity.

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

FigureFigure 12. Curvature12. Curvature 1 and 1 and curvature curvature 2 2 samples samples microstructure on on the the insi insidede and and outside outside of the of fifth the ﬁfth layer.layer. (a) Curvature (a) Curvature 1 sample 1 (samplec) inside; inside; (b )(b Curvature) Curvature 1 1 sample sample outside; outside; ((cc)) CurvatureCurvature(d) 2 2 sample sample inside; inside; (d) (d) Curvature 2 sample outside. FigureCurvature 12. Curvature 2 sample outside. 1 and curvature 2 samples microstructure on the inside and outside of the fifth

layer. (a) Curvature 1 sample inside; (b) Curvature 1 sample outside; (c) Curvature 2 sample inside; (d) Curvature 2 sample outside.

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4. Conclusions This investigation prepared stainless-steel and carbon steel thin-walled specimens with different curvatures, by a wire arc additive manufacturing method. Studies shows that trajectory curvature has an apparent influence on the microstructure and performance of thin-walled samples, in different local regions, which can be summarized as:

The top layer has a slow cooling rate. More carbides particles are precipitated at the grain • boundaries, rather than within the grains, and the microhardness value is slightly lower than that of the middle layer. Different trajectory curvature leads to a difference in microstructure and properties for inner and • outer regions, which originates from the difference in heat input, due to the difference in the actual welding traveling speed between the inside and outside regions. As curvature increases, the enhanced effect of the magnetic arc blow reduces the difference in heat • input, due to the difference in actual welding traveling speed between the inside and outside areas.

Author Contributions: Investigation, S.Y. and Z.B.; methodology, Z.T. and X.H.; software, Y.C.; writing—original draft, T.F.; writing—review & editing, L.W. Funding: This research was funded by National Natural Science Foundation of China No. 50901033. Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. 50901033) and funds from the Hubei Provincial Key Laboratory of Green Materials for Light Industry ((2013)2-6, (2013)2-14). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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