Low-Roughness-Surface Additive Manufacturing of Metal-Wire Feeding with Small Power

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Article Low-Roughness-Surface Additive Manufacturing of Metal-Wire Feeding with Small Power

Bobo Li 1,2 , Bowen Wang 1,2, Greg Zhu 2,*, Lijuan Zhang 2 and Bingheng Lu 1,2,*

1 School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (B.L.); [email protected] (B.W.) 2 National Innovation Institute of Additive Manufacturing, No. 997, Shanglinyuan 8th Road, Gaoxin District, Xi’an 710300, China; [email protected] * Correspondence: [email protected] (G.Z.); [email protected] (B.L.)

Abstract: Aiming at handling the contradiction between power constraint of on-orbit manufacturing and the high energy input requirement of metal additive manufacturing (AM), this paper presents an AM process based on small-power metal ﬁne wire feed, which produces thin-wall structures of height-to-width ratio up to 40 with core-forming power only about 50 W. In this process, thermal resistance was introduced to optimize the gradient parameters which greatly reduces the step effect of the typical AM process, succeeded in the surface roughness (Ra) less than 5 µm, comparable with that obtained by selective laser melting (SLM). After a 10 min electrolyte-plasma process, the roughness of the fabricated specimen was further reduced to 0.4 µm, without defects such as pores and cracks observed. The ultimate tensile strength of the specimens measured about 500 MPa, the relative density was 99.37, and the Vickers hardness was homogeneous. The results show that the

proposed laser-Joule wire feed-direct metal deposition process (LJWF-DMD) is a very attractive solution for metal AM of high surface quality parts, particularly suitable for rapid prototyping for

Citation: Li, B.; Wang, B.; Zhu, G.; on-orbit AM in space. Zhang, L.; Lu, B. Low-Roughness-Surface Additive Keywords: additive manufacturing; surface roughness; direct metal deposition; wire feed; ﬁne metal Manufacturing of Metal-Wire Feeding wire; laser and Joule heating with Small Power. Materials 2021, 14, 4265. https://doi.org/10.3390/ ma14154265 1. Introduction Academic Editor: Antonio Santagata In recent years, metal additive manufacturing gradually developed into a new strategic manufacturing technology, which plays an important role in the aerospace manufacturing Received: 28 June 2021 industry [1–4]. In metal additive manufacturing (AM), raw materials include metal powder Accepted: 27 July 2021 Published: 30 July 2021 and metal wires. Currently, metal material manufacturing technology research mainly focuses on powder-based process and wire-feed (WF)-based process. The powder-based

Publisher’s Note: MDPI stays neutral process was first widely developed and applied in manufacturing. Its representative with regard to jurisdictional claims in technology is selective laser melting (SLM) [5,6]. SLM was the first developed, and after published maps and institutional affil- improvements, is most widely used in sophisticated fields such as aerospace and medical iations. engineering. The SLM is mature and has high printing precision, because of the use of small powder sizes (15–40 µm) and optimization of process parameters that enable the surface roughness of parts to be reduced significantly, to 6–20 µm for SLM techniques [7,8]. However, it has disadvantages, e.g., expensive raw materials, the lower utilization efficiency of materials (20–30%), and poor density [9,10] and so on. More importantly, the SLM cannot Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. be used in a microgravity environment where the motion of the powdered particles is This article is an open access article beyond control. distributed under the terms and Hence, some researchers shifted their focus to wire feed techniques, which, when conditions of the Creative Commons compared to SLM, has lower cost of raw materials, higher utilization efficiency (nearly Attribution (CC BY) license (https:// 100%), and lower impact on the environment [11–13]. The weakness of wire-feed-based creativecommons.org/licenses/by/ AM is that its surface quality is much lower than that obtained by SLM. In most of the 4.0/). reported research works, metal wires of a diameter of about 1 mm are used, and the width

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of deposition is 5 to 10 times the diameter of the wire [14,15]. Using SLM, the layer height and thickness varies between 5 and 19 mm, requiring a considerable amount of subtractive post-process, when used to fabricate high-finish parts. The roughness is a significant index for evaluating AM parts. The lower roughness causes easier fitting, more erosion-resistance, and better sealing; thus, alleviating wear-out and prolonging service life [16–18]. Moreover, with lower roughness, the burden of the post-process is reduced greatly, and some AM parts can even be directly fitted. Lower roughness can further save the material loss by subtractive post-process. There are three factors that make the SLM obtain lower roughness: small size of powder, high-precision heat source, and process parameter optimization. Aiming at these three factors, the study was improved correspondingly. Currently, the commonly used heat sources include plasma, electric arc, electron beam, laser and compound heat sources [19,20]. To fabricate high-finish parts, laser heating is the first choice for heat source. When well-focused, the light spot of the laser is tiny, the heating-power per unit area is very high, and the energy is released intensely. However, the electro-thermal conversion rate of laser is rather low, thus requiring a large-power laser device for metal AM. Demir AG al. [21] used a fine-wire AM technique to build thin-wall structures with aspect ratios up to 20, with a 301 stainless steel 0.5 mm wire as the raw material and a pulse laser heat source of peak power 5 KW. Shaikh MO et al. [22], using 0.1 mm stainless steel wires and a pulse laser, 6 KW peak power, fabricated a metal part with a roughness (Ra) of 8–16 µm. These two studies demonstrated that laser-heated fine-wire AM can be used to produce thin-walled parts with good surface morphology. However, these fine wire AM need large laser-power. Most studies have focused on increasing the deposition efficiency and cost performance [23,24], and less attentions have been focused on obtaining good AM morphology under limitation of input of small heat power. Therefore, to yield high precision metal for on-orbit manufacturing, the crucial issue is to solve the power deficiency of small single laser. A fine-wire-based direct metal deposition (DMD) technique was developed in this study, using a 0.3 mm-diameter 316 stainless steel wire as the raw material and laser and Joule heating as a compound heat source, process parameters were adjusting layer-by-layer. When Joule heating wire during processing, it can greatly reduce the power requirement of laser, thus the total heat input was controlled and the heat in the forming process was effectively managed. Using this method, 316 L thin-walled specimens of an aspect ratio up to 40 were successfully fabricated. The surface roughness, microstructure, density, hardness, and mechanical properties of specimens were tested, and the main factors affecting the surface roughness were analyzed. It is expected that, with its small size and light weight, the laser-Joule wire feed-direct metal deposition (LJWF-DMD) equipment is a premium solution for in situ manufacturing in space.

2. Materials and Methods 2.1. Materials Both the metal wire and the baseplate used in this study were fabricated of 316 L stainless steel. The mass percent of the steel is shown in Table1.

Table 1. Chemical Compositions of 316 L stainless steel.

Elements Fe Cr Mn Mo Ni Si C Wt% 64.447 17.3 1.74 2.66 13.1 0.73 0.023

2.2. Experimental Design and Setup An experimental platform based on laser-Joule wire feed-direct metal deposition (LJWF-DMD) was built in this study. Figure1 is a schematic of the platform, which consisted of a laser unit with a laser head (IPG, Oxford, MA, USA), a camera with a Materials 2021, 14, x FOR PEER REVIEW 3 of 17

Materials 2021, 14, 4265 3 of 16

consisted of a laser unit with a laser head (IPG, Oxford, MA, USA), a camera with a fill lampﬁll lamp(Helica,(Helica, Shenzhen, Shenzhen, China China),),, a wire a feeder, wire feeder, a motion a motion platform, platform, a Joule aheating Joule heatingpower supplypower and supply a baseplate and a baseplate preheating preheating temperature temperature control unit. control Laser unit. and LaserJoule heat and were Joule usedheat as were the usedcompound as the compoundheat source heatin the source printing in the process, printing and process, the baseplate and the preheating baseplate unitpreheating provide unitd a stable provided initial a stabletemperature. initial temperature. Oxidization during Oxidization the printing during process the printing was preventedprocess was by prevented placing the by entire placing platform the entire in aplatform DELLIX in standard a DELLIX glove standard box ( glove(DELLIX, box C((DELLIX,hengdu, China Chengdu,) in which China) the in oxygen which the content oxygen and content moisture and content moisture were content both werebelow both 10 ppm.below 10 ppm.

FigureFigure 1. 1. SchematicSchematic of of the the LJWF LJWF-DMD-DMD AM AM system. system.

AA photo photo of of the the experiment experiment device device is is shown shown in in Figure Figure 22a.a. A laser head, wire feederfeeder andand camera camera are are mounted mounted on on the the sliding sliding blocks blocks of of the the fixture. fixture. The The framework framework is is fabricated fabricated of aluminum. The base is a porous optical slab that is easy to install and adjust. A YLP- of aluminum. The base is a porous optical slab that is easy to install and adjust. A YLP- series fiber laser (IPG, MassachusettsdUSA) is one of the major heat sources for FW-DMD series fiber laser (IPG, MassachusettsdUSA) is one of the major heat sources for FW-DMD manufacturing. In Figure2b is the small (360 mm × 300 mm × 110 mm) and light (5 kg) manufacturing. In Figure 2b is the small (360 mm × 300 mm × 110 mm) and light (5 kg) laser unit, which is equipped with an independent air-cooling system. After collimation laser unit, which is equipped with an independent air-cooling system. After collimation and focusing, the focal length of the laser header is 250 mm and the diameter of the focal and focusing, the focal length of the laser header is 250 mm and the diameter of the focal spot is 80 µm. Since the wire diameter is 300 µm, the laser spot should be negatively spot is 80 μm. Since the wire diameter is 300 μm, the laser spot should be negatively de- defocused to 350 µm so that the laser spot can cover the wire fully with adequate energy focused to 350 μm so that the laser spot can cover the wire fully with adequate energy density under the limited power of 50 W only. A Joule heating source was used mainly for densitymelting under wire. Inthe Figure limited2c, power the power of 50 supply W only. of A the Joule Joule heating heating source source was is a used customized mainly forcurrent melting source wire. with In Figure an adjustable 2c, the power range supply of 0–30 of A the and Joule a power heating less source than 50is a W. custom- Three- izeddimensional current source (3D) movement with an adjusta of theble baseplate range of is realized0–30 A and by meansa power of less a ball-screw than 50 W. three-axis Three- dimensionalmotion platform (3D) movement with a STEVAL-3DP001V1 of the baseplate (STMicroelectronics,is realized by means Geneva,of a ball- Switzerland)screw three- axismain motion control platform unit. A stablewith a high-temperature STEVAL-3DP001V1 field (STMicroelectronics was created on the,baseplate Geneva, Switzer- by using landa pre-heating) main control unit consistingunit. A stable of ceramic high-temperature heating elements, field was heat-isolating created on the mica baseplate sheets and by usinga proportional-integral-derivative a pre-heating unit consisting of (PID) ceramic temperature-control heating elements, power heat-isolating supply, asmica shown sheets in andFigure a proportional2d, which can-integral provide-derivative a maximum (PID) heating temperature temperature-control of power 600 ◦ Csupply, and can as shown be real- intime Figure monitored 2d, which and can controlled provide to a maintain maximum the heating temperature temperature at a constant of 600 level. °C and To enhance can be realimage-time resolution, monitored the and laser controlled welding to camera maintain is equippedthe temperature with a blue-lightat a constant fill level. lamp To for enhancetracking image the laser-wire resolution, and the observing laser welding the printing camera areasis equipped in real with time. a Ablue short-range-light fill lamp wire fordelivery tracking system the laser was-wire designed and observing to feed fine the wire printing of 0.3 areas mm in in real diameter time. smoothly,A short-range and wiresuppress delivery the disturbancessystem was designed and ensure to accuratefeed fine aligning wire of 0.3 of the mm laser in diameter beam with smoothly, the fine wire.and suppressThis system the isdisturbances comprised ofand a motorensure withaccurate drive, aligning a wire guideof the andlaser laser-wire beam with alignment the fine wire.adjustment This system mechanisms. is comprised The abovementioned of a motor with laserdrive, header, a wire wire guide feeder and andlaser camera-wire align- were mentmounted adjustment on the slidingmechanisms. blocks ofThe an abovementioned arc bracket to facilitate laser header, implementation wire feeder of experimentsand camera from different orientations (Figure2e).

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Materials 2021, 14, 4265 were mounted on the sliding blocks of an arc bracket to facilitate implementation4 of 16 of experiments from different orientations (Figure 2e).

FigureFigure 2. Details 2. Details of the of LJWF the LJWF-DMD-DMD AM AM system. system. (a (a) )Overview Overview ofof testtest equipment. equipment. (b ()b Laser) Laser device. device (c). Joule(c) Joule heating heating power power supply.supply. (d) Baseplate (d) Baseplate temperature temperature control control system. system. ( (ee)) Laser Laser-wire-wire alignmentalignment adjustment adjustment mechanism. mechanism.

TableTable 22 gives gives main main the the parameters parameters and adjustment and adjustment ranges, which, ranges, determined which, through determined a lot of experiments, include the laser power (PL), Joule heating power (PJ), laser incident through a lot of experiments, include the laser power (PL), Joule heating power (PJ), laser angle (A1), wire feeding angle (A2), wire feeding speed (VW) and moving speed (VM) incidentrequired angle to ensure (A1), wire AM stability.feeding angle A key ( parameterA2), wire feeding (K) was definedspeed ( asVW the) and ratio moving of wire speed (VMfeeding) required speed to toensure moving AM speed, stability. i.e., the A equation:key parameter (K) was defined as the ratio of wire feeding speed to moving speed, i.e., the equation: Table 2. Characteristics of LJWF-DMD AM system. Table 2. Characteristics of LJWF-DMD AM system. Parameter Value LaserParameter system type Model YLPN-WELD-DEM0-2Value LaserLaser system type type Model Ytterbium YLPN fiber-WELD laser -DEM0-2 Laser modetype CW(continuousYtterbium wave)fiber laser EmissionLaser wave mode length CW(continuous 1064 nm wave) Laser Max power 50 W EmissionLaser incidence wave length angle 50–801064◦ nm LaserWire feeding Max power angle 30–6050◦ W LaserMin. incidence beam diameter angle 80 µ50°m –80° MaximumWire feeding moving angle rate 300 mm/min30°–60° MaximumMin. beam wire diameter feed rate 600 mm/min80 μm Wire feeding direction Front MaximumWire diameter moving rate 0.3300 mm mm/min MaximumShielding wire gasfeed rate 600 Ar mm/min WireOxygen feeding content direction ≤10 ppmFront K = VW (1) Wire diameterVM 0.3 mm Shielding gas Ar ExperimentsOxygen were content carried out for single-layer and multi-layer≤10 depositions. ppm In view of different heating effects under different thermal environments, the parameters were 푉푊 progressively changed for퐾 = a different number of layers. Studies on the influence of the wire (1) 푉 feed direction on printing quality푀 [25–27] already showed that front wire feeding produces parts with the highest surface smoothness. Hence, front wire feeding was adopted in thisExperiments study. were carried out for single-layer and multi-layer depositions. In view of different heating effects under different thermal environments, the parameters were progressively changed for a different number of layers. Studies on the influence of the wire feed direction on printing quality [25–27] already showed that front wire feeding

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Water content ≤10 ppm Maximum Joule power 50 W Baseplate temperatures ≤600 °C

The maximum optical power during actual printing was measured to be 50 W on a PRIMES Cube laser power meter.

2.3. Characterization The surface roughness of a thin-walled specimen was measured mechanically and optically using a MarSurf M300C roughness meter and a Smartproof 5 laser scanning confocal microscope (Zeiss, Jena, Germany), respectively, according to ISO4287. The dimensions of the multi-layer thin-walled structure were measured using an NSCING Vernier caliper (NscingEs, Shuzhou, China) and a digital micrometer with an accuracy of 0.001 mm. The Vickers hardness of sections of the thin-walled part at different heights was measured using a Mitutoyo HM-200 automatic Vickers hardness tester (Mitutoyo, Sanfeng, Japan) 10 s after the application of a 0.2-N load. A universal material testing machine (INSTRON 5982, Norwood, MA, USA) was used to test the tensile strength and elongation at the fracture of the specimens. The specimens fractured by tensile failure were imaged using a JSM-7900F ﬁeld emission scanning electron microscope (JEOL, Tokyo, Japan) provided by JEOL. A mud saw and abrasive paper were used to prepare multi- layer single-track thin-walled cross-sections for imaging with an Axio Ver.A1 inverted metallurgic microscope (Zeiss, Jena, Germany). The image processing software ImageJ was used to measure the widths, heights, sectional areas and wetting angles of the cross- sections. The density of the thin-walled part was measured using a DahoMeter DH-220MN (Dahometer, Shenzhen, China) electronic density and speciﬁc gravity tester based on the Archimedes method of water displacement; the buoyancy and volume of an object in water are equal in magnitude to the weight and volume of water displaced, respectively. Hence, the density of the specimen is obtained as follows:

mw = m1 − m2 (2)

mw Vw = (3) ρw

m1 m1 m1 ρ1 = = m −m = ρw (4) Vw 1 2 m1 − m2 ρw

where mw is the mass of water displaced by the tested specimen (g); m1 is the specimen −3 mass in air (g); m2 is the specimen mass in water (g); ρw is the density of water (g·cm ); 3 Vw is the volume of water displaced by the specimen (cm ); and ρ1 is the specimen density (g·cm−3).

3. Results 3.1. Single-Layer Deposition The main factors that affected the printing quality during single-track formation were PL, PJ, A1, A2, VW and VM, see Figure3. Figure3a shows that the well-matched input energy and wire feeding produces good printing quality. On the other hand, excessive energy input and low feed speed resulted in premature wire melting, where the wire melted into small balls before entering the molten pool and the printing process was disrupted as Figure3b indicates. Figure3c shows that insufﬁcient power input, higher feed speed or lower moving rate (larger K) caused the wire to bend and deposit, resulting in the failure of the process. If the input energy is too low, interlayer bonding cannot be achieved even though the wire melts. It should be noted that the temperature of the baseplate also affects the quality. The specimen cross-section was analyzed to assess the single-track part quality. Figure4a shows a typical single-track section. The inﬂuential parameters include the Materials 2021, 14, x FOR PEER REVIEW 6 of 17

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single-track width (W), single-track height (H), printing area (Ac), molten area (As) and wetting angle (α), and dilution rate (D) which is deﬁned as As/(Ac + As). The α should be Materials 2021, 14, x FOR PEER REVIEW 6 of 17 maintained as small as possible to facilitate good overlap between multiple single tracks during transverse splicing to prevent pore formation during printing.

Figure 3. Effect of different parameters on the morphology of specimens in single-track LJWF-DMD AM. (a) Successful single track. (b) Discontinuity and spheroidization. (c) Wire bending and deposition.

If the input energy is too low, interlayer bonding cannot be achieved even though the wire melts. It should be noted that the temperature of the baseplate also affects the quality. The specimen cross-section was analyzed to assess the single-track part quality. Figure 4a shows a typical single-track section. The influential parameters include the single-track

width (W), single-track height (H), printing area (Ac), molten area (As) and wetting angle (α),Figure and 3. 3. dilutionEffectEffect of of differentrate different (D) parameterswhich parameters is defined on onthe the morphologyas As/(Ac morphology + of As). specimens of The specimens α should in single in be single-track-track maintained LJWF- LJWF-DMD as AM. (a) Successful single track. (b) Discontinuity and spheroidization. (c) Wire bending and depo- smallDMD as AM. possible (a) Successful to facilitate single good track. overlap (b) Discontinuity between multiple and spheroidization. single tracks (c )during Wire bending trans- sition. verseand deposition. splicing to prevent pore formation during printing. If the input energy is too low, interlayer bonding cannot be achieved even though the wire melts. It should be noted that the temperature of the baseplate also affects the quality. The specimen cross-section was analyzed to assess the single-track part quality. Figure 4a shows a typical single-track section. The influential parameters include the single-track width (W), single-track height (H), printing area (Ac), molten area (As) and wetting angle (α), and dilution rate (D) which is defined as As/(Ac + As). The α should be maintained as small as possible to facilitate good overlap between multiple single tracks during transverse splicing to prevent pore formation during printing.

Figure 4. LJWF-DMD single-track cross-sections. (a) Geometry of the single-track section. (b) Single- track sections obtained using a single-laser heat source. (c) Single-track section obtained using a laser Figure 4. LJWF-DMD single-track cross-sections. (a) Geometry of the single-track section. (b) Single- trackandJoule sections heating. obtained (d) Single-track using a single sections-laser obtained heat source. at different (c) Single baseplate-track section heating obtained temperatures. using a laser and Joule heating. (d) Single-track sections obtained at different baseplate heating temperatures. The single tracks formed using various parameters were cross-sectioned and fabricated into specimens. The sectional morphology of the polished specimens was observed by an opticalThe single microscope tracks formed to check using for defects.various parameters Comparisons were were cross observed-sectioned between and fabri- the catedsingle-track into specimens. sections formedThe secti usingonal morphology compound heat of the sources polished and specimens those using was only observed single bylaser. an GivenopticalP microscopeL = 50 W, VW to= check 15 mm/min for defects. and V ComparisonsM = 15 mm/min, were using observed single between laser, the the left singletrack- hastrack a verysections small formed contact using area withcompound the baseplate, heat sources and the and right those one using does only not contact single at all, as shown in Figure4b, i.e., insufﬁcient energy results in poor bonding between the laser.Figure Given 4. LJWF P-LDMD = 50 singleW, VW-track = 15 crossmm/min-sections. and ( aV) MGeometry = 15 mm/min, of the single using-track single section. laser, (b the) Single left- wire and baseplate. Using the same parameters, after introducing a Joule heating current tracktrack hassections a very obtained small usingcontact a single area -withlaser theheat baseplate, source. (c) and Single the-track right section one does obtained not contact using a laserI = 10 andA andJoule raising heating. the (d temperature) Single-track ofsections the baseplate obtained toat 300different◦C, tracks baseplate bond heating well withtempera- the at all, as shown in Figure 4b, i.e., insufficient energy results in poor bonding between the tures.baseplate, without defects such as pores and cracks, as shown in Figure4c. Figure4d shows wire and baseplate. Using the same parameters, after introducing a Joule heating current the variation of the sectional morphology of specimens with the baseplate temperature I = 10 A and raising the temperature of the baseplate to 300 °C, tracks bond well with the increasingThe single and thus tracks decreasing formed using the thermal various gradient parameters between were the cross baseplate-sectioned and theand molten fabri- baseplate, without defects such as pores and cracks, as shown in Figure 4c. Figure 4d poolcated can into effectively specimens. improve The secti theonal spreading morphology of the singleof the track:polished both specimens width (W), was and observed dilution shows the variation of the sectional morphology of specimens with the baseplate (D),by an increase, optical whereasmicroscope height to check (H), andfor defects. wetting Comparisons angle (α), decrease. were observed When α isbetween below 90the◦, singleits section-track is sections smaller formed than a semicircle,using compound and the heat single-track sources and can those be used using for only transverse single laser.multi-pass Given splicing. PL = 50 W, VW = 15 mm/min and VM = 15 mm/min, using single laser, the left track has a very small contact area with the baseplate, and the right one does not contact at all, as shown in Figure 4b, i.e., insufficient energy results in poor bonding between the wire and baseplate. Using the same parameters, after introducing a Joule heating current I = 10 A and raising the temperature of the baseplate to 300 °C, tracks bond well with the baseplate, without defects such as pores and cracks, as shown in Figure 4c. Figure 4d shows the variation of the sectional morphology of specimens with the baseplate

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temperature increasing and thus decreasing the thermal gradient between the baseplate and the molten pool can effectively improve the spreading of the single track: both width (W), and dilution (D), increase, whereas height (H), and wetting angle (α), decrease. When Materials 2021, 14, 4265 α is below 90°, its section is smaller than a semicircle, and the single-track can be used7 of for 16 transverse multi-pass splicing.

3.2. Thin Thin-Wall-Wall Deposition To examine whetherwhether the proposed techniquetechnique is valid,valid, thin-walledthin-walled specimens were prepared on a a 2 2 mm mm-thick-thick baseplate baseplate using using a alaser laser power power of of 50 50 W W,, Joule Joule heating heating current current of 10 A and a baseplate temperature of 500 °C◦ (Figure 5a). Parameters VW and VM were ad- of 10 A and a baseplate temperature of 500 C (Figure5a). Parameters VW and VM were justedadjusted according according to the to thenumber number of the of layers. the layers. Figure Figure 5a shows5a shows three three150-layer 150-layer thin-walled thin- walledparts corresponding parts corresponding to different to different K (0.8, K1 and(0.8, 1.5). 1 and The 1.5). specimen The specimen dimensions dimensions were meas- were ured.measured. The 150 The-layer 150-layer deposition deposition was 24 was mm 24 high mm at high K = at1.5K (Figure= 1.5 (Figure 5b). The5b). thickness The thickness of the specimenof the specimen was measured was measured at different at different points, points, being being0.589 0.589mm on mm average. on average. Its aspect Its aspect ratio reachedratio reached over over 40. In 40. Figure In Figure 5d,5 thed, the thickness thickness was was measured measured at at eight eight points, which are are equally spacedspaced atat thethe upper upper and and lower lower portion portion of of a widera wider thin-wall. thin-wall. The The measurements measurements are aregiven giv inen Table in Table3, with 3, with the average the average wall thicknesswall thickness 0.581 0.581 mm, averagemm, average deviation deviation 0.017 0.017 mm, mm,and rangeand range 0.047 0.047 mm. mm. These These results results show show the specimens the specimens thus formedthus formed using using laser pluslaser Joule plus Jouleheating heating with awith uniform a uniform thickness thickness and good and dimensional good dimensional stability. stability. Figure5 c,fFigure show 5c that,f show the thatformation the formation of specimens of specimens can be performed can be performed with different with different inclination inclination angles toangles the X to- and the YX-axes,- and Y respectively.-axes, respectively.

Figure 5. 5. LJWFLJWF-DMD-DMD thin thin-walled-walled parts. parts. (a) (Partsa) Parts formed formed using using various various values values of K. ( ofb) KHeight.(b) Height measurement.measurement. (c) Thin (c)-walled Thin-walled part inclined part inclined towards towards the Y- theaxis.Y (-axis.d) Measuring (d) Measuring points. points. (e) Thickness (e) Thickness measurement.measurement. (f) Thin (f)- Thin-walledwalled part inclined part inclined towards towards the X the-axis.X-axis.

Table 3.3. The 150-layer150-layer thin-walledthin-walled partpart thicknessthickness measurementsmeasurements (in(in mm),mm),K K= = 1.1.

NumberNumber 11 2 3 4 upperupper 0.5950.595 0.589 0.602 0.604 lowerlower 0.5680.568 0.573 0.563 0.557

3.3. Surface Finish 3.3. Surface Finish Surface roughness is one of the most important characteristics of AM parts. Many Surface roughness is one of the most important characteristics of AM parts. Many interdependent factors and parameters have significant influence on the surface rough- interdependent factors and parameters have significant influence on the surface roughness. ness. The average surface roughness (Ra), is the mean variation in distance of a surface The average surface roughness (Ra), is the mean variation in distance of a surface profile to profile to the measurement center line, and the maximum deviation (Rz), is the distance the measurement center line, and the maximum deviation (Rz), is the distance between the highest and lowest points on the contour [28]. These two indexes, Ra and Rz, are the most commonly used indicators of surface roughness. It is known that layer-by- layer material addition creates an inherent stair-step effect on the test specimen surface. Figure6a,b show that both the front (marked by a red dot) and back surfaces have the same morphology. A two-dimensional (2D) optical microscopy image of the stair-step effect under a 20× objective is given in Figure6c, and its enlarged optical microscopy image in Materials 2021, 14, x FOR PEER REVIEW 8 of 17

between the highest and lowest points on the contour [28]. These two indexes, Ra and Rz, are the most commonly used indicators of surface roughness. It is known that layer-by- layer material addition creates an inherent stair-step effect on the test specimen surface. Figure 6a,b show that both the front (marked by a red dot) and back surfaces have the Materials 2021, 14, 4265 same morphology. A two-dimensional (2D) optical microscopy image of the stair-step ef-8 of 16 fect under a 20× objective is given in Figure 6c, and its enlarged optical microscopy image in Figure 6d. The image shows clearly the lines of deposition layers on the thin-walled part. The contour is cyclical with the height of each layer, approximately 0.11 mm. Ac- cordingFigure to6 d.measurement The image shows specifications, clearly the the lines corresponding of deposition sampling layers on length the thin-walled is 1 mm. As part. theThe specimen contour surface is cyclical is not with perfectly the height even of and each the layer, roughness approximately varies at 0.11 different mm. According points, a to singlemeasurement sampling length specifications, could not the indicate corresponding accurately sampling the surface length roughness is 1 mm. As as the a whole. specimen Therefore,surface ismultiple not perfectly sampling even lengths and the were roughness used here varies at at the different front and points, back a singleof the samplingspeci- mens.length The could Ra and not Rz indicate were measured accurately for the both surface the roughnessfront and asback a whole. sides of Therefore, two 150- multiplelayer thinsampling-walled parts lengths formed were at used K = here1 and at K the = 1.5. front Table and 4 back lists ofhorizontal the specimens. and vertical The Ra meas-and Rz urementswere measured at three points. for both It thecan front be seen and the back roughness sides of two is very 150-layer low in thin-walled the horizontal parts direc- formed K K tion,at indicating= 1 and printing= 1.5. Table was 4smoothly lists horizontal completed and. verticalOn the measurementsother hand, in atthe three vertical points. di- It can be seen the roughness is very low in the horizontal direction, indicating printing was rection, the stair-step effect is significant and the roughness is much larger. It is noted that, smoothly completed. On the other hand, in the vertical direction, the stair-step effect is with K increased, the stair-step effect and hence roughness become worse, although the significant and the roughness is much larger. It is noted that, with K increased, the stair-step printing efficiency improves. effect and hence roughness become worse, although the printing efficiency improves.

Figure 6. Surface morphology of LJWF-DMD-printed specimens, (a) front view, (b) back view, (c) 2D Figureoptical 6. Surface microscopy morphology stair-step of effect LJWF image-DMD (1)-printed (d) and specimens enlarged 2D, (a) optical front view microscopy, (b) back stair-step view, ( effectc) 2D optical microscopy stair-step effect image (1) (d) and enlarged 2D optical microscopy stair-step image (2). effect image (2).

TableTable 4. Surface 4. Surface roughness roughness of LJWF of LJWF-DMD-DMD AM AM thin thin-walled-walled parts, parts, mechanically mechanically measured measured.. µ µ K K DirectionDirection Ra (μm)Ra ( m) Rz (μm)Rz ( m) 1 1Horizontal Horizontal 0.7 0.74.2 4.2 1 1Horizontal Horizontal 0.8 0.83.9 3.9 1 Horizontal 0.7 3.4 1 1Horizontal Vertical 0.7 2.33.412.1 1 1Vertical Vertical 2.3 2.212.1 10.8 1 1Vertical Vertical 2.2 2.410.8 12.1 1 1.5Vertical Horizontal 2.4 2.012.1 11.6 1.5 Horizontal 1.4 12.6 1.5 Horizontal 2.0 11.6 1.5 Horizontal 2.3 9.9 1.5 1.5Horizontal Vertical 1.4 3.812.6 17.9 1.5 1.5Horizontal Vertical 2.3 3.39.917.3 1.5 1.5Vertical Vertical 3.8 3.817.9 18.4

The 3D surface morphology of the thin-walled part formed at K = 1 was observed under a laser scanning confocal microscope and is shown in Figure7a. Sections were extracted along the X- and Y-axes, i.e., in the horizontal and vertical (deposition) directions, respectively. The line roughness measured in both directions, with distortion corrected and noise removed, are given in Figure7b,c. The values given by Smartproof 5 are close to those listed in Table4, indicating the data are credible. Figure7d shows the contour lines along the Y-axis (the Materials 2021, 14, x FOR PEER REVIEW 9 of 17

1.5 Vertical 3.3 17.3 1.5 Vertical 3.8 18.4

The 3D surface morphology of the thin-walled part formed at K = 1 was observed under a laser scanning confocal microscope and is shown in Figure 7a. Sections were extracted along the X- and Y-axes, i.e., in the horizontal and vertical (deposition) directions, respectively. The line roughness measured in both directions, with distortion corrected and noise removed, are given in Figure 7b,c. The values given by Smartproof 5 are close to those listed in Table 4, indicating the data are credible. Figure 7d shows the contour lines along the Y-axis (the printing direction) on the extracted surface. As shown, within the 1 mm range, nine crests and troughs are observed, corresponding to the stair-step effect caused by deposition layer by layer, thus making the contour vary cyclically from layer to layer. The surface roughness (in 3D) is measured and calculated based on ISO4287. The measurements are given in Figure 7e, including the surface arithmetic mean height (Sa), the maximum height (Sz), the root-mean-square (RMS) height (Sq), the skewness (Ssk), the kurtosis (Sku), the peak (Sp), and the vale (Sv) [29]. These data demonstrate that the proposed laser and Joule-heating technique (LJWF-DMD) can effectively reduce

Materials 2021the, 14, 4265stair-step effect and improve the surface quality of thin-walled parts. For comparison, 9 of 16 the data on the reduction in surface roughness by LJWF-DMD and by other processes are given in Table 5.

printing direction) on the extracted surface. As shown, within the 1 mm range, nine crests Table 5. Comparison of the proposed LJWF-DMD process with some research works from the liter- and troughs are observed, corresponding to the stair-step effect caused by deposition layer ature. by layer, thus making the contour vary cyclically from layer to layer. The surface roughness Materials Process(in 3D) is measuredSurface and Roughness calculated based on ISO4287. TheReference measurements are given in SLMFigure 7e , includingSa the (4 surface–10 μm) arithmetic mean heightImade, (Sa K.), et the al. maximum(2018) [30] height (Sz), the Powder SLMroot-mean-square Ra (3– (RMS)4 μm); Rz height (16–20 (S μq),m) the skewness (SskMatras), the, A kurtosis. (2020) [31 (Sku] ), the peak (Sp), EBMand the vale (Sv)[29Ra]. (1 These8–22 μ datam) demonstrate thatBorrelli, the proposed R. et al. (2019) laser [17] and Joule-heating WAAMtechnique (LJWF-DMD)Ra (200 can μm) effectively reduce the stair-stepXiong, J. et effect al. (2017) and [ improve32] the surface Wire FW-qualityLMD of thin-walledRa parts. (8–16 μ Form) comparison, theMuhammad, data on the O.S. reduction et al. (2019) in surface [22] roughness LJWFby-DMD LJWF-DMD Ra (3–5 andμm); byRz( other4–20 μ processesm); Sa (4–8 are μm given) in Table5.This study

Figure 7. Surface roughness of LJWF-DMD thin-walled part. (a) 3D confocal microscopy images. (b) Line roughness Figure 7. Surface roughness of LJWF-DMD thin-walled part. (a) 3D confocal microscopy images. (b) Y c X d e along Line-axis. roughness ( ) Line roughness along Y-axis. along (c) -axis.Line roughness ( ) Surface along contour X-axis. extracted (d) Surface in the printing contour direction. extracted ( in) Specimenthe surfaceprinting roughness. direction. (e) Specimen surface roughness.

Table 5. Comparison of the proposed LJWF-DMD process with some research works from the literature.

Materials Process Surface Roughness Reference SLM Sa (4–10 µm) Imade, K. et al. (2018) [30] Powder SLM Ra (3–4 µm); Rz (16–20 µm) Matras, A. (2020) [31] EBM Ra (18–22 µm) Borrelli, R. et al. (2019) [17] WAAM Ra (200 µm) Xiong, J. et al. (2017) [32] Wire FW-LMD Ra (8–16 µm) Muhammad, O.S. et al. (2019) [22] LJWF-DMD Ra (3–5 µm); Rz (4–20 µm); Sa (4–8 µm) This study

3.4. Hardness and Density The thin-walled parts formed at K = 1 and K = 1.5 were sectioned radially and fabricated into specimens. Figure8a,b are optical micrographs of the sectional morphology of the polished specimens. The Vickers hardness was measured for sections at different heights of the thin-walled parts formed at K = 1 and K = 1.5. While having the same number of deposition layers (150), parts with different values of K have different heights. The values of hardness measured at 0.5 mm intervals are given in Figure8. The hardness corresponding to K = 1 and K = 1.5 is approximately 170 and 180 HV, respectively, almost Materials 2021, 14, x FOR PEER REVIEW 10 of 17

3.4. Hardness and Density The thin-walled parts formed at K = 1 and K = 1.5 were sectioned radially and fabricated into specimens. Figure 8a,b are optical micrographs of the sectional morphology of the polished specimens. The Vickers hardness was measured for sections at different heights of the thin-walled parts formed at K = 1 and K = 1.5. While having the same number Materials 2021, 14, 4265 of deposition layers (150), parts with different values of K have different heights. The10 val- of 16 ues of hardness measured at 0.5 mm intervals are given in Figure 8. The hardness corresponding to K = 1 and K = 1.5 is approximately 170 and 180 HV, respectively, almost on a par with that of parts processed by WF AM [33]. The hardness is uniformly distributed on a par with that of parts processed by WF AM [33]. The hardness is uniformly distributed along the height, with a small bump around the center of the height. Using laser plus along the height, with a small bump around the center of the height. Using laser plus Joule-heating DMD printing can also achieve competent and evenly-distributed hardness. Joule-heating DMD printing can also achieve competent and evenly-distributed hardness.

FigureFigure 8. 8. HardnessHardness distribution distribution at at different different heights heights of of thin thin-walled-walled parts parts with with different different KK. .Section Section of of thinthin-walled-walled part part at at ( (aa)) KK == 1 1and and (b (b) )KK == 1.5. 1.5.

TheThe mass mass of of the the test specimen, whichwhich isis measuredmeasured on on the the abovementioned abovementioned Archimedes Archime- desprinciple principle of waterof water displacement displacement (Equations (Equations (2)–(4)), (2)–(4 was)), was 4.9077 4.9077 g and g and 4.2888 4.2888 g in g air in andair −3 andwater, water, respectively. respectively. Given Given the waterthe water density density of 1 of g· cm1 g·cm, the−3, the density density of theof the test test specimen speci- −3 menwas was calculated calculated to be to 7.93 be g7.93·cm g·cm. Compared−3. Compared to 316 to L31 stainless6 L stainless steel steel with with the standard the standard value · −3 valueof density of density 7.98 g 7.98cm g·cm, the−3, relative the relative density density of the of specimen the specimen formed formed using using laser laser and Jouleand Jouleheating heating reached reached 99.37%, 99.37%, much much better thanbetter that than of that the specimensof the specimens produced produced by SLM by AM SLM [34]. AM3.5. [3 Tensile4]. Test 3.5. TensileThe tensile Test strength and elongation at fracture are two key indexes. The tensile strength is the stress when the part under test is subjected to the maximum plastic defor- The tensile strength and elongation at fracture are two key indexes. The tensile mation. The elongation at fracture of a part is the ratio of its elongated length after fracture strengthto its original is the length.stress when the part under test is subjected to the maximum plastic deformation.Tensile The elongation tests were at conducted fracture of following a part is the ASTM ratio E8, of onits elongated a servo-electric length INSTRON5982 after fracture toframe its original equipped length. with an INSTRON 50-kN load cell, at the room temperature of 250 ◦C. Five samplesTensile were tests taken were each conducted in the vertical following direction, ASTM alongE8, on the a servo building-electric direction INSTRON5982 (BD), and framehorizontal equipped direction, with see an FigureINSTRON9a. Samples, 50-kN load dog bone-shaped,cell, at the room were temperature clamped by of both 250 ends, °C. Fiveas shown samples in Figure were 9takenb. The each extension in the ratevertical was direction, 0.15 mm/min along− 1theand building load-up direction was performed (BD), anduntil horizontal the samples direction, were fractured.see Figure The9a. Sam typicalples, tensile dog bone properties-shaped, of were the thin-walledclamped by partsboth ends,produced as shown by LJWF-DMD in Figure 9b.are givenThe extension in Figure rate9c. The was stress/strain 0.15 mm/min curves−1 and show load- thatup was the ultimate tensile strength (UTS) of horizontal samples reach to 612.2 ± 24.2 MPa, much higher than that of the vertical samples, 514.1 ± 17.1 MPa. In the respect of elongation to failure, however, horizontal samples are slightly weaker than vertical ones, 90.1 ± 5.9% vs.

98.2 ± 1.7%. It should be noted that even the lowest UTS, 514.1 ± 17.1 MPa, is comparable with that of the specimens fabricated of forged 316 stainless steel [35]. More signiﬁcantly, the ductility or elongation to failure of the parts produced by using LJWF-DMD reaches 90% and over, superior to that of the parts using SLM AM [36,37]. In the scanning electron micrograph of the fracture shown in Figure9d, there are a lot of ﬁne even-sized dimples, with no defects such as cracks and pores observed. Materials 2021, 14, x FOR PEER REVIEW 11 of 17

performed until the samples were fractured. The typical tensile properties of the thin- walled parts produced by LJWF-DMD are given in Figure 9c. The stress/strain curves show that the ultimate tensile strength (UTS) of horizontal samples reach to 612.2 ± 24.2 MPa, much higher than that of the vertical samples, 514.1 ± 17.1 MPa. In theMaterials respect 2021 of, 14 , x FOR PEER REVIEW 12 of 17 elongation to failure, however, horizontal samples are slightly weaker than vertical ones, 90.1 ± 5.9% vs. 98.2 ± 1.7%. It should be noted that even the lowest UTS, 514.1 ± 17.1 MPa, is comparable with that of the specimens fabricated of forged 316 stainless steel [35]. More polishing time is longer. The roughness of the AM parts after electrolyte-plasma pro- significantly, the ductility or elongation to failure of the parts produced by using LJWF- cessing are given in Table 6. DMD reaches 90% and over, superior to that of the Materialsparts using 2021, 14SLM, x FOR AM PEER [3 6REVIEW,37]. In the 12 of 17 Materials 2021, 14, 4265 scanning electron micrograph of the fracture shown in Figure 9d, there are a lot of 11fine of 16 Table 6. Specimen images and surface roughness at different time points. even-sized dimples, with no defects such as cracks and pores observed. Time 0 min 2 min 4 min 6 min 8 min 10 min polishing time is longer. The roughness of the AM parts after electrolyte-plasma processing are given in Table 6.

SpecimensTable 6. Specimen images and surface roughness at different time points. Time 0 min 2 min 4 min 6 min 8 min 10 min Materials 2021, 14, x FOR PEER REVIEW 12 of 17

Ra (μm) 3.6 2.5 1.3 0.7 0.5 0.4 Specimens polishing time is longer. The roughness4. ofDiscussion the AM parts after electrolyte-plasma processing are given in Table 6. 4.1. Utilization of Joule Heating

Ra (μm)Table 6. Specimen3.6 images and2.5 surface roughness at1.3 different A high time-power points0.7 laser. printing unit0.5 is not suitable for0.4 outer space processing due to its low electro-light conversion rate. On the other hand, low-power laser units, such as the Time 0 min 2 min 4 min 6 min 8 min 10 min Materials 2021, 14, x FOR PEER REVIEW one used in this study, cannot12 of 17 provide sufficient energy and fails to bond the formed sin- 4. Discussion gle track and the baseplate effectively, as shown in Figure 4b. In this paper, a Joule heating 4.1. Utilization of Joule Heatingsource is introduced, which can remedy the energy shortage with only a single-laser heat A high-power laser printing unit is not suitable for outer space processing due to its Specimens polishing time is longer. The roughness of the AM partssource. after electrolyteIt is known-plasma that, in pro- a Joule heating source, electricity is directly converted into low electro-light conversion rate. On the other hand, low-power laser units, such as the cessing are given in Table 6. heat, without intermediate energy conversion, and its energy utilization rate can reach one used in this study, cannot provide sufficient energy and fails to bond the formed sin- 100% [39]. The power supply provides a constant current flowing through the resistance Figure 9. Tensile test and strength measurementTable of LJWF-DMD 6. Specimen 316 images L specimens. and surface (a) Samples roughnessgle intrack ver- at and different the baseplate time pointsbetween effectively,. the conductive as shown innozzle Figure and 4b. the In thisbaseplate. paper, Becausa Joule eheating the contact resistance between Figure 9. Tensile test and strength measurement of LJWF-DMD 316 L specimens. (a) Samples in Ra (μm) 3.6 2.5 source is1.3 introduced, whichthe0.7 wirecan remedyand the thebaseplate 0.5energy is shortage large and with0.4 the only current a single is constant,-laser heat the largest amount of heat verticaltical and and horizontal horizontal directions. directions (b). Geometrical(b) Geometrical dimensions dimensions of the tensile of the specimen. tensile specimen(c) Stress/strain. (c) Materials 2021, 14, x FOR PEERTime REVIEW 0 min 2 min 4 min source. 6It min is known that, 8in min a Joule12 of 17heating 10 source, min electricity is directly converted into Stress/straincurves. (d) curves Scanning. (d) electron Scanning micrograph electron micrograph of fractures. of fractures. is generated in the contact area and the wire tip is melted first and fed into the molten 4. Discussion heat, without intermediatepool energy which conversion,is generated and by the its laserenergy heat utilizationing. Moreover, rate can the reach extremely hot metal wire in- 3.6.3.6. Post Post-Processing-Processing 4.1. Utilization of Joule100% Heating [39]. The power supplycreases provides the rate aat constant which the current metal flowing absorbs through the laser the energy resistance [40,41 ] and more energy can polishing time is longer. The roughness of the AMbetween parts afterthe conductive electrolyte benozzle-plasma put into and pro- the the pool, baseplate. thereby Becaus enhancinge the contact the energy resistance utilization between as a whole. As the hardness WithWith low low surface surfaceSpecimens roughness roughness obtained obtained by using by using LJWF LJWF-DMD,-DMD, the burdenA the high burden -ofpower post of -laserpro- post- printing unit is not suitable for outer space processing due to its cessing are given in Table 6. the wire and the baseplateof theis large hot andwire the reduces current and is constant,the bridging the largestimproves, amount the processof heat of deposition becomes cessingprocessing of the of formed the formed parts parts can canbe reduced, be reduced, at least at least avoiding avoiding using usinglow conventional electro conventional-light cuttingconversion cutting rate. On the other hand, low-power laser units, such as the is generated in the contactstable area and and smooth the wire [42 ,4tip3], is thus melted achieving first and higher fed surfaceinto the quality. molten During multi-layer dep- machining.machining. For For the the post post-processing-processing of ofparts parts built built by by LJWF LJWF-DMD,-DMD,one electrolyte electrolyte-plasma used in- plasmathis study, pro- pro- cannot provide sufficient energy and fails to bond the formed sin- Table 6. Specimen images and surface roughness at differentpool time which points . is generatedosition, by the laserusing heat the ing.low -Moreover,power laser the unit extremely in continuous hot metal mode wire can in- ensure the formation of a cessingcessing is issufficient sufﬁcient and and offers offers a a variety variety of advantages, withwith nono limitationlimitationgle track on onand the the the shape shape baseplate of of the effectively, as shown in Figure 4b. In this paper, a Joule heating Ra (μm) 3.6 2.5 1.3 creases the0.7 rate at whichshallow the0.5 metal and absorbs steady themolte0.4 laser n pool energy [44 ],[40 thereby,41] and reducing more energy the quantity can of the heat accumu- Materials 2021, 14, xthe FORparts, parts, PEERTime simultaneous REVIEWsimultane 0ous min processing processing of of both2 both min exterior exterior and and interior,4 interior, min applicable applicablesource tois6 min mosttointroduced, most metals, metals, which and 8 min can 12 remedy of 17 the10 energy min shortage with only a single-laser heat be put into the pool, therebylated enhancing in subsequent the energylayers. Therefore,utilization theas a parameters whole. As the are hardnessadjusted to control the heat input andhigh high efﬁciency efficiency without without requiring requiring pre-treatment. pre-treatment. In addition, In addition, duringsource. during the It postis the known post processing, pro-that, in a Joule heating source, electricity is directly converted into 4. Discussion of the hot wire reducesand and heat the accumulationbridging improves, in order the to process prevent of collapse deposition and deformationbecomes during AM. cessing,there are there no micro-cracksare no micro and-cracks residual and residual stress generated stress generated because the becauseheat, part without surface the part intermediate is subjectedsurface energy conversion, and its energy utilization rate can reach to only micro force. Therefore, electrolyte-plasma4.1. Utilization processing of isJoule100% particularly Heating [39]. The suitable powerstable supply forand smooth provides [42 ,4a3 constant], thus achieving current flowing higher surface through quality. the resistance During multi-layer dep- is subjected topolishing only micro time force. is longer.Therefore, The electrolyte roughness- plasma of the AMprocessing parts after is particularly electrolyte -plasma pro- polishingSpecimens parts with complex shapes, and it is a prosperousbetween potential the post-processing conductiveosition, nozzle using theand low the- powerbaseplate.4.2. Major laser Becaus Influentialunit ine thecontinuous F contactactors on resistancemode the Morphology can betweenensure of the the formationSingle-Track of Partsa suitable for polishingcessing areparts given with in complex Table 6. shapes, andA hitigh is -apower prosperous laser printing potential unit post is -not suitable for outer space processing due to its technique in AM [38]. shallow and steady molten pool [44], thereby reducing the quantity of the heat accumu- processing technique in AM [38]. low electro-light theconversion wire and rate. the baseplateOn the other is large hand, and low the-power currentWhen laser is laser constant, units, power, such the Joule aslargest the heating amount power of heat and baseplate temperature are constant, the In electrolyte-plasma processing, the specimen is the anode and the polishinglated solution in subsequent layers. Therefore, the parameters are adjusted to control the heat input In electrolyteTable 6.- Specimenplasma processing, images and thesurface specimen roughnessone usedis the at indifferent anode this study,is andtime generated the points cannot polishing. inprovide the solu-contact sufficient area energyand the and singlewire fails tip-track to is bond melted parameters the first form andareed sin-correlated fed into the with molten the K and VM value. The relationships between and bath is the cathode. A voltage between 200 and 400 V is applied and controlled.and heat The accumulation in order to prevent collapse and deformation during AM. tionRa and (μm) bath is the cathode.3.6 A voltage2.5 between gle200 track and1.3 400and Vthe is pool baseplateapplied which 0.7and effectively, is controlled. generated as shown by0.5 the inlaser Figure heat 4b.theming. In0.4 Moreover, thiswere paper, determined the a Joule extremely byheating multiple hot metal regression wire in- analyses on nine sets of experiment data in Time product0 min from the2 chemical min reaction4 adheres min to the metal6 min surface and is removed8 min by using10 min The product from the chemical reaction adheressource to the ismetal introduced, surfacecreases whichand the is canrateremoved remedyat which by the the energy metal shortageabsorbsTable thewith 7laser. onlyThey energy a are single: [40-laser,41] andheat more energy can electric discharge. When the removal rate exceeds the production rate, polishing effect4.2. occurs. Major Influential Factors on the Morphology of the Single-Track Parts using electric discharge. When4. theDiscussion removal ratesource. exceeds It isthe known productionbe put that, into inrate, thea Joule polishingpool, heating thereby source, enhancing electricity the energy is directly utilization converted as a whole. into As the hardness In this study, a 316 L thin-walled part printed by LJWF-DMD was put to electrolyte-plasmaWhen laser power, Joule heating power and baseplateH = temperature10.7 + 1.08 * VareM + constant, 249 * K the (5) effect occurs. In this study, a 316 L thin-walledheat, part withoutprinted byintermediate ofLJWF the -hotDMD wireenergy was reduces put conversion, to and th ande bridging its energy improves, utilization the processrate can ofreach deposition becomes 4.1. Utilization of Jouleµ Heatingµ polishing, and the Ra value decreased from 3.6 100%m to [3 0.49]. Them for powerstable ten minutes. supply and smooth Itprovides issingle expected [42,4 a-3track constant], thus parameters achieving current are flowinghigher correlated surface through with quality. the the resistance K During and VM Wmulti value. = 276−0.549-layer The relationshipsdep- * VM + 51.6 between * K (6) Specimens electrolyte-plasma polishing, and theA hRaigh value-power decreased laser printing from 3.6unit μm is notto 0.4 suitable μm for for ten outer space processing due to its that the surface roughness could be decreasedbetween further if the the conductive polishingosition, time nozzleusing is the longer.andthem low the- Thepowerwerebaseplate. determined laser Becaus unit in eby continuousthe multiple contact regression resistancemode can analysesbetweenensure the on formation nine sets of of experiment a data in minutes. It is expected that thelow surface electro - rouglighthness conversion could rate. be decreased On the other further hand, if low the- power laser units, such as the α = 5.35 + 1.367 * VM + 104 * K (7) roughness of the AM parts after electrolyte-plasmathe processingwire and the are shallowbaseplate given in and Tableis largesteady6. Table and molte the 7. They ncurrent pool are [4 is: 4 ],constant, thereby thereducing largest the amount quantity of heat of the heat accumu- one used in this study, cannot provide sufficient energy and fails to bond the formed sin- is generated in thelated contact in subsequent area and layers.the wire Therefore, tip is melted the aparameterss firstshown and in fedare Figure adjustedinto 10. the From moltento control Equations the heat (5)– input(7) and Figure 10 it is clear that, the slower the Table 6. Specimen images and surfacegle roughness track and at the different baseplate time effectively, points. as shown in Figure 4b. In this paper, a Joule heatingH = 10.7 + 1.08 * VM + 249 * K (5) Ra (μm) 3.6 2.5 1.3 pool which0.7 is generatedand heat by accumulation0.5 the laser heat ining. order0.4 Moreover, to prevent thebaseplate collapse extremely is and moving, hot deformation metal the wire larger duringin- the amountAM. of energy input per unit time, and the higher source is introduced, which can remedy the energy shortage with only a single-laser heat creases the rate at which the metal absorbs the laser energythe [ 40temperature,41W] and= 276−0.549 more is in energy the * V printingM +can 51.6 * area K and the wire is melted more(6) completely, resulting Time 0 min 2 min 4 minsource. It is known 6 min that, in a Joule 8 min heating source, 10 min electricity is directly converted into 4. Discussion be put into the pool,4.2. therebyMajor Influential enhancing Factors the energy on the utilizationMorphologyin a as singleof a the whole. Single-track As- Tracksection the hardnessParts wi th larger width (W), smaller height (H), and smaller wetting heat, without intermediate energy conversion, and its energy utilization rate canα reach = 5.35 + 1.367 * VM + 104 * K (7) 4.1. Utilization of Joule Heating of the hot wire reducesWhen and laser the bridgingpower, Joule improves, heating the power processangle, and α .of baseplateOn deposition the other temperature becomeshand, under are constant,the same theconditions, when the parameter K becomes 100% [39]. The power supply provides a constant currentas shown flowing in Figure through 10. From the resistanceEquations (5)–(7) and Figure 10 it is clear that, the slower the A high-power laser printing unitstable is andnot suitablesmoothsingle for[42 ,4outer-track3], thus space parameters achieving processing are higher correlated due surface to its with quality.larger, the K Duringthe and amount VM multivalue. of-layer Thewire relationshipsdep- to feed and betweento melt per unit time increases, and H, W and α between the conductive nozzle and the baseplate. Becausbaseplatee the is contact moving, resistance the larger between the amount of energy input per unit time, and the higher Specimens low electro-light conversion rate. osition,On the otherusing hand,thethem low low- powerwere-power determined laser laser unit units, in by continuous multiplesuch as theregression mode can analyses ensure the on formationnine sets of of experiment a data in the wire and the baseplate is large and the current isthe constant, temperature the largest is in the amount printing of heatarea and the wire is melted more completely, resulting one used in this study, cannot provideshallow sufficient and steady energyTable molte and 7. Theyn fails pool are to [4 :bond 4], thereby the form reducinged sin- the quantity of the heat accumu- is generated in the contact area and the wire tip isin melted a single first-track and section fed into wi theth larger molten width (W), smaller height (H), and smaller wetting gle track and the baseplate effectively,lated as in shown subsequent in Figure layers. 4b. Therefore,In this paper, the a parameters Joule heating are adjusted to control the heat input pool which is generated by the laser heating. Moreover,angle, the α. Onextremely theH other= 10.7 hot hand, +metal 1.08 under *wire VM + in- the249 same * K conditions, when the parameter(5) K becomes source is introduced, which can remedyand heat the accumulation energy shortage in order with onlyto prevent a single collapse-laser heat and deformation during AM. Ra (µm) 3.6 2.5 1.3creases the rate at 0.7 which the metal 0.5 absorbs the laser 0.4larger, energy the [40 amount,41] and of more wire energyto feed can and to melt per unit time increases, and H, W and α W = 276−0.549 * VM + 51.6 * K (6) source. It is knownbe putthat, into in thea Joule pool, heating thereby source, enhancing electricity the energy is directly utilization converted as a whole. into As the hardness heat, without intermediate energy4.2. conversion, Major Influential and its F actorsenergy on utilization the Morphology rate canof the reach Single -Track Parts of the hot wire reduces and the bridging improves, the process αof = deposition5.35 + 1.367 becomes* VM + 104 * K (7) 100% [39]. The powerstable supply and smooth provides [42,4When a3 constant], thus laser achieving currentpower, flowingJoulehigher heating surface through power quality. the andresistance During baseplate multi temperature-layer dep- are constant, the as shown in Figure 10. From EquationsM (5)–(7) and Figure 10 it is clear that, the slower the between the conductiveosition, nozzleusing theandsingle low the- powerbaseplate.-track parameterslaser Becaus unit ine are continuousthe correlated contact resistancemode with canthe betweenensureK and V the value. formation The relationships of a between baseplate is moving, the larger the amount of energy input per unit time, and the higher the wire and the shallowbaseplate and is large steadythem and molte thewere ncurrent pooldetermined [4 is4 ],constant, thereby by multiple thereducing largest regression the amount quantity analyses of heat of the on heatnine accumu-sets of experiment data in the temperature is in the printing area and the wire is melted more completely, resulting is generated in thelated contact in subsequent area andTable layers.the 7 wire. They Therefore, tip are is: melted the parameters first and fedare adjustedinto the moltento control the heat input pool which is generated by the laser heating. Moreover,in a single the extremely-track section hot metal with largerwire in- width (W), smaller height (H), and smaller wetting and heat accumulation in order to prevent collapseH =and 10.7 deformation + 1.08 * VM +during 249 * K AM. (5) creases the rate at which the metal absorbs the laserangle, energy α. On[40 ,the41] otherand more hand, energy under can the same conditions, when the parameter K becomes be put into the pool,4.2. therebyMajor Influential enhancing Factors the energy on the utilizationlarger,Morphology the asamount of a the Wwhole. Single= 276−0.549of Aswire-Track the to hardness* Parts feedVM + and 51.6 to * Kmelt per unit time increase(s6,) and H, W and α of the hot wire reduces and the bridging improves, the process of deposition becomes When laser power, Joule heating power andα baseplate = 5.35 + 1.367 temperature * VM + 104 are * Kconstant, the (7) stable and smooth [42,43], thus achieving higher surface quality. During multi-layer dep- single-track parameters are correlated with the K and VM value. The relationships between as shown in Figure 10. From Equations (5)–(7) and Figure 10 it is clear that, the slower the osition, using thethem low- powerwere determined laser unit in by continuous multiple regression mode can analyses ensure the on formation nine sets of of experiment a data in baseplate is moving, the larger the amount of energy input per unit time, and the higher shallow and steadyTable molte 7. Theyn pool are [4: 4], thereby reducing the quantity of the heat accumulated in subsequent layers. Therefore,the thetemperature parameters is arein the adjusted printing to controlarea and the the heat wire input is melted more completely, resulting and heat accumulation in order toin prevent a single collapse-trackH =sectionand 10.7 deformation + wi1.08th *larger VM +during 249width * K AM. (W), smaller height (H), and(5) smaller wetting angle, α. On the other hand, under the same conditions, when the parameter K becomes W = 276−0.549 * VM + 51.6 * K (6) 4.2. Major Influential Factors on the Morphologylarger, the amountof the Single of wire-Track to Parts feed and to melt per unit time increases, and H, W and α When laser power, Joule heating power andα baseplate = 5.35 + 1.367 temperature * VM + 104 are * Kconstant, the (7) single-track parametersas shown are in correlated Figure 10. with From the Equations K and VM (value.5)–(7) Theand relationshipsFigure 10 it is between clear that, the slower the them were determinedbaseplate by multipleis moving, regression the larger analyses the amount on nine of energysets of experimentinput per unit data time, in and the higher Table 7. They arethe: temperature is in the printing area and the wire is melted more completely, resulting

in a single-trackH section= 10.7 + wi1.08th *larger VM + 249width * K (W), smaller height (H), and(5) smaller wetting angle, α. On the other hand, under the same conditions, when the parameter K becomes larger, the amountW = 276−0.549of wire to * feedVM + and 51.6 to* Kmelt per unit time increase(6s,) and H, W and α

α = 5.35 + 1.367 * VM + 104 * K (7) as shown in Figure 10. From Equations (5)–(7) and Figure 10 it is clear that, the slower the baseplate is moving, the larger the amount of energy input per unit time, and the higher the temperature is in the printing area and the wire is melted more completely, resulting in a single-track section with larger width (W), smaller height (H), and smaller wetting angle, α. On the other hand, under the same conditions, when the parameter K becomes larger, the amount of wire to feed and to melt per unit time increases, and H, W and α Materials 2021, 14, 4265 12 of 16

4. Discussion 4.1. Utilization of Joule Heating A high-power laser printing unit is not suitable for outer space processing due to its low electro-light conversion rate. On the other hand, low-power laser units, such as the one used in this study, cannot provide sufficient energy and fails to bond the formed single track and the baseplate effectively, as shown in Figure4b. In this paper, a Joule heating source is introduced, which can remedy the energy shortage with only a single-laser heat source. It is known that, in a Joule heating source, electricity is directly converted into heat, without intermediate energy conversion, and its energy utilization rate can reach 100% [39]. The power supply provides a constant current flowing through the resistance between the conductive nozzle and the baseplate. Because the contact resistance between the wire and the baseplate is large and the current is constant, the largest amount of heat is generated in the contact area and the wire tip is melted first and fed into the molten pool which is generated by the laser heating. Moreover, the extremely hot metal wire increases the rate at which the metal absorbs the laser energy [40,41] and more energy can be put into the pool, thereby enhancing the energy utilization as a whole. As the hardness of the hot wire reduces and the bridging improves, the process of deposition becomes stable and smooth [42,43], thus achieving higher surface quality. During multi-layer deposition, using the low-power laser unit in continuous mode can ensure the formation of a shallow and steady molten pool [44], thereby reducing the quantity of the heat accumulated in subsequent layers. Therefore, the parameters are adjusted to control the heat input and heat accumulation in order to prevent collapse and deformation during AM.

4.2. Major Inﬂuential Factors on the Morphology of the Single-Track Parts When laser power, Joule heating power and baseplate temperature are constant, the single-track parameters are correlated with the K and VM value. The relationships between them were determined by multiple regression analyses on nine sets of experiment data in Table7. They are: H = 10.7 + 1.08 ∗ VM + 249 ∗ K (5)

W = 276 − 0.549 ∗ VM + 51.6 ∗ K (6)

α = 5.35 + 1.367 ∗ VM + 104 ∗ K (7) as shown in Figure 10. From Equations (5)–(7) and Figure 10 it is clear that, the slower the baseplate is moving, the larger the amount of energy input per unit time, and the higher the temperature is in the printing area and the wire is melted more completely, resulting in a single-track section with larger width (W), smaller height (H), and smaller wetting angle, α. On the other hand, under the same conditions, when the parameter K becomes larger, the amount of wire to feed and to melt per unit time increases, and H, W and α increase linearly with K. Hence, K and VM exert signiﬁcant effect on the regularity of the single-track dimensions and morphology.

4.3. Optimization of Thin-Walled Part Printing Parameters For the proposed LJWF-DMD to be performed successfully, the input-power, wire- feeding speed (VW), baseplate moving speed (VM) and the baseplate temperature should be well-coordinated. Given a limited total power input, the parameters K and VM should be adjusted properly to ensure the evenness over total number of layers. It is also noted that inappropriate layer height (∆h) could cause the print head to hit and even damage the thin-wall of the part in process, resulting in failure. The scheme of parameter adjustment is created based on the heat transfer principles. Assuming that the wire, the baseplate with preheating plate are homogeneous, the Fourier law for heat transfer by conduction during the experimental process is modelled as follows:

∂T Q = −kA (8) 1 1 ∂x Materials 2021, 14, x FOR PEER REVIEW 13 of 17

Materials 2021, 14, 4265 13 of 16

increase linearly with K. Hence, K and VM exert significant effect on the regularity of the

single-track dimensionswhere andQ1 ismorphology. the power of heat transfer by conduction (W); k is the thermal conductivity −1 ◦ −1 2 (W·m · C ); and A1 is the heat transfer area (m ). Table 7. Single-track process parameters: H, W and α. Table 7. Single-track process parameters: H, W and α. Number 1 2 3 4 5 6 7 8 9 Number 1 2 3 4 5 6 7 8 9 K 1 1 1 0.8 0.8 0.8 0.5 0.5 0.5 K 1 1 1 0.8 0.8 0.8 0.5 0.5 0.5 VM (mm/min) 5 10 15 5 10 15 5 10 15 V (mm/min) 5 10 15 5 10 15 5 10 15 M H (μm) 262 271 272 218 224 228 137 147 150 H (µm) 262 271 272 218 224 228 137 147 150 W (µWm) (μm) 325 325 323 323321 321314 314312 312309 309300 300297 297294 294 α (◦)α (°) 116 116 124 124132 13296 96100 100106 10664 6472 7279 79

Figure 10. Multiple regression analysis for H, W and α of single-track versus K and VM. Figure 10. Multiple regression analysis for H, W and α of single-track versus K and VM. The thermal convection at the part surface satisfies Newton’s law of cooling: 4.3. Optimization of Thin-Walled Part Printing Parameters Q = hA (T − T ) (9) For the proposed LJWF-DMD to be performed2 successfully,2 2 the input-power, wire- feeding speed (VW), baseplateThe radiation moving heat speed loss at ( theVM part) and surface the baseplate satisfies the temperature Stefan-Boltzmann should law: be well-coordinated. Given a limited total power input, the parameters K and VM should = 4 − 4 be adjusted properly to ensure the evenness overQ 3totalεσ Anumber2 T T 2of layers. It is also noted (10) that inappropriate layer height (Δh) could cause the print head to hit and even damage where Q2 is the power of convective heat transfer (W); h is the coefficient of convective −2 ◦ −1 2 the thin-wall of the partheat transferin process, (W·m resulting· C ); A in2 isfailure. the area of convective and radiation heat transfer (m ); The scheme of parameterQ3 is the power adjustment of radiative is heatcreated transfer based (W); onσ is the the Stefan-Boltzmannheat transfer principles. constant; ε is the ◦ Assuming that the wire,relative the radiance; baseplate and withT2 is preheating the ambient temperatureplate are homoge ( C). neous, the Fourier In Equation (8), the heat transfer velocity is directly proportional to the heat transfer law for heat transferarea by andconduction the temperature during gradient. the experimental The energy provided process by theis modelled baseplate pre-heating as fol- sys- lows: tem increases the initial temperature (T0) of the thin-walled part printing area. Figure 11b shows that, at the first layer, the heat휕푇 (Q1) is dissipated through conduction directly to the baseplate, a large metal푄 sheet.1 = − As푘퐴 more1 and more layers are built up, the path of conductive(8) heat, Q1, changes, dissipating downward,휕푥 and the thermal resistivity of the thin-walled part increases along the height of the part, thus making it difficult for the laser heat to where Q1 is the power of heat transfer by conduction (W); k is the thermal conductivity −1 −1 transfer, resulting in heat accumulation2 which causes the molten pool to become larger and (W·m ·°C ); and A1 evenis the to heat collapse. transfer Equations area (9) (m and). (10) show that as the surface area of the part increases The thermal convectionin the process, at the convective part surface and radiative satisfies heat Newton transfer’ graduallys law of takecooling: dominance over conduction. When a heat balance was reached between energy input and heat dissipation via conduction, convection푄2 and= hA radiation,2(푇 − 푇 the2) dimensions of the molten pool settles( at9) steady The radiation heatvalues, loss without at the increasingpart surface as more satisfies and more the layersStefan build-Boltzmann up. To ensure law: the dimensional accuracy and surface roughness, the printing parameters must be adjusted. Among them, the easiest way to expedite the energy4 allocation4 and dimensional stability of the molten 푄3 = 휀𝜎퐴2(푇 − 푇2 ) (10) pool is changing VM. Experiments indicate that progressively adjusting VM layer by layer where Q2 is the power(∆h )of can convective yield desired heat results. transfer Figure (W);11a shows h is the V coefficientM for each layer, of convective with the thickness of the baseplate at 2 mm, ∆h 0.11 mm at K = 1 and 0.15 mm at K = 1.5, laser power of heat transfer (W·m−2·°C −1); A2 is the area of convective and radiation heat transfer (m2); Q3 50 W, Joule heating current of 10A, and baseplate temperature of 500 ◦C. The value of is the power of radiative heat transfer (W); σ is the Stefan-Boltzmann constant; ε is the relative radiance; and T2 is the ambient temperature (°C ). In Equation (8), the heat transfer velocity is directly proportional to the heat transfer area and the temperature gradient. The energy provided by the baseplate pre-heating system increases the initial temperature (T0) of the thin-walled part printing area. Figure 11b shows that, at the first layer, the heat (Q1) is dissipated through conduction directly to the baseplate, a large metal sheet. As more and more layers are built up, the path of conductive heat, Q1, changes, dissipating downward, and the thermal resistivity of the thin- walled part increases along the height of the part, thus making it difficult for the laser heat

Materials 2021, 14, x FOR PEER REVIEW 14 of 17

to transfer, resulting in heat accumulation which causes the molten pool to become larger and even to collapse. Equations (9) and (10) show that as the surface area of the part increases in the process, convective and radiative heat transfer gradually take dominance over conduction. When a heat balance was reached between energy input and heat dissipation via conduction, convection and radiation, the dimensions of the molten pool settles at steady values, without increasing as more and more layers build up. To ensure the dimensional accuracy and surface roughness, the printing parameters must be adjusted. Among them, the easiest way to expedite the energy allocation and dimensional stability of the molten pool is changing VM. Experiments indicate that progressively adjusting VM layer by layer (Δh) can yield desired results. Figure 11a shows the VM for each layer, with Materials 2021,the14, 4265thickness of the baseplate at 2 mm, Δh 0.11 mm at K = 1 and 0.15 mm at K = 1.5, laser 14 of 16 power of 50 W, Joule heating current of 10A, and baseplate temperature of 500 °C . The value of VM is increased progressively from beginning until the 17-th layer, and is not adjusted more afterwards. The dimensional accuracy and roughness of the specimen VM is increased progressively from beginning until the 17-th layer, and is not adjusted formed by the LJWFmore-DMD afterwards. are shown The dimensional in Figures 5 accuracy–7, indicating and roughness a desirable of the result specimen can be formed by obtained by optimizingthe LJWF-DMD crucial parameters, are shown in V FiguresM, Δh and5–7, K indicating. Although a desirable there are result many can other be obtained factors involved byin optimizingthe process crucial that affect parameters, the temperatureVM, ∆h and Kfield. Although distribution there are in manythe heat other factors transfer system, itinvolved is feasible in the to adjust process certain that affect parameters the temperature to control ﬁeld the distribution dimensions in the of the heat transfer molten pool whensystem, heat balance it is feasible is reached. to adjust Under certain parametersheat balance, to controlthe thermal the dimensions resistance of is the molten pool when heat balance is reached. Under heat balance, the thermal resistance is almost almost unchanged and thus the process parameters are adjusted more. To sum, optimiz- unchanged and thus the process parameters are adjusted more. To sum, optimizing energy ing energy allocationallocation and andadjusting adjusting crucial crucial parameters parameters cancan greatly greatly increase increase the the dimensional dimen- accuracy sional accuracy andand roughness, roughness, andand is is expected expected to to ﬁnd find wide wide practical practical applications. applications.

Figure 11. (a) Relationship between the moving speed at each layer of the thin-walled part. (b) Heat dissipation of the 1-st layer. (cFigure) Heat dissipation11. (a) Relationship of several layers.between the moving speed at each layer of the thin-walled part. (b) Heat dissipation of the 1-st layer. (c) Heat dissipation of several layers. 5. Conclusions 5. Conclusions A direct metal deposition additive manufacturing (DMD AM) with laser and Joule A direct metalheating deposition combination additive is proposed manufacturing and studied (DMD experimentally. AM) with By usinglaser thisand method Joule of LJWF- DMD, high quality of surface of the thin-walled parts can be achieved with low input-power. heating combination is proposed and studied experimentally. By using this method of The dimensions and physical properties of single-track thin-walled parts were measured and LJWF-DMD, hightested. quality From of the surface analysis of and the experiment thin-walled results, parts the can conclusions be achiev areed as with follows: low input-power. The dimensions and physical properties of single-track thin-walled parts (1) Introducing a Joule heating source decreases the demand for laser power, and thus were measured and tested.reduces From the totalthe analysis printing powerand experiment and heat accumulation results, the conclusions and increases are the surface as follows: quality. The maximum optical power during actual printing measured 50 W. (1) Introducing(2) a JouleThin-walled heating source parts formed decreases by using the demand the LJWF-DMD for laser have power, aspect and ratios thus as high as 40, uniform thickness and average deviation as low as approximately 0.017 mm. reduces the total printing power and heat◦ accumulation and increases the surface quality. The maximumThin-walled optical parts power with during 66 inclinations actual printing are built. measured No visible 50 defects, W. such as pores and cracks, are observed on all the specimens. (2) Thin-walled parts formed by using the LJWF-DMD have aspect ratios as high as 40, (3) Progressively adjusting the parameters can improve the surface quality of the thin- uniform thicknesswalled and parts.average The deviation surface roughness as low (asRa )approximately below 5 µm is far 0.017 smaller mm. than Thin that- obtained walled parts withusing 66° WF-basedinclinations metal are AM built. techniques No visible and ondefects, a par withsuch that as obtainedpores and using SLM. cracks, are observedThe horizontalon all the tensilespecimens. strength is approximately 500 MPa; the elongation at fracture is 90%; even and ﬁne dimples are observed on the fracture surface; the hardness is approximately 170 HV and is uniformly distributed at different heights; and the density is 99.37%. The LJWF-DMD is competent with the conventional processing techniques in terms of physical properties. (4) Low surface roughness alleviates the burden of post-processing the formed parts. Us- ing electrolyte-plasma processing can greatly reduce the specimen surface roughness, from 3.6 to 0.4 µm in ten minutes only. (5) The proposed printing technique, LJWF-DMD, is expected to be applied in cases where only low-power laser is available while the required surface quality is higher, e.g., AM in outer space. Materials 2021, 14, 4265 15 of 16

Overall, the thin-wall specimens fabricated by LJWF-DMD AM technique exhibit good surface quality and better density, hardness and mechanical properties. Future studies will base on multi-bead overlapping and complex structure process of LJWF-DMD AM. Additionally, the energy optimization strategy of multi-heat sources in the process by simulation can be considered.

Author Contributions: Conceptualization, B.L. (Bobo Li) and B.L. (Bingheng Lu); methodology, B.L. (Bobo Li), L.Z and G.Z.; investigation, B.L. (Bobo Li), L.Z. and B.W.; data curation B.L. (Bobo Li) and B.W.; project administration, G.Z. and B.L. (Bingheng Lu); resources, B.L. (Bingheng Lu); supervision, L.Z.; writing—original draft, B.L. (Bobo Li); writing—review and editing, B.L. (Bobo Li) and G.Z.; funding acqui- sition, B.L. (Bingheng Lu). All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Key Research and Development Program of Shaanxi province (Grant No. 2021GY-300) and Guangdong Basic and Applied Research Foundation (Grant No. 2019B1515130005). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available upon reasonable request from the corresponding author. Acknowledgments: This work was supported by the National Innovation Institute of Additive Manufacturing, China. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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