metals

Review Research Status and Progress of Welding Technologies for Molybdenum and Molybdenum Alloys

Qi Zhu 1, Miaoxia Xie 2,*, Xiangtao Shang 2, Geng An 1,3, Jun Sun 3, Na Wang 1, Sha Xi 1, Chunyang Bu 1 and Juping Zhang 1

1 Technical Center, Jinduicheng Molybdenum Co., Ltd., Xi’an 710077, China; [email protected] (Q.Z.); [email protected] (G.A.); [email protected] (N.W.); [email protected] (S.X.); [email protected] (C.B.); [email protected] (J.Z.) 2 School of Mechanical and Electrical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China; [email protected] 3 State key laboratory for mechanical behavior of materials, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] * Correspondence: [email protected]; Tel.: +86-181-4902-5125



Received: 28 December 2019; Accepted: 14 February 2020; Published: 20 February 2020


Abstract: Owing to its potential application prospect in novel accident tolerant fuel, molybdenum alloys and their welding technologies have gained great importance in recent years. The challenges of welding molybdenum alloys come from two aspects: one is related to its powder metallurgy manufacturing process, and the other is its inherent characteristics of refractory metal. The welding of powder metallurgy materials has been associated with issues such as porosity, contamination, and inclusions, at levels which tend to degrade the service performances of a welded joint. Refractory metals usually present poor weldability due to embrittlement of the fusion zone as a result of impurities segregation and the grain coarsening in the heat-affected zone. A critical review of the current state of the art of welding Mo alloys components is presented. The advantages and disadvantages of the various methods, i.e., electron-beam welding (EBW), tungsten-arc inert gas (TIG) welding, laser welding (LW), electric resistance welding (ERW), and brazing and friction welding (FW) in joining Mo and Mo alloys, are discussed with a view to imagine future directions. This review suggests that more attention should be paid to high energy density laser welding and the mechanism and technology of welding Mo alloys under hyperbaric environment.

Keywords: molybdenum alloy; welding; status; progress

1. Introduction Molybdenum (Mo) and Mo alloys show characteristics such as high melting point, good high-temperature strength, high wear resistance, high thermal conductivity and low resistivity, low coefficient of linear expansion, high elastic modulus, and good corrosion resistance [1]. Based on this, they have irreplaceable functions and application demands in the fields like the defense industry, aerospace, electronic information, energy, chemical defense, metallurgy, and nuclear industry. However, Mo and Mo alloys are hard and brittle materials in nature, so their weldabilities are generally poor [2]. There are two main sources of molybdenum brittleness: one is the intrinsic brittleness of molybdenum, and the other is the enrichment of interstitial impurities in the grain boundary. Oxygen is the most important impurity element in the grain boundary which affects the embrittlement of molybdenum. The solubility of oxygen in molybdenum at room temperature is less than 0.1 ppm, which forms relatively volatile molybdenum oxide at the grain boundary, which greatly reduces the bond strength of grain boundary. After melting and welding of high-performance molybdenum alloy, the weld forms

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Metalsthan 2020 0.1 , ppm,10, 279 which forms relatively volatile molybdenum oxide at the grain boundary, which2 of 16 greatly reduces the bond strength of grain boundary. After melting and welding of high-performance molybdenum alloy, the weld forms an as-cast structure with coarse grains, the anheat as-cast-affected structure zone form withs coarse a coarse grains, recrystallization the heat-affected structure, zone forms the impurity a coarse recrystallizationelements are enriched structure, in the impuritygrain boundary, elements and are the enriched strength in theand grain toughness boundary, of the and weld the strengthand heat and-affected toughness zone ofare the greatly weld andreduced heat-a [3ff–ected5]. To zone extend are greatly application reduced field [ 3 of–5 ]. Mo To and extend Mo application alloys, researchers field of Mo world andwi Mode alloys, have researchersconducted a worldwide lot of studies have on conducted their welding a lot ofand studies relevant on theirkinds welding of literature and relevant have been kinds reported of literature since havethe 1970s been [1] reported. since the 1970s [1]. In recentrecent years,years, the the global global nuclear nuclear industry industry and and scientific scientific community community have have been been aware aware that that a new a fuelnew system,fuel system, that is,that accident is, accident tolerant tolerant fuel (ATF) fuel (ATF) needs needs to be developedto be developed [6,7]. Such[6,7].a Such fuel systema fuel system needs toneeds be able to be to withstandable to withstand severe accident severe accident conditions conditions and slow and down slow the ratedown of the deterioration rate of deterioration over a long periodover a long of time, period to provide of time, more to provide valuable more time valuable for people time to for take people emergency to take measuresemergency and measure greatlys reduceand greatly the risks reduce of leakagethe risks of of radioactive leakage of materials.radioactive Therefore, materials. Mo Therefore, alloy is listedMo alloy as one is listed of the as main one candidateof the main materials candidate for materials ATF cladding for ATF by cladding the global by nuclear the global industry nuclear [8]. industry In this context, [8]. In this the weldingcontext, technologiesthe welding technologies for Mo and Mofor alloysMo and have Mo attractedalloys hav widee attracted attention wide of attention researchers of inresearchers China and in a China lot of newand a progress lot of new has progress been achieved has been in recentachieved years. in recent years.

2. Analysis on W Weldabilityeldability of Mo and Mo AlloyAlloy 2.1. Room-Temperature Brittleness 2.1. Room-Temperature Brittleness Ductility of most of Mo alloys varies with temperature to make the materials change from ductile Ductility of most of Mo alloys varies with temperature to make the materials change from fracture to brittle fracture in a very small temperature range. The ductile-brittle transition temperature ductile fracture to brittle fracture in a very small temperature range. The ductile-brittle transition of pure Mo ranges from approximately 140 C to 150 C, resulting in difficulties in intensive processing, temperature of pure Mo ranges from approximately◦ ◦ 140 °C to 150 °C, resulting in difficulties in low product performance, and limited application fields [1]. Such brittleness is known as intrinsic intensive processing, low product performance, and limited application fields [1]. Such brittleness is brittleness of Mo, which is mainly determined by an electron distribution characteristic that the known as intrinsic brittleness of Mo, which is mainly determined by an electron distribution outermost and sub-outermost electrons of its atoms are half full. characteristic that the outermost and sub-outermost electrons of its atoms are half full. Mo and Mo alloy have a high melting point, good thermal conductivity, high recrystallization Mo and Mo alloy have a high melting point, good thermal conductivity, high recrystallization temperature, no allotropy transformations in solid state, and low density of bcc crystal structure. temperature, no allotropy transformations in solid state, and low density of bcc crystal structure. Due to these characteristics, weld seam and heat-affected zone (HAZ) is large and grains are seriously Due to these characteristics, weld seam and heat-affected zone (HAZ) is large and grains are coarsened after welding (Figure1), so that interstitial impurities, such as C, N, and O are fully di ffused seriously coarsened after welding (Figure 1), so that interstitial impurities, such as C, N, and O are and enriched on grain boundaries, resulting in greatly weakened bonding strength of grain boundaries, fully diffused and enriched on grain boundaries, resulting in greatly weakened bonding strength of like Figure2 that the fracture of laser welded Mo alloy contains a lot of MoO 2. Under the joint effects grain boundaries, like Figure 2 that the fracture of laser welded Mo alloy contains a lot of MoO2. of intrinsic brittleness of the materials and segregation of impurities at grain boundaries, sensitivity of Under the joint effects of intrinsic brittleness of the materials and segregation of impurities at grain welding cracks is high and strength, plasticity, and ductility of Mo and Mo alloy joints are poor [9,10]. boundaries, sensitivity of welding cracks is high and strength, plasticity, and ductility of Mo and Mo Therefore, molybdenum and molybdenum alloy parts or structures are usually manufactured by alloy joints are poor [9,10]. Therefore, molybdenum and molybdenum alloy parts or structures are powder metallurgy rather than welding. usually manufactured by powder metallurgy rather than welding.

Figure 1. (a) Electron backscatter diffraction diffraction ( (EBSD)EBSD) image of cross section of Mo joint achieved by laser welding,welding, and (b)) enlarged viewview ofof areaarea ((aa)[) [1010].].

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FigureFigure 2.2. ((aa))) SEMSEMSEM image imageimage of ofof the thethe fracture fracturefracture of ofof Mo MoMo joint jointjoint with withwith plenty plentyplenty of MoO ofof MoOMoO2 at2 grain2 atat graingrain boundary, boundaryboundary and,, ( andband) TEM ((bb)) TEMbrightTEM brightbright field image fieldfield imageimage of the ofof weld thethe weld beadweld ofbeadbead the ofof Mo thethe joint MoMo [ joint10joint]. [[1010]]..

2.2.2.2. PP Poreoreore D DefectsDefectsefects Furthermore,Furthermore, owing owing owing to to to the thethe powder powderpowder metallurgy metallurgy metallurgy process process process that canthatthat yield cancan fine yield yield grain fine fine structure grain grain structure structure without withoutpreferredwithout preferredpreferred orientation, orientation,orientation, the refractory thethe refractoryrefractory metal work metalmetal blanks workwork are blanksblanks usually areare preparedusuallyusually preparedprepared by utilizing byby utilizingutilizing powder powdermetallurgypowder metallurgy metallurgy method. This method. method. leads Thisto This that leads leads the material to to that that contains the the material material micropores containscontains and micropores micropores impurity elements, and and impurity impurity and its elements,compactnesselements, and and is its its not compactness compactness comparable to is is that not not ofcomparable comparable the materials to to that thatproduced of of the the by materials materials smelting produced produced metallurgy. by by Therefore, smelting smelting metallurgy.themetallurgy. problem Therefore, Therefore, of high rate the the of poreproblem problem defects of of highhigh (Figure rate rate3) generallyof of pore pore defects defects appears ((FF inigureigure fusion-welded 3)3) generallygenerally Mo appears appears and Mo in in fusionalloys.fusion--weldedwelded Particularly, MoMo andand the high-pressureMoMo alloys.alloys. PParticularly,articularly, residual gases thethe highhigh in the--pressurepressure micropores residualresidual are the gasesgases most inin harmful. thethe microporesmicropores During aretheare thethe welding mostmost process,harmful.harmful. theseDuringDuring high-pressure thethe weldingwelding gasesprocess,process, can thesethese expand highhigh rapidly--pressurepressure in gases thegases molten cancan expandexpand pool after rapidlyrapidly being inin thereleasedthe molten molten into pool pool the high-temperature after after being being released released molten into into pool, the the which high high--temperaturetemperature seriously deteriorates molten molten pool, pool, the quality which which of seriously seriously welded dejointsdeterioratesteriorates of Mo the andthe qualityquality Mo alloys ofof weldedwelded [11–13]. jointsjoints ofof MoMo andand MoMo alloysalloys [[1111––1313]]..

FigureFigure 3. 3. TypicalTypicalTypical porosity porosity morphology morphology in in Mo Mo jointsjoints produced produced by b byy laser laser laser welding: welding welding:: ( (aa)) orthogonalorthogonal experimentexperiment 1# 1# 1# parameter parameter parameter sample, sample sample (b,), orthogonal((bb)) orthogonal orthogonal experiment experiment experiment 2# parameter 2# 2# parameter parameter sample, and sample sample (c) orthogonal,, andand ( (cc)) orthoexperimentorthogonalgonal experimentexperiment 3# parameter 3#3# sample. parameterparameter [11 ].samplesample.. [[1111]]..

33.3.. ResearchResearch PP Progressrogressrogress inin W WeldingWeldingelding of of Mo Mo and and Mo Mo AlloysAAlloyslloys AtAt present,present, the thethe welding weldingwelding methods methodsmethods for forfor Mo MoMo and andand Mo MoMo alloys alloysalloys mainly mainlymainly include includeinclude tungsten-arc tungstentungsten inert--arcarc inert gasinert (TIG) gasgas welding,(TIG)(TIG) welding, welding, electron-beam electron electron welding--beambeam welding welding (EBW), laser (EBW), (EBW), welding, laser laser electric welding, welding, resistance electric electric welding r resistanceesistance (ERW), welding welding brazing, (ERW), (ERW), and brazingfrictionbrazing,, welding. andand frictionfriction welding.welding.

3.1.3.1. EBWEBW PanPan et et et al. al. al. [ 14 [1 [1]44 studied]] studied studied EBW EBW EBW for pure for for pure pure Mo with Mo Mo a with with thickness aa thicknessthickness of 1.5 mm of of obtained 1.5 1.5 mm mm by obtained obtained powder by by metallurgy. powder powder metallurgy.Themetallurgy. results show TheThe resultsresults that the showshow faster thatthat the thethe welding fasterfaster speed thethe weldingwelding is, the small speedspeed the is,is, grain thethe smallsmall size the andthe graingrain the less sizesize the andand interstitial thethe lessless theimpurities.the interstitial interstitial By increasing impurities. impurities. welding By By increasing increasing speed and welding welding reducing speed speed welding and and reducing reducing heat input, welding welding the ductility heat heat input, input,of welded thethe ductilityjointductility of Mo ofof canweldedwelded be significantly jointjoint ofof MoMo improved. cancan bebe significantlysignificantly Vacuum degree improved.improved. significantly VacuumVacuum aff ectsdegredegre ductile-brittleee significantlysignificantly transition affectsaffects ductiletemperature,ductile--brittlebrittle whereas transitiontransition the temperature, temperature, decomposition whereaswhereas of oxides thethe on decompositi decompositi the surfaceonon of of of the oxides oxides workpiece on on the the during surface surface welding of of the the has great influence on the vacuum degree. When a vacuum degree increases from 10 4 mm Hg to workpieceworkpiece during during welding welding has has great great influence influence on on thethe vacuumvacuum degree. degree. When When aa vacuumvacuum− degree degree 10 5 mm Hg, the upper limit of ductile-brittle transition temperature of the welded joint decreases increasesincreases− fromfrom 10−10−44 mmmm HgHg toto 1010−−55 mmmm Hg,Hg, thethe upperupper limitlimit ofof ductileductile--brittlebrittle transitiontransition temperaturetemperature offromof thethe ~150 weldedwelded◦C to jointjoint ~100 decreasesdecreases◦C. Pan et fromfrom al.’s analysis~~150150 °C°C oftoto the~~100100 change °C.°C. PanPan in joint etet al.al. performance’’ss analysisanalysis ofof through thethe changechange the impurity inin jointjoint performancecompositionperformance andthroughthrough impurity thethe impurityimpurity content compositio ofcompositio the gas releasednn andand impurityimpurity after the contentcontent melting ofof of thethe the gasgas material releasedreleased left afterafter a deep thethe meltingimpressionmelting ofof thethe on materialmaterial the authors. leftleft aa Yang deepdeep et impressionimpression al. [15] welded onon thethe pure authors.authors. Mo plates YangYang with etet al.al. thickness [[1155]] weldedwelded of 16 purepure mm MoMo by platesplates using withwith thicknessthickness ofof 1616 mmmm byby usingusing EBW.EBW. TheThe resultsresults demonstratedemonstrate thatthat thethe highesthighest strengthstrength ofof thethe weldedwelded jointjoint ssubjectedubjected toto heatheat treatmenttreatment atat 11100100 °C°C wawass foundfound inin thethe weldweld seam.seam. TensileTensile fracturesfractures ofof

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EBW. The results demonstrate that the highest strength of the welded joint subjected to heat treatment the welded joint are shown in the weld seam and present morphology of cleavage fracture. In at 1100 C was found in the weld seam. Tensile fractures of the welded joint are shown in the weld addition,◦ Zheng et al. [16] welded pure Mo materials with thickness of 16 mm through vacuum seam and present morphology of cleavage fracture. In addition, Zheng et al. [16] welded pure Mo EBW. As can be seen from the above research results, the results illustrate that grains in weld seam materials with thickness of 16 mm through vacuum EBW. As can be seen from the above research of Mo by EBW grow rapidly. results, the results illustrate that grains in weld seam of Mo by EBW grow rapidly. Morito et al. (1989) [17] found that a joint of molybdenum-titanium-zirconium (TZM) alloys Morito et al. (1989) [17] found that a joint of molybdenum-titanium-zirconium (TZM) alloys welded using EBW at room temperature always shows brittle fracture. However, at temperature welded using EBW at room temperature always shows brittle fracture. However, at temperature higher higher than 300 °C, the joint always presents ductile fracture, and there is obvious necking than 300 C, the joint always presents ductile fracture, and there is obvious necking phenomenon phenomenon◦ before fracturing. In addition, the research demonstrates that carbonization and heat before fracturing. In addition, the research demonstrates that carbonization and heat treatment after treatment after welding can effectively raise the strength of the joint of Mo alloys, which is primarily welding can effectively raise the strength of the joint of Mo alloys, which is primarily attributed to an attributed to an increase of the grain boundary cohesion due to the effective carbon segregation and increase of the grain boundary cohesion due to the effective carbon segregation and precipitation. [18]. precipitation. [18]. Morito et al. (1998) [19] found that the strength and ductility of a welded joint of Morito et al. (1998) [19] found that the strength and ductility of a welded joint of Mo alloys obtained Mo alloys obtained through EBW can rise after increasing rhenium (Re) content. The reason is that throughtwo-phase EBW microstructure can rise after increasing is formed rhenium in the (Re) weld content. zone with The reason enhanced is that Re two-phase content. microstructureBased on the isthermal formed simulation in the weld test, zoneMorito with et al. enhanced (1997) [20 Re] compared content. Basedductilities on theof HAZs thermal when simulation welding test,Mo Morito et al. (1997) [20] compared ductilities of HAZs when welding Mo alloys (Mo 99.9 wt%) under alloys (Mo > 99.9 wt%) under two conditions of thermal treatment, i.e., furnace cooling> and quick twocooling conditions through of quenching. thermal treatment, It is found i.e., that furnace quick cooling cooling and after quick welding cooling can throughsignificantly quenching. reduce Itthe is foundductility that of quickHAZ coolingof the welded after welding joint of can Mo significantly alloys, mainly reduce because the ductilitygrain-boundary of HAZ segregation of the welded in jointHAZ of is Mo more alloys, significant mainly because under grain-boundary quick cooling segregation through in quenching. HAZ is more Stütz significant et al. under (2016) quick [21] coolingsystematically through studied quenching. the infl Stützuences et al. of (2016) parameters [21] systematically of EBW process studied on sizes the influencesof fusion zone of parameters (FZ) and ofHAZ, EBW size process of grains on sizesin FZ ofand fusion HAZ, zone sensitivity (FZ) and to HAZ,pores and size cracks of grains in butt in FZ welded and HAZ, joint sensitivityof TZM alloy to poreswith thickness and cracks of in 2 buttmm. welded The results joint ofshow TZM that alloy pore with defects thickness are serious of 2 mm. when The resultswelding show heat that input pore is defectslarge. Small are serious heat input when can welding not only heat inhibit input ispores, large. but Small also heat obviously input can reduce not onlygrain inhibit size in pores, FZ. The but alsostrength obviously of the reducejoint welded grain size by EBW in FZ. can The reach strength 50–77% of the: that joint of welded the base by metal EBW can(BM). reach Therefore, 50–77%: Stütz that ofet al. the pointed base metal out (BM).that itTherefore, was necessary Stütz to et study al. pointed EBW in out terms that itof was filling necessary materials to studyand alloying EBW in metals terms ofin fillingthe weld materials seam, which and alloying has important metals inguiding the weld significance seam, which for improving has important mechanical guiding properties significance of formolybdenum improving alloy mechanical joints. propertiesRecently, Chen of molybdenum et al. (2018) alloy [22] joints. conducted Recently, electron Chen beam et al. welding (2018) [22 of] conductedmolybdenum electron and found beam that welding the even of molybdenum tensile strength and of found the joints that thewas even 280 MPa, tensile and strength the fracture of the jointsposition was was 280 located MPa, and in the the fracture weld, which position was was a locatedbrittle infracture the weld,, the which fracture was location a brittle is fracture, shown the in fractureFigure 4. location It was determined is shown in as Figure quasi4-.cleavage It was determined fracture. Pore as quasi-cleavage and crack defects fracture. were observed Pore and crackin the defectsweld zone. were The observed pores inwere the formed weld zone. by the The oxygen pores were that formedwas not by escaping the oxygen the thatmolten was pool. not escaping Cracks thewere molten confirmed pool. as Cracks solidification were confirmed cracks and as solidificationlow plastic embrittlement cracks and low cracks. plastic embrittlement cracks.

Figure 4. 4. CrossCross-sectional-sectional morphology morphology of of the the weldweldeded joints produced by electron-beamelectron-beam welding (EBW)(EBW) weldingwelding (a()a) pore pore defects defects diagram diagram with with improved improved parameters, parameters (b), crystal (b) crystal cracks, cracks (c) low, (c plastic) low embrittlementplastic embrittlement crack, andcrack (d,) and forming (d) forming after optimized after optimized process. process. [22]. [22].

In addition, Chen et al. (2019) [23] found that during welding with a 0.6 mm beam deflection to Kovar, the weld zone exhibits equiaxed crystals. Compared with the columnar crystal during

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MetalsIn 2020 addition,, 10, x FOR Chen PEER etREVIEW al. (2019) [23] found that during welding with a 0.6 mm beam deflection5 of to16 Kovar, the weld zone exhibits equiaxed crystals. Compared with the columnar crystal during welding withoutwelding beamwithout deflection, beam deflection, the microstructure the microstructure of the weld of zone the transformsweld zone intotransforms an equiaxed into crystalan equiaxed when weldingcrystal when with we a 0.6lding mm with beam a o0.6ffset mm to Kovar.beam offset The morphology to Kovar. The of themorphology reaction layer of the alters reaction due tolayer the altersbeam deflection, due to the which beam escalates deflection, its toughness. which escalates No cracks its toughness.are observed No in the cracks heat-a areff ected observed zone inon the heatmolybdenum-affected zone side. on The the tensile molybdenum strength side. of the The joint tensile increases strength resulting of the joint from incr theeases beam resulting deflection from to theKovar, beam which deflection exceeds to 260Kovar, MPa which when exceeds the beam 260 deflection MPa when is 0.6the mm. beam deflection is 0.6 mm.

3.2. TIG TIG W Weldingelding Wang et et al. al. [24 []24 studied] studied the TIGthe weldingTIG welding of TZM of alloy. TZM The alloy. results The demonstrate results demonstrate that well-formed that weldwell-formed seam can weld be obtained seam can under be obtained proper welding under currentproper (aswelding is shown current in Figure (as is5), shown welding in speed, Figure and 5), weldingargon gas speed flow,, betterand argon welding gas process flow, better parameters: welding welding process speed parameters: 4 mm/s, argon welding flow speed rate 10 4 mm/s,L/min, weldingargon flow current rate 10 should L/min, be welding controlled current at ~210 should A. Jiangbe controlled et al. [25 at] researched~210 A. Jiang TIG et weldingal. [25] researched of Mo-Cu TIGcomposite welding materials of Mo-Cu and composite stainless steelmaterials filled and with stainless Cr-Ni wires. steel filled with Cr-Ni wires.

Figure 5. MicrostructureMicrostructure of of weld weld joint joint ( (aa)) welding seam seam zone zone,, ( (b)) heat heat-a-affectedffected zone zone,, and ( c) TZM martixmartix.. [ 2244]]..

Wang [2626] found that there are fewerfewer pores in weld seams obtained by EBW or TIG welding of smelting Mo Mo alloy, alloy, whereas whereas there there are are more more pores pores in in weld weld seams seams of ofpure pure Mo Mo or orMo Mo alloys alloys obtained obtained by powderby powder metallurgy. metallurgy. The Theaddition addition of C ofcan C improve can improve plasticity plasticity of powder of powder-metallurgy-processed-metallurgy-processed weld weldseam seamof pure of pureMo or Mo Mo or Moalloys alloys by reducin by reducingg content content of ofmolybdenum molybdenum oxide oxide at at grain grain boundary, boundary, and significantlysignificantly reduce pores in the weld seam. In In addition, addition, adding adding Ti Ti and hafnium (Hf) (Hf) into the weld seam can decreasedecrease centerlinecenterline cracks cracks and and pores pores in in weld weld seam seam of powder-metallurgy-processedof powder-metallurgy-processed pure pure Mo, Mo,so that so th tensileat tensile fracture fracture transfers transfers from from weld weld seam seam to HAZ. to HAZ. Matsuda etet al.al. [ 27[2]7 studied] studied EBW EBW and and TIG TIG welding welding of TZM of alloyTZM withalloy thickness with thickness of a 1.5 mmof a prepared 1.5 mm preparedthrough powder through metallurgy. powder The metallurgy. research shows The research that large shows welding that heat large input welding can significantly heat input decrease can significantlyductility of a welded decrease TZM duc alloytility joint of a and welded the ductile-brittle TZM alloy transition joint and temperature the ductile of-brittle the welded transition joint temperatureobtained through of the TIG welded welding joint is obtained ~120 ◦C higherthrough than TIG that welding of EBW. is Furthermore,~120 °C higher they than also that found of EBW. that Furthermore,pore defects only they appear also around found thatthe arc pore starting defects and only arc extinguishing appear around positions the arc in weld starting seam and during arc extinguishingTIG welding, whereaspositions pore in weld defects seam greatly during increase TIG welding, in weld seam whereas during pore EBW defects welding greatly in theincrease vacuum in weldenvironment. seam during Kolarikova EBW et welding al. (2012) in [28 the] investigated vacuum environment. EBW and TIG Kolarikova welding of et pure al. (2012) Mo sheets. [28] Widthsinvestigated of FZs EBW in joints and obtainedTIG welding through of pure EBW Mo and sheets. gas tungsten Widths arcof weldingFZs in joints (GTAW) obtained separately through are EBW0.8 mm and and gas 1.7 tungsten mm, whereas arc welding HAZs (GTAW) are significantly separately di ffareerent 0.8 inmm width and (1.41.7 mm, mm andwhereas 35 mm). HAZs Grain are significantlysizes in FZ and different HAZ in in the width EBW (1.4 joint mm are and obviously 35 mm). smallerGrain sizes than in those FZ and in the HAZ GTAW in the joint, EBW indicating joint are obviouslythat EBW withsmaller high than energy those density in the is GTAW more suitable joint, indicating for welding that Mo EBW than with GTAW high energy density is more suitable for welding Mo than GTAW 3.3. Laser Welding 3.3. LaserLiu etW al.elding (2016) [29] studied continuous-wave Nd:yttrium-aluminum-garnet (YAG) laser welding of anLiu overlap et al. joint (2016) of Mo-Re [29] studied alloy (50Mo:50Re) continuous with-wave the Nd:yttrium thickness of-aluminum 0.13 mm- preparedgarnet (YAG) by powder laser weldingmetallurgy. of an After overlap welding, joint of cracks Mo-Re appear alloy at (50Mo the bonding:50Re) wit interfaceh the thickness of FZ, and of 0.13 many mm large prepared pores areby powderobserved metallurgy. at the bonding After interface welding, of cracks FZ, with appear the diameterat the bonding being interface ~15–20% of of FZ, the thicknessand many of large BM poresplates. are Microscopic observed at analysis the bonding results interface of fracture of presentFZ, with that the intergranular diameter being fracture ~15–20% occurs of the and thickness there are of BM plates. Microscopic analysis results of fracture present that intergranular fracture occurs and there are a large number of dark compounds in the grains and on the grain boundary, as is shown in Figure 6, and the composition analysis showed that the content of C and O in these compounds were 30% and 15%, respectively. It is believed that coarse microstructures and harmful impurity elements

Metals 2020, 10, 279 6 of 16 aMetals large 2020 number, 10, x FOR of PEER dark REVIEW compounds in the grains and on the grain boundary, as is shown in Figure6 of 616, Metals 2020, 10, x FOR PEER REVIEW 6 of 16 and the composition analysis showed that the content of C and O in these compounds were 30% and cause the hardening of bonding interface and intergranular fracturing of the joint. The study of Lin 15%, respectively. It is believed that coarse microstructures and harmful impurity elements cause the (2013)cause the [3] hardening demonstrates of bonding that weldi interfaceng conductive and intergranular elements fracturing of needle of -theshaped joint. pureThe study Mo withof Lin a hardening of bonding interface and intergranular fracturing of the joint. The study of Lin (2013) [3] diameter(2013) [3 ]of demonstrates 0.5 mm using that pulse weldi Nd:YAGng conductive laser welding elements instead of of needle ERW- shapedcan significantly pure Mo raise with the a demonstrates that welding conductive elements of needle-shaped pure Mo with a diameter of 0.5 mm strengthdiameter of of the 0.5 joint. mm using pulse Nd:YAG laser welding instead of ERW can significantly raise the using pulse Nd:YAG laser welding instead of ERW can significantly raise the strength of the joint. strength of the joint.

Figure 6. SEM observation of 50Mo-50Re overlap joint achieved by laser welding: (a) cross section Figureand and 6.6. ( bSEMSEM) fracture observationobservation surface of of [2 50Mo-50Re 950Mo]. -50Re overlap overlap joint joint achieved achieved by by laser laser welding: welding: (a) cross(a) cross section section and and (andb) fracture (b) fracture surface surface [29]. [29]. Kramer et al. (2013) [4] studied EBW and pulse Nd:YAG laser welding of Mo-44.5% Re alloy sheetsKramer with a et thickness al. (201 (2013)3 )of [ [ 440.5]] studied studied mm. The EBW EBW research and and pulse pulse shows Nd:YAG Nd:YAG that a laser Mo-44.5% welding Re alloyof Mo-44.5%Mo joint-44.5% welded Re alloy by sheetsEBW is with well a- formed, thickness without of 0.5 mm.mm. defects The The including research research poresshows shows and that that cracks. a a Mo Mo-44.5%- 44.5% Cracks Re are alloy found joint in welded FZ in theby EBWlaser- welded is well-formed,well- formed,Mo-44.5% without Re alloy defects joint and including the micromorphology pores and cracks. of Cracksbrittle Cracks fracture are are found found is shown in in FZ in the laser-weldedfracturelaser-welded of the Mo Mo-44.5% laser-44.5%-welded Re alloy joint joint after andtesting the mechanical micromorphology performances. of brittle Chatterjee fracture is et shown al. (2016) in the[5] fractureresearched of thetheEBW laser-weldedlaser and-welded Nd:YAG jointjoint laser afterafter-TIG testingtesting hybrid mechanicalmechanical welding for performances.performances. butt welded joint Chatterjee of wrought et al.al. TAM (2016) alloy [5] researched(Ti 0.50 wt%, EBW Zr 0.08 and Nd:YAG wt% and laser-TIGlaserC 0.04-TIG wt%) hybrid with welding thickness for of butt 1.2 welded mm. Grain joint sizes of wrought in FZ and TAM HAZ alloy in (Tithe 0.500.50 EBW wt%, joint Zr are 0.08 0.08 obviously wt% wt% and and small,C C 0.04 0.04 whichwt%) wt%) with withare ~thickness55% thickness and of ~ of 65%1.2 1.2 mm. of mm. those Grain Grain in sizes the sizes injoint inFZFZ and obtained and HAZ HAZ byin inNd:YAGthe the EBW EBW laser joint joint- TIG are are hybrid obviously obviously welding. small, small, The which which weld are width are ~55% ~55% of andEBW and ~ and ~65%65% hybrid of of those those welding in in the the method joint joint obtained is ~1.4 mm by Nd:YAGand 2.6 mm laser-TIGlaser respectively,-TIG hybridhybrid but welding.welding. in both The cases, weld the width width of of EBW HAZ and is ~ 1.5 hybrid times welding of the weld method width. is ~1.4~ 1.4Results mm andof tensile 2.6 mm test respectively, demonstrate butbut that inin both bothstrengths cases,cases, of thethe joints widthwidth prepared ofof HAZHAZ isbyis ~1.5~ Nd:YAG1.5 times oflaser the-TIG weld hybrid width. welding Results ofand tensiletensile EBW testaretest demonstrateaboutdemonstrate 41% and that that 47% strengths strengths that of ofBM. of joints jointsThe prepared fracture prepared morphology by by Nd:YAG Nd:YAG laser-TIGis shownlaser-TIG hybridin F hybridigure welding 7, welding the andtwo EBWjointsand EBW arehardly about are showabout 41% any and41% tensile 47%and that47% plasticity ofthat BM. of inTheBM. the fractureThe tensile fracture morphologytest, morphologyand the isshrinka shown is shownge in and Figure inelongation F7igure, the two7, ofthe joints cross two hardlysectionsjoints hardly show are almost anyshow tensile any zero, tensile plasticity while plasticity the in elongation the tensile in the test,tensileof BM and test,is the up and shrinkage to 8.4%.the shrinka Figure and elongationge 8 and shows elongation ofthe cross presence sectionsof cross of arelargesections almost grains are zero, almost and while nearly zero, the uniformwhile elongation the distribution elongation of BM is up of to BM a 8.4%. second is up Figure to phase 8.4%.8 shows within Figure the the 8 presence shows gain, typicalthe of largepresence volume grains of andfractionlarge nearly grains estimated uniform and nearlyfrom distribution these uniform micro of distribution agraphs second shows phase of that awithin second the theoxide phase gain, phase typical within is nearly volume the gain, 10 fractionpct typical of the estimated volume fromfraction.fraction these estimated micrographs from these shows micro that thegraphs oxide shows phase that is nearlythe oxide 10 pctphase of theis nearly volume 10 fraction. pct of the volume fraction.

Figure 7. FractographsFractographs of of ( (aa)) the the parent mmetal,etal, ( (b)) EB weld joint,joint, and (c) LGTHW Joint of tensile tensile samples.Figure 7. Change Fractographs in the ofmorphology (a) the parent and the metal presence , (b) EB of weld sharp joint faceting, and in(c) samples LGTHW containingc ontaini Joint ofng tensile weld jointssamples. could Change be noticed in the [ 5morphology].]. and the presence of sharp faceting in samples containing weld joints could be noticed [5].

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FigureFigure 8. 8. 8. TEMTEMTEM micrographs micrographs micrographs showing showing showing (a ) ( (a thea)) the the presence presence presence of the of of precipitates the the precipitates precipitates within within within the matrix. the the matrix. matrix. Inset shows Inset Inset showstheshows magnified thethe magnifiedmagnified view; ( bviewview) Magnified;; ((bb)) MagnifiedMagnified view showing viewview sshowinghowing the presence thethe presencepresence of Mo-oxide ofof MoMo phase--oxideoxide in phasephase the weld inin thethe region. weldweld region(regionc) The.. presence((cc)) TheThe prpr ofesenceesence needle-shaped ofof needleneedle--shaped longshaped oxides longlong may oxidesoxides be may noticedmay bebe noticed [noticed5]. [[55]]..

AnAn et et al. al. (2018) (2018) (2018) [ [ [303030]]] carried carried carried out out out laser laser laser lap lap lap welding welding of of fuel fuel cladding cladding and and end end plug plug made made made of of of molybdenummolybdenum (Mo) (Mo) (Mo) alloy. alloy. alloy. Under Under Under the the the optimum optimum optimum processing processing processing conditions, conditions, conditions, the tensile the the tensile tensile strength strength strength of the welded of of the the weldedjointwelded reached jointjoint reachreach 617 MPa,eded 617617 taking MPa,MPa, taking uptaking 82.3% upup 82.3% that82.3% of thatthat the ofof base thethe metal. basebase metal.metal. Recently, Recently,Recently, Zhang ZhangZhang et al. etet (2019)al.al. (2019)(2019) [9] [successfully[99]] successfullysuccessfully enhanced enhancedenhanced the thethe mechanical mechanicalmechanical performance performanceperformance of ofof fusion fusionfusion zone zonezone in inin laser laserlaser beam beambeam weldingweldingwelding jointjoint of of molybdenummolybdenum alloyalloyalloy by byby solid solidsolid carburizing. carburizing.carburizing. As AsAs is is shownis shownshown in Figureinin FigureFigure9 the 99 tensiletthehe tensiletensile strength strengthstrength of carburized ofof carburizedcarburized weld weldjointsweld rosejoints joints by rose rose 426% by by compared 426% 426% compared compared with that with with of uncarburized that that of of uncarburized uncarburized weld joints. weld weld The TEM joints. joints. images TheThe TEM ofTEM molybdenum images images of of molybdenumoxidemolybdenum particles oxideoxide in the particlesparticles FZ of LW inin the jointthe FZFZ show ofof LWLW that jointjoint the showshow particles thatthat were thethe particlesparticles shown as werewere lenticular shownshown orasas elongatedlenticularlenticular orblocksor elongatedelongated under blocks TEMblocks observation underunder TEMTEM (Figure observationobservation 10a) and (F(Figureigure judged 10a)10a) from andand the judgedjudged diffraction fromfrom pattern thethe diffractiondiffraction as MoO 2patternpattern. And the asas MoOTEMMoO22 images.. AndAnd thethe of TEM molybdenumTEM imagesimages ofof carbide molybdenummolybdenum particles carbidecarbide in the particles FZparticles of SC-150 inin thethe joint FZFZ ofof show SCSC--150 that150 jointjoint the particles showshow thatthat were thethe particlesshownparticles as werewere circular shownshown pattern asas circularcircular under TEM patternpattern observation underunder TEMTEM (Figure observationobservation 10b) and judged(Figure(Figure from 10b)10b) theandand di judgedjudgedffraction fromfrom pattern thethe diffractiasdiffracti Mo2C.onon patternpattern asas MoMo22C.C.

Figure 9. The effect of C addition on tensile strength of the Mo alloy joints [9]. FigureFigure 9.9. TThehe effecteffect ofof CC additionaddition onon tensiletensile strengthstrength ofof thethe MoMo alloyalloy jointsjoints [[99]]..

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(021) (204) (021) (202)(204) (402) (020) (001) (a)(a) (202) (402) (b)(b) (020) (001) (000) (000) (000) (000) Mo C MoO2 Mo 2C MoO2 2

Carbide (Mo2C) Carbide (Mo2C) MoO MoO 2 (020) 2 (020) GrainGrain boundaryboundary GrainGrain boundaryboundary 200nm200nm 500nm500nm

FigureFigure 10. 10. TEMTEMTEM images images images of of of ( (a (aa)) ) molybdenum molybdenum molybdenum oxide oxide oxide particles particles particles in in in the the the FZ FZ FZ of of of LW LW LW jointjoint and and ( (bb)) ca carbidecarbiderbide particlesparticles in in the the FZ FZ of of SC SC-150SC--150150 joint joint [ [99]].]..

XieXie etet al.al. (2(2 (2019)019)019) [[ [121212]]] foundfound found thatthat that lowlow low heatheat heat inputinput input (i.e.,(i.e., (i.e., highhigh high weldingwelding welding speed)speed) speed) resultedresulted resulted inin in significantlysignificantly significantly refinedrefinedrefined grains grains inin thethe fusionfusion zonezone (FZ)(FZ) ofof fiberfiber fiber laserlaser weldedwelded n nano-sizednanoano--sizedsized rarerare earthearth oxideoxide particlesparticles andand sssuperfineuperfineuperfine crystalcrystal microstructuremicrostructuremicrostructure (NS) ((NSNS) Mo) MoMo joints, jointsjoints the,, thethe cross-sectional crosscross--sectisectionalonal microstructures microstructuresmicrostructures of the ofof NSthethe Mo NSNS alloy MoMo alloyalloylaser laser weldinglaser weldingwelding joints jointsjoints is shown isis shownshown in Figure inin FigureFigure 11. When 11.11. WhenWhen welding weldingwelding heat heatheat input inputinput decreased decreaseddecreased from fromfrom 3600 36003600 J/cm J/cmJ/cm (i.e., (i.e.,1.2(i.e., kW, 1.21.2 20kW,kW, cm 2020/min) cm/min)cm/min) to 250 toto J/ cm 250250 (i.e., J/cmJ/cm 2.5 (i.e.,(i.e., kW, 2.52.5 600 kW,kW, cm / 600min),600 cm/min),cm/min), the tensile thethe strength tensiletensile strength ofstrength welded ofof joints weldedwelded increased jointsjoints increasedincreasedfrom ~250 fromfrom MPa ~~250 to250 ~570 MPaMPa MPa. toto ~~570570 They MPa.MPa. also TheyThey found alsoalso that foundfound laser thatthat welding laserlaser weldingwelding of NS-Mo ofof NSNS under--MoMo underunder low heat lowlow input heatheat inputinputsignificantly significantlysignificantly reduced reducedreduced the porosity thethe porosityporosity defects defectsdefects in the inin fusion thethe fusionfusion zone (2019) zonezone (2019)(2019) [13]. [[1313].].

FigureFigure 11.11. TheTheThe crosscross cross-sectional--sectionalsectional microstructuresmicrostructures microstructures ofof of thethe the NSNS NS MoMo Mo alloyalloy alloy laserlaser laser weldingwelding welding joints.joints. joints. HeatHeat Heat ininput:put: input: ((aa)) 250250(a) 250J/cmJ/cm J/ cmandand and ((bb)) 3600(3600b) 3600 J/cmJ/cm J/ cm [[1212] [].12. ].

InIn thethe studystudy ofof Zhang Zhang etet al. al. (2019)(2019) [[ 1010],], titanium titanium waswas selectedselected asas anan alloyingalloying elementelement toto to reducereduce brittlenessbrittleness ofof laserlaser weldweld beads beads in in molybdenum molybdenum “ “cladding-end“claddingcladding--endend plug plug”plug”” socket socket joints. joints. BrazingBrazing waswas alsoalso performedperformed to to enhanc enhanceenhancee jointjoint strength.strength.strength. TheTheThe combined combinedcombined structure structurestructure of ofof laser laserlaser welding weldingwelding and andand brazing brazingbrazing is isis shown shownshown in ininFigure FigureFigure 12 ,12,12, joints jjointsoints with withwith the thethe same samesame strength strengthstrength as base asas basebase material materialmaterial and andand hydraulic hydraulichydraulic bursting burstingbursting pressure pressurepressure of 60 ofof MPa 6060 MPawereMPa werewere produced producedproduced using usingusing a combination aa combinationcombination of the ofof two thethe methods. twotwo methods.methods.

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Figure 12. Tracing of the metal elements in the weld bead of the Mo-0.03Ti-B joint: (a) schematic FigurediagramFigure 12. 12. of TracingTracinthe locationg of of the the where metal metal Zr elements elements was added in in the. the(b) weld weldAnalysis bead bead of ofof Mo, the the Zr Mo-0.03Ti-B Mo, and-0.03Ti Ti on-B joint:the joint: cross ( (aa)) schematicsectionschematic of diagramthediagram joint ofbyof theEDSthe location location surface where scanningwhere Zr Zr was. was(c) added.A addednalysis (.b (of)b Analysis )Zr A nalysisin zone of of Mo,A Mo,in Zr,panel Zr and, and (b Ti) onbyTi theEDSon the cross surface cross section scanning.section of the of joint[the10] .joint by EDS by EDS surface surface scanning. scanning (c) Analysis. (c) Analysis of Zr of in Zr zone in zone A in A panel in panel (b) by (b EDS) by surfaceEDS surface scanning. scanning. [10]. [10]. Liu et al. (2019) [11] found that welding cycles had a significant influence on the porosity Liu et al. (2019) [11] found that welding cycles had a significant influence on the porosity ratio ratio of fusion zone (FZ), whereas the amplitude and frequency of laser power waveform slightly of fusionLiu et zone al. (2019) (FZ) ,[ 11w]hereas found thethat amplitudewelding cycles and had frequency a significant of laser influence power on waveform the porosity slightly ratio influenced the porosity. Moreover, Zr added in a molten pool can be preferentially reacted with O to influencedof fusion zonethe porosity. (FZ), w Moreover,hereas the Zr amplitude added in a and molten frequency pool can of be laser preferentially power waveform reacted with slightly O to generateinfluenced ZrO the2, porosity. which can Moreover, inhibit the Zr precipitation added in a molten of volatile pool MoOcan be2 to preferentially thus suppress reacted the generation with O to generate ZrO2, which can inhibit the precipitation of volatile MoO2 to thus suppress the generation of metallurgy-induced pores, reconstructed 3D transparent distribution of pores in joints is shown ofgenerate metallurgy ZrO-2induced, which canpores inhibit, reconstructed the precipitation 3D transparent of volatile distribution MoO2 to thusof pores suppress in joints the is generation shown in in Figure 13. Zhang et al. (2019) [7] conducted laser seal welding of end plug to thin-walled Figureof metallurgy 13. Zhang-induced et al.pores (2019), reconstructed [7] conducted 3D transparent laser seal distribution welding of of endpores plug in joints to thinis shown-walled in nanostructured high-strength molybdenum alloy cladding with a zirconium interlayer and tensile nanostructuredFigure 13. Zhang high et-strength al. (2019) molybdenum [7] conducted alloy lasercladding seal with welding a zirconium of end interlayer plug to thinand -walledtensile strength of the achieved welded joints matched that of the base metal. Note that by taking advantage strengthnanostructured of the high achieved-strength welded molybdenum joints matched alloy cladding that of with the basea zirconium metal. Noteinterlayer that andby takingtensile of the metallurgical characteristics of molybdenum and its high melting point and high thermal advantagestrength of of thethe achievedmetallurgical welded characteristics joints matched of molybdenum that of the and base its metal.high melting Note pointthat byand taking high conductivity, Zhang et al. [9–11] put forward a systematic strategy that can effectively solve the thermaladvantage conductivity, of the metallurgical Zhang et al.characteristics [9–11] put forward of molybdenum a systematic and strategy its high that melting can effectively point and solve high problems of porosity and embrittlement in welding fuel cladding made of molybdenum alloys. More thethermal problems conductivity, of porosit Zhangy and etembrittlement al. [9–11] put in forward welding a systematicfuel cladding strategy made that of molybdenum can effectively alloys. solve importantly, the results of tensile test and hydraulic bursting show that the molybdenum alloy cladding Morethe problems importantly, of porosit the resultsy and embrittlement of tensile test in and welding hydraulic fuel cladding bursting made show of that molybdenum the molybdenum alloys. prepared by this method have excellent performance, which completely eliminates the doubts about alloyMore cladding importantly, prepared the results by this of method tensile have test andexcellent hydraulic performance, bursting which show completely that the molybdenum eliminates the welding quality of molybdenum alloy fuel cladding, and is of great significance to the promotion thealloy doubts cladding about prepared the welding by this method quality have of molybdenum excellent performance, alloy fuel which cladding, completely and is elimin of greatates and application of molybdenum alloy accident-tolerant fuel cladding, the results of tensile test and significancethe doubts to about the promotion the welding and qualityapplication of molybdenumof molybdenum alloy alloy fuel accident cladding,-tolerant and fuel is cladding of great, hydrostatic test are shown in Figure 14. thesignificance results of to tensile the promotion test and hydrostatic and application test are of shown molybdenum in Figure alloy 14. accident-tolerant fuel cladding, the results of tensile test and hydrostatic test are shown in Figure 14.

Figure 13. ReconstructedReconstructed 3D 3D transparent transparent distribution distribution of pores of pores in the in thethree three joints joints achieved achieved under under (a) P (=Figurea )P1.2 =kW 1.213.; ( b kW;Reconstructed) average (b) average power 3D power =transparent 1.2 kW,= 1.2 Amplitude kW,distribution Amplitude = 300 of poresW,= 300 frequency in W, the frequency three = 50 joints HZ,= 50Nachieved = HZ, 2 cycles N under= 2; and cycles; (a ()c P) anda=verage 1.2 ( ckW) average power; (b) average power = 1.2 power kW,= 1.2 a kW,mplitude= 1.2 amplitude kW, Amplitude= 300= W,300 frequency W,= 300 frequency W, frequency = 150= 150 HZ, = HZ, 50 N HZ, N = = 2 N2 cycles, cycles,= 2 cycles adding adding; and Zr, Zr,(c) respectively.average power [[1111]. ]. = 1.2 kW, amplitude = 300 W, frequency = 150 HZ, N = 2 cycles, adding Zr, respectively. [11].

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(a) (b)

) 600

MPa Tube ( Mo tube 400 Necking Addition Ti by type B (Mo-1Ti-B) 200 Weld bead Rod

Tensile stress Tensile 0 0 10 20 30 Engineering stain (%)

(c) (d) )

60 Hydraulic bursting test MPa

40

Rod Hydrostatic test Tube 20

0 Tube Hydraulic pressure( Hydraulic 0 300 600 900 1200 Time(s)

Figure 14. ((aa)) Strength Strength-displacement-displacement curves of Mo Mo-0.03Ti-B-0.03Ti-B and pure Mo tube; ( b) images of the joint after fracture testing; ( c) hydrostatic test curve and hydraulic bursting test curve for the the molybdenum molybdenum joint of Mo Mo-0.03Ti-B;-0.03Ti-B; and ((d)) imagesimages ofof jointjoint afterafter hydraulichydraulic burstingbursting testingtesting [[10]10]..

In the studystudy ofof Wang Wang et et al. al. (2019) (2019) [31 [31], laser], laser beam beam off setoffset welding welding was was used used to join to purejoin pure Mo and Mo 304L. and 304LThe results. The results demonstrated demonstrated that tensile that strengthtensile strength of the joints of the could joints be increasedcould be increased to ~280 MPa to ~ by280 presetting MPa by presettinga nickel (Ni) a foilnickel at Mo(Ni)/304 foil L at interface Mo/304 and L interface shifting laserand shifting beam to laser 304L, beam whereas to 304L, the tensile whereas strength the tensile of the strengthsample without of the sample Ni foil is without only 112 Ni MPa, foil and is only the results112 MP ofa tensile, and the test results is shown of intensile Figure test 15. is In shown the study in Figureof Lu et 15 al.. In (2018) the [ study32,33], of by Lu adding et al. zirconium (2018) [32 (Zr),33], to by the adding molten zirconium pool, ultimate (Zr) tensile to the strength molten (UTS) pool, ultimateof the dissimilar tensile strength joint of titanium (UTS) of and the molybdenumdissimilar joint was of increasedtitanium fromand molybdenum about 350 MPa was to ~470increased MPa, whichfrom about reached 350 more MPathan to ~470 90% MPa of that, which of the reached Ti base metalmore (BM).than 90% Zhou of etthat al. of (2018) the [Ti34 base] observed metal cracks(BM). Zhouin dissimilar et al. (2018) laser [3 welding4] observed of tantalum cracks in to dissimilar molybdenum laser and welding pointed of outtantalum that solidification to molybdenum cracking and pointedtendency out of that Mo wassolidification the main cracking reason for tendency crack initiation of Mo was in thethe Ta main/Mo reason joint. Ningfor crack et al. initiation (2019) [35 in] thestudied Ta/Mo the potentialjoint. Ning of laser et al. welding (2019) [3of5 0.5] studied mm-thick the Titanium-zirconium-molybdenum potential of laser welding of 0.5 (TZM) mm-thick alloy Titaniumin a lap welding-zirconium configuration.-molybdenum They (TZM) found alloy that in introducing a lap welding an interface configuration. gap of They0.09 mm found had that the introducingmost positive an e ff interfaceect in reducing gap of the 0.09 porosity mm had compared the most to usingpositive helium effect gas, in reducingdifferent shielding the porosity gas comparedflow rates, to adding using alloy helium elements, gas, different and diff erent shielding heat gas input flow rates. rates, Liu adding et al. (2019) alloy [element36] compareds, and differentthe micro-structures, heat input properties,rates. Liu et and al. residual (2019) stresses [36] compared of the welded the micro girth-structures, joints achieved properties, at different and residualpreheating stresses temperatures of the welded and found girth joints that the achieved tensile at strength different reached preheat aing maximum temperatures at the preheatingand found thattemperature the tensile of 673 strength K, which reached was approximately a maximum at 50% the that preheating of the base temperature metal. Gao of et 673 al. (2020) K, which [37] was also approximatelystudied the effect 50% of that laser of o fftheset base on microstructure metal. Gao et andal. (2020) mechanical [37] also properties studied ofthe laser effect welding of laser of offset pure onmolybdenum microstructure to stainless and mechanic steel. Asal theproperties laser beam of laser shifts welding from the of Mopure side molybdenum to the stainless to stainless steel side, steel. the Asformation the laser of beam welding shifts defects from the and Mo Fe-Mo side to intermetallic the stainless compounds steel side, the (IMCs) formation are eff ofectively welding restricted defects andbecause Fe-Mo of the intermetallic decrease amount compounds of molten (IMCs) Mo. Consequently, are effectively the restricted tensile strength because of of joints the increaseddecrease amountfirst and of then molten decreased Mo. Consequently, in the laser o theffset tensile range strength of 0.2–0.5 of mm.joints Theincreased highest first tensile and then strength decreased of the injoints the islaser 290 offset MPa at range the laser of 0.2 off–set0.5 ofmm. 0.3 The mm. highest tensile strength of the joints is 290 MPa at the laser offset of 0.3 mm.

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Metals 2020, 10, x FOR PEER REVIEW 11 of 16

(a) (b) (c) Figure 15. 15. ((aa)) Fracture Fracture cross cross section section of of the the joint joint without without Ni, Ni (b,) ( fractureb) fracture cross cross section section of the of joint the with joint Ni,without and (Nic) tensile, and (c results) tensile of results the two of jointsthe two [31 joints.]. [31].

3.4. ERW Xu et al. (2007) [3 [388] investigated the optimization of the resistance spot welding processprocess of an overlapoverlap joint of 50Mo-50Re50Mo-50Re (wt%) alloy with thickness of 0.127 mm. The study found that the longer the time of application of upsetting force after power off off is, the higher the strength of the joint and the better the ductility. When time of applying upsettin upsettingg force after power off off increases from 50 ms to 999 ms,ms, bearing capacity of the joint rises from 100 N to 113 N andand fracturefracture mode changes from brittle intergranularintergranular fracturefracture toto dimple dimple fracture. fracture. This This is is because because after after power power o ffoff,, the the increase increase of of time time of applyingof applying upsetting upsetting force force can can accelerate accelerate the the cooling cooling rate rate of weldof weld seam, seam, thus thus inhibiting inhibiting segregation segregation of Moof Mo at grainat grain boundaries. boundaries. It is It also is also found found from from the studythe study that that the defects the defects of large-size of large pores-size pores appear appear in FZ ofin the FZ joint of the under joint various under welding various conditions,welding conditions, because there becaus aree micropores there are micropores in powder metallurgy in powder materialsmetallurgy and materials is shown and in is Figure shown 16 in. Figure 16.

Figure 16. (a,b) The shapeshape and microstructuremicrostructure of the nuggetsnuggets welded using the electrodeselectrodes shown in ((c,,d),), respectively.respectively. The horizontal arrows show the interfaces between two workpieces in panels ( a) and (b)[) [3388].].

Elizabeth E. E. Ferrenz Ferrenz et al. et [39 al.] controlled[39] controlled welding welding quality quality by using by double using pulse double current pulse waveforms current inwaveforms resistance in spot resistance welding spot of Mo welding and T-Re of alloyMo and wires. T-Re The alloy first wires. pulse currentThe first is smallpulse andcurrent mainly is small used toand remove mainly oxide used film, to remove while the oxi secondde film, pulse while is usedthe second for welding pulse withis used a larger for welding current. with a larger current. 3.5. Brazing 3.5. BrazingXia et al. (2017) [40] studied vacuum brazing for an overlap joint of 50Mo-50Re alloy with a thicknessXia et of al. 0.06 (2017) mm. [40 BM] studied is prepared vacuum by brazing utilizing for powder an overlap metallurgy. joint of The50Mo brazing-50Re alloy filler with metal a isthickness Ni-Cr-Si-B of 0.06 (Ni-19Cr-7.3Si-1.5B mm. BM is prepared wt%) withby utilizing a melting powder temperature metallurgy. range The of 1081brazing to 1136filler◦ C.metal After is heatNi-Cr preservation-Si-B (Ni-19Cr for-7.3Si 20 min-1.5B at wt%) brazing with temperature a melting temperature of 1200 ◦C, range the well-formed of 1081 to 1 brazing136 °C. After seam heat was obtained,preservation without for 20 defects, min suchat brazing as microcracks temperature and pores. of 1200 However, °C, the CrB well and-formed NiSi2 brazingbrittle intermetallic seam was obtained, without defects, such as microcracks and pores. However, CrB and NiSi2 brittle 1 intermetallic compounds are formed at the center of the brazing seam. Song et al. (2015) [41] studied

vacuum brazing for overlap joint of TZM alloy (Ti 0.50 wt%, Zr 0.08 wt%, and C 0.04 wt%) with thickness of 3 mm. The brazing filler metal is Ti-28Ni (wt%) eutectic filler metal with melting

Metals 2020, 10, 279 12 of 16

Metals 2020, 10, x FOR PEER REVIEW 12 of 16 compounds are formed at the center of the brazing seam. Song et al. (2015) [41] studied vacuum temperaturebrazing for overlapin the range joint of TZM940 to alloy 980 °C. (Ti 0.50The wt%,range Zrof 0.08brazing wt%, temperature and C 0.04 is wt%) 1000 with–1160 thickness °C and vacuumof 3 mm. degree The brazing is ~1.33 filler MPa. metal The shear is Ti-28Ni strength (wt%) of eutecticthe brazing filler joint metal preserve with meltingd for 600 temperature s at 1080 °C in reachesthe range ~107 of 940 MPa. to 980 The◦ C. shear The range fracture of brazing shows thetemperature morphology is 1000–1160 of quasi◦-Ccleavage and vacuum transgranular degree is fracture.~1.33 MPa. The shear strength of the brazing joint preserved for 600 s at 1080 ◦C reaches ~107 MPa. The shear fracture shows the morphology of quasi-cleavage transgranular fracture. 3.6. Friction Welding 3.6. Friction Welding Fu et al. [42] studied friction welding of Mo alloy and die steel. The results demonstrate that thermalFu couplin et al. [42g] during studied the friction friction welding welding of Moprocess alloy is and conducive die steel. to Thegrain results refinement demonstrate near seam that andthermal closure coupling of pores during in TZM the frictionpowder welding alloy. According process is to conducive strict welding to grain specifications, refinement near a good seam joint and withoutclosure ofdefects pores can in TZM be obtained powder alloy.through According friction towelding. strict welding Yazdanian specifications, et al. [43] aresearched good joint friction without stirdefects welding can beof obtainedpure Mo throughplate (99.5 friction wt%) welding.with thickness Yazdanian of 1.5 et mm al. [by43 ]utilizing researched a st frictionir-welding stir weldinghead of Iridiumof pure Mo(Ir)- plateRe alloy. (99.5 The wt%) strength with thickness of butt welded of 1.5 mm joint by utilizingprepared a at stir-welding rotation speed head of of 1000 Iridium rpm (Ir)-Re and weldingalloy. The speed strength of 100 of mm/min butt welded reaches joint 86% prepared that of at BM rotation and the speed joint of is 1000 fractured rpm and in HAZ welding in a speed tensile of test.100 mmReheis/min et reachesal. (2014) 86% [44] that investigated of BM and continuous the joint is drive fractured friction in welding HAZ in aof tensile a TZM test. alloy Reheis tube with et al. an(2014) outer [44 diameter] investigated of 55 mm continuous and wall drive thickness friction of welding 7.5 mm. ofA awell TZM-formed alloy tube joint with is obta anined outer under diameter the optimizedof 55 mm andprocess wall parameters thickness of, and 7.5 mm.its tensile A well-formed strength at joint room is obtained-temperature under is theequivalent optimized to processthat of BM,parameters, whereas and its elongation its tensile strength is ~50% atlower room-temperature than that of BM. is equivalent Ambroziak to et that al. (2011) of BM, [4 whereas5] studied its continuouselongation isdrive ~50% friction lower welding than that of of a BM. refract Ambroziakory metal et bar al. with (2011) the [45 diameter] studied of continuous 30 mm under drive differentfriction weldingcombina oftions, a refractory such as Mo metal-Mo, bar TZM with-TZM, the diameter TZM-V, ofTZM 30 mm-Ta, underMo-Nb di, andfferent TZM combinations,-NB. In the wholesuch as welding Mo-Mo, process, TZM-TZM, the sample TZM-V, was TZM-Ta, immersed Mo-Nb, in IME82 and TZM-NB.oil (Figure In 17) the to wholeprevent welding the workpiece process, fromthe sample being was polluted immersed by environmental in IME82 oil (Figure gas at 17 high) to prevent temperature. the workpiece The results from beingpresent polluted that the by wellenvironmental-formed welded gas at joint high with temperature. fine grains The can results be obtained present for that each the combination well-formed under welded reasonable joint with technologicalfine grains can conditions. be obtained for each combination under reasonable technological conditions.

FigureFigure 17. 17. SSchemecheme of of friction friction welding welding in in IME82 IME82 oil oil [4 [455]]..

RecentRecently,ly, Stütz Stütz et et al. al. (2016) (2016) [4 [466]] realized realized continuous continuous drive drive friction friction welding welding of of a a pure pure Mo Mo tube tube withwith wall wall thickness thickness of of 10 10 mm mm and and outer outer diameter diameter of of 130 130 mm mm (Figure (Figure 18) 18).. In In the the study study of of Stutz Stutz et et al. al. (2018)(2018) [4 [477,,4488],], continuous continuous dynamic dynamic recrystallization recrystallization and and the the compe competingting dynamic dynamic recovery recovery were were observedobserved as as key key mechanisms; mechanisms; i intensiventensive subgrain subgrain formation formation and and the the onset onset of of recrystallization recrystallization played played thethe major major role role on on the the microstructure microstructure modification modification due due to to rotary rotary friction friction welding. welding. Grain Grain refinement refinement is is observedobserved in in the the weld weld interfa interfacece for for the the TZM, TZM, whereas whereas coarse coarse grains grains are are observed observed in in the the same same zone zone for for pure Mo but comparable crystallographic texture is observed for both materials, the result is shown in Figure 19.

Metals 2020, 10, 279 13 of 16 pure Mo but comparable crystallographic texture is observed for both materials, the result is shown in Figure 19. MetalsMetals 2020 2020, ,10 10, ,x x FOR FOR PEER PEER REVIEW REVIEW 1313 of of 16 16

FigureFigure 18. 18. Medium MediumMedium-size--sizesize friction friction welded welded MoMo Mo-tube--tubetube (OD: (OD: 150 150 mm, mm, ID: ID: 130m 130mm)130mm)m) [4 [[44666]].]. .

Figure 19. Detail of the microstructure friction welded samples: Overview of (a) Mo; overview of (b) FigureFigure 19. 19. DetailDetail of of the the microstructure microstructure friction friction welded welded samples: samples: Overview Overview of ( ofa) Mo (a); Mo; overview overview of (b of) TZM; (c) zone (1) of panel (a); (d) zone (2) of panel (a); (e) zone (3) of panel (a); (f) zone (4) of panel TZM(b) TZM;; (c) zone (c) zone (1) of (1) panel of panel (a); (d (a)); zone (d) zone(2) of (2)panel of panel (a); (e (a);) zone (e) (3) zone of (3)panel of panel(a); (f) (zonea); (f )(4) zone of panel (4) of (a); (g) zone (1) of panel (b); (h) zone (2) of panel (b); (i) zone (3) of panel (b); (j) zone (4) of panel (b). (panela); (g) (zonea); (g (1)) zone of panel (1) of (b panel); (h) zone (b); ( h(2)) zoneof panel (2) of(b); panel (i) zone (b); (3) (i) of zone panel (3) ( ofb); panel (j) zone (b); (4) (j )of zone panel (4) (b of). [48]. [4panel8]. (b). [48].

4.4. SummarySummary andand PP Prospectsrospectsrospects

4.1. Advantages and Disadvantages of Various Methods for Welding Mo and Mo Alloys 4.1.4.1. AdvantagesAdvantages andand DDisadvantagesisadvantages ofof VVariousarious MMethodsethods forfor WWeldingelding MoMo andand MoMo AAlloyslloys Mo and Mo alloys are commonly used as high-temperature structural materials under harsh MoMo andand MoMo alloysalloys areare commonlycommonly usedused asas hhighigh--temperaturetemperature structuralstructural materialsmaterials underunder harshharsh working conditions. Due to low welding temperature and uniform heating of weldment, brazing workingworking conditions.conditions. DueDue toto lowlow weldingwelding temperaturetemperature andand uniformuniform heatingheating ofof weldment,weldment, brazingbrazing incurs small deformation and is easy to ensure the accurate size of weldment. However, strength and incursincurs smallsmall deformationdeformation andand isis easyeasy toto ensureensure thethe accurateaccurate sizesize ofof weldment.weldment. However,However, strengthstrength heat resistance of brazing seam are lower than those of BM and its performance at high temperature andand heat heat resistance resistance ofof brazing brazing seam seam are are lower lower than than those those of of BM BM and and its its performance performance at at high high temperatureistemperature generally inferior is is generally generally to that inferior inferior of joints to to obtained that that of of jointsby joints fusion obtained obtained welding. by by fusion High fusion electric welding. welding. conductivity High High electric electric and yield strength at high temperature of Mo worsen its weldability in ERW. In friction welding, the conductivityconductivity andand yieldyield strengthstrength atat highhigh temperaturetemperature ofof MoMo worsenworsen itsits weldabilityweldability in in ERW.ERW. IInn frictionfriction welding,instrumentwelding, thethe is instrumentinstrument worn seriously isis wornworn and seriously anseriously irreparable andand an keyholean irreparableirreparable is formed keyholekeyhole in the isis weld formedformed seam inin whenthethe weldweld lifting seamseam the stir-welding head out from the workpiece after welding. Moreover, corrosion resistance of the weld whenwhen lifting lifting the the stir stir--weldingwelding head head out out from from the the workpiece workpiece after after welding. welding. Moreover, Moreover, corrosion corrosion resistanceseamresistance reduces ofof thethe and weldweld it is seam diseamfficult reducesreduces to clamp andand the itit isis components difficultdifficult toto clamp ofclamp the thin-walledthethe componentscomponents tube. ofof The thethe high thinthin- thermal-walledwalled conductivity, the significant tendency of grain coarsening, and grain-boundary embrittlement of Mo tube.tube. The The high high thermal thermal conductivity, conductivity, thethe significantsignificant tendency tendency of of grain grain coarsening coarsening,, andand grainalloysgrain--boundaryboundary determining embrittlementembrittlement that welding ofof usingMoMo alloysalloys a heat determindetermin sourceing withing thatthat high weldingwelding power usingusing density aa heatheat has sourcegreatsource advantages withwith highhigh powerforpower Mo densitydensity and Mo hashas alloys. greatgreat EBW advanadvan withtagestages high forfor energy MoMo andand density MoMo alloys.alloys. can be EBWEBW used with forwith welding highhigh energyenergy refractory densitydensity alloys cancan and bebe useddiusedfficult-to-weld forfor weldingwelding materials, refractoryrefractory and alloysalloys shows andand fast difficultdifficult welding--toto--weld speed,weld materialsmaterials small HAZ,,, andand and showsshows small fastfast welding weldingwelding stress speed,speed, and smallsmall HAZHAZ,, andand smallsmall weldingwelding stressstress andand deformation.deformation. TheThe vacuumvacuum environmentenvironment forfor EBWEBW cancan notnot ononlyly preventprevent thethe moltenmolten metalmetal fromfrom beingbeing pollutedpolluted byby gases,gases, suchsuch asas oxygenoxygen andand nitrogen,nitrogen, butbut alsoalso facilitatefacilitate thethe degassingdegassing andand purificationpurification ofof metalmetal inin weldweld seam.seam. Therefore,Therefore, EBWEBW isis widelywidely usedused inin weldingwelding of of Mo Mo and and Mo Mo alloys. alloys. However, However, EBW EBW has has shortcomings, shortcomings, su suchch as as complex complex process, process, low low efficiency,efficiency, limitedlimited sizesize andand shapeshape ofof weldmentweldment byby thethe vacuumvacuum chamber,chamber, and and susceptibilitysusceptibility toto thethe

Metals 2020, 10, 279 14 of 16 deformation. The vacuum environment for EBW can not only prevent the molten metal from being polluted by gases, such as oxygen and nitrogen, but also facilitate the degassing and purification of metal in weld seam. Therefore, EBW is widely used in welding of Mo and Mo alloys. However, EBW has shortcomings, such as complex process, low efficiency, limited size and shape of weldment by the vacuum chamber, and susceptibility to the interference of stray electromagnetic field and X-ray radiation produced during welding. In recent years, fiber laser and Disk laser technologies with excellent beam quality have developed rapidly, providing opportunities and necessary conditions for the breakthrough and development of welding technologies for Mo and Mo alloys. Laser welding not only has many advantages afforded by heat sources with high energy beams, such as high-power density and small heat input, but also can be carried out in an open environment. In the meanwhile, it is more difficult to protect the high temperature zone and control pore defects in weld seam during laser welding of Mo and Mo alloys compared with EBW. In addition, note that the last welding spot of the nuclear fuel rod needs to be welded under hyperbaric environment to encapsulate the hyperbaric inert gas, so the mechanism and technology of laser welding molybdenum alloy under hyperbaric environment is a blank field to be studied urgently.

4.2. Demands and Prospects In recent years, ATF fuel has gained attention in technical research and development in the international community of nuclear energy. Nuclear energy agencies in these countries, such as the United States, France, South Korea, and Japan, take ATF as their next key development direction. In 2016, Professor Stephen Zinkel, an academician of the National Academy of Engineering in the United States, pointed out that ATF fuel is profoundly influencing the development direction of science and technology of nuclear energy and will change “game rules” for nuclear safety and nuclear power industry in the world. High-performance Mo alloy is a main alternative material for the next generation of ATF cladding materials. Although it has excellent ductility, its weldability remains to be solved, so development of reliable welding technology for Mo alloy has become a pressing need. Studying problems, such as embrittlement and pore defects in welding of Mo and Mo alloys and exploring new methods and mechanisms for controlling welding quality, not only has important theoretical significance, but also shows important engineering application significance.

Author Contributions: Q.Z. and M.X. wrote the major part of the review; X.S., G.A., J.S., N.W., S.X., C.B., and J.Z. participated on the concept of review and made the corrections/suggestions for improvement. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (Grant No. 51775416). Conflicts of Interest: The authors declare no conflict of interest.

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