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Sep 28, 2018 - Schutz, R.W.; Watkins, H.B. Recent developments in titanium alloy application in the energy industry. Mater. Sci. Eng. A 1998, 243, 305–315.
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Influence of Beam Offset on Dissimilar Laser Welding of Molybdenum to Titanium Linjie Zhang *, Guangfeng Lu *, Jie Ning, Liangliang Zhang, Jian Long and Guifeng Zhang State Key Laboratory of Mechanical Behavior for Materials, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (J.N.); [email protected] (Lia.Z.); [email protected] (J.L.); [email protected] (G.Z.) * Correspondence: [email protected] (Lin.Z.); [email protected] (G.L.); Tel.: +86-29-8266-3115 (Lin.Z.) Received: 27 August 2018; Accepted: 21 September 2018; Published: 28 September 2018

 

Abstract: Dissimilar joining of molybdenum (Mo) to titanium (Ti) is of great significance to the design and fabrication of high-temperature facilities. However, few reports were found about fusion joining of these two metals. The objective of this paper is to assess the feasibility of laser beam welding (LBW) of 2 mm-thick molybdenum and titanium. The effects of laser beam offset on the laser dissimilar joint of pure molybdenum to pure titanium were analyzed in terms of microstructure, chemical composition, microhardness, and tensile behavior. The results showed that the weld appearance improved with the increase of the offset. The fusion zone was strengthened because of the solid solution of these two elements. The mechanical properties of samples increased firstly and then decreased with the increasing of offset. When the laser beam irradiated on the titanium plate and the center of the laser spot was 0.5 mm away from the Mo/Ti interface, the joint performed the highest tensile strength, which was about 70% that of titanium base metal. LBW was demonstrated to be a promising method to join dissimilar Mo/Ti joint. Keywords: dissimilar joint; laser beam welding; beam offset; pure molybdenum; pure titanium TA2

1. Introduction Molybdenum belongs to the refractory metals and shows good high-temperature mechanical performance and corrosion resistance. It is therefore widely used in fields such as aerospace, electrical industry, chemical industry, and nuclear industry [1]. Titanium, as an important structural material, has high strength, low density, and good corrosion resistance and is widely utilized in industries like aeronautics, chemical, energy, and marine engineering [2,3]. The melting point of molybdenum is about 1000 ◦ C higher than that of titanium, and its coefficient of thermal expansion is about 6 times that of titanium [4]. According to the Ti-Mo diagram [5], the molybdenum shows infinite solubility in β-titanium and limited solubility in α-titanium, and no intermetallics formed in the solid solution of these two metals. In addition, solid solution strengthening effects occurred when titanium and molybdenum mixed with each other [6]. In practice, the dissimilar welding of different metals can reduce structure weight and save production cost. In recent years, there are more and more researches on dissimilar welding of different metals [7–14]. Chen [7] studied the friction stir welding of dissimilar metals, i.e., titanium alloy and aluminum alloy. Through their study, it was found that TiAl3 phase appeared on the interface and the maximum tensile strength reached 62% that of the base material (BM) of aluminum alloy. Tomashchuk [8] investigated the electron beam welding of titanium alloy to stainless steel with an interlayer of copper foil. The results showed that when electron beams were concentrated on titanium plates, a large number of brittle phases occurred in the interface. However, when an electron beam was Materials 2018, 11, 1852; doi:10.3390/ma11101852

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focused on the steel plate, brittle intermetallic compounds were inhibited and the specimens could reach maximum mechanical performance. Ambroziak [4] researched the feasibility of friction welding of molybdenum with other metals, like vanadium, titanium, and tantalum. The results demonstrated that by using friction welding, the dissimilar joints of molybdenum to other metals like vanadium, titanium, and tantalum were successfully achieved and intermetallic phases were not found in welding seam zone. Chang [9] brazed molybdenum with Ti-6Al-4V by using Ti-15Cu-15Ni as the brazing filler metal. A microstructure test indicated that the brazing zone mainly consisted of titanium-rich phases and most brazed specimens fractured in the molybdenum matrix. Du et al. [13] successfully welded the dissimilar joint of 2205DSS to Q235 with laser beam welding. It is found that the welding seam consisted of a martensitic phase and a small amount of residual austenite. However, the HAZ of 2205DSS was mainly comprised of ferrite, and HAZ of Q235 side consisted of a coarse-grained zone and a fine-grained zone. Although there are a lot of studies on welding of dissimilar metals in recent years, the studies on dissimilar welding of molybdenum and titanium are scarcely carried out. The technical complexity of solid-phase welding and poor fatigue performances of brazing seams limit the applications of these two welding methods in industrial production [15]. And the application of the most widely used welding method in industry, namely the fusion welding, into dissimilar joining Mo/Ti structures, is rarely reported. Laser welding, as a high-energy beam welding method, has numerous advantages, like low heat input and high energy density [16–22], so it is very suitable for welding refractory metals. Furthermore, there are more and more investigations on laser welding of dissimilar metals in recent years [23–26]. Zhou [23] studied the dissimilar laser welding of molybdenum and tantalum and found that the generation of cracks on tantalum/molybdenum joints was mainly attributed to the poor weldability of molybdenum. Sun [24] investigated the laser welding of AA6013 aluminum alloys and Q235 low-carbon steels by using ER4043 welding wires. Based on the study, it was found that the thickness of Fe-Al intermetallic compound layers changed with different welding parameters and the specimen fracture occurred in the brazing interface. Song [25] studied the influences of laser offset on Ti6Al4V/A6061 dissimilar welding. The results showed that with the increase of offset, the thickness of intermetallic compound layers decreased and the mechanical performances of the specimens increased. Casalino et al. [26] successfully welded AA5754 and T40 with Yb-YAG laser welding and found that the laser offset significantly affected the ultimate tensile strength of the joints. This study explored the feasibility of laser beam welding of Mo/Ti dissimilar joints with emphasis on the role of laser beam offsets. The macromorphology and micromorphology of welding seams were observed. By using the energy dispersive X-ray spectrometer (EDX), element distribution in the welding seam zone was tested. The micro Vickers hardness tester and the universal tensile testing machine were used to test microhardness and mechanical performance, respectively. Moreover, the fracture morphology of the specimens was observed by using the scanning electron microscope (SEM). 2. Experiment Pure molybdenum and pure titanium TA2, used in the experiment and the microstructures of the BMs, are shown in Figure 1. The BM of molybdenum was comprised of rolled grains, while the BM of titanium consisted of equiaxed crystals. Five welded joints with different offsets were archived and compared. The welding parameters are shown in Table 1. Laser beam offset indicated the distance between laser spot center on the plates and the Mo/Ti interface. When the center of the laser spot was irradiated on the molybdenum, the offset was recorded as a negative value, while on the titanium plate, the offset was recorded as a positive number. A different offset means different energy distribution on the two plates. In the welding process, molybdenum and titanium plates were preheated to 450 ◦ C and maintained for several seconds with a heating device before welding in argon atmosphere, as shown in Figure 2. After the welding, the heating was stopped and specimens were cooled in the shielding gas. Figure 3 displayed the setting of equipment used in welding test. A sliding table with a regulation precision of 0.01 mm was utilized to change the distance from the laser spot center to Mo/Ti interface

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Moreover, argon was used as a shielding gasspecimens in the welding process. Five different offsets, and the heating device was placed under the for preheating. Moreover, argon was i.e., used−0.25 as a mm, 0 mm, +0.25 mm, +0.5 mm, and +0.75 mm were employed, as shown in Figure 2b. Moreover, argon waswelding used as process. a shielding in the welding process. offsets, −0.25 shielding gas in the Fivegas different offsets, i.e., −0.25Five mm,different 0 mm, +0.25 mm,i.e., +0.5 mm, mm, mm, mm +0.25were mm,employed, +0.5 mm, and +0.75 mm were 2b. employed, as shown in Figure 2b. and0+0.75 as shown in Figure

Figure 1. Cross-sectional microstructure of the BMs of (a) Molybdenum and (b) Titanium. Figure Figure1.1.Cross-sectional Cross-sectionalmicrostructure microstructureofofthe theBMs BMsofof(a) (a)Molybdenum Molybdenumand and(b) (b)Titanium. Titanium. Table 1. Parameters used in laser beam offset welding of dissimilar Mo/Ti joint. Table 1. Parameters used in laser beam offset welding of dissimilar Mo/Ti joint. Table 1. Parameters used in laser beam offset welding of dissimilar Mo/Ti joint.

Power Welding Speed Power Welding Welding (W) (m/min) Power SpeedSpeed Specimen Specimen (W) (m/min) 1 4000 1.5 (W) (m/min) 1 4000 4000 1.5 1.5 12 4000 1.5 2 4000 1.5 4000 1.5 23 1.5 3 4000 4000 1.5 4000 4000 1.5 1.5 34 1.5 4 4000 4000 4000 1.5 1.5 5 4000 45 1.5 5 4000 1.5 Specimen

Defocusing Amount Laser Offset Defocusing Offset (mm) (mm) Defocusing Amount LaserLaser Offset Amount (mm) (mm) 4 −0.25 (mm) (mm) 4 4 −0.25 0 4 −0.25 4 0 4 +0.25 44 +0.25 0 +0.5 44 4 +0.5 +0.25 +0.75 44 4 +0.75 +0.5 4 +0.75

2. The schematic illustration of (a) laser beam offset welding and (b) spot size and and position at Figure 2. different offsets. different offsets. Figure 2. The schematic illustration of (a) laser beam offset welding and (b) spot size and position at different offsets.

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Figure 3. Experimental set-up for argon shielding and preheating.

The cross-sections of the welded specimens were etched with molybdenum corrodent (i.e., Nitric acid: Sulfuric acid: Water Water == 5:2:3, 5:2:3, by by volume, volume, JHD, JHD, Guangdong, Guangdong, China) and titanium corrodent (Kroll reagent, JHD, Guangdong, China), successively. The The surface surface morphology morphology of welding seams and micromorphology of cross-sections were observed observed by using the stereomicroscope stereomicroscope (SAIKEDIGITAL, (SAIKEDIGITAL, Shenzhen, China) thethe metallurgical microscopes (Nikon(Nikon MA200,MA200, Nikon, Tokyo, separately. China)and and metallurgical microscopes Nikon,Japan) Tokyo, Japan) The element distribution in the FZ was tested by utilizing the EDX (JEOL, Tokyo, Japan). Moreover, separately. The element distribution in the FZ was tested by utilizing the EDX (JEOL, Tokyo, Japan). the micro Vickers hardness tester was employed to achieve the distribution microhardness Moreover, the micro Vickers hardness tester was employed to achieve ofthe distribution on of cross-sections The of loading force The and loading durationforce timeand were 200 gf and s, respectively. microhardnessofonspecimens. cross-sections specimens. duration time15 were 200 gf and The test wasThe carried outtest by was using the universal tensilethe testing machine (CSS-88100, SINOMACH, 15 s,tensile respectively. tensile carried out by using universal tensile testing machine (CSSBeijing, China) at a Beijing, constantChina) drawing speed ofdrawing 1 mm/min. the tensile tests, the fracture 88100, SINOMACH, at a constant speedAfter of 1 mm/min. After the tensile tests, morphology was observed through the SEM (JEOL, Tokyo, Japan). the fracture morphology was observed through the SEM (JEOL, Tokyo, Japan). 3. Results and and Discussion Discussion 3. Results 3.1. Surface Morphologies of Dissimilar Mo/Ti Joints 3.1. Surface Morphologies of Dissimilar Mo/Ti Joints Figure 4 shows the top surface morphology and the bottom surface morphology of welding seams Figure 4 shows the top surface morphology and the bottom surface morphology of welding achieved under different laser offsets. As demonstrated in this image, laser offset greatly affected seams achieved under different laser offsets. As demonstrated in this image, laser offset greatly the formation of welding seams. When offsets were −0.25 mm and 0 mm, severe transverse cracks affected the formation of welding seams. When offsets were −0.25 mm and 0 mm, severe transverse were found on both top and bottom surfaces of welding seams and the surface of welding seams was cracks were found on both top and bottom surfaces of welding seams and the surface of welding rough and no welding ripples occurred. When the offset was +0.25 mm, there were no obvious cracks seams was rough and no welding ripples occurred. When the offset was +0.25 mm, there were no on both surfaces of welding seams, but welding ripples were also not obvious. When laser offsets obvious cracks on both surfaces of welding seams, but welding ripples were also not obvious. When were +0.5 mm and +0.75 mm, a favorable morphology of weld surfaces was formed, showing obvious laser offsets were +0.5 mm and +0.75 mm, a favorable morphology of weld surfaces was formed, welding ripples and metallic luster. It can be concluded that with gradually increased laser offset, the showing obvious welding ripples and metallic luster. It can be concluded that with gradually formation of welding seams was improved substantially. increased laser offset, the formation of welding seams was improved substantially.

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Figure (a,c,e,g,i), the Figure 4. 4. Effects Effects of of laser laser beam beam offset offset on on the thesurface surface morphologies morphologies of of Mo/Ti Mo/Ti joints. joints. (a,c,e,g,i), the top top surface morphologies of welded joints under the offsets of − 0.25 mm, 0 mm, +0.25 mm, +0.5 mm, surface morphologies of welded joints under the offsets of −0.25 mm, 0 mm, +0.25 mm, +0.5 mm, +0.75 +0.75 mm respectively; (b,d,f,h,j), the bottom surface morphologies of welded joints under offsets mm respectively; (b,d,f,h,j), the bottom surface morphologies of welded joints under thethe offsets of of − 0.25 mm, 0 mm, +0.25 mm, +0.5 mm, +0.75 mm respectively. −0.25 mm, 0 mm, +0.25 mm, +0.5 mm, +0.75 mm respectively.

3.2. Microstructure and Element Distribution 3.2. Microstructure and Element Distribution Figure 5 shows the cross-section morphologies of Mo/Ti joints. As displayed in Figure 5a,b, Figure 5 shows the cross-section morphologies of Mo/Ti joints. As displayed in Figure 5a,b, when laser offsets were −0.25 mm and 0 mm, pores appeared in the FZ. When offset was −0.25 mm, when laser offsets were −0.25 mm and 0 mm, pores appeared in the FZ. When offset was −0.25 mm, plenty of pores appeared and were dispersedly distributed in the whole FZ. While offset was 0 mm, plenty of pores appeared and were dispersedly distributed in the whole FZ. While offset was 0 mm, a few pores concentrated around Mo/FZ interface. It is mainly related to the fact that the amount a few pores concentrated around Mo/FZ interface. It is mainly related to the fact that the amount of of molybdenum melted in the molten pool scaled with the beam offset. Because of the pre-existing molybdenum melted in the molten pool scaled with the beam offset. Because of the pre-existing porosity, contamination, and inclusions of molybdenum plate associated to the powder metallurgy porosity, contamination, and inclusions of molybdenum plate associated to the powder metallurgy process, a higher proportion of molybdenum in the molten pool would result in more porosity defects process, a higher proportion of molybdenum in the molten pool would result in more porosity defects in the FZ of Mo/Ti joints. Therefore, when laser offsets increased from −0.25 mm to +0.75 mm with in the FZ of Mo/Ti joints. Therefore, when laser offsets increased from −0.25 mm to +0.75 mm with an an increment value of 0.25 mm, the number of pores decreased monotonically. It can be seen from increment value of 0.25 mm, the number of pores decreased monotonically. It can be seen from Figure Figure 5 that the Mo/FZ interface was almost straight, while the FZ/Ti interface was highly curved. 5 that the Mo/FZ interface was almost straight, while the FZ/Ti interface was highly curved. Obviously, the straight Mo/FZ interface could be explained by the high melting point and high thermal Obviously, the straight Mo/FZ interface could be explained by the high melting point and high conductivity of molybdenum. In the welding process, the liquid metal in the molten pool overflowed thermal conductivity of molybdenum. In the welding process, the liquid metal in the molten pool onto the surface of molybdenum plates and solidified there, as shown in Figure 5. Such a phenomenon overflowed onto the surface of molybdenum plates and solidified there, as shown in Figure 5. Such gradually weakened with the increase of laser offset. Figure 6a,b displays the HAZs in molybdenum a phenomenon gradually weakened with the increase of laser offset. Figure 6a,b displays the HAZs plate and titanium plate, respectively. In the HAZ of molybdenum, recrystallization occurred and in molybdenum plate and titanium plate, respectively. In the HAZ of molybdenum, recrystallization equiaxed grains were observed, while martensitic structure was found in the HAZ of titanium. occurred and equiaxed grains were observed, while martensitic structure was found in the HAZ of titanium.

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Figure 5. Effects of laser beam offset on the cross-sectional morphologies of Mo/Ti joints. (a–e), the Figure onon thethe cross-sectional morphologies of Mo/Ti joints. (a–e),(a–e), the Figure 5. 5. Effects Effectsofoflaser laserbeam beamoffset offset cross-sectional morphologies of Mo/Ti joints. cross-sections of welded joints under the offsets of −0.25 mm, 0 mm, +0.25 mm, +0.5 mm, +0.75 mm cross-sections of welded joints joints underunder the offsets of −0.25ofmm, 0 mm, mm,+0.25 +0.5 mm, the cross-sections of welded the offsets −0.25 mm,+0.25 0 mm, mm, +0.75 +0.5 mm mm, respectively. respectively. +0.75 mm respectively.

Figure6. 6. Microstructure Microstructure of of the the HAZs HAZs (a) (a) at at molybdenum molybdenum side side and and (b) (b) at at titanium titanium side. side. Figure Figure 6. Microstructure of the HAZs (a) at molybdenum side and (b) at titanium side.

Figure 7 shows map scanning results of the element distribution on the cross-sections of Mo/Ti Figure 7 shows map scanning results of the element distribution on the cross-sections of Mo/Ti Figure 7 showsthat mapboth scanning results of the elementdistributed distribution of Mo/Ti joints. It is obvious Mo and Ti were uniformly inon thethe FZscross-sections after laser beam offset joints. It is obvious that both Mo and Ti were uniformly distributed in the FZs after laser beam offset joints. It is obvious that both Mo and Ti were uniformly distributed in the FZs after laser beam welding. Figure 8 shows the line scanning paths and corresponding elements distribution offset in the welding. Figure 8 shows the line scanning paths and corresponding elements distribution in the welding. Figure shows joints. the line scanning paths elements distribution in the HAZs and FZs of8 Mo/Ti It can be seen fromand the corresponding figure that for all specimens, the contents of HAZs and FZs of Mo/Ti joints. It can be seen from the figure that for all specimens, the contents of HAZs and FZs of Mo/Ti joints. It can be seen from the figure that for all specimens, the contents of molybdenum and titanium changed gently in the FZ of welding seams, while the contents changed molybdenum and titanium changed gently in the FZ of welding seams, while the contents changed molybdenum titanium changedOwing gentlytoinathe FZ of welding seams, while the contents changed greatly at the and FZ/HAZ interfaces. higher content of molybdenum in the FZ, when laser greatly at the FZ/HAZ interfaces. Owing to a higher content of molybdenum in the FZ, when laser greatly at the FZ/HAZ interfaces. a higherchanged content gradually of molybdenum theMo/FZ FZ, when laser offset was −0.25 mm, the content Owing of bothto elements aroundinthe interface. offset was −0.25 mm, the content of both elements changed gradually around the Mo/FZ interface. offset was −0.25 thefour content of both the elements changed thedrastically Mo/FZ interface. However, for themm, other specimens, contents of bothgradually elementsaround changed around However, for the other four specimens, the contents of both elements changed drastically around the However, forinterface. the other four specimens, thedetected contentsin of the bothHAZ elements changed drastically around the the Mo/FZ No titanium was of molybdenum. Furthermore, the Mo/FZ interface. No titanium was detected in the HAZ of molybdenum. Furthermore, the contents Mo/FZ interface. No titanium was detected in the HAZ of molybdenum. Furthermore, the contents contents of both elements around the interface between titanium and the FZ changed gently. Figure 9 of both elements around the interface between titanium and the FZ changed gently. Figure 9 of both elements around the between titanium and the FZ changed gently. Figure 9 quantitatively demonstrates theinterface change trend of molybdenum content in the FZ. With the increasing quantitatively demonstrates the change trend of molybdenum content in the FZ. With the increasing quantitatively change of molybdenum content the FZ. With increasing of offset from demonstrates −0.25 to +0.75the mm by antrend increment value of 0.25 mm, in the contents ofthe molybdenum of offset from −0.25 mm to +0.75 mm by an increment value of 0.25 mm, the contents of molybdenum of offset from −0.25 mm to +0.75 mm by an increment value of 0.25 mm, the contents of molybdenum in the FZs in terms of atomic percent (i.e., Mo ) were ranked as: Moat > 50%, 30% > Moat > 20%, in the FZs in terms of atomic percent (i.e., Moat) at were ranked as: Moat > 50%, 30% > Moat > 20%, 20% > in the>FZs in >terms atomic percent (i.e.,and Mo1.5% at) were ranked as: Moat > 50%, 30% > Moat > 20%, 20% > 20% Moat 15%,of 3.5% > Mo > 1.5%, > Mo > 0.5%. Moat > 15%, 3.5% > Moat > 1.5%,atand 1.5% > Moat > 0.5%.at Moat > 15%, 3.5% > Moat > 1.5%, and 1.5% > Moat > 0.5%.

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Figure 7. Map Map scanning results of elementdistribution distribution for for Mo/Ti joints obtained at various various laserlaser beambeam Figure 7. scanning results element distribution joints obtained at laser beam Figure 7. Map scanning results ofof element forMo/Ti Mo/Ti joints obtained at various offsets. (a,d,g,j,m), the SEM imageofof ofscanning scanning area area of of joints joints −0.25 mm, mm, +0.25 mm,mm, +0.5 mm, mm, mm, offsets. (a,d,g,j,m), SEM image scanning area mm, 00 mm, +0.25 mm, +0.5 offsets. (a,d,g,j,m), thethe SEM image of joints−0.25 −0.25 mm, 0 mm, +0.25 +0.5 +0.75 mm respectively; (b,e,h,k,n), Mo distribution of joints −0.25 mm, 0 mm, +0.25 mm, +0.5 mm, mm, +0.75 mm respectively; (b,e,h,k,n), Mo distribution of joints −0.25 mm, 0 mm, +0.25 mm, +0.5 mm, +0.75 mm respectively; (b,e,h,k,n), Mo distribution of joints −0.25 mm, 0 mm, +0.25 mm, +0.5 +0.75 mm mm respectively; respectively; (c,f,i,l,o), (c,f,i,l,o), Ti Ti distribution distribution of of joints joints −0.25 −0.25 mm, mm, 00 mm, mm, +0.25 +0.25 mm, mm, +0.5 +0.5 mm, mm, +0.75 +0.75 +0.75 +0.75 mm respectively; (c,f,i,l,o), Ti distribution of joints −0.25 mm, 0 mm, +0.25 mm, +0.5 mm, mm respectively. respectively. mm +0.75 mm respectively.

Figure 8. Line Line scanning results of elementdistribution distribution for for Mo/Ti joints obtained at various various laserlaser beambeam Figure 8. scanning results element distribution joints obtained at laser beam Figure 8. Line scanning results ofof element forMo/Ti Mo/Ti joints obtained at various offsets. (a) the line scanning result of element distribution of joint −0.25 mm; (b) the line scanning offsets. (a) the line scanning result of element distribution of joint −0.25 mm; (b) the line scanning offsets. (a) the line scanning result of element distribution of joint −0.25 mm; (b) the line scanning result of of element element distribution distribution of joint 00 mm; mm; (c) (c) the the line line scanning result of of element element distribution distribution of of joint joint joint resultresult of element distribution of of joint 0 mm; (c) the linescanning scanningresult result of element distribution of joint +0.25 mm; (d) the line scanning result of element distribution of joint +0.5 mm; (e) the line scanning result of element distribution of joint +0.75 mm.

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+0.25 mm; (d) the line scanning result of element distribution of joint +0.5 mm; (e) the line scanning 8 of 15 result of element distribution of joint +0.75 mm.

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Figure molybdenum Mo/Ti Figure9.9.EDX EDXresults resultsofofthe thecontent contentofof molybdenuminin Mo/Tijoints jointsobtained obtainedatatvarious variouslaser laserbeam beam offsets (the numbers (1–10) in the blue boxes in this figure represent the positions of point composition offsets (the numbers (1–10) in the blue boxes in this figure represent the positions of point composition analysis). analysis).(a) (a)point pointcomposition compositionanalysis analysisresult resultofofwelded weldedjoint joint−0.25 −0.25mm; mm;(b) (b)point pointcomposition composition analysis analysisresult resultofofwelded weldedjoint joint0 0mm; mm;(c)(c)point pointcomposition compositionanalysis analysisresult resultofofwelded weldedjoint joint+0.25 +0.25mm; mm; (d) composition analysis result of of welded joint +0.5+0.5 mm; (e) (e) point composition analysis result of (d)point point composition analysis result welded joint mm; point composition analysis result welded joint +0.75 mm; (f) the comparison of point composition analysis results of five welded joints. of welded joint +0.75 mm; (f) the comparison of point composition analysis results of five welded

joints.

3.3. Microhardness

3.3. Figure Microhardness 10 displays the microhardness distribution along the center line of the thickness direction. Except for the specimens with the offset being +0.75 mm, the FZs of other specimens showed Figure 10 displays the microhardness distribution along the center line of the thickness direction. higher microhardness than the BM of molybdenum, which mainly benefited from solid solution Except for the specimens with the offset being +0.75 mm, the FZs of other specimens showed higher strengthening effects [6]. When the offset was −0.25 mm, the microhardness of FZ approaching to the microhardness than the BM of molybdenum, which mainly benefited from solid solution interface between titanium and FZ reduced largely, which was induced by the decrease of content of strengthening effects [6]. When the offset was −0.25 mm, the microhardness of FZ approaching to the molybdenum elements at the position (Figure 8a). The microhardness of the HAZ of titanium increased interface between titanium and FZ reduced largely, which was induced by the decrease of content of due to the martensitic phase transformation, but owing to the recrystallization, the microhardness of molybdenum elements at the position (Figure 8a). The microhardness of the HAZ of titanium the HAZ of molybdenum declined in comparison with the molybdenum BM. increased due to the martensitic phase transformation, but owing to the recrystallization, the microhardness of the HAZ of molybdenum declined in comparison with the molybdenum BM.

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FigureFigure 10. Effects of oflaser offsetonon microhardness distribution the cross-sections 10. Effects laser beam beam offset microhardness distribution on the on cross-sections of Mo/Ti of Mo/Tijoints. joints.

3.4. Tensile Strength and Fracture Observation 3.4. Tensile Strength and Fracture Observation The shape and sizes of tensile specimens and testsare areshown shown Figure 11. Three The shape and sizes of tensile specimens andresults resultsof of tensile tensile tests in in Figure 11. Three specimens werewere tested under each offset and thetensile tensile results of one group specimens tested under each offset andFigure Figure11b 11b displays displays the results of one group of of specimens. The maximum tensile strength the specimens wasMPa, 350 MPa, reaching the specimens. The maximum tensile strength of theofspecimens was 350 reaching 70% 70% of theofstrength of the BM of titanium. shows the offset, the tensile strength of the strength BM of titanium. Figure 11c Figure shows11c that withthat thewith risethe of rise the of offset, the tensile strength of the of the specimens first increased and then decreased. When offset was +0.5 mm, the specimens showed the specimens first increased and then decreased. When offset was +0.5 mm, the specimens showed largest tensile strength of about 350 MPa. Table 2 shows the yield strength, ultimate strength, and largestthe tensile strength of about 350 MPa. Table 2 shows the yield strength, ultimate strength, and elongation rate of a group of welded joints and BMs. Because of the dimensions of tensile specimens, elongation rate of a group of welded joints and BMs. Because of the dimensions of tensile specimens, working easily occurred for the titanium base metal of a welded joint during a tensile test, which working easily occurred for the titanium base metal of a welded joint during a tensile test, which caused a higher yield strength and lower elongation rate of joint +0.5 mm than the titanium base causedmetal. a higher yield strength loweranelongation rateelongation of joint +0.5 mm than the titanium base metal. All the welded jointsand showed unmeasurable rate. 2018, 11, x FOR PEER REVIEW 10 of 15 All theMaterials welded joints showed an unmeasurable elongation rate.

Figure 11. Cont.

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Figure 11. The shape and sizes of tensile specimens (a) and results of tensile tests: (b) tensile test results Figurejoints 11. The and sizes of tensile specimens (a) and results of tensile tests: joint (b) tensile of Mo/Ti andshape the BM of titanium, (c) variation of average tensile strength of Mo/Ti againsttest laser beamofoffset. results Mo/Ti joints and the BM of titanium, (c) variation of average tensile strength of Mo/Ti joint against laser beam offset. Table 2. Tensile result of a group of welded joints and BMs. Specimen

Table 2 Tensile result of a group of welded joints and BMs. Elongation (%) Yield Stress (Mpa) Ultimate Stress (Mpa)

−0.25 mm Specimen Yield Stress (Mpa) Ultimate59.58 Stress (Mpa) Elongation (%) 0 mm 112.59 156.48 −0.25 mm 59.58 +0.25 mm 136.17 189.33 0+0.5 mmmm 112.59 156.48 308.84 346.58 - +0.25 mm 136.17 189.33 +0.75 mm 50.22 - 260.66 500.40 15 +0.5Ti-BM mm 308.84 346.58 Mo-BM 780.64 4.5 +0.75 mm - 625.55 50.22 Ti-BM 260.66 500.40 15 Mo-BM 625.55 4.5 Figure 12 shows the tensile fracture path of the welded 780.64 specimens. When offset was − 0.25 mm, due to the presence of defects, such as pores and cracks, the specimen fractured in the FZ, as indicated Figure 12 shows the tensile12. fracture ofwas the welded specimens. offset −0.25 mm, by the A-A cross-section in Figure Whenpath offset 0 mm, +0.25 mm orWhen +0.5 mm, thewas specimens due to the presence such as poresAs and cracks,ofthe specimen fractured in the FZ, as indicated fractured in the HAZsofofdefects, the molybdenum. a result the decreasing of its microhardness, the HAZ of molybdenum became the weak area of the joint. When the offset was +0.75 mm, the specimen fractured in the interface between the FZ and the molybdenum plate due to lack of fusion. Figures 13–17 display SEM images of tensile fractures of Mo/Ti joints. When offset was −0.25 mm, both pores and cracks running through the thickness direction of the specimens were found on the fracture surface, as can be seen in Figure 13a. The specimen fracture was mainly comprised of cleavage steps. When the offset was 0 mm, micro-cracks were found in the fracture and the fracture mainly presented the characteristics of intergranular fractures, as demonstrated in Figure 14. When the offsets were +0.25 mm or +0.5 mm, the fractures also mainly presented the characteristics of intergranular fractures, as shown in Figures 15 and 16. When offset was +0.75 mm, as shown in Figure 17, the middle part in the thickness direction of the fracture was planar, and its content of titanium was up to about 99.27%, which was similar to the content of the FZ. It is believed that the lack of fusion occurred at the interface of Mo/FZ when the offset was +0.75 mm.

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by the A-A cross-section in Figure 12. When offset was 0 mm, +0.25 mm or +0.5 mm, the specimens fractured in the HAZs of the molybdenum. As a result of the decreasing of its microhardness, the HAZ of molybdenum became the weak area of the joint. When the offset was +0.75 mm, the specimen Materials 2018, 11, 1852 11 of 15 fractured in the interface between the FZ and the molybdenum plate due to lack of fusion.

Figure12. 12.Fracture Fracturepaths pathsof ofMo/Ti Mo/Tijoints jointsproduced producedunder undervarious variouslaser laserbeam beamoffsets. offsets.(a,d,g,j,m) (a,d,g,j,m)top top Figure surfacemorphologies morphologiesofofbroken brokenjoints; joints;(b,e,h,k,n) (b,e,h,k,n)cross-sectional cross-sectionalmorphologies morphologiesofofbroken brokenjoints; joints; surface (c,f,i,l,o)magnified magnifiedregions regionsin inthe thered redboxes boxesin inpanel panelb, b,e, e,h, h,k, k,nnrespectively. respectively. (c,f,i,l,o)

Figures 13–17 display SEM images of tensile fractures of Mo/Ti joints. When offset was −0.25 mm, both pores and cracks running through the thickness direction of the specimens were found on the fracture surface, as can be seen in Figure 13a. The specimen fracture was mainly comprised of cleavage steps. When the offset was 0 mm, micro-cracks were found in the fracture and the fracture mainly presented the characteristics of intergranular fractures, as demonstrated in Figure 14. When the offsets were +0.25 mm or +0.5 mm, the fractures also mainly presented the characteristics of intergranular fractures, as shown in Figure 15 and 16. When offset was +0.75 mm, as shown in Figure 17, the middle part in the thickness direction of the fracture was planar, and its content of titanium was up to about 99.27%, which was similar to the content of the FZ. It is believed that the lack of fusion occurred at the interface of Mo/FZ when the offset was +0.75 mm.

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Figure 13. SEM images of tensile fracture when the offset is −0.25 mm ((a) Overall morphology; (b,c) Figure 13. SEM images of tensile when the offset is −0.25 mm ((a) Overall morphology; (b,c) Magnified images of region B andfracture C in panel (a)). 13.images SEMimages images when offset is −0.25 ((a) Overall morphology; Figure 13. SEM ofoftensile when offset is −0.25 mm mm ((a) Overall morphology; (b,c) Magnified of region Btensile andfracture Cfracture in panel (a)).thethe (b,c) Magnified images of region and C in panel Magnified images of region B andB C in panel (a)). (a)).

Figure 14. SEM images of tensile fracture when the offset is 0 mm ((a) Overall morphology; (b) Figure SEM images of of tensile fracture the the offset is 0ismm ((a) ((a) Overall morphology; (b) Figure 14. 14.images SEM images tensile fracture when offset 0 mm Overall morphology; Magnified of region B in panel (a)). when Figure 14. SEM images of tensile fracture when the offset is 0 mm ((a) Overall morphology; (b) Magnified images of region B inBpanel (a)).(a)). (b) Magnified images of region in panel Magnified images of region B in panel (a)).

Figure of of tensile fracture when thethe offset is 0.25 mmmm ((a) ((a) Overall morphology; (b) Figure 15. 15.SEM SEMimages images tensile fracture when offset is 0.25 Overall morphology; Figure 15. images SEMimages images tensile fracture when the offset is 0.25 mm ((a) Overall morphology; (b) Magnified of region B inBpanel (a)).(a)). (b) Magnified ofof region in panel Figure 15. images SEM images of tensile fracture Magnified of region B in panel (a)). when the offset is 0.25 mm ((a) Overall morphology; (b) Magnified images of region B in panel (a)).

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Figure 16. SEM images of tensile fracture when the offset is 0.5 mm ((a) Overall morphology; (b) Figure 16. images SEM ofoftensile fracture thethe offset is 0.5 mmmm ((a)((a) Overall morphology; (b) SEMimages images fracture when offset is 0.5 Overall morphology; Magnified of region B tensile in panel (a)). when Magnified images of region B inBpanel (a)).(a)). (b) Magnified images of region in panel

Figure images of of tensile fracture when the the offset is +0.75mm (a) overall morphology; (b,c) Figure 17. 17.SEM SEM images tensile fracture when offset is +0.75mm (a) overall morphology; Figure 17. images SEM images images ofregion tensile fracture (b,c) Magnified of and C inwhen panel (a).offset is +0.75mm (a) overall morphology; (b,c) Magnified of region B andBC in panel (a). the Magnified images of region B and C in panel (a).

In conclusion, performance of the joints was related the poor In conclusion,the themechanical mechanical performance ofMo/Ti the Mo/Ti joints was torelated toweldability the poor In conclusion, the mechanical performance of the Mo/Ti joints was related to was the small poor of molybdenum and wetting of liquidofTiliquid on solid offsetoffset was small (i.e., weldability of molybdenum andphenomena wetting phenomena Ti onMo. solidWhen Mo. When weldability of molybdenum wetting phenomena of seams liquid Ti on When offset was When small −0.25 mmmm and 0 mm), the pores and cracks in weld seams reduced thesolid mechanical performance. (i.e., −0.25 and 0 mm), theand pores and cracks in weld reduced theMo. mechanical performance. (i.e., −0.25 mm and+0.25 0 mm), the and cracks in weld seams reduced the performance. offsets were +0.25 mm or +0.5 mm,mm, thethe formation of of pores and ininmechanical the specimens When offsets were mm orpores +0.5 formation pores andcracks cracks theFZ FZ of of the the specimens When offsets were +0.25 mm or +0.5 mm, the formation of pores and cracks in the FZ of the was obviously obviously inhibited. inhibited. Besides Besidesat atthe theinterface interfacebetween betweenMo Moand andFZ, FZ, the the liquid liquid titanium titaniumspecimens and solid solid was and was obviously inhibited. Besides at the interface between Mo and FZ, the liquid titanium and solid molybdenum showed relatively better wettability, which can be proved by the fracture positon of molybdenum showed relatively better wettability, which can be proved by the fracture positon of molybdenum showed relatively better wettability, which can be proved by the fracture positon of these two welded of of specimens increased. In addition, when offset was these welded joints. joints. As Asaaresult, result,the thestrength strength specimens increased. In addition, when offset these two welded joints. As a result, the strength of specimens increased. In addition, when offset +0.25+0.25 mm, mm, the heat was much closer closer to the molybdenum plate, greatly influencing the HAZthe of was the source heat source was much to the molybdenum plate, greatly influencing was +0.25 mm, the heat source was much closer to the molybdenum plate, greatly influencing the molybdenum. That might result in the lower mechanical performance of specimens produced under HAZ of molybdenum. That might result in the lower mechanical performance of specimens produced HAZ of molybdenum. might result in the lower mechanical specimens produced the offset of +0.25 inmm comparison with those under the offset of +0.5ofmm. when offset under the offset of mm +0.25That in comparison with those under theperformance offset +0.5ofHowever, mm. However, when under the offset of +0.75 +0.25 mm with those under theand offset +0.5 mm. However, when was too large (i.e., mm),in atcomparison theatinterface between Mo and FZ, the molybdenum plate was not offset was too large (i.e., +0.75 mm), the interface between Mo FZ,of the molybdenum plate was offset was too large (i.e., +0.75 mm), at the interface between Mo and FZ, the molybdenum plate was completely wetted and the lack of fusion occurred, which caused the decrease of strength of welded not completely wetted and the lack of fusion occurred, which caused the decrease of strength of not completely wetted and theaddition, lack ofInfusion occurred, which caused ofand strength of joints under laser this offset. In around the around interface between Mothe anddecrease FZ, the welded jointsthis under laser offset. addition, the interface between Momicrostructure FZ, the welded joints under thisquietly laser offset. In quietly addition, around the itlow interface between Mo FZ, the and microhardness different and it caused the and very ductility of welded joints. microstructure and were microhardness were different caused the very low and ductility of microstructure and microhardness were quietly different and it caused the very low ductility of welded joints. welded joints.

4. Conclusions 4. Conclusions

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4. Conclusions In this work, the influence of laser offset on the weld appearances, alloy element distribution, microhardness, and mechanical properties of laser welded Mo/Ti joints were studied. The main conclusions can be summarized as follows: (1) Poor weldability of the molybdenum plate, produced by powder metallurgy, greatly affected the mechanical performance of the Mo/Ti joints. Beam offset had a significant influence on the amount of melted molybdenum and therefore plays a crucial role in laser welding of Mo/Ti joints. (2) When a laser was illuminated on the molybdenum plates, many pores, accompanied with macro-cracks, appeared in the FZ of welded specimens. When laser spots moved towards the titanium plates, macro-cracks disappeared and the formation of pores in the FZs of welded specimens was inhibited. (3) When laser spots moved from molybdenum plates to titanium plates, the tensile strength of the specimens first increased and then decreased. When offset was +0.5 mm, the specimens showed the maximum tensile strength of about 350 MPa, which was about 70% that of the BM of titanium plate. The HAZ of molybdenum and the interface between molybdenum plate and FZ were the weakest region of Mo/Ti joint. At last, although it has been demonstrated that the laser beam welding method has the potential to achieve sound Mo/Ti joints, future work to improve the mechanical performance of the joints and the investigation about how the offset affect the wetting behavior of liquid titanium on solid molybdenum is still needed. On the one hand, since that the weakest region of the Mo/Ti joint, which showed the maximum tensile strength of about 350 MPa, was the HAZ of molybdenum, strengthening the HAZ of molybdenum might be an effective way to further improve the strength of laser welded Mo/Ti joint. On the other hand, the mechanism concerning how the offset affects the wetting behavior helps in better understanding the correlation between the laser offset and the strength. Author Contributions: Conceived and designed the experiments, L.Z.; Performed the experiments, G.L.; Analyzed the data, J.N., L.Z., J.L. and G.Z.; Writing, L.Z. and G.L.; All authors have discussed the results, read and approved the final manuscript. Funding: This work was supported by the National Natural Science Foundation of China (Grant No. 51775416 and Grant No. 61773165), the National Thousand Talents Program of China (Grant No. WQ2017610446) and the Natural Science Foundation of Shaanxi Province (Grant No. 2017JM5069). Conflicts of Interest: The authors declare no conflict of interest.

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