Microstructural evolution and mechanical properties

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Apr 12, 2016 - However, material cost is the biggest barrier for further application considered the ... Inter Gas welding process (TIG) [9,10], Electron Beam Melting (EBM) ... structure formed by pulsed plasma arc welding, of which reason is.

Materials and Design 102 (2016) 30–40

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Microstructural evolution and mechanical properties of Ti-6Al-4V wall deposited by pulsed plasma arc additive manufacturing J.J. Lin a,b,⁎, Y.H. Lv b, Y.X. Liu b, B.S. Xu a,b, Z. Sun b, Z.G. Li a, Y.X. Wu a a b

Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai 200240, China National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China

a r t i c l e

i n f o

Article history: Received 18 January 2016 Received in revised form 5 April 2016 Accepted 6 April 2016 Available online 12 April 2016 Keywords: Microstructural evolution Mechanical properties Ti-6Al-4V alloy Pulsed plasma arc additive manufacturing

a b s t r a c t Pulsed plasma arc additive manufacturing (PPAM) is a novel additive manufacturing (AM) technology due to its big potential in efficiency, convenience and cost-savings comparing with other AM process. In this research, several Ti-6Al-4V thin walls were deposited by an optimized weld wire-feed PPAM process, in which the heat input was gradually decreased layer by layer. The deposited thin wall consisted of various morphologies with different microstructure, such as epitaxial growth of prior β-grains, martensite and horizontal layer bands of Widmanstätten, which depend on the heat input, multiple thermal cycles and gradual cooling rate in the deposition process. Reducing heat input of each bead and using pulsed current in the PPAM process, the microstructure of thin wall was refined. Meanwhile, the thin wall was strengthening and toughening. The average yield strength (YS) and ultimate tensile strength (UTS) reach 909 MPa and 988 MPa, respectively, and elongation reaches about 7.5%. The thin wall exhibited excellent performance in the aeronautical applications owing to the high values of mechanical properties in the room temperature. © 2016 Published by Elsevier Ltd.

1. Introduction Titanium alloys, particularly Ti-6Al-4V, have been widely used in aerospace, aircraft, automotive, biomedical and chemical industries due to their excellent combination of strength and fracture toughness, low density, as well as very good corrosion resistance [1,2,3]. However, material cost is the biggest barrier for further application considered the output of lower value and high price sensitive products [4], the main reason is their poor machinability caused by the low thermal conductivity and high chemical reactivity with cutting tool materials [5]. Fortunately, additive manufacturing (AM) technologies offer the potential to reduce cost, energy consumption and carbon footprint [6,7,8]. With a big potential in efficiency, convenience and cost-savings deposited layer by layer, plasma arc additive manufacturing (PAM) is a novel AM technology. The typical AM technologies include Tungsten Inter Gas welding process (TIG) [9,10], Electron Beam Melting (EBM) [11,12,13], Laser Beam Deposition (LBD) [14,15,16]. Both of EBM and LBD processes are characterized by large temperature gradient and high cooling rate, each of processes is most suitable to precise parts or particular applications. Generally, a certain thinness component is larger in width than the spot diameter of laser beam or electrical beam, thus both processes need multiple pass deposit for large components, which result in the build-up of the high stress concentration [10]. Besides, a rapid solidification gives rise to the occurrence of segregation and the

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.matdes.2016.04.018 0264-1275/© 2016 Published by Elsevier Ltd.

presence of coarse prior β-grain [11,14,17,18]. Particularly, the narrow prior β-grain with large length-width ratio always grows up to the top surface of deposited parts, which results to obvious anisotropy. Therefore, the parts normally need to be heat treated after deposition or hot isostatically pressed [19,20]. Table 1 gives a summary of reported advantages of PAM in comparison with other technologies. From the standard of measured length of columnar β grains in samples deposited by AM technologies, it clearly shows that LBM is the best deposition process, and EBM is much better than TIG, while the PAM process is still unclearly. Pulsed plasma arc additive manufacturing (PPAM) is a new AM techniques, because suitable pulsed plasma can be used to refine the deposited microstructure. Moreover, PPAM has advantages of efficiency for the manufacture of high-cost structural components, such as disks and blades of aircraft gas turbine, which are normally produced by costly titanium alloys. These components can be fabricated using one beam only by PPAM process. On the other hand, the energy density of plasma arc is close to laser, much higher than gas tungsten arc welding. The temperature zone of plasma arc transferred to the work piece in the concentrated beam can reach about 10,000 K–16,000 K [21]. Furthermore, the cost of plasma arc power source decreases to 1/7 of the price of laser beam or 1/15 of the price of electron beam [22], as is shown in Table 1. In PAM process, the wire or the powder is fed into the melt pool produced by constant plasma arc source (DC), pulsed plasma arc source as well as variable polarity current which is usually used on welding aluminum alloys in order to wipe off the oxide [24]. Particularly, compared to the constant plasma process, PPAM technology manufactures

J.J. Lin et al. / Materials and Design 102 (2016) 30–40 Table 1 Summary of reported advantages of PAM compared with other technologies [22,23]. AM technology

Length of β grains

Cost of power supply

Laser beam Electron beam TIG Plasma arc

0.5–1.5 mm (0) 1.4–5.0 mm (−) 15–35 mm (− −) (maybe +, 0, or −)

$500,000 (−) $1,000,000 (− −) b$7000 (+) $7000 (0)

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was 2000 × 1500 × 8 mm (length × width × height) hot rolled plates. The plates were grounded by 150 to 400 grit SiC papers and then degreased by acetone and ethanol before being used. A TransTig 4000 Job G/F and A Plasma module 10 were used as the power supply with a KD7000 D-11 wire feeder. The plasma arc welding head was attached to a 6-axis KUKA robot linked to a 2-axis table. The PPAM process was carried out with inter gas (argon) shield.

(+) = Good, (0) = neutral, (−) negative, and (− −) more negative.

2.2. Research methodology components with variational peak current and base current, making it is possible to work with higher current peak value without increasing the average heat input to the melted material, as is shown in Fig. 1. The average current can be calculated by: Iav ¼

Ip  tp þ Ib  tb tp þ tb

where Iav-average current, Ip-peak current(A), Ib-base current(A), tppeak current, and tb-base current. The PPAM process produces a more constricted arc and lower heat input compared to DC power source. In addition, previous researches have confirmed that it can refine the structure formed by pulsed plasma arc welding, of which reason is that the process enables the melt pool to agitate more tempestuously during welding [25]. Using AM technologies to fabricate components will experience complex cyclic thermal history, there is necessity to understand the microstructure relevant to processing and properties, as well as the development of an AM database of materials processing. However, using the PPAM process is rarely used formerly, and its precision is in lower level. Additionally, PPAM process needs increasing cooling time as the layers deposited because the heat input value of each deposited layer keeps constant, which will easily result in heat accumulation [22,26]. In especial, the effect of microstructural evolution on mechanical property remains unclear for Ti-6Al-4V alloy deposited by the PPAM. In this paper, the microstructural evolution of and mechanical properties of Ti-6Al-4V wall deposited by wire-feed PPAM is studied. The interactive effects among microstructural evolution, deposition process and mechanical properties should be clarified for the deposited thin wall without cooling substrate. It is expected that the mechanical properties of directly deposited walls at room temperature to be equal to or exceed the standard level of casting and forging. 2. Experimental procedure 2.1. Experimental material and equipment A 1.0 mm diameter Ti-6Al-4V wire (ERTi-5) was used for the deposited process, and namely chemical composition was 0.02 C, 0.14 O, 0.01 N, 0.007 H, 0.07 Fe, 6.11 Al, 3.95 V, Bal·Ti (in weight %). The substrate

With a 16-layer deposition, the total height with one bead width is shown in Fig. 2. In each specific trial, Ti-6Al-4V wire was deposited layer by layer onto the substrate by PPAM. The scan direction of each layer was built by adopting a single X-coordinate, moreover, each additive height (ΔZ) was set at 1.5 mm in positive Z-coordinate direction. Heat input of bead was gradually decreased layer by layer, and each layer would be deposited only if the temperature of the previous layers fell below 300 °C to prevent air oxidation. Thermocouple probes were inserted into the substrate, of which were distance from the deposited surface was 5 mm. The main processing parameters were constant: pulse frequency was 70 Hz, travel speed (Ts) was 0.25 m/min, Wire Feed Speed (WFS) was 3.5 m/min, plasma gas was 0.2 L/min, and argon atmosphere was 20–30 L/min. The current parameters of each layer and other experimental settings are detailed in Table 2. For microstructure evolution study, a certain number of trials were conducted, of which depositions ranged from one layer to four layers, respectively, as shown in Fig. 2 (set by L1 to L4 parameters). According to ASTM E8-04, the tensile coupons were extracted in deposited direction, as shown in Fig. 2. The dynamic strain gauge extensometer were set up to measure 0.2 pct yield stress (Rp0.2) and ultimate tensile stress (UTS) with a displacement rate of 0.5 mm/min, which had a gage length of 25 mm, and attached to each specimen. All the tensile property tests were carried out by an AG-25KNIS machine. The tested Vickers hardness coupons were incised from the formed thin wall, 20 mm away from the deposition starting point, and it was along the additive direction. The coupons of microstructure were obtained in this same way, as is shown in Fig. 2. Those microstructure analyzing coupons were polished with SiC papers (150,400,600,800,1000 grit) by a BUEHLER machine, then they were electrolytic polished in ethanol (purity 95%) 94%+ perchloric acid (purity 60%) 6% for about 3–5 min at a low temperature. After that, they were etched for 40–60 s in kroll reagent (1 ml HF + 2 ml HNO3 + 50 ml H2O) in order to emerge a flesh condition for observation. According to DIN EN 6507 on cross-sections of additive direction with 100 g (HV0.1) on Tukon 1102(Wilson, BUEHLER), the Vickers hardness of different microstructures were measured in three locations. Microstructure and fracture surfaces of selected typical specimen were examined by using a FEI Nova NanoSEM50 scanning electron microscope (SEM) equipped with an oxford X-Max energy dispersive X-ray (EDS), and microstructural date were

Fig. 1. Schematic representation of the plasma arc deposited process (a) and measured current (b).

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Fig. 2. Schematic drawing of thin-wall deposited by PPAM process.

collected by Electron Back-Scattered Diffraction (EBSD), and analyzed by TSL OIM Collection 5. 3. Results 3.1. Macrostructure Fig. 3 shows the profile of Ti-6Al-4V wall with 16-layer high and one-bead wide deposited by PPAM. The prior β grains with different columnar sizes grow perpendicularly from substrate across multiple deposition layers until the top region (Fig. 3a). Meanwhile, the epitaxial growth of prior columnar β-grains has different growth directions (Fig. 3a and d). The average length of columnar gain is about 3.43 mm, in which the maximum length is about 6.9 mm in the top region and the minimum is about 0.86 mm in the bottom region, as shown in Table 3. The morphology of horizontal layer bands (LBs) are revealed in parallel with deposited layer. Moreover, compared to other microstructure regions, the LBs are made up of fine white strips and back straps under optical microscope (OM) observation. The distance of two nearby parallel LBs is about 1.4 ± 0.40 mm (Table 3), which is marked in single bigger arrow (Fig. 3a, c and d). However, the LBs in last three depositions cannot be observed, and the height of the three layers is 6.28 mm, as is shown in Fig. 3a and b (the top region of the deposited wall). The similar structures of LBs have been reported in researches of ALM by laser [16] as well WAAM by arc [27]. 3.2. Microstructure evolution According to the sizes and arrangements of the two phases of hexagonal close-packed structure (hcp)(α) and body-centered cubic(bcc)(β), the microstructure of Ti-6Al-4V can be described as equiaxed structure, martensite α′, Widmanstätten structure, and “basket-wave” structure in the β matrix [28]. Moreover, the shape and formed α phase are described as primary α, secondary α, acicular α, lamellar, massive α, and Table 2 Details of current parameters of each layer deposited by PPAM process. Layers

L1–L3 L4–L6 L7–L10 L11–L14 L15–L16 a b c d

Parameters of PPAM Ipa(A)

Ibb(A)

Dcyc(%)

Iavd(A)

U(V)

250 250 250 230 230

40%Ip 40%Ip 40%Ip 40%Ip 30%Ip

50% 40% 30% 30% 30%

175 160 145 133 117

19.6 19.5 19.3 18.9 18.5

The peak current of pulsed plasma arc. The base current of pulsed plasma arc. Duty cycle. Average of current is equal to Ip × Dcy + Ib × (100% − Dcy).

colony α. Different microstructures described as fine or coarse are mostly related to the thermal history which is mainly decided by heat treatment process. Martensite α′ structure and the fine basket-wave structure can be discerned by OM. Usually, martensite α′ structure is characterized by a rectangular grid structure [29], while fine basketwave structure is described a new α plates nucleating α plate that tend to grow nearly perpendicular to near plate [30]. 3.2.1. Microstructure evolution of layer band region and EDS measurement Fig. 4 shows SEM of LB in the middle region of a deposited wall (Fig. 4a), Fig. 4P1–P4 and P5–P8 are respectively located in two different microstructures, the dimmed martensite α′ structure (Fig. 4P1, P2, P3, P4) described as rectangular grid is named of nominal martensite α′ or fine Widmanstätten structure, while coarser basket-wave is described likely basket (Fig. 4P5, P6, P7, P8). Especially, Fig. 4P1, P2 and P5 all show typical zones of LB. Specially, all of the microstructures at the middle wall have formed fully α lamellar structure observed by SEM (Fig. 4P1, P2, P5), including fine Widmanstätten (0.28–0.52 μm) and coarse basket-wave (0.35–0.59 μm), which include α rods and α dots in the transformed-β matrix. As shown in Fig. 4a, obviously, the average width of LB is about 278 μm, however, the microstructures of within LBs are coarser. And α colony with larger length lamellar and bigger α parts (e.g. Fig. 4P2) are observed. In the area nearby LB, basket-wave structures within LB are coarser than the upper LB (compared to Fig. 4P1 and P5). The EDS results of LBs in middle wall are shown in Table 4. The contents of main elements of Al and V of Ti-6Al-4V are in the ASTM-B 367-13 ranges [31]. The average volume of Al element in norminal martensite α′ region of spectrums exceeds that in basketwave region, and the area containing the maximum amount of Al is in the upper LB (Fig. 4P1). The average volume of V element in norminal martensite α′ region of spectrums are lower than basket-wave region, and the area with the maximum V amount is in the LB (Fig. 4P6). 3.2.2. Evolution of normal microstructure compared to layer band and EBSD analysis Fig. 5 shows optical microstructure of deposited wall, and martensite α′ structure, Widmanstätten structure and basket-wave structure are observed. The different microstructures are divided by prior β-grain boundaries. Furthermore, acicular α and colony α are displayed in different morphologies within grains or at prior β-grain boundaries separately. With additive layers deposited by PPAM, Martensite α′ structure turns into dim which is generally termed as acicular α in a matrix of transformed β [32], and coarse basket-wave can be observed in middle region (Fig. 5b). Additionally colony α is presented at prior β-grain boundary or in matrix of transformed β. Furthermore, prior β grain boundaries are observed slightly coarser and clearer than that in bottom region. Compared to the bottom region, the finer Widmanstätten structures with parallel align acicular α,

J.J. Lin et al. / Materials and Design 102 (2016) 30–40

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Fig. 3. The profile of Ti-6Al-4V wall shows (a) the columnar prior-β grains, layer high and morphology of layer band, (b) the top region, (c) the middle region and (d) the bottom region.

colony α, or a part of colony α grow along the prior β-grain boundary in the top region. The sequential deposition of layers has repeatedly melted, and been heat treated in the middle region of the wall, contributing to specially characteristic microstructure. Orientation map shows the orientation relation between different α lamellae with different color and different orientation. Generally, parallel neighbor lamellae seem to fuse together and form a larger region (or α colonies), which are called relatively large grains in the orientation map. As shown in Fig. 6, the Y–Z plane in the middle region represents prior β grains at triple point, while Fig. 7 shows the internal prior β gain area from the middle region. Different orientation prior β grains include martensite α′ structure and basket-wave structure, which can be evaluated by different microstructures (Fig. 6a). The distribution of misorientation observed in Figs. 6a1 and 7a1 tends to be about 60 deg.

Table 3 Feature analyzed for macrostructure of wall deposited by PPAM. Feature

Between distance the LBs Width of β grains Length of β grains

PPAM Average ± standard deviation

Min

Max

1.40 ± 0.40 mm 2.00 ± 0.66 mm 3.43 ± 2.58 mm

0.87 mm 1.00 mm 0.86 mm

2.13 mm 3.20 mm 6.90 mm

Apparently, the internal grains of martensite α′ with long streaks of lamellae (Fig. 7b1) are finer than the triple prior β grains (Fig. 6b1), but the prior β grains at triple point have more fraction volume of lowangle grain boundaries. A possible explanation is that small angle misorientation between the prior β grain boundaries could form in more different directions. Frequently, basket-wave structure have larger fraction volume of β phase than the martensite α′ (Figs. 6d and 7d). It is noteworthy that the main 〈11−20〉 poles split into triangular stars or rhombic patterns in Fig. 7e, while the 〈0001〉 pole directions are unobserved to be the same patterns. 3.3. Mechanical properties 3.3.1. Microhardness Fig. 8 shows schematic illustration of hardness measurement on the profile of wall. Three typical regions are selected for hardness measurement, which are the top region, the middle region and the bottom region. All the selected regions have different characteristics of microstructure, including the middle LB microstructure, the upper LB microstructure, and the lower LB (Fig. 8b) microstructure. Moreover, three test points with different microstructures measured. In different regions of LBs, Fig. 8c shows that the average hardness value at the bottom region increases from the first LB to the second. Respecting to the whole wall, the average hardness in the as-built condition is not obviously different. The average microhardness

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Fig. 4. Microstructure of layer band region in the middle region of the deposited wall, (a) optical microscopy, the points indicate different place of the EDS micro-analysis; (P1), (P2) and (P5) SEM photos showing two different microstructures.

Table 4 The EDS measurement of LBs in middle wall and summary of the width of α plates. Spectrums of PPAM

Weight of elements/%

Microstructure

Al

V

Ti, et al.

3.85

Other

P5

6.25 (max) 5.97 6.11 6.02 (min) 6.10

P6

6.08

P1 P2 P3 P4

3.91 4.17 4.18 4.13

4.21 (max) P7 6.10 4.05 P8 6.09 4.18 ASTM-B 367-13 5.5–6.75% 3.5–4.5%

Width of α lamellar (μm)

Fine 0.28–0.5 Widmanstätten 0.33–0.52 – – Coarse basket-wave

0.35–0.59 (max) – – – –

measurement result is shown in Fig. 8c, which almost matches the hardness level of ASTM B367-13 [31]. Specially, the microhardness in middle LB (Fig. 8c, marked by single arrows) is slightly lower than that in the upper LB and the lower LB, but it gently increases near the secondary LB.

3.3.2. Tensile properties Typical tensile properties of the wall are presented in Table 4. The average yield strength (YS) of tested coupons is 909 ± 13.6 MPa, ultimate tensile strength (UTS) is 988 ± 19.2 MPa, and Elongation is 7.5% ± 0.5, respectively. The mechanical properties are higher level than that of ASTM B367-13 for casting, but the elongation values are 10% lower compared to the ASTM B381-13 [33] level for wrought. Compared to the mechanical properties of typical AM technologies, the tensile properties of the wall deposited by PPAM can obtain better

properties (Table 5). However, the strain of failure is lower than some others typical AM technologies via stress-relieved or annealed conditions. The mechanical property can be summarized as higher strength but lower ductility in as-built condition. Fig. 9 shows the SEM fractograph of typical tensile test at the bottom region and the middle region. The coupons present typical fractograph of dimple rupture shown in Fig. 9a1, b1. The arrows without filled back line are used to note the crack growth orientation. It can be seen that the crack growth at the bottom is aligned with additive deposition (i.e. Z direction), while the crack growth in the middle is vertical to the fracture surface (i.e. Y direction). With respect to the shape and depth of the dimples, the fractograph of the coupon shows elongated dimples and tear ridges (Fig. 9a1, noted with white arrow). In addition, the rupture surface of the middle region shows mixture of coarse and fine dimples, which are larger and deeper than those of dimples of the sample in the bottom region (Fig. 9b1, noted with white arrow). 4. Discussion 4.1. Microstructural characteristics 4.1.1. Evolution of the columnar grains and the layer bands The AM process fabricates a component by means of direct energy input to melt the previous deposition layer. Heat input has a significant effect on thermal gradient, nucleation and the growth rate of grains. Prior β-grain varies in shape from nearly globular to large columnar, and perpendicularly grow up across multiple deposition layers. Such phenomenon of perpendicular strip of columnar prior β-grains was also observed in the laser additive layer manufacturing (LAM) or weld wire arc additive manufacturing (WAAM) [27,34]. Fig. 10 shows profile of Ti-6Al-4V wall deposited By PPAM, including one layer, three layers, four layers and single bead, respectively.

Fig. 5. Optical micrograph of normal microstructure compared to layer band of the wall from (a) the bottom region; (b) the middle region; (c) the top region.

J.J. Lin et al. / Materials and Design 102 (2016) 30–40

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Fig. 6. The EBSD of grain boundary at triple microstructure from the middle region of the wall. (a) Orientation map with inverse pole figure; (a1) the distribution of misorientation angle; (b) unique grain color map; (b1) grain size distribution; (c) image quality;(d) distributed phase map.

Varying prior columnar large β grains almost perpendicularly grow across multiple deposited layers from the substrate to the top layer. However, the prior columnar β-grain of one deposited layer and three deposited layers (Fig. 10a, b) grow thought the whole deposited layers from the substrate to the top border, which incline along the highest temperature gradient during deposited. Meanwhile, the prior columnar β-grain epitaxially grows in different directions when deposited after four layers (Fig. 10c), i.e., thus different growth directions are observed. The appearance evolution demonstrates that the first layer deposition has the maximum temperature gradient, in which there have formed long and narrow columnar β-grains (Fig. 10a and b, marked by arrow). However, it can be seen epitaxial growth grains deviating from the direction of maximum temperature gradient (Fig. 10c, marked by arrow) due to the

competitive relation among the growth of prior β-grains with additive layers [34]. The epitaxial growth of prior β-grains is in direction identical with additive height (ΔZ), because strong cooling gradient leads to the preferred growth or easy-growth direction [35,36]. Especially, the grains of additive manufactured thin-wall components tend to grow perpendicularly to the prior layer. With additive layers deposited, there is competitive relationship among the different growing orientations of prior β-grain. The explanation is that the epitaxial growth of the prior βgrain is preferential in the direction of the maximum temperature gradient. In a word, there are two factors contributing to the prior βgrains growing behavior. Firstly, from the summary of the average heat input as listed in Table 6, it knows that the heat input decreases by 0.75 KJ/cm step by

Fig. 7. The EBSD of internal grain from the middle region of the wall. (a) Orientation map with inverse pole figure; (a1) the distribution of misorientation angle; (b) unique grain color map; (b1) grain size distribution; (c) image quality; (d) distributed phase map; (e) pole figure.

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Fig. 8. Schematic illustration of hardness measurement on the profile of the wall (a and b); microhardness of layer bands (bottom, middle and top region) of the wall (c): the zone of within LB was marked by single arrows; ASTM B367-13 was marked by dashed line.

Table 5 Mechanical properties of the walls deposited by PPAM vs. other typical AM technologies. Part

Condition

Deposited direction

UTS(MPa)

0.2%YS(MPa)

%EL

Average ± standard deviation PPAM ASTM B367-13 (cast) ASTM B381-13 (wrought) EBM (Ref. [11]) EBM (Ref. [11]) EBM (Ref. [12]) LBM (Ref. [9]) LBM (Ref. [14]) TIG (Ref. [9])

As-built As-cast – As-built As-built 620–690 °C As-built/600 °C/840 °C As-built As-built 600 °C/840 °C

H – – H V – H or V H or V H or V

988 ± 19.2 895 895 833 ± 22 851 ± 19 1180 872–940 790–960 930–940

909 ± 13.6 825 828 783 ± 15 812 ± 12 – 791–874 697–884 791–874

H means horizontal; V means vertical.

Fig. 9. The SEM fractograph of typical tensile test at room temperature: (a, a1) the bottom region; (b, b1) the middle region.

7 ± 0.5 6 10 2.7 ± 0.4 3.6 ± 0.9 16–25% 4.2–12.5 5–12 6.6–20.5

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Fig. 10. Coupons produced by PPAM: (a) 1 layer is 1.7 mm high; (b) added3 layers is 3.7 mm high; (4) added 4 layers is 6.4 mm high, the columnar prior-β grains growth directions are indicated by arrow.

step with the increase of deposition, which can effectively reduce the energy accumulation of melting layer during deposition process. Secondly, Hirate Y's research work has demonstrated that the pulsed welding arc with frequencies ranging from 1 to 100 Hz have diverse effects on gain refinement for metal melt pool [37]. Pulsed frequency can generate a stirring force towards the metal poor, and then plays the role of refining the grains and microstructure [38], i.e. the peak current and the base current afford for a temperature gradient G, so that the

nucleation can easily occur in the front of dendrite growth, as shown in Fig. 11. In the case of the two reasons above, there appear different orientational grains. Consequently, the PPAM process can effectively reduce the average length-width ratio of the columnar gains, and force the prior β grains to growth epitaxially in a row from bottom to top. That means the PPAM process can be substantially reduce the anisotropy to the deposited components, as shown in Fig. 11.

Fig. 11. The effect of pulse frequency on the freezing range (ΔT). A constant temperature gradient G and cooling rate are assumed.

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be calculated by using the Rosenthal [39] Eq. (1). 2πðT−T0ÞkR −VðR−xÞ ¼ exp Q 2α

ð1Þ

where T is liquid line, T0 is substrate or prior deposited layer, Q is heat input, v is travel speed, k is conductivity, R is the radius of T, x is reference point, α is thermal diffusivity rate. In order to calculate the z of cooling rate, it is assumed that R equals X equals Z. The cooling rate is calculated by Eq. (2)

Fig. 12. α and β phase orientation map of pole figure.

The horizontal LBs can be recognized in the wall deposited by PPAM, but they are not observed in the top three layers, and similar phenomena have been declared in researches of ALM by laser [16,34]. Similarity, referring to the Fig. 10, it is not seen in the third layer, but LB can be observed after deposition of the fourth layer (Fig. 10c, marked by arrow). So the LB is relevant to the effect of the later deposition. In essence, the cause of the occurrence of LBs after depositing 4 layers is related to the multiple thermal cycles and gradual cooling. Plasma arc has higher heat input compared with LAM [34]. The energy density of plasma arc is between the values of laser and gas tungsten arc, and the plasma arc is transferred to the work piece as a concentrated beam in the temperature of about 10,000 K–16,000 K temperature zone, that is near to the energy density of laser [21], so plasma arc can increase the speed of atom diffuses. Respectively, the LBs do not coincide with deposited layers because of repeated heat input across the β/α + βtransus line (995 °C [1]). The martensite α′ may turn into α + β structure (Fig. 4) by subjecting these regions to elevated temperatures within the α + β phase field. The multiple thermal cycles promote to decomposition of martensite α′ and growth of α. Furthermore, after depositing four layers which covers about 6 mm from the first LB, the LB is no observed after three deposited layers (3.7 mm), i.e. in the distance of about 6.4 mm from the first LB, there may form a new LB, in consequence, horizontal LB cannot be observed in the last three deposited layers at the top of the wall (Fig. 3). It can be considered that a new layer is formed. 4.1.2. Evolution of normal microstructural characteristics Thermocouple probes were inserted in the substrate distanced from the deposited layer about 5 mm. The cooling rate of deposited layer can

     ðT−T0Þ2 ∂T ∂T ∂T ¼ ¼ −2πkv Q ∂t z ∂z t ∂t z

ð2Þ

The bottom temperament (T0) is assumed at 25 °C. The deposited process could be cut off until the temperament of the previous layer had fallen to T0 which equals 150 °C. Other setting are k = 27 J/msK, and set v = 0.25 m/min, the β-transus of T = 995 °C, Q = η ∗ Iav ∗ U, where η = 0.8 [40]. The cooling rate is 260.7 °C/s at the bottom region, and 257.6 °C/s at the top region. Cooling rate above 410 °C/s can generate a fully martensitic microstructure, and the mixture of martensitic microstructure and massive transformation are observed between 410 and 20 °C s, this transformation is gradually replaced by diffusion controlled Widmanstätten α formation with decreasing cooling rate [41]. The accumulation of heat input promotes to reduce the cooling rate, resulting in the finer Widmanstätten structure with parallel align acicular α in the top region, and martensite α′ decomposes to α rods and dots or α colony under the multiple thermal cycles in the middle region(Fig. 4), i.e. repeated heat treatment. Moreover, the grain boundaries have high energy so that there tend to form α colony. 4.2. Spatial orientation distributions The distribution of misorientation of the prior β grains at triple region and the internal grain tend to be about 60 deg, shown in Figs. 6a1 and 7a1 (error limited in both angular resolution and step scan). Small volume of low-angle grain orientation locates in grain boundary due to multiple thermal cycles. To avoid interference, the EBSD analysis of the internal grain is used for more accuracy, as is shown in Fig. 12 (from the Fig. 7a). As shown in Fig. 12, phase transformation of the wall deposited by PPAM follows the Burgers orientation relationship, which matches the equation [3], i.e. any of the 6{110} planes can transfer

Fig. 13. Schematic stereographic projection in typical four cases of hexagonal system.

J.J. Lin et al. / Materials and Design 102 (2016) 30–40

the occurrence of elongated dimples or tear ridges in fractograph (Fig. 9a1).

Table 6 Summary of heat input of each layer deposited by PPAM. Layers

Parameters of PPAM Iav(A)

U(V)

Pa(KW)

P/vb(KJ/cm)

L1–L3 L4–L6 L7–L10 L11–L14 L15–L16

175 160 145 133 117

19.6 19.5 19.3 18.9 18.5

3.43 3.12 2.79 2.51 2.16

8.23 7.48 6.71 6.03 5.19

5. Conclusions The Ti-6Al-4V thin wall deposited by the advanced weld wire-feed PPAM process exhibits excellent mechanical properties. The following results are obtained from the systematic investigation on the microstructural evolution.

v means deposited speed. a Power of plasma arc. b Rate of heat input.

into a {0001} basal plane. On the other hand, one of three 〈11–20〉 directions in one basal plane would be parallel to a 〈111〉 direction of the original {110} plane. For geometrical reasons, the angle between these possible orientations that two different 〈111〉 (//〈11 − 20〉) directions lie in one original {110} is 10.5 deg, i.e. {0001} basal plane of two variants orientations is possible. Frequently, only three different 〈0001〉 poles with only one variant of the two possible 〈11–20〉 orientations dominate [42]. Therefore, according to different color in orientation map (Fig. 7), the different 〈11–20〉 poles in a same original {110} plane form triangular stars or rhombic patterns shown in Fig. 13. It means that there are three dominating orientations out of twelve possibilities, as observed in Fig. 7e. f0001g==f110g and h11−20i==h111i:

39

ð3Þ

(1) Both of the size and growth direction of prior β grains are affected by the gradually reduced heat input of the pulsed plasma arc. (2) The formation of horizontal LBs is subjected to the influence of four thermal cycles at least in the PPAM process, and insufficient thermal cycles lead to the absence of LBs in the top of thin wall. (3) The phase transformation follows the Burgers orientation relationship, and three orientations out of the twelve possibilities occupy dominating orientations, which finally form triangular stars or rhombic patterns. (4) The low hardness of LB and relatively low elongation are caused by the formation of α colonies. However, the mechanical properties of the deposited wall at room temperature exceed the standard of casting wall, and the strength is even higher than that of forged wall.

Acknowledgments The research is performed with financial assistance from the military plan projects of China, No. 613213.

4.3. The effect of microstructure on mechanical properties 4.3.1. The effect of microstructure on microhardness In the PPAM process depositing wall, multiple thermal cycles promote the decomposition of martensite α′, so longer α rods (obviously in Fig. 4P2) and a large number of dots (compared to the Fig. 4P1, P2 and P5) appear. With the increase of α lamellar, α lamellar grows along with grain boundary or band layers, becoming α colony which leads to the increase of size of actual slip on strain, and the growth size of α colony may be larger than 10 μm (Figs. 6b1 and 7b1). The macrostructure, thus, possibly exits actual slip due to the larger α colony under tensile test, causing the elongated dimples and tear ridges in fractograph. That is why the hardness of upper LB is lower than that of LB. Moreover, the hardness tendency at the LB region is in similar value, the LB region hardness in top is slightly lower than that in middle, because in the top region, there are longer lengths of α lamellars. However, there is no obviously change on the hardness from the bottom to the top, the most extend hardness of decline appears in bottom, because the heat input of prior number layers in bottom is larger than those of the later deposited layers, which lead to the increase of the dilution rate. 4.3.2. The effect of microstructure on tensile properties The wall deposited by PPAM normally has high tensile strength but low elongation (as listed in Table 5). There are two reasons. On the one hand, the small deviation of the average heat input restrains the growth of prior β grain and α lamellar in the PPAM process, so that the microstructure is refined. The fine microstructure always exhibits ductile facture with a dimple fractograph according to the Hall-Petch relation. On the other hand, the contribution of fine microstructures of α lamellar to the tensile strength is more significant than that of the prior β grains [43], for the details of finer microstructure of α lamellar as shown in Table 4. However, the low elongation due to the parallel acicular α or the coarsened α colonies, specially the α colonies growing along with the prior β-grain boundaries or band layers, which leads to

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