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Microstructure and mechanical properties of hybrid fabricated Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy by laser additive manufacturing
Materials Science & Engineering A 607 (2014) 427–434
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and mechanical properties of hybrid fabricated Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy by laser additive manufacturing Yanyan Zhu a, Jia Li a,b, Xiangjun Tian a,b, Huaming Wang a,b, Dong Liu a,b,n a
Laboratory of Laser Materials Processing and Manufacturing, Beihang University, 37 Xueyuan Road, Beijing 100191, China Engineering Research Center of Ministry of Education on Laser Direct Manufacturing for Large Metallic Components, Beihang University, 37 Xueyuan Road, Beijing 100191, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 November 2013 Received in revised form 19 February 2014 Accepted 5 April 2014 Available online 13 April 2014
The hybrid fabricating technique by laser additive manufacturing provides an attractive potential for manufacturing titanium alloy components. Microstructure, micro-hardness and room tensile mechanical properties of hybrid fabricated TC11 titanium alloy sample were examined. Results show that the hybrid manufactured sample consists of three typical zones: the laser additive manufactured zone (LAMZ), the wrought substrate zone (WSZ), and the bonding zone without any metallurgical defects. Superfine basket-wave microstructure forms in LAMZ and heat affected zone (HAZ) due to the rapid cooling rate. No obvious grain growth or recrystallization occurs in the HAZ. A special bimodal microstructure consisting of coarse fork-like primary α and fine β transformed microstructure is found in the transition zone due to the heat effect in α þβ region. The hybrid fabricated TC11 sample has good mechanical properties with tensile strength of 1033713 MPa and elongation of 6.87 0.2%. The fracture of hybrid sample occurs in the substrate in tensile testing, meaning that the bonding zone has better mechanical properties than the substrate. & 2014 Elsevier B.V. All rights reserved.
Keywords: Laser additive manufacturing Titanium alloy Microstructure Heat affected zone Mechanical properties
1. Introduction Due to their high specific strength and excellent corrosion resistance, titanium alloys have found many applications in aeronautical, petrochemical and biomedical industries in recent years [1–3]. A typical αþβ titanium alloy, Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (named TC11 in China and BT9 in Russia), has excellent comprehensive mechanical properties at room and high temperatures. It has been extensively used in the key structural components of aircraft engines like compressor disk and blade [4–6]. The mechanical properties of this alloy are sensitively dependent on its microstructures, which are dominantly decided by hot working processes and post-heat treatments. At present, aerospace structural TC11 components are mostly fabricated by conventional costly wrought base processes at a high buy to fly ratio and low material utilization ratio [4,7–9]. Laser additive manufacturing is one of the new techniques that fabricate full density near net shape metal components directly
n Corresponding author at: Laboratory of Laser Materials Processing and Manufacturing, Beihang University, 37 Xueyuan Road, Beijing 100191, China. Tel.: þ 86 10 8233 9691; fax: þ86 10 8233 8131. E-mail addresses: [email protected] (Y. Zhu), [email protected] (D. Liu).
http://dx.doi.org/10.1016/j.msea.2014.04.019 0921-5093/& 2014 Elsevier B.V. All rights reserved.
from metal powders [10]. During LAM process, fine metal powders are fed into a molten pool produced by a sharply-focused and high energy laser beam. The working table moves along the pre-set trajectory, then three dimensional components are fabricated layer by layer on the substrate [11–13]. In comparison to conventional wrought base techniques, the LAM process has some remarkable advantages: reduction of production cycle and cost, high material utilization ratio and excellent flexibility. It has a great potential for the manufacturing of large complex metal components especially for the difficult to process materials like titanium alloys [11–15]. Moreover, owing to the fine rapid solidification microstructures, the LAM titanium alloys have excellent mechanical properties comparable or even superior to the wrought materials [15–18]. Generally, the conventional wrought base processes are suitable for manufacturing regular bulk parts, while the LAM technique has superior flexibility of fabricating complex parts [15,18]. When manufacturing large and complex parts, the advantages of the two techniques can be well combined by adding special features to local areas of regular wrought base by the LAM technique [19–22]. During this process, well designed wrought substrate is chose and retained as part of the final component after the LAM process finishing, which is different from the previous reported typical LAM process in which the simple substrate is just a supporter and would be removed [10–14]. This is a special fabrication process
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based on LAM, named the hybrid fabrication (HF) technique [21]. Just like the fabrication of the aero-engine TC11 titanium alloy blisk (integrally bladed disk), the HF technique can be done as follows: a disk is forged first and then blades are added at the rim of the disk by LAM process in which the forged disk is used as substrate and retained. There is no doubt that the HF technique can further increase the material utilization ratio and reduce the cost. It also has a great potential for the manufacturing of dual- or multi-functional components [21]. The hybrid fabrication technique by LAM seems like an advanced form of laser repairing or laser welding. One common concern of the hybrid fabricating technique is the mechanical property of the final part, which is dominantly dependent on the microstructure of the final part especially in the joint zone (including the heat affected zone) [21,23–25]. Richter reported that the laser cladding Ti6242 titanium alloy had good mechanical properties. The laser cladding process they used was quite similar to the HF technique in this paper [20]. Wang studied the microstructure and mechanical properties of hybrid fabricated stainless steel [21]. Wang and Tan found that obvious grain growth occurred in the HAZ of the parent metal during the welding process of TC11 titanium alloy, which deteriorated the mechanical properties of the welded samples [25,26]. However, most of the previous reports about TC11 titanium alloy were focused on the wrought or welding processes [25–29]. The investigations of LAM TC11 titanium alloy were very limited [30]. Therefore, it is very urgent and necessary to investigate the microstructure and mechanical properties of hybrid fabricating TC11 titanium alloy. In order to obtain a deep understanding of the hybrid fabricating process of TC11 titanium alloy, microstructure, micro-hardness and room temperature tensile properties of the hybrid fabricated sample were investigated in this paper. The microstructure evolution in the heat affected zone during the hybrid fabricating process was discussed.
Fig. 1. Schematic illustration of the hybrid fabricating process by laser additive manufacturing.
2. Experiments Considering the microstructure homogeneity of the final component, typical β forged TC11 sample was chosen as the substrate because the lamellar microstructure is closer to the LAM microstructure in comparison with equiaxed or bi-model microstructure. The geometry size of the substrate is 200 mm 40 mm 80 mm and the surface of the substrate was sandblasted before LAM. The LAM process was carried out on the in-house developed LAM system equipped with a GS-TFL-8000 CO2 laser (maximum output power 8 kW), a BSF-2 power feeder together with a co-axial powder delivery nozzle, a HNC-21M computer numerical control (CNC) multi-axis motion system, and an argon purged processing chamber with oxygen content less than 100 ppm. The processing parameters were as follows: laser beam power 6 kW, scanning speed 800-1200 mm/min, beam diameter 4–6 mm, powder delivery rate 500-1000 g/h, overlap ratio 30–50%. The scanning path was back and forth mode, meaning that the scanning directions of adjacent tracks were opposite. A schematic illustration of the LAM process for hybrid fabricating was shown in Fig. 1. Under these parameters, a TC11 plate was fabricated with the same size as the substrate in a track-by-track and layer-by-layer way, as shown in Fig. 2. The newly hybrid fabricated sample was tempered at 530 1C for 4 h in order to eliminate the residual stress. Metallographic samples were prepared using standard metallurgical procedures. A mixture solution of HF: HNO3: H2O with a ratio of 1:6:43 was used as etching agent. Then microstructures of different zones of hybrid fabricated TC11 sample were characterized by optical microscopy (OM) and scanning electron microscopy (SEM). Micro-hardness of the hybrid fabricating sample was
Fig. 2. The hybrid fabricated TC11 bulk and the three kinds of specimens for room tensile test.
measured by using HAZ-1000 Vikers tester with 500 g load and 10 s dwelling time. For easier comparison, room temperature tensile samples of different zones from the same hybrid fabricating TC11 plate were prepared: the pure LAM sample (L), the pure wrought sample (W) and the hybrid sample with the bonding interface in the middle (WþL), as depicted in Fig. 2. The principle axis of the specimens was parallel to the deposition direction and vertical to the texture direction. The tensile test was carried out according to the test standard of ISO 6892:1998 with 25 mm gauge length and 71 mm total length specimen. Three samples of each zone were tested for an average.
3. Results and discussion The aim of this paper is to investigate the microstructure and mechanical properties of hybrid fabricated TC11 titanium alloy. Firstly, the microstructures of the laser additive manufacturing zone and wrought substrate zone were given. Secondly, the microstructure in the heat affected zone and the formation mechanism were studied. Finally, micro-hardness and room temperature tensile properties were tested and analyzed.
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Fig. 3. Macrostructure of the hybrid fabricating TC11 sample consisting of three different zones.
3.1. Microstructure characterization The macrostructure of the hybrid fabricated TC11 sample is shown in Fig. 3. The deposition direction is perpendicular to the texture direction of the wrought substrate. Three obviously different zones with different grain morphologies exist in the sample: the laser additive manufacturing zone (LAMZ) with columnar grains along the deposition direction, the wrought substrate zone (WSZ) with coarse elongated grains along the texture direction and the heat affected zone (HAZ). There is a good metallurgical bonding between the LAMZ and the WSZ. Typical metallurgical defects such as gas porosities, lack of fusion porosities or cracks are not found in the bonding zone. The HAZ has the same contrast with the LAMZ but the same grain size and morphology with the WSZ. The width of HAZ is about 1.5–2 mm, which is a little narrower than that in the electron beam welded TC11 sample [25]. Microstructures of the substrate zone and laser additive manufacturing zone are shown in Fig. 4. In WSZ, typical coarse widmanstätten microstructure exists within the coarse elongated grains. The average grain size can be several millimeters. The width of lamellar α is approximately 1.4 μm and the volume fraction of α phase is about 84%. This microstructure can be obtained easily in the final steps of the processing route by an annealing treatment in the β phase field with a subsequent relatively slow cooling rate [31]. As for the LAMZ, superfine basket-wave microstructure forms within the columnar grains due to the high temperature gradient and the rapid cooling rate in the LAM process. Moreover, owing to the character of intense epitaxial growth of titanium alloys and the large temperature gradient, the columnar grains can grow consecutively through many layers (several millimeters). However, the average width of these columnar grains is just about 100–300 μm. This is a common phenomenon in laser additive manufactured titanium alloys except for some particular conditions [17,32]. The width of the lamellar α is only about 0.4 μm, which is much finer than that of the WSZ. The volume fraction of α phase is about 77%, also a little lower than that of the WSZ.
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Further observation of the bonding region is shown in Fig. 5. The nominal interface between the WSZ and LAMZ is marked as white dot line in Fig. 5(a), which is the original surface of the substrate before LAM process. The first layer (N1) of LAMZ is clearly indicated by the layer band. The layer bands (indicated by white arrows in Fig. 5(a)) regularly distribute in the whole LAMZ, which form just below every layer or melting pool. A few authors have studied the formation mechanism of layer band and Liu's theory seems more persuasive [12,33]. He thought that the high homogenization in composition and sudden change of temperature gradient in the layer band region were the reasons for its formation in LAM TC18 titanium alloy. The first layer band, which exists just below the nominal interface, can be considered as the true interface between the LAMZ and the HAZ. Fig. 5(b) shows the microstructure of the N1 layer. Microstructures of the near and far regions from the true interface in the HAZ are shown in Fig. 5 (c) and (d), respectively. They are all basket-wave microstructures consisting of superfine lamellar α and β, which are clearly distinguishable from that of the WSZ. In comparison with the nominal basket-wave microstructure in LAMZ (Fig. 4(f)), the α platelets of the N1 layer and the HAZ are a little finer, indicating a faster cooling rate. At the beginning stage of LAM, the temperature of the substrate is low and a rapid cooling rate can be obtained. With the proceeding of LAM process, much heat of the melting pool dissipates into substrate, making the temperature of WSZ higher and higher. The higher temperature substrate makes the cooling rate a little slower than that of the beginning stage. Therefore finer α lamellas are obtained in the N1 and HAZ than that in the main body of LAMZ. In Fig. 5, an extremely important character must be mentioned here is that no transition zone or gradient change exists between LAMZ and HAZ. The microstructure of the majority of HAZ is homogeneous and no recrystallisation or grain growth is found. This is completely different from the microstructures obtained by the typical welding techniques, in which obvious grain growth in the HAZ were reported [25–27]. The reason will be analyzed later in the discussion part. However, an obvious gradual change in microstructure occurs in the very narrow region (about 200 μm) between the HAZ and the WSZ (position D in Fig. 5(a)). The microstructures in this transition zone (TZ) are shown in Fig. 6. Instead of widmanstätten or basket-wave microstructure, the transition zone consists of a special bimodal microstructure with fork-like primary α (αp) and superfine β transformed microstructure (lamellar secondary α (αs) and retained β). The volume fraction of primary α increases gradually from the pure HAZ to the pure WSZ (from positions b to f in Fig. 6(a)) and the volume fraction of β transformed microstructure changes on the opposite. In the nearer HAZ zone, the superfine lamellar αs þβ microstructure is almost identical with that of the HAZ except for some coarser αp (Fig. 6(b)). While it is another situation in the nearer WSZ zone, only a little β transformed microstructure can be observed due to the high content of αp (Fig. 6(e)). Microstructure in Fig. 6(f) is almost the same with that of the WSZ (Fig. 4(c)) except for a few broken and coarsened β lamellas. And the forklike morphology at the corner of αp gradually disappears when the volume fraction of αp increases to a certain extent. How does this microstructure evolve? The microstructure evolution process will be discussed in detail in the discussion part. 3.2. Micro-hardness profile Fig. 7 shows the micro-hardness profile of the hybrid fabricated TC11 sample. The micro-hardness of the LAMZ is about 50 HV higher than that of the substrate. In addition to the narrow transition zone (TZ) between WSZ and HAZ, the HAZ exhibits the same micro-hardness as the LAMZ. In the TZ the micro-
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Fig. 4. Microstructures of the wrought substrate zone (a)–(c) and laser additive zone (d)–(f).
hardness shows a sharply dropping from 385 HV to 335 HV. The characteristics of micro-hardness are substantially decided by the microstructures. As discussed above, both the HAZ (TZ exception) and the LAMZ consist of the same superfine basket-wave microstructure, which is much finer and has more α/β interfaces than widmanstätten microstructure in the WSZ. Another reason is that the solid solubility in LAMZ is higher than that in the WSZ due to the rapid solidification effect and the effect of solution strengthening is a little stronger. Therefore a higher micro-hardness in LAMZ is obtained. In the transition zone, the volume fraction of superfine lamellar β transformed microstructure decreases drastically from the pure HAZ to the pure WSZ, resulting in a sharp decline in micro-hardness. 3.3. Room tensile mechanical properties Room temperature tensile properties of the three zones of the hybrid fabricated TC11 sample are shown in Table 1. Fig. 8 shows the typical engineering stress–strain curves for different samples. Both the strength and the ductility of the LAM sample are superior to those of the wrought substrate sample. The tensile strength of the hybrid sample falls between those of the LAM sample and the wrought sample. However, the ductility of the hybrid sample is a little worse than that of the wrought sample. All the hybrid
samples fracture at the substrate zone, far away from the HAZ (as depicted in Fig. 9), which indicates that the bonding zone has a higher strength than that of the wrought substrate. Fig. 10 shows the fracture surface of the tensile specimen of L and W samples. The fracture surface of Wþ L sample is not given here because they exhibit similar characteristics with that of the W samples. The fracture surface of L sample consists of fibrous zone and shear rupture zone, but no obvious shear rupture zone exists in the fracture surface of W sample. Magnification image shows a dimpled morphology in the L sample and mixed dimple and cleavage mode in the W sample, indicating a better ductility in the L sample. Generally accepted, in contrast to bimodal or equiaxed microstructures, the widmanstätten microstructure exhibits the worst ductility because of its coarser grain size, larger αþβ colony and larger width of α platelets. The laser additive manufacturing zone consists of superfine basket-wave microstructure which has more α/β interfaces to hinder the dislocation slipping and reduce the slipping distance during tensile test, leading to a higher strength than that of substrate. The reason that the hybrid sample (W þL) exhibits the least elongation can be explained by the mechanical behaviors as follows. In the tensile test of the Wþ L sample, when the tensile stress increases to the yield strength of the W sample (912 710 MPa), the W half begins to yield and obvious plastic
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Fig. 5. Microstructures in the bonding region (a) overview; (b)–(d) high magnification microstructures of the position A, B and C in (a), respectively.
Fig. 6. Microstructure in the transition zone between HAZ and WSZ: (a) overview (b)–(f) microstructures of the corresponding positions b–f in (a).
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deformation occurs. However, at the moment, no obvious deformation occurs in the L half because the tensile stress has not reached its yield strength (1016 712 MPa). When the stress increases to the yield strength of L sample, plastic deformation begins in the L half of the Wþ L sample. Unfortunately, soon the stress exceeds the tensile strength of the W sample and fracture happens in the W half of the Wþ L sample. At this time, only little plastic deformation has occurred in the L half. Therefore the half W part (35.5 mm length) contributes the majority of the total elongation of the Wþ L sample, which is inevitably less than that obtained by total length tensile specimen (standard 71 mm length). In general, the bonding zone of the hybrid fabricated TC11 titanium alloy sample has a higher strength and less elongation than those of the wrought substrate. To some extent, the β wrought substrate has limited the final mechanical properties of the hybrid fabricated TC11 sample. On one hand, by post-heat treatment, the comprehensive mechanical properties of the hybrid fabricated sample may be further increased. On the other hand, as we all know the αþβ forged sample has better comprehensive mechanical properties than the β wrought sample [4,31]. If the primary consideration of the microstructure homogeneity of final component was ignored and the αþβ forged sample was used as substrate, it can be supposed that the mechanical properties of the hybrid fabricated TC11 titanium alloy sample should be improved. Further studies can be executed deeply in these directions.
morphology of αp is due to preferentially growth of αs from the corner of αp when the laser melting deposited TC4 titanium alloy is annealed in the upper α þβ region [34]. Therefore it is very forceful to draw a conclusion that the transition zone has suffered αþβ annealing treatment and the temperature drops gradually from the pure HAZ to the pure WSZ during the LAM process. This may be caused by the thermal treatment of the moving melting pool. As for the hybrid fabricating process, the microstructure evolution in the HAZ and the transition zone is schematically illustrated in Fig. 11. In this sketch map, the transition zone (αþβ region in Fig. 11) is amplified artificially for sake of clear explanation, which is just 200 μm width in true. The true width of the β region is about 1800 μm, which is nine times of the αþβ region. When the laser begins scanning on the substrate, the high temperature and high energy melting pool rapidly heats the substrate zone beneath it to a high level, which would inevitably result in a considerable microstructure change in this zone (HAZ). Specifically, in the region near the melting pool where the peak
3.4. Discussion From the above results, the microstructure in the HAZ is different from the previous reports obtained by typical welding process. The formation mechanism is discussed in this part. As is known to us all, when αþβ and near α titanium alloys are heated to high temperature region, the low temperature stable phase α will transform to high temperature stable phase β. Typical bimodal microstructure can be obtained by subtransus annealing in the upper αþβ region. The higher the annealing temperature is, the lower the content of primary α is [31]. Lu found that the folk-like
Fig. 7. Micro-hardness profile of the hybrid fabricating TC11 titanium alloy sample.
Fig. 8. Room temperature tensile engineering strain–stress curves for different samples.
Fig. 9. Hybrid samples (Wþ L) after tensile test showing the fracture position.
Table 1 Room temperature tensile properties of the hybrid fabricating TC11 titanium alloy. Samples
UTS, MPa
YTS, MPa
EL, %
RA, %
Hybrid (Wþ L) Laser additive manufacturing (L) Wrought substrate (W)
10337 13 1103 7 12 982 7 10
9757 6 10167 12 9127 10
6.8 70.2 14.0 71.1 8.9 72.7
16.0 7 2.8 28.7 7 1.9 18.7 7 2.1
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Fig. 10. Fracture surface of the tensile specimen (a) and (b) overview and magnification image of the L specimen; (c) and (d) overview and magnification image of the W specimen.
Fig. 11. Schematic illustration of the microstructure evolution in the HAZ during LAM process.
temperature is above Tβ (β region), the coarse widmanstätten microstructure would all transforms to single β phase. While in the region (αþβ region) where the peak temperature is below Tβ, the widmanstätten microstructure transforms to a mixture of rodlike retained α (αp) and β phase. The lower the temperature is, the more the content of retained αp is. When the temperature drops to a certain degree in the region far from the melting pool, it has little effect on the WSZ and the coarse widmanstätten microstructure remains. When the laser moves away, the heat energy in the melting pool and HAZ dissipates to the surrounding substrate rapidly and the temperature drops drastically. In the single β region including the melting pool zone and β region of the HAZ, numerous secondary α (αs) nucleate and grow fast due to the rapid
cooling rate and large undercooling degree. Eventually the fine basket-wave microstructure forms. This is the reason that no transition zone exists between the N1 and the HAZ. In the αþβ region, the supersaturated β phase transforms into ultrafine lamellar αs þ β microstructure during rapid cooling process, meanwhile the retained rod-like αp grow and coarsen into fork-like primary αp. Finally the α þβ region transforms into bimodal microstructure consisting of fork-like αp and superfine lamellar β transformed microstructure. When the volume fraction of αp increases, the fork-like morphology is difficult to obtain due to the relatively small proportion of β phase [34]. Therefore, no forklike morphology at the end of αp is observed in the α þβ region nearer the WSZ. From the above discussing, the HAZ has also suffered heat treatment in single β phase field just like that in typical welding process. Then why does not grain growth occur? First of all, in the previous studies almost all the TC11 base metal are consist of fine equiaxed grains, which are manufactured by typical forging process in αþβ phase field [25–27]. However in this paper, the substrate is fabricated by forging process in single β phase field. For most αþβ titanium alloys, β forged sample has the best thermal stability [31,35]. Short time treatment in the β region has little effect on the grain morphology. Secondly, the thickness of the wrought substrate is larger than those in the previous researches. Therefore the heat energy in the HAZ could dissipate to the substrate body more fast, which can reduce the dwelling time efficiently in the peak temperature.
4. Conclusions (1) Hybrid fabricated TC11 sample consists of three distinguished zones: the laser additive manufacturing zone, the wrought substrate zone and the bonding zone. There is a good metallurgical bonding between the LAM zone and substrate.
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(2) Superfine basket-wave microstructure within coarse prior β grains forms in the laser additive manufacturing zone, resulting in superior tensile properties. (3) No grain growth is found in the HAZ. A special bimodal microstructure with coarser fork-like primary α and finer β transformed microstructure forms in the transition zone of the HAZ. Micro-hardness in the transition zone increases noticeably from the WSZ to the LAMZ. (4) The tensile strength and elongation of hybrid sample are 1033713 MPa and 6.870.2%, respectively. The tensile strength falls between the higher strength LAM sample and the lower strength wrought sample.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and mechanical properties of hybrid fabricated Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy by laser additive manufacturing Yanyan Zhu a, Jia Li a,b, Xiangjun Tian a,b, Huaming Wang a,b, Dong Liu a,b,n a
Laboratory of Laser Materials Processing and Manufacturing, Beihang University, 37 Xueyuan Road, Beijing 100191, China Engineering Research Center of Ministry of Education on Laser Direct Manufacturing for Large Metallic Components, Beihang University, 37 Xueyuan Road, Beijing 100191, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 November 2013 Received in revised form 19 February 2014 Accepted 5 April 2014 Available online 13 April 2014
The hybrid fabricating technique by laser additive manufacturing provides an attractive potential for manufacturing titanium alloy components. Microstructure, micro-hardness and room tensile mechanical properties of hybrid fabricated TC11 titanium alloy sample were examined. Results show that the hybrid manufactured sample consists of three typical zones: the laser additive manufactured zone (LAMZ), the wrought substrate zone (WSZ), and the bonding zone without any metallurgical defects. Superfine basket-wave microstructure forms in LAMZ and heat affected zone (HAZ) due to the rapid cooling rate. No obvious grain growth or recrystallization occurs in the HAZ. A special bimodal microstructure consisting of coarse fork-like primary α and fine β transformed microstructure is found in the transition zone due to the heat effect in α þβ region. The hybrid fabricated TC11 sample has good mechanical properties with tensile strength of 1033713 MPa and elongation of 6.87 0.2%. The fracture of hybrid sample occurs in the substrate in tensile testing, meaning that the bonding zone has better mechanical properties than the substrate. & 2014 Elsevier B.V. All rights reserved.
Keywords: Laser additive manufacturing Titanium alloy Microstructure Heat affected zone Mechanical properties
1. Introduction Due to their high specific strength and excellent corrosion resistance, titanium alloys have found many applications in aeronautical, petrochemical and biomedical industries in recent years [1–3]. A typical αþβ titanium alloy, Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (named TC11 in China and BT9 in Russia), has excellent comprehensive mechanical properties at room and high temperatures. It has been extensively used in the key structural components of aircraft engines like compressor disk and blade [4–6]. The mechanical properties of this alloy are sensitively dependent on its microstructures, which are dominantly decided by hot working processes and post-heat treatments. At present, aerospace structural TC11 components are mostly fabricated by conventional costly wrought base processes at a high buy to fly ratio and low material utilization ratio [4,7–9]. Laser additive manufacturing is one of the new techniques that fabricate full density near net shape metal components directly
n Corresponding author at: Laboratory of Laser Materials Processing and Manufacturing, Beihang University, 37 Xueyuan Road, Beijing 100191, China. Tel.: þ 86 10 8233 9691; fax: þ86 10 8233 8131. E-mail addresses: [email protected] (Y. Zhu), [email protected] (D. Liu).
http://dx.doi.org/10.1016/j.msea.2014.04.019 0921-5093/& 2014 Elsevier B.V. All rights reserved.
from metal powders [10]. During LAM process, fine metal powders are fed into a molten pool produced by a sharply-focused and high energy laser beam. The working table moves along the pre-set trajectory, then three dimensional components are fabricated layer by layer on the substrate [11–13]. In comparison to conventional wrought base techniques, the LAM process has some remarkable advantages: reduction of production cycle and cost, high material utilization ratio and excellent flexibility. It has a great potential for the manufacturing of large complex metal components especially for the difficult to process materials like titanium alloys [11–15]. Moreover, owing to the fine rapid solidification microstructures, the LAM titanium alloys have excellent mechanical properties comparable or even superior to the wrought materials [15–18]. Generally, the conventional wrought base processes are suitable for manufacturing regular bulk parts, while the LAM technique has superior flexibility of fabricating complex parts [15,18]. When manufacturing large and complex parts, the advantages of the two techniques can be well combined by adding special features to local areas of regular wrought base by the LAM technique [19–22]. During this process, well designed wrought substrate is chose and retained as part of the final component after the LAM process finishing, which is different from the previous reported typical LAM process in which the simple substrate is just a supporter and would be removed [10–14]. This is a special fabrication process
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based on LAM, named the hybrid fabrication (HF) technique [21]. Just like the fabrication of the aero-engine TC11 titanium alloy blisk (integrally bladed disk), the HF technique can be done as follows: a disk is forged first and then blades are added at the rim of the disk by LAM process in which the forged disk is used as substrate and retained. There is no doubt that the HF technique can further increase the material utilization ratio and reduce the cost. It also has a great potential for the manufacturing of dual- or multi-functional components [21]. The hybrid fabrication technique by LAM seems like an advanced form of laser repairing or laser welding. One common concern of the hybrid fabricating technique is the mechanical property of the final part, which is dominantly dependent on the microstructure of the final part especially in the joint zone (including the heat affected zone) [21,23–25]. Richter reported that the laser cladding Ti6242 titanium alloy had good mechanical properties. The laser cladding process they used was quite similar to the HF technique in this paper [20]. Wang studied the microstructure and mechanical properties of hybrid fabricated stainless steel [21]. Wang and Tan found that obvious grain growth occurred in the HAZ of the parent metal during the welding process of TC11 titanium alloy, which deteriorated the mechanical properties of the welded samples [25,26]. However, most of the previous reports about TC11 titanium alloy were focused on the wrought or welding processes [25–29]. The investigations of LAM TC11 titanium alloy were very limited [30]. Therefore, it is very urgent and necessary to investigate the microstructure and mechanical properties of hybrid fabricating TC11 titanium alloy. In order to obtain a deep understanding of the hybrid fabricating process of TC11 titanium alloy, microstructure, micro-hardness and room temperature tensile properties of the hybrid fabricated sample were investigated in this paper. The microstructure evolution in the heat affected zone during the hybrid fabricating process was discussed.
Fig. 1. Schematic illustration of the hybrid fabricating process by laser additive manufacturing.
2. Experiments Considering the microstructure homogeneity of the final component, typical β forged TC11 sample was chosen as the substrate because the lamellar microstructure is closer to the LAM microstructure in comparison with equiaxed or bi-model microstructure. The geometry size of the substrate is 200 mm 40 mm 80 mm and the surface of the substrate was sandblasted before LAM. The LAM process was carried out on the in-house developed LAM system equipped with a GS-TFL-8000 CO2 laser (maximum output power 8 kW), a BSF-2 power feeder together with a co-axial powder delivery nozzle, a HNC-21M computer numerical control (CNC) multi-axis motion system, and an argon purged processing chamber with oxygen content less than 100 ppm. The processing parameters were as follows: laser beam power 6 kW, scanning speed 800-1200 mm/min, beam diameter 4–6 mm, powder delivery rate 500-1000 g/h, overlap ratio 30–50%. The scanning path was back and forth mode, meaning that the scanning directions of adjacent tracks were opposite. A schematic illustration of the LAM process for hybrid fabricating was shown in Fig. 1. Under these parameters, a TC11 plate was fabricated with the same size as the substrate in a track-by-track and layer-by-layer way, as shown in Fig. 2. The newly hybrid fabricated sample was tempered at 530 1C for 4 h in order to eliminate the residual stress. Metallographic samples were prepared using standard metallurgical procedures. A mixture solution of HF: HNO3: H2O with a ratio of 1:6:43 was used as etching agent. Then microstructures of different zones of hybrid fabricated TC11 sample were characterized by optical microscopy (OM) and scanning electron microscopy (SEM). Micro-hardness of the hybrid fabricating sample was
Fig. 2. The hybrid fabricated TC11 bulk and the three kinds of specimens for room tensile test.
measured by using HAZ-1000 Vikers tester with 500 g load and 10 s dwelling time. For easier comparison, room temperature tensile samples of different zones from the same hybrid fabricating TC11 plate were prepared: the pure LAM sample (L), the pure wrought sample (W) and the hybrid sample with the bonding interface in the middle (WþL), as depicted in Fig. 2. The principle axis of the specimens was parallel to the deposition direction and vertical to the texture direction. The tensile test was carried out according to the test standard of ISO 6892:1998 with 25 mm gauge length and 71 mm total length specimen. Three samples of each zone were tested for an average.
3. Results and discussion The aim of this paper is to investigate the microstructure and mechanical properties of hybrid fabricated TC11 titanium alloy. Firstly, the microstructures of the laser additive manufacturing zone and wrought substrate zone were given. Secondly, the microstructure in the heat affected zone and the formation mechanism were studied. Finally, micro-hardness and room temperature tensile properties were tested and analyzed.
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Fig. 3. Macrostructure of the hybrid fabricating TC11 sample consisting of three different zones.
3.1. Microstructure characterization The macrostructure of the hybrid fabricated TC11 sample is shown in Fig. 3. The deposition direction is perpendicular to the texture direction of the wrought substrate. Three obviously different zones with different grain morphologies exist in the sample: the laser additive manufacturing zone (LAMZ) with columnar grains along the deposition direction, the wrought substrate zone (WSZ) with coarse elongated grains along the texture direction and the heat affected zone (HAZ). There is a good metallurgical bonding between the LAMZ and the WSZ. Typical metallurgical defects such as gas porosities, lack of fusion porosities or cracks are not found in the bonding zone. The HAZ has the same contrast with the LAMZ but the same grain size and morphology with the WSZ. The width of HAZ is about 1.5–2 mm, which is a little narrower than that in the electron beam welded TC11 sample [25]. Microstructures of the substrate zone and laser additive manufacturing zone are shown in Fig. 4. In WSZ, typical coarse widmanstätten microstructure exists within the coarse elongated grains. The average grain size can be several millimeters. The width of lamellar α is approximately 1.4 μm and the volume fraction of α phase is about 84%. This microstructure can be obtained easily in the final steps of the processing route by an annealing treatment in the β phase field with a subsequent relatively slow cooling rate [31]. As for the LAMZ, superfine basket-wave microstructure forms within the columnar grains due to the high temperature gradient and the rapid cooling rate in the LAM process. Moreover, owing to the character of intense epitaxial growth of titanium alloys and the large temperature gradient, the columnar grains can grow consecutively through many layers (several millimeters). However, the average width of these columnar grains is just about 100–300 μm. This is a common phenomenon in laser additive manufactured titanium alloys except for some particular conditions [17,32]. The width of the lamellar α is only about 0.4 μm, which is much finer than that of the WSZ. The volume fraction of α phase is about 77%, also a little lower than that of the WSZ.
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Further observation of the bonding region is shown in Fig. 5. The nominal interface between the WSZ and LAMZ is marked as white dot line in Fig. 5(a), which is the original surface of the substrate before LAM process. The first layer (N1) of LAMZ is clearly indicated by the layer band. The layer bands (indicated by white arrows in Fig. 5(a)) regularly distribute in the whole LAMZ, which form just below every layer or melting pool. A few authors have studied the formation mechanism of layer band and Liu's theory seems more persuasive [12,33]. He thought that the high homogenization in composition and sudden change of temperature gradient in the layer band region were the reasons for its formation in LAM TC18 titanium alloy. The first layer band, which exists just below the nominal interface, can be considered as the true interface between the LAMZ and the HAZ. Fig. 5(b) shows the microstructure of the N1 layer. Microstructures of the near and far regions from the true interface in the HAZ are shown in Fig. 5 (c) and (d), respectively. They are all basket-wave microstructures consisting of superfine lamellar α and β, which are clearly distinguishable from that of the WSZ. In comparison with the nominal basket-wave microstructure in LAMZ (Fig. 4(f)), the α platelets of the N1 layer and the HAZ are a little finer, indicating a faster cooling rate. At the beginning stage of LAM, the temperature of the substrate is low and a rapid cooling rate can be obtained. With the proceeding of LAM process, much heat of the melting pool dissipates into substrate, making the temperature of WSZ higher and higher. The higher temperature substrate makes the cooling rate a little slower than that of the beginning stage. Therefore finer α lamellas are obtained in the N1 and HAZ than that in the main body of LAMZ. In Fig. 5, an extremely important character must be mentioned here is that no transition zone or gradient change exists between LAMZ and HAZ. The microstructure of the majority of HAZ is homogeneous and no recrystallisation or grain growth is found. This is completely different from the microstructures obtained by the typical welding techniques, in which obvious grain growth in the HAZ were reported [25–27]. The reason will be analyzed later in the discussion part. However, an obvious gradual change in microstructure occurs in the very narrow region (about 200 μm) between the HAZ and the WSZ (position D in Fig. 5(a)). The microstructures in this transition zone (TZ) are shown in Fig. 6. Instead of widmanstätten or basket-wave microstructure, the transition zone consists of a special bimodal microstructure with fork-like primary α (αp) and superfine β transformed microstructure (lamellar secondary α (αs) and retained β). The volume fraction of primary α increases gradually from the pure HAZ to the pure WSZ (from positions b to f in Fig. 6(a)) and the volume fraction of β transformed microstructure changes on the opposite. In the nearer HAZ zone, the superfine lamellar αs þβ microstructure is almost identical with that of the HAZ except for some coarser αp (Fig. 6(b)). While it is another situation in the nearer WSZ zone, only a little β transformed microstructure can be observed due to the high content of αp (Fig. 6(e)). Microstructure in Fig. 6(f) is almost the same with that of the WSZ (Fig. 4(c)) except for a few broken and coarsened β lamellas. And the forklike morphology at the corner of αp gradually disappears when the volume fraction of αp increases to a certain extent. How does this microstructure evolve? The microstructure evolution process will be discussed in detail in the discussion part. 3.2. Micro-hardness profile Fig. 7 shows the micro-hardness profile of the hybrid fabricated TC11 sample. The micro-hardness of the LAMZ is about 50 HV higher than that of the substrate. In addition to the narrow transition zone (TZ) between WSZ and HAZ, the HAZ exhibits the same micro-hardness as the LAMZ. In the TZ the micro-
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Fig. 4. Microstructures of the wrought substrate zone (a)–(c) and laser additive zone (d)–(f).
hardness shows a sharply dropping from 385 HV to 335 HV. The characteristics of micro-hardness are substantially decided by the microstructures. As discussed above, both the HAZ (TZ exception) and the LAMZ consist of the same superfine basket-wave microstructure, which is much finer and has more α/β interfaces than widmanstätten microstructure in the WSZ. Another reason is that the solid solubility in LAMZ is higher than that in the WSZ due to the rapid solidification effect and the effect of solution strengthening is a little stronger. Therefore a higher micro-hardness in LAMZ is obtained. In the transition zone, the volume fraction of superfine lamellar β transformed microstructure decreases drastically from the pure HAZ to the pure WSZ, resulting in a sharp decline in micro-hardness. 3.3. Room tensile mechanical properties Room temperature tensile properties of the three zones of the hybrid fabricated TC11 sample are shown in Table 1. Fig. 8 shows the typical engineering stress–strain curves for different samples. Both the strength and the ductility of the LAM sample are superior to those of the wrought substrate sample. The tensile strength of the hybrid sample falls between those of the LAM sample and the wrought sample. However, the ductility of the hybrid sample is a little worse than that of the wrought sample. All the hybrid
samples fracture at the substrate zone, far away from the HAZ (as depicted in Fig. 9), which indicates that the bonding zone has a higher strength than that of the wrought substrate. Fig. 10 shows the fracture surface of the tensile specimen of L and W samples. The fracture surface of Wþ L sample is not given here because they exhibit similar characteristics with that of the W samples. The fracture surface of L sample consists of fibrous zone and shear rupture zone, but no obvious shear rupture zone exists in the fracture surface of W sample. Magnification image shows a dimpled morphology in the L sample and mixed dimple and cleavage mode in the W sample, indicating a better ductility in the L sample. Generally accepted, in contrast to bimodal or equiaxed microstructures, the widmanstätten microstructure exhibits the worst ductility because of its coarser grain size, larger αþβ colony and larger width of α platelets. The laser additive manufacturing zone consists of superfine basket-wave microstructure which has more α/β interfaces to hinder the dislocation slipping and reduce the slipping distance during tensile test, leading to a higher strength than that of substrate. The reason that the hybrid sample (W þL) exhibits the least elongation can be explained by the mechanical behaviors as follows. In the tensile test of the Wþ L sample, when the tensile stress increases to the yield strength of the W sample (912 710 MPa), the W half begins to yield and obvious plastic
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Fig. 5. Microstructures in the bonding region (a) overview; (b)–(d) high magnification microstructures of the position A, B and C in (a), respectively.
Fig. 6. Microstructure in the transition zone between HAZ and WSZ: (a) overview (b)–(f) microstructures of the corresponding positions b–f in (a).
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deformation occurs. However, at the moment, no obvious deformation occurs in the L half because the tensile stress has not reached its yield strength (1016 712 MPa). When the stress increases to the yield strength of L sample, plastic deformation begins in the L half of the Wþ L sample. Unfortunately, soon the stress exceeds the tensile strength of the W sample and fracture happens in the W half of the Wþ L sample. At this time, only little plastic deformation has occurred in the L half. Therefore the half W part (35.5 mm length) contributes the majority of the total elongation of the Wþ L sample, which is inevitably less than that obtained by total length tensile specimen (standard 71 mm length). In general, the bonding zone of the hybrid fabricated TC11 titanium alloy sample has a higher strength and less elongation than those of the wrought substrate. To some extent, the β wrought substrate has limited the final mechanical properties of the hybrid fabricated TC11 sample. On one hand, by post-heat treatment, the comprehensive mechanical properties of the hybrid fabricated sample may be further increased. On the other hand, as we all know the αþβ forged sample has better comprehensive mechanical properties than the β wrought sample [4,31]. If the primary consideration of the microstructure homogeneity of final component was ignored and the αþβ forged sample was used as substrate, it can be supposed that the mechanical properties of the hybrid fabricated TC11 titanium alloy sample should be improved. Further studies can be executed deeply in these directions.
morphology of αp is due to preferentially growth of αs from the corner of αp when the laser melting deposited TC4 titanium alloy is annealed in the upper α þβ region [34]. Therefore it is very forceful to draw a conclusion that the transition zone has suffered αþβ annealing treatment and the temperature drops gradually from the pure HAZ to the pure WSZ during the LAM process. This may be caused by the thermal treatment of the moving melting pool. As for the hybrid fabricating process, the microstructure evolution in the HAZ and the transition zone is schematically illustrated in Fig. 11. In this sketch map, the transition zone (αþβ region in Fig. 11) is amplified artificially for sake of clear explanation, which is just 200 μm width in true. The true width of the β region is about 1800 μm, which is nine times of the αþβ region. When the laser begins scanning on the substrate, the high temperature and high energy melting pool rapidly heats the substrate zone beneath it to a high level, which would inevitably result in a considerable microstructure change in this zone (HAZ). Specifically, in the region near the melting pool where the peak
3.4. Discussion From the above results, the microstructure in the HAZ is different from the previous reports obtained by typical welding process. The formation mechanism is discussed in this part. As is known to us all, when αþβ and near α titanium alloys are heated to high temperature region, the low temperature stable phase α will transform to high temperature stable phase β. Typical bimodal microstructure can be obtained by subtransus annealing in the upper αþβ region. The higher the annealing temperature is, the lower the content of primary α is [31]. Lu found that the folk-like
Fig. 7. Micro-hardness profile of the hybrid fabricating TC11 titanium alloy sample.
Fig. 8. Room temperature tensile engineering strain–stress curves for different samples.
Fig. 9. Hybrid samples (Wþ L) after tensile test showing the fracture position.
Table 1 Room temperature tensile properties of the hybrid fabricating TC11 titanium alloy. Samples
UTS, MPa
YTS, MPa
EL, %
RA, %
Hybrid (Wþ L) Laser additive manufacturing (L) Wrought substrate (W)
10337 13 1103 7 12 982 7 10
9757 6 10167 12 9127 10
6.8 70.2 14.0 71.1 8.9 72.7
16.0 7 2.8 28.7 7 1.9 18.7 7 2.1
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Fig. 10. Fracture surface of the tensile specimen (a) and (b) overview and magnification image of the L specimen; (c) and (d) overview and magnification image of the W specimen.
Fig. 11. Schematic illustration of the microstructure evolution in the HAZ during LAM process.
temperature is above Tβ (β region), the coarse widmanstätten microstructure would all transforms to single β phase. While in the region (αþβ region) where the peak temperature is below Tβ, the widmanstätten microstructure transforms to a mixture of rodlike retained α (αp) and β phase. The lower the temperature is, the more the content of retained αp is. When the temperature drops to a certain degree in the region far from the melting pool, it has little effect on the WSZ and the coarse widmanstätten microstructure remains. When the laser moves away, the heat energy in the melting pool and HAZ dissipates to the surrounding substrate rapidly and the temperature drops drastically. In the single β region including the melting pool zone and β region of the HAZ, numerous secondary α (αs) nucleate and grow fast due to the rapid
cooling rate and large undercooling degree. Eventually the fine basket-wave microstructure forms. This is the reason that no transition zone exists between the N1 and the HAZ. In the αþβ region, the supersaturated β phase transforms into ultrafine lamellar αs þ β microstructure during rapid cooling process, meanwhile the retained rod-like αp grow and coarsen into fork-like primary αp. Finally the α þβ region transforms into bimodal microstructure consisting of fork-like αp and superfine lamellar β transformed microstructure. When the volume fraction of αp increases, the fork-like morphology is difficult to obtain due to the relatively small proportion of β phase [34]. Therefore, no forklike morphology at the end of αp is observed in the α þβ region nearer the WSZ. From the above discussing, the HAZ has also suffered heat treatment in single β phase field just like that in typical welding process. Then why does not grain growth occur? First of all, in the previous studies almost all the TC11 base metal are consist of fine equiaxed grains, which are manufactured by typical forging process in αþβ phase field [25–27]. However in this paper, the substrate is fabricated by forging process in single β phase field. For most αþβ titanium alloys, β forged sample has the best thermal stability [31,35]. Short time treatment in the β region has little effect on the grain morphology. Secondly, the thickness of the wrought substrate is larger than those in the previous researches. Therefore the heat energy in the HAZ could dissipate to the substrate body more fast, which can reduce the dwelling time efficiently in the peak temperature.
4. Conclusions (1) Hybrid fabricated TC11 sample consists of three distinguished zones: the laser additive manufacturing zone, the wrought substrate zone and the bonding zone. There is a good metallurgical bonding between the LAM zone and substrate.
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(2) Superfine basket-wave microstructure within coarse prior β grains forms in the laser additive manufacturing zone, resulting in superior tensile properties. (3) No grain growth is found in the HAZ. A special bimodal microstructure with coarser fork-like primary α and finer β transformed microstructure forms in the transition zone of the HAZ. Micro-hardness in the transition zone increases noticeably from the WSZ to the LAMZ. (4) The tensile strength and elongation of hybrid sample are 1033713 MPa and 6.870.2%, respectively. The tensile strength falls between the higher strength LAM sample and the lower strength wrought sample.
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