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The influence of heat input on microstructure and porosity during laser cladding of Invar alloy
Optics and Laser Technology 113 (2019) 453–461
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
The influence of heat input on microstructure and porosity during laser cladding of Invar alloy
T
⁎
Xiaohong Zhana, , Chaoqi Qia, Zhuanni Gaoa, Deyong Tiana, Zhengdong Wangb a b
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China Jiangxi Changxing Aviation Equipment Co., Ltd, Jingdezhen 333000, China
H I GH L IG H T S
microstructure evolution of laser cladded Invar alloy was conducted. • The is a dimension discrepancy at different region near the layer lines. • There porosity types and formation were investigated. • The • This paper shows different microstructure and porosity under different heat input.
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser cladding Invar alloy Heat input Porosity Microstructure
The Invar alloy coatings were prepared using laser cladding technology. The effects of heat input on the coatings size, microstructure and porosity of laser cladding layers were investigated by optical microscope (OM), scanning electron microscope (SEM) and electronic differential system (EDS). In this analysis, the microstructure of coatings consist of elongate cellular crystals and equiaxed cellular crystals. The results showed that the size, HAZ and crystals size of the samples clearly show a noteworthy increase by improving the heat input from 81.4 to 230 J/mm2. Meanwhile, an interesting phenomenon was discussed that the subgrain size near the layer line were extraordinarily distinct from the normal. The pores in the coatings are considered to be divided into two categories. The one is fusion lack porosity which is caused by the unfinished powders and rapid solidification. The other one, bubble porosity, is composed of trapped shielding gas and metal vapor. It is demonstrated that the porosity factors increase until a threshold heat input is reached and then decrease as the heat input increases. In this paper, the heat input of 126 J/mm2 is more suitable to be applied in laser cladded Invar alloy.
1. Introduction Fiber-reinforced thermoplastics (FRTP) have been widely and successfully used in the advanced aerospace field due to the high strength and low proportion [1–3]. Molding is required during the FRTP forming. However, the mold produced using the traditional materials, such as carbon steel and aluminum alloy, cannot satisfy the demand of high surface quality and high dimensional precision. The FRTP mold prepared with Invar alloy has attracted increasing attention in recent years, owing to the excellent mechanical properties in cryogenic environment and similar coefficient of thermal expansion with FRTP [4–8]. The FRTP is prepared by the autoclave technology which makes the Invar molds withstand alternating thermal load. After the molds fails, which is caused by the thermal load, they need to be repaired.
⁎
However, the high-quality molds repaired by the traditional overlaying processes are technically difficult. Through rapidly melting and solidifying metal powders on the substrate, the laser cladding is an especially attractive candidate technology in repairing the deactivated Invar molds with high density, accuracy and excellent performance [9,10]. Recently, considerable improvements on laser cladding process have been reported where process parameters, properties, microstructure, defects and numerical simulation were discussed [11–13]. Costa [14] presented a thermo-kinetic laser cladding model coupling finite element heat transfer calculations with transformation kinetics and quantitative property-structure relationships to study the cyclic thermal fluctuations. According to [15], the microstructure and properties of the laser cladded Inconel 625 superalloy were investigated, which were free from relevant defects such as cracks, bonding error and porosity.
Corresponding author. E-mail address: [email protected] (X. Zhan).
https://doi.org/10.1016/j.optlastec.2019.01.015 Received 10 November 2017; Received in revised form 6 November 2018; Accepted 8 January 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 113 (2019) 453–461
X. Zhan et al.
Table 1 Chemical compositions of Invar alloy substrate and deposited powder. wt%
Ni
C
Si
Mn
P
S
Cr
Co
Fe
Substrate Powder
35.5–36.5 35.92
≤0.01 0.025
≤0.2 0.07
0.2–0.6 0.27
≤0.007 0.0087
≤0.002 0.001
≤0.15 0.20
≤0.4 0.47
Bal Bal.
Fig. 1. Experimental principle and equipment of coaxial LMD experiments: (a) experimental principle; (b) fiber laser; (c) water-cooling machine; (d) XSL-ST powder feeder; (e) KUKA robot. Table 2 The experiment parameters of laser cladded Invar ally. Case
Laser power W)
Scanning speed (m/min)
Powder feeding rate (g/min)
Table 3 Heat input and coatings size of different experiment parameters. Beam diameter (mm)
Case 2
1 2 3
1900 2100 2300
7 5 3
7 7 7
Heat input (J/mm ) The height of coatings (mm) The width of coatings (mm) HAZ width of coatings (μm)
2 2 2
1
2
3
81.4 0.7 4.5 388
126 1.3 5.4 511
230 1.6 5.8 628
subsequent research, the influence of the process parameters on the Invar 36 using the selective laser melting process was discussed by measuring the density, microstructures, and surface features [18]. Zhan et al. had a comprehensive research on the CET simulation, thermal field, subgrain and element distribution of laser cladded Invar alloy [19–22]. Nevertheless, the structural integrity (defects such as porosity
However, the report on laser cladding of Invar alloy is lacking although there are a couple of reports on selective laser melt of Invar alloy. Qiu et al. [16] investigated the microstructure and properties of selective laser-melted Invar36. Based on the research foundation of the Qiu, Harrison et al. [17] discovered that the unique low thermal expansion property of Invar is retained after LMD processing. In
Fig. 2. Macroscopic appearance of surface and cross section of Invar alloy by LMD: (a) case 1, surface formation; (b) case 2, surface formation; (c) case 3, surface formation; (d) case 1, cross section; (e) case 2, cross section; (f) case 3, cross section. 454
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Fig. 3. Characteristic microstructures of Invar alloy part by laser cladding: (a) microstructure of cross section; (b) microstructure at the top of coatings; (c) microstructure at the bottom of coatings; (d) microstructure of substrate.
Fig. 4. Characteristic microstructures near the layer line: (a) microstructure near the layer lines; (b) metallographic image of the localized region marked in (a).
and cracking), microstructure and heat inputs of laser cladded Invar alloy are rarely revealed. The laser cladded Invar alloy are now at the position to begin. With the previous researches, the microstructure and porosity have a fundamentally effect on the mechanical properties. In this paper, we investigated the influence of heat input on the Invar alloy during laser cladding. Particularly, the analysis of microstructure characteristics, porosity formation and chemical composition of cladding layer have been performed with the aim of investigating the microstructure and porosities of cladding layer under different heat input.
2.2. Experimental equipment
2. Experiment
2.3. Experimental methods
2.1. Experimental materials
After a number of orthogonal experiments (considering the laser power, scanning speed and powder feeding rate), the optimal forming and significant deposited specimens are selected as the research objects of this paper, whose basic parameters are given in Table 2. The oil and impurities on the surface of substrate were cleaned thoroughly with mechanical and chemical cleaning methods. Thereafter, the substrate was dried in a heat-treatment oven at 300 °C for 1 h to remove the residual chemical reagent. During the laser cladding process, the 99.999% argon gas was used as the shielding gas
The single-layer and single-pass coatings were fabricated by a laser cladding system, equipped with a XSL-ST powder feeder, KUKA robot, and a fiber laser manufactured by IPG Photonics Corporation et al, as illustrated in the schematic experimental design shown in Fig. 1(a). In particular, the high power fiber laser was used to melt the deposited powder on the substrate surface. The powder was injected by the powder feeder with a coaxial nozzle. Fig. 1(b)–(e) shows the primary experimental equipment used in this study.
Invar alloy powder was produced using high energy ball milling process with the spherical particle size between 100 and 150 μm. The powder was dried in a vacuum oven at 150 °C for 3 h to remove the absorbed water. Invar alloy was chosen as the substrate. The specimen was cut into a rectangle shape with dimensions of 40 mm 30 mm 19 mm (width length thickness). Table 1 shows the chemical compositions of substrate and deposited powder respectively. 455
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Fig. 5. Cross-section microstructure of Invar coatings with different heat input: (a) HAZ of Case 1; (b) HAZ of Case 2; (c) HAZ of Case 3; (d) elongated cellular crystals of Case 3; (e) elongated cellular crystals of Case 2; (f) elongated cellular crystals of Case 1.
speed decreased and laser power increased, with the identical beam diameter. The higher heat input can easily result in an unstable laser cladding progress and intensified convection inside the molten pool. Relative width and height of the coatings are measured in Table 3. According to Fig. 2 and Table 3, the size of the coatings clearly show a significant increase by improving the heat input from 81.4 to 230 J/ mm2. That is because the increase of heat input provides enough energy for the melting of deposited powder and substrate. However, the increments of the coating size decreases gradually with the heat input increases, which is attributed to the finite powder.
and the trail blowing to shield the molten pool from oxidation at a blow rate of 15 L/min. Furthermore, the laser beam was always kept perpendicular to the substrate surface. Cross sections and longitudinal sections of the samples, which were cut from each of the coatings for revealing the peculiar microstructure and porosity, are prepared through the mechanical polishing by a MPD2W double disc metallographic polishing machine. The specimens were etched with a solution of 30 vol% HCl, 10 vol% HNO3 and 60 vol% distilled water. The microstructure and porosity of processed samples were also characterized utilizing optical microscopy (OM) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscope (EDS).
3.2. Microstructure 3. Results and discussion Cross-sectional observations, presented in Fig. 3, present the OM morphology and microstructure of the Invar coating under a heat input of 81.4 J/mm2. As illustrated, the Invar alloy substrate is composed by austenite grains. It is obvious that the microstructure gradually changed from the fusion line to the surface, as shown in Fig. 3(a). The microstructure mainly contain relative coarse columnar grains, which grow epitaxial from the substrate perpendicular to the fusion line. Note that the columnar to equiaxed transition occurs as the distance away from the fusion line raised. Fig. 3(b) represents that the subgrains of equiaxed grains are the equiaxed cellular crystals in various sizes. The microstructure within the columnar grains consists of elongate cellular crystals with dissimilar direction, as shown in Fig. 3(c). It is noteworthy that series of layer lines, which spread roughly parallel to the fusion line, are observed. These layer lines are a result of the latent heat release in the process of crystallization, periodic change of the heat input, and uneven distribution of the chemical compositions
3.1. Macro morphology As shown in Fig. 2, the experimental parameters selected in this paper could gain access to a full fusion metallurgy of Invar alloy parts with no crack and obvious porosity. The parabolic configuration of the coatings subjects to surface tension, which maintains a stable molten pool. Meanwhile, the solidified molten pool consist entirely of liquid Invar alloy and significantly affected by heat input. Therefore, it is necessary to conduct the heat input during laser cladding process. The heat input (specific energy) is measured through
Q=
P , D×v
(1)
here P is laser power, D is beam diameter, and v is scanning speed. The Table 3 indicates a significantly elevated heat input as the scanning 456
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Fig. 6. Longitudinal section microstructures of Invar parts with different parameters: (a) longitudinal section microstructure of Case 1; (b) metallographic image of the localized region marked in (a). (c) longitudinal section microstructure of Case 2; (b) metallographic image of the localized region marked in (c). (e) longitudinal section microstructure of Case 3; (b) metallographic image of the localized region marked in (f).
[20]. Generally, the subgrain size are nearly unchanged in the same grain. However, there is an interesting phenomenon that the subgrain size at different regions near the layer line (above the line, on the line and below the line) are extremely diverse, as shown in Fig. 4. Meanwhile, the distinct brightness is seen around the layer line. The heat accumulation on the layer line is too large, resulting in the crystals of region 3 are the maximum, even melted into one. The pre-solidified crystals near the line are coarsened due to the latent heat release on the layer line, which is similar to the formation of the coarse HAZ. The region 5 can be assumed as the substrate, which is not affected by the latent heat. In contrast to region 4, the post-solidified crystals, which is
Table 4 Dendrite spacing with different heat input. Case
Heat input (J/mm2)
Dendrite spacing on cross-section (μm)
Dendrite spacing on longitudinal section (μm)
1 2 3
211 350 767
8 12.5 20
5 5.9 7.7
Fig. 7. Porosities in Invar alloy coatings: (a) bubble porosity; (b) fusion lack porosity. 457
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Fig. 8. The EDS results of LMD porosities: (a) local enlargement; (b) EDS results detected at marked point 1; (c) EDS results detected at marked point 2; (d) EDS results detected at marked point 3.
also near the line, are finer than line’s. This is attributed to that the crystals growth on the line absorbs enough heat input and result in excessive undercooling. The phenomenon decreases gradually as the molten pool solidification, so that the crystals size of region 1 returns to the origin. It is interesting to find that the layer lines are always observed in iron-nickel alloys, instead of the other alloy. The cross-section and longitudinal section microstructures of Invar parts developed under different processing conditions are provided in Figs. 5 and 6 respectively, showing the influence of heat input on microstructure of the Invar alloy coatings. A significant increase in HAZ widths from 388 μm to 628 μm is noticed with an increase in heat input from 81.4 to 230 J/mm2. Furthermore, analogous increasing is also exhibited in terms of the elongate cellular crystal size. For quantitative characterization of the variations, statistics are carried on the cellular crystal crossed by a certain line with the same length in each sample, perpendicular to the crystal growth direction. Particularly, the middle crystals are selected. The average value of dendrite spacing is measured through
λ=
L , n
caused by layer line, leads to uneven energy distribution of the coatings. 3.3. Porosities A majority of porosities can be eliminated effectively under shielding gas during laser cladding process. However, there are still a small number of residual porosities in coatings. In order to control the porosity in specimens, the porosity factor with different heat input and porosity formation have been studied. Fig. 7 shows the individual bubble porosity and fusion lack porosity within the Invar coating, respectively. It is illustrated that the size of the bubble porosity reached a diameter of 30 µm, which is much bigger than the fusion lack porosity, characterized by irregular shape on the order of microns and are uniformly distributed over the cross section. Substantially, the fusion lack porosity is the pores between unmelted powders, which cannot be completely filled with molten pool, so that the pores are characterized by irregularities. The theory has been proved by EDS results, as shown in Fig. 8(a)–(c). The results show that there is little difference between point 1 (substrate) and point 2 (fusion lack porosity), which both are purely Invar alloy. Compared with the fusion lack porosity, the bubble porosity has a more complicated formation during laser cladding. In order to reveal the formation mechanism and classification of the bubble porosity in the coatings, the chemical compositions in the porosity interface are also detected, of which the results are shown in Fig. 8. It is interesting to find a great distinction in content (Mn, Fe and Ni) between the substrate and bubble porosity interface. At the point of impact between the powder and the molten pool, the shielding gas can become sufficiently entrained in the laser cladding process. In the type of bubble porosity, shielding gas and metal vapor mingle deficiently. And the vapor is unevenly condensed on the inner wall of porosity with the rapid cooling rate, which results in the variation of atomic percent.
(2)
with L and n, respectively, being the length of the test line and the number of counted cellular crystals. It is observed that the elongate cellular crystal become considerably coarsen with the increase of heat input (Table 4). This is attributed to the growth of subgrains, which is caused by the more energy (higher heat input). Generally, the coarse crystals are formed with long heating time. Therefore, the crystals are coarse at the bottom of coatings and gradually thin as the surface of the coating is approached. However, the research in this paper is more complicated than the normal, as shown in Fig. 5(d). It is explicit that the crystals size is irrelative to the distance from fusion line, which is attributed to the above mentioned layer lines. The periodic heat input, 458
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Fig. 9. Porosity levels in cross section: (a) Case 1; (b) Case 2; (c) Case 3.
and be dragged below the surface of the molten pool. In order to get a low-porosity coatings, the heat input should be controlled.
Figs. 9 and 10 show the porosity factor of cross section and longitudinal section, respectively. The porosity factors of the three coatings are calculated as shown in Table 5, where the porosity with shortest diameter 2 μm is taken into consideration. It is observed that the porosity with a radius of 45 μm is the maximum, which is shown in Case 3 (heat input is 230 J/mm2). This is attributed that higher heat input provide more metal vapor for the porosity formation. The results show that the coating get a low-porosity when the heat input is at the value of 126 J/mm2. According to the formation of porosity, the significant factors that affect formation of pores are the appearance of entrapped gas and incompletely melted powders. Moreover, laser cladding is the extraordinary rapid solidification process, featured with high heating and cooling speeds. Therefore, the liquid molten pool exist for a shorter time and have a higher possibility of gas entrapment with the lower heat input. On the other hand, a lower heat input result in unfinished powders, the gap between which cannot be filled timely. And the fusion lack porosity is formed. Hence, it is generally put forward that a higher heat input would reduce the rate of solidification, allowing the gas bubbles to escape from the molten pool before the solidification. The conclusion will demonstrate that there is a decrease in porosity as the heat input excessively increases. However, Fig. 7 (b)-(c) show that porosity factors increases with excessive heat input, which can be explained by the Marangoni flow. As the dynamics of this flow increases with increased heat input, there is a greater chance that the gas, entrained with the powder particles, would become trapped in the flow,
4. Conclusions Invar ally coatings were produced by laser cladding with different heat input. A comprehensive research on the microstructure evolution and porosity formation mechanism was presented. Meanwhile, the effect of heat input on macro morphology, microstructure and porosity factor of specimens were investigated. The conclusions have been drawn as follows: 1. The coatings microstructure consist of equiaxed cellular crystals and elongated cellular crystals. And there is a gradual microstructure transition, the elongated cellular crystals to the equiaxed cellular crystals, from the fusion line to the surface. Note that the crystals size under high heat input was greater than under the low heat input. Analogous increasing is also exhibited in terms of the coatings size and HAZ width of the samples. 2. The pores in the laser cladded Invar alloy are composed of fusion lack porosity and bubble porosity. In particular, the liquid Invar alloy has a short time to fill the gap between unfinished powders with the rapid solidification. And the fusion lack porosity is formed. The bubble porosity consists of shielding gas and metal vapor. 3. The heat input plays a key role on the porosity of the coatings. A low 459
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Fig. 10. Porosity levels in longitudinal section: (a) Case 1; (b) Case 2; (c) Case 3. Table 5 Porosity factors with different heat input. Case
1
2
3
Cross section longitudinal section
0.64% 0.92%
0.31% 0.32%
1.21% 0.63%
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heat input provides an opportunity for melting powders deficiently and solidifying molten pool rapidly. However, the excessive heat input will increase the bubble porosity factor, which is relative to the Marangoni flow. Acknowledgement The authors gratefully acknowledge the financial support of the National Commercial Aircraft Manufacturing Technology Research Center Innovation Fund of China (COMAC-SFGS-2017-36736), the Foundation of the Graduate Innovation Center, Nanjing University of Aeronautics and Astronautics (No. kfjj20180604) and Jiangxi Changxing Aviation Equipment Co., Ltd. Project (No. SQ2013ZOC500007). References [1] W. Krenkel, F. Berndt, C/C–SiC composites for space applications and advanced
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X. Zhan et al. simulation using finite element analysis, Acta Mater. 53 (14) (2005) 3987–3999. [15] G.P. Dinda, A.K. Dasgupta, J. Mazumder, Laser aided direct metal deposition of Inconel 625 superalloy: microstructural evolution and thermal stability, Mater. Sci. Eng. A 509 (1–2) (2009) 98–104. [16] C. Qiu, N.J.E. Adkins, M.M. Attallah, Selective laser melting of Invar 36: microstructure and properties, Acta Mater. 103 (2016) 382–395. [17] N.J. Harrison, I. Todd, K. Mumtaz, Thermal expansion coefficients in Invar processed by selective laser melting, J. Mater. Sci. 52 (17) (2017) 10517–10525. [18] M. Yakout, A. Cadamuro, M.A. Elbestawi, The selection of process parameters in additive manufacturing for aerospace alloys, Int. J. Adv. Manuf. Technol. 2 (2017) 1–18.
[19] X.H. Zhan, C.Q. Qi, J.J. Zhou, Effect of heat input on the subgrains of laser melting deposited Invar alloy, Optics Laser Technol. 109 (2019) 577–583. [20] X.H. Zhan, J.J. Zhou, C.Q. Qi, The influence of heat input on the microstructure and solute segregation mechanism of invar alloy laser melting deposition process, Mater. Res. Exp. 5 (11) (2018). [21] X.H. Zhan, X. Lin, Z.N. Gao, Modeling and simulation of the columnar-to-equiaxed transition during laser melting deposition of Invar alloy, J. Alloys Comp. 755 (30) (2018) 123–134. [22] X.H. Zhan, Y. Meng, J.J. Zhou, Quantitative research on microstructure and thermal physical mechanism in laser melting deposition for Invar alloy, J. Manuf. Process. 31 (2018) 221–231.
461
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
The influence of heat input on microstructure and porosity during laser cladding of Invar alloy
T
⁎
Xiaohong Zhana, , Chaoqi Qia, Zhuanni Gaoa, Deyong Tiana, Zhengdong Wangb a b
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China Jiangxi Changxing Aviation Equipment Co., Ltd, Jingdezhen 333000, China
H I GH L IG H T S
microstructure evolution of laser cladded Invar alloy was conducted. • The is a dimension discrepancy at different region near the layer lines. • There porosity types and formation were investigated. • The • This paper shows different microstructure and porosity under different heat input.
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser cladding Invar alloy Heat input Porosity Microstructure
The Invar alloy coatings were prepared using laser cladding technology. The effects of heat input on the coatings size, microstructure and porosity of laser cladding layers were investigated by optical microscope (OM), scanning electron microscope (SEM) and electronic differential system (EDS). In this analysis, the microstructure of coatings consist of elongate cellular crystals and equiaxed cellular crystals. The results showed that the size, HAZ and crystals size of the samples clearly show a noteworthy increase by improving the heat input from 81.4 to 230 J/mm2. Meanwhile, an interesting phenomenon was discussed that the subgrain size near the layer line were extraordinarily distinct from the normal. The pores in the coatings are considered to be divided into two categories. The one is fusion lack porosity which is caused by the unfinished powders and rapid solidification. The other one, bubble porosity, is composed of trapped shielding gas and metal vapor. It is demonstrated that the porosity factors increase until a threshold heat input is reached and then decrease as the heat input increases. In this paper, the heat input of 126 J/mm2 is more suitable to be applied in laser cladded Invar alloy.
1. Introduction Fiber-reinforced thermoplastics (FRTP) have been widely and successfully used in the advanced aerospace field due to the high strength and low proportion [1–3]. Molding is required during the FRTP forming. However, the mold produced using the traditional materials, such as carbon steel and aluminum alloy, cannot satisfy the demand of high surface quality and high dimensional precision. The FRTP mold prepared with Invar alloy has attracted increasing attention in recent years, owing to the excellent mechanical properties in cryogenic environment and similar coefficient of thermal expansion with FRTP [4–8]. The FRTP is prepared by the autoclave technology which makes the Invar molds withstand alternating thermal load. After the molds fails, which is caused by the thermal load, they need to be repaired.
⁎
However, the high-quality molds repaired by the traditional overlaying processes are technically difficult. Through rapidly melting and solidifying metal powders on the substrate, the laser cladding is an especially attractive candidate technology in repairing the deactivated Invar molds with high density, accuracy and excellent performance [9,10]. Recently, considerable improvements on laser cladding process have been reported where process parameters, properties, microstructure, defects and numerical simulation were discussed [11–13]. Costa [14] presented a thermo-kinetic laser cladding model coupling finite element heat transfer calculations with transformation kinetics and quantitative property-structure relationships to study the cyclic thermal fluctuations. According to [15], the microstructure and properties of the laser cladded Inconel 625 superalloy were investigated, which were free from relevant defects such as cracks, bonding error and porosity.
Corresponding author. E-mail address: [email protected] (X. Zhan).
https://doi.org/10.1016/j.optlastec.2019.01.015 Received 10 November 2017; Received in revised form 6 November 2018; Accepted 8 January 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 113 (2019) 453–461
X. Zhan et al.
Table 1 Chemical compositions of Invar alloy substrate and deposited powder. wt%
Ni
C
Si
Mn
P
S
Cr
Co
Fe
Substrate Powder
35.5–36.5 35.92
≤0.01 0.025
≤0.2 0.07
0.2–0.6 0.27
≤0.007 0.0087
≤0.002 0.001
≤0.15 0.20
≤0.4 0.47
Bal Bal.
Fig. 1. Experimental principle and equipment of coaxial LMD experiments: (a) experimental principle; (b) fiber laser; (c) water-cooling machine; (d) XSL-ST powder feeder; (e) KUKA robot. Table 2 The experiment parameters of laser cladded Invar ally. Case
Laser power W)
Scanning speed (m/min)
Powder feeding rate (g/min)
Table 3 Heat input and coatings size of different experiment parameters. Beam diameter (mm)
Case 2
1 2 3
1900 2100 2300
7 5 3
7 7 7
Heat input (J/mm ) The height of coatings (mm) The width of coatings (mm) HAZ width of coatings (μm)
2 2 2
1
2
3
81.4 0.7 4.5 388
126 1.3 5.4 511
230 1.6 5.8 628
subsequent research, the influence of the process parameters on the Invar 36 using the selective laser melting process was discussed by measuring the density, microstructures, and surface features [18]. Zhan et al. had a comprehensive research on the CET simulation, thermal field, subgrain and element distribution of laser cladded Invar alloy [19–22]. Nevertheless, the structural integrity (defects such as porosity
However, the report on laser cladding of Invar alloy is lacking although there are a couple of reports on selective laser melt of Invar alloy. Qiu et al. [16] investigated the microstructure and properties of selective laser-melted Invar36. Based on the research foundation of the Qiu, Harrison et al. [17] discovered that the unique low thermal expansion property of Invar is retained after LMD processing. In
Fig. 2. Macroscopic appearance of surface and cross section of Invar alloy by LMD: (a) case 1, surface formation; (b) case 2, surface formation; (c) case 3, surface formation; (d) case 1, cross section; (e) case 2, cross section; (f) case 3, cross section. 454
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Fig. 3. Characteristic microstructures of Invar alloy part by laser cladding: (a) microstructure of cross section; (b) microstructure at the top of coatings; (c) microstructure at the bottom of coatings; (d) microstructure of substrate.
Fig. 4. Characteristic microstructures near the layer line: (a) microstructure near the layer lines; (b) metallographic image of the localized region marked in (a).
and cracking), microstructure and heat inputs of laser cladded Invar alloy are rarely revealed. The laser cladded Invar alloy are now at the position to begin. With the previous researches, the microstructure and porosity have a fundamentally effect on the mechanical properties. In this paper, we investigated the influence of heat input on the Invar alloy during laser cladding. Particularly, the analysis of microstructure characteristics, porosity formation and chemical composition of cladding layer have been performed with the aim of investigating the microstructure and porosities of cladding layer under different heat input.
2.2. Experimental equipment
2. Experiment
2.3. Experimental methods
2.1. Experimental materials
After a number of orthogonal experiments (considering the laser power, scanning speed and powder feeding rate), the optimal forming and significant deposited specimens are selected as the research objects of this paper, whose basic parameters are given in Table 2. The oil and impurities on the surface of substrate were cleaned thoroughly with mechanical and chemical cleaning methods. Thereafter, the substrate was dried in a heat-treatment oven at 300 °C for 1 h to remove the residual chemical reagent. During the laser cladding process, the 99.999% argon gas was used as the shielding gas
The single-layer and single-pass coatings were fabricated by a laser cladding system, equipped with a XSL-ST powder feeder, KUKA robot, and a fiber laser manufactured by IPG Photonics Corporation et al, as illustrated in the schematic experimental design shown in Fig. 1(a). In particular, the high power fiber laser was used to melt the deposited powder on the substrate surface. The powder was injected by the powder feeder with a coaxial nozzle. Fig. 1(b)–(e) shows the primary experimental equipment used in this study.
Invar alloy powder was produced using high energy ball milling process with the spherical particle size between 100 and 150 μm. The powder was dried in a vacuum oven at 150 °C for 3 h to remove the absorbed water. Invar alloy was chosen as the substrate. The specimen was cut into a rectangle shape with dimensions of 40 mm 30 mm 19 mm (width length thickness). Table 1 shows the chemical compositions of substrate and deposited powder respectively. 455
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Fig. 5. Cross-section microstructure of Invar coatings with different heat input: (a) HAZ of Case 1; (b) HAZ of Case 2; (c) HAZ of Case 3; (d) elongated cellular crystals of Case 3; (e) elongated cellular crystals of Case 2; (f) elongated cellular crystals of Case 1.
speed decreased and laser power increased, with the identical beam diameter. The higher heat input can easily result in an unstable laser cladding progress and intensified convection inside the molten pool. Relative width and height of the coatings are measured in Table 3. According to Fig. 2 and Table 3, the size of the coatings clearly show a significant increase by improving the heat input from 81.4 to 230 J/ mm2. That is because the increase of heat input provides enough energy for the melting of deposited powder and substrate. However, the increments of the coating size decreases gradually with the heat input increases, which is attributed to the finite powder.
and the trail blowing to shield the molten pool from oxidation at a blow rate of 15 L/min. Furthermore, the laser beam was always kept perpendicular to the substrate surface. Cross sections and longitudinal sections of the samples, which were cut from each of the coatings for revealing the peculiar microstructure and porosity, are prepared through the mechanical polishing by a MPD2W double disc metallographic polishing machine. The specimens were etched with a solution of 30 vol% HCl, 10 vol% HNO3 and 60 vol% distilled water. The microstructure and porosity of processed samples were also characterized utilizing optical microscopy (OM) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscope (EDS).
3.2. Microstructure 3. Results and discussion Cross-sectional observations, presented in Fig. 3, present the OM morphology and microstructure of the Invar coating under a heat input of 81.4 J/mm2. As illustrated, the Invar alloy substrate is composed by austenite grains. It is obvious that the microstructure gradually changed from the fusion line to the surface, as shown in Fig. 3(a). The microstructure mainly contain relative coarse columnar grains, which grow epitaxial from the substrate perpendicular to the fusion line. Note that the columnar to equiaxed transition occurs as the distance away from the fusion line raised. Fig. 3(b) represents that the subgrains of equiaxed grains are the equiaxed cellular crystals in various sizes. The microstructure within the columnar grains consists of elongate cellular crystals with dissimilar direction, as shown in Fig. 3(c). It is noteworthy that series of layer lines, which spread roughly parallel to the fusion line, are observed. These layer lines are a result of the latent heat release in the process of crystallization, periodic change of the heat input, and uneven distribution of the chemical compositions
3.1. Macro morphology As shown in Fig. 2, the experimental parameters selected in this paper could gain access to a full fusion metallurgy of Invar alloy parts with no crack and obvious porosity. The parabolic configuration of the coatings subjects to surface tension, which maintains a stable molten pool. Meanwhile, the solidified molten pool consist entirely of liquid Invar alloy and significantly affected by heat input. Therefore, it is necessary to conduct the heat input during laser cladding process. The heat input (specific energy) is measured through
Q=
P , D×v
(1)
here P is laser power, D is beam diameter, and v is scanning speed. The Table 3 indicates a significantly elevated heat input as the scanning 456
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Fig. 6. Longitudinal section microstructures of Invar parts with different parameters: (a) longitudinal section microstructure of Case 1; (b) metallographic image of the localized region marked in (a). (c) longitudinal section microstructure of Case 2; (b) metallographic image of the localized region marked in (c). (e) longitudinal section microstructure of Case 3; (b) metallographic image of the localized region marked in (f).
[20]. Generally, the subgrain size are nearly unchanged in the same grain. However, there is an interesting phenomenon that the subgrain size at different regions near the layer line (above the line, on the line and below the line) are extremely diverse, as shown in Fig. 4. Meanwhile, the distinct brightness is seen around the layer line. The heat accumulation on the layer line is too large, resulting in the crystals of region 3 are the maximum, even melted into one. The pre-solidified crystals near the line are coarsened due to the latent heat release on the layer line, which is similar to the formation of the coarse HAZ. The region 5 can be assumed as the substrate, which is not affected by the latent heat. In contrast to region 4, the post-solidified crystals, which is
Table 4 Dendrite spacing with different heat input. Case
Heat input (J/mm2)
Dendrite spacing on cross-section (μm)
Dendrite spacing on longitudinal section (μm)
1 2 3
211 350 767
8 12.5 20
5 5.9 7.7
Fig. 7. Porosities in Invar alloy coatings: (a) bubble porosity; (b) fusion lack porosity. 457
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Fig. 8. The EDS results of LMD porosities: (a) local enlargement; (b) EDS results detected at marked point 1; (c) EDS results detected at marked point 2; (d) EDS results detected at marked point 3.
also near the line, are finer than line’s. This is attributed to that the crystals growth on the line absorbs enough heat input and result in excessive undercooling. The phenomenon decreases gradually as the molten pool solidification, so that the crystals size of region 1 returns to the origin. It is interesting to find that the layer lines are always observed in iron-nickel alloys, instead of the other alloy. The cross-section and longitudinal section microstructures of Invar parts developed under different processing conditions are provided in Figs. 5 and 6 respectively, showing the influence of heat input on microstructure of the Invar alloy coatings. A significant increase in HAZ widths from 388 μm to 628 μm is noticed with an increase in heat input from 81.4 to 230 J/mm2. Furthermore, analogous increasing is also exhibited in terms of the elongate cellular crystal size. For quantitative characterization of the variations, statistics are carried on the cellular crystal crossed by a certain line with the same length in each sample, perpendicular to the crystal growth direction. Particularly, the middle crystals are selected. The average value of dendrite spacing is measured through
λ=
L , n
caused by layer line, leads to uneven energy distribution of the coatings. 3.3. Porosities A majority of porosities can be eliminated effectively under shielding gas during laser cladding process. However, there are still a small number of residual porosities in coatings. In order to control the porosity in specimens, the porosity factor with different heat input and porosity formation have been studied. Fig. 7 shows the individual bubble porosity and fusion lack porosity within the Invar coating, respectively. It is illustrated that the size of the bubble porosity reached a diameter of 30 µm, which is much bigger than the fusion lack porosity, characterized by irregular shape on the order of microns and are uniformly distributed over the cross section. Substantially, the fusion lack porosity is the pores between unmelted powders, which cannot be completely filled with molten pool, so that the pores are characterized by irregularities. The theory has been proved by EDS results, as shown in Fig. 8(a)–(c). The results show that there is little difference between point 1 (substrate) and point 2 (fusion lack porosity), which both are purely Invar alloy. Compared with the fusion lack porosity, the bubble porosity has a more complicated formation during laser cladding. In order to reveal the formation mechanism and classification of the bubble porosity in the coatings, the chemical compositions in the porosity interface are also detected, of which the results are shown in Fig. 8. It is interesting to find a great distinction in content (Mn, Fe and Ni) between the substrate and bubble porosity interface. At the point of impact between the powder and the molten pool, the shielding gas can become sufficiently entrained in the laser cladding process. In the type of bubble porosity, shielding gas and metal vapor mingle deficiently. And the vapor is unevenly condensed on the inner wall of porosity with the rapid cooling rate, which results in the variation of atomic percent.
(2)
with L and n, respectively, being the length of the test line and the number of counted cellular crystals. It is observed that the elongate cellular crystal become considerably coarsen with the increase of heat input (Table 4). This is attributed to the growth of subgrains, which is caused by the more energy (higher heat input). Generally, the coarse crystals are formed with long heating time. Therefore, the crystals are coarse at the bottom of coatings and gradually thin as the surface of the coating is approached. However, the research in this paper is more complicated than the normal, as shown in Fig. 5(d). It is explicit that the crystals size is irrelative to the distance from fusion line, which is attributed to the above mentioned layer lines. The periodic heat input, 458
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Fig. 9. Porosity levels in cross section: (a) Case 1; (b) Case 2; (c) Case 3.
and be dragged below the surface of the molten pool. In order to get a low-porosity coatings, the heat input should be controlled.
Figs. 9 and 10 show the porosity factor of cross section and longitudinal section, respectively. The porosity factors of the three coatings are calculated as shown in Table 5, where the porosity with shortest diameter 2 μm is taken into consideration. It is observed that the porosity with a radius of 45 μm is the maximum, which is shown in Case 3 (heat input is 230 J/mm2). This is attributed that higher heat input provide more metal vapor for the porosity formation. The results show that the coating get a low-porosity when the heat input is at the value of 126 J/mm2. According to the formation of porosity, the significant factors that affect formation of pores are the appearance of entrapped gas and incompletely melted powders. Moreover, laser cladding is the extraordinary rapid solidification process, featured with high heating and cooling speeds. Therefore, the liquid molten pool exist for a shorter time and have a higher possibility of gas entrapment with the lower heat input. On the other hand, a lower heat input result in unfinished powders, the gap between which cannot be filled timely. And the fusion lack porosity is formed. Hence, it is generally put forward that a higher heat input would reduce the rate of solidification, allowing the gas bubbles to escape from the molten pool before the solidification. The conclusion will demonstrate that there is a decrease in porosity as the heat input excessively increases. However, Fig. 7 (b)-(c) show that porosity factors increases with excessive heat input, which can be explained by the Marangoni flow. As the dynamics of this flow increases with increased heat input, there is a greater chance that the gas, entrained with the powder particles, would become trapped in the flow,
4. Conclusions Invar ally coatings were produced by laser cladding with different heat input. A comprehensive research on the microstructure evolution and porosity formation mechanism was presented. Meanwhile, the effect of heat input on macro morphology, microstructure and porosity factor of specimens were investigated. The conclusions have been drawn as follows: 1. The coatings microstructure consist of equiaxed cellular crystals and elongated cellular crystals. And there is a gradual microstructure transition, the elongated cellular crystals to the equiaxed cellular crystals, from the fusion line to the surface. Note that the crystals size under high heat input was greater than under the low heat input. Analogous increasing is also exhibited in terms of the coatings size and HAZ width of the samples. 2. The pores in the laser cladded Invar alloy are composed of fusion lack porosity and bubble porosity. In particular, the liquid Invar alloy has a short time to fill the gap between unfinished powders with the rapid solidification. And the fusion lack porosity is formed. The bubble porosity consists of shielding gas and metal vapor. 3. The heat input plays a key role on the porosity of the coatings. A low 459
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Fig. 10. Porosity levels in longitudinal section: (a) Case 1; (b) Case 2; (c) Case 3. Table 5 Porosity factors with different heat input. Case
1
2
3
Cross section longitudinal section
0.64% 0.92%
0.31% 0.32%
1.21% 0.63%
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