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Comparative study on laser brazing and furnace brazing of Inconel 718 alloys with silver based filler metal
Optics & Laser Technology 68 (2015) 165–174
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Comparative study on laser brazing and furnace brazing of Inconel 718 alloys with silver based filler metal A. Khorram, M. Ghoreishi n Department of Mechanical Engineering, KNToosi University of Technology, P.O. Box 19395-1999, Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 4 May 2014 Received in revised form 27 November 2014 Accepted 30 November 2014
Laser brazing and furnace brazing of Inconel 718 alloy with silver based filler metal are investigated in this study. Laser brazing was performed using a 400 W pulsed Nd: YAG laser with varying laser powers, speeds and pulse widths. A central composite design (CCD) including five levels of factors was employed to design the experiments. Furnace brazing was performed in vacuum pressure 5 10 5 mbar at 710 1C. The cross sections of brazed joints were examined using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction analyzer (XRD). Microhardness and tensile test were performed for investigation of mechanical properties. The results indicate that laser brazed joints consist of α-Ag solid solution, ά-Cu solid solution surrounded by the α-Ag solid solution and eutectic structure while the furnace brazed joints mainly consist of α-Ag solid solution and ά-Cu solid solution. The furnace brazed joints show the average tensile strength of 348.5 MPa. However, maximum tensile strength of laser brazed joints was 338 MPa, which is approximately 3% lower than furnace brazed joints. The average microhardness for laser brazed joint was 150 HV compared to 120 HV for furnace brazed joint. This higher value of hardness can be attributed to existence of eutectic phase in laser brazed test pieces. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Laser brazing Furnace brazing Inconel 718
1. Introduction Inconel 718 is a vacuum-melted, precipitation-hardened nickelbase alloy that is used for corrosion and heat resistant applications. It can be joined easily by a wide variety of welding and brazing processes and excels in its resistance to strain-age cracking [1–3]. Chemical and metallurgical properties of the alloy and anticipated service conditions are important factors that must be taken into account in the brazing process and its technique. An important precaution is to attack by both sulfur and low-melting-point metals and liquid metal embrittlement in contact with molten brazing filler metals [2]. Nickel and nickel alloys can be brazed with many commercially available brazing alloys (BAg, BCu, BAu, and BNi) that among them, silver based filler metals (BAg) are suitable choices for this purpose [2–4]. Silver based filler metals such as Bag-8 are used for similar joining of TiAl, Ti–6Al–4V and dissimilar joining of Ti–6Al–4V with TZM by infrared brazing and furnace brazing procedures [5–7]. Also, the effect of varying indium contents on melting temperatures and braze ability of silver based filler metals (Ag–Cu–In alloys) on copper substrate have been investigated [8]. Laser welding–brazing is widely used for joining titanium to aluminum and many investigations have been performed for improving the interfacial reaction layer [9–11]. Microstructure and mechanical properties of laser welded–brazed Mg/mild steel and Mg/stainless steel
n
Corresponding author.
http://dx.doi.org/10.1016/j.optlastec.2014.11.026 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
joints using an Mg based filler metal and zinc coated steel (DP600) with aluminum alloy (AA6016) using a Zn based filler metal have been studied by researchers [12,13]. In another article [14] fatigue properties of laser-brazed joints of dual phase and transformation induced plasticity steel with a copper–aluminum filler metal have been investigated. Laser brazing of steel to aluminum with various filler metals (aluminum and zinc based filler metal) and temperature control have been investigated by Mathieu et al. [15–17]. Nagatsuka et al. [18] performed laser brazing for joining isotropic graphite to WC–Co alloy. An effect of Ti, serving as an activator in a eutectic Ag–Cu alloy filler metal, has been studied on the joint strength and the interface structure of the joint. In recent years, laser brazing is also used for joining mineral materials to themselves and metal materials such as steel [19,20]. In previous work, laser brazing parameters such as pulse frequency, pulse width, speed, gap distance and preheating were varied and the effects of these changes on wetting angle, spreading and depth of flow were investigated [21]. The current investigation concentrates on comparison between microstructure and interdiffusion of laser brazing and furnace brazing of Inconel 718 joints. Also tensile test and microhardness across these joints are comprehensively studied.
2. Experimental procedure The base metal used in the experiment was Inconel 718 sheet with dimensions of 120 mm length 25 mm width 1 mm
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thickness and its chemical composition is presented in Table 1. One side of samples was ground to provide a channel for flow of melted filler metal and also the samples degreased prior to brazing to clean the surfaces. Silver based wire, one commercial brazing filler metal, with diameter of 1 mm was used for joining and its chemical composition is shown in Table 2. Its solidus and liquidus temperatures are 618 1C and 652 1C. Brazing range is 652–760 1C. The laser source was a square standard pulse wave Nd: YAG pumped by lamps, with a maximum mean laser power of 400 W. The laser parameters could be adjusted between 1 and 1000 Hz for pulse frequency, 0.2–20 ms for pulse duration, and 0–40 J for pulse energy but any arbitrary combination of pulse energy and pulse frequency should not be used because average laser power could not be more than 400 W. The three lenses were used as the focusing optical system with 75 mm focal length and 250 mm minimum spot size. In this investigation the pulse frequency and preheating temperature were set at 100 Hz and 350 1C respectively. The laser beam was focused on the filler wire and irradiated on it at an angle of 901. Argon was used as shielding gas at flow rate of 30 L/min. The design of experiments was performed using a central composite design (CCD) including five levels of factors. Laser power, speed and pulse width were considered as independent input variables in this design. Laser input variables and designed experiments are shown in Tables 3 and 4. Furnace brazing was performed with pressure vacuum 5 10 5 mbar. For this purpose, the parts were heated in a vacuum furnace up to 710 1C for 60 min (heating rate: 15.5 1C/min).Then the samples were maintained in this temperature (710 1C) for approximately 5 min. The samples were cooled rapidly at the room temperature in a vacuum furnace. The rapid cooling was done by purge of inert gas (argon) in the vacuum furnace. Three samples were furnace brazed for evaluating the process. Fig. 1 shows the schematic geometry used for laser brazing. Prior to the brazing, the edge and the surface of the samples were cleaned with acetone. Filler wire was placed on the surface of work piece prior to performing the experiments. Tensile test was performed by a universal testing machine with a cross-head speed of 1 mm/min according to ASTM E8M standard to evaluate the joint strength of the samples. Microhardness test was carried out using a Vickers microhardness tester with a load of 100 g according to ASTM E384 standard. The structural analysis of the brazed joints was performed using an X-Ray diffract meter (XRD) with Cu Kα as an X-ray source. The cross section of the brazed joints was mounted and etched with the chemical composition of 1 g CrO3 þ1 ml H2SO4 þ 500 ml deionized H2O. The etching time was approximately between 20 and 30 s. The mounted samples were checked using a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS).
3. Results and discussion 3.1. Microstructure of the laser brazed joints Fig. 2 shows the cross section of laser brazed samples performed at various conditions. In this figure, it can be seen that the filler
metal exhibits good wettability on the base metal. The wetting angle changes from 161 to 191, while the range of spreading changes within 3300–3600 mm. The joint interfaces were sound and no defects appeared at the brazing seam. Fig. 3 shows the SEM images of laser brazed joints at various conditions. The dominant chemical composition of the filler metal is silver, so it is predicted that the structure of brazed joint is comprised of silver. The microstructure of the laser brazed joint was approximately constant in different conditions. The Ag, Cu, Zn, and Sn diffuse from filler wire to the base metal and they are alloyed with small amount of base metal. Also, Ni, Cr, and Fe diffuse from base metal to filler metal as shown in Fig. 4.
Table 2 Chemical composition of filler metal. Elements
Ag
Cu
Zn
Sn
Weight percentage (Wt%)
57
22
16.5
4.5
Table 3 Laser input variable. Level (coded)
Laser power (P) (W) Speed (S) (mm/s) Pulse width (P.W.) (ms)
1.68 1 0 1 1.68
180 184.8 195 205.2 210
1.33 1.55 2 2.45 2.67
1.4 1.52 1.8 2.08 2.2
Table 4 Design matrix with coded independent process variables. Run order
Pulse width (ms)
Speed (mm/s)
Laser power (W)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Optimum setting
0 0 0 0 0 0 1 1 1 1 1 0 1 0 1.68 0 1 1 1.68 0 0.355
1.68 0 0 0 0 0 1 1 1 1 1 0 1 0 0 1.68 1 1 0 0 0.450
0 0 0 0 1.68 1.68 1 1 1 1 1 0 1 0 0 0 1 1 0 0 1.002
Table 1 Chemical composition of Inconel 718. Elements Weight percentage (Wt%) Elements Weight percentage (Wt%)
C 0.07 Al 0.66
Mn 0.08 Co 0.23
Si 0.16 TAt 0.008
P 0.015 B 0.0028
S 0.002 Cu 0.07
Cr 18.03 Fe Balanced
Ni 53.16 Mo 3.11
Nb 5.48 Ti 1.15
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Fig. 1. Schematic geometry of laser brazing: (a) before laser brazing with filler wire and (b) after laser brazing.
Fig. 2. Cross section of laser brazed joints: (a) sample 2: power of 195 W, pulse width of 1.8 ms, speed of 2 mm/s, (b) sample 7: power of 205.2 W, pulse width of 1.52 ms, speed of 2.45 mm/s, (c) sample 11: power of 205.2 W, pulse width of 1.52 ms, speed of 1.55 mm/s, (d) sample 18: power of 205.2 W, pulse width of 2.08 ms, speed of 1.55 mm/s and (e) optimum sample: power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s.
Figs. 5–8 show SEM and EDS analysis results of laser brazing in different conditions. The laser brazing shows a very rapid thermal cycle so that interfacial reactions between the molten filler metal and base metal are not prominent. The average thickness of continuous reaction layer for samples 2, 7, 11 and 18 is approximately 2.5 μm while the thickness of this layer for optimum work piece is 3 μm.
It indicates atom diffusion in spite of short reaction time and the fast thermal cycle. Ag solid solution is dominant phase and observed throughout the brazed joint as marked C and D in Fig. 5. Cu solid solution phase is also observed in brazed joint as marked B in Fig. 5. Eutectic phases exist in brazed joint which could not be determined by EPMA because of their small sizes. Therefore,
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Fig. 3. SEM images of laser brazed joints: (a) sample 2: power of 195 W, pulse width of 1.8 ms, speed of 2 mm/s, (b) sample 7: power of 205.2 W, pulse width of 1.52 ms, speed of 2.45 mm/s, (c) sample 11: power of 205.2 W, pulse width of 1.52 ms, speed of 1.55 mm/s, (d) sample 18: power of 205.2 W, pulse width of 2.08 ms, speed of 1.55 mm/s and (e) optimum sample: power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s.
XRD was employed to determine the phases in brazing seam, as seen in Fig. 9. The brazing seams have silver solid solution (α-Ag), cupper solid solution (ά-Cu), AgZn, Ag3Sn, Cu41Sn11, Cu5Zn8, and Cu6Sn5 phases. The Inconel 718 is partially dissolved into molten filler metal so the molten filler metal closed to Inconel 718 is rich in Ni, Fe and Cr. Dissolution of substrate during laser brazing is minimized due to
its rapid thermal cycle. According to binary alloy phase diagrams, silver can alloy with Ni and Fe and forms a solid solution. Also Cu can alloy with Ni, Fe and Cr and forms a solid solution [22]. The experiments show the reaction layer for laser brazed sample at power of 205.2 W, pulse width of 2.45 ms and speed of 1.52 mm/s (marked A) mainly consists of Ag, Cu, Ni and Fe. The composition of the reaction layer for laser brazed samples is
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approximately constant as shown in Figs. 5–8. As it can be seen silver content in this layer is more than the other elements. This phenomenon can be attributed to short time of the process and more content of silver in initial filler metal. 3.2. Microstructure of furnace brazing joints The wetting behavior of the filler material on substrate is shown in Fig. 10. The range of wetting angle and spreading changes within 7–141 and 5000–8800 mm, respectively. An excellent adhesion can be achieved with the filler metal. The spreading width of furnace brazing is approximately twice the spreading width of laser brazing, while the wetting angle of furnace brazing is approximately one and a half-times lower than the wetting angle of laser brazing. There is more time in furnace brazing than the laser brazing so Ni reacts with the Cu component of the filler metal. It does not react with Ag according to binary alloy phase diagrams. Depletion of the Cu content from the molten braze due to Cu motions from molten filler metal (more concentration) to the base metal (little concentration) results in chemical composition of the molten filler metal which deviates from eutectic phase to Ag-rich composition. So eutectic phase decreases in the brazed joint and the Ag solid solution phase is observed throughout the brazed joint after furnace brazing. Therefore microstructures of furnace brazed samples are different from the laser brazed ones. Figs. 11, 13 and 14 show existence of copper in vicinity of the reaction layer. The furnace brazed joints are primarily comprised of Ag solid solution matrix, Cu solid solution and tiny amount of eutectic phase. The Ag solid solution matrix dominates in the furnace brazed joint as illustrated in Figs. 11 and 12. It can be seen in Fig. 11, there is a gap between filler metal and base metal which is caused by surface pollution of the base metal. Concentration profiles of the main elements obtained from EDS line scan across the interface between the brazing seam and the base metal are indicated in Fig. 13. Cu, Ag, Zn and Sn would diffuse from liquid filler metal to the interface, since atoms tend to diffuse from high to low concentrations. As shown in the figure, the Ni, Fe and Cr content increased across the interface, while the Cu, Ag, Sn and Zn content varied in the opposite direction. Some fluctuations on the concentration curves were found in the brazing seam. The main reason can be attributed to Cu solid solution phase causing an increase in Cu content. In furnace brazing, the dissolution of Inconel 718 into the molten braze is significantly enhanced due to increasing of brazing time as demonstrated in Fig. 14. Therefore, chemical composition of reaction layer in furnace brazed joint is different from the laser brazed joint. The reaction layer exhibits a continuous and uniform morphology and the thickness of this layer is approximately 4.5 μm. It must be mentioned that erosion occurs in the furnace brazing due to longer time of this process than the laser brazing process time as can be seen in Figs. 13 and 14. 3.3. Mechanical properties
Fig. 4. Typical SEM–EDS line scan results: (a) sample 2: power of 195 W, pulse width of 1.8 ms, speed of 2 mm/s, (b) sample 7: power of 205.2 W, pulse width of 1.52 ms, speed of 2.45 mm/s, (c) sample 11: power of 205.2 W, pulse width of 1.52 ms, speed of 1.55 mm/s, (d) sample 18: power of 205.2 W, pulse width of 2.08 ms, speed 1.55 of mm/s and (e) optimum sample: power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s.
Fig. 15 shows Vickers microhardness profile for laser brazed joint at power of 205.2 W, pulse width of 2.08 ms and speed of 1.55 mm/s and furnace brazed joint. The values of microhardness were approximately similar for all test pieces in the laser brazing process because of rapid cooling cycle, so only the value of microhardness for one test piece is presented. The values of microhardness in laser brazed joints are approximately 25% higher than furnace brazed joints. This higher value of hardness can be attributed to existence of eutectic phase in laser brazing samples. Table 5 shows tensile strength of brazed specimens with different laser brazing conditions. It is obvious that the filler metal
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Fig. 5. SEM and EDS analysis results of the laser brazed sample 7 (power of 205.2 W, pulse width of 1.52 ms, speed of 2.45 mm/s).
Fig. 6. SEM and EDS analysis results of the laser brazed sample 11 (power of 205.2 W, pulse width of 1.52 ms, speed of 1.55 mm/s).
Fig. 7. SEM and EDS analysis results of the laser brazed sample 18 (power of 205.2 W, pulse width of 2.08 ms, speed of 1.55 mm/s).
Fig. 8. SEM and EDS analysis results of the optimum laser brazed sample (power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s).
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Fig. 9. XRD patterns for laser brazed joint.
Fig. 10. Cross section of furnace brazed samples (a) sample 1, (b) sample 2 and (c) sample 3.
Fig. 11. SEM images of furnace brazed joint (sample 1).
spreading on substrate and filling the gap affect tensile strength values. As the filler metal spreads completely on the substrate and filled the gap, thickness of reaction layer and alloying the elements are the most important parameters for the joint strength.
The gap was not filled in several samples such as 1, 2, 3, 4 and 20. So the tensile strength of these samples is low. Three examples of these samples are represented in Table 5. When the gap is filled by molten filler metal (samples 7, 11, 13, and 18, optimum), higher tensile strength can be obtained. Experimental results show that spreading value more than 3450 μm does not affect tensile strength significantly. The maximum tensile stress 338 MPa is obtained for the laser brazed specimen at power of 205.21 W, pulse width of 1.7 ms and speed of 1.8 mm/s. The higher value of tensile strength in optimum sample can be attributed to better joint geometry and more thickness of reaction layer (3 μm in optimum laser brazed sample versus 2.5 μm in other samples). The tensile strength of furnace brazed specimens is shown in Table 6. There is a gap between filler metal and base metal in sample 1 (as can be seen in Fig. 11) that causes low tensile strength. The average tensile strength for the brazed samples was calculated when the gap was filled with molten filler metals and there was not any defect. The average tensile strength of the furnace brazed specimens was 348.5 MPa. So the tensile strength of optimum laser brazed sample is approximately 3% lower than the average of tensile strength for furnace brazed samples. This higher
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Fig. 12. SEM images with high magnification (a) furnace brazed joints (sample 2) and (b) laser brazed joint at power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s (optimum sample).
Fig. 13. SEM–EDS line scan results of the furnace brazed joints (a) sample 2 and (b) sample 3.
4. Conclusion
Therefore, laser brazing is superior to the traditional furnace brazing process. 2) In laser brazing, the thickness of continuous reaction layer in optimum work piece is approximately 3 μm while the reaction layer thickness in furnace brazing is 4.5 μm. 3) For the laser brazed specimens, Ag solid solution matrix, Cu solid solution and eutectic phase are observed while Ag solid solution and Cu solid solution are dominant phases in furnace brazed samples.
1) Processing time in laser brazing is in order of few seconds compared with few hours in conventional furnace brazing. Additionally, in laser brazing there is no need to use closed chamber.
4) In furnace brazing, decreasing of eutectic phase from the brazing seam is primarily attributed to more consumption of copper from the molten filler metal due to Cu motion to reaction layer and its reaction with Ni.
value of tensile strength can be attributed to more thickness of reaction layer (4.5 μm in furnace brazed samples versus 3 μm in optimum laser brazing sample) and copper atomic diffusion which latter results in better bonding between the base metal and the filler metal.
A. Khorram, M. Ghoreishi / Optics & Laser Technology 68 (2015) 165–174
173
340 320 300 280 260 240 220 200 180 160 140 120 100 80
300
Furnace brazed zone
Laser brazed zone
280 Base metal zone
Hardness(HV)
Hardness(HV)
Fig. 14. SEM and EDS analysis results of the furnace brazed joints (a) sample 2 and (b) sample 3.
260
Base metal zone
240 220 200 180 160 140
0
50
100 150 L(μm)
200
120
250
0
50
100 150 L(μm)
200
250
Fig. 15. Microhardness profile of (a) furnace brazed joint and (b) laser brazed joint at power of 205.2 W, pulse width of 2.08 ms, speed of 1.55 mm/s (sample 18).
Table 5 Tensile strength of laser brazing samples.
Table 6 Tensile strength of furnace brazed samples.
Sample no.
Tensile strength (MPa)
Elongation (%)
Sample no.
Tensile strength (MPa)
Elongation (%)
2 4 7 11 13 18 20 Optimum setting
177 179 302.89 310.70 314.20 306.87 176 338.57
3 3 6.5 7 7 6.5 3 9
1 2 3
190 349 348
3.5 9.5 9.5
5) The laser brazed joint at power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s demonstrates the highest tensile strength of 338 MPa while furnace brazed joints show the average tensile strength of 348.5 MPa. 6) The average microhardness for laser brazing joint is 150 HV while the average microhardness for furnace brazing joint is
120 HV. This higher value of hardness can be attributed to existence of eutectic phase in the laser brazing test pieces.
References [1] Richard CR, Jackson JL, Bakuckas J, Thompson S. Metallic materials properties development and standardization (MMPDS). Washington, DC: U.S. Department of Transportation, Federal Aviation Administration Office of Aviation Research; 2003.
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[2] AWS Committee on Brazing and Soldering. Brazing handbook. 4th edition. Florida: American Welding Society; 2002. [3] ASM International Handbook Committee. ASM specialty handbook: nickel, cobalt and their alloys. Ohio: ASM International; 2000. [4] Schwartz MM. Brazing. 2nd edition. . Ohio: ASM International; 2003. [5] Shiue RK, Wu SK, Chen SY. Infrared brazing of TiAl intermetallic using BAg-8 braze alloy. Acta Mater 2003;51:1991–2004. [6] Du YC, Shiue RK. Infrared brazing of Ti–6Al–4V using two silver-based braze alloys. J Mater Process Technol 2009;209:5161–6. [7] Chan HY, Liaw DW, Shiue RK. The microstructural observation of brazing Ti–6Al–4V and TZM using the BAg-8 braze alloy. Int J Refract Met Hard Mater 2004;22:27–33. [8] Sisamouth L, Hamdi M, Ariga T. Investigation of gap filling ability of Ag–Cu–In brazing filler metals. J Alloys Compd 2010;504:325–9. [9] Möller F, Grden M, Thomy C, Vollertsen F. Combined laser beam welding and brazing process for aluminum titanium hybrid structures. Phys Procedia 2011;12:215–23. [10] Chen S, Li L, Chen Y, Dai J, Huang J. Improving interfacial reaction nonhomogeneity during laser welding–brazing aluminum to titanium. Mater Des 2011;32:4408–16. [11] Chen Y, Chen S, Li L. Influence of interfacial reaction layer morphologies on crack initiation and propagation in Ti/Al joint by laser welding–brazing. Mater Des 2010;31:227–33. [12] Li L, Tan C, Chen Y, Guo W, Song F. Comparative study on microstructure and mechanical properties of laser welded–brazed Mg/mild steel and Mg/stainless steel joints. Mater Des 2013;43:59–65.
[13] Dharmendra C, Rao KP, Wilden J, Reich S. Study on laser welding–brazing of zinc coated steel to aluminum alloy with a zinc based filler. Mater Sci Eng A 2011;528:1497–503. [14] Janssen MHE, Hermans MJM, Janssen M, Richardson IM. Fatigue properties of laser-brazed joints of dual phase and transformation induced plasticity steel with a copper–aluminum consumable. Mater Des 2010;31:3922–8. [15] Mathieu A, Pontevicci S, claudeViala J, Cicala E, Matte S, Greve D. Laser brazing of a steel/aluminum assembly with hot filler wire (88% Al, 12% Si). Mater Sci Eng A 2006;435–436:19–28. [16] Mathieu A, Shabadi R, Deschamps A, Suery M, Mattei S, Grevey D, et al. Dissimilar material joining using laser (aluminum to steel using zinc-based filler wire). Opt Laser Technol 2007;39:652–61. [17] Mathieu A, Mattei S, Deschamps A, Martin B, Grevey D. Temperature control in laser brazing of a steel/aluminium assembly using thermographic measurements. NDT & E Int 2006;39:272–6. [18] Nagatsuka K, Sechi Y, Miyamoto Y, Nakata K. Characteristics of dissimilar laserbrazed joints of isotropic graphite to WC–Co alloy. Mater Sci Eng B 2012;177:520–3. [19] Rohde M, Sudmeyer I, Urbanek A, Torge M. Joining of alumina and steel by a laser supported brazing process. Ceram Int 2009;35:333–7. [20] Heilmann F, Rixecker G, Börner FD, Lippmann W, Hurtado A. Fe2O3-doped forsterite ceramics as a joining partner for ZrO2 in a laser brazing process. J Eur Ceram Soc 2009;29:2783–9. [21] Khorram A, Ghoreishi M, Torkamany MJ, Bali MM. Laser brazing of inconel 718 alloy with a silver based filler metal. Opt Laser Technol 2014;56:443–50. [22] ASM international Handbook Committee. ASM handbook, alloy phase diagrams. The United States of America: ASM International; 1998.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Comparative study on laser brazing and furnace brazing of Inconel 718 alloys with silver based filler metal A. Khorram, M. Ghoreishi n Department of Mechanical Engineering, KNToosi University of Technology, P.O. Box 19395-1999, Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 4 May 2014 Received in revised form 27 November 2014 Accepted 30 November 2014
Laser brazing and furnace brazing of Inconel 718 alloy with silver based filler metal are investigated in this study. Laser brazing was performed using a 400 W pulsed Nd: YAG laser with varying laser powers, speeds and pulse widths. A central composite design (CCD) including five levels of factors was employed to design the experiments. Furnace brazing was performed in vacuum pressure 5 10 5 mbar at 710 1C. The cross sections of brazed joints were examined using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction analyzer (XRD). Microhardness and tensile test were performed for investigation of mechanical properties. The results indicate that laser brazed joints consist of α-Ag solid solution, ά-Cu solid solution surrounded by the α-Ag solid solution and eutectic structure while the furnace brazed joints mainly consist of α-Ag solid solution and ά-Cu solid solution. The furnace brazed joints show the average tensile strength of 348.5 MPa. However, maximum tensile strength of laser brazed joints was 338 MPa, which is approximately 3% lower than furnace brazed joints. The average microhardness for laser brazed joint was 150 HV compared to 120 HV for furnace brazed joint. This higher value of hardness can be attributed to existence of eutectic phase in laser brazed test pieces. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Laser brazing Furnace brazing Inconel 718
1. Introduction Inconel 718 is a vacuum-melted, precipitation-hardened nickelbase alloy that is used for corrosion and heat resistant applications. It can be joined easily by a wide variety of welding and brazing processes and excels in its resistance to strain-age cracking [1–3]. Chemical and metallurgical properties of the alloy and anticipated service conditions are important factors that must be taken into account in the brazing process and its technique. An important precaution is to attack by both sulfur and low-melting-point metals and liquid metal embrittlement in contact with molten brazing filler metals [2]. Nickel and nickel alloys can be brazed with many commercially available brazing alloys (BAg, BCu, BAu, and BNi) that among them, silver based filler metals (BAg) are suitable choices for this purpose [2–4]. Silver based filler metals such as Bag-8 are used for similar joining of TiAl, Ti–6Al–4V and dissimilar joining of Ti–6Al–4V with TZM by infrared brazing and furnace brazing procedures [5–7]. Also, the effect of varying indium contents on melting temperatures and braze ability of silver based filler metals (Ag–Cu–In alloys) on copper substrate have been investigated [8]. Laser welding–brazing is widely used for joining titanium to aluminum and many investigations have been performed for improving the interfacial reaction layer [9–11]. Microstructure and mechanical properties of laser welded–brazed Mg/mild steel and Mg/stainless steel
n
Corresponding author.
http://dx.doi.org/10.1016/j.optlastec.2014.11.026 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
joints using an Mg based filler metal and zinc coated steel (DP600) with aluminum alloy (AA6016) using a Zn based filler metal have been studied by researchers [12,13]. In another article [14] fatigue properties of laser-brazed joints of dual phase and transformation induced plasticity steel with a copper–aluminum filler metal have been investigated. Laser brazing of steel to aluminum with various filler metals (aluminum and zinc based filler metal) and temperature control have been investigated by Mathieu et al. [15–17]. Nagatsuka et al. [18] performed laser brazing for joining isotropic graphite to WC–Co alloy. An effect of Ti, serving as an activator in a eutectic Ag–Cu alloy filler metal, has been studied on the joint strength and the interface structure of the joint. In recent years, laser brazing is also used for joining mineral materials to themselves and metal materials such as steel [19,20]. In previous work, laser brazing parameters such as pulse frequency, pulse width, speed, gap distance and preheating were varied and the effects of these changes on wetting angle, spreading and depth of flow were investigated [21]. The current investigation concentrates on comparison between microstructure and interdiffusion of laser brazing and furnace brazing of Inconel 718 joints. Also tensile test and microhardness across these joints are comprehensively studied.
2. Experimental procedure The base metal used in the experiment was Inconel 718 sheet with dimensions of 120 mm length 25 mm width 1 mm
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thickness and its chemical composition is presented in Table 1. One side of samples was ground to provide a channel for flow of melted filler metal and also the samples degreased prior to brazing to clean the surfaces. Silver based wire, one commercial brazing filler metal, with diameter of 1 mm was used for joining and its chemical composition is shown in Table 2. Its solidus and liquidus temperatures are 618 1C and 652 1C. Brazing range is 652–760 1C. The laser source was a square standard pulse wave Nd: YAG pumped by lamps, with a maximum mean laser power of 400 W. The laser parameters could be adjusted between 1 and 1000 Hz for pulse frequency, 0.2–20 ms for pulse duration, and 0–40 J for pulse energy but any arbitrary combination of pulse energy and pulse frequency should not be used because average laser power could not be more than 400 W. The three lenses were used as the focusing optical system with 75 mm focal length and 250 mm minimum spot size. In this investigation the pulse frequency and preheating temperature were set at 100 Hz and 350 1C respectively. The laser beam was focused on the filler wire and irradiated on it at an angle of 901. Argon was used as shielding gas at flow rate of 30 L/min. The design of experiments was performed using a central composite design (CCD) including five levels of factors. Laser power, speed and pulse width were considered as independent input variables in this design. Laser input variables and designed experiments are shown in Tables 3 and 4. Furnace brazing was performed with pressure vacuum 5 10 5 mbar. For this purpose, the parts were heated in a vacuum furnace up to 710 1C for 60 min (heating rate: 15.5 1C/min).Then the samples were maintained in this temperature (710 1C) for approximately 5 min. The samples were cooled rapidly at the room temperature in a vacuum furnace. The rapid cooling was done by purge of inert gas (argon) in the vacuum furnace. Three samples were furnace brazed for evaluating the process. Fig. 1 shows the schematic geometry used for laser brazing. Prior to the brazing, the edge and the surface of the samples were cleaned with acetone. Filler wire was placed on the surface of work piece prior to performing the experiments. Tensile test was performed by a universal testing machine with a cross-head speed of 1 mm/min according to ASTM E8M standard to evaluate the joint strength of the samples. Microhardness test was carried out using a Vickers microhardness tester with a load of 100 g according to ASTM E384 standard. The structural analysis of the brazed joints was performed using an X-Ray diffract meter (XRD) with Cu Kα as an X-ray source. The cross section of the brazed joints was mounted and etched with the chemical composition of 1 g CrO3 þ1 ml H2SO4 þ 500 ml deionized H2O. The etching time was approximately between 20 and 30 s. The mounted samples were checked using a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS).
3. Results and discussion 3.1. Microstructure of the laser brazed joints Fig. 2 shows the cross section of laser brazed samples performed at various conditions. In this figure, it can be seen that the filler
metal exhibits good wettability on the base metal. The wetting angle changes from 161 to 191, while the range of spreading changes within 3300–3600 mm. The joint interfaces were sound and no defects appeared at the brazing seam. Fig. 3 shows the SEM images of laser brazed joints at various conditions. The dominant chemical composition of the filler metal is silver, so it is predicted that the structure of brazed joint is comprised of silver. The microstructure of the laser brazed joint was approximately constant in different conditions. The Ag, Cu, Zn, and Sn diffuse from filler wire to the base metal and they are alloyed with small amount of base metal. Also, Ni, Cr, and Fe diffuse from base metal to filler metal as shown in Fig. 4.
Table 2 Chemical composition of filler metal. Elements
Ag
Cu
Zn
Sn
Weight percentage (Wt%)
57
22
16.5
4.5
Table 3 Laser input variable. Level (coded)
Laser power (P) (W) Speed (S) (mm/s) Pulse width (P.W.) (ms)
1.68 1 0 1 1.68
180 184.8 195 205.2 210
1.33 1.55 2 2.45 2.67
1.4 1.52 1.8 2.08 2.2
Table 4 Design matrix with coded independent process variables. Run order
Pulse width (ms)
Speed (mm/s)
Laser power (W)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Optimum setting
0 0 0 0 0 0 1 1 1 1 1 0 1 0 1.68 0 1 1 1.68 0 0.355
1.68 0 0 0 0 0 1 1 1 1 1 0 1 0 0 1.68 1 1 0 0 0.450
0 0 0 0 1.68 1.68 1 1 1 1 1 0 1 0 0 0 1 1 0 0 1.002
Table 1 Chemical composition of Inconel 718. Elements Weight percentage (Wt%) Elements Weight percentage (Wt%)
C 0.07 Al 0.66
Mn 0.08 Co 0.23
Si 0.16 TAt 0.008
P 0.015 B 0.0028
S 0.002 Cu 0.07
Cr 18.03 Fe Balanced
Ni 53.16 Mo 3.11
Nb 5.48 Ti 1.15
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Fig. 1. Schematic geometry of laser brazing: (a) before laser brazing with filler wire and (b) after laser brazing.
Fig. 2. Cross section of laser brazed joints: (a) sample 2: power of 195 W, pulse width of 1.8 ms, speed of 2 mm/s, (b) sample 7: power of 205.2 W, pulse width of 1.52 ms, speed of 2.45 mm/s, (c) sample 11: power of 205.2 W, pulse width of 1.52 ms, speed of 1.55 mm/s, (d) sample 18: power of 205.2 W, pulse width of 2.08 ms, speed of 1.55 mm/s and (e) optimum sample: power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s.
Figs. 5–8 show SEM and EDS analysis results of laser brazing in different conditions. The laser brazing shows a very rapid thermal cycle so that interfacial reactions between the molten filler metal and base metal are not prominent. The average thickness of continuous reaction layer for samples 2, 7, 11 and 18 is approximately 2.5 μm while the thickness of this layer for optimum work piece is 3 μm.
It indicates atom diffusion in spite of short reaction time and the fast thermal cycle. Ag solid solution is dominant phase and observed throughout the brazed joint as marked C and D in Fig. 5. Cu solid solution phase is also observed in brazed joint as marked B in Fig. 5. Eutectic phases exist in brazed joint which could not be determined by EPMA because of their small sizes. Therefore,
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Fig. 3. SEM images of laser brazed joints: (a) sample 2: power of 195 W, pulse width of 1.8 ms, speed of 2 mm/s, (b) sample 7: power of 205.2 W, pulse width of 1.52 ms, speed of 2.45 mm/s, (c) sample 11: power of 205.2 W, pulse width of 1.52 ms, speed of 1.55 mm/s, (d) sample 18: power of 205.2 W, pulse width of 2.08 ms, speed of 1.55 mm/s and (e) optimum sample: power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s.
XRD was employed to determine the phases in brazing seam, as seen in Fig. 9. The brazing seams have silver solid solution (α-Ag), cupper solid solution (ά-Cu), AgZn, Ag3Sn, Cu41Sn11, Cu5Zn8, and Cu6Sn5 phases. The Inconel 718 is partially dissolved into molten filler metal so the molten filler metal closed to Inconel 718 is rich in Ni, Fe and Cr. Dissolution of substrate during laser brazing is minimized due to
its rapid thermal cycle. According to binary alloy phase diagrams, silver can alloy with Ni and Fe and forms a solid solution. Also Cu can alloy with Ni, Fe and Cr and forms a solid solution [22]. The experiments show the reaction layer for laser brazed sample at power of 205.2 W, pulse width of 2.45 ms and speed of 1.52 mm/s (marked A) mainly consists of Ag, Cu, Ni and Fe. The composition of the reaction layer for laser brazed samples is
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approximately constant as shown in Figs. 5–8. As it can be seen silver content in this layer is more than the other elements. This phenomenon can be attributed to short time of the process and more content of silver in initial filler metal. 3.2. Microstructure of furnace brazing joints The wetting behavior of the filler material on substrate is shown in Fig. 10. The range of wetting angle and spreading changes within 7–141 and 5000–8800 mm, respectively. An excellent adhesion can be achieved with the filler metal. The spreading width of furnace brazing is approximately twice the spreading width of laser brazing, while the wetting angle of furnace brazing is approximately one and a half-times lower than the wetting angle of laser brazing. There is more time in furnace brazing than the laser brazing so Ni reacts with the Cu component of the filler metal. It does not react with Ag according to binary alloy phase diagrams. Depletion of the Cu content from the molten braze due to Cu motions from molten filler metal (more concentration) to the base metal (little concentration) results in chemical composition of the molten filler metal which deviates from eutectic phase to Ag-rich composition. So eutectic phase decreases in the brazed joint and the Ag solid solution phase is observed throughout the brazed joint after furnace brazing. Therefore microstructures of furnace brazed samples are different from the laser brazed ones. Figs. 11, 13 and 14 show existence of copper in vicinity of the reaction layer. The furnace brazed joints are primarily comprised of Ag solid solution matrix, Cu solid solution and tiny amount of eutectic phase. The Ag solid solution matrix dominates in the furnace brazed joint as illustrated in Figs. 11 and 12. It can be seen in Fig. 11, there is a gap between filler metal and base metal which is caused by surface pollution of the base metal. Concentration profiles of the main elements obtained from EDS line scan across the interface between the brazing seam and the base metal are indicated in Fig. 13. Cu, Ag, Zn and Sn would diffuse from liquid filler metal to the interface, since atoms tend to diffuse from high to low concentrations. As shown in the figure, the Ni, Fe and Cr content increased across the interface, while the Cu, Ag, Sn and Zn content varied in the opposite direction. Some fluctuations on the concentration curves were found in the brazing seam. The main reason can be attributed to Cu solid solution phase causing an increase in Cu content. In furnace brazing, the dissolution of Inconel 718 into the molten braze is significantly enhanced due to increasing of brazing time as demonstrated in Fig. 14. Therefore, chemical composition of reaction layer in furnace brazed joint is different from the laser brazed joint. The reaction layer exhibits a continuous and uniform morphology and the thickness of this layer is approximately 4.5 μm. It must be mentioned that erosion occurs in the furnace brazing due to longer time of this process than the laser brazing process time as can be seen in Figs. 13 and 14. 3.3. Mechanical properties
Fig. 4. Typical SEM–EDS line scan results: (a) sample 2: power of 195 W, pulse width of 1.8 ms, speed of 2 mm/s, (b) sample 7: power of 205.2 W, pulse width of 1.52 ms, speed of 2.45 mm/s, (c) sample 11: power of 205.2 W, pulse width of 1.52 ms, speed of 1.55 mm/s, (d) sample 18: power of 205.2 W, pulse width of 2.08 ms, speed 1.55 of mm/s and (e) optimum sample: power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s.
Fig. 15 shows Vickers microhardness profile for laser brazed joint at power of 205.2 W, pulse width of 2.08 ms and speed of 1.55 mm/s and furnace brazed joint. The values of microhardness were approximately similar for all test pieces in the laser brazing process because of rapid cooling cycle, so only the value of microhardness for one test piece is presented. The values of microhardness in laser brazed joints are approximately 25% higher than furnace brazed joints. This higher value of hardness can be attributed to existence of eutectic phase in laser brazing samples. Table 5 shows tensile strength of brazed specimens with different laser brazing conditions. It is obvious that the filler metal
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Fig. 5. SEM and EDS analysis results of the laser brazed sample 7 (power of 205.2 W, pulse width of 1.52 ms, speed of 2.45 mm/s).
Fig. 6. SEM and EDS analysis results of the laser brazed sample 11 (power of 205.2 W, pulse width of 1.52 ms, speed of 1.55 mm/s).
Fig. 7. SEM and EDS analysis results of the laser brazed sample 18 (power of 205.2 W, pulse width of 2.08 ms, speed of 1.55 mm/s).
Fig. 8. SEM and EDS analysis results of the optimum laser brazed sample (power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s).
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Fig. 9. XRD patterns for laser brazed joint.
Fig. 10. Cross section of furnace brazed samples (a) sample 1, (b) sample 2 and (c) sample 3.
Fig. 11. SEM images of furnace brazed joint (sample 1).
spreading on substrate and filling the gap affect tensile strength values. As the filler metal spreads completely on the substrate and filled the gap, thickness of reaction layer and alloying the elements are the most important parameters for the joint strength.
The gap was not filled in several samples such as 1, 2, 3, 4 and 20. So the tensile strength of these samples is low. Three examples of these samples are represented in Table 5. When the gap is filled by molten filler metal (samples 7, 11, 13, and 18, optimum), higher tensile strength can be obtained. Experimental results show that spreading value more than 3450 μm does not affect tensile strength significantly. The maximum tensile stress 338 MPa is obtained for the laser brazed specimen at power of 205.21 W, pulse width of 1.7 ms and speed of 1.8 mm/s. The higher value of tensile strength in optimum sample can be attributed to better joint geometry and more thickness of reaction layer (3 μm in optimum laser brazed sample versus 2.5 μm in other samples). The tensile strength of furnace brazed specimens is shown in Table 6. There is a gap between filler metal and base metal in sample 1 (as can be seen in Fig. 11) that causes low tensile strength. The average tensile strength for the brazed samples was calculated when the gap was filled with molten filler metals and there was not any defect. The average tensile strength of the furnace brazed specimens was 348.5 MPa. So the tensile strength of optimum laser brazed sample is approximately 3% lower than the average of tensile strength for furnace brazed samples. This higher
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Fig. 12. SEM images with high magnification (a) furnace brazed joints (sample 2) and (b) laser brazed joint at power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s (optimum sample).
Fig. 13. SEM–EDS line scan results of the furnace brazed joints (a) sample 2 and (b) sample 3.
4. Conclusion
Therefore, laser brazing is superior to the traditional furnace brazing process. 2) In laser brazing, the thickness of continuous reaction layer in optimum work piece is approximately 3 μm while the reaction layer thickness in furnace brazing is 4.5 μm. 3) For the laser brazed specimens, Ag solid solution matrix, Cu solid solution and eutectic phase are observed while Ag solid solution and Cu solid solution are dominant phases in furnace brazed samples.
1) Processing time in laser brazing is in order of few seconds compared with few hours in conventional furnace brazing. Additionally, in laser brazing there is no need to use closed chamber.
4) In furnace brazing, decreasing of eutectic phase from the brazing seam is primarily attributed to more consumption of copper from the molten filler metal due to Cu motion to reaction layer and its reaction with Ni.
value of tensile strength can be attributed to more thickness of reaction layer (4.5 μm in furnace brazed samples versus 3 μm in optimum laser brazing sample) and copper atomic diffusion which latter results in better bonding between the base metal and the filler metal.
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340 320 300 280 260 240 220 200 180 160 140 120 100 80
300
Furnace brazed zone
Laser brazed zone
280 Base metal zone
Hardness(HV)
Hardness(HV)
Fig. 14. SEM and EDS analysis results of the furnace brazed joints (a) sample 2 and (b) sample 3.
260
Base metal zone
240 220 200 180 160 140
0
50
100 150 L(μm)
200
120
250
0
50
100 150 L(μm)
200
250
Fig. 15. Microhardness profile of (a) furnace brazed joint and (b) laser brazed joint at power of 205.2 W, pulse width of 2.08 ms, speed of 1.55 mm/s (sample 18).
Table 5 Tensile strength of laser brazing samples.
Table 6 Tensile strength of furnace brazed samples.
Sample no.
Tensile strength (MPa)
Elongation (%)
Sample no.
Tensile strength (MPa)
Elongation (%)
2 4 7 11 13 18 20 Optimum setting
177 179 302.89 310.70 314.20 306.87 176 338.57
3 3 6.5 7 7 6.5 3 9
1 2 3
190 349 348
3.5 9.5 9.5
5) The laser brazed joint at power of 205.21 W, pulse width of 1.7 ms, speed of 1.8 mm/s demonstrates the highest tensile strength of 338 MPa while furnace brazed joints show the average tensile strength of 348.5 MPa. 6) The average microhardness for laser brazing joint is 150 HV while the average microhardness for furnace brazing joint is
120 HV. This higher value of hardness can be attributed to existence of eutectic phase in the laser brazing test pieces.
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