- Email: [email protected]
Q235 diffusion bonding
Journal of Colloid and Interface Science 288 (2005) 521–525 www.elsevier.com/locate/jcis
Division and microstructure feature in the interface transition zone of Fe3Al/Q235 diffusion bonding Yajiang Li a,b,∗ , Juan Wang a , Yansheng Yin a , Haijun Ma a a Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China b National Key Lab of Advanced Welding Technology, Harbin Institute of Technology, Harbin 150001, China
Received 28 January 2005; accepted 28 February 2005 Available online 19 April 2005
Abstract The microstructure near a diffusion interface was studied by means of scanning electron microscopy and electron probe microscopy, and the results indicated that the interface transition zone of Fe3 Al/Q235 dissimilar materials was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the sides of the interface. Microstructures of the interface and base materials were interlaced to form the microstructure of layer characteristic. With increased heating temperature and holding time, the width of the Fe3 Al/Q235 interface transition zone increased and the microstructure gradually became coarse. The microhardness in the diffusion transition zone was decreased and there was a peak value at the diffusion interface. The distribution of Al, Fe, and Cr in the interface transition zone was increased or decreased monotonically with some local concentration fluctuation. There was nearly no change in the concentration of C element near the interface. 2005 Published by Elsevier Inc. Keywords: Interfaces; Intermetallic compounds; Electron microscopy; Diffusion; Microstructure
1. Introduction The development of modern microanalysis technology facilitated research on microstructure and elemental distribution in welding fusion zones [1–3]. Formation of diffusion bonding joints determined the mutual diffusion of elements near the interface without a welded metal and fusion zone, which was obviously different from a fusionwelded joint. There was a transition zone in diffusion bonding joints, in which the microstructure was different from that of base materials, especially for base materials A/B with different composition and performance. Therefore, the qualities of the diffusion bonding joint were affected greatly by the microstructure in the interface transition zone. However, up to now, research about the microstructure performance in the diffusion bonding joint has been limited, es* Corresponding author. Fax: +86 0531 2609496.
E-mail address: [email protected] (Y. Li). 0021-9797/$ – see front matter 2005 Published by Elsevier Inc. doi:10.1016/j.jcis.2005.02.094
pecially for the diffusion bonding joint of Fe3 Al and steel dissimilar materials [4–6]. Identifying the relations among division of the interface transition zone, microstructure features, and distribution of the elements near the interface was fundamental to controlling the microstructure performance and improving the qualities of the diffusion bonding joint. The microstructure and elemental distribution in the diffusion bonding zone of Fe3 Al/Q235 dissimilar materials were analyzed by means of scanning electron microscopy (SEM) and electron probe microscopy (EPMA). A division for the interface transition zone of Fe3 Al and steel dissimilar materials was put forward and microstructure characteristics of the different regions of the bonding joint were researched. Effects of technological parameters (heating temperature T , holding time t, pressure P , etc.) on microhardness, microstructure performance, and elemental distribution near the interface were analyzed. An Fe3 Al/Q235 diffusion bonding joint with excellent microstructure performance can be obtained by controlling technological parameters.
522
Y. Li et al. / Journal of Colloid and Interface Science 288 (2005) 521–525
2. Experimental The materials used in the test are Fe3 Al intermetallic and Q235 low-carbon steel. Fe3 Al is melted by a vacuum induction furnace and then fabricated into a plate by hot-roll technology; its chemical composition and thermophysical performance are shown in Table 1. Composition of Q235 steel (%) is C 0.14, Si 0.10, Mn 0.5%. The oxidation film on the surface of Fe3 Al and Q235 steel was eliminated by mechanical and chemical methods before the diffusion bonding. Plates of Fe3 Al and Q235 steel were overlapped and then welded by Workhorse II vacuum diffusion bonding equipment. Technological parameters of the test were heating temperature T = 960–1080 ◦ C, holding time t = 15–60 min, pressure P = 10–17 MPa, and vacuum degree 4.5 × 10−5 Pa. Some specimens, including the Fe3 Al/Q235 diffusion interface, were cut out by means of a line-cutting machine and then the specimens were made into a series of metallographic samples. The distribution of Al and Fe near Fe3 Al/Q235 interface is measured by means of electron probe microscopy. Microhardness distribution near the Fe3 Al/Q235 interface was measured by means of a Shimadzu-type microscalerometer using a 25-g load and a load time of 12 s.
from that of the base near the contact interface. This region where the microstructure was different from the base is called the interface transition zone. The interface transition zone of A/B dissimilar materials was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the two sides of the interface. After the diffusion bonding of Fe3 Al and Q235 steel, the microregion formed near the original contact interface is called a mixed transition region. The feature region between the mixed transition region and Fe3 Al (or Q235 steel) is called the Fe3 Al side transition region (or Q235 side transition region). Microstructure features and divisions of the Fe3 Al/Q235 interface transition zone are shown in Fig. 2. Width of the interface transition zone was determined by original contact interface and technological parameters (heating temperature T , holding time t, pressure P , etc.) in diffusion bonding of A/B dissimilar materials. The closer the original contact interface was, the higher the heating temperature of diffusion bonding, and the longer the holding time, the wider the interface transition zone was. SEM observation indicated that mutual diffusion of elements near the interface brought about disappearance of the original con-
3. Results and analysis 3.1. Division of diffusion joint zone of Fe3 Al/steel dissimilar materials Owing to the difference in corrosion resistance of Fe3 Al intermetallic and Q235 carbon steel, the Fe3 Al side of diffusion joint was etched by HNO3 + HCl solution (HNO3 :HCl = 1:3) and the Q235 steel side of diffusion joint was etched by 3% HNO3 alcohol solution. Microstructure of the Fe3 Al/ Q235 diffusion bonding joint was observed by a JXA-840 scanning electron microscope. Microstructure characteristics of the Fe3 Al/Q235 interface and diffusion transition zone, taken by SEM, are shown in Fig. 1. During the diffusion bonding, elements from both sides of A/B base materials diffused to the contact interface. When diffusion elements reached an arriving certain concentration, a diffusion reaction appeared and formed a diffusion reaction layer near the interface, whose microstructure was different
Fig. 1. Interface microstructure feature of the Fe3 Al/Q235 diffusion bonding joint: (a) microstructure; (b) SEM features.
Table 1 Chemical composition and thermophysical performance of Fe3 Al intermetallic Chemical compositions (%) Fe Al
Cr
Nb
Zr
B
Ce
80.2
2.50
1.13
0.28
0.09
0.15
Thermophysical performances Structure Order critical temperature (◦ C)
15.71
Young’s modulus (GPa)
Melting point (◦ C)
Coefficient of heat expansion (10−6 K−1 )
Density (g cm−3 )
Tensile strength (MPa)
Elongation (%)
Hardness HRC
DO3
140
1540
11.5
6.72
455
2
29
540
Y. Li et al. / Journal of Colloid and Interface Science 288 (2005) 521–525
523
(a)
(b) Fig. 2. Divisions of interface transition zone in the diffusion bonding joint of Fe3 Al and Q235 steel: (a) microstructure near the interface; (b) divisions of the transition zone.
tact interface, and a diffusion transition zone between Fe3 Al and Q235 steel was formed. Owing to diffusion of elements, microstructure and phase structure near Fe3 Al or Q235 steel changed to a certain extent, and two transition regions near A or B base materials (namely, Fe3 Al transition region and Q235 transition region) and a mixed transition region were formed. Microstructures of the Fe3 Al/Q235 interface transition zone and base materials were mutually interlaced to form the layer microstructure characteristics. Microstructure of the mixed transition region extended continually. Owing to the mutual diffusion of Al and Cr at the two sides of the interface, microstructure near the interface of the diffusion bonding had a larger change. Grain size near the bonding interface was evaluated by means of an XQF-2000 microimage analyzer. According to the formula D 2 = 1/2N +3 (D is grain diameter, N is grain degree), measured and calculated results of grain size in the interface transition zone of Fe3 Al/Q235 diffusion bonding are shown in Table 2. Grain diameter decreased from 250 to 112 µm in crossing the interface from Fe3 Al base to Q235 steel. Microstructure in the Fe3 Al/Q235 interface transition zone was finer than that of Fe3 Al base. This is favorable to improving strength performance of the Fe3 Al/Q235 diffusion bonding joint.
Fig. 3. Effect of technological parameters on width of the diffusion bonding transition zone for Fe3 Al/Q235 interface: (a) effect of heating temperature; (b) effect of holding time.
3.2. Effect of technological parameters on microstructure of interface transition zone With the increasing of heating temperature and holding time, the width of the interface transition zone of Fe3 Al/Q235 diffusion bonding increased and the microstructure coarsened gradually because of element diffusion near the interface. Effects of technological parameters on width of the Fe3 Al/Q235 transition zone, measured under the microscope, are shown in Fig. 3. Width of the Fe3 Al/Q235 transition zone increased to 32 µm for heating temperature 1080 ◦ C and holding time 60 min. It can be predicted according to the measured results that width of the interface transition zone would still increase if the heating temperature were further raised (see Fig. 3a). But the microstructure near the interface would coarsen if the heating temperature were higher. This is not favorable to microstructure and mechanical properties of the diffusion bonding joint. Therefore, the heating temperature should be limited suitably during the diffusion bonding. Width of the interface transition zone increases gradually with holding time (when t < 45 min). Width of the inter-
Table 2 Grain size in the interface transition zone of Fe3 Al/Q235 diffusion bonding Position
Fe3 Al base
Fe3 Al side transition region
Mixed transition region
Q235 side transition region
Grain degree Grain diameter (µm)
1.2 250
1.5 210
2.5 173
3.3 112
524
Y. Li et al. / Journal of Colloid and Interface Science 288 (2005) 521–525
mixed transition region appeared peak value (HM550). Diffusion of Al atoms near the interface promotes the Fe3 Al phase to undergo disorder transformation resulting in decreased microhardness near the Fe3 Al side transition region. Elements near the interface when heating temperature is lower (T = 1020 ◦ C) cannot sufficiently diffuse and gather together to form a phase structure of greater hardness. For heating temperature 1060 ◦ C but holding time 30 min, microhardness in the interface transition zone of Fe3 Al/Q235 diffusion bonding decreased because a diffusion reaction near the interface results in forming a different phase structure. A proportion of 13.9–20% Al in the Fe–Al alloy at room temperature forms an Fe3 Al phase having a superlattice structure and 20–36% Al forms an FeAl phase [7]. For heating temperature 1060 ◦ C and holding time 45–60 min, the joint zone of Fe3 Al/Q235 diffusion bonding did not show an obvious high hardness brittle phase and microhardness in the diffusion transition zone was even (HM460–540). This is favorable to increase toughness and prevent cracks in the joint zone of diffusion bonding. Fig. 4. Microhardness distribution in the transition zone of Fe3 Al/Q235 diffusion bonding: (a) measured point; (b) microhardness distribution.
face transition zone does not increase after the holding time t > 60 min (see Fig. 3b). In the initial stage of the holding temperature, effect of the holding time on element diffusion is greater, and the element diffusion increases with prolongation of the holding time. When t > 60 min, effect of the holding time on element diffusion decreases, and an interface transition zone that has a different microstructure is formed. But, if the holding time was too long, microstructure near the interface would grow up and coarsen to affect the mechanical properties of the diffusion bonding joint. Heating temperature and holding time determined microstructure performance of the interface transition zone. In order to obtain an excellent diffusion bonding joint combines sufficient atomic diffusion and good microstructure performance, technological parameters should be coordinately controlled. The test results indicated that suitable technological parameters of the Fe3 Al/Q235 diffusion bonding are heating temperature T = 1040–1060 ◦ C, holding time t = 45–60 min, and pressure P = 12–15 MPa. In order to ascertain the change of the microstructure performance of the diffusion bonding joint, microhardness near the Fe3 Al/Q235 interface was measured by means of a microscalerometer. The measured points and microhardness distribution near the interface of Fe3 Al/Q235 diffusion bonding are shown in Fig. 4. Microhardness of the Fe3 Al base after diffusion bonding was HM490 and microhardness of Q235 steel was HM350. Microhardness in the interface transition zone of Fe3 Al/Q235 diffusion bonding changed with technological parameters of diffusion bonding. For heating temperature 1020 ◦ C and holding time 60 min, microhardness in the
3.3. Element distribution near the interface of Fe3 Al/Q235 diffusion bonding Microstructure performance in the interface transition zone of Fe3 Al/Q235 diffusion bonding is determined by element diffusion near the interface. Distribution of Al, Fe, Cr, and C in the transition zone, under the conditions T = 1060 ◦ C, t = 60 min, and P = 15 MPa, was measured by means of EPMA. Measurement range in the transition zone was 30 µm. Measured results of EPMA line scanning of alloy elements (Al, Fe, Cr, C) near the interface are shown in Fig. 5. EPMA measured results indicated that distribution of Al, Fe, and Cr in the interface transition zone increased or decreased monotonically with concentration fluctuation in the local region; see Figs. 5a–5c. By calculation according to Al and Fe content, phase structure in the interface transition zone is composed mainly of an Fe3 Al phase and an α-Fe (Al) solid solution. There was a Cr-rich region. Cr richness of the local region is a reason that a microhardness peak value appears in the interface transition zone. C element diffusion is relative to solid solution temperature of carbide formation near the Fe3 Al/Q235 interface. At higher than carbide-forming temperature, concentration gradient pushes forward diffusion. At lower than carbideforming temperature, activity gradient pushes forward diffusion. EPMA analysis indicated that there was nearly no change in the concentration of C near the interface from the Fe3 Al side to the Q235 steel side (see Fig. 5d). Diffusion migration of C was very small. Concentration difference and activity difference between Fe3 Al and Q235 steel is so small that diffusion migration of C near the interface is reduced.
Y. Li et al. / Journal of Colloid and Interface Science 288 (2005) 521–525
525
Fig. 5. Measured results of element distribution in the transition zone of Fe3 Al/Q235 diffusion bonding: (a) Al, (b) Fe, (c) Cr, (d) C.
4. Conclusions The interface transition zone of the Fe3 Al/Q235 diffusion bonding was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the two sides of the interface. With increased heating temperature and holding time, the width of Fe3 Al/Q235 interface transition zone increased and the microstructure coarsened gradually. The width of the interface transition zone increased from 22 to 32 µm when the heating temperature increased from 1000 to 1080 ◦ C with holding time 60 min. Microhardness of the interface transition zone near the Fe3 Al side decreased and the peak value of microhardness appeared at the diffusion interface. The maximum microhardness of the Fe3 Al/Q235 interface was HM530 under conditions of heating temperature 1060 ◦ C and holding time 60 min. EPMA analysis indicated that distribution of Al, Fe, and Cr in the interface transition zone increased or decreased monotonically with fluctuation in the local region from the Fe3 Al side to the Q235 steel
side, and there was nearly no change in the concentration of C near the interface.
Acknowledgments This project was supported by the National Natural Science Foundation of China (50375088) and the Shandong Province Natural Science Foundation (Y2003F05).
References [1] [2] [3] [4] [5] [6] [7]
W.F. Savage, E.F. Nippe, E.S. Szekeres, Weld. J. 55 (1976) 260. Y. Shi, Y. Zhang, Trans. China Weld. Ins. 6 (1985) 177. C.X. Pan, Mater. Charact. 33 (1994) 87. T. Helander, J. Agren, Metall. Mater. Trans. 28A (1997) 303. Y. Li, Z. Zou, B. Zhou, J. Mater. Sci. Technol. 17 (2001) 338. Y.J. Li, H.Q. Wu, J. Wang, Y.S. Yin, Mater. Sci. Technol. 19 (2003) 227. Y. Li, Y. Zhang, Y. Liu, J. Mater. Sci. 30 (1995) 2635.
Division and microstructure feature in the interface transition zone of Fe3Al/Q235 diffusion bonding Yajiang Li a,b,∗ , Juan Wang a , Yansheng Yin a , Haijun Ma a a Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China b National Key Lab of Advanced Welding Technology, Harbin Institute of Technology, Harbin 150001, China
Received 28 January 2005; accepted 28 February 2005 Available online 19 April 2005
Abstract The microstructure near a diffusion interface was studied by means of scanning electron microscopy and electron probe microscopy, and the results indicated that the interface transition zone of Fe3 Al/Q235 dissimilar materials was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the sides of the interface. Microstructures of the interface and base materials were interlaced to form the microstructure of layer characteristic. With increased heating temperature and holding time, the width of the Fe3 Al/Q235 interface transition zone increased and the microstructure gradually became coarse. The microhardness in the diffusion transition zone was decreased and there was a peak value at the diffusion interface. The distribution of Al, Fe, and Cr in the interface transition zone was increased or decreased monotonically with some local concentration fluctuation. There was nearly no change in the concentration of C element near the interface. 2005 Published by Elsevier Inc. Keywords: Interfaces; Intermetallic compounds; Electron microscopy; Diffusion; Microstructure
1. Introduction The development of modern microanalysis technology facilitated research on microstructure and elemental distribution in welding fusion zones [1–3]. Formation of diffusion bonding joints determined the mutual diffusion of elements near the interface without a welded metal and fusion zone, which was obviously different from a fusionwelded joint. There was a transition zone in diffusion bonding joints, in which the microstructure was different from that of base materials, especially for base materials A/B with different composition and performance. Therefore, the qualities of the diffusion bonding joint were affected greatly by the microstructure in the interface transition zone. However, up to now, research about the microstructure performance in the diffusion bonding joint has been limited, es* Corresponding author. Fax: +86 0531 2609496.
E-mail address: [email protected] (Y. Li). 0021-9797/$ – see front matter 2005 Published by Elsevier Inc. doi:10.1016/j.jcis.2005.02.094
pecially for the diffusion bonding joint of Fe3 Al and steel dissimilar materials [4–6]. Identifying the relations among division of the interface transition zone, microstructure features, and distribution of the elements near the interface was fundamental to controlling the microstructure performance and improving the qualities of the diffusion bonding joint. The microstructure and elemental distribution in the diffusion bonding zone of Fe3 Al/Q235 dissimilar materials were analyzed by means of scanning electron microscopy (SEM) and electron probe microscopy (EPMA). A division for the interface transition zone of Fe3 Al and steel dissimilar materials was put forward and microstructure characteristics of the different regions of the bonding joint were researched. Effects of technological parameters (heating temperature T , holding time t, pressure P , etc.) on microhardness, microstructure performance, and elemental distribution near the interface were analyzed. An Fe3 Al/Q235 diffusion bonding joint with excellent microstructure performance can be obtained by controlling technological parameters.
522
Y. Li et al. / Journal of Colloid and Interface Science 288 (2005) 521–525
2. Experimental The materials used in the test are Fe3 Al intermetallic and Q235 low-carbon steel. Fe3 Al is melted by a vacuum induction furnace and then fabricated into a plate by hot-roll technology; its chemical composition and thermophysical performance are shown in Table 1. Composition of Q235 steel (%) is C 0.14, Si 0.10, Mn 0.5%. The oxidation film on the surface of Fe3 Al and Q235 steel was eliminated by mechanical and chemical methods before the diffusion bonding. Plates of Fe3 Al and Q235 steel were overlapped and then welded by Workhorse II vacuum diffusion bonding equipment. Technological parameters of the test were heating temperature T = 960–1080 ◦ C, holding time t = 15–60 min, pressure P = 10–17 MPa, and vacuum degree 4.5 × 10−5 Pa. Some specimens, including the Fe3 Al/Q235 diffusion interface, were cut out by means of a line-cutting machine and then the specimens were made into a series of metallographic samples. The distribution of Al and Fe near Fe3 Al/Q235 interface is measured by means of electron probe microscopy. Microhardness distribution near the Fe3 Al/Q235 interface was measured by means of a Shimadzu-type microscalerometer using a 25-g load and a load time of 12 s.
from that of the base near the contact interface. This region where the microstructure was different from the base is called the interface transition zone. The interface transition zone of A/B dissimilar materials was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the two sides of the interface. After the diffusion bonding of Fe3 Al and Q235 steel, the microregion formed near the original contact interface is called a mixed transition region. The feature region between the mixed transition region and Fe3 Al (or Q235 steel) is called the Fe3 Al side transition region (or Q235 side transition region). Microstructure features and divisions of the Fe3 Al/Q235 interface transition zone are shown in Fig. 2. Width of the interface transition zone was determined by original contact interface and technological parameters (heating temperature T , holding time t, pressure P , etc.) in diffusion bonding of A/B dissimilar materials. The closer the original contact interface was, the higher the heating temperature of diffusion bonding, and the longer the holding time, the wider the interface transition zone was. SEM observation indicated that mutual diffusion of elements near the interface brought about disappearance of the original con-
3. Results and analysis 3.1. Division of diffusion joint zone of Fe3 Al/steel dissimilar materials Owing to the difference in corrosion resistance of Fe3 Al intermetallic and Q235 carbon steel, the Fe3 Al side of diffusion joint was etched by HNO3 + HCl solution (HNO3 :HCl = 1:3) and the Q235 steel side of diffusion joint was etched by 3% HNO3 alcohol solution. Microstructure of the Fe3 Al/ Q235 diffusion bonding joint was observed by a JXA-840 scanning electron microscope. Microstructure characteristics of the Fe3 Al/Q235 interface and diffusion transition zone, taken by SEM, are shown in Fig. 1. During the diffusion bonding, elements from both sides of A/B base materials diffused to the contact interface. When diffusion elements reached an arriving certain concentration, a diffusion reaction appeared and formed a diffusion reaction layer near the interface, whose microstructure was different
Fig. 1. Interface microstructure feature of the Fe3 Al/Q235 diffusion bonding joint: (a) microstructure; (b) SEM features.
Table 1 Chemical composition and thermophysical performance of Fe3 Al intermetallic Chemical compositions (%) Fe Al
Cr
Nb
Zr
B
Ce
80.2
2.50
1.13
0.28
0.09
0.15
Thermophysical performances Structure Order critical temperature (◦ C)
15.71
Young’s modulus (GPa)
Melting point (◦ C)
Coefficient of heat expansion (10−6 K−1 )
Density (g cm−3 )
Tensile strength (MPa)
Elongation (%)
Hardness HRC
DO3
140
1540
11.5
6.72
455
2
29
540
Y. Li et al. / Journal of Colloid and Interface Science 288 (2005) 521–525
523
(a)
(b) Fig. 2. Divisions of interface transition zone in the diffusion bonding joint of Fe3 Al and Q235 steel: (a) microstructure near the interface; (b) divisions of the transition zone.
tact interface, and a diffusion transition zone between Fe3 Al and Q235 steel was formed. Owing to diffusion of elements, microstructure and phase structure near Fe3 Al or Q235 steel changed to a certain extent, and two transition regions near A or B base materials (namely, Fe3 Al transition region and Q235 transition region) and a mixed transition region were formed. Microstructures of the Fe3 Al/Q235 interface transition zone and base materials were mutually interlaced to form the layer microstructure characteristics. Microstructure of the mixed transition region extended continually. Owing to the mutual diffusion of Al and Cr at the two sides of the interface, microstructure near the interface of the diffusion bonding had a larger change. Grain size near the bonding interface was evaluated by means of an XQF-2000 microimage analyzer. According to the formula D 2 = 1/2N +3 (D is grain diameter, N is grain degree), measured and calculated results of grain size in the interface transition zone of Fe3 Al/Q235 diffusion bonding are shown in Table 2. Grain diameter decreased from 250 to 112 µm in crossing the interface from Fe3 Al base to Q235 steel. Microstructure in the Fe3 Al/Q235 interface transition zone was finer than that of Fe3 Al base. This is favorable to improving strength performance of the Fe3 Al/Q235 diffusion bonding joint.
Fig. 3. Effect of technological parameters on width of the diffusion bonding transition zone for Fe3 Al/Q235 interface: (a) effect of heating temperature; (b) effect of holding time.
3.2. Effect of technological parameters on microstructure of interface transition zone With the increasing of heating temperature and holding time, the width of the interface transition zone of Fe3 Al/Q235 diffusion bonding increased and the microstructure coarsened gradually because of element diffusion near the interface. Effects of technological parameters on width of the Fe3 Al/Q235 transition zone, measured under the microscope, are shown in Fig. 3. Width of the Fe3 Al/Q235 transition zone increased to 32 µm for heating temperature 1080 ◦ C and holding time 60 min. It can be predicted according to the measured results that width of the interface transition zone would still increase if the heating temperature were further raised (see Fig. 3a). But the microstructure near the interface would coarsen if the heating temperature were higher. This is not favorable to microstructure and mechanical properties of the diffusion bonding joint. Therefore, the heating temperature should be limited suitably during the diffusion bonding. Width of the interface transition zone increases gradually with holding time (when t < 45 min). Width of the inter-
Table 2 Grain size in the interface transition zone of Fe3 Al/Q235 diffusion bonding Position
Fe3 Al base
Fe3 Al side transition region
Mixed transition region
Q235 side transition region
Grain degree Grain diameter (µm)
1.2 250
1.5 210
2.5 173
3.3 112
524
Y. Li et al. / Journal of Colloid and Interface Science 288 (2005) 521–525
mixed transition region appeared peak value (HM550). Diffusion of Al atoms near the interface promotes the Fe3 Al phase to undergo disorder transformation resulting in decreased microhardness near the Fe3 Al side transition region. Elements near the interface when heating temperature is lower (T = 1020 ◦ C) cannot sufficiently diffuse and gather together to form a phase structure of greater hardness. For heating temperature 1060 ◦ C but holding time 30 min, microhardness in the interface transition zone of Fe3 Al/Q235 diffusion bonding decreased because a diffusion reaction near the interface results in forming a different phase structure. A proportion of 13.9–20% Al in the Fe–Al alloy at room temperature forms an Fe3 Al phase having a superlattice structure and 20–36% Al forms an FeAl phase [7]. For heating temperature 1060 ◦ C and holding time 45–60 min, the joint zone of Fe3 Al/Q235 diffusion bonding did not show an obvious high hardness brittle phase and microhardness in the diffusion transition zone was even (HM460–540). This is favorable to increase toughness and prevent cracks in the joint zone of diffusion bonding. Fig. 4. Microhardness distribution in the transition zone of Fe3 Al/Q235 diffusion bonding: (a) measured point; (b) microhardness distribution.
face transition zone does not increase after the holding time t > 60 min (see Fig. 3b). In the initial stage of the holding temperature, effect of the holding time on element diffusion is greater, and the element diffusion increases with prolongation of the holding time. When t > 60 min, effect of the holding time on element diffusion decreases, and an interface transition zone that has a different microstructure is formed. But, if the holding time was too long, microstructure near the interface would grow up and coarsen to affect the mechanical properties of the diffusion bonding joint. Heating temperature and holding time determined microstructure performance of the interface transition zone. In order to obtain an excellent diffusion bonding joint combines sufficient atomic diffusion and good microstructure performance, technological parameters should be coordinately controlled. The test results indicated that suitable technological parameters of the Fe3 Al/Q235 diffusion bonding are heating temperature T = 1040–1060 ◦ C, holding time t = 45–60 min, and pressure P = 12–15 MPa. In order to ascertain the change of the microstructure performance of the diffusion bonding joint, microhardness near the Fe3 Al/Q235 interface was measured by means of a microscalerometer. The measured points and microhardness distribution near the interface of Fe3 Al/Q235 diffusion bonding are shown in Fig. 4. Microhardness of the Fe3 Al base after diffusion bonding was HM490 and microhardness of Q235 steel was HM350. Microhardness in the interface transition zone of Fe3 Al/Q235 diffusion bonding changed with technological parameters of diffusion bonding. For heating temperature 1020 ◦ C and holding time 60 min, microhardness in the
3.3. Element distribution near the interface of Fe3 Al/Q235 diffusion bonding Microstructure performance in the interface transition zone of Fe3 Al/Q235 diffusion bonding is determined by element diffusion near the interface. Distribution of Al, Fe, Cr, and C in the transition zone, under the conditions T = 1060 ◦ C, t = 60 min, and P = 15 MPa, was measured by means of EPMA. Measurement range in the transition zone was 30 µm. Measured results of EPMA line scanning of alloy elements (Al, Fe, Cr, C) near the interface are shown in Fig. 5. EPMA measured results indicated that distribution of Al, Fe, and Cr in the interface transition zone increased or decreased monotonically with concentration fluctuation in the local region; see Figs. 5a–5c. By calculation according to Al and Fe content, phase structure in the interface transition zone is composed mainly of an Fe3 Al phase and an α-Fe (Al) solid solution. There was a Cr-rich region. Cr richness of the local region is a reason that a microhardness peak value appears in the interface transition zone. C element diffusion is relative to solid solution temperature of carbide formation near the Fe3 Al/Q235 interface. At higher than carbide-forming temperature, concentration gradient pushes forward diffusion. At lower than carbideforming temperature, activity gradient pushes forward diffusion. EPMA analysis indicated that there was nearly no change in the concentration of C near the interface from the Fe3 Al side to the Q235 steel side (see Fig. 5d). Diffusion migration of C was very small. Concentration difference and activity difference between Fe3 Al and Q235 steel is so small that diffusion migration of C near the interface is reduced.
Y. Li et al. / Journal of Colloid and Interface Science 288 (2005) 521–525
525
Fig. 5. Measured results of element distribution in the transition zone of Fe3 Al/Q235 diffusion bonding: (a) Al, (b) Fe, (c) Cr, (d) C.
4. Conclusions The interface transition zone of the Fe3 Al/Q235 diffusion bonding was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the two sides of the interface. With increased heating temperature and holding time, the width of Fe3 Al/Q235 interface transition zone increased and the microstructure coarsened gradually. The width of the interface transition zone increased from 22 to 32 µm when the heating temperature increased from 1000 to 1080 ◦ C with holding time 60 min. Microhardness of the interface transition zone near the Fe3 Al side decreased and the peak value of microhardness appeared at the diffusion interface. The maximum microhardness of the Fe3 Al/Q235 interface was HM530 under conditions of heating temperature 1060 ◦ C and holding time 60 min. EPMA analysis indicated that distribution of Al, Fe, and Cr in the interface transition zone increased or decreased monotonically with fluctuation in the local region from the Fe3 Al side to the Q235 steel
side, and there was nearly no change in the concentration of C near the interface.
Acknowledgments This project was supported by the National Natural Science Foundation of China (50375088) and the Shandong Province Natural Science Foundation (Y2003F05).
References [1] [2] [3] [4] [5] [6] [7]
W.F. Savage, E.F. Nippe, E.S. Szekeres, Weld. J. 55 (1976) 260. Y. Shi, Y. Zhang, Trans. China Weld. Ins. 6 (1985) 177. C.X. Pan, Mater. Charact. 33 (1994) 87. T. Helander, J. Agren, Metall. Mater. Trans. 28A (1997) 303. Y. Li, Z. Zou, B. Zhou, J. Mater. Sci. Technol. 17 (2001) 338. Y.J. Li, H.Q. Wu, J. Wang, Y.S. Yin, Mater. Sci. Technol. 19 (2003) 227. Y. Li, Y. Zhang, Y. Liu, J. Mater. Sci. 30 (1995) 2635.