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Experimental study of hot cracking at circular welding joints of 42CrMo steel
Optics and Laser Technology 97 (2017) 327–334
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Experimental study of hot cracking at circular welding joints of 42CrMo steel Yan Zhang a,b, Genyu Chen a,c,⇑, Binghua Chen c, Jinhai Wang c, Cong Zhou a a
State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, PR China College of Mechanical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, PR China c Han’s Laser Technology Co., Ltd., Shenzhen 518103, PR China b
a r t i c l e
i n f o
Article history: Received 28 November 2016 Received in revised form 23 May 2017 Accepted 14 July 2017
Keywords: 42CrMo Laser-MAG hybrid welding Circular welding Hot cracking
a b s t r a c t The hot cracking at circular welding joints of quenched and tempered 42CrMo steel were studied. The flow of the molten pool and the solidification process of weld were observed with a high-speed video camera. The information on the variations in the weld temperature was collected using an infrared (IR) thermal imaging system. The metallurgical factors of hot cracking were analyzed via metallographic microscope and scanning electron microscope (SEM). The result shows that leading laser laser-metal active gas (MAG) hybrid welding process has a smaller solid–liquid boundary movement rate (V SL ) and a smaller solid–liquid boundary temperature gradient (GSL ) compared with leading arc laser-MAG hybrid welding process and laser welding process. Additionally, the metal in the molten pool has superior permeability while flowing toward the dendritic roots and can compensate for the inner-dendritic pressure balance. Therefore, leading laser laser-MAG hybrid welding process has the lowest hot cracking susceptibility. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Because automobile pistons now bear significantly heavier mechanical loads and thermal loads, conventional aluminum pistons cannot meet reliability requirements, and thus, forged steel pistons have emerged and have gradually gained wide application. Quenched and tempered 42CrMo steel is one of the primary manufacturing materials of steel pistons [1]. Fig. 1 shows the structure of a certain type of quenched and tempered 42CrMo steel piston, and its weld type is circular welding. 42CrMo material has high carbon content and alloy content. The recommended formula of the International Institute of Welding (IIW) reveals that the carbon equivalent (Ceq) of this type of steel is approximately 0.87%, which indicates that 42CrMo steel has poor weldability and weld defects, such as pores and cracks, can easily occur. Currently, popular welding methods are electronic beam welding and friction welding. In related studies, a neodymium-doped yttrium aluminum garnet (Nd:YAG) continuous-wave laser has been used to weld heterogeneous metals, such as nickel-based superalloy K418 and 42CrMo alloy steel, to investigate how welding parameters affect the surface morphology and fusion depth of welds and to analyze the metallographic organization and phase composition of welds ⇑ Corresponding author at: State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, PR China. E-mail address: [email protected] (G. Chen). http://dx.doi.org/10.1016/j.optlastec.2017.07.018 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
[2–5]. In other studies, numerical simulations and experimental analyses of laser lap welding have been performed on heterogeneous metals, such as Ti6Al4V and 42CrMo [6]. The laser keyhole welding of 42CrMo in air and argon atmospheres has been studied both experimentally and numerically, and found that a small amount of oxygen could significantly modify the weld pool dimension, while keeping the temperature history of materials, and thus the microstructure, in the fusion zone and heat-affected zone unchanged [7]. The primary challenge of the welding process for circular welding of quenched and tempered 42CrMo steel is that hot cracking can easily occur at the weld joints. However, there are a limited number of relevant studies. The research about hot cracking rate in Al fiber laser welding under various process conditions found that, The increase of closest trim edge (L) and welding speed (v) resulted in the decrease of hot cracking, but increasing laser power (P) would tend hot cracking to increase [8]. WC particles (WCp) reinforced Fe-based metal matrix composites (WCp/Fe) were manufactured by laser melting deposition (LMD) technology to investigate the characteristics of cracks formation, and the result indicated that the cracks initiated inside WC particle and propagated along the eutectic phase in the matrix [9]. DZ4125 superalloy is prepared by laser solid forming (LSF) method under the heat input between 50 J/mm and 150 J/mm, it is shown that the liquation cracks occurred in LSFed sample with the lower heat input, the propagation extent of the cracks can be reduced with increasing the heat input [10]. The formation
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Fig. 1. The structure of 42CrMo steel piston.
mechanism of hot cracking is related to metallurgical factors, stress, strain, and strain rate (e_ ) [11]. The Rappaz-Drezet-Germaud model indicates that the initiation of hot cracking is caused by the inability of the molten material to compensate for the pressure drop at the dendritic roots [12], the inner-dendritic pressure drop is induced by the e_ transverse to the columnar dendrites. Hot cracking is initiated when this pressure falls below a specific material value for cavitation. The pressure compensation to avoid hot cracking is determined by the permeability of the molten material to flow to the dendritic roots. The hot cracking sensitivity coefficient is based on the inner-dendritic compensation balance with increasing e_ at a certain fraction of solid phase [13,14]. A higher e_ results in a lower critical strain for hot cracking formation [15–17]. Welding strain is caused by the external load, solidification contraction, and thermal contraction [18]. Lower welding speed and V SL help lessen the formation of hot cracking [19–22]. Additionally, reducing GSL during welding helps prevent hot cracking formation [19]. However, reducing e_ may trigger other mechanisms of hot cracking. The length of the vulnerable semi-solid increases for lower GSL [23]. When other parameters are fixed and the welding speed increases, although V SL increases, which is due to the lower welding thermal input, the number of hot cracking decreases [24]. Metallurgical factors also contribute to hot cracking formation. A welding test on 6061-T6 aluminum alloy showed that a smaller V SL increases the solidification temperature range and results in more solidification segregation at the crystal boundaries; thus, a hot cracking is more likely to occur [25]. In this research, a welding test to simulate the circular welding on quenched and tempered 42CrMo steel is performed. A highspeed camera is used to observe fluid flow in the molten pool and weld solidification during the welding process to calculate V SL . An IR thermal imaging device is used to collect the information on the variations in the weld temperature during this period to calculate GSL . A metallographic microscope and a SEM are used to investigate metallurgical factors for hot cracking. Three circular welding processes, laser welding, leading laser laser-MAG hybrid welding, and leading arc laser-MAG hybrid welding, are compared to investigate the possibility of hot cracking formation at the weld joints. 2. Experiments 2.1. Material and equipment A YLS-10000 fiber laser system with a Precitec laser welding head was used in the experiment. The parameters of the fiber laser system were as follows: rated output power: 10.0 kW, working mode: continuous, emitted laser wavelength: 1.07 µm. In addition, a Fronius TPS 5000 arc welding system and a Fronius arc welding
gun were used. The direct-current reverse-polarity arc mode was selected. Based on the welding system’s built-in expert database, the wire feed rate, current and voltage were collectively tuned to control the welding process, and the wire feed rate was used as the representative parameter. A KUKA KR60HA robot was used to control the experimental laser welding process. A Photron SA4 complementary metal–oxide–semiconductor high-speed video camera with a self-adaptive exposure time adjustment system was used to capture high-speed images of the deep penetration laser welding process. In addition, an 808 nm semiconductor laser system was also used as an auxiliary light source for the filming of the molten weld pool. An FLIR A615 IR thermal imaging system was used to acquire temperature variation signals during the welding process. Specimens were prepared on a DK7745 computer numerically controlled electrical discharge wire-cutting machine and mounted using a ZXQ-5hs specimen mounting press machine. After each specimen was polished and ground using an MP-2A polish-grinding machine and cleaned using a KQ5200B ultrasonic cleaner, the microscopic crack morphology was observed using a TESCAN MIRA3 LMU field-emission SEM instrument. The chemical composition around the crack was determined using an Oxford X-Max 20 EDS system. Because 42CrMo steel contains relatively large amounts of alloying elements to ensure its hardenability, quenched and tempered 42CrMo steel has a relatively high strength and hardness and a yield strength of as high as 880–1176 MPa. Esab ER80S-G welding wires (diameter: 1.0 mm) were used in the experiment. Table 1 presents the chemical composition of quenched and tempered 42CrMo steel and ER80S-G welding wires. 2.2. Experimental procedure The experimental specimens were 42CrMo steel plates (thickness: 7 mm; length: 100 mm; width: 30 mm). The steel plates were subjected to bead-on-plate welding using the laser welding and laser–MAG hybrid welding process. A Y-shaped groove with an angle of 60° and a depth of 2 mm was produced on each steel plate prior to welding (Fig. 2). The groove type and size can affect not only the amount of weld material consumed but also the weld quality. In addition, the groove also affects the welding heat input, the phase-change process of the weld and the grain size and hardness [26]. Prior to welding, each specimen was carefully cleaned with acetone, after which the specimen was preheated to 350 °C in a KSY-14-16 three-phase silicon-controlled temperature control box and was allowed to remain at 350 °C for 3 h. The weld area was protected by means of the paraxial blowing of a shielding gas. The shielding gas of laser welding was pure Ar and The shielding gas of laser–MAG hybrid welding was a mixture of Ar (80%) and CO2
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Y. Zhang et al. / Optics and Laser Technology 97 (2017) 327–334 Table 1 The chemical composition of 42CrMo and welding wires (wt.%).
42CrMo ER80S-G
C
Si
Mn
S
P
Cr
Ni
Cu
Mo
0.44 0.09
0.23 0.65
0.76 1.0
0.017 0.015
0.011 0.01
0.99 –
0.042 –
0.04 –
0.17 0.45
Fig. 2. The sketch of groove.
(20%). The addition of CO2 can help to prevent pore formation in the molten pool, increase the wettability of the molten pool material, control the metal vapor during welding and eliminate undercut defects [27]. Three welding experiments were conducted under each combination of parameters. Afterward, A phoenix v/tome/x m 300 kV industrial computerized tomography (CT) system was used to nondestructive detection. In this test, circular welding of quenched and tempered 42CrMo steel is investigated. Because the circular weld has a relatively large diameter, to facilitate the test, a specific arc segment of a circular weld is approximated as a straight line. Therefore, this test is designed to simulate 42CrMo steel circular welding via straight welding. The test of the welding process is shown in Fig. 3. The welding begins at point A of the plate, and welding follows a straight line to point B; then, the welding head is raised and travels back to point C, and the welding head pauses for a period of time (the pause time is calculated from the circumference of the circular weld and welding speed) and then continues welding from point C to point D. Among these points, segments A to D are re-melting area of the weld joint. Observations of the flow of the molten pool, the solidification of the weld metals and the weld temperature during the welding process were collected in real time using a high-speed video camera and an IR thermal imaging system. Fig. 4 shows the setup of the experimental platform.
3. Results and discussion 3.1. Nondestructive CT scanner test result In the test, the hot cracking sensitivities of quenched and tempered 42CrMo steel at the circular weld joints are compared and analyzed for three welding processes: Leading laser laser-MAG hybrid welding, leading arc weld laser-MAG hybrid welding, and pure laser welding. Among them, the two hybrid welding processes have identical parameters (defocusing distance: +2 mm; laser power: 4200 W; laser-arc distance: 3.5 mm; welding speed: 0.75 m/min; wire feed rate: 5 m/min; shielding gas flow rate: 15 L/min). To ensure complete penetration and weld shape quality,
Fig. 3. The sketch of welding process.
Fig. 4. The Welding Process of Experimental Platform.
defocusing distance for pure laser welding was set to +7 mm, the laser power was set to 5200 W, and the welding speed was set to 0.75 mm/min. For simplification, in the following section, the parameters for leading laser laser-MAG hybrid welding are referred to as type I process parameters, the parameters for leading arc laser-MAG hybrid welding are referred to as type II process parameters, and the parameters for laser welding are referred to as type III process parameters. Table 2 lists the results of the nondestructive CT test at the circular weld joints for the three welding processes. Table 2 shows that the type I process is the optimum process, which introduced no cracks at the circular weld joints. By comparison, type II and type III processes produced hot cracking at the joints. 3.2. e_ and hot cracking formation mechanism
e_ is closely related to hot cracking sensitivity of the weld. Reducing the e_ in the welding process helps eliminate hot cracking [19–22]. The e_ in the welding process is estimated via formula (1) [19].
e_ ¼ bT T_ SL ¼ bT jGSL V SL j
ð1Þ
In formula (1), e_ is the strain rate; bT is the thermal contraction
coefficient of the welding material; T_ SL is the solid-liquid boundary cooling rate; GSL is the solid-liquid boundary temperature gradient; and V SL is the solid-liquid boundary movement rate. Because all three processes used quenched and tempered 42CrMo steel as the welding material, the processes have identical thermal contraction coefficients bT . Formula (1) shows that the welding process parameters can be changed to reduce GSL and V SL and thus reduce the hot cracking sensitivity of welds. 3.2.1. V SL During most of the weld solidification process, V SL is stable. A smaller or larger V SL is only found during the initial or final phase of solidification [28]. By the high-speed camera, Table 3 shows the dynamic process of the flow of the molten pool at the circular weld joints and the weld solidification process of the quenched and tempered 42CrMo steel during the three welding processes. The time interval between the images is 100 ms. The curves in the diagram show the molten pool border line and solid-liquid boundary.
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Table 2 The results of three welding processes. Serial number
Weld appearance
CT photograph
CT photograph
Type I
Crack-free
Type II
Hot cracking
Type III
Hot cracking
Table 3 The flow of the molten pool and the weld solidification process.
Type I
Type II
Based on Table 3, the calculation shows that when the type I process is applied, V SL during the weld solidification process is approximately 2.68 mm/s; when the type II process is applied, V SL is approximately 6.25 mm/s; and when the type III process is
Type III
applied, V SL is approximately 5.35 mm/s. A comparison of V SL from the three processes is shown in Fig. 5. It is worth noting that the V SL calculation method ignores the weld solidification rate in the longitudinal and depth directions; the calculation is only based on the
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3.2.2. GSL Formula (1) shows that besides V SL , GSL is another influential factor for e_ . The value of GSL is calculated via the information on the variations in the weld temperature during the welding process measured by the IR thermal imaging device. Fig. 6 shows the temperature-time curves captured by the IR thermal imaging device at the circular weld joints of 42CrMo steel using the three different processes. Among them, the black solid line is the type I process temperature-time curve, the blue dotted line is the type II process temperature-time curve, and the red dash-dotted line is the type III process temperature-time curve. The temperature-time curves in Fig. 6 show that the type I process weld has the longest cooling time, i.e., it has the smallest weld cooling rate, whereas the type II process and type III process have a larger weld cooling rate. The type I process has a peak temperature of 2240.35 °C at 3.10 s. When the temperature drops to 1200 °C, the corresponding time is 9.94 s, and the average cooling rate (V cool ) is approximately 152 °C/s. The type II process has a peak temperature of 2450.29 °C at 1.97 s. When the temperature drops to 1200 °C, the corresponding time is 5.60 s, and V cool is approximately 344 °C/s. The type III process has a peak temperature of 1884.06 °C at 2.21 s. When the temperature drops to 1200 °C, the corresponding time is 5.13 s, and V cool is approximately 234 °C/s. The welding speed was 0.75 mm/min in all three processes. Therefore, the GSL in the type I process is approximately 12,168 °C/m, the GSL in the type II process is approximately 27,555 °C/m, and the GSL in the type III process is approximately 18,742 °C/m. In other words, the GSL in the type I process is smaller than the GSL in the type II and type III processes. A comparison of the GSL in the three processes is shown in Fig. 7. The analysis in Section 3.2.1 and 3.2.2 shows that the type I process has a smaller V SL and GSL compared with the type II and type III processes. Formula (1) shows that the type I process has a considerably smaller e_ compared with the type II process and type III processes; therefore, it has the smallest hot cracking sensitivity.
Fig. 5. The comparison of V SL from the three processes.
2500 2300
the type I process the type II process the type III process
2100
Temperature (°C)
1900
331
1700 1500 1300 1100 900 700 500 300 100 1
2
3
4
5
6
Time (s)
7
8
9
Fig. 6. The temperature–time curves of three different processes.
Fig.7. The comparison of GSL from the three processes.
horizontal solidification rate of the molten pool. Furthermore, because the molten pool solid-liquid boundary displacement is within the microscopic scale, the measurement results will contain certain errors.
3.3. Metallurgical factors 3.3.1. Metallographic analysis Metallurgical factors are also closely related to hot cracking [25]. Fig. 8 shows the metallographic diagram and SEM image of the hot cracking from the type II process. Fig. 8 shows that the crack opening is along the columnar crystal boundary, and fractures separate along the inner-dendritic liquid membrane, as shown in the SEM images; i.e., a thin liquid membrane layer formed at the time of crack formation. Fig. 9 shows the metallographic diagram and SEM image of the hot cracking from the type III process. Fig. 9 shows that in pure laser welding, there is a significant amount of tensile stress on the welds during the solidification period; open cracks at the crater cannot be filled by a sufficient amount of liquid metal, and thus, crater cracks are formed. Pressure compensation against hot cracking formation is determined by the permeability of the molten pool fluid flowing toward the dendritic roots. Superior permeability means that it is more difficult to form hot cracking in welds. The permeability parameter (K) is calculated via formula (2) [29,30]:
K¼
k2 ð1 f s Þ 2 180 fs
3
ð2Þ
In formula (2), k is the secondary dendrite arm spacing, and f s is the representative fraction of the solid phase. Assume that the three welding processes have the same solid phase representative fraction f s ; then, a larger secondary dendrite arm spacing (k) and a larger K are beneficial to hot cracking suppression.
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Fig. 8. The metallographic diagram and SEM image of the hot cracking from the type II process.
Fig. 9. The metallographic diagram and SEM image of the hot cracking from the type III process.
The fitting function of k versus cooling rate for the 42CrMo steel is as follows [31]:
k ¼ 34:8 V 0:26 cool
ð3Þ
where k is the secondary dendrite arm spacing (lm), and V cool is the weld cooling rate (°C/s). Formula (3) shows that there is an exponential relationship between k and V cool . Based on formula (3), k of the type I process is approximately 9.43 lm, k of the type II process is approximately 7.62 lm, and k of the type III process is approximately 8.42 lm. Formula (2) shows that the type I process has a larger K compared with the type II process and type III process and also has the more balanced pressure compensation for the molten pool fluid flowing toward the dendritic roots. Therefore, it is least likely that hot cracking will form at the welds. 3.3.2. Molten pool dynamic behavior and molten pool material permeability Table 4 shows the flow of the molten pool captured by the highspeed camera when the type I process and type II welding process are applied. When the type I process is applied, the metal in the molten pool initially bypasses the laser keyhole from the front margin of the molten pool, then flows toward the keyhole from the trailing edge, and stops around there (as shown in Table 4 (a)). When the type II process is applied, the metal in the molten pool bypasses the keyhole and flows toward the rear of the molten
pool, i.e., the molten pool surface flows from the keyhole to the rear of the molten pool (as shown in Table 4(b)). In Table 4(a), inward flow of the molten pool in type I welding is beneficial to filling of the keyhole by the molten pool metal during the weld solidification phase, to the molten pool flow toward the dendritic roots, and to compensation of the inner-dendritic pressure balance to prevent hot cracking from forming at the weld. 4. Conclusions In this research, a welding test to simulate the circular welding on quenched and tempered 42CrMo steel is performed to compare and investigate the possibility of hot cracking formation at the weld joints in three welding processes: laser welding, leading laser laser-MAG hybrid welding, and leading arc laser-MAG hybrid welding. The following are the primary conclusions: (1) Industrial nondestructive CT test results for the weld show that when leading laser laser-MAG hybrid welding is applied to the circular welding on quenched and tempered 42CrMo steel, hot cracking do not form at the weld joints; in comparison, when leading arc laser-MAG hybrid welding or laser welding are applied, hot cracking form at the weld joints. (2) The molten pool flow and weld solidification during the welding process was captured by a high-speed camera to calculate V SL . The information on the variations in the weld
Y. Zhang et al. / Optics and Laser Technology 97 (2017) 327–334
333
Table 4 The flow of the molten pool during leading laser and leading arc.
temperature were collected via an IR thermal imaging device to calculate GSL . The result shows that leading laser laser-MAG hybrid welding has a smaller V SL and GSL compared with leading arc laser-MAG hybrid welding and laser welding; i.e., leading laser laser-MAG hybrid welding results in a far smaller e_ compared with the other two processes and has the lowest hot cracking sensitivity. (3) An analysis of the metallurgical factors for the three processes showed that compared with the other two processes, leading laser laser-MAG hybrid welding has the largest k; the molten pool fluid flowing toward the dendritic roots also exhibited the strongest permeability. These factors are beneficial to the inner-dendritic pressure balance compensation and in preventing hot cracking in the weld.
Acknowledgments The authors are grateful for the financial support provided by the National Natural Science Foundation of China (project number: 51175165) and the Key National Science and Technology Project (project number: 2013ZX04001131). In addition, the authors also wish to thank Mr. Hongzhong Liu from Shandong Binzhou Bohai Piston Co., Ltd., for his assistance with the industrial CT examination of the experimental weld workpieces.
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Full length article
Experimental study of hot cracking at circular welding joints of 42CrMo steel Yan Zhang a,b, Genyu Chen a,c,⇑, Binghua Chen c, Jinhai Wang c, Cong Zhou a a
State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, PR China College of Mechanical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, PR China c Han’s Laser Technology Co., Ltd., Shenzhen 518103, PR China b
a r t i c l e
i n f o
Article history: Received 28 November 2016 Received in revised form 23 May 2017 Accepted 14 July 2017
Keywords: 42CrMo Laser-MAG hybrid welding Circular welding Hot cracking
a b s t r a c t The hot cracking at circular welding joints of quenched and tempered 42CrMo steel were studied. The flow of the molten pool and the solidification process of weld were observed with a high-speed video camera. The information on the variations in the weld temperature was collected using an infrared (IR) thermal imaging system. The metallurgical factors of hot cracking were analyzed via metallographic microscope and scanning electron microscope (SEM). The result shows that leading laser laser-metal active gas (MAG) hybrid welding process has a smaller solid–liquid boundary movement rate (V SL ) and a smaller solid–liquid boundary temperature gradient (GSL ) compared with leading arc laser-MAG hybrid welding process and laser welding process. Additionally, the metal in the molten pool has superior permeability while flowing toward the dendritic roots and can compensate for the inner-dendritic pressure balance. Therefore, leading laser laser-MAG hybrid welding process has the lowest hot cracking susceptibility. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Because automobile pistons now bear significantly heavier mechanical loads and thermal loads, conventional aluminum pistons cannot meet reliability requirements, and thus, forged steel pistons have emerged and have gradually gained wide application. Quenched and tempered 42CrMo steel is one of the primary manufacturing materials of steel pistons [1]. Fig. 1 shows the structure of a certain type of quenched and tempered 42CrMo steel piston, and its weld type is circular welding. 42CrMo material has high carbon content and alloy content. The recommended formula of the International Institute of Welding (IIW) reveals that the carbon equivalent (Ceq) of this type of steel is approximately 0.87%, which indicates that 42CrMo steel has poor weldability and weld defects, such as pores and cracks, can easily occur. Currently, popular welding methods are electronic beam welding and friction welding. In related studies, a neodymium-doped yttrium aluminum garnet (Nd:YAG) continuous-wave laser has been used to weld heterogeneous metals, such as nickel-based superalloy K418 and 42CrMo alloy steel, to investigate how welding parameters affect the surface morphology and fusion depth of welds and to analyze the metallographic organization and phase composition of welds ⇑ Corresponding author at: State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, PR China. E-mail address: [email protected] (G. Chen). http://dx.doi.org/10.1016/j.optlastec.2017.07.018 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
[2–5]. In other studies, numerical simulations and experimental analyses of laser lap welding have been performed on heterogeneous metals, such as Ti6Al4V and 42CrMo [6]. The laser keyhole welding of 42CrMo in air and argon atmospheres has been studied both experimentally and numerically, and found that a small amount of oxygen could significantly modify the weld pool dimension, while keeping the temperature history of materials, and thus the microstructure, in the fusion zone and heat-affected zone unchanged [7]. The primary challenge of the welding process for circular welding of quenched and tempered 42CrMo steel is that hot cracking can easily occur at the weld joints. However, there are a limited number of relevant studies. The research about hot cracking rate in Al fiber laser welding under various process conditions found that, The increase of closest trim edge (L) and welding speed (v) resulted in the decrease of hot cracking, but increasing laser power (P) would tend hot cracking to increase [8]. WC particles (WCp) reinforced Fe-based metal matrix composites (WCp/Fe) were manufactured by laser melting deposition (LMD) technology to investigate the characteristics of cracks formation, and the result indicated that the cracks initiated inside WC particle and propagated along the eutectic phase in the matrix [9]. DZ4125 superalloy is prepared by laser solid forming (LSF) method under the heat input between 50 J/mm and 150 J/mm, it is shown that the liquation cracks occurred in LSFed sample with the lower heat input, the propagation extent of the cracks can be reduced with increasing the heat input [10]. The formation
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Fig. 1. The structure of 42CrMo steel piston.
mechanism of hot cracking is related to metallurgical factors, stress, strain, and strain rate (e_ ) [11]. The Rappaz-Drezet-Germaud model indicates that the initiation of hot cracking is caused by the inability of the molten material to compensate for the pressure drop at the dendritic roots [12], the inner-dendritic pressure drop is induced by the e_ transverse to the columnar dendrites. Hot cracking is initiated when this pressure falls below a specific material value for cavitation. The pressure compensation to avoid hot cracking is determined by the permeability of the molten material to flow to the dendritic roots. The hot cracking sensitivity coefficient is based on the inner-dendritic compensation balance with increasing e_ at a certain fraction of solid phase [13,14]. A higher e_ results in a lower critical strain for hot cracking formation [15–17]. Welding strain is caused by the external load, solidification contraction, and thermal contraction [18]. Lower welding speed and V SL help lessen the formation of hot cracking [19–22]. Additionally, reducing GSL during welding helps prevent hot cracking formation [19]. However, reducing e_ may trigger other mechanisms of hot cracking. The length of the vulnerable semi-solid increases for lower GSL [23]. When other parameters are fixed and the welding speed increases, although V SL increases, which is due to the lower welding thermal input, the number of hot cracking decreases [24]. Metallurgical factors also contribute to hot cracking formation. A welding test on 6061-T6 aluminum alloy showed that a smaller V SL increases the solidification temperature range and results in more solidification segregation at the crystal boundaries; thus, a hot cracking is more likely to occur [25]. In this research, a welding test to simulate the circular welding on quenched and tempered 42CrMo steel is performed. A highspeed camera is used to observe fluid flow in the molten pool and weld solidification during the welding process to calculate V SL . An IR thermal imaging device is used to collect the information on the variations in the weld temperature during this period to calculate GSL . A metallographic microscope and a SEM are used to investigate metallurgical factors for hot cracking. Three circular welding processes, laser welding, leading laser laser-MAG hybrid welding, and leading arc laser-MAG hybrid welding, are compared to investigate the possibility of hot cracking formation at the weld joints. 2. Experiments 2.1. Material and equipment A YLS-10000 fiber laser system with a Precitec laser welding head was used in the experiment. The parameters of the fiber laser system were as follows: rated output power: 10.0 kW, working mode: continuous, emitted laser wavelength: 1.07 µm. In addition, a Fronius TPS 5000 arc welding system and a Fronius arc welding
gun were used. The direct-current reverse-polarity arc mode was selected. Based on the welding system’s built-in expert database, the wire feed rate, current and voltage were collectively tuned to control the welding process, and the wire feed rate was used as the representative parameter. A KUKA KR60HA robot was used to control the experimental laser welding process. A Photron SA4 complementary metal–oxide–semiconductor high-speed video camera with a self-adaptive exposure time adjustment system was used to capture high-speed images of the deep penetration laser welding process. In addition, an 808 nm semiconductor laser system was also used as an auxiliary light source for the filming of the molten weld pool. An FLIR A615 IR thermal imaging system was used to acquire temperature variation signals during the welding process. Specimens were prepared on a DK7745 computer numerically controlled electrical discharge wire-cutting machine and mounted using a ZXQ-5hs specimen mounting press machine. After each specimen was polished and ground using an MP-2A polish-grinding machine and cleaned using a KQ5200B ultrasonic cleaner, the microscopic crack morphology was observed using a TESCAN MIRA3 LMU field-emission SEM instrument. The chemical composition around the crack was determined using an Oxford X-Max 20 EDS system. Because 42CrMo steel contains relatively large amounts of alloying elements to ensure its hardenability, quenched and tempered 42CrMo steel has a relatively high strength and hardness and a yield strength of as high as 880–1176 MPa. Esab ER80S-G welding wires (diameter: 1.0 mm) were used in the experiment. Table 1 presents the chemical composition of quenched and tempered 42CrMo steel and ER80S-G welding wires. 2.2. Experimental procedure The experimental specimens were 42CrMo steel plates (thickness: 7 mm; length: 100 mm; width: 30 mm). The steel plates were subjected to bead-on-plate welding using the laser welding and laser–MAG hybrid welding process. A Y-shaped groove with an angle of 60° and a depth of 2 mm was produced on each steel plate prior to welding (Fig. 2). The groove type and size can affect not only the amount of weld material consumed but also the weld quality. In addition, the groove also affects the welding heat input, the phase-change process of the weld and the grain size and hardness [26]. Prior to welding, each specimen was carefully cleaned with acetone, after which the specimen was preheated to 350 °C in a KSY-14-16 three-phase silicon-controlled temperature control box and was allowed to remain at 350 °C for 3 h. The weld area was protected by means of the paraxial blowing of a shielding gas. The shielding gas of laser welding was pure Ar and The shielding gas of laser–MAG hybrid welding was a mixture of Ar (80%) and CO2
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Y. Zhang et al. / Optics and Laser Technology 97 (2017) 327–334 Table 1 The chemical composition of 42CrMo and welding wires (wt.%).
42CrMo ER80S-G
C
Si
Mn
S
P
Cr
Ni
Cu
Mo
0.44 0.09
0.23 0.65
0.76 1.0
0.017 0.015
0.011 0.01
0.99 –
0.042 –
0.04 –
0.17 0.45
Fig. 2. The sketch of groove.
(20%). The addition of CO2 can help to prevent pore formation in the molten pool, increase the wettability of the molten pool material, control the metal vapor during welding and eliminate undercut defects [27]. Three welding experiments were conducted under each combination of parameters. Afterward, A phoenix v/tome/x m 300 kV industrial computerized tomography (CT) system was used to nondestructive detection. In this test, circular welding of quenched and tempered 42CrMo steel is investigated. Because the circular weld has a relatively large diameter, to facilitate the test, a specific arc segment of a circular weld is approximated as a straight line. Therefore, this test is designed to simulate 42CrMo steel circular welding via straight welding. The test of the welding process is shown in Fig. 3. The welding begins at point A of the plate, and welding follows a straight line to point B; then, the welding head is raised and travels back to point C, and the welding head pauses for a period of time (the pause time is calculated from the circumference of the circular weld and welding speed) and then continues welding from point C to point D. Among these points, segments A to D are re-melting area of the weld joint. Observations of the flow of the molten pool, the solidification of the weld metals and the weld temperature during the welding process were collected in real time using a high-speed video camera and an IR thermal imaging system. Fig. 4 shows the setup of the experimental platform.
3. Results and discussion 3.1. Nondestructive CT scanner test result In the test, the hot cracking sensitivities of quenched and tempered 42CrMo steel at the circular weld joints are compared and analyzed for three welding processes: Leading laser laser-MAG hybrid welding, leading arc weld laser-MAG hybrid welding, and pure laser welding. Among them, the two hybrid welding processes have identical parameters (defocusing distance: +2 mm; laser power: 4200 W; laser-arc distance: 3.5 mm; welding speed: 0.75 m/min; wire feed rate: 5 m/min; shielding gas flow rate: 15 L/min). To ensure complete penetration and weld shape quality,
Fig. 3. The sketch of welding process.
Fig. 4. The Welding Process of Experimental Platform.
defocusing distance for pure laser welding was set to +7 mm, the laser power was set to 5200 W, and the welding speed was set to 0.75 mm/min. For simplification, in the following section, the parameters for leading laser laser-MAG hybrid welding are referred to as type I process parameters, the parameters for leading arc laser-MAG hybrid welding are referred to as type II process parameters, and the parameters for laser welding are referred to as type III process parameters. Table 2 lists the results of the nondestructive CT test at the circular weld joints for the three welding processes. Table 2 shows that the type I process is the optimum process, which introduced no cracks at the circular weld joints. By comparison, type II and type III processes produced hot cracking at the joints. 3.2. e_ and hot cracking formation mechanism
e_ is closely related to hot cracking sensitivity of the weld. Reducing the e_ in the welding process helps eliminate hot cracking [19–22]. The e_ in the welding process is estimated via formula (1) [19].
e_ ¼ bT T_ SL ¼ bT jGSL V SL j
ð1Þ
In formula (1), e_ is the strain rate; bT is the thermal contraction
coefficient of the welding material; T_ SL is the solid-liquid boundary cooling rate; GSL is the solid-liquid boundary temperature gradient; and V SL is the solid-liquid boundary movement rate. Because all three processes used quenched and tempered 42CrMo steel as the welding material, the processes have identical thermal contraction coefficients bT . Formula (1) shows that the welding process parameters can be changed to reduce GSL and V SL and thus reduce the hot cracking sensitivity of welds. 3.2.1. V SL During most of the weld solidification process, V SL is stable. A smaller or larger V SL is only found during the initial or final phase of solidification [28]. By the high-speed camera, Table 3 shows the dynamic process of the flow of the molten pool at the circular weld joints and the weld solidification process of the quenched and tempered 42CrMo steel during the three welding processes. The time interval between the images is 100 ms. The curves in the diagram show the molten pool border line and solid-liquid boundary.
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Table 2 The results of three welding processes. Serial number
Weld appearance
CT photograph
CT photograph
Type I
Crack-free
Type II
Hot cracking
Type III
Hot cracking
Table 3 The flow of the molten pool and the weld solidification process.
Type I
Type II
Based on Table 3, the calculation shows that when the type I process is applied, V SL during the weld solidification process is approximately 2.68 mm/s; when the type II process is applied, V SL is approximately 6.25 mm/s; and when the type III process is
Type III
applied, V SL is approximately 5.35 mm/s. A comparison of V SL from the three processes is shown in Fig. 5. It is worth noting that the V SL calculation method ignores the weld solidification rate in the longitudinal and depth directions; the calculation is only based on the
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3.2.2. GSL Formula (1) shows that besides V SL , GSL is another influential factor for e_ . The value of GSL is calculated via the information on the variations in the weld temperature during the welding process measured by the IR thermal imaging device. Fig. 6 shows the temperature-time curves captured by the IR thermal imaging device at the circular weld joints of 42CrMo steel using the three different processes. Among them, the black solid line is the type I process temperature-time curve, the blue dotted line is the type II process temperature-time curve, and the red dash-dotted line is the type III process temperature-time curve. The temperature-time curves in Fig. 6 show that the type I process weld has the longest cooling time, i.e., it has the smallest weld cooling rate, whereas the type II process and type III process have a larger weld cooling rate. The type I process has a peak temperature of 2240.35 °C at 3.10 s. When the temperature drops to 1200 °C, the corresponding time is 9.94 s, and the average cooling rate (V cool ) is approximately 152 °C/s. The type II process has a peak temperature of 2450.29 °C at 1.97 s. When the temperature drops to 1200 °C, the corresponding time is 5.60 s, and V cool is approximately 344 °C/s. The type III process has a peak temperature of 1884.06 °C at 2.21 s. When the temperature drops to 1200 °C, the corresponding time is 5.13 s, and V cool is approximately 234 °C/s. The welding speed was 0.75 mm/min in all three processes. Therefore, the GSL in the type I process is approximately 12,168 °C/m, the GSL in the type II process is approximately 27,555 °C/m, and the GSL in the type III process is approximately 18,742 °C/m. In other words, the GSL in the type I process is smaller than the GSL in the type II and type III processes. A comparison of the GSL in the three processes is shown in Fig. 7. The analysis in Section 3.2.1 and 3.2.2 shows that the type I process has a smaller V SL and GSL compared with the type II and type III processes. Formula (1) shows that the type I process has a considerably smaller e_ compared with the type II process and type III processes; therefore, it has the smallest hot cracking sensitivity.
Fig. 5. The comparison of V SL from the three processes.
2500 2300
the type I process the type II process the type III process
2100
Temperature (°C)
1900
331
1700 1500 1300 1100 900 700 500 300 100 1
2
3
4
5
6
Time (s)
7
8
9
Fig. 6. The temperature–time curves of three different processes.
Fig.7. The comparison of GSL from the three processes.
horizontal solidification rate of the molten pool. Furthermore, because the molten pool solid-liquid boundary displacement is within the microscopic scale, the measurement results will contain certain errors.
3.3. Metallurgical factors 3.3.1. Metallographic analysis Metallurgical factors are also closely related to hot cracking [25]. Fig. 8 shows the metallographic diagram and SEM image of the hot cracking from the type II process. Fig. 8 shows that the crack opening is along the columnar crystal boundary, and fractures separate along the inner-dendritic liquid membrane, as shown in the SEM images; i.e., a thin liquid membrane layer formed at the time of crack formation. Fig. 9 shows the metallographic diagram and SEM image of the hot cracking from the type III process. Fig. 9 shows that in pure laser welding, there is a significant amount of tensile stress on the welds during the solidification period; open cracks at the crater cannot be filled by a sufficient amount of liquid metal, and thus, crater cracks are formed. Pressure compensation against hot cracking formation is determined by the permeability of the molten pool fluid flowing toward the dendritic roots. Superior permeability means that it is more difficult to form hot cracking in welds. The permeability parameter (K) is calculated via formula (2) [29,30]:
K¼
k2 ð1 f s Þ 2 180 fs
3
ð2Þ
In formula (2), k is the secondary dendrite arm spacing, and f s is the representative fraction of the solid phase. Assume that the three welding processes have the same solid phase representative fraction f s ; then, a larger secondary dendrite arm spacing (k) and a larger K are beneficial to hot cracking suppression.
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Fig. 8. The metallographic diagram and SEM image of the hot cracking from the type II process.
Fig. 9. The metallographic diagram and SEM image of the hot cracking from the type III process.
The fitting function of k versus cooling rate for the 42CrMo steel is as follows [31]:
k ¼ 34:8 V 0:26 cool
ð3Þ
where k is the secondary dendrite arm spacing (lm), and V cool is the weld cooling rate (°C/s). Formula (3) shows that there is an exponential relationship between k and V cool . Based on formula (3), k of the type I process is approximately 9.43 lm, k of the type II process is approximately 7.62 lm, and k of the type III process is approximately 8.42 lm. Formula (2) shows that the type I process has a larger K compared with the type II process and type III process and also has the more balanced pressure compensation for the molten pool fluid flowing toward the dendritic roots. Therefore, it is least likely that hot cracking will form at the welds. 3.3.2. Molten pool dynamic behavior and molten pool material permeability Table 4 shows the flow of the molten pool captured by the highspeed camera when the type I process and type II welding process are applied. When the type I process is applied, the metal in the molten pool initially bypasses the laser keyhole from the front margin of the molten pool, then flows toward the keyhole from the trailing edge, and stops around there (as shown in Table 4 (a)). When the type II process is applied, the metal in the molten pool bypasses the keyhole and flows toward the rear of the molten
pool, i.e., the molten pool surface flows from the keyhole to the rear of the molten pool (as shown in Table 4(b)). In Table 4(a), inward flow of the molten pool in type I welding is beneficial to filling of the keyhole by the molten pool metal during the weld solidification phase, to the molten pool flow toward the dendritic roots, and to compensation of the inner-dendritic pressure balance to prevent hot cracking from forming at the weld. 4. Conclusions In this research, a welding test to simulate the circular welding on quenched and tempered 42CrMo steel is performed to compare and investigate the possibility of hot cracking formation at the weld joints in three welding processes: laser welding, leading laser laser-MAG hybrid welding, and leading arc laser-MAG hybrid welding. The following are the primary conclusions: (1) Industrial nondestructive CT test results for the weld show that when leading laser laser-MAG hybrid welding is applied to the circular welding on quenched and tempered 42CrMo steel, hot cracking do not form at the weld joints; in comparison, when leading arc laser-MAG hybrid welding or laser welding are applied, hot cracking form at the weld joints. (2) The molten pool flow and weld solidification during the welding process was captured by a high-speed camera to calculate V SL . The information on the variations in the weld
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Table 4 The flow of the molten pool during leading laser and leading arc.
temperature were collected via an IR thermal imaging device to calculate GSL . The result shows that leading laser laser-MAG hybrid welding has a smaller V SL and GSL compared with leading arc laser-MAG hybrid welding and laser welding; i.e., leading laser laser-MAG hybrid welding results in a far smaller e_ compared with the other two processes and has the lowest hot cracking sensitivity. (3) An analysis of the metallurgical factors for the three processes showed that compared with the other two processes, leading laser laser-MAG hybrid welding has the largest k; the molten pool fluid flowing toward the dendritic roots also exhibited the strongest permeability. These factors are beneficial to the inner-dendritic pressure balance compensation and in preventing hot cracking in the weld.
Acknowledgments The authors are grateful for the financial support provided by the National Natural Science Foundation of China (project number: 51175165) and the Key National Science and Technology Project (project number: 2013ZX04001131). In addition, the authors also wish to thank Mr. Hongzhong Liu from Shandong Binzhou Bohai Piston Co., Ltd., for his assistance with the industrial CT examination of the experimental weld workpieces.
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