- Email: [email protected]
similar weld joints between DP780 and DP980 steels processed by fiber laser welding
G Model
ARTICLE IN PRESS
JMST-1046; No. of Pages 11
Journal of Materials Science & Technology xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Journal of Materials Science & Technology journal homepage: www.jmst.org
Microstructure and properties in dissimilar/similar weld joints between DP780 and DP980 steels processed by fiber laser welding Hongshuang Di a , Qian Sun a , Xiaonan Wang b,∗ , Jianping Li a a b
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China Shagang School of Iron and Steel, Soochow University, Suzhou 215021, China
a r t i c l e
i n f o
Article history: Received 24 November 2016 Received in revised form 11 February 2017 Accepted 19 February 2017 Available online xxx Keywords: Dual phase steel Laser welding Dissimilar weld joints Microstructure Mechanical properties Formability
a b s t r a c t The microstructure and mechanical properties (strength, fatigue and formability) of dissimilar/similar weld joints between DP780 and DP980 steels were studied. The microstructure in fusion zone (FZ) was lath martensite (LM), and alloying elements in the FZ were uniformly distributed. The hardness in the FZ of dissimilar weld joint was similar to the average value (375 HV) of the two similar weld joints. The microstructural evolution in heat affected zone (HAZ) of dissimilar/similar weld joints was as follows: LM (coarse-grained HAZ) →finer LM (fine-grained HAZ) →M-A constituent and ferrite (intercritically HAZ) →tempered martensite (TM) and ferrite (sub-critical HAZ). Lower hardness in intercritically HAZ and sub-critical HAZ (softening zones) was observed compared to base metal (BM) in dissimilar/similar weld joints. The size of softening zone was 0.2–0.3 mm and reduction in hardness was ∼7.6%–12.7% of BM in all the weld joints, which did not influence the tensile properties of weld joints such that fracture location was in BM. Formability of dissimilar weld joints was inferior compared to similar weld joints because of the softening zone, non-uniform microstructure and hardness on the two sides of FZ. The effect of microstructure on fatigue life was not influenced due to the presence of welding concavity. © 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction Dual phase (DP) steels, consist of ferrite (soft phase) and martensite (hard phase), and have good mechanical strength, high work hardening rate, low yield ratio, high uniform and total elongation, high strain energy absorption and excellent formability [1–4]. Based on these characteristics, DP steels are widely used in automotive applications to realize energy savings, reduction in emission and light weight [5,6]. Laser welding is an advanced joining technology that is used in automotive applications, because of high power, low maintenance cost and high efficiency [7–9]. Furthermore, because of ease of automation and flexibility, laser welding is being considered to replace potentially other popular joining processes such as resistance spot welding [10] and friction stir welding [11]. A number of studies have been carried out on weld metals using laser welding such as DP steels and other metals [12–14]. The effects of weld line position and geometry on the formability of laser welded high strength low alloy (HSLA) and DP steels blanks
∗ Corresponding author. E-mail address: [email protected] (X. Wang).
were studied by Li et al. [15]. They observed that the softening zone of the heat affected zone was the dominant factor in controlling formability of DP 980 weld joints. Moreover, welds of DP600 failed consistently in the region where severe softening occurred. The tensile properties and microstructure of dissimilar laser weld joints of TWIP/TRIP, DP/TRIP and DP/22MnB5 were studied by Rossini et al. [16]. The laser welding of high strength low alloy and DP 980 was studied by Parkes et al. [17], and they noted that the presence of softening zone occurred on DP980 side and there was no effect on tensile properties and fatigue failure occurred in the welded area. Saha et al. [18] studied the microstructure-property relationship in fiber laser welded DP and HSLA steels. While, Parkes et al. [19] studied tensile properties of fiber laser weld joints of HSLA and DP steels at warm and low temperatures. The mechanical response along different tensile directions of laser-welded dissimilar blanks between DP780 and DP1180 steels was studied by Gong et al. [20]. Microstructure and mechanical properties of laser welded dissimilar DP600 and DP980 steels weld joints were studied by Farabia et al. [21]. In summary, the effect of softening zone on properties of weld joints depends on the degree of softening and size of the zone, which are governed by laser heat input and welded materials [15,18,22–26]. Thus, dissimilar/similar laser welding was carried
http://dx.doi.org/10.1016/j.jmst.2017.09.001 1005-0302/© 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model
ARTICLE IN PRESS
JMST-1046; No. of Pages 11
H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
2
Table 1 Chemical composition of experimental steels (wt%). Materials
C
Si
Mn
Ti
Cr
Mo
Al
Fe
CE
DP980 DP780
0.09–0.11 0.07–0.09
0.3–0.5 0.1–0.2
1.6–1.8 1.6–1.8
0.01–0.03 –
0.3–0.5 0.2–0.4
0.2–0.4 0.1–0.3
0.04–0.06 0.3–0.5
Bal. Bal.
0.584 0.499
Fig. 1. Sample drawings of property testing: (a) tensile test, (b) fatigue test, (c) erichsen test.
out at a constant heat input between DP780 and DP980 steels, with the objective to further understand the effect of softening zone on the properties of weld joints. Furthermore, the differences between microstructure and properties of dissimilar and similar weld joints were studied. 2. Experimental 2.1. Materials Cold-rolled steel sheets of DP780 and DP980 of 1.5 mm thickness were used in the study described here. The chemical composition is listed in Table 1. The two base metals had identical alloying elements but their content was different. The carbon equivalent (CE) was 0.584% and 0.499%, respectively.
2.4. Tensile, formability and fatigue properties Tensile tests in the direction perpendicular to the weld joint was carried out at a constant strain rate of 3 mm/min using DNS-100 universal material testing machine. Fatigue tests were conducted using GPS100 high frequency fatigue testing machine at a stress ratio of R = 0.1 with a stress amplitude of 540 MPa (room temperature). Erichsen test was conducted using CTM604 Erichsen test machine, with a loading force of 15 kn and a test speed of 15 mm/min. FZ was in the center of all the samples and dimensions are presented in Fig. 1. 3. Results and discussion 3.1. Weld morphology
Samples of dimensions 80 mm × 80 mm were welded using IPG YLS-6000 laser. The welding speed was 5.0 m/min and laser power was 2 kW. Laser spot diameter was 0.3 mm and pure argon gas was used for shielding.
Fig. 2 depicts the macroscopic morphology of dissimilar and similar weld joints. Welding concavity was observed in all the weld joints, but the evidence of concavity was less apparent in the case of similar DP980-DP980 weld joints (Fig. 2(c)). This had very small effect on tensile properties and formability of weld joints but seriously impacted the fatigue life, as described below.
2.3. Microstructure and hardness
3.2. Microstructure
Samples perpendicular to weld joint were polished and etched with 4% nital to study the microstructure of the weld joints using SU5000 scanning electron microscope (SEM). HV-1000 microhardness tester was used to measure the microhardness of weld joints at a load of 300 N and a loading time of 10 s. The distance from the welded surface was 0.4 mm and spacing between two indentations was 0.1 mm.
3.2.1. Microstructural evolution Generally, weld joint consists of fusion zone (FZ) and heat affected zone (HAZ). The weld metal was melted at high temperature and rapidly solidified to form FZ, whereas the HAZ formation was heated and cooled without melting. In dissimilar/similar DP780 and DP980 steels welding process, the HAZ can be divided into four zones: coarse-grained HAZ (CG-HAZ), fine-grained HAZ
2.2. Welding methods
Fig. 2. Macroscopic morphology of similar and dissimilar weld joints: (a) DP780-DP780, (b) DP980-DP980, (c) DP780-DP980.
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
3
Fig. 3. Overview of microstructural evolution of dissimilar weld joints.
Fig. 4. Microstructural evolution of DP780 side of dissimilar weld joints: (a) BM, (b) SC-HAZ, (c) IC-HAZ, (d) FG-HAZ, (e) CG-HAZ, (f) FZ.
(FG-HAZ), intercritical HAZ (IC-HAZ) and sub-critical HAZ (SC-HAZ) because of differences in local peak temperature from the center of FZ. Similar classification has been previously suggested [27]. In dissimilar welding, the microstructural evolution zone consisted of DP 780 base metal (BM), DP 780 HAZ, FZ, DP 980 HAZ and DP 980 BM.
Fig. 3 shows an overall view of dissimilar weld joints, where each zone can be clearly depicted. Figs. 4 and 5 are the micrographs of microstructural evolution of dissimilar weld joints between DP780 and DP980 steels. It was observed that the microstructure in FZ of
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11 4
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
Fig. 5. Microstructural evolution of DP980 side of dissimilar weld joints: (a) BM, (b) SC-HAZ, (c) IC-HAZ, (d) FG-HAZ, (e) CG-HAZ.
similar DP780-DP780 and DP980-DP980 weld joints was identical to dissimilar DP780-DP980 weld joints. The microstructure of DP780 BM consisted of ferrite (F) and a number of block martensite (M) dispersed at the ferrite boundary (Fig. 4(a)). In SC-HAZ, the local peak temperature was below Ac1 and austenitization did not take place such that the martensite in the base metal transformed into tempered martensite (TM) during subsequent heating and cooling (Fig. 4(b)). It was also observed that the ferrite phase of the base metal was not changed. In IC-HAZ, the local peak temperature was below Ac3 but above Ac1 , martensite and a part of ferrite in the base metal was austenitized and on cooling transformed into ferrite and M-A constituent, and the grain size of transformed ferrite was finer (Fig. 4(c)) compared to the base metal. In FG-HAZ, the local peak temperature was above Ac3 , and the base metal was completely austenitized to form equiaxed austenite grain structure and transformed into lath martensite (LM) via shear transformation (Fig. 4(d)). The microstructure of CG-HAZ was also lath martensite (LM) (Fig. 4(e)) and the transformation mechanism was similar to FG-HAZ, but grain size was coarse because of higher local peak temperature. Coarse LM was observed in FZ characterized by a typical solidification microstructure (Fig. 4(f)).
On DP980 side of dissimilar weld joints, the microstructure in HAZ was similar to DP 780 side but grain size was different. Higher fraction of martensite and finer ferrite was observed in DP 980 BM compared to DP780 BM (Fig. 5(a)), leading to more martensite and smaller ferrite grain size was observed in SC-HAZ and IC-HAZ of DP980 side (Fig. 5(b) and (c)). The microstructure of FG-HAZ and CG-HAZ was LM with different grain size (Fig. 5(d) and (e)). 3.2.2. Distribution of alloying elements Fig. 6 describes the elemental distribution in dissimilar weld joints. The alloying elements were identical in both the base metal but their content was different (Table 1), especially Al and Si (Si: 0.3–0.5% (DP980), 0.1–0.2% (DP780) and Al: 0.04–0.06% (DP980), 0.3–0.5% (DP780)). EDS line scan was performed across the crosssection of dissimilar weld joints. The Al-content decreased from DP780 side to DP980 side, whereas Si-content was increased (Fig. 6(a)). In order to clearly illustrate the distribution of alloying elements in FZ, EDS line scan was also carried out across the cross-section of FZ (Fig. 6(b)). The results showed that Al and Si distribution was uniform in FZ although there was significant difference in the content in both the base metals. This suggests a good
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
5
Fig. 6. EDS line scanning analyse of dissimialr weld joints: (a) elemental distribution of full weld joints, (b) elemental distribution of FZ.
IC-HAZ, which is analyzed in Section 3.2.1. It resulted that the hardness was decreasing compared to BM. Moreover, the width of the softening zone on both sides of the dissimilar weld joints was narrow, i.e. ∼0.2 mm (DP780 side) and ∼0.3 mm (DP980 side), respectively. Identical results were observed in the case of similar DP780-DP780 and DP980-DP980 weld joints. Heat input and laser power density have a strong impact on the softening behavior of HAZ in laser welded DP steels [19]. Eqs. (1) and (2) are the corresponding relationship: H= Pd =
Fig. 7. Hardness distribution on cross-section of all weld joints.
and uniform mixing of all the elements in FZ during laser welding. Similar elemental distribution has been observed earlier [22]. 3.3. Mechanical properties 3.3.1. Hardness Fig. 7 illustrates the distribution of hardness across the crosssection of dissimilar and similar weld joints. The average hardness of DP780 and DP980 base metal (BM) was measured to be ∼235 HV and ∼315 HV, respectively. In dissimilar weld joints, softening in HAZ of DP780 side was not obvious with ∼18 HV reduction in Vickers hardness, which was 7.6% of DP780 BM. While, reduction in hardness of ∼40 HV was observed on DP980 side, 12.7% of DP980 BM. All the hardness results in HAZ on both sides of dissimilar DP780-DP980 weld joints were identical to the HAZ of similar DP780-DP780 and DP980-DP980 weld joints. From Fig. 7, it may be noted that the softening zone was confirmed in IC-HAZ and SC-HAZ. Similar results were reported previously [23,28]. Microstructure is an aspect that determines microhardness [29]. Tempered martensite in SC-HAZ led to a decrease in hardness compared to BM. Meanwhile, martensite was transformed into M-A constituent in
P
(1)
v 4P d2
(2)
where H is heat input (J/mm), P is laser power (2 kW), v is welding speed (5 m/min), Pd is laser power density (J/mm2 ), d is diameter of laser spot (0.3 mm). Generally, higher heat input and lower laser power density, leads to wider softening zone. For identical weld material DP980, greater width (∼0.5 mm) of softening zone has been reported [14,15] with similar heat input but lower laser power density (P was 6 kW, v was 16 mm/min, d was 0.6 mm). From Fig. 7, it may also be noted that the average hardness of fusion zone (FZ) for similar DP780-DP780 weld joints was ∼356 HV, lower than similar DP980-DP980 FZ (∼385 HV). Higher martensite fraction led to higher average hardness of DP980 BM (Fig. 5(a)), which enhanced the average hardness of FZ. Furthermore, the average hardness of FZ in dissimilar weld joints was ∼375 HV, which can be considered as the average of two similar weld joints, as calculated by Eq. (3).
’ 780 980 HVFZ = HVFZ + HVFZ /2
(3)
’ where HVFZ is average hardness of FZ in dissimilar weld 780 980 are average hardness of FZ in similar joints, HVFZ and HVFZ DP780-DP780 and similar DP980-DP980 weld joints, respectively. ’ was 371 HV with 1% error. According to Eq. (3), the calculated HVFZ Similar studies have been previously conducted [22]. Thus, Eq. (3) can be used to predict the average hardness of FZ of dissimilar weld between different DP steels. The reason that the hardness in the fusion zone of dissimilar DP780-DP980 weld joints was approxi-
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model
ARTICLE IN PRESS
JMST-1046; No. of Pages 11
H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
6
Fig. 8. Stress-strain plot and macroscopic fracture morphology of all the samples: (a) stress-strain plot, (b) macroscopic fracture morphology.
Table 2 Tensile test results of different materials. Materials
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
n
Fracture location
DP780-980 DP780-780 DP980-980 DP780 BM DP980 BM
905 875 1080 885 1080
555 525 695 525 690
11.4 17.2 12.7 18.7 12.5
0.10 0.13 0.11 0.14 0.12
780 BM BM BM BM BM
mately the average of two fusion zones of similar DP780-DP780 and DP980-DP980 weld joints is related to the identical microstructure (lath martensite) and uniform distribution of alloying elements.
3.3.2. Tensile properties Table 2 lists tensile properties data for all the samples. In similar weld joints, tensile strength and yield strength were close to the respective base metal (BM). The elongation of similar DP780-DP780 and DP980-DP980 weld joints were 17.2% and 12.7%, respectively, and similar to BM values of 18.7% and 12.5%, respectively. In dissimilar DP780-DP980 weld joints, the plastic deformation of the samples was concentrated on DP780 side (7 mm) and to some extent on DP980 side (1 mm) as listed in Table 3. The fraction of ferrite in DP 780 (64%) is more than DP 980 (55%), which has better ductility and lower strength. During tensile process, plastic deformation prior occurred on one side with higher ferrite fraction. Thus, most of plastic deformation was shared by DP780 side. The elongation of DP780-DP980 weld joint was lowest (11.4%) compared to other weld joints. The tensile strength and yield strength were 905 MPa and 555 MPa, respectively, slightly lower than DP980 BM (1080 MPa and 695 MPa) but similar to DP780 BM (885 MPa and 525 MPa). Fig. 8 illustrates the engineering stress-engineering strain plots and macroscopic fracture morphology of the samples, and Fig. 9 presents the microstructure morphology of fracture surface of DP780-DP780 weld joints. A number of dimples were observed, implying ductile fracture. All the curves were smooth with no yield point. The curves of similar weld joints (DP780-DP780, DP980DP980) were similar to corresponding BM. While, the curve of dissimilar weld joints (DP780-DP980) was between that of two similar weld joints (DP780-DP780, DP980-DP980) (Fig. 8(a)). This suggested that the applied load was shared between the two base metals, fusion zone and two HAZs in dissimilar weld joints. Similar results were reported by Gong et al. [20]. The fracture of dissimilar and similar weld joints was located in BM (Fig. 8(b)), because of restraint in fusion zone (FZ) with high strength in conjunction
Fig. 9. Microscopic fracture morphology of DP780-DP780 tensile samples.
with a narrow softening zone (∼0.2 to 0.3 mm) in HAZ. However, in previous studies, the tensile fracture location of DP980 weld joints was in HAZ because of the presence of severe softening zone (∼1.1 to 2.0 mm) [23]. The underlying reason is that high power intensity, high welding speed and lower heat input were selected in our study. Thus, it is important to control the softening zone in HAZ for controlling the tensile properties of weld joints. 3.4. Formability Erichsen test results of similar and dissimilar weld joints are presented in Table 4. It can be seen that Erichsen values (IE) of all the weld joints were inferior compared to the corresponding base metal (BM). Hardening index (n) measured in the tensile test is an important factor to evaluate the formability of sheet metals. Generally, the greater n values, the better is the formability. In Table 2, n value of similar/dissimilar weld joints was: 0.10 (DP780-DP980) <0.11 (DP980-DP980) <0.13 (DP780-DP780). Thus, the formability of similar/dissimilar weld joints was: DP780-DP980
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model
ARTICLE IN PRESS
JMST-1046; No. of Pages 11
H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
7
Table 3 Two sides of tensile sample of dissimilar weld joints between DP780 and DP980.
Non-tensile After-tensile
Length of DP980 side (mm)
Elongation percentage of DP980 side (%)
Length of DP780 side (mm)
Elongation percentage of DP780 side (%)
Fracture location from FZ center (mm)
35 36
– 12.5
35 42
– 82.5
– 23
Fig. 10. Microscopic morphology of fracture for all the weld joints: (a) DP780-DP780, (b) DP980-DP980, (c) DP780-DP980.
Table 4 Erichsen test results. Materials
IE (mm)
Fracture location
DP780-DP980 DP780- DP780 DP980- DP980 DP780 BM DP980 BM
6.665 7.994 6.982 8.718 8.148
DP780 HAZ FZ HAZ – –
was inferior, with an IE of 6.67 mm. DP980-DP980 was intermediate, with an IE of 6.98 mm. DP780-DP780 was superior, with an IE of 7.99 mm. The difference in the formability was related to the microstructure and hardness distribution of weld joints, it was analyzed in the following. In Erichsen test, the principal strain can be divided into two components: longitudinal and transverse, one can consider weld line as longitudinal direction in the weld joint test. The hardness and strength of fusion zone (FZ) was higher than BM but the ductility was low, offering a restraint in the longitudinal direction. Thus, formability was limited by this restraint leading to a lower formability for weld joints compared to BM which was a monolithic blank with no restraint biaxial stretch, and failure always initiated in FZ [30]. Fig. 10 presents the macroscopic morphology of fracture for all the weld joints. In similar DP780-DP780 weld joints, the crack initiated in FZ and propagated into BM, uniform deformation along FZ was observed and necking occurred in the fracture zone (Fig. 10(a)). The microstructure of FZ was LM, which had higher hardness and strength, the transverse deformation of the joint was restrained
effectively. Thus, IE value of similar DP780-DP780 weld joints was only reduced by 8.3% compared to BM. However, in similar DP980-DP980 weld joints, the crack initiated in HAZ and propagated parallel to FZ with local necking (Fig. 10(b)). As mentioned in Section 3.3.1, the softening zone in DP980 HAZ (∼0.3 mm, 12.7% hardness reduction) was more severe than DP780 HAZ (∼0.2 mm, 7.6% hardness reduction) The reason is that more fraction of tempered martensite. With the load applied, stress concentration was easy to produce in the softening zone. And, hardness difference between tempered martensite and ferrite was the reason that the crack initiation occurred in softening zone until fracture in the case of transverse strain in similar DP980-DP980 weld joints. In dissimilar DP780-DP980 weld joints, FZ departed from the original location toward DP980 side (Fig. 10(c)). Significant deformation occurred on DP780 side as compared to DP980 side. This is related to the strength because the yield strength of DP780 BM was less than DP980 BM. Necking was observed on DP780 side, while almost no deformation was observed on the DP980 side. In the case of large deformation on DP780 side, localized softening zone with lower strength compared to the adjacent material provided a fracture location, although there was significant softening on the DP980 side. A similar observation was made by Bandyopadhyay et al. [31]. The asymmetrical deformation, softening zone and restrain from FZ, was the reason for lowest IE in dissimilar weld joints among all the Erichsen test samples. To further explore the effect of microstructure on crack propagation, the microstructure and voids in the region close to crack propagation were observed by SEM. Fig. 11 describes the morphology of the fracture surface for all the weld joints. As men-
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
8
Fig. 11. Microscopic morphology of fracture for all the weld joints: (a) and (b) DP780-DP780, (c) DP980-DP980, (d) DP 780-DP980, (e) and (f) SC-HAZ.
tioned, the crack initiation was in FZ in similar DP780-DP780 weld joints. Martensite laths were elongated and voids were generated between laths (Fig. 11(a) and (b)). When load was applied to samples, voids provided sites for crack to nucleate and propagate until fracture. In DP780-DP980 and DP980-DP980 weld joints, crack initiation was in SC-HAZ (Fig. 11(c) and (d)), where tempered martensite (TM) and ferrite grains experienced deformation and voids were observed. The magnified micrographs illustrated that voids initiated in the hard TM phase and propagated along the interface between ferrite and TM (Fig. 11(e) and (f)), because large deformation can be sustained by ferrite, whereas TM experienced less deformation. Thus, this inhomogeneity in the plastic behavior of the two phases led to void initiation at the interface [32,33]. The failure mechanism is similar to that discussed by Cingara et al. [32] for DP steels. 3.5. Fatigue properties Table 5 summarizes the fatigue results of all the samples at a stress amplitude of 540 MPa. In this case, failure of DP780 BM and DP980 BM did not occur. However, lowest fatigue life cycle was
Table 5 Fatigue results of all the samples. Materials
(MPa)
Fatigue life cycle (N)
Fatigue Fracture location
DP780- DP980 DP780- DP780 DP980- DP980 DP780 BM DP980 BM
540 540 540 540 540
42355 44836 115824 10000222 10000222
FZ FZ FZ No failure No failure
obtained in dissimilar DP780-DP980 weld joints. Alam et al. [34] pointed that welding concavity had a significant impact on fatigue life of weld joints as compared to the softening zone. As mentioned above, softening zone and welding concavity (Figs. 2 and 6) were observed in dissimilar and similar weld joints, and welding concavity of DP980-DP980 weld joint was less apparent. As a consequence, the fatigue life cycle of DP980-DP980 weld joints was highest compared to other weld joints in spite of the presence of significant softening zone. The localized stress concentration was easily generated in the welding concavity, which deteriorated the fatigue life of weld joints [35]. Thus, the fracture location was in FZ of all the weld joints (Fig. 12). Additionally, similar DP980-DP980 weld joints had
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
9
Fig. 12. Macroscopic of fatigue fracture of all the samples: (a) DP780- DP780, (b) DP980- DP980, (c) DP780- DP980.
higher strength, which ensured the fatigue life. The effect of softening zone on fatigue life of weld joints needs further and detailed study. Fig. 13 presents the fatigue fracture for all the weld joints. The crack initiation was observed in welding concavity (Fig. 13(a–c)), which can easily result in the occurrence of persistent slip bands (PSB) (Fig. 13(d)). Secondary cracks were generated in the crack propagation zone, indicated by arrows in Fig. 13(e).
4. Conclusions (1) The microstructure of FZ of three types of weld joints was lath martensite. The microstructure of HAZ of dissimilar and similar weld joints was identical: lath martensite (CG-HAZ, FG-HAZ) →M-A constituent and ferrite (IC-HAZ) →tempered martensite and ferrite (SC-HAZ). Finer ferrite and more martensite were observed in IC-HAZ and SC-HAZ of DP980 side. (2) The hardness in the HAZ of DP780-DP780 weld joints was lower than DP980-DP980 weld joints, and identical values were obtained in the HAZs of dissimilar DP780-DP980 weld joints. Softening zone was observed in SC-HAZ (tempered martensite) and IC-HAZ (martensite transformed into M-A constituent and ferrite). Hardness of fusion zone (FZ) of dissimilar weld joints was ∼375 HV, higher than similar DP780-DP780 (∼357 HV) but
lower than similar DP980-DP980 FZ (∼386 HV), which is the average value of both the fusion zones of similar weld joints. (3) Tensile fracture of dissimilar and similar weld joints occurred in the base metal. The tensile strength and yield strength of dissimilar weld joints were 905 MPa and 555 MPa, respectively, lower than DP980-DP980 but similar to DP780-DP780 weld joints. The deformation in dissimilar DP780-DP980 weld joint was concentrated on DP780 side with 7 mm, compared to 1 mm extension on DP980 side, which resulted in lowest elongation (11.4%) compared to other weld joints. Softening zone did not influence the tensile property of weld joints. However, the effect of softening zone on fatigue life of weld joints was not observed because of the presence of welding concavity. (4) The formability of dissimilar DP780-DP980 weld joints was more inferior, compared to similar DP780-DP780 and DP980DP980 weld joints. This behavior is related to asymmetrical deformation and different softening zone on both sides, and restrain from the fusion zone. The crack initiated in the softening zone of DP780 side and propagated parallel to the fusion zone. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51274063 and 51305285),
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
10
Fig. 13. Microscopic of fatigue fracture of all the weld joints: (a) DP780- DP780, (b) DP980- DP980, (c) DP780, (d) crack initiation zone, (e) crack propagation zone.
the National Program on Key Basic Research Project (Grant No. 2011CB606306-2), the Open Research Fund from the State Key Laboratory of Rolling and Automation, Northeastern University (Grant No. 2016005) and the Project Funded by China Postdoctoral Science Foundation (Grant No. 2016M601877).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.H. Lee, S.H. Park, H.S. Kwon, G.S. Kim, C.S. Lee, Mater. Des. 64 (2014) 559–565. A. Lesnykh, S. Golovatiy, D. Valovik, Mater. Today: Proc. 2S (2015) S667–S671. L.R. Bhagavathi, G.P. Chaudhari, S.K. Nath, Mater. Des. 32 (2011) 433–440. K. Mahesh, S. Sankaran, P. Venugopal, J. Mater. Sci. Technol. 28 (12) (2012) 1085–1094. A. Fallahi, J. Mater. Sci. Technol. 18 (2002) 451–454. L. Schemmann, S. Zaefferer, D. Raabe, F. Friedel, D. Mattissen, Acta Mater. 95 (2015) 386–398. L. Quintino, A. Costa, R. Miranda, D. Yapp, V. Kumar, C.J. Kong, Mater. Des. 28 (2007) 1231–1237. M. Zhang, X.N. Wang, G.J. Zhu, C.J. Chen, J.X. Hou, S.H. Zhang, H.M. Jing, Acta Metall. Sin. (Engl. Lett.) 27 (2014) 521–529. J. Canning, Opt. Laser Eng. 44 (2006) 647–676. H. Ghazanfari, M. Naderi, Acta Metall. Sin. (Engl. Lett.) 26 (2013) 635–640. Y. Hovanski, M.L. Santella, G.J. Grant, Scr. Mater. 57 (2007) 873–876. G.M. Xie, H.B. Cui, Z.A. Luo, W. Yu, J. Ma, G.D. Wang, J. Mater. Sci. Technol. 32 (2016) 326–332.
[13] J. Yang, Y. Li, H. Zhang, W. Guo, D. Weckman, N. Zhou, Metall. Mater. Trans. A 46 (2015) 5149–5157. [14] C.C. Chang, C.P. Chou, S.N. Hsu, G.Y. Hsiung, J.R. Chen, J. Mater. Sci. Technol. 26 (2010) 276–282. [15] J. Li, S.S. Nayak, E. Biro, S.K. Panda, F. Goodwin, Y. Zhou, Mater. Des. 52 (2013) 757–766. [16] M. Rossini, P.R. Spena, L. Cortese, P. Matteis, D. Firrao, Mater. Sci. Eng. A 628 (2015) 288–296. [17] D. Parkes, W. Xu, D. Westerbaan, S.S. Nayak, Y. Zhou, F. Goodwin, S. Bhole, D.L. Chen, Mater. Des. 51 (2013) 665–675. [18] D.C. Saha, D. Westerbaan, S.S. Nayak, E. Biro, A.P. Gerlich, Y. Zhou, Mater. Sci. Eng. A 607 (2014) 445–453. [19] D. Parkes, D. Westerbaan, S.S. Nayak, Y. Zhou, F. Goodwin, S. Bhole, D.L. Chen, Mater. Des. 56 (2014) 193–199. [20] H.Y. Gong, S.F. Wang, P.K.Y.P. Korkolis, Mater. Des. 90 (2016) 1115–1123. [21] N. Farabia, D.L. Chen, Y. Zhou, J. Alloys Compd. 509 (2011) 982–989. [22] H.Q. Zhang, A. Wei, X.M. Qiu, J.H. Chen, Mater. Des. 54 (2014) 443–449. [23] J.F. Wang, L.J. Yang, M.S. Sun, T. Liu, H. Li, Mater. Des. 97 (2016) 118–125. [24] Y. Liu, D.Y. Dong, L. Wang, X. Chu, P.F. Wang, M.M. Jin, Mater. Sci. Eng. A 627 (2015) 296–305. [25] V.H. Baltazar-Hernandez, S.S. Nayak, Y. Zhou, Metall. Mater. Trans. A 42 (2011) 3115–3129. [26] M. Xia, E. Biro, Z. Tian, Y.N. Zhou, ISIJ Int. 48 (2008) 809–814. [27] J.F. Wang, L.J. Yang, M.S. Sun, T. Liu, H. Li, Mater. Des. 90 (2016) 642–649. [28] N. Farabi, D.L. Chen, J. Li, Y. Zhou, S.J. Dong, Mater. Sci. Eng. A 527 (2010) 1215–1222. [29] X.N. Wang, C.J. Chen, H.S. Wang, S.H. Zhang, M. Zhang, X. Luo, J. Mater. Process. Technol. 226 (2015) 106–114.
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
[30] M.S. Xia, M.L. Kuntz, Z.L. Tian, Y. Zhou, Sci. Technol. Weld. Join. 13 (2008) 378–387. [31] K. Bandyopadhyay, S.K. Panda, P. Saha, V.H. Baltazar-Hernandez, Y.N. Zhou, Mater. Sci. Eng. A 652 (2016) 250–263. [32] G. Avramovic-Cingara, C.A. Saleh, M.K. Jain, D.S. Wilkinson, Metall. Mater. Trans. A 40 (2009) 3117–3127.
11
[33] N. Saeidi, F. Ashrafizadeh, B. Niroumand, M.R. Forouzan, F. Barlat, Mater. Sci. Eng. A 644 (2015) 210–217. [34] M.M. Alam, J. Karlsson, A.F.H. Kaplan, Mater. Des. 32 (2011) 1814–1823. [35] J. Roesler, H. Harders, M. Baeker, Mechanical Behavior of Engineering Materials: Metals, Ceramics, Polymers, and Composites, 1st ed., Springer, New York, 2007.
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
ARTICLE IN PRESS
JMST-1046; No. of Pages 11
Journal of Materials Science & Technology xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Journal of Materials Science & Technology journal homepage: www.jmst.org
Microstructure and properties in dissimilar/similar weld joints between DP780 and DP980 steels processed by fiber laser welding Hongshuang Di a , Qian Sun a , Xiaonan Wang b,∗ , Jianping Li a a b
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China Shagang School of Iron and Steel, Soochow University, Suzhou 215021, China
a r t i c l e
i n f o
Article history: Received 24 November 2016 Received in revised form 11 February 2017 Accepted 19 February 2017 Available online xxx Keywords: Dual phase steel Laser welding Dissimilar weld joints Microstructure Mechanical properties Formability
a b s t r a c t The microstructure and mechanical properties (strength, fatigue and formability) of dissimilar/similar weld joints between DP780 and DP980 steels were studied. The microstructure in fusion zone (FZ) was lath martensite (LM), and alloying elements in the FZ were uniformly distributed. The hardness in the FZ of dissimilar weld joint was similar to the average value (375 HV) of the two similar weld joints. The microstructural evolution in heat affected zone (HAZ) of dissimilar/similar weld joints was as follows: LM (coarse-grained HAZ) →finer LM (fine-grained HAZ) →M-A constituent and ferrite (intercritically HAZ) →tempered martensite (TM) and ferrite (sub-critical HAZ). Lower hardness in intercritically HAZ and sub-critical HAZ (softening zones) was observed compared to base metal (BM) in dissimilar/similar weld joints. The size of softening zone was 0.2–0.3 mm and reduction in hardness was ∼7.6%–12.7% of BM in all the weld joints, which did not influence the tensile properties of weld joints such that fracture location was in BM. Formability of dissimilar weld joints was inferior compared to similar weld joints because of the softening zone, non-uniform microstructure and hardness on the two sides of FZ. The effect of microstructure on fatigue life was not influenced due to the presence of welding concavity. © 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction Dual phase (DP) steels, consist of ferrite (soft phase) and martensite (hard phase), and have good mechanical strength, high work hardening rate, low yield ratio, high uniform and total elongation, high strain energy absorption and excellent formability [1–4]. Based on these characteristics, DP steels are widely used in automotive applications to realize energy savings, reduction in emission and light weight [5,6]. Laser welding is an advanced joining technology that is used in automotive applications, because of high power, low maintenance cost and high efficiency [7–9]. Furthermore, because of ease of automation and flexibility, laser welding is being considered to replace potentially other popular joining processes such as resistance spot welding [10] and friction stir welding [11]. A number of studies have been carried out on weld metals using laser welding such as DP steels and other metals [12–14]. The effects of weld line position and geometry on the formability of laser welded high strength low alloy (HSLA) and DP steels blanks
∗ Corresponding author. E-mail address: [email protected] (X. Wang).
were studied by Li et al. [15]. They observed that the softening zone of the heat affected zone was the dominant factor in controlling formability of DP 980 weld joints. Moreover, welds of DP600 failed consistently in the region where severe softening occurred. The tensile properties and microstructure of dissimilar laser weld joints of TWIP/TRIP, DP/TRIP and DP/22MnB5 were studied by Rossini et al. [16]. The laser welding of high strength low alloy and DP 980 was studied by Parkes et al. [17], and they noted that the presence of softening zone occurred on DP980 side and there was no effect on tensile properties and fatigue failure occurred in the welded area. Saha et al. [18] studied the microstructure-property relationship in fiber laser welded DP and HSLA steels. While, Parkes et al. [19] studied tensile properties of fiber laser weld joints of HSLA and DP steels at warm and low temperatures. The mechanical response along different tensile directions of laser-welded dissimilar blanks between DP780 and DP1180 steels was studied by Gong et al. [20]. Microstructure and mechanical properties of laser welded dissimilar DP600 and DP980 steels weld joints were studied by Farabia et al. [21]. In summary, the effect of softening zone on properties of weld joints depends on the degree of softening and size of the zone, which are governed by laser heat input and welded materials [15,18,22–26]. Thus, dissimilar/similar laser welding was carried
http://dx.doi.org/10.1016/j.jmst.2017.09.001 1005-0302/© 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model
ARTICLE IN PRESS
JMST-1046; No. of Pages 11
H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
2
Table 1 Chemical composition of experimental steels (wt%). Materials
C
Si
Mn
Ti
Cr
Mo
Al
Fe
CE
DP980 DP780
0.09–0.11 0.07–0.09
0.3–0.5 0.1–0.2
1.6–1.8 1.6–1.8
0.01–0.03 –
0.3–0.5 0.2–0.4
0.2–0.4 0.1–0.3
0.04–0.06 0.3–0.5
Bal. Bal.
0.584 0.499
Fig. 1. Sample drawings of property testing: (a) tensile test, (b) fatigue test, (c) erichsen test.
out at a constant heat input between DP780 and DP980 steels, with the objective to further understand the effect of softening zone on the properties of weld joints. Furthermore, the differences between microstructure and properties of dissimilar and similar weld joints were studied. 2. Experimental 2.1. Materials Cold-rolled steel sheets of DP780 and DP980 of 1.5 mm thickness were used in the study described here. The chemical composition is listed in Table 1. The two base metals had identical alloying elements but their content was different. The carbon equivalent (CE) was 0.584% and 0.499%, respectively.
2.4. Tensile, formability and fatigue properties Tensile tests in the direction perpendicular to the weld joint was carried out at a constant strain rate of 3 mm/min using DNS-100 universal material testing machine. Fatigue tests were conducted using GPS100 high frequency fatigue testing machine at a stress ratio of R = 0.1 with a stress amplitude of 540 MPa (room temperature). Erichsen test was conducted using CTM604 Erichsen test machine, with a loading force of 15 kn and a test speed of 15 mm/min. FZ was in the center of all the samples and dimensions are presented in Fig. 1. 3. Results and discussion 3.1. Weld morphology
Samples of dimensions 80 mm × 80 mm were welded using IPG YLS-6000 laser. The welding speed was 5.0 m/min and laser power was 2 kW. Laser spot diameter was 0.3 mm and pure argon gas was used for shielding.
Fig. 2 depicts the macroscopic morphology of dissimilar and similar weld joints. Welding concavity was observed in all the weld joints, but the evidence of concavity was less apparent in the case of similar DP980-DP980 weld joints (Fig. 2(c)). This had very small effect on tensile properties and formability of weld joints but seriously impacted the fatigue life, as described below.
2.3. Microstructure and hardness
3.2. Microstructure
Samples perpendicular to weld joint were polished and etched with 4% nital to study the microstructure of the weld joints using SU5000 scanning electron microscope (SEM). HV-1000 microhardness tester was used to measure the microhardness of weld joints at a load of 300 N and a loading time of 10 s. The distance from the welded surface was 0.4 mm and spacing between two indentations was 0.1 mm.
3.2.1. Microstructural evolution Generally, weld joint consists of fusion zone (FZ) and heat affected zone (HAZ). The weld metal was melted at high temperature and rapidly solidified to form FZ, whereas the HAZ formation was heated and cooled without melting. In dissimilar/similar DP780 and DP980 steels welding process, the HAZ can be divided into four zones: coarse-grained HAZ (CG-HAZ), fine-grained HAZ
2.2. Welding methods
Fig. 2. Macroscopic morphology of similar and dissimilar weld joints: (a) DP780-DP780, (b) DP980-DP980, (c) DP780-DP980.
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
3
Fig. 3. Overview of microstructural evolution of dissimilar weld joints.
Fig. 4. Microstructural evolution of DP780 side of dissimilar weld joints: (a) BM, (b) SC-HAZ, (c) IC-HAZ, (d) FG-HAZ, (e) CG-HAZ, (f) FZ.
(FG-HAZ), intercritical HAZ (IC-HAZ) and sub-critical HAZ (SC-HAZ) because of differences in local peak temperature from the center of FZ. Similar classification has been previously suggested [27]. In dissimilar welding, the microstructural evolution zone consisted of DP 780 base metal (BM), DP 780 HAZ, FZ, DP 980 HAZ and DP 980 BM.
Fig. 3 shows an overall view of dissimilar weld joints, where each zone can be clearly depicted. Figs. 4 and 5 are the micrographs of microstructural evolution of dissimilar weld joints between DP780 and DP980 steels. It was observed that the microstructure in FZ of
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11 4
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
Fig. 5. Microstructural evolution of DP980 side of dissimilar weld joints: (a) BM, (b) SC-HAZ, (c) IC-HAZ, (d) FG-HAZ, (e) CG-HAZ.
similar DP780-DP780 and DP980-DP980 weld joints was identical to dissimilar DP780-DP980 weld joints. The microstructure of DP780 BM consisted of ferrite (F) and a number of block martensite (M) dispersed at the ferrite boundary (Fig. 4(a)). In SC-HAZ, the local peak temperature was below Ac1 and austenitization did not take place such that the martensite in the base metal transformed into tempered martensite (TM) during subsequent heating and cooling (Fig. 4(b)). It was also observed that the ferrite phase of the base metal was not changed. In IC-HAZ, the local peak temperature was below Ac3 but above Ac1 , martensite and a part of ferrite in the base metal was austenitized and on cooling transformed into ferrite and M-A constituent, and the grain size of transformed ferrite was finer (Fig. 4(c)) compared to the base metal. In FG-HAZ, the local peak temperature was above Ac3 , and the base metal was completely austenitized to form equiaxed austenite grain structure and transformed into lath martensite (LM) via shear transformation (Fig. 4(d)). The microstructure of CG-HAZ was also lath martensite (LM) (Fig. 4(e)) and the transformation mechanism was similar to FG-HAZ, but grain size was coarse because of higher local peak temperature. Coarse LM was observed in FZ characterized by a typical solidification microstructure (Fig. 4(f)).
On DP980 side of dissimilar weld joints, the microstructure in HAZ was similar to DP 780 side but grain size was different. Higher fraction of martensite and finer ferrite was observed in DP 980 BM compared to DP780 BM (Fig. 5(a)), leading to more martensite and smaller ferrite grain size was observed in SC-HAZ and IC-HAZ of DP980 side (Fig. 5(b) and (c)). The microstructure of FG-HAZ and CG-HAZ was LM with different grain size (Fig. 5(d) and (e)). 3.2.2. Distribution of alloying elements Fig. 6 describes the elemental distribution in dissimilar weld joints. The alloying elements were identical in both the base metal but their content was different (Table 1), especially Al and Si (Si: 0.3–0.5% (DP980), 0.1–0.2% (DP780) and Al: 0.04–0.06% (DP980), 0.3–0.5% (DP780)). EDS line scan was performed across the crosssection of dissimilar weld joints. The Al-content decreased from DP780 side to DP980 side, whereas Si-content was increased (Fig. 6(a)). In order to clearly illustrate the distribution of alloying elements in FZ, EDS line scan was also carried out across the cross-section of FZ (Fig. 6(b)). The results showed that Al and Si distribution was uniform in FZ although there was significant difference in the content in both the base metals. This suggests a good
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
5
Fig. 6. EDS line scanning analyse of dissimialr weld joints: (a) elemental distribution of full weld joints, (b) elemental distribution of FZ.
IC-HAZ, which is analyzed in Section 3.2.1. It resulted that the hardness was decreasing compared to BM. Moreover, the width of the softening zone on both sides of the dissimilar weld joints was narrow, i.e. ∼0.2 mm (DP780 side) and ∼0.3 mm (DP980 side), respectively. Identical results were observed in the case of similar DP780-DP780 and DP980-DP980 weld joints. Heat input and laser power density have a strong impact on the softening behavior of HAZ in laser welded DP steels [19]. Eqs. (1) and (2) are the corresponding relationship: H= Pd =
Fig. 7. Hardness distribution on cross-section of all weld joints.
and uniform mixing of all the elements in FZ during laser welding. Similar elemental distribution has been observed earlier [22]. 3.3. Mechanical properties 3.3.1. Hardness Fig. 7 illustrates the distribution of hardness across the crosssection of dissimilar and similar weld joints. The average hardness of DP780 and DP980 base metal (BM) was measured to be ∼235 HV and ∼315 HV, respectively. In dissimilar weld joints, softening in HAZ of DP780 side was not obvious with ∼18 HV reduction in Vickers hardness, which was 7.6% of DP780 BM. While, reduction in hardness of ∼40 HV was observed on DP980 side, 12.7% of DP980 BM. All the hardness results in HAZ on both sides of dissimilar DP780-DP980 weld joints were identical to the HAZ of similar DP780-DP780 and DP980-DP980 weld joints. From Fig. 7, it may be noted that the softening zone was confirmed in IC-HAZ and SC-HAZ. Similar results were reported previously [23,28]. Microstructure is an aspect that determines microhardness [29]. Tempered martensite in SC-HAZ led to a decrease in hardness compared to BM. Meanwhile, martensite was transformed into M-A constituent in
P
(1)
v 4P d2
(2)
where H is heat input (J/mm), P is laser power (2 kW), v is welding speed (5 m/min), Pd is laser power density (J/mm2 ), d is diameter of laser spot (0.3 mm). Generally, higher heat input and lower laser power density, leads to wider softening zone. For identical weld material DP980, greater width (∼0.5 mm) of softening zone has been reported [14,15] with similar heat input but lower laser power density (P was 6 kW, v was 16 mm/min, d was 0.6 mm). From Fig. 7, it may also be noted that the average hardness of fusion zone (FZ) for similar DP780-DP780 weld joints was ∼356 HV, lower than similar DP980-DP980 FZ (∼385 HV). Higher martensite fraction led to higher average hardness of DP980 BM (Fig. 5(a)), which enhanced the average hardness of FZ. Furthermore, the average hardness of FZ in dissimilar weld joints was ∼375 HV, which can be considered as the average of two similar weld joints, as calculated by Eq. (3).
’ 780 980 HVFZ = HVFZ + HVFZ /2
(3)
’ where HVFZ is average hardness of FZ in dissimilar weld 780 980 are average hardness of FZ in similar joints, HVFZ and HVFZ DP780-DP780 and similar DP980-DP980 weld joints, respectively. ’ was 371 HV with 1% error. According to Eq. (3), the calculated HVFZ Similar studies have been previously conducted [22]. Thus, Eq. (3) can be used to predict the average hardness of FZ of dissimilar weld between different DP steels. The reason that the hardness in the fusion zone of dissimilar DP780-DP980 weld joints was approxi-
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model
ARTICLE IN PRESS
JMST-1046; No. of Pages 11
H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
6
Fig. 8. Stress-strain plot and macroscopic fracture morphology of all the samples: (a) stress-strain plot, (b) macroscopic fracture morphology.
Table 2 Tensile test results of different materials. Materials
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
n
Fracture location
DP780-980 DP780-780 DP980-980 DP780 BM DP980 BM
905 875 1080 885 1080
555 525 695 525 690
11.4 17.2 12.7 18.7 12.5
0.10 0.13 0.11 0.14 0.12
780 BM BM BM BM BM
mately the average of two fusion zones of similar DP780-DP780 and DP980-DP980 weld joints is related to the identical microstructure (lath martensite) and uniform distribution of alloying elements.
3.3.2. Tensile properties Table 2 lists tensile properties data for all the samples. In similar weld joints, tensile strength and yield strength were close to the respective base metal (BM). The elongation of similar DP780-DP780 and DP980-DP980 weld joints were 17.2% and 12.7%, respectively, and similar to BM values of 18.7% and 12.5%, respectively. In dissimilar DP780-DP980 weld joints, the plastic deformation of the samples was concentrated on DP780 side (7 mm) and to some extent on DP980 side (1 mm) as listed in Table 3. The fraction of ferrite in DP 780 (64%) is more than DP 980 (55%), which has better ductility and lower strength. During tensile process, plastic deformation prior occurred on one side with higher ferrite fraction. Thus, most of plastic deformation was shared by DP780 side. The elongation of DP780-DP980 weld joint was lowest (11.4%) compared to other weld joints. The tensile strength and yield strength were 905 MPa and 555 MPa, respectively, slightly lower than DP980 BM (1080 MPa and 695 MPa) but similar to DP780 BM (885 MPa and 525 MPa). Fig. 8 illustrates the engineering stress-engineering strain plots and macroscopic fracture morphology of the samples, and Fig. 9 presents the microstructure morphology of fracture surface of DP780-DP780 weld joints. A number of dimples were observed, implying ductile fracture. All the curves were smooth with no yield point. The curves of similar weld joints (DP780-DP780, DP980DP980) were similar to corresponding BM. While, the curve of dissimilar weld joints (DP780-DP980) was between that of two similar weld joints (DP780-DP780, DP980-DP980) (Fig. 8(a)). This suggested that the applied load was shared between the two base metals, fusion zone and two HAZs in dissimilar weld joints. Similar results were reported by Gong et al. [20]. The fracture of dissimilar and similar weld joints was located in BM (Fig. 8(b)), because of restraint in fusion zone (FZ) with high strength in conjunction
Fig. 9. Microscopic fracture morphology of DP780-DP780 tensile samples.
with a narrow softening zone (∼0.2 to 0.3 mm) in HAZ. However, in previous studies, the tensile fracture location of DP980 weld joints was in HAZ because of the presence of severe softening zone (∼1.1 to 2.0 mm) [23]. The underlying reason is that high power intensity, high welding speed and lower heat input were selected in our study. Thus, it is important to control the softening zone in HAZ for controlling the tensile properties of weld joints. 3.4. Formability Erichsen test results of similar and dissimilar weld joints are presented in Table 4. It can be seen that Erichsen values (IE) of all the weld joints were inferior compared to the corresponding base metal (BM). Hardening index (n) measured in the tensile test is an important factor to evaluate the formability of sheet metals. Generally, the greater n values, the better is the formability. In Table 2, n value of similar/dissimilar weld joints was: 0.10 (DP780-DP980) <0.11 (DP980-DP980) <0.13 (DP780-DP780). Thus, the formability of similar/dissimilar weld joints was: DP780-DP980
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model
ARTICLE IN PRESS
JMST-1046; No. of Pages 11
H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
7
Table 3 Two sides of tensile sample of dissimilar weld joints between DP780 and DP980.
Non-tensile After-tensile
Length of DP980 side (mm)
Elongation percentage of DP980 side (%)
Length of DP780 side (mm)
Elongation percentage of DP780 side (%)
Fracture location from FZ center (mm)
35 36
– 12.5
35 42
– 82.5
– 23
Fig. 10. Microscopic morphology of fracture for all the weld joints: (a) DP780-DP780, (b) DP980-DP980, (c) DP780-DP980.
Table 4 Erichsen test results. Materials
IE (mm)
Fracture location
DP780-DP980 DP780- DP780 DP980- DP980 DP780 BM DP980 BM
6.665 7.994 6.982 8.718 8.148
DP780 HAZ FZ HAZ – –
was inferior, with an IE of 6.67 mm. DP980-DP980 was intermediate, with an IE of 6.98 mm. DP780-DP780 was superior, with an IE of 7.99 mm. The difference in the formability was related to the microstructure and hardness distribution of weld joints, it was analyzed in the following. In Erichsen test, the principal strain can be divided into two components: longitudinal and transverse, one can consider weld line as longitudinal direction in the weld joint test. The hardness and strength of fusion zone (FZ) was higher than BM but the ductility was low, offering a restraint in the longitudinal direction. Thus, formability was limited by this restraint leading to a lower formability for weld joints compared to BM which was a monolithic blank with no restraint biaxial stretch, and failure always initiated in FZ [30]. Fig. 10 presents the macroscopic morphology of fracture for all the weld joints. In similar DP780-DP780 weld joints, the crack initiated in FZ and propagated into BM, uniform deformation along FZ was observed and necking occurred in the fracture zone (Fig. 10(a)). The microstructure of FZ was LM, which had higher hardness and strength, the transverse deformation of the joint was restrained
effectively. Thus, IE value of similar DP780-DP780 weld joints was only reduced by 8.3% compared to BM. However, in similar DP980-DP980 weld joints, the crack initiated in HAZ and propagated parallel to FZ with local necking (Fig. 10(b)). As mentioned in Section 3.3.1, the softening zone in DP980 HAZ (∼0.3 mm, 12.7% hardness reduction) was more severe than DP780 HAZ (∼0.2 mm, 7.6% hardness reduction) The reason is that more fraction of tempered martensite. With the load applied, stress concentration was easy to produce in the softening zone. And, hardness difference between tempered martensite and ferrite was the reason that the crack initiation occurred in softening zone until fracture in the case of transverse strain in similar DP980-DP980 weld joints. In dissimilar DP780-DP980 weld joints, FZ departed from the original location toward DP980 side (Fig. 10(c)). Significant deformation occurred on DP780 side as compared to DP980 side. This is related to the strength because the yield strength of DP780 BM was less than DP980 BM. Necking was observed on DP780 side, while almost no deformation was observed on the DP980 side. In the case of large deformation on DP780 side, localized softening zone with lower strength compared to the adjacent material provided a fracture location, although there was significant softening on the DP980 side. A similar observation was made by Bandyopadhyay et al. [31]. The asymmetrical deformation, softening zone and restrain from FZ, was the reason for lowest IE in dissimilar weld joints among all the Erichsen test samples. To further explore the effect of microstructure on crack propagation, the microstructure and voids in the region close to crack propagation were observed by SEM. Fig. 11 describes the morphology of the fracture surface for all the weld joints. As men-
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
8
Fig. 11. Microscopic morphology of fracture for all the weld joints: (a) and (b) DP780-DP780, (c) DP980-DP980, (d) DP 780-DP980, (e) and (f) SC-HAZ.
tioned, the crack initiation was in FZ in similar DP780-DP780 weld joints. Martensite laths were elongated and voids were generated between laths (Fig. 11(a) and (b)). When load was applied to samples, voids provided sites for crack to nucleate and propagate until fracture. In DP780-DP980 and DP980-DP980 weld joints, crack initiation was in SC-HAZ (Fig. 11(c) and (d)), where tempered martensite (TM) and ferrite grains experienced deformation and voids were observed. The magnified micrographs illustrated that voids initiated in the hard TM phase and propagated along the interface between ferrite and TM (Fig. 11(e) and (f)), because large deformation can be sustained by ferrite, whereas TM experienced less deformation. Thus, this inhomogeneity in the plastic behavior of the two phases led to void initiation at the interface [32,33]. The failure mechanism is similar to that discussed by Cingara et al. [32] for DP steels. 3.5. Fatigue properties Table 5 summarizes the fatigue results of all the samples at a stress amplitude of 540 MPa. In this case, failure of DP780 BM and DP980 BM did not occur. However, lowest fatigue life cycle was
Table 5 Fatigue results of all the samples. Materials
(MPa)
Fatigue life cycle (N)
Fatigue Fracture location
DP780- DP980 DP780- DP780 DP980- DP980 DP780 BM DP980 BM
540 540 540 540 540
42355 44836 115824 10000222 10000222
FZ FZ FZ No failure No failure
obtained in dissimilar DP780-DP980 weld joints. Alam et al. [34] pointed that welding concavity had a significant impact on fatigue life of weld joints as compared to the softening zone. As mentioned above, softening zone and welding concavity (Figs. 2 and 6) were observed in dissimilar and similar weld joints, and welding concavity of DP980-DP980 weld joint was less apparent. As a consequence, the fatigue life cycle of DP980-DP980 weld joints was highest compared to other weld joints in spite of the presence of significant softening zone. The localized stress concentration was easily generated in the welding concavity, which deteriorated the fatigue life of weld joints [35]. Thus, the fracture location was in FZ of all the weld joints (Fig. 12). Additionally, similar DP980-DP980 weld joints had
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
9
Fig. 12. Macroscopic of fatigue fracture of all the samples: (a) DP780- DP780, (b) DP980- DP980, (c) DP780- DP980.
higher strength, which ensured the fatigue life. The effect of softening zone on fatigue life of weld joints needs further and detailed study. Fig. 13 presents the fatigue fracture for all the weld joints. The crack initiation was observed in welding concavity (Fig. 13(a–c)), which can easily result in the occurrence of persistent slip bands (PSB) (Fig. 13(d)). Secondary cracks were generated in the crack propagation zone, indicated by arrows in Fig. 13(e).
4. Conclusions (1) The microstructure of FZ of three types of weld joints was lath martensite. The microstructure of HAZ of dissimilar and similar weld joints was identical: lath martensite (CG-HAZ, FG-HAZ) →M-A constituent and ferrite (IC-HAZ) →tempered martensite and ferrite (SC-HAZ). Finer ferrite and more martensite were observed in IC-HAZ and SC-HAZ of DP980 side. (2) The hardness in the HAZ of DP780-DP780 weld joints was lower than DP980-DP980 weld joints, and identical values were obtained in the HAZs of dissimilar DP780-DP980 weld joints. Softening zone was observed in SC-HAZ (tempered martensite) and IC-HAZ (martensite transformed into M-A constituent and ferrite). Hardness of fusion zone (FZ) of dissimilar weld joints was ∼375 HV, higher than similar DP780-DP780 (∼357 HV) but
lower than similar DP980-DP980 FZ (∼386 HV), which is the average value of both the fusion zones of similar weld joints. (3) Tensile fracture of dissimilar and similar weld joints occurred in the base metal. The tensile strength and yield strength of dissimilar weld joints were 905 MPa and 555 MPa, respectively, lower than DP980-DP980 but similar to DP780-DP780 weld joints. The deformation in dissimilar DP780-DP980 weld joint was concentrated on DP780 side with 7 mm, compared to 1 mm extension on DP980 side, which resulted in lowest elongation (11.4%) compared to other weld joints. Softening zone did not influence the tensile property of weld joints. However, the effect of softening zone on fatigue life of weld joints was not observed because of the presence of welding concavity. (4) The formability of dissimilar DP780-DP980 weld joints was more inferior, compared to similar DP780-DP780 and DP980DP980 weld joints. This behavior is related to asymmetrical deformation and different softening zone on both sides, and restrain from the fusion zone. The crack initiated in the softening zone of DP780 side and propagated parallel to the fusion zone. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51274063 and 51305285),
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
10
Fig. 13. Microscopic of fatigue fracture of all the weld joints: (a) DP780- DP780, (b) DP980- DP980, (c) DP780, (d) crack initiation zone, (e) crack propagation zone.
the National Program on Key Basic Research Project (Grant No. 2011CB606306-2), the Open Research Fund from the State Key Laboratory of Rolling and Automation, Northeastern University (Grant No. 2016005) and the Project Funded by China Postdoctoral Science Foundation (Grant No. 2016M601877).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.H. Lee, S.H. Park, H.S. Kwon, G.S. Kim, C.S. Lee, Mater. Des. 64 (2014) 559–565. A. Lesnykh, S. Golovatiy, D. Valovik, Mater. Today: Proc. 2S (2015) S667–S671. L.R. Bhagavathi, G.P. Chaudhari, S.K. Nath, Mater. Des. 32 (2011) 433–440. K. Mahesh, S. Sankaran, P. Venugopal, J. Mater. Sci. Technol. 28 (12) (2012) 1085–1094. A. Fallahi, J. Mater. Sci. Technol. 18 (2002) 451–454. L. Schemmann, S. Zaefferer, D. Raabe, F. Friedel, D. Mattissen, Acta Mater. 95 (2015) 386–398. L. Quintino, A. Costa, R. Miranda, D. Yapp, V. Kumar, C.J. Kong, Mater. Des. 28 (2007) 1231–1237. M. Zhang, X.N. Wang, G.J. Zhu, C.J. Chen, J.X. Hou, S.H. Zhang, H.M. Jing, Acta Metall. Sin. (Engl. Lett.) 27 (2014) 521–529. J. Canning, Opt. Laser Eng. 44 (2006) 647–676. H. Ghazanfari, M. Naderi, Acta Metall. Sin. (Engl. Lett.) 26 (2013) 635–640. Y. Hovanski, M.L. Santella, G.J. Grant, Scr. Mater. 57 (2007) 873–876. G.M. Xie, H.B. Cui, Z.A. Luo, W. Yu, J. Ma, G.D. Wang, J. Mater. Sci. Technol. 32 (2016) 326–332.
[13] J. Yang, Y. Li, H. Zhang, W. Guo, D. Weckman, N. Zhou, Metall. Mater. Trans. A 46 (2015) 5149–5157. [14] C.C. Chang, C.P. Chou, S.N. Hsu, G.Y. Hsiung, J.R. Chen, J. Mater. Sci. Technol. 26 (2010) 276–282. [15] J. Li, S.S. Nayak, E. Biro, S.K. Panda, F. Goodwin, Y. Zhou, Mater. Des. 52 (2013) 757–766. [16] M. Rossini, P.R. Spena, L. Cortese, P. Matteis, D. Firrao, Mater. Sci. Eng. A 628 (2015) 288–296. [17] D. Parkes, W. Xu, D. Westerbaan, S.S. Nayak, Y. Zhou, F. Goodwin, S. Bhole, D.L. Chen, Mater. Des. 51 (2013) 665–675. [18] D.C. Saha, D. Westerbaan, S.S. Nayak, E. Biro, A.P. Gerlich, Y. Zhou, Mater. Sci. Eng. A 607 (2014) 445–453. [19] D. Parkes, D. Westerbaan, S.S. Nayak, Y. Zhou, F. Goodwin, S. Bhole, D.L. Chen, Mater. Des. 56 (2014) 193–199. [20] H.Y. Gong, S.F. Wang, P.K.Y.P. Korkolis, Mater. Des. 90 (2016) 1115–1123. [21] N. Farabia, D.L. Chen, Y. Zhou, J. Alloys Compd. 509 (2011) 982–989. [22] H.Q. Zhang, A. Wei, X.M. Qiu, J.H. Chen, Mater. Des. 54 (2014) 443–449. [23] J.F. Wang, L.J. Yang, M.S. Sun, T. Liu, H. Li, Mater. Des. 97 (2016) 118–125. [24] Y. Liu, D.Y. Dong, L. Wang, X. Chu, P.F. Wang, M.M. Jin, Mater. Sci. Eng. A 627 (2015) 296–305. [25] V.H. Baltazar-Hernandez, S.S. Nayak, Y. Zhou, Metall. Mater. Trans. A 42 (2011) 3115–3129. [26] M. Xia, E. Biro, Z. Tian, Y.N. Zhou, ISIJ Int. 48 (2008) 809–814. [27] J.F. Wang, L.J. Yang, M.S. Sun, T. Liu, H. Li, Mater. Des. 90 (2016) 642–649. [28] N. Farabi, D.L. Chen, J. Li, Y. Zhou, S.J. Dong, Mater. Sci. Eng. A 527 (2010) 1215–1222. [29] X.N. Wang, C.J. Chen, H.S. Wang, S.H. Zhang, M. Zhang, X. Luo, J. Mater. Process. Technol. 226 (2015) 106–114.
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001
G Model JMST-1046; No. of Pages 11
ARTICLE IN PRESS H. Di et al. / Journal of Materials Science & Technology xxx (2017) xxx–xxx
[30] M.S. Xia, M.L. Kuntz, Z.L. Tian, Y. Zhou, Sci. Technol. Weld. Join. 13 (2008) 378–387. [31] K. Bandyopadhyay, S.K. Panda, P. Saha, V.H. Baltazar-Hernandez, Y.N. Zhou, Mater. Sci. Eng. A 652 (2016) 250–263. [32] G. Avramovic-Cingara, C.A. Saleh, M.K. Jain, D.S. Wilkinson, Metall. Mater. Trans. A 40 (2009) 3117–3127.
11
[33] N. Saeidi, F. Ashrafizadeh, B. Niroumand, M.R. Forouzan, F. Barlat, Mater. Sci. Eng. A 644 (2015) 210–217. [34] M.M. Alam, J. Karlsson, A.F.H. Kaplan, Mater. Des. 32 (2011) 1814–1823. [35] J. Roesler, H. Harders, M. Baeker, Mechanical Behavior of Engineering Materials: Metals, Ceramics, Polymers, and Composites, 1st ed., Springer, New York, 2007.
Please cite this article in press as: H. Di, et al., J. Mater. Sci. Technol. (2017), http://dx.doi.org/10.1016/j.jmst.2017.09.001