Microstructure and properties in dissimilar/similar weld joints between DP780 and DP980 steels processed by fiber laser welding

doi:10.1016/j.jmst.2017.09.001

摘要/Abstract

Abstract:

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.

Key words: Dual phase steel, Laser welding, Dissimilar weld joints, Microstructure, Mechanical properties, Formability

. [J]. 材料科学与技术, 2017, 33(12): 1561-1571.

Di Hongshuang, Sun Qian, Wang Xiaonan, Li Jianping. Microstructure and properties in dissimilar/similar weld joints between DP780 and DP980 steels processed by fiber laser welding[J]. J. Mater. Sci. Technol., 2017, 33(12): 1561-1571.

图/表 18

Table 1 Chemical composition of experimental steels (wt%).

Materials	C	Si	Mn	Ti	Cr	Mo	Al	Fe	CE
DP980	0.09-0.11	0.3-0.5	1.6-1.8	0.01-0.03	0.3-0.5	0.2-0.4	0.04-0.06	Bal.	0.584
DP780	0.07-0.09	0.1-0.2	1.6-1.8	-	0.2-0.4	0.1-0.3	0.3-0.5	Bal.	0.499

Fig. 1. Sample drawings of property testing: (a) tensile test, (b) fatigue test, (c) erichsen test.

Fig. 2. Macroscopic morphology of similar and dissimilar weld joints: (a) DP780-DP780, (b) DP980-DP980, (c) DP780-DP980.

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.

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.

Fig. 6. EDS line scanning analyse of dissimialr weld joints: (a) elemental distribution of full weld joints, (b) elemental distribution of FZ.

Fig. 7. Hardness distribution on cross-section of all weld joints.

Table 2 Tensile test results of different materials.

Materials	Tensile strength (MPa)	Yield strength (MPa)	Elongation (%)	n	Fracture location
DP780-980	905	555	11.4	0.10	780 BM
DP780-780	875	525	17.2	0.13	BM
DP980-980	1080	695	12.7	0.11	BM
DP780 BM	885	525	18.7	0.14	BM
DP980 BM	1080	690	12.5	0.12	BM

Table 3 Two sides of tensile sample of dissimilar weld joints between DP780 and DP980.

	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)
Non-tensile	35	-	35	-	-
After-tensile	36	12.5	42	82.5	23

Fig. 8. Stress-strain plot and macroscopic fracture morphology of all the samples: (a) stress-strain plot, (b) macroscopic fracture morphology.

Fig. 9. Microscopic fracture morphology of DP780-DP780 tensile samples.

Table 4 Erichsen test results.

Materials	IE (mm)	Fracture location
DP780-DP980	6.665	DP780 HAZ
DP780- DP780	7.994	FZ
DP980- DP980	6.982	HAZ
DP780 BM	8.718	-
DP980 BM	8.148	-

Fig. 10. Microscopic morphology of fracture for all the weld joints: (a) DP780-DP780, (b) DP980-DP980, (c) DP780-DP980.

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.

Table 5 Fatigue results of all the samples.

Materials	σ (MPa)	Fatigue life cycle (N)	Fatigue Fracture location
DP780- DP980	540	42355	FZ
DP780- DP780	540	44836	FZ
DP980- DP980	540	115824	FZ
DP780 BM	540	10000222	No failure
DP980 BM	540	10000222	No failure

Fig. 12. Macroscopic of fatigue fracture of all the samples: (a) DP780- DP780, (b) DP980- DP980, (c) DP780- DP980.

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.

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