Experimental and numerical study on local mechanical properties and failure analysis of laser welded DP980 steels
Introduction
The environment friendly vehicles demand less fuel consumption as well as lower CO2 emissions. The ever-increasing use of advanced high strength steel (AHSS) is a promising way of lightening car body in the automotive industry. Ferrite-martensite dual-phase (DP) steel is one of the AHSS families widely adopted for body-in-white [1], [2]. DP steels consist of martensite islands embedded in ferritic matrix. These martensite islands contribute to the strength of DP steel, while the ductility arises from ferrite [3]. The combination of martensite and ferrite offers higher initial work hardening rate along with considerably uniform elongation compared to conventional steels [4]. Welding is the mostly used joining technique in the automobile industry. Among various welding methods, laser welding is playing a vital role in the joining of AHSS due to its flexibility and high energy density [5]. The microstructures of joint are transformed locally under welding thermal cycles, and then the mechanical properties change correspondingly. The safety of vehicles is closely related to the mechanical behavior of welded joint. Thus, characterizing and understanding of local microstructures and mechanical properties of welded joints is important for modeling and predicting the overall mechanical behavior of DP welded joints [6].
Extensive studies have been conducted on the mechanical performance of fusion zone (FZ) and HAZ in terms of AHSS welds. Conventional experiment methods to characterize local properties have been reported using mini tensile samples, which are machined directly from FZ and HAZ of the joint, respectively [6], [7]. However, laser welding provides much narrower FZ and HAZ compared with conventional arc welding, making such mini tensile samples are difficult to machine because of insufficient FZ and HAZ in the joints. On the other hand, it is inaccurate for assuming a homogeneous property of HAZ using mini tensile sample method, as the HAZ microstructure varies along with the thermal gradient in real situation [8]. The tensile test based on a “rule of mixture” is another method commonly adopted to extract properties of the FZ and HAZ based on the assumption of iso-strain, of which the weld metal is parallel to the loading direction [9]. Lee et al. extracted the average mechanical properties of the weld bead and the HAZ of tailed-welded blanks via the subsize samples [10]. Abdullah et al. obtained the weld properties from tailed-welded blank using a similar approach [11]. In their study, four different sized samples were tested, and more accurate results were found using smaller samples owing to the larger proportion of weld in the cross section of sample. The tensile test combined with the “rule of mixture” is an appealing method for obtaining local properties due to its simplicity, but only the average properties of the weld or the HAZ could be acquired. As a result, the property variation of the HAZ was ignored, whereas it is important for numerical simulation.
Local constitutive behavior of HAZ could be obtained by scaling stress-strain curve according to the hardness profile, where the variation of the HAZ properties was considered [12], [13], [14]. For example, Pavlina and Van Tyne found that the tensile strengths of steels over the range of 450–2350 MPa presented a linear correlation with hardness [15]. However, Rojek et al. pointed out that this method underestimates the yield stress in the range of small plastic deformation and overestimates the yield stress in the range of large plastic deformation [16]. The newly developed digital image correlation (DIC) has been applied in measuring local strain during tensile test [17], [18], [19]. The corresponding local stress is based on the iso-stress assumption of the whole sample. Only part of the stress-strain curve can be obtained for the hardened zone, as little plastic deformation would occur there.
Thermal simulation is an effective method with precise temperature control that can reproduce a relatively large HAZ samples with homogeneous microstructure. This feature allows for conventional mechanical properties test of HAZ [20]. Goodall et al. have examined the toughness of thermal simulated HAZ of arc-welded X80 line pipe steel by Charpy impact test [21]. Dancette et al. have investigated the local microstructure and constitutive behavior of spot welds of DP steels experimentally with a Gleeble 3500 thermo-mechanical simulator [22]. However, the welding thermal cycles are difficult to measure for the thermal simulation, as the HAZ is too narrow to locate several thermocouples at different positions accurately. In this work, therefore, the thermal cycles experienced during laser welding of DP980 steel were identified using finite element (FE) analysis. The thermal cycles were then reproduced using thermo-mechanical simulator to investigate the local constitutive behaviors of the joint. The overall tensile behavior of the joint was evaluated by experiment and FE analysis. Particular attention was paid to the influence of heat input on the failure location of the joint.
Section snippets
Materials and laser welding process
Hot dip galvanized DP980 steel with 1.2 mm thickness was used in the experiments. The chemical compositions and mechanical properties of the base metal (BM) are shown in Table 1. The milling machined steel sheets (100 mm×200 mm) were welded in butt-joint configuration, as shown in Fig. 1a. Laser welding was conducted using a Nd:YAG laser system (TRUMPF HL4006D), with the welding parameters presented in Table 2. Low heat input joint was welded at the welding speed of 4 m/min, and high heat input was
Local thermal cycles
Fig. 3a presents a correspondence between the observed microstructure and the calculated temperature field of weld cross section with low heat input. The peak temperature in the FZ exceeded the boiling temperature of BM resulting in key-hole effect during laser welding. On both sides of the FZ, the inhomogeneous HAZ was divided into four zones as the descending of peak temperature, namely coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), inter-critical HAZ (ICHAZ) and sub-critical HAZ
FE modeling
To investigate the effect of heat input on tensile behavior of DP980 steel joint, both low and high heat input welded joints were simulated, respectively. To simplify the FE analysis of tensile behavior of butt joints, following basic assumptions were applied:
- (1)
The butt-welded joint consisted of FZ, HAZ and BM, and the HAZ could be further divided into CGHAZ, FGHAZ, ICHAZ and SCHAZ.
- (2)
The mechanical properties of each zone were uniform.
Detailed widths of each zone of the joint were measured and
Conclusions
- (1)
The numerical simulation of laser welding showed that the t8/5 at HAZ of low heat input and high heat input are approximately 0.62 s and 4.35 s, respectively. The microstructure and hardness of thermal simulated HAZ exhibited similar characters to that of experimental HAZ.
- (2)
The UTS of the low heat input and high heat input welded joint reached 99.7% and 95.6% of BM, respectively. The fine grained martensite of FGHAZ contributed to the highest YS and UTS across joint, and the ICHAZ had the lowest
Acknowledgements
This work was supported by the International Science and Technology Cooperation Program of China (No. 2013DFR50590, 2015DFA51460), the National Natural Science Foundation of China (Nos. 50705050, 51605019 and 51675030).
References (30)
- et al.
Tensile and fatigue properties of fiber laser welded high strength low alloy and DP980 dual-phase steel joints
Mater. Des.
(2013) - et al.
Tensile properties of fiber laser welded joints of high strength low alloy and dual-phase steels at warm and low temperatures
Mater. Des.
(2014) - et al.
The influence of microstructure and composition on the plastic behaviour of dual-phase steels
Acta Mater.
(2014) - et al.
Microstructure and mechanical properties of laser welded dissimilar DP600/DP980 dual-phase steel joints
J. Alloy. Compd.
(2011) - et al.
Laser, tungsten inert gas, and metal active gas welding of DP780 steel: comparison of hardness, tensile properties and fatigue resistance
Mater. Des.
(2014) - et al.
Study on the strength and failure modes of laser welded galvanized DP980 steel lap joints
J. Mater. Process. Technol.
(2014) - et al.
On the evolution of local material properties and residual stress in a three-pass SA508 steel weld
Acta Mater.
(2012) - et al.
Tensile testing for weld deformation properties in similar gage tailor welded blanks using the rule of mixtures
J. Mater. Process. Technol.
(2001) - et al.
Experimental and numerical study on formability of friction stir welded TWB sheets based on hemispherical dome stretch tests
Int. J. Plast.
(2009) - et al.
Tensile testing for weld deformation properties in similar gage tailor welded blanks using the rule of mixtures
J. Mater. Process. Technol.
(2001)
Numerical study of strengths of spot-welded joints of steel
Mater. Des.
Overload analysis and fatigue life prediction of spot-welded specimens using an effective J-integral
Mech. Mater.
Determination of mechanical properties of the weld zone in tailor-welded blanks
Arch. Civ. Mech. Eng.
Determination of local constitutive properties of aluminium friction stir welds using digital image correlation
Mater. Des.
Numerical and experimental analysis of residual stresses in full-penetration laser beam welding of Ti6Al4V alloy
Rare Met. Mater. Eng.
Cited by (33)
A comparative study of conventional, dynamic rotation and heat-assisted friction stir welding of Ti-6Al-4V plates to reduce welding defects
2024, Journal of Materials Processing TechnologyA comparative study of microstructure and corrosion resistance in DP980 steel joints under different laser welding method
2023, Journal of Materials Research and TechnologyA review on heat affected zone softening of dual-phase steels during laser welding
2023, Journal of Manufacturing ProcessesLocal microstructure and mechanical characteristics of HAZ and tensile behavior of laser welded QP980 joints
2022, Materials Science and Engineering: AInfluence of chemical composition and thermomechanical treatment of low-carbon steels on the microstructure and mechanical properties of their laser welded joints
2022, Materials Science and Engineering: ACitation Excerpt :In laser welding, both heating and cooling conditions determine the microstructure in formed joints. In this regard, the influence of its parameters (power and welding speed) on the microhardness level in laser welds of low-carbon steels has been reported in Refs. [7,8,10]. It has been shown that it is impossible to avoid the formation of quenched martensitic microstructures in their weld metal (WM) [7,10].
Simultaneously improved the strength dramatically and eliminated HAZ softening of DP980 steel pulsed-arc welding joints by PWHT
2022, Materials Science and Engineering: A