It is often said that more than 50% of a country's GDP is related to welding in one way or another. As far as body structure is concerned, welding is the main operation process. In the previous body design, the most typical material was low carbon steel, and these automobile parts were assembled by resistance spot welding (RSW), realizing the scene of high speed and high output in the demand process of automobile manufacturers. However, due to the refueling scheme, other welding methods in the industry are becoming more and more popular. Among them, laser welding, which is easy to be automated and flexible, has gained a reputation in metal joining. Welding DP steel involves other welding processes, such as resistance spot welding (RSW)[ 1 1], laser spot welding [12], gas shielded metal arc welding (GMAW)[ 13] and friction stir welding [14]. Therefore, it is necessary to analyze its mechanical properties. The laser weldability of dual-phase steel was studied, such as tensile properties, welding effect and very limited fatigue properties. In the application program, structural laser welded joints are easy to fail under cyclic load, so it is necessary to characterize the fatigue strength of welded joints. Previous studies show that the welding of DP steel leads to the formation of a soft zone outside the heat affected zone (HAZ), and the mechanical properties of welded joints have a significant impact in this zone (17, 18). By forming this softening zone (17-20), the tensile properties and formability of welded joints are seriously hindered. In this study, the tensile test was carried out and the same result was obtained. Then the question is whether the soft belt will reduce the fatigue strength. Detailed research shows that the influence of fatigue performance and the failure behavior of soft strip are the shortcomings of dual-phase steel welded joints. It is found that in order to effectively apply DP steel, it is very important to understand its fracture characteristics and mechanism under monotonic and cyclic loads. Therefore, the purpose of this study is to evaluate the mechanical properties of laser welded joints of dual-phase steel under two kinds of monotonous and alternating loads. 2 materials and experimental process 2. 1 materials this DP600 steel has a thickness of 1.2 mm and a zinc diffusion (GA) coating (46 g/m2 at the top and 47 g/m2 at the bottom). The chemical composition of the base metal is given in table 1. Table 1 Chemical composition (wt%) of DP600 steel selected in this study.
CMnSiAlMoCrCuS
0.09 1.84 0.36 0.05 0.0 1 0.02 0.03 0.005
2.2 Laser welding Laser welding is through the use of diode lasers, and the parameters used in the current research are shown in Table 2. The head of this semiconductor laser, Nuvonyx ISL4000L, is mounted on the arm of Panasonic VR6 robot. The rectangular size of the beam is 12mm× 0.9mm, and the focal length is 90 mm. At the same time, the diode laser is limited to the welding conduction mode because of the power density. Ultra-high purity argon with the flow rate of 14.2 l/min is used as shielding gas to weld on the surface of the sample. Weld a penetration weld on the metal template at the welding speed of 1 m/min. Table 2
Laser power of laser source (kW) welding speed (m/min) focal length (cm) beam size (mm2)
nuvonyx ISL-4000 diode 4 1 9 12×0.9
2.3 Microstructure and microhardness test: DP600 steel was cut in three different directions. Namely, longitudinal direction, transverse direction and short transverse direction. Then, these specimens are made by metallographic inlay, grinding, polishing and etching with 2% nitric acid etchant. Then check the microstructure of the cross section of the welded sample. The change of weld microstructure was observed by optical microscope combined with Clemex image analysis system. Vickers microhardness test was carried out on the uncorroded samples. The load used in the test is 500gm, and the holding time is 15s. All values are taken as the average of series samples in three directions. Observe the center of the fusion zone carefully with a microscope and determine the geometric specifications of the weld. All geometric dents are enough to avoid any potential influence caused by the strain field of adjacent notches. Figure 1: Geometry and dimensions of tensile and fatigue specimens used in current research. 2.4 tensile test ASTM-E8M is used for tensile test instead of sample. The geometry of the tensile specimen is shown in figure 1. Treat the welding sample perpendicular to the welding direction. All test samples swing slightly along the loading direction, and the final value is 600. The tensile test was carried out at room temperature with a fully automatic universal tensile testing machine. The strain rate of the current tensile test is 0.0 1 s? 1, 0.00 1s 1, 0.000 1s- 1 and 0.0000 1 s? 1。 The strain measurement test was carried out with a 25 mm extensometer. Test at least two samples at each strain rate. The compensated yield strength, ultimate tensile strength and toughness (elongation) of 0.2% were evaluated.
2.5 The fully computerized 880 1 electro-hydraulic test system is used for fatigue test and metallographic fatigue study, with load control and stress amplitude exceeding 6. These tensile test samples are tested by two or more samples with the same geometry and size at each stress level. What is the stress ratio? Equal to 0. 1. All test sine waves and frequencies are selected as 50 Hz. After the fatigue test, JSM-6380-lv scanning electron microscope equipped with Oxford energy dispersive X-ray spectroscopy system and 3 d microscope was used to analyze and inspect the base metal and welded joint to determine the initiation position and propagation mechanism of fatigue crack. 3 results and discussion 3. 1 microhardness distribution and microstructure change figure 2 shows the microhardness distribution of laser welded joints of DP600 steel. It was observed that the hardness of the fusion zone (FZ) was significantly higher than that of the base metal, which was about 65438 0.5 times. Scanning electron microscope (SEM) shows that the obvious fusion zone (FZ) at this stage mainly includes martensite (M) and some side ferrite and bainite (Figure 3(a)). Martensite (FZ) formed in the fusion zone rapidly cools the hardness of the molten pool during laser welding, and the heat affected zone changes. Due to the formation of martensite, the hardness near the melting zone is higher than that near the base metal. A similar result is given in [2 1]. An external heat affected zone with hardness lower than that of the base metal is observed near this zone, which is called soft zone, as shown in Figure 2. The existence of soft bands is mainly due to the pre-existing tempered martensite [3, 18, 19]. As shown in Figure 3(b), tempered martensite (TM) and bainite ferrite matrix, plus some pre-existing retained austenite. Similar results were found in other grades of welded DP steel (17,18,20). This soft zone will adversely affect the mechanical properties of laser welded DP600 steel joints, which will be seen in later chapters. The constant hardness of almost the whole material is observed in the matrix material, and the corresponding microstructure of the matrix material includes martensite, ferrite matrix and part of retained austenite (Figure 3(c)). Figure 2: Micro-hardness of typical DP600 steel laser welded joint section Figure 3: SEM micrograph shows the microstructure changes of DP600 steel joint, in which (a) fusion zone, (b) external heat affected zone (soft zone) and (c) matrix metal, where M, F, B and TM respectively represent martensite, bainite ferrite and tempered martensite. Figure 4. 4. Engineering stress-strain curve test. At different strain rates, DP600 steel is (a) base material and (b) welded joint.
Figure 5. Typical failure position of tensile specimen of laser welded joint of DP 600 steel 3.2 Tensile properties Figure 4 shows two engineering stress-strain curves of base metal and welded joint. It is interesting to find that the stress-strain curve of DP600 base material is smooth, and all strain rates (Figure 4(a)) are continuous, while the welded DP600 joint shows yield point phenomenon at all strain rates, as shown in Figure 4(b). All the welding samples in the outer HAZ failed, and an example is shown in Figure 5, where vertical lines mark welding. Careful observation in the tensile test shows that yield occurs in the softening zone, and then in this zone (i.e. the external heat affected zone), most plastic deformation accumulates until it finally fails. The yield strength of welded joints is higher than that of base metal, but the tensile strength of welded specimens is slightly lower than that of base metal (Figure 6). Although these two yield strengths and ultimate tensile strength are slightly increased to improve the strain rate, as far as the strain rate is concerned (Figure 7), ductility does not show any slight change. However, the fracture modes of all tensile test samples were observed, and the elongation decreased due to welding (Figure 7). Figure 6. Ultimate test of yield strength and tensile strength of laser welded joints. DP600 steel under different strain rates. The yield point phenomenon in welding samples may be due to the gap diffusion that may occur during laser welding. The energy of the laser generated at high temperature to diffuse carbon (or nitrogen) iron atoms to other positions is slightly lower than that of another real edge dislocations plane atom. The elastic interaction is so strong that the air mass of impurity atoms becomes completely saturated and condenses into a row of atoms along the dislocation of the core. When such an example is loaded with fixed insertion dislocations (i.e., welding samples in this study), higher stress is needed to start dislocation movement and plastic deformation. As a result, the yield strength after laser welding becomes higher, as shown in fig. 6. Dislocation lines may slip away from the influence of solute atoms at low pressure, showing the yield point phenomenon, as shown in Figure 4(b). This yield point is the yield after the load drops, and some detailed descriptions can be found in references [22-24]. Figure 7. Ductility test of laser welded joints of DP 600 steel at different strain rates. The fracture characteristics of welded joint and base metal are basically similar. Cup-shaped depression fracture is the main feature of fracture, which represents ductile fracture mode. An example of a typical SEM micrograph of a welding fracture can be seen in fig. 8. The center of the fracture surface (Figure 8(a)) mainly contains grain pits, showing typical fracture caused by simple tensile load. The fracture near the edge (Figure 8(b)) shows the combination of two equiaxed and shear pits, because it has a new indentation of equiaxed pits on the grain surface, which has a slender parabolic shape. This means that the shearing motion occurs in the area with tensile load.
Figure 8. Scanning electron microscope photos of fracture surfaces of typical welded joints in tensile specimens show that the strain rate is 1× 103 s 1. Surface (a) is in the center, and (b) is near the surface. 3.3 Fatigue test of fatigue performance control load shows that when the fatigue limit of welded joint is slightly lower than that of base metal, the fatigue life of base metal and welded joint is almost the same under high stress amplitude, as shown in Figure 9. The results show that a slight decrease in the hardness of the external heat affected zone (Figure 2) is not enough to reduce the fatigue strength at higher stress amplitude. This is because the high stress amplitude destroys the soft zone with potential negative effects. The fatigue limit and calculated fatigue of these two materials are shown in Table 3. The fatigue limit of the welded joint is lower than 12.5% of the base metal, and then the base metal with a fatigue rate of 0.28 is compared with the base metal with a fatigue rate of 0.32 to obtain the welded joint. This kind of soft band with negative effect in the external heat affected zone of welding specimen slightly reduces the fatigue limit or fatigue ratio, and the pressure amplitude near the fatigue limit zone can not be ignored. The following Basquin equation is used to fit fatigue data. Is the amplitude of alternating stress. Is the fatigue strength coefficient of pressure closure at 2N= 1. N is the number of fatigue failure cycles, 2N is the reverse load failure coefficient, and b is the fatigue strength index. Table 4 gives the parent metal's? And b value and welding sample. This is to see that the fatigue strength coefficient of welded joints is higher, but the absolute value of fatigue strength index will increase to get a slightly shorter fatigue life (because a smaller B value corresponds to a longer fatigue life [24]). Did the result not work as well? And b, as evidenced in fig. 9. Figure 9. S-n curve of 9. DP600 steel substrate and welded joint, R = 0. 1, 50 Hz, room temperature, where the data point arrow indicates the sample jumping. 3.4 Location and Mechanism of Fatigue Fracture With regard to the location of fatigue fracture, it was observed that all welding samples failed in the external heat affected zone at a pressure amplitude higher than 250 MPa, and the measured parts in the parent metal samples failed. However, under the pressure amplitude of 250 mpa or less, the failure of all welding samples and parent metal samples is far from the intermediate measurement part. These results are very consistent. In the S-N diagram of welded joint and base metal, there is a turning point at the pressure amplitude of about 250 MPa, as shown in Figure 9. Two different stress amplitude test ranges can be seen at the defect position of the welding sample, as shown in Figure 10. The possible reason and influence is that the cyclic strengthening mechanism involves inducing martensitic transformation [25, 26, 27] deformation. The microstructure of this DP steel contains a small amount of retained austenite. Under cyclic load, the retained austenite transforms into martensite, which makes the steel have additional strengthening effect. These martensite particles are considered to promote the generation of dislocations and control the cyclic deformation [28]. At a higher stress amplitude, dislocations in the LCF region of the test sample can overcome the obstacle of martensite stress, that is, it exceeds the resistance caused by martensite and produces more cumulative damage in the measurement part of the test sample. The reason for this phenomenon is that there is a high level of stress amplitude in the defective parts of base metal and welded joint. Due to the low hardness, the fatigue failure of welded samples is more obvious in the external heat affected zone (Figure 2). However, at low pressure amplitude, this phenomenon lasted for a long time in HCF region, and the stress could not interact with the resistance of martensite to overcome chaos. The notch effect caused by potential stress concentration makes the area near the measurement site the weakest area. Generally, the notch effect indicates that the HCF region is longer and has a shorter life than the LCF region (29).
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Microstructure and properties of dual-phase steel DP600
Microstructure and mechanical properties of laser welded joints of DP600 (dual-phase steel)
abstract:
Dual-phase steel (DP) has higher tensile strength, better initial work hardening and greater elongation than traditional steel, while reducing fuel consumption and greenhouse gas emissions, and is widely used in automobile industry. Welding and connection must be involved in such application process, but this will lead to local deformation of materials and a series of problems of potential safety and reliability under cyclic load. The purpose of this study is to evaluate the microstructure changes of DP600 steel after laser welding and its influence on tensile and fatigue properties. Laser welding leads to a significant increase in the hardness of the fusion zone, but it also forms a soft zone in the shape of the heat affected zone (HAZ). Although the ductility decreases and the yield strength increases after welding, the ultimate tensile strength remains almost unchanged. Although the fatigue limit between the base metal and the welded joint with almost the same fatigue life under high stress amplitude decreases slightly after welding, tensile fracture and fatigue failure occur in the external heat affected zone with high stress amplitude. It is observed that the fatigue crack initiation zone appears on the surface of the sample, and crack propagation is a characteristic crack formation mechanism. Pits and deformation zones are also observed in the fast propagation region.