289-paper-激光焊接动态试验.doc

Evaluating Strength and Toughness Properties of Laser Weldment by Dynamic TestingFANG Jian(Baosteel Research Institute, Baoshan Iron & Steel Co., Ltd., Shanghai 201900, China) Abstract: The strength and toughness properties of a laser beam welded joint from hot rolled DP600 steel plate were evaluated by Vickers hardness and instrumented Charpy impact testing. Based on the impact force-displacement curve, Critical Crack Length, i.e. CCL and dynamic crack extension resistance curve, i.e. J-Da were determined, describing the welding joints crack initiation toughness and crack growth behavior responding to the dynamic loading. Furthermore, tearing module derived from the slope of J-Da curve was discussed with the link to the material microstructure mechanism affected by the thermodynamic process of laser beam welding.Key words: laser beam welding, instrumented Charpy impact, dynamic testing1. IntroductionDue to the convenient manufacturing process and acquired excellent mechanical properties of welding joint, laser beam welding technique has been widely used in the automotive, ship building and oil transportation pipeline industry so far. In common, the mechanical performance of metallic material may be affected by the thermodynamic factor corresponding to the kinds of welding process. Moreover, It is taken into more and more account that the mechanical properties of structural component consisting of basal material and joint part should be assured even under the service condition, with the requirement of structural integrity and safety for auto framework and ship hull design. It was reported as an example in ThyssenKrupp Co., AHSS including Dual Phase (DP) and Transformation Induced Plasticity (TRIP) steels with their Laser welding components should be evaluated with low temperature and dynamic loading condition to simulate the strength and toughness response in-service1. Besides conventional tensile, hardness and impact testing, some other modern experimental techniques should be applied for characterization of welding joints comprising temperature factor and intermediate strain rate loading condition, which maybe of practical value to process optimization, material selection and quality simulation of laser beam welding.In the present work, subsize Charpy specimens, sampled from various parts of laser welding joint composed of the same hot rolled DP steel plate, were prepared for instrumented impact testing. Combined with the Elastic Plastic Fracture Mechanical (EPFM) analysis, by using BAOSTEEL exclusive ImpactPlus 2.0, the influence of laser welding onto the strength and toughness properties of DP steel were experimentally investigated.2. PrinciplesDerived from the advantage of low cost and reliability of conventional Charpy testing, instrumented impact testing, i.e. IIT is believed as an industrial dynamic testing method for evaluating materials response to dynamic loading with temperature variation. By recording the force signal induced by the interaction between tup and Charpy specimen during contact, IIT technique could transform the absorbed impact energy, Et into the integration of force against loading point displacement, as shown in Fig.1. Moreover, The characteristic force points including Fgy, Fm, Fu and Fa could also be marked on the force-displacement curve as the boundary of specific fracture subdivision, which provides the threshold of computer assisted EPFM analysis such as Single Specimen Method2 and Forced Based Analysis3 with the help of ImpactPlus 2.0.Figure 1 Curve of force-displacement recorded by instrumentation method with fitted Key Curve and the illustration of the position for general yield pointThen the total impact fracture could be subdivided into crack formation and propagation sections. In the present paper, Critical Crack Length, i.e. CCL and dynamic crack extension resistance curve, i.e. J-Da are calculated to describe material crack initiation and resistance properties.CCL is defined as the ratio of J-integral fracture toughness to flow yield stress.(1)As regards the Chapy impact fracture, it is of more practice to obtain Jd by employing modified Rice equation, in which only crack formation energy Ei is required, with the hypothesis that Fm is nominally defined as the crack initiation.(2)Another point should be addressed about the determination of syd, considering the cost and complexity of uniaxial dynamic loading techniques. In the preliminary study, the correlation between syd and general yield force, Fgy, described by Server equation4, is proved to be satisfactory, which means by using IIT technique, Jd and syd could be acquired at the same time from the curve output. (3)Concerning the crack propagation and fracture resistance properties, J-Da curve was established on the concept of “Key Curve” method to indicate the contribution of energy consumption to the stable crack growth, resisting instability of cleavage fracture, as Eq.(4) and (5).(4)(5)3. ExperimentsBMHardnessTesting surfaceWACharpy specimenSubsize Charpy specimens (2.51055mm) were sampled from the laser beam welding joint composed of two same DP600 hot rolled plate steel with the thickness of 3.7 mm. With the aim to distinguish the properties of Base Material, i.e. BM, Heat affecting Zone, i.e. HAZ and Welding Area, i.e. WA, V-notch of the specimens were machined in the thickness direction located in the marked three areas respectively as shown in Fig.2. Instrumented impact testing was carried out at 20 and 40. For evaluation the strength of laser welding joint, the distribution of Vickers hardness was also determined on the testing surface perpendicular to the welding direction.Figure 2 Laser welding joint and the specimen sampled from the plate for Charpy impact testing.4. Results and discussions4.1 Hardness PropertiesThe boundary of welding area was clearly marked by acid corrosion. Then the distribution of hardness on the testing surface perpendicular to the welding direction were determined by means of Vickers along the path from the center of WA to both sides of BM across HAZ every 0.5mm. HAZHAZWABMBMFigure 3 Hardness distribution profile of laser beam welded joints of DP600 plate.Similar to the conventional welding joint, In Fig.2 the hardness values show the tendency of stepping down from 340HV level of WA to approximate the half, i.e. 180HV of BM, which indicates the strength quenching effect caused by the local temperature rising and immediate air cooling thermodynamics occur in the fusion zone controlled by laser beam welding mechanism. Commonly the improvement of the strength is equal to the degradation of toughness property for a given metallic material, which needs more concern especially in the industrially application, considering the factor of temperature and loading rate etc. The results of Et energy, PSF, characteristic force from force-displacement curves of WA, HAZ and BM batches of specimen by instrumented impact testing at 20 and 40 were listed in the Table 1 and illustrated in Fig.4.4.2 Dynamic Strength And Toughness PropertiesAll the impact fracture surface of specimens at ambient and low temperature show the ductile fracture character, implied by the 100% PSF and the lack of Fu and Fa on the IIT curves. Same as the dropping tendency of hardness property, the impact toughness decreases from the high level of WA, almost double higher of BM, which reveals that with the promotion of strength property, the toughness of DP 600 were also upgraded simultaneously. Moreover the slight difference of Et between HAZ and BM proves that during the contact of laser beam on the welded material, heat energy concentrated and gradient of heat decaying were constrained in the shallow band, avoiding the appearance of complexity composition in HAZ compared to the conventional arc and multipass welding.On the other hand, as listed in the Table 1 the conventional results of impact toughness are quite same, the influence of temperature on the crack initiation and fracture resistance and the distinction of relative fracture mechanism linked to the relative parts of laser weldment are hardly investigated only on the basis of the series of Et and PSF results. According to the principle of CCL and J-Da introduced above, EPFM analysis on the Charpy specimen with the three points bending fracture model were performed in the study.Table 1 Summary of the impact toughness results and the parameters of Elastic-Plastic Fracture Mechanics analysisNo.Et(J)Ei (J)EP(J)PSF (%)Fgy (kN)Fm (kN)syd (MPa)Jd (J/mm2)CCL (mm)Welding AreaWA-12051.821.929.91002.924.87546.21.602.9WA-2-4048.821.027.81003.135.06585.61.532.6Heat Affecting ZoneHAZ-12031.311.320.01002.814.16524.40.821.6HAZ-2-4031.110.620.51003.224.53601.50.781.3Base MaterialBM-12029.5 8.421.11003.224.15601.70.611.0BM-2-4026.87.119.71003.894.44727.40.520.7Figure 4 Force-displacement curves of specimens from WA, HAZ and BM parts at ambient and low temperature.In spite of the close value of Et energy for WA, HAZ and BM batches, the duration to the occurrence of Fm is obviously shorten with the increase of Fm level itself, which is a tendency to the elastic fracture character. As a proof, CCL results were calculated respectively as listed in the Table 1. At ambient, CCL of WA was 2.9mm, HAZ 1.6mm and BM 1.0mm, same magnitude of 0.3mm but quite different decay percentage, i.e. 10% for WA, 19% for HAZ and 30% for BM were shrinked with the temperature lowing down. As the meaning of ratio of fracture toughness to yield stress, CCL is directly regarded as the crack toughness proportional to the size of the plastic zone under the notch root and the resistance against in-plane strain fracture due to the crack blunting behavior and plastic zone deformation. The smaller the CCL reaches, the more probably cleavage fracture may occur. It could be briefly summarized that the crack initiation toughness of DP 600 as studied was improved to be several times higher than that of original status by laser welding. In addition the susceptibility of embrittlement effect to the low temperature is of negligibility for the fusion welding area.Figure5 Dynamic crack extension resistance curves of specimens from relative parts of welding joint.J-Da curves were plotted by using ImpactPlus 2.0 software, shown in Fig.5. The solid line represents the testing at ambient and dashed ones for low temperature. At a given temperature, the crack extension resistance of WA batch is the best with the J-Da curves located above, while the position of HAZ and BM are near to each other. As regards the low temperature, three batches of curves are all dropping with a parallel offset. It need to be concerned that the curve of HAZ at 40 is a superposition of BM at 20. It is the laser beam welding that significantly enhances the uncontrolled crack spreading resistance contributed by the accumulation of absorbed impact energy, avoiding the occurrence of fracture with instability, which is also found to be of dependence on the temperature.It should be carefully considered that the J-Da curve of WA batch have the individual feature of two segments, the initial linear steep section followed by rising slow down in comparison of HAZ and BM batches, which is also maintained even at low temperature. At the stage of slow rising period, the form and slope of three batches are quite identical.(6)Tearing Modulus5, Tmat is calculated as the differentiation of J-integral toughness against crack length, equal to the slope of J-Da curve, see Eq.(6). From the Fig.5, Tmat of WA specimens were keeping at a higher level until the crack growth reaches 1mm, after which the Tmat level collapsed with the crack path across the width of specimen to a complete fracture. No obvious initial high level of Tearing Modulus was found related to HAZ and BM batches.By determination of Tmat, the correlation between microstructure and crack extension resistance could be presumed as followed. The welding fusion pool may be formed in the narrow welding area due to the heat input of laser, where the original crude microstructures and secondary particles of as studied DP 600 steel plate maybe austenized immediately. Owe to the precondition of chemical composition contained by DP 600, martensite and secondary strengthening particles are believable to re-precipitate in the WA with refined morphology and homogeneous arrangement among the ferrites during the air cooling period, which is a key mechanism related to the combination promotion of strength and crack initiation and resistance toughness properties for WA of DP 600 steel after welding. Once the cooling rate is adjusted to an optimization, retained austenite will also appear with transformed ferrite and martensite, which is proved to be as a buffer adsorbing the loading energy and beneficial to blunt the stress intensity level of crack tip. 5. ConclusionsThe strength and toughness properties of a DP 600 steel plate joint after laser beam welding was evaluated by means of Vickers hardness testing and instrumented impact testing. As a result, the combination of strength and toughness is improved simultaneously. Based on the force-displacement curve from IIT, CCL parameter and J-Da curve were employed in the study to assess the metallic crack initiation and fracture resistance toughness.The discrepancy between HAZ and BM of laser welding joint was of slight significance, while the dynamic crack initiation toughness of WA were multiple times higher than BM. With the temperature low down, the embrittlement effect of WA was minimized in comparison with HAZ and BM.The crack extension resistance behavior of WA was featured on the