Formability of laser tailor welded blanks in diffe

2022-10-15
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Formability of laser tailor welded blanks under different combinations

in this experiment, laser tailor welded tests were carried out on steel plates of equal thickness/unequal thickness/the same material/different materials. The laser uses a CO2 fast axial flow gas laser. The focal length of the focusing lens is 220mm, and the process auxiliary gas is helium. The output mode of laser is Q mode

the performance data of the steel used for the test are shown in Table 1:

in this study, different test combinations such as the same thickness/the same material, the same thickness/different material, the different thickness/the same material, and the different thickness/different material were carried out for laser butt welding test. The material combinations are shown in Table 2

laser butt welding requires high edge quality of butt joint materials. At present, before laser welding at home and abroad, there are several methods of using high-precision shear for the preparation of material ends: using mechanical rolling method; Using laser cutting method; Using double beam method; Use fine blanking method, etc. Each method has its advantages and disadvantages. In this experiment, the edge preparation is carried out by laser cutting before laser butt welding. After laser cutting, the edge quality fully meets the requirements of laser butt welding

results and discussion

1 Mechanical properties of laser welded joints

during the transverse tensile test of laser butt welding, all fractures were at the base metal, and the measured properties could not reflect the condition of the welded joint. In longitudinal tension, the laser weld is distributed along the length of the sample, which can reflect the situation of the laser tailor welded joint. During longitudinal tension, the changes of yield strength and tensile strength of laser welded joints under four different combinations are shown in Figure 1

it can be seen from the test results that its strength is higher than that of the base metal to a certain extent. This is because the formation process of laser welding seam is a rapid cooling process, its strength and hardness must be higher than the base metal to a certain extent, and the plasticity will be reduced to a certain extent. It can also be seen from the test results that the thickness ratio has little effect on the laser welding strength, while the difference of material grade has a greater effect on the laser butt welding strength. After laser tailor welding of different grades of materials, the strength of the welded joint mainly depends on the materials with low strength grades, rather than the average strength of the two materials

therefore, ABS (styrene butadiene acrylonitrile polymer) and hips (impact resistant polystyrene) materials are mainly used

2 Cupping test

cupping test determines the maximum forming depth when the specimen breaks. In this study, the decrease of the cupping value of the laser welded joint compared with the base metal under four different conditions is compared, as shown in Figure 2. The reduction of cupping value is based on the steel plate on the side with thinner thickness or weaker strength. The results show that the cupping value of laser welding is lower than that of base metal to a certain extent, and the change of thickness ratio of tailor welded steel plate has a great influence on the reduction of cupping value. Figure 3 shows the effect of yield strength on the reduction of cupping value. From the results, it can be seen that the effect is small

3. Limit vault height

when measuring the limit vault height, make the material in the pure tension deformation state, and measure the forming depth when fracture occurs. In this study, the effect of yield strength on the ultimate crown height is compared, as shown in Figure 4. It can be seen from the results that the ultimate vault height at the laser welded joint is lower than that of the base metal to a certain extent, and the yield strength has little effect

in this study, the reduction rate of the ultimate vault height at the laser welded joint compared with the base metal under four different combinations is also obtained, as shown in Figure 5. It can be seen from the results that, compared with the base metal, the reduction value of the limit vault height of the laser welded joint is the largest when the thickness difference/different materials are different, and the limit vault height of the other three combinations also decreases to a certain extent

4. Reaming test

this test measures the relative difference between the aperture when the hole edge cracks and the initial aperture to evaluate the flanging performance of the material. The figure below shows the reduction rate of the reaming rate of the laser welded joint compared with the base metal under the four combinations. From the results, it can be seen that the reduction of the reaming rate is greater than the reduction of the limit vault height, and the maximum reduction of the reaming rate is in the case of different thickness/same material

Figure 7 shows the effect of yield strength on the hole expansion rate. From the test results, it can be seen that the hole expanding rate of laser welding is much lower than that of base metal

5. Forming limit diagram (FLD)

print the lattice of laser tailor welded blanks in the fifth combination. After forming, measure the long and short axes of the deformed lattice with a tool microscope to draw the forming limit diagram (FLD), as shown in Figure 8. The results show that due to the influence of the weld, the material deformation is restrained, and the principal strain decreases significantly. The FLD0 substrate in the forming limit diagram is 44%, and the tailor welded blanks are 30%. It can be seen that the formability of the weld joint is reduced to a certain extent compared with the base metal after laser tailor welding of cold-rolled steel plate, so its position on the parts should avoid the most complex part of forming. At the same time, it can be seen from the figure that the maximum strain of the dangerous area considering the factors such as simple peripheral circuit, good reliability, low power requirements and economy is 14%, so there is still 16% strain allowance at the tailor welded blanks, which shows that the laser tailor welded blanks can still meet the stamping requirements of automobile plates

conclusion

through the experimental study on the mechanical properties and formability of laser welded joints with good shear modulus of cold-rolled steel plates under different combinations, the results show that there is one item beyond the requirements:

1. The strength of the welded joints after laser welding is higher than that of the base metal to a certain extent. The change of thickness ratio has less effect on strength than that of material grade ratio. The strength of laser welded joints with different strength levels mainly depends on the materials with low strength levels

2. The formability of the welded joint is lower than that of the base metal to a certain extent. With the increase of the thickness difference and strength difference of tailor welded blanks, the formability decreases. (end)

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