| In order to satisfy the higher demands of fast-growing automobile, such as reducing weight, inducing the oil consumption, lowering the emission and adding security, colleagues of steel industry are sparing no effort to have reform and innovation; as a result a new type of steel which has greater comprehensive properties is developed. TWIP (Twinning Induced Plasticity) has high plasticity, strain hardening rate and intensity. Besides, the impact absorbing energy of this steel is twice that of common high-strength steel. These superior performances can well meet the demands of modern automobile industry. Therefore, TWIP has wide application prospects in automotive field in the future.The welding procedure process which has high productivity and excellent product quality can save materials and improve working conditions drastically, so it is widely used in the filed of mechanical manufacturing. According to the statistics of industrially developed nations, steels that can be used after welding make up forty-five percent of all steels. For this reason, it is very necessary to conduct a research on the weld ability of TWIP. The author choosed three TWIP steel plates with thickness of lmm, while the ingredients were different. And used the500w pulsed solid laser and the automatic TIG welding machine to weld the TWIP steel with the thickness of1mm. Using observation of the microstructure, stretching, cupping, fracture scanning, micro-hardness test, X-ray diffraction, electrochemical corrosion test, the author explored the microstructure and properties of welded joints at the same time compared these two welding technologies’effect on comprehensive properties of TWIP. The main conclusions are as follows:1. Three kinds of TWIP steels which were welded by laser were numbered1#,2#and3#. Through comparing the structure property of welded joints of these three steels, the following conclusion can be drawn:Laser joints’macro welds of the three steels are wide at the top and narrow at the bottom. Based on the center of welds, they distributed symmetrically. All of them have less noticeable heat affected zones(HAZ). In the center of the welds appear small equiaxed crystals. Columnar crystals with the same direction form in the place where welds meet fusion lines forms. Welded joints shape well. The distribution trends of hardness of these joints were much the same in a horizontal direction. The hardness number was highest in the range of welding centerline, and was lowest in the HAZ. The matrix hardness of3#was the highest as180HV. In the weld depth, their hardness numbers all appear U-shape.1#has the highest at the bottom—432HV.2#and3#have highest hardness at the top; they are422HV and400HV respectively. After stretching, The laser welded joint of number2#fractured in the near line section of the matrix, while for number1#and number3#, welded joints fractured in the welding centerline, the elastic deformation stages of three welded joints are not obvious and have no apparent necking phenomena when fracturing, Experiments indicate that the product of strength and ductility of number2#was the biggest—41310MPa·%. Therefore, number2#had superior laser joints; The joints’tensile fracture of1#and2#which were welded by laser belong to the ductile fracture, on the opposite3#was brittle fracture; The result can be obtained through cupping test that number1#and2#were superior to3#from the perspective of matrix and welding line’s plasticity deformation after laser welding.2. Three kinds of TWIP steels which were welded by TIG. Through comparing the structure property of welded joints of these three steels, the following conclusion can be drawn:The TIG weld center of the organization of number1#was the small equiaxed grains, while for number2#and number3#, both were the typical cellular dendrite; The TIG welded joints’hardness of the number1#was similar to2#as165HV, higher than the number3#; After tensile test, The TIG welded joints of number1#and2#fractured in the HAZ, while for number3#, welded joints break in the welding centerline, Three welded joints had no apparent necking phenomena when fracturing and the tensile strength were less than the base metal; The TIG welded joints’tensile fracture of2#belong to the ductile fracture, on the opposite1#and3#were brittle fracture; The Erichsen value of number1#and number2#increase substantially quite, while the number3#increased maximum, the TIG welded joints stamping performance were good.3. Through comparing the structure property of the2#which welded by laser welding and TIG welding, the following conclusion can be drawn:The plastic deformation capability of TIG welding specimens was much lower than the laser welding specimens. The average micro-hardness of laser welding joints was higher than the TIG welding joints. New phase wasn’t discovered in TWIP steel welded by laser welding and TIG welding, and remained fully austenite structure. The corrosion rate of matrix with welding slowed down, and the corrosion resistance of the TIG welding sample was the best. |
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