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Study on Microstructures and Properties of Laser Welding Joints of Automotive Dual-Phase Steel DP780

Author: YuQun
Tutor: ChenZuo
School: Beijing University of Technology
Course: Optical Engineering
Keywords: DP780 Laser welding Microstructure Mechanical properties
CLC: TG456.7
Type: Master's thesis
Year: 2010
Downloads: 330
Quote: 2
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Abstract


Low-cost dual-phase steel, the work-hardening rate, the yield ratio is low, and can be continuously yield having excellent press formability and has been widely used in the field of automobile manufacturing. Compared with traditional welding methods, laser welding of duplex steel has energy concentrated softening zone narrow, high production efficiency advantages, is becoming the primary means of dual-phase steel welding. The actual components of the dual-phase steel applications, the joint microstructure and properties must meet the demand. So the research the duplex steel laser welded joints organizational performance of great significance for the understanding of the two-phase welding and to promote the practical application of laser welding of duplex steel. Firstly 4000W disc-type Yb: YAG solid-state lasers, the use of the principle of laser deep penetration laser butt welding research on advanced high strength steel DP780 1.2mm thick. Analytical testing methods and the use of optical microscopy, scanning electron microscopy, transmission electron microscopy to study the microstructure of welded joints, and summarizes the laser welding parameters on microstructure and mechanical properties of the joints were investigated. Found by scanning electron microscopy, weld low carbon bainitic lath martensite and retained austenite lath martensite presence of greatly improving the micro-hardness of the weld. Color metallographic analysis technique for determining the content of martensite in the weld test, first determine the weld in the volume fraction of the martensite and residual austenite, and then further measured by micro X-ray diffraction analysis techniques, the content of the residual austenite, and ultimately determine the volume fraction of martensite of the weld. Experimentally derived: the welding speed is increased, so that the increase in volume fraction of martensite, which provide a greater degree of subcooling is because of the larger cooling rate so that more low carbon lath martensite phase transition. Weld martensite, bainite and retained austenite morphology was also observed by transmission electron microscopy and found to improve the welding speed, the martensite and bainite width decreases. The micro-hardness of the weld is more than 1.6 times the base material, welded joints soften the heat-affected zone, the formation of the softening zone is stored by the controlled rolling processing duplex steel distortion energy release due to the weld thermal cycle under the action of out and deformation recovery and recrystallization. The tensile test, the welded joint fracture of the base material rather than the softening zone, because the laser welding the narrow softening zone and microhardness decreased only about 1%. Cupping experiments, the laser power is 2000W, the welding speed of 2m/min, 4m/min, welded joints tear occurs in the vertical welds, the weld hardness values ??increased connector forming the main reason for decreased ability . Laser power of 4000W, the welding speed of 2m/min, tearing along the softening zone occurs at this time, lead to a decreased capacity of the welded joints forming wide softening zone. 3500W Slab CO2 laser lap welding experiment, the use of the inner layer of He gas, the outer Ar gas double gas shielded. The study showed that with the increase in welding speed, weld width is narrowed, and the maximum shear force of the welded joints is reduced, but the shear strength. This is because the increase in welding speed can make low toughness in the weld the bainite content is reduced, thereby improving the strength of the weld. By optimizing the form of lap welding can increase the maximum shear force of the welded joints.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Welding, metal cutting and metal bonding > Welding process > Special welding > Laser welding
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