Effect of laser welding on microstructural evolutions and mechanical properties of dual phase steels

نوع: Type: thesis

مقطع: Segment: masters

عنوان: Title: Effect of laser welding on microstructural evolutions and mechanical properties of dual phase steels

ارائه دهنده: Provider: Pegah Rashidi

اساتید راهنما: Supervisors: Mr Yousef Mazaheri-Mr Mohsen Sheikhi

اساتید مشاور: Advisory Professors: Mr Mahmood Moradi

اساتید ممتحن یا داور: Examining professors or referees: Mr Meisam Nouri- Mr Omid Imantalab

زمان و تاریخ ارائه: Time and date of presentation: 06-03-2023

مکان ارائه: Place of presentation: Amphitheater

چکیده: Abstract: In this research, the effect of different welding speeds and powers on microstructural evolutions and mechanical properties of DP780 dual-phase steel welded by fiber laser process was investigated. Structural analysis of the weld metal showed that it contains ferrite, bainitic ferrite and martensite. Also, three different microstructural regions were observed in the microstructure of all samples. Of course, the heat-affected zone was also divided into three sub-zones. Fiber laser welding was performed with three different powers of (250-300-350)W and two speeds (180 and 270) mm/min. According to the observed investigations, with the change of welding speed in fixed welding power, the size and type of grains changed, especially under the UCHAZ zone, but in constant welding speed, when the welding power was changed, only the difference in grain size was observed. The penetration depth of the weld pool decreased with increasing welding speed. Also, increasing the welding speed decreased the width of the FZ and HAZ zones. Also, weld pools were observed to be deeper with increasing welding power, and the type of weld observed was recorded as the type of key hole weld. Changes in microhardness values along the weld line were correlated with microstructural characteristics. Changing the welding speed did not have a significant effect on the hardness changes; But the increase in welding power increased the hardness, especially in the FZ zone. Tensile tests showed common characteristics of dual-phase steels such as continuous yielding and low yielding ratio. The strength of low carbon steel with dual-phase welded microstructure was more than 800 MPa (about 110.1% higher than the original sample, 742 MPa) along with excellent Elongation compared to the original sample. Changes in welding speed at constant power also increased strength and Elongation. The failure of the samples after the tensile test also occurred mostly in BM and in some samples in SCHAZ. Also, changes in welding speed and power on input heat were also investigated. Increasing the welding speed at constant power was the ideal mode to achieve low input heat and record better mechanical properties. The present study showed significant values of strength-Elongation balance of welded dual-phase steels (UTS*Elongation>21000 MPa%) compared to previous works and commercial dual-phase steels such as DP600, DP780 and DP800

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