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Study on Properties of Laser Welded T-lap Joint of High Strength Low Alloy Steel

Author: GuoXiaoJun
Tutor: LiZhuGuo
School: Shanghai Jiaotong University
Course: Materials Science
Keywords: laser stake welding T-lap joint microstructure mechanical properties
CLC: TG456.7
Type: Master's thesis
Year: 2012
Downloads: 192
Quote: 2
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Abstract

The main application backgrounds of this paper are the development of advanced lightweight structural materials in the shipbuilding industry, and its research subjects are laser welding T-lap joint of high strength low alloy steel. Combined with the advanced manufacturing methods of laser welded sandwich structure panels abroad, the influence of laser welding parameters were studied under high power CO2 laser to achieve proper dimensions, then microstructure and mechanical properties of T-lap joints were also analyzed. Root gap’s influence on section dimensions and torsion performance were especially researched.The laser penetration welding experiment of 10CrNiMnMoV steel shows that appropriate macrograph and section dimensions can be achieved under matching parameters between laser power and heat input; the increasing of laser power is conducive to increase laser penetration. After the laser power is selected, appropriate root width could be obtained where the heat input slightly higher than the deep penetration threshold: 2.4 kJ / cm, and the root width maintain stable even if heat input keeps increasing; defocus amount of -2 mm is selected cause it has important influence on weld penetration and section morphology. In addition, root gap between the web plate and core will affect the stability of welding pool, which is not conducive to stable section dimensions, and may lead to other defects like surface collapsing and porosity etc.Microstructure analysis showed that, seam center of 10CrNiMnMoV steel consists of typical columnar grain, low-carbon martensitic is form the transformation of austenite due to rapid cooling process, and fine grain organization will help improve the joint strength. Joint area is consists of coarse grain zone, fine grain zone, and mushy zone. Width of heat-affected zone is about 1 mm, which is narrow comparing to the joint size, and it is conductive to good application performance.Microhardness test of T-lap joint under optimized parameter indicates that horizontal T-lap joint hardness distribution was saddle-shaped under optimized welding parameters. The hardness of seam is about 320 HV due to appearance of fine martensite, which is largely improved compared with 180 HV of raw materials. The width of heat-affected zone is narrow, where hardness appears sudden drawdown in this area. Heat input of experimental parameters have limited impact on microstructure, then microhardness of same testing spots have little changes.T-lap joint torsion tests show that root width plays critical role when joint are under torsional loading, T-lap joints can withstand greater torque with wider root width. Torque reaches 100 N·m when root width increasing to 1 mm. Root gap have negative effect on torsion performance and section dimensions, although it may slightly increase root width, thus root gap should be avoided during welding process.

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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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