Study On The Welding Process And Mechanical Property Of The Joint Achieved By Narrow Gap Laser Beam Welding With Filler Wire For Thick High Strength Steel Plate

Narrow gap laser beam welding process of thick plates with filler wire has been investigated in the paper. Defocus laser beam is adopted to act on the sidewall and bottom of groove at the same time to form a heat conduction mode molten pool, so that a single pass multi-layer welding method is utilized for realizing the effective connection of heavy section.First of all, laser welding surfacing and in the narrow gap with filler wire are carried out respectively using 12 mm thick CCS-B marine low-carbon steel. Forming influence law on weld seam of each parameter of laser welding with filler wire is studied and summarized. What's more, parameters are constantly optimized and applied in thicker plates. The results show that laser power and defocusing amount make greater contributions to penetration depth. With laser power increasing, penetration depth increases with it, and with defocusing amount decreasing, that is to say that laser power density increasing, penetration depth increasing; besides, welding speed and a matching feeding speed play a significant influence on the stability of welding process. Different matching between parameters may contribute to different weld surface morphology, which is mainly divided into three modes: weld surface raises up like spherical surface; a right angle forms between weld surface and sidewalls; weld surface is concave. The results show that concave surface morphology benefits to the decrease of lack of fusion when welding the next pass with filler wire.30mm thick EQ70 high strength low alloy steel is welded with laser welding with filler wire using the optimized parameters and the microstructure and mechanical properties are observed and analyzed after welding, and the results show that the microstructure going through only once thermal cycle mainly consists of upper bainite and acicular ferrite as well as a handful of granular bainite. Besides, hardness test and tensile property test are carried out. The hardening tendency of weld seam in laser autogenous welding is great and hardness value is high as a result of a function of quick heating and cooling. What's more, hardness in weld seam and HAZ decrease greatly and tend to homogenization when having gone through thermal cycle twice. Fracture of tensile specimens occurs in base metal without lack of fusion, which indicates that the strength of the weld seam with narrow gap laser welding is higher than that of base metal; analysis of the tensile fracture micro-morphology indicates that oxides is a significant factor of causing lack of fusion, which leads to the tensile fracture occurring in weld seam region.This paper compares joint achieved by narrow gap laser welding with that by metal active gas welding, and residual stress are also tested and compared. The results show that embrittlement and softening behavior happens in the joint obtained by metal active gas welding, which contributes to the tensile fracture occurring in weld seam region; whereas the tensile fracture of joint without lack of fusion obtained by narrow gap laser welding happens in base metal, which indicates that strength of the weld is higher than that of base metal. Residual stress test indicates that residual stress peak with narrow gap laser welding decreases greatly, and at the same time, tensile stress region is relatively small.