Root formation and metallurgical challenges in laser beam and laser-arc hybrid welding of thick structural steel

Ivan Bunaziv,Cato Dørum,Xiaobo Ren,Pasi Suikkanen,Odd M Akselsen,Magnus Eriksson,Bård Nyhus,Steen Erik Nielsen

doi:10.1007/s00170-021-07453-0

Abstract

Single-pass laser beam welding (LBW) of steel components with wall thickness of > 10 mm is of high interest due to enhanced productivity. Deep penetration LBW provides excessive hardness and certain quality issues such as root humping in flat position, which is associated with disability of surface tension to sustain melt dropout. High hardness is associated with fast cooling rates and shortage of filler wire transportation to the root of the fusion zone. Use of laser-arc hybrid welding (LAHW) can promote acicular ferrite by adding filler metal and additional heat input from the arc. However, LAHW may promote humping and adjustment of many parameters is required hindering its application. In this work, a 16 kW disk laser was used in butt welding of 12 mm and 15 mm thick plates with different bevelling geometries. Root humping occurred within a wide range of process parameters providing narrow process window. Twelve millimeter thick plates were successfully welded with a single-pass technique providing good quality of root by using zero air gap regardless bevelling geometry. Welding of 15 mm plates was more challenging, and the process was sensitive even with a slight parameter change. Improved results were achieved with application of small air gap. Acceptable hardness in both weld metal and heat affected zone (< 290 HV) was achieved for both plate thicknesses providing good toughness of minimum 27 J at −50°C.

Highlights

Steel plates with thickness of > 10 mm are widely used in a variety of industries like shipbuilding, oil and gas, and construction
Preliminary experiments were performed with variation of the focal point position from 0 mm to +50 mm by using autogenous Laser beam welding (LBW)
There is a possible process window, characterized by a smooth root appearance with a slight reinforcement, when there is a balance between laser power and focal point position (FPP) to obtain full penetration and the occurrence of melt ejections

Summary

Introduction

Steel plates with thickness of > 10 mm are widely used in a variety of industries like shipbuilding, oil and gas, and construction. The same phenomenon was noted by Frostevarg [6] and surface tensional forces were identified as the main factor in redirecting melt flow to prevent humping. The recoil pressure, surface tension, and gravity were identified as main forces to be involved in humping development during experimental and numerical studies in single-pass autogenous LBW of 12 mm thick stainless steel [20]. Tang et al [21] showed that LAHW using leading arc (laser beam is behind) setup provided less humping due to smaller droplet size and geometry of weld pool reducing ability of surface tension to sustain dropout in case of LAHW of 12 mm thick carbon steel. As a result, humping is very intricate phenomenon and still not well understood, especially in LAHW due to many parameters involved. More studies should be done on this issue, especially for thicker than 10 to 12 mm steel plates

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