TWI Industrial Member Report Summary 862/2007
By C M Allen
Background
Hybrid laser-arc welding processes were originally suggested as long ago as the late 1970s (1,2). Interest in and development of the hybrid laser-arc process has been renewed over the past ten years, primarily based on its advantages over laser welding when making poorly fitting joints, and has found industrial application in shipbuilding, in C-Mn steel panel lines (3), using high power CO2 lasers (e.g. 12kW output power). Reported advantages (4,5,6) compared to laser welding, in addition to increased tolerance to fit-up, include higher productivity, reduced heat input, increased penetration and higher weld quality, the latter often determined by weld metal metallurgy which can be adjusted through the addition of filler wire.
In hybrid welds, inhomogeneous mixing in the weld metal between the filler addition from the MAG process and the parent material can be one factor that could contribute to a through-thickness variation in mechanical properties (e.g. strength or toughness), or an increased tendency to solidification cracking, particularly in the weld root.
Inhomogeneous mixing in the weld metal is most likely to occur when a distinct difference is observed in the shape of the weld through its thickness, between a more 'arc like' weld cap, typically a few millimetres in depth, to a more 'laser like' narrow weld root. This can happen in both full penetration and partial penetration welds, irrespective of laser source used and material being welded, where full penetration welds are typical of butt joints between plates and partial penetration welds, more typical of T, lap and edge joints. This difference in weld shape and character through its thickness would depend on the laser heat input compared to that of the arc, and the plate thickness, but would typically be at its most pronounced in plates of such a thickness that the laser was being used at the top of its penetration capability, as arguably, with these conditions, there is the least scope for adjustment of laser parameters to produce a more uniform width weld.
This work analyses weld metal mixing in four selected test cases; namely in three partial penetration melt runs on plate, and one full penetration butt weld, all made in 8mm thick structural steel plate, grade S275 to BS EN 10025-2, using a Nd:YAG laser-MAG hybrid welding process. A thickness of 8mm was chosen to represent the top of the penetration capability of the 4kW Nd:YAG laser chosen for this work. If homogeneous mixing is observed in this work using the Nd:YAG laser close to its penetration limit, the case for mixing in properly optimised full and partial penetration hybrid welding procedures in C-Mn steels, irrespective of the laser source used, can be more confidently stated in the future.
Objective
To determine if mixing in the weld metal is occurring between the filler addition from the MAG process and the parent material when hybrid Nd:YAG-MAG welding partial penetration and full penetration joints in 8mm thick S275 structural steel.