Investigating defects caused by narrow weld pool shapes in deep penetration welding

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Marcel Bachman, of BAM, describes the behaviour and influence of weld pool shape when joining thick materials

Laser beam welding is a widely used joining technique in many industrial applications. This is mainly due to its many unique advantages, especially compared to conventional arc welding processes. These advantages include, among others, highly concentrated energy deposition, low total heat input and a capacity to penetrate deep into the material while causing only small welding distortions. 

However, at the same time, the small dimension of the laser spot, high solidification rates, and small dimensions of the weld pool itself can provoke issues regarding the assembly tolerances of the workpiece, the hot-cracking phenomena, as well as keyhole-induced bubbles escaping from the melt. 

Weld pool shapes in laser beam welding are elongated at the external, free surfaces under the action of the main driving forces in the melt – such as recoil pressure and surface tension forces – while being shorter in the internal areas of the weld pool. This leads to a regular solidification sequence from the internal zones toward the free surfaces, e.g. from the bottom to the top in partial penetration welding. However, in recent studiesreported in the literature [1, 3] and seen in the experimental and numerical investigations of BAM Bundesanstalt für Materialforschung und -prüfung in Berlin [3, 4], it was found that an internal narrowing phenomenon can occur that is often accompanied by a distinct bulging of the weld bead in deeper zones, see Figure 1.

As the internal behaviour of the melt during the process is hardly optically accessible, several numerical models and experimental techniques were established to visualise the mechanisms of the formation of the bulging and the narrowing phenomenon and to reveal the consequences on the solidification sequence, pore formation, and filler metal dilution.

Results

The untypical weld pool shape we describe can be seen in Figure 1(b) and (c), where 10mm-thick AISI 304 was welded using a 1.1mm-thick NiCr20Mo15 filler wire. The upper weld pool region is dominated by tangential Marangoni forces, which lead to elongation because of the high temperature gradient between the hotter keyhole region and the relatively cold area at the weld pool rear. Thereby, a clockwise vortex is formed right behind the keyhole. In contrast, in the weld pool bottom area, the melt flow is dominated by the evaporation process due to the strong local absorption of the laser energy. The laser energy experiences multiple reflection events, every time transferring a part of the ray energy to the melt, which ends in a discontinuous distribution of the evaporation-induced recoil pressure and an unstable keyhole characteristic. The main vortex direction there is counterclockwise. In combination with the flow direction in the upper weld pool area, a narrowing zone forms at around half-depth of the weld pool. In this necking area, the mushy zone becomes untypically thick as the temperature gradient is comparably low there. This behaviour was observed especially for higher weld specimen thicknesses above 6mm.

Figure 1: (a) Experimental and numerical cross sections of wire feed LBW of 10mm-thick AISI 304 with a 1.1mm-thick filler wire NiCr20Mo15. Laser power 6.5kW, welding speed 1.3m/min, wire feeding speed: 2.1m/min. (b) Longitudinal cross-section taken from a setup of steel and quartz glass for optical inspection of the weld from the side. Figure rearranged from [6]. 

This narrowing phenomenon in the middle area of the weld bead leads to a solidification sequence that differs from the standard solidification direction in partial penetration welding from the bottom to the top and from the sides to the centre of the weld, see Figure 2(a). There, induced by the narrow weld zone in the weld pool centre, solidification occurs first in the middle of the weld pool, leaving liquid regions on the top and bottom. The remaining melt in the bottom cannot balance the tensile strain arising during solidification, which increases the local susceptibility to hot-cracking significantly. Note here the corresponding positions of the remaining liquid in the solidification sequence in Figure 2(a) and the bulge in Figure 1(b) and (c). Similar behaviourwas observed in studies for different austenitic stainless steel grades and unalloyed construction materials, as well as for different plate thicknesses, full and partial penetration cases, and also for welding with different fibre and disk laser systems [5].

Figure 1: (c) Weld cross section and flow routes during welding of AISI 304 with filler wire. Figure rearranged from [6].

Another consequence of the narrowing of the weld pool is a direct blocking effect on the flow field. The numerical results indicate that the vertical flow velocities, which are essential for the homogeneous mixing of the melt, are significantly reduced. In consequence, the liquid filler metal is brought to the welding process from the top side, e.g. by a cold wire, and cannot reach the bottom area of the weld pool. As a result, the desired metallurgical effect on the final weld leads to different mechanical properties in the different regions of the weld pool. This incomplete mixing behaviour can be seen in Figure 2(b) for a welding example with a nickel-rich filler wire and a standard AISI 304 stainless steel. Note again the coincidence of the position of the narrowing of the weld bead and the position of the jump in the nickel distribution at around half penetration depth.

Straightforwardly, the narrow region in the weld pool also has a detrimental effect on the remaining porosity in the final weld. As mentioned earlier, the keyhole tip tends to produce bubbles by periodically collapsing and reopening. These bubbles are then trapped in the lower bulging region correspondingly when their escape routes are blocked, thus having a significantly lower chance to reach the upper surface of the weld pool, leaving them as remaining pores, see Figure 2(c).

Conclusion

In the current study, an untypical weld pool shape during high-power laser beam welding and its influence on different weld defects was investigated via several numerical models and experimental techniques. It was discovered that the observed narrowing is often accompanied by a distinct bulging of the lower part of the weld pool during partial penetration welding, or in the middle part in the full penetration case. The boundary between different recirculation zones in the melt leads to a spreading of the mushy zone, and consequently to an untypical solidification sequence. Hereby, the hot-cracking formation can be provoked as the remaining local liquid zones have low resistance against tensile forces. 

Figure 2: (a) Solidification sequence caused by the narrowing phenomenon. (b) Nickel distribution in the longitudinal section of the weld. (c) Projection of the porosity distribution in welding direction on the weld cross section from μ-CT measurements. Figure rearranged from [6].

Another effect of the narrowing is a non-homogeneous mixing of added filler wire in the thickness direction of the weld by blocking the vertical flow routes leading to deteriorated mechanical properties of the final weld. It was also shown that this blocking not only diminishes the ability of diluted filler material to reach the lower weld pool regions, but also traps bubbles due to instabilities at the keyhole-tip where they are then frozen during solidification as pores.

Since the observations were made particularly in the thickness range above 6mm, our findings may be relevant especially for welding applications in the oil and gas, shipbuilding and aerospace industries, as well as for the welding of thick-walled pipelines or crane constructions. Furthermore, our results highlight the importance of optimising welding parameters, as well as the need for conducting quality control of welded joints. This is especially because the investigated effects can lead to welding defects that are not optically accessible during processing, and potentially cause component failure at a later stage.

This work is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Project Nos. 411393804 (BA 5555/5-2), 416014189 (BA 5555/6-1), and 466939224 (BA 5555/9-1).

Marcel Bachmann is the team leader of the welding simulation group at BAM’s welding technology division.

This article was co-authored by Xiangmeng Meng, Antoni Artinovand Michael Rethmeier, of BAM

References

[1] H. Wang, M. Nakanishi, Y. Kawahito, ‘Dynamic balance of heat and mass in high power density laser welding’, Optics Express 26, 6392–6399 (2018).

[2] Y. Feng, X. Gao, Y. Zhang, C. Peng, X. Gui, Y. Sun, X. Xiao, ‘Simulation and experiment for dynamics of laser welding keyhole and molten pool at different penetration status’, International Journal of Advanced Manufacturing Technology 112, 2301–2312 (2021).

[3] A. Artinov, N. Bakir, M. Bachmann, A. Gumenyuk, S.-J. Na, M. Rethmeier, ‘On the search for the origin of the bulge effect in high power laser beam welding’, Journal of Laser Applications 31, 022413 (2019).

[4] A. Artinov, X. Meng, M. Bachmann, M. Rethmeier, ‘Study on the transition behavior of the bulging effect during deep penetration laser beam welding’, International Journal of Heat and Mass Transfer 184, 122171 (2022).

[5] A. Artinov, X. Meng, N. Bakir, Ö. Üstündağ, M. Bachmann, A. Gumenyuk, M. Rethmeier, ‘The bulging effect and its relevance in high power laser beam welding’, IOP Conference Series: Materials Science and Engineering 1135, 012003(2021).

[6] Bachmann et al., ‘Elucidation of the Bulging Effect by an Improved Ray-Tracing Algorithm in Deep Penetration Wire Feed Laser Beam Welding and Its Influence on the Mixing Behavior’, Advanced Engineering Materials 24, 2101299 (2022).

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