On weldability of thick section austenitic stainless steel using laser processes
Karhu, Miikka (2019-10-25)
Väitöskirja
Karhu, Miikka
25.10.2019
Lappeenranta-Lahti University of Technology LUT
Acta Universitatis Lappeenrantaensis
School of Energy Systems
School of Energy Systems, Konetekniikka
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Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-335-417-3
https://urn.fi/URN:ISBN:978-952-335-417-3
Tiivistelmä
Laser welding and its different process variations using filler metal addition have increasingly become the preferred joining technology of the metal structure fabrication industry.
The essential characteristics of laser welding methods enable deep and narrow welds to be produced at high welding speed, which is beneficial for productivity enhancement. However, these inherent process characteristics can cause weldability issues in certain applications. Such challenges include solidification cracking susceptibility of welds produced in thick section joints of rigid constructional arrangements. In keyhole mode laser-arc hybrid multi-pass welding applications, the width of the groove geometry is limited because of the narrow fusion zone produced. In single-side thick section multipass welding, this groove width restriction limits the applicability of root pass welding and, consequently, limits maximum usable joint thickness. Moreover, inhomogeneous distribution and mixture of filler metal across the fusion zone has been encountered in deep and narrow single-pass laser-arc hybrid and laser cold-wire welds where an overalloyed filler metal is needed for alloying purposes. Inhomogeneous filler metal mixing can have an adverse effect on weld metal corrosion resistance and ductility properties, and in certain cases can cause enhanced susceptibility to weld solidification cracking.
The research scope of this article-based doctoral dissertation covers scientific study of welding technology development and improvement of laser-arc hybrid and cold-wire welding of thick section austenitic stainless steels. The research focuses on weldability, in particular, assessment of solidification cracking in multi-pass laser-arc hybrid welding, enhancement of the process capabilities of thick-section welding by defocusing of the laser beam in laser-arc hybrid and cold-wire processes, and study of mixing behavior in thick section laser welding with filler addition, especially improvements to mixing homogeneity as a result of appropriate combinations of groove geometry and process parameters.
The research objective was to develop a self-restraint test set-up and verify whether the set-up is able to provide conditions that promote solidification cracking in multi-pass laser-arc hybrid welding of thick section austenitic stainless steel joints with different groove geometries. A further objective was to develop a defocusing technique for thicker weld joint filling and to investigate how filler metal mixing phenomena change as a result of different torch orientation, groove geometry, specific defocusing technique and laser welding method, for example, laser arc-hybrid and laser cold-wire processes.
The research methods used comprised experimental investigations, numerical simulation studies and theoretical background studies. The laser types considered in the dissertation are solid-state lasers operating in continuous wave mode and at 1 micrometre wavelength. The welded joint thicknesses studied in the experiments were between 10 mm and 60 mm.
A self-restraint welding test sample set-up was developed to help analysis of solidification cracking susceptibility in multi-pass laser welding. The developed test set-up enables assessment of the weldability of thick-section austenitic stainless steels using laser-arc hybrid multi-pass welding with respect to the base metal chemistry, produced weld geometry, base metal dilution, filler metal selection and overall propensity to weld metal solidification cracking susceptibility. The developed defocusing technique for multi-pass procedures offers a new alternative approach to enhancing efficiency in intermediatepower laser welding of thick sections. The results of the mixing studies provide new knowledge about the mixing behavior of weld metals produced in thick section joints using laser-arc hybrid and laser cold-wire welding. Furthermore, the results produce enhanced understanding of the effect of welding parameters, for example, the effects of filler wire feeding configuration and groove geometry on filler metal mixing homogeneity and distribution intensity throughout the fusion zone of thick section welds.
The essential characteristics of laser welding methods enable deep and narrow welds to be produced at high welding speed, which is beneficial for productivity enhancement. However, these inherent process characteristics can cause weldability issues in certain applications. Such challenges include solidification cracking susceptibility of welds produced in thick section joints of rigid constructional arrangements. In keyhole mode laser-arc hybrid multi-pass welding applications, the width of the groove geometry is limited because of the narrow fusion zone produced. In single-side thick section multipass welding, this groove width restriction limits the applicability of root pass welding and, consequently, limits maximum usable joint thickness. Moreover, inhomogeneous distribution and mixture of filler metal across the fusion zone has been encountered in deep and narrow single-pass laser-arc hybrid and laser cold-wire welds where an overalloyed filler metal is needed for alloying purposes. Inhomogeneous filler metal mixing can have an adverse effect on weld metal corrosion resistance and ductility properties, and in certain cases can cause enhanced susceptibility to weld solidification cracking.
The research scope of this article-based doctoral dissertation covers scientific study of welding technology development and improvement of laser-arc hybrid and cold-wire welding of thick section austenitic stainless steels. The research focuses on weldability, in particular, assessment of solidification cracking in multi-pass laser-arc hybrid welding, enhancement of the process capabilities of thick-section welding by defocusing of the laser beam in laser-arc hybrid and cold-wire processes, and study of mixing behavior in thick section laser welding with filler addition, especially improvements to mixing homogeneity as a result of appropriate combinations of groove geometry and process parameters.
The research objective was to develop a self-restraint test set-up and verify whether the set-up is able to provide conditions that promote solidification cracking in multi-pass laser-arc hybrid welding of thick section austenitic stainless steel joints with different groove geometries. A further objective was to develop a defocusing technique for thicker weld joint filling and to investigate how filler metal mixing phenomena change as a result of different torch orientation, groove geometry, specific defocusing technique and laser welding method, for example, laser arc-hybrid and laser cold-wire processes.
The research methods used comprised experimental investigations, numerical simulation studies and theoretical background studies. The laser types considered in the dissertation are solid-state lasers operating in continuous wave mode and at 1 micrometre wavelength. The welded joint thicknesses studied in the experiments were between 10 mm and 60 mm.
A self-restraint welding test sample set-up was developed to help analysis of solidification cracking susceptibility in multi-pass laser welding. The developed test set-up enables assessment of the weldability of thick-section austenitic stainless steels using laser-arc hybrid multi-pass welding with respect to the base metal chemistry, produced weld geometry, base metal dilution, filler metal selection and overall propensity to weld metal solidification cracking susceptibility. The developed defocusing technique for multi-pass procedures offers a new alternative approach to enhancing efficiency in intermediatepower laser welding of thick sections. The results of the mixing studies provide new knowledge about the mixing behavior of weld metals produced in thick section joints using laser-arc hybrid and laser cold-wire welding. Furthermore, the results produce enhanced understanding of the effect of welding parameters, for example, the effects of filler wire feeding configuration and groove geometry on filler metal mixing homogeneity and distribution intensity throughout the fusion zone of thick section welds.
Kokoelmat
- Väitöskirjat [1099]