19/05/2021

Laser beam source and laser-induced plasm

Laser beam source and laser-induced plasma on the surface of a TIG-welded duplex stainless steel sample

Source: BAM

Project period

01/12/2020 - 21/11/2023

Project type

Project status

Current

Description

Project objectives: increase the prediction accuracy of the weld microstructure of duplex stainless steels and adapt the WRC-1992 diagram with respect to neglected alloying elements and cooling rates.

Location

Bundesanstalt für Materialforschung und -prüfung
Unter den Eichen 87
12205 Berlin

Lukas Quackatz

Lukas Quackatz during measurement preparation

Source: BAM

Duplex stainless steels exhibit microstructural changes during welding that lead to uneven phase ratios and changes in chemical composition. Structural changes in these two-phase materials can lead to solidification cracks, increase in susceptibility to corrosion, lower ductility and critical strength values. Laser-induced breakdown spectroscopy (LIBS) enables time- and spatially-resolved in situ measurement of both temperature and chemical composition during welding.

Ein Pfeil in der Mitte einer Zielscheibe

Source: BAM

The aim of the research project is to increase the accuracy of the prediction of the weld microstructure of duplex stainless steels and thus to revise the WRC-1992 diagram. For this purpose, measurements of the chemical composition as well as electron temperature calculations are realised by means of LIBS. With the help of the electron temperature data, it is possible for the first time to calculate the cooling rate without an additional measuring device and to analyse the cooling curves of the solidifying weld metal in the weld pool.

Stilisierter Programmablaufplan

Source: BAM

LIBS is a spectroscopic method that can be used to determine elemental compositions. Short, high-energy laser pulses ablate a small volume (< 0.1 mm3) of the bombarded material and ionise it to form a plasma. The decayed plasma emits element-specific light. The simple experimental set-up and the fast, non-destructive analysis procedure are the distinguishing features of LIBS analysis. Element analyses can be carried out without contact and without pre-treatment of the samples. Liquid as well as solid and gaseous substances can be analysed.

Händeschütteln

Source: BAM


Funding

The project is funded by the German Research Foundation (DFG), project number 442001176.

insituLIBS - in situ investigation of phase distribution and melt pool solidification of duplex stainless steels using laser-induced breakdown spectroscopy (LIBS)

Challenges of welding duplex stainless steels

Welding processes are the most widely used manufacturing processes for duplex stainless steels. Due to thermal cycling, an unbalanced α/γ ratio may occur in the fusion zone (FZ) or the heat affected zone (HAZ) of the weld joint. This lack of balance results in a significant reduction of the local chemical and mechanical properties. Even the slightest element burn-off during welding can cause critical changes in the chemical composition and phase ratio, resulting in significant cracking problems in safety-relevant components.

Due to the outstanding physical properties of duplex stainless steels, they are used in almost all branches of industry. The microstructure of duplex stainless steels has a balanced ratio of α-ferrite and γ-austenite. The main alloying elements are chromium and nickel. Various other alloying elements increase the corrosion resistance and contribute to the strength increase. The right balance between α-forming elements (Cr, Mo, Ti, Nb, Si, Al) and γ-forming elements (Ni, Mn, C, N) ensures the balanced proportion of both phases.

Weld seam monitoring

Laser-induced breakdown spectroscopy (LIBS) offers a promising option for in situ weld monitoring. In LIBS analysis, a high-energy pulsed laser generates a plasma on the material surface. The expansion of the plasma causes electromagnetic radiation. This radiation is element-specific and can be used to chemically characterise the tested material after a reference analysis. The solidification sequence of the material correlates directly with the temperature in the melting bath. It is therefore necessary to monitor this temperature in the welding process to be able to make immediate parameter adjustments. With the help of the calculation of electron temperatures in the laser plasma, the temperature can be determined with time and spatial resolution without additional measuring equipment. Following the measurement process, spectra are generated and evaluated for each individual measurement point. After subtracting the background radiation, the intensities of the elements of interest can be filtered out.

LIPS spectrum of a duplex stainless steel 1.4162

LIPS spectrum of a duplex stainless steel 1.4162 with visualisation of a characteristic chrome line

Source: BAM

Project coordination

Bundesanstalt für Materialforschung und -prüfung (BAM)
Division Weld Mechanics

Funding

The project is funded by the German Research Foundation (DFG), project number 442001176.

DFG-Logo

DFG-Logo

Source: DFG