Elsevier

Applied Surface Science

Volume 257, Issue 9, 15 February 2011, Pages 4113-4122
Applied Surface Science

Analysis of a fibre laser welding case study, utilising a matrix flow chart

https://doi.org/10.1016/j.apsusc.2010.11.185Get rights and content

Abstract

For fibre laser welding of an eccentric corner joint, the quality of the resulting weld cross section was studied with respect to the dependence on process parameters like lateral laser beam alignment, beam inclination, focal plane position or welding speed. The complex load situation of the support beamer was simplified to bending of one corner. Due to fatigue load, the weld properties causing the peak stress are essential, in particular the top and root shape of the weld cross section. For the parameters varied, the resulting shapes were categorized into different top and root classes, determined by certain key dimensions, considering also welding defects like undercuts. The shapes are boundary conditions for Finite Element Analysis of the joint under load for quantitative comparative analysis of the maximum stress. As two high strength steel grades were joined, the hardness transition across the weld was of interest, too. High speed imaging of the weld pool surface shape provided additional information on the relation between the parameter input and quality output. The different trends identified were discussed and guidelines were derived. As the systematic documentation of results is unsatisfactory in welding, a new method was developed and applied for the first time, called the Matrix Flow Chart. It enables an illustrative view on the resulting welding trends in a combined manner and is extendable by other researchers.

Research highlights

▶ Improved documentation and generalization of knowledge is desired. ▶ For varied parameters, shapes are categorized into top and root surface classes. ▶ The Matrix Flow Chart is developed to increase the pictoral presentation of results. ▶ Different trends of defects and shapes are found by changing the input parameters. ▶ Formulation of trends in the MFC is made as a generalization starting point.

Introduction

The quality of a weld is regarded from two points of view. During manufacturing the process parameters chosen determine the welding process and in turn the weld quality. When the welded product is in service, the weld tries to resist the load conditions (beside other criteria) for which poor weld quality can cause fracture.

Laser welding is characterized by superior abilities compared to conventional welding (e.g. resistance spot or electric arc welding), like enabling high precision and high quality, but is held back by high investment costs and difficulties in achieving the high quality in a robust manner. The process window is often narrow and the physical mechanisms of the process are complicated and not fully understood. One application of high potential as an enabler technology is laser welding of high strength steel of different grades, as traditional methods soften the steel in the heat affected zone (HAZ). Due to its high speed and the narrow interaction zone, laser welding creates a narrow HAZ, often even without softening, or at least of little contribution as a narrow part of the overall weld material compound under mechanical load. Laser hybrid welding of ultra high strength steel up to a tensile strength of 1700 MPa has proven to maintain its properties, while during tensile testing the cracking took place in the base material [1].

Besides the metallurgical material properties, essential for the mechanical behaviour of a weld under load is its geometry, particularly the top and root surface shapes, moreover welding defects. In Fig. 1 common laser welding defects in metals are shown, according to the standards [2], accompanied by possible explanations of their causes.

Different welding defects have to be suppressed to achieve a quality that can handle the loading situations that the final product will have to endure. Due to their good beam quality even at high power the recently emerging fibre and disc lasers have made it possible to increase the welding performance. Unfortunately those laser welds are often accompanied by heavy spatter ejection, a defect which has to be suppressed [3]. For monitoring of welding defects a promising method are photodiodes [4], [5] or cameras [5]. Advanced methods to analyze the melt pool and vapour flow during laser welding are high speed imaging [3], [6], [7] or Finite Element Analysis (FEA) [8]. Recently, it was shown that combinations of analysis methods can be a powerful tool, e.g. FEA and high speed imaging [8], X-ray imaging of pore formation followed by modelling [6] or analysis of photodiode monitoring by continuing thermal emission modelling from high speed imaging evidence of the weld pool dynamics [7].

Also the limits of acceptable weld surface properties are defined by the standards, but are still unsatisfactory as the knowledge for load situations is so far not enough generalized. The shape of laser welds was often studied [9], but not in a systematic and generalised context between publications. Typical for laser welding are nail-shaped cross sections, often generated by Marangoni convection depending on the weld pool geometry and also on the shielding gas type [10]. For the new generation of high brightness lasers (Yb:fibre laser, disc laser), spiking can disturb the root shape [11]. Numerical simulation of the heat and fluid flow during laser welding has been studied by several authors [12] to understand the weld surface motion and its resolidification, governing the weld shape. Laser hybrid welding is a complex process owing to its combination with a gas metal arc, but it enables to shape the root (by the laser) and the top (by the electric arc) separately, as studied systematically for a low C-steel fillet joint with different gaps and chamfers, where undercut was likely to take place [13]. For laser hybrid welding of an eccentric stainless steel butt joint a detailed investigation of the top surface was carried out [14]. Little toe radius was the primary criterion (being also a new welding standard guideline) for fatigue crack initiation, but surface ripples by resolidification could prevail and shift the cracking location, guiding the cracks along the ripples. For fatigue bending load either the top or the root often is under compression and thus uncritical, while the other side governs the fatigue crack initiation and propagation essentially by its shape. An overview of fatigue analysis of welds is given in [15], and of fatigue properties and cracking behaviour in [16], [17].

Despite a large number of experiments, case studies and applications of welding (including laser welding) carried out in the past, the transfer of knowledge to other applications is difficult. Even large welding databases were reported to be unsatisfactory since each parameter situation is different, i.e. it is difficult that one point in the N-dimensional parameter space (e.g. N = 20–30 parameters) learns from another. Design of Experiments is a systematic method for solving N-dimensional problems, but was successfully applied in laser welding only in limited process parameter domains and when supported by an expert (additional intuitive skills), as the process is too complex and trends are not monotonic. Despite these limitations in transferring knowledge and the desire for improvements, new approaches and solutions are hardly addressed.

Inspiration from other disciplines is appreciated. For example in machining a management information system was developed [18]. While many solutions have been developed, like for geometric properties such as shape identification, the treatment of complex and abstract data such as parameter combination in a multidimensional space is rare, and strategies are not obvious. Within genetics developed artificial neural networks have been developed, connecting input-properties with selected output-criteria, weighed and marked [19]. Increased treatment and visualization of data depends on the human cognitive ability and disability [20], e.g. the meaning of colours for our perception. Regarding welding and usage of knowledge transfer visualization, perception and cognition are barely mentioned but can be the key for unravelling a new way of how to transfer knowledge in a new and more efficient manner.

The present article shows, discusses and categorizes the results of the weld quality for systematic variation of the parameters, during dissimilar fibre laser welding of two high strength steel grades. In addition, a method was developed to present and document the results in a new, extendable manner, hopefully easier to survey.

Section snippets

Methodological approach

A certain joint and material situation was under investigation, to be fibre laser welded. For a preferred laser beam power, several (mainly geometrical) process parameters were varied in a selective manner, to keep the number of experiments limited, but systematic. The resulting weld cross sections were commonly arranged in a comparable manner, suitable for predicting trends of shapes in the parameter vicinity. As fatigue bending across the weld is the load criterion applied, the top and root

Results and discussion

From the experiments made with the parameters in Table 3 the following results where obtained:

Generalized knowledge documentation

From the experiments, trends were identified, as partially discussed above. These trends are documented in the new Matrix Flow Chart (MFC) developed, as shown in Fig. 11. In Fig. 11(a–c) the resulting weld quality according to Fig. 3, Fig. 6, Fig. 7 is shown in dependence of the process parameters varied in three different output formulations (defects, top/root classes, top/root dimensions). Note that the “OR”-logics is applied here. The method can also be applied to process-internal

Conclusions

The following conclusions can be drawn:

  • (i)

    Improved documentation and generalization of knowledge is desired, e.g. for transferring welding results systematically; multidimensional and graphical result arrangement facilitates the recognition of trends

  • (ii)

    When varying 4 geometrical laser beam parameters for laser welding of an eccentric corner joint, 5 top and 5 root shape categories were distinguished, having different essential stress raiser impact under load; from clear trends unexplored parameter

Acknowledgements

The authors are grateful for funding by VINNOVA – The Swedish Governmental Agency for Innovation Systems (project LOST, no. 2006-00563) and by the Knut and Alice Wallenberg Foundation (Fibre Laser, project no. KAW 2007-0119). The contributions from the Swedish industrial and academic partners involved are highly appreciated.

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