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- TIG welding (2)
- Welding (2)
- 4043 Filler Dilution (1)
- 6060 Aluminium (1)
- Acicular ferrite (1)
- Al-Mg-Si Alloys (1)
- Arc Welding (1)
- Austenitic stainless steel (1)
- Bridge hangers (1)
- Charpy toughness (1)
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Within the framework of an AiF (The German Federation of Industrial Research Associations) project, investigations were conducted into the solidification behaviour of various high-alloy austenitic steels in laser and laser-GMA hybrid welding. For comparison purposes, reference was made to pure gas metalarc welding tests. It was possible to raise the welding speeds in hybrid welding compared with the individual gas metal-arc (GMA) and laser welding processes. The central subjects of the investigations were the type of solidification and the connected hot cracking behaviour during welding. In this respect, particular attention was paid not only to the kinetic effects during solidification which may be caused by the high solidification rates in laser and hybrid welding but also to their consequences.
The new multi-element alloying concept of systems with defined entropy (HEA — high-entropy alloy or MEA — medium-entropy alloy) is increasing in material research interest. Improved properties or combinations of properties are shown by several systems. Thus, the resulting microstructures and production of HEA/MEA as well as properties have been primarily investigated so far. Furthermore, processing is a key issue to transfer HEA/MEA systems to real components. Since welding is the most important joining process for metals, it is crucial to investigate the influence of welding to guarantee component integrity. Since most HEA are made of expensive alloying elements such as Co or Ni, they will not be used entirely as structural materials. Thus, it can be advantageous to weld conventional alloys such as austenitic stainless steels with the HEA and MEA to produce components that are both application-oriented and economically viable. Therefore, in this paper, first results of dissimilar metal welding, by tungsten inert gas (TIG) and friction stir welding (FSW), of a CoCrFeMnNi HEA as well as a CoCrNi MEA with a conventional AISI 304 austenitic stainless steel are presented. The focus is on the microstructure formation due to the two welding processes. The results of TIG welding show a dendritic microstructure, whereas in FSW both materials are stirred but still coexist.
Microcrack formation during gas metal arc welding of high-strength fine-grained structural steel
(2014)
The recent development of high-performance-modified spray arc processes in gas metal arc welding due to modern digital control technology and inverter power sources enables a focused spray arc, which results in higher penetration depths and welding speed. However, microcracks occurred in the weld metal while approaching the process limits of the modified spray arc, represented by a 20-mm double layer DV-groove butt-weld. These cracks were detected in structural steel exhibiting a yield strength level of up to 960 MPa and are neither dependent on the used weld power source nor a consequence of the modified spray arc process itself. The metallographic and fractographic investigations of the rather exceptional fracture surface lead to the classification of the microcracks as hot cracks. The effects of certain welding parameters on the crack probability are clarified using a statistical design of experiment. However, these microcracks do not impact the design specification for toughness in the Charpy V-notch test (absorbed energy at -40 °C for the present material is 30 J).
Multiple principal element alloys encompass the well-known high entropy alloys (HEA). The alloy system represents a new class of materials consisting of at least three alloying elements, each containing 5 to 35 at.%. Thus, this alloying concept differs fundamentally from conventional materials such as steel or nickel alloys. For this purpose, the alloying elements are specifically selected, the microstructures are adjusted in a single-phase and, in some cases, multi-phase manner. In particular, conflicting goals, such as the trade-off between strength and ductility in conventional steels, are overcome. However, in the last 20 years, the focus has been on material synthesis. With the increase in available material quantities, the focus is now on processing issues such as joining and welding processes. The weldability of HEAs has received very little attention so far. The experience with dissimilar metal welds is completely lacking but is essential for the application of these materials in combination with conventional materials. The present study presents comprehensive experimental results on the weldability of an equimolar CoCrFeMnNi-HEA in cold-rolled and heat-treated condition, which was joined by tungsten inert gas welding to an austenitic steel AISI 304. The mechanical properties of the dissimilar metal welds were characterized by cross-weld tensile samples, whereas the local deformation in the weld of the different welding zones was measured by digital image correlation. In accordance with the respective initial HEA condition (cold-rolled vs. heat-treated), the local strain behavior was divergent and influenced the global mechanical properties of both DMW types. Nonetheless, the experiments provided proof in principle of the weldability for dissimilar joints of the CoCrFeMnNi-HEA welded to conventional materials like austenitic stainless steels ensuring a corresponding capability for mechanical loading. This allows further considerations on the application of these innovative materials.
Influence of Surface Preparation on Cracking Phenomena in TIG-Welded High and Medium Entropy Alloys
(2022)
Multi-element systems with defined entropy (HEA—high entropy alloy or MEA—medium entropy alloy) are rather new material concepts that are becoming increasingly important in materials research and development. Some HEA systems show significantly improved properties or combinations of properties, e.g., the overcoming of the trade-off between high strength and ductility. Thus, the synthesis, the resulting microstructures, and properties of HEA have been primarily investigated so far. In addition, processing is crucial to achieve a transfer of potential HEA/MEA materials to real applications, e.g., highly stressed components. Since fusion welding is the most important joining process for metals, it is of vital importance to investigate the weldability of these materials. However, this has rarely been the subject of research to date. For that reason, in this work, the weldability depending on the surface preparation of a CoCrFeMnNi HEA and a CoCrNi MEA for TIG welding is investigated. The fusion welding of longer plates is described here for the first time for the CoCrNi alloy. The welds of both materials showed distinct formation of cracks in the heat affected Zone (HAZ). Optical and scanning electron microscopy analysis clearly confirmed an intergranular fracture topography. However, based on the results, the crack mechanism cannot be conclusively identified as either a liquid metal embrittlement (LME) or hot cracking-like liquid film separation.
Unexpected wind-induced vibrations of the hangers have caused an early fatigue crack on specific steel components and joints of a railway bridge over the Elbe River at Lutherstadt Wittenberg, Germany. During regular periodic inspection a fatigue crack of approximately 240 mm length was found near a butt weld of the longest hanger. The hanger was immediately secured by welded butt straps across the crack. Based on experimental investigations of hanger vibrations additional bracings were added between the hangers to avoid wind-induced vibrations. The weld heat influence zone which was affected by high cyclic stresses was replaced by new material.
Nevertheless it was impossible to determine sufficient remaining service life for those remaining bridge components that were exposed to extreme high real load cycles. The grinding of the affected steel surfaces was the key element of the remedial actions. Furthermore, additional fracture mechanic calculations were carried out in order to assess the remaining service life of the welded joints. In this respect, the calculation approach used by Deutsche Bahn AG was compared to further procedures from the mechanical engineering field. These investigations showed that the studied, repaired components have both, bearing and fatigue capacities within the validity of standards.
Effect of cooling rate on microstructure and properties of microalloyed HSLA steel weld metals
(2015)
Two high strength Nb/Ti microalloyed S690QL steels were welded with identical filler material, varying welding parameters to obtain three cooling rates: slow, medium and fast cooling. As cooling rate increased, the predominantly acicular ferrite in Nb weld metal (WM) is substituted by bainite, with a consequence of obvious hardness increase, but in Ti WM, no great variation of acicular ferrite at all cooling rates contributed to little increment of hardness. The transition between bainite and acicular ferrite has been analysed from the point view of inclusions characteristics, chemical composition and cooling rate. Excellent Charpy toughness at 233 K was obtained with acicular ferrite as predominantly microstructure. Even with bainite weld of high hardness, the toughness was nearly enough to fulfill the minimal requirements. WM for Ti steel showed to be markedly less sensitive to the variations of cooling rate than that for Nb steel.
Al-Mg-Si alloys are known to be highly susceptible to solidification cracking except when using an appropriate filler metal (e.g., 4343 or 5356), although the amount of dilution required to avoid cracking has never been a well-defined quantity. The aim of the present study is to determine a relationship between filler metal dilution, local strain rate conditions, and cracking susceptibility. Making use of the controlled tensile weldability (CTW) test and local strain extensometer measurements, a boundary has been established between crack and non-crack conditions for different local strain rates and dilution amounts, holding all other welding parameters constant. This boundary, presented in the form of critical strain rate-dilution map, defines the critical amount of 4043 filler metal required to avoid solidification cracking when arc welding 6060 aluminium, depending upon local strain rate.