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Schlagworte
- Laser beam welding (6)
- Solidification cracking (4)
- High power laser beam welding (3)
- Hot cracking test (3)
- Keyhole (3)
- Local reduced pressure (3)
- Mobile vacuum (3)
- Hot cracking (2)
- Vapour plume (2)
- Additive Manufacturing (1)
Solidification cracking phenomena taking place under Controlled Tensile Weldability (CTW) test conditions have already been investigated both experimentally and numerically via FEA in order to get a better understanding of the mechanisms of hot crack formation during laser beam welding of austenitic steel grades. This paper develops a three dimensional finite element model employing the contact elements technique to simulate the formation and propagation of solidification cracks during laser full penetration welding of fully austenitic stainless steel 1.4376. During the experimental procedure the resulting strain and displacement directed to the laser beam in the Close vicinity of the weld pool was measured at the surface of the workpiece using a Digital Image Correlation (DIC) technique with an external diode laser as an illuminating source. Local strain fields, global loads and crack lengths predicted by the model are in good Agreement with those observed in experiments.
The presented apparatus enables laser beam welding of thick materials under local reduced pressure conditions, thus improving the quality of welds and reducing the laser beam power necessary for complete penetration welding. The vacuum cap presented in this article uses a local reduced ambient pressure environment in a tight zone around the welding area and, in contrast to a conventional vacuum chamber, it is movable in the welding direction. The mobile installation is very compact and reaches pressure values of around 200 mbar. The reduced pressure in the vacuum cap is sufficient to generate 50 % higher penetration depth in comparison to welding under ambient pressure conditions. The low pressure around the keyhole reduces the vapour-plasma plume and therefore prevents a defocusing and scattering of the laser radiation. This allows to raise the amount of laser beam power entering the keyhole as well as the effective power density.
The development of the vapour plume during the laser beam welding of metals with propagation directions along the laser beam axis leads to a reduction of the absorbed laser power in the keyhole. These unfavourable effects can be significantly mitigated by the application of a reduced ambient pressure thus decreasing the interaction between the laser beam and evaporation products, which leads to more stable welding process and results in increased penetration depth. A conventional technique use a vacuum chamber principle, similar to that, used for electron beam welding. Application of this technique is restricted by the camera size and reduces thus the advantages of this approach especially for large components. We demonstrate a possibility of mobile local vacuum application, which allows to generate a reduced pressure only in small region around the keyhole. By using of specially designed mobile pressure lock which can be moved along the welding direction absolute pressure of around 200 mbar could be obtained. This is sufficient to increase the welding depth by around 50%. Coupons from S355 were welded with an incremental laser power from 5 kW to 12 kW at atmospheric pressure and compared to those welded at ambient pressure of 200 mbar. The evaluation of the longitudinal section revealed an increase of the welding depth by about 35%. Furthermore, the welding trials in butt joint configuration on 15 mm thick plates at various laser beam power performed for atmospheric and reduced ambient pressure of around 200 mbar. The increase in welding depth up to 40% was established for reduced pressure. Particularly, low welding speeds under reduced pressure were especially advantageous in terms of increasing the welding depth.
Welding is one of the most critical operations for the construction of reliable metal structures in everything from ships to reactor vessels. When welds fail, the entire structure often fails—so expectations on weld quality have never been higher. Any process that uses a localized heat source, such as welding, is likely to result in some distortion. The welding process of very thick metal components is not inherently stable and is barely controllable without external forces.
Das Laser-MSG-Hybridschweißverfahren führt bei untersuchten hochfesten Pipelinestählen API X80 und X120 reproduzierbar zu Schweißverbindungen mit anforderungsgerechten Zähigkeitseigenschaften.
Eine metallurgische Beeinflussung der Schweißnahtzähigkeit ist durch eine gezielte Auswahl des Zusatzwerkstoffes möglich, wobei die maximale Eindringtiefe des Zusatzwerkstoffes in die Tiefe der Laserhybridnaht zu beachten ist.
Die maximal erzielbare Eindringtiefe des Zusatzwerkstoffes ist auf ca. 14 mm begrenzt. Die eingesetzte Art des MSG-Lichtbogens hatte keinen erkennbaren Einfluss auf die Aufmischung im Laseranteil der Laserhybridnaht.
Die besseren Ergebnisse der Kerbschlagzähigkeit konnten mit Metallpulverdrähten erreicht werden.
Mit den erzielten gemittelten Werten der Schlagarbeit: ca. 200 J bei -60°C für X80 und ca. 53 J bei -40°C für X120 werden Anforderungen der Norm API 5L und DIN EN 10208-2 für die beiden untersuchten Grundwerkstoffe erfüllt.
Das Laser-Pulver-Auftragschweißen als additives Fertigungsverfahren ermöglicht einen endformnahen Aufbau von Bauteilen. Ein Zielkonflikt besteht zwischen der Forderung nach hoher Aufbaurate und hoher Endformnähe, welcher von der Schweißraupengröße wesentlich beeinflusst wird. In dieser Veröffentlichung wird das Laser-Pulver-Auftragschweißen eingesetzt, um komplexe Formen additiv aufzubauen. Am Beispiel eines Tannenbaumprofiles werden unterschiedliche Einflussfaktoren dargestellt. Dazu gehören die Raupengeometrie, die Überlappung einzelner Raupen, die Verwendung unterschiedlicher Aufbaustrategien und die Teilung des Gesamtkörpers in Teilkörper. Der Zielkonflikt wird durch die Herstellung von Probekörpern mit unterschiedlichen Steigungswinkeln an den Seitenflächen verdeutlicht. Die Ergebnisse zeigen eine verbesserte Endformnähe in Bereichen flacher Steigung beim Einsatz kleiner Schweißraupen. Im Vergleich dazu erlauben die Schweißparameter der großen Raupen eine 5-fach höhere Aufbaurate. Bei einer Raupenüberlappung kleiner und großer Raupengeometrien innerhalb einer Lage treten Anbindungsfehler auf. Strategien zur Behebung dieses Fehlers durch Anpassung der Schweißreihenfolge werden in dieser Veröffentlichung aufgezeigt. Diese Erfahrungen werden genutzt, um einen Gesamtkörper aus Teilkörpern unterschiedlicher Raupengeometrien zu fertigen.
The development of the vapour plume during the laser beam welding of metals with propagation directions along the laser beam axis leads to a reduction of the absorbed laser power in the keyhole. The main reasons are the scattering of the laser radiation on the condensed phase of metallic vapour as well as the defocusing of the laser beam due to the density gradient in the vapour plume. These unfavourable effects can be significantly mitigated by the application of a reduced ambient pressure thus decreasing the interaction between the laser beam and evaporation products, which leads to more stable welding process and results in increased penetration depth, as known from previous studies. A conventional technique use a vacuum chamber principle, similar to that, used for electron beam welding. Application of this technique is restricted by the chamber size and reduces thus the advantages of this approach especially for large components. We demonstrate a possibility of mobile local vacuum application, which allows to generate a reduced pressure only in small region around the keyhole. By using of specially designed mobile pressure lock which can be moved along the welding direction absolute pressure of around 200 mbar could be obtained. This is sufficient to increase the welding depth by around 50%. Coupons from S355 were welded with an incremental laser power from 5 kW to 12 kW at atmospheric pressure and compared to those welded at ambient pressure of 200 mbar. The evaluation of the longitudinal section revealed an increase of the welding depth by about 35%. Furthermore, the welding trials in butt joint configuration on 15 mm thick plates at various laser beam power performed for atmospheric and reduced ambient pressure of around 200 mbar. The increase in welding depth up to 40% was established for reduced pressure. Particularly, low welding speeds under reduced pressure were especially advantageous in terms of increasing the welding depth.
Laser welding is a widely established manufacturing process in many industry sectors. Solidification cracking represents one of the most inadequately solved problems in welding and has major economic implications. The avoidance of hot crack forms for most fusion welding processes poses a key challenge for an important range of metallic construction materials and affects not only the manufacturers of welding equipment and material manufacturers, but also a large number of customers using welding technologies, as well as welding technical standardization and research. Solidification cracking susceptibility was examined with the help of the Controlled Tensile Weldability Test (CTW) developed by Federal Institute for Materials Research and Testing (BAM), Berlin. The test is based on the fact that hot crack formation depends on a critical strain that emerges within a critical temperature range, the so called brittle temperature range (BTR). Using this test and defined investigation programme a centreline solidification crack was generated. By controlling the applied strain during the laser beam welding process, it was possible to determine the critical strain and strain rate that led to solidification cracking formation. The hot cracking susceptibility of the tested stainless steels was qualified and quantified. The results demonstrate that the crack length increases with increasing applied strain. Furthermore, the strain rate has a significant influence on the formation of the solidification crack.
Laser welding is a widely established manufacturing process in many industry sectors. Solidification cracking represents one of the most inadequately solved problems in welding and has major economic implications. The avoidance of hot crack is for most fusion welding processes a key challenge for an important range of metallic construction materials and affects not only the manufacturers of welding equipment and material manufacturers, but also a large number of customers using welding technologies, as well as welding standardization and research. In this study a new investigation programme has been developed to qualify the hot cracking susceptibility of a variety of austenitic stainless steels. The results show the possibility of using this technique to determinate the critical values that occur with initiation of solidification cracking during laser beam welding
Full penetration high power bead-on-plate laser beam welding tests of up to 20 mm thick 2205 duplex steel plates were performed in PA position. A contactless inductive electromagnetic (EM) weld pool support system was used to prevent gravity drop-out of the melt. Welding experiments with 15 mm thick plates were carried out using IPG fiber laser YLR 20000 and Yb:YAG thin disk laser TruDisk 16002. The laser power needed to achieve a full penetration was found to be 10.9 and 8.56kW for welding velocity of 1.0 and 0.5 m min(-1), respectively. Reference welds without weld pool support demonstrate excessive root sag. The optimal value of the alternating current (AC) power needed to completely compensate the sagging on the root side was found to be approximate to 1.6 kW for both values of the welding velocity. The same EM weld pool support system was used in welding tests with 20 mm thick plates. The laser beam power (TRUMPF Yb:YAG thin disk laser TruDisk 16002) needed to reach a full penetration for 0.5 m min(-1) was found to be 13.9 kW. Full penetration welding without EM weld pool support is not possible-the surface tension cannot stop the gravity drop-out of the melt. The AC power needed to completely compensate the gravity was found to be 2 kW. (C) 2016 Laser Institute of America