Welding | EPFL Graph Search

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as brazing and soldering, which do not melt the base metal (parent metal). In addition to melting the base metal, a filler material is typically added to the joint to form a pool of molten material (the weld pool) that cools to form a joint that, based on weld configuration (butt, full penetration, fillet, etc.), can be stronger than the base material. Pressure may also be used in conjunction with heat or by itself to produce a weld. Welding also requires a form of shield to protect the filler metals or melted metals from being contaminated or oxidized. Many different energy sources can be used for welding, including a gas flame (chemical), an electric arc (electrical), a laser, an electron beam, friction, and ultrasound. While often an industrial pr

Simulation of the welding process of steel tube joints made of S355 and S690

Janna Krummenacker

Welding is an important joining method for the work with metals. During the welding process, the material is heated and deforms plastically which leads a change in microstructure in the heat affected zone, to complex residual stress distributions, and distortions. The residual stresses can have a major impact, among other phenomena, on the fatigue resistance of a welded structure. The aim of this work was to develop a model with the finite element software Abaqus for the simulation of a welding process for butt welded tubes and the calculation of the resulting welding residual stress field. The residual stress field depends on several input parameters, like the geometry, the heat input, the welding speed, the material data, and the number of weld passes. The chosen dimensions of the tubes are realistic for tubular steel bridge constructions. Since, no experimental welding procedures were conducted, the welding parameters like the heat input and the welding speed were estimated in order to obtain a realistic temperature distribution in the weld and the heat affected zone. The assignment asked to place a main focus on the influence of the material data and the number of weld passes. Within the framework of this work, tensile tests at three different temperatures were conducted in order to find the deformational behavior of the two different steel grades S355 and S690. The results show generally good agreement with the data given by the European Standard for the structural fire design of steel structures [11], that is noted Eurocode 3 (EC3) in the course of this work. According to the test results, the yield strength at room temperature are underestimated by the Eurocode 3. Based on the experiments and the data given in the Eurocode 3, the influence of the mechanical properties were studied in several Abaqus models. For the description of the temperature dependent material behavior, a bilinear stress-strain curve was implemented. The resulting stress distributions show a similar trend, but the offset values between the stress curves of two models are dependent on the position in circumferential direction and the distance in axial direction. During the literature study, different welding simulation programs were compared. With specific welding simulation programs, mostly two-dimensional models can be realised. A major advantage of these programs is, that phase transformations can easily be considered and therefore a more realistic material model can be implemented provided that the temperature dependant behavior of the different microstructural phases are known. The influence of the number of weld passes was studied by comparing a single- and a multi-pass model with the material data from the Eurocode 3. The total heat input and the welding speed was the same for all the models. The tendency of the resulting stress distributions varied greatly and therefore implies that a multi-pass model cannot be approximated by a single-pass model.

2011

Soudage laser or-acier

Denis Favez

In jewelry manufacturing, joining dissimilar materials is usually achieved by brazing or soldering, in which the materials comprising the joint are heated to a suitable temperature in the presence of a filler metal having a liquidus below the solidus of the base metals. This simple and cost-effective method has however several disadvantages, including undesired phases transformations in the bulk base metals during heat treatment. An alternative joining methodology is laser beam welding. Due to its high energy density, a laser beam is able to melt locally the interface, without affecting the bulk microstructure of the parts to be joined. With this method, however, it is necessary to have a thorough understanding of microstructure formation during solidification. In this thesis, the process of laser welding an 18 carat yellow gold to a superaustenitic stainless steel has been studied and developed. In order to understand the microstructure formation during this welding process, a fundamental metallurgical analysis of the gold-steel system was first carried out, partially based on the binary Au-Fe elemental system. Following this, laser welds have been characterized and a concept for gold-steel welding proposed and successfully tested. First, Differential Thermal Analysis (DTA) and directional solidification in a Bridgman-type furnace followed by rapid quenching were used to acquire an understanding of the microstructure evolution upon cooling in Au-Fe alloys. During the (γ-Fe) primary phase solidification, micro- and macrosegregation phenomena influence the final composition profile. The peritectic phase forms rapidly and instabilities develop during solid-phase peritectic transformation. Below the peritectic temperature, solid phases decompose in a miscibility gap, leading to continuous or discontinuous precipitation, depending on cooling conditions. Finally, (α-Fe) ferrite precipitation gives insights on orientation relationships between primary and peritectic phases. Second, yellow gold-stainless steel alloys have been studied at low cooling rates by DTA. By combination of these observations with the Au-Fe metallurgical analysis, the microstructure formation in gold-steel laser welding could be explained. It appears that the microstructure formed during laser welding is strongly related to the local melt pool composition. Three different cases were identified: (1) Due to its small solidification interval and little segregation, goldrich liquid solidifies quickly as columnar grains, avoiding formation of cracks or porosities. (2) Due to the high cooling rate, liquid of intermediate composition is brought rapidly to low temperature, reaching the metastable liquid miscibility gap, resulting in demixing into two liquids. (3) A liquid containing mainly iron will solidify with a dendritic microstructure rejecting large amount of gold in the interdendritic liquid. In this last case, this large solidification interval combined with the considerable stresses around the weld pool leads certainly to cracking. Based on the above analysis, a concept of gold-steel laser welding can be proposed. By focusing the laser beam on the gold side of the interface, a growing liquid pool can be formed in this metal. When the melt pool reaches the steel surface, a layer of peritectic phase will form, without melting much steel. Since the liquid pool contains mainly gold, hot cracking is avoided. After solidification, the peritectic layer ensures a smooth transition between the mechanical properties of gold and steel. This concept has been successfully applied on a number of real pieces, forming sound welds with suitable properties. This thesis has also shown that a fundamental metallurgical approach is a powerful tool for complex welding development. It allows not only to develop a viable industrial solution, but also to acquire a knowledge that can be used to expand the process to other materials or different geometries.

EPFL2009

Design optimization of the primary coil for induction heating

Andreas Svejgaard Jensen

In this project we investigates a new design method for an optimized primary coil design for induction heating. Induction heating is used in many different application in the industry to heat up metallic materials. There could be many different reasons for the desired heating, including applications of surface treatment, preheating/postheating workpieces in a welding process or as in our specific case, to heat Shape Memory Alloys (SMA) to drive a phase change.