Pascal Paillard

TIG and A-TIG welding experimental investigations and comparison to simulation Part 1: Identification of Marangoni effect

Abstract In the present work, the chemical mechanism was shown for tungsten inert gas (TIG) and active TIG (A-TIG) welding. The results obtained with a two-dimensional axial symmetric model developed to simulate the flow behaviour in the meting pool on a stainless steel disc (304L) melted by a stationary heat source were presented to show the influence of Marangoni convection combined with Lorentz forces. This study shows the influence of the addition of an activating flux on the geometric characteristics of the weld beads in A-TIG welding.

TIG and A-TIG welding experimental investigations and comparison with simulation

Abstract In this part II, the comparison of physical mechanisms between tungsten inert gas (TIG) and active TIG (A-TIG) welding is shown. The plasma was monitored by a high speed camera to present the arc constriction phenomena while passing from TIG to A-TIG. The elemental analysis and the arc temperature measured by optical emission spectroscopy were performed according to the type of welding and the different fluxes in A-TIG welding. The two-dimensional axial symmetric model presented in part I was used to simulate the flow behaviour in the melting pool realised on a stainless steel disc (304L) melted by a stationary heat source and to study the influence of energy density.

Quantitative infrared analysis of welding processes: temperature measurement during RSW and CMT-MIG welding

Abstract This article deals with temperature characterisation by infrared thermography during welding. To demonstrate the versatility of this technique, two welding processes were investigated with different materials: cold metal transfer metal inert gas welding of al alloy and resistance spot welding of galvanised steel. Two experimental methods were used to characterise temperature without requiring implementation of thermocouples. The validity of these methods is demonstrated by metallurgical characterisations. The error on the measured values of temperature is assessed regarding the initial uncertainty on the apparent surface emissivity. The results show that it is possible to obtain absolute temperature values, cooling rates and informative visualisation of temperature gradients with a reasonable uncertainty.

Structural and chemical evolution of super duplex stainless steel on activated tungsten inert gas welding process

Abstract In the present work, the welding parameters of tungsten inert gas (TIG) and activated tungsten inert gas (ATIG) welding processes were compared on duplex stainless steel with two protective gases (Ar and ArHeN2). The addition of an activating flux can improve the TIG welding process. The ATIG process involves an increase in the penetration as well as a reduction in the number of passes for a thickness higher than 2 mm for stainless steel or other metallic materials. Metallographic observations, chemical analyses and mechanical tests were performed. The results show the importance of the welding parameters, the protective gas and the use of a flux on the characteristics of the weld beads, its microstructure, its hardness and its behaviour in corrosion after welding.

Hybrid laser-metal active gas welding of S460ML steel plates: Weldability and consequences of dropping on weld quality

This work investigates the weldability of S460ML steel using a hybrid laser-metal active gas (MAG) process and the effect of dropping on material properties. Dropping is the creation of regularly scattered drops on the back side of thick plates during full penetration hybrid laser-MAG welding. As these drops can reach several millimeters in diameter, it is an unacceptable defect since it lowers esthetic properties. Besides the geometrical aspect, mechanical properties are also affected by dropping. In order to check the consequences of dropping on weld quality: metallographic, mechanical, and dilution analyses have been performed. Results confirm the good weldability of S460ML using hybrid welding with proper welding parameters. If dropping occurs, it enhances heterogeneity in chemistry and microstructure, and welding defects such as porosity and undercuts. However, these properties and defects remain tolerable. Drops can be removed by grinding, making the weld acceptable.

Influence of recovery on dynamic segregation of sulphur to surface of cold rolled nickel

Abstract It is now well established that the kinetics of impurity segregation is greatly increased by the defect annihilation process occurring during the return to equilibrium of a material initially in a metastable state (quenched, irradiated, cold worked). During the return to equilibrium of a cold worked material, i.e. during a recrystallisation anneal, both the recovery and recrystallisation stages contribute to the acceleration of the segregation kinetics. In this work, an attempt is made to study the contribution of each stage separately, with particular emphasis on the recovery mechanisms of cold worked pure nickel and the way these mechanisms affect sulphur segregation taking place in this material. For this purpose, recrystallisation and segregation anneals at 455°C were carried out on lightly (0.25 true strain) and heavily (0.8 true strain) cold rolled nickel. It was found that sulphur segregation in the lightly deformed material took place entirely during the incubation time preceding recrystallisation, i.e. during the recovery stage. The process is thought to involve dislocation pipe diffusion for which a diffusion coefficient of 6.4×10-9 m2 s-1 was estimated. In the heavily deformed material, segregation was concomitant with recrystallisation and the mechanism thought to be most probable is that involving impurity drag.

Metallurgical Study of Friction Stir Welded High Strength Steels for Shipbuilding

Friction Stir Welding (FSW) is one of the most recent welding processes, invented in 1991 by The Welding Institute. Recent developments, mainly using polycrystalline cubic boron nitride (PCBN) tools, broaden the range of use of FSW to harder materials, like steels. Our study focused on the assembly of high yield strength steels for naval applications by FSW, and its consequences on the metallurgical properties. The main objectivewas to analyze the metallurgical transformations occurring during welding. Welding tests were conducted on three steels: 80HLES, S690QL and DH36. For each welded sample, macrographs, micrographs and micro-hardness maps were performed to characterize the variation of microstructures through the weld.

Embrittlement of Steels by Liquid Zinc: Crack Propagation after Grain Boundary Wetting

This study characterizes the mechanical behavior of an advanced multiphase high strength steel by means of high temperature tensile testing. The results show a drastic reduction of the maximum tensile elongation from around 700 °C up to 950°C. Scanning electron microscopy investigations show that the temperature range for embrittlement is correlated with the total wetting of steel grain boundaries. Under external strain, crack propagates along the grain boundaries according to a mechanism that leads to the presence of nanometer-thick films of Zn at the crack tip, as shown by fine X-ray spectroscopy analyses. The effective temperature range for embrittlement is discussed. Mechanisms of i) external stress-free wetting, and ii) atomic-scale crack propagation, are today under discussion in the light of the literature, regarding in particular recent experimental results and theory about grain boundary wetting, intergranular penetration, and the correlation between surface energy and crack propagation rate.

Evaluation Of Four Welding Arc Processes Applied To 6061 Aluminium Alloy

At a time when greenhouse gas emissions must be reduced, the use of the aluminium alloys is expanding, in particular in the transportation industry. In order to extend the possibilities of aluminium assembly design, new Metal Inert Gas (MIG) welding processes have been conceived. They work at lower temperatures than usual arc processes (classic MIG or Tungsten Inert Gas). This study compares four arc welding processes, applied to the 6061 aluminium alloy. These four weld processes have been studied through the metallurgical analysis of the weld beads. Metallography, micro‐hardness testings, X Ray radiography have been carried out on the produced weld beads. The processes are classified according to the quality of the beads like geometry of beads, size of the heat affected zone and presence of defects.

Gallium Distribution in Gallium-Coated Aluminum for Brazing Application

This work deals with the study of a new aluminum brazing process, called Galluminium. This technique, consisting in using gallium as a solder, is advantageous since it enables to braze at ambient air without flux. Indeed if the gallium coating is done mechanically, it descales the alumina layer and acts as a barrier against further reoxidation. Nevertheless, liquid gallium causes a severe aluminum embrittlement since it penetrates into the aluminum grain boundaries. We will show in this paper that this phenomenon has only a little impact on the mechanical resistance of the brazed joints since the amount of deposited gallium is enough low to avoid a severe embrittlement and the heat treatment (from 200°C to 600°C during several minutes) during brazing process dissolves gallium in the bulk.