Welding in the World

On the field of the hybrid welding processes, a rapid technological development can currently be seen with the emphasis more and more laid on the combination of arc and laser beam processes. In this way, the main fields of the development are concentrating on such hybrid processes that are based on the application of a solid-state laser. The reason for this in the first place is that together with modern solid-state lasers (fibre or disc lasers), beam sources are available that provide high output while at the same time offering an excellent beam quality. Performing technological examinations for different materials and welding tasks is directly connected with the weld qualities and strengths produced depending on the type of joint and application. This presentation will serve as an overview on the results of the examinations on hybrid welded joints both concerning their special properties and their strength behaviour as well as the state of the development of regulations for the classification and evaluation of such welds.

In the present work, transient liquid phase bonding (TLP) of stainless steel 316 L to Ti-6Al-4 V using simultaneously both Cu and Ni interlayers was performed and effect of bonding temperature (950 to 1050 °C) on microstructure and mechanical properties of the joints was studied. The joint zones were analyzed using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). Microhardness and shear strength tests were also applied to evaluate the mechanical properties of the joints. The results showed that various eutectic phases and intermetallic compounds were formed at the interface; however, diversity of intermetallics was different in the joint zone for each specimen which can be due to the bonding temperature and type of eutectic phase transformation. The more increase of bonding temperature to higher than 1000 °C, the more deterioration in mechanical properties of the bonded joints is led so that the shear strength decreased from 385 MPa (maximum value) to 257 MPa.

Pulsed currents of various shapes have been employed to control the metal transfer phenomena. In the present study, a simulation model including both the arc plasma and the metal transfer is constructed, and their behaviors in pulsed-MIG arc welding are numerically investigated. When the peak current is set to 450 A and the peak time is set to 1.5 ms, only a single droplet is transferred per pulse. The numerical model can indicate the metal transfer and arc plasma behavior depending on the pulse shape. The temperature of the arc plasma increases rapidly at the early phase of the peak time, and consequently, the temperature of the wire electrode increases. After that, a large amount of the metal vapor generates from the wire tip, and the arc temperature decreases. These behaviors are periodic and can be controlled through the pulse shape. In addition, the appropriate pulse frequency depends on the surface tension of the wire electrode. This result shows that balance of the surface tension and the electromagnetic force is important to determine the droplet behavior. Therefore, in controlling the welding process, it is important to consider the properties of both the welding power source and the welding material.

Friction Stir Spot Welding (FSSW) is a variant of the Friction Stir Welding (FSW) process and has been successfully used in industrial applications. During the FSSW process, thermal inputs due to friction and deformation are commenced simultaneously, as the non-consumable rotating tool plunges into the workpiece to be welded. Various assumptions and hypotheses for mechanisms of heat generation and material deformation during FSW/FSSW process are reported, but a consensus is still to be reached. The joining quality is mainly dependent upon the material flow in this solid state joining technique. The material flow and deformations in the near and far fields of the weld are directly affected by the temperature-sensitive mechanical properties. Therefore, a comprehension of thermo-mechanical responses are of high importance from the viewpoints of parameter optimization and understanding of the mechanisms. The FSSW process is experimentally and theoretically studied to address these issues of the mechanism of heat generation and coupled thermo-mechanical response of the workpiece, as well as the effects of tool rotation and plunge speeds. For theoretical studies, a 3-dimensional, physical-based FEM (Finite Element Method) model is developed using commercial code. For heat generation, friction and deformation-based formulations are used. For material responses, thermal and strain rate-sensitive, elastic-plastic data are employed by a constitutive Johnson Cook material model and thermo-mechanical behaviour is analyzed with respect to experimental observations. To cope with high calculation time and distortion of the mesh, built-in features of the code, mass scaling, ALE (Arbitrary Lagrangian Eulerian) and mesh re-mapping were used. As a result of this work, a basic platform in the form of a physical-based, coupled, thermo-mechanical model is developed. With the help of this model, effects of process parameters on the temperature — displacement behaviour of the workpiece are studied. The role of interaction conditions at the tool-workpiece interface is emphasized and a simplified conceptual mechanism for effects of process variables on the physical phenomena is presented.

Carbon steels can be sensitive to solidification cracking. Predicting their susceptibility to solidification cracking can save cost and time compared to testing, and it can be very useful for designing new steels or welds. Crack susceptibility predictions were made for carbon steels using the recently proposed simple susceptibility index for cracking during solidification, maximum │dT/d(fS)1/2│ near the end of solidification (T temperature and fs fraction solid). T vs. (fS)1/2 curves of the carbon steels were calculated by three different solidification models: equilibrium, Scheil, and Scheil with back diffusion of the available commercial thermodynamic software. The crack susceptibility predictions based on these solidification models were compared to various crack susceptibility test results of carbon steels, and the predictions based on Scheil with back diffusion were found consistent with the most of the crack susceptibility test results. Solidification temperature ranges of the carbon steels, determined based on the solidification models of equilibrium, Scheil, and Scheil with back diffusion, were used to explain the crack susceptibility predictions. The role of the alloying elements of the carbon steels in solidification cracking susceptibility was discussed.

Reduced pressure laser welds were made using a 6-kW commercial fiber-laser system on Ti-6Al-4 V and compared to electron beam welds of the same beam diameters as measured by beam diagnostics. The laser welds showed keyhole characteristics under easily achievable mechanical pumped vacuum levels of 1 mbar pressure that nearly matched the electron beam weld penetrations made at 9 × 10–5 mbar vacuum. Ti-6Al-4 V alloys were used to represent refractory metals such as vanadium, tantalum, zirconium, or molybdenum that require vacuum or highly protective inert gas protection systems to prevent adverse interactions with air and can be difficult to weld under non-vacuum conditions. Results show that laser weld depths of 20 mm with aspect ratios of 17:1 can be made under what appears to be stable keyhole behavior as the result of reduced pressure. The effect of fiber diameter was examined using 0.1-, 0.2-, and 0.3-mm fibers, showing that small spot sizes can easily be achieved at long focal length lenses of 400 and 500 mm. The 0.1- and 0.2-mm fibers produced keyhole welds with minimal amounts of porosity, which was only present at 2 kW or higher, while the 0.3-mm fiber produced keyhole welds with more rounded roots that were porosity free as shown by radiography up to the maximum power of 6 kW. Correlations between weld depth and processing conditions are presented for the reduced pressure laser. These results are directly compared to electron beam welds, facilitating design of future reduced pressure laser systems targeted for deep weld penetrations historically developed for electron beams.

This study is aimed at developing a systematic method for evaluation and prediction of solidification cracking in laser dissimilar welded joints (laser DWJ). The initiation of solidification cracking was observed directly by using a high speed camera during U-type hot cracking testing while laser welding. The high temperature ductility curves were obtained based on the local critical strains of the solidification cracks, measured by investigating each frame of the observation film. The distribution of residual liquid metal during solidification was investigated by using both high magnification and in-situ observation. Moreover, material properties at the elevated temperatures were measured by using a developed tensile test. The weld strain during laser welding was calculated by 3D finite element analysis by using the material properties obtained. Consequently, all the results helped to provide a comprehensive understanding of solidification cracking in laser DWJ.