Comparative Study of Laser-Arc Hybrid Welding for AA6082-T6 Aluminum Alloy with Two Different Arc Modes

The effects of arc modes on laser-arc hybrid welding for AA6082-T6 aluminum alloy were comparatively studied. Two arc modes were employed: pulsed metal inert gas arc and cold metal transfer arc. The results indicated that joints without porosity, undercutting, or other defects were obtained with both laser-pulsed metal inert gas hybrid welding (LPMHW) and laser-cold metal transfer hybrid welding (LCHW). Spatter was reduced, and even disappeared, during the LCHW process. The sizes of equiaxed dendrites and the width of the partially melted zone in the LPMHW joint were larger than those in the LCHW joint. The microhardness in each zone of the LPMHW joint was lower than that of the LCHW joint. The softening region in the heat-affected zone of the LPMHW joint was wider than that of the LCHW joint. The tensile strength of the LCHW joint was higher than that of the LPMHW joint. For the two joints, the fractures all occurred in the softening region in the heat-affected zone, and the fracture morphologies showed ductile fracture features. The dimples in the fractograph of the LCHW joint were deeper than those of the LPMHW joint.

A Study of Deformation Behavior of Double Jointing Girth Welds

Demand for double jointing technology is increasing to improve pipeline construction productivity. Submerged arc welding (SAW) utilized for double jointing is likely to cause a much wider heat affected zone (HAZ) than those of typical field welding by gas metal arc welding (GMAW), and it should be taken into account for strain-based design of high strength line-pipes. However, guidelines for SAW welds properties to ensure strain capacity of high strength line-pipes such as X80 have not been established yet. In this study, a submerged arc weld joint was produced using tensile strength (TS) over-matching welding consumable. API standard type transverse weld tension test was conducted to measure local elongation at weld metal, HAZ, and base material. Elongation at weld metal increases prior to base material, but soon after that elongation at the HAZ softening region and base material adjacent to the HAZ catch up with the elongation in the weld metal, and finally, deformation concentrates at the HAZ softening region before final fracture. Deformation behavior of the joint was analyzed to verify applicability to double jointing girth welds for strain-based design. From finite element (FE) analysis of notched wide plate test which characterizes tensile strain capacity of a pipeline, it is suggested that ductile crack would not initiate before base material start necking in this particular TS over-matching weld joint in which the defect size is 1mm of notch depth and 25mm of notch length. Thus, the weld joint would be applicable for double jointing girth welds based on strain-based design.

A Study on the Prediction of Deformations of Welded Ship Structures

Welding deformations injure the beauty of appearance of a structure and decrease its buckling strength. In addition, welding deformations cause errors during the assembly of the structure and prevent increase of productivity. Welding deformations of real structures are complicated and the accurate prediction of welding deformations has been a difficult problem. This study presents a method to predict welding deformations of large structures accurately and practically based on the accumulated research results. The presented method uses the finite element method combining the inherent strain theory and the experimental results for accurate and efficient analysis. The weld joint is assumed to be divided into 3 regions: inherent strain region, material softening region and base metal region. The simplified elasto-plastic analysis method is introduced and representative material values during the thermal elasto-plastic process are used in this method. It is revealed from the analysis results for the simple welded model that the inherent strains are changed by joint restraint as well as heat input and elastic modulus of the inherent strain region, and the material softening region must be different from that of the base metal region. These results have been supported by the experimental data. The method proposed in this study can be combined with the commercial finite element analysis software to predict reasonably an efficiently the welding deformations of large structures.

Metallurgical Assessment of the Softened HAZ Region During Multipass Welding

CrMoV steels are used in high temperature and high stress sections of power plant members; their good creep resistance is impaired by welding done during fabrication of assemblies and weld repair of service damaged rotors. Occurrence of a “softening” (“tempered”) region in the grain refined heat-affected zone/intercritical heat-affected zone, has become the limiting factor in the life extension of weld repaired high pressure/intermediate pressure steam turbine rotors. This study focuses on the effect that multiple thermal cycles have on the development of this softened region. Work was conducted on real weldments and with simulated heat-affected zones produced with the Gleeble thermomechanical simulator and by isothermal furnace heat treatments. The thermal cycle at the softening region in the actual weldment was measured and reproduced during simulation; it was estimated that the peak temperature at this location was just above the intercritical A1 temperature. Softening occurred before any changes in microstructure could be detected with the light microscope. Carbide coarsening, shown by limited TEM analysis, and the likely dissolution of some of the carbides, most probable, contributed to reduce the microhardness values. [S1087-1357(00)70202-4]