Defect Formation During High Power Laser Welding Of A Creep Resistant Nickel Alloy

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Total Pages : 76 pages
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Download or read book Laser-Arc Hybrid Welding of Thick Section Ni-base Alloys - Advanced Modeling and Experiments written by and published by . This book was released on 2015 with total page 76 pages. Available in PDF, EPUB and Kindle. Book excerpt: Hybrid laser-arc welding of nickel-base alloys can increase productivity and decrease costs during construction and repair of critical components in nuclear power plants. However, laser and hybrid welding of nickel-base alloys is not well understood. This project sought to understand the physical processes during hybrid welding necessary to fabricate quality joints in Alloy 690, a Ni- Cr-Fe alloy. This document presents a summary of the data and results collected over the course of the project. The supporting documents are a collection of the research that has been or will be published in peer-reviewed journals along with a report from the partner at the national lab. Understanding the solidification behavior of Alloy 690 is important for knowing the final properties of the weldment. A study was undertaken to calculate the solidification parameters, such as temperature gradient, solidification rate, and cooling rate in Alloy 690 welds. With this information and measured cell and dendrite arm spacings, an Alloy 690 map was constructed to guide process parameter development and interpret fusion zones in later hybrid welds. This research is contained in "Solidification Map of a Nickel Base Alloy." The keyhole formed under high laser intensity gives the hybrid welding technique the greater penetration depths compared to arc welding. However, keyhole behavior can form defects in the material, so knowing transient keyhole characteristics is important. With international collaborators, a study was undertaken to validate a new process monitoring tool known as inline coherent imaging (ICI), which is able to measure the keyhole depth with spatial and temporal resolutions on the order of 10 microns and 10 microseconds. ICI was validated for five alloy systems, including Alloy 690. Additionally, the keyhole growth rates at the start of welding were measured with unprecedented accuracy. This research is contained in "Real Time Monitoring of Laser Beam Welding Keyhole Depth by Laser Interferometry." During full penetration welding of thick sections, root defects can form, which result in unacceptable weld quality. A study was undertaken to determine the competing forces in root defect formation by independently changing the weight forces and surface tension forces. The weight force was altered by changing the plate thickness, and the surface tension force was altered by changing the surface condition at the bottom surface. Root defects do depend on these two forces. This research is contained in "Mitigation of Root Defect in Laser and Hybrid Laser-Arc Welding." Validation of the hybrid laser-arc model is necessary to properly model heat and mass transfer and fluid flow in Alloy 690 hybrid welds. Therefore, the developed model was validated for low carbon steel. Temperatures calculated by the model were included into a microstructural model in order to calculate the phase fractions. Process maps were developed for the selection of welding parameters to avoid martensite formation. This research is contained in "Fusion Zone Microstructure in Full Penetration Laser-Arc Hybrid Welding of Low Alloy Steel." Alloy 690 suffers from ductility dip cracking, a form of hot cracking. This type of cracking inhibits the use of multipass welding to join Alloy 690. Our partners at ORNL performed some hot ductility testing with Alloy 690 samples using digital image correlation. The results of this work is contained in the report "Summary of 690 ductility dip cracking testing using Gleeble and digital image correlation." Macro-porosity is a limiting factor in the widespread deployment of laser and hybrid laser-arc welding for construction and repair of nuclear power plant components. Keyhole instability and fluctuation results in the formation of large bubbles, which become trapped at the advancing solid- liquid interface as pores. Laser and hybrid laser-arc welds were fabricated for a range of conditions. Porosity levels in the welds were measured in X-ray ...