Author

Sin-Jang Chen

Date

1-1-1991

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Materials Science and Engineering

Institution

Oregon Graduate Institute of Science & Technology

Abstract

At present, titanium alloys can not be welded in thick sections (>25mm) with commercial welding processes such as submerged arc, gas metal arc and flux cored arc welding because of titanium's excessive high reactivity and resistivity at high temperatures. As a result, the only methods to weld titanium alloys in thick section are electron beam welding which is not practical for large structures and gas tungsten arc welding which is extremely labor intensive. Thus, a substantial need for cost-effective welding of titanium alloys exists. In adapting the electroslag technique for joining titanium alloys, many fundamental problems had to be overcome. As a result, the mechanisms of ohmic heat generation in both the slag and electrode were studied. Furthermore, the electrochemical effects between the slag and titanium filler metal were analyzed in order to model the heat generation mechanism in electroslag processing for application to both consumable guide and non-consumable guide ESW. Frequent experimental comparisons between ESW of titanium and steel (for reference) were drawn to better verify the models and predictive equations. An electric path model was developed to explain the overheating of the electrode and guide plate in a consumable guide ESW process. Equations were derived to explain the mechanism of consumable guide ESW process using a low resistivity CaF[subscript 2] flux. Also a mathematical model incorporating the effect of ohmic heating on the electrode melting rate was derived to predict the arcing instability occurring in a non-consumable guide ESW of titanium alloys. As a result of this research and for the first time outside the Soviet Union, the consumable guide electroslag welding (ESW) process has been developed to weld thick-section Ti-6Al-4v and Ti-6211 plates. This joining technique utilized a constant voltage AC power source, pure CaF[subscript 2] flux and argon shielding over the slag pool. The resulting weld deposited on 25 mm (1") and 50 mm (2") thick titanium plates were sound and virtually defect-free. Mechanical properties such as tensile properties, hardness, CVN impact toughness of the welds were investigated. Mechanical properties were correlated to the microstructures and interstitial content of the welds. A model to determine the influence of interstitial elements and grain size on the tensile ductility and toughness of titanium alloys was also developed.

Identifier

doi:10.6083/M49P2ZKG

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