Abstract

Difficult-to-weld steels are ferrous alloys that are characterized by a low thermal conductivity, and large thermal expansion coefficient. These intrinsic features contribute to a high level of distortion and cracking susceptibility during joining of these types of steels. In an effort to address the issues associated with difficult-to-weld steels, highly concentrated beam spots like electron and laser beam welding were developed. Usage of tightly focused heat sources have been accompanied by several challenges. An extremely precise fit-up requirement was considered as the most significant issue corresponding to application of either laser or electron beam. Recently, it was found that the combination of arc and laser in close proximity could lead to development of a new technology called hybrid laser/arc welding (HLAW). Simultaneously, HLAW takes advantage of arc-induced wider molten pool and laser-induced deeper penetration capabilities. Both of which would be mutually exclusive. However, application of this technology in the case of real usage is limited by the high complexity of the process and existence of numerous variables. Furthermore, humping, formation of porosity, and weak corrosion resistance of the welding region have been recognized as the principal issues concerning HLAW joints. In this regard, the major objective of the current Ph.D. research project was to comprehend the difficulties surrounding HLAW of difficult-to-weld steels as well as implementation of associated practical guidelines to make the HLAW process more practical for widespread industrial applications.

High-strength quenched and tempered steels (HSQTSs) are one of the big group of alloyed steels that are vastly used in shipbuilding industry. These types of steels are typically prone to cold cracking due to formation of the hard-martensitic phase during solidification. The common way to weld this type of alloyed steel is to use preheating process integrated with the multi-pass gas metal arc welding. However, that method generated a large level of thermally-induced tensile residual stress and distortion coupled with a large softened area in the vicinity of the fusion zone. Addressing the above issues, a feasibility study of hybrid laser/arc welding of 8-mm-thick HSQTS in butt and T-joint configurations was conducted experimentally and numerically. This chapter focused on developing the processing parameters to produce a sound full-penetrated weld in a single pass. Characterization techniques, including microstructural analysis, hardness, and tensile testing were employed to get a better understanding of the quality and mechanical integrity of the weld. Furthermore, an experimentally-calibrated thermomechanical simulation using SYSWELD commercial software, was introduced to analyze the weld-caused residual stress fields and distortion.

Austenitic stainless steel (ASS) is another subcategory of difficult-to-weld steels due to low thermal conductivity and large thermal expansion coefficient. In practice, stainless steel is majorly found and applied in circular hollow sections, thereby making ASS welding necessary. Thus, orbital welding of ASS pipes are an inseparable part of any manufacturing process. Currently, multi-pass arc-based welding process is a widely developed process for girth-welded pipe joints. However, a large heat affected zone (HAZ) is the main concern regarding the welds that are produced by arc-based processes. It was found that excessively wide HAZ makes additional heat treatment process necessary to retain the microstructure and required surface properties. Addressing the above problem, the feasibility of orbital welding of AISI304L stainless steel tubes by HLAW in two relative positions of the laser with respect to the arc to attain a free pore weld was studied. The effect of welding speed on the formation of porosity were also investigated. To obtain a rough estimation about expansion of HAZ throughout the base metal the heat distribution in the welding region was evaluated through thermal simulation using commercial ANSYS code. The micro-hardness and the tensile strength of joints under optimal conditions were analyzed to reveal the relation between microstructural and mechanical attributes.

The weld-induced heterogonous microstructure of austenitic stainless welds is generally considered to be the weakest area when exposed to a corrosive environment. This conclusion is drawn because of micro-segregation of the main allying elements such as Cr and Ni during the solidification. One of the most important attributes of HLAW process is the addition of filler metal that gives the weld a remarkable advantage over autogenous laser welding. Based on this advantage, it is expected that the proper filler wire will maintain the corrosion resistance of the fusion zone. Accordingly, the effect of wire type on microstructural alteration and corrosion resistance of AISI304 stainless steel joints produced by HLAW was studied. To gain a deeper understanding, an integrated characterization of fusion zone using the scanning electron microscope (SEM), X-ray diffraction (XRD), and cyclic potentiodynamic polarization analysis (CPPA) was conducted to quantitatively evaluate the corrosion resistance and explain the formation and growth of corrosion pit mechanisms.

Degree Date

Spring 2019

Document Type

Thesis

Degree Name

Ph.D.

Department

Mechanical Engineering

Advisor

Dr. Radovan Kovacevic

Subject Area

Mechanical Engineering

Included in

Manufacturing Commons

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