Zhili Feng

The root cause of post-weld baking on the mechanical performance of Al-steel dissimilar resistance spot welds (RSWs) has been determined by machine learning (ML) and finite element modeling (FEM) in this study. A deep neural network (DNN) model was constructed to associate the spot weld performance with the joint attributes, stacking materials, and other conditions, using a comprehensive experimental dataset. The DNN model positively identified that the post-weld baking reduces the joint performance, and the extent of degradation depends on the thickness of stacking materials. A three-dimensional finite element (FE) model was then used to investigate the root cause and the mechanism of the baking effect. It revealed that the formation of high thermal stresses during baking, from the mismatch of thermal expansion between steel and Al alloy, causes damage and cracking of the brittle intermetallic compound (IMC) formed at the interface of the weld nugget during welding. This in turn re...

Objectives • Quantify and develop the knowledge base for hydrogen permeability and embrittlement of pipeline steels and their welds under high-pressure gaseous hydrogen exposure relevant to hydrogen gas transmission pipeline. • Optimize the base metal and weld metal composition and microstructure to avoid excessive hydrogen permeation and increase service performance of hydrogen pipelines. • Evaluate welding technologies suitable for joining high-pressure hydrogen pipelines. • Develop a risk assessment based approach to manage the integrity and safety of hydrogen pipelines including weld joints.

Objective • Develop the technological basis for friction stir welding and processing (FSW/P) of advanced high-strength and lightweight materials for the automotive industry. • Gain fundamental understanding of the relationships between workpiece and tool material properties during FSW/P. • Characterize the mechanical properties and microstructures of joints. • Correlate the proprieties and microstructures produced by FSW/P to the process conditions.

Friction stir processing is a novel solid-state process to modify microstructures and their properties by intense, localized plastic deformation. However, little research has been reported for microstructure evolutions of advanced high-strength steels during the process. The present work focuses on the study of transient microstructure changes and local mechanical properties for friction stir spot processed dual-phase (DP) 980 MPa grade steel (DP980) under different peak temperatures. A pinless silicon nitride ceramic tool was used to produce relatively simple material deformation and flow near the tool. Friction stir spot processed steel samples were characterized by optical and electron microscopies. Furthermore, Vickers microhardness and nano-indentation measurements were used to study local mechanical properties for correlation with microstructures. A swallow layer of refined grains (<0.6 µm) was obtained with a low peak temperature (under 400 °C), whereas higher peak tempera...

Transient distortion of thin plate in the welding process usually has a complicated mode and large magnitude. Quantitative measurement and prediction of full-field distortion are challenging and rarely reported up to now. In this study, the out-of-plane distortion of a thin plate during the Tungsten Inert Gas (TIG) welding process was measured using the digital image correlation (DIC) method. A simulation model based on thermal elastic–plastic finite element method (FEM) and DIC measured geometric imperfection were developed for accurate prediction of the transient welding distortion. The numerical results and experimental data agreed very well in both out-of-plane deformation modes and magnitudes of the plate at different stages of welding. The maximum out-of-plane distortion was larger than 4 mm during welding which can cause instability of arc length and heat input. The distance change between welding torch and plate surface was investigated under different initial deflections of...

ABSTRACT Friction stir spot welding was used to join two advanced high-strength steels using polycrystalline cubic boron nitride tooling. Numerous tool designs were employed to study the influence of tool geometry on weld joints produced in both DP780 and a hot-stamp boron steel. Tool designs included conventional, concave shouldered pin tools with several pin configurations; a number of shoulderless designs; and a convex, scrolled shoulder tool. Weld quality was assessed based on lap shear strength, microstructure, microhardness, and bonded area. Mechanical properties were functionally related to bonded area and joint microstructure, demonstrating the necessity to characterize processing windows based on tool geometry.

Develop a high-fidelity cost modeling tool for composite • pressure vessels designed based on relevant industry standards and codes Quantify the significant cost reduction attainable by • composite vessel technology through the optimal use of steels and concretes and the optimization of vessel geometry Demonstrate a novel steel vessel manufacturing • technology based on ORNL-patented multi-pass, multilayer friction stir welding (MM-FSW) of thick steel section

DOE Scientific ... Publication Date, 1997 Jan 03. OSTI Identifier, OSTI ID: 486147; Legacy ID: DE97003116. Report Number(s), CONF-970726--3. DOE Contract Number, AC05-96OR22464. DOI, 10.2172/486147. Other Number(s), Other: ON: DE97003116; TRN: TRN: AHC29713% ...

Welding residual stresses are a key concern in the fabrication and use of structural components containing welds. Residual stresses in welds are caused by non-uniform expansion and shrinkage of differently heated zones during the thermal transient of a weld pass. In some alloys, solid state phase transformations occurring during the welding transient contribute additional residual stresses. Manufacturing problems arising from

ABSTRACT There are over 100 nuclear power plants operating in the U.S. that range in age from 15 to 40 years old. May plants have obtained license extension for operation to 60 years, and operation beyond that is currently being assessed. As plants age, it is anticipated that some internal components may require repair by welding. Welding repair of irradiated nuclear reactor materials (such as austenitic stainless steels) is especially challenging because of the existence of large amounts of helium in the steel matrix after intense neutron exposure. Under the influence of high temperatures and high tensile stresses during welding, rapid formation of large helium bubbles can occur at grain boundaries, resulting in intergranular cracking in the heat-affected zone (HAZ). In this study, a refined helium bubble nucleation and growth kinetic model is developed based on the literature empirical model in the form of closed-form formulas. Especially, the material kinetic parameters needed by the empirical model are refined using a mesoscale master equation model for helium-vacancy cluster evolution in austenite matrix. Experimental results of high-temperature tensile tests of irradiated steel are used to validate the refined model, where the dependence of bubble size on the angle between grain boundary orientation and primary tensile stress direction are discussed. Finally, a three-dimensional thermal-stress model incorporating the refined helium model is used to calculate the transient temperature and stress conditions and the resulting helium bubble growth kinetics under those transients. The predicted bubble size distribution is compared with the experimental data available in the open literature. The effect of welding parameters on the cracking tendency is evaluated using the numerical model. The quantitative knowledge of helium bubble growth serves as much-needed prerequisite for understanding and mitigation of helium induced intergranular cracking during welding repair of irradiated nuclear reactor materials.