Modeling of Wire Arc Additive Manufacturing
Finite Element Modeling of WAAM Processes: Developed a finite element model to simulate the thermal and mechanical behavior of Wire Arc Additive Manufacturing (WAAM) for Inconel 718 on A36 steel, focusing on the impact of process parameters on residual stress (RS).
Model Verification and Experimental Comparison: Verified the model using temperature data from thermocouples and microstrain measurements from Neutron Diffraction (ND), ensuring accurate simulation of thermal and mechanical behaviors.
Impact of Process Parameters on Residual Stress: Investigated the effects of torch travel speed and interlayer dwell time on RS, finding that reducing dwell time results in more uniform stress profiles and a deeper high-stress region in the build direction.
Neutron Diffraction Analysis of Dissimilar Welds
Residual Stress Investigation via Neutron Diffraction: Measured residual strain distributions in similar and dissimilar welds using neutron diffraction, enabling the calculation of three-dimensional stress states to explore the impact of martensitic phase transformation and material properties on residual stress (RS) distribution.
Distinct Residual Stress Trends: Identified smaller longitudinal RS in the low-carbon steel side of dissimilar welds compared to similar welds, and observed a unique transverse RS trend in dissimilar welds, with tensile RS reaching the base metal's yield strength.
Weld Zone Analysis: Characterized the weld zone using optical microscopy and hardness testing, providing insights into phase transformation effects on residual stress in dissimilar welds.
Additive vs. Conventional Manufacturing
Comparison of Additive and Conventional Manufacturing: Investigated the mechanical properties of 17-4PH stainless steel produced through laser powder bed fusion (L-PBF) and traditional wrought methods, focusing on tensile strength, microhardness, and nanoindentation results.
Impact of Heat Treatment on Properties: Studied the effects of solution annealing and aging on both L-PBF and wrought 17-4PH parts, revealing significant improvements in modulus and hardness in both manufacturing methods after heat treatment.
Ductility and Strength Differences: Despite similar improvements in hardness and modulus, L-PBF parts exhibited lower ductility and ultimate strength compared to traditionally manufactured counterparts, highlighting the challenges of additive manufacturing in achieving comparable mechanical performance.
Residual Stress Mitigation in Dissimilar Welds
Finite Element Analysis of Residual Stress: Employed a finite element analysis (FEA) model to analyze residual stress (RS) formation in autogenous gas tungsten arc (GTA) dissimilar welds between 304 austenitic stainless steel and A36 low-carbon steel.
Material Properties' Role in RS Formation: Isolated the effect of individual mechanical properties on RS formation by systematically changing the properties of 304 stainless steel to those of A36 in a controlled simulation environment.
Mechanical Tensioning as a Stress Reduction Method: Demonstrated that applying mechanical tensioning during the welding process reduces longitudinal RS in the weld zone (WZ) by an amount equivalent to the applied tensioning load, with the effect persisting after cooling and load removal.
Mechanisms of Stress Evolution: Identified the mechanisms of RS formation and mitigation by comparing stress evolution in dissimilar welds under tensioned and free conditions, providing insights into practical stress management techniques for dissimilar welds.
Carbon Fiber-Reinforced Composites in 3D Printing
Impact of Compaction on Fracture Toughness: Examined the effect of post-printing compaction at elevated temperatures on the fracture toughness of carbon fiber-reinforced polyamide composites, finding reduced Mode I fracture toughness in compacted samples compared to unconditioned samples.
Fracture Behavior Analysis: Investigated both intralayer and interlayer fractures in FDM composites, showing that interlayer cracks were more unstable, while intralayer fractures consumed more energy due to polymer plasticity.
Crystallinity and Brittleness: Highlighted the trade-off between reduced void content and increased crystallinity, with compacted samples exhibiting more brittle behavior due to higher crystallinity, while unconditioned samples benefited from the plasticity of nylon.
Gravity on Weld Microstructure
Effect of Gravity on Weld Fusion Zones: Demonstrated that increased gravity levels, inducing stronger buoyancy-driven flow, result in wider but shallower weld fusion zones in stainless steel alloy 316 during spot gas tungsten arc (GTA) welding.
Microstructural Insights at High Gravity: Revealed minimal differences in dendrite spacing between 1G and 3.6G samples but observed enhanced convection breaking more dendrite tips near the fusion boundary at higher gravity levels.
Mechanical Properties Under Varied Gravity: Tensile tests showed no significant difference in peak load between 1G and 3.6G welded samples, though higher gravity samples exhibited slightly reduced elongation, possibly linked to broken dendrite tips.
Transient and Residual Stress in Dissimilar Welds: Modeling and Experiment
Transient and Residual Strain Measurement: Utilized non-destructive strain gauge measurements to study transient strains and residual stress (RS) during gas tungsten arc (GTA) welding of dissimilar materials, offering valuable insights into how RS is reduced during the welding process.
Finite Element Modeling and Experimental Verification: Developed and verified a 3D finite element (FE) model using neutron diffraction measurements, examining the formation of residual stress in a 304-1018 dissimilar weldment during cooling.
Curvature and Strain Measurement Challenges: Explored the impact of weld curvature on strain measurements in dissimilar welds, highlighting differences compared to similar welds, and discussed the limitations and assumptions in strain measurement and stress state determination.
CO₂ Rapid Cooling Technique
CO₂ Rapid Cooling for Residual Stress Reduction: Investigated the effectiveness of CO₂ rapid cooling in reducing residual stress (RS) in both similar and dissimilar metals welds, with a focus on addressing the challenges posed by thermal expansion mismatches in dissimilar metal welds (DMWs).
Thermomechanical Modeling and Experimental Validation: Developed a thermomechanical numerical model to study the impact of CO₂ cooling parameters such as nozzle power, radius, off-center position, and location, validating the results with neutron diffraction measurements.
Asymmetric Residual Stress Distribution in DMWs: Found that while CO₂ cooling significantly reduced RS in both weld types, it is most effective when applied asymmetrically in DMWs, as the RS distribution in these welds is not uniform, suggesting the need for tailored cooling strategies.