| Abstract Scope |
Motivation: The pursuit of optimum performance in engineered components is often framed as a quest for microstructural homogeneity. Homogeneity offers advantages such as predictability, reproducibility, and simplified modeling. Traditionally, high-performance components produced by casting and forging have relied on extensive heat treatments to minimize chemical segregation and eliminate defects, thereby promoting uniformity. In contrast, the welding industry has long operated within the reality of microstructural gradients and localized imperfections, developing design strategies that accommodate rather than eliminate these features. Remarkably, this duality has coexisted within the materials and manufacturing community, enabling some of the field’s most significant engineering accomplishments. Yet nature—and now additive manufacturing—points to a compelling alternative: that controlled microstructural heterogeneity and engineered residual stress distributions may enhance performance by exploiting local property gradients. This paradigm shift prompts a fundamental question: Should we develop new design methodologies that intentionally incorporate microstructural heterogeneity across multiple length scales, aligned with the functional requirements of the part? In this talk, we will answer this question in three parts based on recent literature and also historical, physical, and mechanical metallurgy literature.
Microstructure Heterogeneity and Residual Stresses in Welds and AM: Extensive literature review of both welding and additive manufacturing literature confirms that the microstructural heterogeneity cannot be ignored. As mentioned earlier, most of the existing welding design codes do consider and design around this. In addition, most of the welding metallurgists have rationalized the microstructural heterogeneity based on thermal signature and also correlated the microstructural entropy to the scatter in Charpy toughness. Furthermore, researchers have also shown that the weld heterogeneity must be defined based on both microstructure and hardness variations. Although the microstructural heterogeneity in AM is indeed similar to welds, additional complexity arises due to variability in AM geometry and build strategies. To address these uncertainties, current pragmatic industrial practice focuses on homogenization by post-process heat treatments. This additional post-process heat treatment often defeats the business case for the deployment of AM to industrial applications.
Prediction/Control/Design of Microstructural and Residual Stress Distribution. An alternative emerging design approach is to produce engineering components with the inherent assumption that we can measure/predict/control the spatial distribution of imperfection, microstructural distribution, and residual stresses within AM components. This approach is indeed known for increasing the fatigue life of welded structures by inducing compressive stresses in specific geometric locations either with peening or low-temperature transformation wires. In this part of the talk, we will focus on the feasibility of using computational design and in-situ tools to arrive at similar prediction and control of heterogeneity in AM structures and also challenges in adopting this method for qualification and certification.
Lessons from Nature and Future Directions: Careful observation of the building of habitats of insects and animals shows that nature is indeed comfortable with heterogeneity and on-the-fly changes to overall design with simple hexagonal motifs. For example, paper wasps build their nests with not homogeneous material, rather forcing them to use a heterogeneous distribution of materials. In these examples, the natural processes handle the heterogeneity through localization, integration, and feedback.
Based on the above discussions, perhaps our goal is not an optimum steel weld microstructure that is uniformly bainitic or acicular ferrite, but one that is heterogeneous by design, with microstructural gradation aligned to service demands—echoing nature’s way of embracing controlled disorder for superior function. |