Digital Image Correlation for Determination of Weld and Base Metal Constitutive Behavior

Over several years - with funding from the aerospace industry - researchers at the University of South Carolina used digital image correlation technology from Correlated Solutions to investigate the mechanical properties of Friction Stir Welds (FSW). Popular in the aerospace industry because of its effectiveness with low-yield stress materials (aluminum), FSW is a process for joining two facing workpieces without melting the workpiece material. Problems arise when cracks form along the banded microstructure of the weld under load. The goal of the engineers at USC was to understand the behavior of the materials to provide suggestions for improving the welding process. The following succession of articles describe the research from the initial discovery of fracturing along FSW welds which led directly to tension and sheer testing using the VIC-2D system to explore how the production parameters impact the properties of the welded materials.

Digital image correlation for determination of weld and base metal constitutive behavior

AP Reynolds, F Duvall. Welding Journal 78, 355-60.

Abstract:

To understand and accurately model the mechanical response of welds, it is critical to have constitutive data for the various microstructural regions that make up the weld, e.g., the fusion zone (or dynamically recrystallized zone, DRZ, in friction stir welding), the heat-affected zone (HAZ) and the base metal. Methods currently used for determining the mechanical response of the regions that constitute the weld include testing of miniature specimens excised from the weld region (Refs. 1, 2), instrumented ball indentation testing (Refs. 3, 4) and testing of bulk material produced by weld thermal simulation (Ref. 2). Each method has its advantages and disadvantages. This article describes a new method for obtaining a substantial amount of the data required to determine the constitutive behavior for all of the materials comprising a weld by performing one tensile test of a transversely loaded weld.

Mode I fracture and microstructure for 2024-T3 friction stir welds

MA Sutton, AP Reynolds, B Yang, R Taylor. Materials Science and Engineering: A 354 (1-2), 6-16.

Abstract:

Detailed microstructural studies and mode I fracture experiments have been performed on both base material and three families of friction stir welds (FSWs) in 7 mm thick, 2024-T3 aluminum plate, designated hot, medium and cold due to the level of nominal heat input during the joining process. Microstructural studies indicate that the FSW nugget grain structure is relatively uniform in all welds, with a banded microstructure existing on horizontal cross-sections traversing the weld region; the spatial wavelength of the bands corresponds to the tool advance per revolution. The microstructural bands have elevated particle concentrations, with the particles having the same elemental content as base metal impurities, implying that the FSW process is responsible for the observed particle redistribution and microstructural banding. Furthermore, particle redistribution in all welds resulted in (a-1) particle size and volume fraction reduction on the advancing side of the weld nugget and (a-2) an attendant increase in particle volume fraction on the retreating side of the weld nugget. Finally, results indicate that hardness minima are present in the heat affected zone (HAZ) outside of the weld nugget on both the advancing side and retreating side for all welds, with the hot weld having the lowest overall weld hardness. Results from mode I fracture tests indicate that the measured critical COD at a fixed distance behind the crack tip is a viable fracture parameter for FSW joints that is capable of correlating the observed load-crack extension response for both the base metal and all FSWs. In addition, critical COD measurements indicate that FSW joints have a through-thickness variation in fracture resistance. Finally, the observed ductile crack growth path (which remained in the FSW region for all FSW joints) can be correlated with the locations of hardness minima, with microstructure variations affecting local fracture processes and the corresponding crack path.

Mixed mode I/II fracture of 2024-T3 friction stir welds

MA Sutton, AP Reynolds, B Yang, R Taylor. Engineering Fracture Mechanics 70 (15), 2215-2234.

Abstract:

For the first time, a series of mixed mode I/II fracture experiments have been performed on both base material and three families of friction stir welds (FSWs) in 6.4 mm thick, 2024-T351 aluminum plate; the FSW joints are designated hot, medium and cold due to the level of nominal weld energy input per unit weld length (specific weld energy) during the joining process.

Results from the fracture tests indicate that the measured critical crack opening displacement (COD) at a fixed distance behind the crack tip properly correlates both load-crack extension response and microstructural fracture surface features for both the base metal and all FSWs, providing measure of a quantitative fracture toughness. The COD values also indicate that transition from mode I to mode II dominant crack growth occurs at lower loading angles for FSW joints having higher specific weld energy input, with a truly mixed mode I/II COD measured during crack growth in the medium FSW joint. Using results from recent detailed FSW metallographic studies, specific features in the fracture process are correlated to the FSW microstructure. Finally, the observed ductile crack growth path in all three welds tends to exit the under-matched FSW weld region as the far-field applied shear loading is increased, with the medium FSW being the only case where the flaw remained within the FSW region for all combinations of shear and tensile loading.

Banded microstructure in 2024-T351 and 2524-T351 aluminum friction stir welds: Part II. Mechanical characterization

B Yang, J Yan, MA Sutton, AP Reynolds. Materials Science and Engineering: A 364 (1-2), 55-65.

Abstract:

A series of micro-mechanical experiments have been performed to quantify how the friction stir welding (FSW) process affects the material response within the periodic bands that have been shown to be a common feature of FSW joints. Micro-mechanical studies employed sectioning of small samples and micro-tensile testing using digital image correlation to quantify the local stress–strain variations in the banded region.

Results indicate that the two types of bands in 2024-T351 and 2524-T351 aluminum FSW joints (a) have different hardening rates with the particle-rich bands having the higher strain hardening exponent, (b) exhibit a periodic variation in micro-hardness across the bands and (c) the individual bands in each material have the same initial yield stress.