Diana A. Lados

Fatigue crack growth mechanisms at the microstructure scale in Al-Si-Mg cast alloys: Mechanisms in regions II and III

The fatigue crack growth behavior in Regions II and III of crack growth was investigated for hypoeutectic and eutectic Al-Si-Mg cast alloys. To isolate and establish the mechanistic contributions of characteristic microstructural features (dendritic α-Al matrix, eutectic phases, Mg-Si strengthening precipitates), alloys with various Si content/morphology, grain size level, and matrix strength were studied; the effect of secondary dendrite arm spacing (SDAS) was also assessed. In Regions II and III of crack growth, the observed changes in the fracture surface appearance were associated with changes in crack growth mechanisms at the microstructural scale (from a linear advance predominantly through primary α-Al to a tortuous advance exclusively through Al-Si eutectic Regions). The extent of the plastic zone ahead of the crack tip was successfully used to explain the changes in growth mechanisms. The fatigue crack growth tests were conducted on compact tension specimens under constant stress ratio,R=0.1, in ambient conditions.

Processing, Microstructure, and Residual Stress Effects on Strength and Fatigue Crack Growth Properties in Friction Stir Welding: A Review

The purpose of this review is to provide a comprehensive overview of friction stir welding (FSW), as well as to introduce current research and applications involving this relatively new process. FSW is a new, efficient way of joining metal alloys that are considered unsuitable for welding via conventional fusion joining methods, and is capable of welding dissimilar metals with ease. This process also has the benefit of being solid-state, which mitigates the need for liquid filler metals that are common with conventional fusion welding techniques. This review will examine different facets of the FSW process, exploring the resulting static and dynamic properties and factors that influence these properties including weld zone boundaries, grain refinement, residual stress, and addition of reinforcing particles. Highlights of current research in this area and applications of this process in various industries will also be presented and discussed.

Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and Kmax effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter ΔK–Kmax approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications.

Effects of microstructure on the fatigue crack growth behavior of light metals and design considerations

Fatigue crack growth mechanisms of long and small cracks were investigated in cast and wrought aluminum and titanium alloys with various microstructures (as-cast A535, 6061-T61, and mill-and beta annealed Ti-6Al-4V). In addition, friction stir welded and cold spray processed 6061-T61 were also investigated. The effects of microstructure on the fatigue crack growth response of each material were evaluated. Long crack growth data were generated on compact tension specimens at low and high stress ratios R=0.1 and 0.7, respectively. Small crack growth testing was performed on corner and surface flaw tension specimens at low stress ratio, R=0.1. Fatigue crack growth mechanisms at the microstructural scale of the materials were identified and will be discussed. Closure corrections were applied to long crack growth data, and the results were compared to experimental small crack growth data. Models for small crack growth predictions from long crack growth data will also be presented and discussed.

Friction Stir Welding in Wrought and Cast Aluminum Alloys: Heat Transfer Modeling and Thermal History Analysis

Friction stir welding (FSW) is a technique that can be used for materials joining and local microstructural refinement. Owing to the solid-state character of the process, FSW has significant advantages over traditional fusion welding, including reduced part distortion and overheating. In this study, a novel heat transfer model was developed to predict weld temperature distributions and quantify peak temperatures under various combinations of processing parameters for different wrought and cast Al alloys. Specifically, an analytical analysis was first developed to characterize and predict heat generation rate within the weld nugget, and then a two-dimensional (2D) numerical simulation was performed to evaluate the temperature distribution in the weld cross-section and top-view planes. A further three-dimensional (3D) simulation was developed based on the heat generation analysis. The model was validated by measuring actual temperatures near the weld nugget using thermocouples, and good agreement was obtained for all studied materials and conditions.

Friction Stir Welding in Wrought and Cast Aluminum Alloys: Weld Quality Evaluation and Effects of Processing Parameters on Microstructure and Mechanical Properties

Friction stir welding (FSW) is a solid-state process widely used for joining similar and dissimilar materials for critical applications in the transportation sector. Understanding the effects of the process on microstructure and mechanical properties is critical in design for structural integrity. In this study, four aluminum alloy systems (wrought 6061-T651 and cast A356, 319, and A390) were processed in both as-fabricated and pre-weld heat-treated (T6) conditions using various processing parameters. The effects of processing and heat treatment on the resulting microstructures, macro-/micro-hardness, and tensile properties were systematically investigated and mechanistically correlated to changes in grain size, characteristic phases, and strengthening precipitates. Tensile tests were performed at room temperature both along and across the welding zones. A new method able to evaluate weld quality (using a weld quality index) was developed based on the stress concentration calculated under tensile loading. Optimum processing parameter domains that provide both defect-free welds and good mechanical properties were determined for each alloy and associated with the thermal history of the process. These results were further related to characteristic microstructural features, which can be used for component design and materials/process optimization.

Friction Stir Processing in Wrought and Cast Aluminum Alloys

Friction Stir Processing (FSP) of various aluminium alloys including wrought 6061 and cast A356, 319, and A390 have been systematically investigated in this study. The effects of processing on microstructure, hardness, tensile properties, and fatigue crack growth behaviour of the alloys were studied. The alloys were judiciously selected to understand the effects of Si level, type, and morphology, and to evaluate the contributions of different secondary phases and strengthening precipitates. Individual and combined effects of these microstructural features were also assessed. The results will be presented and discussed.

Friction Stir Welding: a Versatile Process for Light Metal Applications

Friction stir welding is a solid-state process that could be beneficially used for joining and repair of light metal alloys in transportation and defense applications. In this study, various applications, processes, and resulting properties of friction stir welds have been explored. First, the effects of various processing parameters on the resulting weld microstructures were studied. Second, tensile properties and fatigue crack growth mechanisms in friction stir welded 6061-T6 alloys were investigated. Fatigue crack propagation responses of the base and friction stir processed materials were studied in ambient conditions using compact tension specimens and multiple stress ratios, R=0.1, 0.5, and 0.7. Third, various exploratory studies were conducted to determine the feasibility of novel friction stir welding techniques for joining of dissimilar materials, porosity reduction, creation of in-situ metal-matrix composites for local reinforcement, and welding of die casting alloys. These findings will be systematically presented and discussed.

Fatigue Crack Propagation Mechanisms of Long and Small Cracks in Al-Si-Mg and Al-Mg Cast Alloys

Fatigue crack growth of long and small cracks was investigated for various Al-Si-Mg and Al-Mg cast alloys. Low residual stress was ensured during processing to concentrate on microstructural effects on crack growth. Compact tension and single edge tension specimens were fatigue crack growth tested at room temperature and stress ratio, R = 0.1. Microstructure related mechanisms were used to explain the near-threshold behaviour and crack growth response in Regions II and III for each material considering relevant microstructural features such as SDAS, grain size, and volume fraction and morphology of eutectic Si. Threshold behaviour of long cracks is attributed to closure-dependent mechanisms. In Regions II and III, the changes in crack growth mechanisms were explained through correlations between the extent of the plastic zone ahead of the crack tip and material-specific microstructural damage. Threshold behaviour of small cracks is explained through closure-independent mechanisms, specifically through the barrier effects of controlling microstructural characteristics specific to each material. Recommendations for integrating materials knowledge in structural design for fatigue performance are given.

Friction Stir Welding of Dissimilar Al/Al and Al/Non-Al Alloys: A Review

Friction stir welding is a solid-state welding technique that has many advantages over traditional fusion welding, and has been widely adopted in the aerospace and automotive industries. This article reviews research developments in friction stir welding of dissimilar alloys systems, including combinations of aluminum alloys with Mg alloys, Cu, and steel. Microstructural evolution, hardness, tensile and fatigue properties, residual stresses, and corrosion behavior of dissimilar welds will be reported. The effects of processing parameters such as tool rotation and traverse speeds, tool position, material position, and tool geometry on the weld quality are also presented. Discussions on future research directions in friction stir welding will also be provided in the context of existing literature and future high-integrity applications.