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The formation of sub-surface defects during friction stir welding has limited the adoption of the process in high volume production. A potential exists to eliminate/reduce the need for costly post weld inspection of such defects through the development of an in-process defect detection method based on a measured process output. The current state of in-process defect detection consists primarily of applying “black box” methods of correlating process outputs to defect occurrence without a fundamental physical understanding of what is producing the change in the output. This approach constrains the application of such methods when altering any aspect of the friction stir welding process. The current study seeks to provide a fundamental physical explanation as to what is driving the oscillation of friction stir welding process forces at the tool rotational frequency, as well as what is occurring when the tool interacts with defects and the oscillatory process forces are altered. A novel understanding was enabled through the synchronization of force measurements with angular position measurements of tool features. The results suggest that the eccentric motion of the tool and/or the rotation of a slanted shoulder surface are the primary drivers of process force oscillations. A fundamental explanation of the interaction between features on the tool probe and defective volumes has been proposed. The physical understanding helps to explain how altering the process will alter the force transients on which the detection method is based, which will enable a more robust and transferable method.

The objectives of this work are to investigate power and energy consumption of friction stir welding over a wide range of welding parameters and to provide a welding parameter map synthesized with their relations to welding quality. Friction stir welding has been known as an energy efficient welding technology because the whole process takes place in the solid-state, i.e., no excessive heat is needed to melt the workpiece. However, the study of the energy consumption and resulting welding quality across a wide parameter window has not been established inclusively. In this work, a series of 6061-T6 aluminum butt welds were created by friction stir welding across various welding parameters (spindle speeds and weld speeds). In order to understand the overall power and energy consumption, both mechanical process power and electrical wall plug power were measured on the 3-axis CNC mill used to perform the welds. Additionally, temperatures near the workpiece surface and weld roots were measured. The resulting weld quality was assessed by conducting tensile tests of weld cross-sections to determine the location of failures. The results of the power/energy, temperatures, and tensile tests were incorporated, producing an inclusive welding parameter map. The welding parameter map helps identify minimum power and minimum energy consumption points within the process constraints needed for maintaining good welding quality. The general approach presented in this paper can be applied to different welding setups and conditions.

The objective of this work is to develop a method for detecting the creation of discontinuities ( i.e. , voids, volume defects) during friction stir welding. Friction stir welding is inherently cost effective, however, the need for significant weld inspection can make the process cost prohibitive. A new approach to weld inspection is required in which an in-situ characterization of weld quality can be obtained, reducing the need for post-process inspection. To this end, friction stir welds with subsurface voids and without voids were created. The subsurface voids were generated by reducing the friction stir tool rotation frequency and increasing the tool traverse speed to create “colder” welds. Process forces were measured during welding, and the void sizes were measured post-process by computerized tomography ( i.e. , 3D X-ray imaging). Two parameters, based on frequency domain content and time-domain average of the force signals, were found to be correlated with void size. Criteria for subsurface void detection and size prediction were developed and shown to be in good agreement with experimental observations. With the proper choice of data acquisition system and frequency analyzer the occurrence of subsurface voids can be detected in real time.

Friction stir forming (FSF) refers the process variant of friction stir welding (FSW) where a joint is achieved through the creation of mechanical interlocking between two constituents by plasticizing one material through the use of the FSW process and in turn forcing said material to flow into the second material. In order to increase the understanding of material flow within this process, a method of quantifying the plasticity of flowing material during the FSF process is needed. A method is proposed in which a capillary hole is drilled into a backing plate used for FSW, and material is extruded through the capillary during processing. The flow of material is characterized and related back to an effective viscosity using the models that form the basis of a capillary rheometer. Initial testing shows promising agreements with CFD simulations of the FSW process (effective viscosity values within the range of 10 5 to 5×10 6 Pa-s). Furthermore, a promising initial agreement is shown when using effective viscosity values determined from the capillary method in a previously derived model that describes the infiltration for carbon fiber with plasticized aluminum produced by the FSW/FSF process.

The objective of this work is to investigate the effects of welding parameter variation and backing plate diffusivity on energy consumption in friction stir welding (FSW). FSW is a solid-state welding process, where joints are created below the solidus temperature of the workpiece. As a result, FSW is known for being energy efficient and environmentally friendly. However, a method of quantifying the energy consumption in FSW, for a variety of operating conditions, has not been well established. In order to understand the energy consumption of FSW, both process power and wall plug power were measured on a 3-axis CNC mill at various feed rates, spindle speeds, and with different backing plates. The results of monitoring the process power suggest that using a low thermal diffusivity backing plate has a beneficial effect on the instantaneous power consumption and on the total energy consumption during a weld. Conversely, tensile tests also showed that welds over a high thermal diffusivity backing plate consistently failed inside the weld zone, corresponding to a lower welding temperature and lack of penetration in the weld zone. These results suggest that using a low thermal diffusivity backing plate can both reduce energy consumption during FSW while also helping to achieve full penetration welds, while high feed rate is good for both welding productivity and reducing energy consumption per length of weld.

The objective of the work is the reduction and eventual avoidance of post welding inspections that are currently needed to ensure defect-free welds. An analytical thermal model of the FSW process along with an analytical disturbance model is developed. This disturbance model relates defect formation to variations in the measured temperature and is based on experimental process identification. A dynamic disturbance observer computes an estimate of the disturbance signal, which is further processed in order to provide information about the presence of defects along the weld. Experiments for one kind of disturbance verify that the observer shows good tracking behavior.

In this paper a shared control strategy is presented that allows a skilled operator to identify irregularities that occur during robotic friction stir welding (FSW) and assist the robotic system in producing an appropriate response. Human operators are adept at identifying disturbances; however, the complexity of the friction stir welding process makes it difficult for the operator to respond. While examining the capabilities of shared control in friction stir welding, this paper focuses on responding to defects that are caused by a lack of workpiece material during butt welding, such as gaps. A compensation strategy is presented that combines the human operator's perceptual strengths with an automated procedure for adjustment of the process parameters (i.e. travel angle and plunge depth). Experiments comparing four control strategies are performed while welding 5083-H116 aluminum. Through our experiments we demonstrate that if the FSW control task is appropriately shared between the human operator and the computer control system, the weld quality (strength) can be improved (from 9 ksi to 31 ksi for a gap size of 2.5 mm) as compared with the nominal case in which no corrections are made.