Papers by Keyword: Friction Stir Processing (FSP)

Abstract: The solid-state process of friction stirring is increasingly applied to weld or process Aluminum alloy 5052, which is essential to various applications, such as marine, aerospace, and automotive. Friction stirring typically induces microstructural changes and grain refinement, affecting the processed material's constitutive response. Applications involving friction-stir processed aluminum alloy 5052 might be subjected to impact and high-strain-rate loadings. Accordingly, this work investigates the effect of friction stir processing on the high-strain-rate behavior of aluminum alloy 5052. A Split Hopkinson Pressure Bar (SHPB) system is used to experimentally measure the high-strain-rate compressive response of friction-stir processed aluminum alloy 5052 at strain rates ranging between 2700 s^-1 to 5000 s^-1. A high-speed imaging system and the digital image correlation technique were used to measure full-field strain fields. Results showed that friction stir processed samples exhibit lower yield strength (less by 20.8% at strain rate 5000/s) than their unprocessed counterparts at the same strain rate. However, friction stir processed samples exhibited a higher hardening rate.

Abstract: Crack repairing of aluminum alloys is done using conventional welding techniques or mechanical methods, which results in the redundancy of mechanical properties due to defects formation. Friction Stir Welding/Processing (FSW/FSP) is a solid-state joining technique which is used to join various different similar and dissimilar metals, along with the fabrication of surface composites to cater the mentioned problem. The objective of this study is to repair the crack produced in 6061 aluminum alloy by the reinforcement of ceramic particles, Al₂O₃ and B₄C, to further increase the efficiency of the joint along the crack line. Weld parameters, equipment used and the processing conditions are emphasized. The mechanical testing and the characterization of the weld as well as base metal was done and compared using tensile testing, micro hardness test and microstructural analysis. X-Ray Diffraction (XRD) was performed for crystallinity and intermetallic study. The dispersion of the particles was investigated using Field Emission Scanning Electron Microscope (FESEM). The crack in the Al-6061 was effectively repaired using FSP. The reinforced samples showed improved mechanical properties as compared to non-reinforced ones.

Abstract: It is widely accepted that the dominant deformation mechanism of fine-grained superplasticity is through grain boundary sliding (GBS) that occurs in fine-grained materials. However, it has been reported that in “Class I” solid solution alloys, superplastic-like behavior controlled by trans-granular deformation occurs by solute drag creep. In this study, we have investigated superplastic behavior in a fine-grained aluminum solid solution alloy with a thermally unstable microstructure. To obtain fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum (Al−Mg) alloy. An equiaxial fine-grained microstructure with a grain size of 7.4 μm was obtained after FSP; however, this microstructure was unstable at high temperatures. Generally, for fine-grained superplasticity or GBS to occur or continue, the fine-grained microstructure must be smaller than 10 μm during high-temperature deformation. However, a large elongation of over 200% was observed at high temperatures despite the occurrence of grain growth. From microstructural observations, it was determined that a fine-grained microstructure is maintained in the early stage of deformation, but at strain levels greater than 100%, trans-granular deformation occurs. The microstructural feature of this trans-granular deformation is similar to the deformation microstructure of solute drag creep observed in “Class I” solid solution alloys. This indicates that a change in the deformation mechanism from GBS to solute drag creep takes place during high-temperature deformation. Here, based on our observations on our model system, which is a thermally unstable aluminum solid solution alloy, we discuss the possibility of a superplastic elongation occurring by means of a transition of the deformation mechanism.

Abstract: Recent Al7075 severe friction stir processing (FSP) data gave new insights regarding the relationship among processing, microstructure and high temperature behaviour. Grain boundary sliding, GBS, usually operates with fine, equiaxed and highly misoriented grains although, so far, the variable misorientation is missing from the constitutive equation. A collection of very fine microstructures comprising various grain size and misorientation values is employed to evidence the relative importance of grain size vs misorientation in the superplastic behaviour of the processed alloy. This relationship is included into a new GBS constitutive equation incorporating the average misorientation as a variable.

Abstract: In this study, the authors evaluated pressure distribution on a backing plate in friction-stir processing (FSP) utilizing an embedded pressure pin connected to a load sensor. They conducted FSP on aluminum alloy plates repeatedly offsetting the path-lines from the center of the pin and recorded change of forming pressure with tool position, which was compiled from the bearing load of the pin. The authors mapped the results to visualize the two-dimensional contact pressure distribution on a backing plate during FSP. They then compared the height distribution of the wall fabricated by friction-stir forming (FSF) utilizing a die having a groove with the observed distribution of pressure. Consequently, maximum pressure was observed beneath the rim of the tool probe at the retreating side (RS), and the highest points of the wall were observed at the RS.

Abstract: Friction Stir Processing is a solid state process with the ability to modify microstructure and refine grain sizes of the material without melting and uniformly disperse reinforcement particles in the material matrix resulting in further improvements in the mechanical properties. In this study, it was used to disperse Al₂O₃ reinforcement particles of different sizes. Uniform dispersion of the reinforcements was achieved in the aluminium matrix with significant reduction in grain size were observed via SEM and EBSD. Improvement in Vicker’s micro hardness was observed after FSP.

Abstract: Friction stir processing (FSP) is one of the severe plastic deformation (SPD) processes. It has been reported that SPD-processed Al with various purities attained a minimum grain size when Zener-Hollomon parameter is larger than 10¹⁶ s^-1. The minimum grain size is different by purity level and alloying elements. We investigated the influence of Fe solute atoms on grain refinement of high-purity Al on the condition that Zener-Hollomon parameter was larger than 10¹⁶ s^-1. FSP was conducted on Al-0.01%Fe, which was fabricated by using 5N Al (99.999% purity). FSP-ed Al-0.01%Fe exhibits the minimum grain size of 1.4 μm, although high-purity aluminums with more than 99.998% exhibits much larger minimum grain sizes of 30-40 μm. Only 101 at.ppm Fe played a critical role in the grain refinement of pure aluminums.

Abstract: Friction stir processing (FSP) is a microstructural modification technique. In FSP, the material undergoes intense plastic deformation, yielding a dynamically recrystallized fine grain structure. One of the most important issues that need to be tackled in this field is the lack of predictive tools. That enables the selection of the optimum parameters required to achieve the desired modifications on the mechanical properties of the processed materials. In this study, the effects of different FSP parameters (rotational and translational speeds) on the resulting micro-hardness of friction stir processed AZ31 magnesium sheets are examined. Variations of micro-hardness with longitudinal and through-thickness positions are also investigated. Artificial neural networks (ANNs) are used to model and predict the resulting micro-hardness.

Abstract: In this research, one solid state processing technique, friction stir processing, is applied to modify the AZ61 magnesium alloy billet. The FSP modified AZ61 alloy could be refined to 3-8 μm via the dynamic recrystallization during processing. The AZ61 magnesium alloy billet with 75μm grain size could be refined to about 7.5μm by four-pass friction stir processing. The hardness of the stirred zone could increase to around 70-80 after friction stir processing, and after a further compressive strain of about 3% could raise the hardness to 81. The ductility of the weld direction specimens of the modified alloy could have a 235% elongation at 300°C and 1x10^-4 s^-1. The grain boundary sliding (GBS) might be the dominant deformation mechanism during superplastic deformation.

Abstract: Utilizing a proper material model for describing the mechanical behavior of any material is key for a successful simulation of friction stir processing (FSP) where temperature, strain, and strain rate gradients vary abruptly within, and when moving away, from the stirring zone. This work presents a comparison of how faithfully do three different constitutive equations reproduce the state variables of strain, strain rate, and temperature in an FEM simulation of a test-case FSP (1000 rpm spindle speed, and 90 mm/min feed). The three material models considered in this comparison are namely: Johnson-Cook (JC), Sellars-Tegart (ST), and Zerilli-Armstrong (ZA). Constants for these constitutive equations are obtained by fitting these equations to experimental mechanical behavior data collected under a range of strain rates and temperatures of twin-rolled cast wrought AZ31B sheets.It is widely recognized that JC-based models over predicts stress values in the stir zone whereas ST-based models are incapable of capturing work hardening outside of the stir zone. Therefore, a ZA model, being a physical based-HCP specific model, is hereby investigated for its suitability as a material model that would overcome such drawbacks of JC-and ST-based models. The equations from the constitutive models under consideration are fed into an FEM model built using DEFORM 3D to simulate the traverse phases of a friction stir process. Amongst these three material models, comparison results suggest that the HCP-specific ZA model yield better predictions of the state variables: strain, strain rate, and temperature, and, consequently, the estimated values for flow stresses.