Showing papers on "Friction stir processing published in 2012"

Composite fabrication using friction stir processing—a review

Abstract: Composite manufacturing is one of the most imperative advances in the history of materials. Nanoparticles have been attracting increasing attention in the composite community because of their capability of improving the mechanical and physical properties of traditional fiber-reinforced composites. Friction stir processing (FSP) has successfully evolved as an alternative technique of fabricating metal matrix composites. The FSP technology has recently shown a significant presence in generation of ex situ and in situ nanocomposites. This review article essentially describes the current status of the FSP technology in the field of composite fabrication with the main impetus on aluminum and magnesium alloys.

Fabrication of aluminum-alumina metal matrix composites via cold gas dynamic spraying at low pressure followed by friction stir processing

Abstract: Cold gas dynamic spraying at low pressure (1 MPa gage or 150 psig) was used to fabricate Al–Al2O3 metal matrix composite (MMC) coatings onto 6061 Al alloy. The powder contained Al powder admixed with −10 μm Al2O3 in fractions up to 90 wt.%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted to determine the microstructure, properties, and volume fraction of reinforcing particles in the coatings. The coatings were then friction-stir processed (FSP) at tool rotation speeds of 894 or 1723 RPM using a flat cylindrical tool. The Al2O3 content and hardness of the final MMC coatings increased with increasing fractions of Al2O3 particles in the feedstock powder, resulting in a maximum Al2O3 content of 48 wt.% and a hardness of 85 HV of the as-sprayed coating when 90 wt.% Al2O3 was used in the feed powder blend. After FSP, the hardness of the MMC increased to a maximum of 137 HV. The as-sprayed coatings contained Al2O3 particles that were segregated between the Al particles, and FSP was effective in dispersing these Al2O3 particles and decreasing their mean free path. It was suggested that this re-distribution and Al2O3 particle size refinement during FSP improved the hardness of the MMC coatings.

Microstructure and sliding wear behavior of AA6360/(TiC + B4C) hybrid surface composite layer synthesized by friction stir processing on aluminum substrate

Abstract: Friction stir processing (FSP) has evolved as a viable technology to fabricate surface composite layers. Hybrid composites have attracted the attention of researchers to minimize the wear of counterface materials. In this work, AA6360/(TiC + B4C) hybrid surface composite layers (SCLs) with various volume ratios of TiC and B4C were synthesized by FSP. The particles were compacted into a groove of width 0.5 mm, depth 5.5 mm and length 100 mm cut on the surface of the aluminum plate. The FSP was carried out at a tool rotational speed of 1600 rpm, traverse speed of 60 mm/min and axial force of 8 kN. Two passes were applied in opposite directions. The microstructure and sliding wear behavior of the fabricated SCLs were evaluated. TiC and B4C particles were distributed homogeneously in the SCLs. Both the particles behaved as one type of reinforcement particle during FSP. There was little or no comminution of ceramic particles. Addition of TiC and B4C particles enhanced the wear resistance of AA6360. 50%TiC + 50% B4C hybrid SCL exhibited lowest wear rate due to the formation of a thin tribo film. Detailed characterization of the worn surfaces and the wear debris was also reported.

Effect of friction stir processing on microstructure and mechanical properties of aluminium

Abstract: Commercially pure aluminium was subjected to friction stir processing (FSP) to study the microstructure developed and its effects on the mechanical properties. Friction stir processing refined the grain size to 3 μm in a single pass from the starting coarse grain size of 84 μm. Electron backscattered diffraction (EBSD) results showed occurrence of dynamic recrystallization and also revealed existence of different orientations within the stir zone and across the transition zone. Transmission electron microscopy (TEM) revealed fine grains with well defined boundaries. The arrangement and absorption of dislocation into the sub-grain boundaries, formed by dynamic recovery, was also revealed by TEM. The yield strength of the material was improved by a factor of 2.4 after FSP owing to grain refinement. The most important feature of the friction stir processed material was that even after this significant improvement in strength there was little loss of ductility. The hardness also improved by 34% with the peak hardness being observed towards the advancing side.

In situ synthesizing Al3Ni for fabrication of intermetallic-reinforced aluminum alloy composites by friction stir processing

Abstract: Ultrafine-grained in situ synthesized Al3Ni particulate-reinforced composites were fabricated by friction stir processing (FSP) introduced Ni powder into the stirred zone of 1100-H14 aluminum alloy. The microstructures and the compositions of the composites were analyzed by SEM, EDS and XRD. The microhardness and ultimate tensile strength (UTS) were measured. The XRD and EDS analyses showed that the Al–Ni in situ synthesizing product was Al3Ni. When the specimen was stirred 2 passes, the formed Al3Ni was tiny to be detected. Al3Ni subsequently became apparent when stirring 4 and 6 passes and the fine Al3Ni particles were dispersed homogeneously in the composites, which caused significant increases of the microhardness and UTS of the composites. The effective Gibbs free energy change of formation model was proposed to predict the Al–Ni compound formation at solid-state interface and the calculation combined with kinetic factors showed that Al3Ni was the product, which supports the experimental observation.

Mechanical Properties, Corrosion Resistance, and Microstructural Changes during Friction Stir Processing of 5083 Aluminum Rolled Plates

Abstract: Friction stir processing is a solid-state process to modify microstructure and mechanical properties of sheet metals and as-cast materials. In this process, stirring action of the tool causes the material to intense plastic deformation that yields a dynamic recrystallization. In this study, the effect of friction stir processing (FSP) and process parameters on microstructure, mechanical properties, wear resistance, and corrosion behavior of AA5083-O has been investigated. Results show that FSP leads to finer and homogenized grain structure, as well as changes in hardness, corrosion resistance, and wear resistance of the material.

Grain size and texture effects on deformation behavior of AZ31 magnesium alloy

Abstract: Cold rolled AZ31 magnesium alloy sheet was subjected to friction stir processing to generate four average grain sizes ranging from 0.8 to 9.6 μm. The processed material exhibited a strong basal fiber texture with the c -axis tilted about 35–55° towards the processing direction. The grain size and texture dependence of mechanical behavior were evaluated by using tensile testing along two orthogonal directions. Remarkably high ductility of ∼65% was achieved in relatively coarse grained material that fractured without developing necking when tested in the processing direction. The ductility decreased significantly to ∼10% for ultrafine grained material as the tensile yield strength increased from ∼53 MPa to ∼180 MPa. Grain size had limited influence on ductility of processed material tested in transverse direction, but reduced the uniform elongation to ∼2% for ultrafine grained material which exhibited ∼320 MPa yield strength. Accompanying the significant anisotropy in tensile strength in two directions, the deformation of processed AZ31 in the processing direction was mainly accommodated through basal slip and extension twinning (except for ultrafine grained material); however, the deformation of material in transverse direction was dominated by non-basal slip. Influences of grain size and texture on mechanical behavior were studied in terms of work-hardening and deformation mechanisms.

Fabrication of functionally graded aluminum foam using aluminum alloy die castings by friction stir processing

Abstract: Functionally graded aluminum (FG) foam with varying pore structure was fabricated by utilizing friction stir processing. Die castings containing a large amount of gas were used as the starting material. The obtained FG foam had two different deformation stages and two plateau regions with a seamless bonding interface.

High tensile ductility via enhanced strain hardening in ultrafine-grained Cu

Abstract: Low tensile ductility owing to the insufficient strain hardening is the main drawback for ultrafine-grained (UFG) materials, which restricts their practical applications. Here, via a simple friction stir processing technique with additional cooling, we prepared UFG Cu with high strength and tensile ductility. Enhanced strain hardening capacity, which is effective in blocking and accumulating dislocations, was achieved in the present recrystallized UFG microstructure. The enhanced strain hardening capacity is attributed primarily to the low dislocation density, and the presence of large fraction of high angle grain boundaries and a certain amount of coherent twin boundaries. This work provides a strategy for designing UFG materials with good mechanical properties. (C) 2011 Elsevier B.V. All rights reserved.

Transient Heat and Material Flow Modeling of Friction Stir Processing of Magnesium Alloy using Threaded Tool

Abstract: A three-dimensional transient computational fluid dynamics (CFD) model was developed to investigate the material flow and heat transfer during friction stir processing (FSP) in an AZ31B magnesium alloy. The material was assumed to be a non-Newtonian viscoplastic fluid, and the Zener-Hollomon parameter was used to describe the dependence of material viscosity on temperature and strain rate. The material constants used in the constitutive equation were determined experimentally from compression tests of the AZ31B Mg alloy under a wide range of strain rates and temperatures. A dynamic mesh method, combining both Lagrangian and Eulerian formulations, was used to capture the material flow induced by the movement of the threaded tool pin. Massless inert particles were embedded in the simulation domain to track the detailed history of material flow. The actual FSP was also carried out on a wrought Mg plate where temperature profiles were recorded by embedding thermocouples. The predicted transient temperature history was found to be consistent with that measured during FSP. Finally, the influence of the thread on the simulated results of thermal history and material flow was studied by comparing two models: one with threaded pin and the other with smooth pin surface.

Microstructure and mechanical properties of steel/TiC nano-composite surface layer produced by friction stir processing

Abstract: A steel/TiC nano-composite surface layer with ultra fine grains of less than 600 nm was fabricated on mild steel substrate by introduction of nano-sized TiC powder into the stir zone employing four passes of friction stir processing. TiC clusters were formed after the first pass. Sequential break-up of clusters and refinement of matrix grains were caused by subsequent FSP passes. A near uniform dispersion of nano-sized TiC particles was achieved after the fourth pass. The fabricated nano-composite layer exhibited a maximum micro hardness value of ~ 450HV; this is much greater than 185 and 130HV of the friction stir processed layer without introduction of TiC powder and as-received substrates, respectively. Moreover, a significant improvement in wear resistance of the nano-composite layer was observed as compared with that of the as-received substrate. The enhanced properties are attributed to the uniform dispersion of hard nano-sized TiC reinforcements in a matrix of ultra fine dynamically recrystallized grains.

Microstructure and mechanical property of nano-SiCp reinforced high strength Mg bulk composites produced by friction stir processing

Abstract: Friction stir processing has been applied to fabricate SiC–Mg bulk composites in this study. AZ63 magnesium alloy, a kind of commercial engineering materials, was selected as base metal. SiC nanoparticles with average size of 40 nm were selected as reinforced particles. After being ultrasonic dispersed in ethanol and friction stir processed with base metal, the SiC particles were uniformly dispersed. Friction stir processing without filling any particles was also applied to base metal as a comparison group. Microstructure evolution was observed by optical microscope and scanning electron microscope. Fine and uniform nugget zone were found both in comparison group and composite. The phases of the material were determined by X-ray diffraction. Transmission electron microscopy observation was conducted to study the condition of SiC nanoparticles. SiC particles were found both inside the grain and at the grain boundary. No micro-sized particle agglomeration was observed in the composite. Vicker hardness and tensile test were carried out to study the mechanical properties of the composite. The average Vicker hardness of the base metal, comparison group and composite were 80 Hv, 85 Hv and 109 Hv respectively. The ultimate tensile strength of the composite reached 312 MPa. Compared with 160 MPa of the as-casted Mg alloy, 263 MPa of the comparison group, the effect of nanoparticles on strength increase was significant.

On the role of cooling and tool rotational direction on microstructure and mechanical properties of friction stir processed AZ91

Abstract: This paper deals with an experimental investiga- tion focused on the effects of water cooling treatment, friction stir processing pass number, and tool rotational direction on the microstructure and mechanical properties of friction stir processed AZ91 magnesium alloy. Specimens were produced using different combinations of process parameters. Parallel to increasing the amount of oxide par- ticles in the processed area, water cooling was found to reduce the final grain size and enhance their hardness and strength. Changing the rotational direction in each pass reduces the grain size severely (from 150 to ~4 μm) and increases the hardness (from 63 to 98 HV) and strength (from ~130 to ~250 MPa). However, no significant differ- ence was found in wear resistance of the specimens pro- duced with different process parameters.

Influence of texture on superplastic behavior of friction stir processed ZK60 magnesium alloy

Abstract: Commercial ZK60 extruded plate was subjected to friction stir processing (FSP). A fine-grained structure 2.9 mu m in grain size with uniformly distributed fine second-phase particles and predominant high-angle grain boundaries of 97% was obtained. A strong basal fiber texture with the (0002) planes roughly surrounding the tool pin surface was developed in the FSP ZK60 alloy. The FSP ZK60 alloy exhibited superplastic behavior at 225-325 degrees C in both the transverse direction (TD) and longitudinal direction (LD), with the superplastic elongation in the LD being higher than that in the TD. A maximum elongation of 1390% was obtained at 300 degrees C and 3 x 10(-4) s(-1) in the LD. The existence of texture was found to influence the superplastic flow stress and plasticity. The texture had a minor influence on the flow stress, due to the texture weakening during superplastic deformation. However, the anisotropy of plasticity existed at all temperatures and strain rates, as a result of the activation of different slip systems in different tensile directions. (c) 2012 Elsevier B.V. All rights reserved.

Improvement of microstructural, mechanical and tribological characteristics of A413 cast Al alloys using friction stir processing

Abstract: In the present investigation, the effect of the friction stir processing (FSP) on the microstructural, mechanical and tribological characteristics of as-cast A413 aluminum alloy was studied. The influence of the tool rotational and traverse speeds on such characteristics was evaluated. The results revealed that FSP has significantly improved the microstructure of the A413 Al alloy by eliminating structural defects such as porosity and Si segregation. The size of the Si particulates as well as the α-Al grains increases with increasing tool rotational speed and/or decreasing the tool traverse speed. The FSPed zones exhibited better mechanical properties as well as dry sliding wear resistance than the as-cast base alloy.