Showing papers on "Friction stir processing published in 2015"

Fabrication of Al5083/B4C surface composite by friction stir processing and its tribological characterization

Abstract: Improved surface properties with the retainment of bulk properties are necessary for a component for enhanced wear characteristics. Friction stir processing (FSP) is used to produce such surface composites. Fabrication of 5083 aluminum alloy with reinforced layers of boron carbide (B 4 C) through FSP was carried out. Micro and nano sized B 4 C particles were used as reinforcements. The friction processed surface composite layer was analyzed through optical and scanning electron microscopical studies. The number of passes and the size of reinforcement play a vital role in the development of surface composites by FSP. Mechanical properties of the friction stir processed surface composites were evaluated through micro hardness and universal tensile tests. The results were compared with the properties of the base metal. The role of reinforcement and number of passes on properties were also evaluated. Tribological performance of the surface composites is tested through pin on disk test. The surface composite layer resulted in three passes with nano particle reinforcement exhibited better properties in hardness, tensile behavior and wear resistance compared to the behavior of the base metal.

Fabrication of metal matrix composites by friction stir processing with different Particles and processing parameters

Abstract: Friction stir processing has been employed to produce metal matrix composites by incorporating reinforcement particles in an Al 5059 matrix. The fabrication method involved repeated friction stir passes on a groove which contained powder reinforcements, with different processing parameters and tool geometries used for each pass. Various particles with sizes from 130 nm to 4.3 μm, and different process parameters were examined to obtain a uniform distribution of particles within the stir zone. Mechanical properties (i.e. tensile and microhardness) of the Al 5059 matrix MMCs reinforced with Al2O3, SiC, and B4C with particle sizes of 130, 250, and 35 nm respectively were compared. Tensile tests showed 11%, 20%, and 38% increases in yield strength compared to the matrix alloy for composites containing nano-scale Al2O3, SiC, and B4C, respectively. When 4.3 μm Al2O3 particles were employed, higher volume fractions could be achieved which resulted in a 32% increase in yield strength compared to the base metal. The average microhardness value within the stir zone increased from 85 HV in the base material to a maximum of 170 HV in the B4C-reinforced composite. Nano-scale particles seem to be more effective to increase hardness by increasing the particle fraction in the produced composites.

Tungsten particle reinforced Al 5083 composite with high strength and ductility

Abstract: Tungsten particles were incorporated into an Al 5083 matrix by friction stir processing (FSP). FSP resulted in uniform dispersion of the tungsten particles with excellent interfacial bonding and more importantly without the formation of any harmful intermetallics. For the first time, the particles penetrated to a depth equal to the full pin length of the tool. A novel aspect of the 5083 Al–W composite is that it showed an improvement of more than 100 MPa in the UTS and at the same time exhibited a high ductility (30%). The ductility was also evident from the well defined dimples in the fracture surface which also revealed the superior bonding between the particles and the matrix. FSP also resulted in substantial grain refinement of the Al matrix. Electron backscatter diffraction (EBSD) and transmission electron microscopy analysis revealed that the fine grains formed by dynamic recrystallization. A gradual transformation from sub-grain to high-angle grain boundaries was observed from EBSD analysis pointing towards the occurrence of a continuous type of dynamic recrystallization process.

Effect of Microstructure on the Deformation Mechanism of Friction Stir-Processed Al0.1CoCrFeNi High Entropy Alloy

Abstract: Grain refinement from several millimeters in as-received (AR) condition to the range of 0.35–15 μm was achieved by friction stir processing (FSP). Due to the sluggish nature of atomic diffusion in high entropy alloys (HEAs), the FSP region exhibited an immense variation in microstructure which was directly attributed to the accumulated plastic strain during FSP. In accordance with the Hall–Petch relationship, yield strength (YS) has increased by a factor of four after grain refinement while maintaining large uniform elongation (UE). The Kocks–Mecking plot indicated different deformation mechanisms operative in both FSP and AR conditions.

Friction Stir Processing of a High Entropy Alloy Al0.1CoCrFeNi

Abstract: High entropy alloys are a new class of metallic materials with a potential for use in structural applications. However, most of the studies have focused on microhardness and compressive strength measurements for mechanical properties determination. This study presents the tensile deformation behavior of a single-phase, face-centered cubic Al0.1CoCrFeNi high entropy alloy (HEA). Friction stir processing was carried out to refine the grain size. Scanning electron microscopy and electron backscatter diffraction were carried out for microstructural examination. The grain size of the alloy was on the order of millimeters in the as-received condition. The average grain size after friction stir processing of the alloy was 14 ± 10 micrometers. The mechanical properties were determined through microhardness measurement and mini-tensile tests. The friction stir processed alloy showed a total elongation of ~75% for the mini-tensile sample used and yield strength of 315 MPa. It is an exceptional combination of strength and ductility. Friction stress was determined to be 174 MPa and the Hall–Petch coefficient was 371 MPa (µm)1/2. Such a high value of Hall–Petch coefficient suggests that grain boundary strengthening can be a very effective strengthening mechanism for the HEA Al0.1CoCrFeNi.

Cryogenic friction-stir processing of ultrafine-grained Al–Mg–TiO2 nanocomposites

Abstract: Submerged friction-stir processing under cryogenic conditions was employed to fabricate ultrafine-grained nanocomposites with enhanced mechanical characteristics. Al–Mg alloy sheet with 3 vol% TiO2 nanoparticles were processed under air (ambient temperature), a water-dry ice medium (~−25 °C), and liquid nitrogen. It is shown that a homogenous distribution of reinforcement particles throughout the metal matrix is attained at a rotational speed of 1400 rpm and a traverse velocity of 50 mm/min after 4 passes. In situ formation of Al3Ti and MgO nanophases during multi-pass processing is shown by transmission electron microscopy. Under the cryogenic cooling condition, ultrafine grains and cellular structures with sizes smaller than 1 μm and 200 nm are attained. It is shown that the formation of an ultrafine-grained structure is accompanied with significant improvement (150–200%) in the mechanical strength. The tensile yield strength of ~170 MPa, elongation of ~22% and Vickers hardness of ~165 HV are attained. A change in the fracture mode from ductile-brittle to fully ductile is presented when the cryogenic processing is employed. A relationship between the grain size and the fracture features is demonstrated. Effects of cooling conditions on the microstructure and mechanical properties of friction stir processed Al-based nanocomposites are addressed.

Reactive friction stir processing of AA 5052–TiO2 nanocomposite: process–microstructure–mechanical characteristics

Abstract: Friction stir processing (FSP) is a solid state route with a capacity of preparing fine grained nanocomposites from metal sheets. In this work, we employed this process to finely distribute TiO2 nanoparticles throughout an Al–Mg alloy, aiming to enhance mechanical properties. Titanium dioxide particles (30 nm) were preplaced into grooves machined in the middle of the aluminium alloy sheet and multipass FSP was afforded. This process refined the grain structure of the aluminium alloy, distributed the hard nanoparticles in the matrix and promoted solid state chemical reactions at the interfaces of the metal/ceramic particles. Detailed optical and electron microscopic studies showed that the microstructural homogeneity was improved with repetition of FSP up to four passes. The average grain size of the nanocomposite was ∼2 μm, while nanometric MgO and Al3Ti particles were formed in situ and homogenously distributed in the metal matrix. Mechanical characterisations showed that the yield strength and e...

The effect of FSP on mechanical, tribological, and corrosion behavior of composite layer developed on magnesium AZ91 alloy surface

Abstract: Friction stir processing (FSP) has been applied to modify the surface characteristics of metals. Development of surface composites through FSP has been addressed by different research studies. During the process, generally hard particles are embedded in the soft matrix through stirring. In the current research, surface composites were developed on the surface of AZ91 magnesium base alloy. SiC and Al2O3 particles were embedded separately in the surface and accordingly two kinds of composites were developed. Different characteristics, namely mechanical, tribological, and corrosion behavior, were analyzed. The results showed that mechanical properties as well as strength, hardness, and ductility of FS-processed samples were higher than the as-received one. It was concluded that wear and corrosion resistance of FS-processed samples were higher than the as-received material. The results also indicated that by increment of pass number, the mechanical properties improved, corrosion resistance increased, and wear rate decreased. The results also showed that samples processed using SiC particles had better mechanical characteristics and corrosion resistance than samples processed using Al2O3 particles, although particle type did not have significant effect on wear rate.

Effects of stored strain energy on restoration mechanisms and texture components in an aluminum–magnesium alloy prepared by friction stir processing

Abstract: Plates of AA5052 (Al–Mg) alloy in both annealed (solution-treated) and wrought (rolled) temper conditions were subjected to friction stir processing (FSP) at various w/v pitch ratios from 4 to 28 rev.min/mm. The role of stored strain energy on the evolution of restoration mechanisms and crystallographic texture components were assessed in terms of microstructural features evaluated using electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) analysis. The results revealed that FSP significantly refined the grain structure and changed the crystallographic micro-texture components. The grain size of the annealed and wrought alloy was reduced from 49.4 and 9.7 μm initial values to 3.3 and 3.6 μm, respectively when w/v=4. Also, the formation of a { 112 } ⟨ 110 ⟩ crystallographic shear texture with a strong B / B ¯ component intensity were observed. The microstructural changes in the annealed alloy were related to the occurrence of discontinuous dynamic recrystallization (DDRX) mechanism, while operation of a static recrystallization (SRX) prior to a continuous (CDRX) mode in the wrought one, revealing the effect of stored strain energy. Evaluations of the mechanical properties also determined enhanced ultimate tensile strength, elongation, and indentation Vickers hardness while preserving the yield stress.

Mechanical properties of ultra-fine grained AZ91 magnesium alloy processed by friction stir processing

Abstract: The relationship of microstructure and mechanical properties in a friction stir processed (FSP) AZ91 magnesium alloy has been analyzed and compared with numerous investigations in the literature. Since the heat generation and sink during FSP drastically influences the final microstructure, several backing devices were used for controlling the stir zone temperature, producing a grain size refinement down to values close to 0.5 μm. This microstructure often determines excellent mechanical properties at room temperature and superplastic behavior at high temperatures. The yield stress at room temperature shows a sharp decrease in the Hall–Petch slope related to a favorable orientation for slip of the basal planes. Noticeable changes in ductility are explained in terms of grain size and texture effects on work hardening behavior which joins both contributions. Finally, the analysis of the tensile tests performed at high temperature, together with the data reported by other authors, have been used to obtain an unitary description of the Grain Boundary Sliding (GBS) mechanism in the AZ91 magnesium alloys.

Effect of the Zener–Hollomon parameter on the microstructure evolution of dual phase TWIP steel subjected to friction stir processing

Abstract: The present work is devoted to investigate the correlation between the Zener–Hollomon parameter and the grain structure of the thermo-mechanically processed dual phase twinning induced plasticity (TWIP) steel utilizing the friction stir processing (FSP) technique. To this end 3 mm thick workpieces were subjected to FSP under rotational speeds of 800–2500 rpm and constant traveling speed of 50 mm/min. Additionally, isothermal hot compression tests were conducted at temperatures in the range of 800–1100 °C under the strain rates of 0.001–0.1 s −1 . Employing the flow stress data acquired from compression tests, the precise value of deformation activation energy ( Q ) was determined through Arrhenius-type constitutive model. The results indicate that increasing the rotational speed from 800 to 2500 rpm has led to Z -value variation between 1.04×10 20 and 0.03×10 20 s −1 . However, the scanning electron microscopy (SEM) characterization shows that the grain size reaches a certain minimum value at 1600 rpm. Three different models have been established to interpret the correlation between Z -value and the size of FSP-induced grains in the case of the experimental TWIP steel.

Effects of nanometric inclusions on the microstructural characteristics and strengthening of a friction-stir processed aluminum–magnesium alloy

Abstract: An aluminum–magnesium alloy was friction-stir processed in the presence of TiO2 nanoparticles which were pre-placed in a groove on the surface to produce a composite. Field emission-scanning and transmission electron microscopy studies show that solid state chemical reactions occur between the Al–Mg matrix and the ceramic particles upon the severe plastic deformation process. The microstructure of the aluminum alloy consists of a coarse grain structure, large complex (Fe,Mn,Cr)3SiAl12 particles, and small Mg2Si precipitates. After friction stir processing, a deformed grain structure containing rod-like Al–Fe–Mn–Si precipitates is attained, along with cuboidal (~100 nm) Cr2 precipitates and spherical (~100 and 5 nm) Mg2Si particles. In the presence of TiO2 nanoparticles, magnesium oxide (MgO) and titanium aluminide (Al3Ti) nanophases are formed. It is shown that these microstructural modifications lead to a significant enhancement in the hardness and tensile strength of the aluminum alloy. The relationship between the microstructural evolution and mechanical properties and the role of hard inclusions are presented and discussed. An analysis based on strengthening models indicates that the yield strength of the nanocomposite is mainly controlled by dislocations and grain boundaries rather than the nano-scale inclusions.

Effect of friction stir processing with B4C particles on the microstructure and mechanical properties of 6061 aluminum alloy

Abstract: In this paper, friction stir processing (FSP) with B4C particles (B4Cp) is used to improve the surface modification of 6061 aluminum alloy. Optical microscopy, scanning election microscopy, and energy-dispersive X-ray analysis have been performed to investigate the microstructure and the distribution of B4Cp. Wear resistance and microhardness were evaluated in detail. It is observed that the increasing number of FSP passes causes a more uniform distribution of B4Cp. The homogeneous distribution of B4Cp was observed in the weld zone, which significantly improved the wear resistance and microhardness of the surface composite layer as compared to those of the as-received Al alloy.

Effect of two-pass friction stir processing on the microstructure and mechanical properties of as-cast binary Al–12Si alloy

Abstract: The effect of two-pass friction stir processing (FSP) on the microstructural evolution, mechanical properties and impact toughness of as-cast Al–12Si alloy was investigated systematically. Severe plastic deformation imposed by FSP resulted in a considerable fragmentation of the needle-shaped eutectic silicon particles into the smaller ones. The length of eutectic Si particles decreased from 27±23 μm to about 2.6±2.4 μm. The average aspect ratio of 6.1±5.1 for eutectic Si particles in the as-cast state decreased to about 2.6±1.0 after FSP with a corresponding increase in their roundness. The hardness, strength, ductility and impact toughness of the alloy increased simultaneously after two-pass FSP. The increase in the yield and tensile strength values after FSP was about 20% and 29%, respectively. The FSPed alloy exhibited 25% elongation to failure and 15% uniform elongation which were almost seven times and five times higher, respectively, than those of the as-cast alloy. The hardness of the alloy increased from 58 Hv0.5 for the as-cast state to about 67 Hv0.5 after FSP. The absorbed energy during impact test increased to about 8.3 J/cm 2 after FSP, which is about seven times higher than that of the as-cast alloy. Improvements in all mechanical properties were mainly attributed to the radical changes of the shape, size and distribution of the eutectic silicon particles along with the breakage and refined of the large α-Al grains during two-pass FSP.