Multi-pass Friction Stir Processing Research Articles

Effect of Multipass Friction Stir Processing on Mechanical and Corrosion Behavior of 2507 Super Duplex Stainless Steel

The microstructure, mechanical properties, and corrosion behavior of 2507 super duplex stainless steel after multipass friction stir processing (FSP) were examined. A significant refinement in grain size of both ferrite and austenite was observed in stir zone resulting in improved yield and tensile strength. Electrochemical impedance spectroscopy and anodic polarization studies in 3.5 wt.% NaCl solution showed nobler corrosion characteristics with increasing number of FSP passes. This was evident from the decrease in corrosion current density, decrease in passive current density, and increase in polarization resistance. Also, the decrease in density of defects, based on Mott-Schottky analysis, further confirms the improvement in corrosion resistance of 2507 super duplex stainless steel after multipass FSP.

Deformation Behavior of Inhomogeneous Layered Microstructure

Multipass friction stir processing (FSP) technique was used to get inhomogeneous layered microstructure in Al 5086 alloy. Different proportions of fine and coarse grain microstructures were generated using FSP. In the present work, material with inhomogeneous layered microstructure generated using FSP was subjected to deformation at temperature of 500°C and at strain rate in the range of 5×10-4 to 1×10-2 s-1. It was observed that the inhomogeneous layered microstructure with more than 50% fine grain microstructure is required to get higher elongation to failure. However material having homogeneous fine grain microstructure showed lower elongation to failure than the material having both fine and coarse grain microstructures. By varying the proportion of fine and coarse grain microstructures higher ductility was obtained at higher strain rate as compared to a material with homogeneous microstructure. It is clear that materials with inhomogeneous microstructure are not following “rule of mixture” type of behaviour.

Effect of Multi-pass Friction Stir Processing on the Electrochemical and Corrosion Behavior of Pure Titanium in Strongly Acidic Solutions

The corrosion behavior of multi-pass friction stir processed (FSP) pure titanium was studied in 0.5 M H2SO4 solutions. Microstructures of treated and untreated samples were characterized using scanning electron microscopy. It was found that the grain size decreased with increasing the number of applied passes of FSP. Electrochemical tests including potentiodynamic polarization measurements and electrochemical impedance spectroscopy showed that three passes of FSP treatments resulted in a Ti sample which exhibited the best passive behavior and had the highest corrosion resistance among all samples in strongly acidic solutions of 0.5 M H2SO4. These improvements can be attributed to the emergence of diverse structural defects and grain refinement induced by FSP treatments. Moreover, Mott–Schottky analysis was performed to investigate the semiconducting properties of passive films. It was found that the semiconducting behavior remained the same after FSP treatments but it reduced donor densities and surprisingly introduced an additional donor level.

On the passive and electrochemical behavior of severely deformed pure Ti through friction stir processing

The effects of local sever plastic deformation through multi-pass friction stir processing on the passive and electrochemical response of pure Ti are studied in a borate buffer solution (pH = 7) in the present paper. Microstructures of treated and untreated samples were characterized using scanning electron microscopy. Results show that with increasing the number of friction stir processing passes, grains became finer in size. Electrochemical tests, including potentiodynamic polarization measurements and electrochemical impedance spectroscopy, showed that the friction stir processed sample provides superior passive behavior with highest corrosion resistance among all samples in the borate buffer solution. These improvements can be attributed to the emergence of diverse structural defects and grain refinement induced by friction stir processing operation. Moreover, Mott–Schottky analysis was performed to assess the semiconducting properties of passive films. It was found that the semiconducting behavior remained the same upon friction stir processing operation but reduced donor densities and provided an additional donor level.

Trade-off among mechanical properties and energy consumption in multi-pass friction stir processing of Al7075 alloy employing neural network–based genetic optimization

Friction stir processing is a novel material processing technique. In this study, neural network–based genetic optimization is applied to optimize the process performance in terms of post-friction stir processing mechanical properties of Al7075 alloy and the energy cost. At first, the experimental data regarding the properties (i.e. elongation, tensile strength and hardness) and the consumed electrical energy are obtained by conducting tests varying two process parameters, namely, feed rate and spindle speed. Then, a numerical model making use of empirical data and artificial neural networks is developed, and multiobjective multivariable genetic optimization is applied to find a trade-off among the performance measures of friction stir processing. For this purpose, the properties like elongation, tensile strength and hardness are maximized and the cost of consumed electrical energy is minimized. Finally, the optimization results are verified by conducting experiments. It is concluded that artificial neural network together with genetic algorithm can be successfully employed to optimize the performance of friction stir processing.

Fabrication of tungsten particles reinforced aluminum matrix composites using multi-pass friction stir processing: Evaluation of microstructural, mechanical and electrical behavior

In this paper, tungsten particles were dispersed into 1060 Al matrix by multi-pass friction stir processing (FSP) to fabricate W/1060Al composites. The effects of the number of FSP passes on the microstructure, mechanical properties and electrical resistivity were investigated. Microstructural observations revealed that increasing the number of FSP passes contributed to W uniform distribution and grain refinement without changing the size and morphology of W particles. Detailed interfacial microstructure analysis indicated that an excellent metallurgical bonding consisting of an element solid solution was formed at the W/Al interface. The mechanical test results showed that, as the number of FSP passes increased, the microhardness values were remarkably enhanced with a more uniform distribution and at the same time both the strength and ductility of FSPed composites were also improved. The maximum microhardness, strength and ductility were achieved in 5-pass FSPed composite reaching to approximate 86 HV, 126MPa and 24%, respectively. It was also found that the W particles reinforced Al composites exhibited excellent electrical conductivity and increasing the number of FSP passes caused a negligible enhancement in the electrical resistivity of FSPed composites.

Influence of multi-pass friction stir processing on wear behaviour and machinability of an Al-Si hypoeutectic A356 alloy

A356 is a widely used Al-Si alloy in automotive and aerospace industries for its good combination of mechanical and tribological properties. When these materials are to be considered for advanced engineering applications, machining becomes inevitable. However, presence of coarse and non-uniformly distributed silicon particles greatly influence the material properties imposing great challenge to its machinability. Our present work is focussed to improve the wear resistance and machinability of this alloy using multi pass friction stir processing (FSP). The wear behaviour of FSPed materials is characterised against metallic and abrasive medium and the machining studies are done by drilling experiments in dry condition. Under metallic wear condition, the wear resistance is maximum for the material subjected to 3 pass FSP and in case of abrasive wear, the maximum wear resistance is observed in 2 pass FSPed material. A decrease in both drilling force and surface roughness is observed with increase in FSP passes upto 2 pass beyond which they increased. Study on edge burr formation during drilling suggests that the entry and exit burrs are minimal for 3 pass FSPed material. A detailed investigation on the observed results is done in correlation with the microstructural evolution and mechanical properties.

The influence of multipass friction stir processing on the corrosion behavior and mechanical properties of zircon-reinforced Al metal matrix composites

Zircon-reinforced Al metal matrix composites were fabricated by friction stir processing (FSP). Composite materials were analyzed to characterize their microstructure, mechanical properties and corrosion resistance. The number of FSP passes was found to have a significant effect on the microstructure and strength of the composite specimens. Multipass friction stir processing of the composite specimens promoted grain refinement in the base metal and resulted in a uniform dispersion of the reinforcement particles. In addition, increasing the FSP pass number from one to three increased the composite density. Also, corrosion resistance of the composites was severely influenced by the number of FSP passes and presence of reinforcement particles. While, corrosion performance of the composite materials was significantly enhanced as the number of FSP passes increased to two and three passes, a more number of FSP passes reduced the corrosion resistance of composite materials. It was found that process parameters could be easily optimized to enhance the mechanical properties and corrosion resistance of the zircon-reinforced Al metal matrix composites during FSP.

Influence of Multi-pass Friction Stir Processing on Microstructure and Mechanical Properties of Die Cast Al–7Si–3Cu Aluminum Alloy

The influence of overlap multi-pass friction stir processing on the microstructure and the mechanical properties, in particular, strength, ductility and hardness of die cast Al–7Si–3Cu aluminum alloy was investigated. It was observed that increase in the number of overlap passes friction stir processing resulted in significant refinement and redistribution of aluminum silicon eutectic phase and elimination of casting porosities. The microstructural refinement by the friction stir processing not only increases the ultimate tensile strength from 121 to 273 MPa, but also increases the ductility as observed by the increase in fracture strain from 1.8% to 10%. Analysis of the fractured surface reveals that the microstructural refinement obtained by friction stir processing plays a vital role in transforming the fracture mode from completely mixed mode to the ductile mode of the fracture with increasing number of passes. The change in the size, shape, morphology and distribution of eutectic silicon particles and elimination of the porosities are the main reasons for the increases in tensile strength and ductility due to friction stir processing.

Friction-stir processing of an AA8026-TiB2-Al2O3 hybrid nanocomposite: Microstructural developments and mechanical properties

In this study, micro- and nano-sized TiB2 and Al2O3 particles were incorporated separately and simultaneously through the AA8026 aluminum base alloy during multi-pass friction stir processing (FSP) with 100% overlapping to fabricate metal matrix mono and hybrid nanocomposites. Various FSP conditions including different rotational speeds (w), traverse velocities (v), and processing pass numbers were assessed to attain a homogenous distribution of reinforcing particles through the Al-metal matrix. Moreover, the impacts of size (micro or nano) and type of reinforcement particles (TiB2 and Al2O3) on the process-ability of single and hybrid nanocomposite systems were examined. Microstructures of different zones and distributions of reinforcing ceramic particles through the Al-matrix under various processing conditions were studied and characterized by using optical (OM), scanning (SEM), and transmission electron microscopy (TEM) techniques, respectively. The main mechanical characteristics of the prepared nanocomposites, such as, indentation Vickers hardness, tensile properties, and wear resistance were measured and compared for all of the various processing conditions. By optimization of the FSP parameters, as a rotational speed of 1600rpm and a traverse velocity of 40mm/min after 4 passes, a uniform AA8026-TiB2-Al2O3 hybrid nanocomposite was attained with significant improvements (~70–100%) in the different mechanical properties. As a result, the tensile yield strength of ~270MPa, elongation of ~4.5%, and indentation Vickers hardness of ~141HV were obtained. Also, the average wear rate was reduced from the 21×10−3mg/m value for the AA8026 base alloy down to 2.6×10−3mg/m for the best processed nanocomposite. A direct relationship between the wear rate and the indentation hardness resistance was demonstrated. Finally, effects of FSP processing conditions and reinforcement particles (type and size) on the microstructure and mechanical properties of the FSPed Al-matrix nanocomposites were addressed and discussed.

Microstructure and Mechanical Properties of a Fine-Grained AZ91 Magnesium Alloy Prepared by Multi-Pass Friction Stir Processing

Friction stir processing (FSP) is a novel severe plastic deformation technique developed in recent years to produce fine-grained structural materials. Through increasing the processing pass, further grain refinement can be achieved. In this paper, the microstructure and mechanical properties of AZ91 magnesium alloy prepared by the single-pass and two-pass FSP were studied. The results showed that the coarse, network-like eutectic β-Mg17Al12 phase was broken into particles and some of them dissolved into the magnesium matrix, and the α-Mg grains were remarkably refined after FSP. The average grain sizes of the single-pass and two-pass FSP alloys were 8.3 μm and 5.8 μm respectively. The ultimate tensile strengths of the specimens were 284.5 MPa and 319.7 MPa, and elongations were 13% and 14.5%, respectively. The improved mechanical properties of the two-pass FSP specimen were mainly attributed to the finer grain size and more homogenized microstructure.

Microstructural Evolution During Multi-Pass Friction Stir Processing of a Magnesium Alloy

A commercial magnesium alloy was processed through multi-pass and multi-directional (unidirectional, reverse, and transverse tool movements) friction stir processing (FSP). Based on the FSP location, the dominant prior-deformation basal texture was shifted along the arc of a hypothetical ellipse. The patterns of deformation texture developments were captured by viscoplastic self-consistent modeling with appropriate velocity gradients. The simulated textures, however, had two clear deficiencies. The simulations involved shear strains of 0.8 to 1.0, significantly lower than those expected in the FSP. Even at such low shear, the simulated textures were significantly stronger. Microstructural observations also revealed the presence of ultra-fine grains with relatively weak crystallographic texture. Combinations of ultra-fine grain superplasticity followed by grain coarsening were proposed as the possible mechanism for the microstructural evolution during FSP.

Particle refinement and fine-grain formation leading to enhanced mechanical behaviour in a hypo-eutectic Al–Si alloy subjected to multi-pass friction stir processing

The present research aims at studying the synergetic effect of pre-process thermal treatment and multi pass friction stir processing (FSP) on microstructure and mechanical properties of a hypo-eutectic Al–Si automotive alloy. The effect of FSP with and without pre-process thermal treatment is studied up to 3 passes and it is found that both the size of silicon particles and aluminium grains become finer with increasing FSP passes. Maximum particle and grain refinement is observed in the material subjected to 3 passes of FSP with pre-process thermal treatment. The thermal treatment prior to FSP led to spheroidization of silicon particles which influenced the particle and grain refinement during FSP to a great extent. The effect of microstructural modification by FSP on the mechanical properties has been studied by micro-hardness and tensile testing. The 3 pass FSPed material without thermal treatment showed maximum ductility of 36.5%. However, maximum tensile strength of 251MPa is obtained in the 3 pass FSPed material with pre-process thermal treatment. The various mechanisms involved in enhancing the mechanical properties are explained by means of fractography results and the microstructural evolution during multiple FSP passes.

Interfacial microstructure and properties of aluminum–magnesium AZ31B multi-pass friction stir processed composite plate

A composite plate of 1060Al and AZ31B was fabricated by multi-pass friction stir processing (MP-FSP). The microstructure and properties of the interface were investigated. Intermetallic compounds (IMCs) containing Al12Mg17 (γ) and Al3Mg2 (β) were formed in the conversion zone (CZ). The morphology of the microstructure in the CZ changed significantly among different travel speeds. Transmission electron microscopy (TEM) of the CZ revealed that the presence of Al12Mg17 (γ) grains in the CZ and the rectangular shaped Al3Mg2 phase with the dimension about 830nm length and 500nm width was detected. Shear tensile test results indicated that the failure load can reach its maximum by decreasing the travel speeds to 4mm/min. Microhardness measurements indicated that mechanical properties were affected by FSP parameters and mainly depended on the formation of IMCs in the CZ. Fracture morphology studies indicated that the presence of IMCs in the CZ controlled the failure mode. In addition, corrosion test results indicated that the corrosion resistance of the 1060Al/AZ31B clad plate was higher than the multi-pass friction stir processed AZ31B and AZ31B parent material.

Influence of Multi-Pass Friction Stir Processing on the Corrosion Behavior of an Al-Mg-Si Alloy

The effect of multi-pass (100% overlap) friction stir processing (FSP) on the NaCl-induced atmospheric corrosion behavior of an extruded 6005-T6 aluminum alloy has been studied. Samples were contaminated with 70 and 200 μg/cm2 NaCl and exposed in the presence of 400 ppm CO2 for a time interval of 200–3200 h. The results showed that increasing the number of passes gives rise to several crucial changes in the microstructure of the processed regions. Gravimetric analyses and morphological inspections of the corroded samples revealed that multi-pass FSP has also a significant impact on the NaCl-induced atmospheric corrosion behavior of the FS processed samples. While increasing the number of FSP passes resulted in a reduced extent of pitting corrosion in the stir zone, it induced a significant pitting corrosion attack in the heat affected zone of the samples, which were shown to be linked to a reduction in the size of intermetallic particles in the stir zone of the multi-pass FSP specimens. The results also showed that the cathodic activity of intermetallic particles in this family of Al alloys is size-dependent.

Preparation of Al2O3/TiO2 particle-reinforced copper through plasma spraying and friction stir processing

In this study, Al2O3/TiO2 particle-reinforced copper matrix surface composites were successfully processed via friction stir processing (FSP). A “sandwich” lap joint setup subjected to multi-pass FSP was adopted. The composite layer with a thickness of ~2200μm was achieved after triple-pass processing. The Al2O3 and TiO2 particles were evenly distributed in copper matrix and the grains were refined due to the stir and crush action of tool. The defects such as pores and interlamellar contacts in the as-sprayed layer disappeared with the particles involved into substrate. Vickers hardness test were implemented and the average hardness value achieved 448HV0.1 after triple-pass processing, which was about 4 times higher than that of the T2 copper. Shearing tensile tests were carried out and the maximum shearing force reached to 7.98kN. Real-time temperature change showed that the traveling speed has little effect on the Tm variation of stirring zone. Furthermore, the softening temperature of FSPed samples was higher than 900°C, which represents a better thermal stability.

Corrosion Property of Al-Alloy 6061-T6 Processed by Multi-pass Friction Stir Processing

The 6061- T6 aluminium alloy was modified by multi- pass friction stir process with air or water cooling. Corrosion behavior of the processed zone was investigated by immersion test and electrochemical corrosion test, as well as optical microscopy, SEM and TEM. The results show that the grain size of stirred zone was successfully refined and its corrosion resistance became better obviously after the base metal was modified by FSP with different cooling methods. Compared with the base metal, the nugget zone exhibited higher free-corrosion potential, smaller corrosion current density and greater corrosion impedance. Compared to FSP with water-cooling, the nugget zone prepared by FSP with air-cooling showed better corrosion resistance.

Influence of multi-pass friction stir processing on the microstructure and mechanical properties of Al-5083 alloy

Samples with one through three passes with 100% overlap were created using friction stir processing (FSP) in order to locally modify the microstructure and mechanical properties of a cold-rolled Al-5083 alloy. A constant traverse speed and two different rotational speeds were used for processing. The results indicated that single-pass FSP caused dynamic recrystallization in the stir zone, leading to equiaxed grains with high angle grain boundaries. The grain size increased with increasing the rotational speed. The product of ultimate tensile strength and total elongation of the stir zone was significantly enhanced compared with that of the base metal. During the annealing heat treatment, abnormal grain growth (AGG) occurred throughout the stir zone when a low rotational speed was adopted. The stir zone produced with a high rotational speed was found to be more resistant to AGG. The application of multiple process passes did not change the grain size and mechanical properties of the stir zone significantly. The expanding of AGG in the stir zone was inhibited as the number of passes was increased.

An experimental study on multi-pass friction stir processing of Al/TiN composite: some microstructural, mechanical, and wear characteristics

TiN is an exceptionally hard and a wear-resistant ceramic. In the present work, the effect of mixing TiN particulates into the Al7075-T651 alloy was studied by employing a novel material fabrication technique called friction stir processing (FSP). The FSP was carried out using three different tool geometries (namely square, triangular, and threaded taper) with an objective to fabricate the Al/TiN composite with an appropriate set of mechanical and wear properties. A number of microstructural, mechanical, and wear tests were carried out in order to characterize the composite. In comparison to the parent metal, each of the composite specimens showed improved wear and friction performance. However, the improvement in the hardness was realized only with the threaded tool and that in the tensile strength was observed with the square tool, thus revealing a tool-property relationship in FSP. Contrarily, the ductility of all of the composite specimens was lower relative to that of the parent metal. A suitable trade-off among various characteristics was realized when FSP was performed using the square tool. The microscopic and energy dispersive spectroscopy (EDS) analyses showed that the dominant wear mechanism in the composite was adhesion. This study is the first report on the FSPed Al/TiN composite.

Ductilizing of a brittle as-cast hypereutectic Al–Si alloy by friction stir processing

Multipass friction stir processing of brittle as-cast hypereutectic Al–Si alloy resulted in bimodal distribution of ultrafine silicon particles in nano-size aluminum matrix. As compared to the as-cast material, the stress–strain curves revealed simultaneous increase in strength by four times and in ductility by 14 times after three overlap multipass FSP. The transformation from brittle cleavage fracture to completely ductile fracture is confirmed by the fractographs of tensile samples taken before and after the friction stir processing.