Investigation of Microstructural and Tribological Properties of AA6082-SiC/n-Al2O3 Hybrid Surface Composites Produced by the Friction Stir Process

This study focuses on the fabrication of AA7475/B4C/Al2O3 hybrid surface composites (HSCs) using multi-pass friction stir processing (FSP) to enhance their microstructure and mechanical properties. The study utilized four numbers of FSP passes with constant tool rotational speed (1300 rev/min), feed rate (45 mm/min), and reinforcement ratio (50% B4C and 50% Al2O3) and investigated the ultimate tensile stress (UTS), tensile elongation (El), and hardness distribution of HSCs. The study identified that multi-pass FSP resulted in significant grain refinement, fragmentation of reinforced particles, and uniform distribution throughout the stir region of the HSCs. For a single-pass FSP, the average grain size at the stir region was 6.5 ± 0.20 µm, whereas for the four-pass process, it decreased to 2.2 ± 0.18 µm. The XRD results revealed that as the number of passes increased, the peak intensity of B4C and Al2O3 phases was consistently reduced, which suggests fragmentation of nanoparticles. The UTS recorded for 1 pass, 2 passes, 3 passes, and 4 passes were 447 MPa, 464 MPa, 481 MPa, and 501 MPa, respectively. The hardness measurements at the SZ were recorded as 162, 174, 183, and 192 HV for 1, 2, 3, and 4 FSP passes, respectively. After reinforcing B4C and Al2O3, AA7475 showed percentage improvements in UTS, El, and hardness of 22.79%, 69.01%, and 38.12%, respectively. Field emission electron microscopy (FESEM) image of tensile fractured surfaces exhibit honeycomb-like coarser dimples, resulting in brittle-ductile mode of fracture for one-pass FSP, while HSCs produced through four-pass FSP display finer dimples, demonstrating ductile fracture mode.

The metal matrix composites combine the metallic properties of a tough and ductile matrix with properties of reinforcement particles, simultaneously develop the functional properties by proper selection of reinforcements for projected applications. However, hard ceramics reinforcements decrease toughness and ductility of soft matrix and restrict their wide applications. The surface metal matrix composites (SMMCs) preserve the matrix properties with added advanced surface properties by reinforcing particles only in the surface layer. The hybrid surface metal matrix composites (HSMMCs) with more than one reinforcement gained attention in material processing due to their noble tribological behavior and surface properties, which cannot be attained in mono composites. Conventional liquid-phase processing techniques to fabricate hybrid surface composites result in the formation of undesirable brittle compounds, detrimental to desirable properties of composites. Friction stir processing (FSP), a solid-state processing technique, has been used by many investigators using different reinforcements to fabricate mono as well as hybrid surface composites. Friction stir processed (FSPed) hybrid surface composites have not been extensively reviewed. The current review provides a comprehensive understanding of the latest developments of FSP in hybrid surface composites manufacturing. This paper review different reinforcement strategies in the fabrication of FSPed hybrid surface composites and also the effects of single-pass, multipass, and change in pass direction on microstructure and resultant properties. Finally, future directions and challenges to FSPed hybrid surface composites are summarized. This review article containing important information on hybrid surface composites fabrication by FSP will be useful to academicians and investigators in the field.

The surface metal matrix composites of AZ31 Mg alloy reinforced with multiwall carbon nanotubes (MWCNTs) have been fabricated through the friction stir processing by a conventional and two stepped tools. The microstructure and mechanical properties of fabricated composites were studied by optical and electron microscopy, microhardness and tensile tests, respectively. The processing has developed a fine-grain structure along with good distribution of reinforcements. The hardness and tensile strength of fabricated MWCNT/AZ31 composites are generally higher than as-received and FSPed samples. The accumulative effect of grain refinement and reinforcing nanotubes is assumed to be the reason for increasing the ductility after friction stir processing. The hardness is nearly doubled for FSPed samples and some more for nanocomposites compared with the as-received sample. The elongation of nanocomposites is about two times greater than that of the as-rolled sample. The speed ratio, pass number and CNT amount are three important factors influencing the resulting microstructure and mechanical properties. The stepped tools also give a more uniform distribution of reinforcement and higher grain refinement.

Increasing demand of lightweight structures with exceptional properties elicits materials processing and manufacturing technologies to tailor blanks in order to achieve or enhance those prerequisite properties. Friction stir processing (FSP) is a solid-state material processing technique, which was derived from friction stir welding (FSW). Initially, FSP was invented to refine the microstructure in way that superplasticity in a material can be achieved. Afterward, FSP has gained much more attraction as a solid-state grain refinement technique to improve the mechanical, tribological, and corrosion properties in a wide range of low strength non-ferrous and high strength steels. FSP is well capable to produce material with microstructure in range of few micron to nanoscale, depending on the processing conditions. Researchers have investigated FSP at different process parameters such as tool rotation and travel speeds, number of passes, and additional cooling in order to evaluate the impact on the resulting properties for different alloys. Recently, FSP has begun to modify the microstructure and properties in hard alloys and superalloys with some modifications in FSP tooling system. Furthermore, FSP has shown great potential to repair or modify the weld or coating structure by microstructure refinement. Therefore, the present review will discuss the state-of-the-art of FSP under the main categories of microstructure evolution, and effect of process parameters. This review also provides a comprehensive summary of research progress on FSP in different materials i.e. aluminum, magnesium, copper, and steels with the contents much emphasized on the microstructure refinement in terms of average grain size and resulting properties like hardness, tensile, wear, and corrosion. Finally, FSP as a new post-processing approach in weld or coating structure has been discussed.

Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

Investigation of Microstructure and Hardness of Mg/TiC Surface Composite Fabricated by Friction Stir Processing (FSP)

Surface composites by friction stir processing: A review

8 - Tribological aspects in friction stir welding and processing

Effect of friction stir processing on microstructural evolution and mechanical properties of nanosized SiC reinforced AA5083 nanocomposites developed by stir casting

Mg-NiTi-based metal matrix composites are appropriate solutions for the two most important goals of material engineers in the present day, i.e., imparting functional behaviour and the light weighting of metallic structures. In recent years, due to its solid-state nature, the development of Mg-based metal matrix composites has largely benefited from friction stir processing. Despite the great effort of researchers in the domain of friction stir welding and processing, finding optimum process parameters for efficient material mixing and consolidation remains a rigorous and exhaustive challenge. Tool offset variation has been seen to aid the integrity and strength of friction stir welds; however, its effect upon the stir zone structure, material flow, particle distribution, and defect formation has not been investigated for friction stir processing. Therefore, the authors employed Mg as the base metal and NiTi shape memory alloy as the reinforcement to the targeted metal matrix composite. The tool offset was linearly varied by tilting the slotted length with respect to the traverse direction. Friction stir processing performed at a rotational speed of 560 rpm and traverse speed of 80 mm/min revealed crucial changes in defect morphology and area, which has been explicated with the quantified variation in tool offset from the advancing side to the retreating side. For the positive offset conditions, i.e., tool offset towards the advancing side, the shape of the tunnelling defect was chiefly convex from the outward direction. Meanwhile, for the negative offset conditions, i.e., tool offset towards the retreating side, the tunnelling defect exhibited a concave outward shape. A transition from rectangular to triangular morphology was also observed as the tool moved from an offset of 1.75 mm in the advancing side to 1.75 mm in the retreating side.

In this study, as-cast Mg-6 wt % Sn alloy is subjected to one-pass and two-pass friction stir processing (FSP). The effect of processing pass on microstructure and mechanical properties of FSP Mg-6Sn alloy is investigated. It is found that one-pass FSP leads to the breakage and partial dissolution of the Mg2Sn phase in the stir zone (SZ) and two-pass FSP leads to the further dissolution and dynamic precipitation of the Mg2Sn phase. Dynamic recrystallization (DRX) takes place in the SZ of an Mg-6Sn alloy undergoing FSP. Compared to one-pass FSP, two-pass FSP brings about further grain refinement in the SZ. A strong {0001} basal texture is developed in the SZ of a Mg-6Sn alloy from FSP and the change of the sample region or processing pass has little influence on the texture. Compared to an as-cast Mg-6Sn sample, one-pass FSP brings about significant improvement in mechanical properties. Two-pass FSP leads to the further increase in yield strength (YS) and ultimate tensile strength (UTS) but elongation (EL) is reduced. The continuous increase in strength is attributed to the grain refinement and the dissolution and dynamic precipitation of Mg2Sn phase achieved by FSP.

The current investigation is aimed to give a thorough explanation of metal matrix composite fabrication by taking different combinations of the reinforcements by using the principles of friction stir technique and impact of the input processing parameters on mechanical properties done by the different researchers. The researchers attempted to actuate a relation between the input process parameters and their output responses. Friction stir welding and processing method uses significantly less energy. For many engineering applications the metal matrix composites replaces the regular used materials because of its unique mechanical and metallurgical properties, stability, durability, resistance to corrosion. The process parameters includes rotational speed, tilt angle, feed and deposition rate have major impact on mechanical properties of fabricated composite surfaces by the friction stir process. The solid state nature of processing method is brought out the enhancement in different properties in surface composites. The pin profile is also having major impact on mechanical properties. The different pin profile generally used are triangular, cylindrical, hexagonal and pentagon. The present study can give a concept of defect free weld having higher and improved mechanical properties on surface composites.

Friction Stir Processing (FSP) has emerged as an excellent processing approach to tailor the microstructural and other properties of cast alloys and composite materials. In the current investigation, a stir-cast magnesium metal matrix composite (MMMC) reinforced with 12 wt% B4C particles was processed using single-pass FSP. FSP was carried out with a simple cylindrical tool pin using different tool rotational speeds of 800, 1000, 1200 rpm and transverse speed of 60 mm/min. Severe deformation occurred during FSP which helped to refine the distribution of the B4C particles and grain refinement of as-cast composites. Microstructural examination showed an appreciable grain refinement from 98 to 5μm because of dynamic recrystallization during FSP. The maximum microhardness of the friction stir processed (FSPed) composites increased from 73 Hv to 105.4 Hv. Pin-on-disk wear tests were conducted under dry sliding conditions in the load range of 1 kg to 5 kg at a 1 m/s sliding velocity. The wear results indicated an enhancement of wear properties of FSPed composites in comparison to as-cast magnesium metal matrix composites. The improved hardness and wear resistance results were pertaining to the remarkable refinement in grain size coupled with refined B4C particulates and uniform distribution. The best results for hardness and wear resistance of the FSPed composite were obtained for a sample processed at a tool rotation speed of 1000 rpm. SEM/EDS of the worn samples was carried out. The wear mechanism was found as slight abrasion and oxidation. After single pass FSP, the surface of Mg/12wt% B4C cast composite showed remarkable changes in worn surface morphology.

Friction Stir Processing (FSP) has emerged as an excellent processing approach to tailor the microstructural and other properties of cast alloys and composite materials. In the current investigation, a stir-cast magnesium metal matrix composite (MMMC) reinforced with 12 wt% B4C particles was processed using single-pass FSP. FSP was carried out with a simple cylindrical tool pin using different tool rotational speeds of 800, 1000, 1200 rpm and transverse speed of 60 mm min−1. Severe deformation occurred during FSP which helped to refine the distribution of the B4C particles and grain refinement of as-cast composites. Microstructural examination showed an appreciable grain refinement from 98 to 5 μm because of dynamic recrystallization during FSP. The maximum microhardness of the friction stir processed (FSPed) composites increased from 73 Hv to 105.4 Hv. Pin-on-disk wear tests were conducted under dry sliding conditions in the load range of 1 kg to 5 kg at a 1 m s−1 sliding velocity. The wear results indicated an enhancement of wear properties of FSPed composites in comparison to as-cast magnesium metal matrix composites. The improved hardness and wear resistance results were pertaining to the remarkable refinement in grain size coupled with refined B4C particulates and uniform distribution. The best results for hardness and wear resistance of the FSPed composite were obtained for a sample processed at a tool rotation speed of 1000 rpm. SEM/EDS of the worn samples was carried out. The wear mechanism was found as slight abrasion and oxidation. After single pass FSP, the surface of Mg/12 wt% B4C cast composite showed remarkable changes in worn surface morphology.

Mechanical surface hardening processes have long been of interest to science and technology. Today, surface modification technologies have reached a new level. One of them is friction stir processing that refines the grain structure of the material to a submicrocrystalline state. Previously, the severe plastic deformation occurring during processing was mainly described from the standpoint of temperature and deformation, because the process is primarily thermomechanical. Modeling of friction stir welding and processing predicted well the heat generation in a quasi-liquid medium. However, the friction stir process takes place in the solid phase, and therefore the mass transfer issues remained unresolved. The present work develops the concept of adhesive-cohesive mass transfer during which the rotating tool entrains the material due to adhesion, builds up a transfer layer due to cohesion, and then leaves it behind. Thus, the transfer layer thickness is a clear criterion for the mass transfer effectiveness. Here we investigate the effect of the load on the transfer layer and analyze it from the viewpoint of the friction coefficient and heat generation. It is shown that the transfer layer thickness increases with increasing load, reaches a maximum, and then decreases. In so doing, the average moment on the tool and the temperature constantly grow, while the friction coefficient decreases. This means that the mass transfer cannot be fully described in terms of temperature and strain. The given load dependence of the transfer layer thickness is explained by an increase in the cohesion forces with increasing load, and then by a decrease in cohesion due to material overheating. The maximum transfer layer thickness is equal to the feed to rotation rate ratio and is observed at the axial load that causes a stress close to the yield point of the material. Additional plasticization of the material resulting from the acoustoplastic effect induced by ultrasonic treatment slightly reduces the transfer layer thickness, but has almost no effect on the moment, friction coefficient, and temperature. The surface roughness of the processed material is found to have a similar load dependence.