Recrystallization Phenomena During Friction Stir Processing of Hypereutectic Aluminum-Silicon Alloy

Microstructural evolution and related dynamic recrystallization phenomena were investigated in overlapping multipass friction stir processing (FSP) of hypereutectic Al-30 pct Si alloy. FSP resulted in the elimination of porosities along with the refinement of primary silicon particles and alpha aluminum grains. These alpha aluminum grains predominantly exhibit high angle boundaries with various degrees of recovered substructure and dislocation densities. The substructure and grain formation during FSP take place primarily by annihilation and reorganization of dislocations in the grain interior and at low angle grain boundary. During multipass overlap FSP, small second phase particles were observed to form, which are accountable for pinning the grain boundaries and thus restricting their growth. During the multipass overlap FSP, the microstructure undergoes continuous dynamic recrystallization by formation of the subgrain boundary and subgrain growth to the grain structure comprising of mostly high angle grain boundaries.     

Peak Stir Zone Temperatures during Friction Stir Processing

The stir zone (SZ) temperature cycle was measured during the friction stir processing (FSP) of NiAl bronze plates. The FSP was conducted using a tool design with a smooth concave shoulder and a 12.7-mm step-spiral pin. Temperature sensing was accomplished using sheathed thermocouples embedded in the tool path within the plates, while simultaneous optical pyrometry measurements of surface temperatures were also obtained. Peak SZ temperatures were 990 °C to 1015 °C (0.90 to 0.97 T _Melt) and were not affected by preheating to 400 °C, although the dwell time above 900 °C was increased by the preheating. Thermocouple data suggested little variation in peak temperature across the SZ, although thermocouples initially located on the advancing sides and at the centerlines of the tool traverses were displaced to the retreating sides, precluding direct assessment of the temperature variation across the SZ. Microstructure-based estimates of local peak SZ temperatures have been made on these and on other similarly processed materials. Altogether, the peak-temperature determinations from these different measurement techniques are in close agreement.     

Microstructure Evolution During Friction Stir Processing and Hot Torsion Simulation of Ti-6Al-4V

Friction stir processing of three variants of Ti-6Al-4V was conducted at processing temperatures both above and below the β-transus. The base metal substrates that were processed included wrought base metal in the α/β-processed and β-processed condition and weld overlay that was deposited using the gas tungsten arc welding process. Friction stir processing below the β-transus for the α/β-processed condition and the weld overlay produced fully equiaxed-α grains with submicron grain size, while in the β-processed condition, elongated equiaxed-α grains and regions of transformed-β with grain size in the 1 to 2 μm range were observed. Friction stir processing above the β-transus was microstructurally evident by a stir zone composed of 10 to 40 μm recrystallized β-grains with either a basket weave or colony structure and a continuous network of α at the grain boundary. Path and normal forces were recorded for in situ processing of Ti-6Al-4V in all three initial conditions. Comparatively, above-transus processing reduced the path force at the tool-to-workpiece interface, while processing below the β-transus caused the path force to increase by ~300 pct. Based on the dimensionless heat input, it appears that the stir zone microstructure is more dependent on spindle speed (RPM) than travel speed and that the heat input parameter is not a good indicator of the processing temperature. Hot torsion testing of α/β-processed Ti-6Al-4V was used as a method for physically simulating the stir zone microstructure produced from friction stir processing. At a strain rate of 2.5 s^?1 (250 RPM rotation rate), the transition from equiaxed-α to a transformed beta microstructure occurred at approximately 1223 K (950 °C). A comparison of FSP and hot torsion microstructures revealed nearly identical matching depending on the selection of hot torsion conditions.     

Experimental Investigation of Magnesium-Base Nanocomposite Produced by Friction Stir Processing: Effects of Particle Types and Number of Friction Stir Processing Passes

In this research, nanosized SiC and Al₂O₃ particles were added to as-cast AZ91 magnesium alloy, and surface nanocomposite layers with ultrafine-grained structure were produced via friction stir processing (FSP). Effects of reinforcing particle types and FSP pass number on the powder distribution pattern, microstructure, microhardness, and on tensile and wear properties of the developed surfaces were investigated. Results show that the created nanocomposite layer by SiC particles exhibits a microstructure with smaller grains and higher hardness, strength, and elongation compared to the layer by Al₂O₃ particles. SiC particles do not stick together and are distributed separately in the AZ91 matrix; however, distribution of SiC particles is not uniform in all parts of the stirred zone (SZ), which causes heterogeneity in microstructure, hardness, and wear mechanism of the layer. Al₂O₃ particles are agglomerated in the different points of matrix and create alumina clusters. However, distribution of Al₂O₃ clusters in all parts of the SZ is uniform and results in a uniform microstructure. In the specimen produced by one-pass FSP and SiC particles, the wear mechanism changes in different zones of SZ due to the nonuniform distribution of particles. However, in the specimen produced by Al₂O₃ particles, the wear mechanism in all parts of the SZ is the same and, in addition to the abrasive wear, delamination also occurs. Increasing FSP pass number results in improved distribution of particles, finer grains, and higher hardness, strength, elongation, and wear resistance.     

Interfacial Reaction during Friction Stir Welding of Al and Cu

Commercially pure copper was joined to a 1050 aluminum alloy by friction stir welding. A specific configuration where the tool pin was fully located in the aluminum plate was chosen. In such a situation, there is no mechanical mixing between the two materials, but frictional heating gives rise to a significant thermally activated interdiffusion at the copper/aluminum interface. This gives rise to the formation of defect-free joints where the bonding is achieved by a very thin intermetallic layer at the Cu/Al interface. Nanoscaled grains within this bonding layer were characterized using transmission electron microscopy (TEM). Two phases were identified, namely, Al₂Cu and Al₄Cu₉ phases. The nucleation and growth of these two phases are discussed and compared to the standard reactive interdiffusion reactions between Cu and Al.     

Physical Simulation of Deformation and Microstructure Evolution During Friction Stir Processing of Ti-6Al-4V Alloy

The feasibility of using high-strain rate (1.475 to 3.942 s^?1) hot-torsion testing with a Gleeble^® thermomechanical simulator was demonstrated for simulating microstructures consistent with friction stir processing (FSP) of Ti-6Al-4V. The tests were performed on α/β-processed base material at temperatures both above and below the β-transus. Various phenomena including the refinement of α- and β-grains, deformation-induced heating, and deformation instabilities were observed. These tests reproduced the range of microstructures that are observed under FSP processing conditions. The testing methodology can be used for generating constitutive material property equations relevant to computational FSP/friction stir welding models.     

The Effect of Concurrent Straining on Phase Transformations in NiAl Bronze During the Friction Stir Processing Thermomechanical Cycle

Equivalent strains up to a value of ≈2.7 were determined by evaluation of the shape changes of the phases in a duplex α(fcc)/β(bcc) microstructure formed ahead of the pin tool extraction site during the friction stir processing (FSP) thermomechanical cycle in a cast NiAl bronze alloy. Correlation of the local strains with volume fractions of the various microstructure constituents in this alloy shows that the concurrent straining of FSP results in acceleration of the α + β → β reaction in the thermomechanically affected zone (TMAZ) ahead of the pin extraction site. The resulting volume fraction of β (as determined by the volume fraction of its transformation products formed during post-FSP cooling) corresponds closely to the volume fraction expected for the peak stir zone temperature measured separately by means of thermocouples embedded within the tool pin profile along the tool path. The stir zone (SZ) in this material exhibits near-equilibrium microstructures despite brief dwells near the peak temperature (T _peak ≈ 0.95T _melt), reflecting large local strains and strain rates associated with this process.     

Influence of Friction Stir Processing Parameters on the Microstructure of Aluminum Foams

In the present study, friction stir processing (FSP) has been used to fabricate aluminium foams. The effects of the number of FSP passes, FSP tool rotational speed, foaming time and temperature on the porosity have been investigated. Aluminium foam with porosity up to 40% was successfully fabricated. In the samples foamed at 923 K (650 °C), a few irregular pores were produced as a result of high aluminium matrix stiffness in this temperature. In general with increase in foaming temperature the porosity increased. However, in the samples foamed for 30 or 60 min, lower porosity was detected at higher foaming temperature. Also, in the samples which were produced with more FSP passes, the foaming time decreased and more uniform pore structure was obtained.     

Optimizing Powder Distribution in Production of Surface Nano-Composite via Friction Stir Processing

Notwithstanding the extensive interest in using friction stir processing (FSP) for producing metal matrix composite (MMC), more uniform powder distribution along the composite zone is still needed. In most studies, one groove is machined out of the specimen, filled with powder, and then processed by identical passes. In this investigation, an innovative technique was used that involved machining out of three gradient grooves with increasing depth from the advancing side to the retreating side instead of using a conventional sample with just a groove. Macro, optical, and scanning electron microscopy (SEM) images and microhardness test were used to evaluate the powder distribution. The images indicated that the most uniform distribution of SiC particles in the whole composite zone was related to a three-gradient grooves sample. Microohardness measurement of a three-gradient grooves sample, carried out along the cross section and perpendicular to the traverse direction of FSP, experiences less fluctuation in hardness compared with other techniques.     

Grain Structure Development During Friction Stir Welding of Single-Crystal Austenitic Stainless Steel

The high-resolution electron backscatter diffraction (EBSD) technique was used to study the grain boundary development and texture evolution during friction stir welding (FSW) in a single-crystal austenitic stainless steel. Strain-induced crystal rotations were found to be induced by simple shear deformation. With the crystal rotations, the single-crystal structure was broken up into a fine-grained polycrystalline aggregate in the stir zone. This process was deduced to be governed by continuous and discontinuous recrystallizations operating during the FSW process. The final texture which evolved in the stir zone was dominated by $ A/\bar{A}\left\{ {111} \right\} \, \langle 110 \rangle $ ideal simple shear orientations.     

Friction Stir Processing for Enhancement of Wear Resistance of ZM21 Magnesium Alloy

Present work pertains to surface modification of the magnesium alloy using friction stir processing (FSP). Silicon carbide and boron carbide powders are used in the friction stir processing of the ZM21 Magnesium alloy. Coating was formed by FSP of the alloy by placing the carbide powders into the holes made on the surface. Surface coating was characterized by metallography, hardness and pin-on-disc testing. Friction stir processed coating exhibited excellent wear resistance and is attributed to grain boundary pinning and dispersion hardening caused by carbide particles. Surface composite coating with boron carbide was found to possess better wear resistance than coating made with silicon carbide. This may be attributed to formation of very hard layer coating of boron carbide reinforced composite on the surface of magnesium alloy. In the present work an attempt has also been made to compare the wear behaviour of surface composite layer on ZM21 Mg alloy with that of conventionally used engineering materials such as mild steel and austenitic stainless steel. Wear data clearly shows that wear resistance of friction stir processed composite layer is better than that of mild steel and stainless steel. This work demonstrates that friction stir processing is an effective strategy for enhancement of wear resistance of magnesium alloys.     

Friction Stir Processing of Aerospace Aluminum Alloy by Addition of Carbon Nano Tube

Aluminum 7075 is an aerospace alloy that has high strength to weight ratio and used most commonly in aeronautic structures. However, low surface properties such as poor wear resistance and surface hardness are the main weaknesses that limit its application in other areas of manufacturing. In the present work an attempt was made to fabricate aluminum based surface nano-composite reinforced with carbon nano tube (CNT) by means of single pass friction stir processing. Firstly, Microstructural evolution, tensile properties, hardness, wear rate and friction coefficient of fabricated surface composite was compared with pure friction stir processed metal and base material. Hereafter, parametric study based on response surface methodology was carried out to find the effect of tool rotary speed, feed rate and amount of MWCNT on tensile strength and wear rate. Optimization based on desirability approach function was also performed to find optimal parameter setting achieving maximum strength and minimum wear rate, simultaneously. The results revealed that the CNT particles significantly homogenized the microstructure of the composite, enhanced tensile properties and hardness and reduced the wear rate and friction coefficient in sliding test. By performing optimization through RSM, it was found that selection of 1250 RPM tool rotary speed, 40 mm/min feed rate and 0.6 g CNT weight caused 20% improvement in tensile strength and wear rate of fabricated composite when compared with base material.     

Microstructural Evolution During Friction Stir Welding of Mild Steel and Ni-Based Alloy 625

Microstructure evolution during friction stir welding (FSW) of mild steel and Ni-based alloy 625 was studied. Regarding the Ni-based alloy, the welding process led to grain refinement caused by discontinuous and continuous dynamic recrystallization, where bulging of the pre-existing grains and subgrain rotation were the primary mechanisms of recrystallization. In the steel, discontinuous dynamic recrystallization was identified as the recovery process experienced by the austenite. Simple shear textures were observed in the regions affected by the deformation of both materials. Although the allotropic transformation obscured the deformation history, the thermo-mechanically affected zone was identified in the steel by simple shear texture components. A new methodology for the study of texture evolution based on rotations of the slip systems using pole figures is presented as an approximation to describe the texture evolution in FSW.     

Wire-Based Friction Stir Processing as a Novel Pathway for Solid-State Surface Alloying of Magnesium

Wire-based friction stir processing is introduced as a solid-state surface alloying strategy for surface alloying of AZ31 magnesium alloy with aluminum, as a key alloying element in magnesium alloys. This technique enables the formation of a defect-free, grain refined and alloyed surface with the increased volume fraction of Mg-Al second phase, and thus, enhanced surface hardness. This simple technique provides a solid-state surface alloying pathway to improve the surface properties of the metallic materials.