Hot deformation behaviour of bimodal sized Al2O3/Al nanocomposites fabricated by spark plasma sintering

The deformation behaviour of bimodal sized Al₂O₃/Al nanocomposites were investigated by hot compression tests conducted in the temperature range 350–500°C and strain rates of 0.001, 0.01 and 0.1 s^–1. The dynamic recrystallisation behaviour of the nanocomposites strongly depended on the forming parameters. The bimodal sized Al₂O₃ particles played a crucial role in the recrystallised microstructure. The addition of bimodal sized Al₂O₃ particles led to a significant increase of activation energy of plastic deformation, corroborating the enhanced resistance of the nanocomposite to hot deformation. This was also reflected by the increased compressive yield strength in the nanocomposite due to both dislocation strengthening caused by n‐Al₂O₃ and preventing the grain growth due to the presence of μ‐Al₂O₃ at grain boundaries. It was found that with the decrease of Z values, local strain induced by deformation was released and the grain size of aluminium matrix gradually increased, indicating that the main softening mechanism of the bimodal sized Al₂O₃/Al nanocomposites was dynamic recrystallisation (DRX). The lower the Z value was, the easier the DRX occurred. The highly beneficial role of the bimodal sized Al₂O₃ reinforcement in improving the high‐temperature performance of aluminium matrix nanocomposite was discussed.     

Wear properties of friction stir processed AISI D2 tool steel

Influences of microstructural and textural properties of friction stir processing (FSP) on dry reciprocating wear properties of AISI D2 tool steel are investigated in this study. The mechanical improvement is attributed not only to the homogenous distribution of very small carbides in a refined matrix, but also to significant development of textures during FSP. The excellent wear resistance is ascribed to nanohardness enhancement of the FSPed steel. Dominant shear components of {111} 〈110〉 and {112} 〈111〉 with the lowest Taylor׳s factor and the high density of close-packed planes formation significantly enhance the wear resistance of FSPed sample at 500 rpm.     

The microstructure and wear behaviour of friction stir processed AISI 430 ferritic stainless steel

ABSTRACT

The microstructure and wear behavior of Friction Stir Processed (FSPed) AISI 430 ferritic stainless steel were analyzed in the present study. FSP was performed with a tool rotation and advancing speeds of 1400?rpm 16?mm/min respectively by employing a tungsten carbide tool. The FSPed microstructure consisted of a mixture of ferrite and martensite. After FSP, microhardness increased with respect to that of the as-received material. The wear resistance of the FS processed material was significantly enhanced if compared to that of the as-received substrate. According to the SEM analyses of the worn surfaces and wear debris, a combination of adhesive wear and delamination was observed in the case of the base metal. The wear mechanism shifted to mild adhesive wear after FSP. The superior wear resistance of the FS processed AISI 430 steel was attributed to the pronounced grain refinement and to martensite formation in the stir zone.     

Effect of tool pin design on the microstructural evolutions and tribological characteristics of friction stir processed structural steel

In this research, friction stir processing (FSP) technique is applied for the surface modification of ST14 structural steel. Tungsten carbide tools with cylindrical, conical, square and triangular pin designs are used for surface modification at rotational speed of 400 rpm, normal force of 5 KN and traverse speed of 100 mm min⁻¹. Mechanical and tribological properties of the processed surfaces including microhardness and wear characteristics are studied in detail. Furthermore, microstructural evolutions and worn surfaces are investigated by optical and scanning electron microscopes. Based on the achievements, all designed pins were successfully applicable for low carbon steel to produce defect-free processed material. By the microstructural changes within the stirred zone, the processed specimen is obtained higher mechanical properties. This is due to the formation of fine grains as the consequence of imposing intensive plastic deformation during FSP; however, this issue is highlighted by using square pin design. In this case, minimum grain size of 5 μm and maximum hardness of 320 VHN, as well as, maximum wear resistance are all examined for the specimen modified by square pin.     

Electron backscattered diffraction characterization technique for analysis of a Ti2AlNb intermetallic alloy

This investigation was conducted to ascertain the benefits of electropolishing after mechanical polishing for electron backscattered diffraction of a Ti₂AlNb intermetallic Ti−21Al−29Nb (at.%) alloy containing the orthorhombic (O) and body-centered-cubic (BCC) phases. Electropolishing was performed at −40 °C in 6% H₂SO₄ methanol solution. Atomic force microscopy was used to measure the surface topography in attempt to correlate nano-scale surface roughness with electron backscatter diffraction pattern quality. The results suggest that mechanically polishing with colloidal silica (SiO₂) or alumina followed by electropolishing is a sufficient surface preparatory technique for producing quality electron backscattered diffraction patterns for O + BCC microstructures. However, poor pattern quality results after mechanically polishing without electropolishing. High-quality orientation maps for O-dominated O + BCC microstructures were only possible through mechanical polishing followed by electropolishing. The data also suggest that surface roughness, on the order of 50 nm, has less effect on pattern quality than subsurface deformation. Overall, removing the near-surface damage was more critical than reduction of topography.     

Numerical analysis of static performance comparison of friction stir welded versus riveted 2024-T3 aluminum alloy stiffened panels

Most researches on the static performance of stiffened panel joined by friction stir welding（FSW） mainly focus on the compression stability rather than shear stability. To evaluate the potential of FSW as a replacement for traditional rivet fastening for stiffened panel assembly in aviation application, finite element method（FEM） is applied to compare compression and shear stability performances of FSW stiffened panels with stability performances of riveted stiffened panels. FEMs of 2024-T3 aluminum alloy FSW and riveted stiffened panels are developed and nonlinear static analysis method is applied to obtain buckling pattern, buckling load and load carrying capability of each panel model. The accuracy of each FEM of FSW stiffened panel is evaluated by stability experiment of FSW stiffened panel specimens with identical geometry and boundary condition and the accuracy of each FEM of riveted stiffened panel is evaluated by semi-empirical calculation formulas. It is found that FEMs without considering weld-induced initial imperfections notably overestimate the static strengths of FSW stiffened panels. FEM results show that, buckling patterns of both FSW and riveted compression stiffened panels represent local buckling of plate between stiffeners. The initial buckling waves of FSW stiffened panel emerge uniformly in each plate between stiffeners while those of riveted panel mainly emerge in the mid-plate. Buckling patterns of both FSW and riveted shear stiffened panels represent local buckling of plate close to the loading corner. FEM results indicate that, shear buckling of FSW stiffened panel is less sensitive to the initial imperfections than compression buckling. Load carrying capability of FSW stiffened panel is less sensitive to the initial imperfections than initial buckling. It can be concluded that buckling loads of FSW panels are a bit lower than those of riveted panels whereas carrying capabilities of FSW panels are almost equivalent to those of riveted panels with identical geometries. Finite elem