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In situ precipitation of primary Si (SiP) particles within the microstructure of Si-containing Zn–22Al alloys, also known as ZA22/SiPcomposites, can improve their hardness and mechanical strength, provided that the size, morphology, and distribution of SiPparticles are optimized. In this study, Ti–B treatment was employed to modify the microstructure and improve the mechanical properties of ZA22–xSi (x?=?4 and 8 wt%) composite. According to the results, Ti–B treatment eliminated the primary a-Al dendrites, reduced the volume fraction of remaining a-Al phase within the microstructure, reduced the size of SiPparticles, and improved their distribution within the matrix. Moreover, the size of SiPparticles is reduced from 25.2?±?9.5 to 30.1?±?4.4 µm in non-modified samples containing 4 and 8 wt% Si to 14.7?±?6.3 and 19.6?±?5.3 µm in the samples modified by Ti–B, respectively. Ti–B addition also substantially refined and homogenized the distribution of the other second phases and porosities. The mechanical properties analyses revealed that, at a given Si concentration, the hardness of the alloys modified by Ti–B was relatively less than that of the non-modified ones. Moreover, the tensile strength and fracture strain of modified ZA22–4Si and ZA22–8Si composites are increased by about 15 and 59, and 60 and 82%, respectively.

The effect of Zn–40Al–2Cu–xSi (x?=?4 and 8 wt-%) tungsten inert gas (TIG) surfaced coatings and multi-pass friction stir processing (FSP) on the tribological properties of Zn–40Al–2Cu alloy was studied. The coatings had a well-modified microstructure comprising of finely dispersed primary Si particles (SiP). These particles act as in-situ reinforcements, improving coating resistance against the friction-induced strains applied during sliding wear. The results of dry sliding wear showed that, at the applied pressures of 0.25, 0.5, and 0.75 MPa, the wear resistance of 4-pass FSPed coatings containing 4 wt-% Si is higher than that of the base alloy by 27, 47, and 57%, respectively. This can be attributed to the substrate strengthening and formation of compacted tribolayers on the surface of FSPed samples.

ABSTRACTThe effect of Zn–40Al–2Cu–xSi (x = 4 and 8 wt-%) tungsten inert gas (TIG) surfaced coatings and multi-pass friction stir processing (FSP) on the tribological properties of Zn–40Al–2Cu alloy was studied. The coatings had a well-modified microstructure comprising of finely dispersed primary Si particles (SiP). These particles act as in-situ reinforcements, improving coating resistance against the friction-induced strains applied during sliding wear. The results of dry sliding wear showed that, at the applied pressures of 0.25, 0.5, and 0.75 MPa, the wear resistance of 4-pass FSPed coatings containing 4 wt-% Si is higher than that of the base alloy by 27, 47, and 57%, respectively. This can be attributed to the substrate strengthening and formation of compacted tribolayers on the surface of FSPed samples.

This work discusses tribological properties of commercial pure (CP) titanium processed by multi-directional forging (MDF) up to six passes at room temperature and 220 °C. For this purpose, wear test was conducted by dry sliding pin-on-disk method on the initial and ultrafine grained samples using different stress magnitudes of 1, 1.5 and 2 MPa. The results showed that wear resistance of CP titanium increases after the first pass of MDF in comparison with the initial condition, irrespective of the applied normal stress. For example, the average wear rate of MDFed samples was decreased about 30% and 24%, after first pass at room temperature and 220 °C, respectively. However, average wear rate of the samples processed by six MDF passes was reduced about 40% at lower normal loads; it was increased about 9% at higher ones as compared to the initial condition. It was also found that the dominated wear mechanisms were abrasive and delaminated at the lower stresses, while the delamination mechanism was intensified and a slight adhesion was observed during the higher applied normal loads.

Equal channel angular pressing is an effective technique to control the texture and microstructure of metals and alloys. Texture and microstructure of an Al-7075 alloy subjected to repetitive equal channel angular pressing through a 90° die were evaluated by X-ray diffractometer and orientation imaging microscopy. It is observed that processing through different routes leads to different types of textures, in both qualitative and quantitative senses. The texture calculation by Labotex software reveals that texture strengthens after the first pass and weakens by progressing ECAP process up to 4 passes. Microstructure investigations show that after 4 passes of equal channel angular pressing via routes BCand A, very fine grains with average grain size of about 700 nm and 1 μm appear, respectively, and most of the grains evolve into arrays of high angle boundaries. The effects of covering the Al-7075 billets with copper tube on texture and microstructure were also studied.

The presence of retained austenite gives rise to deterioration of the wear resistance and fracture strength of Cr-Mo steels in many cases. Thus, the effects of heat treatments including direct quenching, martempering, and austempering on the retained austenite existing in the microstructure of these steels were investigated. Specimens were austenized at 950 °C followed by direct quenching using compressed and still air. The specimens were also isothermally quenched in salt bath at 200 and 300 °C for 2, 8, 30, and 120 min. Microstructures of the specimens were studied using optical microscope (traditional black and white etching as well as color etching), scanning electron microscope (SEM), microhardness tester, and X-ray diffraction (XRD). The results showed that the lowest amount of retained austenite in the microstructure was obtained in the specimens quenched isothermally at 300 °C for 120 min.

The combined effect of extrusion and multi-directional forging (MDF) was investigated on microstructure and mechanical properties of aluminum-based composite with 10, 15, and 20 wt% Mg2Si. In the casted Al–Mg2Si composites, the primary and eutectic Mg2Si particles are generally coarse which lead to decreasing their mechanical properties and formability. Extrusion process was utilized to overcome this shortcoming by breakage of the eutectic structure, reduction of Mg2Si size, and the decrease of casting defects. Then, MDF process was applied up to failure on the extruded composites at room temperature. It led to the morphological modification of primary and eutectic Mg2Si phases and the reduction of their size. It was found that the MDF process resulted in a considerable improvement in hardness and shear strength of materials. This may be related to the reduction in the average size of Mg2Si particles with their uniform distribution. In addition, ultimate shear strength is, respectively, increased from 94, 99, and 81 MPa to 119, 116, and 117 MPa for the 10, 15, and 20 wt% Mg2Si aluminum composites after the final pass of MDF. Meanwhile, the normal displacement of composites is reduced at initial passes and increased by the addition of more pass numbers.