Effect of laser shot peening on precipitation hardened aluminum alloy 6061-T6 using low energy laser

Mechanical properties of engineering material can be improved by introducing compressive residual stress on the material surface and refinement of their microstructure. Variety of mechanical process such as shot peening, water jet peening, ultrasonic peening, laser shot peening were developed in the last decades on this contrast. Among these, lasers shot peening emerged as a novel industrial treatment to improve the crack resistance of turbine blades and the stress corrosion cracking (SCC) of austenic stainless steel in power plants. In this study we successfully performed laser shot peening on precipitation hardened aluminum alloy 6061-T6 with low energy (300 mJ, 1064 nm) Nd:YAG laser using different pulse densities of 22 pulses/mm² and 32 pulses/mm². Residual stress evaluation based on X-ray diffraction sin² ψ method indicates a maximum of 190% percentage increase on surface compressive stress. Depth profile of micro-hardness shows the impact of laser generated shock wave up to 1.2 mm from the surface. Apart from that, the crystalline size and micro-strain on the laser shot peened surfaces have been investigated and compared with the unpeened surface using X-ray diffraction in conjunction with line broadening analysis through the Williamson-Hall plot.     

Enhancement of electromagnetic and microwave absorbing properties of gas atomized Fe-50 wt%Ni alloy by shape modification

In order to increase the electromagnetic parameters and improve the microwave absorbing properties in the range of 1–4 GHz, gas atomized Fe-50 wt%Ni alloys with spherical form were processed in a planetary mill. The morphology, phase composition and saturation magnetization of the FeNi alloy particles were investigated by means of scanning electron microscopy, X-ray diffraction and vibrating sample magnetometer. The complex permittivity, complex permeability and reflection loss of the microwave absorbing material made from Ethylene–Propylene–Diene Monomer rubber, and the Fe-50 wt%Ni alloys were also studied using vector network analyzer and transmission line theory. The results show that the shape of the atomized Fe-50 wt%Ni powders can be modified by mechanical milling. The flaky Fe-50 wt% Ni particles were prepared, and the aspect ratio increases with increasing the milling time from 10 to 30 h. Mechanical milling does not change the phase compositions of the FeNi alloys but decreases the peak intensity and broadens the peak width. The saturation magnetization decreases and the coercivity increases as the milling time increases. The electromagnetic parameters and microwave absorbing properties are enhanced with the increase of the aspect ratio. The rubber absorbers filled with flaky Fe-50 wt%Ni powders milled for 30 h exhibit the low reflection loss in the 1–4 GHz frequency range.     

Interactive contribution of grain size and grain orientation to coercivity of melt spun ribbons

During melt spinning process, the improvement of certain grain orientation and the refinement of grain size with surface velocity have interactive and contradictory effects on the magnetic properties. The contributions of these effects have seldom been taken into account and they were discussed in this paper via Fe-2, 4, 6.5 wt% Si alloys. Heat treatment at 1173 K for 1 h was performed to show the annealing impact. The X-ray diffraction patterns show that the high surface velocity and heat treatment increase the intensity ratio of line (2 0 0) to (1 1 0) of A2 phase. The (2 0 0) line corresponds to (2 0 0) plane in 〈0 0 1〉 direction, easy magnetization direction of α-Fe phase in Fe-Si alloy. The improvement of this grain orientation with the surface velocity decreases the coercivity, which should increase due to the grain refinement. It is revealed that the 〈0 0 1〉 texture promoted by the anisotropic heat release during melt spinning process is one factor to improve the magnetic properties and should be considered when preparing soft magnetic materials.     

Effect of annealing treatment on soft magnetic properties of Fe-6.5 wt% Si wide ribbons

The 25 mm wide ribbons of Fe-6.5 wt% Si alloy have been developed by melt spinning technique, showing sufficient ductility and white silver appearance. Two magnetic transitions take place at 676 and 760 °C due to the formation of B2 ordered phase and A2 disordered paramagnetic phase, respectively. The saturation magnetization of the ribbon is 17.5 kG under the applied field of 12 kG. The as-cast ribbons consist of disordered A2 structure with a low volume of B2 phases while the annealed microstructure comprises a dispersion of B2 domains in the disordered A2 matrix. The alloy shows the enhancement of the soft magnetic properties with a reduction in coercivity from 150 A/m in the as-cast state to 45 A/m in the annealed condition at 850 °C.     

The magnetostriction of Fe-(18−x) at% Ga-x at% Al (3≤x≤13.5) alloys

The ingot of Fe-(18−x) at% Ga-x at% Al (3≤x≤13.5) alloys was prepared from high purity elements using a high vacuum arc melting system. The X-ray diffraction patterns indicated that the alloys were disordered bcc A2 structure. The magnetostriction of the alloys was measured and the effect of partial substitution of Ga with Al on the magnetostriction of the alloys was investigated. Fe-9 at% Ga-9 at% Al alloy, the optimizing magnetostrictive alloy was found in Fe-(18−x) at% Ga-x at% Al (3≤x≤13.5) alloys. The saturated magnetostriction of the directional solidification Fe-9 at% Ga-9 at% Al rod is up to 135 ppm for 0 MPa and 221 ppm for 53 MPa. It was found that the alloy has the high linearity of the magnetostriction curve, the low hysteresis and saturated magnetic field, which suggests the directional solidification Fe-9 at% Ga-9 at% Al alloy is a potential candidate for magnetostrictive actuator and transducer applications.     

Texture, microstructure and magnetic properties of Fe-28Cr-15Co-3.5Mo permanent magnet

Crystallographic texture, microstructure and magnetic properties of Fe-28Cr-15Co-3.5Mo alloy were studied as a function of processing parameters. Texture studies revealed that thermo-magnetic aging leads to the development of ideal Goss type, {1 1 0} 〈0 0 1〉 and cube type {0 0 1} 〈0 1 0〉 textures. The orientation densities of these texture components become stronger after the step-aging treatments. Microstructural features show that improvement in magnetic properties were due to aligning and elongation of ferromagnetic Fe, Co-rich (α₁) particles in the preferred 〈1 0 0〉 directions. Magnetic analysis reveals that magnetic properties of the alloys are directionally dependent and influenced by the choice of thermo-magnetic treatment temperature or time. The maximum values of intrinsic coercive force, remanence and energy product, obtained in the textured magnetic alloy were 68.99 kA/m (867 Oe), 1.12 T (11.2 kG) and 43.2 kJ m³ (5.4 MG Oe), respectively.     

Investigating the magnetic properties of soft magnetic composites based on mechanically alloyed nanocrystalline Fe-5 wt% Ni powders

In this work, the soft magnetic composites (SMCs) of the nanocrystalline Fe-5 wt% Ni powders coated with phenolic resin were studied. The nanocrystalline powders with an average diameter of 10 nm were obtained by mechanical alloying up to 96 h milling in a high-energy planetary ball mill. The microstructure and magnetic properties of the milled powders were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy and a vibrating sample magnetometer. The results of X-ray diffraction showed that the bcc Fe(Ni) solid solution is formed after 24 h milling. Magnetic measurements indicated that the 96 h milled powders with a steady-state grain size of 10 nm have the highest saturation magnetization and the lowest coercivity. The SMCs based on nanocrystalline powders showed higher electrical resistivity and magnetic permeability up to 1 MHz, as compared with the pure iron-based composites. Besides, the nanocrystalline-based SMCs exhibited higher relaxation frequency and a significantly lower loss factor up to 1 MHz.     

The effect of laser remelting in the formation of tunable nanoporous Mn structures on mild steel substrates

Nanoporous manganese was fabricated by a three-step process involving high power laser cladding of a homogeneous Cu₄₀Mn₆₀ alloy coatings onto a mild steel substrate, laser remelting for tuning the grain size and the composition homogeneity followed by selectively electrochemical de-alloying for removal of Cu element and formation of nanoporous Mn. The microstructure and homogeneity of the precursor Cu₄₀Mn₆₀ alloys have a significant influence on the evolution of nanopores during selectively electrochemical de-alloying. Laser remelting can significantly refine the microstructure. The second dendrite arm spacing decreases with increasing of laser remelting scanning speed. A SDAS of 1.17 μm was obtained at the laser scanning speed of 133 mm/s. When the remelting scanning speed reaches 100 mm/s, a nanoporous structure with average pore size less than 100 nm was achieved under optimized dealloying electrode current density about 2 mA/cm². Nanoporous Mn with nanopore sizes ranging from 80 to 130 nm was fabricated by this method. Surface-enhanced Raman scattering characteristics of the nanoporous materials have been investigated. It is found that smaller nanoporosity leads to significant improvements in surface-enhanced Raman scattering.     

Thermal stability of titanium nitride coatings prepared by the mixing technology with laser and plasma

Titanium samples were treated by the mixing technology with laser and plasma (LPN) using different laser power densities. These nitrided samples were then annealed at 473 K, 673 K, 873 K, and 1073 K for 2 h in vacuum, respectively. The samples before and after annealing were characterized at room temperature and compared in terms of microstructure. X-ray diffraction and cross-sectional optical microscopy studies showed that the layer structure of the titanium nitride coating is preserved after annealing at 1073 K when the coating is formed using a laser power density of 8.0 × 10⁵ W/cm². Therefore, titanium nitride coatings produced by LPN demonstrate excellent thermal stability and are potential candidates for high temperature tribological applications.     

Influence of ternary addition of transition elements (Cr,Si and Mn) on the microstructure and magnetic properties of nano-structured CuCo alloy

The current state of studies presents the effect of ternary addition of transition elements such as Mn, Cr and Si (10 wt%) on the mechanically driven non-equilibrium solubility of 40 wt% Co containing Cu–Co alloy. X-ray powder diffraction analysis indicates that addition of Mn has been found to be the most effective in enhancing the solubility and formation of a complete solid solution between Co and Cu in a short duration (30 h) of ball milling. The microstructure of the ball milled CuCoMn alloy was found to be stable after the isothermal annealing up to a temperature of 450 °C for 1 h. The magnetic properties such as magnetic saturation, coercivity and remanence of ball milled CuCo alloy in the presence of Mn significantly altered after annealing in the temperature range 350–650 °C for 1 h. The best combination of magnetic properties of CuCoMn alloy has been found after annealing at 550 °C for 1 h.     

Preparation of anti-corrosion films by microarc oxidation on an Al-Si alloy

Thick ceramic films over 140 μm were prepared on Al-7% Si alloy by ac microarc oxidation in a silicate electrolyte. The film growth kinetics was determined by an eddy current technique and film growth features in different stages were discussed. The microstructure and composition profiles for different thick films were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. Their phase components were determined by X-ray diffraction. The electrochemical corrosion behaviors of bare and coated alloys were evaluated using potentiodynamic polarization curves, and their corrosion morphologies were observed. In the initial stage of oxidation, the growth rate is slow with 0.48 μm/min due to the effect of Si element though the current density is rather high up to 33 A/dm². After the current density has decreased to a stable value of 11 A/dm², the film mainly grows towards the interior of alloy. The film with a three-layer structure consists of mullite, γ-Al₂O₃, α-Al₂O₃ and amorphous phases. By microarc discharge treatment, the corrosion current of the Al-Si alloy in NaCl solution was significantly reduced. However, a thicker film has to be fabricated in order to obtain high corrosion-resistant film of the Al-Si alloy. Microarc oxidation is an effective method to form an anti-corrosion protective film on Si-containing aluminum alloys.     

The microstructure, mechanical and friction properties of protective diamond like carbon films on magnesium alloy

Protective hard coatings deposited on magnesium alloys are believed to be effective for overcoming their poor wear properties. In this work, diamond-like carbon (DLC) films as hard protective films were deposited on AZ91 magnesium alloy by arc ion plating under negative pulse bias voltages ranging from 0 to −200 V. The microstructure, composition and mechanical properties of the DLC films were analyzed by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and nanoindentation. The tribological behavior of uncoated and coated AZ91 magnesium alloy was investigated using a ball-on-disk tribotester. The results show that the negative pulse bias voltage used for film deposition has a significant effect on the sp³ carbon content and mechanical properties of the deposited DLC films. A maximum sp³ content of 33.3% was obtained at −100 V, resulting in a high hardness of 28.6 GPa and elastic modulus of 300.0 GPa. The DLC films showed very good adhesion to the AZ91 magnesium alloy with no observable cracks and delamination even during friction testing. Compared with the uncoated AZ91 magnesium alloy, the magnesium alloy coated with DLC films exhibits a low friction coefficient and a narrow, shallow wear track. The wear resistance and surface hardness of AZ91 magnesium alloy can be significantly improved by coating a layer of DLC protective film due to its high hardness and low friction coefficient.     

Growth and field emission properties of globe-like diamond microcrystalline-aggregate

The globe-like diamond microcrystalline-aggregates were fabricated by microwave plasma chemical vapor deposition (MPCVD) method. The ceramic with a Ti mental layer was used as substrate. The fabricated diamond was evaluated by Raman scattering spectroscopy, X-ray diffraction spectrum (XRD), and scanning electron microscope (SEM). The field emission properties were tested by using a diode structure in a vacuum. A phosphor-coated indium tin oxide (ITO) anode was used for observing and characterizing the field emission. It was found that the globe-like diamond microcrystalline-aggregates exhibited good electron emission properties. The turn-on field was only 0.55 V/μm, and emission current density as high as 11 mA/cm² was obtained under an applied field of 2.9 V/μm for the first operation. The growth mechanism and field emission properties of the globe-like diamond microcrystalline-aggregates are discussed relating to microstructure and electrical conductivity.     

Characterization of ultra-fast deposited polycrystalline graphite by a CO2 laser-assisted combustion-flame method

High deposition rate, 750 μm/min, crystalline graphite was deposited on WC substrates by a CO₂ laser-assisted combustion-flame method at laser powers between 300 and 800 W. The structures, which were identified as pillars, were characterized by various methods. The pillars were cylindrical in shape and grew to a size of approximately 3 mm in length and in a few minutes. The laser power did not affect the overall length of the pillar, but caused changes in the physical shape. X-ray and electron diffraction results revealed the pillars to be crystalline graphite regardless of the laser power. Investigation of the pillars by scanning electron microscopy showed two distinct microstructural areas: an inner core of dense material surrounded by an outer shell of lamellar-like material. The core/shell microstructure was unaffected by the level of CO₂ laser power.     

Characterization of nanocrystalline FeSiCr powders prepared by ball milling

FeSi₁₀Cr₁₀ powder was mechanically alloyed by high energy planetary ball milling, starting from elemental powders. The microstructural and magnetic properties of the milled powders were characterized by scanning electron microscopy, X-ray diffraction, ⁵⁷Fe Mössbauer spectrometry and a vibratory sample magnetometer.After 3 h of milling, the formation of two bcc solid solutions α-Fe₁ (Si, Cr) and α-Fe₂ (Si, Cr) is observed. Their grain sizes decrease with increase in milling time attaining, at 15 h of milling, 23 and 11 nm, respectively. Mössbauer spectra of the milled powder show the presence of two components. One is a ferromagnetic type with a broad sextuplet. Its distribution of hyperfine field is characterized by high and low hyperfine field’s peaks and a mean value of 26.5 T. The other is a single paramagnetic peak. Its low concentration increases to ∼4% at 15 h of milling. These results can be explained by different atomic environments affected by Si or/and Cr elements, as well as the increased disordered grain boundaries.Magnetic measurements of the milled FeSi₁₀Cr₁₀ alloy powder exhibit a soft ferromagnetic character with a decrease of both magnetization at saturation (Ms) and coercive force (Hc) with milling time attaining values of Ms=151 emu/g and Hc=2500 A/m at 30 h of milling time.     

Effects of thermal treatment on femtosecond laser fabricated diffraction gratings in polystyrene

We report the fabrication of efficient, buried diffraction gratings and micro-craters in bulk polystyrene using femtosecond laser direct writing technique. We recorded a maximum diffraction efficiency of 10% for a buried grating fabricated at 1 μJ energy, 1 mm/s speed, and a period of 30 μm. Buried micro-craters, with typical dimensions of ∼2 μm, were achieved at low energies and high scanning speeds. From the field emission scanning electron microscope studies, the observed emission is attributed as due to the inner surface modifications and the debris settled around the voids. The fabricated gratings subjected to heat treatment were tested for the diffraction efficiency and emission at different excitation wavelengths and the observed results are presented. Raman spectra collected from the femtosecond laser modified regions revealed the disappearance of few Raman modes at high peak intensities associated with incident Gaussian laser pulse. Potential applications of these luminescent micro-craters are highlighted.     

Synthesis, characterization, and microwave absorption properties of Fe-40 wt%Ni alloy prepared by mechanical alloying and annealing

Fe-40 wt%Ni alloys with granular shape and flake shape were prepared by a mechanical alloying (MA) and annealing method. The phase composition and morphology of the FeNi alloys, electromagnetic parameters, and microwave absorbing properties of the silicone rubber composite absorbers filled with the as-prepared FeNi alloy particles were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and vector network analyzer. The XRD results indicate that the crystalline structures of the Fe-40 wt%Ni alloys prepared by both one-step and two-step MA processes are face-centered cubic (fcc) Ni (Fe) solid solutions, and the structures can be retained after annealing at 600 °C for 2 h. SEM images show that the FeNi alloy powders for one-step process have a granular shape; however the particles turned into flake form when they were sequentially milled with absolute ethyl alcohol. With the increase in thickness of composite absorber, the reflection loss (RL) decreases, and the peak for minimum reflection loss shifts towards the lower frequency range. Compared to the absorbers filled with the granular FeNi alloy, the absorbers filled with flaky FeNi alloys possess higher complex permittivities and permeabilities and have a lower RL and peak frequency under the same thickness. Microwave absorbing materials with a low reflection loss peak in the range of 1-4 GHz are obtained, and their microwave absorbing properties can be adjustable by changing their thicknesses.     

Characterization of cold-sprayed nanostructured Fe-based alloy

The ball-milled Fe-Si alloy was used as feedstock for deposition of nanocrystalline Fe-Si by cold spraying process. The microstructure of the as-sprayed nanostructured Fe-Si was characterized by using optical microscopy, scanning electron microscopy and transmission electron microscopy. The grain sizes of the feedstock and as-sprayed deposit were estimated based on X-ray diffraction analysis. The microhardness and coercivity of the deposited Fe-Si alloy were characterized. The results showed that the as-sprayed deposit presented a dense microstructure. The mean grain size of the as-deposited Fe-Si was several tens nanometers and comparable to that of the corresponding milled feedstock. The temperature of driving gas presented little effect on the microstructure of cold-sprayed nanostructured Fe-Si deposit. The mechanical alloying induced oxygen contents up to 8 wt% in the feedstocks and subsequent deposits. The microhardness of the deposit reached about 400 Hv. The deposit achieved a high coercivity up to 190 kA/m indicating the potential possibility for applications to recording materials.     

Effect of gun current on the microstructure and crystallinity of plasma sprayed hydroxyapatite coatings

Hydroxyapatite (HA) is a bioactive material because its chemical structure is close to the natural bone. Its bioactive properties make it attractive material in biomedical applications. Gas tunnel type plasma spraying (GTPS) technique was employed in the present study to deposit HA coatings on SUS 304 stainless steel substrate. GTPS is composed of two plasma sources: gun which produces internal low power plasma (1.3-8 kW) and vortex which produces the main plasma with high power level (10-40 kW). Controlling the spraying parameters is the key role for spraying high crystalline HA coatings on the metallic implants. In this study, the arc gun current was changed while the vortex arc current was kept constant at 450 A during the spraying process of HA coatings. The objective of this study is to investigate the influence of gun current on the microstructure, phase crystallinity and hardness properties of HA coatings. The surface morphology and microstructure of as-sprayed coatings were examined by scanning electron microscope. The phase structure of HA coatings was investigated by X-ray diffraction analysis. HA coatings sprayed at high gun current (100 A) are dense, and have high hardness. The crystallinity of HA coatings was decreased with the increasing in the gun current. On the other hand, the hardness was slightly decreased and the coatings suffer from some porosity at gun currents 0, 30 and 50 A.     

Magnetic and transport properties of Cu2MnAl Heusler alloy prepared by rapidly quenched method

The Cu₂MnAl alloy was prepared by rapidly quenched (suction-casting and melt-spinning) methods with various thicknesses of 20, 40 and 1000 μm. The X-ray diffraction (XRD) patterns of the fabricated samples show a single phase of Cu₂MnAl. All the samples reveal soft magnetic behavior with coercivity below 1.6 kA/m and Curie temperature of about 600 K. Resistance of the alloy behaves as a linear function of applied magnetic field. Magnetoresistance (MR) ratio depends on the thickness of the samples and achieves ∼0.8% at the field of 240 kA/m for the sample with thickness of 20 μm. The variation of the properties of the alloy can be interpreted by the difference of energy band structure caused by defects in the alloy.