Kahtan S. Mohammed

Effect of friction stir processing on the microstructure and hardness of an aluminum-zinc-magnesium-copper alloy with nickel additives

The main object of this study is to investigate the effect of friction stir processing (FSP) on the microstructure and hardness of Al-Zn-Mg-Cu alloys that were produced via casting with the addition of 5 wt % nickel. Furthermore, a single-pass FSP with a rotational speed of 1500 rpm and a traveling speed of 40 mm/min was performed on the alloys. The FSP-treated cast alloys were homogenized, aged at 120°C for 24 h, retrogressed at 180°C for 30 min, and then re-aged at 120°C for 24 h. Microstructural evaluations via optical microscopy and scanning electron microscopy, as well as with energy dispersive X-ray spectroscopy were conducted. In addition, X-ray diffraction analysis was performed to detect the intermetallics and phases of the Al-Zn-Mg-Cu-Ni alloys. Before FSP, the microstructural observations indicated the presence of coarse Ni dispersed particles with a precipitate phase within the matrix. After FSP treatment, the grain refinement led to the uniform space distribution of Ni dispersed particles in the stir zone. The Vickers hardness values for the Al-Zn-Mg-Cu-Ni alloy increased after age tempering at T6 and retrogression and re-aging (RRA) treatment because of the increased precipitation and particles dispersity. The hardness of the Al-Zn-Mg-Cu-Ni alloy was enhanced after FSP and a series of heat treatments, especially the RRA process, because of the stirring action of the FSP tool, the grain refinement, the appearance of additional precipitates, and the refinement of dispersed Ni particles.

The Effect of Various Waste Materials’ Contents on the Attenuation Level of Anti-Radiation Shielding Concrete

Samples of concrete contain various waste materials, such as iron particulates, steel balls of used ball bearings and slags from steel industry were assessed for their anti-radiation attenuation coefficient properties. The attenuation measurements were performed using gamma spectrometer of NaI (Tl) detector. The utilized radiation sources comprised 137Cs and 60Co radioactive elements with photon energies of 0.662 MeV for 137Cs and two energy levels of 1.17 and 1.33 MeV for the 60Co. Likewise the mean free paths for the tested samples were obtained. The aim of this work is to investigate the effect of the waste loading rates and the particulate dispersive manner within the concrete matrix on the attenuation coefficients. The maximum linear attenuation coefficient (μ) was attained for concrete incorporates iron filling wastes of 30 wt %. They were of 1.12 ± 1.31×10−3 for 137Cs and 0.92 ± 1.57 × 10−3 for 60Co. Substantial improvement in attenuation performance by 20%–25% was achieved for concrete samples incorporate iron fillings as opposed to that of steel ball samples at different (5%–30%) loading rates. The steel balls and the steel slags gave much inferior values. The microstructure, concrete-metal composite density, the homogeneity and particulate dispersion were examined and evaluated using different metallographic, microscopic and measurement facilities.

The Influence of Nickel and Tin Additives on the Microstructural and Mechanical Properties of Al-Zn-Mg-Cu Alloys

The effects of nickel and nickel combined tin additions on mechanical properties and microstructural evolutions of aluminum-zinc-magnesium-copper alloys were investigated. Aluminum alloys containing Ni and Sn additives were homogenized at different temperatures conditions and then aged at 120°C for 24 h (T6) and retrogressed at 180°C for 30 min and then reaged at 120°C for 24 h (RRA). Comparison of the ultimate tensile strength (UTS) of as-quenched Al-Zn-Mg-Cu-Ni and Al-Zn-Mg-Cu-Ni-Sn alloys with that of similar alloys which underwent aging treatment at T6 temper showed that gains in tensile strengths by 385 MPa and 370 MPa were attained, respectively. These improvements are attributed to the precipitation hardening effects of the alloying element within the base alloy and the formation of nickel/tin-rich dispersoid compounds. These intermetallic compounds retard the grain growth, lead to grain refinement, and result in further strengthening effects. The outcomes of the retrogression and reaging processes which were carried on aluminum alloys indicate that the mechanical strength and Vickers hardness have been enhanced much better than under the aging at T6 temper.

The Role of Direct Chilling, Retrogression and Reaging Treatment on Mechanical Properties of High Strength Aluminum Alloy

The effect of retrogression and reaging heat treatment on microstructure evolution andmechanical properties of 7075 Al alloy in direct chilling casting process was investigated. The subsequent heat treatment process comprised pre-aging at 120°C for 24 h, retrogression at 180°C for 30 min, and then reaging at 120°C for 24 h. By this three-step process, the mechanical properties of the chilled casted samples were substantially improved. The samples retain their high strength at T6 level. They gave yield strength up to 290 MPa, ultimate tensile strength of 386 MPa and elongation of 5.9%. The average value of multiple Vickers hardness tests results were in the range of 210 Hv. The direct chilling process followed by retrogression and reaging heat treatment yielded casts of fine and uniform microstructure as opposed to the microstructure of samples casted by the conventional process.

Effect of milling parameters on the synthesis of Al–Ni intermetallic compound prepared by mechanical alloying

Nanocrystalline Al–Ni intermetallic compounds were synthesized with different percentages of nickel by mechanical alloying (MA) of elemental powders. In all MA runs, the ball-to-powder weight ratio was 10:1, the rotation speed was 350 rpm, and the milling time ranged from 4 to 12 h. The phase evolution and microstructural changes of the powders during MA were investigated by X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray spectroscopy analyses. The crystallite sizes of milled powders were estimated based on the broadening of XRD peaks using the Williamson–Hall formula. Results showed that an optimum MA time of 12 h led to the formation of solid solutions of Al–Zn–Mg and Ni, which can be added to many Al–Ni intermetallic compounds. Furthermore, an Al–Ni intermetallic phase with <20 nm crystallite size was obtained.

Advantages of the Green Solid State FSW over the Conventional GMAW Process

The present work is an experimental comparison between the friction stir welding (FSW) and the conventional gas metal arc welding (GMAW) in joining of Al alloys. Two sets of 3 mm thick aluminum strip pairs were friction stir welded in a regular butting joint configuration. Two rotational speeds of 1750 rpm and 2720 rpm were utilized to perform the FSW process. The axial force and the transverse speed were kept constant at 6.5 KN and 45 mm/min, respectively. Cylindrical tool shoulder and pin geometry were selected. Strip pairs of other similar sets were butt jointed using the conventional GMAW. The welding quality, power input, and macrostructure and microstructure of the butted joints were examined. The types of the fumes and the amount of the released gases were measured and compared. The results showed that the solid state FSW is green, environment-friendly, and of superior welding properties compared to the conventional GMAW.

Radiation Shielding Characteristics of Concretes Incorporates Different Particle Sizes of Various Waste Materials

In this study, the dependence of gamma-ray absorption coefficient on the particulate matter sizes of steel slag, iron fillings and steel balls incorporated concrete were examined. The contents of these fillers in concrete mix was kept constant to 35 wt. %. Only the filler particle size was varied during the tests. The particle size ranged from 0.2mm to 1mm for steel slags and the iron fillings and from 2.5mm to 10mm for the steel balls.The concrete samples were assessed for their anti-radiation attenuation coefficient properties. The attenuation measurements were performed using gamma spectrometer of NaI (Tl) detector. The utilized radiation source was Cs137 radioactive element with photon energy of o.662 MeV. The results showed that gamma-ray attenuation coefficient was inversely proportional to the filler particulate matter size. Likewise the mean free paths for the tested samples were obtained. Maximum linear attenuation coefficient of 1.102±0.263cm-1 was attained for the iron filling.The iron balls and the steel slags showed much inferior values. The concrete incorporates iron filings afforded the best shielding effect. The density, microstructure, homogeneity and particulate distribution of the concrete samples were examined and evaluated using different metallographic, microscopic and measurement facilities.

Study of the feasibility of producing Al–Ni intermetallic compounds by mechanical alloying

Mechanical alloying (MA) was employed to synthesize Al–Zn–Mg–Cu alloys of high weight percentage of the nickel component from the elemental powders of constituents via high-energy ball milling. The mixed powders underwent 15 h of milling time at 350 rpm speed and 10: 1 balls/powder weight ratio. The samples were cold-compacted and sintered thereafter. The sintered compacts underwent homogenization treatments at various temperatures conditions and were aged at 120°C for 24 h (T6). The milled powders and heat-treated Al alloy products were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The crystallite sizes and microstrains of the alloyed powder were estimated via measuring the broadening of XRD peaks using the Williamson–Hall equation. The results have revealed that optimum MA time of 15 h has led to the formation of Al-based solid solutions of Zn, Mg, Cu, and Ni. The outcomes showed that the Vickers hardness of the sintered Al–Zn–Mg–Cu compacts of Ni alloys was enhanced following aging at T6 tempering treatments. Higher compression strength of Al-alloys with the addition of 15% nickel was obtained next to the aging treatment.

Experimental Studies on the Effects of Tin on the Densification of W-Brass Composites

The effects of 1% tin (Sn) addition on the densification of pre-alloyed and pre-mixed W-brass composites were carried out. The green compacts were produced with the pressure of 350MPa and sintered at the temperature of 800°C, 920°C and 1000°C. The Sn addition is aimed at inhibiting the dezincification (selective removal of zinc from an alloy) of the brass component by the elimination of pores and enhances densification. The hardness of the composites increased with increase in temperature, the densification was low at both temperatures while the electrical conductivity remains constant as a result of constant composition in both pre-alloyed and pre-mixed composites. The microstructures revealed pores, which might be as a result of zinc evaporation.

The Effect of Rotational Speed on Flow Behavior and Weld Properties in Friction Stir Welding of Pure Aluminum

Friction stir welding is a solid state green welding technique. Its key benefit is to allow welding of aluminum that cannot be readily done by fusion arc welding. In this study, two pure aluminum strips of 2mm thick were friction stir welded together. For all welding pairs, three rotational speeds of 1000, 1500 and 2000 rpm were used. The traverse speed, axial force and tool geometry were kept constant. Parameters optimization based on the results of the micrographic, macrographic, microhardness and tensile strength, indicated that sound joints with the best mechanical and microstructural properties can be obtained at rotational speed of 1500 rpm and welding transverse speed of 60 mm/min. Microscopic examination and local mechanical properties analysis suggested that mechanical mixing is the major material flow mechanism in the formation of the nugget stirred zone.