Texture Development and Material Flow Behavior During Refill Friction Stir Spot Welding of AlMgSc

The microstructural evolution during refill friction stir spot welding of an AlMgSc alloy was studied. The primary texture that developed in all regions, with the exception of the weld center, was determined to be 〈110〉 fibers and interpreted as a simple shear texture with the 〈110〉 direction aligned with the shear direction. The material flow is mainly driven by two components: the simple shear acting on the horizontal plane causing an inward-directed spiral flow and the extrusion acting on the vertical plane causing an upward-directed or downward-directed flow. Under such a complex material flow, the weld center, which is subjected to minimal local strain, is the least recrystallized. In addition to the geometric effects of strain and grain subdivision, thermally activated high-angle grain boundary migration, particularly continuous dynamic recrystallization, drives the formation of refined grains in the stirred zone.

     

Production and Corrosion Resistance of Thermally Sprayed Fe-Based Amorphous Coatings from Mechanically Milled Feedstock Powders

Mechanically milled FeCrNbB feedstock powders from commercial precursors were used to produce amorphous coatings through two different industrial thermal-spraying techniques: high-velocity oxygen fuel (HVOF) and flame spraying. Microstructure, thermal behavior, and hardness of the coatings and their corrosion resistances in acidic and alkaline chloride-rich media were comparatively studied. HVOF process was effective to produce ~ 200-µm-thick highly amorphous coatings with hardness over than 700 HV_0.3 and low porosity (~ 5 pct). Flame-sprayed ~ 220-µm-thick coatings were nanocrystalline, composed of α-Fe, Fe₂B, FeNbB, and Fe₂O₃ phases and presented hardness of 564 HV_0.3 and ~ 10 pct porosity. Electrochemical measurements indicated that HVOF coatings exhibit higher corrosion resistance than flame-sprayed ones thanks to the higher amorphous content and lower porosity resulting from the former processing route. Electrochemical impedance spectroscopy results demonstrated that amorphous HVOF Fe₆₀Cr₈Nb₈B₂₄ (at. pct) coatings are interesting to protect mild steels such as the API 5L X80 against corrosion in chloride-rich environments.

     

Ordered phases and texture in spray-formed Fe–5 wt%Si

Fe–Si alloys have excellent soft magnetic properties, especially around 12 at.% Si. However, their industrial applications are limited because they lack the ductility required in rolling operations for the fabrication of thin sheets, thus leading to cracking. The brittleness of high silicon alloys is caused by order–disorder reactions at low temperatures. This work involved an analysis of the effect of heat treatment on the crystalline structure of thin sheets of Fe–5 wt%Si alloy obtained in a two-step fabrication route: (1) spray forming of Fe–3.5%Si + 2.0%Sip composite and (2) rolling and heat treatment of the composite to dissolve the silicon and homogenize its content across the thickness of sheet samples. Structural and microstructural analyses indicated the success in fabricating thin sheets of Fe–5 wt%Si alloys with such strategy. The presence of the ordered B2 phase had an important effect on the texture development and therefore on the magnetic properties of these alloys.     

Hot Extrusion of Nanostructured Al-Powder Alloys: Grain Growth Control and the Effect of Process Parameters on Their Microstructure and Mechanical Properties

Two nanostructured aluminum powder alloys (supersaturated Al4.5Cu prepared by mechanical alloying, and Al3.0Fe0.42Cu0.37Mn rich in precipitates and prepared by rapid solidification via gas atomization) were consolidated into bulk material under various processing conditions via hot extrusion. The microstructural modifications and mechanical properties of the consolidated alloys as a function of the extrusion conditions were investigated and are discussed here. The effect of pre-existing precipitates from nonsupersaturated alloy is shown to be more effective for controlling grain growth during consolidation. The increase in the extrusion load, with a concomitant increase in the extrusion rate and decrease in temperature, is shown to lead to microstructural modifications. The differences in mechanical properties measured by compressive tests are also discussed in association with the extrusion parameters. Furthermore, suggestions are given for rationalizing the extrusion rate and temperature for the consolidation of nanostructured aluminum powder alloys via hot extrusion.     

Topological instability,average electronegativity difference and glass forming ability of amorphous alloys

In this paper, we report the remarkable agreement of the glass forming ability of binary alloys with a new criterion that combines the topological instability parameter (λ) and the average electronegativity difference among the elements of an alloy, assuming both exert a synergetic effect. The best glass forming compositions for Zr–Cu and Ti–Ni systems are well predicted by this new approach. Although the new criterion needs further refinement, it is concluded that the proposed approach is a promising and simple tool to guide and reduce the tedious and labour intensive work to find good glass former compositions in metallic systems.     

Microstructure of a recycled AA7050 alloy processed by spray forming followed by hot extrusion and rotary swaging

The aim of this study is to evaluate the microstructure of a recycled AA7050 alloy processed by spray forming followed by hot extrusion and swaging. Machining chips from an aircraft manufacturer were used as raw material. The microstructure was investigated by optical microscopy (MO) and scanning electronic microscopy (SEM). The spray formed deposit was homogenized at 743 K, 8 h, extruded at 693 K with a deformation rate of 0.1 s^–1 and an extrusion rate of 156:1, producing a bar with 8.0 mm in diameter. Due to a specific combination of high extrusion temperature and deformation rate, partial recrystallization occurred and different grain sizes were obtained through the extruded cross section. After extrusion, the 8.0 mm bar was processed by rotary swaging in several steps up to a 2.0 mm final diameter. The resulting microstructure revealed a cold worked structure, with no recrystallization. Tensile tests were performed in both cases and the slant type of fracture reveals that Portevin Le‐Chatelier effect took place in this alloy. Moreover, the efficacy of extrusion and rotary swaging parameters to reduce the porosity, intrinsic to the spray form process, was analyzed, as well the distribution of intermetallic particles.     

Thermodynamic and topological instability approaches for forecasting glass-forming ability in the ternary Al–Ni–Y system

A thermodynamic approach to predict bulk glass-forming compositions in binary metallic systems was recently proposed. In this approach, the parameter γ* = ΔH^amor/(ΔH^inter − ΔH^amor) indicates the glass-forming ability (GFA) from the standpoint of the driving force to form different competing phases, and ΔH^amor and ΔH^inter are the enthalpies for glass and intermetallic formation, respectively. Good glass-forming compositions should have a large negative enthalpy for glass formation and a very small difference for intermetallic formation, thus making the glassy phase easily reachable even under low cooling rates. The γ* parameter showed a good correlation with GFA experimental data in the Ni–Nb binary system. In this work, a simple extension of the γ* parameter is applied in the ternary Al–Ni–Y system. The calculated γ* isocontours in the ternary diagram are compared with experimental results of glass formation in that system. Despite some misfitting, the best glass formers are found quite close to the highest γ* values, leading to the conclusion that this thermodynamic approach can be extended to ternary systems, serving as a useful tool for the development of new glass-forming compositions. Finally the thermodynamic approach is compared with the topological instability criteria used to predict the thermal behavior of glassy Al alloys.     

Amorphous phase formation by spray forming of alloys [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 and Fe66B30Nb4 modified with Ti

This paper describes results obtained by spray forming three iron-based alloys, namely [(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]₉₆Nb₄, Fe₆₅B₃₀Nb₄Ti₁ and Fe₆₃B₂₉Nb₄Ti₄, whose compositions derive from rapid solidification studies, in an attempt to obtain metallic glasses. The [(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]₉₆Nb₄ alloy presented higher glass-forming ability and showed a high fraction of amorphous phase formation up to a depth of 4 mm in the deposit. On the other hand, the spray formed deposits of the Fe₆₅B₃₀Nb₄Ti₁ and Fe₆₃B₂₉Nb₄Ti₄ alloys showed fully crystalline microstructure, despite the fact that the melt spun ribbons were fully amorphous.