Al-to-Mg Friction Stir Welding: Effect of Material Position,Travel Speed,and Rotation Speed

Because joining dissimilar metals is often difficult by fusion joining, interest has been growing rapidly in using friction stir welding (FSW), which is considered a revolutionary solid-state welding process, as a new way to join dissimilar metals such as Al alloys to Mg alloys, Cu, and steels. Butt FSW of Al to Mg alloys has been studied frequently recently, but the basic issue of how the welding conditions affect the resultant joint strength still is not well understood. Using the widely used alloys 6061 Al and AZ31 Mg, the current study investigated the effect of the welding conditions, including the positions of Al and Mg with respect to the welding tool, the tool travel speed, and the tool rotation speed on the weld strength. Unlike previous studies, the current study (1) determined the heat input by both torque and temperature measurements during FSW, (2) used color metallography with Al, Mg, Al₃Mg₂, and Al₁₂Mg₁₇ all shown in different colors to reveal clearly the formation of intermetallic compounds and material flow in the stir zone, which are known to affect the joint strength significantly, and (3) determined the windows for travel and rotation speeds to optimize the joint strength for various material positions. The current study demonstrated clearly that the welding conditions affect the heat input, which in turn affects (1) the formation of intermetallics and even liquid and (2) material flow. Thus, the effect of welding conditions in Al-to-Mg butt FSW on the joint strength now can be explained.     

Nanocomposites of Aluminum Alloys by Rapid Solidification Processing

Aluminium alloys reinforced with transition metal aluminide (Al₃Ti, Al₃Fe, Al₃Ni, etc.) particles possess high specific strength both at ambient and elevated temperature. The improved strength of these alloys are the results of slower coarsening rate of the intermetallic particles due to low diffusivity of the transition metals in aluminium. However, the strength can be enhanced further by refining the microstructure of the alloys to nanometer range. The authors have successfully attempted two important non-equilibrium processing techniques i.e. rapid solidification processing (RSP) and mechanical alloying for the refinement of the microstructure in various aluminium alloys. In this report, authors present a short review of their work on RSP of Al?CTi and Al?CFe alloys to produce nanocomposites.     

Elastic Properties,Thermal Expansion Coefficients,and Electronic Structures of Mg and Mg-Based Alloys

Ab-initio density functional theory (DFT) calculations were performed to study alloying effects on hcp Mg. The alloy solid solution strengthening represented by bond strength enhancement in alloys, elastic properties, thermal expansion coefficients, and electronic structures of Mg-based alloys was investigated. Results show that alloying additions with sp-metal Al and rare earth (RE) Y are capable of increasing the bond strength, with the addition of Y achieving a better effect. The bond strength enhancement due to an RE Y addition is associated with a hybridization between the d-orbital of Y and the p-orbital of the Mg atoms near the Fermi energy, and this was consistent with the electron localized function (ELF) evaluations showing that more localized and stronger covalent bonds are formed between Y and Mg atoms. It is also found that alloying additions of Al, Zn, and Y are not capable of increasing elastic coefficients and moduli, indicating that bond strength enhancement could play a major role in alloy solid solution strengthening in Mg-based alloys. Possible reasons for the elastic properties accompanying the alloying addition are given from the electronic point of view. Furthermore, from the calculated negative Cauchy pressure (C ₁₃ – C ₄₄ < 0), it is concluded that the chemical bonds between Y and Mg atoms show angular characteristics.     

Structural Features of Al–Hf–Sc Master Alloys

Microstructural features of new master alloys of the Al–Hf–Sc system with metastable aluminides with a cubic lattice identical to the lattice of a matrix of aluminum alloys are investigated using optical microscopy, scanning electron microscopy, and electron probe microanalysis. Binary and ternary alloys are smelted in a coal resistance furnace in graphite crucibles in argon. Alloys Al–0.96 at % Hf (5.98 wt % Hf) and Al–0.59 at % Hf (3.77 wt % Hf) are prepared with overheating above the liquidus temperature of about 200 and 400 K, respectively. Alloys are poured into a bronze mold, the crystallization rate in which is ~10³ K/s. Metastable Al₃Hf aluminides with a cubic lattice are formed only in the alloy overheated above the liquidus temperature by 400 K. Overheating of ternary alloys, in which metastable aluminides Al _n (Hf_1–xSc _x ) formed, is 240, 270, and 370 K. Depending on the Hf-to-Sc ratio in the alloy, the fraction of hafnium in aluminides Al _n (Hf_1–xSc _x ) varies from 0.46 to 0.71. Master alloys (at %) Al–0.26Hf–0.29Sc and Al–0.11Hf–0.25Sc (wt %: Al–1.70Hf–0.47Sc and Al–0.75Hf–0.42Sc) have a fine grain structure and metastable aluminides of compositions Al _n (Hf_0.58Sc_0.42) and Al _n (Hf_0.46Sc_0.54), respectively. Sizes of aluminides do not exceed 12 and 7 μm. Their lattice mismatch with a matrix of aluminum alloys is smaller than that for Al₃Sc. This makes it possible to assume that experimental Al–Hf–Sc master alloys manifest a high modifying effect with their further use. In addition, the substitution of high-cost scandium with hafnium in master alloys can considerably reduce the consumption of the latter.     

The Interface of TiB<Subscript>2</Subscript> and Al<Subscript>3</Subscript>Ti in Molten Aluminum

In the grain refinement of aluminum, Al₃Ti and TiB₂ particles are introduced to reduce the casting grain size down to 200 micrometer level, which makes cold working possible. The particles are brought in by the addition of Al-Ti-B-type master alloys. It is generally believed that TiB₂ particles are stable and nucleate α-Al grains in solidification in the presence of titanium in solution from the dissolution of Al₃Ti particles in the master alloys. The titanium in solution either forms Al₃Ti layers on the surface of TiB₂ particles to promote the nucleation of α-Al grains or remains as solute to restrict the growth of α-Al grains in solidification. However, a consensus on a grain refinement mechanism is still to be reached due to the lack of direct observation of the three phases in castings. This paper presents finding of the TiB₂/Al₃Ti interfaces in an Al-Ti-B master alloy. It demonstrates a strong epitaxial growth of Al₃Ti on the surface of TiB₂ particles, a sign of the formation of an Al₃Ti layer on the surface of TiB₂ particles in grain refinement practice. The Al₃Ti layer has a crystal coherency with α-Al and hence offers a substrate for heterogeneous nucleation of α-Al grains. However, the layer must be dynamic to avoid the formation of compounded Al₃Ti and TiB₂ particles leading to the loss of efficiency in grain refinement.     

Reoxidation of Aluminum in Fe-Al-M (M = C,Mn, and Ti) melts with CaO-Al2 O3-Fe t O (3 mass pct) slags

An Fe-0.01 to 0.5 mass pctAl alloy and an Fe-0.003 to 0.71 mass pctAl-1 mass pctM (M = C, Mn, and Ti) alloy were reoxidized with the CaO-Al₂O₃-Fe_tO (3 mass pct) slags at 1873 K in an Al2O3 or CaO crucible for 5 and 60 minutes. The contents of acid-insoluble Al, total O, and alloying element M in metal as well as those of M and FetO in slag were measured as a function of total Al content. On the basis of the present and previous results for Fe-Al-Te alloys, the effect of alloying elements on the degree of supersaturation with respect to the Al2O3 precipitation was studied. As a result, the supersaturation phenomenon was observed in all experiments at 5 minutes, but in the experiments at 60 minutes, it was observed only in Fe-Al and Fe-Al-Ti alloys. No supersaturation was observed in the reoxidation of Si in Fe-0.13 to 0.98 mass pctSi alloys with the CaO-SiO2-FetO (3 mass pct) slags in a CaO crucible at 5 and 60 minutes.     

The Influence of Grain Structure on Intermetallic Compound Layer Growth Rates in Fe-Al Dissimilar Welds

The thickness of the intermetallic compound (IMC) layer that forms when aluminum is welded to steel is critical in determining the properties of the dissimilar joints. The IMC reaction layer typically consists of two phases (η and θ) and many attempts have been made to determine the apparent activation energy for its growth, an essential parameter in developing any predictive model for layer thickness. However, even with alloys of similar composition, there is no agreement of the correct value of this activation energy. In the present work, the IMC layer growth has been characterized in detail for AA6111 aluminum to DC04 steel couples under isothermal annealing conditions. The samples were initially lightly ultrasonically welded to produce a metallic bond, and the structure and thickness of the layer were then characterized in detail, including tracking the evolution of composition and grain size in the IMC phases. A model developed previously for Al-Mg dissimilar welds was adapted to predict the coupled growth of the two phases in the layer, whilst accounting explicitly for grain boundary and lattice diffusion, and considering the influence of grain growth. It has been shown that the intermetallic layer has a submicron grain size, and grain boundary diffusion as well as grain growth plays a critical role in determining the thickening rate for both phases. The model was used to demonstrate how this explains the wide scatter in the apparent activation energies previously reported. From this, process maps were developed that show the relative importance of each diffusion path to layer growth as a function of temperature and time.     

A layer growth mechanism proposed on studies on a gas nitrided AISI 316 L austenitic stainless steel

In this work an austenitic stainless steel AISI 316 L (1.4404) was gas nitrided at 540°C. The nitride layer was characterised by means of X-ray analysis, SEM, TEM and electron probe X-ray analysis. A fine lamellar structure comprising ? and γ' nitrides was formed. Nickel paritioned into γ'-Fe₄N type nitride while chromium partitioned into the ?-Fe_2–3N type nitride. A layer formation mechanism was proposed considering a local partition of the alloying elements Cr and Ni.     

Rare-earth metals in nickel aluminide-based alloys: III. Structure and properties of multicomponent Ni<Subscript>3</Subscript>Al-based alloys

The possibility of increasing the life of heterophase cast light Ni₃Al-based superalloys at temperatures higher than 0.8T _m of Ni₃Al is studied when their directional structure is additionally stabilized by nanoprecipitates, which form upon additional alloying of these alloys by refractory and active metals, and using special methods for preparing and melting of an alloy charge. The effect of the method of introducing the main components and refractory reaction-active and surface-active alloying elements into Ni₃Al-based cast superalloys, which are thermally stable natural composite materials of the eutectic type, on the structure-phase state and the life of these alloys is studied. When these alloys are melted, it is necessary to perform a set of measures to form particles of refractory oxide cores covered with the β-NiAl phase and, then, γ′_prim-Ni₃Al phase precipitates during solidification. The latter phase forms the outer shell of grain nuclei, which provides high thermal stability and hot strength of an intermetallic compound-based alloy. As a result, a modified structure that is stabilized by the nanoprecipitates of nickel and aluminum lanthanides and the nanoprecipitates of phases containing refractory metals is formed. This structure enhances the life of the alloy at 1000 °C by a factor of 1.8–2.5.     

Galvanic cell measurements on supersaturated activities of oxygen in Fe-Al-M (M=C, Te, Mn, Cr, Si, Ti, Zr, and Ce) melts

Using a mullite (3Al₂O₃ · 2SiO₂)-tube and ZrO₂-9 mol pct MgO-plug type solid electrolyte galvanic cells, the activities of supersaturated oxygen in Fe-0.0017 to 0.41 mass pct Al-M (M=C, Te, Mn, Cr, Si, Ti, Zr, and Ce) alloys were measured as a function of total Al or M contents at 1873 K in an alumina crucible. Based on these results, the effects of alloying elements on the supersaturated oxygen activity with respect to alumina precipitation were studied. In the Fe-Al-M (M=C, Te, Mn, Cr, and Si) alloys, the supersaturated oxygen activities for a given Al level approach the equilibrium values with increasing contents of alloying elements. However, the oxygen activities for a given Al level are still supersaturated in the Fe-Al-M (M=Ti, Zr, and Ce) alloys even in the presence of considerable amounts of the alloying elements.     

Effects of trace ytterbium addition on microstructure,mechanical and thermal properties of hypoeutectic Al–5Ni alloy

Over the past decade, the cast aluminum alloys with excellent mechanical and conductivity properties have emerged as potential materials for thermal management. However, the traditional Al–Si based alloys are difficult to make significant breakthrough in conductivity performance. The hypoeutectic Al–5Ni alloy also possesses sound castability and is expected to be applied in thermal management applications. In this study, the effects of ytterbium (Yb element) at 0–0.5 wt% on the microstructures as well as the electrical/thermal conductivity and mechanical properties of the Al–5Ni alloy were systematically investigated. The experimental results indicate that the addition of Yb at a relatively low amount not only reduces the secondary dendrite arm spacing of the α-Al grains, but also modifies the morphology and distribution of eutectic boundary phase. Moreover, it is found that the dosage of Yb at 0.3 wt% in the Al–5Ni alloy can simultaneously improve the yield strength, ultimate tensile strength and electrical/thermal conductivity. The strengthening and toughening of the Al–5Ni alloy are mainly attributed to the decrease of secondary dendrite arm spacing and the improvement of eutectic phases. The transmission electron microscopy/selected area electron diffraction (TEM/SAED) analysis indicates that the ytterbium in Al–5Ni alloy will form Al₃Yb phase, which mainly agglomerates in the Al₃Ni phase region. This phase is helpful to decrease the solubility of impurity elements (e.g., Fe and Si) in the α-Al matrix, which is beneficial to electrical/thermal conductivity. The value of this study lays foundation for manufacturing Al–Ni alloys with high thermal conductivity and acceptable mechanical properties.     

A study on the microstructural evolution of Al-25 at. pct V-12.5 at. pct M (M = Cu,Ni, Mn) powders by planetary ball milling

The alloying behavior of Al-25 at. pct V-12.5 at. pct M (M = Cu, Ni, Mn) by planetary ball milling of elemental powders hours as been investigated in this study. In Al₃V binary system, an amorphous phase was produced after 6 hours and the amorphous phase was mechanically crystallized after 20 hours. The large difference in the diffusivities between Al and V atoms in Al matrix results in the formation of the amorphous phase when the homogeneous distribution of all the elements in a powder was achieved at 6 hours. According to thermal analyses, the amorphous phase in the binary Al₃V was crystallized at 350 °C. The addition of ternary elements (Cu, Ni, Mn) increased the activation energy for the crystallization to D0₂₂ phase by interfering with the diffusion process. Therefore, ternary element addition improved the thermal stability of the amorphous structures. The amorphous phase in the 12.5 at. pct Ni added Al₃V was crystallized to D0₂₂ phase at 540 °C. The mechanical crystallization of the amorphous phase in the ternary element-added Al-V system either occurred later or was not observed during ball milling up to 100 hours. It is thought that the amorphous intermetallic compacts could be produced more easily in ternary element-added alloys by using an advanced consolidation method.     

Thermodynamic effect of alloying addition on <Emphasis Type="Italic">in-situ</Emphasis> reinforced TiB<Subscript>2</Subscript>/Al composites

From the viewpoint of thermodynamics, using the Wilson equation and an extended Miedema model, the effect of the alloying element on the stability of the precipitated phases during the fabrication of in-situ reinforced TiB₂/Al composites was evaluated. The result shows that additions of alloying elements, such as Mg, Cu, Zr, Ni, Fe, V, and La, can promote the formation of Al₃Ti and TiB₂ phases. Particularly, Zr has the most pronounced effect among these alloying elements. In addition, alloying elements can hinder the formation of AlB₂ to a small extent. The calculation results also show that it is easier for magnesium to react with the salts to form TiB₂ than aluminum during the fabrication of in-situ reinforced TiB₂/Al using the flux-assisted synthesis (FAS) technology.     

Rare-earth metals (REMs) in nickel aluminide-based alloys: I. Physicochemical laws of interaction in the Ni-Al-REM and Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>-REM-AE (alloying element) systems

The data on the Ni-Al-R (R = REM Sc, Y, La, lanthanides) binary and ternary systems and the interactions of three rare-earth metals (yttrium, lanthanum, cerium) with the main alloying elements (Ti (Zr, Hf), Cr (Mo, W) that are introduced into Ni₃Al-based VKNA alloys are analyzed. The binary aluminides of REMs in the Ni-Al-R ternary systems are shown to be in equilibrium with neither NiAl nor Ni₃Al. The solid solution of aluminum in RNi₅, which penetrates deep into these ternary systems, is the most stable phase in equilibrium with Ni₃Al. In the NiAl (Ni₃Al)-AE-R systems, REM precipitation (segregation) on various defects and interfaces in nickel aluminides is likely to be the most probable, and REMs are thought to interact with the most active impurities in real alloys (C, O, N), since REMs have a large atomic radius and, thus, are virtually undissolved in nickel, aluminum, and nickel aluminides.     

Influence of Strontium Addition on Microstructure and Mechanical Properties of an Al–10Si–5Cu Alloy

The microstructure and mechanical properties of Al–10Si–5Cu cast alloys with micro-addition of alloying elements (V, Cr and Ni) were studied before and after strontium addition. Samples were examined using the X-Ray diffraction, the optical microscope, the scanning electron microscope and the energy dispersive spectrometer. The results indicated that the α-Al matrix, eutectic Si phase and Al₂Cu phase were the main constituent phases of Al–10Si–5Cu alloys before or after strontium addition. Strontium addition affected the refining of the α-Al grains and transforming the configuration of interdendritic phases. The un-modified alloy showed a brittle nature because of existing brittle and aggregated AlSiMnFe phases. Contributing to the alteration of microstructure in strontium modified alloy, the strength and elongation of the alloy were improved. In addition, the fracture mechanism and crack propagation process were investigated in both the alloys.     

Electronic and structural studies of grain boundary strength and fracture in Ll2 ordered alloys—II. On the effect of third elements in Ni3Al alloy

It was proposed in the Paper I that the grain boundary of the polycrystalline Ni₃Al compound is intrinsically weak due to the heterogeneous bonding environment at the grain boundary region. In this paper, the effect of the substitutional third elements on the mergranular embrittlement of the Ni₃Al compound was systematically investigated at room temperature. Various kinds of third elements ranging from groups IIIa to Vb in the periodic table were selected and alloyed in the Ll₂ structure range. First the phenomenological aspects such as the mechanical tests, metallographic and fractographic observations were described. It was shown that the third elements (Mn and Fe) which have the similar electronic chemical bonding nature with the Ni atom prohibited the grain boundary fracture when they substitute for the Al site. Several factors responsible for these phenomena were considered. As a consequence, the value of the valency difference between the third element and constitutive solvent atom substituted by the third element seems to control the grain boundary strength of the ternary Ni₃Al compound. The grain boundary is strengthened when the value of the valency difference is positive. The possible fracture mechanism was proposed, based on the crystal structures connected with the electronic chemical bonding nature at the grain boundary region. The modification in that the electronic chemical bonding environment of the grain boundary region becomes more homogeneous by the alloying makes the grain boundary stronger.     

Investigation of Microstructure and Mechanical Properties of Steel‐Aluminium Joints Produced by Metal Arc Joining

Joints of zinc coated steel (DX54D+Z200) to aluminium alloys (AW6181‐T4 and AW5182‐H111) were produced by the Cold Metal Transfer (CMT) technique, a modified metal inert gas (MIG) joining process. The aluminium alloy sheets and the AISi3Mn1 filler material are welded, while brazing occurs between the AISi3Mn1 filler and the steel sheet. At the interface between the Al‐based filler and the steel sheet, aluminium‐rich intermetallic Fe_xAl_y‐phases are formed. The comparatively low heat input of the CMT process and the choice of filler composition limit the thickness of the intermetallic phase seam (IMP) to a few micrometers. The structure of the intermetallic phase and its morphology are strongly influenced by the alloying elements (Mn, Si) of the filler. Tensile tests and crash tests of the steel to aluminium alloy joints revealed good mechanical properties.     

Simulation of the sulfide phase formation in a KhN60VT alloy

The conditions of the existence of sulfide phases in Fe–Ni–S alloys and four-component Fe–50 wt % Ni–0.001 wt % S–R (R is an alloying or impurity element from the TCFE7 database) systems are studied using the Thermo-Calc software package and the TCFE7 database. The modification of nickel superalloys by calcium or magnesium is shown to increase their ductility due to partial desulfurization, the suppression of the formation of harmful sulfide phases, and the uniform formation of strong sulfides in the entire temperature range of metal solidification. The manufacturability of superalloys can decrease at a too high calcium or magnesium content because of the formation of intermetallics with a low melting temperature along grain boundaries.     

Influence of alloying elements and grain refiner on microstructure,mechanical and wear properties of Mg-Al-Zn alloys

In the present investigation, the effects of alloying elements (Sn, Pb) and grain refiner (Ag, Zr) on microstructure, mechanical and wear properties of as-cast Mg-Al-Zn alloys were studied. The alloys were prepared through melting-casting route under a protective atmosphere and cast into a permanent mould. The microstructure of the base alloy consisted of α-Mg, Mg₁₇Al₁₂ continuous eutectic phase at the grain boundary and Mg-Zn phase was distributed within the grains. Addition of Sn and Pb suppressed the formation of continuous Mg₁₇Al₁₂ eutectic phase and formed Pb enriched Mg₂Sn precipitates at the grain boundary as well as inside the grain. The Ag and Zr addition to Mg-Al-Zn-Sn-Pb alloy suppressed the Mg₁₇Al₁₂ phase formation and refined the grains leading to improve mechanical properties. Addition of Sn, Pb and grain refiner (Ag, Zr) significantly enhanced the tensile strength and elongation but reduced hardness. The Ag addition imparted best tensile properties, where ultimate tensile strength (UTS) and elongation are 205?MPa and 8.0%, respectively. The fracture surfaces were examined under SEM which revealed cleavage facets and dimple formation. Therefore, the cleavage fracture and dimple rupture were considered as the dominant fracture mechanisms for developed Mg alloys. The cumulative volume loss of Mg alloys increased with sliding distance and applied load. The coefficient of friction decreased with sliding distance. The microscopic observation, analysis of the wear surface and coefficient of friction revealed that the wear mechanism of developed Mg alloys changes from abrasion oxidation to delamination wear.