Microstructure and strengthening mechanism of Mg—5.88Zn—0.53Cu—0.16Zr alloy solidified under high pressure

Mg—5.88Zn—0.53Cu—0.16Zr (wt.%) alloy was solidified at 2—6 GPa using high-pressure solidification technology. The microstructure, strengthening mechanism and compressive properties at room temperature were studied using SEM and XRD. The results showed that the microstructure was refined and the secondary dendrite spacing changed from 35 μm at atmospheric pressure to 10 μm at 6 GPa gradually. Also, Mg(Zn,Cu)₂ and MgZnCu eutectic phases were distributed in the shape of network, while under high pressures the second phases (Mg(Zn,Cu)₂ and Mg₇Zn₃) were mainly granular or strip-like. The solid solubility of Zn and Cu in the matrix built up over increasing solidification pressure and reached 4.12% and 0.32% respectively at 6 GPa. The hardness value was HV 90 and the maximum compression resistance was 430 MPa. Therefore, the grain refinement strengthening, the second phase strengthening and the solid solution strengthening are the principal strengthening mechanisms.     

Mechanical properties of friction welded joint between Ti–6Al–4V alloy and Al–Mg alloy (AA5052)

Abstract

The present paper describes the mechanical properties of a friction welded joint between Ti–6Al–4V alloy and Al–Mg alloy (AA5052). The Ti–6Al–4V/AA5052–H112 joint, made at a friction speed of 27.5 rev s^?1, friction pressure of 30 MPa, friction time of 3.0 s, and forge pressure of 60 MPa, had 100% joint efficiency and fractured in the AA5052–H112 base metal. The Ti–6Al–4V/AA5052–H34 joint, made under the same friction welding conditions, did not achieve 100% joint efficiency and it fractured in the AA5052–H34 base metal because the AA5052–H34 base metal had softened under friction heating. The joints made at low friction speed or using short friction time showed fracture at the welded interface because a sufficient quantity of heat for welding could not be produced. However, the joints made at high friction speed or using long friction time were also fractured at the welded interface: in this instance, the welded interface also had an intermetallic compound layer consisting of Ti₂Mg₃Al₁₈. The Ti–6Al–4V/AA5052–H34 joint made at a friction speed of 27.5 rev s^?1 with friction pressure of 150 MPa, friction time of 0.5 s, and forge pressure of 275 MPa had 100% joint efficiency and fractured in the AA5052–H34 base metal, although the AA5052–H34 side softened slightly. In conclusion, the Ti–6Al–4V/AA5052–H112 joint and Ti–6Al–4V/AA5052–H34 joint had 100% joint efficiency and fractured in the AA5052 base metal when made under the friction welding conditions described above.     

Refinement and strengthening mechanism of Mg−Zn−Cu−Zr−Ca alloy solidified under extremely high pressure

Mg?Zn?Cu?Zr?Ca samples were solidified under high pressures of 2–6 GPa. Scanning electron microscopy and electron backscatter diffraction were used to study the distribution of Ca in the microstructure and its effect on the solidification structure. The mechanical properties of the samples were investigated through compression tests. The results show that Ca is mostly dissolved in the matrix and the Mg₂Ca phase is formed under high pressure, but it is mainly segregated among dendrites under atmospheric pressure. The Mg₂Ca particles are effective heterogeneous nuclei of α-Mg crystals, which significantly increases the number of crystal nuclei and refines the solidification structure of the alloy, with the grain size reduced to 22 μm at 6 GPa. As no Ca segregating among the dendrites exists, more Zn is dissolved in the matrix. Consequently, the intergranular second phase changes from MgZn with a higher Zn/Mg ratio to Mg₇Zn₃ with a lower Zn/Mg ratio. The volume fraction of the intergranular second phase also increases to 22%. Owing to the combined strengthening of grain refinement, solid solution, and dispersion, the compression strength of the Mg–Zn–Cu–Zr–Ca alloy solidified under 6 GPa is up to 520 MPa.     

Solidification microstructure characteristics of Ti–44Al–4Nb–2Cr–0.1B alloy under various cooling rates during mushy zone

Beta-solidifying TiAl alloy has great potential in the field of aero-industry as a cast alloy.In the present work,the influence of cooling rate during mushy zone on solidification behavior of Ti-44Al-4Nb-2Cr-0.1B alloy was investigated.A vacuum induction heating device combining with temperature control system was used.The Ti-44Al-4Nb-2Cr-0.1B alloy solidified from superheated was melted to β phase with the cooling rates of 10,50,100,200,400 and 700 K·min~(-1),respectively.Results show that with the increase in cooling rate from 10 to 700 K·min~(-1),the colony size of α_2/γ lamella decreases from 1513 to48 urn and the solidification segregation significantly decreases.Also the content of residual B2 phase within α_2/γlamellar colony decreases with the increase in cooling rate.In addition,the alloy in local interdendritic regions would solidify in a hypo-peritectic way,which can be attributed to the solute redistribution and enrichment of Al element in solidification.     

Effect of welding processes on joint characteristics of Ti–6Al–4V alloy

Abstract

Although Ti–6Al–4V alloys show reasonable weldability characteristics, the joint properties are greatly influenced by the welding processes. Microstructures and tensile and impact properties of welded Ti–6Al–4V alloy were evaluated for high vacuum electron beam welding, CO₂ laser beam welding and gas tungsten arc welding. The resultant tensile and impact properties of the welded joints are correlated with the weld metal microstructure and hardness. The results indicate that the electron beam welding is more suitable for Ti–6Al–4V sheet welding and the welding seam without defects can be obtained. The full penetration butt welds are obtained by gas tungsten arc welding process, but they have many drawbacks such as wide weld seam, big deformation and coarse grains. Laser beam welding has many advantages such as the narrowest weld seam, the least deformation and the finest grains, but it should be studied again for the reasons of unstable welding technologies and strict condition.     

Microstructural,mechanical, and electrical characterization of directionally solidified Al–Cu–Mg eutectic alloy

The composition of an Al–Cu–Mg ternary eutectic alloy was chosen to be Al–30 wt% Cu–6 wt % Mg to have the Al₂Cu and Al₂CuMg solid phases within an aluminum matrix (α-Al) after its solidification from the melt. The alloy Al–30 wt % Cu–6 wt % Mg was directionally solidified at a constant temperature gradient (G = 8.55 K/mm) with different growth rates V, from 9.43 to 173.3 μm/s, by using a Bridgman-type furnace. The lamellar eutectic spacings (λ_E) were measured from transverse sections of the samples. The functional dependencies of lamellar spacings λ_E (\({\lambda _{A{l_2}CuMg}}\) and \({\lambda _{A{l_2}Cu}}\) in μm), microhardness H _V (in kg/mm²), tensile strength σ_T (in MPa), and electrical resistivity ρ (in Ω m) on the growth rate V (in μm/s) were obtained as \({\lambda _{A{l_2}CuMg}} = 3.05{V^{ - 0.31}}\), \({\lambda _{A{l_2}Cu}} = 6.35{V^{ - 0.35}}\), \({H_V} = 308.3{\left( V \right)^{ - 0.33}}\); σ_T= 408.6(V)^0.14, and ρ = 28.82 × 10^–8(V)^0.11, respectively for the Al–Cu–Mg eutectic alloy. The bulk growth rates were determined as \(\lambda _{A{l_2}CuMg}^2V = 93.2\) and \(\lambda _{A{l_2}Cu}^2V = 195.76\) by using the measured values of \({\lambda _{A{l_2}CuMg}}\), \({\lambda _{A{l_2}Cu}}\) and V. A comparison of present results was also made with the previous similar experimental results.     

Effect of Al on Expansion Behavior of Mg–Al Alloys During Solidification

The cooling curves and the change of contraction/expansion during solidification and cooling were tested by using a selfmade device which could achieve the one-dimensional contraction instead of three-dimensional contraction of the casting.Then, the effects of Al content(0, 1.1, 3, 5, 10, 12.9, 15, 17, 19, 22, 24 and 30 wt%) on the thermal contraction/expansion of the binary Mg-Al as-cast alloys during solidification were obtained. The results showed that expanding instead of contraction was present in Mg-Al alloys with the addition of 0-30 wt% Al during solidification. The values of expansion significantly increased at first and then decreased with the increase in Al content. And the maximum expansion ratio of 0.44%(maximum expansion value: 0.841 mm) was present in the Mg-15 wt% Al alloy. Contraction instead of expansion occurred once the temperature drops to the temperature corresponding to the expansion value in total, indicating the occurrence of a continuous expansion during the solidification process in mushy zone for the Mg alloys with Al addition of 5-30 wt%. The expansion value in total consisted of two parts: the expansions occurring in the liquid-phase zone and mushy zone. The expansion in liquid zone was present in every Mg-Al alloy, and it contributed to the most proportion of the total expansion value when the Al content in Mg-Al alloy was lower than 10 wt% or higher than 22 wt%. However, the total expansion value was mainly determined by the solidification behavior in mushy zone when the Al content was among 10-22 wt% in Mg-Al alloys.     

High cyclic fatigue performance of Al–Cu–Mg–Ag alloy under T6 and T840 conditions

High cyclic fatigue (HCF) behavior of an AA2139 alloy belonging to Al–Cu–Mg–Ag system in T6 and T840 conditions was examined. The T840 treatment involving cold rolling with a 40% reduction prior to peak ageing provides an increase in tensile strength compared with the T6 condition. However, fatigue lifetime for two material conditions was nearly the same since there is weak effect of thermomechanical processing on micro-mechanisms of crack initiation and growth.     

Effect of temperature conditions on grain refinement of Mg–Al alloy under ultrasonic field

The effects of temperature conditions on the grain refinement of a Mg–Al alloy by ultrasonic treatment were investigated. It was found the grain refinement strongly depended on the temperature. When the ultrasonic treatment was performed above the liquidus temperature, the better grain refinement was achieved in the ingot treated at 700 °C rather than at lower temperature. While for the cases of the ultrasonic treatment being ended below the liquidus temperature, the better refinement can be achieved at lower ending temperature. The undendritic structrue of UST ingots is obviously different with the dendritic structure of UMT ingots. The ultrasonic cavitation improves the nucleation temperature, resulting in greater undercooling to benefit the nucleation. The acoustic streaming accelerates the release of latent heat of solidification and shorten the time for grain growth. Both of the two factors contribute to achieve the grain refinement.     

Prediction of the morphology of Mg32(Al,Zn)49 precipitates in a Mg–Zn–Al alloy

A geometrical model has been applied to predict the morphology of faceted Mg₃₂(Al, Zn)₄₉ precipitates in a Mg–Zn–Al alloy using the observed orientation relationship (OR) and the lattice parameters of the precipitates and the matrix as inputs. Planes in rational or in irrational orientations with higher densities of good matching sites are more likely to be preferred, which agrees well with experimental observations.     

Diffusion bonding of γ-TiAl alloy to Ti-6Al-4V alloy under hot pressure

1 Introduction Alloys based on the ?-TiAl intermetallic compounds have been of much interest in recent years as light-mass structural materials for elevated temperature aerospace applications[1- 5]. Successful joining and cost effective fabrication method…     

Oxide film characteristics of Al–7Si–Mg alloy in dynamic conditions in casting

Abstract

'New' oxide film which forms in a very short time in the casting process was studied. Samples for the study were prepared based on a technique in which an oxide–metal 'sandwich' could be made. Alloy A356 (Al–7Si–0.4Mg) was selected for the study. Features such as thickness of the oxide film, its morphology, rigidity and presence of eutectic phase have been examined and shown by SEM study. Possible consequences of the morphology of the oxide film are discussed.     

Kinetics and fracture behavior under cycle loading of an Al–Cu–Mg–Ag alloy

The behavior of aluminum alloy AA2139 subjected to T6 treatment, including solution treatment and artificial aging, has been studied using cyclic loading with a constant total strain amplitude. Upon low-cyclic fatigue in the range of total strain amplitudes ε_ac of 0.4–1.0%, the cyclic behavior of the AA2139-T6 alloy is determined by the processes that occur under the conditions of predominance of the elastic deformation over plastic deformation. The AA2139 alloy exhibits stability to cyclic loading without significant softening. The stress-strained state of the alloy upon cyclic loading can be described by the Hollomon equation with the cyclic strength coefficient K' and the cyclic strain-hardening exponent n' equal to 641 MPa and 0.066, respectively. The dependence of the number of cycles to fracture on the loading amplitude and its components (amplitudes of the plastic and elastic deformation) is described by a Basquin–Manson–Coffin equation with the parameters σ′/E = 0.014, b =–0.123, ε′_f= 178.65, and c =–1.677.     

Strength properties and structure of a submicrocrystalline Al–Mg–Mn alloy under shock compression

The results of studying the strength of a submicrocrystalline aluminum A5083 alloy (chemical composition was 4.4Mg–0.6Mn–0.11Si–0.23Fe–0.03Cr–0.02Cu–0.06Ti wt % and Al base) under shockwave compression are presented. The submicrocrystalline structure of the alloy was produced in the process of dynamic channel-angular pressing at a strain rate of 10⁴ s^–1. The average size of crystallites in the alloy was 180–460 nm. Hugoniot elastic limit σ_HEL, dynamic yield stress σ_y, and the spall strength σ_SP of the submicrocrystalline alloy were determined based on the free-surface velocity profiles of samples during shock compression. It has been established that upon shock compression, the σ_HEL and σ_y of the submicrocrystalline alloy are higher than those of the coarse-grained alloy and σ_sp does not depend on the grain size. The maximum value of σ_HEL reached for the submicrocrystalline alloy is 0.66 GPa, which is greater than that in the coarse-crystalline alloy by 78%. The dynamic yield stress is σ_y = 0.31 GPa, which is higher than that of the coarse-crystalline alloy by 63%. The spall strength is σ_sp = 1.49 GPa. The evolution of the submicrocrystalline structure of the alloy during shock compression was studied. It has been established that a mixed nonequilibrium grain-subgrain structure with a fragment size of about 400 nm is retained after shock compression, and the dislocation density and the hardness of the alloy are increased.     

Enhancing the intergranular corrosion resistance of high Mg-alloyed Al–Mg alloy by Y addition

Microalloying is thought to improve the performance of Al–Mg alloys commonly used in transport applications. The effect of Y addition (0–0.4%) on the microstructure, mechanical properties, and corrosion resistance of Al–9.2Mg–0.7Mn alloy is investigated for potential use in engineering applications. The generation of the β-Al₃Mg₂ phase along the grain boundaries is suppressed in the as-cast alloy due to the formation of the AlMgY ternary phase. The average intergranular corrosion mass loss of the alloy with 0.1% Y addition decreases about 53.1% almost at no expense of mechanical performance in the as-rolled alloy after annealing. Moreover, the alloy with 0.1% Y addition shows the corrosion mass loss about 30.2% lower than the Y-free alloy in the sensitized state. The enhanced corrosion resistance of the alloy can be ascribed to the reduced β-Al₃Mg₂ precipitation along the grain boundaries associated with Y addition.     

Microstructural characteristics and mechanical properties of Al–Mg–Si alloy self-reacting friction stir welded joints

The 5?mm thick Al–Mg–Si alloy was self-reacting friction stir welded using the specially designed tool at a constant rotation speed of 400?rev?min^?1 with various welding speeds. Defect-free welds were successfully obtained with welding speeds ranging from 150 to 350?mm?min^?1, while pore defects were formed in the weld nugget zone (WNZ) at a welding speed of 450?mm?min^?1. Band patterns were observed at the advancing side of WNZ. Grain size and distribution of the precipitated phase in different regions of the joints varied depending on the welding speed. The hardness of the weld was obviously lower than that of the base metal, and the lowest hardness location was in the heat affected zone (HAZ). Results of transverse tensile tests indicated that the defective joint fractured in the WNZ with the lowest tensile strength, while the fracture location of the defect-free joints changed to the HAZ.     

Dynamic recrystallization,texture and mechanical properties of high Mg content Al–Mg alloy deformed by high strain rate rolling

The Al–Mg alloy with high Mg addition (Al–9.2Mg–0.8Mn–0.2Zr-0.15Ti, in wt.%) was subjected to different passes (1, 2 and 4) of high strain rate rolling (HSRR), with the total thickness reduction of 72%, the rolling temperature of 400 °C and strain rate of 8.6 s⁻¹. The microstructure evolution was studied by optical microscope (OM), scanning electron microscope (SEM), electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The alloy that undergoes 2 passes of HSRR exhibits an obvious bimodal grain structure, in which the average grain sizes of the fine dynamic recrystallization (DRX) grains and the coarse non-DRX regions are 6.4 and 47.7 μm, respectively. The high strength ((507±9) MPa) and the large ductility ((24.9±1.3)%) are obtained in the alloy containing the bimodal grain distribution. The discontinuous dynamic recrystallization (DDRX) mechanism is the prominent grain refinement mechanism in the alloy subjected to 2 passes of HSRR.     

Effect of hot-dip aluminizing on the oxidation resistance of Ti–6Al–4V alloy at high temperatures

Hot-dip aluminizing and interdiffusion treatment were used to develop a TiAl₃-rich coating on Ti–6Al–4V alloy. Interrupted oxidation at temperatures from 600 to 900 °C and isothermal oxidation at 700 and 800 °C of the coating were conducted. The coating markedly decreases the oxidation rate in comparison with the alloy at temperatures below 800 °C during the interrupted oxidation. The oxidation kinetics follows parabolic relations at 700 and 800 °C during the isothermal oxidation. A layered structure of Al₂O₃/TiAl₃/TiAl₂/TiAl/alloy from the outside to the inside forms after oxidation at 700 °C without changing the main body of the coating.