Coarse Second-phase Particles Research Articles

Structure refinement and performance balance of high-strength aluminum alloys using an improved thermomechanical treatment process

The microstructure of 7A85 aluminum alloy forgings often contains coarse second-phase particles and grains. To improve the microstructures and balance the properties, we used an enhanced thermomechanical treatment process that included multiaxial hot forging, multiaxial cryoforging (MACF), and three-step aging treatment. We thoroughly investigated the effects of MACF with 1–3 passes on the evolution of microstructure, three-dimensional (3D) mechanical properties at the center and edge, fracture toughness, and corrosion resistance of the alloy. Our findings revealed that multiaxial hot forging had a limited ability to crush the coarse second-phase particles. However, the higher flow stress and compound brittleness of MACF resulted in a significant increase in particle fragmentation and driving force for precipitation. Additionally, under MACF, massive dislocations proliferated due to the inhibition of dynamic recovery, forming large orientation gradient regions, which promote static recrystallization and grain refinement during solution treatment. As the number of MACF passes increased from one to three, the effect of particle crushing initially increased and then plateaued, the 3D grain sizes first decreased and then slightly increased, and the spacing of grain boundary precipitates (GBPs) increased. The dense and uniform precipitates, finest equiaxed grains, a low fraction of coarse particles, and large GBP spacing in MACF2 resulted in the higher strength, highest elongation with minimal anisotropy, superior fracture toughness, and enhanced corrosion resistance. Ultimately, MACF2 demonstrated the best overall performance and good homogeneity, with an average 3D ultimate tensile strength, average 3D yield strength, average 3D elongation, and fracture toughness at the center of 548 MPa, 476 MPa, 13.4%, and 42.5 MPa m1/2, respectively. These values were respectively 4.2%, 1.9%, 50.6%, and 13.9% higher than those of the uncompressed sample.

Investigation of the effects of beam oscillations in electron beam–welded S1100M TMCP steel

The development of thermomechanically controlled processed (TMCP) high-strength steel (HSS) has significantly contributed to designing and developing the intricate structural components. It has broader applications in the cranes and lifting process industry (base frame, crane jibs, and crane columns), trailers, agricultural and forestry machinery, earth-moving equipment, etc. However, the development of new-grade steels with higher tensile strength led to higher requirements for welded joints, and the associated weldability issues have inspired detailed studies on electron beam welding (EBW) with different beam oscillations. Beam oscillation application with EBW processes improves the welding efficiency, weld quality, weld geometry, keyhole, etc., affecting the welded joints mechanical and microstructural properties. Thus, the present study investigates the impact and comparison of various beam oscillations on the microstructural and mechanical properties of EB-welded S1100M steel. The influence of welding parameters on the microstructure of welded joints was analyzed using a scanning electron microscope (SEM) and electron backscattered diffraction (EBSD). The analysis focused on evaluation of grain sizes, morphologies, distributions, and crystallographic orientations of different phase constituents in fusion zone (FZ) and heat-affected zone (HAZ). The mechanical properties were analyzed using hardness, tensile, and Charpy V-notch impact tests. The texture in the FZ is typically random, while the HAZ typically exhibits a strong rolling texture. In general, the cooling rate in EBW is very fast, possibly resulting in a fine-grained structure and reduced formation of coarse second-phase particles in the weld zone. The elliptical beam oscillation showed the highest hardness in HAZ 450 HV10. Elliptical beam oscillation slightly improves the welded joint’s tensile strength, and the impact test showed mixed fracture behavior.

Effect of coarse second-phase particles on mechanical properties of large-scale 2219 Al alloy rings

Large-scale 2219 Al alloy rings are widely used as important parts on launch vehicles and thus have high requirements for the mechanical properties of rings in different direction, which are associated with coarse second-phase particles. Therefore, the objective of this study is to provide a comprehensive and systematic quantitative analysis of the effect degree of the coarse second-phase particles characteristics on the mechanical properties of 2219 Al alloy rings in different direction, so as to guide the design and optimization of the global process based on the relationship between each process and the coarse second-phase particles. Firstly, according to the features of the coarse second-phase particles characteristics in the experimental results, a reliable research scheme with the particles size, percentage and distribution as the study variable was designed. Then, based on the experimental characterization results, a representation volume element modeling method incorporating coarse second-phase particles characteristics was developed. The representation volume element finite element models for different research schemes were established in the ABAQUS/EXPLICIT platform respectively. Next, the comparison of experimental and numerical simulation results verifies that the finite element model with coarse second-phase particles developed in this paper can accurately predict ultimate tensile strength and elongation of the tested specimen. Finally, based on the numerical simulation results of the different research schemes, the effects of different sizes, percentages and distributions of coarse second-phase particles on the mechanical properties of the ring are analyzed to determine the extent to which the mechanical properties can be improved with different coarse second-phase particles characteristics. By exploring the relationship between the manufacture process, coarse second-phase particles characteristics and mechanical properties, the design and optimization of the manufacture process for different mechanical properties requirements are guided. This study is valuable in achieving comprehensive regulation of the mechanical properties of large-scale 2219 Al alloy rings.

Strengthening mechanism of 2219 Al-Cu alloy by room-temperature random vibration

Conventional thermal strengthening treatments face the challenges of high energy consumption, environmental pollution, and low coordination between strength and toughness. This study attempted to suppress these imperfections by proposing a novel approach that employed room-temperature random vibration (RRV). The microstructures were analyzed by scanning electron microscopy and transmission electron microscopy in detail. The 2219 Al-Cu alloy is in a tension–compression bidirectional stress state during random vibration, and its stress state changes with frequency, which induces dislocation movement and multiplication. The dislocation movement eventually develops into a dislocation network, resulting in the dislocation pinning effect. Notably, the dislocation migration promotes the diffusion of solute and forms solute clusters owing to tension–compression deformation. The content of the coarse second phase particles (CSPP) decreased greatly with a small narrow precipitate-free zone (PFZ) width after random vibration treatment. The tensile and yield strength were improved by 13.5% and 17.6%, respectively, after RRV treatment. The ductility increases simultaneously, differing from conventional heat treatment processes, which is attributed to the joint action effect of dislocations, second-phase particles, and solute clusters. The findings confirm a new route for Al-Cu alloys to obtain excellent mechanical properties and provide valuable insight into the strengthening mechanism of room-temperature random vibration.

Effect of three-dimensional deformation at different temperatures on microstructure, strength, fracture toughness and corrosion resistance of 7A85 aluminum alloy

In this study, 7A85 aluminum alloy was subjected to 20% three-dimensional (3D) deformation at different temperatures (−196, 25, 250, and 480 °C), followed by solid solution, water quenching, 3% cold deformation, and retrogression and re-aging (RRA) treatment. The microstructural evolution and 3D mechanical properties, fracture toughness, electrical conductivity, and corrosion resistance of the alloy under different deformation processes were analyzed for comparison. The results indicated that conducting the 3D deformation process prior to solution treatment significantly improved the strength and fracture toughness of the alloy as compared to the undeformed (UD) specimen, and maintained its good corrosion resistance under the RRA treatment. In addition, with the decrease of the deformation temperature, the greater flow stress strongly sheared and crushed the coarse second-phase particles into fine particles, thus facilitating their full dissolution in the aluminum matrix during subsequent solution treatment and increasing the driving force for aging precipitation. Furthermore, more dislocations were introduced at low-temperature deformation, thus providing more nucleation sites for static recrystallization during solution treatment and refining grains. Ultimately, the densest precipitates and the finest equiaxed grains with the narrowest grain boundary precipitate-free zones (PFZs) were acquired for the cryogenic deformation (CD) sample, resulting in the highest ultimate tensile strength (UTS), elongation (EL) with the least anisotropy, and fracture toughness of the CD specimen. The optimal comprehensive performance was obtained for the CD specimen with the average 3D UTS, average 3D yield strength (YS), average 3D EL, and fracture toughness of 545 MPa, 476 MPa, 16%, and 42.49 MPa·m1/2, respectively, which were respectively 2.3%, 0.2%, 18.5%, and 21.4% higher than those of the UD specimen.

Tensile Fracture Behavior of 2A14 Aluminum Alloy Produced by Extrusion Process

In this study, the tensile mechanical properties of a 2A14 aluminum alloy produced by extrusion were tested at room temperature to investigate the tensile fracture behavior. The results showed that the tensile fracture of the alloy was mixed intergranular and transgranular ductile fracture, as numerous coarse second-phase particles were distributed in a band along the loading direction, making it prone to microcracking. This was determined to be the main cause of fracture failure of the alloy. In addition, we observed large α-AlFeMnSi(Cu), Al(Fe,Mn)Cu, AlCuMgSi, and Al2Cu phases in the microstructure of the 2A14 aluminum alloy, and both Al2Cu second phase and precipitation-free phase zone (PFZ) at the grain boundaries were observed, which made the alloy susceptible to fracture failure and reduced the mechanical properties of the alloy.

Texture Evolution and Control of 2524 Aluminum Alloy and Its Effect on Fatigue Crack Propagation Behavior

The influences of cold rolling and subsequent heat treatment on the microstructure evolution of 2524 alloy were investigated using an orientation distribution function (ODF) and electron back-scattered diffraction (EBSD). A preparation method of 2524-T3 aluminum alloy with a strong Brass texture was developed, and its effect on the fatigue properties of the alloy was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that with the increase in cold rolling deformation from 0% to 80%, the volume fractions of Brass, copper, and S textures in the 2524-T3 alloy also increase, especially in the case of Brass and S textures. However, the volume fractions of cube and Goss textures are reduced significantly, especially for cube textures, which are decreased by 57.4%. Reducing coarse second-phase particles (CSPs) is conducive to the formation of a strong deformation texture during cold rolling. A 10% deformation at each rolling pass, followed by a step annealing, helps the preservation of a Brass texture even after solution treatment at 500 °C for 0.5 h, while a large cold deformation followed by high-temperature annealing helps the formation of a strong cube texture. The Brass texture can enhance the strength while decreasing the fatigue crack growth resistance of this alloy.

Effects of warm saddle forging deformation on the reduction of second-phase particles and control of the three-dimensional mechanical properties of 2219 aluminum alloy rings

Severely segregated Al2Cu coarse second-phase particles (CSPPs) and uneven mechanical properties are the main problems in the manufacturing process of 2219 aluminum alloy large rings. A new method for the control of the three-dimensional mechanical properties (TDMPs) of the rings is proposed in the present study. The evolution law of the Al2Cu particles and the regulation mechanism of the TDMPs were investigated. The research revealed that optimizing the warm compression and rolling deformation can effectively reduce the quantity of CSPPs and regulate the TDMPs. With the increase of compression deformation (from 30% to 70%), the degree of the crushing and passivation of CSPPs was found to increase, the area fraction of the CSPPs decreased by 72.6%, and their roundness factor increased from 0.694 to 0.845; in the compression deformation process, high-density dislocations can effectively promote the crushing, dissolution and diffusion of Al2Cu particles, thereby enhancing the precipitation driving force of the strengthening phase during aging, resulting in a 99% increase in the quantity of the θ′ phase. When the compression deformation was 20% at 510 °C and 50% at 240 °C, the tensile strength, yield strength, and elongation of the ring in the radial direction were found to increase from 392 MPa, 287 MPa, and 6.4% to 468 MPa, 358 MPa, and 11.2%, respectively; moreover, the anisotropies of the three indicators of the rings were reduced by 32.6%, 54.0%, and 38.9%, respectively.

Refinement of second-phase particles and grain structures of 2219 Al[sbnd]Cu alloy forgings using an improved thermomechanical treatment process

2219 AlCu alloy components are conventionally fabricated by multiaxial hot forging, but they often contain coarse second-phase particles and grain structures. In this work, an improved thermomechanical treatment process involving multiaxial hot forging, multiaxial cryoforging, and T8 heat treatment (i.e., solid solution, pre-deformation, and artificial aging), was used to refine the coarse microstructures of 2219 AlCu alloy. The microstructure evolution during this process and its effect on the mechanical properties were studied. The results show that the second-phase particles experienced limited break-up during multiaxial hot forging but were significantly broken into finer particles during multiaxial cryoforging. Particle fragmentation gradually increased upon increasing the number of cryoforging passes, which increased the particle/matrix interfacial energy. Meanwhile, ultrafine-grain structures and high dislocation density were formed in the cryoforged sample after six passes, which increased the driving force for recrystallization and thus the number of nucleation sites in the material. Consequently, the grain size decreased to 18.9 μm compared with the 29.5 μm grain size of the sample obtained in one pass after T8 heat treatment. The smaller particle size and increased number of grain boundaries promoted the dissolution of second-phase particles, which increased the density of precipitates during subsequent artificial aging. As a result, the forging obtained by the thermomechanical treatment displayed good mechanical properties, with a yield strength of 375 MPa, ultimate tensile strength of 473 MPa, and elongation of 16.3%.

Influence of cryogenic deformation on second-phase particles, grain structure, and mechanical properties of Al–Cu–Mn alloy

To refine the coarse second-phase particles (Al2Cu) and grains of Al–Cu–Mn alloy, 10% and 20% compression deformation at cryogenic and room temperatures was applied, followed by a T8 heat treatment process consisting of solution treatment, 3% cold deformation, and artificial aging. The microstructures of the compressed and heat-treated samples were investigated using a combination of scanning electron microscopy, X-ray diffractometry, electron backscattering diffractometry, and transmission electron microscopy, with the results showing that coarse Al2Cu particles are fragmented during cryogenic compression. Further, the higher dislocation densities in the cryogenically deformed samples, which lower the activation energies for atomic diffusion, promote the dissolution of particles during subsequent heat treatment. After heat treatment, the volume fraction of coarse particles decreased from 2.5% for a sample processed without deformation, to 0.5% for the sample cryogenically compressed at a ratio of 20%. Meanwhile, the higher dislocation density provides more nucleation sites for recrystallization during heat treatment. Consequently, at 33 μm, the average size of the grains in the 20% cryogenically compressed sample was much smaller than the size of the grains in the uncompressed sample (500 μm). This microstructural refinement increased the ultimate tensile strength, yield strength, and elongation from 445 MPa, 363 MPa, and 7.1% in the undeformed sample, to 471 MPa, 377 MPa, and 13.4%, respectively, in the cryogenically compressed sample.

Role of second-phase particles in the microstructural evolution and mechanical properties of extruded Mg-Gd-Y-Ag-Zr alloys

The role of coarse second-phase particles in the microstructural evolution and mechanical properties of as-extruded Mg-Gd-Y-Ag-Zr alloys was investigated. Results show that the volume fraction of coarse β particles was enhanced by the addition of Ag (0.5 and 1.0) (wt.%) in the Mg-10.5 Gd-5 Y-0.5 Zr (wt.%) alloy. These particles have a face-centered cubic structure phase. In addition to Gd and Y, Ag is also partitioned into particles in the Ag-containing alloys. β particles can promote the dynamic recrystallization via particle-simulated nucleation and effectively suppress grain growth by pinning effects, which are responsible for the grain refinement. With the increase of volume fraction of β particles, the texture and dislocation features have not been changed. Since the coarse β particles can greatly lead to the reduction of grain size, the grain boundary strengthening in Ag-containing alloys with high volume fraction of particles contributes to the significantly improvement of the tensile yield strength. The coupled effect of the increase in the coarse β particle fraction and the reduction in grain size causes a slight decrease in elongation for the Ag-containing alloy.

Role of Sr in Microstructure, Hardness and Biodegradable Behavior of Cast Mg–2Zn–2Ca–0.5Mn (ZXM220) Alloy for Potential Implant Application

Microstructure, hardness and biocorrosion properties of ZXM220–xSr alloys (x = 0, 0.2, 0.4, 0.8 and 1 wt%) were investigated, and the results were compared with each other. Microstructural characterizations were carried out by XRD, optical and scanning electron microscopes. Biocorrosion properties were determined by immersion and electrochemical corrosion tests, performed in a simulated body fluid solution at 37 °C. Compared to the Sr-free alloy, Sr-bearing alloys exhibited a smaller grain size and a higher volume fraction of second-phase particles. The hardness of the alloy was also gradually improved by increasing Sr additions. Biocorrosion properties of the alloy significantly improved by 0.2 wt% Sr addition, and these properties deteriorated with higher additions of Sr due to the strong microgalvanic effect of coarse second-phase particles. Therefore, the ZXM220–0.2Sr alloy can be regarded as a promising candidate for a new biodegradable implant material due to the optimum results of mechanical and biocorrosion properties.

Strengthening of Aluminum Alloy 2219 by Thermo-mechanical Treatment

Strengthening of aluminum alloy 2219 by thermo-mechanical treatment has been compared with artificial aging. Three simple deformation modes including pre-stretching, compression, and rolling have been used in thermo-mechanical treatment. The tensile strength, elongation, fracture feature, and precipitated phase have been investigated. The results show that the strengthening effect of thermo-mechanical treatment is better than the one of artificial aging. Especially, the yield strength significantly increases with a small decrease of elongation. When the specimen is pre-stretched to 8.0%, the yield strength reaches 385.0 MPa and increases by 22.2% in comparison to the one obtained in aging condition. The maximum tensile strength of 472.4 MPa is achieved with 4.0% thickness reduction by compression. The fracture morphology reveals locally ductile and brittle failure mechanism, while the coarse second-phase particles distribute on the fracture surface. The intermediate phases θ″ or θ′ orthogonally precipitate in the matrix after thermo-mechanical treatment. As compared to artificial aging, the cold plastic deformation increases distribution homogeneity and the volume fraction of θ′′ or θ′ precipitates. These result in a better strengthening effect.

Effect of Coarse Second-Phase Particles on Galvanic Corrosion of Anodized 6061 Aluminum Alloy Coupled with C1100 Copper

Effect of Coarse Second-Phase Particles on Galvanic Corrosion of Anodized 6061 Aluminum Alloy Coupled with C1100 Copper

Recrystallization of ECAP-Processed AA4343 Aluminium Alloy Containing Large Second Phase Particles

The objective of this study was to determine the effect of ECAP-processing on the texture and the microstructure development of the deformed and recrystallized material. The commercial purity AA4343 aluminium alloy, containing very large second phase particles (SPP) of pure silicon, was deformed via route C up to 10 passes and then annealed. The samples were characterized by TEM and SEM equipped with high resolution EBSD. It was found that the microstructure of the deformed state in areas far from coarse SPP was very similar to that observed in pure aluminium. The orientations of the new grains were scattered but not random and only specific groups of orientations were observed.

Influence of processing route on microstructure and mechanical properties of two Mg–Ni–Y–RE alloys

The effect of the processing route on the microstructure and mechanical properties of two Mg–Ni–Y–CeMM alloys has been evaluated. The alloys processed by a powder metallurgy route presented higher strength, thanks to the finer grain size and more homogeneous dispersion of second-phase particles. In the alloys prepared by hot extrusion of as-cast rods, hardening induced by coarse second-phase particles was higher due to load transfer from the magnesium matrix towards the particles. Irrespective of the processing route, the absence of decohesion between the different interfaces in the Mg–Ni–Y–CeMM alloys evidenced their inherent strong nature.

Refinement of the microstructure during superplastic deformation of extruded Mg 94Ni 3Y 1.5CeMM 1.5 alloy

The microstructure and mechanical properties of extruded Mg 94 Ni 3 Y 1.5 CeMM 1.5 alloy were evaluated between 250 and 400 °C. The alloy exhibited superplasticity between 300 and 400 °C. Superplastic deformation was characterized by the refinement of coarse second-phase particles and their continuous redistribution in the magnesium matrix. The equiaxed structure of the magnesium matrix in the deformed samples as well as the stress exponent close to 2 indicated that grain boundary sliding was the mechanism controlling the superplastic deformation.

Resistance-Spot-Welded AZ31 Magnesium Alloys: Part I. Dependence of Fusion Zone Microstructures on Second-Phase Particles

A comparison of microstructural features in resistance spot welds of two AZ31 magnesium (Mg) alloys, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), with the same sheet thickness and welding conditions, was performed via optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). These alloys have similar chemical composition but different sizes of second-phase particles due to manufacturing process differences. Both columnar and equiaxed dendritic structures were observed in the weld fusion zones of these AZ31 SA and SB alloys. However, columnar dendritic grains were well developed and the width of the columnar dendritic zone (CDZ) was much larger in the SB alloy. In contrast, columnar grains were restricted within narrow strip regions, and equiaxed grains were promoted in the SA alloy. Microstructural examination showed that the as-received Mg alloys contained two sizes of Al8Mn5 second-phase particles. Submicron Al8Mn5 particles of 0.09 to 0.4 μm in length occured in both SA and SB alloys; however, larger Al8Mn5 particles of 4 to 10 μm in length were observed only in the SA alloy. The welding process did not have a great effect on the populations of Al8Mn5 particles in these AZ31 welds. The earlier columnar-equiaxed transition (CET) is believed to be related to the pre-existence of the coarse Al8Mn5 intermetallic phases in the SA alloy as an inoculant of α-Mg heterogeneous nucleation. This was revealed by the presence of Al8Mn5 particles at the origin of some equiaxed dendrites. Finally, the columnar grains of the SB alloy, which did not contain coarse second-phase particles, were efficiently restrained and equiaxed grains were found to be promoted by adding 10 μm-long Mn particles into the fusion zone during resistance spot welding (RSW).

Improvement of warm formability of Al-Mg sheet alloys containing coarse second-phase particles

Several alloying elements involving Zr, Cu, Zn and Sc were added to Al-Mg sheet alloys in order to obtain an excellent combination of high strength and good high-temperature formability. Microstructural examination showed that coarse intermetallic particles were formed in the microstructure and their amounts changed with variations of the alloying elements. During warm rolling of thermomechanical treatments prior to warm deformation, the coarse particles initiated cracks, decreasing the warm formability. For healing the crack damage and further improving the warm formability, a process of hot isothermal press was developed and optimized to the sheet alloys. With this process, the biaxial stretch formability at 350°C was improved by 22% for an aluminum alloy containing a large amount of coarse particles.

The effect of coarse second-phase particles and fine precipitates on microstructure refinement and mechanical properties of severely deformed Al alloy

The sub-micrometer microstructure developed during severe plastic deformation by equal channel angular pressing in an Al alloy containing a small amount of dispersed intermetallic particles has been examined in material in a variety of solutionised and precipitated states. Microstructural refinement, and corresponding strengthening, occurs faster in these particle-containing materials than in particle-free equivalents. Coarse precipitate and dispersoid particles remain mostly unaffected by the severe deformation, and appear to help stabilise a finer deformation microstructure, while fine precipitates are sheared and apparently dissolved. The extent of microstructural refinement and strengthening depends more on solute content than on particle distributions. The limited effect of dispersoid and precipitate particles may be due, in part, to their presence in only small volume fractions, less than 1% by volume each. Strengthening depends more on dislocation arrangements and densities than on the number of boundaries present.