Microstructure evolution of the transitional region in isothermal local loading of TA15 titanium alloy

The microstructure of the transitional region determines the performance of TA15 component under isothermal local loading forming. To understand the characteristic of microstructure evolution in transitional region, a set of analogue experiments for isothermal local loading were designed and carried out to investigate the microstructure development under different temperatures and complex strain path. It is found that the isothermal local loading does not change the microstructural composition of TA15 alloy with initially equiaxed microstructure in the transitional region, though a small fraction of lamellar α phases appear at temperatures above 950 °C due to reheating and small deformation in the second loading step. Recrystallization takes place in the β matrix but not in the primary equiaxed α phases. The β grains are fine and equiaxed irrespective of strain path due to recrystallization. The existence of lamellar α refines the β grains. Kinked and disordered primary equiaxed α phases are produced under complex strain path. The grain size of primary equiaxed α phases increases slightly due to static coarsening and strain induced grain growth in multi-heat processing. The volume fraction of primary equiaxed α phases is not influenced by strain path but determined by working temperature.     

Materials modeling and simulation of isothermal forging of rolled AZ31B magnesium alloy: Anisotropy of flow

Isothermal forging of a rib–web shape in AZ31B magnesium alloy in the rolling direction was conducted at speeds of 0.01–10 mm s⁻¹ in the temperature range of 300–500 °C with the purpose of validating the results of materials models involving kinetic analysis and processing map. The process was also simulated using finite element method DEFORM to obtain the local values of strain and strain rate. Forging parallel to the rolling direction in the range 375–550 °C and 0.0003–0.3 s⁻¹ under the conditions of dynamic recrystallization (DRX) resulted in a symmetrical cup-shape while at other conditions an elliptical boat-shape was produced with the major axis coinciding with the transverse direction and the minor axis aligned with the normal direction. This anisotropy of flow has been attributed to the strong basal texture in the rolled plate and the dominance of prismatic slip at lower temperatures. In the DRX domain on the other hand, pyramidal slip dominates along with cross-slip as the recovery mechanism, which destroys the initial texture and restores the symmetry of flow. The grain size variation for forgings done in the DRX domain validated the predictions of the material models.     

Study of the dynamic recrystallization of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process via Finite Element Method modeling and microstructure characterization

By integrating the thermomechanically coupled simulation with the mathematically modeling of microstructure evolution using Finite Element Method (FEM), the study of the dynamic recrystallization (DRX) of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process is conducted. Through physical experiment, microstructure characterization and FEM-based microstructure modeling, the DRX behavior of the Ti-alloy in β-forging process is extensively explored. The effects of plastic deformation strain, strain rate and deformation temperature on the DRX of the Ti-alloy in terms of DRX volume fraction, DRX grain size and the average grain size are systematically investigated. The simulation results show that the increase of plastic deformation strain, deformation temperature, and strain rate contributes to the DRX of the alloy. The simulation and experimental results further reveal that the FEM-based microstructure evolution modeling is able to predict the DRX behavior and the microstructure evolution of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process.     

Prediction of flow stress in isothermal compression of Ti60 alloy using an adaptive network-based fuzzy inference system

Isothermal compression of Ti60 titanium alloy at the deformation temperatures ranging from 960 to 1110 °C, the strain rates ranging from 0.001 to 10 s⁻¹ and the height reductions of 60% were carried out on a Gleeble–3800 simulator. An adaptive network-based fuzzy inference system (ANFIS) model has been established to predict the flow stress of Ti60 alloy during hot deformation process. A comparative evaluation of the predicted and the experimental results has shown that the ANFIS model used to predict the flow stress of Ti60 titanium alloy has a high accuracy. The maximum difference and the average difference between the predicted and the experimental flow stress are 13.83% and 5.15%, respectively. The comparison between the predicted results based on the ANFIS model for flow stress and those using the regression method has illustrated that the ANFIS model is more efficient in predicting the flow stress of Ti60 alloy.     

Deformed microstructure evolution in AM60B Mg alloy under hypervelocity impact at a velocity of 5 km s

Deformed microstructure in AM60B Mg alloy under hypervelocity impact at a velocity of 5 km s⁻¹ were investigated through optical microscope, scanning electron microscope and transmission electron microscope. The results show that four deformed zones around the crater can be classified based on the different deformed microstructure, including ultrafine grain zone, ultrafine grain and deformation twin zone, high and low density deformation twin zones. The dislocation slipping, deformation twins and ultrafine grains are the dominant components in the four deformed zones, and the evolution of deformed microstructure is speculated based on the deformed microstructure observed in four zones. Slipping and twinning play a critical role for the formation of the dynamic recrystallized grains, and twinning-induced rotational dynamic recrystallization mechanism is thought to be the main mechanism for the formation of ultrafine grains. The microhardness and dynamic compressive strength in different deformed zones were measured, and the high microhardness and yield strength in ultrafine grain zone should be attributed to the strain hardening and grain refining.     

Predictive equations of the tensile properties based on alloy hardness and microstructure for an A356 gravity die cast cylinder head

One of the main problems in the design of complex Al–Si cast components is the wide variety of mechanical properties in different regions of the castings which is due to the wide range of solidification microstructures, related to the local solidification conditions. There are many papers available on the widely used A356/A357 Al–Si–Mg alloys, however, most experimental data on their tensile or fatigue properties are generally obtained from specimens cast separately or produced under controlled laboratory conditions, that are extremely different from those of industrially cast components. Moreover, most of these data often relate the mechanical properties to only one microstructural parameter, such as solidification defects or secondary dendrite arm spacing, and do not take their simultaneous effect into consideration. For all these reasons, the main problem, in the design phase, is the lack of knowledge of the true local mechanical properties in complex-shaped castings, which often means a conservative approach is necessary, with a consequent increase in thickness and therefore in weight. The aim of this research was to study a complex A356 gravity die cast cylinder head, in order to verify the range of variability of the main microstructural parameters and tensile properties, using specimens directly machined from the casting. The component was heat treated at the T6 condition, and the effect of the delay between quenching and aging on the alloy hardness was also evaluated. Simple experimental equations have been proposed, able to successfully predict the local tensile properties in the casting, when only the most important microstructural parameters and alloy hardness are known. These equations allow the designer to predict the local tensile behaviour without any tensile tests; moreover they can also link the post-processing results of the casting simulation software to the pre-processing phase of the structural ones, with an approach of co-engineered design.     

Constitutive base analysis of a 7075 aluminum alloy during hot compression testing

In the field of deformation process modeling, the constitutive equations may properly represent the flow behavior of the materials. In fact, these valuable relationships are used as a calculation basis to simulate the materials flow responses. Accordingly, in the present study a hot working constitutive base analysis has been conducted on a 7075 aluminum alloy. This has been performed using the stress–strain data obtained from isothermal hot compression tests at constant strain rates of 0.004, 0.04 and 0.4 s⁻¹ and deformation temperatures of 450, 500, 520, 550 and 580 °C up to a 40% height reduction of the specimen. A set of constitutive equations for 7075 Al alloy have been proposed employing an exponent-type equation. The related material constants (i.e., A, n and α) as well as the activation energy Q for each temperature regime have been determined. The correlation of flow stress to strain rate and temperature can be deduced from the proposed equations. Furthermore, a change in deformation mechanism has been realized in the semi-solid temperature range. This has been related to the onset of lubricated flow mechanism during processing.     

Microstructure and mechanical properties of TA15 titanium alloy under multi-step local loading forming

In this paper six different local loading processes were proposed to study the effects of local loading conditions (temperature, deformation degree, loading pass, heats, cooling modes and heat treatment) on the microstructure and mechanical properties of TA15 titanium alloy workpieces including room and high temperature tensile properties, impact property, fracture toughness and high temperature duration property.It is found that it is better to finish the local loading forming in one heating time, if multi-fire forging needed the optimal forging technique as follows: adopting conventional forging (950 °C) at first and then following near-beta forging to control the proportion of the equiaxed primary α phase and the transformed β phase, allocating deformation degree of each loading pass rationally and using WQ cooling mode. Thus the workpiece with good compositive mechanical properties can be obtained.     

Materials modeling and finite element simulation of isothermal forging of electrolytic copper

Isothermal forging of electrolytic copper is modeled using finite element simulation and materials models involving kinetic analysis and processing maps with a view to validate their predictions. Forging experiments were conducted on a rib–web (cup) shape in the temperature range of 300–800 °C and at speeds of 0.01–10 mm s⁻¹. The processing map for hot working of electrolytic copper revealed two domains in the temperature and strain ranges of (1) 400–600 °C and 0.001–0.01 s⁻¹, (2) 650–950 °C and 0.3–30 s⁻¹, where dislocation core diffusion and lattice self-diffusion are the rate-controlling mechanisms, respectively. Finite element simulation using the relevant experimental constitutive equations, predicted load–stroke curves that correlated well with the experimental data. The simulation has shown that there is a strain variation from about 0.4 to 4 in the web and rib regions of the forged component, although the dynamically recrystallized grain structure is fairly uniform, suggesting that dynamic recrystallization (DRX) is not sensitive to strain once the steady state flow is reached. The DRX grain size in the component is linearly dependent on Z and is similar to that predicted by the materials model after discounting for the longer time taken for the component removal.     

The microstructure evolution and nucleation mechanisms of dynamic recrystallization in hot-deformed Inconel 625 superalloy

Hot compressions tests of Inconel 625 superalloy were conducted using a Gleeble-1500 simulator at different strains between 900 °C and 1200 °C with a strain rate of 0.1 s⁻¹. Optical microscope, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the microstructure evolution and nucleation mechanisms of dynamic recrystallization. It was found that both the size and fraction of dynamically recrystallized grains increase with increasing deformation temperature. However, the size of dynamically recrystallized grains almost remains constant with increasing deformation strain. The dominant nucleation mechanism of dynamic recrystallization in Inconel 625 superalloy deformed at 1150 °C is the discontinuous dynamic recrystallization, which is characterized by the bulging of the original grain boundaries accompanied with twining. The continuous dynamic recrystallization characterized by progressive subgrain rotation occurs simultaneously in dynamic recrystallization process, although it can only be considered as an assistant nucleation mechanism at the early stage of hot deformation.     

Forging of Mg–3Al–1Zn–1Ca alloy prepared by high-frequency electromagnetic casting

The 1 wt.%Ca–AZ31 alloy produced by electromagnetic casting (EMC) in presence of electromagnetic stirring (EMS) was extruded and then subjected to the closed-die forging to make a pulley for automobile application. Effective dynamic recrystallization (DRX) took place during the forging process, leading to formation of fully recrystallized grains with the average size of 3–4 μm. High-forging ability and high degree of grain refinement achieved during the forging were attributed to the novel microstructure of the cast composed of small and equiaxed grains with the average size of 50 μm and thin layer (Al, Mg)₂ Ca phase at grain boundaries, which would provide more nucleation sites and a faster rate of recrystallization during deformation by forging as compared to that of the conventionally processed cast composed of large size grains and thick layer (Al, Mg)₂ Ca phase. The forged pulley exhibited the ultimate tensile strength of 273–286 MPa with tensile elongations of 30%. The present result demonstrates a possibility that EMC + EMS techniques can be used in producing magnesium feed stocks with high-forging ability.     

Development of ultrafine grained high strength age hardenable Al 7075 alloy by cryorolling

High strength age hardenable Al 7XXX series alloys are difficult to process by many of the severe plastic deformation processes at room temperature. The Al 7075 alloy has been processed at cryogenic temperature and room temperature up to different rolling strains, in the present work, with the objective of developing a processing strategy to obtain ultrafine grained microstructure with enhanced mechanical properties in the alloy. It has been identified that the Al 7075 alloy samples can be successfully cryorolled to higher strains (up to 3.4) if the reduction per pass is less than 0.3 mm, however it was found to be difficult to deform the samples at room temperature. A cryorolling strain of 3.4 has been found to be desirable for producing the ultrafine grained Al 7075 alloys with the high angle grain boundaries. However, the subgrains are not recrystallized up to this strain in the case of room temperature rolled Al alloys. The strength and hardness of the cryorolled Al 7075 alloy samples are higher than that of the room temperature rolled samples as observed in the present work. The improved strength and hardness of cryorolled samples are due to the grain size effect and higher dislocation density. The reduction in dimple size of cryorolled Al 7075 alloy upon failure confirms the grain refinement and strain hardening mechanism operating in the heavily deformed samples.     

Electrochemical behavior of a lead-free SnAg solder alloy affected by the microstructure array

The aim of this study is to evaluate the electrochemical corrosion behavior of a Sn–Ag solder alloy in a 0.5 M NaCl solution at 25 °C as a function of microstructural characteristics. Different microstructure morphologies, which can be found in Sn–Ag solder joints and that are imposed by the local solidification cooling rate, are evaluated and correlated to the resulting scale of the dendritic matrix and the morphology of the Ag₃Sn intermetallic compound. Cylindrical metallic molds at two different initial temperatures were employed permitting the effect of 0.15 °C/s and 0.02 °C/s cooling rates on the microstructure pattern to be experimentally examined. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical parameters. It was found that higher cooling rates during solidification are associated with fine dendritic arrays and a mixture of spheroids and fiber-like Ag₃Sn particles which result in better corrosion resistance than coarse dendrite arrays associated with a mixture of fibers and plate-like Ag₃Sn morphologies which result from very slow cooling rates.     

Hot compressive deformation behavior of a new hot isostatically pressed Ni–Cr–Co based powder metallurgy superalloy

The hot compressive deformation behavior of a new hot isostatically pressed Ni–Cr–Co based powder metallurgy (P/M) superalloy was studied in the temperature range of 950–1150 °C and strain rate range of 0.0003–1 s⁻¹ using Gleeble-1500 thermal simulator. The dynamic recrystallization-time–temperature (RTT) curve was developed and the constitutive equation of flow stress during hot deformation was established. The results show that the flow stress decreases with increasing deformation temperature and decreasing strain rate. The flow stress represents as the characteristic of dynamic crystallization with the increasing of strain at the deformation temperatures lower than 1100 °C and strain rates higher than 0.0003 s⁻¹. The beginning time of dynamic crystallization has no linear relationship with deformation temperature in the condition of strain rate lower than 0.01 s⁻¹. Besides, the experiments verify that the hyperbolic sine model including the variable of strain reflects the changing law of flow stress during the hot deformation process.     

Prediction of hot compression flow curves of Ti–6Al–4V alloy in α + β phase region

Prediction of material flow behavior is essential for designing the forming process of any material. In this research, experimental flow curves of Ti–6Al–4 V alloy were obtained using the isothermal hot compression test done at 750–950 °C with 50 °C intervals and constant strain rates of 0.001, 0.005 and 0.01 s⁻¹. For prediction of hot deformation flow curves two methods of modeling were applied. In the first method, an entire flow curve was modeled using Sellars equation. In the second one, modeling of a flow curve up to the peak point was carried out with Cingara model, and modeling beyond that was performed with a model developed based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory. The accuracy of each model was examined through a statistical method. Results showed that flow curve modeling using Cingara model and JMAK theory leads to results that are more consistent with the experimental data.     

Effects of zirconium addition on as-cast microstructure and mechanical properties of Mg–3Sn–2Ca magnesium alloy

At present, the mechanical properties of the Mg–3Sn–2Ca magnesium alloy are not satisfying and further enhance needs to be considered via further alloying/microalloying additions. The effects of Zr addition on the as-cast microstructure and mechanical properties of the alloy were investigated by using optical and electron microscopies, differential scanning calorimetry (DSC) analysis, and tensile and creep tests. The results indicate that adding 0.41, 0.76 or 1.18 wt.% Zr can refine the grains of the alloy, and the primary CaMgSn phases in the Zr-containing alloys are changed from coarse needle-like net to relatively fine short block and/or particle-like shapes. As a result, the tensile and/or creep properties of the Zr-containing alloys are improved. Among the Zr-containing alloys, the alloy with the addition of 0.76 wt.% Zr exhibits the relatively optimum mechanical properties.     

Characterization of deformation instability in modified 9Cr–1Mo steel during thermo-mechanical processing

In this study, various existing instability criteria were employed to delineate the unstable flow regions in modified 9Cr–1Mo steel during hot deformation. Experimental stress–strain data obtained from isothermal hot compression tests, in a wide range of temperatures (1123–1373 K) and strain rates (10⁻³–10 s⁻¹), were employed to develop instability maps. The domains of these instability maps were validated through detailed microstructural study. It has been observed that Hart’s stability criterion, Jonas’s criterion and Semiatin’s criterion under-predicts the instability regions in the studied temperatures and strain rates regime. Gegel’s and Alexander’s criteria as well as Murty’s metallurgical instability criterion, on the other hand, found to over-predict the instability domains. The instability map developed based on Dynamic Materials Model criterion has been found to precisely predict the instability domains. This instability map revealed four major unstable domains. Microscopic examination in these domains revealed that the instability is manifested in the specimens either as localized deformation band primarily along one of the diagonal or inhomogeneous distribution of martensite lath in the prior austenite grains.     

Effect of calcium addition on the microstructure and compressive deformation behaviour of 7178 aluminium alloy

Aluminium 7178 alloys containing 1% calcium are used to study the effect of calcium addition on their microstructure and compressive deformation behaviour. The compressive deformation behaviour of aluminium alloy containing 1% calcium is studied at varying strain rates (10⁻²–10/s). The material is prepared using stir casting technique. The yield stress, flow stress and elastic limit are measured from the true stress–strain graph. The strain rate sensitivity and strain-hardening exponent was also determined for each material at different strain rates. Its microstructural characterization reveals that Ca particles act as grain refiners for primary base alloy and helps in improving the strength of the virgin alloy. An empirical relationship has been proposed to predict the flow curve of the alloys as a function of strain and strain rate.     

Material flow and mechanical behaviour of dissimilar AA2024-T3 and AA7075-T6 aluminium alloys friction stir welds

The scope of this investigation is to evaluate the effect of joining parameters on the mechanical properties, microstructural features and material flow of dissimilar aluminium alloys (3 mm-thick AA2024-T3 and AA7075-T6 sheets) joints produced by friction stir welding. Mechanical performance has been investigated in terms of hardness and tensile testing. Material flow using the stop action technique has also been investigated in order to understand the main features of the mixing process. No onion ring formation has been observed; the boundary between both base materials at the stir zone is clearly delineated, i.e., no material mixing is observed. A non-stable rotational flow inside the threads has been identified due to the formation of a cavity on the rear of the pin. Microstructural observation has revealed the development of a recrystallised fine-grained stir zone, with two different grain sizes resulting from the two different base materials.     

Characterization of friction stir welded boron carbide particulate reinforced AA6061 aluminum alloy stir cast composite

Development of welding procedures to join aluminum matrix composite (AMCs) holds the key to replace conventional aluminum alloys in many applications. In this research work, AA6061/B₄C AMC was produced using stir casting route with the aid of K₂TiF₆ flux. Plates of 6 mm thickness were prepared from the castings and successfully butt joined using friction stir welding (FSW). The FSW was carried out using a tool rotational speed of 1000 rpm, welding speed of 80 mm/min and axial force of 10 kN. A tool made of high carbon high chromium steel with square pin profile was used. The microstructure of the welded joint was characterized using optical and scanning electron microscopy. The welded joint showed the presence of four zones typically observed in FSW of aluminum alloys. The weld zone showed fine grains and homogeneous distribution of B₄C particles. A joint efficiency of 93.4% was realized under the experimental conditions. But, FSW reduced the ductility of the composite.