Lower Flow Stress Research Articles

A study on hot-working as alternative post-processing method for titanium aluminides built by laser powder bed fusion and electron beam melting

Intermetallic Titanium Aluminides (TiAl) have been designed for high-temperature lightweight applications. Powder-bed additive manufacturing (AM) processes such as laser powder bed fusion (LPBF) or electron beam melting (EBM) allow near-net-shape production of TiAl components. However, TiAl parts produced by LPBF or EBM do not reach the structural integrity and mechanical properties of forged parts. The main post-processing step of TiAl parts made by AM is hot-isostatic pressing (HIP), which has several disadvantages such as a long process time and an undesired coarsening of the microstructure. High-performance TiAl components such as turbine blades are thus still produced by isothermal forging of preforms made by investment casting and hot isostatic pressing. Due to the low workability of TiAl alloys, often more than one deformation step is required, and the tooling costs and the low material yield make this process chain very expensive. This paper explores the feasibility of combining AM with isothermal forming in order to replace HIP by thermomechanical post-processing. The hot forming behavior of the Ti-43.5Al-4Nb-1Mo-0.1B (at. %) TNM-B1 alloy, manufactured by EBM and LPBF, is analyzed concerning the evolution of the voids and grain sizes and compared to hot isostatic pressing. It is shown that the fine microstructure produced by AM yields a much lower flow stress and faster globularization kinetics in comparison to conventional cast and HIPed material. While HIP experiments are shown to significantly coarsen the microstructure, isothermal hot working is shown to convert the AM microstructure to a dense and refined state. Moderate hot working with total strains of ∼1 of AM pre-forms may thus serve as an alternative process chain to conventional large strains forging of cast pre-forms and to AM + HIP in the series production of high-performance TiAl components.

Superplastic behavior of a nickel-base superalloy containing topologically close-packed phases

Superplastic behaviour of a polycrystalline nickel-base superalloy Ni-47(Cr,Co,W,Al,Ta,Hf)-0.2(C,B) (wt.%) in the fine-grained condition obtained via thermomechanical treatment (TMT) has been studied. TMT transformed a coarse-grained as-cast structure into a homogeneous fine-grained one. Both the initial as-cast and the thermomechanically treated conditions contained not only the matrix γ phase, the hardening γ’ phase and carbides but also topologically close-packed (TCP) phases. In spite of the presence of the TCP phases, the superalloy in the fine-grained condition demonstrated superplastic properties (δ>300% and low flow stresses) at subsolvus temperatures. Superplastic deformation led to some refinement and alignment of the TCP phase particles along the tensile axis.

Analysis of tool wear and deformation in friction stir welding of unequal thickness dissimilar Al alloys

Friction stir welding between plates of unequal thickness, which are made from similar or dissimilar materials, finds wide range of applications in the aerospace and automotive sectors. Friction stir welding of plates made from dissimilar materials having unequal thicknesses is challenging. One of the major challenges is the control of rapid tool degradation which occurs during welding. This work reports a maiden study on tool degradation of high thickness ratio unequal thickness dissimilar material joints made between 6.3 mm thick AA2024-T3 and 2.5 mm thick AA7475-T7 plates. The degradation of friction stir welding tool made of T4 tool steel having tapered cylindrical pin and scrolled shoulder was analyzed. The geometry of tool (before and after welding) was compared; the degradation was categorized, characterized, and analyzed in the light of measured welding temperature, process forces, and process parameters. It was found that the pin undergoes significant degradation in the form of wear and deformation compared to the tool shoulder. The experimental results demonstrated that lower flow stresses caused by higher process temperature leads to lower tool wear and deformation, and vice versa. In addition to temperature and process forces, the surface tilt angle was found to significantly affect the pin deformation. The higher surface tilt angle caused an increase in tool wear and deformation.

On the hot deformation behavior of Ti-6Al-4V made by additive manufacturing

Innovative process chains that use additive manufacturing (AM) to produce pre-forms for hot forging have been proposed recently, but almost no scientific knowledge on the hot deformation behavior of AM material exists. The combination of AM and metal forming may allow for producing pre-forms by AM which can be forged to the final shape in a single forming operation, thus saving tooling costs compared to conventional multi-stage forging processes. In this work, Ti-6Al-4 V samples made from powder using Directed Energy Deposition (DED) and Selective Laser Melting (SLM) are investigated under typical hot working conditions. An increasing cooling rate in AM promotes the formation of martensite. When heated to forming temperature, the martensite decomposes and yields intermediate microstructures with improved hot working properties compared to conventional wrought material. Differences in the flow stress and activation energy for hot working are analyzed for conventional and as-built Ti-6Al-4 V. AM material exhibits both lower flow stresses and a faster globularization rate, which is explained by non-equilibrium phase fractions and the spatial arrangement of the β-phase. A model is proposed to predict flow stresses and microstructure evolution in Ti-6Al-4 V specimens with a conventional and AM microstructure, which takes the spatial arrangement into account. The findings can be used to design new process chains that allow for single-stage net-shape forging of Ti-6Al-4 V parts at reduced forming forces and with improved mechanical properties.

Transient strain age hardening of Al–Mg alloys

The static strain aging response of an Al–Mg alloy is investigated by means of tensile pre-deformation followed by annealing. The mechanical response following re-testing in tension reveals a lower flow stress and raised work hardening rate at yield. The enhanced work hardening rate is shown to be transient, the behaviour returning to that of the non-aged material at high flow stress. In order to decouple the various effects arising from solute segregation and dislocation loss via recovery, a simplified version of the two internal state variable model originally proposed by Kubin and Estrin is introduced. It is proposed that the response, yield strength and work hardening rate, following pre-deformation and annealing can be ascribed to the combined effects of the loss of mobile dislocations and a transient hardening that is interpreted to come from dislocations that have had solute segregated to them.

Effect of biaxial compressive stress state on the microstructure evolution and deformation compatibility of rolled sheet Mg alloy AZ31 at room temperature

Mg alloys usually deform under complex biaxial compressive stress states (e.g. during drawing, forging and extrusion). In order to better understand how the biaxial compressive stress state influences the evolution of the microstructure and the deformation compatibility, the uniaxial and biaxial compression tests of AZ31 rolled sheets were performed at room temperature. The new self-developed biaxial compression devices offered a possibility to obtain the evolution of the microstructures by in situ measurements via electron backscatter diffraction. The analysis of geometrically necessary dislocation and the simulation based on the visco-plastic self-consistent method were used to examine the microscopic behaviors in the sheets during the uniaxial and biaxial compression tests. The results indicated that the reorientation of the texture during the biaxial compression was different from that during the uniaxial compression. The simulated results based on the visco-plastic self-consistent model suggested that the equivalent yield strength under a biaxial compressive stress state was lower than that under a uniaxial stress state. This was due to the higher relative activity of extension twins under biaxial compressive stress state. This also led to a lower geometrically necessary dislocation density in the biaxial compressed sample. The geometrically necessary dislocations were also more homogeneously distributed with a biaxial compressive stress state. The analysis of the activation of all the deformation mechanisms proved that the appearance of extension twinning variants limited the activation of prismatic slip during the uniaxial compression while promoting prismatic slip during the biaxial compression. This changeable interaction between extension twinning and prismatic slip combining with the lower geometrically necessary dislocations, the lower flow stress inferred that the application of the biaxial compressive stress state during processing at room temperature was a way to improve the deformability of Mg alloys.

Effects of low flow and co-occurring stressors on structural and functional characteristics of the benthic biofilm in small streams

Low flow and co-occurring stress is a more and more frequent phenomenon these years in small agricultural streams as a consequence of climate change. In the present study we explored short and longer term structural responses of the stream benthic algae community and biofilm metabolism to multiple stress in small streams applying a semi-experimental approach. We hypothesized that i) a reduction in flow in combination with secondary stress (nutrients and sediments) have immediate effects on the benthic algae community in terms of biomass (chlorophyll a, biovolume), taxonomic and trait (lifeform and size distribution) compositions as well as on metabolism (GPP and CR), and ii) that changes in the benthic algae community persist due to altered environmental settings but that functional redundancy among benthic algae species provides a high level of resilience in metabolism (GPP and CR). Overall, we found that stress imposed by nutrients was less pronounced than stress imposed by fine sediments under low flow, and that nutrient enrichment to some extent mitigated effects of fine sediments. Fine sediment deposition mediated a decline in the fraction of erect algae and/or algae with mucilage stalks but this did not happen under co-occurring stress from both sediments and nutrients. Additionally, fine sediment deposition mediated a decline in GPP of the biofilm, but again this did not happen under co-occurring stress from nutrients. We conclude that 1) the benthic algae community and biofilm metabolism displayed similar resilience to stress imposed by low flow and co-occurring stress from nutrients and sediments on a short and longer time scale and 2) as structure–function adaptations may occur at several trophic levels in the biofilm, more research is needed to explore mechanisms underlying mitigating effects of nutrients in response to sediment deposition under low flow.

Relationship among microstructure, mechanical properties and texture of TA32 titanium alloy sheets during hot tensile deformation

The relationship among microstructure, mechanical properties and texture of TA32 titanium alloy sheets during hot tensile deformation at 800 °C was investigated. In the test, the original sheet exhibited relatively low flow stress and sound plasticity, and increasing the heat treatment temperature resulted in an increased ultimate tensile strength (UTS) and a decreased elongation (EL). The deformation mechanism of TA32 alloy was dominated by high angle grain boundaries sliding and coordinated by dislocation motion. The coarsening of grains and the annihilation of dislocations in heat-treated specimens weakened the deformation ability of material, which led to the increase in flow stress. Based on the high-temperature creep equation, the quantitative relationship between microstructure and flow stress was established. The grain size exponent and α phase strength constant of TA32 alloy were calculated to be 1.57 and 549.58 MPa, respectively. The flow stress was accurately predicted by combining with the corresponding phase volume fraction and grain size. Besides, the deformation behavior of TA32 alloy was also dependent on the orientation of predominant α phase, and the main slip mode was the activation of prismatic 〈a〉 slip system. The decrease of near prism-oriented texture in heat-treated specimens resulted in the enhancement of strength of the material.

Amorphous structure in single-crystal magnesium under compression along the c axis with ultrahigh strain rate

It is well known that twinning deformation plays an important role in the plastic deformation of hexagonal close-packed (HCP) metals because of the insufficient independent slip systems in HCP metals. However, recent works show twinning deformation does not happen in compression in the $c$ axis of the magnesium (Mg) single crystal, and it is not clear that how it deforms with limited dislocation glide in absence of twinning deformation. In the present work, molecular dynamics simulations with well validated modified embedded-atom method potential for Mg are performed to reveal the microscopic deformation behaviors of Mg single crystal compressed in the $c$ axis. We focus on the microstructures and the evolution during both the deformation and the relaxation of the Mg single crystal. Twinning deformation is not observed in the simulations as expected, and dislocations and the amorphous regions are found to be dominant under different loading conditions. The amorphous regions are dominant in the deformation under ultrahigh strain rate, and it results in more homogenous deformation and lower flow stress than dislocation glide. The dislocation glide is dominant under lower strain rate, and the insufficient dislocations may lead to quite inhomogeneous plastic deformation. Furthermore, it is revealed that the amorphous regions consist of irregularly distributed atoms with higher per-atom potential than those in the crystalline state. The amorphous regions thus annihilate rapidly once the high-rate deformation is stopped. The annihilated amorphous regions turn into crystalline ones with dense dislocation networks. The annihilation rate of the amorphous regions decreases with increasing pressure, while it hardly depends on temperature.

Comparison of Prediction Capabilities of Flow Stress by Various Constitutive Equation Models for Hot Deformation of Aluminum Matrix Composites

One of the important objectives of studies on the hot deformation of materials is the development of constitutive equations that represent the flow stress of the material as a function of strain, strain rate, and temperature useful for the structural designers in finite element analysis. While the constitutive analysis is fairly simple for pure metals and dilute alloys at high temperatures and low strain rates (low flow stresses), it can get complicated at high stresses and with the presence of a second phase, such as those in metal matrix composites. In the present work, the hot deformation behavior of aluminum matrix composites was studied by developing constitutive equations using Johnson–Cook (JC), modified JC (m-JC), Arrhenius, and modified Zerilli and Armstrong (m-ZA) models, and the prediction capability of these models was compared based on average absolute error (Δ). Further, the effect of reinforcement on activation energy of these composites was studied by varying parameters, viz., nature of reinforcement, volume fraction, and size. Out of the four constitutive models studied, the average absolute error is lowest for m-JC and Arrhenius models, and the flow stress values predicted by these two models compare well with the experimental data. Material constants of all the constitutive models for silicon carbide particulate (SiCp)-reinforced AA2124 composites with various volume fractions (5–30 %) were evaluated and provided as a database for the numerical simulations of hot deformation of these composites. To validate the constitutive equations, power dissipation maps have been developed for AA2124-15 vol % SiCp-reinforced composites, and it was found that the predicted flow stress data obtained from m-JC and Arrhenius equations could successfully reproduce the efficiency of power dissipation, indicating their better predictability of flow stress over JC and m-ZA models. Constitutive equations are quantified for 28 aluminum alloys and their composites, and physical interpretation of the constants of Arrhenius equation was provided.

Rate dependent behaviour and dynamic strain localisation of three novel impact resilient titanium alloys: Experiments and modelling

The tensile behaviour of three ductile titanium alloys, Ti0.7Al4V, Ti1Al4V and Ti3Al2.5V, conceived for impact containment applications, is characterised at quasi-static, intermediate and high strain rates. Tests have been performed using a screw driven mechanical system and a bespoke in-house developed split Hopkinson tension bar equipped with ultra-high speed photographic equipment. The three alloys present noticeable strain rate sensitivity. Ti1Al4V is characterised by the lowest flow stress and highest ductility of the three alloys. Ti3Al2.5V higher flow stress but lowest strain to failure while Ti0.7Al4V has similar flow stress but larger engineering failure strain compared to Ti3Al2.5V. The dynamic true strain rate, determined by measuring the time history of the minimum cross-section diameter during necking, reaches values up to one order of magnitude higher than the nominal strain rate. This phenomenon, occurring during dynamic strain localisation, is of fundamental importance for understanding and modelling the dynamic behaviour of ductile alloys designed to withstand impact loading. The experimental results are used to determine the material parameters of the Johnson-Cook (JC) and Khan–Huang–Liang (KHL) models, which are incorporated in the ABAQUS/explicit code for finite element simulations of the dynamic tensile tests. It is found that the KHL model predicts the experimentally measured strain field, deformed geometry, effective strain rate and macroscopic force-displacement response during the high strain rate experiments with significantly better agreement than the JC model.

Effects of coarse Al2CuLi phase on the hot deformation behavior of Al–Li alloy

The effects of coarse T1 phase (Al2CuLi), produced by homogenization, on the hot deformation behavior of Al–Li alloy was studied using uniaxial compression. Stress-strain curves, constitutive equations and processing maps were established, and the microstructure was observed by EBSD, SEM and TEM. The homogenized alloy with coarse T1 phase presents lower flow stresses and significantly lower deformation activation energy than the solution treated alloy. Besides, the homogenized alloy exhibits the same hot-working region with the solution treated alloy, and possesses lower power dissipation efficiencies. The EBSD investigation confirms the formation of massive recrystallized grains in the homogenized alloy at 340–460 °C, revealing the coarse T1 phase can promote dynamic recrystallization through particles stimulated nucleation. In contrast, the solution treated alloy is softened dominantly via dynamic recovery at 340–490 °C. This work not only merits the beneficial effects of the coarse T1 phase but also suggests the feasibility of obtaining accelerated recrystallization kinetic and good workability during hot working of Al–Li alloys.

Hot Deformation Behavior and Microstructural Evolution of an Fe-Cr-W-Mo-V-C Steel

The hot compression deformation behavior and microstructural evolution of an Fe-Cr-W-Mo-V-C steel have been investigated by hot compression deformation experiments carried out at 900 °C to 1150 °C and under strain rates varying from 10 to 0.1 s−1. The results revealed that the flow stress decreased with decreasing strain rate, while increasing deformation temperature led to a lower flow stress. An Arrhenius-type equation was used to analyze the effects of the strain rate and deformation temperature on the plastic flow behavior of the steel. Based on this equation and the experimental results, the average activation energy was calculated to be 747.7 kJ/mol. The tested samples were subjected to careful microstructural examinations, with a focus on determination of the dynamic recrystallization (DRX) grain sizes. A straightforward contour map correlating the DRX grain sizes with the different deformation conditions was drawn. According to this map and the microstructural examination results, the optimum hot working parameters enabling us to obtain appropriate DRX microstructures have been identified at 0.1 s−1 for the strain rate and 1100 °C for the deformation temperature.

Superplastic deformation behavior of lamellar microstructure in a hydrogenated friction stir welded Ti-6Al-4V joint

It is rather difficult for Ti alloy welds to be superplastic formed due to their low superplasticity and high flow stress. In this study, the Ti-6Al-4V alloy after hydrogenation with 0.2 wt% hydrogen was subjected to friction stir welding (FSW). The superplastic behavior of the FSW joint was investigated. The stir zone with a fine lamellar microstructure exhibited a largest elongation of 660% at 825 °C and 3 × 10−3 s−1. Besides, the stir zone showed a similar superplasticity of close to 600% to the base material at 825 °C, 3 × 10−4−1 × 10−3 s−1. It is the first report on the superplasticity of FSW Ti alloy joints with hydrogenation. Compared to the FSW Ti-6Al-4V alloy joint without hydrogenation, the hydrogenated FSW joint showed superior superplastic properties, including a high elongation, less than a half flow stress and lower superplastic temperature. These superior superplastic properties were mainly attributed to the globularization of the lamellar microstructure and the high fraction of β phase induced by the hydrogen element. This study provides an effective way to reduce the difficulty of practical superplastic forming for Ti alloy welds, although a dehydrogenation process is still needed to avoid hydrogen embrittlement.

Studies on Titanium Alloys for Aerospace Application

Since the development of the Ti54M titanium alloy in 2003, its application within the aerospace sector has gradually increased due to the combination of properties such as improved forgeability and machinability, low flow stress at elevated temperatures, and superplastic characteristics. However, for the successful exploitation of Ti54M a comprehensive understanding of its mechanical characteristics, microstructure stability, and superplastic behaviour is required. The superplastic forming of titanium alloys is characterised by high deformation at slow strain rates and high temperatures which influence the material microstructure, and in turn, determine the forming parameters. These mechanisms make the prediction of the material behaviour very challenging, limiting its application within the aerospace industry. Even though Ti54M has been commercially available for over 10 years, further studies of its mechanical and superplastic properties are still required with the aim of assessing its applicability within the aerospace industry as a replacement for other commercial titanium alloys. Therefore, in this work a study of the mechanical and superplastic properties of Ti54M, in comparison with other commercial titanium alloys used in the aerospace industry - i.e. Ti-6AL-4V, and Ti-6-2-4-2 - is presented. The final objective of this study, carried out at the Advanced Forming Research Centre (AFRC, University of Strathclyde, UK), is to obtain material data to calibrate and validate a model capable of estimating the behaviour and grain size evolution of titanium alloys at superplastic conditions.

Flow stress and deformation behavior of fine-grained Mg matrix influenced by bimodal size SiCp

Abstract

Characterization of semisolid deformation behavior of a high Zr-containing WE magnesium alloy

The effects of temperature and strain rate on the semisolid deformation behavior of a high Zr WE54 magnesium alloys were studied at temperatures of 570, 595 and 620 °C. A flow stress peak was appeared during deformation of the feedstock experimental material at deformation conditions of 570 °C, 0.010 and 0.100 s−1, and 595 °C, 0.100 s−1. The latter results were connected to the occurrence of dynamic recrystallization in solid globules. Also, these conditions were gauged to be able to activate “lubricated flow” mechanism. At 570 °C and strain rate of 0.001 s−1 as well as 595 °C and rates of 0.010 and 0.001 s−1, the flow stress increases and reaches a plateau, where dynamic recovery is considered as the dominant mechanism. However, at 620 °C under all the strain rates, the flow stress drops for a limited strain range and then increases again. The results show that as the liquid removes from the boundaries between the solid parts, the densification of the globules occurs at higher strains, causing an increase in the flow stress. Comparative analysis points out that the feedstock material exhibits a lower flow stress than as-cast alloy associating with a modified morphology of Zr particles after deformation.

Dynamic transformation of α → β titanium at temperatures below the β-transus in commercially pure titanium

Hot compression tests were carried out on commercially pure titanium grade 2 at temperatures below the β-transus (915 °C). At temperatures of 840, 860, 880, and 900 °C, the β volume fraction increased by 30%, 32%, 36%, and 55%, respectively. The minimum temperature at which dynamic transformation could be induced was 765 °C. The driving force for transformation given by the net softening associated with the formation of the lower flow stress beta phase was calculated to fall in the range between 200 and 500 J/mol.

Rectangular shear clinching for joining of ultra-high strength steel sheets

A rectangular shear clinching process was applied to join two 1180 MPa ultra-high strength steel sheets having low ductility. In the rectangular shear clinching process, the sheets were joined with partial separation in the upper sheet. The sheets were joined by interlock between the upper and lower sheets with upper sheet separation in the short sides of a rectangle, whereas fracture was prevented in the long sides of a rectangle. The die shape was optimized in order to increase the interlock, and the sheets were joined. Then, the strength of the joined sheets by rectangular shear clinching was compared with that of welded sheets. The strength of the clinched sheets was smaller than that of the welded sheets in both the cross-tension and tension-shearing tests. The fatigue limit of the clinched sheets in the cross-tension test was smaller than that of the welded ones, while the limit in the tension-shearing test was larger. It was found that the rectangular shear clinching process without an airtight joint is effective in joining of the ultra-high strength steel sheets having low ductility and high flow stress.

Effect of grain size inhomogeneity of ingot on dynamic softening behavior and processing map of Al-8Zn-2Mg-2Cu alloy

The effect of grain size inhomogeneity of large ingot on the hot deformation behavior of Al-8Zn-2Mg-2Cu alloy were investigated using hot compression tests over a temperature range from 300 °C to 450 °C with strain rate from 0.1 s−1 to 10 s−1. It was found that the grain size (100 μm) in surface layer of the ingot is finer than that of the central layer (>200 μm). The surface specimen exhibited lower flow stress than that of central one in the temperature range from 350 °C to 450 °C and the strain rate range from 0.1 s−1 to 1 s−1. The softening mechanism is dynamic recovery, together with a partial dynamic recrystallization (DRX) at high temperature and low strain rate condition regardless the grain size. The nucleation mechanism of DRX is the strain induced grain boundary migration. Therefore, the DRX fraction is higher in surface specimen because the higher grain boundary density provides more nucleation sites. Based on processing maps, the applicable hot deformation parameters for the large size ingot are determined to be at the temperature range of 440–450 °C and the strain rate of 0.1–0.3 s−1 when taking the grain size inhomogeneity into account.