Effect of different thermomechanical processing on structure and mechanical properties of electron beam melted niobium

Among the refractory metals and alloys, niobium and niobium alloys are used in variety of high temperature applications ranging from light bulbs to rocket engines because of its high melting and boiling point, lower density, good ductility at room temperature and high corrosion resistance. In this paper the effects of different thermomechanical processing on structure and mechanical properties of electron beam melted niobium ingot was investigated. The correlation among the different processing conditions and microstructure as well as mechanical properties have been investigated using optical microscope, SEM, UTM and microhardness testing. The results show that the cold forging response of EB melted ingot was very poor, where as oxidation resistant coated ingot and ingot sealed in evacuated stainless steel jacket were successfully forged at 900 °C. The cast and hot forged EBM niobium ingot was cold rolled without any intermediate annealing. The hot forged, cold rolled and annealed niobium sheets exhibit better strength as compared to cold rolled and annealed niobium sheets. The mechanical properties of all the niobium sheets processed by using different processing conditions are superior to the properties specified by ASTM standard.     

A study on multiple microalloyed forging grade steels

Investigations were conducted for the development of multiple microalloyed air‐hardenable forging grade steels. A number of multiple microalloying combinations were tried for optimisation of the alloy design. In the initial laboratory investigations, the hot forged steels were austenitised at 1100 °C and then forced air‐draft cooled. A range of mechanical properties (Y.S: 350 to 1100 N/mm², UTS: 500 – 1150 N/mm², El.: 25 – 40%) would be developed primarily by varying the microalloying combinations and the carbon equivalent. Further industrial trial was conducted on forged steels. Connecting rods hot‐forged in successive stages after initial soaking at 1175 °C were forced air draft cooled and tempered at 500 °C. There was good correlation between the mechanical properties of the connecting rods and those of laboratory test specimens. An attempt has been made to characterise the microstructures of these steels by transmission electron microscopy.     

Ductile Fracture Behaviour of Hot Isostatically Pressed Inconel 690 Superalloy

Herein we assess the differences in Charpy impact behavior between Hot Isostatically Pressed and forged Inconel 690 alloy over the temperature range of 300 °C to ? 196 °C. The impact toughness of forged 690 exhibited a relatively small temperature dependence, with a maximum difference of ca. 40 J measured between 300 °C and ? 196 °C, whereas the HIP’d alloy exhibited a difference of approximately double that of the forged alloy over the same temperature range. We have conducted Charpy impact testing, tensile testing, and metallographic analyses on the as-received materials as well as fractography of the failed Charpy specimens in order to understand the mechanisms that cause the observed differences in material fracture properties. The work supports a recent series of studies which assess differences in fundamental fracture behavior between Hot Isostatically Pressed and forged austenitic stainless steel materials of equivalent grades, and the results obtained in this study are compared to those of the previous stainless steel investigations to paint a more general picture of the comparisons between HIP vs forged material fracture behavior. Inconel 690 was selected in this study since previous studies were unable to completely omit the effects of strain-induced martensitic transformation at the tip of the Chary V-notch from the fracture mechanism; Inconel 690 is unable to undergo strain-induced martensitic transformation due to the alloy’s high nickel content, thereby providing a sister study with the omission of any martensitic transformation effects on ductile fracture behavior.     

热处理温度对TC10钛合金棒材组织与性能的影响

以不同变形量热锻TC10钛合金棒坯,得到了直径50 mm的钛合金棒材.分别在780、790、800、810、820℃对TC10钛合金棒材进行固溶热处理(保温时间1h,空冷),研究了热处理温度对TC10钛合金棒材组织与性能的影响.研究结果表明,热锻变形量达到70％的TC10钛合金棒材呈现出良好的强度和塑性;在经过温度为8...     

Hot forging of Ti–6Al–4V alloy preforms produced by spark plasma sintering of powders

Abstract

Powder preform forging is a technology that comprises the preparation of near net shape preforms through powder metallurgy and a subsequent hot forging in order to obtain the desired final shape. In this work, two Ti–6Al–4V powder preforms were sintered through spark plasma sintering (SPS) and then hot compressed in a horizontal dilatometer. Varying the temperature of the process, two full density preforms having different microstructures were produced: sintering at 950°C, a plate-like α was obtained, whereas sintering at 1050°C, an acicular α was obtained. The behaviour of the preforms under hot forging has been studied through hot compression tests carried out in a quenching and deformation dilatometer in a range of temperature and strain rates typically used in hot forging this alloy (850–1050°C, 0·01–1 s^?1). Hot workability has been evaluated by measuring the stresses required for deformation and by analysing both the stress–strain curves recorded during testing and the microstructures after deformation. The main microstructural phenomena occurring during hot compression were individuated. The best conditions for the hot forging operation of SPS preform are temperatures above β transus, where the materials are deformed in a regime of dynamic recrystallisation, at every strain rate.     

On hot deformation of aluminium–silicon powder metallurgy alloys

Abstract

The growing field of aluminium powder metallurgy (PM) brings promise to an economical and environmental demand for the production of high strength, light weight aluminium engine components. In an effort to further enhance the mechanical properties of these alloys, the effects of hot upset forging sintered compacts were studied. This article details findings on the hot compression response of these alloys, modelling of this flow behaviour, and its effects on final density and microstructure. Two aluminium–silicon based PM alloys were used for comparison. One alloy was a hypereutectic blend known as Alumix-231 (Al–15Si–2·5Cu–0·5Mg) and the second was an experimental hypoeutectic system (Al–6Si–4·5Cu–0·5Mg). Using a Gleeble 1500D thermomechanical simulator, sintered cylinders of the alloys were upset forged at various temperatures and strain rates, and the resulting stress–strain trends were studied. The constitutive equations of hot deformation were used to model peak flow stresses for each alloy when forged between 360 and 480°C, using strain rates of 0·005–5·0 s^?1. Both alloys benefited from hot deformation within the ranges studied. The experimental alloy achieved an average density of 99·6% (±0·2%) while the commercial alloy achieved 98·3% (±0·6%) of its theoretical density. It was found that the experimentally obtained peak flow stresses for each material studied could be very closely approximated using the semi-empirical Zener–Hollomon models.     

High-Temperature Compression Behavior of Cast and Homogenized IN939 Superalloy

Hot deformation behavior of IN-939 superalloy was investigated in this work. Hot compression experiments were performed at temperatures of 1273 K, 1323 K, 1373 K, and 1423 K (1000 °C, 1050 °C, 1100 °C, and 1150 °C) at strain rates of 0.001, 0.01, 0.1, and 1 s^?1 up to a true strain of 0.8. Then variations in stress-strain curves as well as changes in microstructures of various hot-deformed samples were studied. At 1273 K to 1323 K (1000 °C to 1050 °C), dynamic recovery (DRV), and at 1373 K to 1423 K (1100 °C to 1150 °C), dynamic recrystallization (DRX), were recognized to be the main mechanisms of the alloy softening during hot compression tests. The relationships between flow stress, strain rate, and temperature were mathematically modeled with three well-known equations, and on the basis of those equations, the activation energy of hot deformation was calculated. For improvement of the proposed models, it was necessary to conduct the investigation at two temperature ranges: 1373 K to 1423 K (1100 °C to 1150 °C), in which DRX occurred, and 1273 K to 1323 K (1000 °C to 1050 °C), where DRV as well as γ′ precipitation happened. For each of the temperature ranges, a different value for activation energy was obtained, which in conjunction with the related model, can be used for simulating the deformation behavior of the alloy.     

A Study on the Oxidation Behavior of Nb Alloy (Nb-1 pct Zr-0.1 pct C) and Silicide-Coated Nb Alloys

In the current work, silicide coatings were produced on the Nb alloy (Nb-1 pct Zr-0.1 pct C) using the halide activated pack cementation (HAPC) technique. Coating parameters (temperature and time) were optimized to produce a two-layer (Nb₅Si₃ and NbSi₂) coating on the Nb alloy. Subsequently, the oxidation behavior of the Nb alloy (Nb-1 pct Zr-0.1 pct C) and silicide-coated Nb alloy was studied using thermogravimetric analysis (TGA) and isothermal weight gain oxidation experiments. Phase identification and morphological examinations were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. TGA showed that the Nb alloy started undergoing accelerated oxidation at and above 773 K (500 °C). Isothermal weight gain experiments carried out on the Nb alloy under air environment at 873 K (600 °C) up to a time period of 16 hours exhibited a linear growth rate law of oxidation. In the case of silicide-based coatings, TGA showed that oxidation resistance of silicide coatings was retained up to 1473 K (1200 °C). Isothermal weight gain experiments on the silicide coatings carried out at 1273 K (1000 °C) in air showed that initially up to 8 hours, the weight of the sample increased, and beyond 8 hours the weight of the sample remained constant. The oxide phases formed on the bare samples and on the coated samples during oxidation were found to be Nb₂O₅ and a mixture of SiO₂ and Nb₂O₅ phases, respectively. SEM showed the formation of nonprotective oxide layer on the bare Nb alloy and a protective (adherent, nonporous) oxide layer on silicide-coated samples. The formation of protective SiO₂ layer on the silicide-coated samples greatly improved the oxidation resistance at higher temperatures.     

The fracture behavior of tungsten wire reinforced superalloy composites during isothermal forging

The isothermal forging behavior of a wire reinforced superalloy powder composite has been examined. The material consisted of a Mar-M200 matrix containing 40 pct by volume of tungsten wire and was prepared by hot isostatic pressing. Specimens were deformed by uniaxial compression at constant temperature in the range 1050 °C to 1180 °C, and at constant true strain rates between 10^-5 s^-1 and 10^-1 s^-1. Loading was normal to the direction of wire alignment. Microstructural defects existing in the as-pressed composites are compared with defects in the forged materials. An upper bound forming limit occurs when fibers come into contact. However, microstructural damage occurs at lower strains which depends on temperature and strain rate. Observed and calculated values of peak flow stress are used to define practical forming conditions for the material which should avoid the formation of internal damage at low strains. Formerly with Mechanical and Aeronautical Engineering Department, Carleton University, Ottawa, Ontario, Canada, K1S 5B6.     

Deformation characteristics of isothermally forged UDIMET 720 nickel-base superalloy

The hot deformation behavior of nickel-base superalloy UDIMET 720 in solution-treated conditions, simulating the forging process of the alloy, was studied using hot compression experiments. Specimens were deformed in the temperature range of 1000 °C to 1175 °C with strain rates of 10⁻³ to 1 s⁻¹ and total strain of 0.8. Below 1100 °C, all specimens showed flow localization as shear band through the diagonal direction, with more severity at higher strain rates. A uniform deformation was observed when testing between 1100 °C and 1150 °C with dynamic recrystallization as the major flow softening mechanism above 1125 °C. Deformation above γ′ solvus temperature was accompanied with grain boundary separation. The hot working window was determined to be in the interval 1100 °C to 1150 °C. Thermomechanical behavior of the material was modeled using the power-law, the Sellars-Tegart, and an empirical equation. The flow stress values showed a nonlinear dependence of strain rate sensitivity to strain rate. The analysis indicated that the empirical method provides a better constitutive equation for process modeling of this alloy. The apparent activation energy for deformation was calculated and its variations with strain rate and temperature are discussed.     

Effect of Yttrium Alloying on Intermediate to High-Temperature Oxidation Behavior of Mo-Si-B Alloys

The oxidation behavior of 0.2 Y-alloyed Mo-9Si-8B (at. pct) was investigated in a wide temperature range from 923 K to 1673 K (650 °C to 1400 °C). Formation of a thin yttrium-silicate scale at the outer layer along with the thick silica-rich inner layer containing Y-rich oxide inclusions was detected beyond 1573 K (1300 °C). A substantial improvement in the oxidation resistance of the alloy could be realized at 1073 K to 1273 K (800 °C to 1000 °C) with the addition of yttrium. The formation of a viscous silica-rich protective scale could prevent the permeation of MoO₃ at the initial stages of oxidation at this temperature regime. The growth of the internal oxidation zone followed a parabolic rate at 1273 K to 1673 K (1000 °C to 1400 °C), and the activation energy values calculated for both the outer oxide scale and internal oxidation zone formation indicated the inward diffusion of oxygen as the dominant rate controlling mechanism. The microstructural and kinetic data obtained for internal and external oxidation indicate that yttrium-silicate scale reduces the inward diffusion of oxygen, thereby improving the oxidation resistance of the alloy at high temperatures in any oxidizing environment.     

Microstructure development during conventional and isothermal hot forging of a near-gamma titanium aluminide

The breakdown of the lamellar preform microstructure in the ingot metallurgy near-gamma titanium aluminide, Ti-45.5Al-2Cr-2Nb (atomic percent), was investigated. Microstructures developed during canned, conventional hot forging were compared to those from isothermal hot forging. The higher rate of deformation in conventional forging led to considerably finer and almost completely broken-down structures in the as-forged condition. Several nontraditional approaches, including the isothermal forging of a metastable microstructure (so-called “alpha forging”) and the inclusion of a short static recrystallization anneal during forging, were found to produce a more fully broken-down structure in as-isothermally forged conditions. Despite the noticeable microstructure differences after forging, conventionally and isothermally forged material responded similarly during heat treatment. In both cases, almost totally recrystallized structures of either equiaxed gamma or transformed alpha grains surrounded by fine gamma grains were produced depending on the heat-treatment temperature. Metallography on forged and heat-treated pancake macroslices was useful in delineating small differences in composition not easily detected by analytical methods.     

The kinetics of static globularization of Ti-6Al-4V

The kinetics of the evolution of the lamellar-colony microstructure to an equiaxed morphology during heat treatment of a hot-worked, two-phase titanium alloy were established. For this purpose, the alpha/beta alloy Ti-6Al-4V was isothermally upset forged at 900 °C or 955 °C and subsequently annealed for times ranging from 0.5 to 100 hours. The degree of the breakup of alpha-phase lamellae into lower-aspect-ratio grains during static annealing was measured and related to the imposed strain estimated using finite-element analysis (FEA). The kinetics of the static globularization of the alpha phase were found to depend on the amount of strain and the annealing temperature but were not affected by the specific deformation temperature in the 900 °C to 955 °C range. These results demonstrated that deformation-induced dislocation substructure has a small effect on the static-globularization process. In addition, the relative globularization kinetics at 900 °C and 955 °C were rationalized in terms of classical coarsening theory.     

The Effect of Hot Working on Structure and Strength of a Precipitation Strengthened Austenitic Stainless Steel

The development of microstructure and strength during forging in a γ′ strengthened austenitic stainless steel, JBK-75, was investigated by means of forward extrusion of cylindrical specimens. The specimens were deformed in a strain range of 0.16 to 1.0, from 800°C to 1080°C, and at approximate strain rates of 2 (press forging) and 2 × 10³ s^-1 (high energy rate forging), and structures examined by light and transmission microscopy. Mechanical properties were determined by tensile testing as-forged and forged and aged specimens. The alloy exhibited an extremely wide variety of structures and properties within the range of forging pzrameters studied. Deformation at the higher strain ratevia high energy rate forging resulted in unrecovered substructures and high strengths at low forging temperatures, and static recrystallization and low strengths at high temperatures. In contrast, however, deformation at the lower strain ratevia press forging resulted in retention of the well developed subgrain structure and associated high strength produced at high forging temperatures and strains. At lower temperatures and strains during press forging a subgrain structure formed preferentially at high angle grain boundaries, apparently by a creep-type deformation mechanism. Dynamic recrystallization was not an important restoration mechanism for any of the forging conditions. The results are interpreted on the basis of stacking fault energy and the accumulation of strain energy during hot working. The significance of observed microstructural differences for equivalent deformation conditions (iso-Z, where Z is the Zener-Holloman parameter) is discussed in relation to the utilization of Z for predicting hot work structures and strengths. Aging showed that the γ′ precipitation process is not affected by substructure and that the strengthening contributions, from substructure and precipitation, were independent and additive. Applications for these findings are discussed in terms of process design criteria. Formerly with Rockwell International, Energy Systems Group     

Deformation and unstable flow in hot forging of Ti-6Ai-2Sn-4Zr-2Mo-0.1Si

The stable and unstable plastic flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) has been investigated at temperatures from 816 to 1010 °C (1500 to 1850 °F) and at strain rates from 0.001 to 10 s^-1 in order to establish its hot forging characteristics. In hot, isothermal compression, Ti-6242 with an equiaxed a structure deforms stably and has a flow stress which decreases with straining due to adiabatic heating. With a transformed-β microstructure, unstable flow in hot compression is observed and concluded to arise from large degrees of flow softening caused by microstructural modification during deformation and, to a small extent, by adiabatic heating. Both microstructures have a sharp dependence of flow stress on temperature. Using the concepts of thermally-activated processes, it was shown analytically that this dependence is related to the large strain-rate sensitivity of the flow stress exhibited by the alloy. From lateral sidepressing results, the large dependence of flow stress on temperature was surmised to be a major factor leading to the shear bands occurring in nonisothermal forging of the alloy. Shear bands were also observed in isothermal forging. A model was developed to define the effect of material properties such as flow softening rate and strain-rate sensitivity on shear band development and was applied successfully to predict the occurrence of shear bands in isothermal forging.     

Plastic Flow and Microstructure Evolution during Thermomechanical Processing of a PM Nickel-Base Superalloy

Plastic flow and microstructure evolution during sub- and supersolvus forging and subsequent supersolvus heat treatment of the powder-metallurgy superalloy LSHR (low-solvus, high-refractory) were investigated to develop an understanding of methods that can be used to obtain a moderately coarse gamma grain size under well-controlled conditions. To this end, isothermal, hot compression tests were conducted over broad ranges of temperature [(1144 K to 1450 K) 871 °C to 1177 °C] and constant true strain rate (0.0005 to 10 s^?1). At low temperatures, deformation was generally characterized by flow softening and dynamic recrystallization that led to a decrease in grain size. At high subsolvus temperatures and low strain rates, steady-state flow or flow hardening was observed. These latter behaviors were ascribed to superplastic deformation and microstructure evolution characterized by a constant grain size or concomitant dynamic grain growth, respectively. During supersolvus heat treatment following subsolvus deformation, increases in grain size whose magnitude was a function of the prior deformation conditions were noted. A transition in flow behavior from superplastic to nonsuperplastic and the development during forging at a high subsolvus temperature of a wide (possibly bi- or multimodal) gamma-grain-size distribution having some large grains led to a substantially coarser grain size during supersolvus annealing in comparison to that produced under all other forging conditions.     

Numerical Simulation using Finite Elements to Develop and Optimize Forging Processes

Forging is one of the main processes used to manufacture metal components for a broad range of applications. This occurs mainly because forged products are highly reliable and present superior mechanical properties. However, lately the competitiveness of forged products has been threatened, since the difference between their superior performance and the performance resulting from other processes has lessened continuously. This has obliged the forging industry to invest in optimizing its processes, saving in raw materials and energy. In this context, the use of numerical simulation of the forging process has become an increasingly reliable tool in seeking this optimization. This study uses the commercial software QForm 3D, version 3.2.1.1, to analyse two forging processes, one by hot forging and one by cold. In the case of hot forging, work on a component with axial symmetry is looked at from which a gear is machined. Currently the part is forged in three stages based on an initial billet with a 7.0 kg mass. Forging is performed in a 40 NM mechanical press with an initial temperature of 1200°C. The hot forging process is optimized and this results in a saving of about 5% in material. In the cold forging case it is shown that the process, as designed, results in laps in the final part, and in possible tool failure due to excess load. In both cases, the material used is DIN 1.7131 (16MnCr5) steel.     

Texture Evaluation of a Bi-Modal Structure During Static Recrystallization of Hot-Deformed Mg-Al-Sn Alloy

In this study, Mg-Al-Sn alloy was hot compressed at 523 K (250 °C) and annealed at 623 K (350 °C) for various times. The initial as-deformed microstructure was partially dynamic recrystallized with strain-induced precipitates on the recrystallized grain boundaries. After annealing at 623 K (350 °C), static recrystallization (SRX) of the bimodal microstructure took place where, at this temperature, no static precipitates formed. The goal of this work was to study the effect of dynamic precipitation on the texture evolution during the SRX process. Progressive texture evolution was studied during annealing by electron backscattered diffraction technique through a microstructure-tracking process. It was found that the grain-coarsening mechanism during the early stage of annealing is not totally controlled by the basal-oriented grains. Also, it was found that the dynamic precipitates may have significant influence in the early texture weakening during annealing of a bimodal structure.     

A Novel Heat Treatment for Excavator Dipper Teeth Manufactured from Low‐Carbon Low‐Alloy Steel

This research examined the metallurgy of production of forged excavator teeth manufactured from low‐carbon low alloy steel. A novel partial forging‐remnant‐heat hardening technology was proposed. The critical temperature of isothermal annealing for 40 Cr steel was determined experimentally to be 650°C. Production dipper teeth were produced using this novel technology. Their properties were compared with dipper teeth produced by the alternative routes. The novel technology provided optimum microstructure, good mechanical properties, and a lower economical cost, congruent with the low carbon economy.     

Effects of Tungsten Addition on the Microstructure and Mechanical Properties of Microalloyed Forging Steels

In the current study, the effects of tungsten (W) addition on the microstructure, hardness, and room/low [223 K and 173 K (?50 °C and ?100 °C)] temperature tensile properties of microalloyed forging steels were systematically investigated. Four kinds of steel specimens were produced by varying the W additions (0, 0.1, 0.5, and 1 wt pct). The microstructure showed that the addition of W does not have any noticeable effect on the amount of precipitates. The precipitates in W-containing steels were all rich in W, and the W concentration in the precipitates increased with the increasing W content. The mean sizes of both austenite grains and precipitates decreased with the increasing W content. When the W content was equal to or less than 0.5 pct, the addition of W favored the formation of allotriomorphic ferrite, which subsequently promoted the development of acicular ferrite in the microalloyed forging steels. The results of mechanical tests indicated that W plays an important role in increasing the hardness and tensile strength. When the testing temperature was decreased, the tensile strength showed an increasing trend. Both the yield strength and the ultimate tensile strength obeyed an Arrhenius type of relation with respect to temperature. When the temperature was decreased from 223 K to 173 K (from ?50 °C to ?100 °C), a ductile-to-brittle transition (DBT) of the specimen with 1 pct W occurred. The addition of W favored a higher DBT temperature. From the microstructural and mechanical test results, it is demonstrated that the addition of 0.5 pct W results in the best combination of excellent room/low-temperature tensile strength and ductility.